The photolithography process:
Step 1: Since a silicon wafer might have grown silicon dioxide due to air exposure, the first step is to clean the wafer by etching all of the silicon dioxide from the wafer. To do this, you have to dip the wafer into a hydrogen fluoride (HF) solution, rinse it with deionized water, dry it with hydrogen gas and place the silicon wafer in a hot plate to remove all moisture.
Step 2: Grow silicon dioxide using wet etching and a horizontal tube furnace. Silicon dioxide will now be about 1 micron in thickness throughout the entire water.
Step 3: Add a thin later of HMDS to improve the adhesion of the photoresist layer.
Step 4: Spin coat photoresist layer.
• Positive photoresist: The area in the silicon wafer that was not exposed to UV light gets stronger.
• Negative photoresist: The area in the silicon wafer that was not exposed to UV light gets weaker.
Step 5: Soft bake on a hot plate at 95° C for 1 minute.
Step 6: Load a mask and expose the wafer to UV light.
Step 7: Develop the wafer by dipping it into a photoresist developer, this will remove the weaker photoresist film.
Step 8: Hard bake on a hot plate at 120° C for 1 minute.
Step 9: Dip the wafer into BHF solution to remove silicon dioxide not protected by the leftover photoresist film.
Step 10: Dip the wafer into acetone to strip the leftover photoresist leaving behind only the silicon dioxide pattern that was protected by the photoresist.
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