GB2321063A

GB2321063A - Reactive particle beam sputtering

Info

Publication number: GB2321063A
Application number: GB9700381A
Authority: GB
Inventors: Timothy Wesley Jolly
Original assignee: OXFORD PLASMA TECHNOLOGY Ltd
Current assignee: OXFORD PLASMA TECHNOLOGY Ltd
Priority date: 1997-01-08
Filing date: 1997-01-08
Publication date: 1998-07-15
Also published as: GB9700381D0

Abstract

A method of providing a film on a substrate 22, the method comprising bombarding a target with a primary particle beam 8 produced or derived from a plasma of a primary particle beam source 6 which generates and maintains the plasma by means of electromagnetic energy directed at a discharge chamber, the target being housed in a vacuum chamber 1 containing the substrate; and introducing a reactive gas 25 into the vacuum chamber whereby the reactive gas combines with material from the target to produce a reaction product which sputter deposits as a film on the substrate, wherein the reactive gas is not oxygen. The primary particle beam may comprise the reactive gas. In a further embodiment there is besides the primary particle beam a secondary particle beam which comprises a reactive gas. The reactive gas may be a halogen, eg fluorine and the target may be formed of Ti, Ta, Si, Ca or Hf. A multiple layer film may be formed by bombarding a second target in the chamber with a particle beam.

Description

METHOD AND APPARATUS FOR PROVIDING A FILM ON A SUBSTRATE The present invention relates to a method and apparatus for providing a film on a substrate by sputter deposition induced by a source of energetic particles.

Sputter deposition induced by a source of energetic particles involves bombarding a metal or dielectric target with a broad-beam primary particle source. The particle beam typically comprises a charged particle beam such as an ion beam. The target material is sputtered under the action of the beam and condenses onto a substrate.

An example of such a technique is illustrated in USRe32849. In this case the primary ion beam consists of positively charged argon ions. The target material reacts with an atmosphere of oxygen to produce a reaction product which is deposited on the substrate. The ion beam is produced by a Kaufman-type source: this generates a plasma by running a filament or other electron emitting device (such as a hollow cathode or other electron gun) in a plasma discharge chamber. The emitted electrons collide with argon atoms in the discharge chamber, and this effect is generally enhanced by the addition of a weak magnetic field, typically between 1.OE-3 Tesla and 1.OE-2 Tesla.

Conventional sputter deposition techniques have only used oxygen as a reactive gas.

In accordance with a first aspect of the present invention there is provided a method of providing a film on a substrate, the method comprising bombarding a target with a primary particle beam produced or derived from a plasma of a primary particle beam source which generates and maintains the plasma by means of electromagnetic energy directed at a discharge chamber, the target being housed in a vacuum chamber containing the substrate; and introducing a reactive gas into the vacuum chamber whereby the reactive gas combines with material from the target to produce a reaction product which sputter deposits as a film on the substrate, wherein the reactive gas is not oxygen.

It has been recognised that Kaufman-type ion beam sources have great difficulty in operating in deposition systems which employ reactive gases. This is for two main reasons: firstly the reactive gas will tend to combine with free electrons in the ion source discharge chamber, and quench the plasma; and secondly the reactive gas will tend to corrode the ion source filament or other electronemitting device. These difficulties can just be overcome for the case of oxygen, but only by carefully limiting the amount of oxygen used, and generally by admitting the oxygen into the process chamber and not directly into the ion source. For some other gases, such as fluorine and chlorine, the problems have been intractable, and so such gases have been hard or impossible to use in deposition processes.

The present invention generates a primary particle beam by means of a particle beam source which generates and maintains a plasma by means of electromagnetic energy directed at a discharge chamber. Such a source does not suffer from the problems discussed above, enabling reactive gases other than oxygen to be used. This allows a much greater variety of coatings to be built up on the substrate. The particle beam source typically transfers energy into the discharge chamber by means of a timevarying electric field, or a time-varying magnetic field, or both. Examples of suitable particle sources are inductively or capacitively-coupled radio frequency (RF) ion sources and electron cyclotron resonance (ECR) ion sources, fast atom sources and free radical sources.

The reactive gas may be introduced into the vacuum chamber by an inlet port which provides an atmosphere of the reactive gas in the chamber. In this case the primary particle beam typically comprises an inert gas such as argon. Preferably however the primary particle beam source generates a particle beam which consists partly or wholly of the reactive gas.

In accordance with a second aspect of the present invention there is provided a method of providing a film on a substrate, the method comprising bombarding a target with a primary particle beam comprising a reactive gas, the target being housed in a vacuum chamber containing the substrate, whereby the reactive gas combines with material from the target to produce a reaction product which sputter deposits as a film on the substrate.

Instead of introducing the reactive gas by a conventional method, eg through an inlet port which provides an atmosphere of the reactive gas in the chamber, the method according to the second aspect of the present invention introduces the reactive gas into the chamber in the form of the primary particle beam.

This results in a number of advantages. Firstly, due to the extra energy carried by the reactive gas the reactive gas reacts more efficiently with the material from the target. This may happen at the surface of the target, or it can occur as the film is deposited on the substrate, since reactive gas will be reflected across to the substrate from the target. As a result less reactive gas is required than in a conventional method. Secondly the extra energy also directly improves properties of the film such as density, refractive index and low porosity.

Thirdly the stoichiometry of the film can be controlled by controlling the intensity and composition of the primary particle beam. Fourthly, no secondary supply of reactive gas needs to be controlled.

In accordance with a third aspect of the present invention there is provided a method of providing a film on a substrate, the method comprising bombarding a target with a primary particle beam, the target being housed in a vacuum chamber containing the substrate; and bombarding the target with a secondary particle beam comprising a reactive gas, whereby the reactive gas reacts with material from the target to produce a reaction product which is sputter deposited as a film on the substrate.

The third aspect of the invention provides a further alternative method of introducing the reactive gas into the vacuum chamber, in the form of a secondary beam which bombards the target with the reactive gas.

By aiming the secondary particle beam at the target, a number of benefits are provided. In particular, the addition of the second beam means that the energy of the reactive flux can be controlled independently from the energy of the primary flux. By aiming the secondary particle beam away from the substrate, the risk of contamination from the secondary particle beam reaching the growing film on the substrate is greatly reduced.

In the case where the secondary beam is an ion beam, the secondary beam can be used to charge neutralise the target. The simplest method to ensure neutralization of the target is to run one particle source as a positive ion beam, and the other as a negative ion beam. In this case, the ion beam currents need to be approximately equal.

The third aspect of the invention also extends to apparatus for providing a film on a substrate, the apparatus comprising a vacuum chamber; means for supporting a substrate in the vacuum chamber; means for supporting a target in the vacuum chamber; a primary particle beam source which, in use, bombards the target with a primary particle beam; and a secondary particle source which, in use, bombards the target with a secondary particle beam comprising a reactive gas, whereby the reactive gas reacts with material from the target to produce a reaction product which is sputter deposited as a film on the substrate.

In all three aspects of the present invention a number of types of primary particle beam can be used as discussed below.

The primary and/or secondary particle beam may be produced or derived from a fast neutral atom source which generates a plasma, accelerates ions from the plasma and then neutralises the ions to produce a beam of fast neutral atoms or molecules that, in the case of the primary beam must generally have energies of several hundred electron volts.

Alternatively the primary and/or secondary particle beam may be produced or derived from a free radical source.

In the case of the second and third aspects of the present invention, the primary and/or secondary beam may be produced or derived from any suitable source, such as a Kaufman-type ion beam source. Preferably however the primary and/or secondary beam is produced or derived from a plasma of a particle source which generates and maintains the plasma by means of electromagnetic energy directed at a discharge chamber. This enables reactive gases other than oxygen to be used.

The following comments apply to all three aspects of the present invention.

Preferably the primary and/or secondary particle beam comprises an ion beam. Examples of suitable ion beam sources are RF or ECR ion sources.

Preferably the only reactive gas is that which is introduced into the vacuum chamber by the primary and/or secondary particle beam. In this case, a separate input port for introducing the reactive gas is not required, resulting in a simplified apparatus. In addition, the method is more easily controllable since the only parameter which needs to be controlled is the ion beam source.

The reactive gas may be any suitable gas or mixture of gases chosen for the particular type of film which is required. Typically however the reactive gas comprises an oxidising agent which chemically combines with the material from the target in an oxidation/reduction reaction.

The reactive gas may comprise nitrogen or (in the case of the second or third aspects of the invention only) oxygen. In a particularly preferable embodiment the reactive gas comprises a halogen such as fluorine.

Fluorine is highly reactive and combines very efficiently with most target materials.

The reactive gas may be provided in a variety of forms such as positive or negative ions, free radicals, elemental atoms or molecules (eg. F2) or chemically combined with another element (eg. combined with Xe in the form of XeF2 molecules).

The reactive gas may comprise a single gas, or may comprise a mixture of two or more gases. For example the gas may comprise a mixture of reactive gases (e.g. a mixture of halogens or a mixture of halogen with Oxygen) or a mixture of reactive gas with a non-reactive gas such as Argon or Xenon.

The film provided on the substrate may comprise a single layer film. Preferably however the method is employed in the fabrication of a multiple layer film. In this case the method further comprises bombarding a second target in the chamber with a particle beam whereby a second film comprising material from the second target is sputter deposited on the substrate. The material from the second target may be deposited directly onto the substrate, or may react with the reactive gas which was used to react with the material from the first target (or an alternative reactive gas) to produce a second reaction product, the second film layer comprising the second reaction product.

The target material is chosen for the particular type of film which is required. The target may be a pure element (typically metallic) or may be a dielectric. The target may also comprise the reaction product. In this case the reactive gas is still required to ensure that the correct composition is obtained in the growing layer.

Examples of suitable target materials are Ti,Ta,Si,Ca, Hf etc which can be used to provide layers of the reaction products of these elements (eg fluorides such as CaF2, oxides such as TiO2 or Ta205, nitrides etc).

The film is typically a thin film which is part of an ultra-violet, infrared, visible or X-ray optical component.

Examples are multi-layer dielectric mirrors for laser gyroscopes, mirrors for high power lasers, ultra-high stability beam splitters, ultra low-loss mirrors for gravity wave detectors, X-ray mirrors, or other multi-layer interference components. Layers are typically a quarterwavelength thick, and so can be in the region of lnm thick for X-ray coatings, up to several microns thick for infrared coatings. The number of layers used in the component will depend on the design, and will vary from single layers through to many hundreds of layers for X-ray mirrors.

Multi-layer dielectric mirrors for visible light will typically contain 30-35 layers.

A number of embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a plan view in cross-section showing sputter deposition apparatus for use in a first method according to the first and/or second aspects of the present invention; Figure 2 shows the rf ion source in more detail; Figure 3 is a plan view in cross-section showing sputter deposition apparatus for use in a second method according to the first and/or second aspects of the present invention; Figure 4 is a plan view in cross-section showing sputter deposition apparatus for use in a third method according to the first and/or second aspects of the present invention; and, Figure 5 is a plan view in cross-section showing sputter deposition apparatus according to the third aspect of the invention.

Referring to Figure 1, a vacuum chamber 1 has an outlet port 2 which is connected to a vacuum pump (not shown). Mounted in the chamber 1 is a target holder 3 which has a first target material 4 on one side, and a second target material 5 on the other side. Examples of suitable target materials 4,5 are Ti, Si, Ta, Ca, Hf or oxides, fluorides or chlorides of the above. The choice of target material depends upon the required film. In the position shown in Figure 1, the target material 5 is being bombarded with a beam 7 from rf ion source 6. If three different target materials are required, then the target holder 3 may be triangular, with the three materials on the three sides of the triangle.

Figure 2 is a detailed view, partly in section, of the rf ion source 6. A helical antenna 30 is wrapped around an alumina discharge chamber 31. Gas is introduced into the chamber 31 via inlet tube 32. An electrical isolation device 38 is generally inserted into the gas inlet tube so that DC voltages can be applied to the plasma in the discharge chamber and to the discharge chamber itself without electric current flowing down the walls of the inlet tube (which is otherwise generally made of metal) or through the gas it contains. This device is not essential, but it does enable the gas supply to be grounded for safer operation. Magnets in various configurations may be arranged around the discharge chamber 31. In the case of Figure 2, a pair of solenoidal magnets 33,34 are arranged around the discharge chamber 31. In an alternative the magnets 33,34 may be omitted. RF power is applied to the antenna 30, and the energy couples inductively and capacitively with the plasma in the discharge chamber 31.

In most designs the dimensions are optimised to encourage inductive coupling, as this reduces the amount of sputtering of ion source components, and hence reduces the contamination of the coatings made in the system.

The plasma comprises a mixture of positive ions and electrons. In addition, with some gases, there will be substantial quantities of negative ions (eg with fluorine F+, e- and F-). Positive or negative ions (the polarity depending upon the polarity of the grids 35,36) are extracted from the discharge chamber by screen grid 35 and accelerator grid 36 between which an extraction voltage is applied, to produce the primary ion beam 7. The ion beam 7 has an energy of the order of 1000 electron volts. The ion beam 7 bombards the target material 5, causing the surface of the target material 5 to evaporate and generate a sputtered material 8. The sputtered material 8 typically has an energy of a few electron volts (eg 20 ev).

When the target material 4 is required, the target holder 3 is rotated as illustrated at 20 by 1800 to bring the reverse side of the target into the path of beam 7.

Charge neutraliser 21 comprises an electron beam source 22 which emits an electron beam 23 through a pin hole 24. The electron beam 23 neutralizes the ion beam 7 by forming a mixture of ions and electrons in the beam.

The particles do not join up to form neutral particles, but the neutralized ion beam does not cause the target to charge up to high voltages.

Reactive gas 25 is introduced into the chamber 1 through inlet port 28 from a gas source (not shown) under control of valve 26. The reactive gas chemically reacts with the sputtering beam 8 to provide a reaction product.

Examples of suitable reactive gases are 02, Fl2, C12, XeF2 or a mixture of the above. The rate of flow of the reactive gas 25 is controlled to maintain a required atmosphere of reactive gas in the chamber, ensuring that the reactive gas 25 reacts with the sputtered material 8 to provide a reaction product with a required stoichiometry.

In addition the reactive gas may also be introduced into the discharge chamber 31 of the rf ion source to generate a beam 7 containing ions of the reactive gas.

A planetary substrate holder 9 is mounted in the side of the chamber 1. The substrate holder 9 comprises a base 10 which is mounted on a driven shaft 11. Mounted on the base 10 are two or more substrate holders (planets) 12,13 which are each mounted on respective driven shafts 14,15.

The substrate holders can each carry one or more substrates. A single substrate 27 is shown on substrate holder 13.

By having the faces of the planets and the face of the target oriented vertically, any dust tends to fall to the base of the chamber, and not onto the substrate.

Shutter 16 is mounted adjacent the planetary substrate holder 9. The shutter 16 is either fully closed, covering all planets, 12,13 etc, or it is fully open and exposes all the planets to the coating flux. When the shutter is fully open, most of the flux 8 in practice goes onto one planet as shown, but a small amount goes onto the other planets also.

During deposition, the whole planetary rotates continuously. The main shaft rotates continuously, moving the planets 12,13 around into and out of the flux 8, and the planets rotate on their own shafts 14,15 etc, in order to improve the uniformity. Thus the same coating structure is made simultaneously on all of the planets. When the layer has reached a desired thickness, the target 3 is rotated to bring the second target material into the path of the primary ion beam. A second thin film layer is then built up on the substrate 27 on top of the first thin film layer. This process can then be repeated as desired to provide alternating layers.

Figure 3 illustrates a second vacuum chamber sputter depositing apparatus. The apparatus is similar to the apparatus of Figure 1, and like reference numerals have been used for equivalent components. In this case, the reactive gas comprises at least part of the primary ion beam 40, and is not introduced to the vacuum chamber via a separate inlet. Reactive gas such as F2 or 02 is introduced into quartz discharge chamber 31 via inlet 32. The F2 or 02 molecules are ionized to produce disassociated ions.

The positive or negative ions are extracted from the chamber by grids 35,36, resulting in a primary beam 40 comprising the reactive gas. Alternatively a mixture of gases may be introduced via the single inlet 32 or via separate inlets.

A major benefit of the process illustrated in Figure 3 is that the high energy reactive ions will react much more readily with the target and the growing layer than low energy gas molecules will. This results in coatings of higher quality, and faster deposition rates. This is particularly important in the case of Ta205. The amount of reactive ions delivered can be controlled by measuring the current of the primary beam 40, which is a direct measure of the number of ions delivered.

Figure 4 shows alternative sputter deposition apparatus which is a variant on the apparatus shown in Figure 3. In this case the vacuum chamber 1 contains a single substrate holder 50 which holds one or more substrates 51-54. The substrate holder 50 is mounted on a driven rotary shaft 55. A pair of rf ion sources 56,57 are mounted on opposite sides of the substrate holder 50. The neutralizers for the ion sources that would be needed in practice are omitted from the Figure for clarity. The ion source 56 emits a primary beam 58 which is directed towards a face of triangular target holder 59. The target holder 59 carries first, second and third target materials 60,61,62 on its three faces. In the position shown in Figure 4, the target material 60 is being deposited. Ion source 57 emits an ion beam 63 which is directed towards a second target material holder 64 which similarly carries three target materials on its three faces.

The two ion sources 56,57 can be run simultaneously, but typically the apparatus in Figure 4 is used to coat the substrates 51-54 with a layer from the first target material 60 with the ion beam 58, and then a second layer is built up with the ion beam 63 and second target material 65.

Figure 5 is a plan view in cross-section of a vacuum chamber containing apparatus according to the third aspect of the present invention. In this case, the charge neutraliser 21 has been replaced by a secondary rf ion source 70 which generates a beam 71 of ions which bombard the target material 5. The beam 71 reacts with material on the surface of the target, as well as material which has evaporated from the target under the action of the primary beam 7. The reaction product 8 is sputter deposited on the substrate 27. The ion beam 71 may comprise a variety of reactive gases, such as O , F1 or Cl . By suitably controlling the current supplied by ion beam 71, the charge balance of the target material 5 and the primary particle beam 7 can be maintained without the requirement of a charge neutraliser 21. The ion beam neutralizes the ion beam 5 by forming a mixture of ions of positive and negative charges in the beam. The particles do not join up to form neutral particles, but the neutralized ion beam 5 does not cause the target to charge up to high voltages.

Alternatively, a single or multiple charge neutralizer 21 may be included in the apparatus of Figure 5. In this case it is likely, but not necessary, that both ion sources will be set to generate positive ions, and that the neutralizer or neutralizers will be required to neutralize both of them.

Claims

1. A method of providing a film on a substrate, the method comprising bombarding a target with a primary particle beam produced or derived from a plasma of a primary particle beam source which generates and maintains the plasma by means of electromagnetic energy directed at a discharge chamber, the target being housed in a vacuum chamber containing the substrate; and introducing a reactive gas into the vacuum chamber whereby the reactive gas combines with material from the target to produce a reaction product which sputter deposits as a film on the substrate, wherein the reactive gas is not oxygen.

2. A method according to claim 1 wherein the primary particle beam comprises the reactive gas.

3. A method of providing a film on a substrate, the method comprising bombarding a target with a primary particle beam comprising a reactive gas, the target being housed in a vacuum chamber containing the substrate, whereby the reactive gas combines with material from the target to produce a reaction product which sputter deposits as a film on the substrate.

4. A method of providing a film on a substrate, the method comprising bombarding a target with a primary particle beam, the target being housed in a vacuum chamber containing the substrate; and bombarding the target with a secondary particle beam comprising a reactive gas, whereby the reactive gas reacts with material from the target to produce a reaction product which is sputter deposited as a film on the substrate.

5. A method according to claim 4 wherein the primary particle beam comprises the reactive gas.

6. A method according to claim 4 or 5 wherein the secondary particle beam is produced or derived from a plasma of a particle beam source which generates and maintains the plasma by means of electromagnetic energy directed at a discharge chamber.

7. A method according to any of claims 4 to 6 wherein the secondary particle beam comprises an ion beam which maintains the charge balance of the target and the primary particle beam.

8. A method according to any of claims 3 to 7 wherein the primary particle beam is produced or derived from a plasma of a particle beam source which generates and maintains the plasma by means of electromagnetic energy directed at a discharge chamber.

9. A method according to any of the preceding claims wherein the reactive gas comprises a halogen.

10. A method according to claim 9 wherein the halogen is fluorine.

11. A method according to any of the preceding claims wherein the reactive gas is a mixture of two or more gases.

12. A method according to claim 11 wherein one of the gases comprises an inert gas.

13. A method according to any of the preceding claims including at least one of claims 2 to 4 wherein the only reactive gas is that which is introduced into the vacuum chamber by the primary and/or secondary particle beam.

14. A method according to any of the preceding claims further comprising bombarding a second target in the chamber with a particle beam whereby a second film comprising material from the second target is sputter deposited on the substrate and a multiple layer film is fabricated.

15. A method according to claim 14 wherein the reactive gas reacts with the material from the second target to produce a second reaction product, and the second thin film layer comprises the second reaction product.

16. A method according to any of the preceding claims wherein the primary particle beam is produced or derived from an rf ion source.

17. A method according to any of the preceding claims including at least claim 4 wherein the secondary particle beam is produced or derived from an rf ion source.

18. A method substantially as hereinbefore described with reference to any of the examples shown in the accompanying drawings.

19. Apparatus for providing a film or a substrate comprising a vacuum chamber; means for supporting a substrate in the vacuum chamber; means for supporting a target in the vacuum chamber; a primary particle beam source which, in use, bombards the target with a primary particle beam; and a seccndary particle source which, in use, bombards the target with a secondary particle beam comprising a reactive gas, whereby the reactive gas reacts with material from the target to produce a reaction product which is sputter deposited as a film on the substrate.

20. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.