A set of 4 m wide windows are to be etched into a SiO2 film of thickness 0.8m. a

Question

A set of 4 m wide windows are to be etched into a SiO2 film of thickness 0.8m.

a) Find the dimension of the window, as measured at the top of the oxide, after ideal isotropic
etching (i.e. no overetch). Draw a cross section of the window opening with the resist still on.

b) What is the dimension of the window at the oxide substrate interface.

c) Find the average slope of the window edge.

d) Repeat the above question a), b) and c) if the process was done with a 20% over-etch,
assuming that the silicon is etched at 2% of the oxide etch.

e) what type of Si wafer (orientation) and chemical etchant would be needed to make a perfectly
vertical wet etch for these 4 m wide windows? Comment on the practicality of such an etch

Explanation / Answer

Ans (a)

In Silicon, for all practical purpose, etch rate is not constant. But for ideal case, as described

by the term "IDEAL ISOTROPIC ETCHING" , the dimension of teh window will be just as specified:

4mu m.

Now the substrate is very thin (0.8 mu m) . So a resist window will be necessary.

The diagram : It will be a rectangle showing the film. The breadth will denote the depth of the film.

At the centre of the length will be the window, the rest of the two sides will mark the resist.

Ans (b)

The set up indicates a window has been exposed in the resist. Assuming the exposure to be sharp,

The dimension of the window at the oxide-substrate interface will be same as the specification, if the process is absolutely ideal. However for practical situations, it will be less.

Ans C

The average slope of the window, for ideal case, will be 900.

Ans d

Overetching will involve etching into the resist

This question is containing multiple segments.

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