- Email: [email protected]
Size estimation of vaporized-metal clusters by electron microscope
Nuclear Instruments and Methods in Physics Research B37/38 (1989) 886490 North-Holland, Amsterdam
886
SIZE ESTI~TION
OF VAPORIZED-METAL
CLUSTERS
BY ELECTRON
MICROSCOPE
Hiroaki USUI, Makoto TANAKA, Isao YAMADA and Toshinori TAKAGI Ion BeamEngineering Ex~eri~e~ta~ Luboratory &wto Unioersity, Sakyo, Kyoto 606, Japan
Silver was deposited from a nozzle source and an open source on carbon films cooled by liquid nitrogen. The deposits were observed by an electron microscope in order to estimate the size of clusters generated by the vapor ejection through the nozzle. Depositions of less than 1 ;\ mean thickness were performed with reasonable consistency by the use of a rotating chopper. The surface migration of the atoms was suppressed by encapsulating the deposit with a carbon overcoating. The electron microscopy revealed islands of 10-50 A diameter. The island formation process indicated a significant difference between the nozzle-source and the open-source depositions. The islands formed on the substrate by the nozzle source deposition appear to represent the clusters generated by homogeneous nucleation during the nozzle beam expansion process of the metat vapor. The cluster size is estimated to be several hundred
atoms,
and does not show appreciable
dependence
1. Introduction The ICB (ionized cluster beam) technique has been used to deposit various kinds of films since its development in 39’72. Advantages of the ICB method, such as the production of low-energy ions and the enhancement of atom migration on the substrate surface, were explained by the presence of clusters, which are aggregates of several hundreds to thousands of atoms. The clusters are considered to be generated by homogeneous nucleation when the vaporized source material is ejected out of a crucible in the form of a supersonic nozzle beam. There has been considerable controversy, however, whether metal clusters can be formed by a vapor ejection through a crucible nozzle [1,2]. Theoretical calculations appear to support the feasib~I~ty of forming metal clusters by this method [3-51. Efforts have been made to confirm experimentally the formation of clusters, using energy analyzers [6-g], a time-of-flight mass analyzer [lo] and so on. However, these measurements have limitations due to the difficulty of detecting heavy and slow ions. The experiment described in this article detects the clusters by a trans~ssion electron microscope (TEM). The clusters are deposited on carbon films cooled by liquid nitrogen and observed with a TEM. This method can detect neutral clusters as long as they are collected in a stable form on the substrate. Theeten and coworkers applied this technique for CdTe clusters [lx]. Our previous experiment has also shown that the TEM observation is a useful way to estimate the metal cluster size [12]. This article describes our recent results on the TEM observation of silver clusters obtained with a refined experimental technique. Special attention was paid to deposit small amounts of material with reasonable con0~68-5~3X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
on the crucible
temperature.
trol. This was accomplished by introducing a rotating chopper to reduce the effective deposition time. In previous experiments, it was questionable whether the newly formed deposits remained stable during the warming process from liquid nitrogen to room temperature. To increase stability, we suppressed the surface migration of atoms by encapsulating the deposits with a carbon overcoating. This article compares the results of silver depositions from a nozzle source, which is used in the ICB method, and an open crucible source, which represents the conventional source for vacuum deposition. The effect of the crucible temperature on the cluster size was also studied.
Fig. 1 shows a schematic diagram of the deposition system. A carbon crucible is charged with 99.9% pure silver metal. The nozzle source has a cylindrical nozzle, 1 mm in diameter and 1 mm long, on top of the confinement-type crucible. Depositions are also performed by the open source by removing the top lid of the crucible. The open source essentially generates atomic vapor by Langmuir evaporation. The crucible is heated by bombardment of electrons emitted from a tungsten filament surrounding the crucible. The filament is heated by an ac current, and is biased -100 V from the ground potential to keep the electrons from straying out of the crucible region. An electron accelerating voltage of 1000 V is applied between the crucible and the filament. The crucible temperature can be adjusted by controlling the electron bombardment current. The crucible is Located in a water-cooled jacket beneath a liquid nitrogen shroud. The use of this liquid
H. Usui et al. / Size esrimarion of uaponzed-metal clusters
LECTRON EWTTER FOR RUCIBLE BOMBARDMENT
Fig. 1. Schematic diagram of the apparatus for sample preparation.
nitrogen shroud is effective for condensing the unnecessary part of the silver vapor and for reducing the background pressure in the vacuum chamber. Some portion of the beam is unavoidably ionized in the crucible region by the electrons for crucible heating. Two ion-repelling electrodes, which are both biased 700 V positive to the crucible, are installed in the shroud to remove the ions from the beam. The carbon film substrates are less than 100 A thick and are supported on 200 mesh copper grids. They are commercially available for high-resolution electron microscopy. The substrates are mounted on a copper plate and are evacuated without special treatment. After heating at 400 K for 1 h in a vacuum, they are cooled by liquid nitrogen. The distance from the crucible nozzle to the substrates is about 40 cm. There is a movable mask in front of the substrate holder, which allows deposition on any one of nine substrates mounted on the holder. A quartz crystal oscillator (QCO) thickness monitor is placed near the substrate holder to measure the deposition rate. The deposition time is manually controlled by using a shutter located between the crucible and the shroud. Previous experiments have shown that proper cluster size estimation by this method requires the ability to limit the deposition time to the order of 1 s [12]. However, the manually controlled mechanical shutter is not feasible to control depositions of less than 1 s. This problem becomes serious when making depositions at
887
higher crucible temperatures. In order to make short depositions, a rotating chopper which has two sector slits is installed above the liquid nitrogen shroud. The chopper is driven by a dc motor at a speed of 300 rpm. This reduces the actual deposition time to one fortieth of the opening time of the manual shutter. To measure the deposition rate, the chopper assembly can be retracted from the beam path by a sliding feedthrough. The shutter opening time is measured by detecting the light emitted from the crucible using a photodiode, whose output is recorded by a digital wave memory. After the deposition. the substrate temperature is warmed back to room temperature. The temperature of the substrate holder increases from 77 K to about 250 K in one hour, and later slowly approaches room temperature. The substrates are taken out of the chamber after a half day, and are inspected by electron microscope (Hitachi H-800 Analytical TEM) within two hours of exposing them to the atmosphere. It is questionabte whether the silver deposits on the substrates are stable after warming them back to room temperature. An encapsulating coating might help to stabilize the silver deposits. Some of the samples are encapsulated by depositing thin carbon layers (less than 100 A thick) before warming them back. The carbon is evaporated by flowing a current through a sharpened carbon rod of 0.95 mm diameter. The carbon rod is housed in a cylindrical shield which has a small opening to minimize the radiative heating of the substrates.
3. Results Silver was deposited to mean thicknesses of 0.4-4 A 0 at a deposition rate of 45 A/mm from the nozzle source and the open source respectively. The crucible temperature was 1700 K for the nozzle source and 1460 K for the open source. Fig. 2 shows typical TEM images of the deposits by the nozzle source (a-e) and by the open source (f-j). These samples have been encapsulated with carbon. Small islands, lo-50 A in diameter, are observed uniformly over the substrate. The islands were stable for the electron irradiation during the TEM observation. The samples without carbon encapsulation gave similar images with somewhat better contrast. The size distributions of the islands were visually inspected using a caliper on photographs magnified to a scale of 6 X 105. Fig. 3 indicates the size distributions of the islands shown in fig. 2. At the magnification of 6 x 10’. a granular structure of the carbon substrate is revealed, which causes a background that prohibits the detection of islands smaller than 10 A. Therefore, this measurement inevitably includes considerable inaccuracy for the small diameters. Figs. 4 and 5 show the mean diameter and concentration of the islands, respectively, as a function of mean thickness. The results for VII. EMERGING PROCESSES FOR VLSI
H. Usui et ai. / Sire estimatian ojoaporized-metal
clusters
Fig. 2. Transmission electron micrographs of the Carbon-encapsulated silver deposits formed by the nozzle source (a-e) and by the 0 The mean thickness are (a) 0.38 A, (b) 0.75 A, (c) 1.13 A, (d) 2.25 A and (e) 3.75 A for the nozzJe-source depositions, and (f) 0.40 A, (g) 0.78 A, (h) 1.20 A, (i) 2.44 A and (j) 4.04 A for the open-source depositions. The scale bar in (e)
open source (f-j).
indicates 200 A.
samples without carbon encapsulation are also indicated. In the nozzle source deposition, the mean island diameter is constant and the island concentration increases linearly with the mean thickness for short de-
(a)
NOZZLE SOURCE
(b)
OPEN
positions. When the mean tbiekness exceeds 1 A,, the mean diameter increases with increasing mean thickness while the concentration slightly decreases. With the carbon encapsulation, the mean diameter remains constant and the concentration increases linearly for a mean thickness of up to 2 A. For the thin depositions
SOURCE
I
MEAN THICKNESS
1 MEAN THICKNESS
NOZZLE SOURCE 0 : NON-ENCAPSULATED zto
‘/”
0 : ENCAPSULATED
A/
;: E: t
I
0
~~oDIAMETER(.i)
20
40
DIAMETER&
Fig. 3. Diameter distributions of the carbon-encapsulated islands formed by the nozzle source (a) and by the open source (b) for different mean thicknesses.
0
SOURCE Cl : NON-ENCAPSULATED W: ENCAPSULATED 1 1 ,
OPEN
1 MEAN 2THiCKN:SS
$1
Fig. 4. The mean diameter of islands deposited by the nozzle source (circles) and the open source (squares), respectively. The closed points indicate the results for carbon-encapsulated samples and the open ones are for those without encapsulation.
H. Usui et al. / Size estimation
of vaporized-metal clusters
889
4. Discussion
NOZZLE SOURCE 0 : NON-ENCAPSULATED 0 : ENCAPSULATED
I/ OO
OPEN SOURCE 0 NON-ENCAPSULATED I 1
W ENCAPSULATED I 1 I 5 MEAN *THICKN:SS
(A41
Fig. 5. The island concentration on the substrates deposited by the nozzle source (circles) and the open source (squares), respectively. The closed points indicate the results for carbonencapsulated samples and the open ones are for those without
encapsulation.
the mean diameter and the island concentration are not affected by the encapsulation. For the open source, on the other hand. the mean diameter increases continuously with increasing mean thickness throughout the thickness range studied here. The island concentration increases for very thin depositions and then decreases for mean thicknesses larger than 1 A. The carbon encapsulation reduces the rate at which the island diameter increases. With the encapsulation, it becomes clearer that the island concentration increases while the mean thickness is small, and the increase of concentration is observed for a mean thickness up to 2.4 A. The results were compared for the nozzle-source depositions at the source temperatures of 1540 K, 1610 K, and 1700 K. The equilibrium vapor pressures of silver at these temperatures are 0.45, 1.1 and 3.0 Torr, respectively [13]. The deposition rate changes with the source temperature, and was 4 A/mm at 1540 K, 17 A/mm at 1610 K and 45 A/rnin at 1700 K. However, the mean island diameter and the island concentration showed almost the same dependence on the mean thickness, especially when the mean thickness was less than 1.5 A. In this thickness region, the mean diameter was 0 almost constant at 17 A, and the island concentration increases linearly with increasing mean thickness.
The island formation process of the surface appears to be classified into two regions according to the mean thickness, regardless of the deposition method and carbon encapsulation. The island concentration increases with increasing mean thickness in the smallthickness region, while it decreases in the larger-thickness region. The TEM images suggest that in the latter region the islands may be close enough to each other to grow by their coalescence. It should be noted that the border of the two regions, i.e. the maximum mean thickness for which coalescence does not take place, becomes larger by the carbon encapsulation. This verifies that carbon encapsulation suppresses the surface migration of atoms. There is a significant difference between the nozzle source deposition and the open source deposition. The difference is especially distinctive in the small-thickness region. The mean diameter of islands deposited by the nozzle source remained constant, while that by the open source increased continously with the mean thickness. For the nozzle source, the same mean diameter and island concentration were obtained, regardless of the carbon encapsulation in this thickness region. On the other hand, the carbon encapsulation suppressed the formation and growth of islands by the open-source depositions. The dissimilarity may not be attributed to the difference in crucible temperature, because the nozzle source depositions at different crucible temperatures gave the same results except for the deposition rate. This result suggests that the islands from nozzle- and open-source depositions are formed by different mechanisms. In the open-source deposition, they are created from silver atoms through the normal film formation process, which involves surface nucleation and growth of nuclei by capturing migrating atoms. The islands from the nozzle-source deposition, however, might be formed by sticking of clusters that have been generated by homogeneous nucleation in the nozzle beam expansion process. This interpretation assumes that the clusters deposited on the cold substrate are quenched upon the impingement without breaking off at the surface. The islands formed by short deposition from the nozzle source would represent the individual clusters formed in the gas phase nucleation. The cluster size may be calculated if their shape and density are known. Although it is unlikely that these small islands have the same physical properties as the bulk crystal, the typical cluster size can be roughly estimated to beOabout 150 atoms, assuming that the islands are 17 A diameter spheres of bulk density. The island diameters at a mean thickness of 1.1 A are distributed from 13 to 25 A,, corresponding to cluster sizes of 70 to 480 atoms. This experiment has shown that the cluster size is not affected by the crucible temperature. This result is VII. EMERGING
PROCESSES FOR VLSI
890
H. Usui et al. / Size estimation o~~apori~e~-metal
significant for ICB deposition because the deposition rate can be adjusted independently from the cluster size by changing the crucible temperature. The knowledge concerning the nozzle beam cluster sources for normally gaseous materials has revealed that the cluster size increases with increasing source pressure and with decreasing source temperature [14]. Since the source pressure of the vapo~zed-metal cluster source is determined by the equilib~um vapor pressure, which is an increasing function of the source temperature, the influence of the source temperature might be balanced out by the change of the source pressure.
5. Conclusions A small amount of silver was deposited from a nozzle source and an open source on carbon substrates which were cooled by liquid nitrogen. The use of a rotating chopper facilitated the depositions for mean thicknesses of less than 1 A. TEM observation of the islands formed on the substrate showed a clear distinction between the nozzle-source deposition and the open-source one. It is interpreted that the islands formed by the nozzle source for less than 1 A depositions represent the clusters that are formed when the metal vapor is ejected from the crucible nozzle. Carbon encapsulation was found to be effective in suppressing the surface migration of atoms. However, the encapsulation did not affect the result of cluster size estimation. The clusters are estimated to consist of several hundred atoms, which agrees with previous experiments by other methods [6-lo]. The cluster size did not show substan-
clusters
tial variation in the crucible 1540 to 1700 K.
temperature
range
from
References [l] G.D.
Stein, Proc. Int. Ion Eng. Congr., Kyoto (Inst. Electron. Eng. Japan, Tokyo, 1983) p. 1165. [2] I. Yamada, ibid., p. 1177. [31 I. Yamada, H. Usui and T. Takagi, 2. Phys. D3 (1986) 137. [41 1. Yamada, H. Usui and T. Takagi, J. Phys. Chem. 91 (1987) 2463. 151 S.-N. Yang and T.-M. Lu, J. Appl. Phys. 58 (1985) 541. WI T. Takagi, I. Yamada, A. Sasaki, S. Itoh, M. Ozawa, K. Kodama, K. Tominaga and T. Hattori, Proc. 7th Int. Vat. Congr. and 3rd Int. Conf. on Solid Surfaces (Vienna, 1977) p. 1603. [71 T. Takagi, I. Yamada and A. Sasaki, Inst. Phys. Conf. Ser. no. 38 (1978) ch. 5, p. 229. 181 I. Yamada and T. Takagi, Thin Solid Films 80 (1981) 105. [91 H. Usui, A. Ueda, I. Yamada and T. Takagi, 9th Symp. on Ion Sources and Ion-Assisted Technology, Tokyo (Research Group of Ion Engineering, Kyoto University, 1985) p. 39. IlO A. Ueda, H. Usui, I. Yamada and T. Takagi, ibid., p. 45. A. Rocher WI J.B. Theeten, R. Madar, A. Mircea-RousseI, and G. Laurence, J. Cryst. Growth 37 (1977) 317. I. Yamada and T. Takagi, Proc. WI H. Usui. H. Hashimoto, 11th Symp. on Ion Sources and Ion-Assisted Technology, Tokyo (Research Group of Ion Engineering, Kyoto University, 1987) p. 133. 1131 R.E. Honig, RCA Review (December 1962) 567. I141 O.F. Hagena and W. Obert, J. Chem. Phys. 56 (1972) 1793.
886
SIZE ESTI~TION
OF VAPORIZED-METAL
CLUSTERS
BY ELECTRON
MICROSCOPE
Hiroaki USUI, Makoto TANAKA, Isao YAMADA and Toshinori TAKAGI Ion BeamEngineering Ex~eri~e~ta~ Luboratory &wto Unioersity, Sakyo, Kyoto 606, Japan
Silver was deposited from a nozzle source and an open source on carbon films cooled by liquid nitrogen. The deposits were observed by an electron microscope in order to estimate the size of clusters generated by the vapor ejection through the nozzle. Depositions of less than 1 ;\ mean thickness were performed with reasonable consistency by the use of a rotating chopper. The surface migration of the atoms was suppressed by encapsulating the deposit with a carbon overcoating. The electron microscopy revealed islands of 10-50 A diameter. The island formation process indicated a significant difference between the nozzle-source and the open-source depositions. The islands formed on the substrate by the nozzle source deposition appear to represent the clusters generated by homogeneous nucleation during the nozzle beam expansion process of the metat vapor. The cluster size is estimated to be several hundred
atoms,
and does not show appreciable
dependence
1. Introduction The ICB (ionized cluster beam) technique has been used to deposit various kinds of films since its development in 39’72. Advantages of the ICB method, such as the production of low-energy ions and the enhancement of atom migration on the substrate surface, were explained by the presence of clusters, which are aggregates of several hundreds to thousands of atoms. The clusters are considered to be generated by homogeneous nucleation when the vaporized source material is ejected out of a crucible in the form of a supersonic nozzle beam. There has been considerable controversy, however, whether metal clusters can be formed by a vapor ejection through a crucible nozzle [1,2]. Theoretical calculations appear to support the feasib~I~ty of forming metal clusters by this method [3-51. Efforts have been made to confirm experimentally the formation of clusters, using energy analyzers [6-g], a time-of-flight mass analyzer [lo] and so on. However, these measurements have limitations due to the difficulty of detecting heavy and slow ions. The experiment described in this article detects the clusters by a trans~ssion electron microscope (TEM). The clusters are deposited on carbon films cooled by liquid nitrogen and observed with a TEM. This method can detect neutral clusters as long as they are collected in a stable form on the substrate. Theeten and coworkers applied this technique for CdTe clusters [lx]. Our previous experiment has also shown that the TEM observation is a useful way to estimate the metal cluster size [12]. This article describes our recent results on the TEM observation of silver clusters obtained with a refined experimental technique. Special attention was paid to deposit small amounts of material with reasonable con0~68-5~3X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
on the crucible
temperature.
trol. This was accomplished by introducing a rotating chopper to reduce the effective deposition time. In previous experiments, it was questionable whether the newly formed deposits remained stable during the warming process from liquid nitrogen to room temperature. To increase stability, we suppressed the surface migration of atoms by encapsulating the deposits with a carbon overcoating. This article compares the results of silver depositions from a nozzle source, which is used in the ICB method, and an open crucible source, which represents the conventional source for vacuum deposition. The effect of the crucible temperature on the cluster size was also studied.
Fig. 1 shows a schematic diagram of the deposition system. A carbon crucible is charged with 99.9% pure silver metal. The nozzle source has a cylindrical nozzle, 1 mm in diameter and 1 mm long, on top of the confinement-type crucible. Depositions are also performed by the open source by removing the top lid of the crucible. The open source essentially generates atomic vapor by Langmuir evaporation. The crucible is heated by bombardment of electrons emitted from a tungsten filament surrounding the crucible. The filament is heated by an ac current, and is biased -100 V from the ground potential to keep the electrons from straying out of the crucible region. An electron accelerating voltage of 1000 V is applied between the crucible and the filament. The crucible temperature can be adjusted by controlling the electron bombardment current. The crucible is Located in a water-cooled jacket beneath a liquid nitrogen shroud. The use of this liquid
H. Usui et al. / Size esrimarion of uaponzed-metal clusters
LECTRON EWTTER FOR RUCIBLE BOMBARDMENT
Fig. 1. Schematic diagram of the apparatus for sample preparation.
nitrogen shroud is effective for condensing the unnecessary part of the silver vapor and for reducing the background pressure in the vacuum chamber. Some portion of the beam is unavoidably ionized in the crucible region by the electrons for crucible heating. Two ion-repelling electrodes, which are both biased 700 V positive to the crucible, are installed in the shroud to remove the ions from the beam. The carbon film substrates are less than 100 A thick and are supported on 200 mesh copper grids. They are commercially available for high-resolution electron microscopy. The substrates are mounted on a copper plate and are evacuated without special treatment. After heating at 400 K for 1 h in a vacuum, they are cooled by liquid nitrogen. The distance from the crucible nozzle to the substrates is about 40 cm. There is a movable mask in front of the substrate holder, which allows deposition on any one of nine substrates mounted on the holder. A quartz crystal oscillator (QCO) thickness monitor is placed near the substrate holder to measure the deposition rate. The deposition time is manually controlled by using a shutter located between the crucible and the shroud. Previous experiments have shown that proper cluster size estimation by this method requires the ability to limit the deposition time to the order of 1 s [12]. However, the manually controlled mechanical shutter is not feasible to control depositions of less than 1 s. This problem becomes serious when making depositions at
887
higher crucible temperatures. In order to make short depositions, a rotating chopper which has two sector slits is installed above the liquid nitrogen shroud. The chopper is driven by a dc motor at a speed of 300 rpm. This reduces the actual deposition time to one fortieth of the opening time of the manual shutter. To measure the deposition rate, the chopper assembly can be retracted from the beam path by a sliding feedthrough. The shutter opening time is measured by detecting the light emitted from the crucible using a photodiode, whose output is recorded by a digital wave memory. After the deposition. the substrate temperature is warmed back to room temperature. The temperature of the substrate holder increases from 77 K to about 250 K in one hour, and later slowly approaches room temperature. The substrates are taken out of the chamber after a half day, and are inspected by electron microscope (Hitachi H-800 Analytical TEM) within two hours of exposing them to the atmosphere. It is questionabte whether the silver deposits on the substrates are stable after warming them back to room temperature. An encapsulating coating might help to stabilize the silver deposits. Some of the samples are encapsulated by depositing thin carbon layers (less than 100 A thick) before warming them back. The carbon is evaporated by flowing a current through a sharpened carbon rod of 0.95 mm diameter. The carbon rod is housed in a cylindrical shield which has a small opening to minimize the radiative heating of the substrates.
3. Results Silver was deposited to mean thicknesses of 0.4-4 A 0 at a deposition rate of 45 A/mm from the nozzle source and the open source respectively. The crucible temperature was 1700 K for the nozzle source and 1460 K for the open source. Fig. 2 shows typical TEM images of the deposits by the nozzle source (a-e) and by the open source (f-j). These samples have been encapsulated with carbon. Small islands, lo-50 A in diameter, are observed uniformly over the substrate. The islands were stable for the electron irradiation during the TEM observation. The samples without carbon encapsulation gave similar images with somewhat better contrast. The size distributions of the islands were visually inspected using a caliper on photographs magnified to a scale of 6 X 105. Fig. 3 indicates the size distributions of the islands shown in fig. 2. At the magnification of 6 x 10’. a granular structure of the carbon substrate is revealed, which causes a background that prohibits the detection of islands smaller than 10 A. Therefore, this measurement inevitably includes considerable inaccuracy for the small diameters. Figs. 4 and 5 show the mean diameter and concentration of the islands, respectively, as a function of mean thickness. The results for VII. EMERGING PROCESSES FOR VLSI
H. Usui et ai. / Sire estimatian ojoaporized-metal
clusters
Fig. 2. Transmission electron micrographs of the Carbon-encapsulated silver deposits formed by the nozzle source (a-e) and by the 0 The mean thickness are (a) 0.38 A, (b) 0.75 A, (c) 1.13 A, (d) 2.25 A and (e) 3.75 A for the nozzJe-source depositions, and (f) 0.40 A, (g) 0.78 A, (h) 1.20 A, (i) 2.44 A and (j) 4.04 A for the open-source depositions. The scale bar in (e)
open source (f-j).
indicates 200 A.
samples without carbon encapsulation are also indicated. In the nozzle source deposition, the mean island diameter is constant and the island concentration increases linearly with the mean thickness for short de-
(a)
NOZZLE SOURCE
(b)
OPEN
positions. When the mean tbiekness exceeds 1 A,, the mean diameter increases with increasing mean thickness while the concentration slightly decreases. With the carbon encapsulation, the mean diameter remains constant and the concentration increases linearly for a mean thickness of up to 2 A. For the thin depositions
SOURCE
I
MEAN THICKNESS
1 MEAN THICKNESS
NOZZLE SOURCE 0 : NON-ENCAPSULATED zto
‘/”
0 : ENCAPSULATED
A/
;: E: t
I
0
~~oDIAMETER(.i)
20
40
DIAMETER&
Fig. 3. Diameter distributions of the carbon-encapsulated islands formed by the nozzle source (a) and by the open source (b) for different mean thicknesses.
0
SOURCE Cl : NON-ENCAPSULATED W: ENCAPSULATED 1 1 ,
OPEN
1 MEAN 2THiCKN:SS
$1
Fig. 4. The mean diameter of islands deposited by the nozzle source (circles) and the open source (squares), respectively. The closed points indicate the results for carbon-encapsulated samples and the open ones are for those without encapsulation.
H. Usui et al. / Size estimation
of vaporized-metal clusters
889
4. Discussion
NOZZLE SOURCE 0 : NON-ENCAPSULATED 0 : ENCAPSULATED
I/ OO
OPEN SOURCE 0 NON-ENCAPSULATED I 1
W ENCAPSULATED I 1 I 5 MEAN *THICKN:SS
(A41
Fig. 5. The island concentration on the substrates deposited by the nozzle source (circles) and the open source (squares), respectively. The closed points indicate the results for carbonencapsulated samples and the open ones are for those without
encapsulation.
the mean diameter and the island concentration are not affected by the encapsulation. For the open source, on the other hand. the mean diameter increases continuously with increasing mean thickness throughout the thickness range studied here. The island concentration increases for very thin depositions and then decreases for mean thicknesses larger than 1 A. The carbon encapsulation reduces the rate at which the island diameter increases. With the encapsulation, it becomes clearer that the island concentration increases while the mean thickness is small, and the increase of concentration is observed for a mean thickness up to 2.4 A. The results were compared for the nozzle-source depositions at the source temperatures of 1540 K, 1610 K, and 1700 K. The equilibrium vapor pressures of silver at these temperatures are 0.45, 1.1 and 3.0 Torr, respectively [13]. The deposition rate changes with the source temperature, and was 4 A/mm at 1540 K, 17 A/mm at 1610 K and 45 A/rnin at 1700 K. However, the mean island diameter and the island concentration showed almost the same dependence on the mean thickness, especially when the mean thickness was less than 1.5 A. In this thickness region, the mean diameter was 0 almost constant at 17 A, and the island concentration increases linearly with increasing mean thickness.
The island formation process of the surface appears to be classified into two regions according to the mean thickness, regardless of the deposition method and carbon encapsulation. The island concentration increases with increasing mean thickness in the smallthickness region, while it decreases in the larger-thickness region. The TEM images suggest that in the latter region the islands may be close enough to each other to grow by their coalescence. It should be noted that the border of the two regions, i.e. the maximum mean thickness for which coalescence does not take place, becomes larger by the carbon encapsulation. This verifies that carbon encapsulation suppresses the surface migration of atoms. There is a significant difference between the nozzle source deposition and the open source deposition. The difference is especially distinctive in the small-thickness region. The mean diameter of islands deposited by the nozzle source remained constant, while that by the open source increased continously with the mean thickness. For the nozzle source, the same mean diameter and island concentration were obtained, regardless of the carbon encapsulation in this thickness region. On the other hand, the carbon encapsulation suppressed the formation and growth of islands by the open-source depositions. The dissimilarity may not be attributed to the difference in crucible temperature, because the nozzle source depositions at different crucible temperatures gave the same results except for the deposition rate. This result suggests that the islands from nozzle- and open-source depositions are formed by different mechanisms. In the open-source deposition, they are created from silver atoms through the normal film formation process, which involves surface nucleation and growth of nuclei by capturing migrating atoms. The islands from the nozzle-source deposition, however, might be formed by sticking of clusters that have been generated by homogeneous nucleation in the nozzle beam expansion process. This interpretation assumes that the clusters deposited on the cold substrate are quenched upon the impingement without breaking off at the surface. The islands formed by short deposition from the nozzle source would represent the individual clusters formed in the gas phase nucleation. The cluster size may be calculated if their shape and density are known. Although it is unlikely that these small islands have the same physical properties as the bulk crystal, the typical cluster size can be roughly estimated to beOabout 150 atoms, assuming that the islands are 17 A diameter spheres of bulk density. The island diameters at a mean thickness of 1.1 A are distributed from 13 to 25 A,, corresponding to cluster sizes of 70 to 480 atoms. This experiment has shown that the cluster size is not affected by the crucible temperature. This result is VII. EMERGING
PROCESSES FOR VLSI
890
H. Usui et al. / Size estimation o~~apori~e~-metal
significant for ICB deposition because the deposition rate can be adjusted independently from the cluster size by changing the crucible temperature. The knowledge concerning the nozzle beam cluster sources for normally gaseous materials has revealed that the cluster size increases with increasing source pressure and with decreasing source temperature [14]. Since the source pressure of the vapo~zed-metal cluster source is determined by the equilib~um vapor pressure, which is an increasing function of the source temperature, the influence of the source temperature might be balanced out by the change of the source pressure.
5. Conclusions A small amount of silver was deposited from a nozzle source and an open source on carbon substrates which were cooled by liquid nitrogen. The use of a rotating chopper facilitated the depositions for mean thicknesses of less than 1 A. TEM observation of the islands formed on the substrate showed a clear distinction between the nozzle-source deposition and the open-source one. It is interpreted that the islands formed by the nozzle source for less than 1 A depositions represent the clusters that are formed when the metal vapor is ejected from the crucible nozzle. Carbon encapsulation was found to be effective in suppressing the surface migration of atoms. However, the encapsulation did not affect the result of cluster size estimation. The clusters are estimated to consist of several hundred atoms, which agrees with previous experiments by other methods [6-lo]. The cluster size did not show substan-
clusters
tial variation in the crucible 1540 to 1700 K.
temperature
range
from
References [l] G.D.
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