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A comparison of ion plated and ion assisted coating of aluminium
l/acuum/volume 38/numbers Printed in Great Britain
Extended
8-l O/pages 943 to 944/l
988
0042-207X/88$3.00+.00 Pergamon Press plc
abstracts
A comparison of ion plated and ion assisted coating of aluminium J S Colligon, R D Arnell, R G Milburn, S F Tinston, H Kheyrandish and R I Bates, Centre for Thin Film and Surface Research, University of Salford, Salford MS 4 WT, UK The production ?f well-bonded surface coatings allows the modern engineer considerable flexibility in choice of materials since a material with particular bulk properties may be selected whilst, at the same time, the surface may be adapted to produce better wear, corrosion resistance or other desirable features. A novel method for production of thin coatings involves deposition of material together with simultaneous irradiation by energetic ions. This so-called ion beam assisted deposition produces well-bonded films and, if the ion species is chemically active, such as nitrogen or oxygen, compound films may be formed. Cuomo et al’ have reviewed this field and it is clear that the ion/atom arrival ratio, which can be carefully controlled in this method, has a strong influence on the stoichiometry of the resulting film whilst the ion energy can influence the film structure. Ion beam assisted coating is however limited to treatment of relatively small samples and is a ‘line of sight’ process so that coating of complex shapes presents problems. Plasma assisted techniques (e.g. ion plating) allow deposition on complex shapes and are commercially more economical than ion beam processing methods. Ion plating was first described by Berghaus* and, later, developed by Mattox3. The resulting films often have superior properties in terms of adhesion and structure when compared with evaporated and sputtered films but, unlike the ion beam assisted situation, the parameters are not well defined, nor is it obvious how to control them since a variation of plasma conditions changes many of the bombardment conditions. The present programme is to study the effect of energy deposited per atom during deposition using ion plating and ion assisted film deposition techniques and to make some comparisons between the films resulting from the two techniques. This paper describes preliminary results for aluminium films deposited onto steel substrates, the details of which will be published later. Aluminium films were deposited using a dual ion beam system. Argon ions from a broad beam 3 in. Kaufman source were used to sputter aluminium onto the steel substrate. During deposition the film was bombarded by 20-27 keV Ar+ ions from a cold cathode Penning type of source. Relative arrival rates of ions and atoms were changed to provide different average values of energy in the interval O-800 eV atom-‘. Aluminium films were also deposited in a triode ion plating rig using various deposition parameters. In all cases the samples were sectioned and the film-substrate interface was viewed in a SEM. Argon and oxygen content of the films was measured using an electron probe microanalyser and film hardness was determined using a Knoop microhardness indenter. Films produced by the ion beam assisted deposition showed much finer structure than those formed by ion plating. They were seen to contain argon but no oxygen, the amount of argon increasing with increase in average energy per deposited atom.
Ion plated films on the other hand had no detectable argon or oxygen content. The adhesion of all films was noticeably better than for films that were simply sputtered. The hardness of the films was of similar magnitude but decreased slightly with increase in argon content for the ion beam assisted films. These differences must be attributed to the basic variations in the processes: For example, in ion assisted deposition, the impacting particle energy is shared with other atoms beneath the surface whereas, in ion plating, because of the small mean free path, it is shared in the plasma so that the majority of particles strike the surface with low energy. Thus argon is implanted fairly deeply into the film and trapped during the ion assisted deposition but stays at the surface and is able to escape during ion plating. Another variable will be the different rates of film growth, ion plated films forming at about ten times the rate of ion beam assisted films. Further experiments are being devised to provide ion beam assisted growth where these differences are minimised. To understand these differences better, a new ion plating rig has been designed and is nearing completion. In this the charged and neutral species incident on the forming films -are mass and energy analysed by using an ioniser, a double hemispherical energy analyser and a quadrupole mass spectrometer in tandem. The new apparatus will allow a precise comparison of film properties with the known ion/atom energy distributions that exist in the ion plating system. References ‘J J Cuomo, Nucl Instrum Meth Phys Res, B19/20,963 (1987). 2B Berghaus, UK Patent 510,995 (1938). ‘D Mattox, J appl Phys, 34, 2493 (1963).
Application of Taguchi gallium arsenide
analysis
to PECVD
of tungsten onto
E M Geake and M Middleton, Department of Electronics, University of York, Heslington, York YOI SDD, UK The aim of this work is to identify a suitable gate metallisation for use in a self-aligned gate FET structure’. Since the gate is used as a mask during ion implantation, it must withstand annealing (at temperatures exceeding 800°C for GaAs) without degrading physically or electrically. Tungsten is an obvious choice for this application because of its high temperature stability and low electrical resistivity (bulk value = 5.3 PLRcm). In this study it has been deposited by plasma enhanced CVD using WF, and Hz as reactant gases with Ar as a carrier gas. A statistical method due to Taguch? has been used to optimise the mass (measured in pg cm-’ min-‘) and sheet resistance (in no-‘) of tungsten films deposited. The method quantifies the effects of various controllable input parameters (or ‘factors’) on the mean and variance of the measured ‘responses’, hence highlighting the factors which merit detailed theoretical investigation. The levels at which the factors are set must be chosen carefully: not SO 943
Extended
abstracts
widely that small effects go unnoticed, and not so closely that no effects at all can be detected. Hence, a preliminary experiment must be conducted to aid the choice of factor settings. Factor settings which fit into an orthogonal array are used for the Taguchi analysis; it is important to choose the array to fit the experiment, rather than choosing an array and arranging the experiment to fit into it. Any suspected interactions between factors should be included as additional factors. An advantage of the orthogonal array is that it uses a small fraction of the experimental runs which would be needed for a full factorial design, and infers the results of the remainder’. Our preliminary results revealed that no tungsten could be deposited without a plasma, hence confirming that LPCVD using WF, and Hz does not occur at 200°C. Eliminating the Ar carrier gas increased the film thickness by approximately 10% and reduced the resitivity by 5%, indicating that Ar is incorporated into the films. Additional evidence for this, and for the presence of voids in the films, was that annealing (at 850°C for 5 s in NJ produced a 40% reduction in film thickness, a 55% increase in density and a 65% decrease in resistivity6. It was found that a film
944
3020 8, thick with a resistivity of 13.6 ptR cm could be deposited in the absence of Ar. Electrical characterisation of diodes defined by reactive ion etching in SF, will be reported in a future paper. Acknowledgement This work was funded by the Science and Engineering Council.
Research
References
’ B M Welch, R C Eden and F S Lee, Gallium Arsenide: Materials. Devices, and Circuits (Edited by M J Howes and D V Morgan), Chapter 13. Wiley-Interscience (1985). 2 N Logothetis, GEC J Res, 5 (1987). ‘N Logothetis, Experimental Design and Off-Line Quality Control (Taguchi Method), Statistical Advisory Service, GEC Research Ltd. (1987). - ‘G Taguchi and Y Wu, Introduction to off-line quality control, Central Japan Quality Control Association (1985). 5 G Z Yin and D W Jillie, Solid State Tech&, 127 (1987). ‘Kindly performed by S Bland and S Kitching of STC Technology Ltd.
Extended
8-l O/pages 943 to 944/l
988
0042-207X/88$3.00+.00 Pergamon Press plc
abstracts
A comparison of ion plated and ion assisted coating of aluminium J S Colligon, R D Arnell, R G Milburn, S F Tinston, H Kheyrandish and R I Bates, Centre for Thin Film and Surface Research, University of Salford, Salford MS 4 WT, UK The production ?f well-bonded surface coatings allows the modern engineer considerable flexibility in choice of materials since a material with particular bulk properties may be selected whilst, at the same time, the surface may be adapted to produce better wear, corrosion resistance or other desirable features. A novel method for production of thin coatings involves deposition of material together with simultaneous irradiation by energetic ions. This so-called ion beam assisted deposition produces well-bonded films and, if the ion species is chemically active, such as nitrogen or oxygen, compound films may be formed. Cuomo et al’ have reviewed this field and it is clear that the ion/atom arrival ratio, which can be carefully controlled in this method, has a strong influence on the stoichiometry of the resulting film whilst the ion energy can influence the film structure. Ion beam assisted coating is however limited to treatment of relatively small samples and is a ‘line of sight’ process so that coating of complex shapes presents problems. Plasma assisted techniques (e.g. ion plating) allow deposition on complex shapes and are commercially more economical than ion beam processing methods. Ion plating was first described by Berghaus* and, later, developed by Mattox3. The resulting films often have superior properties in terms of adhesion and structure when compared with evaporated and sputtered films but, unlike the ion beam assisted situation, the parameters are not well defined, nor is it obvious how to control them since a variation of plasma conditions changes many of the bombardment conditions. The present programme is to study the effect of energy deposited per atom during deposition using ion plating and ion assisted film deposition techniques and to make some comparisons between the films resulting from the two techniques. This paper describes preliminary results for aluminium films deposited onto steel substrates, the details of which will be published later. Aluminium films were deposited using a dual ion beam system. Argon ions from a broad beam 3 in. Kaufman source were used to sputter aluminium onto the steel substrate. During deposition the film was bombarded by 20-27 keV Ar+ ions from a cold cathode Penning type of source. Relative arrival rates of ions and atoms were changed to provide different average values of energy in the interval O-800 eV atom-‘. Aluminium films were also deposited in a triode ion plating rig using various deposition parameters. In all cases the samples were sectioned and the film-substrate interface was viewed in a SEM. Argon and oxygen content of the films was measured using an electron probe microanalyser and film hardness was determined using a Knoop microhardness indenter. Films produced by the ion beam assisted deposition showed much finer structure than those formed by ion plating. They were seen to contain argon but no oxygen, the amount of argon increasing with increase in average energy per deposited atom.
Ion plated films on the other hand had no detectable argon or oxygen content. The adhesion of all films was noticeably better than for films that were simply sputtered. The hardness of the films was of similar magnitude but decreased slightly with increase in argon content for the ion beam assisted films. These differences must be attributed to the basic variations in the processes: For example, in ion assisted deposition, the impacting particle energy is shared with other atoms beneath the surface whereas, in ion plating, because of the small mean free path, it is shared in the plasma so that the majority of particles strike the surface with low energy. Thus argon is implanted fairly deeply into the film and trapped during the ion assisted deposition but stays at the surface and is able to escape during ion plating. Another variable will be the different rates of film growth, ion plated films forming at about ten times the rate of ion beam assisted films. Further experiments are being devised to provide ion beam assisted growth where these differences are minimised. To understand these differences better, a new ion plating rig has been designed and is nearing completion. In this the charged and neutral species incident on the forming films -are mass and energy analysed by using an ioniser, a double hemispherical energy analyser and a quadrupole mass spectrometer in tandem. The new apparatus will allow a precise comparison of film properties with the known ion/atom energy distributions that exist in the ion plating system. References ‘J J Cuomo, Nucl Instrum Meth Phys Res, B19/20,963 (1987). 2B Berghaus, UK Patent 510,995 (1938). ‘D Mattox, J appl Phys, 34, 2493 (1963).
Application of Taguchi gallium arsenide
analysis
to PECVD
of tungsten onto
E M Geake and M Middleton, Department of Electronics, University of York, Heslington, York YOI SDD, UK The aim of this work is to identify a suitable gate metallisation for use in a self-aligned gate FET structure’. Since the gate is used as a mask during ion implantation, it must withstand annealing (at temperatures exceeding 800°C for GaAs) without degrading physically or electrically. Tungsten is an obvious choice for this application because of its high temperature stability and low electrical resistivity (bulk value = 5.3 PLRcm). In this study it has been deposited by plasma enhanced CVD using WF, and Hz as reactant gases with Ar as a carrier gas. A statistical method due to Taguch? has been used to optimise the mass (measured in pg cm-’ min-‘) and sheet resistance (in no-‘) of tungsten films deposited. The method quantifies the effects of various controllable input parameters (or ‘factors’) on the mean and variance of the measured ‘responses’, hence highlighting the factors which merit detailed theoretical investigation. The levels at which the factors are set must be chosen carefully: not SO 943
Extended
abstracts
widely that small effects go unnoticed, and not so closely that no effects at all can be detected. Hence, a preliminary experiment must be conducted to aid the choice of factor settings. Factor settings which fit into an orthogonal array are used for the Taguchi analysis; it is important to choose the array to fit the experiment, rather than choosing an array and arranging the experiment to fit into it. Any suspected interactions between factors should be included as additional factors. An advantage of the orthogonal array is that it uses a small fraction of the experimental runs which would be needed for a full factorial design, and infers the results of the remainder’. Our preliminary results revealed that no tungsten could be deposited without a plasma, hence confirming that LPCVD using WF, and Hz does not occur at 200°C. Eliminating the Ar carrier gas increased the film thickness by approximately 10% and reduced the resitivity by 5%, indicating that Ar is incorporated into the films. Additional evidence for this, and for the presence of voids in the films, was that annealing (at 850°C for 5 s in NJ produced a 40% reduction in film thickness, a 55% increase in density and a 65% decrease in resistivity6. It was found that a film
944
3020 8, thick with a resistivity of 13.6 ptR cm could be deposited in the absence of Ar. Electrical characterisation of diodes defined by reactive ion etching in SF, will be reported in a future paper. Acknowledgement This work was funded by the Science and Engineering Council.
Research
References
’ B M Welch, R C Eden and F S Lee, Gallium Arsenide: Materials. Devices, and Circuits (Edited by M J Howes and D V Morgan), Chapter 13. Wiley-Interscience (1985). 2 N Logothetis, GEC J Res, 5 (1987). ‘N Logothetis, Experimental Design and Off-Line Quality Control (Taguchi Method), Statistical Advisory Service, GEC Research Ltd. (1987). - ‘G Taguchi and Y Wu, Introduction to off-line quality control, Central Japan Quality Control Association (1985). 5 G Z Yin and D W Jillie, Solid State Tech&, 127 (1987). ‘Kindly performed by S Bland and S Kitching of STC Technology Ltd.