Charge Trapping in Irradiated SOI Wafers Measured by ... - IEEE Xplore

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 6, DECEMBER 2004

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Charge Trapping in Irradiated SOI Wafers Measured by Second Harmonic Generation Bongim Jun, Member, IEEE, Ronald D. Schrimpf, Fellow, IEEE, Daniel M. Fleetwood, Fellow, IEEE, Yelena V. White, Robert Pasternak, Sergey N. Rashkeev, Francois Brunier, Nicolas Bresson, Marion Fouillat, Sorin Cristoloveanu, Fellow, IEEE, and Norman H. Tolk

Abstract—Total dose effects on silicon on insulator (SOI) UNIBOND wafers are studied via optical second harmonic generation (SHG). This technique is qualitatively compared with the pseudoMOSFET technique for monitoring charges at the interfaces. Optical and electrical methods are combined to separate the contribution of the signal from each interface to the total SHG intensity. Radiation-induced oxide and interface traps increase the interface fields as determined from the SHG signals and the results are compared with electrical measurements. Index Terms—Pseudo-MOSFET, radiation effects, second harmonic generation (SHG), silicon on insulator (SOI), total dose, UNIBOND.

I. INTRODUCTION

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OS TRANSISTORS fabricated on silicon on insulator (SOI) wafers have received great attention because of advantages in device isolation, speed, density, and scalability over bulk silicon devices [1]. Although SOI devices are naturally resistant to transient photocurrents and single event upset, total-dose irradiation may induce a parasitic conduction path at the buried oxide (BOX) interface due to radiation-induced oxide and interface traps [2]. The Si/BOX interface quality has been studied electrically through various techniques such as I-V and C-V measurements. Wafer-level measurements via the pseudo-MOS technique are frequently used for evaluation of partially-processed wafers [3]–[5]. The limitation of this technique is that it damages the active device regions by directly probing the Si-film, and it is limited to characterization of the top interface. Recently, the second harmonic generation (SHG) technique has been proposed as a nondestructive and noninvasive probing technique since the SH signal detects interface without directly conthe electric field at the Manuscript received July 20, 2004. This work was supported in part by Air Force Office of Scientific Research through the MURI Program and the Defense Threat Reduction Agency. B. Jun, R. D. Schrimpf, D. M. Fleetwood, Y. V. White, R. Pasternak, S. N. Rashkeev, and N. H. Tolk are with Vanderbilt University, Nashville, TN 37232 USA (e-mail: [email protected]; ron.schrimpf@vanderbilt. edu; [email protected]; [email protected]; [email protected]; [email protected]; norman. [email protected]). F. Brunier is with SOITEC, Chemin des Franques, Bernin, 38926 Crolles, France (e-mail: [email protected]). N. Bresson and S. Cristoloveanu are with the ENSERG, 38016 Grenoble Cedex 1, France (e-mail: [email protected]; [email protected]). M. Fouillat is with the Department of the IUT, Bordeaux 1 Mesures Physiques, 33405 Talence Cedex, France (e-mail: [email protected]). Digital Object Identifier 10.1109/TNS.2004.839140

Fig. 1. Schematic diagrams of UNIBOND wafer structures used for optical measurements. Incident fundamental beam and selected SHG signals from (a) Si island and (b) BOX region. In (c) we show a schematic diagram of the incident and reflected beams. Doping is p-type for the Si film and substrate (2 10 =cm ); the thickness of the Si film is 72–160 nm; that of the BOX is 145 nm or 230 nm; and the area of the Si islands is 25 mm .

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tacting the surface [6], [7]. Furthermore, the large penetration depth of the optical radiation allows us to use SHG as a sensitive probe of electric field at deeply buried SOI interfaces. Therefore, SHG can be an attractive alternative to investigate carrier dynamics in SOI wafers. In this paper, we apply SHG to characterization of multi-interfacial UNIBOND wafers. Significant effects on charge generation, trapping, and recombination at interfaces are measured for irradiated samples. II. EXPERIMENTAL DETAILS Fig. 1 shows schematic diagrams of the UNIBOND wafers used in this study. The Si wafer with a thermally grown oxide on top is bonded to the new Si substrate. After wafer splitting, a dry etch technique is used to define the Si islands. Both the top Si films and the substrate wafer are p-doped with a doping . Fig. 1(a) illustrates detection concentration of of SHG signals from both the top and bottom interfaces, while Fig. 1(b) illustrates detection of signals from the bottom interface only. Fig. 1(c) shows the angles of the incident, refracted, and reflected beams at each interface for the fundamental beam . Detailed opobtained using Snell’s law, tical properties of the device and beams are discussed in the Appendix. The SHG test was performed using a 5 W Verdi pumped Mira Ti:sapphire laser at a wavelength of 800 nm (1.5 eV) with average power of 600–730 mW. After the reflected fundamental

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Fig. 2. Schematic diagrams of (a) the sample configuration used for electrical characteristics via the pseudo-MOSFET technique and (b) the -MOSFET band structure, depicting positive applied bias to the substrate. Two inner point probes in a 4-point probe system (a) are used as source and drain contacts while the metal wafer holder serves as the gate.

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Si SiO

Fig. 3. Schematic energy band diagram of a = structure with photons incident on the oxide surface. Band bending is not shown in the diagram.

and SHG signals are separated by a prism, the 400 nm wavelength SHG signals are detected by a photomultiplier (PMT) and measured by a photon counter with a 0.1 s temporal resolubetween islands allows examtion. The area of ination of the bottom interface since the beam diameter is ap. At a beam power of 730 mW, the average proximately 40 number of generated photons is . The electrical characteristics were obtained using a 4-point probe and the pseudo-MOSFET ( -MOSFET) technique [3]–[5]. DC characterization of this structure was performed using an HP 4156 semiconductor parameter analyzer. Fig. 2(a) and (b) shows schematic diagrams of the -MOSFET technique and an energy band diagram when positive bias is applied to the substrate, respectively. Total dose tests on these samples were performed with 10 keV x-rays at a dose rate of . The substrates of the structure during 31 irradiation were either DC biased or grounded. Room and high temperature annealing effects after irradiation were studied in an isochronal manner. The presence of any native oxide on the top of the Si-film creates an extra interface with the Si-film, which must be taken into account. III. THEORY Fig. 3 shows a schematic energy band diagram with photons incident on the structure. The laser irradiation generates electron-hole pairs in the Si regions; some of these electrons acquire enough energy to overcome the barrier at the interface and are injected into the oxide. Some of the injected

Fig. 4. Schematic energy band diagram of a multiple interface structure. Photon-induced electron-hole pairs create time-dependent electric fields at each interface contributing to SHG signals independently. Band bending is not shown in the diagram.

electrons are trapped on free surfaces or at defects in the oxide regions. These electrons are responsible for the time-dependent interfaces [6]–[9]. Hole trapping in electric field at the thin oxides is less significant since trapped holes easily recombine with de-trapped electrons from the surface [10]. The time-dependent electric field-induced second-harmonic (EFISH) generation is governed by (1) for a single interface. As is a quasistatic electric field related to expressed in (1)–(3), the effective oxide surface charge density, , which is an inteover the normal gration of oxide volume charge density, axis ( , in this example) to the surface [4]–[6]. (1) (2) (3)

and are the fundamental and SHG signal intenHere , are the third-order susceptibility of silsities, and icon and the effective SHG susceptibility from other sources, respectively. For multi-interfacial structures, as shown in Fig. 4, the total SH intensity includes contributions from all interfaces. The time-dependent electric field is created independently at each interface. is described by (4), Hence, the detected SH intensity which includes the effects of the electric fields at the different interfaces. The electric field generated at each interface contributes to the total SHG intensity independently, yet it also is affected by an externally applied electric field. The SHG intensity including the contribution of the constant applied field is expressed in (5). Depending on the polarity of the external field, it can add to or subtract from the existing field. The subscript “ ” represents the contribution of each interface. (4) (5)

JUN et al.: CHARGE TRAPPING IN IRRADIATED SOI WAFERS

Fig. 5.

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Device structure containing multiple interfaces used for this study.

Fig. 7. Examples of SHG signals from region III for BOX thicknesses of = 145 nm and 230 nm. Region III represents SHG from the interface T between the substrate and the BOX.

Fig. 6. Example of SHG signal from all Si=SiO interfaces, including the interface between a thin native oxide and the Si film. The laser beam was blocked for 2–3 min and unblocked to monitor SHG signal recovery by e-h recombination. (T = 160 nm, T = 145 nm.)

IV. RESULTS AND DISCUSSION A. SHG Signals From Various Interfaces Fig. 5 shows the cross-sectional view of a sample with the different regions identified. Regions I, II, and III contain three, two, and one Si/oxide interfaces, respectively. The nature of each interface is different: 1) the interface between the bottom of the BOX and the Si-substrate was formed by mechanical bonding; 2) the interface between the top of the BOX and the bottom of the Si-body was created through conventional thermal oxidation, and 3) the native oxide was formed on the top of the Si-body by the air ambient. The native oxide can be removed by dipping the sample into a buffered oxide etch solution. The SHG signals from region I are shown in Fig. 6. The large SH intensity detected from region I is contributed mainly by electrons trapped at the free surface of the native oxide layer [3]–[5]. The contributions from the buried interfaces of the BOX are not significant due to the smaller local field and absorption of the SH signal in the Si-body. the SHG signal from each region starts with nonzero At intensity noted as , which indicates the contribution of the , described in (4). The SHG signal time-independent term, from the thin oxide increases with time, as the separated electrons reach the surface and are trapped. The signal reaches satuwhen the time-dependent electric field becomes ration a constant. Now the beam is blocked for 2 min to allow the electrons to transport back to the Si (e.g., by tunneling) and redue to recombicombine. The SHG intensity is reduced by nation while the beam is blocked.

Fig. 8. Schematic energy band diagrams of BOX/Si (a) before and (b) during/ after the photon irradiation.

This is not the case for the thick oxide, since the photo-injected electrons cannot reach the surface. This is clearly shown in Fig. 7 for Region III. Fig. 7 shows SHG signals from two dif, is ferent BOX thicknesses of 230 nm and 145 nm. When larger from the thicker BOX than that from the thinner one, due to the larger charge separation compared to the relatively thinner BOX. Typically, a UNIBOND wafer contains residual positive charges in the BOX, which makes the flatband voltage negative. As time increases, the time-dependent electric field starts compensating the initial field. From the shape of the signal, one can predict the direction of the time-dependent electric field. The charge separation is illustrated in Fig. 8(a) and (b) before and during/after optical radiation, respectively. The electric field at the interface, denoted by an arrow in (a), is compensated by an oppositely-directed electric field due to optically generated charge separation. The time dependence of the signal from the thick oxide interface is caused by charge trapping and detrapping at interfaces and in the oxide [4]. For the thick oxide, e-h recombination during the beam-block periods is smaller than for thin oxides because of the greater time required for charge detrapping and transport. Fig. 9 shows SHG signals from the Si island (region II) obtained from wafers with two different Si-body thicknesses. The signal is smaller from the thicker Si-body wafer due to absorption in the thick Si body; only 27% of the generated SHG intensity at the interface is collected in the detector while the

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Fig. 11. Schematic diagram of a device showing the local fields from both interfaces. The bottom local field is assisted by the externally applied electric field while the top local field is compensated by the external field.

Fig. 9. Examples of SHG signals from region II of samples with BOX = 145 nm and 230 nm. Region II contains two thicknesses of T interfaces (Si film/box and box/substrate).

Fig. 12. Schematic energy band diagrams of substrate/BOX regions. In (b) and (c), the internal fields are compensated and illustrated as light gray.

Fig. 10. Schematic energy band diagrams of Si/BOX/substrate regions. The band bending in the Si body and substrate are reversed when the photon-induced fields become larger than the initial fields at both interfaces.

thinner wafer absorbs 40% of the intensity in the Si body (Appendix). Again, optical absorption in the BOX is negligible. Fig. 10 shows the optically-generated charge separation at both interfaces, which produces a time-dependent electric field comand , from the residual positive pensating the initial fields, charges after the wafer process. for a thick oxide The time-independent SH intensity at is a simple measure of oxide quality, since it is directly related to the local field created by defects and dangling bonds. B. Applied Bias Effect on SH Intensity The signals shown in Fig. 9 contain the EFISH signals generated at both top and bottom interfaces. One can qualitatively separate the contribution of the EFISH signal from each interface to the total SHG intensity by applying an external field across the structure. The externally applied electric field across the BOX adds to the optically induced fields at the interfaces due to the redistribution of the surface charge. The fields at the top and bottom interfaces of the BOX are in opposite directions as illustrated in Fig. 10. Fig. 11 shows a schematic diagram of a device with a positive bias applied to the substrate and subjected to photon radiation simultaneously. The charge redistribution at the interface between the substrate and

Fig. 13. Examples of SHG signals with an external bias applied to the substrate prior to the laser beam illumination; (a) 40 V on the substrate and (b) 40 V = on the substrate with respect to the Si-film (T = 160 nm and T 145 nm).

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the BOX due to the external field is illustrated in Fig. 12. The diagram in Fig. 12(a) shows the interfacial field created by the oxide charges, which is compensated by the externally applied field, as shown in Fig. 12(b). The net field increases due to the photon-generated field, as illustrated in (c). Fig. 13(a) and (b) shows examples of SHG signals obtained from region II with substrate biases of 40 V and 40 V, respectively. Compared to the results without the external field shown in Fig. 9, it is evident that both time-independent and time-dependent interfacial fields are strongly affected by the external fields across the oxide. Moreover, the initial SHG intensity at with is more than two times larger . This implies that the bottom interthan that with face creates a larger time-independent electric field than the top


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Fig. 14. Pre-existing local fields and external fields. The longest arrows illustrate external fields of opposite polarity.

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V characteristics of -MOSFETs with two different Fig. 16. DC I BOX thicknesses. The inset figure describes the application of the pseudoMOSFET technique to an SOI wafer.

Fig. 15. Normalized saturated and initial SHG signals versus substrate applied bias. The beam power was 730 mW and the wavelength of the beam was 800 nm.

interface when the external field is applied across the structure. Fig. 14 shows how the external field compensated the local field. Initially, the field at the top interface was smaller than that at the bottom interface, as shown in (a). However, the external bias of 40 V compensates the field at the bottom interface, while it adds to the field at the top interface. Considering the different nature of the processes used to form the interfaces (that is, conventional oxidation for the top interface and mechanical bonding for the bottom interface), the charge separation is larger at the bottom interface due to more process-related defects, which trap more electrons. The total field at each interface (which is related to the SHG signal) is modulated when the absolute value of the external field is changed. Fig. 15 shows the initial and saturated SHG signals from region II with varying substrate bias for a and . wafer with Note that the initial SHG intensity for 0 V is larger than that for 10 V since the small existing field at the bottom interface is compensated by the external field pointing in the opposite direction. C. Electrical Characteristics via Pseudo-MOSFET ( -MOSFET) Technique The bias dependence of the saturated SHG signals shown in Fig. 15 is analogous to the I-V characteristics commonly obtained using the -MOSFET technique [3]–[5]. Current-voltage characteristics from the devices with two different oxide and Si-film thicknesses are shown in Fig. 16. The inset figure shows a schematic diagram of the -MOSFET technique.

Fig. 17. Electrical and optical characteristics as a function of substrate bias (squares: T ,T , and circles: T , T for both SHG intensity and I ).

= 160 nm = 230 nm

= 145 nm

= 72 nm

Fig. 17 shows a direct comparison of the optical and electrical . Drain current is shown on the right characteristics versus axis and optical SHG intensities on the left axis. The nonzero applied bias required for the minimum SHG intensity results from the presence of charges at the interfaces and in the oxide. The location of the minimum SHG point is nonzero for thinner Si-films and/or thicker BOX layers. The flatband versus voltages were obtained from a plot of [13]. The thinner oxide sample has a lower flatband voltage than the thicker oxide sample, which is the -intercept in the accumulation regime of each curve. The flatband voltages are 1.8 V and 14.2 V for the thinner device and the , respectively. Since the field thicker device is smallest near flatband, the SHG minimum should occur near flatband. D. Total Dose and Annealing Effects on SHG Signals Radiation-induced charge can change the charge distribution in the Si, which directly affects the local fields at the interfaces

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Fig. 20. Fig. 18. Example of SHG signals versus substrate bias before and after the = 230 nm. Beam power = total dose of 5 Mrad. T = 72 nm and T 600 mW.

Fig. 19. Example of SHG signals versus substrate bias before and after a total = 72 nm and T = 230 nm. Beam power = dose of 5 Mrad. T 600 mW.

[11], [12]. Fig. 18 shows the SHG signals from region II of a sample with Si thickness of 72 nm and oxide thickness of (for unbiased irradi230 nm at a total dose of 5 and saturation levels of the SH ination). Both the initial tensities increase regardless of the applied bias, indicating that radiation-induced trapped charges increase the local fields at the interfaces. Applied bias effects on the saturated SHG signal from an irradiated device at different substrate biases were examined. As expected from Fig. 18, the absolute SHG magnitude increased, independent of the value before irradiation. Fig. 19 after a total shows the saturation SHG intensity versus dose of 5 Mrad. The shift in the intensities of the SHG signals after irradiation is analogous to the shift observed in the I-V characteristics obtained from the -MOSFET technique. The results of the total dose test performed on the same device are shown in Fig. 20. The flatband voltage shift is due to radiation induced oxide charges, which also cause an increase in the SHG inten-

T

I-V characteristics obtained from a device with T

= 72:1 nm and

= 230 nm for increasing total dose.

Fig. 21. Examples of SHG signals collected from region II of a device with Si and oxide thicknesses of 125 nm and 143 nm, respectively. The sample was irradiated at 31 krad(SiO )=min with the top and bottom of the structure shorted during irradiation. Annealing was performed for 10 min at selected temperatures.

sity, as shown in Fig. 19. These changes in the SHG signal can be used to obtain information about the oxide trap charges for devices subjected to ionizing radiation. Isochronal annealing was performed after a total dose of ; the results are plotted with the radiation results 2 , the initial and saturation in Fig. 21. At a dose of 2 SHG intensities are changed for 0 V, 40 V and 40 V of substrate bias. At 200 , the 0 V and 40 V curves show full recovery of both initial and saturation SHG intensity, while that has still not recovered. The different temfor perature responses of the interfaces indicate different annealing rates. V. CONCLUSION In this paper, we demonstrate that SHG is a useful, noninvasive technique to characterize the radiation response of multi-interface SOI wafers. This technique provides information about


TABLE I MATERIAL PROPERTIES OF Si AND SiO USED FOR THIS STUDY. (: WAVELENGTH, n : INDEX OF REFRACTION, n : EXTINCTION COEFFICIENT, K: ABSORPTION COEFFICIENT (= 4n =), 1/K: PENETRATION DEPTH)

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the BOX and the Si-film. Therefore, the detected SHG signals contain contributions from both interfaces. ACKNOWLEDGMENT The authors would like to thank Dr. C. Mazure and Dr. F. Allibert from SOITEC for providing SOI wafer samples and helpful discussions. REFERENCES

charge carrier dynamics in SOI structures and gives us information about defect buildup and annealing rates. In particular, it was found that different defect annealing rates were observed at the top and bottom interfaces of irradiated BOXs. APPENDIX The optical properties of each element in a device and laser beam are described in this section. The indices of refraction of materials and the wavelengths of the laser beams used as probes are the primary parameters related to this optical technique that need to be optimized for better understanding of the results. for the waveTable I lists the indices of refraction of Si and lengths used for this study. The penetration depth of the 800 nm light in Si is about , which enables us to investigate the buried interfaces of 10 the tested wafers. On the other hand, the absorption coefficient, , for the 400-nm wavelength SHG signals becomes significant compared to the thicknesses of the top Si films. That is, the penetration depth of the second harmonic is smaller than that of the fundamental beam. The final intensity of the beam can be obtained from the relationship

where is the initial intensity, is the absorption coefficient, , and represents the thickness of the Si-film. defined as , the Note that, from the small absorption coefficient of SHG intensity from the bottom interface loses negligible intensity when the beam travels across the BOX. Furthermore, the spatial separation of the SHG signals from both interfaces is in the range of 140 nm 225 nm, depending on the thickness of

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