Effects of oxygen on selective silicon deposition using disilane

March 1999

Materials Letters 38 Ž1999. 418–422

Effects of oxygen on selective silicon deposition using disilane ¨ ¨ Patricia A. O’Neil a , Mehmet C. Ozturk a

a,)

, Alan D. Batchelor b, Dennis M. Maher

b

Department of Electrical and Computer Engineering, North Carolina State UniÕersity, Box 7911, Raleigh, NC 27695-7911, USA Department of Materials Science and Engineering, North Carolina State UniÕersity, Box 7916, Raleigh, NC 27695-7916, USA

b

Received 11 August 1998; accepted 9 October 1998

Abstract Using Si 2 H 6 in an ultrahigh vacuum rapid thermal chemical vapor deposition reactor, we have investigated the role of high levels of oxygen Ž) 5 = 10y6 Torr. introduced during selective silicon deposition. The effects of oxygen have been investigated with regard to oxygen incorporation, selectivity with respect to thermal SiO 2 , growth rate, and epitaxial quality. The addition of oxygen was found to enhance the inherent process selectivity of Si 2 H 6 to SiO 2 while causing no reduction in the silicon growth rate or measurable oxygen incorporation into the growing film for oxygen pressures below 5 = 10y5 Torr. Contrary to published reports, the silicon film was devoid of the pyramidal defects usually characteristic to highly oxygenated processes. The silicon surface morphology, however, exhibited increased roughness with increasing oxygen partial pressure. The surface roughness is believed to be a result of the high levels of oxygen adsorbed at the initial growth surface. q 1999 Elsevier Science B.V. All rights reserved. PACS: 68.55.y a; 68.55.Jk; 73.61.Cw Keywords: Silicon; CVD; Disilane; Selective silicon deposition

1. Introduction Low-temperature selective silicon deposition has been recently investigated for possible applications in CMOS process integration by using a variety of techniques and process chemistries. We have reported a selective silicon epitaxy process using Si 2 H 6 in an ultrahigh vacuum rapid thermal chemical vapor deposition reactor ŽUHV-RTCVD. w1x. By utilizing chlorine-free Si 2 H 6 or SiH 4 process chemistries, an inherent selectivity is obtained when ultra-clean deposition conditions are employed w2–4x. The silicon deposition, however, is limited to a critical layer thickness or critical growth time above which selec)

Corresponding author

tivity to SiO 2 or Si 3 N4 is lost. Typically, greater process selectivity is desired, and therefore small amounts of chlorinated species are added to the process. An alternative selectivity enhancement mechanism recently discovered by Irene and Basa w5x involves the introduction of oxygen into a Si 2 H 6 based nucleation process. They found that oxygen could effectively enhance the incubation time and suppress nucleation on thermal SiO 2 . A similar study by Unnikrishnan et al., w6x however, showed no evidence of selectivity enhancement with oxygen. Since this study included an additional element to the process, namely chlorine, a disruption to the selectivity enhancement mechanism that was observed by Irene et al. may have occurred.

00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 2 0 0 - 6

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In addition to selectivity, several studies have investigated the role of oxygen in an epitaxial growth environment w6–11x. These studies have primarily dealt with concerns regarding oxygen incorporation, and defect generation for processes utilizing SiH 2 Cl 2 or SiH 4 based chemistries in non-UHV environments. For all of the studies which investigated the epitaxial quality, increases in either the pyramid defect density or the surface roughness was reported with increasing oxygen during the actual growth process or in situ pre-clean w6–10x. However, incorporation of oxygen on epitaxial growth varied greatly between the researchers, and contrasting conclusions were obtained regarding the mechanisms which limit oxygen incorporation at varying process conditions w7,8,11x. It has been agreed upon, however, that variations in the system environment, for example the presence of a boundary layer and a high hydrogen background level, which are both common to increased pressure processing, can greatly affect the sticking efficiency of oxygen and may help to explain the reported variations in oxygen incorporation w9,11x. 2. Discussion The experiments in this study were performed in a UHV-RTCVD reactor designed and constructed at North Carolina State University. UHV-RTCVD combines the ultra-clean environment of a UHV chamber with the rapid thermal switching capability of rapid thermal processing to grow low temperature epitaxial films at rates compatible with single wafer manufacturing. A detailed description of the system can be found elsewhere w2,12x. Deposition was performed on 4 in. Ž100. silicon wafers which were patterned with 1 mm = 1 mm ˚ thermally grown SiO 2 . The openings in 1000 A wafers were cleaned ex situ using a standard RCA solution and the substrate surface was hydrogen passivated immediately prior to loading. This was achieved by dipping the wafer in a dilute 5% HF solution for 15 s followed by a 15-s deionized water rinse. Deposition was controlled by temperature switching during which the temperature ramp rates achieved by the 35 kW Peak Systemse LXU-35 arc lamp have been approximated at around 1508Crs. The process utilized in this study included 130 sccm

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of 10% Si 2 H 6 in He Ž; 40 mTorr., 0 to 1 = 10y3 Torr of pure oxygen, and a growth temperature of 8008C. The oxygen was introduced into the process via a precision leak valve prior to temperature rampup. The oxygen partial pressure was calibrated by using both an ion gauge and a differentially pumped residual gas analyzer in order to insure accurate oxygen partial pressures during the actual growth process. By investigating an inherently nonselective process condition, variations in silicon nucleation on thermal SiO 2 were observed for varying levels of oxygen partial pressure. Percent nuclei coverage was monitored as a measure of selectivity since it would potentially include both changes in nuclei size and density. The conditions used for silicon deposition included 130 sccm of 10% Si 2 H 6 in He, a process pressure of about 40 mTorr, and a growth temperature of 8008C. The incubation time for this process was determined to be about 25 s, provided that no in situ clean was performed prior to deposition. A process time of 27 s was selected to insure that nucleation on the thermal SiO 2 surface would occur when no oxygen was introduced. Following deposition, the nuclei coverage was examined by SEM. All of the nuclei observed by SEM were of diameter 80 nm or less. As illustrated in Fig. 1, the percent nuclei coverage decreased logarithmically with increasing oxygen partial pressure during the process. The added oxygen provided a selectivity

Fig. 1. Percent nuclei coverage has been plotted as a function of oxygen partial pressure during processing. Significant amounts of oxygen in the ambient reduced the overall nuclei coverage on SiO 2 and enhanced selectivity.

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enhancement mechanism and effectively increased the incubation time of the process. This observation is in agreement with those of Irene and Basa w5x but contrary to those found by Unnikrishnan et al. w6x. The reason for which selectivity enhancement is obtained for the nonchlorinated, Si 2 H 6 process can be attributed to several possible factors including the formation of volatile SiO, the passivation of Si terminating nucleation sites on the SiO 2 surface with oxygen w5x, or the initiation of SiO 2 deposition. In a chlorinated ambient, however, it is possible that oxygen passivation andror further SiO 2 deposition is reduced since chlorine itself readily passivates to silicon atoms at temperatures less than 7508C w13x. Furthermore, through thermodynamic equilibrium simulation of the Cl 2 –O 2 –SiŽs. system, the removal of silicon through the formation of SiCl x Ž1 F x F 4., typical of a chlorinated selective silicon deposition process w13x, predominates over adatom removal by SiO formation. Therefore, the increased SiO formation may be masked by the already more favorable SiCl x desorption mechanism. Although the benefits of oxygen for selectivity enhancement in a nonchlorinated silicon deposition process have been demonstrated, the impact of oxygen on the deposited film still needs to be addressed. The thicknesses of the films deposited using varying oxygen process pressures were measured by profilometry and interferometry techniques. The trend for deposited silicon thickness as a function of oxygen partial pressure is shown in Fig. 2. As observed, for oxygen partial pressures below 1 = 10y4 Torr the silicon deposition thickness is unaffected by the oxygen. However, once the oxygen level is increased above 1 = 10y4 Torr, no deposition is detected and therefore Si growth is completely suppressed. The abrupt, rather than gradual decrease in Si thickness implies the presence of an additional reaction or mechanism once an oxygen level greater than 1 = 10y4 Torr is added to the 40 mTorr Si 2 H 6 process. Since the process gases, including oxygen are introduced prior to reaching the absolute growth temperature of 8008C, it is probable that the formation of SiO 2 occurred prior to silicon monolayer growth and therefore suppressed the silicon deposition. Prior studies have indicated a dependence between the oxygen contamination level and the epitaxial quality of the silicon films w6–8,10x. Incorporated

Fig. 2. Silicon thickness plotted as a function of oxygen partial pressure. The silicon growth was unaffected by oxygen for partial pressures lower than 1=10y4 Torr. Above 1=10y4 Torr of oxygen, the silicon growth was fully suppressed.

oxygen has been shown to roughen the surface morphology w6,7x, whereas interfacial oxygen produces pyramidal defects whose density is exponentially proportional to the oxygen dose w8,10x. In this study, oxygen levels greater than 1 = 10y4 Torr completely suppressed any silicon growth. However, using 1 = 10y4 Torr of oxygen, a silicon film filled with irregular voids penetrating to the interface was obtained. This film structure was a direct result of the transition between silicon monolayer growth and SiO 2 formation. Below 1 = 10y4 Torr of added oxygen, no increase in gross defect density was observed; therefore, the results found here contradict the reports by Agnello and Sedgwick w8x and Sedgwick et al. w10x. However, since pyramidal defects result directly from the selectivity of the process to oxide islands at the interface, increased defect densities will only occur for processes utilizing high degrees of selectivity and chlorine w14x. The lack of pyramidal defects within the deposited films of this study therefore implies that the nonchlorinated epitaxial process favors silicon overgrowth of the oxide islands rather than defect formation. Upon closer examination of the resulting films by atomic force microscopy ŽAFM., the surface morphology of the deposited silicon films exhibited increased roughening upon the addition of oxygen. For example, the RMS surface roughness increased from

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˚ with no added oxygen, to 6.9 A, ˚ at an oxygen 1.4 A, partial pressure of 1 = 10y5 Torr. Shown in Fig. 3Ža,b. are the AFM Ž1 mm = 1 mm. phase images of the silicon surfaces resulting from oxygen-free and oxygen-rich environments. Despite the polycrystalline appearance of Fig. 3Žb., the resulting silicon film is epitaxial as observed by cross-sectional transmission electron microscopy. Unnikrishnan et al. obtained similar surface morphologies, however

Fig. 4. For process utilizing 1=10y5 Torr of oxygen, the interfacial oxygen level increased an order of magnitude and potentially caused the surface roughening observed in Fig. 3Žb.. The width of the oxygen peak at the epitaxyrsubstrate interface is enhanced due to SIMS broadening effects. No change in the baseline oxygen level of the silicon film was observed.

claimed bulk polycrystalline films, by using SiH 2 Cl 2 as opposed to Si 2 H 6 . Unnikrishnan et al. w6x attributed the observed structural change to oxygen incorporation within the growing films. Within this work, however, no measurable evidence of oxygen incorporation was detected for films deposited with oxygen partial pressures at or below 1 = 10y5 Torr. As shown in Fig. 4, silicon epitaxy which was deposited at an oxygen partial pressures of 1 = 10y5 Torr produced a film with an oxygen level below the SIMS detection limit of 4 = 10 17 rcm3. The interfacial oxygen level, however, did increase an order of magnitude over the standard epitaxial silicon process in which no in-situ clean was utilized. Therefore, the accumulation of oxygen at the interface promoted the surface roughening detected on the silicon film shown in Fig. 3b.

3. Conclusion

Fig. 3. Ža. AFM 1 mm=1 mm phase images of Ža. an oxygen-free process and Žb. a 1=10y5 Torr oxygenated process.

In summary, the presence of high levels of oxygen during the deposition of silicon by UHV-RTCVD can greatly affect the epitaxial process. Oxygen has been shown to enhance selectivity while degrading film quality and deposition thickness through the formation of interfacial oxygen. For oxygen levels below 1 = 10y5 Torr, however, the degradation in

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film quality is limited to a roughened surface morphology, therefore silicon epitaxial layers, free of pyramidal defects, can be selectively deposited at ˚ Further research, however, thicknesses up to 850 A. is required to determine if similar findings exist for oxygen in a chlorinated selective silicon epitaxy process environment. Acknowledgements The authors would like to acknowledge George Guryamov and Dieter Griffis for the SIMS analysis; and also the NCSU Microelectronics Laboratory staff, S. Muhsin C¸ elik, Ibrahim Ban, and Nemanja Pesovic for their invaluable assistance. This research was partially supported by NSF Engineering Research Centers Program through the Center for Advanced Electronic Materials Processing ŽGrant CDR8721545., the Eaton, and by the NSF Presidential Faculty Fellow award. References ¨ ¨ Appl. Phys. Lett. w1x K.E. Violette, P.A. O’Neil, M.C. Ozturk, 68 Ž1996. 66–68.

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