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Flat panel x-ray image sensors Bob Street Palo Alto Research Center How do they work? – TFTs, sensors, active matrix, direct and indirect detection How are they made? – Materials, devices, patterning How can they be improved? – New directions – polysilicon, single photon detection, printed arrays Flat panel x-ray imaging X-rays Radiography, fluoroscopy, mammography, radiation therapy, CT, quality control, security screening. Up to 40x40 cm active area, 10,000,000 pixels, 16 bit dynamic range, 2000 electron noise Attributes of an image sensor Sensitivity, dynamic range – X-ray conversion, electronic noise, etc Spatial resolution – Pixel size, conversion process X-rays Conversion; x-ray to charge Charge storage Overall size – Pixel count, manufacturing process Detection process Read out speed – Matrix addressing, capacitance, external electronics. Charge readout substrate Two modes of x-ray conversion Indirect detection x-ray e photo-excitation phosphor ionization e.g. CsI recombination visible light a-Si sensor array Good sensitivity (contact imaging). Reduced resolution due to light scattering. Simpler structure and materials. Direct detection x-ray Voltage e photo-excitation photoionization conductor a-Si array Potentially higher sensitivity. Better spatial resolution. More difficult materials. Active matrix addressing gate shift register Data output The pixel capacitance stores the signal charge. The TFT (off) holds the charge on the pixel. The gate lines are addressed one at a time. The TFT (on) passes the signal from the column of pixel to the data line Readout resets the pixel capacitor N2 pixels are read out with 2N contacts bias TFT is on for 10-30 ms and off for 15-1000 ms A-Si:H sensor array (indirect) Side view illumination Bias contact ITO Passivation p i photodiode n TFT a-Si:H data ~1 mm silicon nitride gate TFT gate line data line Bias line Top view photodiode Materials and devices A-Si and poly-Si Thin film transistors Device processing A-Si p-i-n photodiodes Charge collection. VSD, ISD A-Si:H TFTs Mobility 0.5-1 source cm2/Vsec VG mA on-current. Very high on/off ratio (1010) Low threshold voltage Moderate sub-threshold slope – On-off voltage swing is 10-15V Small bias-stress effect TFT current ISD/VD = (W/L)CG mF (VG-VT) Conduction = geometry . mobility . voltage drain channel insulator gate 1.E-06 Source-drain current (A) Passiv. above threshold 1.E-08 W/L = 2 Vds = 5 V 1.E-10 1.E-12 leakage subthreshold 1.E-14 1.E-16 -10 0 10 Gate Voltage (V) 20 TFT Fabrication (an example) Amorphous silicon thin film deposition is scalable to large area Low temperature process for glass substrate Channel, dielectric and passivation are deposited together. 1. Pattern gate electrode Gate Glass Substrate 2. Thin film deposition Nitride passivation – a-Si:H ~50 nm – dielectric ~300nm a-Si:H Glass Substrate SiN gate dielectric TFT Fabrication 3. Self-aligned passivation etch The etch exposes the channel for the contacts Self-aligned for low capacitance. N+ layer for leakage barrier. Metal for low resistance Glass Substrate UV 4. Patterning source/drain contacts Metal N+ a-Si:H Glass Substrate Polysilicon TFT Channel Si (50 nm) Oxide (700 nm) Glass Substrate Laser Laser recrystallization n+, p+ Dielectric, gate, dopant implant Gate passivation, contacts Source Channel Drain Polysilicon TFT 0.01 cm2/Vsec Mobility ~100 CMOS capability Higher leakage current 2 µ= 58 cm /Vs 1E-4 2 µ= 190 cm /Vs Used for driver integration and pixel amplifiers ID ( A ) NMOS, W=50 µm L=30 µm L=15x2 µm VD= 5V 1E-6 – Dual gate PMOS, W=50 µm L=30 µm L=15x2 µm VD= -5V 1E-8 1E-10 1E-12 1E-14 -20 -15 -10 -5 0 VG( V ) 5 10 15 20 a-Si p-i-n photodiodes ITO p+ a-Si:H 10 nm i Reverse bias Large charge collection – Independent of bias – Peak sensitivity at 500-600 nm Low leakage current – leakage mechanisms; bulk, contact, edge a-Si:H 1-2 mm n+ a-Si:H 20 nm metal p-i-n photodiode Charge collection V Charge collection depends on mobility-lifetime (m) product e h X trap – Material property related to trap density FQ m V d 2 1 exp d 2 m V d Values of V when mV/d2= 1 m 1mm film 300 mm film 10-6 cm2/V 0.01V 1000V 10-4 cm2/V 10-4V 10 V 1 FQ 0 a-Si direct det. V Photodiode leakage current Sources of leakage current:– Bulk defects – Contact injection – Edge leakage Sensitive to processing Reduced to 0.1 pA/mm2 Sensor reverse bias current - dependence on passivation Indirect detection arrays Pixel circuit Device requirements – TFT – capacitance Signal to noise High fill factor design Indirect detection Bias voltage Gate line photodiode Storage capacitor a-Si:H TFT Bias lines data lines TFT sensor Data line Gate lines Pixel circuit (simple) A-Si:H p-i-n photodiode provides pixel storage capacitance Fill-factor = area of pixel covered by sensor. 1 fill factor 0 100 200 pixel size (micron) TFT requirements Assumptions: Pixel capacitance 1 pF; 1000 gate lines; 30 fps 2 msec – RON 2 Mohm TFT with W/L 1.5 Current requirement is easily met with mobility 1 cm2/Vsec C Ron 1.E-06 Source-drain current (A) 1. TFT ON Charge must transfer quickly to data line Pixel RONC time-constant 1.E-08 W/L = 2 Vds = 5 V 1.E-10 1.E-12 1.E-14 1.E-16 -10 0 10 Gate Voltage (V) 20 TFT requirements (cont) ROFFC > 100 sec (<1% discharge) ROFF > 1014 W Very low TFT off-current is required – On/off ratio ~108 1.E-06 Source-drain current (A) 2. TFT OFF Charge must remain on the pixel during integration 1.E-08 W/L = 2 Vds = 5 V 1.E-10 1.E-12 1.E-14 1.E-16 -10 0 10 Gate Voltage (V) 20 Capacitance effects TFT parasitic capacitance – Puts feed-through charge on the pixel Gate and data line capacitance – Reduces addressing speed – contributes to noise Low capacitance improves performance – Self-aligned TFTs – Thick isolation layers Many sources of capacitance sensor CS gate CF T CDG CF T VA TFT bias CDB data Electronic noise Sources of noise Data line capacitance to external preamplifiers – Noise = A+bCD – Depends on readout ASIC CD Pixel kTC noise C R Data line resistance Power supply fluctuations – Array capacitance ext. amp. High fill factor sensor arrays Continuous sensor layer – 3-d structure – Improves fill factor – Avoids sensor side walls Sensor Passivation Metal Gate Contact Lateral leakage can be controlled Top view Visible light image A short break Direct detection Direct detection arrays Material requirements Se and HgI2 Sensitivity and loss mechanisms – Charge collection Direct detection array Thick photoconductor replaces phosphor+photodiode Active matrix array with added capacitor gate line electrode TFT bias V electrode TFT capacitor data line Crosssection Direct detection material requirements X-ray absorption – 200-500+ micron thick – High atomic number material Photoconductor Charge collection – high m products Top Metal Low leakage current Low image lag Large area deposition – Amorphous or polycrystalline – Evaporated, sputtered, screen-print… – Continuous film Bottom Metal Passivation S/D Metal TFT Capacitor Ground Gate Line Data Line Material choices:a-Se, PbI2, HgI2, CdZnTe Selenium direct detection Amorphous selenium deposited by vacuum evaporation – Doped with As and Cl to give good electron and hole charge collection Ionization/collection is strongly field dependent – High operating voltages Charge trapping at pixel boundaries – Illumination between frames HgI2 films; a new alternative Vacuum deposition or particlein-binder 10000 – Polycrystalline layer; grain size 20->50 mm – Blocking layer to protect against chemical reactions. 1000 High x-ray absorption Good LSF Low leakage current Several issues yet to resolve 50kV line-spread function 60kV 100 10 1 -0.25 -0.15 -0.05 0.05 0.15 position (mm) Line-spread function of HgI2 0.25 HgI2 x-ray response 6000 High electron charge collection at low bias. Higher sensitivity than other materials Linear response Signal (ADC units) 7000 70kV p 5000 4000 60kVp 3000 2000 60kVp - 5960 ke/mR 70 kVp - 6255 ke/mR 1000 0 0 0.5 Charge collection 1 1 Exposure (mR) 0.8 Charge collection versus bias 250 mm film 0.6 FQ 0.4 0.2 m V d 2 1 exp d mu-tau = 6.10-5 cm2/s 0 0 50 Voltage (V) 100 Good fit to charge collection formula 2 m V Charge collection corrections Three loss components:- 4000 – Electron trapping (m, V) – Small hole contribution (m, V) signal (ADC units) 3500 – Absorption depth (kVp) negative bias 3000 2500 2000 positive bias 1500 1000 80 kVp 500 5000 25kVp, 6mR 150mm screen print 4500 signal (ADC units) 4000 25 kVp 0 0 negative V 30 60 90 120 Bias voltage (V) 3500 Sum of positive and negative bias total electron collection. positive data represents loss 3000 m(e)= 6x10 cm /V m(h)= 4x10-7 cm2/V -6 2500 2000 2 1500 1000 positive V 500 Hole m measured by correcting for electrons 0 0 20 40 60 bias voltage (V) 80 effect. ionization energy (eV) Sensitivity evaluation Effective ionization energy, WEFF – Sensitivity ~ 1/WEFF – Max sensitivity when WEFF = W (=~5 eV) Intrinsic sensitivity approaches theoretical maximum 30 WEFF before and after correction for x-ray absorption HgI2 Bias 30V 25 20 15 10 corrected for x-ray absorption 5 0 40 60 80 energy (kVp) – Losses understood HgI2 25 kVp HgI2 80 kVp Measured WEFF (eV) 7.8 19.6 Absorption loss (bABS) 1.0 0.49 Charge collection loss (bQ) 0.77 0.65 Image lag loss (bLAG) 0.82 0.82 b = bABS bQ bLAG 0.63 0.26 b . WEFF (eV) 4.9 5.1 100 Next generation image sensors? Single photon detection – HgI2 – GEM amplifiers Power Polysilicon arrays – Pixel amplifiers – Integrated drivers New backplane technology – Printed arrays – Organic semiconductors Active area ADCs Readout amplifiers Gate shift register Logic Digital data Single photon detection – HgI2 First detection of single photons by a flat panel solid state detector. Energy resolution needs to be improved – Low hole collection – Noise from dark current 512x512 array 100 mm pixel HgI2 detector 1200 3500 photo-peak 1000 dark 2500 histogram # histogram value 3000 2000 1500 800 600 400 1000 200 500 0 0 80 100 120 ADC channel 140 160 0 10 20 signal (ke) 30 40 GEM detectors with a-Si arrays Single photon detection using GEM (gas electron multiplier) with a-Si:H backplane array – Gain of ~10,000 – Observation of x-ray polarization Example of novel gain structure for single photon detection A-Si array to collect charge Images of 4-20 keV x-ray photons, measured with a-Si array GEM detector Electronic integration with polysilicon Integration of drive electronics – Shift register to drive TFT gates – Output multiplexer/amplifier to simplify readout. One stage of shift register 1mm First polysilicon image sensor array 384x256 array; 90 mm pixel To gate line Polysilicon pixel amplifier Demonstrated in 256x384 pixel array – 3 TFT follower circuit – 800 e noise Needs 3-d sensor structure Reduces sensitivity to external noise sources Poly-Si sensor arrays with pixel amplifier data line Vcc reset bias gate line Sensor on top 3 TFT circuit a-Si TFT arrays fabricated by jet-printing Digital lithography; maskless; software registration Wax ink; feature size control Multi-ejector print-head; high print speed Direct Write Etch Mask Deposit film Print wax mask Jet-printer Print head Heated sample holder Registration camera x-y stage Etch film strip wax Jet-printed a-Si TFT array Gate layer 100 mm Line width 30-40 mm Island layer d s G TFT Registration ~ 5 mm Source/drain layer Printed a-Si TFT array 64×64 matrix addressed array Carrier mobility ~ 0.9 cm2 V-1s-1 Extension to poly-silicon. Could be used for large pixel applications 300 mm Data Line Drain Current (Amps) W/L = 40/80 1E-7 s d Gate Line Vd = 2.5 V Vd = 7.5 V Vd = 15 V 1E-10 1E-13 1E-16 -5 0 5 10 Gate Voltage (Volts) 15 20 Polymer transistors Xerox poly(thiophene) -2.0E-06 Mobility is approaching a-Si Simple deposition – Spin coat, print etc. Unknown stability, lifetime, radiation hardness -1.5E-06 Idrain W/L = 500/120 mm 'spun' on OTS-8 annealed at 150 C m =0.07 cm2V-1s-1 (linear & sat.) ON/OFF ~ 106 -1.0E-06 -5.0E-07 0.0E+00 0 -10 -20 Vsd -30 -40 Printed a-Si Summary - progress in large-area electronics Digital Lithography Poly-Si TFT array Poly-Silicon Medical imaging Printed Organic Arrays AMLCD Organic TFTs Amorphous Si 1980 1990 2000