Content area
Full Text
Negative photoresists are materials that become insoluble in developing solutions when exposed to optical radiation. They were the first systems used to pattern semiconductor devices, and still comprise the largest segment of the photoresist industry because they are widely used to define the circuitry in printed wiring boards. However, the current use of negative resists in the semiconductor industry has been limited by past difficulties in achieving high-resolution patterns. Recent advances in the chemistry of negative-resist systems, however, have provided materials with wide processing latitude and high resolution that are used to manufacture IBM's advanced CMOS devices and to achieve high-aspect-ratio patterns for micromachining applications. This paper provides an overview of the history and chemistry of negative-resist systems and their development in IBM.
1. Introduction
The majority of polymers, when exposed to ultraviolet light in the range from 200 to 300 nm (-4 eV to 6 eV), form "radical species" which can result in cross-linking, increased molecular weight, insolubilization, and film embrittlement. While industries such as the automotive and aerospace have concentrated research activities on inhibiting these reactions to extend the life of plastic coatings and components, the semiconductor industry has used this effect to its advantage to produce polymeric stencils resistant to the acids and bases used to fabricate semiconductor devices and circuitry. As shown in Figure 1, these photosensitive films, which become insoluble in solvents or water-based developers upon exposure to radiation, form "negative" patterns which are used as temporary stencils to delineate many levels of circuitry in semiconductor devices and printed wiring boards (PWBs). This insolubilization can be achieved by using materials which upon UV exposure either 1) increase in molecular weight, or 2) are photochemically rearranged to form new insoluble products. To increase molecular weight, photoinitiators are generally used that can generate free radicals or strong acids to facilitate polymeric cross-linking or the photopolymerization of monomeric or oligomeric species. Without an increase in molecular weight, negative patterns can be achieved by the photochemical formation of hydrophobic or hydrophilic groups which provide preferential solubility between the exposed and unexposed resist film.
Over the past thirty years, chemists and engineers have been able to provide a wide variety of resists, both negative (insolubilize upon exposure to radiation) and positive (solubilize upon exposure...