Negative Photoresists vs Positive Photoresists
Through the mid-1970s, negative resists were dominant in the masking process. While positive resists had been around for over two decades, they were not often used because of their poor adhesion properties. By the 1980s, positive resists were more common, but the transition was not simple. The polarity of the masks had to be switched once positive resists were used, requiring a completely new process. With a negative resist and a light-field masks, the dimensions in the resist is smaller than the mask/reticle dimensions due to light wrapping around the image. With a positive resist and a dark-field mask, the diffraction often widens the image. This change must be considered when the mask/reticle is made as well as during the design of the other masking processes. Another difference between positive and negative resists relates to oxygen. Negative resists react to oxygen that is in the atmosphere, which can result in a thinning of the resist film by up to 20%. Positive resists do not react with oxygen in this way. Positive resists are more expensive then negative resists, but in some cases the extra cost could be offset by higher yields that positive resist could produce. The two types of resists differ when it comes to developing characteristics as well. Negative resists develop in solvents and have a high solubility differential between the polymerized and unpolymerized areas. During developing, the image dimensions remain fairly constant. Positive resists on the other hand, have a lower solubility between the polymerized and unpolymerized areas, thus requiring carefully prepared developer solutions and a temperature control process. Just prior to the completion of the masking process, the photoresist must be removed. In general, the removal of positive resists is easier and occurs in chemicals that are more environmentally sound.
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Van Zant, P. (2000). Microchip fabrication, a practical guide to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.