1. 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

2. 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

3. Attributes of an image sensor
Sensitivity, dynamic range
X-ray conversion, electronic noise, etc
Spatial resolution
Pixel size, conversion process
Overall size
Pixel count, manufacturing process
Read out speed
Matrix addressing, capacitance, external electronics.
Detection process
X-rays
Conversion;
x-ray to charge
Charge storage
Charge readout
substrate

4. Two modes of x-ray conversion
Indirect detection
Direct detection
x-ray
x-ray
Voltage
photo-excitation e e
photo-excitation
phosphor
e.g. CsI
ionization
photo-
conductor
ionization
recombination
visible light
a-Si array
a-Si sensor array
Good sensitivity (contact imaging).
Reduced resolution due to light scattering.
Simpler structure and materials.
Potentially higher sensitivity.
Better spatial resolution.
More difficult materials.

5. Active matrix addressing
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
gate shift register
Data output
bias
TFT is on for ms and off for ms

6. A-Si:H sensor array (indirect)
Side view
illumination
Bias contact
ITO
Passivation p
TFT i
photodiode
~1 mm n
a-Si:H silicon nitride
data
gate
gate line
photodiode
TFT
data line
Bias line
Top view

7. Materials and devices A-Si and poly-Si Thin film transistors
Device processing
A-Si p-i-n photodiodes
Charge collection.

8. A-Si:H TFTs TFT current ISD/VD = (W/L)CG mF (VG-VT)
gate
insulator
channel
Passiv.
source
drain VG
VSD, ISD
Mobility cm2/Vsec
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
above threshold
sub-threshold
TFT current
ISD/VD = (W/L)CG mF (VG-VT)
leakage
Conduction = geometry . mobility . voltage

9. 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.
a-Si:H ~50 nm
dielectric ~300nm
1. Pattern gate electrode
Gate
Glass Substrate
2. Thin film deposition
Nitride passivation
a-Si:H
Glass Substrate
SiN gate dielectric

10. TFT Fabrication The etch exposes the channel for the contacts
3. Self-aligned passivation etch UV
Glass Substrate
The etch exposes the channel for the contacts
Self-aligned for low capacitance.
N+ layer for leakage barrier.
Metal for low resistance
4. Patterning source/drain contacts
N+ a-Si:H
Metal
Glass Substrate

11. Polysilicon TFT Source Channel Drain Gate Laser
Channel Si (50 nm)
Oxide (700 nm)
Glass Substrate
Laser
Laser recrystallization
n+, p+
Dielectric, gate, dopant implant
Gate
passivation, contacts
Source Channel Drain

12. Polysilicon TFT Mobility ~100 cm2/Vsec CMOS capability
Higher leakage current
Dual gate
Used for driver integration and pixel amplifiers

13. a-Si p-i-n photodiodes
ITO
p+ a-Si:H 10 nm
i a-Si:H 1-2 mm
n+ a-Si:H 20 nm
metal
Reverse bias
Large charge collection
Independent of bias
Peak sensitivity at nm
Low leakage current
leakage mechanisms; bulk, contact, edge
p-i-n photodiode

14. Charge collection V
Charge collection depends on mobility-lifetime () product
Material property related to trap density
Values of V when V/d2= 1 eh X
trap d 
1m film
300 m film
10-6 cm2/V
0.01V
1000V
10-4 cm2/V
10-4V
10 V 1 FQ
a-Si direct det. V

15. 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

16. Indirect detection arrays
Pixel circuit
Device requirements
TFT
capacitance
Signal to noise
High fill factor design

17. Indirect detection Pixel circuit (simple)
Bias voltage
photodiode
Bias lines
Storage capacitor
Gate line
data lines
a-Si:H
TFT
TFT
sensor
Data line
Gate lines
Pixel circuit (simple)
pixel size (micron)
fill factor 1
100
200
A-Si:H p-i-n photodiode provides pixel storage capacitance
Fill-factor = area of pixel covered by sensor.

18. TFT requirements 1. TFT ON Charge must transfer quickly to data line
Assumptions: Pixel capacitance 1 pF; gate lines; 30 fps
1. TFT ON
Charge must transfer quickly to data line
Pixel RONC time-constant  2 sec
RON  2 Mohm
TFT with W/L  1.5
Current requirement is easily met with mobility 1 cm2/Vsec C
Ron

19. TFT requirements (cont)
2. TFT OFF
Charge must remain on the pixel during integration
ROFFC > 100 sec (<1% discharge)
ROFF > 1014 W
Very low TFT off-current is required
On/off ratio ~108

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

21. Electronic noise Sources of noise
Data line capacitance to external preamplifiers
Noise = A+bCD
Depends on readout ASIC
Pixel kTC noise
Data line resistance
Power supply fluctuations
Array capacitance
ext. amp. CD C R

22. High fill factor sensor arrays
Gate Contact
Sensor
Metal
Passivation
Continuous sensor layer
3-d structure
Improves fill factor
Avoids sensor side walls
Lateral leakage can be controlled
Top view
Visible light image

23. A short break

24. Direct detection Direct detection arrays Material requirements
Se and HgI2
Sensitivity and loss mechanisms
Charge collection

25. Direct detection array
Thick photoconductor replaces phosphor+photodiode
Active matrix array with added capacitor
electrode
TFT
bias V
gate line
electrode
Cross-section
TFT
capacitor
data line

26. Direct detection material requirements
X-ray absorption
micron thick
High atomic number material
Charge collection
high mt products
Low leakage current
Low image lag
Large area deposition
Amorphous or polycrystalline
Evaporated, sputtered, screen-print…
Continuous film
Top Metal
Photoconductor
Passivation
Bottom Metal
Capacitor
Ground
TFT
S/D Metal
Gate Line
Data Line
Material choices:-
a-Se, PbI2, HgI2, CdZnTe

27. 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

28. HgI2 films; a new alternative
Vacuum deposition or particle-in-binder
Polycrystalline layer; grain size 20->50 m
Blocking layer to protect against chemical reactions.
High x-ray absorption
Good LSF
Low leakage current
Several issues yet to resolve
Line-spread function of HgI2

29. HgI2 x-ray response High electron charge collection at low bias.
Higher sensitivity than other materials
Linear response
Charge collection versus bias
250 mm film
Good fit to charge collection formula

30. Charge collection corrections
Three loss components:-
Electron trapping (mt, V)
Absorption depth (kVp)
Small hole contribution (mt, V)
80 kVp
25 kVp
Sum of positive and negative bias  total electron collection.
 positive data represents loss
Hole mt measured by correcting for electrons

31. Sensitivity evaluation
WEFF before and after correction for x-ray absorption
Effective ionization energy, WEFF
Sensitivity ~ 1/WEFF
Max sensitivity when WEFF = W (=~5 eV)
Intrinsic sensitivity approaches theoretical maximum
Losses understood
HgI2
25 kVp
80 kVp
Measured WEFF (eV)
7.8
19.6
Absorption loss (ABS)
1.0
0.49
Charge collection loss (Q)
0.77
0.65
Image lag loss (LAG)
0.82
 = ABS Q LAG
0.63
0.26
 . WEFF (eV)
4.9
5.1

32. Next generation image sensors?
Single photon detection
HgI2
GEM amplifiers
Polysilicon arrays
Pixel amplifiers
Integrated drivers
New backplane technology
Printed arrays
Organic semiconductors
Active area
Gate shift register
Readout amplifiers
Logic
ADCs
Digital data
Power

33. 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
photo-peak
dark

34. 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

35. Electronic integration with polysilicon
To gate line
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

36. Polysilicon pixel amplifier
gate line
data line
Vcc
reset bias
Demonstrated in 256x384 pixel array
3 TFT follower circuit
800 e noise
Needs 3-d sensor structure
Reduces sensitivity to external noise sources
3 TFT
circuit
Sensor
on top
Poly-Si sensor arrays with pixel amplifier

37. 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
Etch film strip wax
Print wax mask
Heated sample holder
Registration camera
Print head
x-y stage
Jet-printer

38. Jet-printed a-Si TFT array
100 mm
Gate layer
Line width mm
Registration ~ 5 mm
Island layer G d s
Source/drain layer
TFT

39. Printed a-Si TFT array 300 mm s d 64×64 matrix addressed array
Carrier mobility ~ 0.9 cm2 V-1s-1
Extension to poly-silicon.
Could be used for large pixel applications s d
Gate Line
Data Line
300 mm

40. Polymer transistors Mobility is approaching a-Si Simple deposition
Xerox poly(thiophene)
W/L = 500/120 mm
'spun' on OTS-8 annealed at 150 C
m =0.07 cm2V-1s-1 (linear & sat.)
ON/OFF ~ 106
Mobility is approaching a-Si
Simple deposition
Spin coat, print etc.
Unknown stability, lifetime, radiation hardness

41. Printed Organic Arrays
Summary - progress in large-area electronics
Printed a-Si
Digital Lithography
Poly-Si TFT array
Poly-Silicon
Medical imaging
Printed Organic Arrays
AMLCD
Organic TFTs
Amorphous Si