As the fundamental structures of electronic devices become smaller, their capability to withstand the damage inflicted by electrostatic discharge (ESD) becomes less as well. Five years ago, such damage typically began occurring at charge levels of 20 nC. Today, device geometries have shrunk so much that damage begins anywhere from 0.2 nC to 0.5 nC, a 100-fold increase in sensitivity. And the trend will continue.
The enormous costs associated with the effects of electrostatic charge (ESC) are becoming more obvious by the day. To avoid these formidable challenges, a new technology and a more profound understanding of workplace electrostatics are required.
In addition, operator efficiency is being scrutinized. Time-wasters include the traffic jams caused when personnel queue up while testing their ESD gear before entering the wafer fab area.
With this in mind, potential users are considering the capital dollars required for newer, real-time testing at the workstation a worthwhile trade-off, with payback achieved in a relatively short period of time. One solution is a workstation voltage-detection system that provides real-time monitoring
ESD in Submicron Space
With the advent of such hypersensitive devices such as magnetoresistive heads in the disk drive industry and submicron structures in semiconductor devices, many ESD truisms are being overturned at these smaller geometries. Consequently, some of the traditional ESD-protective workplace practices, taken for granted for years, no longer are valid at these levels.
For example, mere spot checking of ionization balance and neutralization of charge no longer is considered acceptable for critical environments. With the control requirements of today’s products and even tighter requirements forecast in the next 12 to 18 months, real-time ionization monitoring at a specific product workstation is considered a must.
No longer are manufacturers willing to assume that the ionization system is performing properly between calibration checks. They require real-time monitoring integrated with information on temperature, humidity, and particle count.
Low-frequency electrostatic fields from power-distribution wiring in the workplace also are commanding serious attention. Peak field strengths of several hundred volts per meter appear to be very common in the workplace. This level is more than sufficient to cause device failure by electrostatic induction.
The wrist straps and conductive footwear as we know them today cannot keep the electrostatic potential below the now required levels of less than 20 V. Even more alarming, certain wrist-strap monitoring schemes are suspected of actually contributing to the degradation or failure of sensitive devices.
This happens because certain wrist-strap monitors apply measurement voltages to split-conductor wrist straps to establish the integrity of the wrist-strap-to-ground path. The procedure may charge the wrist-strap wearers to levels that exceed the capabilities of sensitive structures to withstand damage or, at a minimum, do not allow the system to detect voltages at lower levels (<20 V) now required by some users.A New Approach
A new, two-fold approach to the ESC/ESD problem has been developed. First, an entire work area should be audited, station by station. Not only should this be done to verify that ESD protective devices are performing their intended function, but also to discover other potential ESD problems which may be hidden from obvious consideration.1
The complementary step is to install permanent ESC monitors at each critical workstation to provide continuous voltage detection (Figure 1). This will provide both an immediate alert whenever a pre-set voltage threshold is exceeded and a permanent record of variations and deviations when connected to a data acquisition system for later analysis.
Like conventional equipment, these devices also use split-conductor wrist straps. In this case, however, no voltages are applied to make capacitive- or resistive-type measurements between the two isolated sections of the wrist strap. Half of the wrist strap connects the wearer to ground in the traditional manner, and the other isolated half measures the body voltage of the wearer. If the measured body voltage exceeds a pre-determined level (typically 2 V to 5 V), an alarm sounds.
In addition, a proximity channel and a sensing antenna detect an approaching charged human body. Human intruders into a workspace area are a major contributor to device damage via electrostatic induction.
This same proximity input can perform general-purpose ±5-kV contact electrometer measurements. The inclusion of current-loop and analog-voltage outputs satisfies the recent trend of connecting individual workstations together through data acquisition and facility monitoring systems.
Such plant-wide environmental data acquisition systems have been used in wafer fabs for some time. However, the parameters usually reported were temperature, humidity, gas pressure, and gas flow. Now, ESC/ESD has been added as an environmental variable.
ESC/ESD data acquisition systems will lead to a better understanding of the total effect of electrostatic variables and related protective devices (Figure 2). This is combined with the calculation of the costs associated with ESC/ESD protective measures, or conversely, the costs of the lack of these measures. These systems also will allow proactive oversight in the manufacturing process since it will be easier to track, recognize, and isolate batches of devices that might have been exposed to potentially damaging levels of ESC.
In-Circuit Protection
Another recent development is the static-event detection and protection device. This is basically a magneto-optic detector on a chip that can be incorporated in an ESD path, either as a discrete device or integrated into the input/output structure of another integrated circuit.
When an ESD current flows through the detector, many tiny domains or pixels will be switched on optically, each domain corresponding to a certain current level and polarity. The status of the domains can be read either optically with a polarized microscope or electrically.
The device can be reset externally and, provided it was not destroyed by excessive ESD current, used indefinitely. It is relatively simple and rugged. Availability is in die form or a variety of packages with optical windows.
Summary
What ESD professionals need to focus on today is the net result of the performance of the individual ESD-protective products and its effect on ESC control at the workstation. This is a redirection in philosophy but the next step in fully understanding workstation control. The performance of one ESD-related product at the workstation is not as important alone as is its contribution to the entire ESC/ESD protection equation.
The battle against ESD/ESC goes on, and the weapons become more ingenious, sophisticated, and refined. Still one important message must be repeated again and again: No ESC means no ESD. Since ESC is the root cause of ESD-related damage, that’s where most of the firepower must be directed.
Reference
1. Heymann, S., Newberg, C., Verbiest, N., and Branst, L., “Charge Is the Real Enemy,” Evaluation Engineering, July 1996, pp. 54-55.
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About the Authors
Steve Heymann is president of Novx, 1590 Oakland Rd., San Jose, CA 95131, (800) 728-6689.
Carl Newberg is the ESD program manager in the Enterprise Systems Group at Western Digital, 1599 N. Broadway, Rochester, MN 55906, (507) 286-7122.
Noel Verbiest is the power systems engineer at Cisco Systems., 170 W. Tasman Dr., San Jose, CA 95134, (408) 526-6285.
Lee Branst is president of Caracal Communications, P.O. Box 1513, Redondo Beach, CA 90278, (310) 542-4233.
Copyright 1997 Nelson Publishing Inc.
November 1997