Physics and Engineering of Radiation Detection

448 Chapter 7. Position Sensitive Detection and Imaging

should extend up to the end of both sides. This ensures that all the charge pairs
created in the bulk of the silicon constitute the output signal. The detector in such
a configuration is said to operate in fully- or over-depleted mode, which is the usual
mode of operation of SMSDs. The output signal is read out through aluminum strips
implanted over the n+ strips. The two are however separated by an insulation strip
usually made ofSiO 2. The insulation creates a capacitance between the two strips
and therefore the readout is actually capacitively coupled. In other words, mirror
charges on the aluminum strips are actually seen by the readout circuitry. This de-
sign eliminates the need to install external coupling capacitors, something that can
be hard to implement considering the available space. Fig.7.2.7(a) shows another
layer of p-spray material. The reason for this layer is to discourage oxide buildup
between the strips that could lead to shorts and consequent impairment of position
resolution. Another design that can achieve this is shown in Fig.7.2.7(b), where
instead of a spray, strips of p+ material are implanted between the n+ strips. At
present p-stop design is more popular than p-spray design mainly due to engineering
difficulties.

SiO 2

SiO 2

Al Strip

n−Type Silicon

n+ Implant p−Spray

n−Type Silicon

p+ Implant p−Stop

Al

Al

(b)

(a)

Al Strip

Pitch

Pitch

Signal Out

Signal Out

Figure 7.2.7: Simple one

sided silicon microstrip de-

tector of (a) p-spray and (b)

p-stop types.

For proper functioning each of the strips must be biased such that the detector
gets fully depleted. Supplying bias is, however, not a trivial matter as it involves
distributing the output of a power supply to all the strips through biasing resis-
tors. These bias resistors take space and therefore must be made as small as possi-
ble. The available structures include implanted resistors, polysilicon resistors, and
punch-through resistors; each having their own pros and cons. The particular choice
depends on the type of detector, the radiation environment, and the cost.
The main dimension that determines the spatial resolution of a microstrip de-
tector is its pitchp, which is simply the distance between two consecutive readout