8/20/2019 Fabrication Flowchart BM658
1/2
Biomedical Microsystems BM 658 1
FABRICATION PROCESS
Wafer 1:(a) Starting material, phosphor doped (n) bulk
silicon , 10µ thick with ρ= 3-5
Ω-cm.
(b)
1µ thick SiO2 by oxidation (4h in steam at 1 atm at ~1100°
C).(c) Backside n+ diffusion (P2O5)(d) Deposit
photoresist.
(e) Pattern photoresist by exposure for p+ diffusion
(lithography)
(f) HF + NH4F oxide etch followed by stripping resist
(g) p+ Boron diffusion to realise p-n photodiode
& provision for diode contact(h) LPCVD depositions of
Dielectric layers of Si3 N4-SiO2-Si3 N4, each of
thickness
λ /4=200nm, patterned using liftoff to form a Dielectric
mirror at the operating
wavelength.(i) Deposition & patterning of 200 nm Al by
sputtering. (contact for electrostatic
control of cavity spacing) Additional deposition 2µ for contact
with p+ region
(j) LPCVD deposition & patterning of sacrificial layer
of PolySi , 2 µ thick(k)
LPCVD oxidation(100 Pa, 600 °C) & lithography as described
earlier for SiO2
layer for upper mirror & suspension beams 1µ thick(l)
Additional deposition of 2 µ thick polySi , patterned to provide
strength for
suspension beams, followed by patterned deposition of
SiO2
(m) e beam deposition & patterning of Ti to improve the
Au adhesion to SiO2 (n) Au deposition by sputtering (30
nm) for upper mirror (reflectivity as well as
electrostatic control of sensitivity).
(o) Anisotropic wet etching of sacrificial PolySi in TMAH
solution at ~80°C.
(p) Antireflective coating of LPCVD silicon nitride 200 nm
thick patterned on the backside of the wafer.
(q) e beam deposition of Al 200nm (backside) patterned
through liftoff for
electrostatic control of cavity spacing.
N.S.Ananathakrishnan MTech I Biomedical 05330002
