SILICON PARTICLE DETECTOR Supervised Learning Project Eslikumar Adiandhra 12D260012 Department of Physics, IIT Bombay Guide: Prof. Raghava Varma Department of Physics, IIT Bombay November 8, 2015
Abstract Silicon detectors have been used in almost all the High Energy Physics experiments built in the last half century or so; from LHC experiments to fixed target ones. It has also been used in many specialized detectors in astrophysics, medical imaging etc. The application of this detector technology is mostly used in tracking detectors, i.e., detectors which measure the position of charged particle to determine its energy momentum four vectors used in the physical analysis. The following chapters review the basic fundamentals needed to understand the operation of such detectors and how they are designed.
Acceptance Certificate Department of Physics Indian Institute of Technology, Bombay The Project Report titled Silicon Particle Detector submitted by Eslikumar Adiandhra (12D260012) may be accepted for evaluation. Signature (Supervisor: Prof. Raghava Varma) 2
Declaration Form I, Eslikumar Adiandhra, Roll No. 12D260012, understand that plagiarism is defined as any one or the combination of the following: 1. Uncredited Verbatim copying of individual sentences, paragraphs or illustrations from any source, published or unpublished, including the internet. 2. Uncredited improper paraphrasing of pages or paragraphs (changing a few words or phrases, or rearranging the original sentence order) 3. Credited verbatim copying of a major portion of a paper (or thesis chapter) without clear delineation of who did or wrote what. I have made sure that all the ideas, expressions, graphs, diagrams, etc., that are not a result of my work, are properly credited. Long phrases or sentences that had to be used verbatim from published literature have been clearly identified using quotation marks. I affirm that no portion of my work can be considered as plagiarism and I take full responsibility if such a complaint occurs. I understand fully well that the guide of my seminar report may not be in a position to check for the possibility of such incidences of plagiarism in this body of work. Signature Name: Eslikumar Adiandhra Roll Number: 12D260012 Date : 3
Acknowledgement My experience through this semester has been very smooth and exciting. The texts and papers referred were very lucid and all of the doubts were thoroughly entertained irrespective of how immature they were. I would like to express my deepest gratitude to my supervisor, Prof. Raghava Varma for his excellent guidance, caring and patience. Also, I would like to thank Ankur Agrawal, Dhanashree Shedge and Manoj Jadhav at IIT Bombay for clarifying my doubts. 4
Contents 1 Introduction 6 2 Basics of Silicon Semiconductor Technology 7 2.1 Material Properties of Intrinsic Silicon............ 7 2.2 Extrinsic properties of Silicon................. 9 2.3 The p-n Junction or Diode................... 11 3 Silicon Strip Detector 15 3.1 Working Principle of Silicon Detector............. 15 3.2 Design Basics of Silicon Strip Detector............ 16 3.2.1 Brief Description of Fabrication Technique...... 16 3.2.2 Strip Geometry..................... 17 3.2.3 DC to AC coupled strips................ 17 3.2.4 Biasing of the Strips.................. 18 3.2.5 Breakthrough Protection................ 19 3.2.6 Mask Design using CleWin............... 19 3.2.7 Sequence of Fabrication Process............ 20 3.2.8 Summary........................ 24 5
Chapter 1 Introduction Semiconductor detectors have several advantages over other types of radiation detectors. They possess unique properties that distinguishes them from others: better energy resolution, flexibility of design, tolerance to high energy radiation, fast timing response etc. In semiconductor detector, the electron hole pair produced by the interaction of an ionizing radiation are collected by applying an electric field to provide a signal. The signal strength is directly proportional to the total number of electron hole pairs produced i.e., the total quantity of ionization and therefore to the absorbed energy. (Since, the number of e-h pairs produced is directly proportional to the energy absorbed). The detecting medium should possess the following properties to achieve the discussed factors:- (a) Average energy required to produce an e-h pair should be low so that the number of e-h pairs is large as that determines the energy resolution of the device. (b) The charge carrier should move readily through the detector material, so that their collection time must be smaller than the carrier life time to complete the collection of free charges. (c) Leakage current must be very small even when large electric field ( 1000V.cm 1 ) is applied, such that the tiny signal from the transient current can also be measured. Therefore, semiconductors used are of high resistivity. [1] Silicon detector satisfies all the above criteria for a detecting medium and that explains their wide usage. 6
Chapter 2 Basics of Silicon Semiconductor Technology In this chapter, we will discuss about the material properties of silicon and about semiconductor device physics. 2.1 Material Properties of Intrinsic Silicon Silicon, the most common semiconductor, is a hard solid. At a temperature approaching 0 K the electrons occupy the lowest energy states, so that all states in the valence band are filled and the states in the conduction band are empty and no current can flow. At higher temperatures the thermal energy is high enough to break bonds and lift electrons from the valence band to the conduction band, thus creating a weak conductivity due to free electrons and holes (unoccupied electron states in the valence band). An insulator has a similar structure as a semiconductor, except that the bandgap is much larger (typically >5 ev) resulting in zero occupation probability of the states in the conduction band at room temperature. Metals may either have overlapping valence and conduction bands or a partially filled conduction band. So, we can say that at low temperatures, it acts like an insulator and shows measurable conductance at higher temperatures. Silicon possess all the required properties of a detecting medium. (a) Mean energy required for e-h pair generation is comparatively low for Silicon: 3.61±0.01 ev. [3] (b) Small band-gap ( 1.12 ev at room temperature), electrons can be excited easily to conduction band. (C) Silicon is abundantly available in the form of its oxide SiO 2. So, due to the natural existence of its oxide, external damage, contamination can 7
Figure 2.1: The formation of energy bands as the diamond lattice crystal is formed by bringing isolated silicon atoms together. [2] be restrained. (d) Flexible design: it is possible to fabricate small structures upto a precision of 10 micrometers. Moreover, various geometries have already been developed and tested. Pad and Strip silicon detectors are highly used in energy measuring trackers. Although, silicon is an ideal sensor for various applications but intrinsic silicon will be of minimal use due to the following factors: (a) Transient current of the signal will be dominated by the noise current (leakage current). This is due to the fact that the concentration of intrinsic charge carriers is 10 4 bigger than the concentration of e-h pairs created by MIP (minimum ionizing particles) (b) Intrinsic silicon is not a conductor and therefore, ohmic contacts are needed to extract the signal. 8
Figure 2.2: A simplified model of the electron energy band structure in solids. [4] 2.2 Extrinsic properties of Silicon The intrinsic electronic properties of silicon can be manipulated by replacing silicon atoms from the crystal lattice i.e., by injecting impurity atoms, so-called dopands. Group III elements, which have one electron less than silicon in their outermost shell or group V elements, which have an additional electron can be used to change the number of charge carriers as seen in fig 2.4. Group III elements are called acceptors (they can accept an extra electron) while group V elements are called donors. (they can donate an additional electron). The doping of silicon with a donor (Phosphorus or Arsenic) increases the negative charge carriers. The extra electron introduced will reside in an energy level slightly below the lower conduction band edge E c as seen in figure 2.5. This type of silicon is called n-type silicon. If an acceptor, usually boron (B) is doped with silicon. Boron, a group III element will have three electrons in its outer shell and the missing electrons will act like a hole, thus increasing the number of positive charge carriers. These holes will introduce energy states slightly above the upper valence band energy E v as seen in figure 2.5. Doped silicon with excess number of holes is also called p-type silicon. As mentioned earlier, the concentration of e-h pair in intrinsic silicon pro- 9
Figure 2.3: (a) n-type Si with donor (phosphorus). acceptor (Boron). [3] (b) p-type Si with Figure 2.4: (a) n-type Si with donor (phosphorus). acceptor (Boron). [5] (b) p-type Si with duced by ionizing radiation is less than the concentration of free charge carriers. So, in order to improve the quality of signal and minimize the effects of noise, we either have to increase the concentration of e-h pair produced by MIP by 10 5 10 6 times or restrict the free charge carrier concentration such that it won t interfere with the signal and we can almost 10
get noise-free signal. The first one seems impossible but the latter can be done. In a silicon detector, in order to enhance signal to noise ratio (SNR) i.e., to reduce the leakage current, it is required to minimize the concentration of free charge carriers. For this, negative contact must not inject electrons and positive contact must not inject holes into the bulk medium. This can be achieved by preventing free exchange of electrons and holes between the electrodes and semiconductor(silicon). A very common method of achieving this is by forming p-n junction just beneath the surface of the detector and operating with a reverse bias voltage. These detectors are fabricated by forming a heavily doped p-type layer on the surface of n-type material or vice-versa. 2.3 The p-n Junction or Diode Diode is the simplest semiconductor device. It is an electronic check valve which enables the flow of electric current only in a single direction. If operated in conducting direction, diode will have a low resistance while operated in reverse direction, the resistance is high. A p-n junction is created by joining a p-type and an n-type material. Initially when they are brought into contact, the electrons from n-type region will diffuse into p-type region and recombine with the holes. As the electrons diffusing from the n-type region leaves the donor ions uncompensated, a space charge region near the junction is created. Similarly in the p-type region, the holes are compensated by the electrons and leave the acceptor ions uncompensated. The diffusion of the electrons will be stopped due to the build up of charge until a certain depth of space charge region is created, which is depleted of free charge carriers. The situation in the proximity of the p-n junction is shown in fig 2.6. Prior to contact, the band structure is different in the p-type and n-type material but due to doping, the fermi level E fermi will move towards the conduction band for n-type material and will move towards the valence band for p-type material. At thermal equilibrium, fermi level should lineup. This will lead to a so called built in voltage V bi or diffusion voltage, by shifting the valance and conduction bands as shown in figure 2.6.b. We can calculate V bi using the following equations of charge carrier concentration n = n i exp ( E fermi E ) i K B T (2.1) 11
p = n i exp ( E i E ) fermi K B T (2.2) and setting the majority carrier concentrations equal to the donor and acceptor concentrations: n n = N D p p = N A (2.3) N D N A = n i 2 exp ( E p i E n i KT ) (2.4) V bi = 1 q (Ep i Ei n ) = KT N q ln( D N ) A n 2 i (2.5) The size of the depleted zones in the n-type (d p ) and p-type (d n ) material can be calculated using the following expressions: d p = 2ɛε q e N D N D (N A + N D ) V bi (2.6) d n = 2ɛε q e N A N D (N A + N D ) V bi (2.7) d = d n + d p = 2ɛε(NA + N D ) q e N D N A V bi (2.8) Generally, p-n junctions are usually formed using a low doped material for practical applications, where a certain region gets highly doped towards the opposite type. The p-n junction is formed at the edge of the highly doped region which is in contact to the surrounding bulk with low doping concentration of opposite dopands. In this realistic case where the doping concentration on one side of the junction is significantly higher than on the opposite side (e.g. NA >>ND), equation 2.8 becomes: d = 2ɛε q e N D V bi (2.9) If an external voltage is applied across the p-n junction, the system will not be in the thermal equilibrium anymore. The charge carriers will start to drift according to the electric field and depending on the polarity of the applied voltage, the width of the space charge region will shrink (forward 12
Figure 2.5: Various parameters in the proximity of a p-n junction. a) the donor and acceptor distribution of a partially depleted p-n junction. b) shows the energy band structure. c) the concentration of doping atoms. d) the charge density is shown, e) electric field and f) electric potential. [5] bias) or expand (reverse bias) and we have to replace the built-in voltage 13
V bi with V bi - V : d = 2ɛε(NA + N D ) q e N D N A (V bi V ) (2.10) For the realistic case as discussed above with N A >>N D, equation 2.10 simplifies to d = 2ɛε q e N D (V bi V ) (2.11) 14
Chapter 3 Silicon Strip Detector In this chapter, we will discuss about the working principle of silicon detector and the design process of a silicon strip detector. 3.1 Working Principle of Silicon Detector The operating principle of a semiconductor detector is similar to a gas ionization chamber. In a simple design, the absorbing material is now replaced by semiconductor(silicon) and is covered by a pair of electrodes with an applied voltage. In a more complex configuration, the absorbing material is further broken into precise sizes e.g., strip and pixels so as to localize the path of the particle and thereby track it. The individual segments are basically diodes which are reverse biased to fully deplete the silicon bulk from free charge carriers. Electron-hole pairs are produced in the depleted volume by ionizing radiation. These e-h pairs drift under the influence of applied electric field and induce an electric current in the external circuit. The current measured is proportional to the concentration of e-h pairs created in the depleted volume and the number of created e-h pairs is proportional to the absorbed energy. Figure 3.1: Schematic of the working principle of a silicon detector [6] 15
3.2 Design Basics of Silicon Strip Detector It is possible to detect a particle hitting the detector using the basic principle described in the previous section. But, it won t give us any additional information regarding the site of hit on the detector. By creating smaller sensing elements which are electrically isolated from each other using the same principle, we can achieve this. Strip detector is one such geometry used in detecting the particles. Strip detector have narrow and long asymmetric strips as basic detecting elements. These strips usually extend to the full length of the detector giving only one-dimensional information on the location of the hit on the detector. The width of each strip is in the order of tens of microns, while the pitch between the strips can be from 50 microns to several hundreds of microns. Silicon strip detectors are generally fabricated by passivated planar techniques, which combine the technique of ion implantation and photolithography. Figure 3.2: A schematic drawing of a silicon micro-strip sensor cross section 3.2.1 Brief Description of Fabrication Technique A n-type silicon is taken as a substrate and is then chemically cleaned and oxidised by heating it in O 2 atmosphere at around 1000 o C to have the whole surface passivated. Photolithography techniques are used to remove the selected areas of oxides. The etched areas enable doping of silicon in desired areas (by ion implantation techniques). Radiation damage caused by ion implantation technique is removed by annealing. Finally surface of wafer is metalized by Al and by proper masking desired pattern is generated. The oxide can be left to make a capacitor (integrated coupling capacitors). 16
3.2.2 Strip Geometry In Strip detector, only the upper electrode is segmented while the lower electrode called the backplane covers the backside of detector. The readout electronics is attached to each strip and collects the charges generated by incident particles. The strip pitch depends on the the density of readout channels and spatial resolution. The ratio between width and pitch (width to pitch ratio r wp ) is driven by the balance of low capacitance between the strips. Figure 3.3: 3D-model of a standard silicon strip used in the CMS experiment [7] 3.2.3 DC to AC coupled strips A p-n junction in reverse bias operation will show a small reverse bias current. In an actual detector, each strip exhibits such a small DC current of the order of nano-amperes. However, the reverse bias current can rise up to few micro-amperes per strip in case of heavily irradiated strip detectors. Using current compensation circuits, the amplifiers in the readout chips have to be protected. The high pass filters will remove the DC fraction of the current, allowing only the AC signals to pass to the amplifier. We can also use large capacitors between the implant and the readout connection to incorporate the same functionality. The readout capacitors are created by covering the strips with an aluminum electrode of about equal size above the strip implant, separated by a thin layer of oxide. By minimizing the thickness of the oxide according to C = ɛ A d, we can maximize the capacitance of this parallel-plate capacitor. DC strips are biased from the readout electronics, we need a separate biasing scheme for AC coupled strips. 17
3.2.4 Biasing of the Strips A reverse bias voltage is needed to deplete the underlying sensor bulk from charge carriers. An aluminium metallisation covering the backside will act as contact to the bulk. In order to isolate the strips from each other electrically, each strip is connected to the bias ring using a resistor. These are few ways in which resistors can be implemented: Polysilicon resistor: A resistor made of doped polysilicon is embedded in the sensor. It has a linear ohmic behaviour and is very radiation hard. Figure 3.4: Polysilicon resistor [7] FOXFET: The small gap between the strip implant and the bias ring is covered by a metal-oxide structure. Similar to a Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) where the effective resistance of the gap can be controlled using a small voltage applied to the metal gate. This structure is often referred to as FOXFET, as it utilizes the thick Field Oxide as gate insulator. It is cheaper but less radiation hard than polysilicon resistors. Figure 3.5: FOXFET [7] 18
Punch through: Very similar to a FOXFET but without a gate and a smaller gap. The size of the gap and the electric properties of the oxide define the resistance of the structure. This is also cheaper but less radiation hard than polysilicon resistors. Figure 3.6: Punch through [7] 3.2.5 Breakthrough Protection Guard Ring: The bias voltage supplied to the bias ring will drop to the backside potential in the small area between the edge of the ring implant and the sensors cutting edge. This will introduce high field strengths in an area afflicted by strong lattice deformations and contamination due to the cutting. To lessen the effect of the abrupt change in potential, an additional ring of implant connected to the aluminium above it surrounds the bias ring. These so-called guard rings are floating and ensure a defined drop of the bias voltage along a larger distance. A multi guard ring structure implements several concentric rings, ensuring an even wider spread of the voltage drop in small steps. They are usually all on a floating potential were the potential drop from one ring to the next can be adjusted via a punch through biasing. 3.2.6 Mask Design using CleWin We have used the CleWin software in designing the mask of silicon strip detector. The mask of the detector consists of three layers. First layer 19
consists of guard rings, second layer consists of strips of desired width with a pitch in the range of ten of microns to few hundreds of microns. The third layer fills the empty spaces and also the design of pad detectors is included. The width of strips is typically increased by 5 microns in the second layer as it is overlapped by the metal in the third layer of the mask design. Figure 3.7: Screenshot of a digital design layout for photomask 3.2.7 Sequence of Fabrication Process All the images are taken from [8] 1. Plain Silicon wafer 20
2. Oxidation and first photolithographic step 3. Etching of oxide mask against n+ implantation 4. Phosphorus implantation/diffusion 5. Oxidation and second photolithographic step 21
6. Etching of oxide mask against p+ implantation 7. Boron implantation 8. Thin Readout Oxide (Gate Oxide) 9. Polysilicon deposition and doping 10. Third lithographic step 22
11. Etching of polysilicon 12. Fourth photolithographic step and implantation of the polysilicon heads 13. Fifth photolithographic step 14. Deposition of metal and the sixth photolithographic step 15. Passivation and seventh photolithographic step 23
3.2.8 Summary A detailed study about material properties of silicon and semiconductor device physics of silicon has been done. I studied about the working principle of silicon detectors and design of silicon strip detectors. I have learnt CleWin software used in the designing of detector mask. 24
References [1] Nuclear Radiation Detectors, S.S. Kapoor, V. S. Ramamurthy. Wiley Eastern Limited. [2] S. M. Sze, M.-K. Lee, Semiconductor Devices: Physics and Technology, John Wiley Sons, 2012. [3] Energy for Electron-Hole Pair Generation in Silicon by Electrons and α Particles [4] Adapted from Prof. Dr. Hans-Christian Schultz-Coulon Lecture Notes [5] G. Lutz. Semiconductor Radiation Detectors: Device Physics. Springer, Berlin, 2007. [6] Adapted from lecture notes of Prof. Helmuth Spieler [7] F. Hartmann. Evolution of Silicon Sensor Technology in Particle Physics, volume 231 of Springer Tracts in Modern Physics. Springer, 2009. [8] The New Silicon Strip Detectors for the CMS Tracker Upgrade [9] Design of a Radiation Hard Silicon Pixel Sensor for X-ray Science. [10] Characterization of Single Sided Silicon Microstrip Detector 25