【Integrated Circuit Series】Difference between positive and negative photoresists

2019-08-04

Alpha Economic Research

Photolithography includes two basic processes: negative photolithography and positive photolithography. Negative photolithography replicates the inverse of the pattern on the mask onto the silicon wafer surface; positive photolithography replicates the same pattern on the mask onto the silicon wafer surface. The main difference between these two basic processes is the type of photoresist used.

Negative Photolithography

The basic characteristic of negative photolithography is that after exposure, the photoresist becomes insoluble and hardens due to crosslinking. Once hardened, the crosslinked photoresist cannot be washed away in a solvent because the pattern on the photoresist is the inverse of the pattern on the projection mask. Negative photoresist was the first photoresist used in semiconductor photolithography.

The above figure shows that the mask for negative photoresist is a transparent quartz plate, and the black part of the mask is a deposited chromium film. Since chromium is opaque and does not allow ultraviolet light to pass through, the desired mask pattern can be formed.

For negative photoresist, the areas under the opaque chromium on the mask are not exposed and therefore remain unchanged, with the photoresist remaining soft. When exposed to a developing chemical solvent, it will dissolve. Ultraviolet light passing through the transparent areas of the mask hardens the photoresist, so it will not dissolve in the developer.

Positive Photolithography

Positive photolithography replicates the same pattern onto the silicon wafer surface as the mask. The areas exposed to ultraviolet light undergo a chemical reaction, softening in the developer and becoming soluble in the developer. The exposed positive photoresist areas will be removed in the developer, while the unexposed photoresist under the opaque mask remains on the silicon wafer, as shown in the figure below:

Since the pattern formed on the photoresist is the same as that on the projection mask, this photoresist is called positive photoresist. The remaining photoresist has been hardened before exposure and will remain on the silicon wafer surface as a protective layer for subsequent processes such as etching. The photoresist will be removed after the subsequent processes are completed.

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