Transcript Slides - Indico

Silicon Detectors
XII ICFA School on Instrumentation
Bogotá, Nov. 25th – Dec. 6th, 2013
Part 2
Manfred Krammer
Institute for High Energy Physics, Vienna, Austria
3 Detector Structures
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3.0 Pad Detector
The most simple detector is a large
surface diode with guard ring(s).
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3.1 Microstrip Detector
DC coupled strip detector
Through going charged particles create e-h+ pairs in the depletion zone (about
30.000 pairs in standard detector thickness). These charges drift to the electrodes.
The drift (current) creates the signal which is amplified by an amplifier connected
to each strip. From the signals on the individual strips the position of the through
going particle is deduced.
A typical n-type Si strip detector:
p+n junction:
Na ≈ 1015 cm-3, Nd ≈ 1–5·1012 cm-3
n-type bulk:  > 2 kcm
thickness 300 µm
Operating voltage < 200 V.
n+ layer on backplane to improve
ohmic contact
Aluminum metallization
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3.1 Microstrip Detector
n-type and p-type detectors
 The previous slide explains an n-type detector (detector bulk is ntype silicon)
 Using p-type silicon and exchanging p+ and n+ would give a
perfectly working p-type detector.
 For tradition and production reasons most of the present
detectors are n-type detectors.
 p-type detectors are discussed as base line detectors for the
upgrade experiments CMS and ATLAS, due to their
advantages in the high radiation environment (see later).
 For simplicity I will continue discussing n-type detectors only...
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3.1 Microstrip Detector
AC coupled strip detector
AC coupling blocks leakage current from the amplifier.
Integration of coupling capacitances in
standard planar process.
AC coupled strip detector:
Deposition of SiO2 with a thickness of 100–
200 nm between p+ and aluminum strip
Depending on oxide thickness and strip width
the capacitances are in the range of
8–
32 pF/cm.
Problems are shorts through the dielectric
(pinholes). Usually avoided by a second layer
of Si3N4.
However, the dielectric cuts the bias connection to the strips!
Several methods to connect the bias voltage: polysilicon resistor,
punch through bias, FOXFET bias.
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3.1 Microstrip Detector
Polysilicon bias – 1
Deposition of polycristalline silicon between p+ implants and a common bias
line.
Sheet resistance of up to Rs ≈ 250 k/ .
To achieve high resistor values winding poly structures are deposited.
Depending on width and length a resistor of up to R ≈ 20 M is achieved
(R = Rs·length/width).
Drawback: Additional production
steps and photo lithograpic masks
required.
Cut through an AC coupled strip
detector with integrated poly resistors:
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3.1 Microstrip Detector
Polysilicon bias – 2
Top view of a strip detector with
polysilicon resistors:
CMS-Microstrip-Detektor: Close view of
area with polysilicon resistors, probe
pads, stip ends.
CMS Collaboration, HEPHY Vienna
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3.1 Microstrip Detector
Punch through bias
Punch through effect: Figures show the increase of the depletion zone with increasing
bias voltage (Vpt = punch through voltage).
1.)
2.)
3.)
Advantage: No additional production steps required.
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3.1 Microstrip Detector
FOXFET bias
Strip p+ implant and bias line p+ implant are source and drain of a field effect
transistor - FOXFET (Field OXide Field Effect Transistor).
A gate is implemented on top of a SiO2 isolation.
Dynamic resistor between drain and source can be adjusted with gate
voltage.
FOXFET biasing:
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3.1 Microstrip Detector
Wire bond connection
Ultrasonic welding technique
typically 25 micron bond wire of Al-Si-alloy
Fully-automatized system with automatic
pattern recognition
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3.2 Double Sided Strip Detectors
Principle
Single sided strip detector measures only
one coordinate. To measure second
coordinate requires second detector
layer.
Scheme of a double sided strip
detector (biasing structures not
shown):
Double sided strip detector measures two
coordinates in one detector layer
(minimizes material).
In n-type detector the n+ backside
becomes segmented, e.g. strips
orthogonal to p+ strips.
Drawback: Production, handling, tests
are more complicated and hence double
sided detector are expensive.
•
•
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Holes drift to p+ strips
Electrons drift to n+ strips
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3.2 Double Sided Strip Detectors
n-side separation
Problem with n+ segmentation: Static, positive oxide charges in the Si-SiO2
interface.
These positive charges attract electrons. The electrons form an accumulation
layer underneath the oxide.
n+ strips are no longer isolated from each other (resistance ≈ k).
Charges generated by through going
particle spread over many strips.
No position measurement possible.
Solution: Interrupt accumulation layer
using p+-stops, p+-spray or field
plates.
Positive oxide charges cause electron
accumulation layer.
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3.2 Double Sided Strip Detectors
p+-stops
p+-implants (p+-stops, blocking electrodes) between n+-strips interrupt the
electron accumulation layer.
Interstrip resistance reaches again G.
Picture showing the n+-strips and the p+-stop
structure:
A. Peisert, Silicon Microstrip Detectors,
DELPHI 92-143 MVX 2, CERN, 1992
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J. Kemmer and G. Lutz, New Structures for Position
Sensitive Semiconductor Detectors,
Nucl. Instr. Meth. A 273, 588 (1988)
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3.2 Double Sided Strip Detectors
p-spray
p doping as a layer over the whole surface.
disrupts the e- accumulation layer.
Some companies use a combination of p+ stops and p spray
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3.2 Double Sided Strip Detectors
Field plates
Metal of MOS structure at negative potential compared to the n+-strips displace
electrons below Si-SiO2-interface.
Above a threshold voltage n+-strips become isolated.
Simple realization of AC coupled sensors: Wide metal lines with overhang in the
interstrip region serve as field plates.
A field plate at negative potential
interupts accumulation layer:
n+-strips of an AC coupled detector. The
alumimnum readout lines act as field plates:
A. Peisert, Silicon Microstrip Detectors, DELPHI
92-143 MVX 2, CERN, 1992
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3.2 Double Sided Strip Detectors
2nd metal layer – 1
In the case of double sided strip detectors with
orthogonal strips the readout electronics is
located on two sides (fig. a).
(a)
Many drawbacks for construction and material
distribution, especially in collider experiments.
Electronics only on one side is a preferred
configuration (fig. b).
Possible by introducing a second metal layer.
Lines in this layer are orthogonal to strips and
connect each strips with the electronics (fig. c).
The second metal layer can be realized by an
external printed circuit board, or better
integrated into the detector.
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(b)
how?
(c)
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3.2 Double Sided Strip Detectors
2nd metal layer – 2
3D scheme of an AC coupled double
sided strip detector with 2nd metal
readout lines (bias structure not
shown). The isolation between the two
metal layers is either polyimide or SiO2:
Cross section of the n+ side of an AC
coupled double sided strip detector with 2nd
metal readout lines. Shown is the end of a
strip with the bias resistor:
A. Peisert, Silicon Microstrip Detectors, DELPHI 92-143
MVX 2, CERN, 1992
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3.3 Hybrid Pixel Detectors
Advantage
Double sided strip sensors measure the 2 dimensional position of a particle
track. However, if more than one particle hits the strip detector the measured
position is no longer unambiguous. “Ghost”-hits appear!
True hits and ghost hits in a
double sided strip detector in
case of two particles traversing
the detector:
Pixel detectors produce unambiguous hits!
Measured hits in a pixel detector
in case of two particles
traversing the detector:
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3.3 Hybrid Pixel Detectors
Advantages and disadvantages
Typical pixel size is 50 µm x 50 µm.
If signal pulse height is not recorded, resolution is the digital resolution:
~14 µm (50 µm pixel pitch).
d
… pixel dimension
Better resolution achievable with analogue readout
Small pixel area low detector capacitance (≈1 fF/Pixel)
noise ratio (e.g. 150:1).
Small pixel volume
large signal-to-
low leakage current (≈1 pA/Pixel)
Drawback of hybrid pixel detectors: Large number of readout channels
Large number of electrical connections in case of hybrid pixel
detectors.
Large power consumption of electronics..
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3.3 Hybrid Pixel Detectors
Principle
“Flip-Chip” pixel detector:
On top the Si detector, below the readout chip,
bump bonds make the electrical connection for
each pixel.
S.L. Shapiro et al., Si PIN Diode Array Hybrids for Charged
Particle Detection, Nucl. Instr. Meth. A 275, 580 (1989)
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Detail of bump bond connection.
Bottom is the detector, on top the
readout chip:
L. Rossi, Pixel Detectors Hybridisation,
Nucl. Instr. Meth. A 501, 239 (2003)
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3.3 Hybrid Pixel Detectors
Bump bonding process - 1
1.
Deposition of an “underbump metal layer”,
plasma activated, for a better adhesion of
the bump material.
2.
Photolithography to precisely define areas
for the deposition of the bond material.
3.
Deposition, by evaporation, of the bond
material (e.g. In or SnPb) producing little
“bumps” (≈ 10 µm height).
4.
Edging of photolithography mask leaves
surplus of bump metal on pads.
5.
Reflow to form spheres.
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L. Rossi, Pixel Detectors Hybridisation,
Nucl. Instr. Meth. A 501, 239 (2003)
A typical bump bonding process (array bump bonding) is the following:
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3.3 Hybrid Pixel Detectors
Bump bonding process
Electron microscope pictures before and after the reflow production step.
In bump, The distance between bumps is 100 μm, the deposited indium is 50 μm wide
while the reflowed bump is only 20 μm wide.
C. Broennimann, F. Glaus, J. Gobrecht, S. Heising, M. Horisberger, R. Horisberger, H. Kästli,J. Lehmann, T. Rohe, and S. Streuli, Development of an
Indium bump bond process for silicon pixel detectors at PSI, Nucl. Inst. Met. Phys, Res. A565(1) (2006) 303–308 82
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3.3 Hybrid Pixel Detectors
Bump bonding process – 3
Electron microscope picture of pixel detector with long strip.
Left: Detector chip, right: readout chip with bump bonds applied.
G. Lutz, Semiconductor Radiation Detectors, Springer-Verlag, 1999
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3.4 – 3.9 Other Si Detector Structures
Strip and hybrid pixel detectors are mature technologies employed in
almost every experiment in high energy physics.
Additional interesting silicon detector structures are:
• Charged Coupled Devices (CCD)
• Silicon Drift Detectors (SDD)
• Monolithic Active Pixels (MAPS)
• Depleted Field Effect detectors (DEPFET)
• Silicon On Oxide (SOI)
• 3D detectors
• Avalanche Photo Diodes (APD) and Silicon Photo Multiplier (SiPM)
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3.4 Charge Coupled Devices (CCD)
Principle
Shallow depletion layer (typically 15 m), relatively small signal, the charge is kept
in the pixel and during readout shifted through the columns and through final row to
a single signal readout channel:
Slow device, hence not suitable for fast detectors.
Improvement are developed, e.g. parralel column readout.
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3.4 Charge Coupled Devices (CCD)
The SLD Si pixel vertex detector
The SLD (SLAC, USA) silicon vertex detector used large area CCDs.
Pixel size 20 µm x 20 µm, achieved resolution 4 µm.
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3.5 Silicon Drift Detectors
Principle
Evolution of Silicon Sensor Technology in Particle
Physics, F. Hartmann, Springer Volume 231, 2009
In silicon drift detectors p+ strips and the backplane p+ implantation are used to
fully deplete the bulk. A drift field transports the generated electrons to the readout
electrodes (n+). One coordinate is measured by signals on strips, the second by
the drift time.
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Used for example
in the experiment
ALICE (CERN)
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3.6 Monolithic Active Pixels
CMOS
Scheme of a CMOS monolithic active pixel cell with an NMOS transistor. The N-well
collects electrons from both ionization and photo-effect.
Evolution of Silicon Sensor Technology in Particle Physics,
F. Hartmann, Springer Volume 231, 2009
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3.6 Monolithic Active Pixels
Silicon on Insulator (SOI)
A SOI detector consists of a thick full depleted high resitivity bulk and seperated by
a layer of SiO2 a low resistivity n-type material. NMOS and PMOs transistors are
implemented in the low resitivity material using standard IC methods.
Evolution of Silicon Sensor Technology in Particle Physics, F.
Hartmann, Springer Volume 231, 2009
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3.7 DEPFET Detectors
Principle
The DEPFET detector is a detector with an internal amplification structure.
The n-bulk is fully depleted with a potential minimum below the strips and the
structure of a field effect transistor. The electrons created by a charged
particle accumulate in the potential minimum. The field configuration is such
that the electrons drift underneath the gate of the transistor modifying the
source drain curreny. An active clear is necessary to remove the electrons.
Evolution of Silicon Sensor Technology in Particle Physics, F.
Hartmann, Springer Volume 231, 2009; J. Kemmer and G. Lutz, NIM
A253 (1987) 365
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Used in Belle II and candidate for ILC
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3.8 3D Detectors
Principle
3D detecors are non planar detectors. Deep holes are etched into the silicon and
filled with n+ and p+ material. Depletion is sideways. The distances between the
electrodes are small, hence depletion voltage can be much smaller and charge
carries travel much short distances.
Picture from CNM-IMB (CSIC), Barcelona
Very radiation tolerant detectors, first use in ATLAS IBL layer.
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3.8 3D Detectors
Different approaches
Single column:
Low field region between columns
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Double-sided double column:
High field, but more complicated
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3.9 Avalanche Photo Diode (APD)
Principle
APD are operated in reverse bias mode in the breakdown regime. A photon is
able to trigger an avalanche breakdown. The current increase has to be limted
by a quenching resistor.
R. H. Haitz, J. App.Phys. Vol. 36, No.
10 (1965) 3123
Used for photon detection in calorimeters (e.g the electromagnetic calorimeter
of CMS), in cherenkov counters, etc.
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3.9 Avalanche Photo Diode (APD)
Silicon photo multiplier (SiPM)
SiPM are matrices of APDs:
Front contact
Al
h
Rquenching
ARC

n+
p
n+
p
p+ silicon wafer
Back contact
-Vbias
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SiPM become more and more popular as
replacement for standard photo multiplier.
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4 Performance
Parameter
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4.1 Signal to Noise Ratio
Introduction
 The signal generated in a silicon detector depends essentially only
on the thickness of the depletion zone and on the dE/dx of the
particle.
 The noise in a silicon detector system depends on various
parameters: geometry of the detector, the biasing scheme, the
readout electronics, etc.
Noise is typically given as “equivalent noise charge” ENC. This is
the noise at the input of the amplifier in elementary charges.
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4.1 Signal to Noise Ratio
Noise contributions
The most important noise contributions are:
1.
2.
3.
4.
Leakage current (ENCI)
Detector capacity (ENCC)
Det. parallel resistor (ENCRp)
Det. series resistor (ENCRs)
Alternate circuit diagram of a
silicon detector.
The overall noise is the quadratic sum of all contributions:
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4.1 Noise Contributions
Leakage current - 1
 The detector leakage current comes from thermally generated
electron holes pairs within the depletion region. These charges are
seperated by the electric field and generate the leakage current. The
fluctuations of this current are the source of noise.
In a typical detector system (good detector quality, no irradiation
damage) the leakage current noise is usually negligible.
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4.1 Noise Contributions
Leakage current - 2
Assuming an amplifier with an integration time (“peaking time”) tp
followed by a CR-RC filter the noise contribution by the leakage
current l can be written as:
e .… Euler number (2.718...)
e … Electron charge
Using the physical constants, the leakage current in units of nA and
the integration time in µs the formula can be simplified to:
To minimize this noise contribution the detector should be of high
quality with small leakage current.
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4.1 Noise Contributions
Detector capacity
The detector capacity at the input of a charge sensitive amplifier is
usually the dominant noise source in the detector system.
This noise term can be written as:
The parameter a and b are given by the design of the (pre)-amplifier.
C is the detector capacitance at the input of the amplifier channel.
Integration time tp is crucial, short integration time leads usually to
larger a and b values. Integration time is depending on the accelerator
time structure.
Typical values are (amplifier with ~ 1 µs integration time):
a ≈ 160 e und b ≈ 12 e/pF
To reduce this noise component segmented detectors with short
strip or pixel structures are preferred.
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4.1 Noise Contributions
Parallel resistor
The parallel resistor Rp in the alternate circuit diagram is the bias
resistor. The noise term can be written as:
e .… Euler number (2.718...)
e … Electron charge
Assuming a temperature of T=300K, tp in µs and Rp in M the formula
can be simplified to:
To achieve low noise the parallel (bias) resistor should be large!
However the value is limited by the production process and the
voltage drop across the resistor (high in irradiated detectors).
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4.1 Noise Contributions
Series resistor
The series resistor Rs in the alternate circuit diagram is given by the
resistance of the connection between strips and amplifier input (e.g.
aluminum readout lines, hybrid connections, etc.). It can be written as:
C .… Detector capacity on pF
tp … Integration time in µs
Rs … Series resistor in 
Note that, in this noise contribution tp is inverse, hence a long tp
reduces the noise. The detector capacitance is again responsible for
larger noise.
To avoid excess noise the aluminum lines should have low
resistance (e.g. thick aluminum layer) and all other connections
as short as possible.
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4.1 Signal to Noise Ratio
Summary
To achieve a high signal to noise ratio in a silicon detector system the
following conditions are important:
Low detector capacity (i.e. small element size)
Low leakage current
Large bias resistor
Short and low resistance connection to the amplifier
Long integration time
Obviously some of the conditions are contradictory. Detector and
front end electronics have to be designed as one system. The
optimal design depends on the application.
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4.1 Signal to Noise Ratio
Examples
DELPHI Microvertex (LEP):
CMS Tracker (LHC):
readout chip (MX6):
a = 325 e, b = 23 e/pF, tp = 1.8 µs
readout chip (APV25, deconvolution):
a = 400 e, b = 60 e/pF, tp = 50 ns
2 detectors in series each 6 cm
long strips, C = 9 pF
ENCC = 532 e
2 detectors in series each 10 cm long
strips, C = 18 pF
ENCC = 1480 e
typ. leakage current/strip: I ≈ 0.3 nA
ENCI = 78 e
max. leakage current/strip: I ≈ 100 nA
ENCI = 103 e
bias resistor Rp = 36 M
ENCRp = 169 e
bias resistor Rp = 1.5 M
ENCRp = 60 e
series resistor = 25 
ENCRs = 13 e
series resistor = 50 
ENCRs = 345 e
Total noise: ENC = 564 e (SNR 40:1)
Total noise: ENC = 1524 e (SNR 15:1)
Calculated for the signal of a minimum ionizing particle (mip) of 22500 e.
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4.2 Position Resolution
Introduction
The position resolution – the main parameter of a position detector –
depends on various factors, some due to physics constraints and some
due to the design of the system (external parameters).
Physics processes:
– Statistical fluctuations of the energy loss
– Diffusion of charge carriers
External parameter:
– Binary readout (thresh hold counter) or read out of
analogue signal value
– Distance between strips (strip pitch)
– Signal to noise ratio
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4.2 Position Resolution
Statistical fluctuation of the energy loss – 1
Silicon position detectors are thin (300–500 µm) and absorb only a small
fraction of the total energy of through going particles.
The energy loss dE/dx follows a Landau distribution, an asymmetric
probability function with a long “tail” to large energy deposits.
Example of a mip measured in a 300 µm thick silicon detector:
Pions and Protons:
W. Adam et al., CMS note 1998/092 (1998)
o Most probable energy loss
(Maximum of the distribution):
78 keV in 300 µm ≈ 72 e–h+
pairs per µm
o Mean of the energy loss:
116 keV in 300 µm ≈ 108 e–h+
pairs per µm
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4.2 Position Resolution
Statistical fluctuation of the energy loss – 2
Long tail in energy loss distribution is due to -electrons.
-electrons have a high energy (keV) and are produced by rare, hard
collisions between incident particle and electrons from the detector
material.
Displacement probability (calcuThe probability to produce a lation) of the charge center of gravity
due to -electrons:
electrons is small.
A. Peisert, Silicon Microstrip Detectors,
DELPHI 92-143 MVX 2, CERN, 1992
-electrons have a long track
length in the detector material and
may produce e+h- pairs along the
track.
Dislocate the measured track
Measurement errors in the order of
µm unavoidable
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4.2 Position Resolution
Diffusion – 1
After the ionizing particle has passed the detector the e+h- pairs are
close to the original track.
While the cloud of e+ and h- drift to the electrodes, diffusion widens the
charge carrier distribution. After the drift time t the width (rms) of the
distribution is given as:
with:
D … width “root-mean-square” of the charge carrier distribution
t
k
e
… drift time
… Boltzmann constant
… electron charge
D … diffusion coefficient
T … temperature
 … charge carrier mobility
Note: D  µ and t  1/µ, hence D is equal for e– and h+.
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4.2 Position Resolution
Diffusion – 2
h+ created close to the anode (i.e. the n+ backplane) and e- created
close to he cathode (i.e. the p+ strips or pixels) have the longest drift
path. As a consequence the diffusion acts much longer on them
compared to e- h+ with short track paths.
The signal measured comes from many overlapping Gaussian
distributions.
Drift and diffusion acts on charge carriers:
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Charge density distribution for 5
equidistant time intervalls:
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4.2 Position Resolution
Diffusion – 3
Diffusion widens the charge cloud. However, this has an positive
effect on the position resolution!
charge is distributed over more than one strip, with interpolation
(calculation of the charge center of gravity) a better position
measurement is achievable.
This is only possible if analogue read out of the signal is implemented.
Interpolation is more precise the larger the signal to noise ratio is.
Strip pitch and signal to noise ratio determine the position
resolution.
Larger charge sharing can also be achieved by tilting the detector.
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4.2 Position Resolution
Threshold readout versus analogue readout
Threshold (binary) readout (one strip signal):
position:
x = strip position
resolution:
p
… distance between strips
(readout pitch)
x
… position of particle track
charge center of gravity (signal on two strips):
position:
h
h x +h x
x = x1 + 1 ( x2 - x1 ) = 1 1 2 2
h1 + h2
h1 + h2
resolution:
p
SNR
sx µ
»
x1,x2 … position of 1st and 2nd strip
h1,h2 … signal on 1st and 2nd strip
SNR … signal to noise ratio
A position resolution of a few µm is achievable with analogue readout !
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4.2 Position Resolution
Intermediate strips – 1
The strip pitch determines to a large extend the position resolution. With
small strip pitch a better position resolution is achievable.
small strip pitch requires large number of electronic channels
cost increase
power dissipation increase
A possible solution is the implementation of intermediate strips. These are
strips not connected to the readout electronics located between readout
strips.
The signal from these intermediate strips is transferred by capacitive
coupling to the readout strips.
more hits with signals on more than one strip
Improved resolution with smaller number of readout channels.
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4.2 Position Resolution
Intermediate strips – 2
Scheme of a detector with two intermediate strips. Only every 3rd strip is
connected to an electronics channel. The charge from the intermediate
strips is capacitive coupled to the neighbor strips.
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4.2 Position Resolution
Example – influence of readout pitch and SNR
Example of a detector with strip pith of 25 µm and analogue readout.
The position resolution is plotted as a function of the SNR.
Bottom curve: every strip is connected to the readout electronics
Top curve: every 2nd strip is connected, one intermediate strip
To benefit from intermediate strips
large SNR is required!
A. Peisert, Silicon Microstrip Detectors,
DELPHI 92-143 MVX 2, CERN, 1992
XII ICFA School on Instrumentation, Bogotá, 2013
Manfred Krammer
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