Download Total ionizing dose effects on a radiation-induced BiMOS analog

Vol. 34, No. 1
Journal of Semiconductors
January 2013
Total ionizing dose effects on a radiation-induced BiMOS analog-to-digital converter
Wu Xue(吴雪)1; 2; 3 , Lu Wu(陆妩)1; 2; Ž , Wang Yiyuan(王义元)1; 2; 3 , Xu Jialing(胥佳灵)1; 4 ,
Zhang Leqing(张乐情)1; 2; 3 , Lu Jian(卢健)1; 2; 3 , Yu Xin(于新)1; 2; 3 , Zhang Xingyao(张兴尧)1; 2; 3 ,
and Hu Tianle(胡天乐)1; 4
1 Xinjiang
Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China
Key Laboratory of Electronic Information Materials and Devices, Urumqi 830011, China
3 University of Chinese Academy of Sciences, Beijing 100049, China
4 Xinjiang University, Urumqi 830046, China
2 Xinjiang
Abstract: The total dose effect of an AD678 with a BiMOS process is studied. We investigate the performance
degradation of the device in different bias states and at several dose rates. The results show that an AD678 can
endure 3 krad(Si) at low dose rate and 5 krad(Si) at a high dose rate for static bias. The sensitive parameters to
the bias states also differ distinctly. We find that the degradation is more serious on static bias. The underlying
mechanisms are discussed in detail.
Key words: BiMOS; A/D converter; 60 Co radiation; bias state; dose rate
DOI: 10.1088/1674-4926/34/1/015006
EEACC: 2570K; 2550R
2. Device structure and experimental approach
1. Introduction
As critical components, analog-to-digital (A/D) converters play an important role in space and military electronic systems. However, the electrical parameters of A/D converters in
these systems are affected by various particles in space through
ionizing effects, displacement effects, and single-event effects,
and even causing functional failureŒ1 6 . According to Refs. [7,
8], ionizing irradiation could result in a shift of threshold voltages and an increase of current leakage in MOSFETs, and also
the degradation of gain in bipolar junction transistors (BJTs).
Changes in these parameters can cause degradation of a device’s performance. Such degradation could threaten the reliability of spacecraft electrical systems. Consequently, it is important to research total ionizing dose effects and analyze the
failure models and mechanisms for A/D converters.
Total ionizing dose effects and dose-rate effects of A/D
converters have been studied since the 1990sŒ1 6 . There are
analog blocks and digital blocks in A/D converters and the
electrical characterization testing set-up for them is complex
because changes of only a few millivolts are sufficient to cause
high accuracy A/D converters to exceed specification limits.
So, analyzing the degradation mechanisms is not an easy job.
The key point in evaluating the reliability of a device is choosing the right bias state and dose rate. In a spacecraft, not all
devices are kept in an operational state and different bias states
will result in different radiation responsesŒ1; 2 , so we choose
different bias conditions for evaluating a device in a laboratory. In 1991, Enlow and co-workersŒ9 found that bipolar devices show enhanced low dose rate sensitivity (ELDRS). We
are not sure whether BiMOS A/D converters combining bipolar and CMOS processes exhibit the ELDRS effect or not. In
this paper, we discuss the bias effect and dose rate effect of one
BiMOS A/D converter.
The AD678 is a complete, multipurpose 12-bit monolithic
A/D converter, fabricated in Analog Devices’ BiMOS process, combining a high-precision low-power CMOS logic, low
noise bipolar circuits; and laser-trimmed thin-film resistors to
provide high accuracy. Bipolar devices are generally used in
only a few circuit areas where the low offset voltage and high
transconductance of bipolar devices provide major advantages,
such as the input stage of the comparator. Hence, the usual approach is to design bipolar devices that can be integrated into a
CMOS process with relatively few process steps. The AD678
is designed for an easy board layout, and provides flexibility
for design applications, consisting of a sample-hold amplifier
(SHA), a microprocessor compatible bus interface, a voltage
reference, and clock generation circuitry. A simplified functional block diagram is shown in Fig. 1.
In addition to the SHA and control logic, the device utilizes
a recursive subranging algorithm which includes error correction and a flash converter to achieve high speed and high resolution.
Total dose irradiation experiments were executed with
60
Co -ray sources of 1 1015 Bq and 3.7 1013 Bq at
the Xinjiang Technical Institute of Physics and Chemistry, at
a dose rate of 25 rad(Si)/s (high dose rate) and 0.02 rad(Si)/s
(low dose rate). All the samples were placed in Pb/Al boxes for
accurate irradiation and divided into two bias groups: (1) zero,
all the pins were grounded; and (2) static, which initialized the
devices to a known state, but did not allow it to perform active conversions during the irradiation, as shown in Fig. 2. The
outputs were tied to the GND through a 10 k resistor. After
all irradiations were complete, the devices which irradiated at a
high dose rate were annealed with the same bias condition, and
the annealing time equaled the irradiation time at a low dose
rate to confirm whether it had time dependent effects (TDE).
Ž Corresponding author. Email: [email protected]
Received 10 May 2012, revised manuscript received 5 June 2012
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c 2013 Chinese Institute of Electronics
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Wu Xue et al.
Fig. 1. AD678 functional block diagram.
failed until 100 krad(Si) under zero bias at both dose rates.
More details will be discussed below.
3.1. Radiation response between different bias states
Fig. 2. Static bias state.
All the tests were completed with an off-line-test method regardless of irradiation or annealing on the integrated circuits
testing system according to SJ 20961-2006.
3. Experimental results
There were big differences in the total dose levels of the
converters with different bias conditions and dose rates. Some
parameters failed at levels as low as 5 krad(Si) under a static
bias condition at an HDR, whereas the power consumption
Figure 3(a) describes the relationship between degradation
of power consumption and total dose. It can be shown that with
the increase of irradiation total dose, with static bias state and
HDR, the power consumption sharply increased, whereas under the other conditions, the changes are not so obvious. The
nominal power consumption of the samples is about 560 mW.
Under static bias, it exceeded the maximum specification limit
of 745 mW at about 5 krad(Si) and continuously increased until 10 krad(Si) at HDR, however, it was within its specification limit until reaching the same total dose at LDR. Contrarily, under zero bias, it never exceeded the limit until irradiation was over at both dose rates. So we can be sure that the
degradation of power consumption was not the reason for sample failure. Figure 3(b) shows that when the HDR-irradiated
converters were annealed at room temperature for nearly three
months, there was some recovery for the samples that were under static bias. However for those devices under zero bias, they
kept nearly constant.
The input–output curve represents whether the converter
function is normal or not. For a normal A/D converter, with
the increase of the analog input, the output code will increase
linearly and also as long as the input step is small enough, no
code will be missed. Figures 4 and 5 show the relationships between the input–output curve and the total dose at an LDR. We
do not give the curves at an HDR because the sample degradation trend was the same as that at an LDR. From Figs. 4 and
5, we can see that with different bias states the failure modes
and failure levels were also distinct. Before irradiation, the input–output curves showed a linear relationship. However, it
was found that there were some changes after irradiation. At
levels as low as 3 krad(Si), with higher analog input the curve
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Fig. 3. Power consumption of the AD678.
Fig. 4. The input–output curve of AD678 with LDR.
Fig. 5. The input–output curve of AD678 with LDR.
became abnormal at static bias. After then, with total dose increasing, more obvious changes appeared in the curve. It is very
different with degradation under zero bias. Until 50 krad(Si),
we found some small changes in the curve and, also, the mode
is different, as is shown in Fig. 4. After 50 krad(Si) irradiation, Code 1 was found in the output until the analog input was
at about 37 LSB, and then the curve displayed good linearity.
Also Code 4095 did not appear with a VFS input. That means
parameters E0 (zero error) and EG (gain error) could not be
measured according to their definitions. Although the curves
of E0 and EG are not drawn here, we could know their changes
according to the input–output curves.
The differential non-linearity (DNL) and integral nonlinearity (INL) of one converter, which can be figured out according to the input–output curve, are the most important parameters of A/D converters; changes in the curve can reflect
changes of DNL. The radiation responses of DNL under LDR
are shown in Figs. 6 and 7. It can be seen that DNL is more
sensitive under static bias than zero bias. Figure 6 shows that
DNL exceeded its specification (˙1 LSB) at about 3 krad(Si).
On the other hand, DNL degraded until to 50 krad(Si) at zero
bias, the same as the input–output curve. The DNL characteris-
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Fig. 6. Relationship between the DNL of the A/D converter and total
dose with static bias at 0.02 rad(Si)/s.
Fig. 7. Relationship between the DNL of the A/D converter and total
dose with zero bias at 0.02 rad(Si)/s.
Fig. 8. Relationship between the misscode of the A/D converter and total dose.
tic under HDR showed the same trend as shown in Figs. 6 and
7. With higher analog input, the DNL of AD678 degraded.
3.2. Dose-rate effect
Irradiation will produce oxide-trap and interface-trap
charges. When the oxide-trapped charge, especially the
metastable oxide trapped charge, is dominant to the ICs’ degradation, the device will exhibit TDE; otherwise, it may exhibit
ELDRS. Figure 8 shows the degradation of misscode up to
150 krad(Si) for zero bias and up to 10 krad(Si) for state bias at
HDR and LDR. The misscode, as shown in Fig. 8 for static bias
devices at LDR, exceeded its specification at about 3 krad(Si).
As for static bias devices irradiated under HDR, this parameter
failed at about 5 krad(Si). Eventually, the samples at static bias
were irradiated to 10 krad(Si) for observing the trend. From
Fig. 8, when the samples were radiated to 10 krad(Si), the
degradation at an LDR was more serious than that at an HDR.
For zero bias samples, no obvious difference was found at HDR
and LDR. All devices irradiated at an HDR recovered subse-
quently to annealing at room temperature. Although they did
not recover to their initial values, compared to the degradation
value at LDR, they were smaller enough to make sure that the
misscode had an ELDRS.
4. Analysis and discussion
The AD678 contains different analog blocks and digital
blocks, so it needs to be driven by an analog power supply
(˙12 V) and a digital power supply (C5 V). We can explain
the degradation of power consumption through the changes of
power supply. Three operating currents versus total dose under both bias states at HDR and LDR irradiation is shown in
Fig. 9. Through comparison of various conditions, we found
that at static bias with HDR, the degradation was the most serious. The reasons for this are as follows: (1) for transistors (BJTs
or MOSFETs), power on them could induce an electric field in
the SiO2 oxide layer, which could scan the electrons produced
by radiation out of the oxide layer. (2) It could produce lots of
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Fig. 9. The normalized operating current. (a) C12 V operating current. (b) C5 V operating current. (c)
dose.
12 V operating current versus total
of bipolar devices provide major advantages such as the input stage of the comparator. The architecture of the samples is
pipelined, and the samples contain flash A/D converter blocks.
Figure 10 shows the structure of a flash A/D converter and we
can see that it contains comparator blocks. The performance of
a comparator, including its low offset voltage, offset current,
and transfer curve is very closely related to the DNL and miscode parametersŒ12 . Studies have shownŒ5; 12 that irradiation
will cause offset voltage to increase and output voltage to decrease. According to Fig. 10, they could influence the function
of a decoding logic block and then the output codes. This degradation will change the input–output curve of the converter and
then correspondingly the DNL value, which can be calculated
by using Eq. (1).
DNL.i / D fŒS.i C 1/
Fig. 10. The structure of a flash A/D converter.
electron-hole pairs very quickly under an HDR and do not have
enough time to recombine, so the electrons would be scanned
out of the oxide layer by the electric field immediately. The
remaining holes are transported to the Si–SiO2 interface, captured by the oxide-trap and interface-trap, and form oxide-trap
and interface-trap charges eventually. Comparing the degradation of three operating currents, we concluded that digital current was the most sensitive. After irradiation, the value was
about 13 times more than the initial one. However, the analog supply currents degraded unobviously under LDR irradiation. The digital power was supplied mainly the digital blocks,
which might be processed largely by CMOS transistors in BiMOS converters. Gamma ray and leakage current effects in
the CMOS transistors were more serious than that found in the
BJT, including gate oxide leakage, field oxide leakage, and so
onŒ8 . It is because of this that the degradation of the digital operating current was more severe than in an analog current. On
the other hand, as to BJT, irradiation will also increase the base
current and a produce leakage current, which is much smaller
between the collector and the emitterŒ7 . So it can explain the
changes of Ie . Throughout the irradiation process, no obvious
degradation was found in Ic .
Bipolar devices are generally used in only a few circuit areas, where the low offset voltage and high transconductance
S.i /=LSBg
1;
i D 1; 2; : : : ; 2N
2:
(1)
According to Eq. (1), when DNL(i) D –1, the code i
missed. Combining Refs. [3, 10], we almost make sure that
the comparators in the flash A/D converter degrade more seriously when irradiated at LDR than at HDR. When bipolar
comparators are exposed to ionizing radiation, oxide-trapped
charges and interface states accumulate in the screen oxides
that lie over the surface of the emitter–base junction. The form
of the oxide-trapped charge and interface state are correlated
with the radiation-induced defects in the SiO2 , especially those
near the SiO2 –Si interface and the electric field induced by
irradiation. The space charge modelŒ7;12 14 illustrates radiation under different dose rates. The space field formed by
oxide-trapped charge is different and this could influent the
form of interface state. During radiation at HDR, lots of oxide
trapped charges are produced very quickly, which could form
a space field and prevent the holes or HC moving forward to
the SiO2 –Si interface. Only a small amount of holes and HC
can be transported to the interface and then form an interface
state. However, the amount of electron–hole pairs produced by
irradiation at LDR at unit time was much less than at HDR, so
the space field in the SiO2 oxide layer was a little weak. Consequently, holes had moved to the interface and formed interface
states. That is why the degradation was more serious when irradiated at LDR than at HDR.
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5. Conclusion
The AD678 is very sensitive to gamma ray irradiation, especially under a static bias state. Many parameters which are
very important for A/D converters failed at 10–15 krad. Digital
power current is more sensitive than analog current because of
the increase of leakage in internal CMOS transistors.
Misscode degraded more significantly at LDR and exhibited ELDRS. The failure appears to be due to internal comparators in the converter manufactured by using a bipolar process.
For mixed signal ICs such as A/D converters and D/A converters, it is very hard to evaluate and analyze the degradation mechanisms because there are many complicated analog
blocks and digital blocks inside. So we need more experiments
and sophisticated instruments to figure out which block is the
most sensitive and fatal.
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