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Relaxation of operational amplifier parameters after pulsed electron beam irradiation
Microelectronics Reliability 39 (1999) 1485±1495
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Relaxation of operational ampli®er parameters after pulsed electron beam irradiation C.A. Betty a,*, K.G. Girija a, R. Lal b a
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b Electrical Engineering Department, I.I.T., Powai, Mumbai 400 076, India Received 17 February 1999; received in revised form 12 June 1999
Abstract The relaxation of operational ampli®er parameters (oset voltage and dierential gain) with time after pulsed electron beam irradiation has been studied as a function of total dose and ampli®er type. Four types of operational ampli®ers were studied viz., general purpose bipolar input (mA 741), super-beta transistor input (LM 308), JFET input (LF 356) and MOSFET input (CA 3140) from dierent vendors. The experiments were carried out mainly using 500 ns pulses from a Linear Accelerator. The study, the ®rst of its kind, shows that while the electrical transient at the output of the operational ampli®er recovers in a few milliseconds, relaxation of parameters can take several to several tens of seconds. This relaxation is attributed to the build up and/or anneal of damage in the oxide or at the interface of the internal transistor structures. The change and relaxation of parameters depend on operational ampli®er type and total dose, and can have signi®cant eects in certain application domains as illustrated by the response of a thermocouple ampli®er after pulsed irradiation. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Relaxation; Operational ampli®ers; Pulsed irradiation; LINAC; Time dependent phenomena
1. Introduction The relaxation of device parameters after a pulse of ionizing radiation is of concern to device users designing systems for harsh transient radiation environments. It is well known that electron±hole pairs created in the semiconductor by ionizing radiation recombine within a millisecond. Dispersive transport of holes in the oxide and detrapping from shallow or tunneling type
* Corresponding author. Tel.: +91-22-550-5148; fax: +9122-550-5151. E-mail address: [email protected] (C.A. Betty)
of traps near the interface take times of the order of tens of seconds [1,2]. Proton creation, drift of protons and interface state generation can occur for much longer periods [3,4]. Oxide charges and interface states aect the device parameters and as a consequence there should be relaxation of device parameters. Parameter relaxation eects, after a high dose pulse, are expected to occur in integrated circuits. While the mechanisms responsible for the delayed eects have been studied in detail using MOS capacitors or MOSFETs, their eects on integrated circuit parameters have not been investigated. This study has been carried out to see how operational ampli®er parameters relax with time. The eects of parameter relaxation would be of greater consequence for analog
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Table 1 List of manufacturers and batch/identi®cation numbers Device number
Manufacturer
Type number
Batch/identi®cation number
M3/741/2 M5/741/2 M3/308/1 M6/308/2 M6/308/4 M2/356/2 M3/356/10 M1/3140/5 M4/3140/1
M3-National M5-GSL (India) M3-National M6-Fairchild (Korea) M6-Fairchild (Korea) M2-Motorola M3-National M1-RCA M4-HARRIS
LM 741 CH 741C LM 308 AH UA 308 HC UA 308 HC LF 356 BH LF 356 H CA 3140 CA3140T
S:H8712 X3 M9036 8635K 8724K K8544 950 RCA537 H041
integrated circuits than for digital integrated circuits and should therefore ®rst be investigated for parameter relaxation. In this paper, the eects of a pulse of ionizing radiation on operational ampli®er parameters are being reported. There have been experimental studies of the transient response of operational and dierential ampli®ers after pulsed X-ray or electron beam irradiation [6,7]. These experiments have been compared with device and circuit simulations in which only excess carriers in the semiconductor have been modeled and have shown reasonable ®ts in the time frame under consideration (less than a ms) [5,7,8]. However, this is the ®rst report on the eects of relaxation of operational ampli®er parameters after high dose rate pulsed electron beam irradiation.
2. Experiment Four categories of operational ampli®ers (OA) were studied: general purpose bipolar input (mA 741), superbeta transistor input (LM 308), JFET input (LF 356) and MOSFET input (CA 3140). Epoxy encapsulated DIP devices were susceptible to encapsulation failure and hence only devices in metal-can TO-99 packages were studied. In order to check if the dierences in parameter relaxation due to high dose rate irradiation are due to dierences in technology, lots of ®ve devices from two vendors for each category were used in these studies. While we present data for a particular device, trends are similar for devices from the same vendor except for some exceptions which have been indicated. Table 1 gives details of device number and manufacturer. The devices were mounted on an X±Y stage, con®gured as an inverting ampli®er (gain = ÿ1) and pulsed irradiated by 7 MeV electron pulses from a Linear Accelerator (LINAC). Pulses of 50 and 500 ns were used. The dose per pulse determined by thiocyanate dosimetry was 16.16 Gy(Si) and 27.84 Gy(Si), respect-
ively. The ¯uence per pulse was estimated to be 2.047 1011 and 5 1011 electrons/cm2, respectively. A computer aided measurement system was used for data acquisition and analysis. The block diagram of the measurement setup is given in Fig. 1. The LINAC was triggered by an external pulse which was also used to trigger a 100 Msample/s digitizer in the measurement room. The transient output of the OA, connected by guard driven shielded cable to a fast 120 MHz bandwidth ampli®er housed in a radiation and EMI (Electromagnetic Interference) shielded box in the irradiation room was captured by the digitizer in the measurement room. Operational ampli®er parameters were obtained with the help of a relay matrix and a data acquisition and control card. Standard con®gurations and techniques were used for measurement of the parameters viz., oset voltage (Vos) and open loop gain (Adm) [9]. The power supply used was a 215 V regulated DC supply. In the initial stages of the experiment, an 8 bit ADC card with a software selectable voltage range was used for parameter measurements. Later, to make more sensitive measurements in the case of OAs with very small relaxation of oset voltage we used a 12 bit ADC with a switch-selectable voltage range. Even with a 12 bit ADC, errors in Adm were large and hence CMRR (Common Mode Rejection Ratio) relaxation is not being presented. The operational ampli®er under test was con®gured for parameter measurements after the electrical transient settled down (Fig. 2(a) and (b)). First set of parameter measurements were done after 200 ms after the pulse. Subsequent sets of operational ampli®er parameter measurements were made after 1, 2, 5, 10, 50, and 100 or 500 s after the irradiation pulse. This sequence was repeated after every ®ve pulses till 25 pulses and then after every 25 pulses till the operational ampli®er parameters degraded or there is a `total failure' or the total dose exceeded 5000 Gy(Si). By parameter degradation we mean the parameters going out of acceptable range from the speci®ed value, for e.g. the acceptable range for Vos is 210 mV from the initial
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Fig. 1. Measurement setup.
value. By `total failure' we mean catastrophic failure which makes the output saturate to near rail voltages (railspan collapse). We have thus obtained relaxation of OA parameters after pulsed electron irradiation as a function of total dose results. In this paper, we shall focus on the relaxation of operational ampli®er parameters as a function of total dose. One set of parameter measurements takes about 400 ms because relays are switched and time has to be allowed for settling of the feedback circuits used in these measurements. We have noted that the devices are not in latched-up condition since the transients decay to zero and the operational ampli®er circuit behaves normally without requiring to turn o and turn on the power to the ampli®er under test. Experimentally, it has been veri®ed that the current transients in the power supply last for less than a few tens of microseconds. In Fig. 2(a) and (b), we give the output transients obtained with one pulse of electrons for dierent OAs. The fast buer has an output range of 24 V with a wide bandwidth attenuator at the input of the fast ampli®er and the overall gain from the input of the fast ampli®er to the digitizer is 0.475.
Therefore, an output 24 V corresponds to 28 V. In order to capture the full signal, a pretrigger delay of about 50 ms was allowed. From Fig. 2 it is clear that the electrical transient recovers within a millisecond. When a device is irradiated with a high dose rate pulse, two eects occur: a total dose eect on parameters and a relaxation of parameters. These are shown schematically in Fig. 3. The dierence in the values of the parameter at t 4 1 and the preirradiation value is the total dose damage due to the irradiation pulse. The dierence in the values of the parameter at any time after the irradiation pulse and t 4 1 is the relaxation. The relaxation is, as we broached earlier, due to buildup and/or anneal of damage in the oxide or at the interface. The parameter relaxation plots are normalised using: Normalized Pi
Pi t P i 0
where Pi 0 is the ®rst parameter reading after the corresponding irradiation. The number of pulses i, and the values of the parameter at 0.2 s (Pi 0) are given in
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stants, possibly implying that the controlling processes can be modeled using a ®rst order dierential equation. Each time constant corresponds to the time taken by the respective slope in the parameter relaxation plot. In Table 2, we have given the order of the relaxation time and the number of time constants for the relaxation of oset voltage and dierential gain. 2.1. General purpose bipolar input operational ampli®er (mA 741) The bipolar operational ampli®er mA 741 from two vendors (M3 and M5) were studied and 500 ns pulses were used for parameter relaxation studies. The devices were irradiated without limiting resistors in the power supply leads. M3's devices took longer times for Vos relaxation compared to M5's devices (see Fig. 4(a) and (b)). From Fig. 4(a), we note that the magnitude of variation of Vos with time for M3's devices is not much as compared to M5's devices (Fig. 4(b)). In Fig. 4(a), we can see three time constants for the relaxation. Vos increases slowly with total dose. Adm does not show signi®cant relaxation for both manufacturers. But Adm decreases with total dose. For M3/741/2, Adm decreases from 5 104 to 1.1 104 for 3.5 kGy(Si). For M5's devices, the oset voltage relaxed within 1±2 s (Fig. 4(b)). It is observed that Vos changes with just one pulse of radiation. The inset shows that immediately after the ®rst radiation pulse there is a large
Fig. 2. (a) Transient output signals of bipolar input operational ampli®ers. (b) Transient output signals of FET and MOSFET input operational ampli®ers.
the insets in relevant ®gures. The results are discussed operational-ampli®er-type-wise. In Table 2, we summarise the doses to which the devices were irradiated. For several parameters, we found that the relaxation process has several time con-
Fig. 3. Schematic diagram of parameter relaxation.
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Table 2 Operational ampli®er types, maximum dose given and relaxation parameters Type #
741 308 356 3140 a b
Manufacturer
M3 M5 M3 M6 M2 M3 M1 M4
No. of pulses given
200 (500 ns) 200±325 (500 ns) 40±50 (500 ns) 150 (500 ns) 50 (500 ns) 150 (50 ns) 50 (500 ns) 2±3 (500 ns)
Corresponding total dose in kGy(Si)
Relaxation time and no. of time constants For Vos
For Adm
5.57 5.57±9.0 1.1±1.4a 4.15 1.4b 2.4 1.4a 0.057±0.08a
> 100 s, 2±3 time constants 01 s, 1 time constant 050 s, 2±3 time constants 01 s, 1 time constant 0500 s, 3 time constants 0150 s, 2 time constants > 500 s, 2 time constants Railspan collapse
Little relaxation Little relaxation Little relaxation Little relaxation Little relaxation > 500 s, 2 time constants 0500 s, 2 time constants Railspan collapse
Parameter went out of measurement range. Irradiated with current limiting resistors in the power supply leads.
change in Vos which relaxes nonmonotonically. In Fig. 4(b), there is only one time constant for the Vos vs. time plots. The pre-radiation value of Vos which is positive, changes to negative with total dose. 2.2. Super beta input operational ampli®er (LM 308) The devices were studied for parameter relaxation with 500 ns pulses. Devices from M3 and M6 were irradiated without any current limiting resistor in the power supply leads. The Vos relaxation plot of M3 is given in Fig. 5(a). It can be seen that for M3's devices Vos relaxes sharply after the irradiation pulse for the initial irradiation. However, after 25 pulses (1700 Gy(Si)) there is little relaxation. M3's devices show a nonmonotonic Vos relaxation. Also Vos changes sign with dose. The plot shows that the oset voltage increases in magnitude very fast with total dose. Three time constants are present in the Vos relaxation plot (Fig. 5(a)). For M3's devices, Adm did not relax much till a dose of 12070 Gy(Si) but decreased from 1.4 105 to 9 104. The typical Vos vs. time plot (Fig. 5(b)) for M6's devices shows that Vos changed with the ®rst pulse. In this case also the change in Vos immediately after the irradiation is very large. It took about 1 s for relaxation till a total dose of 1700 Gy(Si). After 1840 Gy(Si), there is no signi®cant relaxation. The shift in oset voltage with total dose is also very large. Only one time constant is seen in Fig. 5(b). In all devices except for M6/308/4, Vos showed a monotonic decrease with total dose. M6/308/4 showed a change in sign for Vos with dose. For all devices, Adm did not show a relaxation pattern, but decreased with total dose. On comparison of irradiation results for these two manufacturers' devices, M6's devices showed a greater amount of parameter relaxation as well as greater
degradation than M3's devices. M3's devices were irradiated upto 4000 Gy(Si) without much degradation in the parameters. 2.3. JFET input operational ampli®er (LF 356) Parameter relaxation studies were carried out on devices from M2 and M3. Since both manufacturers' devices failed catastrophically with a single 500 ns pulse, 50 ns pulses were used. But M2's devices failed even with a 50 ns pulse. Hence M2's devices were irradiated with 500 ns pulses, connecting a 20 O current limiting resistors in the power supply leads and typical plots of parameter relaxation for these devices are also given. Fig. 6(a) gives the Vos relaxation plot for M2's devices. For M3's devices, we got parameter relaxation even with a small pulse (50 ns) (Figs. 6(b) and 7). Fig. 6(a) shows that it takes a longer time for Vos relaxation of M2's devices compared to that from M3 (Fig. 6(b)). For M2's devices there are three time constants (Fig. 6(a)) and for M3's devices there are only two time constants in the Vos relaxation plots (Fig. 6(b)). After 100 s, Vos started decreasing for M3's devices. Similarly in Fig. 7 Adm also increased after about 100 s. Adm relaxation shows two time constants. 2.4. MOSFET operational ampli®er (CA3140) Devices from M1 and M4 were used for parameter relaxation studies. The radiation pulse width used was 500 ns without any current limiting resistor. There was a large variation in gain (Adm shifted from 32094 to 22910 for M4/3140/1) and oset voltage (Vos decreased from 3.99 10ÿ3 to ÿ1.567 10ÿ2 V for M4/3140/1) between the pre- and ®rst post-irradiation measurements. The outputs of M4's devices got saturated to
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Fig. 4. (a) Typical plot of normalised oset voltage vs. time for M3/741/2. (b) Typical plot of normalised oset voltage vs. time for M5/741/2.
power supply voltage (railspan collapse) with the second irradiation pulse. Parameter relaxation was, therefore, studied using only M1's devices. As in the case of JFET input operational ampli®ers, MOSFET operational ampli®ers also show three time constants in the Vos relaxation (see Fig. 8). Adm relaxation shows only two time constants (see Fig. 9). From these two ®gures it can be observed that the shapes of the relaxation plots for Adm and Vos are similar. M1's devices show
Fig. 5. (a) Typical plot of normalised oset voltage vs. time for M3/308/1. (b) Typical plot of normalised oset voltage vs. time for M6/308/2.
improvement in all parameters at low total doses. However, after a certain threshold total dose, all these devices showed a sudden degradation (railspan collapse) in all the parameters. The dose at which this degradation appears varies from device to device and lies in the range 400 to 2000 Gy(Si). M1/3140/5 saturated to the power supply voltage with 20 pulses.
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Fig. 7. Typical plot of normalised open loop gain vs. time for M3/356/10 (12 bit ADC measurement).
technology is required. In the absence of this information, only some generic comments can be made about operational ampli®er response to a pulse of ionizing radiation. All the operational ampli®ers Ð general purpose bipolar and super-b, JFET and
Fig. 6. (a) Typical plot of normalised oset voltage vs. time for M2/356/2 (12bit ADC measurement). (b) Typical plot of normalised oset voltage vs. time for M3/356/10 using a 50 ns pulse (12 bit ADC measurement).
3. Discussion From the results above, it is clear that there is wide variation in the relaxation of operational ampli®er parameters after pulsed 7 MeV electron beam irradiation. To be able to coherently explain all the data obtained in this study, detailed information about layout and
Fig. 8. Typical plot of normalised oset voltage vs. time for M1/3140/5 (12 bit ADC measurement).
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Fig. 9. Typical plot of normalised open loop gain vs. time for M1/3140/5 (12 bit ADC measurement).
MOSFET input operational ampli®ers Ð used in this study had three stages: a dierential input stage, an intermediate stage and an output stage. Therefore there are a minimum of seven devices directly in the signal path that could aect the operational ampli®er parameters. In addition, there are several in the biasing network whose imbalance could aect parameters. One might expect to see the signatures of the relaxation of these devices. With pulsed high dose rate irradiation, we see from the studies that there are both total dose and time dependent eects. The total dose modi®cation and time dependent relaxation of parameters in the signal path and biasing networks bring about the changes we have observed. Therefore we discuss how transistor parameters might relax after pulsed irradiation. Freitag et al. [11] have reported the gain relaxation of transistors due to low dose rate irradiation. But during low dose rate irradiation the mechanisms are quite dierent because both defect generation and annealing process are happening simultaneously. It has been observed that both lateral and substrate pnp transistors which are used in linear ICs show severe gain degradation due to irradiation [12,13]. These transistors are used in internal bias circuits, active loads and output driver stages. Input transistors (npn, super-b, JFET and MOSFET) [12,13,15] also show gain degradation after high/low dose rate irradiation. Relaxation of transistor parameters occurs due to charge trapping and anneal in the oxide and interface
state creation. All these processes continue after the pulsed irradiation is over. Since the processes are oxide and interface processes, regions of devices near the oxide±semiconductor interface are most susceptible. Hence MOSFETs and lateral transistors are especially vulnerable. Buried channel JFETs as used in operational ampli®ers and vertical transistors are not as susceptible, though the intersection of a diused junction with the interface could be a problem area. Because hole trapping and interface state creation Ð either due to trapped hole recombination or proton transport to the interface Ð depend on the number of holes generated in the oxide, thicker oxides would cause larger relaxation eects if covering active areas of devices. The eects of hole trapping and anneal and interface state creation on MOSFETs are well studied. Change in bulk and interface charge aects not only threshold voltage but also transconductance due to reduction of surface mobility because of oxide and interface charge scattering. Fig. 10(a)±(d) give the schematic of the speci®c technologies used for fabrication of internal transistors of the OAs studied. In bipolar transistor, very thick oxides (11 mm) are present over the base±emitter junction of substrate npn and lateral pnp transistors structures (Fig. 10(a) and (b)). After a high-dose-rate pulsed irradiation, a large number of holes (Qh) get trapped in shallow hole traps as well as in deep hole traps present or created in the bulk oxide [16]. Interface states (Nis) are also created at the interface due to irradiation [17]. The holes in shallow traps detrap in less than a second. Most of these detrapped holes during transport towards the interface get trapped in the deep hole traps near the interface [16]. Some of the annihilated holes may form interface states. The holes in deep traps detrap slowly over a few decades in time and when a thermally detrapped hole drops into the silicon valence band as it leaves the oxide, it forms interface states [14]. Shallow hole trapping in the bulk oxide releases protons which diuse and, in the presence of positive bias, drift to the interface where they create interface states. The hole trapping, detrapping and interface state formation cause gain relaxation of the internal transistors via several processes as shown in Fig. 10(a)±(d). This subsequently aects operational ampli®er parameters. In thick oxides, with low ®elds, a signi®cant fraction of the charge appears to be trapped permanently in the bulk region of the oxides which leads to lesser trapped hole recombination [12]. But in thin gate oxides with large electric ®elds, the detrapping of holes from deep traps is faster and this leads to a higher density of interface states [12]. At low total doses the oxide has less number of holes trapped in deep traps and interface states. As the device is exposed to pulses over a length of time, the density of interface states increases. The amount and
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Fig. 10. (a±d) Schematic of the mechanisms which cause gain relaxation of internal transistors.
nature of relaxation get aected in a complex way, depending on the space charge, due to prior total dose damage and subsequent trapping and anneal. The dierential gain relaxation of the transistors in the internal bias circuitry and input pair is responsible for the Vos relaxation in the operational ampli®er circuits [12]. For MOSFET input stages the dierence in threshold voltage and the relaxation of this would dominantly aect Vos. The relaxation of parameters of transistors in internal bias circuit Ð including substrate pnp transistors Ð and active load cause open loop gain relaxation [12]. The longer time taken for Vos relaxation in JFET and MOSFET operational ampli®ers is mainly due to the eect of interface states produced in the input transistors. In bipolar input stage devices, the bulk ®eld oxide traps are causing more parameter relaxation compared to the interface states. But in devices with thin gate oxides, the inter-
face states are causing more relaxation. LF356 which has a buried p-channel input dierential pair and CA3140 which has a p-MOSFET (with holes pulled away from the interface) input pair show an improvement in the Adm plots. As the holes drift away from the oxide±silicon interface there is no signi®cant hole trapping and interface state generation in p-channel MOSFETs. If some holes however do get trapped and subsequently anneal, there would be an improvement in gm. This explains the improvement of Adm in pMOSFET input stage devices. A similar argument could be given for p-buried-channel JFET OAs. However, in this case, there is no high ®eld in the oxide over the buried channel and therefore lesser trapped hole anneal. The dierence in the parameter behaviour of devices from dierent manufacturers can be attributed to the variations in device structures and oxide properties of the internal transistors.
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4. Conclusion We report on the relaxation of operational ampli®er parameters to pulsed electron beam irradiation. While all the operational ampli®er types studied Ð i.e. general purpose, super-û, JFET and MOSFET input Ð showed the relaxation of one or other parameter, the fractional change of parameters is relatively smaller for the general purpose operational ampli®er mA 741. It also has better total dose performance. The eect of parameter shift and relaxation can have considerable eect in certain classes of circuits based on operational ampli®er, such as low drift thermocouple ampli®ers or high gain instrumentation ampli®ers. In Fig. 11, we give a typical output after three 500 ns pulses are given to a high gain (gain = 1000) thermocouple ampli®er based on an OP07. The inputs were grounded. With the sampling rate set at a second, the electrical transient which lasts for hundreds of microseconds, is not captured. However, what is captured is the eect of parameter relaxation. It can be seen that the output takes tens of seconds to settle down due to the relaxation of parameters of the operational ampli®er, in this case dominantly due to oset voltage relaxation. To date, the design of devices and systems for radiation harsh environments have primarily considered total dose eects, prevention of latch up, single event
upset and electrothermal damage. Care is taken to reduce total dose eects in rad-hard devices by suppressing the density of bulk oxide traps or removing them from near the Si±SiO2 interface by appropriate processing. However, the oxide still has a fairly high concentration of hydrogen [10] which causes, in the absence of an oxinitride layer, a slow increase of interface state density under irradiation, especially when there is a ®eld aiding drift of irradiation generated hydrogen ions to the interface [4]. As a consequence, devices are still prone to slow parameter relaxation after a high dose rate pulse. It has been observed that parameter relaxation strongly depends on the history of irradiation. Further studies of the eects of transient radiation on unit devices used in operational ampli®er circuits is required before design changes can be made to fabricate operational ampli®ers that are truly hard to both total dose and dose rate eects.
Acknowledgements Authors are thankful to Dr.J.P.Mittal for his encouragement in this work. Thanks are also extended to Mr. Dipankar Sarkar and Mr. Vijaya Raghavan of Electrical Engineering Department I.I.T. Bombay for their help in the instrumentation.
References
Fig. 11. Typical output response of a high gain thermocouple ampli®er after an electron beam pulse based on OP07 which had its input grounded. (Response after 3rd 500 nanosecond pulse (8 Gy(Si)).)
[1] Ma TP, Dressendorfer PV. Ionizing radiation eects in MOS devices and circuits. New York: Wiley-Interscience, 1989. [2] Messenger, Ash. The eects of radiation on electronic systems. New York: Van Nostrand Reinhold, 1986. [3] Edwin Boesch Jr. H. Time dependent interface trap eects in MOS devices. IEEE Trans Nucl Sci 1988;6:1160. [4] Saks NS, Brown DB. Observation of H+ motion during interface trap formation. IEEE Trans Nucl Sci 1990;37(6):1624. [5] Nicholas JS, Alexander DR, Hittinger GN. Characterization and modeling of the 709 integrated operational ampli®er in an ionizing radiation environment. IEEE Trans Nucl Sci 1969;16:172. [6] Massengill LW, Sherra E Deihl-Nagle. Transient radiation upset simulations of CMOS memory circuits. IEEE Trans Nucl Sci 1984;31(6):1337. [7] Paulos JJ, Bishop RJ, Tur¯inger TL. Radiation induced response of operational ampli®ers in low level transient radiation. IEEE Trans Nucl Sci 1987;34(6):1442. [8] Florian JR, Jacobs RW, Micheletti Paul E, King EE. Improved transient response modeling in ICs. IEEE Trans Nucl Sci 1984;31(6):1402. [9] Analog Circuit Design seminar, Analog Devices Inc., USA, 1984, pp. 1±4, 1±6. [10] Gale R, Feigl FJ, Magee CW, Young DR. Hydrogen mi-
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MOS capacitors with the silicon in inversion. J Appl Phys 1993;73(8):3857. [15] Johnston AH, Swift GM, Rax BG. Total dose eects in conventional bipolar transistors and linear integrated circuits. IEEE Trans Nucl Sci 1994;41:2427. [16] Fleetwood DM, Kosier SL, Nowlin RN, Schrimpf RD, Raber RA, Delausm M, Winokur PS, Wei A, Combs WE, Pease RL. Physical mechanisms contributing to enhanced bipolar gain degradation at low dose rates. IEEE Trans Nucl Sci 1994;41:1871±3. [17] Saks NS, Dozier CM, Brown DB. Time dependence of interface trap formation in MOSFETS following pulsed irradiation. IEEE Trans Nucl Sci 1988;35(6):1168±77.
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Relaxation of operational ampli®er parameters after pulsed electron beam irradiation C.A. Betty a,*, K.G. Girija a, R. Lal b a
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b Electrical Engineering Department, I.I.T., Powai, Mumbai 400 076, India Received 17 February 1999; received in revised form 12 June 1999
Abstract The relaxation of operational ampli®er parameters (oset voltage and dierential gain) with time after pulsed electron beam irradiation has been studied as a function of total dose and ampli®er type. Four types of operational ampli®ers were studied viz., general purpose bipolar input (mA 741), super-beta transistor input (LM 308), JFET input (LF 356) and MOSFET input (CA 3140) from dierent vendors. The experiments were carried out mainly using 500 ns pulses from a Linear Accelerator. The study, the ®rst of its kind, shows that while the electrical transient at the output of the operational ampli®er recovers in a few milliseconds, relaxation of parameters can take several to several tens of seconds. This relaxation is attributed to the build up and/or anneal of damage in the oxide or at the interface of the internal transistor structures. The change and relaxation of parameters depend on operational ampli®er type and total dose, and can have signi®cant eects in certain application domains as illustrated by the response of a thermocouple ampli®er after pulsed irradiation. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Relaxation; Operational ampli®ers; Pulsed irradiation; LINAC; Time dependent phenomena
1. Introduction The relaxation of device parameters after a pulse of ionizing radiation is of concern to device users designing systems for harsh transient radiation environments. It is well known that electron±hole pairs created in the semiconductor by ionizing radiation recombine within a millisecond. Dispersive transport of holes in the oxide and detrapping from shallow or tunneling type
* Corresponding author. Tel.: +91-22-550-5148; fax: +9122-550-5151. E-mail address: [email protected] (C.A. Betty)
of traps near the interface take times of the order of tens of seconds [1,2]. Proton creation, drift of protons and interface state generation can occur for much longer periods [3,4]. Oxide charges and interface states aect the device parameters and as a consequence there should be relaxation of device parameters. Parameter relaxation eects, after a high dose pulse, are expected to occur in integrated circuits. While the mechanisms responsible for the delayed eects have been studied in detail using MOS capacitors or MOSFETs, their eects on integrated circuit parameters have not been investigated. This study has been carried out to see how operational ampli®er parameters relax with time. The eects of parameter relaxation would be of greater consequence for analog
0026-2714/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 9 9 ) 0 0 0 9 0 - 6
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Table 1 List of manufacturers and batch/identi®cation numbers Device number
Manufacturer
Type number
Batch/identi®cation number
M3/741/2 M5/741/2 M3/308/1 M6/308/2 M6/308/4 M2/356/2 M3/356/10 M1/3140/5 M4/3140/1
M3-National M5-GSL (India) M3-National M6-Fairchild (Korea) M6-Fairchild (Korea) M2-Motorola M3-National M1-RCA M4-HARRIS
LM 741 CH 741C LM 308 AH UA 308 HC UA 308 HC LF 356 BH LF 356 H CA 3140 CA3140T
S:H8712 X3 M9036 8635K 8724K K8544 950 RCA537 H041
integrated circuits than for digital integrated circuits and should therefore ®rst be investigated for parameter relaxation. In this paper, the eects of a pulse of ionizing radiation on operational ampli®er parameters are being reported. There have been experimental studies of the transient response of operational and dierential ampli®ers after pulsed X-ray or electron beam irradiation [6,7]. These experiments have been compared with device and circuit simulations in which only excess carriers in the semiconductor have been modeled and have shown reasonable ®ts in the time frame under consideration (less than a ms) [5,7,8]. However, this is the ®rst report on the eects of relaxation of operational ampli®er parameters after high dose rate pulsed electron beam irradiation.
2. Experiment Four categories of operational ampli®ers (OA) were studied: general purpose bipolar input (mA 741), superbeta transistor input (LM 308), JFET input (LF 356) and MOSFET input (CA 3140). Epoxy encapsulated DIP devices were susceptible to encapsulation failure and hence only devices in metal-can TO-99 packages were studied. In order to check if the dierences in parameter relaxation due to high dose rate irradiation are due to dierences in technology, lots of ®ve devices from two vendors for each category were used in these studies. While we present data for a particular device, trends are similar for devices from the same vendor except for some exceptions which have been indicated. Table 1 gives details of device number and manufacturer. The devices were mounted on an X±Y stage, con®gured as an inverting ampli®er (gain = ÿ1) and pulsed irradiated by 7 MeV electron pulses from a Linear Accelerator (LINAC). Pulses of 50 and 500 ns were used. The dose per pulse determined by thiocyanate dosimetry was 16.16 Gy(Si) and 27.84 Gy(Si), respect-
ively. The ¯uence per pulse was estimated to be 2.047 1011 and 5 1011 electrons/cm2, respectively. A computer aided measurement system was used for data acquisition and analysis. The block diagram of the measurement setup is given in Fig. 1. The LINAC was triggered by an external pulse which was also used to trigger a 100 Msample/s digitizer in the measurement room. The transient output of the OA, connected by guard driven shielded cable to a fast 120 MHz bandwidth ampli®er housed in a radiation and EMI (Electromagnetic Interference) shielded box in the irradiation room was captured by the digitizer in the measurement room. Operational ampli®er parameters were obtained with the help of a relay matrix and a data acquisition and control card. Standard con®gurations and techniques were used for measurement of the parameters viz., oset voltage (Vos) and open loop gain (Adm) [9]. The power supply used was a 215 V regulated DC supply. In the initial stages of the experiment, an 8 bit ADC card with a software selectable voltage range was used for parameter measurements. Later, to make more sensitive measurements in the case of OAs with very small relaxation of oset voltage we used a 12 bit ADC with a switch-selectable voltage range. Even with a 12 bit ADC, errors in Adm were large and hence CMRR (Common Mode Rejection Ratio) relaxation is not being presented. The operational ampli®er under test was con®gured for parameter measurements after the electrical transient settled down (Fig. 2(a) and (b)). First set of parameter measurements were done after 200 ms after the pulse. Subsequent sets of operational ampli®er parameter measurements were made after 1, 2, 5, 10, 50, and 100 or 500 s after the irradiation pulse. This sequence was repeated after every ®ve pulses till 25 pulses and then after every 25 pulses till the operational ampli®er parameters degraded or there is a `total failure' or the total dose exceeded 5000 Gy(Si). By parameter degradation we mean the parameters going out of acceptable range from the speci®ed value, for e.g. the acceptable range for Vos is 210 mV from the initial
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Fig. 1. Measurement setup.
value. By `total failure' we mean catastrophic failure which makes the output saturate to near rail voltages (railspan collapse). We have thus obtained relaxation of OA parameters after pulsed electron irradiation as a function of total dose results. In this paper, we shall focus on the relaxation of operational ampli®er parameters as a function of total dose. One set of parameter measurements takes about 400 ms because relays are switched and time has to be allowed for settling of the feedback circuits used in these measurements. We have noted that the devices are not in latched-up condition since the transients decay to zero and the operational ampli®er circuit behaves normally without requiring to turn o and turn on the power to the ampli®er under test. Experimentally, it has been veri®ed that the current transients in the power supply last for less than a few tens of microseconds. In Fig. 2(a) and (b), we give the output transients obtained with one pulse of electrons for dierent OAs. The fast buer has an output range of 24 V with a wide bandwidth attenuator at the input of the fast ampli®er and the overall gain from the input of the fast ampli®er to the digitizer is 0.475.
Therefore, an output 24 V corresponds to 28 V. In order to capture the full signal, a pretrigger delay of about 50 ms was allowed. From Fig. 2 it is clear that the electrical transient recovers within a millisecond. When a device is irradiated with a high dose rate pulse, two eects occur: a total dose eect on parameters and a relaxation of parameters. These are shown schematically in Fig. 3. The dierence in the values of the parameter at t 4 1 and the preirradiation value is the total dose damage due to the irradiation pulse. The dierence in the values of the parameter at any time after the irradiation pulse and t 4 1 is the relaxation. The relaxation is, as we broached earlier, due to buildup and/or anneal of damage in the oxide or at the interface. The parameter relaxation plots are normalised using: Normalized Pi
Pi t P i 0
where Pi 0 is the ®rst parameter reading after the corresponding irradiation. The number of pulses i, and the values of the parameter at 0.2 s (Pi 0) are given in
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stants, possibly implying that the controlling processes can be modeled using a ®rst order dierential equation. Each time constant corresponds to the time taken by the respective slope in the parameter relaxation plot. In Table 2, we have given the order of the relaxation time and the number of time constants for the relaxation of oset voltage and dierential gain. 2.1. General purpose bipolar input operational ampli®er (mA 741) The bipolar operational ampli®er mA 741 from two vendors (M3 and M5) were studied and 500 ns pulses were used for parameter relaxation studies. The devices were irradiated without limiting resistors in the power supply leads. M3's devices took longer times for Vos relaxation compared to M5's devices (see Fig. 4(a) and (b)). From Fig. 4(a), we note that the magnitude of variation of Vos with time for M3's devices is not much as compared to M5's devices (Fig. 4(b)). In Fig. 4(a), we can see three time constants for the relaxation. Vos increases slowly with total dose. Adm does not show signi®cant relaxation for both manufacturers. But Adm decreases with total dose. For M3/741/2, Adm decreases from 5 104 to 1.1 104 for 3.5 kGy(Si). For M5's devices, the oset voltage relaxed within 1±2 s (Fig. 4(b)). It is observed that Vos changes with just one pulse of radiation. The inset shows that immediately after the ®rst radiation pulse there is a large
Fig. 2. (a) Transient output signals of bipolar input operational ampli®ers. (b) Transient output signals of FET and MOSFET input operational ampli®ers.
the insets in relevant ®gures. The results are discussed operational-ampli®er-type-wise. In Table 2, we summarise the doses to which the devices were irradiated. For several parameters, we found that the relaxation process has several time con-
Fig. 3. Schematic diagram of parameter relaxation.
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Table 2 Operational ampli®er types, maximum dose given and relaxation parameters Type #
741 308 356 3140 a b
Manufacturer
M3 M5 M3 M6 M2 M3 M1 M4
No. of pulses given
200 (500 ns) 200±325 (500 ns) 40±50 (500 ns) 150 (500 ns) 50 (500 ns) 150 (50 ns) 50 (500 ns) 2±3 (500 ns)
Corresponding total dose in kGy(Si)
Relaxation time and no. of time constants For Vos
For Adm
5.57 5.57±9.0 1.1±1.4a 4.15 1.4b 2.4 1.4a 0.057±0.08a
> 100 s, 2±3 time constants 01 s, 1 time constant 050 s, 2±3 time constants 01 s, 1 time constant 0500 s, 3 time constants 0150 s, 2 time constants > 500 s, 2 time constants Railspan collapse
Little relaxation Little relaxation Little relaxation Little relaxation Little relaxation > 500 s, 2 time constants 0500 s, 2 time constants Railspan collapse
Parameter went out of measurement range. Irradiated with current limiting resistors in the power supply leads.
change in Vos which relaxes nonmonotonically. In Fig. 4(b), there is only one time constant for the Vos vs. time plots. The pre-radiation value of Vos which is positive, changes to negative with total dose. 2.2. Super beta input operational ampli®er (LM 308) The devices were studied for parameter relaxation with 500 ns pulses. Devices from M3 and M6 were irradiated without any current limiting resistor in the power supply leads. The Vos relaxation plot of M3 is given in Fig. 5(a). It can be seen that for M3's devices Vos relaxes sharply after the irradiation pulse for the initial irradiation. However, after 25 pulses (1700 Gy(Si)) there is little relaxation. M3's devices show a nonmonotonic Vos relaxation. Also Vos changes sign with dose. The plot shows that the oset voltage increases in magnitude very fast with total dose. Three time constants are present in the Vos relaxation plot (Fig. 5(a)). For M3's devices, Adm did not relax much till a dose of 12070 Gy(Si) but decreased from 1.4 105 to 9 104. The typical Vos vs. time plot (Fig. 5(b)) for M6's devices shows that Vos changed with the ®rst pulse. In this case also the change in Vos immediately after the irradiation is very large. It took about 1 s for relaxation till a total dose of 1700 Gy(Si). After 1840 Gy(Si), there is no signi®cant relaxation. The shift in oset voltage with total dose is also very large. Only one time constant is seen in Fig. 5(b). In all devices except for M6/308/4, Vos showed a monotonic decrease with total dose. M6/308/4 showed a change in sign for Vos with dose. For all devices, Adm did not show a relaxation pattern, but decreased with total dose. On comparison of irradiation results for these two manufacturers' devices, M6's devices showed a greater amount of parameter relaxation as well as greater
degradation than M3's devices. M3's devices were irradiated upto 4000 Gy(Si) without much degradation in the parameters. 2.3. JFET input operational ampli®er (LF 356) Parameter relaxation studies were carried out on devices from M2 and M3. Since both manufacturers' devices failed catastrophically with a single 500 ns pulse, 50 ns pulses were used. But M2's devices failed even with a 50 ns pulse. Hence M2's devices were irradiated with 500 ns pulses, connecting a 20 O current limiting resistors in the power supply leads and typical plots of parameter relaxation for these devices are also given. Fig. 6(a) gives the Vos relaxation plot for M2's devices. For M3's devices, we got parameter relaxation even with a small pulse (50 ns) (Figs. 6(b) and 7). Fig. 6(a) shows that it takes a longer time for Vos relaxation of M2's devices compared to that from M3 (Fig. 6(b)). For M2's devices there are three time constants (Fig. 6(a)) and for M3's devices there are only two time constants in the Vos relaxation plots (Fig. 6(b)). After 100 s, Vos started decreasing for M3's devices. Similarly in Fig. 7 Adm also increased after about 100 s. Adm relaxation shows two time constants. 2.4. MOSFET operational ampli®er (CA3140) Devices from M1 and M4 were used for parameter relaxation studies. The radiation pulse width used was 500 ns without any current limiting resistor. There was a large variation in gain (Adm shifted from 32094 to 22910 for M4/3140/1) and oset voltage (Vos decreased from 3.99 10ÿ3 to ÿ1.567 10ÿ2 V for M4/3140/1) between the pre- and ®rst post-irradiation measurements. The outputs of M4's devices got saturated to
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Fig. 4. (a) Typical plot of normalised oset voltage vs. time for M3/741/2. (b) Typical plot of normalised oset voltage vs. time for M5/741/2.
power supply voltage (railspan collapse) with the second irradiation pulse. Parameter relaxation was, therefore, studied using only M1's devices. As in the case of JFET input operational ampli®ers, MOSFET operational ampli®ers also show three time constants in the Vos relaxation (see Fig. 8). Adm relaxation shows only two time constants (see Fig. 9). From these two ®gures it can be observed that the shapes of the relaxation plots for Adm and Vos are similar. M1's devices show
Fig. 5. (a) Typical plot of normalised oset voltage vs. time for M3/308/1. (b) Typical plot of normalised oset voltage vs. time for M6/308/2.
improvement in all parameters at low total doses. However, after a certain threshold total dose, all these devices showed a sudden degradation (railspan collapse) in all the parameters. The dose at which this degradation appears varies from device to device and lies in the range 400 to 2000 Gy(Si). M1/3140/5 saturated to the power supply voltage with 20 pulses.
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Fig. 7. Typical plot of normalised open loop gain vs. time for M3/356/10 (12 bit ADC measurement).
technology is required. In the absence of this information, only some generic comments can be made about operational ampli®er response to a pulse of ionizing radiation. All the operational ampli®ers Ð general purpose bipolar and super-b, JFET and
Fig. 6. (a) Typical plot of normalised oset voltage vs. time for M2/356/2 (12bit ADC measurement). (b) Typical plot of normalised oset voltage vs. time for M3/356/10 using a 50 ns pulse (12 bit ADC measurement).
3. Discussion From the results above, it is clear that there is wide variation in the relaxation of operational ampli®er parameters after pulsed 7 MeV electron beam irradiation. To be able to coherently explain all the data obtained in this study, detailed information about layout and
Fig. 8. Typical plot of normalised oset voltage vs. time for M1/3140/5 (12 bit ADC measurement).
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Fig. 9. Typical plot of normalised open loop gain vs. time for M1/3140/5 (12 bit ADC measurement).
MOSFET input operational ampli®ers Ð used in this study had three stages: a dierential input stage, an intermediate stage and an output stage. Therefore there are a minimum of seven devices directly in the signal path that could aect the operational ampli®er parameters. In addition, there are several in the biasing network whose imbalance could aect parameters. One might expect to see the signatures of the relaxation of these devices. With pulsed high dose rate irradiation, we see from the studies that there are both total dose and time dependent eects. The total dose modi®cation and time dependent relaxation of parameters in the signal path and biasing networks bring about the changes we have observed. Therefore we discuss how transistor parameters might relax after pulsed irradiation. Freitag et al. [11] have reported the gain relaxation of transistors due to low dose rate irradiation. But during low dose rate irradiation the mechanisms are quite dierent because both defect generation and annealing process are happening simultaneously. It has been observed that both lateral and substrate pnp transistors which are used in linear ICs show severe gain degradation due to irradiation [12,13]. These transistors are used in internal bias circuits, active loads and output driver stages. Input transistors (npn, super-b, JFET and MOSFET) [12,13,15] also show gain degradation after high/low dose rate irradiation. Relaxation of transistor parameters occurs due to charge trapping and anneal in the oxide and interface
state creation. All these processes continue after the pulsed irradiation is over. Since the processes are oxide and interface processes, regions of devices near the oxide±semiconductor interface are most susceptible. Hence MOSFETs and lateral transistors are especially vulnerable. Buried channel JFETs as used in operational ampli®ers and vertical transistors are not as susceptible, though the intersection of a diused junction with the interface could be a problem area. Because hole trapping and interface state creation Ð either due to trapped hole recombination or proton transport to the interface Ð depend on the number of holes generated in the oxide, thicker oxides would cause larger relaxation eects if covering active areas of devices. The eects of hole trapping and anneal and interface state creation on MOSFETs are well studied. Change in bulk and interface charge aects not only threshold voltage but also transconductance due to reduction of surface mobility because of oxide and interface charge scattering. Fig. 10(a)±(d) give the schematic of the speci®c technologies used for fabrication of internal transistors of the OAs studied. In bipolar transistor, very thick oxides (11 mm) are present over the base±emitter junction of substrate npn and lateral pnp transistors structures (Fig. 10(a) and (b)). After a high-dose-rate pulsed irradiation, a large number of holes (Qh) get trapped in shallow hole traps as well as in deep hole traps present or created in the bulk oxide [16]. Interface states (Nis) are also created at the interface due to irradiation [17]. The holes in shallow traps detrap in less than a second. Most of these detrapped holes during transport towards the interface get trapped in the deep hole traps near the interface [16]. Some of the annihilated holes may form interface states. The holes in deep traps detrap slowly over a few decades in time and when a thermally detrapped hole drops into the silicon valence band as it leaves the oxide, it forms interface states [14]. Shallow hole trapping in the bulk oxide releases protons which diuse and, in the presence of positive bias, drift to the interface where they create interface states. The hole trapping, detrapping and interface state formation cause gain relaxation of the internal transistors via several processes as shown in Fig. 10(a)±(d). This subsequently aects operational ampli®er parameters. In thick oxides, with low ®elds, a signi®cant fraction of the charge appears to be trapped permanently in the bulk region of the oxides which leads to lesser trapped hole recombination [12]. But in thin gate oxides with large electric ®elds, the detrapping of holes from deep traps is faster and this leads to a higher density of interface states [12]. At low total doses the oxide has less number of holes trapped in deep traps and interface states. As the device is exposed to pulses over a length of time, the density of interface states increases. The amount and
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Fig. 10. (a±d) Schematic of the mechanisms which cause gain relaxation of internal transistors.
nature of relaxation get aected in a complex way, depending on the space charge, due to prior total dose damage and subsequent trapping and anneal. The dierential gain relaxation of the transistors in the internal bias circuitry and input pair is responsible for the Vos relaxation in the operational ampli®er circuits [12]. For MOSFET input stages the dierence in threshold voltage and the relaxation of this would dominantly aect Vos. The relaxation of parameters of transistors in internal bias circuit Ð including substrate pnp transistors Ð and active load cause open loop gain relaxation [12]. The longer time taken for Vos relaxation in JFET and MOSFET operational ampli®ers is mainly due to the eect of interface states produced in the input transistors. In bipolar input stage devices, the bulk ®eld oxide traps are causing more parameter relaxation compared to the interface states. But in devices with thin gate oxides, the inter-
face states are causing more relaxation. LF356 which has a buried p-channel input dierential pair and CA3140 which has a p-MOSFET (with holes pulled away from the interface) input pair show an improvement in the Adm plots. As the holes drift away from the oxide±silicon interface there is no signi®cant hole trapping and interface state generation in p-channel MOSFETs. If some holes however do get trapped and subsequently anneal, there would be an improvement in gm. This explains the improvement of Adm in pMOSFET input stage devices. A similar argument could be given for p-buried-channel JFET OAs. However, in this case, there is no high ®eld in the oxide over the buried channel and therefore lesser trapped hole anneal. The dierence in the parameter behaviour of devices from dierent manufacturers can be attributed to the variations in device structures and oxide properties of the internal transistors.
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4. Conclusion We report on the relaxation of operational ampli®er parameters to pulsed electron beam irradiation. While all the operational ampli®er types studied Ð i.e. general purpose, super-û, JFET and MOSFET input Ð showed the relaxation of one or other parameter, the fractional change of parameters is relatively smaller for the general purpose operational ampli®er mA 741. It also has better total dose performance. The eect of parameter shift and relaxation can have considerable eect in certain classes of circuits based on operational ampli®er, such as low drift thermocouple ampli®ers or high gain instrumentation ampli®ers. In Fig. 11, we give a typical output after three 500 ns pulses are given to a high gain (gain = 1000) thermocouple ampli®er based on an OP07. The inputs were grounded. With the sampling rate set at a second, the electrical transient which lasts for hundreds of microseconds, is not captured. However, what is captured is the eect of parameter relaxation. It can be seen that the output takes tens of seconds to settle down due to the relaxation of parameters of the operational ampli®er, in this case dominantly due to oset voltage relaxation. To date, the design of devices and systems for radiation harsh environments have primarily considered total dose eects, prevention of latch up, single event
upset and electrothermal damage. Care is taken to reduce total dose eects in rad-hard devices by suppressing the density of bulk oxide traps or removing them from near the Si±SiO2 interface by appropriate processing. However, the oxide still has a fairly high concentration of hydrogen [10] which causes, in the absence of an oxinitride layer, a slow increase of interface state density under irradiation, especially when there is a ®eld aiding drift of irradiation generated hydrogen ions to the interface [4]. As a consequence, devices are still prone to slow parameter relaxation after a high dose rate pulse. It has been observed that parameter relaxation strongly depends on the history of irradiation. Further studies of the eects of transient radiation on unit devices used in operational ampli®er circuits is required before design changes can be made to fabricate operational ampli®ers that are truly hard to both total dose and dose rate eects.
Acknowledgements Authors are thankful to Dr.J.P.Mittal for his encouragement in this work. Thanks are also extended to Mr. Dipankar Sarkar and Mr. Vijaya Raghavan of Electrical Engineering Department I.I.T. Bombay for their help in the instrumentation.
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Fig. 11. Typical output response of a high gain thermocouple ampli®er after an electron beam pulse based on OP07 which had its input grounded. (Response after 3rd 500 nanosecond pulse (8 Gy(Si)).)
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