Evaluation of Reliability and Data Retention of an Irradiated Nonvolatile Memory
ABSTRACT Space systems require high reliability nonvolatile memory. This paper analyzes the reliability of an EEPROM for data retention, endurance and radiation performance across multiple die lots.
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 Cited In (3)

Conference Paper: Compendia of Radiation Test Results
[Show abstract] [Hide abstract]
ABSTRACT: TID and SEE data were taken to qualify and evaluate IC devices for radiation susceptibility in the natural space environment. A summary of the test data is presented and discussed.Radiation Effects Data Workshop, 2007 IEEE; 08/2007  [Show abstract] [Hide abstract]
ABSTRACT: We discuss new experimental results on the postradiation annealing of Floating Gate errors in Flash memories with both NAND and NOR architecture. We investigate the dependence of annealing on the program level, linking the reduction in the number of Floating Gate errors to the evolution of the threshold voltage of each single cell. To understand the underlying physics we also discuss how temperature affects the number of Floating Gate errors.IEEE Transactions on Nuclear Science 01/2010; · 1.22 Impact Factor  M. Bagatin, S. Gerardin, G. Cellere, A. Paccagnella, A. Visconti, S. Beltrami, R. HarboeSorensen, A. Virtanen[Show abstract] [Hide abstract]
ABSTRACT: Heavyion irradiation of NAND flash memories under operating conditions leads to errors with complex, datadependent signatures. We present upsets due to hits in the floating gate array and in the peripheral circuitry, discussing their peculiarities in terms of pattern dependence and annealing. We also illustrate single event functional interruptions, which lead to errors during erase and program operations. To account for all the phenomena we observe during and after irradiation, we propose an ldquoeffective cross section,rdquo which takes into account the array and peripheral circuitry contributions to the SEU sensitivity, as well as the operating conditions.IEEE Transactions on Nuclear Science 01/2009; · 1.22 Impact Factor
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Evaluation of Reliability and Data Retention of an Irradiated
Nonvolatile Memory
Phil Layton, Larry Longden and Ed Patnaude
Maxwell Technologies Inc, 9244 Balboa Ave, San Diego, CA 92123, USA
AbstractSpace systems require high reliability nonvolatile
memory. This paper analyzes the reliability of an EEPROM for
data retention, endurance and radiation performance across
multiple die lots.
I.
INTRODUCTION
Long term nonvolatile memory reliability is a potential
concern in space environments. Typical part analysis
includes individual lot TID testing and a separate reliability
analysis. For devices such as nonvolatile memory, which is
frequently used to store critical data such as boot code, the
understanding of the reliability of the device while under
irradiation is important to understand. Data retention is not
typically tested in TID tests. The overall reliability of the
part requires understanding the parametric performance,
functionality and data retention. The authors have unique
access to Maxwell EEPROM manufacturing data, which
includes large numbers of die lot specific TID and reliability
data. This paper will look at the reliability of EEPROMs
using large sampling of TID, data retention and life test
data.
The Hitachi HN58C1001 EEPROM utilizes a Floating
Gate Technology as shown in Figure 1. Each memory cell
location in the array is constructed of a single MOS
transistor with a floating gate. The polysilican gate is
encapsulated in silicon dioxide, which insulates it from the
transistor channel. By charging and discharging the
floating gate, the output of the memory can be maintained
as a logic 0 or logic 1.
With the floating gate charged, the output of the memory
cell will be logic 0. To program the cell to logic 0 the
control gate of the cell is turned on causing current to flow
through the transistor creating enough energy to allow free
electrons to tunnel through the insulator thus charging the
floating gate. Once the gate has been charged it will hold
that charge for longer than ten years with the device in a
powered or unpowered state. This is referred to as “the
data retention time”.
The silicon dioxide material used to insulate the floating
gate is not a perfect insulator. Over time the charge on the
gate will leak off through conductive paths made up of
impurities in the material. In a small percentage of
EEPROM devices there are sufficient impurities in the
insulating material to cause cells to leak off in days or
weeks rather than years. Standard testing and screening
methods will not always find these infant mortalities.
Therefore a more extensive screening methodology is
required to detect these defects for long term data retention
reliability. Maxwell’s data retention screen is intended to
detect these defective parts and remove them from the final
production devices. We are able to use that data to
generate long term data retention and reliability calculations
of this part. With ionizing radiation (TID), electron hole
pairs generated in the oxide layer may also create paths for
leakage. Therefore TID may also affect data retention.
We present data and analysis below to evaluate the Hitachi
HN58C1001 128k x 8 EEPROM for data retention both
before irradiation and after irradiation up to 40 krad(Si).
Endurance evaluation involves testing for failures from
multiple erase and write cycles. The EEPROM is specified
for 10,000 erase and write cycles. We tested devices both at
specification and at 2 times specification after irradiation to
40 krad(Si) to evaluate whether TID induced leakage
degrades endurance.
With large nonvolatile memory requirements, many
applications require an analysis of performance variations
over large sample lots.
To address these concerns, we present three sets of data: 1)
die lot TID test data from 20 different die lots, 2) data
retention tests from production screening and devices that
have undergone irradiation to 40 krad(Si) and data retention
testing, and 3) endurance test on parts that have both
undergone irradiation and data retention tests. These test
show that this device is robust and will meet specifications
in space radiation environments.
Figure 1. Floating Gate Cell
Page 2
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II.
TID TESTING
A. Die lot TID test data
The authors have collected a significant amount of die lot
specific TID data on the EEPROM. Twenty different die
lots were analyzed for variations in performance with TID.
Each die lot was typically tested at 5 volts both in the write
read mode and in the read only mode. We analyzed the 5
volt write read parametric data only as this put the most
stress on the parts by adding an extra write cycle at each test
level. Since the data comes from 20 separate die lot test
over several years, the total dose test levels for each die lot
varied. Overall over 160 parts were irradiated. For each
total dose level, we used a mean point with a standard
deviation from that mean to analyze the data. Table 1
shows the averaged TID test points, the standard deviation
of the test levels from the mean and the number of die lots
with test data at each level (each die lot had approximately 8
irradiated parts). The number of die lots tested at each level
varies due to some tests skipping some of the levels, with
some tests going to higher levels. The higher level tests
were not included in this analysis for consistency. Not all
die lots were tested at each level as shown in Table 1 only
18 lots were tested at each level.
Table 1. EEPROM TID Levels and Standard Deviation
TID
krad(Si)
Standard
Deviation
krad(Si)
Number
of Die
lots
0 0 18
29 3.3 18
43 2.4 18
Looking at the usage of a large number of devices, a one
sided statistical analysis [1] would enable the prediction of
failure or parametric degradation outside of specification
based on the mean and standard deviation. This analysis
calculates the probability a parameter on one device in a
large statistical sample will drift out of specification, based
on a set test sample size. Equation 1 gives a maximum
limiting quantity for a set number of parts tested and
planned to be used based on an average standard deviation
and a set confidence level.
Lmax = m + KTL(n,C,P)S
Where:
Lmax is the maximum limiting quantity
m is the mean
KTL is the tolerance limit and is a function of the sample
size n, desired confidence, C and lot quality (survival
probability) P
S is the standard deviation
(1)
Standby current with CE = VCC showed the greatest
movement of all the parameters measured. Figure 2 shows
the Standby current with CE=VCC with the error bars on
the average line ("avrg") showing the standard deviation for
both the TID levels (x error bars) and the standard
deviation (y error bars) of the measured current. As can be
seen, the average and the Lmax values are well within
operating specification at 43 krad(Si). Figure 3 shows the
leakage current, both the average over the lots and Lmax.
Although there is some increase in leakage current with
TID, the levels are well under the 2 µA limit.
Looking at the 43 krad(Si) there were 144 devices tested.
We analyzed half of those device for parametric drift. For
ICC1 the mean ICC1 was 8.0 µA with a standard deviation
of 4.1 krad(Si). The KTL factor for a probability of 0.99 (lot
quality) and a confidence factor of 90%, is 2 [2]. Lmax is
then 16.1 µA. Therefore one would expect even with a
large 100piece sample size that all devices would be well
within the 20 µA requirement based on a grouping of all the
die lots. Lmax was evaluated for the other tested
parameters and all were within specification after irradiation
to 43 krad(Si).
B. Die Lot Variation
The individual die lots where then analyzed for lot to lot
variation. We looked at each die lot at three different TID
0
100
200
300
400
500
05 10 1520 25 3035 4045
TID krad(Si)
Leakage current (nA)
IIL average
IIL Lmax
ILO 5.5v aver
ILO 5.5V Lmax
Maximum 2µA
Figure 3. Leakage Current as a function of TID and Lmax.
Note the Maximum is 2000nA.
0
5
10
15
20
05101520 2530354045
TID krad(Si)
uA
avrg
Lmax
Max 20 uA
1 Standard
Deviation
Figure 2. ICC1 Standby Current CE=VCC for EEPROM
die lots. Average and Lmax values are shown.
Page 3
PW8
levels 0 krad(Si), 29 krad(Si) and 43 krad(Si). Tables 2
through 4 show the mean of several measured parameters
and the number of standard deviations from the
specification. The number of standard deviation from the
average is measured by the absolute value of the difference
between the specification and the mean divided by the
standard deviation. The standard deviation is a function of
the variance of the data and therefore reflects how tight the
data points are to each other. Therefore although the
averages may be close if there is a large data spread this will
be reflected in the "specmean/σ" columns with a smaller
value then a tightly grouped set of data. High numbers in
the "specmean/σ" columns show very tightly grouped
numbers. For some of the prerad data some values where all
the same and the authors had to add a small deviation to
prevent dividing by zero, any number over a thousand
shows data where all data points were at the mean. This
number also reflects the inherent limitation of the testers,
since some values are binned based on tester sensitivity a
slight change in value may push the "measured" value into
the next measurement bin or tester resolution. If the bins
are large relative to the specification these can result in a
large movement in the "specmean/σ" number that only
reflects the lack of tester resolution.
As the TID increases the lowering "specmean/σ" numbers
shows both the increase in the mean and the increasing
spread of the data. The "specmean/σ" number for ICC
operating and VOL stay consistently high across the three
radiation levels with the lowest value for VOL of 24
actually occurring in the preirradiation test for die lot
G20002. This is a function of testers voltage measurement
resolution as discussed above. Access time doesn't move
much with the lowest distance to the specification being 5
standard deviations for a few die lots at 43 krad(Si).
Standby Current is the closest to specification with one die
lot (G20002 mentioned above) at 29 krad(Si) measuring 1
standard deviation from the specification and two different
die lots measuring 1 deviation from specification at 43
krad(Si). Interestingly Standby Current forG20002 actually
improves at 43 krad(Si), which shows how sensitive this
leakage current measurement is with a specification of only
20 µA. With data and access to large numbers of die lots,
high TID and high margin mission requirements can choose
the appropriate die lot based on data .
III. DATA RETENTION TESTING
Production testing includes a data retention screening. We
used this data in addition to radiation data to evaluate the
EEPROM for data retention performance. All flight lots
are normally put through a burn in to screen out infant
mortality. These parts are then electrically tested again at
room temperature, programmed with a checkerboard
pattern of 55AA and then software protection is enabled.
The parts are then placed in a 150°C oven, unbiased, for
72 hours. The parts are unbiased during this test (worst
case condition) to demonstrate their ability to retain charge
across the floating gate without bias. Upon completion of
the unbiased bake, the patterns are verified on an ATE
tester. Any devices that fail pattern verification can then
Table 2 Prerad die lot measured parameter variation from
specification
Parameter
Limit
Diel Lot
B20079
B20080
B20462
C20384
E20007
E20008
E20055
E20312
E20313
E20315
E20316
F20019
F20020
F20023
F20036
F20049
F20051
G20002
Mean
13.5
13.6
13.9
13.4
12.7
13.2
13.4
13.3
13.2
13.1
13.6
12.7
13.2
13.8
12.9
12.5
12.9
12.6
Mean
3.6
3.7
3.4
3.5
3.6
3.3
3.3
3.9
3.4
3.8
3.6
3.3
3.6
3.3
3.6
3.4
3.8
3.8
Std Dev from
Spec
143
3644
93
3689
300
187
186
293
49
91
144
68
55
110
300
112
295
182
Mean
102
100
96
96
100
100
100
100
97
94
98
95
93
90
100
102
102
99
Std Dev
from Spec
83
75
6797
339
73
66
58
75
277
117
123
37
41
43
6708
114
27
24
Mean
89.2E9
85.1E9
85.1E9
89.5E9
86.9E9
85.1E9
87.2E9
86.2E9
84.9E9
83.7E9
86.7E9
84.4E9
84.8E9
80.5E9
82.5E9
86.5E9
86.7E9
87.0E9
106
176
67
105
680
206
121
150
206
249
158
72
99
152
117
96
235
192
41
115
41
7
239
54
60
112
36
39
15
12
35
74
27
39
9
8
50 mA20 uA
Spec−
400 mV120 ns
Spec −
VOL,Access Time Standby CurrentICC operating
σ
MeanSpec−
σ
Mean
σ
Mean
Table 3. 29krad(Si) die lot measured parameter variation
from specification
Parameter
Limit
Diel Lot
B20079
B20080
B20462
C20384
E20007
E20008
E20055
E20312
E20313
E20315
E20316
F20019
F20020
F20023
F20036
F20049
F20051
G20002
Mean
13.4
13.4
13.8
13.4
12.5
13.2
13.4
13.3
13.0
12.9
13.4
12.4
12.9
13.5
12.7
12.2
12.6
12.3
Mean
4.1
4.5
3.7
5.4
6.3
4.6
4.5
5.0
6.4
4.7
8.6
5.7
5.9
4.6
4.4
5.0
6.7
12.7
Mean
98
97
101
101
105
95
97
96
93
94
108
100
98
97
97
96
102
101
Mean
92.4E9
88.7E9
87.7E9
93.5E9
89.2E9
86.2E9
88.6E9
87.3E9
88.8E9
88.5E9
89.6E9
87.2E9
87.7E9
84.9E9
87.3E9
91.0E9
95.0E9
96.0E9
100
196
53
167
193
212
99
223
244
166
196
70
97
272
116
98
305
196
59
34
39
10
9
20
17
23
7
30
3
4
9
17
15
13
6
1
214
169
53
111
67
171
277
339
34
35
29
50
87
78
69
25
136
99
19
23
23
4
25
28
27
38
18
20
14
7
19
20
22
21
6
9
50 mA
Spec−
20 uA
Spec−
400 mV120 ns
Spec−
ICC operatingStandby CurrentVOL,Access Time
σ
Mean
σ
Mean
σ
MeanSpec −
σ
Mean
Table 4. 43 krad(Si) die lot measured parameter variation
from specification.
Parameter
Limit
Diel Lot
B20079
B20080
C20037
D20128
E20007
E20008
E20055
E20312
E20313
E20315
E20316
F20019
F20020
F20023
F20036
F20049
F20051
G20002
Mean
13.4
13.5
12.8
12.3
12.4
12.9
13.3
13.0
12.9
12.9
13.3
12.2
12.9
13.5
12.6
12.1
12.5
12.2
Mean
4.9
4.7
6.3
5.5
4.2
4.4
4.2
4.8
13.7
7.5
12.9
10.0
9.5
6.0
5.9
7.5
5.7
8.0
Mean
99
94
102
107
105
100
108
98
104
98
107
100
101
98
107
94
100
99
Mean
94.0E9
88.2E9
93.8E9
98.8E9
89.7E9
89.1E9
91.0E9
89.9E9
89.2E9
88.8E9
91.8E9
90.2E9
88.0E9
85.3E9
87.5E9
91.7E9
90.8E9
92.2E9
55
140
72
145
193
167
91
284
166
193
202
66
93
333
107
110
288
182
23
26
23
24
47
23
28
26
1
6
1
3
2
4
7
4
9
3
275
139
98
134
57
43
59
87
55
138
97
47
47
87
81
34
212
168
12
14
5
9
22
22
19
27
17
15
11
9
18
21
21
18
5
11
50 mA
Spec−
20 uA
Spec−
400 mV
Spec−
120 ns
Spec−
ICC operatingStandby CurrentVOL,Access Time
σ
Mean
σ
Mean
σ
Mean
σ
Mean
Page 4
PW8
be counted (and removed from production lots). The parts
are then programmed with all 00’s and software protection
is disabled and the parts are tested again. This data was
then used as part of our analysis.
The 55AA checkerboard pattern of alternating 1’s and 0’ is
widely used in memory testing. This pattern tests for
leakage to the voltage supplies and leakage between
adjacent cells. The maximum rated storage temperature of
the device is 150°C. Since high temperature accelerates
the leakage characteristics of the memory cell, using the
highest allowable temperature
acceleration. The principle of temperature acceleration is
used when determining component reliability. The
Arrhenius model (equation 2) predicts failure based on
time acceleration due to temperature. It is widely used in
the semiconductor industry to establish FIT rates.
The equation takes the form of:
AF = exp {(AE / k)[1/T11/T2]} (2)
WhereAF = Acceleration Factor
AE = Activation Energy
k = Boltzmann's Constant (8.6E5 eV/K)
T1 = Lower Temperature
T2 = higher Temperature
We used the activation energy of 1.1 eV supplied by
Hitachi. A data retention period of ten years is specified
by Hitachi at 55°C and below. Therefore T1 is 328 °K
(55°C ) and T2 is 423 °K (150 °C) . Using these number
and plugging them into equation 2, the data retention test
simulates a 50 year period.
A. Data
Table 5 summarizes the results of the data retention tests
performed on the EEPROM. The first row shows the
results of the production screening test where the parts
were not irradiated. Any parts that fail are then removed
from the production lot and the 100% screened parts are
then used for flight parts (second row). There are a large
number of die lots and parts in this set of data, which can be
used as a benchmark to compare to the irradiated part data.
The last row shows the result of life tests performed on
irradiated parts. The Fit rate [3] and Mean time between
failures (MTBF) for the last three rows show only the upper
bound for FIT rate and lower bound for MTBF, since there
were no failures during the tests. This means the fit rate is
arrived at assuming that if one more part is used there will
be a failure.
gives the highest
Table 5. Data Retention Versus TID
Data
Description
# of
Parts
# of
Failures
Radiation
Level
(krad(Si))
Fit
Rate
Mean Time
between
Failures
(years)
111,450 Screening
Data
8741 4 0 1.024
Production
Lot
8737 0 0 <0.26 >445,600
Radiation
Test
35 0 40 <62 >1,836
B Analysis
1) Nonirradiated Data Retention FIT rate.
The number of failures seen in the nonirradiated parts
was very small (four out of 8741 devices). The calculated
data retention FIT rate for the screening test on non
irradiated parts is 1.02, which is quite small indicating a
mean time between failures of 111,450 years per device.
As an example, if one where to use 100 devices for 10
years, the probability of a data retention failure would be
0.9% or a 99.1% chance of no data retention failure for
these prescreened parts. Since Maxwell screens out those
failures with a 50 year data retention test, actual flight lots
would expect no data retention failures.
2) Data Retention with Radiation
Since there were no data retention failures in the
irradiated parts the data shows that leakage currents
generated by total ionizing dose at 40 krad(Si) are not
sufficient enough to cause a data retention failure.
Therefore data retention is not degraded by TID at 40
krad(Si) .
IV. ENDURANCE TEST
Endurance testing evaluates the ability of the devices to
undergo repeated erase and write cycles. The device is
specified for 10,000 erase/write cycles. We performed a
series of tests summarized in Table 6. The last row test
included three separate stress variables including a 2x
specification endurance test, data retention and irradiation to
40 krad(Si) . In this test the 10 devices went through the
following test sequence:
1) 5000 write cycles with alternating 55AA and AA55
patterns
2) 72 hour 150 °C data retention test with 55AA pattern
3) 72 hour 150 °C data retention test with AA55 pattern
4) 10 krad(Si) irradiation
Steps 1 through 4 where then repeated three more times
to a total of 20,000 write cycles, 8 data retention tests and
40 krad(Si) and finally a full electrical test was performed
on the devices (all passed the electrical tests).
Page 5
PW8
Table 6 Endurance Test Summary
# of
Parts
Write
Cycles
Test
Description
Erase/
# of
Failures
Radiation
Level
(krad(Si))
Post
production
8737 25 0 0
Endurance 10 10,000 0 0
Endurance +
radiation
10 10,000 0 40
2x
Endurance
+Data
retention +
radiation
10 20,000 0 40
The data shows several results: 1) the write cycles do not
contribute to data retention failures. The 4 devices that
failed data retention during prescreening in table 4 went
through a total of 218525 total write cycles spread out over
8741 parts. In table 6 30 parts were subject to a total of
400,000 write cycles without failure. Therefore the erase
write sequence does not induce any damage to the floating
gate cell. 2) endurance is not effected by TID. The last two
rows shows that at 40 krad(Si) irradiation levels the leakage
currents are not significant enough to induce any endurance
or data retention failure.
V. CONCLUSION
A unique set of data is available on a nonvolitile memory
that allows the statistical analysis of this device for TID
across multiple die lots, the effect of TID on data retention
and endurance. Several conclusions can be drawn from the
data.
Die lot electrical performance variation exists as can be seen
from table 1. These variations are small compared to the
parts electrical specifications and are partly a function of the
resolution of test equipment. TID increases the die lot
electrical performance variation. Despite the increase in
data spread with TID, all but one parameter were
significantly below the
specifications to be considered insignificant with regards to
large sample lots and typical mission margin requirements.
Standby current is the only parameter to show any
significant movement relative to the specifications after
irradiation to 43 krad(Si). This parameter stayed within
specification for all 80 tested devices at 43 krad(Si). When
clumping all die as one lot, the large number of tested
devices shows all parameters stayed within specification
after 43 krad(Si) when using a one sided confidence test.
Only when looking at each die lot separately and using the
same one sided confidence test does certain die lots show a
risk of exceeding standby current for large mission lots.
Because of this, Maxwell assigns die lots based on package
level shielding and mission requirements which are a
function of TID and required margins to insure that all
assigned parts meet mission requirements.
maximum performance
The unscreened EEPROM data retention FIT rate is
approximately 1 with a MTBF of 11,450 years. Actual
flight lots are 100% screened to 50 years of data retention so
no parts are expected to fail. Additionally, tests show that
data retention is not effected after irradiation to 40 krad(Si),
which is the uppermost die level requirement for most
environments used in conjunction with Maxwell's shielded
packages.
Endurance tests show that the devices more then meet the
specification requirement
Endurance, data retention in conjunction with TID at 40
krad(Si) test show that endurance and data retention are not
effected by TID at 40 krad(Si).
In summary, an extensive series of tests show that this
device is robust, meeting high reliability requirements in a
space environment with margin.
of 10,000 write cycles.
ACKNOWLEDGMENT
The authors wish to thank Carol Jackson for her help with
some of the statistical analysis.
REFERENCES
[1] Mil Handbook 814 p 90 103
[2] A detailed discussion of calculating onesided KTL
values is provided on
http://www.itl.nist.gov/div898/handbook/prc/section2/prc26
3.htm.
[3] The FIT rate is defined as the expected number of
component failures per 109 (ten to the ninth power, or
1,000,000,000) hours. The FIT rate can be converted to the
MTBF (Mean Time Between Failures) in hours as MTBF =
109/FIT.
NIST's web site at