Radiation damage studies of n-side silicon microstrip detectors

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Nuclear Physics B (Proc. Suppl.) 44 (1995) 475-479

Radiation damage studies of n-side silicon microstrip detectors K. Gill

High Energy Physics, Blackett Laboratory, Imperial College, London, SW7 2AZ, United Kingdom The RD20 Collaboration

Athens, Bonn, Bratislava, CERN, Cracow, MPI Heidelberg, Helsinki, Liverpool, Brunel University London, Imperial College London, Marseille, University of Oslo, SINTEF Oslo, INFN Padova, INFN Roma, Rutherford Appleton Laboratory, LEPSI Strasbourg, INFN Torino, Uppsala, IHEP Vienna, PSI Villigen. The RD20 Collaboration has carried out systematic radiation damage studies of prototype silicon microstrip Iracking detectors for use in LHC experiments. Results are presented for dedicated n-side test-structures showing the effect of irradiation with photons to 7Mrad and fast neutrons to 8x1013 n/cm 2. Both p-stop and MOS fieldplate devices were investigated, each having a range of strip geometries in order to determine optimal configurations for long-term viability and performance.

1. Introduction Silicon microstrip detectors are intended for use in the inner tracking region of both C M S [ l l and ATLAS[2I, the two general purpose experiments proposed for the Large Hadron Collider (LHC) at CERN. The extremely harsh radiation environment expected at the LHC has prompted several groups to carry out systematic studies of the radiation tolerance of silicon detectors[3-5], where the goal is to develop detectors and readout electronics capable of operating for at least 10 years at the LHC. Double sided silicon microstrip detectors offer the opportunity of two-coordinate readout with a minimal increase of material in the tracking system. We have previously published results of systematic testing of p-side detectors[6] and in this paper the results from tests of n-side detectors are presented. Extra structures, such as field-plates[7] and pstops[8], are required on the n-side (ohmic side) devices in order to isolate the n-side strips. We have investigated the radiation hardness of these isolation technologies and have quantified the capacitance penalties associated with the additional isolation structures. 2. N-side isolation techniques Due to positive charge being trapped in the oxide, either as a result of processing or radiation damage, there is an accumulation layer of electrons at the 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(95)00572-2

silicon-oxide interface which effectively shorts the strips on the n-side. Figure 1 illustrates the extra structures required to provide good strip isolation. a) n o i s o l a t i o n

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Fig. 1 N-side strip isolation schemes Field-plates are AC-coupled devices with the readout metal extended beyond the n-type strip. By applying a negative bias to the readout strip relative to the ntype strip, the resulting electric field pushes the

476

K. Gill~Nuclear Physics B (Proc. Suppl.) 44 (1995) 475-479

surface into inversion, repelling the electrons in the accumulation layer. The critical parameter for the field-plate devices is the voltage required across the oxide to ensure good isolation.

Table 1 Ca) 5(~m pitch field-plate device geometries device n + strip metal metal width width overlap (lam) (j.ttn)

qam)

In p-stop devices the accumulation layer is broken due to the depletion around the p-n junction between the strips. The critical parameter is the p-stop doping concentration which must be sufficient to compensate the trapped positive charge in the oxide layer. Both field-plate and p-stop isolation techniques give good performance in low radiation environments, though little data exists on the radiation resistance of p-stop and field-plate isolation. Our previous work[9] has shown that field-plate isolation was viable for radiation doses up to 4Mrad and 4x1013n/cm 2. The results presented here supplement this data as well as describe the radiation tolerance of p-stop isolation. A detailed summary of the n-side device study can be found in [10]. 3 The N-side test structures A range of dedicated microstrip test-structures were produced that allowed evaluation of radiation sensitive device parameters such as interstrip capacitance and isolation and leakage current changes. The devices were designed at Imperial College and the results described in this paper are from devices fabricated at SINTEF, Norway[ll]. Devices from other manufacturers are currently being evaluated. The devices were designed with a range of geometries, at both 50ktrn and 100pro pitch, in order to optimise the choice of microstrip design for LHC applications. Tables 1 and 2 show the different fieldplate and p-stop device geometries. The p-stop devices were designed to include both 'common' p-stops, where the p-stops between nstrips are joined at the ends, and 'individual' p-stops where the p-stop strips between the n-strips were unconnected. The results in this paper are for common p-stops only; a comparison of the two designs is included in [10]. 19 wafers in total were tested with p-stop devices having three doping concentrations (1011, 9x1011, 5x1013 cm-2). The bulk leakage current for the devices was typically 50nA/cm 2.

A B C D

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(b) 100~tm pitch field-plate device geometries device n + strip metal metal width width overlap ([.tm) (~lm) A B C D

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Table 2 (a) 50Jaxn pitch p-stop device geometries device n + strip p+ strip metal width width width (gm) A B C D

5 5 5 10

5 12 20 10

10 10 10 15

(b) 100pro pitch p-stop device geometries device n + strip p+ strip metal width width width (ktm) A B C D 4.

5 25 40 65

65 45 30 5

17 37 52 77

Results

4.1 Field-plate a) Field-plate

results strip isolation

Requiring an interstrip isolation of at least 10M~ allows us to define the 'minimum operating voltage' for the field-plate devices. This voltage is applied both across the AC-coupling for strip isolation, and

K. Gill~Nuclear Physics B (Proc. Suppl.) 44 (1995) 475- 479

the bulk of the device to deplete the detector. Figure 2 shows how this voltage was observed to vary in the 501am pitch devices with 6°Co photon dose up to 7Mrad. Similar results were observed for the 1001.tm pitch devices and in general the interstrip isolation was of the order of 100MfZ for an operating voltage of 100V. >_.. 8o.,~

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0.6pF/cm which appears to saturate at around 100krad. We do not expect any further increase for higher doses. The 1MHz values correspond more closely to the high frequency operation of the L H C amplifiers than the m e a s u r e m e n t s at 100kHz; therefore the larger increase in the capacitance at 100kHz is not of great concern.

c) Microdischarge effects The SDC collaboration has previously observed increased levels of noise[12] in p-side detectors where a voltage was maintained across the coupling capacitors of AC-coupled detectors. This effect was observed to be worse for devices where the readout metal overlapped the implanted strip; a similar situation to the use of field-plates for isolation on the n-side. 10 -5 _

Photon dose (Mrad)

Fig. 2 Change in the field-plate operating voltage with dose 10 -6 _

b) Field plate strip capacitance

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The electronic noise in the L H C readout amplifiers will depend upon the capacitive load of the strips, which is dominated by the interstrip capacitance, i.e. the capacitance of a strip to all of its neighbours. This was measured before and after 6°Co irradiation, with the results shown in figure 3.

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Fig. 3 Effect of radiation on fieId-plate interstrip capacitance It can be seen that at 1MHz there is very little change in the interstrip capacitance up to 200krad, whereas at 100kHz there is an increase of typically

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Fig. 4 Microdischarge effect on leakage current in field-plate devices Figure 4 shows results for the leakage current in field-plate devices as a function of bias voltage, with and without a field across the oxide. Above 120V with the field-plates biased, an increase in leakage current was observed, consistent with microdischarge breakdown. This effect was enhanced after neutron damage and suppressed by subsequent 60Co photon damage[10]. W e t h e r e f o r e c o n c l u d e that microdischarge breakdown occurs in field-plate devices.

5 P-stop results a) P-stop strip isolation Figure 5 shows how the interstrip isolation varied for the different geometries and p-stop doping

478

K. Gill~Nuclear Physics B (Proc. Suppl.) 44 H995) 475~479

densities before irradiation. Immediately, one can conclude that the lowest p-stop doping density was insufficient to provide good interstrip isolation ~IOMD.). lO'

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c) P-stop strip capacitance The interstrip capacitance was again the dominant component in these devices and figure 7 shows how the interstrip capacitance varied with 60Co dose for devices that were previously unirradiated as well as for devices that had been previously irradiated with 8xl013n/cm 2. The following section is limited to a discussion of the results from the highest doped pstop devices as we showed in the previous section that the lower p-stop doping densities were insufficient for operation at the LHC.

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After irradiation with 60Co photons up to 7Mrad, as in figure 6, it was clear that the intermediate p-stop doping density of 9x101 lcm2 was also insufficient for silicon microstrips intended for use at the LHC. Note the saturation effect due to the built up field of the trapped charge in the oxide layer. Neutron irradiation also decreased the strip isolation to around 100Mr2 at 100V bias, for the highest doped p-stops, the decrease saturating after type inversion at 1013n/cm 2.

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Fig. 7 Interstrip capacitance for p-stops as a function of 6°Co photon dose The radiation damage from 60Co photons to previously unirradiated devices causes a larger increase in the interstrip capacitance than in devices previously irradiated beyond inversion with neutrons. Again, the effect of saturation of the oxide damage was observed at around 1Mrad. These results apparently do not entirely agree with previous SDC results[13] which show an overall decrease in the interstrip capacitance from initial values after combined ionising and bulk radiation damage. The change in the interstrip capacitance of our devices due to radiation damage could be linked directly with the radiation induced changes in the pstop voltage characteristics. The dependence of the nearest neighbour capacitance on the p-stop voltage is illustrated in figures 8. For a sufficiently large pstop voltage, the capacitance reaches a mmimum value which depends only upon the geometry of the n-type strip (specifically, its width to pitch ratio).

K. Gill~NuclearPhysics B (Proc. Suppl.) 44 (1995) 475~479

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Fig. 8 Interstrip capacitance for p-stops before irradiation as a function of p-stop voltage The effect of irradiation on the p-stop voltage is such that neutron damage increases, and photon damage decreases, the p-stop voltage at a given bias voltage[14,15]. In devices where the magnitude of the p-stop voltage was greater than around 6V (at 100V bias) before and after ionising damage, the interstrip capacitance at 1MHz and 100V bias therefore did not change significantly with dose. This was indeed the case for the devices irradiated previously with neutrons, as in figure 8. The p-stop voltage at 100V bias was measured to be as high as 30V after inversion with neutrons in the 501am pitch device with the widest p-stops. In contrast, identical structures that were irradiated only with photons were measured to have p-stop voltages less than 6V at 100V bias after irradiation, explaining the larger increase in the interstrip capacitance with ionising dose observed for these devices. 6.

Conclusions

A~ thorough study has been made of n-side microstrips with field-plate and p-stop isolation. Both schemes provide good strip isolation (>10Mf~) before and after radiation doses typical of those expected at LHC. In the case of p-stop devices, only the devices with a doping density of 5xl013/cm 2 provided adequate interstrip isolation after large doses of neutron and photon radiation. The microdischarge effect was confirmed in fieldplate devices. This represents a problem for the

479

operation of field-plate devices at LHC, where high voltages across the oxide would be required to maintain good interstrip isolation. The strip capacitance before irradiation for the fieldplate and p-stop devices was very similar for the same n-strip width. After irradiation with photons, there was almost no change in the capacitance of the field-plate devices at 1MHz; whereas for the p-stop devices, the interstrip capacitance increased by around 25%. This increase was suppressed in devices previously irradiated beyond inversion with neutrons. The capacitance changes in the p-stop devices were found to be connected with radiation induced changes in the p-stop 'punch-through' voltage characteristics. Further results from the n-side study can be found in [10], including signal tests on the devices before and after irradiation, a comparison of p-stop technologies and a comparison of different manufacturers. Acknowledgements

This work was funded by CERN and several national agencies: BWF (Austria), INFN (Italy), NAVF (Norway), KBN (Poland) and SERC (UK). We thank them for their support. We would also like to thank those who provided assistance with the neutron irradiation facility at ISIS and the gamma sources at Imperial College and Cracow. The technical support of the electronic and mechanical workshops of Imperial College was also greatly appreciated. References

[11 CMS Letter of Intent, CERN/LHCC 92-3. [2] Atlas Letter of Intent, CERN/LHCC 92-4. [3] RD20 Status Report, CERN/DRDC 94-39 [4] RD2 Collaboration, NIM A342 (1994) 199. [5] H. Ziock et al., NIM A342 (1994) 96. [6] A. Holmes-Siedle et al., NIM A339 (1994) 511. [7] B.S. Avset et al., IEF.E Trans. Nucl. Sci. NS-37 (1990) 1153. [8] G. Batignani et al., NIM A277 (1989) 147. [9] K. Gill et al., NIM A322 (1992) 177. [101 H-G. Moser et al, RD20 pre-print, submitted to NIM. [11] R. Wheadon, RD20 Note, RD20 TN/2 [12] T. Ohsugi et al., NIM A342 (1994) 22. [13] E. Barberis et al., NIM A342 (1994) 90. [14] K. Gill, RD20 Note, RD20 TN/28. [15] K. Gill and R. Wheadon, RD20 Note, RD20 TN/29.