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Topographic study of thin gold films grown on SiO2
Ultramicroscopy 42-44 (1992) 1317-1320 North-Holland
Topographic study of thin gold films grown
on SiO 2
L. S t o c k m a n , H. V l o e b e r g h s , I. H e y v a e r t , C. V a n H a e s e n d o n c k a n d Y. B r u y n s e r a e d e Laboratorium voor Vaste Stof-Fysika en Magnetisme, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Received 12 August 1991
The surface topography of thin Au films (thickness ~ 10 nm) has been studied by STM in air. The films are obtained by flash evaporation onto liquid-nitrogen cooled, oxidized Si substrates. The polycrystalline layers have a grain size comparable to the thickness. The presence of cracks and holes near the grain boundaries, where the films tend to become discontinuous, is also confirmed by transmission electron microscopy. The film smoothness and continuity can be improved dramatically by evaporating the Au layers in a reduced O 2 or He atmosphere. The topographic information is also reflected by the electrical stability of submicron lines (width comparable to the grain size).
I. Introduction
The quality of submicron metal lines strongly depends on the microscopic structure of the thin-film material, which is used for the fabrication of the lines. We have made a detailed study of the structural properties of fine lines, which are obtained by a combination of electron beam lithography and lift-off techniques [1]. In fig. 1 we show the electron micrograph for a typical Au loop, which is used to study the electron interference occurring on a mesoscopic scale, i.e., on a scale which is comparable to the phase coherence length for the electron waves [2]. For thin metal films, with a thickness of 20 nm and a resistance per square of a few Ohm, the phase coherence length will be of the order of 1 ~ m at low temperatures. The loop in fig. 1 shows pronounced magnetoconductance oscillations, which directly reflect the electron interference effects
Fig. 1. Electron micrograph for a mesoscopic Au loop which is used to study the electron interference effects. The marker corresponds to 1 ~ m .
[31. The loop structures turn out to be extremely effective electrical fuses. The Au loops tend to "explode" near the weak links in the metal lines when parasitic noise voltages are picked up by the measuring circuit. The most obvious candidate for a weak link is a grain boundary which runs across a line segment of the loop. Therefore,
discontinuities in the surface topography of the polycrystalline Au films have to be avoided. Previous STM studies have already focussed on the topography at nanometer level of thin Au films [4,5]. In this contribution, we will show that STM is a very powerful tool to detect the presence of cracks or holes in the fine-line samples. It
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1318
L. Stockman et al. / Topography of thin gold films on SiO 2
is extremely difficult to trace back the loop structure on the oxidized Si substrate. Moreover, the operation of the STM may easily destroy the fine lines. In order to circumvent this problem, we usually obtain our STM pictures for the wider contact pads (1 to 100/~m) which are connected to the loop. These pictures should still provide a reliable image of the grain structure of the submicron lines. Since we wanted to study the influence of spin scattering on the interference occurring in the loop structures, we had to dope the Au films intentionally with a very small amount ( ~ 100 ppm) of Fe impurities. The small amount of Fe impurities is not expected to alter considerably the film topography. In order to obtain homogeneously doped Au layers, a flash evaporation technique is used. For a low background pressure the doped Au films with a thickness ~ 10 nm are not perfectly continuous. This implies the presence of many cracks and holes near the grain boundaries of the polycrystalline structure [6]. We will show that flash evaporation in a residual O 2 or He atmosphere strongly improves the film smoothness and continuity.
2. Experimental procedure The samples were obtained by flash evaporation of small pieces (1 mg) of the bulk Au (alloy) from a resistively heated Mo boat. The substrates were Si wafers on which a 50 nm thick SiO 2 oxide layer had been grown. The deposition was controlled with a quartz-crystal thickness monitor, which was calibrated using a thickness profile meter (Dektak IIA). During the evaporation, the substrate was cooled down to liquid-nitrogen temperature to favor the growth of continuous films. A first series of films were evaporated in a residual pressure of about 5 x 10 - 6 Pa. A second series of Au films were obtained by evaporation in a residual gas atmosphere of a very pure gas, either 0 2 (99.998% purity) or He (99.9999% purity). The film topography was characterized in air using a Nanoscope II (Digital Instruments) STM system, with a single-tube piezo to control the tip
motion. All images were taken with a constant tunneling current of 2 nA and a bias voltage of 25 mg.
We have also used transmission electron microscopy (TEM) in order to obtain additional information concerning the microscopic film structure. The T E M samples were evaporated simultaneously with the STM samples onto electron transparant windows defined in the Si wafer.
3. Experimental results STM images of the films evaporated in a residual pressure of 5 x 10 -6 Pa reveal a hilly topography. The lateral size of the Au grains is of the order of 15 nm. Fig. 2a shows a section of an image for a film with a thickness of 11.6 rim. We find that the rolling surface is disturbed by a series of cracks and holes (black areas), where the films tend to become electrically discontinuous. This is confirmed by the fact that the height difference between top and bottom of the image near the discontinuity exceeds the average film thickness. When the film thickness is increased to 19.5 nm, we again find the hilly structure with deep valleys near the grain boundaries, which tend to indicate that cracks and holes are still present. From the STM measurements we find that the depth of the cracks and holes is limited to a maximum value of about 12 nm, i.e. considerably smaller than the average film thickness. This limited depth can be accounted for by the finite size of the tip, which cannot penetrate completely the film discontinuities [7,8]. As shown in fig. 2b, the T E M measurements confirm the presence of the cracks and holes (white areas) in the film structure. We also find that the density of cracks and holes rapidly decreases with increasing film thickness. For a film thickness exceeding 35 nm the discontinuities have almost completely disappeared. The films, which have been evaporated in a residual gas atmosphere, reveal a much smoother topography. On the other hand, the grain size distribution has become much wider and the grains do no longer appear as spherical particles.
L. Stockman et aL / Topography of thin gold films on SiO 2
1319
3O 20 10 O0
10 50
100
150 NM
Fig. 2. (a) STM picture and linear cross-section for a 11.6 nm thick Au film, deposited in a residual pressure of 5 :x: 10 -6 Pa. The black regions correspond to film discontinuities. (b) TEM picture (the marker correponds to 100 nm) for a 19.5 nm thick Au film, deposited under similar conditions. The white regions correspond to film discontinuities.
O0
100
200 rim
Fig. 3. (a) STM picture and linear cross-section for a 13.3 nm thick Au film, deposited in a partial oxygen atmosphere of 5)< 10 -4 Pa. (b) TEM picture (the white marker corresponds to 100 nm) for a 19.5 nm thick Au film, deposited under the same conditions.
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L. Stockman et al. / Topography of thin gold films on SiO 2
The average grain size is, however, still of the order of the film thickness. Fig. 3a illustrates that for a 13.3 nm thick Au film, which has been evaporated in 0 2 pressure of about 5 x 10 - 4 Pa, the height variations do not exceed 5 nm. For a 19.5 nm thick film, which has also been evaporated in 0 2, the height variations increase to a maximum of 7 nm. The T E M picture for the latter Au film (see fig. 3b) confirms the absence of any discontinuity. The much improved smoothness for the films evaporated in a gas atmosphere may result from two effects. First of all, the collisions between the evaporated Au atoms and the gas particles will reduce the energy of the deposited Au atoms. As a result of their smaller energy, the Au atoms can no longer occupy the positions corresponding to the minimum surface energy and the droplet formation will be inhibited. On the other hand, in the residual gas atmosphere the evaporation source can no longer be considered as a point source. The Au atoms will acquire an isotropic velocity distribution and they do no longer impinge on the surface under a constant angle. We note that a similar improvement of the film smoothness is also observed for films, which are sputtered in an Ar atmosphere. Finally, we find that the flash evaporation in a residual gas atmosphere considerably improves the electrical stability and the lifetime of the fine-line samples. "Explosions" of the samples, caused by the pick-up of parasitic voltages, will now mainly occur in the broader contact area, confirming the homogeneous, continuous film structure of the fine lines.
4. Conclusions We have probed with the STM the surface quality of fine Au lines prepared by flash evaporation on a SiO 2 layer. Samples, which are evaporated in a vacuum ~ 5 × 10 - 6 Pa and have a thickness of about 10 nm, reveal a hilly topogra-
phy, which is disturbed by a series of cracks and holes. As confirmed by transmission electron microscopy, the cracks and holes are still present when the film thickness is increased to 20 nm. At this point, the STM tip can no longer fully penetrate the film discontinuities. The samples, which have been prepared in a reduced 0 2 or He atmosphere, reveal a much improved surface smoothness, while the cracks and holes are no longer present.
Acknowledgements The authors are much indebted to T. Krekels and S. Van Tendeloo for the T E M measurements. They also would like to thank the Belgian Fund for Medical Scientific Research (FGWO), the Belgian Inter-University Attraction Poles (IUAP), as well as the Research Council of the Katholieke Universiteit Leuven for financial support. C.V.H. is a Research Associate of the Belgian National Fund for Scientific Research (NFWO)
References [1] A.H. Verbruggen, P.A.M. Holweg, H. Vloeberghs, C. Van Haesendonck, J. Romijn, S. Radelaar and Y. Bruynseraede, Microelectron. Eng. 13 (1991) 407. [2] S. Washburn and R.A. Webb, Adv. Phys. 35 (1986) 375. [3] A.H. Verbruggen, H. Vloeberghs, P.A.M. Holweg, C. Van Haesendonck, S. Radelaar and Y. Bruynseraede, to be published. [4] R.C. Jaklevic, L. Elie, Weidian Shen and J.T. Chen, Appl. Phys. Lett. 52 (1988) 1656. [5] C. Sch6nenberger, S.F. Alvarado and C. Ortiz, J. Appl. Phys. 66 (1989) 4258. [6] C.A. Neugebauer, in: Handbook of Thin Film Technology, Eds. L.I. Maissel and R. Glang (McGraw-Hill, New York, 1970) ch. 8. [7] G. Reiss, J. Vancea, H. Wittmann, J. Zweck and H. Hoffmann, J. Appl. Phys. 67 (1990) 1156. [8] J.K. Gimzewski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951.
Topographic study of thin gold films grown
on SiO 2
L. S t o c k m a n , H. V l o e b e r g h s , I. H e y v a e r t , C. V a n H a e s e n d o n c k a n d Y. B r u y n s e r a e d e Laboratorium voor Vaste Stof-Fysika en Magnetisme, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Received 12 August 1991
The surface topography of thin Au films (thickness ~ 10 nm) has been studied by STM in air. The films are obtained by flash evaporation onto liquid-nitrogen cooled, oxidized Si substrates. The polycrystalline layers have a grain size comparable to the thickness. The presence of cracks and holes near the grain boundaries, where the films tend to become discontinuous, is also confirmed by transmission electron microscopy. The film smoothness and continuity can be improved dramatically by evaporating the Au layers in a reduced O 2 or He atmosphere. The topographic information is also reflected by the electrical stability of submicron lines (width comparable to the grain size).
I. Introduction
The quality of submicron metal lines strongly depends on the microscopic structure of the thin-film material, which is used for the fabrication of the lines. We have made a detailed study of the structural properties of fine lines, which are obtained by a combination of electron beam lithography and lift-off techniques [1]. In fig. 1 we show the electron micrograph for a typical Au loop, which is used to study the electron interference occurring on a mesoscopic scale, i.e., on a scale which is comparable to the phase coherence length for the electron waves [2]. For thin metal films, with a thickness of 20 nm and a resistance per square of a few Ohm, the phase coherence length will be of the order of 1 ~ m at low temperatures. The loop in fig. 1 shows pronounced magnetoconductance oscillations, which directly reflect the electron interference effects
Fig. 1. Electron micrograph for a mesoscopic Au loop which is used to study the electron interference effects. The marker corresponds to 1 ~ m .
[31. The loop structures turn out to be extremely effective electrical fuses. The Au loops tend to "explode" near the weak links in the metal lines when parasitic noise voltages are picked up by the measuring circuit. The most obvious candidate for a weak link is a grain boundary which runs across a line segment of the loop. Therefore,
discontinuities in the surface topography of the polycrystalline Au films have to be avoided. Previous STM studies have already focussed on the topography at nanometer level of thin Au films [4,5]. In this contribution, we will show that STM is a very powerful tool to detect the presence of cracks or holes in the fine-line samples. It
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1318
L. Stockman et al. / Topography of thin gold films on SiO 2
is extremely difficult to trace back the loop structure on the oxidized Si substrate. Moreover, the operation of the STM may easily destroy the fine lines. In order to circumvent this problem, we usually obtain our STM pictures for the wider contact pads (1 to 100/~m) which are connected to the loop. These pictures should still provide a reliable image of the grain structure of the submicron lines. Since we wanted to study the influence of spin scattering on the interference occurring in the loop structures, we had to dope the Au films intentionally with a very small amount ( ~ 100 ppm) of Fe impurities. The small amount of Fe impurities is not expected to alter considerably the film topography. In order to obtain homogeneously doped Au layers, a flash evaporation technique is used. For a low background pressure the doped Au films with a thickness ~ 10 nm are not perfectly continuous. This implies the presence of many cracks and holes near the grain boundaries of the polycrystalline structure [6]. We will show that flash evaporation in a residual O 2 or He atmosphere strongly improves the film smoothness and continuity.
2. Experimental procedure The samples were obtained by flash evaporation of small pieces (1 mg) of the bulk Au (alloy) from a resistively heated Mo boat. The substrates were Si wafers on which a 50 nm thick SiO 2 oxide layer had been grown. The deposition was controlled with a quartz-crystal thickness monitor, which was calibrated using a thickness profile meter (Dektak IIA). During the evaporation, the substrate was cooled down to liquid-nitrogen temperature to favor the growth of continuous films. A first series of films were evaporated in a residual pressure of about 5 x 10 - 6 Pa. A second series of Au films were obtained by evaporation in a residual gas atmosphere of a very pure gas, either 0 2 (99.998% purity) or He (99.9999% purity). The film topography was characterized in air using a Nanoscope II (Digital Instruments) STM system, with a single-tube piezo to control the tip
motion. All images were taken with a constant tunneling current of 2 nA and a bias voltage of 25 mg.
We have also used transmission electron microscopy (TEM) in order to obtain additional information concerning the microscopic film structure. The T E M samples were evaporated simultaneously with the STM samples onto electron transparant windows defined in the Si wafer.
3. Experimental results STM images of the films evaporated in a residual pressure of 5 x 10 -6 Pa reveal a hilly topography. The lateral size of the Au grains is of the order of 15 nm. Fig. 2a shows a section of an image for a film with a thickness of 11.6 rim. We find that the rolling surface is disturbed by a series of cracks and holes (black areas), where the films tend to become electrically discontinuous. This is confirmed by the fact that the height difference between top and bottom of the image near the discontinuity exceeds the average film thickness. When the film thickness is increased to 19.5 nm, we again find the hilly structure with deep valleys near the grain boundaries, which tend to indicate that cracks and holes are still present. From the STM measurements we find that the depth of the cracks and holes is limited to a maximum value of about 12 nm, i.e. considerably smaller than the average film thickness. This limited depth can be accounted for by the finite size of the tip, which cannot penetrate completely the film discontinuities [7,8]. As shown in fig. 2b, the T E M measurements confirm the presence of the cracks and holes (white areas) in the film structure. We also find that the density of cracks and holes rapidly decreases with increasing film thickness. For a film thickness exceeding 35 nm the discontinuities have almost completely disappeared. The films, which have been evaporated in a residual gas atmosphere, reveal a much smoother topography. On the other hand, the grain size distribution has become much wider and the grains do no longer appear as spherical particles.
L. Stockman et aL / Topography of thin gold films on SiO 2
1319
3O 20 10 O0
10 50
100
150 NM
Fig. 2. (a) STM picture and linear cross-section for a 11.6 nm thick Au film, deposited in a residual pressure of 5 :x: 10 -6 Pa. The black regions correspond to film discontinuities. (b) TEM picture (the marker correponds to 100 nm) for a 19.5 nm thick Au film, deposited under similar conditions. The white regions correspond to film discontinuities.
O0
100
200 rim
Fig. 3. (a) STM picture and linear cross-section for a 13.3 nm thick Au film, deposited in a partial oxygen atmosphere of 5)< 10 -4 Pa. (b) TEM picture (the white marker corresponds to 100 nm) for a 19.5 nm thick Au film, deposited under the same conditions.
1320
L. Stockman et al. / Topography of thin gold films on SiO 2
The average grain size is, however, still of the order of the film thickness. Fig. 3a illustrates that for a 13.3 nm thick Au film, which has been evaporated in 0 2 pressure of about 5 x 10 - 4 Pa, the height variations do not exceed 5 nm. For a 19.5 nm thick film, which has also been evaporated in 0 2, the height variations increase to a maximum of 7 nm. The T E M picture for the latter Au film (see fig. 3b) confirms the absence of any discontinuity. The much improved smoothness for the films evaporated in a gas atmosphere may result from two effects. First of all, the collisions between the evaporated Au atoms and the gas particles will reduce the energy of the deposited Au atoms. As a result of their smaller energy, the Au atoms can no longer occupy the positions corresponding to the minimum surface energy and the droplet formation will be inhibited. On the other hand, in the residual gas atmosphere the evaporation source can no longer be considered as a point source. The Au atoms will acquire an isotropic velocity distribution and they do no longer impinge on the surface under a constant angle. We note that a similar improvement of the film smoothness is also observed for films, which are sputtered in an Ar atmosphere. Finally, we find that the flash evaporation in a residual gas atmosphere considerably improves the electrical stability and the lifetime of the fine-line samples. "Explosions" of the samples, caused by the pick-up of parasitic voltages, will now mainly occur in the broader contact area, confirming the homogeneous, continuous film structure of the fine lines.
4. Conclusions We have probed with the STM the surface quality of fine Au lines prepared by flash evaporation on a SiO 2 layer. Samples, which are evaporated in a vacuum ~ 5 × 10 - 6 Pa and have a thickness of about 10 nm, reveal a hilly topogra-
phy, which is disturbed by a series of cracks and holes. As confirmed by transmission electron microscopy, the cracks and holes are still present when the film thickness is increased to 20 nm. At this point, the STM tip can no longer fully penetrate the film discontinuities. The samples, which have been prepared in a reduced 0 2 or He atmosphere, reveal a much improved surface smoothness, while the cracks and holes are no longer present.
Acknowledgements The authors are much indebted to T. Krekels and S. Van Tendeloo for the T E M measurements. They also would like to thank the Belgian Fund for Medical Scientific Research (FGWO), the Belgian Inter-University Attraction Poles (IUAP), as well as the Research Council of the Katholieke Universiteit Leuven for financial support. C.V.H. is a Research Associate of the Belgian National Fund for Scientific Research (NFWO)
References [1] A.H. Verbruggen, P.A.M. Holweg, H. Vloeberghs, C. Van Haesendonck, J. Romijn, S. Radelaar and Y. Bruynseraede, Microelectron. Eng. 13 (1991) 407. [2] S. Washburn and R.A. Webb, Adv. Phys. 35 (1986) 375. [3] A.H. Verbruggen, H. Vloeberghs, P.A.M. Holweg, C. Van Haesendonck, S. Radelaar and Y. Bruynseraede, to be published. [4] R.C. Jaklevic, L. Elie, Weidian Shen and J.T. Chen, Appl. Phys. Lett. 52 (1988) 1656. [5] C. Sch6nenberger, S.F. Alvarado and C. Ortiz, J. Appl. Phys. 66 (1989) 4258. [6] C.A. Neugebauer, in: Handbook of Thin Film Technology, Eds. L.I. Maissel and R. Glang (McGraw-Hill, New York, 1970) ch. 8. [7] G. Reiss, J. Vancea, H. Wittmann, J. Zweck and H. Hoffmann, J. Appl. Phys. 67 (1990) 1156. [8] J.K. Gimzewski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951.