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Silicon Detectors XII ICFA School on Instrumentation Bogotá, Nov. 25th – Dec. 6th, 2013 Part 2 Manfred Krammer Institute for High Energy Physics, Vienna, Austria 3 Detector Structures XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 1 3.0 Pad Detector The most simple detector is a large surface diode with guard ring(s). XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 2 3.1 Microstrip Detector DC coupled strip detector Through going charged particles create e-h+ pairs in the depletion zone (about 30.000 pairs in standard detector thickness). These charges drift to the electrodes. The drift (current) creates the signal which is amplified by an amplifier connected to each strip. From the signals on the individual strips the position of the through going particle is deduced. A typical n-type Si strip detector: p+n junction: Na ≈ 1015 cm-3, Nd ≈ 1–5·1012 cm-3 n-type bulk: > 2 kcm thickness 300 µm Operating voltage < 200 V. n+ layer on backplane to improve ohmic contact Aluminum metallization XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 3 3.1 Microstrip Detector n-type and p-type detectors The previous slide explains an n-type detector (detector bulk is ntype silicon) Using p-type silicon and exchanging p+ and n+ would give a perfectly working p-type detector. For tradition and production reasons most of the present detectors are n-type detectors. p-type detectors are discussed as base line detectors for the upgrade experiments CMS and ATLAS, due to their advantages in the high radiation environment (see later). For simplicity I will continue discussing n-type detectors only... XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 4 3.1 Microstrip Detector AC coupled strip detector AC coupling blocks leakage current from the amplifier. Integration of coupling capacitances in standard planar process. AC coupled strip detector: Deposition of SiO2 with a thickness of 100– 200 nm between p+ and aluminum strip Depending on oxide thickness and strip width the capacitances are in the range of 8– 32 pF/cm. Problems are shorts through the dielectric (pinholes). Usually avoided by a second layer of Si3N4. However, the dielectric cuts the bias connection to the strips! Several methods to connect the bias voltage: polysilicon resistor, punch through bias, FOXFET bias. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 5 3.1 Microstrip Detector Polysilicon bias – 1 Deposition of polycristalline silicon between p+ implants and a common bias line. Sheet resistance of up to Rs ≈ 250 k/ . To achieve high resistor values winding poly structures are deposited. Depending on width and length a resistor of up to R ≈ 20 M is achieved (R = Rs·length/width). Drawback: Additional production steps and photo lithograpic masks required. Cut through an AC coupled strip detector with integrated poly resistors: XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 6 3.1 Microstrip Detector Polysilicon bias – 2 Top view of a strip detector with polysilicon resistors: CMS-Microstrip-Detektor: Close view of area with polysilicon resistors, probe pads, stip ends. CMS Collaboration, HEPHY Vienna XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 7 3.1 Microstrip Detector Punch through bias Punch through effect: Figures show the increase of the depletion zone with increasing bias voltage (Vpt = punch through voltage). 1.) 2.) 3.) Advantage: No additional production steps required. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 8 3.1 Microstrip Detector FOXFET bias Strip p+ implant and bias line p+ implant are source and drain of a field effect transistor - FOXFET (Field OXide Field Effect Transistor). A gate is implemented on top of a SiO2 isolation. Dynamic resistor between drain and source can be adjusted with gate voltage. FOXFET biasing: XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 9 3.1 Microstrip Detector Wire bond connection Ultrasonic welding technique typically 25 micron bond wire of Al-Si-alloy Fully-automatized system with automatic pattern recognition XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 10 3.2 Double Sided Strip Detectors Principle Single sided strip detector measures only one coordinate. To measure second coordinate requires second detector layer. Scheme of a double sided strip detector (biasing structures not shown): Double sided strip detector measures two coordinates in one detector layer (minimizes material). In n-type detector the n+ backside becomes segmented, e.g. strips orthogonal to p+ strips. Drawback: Production, handling, tests are more complicated and hence double sided detector are expensive. • • XII ICFA School on Instrumentation, Bogotá, 2013 Holes drift to p+ strips Electrons drift to n+ strips Manfred Krammer 11 3.2 Double Sided Strip Detectors n-side separation Problem with n+ segmentation: Static, positive oxide charges in the Si-SiO2 interface. These positive charges attract electrons. The electrons form an accumulation layer underneath the oxide. n+ strips are no longer isolated from each other (resistance ≈ k). Charges generated by through going particle spread over many strips. No position measurement possible. Solution: Interrupt accumulation layer using p+-stops, p+-spray or field plates. Positive oxide charges cause electron accumulation layer. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 12 3.2 Double Sided Strip Detectors p+-stops p+-implants (p+-stops, blocking electrodes) between n+-strips interrupt the electron accumulation layer. Interstrip resistance reaches again G. Picture showing the n+-strips and the p+-stop structure: A. Peisert, Silicon Microstrip Detectors, DELPHI 92-143 MVX 2, CERN, 1992 XII ICFA School on Instrumentation, Bogotá, 2013 J. Kemmer and G. Lutz, New Structures for Position Sensitive Semiconductor Detectors, Nucl. Instr. Meth. A 273, 588 (1988) Manfred Krammer 13 3.2 Double Sided Strip Detectors p-spray p doping as a layer over the whole surface. disrupts the e- accumulation layer. Some companies use a combination of p+ stops and p spray XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 14 3.2 Double Sided Strip Detectors Field plates Metal of MOS structure at negative potential compared to the n+-strips displace electrons below Si-SiO2-interface. Above a threshold voltage n+-strips become isolated. Simple realization of AC coupled sensors: Wide metal lines with overhang in the interstrip region serve as field plates. A field plate at negative potential interupts accumulation layer: n+-strips of an AC coupled detector. The alumimnum readout lines act as field plates: A. Peisert, Silicon Microstrip Detectors, DELPHI 92-143 MVX 2, CERN, 1992 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 15 3.2 Double Sided Strip Detectors 2nd metal layer – 1 In the case of double sided strip detectors with orthogonal strips the readout electronics is located on two sides (fig. a). (a) Many drawbacks for construction and material distribution, especially in collider experiments. Electronics only on one side is a preferred configuration (fig. b). Possible by introducing a second metal layer. Lines in this layer are orthogonal to strips and connect each strips with the electronics (fig. c). The second metal layer can be realized by an external printed circuit board, or better integrated into the detector. XII ICFA School on Instrumentation, Bogotá, 2013 (b) how? (c) Manfred Krammer 16 3.2 Double Sided Strip Detectors 2nd metal layer – 2 3D scheme of an AC coupled double sided strip detector with 2nd metal readout lines (bias structure not shown). The isolation between the two metal layers is either polyimide or SiO2: Cross section of the n+ side of an AC coupled double sided strip detector with 2nd metal readout lines. Shown is the end of a strip with the bias resistor: A. Peisert, Silicon Microstrip Detectors, DELPHI 92-143 MVX 2, CERN, 1992 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 17 3.3 Hybrid Pixel Detectors Advantage Double sided strip sensors measure the 2 dimensional position of a particle track. However, if more than one particle hits the strip detector the measured position is no longer unambiguous. “Ghost”-hits appear! True hits and ghost hits in a double sided strip detector in case of two particles traversing the detector: Pixel detectors produce unambiguous hits! Measured hits in a pixel detector in case of two particles traversing the detector: XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 18 3.3 Hybrid Pixel Detectors Advantages and disadvantages Typical pixel size is 50 µm x 50 µm. If signal pulse height is not recorded, resolution is the digital resolution: ~14 µm (50 µm pixel pitch). d … pixel dimension Better resolution achievable with analogue readout Small pixel area low detector capacitance (≈1 fF/Pixel) noise ratio (e.g. 150:1). Small pixel volume large signal-to- low leakage current (≈1 pA/Pixel) Drawback of hybrid pixel detectors: Large number of readout channels Large number of electrical connections in case of hybrid pixel detectors. Large power consumption of electronics.. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 19 3.3 Hybrid Pixel Detectors Principle “Flip-Chip” pixel detector: On top the Si detector, below the readout chip, bump bonds make the electrical connection for each pixel. S.L. Shapiro et al., Si PIN Diode Array Hybrids for Charged Particle Detection, Nucl. Instr. Meth. A 275, 580 (1989) XII ICFA School on Instrumentation, Bogotá, 2013 Detail of bump bond connection. Bottom is the detector, on top the readout chip: L. Rossi, Pixel Detectors Hybridisation, Nucl. Instr. Meth. A 501, 239 (2003) Manfred Krammer 20 3.3 Hybrid Pixel Detectors Bump bonding process - 1 1. Deposition of an “underbump metal layer”, plasma activated, for a better adhesion of the bump material. 2. Photolithography to precisely define areas for the deposition of the bond material. 3. Deposition, by evaporation, of the bond material (e.g. In or SnPb) producing little “bumps” (≈ 10 µm height). 4. Edging of photolithography mask leaves surplus of bump metal on pads. 5. Reflow to form spheres. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer L. Rossi, Pixel Detectors Hybridisation, Nucl. Instr. Meth. A 501, 239 (2003) A typical bump bonding process (array bump bonding) is the following: 21 3.3 Hybrid Pixel Detectors Bump bonding process Electron microscope pictures before and after the reflow production step. In bump, The distance between bumps is 100 μm, the deposited indium is 50 μm wide while the reflowed bump is only 20 μm wide. C. Broennimann, F. Glaus, J. Gobrecht, S. Heising, M. Horisberger, R. Horisberger, H. Kästli,J. Lehmann, T. Rohe, and S. Streuli, Development of an Indium bump bond process for silicon pixel detectors at PSI, Nucl. Inst. Met. Phys, Res. A565(1) (2006) 303–308 82 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 22 3.3 Hybrid Pixel Detectors Bump bonding process – 3 Electron microscope picture of pixel detector with long strip. Left: Detector chip, right: readout chip with bump bonds applied. G. Lutz, Semiconductor Radiation Detectors, Springer-Verlag, 1999 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 23 3.4 – 3.9 Other Si Detector Structures Strip and hybrid pixel detectors are mature technologies employed in almost every experiment in high energy physics. Additional interesting silicon detector structures are: • Charged Coupled Devices (CCD) • Silicon Drift Detectors (SDD) • Monolithic Active Pixels (MAPS) • Depleted Field Effect detectors (DEPFET) • Silicon On Oxide (SOI) • 3D detectors • Avalanche Photo Diodes (APD) and Silicon Photo Multiplier (SiPM) XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 24 3.4 Charge Coupled Devices (CCD) Principle Shallow depletion layer (typically 15 m), relatively small signal, the charge is kept in the pixel and during readout shifted through the columns and through final row to a single signal readout channel: Slow device, hence not suitable for fast detectors. Improvement are developed, e.g. parralel column readout. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 25 3.4 Charge Coupled Devices (CCD) The SLD Si pixel vertex detector The SLD (SLAC, USA) silicon vertex detector used large area CCDs. Pixel size 20 µm x 20 µm, achieved resolution 4 µm. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 26 3.5 Silicon Drift Detectors Principle Evolution of Silicon Sensor Technology in Particle Physics, F. Hartmann, Springer Volume 231, 2009 In silicon drift detectors p+ strips and the backplane p+ implantation are used to fully deplete the bulk. A drift field transports the generated electrons to the readout electrodes (n+). One coordinate is measured by signals on strips, the second by the drift time. XII ICFA School on Instrumentation, Bogotá, 2013 Used for example in the experiment ALICE (CERN) Manfred Krammer 27 3.6 Monolithic Active Pixels CMOS Scheme of a CMOS monolithic active pixel cell with an NMOS transistor. The N-well collects electrons from both ionization and photo-effect. Evolution of Silicon Sensor Technology in Particle Physics, F. Hartmann, Springer Volume 231, 2009 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 28 3.6 Monolithic Active Pixels Silicon on Insulator (SOI) A SOI detector consists of a thick full depleted high resitivity bulk and seperated by a layer of SiO2 a low resistivity n-type material. NMOS and PMOs transistors are implemented in the low resitivity material using standard IC methods. Evolution of Silicon Sensor Technology in Particle Physics, F. Hartmann, Springer Volume 231, 2009 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 29 3.7 DEPFET Detectors Principle The DEPFET detector is a detector with an internal amplification structure. The n-bulk is fully depleted with a potential minimum below the strips and the structure of a field effect transistor. The electrons created by a charged particle accumulate in the potential minimum. The field configuration is such that the electrons drift underneath the gate of the transistor modifying the source drain curreny. An active clear is necessary to remove the electrons. Evolution of Silicon Sensor Technology in Particle Physics, F. Hartmann, Springer Volume 231, 2009; J. Kemmer and G. Lutz, NIM A253 (1987) 365 XII ICFA School on Instrumentation, Bogotá, 2013 Used in Belle II and candidate for ILC Manfred Krammer 30 3.8 3D Detectors Principle 3D detecors are non planar detectors. Deep holes are etched into the silicon and filled with n+ and p+ material. Depletion is sideways. The distances between the electrodes are small, hence depletion voltage can be much smaller and charge carries travel much short distances. Picture from CNM-IMB (CSIC), Barcelona Very radiation tolerant detectors, first use in ATLAS IBL layer. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 31 3.8 3D Detectors Different approaches Single column: Low field region between columns XII ICFA School on Instrumentation, Bogotá, 2013 Double-sided double column: High field, but more complicated Manfred Krammer 32 3.9 Avalanche Photo Diode (APD) Principle APD are operated in reverse bias mode in the breakdown regime. A photon is able to trigger an avalanche breakdown. The current increase has to be limted by a quenching resistor. R. H. Haitz, J. App.Phys. Vol. 36, No. 10 (1965) 3123 Used for photon detection in calorimeters (e.g the electromagnetic calorimeter of CMS), in cherenkov counters, etc. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 33 3.9 Avalanche Photo Diode (APD) Silicon photo multiplier (SiPM) SiPM are matrices of APDs: Front contact Al h Rquenching ARC n+ p n+ p p+ silicon wafer Back contact -Vbias XII ICFA School on Instrumentation, Bogotá, 2013 SiPM become more and more popular as replacement for standard photo multiplier. Manfred Krammer 34 4 Performance Parameter XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 35 4.1 Signal to Noise Ratio Introduction The signal generated in a silicon detector depends essentially only on the thickness of the depletion zone and on the dE/dx of the particle. The noise in a silicon detector system depends on various parameters: geometry of the detector, the biasing scheme, the readout electronics, etc. Noise is typically given as “equivalent noise charge” ENC. This is the noise at the input of the amplifier in elementary charges. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 36 4.1 Signal to Noise Ratio Noise contributions The most important noise contributions are: 1. 2. 3. 4. Leakage current (ENCI) Detector capacity (ENCC) Det. parallel resistor (ENCRp) Det. series resistor (ENCRs) Alternate circuit diagram of a silicon detector. The overall noise is the quadratic sum of all contributions: XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 37 4.1 Noise Contributions Leakage current - 1 The detector leakage current comes from thermally generated electron holes pairs within the depletion region. These charges are seperated by the electric field and generate the leakage current. The fluctuations of this current are the source of noise. In a typical detector system (good detector quality, no irradiation damage) the leakage current noise is usually negligible. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 38 4.1 Noise Contributions Leakage current - 2 Assuming an amplifier with an integration time (“peaking time”) tp followed by a CR-RC filter the noise contribution by the leakage current l can be written as: e .… Euler number (2.718...) e … Electron charge Using the physical constants, the leakage current in units of nA and the integration time in µs the formula can be simplified to: To minimize this noise contribution the detector should be of high quality with small leakage current. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 39 4.1 Noise Contributions Detector capacity The detector capacity at the input of a charge sensitive amplifier is usually the dominant noise source in the detector system. This noise term can be written as: The parameter a and b are given by the design of the (pre)-amplifier. C is the detector capacitance at the input of the amplifier channel. Integration time tp is crucial, short integration time leads usually to larger a and b values. Integration time is depending on the accelerator time structure. Typical values are (amplifier with ~ 1 µs integration time): a ≈ 160 e und b ≈ 12 e/pF To reduce this noise component segmented detectors with short strip or pixel structures are preferred. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 40 4.1 Noise Contributions Parallel resistor The parallel resistor Rp in the alternate circuit diagram is the bias resistor. The noise term can be written as: e .… Euler number (2.718...) e … Electron charge Assuming a temperature of T=300K, tp in µs and Rp in M the formula can be simplified to: To achieve low noise the parallel (bias) resistor should be large! However the value is limited by the production process and the voltage drop across the resistor (high in irradiated detectors). XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 41 4.1 Noise Contributions Series resistor The series resistor Rs in the alternate circuit diagram is given by the resistance of the connection between strips and amplifier input (e.g. aluminum readout lines, hybrid connections, etc.). It can be written as: C .… Detector capacity on pF tp … Integration time in µs Rs … Series resistor in Note that, in this noise contribution tp is inverse, hence a long tp reduces the noise. The detector capacitance is again responsible for larger noise. To avoid excess noise the aluminum lines should have low resistance (e.g. thick aluminum layer) and all other connections as short as possible. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 42 4.1 Signal to Noise Ratio Summary To achieve a high signal to noise ratio in a silicon detector system the following conditions are important: Low detector capacity (i.e. small element size) Low leakage current Large bias resistor Short and low resistance connection to the amplifier Long integration time Obviously some of the conditions are contradictory. Detector and front end electronics have to be designed as one system. The optimal design depends on the application. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 43 4.1 Signal to Noise Ratio Examples DELPHI Microvertex (LEP): CMS Tracker (LHC): readout chip (MX6): a = 325 e, b = 23 e/pF, tp = 1.8 µs readout chip (APV25, deconvolution): a = 400 e, b = 60 e/pF, tp = 50 ns 2 detectors in series each 6 cm long strips, C = 9 pF ENCC = 532 e 2 detectors in series each 10 cm long strips, C = 18 pF ENCC = 1480 e typ. leakage current/strip: I ≈ 0.3 nA ENCI = 78 e max. leakage current/strip: I ≈ 100 nA ENCI = 103 e bias resistor Rp = 36 M ENCRp = 169 e bias resistor Rp = 1.5 M ENCRp = 60 e series resistor = 25 ENCRs = 13 e series resistor = 50 ENCRs = 345 e Total noise: ENC = 564 e (SNR 40:1) Total noise: ENC = 1524 e (SNR 15:1) Calculated for the signal of a minimum ionizing particle (mip) of 22500 e. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 44 4.2 Position Resolution Introduction The position resolution – the main parameter of a position detector – depends on various factors, some due to physics constraints and some due to the design of the system (external parameters). Physics processes: – Statistical fluctuations of the energy loss – Diffusion of charge carriers External parameter: – Binary readout (thresh hold counter) or read out of analogue signal value – Distance between strips (strip pitch) – Signal to noise ratio XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 45 4.2 Position Resolution Statistical fluctuation of the energy loss – 1 Silicon position detectors are thin (300–500 µm) and absorb only a small fraction of the total energy of through going particles. The energy loss dE/dx follows a Landau distribution, an asymmetric probability function with a long “tail” to large energy deposits. Example of a mip measured in a 300 µm thick silicon detector: Pions and Protons: W. Adam et al., CMS note 1998/092 (1998) o Most probable energy loss (Maximum of the distribution): 78 keV in 300 µm ≈ 72 e–h+ pairs per µm o Mean of the energy loss: 116 keV in 300 µm ≈ 108 e–h+ pairs per µm XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 46 4.2 Position Resolution Statistical fluctuation of the energy loss – 2 Long tail in energy loss distribution is due to -electrons. -electrons have a high energy (keV) and are produced by rare, hard collisions between incident particle and electrons from the detector material. Displacement probability (calcuThe probability to produce a lation) of the charge center of gravity due to -electrons: electrons is small. A. Peisert, Silicon Microstrip Detectors, DELPHI 92-143 MVX 2, CERN, 1992 -electrons have a long track length in the detector material and may produce e+h- pairs along the track. Dislocate the measured track Measurement errors in the order of µm unavoidable XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 47 4.2 Position Resolution Diffusion – 1 After the ionizing particle has passed the detector the e+h- pairs are close to the original track. While the cloud of e+ and h- drift to the electrodes, diffusion widens the charge carrier distribution. After the drift time t the width (rms) of the distribution is given as: with: D … width “root-mean-square” of the charge carrier distribution t k e … drift time … Boltzmann constant … electron charge D … diffusion coefficient T … temperature … charge carrier mobility Note: D µ and t 1/µ, hence D is equal for e– and h+. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 48 4.2 Position Resolution Diffusion – 2 h+ created close to the anode (i.e. the n+ backplane) and e- created close to he cathode (i.e. the p+ strips or pixels) have the longest drift path. As a consequence the diffusion acts much longer on them compared to e- h+ with short track paths. The signal measured comes from many overlapping Gaussian distributions. Drift and diffusion acts on charge carriers: XII ICFA School on Instrumentation, Bogotá, 2013 Charge density distribution for 5 equidistant time intervalls: Manfred Krammer 49 4.2 Position Resolution Diffusion – 3 Diffusion widens the charge cloud. However, this has an positive effect on the position resolution! charge is distributed over more than one strip, with interpolation (calculation of the charge center of gravity) a better position measurement is achievable. This is only possible if analogue read out of the signal is implemented. Interpolation is more precise the larger the signal to noise ratio is. Strip pitch and signal to noise ratio determine the position resolution. Larger charge sharing can also be achieved by tilting the detector. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 50 4.2 Position Resolution Threshold readout versus analogue readout Threshold (binary) readout (one strip signal): position: x = strip position resolution: p … distance between strips (readout pitch) x … position of particle track charge center of gravity (signal on two strips): position: h h x +h x x = x1 + 1 ( x2 - x1 ) = 1 1 2 2 h1 + h2 h1 + h2 resolution: p SNR sx µ » x1,x2 … position of 1st and 2nd strip h1,h2 … signal on 1st and 2nd strip SNR … signal to noise ratio A position resolution of a few µm is achievable with analogue readout ! XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 51 4.2 Position Resolution Intermediate strips – 1 The strip pitch determines to a large extend the position resolution. With small strip pitch a better position resolution is achievable. small strip pitch requires large number of electronic channels cost increase power dissipation increase A possible solution is the implementation of intermediate strips. These are strips not connected to the readout electronics located between readout strips. The signal from these intermediate strips is transferred by capacitive coupling to the readout strips. more hits with signals on more than one strip Improved resolution with smaller number of readout channels. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 52 4.2 Position Resolution Intermediate strips – 2 Scheme of a detector with two intermediate strips. Only every 3rd strip is connected to an electronics channel. The charge from the intermediate strips is capacitive coupled to the neighbor strips. XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 53 4.2 Position Resolution Example – influence of readout pitch and SNR Example of a detector with strip pith of 25 µm and analogue readout. The position resolution is plotted as a function of the SNR. Bottom curve: every strip is connected to the readout electronics Top curve: every 2nd strip is connected, one intermediate strip To benefit from intermediate strips large SNR is required! A. Peisert, Silicon Microstrip Detectors, DELPHI 92-143 MVX 2, CERN, 1992 XII ICFA School on Instrumentation, Bogotá, 2013 Manfred Krammer 54