Damage in hydrogen plasma implanted silicon

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1 JOURNAL OF APPLIED PHYSICS VOLUME 90, NUMBER 4 15 AUGUST 2001 Damage in hydrogen plasma implanted silicon Lianwei Wang, a) Ricky K. Y. Fu, Xuchu Zeng, and Paul K. Chu b) Department of Physics and Materials Sciences, City University of Hong Kong, Tat Chee Avenue, Kowlong, Hong Kong W. Y. Cheung and S. P. Wong Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received 20 November 2000; accepted for publication 4 June 2001 The damage and defects created in silicon by hydrogen plasma immersion ion implantation PIII are not the same as those generated by conventional beamline ion implantation due to the difference in the ion energy distribution and lack of mass selection in PIII. Defect generation must be well controlled because damage in the implanted and surface zones can easily translate into defects in the silicon-on-insulator structures synthesized by the PIII/wafer bonding/ion-cut process. The defect formation and its change with annealing temperature were investigated experimentally employing channeling Rutherford backscattering spectrometry, secondary ion mass spectrometry, and atomic-force microscopy. We also calculated the damage energy density of the three dominant hydrogen species in the plasma H,H 2, and H 3 as well as displacement of silicon atoms in the silicon wafer. H 2 creates the most damage because its damage energy density is very close to the silicon threshold energy. The effects of atmospheric gaseous impurities unavoidably coimplanted from the overlying plasma are also modeled. Even though their concentration is usually small in the plasma, our results indicate that these gaseous impurities lead to significant silicon atom displacement and severe damage in the implanted materials American Institute of Physics. DOI: / I. INTRODUCTION Silicon on insulator SOI is an important technology for radiation-hardened integrated circuits as well as low-power, low-voltage, and high-temperature microelectronics. In fact, with the introduction of consumer microelectronic chips fabricated using SOI, SOI is now regarded to be not just the material of the future but also the material of the present. 1,2 Compared to the more mature SOI technologies such as separation by implantation of oxygen SIMOX 3 and smart-cut, 4 synthesis of SOI materials by plasma immersion ion implantation PIII combined with wafer bonding and ion cut is an attractive approach due to the high efficiency of PIII and relatively inexpensive instrumentation. 5 In fact, the cost saving is more significant for 300 mm silicon wafers as the PIII processing time is independent of the wafer dimension. There are many intrinsic differences between PIII and conventional beamline ion implantation. For example, in PIII, all ion species in the overlying plasma are implanted when a negative voltage pulse is applied to the silicon wafer and the ion energy distribution depends on factors such as the sample voltage wave form, plasma sheath propagation, collision effects, and hardware-related issues such as displacement current and cable capacitance. In contrast, the ion a Also affiliated with Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai , China and currently on leave at Delft University of Technology, DIMES TC, Feldmannweg 17, P.O. BOX 5053, 2600 GB, Delft, Netherlands. b Author to whom correspondence should be addressed; electronic mail: paul.chu@cityu.edu.hk beam in beamline ion implantation is mass and energy selected to comprise only one ion species with a specific energy. Therefore, the formation and effects of damage and defects in hydrogen plasma implanted silicon are expected to be different from those of beamline ion implantation. The damage in the hydrogen plasma implanted wafer can conceivably translate into defects in the SOI structure produced by PIII/ion-cut/wafer bonding and impact the yield of the fabrication process. In order to investigate this problem, the following factors must be considered: 6,7 1 The ion implant dose required for the hydrogen PIII layer transfer process is quite large high cm 2 to low cm 2. Therefore, significant crystal damage results in spite of the small hydrogen mass. 2 Different hydrogen ion species exist in the plasma, mainly, H,H 2, and H 3, and all of them are coimplanted into the silicon wafer. The damage profile is different for each ion species due to the difference in the net impact energy for example, each H atom in the H 3 molecular ion possesses 1/3 of the kinetic energy of the H atom in the H atomic ion and dose for instance, there are three hydrogen atoms in each H 3 molecular ion. 3 Since PIII machines are typically not designed for ultra-high-vacuum UHV operation, there are residual oxygen, nitrogen, water, and other atmospheric species in the vacuum chamber. The plasma thus contains some of these ions and they are unavoidably coimplanted into the silicon wafer together with hydrogen. The damage caused by these residual gas species must be taken into account /2001/90(4)/1735/5/$ American Institute of Physics

2 1736 J. Appl. Phys., Vol. 90, No. 4, 15 August 2001 Wang et al. 4 The ion energy distribution is broad due to multiple ion species as well as the low-energy component arising from the nonzero rise and fall times of the sample voltage pulse. In general, a surface dislocation density of less than 50 cm 2 is desired in a production environment. 1 Hence, damage in the plasma implanted wafer must be carefully controlled in each step during the manufacturing process. In this work, the damage characteristics of Si after hydrogen PIII are investigated experimentally using secondary ion mass spectrometry SIMS, channeling Rutherford backscattering spectrometry RBS/C, and atomic-force microscopy AFM. We also use a relatively simple model to derive the damage energy density of the three hydrogen ions as well as gaseous impurity ions. II. EXPERIMENT Boron-doped p-type Si 100 with a resistivity of cm was hydrogen plasma implanted using our semiconductor PIII instrument. 8 The base pressure in the vacuum chamber was Torr. Before implantation, highpurity hydrogen gas was bled into the chamber to establish a working pressure of Torr. The PIII experiments were conducted at a bias voltage of 25 kv, current of A, pulsing frequency of 200 or 300 Hz, and pulse duration of 30 s. The hydrogen implant dose was calculated based on the SIMS results using a relative sensitivity factor derived from standard ion implant materials. A simple wet cleaning process was carried out to remove some surface contaminants before the SIMS measurements. The damage characteristics of the as-implanted and annealed samples 200 and 400 C were assessed using helium RBS/C. The incident energy was 2 MeV and the backscattering angle was 170. The surface morphology of the samples was studied by AFM and it was conducted using a Park Instrument SPM machine at room temperature and atmospheric pressure. III. RESULTS AND DISCUSSION In PIII, there is no mass selection and this is one of the reasons why PIII boasts a high ion flux and throughput. However, it also means that all the ions in the plasma are implanted into the wafer simultaneously. A typical hydrogen plasma consists of three dominant ions, H,H 2, and H 3, and their implantation into Si can be easily verified by fitting the SIMS hydrogen depth profile. According to the molecular ion implantation theory, 9 the net ion energy is given by Em/M, where E is the energy of the molecular ion, m is the mass of the atom, and M is the total mass of the molecule. Hence, the hydrogen atoms in these three ion species have different net implant energies. For example, each hydrogen atom in the H 3 molecular ion has 1/3 of the implant energy and, consequently, smaller penetrating depth. Since the nuclear stopping power is a function of the ion energy, the damage profile is different for the three hydrogens containing ions. In our fits, we use the following simple relationship: FIG. 1. Hydrogen depth profile acquired by SIMS from hydrogen plasma implanted silicon 300 Hz pulsing frequency and 30 min implantation time and the theoretical fit using overlapping H,H 2, and H 3 Gaussian distributions. Hydrogen diffusion that causes broadening of the half width and depth shift nm due to the surface treatment has been considered. The high surface peak in the SIMS profile is a measurement artifact. n N x i 1 d i 2 pi exp x R 2 pi 2, 1 2 pi where N(x) is the density of H ions at distance x from the surface, d i is the dose of the ith species, R pi is the project range of the ith species, and pi is the straggle which can be obtained from TRIM simulation. However, considering the heating effects during implantation, hydrogen diffusion needs to be taken into account as well. Moreover, surface oxidation during PIII and the surface treatment before SIMS measurement may cause a small shift in the depth profile measurement. Figure 1 depicts the hydrogen SIMS depth profile of the sample implanted at 300 Hz for 30 min, and the integrated ion dose was calculated to be cm 2. This dose is typical of the ion-cut process by PIII. Using TRIM and Eq. 1, we fit the SIMS data and the doses are determined to be , , and cm 2 for H,H 2 and H 3, respectively. The sum of the ion doses from the fit is cm 2. The difference between the fitted value and measured result is due to surface hydrogen that contributes to the SIMS result but not to the modeled value. To investigate the damage, we derive the damage energy density using these experimentally determined doses and the following relationship: 10 e d x D de d x, 2 N Si dx where e d (x) is the damage energy density in ev/atom, D is the dose, N Si is the atomic density of Si ( /cm 3, and de d (x)/dx is the nuclear stopping energy, which can be directly calculated by TRIM. The calculated damage energy densities for H, H 2, and H 3 are 2.1, 15, and 12.6 ev/ atom, respectively, and the simulated damage atomic displacement distribution is exhibited in Fig. 2. Here, we consider that the atomic displacement is mainly caused by

3 J. Appl. Phys., Vol. 90, No. 4, 15 August 2001 Wang et al FIG. 2. Calculated composite damage distribution dpa displacement per atom at an implantation voltage of 25 kv using H,H 2, and H 3 with doses of , , and cm 2, respectively. nuclear stopping. 11,12 Comparing the composite damage energy density of H,H 2, and H 3 with the threshold energy of Si about 15 ev, our hydrogen PIII process causes significant damage to the crystal structure of silicon and can render the region in the vicinity of the projection range R p amorphous. It should also be pointed out that a high concentration of implanted hydrogen will cause high pressure or stress within the Si crystal, contributing to additional damage to the crystal structure. 13 Figure 3 shows the RBS/C spectra of the sample implanted at 200 Hz and for 30 min, and those of the sample annealed at 400 C are displayed in Fig. 4. Comparison between the random and channeled spectra shows that the damage layer in both samples is almost amorphous and the thickness is approximately 170 nm. The damage profile acquired from a smaller implant dose sample is exhibited in Fig. 5. However, such a small implantation dose is inadequate for effective microcavity formation and ion cut, and so there is no practical need to investigate the latter case. Figure 5 shows that a lower dose implant 25 kv, 200 Hz, 10 min leads to a thinner damage layer about 70 nm in thickness. FIG. 4. RBS/C spectrum acquired from the hydrogen PIII sample shown in Fig. 2 after annealing at 400 C for 2 h. In this case, the damage caused by residual gaseous impurities is relatively significant, and this issue will be discussed later in this article. Piatkowska, Gawlik, and Jagielski 14 studied the relationship between the surface morphology and hydrogen implant dose in beamline ion implantation. They, however, did not present detailed results on the change of surface morphology with temperature. Here, we show the surface morphological change in the as-implanted and annealed samples. Figure 6 depicts the AFM topographic maps of the as-implanted and annealed hydrogen PIII samples, and the change in the surface roughness as indicated by our root-mean-square rms calculation in a 5 m 5 m region for different annealing temperatures is shown in Table I. Based on the observed increase in the surface roughness with the anneal temperature, the hydrogen movement or coalescence process during annealing plays an important role in the surface morphology, even though the change in the RBS channeling behavior after FIG. 3. Channeling RBS RBS/C spectrum acquired from a hydrogen PIII sample 300 Hz and 30 min. FIG. 5. RBS/C spectrum acquired from a lower dose sample 200 Hz and 10 min.

1738 J. Appl. Phys., Vol. 90, No. 4, 15 August 2001 Wang et al. FIG. 6. AFM topographical map of the hydrogen PIII samples 300 Hz and 30 min : a as implanted and b after annealing at 400 C for 2 h.

4 1738 J. Appl. Phys., Vol. 90, No. 4, 15 August 2001 Wang et al. FIG. 6. AFM topographical map of the hydrogen PIII samples 300 Hz and 30 min : a as implanted and b after annealing at 400 C for 2 h. annealing at 400 C is not obvious. It should be noted that our work focuses on temperature at or below 400 C because in the hydrogen PIII/ion-cut process, an implantation temperature of 400 C or higher will cause hydrogen bubble formation or blistering and, consequently, premature exfoliation. As an interstitial atom with a dissolution enthalpy of 0.8 ev, hydrogen displays an extraordinary chemical reactivity to silicon, leading to the formation of point and extended defects irrespective of its atomic or molecular state. Cerofolini et al. 15 investigated the bubble formation in H and He implanted Si and deduced the change of the enthalpy in the Si H 2 system with respect to temperature, hydrogen concentration, and other factors. One of the important factors influencing the surface morphology is internal stress caused by the high pressure exerted by hydrogen in the microvoids. This stress leads to an increase of the surface roughness, and finally, surface blistering at a high enough temperature or under mechanical stress. Normally, for a Si wafer implanted with a hydrogen dose higher than cm 2, surface blistering will occur and become visible when the wafer is heated to 450 C. At a higher implant dose, the required temperature is lower and this is also true in the case of boron coimplantation. Figure 6 b confirms the gradual change in the surface morphology that eventually leads to surface blistering. For the ion-cut process, the generation of a sufficient amount of bubbles or microcavities is necessary. However, since the damage and defects cannot be annealed out at low temperature, a paradox is created when a high-temperature treatment before wafer bonding is not feasible. The broad damage zone buried in the SOI structure will affect recrystallization of the transferred layer during the subsequent solid-state reaction at high temperature. One can argue that if hydrogen PIII is conducted on a silicon wafer with a pregrown thin surface oxide, the damage region on the surface can be confined to the oxide that can be removed prior to wafer bonding. However, our RBS results Figs. 3 5 show that the damage zone stretches all the way from the vicinity of the ion-projected range to close to the surface. In addition, oxygen recoil may cause other problems and the damage created in the bulk cannot be circumvented totally. Figure 5 shows that surface damage is still serious even when the dose is lower. According to our previous studies, under typical conditions, contaminants such as oxygen, nitrogen, and carbon in the plasma constitute a few percent of the total ion current in hydrogen PIII. 16 This is because PIII equipment is usually not designed for UHV conditions. For a total ion implant dose in the high to low cm 2, the oxygen, nitrogen, and carbon ion doses may be close to or exceed cm 2. Based on our experimental results at 25 kv for an oxygen dose of cm 2, we calculate damage energy densities of 29.3 and 33.5 ev/atom for O and TABLE I. Root-mean-square rms surface roughness values of the asimplanted sample and samples annealed at 200, 300, and 400 C. As implanted 200 C 300 C 400 C Surface roughness rms nm FIG. 7. Calculated damage distribution profile of Si atoms dpa displacement per atom in 25 kv oxygen plasma implanted samples.

5 J. Appl. Phys., Vol. 90, No. 4, 15 August 2001 Wang et al TABLE II. Calculated damage energy densities of carbon and nitrogen N and N 2 in Si implantation energy 25 kev. Ion species C N N 2 Implantation energy kev Implantation dose ions cm Projection range R p / 75.7/ / /17.6 longitudinal straggling nm Damage energy density ev/atom O 2, respectively. The simulated displacement of Si atoms is shown in Fig. 7. The calculated damage energy densities of nitrogen and carbon are listed in Table II for the same implant conditions 25 kv. In spite of their small percentage, the presence of gaseous contaminants can significantly increase the damage. These contaminant ions are heavier and the damage zone is closer to the surface. Hence, the RBS/C spectrum shown in Fig. 5 makes sense. This damage region is buried in the SOI structure after layer transfer, that is, close to the buried oxide, and affects recrystallization even more severely. This contamination issue must be addressed properly in experiments, for instance, by using better vacuum and pumping devices. IV. CONCLUSION The damage in hydrogen plasma implanted Si has been investigated experimentally and theoretically. Our RBS/C results indicate that the damaged layer is quite broad, and different from that observed in single-energy beamline ion implantation in which the damage only occurs near the projected range of the implanted ions. The broadness of the damage zone is attributed to different ion species from the plasma implanted to different depths. Finally, we discuss the contribution of inevitable coimplanted gaseous contaminants. Even though they are small in percentage, our damage energy density and silicon atom displacement calculations reveal the severity of the effects, and care must, therefore, be exercised in reducing the amount of these gaseous species in PIII equipment. ACKNOWLEDGMENTS The work was jointly supported by the Hong Kong Research Grants Council CERG Grant No or CityU Grant Nos. 1003/99E and or CityU Grant No. 1032/ 00E, the City University of Hong Kong SRG Grant No , and the Chinese NSF Grant No J.-P. Colinge, Silicon-on-Insulator Technology, Materials to VLSI, 2nd ed. Kluwer Academic, Dordrecht, S. Cristoloveanu and S. S. Li, Electrical Characterization of Silicon-on- Insulator Materials and Devices Kluwer Academic, Dordrecht, L. Alles, P. Dolan. M. J. Anc, P. Allen, F. Cordts, and T. Nakai, Proc. IEEE Int. SOI Conf. 1997, p.10 unpublished. 4 M. Bruel, B. Aspar, B. Charlet, C. Maleville, T. Poumeyrol, A. Soubie, A. J. Auberton-Herve, J. M. Lamure, T. Barge, F. Metral, and S. Trucchi, Proc. IEEE Int. SOI Conf. 1995, p.178 unpublished. 5 W. G. En, I. J. Malik, M. A. Bryan, S. Farrens, F. J. Henley, N. W. Cheung, and C. Chan, Proc. IEEE Int. SOI Conf. 1998, p.163 unpublished. 6 P. K. Chu, S. Qin, C. Chan, N. W. Cheung, and L. A. Larson, Mater. Sci. Eng., R. R17, Z. Fan, Q. C. Chen, P. K. Chu, and C. Chan, IEEE Trans. Plasma Sci. 26, P. K. Chu, S. Qin, C. Chan, N. W. Cheung, and P. K. Ko, IEEE Trans. Plasma Sci. 26, S. Prussin, in Ion Implantation in Semiconductors, edited by S. Namba Plenum, New York, 1975, p J. Ziegler, J. Biersack, and U. Littmar, in The Stopping and Range of Ions in Solids Pergamon, New York, S. Tian, M. F. Morris, S. J. Morris, B. Obradovic, G. Wang, A. F. Tasch, and C. M. Snell, IEEE Trans. Electron Devices 45, W. Bohmayr, A. Burenkov, J. Lorenz, H. Ryssel, and S. Selberherr, IEEE Trans. Comput.-Aided Des. 17, G. F. Cerofolini, G. Calzolari, F. Corni, C. Nobili, G. Ottaviani, and R. Tonini, Mater. Sci. Eng., B 71, A. Piatkowska, G. Gawlik, and J. Jagielski, Appl. Surf. Sci. 141, G. F. Ceroflini, F. Corni, S. Frabboni, C. Nobili, G. Ottaviani, and R. Tonini, Mater. Sci. Eng., R. 27, Z. Fan, X. Zeng, P. K. Chu, C. Chan, and M. Watanabe, Nucl. Instrum. Methods Phys. Res. B 155,

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