Static Control in Desiccator Cabinets

• What is electrostatic discharge?
• How is it generated?
• How does it damage microelectronic devices?
• How can it be controlled?
• What are the advantages and disadvantages of static control methods available today?

Why all the Static?
Static electricity is an excess or deficiency of electrons on a surface. The total number of deficient or excess electrons determines the charge on that surface. A surface exhibiting an excess of electrons is charged negatively, and an electron-deficient surface is charged positively.

How are Electrostatic Charges Generated?
Electrostatic charges are usually generated by friction between and/or separation of two dissimilar materials, at least one of which is a nonconductor or a poor conductor of electricity. The accumulated charge (static) resides on the surface of, rather than within, the charged nonconductive object.

Electrostatic discharge (ESD) occurs when an accumulated charge (which may be present on an operator, the operator's clothing, or on any other non-conductive object) is discharged through a conductor (which may be part of a wafer, a chip, or a packaged device), and it may occur whenever the electrostatic potential corresponding to accumulated charge is great enough.

How Do Electrostatic Charges Damage Sensitive Devices?
Microelectronic devices are degraded or destroyed by static in two ways: by excessive voltage and by excessive current. An operator can easily damage the devices he is working with in either of these ways. A voltage-sensitive device may be damaged by exposure to the extremely high field intensity surrounding a highly-charged person (charges may reach as high as 5000V). On the other hand, if he touches the device, an operator may pass a transient discharge directly through it. This electrostatic discharge (ESD) produces such a high current density that it can overheat and even melt current-sensitive circuitry.

Manufacturers have taken steps to combat these problems throughout the manufacturing process, but until recently, little was done to ensure static-free storage. Desiccator cabinets provide protection against contaminants, but nearly all are made of acrylic, one of the most prolific generators of ESD.

Why Use Acrylic in Desiccator Manufacturing?
In critical contamination-control and static-control applications, you shouldn't.

Acrylic features many attractive characteristics: it allows easy and inexpensive fabrication, permits optimal viewing of stored parts, and stands up to the most rugged manufacturing and stockroom conditions. Yet it also generates static charges. Not only does this static generation threaten sensitive components, but it also makes your desiccator a dust magnet. Particles that cling to charged acrylic surfaces threaten parts, diminish visibility, and are difficult to remove.

Hence, acrylic desiccators can be a dangerous place to store microelectronic parts. For this reason, transparent static-dissipative plastics (such as Terra's static-dissipative PVC) are becoming the industry standard in cleanroom applications.

How Can Electrostatic Discharge be Controlled?
The basic concept behind static protection for microelectronic components is the prevention of static buildup whenever possible and the quick, reliable removal of already existing charges. The means by which these charges are removed depends on whether the charged object is a conductor or a nonconductor (insulator). Both types of materials coexist throughout the semiconductor manufacturing environment.

Once a nonconductive surface has obtained a charge, the only way it can be neutralized is by means of an ion source. Both positive and negative surface charges are eliminated on nonconductive surfaces by ions of the opposite polarity which neutralize surface charges. Static decay times are directly related to the number of airborne ions; the more negative ions, the faster the decay time for positive charges (and vice versa).

For conductive materials, Terra's Faraccator™ concept guarantees 100% ESD protection within a storage area.

Terra has developed a complete line of noncontaminating stainless steel shelves, trays, and drawers that guarantee the absolute conductivity required for a Faraday cage. These durable units come in standard sizes to fit all of our desiccator cabinets and provide organized storage—both inside and outside the desiccator.

Do these Units Provide the Same Static-Proof Protection when Removed?
Absolutely. Shelves and trays interlock to form sturdy, static-free carrying containers. Even though the unit is not grounded outside the desiccator, its own capacitance lets it absorb a great deal of static charge without affecting stored material. Our grounded, chrome-plated racks even let you stack these storage—both maintaining a static-free environment.

Terra also offers a full line of Faraccator™ storage cabinets that incorporate slide-out conductive drawers and stainless steel inlays. Like the trays and shelves, these drawers continue to shield against ESD when removed and used as carrying containers.

Can Terra Desiccators be Retrofitted to Deliver the ESD-Free Protection of a Faraccator™?
Easily. For each of our desiccators we offer chrome-plated racks that accept standard trays and shelves and, connected to a suitable ground, deliver complete electrostatic protection.

Our GroundWatch™ device helps to enhance this protection by continuously monitoring the integrity of the ground connection of an indefinite number of desiccators. The minute a ground line is disrupted, visible and audible alarms alert you to the problem before stored parts can be damaged.

Which Method of ESD Control is Best Today?
The answer depends on the type of material you need to protect.
If stored material is conductive or dissipative (material placed in conductive bags or boxes, conductive waffle packs, etc.)

1. Take advantage of Terra systems based on the Faraday concept.

   Material up to 4" high

   Faraccators Security Faraccators

   Larger Materials

   Stainless Steel Desiccators Double Agent Desiccators Desiccators with Stainless Steel Shelving

2. Select Dissipative PVC Desiccators in conjunction with stainless steel shields, shelves, and trays.

If stored material is nonconductive (plastic boxes, nonconductive waffle packs, etc.), or

If stored material is already charged and must be neutralized, we recommend ionizing equipment in addition to the safety measures listed above. For additional protection, use conductive trays in conjunction with ionizing nozzles whenever possible.