Download Monolithic Active Pixel Detectors

Fast position resolution silicon detectors
Gregor Kramberger , DESY
OUTLINE
•General properties of position sensitive detectors
•Column Parallel CCD (CPCDD)
•Monolithic Active Pixel Detectors (CMOS imager)
•Depleted Field Effect Transistor Detectors (DEPFET)
•Hybrid Active Pixel Detectors (HAPS)
Particle tracking
Imaging
3.6 eV/pair
p+
up
to 1
mm
p+
n-Si
n+
n+
U
each particle leaves a track
up
to 1
mm
Only some photons interact
dP
 x
 exp   
dx
 
collection time ~ns order
p+
n-Si
n-Si
U
  25m (5.9 keV)
up
to 1
mm
n+
U
U
Signal
Signal
Position can be determined – center of gravity
To obtain good position resolution – high S/N ratio
N
Segmentation options:
•low particle (photons) rate – strip detectors
Read-out channels/detector module: N+M
When read-out in:
series: N+M time units
parallel: bigger of [N,M]
M
Only projections can be obtained.
Bottom side perpendicular segmentation
or two detectors (each 1D) can be used!
•high particle (photons) rate – pixel detectors
N
Read-out channels/detector module: NxM
When read-out in:
series: NxM time units
parallel: bigger of [N,M]
Usually many detector modules are put
together to cover the image area!
(silicon is produced in silicon wafer – nowadays already 8” inch)
M
15 cm diameter
>1m
Slicing the rod in typically
300 m slices
(wafers)
Detectors diced out of
silicon wafers
Similar processing as for microelectronics
lithographic steps, etching, implantations, PolySi filling
What do we want from one module?
A large high resolution “picture” as fast and easy as possible!
few 10 cm2
few m
picture taking rate
(frame rate) up to
few 10 kHz
With a lot of
effort can be
achieved!
without severe restriction on operating conditions,
electronics (voltage supply etc) needed
One of the following requirements can be even more
demanding on expense of the others!
This talk will try to illustrate
different detector options for
particle tracking at LC and to
some extent their ability to be
exploited for X-ray detection!
Many common points: with
modification same detector
concepts can be used for both
The further discussion will refer only to pixel detectors!
One can not explain everything -> only the concepts will be presented !
things specific to tracking at LC (material budget – thinning of the detectors, power consumption,
mechanics, cooling … will be left out)
Division of fast position sensitive detectors with respect to operational principle!
• Charge Coupled Devices (CP CCD)
(RAL, Oxford, Liverpool)
•Active Pixel Sensor (APS)
- Depleted Field Effect Transistor Detector
(MPI München, Mainz, Bonn)
- Monolithic Active Pixel Sensors (MAPS, CMOS imager)
(IRES Strasbourg, DESY, NIKHEF)
- Hybrid Active Pixel Sensors (HAPS)
(Warshaw, Krakow, Insubria)
(widely used technology for pixel detectors in HEP)
Different
collaborations
working on LC
vertex detector
(plots taken
from their
presentations)
CPCCD - Principles of operation
oxide
p1
p2
p3
CCD is an array of capacitors
metal
gates
buried channel CDD
•potential minimum moved from the surface by n+
•collected charge is a combination of drift and diffusion
(drift much faster – high resistive epi-Si)
•p/p+ edge works as a reflection layer
•MOS gate is superimposed on top of the n+ layer
•the depleted region is controlled by the voltage applied
to the electrodes (p1,p2,p3)
U
dep.
p1
p2
(epi)
V
(bulk)
p3
p+ implants
Fully depleted
p
p+
n-
p+
bulk
PNP CDD
•Instead of MOS a p-n-p structure is formed
•Larger volume can be depleted and by that higher
photon detection efficiency at larger energies
( used for XMM – Newton )
CPCCD - Readout
Asymmetrical
n+ doping
both direction of charge transfer possible
V
Only one direction of charge transfer possible
p1
sine clocks
p1
p2
p2
p3
Typical frequency 5 MHz
Classical CCD: slow –
not appropriate for high
frame read-out rates
reset
Source follower
One can use higher frequency in
horizontal shift register – some
gain in speed, but not enough!
(1st stage of amplification)
2nd stage amplification follows
Different solution is needed!
“Classic CCD”
Readout time=NxM/nout
different frequencies can be used in
horizontal shift register
TESLA VXD – requires 50 s read-out/frame in layer 1 – huge challenge !
read-out of column with ~2500 pixels (10x1.3 cm2) in 50 s (20 kHz frame rate)
1st layer ladder read-out
650 20 m pixels
CCD VXD
read-out
1.3 cm
S=13
cm2
2500 20 m pixels
2500 20 m pixels
10 cm
Fast readout speed only with Column parallel readout new
design – first in the world !
• Serial register omitted
• 50 Mpixels/sec from each column
“Column Parallel CCD”
Readout time=N/nout
• Image section clocked at high frequency
• Each column has its own ADC/amplifier
(compare to classic CCD)
Fast CPCCD – considerations
•
•
•
•
imagine feeding large capacitance (2-3 nF/cm2) at 50MHz:
•
Low resistivity gates are required - Polysilicon gates replaced by metallized gates (30%
variation in clock amplitudes over CCD - simulations)
•
Low voltage clocks up to max. 3 V amplitude - to reduce the power heating
high resistive epi-Si to have large area depleted and therefore fast collection
well capacity ~ 20000 e
n+ implant design to enhance charge transfer between cells
+some other things ……
Two phase, 50 MHz design
pixel size 20 μm  20 μm;
Metallized gates + field enhance
implant
Metallized gates
SF
DC
PolySi gates + field enhance
implant
PolySi gates
wire bonds
World’s 1st CPCCD
prototype fully
designed and
operational at 50
MHz
Source follower
( needs reset )
or
Direct Coupling ?
Wire/bump bond
pads
Charge
Amplifiers DC
Voltage
Amplifiers SF
Read-out chip (CPR-1 chip, 0.25 mm CMOS, 50 MHz)
250 5-bit flash ADCs
 Both charge and Voltage amplifier for DC or SF coupling
 5 bit flash ADC
 buffer FIFO
Next iterations will have also:
Gain eq. between columns, CDS, clustering ,data sparsification
Wire/bump bond
pads
FIFO
Problem of CCDs
Charge collection efficiency (generated charge clocked through the detector)
Q  Q0 (1  CTI )
n
CTI denotes the loss of charge when
shifted from one cell to another
TRAPS  - imperfections in Si crystal (capture charge during transfer)
radiation induced – radiation damage
Charge losses must be very small: CTI~0.0001 with n=2000 Q/Q0=82%
How to reduce CTI:
•2-phase CCD (smaller V)
•notch CCD –additional implant
(smaller V Q sees less traps)
•fast CCD
(Q has no time to get trapped) – implementation field enhanced implants
•pre-injection of dark current to fill the traps
•proper operational temperature
CPCCD - Prototype performance
Previous prototype – off-shell 3 phase CCD driven at 50 MHz
at –70 with pp 3V amplitudes
55Fe
spectrum
  2.8 ADC
Pros CP-CCD
•Proven technology – a lot of experience
•with low resistive epi-Si or PNP-CCD large effective thickness can be reached – good for
imaging
•large homogeneity of charge collection
•small pixel sizes of order (20x20 m2) – good spatial resolution < 5 m
Cons CP-CCD
•High costs and limited vendor choice ( only MTech is working on them )
•Radiation hardness is questionable (charge transfer over entire detector – CTI degradation)
•detectors may need to be operated at lower temperatures
MAPS (CMOS imager)- principles of operation
-no HV
-operation voltage set
by CMOS process
fill factor = 100%
through the center of N well
through the center of P well
• double-well CMOS process with epitaxial layer
• the charge generated by the impinging particle is reflected by the potential barriers due to
doping differences and collected by thermal diffusion by the n-well/p-epi diode
•large charge spreading (signal shared over many pixels)
•“slow” charge collection (t~100 ns – depends on epi-Si)
• integration of the circuitry electronics on the same sensor substrate (1st stage of amplification)
•useable for detection off photons with few keV (limit set by epi-layer thickness)
Relaxation of excess charge after particle passage
Particle track
0 nsec
1 nsec
10 nsec
20 nsec
0 nsec
1 nsec
10 nsec
20 nsec
If r of the substrate is high significant contribution of
diffused charge from substrate to the total charge
MAPS - Readout
Reset
(common row)
M2 gate
potential
Collection
(int. time=frame rate)
Output
Reset…
Current prototype’s
clock speed 40 MHz
At present prototypes only reset and
clock signals are needed
Last amplification stage common to all
Full analog information
(all pixel) is read-out in
series - slow
Simple readout scheme for the first
prototypes (5 up to now)
•a reset cycle (all pixels) – common row reset
•cell output is amplified - physical signal: two frames are read-out and subtracted – CDS
CDS : get rid of FPN, reset noise, 1/f noise
Signal/Noise ratio
for given event
“Large” pixel size and deep submicron technology -> integration of high chip functionality
Chip layout
MIMOSA 6 (being manufactured)
• pixel pitch 28x28 m2
• 1 array 30x128 pixels – 29 transistors/pixel – instead of 3
New features:
• columns read-out in parallel - max. clock freq.: 30 MHz (CP)
• Also 2nd stage amplification and CDS done on-pixel!
• ADC conversion done at the edge of columns
• data sparsification integrated at the edge off the chip – zero
suppression
AC coupling
capacitor
CP read out
Single pixel
layout
6 clock cycles for row
Charge
storage
capacitors
1st layer ladder read-out
optimistic
Column read-out
pixels
~5-8 clock cycles
will be needed for
processing the
signal for each pixels
in each column
Analog and digital electronics
10 cm
R/O parallel to the short side
of the detector: for TESLA ~
200 pixel (0.5 cm)/50s
Analog and digital electronics
Column read-out
~1 cm
less optimistic
pixels
Analog and digital electronics
MAPS – prototype performance
5 prototypes build so far in different technologies (deep sub ), different pixel sizes,
clock frequencies and epilayer thicknesses
MIMOSA 5 – large size detector - standard 0.6 m CMOS of AMS with 14 m thick
EPI layer (1014 cm-3), pixel 17x17 m2, well capacity > 10000 e
chip size
1.73x1.73 cm2
Wafer view
~10% of the total surface
max. CMOS die size 2x2 cm2
Pixel read-out direction
•First stitched ladders of few neighboring chips are produced (100 m between chips – can
be reduced to 1 m – almost no dead area)
• simple serial frame read-out – 150 Hz frame rate (full analog information read-out)
• problem with fabrication yield!
55Fe
calibration
MIMOSA I CMOS 0.6 m
IMOSA II CMOS 0.35 m
1 diode – 14.6 V/eENC = 14 e- @1.6 ms f. rate
1 diode rad. tol.– 22.9 V/eENC = 12 e- @0.8 ms f. rate
Large scale prototype test in pion beam
To large extent leakage current contribution
(shorter frame rate should reduce noise to around 10e)
T=0oC
Pros MAPS
•low cost (production of 6 6” wafers for example 44000 Euros, 9 USD/cm2 is expected)
•standard CMOS process – profiting from huge progress in microelectronic industry: convenient way of
design - standard software tools, design kits and libraries, high yield, low power consumption
•Radiation hardness – up to few 100 kRad less than 10% degradation in collected charge
•low noise due to small gate capacitance (few fF) – theoretically few e - few 10 e
•signal processing (1st and 2nd stage amplification) in each individual pixel is possible -> good S/N at
high speed
•homogeneity of charge collection >97%
•pixel sizes of order (25x25 m2) – could be limited by integration of large number of transistors
Cons MAPS
•limited epi-layer thickness and by that usability for detection of photons (deep sub  maybe no epi)
•requirement for 8 metal layers and also analog design rules for CMOS – not very easily found
•potential danger of very deep sub-micron technology (trench isolation – charge trapping)
DEPFET - Principles of operation
•p-channel JFET or MOSFET integrated on high-ohmic, sidewards-depleted, n-substrate
•a local potential minimum is formed by S/D potentials aided by a deep n implantation
(punch-through bias of the pixels)
• electrons are collected in an internal gate close to the surface (collection time few ns)
• the transistor channel current is modulated by charge collected in the internal gate
• the device can be switched on/off by an external (top) gate
pulsed clear: pixel dead time < 1% of
measuring time
Internal gate fills up with:
•signal charges
•thermally generated charges (leakage
current)
DEPFET - Readout
gate
DEPFET- matrix
reset
off
off
on
reset
off
off
Current of pixel i,j
sample Iped+Isig
sample Iped
Reset row i
nxm
pixel
off
off
VGATE, ON
VGATE, OFF
IDRAIN
drain
Gate row i
VCLEAR, ON
VCLEAR, OFF
VCLEAR-Control
0 suppression
output
random access to pixels !
DEPFET Column parallel read-out mechanism:
•switch on one row through gate contacts and take
pedestal current + signal current
•reset the row
•switch row on again and take pedestal current
•subtract the signal-pedestal
•repeat for all rows
•do CDS
Now: 20 s – 50 kHz
TESLA: 20 ns – 50 MHz
a very ambitious but
achievable goal
Electronics requirement
•Current read-out (@ drain)
•Current memory cells
Using different readout scheme the frame read-out time can be reduced to as low as
10 s (100 kHz frame rate)
Drawback is larger power consumption:
current prototype
development:
128x128 pixels
array clocked at
50 MHz
expected noise < 100 e (at root temperature) with
resolution of < 5m
DEPFET – prototype performance
DEPFET’s field of use – beside tracking in particle physics:
•low energy X ray astronomy XEUS (0.1-30 keV sensitivity)
•Medical imaging (autoradiography) - BIOSCOPE (64x64 pixel array of 50x50 m2)
single pixel device ENC=4-5 e
matrix – 69 e (35oC)
1 kHz frame rate - 50 kHz line
55Fe
(6 keV) - 37 lp/mm ~ 6.7 m
109Cd
(22 keV) - 57 lp/mm ~ 4.3 m
6  threshold
75 m tungsten plate
Imaged with sources
projections
Real time – space and energy resolution!
(different markers can be separated)
Pros DEPFET
•low noise due to small gate capacitance (few 10 fF) – theoretically few e
•external amplification only in the 2. stage what leads to good signal/noise
•the thickness of the substrate can be large - higher efficiency for photons!
•Homogeneity of charge collection >95% (achieved with prototypes so far)
•pixel sizes of order (25x25 m2)
•Non-linearities < 0.1% within large dynamic range
•Radiation hard (deep submicron technology, rad-hard design rules) ?????
Drawback
•High cost (8 implantations, 15 masks, 200 technological steps)
•less flexible – not suitable for any vendor
HAPS - Principles of operation
All major HEP experiments concept
At LHC MHz frame rate – with noise around 200 e
•fast read-out (charge collection times of ns order)
chip
•each pixel has its own read-out amplifier
Charge
sensitive
preamplifiers
p+
sensor
n-
n+
•Detection of low-energy photons from the back –
large thickness – up to 1 mm - can be depleted
•The read-out chip is mounted directly on top of the
pixels (bump-bonding)
Problem is in assembly of pixel detector-hybrid:
bump bonding – alignment
This limits the pixel detector resolution – minimal
bump-bondable size – pixel area limited by the readout chip!
Large pixel capacitance – higher noise
BUT…, no limit on read-out chip design
•possible to detect few keV photons from the back
Pixel detector with interleaved pixels
Readout pixel
Interleaved pixel
Polyresistor
readout pitch = n x pixel pitch
Large enough to
house the VLSI
front-end cell
p+
n
Small enough
for an effective
sampling and
good spatial
resolution
Charge carriers generated underneath one of the interleaved pixel cells induce a signal on the capacitively
coupled read-out pixels, leading to a spatial accuracy improvement by a proper signal interpolation.
Silicon On Insulator (SOI) detector
Detector: conventional p+-n, DC-coupled: Electronics: conventional bulk MOS technology
INSULATOR
Detector  handle wafer
– High resistive
– 300 m thick
Electronics  active layer
– Low resistive
– 1.5 m thick
SUCIMA
HAPS - Prototype performance
The read-out pixels were wire bonded to
the readout electronics chip.
The BELLE experiment amplifiers and
readout chain were used
880 nm laser used to determine CCE
<80 m spot size – scan performed with 2D stage
cell size = 100 m
60 m
Interleaved
Readout
Max charge loss ~ 40%
In good agreement with estimated values for
capacitive network
Resolution between 3-10 m
Pros HAPS
•Proven technology – a lot of experience
•Very fast - up to few MHz frame rate
•good homogeneity of charge collection
•no problems with radiation hardness (can sustain 3 orders of magnitude larger doses than others)
•large thickness can be depleted – good efficiency for photons
•Independent design of the read-out chip
Cons HAPS
•High cost (ATLAS and CMS estimation)
•Complicated assembly – alignment of hybrids and detectors
•higher noise (large pixel capacitance)
SUMMARY
There is a bright future for silicon in the field of particle
detectors!
With new ideas coming and microelectronics industry growing
… sky is the limit