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Preparation of W-Th films by ion beam sputtering
782
Nuclear
Preparation M.
Instruments
and Methods
in Physics
Research
B55 (1991) 782-784 North-Holland
of W--Th films by ion beam sputtering
Griepentrog, R.-A. Noack, M. Rose~gart~~ and W. Schneider
Departmentof Physics,H~~oldt University, Berlin 1040, Germany
In the present work ion beam sputtering of polycrystalline W/Th targets with a well-defined Th content by 12 keV Xe+ ions was used for the preparation of W-Th films in order to produce a new material for cathodes in light-emitting devices. The films, deposited on silicon and AlsOs, were investigated by secondary ion mass spectrometry (SIMS), electron probe micro-analysis (EPMA), and Auger electron spectroscopy (AES). The films are close-packed and have the density of tungsten bulk material. It was established that the thorium distribution in the films is homogeneous. The thorium is always activated. The W/Th ratio is proportional to the thorium content in the target. Because of the poor vacuum conditions during the deposition (lop4 Pa) oxygen and carbon were found in the films. The films are mostly amorphous and seem to be a new exciting cathode material with interesting properties.
1. Introduction The life-time of cathodes in light-emitting devices is in general limited by the erosion rate of the material used. At present powder-metallurgically made polycrystalline tungsten is often used for the production of cathodes because of the interesting thermal and electrical properties of tungsten. To obtain lower erosion rates and better emission characteristics the tungsten matrix is doped by an activator, i.e. a material with a low work-function (e.g. thorium oxide or barium oxide) [l]. Investigations of thoriated tungsten cathodes (21 show that the thorium oxide forms irregularly ~stributed particles with a diameter of a few micrometers. Size, concentration, and distribution of the activator particles determine the ignition, emission, and erosion behaviour of the cathodes. The intention of the present paper is to show that W-Th films produced by ion beam sputter deposition (IBSD) are an interesting material for improving cathode performance. IBSD is a well established deposition technique [3,4]. In IBSD a collimated beam of energetic noble gas ions strikes a target. The sputtered particles impinge on a nearby substrate and form a thin film there. According to the energy ~st~bution of the sputtered particles in IBSD the energy of the particles impinging on the substrate is much higher than in conventional evaporation processes. The sputtered particles leave the target with a near-cosine distribution. The energy and angular distribution of the sputtered particles depend on the energy, mass, and angle of incidence of the sputtering ions, and on the mass, chemical bonding states, and crystalline structure of the target [S]. There are different interactions between the impinging particles, the substrate surface, and the grow0168-583X/91/$03.50
film [6,7]. Using highly energetic particles for film formation even at low temperatures films with good adhesion, high density, and low contamination can be prepared [8,9]. The sputtering of a target by energetic ion bombardment often results in the reflection of a significant fraction of incident ions as energetic neutrals into the growing film. These energetic neutrals also influence the growing film [lo]. There are some additional advantages of IBSD in comparison with other sputtering techniques, for example, lower background pressure, independent control of the ion energy and ion current density, no interaction between substrate and plasma, and better control of the process. ing
Q 1991 - Elsevier Science Publishers
2. Experimental In this work ion beam sputtering by 12 keV Xe+ ions was used. The beam was 8 mm in diameter. The current density of the ion beam was I mA/cm2. The normal of the target was oriented at 45 o with respect to the primary ion beam. Substrates for deposition were mounted on a sample holder placed 25 mm from the target. The deposition chamber was pumped by an ion getter pump. The pressure within the vacuum chamber was 1 x lop4 Pa and 5 x 1O-4 Pa during ion source operation. W-Th films with thicknesses from 50 to 250 nm were deposited on crystalline silicon and on polycrystalline Al,O, substrates. The surface of the silicon substrates was polished. The surface of the Al,O, substrates showed a high roughness. Poly~sta~ne tungsten with different contents of thorium oxide (1.8 to 10 mass%) was used as target material. Various methods for surface analysis have been used for investigating
B.V. (North-Holland)
783
M. Griepentrog et al. / Preparation of W-Th films
formation processes and properties of W-Th films by determining composition, structure and chemical bonding states. The techniques mostly used are electron probe microanalysis (EPMA), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). These techniques have both special features and disadvantages in investigations of thin films.
3. Results and discussion As shown by studies of scanning electron microscopy the W-Th films on Al,O, and Si are continuous. The films on the Al,O, substrate have the same roughness as the substrate. A direct determination of the film thickness is thus not possible. The films deposited on Si are characterized by a smooth surface and a high adhesion to the substrate surface. The thickness of films was determined by a surface profiling device due to masking a fixed area of the substrate surface during deposition. A symmetric distribution of film thickness with a gradient of lO%/cm was found. The maximum of the distribution is located opposite the centre of the area sputtered by the Xe+ ion beam. The average film growth rate is 1.2 nm/min for W-Th films where the film is thickest. The films show good electrical conductivity. The specific resistance determined from four-point probe measurements is two orders of magnitude higher than that of metallic tungsten (5.5 l~,fi cm). Because the application of EPMA, SIMS, and AES for the characterization of thin films on rough Al,O,
substrates is complicated and not always possible. Results are limited, in some cases, to films on Si substrates. The SIMS depth profiling for W and Th allows an assessment of the homogeneity of the films. The depth profiles exhibit an increase in secondary ion signals at the surface and at the interface between film and substrate. Constant signals of secondary ions of the bulk species indicate the homogeneous nature of the film. Investigations by SIMS, AES, and measurements with an electron probe microanalyzer showed that there are no additional impurities in the layers with the exception of carbon (< 10 at.%) and oxygen (< 3 at.%). Measurements by X-ray photoelectron spectroscopy (XPS) carried out for some samples indicate the existence of thorium oxide, various tungsten oxides and tungsten carbides in the tungsten film. The thorium oxide will work as the activator. It is possible to obtain information concerning the lateral homogeneity of the distribution of activators by X-ray microanalysis [2]. These investigations show that the thorium oxide is distributed totally homogeneously through the films. Precipitates are not traceable (fig. 1). The concentration of activators in the films can be determined by means of X-ray microanalysis [ll]. The concentration of Th in a W-Th film produced by IBSD of a polycrystalline tungsten target with 5% thorium oxide (mass %) was determined to be (4 f l)% Th. In general, the concentration of activator in the films is proportional to the content of the activator in the target material. Measurements of the cathodoluminescence signal (CL) show an almost homogeneous distribution.
10
i
4-
0.6
0
20
40
60
80 x/pm-
100
O&t
4300
02 -
-100
0
t
I
,e
2000
4000
,kTh 6000
8000
0
x/pm-
Fig. 1. X-ray microanalysis of the W and Th distribution in the films (k-values for the W,, and the Th,, line; E = 25 keV) along the surface (a) of a WTh target (distance between the measuring points 1 nm); and (b) of a W-Th film on Si (target WTh 50) and the resulting distribution of the thickness d (distance between the measuring points 100 pm). VI. MATERIALS
SCIENCE
784
M. Griepentrog et al. / Preparation
This indicates that the activators in the films are crystalline without needing to heat to high temperatures [2]. If the thickness of the film is known, the density of the deposited film can be determined using the AES concentration data. The density of the deposited films of about 16 g/cm3 is near to the density of bulk tungsten (19.3 g/cm3).
4. Conclusions The ion beam sputter deposition has proven to be a useful technique for producing a new material for cathodes with exciting properties. However, compared to other deposition techniques, several problems remain to be solved, such as low deposition rates and the problem of contamination.
References [l] V.M. Amosow, B.A. Karelm and V.V. Kubyskin, Material for Cathodes on the Basis of High Melting Metals (Metalurgija, Moscow, 1976) in Russian.
of W-Th
films
[2] H. Diisterhbft, W. Schneider and V. Steinbrtick, Exp. Tech. Phys. 25 (1981) 447. [3] P. Mazzoldi and G.W. Arnold (eds.), Ion beam modification of insulators, in: Beam Modification of Materials, vol. 2 (Elsevier, New York, 1987). [4] T. Itoh (ed.), Ion beam assisted film growth, in: Beam Modification of Materials, vol. 3 (Elsevier, New York, 1989). [5] R. Behrisch (ed.), Sputtering by Ion Bombardment Physics and Application, (Springer, Berlin 1981). [6] T. Tagaki, Proc. Int. Ion Engineering Congress - ISIAT ‘83, Kyoto, 1983, ed. T. Tagaki (Institute of Electrical Engineerings of Japan, Tokyo, 1983) p. 785. [7] T. Tagaki, J. Vat. Sci. Technol. A2 (1984) 382. [8] J.M.E. Harper, Solid State Technol. 30 (1987) 129. [9] J.E. Greene, Solid State Technol. 30 (1987) 115. [Xl] E. Kay, F. Parmigiani and W. Parrish, J. Vat. Sci. Technol. A5 (1987) 44. [ll] W. Weisweiler and J. Eck, Beitr. Elektronmikr. Direktabb. Oberfl. 18 (1985) 1.
Nuclear
Preparation M.
Instruments
and Methods
in Physics
Research
B55 (1991) 782-784 North-Holland
of W--Th films by ion beam sputtering
Griepentrog, R.-A. Noack, M. Rose~gart~~ and W. Schneider
Departmentof Physics,H~~oldt University, Berlin 1040, Germany
In the present work ion beam sputtering of polycrystalline W/Th targets with a well-defined Th content by 12 keV Xe+ ions was used for the preparation of W-Th films in order to produce a new material for cathodes in light-emitting devices. The films, deposited on silicon and AlsOs, were investigated by secondary ion mass spectrometry (SIMS), electron probe micro-analysis (EPMA), and Auger electron spectroscopy (AES). The films are close-packed and have the density of tungsten bulk material. It was established that the thorium distribution in the films is homogeneous. The thorium is always activated. The W/Th ratio is proportional to the thorium content in the target. Because of the poor vacuum conditions during the deposition (lop4 Pa) oxygen and carbon were found in the films. The films are mostly amorphous and seem to be a new exciting cathode material with interesting properties.
1. Introduction The life-time of cathodes in light-emitting devices is in general limited by the erosion rate of the material used. At present powder-metallurgically made polycrystalline tungsten is often used for the production of cathodes because of the interesting thermal and electrical properties of tungsten. To obtain lower erosion rates and better emission characteristics the tungsten matrix is doped by an activator, i.e. a material with a low work-function (e.g. thorium oxide or barium oxide) [l]. Investigations of thoriated tungsten cathodes (21 show that the thorium oxide forms irregularly ~stributed particles with a diameter of a few micrometers. Size, concentration, and distribution of the activator particles determine the ignition, emission, and erosion behaviour of the cathodes. The intention of the present paper is to show that W-Th films produced by ion beam sputter deposition (IBSD) are an interesting material for improving cathode performance. IBSD is a well established deposition technique [3,4]. In IBSD a collimated beam of energetic noble gas ions strikes a target. The sputtered particles impinge on a nearby substrate and form a thin film there. According to the energy ~st~bution of the sputtered particles in IBSD the energy of the particles impinging on the substrate is much higher than in conventional evaporation processes. The sputtered particles leave the target with a near-cosine distribution. The energy and angular distribution of the sputtered particles depend on the energy, mass, and angle of incidence of the sputtering ions, and on the mass, chemical bonding states, and crystalline structure of the target [S]. There are different interactions between the impinging particles, the substrate surface, and the grow0168-583X/91/$03.50
film [6,7]. Using highly energetic particles for film formation even at low temperatures films with good adhesion, high density, and low contamination can be prepared [8,9]. The sputtering of a target by energetic ion bombardment often results in the reflection of a significant fraction of incident ions as energetic neutrals into the growing film. These energetic neutrals also influence the growing film [lo]. There are some additional advantages of IBSD in comparison with other sputtering techniques, for example, lower background pressure, independent control of the ion energy and ion current density, no interaction between substrate and plasma, and better control of the process. ing
Q 1991 - Elsevier Science Publishers
2. Experimental In this work ion beam sputtering by 12 keV Xe+ ions was used. The beam was 8 mm in diameter. The current density of the ion beam was I mA/cm2. The normal of the target was oriented at 45 o with respect to the primary ion beam. Substrates for deposition were mounted on a sample holder placed 25 mm from the target. The deposition chamber was pumped by an ion getter pump. The pressure within the vacuum chamber was 1 x lop4 Pa and 5 x 1O-4 Pa during ion source operation. W-Th films with thicknesses from 50 to 250 nm were deposited on crystalline silicon and on polycrystalline Al,O, substrates. The surface of the silicon substrates was polished. The surface of the Al,O, substrates showed a high roughness. Poly~sta~ne tungsten with different contents of thorium oxide (1.8 to 10 mass%) was used as target material. Various methods for surface analysis have been used for investigating
B.V. (North-Holland)
783
M. Griepentrog et al. / Preparation of W-Th films
formation processes and properties of W-Th films by determining composition, structure and chemical bonding states. The techniques mostly used are electron probe microanalysis (EPMA), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). These techniques have both special features and disadvantages in investigations of thin films.
3. Results and discussion As shown by studies of scanning electron microscopy the W-Th films on Al,O, and Si are continuous. The films on the Al,O, substrate have the same roughness as the substrate. A direct determination of the film thickness is thus not possible. The films deposited on Si are characterized by a smooth surface and a high adhesion to the substrate surface. The thickness of films was determined by a surface profiling device due to masking a fixed area of the substrate surface during deposition. A symmetric distribution of film thickness with a gradient of lO%/cm was found. The maximum of the distribution is located opposite the centre of the area sputtered by the Xe+ ion beam. The average film growth rate is 1.2 nm/min for W-Th films where the film is thickest. The films show good electrical conductivity. The specific resistance determined from four-point probe measurements is two orders of magnitude higher than that of metallic tungsten (5.5 l~,fi cm). Because the application of EPMA, SIMS, and AES for the characterization of thin films on rough Al,O,
substrates is complicated and not always possible. Results are limited, in some cases, to films on Si substrates. The SIMS depth profiling for W and Th allows an assessment of the homogeneity of the films. The depth profiles exhibit an increase in secondary ion signals at the surface and at the interface between film and substrate. Constant signals of secondary ions of the bulk species indicate the homogeneous nature of the film. Investigations by SIMS, AES, and measurements with an electron probe microanalyzer showed that there are no additional impurities in the layers with the exception of carbon (< 10 at.%) and oxygen (< 3 at.%). Measurements by X-ray photoelectron spectroscopy (XPS) carried out for some samples indicate the existence of thorium oxide, various tungsten oxides and tungsten carbides in the tungsten film. The thorium oxide will work as the activator. It is possible to obtain information concerning the lateral homogeneity of the distribution of activators by X-ray microanalysis [2]. These investigations show that the thorium oxide is distributed totally homogeneously through the films. Precipitates are not traceable (fig. 1). The concentration of activators in the films can be determined by means of X-ray microanalysis [ll]. The concentration of Th in a W-Th film produced by IBSD of a polycrystalline tungsten target with 5% thorium oxide (mass %) was determined to be (4 f l)% Th. In general, the concentration of activator in the films is proportional to the content of the activator in the target material. Measurements of the cathodoluminescence signal (CL) show an almost homogeneous distribution.
10
i
4-
0.6
0
20
40
60
80 x/pm-
100
O&t
4300
02 -
-100
0
t
I
,e
2000
4000
,kTh 6000
8000
0
x/pm-
Fig. 1. X-ray microanalysis of the W and Th distribution in the films (k-values for the W,, and the Th,, line; E = 25 keV) along the surface (a) of a WTh target (distance between the measuring points 1 nm); and (b) of a W-Th film on Si (target WTh 50) and the resulting distribution of the thickness d (distance between the measuring points 100 pm). VI. MATERIALS
SCIENCE
784
M. Griepentrog et al. / Preparation
This indicates that the activators in the films are crystalline without needing to heat to high temperatures [2]. If the thickness of the film is known, the density of the deposited film can be determined using the AES concentration data. The density of the deposited films of about 16 g/cm3 is near to the density of bulk tungsten (19.3 g/cm3).
4. Conclusions The ion beam sputter deposition has proven to be a useful technique for producing a new material for cathodes with exciting properties. However, compared to other deposition techniques, several problems remain to be solved, such as low deposition rates and the problem of contamination.
References [l] V.M. Amosow, B.A. Karelm and V.V. Kubyskin, Material for Cathodes on the Basis of High Melting Metals (Metalurgija, Moscow, 1976) in Russian.
of W-Th
films
[2] H. Diisterhbft, W. Schneider and V. Steinbrtick, Exp. Tech. Phys. 25 (1981) 447. [3] P. Mazzoldi and G.W. Arnold (eds.), Ion beam modification of insulators, in: Beam Modification of Materials, vol. 2 (Elsevier, New York, 1987). [4] T. Itoh (ed.), Ion beam assisted film growth, in: Beam Modification of Materials, vol. 3 (Elsevier, New York, 1989). [5] R. Behrisch (ed.), Sputtering by Ion Bombardment Physics and Application, (Springer, Berlin 1981). [6] T. Tagaki, Proc. Int. Ion Engineering Congress - ISIAT ‘83, Kyoto, 1983, ed. T. Tagaki (Institute of Electrical Engineerings of Japan, Tokyo, 1983) p. 785. [7] T. Tagaki, J. Vat. Sci. Technol. A2 (1984) 382. [8] J.M.E. Harper, Solid State Technol. 30 (1987) 129. [9] J.E. Greene, Solid State Technol. 30 (1987) 115. [Xl] E. Kay, F. Parmigiani and W. Parrish, J. Vat. Sci. Technol. A5 (1987) 44. [ll] W. Weisweiler and J. Eck, Beitr. Elektronmikr. Direktabb. Oberfl. 18 (1985) 1.