Photoresists Shaping the Future of Electronics

By Cynthia Challener

The advancement of semiconductor technology correlates directly with the ability of photoresist companies to develop products that can be used to pattern circuits on silicon wafers at ever smaller dimensions. Polymer composition has changed as new technology nodes have been approached and surpassed. Interest in silicon-based polymers is growing as the limits of organic polymer technology are reached.

The need for antireflective coatings has also grown as circuit size has diminished. Immersion photolithography, the latest technology adopted by the semiconductor industry, now presents additional challenges to photoresist producers. Photoresists contain polymers sensitive to light that are laid down on the silicon wafer ( typically spun on). A photomask of the desired circuit pattern positioned three to six feet away from the wafer is used to project an image at 4X reduction onto the wafer surface through a multi-million dollar lens.

The uncovered areas of the polymer are exposed to specific patterns of light at a given wavelength, causing chemical and physical changes that make it either soluble in a developesolution (positive tone) or resistant to the developer (negative tone). After the reacted polymer is removed, the wafer not protected by the remaining photoresist is etched to create the integrated circuit pattern.

The process is repeated multiple times to create the final chip structure. Most integrated circuits are fabricated using positive tone photoresists. Metal ion-free developers, which contain only parts per billion of metal ions, are composed largely of deionized water and the base tetra methyl ammonium hydroxide (TMAH). The purity of the developers is critical, as the presence of iron, sodium, potassium, and other metal ions in developers can cause leakage currents in gate oxides, hindering CMOS device performance.

The concentration of the TMAH varies depending on the photoresist formulation, and some developers also contain surfactants for improved surface wetting properties.