Origin of perfect selectivity of Si epitaxial growth through disilane molecularbeamepitaxy using synchrotron radiationYuichi Utsumi and Housei Akazawa Citation: Journal of Applied Physics 77, 2211 (1995); doi: 10.1063/1.358804 View online: http://dx.doi.org/10.1063/1.358804 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/77/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Synchrotronradiationinduced photodoping using disilane molecularbeam epitaxy: Lowtemperature highdoping of B J. Appl. Phys. 79, 717 (1996); 10.1063/1.360816 Effects of synchrotron radiation irradiation on selective Si epitaxial growth by disilane molecular beamepitaxy J. Vac. Sci. Technol. B 13, 936 (1995); 10.1116/1.588209 Perfectly selective Si epitaxial growth due to synchrotron radiation irradiation during disilane molecularbeam epitaxy Appl. Phys. Lett. 62, 1647 (1993); 10.1063/1.108614 Synchrotron radiation excited Si epitaxial growth using disilane gas source molecular beam system Appl. Phys. Lett. 58, 2776 (1991); 10.1063/1.104783 Selective growth condition in disilane gas source silicon molecular beam epitaxy Appl. Phys. Lett. 52, 2242 (1988); 10.1063/1.99654

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Sun, 21 Dec 2014 15:28:02

Origin of perfect selectivity of Si epitaxial growth through molecular-beam epitaxy using synchrotron radiation

Yuichi Utsumi and Housei Akazawa NTT LSI Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa-ken, 24341, Japan

(Received 27 June 1994; accepted for publication 8 November 1994)

It has been reported that selective Si epitaxial growth between Si and SiO, substrates can be maintained irrespective of the growth period by synchrotron radiation (SR) during disilane molecular-beam epitaxy. The effects of SR on the initial stage of the polySi growth on SiOa are investigated as they relate closely to the appearance of perfect selectivity. It is observed that SR changes the surface composition of SiOs and directly affects the incubation period of Si nucleation ending in poly-Si film formation on SiOs. A temperature above 700 “C is necessary for perfect selectivity since the SiO, surface becomes Si rich below 700 “C due to radiation. The incubation period dependence on the pause/irradiation period in intermittent SR suggests that a photothermal reaction completely removes Si nuclei from SiOz and brings about perfect selectivity. 0 1995 American Institute of Physics.

Selective epitaxial growth (SEG) of Si, enabling Si deposition only on Si but not on SiO,, is a promising tech- nique for the fabrication of fine self-aligned structures for use in high-speed bipolar Si devices.

SEG through gas-source molecular-beam epitaxy (GSMBE) using silane or disilane has been widely studied.‘,* Using this technique, selective epitaxial films with few sur- face defects can be fabricated at low temperatures with high thickness controllability. However, SEG through GSMBE has shown less than perfect results in that it eventually breaks down after an incubation period (IP) of Si nucleation that ends in polycrystalline Si (poly-Si) tilm formation on the SiOz surface. This problem arises mainly because Si nucle- ation and its coalescence on SiO? cannot be completely sup- pressed in conventional GSMBE. To attain perfect selectivity with an infinite IP, the SiOa surface must be modified to prevent Si nucleation or the Si nuclei formed on the SiO, surface must be removed to prevent the growth of a continu- ous poly-Si film. It has been reported that perfect SEG can be achieved3 by combining GSMBE and synchrotron radia- tion (SR). The photon energies of SR are most suitable for inducing surface chemical reactions.4’5 It was assumed in a previous report that the SiO, surface was modified into a surface which had no Si dangling bond, therefore, adsorbed dissociation of S&H6 could not occur with the aid of SR- stimulated Si02 evaporation.

However, in our previous experiments, the assumed mechanism was not totally verified. This communication in- vestigates the effects of SR on the initial stage of the poly-Si growth on SiOs surfaces as they relate closely to the appear- ance of perfect selectivity between the Si and SiO, substrate. It is confirmed that perfect SEG does not result from the modification of the SiOz surface. On the basis of the IP de- pendence on the pause/radiation period of intermittent SR, it is verified that perfect SEG results from the complete re- moval of Si nuclei from SiO, surface due to SR. It is also proposed that the role of the radiation is to enhance Si02 reduction through by electronic excitation of the SiOs.

The BLlC beam line of the 2.5 GeV Photon Factory storage ring at the National Laboratory for High Energy

Physics is used in the experiment. The reaction apparatus is described in detail elsewhere.6 Silicon (100) substrates cov- ered partially with thermally grown Si02 patterns were used for selective growth. Before growing the epitaxial Si films, the native oxide in the Si windows was removed by heating the substrate at 980 “C for 10 min. Disilane gas with a purity of 99.99% was fed into the reaction chamber while a SR beam with a wavelength of from 1 to 100 run and a peak at 10 nm perpendicularly radiated the sample surface. The av- erage storage ring current during the experiments wasp 3 10 mA, which provided 3X lOI photons/s/cm”. Distinctions be- tween SEG and non-SEG in a series of experiments were inferred mainly from reflection high-energy electron diffrac- tion (RHEED) patterns observed immediately after growth in the reaction chamber. When growth was selective, no poly- crystalline Si grew on the SiO, and RHEED yielded a halo pattern characteristic of the SiO, surface. In the case of non- SEG, the RHEED for the SiO, region showed a ring struc- ture since a polycrystalline Si layer grew on the SiO,. SEG was also substantiated by secondary-ion-mass spectroscopy (SIMS), scanning electron microscope (SEM), or Auger elec- tron spectroscopy (AES).

To clarify differences in the characteristics of SR- induced selective epitaxial growth (SRSEG) and SEG through GSMBE (without SR), the temperature dependence of the IP was measured for samples with and without SR, as shown in Fig. 1. The 1P was determined by measuring the time from the beginning of growth to the appearance of the RHEED ring pattern of poly-Si on the SiO,. For SEG with- out SR, the incubation period versus temperature curve has a minimum at 700-750 “C, which is characteristic of SEG by GSMBE.2 In this case the IP increases as the temperature decreases at lower temperatures since the surface decompo- sition rate of S&H6 decreases. At temperatures above the minimum it increases as the temperature increases. At high temperatures, the Si02 surface is gradually reduced through adsorbed Si atoms decomposed from Si2H6, and the ad- sorbed Si atoms are evaporated in the form of volatile SiO.’ In this way, Si nucleation on the SiO, is suppressed at higher substrate temperatures. Perfect selectivity, however, is not

J. Appl. Phys. 77 (CT), 1 March 1995 0021-8979/95/77(5)/221 l/3/$6.00 6 1995 American Institute of Physics 2211 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Sun, 21 Dec 2014 15:28:02

Ring Current 17; 3lOmA

f3 E 2 100 f5 2 5 s

IO -400 500 600 700 800 900

SUBSTRATE TEMPERATURE (‘+I 1

FIG. 1. Temperature dependence of the IP using disilane pressure as a parameter for the cases in which SR was radiated (open symbols) and not radiated (solid symbols) during SEG. The disilane pressure was set at 3X 1O-5 and 6X lo+ Torr.

obtained in this temperature regime for SEG through GSMBE. The IP is lower for SRSEG than for SEG through GSMBE at low temperatures. We found this to be related to the collapse of the SiO, surface stoichiometry due to SR- stimulated SiO, evaporation. Figure 2 shows the temperature dependence of AES spectra of SiOz surfaces obtained after the SiO, film was etched to 300 nm using SR-stimulated evaporation in an ultrahigh vacuum. This SR-stimulated SiO, evaporation is caused by oxygen atom displacement into an interstitial site due to electron excitations followed by thermal desorption of SiO molecules.6 The appearance of a

etching depth 300nm

40 60 ilO 100 Electron Energy (eV)

FIG. 2. Temperature dependence of the Auger spectra of SiOz surfaces obtained after the SiO? film was etched to 300 nm using SR-stimulated evaporation in an ultrahigh vacuum.

2212 J. Appl. Phys., Vol. 77, No. 5, 1 March 1995

92 eV Si peak in the figure shows that the Si02 surface becomes Si rich below 700 “C. This is due to the preferential desorption of oxygen from the SiO, surface through SR. These results agree with those obtained in another study.7 The results indicate that the abundance of Si dangling bonds on Si-rich SiOz surfaces promotes S&H, decomposition and hastens the nucleation, as shown in Fig. 1. Thus, the SR increases the Si dangling bond on SiO, below 700 “C. It was also observed that SiO, samples of which the surface was pm-irradiated (10 min) above 700 “C had a finite IP of ther- mal selective growth by disilane although the IP was pro- longed more than twa times. These show that a few Si dan- gling bonds still exist on SiO, surface after SR, that is, perfect selectivity does not result from the modification of the SiO, surface by SR. As shown in Fig. 2, the stoichiom- etry of the SiO;? surfaces is maintained above 700 “C. In addition, the IP increases monotonically with the substrate temperature and SEG is sustained as long as the Si02 film exists above 700 “C (Fig. 1). Thus, temperatures above 700 “C are necessary for perfect SRSEG since the formation of a Si-rich surface causes Si nucleation. These results show the distinctive features of SRSEG which could not be ob- served for conventional SEG through GSMBE. From the re- sults shown in Figs. 1 and 2 we conclude that this distinct difference between SRSEG and SEG through GSMBE is caused by SR-induced chemical reactions such as interface reactions between adsorbed Si atoms and surface SiO, mol- ecules.

To clarify the origin of perfect SEG, it is important to investigate the effects of SR on the initial stage of poly-Si growth on SiO,, as they relate closely to the appearance of perfect selectivity. For perfect selectivity, Si nucleation has to be completely suppressed or Si nuclei must be removed from the SiOs surface: otherwise, they would cause the growth of continuous poly-Si film on the SiO, surface. The effects of SR on the annihilation of Si nuclei is investigated by intermittently radiating the substrate. One cycle in inter- mittent SR consists of a pause followed by a radiation pe- riod. The substrate was exposed to disilane gas during inter- mittent radiation. During the pause between exposures, dissociative adsorption of disilane on the SiO, surface pro- ceeds and the density of Si nuclei increases. If these nuclei are not removed from SiO, through SR between pauses the density continuously increases, they coalesce and grow, and finally a continuous poly-Si film is formed, causing selectiv- ity to break down. If Si nuclei formed during pauses are completely removed during radiation, the nuclei density does not increase. As a result, perfect selectivity is obtained under intermittent radiation. The removal of Si nuclei through SR was confirmed by measuring the incubation period under various conditions in intermittent radiation. The incubation period was measured, including the radiation period. Figure 3 shows the radiation period dependence of the incubation period at two substrate temperatures. The pause was fixed at 40 min, time enough for Si nucleation to occur, but not enough for continuous poly-Si film to form at 750 “C!. This was confirmed by AES, RHEED measurement, and SEM observations of SiOs surfaces. It is confirmed that the IP increases with the irradiation period at 750 “C and selectivity

Y. Utsumi and H. Akazawa

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Sun, 21 Dec 2014 15:28:02

40min

SR ON -+ k-- n ‘f-l r-!

i?i 310mA Si,H, 3 X 1 Os5Torr SUBSTRATE TEMF

0 750°C 0 650°C

1 I , I

0 1 2 IRRADIATION PERIOD ( min )

1 min

SR L5-Lh-L

c

PIG. 3. Radiation period dependence of the IP under intermittent SR using the substrate temperature as a parameter. The disilane pressure was set at 3X 10T5 Tom The pause was fixed at 40 min.

J 0 20 40 60 80

PAUSE PERIOD ( min )

is maintained as long as the SiOp film exists at radiation periods longer than 2 min. At 650 “C, however, a 15 nm continuous poly-Si film grew on the SiO, surface during a 40 min pause. Once a continuous poly-Si film forms on the SiOa, it cannot be removed using SR. Next, the IP was mea- sured at a fixed 1 min radiation period. Figure 4 shows the pause dependence of the IP for various substrate tempera- tures. From the figure it is clear that the IP increases mono- tonically as the pause is shortened and perfect selectivity is obtained for pauses under 30 min. At a substrate temperature of 650 “C, perfect selectivity was not obtained since the SiOa surface became Si rich due to SR (Fig. 2). These results prove that Si nuclei formed during pauses are completely removed through a SR-induced photothermal reaction.

According to these results, the probable reaction for per- fect Si nuclei removal from the SiO, surface is proposed. Considering the fact that the temperature region above which the stoichiometry of the SiOz surface is maintained coincides with the region above which Si nuclei on the SiO, surface are removed, it is inferred that any photothermal reaction would simultaneously cause both phenomena. It is specu- lated that the yield of SiO, reduction through adsorbed Si atoms is significantly enhanced above 700 “C due to the electronical excitation of SiO, molecules caused using SR. The Si nuclei on SiO, are assumed to change into volatile

FIG. 4. Pause period dependence of the JP under intermittent SR using the substrate temperature as a parameter. ‘The disilane pressure was set at 3X 10u5 Tom The radiation period was fixed at 1 min.

SiO through the photothermal reduction of SiO,. As a result, the Si nuclei are removed from the SiO, surface. By assum- ing this photothermal reaction, all the principal features of SRSEG can be qualitatively explained.

In conclusion, no Si nucleation occurs on SiOz irrespec- tive of growth time at substrate temperatures above 700 “C using SR during GSMBE using disilane; however, Si nucle- ation is promoted below 700 “C since the SiOz surface be- comes Si rich due to SR. From the experimental results for SRSEG with intermittent SR, it is verified that perfect selec- tivity results from- the elimination of Si nuclei on the SiOz surface through photothermal reaction.

The authors would like to thank Dr. K. Izumi, Dr. K. Murase, and Dr. H. Kinoshita for their continuous encour- agement and useful suggestions throughout this work.

‘H. Hirayama, T. Tatsumi, and N. Aizaki, Appl. Phys. Lett. 52, 2242 (1988).

‘K. Aketagawa, T. Tatsumi, and J. Sakai, Appl. Phys. Lett. 59, 173.5 (1991). ‘Y. Utsumi, H. Akazawa, M. Nagase, T. Urisu, and I. Kawashima, Appl. Phys. Len. 62, 1647 (1993).

“H. Kyuragi, and T. Urisu, J. Appl. Phys. 61, 2035 (1987). 5Y. Utsumi, J. Takahashi, and T Urisu, J. Vat. Sci. Technol. B 9, 2507

(1991). 6Y Utsumi, J. Takahashi, H. Akazawa, and T. Urisu, Jpn. J. Appl. Phys. 30,

3195 (1991). 7H. Akazawa, M. Nagase, and Y. Utsumi, Nucl. Instrum. Methods B 91,

644 (1994).

J. Appl. Phys., Vol. 77, No. 5, 1 March 1995 Y. Utsumi and H. Akazawa 2213 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Sun, 21 Dec 2014 15:28:02