Detecting ESD Events in Automated Processing Equipment
Arnie Steinman, Joseph C. Bernier, Donald Boehm, Thomas Albano, Wayne Tan, and Donald L. Pritchard
ESD events can cause device damage and equipment malfunctions in automated processing equipment. Several test methods are available that can detect these charges.
The laws of physics are the same everywhere. Static-charge generation is unavoidable whenever any materials come into contact. Without a static control program, the problems caused by static charge are also unavoidable. The most common problem caused by static charge is electrostatic discharge (ESD).
ESD is the rapid, uncontrolled transfer of charge between objects at different potentials. This results in damaged semiconductor integrated circuits, photomask defects, magnetoresistive read head defects in disk drives, and drive circuit failures for flat-panel displays. ESD also creates a significant amount of electromagnetic interference (EMI). Often mistaken for software errors, EMI resulting from ESD interrupts the operation of production equipment. This is particularly true of equipment that depends on high-speed microprocessors for control. Results include unscheduled downtime, increased maintenance requirements, and frequently, product scrap. Technology trends toward smaller device geometries, faster operating speeds, and increased circuit density make ESD problems worse.1
Solving the ESD problem has become essential to achieving high production yields in modern electronics manufacturing. Static control programs exist for every situation from the silicon factory to the printed circuit board (PCB) assembly and test program. In circuits designed to operate at lower and lower voltages, charges resulting in voltages as low as 50 V can damage or destroy an electronic devicepaving the way for product failure.
For many years, static control programs concentrated on protecting components from the charge generated on the personnel that handled them. Many static control methodsincluding wrist and heel straps, dissipative shoes and flooring, and garmentswere devised to control the charge on people. Increasingly, however, automated equipment controls the production of electronic components, and personnel never come into contact with static-sensitive devices. Solving the ESD problem means ensuring that ESD events do not occur in the equipment used to manufacture and test electronic components.
ESD Hazards in Equipment
The primary method of static-charge generation is triboelectric charging, which occurs when materials are in contact and in motion with respect to each other. It is hard to imagine how this might be prevented in automated equipment, where both the equipment parts and the product are constantly in motion. ESD occurs when charged equipment parts contact the product or when charged products contact grounded equipment parts. A successful static control program for equipment must prevent both of these types of events from occurring. Also, once an object is charged, it can induce charge on nearby conductive objects. This is most obvious when charge is generated on the insulating material of the component package. Charge is then induced on the component leads attached to the internal circuitry. If the leads touch a grounded surface, an ESD event will occur that could damage the component.
Several test methods are available that can determine sensitivity of components to ESD events in equipment. Machine model (MM) ESD testing measures the sensitivity to discharges that might occur from ungrounded conductive equipment parts. A 200-pF capacitor is charged to a known voltage and then discharged through a zero-resistance path into the leads of the component being tested. Charged-device model (CDM) testing simulates the ESD event that occurs when the device package becomes charged, and then the device leads contact a conductive equipment part. In this case, the device itself is charged to a known voltage and the device leads are connected through a 1-W path to ground (see Figure 1). There is little or no established correlation between the safe discharge levels of these test methods or with any other common ESD-related component tests. In most cases, however, modern components are not sensitive below 200-V CDM discharges. There are, of course, occasional exceptions to this level.2,3
Figure 1. Machine model and charged-device model schematics.
Static Control in Equipment
An effective static control program in equipment should be designed so that it prevents both MM and CDM ESD events. Any static control program should start with grounding all materials that might come close to, or in contact with, the static-sensitive components. This prevents the generation of static charge on machine components and eliminates them as a source of charge-creating ESD events. Care must be taken in a grounding program to ensure that moving equipment parts remain grounded when they are in motion. In some cases, static-dissipative materials may be substituted for conductive materials where flexibility, thermal insulation, or other properties not available in conductive ma-terials are needed. If charging of components is unavoidable, static-dissipative materials may be used to slow the resulting discharges and therefore prevent component damage.
In fact, component charging that results in CDM ESD events is the more difficult problem to solve. Most electronic components contain insulators as part of their design or packaging. The epoxy packages of ICs and the substrates of PCBs are the most obvious examples. Handling these insulating materials inevitably generates static charge, and grounding the materials does not remove this charge. If charge generation is unavoidable, air ionization is the only effective method of neutralizing the charge on insulators or isolated conductors. To neutralize the charge that causes CDM ESD damage, ionizers are typically mounted in the load stations and process chambers of automated equipment.
Verifying Equipment Static Control
In a static control program, two basic requirements must be demonstrated. First, are all components in the product-handling path connected to ground? Second, as the product passes through the equipment, is it handled in a way that does not generate static charge above an acceptable level on the component? Assistance in answering these questions is provided by an ESD Association Standard Practice, EOS/ESD DSP10.11999.4 This document contains test methods to verify the integrity of the ground path to equipment parts, as well as to determine whether the product is being charged as it passes through the equipment. The test methods are applicable during the original design of the equipment and during acceptance testing by the end-user.
Although the test methods in DSP10.1 can also be used for periodic verification of the equipment performance, they have one drawback. Automated equipment must be taken off-line to do the testing. To avoid lost production time, periodic testing is often forfeited to maintain product throughput. Testing usually occurs only when product losses reach a level that causes concern. In today's high-speed manufacturing environment, product losses caused by ESD hazards can quickly become very expensive. It is clear that test methods are needed that can be performed without altering or disturbing equipment operation.
ESD and EMI
When ESD occurs, the discharge time is usually 10 nanoseconds or less. Discharging energy in such a short interval results in the generation of broadband electromagnetic radiation, as well as the heat that damages electronic components.5 This electromagnetic radiationespecially in the 10 MHz to 2 GHz frequency rangecan affect the operation of production equipment. Much of today's complex equipment is controlled by microprocessors that operate in the same frequency range as the EMI from ESD events. ESD events cause a variety of equipment operating problems including stoppages, software errors, testing and calibration inaccuracies, and mishandling, all of which can cause physical component damage. EMI can be either radiated or conducted over long distances, so identifying the EMI source is often difficult. It may not even occur in the equipment experiencing the operating problem, and it tends to be random in nature.
Component damage due to ESD is a more serious problem because it tends to be repetitive, rather than random. A machine action that charges a component will generally charge all components being handled. At some later point in the equipment's operation, the component contacts ground, and an ESD event occurs. As with any ESD event, EMI is generated. However, this EMI can be detected with appropriate instrumentation.
Using EMI Locators
When ESD is the suspected cause of component damage or equipment problems, detecting the EMI generated by the ESD event can help isolate the problem. Such testing can determine whether static charge has been generated, and it serves as a measurement point to ascertain whether any static control methods have been successful. Because such testing is a measure of dynamic operating conditions, it can prevent time wasted trying to find the cause of random machine stoppages or ESD-damaged components. It is usually not necessary to interrupt equipment operations to make measurements.
EMI locators, which are available in a number of different forms, can be used for this testing. In its simplest form, an EMI locator consists of an AM radio tuned off station. A popping noise is heard when an ESD event occurs. The most complex form consists of a wideband (>1 GHz) digital storage oscilloscope with a set of appropriate antennas, probes, and software. A number of methods have been devised that use multiple locations of this equipment to pinpoint the precise location of an ESD event.
Several papers have been published detailing how an antenna, typically a monopole antenna in the range of 550 mm in length, can be used to detect the presence of ESD pulses in a local area.69 A set of antennas can be used not only to detect the presence of an ESD event, but also to determine the location of the pulse in three dimensions.10,11 Using the same concept as a global positioning system, the difference in the arrival times of the signal is directly related to the difference in the distance of each antenna from the ESD source. With the time deltas and the locations of the antennas known, the location of the spark can be uniquely identified. Waveforms from a set of three antennas are shown in Figure 2. The location of the CDM ESD event that produced these waveforms can be identified using an appropriate analysis program.
Figure 2. Example of radiated EMI from a CDM ESD event detected by a set of three antennas. Source: Joe Bernier at Intersil.
Most other types of EMI-locating equipment consist of high-frequency receiving circuitry followed by level detectors to determine the signal's magnitude. For detecting EMI from ESD events, the equipment should have some way of differentiating between the short impulse of EMI from the ESD event and the continuous high-frequency radiation of other EMI sources. The sidebar "EMI Locators" at the bottom of this page describes several types available.
One caution needs to be observed when using EMI locators to detect ESD events that cause component damage. The signal received is generated in areas usually surrounded by grounded metal components. The signal may have to pass through equipment panels and travel some distance through the air before it reaches the detector. There may be other radio-frequency sources and reflecting or absorbing materials in the area. It is difficult to establish any correlation between the amplitude of the signal received by the EMI locator and the energy in the ESD event that produced the signal. The EMI locator primarily determines that an ESD event took place. The locator indicates that a particular static control method has eliminated the ESD event. It should not be assumed that every ESD event detected results in damage to components or equipment problems. Additional testing is needed to establish this connection.
Detecting Static Charge and ESD
An EMI locator should be useful in pinpointing the location of an ESD event. If not, it still should have demonstrated whether an ESD event occurred in the process equipment. In either circumstance, the next step is to search for the origin of the charge, and then to apply countermeasures that prevent charge generation or charge retention.
Identifying the presence of static charge in automated equipment presents significant difficulties. Coulombmeters with Faraday cups, ESD indicators, electrostatic fieldmeters, and electrostatic voltmeters are the most commonly used instrumentation for detecting or measuring charge accumulation. Of these, only the coulombmeter with Faraday cup (shown in Figure 3) measures the charge directly. The other instruments (shown in Figure 4) locate charges indirectly by detecting or measuring their electrostatic fields.
Figure 3. Faraday cup for charge measurement.
Figure 4. Indirect charge measurment methods: (a) fieldmeter, (b) static sensor, and (c) voltmeter with probes.
The coulombmeter with Faraday cup is used to measure the charge on small objects directly. The part to be measured is carefully transferred (without generating additional charge) to the interior of the Faraday cup. However, a problem in using the Faraday cup is that the part must be removed from the equipment to make the measurement. This often involves disassembly of the equipment or, at the very least, stopping the equipment. Moreover, it is difficult to accomplish when large objects like PCBs are involved.
Many fieldmeters are handheld instruments designed to measure the strength of the electric field produced by charge on a surface. Handheld instruments are usually not appropriate for locating charged surfaces in equipment that is operating unless the handheld instrument can be suitably affixed in the equipment. It should be remembered that fieldmeters installed close to a charged surface alter the field from the charge on that surface and therefore may give an inaccurate measurement. Figure 4A shows the electric field lines both with and without the fieldmeter in place.
When handheld fieldmeters are used to measure the field on a moving object, it is particularly important to follow an appropriate calibration procedure (see EOS/ESD DSP 10.1). Considerable errors can be introduced when measuring fields from charges on insulating materials.
Static sensors such as the one shown in Figure 4B incorporate very high input-impedance circuitry. These can be used to sense the field generated by a charged part as it moves through the process tool. The sensor should be mounted as close as practical to the part, and it should be no wider than the part being measured to eliminate the effects of field suppression by nearby grounds.
As the part moves in the vicinity of the sensor, the measuring circuit shows any change in the electric field amplitude, indicating whether the part is charged. Although this is a relative measurement, it can be calibrated by making a contact measurement on the charged part and comparing it to the field strength measured. Environmental conditions such as humidity and temperature variations can also affect the level of the electric field measurement and should be taken into account.
Unlike fieldmeters, nullifying any preexisting field is not required, making this technology useful for indicating charge on parts in high-throughput machines. Mounted permanently, static sensors provide continuous monitoring and enable closed-loop control of the equipment's static control system performance.
Electrostatic voltmeters (see Figure 4C) use voltage feedback to their sensor probe housing to null the electric field between the charged surface and the probe. Compared with fieldmeters, this method minimizes capacitive loading of the charged surface and more accurately reports the potential on the charged surface. Nevertheless, it is still difficult to accurately measure a voltage on the surface of a charged insulator. Because the sensor probe housing is at some nonzero voltage, care must be taken in mounting these probes in equipment.
Electrostatic voltmeters and electrostatic fieldmeters featuring small probes can be mounted in critical locations in automatic handling equipment to monitor the charge on parts as they pass by the probe. The probes are small enough to be useful in the small confines of equipment. In-situ calibration of these probes is often necessary because their measurements are affected by the field-suppression effect of grounded surfaces; the size, speed, and distance of the part from the probe; and the orientation of the charged surface with respect to the probe. Care must be taken in identifying locations for the probes to ensure that they make appropriate measurements without interfering with the movement of equipment parts.
Static Event Detectors
The first static event detector (SED) was invented by Zero Static Systems in the late 1980s. The detector is small enough to be placed on circuit boards and detects the current pulse in an ESD event through an antenna or external clip. An amplified signal is processed to produce a reflectance change in a built-in liquid-crystal display. The SED, which is designed to trip at a predetermined threshold voltage, detects ESD transients above the selected amplitude and is not polarity sensitive. The device is reset with a magnet, making it reusable. Unfortunately, the threshold setting does not directly relate to ESD damage in electronic circuits.
Electrostatic Designs introduced a second-generation device called the Static Bug. It employs the well-understood ESD susceptibility of metal oxide semiconductor field-effect transistors (MOSFETs). The test methodology is to amplify an ESD transient creating sufficient energy to destroy the gate oxide. The standard configuration has a 300-V ESD failure threshold, and it can be reused until the ESD level is achieved and the SED fails. This type of SED requires additional instrumentation to determine its status.
Motorola has developed a similar device based on another historically ESD-susceptible device, the metal oxide capacitor (MOSCAP). The current leakage through the device significantly increases if the ESD amplitude is sufficient to damage the MOS structure. Both Electrostatic Designs and Motorola SEDs must be removed from the assembly and inserted into a readout unit to ascertain whether the sensor has recorded an ESD event.
Another type of SED is referred to as the ExMOD (exotic magneto-optical detector).14 This detector employs the Faraday ef-fect to detect and optically record an ESD current transient onto a magneto-optic thin-film detector. The magnetic field from the ESD current alters the film's magnetic state and affects the degree of polarization of visible light reflected from the film. Different thresholds are indicated by varying the distance between the film and the ESD current-carrying conductor. An ExMOD (see Figure 5) can be read using a microscope equipped with a polarizing element. This device can remain in the circuitry to be read, and it can be reset without contact using a very strong magnet.
Figure 5. An ExMOD with several sites indicating ESD events.
An SED, which can be monitored optically as it passes through operating equipment, provides a convenient method for verifying whether given automated equipment is generating static-charge levels that result in ESD damage.
There is little question that static-charge problems continue to result in significant losses in high-technology manufacturing. Implemented properly, static control programs can minimize these losses. Increasingly, static control methods must be applied to the automated equipment that produces the product. In the future, equipment static-related problems will increase and personnel-related static problems will tend to decrease. It will be important to develop new diagnostic methods and measurement equipment for ESD in automated equipment. This article has presented some of the methods currently available.
1.L Levit et al., "It's the Hardware. No, Software. No, It's ESD!" Solid State Technology 42, no. 5 (1999).
2.EOS/ESD STM 5.21997, "Machine Model Device Test Method," ESD Association, Rome, NY.
3.EOS/ESD STM 5.31995, "Charged Device Test Method," ESD Association.
4.EOS/ESD DSP10.11999, "Draft Standard Practice for Protection of Electrostatic Discharge Susceptible ItemsAutomated Handling Equipment," ESD Association.
5.M Honda and T Kawamura, "EMI Characteristics of ESD in a Small Air GapARP Governs the EMI," in Proceedings of the EOS/ESD Symposium (Philadelphia, ESD Association, 1984): 124130.
6.T Takai, M Kaneko, and M Honda, "One of the Methods of Observing ESD Around Electronic Equipment," in Proceedings of the EOS/ESD Symposium (Orlando, FL, ESD Association, 1996): 186192.
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9.D Smith, "A New Type of Furniture ESD and its Implications," in Proceedings of the EOS/ESD Symposium (Lake Buena Vista, FL, ESD Association, 1993): 37.
10.J Bernier, G Croft, and R Lowther, "ESD Sources Pinpointed by Analysis of Radio Wave Emissions," Journal of Electrostatics 44, no. 11 (1998): 149157.
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12.A Fujie, "Pinpointing Sources of Static Electricity with EMI Locator," Parts 1 and 2, Nikkei Electronics Asia, December 1992 and January 1993.
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Arnie Steinman, Joseph C. Bernier, Donald Boehm, Thomas Albano, Wayne Tan, and Donald L. Pritchard are members of the ESD Association StandardsAutomated Handler Workgroup (WG 10). Steinman is chief technology officer for Ion Systems (Berkeley, CA). He can be reached at 510/548-3640, or by e-mail at email@example.com. Bernier is reliability staff engineer for the Semiconductor Product Div., Harris Semiconductor (Melbourne, FL). Boehm is executive vice president for Novx Corp. (San Jose). Albano is a member of the Electrostatic Group for Eastman Kodak Co. (Rochester, NY). Tan is a senior member of the technical staff, Manufacturing Services Group, Quality Engineering, for Advanced Micro Devices (Sunnyvale, CA), and Pritchard is marketing and sales manager for TREK (Medina, NY). This article is based on a paper originally presented at the 1999 EOS/ESD Symposium.
The Lucent Technologies Model T100 ESD event detector provides detection levels of 20 mV and 2 V using a small loop antenna attached to the instrument. It contains a counter to total the number of ESD events above the threshold and alarms to indicate when the number of ESD events exceeds a preset number. The instrument can be left near a piece of equipment to monitor ESD events.
The Sanki Model ES-81V is a small, handheld device that uses a short monopole antenna to detect the high-frequency impulses generated during an ESD event. It provides sensitivity levels of 5 and 120 mV, and separate alarm indicators for each level. Optional longer antennas can extend the device sensitivity to lower levels. Because it is a battery-operated device, it is suitable for surveying an entire production facility for the occurrence of ESD events. Because this EMI locator is small, it can be placed inside operating equipment to detect signals that might otherwise be shielded by the equipment's cover panels. This locator can pinpoint the location of an ESD event, which can then be correlated to particular machine operations. Knowing the location enables identification of ESD events that cause equipment malfunctions as well as those that could damage components.6,12,13
The Credence Technologies EM Eye CTM041 and CTM045 are small, handheld devices that use directional antennas to help locate sources of ESD events, not only by proximity, but also by focusing on the direction of maximum signal strength. Both models include ESD event counters and log the magnitude of these events into memory. The devices include an audio capability to listen to the discharges in order to locate their sources. The CTM045DL, which also keeps a data log of ESD events with time-date stamping, uses a computer interface for data retrieval.
Some equipment is suitable for continuous monitoring, which enables changes in static discharge characteristics to be detected quickly and thus reacted to in shorter time frames. Computer logging of monitor results can help isolate the sources by correlating discharge times with, for example, a new tester setup or a new operator beginning a shift. The Credence Technologies AWARE is a low-cost ESD event monitor that provides continuous monitoring of workstations and equipment. These monitors include sensitivity adjustment, an audio event indicator, an LED event indicator, and networking capability for use as remote ESD event sensors in a monitoring system. An analog output indicates the magnitude of ESD events.
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