Choosing a Polymer
Choosing a polymer typically includes reviewing physical, mechanical, chemical and many other properties that may support or preclude their use in packaging. Materials that were suitable as protection in the older packages, such as epoxies, are no longer suitable to keep the finished product at a minimum thickness, because of the thinness of the layers required in WLP.
Polyimides and related polymers have been the materials of choice since the 70’s. They continue to dominate the market in semiconductor applications because they have offered an excellent set of manufacturing compromises, along with bearable cost and good reliability. Polyimides are known for their excellent chemical, mechanical, thermo-oxidative resistance and adhesive properties. The films do not chip, crack or peel and remain flexible at cryogenic temperatures. However, one deficiency of polyimide is the tendency to absorb some moisture, due to the presence of carbonyl bonds in the polymer chain. Carbonyl bonds allow attack of the polymer by strong bases, such as some harsh photoresist removers.
NASA sponsored the development of numerous aromatic thermosets during space programs in the 60’s. Synthesis of new polymers for high temperature applications continues and remains very dynamic. Many of the thermally stable polymers possess complementary outstanding properties, such as extreme chemical resistance and ease of deposition using a low temperature chemical vapor deposition process, making them candidates for WLP applications.
Reliability issues to look for
The relationships of polymer structure to thermo-oxidative stability, chemistry and mechanical properties of polymers, cross-linking and degradation mechanism at elevated temperatures, are areas of prime interest. The degradation of polymers often limits their useful life. Degradation is the result of an environment-dependent chemical or physical attack. The mechanism can be very complex, since it may involve a number of chemical and physical reactions. UV light, high temperatures, oxidizing atmosphere and chemical attacks from air pollutants, as well as mechanical stress, can all degrade polymers.
Oxidation is the most important mode of degradation in electronic packaging, since the polymer's role is to protect the semiconductor from the environment. This assumes that all potential causes of degradation during fabrication have been resolved. Observation of oxidative degradation in a polymer was first made by Hoffman in 1861 when he discovered that rubber's failure was due to oxygen. Many books have been written since on the subject, but the potential source of problem is still with us.
In practice, the life of a polymer is limited by a deterioration of its mechanical properties. Embrittlement of a polymer causes a gradual change from ductile and elastic behavior to brittle fracturing. Since the oxidation process starts at the surface exposed to air, a brittle oxide skin will facilitate the initiation of stress cracks at low stress levels.
Adhesion of Polymers
Adhesion of polymers to metals has been intensively studied because it is equally important to common household products like paint, tires or fiberglass automobile parts, as it is to passivation layers on silicon wafers.
Several approaches are available to enhance adhesion of polymers to metal or to other polymers or to themselves. Polymers can be modified by small addition of chemicals, by adhesion promoters, or by optimizing the materials interfaces. Another approach is to modify the surface of the substrates to improve the materials properties at their interface.
In some cases, the adhesion is due to interdiffusion of the layers at the boundaries of the materials. In this case, the mechanical strength is limited by the physical and chemical interlocking of the entangled materials. In other cases, the boundary is very sharp, with limited interdiffusion. The adhesion is due to chemical bonding, bond sharing or strong polar interaction at the boundaries.
An adhesion promoter, sometimes called coupling agent, can be defined as a material placed at the interface of two dissimilar materials in order to enhance bonding between the materials, either by interdiffusion or by chemical bonding. Typically, the most common adhesion promoters between polymers and SiO2 are alkoxysilanes. There exist dozens of candidate materials with a methyl or ethyl group that can react with the polymer on one side and the Si-O-Si groups on the other side to achieve the coupling effect between the materials. Silane adhesion promoters are generally recognized to form oxane bonds (M-O-Si), where M is a metal surface, such as Si, Ti, Al, Mg or Fe.
Adhesion promoters can be applied by a variety of techniques: dispersed in an aqueous solution; in an organic solution, for example alcohol; mixed directly in the polymer or vapor deposited. In order to be successful, the adhesion promoter needs to be applied in a very thin layer not exceeding a few monolayers. This is why the adhesion promoter solutions are highly diluted.
Polyimides use adhesion promoters incorporated in the material by the manufacturer or in solutions applied and dried immediately before the application of the polymer. HMDS (hexamethyldisilazane), the common photoresist to silicon dioxide adhesion promoter, is most often applied in vapor form for convenience and better process control.
Control of the surfaces
Because adhesion relies on what happens at the interface of the materials on the molecular level, it is evident that control of the cleanliness of these interfaces is primordial to obtain good adhesion. Repeatability and control of the surface preparation is essential. Strong adhesion of one of the materials to loosely bonded dirt on the other side of the interface will not help.
Surfaces can be examined with powerful surface analytical tools, such as ESCA or AFM, in the initial search for a process and its control; but this is impractical for every day use. A much simpler way to evaluate cleanliness of a surface is available by measuring a liquid to solid contact angle. Measurements can be made from simple homemade laser instruments or from computerized commercial instruments.
A drop of DI water, or of a liquid chosen for its surface tension properties, is applied to the surface and the contact angle shown in the illustration, is measured with the goniometer. The contact angle gives an insight on the physical and chemical state of the surfaces energies. With this method, precise quantitative measurements of the surface tension are complex, nevertheless, a measurement with a single liquid gives a quick, pragmatic and consistent evaluation of the surface, whereas exact knowledge of the surface tension value is superfluous for quality control purpose.