handbook of modern sensors

552 18 Sensor Materials and Technologies
Fig. 18.13. Etching around the boron-doped silicon.
(A)
(B)
(C)
(D)
18.3.2.1.3 Concentration-Dependent Etching
Thinner diaphragms, up to about 20 µm thick, can be produced using boron to stop
the KOH etch (Fig. 18.12B). This is called the concentration-dependent etching. The
thickness of the diaphragm is dependent on the depth to which the boron is diffused
into the silicon, which can be controlled more accurately than the simple, timed KOH
etch. High levels of boron in silicon will reduce the rate at which it is etched in KOH
by several orders of magnitude, effectively stopping the etching of the boron-rich
silicon. The boron impurities are usually introduced into the silicon by a process
known as diffusion.
In addition to the diaphragms, many other structures can be built by the concentration-dependent
etching. A thick oxide mask is formed over the silicon wafer and
patterned to expose the surface of the silicon wafer where the boron is to be introduced
(Fig. 18.13A). The wafer is then placed in a furnace in contact with a boron diffusion
source. Over a period of time, boron atoms migrate into the silicon wafer. Once the
boron diffusion is completed, the oxide mask is stripped off (Fig. 18.13B). A second
mask may then be deposited and patterned (Fig. 18.13C) before the wafer is immersed
in the KOH etch bath. The KOH etches the silicon that is not protected by the mask,
and it etches around the boron-doped silicon (Fig. 18.13D). Boron can be driven into
the silicon as far as 20 µm over periods of 15–20 h; however, it is desirable to keep the
time in the furnace as short as possible. Concentration-dependent etching can also be
used to produce narrow bridges or cantilever beams. Figure 18.14A shows a bridge,
defined by a boron diffusion, spanning a pit that was etched from the front of the
wafer in KOH. A cantilever beam (a bridge with one end free) produced by the same
method is shown in Fig. 18.14B. The bridge and beam project across the diagonal of
the pit to ensure that they will be etched free by the KOH. More complex structures
are possible using this technique, but care must be taken to ensure that they will be
etched free by the KOH.