An attempt was made to improve the sensitivity and heat resistance of positive photoresist applicable to very large‐scale integrated (VLSI) semiconductor processing. It was found out that, in general, when the lithographic sensitivity of resist was made higher, film thickness retention and heat resistance were lowered. A study was made on how to raise sensitivity without decrease in film thickness retention and heat resistance. It was necessary to design a new type of novolac resin that has a molecular structure and a molecular weight different from the existing materials. Many kinds of cresol‐formaldehyde type novolac 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 characteristics in sensitivity, film thickness retention, and/or heat resistance were obtained. It is also remarkable that the introduction of the new polymer design enabled to make the allowance of the operating conditions much wider at various points of resist work, including prebaking temperature, light exposure time, developing time, and developing temperature, compared to the present commercially available products. This new material not only makes it possible to raise the throughput of the stepper exposure machine and developing machine, but the wider allowance of processing conditions assures higher yield of the circuits.