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MEM 417_Lab_I-Photolithography

MEM 417_Lab_I-Photolithography - Photolithography is a...

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17 – Micro Fabrication FALL 08 Lakir Patel Drexel University FALL 08 LAB – I PHOTOLITHOGRAPHY
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Photolithography is a process of transferring geometric shapes on a mask to the surface of a silicon wafer. For a Photolithographic process to occur, wafers must be cleaned, barrier layer formed, photoresist application applied, soft bake, mask alignment, exposure and development and finally hard baking. (ECE, GA Tech, MEMs Lab) The first lab was primarily to develop an understanding of Photolithography with thin positive photoresist and a thick negative photoresist. For positive resists, the resist is exposed with UV light wherever the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. In other words, "whatever shows, goes." The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer. Negative resists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photoresists, therefore, contain the inverse (or photographic "negative") of the pattern to be transferred. The figure below shows the pattern differences generated from the use of positive and negative resist. (GA TECH, MEMs Lab, 2008) For our photolithography lab, we utilized a 3” silicon wafer, about 500 m thick, which was μ purchased, to minimize the contamination and was opened under a lab hood to further reduce the dust particles from settling on the surface, which may interact negatively with the photolithographic process. The substrate was cleaned with Piranha Solution, which is a blend of Sulfuric Acid and Hydrogen Peroxide, for this lab, the solution ratio was 3 : 1. Because the mixture is a strong oxidizer, it will remove all the organic matter on the substrate. The solution mix is highly endothermic, the solution must be allowed to cool and gases resulting from it allowed to dissipate, for safety precautions. The photoresist
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MEM 417_Lab_I-Photolithography - Photolithography is a...

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