Silicon Detectors !
Manfred Krammer!
Institute of High Energy Physics, Vienna, Austria!
Silicon Detectors
Content!
2 Basics!
3 Detector Structures!
4 Performance!
6 The CMS Inner Tracker!
M. Krammer, Praktikum 2010/11!
Silicon Detectors!
1!
2 Basics!
M. Krammer, Praktikum 2010/11!
Silicon Detectors!
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2.1 Materials
Elemental semiconductors!
!  Germanium: Used in nuclear physics, due to small band gap
(0.66 eV) needs cooling (usually done with liquid nitrogen at 77 K)!
!  Silicon: Standard material for vertex and tracking detectors in high
energy physics, can be operated at room temperature, synergies with
micro electronics industry.!
!  Diamond (CVD or single crystal): Large band gap, requires no
depletion zone, very radiation hard, drawback is a low signal and high
cost!!
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2.1 Materials
Compound semiconductors!
Compound semiconductors consist of two (binary semiconductors) or
more atomic element. Depending on the column in the periodic system
of elements one differentiates between IV-IV- (e.g. SiGe, SiC), III-V-,
und II-VI compounds!
!  important III-V compounds:!
–  GaAs: Faster and probably more radiation resistant than Si. !
!Drawback is less experience in industry and cost.!
–  GaP, GaSb, InP, InAs, InSb, InAlP!
!  important II-VI compounds: !
–  CdTe: High atomic numbers (48+52) hence very efficient to detect
photons.!
–  ZnS, ZnSe, ZnTe, CdS, CdSe, Cd1-xZnxTe, Cd1-xZnxSe!
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2.2 Material Properties
Bond model of semiconductors!
Example of column IV elemental semiconductor (2dim projection) :!
T = 0 K!
T > 0 K!
Valence electron!
Conduction electron!
!  Each atom has 4 closest neighbors, the 4 electrons in the outer shell are
shared and form covalent bonds.!
!  At low temperature all electrons are bound!
!  At higher temperature thermal vibrations break some of the bonds " free ecause conductivity (electron conduction)!
!  The remaining open bonds attract other e- " The holes change position
(hole conduction)!
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2.2 Material Properties
Energy bands: isolator–semiconductor–metal!
In an isolated atom the electrons have only discrete energy levels. In solid state
material the atomic levels merge to energy bands. In metals the conduction and
the valence band overlap, whereas in isolators and semiconductors these levels
are separated by an energy gap (band gap). In isolators this gap is large.!
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2.2 Material Properties
Intrinsic carrier concentration!
!  Due to the small band gap in semiconductors electrons already occupy!
the conduction band at room temperature.!
!
!  Electrons from the conduction band may recombine with holes.!
!
!  A thermal equilibrium is reached between excitation and recombination!
Charged carrier concentartion ne = nh = ni !
This is called intrinsic carrier concentration:!
ni =
3
$ Eg '
$ Eg '
2
NC NV " exp&#
) * T " exp&#
)
2kT
2kT
%
(
%
(
In ultrapure silicon the intrinsic carrier concentration is1.45·1010 cm-3. !
!
With approximately 1022 Atoms/cm3 about 1 in 1012 silicon atoms is ionised.!
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2.3 Constructing a Detector
The ideal semiconductor detector!
One of the most important parameter of a detector is the signal to noise ratio
(SNR). A good detector should have a large SNR. However this leads to two
contradictory requirements:!
#  Large signal
" low ionisation energy " small band gap!
#  Low noise
" very few intrinsic charge carriers " large band gap!
An optimal material should have Eg ≈ 6 eV. !
In this case the conduction band is almost empty at room temperature and the
band gap is small enough to create a large number of e-h+ pairs through
ionisation.!
Such a material exist, it is Diamond. However even artificial diamonds (e.g. CVD
diamonds) are too expensive for large area detectors.!
M. Krammer, Praktikum 2010/11!
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2.3 Constructing a Detector
Estimate SNR in an intrinsic silicon detector!
Let s make a simple calculation for silicon: !
!
!
!
!Mean ionization energy I0 = 3.62 eV, mean energy loss per flight path
!dE/dx = 3.87 MeV/cm, intrinsic charge carrier density at T = 300 K
!ni = 1.45 · 1010 cm-3. !
Assuming a detector with a thickness of d = 300 µm and an area of A = 1 cm2.!
"
Signal of a mip in such a detector:!
!
dE dx " d 3.87 "10 6 eV cm " 0.03cm
=
# 3.2 "10 4 e$h +$pairs
I0
3.62eV
" !Intrinsic charge carrier in the same volume (T = 300 K):!
n i d A = 1.45 "1010 cm-3 " 0.03cm"1cm2 # 4.35 "10 8 e$h +$pairs
!
!
" !Number of thermal created e–h+-pairs are four orders of magnitude larger
than signal!!!!
Have to remove the charge carrier! !
!
!" Depletion zone in reverse biased pn junctions!
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2.4 Doping
Bond model: n-doping in Si!
Doping with an element 5 atom (e.g. P, As, Sb). The 5th valence electrons is
weakly bound.!
The doping atom is called donor!
M. Krammer, Praktikum 2010/11!
The released conduction electron
leaves a positively charged ion!
Silicon Detectors!
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2.4 Doping
Bond model: n-doping in Si!
The energy level of the donor is just below the edge of the conduction band. At
room temperature most electrons are raised to the conduction band.!
The fermi level EF moves up. !
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2.4 Doping
Bond model: p-doping in Si!
Doping with an element 3 atom (e.g. B, Al, Ga, In). One valence bond
remains open. This open bond attracts electrons from the neighbor atoms.!
The doping atom is called acceptor. !
M. Krammer, Praktikum 2010/11!
The acceptor atom in the lattice is
negatively charged.!
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2.4 Doping
Bond model: p-doping in Si!
The energy level of the acceptor is just above the edge of the valence band. At
room temperature most levels are occupied by electrons leaving holes in the
valence band.!
The fermi level EF moves down. !
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2.5 The p-n Junction
Creating a p-n junction!
At the interface of an n-type and p-type semiconductor the difference in the fermi
levels cause diffusion of surplus carries to the other material until thermal equilibrium
is reached. At this point the fermi level is equal. The remaining ions create a space
charge and an electric field stopping further diffusion.!
The stable space charge region is free of charge carries and is called the depletion
zone.!
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2.5 The p-n Junction
Operation with forward bias!
p-n junction with forward bias!
Applying an external voltage V with the anode
to p and the cathode to n e- and holes are
refilled to the depletion zone. The depletion
zone becomes narrower.!
!
!
The potential barrier becomes smaller by eV
and diffusion across the junction becomes
easier. The current across the junction
increases significantly.!
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2.5 The p-n Junction
Operation with reverse bias!
p-n junction with reverse bias!
Applying an external voltage V with the cathode
to p and the anode to n e- and holes are pulled
out of the depletion zone. The depletion zone
becomes larger.!
!
!
The potential barrier becomes higher by eV and
diffusion across the junction is suppressed. The
current across the junction is very small
leakage current .!
" That s the way we operate our semiconductor detector!!
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2.5 The p-n Junction
Width of the depletion zone!
Example of a typical p+-n junction in a silicon detector:!
Effective doping concentration Na = 1015 cm–3 in p+ region and Nd = 1012 cm–3 in
n bulk.!
Without external voltage:!
Wp = 0.02 µm !
Wn = 23 µm!
Applying a reverse bias voltage of 100 V:!
Wp = 0.4 µm !
Wn = 363 µm!
Width of depletion zone in n bulk:!
W "
1
with! " =
e µ Neff
2# 0# r µ$V
M.!Krammer, Praktikum 2010/11!
!
p+n junction!
V !… !External voltage!
" !… !specific resistivity!
µ  !… !mobility of majority charge carriers!
Neff!… !effective doping concentration!
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2.5 The p-n Junction
Current-voltage characteristics!
Typical current-voltage of a p-n junction (diode): exponential current increase
in forward bias, small saturation in reverse bias.!
!
*
#eV & Ideal diode equation:!
I0 … reverse saturation current!
I = I 0 " ,exp% ( ) 1/
$ kT ' .
+
!
S.M. Sze, Semiconductor Devices , J. Wiley & Sons, 1985!
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2.6 Detector Characteristics
Leakage Current!
A silicon detector is operated with reverse bias, hence reverse saturation
current is relevant (leakage current). This current is dominated by thermally
generated e-h+ pair. Due to the applied electric field they cannot recombine and
are separated. The drift of the e- and h+ to the electrodes causes the leakage
current. !
Measured detector leakage current, CMS
strip detector (measurement at room
temperature):!
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2.6 Detector Characteristics
Depletion Voltage!
reverse bias voltage V [V]!
The depletion voltage is the minimum voltage at which the bulk of the sensor is
fully depleted. The operating voltage is usually chosen to be slightly higher
(overdepletion).!
High resistivity material (i.e. low doping) requires low depletion voltage.!
Depletion voltage as a function of the
material resistivity for two different
detector thicknesses (300 µm, 500 µm).!
resistivity " [kOhm cm]!
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2.6 Detector Characteristics
Capacitance of a detector!
For a typical Si p-n junction (Na >> Nd >> ni) the detector capacitance is given
as:!
" !… !specific resistivity of the bulk!
"0"r
µ !… !mobility of majority charge carrier!
C=
$A
V !… !bias voltage
2µ# V
!!
A !… !detector surface!
!
!
!
Measured detector capacitance as a
function of the bias voltage, CMS strip
detector:!
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3 Detector Structures!
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3.1 Microstrip Detector
DC coupled strip detector!
Through going charged particles create e-h+ pairs in the depletion zone (about
30.000 pairs in standard detector thickness). These charges drift to the electrodes.
The drift (current) creates the signal which is amplified by an amplifier connected
to each strip. From the signals on the individual strips the position of the through
going particle is deduced. !
!
A typical n-type Si strip detector:!
! p+n junction: !
Na ≈ 1015 cm-3, Nd ≈ 1–5·1012 cm-3!
!  n-type bulk: ! > 2 k#cm
" thickness 300 µm !
!  Operating voltage < 200 V.!
!  n+ layer on backplane to improve
ohmic contact!
!  Aluminum metallization !
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3.1 Microstrip Detector
AC coupled strip detector!
AC coupling blocks leakage current from the amplifier.!
!  Integration of coupling capacitances in
standard planar process.!
AC coupled strip detector:!
!  Deposition of SiO2 with a thickness of 100–
200 nm between p+ and aluminum strip!
!  Depending on oxide thickness and strip width
the capacitances are in the range of
8–
32 pF/cm.!
!  Problems are shorts through the dielectric
(pinholes). Usually avoided by a second layer
of Si3N4.!
Several methods to connect the bias voltage: polysilicon resistor,
punch through bias, FOXFET bias.!
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3.1 Microstrip Detector
Polysilicon bias – 1!
!  Deposition of polycristalline silicon between p+ implants and a common bias
line. !
!  Sheet resistance of up to Rs ≈ 250 k#/$. Depending on width and length a
resistor of up to R ≈ 20 M# is achieved (R = Rs·length/width).!
!  To achieve high resistor values winding poly structures are deposited.!
!  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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3.3 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)!
M. Krammer, Praktikum 2010/11!
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 – 2!
Electron microscope pictures before and after the reflow production step. !
PbSn bump, diameter 25 µm, pitch: 50 µm, reflow by heating to app. 300 °C.!
L. Rossi, Pixel Detectors Hybridisation, Nucl. Instr. Meth. A 501, 239 (2003)!
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4 Performance!
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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:!
ENC =
2
2
ENCC2 + ENCI2 + ENCRp
+ ENCRs
!
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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:!
ENCC = a + b " C
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 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!
!  Usually 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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4.2 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.!
o  Most probable energy loss
(Maximum of the distribution):
78 keV in 300 µm " ≈ 72 e–h+
pairs per µm!
Pions and Protons:!
o  Mean of the energy loss:
116 keV in 300 µm " ≈ 108 e–h+
pairs per µm!
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W. Adam et al., CMS note 1998/092 (1998)!
!  Example of a mip measured in a 300 µm thick silicon detector:!
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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:!
M. Krammer, Praktikum 2010/11!
Charge density distribution for 5
equidistant time intervalls: !
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4.2 Position Resolution
Threshold readout versus analogue readout!
!  Threshold readout (one strip signal):!
" position:!
" resolution: !
x = strip position
p
p
"x #
12
x
!… !distance between strips
(readout pitch)!
!… !position of particle track!
!
!  charge center of gravity (signal on two strips):!
!
"  position: !
!
!
h1
hx +h x
x = x1 +
(x 2 " x 1) = 1 1 2 2
h1 + h 2
h1 + h 2
"  resolution: !
!
M. Krammer, Praktikum 2010/11!
!!
"x "
#
p
SNR
Silicon Detectors!
x1,x2 … position of 1st and
2nd strip!
h1,h2 … signal on 1st and
2nd strip!
SNR … signal to noise ratio!
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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!
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6.3 The CMS Inner Tracker!
!
!
!!
!
!
!
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The CMS Inner Tracker!
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6.3 The CMS Inner Tracker
The CMS Experiment!
CMS
A Compact Solenoidal Detetor for LHC
MUON CHAMBERS
INNER TRACKER
E.M. CRYSTAL CAL.
HADRON CAL.
VERY FORWARD
CALORIMETER
SUPERCONDUCTING COIL
Total Weight
:
Overall diameter :
Overall length :
Magnetic field :
12,000 t.
14.00 m
20.00 m
4 Tesla
RETURN YOKE
jlb
CMS 1000
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6.3 The CMS Inner Tracker
Layout of the detector!
Outer Barrel –TOB
Pixel
End cap –TEC
Inner Barrel –TIB
2,4 m
Inner Disks –TID
Volume 24.4 m3
temperature -10 0C
dry atmosphere
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6.3 The CMS Inner Tracker
Some numbers!
Strip detector:
!~200 m2 of silicon sensors
!24,244 single silicon sensors
!15,148 modules
!9,600,000 strips $ electronics channels
!75,000 read out chips (APV25)
!25,000,000 Wire bonds!
!
Pixel detector:!
!1 m2 detector area!
!1440 pixel modules!
!66 million pixels!
!
Industrial type of production in many laboratories worldwide.!
!Largest Silicon detector built so far !!
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6.3 The CMS Inner Tracker
A worldwide collaboration!
55 institutes from 9 countries (Austria, Belgium, Finland, !France, Germany, Italy, Switzerland, UK, USA)
About 500 scientists and engineers involved in the design and construction!
of the CMS inner tracker.!
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6.3 The CMS Inner Tracker
Example of strip module geometries!
27 mechanical different modules + 2 types of alignment modules !
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Tracker Insertion!
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6.3 The CMS Inner Tracker
The pixel detector!
~50 cm!
~1 m!
~40 cm!
Pixel size: 100 µm x 150 µm!
Barrel Pixel:
3 barrel layers at r of 4.3, 7.3, 10.4 cm
11520 chips (48 million pixels)!
Forward Pixel:
4 disks at z of ±35.5 and ±46.5 cm
4320 chips (18 million pixels)
Modules dilted by 20o for better charge sharing
!
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6.3 The CMS Inner Tracker
The pixel detector - construction!
Hybrid Pixel Technology: Readout chips (ROCs) bump bonded to high resistivity silicon sensor!
!
Barrel Pixel sensor wafer:
!
Bump bonds:
Thickness 270 µm
n+-pixels in n-bulk with p-spray (CIS) or p-stop (SINTEF) Bump bonding process
Barrel: Indium, In-house at PSI
Forward: PbSn at two vendors (RTI, IZM)
!
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6.3 The CMS Inner Tracker
Barrel pixel: module design!
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6.3 The CMS Inner Tracker
A barrel pixel module !
!
The barrel pixel consists of about 800 modules
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6.3 The CMS Inner Tracker
A half disc of the forward pixel detector!
Forward Pixel: consists of 672 plaquettes
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!
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6.3 The CMS Inner Tracker
PIxel insertion!
Barrel Pixel!
Forward Pixel!
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6.3 The CMS Inner Tracker
Possible discoveries at the LHC: Higgs!
A promising discovery channel for a light Standard Model Higgs Boson is!
ttH"jjb l%b bb:!
Final state 6 jets (4 b-jets), 1 lepton, !
1 neutrino (missing Energy):!
…a superb task for the Silicon detector…!
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6.3 The CMS Inner Tracker
Simulation from CMS: Higgs!
Simulation of a 120 GeV Higgs decaying inside the CMS Silicon tracker:!
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