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Figure 9. Photocurrent induced (at maximum laser power) vs.transistor lentgh (W=10um). — Figure 9 Photocurrent induced (at maximum laser power) vs.transistor lentgh (W=10um).

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Figure 1. Measured I-V curve of P+/Nwell junction vs. laser power. The Device Under Test (DUT) is embedded in a 90nm technology test structure and its surface is about 200um?. The experimentation was realized on an i-Phemos Hamamatsu system. The laser spot was positioned in the center of the junction and laser power was adjustable below 100mW. Current-voltage (I-V) characteristic was measured for different laser power, and presented on Figure 1. When the PN junction is reverse-biased under 0.6V, the photocurrent is almost constant independently of applied voltage since the SCR width does not vary significantly. The induced photocurrent is higher than the junction reverse leakage and depends on laser power. Then, when applied voltage raises over 0.6V, the SCR width starts to decrease and consequently the amount of induced photocurrent is also reduced. When the applied voltage reaches diode built-in potential (black cross on Figure 1), the SCR decreases, the induced photocurrent is therefore reduced and becomes negligible with respect to the junction forward current. Moreover, the so far negligible resistances of the P and N-type regions start to play a significant role in forward mode and get reduced under the illumination, which leads to a current increase with laser intensity.

Figure 3. Simulated I-V curve of P+/Nwell junction vs. laser power. I-V characteristics of the junction are simulated in function o: the intensity (from 100 W.cm? to 500 W.cm7”) of the 1064 nm smooth wave simulator (Figure 3). Results are correlatec to measurements, currents levels are not identical tc measurements because TCAD simulator is not calibrated to fit silicon results, it is only used to have trends.

Figure 2. Structure used for junctions TCAD simulations.

Figure 4. Long channel transistor currents vs. laser power (transistor in OFF state). A ma aa OFF state: A 10um 3 x © 10m PMOS transistor is biased as follows: Gate, Source and Nwell are biased at 1.2V and the Drain is grounded. Evolution of these four electrodes currents in function of the laser power is given in Figure 4. Obviously the gate current is always equal to zero. The current conservation law is respected since the sum of all currents is always equal to zero. Moreover, the same quantity of current is approximately generated in Drain/Substrate and Source/substrate junctions.

Figure 5. Long channel transistor currents vs. laser power (transistor in ON state). decreases since the starting up in conduction of the channel eads an additional current which opposes to the circulation of Source photocurrent. On the other hand, |Igain| increases because it comes to be added to the normal circulation of electrons in the channel (due to its starting in conduction). Finally, the same quantity of photocurrent is induced in the transistor in both ON and OFF states (Figure 6): approximately 30 uA in Source-Nwell or Drain-Nwell junctions and 65 wA in Nwell-Psubstrate junction (at maximum laser power).

Figure 6. Long channel transistor photocurrent generation difference between OFF and ON states (in this case subtraction between the current values obtained with and without laser stimulation).

Figure 7. Short channel transistor currents vs. laser power (transistor in OFF state). OFF state: A 10um x 0.09um PMOS transistor is used. Evolution of electrodes currents in function of the laser power is presented in Figure 7. The quantity of photocurrent induced is approximately three times smaller than in the long channel transistor case (for example Iprain-short channel= ~8-1WA and Ipyain-

Figure 8. Short channel transistor currents vs. laser power (transistor in ON state).

Figure 10. Long channel transistor currents vs. laser power (transistor in OFF state). OFF state: The same experiment than in part A is done with the p-Substrate grounded. Results are presented in Figure 10. Gate current is always equal to zero, current conservation law is respected. When the p-Substrate is grounded, photocurrents collected at Drain and Source are small compared to the photocurrent induced in Nwell-Substrate junction. In this case we can consider that the laser stimulation has a dominating effect only on the Nwell-Substrate junction.

Figure 11. Long channel transistor currents in function of laser power (transistor in ON state).

Figure 12. Long channel transistor photocurrent generation difference between OFF and ON states (in this case subtraction between the current values obtained with and without laser stimulation).

Figure 13. Doping concentration mapping and SCR displayed in white lines (a) and doping profile of the mapping along the white dashed line (b)

Figure 14. Short channel transistor currents in function of laser power (transistor in OFF state). channel -OnA, Tywelt-short channel 1.6mA and TNwell-long channel— 1.48mA).

Figure 15. Short channel transistor currents in function of laser power (transistor in ON state). In conclusion, the same magnitudes of photocurrent seem to be induced whether the transistor is short or long channel, in ON or OFF state.

Figure 16. Measurements of photocurrent induced (at maximum laser power) in function of transistor lentgh (W=10um). In conclusion, when the p-substrate of a long channel PMOS transistor is grounded, the same quantity of photocurrent is theoretically induced whether the transistor is short or long channel. In practice, more photocurrent is induced in transistors owning gate length smaller than the laser spot diameter.

Figure 17. Simulation of photocurrent induced (at maximum laser power) in function of transistor lentgh (W=10pm).

Conclusions