Richard W. Campbell
Manager Product Development
Quadrant Engineering Plastic
Products Reading, Pa.
There's no question that designers must always assess how candidate materials will stand up to the chemicals, temperature extremes, and stresses they will see in service. This is doubly true for semiconductor fab-line components. During development, some often-used materials including silica, copper, aluminum, quartz, and epoxy will likely top the list of candidates due to their good mechanical and chemical-resistance properties.
But there's a host of thermoplastics (TPs) that are in widespread use on semicon fab lines. High-performance TPs generally aren't in the products that go out the fab door, but are critical to the processing of these devices. TPs have a range of electrical resistivities (for preventing static sparks) that is tough to find in other materials. They also handle highly aggressive chemicals and withstand extremes in temperatures.
THE RIGHT PLASTIC
Though it may sound obvious, designers evaluating materials should always ask how a material will be used and what it will be expected to do. Factors to consider include temperature, the chemicals present, dimensional stability, electrical requirements, machinability, mechanical aspects, and ionic purity.
Temperature: Several things can happen to plastics as temperatures rise. Of course, they can soften and melt at the extreme. But temperature also causes dimensional changes defined by the coefficient of linear thermal expansion, or CLTE. The plastic matrix and the type and concentration of fillers employed determines CLTE. High-aspect fiber reinforcements generally reduce CLTE and make dimensions more stable.
Amorphous plastics including polyvinylidene difluoride (PVDF), polycarbonate (PC), polyetherimide (PEI), polyamideimide (PAI), polyimide (PI), polybenzimidazole (PBI) lose >95% of their strength at their glass-transition (Tg) temperature and can begin to flow.
In contrast, crystalline plastics such as polyetheretherketone (PEEK) and polyphenylene sulfide (PPS), can lose 50% of their strength at Tg. But they will still retain predictable mechanical properties up to their melting point. Fortunately, there is a good way to see at a glance how well a material will stand up under stress and elevated temperatures: DMA (dynamic mechanical analysis) charts can be helpful in such evaluations.
A material's heat-deflection temperature (HDT) and continuous-use temperature rating (CUTR) both characterize how well it will resist heat. HDT is an indication of softening temperature. It is generally accepted as a maximum short-term temperature limit for moderately to highly stressed, unconstrained components. Material suppliers generally report CUTR as the temperature where significant, permanent physical property and chemical degradation begins after longterm exposure.
Reinforcing fillers raise HDT, especially for crystalline plastics such as PPS and PEEK. But the HDT for amorphous materials including PVDF, PC, PEI, PAI, PI, and PBI is usually a few degrees below Tg and independent of any reinforcement or its concentration. Designers can estimate HDT as the point at which the DMA curve equals 150 to 200 kpsi.
Chemical environment: Although strong acids, bases, and organic solvents found in processing affect some plastics, others stand up well to these materials. PVDF and crystalline highperformance plastics such as PEEK and PPS are in the latter group. They are virtually unaffected and therefore can be used in the presence of severe chemicals. Even the extremely aggressive and strongly oxidizing acid Piranha hardly affects PAI and PPS. PVDF also shows excellent resistance to this acid.
Polymer producers provide information about chemical compatibility, but caution that it is only a guideline. It may be tough to predict how well a particular plastic will stand up in a given application. Chemical concentration, temperature, time, and stress all bear on suitability for use. It is strongly recommended that designers test candidate materials under end-use conditions to see what happens.
Plasma-etch resistance is a specific type of chemical resistance. Fab lines often employ plasmas to remove organic materials from wafers. These organics can include oils and greases from machining and handling as well as photoresists from selective metal deposition.
Fab equipment should avoid using metal components such as retainers and screws to prevent contamination from metal ions. That's where special high-purity plastics with ionics of <1 ppm (parts/million) come into play. Unfortunately, plasmas are highenergy and aggressive chemicals. They are not discriminating and thus will also attack surfaces of plastics. The etch rates in the four basic types of plasma were detailed in MACHINE DESIGN, Semiconductor Manufacturing Equipment Supplement of July 12, 2001.
Moisture absorption: Next-generation chip packages rely on connections with fine-pitch spacings. But these high-density connections are pushing the limits of processing equipment that contains burn-in and test sockets (BiTS). So dimensional stability has become more critical. And some of this production has moved offshore where environmental controls are not as rigorous as in U.S. fabs. Some plastics available in those countries are prone to absorbing moisture and swelling.
Imidized materials (PEI, PAI, and PI) typically are more hygroscopic and thus have poorer dimensional stability than crystalline polymers (PPS and PEEK). So materials suppliers are basing the latest generation of electrostatic dissipative (ESD) materials on PEEK (Semitron ESd 480 and ESd 490HR).
For example, in the widely used 105 to 109 range, Ultem PEIbased Semitron ESd 420 may be best if the application will see 200°C. But this material absorbs more moisture than others. The less moisture sensitive PEEKbased ESd 480 grade may be a better choice to 150°C.
Electrical aspects: Nearly all plastics are inherently good insulators. Although this is ideal for electrical insulation, these same insulative properties can be negatives in semicon processing.
Static charges can build up on the surface of insulators, and insulators provide no pathway for electrons to safely dissipate slowly. Consequently, discharges may take the form of violent arcing or sparking and may destroy circuitry on an expensive wafer.
Worse, the damage may be latent and could make the device fail in the field later on. One way of not keeping static in check is to add metal or carbon fibers, graphite, or conductive carbon black to the polymer. It's been shown repeatedly that these measures aren't effective. The resistivity can't be well controlled and the typical result is a highly conductive material. At voltages >500 V sudden static discharges can be highly destructive.
Static must dissipate in a slow, controlled rate to prevent damaging chips. The ideal discharge rate depends on the application and can be a trade-off between rate or surface resistivity and allowable current bleed. Designers can use Ohm's Law to compute these factors and determine the right compromise. Proprietary technology can now tailor the properties of ESD materials to target specific resistivity ranges. Designers, for example, can now spec materials with intermediate resistivities (105 to 109 Ω/square) for handlers and specify materials with higher values for nests and sockets where pin-to-pin bleed-off may be more of a factor.
Machinability can also factor into material selection. Suppliers can stress relieve a TP to improve machinability. In general, glass and carbon-reinforced grades are considerably more abrasive on tooling than unfilled grades. They are also more notch sensitive during machining. But reinforced grades are commonly more stable during machining.
Celazole PBI can be challenging to fabricate because of its extreme hardness. This material must be machined with carbide and polycrystalline diamond tools.
Mechanical aspects: For components going into structural applications, it's important to compare mechanical requirements to published data sheet properties. In ESD materials, for example, acetalbased Semitron ESd 225 is a widely used low-cost static dissipative material. But it has less strength than other materials, which may keep it out of highly stressed or load-bearing applications. Here, a fiber-reinforced material such as ESd 420, ESd 480, ESd 490, or ESd 520 would be twice as strong. The reinforcing fibers, as stated earlier, also have the benefit of reducing CLTE.
Purity: Valves, seals, tanks, and manifolds must be made from highly pure materials so there's no chance of contaminating the high purity liquids needed by semicon fabs. High purity is also a must for components that will see ablation from plasma etching and other cleaning processes.
There are two ways of gauging purity here: Leaching for the former and total composition for the latter. Leaching takes place in a variety of liquids from UHP water to Piranha. Technicians measure the concentration of ions, which are soluble in the test solution to parts/billion levels after some period of time and at a specified temperature. Insoluble constituents left behind in the plastic are not considered sources of contamination and therefore are not measured.
Total compositional tests determine all constituents, soluble or not. It is assumed that anything present in the plastic can and will release into the chamber during etching. The target is generally <1 ppm for each of "16 Critical Elements," especially the mobile ions such as sodium, lithium, iron, and aluminum. Special grades of high purity materials, such as Ketron UHP, PEEK 320, Symalit PVDF, Lexan PC, and Duratron XP, achieve this target.
Finally, another test of purity is the degree of outgassing. Volatiles evolved from a material, and which might potentially deposit on the surface a wafer, are quantified using tests such as ASTM E-595.
Quadrant Engineering Plastic Products, (800) 366-0300, www.quadrantepp.com