Adhesive technology for photonics
Abstract:
Optical fiber communication systems involve generation, guiding and control of light. In such systems, optical devices can be made using different materials, and they are generally bonded with optical fibers using various types of adhesive. Among the myriad types of adhesive, polymeric optical adhesives offer a great potential for low cost and mass production in both device fabrication and component packaging. The performance of photonic components depends mainly on manufacturing process limitations and selection of compatible adhesive materials for the process. This chapter provides an overview of both fiber packaging and device fabrication issues. First, packaging processes of photonic devices are briefly introduced, together with major failure issues. Then the materials and fabrication issues of polymeric photonic devices are discussed. These critical issues need to be addressed in order to reliably manufacture these adhesive-based photonic devices and components.
8.1 Introduction
Owing to the increasing and high demand for photonic components for handling our increasingly networked society, tremendous efforts have been made to open the door for technologies that meet the economic criteria, technical specifications and rapid manufacturing rate requirements without sacrificing high performance.1 Polymeric adhesives are being increasingly used for a variety of applications in photonics because of their structural flexibility, easy processing and fabrication capabilities at low cost and high yields.2 However, as a relatively new technology, polymeric adhesives have some limitations and drawbacks in this application. Therefore, it is very important to understand their limitations and how to use them optimally. The development of an adhesive with advantageous optical, mechanical and thermal properties, which will facilitate the use of adhesive bonding in the joining of various components with different characteristics, is essential. Nevertheless, no single adhesive can be used for a wide variety of applications, and thus one of the essential tasks is finding the most suitable adhesive for a specific application. The initial desirable properties include (i) high bond strength, (ii) fast curing rate, (iii) high stability, (iv) uniform film formation, and (v) low stress generation.3 Therefore, the main objective of this chapter 214 is to outline the issues related to adhesive applications in photonics with a general emphasis on their process optimization in fabricating planar photonic devices and their packaging with optical fiber links.
There are some fundamental differences in the use of adhesives for photonic and for electronic applications. For electrical interconnection, these concerns are related to the presence of stresses and the issue of structural integrity, as well as the need to maintain high electrical conductivity; in optical applications, they are mainly characterized by the refractive index of the component material, and aiming at minimizing any light transmission losses.4
8.2 The major characteristics of adhesives for photonic applications
Polymeric adhesives are versatile, and very importantly, they are able to spread and interact on the surface of the substrate material. For this highly technical application, adhesives are generally designed for both performance and processability. As a result, they can dramatically differ in their characteristics, even if their chemistries are relatively similar. Such types of adhesives should have an appropriate molecular structure to provide controllable optical properties that they need to develop for various optical devices and assemblies. The major characteristics of adhesives for photonic applications are briefly explained in the following:5
8.2.1 Viscosity
Viscosity is defined as the resistance of a fluid to flow. It is very important in the case of thin film deposition, and in dispensing through capillary processes. The viscosities of adhesives range from very liquid (low viscosity) to viscous (high viscosity) and are very sensitive to temperature. It is important to control the fluid flow during the packaging and to control the film thickness in device fabrication.
8.2.2 Curing profile
The adhesive is generally supplied in the form of a reactive, cross-linkable monomer blend. The process of converting it from a liquid to a solid state is termed ‘curing’. The adhesive is generally cured by heat, or by light, or even by a combination of both (known as dual cure). The cure schedule is a combination of the temperature or power and the amount of time to which an adhesive must be exposed in the curing environment for ‘complete’ curing. Different cure schedules result in different properties of the cured adhesive. Different cure schedules as well as different adhesives are necessary for specific applications. The optimal curing condition depends on many variables and is often optimized through testing and experience.
8.2.3 Refractive index
Refractive index (RI or n) is of critical importance for photonics applications such as optical waveguide devices and their packages. It is an important property in determining the light harvesting efficiency of fibers and other devices. However, substituents and backbone atoms that control the refractive index also effect Raleigh scattering losses, and mechanical, thermal and surface properties of the system.6 The optical design of the components and packages is totally based on their unique refractive index characteristics and good optical clarity.
8.2.4 Transparency
Specific applications require transparency windows in specific regions of the spectrum, from the UV/visible to the near infrared. Intrinsic losses result from the physical and chemical structure of the polymer, due to absorption and scattering. Absorption of light in the ultraviolet, visible and near infrared regions of the electromagnetic spectra is directly related to polymer chemistry. Extrinsic losses in optical fibers and waveguides arise from impurities and additives that absorb light, as well as from inclusions and core-cladding interface imperfections that scatter light.
8.2.5 Optic coefficient
The thermo-optic (TO) and electro-optic (EO) coefficients of polymers play a vital role in determining a device’s performance. A large thermo-optic coefficient can favor the reduction of power consumption for both switching and attenuating devices because it corresponds to a small temperature change and thus a small power input for causing the necessary change in the refractive index of polymer optical device.7 Devices based on a large electro-optic response also offer greatly increased rates of information transmission by enhancing optical network speed, capacity and bandwidth for data networking and telecommunications.8
8.2.6 Adhesive strength
High adhesive strength is a critical parameter for multi-layer interconnections that are sensitive to shocks encountered during fabrication, handling and lifetime.9 Therefore, the most important test for any adhesive is that it should give joints that are strong and durable. Although ways do exist of assessing the quality of joints by ultrasonic non-destructive testing, the ultimate test is to measure the force or energy needed to break the joint.
8.2.7 Moisture resistance
Usually, polymeric adhesive materials are sensitive to the surrounding environment, such as high humidity. Small amount of absorbed moisture can change in the refractive index, optical transmission, glass transition temperature and mechanical integrity of this class of materials.10 Thus, device performance may easily be affected by changes in those main critical parameters. Moisture resistance is determined by exposing the cured adhesive to humid conditions or submerging it in a water bath for a defined period of time and temperature. It is expressed as the percent gain when the initial weight and the weight after the moisture exposure are compared.
8.2.8 Operating temperature
Polymeric adhesive materials can easily degrade or vaporize under high temperatures. Therefore, some important borderline temperatures should be distinguished for each type of polymeric adhesive. These are: (i) the maximum continuous operating temperature at which the adhesive can be exposed for an unlimited period of time without any damage; (ii) the maximum intermittent temperature at which it is possible to exceed the maximum continuous operating temperature for a short period (up to several hours) without noticeable damage to the adhesive; (iii) the degradation temperature which is a good indicator of the thermal stability of the adhesive material; (iv) the glass transition temperature (Tg), at which the cured adhesive transitions from a ‘glass-like hard’ to a ‘rubber-like soft’ state. Below the Tg, the adhesive is hard and rigid, and above the Tg it becomes rubber-like (comparable to a pencil eraser).5
8.3 Types of adhesive used in photonics
There is a wide selection of polymeric adhesives available for various applications and no simple way to classify the optical adhesives. They can be categorized according to their chemical family, according to their cure method, or according to their function. In all cases, there is some overlap among the adhesives types. In the fiber-optics industry, the adhesives are mostly based on epoxy or acrylate types. The term ‘epoxy’ refers to a chemical group consisting of an oxygen atom bonded to two carbon atoms forming a ring structure. The simplest epoxy is a three-member ring structure known by the term ‘alpha-epoxy’ or ‘1,2-epoxy’. The term acrylate refers to a chemical group consisting of a carbon–carbon double bond bonded to an ester functional group –COOR. Other types of adhesive are urethanes, silicones, cyanoacrylates, etc.
On the basis of their curing method, they can be classified as thermally curable, light curable or moisture curable.
Based on the adhesive usage, they can be classified as fixing adhesive, bonding adhesive, mounting adhesive, temporary adhesive, structural adhesive, locking adjustable adhesive, coating adhesive, film adhesive or sealant adhesive.
8.4 Major applications of adhesives in photonics
In the photonics industry, adhesives are mainly used for two different purposes: (i) for packaging of photonic components, and (ii) for fabricating planar photonic devices. Other applications include the encapsulation of LEDs, providing conductive media, hosting other active materials, and as anti-reflective coatings for solar cells, displays and contact lenses. This chapter will discuss the first two (main) applications of adhesives in photonic applications.
8.5 Adhesives for photonic packaging
Packaging, and in particular pigtailing, is becoming an increasingly important issue as optical networks move from the wide area network (WAN) domain to the local area network (LAN) domain, with a resultant pressure to increase the production and reduce the cost.11 In general, photonic packaging fulfils two functions:
(i) To provide robustness for the optical component in order to enable it to survive in different application environments such as temperature, humidity and mechanical stress.
(ii) To provide connections between photonic chips and optical fibers so that the optical signals can be transmitted between them.
8.5.1 Typical example of photonic packaging
The passive optical power splitter is one of the key elements in a passive optical network (PON), which equally splits the signal power from the optical line terminal (OLT) in the central office (CO) to each optical network unit (ONU). It can branche and couple waves without converting optical transmissions into electric signals, for connecting households to telecommunication carriers in optical communication networks.12 Planar lightwave circuits (PLC) can provide various key practical devices for such optical networks because of their suitability for large-scale integration, long-term stability, and mass- production capability.13 In order to utilize the integration capability of PLC devices, the input and output fibers have to be connected to the PLCs. The fiber connected splitters are required to exhibit not only high optical performance (such as low loss, wavelength flatness and low polarization dependence) but also long-term reliability. However, to attain a reliable low loss splitter, connection methods must be precise and meticulous.14
Figures 8.1a and b show typical schematic configurations of an unpackaged and a packaged PLC-type optical splitter, respectively. The unpackaged 1 × 8 PLC optical splitter includes a PLC (splitter) chip, and single-channel (input) and eight-channels (output) fiber arrays. The three parts are mounted by an adhesive, as shown in Fig. 8.1a. However, in a packaged device, the mounted optical splitter is secured in its housing by applying fixing adhesive between the Al fixing blocks and the fibers. Moreover, an adhesive is used to bond the rubber boots and the end of fiber arrays in the housing, as shown in Fig. 8.1b. It is very clear that different types of adhesive are needed for bonding different parts of the PLC package. In fact, a large number of different materials are ultimately required for assembling the splitter packages. Table 8.1 shows the physical properties of materials used in an adhesive-based optical splitter package. Figure 8.2 shows the appearance of a bonded PLC optical splitter package in an aluminum package.
8.5.2 Advantages of adhesive bonding in photonic packaging
Until now, soldering, laser welding and adhesive bonding methods have been employed for the assembly of optical fiber arrays. The soldering process’s need for metallization and high heat loads can affect reliability. Laser welding is regarded as having long-term stability, but it also requires metallization, high power lasers, and careful control of a variety of parameters. Adhesives in photonic packaging have many advantages in both cost and convenience as compared to the above-mentioned alternative conventional methods.
Currently, adhesives offer advantages in terms of mass-productivity and low-cost.15 The adhesives described not only perform the function of bonding, but also have the high degree of light transmittance and other properties required to form a bond most suitable from an optics point of view. They can also be cured by both heat and by light, without affecting the fiber alignment. Light curing provides a number of economic advantages: rapid through-cure, low energy requirements, room temperature treatment, and non-polluting and solvent-free formulations. In this way, heat-sensitive materials in the assembly are not damaged during cure.11
8.5.3 Packaging process of photonic devices
The low-cost packaging process of photonics devices is a challenging task, because the design rules for the packaging are significantly more complex than those found in the semiconductor industry. For semiconductors, advances in wafer processing technology have resulted in a packaging process that is automated and planar. However, for optical components, the front-end process is significant, and the controlled assembly processes and critical assembly tolerances create a challenge during the package design. The selection of a particular method depends on a series of criteria, which include reliability, temperature excursions of subsequent packaging processes, package materials used, and constraints imposed by active alignment and automated assembly. Therefore, the selection of appropriate adhesive materials and methods of attachment are very important in determining the stability and reliability of the packaged device.11 A schematic of the typical adhesive-based bonding process of an eight channel fiber array is shown in Fig. 8.3. The packaging process consists mainly of alignment, adhesive dispensing and adhesive curing.
Alignment methods
There are two main alignment methods in device packaging. They are: (i) Active alignment technique: In this method during the assembly process, coupled power from a photonic component is monitored by an optical power meter. The alignment accuracy is optimized by maximizing the measured coupled optical power. The technique applies iterative movements of the components while monitoring the coupled output power to acquire precise positioning. This technique is typically slow and therefore increases the production cost in both semi and fully automated packaging.16 (ii) Passive alignment technique: In this technique, all components are assembled without electronically activating the dies and measuring the light output. Passive alignment determines the aligned position by fiducial marking, device features or by mechanical precision fixture. As a result, the techniques decrease the assembly steps and reduce the manufacturing complexity. Thus, the technique is suitable for mass production due to the fact that it is intrinsically simple and fast.17
Adhesive dispensing
Nowadays, various types of adhesive dispensing tools are available for use in photonic applications. They are hand syringes, micropens, ink jets, dispense jets, and auger valves. Manual adhesive dispensing is not suitable for fast assembling processes. Therefore, the dispensing technique must be compatible with the components assembly process and rates. In addition, the continuous miniaturization of photonic components and devices requires new micro and nano-dispensing techniques. One of the promising techniques for such micro- and nano-dispensing is the valve technique, which provides a dispensing system with accurate control of the dispensing needle tip, as well as improved repeatability of the deposited materials.18
Adhesive curing
The curing process of the adhesive is an important factor in determining the performance of the component. Traditional thermal curing may not be suitable for adhesive curing in photonic applications. Most optical adhesives are photosensitive and require a certain type of light-source for radiation curing. Others are moisture-sensitive and may require a particular level of humidity for curing. Some adhesives can be cured in few seconds and others may require several days to achieve a full cure. They may be curable at room temperature or may require an elevated temperature. Sometimes there is a need to compromise between the cure schedules of cure time and temperature. However, the final properties of the cured adhesive often depend upon the details of the curing process and schedule.19
8.5.4 Major failure issues for photonic packaging
Failure is normally identified by adhesion failure, an increase in delta core pitch and insertion loss. The major failure issues are discussed below:
Surface contamination
As the integrated optical components are getting smaller with the use of advanced materials, contaminant-free active surfaces are crucial to obtain high-yield reliable products. Therefore, an important part for product reliability achievement is the control of contamination to ensure good bondability between various mating surfaces.20 Figure 8.4 shows contamination-induced delamination between the V-groove fiber block shoulder and the cover lid. Contaminants may be introduced in the packages during the fabrication and also from the environment. Contamination is caused mainly by poor process control and is present in the form of residues, mold release agents, anti-oxidants, carbon residues or other organic compounds on the bonding surface. The contaminant may case serous defects, such as:
8.4 Optical microscopic photograph of contamination-induced delamination between V-groove fiber block shoulder and cover lid.
• Greatly degraded bonding surface of the photonic package.
• Weakened adhesion force resulting in poor performance. (When such a phenomenon occurs, an optical fiber may readily slip off from the substrate due to adhesion failure.)
• Osmotic pressure built up at the contaminant surface, initiating delamination.
• Less hydrogen bonding between adherents causing a decrease in the adhesion strength. (The adhesion depends strongly on hydrogen bonds between the oxygen atoms on the quartz layer and the hydrogen atoms of the bonding adhesive). Hence, the total adhesion depends strongly on the cleanliness of the bonded surface and the availability of oxygen atoms to form hydrogen bridges.21
Therefore, developing suitable photonic packages to minimize deterioration, delamination, cracking or peeling is essential. Because impurities must be thoroughly removed before the bonding process in order to eliminate the delamination problem and enhance the adhesion of photonic packages, plasma treatment-based surface modification is used for surface cleaning and increased surface roughness before bonding. Plasma’s physical and chemical energy can be used to remove micron-level contamination. Also, roughening of the surface will increase the total contact area at interfaces, which significantly increases adhesion between the adhesive and the substrate.22
Entrapped air bubbles
Air bubbles entrapped in the adhesive are also a concern for reliable adhesion. Air bubbles may be entrapped during the adhesive flow process. Figure 8.5 shows optical microscopic photos of entrapped air bubbles in an adhesive-based eight channel fiber array package. Trapped air bubbles are best avoided because they can cause adhesive delamination when the array is exposed to temperature cycling. Such defects also provide a propagation path for stress cracking. The voids can nucleate at the interface and propagate through the interconnection, resulting in a loss of adhesion and maybe failure under low force.23
Misalignment
The position of the optical device in passive alignment is defined by the geometry of the device. The impact of this misalignment can be described by the position of the packaged fiber in the V-groove as shown in Fig. 8.6. If the fiber makes contact with the V-groove side walls, the center of the fiber can be located, as the radius is known.11 Therefore, the surface feature of the V-groove side wall is another significant factor affecting the fiber alignment. If the center of the fiber has an offset less than 2 mm to the theoretical center position, the fiber is declared as an aligned fiber. The buoyancy of the adhesive under the fiber can cause it to float upwards. Usually the cover plate is used to press the fiber against the side-wall of the V-groove. However, the optical properties are affected if the pressing process is not well-controlled. The stress applied by the pressing cover may deform and even damage the optical fiber. Moreover, the pressing process may introduce voids and bubbles in the adhesive and this leads to reliability problems.23
Uneven curing of the adhesive
Uniform adhesive curing and uniform adhesive bondline are essential for minimizing the stress within the fiber package as well as for preventing actual failure. Uneven curing of adhesive in the package can generate high interfacial stresses upon heating or cooling of the structure during fabrication, assembly, or in field use. Propagation of the resulting delamination along an interface can degrade or destroy the functionality of the system. Therefore, interfacial delamination due to the uneven curing of adhesive, is one of the primary concerns in photonic package designs. Alignment and shrinkage of the fiber array also depend on effective light-ray penetration during the adhesive curing process. Due to the complexity of interconnects, it is also interesting to consider how illuminated light propagates through the uneven interfaces. High light-reflectance from any interface of the assembly reduces the light intensity for the next layers and induces uneven curing of adhesive. Shadowing due to light bending or optical element shape can also cause incomplete or uneven curing.24
The uneven-curing-induced delamination effect has been extensively studied.10 The shaded area and delamination were much greater when light was exposed from the bottom side of the V-groove rather than from the top side, as compared in Fig. 8.7. This is due to the geometrical shape of the bonding element. Minimum shaded area and delamination were found at the middle fiber when light was exposed from the top side. These were greater when the light was exposed from the bottom side, and were at a maximum at the outermost fiber. These effects are very severe for large value of An, the refractive index difference between the adhesive and cladding materials. It is concluded that that the delamination problem can be minimized by using a UV-curable adhesive having the same or slightly higher reflective index than that of the cladding material. It is also recommended to light expose from the top side, and the lower pitch V-groove is preferred for fiber packaging. This type of fiber array results in a more reliable assembly and also increases productivity.
Curing shrinkage
Shrinkage is the volume prior to curing compared with the volume attained when fully cured. This volume shrinkage will cause a micro shift between the adhesive surface and the device. Movement or relaxation of the fiber bond joint during the cure process is a typical causes of this failure mode. During the long post-cure cycle, most adhesives that have been gelled only, will relax as they begin to heat, and transform into a fully cross-linked polymer only after passing through a period of lower viscosity and relaxation.25
Coefficient of thermal expansion mismatch
When packaged, the coefficient of thermal expansion (CTE) of the adherents, fiber and bonding materials have to be well matched. If there is a difference in CTE among the constituent materials, stresses and strains in the packages are bound to occur. The stress concentration of the bonding materials in the V-groove caused by these phenomena cannot sufficiently adapt to the thin bonding layer. The stress caused by the bond increases particularly with a larger CTE mismatch and higher Young’s modulus of the adhesive.26 Alternatively, increasing humidity causes expansion and relaxation of the adhesive.27 Any misalignment among the optics will cause optical loss, resulting in out-of-specification product.
The effect of CTE of the adhesive on reliability has been investigated.21’28 Two adhesives of different CTE were used.21 The samples were subjected to thermal shock and a highly-accelerated stress test (HAST) for the reliability study. Interfacial delamination, delta core pitch and insertion loss measurements were used to characterize the packages. The adhesive having less CTE mismatch with the constituent materials has showed the best performance. In order to reduce such degradation in the performance of the fiber array assembly, it is recommended to select bonding adhesives having a close CTE match with the bonding substrate.
In an unpackaged optical device, the CTE mismatch-induced stress can relax without increasing insertion loss during the environmental test. However, in the packaged device, the stress cannot release and increases the insertion loss. The CTE mismatch effect also induces fiber bending inside packages, which ultimately initiates fiber cracks. Numerical simulation has been used to confirm the experimental results and interpretation. These numerical calculations were in good agreement with the experimental measurements.28
8.6 Adhesives used in photonic devices
The use of photons instead of electrons to transmit communication signals offers significantly greater bandwidth and speed. Therefore, particularly for applications involving high-speed processing or communications, photonic devices provide a significant performance edge. Optical waveguide is an alternative to copper wire, which is used to carry signals for voice and data. Waveguide technology shares a number of advantages for optical interconnections. It provides well-controlled and accurately directed connections for optical transmission. They do not require impedance matching, as do-high speed electrical interconnections, and thus support extremely high bandwidths. It can also provide additional functions to enhance the coupling efficiency or to incorporate component redundancy.29’30
8.6.1 Example of a photonic device
Most optical devices including lasers, modulators, switches, power splitters, directional couplers and filters are in the form of optical waveguides. Optical waveguides are dielectric structures where the central material, called the core, is surrounded by another material, called the cladding, of a lower refractive index. Figure 8.8 schematically shows a typical structure of a fully buried or embedded optical waveguide. It shows that the fabrication process of the optical waveguide includes the deposition of a multi-layered thin-film structure. This multi-layered structure supports electromagnetic waves, which are guided in the core region as the wave propagates along the z-direction. These waves are stable and their energy generally stays mostly within the core region.31
8.6.2 Materials issues for photonic devices
Several light-guiding inorganic materials capable of multiple functions (such as modulation, switching/attenuation, wavelength conversion, and amplification) are under intensive investigation, including lithium niobate (LiNbO3), silicon dioxide (SiO2) on silicon, and III-V compound semiconductors. Such materials exhibit polarization-dependent loss for many active and passive devices. Sharp and polarized absorption and emission lines result in an amplification bandwidth too narrow to be useful for multiple wavelength systems such as wavelength division multiplexers and bit-parallel multi-wavelength interconnects. Such characteristics limit their use for optical gain circuits in specific fiber systems unless polarization converters are used. These materials are strictly substrate selective due to the lattice matching required for single-crystal thin-film growth. Finally, the fabrication costs associated with these materials are very high, which seriously jeopardizes the commercialization of the end products.32 As an alternative, organic adhesives, typically composed of polymers, offer many advantages over the aforementioned materials for the fabrication of optical devices.33 These include:
• Polymers are usually in an amorphous state that can provide a wider bandwidth of amplification if an appropriate gain mechanism is identified.
• The microstructure can be easily engineered to provide desired optical parameters such as bandwidth of transparency, high electro-optic (EO) coefficient values, and temperature stability for specific applications.
• The thermo-optic (TO) coefficient (∆n/∆T) of polymeric material is an order of magnitude more than that of SiO2; as a result, a polymer-based thermal optical switch can potentially perform both switching and variable attenuation functions simultaneously. In the case of metropolitan-area-network applications that require arrays of optical switching devices with millisecond switching times, a combined switch and variable optical attenuator is very attractive from the viewpoint of cost and power consumption.
• Unlike any of the inorganic materials that cannot be transferred to other substrates, the polymeric passive and active devices proposed herein can be easily integrated on any surface of interest.
Of the different kinds of polymer used in this application, epoxy is the best suited due to: 34
• Low cost compared with acrylic and polyimide based polymers.
• High thermal stability. The resins have a high Tg (ca 200 °C) and are therefore expected to have good thermal stability.
• Precise controllability of refractive index. By mixing several epoxy resins together, the refractive index can be precisely controlled in the region of 1.48–1.60 with a 0.001 order of accuracy.
• Photo-curable property. The resin can be easily patterned with a conventional mask process using a UV light source.
• Acceptable optical loss. This issue can be minimized when epoxy is used as the cladding layer.
8.6.3 Processing issues in device fabrication
Recent advances in the development of organic molecules and polymeric epoxy adhesives with high optical quality and performance have led to a maturing of the PLC device field and have brought commercialization closer. However, the physical, optical and mechanical properties of the epoxy adhesive are very critical for reliable fabrication and mainly depend on the processing. They are all interrelated and it is not possible to look at one property alone to determine suitability. A balance of these properties is obviously needed for reliable fabrication of PLC devices. Only a clearer fundamental understanding of the deposition, fabrication and degradation mechanisms can allow manufacturers to develop highly reliable, low-cost and better performance polymer photonic devices.35 The performance of polymeric epoxy adhesive-based PLC devices depends mainly on the following issues:
Thin film deposition
Polymeric thin film is the fundamental building block of polymer-based optical devices. One of the simplest and most common techniques of applying thin polymers films onto wafers is spin coating. The process solely involves the dispensing of an excessive amount of fluid onto a stationary or slowly spinning substrate and then spinning it at high speed. Therefore, it is useful to understand the behavior of complex mixed solutions under conditions of rapid fluid flow and convectively-driven evaporation that occur during spin coating.36
Curing conditions
The curing of epoxy adhesive is of a great importance in applications. It is a process of conversion from a liquid to a solid state, accomplished by a chemical reaction in the epoxy resin. Therefore, curing is believed to be critical to develop the ultimate mechanical and optical performance of polymer devices. A minimum degree of curing is needed to provide a certain level of that performance. An under-cured adhesive is not optimized to acquire those performances, especially in regards to environmental resistance and dimensional properties. However, over-cured adhesives also become brittle, resulting in greater stress on the adhesive bond and interfaces.37,38
Stability
In this application, after the deposition of the initial adhesive layer, additional heating and chemical etching is required for the fabrication of the subsequent step of photonic device manufacture. Therefore, the polymeric adhesive material should have sufficient thermal and chemical stability to withstand typical fabrication processing and operating conditions with good performance.39 Knowledge of stability, degradation and mode of decomposition under the influence of heat and chemical solution is very important in process optimization. The threshold gives an indication of the ultimate processing conditions that can be used during the subsequent fabricating and operating processes. A proper understanding of potential degradation mechanisms can greatly aid the appropriate selection of material and process parameters in the fabrication and extend the outdoor longevity of the product.40’41
Mechanical strength
The curing conditions and stability of the polymeric adhesive film have an influence on the strength of the adhesion of the coated film to the substrate. High adhesion strength is a critical parameter of multi-layer interconnections, which are fragile to the shocks encountered during fabrication, handling and lifetime. Concerning this technology, surface finish or surface roughness is another important parameter that controls the state of adhesion. The interfacial strength also depends on the environmental and processing conditions of subsequent fabrication and operation processes.42 Thermo-mechanical failures are caused by stresses and strains generated within an optical device due to mismatch in the coefficient of thermal expansion (CTE) among different materials during thermal loading from the environment or internal heating in service operation.25
Surface condition of adherend
Continuous effort is being made to improve interfacial adhesion in polymeric adhesive-based PLC devices. Interfacial strength is fundamentally related to surface attachment, and the properties and condition of the adherend surface are of paramount importance. The silicon wafer is commonly used as a substrate for the fabrication of photonic devices. The main advantages of silicon wafers is that it is easy to cut into pieces as the samples need to be sectioned to the area of interest, or for easier handling, and therefore there is no need to polish the end faces of the devices. However, the adhesion of polymeric adhesives to the silicon surface is comparatively poor.43 An oxidized silica-containing (thin layer of silica on silicon) silicon wafer can be use as a substrate and is becoming very attractive due to its matching refractive index with optical polymers. Thermal oxidation of silicon is easily achieved by heating the substrate to temperatures typically in the range of 900-1200 °C. Another potential alternative is to use a thin metal layer on a silicon wafer to improve the adhesion of the polymeric adhesive to the substrate. A thin metal layer can be deposited rapidly on silicon wafers by a typical deposition process such as sputtering, or thermal or E-beam evaporation. Thus, a careful treatment and modification of the surface before the adhesive deposition is essential for realizing a strong interfacial bond.27
8.6.4 Fabrication techniques of planar polymer photonic devices
In this application, the critical issue is the ability to pattern the micro or nanostructure in a high-throughput and cost-effective manner. The patterning of polymer photonic films can be done by many techniques. Typical techniques that have attracted great attention in recent years are given in Fig. 8.9 and the limitations are briefly discussed as follows:
8.9 Overview of the fabrication processes of a polymer channel waveguide using different techniques.
Photolithography technique
The very common photolithography process can be applied to the simple polymeric material but still faces many problems regarding the material and process. The technique includes many steps such as metal deposition as a hard mask, photoresist deposition and patterning, reactive ion etching, and, finally, metal and photoresist etching. The problems associated with the process include chemical attack during the metal and photoresist etching, roughness, and stress-induced scattering loss.44
Embossing technique
The embossing technique is appropriate for mass production and fabrication of high-precision polymer microstructures for optical components. The method additionally needs application of optimum pressure and temperature/UV to the polymer. Therefore, it is essential to consider how mixed solutions behave under the conditions of applied rapid pressure and temperature/UV through the embossing master. Inevitably, the replication process can introduce an additional roughness to the waveguides, which adds significant scattering loss contribution to the overall waveguide loss. The method, however, has also some limitations regarding the achievable aspect ratio and is not suitable for constructing vertical waveguide interconnects among multiple layers. Until now, the problem of fine alignment has not yet been solved. The fabrication of an ultra-high resolution mold is also a difficult task. Fabrication of reliable and cheap imprint devices is therefore clearly another challenge.45
Direct laser writing technique
The direct UV or E-Beam writing technique has also recently been used since it is much simpler for the fabrication of complex devices. The technique involves beam irradiation on photosensitive material for waveguide fabrication, which locally changes the refractive index of polymer film. However, the overall performance of the optical device mainly depends on beam intensity, wavelength, spot size, scan speed and energy fluencies. Parameters such as post-exposure baking time, vertical and horizontal resolution, hardness and sensitivity are also all very crucial. Sophisticated equipment and numerous beams scanning a large area are required.46–49
Laser ablation technique
Laser ablation is a low-cost manufacturing technique and is compatible with current PCB production for optical interconnections. However, depending on the laser wavelength and the material, this can have the characteristics of ablative photodecomposition, or rapid heating and vaporization. These processes may introduce material degradation. Sidewall roughness induced scattering loss is also an issue with this technique.50
There are also some other ways of fabricating polymer optical waveguides, such as injection molding,51 diffusion and ion exchange,52 etc. All the techniques have some sort of process induced limitations. Therefore, there is a major need for practical as well as scientific information about the proper handling and properties of polymeric materials for the fabrication of reliable photonics products. Recent studies38,43,53 have focused mainly on proper polymer processing and the fabrication of reliable photonics devices. The challenges are to achieve compatibility with other electro-optic systems and processes, end-use reliability, reproducibility, stability, acceptable performance, and cost, commensurate with added value.
8.7 Typical challenges for reliable fabrication of photonic devices
The most typical challenges for the production of reliable polymer photonic devices are uniform adhesive curing, higher stability, higher adhesion strength, and environmental reliability. They can all cause device failure at different stages. Figure 8.10 is an example of the interfacial failure of a channel waveguide. The sample was prepared by a photolithography technique using benzocyclobutane (BCB) as core, and silica and epoxy as the lower and upper cladding material respectively. The figure shows that there is considerable delamination between the polymer films at the interfaces. To find out the root cause of the delamination or failure, extensive research work has already been done, which included substrate surface analysis and optimum polymer curing. It was found that the bulk polymer on the silicon substrate showed good behavior and no interfacial delamination. However, the spin-coated epoxy adhesive showed degraded properties for the fabrication of waveguide device where most failures often occur. Figure 8.11 shows a typical failure and peeling out of the thin film from the substrate. Therefore, the spin-coated polymer films have been extensively studied. Some of the recent findings are summarized as follows:
8.7.1 Degree of curing of spin-coated polymeric films
Thin polymer adhesive films can be deposited by a variety of techniques with different complexity and applicability. The choice of deposition technique depends upon the physicochemical properties of the material, the film quality requirements and the substrate being coated. The final properties of these films also depend on their morphologies, which are largely affected by the polymer chain orientation and the state of aggregates. One of the simplest and most common techniques of applying thin films onto wafers is spin coating.
The degree of curing of a spin-coated polymer film (over three locations of the deposited sample indicated in Fig. 8.12) was measured using the same FTIR method. Figure 8.13 shows the effect of spin coating on the curing rate of the epoxy adhesive at the different locations for different UV exposure times. The figure shows that the curing rate decreases abruptly for the spin-coated adhesive film compared with the adhesive that was not spin-coated. The degree of curing also varies at different locations of the spin-coated substrate. At the center of the substrate it is higher than at the other locations. The slower reaction rate is due mainly to changing the material properties during spinning. Spin-coating involves transient fluid flow and mass transfer in a medium that experiences drastic changes in properties. Both fluid viscosity and solvent-solute diffusivity can change by several orders of magnitude from the initial stages.54
8.13 Effect of spin coating on the curing behavior of epoxy adhesive at different locations of the substrate for different UV exposure time (min).
Factors affecting the curing rate of spin-coated polymer film
The spin-coated liquid usually contains a volatile solvent that evaporates during spinning, leaving behind a thin solid film. The solvent during spin coating is most influential for obtaining good spin-coated films. Its evaporation has a tremendous influence on the control of the final film thickness after mass transfer and spreading of the solvent.55 Evaporation rates are influenced by such factors as the temperature of the fluid, ambient temperature, heat conductivity, molecular association, molecular weight, vapor pressure, surface tension, humidity, latent heat of evaporation and vapor density. A high degree of uniformity in the evaporation rates leads to a similar uniformity in the final polymer film.56
Solvent evaporation couples liquid-phase diffusive transport of solvent toward the interface to gas-phase convective mass transport of solvent away from the interface. The solvent vapor is carried away in a stream of air that is drawn down axially toward the surface and driven radially across it by a centrifugal pumping action created by the spinning substrate.55 Solvent is continually evaporating from the film, altering the material properties (e.g. viscosity, diffusivity, vapor pressure, surface tension) and eventually producing a film of finite thickness. As a result, the reaction rate of the spin-coated adhesive is decreased due to the following two factors.
(i) Solvent evaporation. Owing to evaporation during spinning, the viscosity of the adhesive increases. Increased viscosity reduces the mobility of the polymer molecules and slows the reaction rate. The number of solvent molecules also decreases and the free volume increases.57 Owing to the lack of solvent molecules, the reaction rate and the total amount of reacted species is reduced, hence the lower curing degree of spin-coated adhesive films.
(ii) Composition gradient. Evaporation also establishes a composition gradient at the surface as volatile species leave and less volatile components are left behind.58 The top surface becomes a high volatile species-rich layer and the bottom is of low volatile species. Constituents need to further inter-diffuse for chemical cross-linking reaction during the UV exposure. This takes additional time which decreases the curing reaction rate substantially compared to the adhesive that has not been subjected to the spinning process.
The curing degree of a spin-coated adhesive is lower than that without spinning, even after a long (10 min) UV exposure. At the center of the substrate, the rate of evaporation is low due to the low angular momentum of the fluid. On the other hand, higher evaporation rates are caused by the combination of both radial and axial flow at the edge of the substrate. Therefore, the evaporation, as well as the composition gradient, varies from the center to the edge of the substrate. They are lower at the center and higher at the edge. The cross-linking reaction rate is higher in the center (Location 1) than at other locations of the substrate. Owing to the variation in curing degree and surface tension, the topography also varies at different locations of the spin coated adhesive film. Figure 8.14 shows the surface topography at the corner of the spin-coated adhesive film after curing for 10 min. The figure clearly shows how the topography differs from the center to the corner due to the spinning process.
Before curing, the fluid properties dictate a constant angle at the solid–liquid–gas interface. The result is a thick edge bead confined at the wafer’s edge. Another reason for such a film pattern is the increased friction with air at the periphery, resulting in an increased evaporation rate that causes a dry skin to form at the edge and impedes fluid flow. As a result, the fluid in the center of the substrate still being driven out by centrifugal force begins to flow over the dry film and dries, resulting in a build-up of the edge bead.59 The thickness and the viscosity of the edge bead is very high compared to other locations, so the degree of curing at Location 3 of the spin-coated adhesive film is very low.60 As the free volume shrinks during the curing of the film, the film size is reduced and a few millimeters of the substrate becomes unoccupied by the adhesive.
The uneven curing induces the internal stresses, shrinkage and interfacial delaminations in the devices. This type of adhesive should not be used in spin-coating for the fabrication of polymeric thin films of optical devices. Therefore, when designing coating solutions, solvents should be selected with relatively low vapor pressures to reduce the solvent evaporation; also to reduce the composition gradient during spinning and the variation of curing speed observed after spin coating. This is one of the major criteria for selecting a spin-coating solution for the fabrication of thin films for PLC devices.
8.7.2 Stability of spin-coated polymeric adhesive films
The curing reaction rate of a spin-coated epoxy adhesive is much slower than that of one without spinning. The reaction rate at the center of the substrate is also higher than at other locations. The slower reaction rate is mainly due to changing material properties during spinning.38 These changes, as well as the curing degree or cross-linking density of the cured material, greatly influence the stability of the adhesive.
Thermal stability
Thermo-gravimetric analysis (TGA) can be used to explore the thermal stability of spin-coated epoxy adhesive. Figure 8.15 shows a typical TGA diagram (weight loss during the temperature rise) of a cured epoxy adhesive (Samples A and B) that was not spin coated. The weight-loss profile for those two samples was similar throughout the weightloss process and differed only in initial degradation temperature. A lower initial degradation temperature was observed for Sample A (70 °C) than for Sample B (100 °C). Because Sample A was not post-cured by heat after UV curing, Sample B was more stable than Sample A. Sample A may have contained absorbed moisture in the adhesive existing in a state of free or loosely-bound water which started to evaporate at low temperature. It also indicates that UV light energy only is not sufficient for optimum curing of UV-curable epoxy adhesive, which needs post thermal exposure for moisture evaporation. The major concern involving the presence of moisture at elevated temperatures is hydrolytic degradation. Hydrolysis is a chemical change, which occurs when moisture is present above or near the glass transition temperature (Tg) of the polymer. Hydrolytic degradation causes random chain scissions to occur, which brings about a reduction in molecular weight and, in turn, a reduction in the mechanical integrity of the cured adhesive. During post thermal exposure of an anhydride-cured epoxy, the cross-linking density also increases.61 The anhydride group and hydroxyl group of the resin react to form ester cross-links. These ester networks are more thermo-stable than other types of linkage.62
The thermal stability of a spin-coated adhesive film (Sample C, over three locations of Fig. 8.12) was also measured using the same TGA method. Figure 8.16 shows the corresponding TGA curve of Sample C at three different locations alongside the TGA curve of Sample B. A comparison of the stability change due to the spin-induced degradation of the epoxy adhesive determined here with other previously found characteristic temperatures is shown in Table 8.2. The results show that the thermal stability decreased for the spin-coated adhesive film compared with the adhesive that was not spin coated.
Thermal stability also varied at different locations of the spin-coated substrate. At the center of the substrate it was higher than at the other locations. The lower thermal stability is mainly due to changes in material properties to various degrees at various location of the spin-coated epoxy adhesive during the spinning process. As previously described, the spin-coated liquid usually contains a volatile component that evaporates during spinning, leaving behind a thin solid film. Solvent evaporation from the film alters the material properties (e.g. viscosity, diffusivity, vapor pressure, surface tension). The high degree of uniformity in the evaporation rates leads to a similar uniformity in the final polymer film properties.57
Chemical stability
Chemical stability is the material’s ability to withstand change from chemical contact. This issue, involving corrosive fluid exposure, should be evaluated to ensure chemically stable polymeric thin films for optical waveguides. An immersion test shows that the spin-coated epoxy surfaces deteriorate to various degrees at various location of the coated epoxy adhesive. Figure 8.17a shows an optical micrograph of Sample B after immersion in a nickel etchant. It is very clear that the without-spin sample surface was chemically stable in that solution with almost no change in surface morphology. However, the spin-coated sample (Sample C) showed different results. Figure 8.17b–d displays the optical micrographs of different locations on the spin-coated cured adhesive after immersion in the etchant. The chemical attack led to porosity at the polymer interface, which resulted in an excessive increase of optical loss. However, it was more stable in a chromium etchant than in the nickel. Figure 8.18 shows the optical micrograph of Sample C after the immersion in chromium etchant. The spinning also affected the refractive index of the cured adhesive. Figure 8.19 shows the refractive index of the spin-coated adhesive (Sample C) before and after immersion in chemical solutions. Before immersion, a higher refractive index was found at the corner and after immersion the higher drop also indicated lower stability at that portion. Moreover, the change of refractive index in the chromium etchant was lower than that in the nickel.
8.17 Optical micrograph of the cured epoxy after immersion in nickel (Ni) metal etchant: (a) without spin, (b) spin-coated Position 1, (c) spin-coated Position 2, (d) spin-coated Position 3.
Factors affecting the stability of spin-coated polymer film
Earlier studies have found that the curing reaction rate of a spin-coated epoxy adhesive is much slower than that of the adhesive without spinning. The reaction rate at the center (Location 1) is also higher than at other locations on the substrate. The slower reaction rate is due mainly to changing the material properties during spinning. It clearly indicates the spin-induced degradation of spin-coated epoxy adhesive during the spinning. Compared with the no-spin material, the thermal and chemical stability decrease is due mainly to:
(i) Mechanical degradation during spinning. Under the influence of spinning forces, intermolecular interactions between certain molecules at certain sites of the polymer are disrupted. Spin-induced main chain rupture might also initiate the de-polymerization of linear polymers.63 This type of chain scission plays a significant role in the thermal and chemical stability of polymeric materials.64 During the heating or chemical etching, less energy is needed to fragmentize the polymer, hence there is a lower stability in spin-coated polymeric film.
(ii) Changes in the ratio of resin and hardener. Epoxy resin is more volatile than the hardener (amine). Therefore, the evaporated amount of epoxy resin is larger than that of the amine during the spinning. As a result, the correct mix ratio of amine and epoxy, to ensure the complete reaction of that reactive component is disrupted. Since the amine molecules ‘co-react’ with the epoxy molecules in a fixed ratio, unreacted amine remains within the matrix after the cure reaction; this alters and affects the final properties of the adhesive.
(iii) Void formation. Owing to the variation in spinning force, surface tension and the topography also vary at different locations in the spin-coated adhesive film. Figure 8.14 clearly shows how the topography differs from the center to the corner, due to the spinning process. As shown, there are a large number of voids at the corner and almost no voids at the center of the substrate. The void formed during the spinning and curing of the epoxy adhesive released from the coated adhesive layer at the heating or etching step and induced the excessive weight loss or porosity.
(iv) Cross-linking density. Lower curing degree, higher mechanical degradation and more void formation decreases the cross-link density. Decreasing cross-link density – that is, increasing the distance between reactive sites, usually has the effect of reducing thermal and chemical resistance by decreasing the compressive and tensile modulus as well as the impact strength. Less thermal or chemical energy is required to degrade a given mass into its volatile products.65
At the periphery of the substrate, the polymer experiences the highest spinning force, more voids and lower cross-link density. Therefore, the lowest thermal and chemical stability is observed at the periphery of spin-coated adhesive due to the rapid fluid flow and convectively-driven evaporation that occur during spin coating. On the other hand, at the center of the substrate, the highest stability is observed due to low stress, no voids and higher cross-linking density.
To overcome the problems, it is recommended that lower spin speeds be used during spin coating. Also less volatile reactive components of the spin coating solutions are preferred, having higher intermolecular forces that allow a greater part of the thinning behavior to occur without significant degradation of materials during the spinning process. Adhesives were also found physically and chemically more stable in chromium etchant solution than in nickel etchant. Therefore, chromium is also proposed to be used as the hard mask in photolithography processes during the fabrication of PLC devices.
8.7.3 Interfacial adhesion of spin-coated polymeric films
As the curing reaction rate of spin-coated epoxy adhesive is much slower than that of the adhesive without spinning, the reaction rate at the center (Location 1) is also higher than at other locations on the substrate. The slower reaction rate is due mainly to the material’s changing properties during spinning. Lower thermal and chemical stability for the spin-coated adhesive was also found due to rapid fluid flow and convectively-driven evaporation that occur during spin coating. This was also the case at the periphery of the spin-coated polymer substrate due to the highest spinning force experienced, more voids and low cross-link density. On the other hand, at the center of the substrate, higher stability was observed due to the lowest stress, no voids and higher cross-linking density. These results clearly indicate the spin-induced degradation of the adhesive during the spinning.53
Different adhesion strength at different location
The properties of an adhesive depend upon the degree of cross-linking or completion of the curing reaction. During the curing, the polymer chains become locked together and their movement consequently becomes restricted. Cross-linked polymer chains are chemically bound together to give a three-dimensional ‘chicken wire’ molecular structure or chemical network. The higher the curing degree, the stronger the chemical bonding and the better adhesion strength at the adhesive interface. Shear strength was measured for investigating the interfacial adhesion of the spin-coated epoxy layer on a silicon substrate. The result shows that the adhesion strength is higher at the center and lower at the boundary of the substrate. Figure 8.20 shows the average shear strength of the deposited and cured adhesive layer at three different positions; the figure shows results for the heat-exposed adhesive and also the non-heat-exposed adhesive. Different adhesion strengths were found at different positions of the same sample. As the curing degree and stability of the adhesive are higher at the center and lower at the border side, consequently, the adhesion strength is also higher at the center and lower at the border side.
Interfacial adhesion after heat exposure
After heat exposure, interfacial adhesion decreases substantially at all locations of the substrate. The heat exposure was performed at a high temperature, 250 °C, which is 130 °C above the Tg (120 °C) of the adhesive material. The epoxy adhesive exhibits a lower coefficient of thermal expansion (CTE) (64 ppm/°C) below its Tg than that above its Tg (143 ppm/°C). The difference between the CTE of the epoxy adhesive and silicon is also very high (4 ppm/°C for silicon vs. 143 ppm/°C at higher temperature). Therefore, the adhesion strength of the heat-exposed sample substantially decreased due to the following reasons:
(i) Above Tg, the amorphous or semi-crystalline polymer is transformed to a rubbery viscous state which lowers the mechanical integrity of the adhesive.
(ii) When the temperature rises in the solid, it expands, and this thermal expansion is directly proportional to the CTE of the material, its length and the temperature change. In this bi-material ‘sandwich’ structure, the layers are rigidly connected to each other. Therefore, when they attempt to expand in accordance with their CTE, each layer imposes a force along the interface to expand in an identical manner, and causes stresses to appear. The generation of these stresses may be understood from the sketch presented in Fig. 8.21. The generated stresses at the interface decrease the adhesion strength.
8.21 Free-body diagram of bi-material structure. Ta and TS are the tensile strengths of the adhesive and the substrate respectively; Ma and MS are bending moments of the adhesive and substrate respectively.
(iii) Figure 8.21 also shows that all the stresses are ultimately concentrated at the border side. Therefore, the highest stresses are experienced at the border side and the lowest at the center. Due to the highest developed stresses at the interfaces of the border side, the loss of mechanical strength is also higher in that portion. As a result, the highest drop of interfacial adhesion is observed at that position.
It follows that the adhesive should not be exposed at above Tg during the subsequent fabrication process or in its operating life. In other words, an adhesive material with a Tg lower than the curing temperature of the waveguide core material should not be used as the lower cladding of an optical waveguide. (Otherwise the adhesive has to be processed at or above its Tg thus weakening its adhesive strength.)
Interfacial adhesion on plasma-treated substrates
Considering the mechanical interlocking theory of adhesion, the substrate was plasma-treated to increase the surface roughness and adhesion strength. Figure 8.22 shows a 3-D AFM image of untreated and plasma-treated substrate surfaces for different plasma conditions. Table 8.3 shows the surface roughness data for these different pre-treatment conditions.
Table 8.3
Surface roughness of silicon substrate for different surface conditions
Rp-v, height difference between the highest and lowest point in the measured region.
8.22 3-D AFM image of untreated and plasma-treated substrate surface for different plasma conditions: (a) without plasma treatment, (b) 100% SF6 plasma treatment condition, (c) 90%-SF6 + 10% O2 plasma treatment condition, and (d) 80 % SF6 + 20% O2 plasma treatment condition.
It is well known that plasma etching increases surface roughness. However, it has been found that surface roughness decreases after etching in the highly reactive process gas, sulfur hexafluoride (SF6, 100%), but increases again with the addition of oxygen to the gas. The following is a discussion of the mechanism of plasma etching of the silicon substrate in order to increase the surface roughness using SF6 or mixture of SF6 + O2. Etching in SF6 (100%) causes the gas phase to consist of F and SFx (1 ≤ x ≤ 5) formed by electron impact dissociation. Its interaction with the silicon surface causes the formation of a non-volatile, thin fluorosilane SiFx layer (1 ≤ x ≤ 4) with a thickness of 1 to 3nm. After SF6 etching, the thin fluorosilane (SiFx) layer covers the silicon surface and decreases the roughness of the substrate. However, if O2 is added to the feed gas, besides F and SFx, sulfur oxyfluorides (SO2F2, SOF2, SOF4) are also produced in the discharge. The formation of oxyfluorides is due to the reaction of oxygen with SFx radicals. The oxyfluorides are powerful etching agents. Thus they inhibit the formation of fluorosilane on top of the substrate and increase the surface roughness by etching. More O2 added to the feed gas results in higher surface roughness.66
It is also well known that increasing surface roughness by plasma etching usually improves adhesion. However, the adhesion strength after plasma treatment has been shown to be lower than that of samples with no plasma treatment. Figure 8.23 shows the average shear strength at different positions, with and without plasma surface treatment. With such deterioration in the adhesion strength, it is necessary to investigate the mechanism behind it.
8.23 The average shear strength at different positions for untreated samples and samples with plasma surface treatment.
The mechanisms for adhesion include physical adsorption (van der Waals force), chemical bonding (covalent, ionic or hydrogen bonds), diffusion (inter-diffusion of polymer chains), and mechanical interlocking of irregular surface. With regards to adhesion mechanisms in the absence of surface treatment, mechanical interlocking has little role because the substrate surface was found to be very smooth (Ra = 20.2 Å). The role of inter-diffusion of polymer chains or other strong bonds, such as covalent or ionic bonding, are not considered here, as there was no surface deformation and smearing in the fracture surfaces. Therefore, only the Van der Waals force and hydrogen bonds are considered to be responsible for the adhesive bonding in the untreated condition. Although after plasma etching, surface roughness increased, the adhesion strength decreased. The probable reason may be.
(i) The roughness of substrates is generally a contributing factor only if the coating penetrates completely into all the irregularities of the surface and wets the surface. Failure to completely penetrate can lead the less coating-to-interface contact than the corresponding geometric area and will leave voids between the coating and substrate. The increased roughness can then lead to decreased adhesion, since trapped air bubbles in these voids allow an accumulation of moisture.
(ii) During the plasma etching, the surface structure may changes chemically, which may suppress the mechanical interlocking effect and reduce the adhesion strength.43
It is recommended that adhesives which exhibit different adhesion strengths at different parts of the substrate should not be used. The internal stresses developed in the devices may damage the functionality of these systems. An adhesive material with Tg lower than the curing temperature of the waveguide core material should also not be used for the lower cladding of an optical waveguide. An adhesive material with higher Tg is recommended. Silicon substrates that were plasma-treated to improve adhesion were found to be inefficient in increasing the adhesion strength. Lower adhesion strength was unexpectedly observed after plasma treatment, even for greater surface roughness. The changes caused by plasma etching of the silicon wafer surface are not yet clearly understood.
8.7.4 Interfacial adhesion on different substrates
Different adhesion strengths were found for different type of sample and process conditions. Even for the same substrate and process condition, different interfacial adhesions were found on different portions of the substrate.43 However, the differences within the same substrate were not large and therefore the distribution did not show up well. Here, the results present the averages (solid bar) and the full distribution of the data values (36 samples for each bar) of the interfacial adhesion for each corresponding substrate structure with respect to a specific process condition. Figure 8.24 shows a comparison of shear strength for three different types of substrate structure with different types of process condition. The results are summarized below:
(i) Among the surfaces evaluated, the thin sputtered Cr-containing surface had the highest strength, followed by oxidized SiO2 surfaces. Lower adhesion strength was observed for as-received silicon wafers.
(ii) The adhesion strength slightly decreased with silicon and silica surfaces but increased with the Cr surface due to applying heat treatment on the spin-coated thin adhesive layer before fabrication of the shear button.
(iii) However, the interfacial adhesion decreased substantially due to exposure to a damp heat condition (75 °C/95% RH/168 hours) after shear button preparation, with all substrates. The degradation was much higher with the silicon substrate than with the silica and Cr containing substrate.
To understand the variations in the adhesion strength on different substrates, it is necessary to undertake a morphology study of the substrate surfaces. In these experiments, the morphological study was carried out under an atomic force microscope (AFM). Among the morphology, one of the known variables affecting the adhesion is surface roughness, because of its effect on wetting at the bonding surfaces and mechanical interlocking. Therefore it was measured to quantify the surface morphology. Three-dimensional AFM images of three substrate surfaces are also shown in Fig. 8.25.
8.25 3-D AFM image of substrate surface for different surface treatments: (a) as-received silicon wafer, (b) oxidized SiO2 on silicon wafer and (c) sputtered chromium on silicon wafer.
Interfacial adhesion on silicon surface
With regards to mechanisms of adhesion, mechanical interlocking has little role for adhesion on the silicon substrate because the substrate surface was found to be very smooth (average surface roughness, Ra = 1.81 Å). The role of inter-diffusion of polymer chains or other strong bonds such as covalent or ionic bonding are not considered here, as the silicon surface is very inactive to the polymeric adhesive and there was no deformation or smearing on the fracture surfaces. Only weak bonds such as Van der Waals force or hydrogen bonds are considered to be responsible for the bonding on the as-received silicon wafer.43
Interfacial adhesion on silica surface
The average surface roughness (Ra = 3.32 Å) was higher than on the silicon surface and therefore the role of mechanical interlocking for adhesion was higher. Beside this, covalent bonds are formed during the polymerization, since functionalities such as hydroxyl groups are generated during the epoxy curing reaction.67 The hydroxyl produces polymeric chains of–Si(OH)2-O-Si(OH)2-OH groups with the silica surface which can link up in many different ways to form a three dimensional network and increase the adhesion strength.68
Interfacial adhesion on Cr surface
Among the surfaces evaluated, the thin sputtered Cr-containing surface had the highest average surface roughness (Ra = 4.71 Å) and this contributed to its higher adhesion strength. Also, for polymer–metal interfaces, other major adhesion mechanisms such as diffusion (or inter-diffusion), Lifshitz–van der Waals interaction, molecular interaction (acid–base interaction) and chemical adhesion (covalent bond) should have contributions for higher adhesion strength.69
8.7.5 Effect of heat treatment on the interfacial adhesion
Heat exposure was performed at a high temperature (275 °C). Above its Tg, the amorphous or semi-crystalline polymer is transformed to a rubbery, viscous state and this reduces the mechanical integrity of the adhesive. Therefore, the adhesion strength of a heat-exposed sample normally decreases. In the case of silicon and silica surfaces, it was also found that the interfacial adhesion decreased due to the heat treatment of the spin-coated adhesive film. But the decreased adhesion strength on silicon surface is not high, because, the adhesive used here had a higher Tg. Also, we used a lower spin speed to reduce the spin-induced degradation. Therefore, it is confirmed that an adhesive with a high glass transition temperature and use of a low spin speed is appropriate to reduce the degradation of the polymer photonic devices.
However, for the sputtered Cr surfaces, the adhesion increased after heat treatment. The possible reason for this is that chemicals bonds are formed at the interface, usually as a result of a charge transfer from the metal to the polymer. The metals (Ti, Cr, Zr, Al) can strongly bond to oxidized (due to heat treatment at 275 °C) polymer surfaces; when the hydrophilic groups on the polymer surface make contact with the metal layer, electrons are transferred from the metal to the hydrophilic groups, resulting in the formation of a charge transfer complex, which enhances the adhesion between the metal and the polymer.69
[8.1]
Formation of Cr(III) oxide by a redox reaction between Cr and oxygen-containing functional groups of the polymer will increase the adhesion strength. Also there is a higher possibility of typical inter-diffusion between the metal–polymer interfaces increasing the adhesion strength.12,16
Effect of damp heat on the interfacial adhesion
Epoxy-based adhesives absorb moisture and experience hygroscopic swelling in humid environments, hence degrading the adhesion strength and elasticity. Any uncured adhesive can be severely attacked by moisture during the reliability test. Such hydrolytic attack breaks the ester linkages (′R–(C=O)–OR′) of the polymer chains and creates two new end groups, a hydroxyl and a carbonyl. Hydrolyzation of the adhesive would appear to weaken its mechanical strength and adhesion to the substrate. The reduced adhesive strength also induces delamination on that interface. However, the reduction in adhesion strength is very much larger on silicon surfaces, followed by oxidized SiO2 surfaces. A smaller reduction in adhesion strength was observed for the Cr surface during the temperature humidity test, because the formation of chemical bonds at the metal–adhesive interface by charge transfer is stronger than both the covalent bond at silica–adhesive interfaces and other weak bonds such as Van der Waals force or hydrogen bonds in the silicon-adhesive interface. Therefore, the Cr-containing interface is more reliable in a humid environment compared with the other two interfaces.
It is recommended that a thin metal layer (such as Cr) on the silicon wafer be used to increase the adhesion and reliability of polymer photonic devices. Oxidized silica on the silicon wafer is an alternative choice at the expense of reducing adhesion. However, using a silica layer has the advantage over using a Cr layer in that one fabrication step can be reduced, since the silica layer itself can effectively act as the lower cladding of the device.
8.8 Conclusions
Polymeric adhesives have gained much attention, and significant technological progress has been made recently in the photonic industry in order to meet the requirements of high-speed and large-capacity transmission of information at low cost. Thus the manufacturing infrastructure and packaging process for polymer-based photonic devices is fully deployed and is capable of meeting the demands of current optical modules. It is, however, essential to obtain a better understanding of the materials and process optimization in manufacturing adhesive-based photonic components. Within each area, major research findings and recommendations have been given in this chapter. These insights should be very useful for adhesive manufacturers in formulating better polymers with favorable performance for this application. In contrast, the industry is still learning about polymer materials and the process technologies have yet to be fully developed. It is believed that only after adhesive materials and processes have been broadly understood, incorporated and the manufacturing infrastructure built, will polymeric adhesives be widely used and eventually replace other conventional materials and techniques. Therefore, there is still a long way to go in terms of polymer advancement; the corresponding process development and infrastructure must be built before polymeric adhesives become a vital part of the PLC device. However, from polymer evolution history and the current explosive technological breakthroughs, it is certain that polymeric adhesives will be more widely utilized and will play an important role in the photonic industry.
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