Download TFT - Bahnhof

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 mV/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