handbook of modern sensors

18.1 Materials 535
etching is a key technology for the micromachining of miniature three-dimensional
structures in silicon. Two etching systems are of practical interest. One is based on
ethylenediamine and water with some additives. The other consists of purely inorganic
alkaline solutions like KOH, NaOH, or LiOH.
Forming the so-called polysilicon (PS) materials allows one to develop sensors
with unique characteristics. Polysilicon layers (on the order of 0.5 µm) may be formed
by vacuum deposition onto oxided silicon wafer with an oxide thickness of about 0.1
µm [2]. Polysilicon structures are doped with boron by a technique known in the
semiconductor industry as LPCVD (low-pressure chemical vapor deposition).
Figure 18.1A shows the resistivity of boron-doped LPCVD polysilicon in a comparison
with SCS. The resistivity of PS layers is always higher than that of a singlecrystal
material, even when the boron concentration is very high. At low doping
concentrations, the resistivity climbs rapidly, so that only the impurity concentration
range is of interest to a sensor fabrication. The resistance change of PS with
temperature is not linear. The temperature coefficient of resistance may be selected
over a wide range, both positive and negative, through selected doping (Fig. 18.1B).
Generally, the temperature coefficient of resistance increases with decreased doping
concentration. The resistance at any given temperature of a PS layer may be found
from
R(T ) = R 20 e α R(T −T 0 ) , (18.1)
where
α R = 1 dR(T 0 )
R 20 dT
(A)
(B)
Fig. 18.1. Specific resistivity of boron-doped silicon (A); temperature coefficient of resistivity
of silicon for different doping concentrations (B).