1) Ion implantation is an important technique for semiconductor processing. To produce a p-n junction in an n-type (1015 cm-3) substrate, it is necessary to implant with boron or another species.
(A) For an implant energy of 50 keV and a peak boron concentration of 5x1018 cm-3, sketch the boron profile (calculate for five points) and determine the junction depth(s).
(B) Is this sufficient (energy, dose) to make the material amorphous? Explain
(C) Germanium is sometimes used to pre-amorphize a silicon wafer. What energy and what dose is required to make the layer amorphous by the Ge implant (I know that you don't have the implant curve for Ge in silicon below - improvise) so that the maximum implanted Ge concentration location matches the maximum boron concentration from part a)?
(D) What is the change in junction depth in (A) change if annealed at (i) 1000 °C for 30 minutes; (ii) 550 °C for 30 minutes? Answer quantitatively although you may estimate (show your reasoning) an answer for (ii). Please note there is a graph at the end of the test that may be of help.
(E) What benefits accompany amorphization? Answer qualitatively.
2) AlxGa1-xAs is an alloy semiconductor that can be grown by either OMVPE or MBE.
(i) a) (a) Define OMVPE and MBE.
b) What are the key benefits of OMVPE compared to halide or hydride VPE?
(ii) Describe two main points about each technique (MBE and OMVPE) from the ‘tour'.
(iii) The graph below shows a RHEED pattern during MBE for a structure that consists of the deposition of a layer of GaAs, followed by AlxGa1-xAs, followed by AlAs. What is the thickness of the AlAs and the GaAs layers ( in "monolayers")? What is the alloy composition?
(iv) If the Al cell temperature during that MBE growth is 1000 °C, determine the temperature of the Ga cell to achieve the behavior in the RHEED (or relative fluxes) data assuming that the Al (MW = 27) and Ga (MW=69) cells are otherwise identical. You may neglect the explicit dependence of the flux on temperature for this comparison (in other words, a certain temperature will correspond to a certain vapor pressure. However, there is a separate temperature term in the denominator. Ignore the term in the denominator.)
3) A p-type <111> oriented silicon wafer (1015 cm-3 ) is placed in a wet oxidation system to grow a field oxide of 600 Å at 1000 °C.
i) Determine the time required to grow the field oxide.
ii) After this first oxidation, the oxide over region A is removed. Then a diffusion step is carried out in which phosphorus is diffused into the wafer and the surface concentration of phosphorus is 3x1020 cm-3 . Next, a dry oxidation step is performed at 900 °C for 60 minutes. Find the thickness of the oxide over region A and the new thickness of the oxide over the other area (B in the figure below). Ignore any chemical or structural differences in the field oxide and gate oxide in terms of the oxidation kinetics. Explain why wet oxidation is used for the field oxide and dry oxidation is used for the gate oxide.
iii) Explain the difference between mass-transport limited oxidation and reaction-rate limited oxidation. Under what oxide growth conditions would each of these be expected?
4) True/False / Fill-in-the blanks / short answers
a) The typical precursors for silicon VPE growth are
b) Is Ni or P a faster diffuser in Si? Why?
c) A high phosphorus concentration in a silicon wafer will increase the oxidation rate during the growth of a thick oxide. T / F
d) Mass separation is achieved in ion implantation by electrostatics. T F
e) PAC stands for:
f) The ratio of the oxidation linear reaction rate constants B/A (111) to B/A (100) isrelated to:
g) I-line refers to:
h) Draw epitaxy growth rate vs. inverse temperature graphs for a system whose formation reaction is (a) endothermic and (b) exothermic
