Anna Macchiolo, PQC Firenze Update on the Inter-strip Resistance problem in the recent STM production PQC Firenze: Mirko Brianzi, Anna Macchiolo, Florentina Manolescu, Matteo Sani Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze Bulk N-- 508 m Thermal oxide Vapox PSG Aluminum N+ Passivation (PSG) Backside aluminum STM explanation: one of the two concurring effects in the diminished inter-strip isolation is the decrease of the fixed charge density in the inner oxide layer The recent production has a positive charge density into this triple oxide layer of: 5E9 ÷ 1E10 charges/cm2 (used to be NF = 5E10 ÷ 1.0E11 charges/cm2) This reflect also in the lower Vfb that we measure in the MOS for the last shipments PISA 115-121 Used to be around 5 Volts Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze The same effect explains the decrease of the surface generated current measured on the GCD The average surface current used to be around 40 pA PISA 110-114 Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze The second cuncurring effect is a slightly increase in the negative charges of the passivation oxide that is not measured in the MOS and GCD (since it is not deposited under the Aluminum layer). These negative charges are no more compensated by the positive ones in the inner oxide. Passivation oxide (PSG) N+ Al P+ + + + + + + + + + - - - - - - - - - - - - + + Backside aluminum Bulk N-- Two actions taken in STM: Removal of the equipment that introduced an excessive negative charge Introduction of a gate on the R-int value Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze STM measures R-int on its own test-structure (placed in one of three the not-standard half moons) with the back kept floating since this is believed to be the worst conditions. The STM test-structure corresponds to 2 squares of inter-strip material. The CMS CAP-TS-DC to 0.015 squares Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze Comparison between the measurements on the CMS and STM test-structures: these batches were delivered in October and November before the introduction of the gate on Rint The STM results have been rescaled taking into account the different number of squares STM Rint gate The value of the STM Rint gate is put at 1E08 relative to the direct measurement on the RS_ISO structure (1E 05 after rescaling to compare with our measurements) The gate identifies the three batches that are affected according to the standard CMS criteria Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze Measuring the CMS test-structure with the back left floating improves the comparison with the STM Rint measurements STM Rint gate Leaving the back floating the sensitivity to the presence of negative charges in the passivation oxide is enhanced The STM gate is reliable Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze The last measured batches (@ Vbias=20 V ) show a clear improvement in the R-int performance N.B. The values inserted in the plot are obtained after the full PQC scan Good at 50 V bias Good at 50 V bias Good at 50 V bias Good at 100V bias Good at 100V bias 1 GΏ Delivery date Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze Dependance on the humidity of the Rint behaviour Some affected structures from old batches have undergone a thermal treatment (TT): kept in the oven for 3 hours at 90o . 1,00E+03 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08 1,00E+09 1,00E+10 1,00E+11 1,00E+12 2 4 6 8 10 12 Structure number Rint (Ohm) Rint before TT Rint after TT Rint after TT and one week in dry environment For very low values of the Rint before TT the improvement is mainly in the bias voltage at which the RInt value comes back within our specification (typically from 200 to 100 V) STM Rint gate The results are stable after TT and one week at low humidity in the dry cupboard. This constitutes a warning on the ambiental conditions at which the sensors are stored: humidity affect the Rint behaviour Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze A proposal In view of the better performance of the latest STM production in terms of Rint and the reliability of the STM gate on this parameter we would benefit of a change in our Rint acceptance criteria Rint > 1 G Ώ @ 100 V This would simplify our current procedure: for the structures failing our present cut we now increase in a time-consuming way the bias voltage starting from 20 V up to the voltage where the measured Rint is larger than 1 G Ώ (and in the latest STM production this voltage is always lower than 100 V) 100 V is a safe value since it is considerably lower than the bias voltage foreseen for the sensors in the normal tracker operation (300 V) Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze
Anna Macchiolo, PQC Firenze R-int measured on CAP-TS-DC @ 20 V bias corresponding to the sensors used in the US to build the R-int affected modules Module # I Sensor # R-int Voltage at which RInt > 1 G II Sensor # 5036 32016112 29 M 50 V 32016113 20 V 112 G 5041 32016111 808 K 32016101 228 G 5077 32016114 178 K 100 V 32016103 150 V 146 K 5100 33600223 140 K 200 V 33215723 138 K 5187 32016120 169 M 32016117 5 M Sensor Meeting, Tracker Week, 11/04/02 Anna Macchiolo, PQC Firenze