Measurement of Dose Rate Dependence of Radiation Induced Damage to the Current Gain in Bipolar Transistors 1

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1 Measurement of Dose Rate Dependene of Radiation Indued Damage to the Current Gain in Bipolar Transistors 1 D. Dorfan, T. Dubbs, A. A. Grillo, W. Rowe, H. F.-W. Sadrozinski, A. Seiden, E. Spener, S. Stromberg, R. Wihmann SCIPP, University of California, Santa Cruz, CA 9564 USA N. E. Ipe, S. Mao Stanford Linear Aelerator Center, Stanford, CA 9439 USA Abstrat We report the study of radiation indued hange in the urrent gain of bipolar transistors for three different gamma dose rates. The dose rates differed by a fator of 6 with the lowest lose to that antiipated for the LHC, and the highest at a rate we have been routinely using for radiation damage tests. The maximum dose attained was 2kRad, whih is high enough to ompare with other measurements. The importane of annealing high dose rate data is demonstrated. I. INTRODUCTION One of the major hallenges for High Energy Physis experimentation at the future Large Hadron Collider (LHC) at CERN is the expeted radiation damage to eletroni devies. For example, the front-end eletronis (E) of the silion traking devie SCT in the ATLAS experiment is expeted to aumulate a total dose of approximately 1 MRad over an antiipated lifetime of 1 years [1]. Previous studies have examined the effet of total irradiation dose on bipolar transistors and eletronis, and found lowered urrent gain for inreased doses [2,3]. Reently, new results have shown that for a given dose, more damage is indued if the irradiation is performed at a lower dose rate [4-1]. This effet may have a signifiant impat on eletronis being developed for the readout of the silion traking detetors for use at the LHC. Radiation damage to bipolar transistors is haraterized by a rapid derease of the urrent gain in the first few hundred krad, with a leveling off at muh larger total doses. While it has been demonstrated that the transistors will still operate adequately after 1 MRad when irradiated at a dose rate of, (yielding a urrent gain of about 5 in typial npn transistors [3]), there is onern that the low irradiation rate at the LHC of about.5rad/s may lead to inreased damage suh that the eletronis are no longer funtional for their designed lifetime. The existene of dose rate effets has been observed in radiation damage to CMOS transistors [11] and silion detetors [12], and in both ases they are explained by annealing whih ours during and after the irradiation. Reent measurements [4-1] show large dose rate dependene beginning at a relatively low total dose of about 1kRad. We thus deided to test dose rate dependene 1 This work was supported in part by the US. Department of Energy. systematially up to 2kRad over a period of a few weeks, whih yields a dose rate similar to that expeted at the LHC. Our previous measurements [2,3] indiated that a large fration of the damage ours early in the irradiation. It should be pointed out that in Refs. [4-1], the radiation damage was investigated at extremely low urrents and orrespondingly small V BE. In this investigation, we will onentrate on damage at onstant olletor urrent I C, whih tends to be kept stabile in bipolar iruits, and determine the hange in urrent gain at realisti olletor urrents I C. In this study we have irradiated 32 transistors from the Maxim CB2 proess idential to those whih are urrently being used in the prototype CA-M bipolar hip designed for the silion traker of the ATLAS experiment [13]. We have exposed these transistors to three different rates of gamma's from a 6 Co soure at the Santa Cruz Institute for Partile Physis (SCIPP), inluding the expeted dose rate for the LHC of.5rad/s, and at the highest dose rate of the soure (), employed in previous measurements [2,3]. Measurements of indued damage to the urrent gain of the transistors were performed at regular intervals throughout the irradiation to a total dose of 2 krad. In addition, areful studies of the effet of annealing under ontrolled onditions were obtained. II. TEST STRUCTURES AND IRRADIATION The Maxim CB2 was developed by Maxim as an enhanement to the Tektronix SHPi proess by adding vertial pnp strutures. The proess is a fully omplementary bipolar proess whih uses oxide isolation. The typial p+ isolation is replaed with a reessed oxide isolation to redue the olletor area. This proess is designed for superior performane and flexibility. The CB2 proess uses polysilion emitters in both the pnp and npn transistors and typially shows d gains (h) of about 9. The various size transistors are arranged in standard test keys inluded on the wafers of CA-M VLSI hips. Table 1 summarizes the various transistors used in this study. The referene urrent listed in the table is the nominal urrent at whih the transistor is run in the CA-M hip. To haraterize the transistors we performed forward Gummel plots following the speifiations of the CB2 proess published by Maxim in their design doumentation to extrat the d parameters and harateristis of the various BJT's under test [14].

2 Table 1 Summery of MAXIM CB2 Transistors. Devie Emitter size Emitter # Ref. urrent NPN1 1.x3.2 µm µa NPN2 1.x7.2 µm 2 1 1µA NPN8 1.x7.2 µm 2 4 4µA NPN32 1.x31.2 µm µA PNP1 1.x3.2 µm 2 1 5µA PNP2 1.x7.2 µm 2 1 1µA PNP8 1.x31.2 µm 2 1 4µA PNP16 1.x31.2 µm 2 2 8µA The photon irradiation took plae at the 6 Co soure at UC Santa Cruz. The devies were irradiated to a total dose of 2 krad at three different rates: 3,.1 and.5rad/s. Two opti-hromi dosimeters [15] were used to alibrate the dose rate at eah of the three different devie and exposure positions. The measured dose rate at the different positions agreed to better than 5%. We estimate the relative error on the dose between different dose rates to be 5%. In all ases the transistors were biased with an emitter urrent of approximately 1 µa during the irradiation at a temperature of 2C. During the annealing period, the devies were stored in the same loation to eliminate temperature effets to skew the data, but without applied bias. Forward Gummel plots were obtained periodially throughout the irradiation for the lower dose rates. Table 2 summarizes the irradiation and annealing history of the devies under test. A. Annealing III. RESULTS During the measurements taken throughout the irradiation, as indiated in Table 2, we looked arefully for room temperature annealing of 1 week, 2 weeks and 3 weeks respetively. The data shows appreiable detrimental post-rad. annealing, with the radiation damage inreasing by a large fator within the first week after the exposure for strutures irradiated with high dose rate, but with very little hange afterwards. However, for the test strutures irradiated with the lower dose rates, little hange appears between measurements taken diretly after the irradiation, and those performed a week later. Thus we an onlude that for exposures lasting muh longer than one week, the radiation damage in low rate exposures will have annealed out during the irradiation time and the measurements taken immediately after the final irradiation will reflet the stable radiation damage. For the 1Mrad irradiation of Ref. 3 at, the exposure time was 5.5 weeks, so the error was less than 2%, assuming an anneal time of less than one week. The data taken during the irradiation, on the other hand, would have systematially underestimated the damage. In the following, we will ompare data of low and high dose rate exposures after annealing times of up to 13 weeks. Table 2 Irradiation history for CB2 Test Strutures. Devie Rate (Rad/s) Dose (krad) Anneal Time (hrs) B. Change of Current gain as a funtion of Dose During irradiation, we measured the urrent gain h as a funtion of olletor urrent for all eight of the transistors at the dose and anneal points listed in Table 2. They orrespond to values of V BE >=.7V. They were fit with a seond order polynomial in log (I C ), whih desribes the data very well: h = A + B*ln(I) + C*ln(I) 2. (1) For omparison of the radiation indued damage between different transistors and differing rates of irradiation, the data for different transistors ould not be ompared diretly beause of the differene in pre-rad urrent gains. Instead we ompared the perent hange form the pre-irradiation urrent gain at the nominal olletor urrents speified in Table1.

3 MAXIM I npn1rad/s npn1-.1rad/s npn1- npn1- = 5miroA MAXIM I = 1miroA npn2rad/s npn2-.1rad/s npn2- npn MAXIM I MAXIM I pnp2rad/s pnp2-.1rad/s pnp2- pnp2- = 5miroA pnp1rad/s.1 pnp1-.1rad/s pnp1-.5 pnp = 1miroA MAXIM I = 4miroA npn8rad/s npn8-.1rad/s npn8- npn MAXIM I Rad/s.1 Rad/s = 4miroA MAXIM I = 16miroA npn32rad/s npn32-.1rad/s npn32- npn MAXIM I.5 Rad/s.1 Rad/s = 8miroA Figure 1: Relative hange in transistor urrent gain vs. dose for different dose rates for the four npn transistor at saled olletor urrents I. Figure 2: Relative hange in transistor urrent gain vs. dose for different dose rates for the four pnp transistor at saled olletor urrents I.

4 Figures 1 and 2 summarize the results for 8 transistor sizes. Eah figure ontains data from all three dose rates for a given size transistor. For data orresponding to the same total dose, the one with the longest anneal time is displayed. Error bars representing our estimate of the relative dose unertainties (5%) are shown. The error bars in the vertial sale are of the order 1%. The smallest transistors (npn1, pnp1) show similar trends as the other 6 transistor sizes, but have speial features whih we will investigate further in the future. The npn transistors show a larger initial gain than the pnp transistor, however, the relative damage in the npn transistors appears larger, espeially at the lower olletor urrents. These trends are ommon aross the 32 transistors tested. The inreased radiation hardness of the pnp transistors has not been seen in previous Maxim C-Pi or SHPi proesses. For the pnp transistors, we observe for low dose an initial inrease in the urrent gain, followed by the expeted derease for larger doses. C. Dose Rate Dependene In Figures 1 and 2, a line onneting the origin with the 2kRad dose point for the 3. Rad/s dose rate is displayed. A deviation from this line for the data with lower dose rates will indiate a dose rate dependene of the radiation damage. We an first ompare the points of the two test strutures irradiated with the high dose rate of, whih had different histories of irradiation and annealing, as listed in table 2. At a total dose of 22kRad, they agree with eah to 3% for the npn (Fig. 1) and muh better for the pnp transistors (Fig. 2). This level of agreement is a measure of the sensitivity of our test. Sine the low dose rate data deviate from the high dose rate data at 2kRad by about the same amount, we an state that the maximal dose rate effet is less or of the order of 3%. In general, the initial low dose points deviate more from the expetation than the final point. The npn and the pnp show different trends, with the pnp's generally lying above the expetation. We attribute this to the fat that the data taken at early times did not have time to anneal out ompletely. For a given radiation dose, it is apparent that the indued damage for a pnp transistor is roughly half that of a npn transistor, as mentioned before. This makes the use of pnp's more attrative for appliations where the urrent gain is affeted by radiation. A omparison of the dose rate dependene at the highest total dose point and more than 1 weeks of annealing is shown in Figs. 5 and 6. Here the relative hange in urrent gain is h = [h (22kRad)-h (krad)] (krad) (2) shown as a funtion of the olletor urrent. The urves are alulated using the fits to the forward Gummel plots mentioned above [eq. (1)]. The values for the two lower dose rates are saled linearly by 1% to aount for the different total dose. The onlusions from these plots are the same as from the figures showing the dose dependene (Figs. 1 and 2): the radiation damage in the pnp is about half of that in the npn, and the satter of the two high dose rate urves are as large as between the high and the low dose rate sets, of about 3%. IV. CONCLUSIONS We have investigated the dose rate dependene of radiation indued damage to the urrent gain of bipolar Maxim CB2 transistors at realisti olletor urrents. Test strutures were irradiated with gamma s from a 6 Co soure at dose rates differing by a fator 6 up to a total dose of 2kRad Colletor Current IC [A] Colletor Current IC [A] Colletor Current IC [A] Figure 3: Relative hange in transistor urrent gain for npn transistors vs. olletor urrent I at the maximum dose of 22kRad for different dose rates. Note the vertial offset in the npn32 data.

5 Colletor Current IC [A] -.4.1Rad/s Colletor Current IC [A] Colletor Current IC [A] Figure 4: Relative hange in transistor urrent gain for pnp transistors vs. olletor urrent I at the 22kRad dose. No evidene for rate dependene was found in the region of.5rad/s to. The signifiane of the former is that it is at the antiipated dose rate at the LHC, and the one of the latter that it is the dose rate of our Co soure, used before frequently for our radiation damage studies. We find that for higher dose rates the total damage to the transistor urrent gain is not apparent until an annealing period of about a week at room temperature has elapsed. Measurement taken diretly after irradiation would yield redued damage effets for these higher rate exposures [12]. Earlier long-term studies of several weeks [2,3] are thus only marginally affeted. To a level of 3%, the total indued damage appears to be dependent on dose reeived, at least for the dose rates employed in this study. A diret omparison with previous measurements indiating dose rate dependene of the radiation damage in bipolar iruits [4-1] is diffiult beause they were determined in very different urrent regimes, i.e. at very muh lower urrents where the radiation damage is muh larger and surfae effets are large and possibly tehnology dependent. V. RERENCES [1] ATLAS Inner Detetor TDR, CERN/LHCC/97-16/17. [2] E. Barberis et al., Radiation Hardness Measurements on Components of a full Custom Bipolar Proess, SCIPP 92/38, Aug [3] A. A. Grillo et al., Radiation Hardness Measurements on MAXIM full Custom Bipolar Proesses C-Pi and CB2, SCIPP 96/34, Aug [4] E. W. Enlow et al., Response of Advaned Bipolar Proesses to Ionizing Radiation, IEEE Trans. Nul. Si., vol. 38, pp. 1242, [5] R. N. Nowlin et al., Hardness-Assurane and Testing Issues for Bipolar/BiCMOS Devies, IEEE Trans. Nul. Si., vol. 4, pp. 1686, [6] A. H. Johnston et al., Enhaned Damage in Linear Bipolar Integreated Ciruits at Low Dose Rate, IEEE Trans. Nul. Si., vol. 42, pp. 165, [7] D. M. Shmidt et al., Modeling Ionizing Radiation Indued Gain Degradation of the Lateral PNP Bipolar Juntion Transistor, IEEE Trans. Nul. Si., vol. 43, pp. 332, [8] H. Barneby et al., Analysis of Bipolar Linear Ciruit Response Mehanisms for High and Low Dose Rate Total Dose Irradiations, IEEE Trans. Nul. Si., vol. 43, pp. 34, [9] A. H. Johnston et al., Enhaned Damage in Bipolar Devies at Low Dose Effets at Very Low Dose Rates, IEEE Trans. Nul. Si., vol. 43, pp. 349, [1] S. MClure et al., Dependene of Total Dose Response of Bipolar Linear Miroiruits on Applied Dose Rate, IEEE Trans. Nul. Si., vol. 41, pp. 2544, [11] D. W. Fleetwood et al., A Reevaluation of worst-ase Postirradiation Response for hardened MOS Transistors, IEEE Trans. Nul. Si., vol. 34, pp. 1178, [12] H. Ziok et al., Nul. Instr. Methods, vol. A342, pp. 96, [13] Ref. 1 and ATLAS-SCT Projet Speifiation, Projet Name: CA-M, Version 3.1. [14] MAXIM Quikhip8 IC Design Doumentation, 1994, Beaverton, Oregon 973. [15] Opti-hromi dosimeters of type FWT 7-4M, Bath -1 from Far West Tehnology In., 33-D S. Kellogg, Goleta, CA 9317, USA. [16] N. E. Ipe et al., High Level Dosimetry at SLAC, SLAC-Pub 5686, Nov 1991.