6.2.2012
Paul Dolejschi
Characterisation of DSSD interstrip parameters
BELLE II SVD-PXD Meeting
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Characterisation of DSSD interstrip parameters
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QTC-Setup
• switching-system• LCR-meter
(measurement of capacitance)
• 2 SMUs (Bias-Voltage, Resistance)
• electrometer (current)• needles, chuck, table• LabView-software• Completley automated
setup
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Characterisation of DSSD interstrip parameters
What have we tested?• Global parameters:
– IV-Curve: Dark current, Breakthrough
– CV-Curve: Depletion voltage, Total Capacitance
• Strip Parameters e.g.– strip leakage current Istrip
– poly-silicon resistor Rpoly
– coupling capacitance Cac
– dielectric current Idiel
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Characterisation of DSSD interstrip parameters
Switching Scheme (Vienna)
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Characterisation of DSSD interstrip parameters
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Validation of oxide thickness
SEM result: 355nm average from C_ac measurement: 354.2 nmMicron average: 391.8
metal layer
implant
oxide
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Characterisation of DSSD interstrip parameters
Interstrip measurements
• Interstrip Capacitance
– Comparison of Frequency dependent measurements on
• Hamamatsu barrel sensors
• CMS-test structure
• Interstrip Resistance
– Hamamatsu• Barrel sensors• 4 batches
– Micron• Wedge sensors• 2 batches, p-stop/p-spray
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Characterisation of DSSD interstrip parameters
Interstrip Capacitance
• Capacitance between– Implants (p+/n+)
• Charge Sharing
– Metal layers (Al)• Cross Talk, Signal to
noise
– Metal layer and implant (AC coupling)
• Separates strip leakage current from readout electronics
→ Electrical Network!
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Characterisation of DSSD interstrip parameters
Interstrip Capacitance
• Different measurement methods– Contacting Implants only (via DC pads)
– Contacting metal layer only (via AC pads)
– Contacting both implants and metal layer• Additional option: Measuring 1, 2 or 4 neighbouring strips
• Slightly different result for each method and/or sensor type – AC or DC coupled structures, different strip length, bias-
resistor,…
– Try to distinguish different contributions of capacitances, restistors etc…
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Characterisation of DSSD interstrip parameters
Frequency dependent interstrip capacitance measurement
LCR-meter measures impendance and phase at the same time and then computes capacitance with chosen equivalent circuit.
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Characterisation of DSSD interstrip parameters
Comparison of different measurement types
Strip length 12cm
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Characterisation of DSSD interstrip parameters
Comparison of different measurement types
Strip length 1cm
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Characterisation of DSSD interstrip parameters
Influence of polysilicon resistor
High pass filter
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Characterisation of DSSD interstrip parameters
Unknown effect of implants in low frequency region
Frequency dependent interstrip capacitance measurement
High frequency: no contribution of implants if strips are long
Low frequency: no contribution of metal layer because of high pass filter
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Characterisation of DSSD interstrip parameters
Conclusion
• High Frequencies:– Above a certain frequency only a small length of the
implant contributes to the capacitance
– The capacitance between the metal layers dominates the observed value when both AC and DC pads are contacted
• Low Frequencies:– Presence of a polysilicone resistor influences low
frequency region high pass filter for metal layers if R_poly is low
– Unknown effect of implants in low frequency region
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Characterisation of DSSD interstrip parameters
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Interstrip Resistance - Measurement Principle
• DC pad #X kept on ground, voltage applied to DC pad #X+1, electromenter measures current on pad #X
• Don‘t want to measure series connection of poly-resistances
• R-poly can be measured at the same time
Strip X
Strip X+1
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Characterisation of DSSD interstrip parameters
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• Usually five voltage steps, slope of the IV curve represents 1/R
• Typical ΔI: 5-20pA
• Typical R_int: 50-200GΩ
• Intersection of R-poly curve at y=0 reveals current of next strip
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Characterisation of DSSD interstrip parameters
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Fit fails sometimes (often)
failed fit„Fit ok“
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Characterisation of DSSD interstrip parameters
Measurement with 3rd SMU for compensation
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introduces current forI_strip compensation
Keeping electrometer in lowestpossible range (200 pA)!
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Characterisation of DSSD interstrip parameters
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„Ideal stripscan“
• Interstrip resistance and polysilicon resistor measured at same time
• Value plotted for each strip
• More than 90% „fit ok“ in this exapmple
• Measurement success
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Characterisation of DSSD interstrip parameters
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Hamamatsu n-sides
• n-side– Similarity in shape– new measurement method
using 3rd SMU for I_strip-compensation (+guarded positioners) - no improvement
– Measurement accuracy high enough to measure >1TΩ
• Similarity between Hamamatsu sensors (all 4 batches)
• Independent of „direction“ of stripscan
HPK #4
HPK #80
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Characterisation of DSSD interstrip parameters
Hamamatsu n-sides
• The higher the strip number, the higher the resistance
• „mean dI“: – after the voltage is applied,
it takes some time (sec) until current is stable
– Difference between first and final value = „mean dI“
– Can be positive or negative– „responsible“ for higher
resistance?
current
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Characterisation of DSSD interstrip parameters
Hamamatsu n-sides
~50% „Fit ok“
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Characterisation of DSSD interstrip parameters
Hamamatsu n-sides
~50% „Fit ok“
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Characterisation of DSSD interstrip parameters
Hamamatsu n-sides
~96% „Fit ok“
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Characterisation of DSSD interstrip parameters
Other frequently onserved effects
• Mainly on Micron p-sides
• „Fit ok“ below 5% (averaged over all sensors from same batch)
• Well reproduceable
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Characterisation of DSSD interstrip parameters
Statistics
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Characterisation of DSSD interstrip parameters
Conclusion
• The overall detector performances (dark current, depletion voltage, radiation hardness,…) are ok, but interstrip resistance measurement is not fully understood
– Reproducable effects on Hamamatsu n-sides and Micron p-sides
– Improvement with growing batch number– Measurement impossible on noisy strips – Effects possibly caused by pn-junction effects,
simulation required