Page 403 - Sami Franssila Introduction to Microfabrication

382 Introduction to Microfabrication

3. See Figure 5.6. The answer will inevitably be a 11. Etching
rough estimate only.
4S. Keep the deposition time constant. 1. Look for volatile etch products. Remember that mask
selectivity depends heavily on the etched depth: if
100 nm is to be etched, even 1:1 mask selectivity is
7. Thin-film Growth and Structure more than enough for a 1 µm thick mask.
2. Graphical solution will reveal something.
1. Layer thicknesses and wavelengths are coupled. 3. Remember Arrhenius behaviour.
2. Radius of curvature relates to bow. Typical wafer- 4. Over-etch time is determined mainly by the step
thickness specification is ±25 µm; how does this height; not by film or etch non-uniformity.
affect your result? 6. Calculate via masses: before porosification, after
3. E = hν and c = λν porosification and after etching the porous layer
7. R. Shohji et al: High-reliability tungsten-stacked away.
via process with fully converted TiAl 3 formation 7. Pore size and resistivity are connected, and you can
annealing, IEEE TSM 12 (1999), p. 302 come to a range of possible resistivities.
9. Typical chrome thickness is 100 nm.

8. Pattern Generation
12. Cleaning
1. Equation 2.9 gives the electron stopping range in −5
solids. Two significant digits suffice. 1. 10 monolayers are difficult to visualize: try
2. Plot this as a function of resist thickness. What another viewpoint: how far are the iron atoms from
do you expect to be the practical minimum resist each other?
thickness? 2. Use a simple model molecule, like Si-O-Si(CH 3 ) 3 for
3. Which process is limiting the writing time? organic contamination.
4. See T.R. Groves: Theory of beam-induced substrate 3. This is an order of magnitude question; try a few
heating, J.Vac.Sci.Tech. B 14 (6) (1996) values for phosphorous content (say, 1%, 0.1%,
6. Compare raster scan and vector scan. 0.01%) to get a feeling if there is any problem at all.
4. Table 12.1 helps, and tank volumes can be guesti-
mated.
5. How do CVD and evaporation compare?
9. Optical Lithography
6. An order of magnitude question. Even if you can find
1. How far down can you reduce the wavelength and a literature value for sweat salt content, you have
still call it optical? What is the minimum resist to guestimate the droplet size all by yourself. This
thickness conceivable? 0.1 ppb specification is for 0.13 µm CMOS.
2. Plot this as a function of gap: 20 µm is typical of
X-ray lithography systems.
3. All misalignment tolerance cannot be used to com- 13. Oxidation
pensate for thermal expansion: some have to be 1. The original 1 µm oxide will also grow dur-
reserved for mechanical warpage and optical aber- ing oxidation.
rations. 2. Quadrupling time will result in double thickness, in
5. Equation 9.1 gives an estimate for resolution, but the parabolic regime.
thick resist profile may be far from ideal. 4. This is a rather heavily doped poly.
7S. Is segregation similar in dry and wet oxidation?

10. Lithographic Patterns
14. Diffusion
1. Choose a few wafer sizes and resist thicknesses, and
then calculate some rough estimates. 1. Graphical approximation according to Equation
3. You can have a high absorption coefficient in a very 14.7.
thin imaging layer. 4. Equations 14.3 and 14.4 can be used to get a feeling
6. You can calculate resist thickness from the standing for the order of magnitude.
wave period. 5S. Explore this over a range of temperatures.