11. (8 pts) A technician was tasked with diffusing phosphorus into a silicon wafer. After performing the procedure, he noticed that he had misread the instructions: he was supposed to dope the wafer to 10.0ppm, but didn't see the decimal point and doped it to 100ppm. He came to you asking what to do. You said, "Easy - pump a vacuum on the Si, heat it up, and the phosphorus will diffuse back out. Stop when you get down to 10 ppm." He then asked, "How long will that be? If the wafer is 100m thick, and he heats the wafer up to 1100C, where the diffusion coefficient for phosphorus is 6.51011cm2s1, what should you tell him? Clearly there will be a phosphorus concentration gradient inside the wafer, so just tell him when 10ppm is the average value