Though it is an old problem, the dissipation of static discharge is under intense scrutiny by plastics suppliers. They are now fielding materials better able to keep test equipment out of trouble.
RICHARD W. CAMPBELL
DSM Engineering Plastic Products
It's no secret that even small currents at high potentials can damage or destroy an IC. Those familiar with manufacturing semiconductors know just how insidious damage from static discharge can be. A few seemingly innocent actions on the part of an uninformed handler can create catastrophe for a chip. Just walking an ungrounded IC through a static-electric field can make it a candidate for a landfill. Further, static-electric fields can attract dust to component surfaces and the contamination that results may cause damage as well.
Heading off such difficulties is not just a matter of dissipating charge by using a noninsulating material. Sufficiently conductive material that touches a component can also cause a discharge, creating a short or current leakage between leads.
On the other hand, material that's too insulative cannot dissipate destructive charges at all. A chip touching such material can be damaged or destroyed by way of the field-induced charged device model. In this failure mode, the semiconductor component can produce and hold high static charge. The charge, with its electric field, can polarize electrical charges on the conductive elements of the semiconductor device. As the device touches or comes close to a metal component, its polarized mobile charges quickly discharge and cause damage.
The cure for such woes is an electrostatic dissipative (ESd) material. There are several plastics that are in this category. Though nearly all plastics are inherently insulative, materials suppliers have put a lot of effort in recent years into tailoring their electrical properties and increasing their versatility.
Materials that touch sensitive electronic components must be dissipative. Equally important, they must be able to discharge static electricity in a controlled manner. The commonly accepted resistance range for an ESd material is 104 to 1011 Ω (105 to 1012 Ω/square surface resistivity).
Common sense might lead you to select a product with a low resistance to assure that a charge will be dissipate quickly and completely. Surprisingly, this may not always be the best choice. For example, nests and sockets used in backend test fixtures for ICs must be sufficiently insulative to minimize the cross-talk, or bleed, between adjacent leads. But they must be conductive enough to prevent static build-up which can arise during handling of electronic devices. For these applications, a resistance of 109 to 1011 Ω (1010 to 1012 Ω/square surface resistivity) is usually the target.
Other applications, such as wafer boats, often use a more conductive material. But there is a risk here of dissipating a charge too quickly. This can be harmful in some applications because it lessens the protection from charged device model ESD events.
Static decay rate is often used as a criterion for ESD protection. MIL-B-81705C and FTMS 4046 are the generally accepted protocols for measuring the time required for a surface charge to be dissipated. Although there may be general agreement between surface resistance and static decay rate, there is no quantitative formula for predicting one from the other. A time of 2 sec, maximum, is the standard for the dissipation of a 5-kV potential applied to a surface, if the material is to be considered ESd.
Also critical is that the material remain electrically stable. It is unacceptable for a dissipative material to become more conductive after a discharge event. Instability can lead to variable performance and poor process yields. There can be problems above 200 V such as dielectric breakdown of the polymer matrix, making the part immediately and irreversibly conductive. Further, it's unlikely that adjusting the level of the conductive additive in the ESd material will avoid such difficulties. The relationship between surface resistivity and additive concentration is steep. Thus tweaking the recipe is unlikely to produce a consistent product.
Another potential problem for ESd materials is sloughing, which happens when repeated surface abrasion produces conductive "dust." Commonly associated with carbon black, sloughing of conductive particles is something to be avoided around workstations.
Conductive plastics are well suited to applications demanding fast static discharge. Typical applications include handling trays, wafer-handling combs, and test sockets. Carbon-fiber-filled plastics are available with surface resistivities ranging from 102 to 106 Ω/sq.
At temperatures up to 410°F, carbon-fiber-filled polyetherimide (PEI) has stable mechanical and electrical properties, and parts made of the material are dimensionally stable. ESd PEI has a static discharge rate exceeding 0.05 sec.
Semiconductor manufacturers often want a material that exhibits quick and dependable static decay but not fast enough to damage the device.
Various plastics come in the electrostatic dissipative (ESd) range. They offer various combinations of heat resistance, strength, and dimensional stability. For example, ESd acetal has a heat-deflection temperature of 225°F and a surface resistivity of 1010 Ω/sq. The material is widely used for wafer combs, flat finders, and vacuum wand tips.
ESd properties in an acetal come by virtue of alloying the polymer with an intrinsically dissipative polymer (IDP). The ESd acetal has a static decay rate below 0.1 sec at 12% RH.
ESd polyethersulfone (PES) has a special combination of properties: static dissipation, low coefficient of expansion, high strength and heat resistance, and doesn't slough conductive dust. The material has a tensile modulus of 550,000 psi, a heat-deflection temperature (at 264 psi) of 420°F, and a surface resistivity in the intermediate range of 106 to109 Ω/sq
ESd PES, which uses carbon fiber for its ESd properties, frequently serves in fixtures for handling silicon wafers and other electronic components in hard-drive manufacturing and assembly processes. The material has a good rate of discharge (less than 1-sec static decay rate). It also has a low coefficient of thermal expansion, high compressive strength, and good wear resistance. Perhaps most important, it is nonsloughing so it doesn't particulate significantly in handling applications.
ESd fluoropolymer has a heat-deflection temperature of 500°F and a surface resistivity of 1010 to1012 Ω/sq. The material is used as thermal insulation for test chambers, test fixtures, and boat inserts. It's also well suited for aggressive wet chemical environments, such as wafer etching and cleaning. It has a static decay rate below 2 sec at 12% RH. Because it is a relatively soft material, it is not recommended for machining into thin-wall sections or intricate designs.
ESd polyamide-imide (PAI) has a heat deflection temperature of 520°F surface resistivity of 1010 to1012 Ω/sq. It has a low coefficient of thermal expansion, high compressive strength, good wear resistance and can be machined to tight tolerances. These are all important for nests and sockets used in testing semiconductor devices. ESd PAI also has outstanding dielectric properties: a dissipation factor of 0.18151 and a dielectric constant of 5.764, both at 1 MHz.
Static-dissipative nests, sockets, and contactors for backend testing semiconductor equipment, and other device-handling components are made of ESd PAI.
Carbon fiber is used in ESd PAI, which has a static decay rate below 2 sec at 12% RH. Further, like all other materials mentioned here, ESd PAI retains its surface resistivity at electrical forces greater than 1 kV. Other potential products experience dielectric breakdown when exposed to voltages exceeding 100 V, causing unreliable performance.
Semitron ESd 520HR doesn't slough and is a high-temperature material with a surface resistivity between 1010 to 1012 Ω/sq and no dielectric breakdown. The latter is demonstrated by measuring the surface resistivity in a series of voltages, increasing to 1 kV and then decreasing back to 100 V, without moving the test probe. Most materials display a hysteresis effect in this test that arises from an irreversible dielectric breakdown, resulting in an inability to protect the ESD-sensitive device being processed.
Surface resistivity of common
Resistivity level, Ω/sq