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Single-event correlation between heavy ions and 252Cf fission fragments
Nuclear Instruments and Methods in Physics Research B45 (1990) 714-717 North-Holland
714
Section XII. Medical therapy and VSLI SINGLE-EVENT
CORRELATION
soft fails (SEU)
BETWEEN
HEAVY IONS AND 252Cf FISSION FRAGMENTS
John S. BROWNING Radiation Applications
Division, Sandia National Laboratories,
Albuquerque,
NM 87185, USA
The results of a correlation study show that 252Cf sources can replace accelerators for single-eventtests, provided that the depth of penetration and the linear energy transfer required of the heavy ions fall within the spectrum of the 252Cf fission fragments.
1. Introduction Considerable effort and expense is required to test microelectronics for single events using heavy-ion beams produced by large accelerators. Depending on the number of test conditions (for example, angle of incidence, temperature, bias, total dose and decoupling resistance), the cost of a single-event characterization of a single-part type ranges from $10000to $100000. Single-event tests using 252Cf fission fragments are much less expensive and more readily available then accelerator testing. (The *‘*Cf single-event test system described below can be assembled for less than $50000.) However, the uncertainties in the test results associated with the range of linear energy transfer (LET) and the depth of penetration in microelectronics materials severely limit the usefulness of 252Cf testing [l]. In this paper a method for reducing the uncertainty in 252Cf test data is presented. Based on a correlation between accelerator and 252Cf test data it is shown that the 252Cf single-event tests can produce results equivalent to single-event tests performed at accelerators. The basic parameters needed from single-event tests to calculate the event rates in space environments are the threshold LET that the cosmic ray must have to cause a single event in the microcircuit, and the saturation cross section (total area sensitive to single events) of the microcircuit [2]. These parameters are usually extracted from data taken at a heavy-ion accelerator. The experimental cross section (number of single events divided by the angle-corrected particle fluence) is plotted as a function of effective LET (LET times the secant of the angle of incidence of the ion beam; zero degrees is normal incidence). From this plot the threshold LET for upset and the saturation cross section are determined. Also, the experimental cross section curve is often an important source of device modeling information. For example, the experimental cross section of a CMOS SRAM was used to distinguish between single events caused by ion strikes in the p-drain or n-drain transistor areas of a memory cell [3]. 0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
As a source of heavy ions for single-event tests, 252Cf provides a wide range of fragment masses and energies. A range of effective LETS from 22 to 75 MeVcm2/mg can be obtained by degrading the fragment energies by passing them through various thickness of an energy absorber (air in the work described below), or by varying the angle of incidence. Over this range of LETS the fission fragments will have a minimum penetration depth of at least 5 pm in silicon, a thickness comparable to a typical sensitive layer in many microcircuit technologies. Because 252Cf produces wide LET spectra, the fission fragments cannot be used to directly measure a single-event cross section as a function of LET. A method of unfolding the 252Cf LET spectra to obtain a curve of single-event cross section vs LET is now presented.
2. Experimental details Fig. 1 is a diagram of the 252Cf single-event test system used to perform the work described below. It consists of a small vacuum chamber (Spectrum Sciences model 5005) in which the 252Cf source is mechanically mounted such that the distance between the source and a test head in the bottom of the chamber can be varied
Fig. 1. 252Cf single-eventtest system.
715
J.S. Browning /Single-event correlation between heavy ions and 252Cffission fragments
’ t wt
*
1.4
1.2
w3-J
(26”) l/COSINE
’
1.6
-
’
1.8
OF ANGLE
OFF
’
’ :sw
60’) NORMAL
Fig. 3. 252Cf fission-fragment LET spectra that can be obtained over a 5 pm depth in silicon by varying the amonnt of energy absorber (air at STP). The 10% and 90% cnmtdative fractions as we11 as the average value of the LET average over the 5 nrn depth are shown.
The experimental cross section u is estimated by dividing the number of events recorded by the DUT test circuit by the angle-corrected particle fluence. The particle fluence is determined by multiplying the exposure time with the measured particle flux.
3. Method for reducing 252Cf test data Fig. 2. Interior of the vacnnm chamber showing (1) 252Cf source bolder and positioner, (2) DUT test head and (3) SSD and positio~ng arm.
and the angIe of incidence of the fission fragments on the device under test (DUT) can be varied from 0 o to 60 O. Fig. 2 is a photograph taken inside the vacuum chamber and shows these details. By varying the angle of incidence a range of effective LETS from 37 to 75 MeVcm2/mg can be obtained over a 5 pm depth in silicon. A small leak valve (LV) permits a controlled amount of air to enter the chamber. 3y using air to degrade the energy of normal-incidence fission fragments, a range of LETS from 37 to 22 MeVcm’/mg can be obtained over the 5 pm depth. A solid-state detector (SSD), shown in fig. 2, is used to measure the flux of particles from the 252Cf source. A small microscope replaces the *‘*Cf source to calibrate the distances from the source to the SSD and from the source to the DUT. (When the microscope is in focus on the surface of the SSD or the dehdded DUT the distances are the same.) Analysis of the pulse height output of the SSD is used to calibrate the vacuum gauge. Degradation in the energy of the ***Cf heavy- and light-fragment spectral peaks indicate the amount of air that is present in the chamber.
When the angle of incidence or the energy of the fission fragments are varied, the LET spectrum is shifted slightly. This shift in spectra is shown in figs. 3 and 4 (the vertical lmes indicate several experimental variations). The variation can be selected such that the LET spectra overlap. For any variation i, the experimental cross section u is related to the single-event cross section u(L) by
ui=
Cwi,jafLj)> j
where wi,i is the fraction of the fission fragment spec-
THICKNESS
OF AIR cn STP
(cm)
Fig. 4. 252Cf fission-fragment effective LET spectra that can be obtained over a 5 pm depth in s&con by varying the angle of incidence of the fission fragments. XII. MEDICAL
THERAPY
716
J.S. Browning / Single-event correlation between heavy ions and 2J2Cffission fragments
trum with a LET equal to Lj (L is the midpoint of the interval of overlap of the LET spectra for two experimental variations). Thus
Table 1 Threshold LET [MeVcm2/mg] for upset in the 16 k-bit SRAM samples (the threshold LET is defined as the value of LET that gives a cross section of 10W6 cm*) Decoupling resistance
Heavy ion
2s2Cf
40 46 72
39 45 68
Ml 73 82 130 %3=~13,224&*)
++%3*23~t~23)
‘t%,254L25)
+
fw,3,244~24)
W*3,2dL2d* (2)
The
system of equations (2) can be written in matrix
form as
f”il= [wi,jl[u]
(3)
with solution
(4) 4. Experimental verification of the method As discussed above, the basic parameters needed from single-event tests to calculate the event rate in space particle environments are the threshold LET for the event to occur and the saturation cross section. These parameters are usually obtained from a cross section vs LET curve measured at a heavy-ion accelerator. The following experimental results show that the same parameters can be extracted from 252Cf test data, provided that the threshold LET of the single event and the required depth of penetration of the heavy ions are within the range of the Z52Cf fission fragments. In the tests described below, the Sandia SA3240 CMOS static random-access memory (SRAM) was used to correlate accelerator heavy-ion data with 252Cf data. The SA3240 has been extensively characterized and
modeled for single-event upset [3,4]. The sensitive transistor drain areas of the SRAM are 3 pm deep, and they are covered by a 1 pm layer of burnt and a 1 pm layer of silicon dioxide. Thus the depth of penetration required of heavy ions that upset the SRAM is within the range of the 252Cf fission fragments. For SRAM, devices with decoupling resistors of 0.130, 73, 82 and 130 k(;2 were used in this study. In Fig. 5 the cross sections obtained from a 140 MeV bromine ion beam, the experimental 252Cf cross section and the cross section calculated from the 252Cf test data are compared. As can be seen, the agreement between the heavy-ion data (squares) and a(L) (di~o~ds) is very good. Similarly, in table 1 the threshold LET for upset from heavy ions compares well with the threshold LET extracted from 252Cf test data. The cross section vs LET curve is often an important soume of device modeling info~ation. In ref. f3] the cross-section curve was used to distinguish between the p- and n-drain upset mechanisms. This information was used to determine the transient-voltage time constants for ion strikes in either of the sensitive areas. In fig. 6 it can be seen that the same modeling isolation could
10
30
EFFECTIVE
38
40
42
44
EFFECTIVE Fig.
LET
46
48
SO
52
54
56
LET (Me’+cm2/mg~
5. Cross section for single-event upset as a function of for a 16 kbit SLUM with a memory cell decoupling resistance of 73 kP.
so
70
LET WA’-cm*
90
/mg)
Fig. 6. Cross section for single-event upset as a function of LET for a 16 kbit SRAM with a memory cell decoupling resistance of 130 Q. The cross section reaches a plateau between 200 and 30 MeVcm’/mg, corresponding to a saturation of the upsets caused by p-channel transistor dram bits, and then increases to a second plateau at 75 MeVcm2/mg, corresponding to the onset and subsequent saturation of the upsets caused by n-channel transistor drain hits.
J.S. Browning / Single-event correlation between heuuy ions and 252Cffission fragments have been obtained from a 252Cf test. There is good agreement between the heavy-ion data (squares) and the 252Cf data (diamonds) that show the p-drain plateau and the onset of upset from the n-drain strikes. Eventually the combined p- and n-drain areas are approached by both cross-section curves at appro~ately the same value of LET.
717
The author wishes to thank the Center for Radiation Hardened Microelectronics for providing the necessary facilities at Sandia National Laboratories. Thanks are also due to Dr. D.B. Holtkamp of Los Alamos National Laboratory for many valuable discussions about how to model the spont~eous-fission process, and to Dr. J.F. Ziegler of IBM Research for use of the TRIM computer code.
5. Conclusions The single-event cross section as a function of linear energy transfer can be obtained from 252Cf tests data. Fission fragments can replace heavy ions obtained from large accelerators, provided that the range of linear energy transfer, and the depth of penetration in the microcircuit under test, required for the single event to occur can be produced by the fragments. A single-event test using a 252Cf source costs only a small fraction of the cost of an equivalent test performed at a large accelerator.
References [l] J.T. Blandford and J.C. Pickel, IEEE Trans. Nucl. Sci. NS-32 (1985) 4286. [2] T.K. Sanderson et al., Electron. Lett. 19 (1983) 373. [3] H.T. Weaver et al., IEEE Trans. Electron Devices ED-35 (1988) 1117. 141 J.S. Browning et al., IEEE Trans. Nucl. Sci. NS-32 (1985) 4133.
XII. MEDICAL
THERAPY
714
Section XII. Medical therapy and VSLI SINGLE-EVENT
CORRELATION
soft fails (SEU)
BETWEEN
HEAVY IONS AND 252Cf FISSION FRAGMENTS
John S. BROWNING Radiation Applications
Division, Sandia National Laboratories,
Albuquerque,
NM 87185, USA
The results of a correlation study show that 252Cf sources can replace accelerators for single-eventtests, provided that the depth of penetration and the linear energy transfer required of the heavy ions fall within the spectrum of the 252Cf fission fragments.
1. Introduction Considerable effort and expense is required to test microelectronics for single events using heavy-ion beams produced by large accelerators. Depending on the number of test conditions (for example, angle of incidence, temperature, bias, total dose and decoupling resistance), the cost of a single-event characterization of a single-part type ranges from $10000to $100000. Single-event tests using 252Cf fission fragments are much less expensive and more readily available then accelerator testing. (The *‘*Cf single-event test system described below can be assembled for less than $50000.) However, the uncertainties in the test results associated with the range of linear energy transfer (LET) and the depth of penetration in microelectronics materials severely limit the usefulness of 252Cf testing [l]. In this paper a method for reducing the uncertainty in 252Cf test data is presented. Based on a correlation between accelerator and 252Cf test data it is shown that the 252Cf single-event tests can produce results equivalent to single-event tests performed at accelerators. The basic parameters needed from single-event tests to calculate the event rates in space environments are the threshold LET that the cosmic ray must have to cause a single event in the microcircuit, and the saturation cross section (total area sensitive to single events) of the microcircuit [2]. These parameters are usually extracted from data taken at a heavy-ion accelerator. The experimental cross section (number of single events divided by the angle-corrected particle fluence) is plotted as a function of effective LET (LET times the secant of the angle of incidence of the ion beam; zero degrees is normal incidence). From this plot the threshold LET for upset and the saturation cross section are determined. Also, the experimental cross section curve is often an important source of device modeling information. For example, the experimental cross section of a CMOS SRAM was used to distinguish between single events caused by ion strikes in the p-drain or n-drain transistor areas of a memory cell [3]. 0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
As a source of heavy ions for single-event tests, 252Cf provides a wide range of fragment masses and energies. A range of effective LETS from 22 to 75 MeVcm2/mg can be obtained by degrading the fragment energies by passing them through various thickness of an energy absorber (air in the work described below), or by varying the angle of incidence. Over this range of LETS the fission fragments will have a minimum penetration depth of at least 5 pm in silicon, a thickness comparable to a typical sensitive layer in many microcircuit technologies. Because 252Cf produces wide LET spectra, the fission fragments cannot be used to directly measure a single-event cross section as a function of LET. A method of unfolding the 252Cf LET spectra to obtain a curve of single-event cross section vs LET is now presented.
2. Experimental details Fig. 1 is a diagram of the 252Cf single-event test system used to perform the work described below. It consists of a small vacuum chamber (Spectrum Sciences model 5005) in which the 252Cf source is mechanically mounted such that the distance between the source and a test head in the bottom of the chamber can be varied
Fig. 1. 252Cf single-eventtest system.
715
J.S. Browning /Single-event correlation between heavy ions and 252Cffission fragments
’ t wt
*
1.4
1.2
w3-J
(26”) l/COSINE
’
1.6
-
’
1.8
OF ANGLE
OFF
’
’ :sw
60’) NORMAL
Fig. 3. 252Cf fission-fragment LET spectra that can be obtained over a 5 pm depth in silicon by varying the amonnt of energy absorber (air at STP). The 10% and 90% cnmtdative fractions as we11 as the average value of the LET average over the 5 nrn depth are shown.
The experimental cross section u is estimated by dividing the number of events recorded by the DUT test circuit by the angle-corrected particle fluence. The particle fluence is determined by multiplying the exposure time with the measured particle flux.
3. Method for reducing 252Cf test data Fig. 2. Interior of the vacnnm chamber showing (1) 252Cf source bolder and positioner, (2) DUT test head and (3) SSD and positio~ng arm.
and the angIe of incidence of the fission fragments on the device under test (DUT) can be varied from 0 o to 60 O. Fig. 2 is a photograph taken inside the vacuum chamber and shows these details. By varying the angle of incidence a range of effective LETS from 37 to 75 MeVcm2/mg can be obtained over a 5 pm depth in silicon. A small leak valve (LV) permits a controlled amount of air to enter the chamber. 3y using air to degrade the energy of normal-incidence fission fragments, a range of LETS from 37 to 22 MeVcm’/mg can be obtained over the 5 pm depth. A solid-state detector (SSD), shown in fig. 2, is used to measure the flux of particles from the 252Cf source. A small microscope replaces the *‘*Cf source to calibrate the distances from the source to the SSD and from the source to the DUT. (When the microscope is in focus on the surface of the SSD or the dehdded DUT the distances are the same.) Analysis of the pulse height output of the SSD is used to calibrate the vacuum gauge. Degradation in the energy of the ***Cf heavy- and light-fragment spectral peaks indicate the amount of air that is present in the chamber.
When the angle of incidence or the energy of the fission fragments are varied, the LET spectrum is shifted slightly. This shift in spectra is shown in figs. 3 and 4 (the vertical lmes indicate several experimental variations). The variation can be selected such that the LET spectra overlap. For any variation i, the experimental cross section u is related to the single-event cross section u(L) by
ui=
Cwi,jafLj)> j
where wi,i is the fraction of the fission fragment spec-
THICKNESS
OF AIR cn STP
(cm)
Fig. 4. 252Cf fission-fragment effective LET spectra that can be obtained over a 5 pm depth in s&con by varying the angle of incidence of the fission fragments. XII. MEDICAL
THERAPY
716
J.S. Browning / Single-event correlation between heavy ions and 2J2Cffission fragments
trum with a LET equal to Lj (L is the midpoint of the interval of overlap of the LET spectra for two experimental variations). Thus
Table 1 Threshold LET [MeVcm2/mg] for upset in the 16 k-bit SRAM samples (the threshold LET is defined as the value of LET that gives a cross section of 10W6 cm*) Decoupling resistance
Heavy ion
2s2Cf
40 46 72
39 45 68
Ml 73 82 130 %3=~13,224&*)
++%3*23~t~23)
‘t%,254L25)
+
fw,3,244~24)
W*3,2dL2d* (2)
The
system of equations (2) can be written in matrix
form as
f”il= [wi,jl[u
(3)
with solution
(4) 4. Experimental verification of the method As discussed above, the basic parameters needed from single-event tests to calculate the event rate in space particle environments are the threshold LET for the event to occur and the saturation cross section. These parameters are usually obtained from a cross section vs LET curve measured at a heavy-ion accelerator. The following experimental results show that the same parameters can be extracted from 252Cf test data, provided that the threshold LET of the single event and the required depth of penetration of the heavy ions are within the range of the Z52Cf fission fragments. In the tests described below, the Sandia SA3240 CMOS static random-access memory (SRAM) was used to correlate accelerator heavy-ion data with 252Cf data. The SA3240 has been extensively characterized and
modeled for single-event upset [3,4]. The sensitive transistor drain areas of the SRAM are 3 pm deep, and they are covered by a 1 pm layer of burnt and a 1 pm layer of silicon dioxide. Thus the depth of penetration required of heavy ions that upset the SRAM is within the range of the 252Cf fission fragments. For SRAM, devices with decoupling resistors of 0.130, 73, 82 and 130 k(;2 were used in this study. In Fig. 5 the cross sections obtained from a 140 MeV bromine ion beam, the experimental 252Cf cross section and the cross section calculated from the 252Cf test data are compared. As can be seen, the agreement between the heavy-ion data (squares) and a(L) (di~o~ds) is very good. Similarly, in table 1 the threshold LET for upset from heavy ions compares well with the threshold LET extracted from 252Cf test data. The cross section vs LET curve is often an important soume of device modeling info~ation. In ref. f3] the cross-section curve was used to distinguish between the p- and n-drain upset mechanisms. This information was used to determine the transient-voltage time constants for ion strikes in either of the sensitive areas. In fig. 6 it can be seen that the same modeling isolation could
10
30
EFFECTIVE
38
40
42
44
EFFECTIVE Fig.
LET
46
48
SO
52
54
56
LET (Me’+cm2/mg~
5. Cross section for single-event upset as a function of for a 16 kbit SLUM with a memory cell decoupling resistance of 73 kP.
so
70
LET WA’-cm*
90
/mg)
Fig. 6. Cross section for single-event upset as a function of LET for a 16 kbit SRAM with a memory cell decoupling resistance of 130 Q. The cross section reaches a plateau between 200 and 30 MeVcm’/mg, corresponding to a saturation of the upsets caused by p-channel transistor dram bits, and then increases to a second plateau at 75 MeVcm2/mg, corresponding to the onset and subsequent saturation of the upsets caused by n-channel transistor drain hits.
J.S. Browning / Single-event correlation between heuuy ions and 252Cffission fragments have been obtained from a 252Cf test. There is good agreement between the heavy-ion data (squares) and the 252Cf data (diamonds) that show the p-drain plateau and the onset of upset from the n-drain strikes. Eventually the combined p- and n-drain areas are approached by both cross-section curves at appro~ately the same value of LET.
717
The author wishes to thank the Center for Radiation Hardened Microelectronics for providing the necessary facilities at Sandia National Laboratories. Thanks are also due to Dr. D.B. Holtkamp of Los Alamos National Laboratory for many valuable discussions about how to model the spont~eous-fission process, and to Dr. J.F. Ziegler of IBM Research for use of the TRIM computer code.
5. Conclusions The single-event cross section as a function of linear energy transfer can be obtained from 252Cf tests data. Fission fragments can replace heavy ions obtained from large accelerators, provided that the range of linear energy transfer, and the depth of penetration in the microcircuit under test, required for the single event to occur can be produced by the fragments. A single-event test using a 252Cf source costs only a small fraction of the cost of an equivalent test performed at a large accelerator.
References [l] J.T. Blandford and J.C. Pickel, IEEE Trans. Nucl. Sci. NS-32 (1985) 4286. [2] T.K. Sanderson et al., Electron. Lett. 19 (1983) 373. [3] H.T. Weaver et al., IEEE Trans. Electron Devices ED-35 (1988) 1117. 141 J.S. Browning et al., IEEE Trans. Nucl. Sci. NS-32 (1985) 4133.
XII. MEDICAL
THERAPY