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Interstrip PT Updates

Interstrip PT Updates. John Wright. Introduction. In DC PT tests it was previously assumed that PT occurred between the implant and the bias rail only, most likely at both ends of the implant.

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Interstrip PT Updates

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  1. Interstrip PT Updates John Wright

  2. Introduction • In DC PT tests it was previously assumed that PT occurred between the implant and the bias rail only, most likely at both ends of the implant. • This interpretation was supported by tests showing two tiered PT resistance when a sensor was grounded through an implant adjacent to an implant that was used to perform voltage sweeps. • The tests show that in the range ~-50V to ~-90V interstrip resistance is ~1.5MΩ, confirming that one strip has punched through and one has not, and that the interstrip resistance is significantly higher than the path through one implant to bias PT and then through a bias resistor. • However, tests performed biasing through the bias ring, holding one implant at to ground, and performing a voltage sweep on an adjacent implant broke sensors, indicating that the effect of the neighboring implant was not yet completely understood.

  3. Experiment Description • A new test was performed to further investigate interstrip PT. • A sensor was biased through the bias ring, and a test voltage was applied to one implant. The induced voltage on an adjacent implant was measured with a high impedance instrument. The current flowing from the test strip to the bias ring was also measured. • The implant to which the test voltage was applied will henceforth be labeled Implant 1. The nearest neighboring implant to Implant 1, from which voltage measurements are taken will be labeled Implant 2. The next neighboring implant will be labeled Implant 3. Currents and voltages will be labeled similarly i.e. the voltage on Implant 2 will be labeled Voltage 2 or V2. • It is assumed that all voltages and currents are symmetric about Implant 1. All measurements were made near the middle of the sensor. • All sensors were biased at 200V.

  4. Analysis • Using the known current from Implant 1 to ground we can infer the current from implant 2 to ground. This is because the current leaving the Implant 1 for some given voltage on the Implant 2 should be equal to the measured current leaving the Implant 1 when the Implant 1 was at that same voltage. • The current from the Implant 2 to ground must be coming from the test implant, and therefore this current is equal to the interstrip current. • By a similar process I3 can be calculated. No appreciable current should be flowing to Implant 4 for all sensors tested. • By subtracting 2*(I2+I3) from Itest we receive I1. By using I1 to calculate I2 and I3 we ensure that the interstrip current is not included in the approximation of the current from the implant to the bias ring. • Using V1, V2, V3, I1, I2, I3 we can calculate all of the relevant resistances.

  5. Results • Wafer 71 Zone 1, 2, 3, 4D, and 6 as well as Wafer 50 Zone 2, 3, and 6 have been tested • In general the Interstrip PT effect contributes significantly to the the total resistance from Implant 1 to ground. • All data and analysis, including plots, can be found in the spreadsheet “DC_PT_neighbor.xls”

  6. Plot shows comparison between Rtotal (Vtest/Itest) and R1eff as defined on slide 5 for several sensors (same sensor->same color) @ 100V test voltage. • The data illustrate the contribution of the interstrip resistance.

  7. Comparison to Laser Data • Previous tests have shown that during laser irradiation a broad profile of implants surrounding the focus of the laser beam are brought to high voltages. • In DC tests, only implants up to two strips away achieve significant voltages. • Therefore the broad profile seen during laser irradiation is not due to interstrip punch-through. A possible explanation of the profile is diffusion of charge carriers within the bulk of the sensor during a laser test. • Therefore the circuit from the implant to ground during a laser test is different from the circuit of the DC tests: in a laser test the neighbors of the target strip also collect charge and achieve high voltages and the voltage difference between neighbors is insufficient for interstrip PT, while in DC tests interstrip PT occurs and significantly lowers the effective implant to ground resistance of a single test implant. • Therefore the implant to ground resistance measured in laser tests should correspond to R1eff, while in DC tests Rtotal is measured. The plots on the next slide compare the measured resistance from laser tests to R1eff and Rtotal from DC tests.

  8. Comparison to Laser Data cont.

  9. An Odd Resistor • The interstrip resistance behaves oddly. The top plot shows the interstrip resistance from Implant 1 to Implant 2 as a function of the potential difference between the two implants (ΔV). • ΔV decreases after interstrip PT. V2 increases after interstrip PT. Increasing V2 means increasing I2. I2 must come from interstrip PT from Implant 1 to Implant 2. Therefore I2 increases as ΔV decreases, which means R interstrip pt 1->2 must decrease as ΔV increases. • When R Interstrip pt 1->2 is looked as a function of I2+I3 (Both I2 and I3 must flow from Implant 1 to Implant 2) different sensors exhibit similar behaviors with the exception of BZ1, which lacks interstrip isolation. • The odd behavior of the interstrip resistance could be indicative of false assumptions in the analysis. • Thermal runaway might provide another explanation

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