Introduction
To begin with, we have chosen this topic due to the fact that silicon radiation detectors are one of the main type of particle detectors used in the radiation detection industry nowadays. Hence, it is crucial for an engineer in this field to know the history of silicon production as well as the development process and concrete examples of the different types of detectors.
In this wiki page we are studying the rapid progress of the silicon radiation detectors, the design of the single sided stereo angle silicon strip detector, CdTe and CdZnTe (CZT) pixel detectors and the development of the detectors in the VTT Technical Research Centre of Finland. The information was collected from Institute of Electrical and Electronics Engineers/Institution of Electrical Engineers (IEEE/IEE) Electronic Library online.
History of silicon radiation detectors
According to Rehak (2003), silicon is the main material which is used in electronics and radiation detection industries. Moreover, there was a rapid development in the field of silicon radiation detectors in the last decade, which could be noted as a considerable progress after 1982, when planar technology was invented. [1.] Furthermore, according to Orava et al (2007), even today new development activity is initiated with introduction of 3D detector technologies, active edge silicon processing and other perspective inventions in the field. [3.] It has to be said that there is always interaction between electronics industry and silicon radiation detectors because they both use silicon as the main component. The main point is that silicon used for radiation detectors should have 3-4 times more resistivity and lower density of traps than the one used for electronics. [1.]
Interestingly, almost all radiation detectors before 1982 were lithium-drifted (Figure 1). Lithium doping was used in order to increase the thickness of the detector and thus to decrease the capacitance and apply reasonable voltages which would lead to lower noise levels.
Figure 1. Lithium drifted silicon detector. Reprinted from Rehak (2003) [1,2494].
However, the main disadvantage was the instability of the surfaces along the field of the detector and working temperature of about 77K. Obviously, silicon radiation detectors which could be used at room temperature were drastically needed. [1.]
Planar technology was introduced in 1982, which led to the rapid progress in development of the silicon detectors. Among others, strip detectors were invented and matched to meet the requirements of the Large Hadron Collider (LHC) developed at The European Organization for Nuclear Research (CERN). Main advantages were lower noise, smaller capacitance, room working temperature, easier production, reduced cooling requirements and larger area coverage. [1;3]
Strip silicon radiation detectors
Theory and design
Silicon detectors consist of n type material while having p type aluminium strips on the surface which are separated by a thin insulator. At the same time there is an electric field between the p strips and the n type material. When a radiation particle passes the detector, silicon atoms are ionized and free electrons leave these atoms with an electron vacancy (holes). After they reach the p type strip they are collected and then create a charge on the aluminium strip which is possible to be measured with special electronics.
Orava et al (2007) proposes a new type of the strip silicon detector (Figure 2) which implements cross-connected short strips interlaced by long strips. The main advantage is that better measuring resolution is achieved, which requires much less accuracy [3].
Figure 2. Layout of the single sided stereo angle silicon strip detector. Reprinted from Orava et al (2007) [3,227].
Furthermore, detector is designed in a way that every second strip is long while short strips are crossing them which helps to register particles at better angles to the surface of the sensor. Consequently, the high efficiency and proper special resolution are achieved. [3].
Development in Finland
VTT Technical Research Centre of Finland has been developing strip silicon radiation detectors for many years. What is more, VTT have been closely working with CERN which conducts the most important experiments on particle physics in the world.
Wafer is a very thin slice of silicon crystal which is used to produce the actual detectors. The production line at VTT can make wafers up to 200 mm in diameter using planar technology. Furthermore, the thickness of the strip detectors is only 150 μm or 300 μm which can be implemented for different bias configurations. Area of the detectors is from 5 x 5 cm2 to 1 x 1 cm2. Interestingly, the modern fabrication techniques such as etching, planarization, lithography and bonding are applied. The most important thing is that leakage currents of the detectors are under few nanoamperes, which makes it acceptable for radiation detection. [2].
Conclusion
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References
Rehak, P. ; Brookhaven Nat. Lab., Upton, NY, USA. Silicon radiation detectors [online]. Nuclear Science Symposium Conference Record, 2003 IEEE (Volume:5 ); 19-25 Oct. 2003, 2492-2497.
URL:
http://ieeexplore.ieee.org.ezproxy.metropolia.fi/stamp/stamp.jsp?tp=&arnumber=1352629
Accessed 16 March 2013.
Virolainen, T. ; Kamarainen, V. ; Ji, F. ; Garcia, F. ; Orava, R. ; van Remortel, N. ; Santala, M. Silicon radiation detector development at VTT [online]. Nuclear Science Symposium Conference Record, 2007. NSS '07. IEEE (Volume:2 ); Oct. 26 2007-Nov. 3 2007, 1494-1497.
URL:
http://ieeexplore.ieee.org.ezproxy.metropolia.fi/stamp/stamp.jsp?tp=&arnumber=4437282
Accessed 16 March 2013.
Huhtinen, M. ; Res. Inst. for High Energy Phys., Helsinki, Finland ; Orava, R. ; Pimia, M. ; Tuuva, T. Single sided stereo angle silicon strip detector [online]. Nuclear Science, IEEE Transactions on (Volume:40 , Issue: 4 ); Aug 1993, 227-229.
URL:
http://ieeexplore.ieee.org.ezproxy.metropolia.fi/stamp/stamp.jsp?tp=&arnumber=256575
Accessed 16 March 2013.
- Tom Schulman (2006) Si, CdTe and CdZnTe radiation detectors for imaging applications. University of Helsinki, Finland; June 19, 2006.