Unless otherwise stated, all transient capture measurements were performed using a large-area (50 µm x 50 µm) bulk silicon n-well over p-substrate diode fabricated using Jazz Semiconductor’s CA18HD process. This is a 180 nm non-epitaxial bulk CMOS pro- cess featuring dual wells. While the transient response of a large-area diode is not neces- sarily representative of the transient response of modern, highly-scaled technologies, there are practical reasons for studying the transient response (and underlying charge-collection mechanisms) of a large junction; specific examples include single-event latchup, multiple- bit upsets, and other phenomena that could be attributed to well potential modulation ef- fects. A thorough understanding of large-junction response is also helpful (and potentially necessary) for understanding related phenomena in small junctions, where interpretation of the response can be complicated by small feature sizes. A discussion of the transient response for a smaller junction is included in Chapter 5.
A to-scale, recolored image of the diode from its GDSII layout file is shown in Fig- ure 4.1. The light blue and dark blue vertical stripes correspond to p-well and n-well con- tacts, respectively. The horizontal blue stripes are metal lines that do not contact the silicon.
Figure 4.1: A to-scale, top-down view of the tested device taken from a GDSII layout file.
Locations pertinent to the TPA measurements are shown.
The vertical gray stripe near the bottom of the image represents a metal line that provides bias to the n-well contacts. The dark red region corresponds to the n-well diffusion. Sev- eral locations are labeled on the figure, which are significant for the TPA measurements described later. The “center” position is the center of the n-well diffusion. The “LC” and
“RC” labels denote the locations of the left and right sets of n-well and p-well contacts, respectively. The “start” and “stop” labels indicate the TPA laser scan beginning and end points.
The location and magnitude of each doping concentration was determined through spreading resistance measurements. The n-well is approximately 800 nm deep and has a peak doping of approximately 5x1017 cm−3. This is a blanket p-well technology, there- fore at any location where n-type dopants are not specifically implanted, the material is p-type. The peak p-well doping is approximately 4x1017cm−3. The p-substrate has a dop- ing of approximately 2x1015 cm−3 and contains no buried layers. The diode is covered by approximately 11 µm of SiO2 overlayer material and polysilicon fill. The overlayers over the active diffusion have no metal fill. A representative cross-section of the device produced during device-level simulations is shown in Figure 4.8 (the simulation setup and
results are discussed in Section 4.2). For all measurements, the diode was reverse biased at 1 V unless otherwise stated. The diode’s reverse bias leakage current did not exceed 10 nA throughout the duration of the heavy ion irradiations or the TPA measurements.
Transients were recorded using a high-speed Tektronix TDS 6124C oscilloscope with a 12 GHz bandwidth and 25 ps sampling resolution. Devices were packaged similarly to what was described earlier (see Figure 3.3). The experimental setup was otherwise identical to what was described in Chapter 3.
Heavy ion irradiations were performed in vacuum using Lawrence Berkeley National Laboratories’ 88-inch cyclotron facility. All ions used for the experiments were from the 4.5 MeV/u cocktail. The incident LET values in Si for each ion species are as follows (in units of MeV-cm2/mg): Ne, 5.8; Ar, 14; Y, 46; Ag, 58; Ta, 87. Each ion’s range in silicon is approximately 50µm.
Laser irradiations were performed at Vanderbilt University using an optical parametric generator operating at a wavelength of 1260 nm with a nominal pulse width of approxi- mately 150 fs. The wavelength is sub-bandgap for silicon, resulting in carrier generation by TPA. The incident laser pulse energy is monitored in real time using a calibrated InGaAs photodiode. The beam is focused through a 100x (NA 0.5) objective. For all measurements, the device under test was mounted on an automated precision linear stage with a minimum step size of 0.1 µm in the X, Y, and Z directions. More information about this setup is included in Appendix B.
The charge generation spot size in silicon was measured in a manner consistent with a typical knife-edge optical spot size measurement [60]. For this measurement, a separate large-area bulk-silicon n-well over p-substrate diode fabricated on the same wafer as the test structure shown in Figure 4.1 was used. Half of the diode’s active area is covered by overlayer metal on metal layer one, while the other half is covered by SiO2and polysilicon fill only. Because the wavelength of the beam is sub-bandgap for silicon, the measurement yields the charge generation spot size, rather than the true optical spot size. Having the
−4 −3 −2 −1 0 1 2 3 0.5
1 1.5 2 2.5
Z−position (µm)
10−90 Spot Size (µm)
Figure 4.2: The results of the knife-edge measurement at various focusing depths around the beam waist. The charge generation spot size at the waist is approximately 1.2µm.
metal “knife-edge” embedded in the device overlayers, and thus as close as possible to the active silicon, helps to limit the effects of diffraction on the measurement results. This is a similar approach to earlier work concerning measurements of the TPA effective spot size in a bulk silicon device [57].
For this work, the charge generation spot size in the vicinity of the beam waist was measured. This was determined by focusing the beam on top of the metal coverage and then moving in small steps from the region covered by metal into the region with no metal coverage. The TPA-induced current transients were recorded at each position, and their peak values were plotted as a function of position. The convention chosen here for the spot size is the distance between 10% and 90% of the maximum peak current when it is plotted as a function of position. Repeating this scanning procedure at various focusing depths around the waist and determining the spot size for each scan produces the data shown in Figure 4.2. As indicated in the figure, the value for the charge generation spot size at the beam waist is approximately 1.2µm.
(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.