Clinton David Snyder

Airborne chemical contamination of a chemically amplified resist

We have found that the performance of the t-BOC/onium salt resist system is severely degraded by vapor from organic bases. This effect is very pronounced and can be observed when the coated wafers stand for 15 minutes in air containing as little as 15 parts per billion (ppb) of an organic base. The observed effect, caused by this chemical contamination, depends on the tone of the resist system. For negative tone systems the UV exposure dose, required to obtain the correct linewidth, increases. While for the positive tone system, one observes the generation of a skin at the resist-air interface. Both effects are caused by the photogenerated acid being neutralized by the airborne organic base. There are a wide variety of commonly used materials which can liberate trace amounts of volatile amines and degrade resist performance. For example, fresh paint on a laboratory wall can exhibit this detrimental effect. These effects can be minimized by storing and processing the resist coated wafers in air that has passed through a specially designed, high efficiency carbon filter. The implementation of localized air filtration, to bathe the resist in chemically pure air, enabled this resist system to operate in a manufacturing environment at a rate of 100 wafers/hour.

Influence of polymer properties on airborne chemical contamination of chemically amplified resists

We describe here an extensive study of the relation between polymer structure and rates of NMP uptake for a set of polymer films with a very broad range of properties. NMP labeled using radioactive 14C was introduced at a concentration of 12 ppb into a stream of purified air. The film of interest was then immersed in the airstream for a predetermined time under controlled conditions. The radionuclide serves as a tag which allows precise quantitation of NMP absorbed by the polymer film. NMP uptake rates measured using this method are shown to correlate with polymer physical properties in a straightforward manner. The observed trends are in accord with those expected on the basis of the solution diffusion model of polymer permeation. These data provide a basis for the rational design of polymers for chemically amplified resists which are insensitive to airborne contaminants.