Laser simulation of single-particle effects - Nuclear Science, IEEE

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ON NUCLEAR SCIENCE, VOL. 39, NO. 6 , DECEMBER 1992

Laser Simulation of Single-Particle Effects C.A. Gossett, B.W. Hughlock, and A.H. Johnston High Technology Center Boeing Defense and Space Group Seattle, WA 98119 doubled Nd:YAG lasers (0.53 pm), have quite shallow penetration depths. Figure 1 compares d e r generation of these lasers as a function of depth into silicon material. The Earlier work has indicated that pulsed picosecond lasers penetration of a 0.53 pm laser is extremely shallow and most can produce charge tracks in semiconductors which are of the energy is absorbed very near the surface. The 0.9 pm similar to the charge tracks produced by heavy ions[1.21, and dye laser has a penetration depth of approximately 19 pm that in many cases this allows a laser to adequately simulate which, as will be shown later, is insufficient for reproducing single-particleeffects with greater convenience and lower cost single-particle effects in lightly doped materials where charge compared to particle accelerators. Although there are may be collected over depths of 30-40 pm. similarities in the charge generation from lasers and heavy The usual method of applying lasers for simulation of ions, differences in the radial characteristics of the resulting single-particle effects in integrated circuits involves using a ionization tracks can affect the equivalence between them. microscope objective to focus the beam to a small spot. This Several factors make it difficult to calibrate laser intensity for provides a way to determine the location of the laset pulse on testing or screening purposes including optical reflection the circuit, However, due to the relationship between losses and the spreading of the light within the semiconductor numerical aperture and resolution, a small spot size can only due U, refraction. In addition, the carrier density in the be obtained by using objective lenses with large incident ionization track produced by the laser can be two or more angles. Specifically, the numerical aperture, N.A., equal to orders of magnitude less than that for an ion back This may the product of the index of refraction and the sine of the angle have serious implications on the suitability of the laser to of incidence, determines the resolution, R through the reproduce the charge funnelling process characteristic of relationship: heavy ions. Diffused charge may also be greater when a laser is used due to increased penetration depth compared to accelerator produced ion beams. Although it is possible to calibrate a laser with heavy-ion experiments on a specific set For example, the minimum spot size would be approximately of devices, the issue of track density and its effect on prompt 1.6 pm in diameter for a numerical aperture of 0.4. Thus when focussed to a small spot, the laser beam enters the charge has not been thoroughly examined This paper examines laser simulation in more detail, device a relatively large angles. This has two important investigating the effects of doping density and charge effects. First, it increases surface reflection losses and makes collection depth on funnelling and charge collection, and it much more difficult to calculate reflection losses since the accounting for the spreading of the focussed laser beam within simple formulas for normal incidence no longer apply. the silicon material. Calculations using the device simulation Second, it causes the laser beam to diverge with increasing code PISCES are compared for heavy ions and lasers to distance into the material due to refraction. investigate the accwacy of laser simulation of single-particle effects. Experimental charge collection measurements with 1-.o- I I heavy ions are used to establish the accuracy of the PISCES simulations.

INTRODUCTION

E T

LASER PROPERTIES

normalird t0.ud.c.

Although lasers with various wavelengths have been used for single-particle upset simulation, a pulsed picosecond Nd:YAG laser with wavelength 1.06 pm is probably best suited for realistic simulation results. At a wavelength of 1.06 pm the absorption coefficient in silicon is approximately 40cm-l at room temperature and hence the l/e penetration depth is roughly 250 pm. By contrast, lasers with shorter wavelengths, for example dye lasers (0.9 pm) or frequency

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Figure 1. Laser absorption as a function of depth into silicon for wavelengths 053.0.9, and 1.06 p.

@018-9499~92$03.00 0 1992 IEEE

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For normal incidence, surface reflection will be about 30% at an aidsilicon interface. In practice, there is a surface layer of Si02 which further complicates surface reflection. The thickness of the Si02 layer may cause constructive or destructive interference, which also depends on the angle of incidence. The net effect is a relatively large uncertainty in surface reflection. This must be measured or realistically calculated in order to get reasonable agreement between heavy ion and laser results. Note that surface reflection is also affected by beam polarization which is an additional complication. The high index of refraction in silicon causes the angle at which the laser diverges within the material to be much lower than the incident angle. Nevertheless, for a highly focussed spot at the surface, the spot sue increases substantially with incmsing distance into the material. For a numerical aperture of 0.4 as above and spot size of 1.6 pm, the beam diameter will have increased to 2.7 pm diameter at a depth of 5 pm. Laser charge generation is affected also by the energy and energy density. Calibration methods must take into account all of these factors: refraction, reflection, and energy density.

HEAVY ION EXPERIMENTS

The test devices were originally designed as large area latchup test structures. For the measurements reported here they were connected as large reverse biased diodes. Charge collection measurements as a function of bias were obtained for junctions diffused into the substrate and for junctions diffused into a well. The contacVsubstrate junctions were rectangular rings 4 p n wide with outside dimensions 180 pm by 96 pm. The contacVwel1 junctions had rectangular dimensions 106 pm by 28 pm. The devices were fabricated with 2 micron p- and n-well processes and arrays of 1449 devices were connected in parallel on each die to provide large sensitive areas for charge collection measurements. Realistic doping profiles were obtained from spreading resistance measurements. Table 1. Ion Parameters Ion

Energy

B C

Charge collection measurements were performed at the University of Washington tandem Van de Graaff accelerator using the ions listed in Table 1. The test devices were connected through a charge sensitive preamplifier to a pulse shaping amplifier and then to a multichannel analyzer. The charge collection measurements were calibrated with measurements of the energy deposited by the incident particles in a silicon surface barrier detector located adjacent to the test devices.

m) 23 43 36 22 21

18 37 58

F Si Cl

Incident LET meVcm2/m g)

Range

65

79

2.7 2.7 5.5 12.3 16.4

The charge collected in an n+/p-substrate junction as a function of bias for the ions listed in Table 1 is shown on the left side of Fig. 2, while that measured for the p+/n-well junction is shown on the right. Note the factor of two difference in vertical scales. The well doping of the device was 5 ~ 1 0 ~cm-3 7 while the substrate doping was 7x1Ol4 cm-3.

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Figure 2. Charge collected for ions listed in Table 1 as a function of applied bias. Left Charge collected in a n+/psubstrate junctions. Right: Charge collected in a p+/n-well junction.

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The voltage dependence of the collected charge for the n+/p-substratehas a similar shape as that observed by M c h and Oldham [3,41. The charge collected in the p+/n-well junction is much less voltage dependent. In addition, the charge collected in the p+/n-well junction scales linearly with ion linear energy transfer (LET), while the charge collected in the n+/p-substrate junction has a much less straightforward LET dependence. This is illustrated further in Figure 3. The incident energies of the carbon and boron ions listed in Table 1 were chosen to compare results for ions with the same LET but with ranges in silicon differing by nearly a factor of 2. The amount of charge collected in the n+/p-substrate junction is clearly sensitive to the effect of ion range while that collected in the pe/n-well junction is nearly independent of range. I

1

mobility were applied. Of particular importance is the inclusion of carriercarrier scattering in the version of PISCES used. Carrier-carrier scattering has large effects on the radial and t h e dependent diffusion of charge, particularly for the very high carrier concentrations found along the ionization tracks produced by the charged particles or laser beam. Indeed the lack of success of the earlier work of Knudson and Campbell [6] to reproduce measured charge collection waveforms may have been due to the absence of caniercarrier scattering in the version of the code used by those authors. Path of incident particle

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Doping densities: n+ contact = p- substrate

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= 7 x IOl4 cm-3 (Case 1) 7 x 1015 cm-3 (Case 2) 7 x 1016 ~ 1 1 7 (Case .~ 31

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Figure 3. Charge collected in pt/n-well and nt/psubstrate junctions for ions of varying LET. See Table 1. Applied bias of 5 volts.

MODELING The semiconductor device simulation code PISCES was used to examine the suitability of laser simulation of singleparticle effects by comparing the charge collection from a moderately heavy particle with that from a laser. The continuity equations for holes and electrons and Poisson's equation are solved self-consistently in PISCES for the electrostatic potential and the electron and hole concentrations. The geometry chosen for the modeling reported in this work is shown in Figure 4. The geometry was chosen to be cylindrically symmetric about the ionization track. For this case, the 2dimensional calculation was effectively 3-dimensional. The version of PISCES used was that from Technology Modeling Associates version 9033 [5]. Auger and ShockleyRead-Hall recombination, as well as band gap narrowing were included. Models for both field and concentration dependent

Figure 4. Device geometry used for charge collection modeling.

The results of measured charge collection in the n-well process test structure are compared with PISCES predictions in Figure 5. The finite ion range and linear energy transfer dependence on depth were included in the calculations. Quite good agreement is observed, with PISCES reproducing the light ion results quite well and overestimating the charge collection for the heavier ions by approximately 20%. Note that the simple cylindrical geometry described above was used for the calculations although the actual geometry was a complicated 3-dimensional one. Having established the success of PISCES in reproducing measured charge collection results, one may now apply PISCES simulations to compare charge collection from heavy ions and lasers. Shown in Figure 6a are contours of electron density and electrostatic potential calculated for ion and laser tracks 10 picoseconds after incidence. For both the ion and l a w tracks the LET as a function of depth into the silicon was assumed to be constant. The incident laser beam spot size was assumed to be 1.5 gm in diameter with a divergence of 7' in the silicon. The device modeled was a -substrate device with a rather low doping density of 7x10p4 ~ m - ~As. mentioned earlier, cylindrical symmetry about the ion or laser track was assumed. These tracks appear along the left edges of the

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figure. The dashed curves extending primarily in the vertical The central track direction are contours of electron densi density for the ion track is roughly 2 ~ 1 0 ~ 9 while for the laser it is more than two orders of magnitude lower, approximately I x ~ O ~ ~The effect of the divergence of the laser beam within the silicon is evident. 4.0

3*5

1

t

I 0 PISCES

Measured 0

that funnelling was exhibited by lasers with extremely high LET, greater than 50 MeV-cm2/mg. The present results are in agreement with that work. That is, for extremely high LET where the central Carrier density in the ionization track is much larger than the background doping density, charge funnelling with a laser is expected in spite of the very large track diameter limited by focussing of the laser beam. However, looking ovec a broader range of LET and doping density, particularly for the high doping densities required for aggressively scaled devices, this will clearly not be the case. For high doping density or low LET the laser does not reproduce heavy ion results primarily due to the effect of substantially reduced d e r density in the ionization track.

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Incident Energy IMeV)

Ions:

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Figure 5. Comparison of calculated and measured total charge collection for boron. carbon, fluorine, silicon and chlorine ions. See Table 1.

The solid cuives in Figure 6 =present contours of constant electrostatic potential. One observes clear evidence of funnelling along the ion and laser tracks, that is, a strong distortion of the potential contours along the track. Contours of electron density and electrostatic potential 0.5 nanoseconds after incidence are shown in Figure 6b. At these times the funnelling has essentially collapsed and the induced charge density along the particle or laser track has diffused radially. The results of PISCES calculations indicate that the prompt charge collected for the laser is in good agreement with that from the heavy ion while the diffused charge collected from the laser track is reduced by approximately 20%. Thus for the low doping density device modeled here, the laser does appear to repmduce the funnelling effect which would be observed for a heavy ion, and the prompt charge collected is in good agreement. In contrast, funnelling is not observed for the laser simulation for high doping density. For example, electron density and potential contours 10 picoseconds after a laser strike are shown in Figure 7 for a device with substrate doping of 7x1Ol6 In this case no funnelling is observed for the laser track. The results of the present work indicate that in certain cases, for low doping density or high effective LET, that a laser of an appropriate wavelength can be used to simulate single-particle effects in silicon. In previous work, Buchner, et al[7], concluded from total charge collection measurements

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Figure 7. Calculated contours of electron density and tential for laser track 10 ps after incidence. Substrate doping 7 x l O G :3

Another important factor in comparing heavy ions and laser simulation is ion range. Most experimental work on charge collection has included use of ions with ranges of 1525 pm [3,4,8], with the assumption that the prompt charge collected would be unaffected as long as the ion range exceeded the funnelling depth. However the range of ions encountered in space environments and the effective range of a 1.06 pm wavelength laser in silicon far exceed the funnelling depth. Figure 8 shows PISCES calculations of prompt and total collected charge for ions with various ranges, but with constant LET of 3 MeV-cm2/m along the particle range. A subslrate doping of 7x1Ol4 cm’ $ for a diode reverse biased at 5 volts was assumed. The modeling mults indicate that the prompt charge does not saturate until the range of the ion exceeds by about 50% the funnelling depth estimated from the ratio of carrier mobilities. In order to reproduce the total charge that would be collected, an ion range roughly twice the estimated funnelling depth would be required for the doping density and LET assumed here. These effects become more

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Particle

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Figure 6. Calculated contours of electron density in electrons/cm3 (dashed curves) and potential in volts (solid curves) for ionization tracks (along left edges, axes of cylindrical symmetry) from LET = 3 MeV-cm2/mg "particles"and laser beams. Substrate doping 7x1014 c m 3 . (a) Contours 10 ps after incidence. (b) Contours 0.5 ns after incidence.

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pronounced for higher LET. Thus an important conclusion of the present work is that care must be taken in selecting laboratory ions with sufficient range for simulation of singleprtrticle effects in a space environment where the ion ranges can be quite long, or for calibration of laser intensity in a specific device. 1.0

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t Funneling depth based on mobility ratio I

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120'

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Figure 8. Calculated pmmpt and total charge as a function of range for "particles" with constant LET = 3 MeVcmqmg.

calibration of effective laser LET, particularly for devices in which the shunt effect is observed with ions.

CONCLUSIONS The semiconductor device simulation code PISCES was used to examine the suitability of laser simulation of singleparticle effects in simple silicon diodes. For low doping density devices or high effective linear energy transfer, laser simulation of single-particle effects appears to be valid. However, care must be taken to choose ions with sufficiently long range in order to calibrate the laser intensity and corresponding effective LET. Calibration of effective laser LET compared to heavy ions should be performed by charge collection measurements with structures embedded in wells where differences between heavy ions and lasers due to the density of the ionization track, variation of charge deposition with depth, and the dependence of collected charge on effective range will be minimized. For high doping density or low effective LET, lasers do not reproduce the charge funnelling effects observed with heavy ions. The results of modeling predictions mdicate that ions of range much larger than the expected funnelling depth for a device should be used for laboratory simulation of space environment particles which may have quite long ranges in silicon.

REFERENCES

The modeling and heavy-ion charge collection data [ l ] S.P. Buchner. D. Wilson, K. Kang, D. Gill, and J.A. Mazer, discussed above have serious implications for attempts to "Laser Simulation of Single Event Upsets", IEEE Trans. Nucl. calibrate the intensity or effective LET of a laser relative to Sci., Vol. 34, pp. 1228-1233, December 1987. heavy ions. The results illustrated in Figures 2 and 3 indicate that charge collection for the n+/psubstrate junction is clearly [2) A.K. Richter and I. Arimura, "Simulation of Heavy Charged Particle Tracks Using Focused Laser Beams", IEEE Trans. sensitive to factors such as ion range and applied bias, while Nucl. Sci., Vol. 34, pp. 1234-1239, December 1987. charge collection in the contact/well junction appears to be limited by the well depth and is linear with incident LET. [3] F.B. McLean and T.R. Oldham, "Charge Funnelling in N- and PTherefore one recommendation of the present work is that Type Si Substrates", IEEE Trans. Nucl. Sci., Vol. NS-29, pp. 2018-2023, December 1982. charge collection for structures embedded in a well should be used to calibrate laser intensity compared to heavy ions. [4] T.R. Oldham. F.B. McLean, and J.M. Hartman. "Revised Complication of laser versus heavy-ion calibration due to the Funnelling Calculations for Heavy Particles with High W&", inherent differences in the carrier density in the ionization IEEE Trans. Nucl. Sci.. Vol. NS-33, pp. 1646-1650, December track, divergence of the laser beam within the material, 1986. differences in the depth dependence of the charge deposition and the effects of ion and effective laser range on totai and [ 5 ) Technology Modeling Associates PISCES version 9033, Technology Modeling Associates, Palo Alto, CA. prompt charge collection may be reduced by calibration using structures embedded in wells. Due to the limitation of charge [6] AB. Knudson and A.B. Campbell, "Comparison of Experimental collection depth by the well itself, the effects of the Charge Collection Waveforms with PISCES Calculations", IEEE Trans. Nucl. Sci., Vol. 38, pp. 1540-1545. December differences mentioned above are significantly reduced. 1991. Note that the "ion shunt effect" [9,10] has neither been observed experimentally nor modeled in this work. Based on (71 S. Buchner, A. Knudson, K. Kang,and A.B. Campbell. "Charge the results discussed above, for ions and devices where the ion Collection from Focussed Picosecond Laser Pulses", IEEE shunt effect is not observed, one would expect that the above Trans. Nucl. Sci.. Vol. 35, pp. 1517-1522.December 1988. conclusions and recommendations regarding calibration of laser effective LET using junctions embedded in wells would [8] R.S. Wagner, N. Bourdes, J.M. Bradley, C.J. Maggiore, AX. Knudson, and A.B. Campbell, "Alpha-, Boron-, Silicon-, and remain valid. Further modeling and experimental work may Iron- Ion-Induced Current Transients in Low Capacitance be needed to quantify the importance of the shunt effect in Silicon and GaAs Diodes", EEE Trans. Nucl. Sci.. Vol. 35, pp. 1578-1584,December 1988.

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191 A.R. Knudsen, A.B. CampbeU, P. Shapiro, WJ. Stapor. E.A. Wolicki, EL.Petersen. SE. Diehl-Nagle, J. Hauser, and P.V. Dressendorfer, "Charge Collection in Multilayer Structures", IEEE Trans. Nucl. Sci.. Vol. NS-31,pp. 1149. December 1984.

[lo] A.R. Knudsen. A. B. Campbell, J.R. Hauser, M. Jesse, WJ. Stapor,and P. Shapiro, "Charge Transport By the Ion Shunt Effect", IEEE Trans. Nucl. Sci., Vol. NS-33. pp. 1560, December 1986.

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