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Microscopic in situ observation of diamond growth by dark-field scanning laser microscopy and object scanning
Journal of Crystal Growth 187 (1998) 421—425
Microscopic in situ observation of diamond growth by dark-field scanning laser microscopy and object scanning R. Bremensdorfer, O. Weis* Abteilung Festko( rperphysik, Universita( t Ulm, D 89069 Ulm, Germany Received 10 November 1997; accepted 11 December 1997
Abstract We report on the first microscopic in situ observation of diamond growth under dark-field conditions using a confocal scanning laser microscope. The main intention of this paper is to describe and demonstrate this method which is also applicable to investigate other growth processes in a hostile environment. Dark-field conditions were achieved by tilting the sample plane to such an extent that specularly reflected laser light could not enter the objective. A piezoelectrically driven raster motion of the sample was applied. The measured lateral resolution was better than 1 lm. The growth of a diamond film was observed over a period of 22 h. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 42.30; 68.55; 78.65 Keywords: In situ growth observation; Confocal scanning laser microscope; Dark-field conditions; Diamond growth
1. Introduction Microscopic in situ observation of diamond growth would be of advantage to study directly the influence of different growth parameters. Such a study is hindered by the hostile environmental conditions present during the usual chemical vapor deposition (CVD). These are: a sample temperature in the range 850—950°C, a methane-hydrogen gas at a pressure of about 40 mbar, and either a hot filament of about 2000°C or a microwave plasma. Recently, we have shown that scanning laser microscopy (SLM) in combination with a large
* Corresponding author. Tel.: #49 731 383704.
working distance reflecting objective can be used for the in situ investigation of diamond growth [1]. These investigations were done in a bright-field using the beam scanning method in the form of objective scanning [2]. To further improve the lateral resolution and depth sensitivity, we employed in the present paper a different configuration known as confocal scanning laser microscopy (CSLM). This configuration is obtained by putting a pinhole as a detector aperture in the image of the laser focus [3]. As a result, the previously used objective scanning had to be replaced by sample scanning. Here, the diffraction-limited laser focus on the axis of the objective stays fixed in space whereas the sample surface is moved in a raster motion through the laser focus.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 8 8 3 - X
422
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
We were especially interested to observe the nucleation sites of diamond and the growth of the microcrystallites on silicon wafers. The wafers were prepared by scratching with nominal 1 lm diamond particles. We found that the specular reflections from the plane silicon wafer strongly disturbed our observations. Therefore, we used a dark-field configuration by using an angle of incidence sufficiently large to guarantee that the specularly reflected light could not enter the objective. In the next section, we will give a detailed description of the experimental setup. In Section 3, we will present a series of microphotographs of the time dependence of diamond growth at a chosen position over a time interval of 22 h. Finally, conclusions are drawn in Section 4 from the obtained results.
2. Experimental setup The experimental setup is shown schematically in Fig. 1. Our confocal scanning laser microscope used an argon-ion laser of 488 nm wavelength with a dominant TEM laser mode. The laser beam 00 was focused in a spatial filter onto a diaphragm with a pinhole of 1 lm diameter. After passing a microscope objective, a parallel beam of 8 mm
diameter with a nearly homogeneous intensity distribution was obtained. This beam was focused onto the sample surface by a Schwarzschild-type reflecting objective (Model 25-0506, Ealing) which had a numerical aperture of NA"0.28 and the relatively large working distance of 24 mm. The scattered light emerging from the focal point on the sample was collected by the same reflecting objective. After passing a beamsplitter, the light was projected by means of a second microscope objective onto the 1 lm detector pinhole in front of the detecting photodiode. In ex situ experiments, lateral (Rayleigh) resolutions of 0.7—1 lm were measured in this confocal arrangement using the parallel metal lines of unsealed 4 MBit memories as test objects. The observed depth resolution of the microscope was about 11 lm. The raster motion of the glowing diamond sample is best done by two crossed piezoceramic translators. In general, these translators may be placed either inside or outside the CVD reactor. However, since we were working at a gas pressure of about 40 mbar where an electric discharge may occur at the piezoceramic translators, we had to arrange the piezoceramic translators outside the reactor. Therefore, the motion of each translator had to be transferred to the sample by a vacuum tight feed through. Each feed through was made by
Fig. 1. Schematic diagram of the dark-field confocal scanning laser microscope and the crystal-growth equipment.
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
soldering two circular corrugated bronze membranes (50 lm thick) at the outer edges to an annular inset and at the inner edges to a movable piston (see Fig. 1a and Fig. 1b). To compensate forces due to fluctuations of the pressure difference between inside and outside the CVD reactor, two identical feed throughs at opposite sides were applied. We used piezoceramic translators (Model P-245.70, Physik Instrumente GmbH) with a linearized maximal travel of 120 lm. The sample holder was cut from a V2A block in a shape that combined the demands for high stiffness, low mass, and extremely low heat conduction. The movement was constrained to the raster plane by means of four steel wires which supported the sample holder. We measured the fundamental mechanical resonance frequency of the assembly at 318 Hz. Since the raster scan was performed with 6 lines/s, this resonance frequency gave no practical limitations to scanning. The substrate temperature of about 800°C was produced in the focus of a heater lamp (Model HLX 64635, Osram). The actual temperature of the silicon substrate (5]8]0.5 mm3) was measured by a thermocouple. The activation of the feed gas for the CVD process was done by means of a electrically heated tungsten filament to about 2000°C. We used the filament of a 50 W halogen lamp (Halostar 64440, Osram). Due to the confocal arrangement the light emission generated by the heated filament and the sample itself was suppressed very effeciently. This eliminated the need for a 488 nm filter initially placed in front of the detector. The strong heat production in the CVD reactor made it necessary to use water cooling for the reactor walls and for the glass windows. Since the flowing water between the windows lead to the formation of schlieren, the water cooling of the windows was interrupted during the time it took to scan a picture which was about 4 min for the acquisition of 256]256 pixels. We used a 12 bit A/D converter. By tilting the CVD reactor, the normal of the silicon substrate could be oriented more than 16° against the optical axis of the Schwarzschild objective. Under these conditions, all of the reflected laser light was made to miss the receiving aperture of the Schwarzschild objective, i.e. we were working under dark-field conditions.
423
3. In situ observations In order to demonstrate the feasibility of the presented method to perform optical in-situ observations of crystal growth, Fig. 2 shows, as an example, several stages of the growth of diamond crystallites on a diamond polished (1 0 0) silicon substrate during a 22 h lasting growth experiment [4]. The growth parameters are listed in the caption of Fig. 2. They were kept constant as close as possible during the experiment. It is well known [5] that both, growth rate and morphology, depend on substrate temperature in hot-filament deposition of diamond on nondiamond substrates. However, to some extent, the formation of tungsten carbide at the hot filament was influencing the diamond growth in an uncontrollable manner. First, the resistance, the surface area, and the emission coefficient were gradually changing by the carbide formation which in turn lowered the filament temperature. Secondly, the distance to the substrate was changing due to the deformations of the filament. The raster area of 256]256 pixels had a linear dimension of 115 lm and was chosen at a position near the hot filament. Due to small hysteresis effects at both ends of the raster lines, the photomicrographs of Fig. 2 have been restricted to a clip of 220]220 pixels corresponding to linear dimension of about 100 lm. The images are given in 256 gray levels. At the beginning of the experiment, the scattered light intensity from the substrate was very weak. Obviously, there were two kinds of contributions to scattering. First, scattering at parallel groves which, in the micrographs, run horizontally across the field of view. These groves were generated by the pretreatment, namely the polishing with nominal 1 lm diamond powder. During the course of the CVD process the groves were reduced in depth, probably by etching. The second contribution to scattering came from heavy subsurface plastic deformations that appear mainly as oblique lines. These lines were produced by a prepolishing process with larger particles. The traces remained visible even 10 h after starting the CVD process and finally disappeared only due to the coverage with diamond crystallites. About 7.5 h after beginning the experiment, we observed an enhancement of scattered light at
424
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
425
Nevertheless, it can turn into strongly reflected light if a crystal face is oriented in the right way to produce a specular reflection passing through the aperture of the objective.
4. Conclusions By using, as an example, the polycrystalline diamond growth on silicon, we were able to demonstrate in this work the following main results:
Fig. 3. Scanning electron micrograph of the as grown polycrystalline diamond film.
several positions at which we could identify crystallites 1 h later. The number of crystallites increased in the following and after 15 h, a nearly continuous diamond layer appeared. Due to the limited lateral resolution of our CSLM, the first stage of diamond growth could not be observed. Especially, the nucleation process which is characterized by an incubation time [6] could not be observed. Whereas the nucleation density is mainly determined by the morphology of the surface, the nucleation rate depends in addition on the growth conditions [7]. In the experiment presented here, we observed a continuous occurrence of new crystallites. Secondary nucleation occurred in later growth stages. We stopped the CVD process after 22 h and investigated the diamond film with a scanning electron microscope (SEM). The film thickness was about 11 lm. As expected, the size of the crystallites differed considerably (Fig. 3). When comparing the SEM picture with those of the CSLM, one has to take into consideration that the detected laser light is, in general, only weak diffusely scattered light. $&&&&&&&&&&&&&&&&&&&&&&&&&& Fig. 2. In situ microphotographs of different stages of diamond growth on silicon obtained with the dark-field confocal scanning laser microscope. The CVD parameters were: feed gas 1% methane in hydrogen, gas pressure 45 mbar, flow rate 20 mm/min, and substrate temperature about 800°C.
1. Optical microscopic in situ observation of crystal growth is possible in a hostile environment using a confocal scanning laser microscope. 2. We were able to achieve a lateral resolution better than 1 lm. This can be improved to 0.2—0.5 lm by applying an objective of higher numerical aperture. 3. Dark-field conditions were used to eliminate the strong specularly reflected light from the plane substrate. This is especially of advantage in the early growth stage, where only crystallites of submicron size are present and therefore the scattered light is very weak.
Acknowledgements Part of this work has been supported by Deutsche Forschungsgemeinschaft.
References [1] R. Bremensdorfer, O. Weis, Diamond Relat. Mater. 3 (1994) 741. [2] D.K. Hamilton, T. Wilson, J. Phys. E: Sci. Instrum. 19 (1986) 52. [3] T. Wilson (Ed.), Confocal Microscopy, Academic Press, London, 1990. [4] S. Matsumoto, Y. Sato, M. Tsutumi, N. Setaka, J. Mater. Sci. 17 (1982) 3106. [5] D.W. Keon, J.Y. Lee, J. Appl. Phys. 69 (1991) 8329. [6] S.J. Harris, A.M. Weiner, Th.A. Perry, J. Appl. Phys. 70 (1991) 1385. [7] G. Popovici, M.A. Prelas, Phys. Status Solidi (a) 132 (1992) 233.
Microscopic in situ observation of diamond growth by dark-field scanning laser microscopy and object scanning R. Bremensdorfer, O. Weis* Abteilung Festko( rperphysik, Universita( t Ulm, D 89069 Ulm, Germany Received 10 November 1997; accepted 11 December 1997
Abstract We report on the first microscopic in situ observation of diamond growth under dark-field conditions using a confocal scanning laser microscope. The main intention of this paper is to describe and demonstrate this method which is also applicable to investigate other growth processes in a hostile environment. Dark-field conditions were achieved by tilting the sample plane to such an extent that specularly reflected laser light could not enter the objective. A piezoelectrically driven raster motion of the sample was applied. The measured lateral resolution was better than 1 lm. The growth of a diamond film was observed over a period of 22 h. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 42.30; 68.55; 78.65 Keywords: In situ growth observation; Confocal scanning laser microscope; Dark-field conditions; Diamond growth
1. Introduction Microscopic in situ observation of diamond growth would be of advantage to study directly the influence of different growth parameters. Such a study is hindered by the hostile environmental conditions present during the usual chemical vapor deposition (CVD). These are: a sample temperature in the range 850—950°C, a methane-hydrogen gas at a pressure of about 40 mbar, and either a hot filament of about 2000°C or a microwave plasma. Recently, we have shown that scanning laser microscopy (SLM) in combination with a large
* Corresponding author. Tel.: #49 731 383704.
working distance reflecting objective can be used for the in situ investigation of diamond growth [1]. These investigations were done in a bright-field using the beam scanning method in the form of objective scanning [2]. To further improve the lateral resolution and depth sensitivity, we employed in the present paper a different configuration known as confocal scanning laser microscopy (CSLM). This configuration is obtained by putting a pinhole as a detector aperture in the image of the laser focus [3]. As a result, the previously used objective scanning had to be replaced by sample scanning. Here, the diffraction-limited laser focus on the axis of the objective stays fixed in space whereas the sample surface is moved in a raster motion through the laser focus.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 8 8 3 - X
422
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
We were especially interested to observe the nucleation sites of diamond and the growth of the microcrystallites on silicon wafers. The wafers were prepared by scratching with nominal 1 lm diamond particles. We found that the specular reflections from the plane silicon wafer strongly disturbed our observations. Therefore, we used a dark-field configuration by using an angle of incidence sufficiently large to guarantee that the specularly reflected light could not enter the objective. In the next section, we will give a detailed description of the experimental setup. In Section 3, we will present a series of microphotographs of the time dependence of diamond growth at a chosen position over a time interval of 22 h. Finally, conclusions are drawn in Section 4 from the obtained results.
2. Experimental setup The experimental setup is shown schematically in Fig. 1. Our confocal scanning laser microscope used an argon-ion laser of 488 nm wavelength with a dominant TEM laser mode. The laser beam 00 was focused in a spatial filter onto a diaphragm with a pinhole of 1 lm diameter. After passing a microscope objective, a parallel beam of 8 mm
diameter with a nearly homogeneous intensity distribution was obtained. This beam was focused onto the sample surface by a Schwarzschild-type reflecting objective (Model 25-0506, Ealing) which had a numerical aperture of NA"0.28 and the relatively large working distance of 24 mm. The scattered light emerging from the focal point on the sample was collected by the same reflecting objective. After passing a beamsplitter, the light was projected by means of a second microscope objective onto the 1 lm detector pinhole in front of the detecting photodiode. In ex situ experiments, lateral (Rayleigh) resolutions of 0.7—1 lm were measured in this confocal arrangement using the parallel metal lines of unsealed 4 MBit memories as test objects. The observed depth resolution of the microscope was about 11 lm. The raster motion of the glowing diamond sample is best done by two crossed piezoceramic translators. In general, these translators may be placed either inside or outside the CVD reactor. However, since we were working at a gas pressure of about 40 mbar where an electric discharge may occur at the piezoceramic translators, we had to arrange the piezoceramic translators outside the reactor. Therefore, the motion of each translator had to be transferred to the sample by a vacuum tight feed through. Each feed through was made by
Fig. 1. Schematic diagram of the dark-field confocal scanning laser microscope and the crystal-growth equipment.
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
soldering two circular corrugated bronze membranes (50 lm thick) at the outer edges to an annular inset and at the inner edges to a movable piston (see Fig. 1a and Fig. 1b). To compensate forces due to fluctuations of the pressure difference between inside and outside the CVD reactor, two identical feed throughs at opposite sides were applied. We used piezoceramic translators (Model P-245.70, Physik Instrumente GmbH) with a linearized maximal travel of 120 lm. The sample holder was cut from a V2A block in a shape that combined the demands for high stiffness, low mass, and extremely low heat conduction. The movement was constrained to the raster plane by means of four steel wires which supported the sample holder. We measured the fundamental mechanical resonance frequency of the assembly at 318 Hz. Since the raster scan was performed with 6 lines/s, this resonance frequency gave no practical limitations to scanning. The substrate temperature of about 800°C was produced in the focus of a heater lamp (Model HLX 64635, Osram). The actual temperature of the silicon substrate (5]8]0.5 mm3) was measured by a thermocouple. The activation of the feed gas for the CVD process was done by means of a electrically heated tungsten filament to about 2000°C. We used the filament of a 50 W halogen lamp (Halostar 64440, Osram). Due to the confocal arrangement the light emission generated by the heated filament and the sample itself was suppressed very effeciently. This eliminated the need for a 488 nm filter initially placed in front of the detector. The strong heat production in the CVD reactor made it necessary to use water cooling for the reactor walls and for the glass windows. Since the flowing water between the windows lead to the formation of schlieren, the water cooling of the windows was interrupted during the time it took to scan a picture which was about 4 min for the acquisition of 256]256 pixels. We used a 12 bit A/D converter. By tilting the CVD reactor, the normal of the silicon substrate could be oriented more than 16° against the optical axis of the Schwarzschild objective. Under these conditions, all of the reflected laser light was made to miss the receiving aperture of the Schwarzschild objective, i.e. we were working under dark-field conditions.
423
3. In situ observations In order to demonstrate the feasibility of the presented method to perform optical in-situ observations of crystal growth, Fig. 2 shows, as an example, several stages of the growth of diamond crystallites on a diamond polished (1 0 0) silicon substrate during a 22 h lasting growth experiment [4]. The growth parameters are listed in the caption of Fig. 2. They were kept constant as close as possible during the experiment. It is well known [5] that both, growth rate and morphology, depend on substrate temperature in hot-filament deposition of diamond on nondiamond substrates. However, to some extent, the formation of tungsten carbide at the hot filament was influencing the diamond growth in an uncontrollable manner. First, the resistance, the surface area, and the emission coefficient were gradually changing by the carbide formation which in turn lowered the filament temperature. Secondly, the distance to the substrate was changing due to the deformations of the filament. The raster area of 256]256 pixels had a linear dimension of 115 lm and was chosen at a position near the hot filament. Due to small hysteresis effects at both ends of the raster lines, the photomicrographs of Fig. 2 have been restricted to a clip of 220]220 pixels corresponding to linear dimension of about 100 lm. The images are given in 256 gray levels. At the beginning of the experiment, the scattered light intensity from the substrate was very weak. Obviously, there were two kinds of contributions to scattering. First, scattering at parallel groves which, in the micrographs, run horizontally across the field of view. These groves were generated by the pretreatment, namely the polishing with nominal 1 lm diamond powder. During the course of the CVD process the groves were reduced in depth, probably by etching. The second contribution to scattering came from heavy subsurface plastic deformations that appear mainly as oblique lines. These lines were produced by a prepolishing process with larger particles. The traces remained visible even 10 h after starting the CVD process and finally disappeared only due to the coverage with diamond crystallites. About 7.5 h after beginning the experiment, we observed an enhancement of scattered light at
424
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
R. Bremensdorfer, O. Weis / Journal of Crystal Growth 187 (1998) 421—425
425
Nevertheless, it can turn into strongly reflected light if a crystal face is oriented in the right way to produce a specular reflection passing through the aperture of the objective.
4. Conclusions By using, as an example, the polycrystalline diamond growth on silicon, we were able to demonstrate in this work the following main results:
Fig. 3. Scanning electron micrograph of the as grown polycrystalline diamond film.
several positions at which we could identify crystallites 1 h later. The number of crystallites increased in the following and after 15 h, a nearly continuous diamond layer appeared. Due to the limited lateral resolution of our CSLM, the first stage of diamond growth could not be observed. Especially, the nucleation process which is characterized by an incubation time [6] could not be observed. Whereas the nucleation density is mainly determined by the morphology of the surface, the nucleation rate depends in addition on the growth conditions [7]. In the experiment presented here, we observed a continuous occurrence of new crystallites. Secondary nucleation occurred in later growth stages. We stopped the CVD process after 22 h and investigated the diamond film with a scanning electron microscope (SEM). The film thickness was about 11 lm. As expected, the size of the crystallites differed considerably (Fig. 3). When comparing the SEM picture with those of the CSLM, one has to take into consideration that the detected laser light is, in general, only weak diffusely scattered light. $&&&&&&&&&&&&&&&&&&&&&&&&&& Fig. 2. In situ microphotographs of different stages of diamond growth on silicon obtained with the dark-field confocal scanning laser microscope. The CVD parameters were: feed gas 1% methane in hydrogen, gas pressure 45 mbar, flow rate 20 mm/min, and substrate temperature about 800°C.
1. Optical microscopic in situ observation of crystal growth is possible in a hostile environment using a confocal scanning laser microscope. 2. We were able to achieve a lateral resolution better than 1 lm. This can be improved to 0.2—0.5 lm by applying an objective of higher numerical aperture. 3. Dark-field conditions were used to eliminate the strong specularly reflected light from the plane substrate. This is especially of advantage in the early growth stage, where only crystallites of submicron size are present and therefore the scattered light is very weak.
Acknowledgements Part of this work has been supported by Deutsche Forschungsgemeinschaft.
References [1] R. Bremensdorfer, O. Weis, Diamond Relat. Mater. 3 (1994) 741. [2] D.K. Hamilton, T. Wilson, J. Phys. E: Sci. Instrum. 19 (1986) 52. [3] T. Wilson (Ed.), Confocal Microscopy, Academic Press, London, 1990. [4] S. Matsumoto, Y. Sato, M. Tsutumi, N. Setaka, J. Mater. Sci. 17 (1982) 3106. [5] D.W. Keon, J.Y. Lee, J. Appl. Phys. 69 (1991) 8329. [6] S.J. Harris, A.M. Weiner, Th.A. Perry, J. Appl. Phys. 70 (1991) 1385. [7] G. Popovici, M.A. Prelas, Phys. Status Solidi (a) 132 (1992) 233.