Characterisation of DSSD interstrip parameters

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

3

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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