A guide to static safe polymersfor automated handling equipment

02/01/1997

A guide to static-safe polymers for automated handling equipment

Richard W. Campbell, DSM Engineering Plastic Products, Reading, Pennsylvania and Wayne Tan, Advanced Micro Devices, Sunnyvale, California

Electrostatic charges can, however, be generated on these materials, typically due to the turbulent flow of contaminants in the air, whether at atmospheric pressure or compressed. Fields from this charge can be in the range of several thousand volts. Product flows along the equipment`s tracks must pass through these electrostatic fields. The fields can induce a charge on the product, which is then damaged when it makes contact with the grounded surface. The negative consequences of this field-induced charged device model (CDM) damage are well known [1, 2]. Conventional ESD-protective materials, including the traditional antistatic agent additive types, have been partially successful in reducing the hazards but have not provided the protection needed for highly demanding applications. Polymers used for components in device handling equipment can be

1) conductive (<106 W) if the components do not contact the semiconductor devices

2) antistatic or static dissipative (106-1010 W) if used as sliding rails, and

3) highly resistant (1010-1012 W) if used as test sockets.

An environmental test handler depends on polymeric thermal insulating materials to maintain the hot or cold environment required for the electrical stress test of products. Use of inherent static dissipative and antistatic polymers in automated handling equipment in semiconductor manufacture can minimize ESD hazards and improve product yield. ESD-protective additive technologies for static control in polymeric materials divide into at least four basic groups: hygroscopic agents, conductive particulates, metallocenes, and conductive polymers.

Figure 1.. Types of hygroscopic agents in plastics.

The 1996 Modern Plastics Encyclopedia lists over 250 antistats and static dissipative materials, so there is no shortage of candidate resin systems, though chemical compatibility and suitability for the intended end-use environment narrow the field significantly. Each of these material classes has advantages and disadvantages for use in semiconductor processing and testing, so the selection must be done judiciously. For this article, we do not include surface treatments, such as vacuum metallized or metal-based coatings. They require secondary operations and are generally more conductive than optimal for a typical charged device dissipation path to ground in automated device handling equipment.

Hygroscopic surfactants

Hygroscopic surfactants or antistats, such as tertiary fatty acids and their amines or salts, quaternary ammonium salts, monoalkyl glycerides, alkyl phosphonates, and sulfonamides (all of which comprise the majority of the additives listed in the Modern Plastics Encyclopedia) are relatively inexpensive. Hygroscopic surfactants can be incorporated at low concentrations (a few ppm to a few percent) into a polymer during processing or applied topically at a later time. Typically, though, their limited thermal stabilities restrict their use to polyolefins (polyethylene or polypropylene), acrilonitrile-butadiene-styrene (ABS), vinyls or styrenics, all of which are processed and used at relatively low temperatures. A further potential limitation is chemical reactivity, which can degrade more complex polymers, such as polyesters or nylons during melt processing.

Since this class of antistats functions by migrating to the surface, or "blooming," and attracting a conductive film of surface moisture (Fig. 1), the effectiveness of hygroscopic surfactants depends on maintaining a minimum ambient humidity and limited time of use. The antistat effect is thus primarily a surface phenomenon, and there will often be a significant discrepancy between the surface and volume resistivity measurements, in contrast to other additives that are more uniformly dispersed throughout the material. Being migratory, the hygroscopic surfactants also present the potential for outgassing at elevated temperatures, and subsequent deposition on cooler surfaces, which is undesirable in a cleanroom.

Successful applications of hygroscopic surfactants include carpeting, fabrics, packaging, or disposables, where cost considerations may outweigh durability concerns. These surfactants, however, are in general too transient and too dependent on ambient humidity for widespread use in the IC industry.

Conductive particulates

Other internal additives widely used to increase the conductivity of polymers, especially high-temperature polymers, include conductive particulates such as carbon black, carbon fibers, and metallic fibers. The effectiveness of conductive particulates, as measured by surface or volume resistivity, is a function of the additive concentration. Required concentrations are much higher than for the hygroscopic agents, often up to 20% loading. In the case of carbon fibers, the high concentration can add significant cost to the product, although the expenditure can be easily justified based on the value of the electronic devices being processed. Local variations in the concentration, dispersion, or orientation of the additives can result in one section of a part being insulative (>1013 W) while a nearby section can be fully conductive (<104 W).

Although conductive particulates can easily give highly conductive polymers, it is difficult to attain reliably the intermediate conductivities required for ESD-safe use. The percolation curve (effect vs. concentration, Fig. 2) is very steep in the range of interest (1010-1012 W). Contamination of device handling equipment is also an issue, especially in carbon black-filled components. Abrasion and wear can release highly conductive particles, a process known as "sloughing."

Figure 2. Surface resistivity vs. loading of conductive additives.

Figure 3. Effect of high voltage on CF plastics, a) before high voltage arcing; b) after dielectric breakdown. Carbonaceous formation reduces resistivity of sample. The "stars" refer to points where arcing of the material takes place.

DSM Engineering Plastic Products and AMD have also found that carbon fiber-filled polymers are seriously affected by dielectric breakdown of the resin matrix. Typically, when the concentration and orientation are carefully controlled, a static dissipative polymer, such as carbon fiber-filled PEEK, exhibits surface resistivity in the target 108-1012 W/square range, as measured by low voltage testing. When the polymer is tested at higher voltages (>100 V), however, the resistance drops precipitously and irreversibly. This behavior can be explained by the dielectric breakdown of the thin film or sheath of polymer separating the carbon fibers. Since the typical dielectric strength of most plastics is 16-24 kV/mm (400-600 V/mil) and fiber-to-fiber separation is < 2.5 mm (0.1 mil), the dielectric strength is easily exceeded. Since most polymers break down to form carbonaceous residues at elevated temperatures, the observed increase in conductivity can be explained by the formation of a new conductive pathway. Although we have not conclusively pinpointed a carbon track in the midst of a carbon fiber matrix, support for this hypothesis has been gained through testing of carbon fiber composites of resins that do not form carbonaceous breakdown products; their resistivities are stable to applied voltages up to 1000 V (Fig. 3).

The length of the fiber is also an issue with polymers containing carbon fibers. Often, the fiber is long compared to the separation of leads on electronic devices, so a single fiber can form an undesirable conductive bridge. Bridging is especially probable on machined polymer components, where the semi-insulative polymer matrix is mechanically removed, exposing bare conductive fibers.

Despite the limitations, carbon black-filled polymers are used extensively in wafer and chip processing to transfer components outside the cleanroom environment for, say, cleaning and inspection. Typically, the filled polymers are relatively small molded trays or similar articles [3]. Likewise, carbon fiber-filled polymers based on high-temperature engineering thermoplastics exhibit outstanding mechanical properties and are excellent candidates for applications requiring durability, chemical resistance, and structural strength at elevated temperatures in materials with a surface resistivity below 106 W/square. Carbon fiber-filled PEEK and carbon fiber-filled Ultem polyetherimide are among the several conductive polymers available in the form of machinable stock shapes or injection-molded parts.

Metallocenes

Metallocenes, which are currently receiving extensive attention for their uses as polyolefin catalysts, are also reported to be effective static-control additives for polymers [4]. One such metallocene, bis(methyl)cyclopentadianyl cobalt, provides very effective - though expensive - static dissipation at levels of 8-15%.

Although most metallocenes have displayed low thermal stability, limiting their use to polyolefins or other polymers processed below 2008C, developmental titanates and zirconates have been shown to be stable in excess of 2608C. Reportedly, specific combinations of neoalkoxy titanates, and zirconates form an internal circuitry through their orientation into alternating bipolar charge layers. This structure results in a very narrow band gap, and therefore low resistance to electron transfer [5] (Fig. 4). Once formed, these oriented layers are stable and nonmigratory, but generally very strongly colored.

Figure 4. Charge hand-off in ether oxygen molecules.

Metallocene static-control additives are the most expensive of the four approaches to ESD protection, but may be the best choice for surface resistivities in the 108-1012 W/square range for high-temperature polymers.

Conductive polymers

Inherently conductive or dissipative polymers (ICP or IDP) can be made, i.e., no static-control additive is needed. Although sometimes used alone, ICPs and IDPs are generally more functional as additives to form polymer alloys with other resins. Typically, the alloys form an interpenetrating network, where the base polymer is selected to optimize performance in the thermal, mechanical, and chemical environments in which the component is used. Examples of highly conjugated electrically conductive polymers include polyacetylene and charge transfer complexes of tetracyanoquinodimethane with a N-containing aromatic electron donor.

IDPs are less conductive than ICPs and work by a different mechanism. Instead of relying on conjugated unsaturation in the chains for charge transfer, IDPs contain ether oxygens in the backbones (e.g. polyethers or polyetherurethanes), which complex with cationic species to form ionically conductive solid state solutions of polymers and inorganic salts, e.g., functionalized polyethylene oxide (PEO) and lithium iodide [6], thus creating a medium for ionic charge transfer. Chain flexibility of the polyethers and polyurethanes allows charges to be transferred from one ether oxygen link to another (Fig. 5). When a melt blend of the additive and a substrate is made, the polyether or polyurethane forms separate, interconnected, elongated domains, which conduct electrostatic charges in a controlled fashion.

Examples of IDP technology include thermoplastic polyurethanes, such as a copolymer formed by reacting an ethylene ether oligomer glycol intermediate with a nonhindered diisocyanate and an aliphatic extender glycol [7] or a copolymer consisting of 60-95% polyethylene oxide and 5-40% epichlorohydrine components [8]. Doping with active cationic species can further reduce the surface resistivity to as low as 104 W/square, although 109 W/square is more typical and practical.

Figure 5. Inherent dissipative polymer approach. The darker material in the background represents the base material while the lighter parts correspond to the inherently dissipative polymer (IDP). The lines refer to the points of phase separation.

Since IDPs are high molecular weight polymers, they are stable and will not migrate, outgas, or leach out; they are noncorrosive to metals. Their conductivity is essentially independent of humidity, so parts meet FTMS 101C Method 4046, Electrostatic Properties of Materials, a static decay test using the 1% cutoff (in much less than the 2 sec limit) at 15% relative humidity. After several years, little or no change in the material`s resistance, i.e., static dissipating ability, has been detected. IDPs are colorless, so unlike carbon black or carbon fiber-filled polymers, the polymer alloy can be pigmented in a range of desired colors depending on the application. This is obviously not an option with carbon black or carbon fiber-filled polymers.

Although limited by thermal stability to 2508C, IDPs are used extensively as additives in a number of low- to mid-range thermoplastic matrices, including polyolefins, polyesters, nylons, and polyacetals. They can be melt blended and processed into a variety of end-use products exhibiting surface resistivities typically 108-1010 W/square. IDP-based conductive polymers have found many successful applications in device-handling equipment. Stock shapes can be extruded from these polymers for the precision machining of a multitude of components.

Conclusion

This article has addressed the technologies available for imparting static control characteristics to the thermoplastics used in making components for automated device-handling equipment. Although there may be a number of options available, each has advantages and limitations. Therefore, the end-use environment and performance expectations must be thoroughly understood before a selection of the base resin and the optimum additive system is made.

RICHARD CAMPBELL has a BA in chemistry and a PhD in polymer science and engineering from the University of Massachusetts. He is the manager of product development at DSM Engineering Plastic Products, where he heads a group focused on developing high-performance engineering thermoplastic stock shapes for the semiconductor industry. He presented a paper at EOS/ESD `95 and has 20 US patents. DSM Engineering Plastic Products, PO Box 14235, Reading, PA 19612-4235; ph 610/320-6970, fax 610/320-6591, e-mail [email protected].

WAYNE TAN holds BSME, BSEE, and MSEE degrees and is a senior member of the technical staff at Advanced Micro Devices in Sunnyvale, California. He has presented more than 15 papers worldwide and received the Best Presentation Award at the 1995 EOS/ESD Symposium. His articles on ESD control appear in Circuit Assembly and PCB (Italy) trade magazines.

References

1. J. Bernier, B. Hesher, "ESD Improvements for Familiar Automated Handlers," EOS/ESD Symposium Proceedings, 1995.

2. W. Tan, A. Steinman, "Reducing Static Related Defects in Semiconductor Test Handlers," Third Annual Man. Test Conf., SEMICON West 94, July 1994.

3. W. Tan, "Carbon Loaded Device Handling Trays: Analysis and Measurements," EOS/ESD Symposium Proceedings, 1995.

4. US Patent 4,715,968, G. Sugarman, S.J. Monte, Kenrich Petrochem., Dec. 1987.

5. S.J. Monte, G. Sugarman, "Unfilled Plastics - A New Application for Coupling Agents," SPE RETEC, PMAD Division, Akron, Ohio, Nov. 1986.

6. US Patent 5,346,959, P.M.Goman, L.R. Stebbins, K. Udipi, Monsanto Corp, Sept. 13, 1994.

7. US Patent 5,159,053; E.G. Kolycheck, E.A. Mertzel, F.R. Sullivan, BF Goodrich; Oct. 27, 1992.

8. US Patent 4,931,506; S.H. Yu, B.F. Goodrich, June 5, 1990.