Jo Beck 
6/28-8/05
2004
Silicon Strip Detectors

This internship has involved working with silicon strip detectors like the ones that will be used in the Gamma-ray Large Area Space Telescope (GLAST). The strip detectors will, specifically, serve as precision trackers in the large area telescope. This tracker consists of a four by four arrangement of stacks of silicon strip detectors and lead converter sheets. There are nineteen pairs of silicon planes in each stack, with the pairs of silicon strips perpendicular to each other. Gamma-rays strike the lead, causing an electron and a positron to be emitted. When the silicon is hit with these particles, the trajectories of the particles can be traced in three dimensions; two from the perpendicular silicon strips and the third from the tracks of the particles through the stacks.
	In 2007, when GLAST is launched, it will be used to map the sky with respect to gamma-ray emissions. Many different gamma-ray sources, such as gamma-ray bursts, blazers, pulsars, and solar flares, will be studied in order to be better understood.
	Working with graduate student Jessica Metcalfe, one of our first tasks was to take the total bulk capacity measurement of a silicon strip detector in relation to the voltage. The setup to take this measurement involved placing one probe tip on the bias pad. The other probe tip was in contact with the copper plate the SSD was resting on to ground it. We then ran different voltages through the bias probe from 10v to 100v.
The CV curve that we graphed for this test did not turn out as we would have expected it to based on previous tests by other people. 

	The next step was to redo the measurement to see if we could explain why our results were so far off of what was expected. One of the probe tips we were using in our setup was bent, and we suspected it was not making good electrical contact with the silicon strip detector. After replacing the probe tip, we redid the test and this time came up with a more reasonable graph for the CV curve. 
We found that there was a frequency dependence for the capacitance. Because we had misinformation, we thought this dependence was not supposed to occur. We measured the bulk capacity of a photodiode vs the current to see if the same frequency dependence was present in order to determine if it was being caused by something in our setup. The graphs we produced from this data appeared normal and did not show a frequency dependence. 
	At this point, past research was found showing that finding a frequency dependence for silicon strip detectors when measuring capacitance was normal. We were able to use our capacitance measurements to make a graph of voltage vs 1/(capacitance)^2. We then used this to determine the depletion voltage. Depletion voltage is the voltage at which 1/C2(V)-1/C2(V-5)<0.039 1/nF2. Assured that our setup was working as expected and that we were comfortable working with the equipment, we moved on to a different detector.
This time we wanted to compare the performance of a skinny detector to that of a baby detector. The wafer we were using had both of these on it.
	The first thing we wanted to do with the detectors was to measure the IV curve for each of them. To do this, we needed two probes. One probe, which we called SMU1 and supplied variable voltage, was touched down on the copper plate that the silicon strip detector was resting on. The other probe, SMU2, was constant at zero volts and was touched down on the bias ring. The IV curve was measured from 0 volts through 100 volts. We repeated this procedure for both the skinny detector and the baby detector. 
	The next measurement we wanted to take was front-to-back capacitance, but first we had to do corrections so that our data would be accurate. The short correction required two probes, both touched down on the backplane. Only SMU1 was set up to have 50 volts. We set the compliance of the semiconductor parameter analyzer at 10μA. After the short correction was completed, we performed the open correction. For this one, one probe was still touching the backplane, but the other was over the bias ring, near the pad, but not touching it. We had 50 volts going to the backplane probe. The precision LCR meter performed all of the corrections after we had the probes set up.
	Once the corrections were completed, all we had to do to measure capacitance was move the bias ring probe down until it was touching the pad, then we recorded the capacitance of each detector (skinny and baby) at frequencies from 1kHz to 1MHz and voltages from 10 volts to 100 volts. 
	The final comparison we wanted was of the interstrip capacitance for each detector. This measurement required three additional probes and the same short and open corrections as the front-to-back capacitance measurement. One of the probes was SMU1, and it would be touched down on the backplane. The second was SMU2, and it would be touched down on the bias ring of the detector. Two others were set up for low potential, and were intended to be touched down on the outer of three neighboring silicon strips, while the fifth probe was connected to the high potential and was designated for the center of the three strips.
	Once we had the probes hooked up appropriately, we had to perform the corrections. For the short correction, four of the probes were set to touch the copper plate, with only SMU2 touched down on the bias ring. For the open correction, SMU1 remained on the backplane and SMU1 stayed on the bias ring, but the other three were moved onto the pads of neighboring silicon strips. The low potential probes were on the pads of the two outer silicon strips. The center strip had the high potential probe near it, but not touching it.
	After the corrections were done, we touched the high potential probe down on the center silicon strip and were ready to measure the interstrip capacitance. We did this in much the same way as we did the front-to-back capacitance, recording the capacitance at the same range of frequencies and voltages. 
	With the data we collected from this set of measurements, we were able to compare the baby detectors to the skinny detectors. 
These are examples of the variety of measurements we have done with different silicon strip detectors. They have helped us become comfortable with the detectors and the equipment. This familiarity will be necessary when Jessica moves on to do more complex work with the silicon strip detectors.