CHEMICAL FILTER
The problem of corrosion and odor contamination in industrial environment at manyfactories. it is not caused by a single factor and can not be resolved by a single solution.
SEMl standard F21-95 provides a method of classifying microelectronic cleanrooms withrespect to Airbore Molecular Contamination(AMC)levels. This is for gas phasecontaminants,analogous to Federal standard 209E for particulate contamination.SEMstandard F21-95 classifies a cleanroom based upon the category of the gas-phasecontaminant,and the allowable concentration level for each category.
The four categories of gas-phase contaminants:
1.Acids(category A):
Corrosive gases which react chemically as acids (an electronacceptor)
2.Bases(category B):Corrosive qasses which react chemically as bases (an electron donor)
3.Condensables (category C):Contaminants whose boiling point are typically above room temperature and arecapable of condensing on the wafer surface.
4.Dopants (category D):
Contaminants that modify the electrical properties of semiconductor material.
The SEMl F21-95 classification is identified by the letter “M",followed by the categorydesignator,then the allowable contaminant concentration in part per trilion(ppt).Folexample,a classification of MA-10is interpretedas having a maximum allowableconcentration of 10 ppt of all acid gases in the room air.
Classification of Airborne Molecular Contaminants
| Material Category | 1 ppt | 10 ppt | 100 ppt | 1,000 ppt | 10,000 ppt |
| Acids | MA-1 | MA-10 | MA-100 | MA-1000 | MA-10,000 |
| Bases | MB-1 | MB-10 | MB-100 | MB-1000 | MB-10,000 |
| Condensables | MC-1 | MC-10 | MC-100 | MC-1000 | MC-10,000 |
| Dopants | MD-1 | MD-10 | MD-100 | MD-1000 | MD-10,000 |
"SEMlF21-95:Classification of Airborne Molecular Contaminant Levels in CleanEnvironments".1 ppb=1,000 ppt,and 1ppm=1,000,000 ppt.
The allowable AMC (airborne molecular contaminants) concentration depends uponthe class of cleanroom,the process used,and the equipment employed.
Projected AMC limits for the 0.25 um Process
| Process Step | MAX Sit Time | MA(ppt) | MB (ppt) | MC (ppt) | MD (ppt) |
| Pre-gateoxidation | 4 hrs | 13,000 | 13,000 | 1,000 | 0.1 |
| Salicidation | 1 hrs | 180 | 13,000 | 35,000 | 1,000 |
| Contact Formation | 24 hrs | 5 | 13,000 | 2,000 | 100,000 |
| DUV Photorithagraphy | 2 hrs | 10,000 | 1,000 | 100,000 | 10,000 |
Typical Contaminants and Concentrations in Cleanroom
| Category | Contaminants | Concentrationrangr,ppt Low High |
Acids | Hydrochloric Acid | 20,000 | 400，000 |
| Hydrofluoric Acid | 40,000 | 250,000 |
| Nitric Acid | 20,000 | 250,000 |
| Sulfuric acid | 10,000 | 300,000 |
| Phosphoric acid | 10,000 | 400,000 |
| Acetic acid | 10,000 | 250,000 |
| Nitrogen dioxide | 30,000 | 300,000 |
| Sulfur dioxide | 10,000 | 150,000 |
Bases | Ammonia | 10,000 | 200,000 |
| NMP | 20,000 | 300,000 |
Condensables | Acetone | 20,000 | 500,000 |
| Toluene | 10,000 | 250,000 |
Dopants | Boric acid | ND | 200,000 |
| Phosphor ous | ND | 5,000 |
| Arsine | ND | 50,000 |
Four main techniques for controlling odor and gas contaminants:masking,combustion,.ventilation and removal.Media pellets are used to control gas contaminants by adsorp-tion and oxidation.Chemical filter can be removed molecular contaminants by adsorbingand absorbing principles.
To purify air by elimianting particulate material,gaseous and vaporous compoundParticulate filters do not have the effective on undesirable gases and vapors.
When gaseous or liquid molecules reach the surface of an adsorbent and remain withoutany chemical reaction,the phenomenon is called physical adsorption or physisorption.
The carbon can be impregnated with other chemicals to improve certain gases of lowadsorptive capacity by untreated carbon.These chemicals react with the gases to reducethe contaminant concentration.
Another way to delete specific gases is to catch and react with them.This process is calledchemisorption.
Many adsorptive materials are available;each of them with special affinity for certainvapors.
The surface area increased when the dimensions of carbon are getting smaller.The particlesize can be very small,even though it just becomes carbon dust.lt also creates problems ofescaping into the air steam,or packing tightly caused high pressure drop. The optimal sizemust be determined by experimentation.
In molecular contaminant system,initial contaminant removal efficiency doesn' t depend onweight,carbon particle size,and carbon activity,Total surface area and surface chemistry aretwo important parameters for determining initial efficiency.
If air pollution is very humid (like on a rainy day),the moisture can be adsorbed competitivelywith targeted contaminated gases for carbon capacity,However.f the reaction ischemisorbed,moisture can increase reaction rapidly.
Temperature also can affect the rate of adsorption when temperature is gettinghigher.it is harder to attract the contaminants to the carbon.
