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Technology Stocks : PLASMA THERM (PTIS)
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To: C. Shivery who wrote (275)12/24/1997 11:19:00 AM
From: C. Shivery  Read Replies (1) of 509
 
Photolithography.

Photolithography is the basic technique used to define the shape of micromachined structures in the three techniques outlined below. The technique is essentially the same as that used in the microelectronics industry, which will be described here. The differences in the photolithographic techniques for Excimer laser micromachining and LIGA will be outlined in the relevant sections.

Figure 1a shows a thin film of some material (eg, silicon dioxide) on a substrate of some other material (eg, a silicon wafer). It is desired that some of the silicon dioxide (oxide) is selectively removed so that it only remains in particular areas on the silicon wafer (figure 1f).

Firstly a mask is produced. This will typically be a chromium pattern on a glass plate. The wafer is then coated with a polymer which is sensitive to ultraviolet light (figure 1b), called a photoresist. Ultraviolet light is then shone through the mask onto the photoresist (figure 1c). The photoresist is then developed which transfers the pattern on the mask to the photoresist layer (figure 1d).

There are two types of photoresist, termed positive and negative. Where the ultraviolet light strikes the positive resist it weakens the polymer, so that when the image is developed the resist is washed away where the light struck it - transferring a positive image of the mask to the resist layer. The opposite occurs with negative resist. Where the ultraviolet light strikes negative resist it strengthens the polymer, so when developed the resist that was not exposed to ultraviolet light is washed away - a negative image of the mask is transferred to the resist.

A chemical (or some other method) is then used to remove the oxide where it is exposed through the openings in the resist (figure 1e). Finally the resist is removed leaving the patterned oxide (figure 1f).
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