Capacitance-voltage and thermally stimulated current methods are used to investigate radiation induced charge trapping in bipolar base oxides. Results are compared with models of oxide and interface trap charge buildup at low electric fields

Fleetwood, D. M.

Riewe, L. C.

Witczak, Schrimpf, R. D.

UNT Digital Library

s& lvBg&- Dya& p', - -  RADIATION-INDUCED CHARGE TRAPPING IN BIPOLAR BASE OXIDES D. M. Fleetwood, L. C. Riewe Sandia National Laboratories P. 0. Box 5800, MS 1083 Albuquerque, NM 87185-1083 USA Phone (505) 844-1825; Fax (505) 844-2991 e-mail: dmfleet@smtplink.mdl.sandia.gov RAR 1 1 t953 O,S7 I  and S. C. Witczak and R. D. Schrimpf University of Arizona Dept. of Electrical and Computer Engineering Tucson, AZ 85721 USA Phone (520) 621-8491 .-- 35-wd Abstract Capacitance-voltage and thermally-stimulated-current methods are used to investigate radiation in- duced charge trapping in bipolar base oxides. Results are compared with models of oxide and interface trap charge buildup at low electric fields. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness. or usefulness of any information, apparatus, product, or proccss disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. * Work performed at Sandia National Laboratories was supported by the Defense Nuclear Agency through its hardness assurance program, and by the Department of Energy through Contract No. DE- AC04-94AL85000. Work performed at the University of Arizona was supported by DNA and NSWC- Crane through a contract with h4RC. ER Introduction Many types of bipolar devices show significantly greater gain degradation in low-dose-rate radiation environments than in high-rate environments. Several physical mechanisms have been proposed to ac- count for this behavior. These include (A) space charge effects in the base oxide associated with metas- tably trapped holes [ 13, (B) reduced recovery rates due to retarded hole transport in thick oxides at low electric fields [2,3], (C) enhanced electron-hole recombination due to electrons in shallow traps in the base oxide [4], and @) delayed buildup of near-midgap interface traps at the surface of the base [5,6]. Mechanisms (A) and (C) have been discussed primarily in the context of NPN transistor results, while (B) and @) have been discussed primarily for PNP transistors. There are compelling reasons to believe that NPN and PNP structures would show qualitatively different radiation responses even if the trapped charge densities in the base oxides were identical [6-91. However, to date there has not been a compari- son of the charge trapping properties of the base oxides used in different bipolar processes. In this summary we compare the radiation response of two types of capacitors processed to simulate base oxides in development versions of Analog Devices’ RF25 and XFCB processes [10,11]. Enhanced low-rate gain degradation in the RF25 (formerly called ADRF) process is observed primarily in lateral and substrate PNP’s; in the XFCB process, it is observed in vertical NPN’s. The base oxides differ sig- nificantly in the two processes. For the development RF25 devices we have evaluated, a 600-nm wet thermal oxide is employed. For the XFCB oxide, a 540 nm deposited oxide is stacked on a 60 nm ther- mal oxide over the base. Both oxides see base implantation and high-temperature annealing steps, mak- ing them quite susceptible to radiation-induced charge trapping [ 11. High-frequency capacitance-voltage (C-V) and thermally-stimulated-current (TSC) test methods [ 121 were used to evaluate radiation-induced charge trapping in these two types of base oxides. Results are compared with mechanisms A-D above. Experimental Results Figure 1 shows radiation induced net-oxide (ANoJ and interface-trap (ANiJ charge densities inferred from C-V measurements on capacitors simulating RF25 and XFCB base oxides. ANot was estimated from midgap C-V shifts, and ANjt was estimated from midgap to flatband stretchout [13]. ANot is sli htly higher in the XFCB base oxide (- 3.0 x 10” cm-2 at 200 krad) than the RF25 base oxide (- 2.3 x 10 cm-2). However, ANit is much larger in the RF25 oxides (- 2.9 x 10” cm-2 at 200 krad) than the XFCB oxides (- 0.25 x 10” cm-*). These results are consistent with device studies showing that oxide charge is the dominant factor that determines the radiation response of XFCB devices (e. g., Mechanisms A-C) [7-91, with interface traps (Mechanism D) being more important in RF25 devices [6]. ?l That the RF25 base oxides show such large densities of net oxide-trap charge and interface traps suggests they are quite useful to study in detail to examine the potential effects of both types of charge on bipolar base oxide radiation response. Figure 2 shows inferred flatband voltage shifts (AV,) and components due to oxide traps and interface traps (AVot and AViJ from C-V measurements on RF25 base oxides as a function of radiation dose rate. AVot and AVjt show similar dose-rate dependenciis, so they contribute equally to the dose-rate dependence of AV,. It is interesting that the transition between trap densities characteristic of “high rate” response and those reflecting “low rate” response in Fig. 2 occurs between - 10 and 100 rad(Si02)/s for these 600 nm oxides. This is the same region of dose rates in which this transition occurs for the much thinner (- 60 nm thermal) RBCMOS base oxides studied in Ref. [ 13, strongly suggesting that a simple difference in recovery rates due to differences in oxide thick- ness (Mechanism B) [2,3] cannot account for the dose rate effects in these oxides. If Mechanism B dominated the device response, the transition region should occur at much lower dose rates for the 600- nm RF25 base oxides than the 60-nm RBCMOS oxides. Figure 3 shows TSC results for RF25 base oxides irradiated at 320 and 0.83 rad(Si02)/s. The TSC is a measure of the total density of radiation-induced trapped positive charge (assumed primarily to be holes) that is emitted and transported across the oxide [12]. Despite their very different levels of net ox- ide-trap charge (Fig. 2), the TSC is nearly identical in the two cases in Fig. 3! This means that the re- duction in net oxide-trap charge at high dose rates in these devices is not due to a difference in the total number of trapped holes in the base oxide. Instead, it is caused by a different number of compensatin electrons at high rates than at low rates. Integrating the TSC curves in Fig. 3, we find - 2.5 x 1OI2 cm- holes tra ped after either 320 or 0.83 rad(Si02)/s irradiation. The net oxide-trap charge density is - 1.7 x 10" cm- for the high rate case, and - 4.6 x 10" cm-2 in the low-rate case (see Fig. 2). This implies that - 93 % of the radiation-induced trapped positive charge is compensated by trapped electrons in the high rate case, and - 82 'YO is compensated in the low-rate case. Thus, it is evidently not a difference in trapped-hole density due to enhanced electron-hole recombination (Mechanism C) that causes the dose- rate response in these base oxides. It is the relatively small difference between the number of compen- sating electrons in the two cases that leads to the large difference in ANop This is consistent with Mechanism A and the results of Ref. [ 11, and reinforces the crucial role that electron trapping plays in determining the radiation response of bipolar base oxides [1,4]. Finally, it is certainly quite interesting that ANit would scale similarly to the net oxide-trap charge density in Fig. 2, for example, as opposed to the total oxide-trap charge density. Reasons for this connection will be explored in the full paper. 5 Y Similar trends for ANot and ANit were observed with increasing temperature during 10-keV x-ray irradiation (22 to 125OC) as with decreasing dose rate, as shown in Fig. 4. That is, both the net oxide- trap charge and interface-trap density increase with increasing radiation temperature (at least to - 125°C) for a fixed high dose rate, and increase with decreasing dose rate for room temperature irradiations. Similar trends have been observed with Co-60 irradiations as a function of temperature. TSC measure- ments (which will be shown in the full paper) again show that, at higher temperatures, the total number of trapped holes is not significantly increased, but the number of holes compensated by trapped electrons decreases, just as for the dose-rate data of Figs. 2 and 3. This supports hardness assurance techniques that rely on elevated temperature irradiations to simulate low-rate response for these types of devices [1,14]. Moreover, it is encouraging that both the oxide-trap charge and interface-trap charge components are enhanced similarly by increasing temperature, suggesting that using elevated-temperature irradiations to simulate low-rate response does not depend critically on whether oxide traps [1,7-91 or interface traps [5,6,13,14] lead to the enhanced low-rate response, at least for these devices. Summary and Conclusions We have found that interface traps play a much more significant role in determining the radiation response of capacitors simulating base oxides for Analog Devices' RF25 development process than their XFCB process. This is consistent with device studies suggesting interface traps dominate the dose-rate response of the RF25 process [6], but oxide traps dominate the response of the XFCB process [7-91. The enhancement of the net oxide-trap charge for the RF25 process at low dose rates and/or high tempera- tures is primarily due to differences in the number of compensating trapped electrons in the base oxides in the two cases. Results are consistent with a model that depends on space charge effects to modi@ the distribution of trapped holes in the base oxides, which in turn leads to differences in the number of com- pensating trapped electrons [l]. Results for these oxides do not appear to be consistent with models that depend on differences in recovery rates in thick and thin oxides at low electric fields [3], or on a reduc- tion in the number of trapped holes due to enhanced recombination of radiation-induced holes with elec- trons in shallow traps [4]. However, these mechanisms may be more significant in other bipolar base oxides. Additional TSC and C-V results on these and other thick oxides will be presented in the full pa- per, along with their implications for models of enhanced bipolar gain degradation at low dose rates. Acknowledgments We thank R. L. Pease, M. DeLaus, D. M. Schmidt, P. S. Winokur, W. L. Warren, J. R. Schwank, and K. F. Galloway for stimulating discussions, and R. A. Reber, Jr. for technical assistance. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. D. M. Fleetwood, S. L. Kosier, R. N. Nowlin, R D. Schrimpf, R. A. Reber, Jr., M. DeLaus, P. S. Winokur, A. Wei, W. E. Combs, and R. L. Pease, “Physical Mechanisms Contributing to Enhanced Bipolar Gain Degradation at Low Dose Rates,” IEEE Trans. Nucl. Sci. & 1871 (1994). H. E. Boesch and F. B. McLean, “Hole Transport and Trapping in Field Oxides,” IEEE Trans. Nucl. Sci. 32,3940 (1985). A. H. Johnston, B. G. Rax, and C. I. Lee, “Enhanced Damage in Linear Bipolar ICs at Low Dose Rate,” IEEE Trans. Nucl. Sci. 42, 1650 (1995). V. V. Belyakov, V. S. Pershenkov, A. V. Shalnov, I. N. Shvetzov-Shilovsky, “Use of MOS Structures for the Investigation of Low-Dose-Rate Effects in Bipolar Transistors,” IEEE Trans. Nucl. Sci. Q, 1660 (1995). S. McClure, R. L. Pease, W. Will, and G. Peny, “Dependence of Total Dose Response of Bipolar Linear Microcircuits on Applied Dose Rate,” IEEE Trans. Nucl. Sci. & 2544 (1994). D. M. Schmidt, D. M. Fleetwood, R. D. Schrimpf, R. L. Pease, R J. Graves, G. H. Johnson, K. F. Galloway, and W. E. Combs, “Comparison of Ionizing-Radiation-Induced Gain Degradation in Lateral, Substrate, and Vertical PNP BJT’s,” IEEE Trans. Nucl. Sci. Q, 1541 (1995). S. L. Kosier, R. D. Schrimpf, R. N. Nowlin, D. M. Fleetwood, M. DeLaus, R. L. Pease, W. E. Combs, A. Wei, and F. Chai, “Charge Separation for Bipolar Transistors,” IEEE Trans. Nucl. Sci. 4Q, 1276 (1993). A. Wei, S. L. Kosier, R. D. Schrimpf, D. M. Fleetwood, and W. E. Combs, “Dose-Rate Effects on Radiation-Induced BJT Gain Degradation,” Appl. Phys. Lett, &l, 1918 (1994). S. L. Kosier, A. Wei, R. D. Schrimpf, D. M. Fleetwood, M. DeLaus, R. L. Pease, and W. E. Combs, “Physically Based Comparison of Hot-Carrier-Induced and Ionizing-Radiation-Induced Degradation in BJT’s,” IEEE Trans. Electron Dev. a 436 (1995). S. Feindt, J.-J. J. Hajar, J. Lapham, and D. Buss, “XFCB: A High Speed Complementary Bipolar Process on Bonded SOI,” IEEE BCTM Tech. Digest, 264 (1992). K. 0. P. Garone, C. Tsai, B. Scharf, M. Higgins, D. Mai, C. Kermarrec, and J. Yasaitis, “A Double Polysilicon Self-Aligned npn Bipolar Process (ADRF) with Optional NMOS Transistors for RF and Microwave Applications,” IEEE BCTM Tech. Digest, 221 (1994). R. A. Reber, Jr. and D. M. Fleetwood, “TSC Measurements of SiO, Defect Density and Energy in Irradiated MOS Capaci- tors,” Rev. Sci. Instrum. P. S. Winokur, J. R. Schwank, P. J. McWhorter, P. V. Dressendorfer, and D. C. Turpin, “Correlating the Radiation Re- sponse of MOS Capacitors and Transistors,” IEEE Trans. Nucl. Sci. a 1453 (1984). R. D. Schrimpf, R. J. Graves, D. M. Schmidt, D. M. Fleetwood, R L. Pease, W. E. Combs, and M. DeLaus, “Hardness Assurance Issues for Lateral PNP Bipolar Junction Transistors,” IEEE Trans. Nucl. Sci. Q, 1641 (1995). 5714 (1992). 600 nm Oxides 0 50 100 150 200 250 Dose [krad(Si02)] Figure 1. Radiation-induced trapped charge density vs. dose for XFCB and RF25 base oxide capacitors. Devices were irradiated at room temperature with IO-keV x rays at 320 rad(SiO,)/s and 0 V bias. (-10 V Shift= 3.8 x IO"'  m i 2 )  m 0 .C -15 v) Q UJ -20 m 3 -25 -30 -35 h Y -0.1 I 10 100 1000 Dose Rate [rad(SiOz)/s] Figure 2. Flatband voltage shifts and components due to net-oxide and interface-trap charge vs. dose rate for RF25 base oxide capacitors. Devices were irradiated at room temperature with IO-keV x rays at 0 V bias to 200 krad(Si0J. A conversion factor from voltage shift to trap density is provided for the net oxide-trap charge density and the interface-trap density. 0.5 0.4 0.3 0.2 0.1 n - 0 50 100 150 200 250 300 350 Temperature ( O C )  Figure 3. TSC corrected for background leakage for RF25 base oxides irradiated to 200 krad(Si0J with IO-keV x rays at 0 V at room temperature. The TSC ramp rate was - 7"C/min (Ref. [12]), and the TSC bias was -60 V. " 1  (-10 V Shift = 3.8 x l o l l  m i 2 )  0 -20 I . . . I . . . I . . . l . . . I . . . 1 .  0 20 40 60 80 100 120 140 Temperature ( O C )  .I Figure 4. Flatband voltage shifts and components due to net-oxide and interface-trap charge vs. radiation temperature for RF25 base oxide capacitors. Devices were irradiated at 315 rad(Si0Js with 10-keV x rays at 0 V bias to 50 krad(Si0J. A conversion factor from voltage shift to trap density is provided for the net oxide-trap charge density and the interface-trap density. - -. 

Radiation-induced charge trapping in bipolar base oxides

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Radiation-induced charge trapping in bipolar base oxides

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