Page 145 - Sami Franssila Introduction to Microfabrication

124 Introduction to Microfabrication

2.0 Electropolishing
Log i (mA/cm 2 ) 1.0 Transition region HF

0
Porous silicon
Si Si Pt Pt
−1.0
−1.0 −0.5 0 0.5 1.0 1.5
Log [HF] (vol %)
(a) (b)
Figure 11.5 (a) Regimes of silicon anodic etching in HF: porous silicon formation and electropolishing. Reproduced
from Collins, S.D. (1997), by permission of Electrochemical Society Inc; (b) Electrochemical etching set-up

10000
p-type
n-type
1000 Macro
Porediameter (nm) 100

10 Meso

Micro S4700 1.5 kV 7.6 mm × 8.21k SE(L) 3/31/03
1 5.00 µm
0.001 0.01 0.1 1 10 100
Resistivity (ohm cm)
(a) (b)
Figure 11.6 (a) Pore size ranges of electrochemically etched silicon: macroporous, mesoporous and microporous
regimes. Reproduced from Lehmann, V. (1995), by permission of IEEE; (b) 50 nm pore size (with a micron particle).
SEM micrograph courtesy Eero Haimi, Helsinki University of Technology

Illumination contributes to hole concentration in high resistivity material has to be used. If pore formation
n-silicon (but not in p-type Si) and a very wide range starts from an unobstructed surface, a random pore array
of pore sizes from 0.2 to 20 µm can be etched by results. If initial pits are prepared by lithography and
varying electrolyte concentration, current density and etching, pores can be arranged at will.
illumination (Figure 11.6). As a rule of thumb, pore There are a couple of drawbacks in electrochemical
diameter in micrometres is half the resistivity in ohm- etching (and deposition): electrical contact has to be
cm: for 1 µm pores, 2 ohm-cm n-silicon is suitable. For made to the wafer backside, and this contact has to
small pores, low resistivity is needed; for large pores, tolerate the etchant. Concentrated HF (49%) is often