EE143quiz1-wed-ans

EE143quiz1-wed-ans - reached): (2 points) Answer Key We use...

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EE 143: Lab Quiz 1 Fall 2008, University of California, Berkeley Name: Section: Wed 10-1 1. Which type of photoresist (positive or negative) do we use in this lab? What happens to this type of resist in the areas that are exposed to light? (2 points) 2. a) Which type of oxidation (wet or dry) was used to grow the field oxide on the bare wafer? Why? (1 point) b) Which type did we use to grow the gate oxide ? Why? (1 point) 3. According to the standard recipe, the spinner setting is 3000 rpm and 30 sec. If you instead increase it to 4000 rpm for 30 sec, you would get: (2 points) thicker resist coating uneven resist coating thinner resist coating 4. During oxidation, we pushed wafers into the furnace very slowly. Why should we do this? (2 points) 5. Given the following mask and wafer cross section, draw a single cross section showing the final state after all of the following (and only the following) have been done: exposing, developing, hard bake, HF wet etching (at least until bare Si is
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Unformatted text preview: reached): (2 points) Answer Key We use positive photoresist. The areas exposed to light become weakened and are dissolved in developer. Wet oxidation was used, since we needed a very thick layer that didnt have to be high quality. Wet oxidation grows more quickly, but produces lower quality oxide. We used dry oxidation since we wanted to controllably grow a thin, high quality oxide layer. Dry oxidation is slower, but produces a denser oxide with better electrical performance. Pushing the wafers and boat into the hottest part of the furnace (the middle) too quickly could result in thermal shock, causing warping or cracking of the wafers or the boat due to uneven thermal expansion. HF wet etch is isotropic: lateral etch = vertical etch PR doesnt get etched, and we havent stripped it yet. This leads to undercut and a curved SiO 2 profile. The faster rotation rate creates a thinner coating in the same amount of time....
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