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In situ monitoring of HFCVD diamond growth on nickel using soft X-ray emission spectroscopy
Vacuum/volume
Pergamon 0042-207x(95)00104-2
46/numbers &IO/pages 1053 to 105411995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00
In situ monitoring of HFCVD diamond growth using soft X-ray emission spectroscopy
on nickel
P Skytt, E J.ohansson, J-O Carlsson, N Wassdahl and J Nordgren, Angsfriim Consorfium for thin film processes, Uppsala University, Box 530, S-75 12I Uppsala, Sweden
It is well known that by analysing the C-K-emission from lowpressure grown diamond films that it is possible to extract information of the types of bonding present. A HFCVD reactor has been constructed which together with a small soft X-ray emission spectrometer is suitable for deposition studies in situ. First results are presented using a high energy electron beam as excitation source taking data when the deposition reaction is switched off. Data on the early carbon layer formed on Ni in a diamond deposition system is presented. The results show that the experimental system can be useful for studying different properties of the growth.
The growth of diamond thin films in low pressure vapour environments has attracted a lot of attention over the last years. A variety of growth processes have been developed and the majority of them are Chemical Vapour Deposition (CVD) based. There exist few analysis methods that can be used in situ in the growth chamber. This is mainly due to the high pressure and temperatures present. Nevertheless, studies have been made to measure properties like stress using laser reflections, optical constants using spectroscopic ellipsometry and phase identification using Raman signatures on a growing film. None of these methods have been able to directly probe the electronic structure of the grown film that can be obtained by for example analysing the C-K Soft X-ray Emission (SXE) from the films’. This study presents first results from an in situ experiment recording SXE spectra to study the early carbon film formation on Ni substrates. The deposition experiments were performed using a specially built reaction chamber in stainless steel with ports for a soft X-ray spectrometer’, electron-gun, pyrometer viewport and feedthrews for gas and electrical connections. The system is illustrated by a schematic picture in Figure 1. The soft X-ray spectrometer is a costume built instrument dedicated for thin film deposition monitoring. It consists of a spherical grating and a Rawland mounted slit and 2-D detector. One of the key features of the instrument is the physical dimensions and that it can be mounted directly hanging in a standard UHV flange with no other support. The only part of the spectrometer exposed to the deposition environment is the entrance slit sticking in 100 mm from the mounting flange. The detector requires a pressure better than 10m6 torr to operate and therefore the spectrometer is turbomolecularly pumped. The connection to the deposition chamber is equipped with a valve with thin foils that can withstand pressure differences in the lo-’ torr range to be able to operate in typical deposition process environments. The foils have to be very thin (1000 A) so as not to absorb too many soft X-rays. For details about the instrument see Skytt et UP. The electron gun is
Substrate and filament
Gas inlet
I
m-60 pump Figure1. Schematic figure of the deposition/analysis chamber.
situated at right angles to the optical axis of the spectrometer and was typically operated at 4 kV yielding a total current in the 1 mA range. The electron beam could be steered by means of an electric dipole lens to optimise the signal. In this set-up the substrate was situated at 30” gracing incidence with respect to both incoming electron beam and direction of the optical axes of the spectrometer. The deposition of the carbon film on Ni was made using standard hot filament technique. The heated Tantalum filament was situated about 8 mm above the substrate surface in a gas mixture of hydrogen and C,H, at a total pressure of 30 torr measured by a capacitance manometer. The gas flows were for 100 seem hydrogen and 0.4 seem for C,H,. The temperature of 1053
P Skyft et al: HFCVD diamond growth on nickel
the substrate was measured with a pyrometer to 900%. The experiments were obtained in a sequential mode. A deposition of 40 min was followed by a pump down, to lower than 1 - 10 ’ torr, to be able to record SXE spectra. Ijuring recording of the SXE
O-
I
I 270
I 280 Emission
Figure 2. The
I 290
I 300
rl
310
Exlergy (eV)
spectrum with filled squares
are from the film before the deposition, showing only the Ni L,,, emission in third order dilliaction. The open circle spectrum afterwards is recorded after a40 min deposition and keeping the sample temperature at 9OO'C. The solid lines are after cooling the sample down lo 600°C. The inset displays extracted C-K
emission from the two after-deposItion spectra.
1054
the filament cttrrent was ad.justed so that the substrate temperature was kept at specific tcmpcratures. Figure 2 displays X-ray spectra recorded before and after the deposition. The spectra before show only the Ni L,,, emission in 3 : d order diffraction with no traces of any carbon. Directly after the deposition the spectra with open circles were recorded while keeping the temperature at 9OO’C. One observes an increased intensity of C-K-emission. It overlaps unfortunately with the Ni signals in the high energy region but a true C spectra can be extracted by subtracting out the Ni signal (Figure 2). When lowering the temperature of the sample to 600°C the C intensity has increased by about a factor of two, suggesting that C from the bulk migrates from the bulk out IO the surface to form the final film. A spectra of the film at room temperature shows no differences from the spectra recorded at 600°C suggesting that most of the carbon moves at higher temperatures than 600°C. By recording spectra with short recording times one observes a Cpeak growing with time suggesting that WC have a possibility to measure the rate of migration ofcarbon to the surface. One thing that should be kept in mind is that when using 4 kV electron excitation the information depth of the measured soft X-rays is in the order of 500 A. The extracted (1-K spectra showed in the inset have different line shapes indicating that the C atoms probed at the different times have a different chemical environment. Recording spectra of standard diamond and graphite samples at the same experimental resolution say that the final film is mainly graphite while the initial carbon directly after the deposition is neither graphitic nor diamond-like. spectra
References
’ P Sky& E Johansson. T Wiell, J-H Guo, J 0 Carson, N Wassdahl and J Nordgrcn, Diatnond tmd Rrlatcd Murerials, 3, 1 (1994). *P Skytt et al. unpubli&ed, To be prcscnted at SPIE, San Diego (1994).
Pergamon 0042-207x(95)00104-2
46/numbers &IO/pages 1053 to 105411995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00
In situ monitoring of HFCVD diamond growth using soft X-ray emission spectroscopy
on nickel
P Skytt, E J.ohansson, J-O Carlsson, N Wassdahl and J Nordgren, Angsfriim Consorfium for thin film processes, Uppsala University, Box 530, S-75 12I Uppsala, Sweden
It is well known that by analysing the C-K-emission from lowpressure grown diamond films that it is possible to extract information of the types of bonding present. A HFCVD reactor has been constructed which together with a small soft X-ray emission spectrometer is suitable for deposition studies in situ. First results are presented using a high energy electron beam as excitation source taking data when the deposition reaction is switched off. Data on the early carbon layer formed on Ni in a diamond deposition system is presented. The results show that the experimental system can be useful for studying different properties of the growth.
The growth of diamond thin films in low pressure vapour environments has attracted a lot of attention over the last years. A variety of growth processes have been developed and the majority of them are Chemical Vapour Deposition (CVD) based. There exist few analysis methods that can be used in situ in the growth chamber. This is mainly due to the high pressure and temperatures present. Nevertheless, studies have been made to measure properties like stress using laser reflections, optical constants using spectroscopic ellipsometry and phase identification using Raman signatures on a growing film. None of these methods have been able to directly probe the electronic structure of the grown film that can be obtained by for example analysing the C-K Soft X-ray Emission (SXE) from the films’. This study presents first results from an in situ experiment recording SXE spectra to study the early carbon film formation on Ni substrates. The deposition experiments were performed using a specially built reaction chamber in stainless steel with ports for a soft X-ray spectrometer’, electron-gun, pyrometer viewport and feedthrews for gas and electrical connections. The system is illustrated by a schematic picture in Figure 1. The soft X-ray spectrometer is a costume built instrument dedicated for thin film deposition monitoring. It consists of a spherical grating and a Rawland mounted slit and 2-D detector. One of the key features of the instrument is the physical dimensions and that it can be mounted directly hanging in a standard UHV flange with no other support. The only part of the spectrometer exposed to the deposition environment is the entrance slit sticking in 100 mm from the mounting flange. The detector requires a pressure better than 10m6 torr to operate and therefore the spectrometer is turbomolecularly pumped. The connection to the deposition chamber is equipped with a valve with thin foils that can withstand pressure differences in the lo-’ torr range to be able to operate in typical deposition process environments. The foils have to be very thin (1000 A) so as not to absorb too many soft X-rays. For details about the instrument see Skytt et UP. The electron gun is
Substrate and filament
Gas inlet
I
m-60 pump Figure1. Schematic figure of the deposition/analysis chamber.
situated at right angles to the optical axis of the spectrometer and was typically operated at 4 kV yielding a total current in the 1 mA range. The electron beam could be steered by means of an electric dipole lens to optimise the signal. In this set-up the substrate was situated at 30” gracing incidence with respect to both incoming electron beam and direction of the optical axes of the spectrometer. The deposition of the carbon film on Ni was made using standard hot filament technique. The heated Tantalum filament was situated about 8 mm above the substrate surface in a gas mixture of hydrogen and C,H, at a total pressure of 30 torr measured by a capacitance manometer. The gas flows were for 100 seem hydrogen and 0.4 seem for C,H,. The temperature of 1053
P Skyft et al: HFCVD diamond growth on nickel
the substrate was measured with a pyrometer to 900%. The experiments were obtained in a sequential mode. A deposition of 40 min was followed by a pump down, to lower than 1 - 10 ’ torr, to be able to record SXE spectra. Ijuring recording of the SXE
O-
I
I 270
I 280 Emission
Figure 2. The
I 290
I 300
rl
310
Exlergy (eV)
spectrum with filled squares
are from the film before the deposition, showing only the Ni L,,, emission in third order dilliaction. The open circle spectrum afterwards is recorded after a40 min deposition and keeping the sample temperature at 9OO'C. The solid lines are after cooling the sample down lo 600°C. The inset displays extracted C-K
emission from the two after-deposItion spectra.
1054
the filament cttrrent was ad.justed so that the substrate temperature was kept at specific tcmpcratures. Figure 2 displays X-ray spectra recorded before and after the deposition. The spectra before show only the Ni L,,, emission in 3 : d order diffraction with no traces of any carbon. Directly after the deposition the spectra with open circles were recorded while keeping the temperature at 9OO’C. One observes an increased intensity of C-K-emission. It overlaps unfortunately with the Ni signals in the high energy region but a true C spectra can be extracted by subtracting out the Ni signal (Figure 2). When lowering the temperature of the sample to 600°C the C intensity has increased by about a factor of two, suggesting that C from the bulk migrates from the bulk out IO the surface to form the final film. A spectra of the film at room temperature shows no differences from the spectra recorded at 600°C suggesting that most of the carbon moves at higher temperatures than 600°C. By recording spectra with short recording times one observes a Cpeak growing with time suggesting that WC have a possibility to measure the rate of migration ofcarbon to the surface. One thing that should be kept in mind is that when using 4 kV electron excitation the information depth of the measured soft X-rays is in the order of 500 A. The extracted (1-K spectra showed in the inset have different line shapes indicating that the C atoms probed at the different times have a different chemical environment. Recording spectra of standard diamond and graphite samples at the same experimental resolution say that the final film is mainly graphite while the initial carbon directly after the deposition is neither graphitic nor diamond-like. spectra
References
’ P Sky& E Johansson. T Wiell, J-H Guo, J 0 Carson, N Wassdahl and J Nordgrcn, Diatnond tmd Rrlatcd Murerials, 3, 1 (1994). *P Skytt et al. unpubli&ed, To be prcscnted at SPIE, San Diego (1994).