Testing of surface properties pre-rad and post-rad of n-in-p silicon sensors for very high radiation environment

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Abstract

We are developing n+-in-p, p-bulk and n-readout, microstrip sensors as a non-inverting radiation hard silicon detector for the ATLAS Tracker Upgrade at the super LHC experiment. The surface radiation damages of the sensors fabricated by Hamamatsu Photonics are characterized on the interstrip capacitance, interstrip resistance and punch-through protection evolution. The detector should provide acceptable strip isolation, exceeding the input impedance of the signal readout chip ∼1 kΩ, after the integrated luminosity of 6 ab−1, which is twice the luminosity goal.

Introduction

The silicon microstrip detector continues to play an essential role in high-energy experiments for its ability of precision tracking. The detector at the planned Super LHC (large hadron collider) is required to remain operational up to the integrated luminosity of 3000 fb−1 with the instantaneous luminosity of 1×1035 cm−2 s−1. In order to cope with ten-fold increase in instantaneous luminosity beyond the design value of the LHC, currently under commissioning, the ATLAS collaboration is investigating an inner tracking system based fully on semiconductor devices. The segmentation is varied in radius R, the innermost being the pixel, followed by short (2.4 cm) and long (9.7 cm) microstrip detectors. The radiation activity [1] with a safety factor of two multiplied is (7−11)×1014 1 MeV neq./cm2 for the short strip (R=38 cm) and 3−6×1014 1 MeV neq/cm2 for the long strip (R=85 cm) regions, where the two numbers in the parentheses are the fluence values at the central and forward regions. The charged particle contribution to the fluence is similar to the neutral at R∼28 cm in the central region but decreases with R and in the forward region to typically 20% of the total. It is therefore important to investigate the damages due to both charged particles and neutrons. Both neutrons and protons displace silicon atoms via non-ionizing energy losses, which results in a bulk damage. Protons in addition ionize the atoms in their path that leads to permanent damage at the sensor surface.

The ATLAS R&D group “Development of non-inverting silicon strip detectors for the ATLAS ID upgrade” is formed to develop radiation hard tracking detectors based on the p-bulk microstrip [2] technology. Since the radiation induced impurity in silicon acts as an acceptor, the n+-on-p device is non-inverting. This allows us to operate the sensors at partial depletion when obliged. The experience of adopting p-bulk silicon for particle tracker is limited. This is in part because additional strip isolation structure is required for individual strip signal readout to prevent mobile electrons to be accumulated between strips. The R&D group is evaluating the sensors fabricated by Hamamatsu Photonics using commercially available p-type wafers. We report the surface damage including the decrease in strip isolation and punch-through voltage evolution. The bulk damage is reported in Ref. [3].

One of the most pressing issue for n-in-p strip sensors are the interstrip characteristics before and after ionizing radiation, since the electron accumulation layer on the surface needs to be compensated for large fluences and dose levels.

Previous studies with p-type sensors, e.g. within the context of RD50, were done using p-spray to isolate the n-strips [4]. This study uses “mini-SSDs” (∼1 cm long) produced by Hamamatsu Photonic (HPK) within the ATLAS upgrade program. The isolation is done with p-stops of varying geometry, p-spray and both combined with p-doses (concentration) varying from 0 up to 2×1013 p/cm2.

Section snippets

Samples and irradiation

The sample sensors were fabricated using 15 cm wafers with 〈1 0 0〉 crystal orientation and 320 μm thickness. The wafers we report in this paper are FZ grown (FZp wafers in Ref. [5]) having fewer defects than normal FZ wafers. The R&D group continues to evaluate other commercially available p-type wafers [5]. The strip pitch is 74.5 μm. Details of the design including strip isolation structures are described in Ref. [2]. The performance of main sensors, 97.5 mm2, is reported elsewhere [6]. The

Strip isolation structures

It is thought that n-on-p detectors are more sensitive to surface effects than p-on-n detectors. One concern is the risk that the fixed oxide charges in the Si–SiO2 interface would lead to a conductive layer of electrons at the surface [8]. Within the project ATLAS07 for the ATLAS upgrade different structures for mini-SSDs have been produced. The different structures use the concept of preventing those damages by surface treatments, positive doped implants (p-impurities) in form of p-stop or

Interstrip resistance and capacitance measurements

The interstrip resistance and capacitance are important parameters used to characterize the effects of surface radiation damage of silicon strip detectors. The interstrip resistance is important for strip isolation, so that a sufficiently high interstrip resistance can prevent signal sharing between neighbors which could lead to degradation of the position resolution. The interstrip capacitance is the main contributor of noise in between strips. A properly functioning detector should thus try

Punch-through protection

Ac-coupled sensors are susceptible to very large voltages between the metal readout traces (held to ground through the front-end electronics) and the strip implants in the case of large charge accumulation in the bulk, for instance in the case of beam losses [9]. When sufficient charge is deposited in the detector, the electric field can collapse causing the implants and backplane of the sensor to float to unknown voltages. These large voltages on the implants can reach the order of half the

Results

To first order, the interstrip resistance does not seem to depend on the specific zone, but instead depends only on the total amount of p-dose applied to the surface (through p-stop or p-spray), as seen in Fig. 6. There is an obvious correlation between the total amount of p-dose applied and the value of interstrip resistance after irradiation, with a higher total p-dose resulting in better post-rad strip isolation (higher interstrip resistance), which is illustrated in Fig. 7.

The interstrip

Summary and discussion

We are designing radiation hard silicon microstrip detector for the ATLAS Inner Detector Upgrade at the super LHC. P-bulk sensor is a good candidate for such a very high radiation environment reaching 1×1015 1 MeV neq/cm2.

The interstrip resistance decreases and interstrip capacitance increases after irradiation. To first order, the interstrip resistance does not depend on the specific zone of the detector, but instead depends on the total dose of the p-impurities applied. The higher the total

Acknowledgements

We would like to express our thanks to K. Yamamura and S. Kamada of Hamamatsu Photonics K.K. for helpful comments on the design of the sensor. The team from CYRIC, Tohoku University for conducting excellent irradiations. The research was partly supported by Ministry of Education, Youth and Sports of the Czech Republic, the German Federal Ministry of Education and Research, the Japan Grant-in-Aid for Scientific Research (A) (Grant no. 20244038), (C) (Grant no. 20540291), and on Priority Area

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