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Ion beam deposition of zinc sulphide films
Short received
communication
26 September
1976
Ion beam deposition of zinc sulphide films
the substrates prior to film deposition and in all cases a 30 minute bombardment with 20pA of 300 eV argon ions onto a defined 5 mm diameter spot on the substrate was carried out. Following cleaning, the zinc sulphide was evaporated into the plasma inside the expansion cup and a sudden increase in ion current was observed. In the case of the glass substrates the current to the defining aperture was monitored. The increase in current was taken as an indication that deposition in the form of ionised particles was playing a significant role in the formation of the films. Deposition was stopped following the formation of a transparent film. The three films obtained on germanium substrates were observed to be uniform and free from marks and the best film showed low optical absorption over the range of measurement from 2 to I5 pm. The optical characteristics of this film were similar to those expected for a good quality zinc sulphide film prepared by conventional techniques. Higher absorption was observed, particularly at shorter wavelengths, for the other two films indicating that some free zinc may have been present. That films with the correct stoichiometry can be obtained using this method was confirmed by the Rutherford scattering measurements carried out on the glass substrates. Although abrasion tests indicated that the ion beam deposited films were no better than those obtained using conventional evaporation in conjunction with sputter cleaning, it is considered that the preliminary experiments have established the feasibility of ion beam deposition of a binary compound. An experimental system capable of extending this work to carry out a more detailed study of the mechanisms involved in the deposition of films using ion beam techniques is now in an advanced stage of construction.
The increasing USC of thin films in optical and electronic applications has led to a demand for the development of deposition techniques suitable for a wide range of film-substrate combinations. One of the major problems associated with film technology is the failure of many films to adhere satisfactorily to the desired substrate and techniques, such as ion plating’ and ion beam deposition’ have been developed to overcome this problem by increasing the energy with which atoms (or ions) of the film material arrive at the substrate. In ion plating the substrate is bombarded with ions and atoms of ill-defined energy of both argon, the usual support gas, and the film material. Although films with excellent mechanical properties have been obtained using this method. the reasons for the success are not fully understood and, for example, the role of the support gas in determining such features as film hardness and uniformity is difficult to evaluate or, in fact, investigate, since correct discharge conditions can only be obtained over a limited range of pressure. In contrast to this, ion beam deposition in which the film material is ionised in a suitable ion source and extracted into a low pressure region prior to deposition allows precise control over parameters such as deposition energy and gas pressure in the deposition region. The successful application of this technique to the deposition of carbon films on a variety of substrate? has clearly illustrated its potential and the present communication describes preliminary experiments designed to investigate the properties of films of the binary compound, zinc sulphide, deposited by this method. Films were deposited onto glass and germanium substrates and studied utilising Rutherford backscattering of I.5 MeV helium ions and optical absorption measurements, since it was originally expected that dissociation of the zinc sulphide in the ion source would lead to the formation of non-stoichiometric films. The zinc sulphide beam was obtained from a conventional duoplasmatron ion source fitted with a molybdenum expansion cup similar to that used by Masic el# to produce oxygen and copper ion beams. For the purpose of the preliminary tests, a simple ion optical system comprising a conical extraction electrode followed by a single focussing cylinder was used. The extraction aperture in the expansion cup was 1.5 mm in diameter and the distance between this aperture and the substrate was 3 cm. Since no attempt was made to deflect the beam prior to deposition, it is likely that conventional evaporation played some part in the formation of the films. The ion source was run using argon as the support gas and the zinc sulphide powder initially inserted into the expansion cup was vapourised by electron bombardment with 2 keV electrons emitted from a tungsten filament wound outside the cup. This arrangement allowed convenient sputter cleaning of
Vacuum/volume
27/number
1.
Pergamon
PressfPrinted
in Great
Acknowledgements We would like to thank Mr Monachan of Barr & Stroud Ltd for discussions during the course of the experiments and for carrying out the optical measurements.
J A Van den Berg and D G Armour Departmenr of Electrical University of Salford, Salford, 5M 4WT
Engineering.
References ‘D M Mattox, J Vnc Sci Technol 10, 1973, 47. %i Aisenberg and R Chabot, J Appl Phys, 42,1971,2953. ‘S Aisenberg, 15th National Vacuum Symposium, Pittsburgh,
(1968). *R Masic,
J M Sautter
and R J Warnecke,
N~cl Znstr Meth,
Pa.
71,1969,
339. Britain
27
communication
26 September
1976
Ion beam deposition of zinc sulphide films
the substrates prior to film deposition and in all cases a 30 minute bombardment with 20pA of 300 eV argon ions onto a defined 5 mm diameter spot on the substrate was carried out. Following cleaning, the zinc sulphide was evaporated into the plasma inside the expansion cup and a sudden increase in ion current was observed. In the case of the glass substrates the current to the defining aperture was monitored. The increase in current was taken as an indication that deposition in the form of ionised particles was playing a significant role in the formation of the films. Deposition was stopped following the formation of a transparent film. The three films obtained on germanium substrates were observed to be uniform and free from marks and the best film showed low optical absorption over the range of measurement from 2 to I5 pm. The optical characteristics of this film were similar to those expected for a good quality zinc sulphide film prepared by conventional techniques. Higher absorption was observed, particularly at shorter wavelengths, for the other two films indicating that some free zinc may have been present. That films with the correct stoichiometry can be obtained using this method was confirmed by the Rutherford scattering measurements carried out on the glass substrates. Although abrasion tests indicated that the ion beam deposited films were no better than those obtained using conventional evaporation in conjunction with sputter cleaning, it is considered that the preliminary experiments have established the feasibility of ion beam deposition of a binary compound. An experimental system capable of extending this work to carry out a more detailed study of the mechanisms involved in the deposition of films using ion beam techniques is now in an advanced stage of construction.
The increasing USC of thin films in optical and electronic applications has led to a demand for the development of deposition techniques suitable for a wide range of film-substrate combinations. One of the major problems associated with film technology is the failure of many films to adhere satisfactorily to the desired substrate and techniques, such as ion plating’ and ion beam deposition’ have been developed to overcome this problem by increasing the energy with which atoms (or ions) of the film material arrive at the substrate. In ion plating the substrate is bombarded with ions and atoms of ill-defined energy of both argon, the usual support gas, and the film material. Although films with excellent mechanical properties have been obtained using this method. the reasons for the success are not fully understood and, for example, the role of the support gas in determining such features as film hardness and uniformity is difficult to evaluate or, in fact, investigate, since correct discharge conditions can only be obtained over a limited range of pressure. In contrast to this, ion beam deposition in which the film material is ionised in a suitable ion source and extracted into a low pressure region prior to deposition allows precise control over parameters such as deposition energy and gas pressure in the deposition region. The successful application of this technique to the deposition of carbon films on a variety of substrate? has clearly illustrated its potential and the present communication describes preliminary experiments designed to investigate the properties of films of the binary compound, zinc sulphide, deposited by this method. Films were deposited onto glass and germanium substrates and studied utilising Rutherford backscattering of I.5 MeV helium ions and optical absorption measurements, since it was originally expected that dissociation of the zinc sulphide in the ion source would lead to the formation of non-stoichiometric films. The zinc sulphide beam was obtained from a conventional duoplasmatron ion source fitted with a molybdenum expansion cup similar to that used by Masic el# to produce oxygen and copper ion beams. For the purpose of the preliminary tests, a simple ion optical system comprising a conical extraction electrode followed by a single focussing cylinder was used. The extraction aperture in the expansion cup was 1.5 mm in diameter and the distance between this aperture and the substrate was 3 cm. Since no attempt was made to deflect the beam prior to deposition, it is likely that conventional evaporation played some part in the formation of the films. The ion source was run using argon as the support gas and the zinc sulphide powder initially inserted into the expansion cup was vapourised by electron bombardment with 2 keV electrons emitted from a tungsten filament wound outside the cup. This arrangement allowed convenient sputter cleaning of
Vacuum/volume
27/number
1.
Pergamon
PressfPrinted
in Great
Acknowledgements We would like to thank Mr Monachan of Barr & Stroud Ltd for discussions during the course of the experiments and for carrying out the optical measurements.
J A Van den Berg and D G Armour Departmenr of Electrical University of Salford, Salford, 5M 4WT
Engineering.
References ‘D M Mattox, J Vnc Sci Technol 10, 1973, 47. %i Aisenberg and R Chabot, J Appl Phys, 42,1971,2953. ‘S Aisenberg, 15th National Vacuum Symposium, Pittsburgh,
(1968). *R Masic,
J M Sautter
and R J Warnecke,
N~cl Znstr Meth,
Pa.
71,1969,
339. Britain
27