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ESD issues in Cleanrooms
 

As electrostatic discharge (ESD) thresholds increase, knowledge of ESD standards and control in cleanrooms is a major concern in both microelectronics and medical device manufacturing

Background
A cleanroom is a hyper-clean environment achieved through the control of ventilation, filtration, temperature, humidity, air pressure, and ionization as well as materials in the room and other parameters. The quality of cleanliness in a cleanroom is significantly determined by the amount of particles floating in the air. The preferred unit of measurement is the micron, which is equal to one millionth of a meter. One micron equals approximately 0.000039th of an inch.

The method most universally accepted to classify cleanrooms comes from ISO 14644 Standard, which uses 0.1 microns per cubic meter. ISO now replaces the Federal Standard 209-1992 (Table 1), which focuses on particles equal to and greater than 0.5 microns per cubic foot of air. Therefore, a Class 100 cleanroom would have a maximum allowable number of 100 particles equal to or greater than 0.5 microns per cubic foot of air, and ISO classifications would be Class 5, which is 3,520 particles/meter cubed (p/m3) equal to or greater than 0.5 particles per cubic meter.

Electrostatic charge generation control is also necessary in cleanrooms as it can reduce yield, disrupt automatic equipment from electromagnetic interference (EMI), and reduce surface particle collection by electrostatic attraction (ESA). ESA is becoming more of a concern as static charges have been seen to actually bond contaminants to surfaces of products or tooling. For example, for a 4” wafer, charged to 1000 volts, a particle of 1 micron in diameter would have the bonding force of over 830,000 pounds per square inch. Therefore, electrostatic attraction between charged objects and particles can be quite strong compared to gravitational, aerodynamic, or adhesion forces.

With the heavy use of insulating materials such as glass, Teflon, and polymers, items can become highly charged. Grounded workstations are a potential concern, as is the use of stainless steel work surfaces compared to static dissipative work surfaces. Conductive work surfaces can be considered a current-carrying hazard to people and ESD-sensitive devices.

The cleanroom classification is illustrated in Table 2. By U.S. law, Federal Standard 209E can be superseded by new international standards. The 209E standard will be phased out and replaced globally by ISO 14644 [1].

ISO Class 8
(Fed Std. Class 100,000)
An ISO Class 8 cleanroom has no more than 3,520,000 particles per cubic meter equal to and larger than 0.5 microns. For ISO Class 8, the Fed Std. 209E rating is Class 100,000.

In a Class 100,000 (ISO Class 8) cleanroom, amine-free antistatic and carbon products are extensively used. Corrugated containers are seen in the facility, as well as garments and shoes with heel grounders. A Class 100,000 cleanroom is not always a well-controlled environment. Masks or gloves are not required. Items shipped into an ISO Class 8 cleanroom do not require cleaning or bagging in advance.

ISO Class 7
(Fed Std. Class 10,000)
An ISO Class 7 cleanroom has no more than 352,000 particles per cubic meter equal to and larger than 0.5 microns. For ISO Class 7, the Fed Std. 209E rating is Class 10,000. Masks are not used, but wearing cleanroom smocks, booties, hairnets and latex gloves are required antistatic products and carbon-loaded materials are acceptable in this environment. Extensive use of ionization is concentrated in specific areas. Heel grounders or booties are acceptable. Likewise, cross-linked cleanroom specific products are acceptable. Polyethylene antistatic foams are sometimes used with antistatic-type work carriers.

ISO Class 6
(Fed Std. Class 1000)
An ISO Class 6 cleanroom has no more than 35,200 particles per cubic meter equal to and larger than 0.5 microns. For ISO Class 6, the Fed Std. 209E rating is Class 1,000.

In an ISO Class 6 cleanroom, full body ESD garments are required with booties utilizing a grounding mechanism as heel grounders or an affixed grounding strip. Also, static dissipative suits are permissible and used as needed. Carbon loaded materials are avoided unless the products pass a battery of tests for particle count, outgassing, and chemical compatibility. Products that enter the cleanroom are cleaned and double bagged prior to being sent into an ISO Class 6 environment. Gloves and facemasks are used. After only one use, garments must be laundered before reentering an ISO Class 6 cleanroom. New or laundered garments must be bagged before entrance into the gowning room. Antistatic products can be used if they pass contamination-generating tests. Extensive use of ionization is used to reduce particle attraction or to minimize ESD related issues.

ISO Class 5
(Fed Std. Class 100)
An ISO Class 5 cleanroom has no more than 3,520 particles per cubic meter equal to and larger than 0.5 microns. For ISO Class 5, the Fed Std. 209E rating is Class 100.
Extensive use of ionization is recommended in an ISO Class 5 environment. A worker is required to wear cleanroom garments that cover his or her body, head, face, hands, and eyes. An increased number of ionization units can be observed at the workstations and in manufacturing areas. Contamination-free carbon or amine-free topical antistats are minimally used in an ISO Class 5 cleanroom due to contamination issues. Inherently conductive polymer-based products are widely utilized due to their humidity independence and ability to be IPA rinsed without losing favorable ESD properties. Items being introduced into an ISO Class 5 cleanroom are to be cleaned and double bagged, prior to entrance of the gowning room. Gloves are worn with no skin exposure. The use of ESD vinyl mats for work surfaces are limited unless the material does not contain antistats with amines that will migrate or transfer onto objects placed onto the surface. Conventional non-cleanroom paper and pens must not be used in this environment. A company may need to employ one or more full time contamination control engineers to manage both ESD and contamination issues. Large equipment which is difficult to double bag must be 70% IPA wiped or deionized water bathed before being introduced into the cleanroom.

ISO Class 4
(Fed Std. Class 10)
An ISO Class 4 cleanroom has no more than 352 particles per cubic meter equal to and larger than 0.5 microns. For an ISO Class 4, the Fed Std. 209E rating is Class 10. In an ISO Class 4 cleanroom, a full bunny suit with a miniature air circulation unit is required. Ionization at all workstations and manufacturing centers is considered standard work practice. The company may employ a full time ESD and contamination control engineer. Materials must be 70% IPA or deionized water bathed before entrance into the work areas. The same practices in an ISO Class 5 environment will be employed in an ISO Class 4 cleanroom. Consumables have more rigid ESD and contamination requirements for use in an ISO Class 4 environment.

ISO Class 3
(Fed Std. Class 1)
An ISO Class 3 cleanroom has no more than 35 particles per cubic meter equal to and larger than 0.5 microns. For an ISO Class 3, the Fed Std. 209E rating is Class 1.

People do not work directly in an ISO Class 3 cleanroom environment. Only ionizers with specialized low particle generating emitter points or newer generation ionizers such as Alpha are used to ESD control issues. Newer generation ionizers for the control of ESA issues must not generate contaminants and are monitored for particle count on a continuous basis.

Standard ionization with moving parts pose a contamination generator in an environment held to the highest standards for cleanliness. Alpha particle or X-Ray ionizers may be utilized after passing a rigorous qualification sequence. Equipment tends to generate particles, therefore special attention is given to eliminate or divert possible sources. A company may employ one or more contamination control engineers and an ESD engineer if components generate sensitive electrostatic field generated events.

Materials must be 70% IPA wiped or distilled water bathed before introduction into the cleanroom. It is important that the purchasing department issues purchase orders requiring guarantees or certification from the supplier for cleanliness and/or ESD conformance of their products.

Tools for the Control of ESD and ESA Issues
The reader should gain a greater understanding of cleanroom classifications and ESD and ESA measures to not only control contamination, but also static electricity concerns. ESD control measures have traditionally been utilized for defense, electronics assembly, semiconductor, wafer manufacturing, and the disk drive industry. In the ESA control arena, fiber optics, medical, pharmaceutical, wafer, and disk drive companies have stepped up programs to control ESA in their cleanrooms. Controlling both ESD events and static electricity in the cleanroom is important for all these industries. However, depending upon the sensitivity level of electronic components or MR heads, ESD may be of greater concern whereas ESA could weigh more heavily in wafer manufacturing and in areas where contamination control is foremost.

Advancements in ESD Standards and ESD/ESA measurement instrumentation combined with static control technology provide the proper tools for evaluating materials, equipment, personnel and, most recently, ESD events and ESA fields.

ESD Control Measures in the Cleanroom
We will now discuss the ESD standards or industry evaluation practices for measuring the effectiveness of selected products used in the control of ESD events or ESA for a representative selection of the following products and materials:
1. Flooring
2. Workstations
3. Conveyors
4. Racks
5. Plexiglas or Polycarbonate Enclosures
6. Transfer Carts
7. Chairs
8. Garments
9. Ionizations
10. ESD Monitoring
11. ESD Materials

1) Polymer Totes
2) Vacuumed Formed Trays
3) Gloves
4) Suction Cups
5) Barrier or Static Shielding Bags
6) Paper
7) Films
8) Vacuum Hoses
9) Wafer Boats

Surface Resistance versus Relative Humidity (RH)
According to ANSI/ESD S541-2003 (Packaging and Materials ESD Standard), surface resistance measurements at 1.0 x 1011 ohms for materials are considered insulators. In the insulative range, materials become nonconductive and hold static charges for several seconds or more. Surface resistance rises and falls with relative humidity and the RH at 1.0 x 1011 ohms is the cut-off for retention of static dissipative properties. A low cut-off is desired for packaging materials because dry air may be encountered in shipping and handling. During unpacking, triboelectric charges (two surfaces rubbing together) not drained to ground could cause damage by field induction.

Table 3 lists possible charge generation characteristics at various relative humidities. ANSI/ESD S20.20-1999 recommends a target RH between 30% - 70%. Below 30% RH, materials have a greater tendency to charge. The ESDA (Electrostatic Discharge Association) standard for surface resistance (ANSI/ESD STM11.11-2001) recommends the evaluation of planar materials at 12% +/-3% RH @730F +/-50F after 48 hours of preconditioning. Therefore, a relationship between relative humidity and surface resistance exists for ESD materials that are not humidity independent. A moisture barrier bag’s (MBB) ESD characteristics represent an excellent example to understand the relationship between relative humidity and surface resistance or a material’s ability to remain static dissipative at low RH.

When dry air or nitrogen is used to draw a vacuum for removal of moisture in a MBB combined with a desiccant package, the RH inside the bag can reach 4% or below. Measuring surface resistance of the bag after 48-72 hours of preconditioning will allow the opportunity of selecting a material that has a greater likelihood of maintaining charge free ESD properties when the contents are removed from the package. Figure 1 illustrates the ANSI/ESD STM11.11-2001 test method. Figure 2 demonstrates how one result can vary from another.

ANSI/ESD STM11.11-2001 can be used to evaluate wafer boat carrier totes, wafer separators, conveyors, cleanroom paper, films, Plexiglas or polycarbonate type enclosures, barrier or static shielding bags, cleanroom ESD foam, polymer hinged cases and other products.

ANSI/ESD STM11.13-2004 for Small Profile
Materials
Due to limited space, not all resistance measurement test methods will be reviewed in this article. However, it is important to discuss the newly released ANSI/ESD STM11.13-2004 (standard test method) for cleanroom materials that fall outside the measurement range of a concentric ring fixture as employed by ANSI/ESD STM11.11-2001. Figure 3 illustrates an evaluation method for static dissipative vacuum or suction cups. When the polymer structure becomes worn after too many uses, the surface resistance of the cups becomes insulative (>1.0 x 1011 ohms).

Thermoformed or vacuum formed trays are common throughout cleanroom environments. Many trays are considered complex in construction. A concentric ring fixture used to measure planar materials is not capable of measuring “draws” of a vacuum formed tray. Therefore, ANSI/ESD STM11.13-2004 is a useful tool for measuring gloves, trays, suction cups, wafer boats, films and other small profile polymers.

Typical passing results are listed in Table 4 following format for results secured from the measurement of trays.

ANSI/ESD Association S7.1-2005 for Flooring Areas
ANSI/ESD Association S7.1-2005 is an excellent evaluation method for measuring the point-to-point and resistance-to-ground (RTG) resistance for evaluating a floor. Conductive (permanent ESD characteristics) floors are designed to measure at or below 1.0 x 106 ohms whereas topically treated floors’ point-to-point resistance is between 1.0 x 106 ohms to 1.0 x 1.0 x 109 ohms. A floor should be adequate in preventing a charge from triboelectrification, but may act poorly as a ground for persons wearing insulative shoes. Figure 4 illustrates a point to point resistance in accordance with the ANSI/ESD S7.1-2005 testing method.
Figure 5 represents the measurement of a floor or workstation from the surface to a groundable point.

Regarding Workstations per ANSI/ESD STM4.1-1997: Current Carrying Capability
An alternating current of 4-21mA causes reflex action, 21-40 mA causes muscular inhibition and 40-100 mA causes respiratory block. Workstations with readings below 1.0 x 106 ohms may be too conductive and are considered a charged device model (CDM) or current carrying hazard. All stainless steel carts, tables and benches should have static dissipative cleanroom laminates on the surface.

On pages 155-156 of ESD from A to Z in accordance with DOD-Handbook-263 an antistatic top may drain too slowly, whereas conductive tops are an electrical and or safety hazard. The conductive surface could damage “charged” ESD items by providing the means for a rapid discharge or a fast drain to ground. Surface resistance for a workstation should be between 1.0 x 106 ohms to 1.0 x 109 ohms to be safe with a 1 Megohm resistor connection to ground. Again, Figure 5 illustrates the Resistance to Ground technique whereas Figure 6 illustrates the ANSI/ESD S7.1-2005 Point to Point testing protocol.

Crypto Charges
These hidden charges are an example of stored energy. Crypto charges are difficult to detect with an electrostatic field meter and occur on packaging materials with a buried conductive layer that suppresses the voltage on a charged nonconductive surface. The surface of a conductor comes into contact with a nonconductive surface and a field transfers by induction. Contact compound induction (CCI) can impart several thousand volts and cause ESD damage.

An electrostatic field meter is employed to measure the Plexiglas window which is in close proximity to “pick and place” wafers during a robotic process. The window measured 147,400 volts (Figure 7) at a distance of 1.0 inches.

The wafers were charged when passed in proximity to the Plexiglas structure. When the window was treated with a semi-transparent film, attraction of particles on the wafers was minimized.

This measurement technique would have been ineffective to pin point charge, therefore a non-contact computer interfaced meter was employed as shown in Figure 8.

A non-contact voltage probe and meter (Figure 8) can be used for measuring charge on disks, wafers, vacuum formed trays, and other objects. Figure 9 illustrates actual results of measuring charge on a pink polymer antistatic film substrate.

Faraday Cup Measurements per ESDA Adv. 11.2-1995
Another method for measuring residual charge on materials is the use of a Faraday Cup. Measurements less than 1nC/pF (100 volts) have been acceptable for polymer materials in the semiconductor industry.

The tray pictured in Figure 10 was subjected to ionization for removal of charges that may have been present even after being grounded. In this case, the tray was charged to +/-1000 volts and then grounded after free falling into the Faraday Cup. The tray was capable of draining residual charges and the final results measured <1nC/pF.

Static Decay
This test measures the rate of decay of a charged isolated object at 10 percent of its original value. Federal Test Method Standard No. 101C, Test Method Number 4046.1 specifies that the charged object at +/- 5000 volts should drain the voltage to +/- 500 volts in less than 2.0 seconds. Recently, +/-1000 volts to +/-100 volts has been incorporated to measure static decay in less than 2.0 seconds. Therefore, this test represents a material’s ability to dissipate induced voltage with proper grounding. However, this test method has difficulty with materials of complex construction. Such materials are ESD convoluted foams, vacuum formed polymers and small items that can be too large to fit in the static decay equipment’s fixturing.

According to Section 30.5 of the Military Handbook-263A, Appendix, H, this test does not always typify real world events. However, it is relatively effective for correlation purposes in a controlled environment. A modification of the Fed Std. 101C, Method 4046.1, is widely used in the disk drive and semiconductor industry with a charge plate monitor (Figure 11).

Figure 12 illustrates a test being performed on cleanroom gloves. A passing score is decay from +/-1000 volts to +/-100 volts in less than 2.0 seconds. For the disk drive industry, a lower limit may be established from +/-1000 volts to +/-3 volts in less than 2.0 seconds.

ANSI/ESD STM2.1-1997 for ESD Garments
It is important to utilize garments that do not store charge when worn by personnel in accordance with ANSI/ ESD STM2.1-1997 “for the Protection of Electrostatic Susceptible Items- Garments.” This section will feature a garment material that was subjected to ANSI/ESD STM11.11-2001 for Surface resistance measurements at 73°F +/-5° @ 12% RH +/-3% RH after 48-72 hours of preconditioning.

Point-To-Point Resistance
ANSI/ESD STM 2.1-1997 employs the use of two NFPA 5-lb weights to evaluate panel to panel and sleeve-to-sleeve characteristics. Special ESD STM2.1 compliant clips are used for cuff-to-cuff measurements. A target range of 1.0 x 106 ohms to 1.0 x 109 ohms is desired. Said product is evaluated on an insulative test bed.

Another examination (not a standard, but an industry practice) employed by some organizations involves the placement of a non-contact volt meter probe inside a garment. A charged object is brought in proximity to a grounded garment. There should be minimal or no penetration of an electrostatic field inside the garment. A ZEROSTAT gun capable of generating several thousand volts is fired directly at the non-contact voltage probe (outside the garment) for a peak voltage measured at +5,355 and -7,185 volts. Said garment measured +15 volts and -40 volts when the non-contact voltage probe was placed inside the gown after being subjected to the ZEROSTAT firing exercise.

Ionization Measurements per ANSI/ESD STM3.1 -2000
Due to its relative polarity, a statically charged object in the vicinity of ionization will attract or repel ions. When ions intermix with charged surfaces, the neutralization process takes place. In the majority of applications, reduction of a static charge to a few hundred volts is sufficient to eliminate dust attraction. However, with MR heads, a few volts can have disastrous consequences. A new generation of ionizers constantly monitors ion output to ensure neutralization at less than +/-1.0 volts to reduce the risk of damage to MR heads. However, if an ionizer is out of balance, it is also possible to generate static charges in materials or on objects through induction.

ESA results from airborne particles that are attracted to charged surfaces of opposite polarities. This can be a major cause for rejects in the pharmaceutical, medical, disk drive, photomasking, fiber optics, and defense industries.

The electronics industry primarily addresses charge generation by the use of grounded conductive or static dissipative materials in the work place using ANSI/ESD S541-2003 for compliant packaging in conformance to ANSI/ESD S20.20-1999.

In cleanrooms, laminar air flows can generate electrostatic fields. Tribocharging from contact separation within equipment can compromise an ESD sensitive component when placed onto a conductive surface to generate a spark or discharge. Ionization used in combination with HEPA filtration is utilized extensively in cleanrooms from ISO Class 2 – 4 cleanrooms. Likewise, ISO Class 5 – 8 cleanrooms will position ionization at workstations in order to reduce charges on insulators. For the ionization evaluation process, ANSI/ESD STM3.1-2000 is an effective tool in the measurement of a unit’s performance for static decay and balance.

A 6” x 6” 20pF charge plate is positioned under the ionizer to measure the effective balance and decay at nine separate positions.

In Figure 13, the balance results were under +/-1.0 volts with the use of a feedback controller. Other ionization controller systems monitor ion balance, decay, humidity, and particle count.

In short, the control of electrostatic fields and events is a most difficult task. However, the use of ESD testing protocols and understanding ESD material characteristics can assist in the development of an ESD/ESA Preventative program that incorporates procedures for qualifying products and materials, while insuring that outside or self auditing takes place on a regular basis. One source of charge generation is with polymers. By utilizing the newly released ESD Packaging and Materials Document (ANSI/ESD S541-2003), a good first step can be accomplished in the implementation of a cleanroom program to not only reduce ESA (electrostatic attraction), but also control ESD events for ISO Class 1-8 cleanroom environments.

One must verify a supplier’s claims for both ESD compliance and product cleanliness. Qualifying products with a specification sheet alone can lead to disappointment without in house or 3rd party verification. There are many case histories of ESD and cleanroom products being purchased without the proper qualification protocols for the specific class of cleanroom only to be discarded or replaced when materials or products are found to be non compliant.

References:
1 The U.S. General Services Administration (GSA) released a , “Airborne Particulate Cleanliness Classes in cleanrooms and Clean Zones,” on November 29, 2001. Note: The IEST address changed after issuance of the notice. The new address is 5005 Newport Drive, Suite 506, Rolling Meadows, IL 60008-3841, http: //www.iest.org.