6.4 Results and Discussion
6.4.1 Experimental Results
Results of the backside TPA measurements are shown in Figure 6.3. Figure 6.3a is a top view of the device taken from its GDSII layout file. The drain and source designations assume an off-state bias condition. That is, Vdrain= 0 V, Vwell= 1 V, Vsource=1 V, Vgate = 1 V, and Vsub= 0 V. Three locations shown in Figure 6.3a are labeled with the letters b, c, and d. The locations of these letters represent the approximate strike location that produced the transients shown in Figures 6.3b-d. For the measurements shown in the figure, the laser pulse energy was approximately 300 pJ. The laser spot was visually focused on the surface of the device, similar to the procedure described in Chapter 3.
Figure 6.3b shows the transient response measured at the well, source, and drain termi- nals when the laser spot is centered in the source region of the device (i.e., placed at location b in Figure 6.3). At first glance, it appears that charge is not being appropriately conserved (i.e., the integrals of the transients shown in each plot do not sum to zero). However, re- call that the substrate current was not monitored during these measurements. The substrate
Figure 6.3: Results of the backside TPA measurements for the large PMOSFET device.
The top-side view of the device from its GDSII layout file is shown in a). The source, gate, and drain have been labeled. The letters shown in these regions (b, c, and d) correspond to the approximate location of the laser strikes that produced the transients shown in figures b, c, and d respectively.
current would appropriately account for charge conservation during the measurements.
As was typical of the results described in Chapter 4, the well transients shown in Fig- ure 6.3 are positive-going, which is indicative of the collection of electrons at the well/sub- strate junction. The source transient is negative-going, indicating the collection of holes at the source/well junction. This is an obvious result when it is compared to earlier re- sults for the n-well/p-substrate diode. The collection of holes should result in a negative- going transient because the collection of electrons at the well/substrate junction results in a positive-going transient. Taken alone, the direction of any particular transient might not be
especially meaningful; however, their direction relative to one another is important as it can be used to interpret charge collection mechanisms at a particular junction. For example, a positive-going well/substrate transient for this particular experimental setup is indicative of electron collection at that junction (which is apparent from discussions in earlier chapters).
Therefore, a negative-going transient at the source terminal should be expected, due to the collection of holes at the source/well junction.
The large transients seen at the struck junctions are due to the direct collection of charge carriers generated by TPA at those junctions. However, in addition to these transients, Figures 6.3b-d show small positive-going transients occurring at junctions that were not struck directly. For example, Figure 6.3b shows a small positive-going drain transient following a strike to the source. A similar positive-going source transient can also be seen for strikes to the drain (Figure 6.3d), and a strike to the gate region of the device can produce small positive-going transients at both the source and the drain. These transients are shown in greater detail in Figure 6.4, which shows the same transients as are shown in Figure 6.3b-d, only with reduced axis limits to emphasize the smaller transients.
There is a small positive-going transient measured at the drain terminal. The fact that it is opposite in polarity to the transient measured at the source terminal (which is due to the collection of holes), might lead one to conclude that the drain is injecting holes across the drain/well potential barrier. However, this is likely an erroneous conclusion.
The voltage drop across the drain/well junction is approximately 1.7 V, which is the applied voltage at the well plus the built-in voltage drop across the junction. Following a carrier generation event in the n-well, well potential modulation effects can significantly lower the well potential in the region surrounding the carrier generation. Earlier work has shown that, in some cases, the potential can be lowered enough to forward bias individual p-n junctions in the well [58]. However, for the particular biases voltages used here, the well potential cannot be lowered enough near the drain/well junction to ever forward bias the drain/well junction. Said another way, the well potential near the drain cannot go lower
(a) Source Hit (b) Gate Hit
(c) Drain Hit
Figure 6.4: The same transients shown in Figures 6.3b-d for a TPA hit to the source, gate, and drain of the large PMOSFET. Here, the axis limits have been reduced to emphasize the smaller transienst shown in Figures 6.3.
than the applied voltage at the drain following a strike to the source, which eliminates the possibility of significant carrier injection by the drain following a strike to the source. This is shown below using device-level TCAD simulations.
Figure 6.5 shows identically measured transients to Figures 6.3b-d, only under broad- beam 10 MeV/u Xe irradiation instead of backside TPA exposure. Due to the nature of broadbeam heavy ion testing, the only way to determine where an ion struck on the de- vice is through interpretation of the device response. Using the TPA measurement results
(a) 10 MeV/u Xe Drain Strike (b) 10 MeV/u Xe Source Strike
Figure 6.5: A comparison of transients resulting from 10 MeV/u Xe irradiation on the large PMOSFET device. a) shows a drain strike while b) shows a source strike. Strike locations were determined from TPA measurement results.
described earlier, we can infer that any ion strike giving rise to a positive well transient, negative drain transient, and positive source transient is due to an ion strike to the drain region. Similar reasoning can be applied to identifying current transients that are the result of an ion strike to the source region. While we cannot be certain where in the drain or source region a particular ion struck, we can know by the device response that a strike did occur somewhere within those regions. What results is Figure 6.5a, which shows current transients resulting from a Xe strike to the drain, and Figure 6.5b, which shows current transients resulting from a strike to the source. The bias condition for the measurements shown in the figure as the same as that for the measurements shown in Figure 6.3.
The key result of Figure 6.5 is that the same response seen during TPA testing is also observed during heavy-ion irradiation. Well potential modulation effects can induce a tran- sient response at junctions that were not directly hit by the ion strike. The impact of well potential modulation on the overall transient response of this device is investigated through device-level TCAD simulations in the section that follows.