CN113174572A - Improved carrier gas cluster source generation method and device - Google Patents

Abstract

A carrier gas cluster source apparatus for providing clusters to a target, comprising: a beam source device for generating a cluster beam, the beam source device including an evaporation source device for emitting an atomic beam and a gas mixing chamber, wherein a carrier gas is mixed with the atomic beam to form a cluster beam having atomic clusters mixed with atomic clusters; a carrier gas flow regulating device for regulating the cluster bundle after forming the clusters so that substantially all of the clusters are in a liquid state; and a transmitting device that transmits the clusters of the conditioned cluster beam to the target in such a way that substantially all of the clusters are in a liquid state before striking the target.

Description

Improved carrier gas cluster source generation method and device

Technical Field

The present invention relates to the deposition of thin films, and more particularly to a clustered source.

Background

The deposition of thin films on substrates is an important manufacturing and research tool in many fields. For example, microelectronic devices are prepared by depositing successive film layers onto a substrate to obtain specific electronic properties of the composite material. Photosensitive devices (such as vidicons and solar cells) are fabricated by depositing thin films of photosensitive materials onto substrates. The optical properties of the lens can be improved by depositing a thin film on its surface. Of course, these examples illustrate only thousands of applications for thin film deposition techniques.

In applications requiring high quality thin films, a typical highly controlled thin film deposition process is to form the thin film by successively depositing monolayers of the thin film, each one atom thick. The mechanics of the deposition process are best considered in atomic terms. In such processes, generally, the surface of the substrate must be carefully cleaned, since the small contaminant mass and even the contaminant atoms can significantly impede the deposition of the desired highly finished film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, e-beam evaporation, sputtering or chemical vapor deposition, to name a few.

In another technique for depositing thin films, ionized clusters of atoms are formed in a cluster deposition apparatus. These clusters typically have approximately 1000-2000 atoms each. The clusters are ionized and then accelerated toward the substrate target by a potential that imparts energy to the clusters equal to the acceleration voltage multiplied by the ionization level of the clusters. Upon reaching the substrate surface, these clusters can break down under impact into atoms that are free to move on the surface. The energy of each atomic fragment remaining after decomposition is equal to the total energy of the cluster divided by the number of atoms in the cluster. Thus, the clusters prior to decomposition have a relatively high mass and energy, while each atom remaining after decomposition has a relatively low mass and energy. The energy of the atoms deposited on the surface makes them mobile on the surface, so that they can migrate to kinks or holes that may be present on the surface. The deposited atoms stay in the defect, thereby eliminating the defect and increasing the perfection and density of the film. Other methods using clusters have been developed, and thin film deposition using cluster beams is a promising commercial film fabrication technique.

The cluster source that generates the clusters is a key component of the cluster beam deposition apparatus. The cluster source should produce high quality clusters of a selected size range and exhibit high cluster formation efficiency. That is, the cluster bundle should have a large portion of the mass of the bundle rather than atoms, otherwise the beneficial effects of using the cluster would be lost. The cluster source should also provide a cluster bundle in which the clusters are in the appropriate energy state.

One type of cluster source is a carrier gas cluster source, where a stream of atoms to be condensed into clusters is emitted from a crucible into a gas mixing chamber. In the gas mixing chamber, a carrier gas is mixed with the atom stream, quenching the atoms to supersaturation and forming clusters. Clusters coming out of the source enter the vacuum, are ionized and accelerated towards the target.

Carrier gas cluster sources have two important advantages over surface-grown cluster sources, the most important class of sources. Carrier gas cluster sources have higher cluster formation efficiency, resulting in a large fraction of the mass of the cluster beam being clusters rather than atoms. Second, carrier gas cluster sources can be used to form cluster beams of very high melting point materials (e.g., refractory metals), which surface growth cluster sources cannot achieve.

However, it has been experimentally observed that films deposited using conventional carrier gas cluster sources tend to be particulate and rough. As a result, the film is not suitable for many types of demanding applications, such as microelectronic devices. It is desirable to improve the quality of deposited films obtained from carrier gas cluster sources while retaining the advantages in terms of efficiency and versatility currently enjoyed by these sources. The present invention fulfills this need, and further provides related advantages.

Disclosure of Invention

It is an object of the present invention to provide an improved carrier gas cluster source that produces clusters tailored to deposit high quality films on substrates. The source retains a high cluster formation efficiency because the formation process is not substantially altered. The source is also capable of producing clusters of high temperature material. The improved construction is fully compatible with existing cluster deposition systems and can be economically constructed.

According to the present invention, a carrier gas cluster source for providing clusters to a target comprises a beam source device for generating a cluster beam, the beam source device comprising an evaporation source emitting an atom beam and a gas mixing volume, wherein a carrier gas is mixed with the atom beam to form a cluster beam, the clusters of the beam being mixed with a carrier gas flow. And adjusting means for adjusting the cluster bundle so that substantially all of the clusters are in the liquid state. A transmitting means for transmitting the adjusted beam to the target before striking the target before all of the adjusted beam is in the liquid state.

In a preferred embodiment, the carrier gas cluster source comprises a beam source comprising a gas mixing chamber and a crucible that emits a stream of atoms of the cluster material into the mixing chamber, wherein the stream of atoms and the carrier gas mix to form a cluster beam of cluster material mixed with the carrier gas. The drift tube, the cluster beam and the carrier gas pass after exiting the beam source. A heater for heating the drift tube; previous studies of clusters generated by carrier gas cluster sources and films generated when such clusters impact a target have shown that at least a portion of the clusters in one of the clusters are in a solid state. In the solid state, as that term is used herein, the material is concentrated into clusters and additionally exhibits the crystallinity characteristic of bulk solid materials. Clearly, when such a solid cluster impacts the target, the cluster does not disintegrate without scattering atoms around the surface in the desired manner. As a result, the acrylic clusters leave a particulate surface structure on the film. As more clusters are deposited, the granularity is preserved, resulting in a degradation of the film quality.

As the flowing gas stream quenches the vapor emitted from the vapor source, clusters form in the carrier gas cluster source, causing the vapor to become supersaturated in the gas stream and form clusters. Quenching must be performed rapidly at low gas temperatures or the clusters will only grow to an undesirably small size. Another consequence of quenching with gas at lower temperatures is that the clusters exhibit solid crystallinity.

In the present invention, the clusters are conditioned to a liquid state after formation. As used herein, "liquid" is a term that refers to a condensed state of a substance in which there is substantially no crystallinity, similar to a bulk liquid substance.

One way to adjust the cluster is to increase its temperature, which is reflected in the random kinetic energy of the atoms in the cluster, rather than the directional kinetic energy generated as the cluster moves toward the target. This increased energy overcomes the tendency of the clusters to become crystalline at low temperatures. In a preferred embodiment, the clusters are passed through a diffusion tube maintained at a sufficiently high temperature to heat the clusters to a temperature at which the crystallinity disappears and the clusters are in a liquid state. Heating is accomplished by an efficient conduction process by the presence of a carrier gas intermixed with the clusters within the diffuser.

In fact, all clusters are in this liquid state, but some of the largest clusters may not be heated sufficiently to remove crystallinity. Large clusters can lead to an undesirable graininess if the target is to be reached. However, in practice, the largest clusters are removed from the cluster bundle by the mass separator, and there is no problem with the quality of the film.

After the cluster is adjusted to the appropriate amorphous state, it must be kept in this state until the target is affected. The transport apparatus used to achieve this conservation separates and removes most of the carrier gas from the beam. After removal of the carrier gas, the clusters move under a near vacuum. Under such a vacuum, conductive and convective heat losses to the clusters are almost eliminated. The radiation heat loss of the tiny clusters in the vacuum is also small. As a result, the temperature state of the clusters held in the conditioning apparatus is maintained while the clusters are subjected to subsequent ionization, mass separator, accelerator and focusing assembly (if any) in vacuum. The clusters that reach the target maintain the temperature and the amorphous, loosely bound state established in the conditioning apparatus.

In other words, the invention is embodied in a method of providing high quality bunching to impact a target, the method including the step of forming the bunches by mixing a carrier gas with a stream of atoms. Conditioning the clusters in the cluster bundle so that substantially all of the clusters are in the liquid state; and emitting the conditioned cluster bundle to the target with substantially all of the clusters in a liquid state prior to impacting the target.

It will now be appreciated that the cluster source of the present invention provides an important advance in the art of carrier gas cluster sources. The high cluster formation efficiency and versatility of existing carrier gas cluster sources is retained, but in addition, the clusters are mediated into an amorphous liquid state, thereby rendering the deposited film smooth and free of particulates and other defects of deposition. Thus, the present invention allows carrier gas cluster sources to be used in competition with other cluster sources. Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

Drawings

FIG. 1 is a schematic view of a carrier gas cluster deposition apparatus.

Fig. 2 is a side cross-sectional view of a carrier gas cluster source according to the present invention.

Detailed Description

A typical cluster deposition apparatus 10 is shown in fig. 1. Reference is now made to fig. 1 for an illustration of the type of system in which the present invention finds application. The deposition apparatus 10 includes a cluster source 12 that generates a cluster beam 14. Cluster bundle 14 is composed of clusters of various sizes and some non-clustered atoms. The clusters and atoms of cluster beam 14 are ionized in ionizer 16, which is typically tuned to have a single charge per cluster and atom. That is, a cluster with 2000 atoms will only have a single charge, as will a single atom.

It is desirable that only clusters of a narrow size range reach the target 18. Thus, the cluster bundle 14 passes through a mass separator 20, which mass separator 20 separates clusters significantly larger or smaller than the selected size, thereby allowing only clusters that reach the desired size to arrive. Finally, the cluster beam 14 is excited by the accelerator electrode 21, and then the cluster beam 14 is focused and deflected by the lens and deflection plate 22, so that a pattern of clusters can be written on the entire surface of the target 18. The deposition apparatus is placed in a vacuum chamber 24 that maintains the target and the bunches in a vacuum.

The present invention is embodied in a cluster source and is shown in fig. 2. In fig. 2, this is referred to as a modified cluster source 30. The cluster source 30 includes a crucible 32 into which a supply 34 of material to be transported as clusters to the target 18 is placed. Such materials may be metallic or non-metallic. The crucible 32 and the supply of material 34 therein are heated by any suitable means, illustrated here as a resistive winding 36 surrounding the crucible 32. The crucible is a closed container through which material vapors escape except for the hole 38. The crucible 32 is surrounded by a heat shield 40 to maintain the temperature of the crucible 32 and to prevent heat dissipation from other portions of the heating source 30.

Vapor from the vaporized material of the crucible 32 passes through the apertures 38, openings 42 in the heat shield 40, and into the gas mixing chamber 44, the walls of which are maintained at about ambient temperature. Carrier gas streams are introduced into the gas mixing chambers 44 from gas inlets 46, respectively. The vaporized vapor is at a temperature determined by the vaporization of crucible 32. The temperature of the carrier gas is much lower than the temperature of the vaporized vapor, and typically, at room temperature, when the vapor and carrier gas are mixed, the vapor atoms rapidly cool to supersaturation, thereby promoting cluster formation in the gas mixing chamber 44 with a relatively small volume. Instead, the clusters grow to much smaller sizes. This result is undesirable because in most cases, clusters of about 2000 atoms are preferred per cluster.

The rapid cooling of the vapor to form clusters results in clusters having a lower temperature. That is, the clusters form agglomerates having a crystalline or partially crystalline structure that do not readily disintegrate upon impact on the target 18 without the use of the present invention.

In the mixing chamber 44, clusters of various sizes are formed from a portion of the evaporated mass, while some of the evaporant remains as unclustered atoms. The vaporized atoms, clusters, and carrier gas mix together to form a mixed beam 48, which mixed beam 48 flows from the gas inlet to the vacuum of the vacuum chamber 24.

The mixed beam 48 enters a drift tube 50, which is a hollow cylindrical tube that is externally heated by a resistance heater 52. As the mixed beam passes through the drift tube 50, the clusters are conditioned so that substantially all of the clusters are converted to a liquid state with little crystallization. Heating of the clusters is achieved primarily by conduction and convection from the heated walls of the drift tube 50 through the medium of the mixed carrier gas.

The temperature required to achieve the liquid or disordered state in the cluster is different from the bulk melting temperature of the materials making up the cluster. Due to its small size, the clusters become liquid when heated at temperatures typically well below the melting temperature of the bulk. For example, at about two-thirds of the bulk melting temperature, about 1000 atoms of the metal cluster are expected to be liquid, while at about two-thirds of the melting temperature, 100 atoms of the metal cluster are expected to be liquid. The required temperature that must be reached as the cluster passes through the drift tube 50 therefore depends on the material of the cluster and the size of the cluster impinging on the target. The maximum temperature of the drift tube 50 is preferably maintained near the temperature required to liquefy the cluster, as higher temperatures may cause atoms to re-evaporate from the cluster.

The required operating temperature of the drift tube under certain conditions is best determined by a set of tests in which the degree of crystallinity of the clusters is measured in flight by diffraction means, or by observing the quality of the deposited film, since the operating temperature of the drift tube is gradually increased.

The length of the drift tube 50 must be large enough to heat the clusters, but not so large that the clusters deposit droplets on the inner wall of the drift tube. The minimum length required depends on the diameter of the drift tube 50, the initial temperature of the clusters and the final temperature required, the nature of the clusters, the size of the clusters, the nature of the carrier gas and the flow rate. An approximation of 2000 atomic silver clusters was initially calculated at ambient temperature and finally at 1000 ° K in argon at a flow rate of 100 cm/sec; the cluster adjacent the drift tube wall reaches a wall temperature within about 0.1 cm and heat diffusion to the remaining clusters requires a distance approximately equal to the diameter of the drift tube. Thus, the minimum length of the drift tube 50 is determined to be about the diameter of the drift tube so that clusters throughout the tube reach the equilibrium wall temperature.

The maximum length of the drift tube is limited by the condensation of droplets of the clustered material on the heated walls. From observations of cluster condensation in similar systems, the maximum length is believed to be about five times the diameter of the drift tube.

Thus, it is believed that for optimal steady state operation, the length of the drift tube 50 should be about one to about five times the diameter of the drift tube. These design parameters reflect the presently preferred methods and embodiments, and are not to be considered critical limitations.

After the clusters have passed through the drift tube 50 and adjusted to the appropriate temperature, the clusters must be introduced into the vacuum of the vacuum chamber 24 so that the pumping capacity of the chamber is not overloaded, thereby allowing the carrier to separate and remove the gas from the cluster bundle 14 to the extent reasonably possible. The cluster gas must be separated to prevent sufficient cooling of the clusters that have been conditioned in the drift tube 50. The presence of the carrier gas facilitates heat transfer to the clusters in the heated drift tube 50 by conduction and convection and, if present. The same mechanism will promote heat transfer of the clusters through the flight from the cluster source 30 to the target 18. By removing the carrier gas from the cluster beam, the heat lost from the cluster by conduction and convection is virtually eliminated. Since the heat loss from the radiation from the small clusters is small, the total heat loss from the clusters is negligible in their way to the target.

The carrier gas is separated and removed from the bundle by expanding the bundle through a nozzle 54 located downstream of the drift tube 50. During the expansion, the trajectories of the heavy clusters do not change significantly, and the cluster continues to remain unchanged. On the other hand, the trajectories of the light atoms or molecules of the carrier gas are deflected radially outwards in the free expansion. Thereby separating the previously homogeneously mixed clusters and carrier gas into a radial distribution, wherein the clusters are mainly located in the center of the distribution.

As the beam distribution changes, atoms or molecules of the carrier gas can be removed from the beam by any of several techniques. A preferred method is to provide a skimmer 56 that deflects the radially outward portion of the beam, i.e., the carrier gas, and directs the deflected carrier gas to a pumping port 58. This approach to the skimmer 56 is not considered to remove all of the carrier gas from the clusters, but successfully removes a sufficient amount of carrier gas to reduce the subsequent heat from the clusters to an acceptably low level. The cluster beam 14 emitted from the cluster source 30 then passes through the remaining deposition apparatus in the manner previously described. The liquid state of the clusters does not alter or hinder the function of the ionizer 16, the mass separator 20, the accelerator 21 or the deflector 22.

In operating the cluster source 30, the carrier gas flow rate must be controlled as an operating parameter to maintain a high cluster formation efficiency so that a large portion of the mass of material evaporated from the crucible 32 is transported as clusters. Heat from the drift tube 50 tends to diffuse upstream (as opposed to mass flow) toward the gas mixing chamber 44. This thermal diffusion increases the mixing temperature between the vaporized atoms and the carrier gas, reducing the efficiency of cluster formation. On the other hand, the mass flow of carrier gas transfers heat in a downstream direction. Thus, the minimum steady state working gas flow rate can be calculated approximately by equalizing the upstream and downstream thermal diffusivities. For the operating parameters discussed above, the required gas flow rate was calculated to be about 140 cm/sec, which is well within the capabilities of the system. If the carrier gas flows at a lower rate, a net heat flow in the upstream direction to the gas mixing chamber 44 is expected, which will fail steady state operation by increasing the mixing temperature and reducing cluster formation efficiency.

By operating the beam focusing source in a pulsed manner, upstream heat flow problems can be reduced or avoided. By preventing upstream heat flow using a cooling device (e.g., 35 optional cooling tubes 60 placed between the mixing chamber 44 and the drift tube 50), the source can be operated continuously at lower carrier gas flow rates. The cooling tubes 60 are in the opposite manner to the drift tubes 50 by extracting heat that would otherwise diffuse upstream from the airflow to the cooling chamber to the upstream of the mixing chamber 44.

In a particular example, the production of about angstrom diameter silver clusters is accomplished by placing silver into crucible 32 and heating crucible 32 to about 1500 ° K to emit a vaporized atomic stream. Argon gas at room temperature was introduced at a pressure of about 0.5Torr and a flow rate of about 150 cm/sec and mixed with the silver vapor in the gas mixing chamber 44. The drift tube was 0.6 cm in diameter and 2 cm long and held at approximately 830 ° K (two-thirds of the melting point). The clusters exiting the source 30 are expected to be completely liquid.

Thus, the improved cluster source of the present invention improves the quality of deposited films produced using carrier gas cluster sources by adjusting the cluster to liquid and temperature such that the cluster is liquid and substantially free of crystalline order. The cluster source retains the high cluster forming efficiency and versatility of the existing cluster source, while the clusters significantly improve the quality of the film.

Claims (10)

1. A carrier gas cluster source apparatus for providing clusters to a target, comprising: a beam source device for generating a cluster beam, the beam source device including an evaporation source device for emitting an atomic beam and a gas mixing chamber, wherein a carrier gas is mixed with the atomic beam to form a cluster beam having atomic clusters mixed with atomic clusters; a carrier gas flow regulating device for regulating the cluster bundle after forming the clusters so that substantially all of the clusters are in a liquid state; a transmitting device that transmits the clusters of the conditioned cluster beam to the target in such a way that substantially all of the clusters are in a liquid state before impacting the target.

2. The cluster source apparatus of claim 1, wherein the adjusting means comprises means for adjusting the temperature of the cluster.

3. The cluster source apparatus of claim 1 wherein the conditioning device is a drift tube through which the cluster beam passes; the conditioning apparatus also includes a heater for the drift tube.

4. The cluster source of claim 1, wherein the evaporation source is a crucible and has an aperture therein for emitting an atomic beam; with a heater therefor.

5. The cluster source apparatus of claim 1 wherein the transport means comprises means for removing at least a portion of the carrier gas from the cluster bundle; for removing a portion of the carrier gas from the cluster beam.

6. The cluster source apparatus of claim 1 wherein the transport device comprises an expansion nozzle.

7. The cluster source apparatus of claim 1, wherein the transport device comprises a skimmer.

8. A carrier gas cluster source apparatus, comprising: a beam source comprising a gas mixing chamber and a crucible that emits a stream of atoms of a cluster material into the mixing chamber, wherein the stream of atoms and a carrier gas mix such that a cluster beam of cluster material mixes with the carrier to form a gas;

a drift tube through which the cluster beam and carrier gas pass after exiting the beam source;

a heater for heating the drift tube; and a nozzle through which the cluster beam and carrier gas pass after exiting the drift tube;

the length of the drift tube is about one to about five times the diameter of the drift tube;

further comprising a cooling tube through which the cluster beam and carrier gas pass before entering the drift tube, the cooling tube being located between the drift tube and the beam source.

9. A method of providing a high quality beam bunch for impinging a target to form a carrier gas cluster source, comprising the steps of: forming a cluster beam by mixing a carrier gas with an atomic flow;

conditioning the clusters in the cluster bundle so that substantially all of the clusters are in the liquid state; and emitting the conditioned cluster bundle to a target, substantially all clusters being in a liquid state prior to impacting the target;

the carrier gas is an inert gas;

said conditioning step is accomplished by heating said clusters to a temperature wherein said clusters are in a condensed state but without a crystallization order;

wherein the step of conditioning the clusters comprises passing the clusters through a drift tube to heat the clusters.

10. The method of claim 9, wherein the delivering step includes the step of separating and removing at least a portion of the carrier gas from the clusters.

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