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BME%20551%20Homework%202%20solution - Homework#2 BME 551 1...

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Homework #2 BME 551 1-21-09 1. Contrast wet vs. dry etching. Wet Etching Dry Etching Phase Liquid Gas/Vapour Selectivity High Low Anisotropy Anisotropic/Isotropic Isotropic (Anisotropic possible) Etch rate Faster Slower Cost Low High Set Up Simple Bench top Vacuum Chamber Contamination High (during drying process Low Feature size Larger finer Etc Etc. Etc. 2. Define plasma etching. Is it isotropic or anisotropic? Highly ionized gas (Plasma) containing local ionic charges with long enough lifetime can be generated by striking a high voltage ( > 1 kV) electrical discharge through a low pressure system ( ~100s mTorr). Random movements of the ions inside the chamber cause etching of the material via chemical reactions and forming gaseous by-products. Plasma etching is highly isotropic. 3. What etching processes will be considered for fabricating structures with high aspect ratios? Dry etching (DRIE) & LIGA 4. What chemicals are used for chemical etching? HF, HNO3, H3PO4, KOH, EDP etc etc.
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5. Define DRIE. Deep Reactive Ion Etching: Highly anisotropic physical etching due to ion bombardment along the electrical field on the horizontal surface can etch deep channels and pits (up to few tens of microns deep) with nearly vertical walls and of arbitrary shape. Unlike anisotropic wet etching, DRIE etching is not controlled by the relative etch rates of the silicon crystal planes. 6. Define e-beam lithography. Electron beam lithography is a prevalent technique for the fabrication of planar nanostructures. In a first step, a layer of PMMA positive-resist is spin coated onto a substrate. That followed, the beam of a computer controlled electron microscope is used to "write" the desired pattern into the resist. The exposed areas of the resist are dissolved during development resulting in a masque for the subsequent metal-evaporation process. Finally, the unexposed resist is removed with acetone. 7.
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BME%20551%20Homework%202%20solution - Homework#2 BME 551 1...

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