Q: Products and their manufacturing technology are constantly evolving. The result has been increased device functionality, but it has also resulted in lower Electrostatic Discharge (ESD) withstand voltages for products. Are new types of measurements needed to assure that a manufacturing process can safely handle these products?
A: High technology manufacturing continues to evolve at a rapid pace. In semiconductors, it has brought increased device functionality through small device feature sizes and increased operating speeds. Profitability has improved through the increases in wafer sizes, with today’s 300 mm wafers expected to be replaced with 450 mm wafers over the next decade. In disk drives, we have seen ever increasing storage density in smaller and smaller packages.
New manufacturing methods keep pace with the technology change in the products. Technology changes have made the control of static charge levels on and around the product critically important. Smaller device geometries and the magneto-resistive (MR) heads of disk drives require control of static charge to very low levels. The ESD Association ESD Technology Roadmap gives insights into trends in device technology that are driving future predictions of ESD sensitivity. [1] Control of particles in manufacturing areas has always been needed, but smaller devices features mean smaller particles have the potential to cause defects, and they are more easily attracted to charged surfaces. Increased automation in manufacturing increases the likelihood of equipment problems caused by static discharges. At the same time, equipment complexity makes the application of static control methods more difficult.
Static control programs are established to protect ESD-sensitive products. ESD standards define methods to verify the performance of static control program elements, typically assuring that resistance of these elements falls in a specified range. Unfortunately, they do not measure residual charge or voltage that may be present on the product or equipment, despite the use of static control methods. An enhanced measurement technique, referred to as Process Risk Assessment, makes measurements to demonstrate that the static control program has effectively limited static charge or voltage on personnel, equipment, or product. It can also demonstrate that the static control program has eliminated the occurrence of ESD events. [2]
Process risk analysis demonstrates that there is no place in the process where charge or voltage is generated on personnel or devices that exceeds the device Human Body Model (HBM) or Charged Device Model (CDM) ESD sensitivity. Process risk analysis can identify locations in manufacturing where there may be ESD hazards to devices. For HBM ESD, it will identify those operations where personnel handle devices, whether individually or on circuit boards, and could cause a discharge to the devices. For CDM ESD or Charged Board Events (CBE), it will identify both the source of the charge in the vicinity of the device, as well as the location where the device may contact ground and cause a discharge.
Once the locations of ESD hazards have been identified, appropriate measurements need to be made of the charge or voltage present on personnel or devices. The current standard for measuring charge generation on people, ANSI/ESD STM97.2, uses the Charge Plate Monitor as a high impedance measuring device. It monitors the voltage generated on people connected to the isolated conductive plate of the instrument. The voltage is recorded and analyzed to determine if it is a potential HBM ESD hazard by exceeding the HBM withstand voltage of the devices being handled. [3]
Typically, presence of charge is measured in a work area by determining the resulting electric field from the charge using an electrostatic fieldmeter. Since the meter is “distance sensitive” and sees an area about 10 cm (4 inches) in diameter at the 2.5 cm (1 inch) calibration distance, it is useful only on large objects. For smaller objects, an electrostatic voltmeter is preferred as it makes measurements at much smaller distances (<5 mm = 0.2 inches). Both instruments are used to identify the location of charge that might create an ESD hazard.
The current version of ANSI/ESD S20.20 contains three specifications for the allowable charge or voltage on objects in the work area. [4] Objects whose electric field measures greater than 125 volts/inch must be kept at least 2.5 cm (1 inch) from sensitive product. Objects whose electric field measures greater than 2000 volts/inch must be kept at least 30 cm (12 inches) from the sensitive product. Finally, if there are any isolated conductors in the work area, the voltage on these conductors must be kept under 35 volts. It should be remembered that this document is designed to protect 100 volt HBM and 200 volt CDM sensitive devices. In many cases, products to be protected may have lower ESD withstand voltages. More important, the location and charge level of objects may vary from day-to-day during manufacturing operations. A successful static control program will have elements to assure that there are no charged objects close to sensitive product.
But the true ESD hazard is the presence of voltage on a product itself that results in an ESD event when the product touches ground. Measuring the presence of charge does not determine the actual voltage on the product. It is possible to measure the voltage on the pins of a device with a high impedance, contacting digital voltmeter (HIDVM). This instrument, possessing both a high input impedance (>1014 ohms) and a low input capacitance (<0.01 pF), is able to measure the voltage on very small objects like IC pins, without altering the voltage. This voltage can then be compared with known product ESD sensitivities. While providing useful information, a drawback of this measurement method is that it can only be done when the manufacturing process has been stopped. It does not necessaily characterize what is happening in a moving manufacturing process.
The other indication of ESD hazards is the ESD event itself. The presence of RF radiation from an ESD event may be indicated on an ESD Event Detector. These types of instruments may be used to find the location, and in some cases, the magnitude of ESD events in manufacturing. They can indicate where ESD control methods need to be applied. At the end of the risk analysis process, event detectors may be used to indicate that the ESD mitigation has been successful in eliminating ESD events from the manufacturing process.
Technology innovation is ongoing and manufacturing technology must keep up to assure efficiency and profitability. Static control methods may be tested according to industry standards, but additional process risk assessment methods are needed to assure that the static control elements have been successful in mitigating the ESD risks. ESD Association Working Group 17 is involved in ongoing work to release a standard practice for Process Assessment methods. Contact the ESD Association (www.esda.org) to become involved in this and other ESD standards activities.
References
All available from the ESD Association (www.esda.org)
- Electrostatic Discharge (ESD) Technology Roadmap
- ESD TR17.0-01-15: ESD Process Assessment Methodologies in Electronic Production Lines – Best Practices used in Industry
- ANSI/ESD STM 97.2: Test methods for the measurement of the voltage on a person in combination with floor materials and static control footwear, shoes, or other devices.
- ANSI/ESD S20.20: ESD Association Standard for the Development of an Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies, and Equipment (Excluding Electrically Initiated Explosive Devices)
This article was prepared by Arnold Steinman, M.S.E.E. based on presentations made at ESDA’s Manufacturing Symposia.
Arnold Steinman is a consultant in electrostatics, static charge control, and ionization, utilizing the knowledge and experience gained in 25+ years as chief technology officer for Ion Systems, Alameda, CA. He holds four patents covering air ionizer technology. Steinman graduated from the Polytechnic Institute of Brooklyn, receiving both BSEE and MSEE degrees. He is an ESDA certified ESD program manager and an iNARTE certified ESD engineer.
Founded in 1982, EOS/ESD Association, Inc. is a not for profit, professional organization, dedicated to education and furthering the technology Electrostatic Discharge (ESD) control and prevention. EOS/ESD Association, Inc. sponsors educational programs, develops ESD control and measurement standards, holds international technical symposiums, workshops, tutorials, and foster the exchange of technical information among its members and others.
