Fitzgerald B. Brarnwell, Richard E. Zadjura, and Charles Paley Brooklyn College, CUNY Brooklyn. New York 11210 and S u s a n R. Fahrenholtr Bell Laborator~es Murray Hill. New Jersey 07974
A Positive Photoresist The photochemical Wolff rearrangement
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In the last ten years, the use of photoresists has been of great significance in the electronici and communications industries. They are universally used in the fabrication of miniaturized inteerated circuits needed for the oroduction of oocket cal" culators, computers, telecommunications devices, and a host of consumer oriented products and services (1,2). A photoresist is a mixture of chemicals generally containing a filmforming polymer and one or more photosensitive compounds. The term photoresist arises from the essential photochemical behavior and chemical resistance of this mixture. When exposed to light of the proper wavelength, a photoresist undergoes photochemical reactions which alter the molecular structure of some of its components and change its solubility. If exposure to light imparts increased chemical solubility, the photoresist is called a positive photoresist. Conversely, if exposure to light imparts reduced solubility, the photoresist is called a negative photoresist. The fahrication of an integrated circuit is a complex process which depends on the construction of a precise pattern of electronic components, for example, metal-oxide-semiconductor (MOS) devices. Critical distances in commercially available integrated circuits are often measured in micrometers. The figure schematically shows the key role played by photoresists in the fabrication process for MOS devices. In this example, the photoresist is used to form a well-defined image of silicon dioxide on silicon. The photoresist is uniformly deposited on a thin layer of silicon dioxide above a silicon suhstrate and irradiated through a mask. Illuminated areas undergo a photochemically initiated solubility change. The insoluble nortion of the ohotoresist Drotects the silicon dioxide-silicon substrate from chemical attack. The soluble portion of the ohotoresist is chemicallv removed usina the appropriate solvent to leave areas of the &con dioxide-~ilicon&h.&ate which are subject to chemical attack. The exposed silicon dioxide layer can be removed by processing methods such as acid etching, plasma etching, or ion milling to form a welldefined image of silicon dioxide on the silicon suhstrate. Subsequent processing steps in the fahrication of an MOS integrated circuit include doping the silicon dioxide-silicon substrate, e.g. with phosphorous, and vacuum deposition of aluminum andlor other metals. The use of photoresists and subsequent processing steps is repeated many times in the manufacture of a typical integrated circuit which may contain as many as 10,000 electronic components on a few millimeters square silicon wafer. Presently, the image resolution of such photoresists is 2 pm. We have develoved an undergraduate experiment in which thv formulation and phutorhemicnl twhariur o i a pusiiive photore
