4.3 Results and Discussion
4.3.1 Device Response to Strikes Near the Contacts
(a) Backside TPA transients (b) 10 MeV/u Xe transients
Figure 4.3: A comparison of transients at different strike locations on the DUT for both TPA pulses and 4.5 MeV/u Ta ions.
To mitigate the effect of fluctuating pulse energy on the interpretation of the results, any transients resulting from laser pulses with energies outside one standard deviation of the mean pulse energy were not considered in the analysis. For this work, the mean pulse energy is reported. The laser pulse energy that produced any single device-level transient shown is within 5% of the reported mean.
Figure 4.3 shows a comparison of 4.5 MeV/u Ta induced transients and TPA-induced transients. The TPA-induced transients have been used to deduce the strike locations of the ion-induced transients. Three distinct groupings are apparent in Figure 4.3. One grouping, labeled “Outside junction” shows transients resulting from strikes that missed the junction, but still had significant carrier diffusion from the strike location to the depletion region to produce a measurable transient current response. Correlating the TPA-induced transients to their strike locations shows that the transients with the greatest peak current are the result of TPA pulses incident on or very near the contacts (the LC and RC positions shown in Figure 4.1). Strikes away from the contacts, toward the center of the device, produce transients with lower peak current values that are significantly wider in time. Various scans over the same area shown in Figure 4.1 at different laser pulse energies reveal that strikes near the contacts cause the greatest peak current, regardless of the incident laser pulse energy. This change in device response to strikes near the contacts can be used to infer the location of the broadbeam ion strikes.
For the transients shown in Figure 4.3, the strikes near the center resulted in the greatest total collected charge. For the 4.5 MeV/u Ta, the center strike resulted in 7.8 pC of total collected charge while the strike near the contacts resulted in 6.5 pC of total charge collec- tion. Because the contacts are located near the edge of the n-well diffusion, strikes there could result in a significant amount of the total generated carrier density diffusing away from the junction after the strike. For strikes to the center, the junction is situated above the generated carrier density, therefore charge generated beneath the junction by the ion strike has an increased likelihood of being collected as it diffuses outward from the strike
(a) (b)
(c)
Figure 4.4: The simulated device used for determining the significance of the n-well contact for determining the device response and the simulated ion strike direction and location.
(a) shows the simulated device and its dimensions and doping types. (b) is an expanded view of the region contained in the black square shown in (a) that denotes the applied bias voltages and strike location. (c) is a cutplane through the device showing the location and direction of the simulated ion strike as well as a cutline indicating the location of the potential cutlines shown in Figure 4.5.
location.
The transients with higher peak currents near the contacts can be explained in terms of well potential modulation (WPM) phenomenon [58, 61–63]. Ion strikes in an n-well can significantly modulate the n-well potential. It has been shown that increasing the n-well contact area and density can mitigate this effect by providing a shorter, lower-resistance
−60 −40 −20 0 20 40 60
−0.5 0 0.5 1 1.5
X Location (µm)
Electrostatic Potential (V)
Pre−Strike 0.5 ns 1.0 ns 5.0 ns
(a)
−60 −40 −20 0 20 40 60
−0.5 0 0.5 1 1.5
X Location (µm)
Electrostatic Potential (V)
Pre−Strike 0.5 ns 1.0 ns 5.0 ns
(b)
Figure 4.5: Electrostatic potential along a cutline through the entire width of the n-well at three time points following a simulated ion strike. The ion strike is located at approximately -27 µm along the horizontal axis. The simulated devices used in (a) and (b) are dentical, except that (a) simulates the contacts as they are found in the real device, while the left-most topside contacts have been removed in (b).
path for generated carriers to leave the well (thus limiting their ability to significantly mod- ulate the n-well potential) [58, 64, 65]. For the diode described here, the n-well contacts are only near the left and right (when referring to Figure 4.1) sides of the n-well. For strikes near the n-well contacts, their presence serves to hold the potential very near the bias volt- age, limiting WPM effects. It also provides an efficient path for excess electrons to leave the well. Both of these effects result in a transient with a shorter duration and a higher peak for strikes near the n-well contacts.
The significance of striking near the contacts for the current transient response was investigated through device-level simulations using the Synopsys Sentaurus TCAD tools [66]. Because these simulations required the ion strike to pass through the contacts, rather than the center of the device, full three-dimensional simulations were used. The simulated device is shown in Figure 4.4a. Figure 4.4b is an expanded view of the region inside the black rectangular box shown in Figure 4.4a. The simulated device was designed to represent the device shown in Figure 4.1. The contacts are 1 µm wide, 1.6µm apart, and run the entire length of the n-well diffusion.
The ion used for these simulations was 4.5 MeV/u Ta. TRIM was used to determine the charge density of the ion track along a linear coordinate in the direction of the ion strike (i.e, the LET as a function depth in the silicon) [67]. This information was used to define the heavy-ion induced carrier generation in the simulator. This allows for a better approximation to the heavy-ion induced charge generation than assuming a constant LET along the entire track length would. The simulated ion strike was assigned a Gaussian radial profile with a characteristic diameter of 100 nm. The ion strike was normal to the top surface of the device. Carrier mobility, recombination, and carrier-to-carrier scattering models were identical to those used in Chapter 3. The ion strike was normal to the device surface and originated at the location shown in Figures 4.4b and 4.4c. For all simulations, the device was reverse biased at 1 V.
To determine the influence of the contacts on the device response, different contact schemes were simulated. In each scheme, only the left-most contacts were altered (i.e., the contacts shown in the black rectangle of Figures 4.4a). The right-most contacts on the top surface, and the backside p-substrate contact were identical for all simulations.
The influence of the contacts is most readily observed in plots of the electrostatic po- tential in the n-well. To demonstrate this, Figure 4.5 shows the potential along a cutline that runs parallel to the top of the device and passes through the center of the n-well and ion track (see Figure 4.4c). In the figure, identical strike and bias conditions are shown for two different contact schemes. For the simulation results shown in Figure 4.5a, the contacting scheme was identical to that shown in Figure 4.4a. In other words, the topside n-well and p-well contacts are present as they can be found in the real device. In Figure 4.5b, the left- most topside contacts have been removed from the simulated device. Otherwise, the two simulated devices are identical. For both sets of simulations, the ion strike is positioned at -27µm, the center of the device is at 0 µm, the right-most contacts are at 27µm, and the left-most contacts (if present) are located at approximately -27µm.
Figure 4.5a shows the potential at various times during the strike when both of the left-
most contacts are present. The presence of the n-well contact serves to hold the potential very near the bias voltage (which is shown here as the applied bias plus the built-in junction voltage) at the strike location throughout the duration of the strike. The n-well contact also provides a nearby sink for excess carriers in the n-well. Figure 4.5b shows the potential along an identical cutline as Figure 4.5a, only with the p-well and n-well contacts on the top left side of the device removed. Without the presence of the n-well contact, the po- tential in the well rapidly collapses. Without the n-well contact to provide a hard tie to the bias voltage, a potential gradient forms between the generated carrier density and the n-well contact on the other side of the well. This potential drop sets up a field that serves to accelerate electrons toward the n-well contact. In the context of the results shown in Figure 4.3, strikes to the center of the junction would lead to a similar well potential col- lapse as the one shown in Figure 4.5b due to the strike location being far from either of the topside n-well contacts. This is consistent with the typical description of WPM effects.
Device-level WPM effects are discussed in greater detail in Chapter 6.