1. 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

2. 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/cm (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
Used to be around 5 Volts
Sensor Meeting, Tracker Week, 11/04/02
Anna Macchiolo, PQC Firenze

3. 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
Sensor Meeting, Tracker Week, 11/04/02
Anna Macchiolo, PQC Firenze

4. 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

5. 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 squares
Sensor Meeting, Tracker Week, 11/04/02
Anna Macchiolo, PQC Firenze

6. 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 1E08 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

7. 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

8. 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

9. 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

10. 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 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

11. Anna Macchiolo, PQC Firenze
R-int measured on 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
29 M
50 V
20 V
112 G
5041
808 K 
228 G 
5077
178 K 
100 V
150 V
146 K 
5100
140 K 
200 V
138 K 
5187
169 M
5 M
Sensor Meeting, Tracker Week, 11/04/02
Anna Macchiolo, PQC Firenze