It has been shown that n-type readout planar sensors are more radiation tolerant at large doses when compared to p-type readout devices [193, 194]. This is due to the shorter collection time of electrons with respect to holes, thus, reducing the total charge trapping. Charge trapping increases with fluence to the point of making the charge carrier life time shorter than the collection time and therefore, the collection distance shorter than the actual thickness of the device. After a fluence of 1·1016neq/cm2, assuming a bias
voltage high enough to saturate carrier mobility, the total collection distance in silicon is ∼ 24µm using a trapping time calculated according to the parameterisation in [152]. In this situation, where the collection distance is significantly shorter than the actual device thickness, thinner detectors could have an advantage with respect to thicker devices because of higher electric fields at the same bias voltage. They would also be less susceptible to charge trapping , because of increased carrier mobility and should have an increased charge multiplication effect due to the larger electric fields. To study this, a selection of Micron produced, 1x1cm2 n-in-p micro-strip sensors of thicknesses, 106, 144 and 296µm have been characterised as a function of fluence and bias voltage.
Before irradiation all devices were tested with the ALiBaVa system to measure their response with respect to a MIP to be used as reference values. Figure 10.21 shows the results for the 106, 144 and 296µm thick devices. All devices are operated above their full depletion voltage to guarantee the maximum charge is collected, a saturation in the collected charge for each device is seen as indicated by the flat distributions.
100 200 300 400 500 600 700 5 10 15 20 25 30 C o l l e ct e d C h a r g e ( ke ) Voltage (V) 106µm 144µm 296µm
Figure 10.21: Signal as a function of bias voltage for 50, 106, 144 and 296µm thick planar silicon sensors.
10.5.1 Irradiation and Characterisation
Devices were irradiated to 2, 5, 10 and 20·1015neq/cm2at the Birmingham irradiation
facility. The ALiBaVa system was used to measure each device response in the usual way. Current-voltage (IV) measurements were performed along side the ALiBaVa measurements. Throughout storage and measurement all devices were cooled to - 25◦C. The total annealing time for each device has been estimated to be around 1 day (at 20◦C).
Figure 10.22 shows the measured IV characteristics for all devices at each fluence as a function of bias voltage. All the leakage currents have been corrected to a temperature of - 25 ◦C.
Figure 10.23 shows the CC(V) results for each device thickness and for fluences up to 20·1015n
eq/cm2. At low doses the thinner sensors collect less charge than the
thicker sensors, as expected. At 2·1015n
eq/cm2the 144 and 296µm thick sensors behave
2x10 15 n eq cm 2 - 106 µm 2x10 15 n eq cm 2 - 144 µm 2x10 15 n eq cm 2 - 296 µm 5x10 15 n eq cm 2 - 106 µm 5x10 15 n eq cm 2 - 144 µm 5x10 15 n eq cm 2 - 296 µm 10x10 15 n eq cm 2 - 106 µm 10x10 15 n eq cm 2 - 144 µm 10x10 15 n eq cm 2 - 296 µm 20x10 15 n eq cm 2 - 106 µm 20x10 15 n eq cm 2 - 144 µm 20x10 15 n eq cm 2 - 296 µm 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 L e a ka g e C u r r e n t ( n A ) Voltage (V)
Figure 10.22: Measured current-voltage (IV) characteristics of 106, 144 and 296µm thick
devices after irradiation to 2, 5, 10 and 20·1015neq/cm2all cooled to - 25◦C.
seen at 5·1015neq/cm2, the thicker sensors collect the highest charge but the difference
in collected charge between the 296 and 106µm devices is severely reduced. After very high doses, 10·1015n
eq/cm2and 20·1015neq/cm2, the thinner sensors
perform better. At 10·1015n
eq/cm2the 144µm shows the best performance relative to
peak charge collected while at 20·1015n
eq/cm2the 106µm sensors performs the best.
The collected charge of all devices across the full fluence range (2·1015neq/cm2→
20·1015neq/cm2) at 600 V and 900 V is shown in Figure 10.24a and Figure 10.24b,
2x10 15 n eq cm 2 - 106 µm 2x10 15 n eq cm 2 - 144 µm 2x10 15 n eq cm 2 - 296 µm 200 400 600 800 1000 1200 0 5 10 15 20 25 C o l l e ct e d C h a r g e ( ke ) Voltage (V) (a) 5x10 15 n eq cm 2 - 106 µm 5x10 15 n eq cm 2 - 144 µm 5x10 15 n eq cm 2 - 296 µm 200 400 600 800 1000 1200 0 5 10 15 C o l l e ct e d C h a r g e ( ke ) Voltage (V) (b) 10x10 15 n eq cm 2 - 106 µm 10x10 15 n eq cm 2 - 144 µm 10x10 15 n eq cm 2 - 296 µm 200 400 600 800 1000 1200 0 3 6 9 12 C o l l e ct e d C h a r g e ( ke ) Voltage (V) (c) 20x10 15 n eq cm 2 - 106 µm 20x10 15 n eq cm 2 - 144 µm 20x10 15 n eq cm 2 - 296 µm 200 400 600 800 1000 1200 0 2 4 6 8 10 C o l l e ct e d C h a r g e ( ke ) Voltage (V) (d)
Figure 10.23: CC(V) dependance for 106, 144 and 296µm thick n-in-p sensors as a function
of fluence. (a) 2·1015neq/cm2, (b) 5·1015neq/cm2, (c) 10·1015neq/cm2and
(d) 2·1015neq/cm2.
10.5.2 Conclusion
In conclusion, thinner devices are found to be more radiation tolerant than thicker devices at the same operating voltage. At lower fluences the peak signal measured by thicker devices is greater than the thinner devices. However, the response of thinner devices varies significantly less with irradiation, yielding a superior signal after the highest fluences. Additionally, the use of thinner devices reduces the amount of material within the tracker, reducing the multiple scattering effect of low energy particles.
(a) (b)
Figure 10.24: Charge collected as a function of fluence for 106, 144 and 296µm thick sensors at (a) 600 V and (b) 900 V.