JP2001158956A - Cluster generation method and apparatus - Google Patents

Abstract

(57) [Summary] [Problem] To generate clusters of uniform size. SOLUTION: In a cluster generation space filled with an inert gas, a target serving as a raw material of a cluster is evaporated. This causes a state in which the evaporated atoms of the target expand. The expanding region and the shock wave generated in the inert gas region due to the expansion or the shock wave reflected by the shock wave reflected on the wall of the cluster generating space form a cluster generating region, which is a mixed region thereof. A cluster is generated in the cluster generation area, and the generated cluster is extracted from the cluster generation space to an external space.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for generating atomic clusters by laser ablation, which are used for forming a thin film by depositing clusters on a substrate.

[0002]

2. Description of the Related Art As a conventional cluster generating apparatus, as shown in FIG.
101, target material 102, target holder 103
And an inert gas supply source 104 such as helium, and a skimmer 105, all of which are housed in a vacuum chamber.

The target material 102 is evaporated by irradiating the target material 102 with a pulse laser.
The evaporated atoms form clusters in the inert gas, and the flow created by the pressure difference between the inert gas source and the vacuum causes
It becomes a cluster beam. Skimmer 10 installed downstream
5 has the function of extracting only clusters with axial velocity.

[0004]

The conventional apparatus has a drawback that the time for irradiating the target 102 with the pulse laser is finite, and the size of the grown cluster differs depending on the time of evaporation from the target 102. According to mass spectrometry experiments, the spread of the cluster size distribution is affected by the length of the observation time, and the longer the observation time, the greater the spread of the size distribution. Therefore, if the cluster sampling time is limited shortly after laser irradiation, clusters of uniform size can be obtained, but sufficient flux of a uniform cluster beam could not be obtained with the conventional apparatus (Fig. 9, clusters C of large and small particle size are schematically shown).

FIG. 10 shows the relationship between the residence time of cluster particles and the size distribution of formed particles. This is obtained by an experiment performed with the above-described conventional apparatus.
According to FIG. 10, it can be seen that the distribution of cluster particles is substantially constant in a range where the residence time is large. On the other hand, in the range where the residence time is short, the particle distribution becomes smaller as the residence time becomes shorter. Therefore, it can be understood that in order to obtain a cluster having a small particle distribution, that is, a cluster having a uniform size, it is necessary to temporally define the generation process of the cluster.

Therefore, a device for obtaining a cluster beam having a sufficient flux and a uniform size is required.

[0007] The size, internal energy and electronic state of the cluster to be obtained are uniquely determined by the initial thermodynamic state of the field generated by the cluster and the subsequent thermodynamic state during the growth of the cluster. Therefore, in order to generate a cluster beam with a large flux having a uniform size, it is necessary to make the thermodynamic conditions of the field generated by the cluster uniform and to maintain such a field for a certain period of time.

It is an object of the present invention to provide a specific cluster generation method and apparatus for realizing uniform thermodynamic conditions in time and space as described above.

[0009]

According to the cluster generating method of the present invention, in a cluster generating space filled with an inert gas, a target serving as a raw material of the cluster is evaporated to generate a state in which the evaporated atoms of the target expand. , An expanding region and a shock wave generated in the inert gas region due to the expansion or a reflected shock wave reflected from the wall of the cluster generation space to form a cluster generation region, which is a mixed region of these, and the inside of the cluster generation region And generating the cluster from the cluster generation space to the external space.

According to the cluster generation method of the present invention, the mixed region of the gas evaporating from the target and the inert gas is generated by the interference between the reflected shock wave and the contact surface. In this mixed region, thermodynamic conditions are maintained uniformly in space and time, so that clusters with a more uniform size than before can be extracted with a sufficient flux.

In the above cluster generation method, it is preferable that the target is instantaneously evaporated by external light.

Further, it is preferable that the target is arranged on a movable portion whose position or direction can be finely adjusted, and that the target position or direction is changed by the movable portion and the irradiation position of the irradiation light is changed every time the target is evaporated. .

The above cluster generation method is embodied by the following apparatus, which comprises a cluster generation space in which a target serving as a raw material of a cluster is arranged, and an inert gas supply for supplying an inert gas into the cluster generation space. Means, a state generating means for evaporating the target, and a reflector for reflecting a shock wave propagating in an inert gas generated by evaporation of the target.

[0014] In the above-mentioned cluster generating apparatus, it is preferable that the state generating means is linear light to be introduced from the outside through an introducing hole which connects the cluster generating space to the outside.

Further, it is preferable to dispose the target on a movable portion whose position or direction can be finely adjusted.

Preferably, the introduction hole is cooled by a cooling means.

It is preferable that the introduction hole is provided in a low thermal expansion material or a high melting point material, or a low thermal expansion material and a high melting point material.

It is preferable that the shape of the reflection plate is a concave portion having a curved surface with respect to the target direction.

Further, it is preferable that the curved surface is a part of the surface of the spheroid having one focus at or near the target surface.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 which will be described below with reference to the drawings, an embodiment of the present invention, a cluster generating apparatus includes a laser generator 11, a skimmer 12, and an apparatus body 13. I have. These laser generators 11,
The skimmer 12 and the device body 13 are housed in a vacuum chamber (not shown).

The laser generator 11 emits a YAG laser beam. The skimmer 12 has a cone-shaped slit 14 for extracting a gas flow having a translation speed only in the axial direction.

As shown in detail in FIG. 2, the apparatus body 13 has a cluster generation space 21 and is cooled by liquid nitrogen to form a cooling cell. On the left side of the cluster generation space 21, the target chamber 22
Is provided, and on the right side thereof, a reflection plate 23 is provided in an inset shape.

The target chamber 22 has a left-end opening horizontal cylindrical shape, has an annular seat 24 on the left end opening edge, and has a circular window 25 communicated with the cluster generation space 21 on the right end. I have. An inert gas passage 26 is provided upward from the target chamber 22, and an inert gas supply pipe 27 is connected to the inert gas passage 26. The inert gas supply pipe 27 is provided with a flow control valve 28 and a Pirani gauge 29.

A spherical holder 31 is accommodated in the target chamber 22 so as to slide on the seat 24. Holder 31
The stepped target hole 32 is formed so as to penetrate the center of the holder. A stepped round bar-shaped target 33 made of silicon, a compression coil spring 34, and a spring retainer 35 are sequentially inserted into the target hole 32. The front end surface of the target 33 is substantially flush with the outer surface of the holder 31 and is exposed through the window 25 into the cluster generation space 21. A rotary shaft 36 is connected to the spring retainer 35. The rotating shaft 36 is
It is inclined about 4 degrees with respect to the horizontal line, that is, the laser optical axis (indicated by θ1 in FIG. 3).
It can rotate by 60 degrees and is movable in the axial direction. Correspondingly, the surface of the target 33 is inclined about 4 degrees so as to be perpendicular to the laser optical axis (indicated by θ2 in FIG. 3), and has an umbrella shape drawing a gentle cone.
Thereby, the irradiation position of the laser on the target surface is shifted little by little.

An annular gas reservoir 41 having a substantially triangular cross section is formed between the peripheral surface of the target chamber 22 and the outer surface of the holder 31 opposed thereto. The gas reservoir 41 communicates with the cluster generation space 21 via the window 25 by a narrow flow passage restriction passage 42 having a narrowed cross section.

The reflection plate 23 is formed in a vertical disk shape. A spherical recess 51 is formed on the inner surface of the reflection plate 23, and a horizontal circular hole 52 is formed at the bottom of the recess 51. The recess 51 and the guide hole 52 are concentric with a horizontal line passing through the center of the holder 31. There is a possibility that the material of the reflection plate 23 undergoes thermal expansion due to shock wave heating, while the cooling cell 13 is cooled, so that the thin shock wave reflection plate 23 may be deformed. In order to prevent this, a low thermal expansion material is used as the material of the reflection plate 23.

Although not shown, the rotating shaft 36 is connected to the coupling via a ceramic bearing. One end of this coupling is connected to a rotation introducing machine, and can rotate the target 33 by 360 °.
The irradiation position of the laser on the surface can be adjusted. The flange of the rotary introducer is connected to the introduction port of the chamber via the bellows, and the target can be retracted through expansion and contraction of the bellows.

The condition of the laser beam is as follows.
It requires power and energy to ablate 33 instantaneously, and a pulsed one is suitable. on the other hand,
It is also necessary to narrow the laser beam so that the laser beam can pass through. A YAG laser beam is suitable for that.

Before irradiating the target 33 with the laser,
It is necessary to confirm that the laser passes through the guide hole 52 of the reflector 23. When the target holder 31 is retracted, the ceramic reflector mounted on the linear introduction device can be inserted. The fact that the laser has passed can be confirmed by irradiating the ceramic reflector with a laser spot.

The target holder 31 is used in close contact with the seat 24 of the cooling cell 13 from the viewpoint of positioning the target 33 and containing the inert gas. For this reason, the contact portion wears, so that the surface of the target holder 31 is protected by silver ion plating.

Further, the inert gas is supplied to the cluster generation space 21.
When supplying the inert gas, it is necessary to maintain the uniform thermodynamic conditions by making the inflow amount and the outflow amount of the inert gas equal so that the state in the space 21 is not disturbed. This object is achieved by the restricted passage 42.

FIGS. 1A and 1B schematically show the timing of laser irradiation and the timing of extracting a cluster beam.

The laser emitted from the laser generator 11 is guided into the cluster generation space 21 through the guide hole 52.

In FIG. 1A, a laser is applied to a target 33.
, And the evaporated atoms are about to fly out of the target 33. At this time, an inert gas is constantly flowing into the cluster generation space 21 through the supply pipe 27, and the inside thereof is maintained at a pressure of about 1 to 10 Torr. A vacuum is maintained outside the cluster generation space 21. Laser irradiation is pulsed, and irradiation time is on the order of several nanoseconds to several tens of nanoseconds.

In FIG. 1B, a cluster beam is generated after the laser irradiation is completed. Due to the laser irradiation, a shock wave is generated in the cluster generation space 21 because the target material rapidly evaporates and expands.
This shock wave is reflected by the reflection plate 23 to become a reflected shock wave, and reverses the traveling direction. Behind the shock wave, there is a contact surface which is a boundary between the evaporating gas of the target material and the inert gas. At this time, a mixed region of the evaporative gas and the inert gas is formed near the contact surface, and in this region, the evaporative gas undergoes rapid cooling and crystallizes to form clusters. In addition, since this mixed region is created by a high-speed air flow, it is not confined to the diffusion phenomenon, and is a confined region having uniform thermodynamic conditions. Therefore, it is expected that a crystal nucleus of a uniform size will be generated from this region, and uniform cluster growth will occur even during the expansion process as a cluster beam.

In FIG. 1B, the shock wave is shown as W. The shock wave W spreads as a part of the spherical surface immediately after the evaporation, but when reaching the vicinity of the reflector 23, it becomes a substantially vertical plane.

As described above, by providing a reflector in a direction perpendicular to the direction of laser irradiation, uniform thermodynamic conditions can be created by using a generated shock wave as a medium.
Creating uniform thermodynamic conditions in the cluster generation region, that is, by making the pressure, density, and temperature of the inert gas uniform in the region, the particle size of the cluster, the internal energy of the cluster, and the electronic state of the cluster The requirement for homogenizing is fulfilled.

FIG. 4 shows an analysis result of a transition state of a mixed region of the evaporated gas and the inert gas of the target sample.
The state of the contact surface and the shock wave described above was obtained by computer simulation. In FIG. 4, the horizontal axis indicates the distance from the target tip surface. In addition, a change in a state progressing with time in order from the top is shown in the vertical axis direction.

First, a shock wave travels from left to right in the inert gas. This is shown as a state where the contact surface and the shock wave travel rightward from t41 to t1477. next,
The shock wave reflects off the right wall. At t1477, the shock wave becomes a reflected shock wave. This reflected shock wave serves to suppress the progress of the interface between the subsequently evaporated atoms and the inert gas, and the interface temporarily stagnates. Since cluster generation is performed in this boundary region, a dense cluster generation region is generated here. A cluster generation region is formed between the contact surface and the curve indicating the reflected shock wave, and this region continues thereafter until time t3333.

Next, referring to FIG. 5, a modified example of the reflection plate will be described. In the following embodiments, portions corresponding to those in the above embodiment are denoted by the same reference numerals.

In this modification, the concave portion 62 of the reflecting plate 61
Has a spheroidal shape. One focus A of ellipse 63
Is on the surface of the target 33, and the other focal point B is
It is at the convergence point of the reflected shock wave.

The reflecting plate 61 thus formed reflects a shock wave generated by laser ablation, and in order to efficiently form a confined region, the shock wave reflected on the spheroidal surface of the recess 62 has one point (spheroidal shape). Focus point B), and interferes with the rear contact surface to create a three-dimensional confinement region. Clusters of uniform size are concentrated in this area.

The size of the spheroid is determined from the following viewpoints. The ellipse is determined by the two focal distances 2a0 and the distance s0 from the focal point to the vertex of the ellipse. The distance 2a0 is the axial distance from the target surface to the convergence point of the reflected shock wave. This is determined in the same manner as the method for determining the “thickness of space” described later. s0 is the distance from the guide hole of the reflector to the convergence point of the reflected shock wave. When these values are specified, the size in the direction perpendicular to the axis is automatically determined.
If s0 is too small, the minor axis of the ellipse becomes very small, and the intensity of the reflected shock wave contributing to cluster generation becomes small.

Basically, the spheroid converges a shock wave incident as a plane wave at the focal point. Therefore, if the minor axis of the ellipse is too long, the curvature of the incident shock wave front cannot be ignored, and eventually the convergence of the reflected shock wave will be weakened.

FIG. 6 shows a further modification of the reflection plate. In this modification, a spherical inner recess 72 is formed at the center of the inner surface of the reflector 71, and an annular outer recess 73 is formed so as to surround the inner recess 72.

The inner recess 72 corresponds to the elliptical recess 62 described above. The outer recess 73 is for preventing the influence of a shock wave reflected on a wall other than the spheroid from reaching the center. Outer recess
Without the 73, there could be a flow towards the inner recess 72, which is prevented by the outer recess 73. The larger or deeper the outer recess 73 is, the better, but it cannot be infinitely large.

FIG. 7 shows a further modification of the reflector. In the reflection plate 81 according to this modification, a horizontal cylindrical portion 83 surrounding the conducting hole 52 and protruding inward is provided at the bottom of the concave portion 82 corresponding to the above-mentioned elliptical surface. Tubular part
The height of 83 corresponds to the depth of the recess 82. Due to the presence of the cylindrical portion 83, the target material can be discharged to the outside of the cluster generation space 21 from the vicinity of one focal point B of the above-mentioned elliptical surface, not from the bottom of the concave portion 82 but from the concave portion.

Next, in order to deepen the understanding of the present invention, the advantages of the present invention will be examined from various angles, including a comparison with the above-mentioned prior art.

FIG. 8 shows the distribution of the number of cluster particles by comparing the conventional apparatus with the apparatus according to the present invention. The spread of the cluster size distribution is generally evaluated by ΔN / N. Here, N is the average value of the cluster constituent molecules in the size distribution, and ΔN is the half width of the size distribution. In the conventional apparatus, △ N / N = 0.5, but in the apparatus of the present invention, △ N / N = 0.5.
N / N = 0.01 is the target. Regarding this target value, the target value: △ N /
Shown as N = 0.01.

As a specific example of the material of the reflection plate, molybdenum can be mentioned. By using molybdenum,
Thermal stress is reduced to about half that of stainless steel, for example. Molybdenum is used not only because it is a low thermal expansion material, but also because molybdenum is a representative of high melting point materials and is commonly used as a heat resistant material.
The same metals as molybdenum include vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), and tungsten (W)
Is mentioned. Each of these has a small coefficient of linear thermal expansion and may be used as a material of the reflector in the present invention.

Consider a specific example of the restriction passage 42. This passage
The purpose of 42 is to not disturb the state of the inert gas in the space 21 inside the cooling cell 31.

In the apparatus of the present invention, the piping to the cooling cell 13
27 is one. If this is directly coupled to the cooling cell, the gas in the space inside the cooling cell will be disturbed, and a geometrically asymmetric flow will occur. In the present invention, the restriction passage 42
Is provided to form an annular gas reservoir 41 between the spherical holder 31 and the cooling cell 13. The conductance of the annular flow path of the gas reservoir 41 is designed to be larger than the conductance of the restriction passage 42. As a result, the gas first fills the gas reservoir 41, and then gradually flows toward the cluster generation space 21 through the restriction passage 42. Thus, an axially symmetric flow is generated, and a flow perpendicular to the traveling direction of the shock wave is unlikely to be generated. The flow perpendicular to the direction of travel of the shock wave may disturb the shape of the shock wave, and is therefore not preferable for creating uniform thermodynamic conditions. On the other hand, in order to make the conditions for cluster generation uniform over time, this space 13
It is necessary to keep the gas pressure in the chamber constant. For this purpose,
A flow control valve 28 is mounted in the flow path of the inert gas. Strictly speaking, the control signal sent to the flow control valve 28
13 Pressure data of the internal space must be sent. However, considering the pressure loss of the gas pipe by the calculation of the flow in the pipe which is usually performed, it is found that the pressure gradient due to the pipe is small under the conditions of the present invention, and the pressure near the flow regulating valve may be monitored. I understand.

As described above, the restriction passage 42 and the flow control valve 28
Thereby, a constant atmospheric pressure can be maintained without disturbing the inert gas in the space 21.

In order to create a "narrow space" (mixed area) in which clusters stagnate, the following precautions are important. The width and thickness of the "narrow space" are determined by the diameter of the guide hole of the reflector and the pressure in the space. It is based on the following items.

(1) No turbulence occurs in the flow in the space where clusters are generated.

(2) The influence of the conductive hole of the reflector on the reflected shock wave is sufficiently small.

The above item (1) is based on the calculation of the Reynolds number of helium gas. The Reynolds number is a reference value for determining whether a flow becomes laminar or turbulent.

For the above-described apparatus of the present invention, the hole diameter is 0.2 m.
m, the pressure is 10 Torr, and the diameter of the space is 60 mm, the Reynolds number is 0.9. Since the turbulence occurs at 2300 or more, the design value in this case is appropriate.

The larger the values of the pore diameter, the atmospheric pressure and the diameter of the space, the larger the Reynolds number. However, the hole diameter is also limited from the standpoint of machining, and if the pressure is reduced too much, shock waves will not be formed. If the space is too small, the generated clusters will stick to the side walls,
There is a problem that the shock wave is reflected on the side wall and disturbs the flow field.
Actually, it is necessary to determine design values in consideration of these matters.

If the thickness of the space of the cluster generation space, that is, the length of the shock wave in the traveling direction is short, the phenomenon ends before the shock wave and the contact surface are separated. Since the shock wave expands three-dimensionally, if the distance is too long, the shock wave will weaken and eventually disappear.

Now, for simplicity, consider a one-dimensional problem. In one-dimensional problems, every wave means a plane wave. In general,
The speed of the shock wave increases as the pressure ratio before and after the shock wave increases. Therefore, when the same evaporation energy is given by the laser, the velocity of the shock wave increases as the atmospheric pressure in the space decreases. Since the reflected shock wave and the contact surface are generated by such a shock wave, these phenomena are naturally affected by the atmospheric pressure. Since the area where clusters are generated occurs near the interference position between the reflected shock wave and the contact surface, this position is related to the atmospheric pressure in the space. The thickness of the space is determined in consideration of the position and timing at which this interference phenomenon occurs. Specifically, the thickness of the "narrow space" is determined so that the phenomenon is on the order of microseconds. For example, Mach in helium gas
When 10 shock waves are generated, the speed is about 10,000 m / s. To capture this in the order of 1 μs, a distance of 10 mm is required.

On the other hand, the influence of the atmospheric pressure on the timing and position of the interference between the reflected shock wave and the contact surface is not known exactly. Further, the interference between the reflected shock wave and the contact surface is a complicated phenomenon that changes its appearance depending on the acoustic impedance of each. Future research is indispensable for understanding such phenomena. By the way, even if the problem of interference between the reflected shock wave and the contact surface is found, it is still difficult to predict the position and timing of the interference because the evaporation energy of the laser varies. Therefore, it is preferable that at least the position can be predicted.

As described above, the shape of the reflection plate is (part of) a spheroid to define the position of the reflected shock wave. If the contact surface is held down by the reflected shock wave focused on the focal point of the spheroid, the interference position between the reflected shock wave and the contact surface, that is, the position of the cluster generation region can be geometrically defined. In addition, due to the convergence effect of the reflected shock wave, it can be expected that the contact surface is reliably stopped without passing the reflected shock wave.

Next, the problem of the above item (2) will be examined.
Of course, in order to obtain an ideal reflected shock wave, it is better not to form a guide hole in the reflector. However, it is necessary to drill a hole to pass the laser.

When the hole is opened, the shape of the reflected shock wave is disturbed. However, it is generally known empirically that a disturbed reflected shock wave restores its shape as it propagates. In other words, the reflected shock wave whose shape has been disturbed by the guide hole is repaired into a plane wave as the wave propagates. The distance at which the reflected shock wave is restored to a plane wave may be on the order of the diameter of the orifice. In the present invention, a phenomenon that is about several mm away from the reflection plate is considered as a problem. From the above, it is considered that a hole of about 0.2 mm does not significantly affect the phenomenon.

[0066]

According to the present invention, the target material is irradiated with a laser to evaporate the target material, and the shock wave generated at this time is reflected by the reflector and stagnated in a narrow space, thereby reducing the generation of clusters. Since the related thermodynamic conditions are temporally and spatially uniform and clusters are generated under such conditions, clusters of uniform size can be generated.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a schematic configuration of a cluster generation device according to the present invention.

FIG. 2 is a partially enlarged sectional view of FIG.

FIG. 3 is a partially enlarged sectional view of FIG. 2;

FIG. 4 is an explanatory diagram showing a cluster generation process.

FIG. 5 is a cross-sectional view corresponding to FIG. 2, showing a modification of the reflection plate of the device.

FIG. 6 is a cross-sectional view and a perspective view showing a further modified example of the reflector.

FIG. 7 is a sectional view and a perspective view showing a further modified example of the reflector.

FIG. 8 is a graph showing a distribution state of clusters.

FIG. 9 is a configuration diagram showing a conventional apparatus.

FIG. 10 is a graph showing the size of cluster particles in relation to the residence time.

[Explanation of symbols]

 13 Instrument body 21 Cluster generation space 23 Reflector 31 Holder 33 Target 52 Guide hole

Continuing from the front page (72) Inventor Yasushi Iwata 1-1-4 Umezono, Tsukuba City, Ibaraki Pref. Within the Institute of Electronics and Technology Research Institute (72) Inventor Han Min 1-1-1 Umezono Tsukuba, Ibaraki Pref. Within the Institute for Electronic Technology Research (72) Inventor Manabu Kiyama 1-1-4 Umezono, Tsukuba, Ibaraki Industrial Technology Institute (72) Inventor Akira Fukuda 1-1-4 Umezono, Tsukuba, Ibaraki Industrial (72) Inventor Shiro Sada, 6-37-2-504 Minamisenju, Arakawa-ku, Tokyo (72) Inventor Makiko Muto 2-12-3, Natsumi, Funabashi-shi, Chiba (72) Inventor Toshio Takitani 1-7-89 Minami Kohoku, Suminoe-ku, Osaka Nippon Tate Shipbuilding Co., Ltd. (72) Inventor Akio Komura 1-7-89 Minami Kohoku, Suminoe-ku, Osaka Nichi Tate Shipbuilding Co., Ltd. F-term (reference) 4K029 CA03 DB20 DD03

Claims (2)

[Claims] In a cluster generation space filled with an inert gas, a target serving as a raw material for a cluster is evaporated to generate a state in which evaporated atoms of the target are expanded. The shock wave generated inside or the shock wave reflected by the wall of the cluster generation space forms a cluster generation region, which is a mixed region of these, and forms clusters in the cluster generation region, and clusters the generated clusters A cluster generation method characterized by extracting from a space to an external space. 2. A cluster generation space in which a target serving as a raw material of the cluster is arranged; an inert gas supply unit for supplying an inert gas into the cluster generation space; a state generation unit for evaporating the target; A reflector for reflecting a shock wave propagating in an inert gas generated by the above, and a cluster generating device comprising: