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Practice_Exam_and_Solutions - Practice Exam and Solutions 1...

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Practice Exam and Solutions 1. Lithography often has to be done over underlying topography on a silicon chip. This can result in variations in the resist thickness as the underlying topography goes up and down. This can sometimes cause some parts of the photoresist image to be underexposed and/or other regions to be overexposed. Explain in terms of the chemistry of the resist exposure process why these underexposure and overexposure problems occur. Answer: Photoresist exposure occurs with a bleaching process. What this means is that the photons are initially absorbed by the PAC in the top layers of the resist and that is therefore where the initial exposure occurs. As the PAC is reacted in these top layers, the resist becomes more transparent (bleaches) and the light then penetrates to the deeper portions of the resist to expose those regions. The time required for complete exposure all the way to the bottom of the resist will thus depend on the resist thickness. Thus if there are thickness variations, there will be problems with overexposure in the thin regions if the exposure is long enough to expose the thick regions. Alternatively, there will be underexposure problems in the thick regions if the exposure is only long enough to expose the resist in the thin regions.
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