1. Axel König, HEPHY Vienna
Developing silicon strip sensors produced on 8-inch wafers for upcoming upgrades of LHC experiments
Photo: © Infineon Technologies AG
IPRD 2016, Siena
Axel König, HEPHY Vienna

2. Content Introduction Strip sensors on 8-inch wafers Radiation hardness
Field effect transistors for p-stop studies
Summary and conclusion
Axel König

3. Content Introduction Strip sensors on 8-inch wafers Radiation hardness
Field effect transistors for p-stop studies
Summary and conclusion
Axel König

4. 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.
Axel König

5. Content Introduction Strip sensors on 8-inch wafers Radiation hardness
Field effect transistors for p-stop studies
Summary and conclusion
Axel König

6. Wafer specifications and split groups
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:
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
Axel König

7. 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
Axel König

8. New set of test structures
Aims:
Process control on a specially confined structure
Suitable for measurements in the laboratory and at the manufacturer
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.
2.2 mm ⨉ 0.6 mm
1 cm ⨉ 1.2 cm
Axel König

9. 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
Axel König

10. Electrical characterization – single strip parameters
More than 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
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
Axel König

11. 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
Axel König

12. 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
Axel König

13. Content Introduction Strip sensors on 8-inch wafers Radiation hardness
Field effect transistors for p-stop studies
Summary and conclusion
Axel König

14. 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
Alexander Dierlamm (KIT)
Axel König

15. 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:
Strong increase in Vdepl directly after irradiation
Vdepl decreases up to a total annealing time of ~ 80 min at 60°C
Vdepl starts to increase again beyond an annealing time of ~ 80 min at 60°C
Leakage current continuously decreases
Sample #1
Sample #2
Axel König

16. 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 𝛼- value
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
Axel König

17. Inter-strip resistance of irradiated samples
Sample #1: 6.22e14 neq/cm2
Inter-strip resistance before irradiation GΩ - 17 GΩ (for p-stop early samples)
Inter-strip resistance measured on sample basis for three strips for each irradiated sensor
Both samples show inter-strip resistances in the MΩ region
Inter-strip resistances after irradiation are too small in general
Related to small inter-strip resistances already present before irradiation
Sample #2: 2.05e15 neq/cm2
Axel König

18. Content Introduction Strip sensors on 8-inch wafers Radiation hardness
Field effect transistors for p-stop studies
Summary and conclusion
Axel König

19. Field effect transistor (FET) test structures
Bias ring
Gate
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
p-stop
Source
Drain
Axel König

20. 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)
Axel König

21. 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 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
Axel König

22. 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
Axel König

23. Variation of parameters
Logarithmic like increase of Vth for increasing p-stop depths
Nearly no change in inter-pad resistance
Extracted values
Threshold voltage from transfer characteristics
Inter-pad resistance of fully depleted FET structure
Parameters varied
P-stop implantation depth
P-stop concentration
Pad distance
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
No significant change in Vth
Exponential decrease of inter-pad resistance up to 60 μm pad distance
Axel König

24. Content Introduction Strip sensors on 8-inch wafers Radiation hardness
Field effect transistors for p-stop studies
Summary and conclusion
Axel König

25. Summary
The world’s first AC-coupled silicon strip sensors produced on 8-inch wafers were characterized
Issues to be resolved:
Non uniformity of strip metallization  spread in coupling capacitance
Adjustment of implantation profiles and doses
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
Axel König

26. 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
Axel König

27. Thank you for your attention!
Axel König

28. Backup
Axel König

29. MOS and Diodes MOS curves very reasonable Flat-band voltage of 1.2 V
Oxide charge density of e10 cm2
P-spray MOS show irregularities in CV-MOS measurements as expected
IV curves of diodes show much better breakthrough behavior than main large sensors
Could be related to defect density and the smaller covered area
p-spray
Axel König

30. 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
Axel König

31. 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
Axel König

32. Preliminary results of the 2nd batch
Homogenous strip aluminum width
No frequency dependence in Cac
Axel König