3.3 Photolithography
Photolithography is used to pattern the wafer surface for defining the exact dimensions of devices and circuits. Although e-beam lithography provides the most advanced technology, this work involved only conventional photolithography using an ultra violet source. This involves spinning on a thin layer of a light sensitive polymer, known as photoresist, on the wafer surface. Ultra-violet light is shone onto this layer through a mask containing the pattern to be transferred. Areas exposed to UV light can then be removed using a developer solution. The remaining pattern on the wafer can now be used for etching the wafer or depositing a metallic layer onto it. Further detail is available in [107].
The thickness of the resist layer is critical. It is usually set by the duration and the speed of the spinner for a given photoresist. In this case Shipley 1400-27 photoresist was used and spun at 4,500 rpm for 35 seconds producing a thickness of approximately 1mm. Best results were achieved for wafer dimensions of roughly 1cm x 1cm.
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