Static charge control has become a necessity in cleanroom environments to enchance the production of semiconductor,disk drive, flatpannel display, medical and optical devices, and any variety of other products. By controlling static charge it is possible to
There is a large body of information concerning the efects and mitigation of static charge on products and equipment.
This article concerns itself with the interactions between static charge, cleanroom personnel and cleanrom operation.
Static chage problems caused by personnel
Personnel movement in any work environment generates static charge triboelectrically. Walking on the floor, touching other materials, and moving objects from one place to another are activities by static charges. Static charge may be found on objects that are handled, on personnel, or both. When personnel or objects become trioelectrically charged, they may transfer the charge directly to products or equipment. This transfer is an ESD event that can damage the product or cause an equipment malfunction. Charged personnel may also cause induction charging of other objects without actually touching them. If a conductive object is brough near a charged person, or any other charged objects, a charge will be induces to flow to, or from, the conductive object contacts a grounded surface. Cleanliess and chemical process requirements in the cleanroom require both a low humidity environment(typically 40percent RH +- 10percent RH)and the use of insulating materials such as glass, Teflon, kynar and other plastic. Cleanliness requires personnel to be enclosed in cleanroom garments, booties, and glove which are also made primarily of insulating materials. Contact between these materials is the primaary source of static charge generated in cleanroom.
Static charge problems for personnel
The amount of static charge generated on an abject is proportional to its surface area. Whenever large surfaces are involved such as 300-mm silicon wafers, large glass pannels, plastic films or extrusions, high levels of static charge result. Although rarely fatal, discharges from highly charged surfaces are unpleasant to personnel will change the way they perform a production step to avoid static discharges.
A more serious problem can coocu when personnel must handle, or contact, charged product around automated equipment. ESD may cause a "sensation shock" or "startle reaction". This unexpected physical motion caused by ab ESD event may place personnel at harzard to moveing machine parts. Where personnel must handle chemicals, unexpected physical motion might result in chemical spills. In areas fine powders must be handled, there is always a concern about explosions that might result from discharges caused bt personnel. Although static charge most often causes damage to sensitive products, the possible effects on personnel should not be overlooked.
Solving Many devices exist for preventing
personnel static problems or dissipating static charge on personnel or the objects they handle. Wrist or heel straps, conductive shoes and floors,smocks,dissipative materials and air ionization are common static control methods in work areas outside cleanrooms. Both contamination control and chemical resistance requirements may make some of these methods difficult or impossible to use in cleanroom.
Most passive static control methods make use of static dissipative or conductive materials that involve additives to insulative base materials. As with any other objects brough into a cleanroom, these materials must demonstrate cleanroom compatibility. This involves testing for particle shedding (particulary since many materials use carbon or metalllic particles or fibers to achieve the desired conductivity) of both new materials and those that have undergone some period of cleanroom use. Static control materials must also pass the sasme outgassing and nonvolatile residue testing as other cleanroom materials.
Particular attention must be paid to emitter point materials used by air ionizers. Metallic or germanium emitter points may be compatible with cleanrooms making medical products or pharmaceuticals, but they may not be appropriate with the more sensitive operation in cleanrooms thet make flat pannel displays or disk drivers. Semiconductor cleanrooms use almost exclusively silicon ion emitter points to avoid the risk of contaminatin.
Wrist straps are the primary method of preventing the buildup of static charge on personnel. There are two main components, the wrist cuff and the ground cord. The cuff is designed to make a reliable continuous connection arond the wearer's wrist, but may or may not be a conductive material itself. It may be combined with a metal plate thet is used to make skin contact on the wearer's wrist. The flexible ground cord connects the cuff asembly to ground and usually incorporates a current limitting resistor for safety.
Cuff types include fabric, plastic and metal, and there is a variety of ground cord styles and connectors between the various components.Comfort and low restriction of movement will certainly be consideratins. Reliability of connection will be important as the wrist strap will be worn inside other cleanroom garments.
In some applications, reliability of the wrist strap is so important thet constant monitoring devices will be used to assure wrist strap performance.
Heel strap may be an alternative to wrist straps in some applications. More common in nonm-cleanroom applications, these devices consist of a grounding strap attached to the outside of the wearer'sche, and a second strap that contacts the wearer's skin inside a shoe or sock. Since contact with a grounded floor is required for the straps to be effective, heel straps should be used on booth of the wearer's feet, providing grounding even while walking. Wearers must maintain at least one foot on the floor at all times. In the heel straps with the cleanroom booties, shoes and floor materials will need to be considered.
Any wrist or heel strap device used in a cleanroom will need to meet cleanroom requirements for low particle shedding, outgassing and chemical compatibility. There may be some flexibility in meeting these requirements for devices that will be worn under cleanroom garments. The type of cleanroom operation may determine whether attaching personnel to a workstation with a wrist strap is feasible.
Shoes,booties and conductive floors
An alternative method of grounding may be needed for mobile personnel where it is not possible to use a wrist strap. In this case, a grounding system consisting of footwear and flooring may be used. It should be recognized that this method may not have the relability and simple verifiability available with wrist straps.
Shoes or other footwear used for this purpose provide a conductive path from the wearer's foot to the sole of the shoe. Depending on the cleanroom, this footwear may or may not be contained in booties. If used for static control, the booties must provide a conductive path from the sole of the bootie. Finally, the floor material of the room must provide a conductive path from the sole of the shes or booties to ground. As with heel straps described previously, at least one of the wearer's feet must be in contact with the flooring at all times with this grounding system.
Typically, the static control footwear will be reserved for use only in the cleanroom. If exposed in the cleanroom. If exposed in the cleanroom compatibility requirements. If enclosed in cleanroom booties, the requirements may not be as stringent. The materials used in both the booties and floor materials to provide a path to ground are exposed in the cleanroom and subject to a significant amount of abrasion. Cleanroom compatibility will need to be demonstrated through wear tests. Maintenance required for the floor materials and multiple washings of the booties will afffect their static control properties.
Cleanroom garments completely enclose personnel to prevent personnel generated particles from entering the cleanroom work area. The materials used in these garments, including shoes or booties, are often insulating high levels of static charge when they contact other materials. Charge is generated both inside and outside the garment as well as onpersonnel wearing the garments. Fields from these charges will affect nearby product and equipment. In many cleanroom operations personnel also wear some type of glove on their hands. This glove will generally charge anything that personnel handle during production,even if the glove itself is made of static dissipative materials.
Static control garments for cleanroom use are made by including some type of conductive path in the garment construction. This may be matallic or carbon fibers woven into textiles, or additive to the garment, glove or bootie materials. A conductive path must be established from the wearer'sbody to both ground and the indivisual garment pieces for these control devices. As noted above, the booties may be an essential part of the oersonnel ground path.
It will be diffcult to use garments as the primary method of personnel grounding. There are many pieces(head cover, 1 or 2 piece bunny suit, boties,gloves,etc,)provided with grounding paths.Establishing a relaible connection to the body of the wwearer will also be diffcult. In addition, cleanroom garments are subject to multiple washings that may ultimately affect the static control properties. There is no current agreement an test metjod to asure the static control provide some shielding from the charge developed on the wearer's body, they should be used together with other, more positive means of personnel grounding.
The importance of personnel issues with regard to static charge control should not be underestimated. In cleanrooms as in other work areas, personnel are the primary sources of the static charge that reaches the product, causing ESD damage, equipment malfunctions and other problems. High static charges may be harzardous to personnel as well. Personnel static control devices reduce the generation and storage og static charge on people, reducing the posibility of static problems.Neutralizing high charges on cleanroom surfaces makes it less likely that discharges will affect cleanroom personnel. As with all materials used in the cleanroom, personnel static control devices must demonstrate their cleanroom compatiblity.
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1. Protect against productcontamination due to electrostatic
attraction and bonding of particulates.
2.Prevent damage done directly to products by electrostatic
discharge(ESD)
3. Prevent the malfunction of production equipment
caused bt ESD
4. Reduce the possibility of injury to personnel
caused by ESD
What is ESD(Electrostatic Discharge)
When there is a difference in electrostatic potential between two charged objects, an exchange of electrostatic energy occurs so that their potentials can become balanced. This exchange of energy is called an Electrostatic Discharge.
An Electrostatic Discharge occurs when there is friction or separa tion. The pulse period is generally short within the units nS ~ uS. When there is a discharge, even at a low voltage below 200V, Gate Oxide destruction or Junction spiking failure may occur. The level of electrostatic discharge that humans can normally feel is above 3000V~4000V. However, most semiconductors can be damaged by a discharge of a level we cannot feel.
The exchange of electrostatic energy between two charged objects is explained by the figure given below. According to their priority, one object loses an electron to become positively charged, and the other object gains an electron to become negatively charged.
Model Explaining ESD HBM (Human Body Model) The Human Body Model ESD is a modeling of a situation where a certain amount of energy from an external electrical power or an electrostatic energy source becomes charged within the human body to become discharged as electrostatic energy. The MIL-STD sets the Resistance as 1500 Ohm, and Capacitance as 100pF for testing. The basic test circuit can be configured as shown below.
The Charged Device Model is when an electric charge within the Package becomes discharged through an external conductive material, and the device is damaged through a self-discharge. This model is different from other electrostatic discharge models in that the Pulse duration is very short, which causes Oxide failures such as oxide or poly-filament breakdown.
