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Permeable diamond membranes
Diamond
and Related
Materials
4 (1995) 1069-1072
Permeable diamond membranes M.C. Salvadori, Y. Miyao, G. Moscati Institute of Physics, Sdo Paul0 University, C.P. 66318, CEP 05389-970, Sdo Paulo, SP, Brazil
Received 19 August 1994; accepted in final form 11 January 1995
Abstract The unique properties of diamond films open up a broad range of potential applications. Permeable diamond membranes would find good use, for example, as filter materials, due to their chemically inert and mechanically resistant nature, and for heat exchangers, since diamond has the highest thermal conductivity of all materials at room temperature. In the work presented here, diamond films were grown by microwave plasma-assisted chemical vapour deposition. The substrate used was molybdenum and the voids were obtained by sprinkling molybdenum powder on the substrate prior to growth. To increase the diamond nucleation, some diamond powder was also sprinkled on the substrate. After growth, the molybdenum was chemically removed. Our results show that it is possible to produce self-supporting permeable diamond membranes (SSPDMs) by chemical vapour deposition, with control of the membrane thickness and the void density and diameter. Keywords: Diamond; Membranes; Free-standing;
CVD microwave
1. Introduction Permeable diamond membranes are highly desirable because of the unique properties of diamond [ 11: (a) it is chemically inert; (b) it is the hardest material known; (c) it has the highest thermal conductivity of all materials (20 W cm-’ K-‘) at room temperature, about five times larger than that of copper; (d) its thermal expansion coefficient is small (0.8 x 10e6 K-l at 293 K); (e) it is transparent from the IR to the visible region; (f) it is a good electrical insulator, but can be doped to produce a semiconductor. Some potential applications for permeable diamond membranes include bacteriological filters, filtration media in purification processes, as heat exchangers and as inert supports for other fragile membranes.
2. Experimental details The substrate used for diamond membrane growth was a molybdenum wafer smoothed by sandpaper. The preparation of the substrate involved the following three steps: (a) cleaning in an acetone ultrasonic bath; (b) sprinkling with 1 pm diamond powder on the surface (submerging the substrate in a mixture of water, alcohol and diamond powder in an ultrasonic bath for 5 min 0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00277-4
and drying in a stove); (c) sprinkling with molybdenum powder (grain size, 60-90 pm) on the surface. Originally, the molybdenum powder had a broad distribution of grain sizes. The grain size was selected by sieves, the first to remove grains smaller than about 60 pm, and the second to retain grains greater than about 90 pm. The equipment used for diamond synthesis was a microwave plasma-assisted chemical vapour deposition system [a]. The deposition parameters were as follows: hydrogen flow rate, 300 standard cubic centimetres per minute (seem); methane flow rate, 1.5 seem (0.5 vol.% methane); chamber pressure, 70 Torr; sample temperature, 850 “C; microwave power, 520 W. After deposition, the diamond surface was scratched by sandpaper to remove the diamond grown on the molybdenum powder. Finally, the molybdenum was chemically removed in a solution of three parts hydrochloric acid to one part nitric acid. The characterization techniques used in this work were scanning electron microscopy (SEM) and Raman spectroscopy.
3. Results and discussion The idea is to create voids in the diamond film by sprinkling powder on the substrate (grain size greater
M. C. Salvadori et al.lDiamond und Related Materials 4 (1995) 1069-1072
1070
than the film thickness) prior to growth. We used a powder of the same material as the substrate, so that the grains could be dissolved by the same process used to remove the substrate. Some diamond powder was also sprinkled on the substrate so as to increase the diamond nucleation. Fig. 1 shows the diamond film surface after growth. It consists of a conventional diamond film and some clusters on the molybdenum grains. If the substrate is dissolved at this stage, a film with small cavities is obtained. To avoid this result, it is necessary to scratch the surface with sandpaper so as to remove the diamond growth on the molybdenum
Fig. 1. Diamond
Fig. 2. Diamond
film grown
film scratched
on the molybdenum
by sandpaper
powder. The scratched surface is shown in Fig. 2, where it can be seen that some diamond clusters on molybdenum grains were not removed. Subsequently, the molybdenum, substrate and powder were chemically removed and a permeable diamond membrane was obtained. Fig. 3 shows a panoramic view of the membrane back side, where the shape is a replica of the substrate, revealing the scratches produced by the sandpaper during substrate preparation. Fig. 4 shows a top view detail of a void of about 60 urn in diameter. The membrane thickness can be controlled by the growth time, but it cannot exceed the grain size of the
substrate
to remove
plus molybdenum
the diamond
growth
powder
sprinkled
on the molybdenum
on it.
powder.
M. C. Salvadori et al./Diamond and Related Materials 4 (1995) 1069-1072
Fig. 3. A panoramic
view of the membrane
Fig. 4. Top view detail of a void of about
powder that produces the voids. Specifically, the membrane shown in Figs. 3 and 4 is about 40 pm thick and its growth time was 24 h. The void density can be controlled by the amount of molybdenum powder sprinkled on the substrate before growth, and the void diameter can be controlled by selecting the powder grain size with sieves. In the case
1071
back side.
60 pm in diameter.
described here, the grain size was between 60 and 90 pm and the void density was about 10 voids mme2. Raman spectroscopy showed the same diamond quality on the top and on the back side. Fig. 5 shows a typical Raman spectrum for a permeable diamond membrane made by the process described here. The presence of diamond is indicated by the peak at 1333 cm-‘, and
M. C. Salvadori et aLlDiamond and Related Materials 4 (1995) 1069-1072
1072
Raman
density and uniformity attainable. Other substrates and template production methods are being considered. In the use of solid materials in science and technology, it is important to have control over the geometry either by forming procedures or by machining. With diamond membranes, due to the difficulties in performing machining after production, it is desirable to explore shaping procedures at the forming stage. Further developments in the improved control of membrane characteristics should result in a new construction element that may find application in different areas of technology in which an inert, self-supporting, permeable membrane is desirable.
shift kni’)
Fig. 5. A typical Raman spectrum for a permeable diamond membrane made by the process described here.
the small broad band near 1530cm-l presence of some amorphous carbon.
4.
indicates the
Conclusions
Our results show that it is possible to produce selfsupporting permeable diamond membranes (SSPDMs) by chemical vapour deposition, controlling the membrane thickness and the void density and diameter. Specifically, in this work, the void diameter was between 60 and 90 pm, the void density was about 10 voids mm-’ and the thickness was about 40 urn.
Acknowledgments This work was supported by the Fundacao de Amparo a Pesquisa do Estado de Sao Paulo and the Instituto de Fisica da Universidade de S%o Paulo, Brazil. We are grateful to Ian G. Brown from Lawrence Berkeley Laboratory for helpful discussions and to Joel Camargo Rubim for the Raman spectroscopy.
References [l]
5.
Forthcoming work
Further studies are in progress to investigate the thickness, area, mechanical strength, void diameter, void
J.C. Angus, F.A. Buck, M. Sunkara, T.F. Groth, C.C. Hayman and R. Gat, Diamond growth at low pressures, Mater, Rex Sot.
Bull., October (1989). [Z] MC. Salvadori, V.P.
Mammana, O.G. Martins and F.T. Degasperi, Plasma-assisted chemical vapor deposition in a tunable microwave cavity, Plasma Sources Sci. Technol., in press.
and Related
Materials
4 (1995) 1069-1072
Permeable diamond membranes M.C. Salvadori, Y. Miyao, G. Moscati Institute of Physics, Sdo Paul0 University, C.P. 66318, CEP 05389-970, Sdo Paulo, SP, Brazil
Received 19 August 1994; accepted in final form 11 January 1995
Abstract The unique properties of diamond films open up a broad range of potential applications. Permeable diamond membranes would find good use, for example, as filter materials, due to their chemically inert and mechanically resistant nature, and for heat exchangers, since diamond has the highest thermal conductivity of all materials at room temperature. In the work presented here, diamond films were grown by microwave plasma-assisted chemical vapour deposition. The substrate used was molybdenum and the voids were obtained by sprinkling molybdenum powder on the substrate prior to growth. To increase the diamond nucleation, some diamond powder was also sprinkled on the substrate. After growth, the molybdenum was chemically removed. Our results show that it is possible to produce self-supporting permeable diamond membranes (SSPDMs) by chemical vapour deposition, with control of the membrane thickness and the void density and diameter. Keywords: Diamond; Membranes; Free-standing;
CVD microwave
1. Introduction Permeable diamond membranes are highly desirable because of the unique properties of diamond [ 11: (a) it is chemically inert; (b) it is the hardest material known; (c) it has the highest thermal conductivity of all materials (20 W cm-’ K-‘) at room temperature, about five times larger than that of copper; (d) its thermal expansion coefficient is small (0.8 x 10e6 K-l at 293 K); (e) it is transparent from the IR to the visible region; (f) it is a good electrical insulator, but can be doped to produce a semiconductor. Some potential applications for permeable diamond membranes include bacteriological filters, filtration media in purification processes, as heat exchangers and as inert supports for other fragile membranes.
2. Experimental details The substrate used for diamond membrane growth was a molybdenum wafer smoothed by sandpaper. The preparation of the substrate involved the following three steps: (a) cleaning in an acetone ultrasonic bath; (b) sprinkling with 1 pm diamond powder on the surface (submerging the substrate in a mixture of water, alcohol and diamond powder in an ultrasonic bath for 5 min 0925-9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00277-4
and drying in a stove); (c) sprinkling with molybdenum powder (grain size, 60-90 pm) on the surface. Originally, the molybdenum powder had a broad distribution of grain sizes. The grain size was selected by sieves, the first to remove grains smaller than about 60 pm, and the second to retain grains greater than about 90 pm. The equipment used for diamond synthesis was a microwave plasma-assisted chemical vapour deposition system [a]. The deposition parameters were as follows: hydrogen flow rate, 300 standard cubic centimetres per minute (seem); methane flow rate, 1.5 seem (0.5 vol.% methane); chamber pressure, 70 Torr; sample temperature, 850 “C; microwave power, 520 W. After deposition, the diamond surface was scratched by sandpaper to remove the diamond grown on the molybdenum powder. Finally, the molybdenum was chemically removed in a solution of three parts hydrochloric acid to one part nitric acid. The characterization techniques used in this work were scanning electron microscopy (SEM) and Raman spectroscopy.
3. Results and discussion The idea is to create voids in the diamond film by sprinkling powder on the substrate (grain size greater
M. C. Salvadori et al.lDiamond und Related Materials 4 (1995) 1069-1072
1070
than the film thickness) prior to growth. We used a powder of the same material as the substrate, so that the grains could be dissolved by the same process used to remove the substrate. Some diamond powder was also sprinkled on the substrate so as to increase the diamond nucleation. Fig. 1 shows the diamond film surface after growth. It consists of a conventional diamond film and some clusters on the molybdenum grains. If the substrate is dissolved at this stage, a film with small cavities is obtained. To avoid this result, it is necessary to scratch the surface with sandpaper so as to remove the diamond growth on the molybdenum
Fig. 1. Diamond
Fig. 2. Diamond
film grown
film scratched
on the molybdenum
by sandpaper
powder. The scratched surface is shown in Fig. 2, where it can be seen that some diamond clusters on molybdenum grains were not removed. Subsequently, the molybdenum, substrate and powder were chemically removed and a permeable diamond membrane was obtained. Fig. 3 shows a panoramic view of the membrane back side, where the shape is a replica of the substrate, revealing the scratches produced by the sandpaper during substrate preparation. Fig. 4 shows a top view detail of a void of about 60 urn in diameter. The membrane thickness can be controlled by the growth time, but it cannot exceed the grain size of the
substrate
to remove
plus molybdenum
the diamond
growth
powder
sprinkled
on the molybdenum
on it.
powder.
M. C. Salvadori et al./Diamond and Related Materials 4 (1995) 1069-1072
Fig. 3. A panoramic
view of the membrane
Fig. 4. Top view detail of a void of about
powder that produces the voids. Specifically, the membrane shown in Figs. 3 and 4 is about 40 pm thick and its growth time was 24 h. The void density can be controlled by the amount of molybdenum powder sprinkled on the substrate before growth, and the void diameter can be controlled by selecting the powder grain size with sieves. In the case
1071
back side.
60 pm in diameter.
described here, the grain size was between 60 and 90 pm and the void density was about 10 voids mme2. Raman spectroscopy showed the same diamond quality on the top and on the back side. Fig. 5 shows a typical Raman spectrum for a permeable diamond membrane made by the process described here. The presence of diamond is indicated by the peak at 1333 cm-‘, and
M. C. Salvadori et aLlDiamond and Related Materials 4 (1995) 1069-1072
1072
Raman
density and uniformity attainable. Other substrates and template production methods are being considered. In the use of solid materials in science and technology, it is important to have control over the geometry either by forming procedures or by machining. With diamond membranes, due to the difficulties in performing machining after production, it is desirable to explore shaping procedures at the forming stage. Further developments in the improved control of membrane characteristics should result in a new construction element that may find application in different areas of technology in which an inert, self-supporting, permeable membrane is desirable.
shift kni’)
Fig. 5. A typical Raman spectrum for a permeable diamond membrane made by the process described here.
the small broad band near 1530cm-l presence of some amorphous carbon.
4.
indicates the
Conclusions
Our results show that it is possible to produce selfsupporting permeable diamond membranes (SSPDMs) by chemical vapour deposition, controlling the membrane thickness and the void density and diameter. Specifically, in this work, the void diameter was between 60 and 90 pm, the void density was about 10 voids mm-’ and the thickness was about 40 urn.
Acknowledgments This work was supported by the Fundacao de Amparo a Pesquisa do Estado de Sao Paulo and the Instituto de Fisica da Universidade de S%o Paulo, Brazil. We are grateful to Ian G. Brown from Lawrence Berkeley Laboratory for helpful discussions and to Joel Camargo Rubim for the Raman spectroscopy.
References [l]
5.
Forthcoming work
Further studies are in progress to investigate the thickness, area, mechanical strength, void diameter, void
J.C. Angus, F.A. Buck, M. Sunkara, T.F. Groth, C.C. Hayman and R. Gat, Diamond growth at low pressures, Mater, Rex Sot.
Bull., October (1989). [Z] MC. Salvadori, V.P.
Mammana, O.G. Martins and F.T. Degasperi, Plasma-assisted chemical vapor deposition in a tunable microwave cavity, Plasma Sources Sci. Technol., in press.