Download Developing silicon strip sensors produced on 8

Photo: © Infineon Technologies AG
IPRD 2016, Siena
Axel König, HEPHY Vienna
Content
• Introduction
• Strip sensors on 8-inch wafers
• Radiation hardness
• Field effect transistors for
p-stop studies
• Summary and conclusion
04.10.2016
Axel König
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Content
• Introduction
• Strip sensors on 8-inch wafers
• Radiation hardness
• Field effect transistors for
p-stop studies
• Summary and conclusion
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Demand for tracking detector upgrades
•
The LHC at CERN is
continuously increasing its peak
luminosity
•
A major upgrade is scheduled
around 2023 during the third long
shutdown (LS3)
 Increase in peak luminosity
by a factor of ~ 5
•
LHC long-term schedule
Necessity for upgrades of the silicon tracking detectors
• Reach the end of their designed lifetime until LS3
 Accumulated radiation damage
• Have to cope with the high luminosity conditions present after LS3
 Higher granularity needed
 More radiation hard sensors
 etc.
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Content
• Introduction
• Strip sensors on 8-inch wafers
• Radiation hardness
• Field effect transistors for
p-stop studies
• Summary and conclusion
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Wafer specifications and split groups
•
•
Specifications
•
Manufactured by Infineon Technologies Austria AG
•
High resistive float-zone p-type base material
•
Resistivity of 7 kΩ cm
•
200 μm physical and active thickness
Split groups
•
22 wafers were delivered to HEPHY
•
Wafers are subdivided into split groups
•
Particular process parameter varied for every group:
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•
P-stop or p-spray strip separation
•
Two different p-spray implantation doses
•
Three different p-stop implantation doses
•
Two different thermal budgets concerning p-stop
i.e. “early” and “late”
•
Five different polysilicon implantation doses
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Layout details
•
•
•
•
Main Sensor
•
Dimension: 15 cm ⨉ 10 cm
•
AC-coupled and biased via polysilicon resistors
•
2032 strips segmented into two parts
•
The design is very similar to the current strip sensor
upgrade design of CMS
Several mini Sensors used for
•
P-stop geometry studies
•
Biasing scheme studies
•
Irradiations purposes
Several conventional test structures
•
Diodes in different sizes
•
Metal-Oxide-Semiconductor (MOS) structures in
different sizes
•
Gate controlled diode
•
…many more
New set of test structures
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New set of test structures
•
•
•
Aims:
1. Process control on a specially
confined structure
2. Suitable for measurements in the
laboratory and at the manufacturer
3. Transfer challenging measurements
to simple ones
Elementary cell is called flute
Implemented Test structures
• Diodes, MOS, GCD
• Meander for sheet resistance
measurements
• Van der Pauw structures
• Field effect transistors in different
designs for p-stop studies
• etc.
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2.2 mm ⨉ 0.6 mm
1 cm ⨉ 1.2 cm
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Electrical characterization – global parameters
• 15 main sensors were characterized
• Global parameters IV and CV
• Most sensors show breakthroughs just above full depletion voltage
• Full depletion voltage of ~ 75 V
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Electrical characterization – single strip parameters
• More than 300.000 measured strips
• Measured parameters per strip
• Single strip current Istrip
• Coupling capacitance Cac
• Polysilicon resistance Rpoly
• Current through the dielectric Idiel
• Results
• Sufficiently small Istrip and Idiel
• No pinholes
• Polysilicon resistance split
groups clearly distinguishable
• Comparable broad distribution in
Cac
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Parameter
Median
Std. dev.
Istrip
184 pA/cm
100 pA/cm
Cac
8.9 pF/cm
1.4 pF/cm
Rpoly (largest)
391 kΩ
33 kΩ
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Inter-strip resistance
• Measured on sample basis only
• One sensor of every p-stop
and p-spray split group
• Inter-strip resistance
measured on 10 strips for
each sensor
• Larger inter-strip resistance for
p-stop “early” split groups
• P-spray and p-stop “late”
samples show similar inter-strip
resistances
 In general, larger inter-strip
resistances are desired
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Investigations concerning coupling capacitance
•
•
Frequency and amplitude dependence
of Cac
• Caused by Schottky contacts at the
strip implantation
Observed broad distribution in Cac
• Caused by non uniform strip
metallization
 Varying areas of the electrodes result
in varying coupling capacitances
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Content
• Introduction
• Strip sensors on 8-inch wafers
• Radiation hardness
• Field effect transistors for
p-stop studies
• Summary and conclusion
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Proton irradiations
•
•
•
•
Two mini sensors were irradiated at KIT in
Karlsruhe with 23 MeV protons
Fluences
• Sample #1: 6.22⋅1014 neq/cm2
• Sample #2: 5.02⋅1015 neq/cm2
Fluences match the expected maximal
particle fluences the CMS Tracker will
experience
Initial investigations:
• Annealing studies
• Change of full depletion voltage with
respect to annealing time
• Current decrease with respect to
annealing time
• Estimation of current related damage rate
• First look at the inter-strip resistance
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Alexander Dierlamm (KIT)
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Annealing studies
•
Both sensors were annealed at a constant
temperature of 60°C for increasing time
durations
•
IV and CV characteristics were measured
after every annealing step
•
Change in full depletion voltage Vdepl
corresponds to the predictions:
1.
Strong increase in Vdepl
directly after irradiation
2.
Vdepl decreases up to a
total annealing time of
~ 80 min at 60°C
Sample #1
3.
•
Sample #2
Vdepl starts to increase
again beyond an annealing
time of ~ 80 min at 60°C
Leakage current continuously
decreases
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Current related damage rate 𝛂
•
General relation: ΔI = 𝛼 Φeg V [1]
•
Instead of 𝛼 the geometrical current related
damage rate 𝛼* is calculated
•
Geometrical volume instead of actual
depleted volume of the sensor is used to
calculate 𝛼*
•
At full depletion, 𝛼* should be equal to 𝛼
•
𝛼* is calculated for every depletion voltage step
•
ΔI was measured after an annealing of 80 min at
60°C and scaled to 20°C measurement
temperature
•
Observations:
•
Calculated 𝛼* is always below the predicted 𝛼value
•
Theory might not be valid anymore for
fluences that large (similar observations made
in [2])
•
Plotting ΔI/V vs. Φeg gives an 𝛼-value of
1.71e-17 A/cm
[1] M. Moll, Radiation damage in silicon particle detectors, (Ph.D.thesis), Universität Hamburg, 1999
[2] S. Wonsak et al., Measurements of the reverse current of highly irradiated silicon sensors, NIMA, 796 (2015), p. 126-130
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Inter-strip resistance of irradiated samples
Sample #1: 6.22e14 neq/cm2
• Inter-strip resistance before irradiation
12 GΩ - 17 GΩ (for p-stop early samples)
2µ
1µ
Strip #1: R
int
= (23.8 ± 0.3) M
W
Strip #2: R
int
= (31.6 ± 0.2) M
W
int
= (5.29 ± 0.04) M
W
-1
2
• Inter-strip resistance measured on sample
basis for three strips for each irradiated
sensor
• Both samples show inter-strip
resistances in the MΩ region
Current (A)
Strip #3: R
0
-1µ
-2µ
-5
-3
-2
0
1
3
4
5
Voltage (V)
Sample #2: 2.05e15 neq/cm2
• Inter-strip resistances after irradiation
are too small in general
2µ
1µ
W
int
= (63 ± 0.9) M
Strip #2: R
int
= (58.5 ± 0.5) M
W
Strip #3: R
int
= (56.5 ± 1.2) M
W
Strip #1: R
Current (A)
• Related to small inter-strip resistances
already present before irradiation
-4
0
-1µ
-2µ
-5
-4
-3
-2
-1
0
1
2
3
4
5
Voltage (V)
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Content
• Introduction
• Strip sensors on 8-inch wafers
• Radiation hardness
• Field effect transistors for
p-stop studies
• Summary and conclusion
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Field effect transistor (FET) test structures
•
•
•
Bias ring
Aim: Investigate p-stop properties using the
threshold voltage Vth obtained from transfer
characteristics of field effect transistors
Main difference to standard field effect transistors:
Two layers of p-stop between source and drain
Simple structure:
• n+ implanted source and drain
• Varying distances of 56 μm, 46 μm, 36 μm
and 26 μm
• Each implanted electrode is surrounded by
p-stop
• Fixed width of 6 μm
• Fixed distance to each electrode of 5 μm
• One common gate electrode
• Gate dielectric consists of SiO2
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Gate
p-stop
Source
Drain
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FET measurement results
•
Transfer characteristics of FET test
structures of every p-stop split group were
measured
•
Expectation: threshold voltage should be
different for every p-stop split group
•
•
Different implantation depths “early”
and “late”
•
Different implantation doses
Results
•
Very similar threshold voltages for all
p-stop late split groups (A, B and C)
•
Increasing threshold voltages for
p-stop early split groups A to C
•
No dependency on pad distance
•
Influence of Schottky contacts on Vth
could be observed (more later)
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Simulating FET structures using Synopsys TCAD
•
Aim: Investigate the usability of FET test structures for p-stop studies by accessing a
larger parameter space using TCAD simulations
•
2D simulations with 260.000 vertices in total
•
Complete FET geometry including biasing
•
Layer thicknesses implemented as observed in SEM images
•
Interface charge implemented according to MOS measurements
•
Gaussian doping profiles and / or 1D-doping profiles gained from SRP measurement
Gate voltage = 6 V
Gate voltage = 12 V
Gate voltage = 30 V
Gate
Source
Drain
Total current density of simulated FETs for varying gate voltages and fixed drain-source voltages. Zoomed into the region of the gate
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Simulations
Measurements
•
Measurements show typical behavior of
Schottky barrier MOSFETs
•
1D-doping profiles of SRP measurements
used for simulation together with TCAD
standard Schottky contact model
•
Transfer characteristics
⨉ Slightly higher drain currents in
simulations
⨉ Smaller Vth in simulations
•
No existing Schottky barrier
 Increasing currents for decreasing pad
distances
•
Output characteristics
⨉ Much higher drain currents in
simulations
 Increasing currents for decreasing pad
distances
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Variation of parameters
•
•
Extracted values
•
Threshold voltage from
transfer characteristics
•
Inter-pad resistance of fully
depleted FET structure
Parameters varied
•
Logarithmic like increase of Vth for increasing
p-stop depths
•
Nearly no change in inter-pad resistance
•
Exponential like increase of Vth for increasing
p-stop concentrations
•
Exponential like increase of inter-pad
resistance for p-stop concentrations up to
1e1016 cm-3
•
P-stop implantation depth
•
P-stop concentration
•
No significant change in Vth
•
Pad distance
•
Exponential decrease of inter-pad
resistance up to 60 μm pad distance
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Content
• Introduction
• Strip sensors on 8-inch wafers
• Radiation hardness
• Field effect transistors for
p-stop studies
• Summary and conclusion
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Summary
• The world’s first AC-coupled silicon strip sensors produced on 8-inch
wafers were characterized
• Issues to be resolved:
1. Non uniformity of strip metallization  spread in coupling
capacitance
2. Adjustment of implantation profiles and doses
3. Process optimizations resulting in higher breakthrough
voltages
•
Irradiated samples show reasonable results and indicate sufficient
radiation hardness
•
Measurements on field effect transistor test structures indicate the
suitability for p-stop characterization purposes
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Outlook
• HEPHY received a second batch silicon strip sensors processed on
8-inch wafers very recently
• Large effort at the manufacturer has been made to resolve the
discussed issues using the input of measurements presented here
• Irradiation studies will continue
• Tuning of TCAD simulations for implementing Schottky contacts
• Tuning of inter-pad resistance simulations for a larger parameter
space
We are close to maturity and looking forward characterizing the
second batch of silicon strip sensors processed on 8-inch
wafers
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Thank you for your attention!
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Backup
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MOS and Diodes
CV MOS IFX
1.6n
• MOS curves very reasonable
1.4n
• Oxide charge density of
2e10 cm2
p-spray
1.2n
Capacitance (F)
• Flat-band voltage of 1.2 V
Wafer No.
01
06
10
15
16
18
20
21
23
24
• P-spray MOS show irregularities in
CV-MOS measurements as
expected
1.0n
800.0p
600.0p
400.0p
200.0p
0.0
-3
-2
-1
0
1
2
3
4
5
6
Voltage (V)
• IV curves of diodes show much
better breakthrough behavior than
main large sensors
 Could be related to defect
density and the smaller covered
area
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Van der Pauw measurements
• Used to determine the sheet resistance of the p-stop
• All p-stop “late” variants show similar sheet resistances
 Corresponds to measurements made on FETs
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Strip measurement indicating Schottky contact
• An IV-curve was measured from DC to DC pad to determine the
resistance of the strip implant
• The shape of the curve indicates an existing Schottky contact
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Preliminary results of the 2nd batch
• Homogenous strip aluminum width
• No frequency dependence in Cac
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