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ESD Polymers in Cleanroom Applications

An Overview by:  Ryan J. Money

Plastics are used extensively...

Plastics are used extensively throughout the semiconductor industry, especially in cases where moderate-temperature processing is required. Plastics are:1) Dimensionally stable at medium and low temperatures2) Provide a somewhat clean & contaminant-free surfaces3) And are not affected by harsh chemicals, acids, or solvents.

Plastics Are Here to Stay-New plastics continue to replace metal parts for wafer handling, processing vessels, parts carriers, and IC test equipment sockets & handlers. However when chosen wrong, Polymers used in cleanrooms are major sources of organic contaminants and Electrical Static Discharge (ESD).

Terms to Consider- Outgassing or Outgas, occurs when a material is placed into a vacuum (very low atmospheric pressure) environment, subjected to heat, and some of the material's constituents are volatilized (evaporated or "outgassed"). ASTM TEST METHOD E595

Resistivity - Electrical measurement which reports the electrical resistance of the material

POLYMERS ARE MOLECULAR STRINGS OF SIMILAR OR DISSIMILAR MONOMERS

EXAMPLES OF POLYMERS WE SEE EVERYDAY:ESD BLACK TOTE BOX: ...POLYPROPYLENEWHEAT STARCH (BREAD): GLUCOSE SUGAR MONOMERSANIMAL PROTEIN (HAMBURGER): AMINO ACID MONOMERSPLASTIC SODA BOTTLE (COKE): PET-POLYETHYLENE TERTHALATEPLASTIC GARBAGE BAG (GLAD BAG): POLYETHYLENEESD WORK MAT: NITRILE RUBBER

There are many different types of carbon Black to select from. Each has different applications and benefits.      

  • OIL BLACK     

  • ACETYLENE BLACK     

  • BONE BLACK

A typical mix would be 10% to 30% powdered conductive carbon black into molten plastic pelletize and would deliver a resistance between 10e6 to 10e2 ohms.By compounding one or more of the following additives into plastic resins, electrical properties can be obtained.      POWDERED METALS: GOLD, SILVER, COPPER, ALUMINUM      POWDERED CARBON: CARBON BLACK, GRAPHITE      ANTISTATS: ETHOXYLATED AMIDO AMINES     

INHERENTLY CONDUCTIVE POLYMERS:

POLYETHER AMIDES POLYANILINES      METAL OXIDE SEMI-CONDUCTORS: TIN OXIDE, VANADIUM OXIDE INDIUM TIN OXIDEINJECTION MOLDED PARTS CAN HAVE HIGH AND LOW POCKETS OF RESISTANCE        MATERIALS HAVE BOTH SURFACE AND VOLUME CONDUCTIVITY        CARBON LOADED MATERIALS HAVE PERMANENT CONDUCTIVITY        CARBON IS A LOW COST ADDITIVE        IT IS DIFFICULT TO PRODUCE PLASTICS THAT ARE IN THE DISSIPATIVE RANGE: E6 OHMS TO E9 OHMS        CARBON SLOUGHS FROM THE MATERIAL. NOT RECOMMENDED FOR CLEAN ROOMS.

Examples of antistats we see every day.     

  • FABRIC SOFTENER FOR THE LAUNDRY:     

  • HAIR CONDITIONER:     

  • HOUSEHOLD CLEANER

HOW ARE ANTISTATS USED?

ADD 1% TO 5% ANTISTATIC CHEMICAL TO MOLTEN PLASTIC AND PELLETIZE TREAT THE SURFACE OF THE PLASTIC MATERIAL WITH A 10% TO 20% WATER SOLUTION OF ANTISTAT. DRY MATERIAL IN AN OVEN TO REMOVE WATER

ANTISTATS USED IN MAKING ESD PRODUCTS ARE: ETHOXYLATED AMIDOAMINES WHICH ARE SPECIFICALY DESIGNED TO BE FREE OF CHLORIDES, TERTIARY AMINES AND OTHER CONTAMINATES THAT CAN CAUSE CORROSION OF METALS AND ATTACK PLASTICS.

NOT RECOMMMENDED FOR CLEANROOMS  

TREATMENT IS CLEAR. USUALLY DOES NOT CHANGE THE APPEARANCE OF THE BASE MATERIAL     

ANTISTATS ARE VERY LOW COST AND EASY TO USE      

  • THEY DO NOT PROVIDE VOLUME CONDUCTIVITY     

  • TYPICAL SURFACE RESISTANCE IS 10-E9 OHMS TO 10-E11 OHMS     

  • ANTISTAT ARE NOT PERMANENT.

  • THEY EVAPORATE

  • TYPICALLY REQUIRE HUMIDITY TO FUNCTION.

  • THEY COMBINE WITH WATER TO MAKE A CIRCUIT.

  • AT VERY LOW HUMIDITY THEY DO NOT DISSIPATE STATIC      

EXAMPLES:

  •  FLOOR FINISH (ESD WAX)      

  • ESD LOTION       S

  • SOME MAT CLEANERS AND RESTORERS

ESD PACKAGING MATERIALS:      

  • BLUE POLY & PINK POLY     

  • CLAM SHELLS     

  • STRETCH WRAP     

  • PINK STYRAFOAM     

  • SOME ESD BAGS

OTHER TYPES OF CONDUCTIVE MATERIAL HAS COME INTO COMMERCIAL USE IN THE LAST 10 YEARS.

 INHERENTLY CONDUCTIVE POLYMERS (ICP'S) - Conduct Electricity by the flow of electrons through their unique molecular Examples of ICP's Used Today Are:       POLYETHER AMIDES      POLYANILINES MIXED METAL OXIDES CHARACTERISTICS OF METAL OXIDE COMPOUNDED PLASTICS+ PLASTICS ARE HUMIDITY INDEPENDENT+ THEY HAVE BOTH SURFACE AND VOLUME CONDUCTIVITY WHICH CAN BE PRECISELY CONTROLLED TO A GIVEN RESISTANCE AT A GIVE VOLTAGE+ METAL OXIDES IMPART PERMANENT CONDUCTIVITY+ METAL OXIDES INCREASE THE STRENGTH OF THE PLASTIC+ METAL OXIDES DO NOT SLOUGH. THESE PLASTICS ARE SUITABLE FOR CLEAN ROOM USE+ BRIGHT COLORS CAN BE ACHIEVED+ METAL OXIDES ARE EXPENSIVE

Static Control What is electrostatic discharge? Static electricity doesn't do any damage until it is discharged - and is then called electrostatic discharge or ESD. Quite simply, electrostatic discharge is a spark.

Requirements for electrostatic discharge

Two things are required to allow ESD:

1) A charge separation.

               And

2) two associated conductors that collect charge and are brought together initiating the discharge.

 

Anti-Static:

Resistivity generally between 1011 and 1012 ohms per square. Initial electrostatic charges are suppressed. May be surface resistive, surface-coated or filled throughout.

Static Dissipative (SD) :

Resistivity generally between 104and 1011 ohms per square. Low or no initial charges -- prevents discharge to from human contact. May be either surface-coated or filled throughout. Usually some form of IDP, coating or carbon particle or carbon fiber filled

Conductive

Resistivity generally below 104 ohms per square. No initial charges, provides path for charge to bleed off. Usually carbon-particle or carbon-fiber filled throughout.( CN)

When to measure

  1. Periodically monitor and train the Staff

  2. Periodically monitor the test Equipment                                       

  3. Periodically monitor the Line for violations

What to measure

Surface ResistanceS20-20 Standard

  1. Volume resistance

  2. Charge Decay Time (1000v to 100v in less than 2 sec)

  3. Energy retention

  4. Transient Current

Tests for Outgassing

ASTM E-595-vacuum outgassing Static headspace GC-MS (Gas Chromatography-Mass Spectrometry) tests at 100°C Dynamic headspace GC-MS "screening tests" at 100°C Room temperature GC-MS "engineering test" methods.

Acceptable Materials

  • PolyEtherEtherKetone (PEEK)

  • Polyimide PolyPhenylene Sulfide (PPS)

  • PolyChloroTetraFluoroEthylene

All have less than -1.85 %TML, 0.00 %CVCM, 0.49% WVR

Typical Industry OutGass Test Limits

The criteria of TML < 1.0% and CVCM < 0.1% has been typically used to screen materials

What are we trying to achieve?

Clean materials with no sloughing, Mechanically stable Temperature resistance

 

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