Blog

 

Static Electricity in Cleanrooms: Affects and Remedies

April 21, 2011

The following is an excerpt from the article “Static Electricity in Cleanrooms” by Lawrence B. Levit, Ph.D., founder of LBL Scientific. He has worked in the static charge control industry for over 15 years. Levit is a senior member of the ESD Association, a senior member of the IEST and sits on the Board of Directors of the SiVa ESD Society and is the chair of WG22 of the IEST. He holds 6 patents in the area of ESD control.

Introduction
Static electricity is often overlooked in cleanroom environments and the results of this oversight can cause a reduction in profitability. In some factories where the effects are not studied and understood, they are dealt with by budgeting a reduction in production yield for unforeseen issues. Static charge and its effects certainly can be understood and it can be kept in check. The purpose of this article is to remove the “Black Magic” from the issue, explain how it affects the manufacturing process and discuss remedies

What is special about a cleanroom?
A cleanroon is a unique environment. For many applications it is regulated to a low relative humidity (RH) to optimize the process. Much of the particulate contamination is excluded by massive amounts of air filtration involving high efficiency particulate air (HEPA) filters. Objects in the cleanroom are wiped down before they can enter into the room. As it turns out, these actions are very good for maintaining an ultra clean environment but they are also the prescription for achieving massive levels of static charge in a cleanroom.

It is well known that static charge is created efficiently in a low humidity environment. It is commonplace for individuals to get stung by a spark when they reach for a doorknob in the winter time. Buildings are heated and lower RH results from the temperature rise. This same effect causes the rate of static charge generation to be higher in a cleanroom, especially a low RH cleanroom, than in a conventional room.

Once static charge has been generated by contact with other materials, nature has several methods to dissipate the charge. The first mechanism is conduction of this electrical charge to ground through any surface contamination on the object. Normally, such contamination, for example oil from a person’s hands, is removed from the object before it enters the cleanroom and the object remains clean because the products are only handled by gloved hands.

The only mechanism remaining for the natural dissipation of surface charge is the presence of ions in the air. These ions are created by naturally occurring radioactivity and by other items like runner water and electrical motors. Unfortunately, ions cannot pass through a HEPA filter – they are attracted electrostatically to the materials of the HEPA filter – so they are removed from the air entering the cleanroom.

To complicate matters further, most cleanrooms employ many insulators such as glass and plastic. Teflon, for example, is an incredibly effective insulator and holds onto its static charge aggressively. Glass and other plastics are also very effective insulator. As a consequence of the above argument, cleanrooms allow static charge to be generated very efficiently and dissipated very poorly. The result is levels of static charge which far exceed those in conventional rooms.

For the full article and to learn how to control ESD in a cleanroom, follow this link to “Static Electricity in Cleanrooms.”

Levit graduated from Case Institute of Technology, Cleveland, Ohio with a BS degree in physics with honors.  In 1970, he earned a Ph.D. in Experimental High Energy Physics from Case Western Reserve University. He can guide you to a more profitable factory by eliminating the sources of ESD damage, mitigating microcontamination issues and identifying EMI from ESD that is effecting your robotics and automated test systems. Contact LBL Scientific by Phone at 925-788-2969   or    EMail

 

ESD Protection in the Field: Field Service Kits

April 7, 2011

ESD Protection in the Field
Static damage is a constant threat anywhere micro-electronic components are used, but these components are at highest risk during servicing. The Transforming Technologies’ FSM2424 Field Service Kit is designed to quickly and reliably remove static charge on the service technician and provide a safe static-free surface to lay parts. The kit includes a 24” x 24” rugged mat with two 8” x 12” pockets for storing tools, an adjustable wrist band, coil cord and 15’ ground cord.
The FSM2424 Field Service Kits eliminates static charge through the principle of grounding. Both the mat and the operator’s wrist band connect to the specialized ground cord which when properly connected to ground, provides a safe path for static charges to drain to ground.

Field Service Kit

Field Service Procedure
1. Upon arrival at the service call site, unpack the field service kit in a convenient area near the equipment to be serviced.
2. Remove any accessories from the pockets and attach the ground cord to the mat via one of the two snaps found in the corner of the mat.
3. Connect the opposite end of the cord to a reliable ground. Common ground locations are water pipes, unpainted equipment frames, and building frames.
4. Slip on the wrist band and adjust the size to fit comfortably around the wrist but tight enough to make contact with the skin at all times. Snap the coil cord to the wrist band.
5. Insert the banana plug of the coil cord into the jack of the ground cord. This connects the wrist band to the same common ground as the work surface.
6. It is now safe to remove and handle static-sensitive components.

Field Service Care
The FSM2424 Field Service Kits can be cleaned with ESD-safe mat and surface cleaner or with a mild detergent and water as needed. Once clean, allow the material to air dry, removing any excess water/moisture with an absorbent cloth. DO NOT use heat of any kind to dry mat. Though inherently flame resistant, the field service products will burn with enough exposure to open flame or other heat sources of sufficient temperature. DO NOT use a clothes dryer, as the excess heat may cause the PVC material to retain the shape it held in the dryer. Most strong solvents are not recommended. AVOID: Halogens, concentrated Oxidizing Agents, Ethers, ketones, and Low-Molecular Weight Solvents derived from the Ether and ketones fami-lies. They include (but are not limited to) TetraHydroFuran (THF), Methyl Ethyl ketone (MEK), Methyl Isobutyl Ketone (MIBK) and Acetone. These chemicals will mar and/or destroy the sur-face of the PVC product (the Ketones), react with it (Oxidizing Agents), or actually dissolve it (THF and other Ethers). Wrist strap can be hand or machined washed.

For information on resistance monitoring, dual conductor grounding products or any general questions, please contact Transforming Technologies at 419-841-9552 or email at info@transforming-technologies.com.

 

Resistance Monitoring and Dual Conductor Workstations

April 4, 2011

Resistance Monitoring
Resistance based monitoring is the most complete wrist strap monitoring system available. Workstations using resistance monitors are almost never at risk. Basic systems use impedance technology and single wire wrist straps which can be easily fooled. If a wrist strap is worn incorrectly, the monitor can still register a “pass” condition or if the wire of the wrist strap is severed the workstation could be put at risk for ESD damage. Dual conductor wrist bands offer fail safe protection because the monitor will alarm if the operator looses contact with one ground wire. It is very rare to loose both ground wires at the same time, so the workstation is still protected even during a alarm condition. The CM2800 series from Transforming Technologies is a high quality resistance monitor and provides continuous monitoring of two operators, two mats, or two grounded workstation tools.

CM2800 constant monitor dual wire

CM2800 Dual Wire Constant Monitor

A New Cost Effective Source
For many years, resistance monitoring and dual conductor products were limited to mainly 3M.  But  Transforming Technologies now offers a value alternative for these premium 3M* dual conductor products. We incorporate the best design elements of expired patent # 5,018,044 with our own experience making premium quality ESD products to produce the WB5000, WB7000 to be used with the resistance monitor CM2800 series. Transforming Technologies and 3M’s dual conductor products are functionally equivalent*. The wrist bands, coil cords and resistance monitors can be used together in any combination!

Dual wire metal wrist band comparison

Both 3M and Transforming Technologies’ Speidel metal band feature two 180 de-gree conductive paths around the wrist, providing maximum skin contact. The expandable band is constructed with two stainless steel back plates and is available in small, medium and large sizes.

fabric-dual conductor wirst band sets

Transforming Technologies and 3M’s fabric wrist strap features a band made of a silver plated, monofilament, continuous thread woven together with elastic nylon to maintain full conductivity, comfort and reliability, while providing rapid and con-tinuous drain of static charge.

Coil Cords
Used in conjunction with dual conductor constant workstation monitors, Transforming Technologies’ CC3000 series dual conductor coil cords provide unmatched reliability and value. A double insulated jacket provides incredible durability and a wide diameter straight plug makes it easy to insert and remove the cord from a remote input jack. The coil cord come standard in 5’, 10’ or 12’ lengths and are functionally equivalent to the 3M coil cords.

dual-conductor-coil-cords*Compatibility with particular resistance monitors should be verified.

For information on resistance monitoring, dual conductor grounding products or any general questions, please contact Transforming Technologies at 419-841-9552 or email at info@transforming-technologies.com.

 

Dual Conductor Wrist Strap Cord Flex Testing For Transforming Technologies

March 31, 2011


Over the past year, Transforming Technologies has worked diligently to create a line of dual conductor grounding products that is competitive in design and value to 3M’s dual line.   Recently our CC3000 Series Dual Conductor Coil Cords were put through an independent test by Affinity Static Control Consulting. Below is an excerpt from their test results.

Introduction:
Tom Watkins of Transforming Technologies, LLC., contacted David E. Swenson of Affinity Static Control Consulting, LLC (Affinity) regarding testing of dual conductor wrist strap cords according to ANSI/ESD S1.1 – Wrist Straps, for flex life. The aforementioned Standard requires a minimum of 16,000 cycles through 120 degrees of rotational Arc while under tension of 1 pound on the cord under test.
Specifications:
Standard 1.1 requires that the minimum life of all samples tested have a flex life of at least 8,000 cycles and that the average of all samples exceed 16,000 cycles. Flex life failure includes an increase in electrical resistance exceeding 25% of the rated resistance value or any evidence of strain relief or cord jacket failure. In the case of the dual conductor cords tested, there is a 1 megohm resistor in each of the conductors installed in the wires at the wrist strap plug end of the cord. The cord continuity is from barrel to barrel and from tip to tip of the modified phono-jack type connectors with an expected resistance of 0.9 kohms to 1.1 megohms. The test apparatus has a capability of testing two cords at one time at a rate of 2220 cycles per hour.

Test Results:
Samples were selected at random from the set of 10 received cords for testing and placed on the tester 2 at a time, starting with the wrist strap plug end first. All the samples tested survived well past the 16,000 cycles required by Standard S1.1. No evidence of strain relief cracking or electrical change was noted on either end of the dual conductor cords until much higher cycle times. The cords, as tested, meet the requirements of S1.1 with regards to strain relief integrity.

Summary:
The dual conductor cords provided by Transforming Technologies, LLC for qualification testing meet and exceed the requirements for flex life established in ANSI/ESD S1.1 – Wrist Straps.  The full report from Affinity can be found here.

 

Constant Monitor Selection Guide: Impedance Vs. Resistance

March 21, 2011

Constant Monitors are excellent tools for static control and should be considered for any grounded workstation.  ESD Constant monitors reduce production costs by eliminating the time spent on testing wrist straps before each shift. Further savings may be realized by reduced ESD damage from malfunctioning wrist straps. But a quick web search results in a dizzying amount of options.  The following article outlines the important considerations when choosing an ESD Constant Monitor.   For more information, please read “Survey of Constant (Continuous) Monitors for Wrist Straps” in the ANSI/ESD S20.20.

Type of Constant Monitors:

There are several types of constant monitors available. Understanding the different technologies used for constant monitoring will make your decision easier.  For the purpose of this article, we will be reviewing the two most common types:  Impedance and Resistance Monitors.

Impedance (or single wire) constant monitors
Impedance monitors use a very low AC voltage to detect a person though a single wrist strap. When a person is attached and the wrist strap is functioning, the monitor is in an unalarmed state. The impedance monitor uses the phase difference between current and voltage to detect changes in impedance of the cord, band and person. A detection circuit is used to reduce false alarms and eliminate adjustments. These types of monitors list anywhere from $30-$100 and any standard wristband and coil cord can be used. The CM400 series from Transforming Technologies are single wire impedance monitors.

Constant-Monitor_CM420-Large

Single Wire Constant Monitor CM420

Resistance (or Dual Wire) constant monitors
This type of monitor is used with a two wire (dual conductor) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the ground loop, consisting of a wire, a person, a wrist-band, and a second wire. If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state. An important feature of the dual wire wrist strap is that even if one conductor is severed, the operator has reliable path-to-ground with other wire.  These types of monitors range from $80-$200 and must use dual wire wrist straps. The CM2800 series from Transforming Technologies area full line a Resistance Monitors and can be used with most commercially available dual wire wrist straps.

CM2800-constant-monitor

CM2800 Constant Monitor

Monitoring Features:

Constant monitors can be very simple, or can be loaded with bells and whistles depending on the application. The following are some common features:

Work surface Monitoring: An option available with some constant monitors is the ability to monitor work surface ground connections.
Tool Monitoring: Constant monitors may also monitor grounded equipment  in a manner similar to work surfaces.
Data logging: Some monitoring systems have data recording capabilities.
Adjustable Alarms: Some monitors allow users to set the alarm parameters in order to satisfy higher ESD standards.
Alarm Accessories: Display modules, external relays and sirens are available for specific applications.

Conclusion:

Constant monitors are reliable, money saving devices used ensure elements of a workstation are grounded. Single wire monitors, such as the CM400 series from Transforming Technologies use single wire wrist straps and impedance technology to ensure grounding.  The CM400 is the most cost effective and monitors one wrist strap.  The CM410 monitors one wrist strap and the work surface. The CM420 is the fully loaded model that monitors two wrist straps and the work surface.  The CM2800 dual wire constant monitor from Transforming Technologies is the top of the line and monitors two wrist straps, two work surfaces, two grounded tools and has user programmable alarm parameters.  The entry level CM2815 monitor uses the same great design of CM2800 and monitors two wrist straps, and two work surfaces.  Use the following graph to help choose your constant monitor:

constant-monitor-selection-guide

Constant Monitor Selection Guide from Transforming Technologies

 

ESTECH 2011: Transforming Technologies Will See You There!

March 15, 2011

Transforming Technologies will be exhibiting at ESTECH 2011.  The annual technical meeting and exposition of IEST will be held May 2-5, 2011, in northwest suburban Chicago.  There is no substitute for peer-to-peer interaction, which you cannot get with the Internet, e-mail, and phone.  ESTECH offers attendees a valuable educational experience with seminars and continuing education courses in the fields of design, test, and evaluation/product reliability; contamination control; aerospace; and nanotechnology. Interact with leaders from these industries, academia, and government at the 57th Annual Technical Meeting and Exposition of IEST.

Find Transforming Technologies at Booth 15!

ESTECH 2011
May 2-5, 2011
Chicago Marriott Schaumburg, Schaumburg, Illinois

The exposition will be held during the Presidential Welcome Reception on Tuesday, May 3
and all-day Wednesday, May 4.

 

Alphaboost™ Electronically-Enhanced Alpha Ionization: The First Ionizer Designed for the 25 nm Node

March 15, 2011

The following article was written by Tom Watkins, President of Transforming Technologies.  The full article can be found here. For more information, please view Cleanroom Air Ionization in a Nano World a presentation prepared by Dr. Larry Levitt of LBL Scientific that discusses Alpha Ionization.

Introduction

As semiconductor fabrication moves to the 25 nm node, ultrafine particles (those under 100 nm) generated by and on corona emitter points now pose a threat to manufacturing. Corona ionizers employ high voltages to emitter point(s) to create an ion current. Cleanroom corona ionizers can be very effective at controlling a wide range of static related issues in semiconductor fabrication, but corona ionizers create micron, sub-micron and deep sub-micron particlesi.

Alpha ionization is an inherently clean and balanced technology that eliminates all particle concerns associated with corona ionization. Alpha technology eliminates airborne molecular contamination (AMC) particle agglomeration risks caused by corona ionizers.

AlphaBoost™ bipolar ionization systems significantly enhance the effective range of P-2042 alpha sources (NRD, Inc.)and offer a practical alternative for static control in ultraclean manufacturing environments. Alphaboost™ uses self-contained, pulse DC fields to repel ions rapidly away from the alpha source. The Alphaboost™ system variable frequency control provides excellent static protection, in multiple congurations.

alphaboost technical drawing
corona ionization tips with particles
Alphaboost™ electronically-enhanced Alpha ionizer
Contamination on the tip of a Corona ionizer

Background

Signicant work has been done to study the mechanisms by which corona ionizers create particlesii. A variety of theories have been put forward on the subject. In summary, there are two dominant mechanisms, both related to the corona process itself. These processes are both a direct consequence of the energy density associated with the intense electric field required to create the ions. The emitter points of a corona ionizer must be quite sharp and the plasma created in the atmosphere immediately adjacent to the tip is characterized by a very high temperature.

In the first process, high temperature effects the physical integrity of the emitter point material and emitter material is sloughed offiii,iv. Emitter sloughing is currently reduced in cleanroom corona ionizers by optimization of the corona emitter point shape and material and applied voltages. These design subtleties lower the temperature of the plasma.

The second process involves the interaction of the corona plasma with airborne molecular contaminants in the cleanroom air. There are many solvents whose vapors become dissolved in the atmosphere and are referred to as AMCs. For example, hexamethyldisilazane (HMDS) is a large molecule used as a wetting agent in the photolithography process. It is well known that ionizers require much more frequent cleaning in a litho bay, but it is less well realized that additional particles are released from corona ionizer points into the cleanroom.

Any AMCs which pass through the corona region will react and agglomerate into small particles which are attracted to the emitter points by the dielectrophoretic force. When the particles contact the tip, some undergo charge exchange and are repelled into the cleanroom. Moving to a broader emitter tip made up of material which has good thermal conductivity lowers the temperature of the plasma and reduces the size of the agglomerated particles. In our experience, it makes the majority of them smaller than can be measured with a conventional laser particle counter. However, they still exist, and can be measured using a condensation nucleus counter. Today, these very small (<100 nm) particles are comparable to or larger than the feature size on modern semiconductors (CD~25 nm). Alpha ionizers utilize polonium 210 (Po210), a naturally occurring element in the soil, plants, vegetables, etc. The source emits 5.3 MeV alpha particles. The alphas move through the air colliding with air molecules as they go. Each time they collide, they give up some of their energy by ionizing the molecules they strike. On the average, each alpha particle travels 3 cm though the air and creates hundreds of thousands of ion pairs along its path before it stops.

At that point, the alpha particle picks up two electrons and becomes a helium atom, and drifts up through the atmosphere. There is no corona, no plasma and no agglomeration. There are no emitter tips to wear. There are no plasma regions and hence no agglomeration of AMCs. The process is inherently clean.

AlphaBoost™ bipolar ionization systems utilize clean, alpha ionization technology and extends the effective range by employing low intensity electric fields to move the ions rapidly away from the alpha source and into the laminar flow of the cleanroom. The nominal, variable pulsed voltage propel the ions and enables the alpha source to provide reliable protection at up to a meter in distance. AlphaBoost™ is designed to provide an ionizer which is safe, clean and requires NO maintenance for a full year.


i M. Blitshteyn and S. Shelton Contamination and erosion of cleanroom air ionizer emitters. Microcontamination 3,8
(August 1986)

ii Yost, Michael, et al., “Method For Measuring Particles From Air Ionization Equipment”, Institute of Environmental
Sciences, 35th Annual Technical Meeting, May 3, 1989

iii K. L. Mittal, Particles on Surfaces: Detection, Adhesion, and Removal, Volume 3, 1995Marcel Dekker, p250

iv Benjamin Liu et al, Characterization of Electronic Ionizers for Clean Rooms, Proceedings of the Technical Meeting of
the IES, 1985

 

Heel Grounders: 1 meg Vs. 2 meg

March 14, 2011

One Meg Vs 2 Meg Heel Grounders

One Meg Vs Two Meg Heel Grounders

Introduction

Have you ever wondered what it means when a heel grounder described as having a “1 meg resistor”? or a 2 meg resistor?  The size of a current limiting resistor is often included in the description  of personal grounding products, but what does it mean and how concerned should I be with the size of the resistor when I am buying heel grounders?  Today we will answer these questions but first some background:

What is a Resistor?

Resistor

A common resistor

A resistor is a component of an electrical circuit that resists the flow of electrical current. A resistor has two terminals across which electricity must pass, and is designed to drop the voltage of the current as it flows from one terminal to the next. A resistor is primarily used to create and maintain a known safe current . Most grounding products have a current limiting resistor and it is most commonly one megohm, rated at least 1/4 watt with a working voltage rating of 250 volts. This ensures that the flowing current will be within safe levels.

Heel Grounders and ESD Protection?
Heel grounders discharge static from a person to ground by connecting the person to a grounded walking surface. A heel grounder is made from a conductive material and is worn on each shoe.  A conductive ribbon placed inside the wearer’s shoe or sock makes electrical contact with the skin through perspiration. The ribbon is joined to a resistor which limits current should accidental exposure to electricity that may  occur. The other end of the resistor is joined to a conductive sole. This sole contacts a grounded ESD floor mat or ESD flooring system. Heel grounders should be worn on both feet to maintain ground contact while walking. UL and OSHA recommends a minimum of 1 megohm resistance to ground (RTG) in order to limit current for safety purposes.

Types of Heel Grounders?
Heel Grounders come in a variety of styles:

  • Cup Style: Made with two reversible soles for longer life span.
  • Foot Grounder: Provides a more complete path-to-ground due to wider contact area and heel-to-toe coverage.
  • Disposable: Strip of conductive material is applied to shoe. Economical and perfect for one time use.
  • Toe Grounder: Ideal for high heels when standard heel grounders won’t fit properly.

Resistors and Heel Grounders
Heel grounders come with either a 1 megohm or 2 megohm resistor. When one foot is on the ground, a 1 megohm heel grounder gives you an RTG of 1 megohm and a 2 megohm heel grounders results in 2 megohm RTG. But when both feet are on the ground, the sum of the resistors yield a RTG of 1/2 that measurement.
For example:

  • When you wear two, 1 megohm heel grounders and have both feet on the floor, your RTG is only 1/2 megohm, NOT 1 megohm!
  • If you wear two, 2 megohm heel grounders an have both feet on the ground, your RTG is 1 megohm.
  • By wearing two heel grounders with a 2 megohm resistor you are complying with UL and OSHA at all times.

This can be sometimes confusing.  So we made a diagram to illustrate:

One Meg resistor vs Two Diagram

Conclusion

Heel grounders are an effective and popular way to ground mobile personnel in ESD areas. Heel grounders use built in resistors to limit any potential current that flows through the device to ensure operator safety.  The size of the resistor that you use depends on the standards of the specific operation,  but by wearing two heel grounders with a 2 megohm resistor you are complying with UL and OSHA at all times.

To learn more about ESD heel grounders and Resistors, visit Transforming Technologies.com