IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 41, NO. 6, DECEMBER 1994 2195

Implications of the Spatial Dependence of the Single-Event-Upset Threshold in SRAMS Measured with a Pulsed Laser

S. Buchnef, J.B. Langworthy, W.J. Stapor, A.B. Campbell Naval Research Laboratory

Washington DC 20375 &

S. Rivet Harris Semiconductor Schaumberg I1 60173

Abstract

Pulsed laser light was used to measure single event upset (SEU) thresholds for a large number of memory cells in both CMOS and bipolar SRAMs. Results showed that small variations in intercell upset threshold could not explain the gradual rise in the curve of cross section versus linear energy transfer (LET). The memory cells exhibited greater intracell variations implying that the charge collection efficiency within a memory cell varies spatially and contributes substantially to the shape of the curve of cross section versus LET. The results also suggest that the pulsed laser can be used for hardness-assurance measurements on devices with sensitive areas larger than the diameter of the laser beam.

I. INTRODUCTION

SEU error rates are calculated by integrating the product of two curves - one relating differential cross section (0) to ion linear energy transfer (LET), and the other relating environmental abundance of ion species to LET.[ 11 Clearly, the shape of o(L) is important in determining the overall error rate, and considerable effort has been devoted to investigating what factors influence the shape.

To determine a&), the circuit is exposed to various energetic accelerator ions, and the number of upsets per unit fluence as a function of incident LET is measured. A simplifying assumption frequently used for most large-area devices is that the depletion layer associated with an SEU- sensitive junction can be regarded as a thin parallelepiped, and charge deposited in the depletion layer by an ion is rapidly collected via drift in the junction electric field. It follows from this simple picture that a&) has the shape of a step function at an LET threshold (Lo). The amount of charge (Q) deposited by an ion in the junction is obtained from the ion’s LET, the length of the track through the depletion layer, and the average energy it takes to produce one elecuon-hole pair. All ions

traversing the parallelepiped with LETs greater than Lo will deposit more charge in the cell than the cell’s critical charge (Q,,), and o(L)=osAT. Ions with LETs smaller than Lo will not cause upsets, and a&) = 0. For this simple model, only two parameters are needed to calculate error rates - Lo and

For many devices, however, 00.) differs significantly from

a gradual increase with increasing LET above Lo, discontinuities, where two different cross sections are measured for ions with the same effective LET, a failure to saturate at high LET values.

a step function, exhibiting,

In fact, error rates calculated from measured values of o versus LET can be up to an order of magnitude lower than error rates using only Lo and qAT. Therefore, to obtain more accurate values of error rate, o(L) must be measured over the appropriate range of LET. Identifying the origins of these differences should enhance our understanding of SEU phenomena and may, ultimately, aid in the design and manufacme of more SEU-immune circuits.

Numerous mechanisms have been proposed to explain the complicated dependence of cross section on LET. For instance, various authors have suggested that,

the gradual rise in 06) is caused by: stochastic processes associated with ion-energy loss in thin layers,[2] cell-to-cell variations in upset threshold due to processing-related variations in cell properties, affecting either the amount of charge collected or the critical charge of the ce11,[3] intracell variations in upset threshold due to deviations from the parallelepiped model,[4] contributions to upset from multiple junctions in a single cell, [3]

the presence of discontinuities has been atuibuted to: - change of ion LET due to energy loss in overlying

materials,[3] - finite size of the depletion depth,[3]

SFA Inc., Landover. MD 1

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- funnel effects,[5] - two ions with vastly different energy but having the

same LET.[q the failure of the cross section to saturate at high LETS is due to: - charge generation outside the junction and

subsequent diffusion of the charge to the junction. VI.

For this work, we concentrate on identifying the causes of the gradual rise of o(L). Previously, Kohler and Koga[81 measured the feedback times that control the upset threshold by measuring the distributions of write times for a 64K and a 256K SRAM, both hardened to SEUs by the addition of decoupling polysilicon resistors. They showed that there was a statistical distribution of write times with a standard deviation of about 8% of the mean, which they attributed to variations in the resistances of the polysilicon decoupling resistors. The SEU threshold for each cell was determined by its decoupling resistor value. Therefore, there is a direct link between the distribution of write times and the distribution of SEU thresholds. Reasonably good fits were obtained between the actual measured values for o(L) and the values calculated from the distribution of write times. For those two devices, the intracell distribution of decoupling resistor values accounts for the shape of a&).

More recently, Cutchin et al. measured the number of cells that upset as a function of ion fluence for a 1K GaAs C- HIGFET SRAM[9]. They selected an ion with an LET that gave a cross section equal to 5% of oSAp Assuming that the individual cell upset thresholds follow a statistical distribution, only 5% of the cells would be expected to upset. However, they found that, at high fluences, the number of errors approached half the total number of cells in the SRAM, suggesting that all the cells could upset at that LET, and that many of them had switched states more than once. Consequently, the slope of oQ for the C-HIGFET was due to intracell rather than intercell variations in Lo.

We have used a pulsed picosecond laser to further investigate the origins of the finite slope of o(L). By selecting older devices with large areas, one can effectively ignore contributions from stochastic processes. and consider, instead, the other three mechanisms. Our approach involved directly measuring the spatial dependence of the upset thresholds in three different types of SRAMs, using a pulse of light focussed to a spot with a diameter of about 1 pm. The devices selected were the 93L422 (4x256) bipolar SRAM, the HM6504 (4Kx1) unhardened CMOS SRAM, and the HM6504RRH (4Kx1) SEU-hardened CMOS SRAM. The small size of the focussed laser beam facilitates the measurement of the spatial dependence of both the intercell and the intracell upset thresholds. All three memories have sensitive areas considerably larger than the size of the laser spot for easy, nondestructive probing. The fact that the laser can be used to

generate SEUs in microcircuits and to provide reliable relative upset thresholds has been demonstrated in a previous publication[lO]. It is, therefore, well suited for measuring the spatial dependence of the upset threshold.

I1 EXPERIMENTAL CONDITIONS

The experimental setup has been described in detail in a previous publication.[ 111 For producing upsets, we selected the frequency-doubled component (A = 0.53 p) of the Nd:YAG laser, instead of the fundamental mode output, because the smaller diameter (“1 p) of the focussed laser light improved the spatial resolution of our measurements, and the larger linear absorption minimized nonlinear effects, such as two- photon absorption and band-gap renormalization. At that wavelength, the light intensity, and, therefore, the carrier density, decreased to l/e of its value at the surface in a distance of 1.5 p, comparable to the thickness of the silicon epitaxial layer. This was sufficient to generate charge in all junctions sensitive to upset. Because, the spatial resolution also depended on the accuracy with which we could position the focussed light on a sensitive junction, we mounted the circuit on an x-y stage having a minimum step size of 0.1 pm. Upset thresholds were determined by measuring the minimum laser pulse energy (E,) needed to produce an upset, and E, was assumed to be proportional to Lo. No attempt was made to calculate the equivalent LET for the laser pulses, because we were interested only in relative values. However, previous measurements with the laser gave upset thresholds that were 50% higher than the published values obtained with ions.[9]

Cell addresses were determined by writing a “1“ to all the cells, upsetting a particular cell with the laser, and reading the address of the cell that upset. Once the cell of interest was identified, all subsequent measurement were done by writing to and reading only that cell.

111. DEVICES TESTED

Two different types of SRAMs were selected for this investigation, each with its own unique characteristics. The 93L422 is an SEU-sensitive bipolar device with an upset threshold of less than 1 MeV-cm2/mg. Its cross section increases gradually over nearly two orders of magnitude in LET.[2] In contrast, the HM6504RRH is an SEU-hardened CMOS SRAM with an upset threshold greater than 80 MeV- cm2/mg due to the addition of decoupling polysilicon resistors. There is no published data for a&) for the HM6504FtRH. However, there is data for the HM6504RH which has the same layout except for the decoupling resistors.[l2] The data shows that the cross section for the HM6504RH rises much more rapidly with LET than for the 93L422. One would expect that the presence of the decoupling resistors in the HM6504RRH part would result in a larger intercell variation in Lo than in the HM6504RH part.

M Une

word une

I I

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Fig. 1 Memory cell for the 93L422 bipolar SRAM. When latched with data, the collector/base and collector/substrate junctions of the "off" uansistor are sensitive to SEU.

i) 93L422

The electrical layout of the 93L422 is shown in Fig. 1. The memory cell contains two bipolar transistors with double emitters and two pull-up resistors, each in parallel with a diode. When latched, one of the transistors is conducting and the other is nonconducting. Ion strikes to the collector/substrate (c/s) or collector/base (c/b) junctions of the "off" transistor will generate upsets. The part's SEU sensitivity is well characterized and exhibits a very low SEU threshold (L0=0.6 MeV-cm2/mg). The c/b junction partially covers the c/s junction and the two compete for charge during exposure to ion or laser beams. Evidently, it would be a gross simplification to assume that the sensitive volumes are parallelepipeds.

A- B

Fig. 2 is a photomicrograph of a single memory cell. The large dark areas are free of metal making it easy for the laser to be used to probe for upsets. Using the laser we were able to show that one half of the cell was sensitive to upsets when loaded with a "1" and the other half was sensitive when loaded with a "0". Furthermore, the input circuitry is designed in such a way that, when all cells are loaded with the same information, the data in adjacent columns is stored on opposite sides of the cells, i.e., with all cells containing the same information, those in even-numbered columns (counting fiom the left side of the chip) are sensitive on the left side of the memory cell whereas those cells in odd-numbered columns are sensitive on the right side.

ii) HM6504RH and HM6504RRH

Both the HM6504RH and the HM6504RRH are CMOS SRAMs with six-transistor memory cells for which the drains of the "off" p- and n-channel transistors are sensitive to upset. Fig. 3 shows the layout of a HM65MRRH memory cell in which the pchannel Uansistor drain is marked. CMOS memories are simpler than bipolar ones because the transistors have drains with only one junction sensitive to upsets. Both devices are constructed in epitaxial layers grown on highly doped substrates to eliminate the possibility of latchup. The highly-doped substrate effectively cuts off charge collection and raises the SEU threshold.[l3] The HM6504RRH part incorporates decoupling polysilicon resistors that slow down the feedback and allow the disturbed voltages to be restored before the cell can upset. Obtaining the required resistor values as well as controlling their uniformity across the chip has

RAIN

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 205 210 215 220 225 230 235 240 245 250 255

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WElBuu FIT TO ON M T A WEIWU FIT TO LASER DATA ICW DATA

n 1 1 i o 100 lo00

UPSET THRESHOLD (ARB. UNITS) LET (MeV-cm2/mg)

Fig. 4 Distribution of energy upset thresholds for the 93L422. The cross-hatched bars are for cells in even columns having a mean of 206 and a standard deviation of 3.76. The solid bars are for cells in odd columns having a mean of 235 and a standard deviation of 7.2.

Fig. 5 Normalized cross section vs LET for the 93LA22. The solid line is a fit to the ion data (circles) using a Weibull curve. The dashed line is a Weibull curve calculated from the distribution of intercell upset thresholdsmeasured with the laser.

been a difficult task and accounts for the variation in write times reported a few years ago.[8] Processing improvements in the intervening years have led to a much more uniform distribution of resistor values. Using the laser to measure the upset thresholds in different cells makes it possible to evaluate how well controlled the resistor values now are. Even for these parts the drains are not simple parallelepipeds, having a large rectangular area joined to a "panhandle" so that the simple parallelopiped approximation is not valid.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

i) 93L422

To compare Lo for different cells, we loaded each cell with a "1" and positioned the focussed laser light at the same location in each cell. Location A (Fig 2) was selected because i t was far from any metallization that might obscure part of the beam, and because the energy to produce an upset had a localized minimum there. For cells in the opposite state, the light was focussed at location B which is the mirror image of A. As previously pointed out, the cells in each column are mirror images of each other, so that a cell in an odd column would upset if the light were located at the opposite side to that of a cell in an even column. Fig. 4 shows a histogram depicting the SEU thresholds for 102 different cells, 49 (cmss- hatched) in even columns and 53 (solid) in odd columns. The data shows a clear bimodal distribution with the lower peak having a mean of 206 (arbitrary energy units) and a standard deviation of 3.76, whereas the higher peak has a mean of 235 and a standard deviation of 7.2. The bimodal distribution results from the 15% difference in E, for even and odd columns that contributes a small amount to the broadening of OK), certainly not sufficient to significantly affect its shape.

This difference was confirmed by comparing the values of E,, measured at location A when the cell contained a "l", with E, measured at the exact opposite side B of the same cell when the cell was loaded with a "0". Evidently, the cells are not totally symmetric. Of major significance is the fact that the standard deviation for both peaks was much less than 5% of the mean, which suggests that the gradual rise in o(L) Cannot be due to large intercell variations in Lo. A significant fraction of the noise (1-2%) in the measured upset levels is due to noise in the energy meter used to measure the laser pulse energy. We did not correct the readings for the energy-meter noise because the total noise was still much smaller than required to explain the shape of a@).

Fig. 5 shows both the data for the cross section as a function of LET for the 93L422 as well as a fit to the data using a Weibull function. The Weibull function describes the probability of failure for a system consisting of an ensemble of identical elements, in which each element can fail, and all elements must function properly for the system as a whole not to fail. The Weibull function is given by:

where osAT is the saturation value of the cross section, Lo is the threshold, and W is a parameter describing the width of the distribution. The Weibull curve is a fit to the experimental data for the 93L422 using the following values [3];

Lo = 0.6 MeV-cm*/mg W = 4.4 MeV-cm2/mg s = 0.7

2100

The mean (m) is given by :

where r is the error function. The standard deviation (A) is given by:

(3)

Substituting the values obtained by fitting the Weibull function to the data points gives a mean of 1.22 MeV-cmz/mg and a standard deviation of 9.05 MeV-cmz/mg. This calculation suggests that L, varies by more than an order of magnitude from cell to cell, which clearly does not agree with the data obtained from the pulsed laser measurements.

If we substitute the mean and standard deviation obtained from the pulsed laser measurements, and solve equations 2 and 3 for the Weibull parameters (assuming s does not change and replacing E, with LJ, we obtain the following Weibull parameters:

La = 0.6 MeV-cmz/mg W = 0.015 MeV-cmz/mg

and the resulting curve is given by the dashed line in Fig. 5 Clearly, the behavior of o(L) must be determined by factors other than intercell upset threshold variation.

Fig. 6 shows the values of the upset threshold measured with the pulsed laser for different locations throughout the memory cell of the 93L422. One can see that the SEU threshold varies greatly from one location to another within the cell. Special care was taken to ensure that at all locations the beam was not obscured by metallization. The carriers are collected at either the collector/substrate junction or the collector/base junction. Because the laser light decays exponentially, its intensity at a depth of 1 . 5 ~ is only 37% of its value at the surface. This decay means that junctions at different distances below the surface will collect different amounts of charge, even if the junctions have the Same sensitivity to SEU. Inside the dashed box both the c/s and the c/b compete for charge, whereas outside the box, only the c/s collects charge. (Depths of junctions are proprietary). Thus, to assess the magnitude of the spatial variation, upset thresholds at different locations within the dashed box should be compared only with thresholds at other locations within the box. Similarly, upset thresholds at positions outside the box should only be compared with upset thesholds at other locations outside the box. The data show that Lo varies significantly both inside and outside the box, strongly suggesting that the inncell variation of La contributes significantly to the gradual rise of OF). If the entire sensitive area were not covered with metal, one could use the laser to generate a contour plot depicting the spatial variation of Lo. from which it would be possible to determine the mean and standard deviations for comparison with the ion data. However, this was not possible in any of the devices tested because of the presence of metal contacts and interconnects that convered some fraction of the sensitive area.

1281 654 519 587 504 576 456 555 5% 57 1 502 929 997 lOs0

2840

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..................

625 630 635 640 645 650 656 660 665 670 675 UPSET THRESHOLD (ARB. UNITS)

Fig. 6 . Spatial distribution of energy upset thresholds for the 93U22 SRAM.The box defined by the broken curve shows the boundary between regions where both the c/b and c/s collect charge (inside the box) and only the c/s (outside the box) collects charge.

Fig. 7 lhtribution of With a mean of 651 and a standard deviation of 11.

Wet thresholds for the HM6504RH

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i i ) HM6504RRH

Intercell and intracell variations in Lo for the HM6504RH and the HM6504RRH SRAMs were also measured using the pulsed laser. Fig. 7 shows the distribution of Lo for 60 ”off“ p R\ channel transistors in the HM6504RH SRAh4 with the light focussed at the center of the drain where there was a local minimum in Lo. (It should be pointed out that there is no relationship between the energy scales used in Figs. 4, 7, and 10, so that relative SEU-immunity cannot be inferred from the values of the energy upset thresholds.) The data show that the standard deviation of Lo was less than 2% of the mean. As for the case of the 93L422, we compared the ion data with the laser data to see whether the variation in intercell values of Lo measured with the laser could account for the gradual increase in o(L). Fig. 8 shows the data points obtained from accelerator testing fitted with a Weibull curve having the following values for the Weibull parameters [31:

Lo = 30.75 MeV-cmz/mg W = 1.4 MeV-cmz/mg s = 1.4

Using the mean (set equal to the ion data mean) and standard deviation measured with the laser, we obtained the dashed curve in Fig. 8. In this case as well, the intercell variations in Lo cannot account for the shape of o(L). Fig. 9 shows the spatial distribution of Lo from which it is clear that the variations measured within the drain range from 591 to 738 energy units, much larger than the intercell variation. In this device, there are no competing junctions and, therefore, no ambiguity in interpreting the spatial variation. We also measured Lo for the nchannel transistors and found a

L I I I I I I l l 1 I I I 1 1 1 1 1

w-1 4 s=l 4 k

n 10 100

LET (MeV-cm‘/mg)

10 um

\1 Fig. 9. Spatial distribution of upset thesholds for the HM6504RH. The area tested is the drain of a p-channel transistor. The shaded areas are metal contacts and interconnects.

minimum value that was about 5/6 of the minimum value measured for the pchannel transistors. Thus, the upset threshold of the entire SRAM is determined primarily by the upset threshold of the n-channel transistors. However, since the drains of the pchannel transistors were larger than the drains of the n-channel transistors and less of the area was covered with metal, we concentrated on measuring Lo in the p-channel transistors. Our results for the HM6504RH show that the intercell variations in Lo are much smaller than the intracell variations, and that a combination of intracell variation and the contributions from two transistors (one n-channel and one p- channel in each memory cell) with different upset thresholds

230 235 240 245 250 255 260 265 270 275 280 285

Upset Threshold

Fig. 8. Normalized cross section vs LET for HM6504RH. The solid line is a fit to the data (circles) using a Weibull function. The dashed line is a Weibull curve calmlad from the mean and standard deviation of the intercell variations in the SEU thresholds me-4

Fig. 10. Distribution of energy upset thresholds for 28 cells in the HM6504RRH SRAM. The average is 259 with a standard deviation

with the laser. of 9.9

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technique for doing hardness assurance would be an important advance. Requirements for hardness assurance are that the technique be non-destructive, convenient, and reliable. By reliable, we mean that any intercell variations in structure that have no effect on Lo measured with an ion beam, should also have no effect on E, measured with the laser beam. For instance, if openings in metal contacts deposited on the surface of the sensitive region vary from cell to cell, they are likely to have a profound effect on the uniformity of E, measured with the laser, but unlikely to affect Lo measured with an ion beam. Dielectric thickness is another parameter that would affect E, measured with the laser much more than Lo measured with a high-energy ion beam. Melinger et. al. have calculated the change in the transmittance as a function of the thickness of SiO,.[14] Their calculations show that for an oxide with a thickness of 400 nm the variation in thickness would have to be about 25% for a maximum variation of 15% in transmitted intensity.

I I - - n /

Fig. 1 1 . Spatial distribution of the upset thresholds in the drain of a p-channel transistor in the HM6504RRH SRAM. The shaded areas are metal interconnects and contacts.

contribute significantly to the gradual rise in o(L). Fig. 10 shows that the intercell distribution of E, measured with the laser for the HM6504RRH is small, with a standard deviation of less than 5% of the mean. Fig. 11 shows the intracell spatial distribution of E, which is, as in the other devices tested, much larger than the intercell distribution. Although one cannot directly compare the laser data with ion data because the latter do not exist, one can say, with a fair degree of certainty, that the shape of a&) would be determined, in large part, by the intracell variation of Lo and the contributions from two different junctions. Thus, even though these SRAM cells contain decoupling resistors, they do not lead to large variations in Lo from cell to cell. This is because the process used to make the chip is well controlled and the resistor values across the chip are, apparently, very uniform.

The fact that there is little variation in Lo for both bipolar and CMOS SRAMs indicates that there is a unique critical charge for the memory as a whole, i.e. each cell has approximately the same critical charge. However, the charge collection efficiency depends on the location of the ion suike in the memory cell. Our results, together with those of Cutchin et. al., suggest that at the onset of upset, only a very small part of each cell is sensitive, and all cells are capable of being upset. With increasing LET, each cell contributes more area to the total cross section, untd at higher LETs, the entire area of each junction is sensitive. At even higher LETs, areas adjacent to the sensitive junctions can also contribute, and the cross section continues to increase, provided the junctions are not isolated from one another.

V. IMPLICATIONS FOR HARDNESS ASSURANCE

At present, SEU-hardness-assurance measurements are done using accelerator ions, which has the disadvantages of limited access and of being destructive. An alternative

The very small variations we measured for the intercell uniformity of E, for all three SRAMs suggests that the pulsed laser can be used for hardness-assurance testing, provided the beam is positioned in the exact same relative location in each cell. Apparently, the factors mentioned above that could affect the amount of transmitted light are, in these cases, not significant By itself, the laser only gives relative values for upset threshold, but absolute measurements can be obtained by first testing the device with a pulsed laser and then with an ion beam. In this way, a fiduciary value is established, against which all subsequent measurements on devices of the same type can be compared.

We should point out that it will be more difficult to test devices with sensitive areas comparable to, or smaller than, the diameter of the laser beam, because small variations in position of the beam from one cell to another may give rise to differences in SEU threshold that are not indicative of the values measured with an ion beam.

VI. CONCLUSION

We have demonstrated with a pulsed laser that the intercell variation in upset threshold for two types of SRAMs are too small to account for the shape of the cross section versus LET curve. Finally, our results show that the pulsed laser can be used for hardness-assurance measurements, provided the focussed laser beam is smaller than the size of the sensitive area being probed. Further work needs to be done in order to determine the limits of the technique for devices with smaller dimensions.

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