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A scanning tunnelling and transmission electron microscopy comparison of the surface structure of evaporated and ion-assisted gold films
Applied Surface Science 47 (1991) 187-191
187
North-Holland
A scanning tunnelling and transmission electron microscopy comparison of the surface structure of evaporated and ion-assisted gold films R. Bartlett, H. Jaeger, B.A. Sexton * CSIRO
Division of Materials
Science and Technology, Locked Bag 33, Clayton,
Victoria 3168, Awtralia
R.P. Netterfield and P.J. Martin CSIRO
Division of Applied Physics, P.O. Box 218, Lindfield,
Received 13 July 1990; accepted for publication
8 September
N.S. W. 2070, Australia 1990
The surface topography of evaporated and ion-assisted deposited gold transmission electron microscopy. Both techniques demonstrate that the average dimensions than that of the evaporated films. The STM is used ion-assisted films. We discuss some of the difficulties in identifying grain thin films.
The technique of ion-assisted deposition (IAD) of thin films has been used in recent years to modify the mechanical and optical properties of thin films on various substrates. Of the modification to the mechanical properties of thin films, the most commonly sought after is increased adhesion of the film to the substrate [1,2], although other effects have been reported such as increased film density [3] and reduced grain size [4]. The optical properties which have been modified include refractive index [3,4] and spectral transmittance and reflectance [5]. The technique of IAD may find useful applications in such diverse fields as microelectronics and optical design. In order to further the applications of IAD, a greater understanding of the surface and bulk modification processes of these films is required. STM has been previously proven to be a useful tool for the study of nucleation and growth of thin conducting films [6-91. Recent STM studies have * To whom correspondence 0169-4332/91/$03.50
should be addressed.
films on silica has been studied by scanning tunnelling and bulk microstructures of the ion-assisted films have lower to compare the surface roughness of the evaporated and sires and grain boundaries from STM and TEM images of
revealed information on surface roughness, defects, steps, dislocations and other surface phenomena. In a recent study of Fe,O, films Schonenberger et al. [lo] combined both TEM and STM measurements to demonstrate a good correlation between crystallite sizes as measured from the two methods. We have recently obtained a good correlation between X-ray diffraction (XRD) and STM measurements of crystallite sizes of aand /3-PbO, films [ll]. There is some interest in comparing STM and TEM as analytical tools for microstructural studies. TEM and reflection electron microscopy (REM) are well advanced techniques and have been used to image atomic scale reconstructions on gold surfaces [12-141 as well as direct imaging of atomic steps in reflection mode [l&16]. Electron microscopy also has the advantage in transmission mode of aiding the location of grain boundaries and internal defects. Some doubts remain, however, concerning the reliability of STM as a tool for interpreting bulk microstructure from
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
188
R. Bartlett et al. / STM and TEM comparison of the structure of evaporated and IAD
a surface profile. Schonenberger et al. [lo] have already discussed the effects of the STM tip radius on the film profiles in their work. In the present paper we combine STM and TEM measurements on a system in which there is a grain size difference. We were particularly interested to see whether grains could be clearly delineated by the surface profiles. In addition, we were also interested in comparing the peak-to-peak surface roughness of both films with the STM. The gold films on silica substrates were prepared in an electron beam evaporator, described previously [17]. The samples were mounted approximately 30 cm from the evaporant source, and there was no thermal control of the substrates during evaporation. The evaporated gold film was deposited at a rate of around 100 nm/min. The ion-assisted film was deposited with 100 eV 0; ions as the bombarding species, with the 0: to-Au arrival rate ratio set at 0.7. The evaporation was stopped when both films became opaque, approximately 50 nm film thickness. The STM was an air model of the single tube scanner design, and has been described elsewhere [18]. Typical imaging conditions were: tip bias -0.1 V, tunnel
Au films
current 0.2 I-IA, with the STM operating in constant current (topographic) mode. The only image enhancement used was background subtraction. The TEM was a JEOL lOOCX, and films were prepared for microscopy by detaching from the silica substrate with hydrofluoric acid and mounting on a carbon film backing. Fig. 1 shows representative examples of the two types of films under investigation as viewed by the TEM. The evaporated gold film, shown in fig. la, is polycrystalline. The majority of grains appear to be in the lo-50 nm size range. Other areas appear to be large grains of 100-200 nm size. Gold films evaporated on silica at room temperature should have a grain size around 50 nm [8]. The appearance of the larger grains in fig. la can be best explained by the preferential (111) orientation of adjacent crystallites during evaporation. In transmission mode, the grain boundaries between these preferentially oriented crystals may be obscured under a particular set of imaging conditions, giving an apparently larger grain size. The IAD film, shown in fig. lb, displays a lower average grain size, with smaller grains in the lo-30 nm size range. The effects of the ion-assisted deposition
Fig. 1. TEM micrographs of gold films on silica, showing the crystalline grain size variation between techniques in this study: (a) Electron-beam evaporation. (b) Ion-assisted deposition.
the different
deposition
R. Bartlett et al. / STM and TEh4 comparison of the structure of evaporated and IAD Au films
are manifested as a disruption of the recrystallization effects which normally occur during evaporative growth. STM results are shown in fig. 2a for the evaporated film and fig. 2c for the IAD film. Such
189
images were representative of many 400 X 400 nm of the samples. The images are shown as a gray scale, with black corresponding to the minimum height in the image and white the maximum. To allow a comparison of the STM and TEM scans
Fig. 2. Comparison of STM and TEM results, displayed at the same ma~fication,
showing the similarities and differences between the two films as detected by the imaging techniques: (a) STM gray scale image of the evaporated fiim. (b) TEM micrograph of the evaporated film, taken under conditions to reveal variations in film thickness, not internal defects. (c) !3TM gray scale image of the IAD film. (d) TEM micrograph of the IAD film.
190
R. Bartlett et al. / STM and TEM comparison of the structure of evaporated and IAD Au films
results, matching TEM micrographs, at the same magnification, but not from the same sample are displayed in fig. 2b for the evaporated film and in fig. 2d for the IAD film. The STM image, fig. 2a, of the evaporated film displays structures uniformly distributed over the surface with typical half-widths of 40-80 nm. We found no evidence of any single crystal grains larger than 100 nm in the STM surface profiles, such as the random larger crystallites observed in the TEM image, fig. la. To resolve this discrepancy, we changed the TEM imaging conditions to maximise the contrast due to film thickness variations [19], and examined the film in more detail. In fig. 2b, the film thickness variations show that the surface of the film has undulations comparable in size and distribution to that seen in the STM image, fig. 2a. Apparently, even the grains larger than 100 nm have surface structure of 40-80 nm. As discussed previously, the surfaces of the larger grains may be divided up into individual smaller grains with separating grain boundaries, not easily seen by the TEM under a single set of imaging conditions. Alternatively, the larger grains, despite being a single crystal, may have multiple mounds on the surface which are imaged by the STM as apparent grain boundaries. We tend to favour the first explanation. The STM and TEM results for the IAD films are easier to interpret. The STM image, fig. 2c, shows structures with typical half-widths of 20-40 nm, with isolated larger structures interspersed across the film. The TEM image, fig. 2d, shows the grains to be of similar dimensions to that of the STM image, in the lo-30 nm range. In this case the TEM appears to give an underestimate of the grain size. This can occur when the film thickness (50 nm) is larger than the crystallite size, with multiple crystallites and grain boundaries being viewed through the film in transmission mode. Note that this is the reverse of the case in fig. la, where the crystallite size exceeds the film thickness. In a previous paper [ll], we studied the surface structure of (Y- and p-PbO,. A correlation with data from other solid state characterisation methods was required to obtain a fuller description of the bulk microstructure of these materials. XRD
Cross Section (nm) (b:
-2 5 E 2.0 .g I”
0
200 Cross Section (nm)
4(
Fig. 3. ComparativeSTM line
scans taken from data shown in figs. 2a and 2c, showing the half-width and height of the surface features of both films: (a) Evaporated gold film. (b) Ion-assisted gold film.
data showed that the average crystallite sizes were 55 nm for o-PbO, and 20 nm for p-PbO,. STM images showed grains in the size range of lo-70 nm, and further analysis estimated the grain sizes to be 35 nm for a-PbO, and 20 nm for p-PbO,, in agreement with the XRD data. It was concluded that the grains imaged by the STM were crystallites of the PbO, emerging at the surface. The surface roughness of the evaporated and IAD films is illustrated in fig. 3, which shows typical line scans taken from the STM data. The evaporated film, fig. 3a, has larger surface features typically 60-80 nm half-width and l-2 nm in height. The IAD film, fig. 3b, has smaller surface features typically of 20-30 nm half-width and l-2 nm in height. We were unable to find a statistically significant difference in the peak-to-peak
R. Bartlett et al. / STM and TEM comparison of the structure of evaporated and IAD Au fihns
surface roughness values of the evaporated and IAD films, averaged over a large number of line scans. The IAD process, therefore, does not produce significantly smoother films, despite the smaller grain size. In conclusion, we have shown that a combination of STM and TEM is required to assess the grain size distribution on thin films. The STM measures only surface profiles, but appears to give a good correlation between the measured and expected values of grain sizes on room temperature evaporated gold films on silica. Initially, the TEM results appeared to contradict the STM profiles, but were rational&d by taking into account the imaging modes employed in the TEM. Where the crystallite size exceeds the film thickness, as in the evaporated case, some grain boundaries can be obscured, resulting in an overestimate of the grain size. Changing the imaging conditions to reveal thickness variations brings the results into line with the STM surface profiles. Similarly, where the grain size is less than the film thickness, as in the IAD films, imaging multiple grains in transmission mode gives an underestimate of the grain size, with the STM profiles showing larger average surface dimensions. When these considerations are taken into account, there are no major .discrepanties between the STM and TEM results on tihe two films. STM has the added advantage of obtaining quantitative surface roughness measurements, and we find no significant difference between the evaporated and IAD gold films o,n silica.
191
References [l] S.S. Nandra, F.G. Wilson and C.D. DesForges, Thin Solid Films 107 (1983) 335. [2] M.J. Mirtich, J. Vat. Sci. Technol. 18 (1981) 186. [3] P.J. Martin, R.P. Netterfield and W.G. Sainty, J. Appl. Phys. 55 (1984) 235. [4] P.J. Martin, H.A. Ma&sod, R.P. Netterfield, C.G. Pacey and W.G. Sainty, Appl. Opt. 22 (1983) 178. [5] G.B. Smith, Appl. Phys. Lett. 46 (1985) 716. (61 J.K. Gimzewski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951. [7] M. Salmeron, D.S. Kaufman, B. Marchon and S. Ferrer, Appl. Surf. Sci. 28 (1987) 279. [S] C.E.D. Chidsey, D.N. Lociacono, T. Sleator and S. Nakahara, Surf. Sci. 200 (1988) 45. [9] R. Emch, J. Nogami, M.M. Dovek, C.A. Lang and C.F. Quate, J. Appl. Phys. 65 (1989) 79. [lo] C. Schonenberger, S.F. Alvarado and C. Ortiz, J. Appl. Phys. 66 (1989) 4258. (111 B.A. Sexton, G.F. Cotterill, S. Fletcher and M.D. Home, J. Vat. Sci. Technol. A 8 (1990) 544. [12] T. Hasegawa, K. Koboyashi, N. Ikarashi K. Takayanagi and K. Yagi, Jpn. J. Appl. Phys. 25 (1986) L366. [13] K. Takayangi, Y. Tanishiro, K. Koboyashi, K. Akiyama and K. Yagi, Jpn. J. Appl. Phys. 26 (1987) L957. [14] N. Ikarashi, K. Kodayashi, H. Koike, H. Hasegawa and K. Yagi, Ultramicroscopy 18 (1988) 195. (151 N. Shimizu, Y. Tanishiro, K. Kobayashi, K. Takayanagi and K. Yagi, Ultramicroscopy 18 (1985) 453. [16] G. Lehmpfuhl and Y. Uchida, Ultramicroscopy 26 (1988) 177. [17] R.P. Netterfield, P.J. Martin, W.G. Sainty, R.M. Duffy and C.G. Pacey, Rev. Sci. Instr. 56 (1985) 1995. [18] B.A. Sexton and G.F. Cotterill, J. Vat. Sci. Technol. A 7 (1989) 2734. 1191 C.J. Rossouw, S.E. Donnelly and D.F. Lynch, Ultramicroscopy 16 (1985) 41.
187
North-Holland
A scanning tunnelling and transmission electron microscopy comparison of the surface structure of evaporated and ion-assisted gold films R. Bartlett, H. Jaeger, B.A. Sexton * CSIRO
Division of Materials
Science and Technology, Locked Bag 33, Clayton,
Victoria 3168, Awtralia
R.P. Netterfield and P.J. Martin CSIRO
Division of Applied Physics, P.O. Box 218, Lindfield,
Received 13 July 1990; accepted for publication
8 September
N.S. W. 2070, Australia 1990
The surface topography of evaporated and ion-assisted deposited gold transmission electron microscopy. Both techniques demonstrate that the average dimensions than that of the evaporated films. The STM is used ion-assisted films. We discuss some of the difficulties in identifying grain thin films.
The technique of ion-assisted deposition (IAD) of thin films has been used in recent years to modify the mechanical and optical properties of thin films on various substrates. Of the modification to the mechanical properties of thin films, the most commonly sought after is increased adhesion of the film to the substrate [1,2], although other effects have been reported such as increased film density [3] and reduced grain size [4]. The optical properties which have been modified include refractive index [3,4] and spectral transmittance and reflectance [5]. The technique of IAD may find useful applications in such diverse fields as microelectronics and optical design. In order to further the applications of IAD, a greater understanding of the surface and bulk modification processes of these films is required. STM has been previously proven to be a useful tool for the study of nucleation and growth of thin conducting films [6-91. Recent STM studies have * To whom correspondence 0169-4332/91/$03.50
should be addressed.
films on silica has been studied by scanning tunnelling and bulk microstructures of the ion-assisted films have lower to compare the surface roughness of the evaporated and sires and grain boundaries from STM and TEM images of
revealed information on surface roughness, defects, steps, dislocations and other surface phenomena. In a recent study of Fe,O, films Schonenberger et al. [lo] combined both TEM and STM measurements to demonstrate a good correlation between crystallite sizes as measured from the two methods. We have recently obtained a good correlation between X-ray diffraction (XRD) and STM measurements of crystallite sizes of aand /3-PbO, films [ll]. There is some interest in comparing STM and TEM as analytical tools for microstructural studies. TEM and reflection electron microscopy (REM) are well advanced techniques and have been used to image atomic scale reconstructions on gold surfaces [12-141 as well as direct imaging of atomic steps in reflection mode [l&16]. Electron microscopy also has the advantage in transmission mode of aiding the location of grain boundaries and internal defects. Some doubts remain, however, concerning the reliability of STM as a tool for interpreting bulk microstructure from
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
188
R. Bartlett et al. / STM and TEM comparison of the structure of evaporated and IAD
a surface profile. Schonenberger et al. [lo] have already discussed the effects of the STM tip radius on the film profiles in their work. In the present paper we combine STM and TEM measurements on a system in which there is a grain size difference. We were particularly interested to see whether grains could be clearly delineated by the surface profiles. In addition, we were also interested in comparing the peak-to-peak surface roughness of both films with the STM. The gold films on silica substrates were prepared in an electron beam evaporator, described previously [17]. The samples were mounted approximately 30 cm from the evaporant source, and there was no thermal control of the substrates during evaporation. The evaporated gold film was deposited at a rate of around 100 nm/min. The ion-assisted film was deposited with 100 eV 0; ions as the bombarding species, with the 0: to-Au arrival rate ratio set at 0.7. The evaporation was stopped when both films became opaque, approximately 50 nm film thickness. The STM was an air model of the single tube scanner design, and has been described elsewhere [18]. Typical imaging conditions were: tip bias -0.1 V, tunnel
Au films
current 0.2 I-IA, with the STM operating in constant current (topographic) mode. The only image enhancement used was background subtraction. The TEM was a JEOL lOOCX, and films were prepared for microscopy by detaching from the silica substrate with hydrofluoric acid and mounting on a carbon film backing. Fig. 1 shows representative examples of the two types of films under investigation as viewed by the TEM. The evaporated gold film, shown in fig. la, is polycrystalline. The majority of grains appear to be in the lo-50 nm size range. Other areas appear to be large grains of 100-200 nm size. Gold films evaporated on silica at room temperature should have a grain size around 50 nm [8]. The appearance of the larger grains in fig. la can be best explained by the preferential (111) orientation of adjacent crystallites during evaporation. In transmission mode, the grain boundaries between these preferentially oriented crystals may be obscured under a particular set of imaging conditions, giving an apparently larger grain size. The IAD film, shown in fig. lb, displays a lower average grain size, with smaller grains in the lo-30 nm size range. The effects of the ion-assisted deposition
Fig. 1. TEM micrographs of gold films on silica, showing the crystalline grain size variation between techniques in this study: (a) Electron-beam evaporation. (b) Ion-assisted deposition.
the different
deposition
R. Bartlett et al. / STM and TEh4 comparison of the structure of evaporated and IAD Au films
are manifested as a disruption of the recrystallization effects which normally occur during evaporative growth. STM results are shown in fig. 2a for the evaporated film and fig. 2c for the IAD film. Such
189
images were representative of many 400 X 400 nm of the samples. The images are shown as a gray scale, with black corresponding to the minimum height in the image and white the maximum. To allow a comparison of the STM and TEM scans
Fig. 2. Comparison of STM and TEM results, displayed at the same ma~fication,
showing the similarities and differences between the two films as detected by the imaging techniques: (a) STM gray scale image of the evaporated fiim. (b) TEM micrograph of the evaporated film, taken under conditions to reveal variations in film thickness, not internal defects. (c) !3TM gray scale image of the IAD film. (d) TEM micrograph of the IAD film.
190
R. Bartlett et al. / STM and TEM comparison of the structure of evaporated and IAD Au films
results, matching TEM micrographs, at the same magnification, but not from the same sample are displayed in fig. 2b for the evaporated film and in fig. 2d for the IAD film. The STM image, fig. 2a, of the evaporated film displays structures uniformly distributed over the surface with typical half-widths of 40-80 nm. We found no evidence of any single crystal grains larger than 100 nm in the STM surface profiles, such as the random larger crystallites observed in the TEM image, fig. la. To resolve this discrepancy, we changed the TEM imaging conditions to maximise the contrast due to film thickness variations [19], and examined the film in more detail. In fig. 2b, the film thickness variations show that the surface of the film has undulations comparable in size and distribution to that seen in the STM image, fig. 2a. Apparently, even the grains larger than 100 nm have surface structure of 40-80 nm. As discussed previously, the surfaces of the larger grains may be divided up into individual smaller grains with separating grain boundaries, not easily seen by the TEM under a single set of imaging conditions. Alternatively, the larger grains, despite being a single crystal, may have multiple mounds on the surface which are imaged by the STM as apparent grain boundaries. We tend to favour the first explanation. The STM and TEM results for the IAD films are easier to interpret. The STM image, fig. 2c, shows structures with typical half-widths of 20-40 nm, with isolated larger structures interspersed across the film. The TEM image, fig. 2d, shows the grains to be of similar dimensions to that of the STM image, in the lo-30 nm range. In this case the TEM appears to give an underestimate of the grain size. This can occur when the film thickness (50 nm) is larger than the crystallite size, with multiple crystallites and grain boundaries being viewed through the film in transmission mode. Note that this is the reverse of the case in fig. la, where the crystallite size exceeds the film thickness. In a previous paper [ll], we studied the surface structure of (Y- and p-PbO,. A correlation with data from other solid state characterisation methods was required to obtain a fuller description of the bulk microstructure of these materials. XRD
Cross Section (nm) (b:
-2 5 E 2.0 .g I”
0
200 Cross Section (nm)
4(
Fig. 3. ComparativeSTM line
scans taken from data shown in figs. 2a and 2c, showing the half-width and height of the surface features of both films: (a) Evaporated gold film. (b) Ion-assisted gold film.
data showed that the average crystallite sizes were 55 nm for o-PbO, and 20 nm for p-PbO,. STM images showed grains in the size range of lo-70 nm, and further analysis estimated the grain sizes to be 35 nm for a-PbO, and 20 nm for p-PbO,, in agreement with the XRD data. It was concluded that the grains imaged by the STM were crystallites of the PbO, emerging at the surface. The surface roughness of the evaporated and IAD films is illustrated in fig. 3, which shows typical line scans taken from the STM data. The evaporated film, fig. 3a, has larger surface features typically 60-80 nm half-width and l-2 nm in height. The IAD film, fig. 3b, has smaller surface features typically of 20-30 nm half-width and l-2 nm in height. We were unable to find a statistically significant difference in the peak-to-peak
R. Bartlett et al. / STM and TEM comparison of the structure of evaporated and IAD Au fihns
surface roughness values of the evaporated and IAD films, averaged over a large number of line scans. The IAD process, therefore, does not produce significantly smoother films, despite the smaller grain size. In conclusion, we have shown that a combination of STM and TEM is required to assess the grain size distribution on thin films. The STM measures only surface profiles, but appears to give a good correlation between the measured and expected values of grain sizes on room temperature evaporated gold films on silica. Initially, the TEM results appeared to contradict the STM profiles, but were rational&d by taking into account the imaging modes employed in the TEM. Where the crystallite size exceeds the film thickness, as in the evaporated case, some grain boundaries can be obscured, resulting in an overestimate of the grain size. Changing the imaging conditions to reveal thickness variations brings the results into line with the STM surface profiles. Similarly, where the grain size is less than the film thickness, as in the IAD films, imaging multiple grains in transmission mode gives an underestimate of the grain size, with the STM profiles showing larger average surface dimensions. When these considerations are taken into account, there are no major .discrepanties between the STM and TEM results on tihe two films. STM has the added advantage of obtaining quantitative surface roughness measurements, and we find no significant difference between the evaporated and IAD gold films o,n silica.
191
References [l] S.S. Nandra, F.G. Wilson and C.D. DesForges, Thin Solid Films 107 (1983) 335. [2] M.J. Mirtich, J. Vat. Sci. Technol. 18 (1981) 186. [3] P.J. Martin, R.P. Netterfield and W.G. Sainty, J. Appl. Phys. 55 (1984) 235. [4] P.J. Martin, H.A. Ma&sod, R.P. Netterfield, C.G. Pacey and W.G. Sainty, Appl. Opt. 22 (1983) 178. [5] G.B. Smith, Appl. Phys. Lett. 46 (1985) 716. (61 J.K. Gimzewski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951. [7] M. Salmeron, D.S. Kaufman, B. Marchon and S. Ferrer, Appl. Surf. Sci. 28 (1987) 279. [S] C.E.D. Chidsey, D.N. Lociacono, T. Sleator and S. Nakahara, Surf. Sci. 200 (1988) 45. [9] R. Emch, J. Nogami, M.M. Dovek, C.A. Lang and C.F. Quate, J. Appl. Phys. 65 (1989) 79. [lo] C. Schonenberger, S.F. Alvarado and C. Ortiz, J. Appl. Phys. 66 (1989) 4258. (111 B.A. Sexton, G.F. Cotterill, S. Fletcher and M.D. Home, J. Vat. Sci. Technol. A 8 (1990) 544. [12] T. Hasegawa, K. Koboyashi, N. Ikarashi K. Takayanagi and K. Yagi, Jpn. J. Appl. Phys. 25 (1986) L366. [13] K. Takayangi, Y. Tanishiro, K. Koboyashi, K. Akiyama and K. Yagi, Jpn. J. Appl. Phys. 26 (1987) L957. [14] N. Ikarashi, K. Kodayashi, H. Koike, H. Hasegawa and K. Yagi, Ultramicroscopy 18 (1988) 195. (151 N. Shimizu, Y. Tanishiro, K. Kobayashi, K. Takayanagi and K. Yagi, Ultramicroscopy 18 (1985) 453. [16] G. Lehmpfuhl and Y. Uchida, Ultramicroscopy 26 (1988) 177. [17] R.P. Netterfield, P.J. Martin, W.G. Sainty, R.M. Duffy and C.G. Pacey, Rev. Sci. Instr. 56 (1985) 1995. [18] B.A. Sexton and G.F. Cotterill, J. Vat. Sci. Technol. A 7 (1989) 2734. 1191 C.J. Rossouw, S.E. Donnelly and D.F. Lynch, Ultramicroscopy 16 (1985) 41.