More metal parts go to composites

Associate Editor

It's no secret that suppliers of engineered materials must work closely with designers to develop products that meet specific needs. This is particularly true for components that serve in caustic environments at elevated temperatures under load -- conditions typical of semiconductor-processing applications where designers are often limited to specifying metal and ceramic.

In the semiconductor industry, however, trends such as larger wafers, more caustic etching solutions, and the need for faster throughput are pushing the mechanical and physical limits of metals and ceramic components used in wafer handling, etching, and polishing. So says Bill Sproesser, development programs manager for DuPont Engineering Polymers, Wilmington, Del.

This has prompted designers to look at alternative materials such as high-performance composites made from polyimides, epoxies, fluoropolymers, and phenolics reinforced with woven, braided, or chopped graphite, carbon, glass, and aramid fibers. Advancements in these raw material forms provide designers a balance of price, processibility, and performance.

For years, designers of semiconductor manufacturing equipment have turned to Vespel composites from DuPont Vespel parts and shapes, Cleveland, when other materials failed. Here are a few of the most recent Vespel composites that replace metals, ceramics, and other engineered thermoplastics in wafer-processing and handling applications.

Light but stiff

Vespel CP grades are fiber-reinforced polymers with good strength-to-weight ratios. They come in a range of options -- laminates, braided reinforcements, and sheet-molding compounds -- and employ a variety of fiber reinforcement singularly or in combination. An ultrahigh modulus carbon-fiber and epoxy combination, CP-9800, is a lightweight laminate used for robotic end effectors that handle semiconductor wafers during processing.

"The laminate provides a highly rigid and vibration-dampening platform that is lighter than traditional ceramic and aluminum end effectors," says Sproesser. Vespel CP-9800 has a density of 1.7 gm/cm³ compared to 2.7 and 4.0 gm/cm³ for aluminum and alumina (Al₂O₃) ceramic, respectively. The composite deflects little under load, improving positional accuracy and stable transfer of large 300-mm wafers. A 3-mm-thick end effector made from the ultrahigh modulus CP-9800 deflects a total of 0.04 in. while carrying a 300-mm wafer. Its Al₂O₃ counterpart deflects 0.07 in.

Designers customize the rigidity of the composite via manipulation of fiber reinforcement and matrix resin selection. Modulus values range from low 23,000 to ultrahigh 46,000 kpsi. The choice of matrix resin (polyimide, phenolic, or epoxy) is also a metric that lets designers tweak composite temperature and chemical resistance. Working temperatures for the various epoxy laminates range from 176 to 446°F. Selection of the highest modulus fiber and the highest temperature epoxy can double the cost of the end effector. Maximum working temperatures can be pushed towards 550°F with the selection of a polyimide instead of epoxy, but again doubling the cost.

Vespel CP-9800 laminates also help boost processing yields. They are less likely to damage wafer surfaces compared to their ceramic counterparts. They are much cleaner than their aluminum analog so there's less chance of wafer contamination during handling. Also, brittle ceramics can shatter from an unintentional impact causing downtime to remove the ceramic particles. Tough composites can endure such impact without shedding debris.

Designers of robots for large, flat-panel-display manufacturers are also looking to replace ceramic end effectors with CP-9800 laminates. That's because the large glass panels are getting bigger and heavier and the end effecter design has reached the mechanical limits of the ceramic. Custom CP-9800 end effectors made from ultrahigh modulus carbon fibers and high-performance resins are tough and strong enough to transport the monolithic (1,500 to 1,800-mm) glass panels from one manufacturing cell to the next. The cross section of the composite end effectors is hollow to reduce weight while still maintaining their rigidity.

Another industry trend that may position composites as the end effecter material of choice, says Sproesser, is the need to add sensors directly into the end effectors to relay a wide range of information such as whether the robot has successfully picked up the wafer or let it go as well as reporting any anomalies in the "health" of the wafer itself. This will let processors increase throughputs and help eliminate defective wafers from the fab line resulting in higher process yields. Ports for the optical sensor can be easily machined in the CP-9800 composite to exacting tolerances.

Hot properties

Another Vespel grade suitable for semiconductor applications is Vespel CR-6100. This sheet-molding composite is a Teflon PFA reinforced with high-tensile-strength carbon fibers oriented in the XY plane. This gives it a lower coefficient of thermal expansion (i.e., good creep resistance) than steel in this plane. From ambient to 500°F its XY CTE is 1.8X10^-6 in./in./°F. Creep resistance in the Z direction is temperature dependent. Values here are 180, 250, and 510 X 10^-6 in./in./°F for temperatures ranging from ambient to 300°F, 300 to 400°F, and 400 to 500°F, respectively.

Other key advantages of the composite are its wide resistance to chemicals at pH values of 0.2 to 14 and excellent wear resistance. The wear rate for Vespel CR-6100 is about one-third that of glass-fiber-reinforced PEEK when tested at 25 ft/min against an unlubricated tri-pin-on AISI carbon-steel disk (16-µin. finish). Additionally, under similar testing conditions the CR-6100 wear rate is about 25 times less than an unfilled PEEK.

Current applications in semiconductor processing include wafer retainer rings for chemical-mechanical planarization (CMP), due to the material's combined chemical and wear resistance compared to unfilled or less chemical-resistant polymers such as polyphenylene sulfide (PPS). These are often specified in less-demanding CMO applications such as oxide CMP.

A thermoplastic Vespel TP grade broadens design freedom for parts used in extreme environments. More complex geometries, parts consolidation, and weight-reduction opportunities come via injection molding, extrusion, and thermoforming. Vespel TP-8005 is an unfilled version that has low outgassing and good resistance to etching compounds. It's used to build chamber liners for wafer processing.

The thermoplastic has a Rockwell E hardness of 47, a room temperature elongation at break of 90%, and a Notched Izod impact of 2 ft-lb/in. Its tensile, flexural, and compressive strengths are 13.3, 19.6, and 21 kpsi, respectively. At temperatures from ambient to 300°F its CTE can range from 2.7 to 2.9 X 10^-5 in./in./°F.

The predominate advantage of using an injection moldable liner is the ability to form a seamless yet flexible one-piece liner that can snap securely in place in the interior of the etching chamber. Other liners are also used in the chamber environment. These designs often include a seam where debris from the etching process collects and later can shed to potentially impact wafer quality. Other designs may even need adhesives to secure the liners to the chamber wall.

Make contact:
Brooks Automation Inc., (978) 262-2400, www.brooks.com
DuPont Vespel parts and shapes, 800-972-7252, www.vespel.com

PERFORMANCE COMPARISON - END EFFECTORS

COMPARATIVE WEAR DATA