Yukikazu Uemura

Novolak Design For High Resolution Positive Photoresists

The mechanism of resolution improvement in novolak-based positive photoresists was investigated from the stand-point of the image formation process. The image formation process in the novolak-quinonediazide system involves the dissolution inhibition in unexposed parts and the dissolution promotion in exposed parts. The 4-(gamma)-value, which is one of the indexes of resolution capabilities, depends greatly on the difference between the solubility of unexposed parts and that of exposed parts, i.e.-the lower dissolution rate in unexposed parts(Ro) and the higher one in exposed parts(Rp) are desirable to obtain high γvalues.

High Resolution Positive Photoresists

An attempt was made to improve the resolution capability of positive photoresists applicable to VLSI semiconductor processing. It was found out that in general, when the contrast (y(gamma)-value) of resists is made higher, the exposure latitude and the mask size reproducibility tend to be lowered. Study was made how to raise Y-value without decrease in the exposure latitude and the mask size reproducibility. It was necessary to design a new type of novolac resin that has a molecular weight and a molecular structure different from the existing materials. Many kinds of cresol-formaldehyde type novolak resins were synthesized and evaluated. Such items as the isomeric structure of cresol, the position of the methylene bond and the influence of molecular weight were investigated. As a result of the optimization of these items, a number of different resist materials that exhibit improved resolution capabilities were obtained. It is also remarkable that the introduction of the new polymer design not only enabled to improve resolution capabilities, but also made the process latitude much wider at various points of resist work including, prebaking temperature, developing time and developing temperature, compared to the so far commercially available products. This new material makes it possible to obtain fine pattern resolution required in VLSI processing. Moreover, wider latitude of processing conditions assures higher yields of the circuits.