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The use of a resist applied before nickel and silver plating can greatly reduce background plating while helping to control plated line width spread on the front side of the solar cell grid. Reduced line spread helps to minimize shadowing and has a positive impact on cell efficiency. EXPERIMENTAL OBJECTIVES(1) Investigate hot melt ink as resist for both hydrofluoric acid etching and metal plating.(2) Create high resolution line widths using inkjet as a plating mask to reduce the light shadowing effect.(3) Eliminate background plating caused by imperfections in the silicon nitride.(3) Optimize the inkjet and plating processes to maximize cell efficiency. BACKGROUND

Divinylsiloxane-bis-benzocyclobutene (DVS-bis-BCB, or BCB) is a well-known dielectric material that has been used in high volume manufacturing for many years (Dow's CYCLOTENE™ 3000/4000-series Advanced Electronic Resins). Typically, the application of these products has been by spincoating or spray coating of the dielectric material from a solvent-based formulation. However, for certain applications - for example, those involving large area, square substrates such as glass panels - it is desirable to be able to apply the BCB-based dielectric material using a dry film coating process, such as vacuum or hot roll lamination. In this paper, we describe the concept of creating a laminate film utilizing DVS-bis-BCB as the primary dielectric material component. In creating the laminate dielectric material, it is important to maintain the unique combination of thermal and electrical properties of DVS-bis-BCB, including high thermal stability, excellent copper barrier properties, low moisture uptake, low dielectric constant, and low dielectric loss. However, DVS-bis-BCB alone is too rigid to produce a high quality laminate film, therefore, it is necessary to modify the formulation to improve flexibility and lamination quality. As the flexibility of the film is increased, higher fracture toughness (K1C) and higher elongation values should result. Novel formulation adjustments to the base DVS-bis-BCB polymer system have resulted in an experimental laminate dielectric product that will be the focus of this discussion. Depending on the application, laminate films can vary in thickness from 2μm to 50μm or even thicker. A typical laminate film construct includes the BCB-based dielectric film with a base sheet of an optically-clear polyester (PET) film and a polyethylene (PE) cover sheet. The DVS-bis-BCB-based laminate can be tuned with appropriate additives to either pattern with UV exposure from a tool such as a Süss MicroTec Mask Aligner (50mJ/cm2 – 100mJ/cm2 exposure energy) or laser pattern with a tool such as a Süss MicroTec 248nm Excimer Laser. Aspect ratios of 1:1.5 have been achieved with a photopatternable DVS-bis-BCB film and aspect ratios of >1:1 are possible with a laser-patterned film. Beside the photopackage added to the film to enable UV patterning, toughening additives have also been incorporated to enhance the fracture toughness (K1C) and elongation properties. K1C for the laminate has been improved to 0.55Mpa·m1/2 vs. 0.35Mpa·m1/2 for DVS-bis-BCB. Elongation has been improved to 13% for the laminate vs. 8% for DVS-bis-BCB. Electrical properties are similar to DVS-bis-BCB. An additional attribute of the laminate dielectric is the ability to tent over and protect vias. Vias >100μm diameter have successfully been tented with a 10μm thick film. A DVS-bis-BCB-based laminate has been demonstrated with continued optimization and evaluation to follow.

There has been significant activity in recent years to develop Non-Conductive Films (NCF), which are also known as Pre-Applied Underfills (PAUF) and Wafer Level Underfills (WLUF), for use in the High Volume Manufacturing (HVM) of 2.5D and 3D packages. They are essentially underfills in laminate film form. Like other underfills, they ensure the integrity of the electrical interconnects in a package by mitigating stress, acting as an adhesive to bind the package together, and encapsulating to protect against moisture and other unwanted materials that can compromise electrical connectivity. NCF's are seen as an alternative to the more traditional capillary underfills, especially in devices with finer pitch, smaller gap, and larger size. Since the NCF can be applied to multiple devices simultaneously, this new technology has an advantage over capillary underfills in HVM. These film-type underfills can be applied to wafers (PAUF, WLUF, NCF) or substrates (PAUF, NCF). Two key attributes of any underfill are no voiding and high reliability. No voiding is an essential requirement for high reliability. Voiding in NCF materials can result from the lamination and thermocompression bonding (TCB) processes. Voiding under the die can manifest from a variety of causes, including some of the following: (1) volatilization of materials in the coating, (2) dimensional changes within the coating during processing, (3) poor fundamental conformation to surface topography during film lamination and (4) ineffective air release during TCB due to un-optimized material flow. Unwanted issues such as reduced adhesion, solder shorting, increased moisture uptake and reduced stress mitigation can result from the presence of voids in the NCF. Pressure cure after joining is one method for eliminating voids in NCF materials. An earlier version of the NCF discussed in this study required pressure to eliminate voids after TCB. Pressure curing, however, adds process steps and is not accepted by all manufacturers. An improved NCF formulation and bonding process has resulted in a void-free NCF that does not require pressure cure for void elimination. This improved version not only addresses voiding after bonding, but also it has been proven to be very reliable in the presence of extreme temperatures, high humidity and temperature cycling. No one test is sufficient to adequately determine the long term reliability of NCF materials so a battery of tests was run to conduct a comprehensive assessment. The reliability evaluation results demonstrate that this newly developed NCF exhibits not only no voiding after TCB without the need for pressure cure, but also high reliability to various forms of temperature and humidity stress testing.

To enable advanced wafer level packaging approaches for devices like MEMS, image sensors or optical elements, wafer-to-wafer bonding processes using structured low temperature curable adhesives are required. A lot of Benzocyclobutene (BCB)-based wafer bonding works have been reported in the past showing a broad range of applications and good performance, but also some limitations such as long bond cycles and high cure temperature of 250 °C. In 2013 new process concepts were demonstrated [1], showing that wafer bond cycle time can be reduced to less than 10 min and a post bond batch cure at temperatures below 200 °C can be used to significantly shrink the overall cost of a BCB-based adhesive wafer bonding process. In order to create a patterned BCB bond layer, photo structuring of CYCLOTENE ® 4000 Resin is one solution. However, due to the decreased flow capability of that material after exposure, high bond forces and extended bonding times during wafer bonding as well as nearly flat surfaces with low topography are required for void-free bonding. To overcome these limitations, an increased material flow capability during wafer bonding is required. In this context non-photo sensitive CYCLOTENE ® 3000 Resin is suitable, since it has excellent flow capability in non-cured state. However, non-cured CYCLOTENE ® 3000 Resin cannot be structured with standard dry etching processes using a photo resist layer as mask. In order to enable patterned adhesive bonding based on CYCLOTENE ® 3000 Resin, alternative structuring methods have to be evaluated. One method was presented in [1] which is transfer printing of CYCLOTENE ® 3000 Resin from a help wafer to topography features of the device wafer. Although very good results were obtained, the method is restricted to applications with significant topography to enable the transfer printing. In this work we focus on a new structuring method for non-cured BCB layers formed from CYCLOTENE ® 3000 Resin. The layers were spin coated, baked and subsequently patterned using a 248 nm excimer laser stepper. The system features a 2.5× mask projection with a resulting exposure field of 6.5 × 6.5 mm2 and allows a direct ablation patterning of polymers. By using this method bond frame structures were patterned into 5 μm thick BCB layers at 200 mm silicon wafers. The wafers with the structured adhesive were bonded at 80 °C and 0.2 MPa for 5 minutes with 200 mm glass wafers. The bonded wafer stacks were subsequently post bond batch cured at 190 °C. Wafer dicing and shear tests of the bonded structures revealed excellent mechanical robustness of the BCB bond frames. The paper will review the new BCB wafer bond processes for supporting short cycle times with special focus on the new patterning approach by laser ablation. Process flow description as well as systematical analysis of pattern reproducibility of the new structuring method is part of the discussion.