The illustration below show a process called LOCOS for Local Oxidation of Silicon. In this a thin film of Silicon Nitride is depostied and patterned into protective pads, in this case 20 microns wide. Silicon Nitride does not let the oxide diffuse through it, so that when an oxidation process is done the oxide only grows in the areas not protected by the silicon nitride, and no oxide grows under the nitride. (4a) Choose a time, temperature combination that will grow 500 nm of wet oxide in the uncovered Si areas in under 80 minutes. Use the chart from the notes for this. (5 marks) (4b) Sketch what the resulting structure looks like in cross section, assuming that oxide does not grow under the nitride area. Be sure to show where the Si/oxide and nitride/Si boundries are (5 marks) (4c) Now assume the nitride is stripped leaving bare silicon there, without affecting the other oxide. Now choose a time, temperature combination that will grow 250 nm of wet oxide in the uncovered Si areas in under 80 minutes. Use the chart from the notes for this. (5 marks) (4d) What is the resulting oxide thickness of the field area (outside the etched area). Now do the same calculations using both A & B coefficents from the notes. (10 marks) (4e) Sketch the resulting structure, showing the oxide/Si interface position at each point, and oxide surface. Note thickness and positions of one layer relative to the other. Show the original Si surface position on the same drawing. (15 marks) (4f) If the same structure had instead been exposed to 1.5 hours of dry oxidation at 1100C what would be the oxide thicknesses in the field and opened areas. Sketch the resulting structure as in part (e). Assume the continued growth of an oxide is not affected by whether the original oxide was wet or dry. (10 marks) Si Nitride Si Nitride 20 um Silicon Substrate 20 um