Processed Silicon Wafer


Concepts Shown:

semiconduction, intrinsic, n-type, p-type


unprocessed silicon wafer, processed silicon wafer, high resolution pictures of processed silicon wafer, 2 plastic cases to hold silicon wafers, 1 3x magnification glass. Cost less than $20.


Setup: The silicon wafers should be available from schools electrical engineering/computer science department. The pictures can be taken with any optical or electron scanning microscope and camera. If desired, mount the pictures on a display board for better viewing by students. Try to take pictures at various levels of magnification (50x, 100x, 200x, 500x etc.) Presentation: Pass around the unprocessed wafer in open plastic cases. Ask if anyone knows what this material is. Explain that the material is very brittle and should be handled with extreme care. Pass around the processed silicon wafer with the magnifying glass. Show high resolution pictures and talk about what they are seeing. At 500x, you can easily see the connections to certain devices such as transistors, resistors, capacitors, and so on. Explain to the students that this is what is meant by solid state and how this type of circuit was previously possible only on a macroscopic level. (Possibly for effect, bring in old circuit board with amplifying tubes, bulbs and other large electronic devices hard wired to it). Note: This demonstration is designed to be shown in conjunction with the Transistor Processing Demonstration.


Solid state is based on the principles behind semiconduction silicon. For example, a transistor is a basic device that makes use of extrinsic semiconduction of both n-type and p-type. Basically, a transistor acts to amplify the signal traveling within the circuit. (For the science behind how a transistor works, see "Transistors as Power Amplifiers" demonstration. Other devices within the solid state make use of the same principles of semiconducting materials. When all the devices are combined in specific combinations developed by engineers, the chip can become the heart and soul of everything from a wristwatch to spaceship circuitry. A typical solid state transistor is about 4 microns long, meaning that a typical 3" wafer contains approximately � million transistors (1991 technology). Because you are dealing on such a small scale, one possible topic to discuss is the problems that occur when dealing on such a microscopic level. The problem of heat dissipation provides a need for better substrate materials (the current material being used today is Al2O3). Diffusion is in no way an exact science, so tolerances must be made much larger than what is predicted in theory. Also defects, one tiny dust particle can cause a defect that will ruin the entire circuit, and this is why clean rooms are used when processing these chips. Because of the extreme sensitivity of these devices to dirt and defects humans cannot even work with them we are just too "dirty". Thus when making high tech devices that use these "chips", robots must be used in our place. [eq].


Tyler vanHouwelingen

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