Automation is bringing sweeping changes to the electronics and semiconductor industries. Much of the work previously done by people in large assembly areas or clean rooms now is completed by machines or robots, often in ultraclean, isolated mini environments. And as production methods change from personnel to equipment, a new approach to static control is needed.
In the past, the emphasis was on protecting product from the personnel that handled it during production. Now we require static-control methods appropriate for automated production.1,2,3
Proper grounding of conductive and static-dissipative materials is of primary importance in designing and using static-safe automated equipment. Unfortunately, the semiconductor fabrication process requires the use of materials that are insulators, such as Teflon and quartz.
Electronic components often are packaged in insulating materials, such as plastic, epoxy and ceramics. A product and its associated production equipment are constructed of materials that easily charge and are constantly in contact and in motion with respect to each other. As a result, the possibilities are limitless for triboelectric or induction charging and the resulting ESD damage or particle contamination.
Discharge of charged product to nearby equipment surfaces not only causes ESD damage to the product, but also the resulting EMI may interrupt the operation of the production equipment. With high-speed automation, equipment problems may be more significant than random ESD-related product damage.4
Ionizers for Static Control
Air ionizers are used in many applications to deal with static charges on insulators. By creating and delivering balanced quantities of both negative and positive ions, ionizers neutralize whatever charges occur in most work areas.
In clean rooms, ionization generally is the most appropriate method of dealing with charges on insulators. Remember that the requirements for using ionizers in production equipment may be quite different from when they are used in rooms or on work benches. The critical differences are the short distances from the ionizer to the product and from grounded surfaces to the ionizer emitter points.
When ionizers are attached to the ceiling of a room, generally 120 cm to 200 cm are between the ionizer emitter points and the product. The ionizer emitter points operate at voltages from 4,000 V to 20,000 V and the electrostatic field from the points drops off rapidly with distance.
It is good engineering practice to prevent product, or objects that might contact the product, from coming any closer than 20 cm to exposed ion emitter points, a practice easily accomplished by ceiling-mounted ionizers. Most benchtop ionizing blowers shield product from electric fields by providing the emitter points with electrostatic shielding in the blower construction.
In production tools, much smaller dimensions are involved. Often, product is within a few centimeters of the emitter points and can easily be charged to high voltages by the electrostatic field of the points. Depending on the separation of the points and the polarity of the voltage applied to them, adjacent surfaces of the product or tool might be charged to opposite polarities. Rather than remove static charge, these exposed points could create charges that otherwise would not exist.
Ionizer emitter points in rooms should be separated from nearby grounded surfaces. This maximizes the supply of useful ions to the work area rather than uselessly to ground, decreasing the required output current of the ionizer. In turn, this reduces problems of ozone, EMI and particle emissions.
Placing unshielded emitter points in the walls of an equipment process chamber, close to grounded surfaces, increases the ionizer emitter current without producing useful ionization. High emitter currents also increase particle contamination.5
Ionizers for Confined Spaces
The predicted growth of mini environments, cluster tools, automated test handlers, pick-and-place equipment and other automation will make ionization an increasingly important static-control method. Here are some basic requirements for an ionizer designed for these applications:
No exposed emitter points.
Emitter points isolated from ground.
Long-term ionizer balance maintained without external sensors.
No particles in clean-room applications.
Ideally, the task is to create balanced, particle-free ionization in a remotely located ionizer and transport the ions to the equipment process chamber or mini environment.
To illustrate how specialized ionization techniques control static in automated production equipment, consider the following applications.
Wafer Transfer Equipment
Many manufacturing processes generate static charges. Operations that involve cleaning insulators are particularly suspect due to the contact and separation of the insulators, solvents and equipment parts.
Preventing the buildup of static charge during cleaning processes is desirable. If this is not possible, neutralizing static charges before or after a cleaning process is advisable.
For example, problems occurred in a semiconductor cassette-to-cassette transfer tool used after a wafer-cleaning process. This equipment transferred a set of wafers from the cleaning-machine cassette to a cassette that carried the wafers to the next step in a photolithography process. Visible ESD events were noted during this transfer, and unexplained random machine shutdowns also were thought to be due to excessive charges on the wafers.
An investigation discovered that room ionization did not work rapidly enough to eliminate the problem without slowing production operations. Instead, an ionizing chamber attached to a U-shaped manifold was installed directly above the area where wafers were handled (Figure 1). Clean dry air flowed through the ionizer and manifold. Air exited through holes in the manifold, bathing the cassette and wafers with ionization. The transfer equipment turned on the ionized air supply when wafers arrived at the cassette transfer tool and waited a short time before beginning the transfer process. This allowed sufficient time to neutralize the charge on the wafers and cassettes.
Twenty seconds of ionized airflow were sufficient to reduce the charge on the wafers to a level where it had no further effect on the operation of the cassette transfer equipment. Equipment interruptions and wafer breakage were eliminated. 6
Pin Electronics in Test Head
Increasingly expensive equipment is required to test and to verify the high-speed operation of complex electronic devices. Higher speeds often mean the test equipment is more susceptible to static discharges.
In this application, a semiconductor manufacturer was testing microprocessors with a high-speed tester using costly gallium-arsenide devices in the test pin electronics. An unreasonable number of test-head failures were discovered in the test equipment. Not only were the repairs costly, but also—and more seriously—the equipment was not available while repairs were underway.
The test head was removed and a special jig was made to mount an electrostatic field meter in place of the test head. The electric field resulting from static charges on the devices was measured.
The test setup, shown in Figure 2, included a 12-bit transient digitizer operating at 20 S/s to digitize the analog output of the field meter. The field from the charge on the parts was recorded on the fly during normal test-handler operation.
The resulting measurement showed a high electrostatic field at the test-socket location resulting from the charge on each of the devices as they passed through the tester, as shown in Figure 3a. The test handler brought the devices into the high-temperature environmental chamber in sets of four, as seen in the test data. It was determined that high levels of static charge were generated on the insulating packages of the devices before they arrived in the test handler and during their movement by the test handler.
Ionization was chosen to continuously neutralize charges generated on the device packages. An ionizer was designed of materials suitable for the temperature extremes of -70oC to +150oC of the environmental test chamber.
The proximity of large grounded conductors (some of which transported the devices) within the test handler could have unbalanced the ionizer operation. This would have resulted in a continued static problem due to the presence of static charge on the devices.
This problem was eliminated and long-term system balance was accomplished by using isolated ionization techniques.5 Sensors and ionizer feedback control systems were not an option in this application.
Measurements from the field meter showed a significant reduction in static-charge levels with the ionizer in operation (Figure 3b). Since some of the charge was arriving at the test-handler load station on the device packages, an ionizing blower also was mounted above the load station. The results with both ionizers in operation are shown in Figure 3c.
Having both ionizers in operation reduced charges on the devices to levels that no longer damaged the test equipment or presented any hazard to the devices. The ionizer installation in the test handler is shown in Figure 4.
Conclusion
As the use of automated equipment increases, control of static charge will become essential. Air ionization is the primary method for controlling static charge in the interior of high-quality production tools and in clean-room environments, particularly when insulators cannot be eliminated. In some cases, it is the only static-control method that can be used.
Ionizers reduce the number of ESD-related product defects occurring during processing in the confined spaces of production equipment. Ionization also maximizes production equipment uptime, increases the time intervals between maintenance and prevents equipment damage due to static-charge accumulation.
References
1. Steinman, A., “Static-Charge—The Invisible Contaminant,” Cleanroom Management Forum, Microcontamination, October 1992.
2. Shu, C.Y. and Tu, L.C., “Designing, Operating a Submicron Facility with Isolation Technology,” Microcontamination, March 1992.
3. Neuber, A., Laub, H. and Yost, M., “Ionization in Mini-Environment Systems,” SEMI Ultraclean Manufacturing Symposium, Austin, TX, February 1992.
4. Tan, W., “Minimizing ESD Hazards in IC Test Handlers and Automatic Trim/Form Machines,” EOS/ESD Symposium Proceedings, September 1993.
5. Steinman, A., “Ionization for Production Tools,” EOS/ESD Symposium Proceedings, September 1994.
6. Rush, J., et al, “Reducing Static Related Defects and Controller Problems in Semiconductor Production Automation Equipment,” Proceedings of the SEMI Ultraclean Manufacturing Symposium, October 1994.
About the Authors
Arnold Steinman is director of engineering development at Ion Systems. Before joining the company in 1983, he was affiliated with Lawrence Livermore and Lawrence Berkeley Laboratories and later was an independent consultant. Currently, Mr. Steinman is a member of the ESD Association Standards Committee and several other standards work groups and leader of the SEMI ESD Task Force. He graduated from the Polytechnic Institute of Brooklyn with B.S.E.E. and M.S.E.E. degrees.
Lawrence B. Levit, Ph.D., is director of technology development at Ion Systems. Previously, he held technical support, sales and marketing roles at LeCroy and Jandel Scientific Software. Mr. Levit graduated from Case Institute of Technology with a B.S. degree in physics and earned a Ph.D. in experimental high-energy physics from Case Western Reserve University. He is a member of and serves on several standards committees of the ESD Association and the Institute of Environmental Sciences.
Ion Systems, 1005 Parker Street, Berkeley, CA 94710, (510) 548-3640.
Copyright 1997 Nelson Publishing Inc.
April 1997