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Silicon homoepitaxy using photochemical vapor deposition: a reflection high energy electron diffraction and transmission electron microscopy study
Materials Science and Engineering, BIO ( 1991 ) 181 - 186
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Silicon homoepitaxy using photochemical vapor deposition: a reflection high energy electron diffraction and transmission electron microscopy study S. Lian, B. Fowler, S. Krishnan, L. Jung and S. Banerjee Microelectronics Research Center, University of Texas, Austin, TX 78712 (U.S.A.)
(Received January 4, 1991 )
Abstract Defect characterization of epitaxial silicon films grown on Si(100) lightly boron-doped wafers by low temperature photochemical vapor deposition using ArF excimer laser decomposition of Si2H6 is discussed. The film morphology and crystallinity were investigated by defect etching-Nomarski optical microscopy, reflection high energy electron diffraction and transmission electron microscopy. The growth parameters such as laser power, Si2H 6 partial pressure and substrate temperature were varied to study the dependence of crystallinity on these parameters. Single-crystal films with a very low defect density were obtained at 0.08-0.7 W laser power, 5-20 mTorr SieH 6 partial pressure and 250-350 °C substrate temperature.
1. Introduction Low temperature semiconductor processing will have a major impact on future ultralargescale integration and silicon-based heterostructure devices because it reduces thermalstress-generated-defects and maintains sharp doping profiles and heterolayer integrity [1, 2]. Several low temperature techniques have been investigated for silicon homoepitaxy in the temperature range of 500-800 °C [3-6]. In our work, ArF excimer laser-enhanced photochemical vapor deposition (PCVD) has been used to achieve silicon epitaxy at temperatures as low as 250 °C. Modified Schimmel etching-Nomarski microscopy, transmission electron microscopy (TEM) and reflection high energy electron diffraction (RHEED) have been used to study the microstructure of silicon films as a function of laser power, substrate temperature and disilane (82H 6) partial pressure.
2. System The schematic diagram of the PCVD system is shown in Fig. 1. The system is composed of a process chamber and a load lock, both made of 0921-5107/91/$3.50
type 304 stainless steel. The wafer can be transferred into the process chamber through a load lock via a magnetically coupled transfer rod. The load lock has a cassette which is capable of holding three wafers at a time. In the process chamber the wafers are held on quartz pins, face down to minimize particulates, and can be heated rapidly from the back from room temperature to 900 °C at ramp rates of 20 °C s-1 by a bank of tungsten-halogen lamps. The heater uses Haynes alloy 214 (trademark of Haynes International, Inc.) as its reflector which is oxidation and corrosion resistant. The heater has a sensor chip with an embedded thermocouple, and the wafer temperature is feedback controlled by a three-phase silicon-controlled rectifier with temperature uniformity across a 4 in wafer at 10 °C. The ultrahigh vacuum load lock chamber equipped with sorption pumps and mechanical pump-backed turbopump has a base pressure of 2 x 10 -8 Torr, The ultrahigh vacuum process chamber is pumped by a 330 I rain -~ Balzers turbopump to a base pressure of 3 x 10-9 Torr. The operating pressures range from 200 to 600 mTorr. The process and base pressures are monitored with a capacitance manometer and ion gauge respectively. The gas network is made of © Elsevier Sequoia/Printed in The Netherlands
182
Mechanical Pump
Roots Blower
Mechanical Pump
3. Experiments
Turbo Pump
The wafers used were lightly boron doped Si(100) with resistivities of 10-15 f2 cm. The wafers were processed as described below to have a polycrystalline silicon (1000A)/oxide (1000 A) stack over a quarter of the wafer which allows measurements of deposited film thickness. The wafers were first Piranha (2:1 H2SO 4 :H202) cleaned and then oxidized at 950 °C in dry oxygen with trichloroethane for 220 min, followed by a nitrogen anneal at 750 °C for 15 h to improve the Si-SiO 2 interface roughness. A 1000 A polycrystalline silicon film was deposited by low pressure chemical vapor deposition at 625 °C on the oxidized wafers. The wafers were then patterned using photolithography and the exposed polycrystalline silicon was stripped using 75:50:1 HNO3:H20:HF(49%) for 8 min followed by a 20:1 HeO:HF(49% ) etch for 4 min to remove the exposed underlying oxide. The polycrystalline silicon/oxide stack was retained over a quarter of the wafer for deposited film thickness measurements. Prior to loading into the growth chamber, the wafers received an ultrasonic solvent clean (trichloroethane, acetone, methanol and deionized water), followed by an RCA clean (5:1:1 H20:H202:NH4OH for 10 min at 70°C, 40:1 H20:HF(49%) dip for 30 s, 5:1:1 H20:H202:HC1 for 10 min at 70 °C), to remove organic, metallic and particulate contaminants [8], and a final 40:1 H20:HF(49%) dip for 1 min and deionized water rinse for 30 s to remove the native oxide. The final H 2 0 : H F dip also hydrogen passivates the wafer surface [9]. The wafers are loaded into a load lock through a nitrogen-purged glove-box to minimize oxygen and moisture contamination of the load lock and subsequently transferred into the ultrahigh vacuum process chamber via a magnetic transfer rod. RHEED, TEM selected-area electron diffraction and modified Schimmel etching-Nomarski microscopy were employed to investigate the film quality. The modified Schimmel etches involved defect etching in a solution of 1:2:1.5 Cr203(0.75 M):HF(49%):H:O typically for 2 s at etch rates of 70 A s- ~. These etchants reveal stacking faults in the epitaxial films. TEM was used to determine dislocation densities. Several growth conditions were studied. Laser power delivered to the wafer was varied from 0.06 to 0.91 W, the S2H6 partial pressure from 5 to 50 mTorr, and the substrate temperature from 200 to 450 °C. These experiments were all performed
ateValve RHEED Gun
Throttl~,~ Valve
Quartz
Residual Gas Analyzer Substrate >strate Heater
~ \-~]
Quartz Window
cimer
Ultra-Hig]h Vz Chamber Gas v~ Manifold Si 2 H6 • GeH4~ B 2 H6 ,PH 3 ,
Load Lock
H2
Transfer Rod Fig. 1. S c h e m a t i c d i a g r a m o f t h e P C V D s y s t e m .
type 316 stainless steel tubing utilizing a fivevalve gas panel with extensive safety, vacuum and nitrogen purge provisions [7]. The gas cylinders are equipped with Nupro VCR valves for ultrahigh vacuum compatibility. The chambers and gas network are baked to 150 °C to achieve ultrahigh vacuum. The process gases (H2, He and SizH6) are flowed through mass-flow controllers and metal getter-type gas purifiers and pumped with a Roots blower backed by a mechanical pump. The pressure is feedback controlled with a Baratron capacitance manometer pressure gauge which automatically controls a throttle valve. A 10 W Questek ArF excimer emitting at 193 nm provides non-thermal excitation to dissociate process gases by pumping in up to 2× 1017 photons pulse -I (20 ns pulse -~ at 200 mJ pulse -l) with a repetition rate of 1-80 Hz. The laser beam is passed through a beam expander to provide a beam 6 cm wide and 2 mm high with 10% uniformity along the width, as determined by a Scientech power meter. The laser shines into the process chamber, tangentially under the wafer through Suprasil quartz windows which have 80% transmittance at 193 nm. The windows are purged with 1600 standard cm 3 min-1 of helium to prevent deposition on them. The process chamber is equipped with a RHEED system for in situ monitoring of silicon surface reconstruction and film crystallinity, and a residual gas analyzer for checking background contamination.
183
at the same total chamber pressure of 600 mTorr, and laser repetition rate of 80 Hz. 4. Results and discussion
It has been proposed that photolytic dissociation of Si2H 6 results in the creation of SiH3SiH and H2 via the 2al g-4S Rydberg transition [10]. However, SiH3SiH has not actually been detected spectroscopically. It has also been suggested that SiH3SiH absorbs a second photon, creating the Sill species in the A2A excited state, and S i l l 3. SiH3SiH, Sill, Sill 3 and Si2H6 undergo heterogeneous and homogeneous reactions, leading to adsorption of Sill 2 on the hydrogenated silicon surface, followed by H 2 evolution leading to silicon film growth. The most important pyrolytic pathway for decomposition of Si2H 6 is the creation of S i l l 4 and Sill 2. A typical R H E E D pattern of an as-loaded Si(100) wafer along the [110] direction is shown in Fig. 2. The R H E E D electron beam energy is 15 keV, with a de Broglie wavelength of 0.1 A [12]. The separation of the streaks are consistent with the Si lattice constant and the chamber geometry. The integral (1 x 1) streaks and Kikuchi lines indicate a clean surface which is dihydride terminated. A clean silicon surface is the key for successful epitaxial growth. For S i 2 H 6 partial pressures less than 40 mTorr, a chamber total pressure less than 1 Torr and a laser beam intensity less than 1017 photons cm-2 pulse -1 , the growth rates are observed to be linearly dependent on laser power and S i 2 H 6 partial pressure. The linear dependence of
growth rates on laser power indicates that the generation rate of precursors by photo dissociation is the growth rate limiting step. Figure 3 shows the R H E E D patterns from three films grown at 0.06, 0.7 and 0.91 W laser power at 322 °C substrate temperature, 5.4 mTorr Si2H6 partial pressure at growth rates of 0.17 A min- l,
!I1 Fig. 2. R H E E D pattern of as-loaded Si(100) wafer along the [110] direction using 15 keV electrons.
Fig. 3. RHEED analysis of silicon films grown at 300 °C substrate temperature and 5.4 mTorr Si2H 6 partial pressure for (a) 0.06 W, (b) 0.7 W and (c) 0.91 W laser powers.
184 0.97 A min 1 and 1.16 ,/k min- l for 1 h, 3 h and 1 h respectively. The films are smooth and single crystal for laser powers between 0.08 and 0.7 W, as determined by the streaky ( 1 x 1 ) rod pattern and Kikuchi lines along the [110] direction during in situ RHEED analysis (Fig. 3(b)). When the laser power is reduced to 0.06 W, the films become polycrystalline and/or amorphous (Fig. 3(a)) as seen from the RHEED picture, where we see a ring-type or featureless diffraction pattern. It is believed that at these low powers the generation rate of precursors is so small that the background carbon and oxygen contamination in the ambient hinders epitaxial growth. For high laser powers (above 0.91 W), the growth rates are so high ( 1.16/k min- 1) that the adatom mobifities are too low at these substrate temperatures (300 °C) to allow single-crystal growth (Fig. 3(c)). Crystalline defects such as stacking faults and dislocation loops are not observed in single-crystal films grown at 0.7 W laser power, after Schimmel etching-Nomarski microscopy and TEM respectively. The dislocation loop density is less than 105 loops cm -2, which is the detection limit of TEM. The morphology of these single-crystal films is smooth since R H E E D does not show any spotty pattern due to three-dimensional diffraction. The growth rate increases linearly with disilane partial pressure in the range 5.4 to 20 mTorr at 300 °C substrate temperature and 0.3 W laser power. Figure 4 shows the R H E E D pattern from samples grown at 5, 20 and 40 mTorr with other parameters held fixed at 300 °C substrate temperature and 0.3 W laser power. The films thicknesses are 150/k, 350/k and 170/k, with growth rates of 0.85/k min -1, 1.94/k min -1 and 2.87/k min-1 respectively. Single-crystal growth can be sustained until the Si2H6 partial pressure is higher than 20 mTorr as shown by a ( 1 x 1) RHEED diffraction pattern along the [110] direction (Figs. 4(a) and 4(b)). For higher Si2H6 partial pressure, polycrystalline silicon films result where RHEED shows a ring-type diffraction pattern (Fig. 4(c)), presumably because of high growth rates. Singlecrystal deposition is achieved even when the Si2H6 partial pressure is as low as 5 mTorr at a growth rate of 0.85/k rain- 1 for 3 h. The distortion of the RHEED streak pattern is due to charging up of the wafer. The TEM micrographs of these films are featureless, and stacking faults and dislocation loops are not detectable. The surface of these single-crystal films are smooth as
Fig. 4. RHEED analysis of silicon films grown at 300 °C substrate temperature and 0.3 W laser power for (a) 5.4 mTorr, (b) 10 mTorr and (c) 30 mTorr Si2H6 partial pressures. indicated by the R H E E D streaky pattern which is due to two-dimensional surface diffraction. Epitaxial growth can be achieved at temperatures between 250 and 350 °C, where the deposition is entirely due to laser enhancement and there is no pyrolytic growth. At even higher tern-
185
peratures (450 °C), there is a thermal component of growth (2 A min-1) over and above photoenhanced growth (2 A min- 1). Figure 5 shows the RHEED diffraction pattern from samples grown at 200, 300 and 400 °C substrate temperature with 20 mTorr S i 2 H 6 partial pressure and 0.22 W laser power. The RHEED diffraction pattern shows clear (1 × 1) streaks and Kikuchi lines for
m Fig. 5. R H E E D analysis of silicon films grown at 20 mTorr Si2H 6 partial pressure and 0.22 W laser power for (a) 200 °C, (b) 300 °C and (c) 400 °C substrate temperatures.
films grown at 300 °C, 20 mTorr Si2H6 partial pressure and 0.2 W laser power (Fig. 5(b)). There is an Arrhenius dependence of growth rate on wafer temperature with an activation energy of 0.08 eV, which is lower than that for pure pyrolysis o f S i 2 H 6. P o l y c r y s t a l l i n e a n d / o r a m o r p h o u s growth is observed when the temperature is lowered to 200 °C, as shown by the ring-type
Fig. 6. (a) TEM micrograph, (b) selected-area electron diffraction pattern and (c) Nomarski micrograph of defectetched silicon films grown at 20 mTorr Si2H ~ partial pressure, 300 °C substrate temperature and 0.22 W laser power.
186
RHEED diffraction pattern (Fig. 5(a)). This is because at low temperatures the atoms do not have sufficient adatom surface mobility and hence cannot overcome the activation energy barrier. Interestingly, polycrystalline deposition results at 400 °C, although single-crystal growth is achieved at a lower temperature (300 °C). At 400 °C, the rapid loss of hydrogen from the hydrogenated silicon surface generates active silicon surfaces that are susceptible to carbon and oxygen contamination from the growth ambient, which hinders epitaxy [12]. Figure 6 shows a TEM micrograph, selectedarea electron diffraction pattern and Schimmel etching-Nomarski microscopy results for a single-crystal film grown under optimal conditions: 0.22 W laser power, 20 mTorr S i 2 H 6 partial pressure and 300 °C substrate temperature. Modified Schimmel etching of films 350 A thick for 2 s at etch rates of 70 A s-1 does not reveal any square or rectangular etch patterns which implies an absence of stacking faults in the films [13]. Since stacking faults are nucleated by damage sites and oxygen or heavy metal precipitates on the silicon substrate, this result confirms the effectiveness of the ex situ cleaning of the wafers. Dislocation loops are not seen by TEM at a magnification of 7 2 0 0 0 x , where the only observable features are thickness fringes due to sample preparation. The observed diffraction pattern confirms that the films are pure silicon and grow epitaxially on Si(100). 5. Conclusions Photolysis of S i 2 H 6 by an ArF excimer laser has been used to grow silicon epitaxial layers at temperatures as low as 250 °C. Although silicon epitaxy by photoexcitation has been achieved before, it has involved either less selective broadband sources (e.g. mercury lamps), higher temperatures (600 °C), non-ultrahigh vacuum
systems (1 x 10 -6 Torr base pressure), different gases ( S i l l 4 o r ultrahigh vacuum-incompatible SiH2F2-SiH2CI2) or direct irradiation of the silicon surface possibly involving pyrolytic as well as photolytic mechanisms, as well as potential for surface damage. The highly selective (193 nm ArF excimer laser excitation) can minimize parallel reaction pathways, thereby improving film morphology. The present approach is unique and has distinct advantages compared with previous techniques. Acknowledgments This work was supported by the National Science Foundation Presidential Young Investigator Award Program and by the Texas Advanced Technology Program. References 1 S. Lian, B. Fowler, D. Bullock and S. Banerjee, Appl. Phys. Lett., 58(1991) 514. 2 L. Breaux, B. Anthony, T. Hsu, S. Banerjee and A. Tasch, Appl. Phys. Lett., 55(1989) 1885. 3 B.S. Meyerson, Appl. Phys. Lett., 48(1986) 797. 4 Y. Ota, Thin Solid Films, 106(1983) 1. 5 T. Donahue and R. Reif, J. Appl. Phys., 57(1985) 2757. 6 T. Yamazaki, S. Watanable and T. Ito, J. Electrochem. Soc., 137(1)(1990) 313. 7 J. F. O'Hanlon and D. B. Fraser, J. Vac. Sci. Technol. A, 6 (3) (1988) 1226. 8 W. Kern, Semicond. Int., (April 1984) 94. 9 T. Takahagi, I. Nagai, A. Ishitani and H. Kuroda, J. AppL Phys., 64(1988)3516. 10 D. Eres, D. Lowndes, D. Geohegan and D. Mashburn,
Laser and Particle Beam Chemical Processing for Microelectronics, Materials Research Society, Symp. Proc., Vol. 101, Materials Research Society, Pittsburgh, PA, 1988, p. 355. 11 D. P. Woodruff and T. A. Delchar, Modern Technology of Surface Science, Cambridge University Press, Cambridge, Cambridgeshire, 1986, p. 71. 12 V. Comello, Semicond. Int., (December 1990) 18. 13 R. H. Finch and H. J. Queisser, J. Appl. Phys., 34 (1963) 406.
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Silicon homoepitaxy using photochemical vapor deposition: a reflection high energy electron diffraction and transmission electron microscopy study S. Lian, B. Fowler, S. Krishnan, L. Jung and S. Banerjee Microelectronics Research Center, University of Texas, Austin, TX 78712 (U.S.A.)
(Received January 4, 1991 )
Abstract Defect characterization of epitaxial silicon films grown on Si(100) lightly boron-doped wafers by low temperature photochemical vapor deposition using ArF excimer laser decomposition of Si2H6 is discussed. The film morphology and crystallinity were investigated by defect etching-Nomarski optical microscopy, reflection high energy electron diffraction and transmission electron microscopy. The growth parameters such as laser power, Si2H 6 partial pressure and substrate temperature were varied to study the dependence of crystallinity on these parameters. Single-crystal films with a very low defect density were obtained at 0.08-0.7 W laser power, 5-20 mTorr SieH 6 partial pressure and 250-350 °C substrate temperature.
1. Introduction Low temperature semiconductor processing will have a major impact on future ultralargescale integration and silicon-based heterostructure devices because it reduces thermalstress-generated-defects and maintains sharp doping profiles and heterolayer integrity [1, 2]. Several low temperature techniques have been investigated for silicon homoepitaxy in the temperature range of 500-800 °C [3-6]. In our work, ArF excimer laser-enhanced photochemical vapor deposition (PCVD) has been used to achieve silicon epitaxy at temperatures as low as 250 °C. Modified Schimmel etching-Nomarski microscopy, transmission electron microscopy (TEM) and reflection high energy electron diffraction (RHEED) have been used to study the microstructure of silicon films as a function of laser power, substrate temperature and disilane (82H 6) partial pressure.
2. System The schematic diagram of the PCVD system is shown in Fig. 1. The system is composed of a process chamber and a load lock, both made of 0921-5107/91/$3.50
type 304 stainless steel. The wafer can be transferred into the process chamber through a load lock via a magnetically coupled transfer rod. The load lock has a cassette which is capable of holding three wafers at a time. In the process chamber the wafers are held on quartz pins, face down to minimize particulates, and can be heated rapidly from the back from room temperature to 900 °C at ramp rates of 20 °C s-1 by a bank of tungsten-halogen lamps. The heater uses Haynes alloy 214 (trademark of Haynes International, Inc.) as its reflector which is oxidation and corrosion resistant. The heater has a sensor chip with an embedded thermocouple, and the wafer temperature is feedback controlled by a three-phase silicon-controlled rectifier with temperature uniformity across a 4 in wafer at 10 °C. The ultrahigh vacuum load lock chamber equipped with sorption pumps and mechanical pump-backed turbopump has a base pressure of 2 x 10 -8 Torr, The ultrahigh vacuum process chamber is pumped by a 330 I rain -~ Balzers turbopump to a base pressure of 3 x 10-9 Torr. The operating pressures range from 200 to 600 mTorr. The process and base pressures are monitored with a capacitance manometer and ion gauge respectively. The gas network is made of © Elsevier Sequoia/Printed in The Netherlands
182
Mechanical Pump
Roots Blower
Mechanical Pump
3. Experiments
Turbo Pump
The wafers used were lightly boron doped Si(100) with resistivities of 10-15 f2 cm. The wafers were processed as described below to have a polycrystalline silicon (1000A)/oxide (1000 A) stack over a quarter of the wafer which allows measurements of deposited film thickness. The wafers were first Piranha (2:1 H2SO 4 :H202) cleaned and then oxidized at 950 °C in dry oxygen with trichloroethane for 220 min, followed by a nitrogen anneal at 750 °C for 15 h to improve the Si-SiO 2 interface roughness. A 1000 A polycrystalline silicon film was deposited by low pressure chemical vapor deposition at 625 °C on the oxidized wafers. The wafers were then patterned using photolithography and the exposed polycrystalline silicon was stripped using 75:50:1 HNO3:H20:HF(49%) for 8 min followed by a 20:1 HeO:HF(49% ) etch for 4 min to remove the exposed underlying oxide. The polycrystalline silicon/oxide stack was retained over a quarter of the wafer for deposited film thickness measurements. Prior to loading into the growth chamber, the wafers received an ultrasonic solvent clean (trichloroethane, acetone, methanol and deionized water), followed by an RCA clean (5:1:1 H20:H202:NH4OH for 10 min at 70°C, 40:1 H20:HF(49%) dip for 30 s, 5:1:1 H20:H202:HC1 for 10 min at 70 °C), to remove organic, metallic and particulate contaminants [8], and a final 40:1 H20:HF(49%) dip for 1 min and deionized water rinse for 30 s to remove the native oxide. The final H 2 0 : H F dip also hydrogen passivates the wafer surface [9]. The wafers are loaded into a load lock through a nitrogen-purged glove-box to minimize oxygen and moisture contamination of the load lock and subsequently transferred into the ultrahigh vacuum process chamber via a magnetic transfer rod. RHEED, TEM selected-area electron diffraction and modified Schimmel etching-Nomarski microscopy were employed to investigate the film quality. The modified Schimmel etches involved defect etching in a solution of 1:2:1.5 Cr203(0.75 M):HF(49%):H:O typically for 2 s at etch rates of 70 A s- ~. These etchants reveal stacking faults in the epitaxial films. TEM was used to determine dislocation densities. Several growth conditions were studied. Laser power delivered to the wafer was varied from 0.06 to 0.91 W, the S2H6 partial pressure from 5 to 50 mTorr, and the substrate temperature from 200 to 450 °C. These experiments were all performed
ateValve RHEED Gun
Throttl~,~ Valve
Quartz
Residual Gas Analyzer Substrate >strate Heater
~ \-~]
Quartz Window
cimer
Ultra-Hig]h Vz Chamber Gas v~ Manifold Si 2 H6 • GeH4~ B 2 H6 ,PH 3 ,
Load Lock
H2
Transfer Rod Fig. 1. S c h e m a t i c d i a g r a m o f t h e P C V D s y s t e m .
type 316 stainless steel tubing utilizing a fivevalve gas panel with extensive safety, vacuum and nitrogen purge provisions [7]. The gas cylinders are equipped with Nupro VCR valves for ultrahigh vacuum compatibility. The chambers and gas network are baked to 150 °C to achieve ultrahigh vacuum. The process gases (H2, He and SizH6) are flowed through mass-flow controllers and metal getter-type gas purifiers and pumped with a Roots blower backed by a mechanical pump. The pressure is feedback controlled with a Baratron capacitance manometer pressure gauge which automatically controls a throttle valve. A 10 W Questek ArF excimer emitting at 193 nm provides non-thermal excitation to dissociate process gases by pumping in up to 2× 1017 photons pulse -I (20 ns pulse -~ at 200 mJ pulse -l) with a repetition rate of 1-80 Hz. The laser beam is passed through a beam expander to provide a beam 6 cm wide and 2 mm high with 10% uniformity along the width, as determined by a Scientech power meter. The laser shines into the process chamber, tangentially under the wafer through Suprasil quartz windows which have 80% transmittance at 193 nm. The windows are purged with 1600 standard cm 3 min-1 of helium to prevent deposition on them. The process chamber is equipped with a RHEED system for in situ monitoring of silicon surface reconstruction and film crystallinity, and a residual gas analyzer for checking background contamination.
183
at the same total chamber pressure of 600 mTorr, and laser repetition rate of 80 Hz. 4. Results and discussion
It has been proposed that photolytic dissociation of Si2H 6 results in the creation of SiH3SiH and H2 via the 2al g-4S Rydberg transition [10]. However, SiH3SiH has not actually been detected spectroscopically. It has also been suggested that SiH3SiH absorbs a second photon, creating the Sill species in the A2A excited state, and S i l l 3. SiH3SiH, Sill, Sill 3 and Si2H6 undergo heterogeneous and homogeneous reactions, leading to adsorption of Sill 2 on the hydrogenated silicon surface, followed by H 2 evolution leading to silicon film growth. The most important pyrolytic pathway for decomposition of Si2H 6 is the creation of S i l l 4 and Sill 2. A typical R H E E D pattern of an as-loaded Si(100) wafer along the [110] direction is shown in Fig. 2. The R H E E D electron beam energy is 15 keV, with a de Broglie wavelength of 0.1 A [12]. The separation of the streaks are consistent with the Si lattice constant and the chamber geometry. The integral (1 x 1) streaks and Kikuchi lines indicate a clean surface which is dihydride terminated. A clean silicon surface is the key for successful epitaxial growth. For S i 2 H 6 partial pressures less than 40 mTorr, a chamber total pressure less than 1 Torr and a laser beam intensity less than 1017 photons cm-2 pulse -1 , the growth rates are observed to be linearly dependent on laser power and S i 2 H 6 partial pressure. The linear dependence of
growth rates on laser power indicates that the generation rate of precursors by photo dissociation is the growth rate limiting step. Figure 3 shows the R H E E D patterns from three films grown at 0.06, 0.7 and 0.91 W laser power at 322 °C substrate temperature, 5.4 mTorr Si2H6 partial pressure at growth rates of 0.17 A min- l,
!I1 Fig. 2. R H E E D pattern of as-loaded Si(100) wafer along the [110] direction using 15 keV electrons.
Fig. 3. RHEED analysis of silicon films grown at 300 °C substrate temperature and 5.4 mTorr Si2H 6 partial pressure for (a) 0.06 W, (b) 0.7 W and (c) 0.91 W laser powers.
184 0.97 A min 1 and 1.16 ,/k min- l for 1 h, 3 h and 1 h respectively. The films are smooth and single crystal for laser powers between 0.08 and 0.7 W, as determined by the streaky ( 1 x 1 ) rod pattern and Kikuchi lines along the [110] direction during in situ RHEED analysis (Fig. 3(b)). When the laser power is reduced to 0.06 W, the films become polycrystalline and/or amorphous (Fig. 3(a)) as seen from the RHEED picture, where we see a ring-type or featureless diffraction pattern. It is believed that at these low powers the generation rate of precursors is so small that the background carbon and oxygen contamination in the ambient hinders epitaxial growth. For high laser powers (above 0.91 W), the growth rates are so high ( 1.16/k min- 1) that the adatom mobifities are too low at these substrate temperatures (300 °C) to allow single-crystal growth (Fig. 3(c)). Crystalline defects such as stacking faults and dislocation loops are not observed in single-crystal films grown at 0.7 W laser power, after Schimmel etching-Nomarski microscopy and TEM respectively. The dislocation loop density is less than 105 loops cm -2, which is the detection limit of TEM. The morphology of these single-crystal films is smooth since R H E E D does not show any spotty pattern due to three-dimensional diffraction. The growth rate increases linearly with disilane partial pressure in the range 5.4 to 20 mTorr at 300 °C substrate temperature and 0.3 W laser power. Figure 4 shows the R H E E D pattern from samples grown at 5, 20 and 40 mTorr with other parameters held fixed at 300 °C substrate temperature and 0.3 W laser power. The films thicknesses are 150/k, 350/k and 170/k, with growth rates of 0.85/k min -1, 1.94/k min -1 and 2.87/k min-1 respectively. Single-crystal growth can be sustained until the Si2H6 partial pressure is higher than 20 mTorr as shown by a ( 1 x 1) RHEED diffraction pattern along the [110] direction (Figs. 4(a) and 4(b)). For higher Si2H6 partial pressure, polycrystalline silicon films result where RHEED shows a ring-type diffraction pattern (Fig. 4(c)), presumably because of high growth rates. Singlecrystal deposition is achieved even when the Si2H6 partial pressure is as low as 5 mTorr at a growth rate of 0.85/k rain- 1 for 3 h. The distortion of the RHEED streak pattern is due to charging up of the wafer. The TEM micrographs of these films are featureless, and stacking faults and dislocation loops are not detectable. The surface of these single-crystal films are smooth as
Fig. 4. RHEED analysis of silicon films grown at 300 °C substrate temperature and 0.3 W laser power for (a) 5.4 mTorr, (b) 10 mTorr and (c) 30 mTorr Si2H6 partial pressures. indicated by the R H E E D streaky pattern which is due to two-dimensional surface diffraction. Epitaxial growth can be achieved at temperatures between 250 and 350 °C, where the deposition is entirely due to laser enhancement and there is no pyrolytic growth. At even higher tern-
185
peratures (450 °C), there is a thermal component of growth (2 A min-1) over and above photoenhanced growth (2 A min- 1). Figure 5 shows the RHEED diffraction pattern from samples grown at 200, 300 and 400 °C substrate temperature with 20 mTorr S i 2 H 6 partial pressure and 0.22 W laser power. The RHEED diffraction pattern shows clear (1 × 1) streaks and Kikuchi lines for
m Fig. 5. R H E E D analysis of silicon films grown at 20 mTorr Si2H 6 partial pressure and 0.22 W laser power for (a) 200 °C, (b) 300 °C and (c) 400 °C substrate temperatures.
films grown at 300 °C, 20 mTorr Si2H6 partial pressure and 0.2 W laser power (Fig. 5(b)). There is an Arrhenius dependence of growth rate on wafer temperature with an activation energy of 0.08 eV, which is lower than that for pure pyrolysis o f S i 2 H 6. P o l y c r y s t a l l i n e a n d / o r a m o r p h o u s growth is observed when the temperature is lowered to 200 °C, as shown by the ring-type
Fig. 6. (a) TEM micrograph, (b) selected-area electron diffraction pattern and (c) Nomarski micrograph of defectetched silicon films grown at 20 mTorr Si2H ~ partial pressure, 300 °C substrate temperature and 0.22 W laser power.
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RHEED diffraction pattern (Fig. 5(a)). This is because at low temperatures the atoms do not have sufficient adatom surface mobility and hence cannot overcome the activation energy barrier. Interestingly, polycrystalline deposition results at 400 °C, although single-crystal growth is achieved at a lower temperature (300 °C). At 400 °C, the rapid loss of hydrogen from the hydrogenated silicon surface generates active silicon surfaces that are susceptible to carbon and oxygen contamination from the growth ambient, which hinders epitaxy [12]. Figure 6 shows a TEM micrograph, selectedarea electron diffraction pattern and Schimmel etching-Nomarski microscopy results for a single-crystal film grown under optimal conditions: 0.22 W laser power, 20 mTorr S i 2 H 6 partial pressure and 300 °C substrate temperature. Modified Schimmel etching of films 350 A thick for 2 s at etch rates of 70 A s-1 does not reveal any square or rectangular etch patterns which implies an absence of stacking faults in the films [13]. Since stacking faults are nucleated by damage sites and oxygen or heavy metal precipitates on the silicon substrate, this result confirms the effectiveness of the ex situ cleaning of the wafers. Dislocation loops are not seen by TEM at a magnification of 7 2 0 0 0 x , where the only observable features are thickness fringes due to sample preparation. The observed diffraction pattern confirms that the films are pure silicon and grow epitaxially on Si(100). 5. Conclusions Photolysis of S i 2 H 6 by an ArF excimer laser has been used to grow silicon epitaxial layers at temperatures as low as 250 °C. Although silicon epitaxy by photoexcitation has been achieved before, it has involved either less selective broadband sources (e.g. mercury lamps), higher temperatures (600 °C), non-ultrahigh vacuum
systems (1 x 10 -6 Torr base pressure), different gases ( S i l l 4 o r ultrahigh vacuum-incompatible SiH2F2-SiH2CI2) or direct irradiation of the silicon surface possibly involving pyrolytic as well as photolytic mechanisms, as well as potential for surface damage. The highly selective (193 nm ArF excimer laser excitation) can minimize parallel reaction pathways, thereby improving film morphology. The present approach is unique and has distinct advantages compared with previous techniques. Acknowledgments This work was supported by the National Science Foundation Presidential Young Investigator Award Program and by the Texas Advanced Technology Program. References 1 S. Lian, B. Fowler, D. Bullock and S. Banerjee, Appl. Phys. Lett., 58(1991) 514. 2 L. Breaux, B. Anthony, T. Hsu, S. Banerjee and A. Tasch, Appl. Phys. Lett., 55(1989) 1885. 3 B.S. Meyerson, Appl. Phys. Lett., 48(1986) 797. 4 Y. Ota, Thin Solid Films, 106(1983) 1. 5 T. Donahue and R. Reif, J. Appl. Phys., 57(1985) 2757. 6 T. Yamazaki, S. Watanable and T. Ito, J. Electrochem. Soc., 137(1)(1990) 313. 7 J. F. O'Hanlon and D. B. Fraser, J. Vac. Sci. Technol. A, 6 (3) (1988) 1226. 8 W. Kern, Semicond. Int., (April 1984) 94. 9 T. Takahagi, I. Nagai, A. Ishitani and H. Kuroda, J. AppL Phys., 64(1988)3516. 10 D. Eres, D. Lowndes, D. Geohegan and D. Mashburn,
Laser and Particle Beam Chemical Processing for Microelectronics, Materials Research Society, Symp. Proc., Vol. 101, Materials Research Society, Pittsburgh, PA, 1988, p. 355. 11 D. P. Woodruff and T. A. Delchar, Modern Technology of Surface Science, Cambridge University Press, Cambridge, Cambridgeshire, 1986, p. 71. 12 V. Comello, Semicond. Int., (December 1990) 18. 13 R. H. Finch and H. J. Queisser, J. Appl. Phys., 34 (1963) 406.