Microelectronic devices are formed by first forming SiO2 on a silicon wafer by chemical vapor deposition (Figure). This procedure is followed by coating the SiO2 with a polymer called a photoresist. The pattern of the electronic circuit is then placed on the polymer and the sample is irradiated with ultraviolet light. If the polymer is a positive photoresist, the sections that were irradiated will dissolve in the appropriate solvent, and those sections not irradiated will protect the SiO2 from further treatment. The wafer is then exposed to strong acids, such as HF, which etch (i.e., dissolve) the exposed SiO2. It is extremely important to know the kinetics of the reaction, so that the proper depth of the channel can be achieved. The dissolution reaction is
SiO2 + 6HF → H2SiF6 + 2H2O
From the following initial rate data, determine the rate law.
Etching Rate (nm/min) | 60 | 200 | 600 | 1000 | 1400 |
HF (wt %) | 8 | 20 | 33 | 40 | 48 |
Figure P7-12B Semiconductor etching.
A total of 1000 thin wafer chips are to be placed in 0.5 dm3 of 20% HF. If a spiral channel 10 μm wide and 10 m in length were to be etched to a depth of 50 μm on both sides of each wafer, how long should the chips be left in the solution? Assume that the solution is well mixed.
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