Transcript Document

ASIPET: a low noise optoelectronic integrated readout with a-Si:H photodiode array for PET

CHIPP Meeting Geneva, 12 June 2008 A. Nardulli ETH, Institut for Particle Physics (IPP), Zürich Switzerland.

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What is PET? The a-Si:H material The Thin Film on ASIC (TFA) technology Quantum Efficiency measurement with laser QE optimization: study of a-Si:H photodiodes deposited on glass and on chip The new photo detector structure Final layout of the ASIPET chip Conclusions A. Nardulli, ETH Zurich 1

What is PET?

(Positron Emission Tomography)

Is a nuclear medical imaging technique which produces an image of a functional process in the body • • • • • Compound like sugars (glucose) are labeled with signal-emitting tracers and are injected into the patient.

The short-lived isotope decays, emitting a positron. After a positronium annihilation process 2 back-to back photons of energy 511KeV are emitted.

The photons are detected by scintillating crystals and read out by a matrix of photo sensors (PMT, APD,…) A computer reassembles the signals into actual images.

Because cancer cells are highly metabolic, they are easily seen by a PET scan.

A. Nardulli, ETH Zurich 2

The a-Si:H material

Hydrogenated amorphous silicon is a tetrahedrally bonded amorphous semiconductor

• Silicon atoms not arranged in an ordered structure Defects such as dangling bonds and distorted Si-Si bonds (in both lengths and angles) • • Defects yield energy levels in the energy gap where e-h recombine Reduced mobility Band edges of the Si are replaced by a broadened tail of states • Hydrogen atoms saturate dangling and weak bonds reducing traps Increases the tolerance to impurities

a-Si:H

a-Si:H is known to be a radiation hard material 

may be attractive for High Energy Physics

A. Nardulli, ETH Zurich 3

Introduction to the project

• The idea is to develop a new photo-sensor technology:  Vertical integration of a:Si-H n-i-p photodiode and a pixel readout chip.

 The a:Si-H is used as photosensor for a PET to detect the output light from the LYSO crystal (420 nm).

Interesting points: – High degree of system integration .

– Low cost.

– High Photo Detection Efficiency after QE optimization – Low bias voltage.

• Critical points are: – Optimization of Quantum Efficiency – Ultra low noise signal retrieval.

– Full depletion of photodiodes.

– Leakage current. for 420 nm wavelength A. Nardulli, ETH Zurich 4

TFA (Thin Film on ASIC) technology

• • Vertical integration technique comprises the deposition of a detecting layer on top of a readout chip.

In our case processing.

TFA technology is achieved by deposition of a hydrogenated amorphous silicon (a-Si:H) film (n-i-p diode structure) on top of an ASIC chip that perform both charge amplification and read-out

Particle a-Si:H Detector Front electrode a-Si:H diode Rear electrode

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Detector Insulation layer/ASIC passivation ASIC ASIC

• Advantages of TFA using a-Si:H : » a-Si:H presents the technological advantage of being deposited at low temperature on large area .

» Simple detector construction, compared to hybrid detector schemes.

» Large potential for system cost reduction.

» No need of bump bonding.

Deposition based on VHF-PE-CVD technique by IMT Neuchatel

A. Nardulli, ETH Zurich 5

The photo-detector structure

•The photons detected in the i-layer create e-h pairs.

•Motion of electrons and holes generated in the depleted region induce a current.

•The non depleted zone in the diode acts as a high resistive layer for the signal, because of the low conductivity of intrinsic a-Si:H •For UV detection a thin layer (~1 μm) coupled with a crystal is in principle fine, but Cdet ~(1/d) 

Photodiode thickness: 10-12 μm

A. Nardulli, ETH Zurich 6

a-Si:H diodes deposited on

glass

: QE results

• • • The top transparent conductive layer is made of indium tin oxide (ITO) and for standard a-Si:H diodes has a thickness of 65 nm which corresponds to a deposition time of 1’45’’ .

Studies performed on samples on glass have shown that reducing the deposition time to 45’’, for the ITO layer, optimizes the quantum efficiency at wavelengths from 350 to 450 nm. This deposition time corresponds to an estimated reduced thickness of the layer of 28 nm. For the reduction of the optical loss the thickness of the p-layer has been reduced. The optimized photodiodes have a deposition time for the p-layer of 4’ to 5’ compare to 10’ for a standard p layer. The thickness of the p-layer has been reduced to approximately 20 nm, while the thickness of the n-layer is between 30 and 40 nm. 100.0

QE (%)

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Measurement of a-Si:H photodiodes deposited on glass before and after treatment of top layers: one can see the increase of QE for shorter wavelengths (pin thickness ~5 μm) 70.0

60.0

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Wavelength (nm)

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A. Nardulli, ETH Zurich 7

The photo-detector prototype

The first photo-detector prototype (MACROPAD chip) originally designed for particle detection. Top view before and after photo-sensor deposition; 4x4mm 2 ASIC with an array of 8x6 octagonal pixels with 150  m pad.

•Implemented in 0.25

•Optimized to detect  m CMOS technology.

•Each channel consists of a charge amplifier and a shaper stage.

0.1fC (625e-)

signal with a measured noise of

30e-

noise:

OPTIMIZED FOR SMALLER PIXELS AND INPUT CAPACITANCE

A. Nardulli, ETH Zurich 8

LASER 405 nm

QE: measurement with light source

The laser, in AC mode is controlled by a pulse generator with TTL signal level and variable pulse width (50-500 ns) Light attenuator ~:300 100 μm Collimator Instead of a crystal, a pulsed laser (λ~405nm) is used to simulate the incoming signal. The laser is mounted together with the attenuator and the 100 μm diameter collimator; it is then centered with the 150 μm width pixel of the a-Si:H photodiode by means of micromanipulators.

A. Nardulli, ETH Zurich 9

Calibration of the light source V

LASER 405 nm Calibrated APD Calibrated amplifier Amplifier calibrated in HSPICE Light attenuator ~300 100 μm collimator Gain 175+/-2 QE=75%+/-3% T under control A test setup using calibrated amplifier and an APD has been used to determine the number of incoming photons from the laser for a given pulse width: this number will be used for the QE calculations for the a-Si:H photodiode deposited on chip.

20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0

Laser calibration w ith APD + CALIBRATED AMPLIFIER

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Pulse Width (ns)

400

t

500 600 A. Nardulli, ETH Zurich 10

a-Si:H diodes deposited on

chip

: QE results

QE~45% QE~77%

line 6 line 5 line 4 line 3

Column

line 2 line 1 1 2 100 90 80 70 60 50 40 30 20 10 0

QE %

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Pixel's Row

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Q E on 10um non optimized photodiode

QE measured at λ=405nm: Average value ~ 45% Non optimized diode deposited on chip line 5 line 3

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Pixel's Raw

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QE %

40 30 20 10 0 QE measured for λ=405nm in average 77% (min=69% max=83%) Optimized diode deposited on chip

Number of photons~4100 +/-120; A good uniformity response is also visible from the study.

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Input specifications for 9-pixel structure

LYSO crystal: 32000 ph/MeV  ~16350ph@511KeV Experimentally on a 2x2mm 2 surface one collect ~40% of the light because of the solid angle, the LYSO-photosensor coupling and reflections  6540ph •6540ph/9 pixel  Number of photo-electrons in input=725 •QE~77% and FillFactor~95% gives a PDE=QE*FF~73% Input charge=725*PDE=530e Using an other crystal with high light output like Lantanium Bromide (LABr3 60.000ph/MeV) could improve the input signal of a factor 4, but it sacrifices spatial resolution to improve energy resolution.

•Pad Width=squares 0.6x0.6 mm^2  Detector Capacitance~10pF A. Nardulli, ETH Zurich 12

Readout electronics of the ASIPET chip

New structure: regulated cascode to enhance the open loop gain.

Noise optimization brings to increase the input bias current with consequent reduction of the output resistance and of the gain of the stage: the regulated cascode guarantees a sufficiently high open loop gain and eliminates the influence of the input capacitance on the dominant pole.

•Rise time~750 ns •Feedback capacitance=6 fF •Bias Current input Transistor= 4.5 mA •Gain preamplifier@1MHz 80 dB •Gain preamplifier & shaper@1MHz 100 dB •Closed loop gain ~800 mV/fC Total noise in simulation (includes flicker, GIC & other thermal contrib.)  ENC~52e for a 490 μm pixel A. Nardulli, ETH Zurich 13

Readout electronics of the ASIPET chip

Linearity for (-1,1) fC Gain is ~800mV/fC Reduction of the amplitude for an increase of the leakage current in the feedback branch A. Nardulli, ETH Zurich 14

Readout electronics of the ASIPET chip

Decoupling capacitors preamplifier CR RC filter A. Nardulli, ETH Zurich 15

4 x 5.5 mm 2

Layout of the ASIPET chip

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Conclusions

Photo-diode:

 QE on optimized samples measured: ~75-80%. PDE~70-75%  P and ITO layer of photodiodes reduced for an optimization of photodiode at 350-450 nm  Study on different thicknesses of the diodes have shown that 10-12 μm will ensure full depletion of the diode without influencing drastically the input detector capacitance  3x3 pixels of 660 μm width gives optimum PDE with 2x2 mm 2 LYSO crystal, but a 4x4 pixels structure minimizes the leakage current.  Input capacitance has been calculated for the different pixel sizes: it varies from 7 to 10 pF for the two structures.

Readout electronics:

 Leakage currents up to 5 nA per channel can be compensated by feedback circuit.

 A mathematical model of the circuit, which agrees with the HSPICE simulations, shows that ENC=50-60e for the 2 structures with a total SNR of 22-26.

 The ASIPET chip is in production (IBM) and will be delivered June-July 2008.

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CR-RC Shaper

The shaper stage consists of a high-pass filter built with component C1 and R1 and an integrator built with two cascaded stage of amplifiers working in common source configuration t~120ns       

sV feed

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gm V

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V feed V R in

outp casc

V V R

feed feed C feed

outp C sV in sV feed in

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V R I input feed feed

  0 0 Output response in the time domain

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2 10 -7 4 10 -7 6 10 -7 8 10 -7 1 10 -6 A. Nardulli, ETH Zurich 18

Noise results and SNR

Expected input signal: 300e- and 545e Simulation noise results:  Series noise 30 and 44e Leakage current expected= 300-500 pA  Parallel noise ~30e- & 24e Total noise in simulation (includes flicker, GIC & other thermal contrib.)  ENC~52e- for the 490um and 62 for the 660 um width pixels Total Signal to Noise ratio is respectively multiplied by 3 and 4 for the 3xe and 4x4 macro pixel structures: i.e. assuming that in the 3x3 structure the 9 pixels get the same amount of light, the 9 signals are summed, while the total rms noise is 3 times higher 3x3: 4x4: SNR pixel ~8-9 SNR crystal ~26 SNR pixel ~5-6 SNR crystal ~22 A. Nardulli, ETH Zurich 19

Light or particles Metallic back contacts CMOS chip

The leakage current measurement

• • • The leakage current is: Dependent from the Electric field and diode thickness.

Strongly influenced by the geometry of the substrate: a-Si:H diodes deposited on non-planar substrates exhibit increased leakage currents due to corner and border effects.

– field concentration at substrate steps, spikes or other sharp surface features Bigger opening on the passivation reduces the total leakage current!

10um - Sample 2

140 120 100 80 Leakage current Measurement (nA) 60 40 20 0 0 10 20 30 40 50

Bias [V]

60 70 80 90 100 A. Nardulli, ETH Zurich 20