Hamamatsu vs SGS Thomson

Hamamatsu vs SGS Thomson Layer00 detectors

Strip Failures	@ 50V	@ 100V	Comments
SGS Thomson (~2.5 detectors measured)	2.8%	3.7%	Those failing at 100V do not subsequently hold any voltage. See example
Hamamatsu	<0.5%	<0.5%	We have no reason to disagree with the manufacturers own results which indicate <0.1% See example

Specifications: < 1.28% bad strips. Max bad strips: 2 (type A), 1 (type B) > 100V coupling capacitor breakdown

NOTE:

Some of the detectors/regions studied show the very long time constant behaviour (as measured in Liverpool and by Ronan and Tara at FNAL). For these we cannot measure the strip failures as at short times the currents are up to mA levels (these are not included in the bad-strip estimates). This behaviour needs further studies as there is clearly some associated `memory' with strips which have been held at high voltage for any length of time, subsequently showing much faster recovery from the application of high voltage.

Strip Resistance	Total (Ohms)	Ohms/cm	Comments
SGS Thomson	280	36	Out of specifications
Hamamatsu	190	24

Specification: < 30 Ohms/cm

I-V characteristics	Reverse current @ 350V [nA]	Reverse current @ 500V [nA]	Comments
SGS Thomson "Type B"	~15	~20
Hamamatsu "Type B"	~50	~50

Specification: 4uA (type A) , < 2uA (type B)

NOTE:

SGS Thomson values look remarkably low (here you find the plot). Anyway, after irradiation the current is dominated by the induced defects in the substrate so starting currents are not terribly significant.

Bias-resistor Values	Det. #1 [kOhms]	Det. #2 [kOhms]	Det. #3 [kOhms]	Comments
SGS Thomson	760+/-20	800+/-20	840+/-20	Out of specification
Hamamatsu	2400+/-100	2400+/-100	2400+/-100

Specification: 2.5 +/- 0.5 MOhms

NOTE:

We measured very low resistor values on the 4th detector (30-40 kOhms). Actually, this result was obtained after the strip quality measurement, when a bias of -100 V was put through the coupling capacitors. Measuring the resistors with the voltage across the oxide resulted in this low value. Grounding the strips and repeating the measurement resulted in a resistor value in good agreement with that reported above.

Interstrip Capacitance	pF cm^-1	Comments
SGS Thomson	~ 0.8	See note
Hamamatsu	~ 1	See note

Specification: < 1.2 pF cm ^-1 interstrip capacitance (The total capacitive load will also include a ~0.2 pF cm^-1 contribution to the backplane.)

NOTE:

The interstrip capacitance is genuinely lower for the SGS Thomson at the frequencies we can study (it looks to get closer to the Hamamatsu values at higher frequency:see plot). There is voltage dependence but a resonable estimate is 1pF/cm for the Hamamatsu and 0.8pF/cm for the SGS Thomson. ATLAS results after irradiation showed that <100> material such as Thomson may be using did show a lower interstrip capacitance than the <111> used by Hamamatsu but with a very strong frequency dependence. The difference reduces with frequency and as far as the actual results with fast electronics are concerned (ie at 40MHz) no appreciable difference in noise was observable for irradiated detectors.

Full depletion voltage:

We have not checked the depletion voltages. We know these are supposed to be well over 100V for the SGS Thomson but this would not in itself a major issue except that the capacitors are not rated to the required depletion voltages. For Hamamatsu all the results on strip failures are at 100V which is well above the required operating voltage of the detectors.

Specification: 60V < V_dep <100V

FINAL REMARKS

Over 2 dozen Hamamatsu detectors to these designs (but much larger area) have been irradiated by ATLAS in charged hadrons at the CERN and KEK PS machines with doses of 3 10¹⁴ p cm^-2. All devices have survived and simply show the expected rise in depletion voltage and reverse currents. Most of these detectors have been studied after irradiation with ATLAS fast read-out and so the noise and any failures on the 768 strips have been looked for. The detector satisfy the ATLAS stringent specifications which include the requirement that the number of strips failing after irradiation remains below 1%. Devices have also been irradiated with neutrons but charged hadrons are mostly what we get in the experiment and the differences are relevant.
Has any similar systematic post-irradiation studies been carried out on SGS Thomson detectors using charged hadrons?
All detectors supplied by Hamamatsu have been put by the company through a rigorous measurement programme and we know from ATLAS that the Hamamatsu QA is completely reliable. Micron are also to be trusted in terms of their measurement QA. What is the SGS Thomson QA?
Other companies (most notable SEIKO for BELLE) have attempted to start up in this business and all of them have failed for the first one or two iterations to get devices that work. SEIKO were dropped by BELLE because of this. Even for a good company there is a learning curve and on this basis the SGS Thomson results look pretty good for a first iteration.