characterisation of dssd interstrip parameters

6.2.2012

Paul Dolejschi

Characterisation of DSSD interstrip parameters

BELLE II SVD-PXD Meeting

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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



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

3



Switching Scheme (Vienna)

4




Validation of oxide thickness

SEM result: 355nm average from C_ac measurement: 354.2 nmMicron average: 391.8

metal layer

implant

oxide



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



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!



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…



Frequency dependent interstrip capacitance measurement

LCR-meter measures impendance and phase at the same time and then computes capacitance with chosen equivalent circuit.



Comparison of different measurement types

Strip length 12cm



Comparison of different measurement types

Strip length 1cm



Influence of polysilicon resistor

High pass filter



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



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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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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• 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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Fit fails sometimes (often)

failed fit„Fit ok“



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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„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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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



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



Hamamatsu n-sides

~50% „Fit ok“



Hamamatsu n-sides

~96% „Fit ok“



Other frequently onserved effects

• Mainly on Micron p-sides

• „Fit ok“ below 5% (averaged over all sensors from same batch)

• Well reproduceable



Statistics



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

characterisation of dssd interstrip parameters

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