For decades photoresists have been used in the printing industry, and in the late 1920s were applied to the printed circuit board (PCB) industry. The technology was adopted by the semiconductor industry when Eastman Kodak and the Shipley Company introduced positive and negative photoresists in the late 1950s. Below we explore the composition of photoresists and the difference between positive and negative photoresists.
Photoresists are designed to respond to specific wavelengths of light and various exposing sources. They are also given specific thermal flow characteristics and formulations to adhere to specific surfaces. Four ingredients make up photoresists. They are polymers, solvents, sensitizers, and additives. Light and energy sensitive polymers give the photoresists their photosensitive properties. In negative photoresists, the polymers are most often the polyisopreme type, while positive photoresists utilize a phenol-formaldehyde polymer. The polymer’s structure changes from soluble to polymerized, or vice versa, when it comes in contact with the exposure source in the aligner. A solvent is the largest ingredient by volume in a photoresist, and makes the resist a liquid. This allows the resist to be applied to the wafer surface as a thin layer. For negative resists the solvent is xylene, while positive resists use either ethoxyethl acetate or 2-methoxyethyl. Sensitizers are added to resists to either control or entice certain reactions of the polymer. They can also be used to broaden or narrow the wavelength response of the photoresist. Bisazide is used as the sensitizer in negative resists, while diazonaphthoquinones are utilized by positive resists. (See Figure Above) Additives are mixed into the resists to achieve specific results. Some possibilities could include dyes to absorb and control light rays in negative resists, or chemical dissolution inhibitor systems in positive resists.
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.