GB2345919A

GB2345919A - Depositing material

Info

Publication number: GB2345919A
Application number: GB0001330A
Authority: GB
Inventors: Paul Vincent Dennis
Original assignee: Marconi Caswell Ltd
Current assignee: Marconi Caswell Ltd
Priority date: 1999-01-20
Filing date: 2000-01-20
Publication date: 2000-07-26
Also published as: GB0001330D0; GB9901093D0; WO2000043564A1

Abstract

A method for depositing a selected material onto a substrate, the selected material not readily adhering to said substrate is disclosed comprises depositing a first material onto the substrate, said first material adhering to the substrate; then depositing the selected material onto the first material, characterised by simultaneously depositing the first and selected materials such as to deposit a graded interface therebetween. Deposition may be effected by directing a beam of particles onto a target to eject particles onto the substrate. The graded interface may be produced by providing a target comprising regions of different materials and effecting relative movement of the beam of particles to the target so as to deposit the first material, to then eject material from both regions of the target to form the graded interface and finally to deposit the selected material. The first material may be germanium oxynitride and the selected material may be hafnium oxynitride or zirconium oxynitride. The substrate may be zinc sulphide or zinc selenide.

Description

METHOD OF, AND APPARATUS FOR, DEPOSITING MATERIALS This invention relates to a method of, and an apparatus for, depositing a selected material onto a substrate, the selected material not readily adhering to said substrate. More especially although not exclusively, the invention concerns depositing coatings of materials for multi-spectral windows.

Infra-red (IR) detectors can be used to determine a value of temperature of a body or to observe objects particularly when visible light levels are low. Such detectors are usually relatively fragile and so they need to be protected from their environment, for example atmospheric conditions and physical contact. Consequently they are usually placed behind windows or domes which are made of materials transparent to the radiation that is being detected. Since many IR window materials are inherently soft and have poor scratch resistance, they suffer damage from water, sand, dust and insect impact, particularly when used in high speed airborne applications. In land based systems a combination of loose particulate matter and window wipers can cause scratches.

Multi-spectral coatings are known which transmit radiation both at IR and visible wavelengths. IR radiation is considered typically to be in the wavelength range of approximately lllm to approximately 14um. This includes a first IR radiation range of Ipm to 5llm and a second IR radiation range of 8 to 14m separated by a radiation range of Sum to 811m which is largely absorbed by the atmosphere. Visible radiation is considered typically to be in the wavelength range 0.3 to lpm. Such coatings, if sufficiently hard, are used to protect windows or other substrates made of soft materials. Examples of such coatings, zirconium oxynitride (ZrON) and hafnium oxynitride (HfON), are described in our UK patent GB 2310218B. Whilst such coatings provide an adequate protective coating and are substantially transparent to both visible and IR radiation they do have an inherent problem of not readily adhering directly to certain substrates, in particular zinc sulphide (ZnS).

To overcome incompatibility between a coating layer and a substrate, or between adjacent coating layers, it is known to use a discrete interlayer, sometimes termed a bonding layer, which is compatible with the materials between which it is placed. In GB 2310218B we teach using germanium, germanium nitride, silicon, hafnium or zirconium oxide or nitride as an interlayer material. Although such a bonding layer can improve the adhesion, or bonding, of ZrON or HfON the inventors have appreciated that such deposited materials still have disadvantages. For example, germanium is only effective as an interlayer for ZrON and has poor transmission in the visible part of the spectrum.

Likewise, silicon has virtually no transmission in the visible part of the spectrum. To improve transmission in the visible part of the spectrum when using silicon or germanium as interlayers it is proposed in GB 2310218B to use a thin interlayer of approximately 0. 1 um thickness. Furthermore whilst hafnium nitride or zirconium nitride have good IR transmission properties, they have limited transmission in the visible part of the spectrum. The converse applies to the oxides of these elements. As a result there is no single bonding material which has good multi-spectral properties.

The present invention arose in an endeavour to provide an improved method and/or apparatus for depositing coatings of, in particular, metal oxynitrides onto a ZnS substrate.

According to the present invention there is provided a method of depositing a selected material onto a substrate, the selected material not readily adhering to said substrate, the method comprising: depositing a first material onto the substrate, said first material adhering to the substrate and then depositing the selected material onto the first material, characterised by simultaneously depositing the first and selected materials such as to produce a graded interface between the materials.

In the context of this patent application"graded"means that the proportion of the two materials varies through the thickness of the interface, there being both materials present in the same depth of the deposited materials. Although the present invention arose in relation to depositing metal oxynitrides onto a zinc sulphide substrate it will be appreciated that the method applies to depositing any material which does not readily adhere to the substrate onto which it is desired to be deposited. For example it is envisaged that the method of the present invention be used in the fabrication of semiconductor circuits and/or optoelectronic devices to deposit novel metallisation materials.

Preferably the method comprises depositing the materials by directing a beam of particles onto a target to eject particles of the target onto the substrate; though other deposition techniques can be used such as evaporation or MOCVD. Advantageously the target comprises regions of different materials and the method further comprises effecting relative movement of the beam of particles to the target such as to deposit the first material, to then eject material from both regions of the target to form the graded interface and finally to deposit the selected material. In a particularly preferred arrangement the target is rotatable and has adjacent faces, or regions, of said different materials and the method further comprises rotating the target between said adjacent faces, or regions, during deposition to deposit the first and selected materials.

To enable certain compounds to be deposited the method preferably further comprises directing a beam of reactive particles onto said substrate during deposition to react with the, or each, deposited material to form a compound therewith. The, or each, beam of particles is preferably derived from an ion beam source.

Preferably the first and selected materials comprise a metal oxynitride. Most preferably the first material comprises germanium oxynitride. This is particularly advantageous when the selected material comprises hafnium or zirconium oxynitride ZrON, HfON and it is desired to deposit these materials onto a zinc sulphide or zinc selenide substrate.

Preferably the deposited selected material comprises a protective coating to an environment and said coating is substantially transparent to visible and infrared radiation.

Although the coating can provide an outer surface for the substrate, it can alternatively comprise an interlayer between the substrate and further coatings, such as an antireflection coating. Such an interlayer may be between 0. 01, um and 1. Opm thick although in some applications it may be thicker, for example in the order of 20, um.

In a preferred application the coating is applied to a window. The window can be a window for a sensor. The term sensor refers to one or more electronic devices for detecting or emitting electromagnetic radiation in the ranges of IR, visible or both.

Preferably the selected material is of a thickness between 2.5nm and 15m although layers of greater thickness, up to 501lm can be deposited. The thickness depends on how the layer is to be used. At the lower end of the range, such a thickness may be used as an interlayer. At the upper end of the range it may be used as a protective coating.

According to a second aspect the invention provides a deposited layer comprising a first layer of the first material which is graded into a layer of the selected material, said layer being deposited in accordance with the method described above. According to a third aspect, the invention provides a radiation source and/or detector system incorporating a window having a deposited layer as described above. Preferably the system comprises one or more devices for detecting electromagnetic radiation in the ranges of IR, visible or both. Preferably the system comprises one or more sources of electromagnetic radiation in the ranges of IR, visible or both. The sensor system can comprise devices which are radiation sources such as lasers.

According to a further aspect of the invention there is provided deposition apparatus for depositing a selected material onto a substrate, the selected material not readily adhering to said substrate, the apparatus comprising: first deposition means for depositing a first material onto the substrate, said first material adhering to the substrate; second deposition means for depositing the selected material onto the first material, characterised in that said deposition means are capable of simultaneously depositing the first and selected materials such as to deposit a graded interface therebetween. Preferably the first and second deposition mean comprises ion beam sputter deposition apparatus.

Advantageously the ion beam sputter deposition apparatus comprises an ion beam source for generating a beam of particles, a respective target onto which said beam of particles is directed such that particles of the target are ejected towards and onto the substrate, said apparatus being characterised by the target having respective regions of different target materials and means for effecting relative movement of the beam of particles to the target such that the beam of particles is simultaneously incident on both regions such as to deposit a graded interface. In a particularly preferred embodiment of the apparatus the target is rotatable and adjacent faces, or regions, of the target comprise the different target materials. Advantageously the deposition apparatus further comprises a further ion beam source which is arranged to direct a beam of reactive particles onto the substrate during deposition to react with the, or each, deposited material to form a compound therewith.

An apparatus and a method of operating said apparatus in accordance with an embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a sputtering system in accordance with the invention; Figure 2 shows part of the sputtering system of Figure 1 in greater detail; Figure 3 shows transmission in the visible part of the spectrum for a coating made in accordance with the method and using the apparatus of the invention; and Figure 4 shows transmission in the IR part of the spectrum for a coating made in accordance with the invention.

Referring to Figure 1 there is shown a sputtering system 2 which performs a process generally known in the art as dual ion beam sputtering (DIBS). The system 2 comprises a vacuum chamber 4 which is evacuated by a turbomolecular pump through an outlet 6 and by a cryopump through an outlet 8. A target assembly 10 is located in the vacuum chamber 4 together with a substrate 12 which is supported on a substrate holder (not shown). The substrate 12 is typically rotated during sputtering.

The target assembly 10 carries at least a pair of circular targets being planar in form and disposed at right angles to each other. It may additionally carry other targets to deposit coatings of other kinds, such as hard coatings and anti-reflection coatings. The target assembly 10 is rotatable in a direction 14 about an axis 15. It is driven by a stepper motor.

The system 2 also comprises two ion beam guns operating at radio frequency. A primary ion beam gun or sputter gun 16 which is fed by a supply 18 of inert gas, for example argon or krypton, produces a focussed primary ion beam 20 which is directed towards the target assembly 10. A secondary ion beam gun or assist gun 22 produces a diffuse secondary ion beam 24 which is directed towards the substrate 12. The assist gun 22 is fed by a gaseous supply 26 of oxygen and nitrogen and thus produces a diffuse beam comprising atomic and ionic oxygen and nitrogen.

The substrate 12 is rotatable about an axis 28 in order to control the uniformity of deposition of material on it. The substrate holder is also tiltable through an arc 30 of 0 to 45 for controlling the composition of the materials deposited. The substrate holder also comprises heating means (not shown) to heat the substrate to a temperature suitable for deposition. During coating the substrate is held at a temperature of about 340 C. A two colour IR pyrometer (not shown) is used to measure the temperature of the substrate.

Figure 2 shows the target assembly 10 and the sputter gun 16 in greater detail. The target assembly has four sides 32, each of which is able to carry a target 34,36 and 38.

Although in its simplest embodiment only two targets of different material are placed adjacent to each other, if there is a difficulty in accommodating them in such a configuration, a foil target 36, for example zirconium or hafnium, can be used to bridge between targets of different materials 34 and 38. Figure 2 also shows focussing of the ion beam 20.

In operation the sputter gun 16 is supplied with argon or krypton gas which is ionised, accelerated to an energy of 500 to 1000eV, and fired at the target assembly 10 as a focussed beam of inert gas ions. Impact of the ions on the target sputters material out of it. Sputtered target material lands on the substrate and coats it.

During the coating operation the target assembly 10 is rotated so that only a first target material is sputtered from one or more of the sub-targets, a mixture of the first and a second target materials are sputtered from two or more of the sub-targets and then the second target material is sputtered from one or more of the sub-targets. Typically, the target assembly will have a germanium sub-target adjacent to a zirconium sub-target and sputtering will firstly be from the germanium target, secondly from both the germanium and zirconium sub-targets and thirdly from the zirconium target. Sputtering whilst the target assembly rotates enables coating layers comprising germanium and zirconium to be graded into each other. Of course, there may be more than two sub-targets and target materials other than germanium and zirconium may be used.

A method according to the invention can conveniently be applied to coat a ZnS substrate with ZrON or HfON hard coatings. If one of the targets on the target assembly is germanium and the other is zirconium or hafnium then rotating the target assembly from an orientation in which the ion beam of the sputter gun is incident on only the germanium sub-target, through an orientation in which the ion beam is incident on both germanium and zirconium or hafnium sub-targets, to an orientation in which the ion beam is incident on only the zirconium or hafnium sub-target will result in a coating being deposited on the ZnS substrate having a graded composition. The face of the coating in contact with the substrate will have a composition of 100% GeON. Within the coating the composition is graded with the proportion of ZrON or HfON relative to GeON increasing towards the outer surface of the coating. At its surface the coating has a composition of 100% ZrON or HfON. Providing a graded coating in this way eliminates the problems associated with adherence at the interface between two different materials. The graded interface is considered to be the key to the present invention.

As target material is being sputtered onto the substrate the assist gun 22 bombards the substrate with oxygen and nitrogen ions to form an oxynitride coating of the target material or materials on the substrate. Of course coating materials other than oxynitrides can be formed. The process can be used to form oxides or nitrides. Alternatively if the targets are metal oxides or metal nitrides, metal oxynitride coatings can be deposited by bombarding the sputtered material with oxygen or nitrogen ions as appropriate.

The ratio of nitrogen to oxygen is chosen to provide optimised spectral or multi-spectral transmission characteristics in the resultant coating.

A Nordiko 3450 dual ion bean sputtering system set to the following parameters can be used to make a coating having a GeON interlayer graded into a ZrON surface layer: Parameter GeON ZrON Pan Angle (% of 45 ) 73% 73% Chamber Pressure 0.5 x 104 torr 0.5 x 104 tOIT Deposition Temperature 340 C 340 C Sputter Gun Gas Flow 30 sccm of Argon 20 sccm of Krypton Sputter Gun Voltage 600 V 700 V Sputter Gun Current 85 mA 90 mA Assist Gun Nitrogen Flow 5 sccm 35 sccm Assist Gun Oxygen Flow 35 sccm 5 sccm Assist Gun Voltage 100 V 100 V Assist Gun Current 40 mA 40 mA In one example the rate of rotation of the target assembly is varied so as to provide a layer of GeON which is 50nm thick carrying a coating of ZrON up to 1. 5pm thick. This involves sputtering from the germanium target alone for 2 minutes, rotating from the germanium target to the zirconium target over 80 seconds and then sputtering from the zirconium target alone for 150 minutes.

Prior to deposition of the coating the substrate may be cleaned by using an inert gas ion beam from the secondary ion gun.

Unlike a conventional DIBS process the rotatable target assembly enables a coating to be applied to the substrate having a grade interface, or mixture, of two target materials.

More importantly, the proportion of the two target materials can vary depending on the rotational orientation of the target assembly relative to the substrate. In this way, by rotating the target assembly as material from it is sputtered by the sputter gun 16, the proportion of the two target materials in the coating varies with coating depth. Where the coating is in contact with the substrate it may comprise the first of the target materials and not the second and at the surface of the coating it may comprise the second of the target materials and not the first. Of course, the coating process can be arranged to provide any proportions of the target materials throughout the depth of the coating. Furthermore, the variation in proportion may not be linear with depth since this depends upon how constantly or otherwise the target assembly rotates and any effects caused by the changing geometry of the target assembly which is directly in the path of the beam of ions 20 coming from the sputter gun.

The invention enables incompatibilities between various substrates, interlayers and coatings to be overcome. For example ZnS is a desirable window material but it is difficult to adhere to. One material which will adhere to it is GeON and so this serves as a suitable interlayer.

If a ZnS substrate is being used for its multi-spectral properties, then the GeON interlayer must be thin because it does not have ideal multi-spectral properties. An interlayer of GeON will not normally be adhered to by either ZrON or HfON. To coat a ZnS substrate with ZrON, a known method referred to as reactive gas flow decay is used. The substrate is first coated with a layer of GeON by reactive ion beam sputtering, and then the supplies of oxygen and nitrogen are turned off, so as to coat the top of the GeON interlayer with a coating of germanium. ZrON will adhere to germanium. However, germanium is opaque to visible radiation and so is not ideally suited to multi-spectral window materials. This known method is not suitable for applying HfON coatings because HfON will not adhere to germanium.

Using the method of the invention metal oxynitride coatings have been deposited on substrate materials such as germanium, ZnS, silicon and fused quartz.

Transmission through a coating applied according to the invention is shown in Figure 3 for the visible part of the spectrum and in Figure 4 for the IR part of the spectrum. The coating is a 1. 511m thick ZrON film bonded by a graded interface to a GeON interlayer which itself has previously been deposited on a ZnS substrate. As shown in the graph the transmission is around 50% in the visible region. Some of the incident light which is not transmitted is due to reflection from the coating surface and the coating/substrate interface. It is anticipated that transmission values approaching 100% could be obtained if the ZrON is used to coat a window which is coated on each of its outer surfaces with an anti-reflection coating.

In some applications it is desirable to coat IR windows with a diamond coating. This provides high erosion and impact resistance due to the exceptional hardness of diamond.

Furthermore, since diamond can transmit radiation in both visible and infra red wavelengths it has excellent transmission properties for use as a coating in this type of application.

A suitable process for applying a diamond coating is plasma enhanced chemical vapour deposition. However, due to the temperature of the process and of the gases involved, (predominantly methane and hydrogen) the plasma chemically attacks many window materials including ZnS and zinc selenide. In order to protect the surface of the window material from attack an interlayer is employed. The interlayer may also promote diamond adhesion and relieve stresses arising from the large thermal expansion mismatch between the diamond coating and the IR window. The process deposits a diamond layer at about 600 C. The decomposition temperatures of ZrON and HfON are 550 C and 625 C respectively. Therefore, the method of the invention enables various substrates to be provided with an interlayer which enables them to be coated with diamond. For a diamond coating process occurring at 600 C, a HfON interlayer would be suitable. A lower temperature process would be necessary to enable ZrON to be suitable.

Although the examples described above refer to use of a GeON interlayer, other interlayer materials can be used. This would depend on the application. For example, it is difficult to adhere aluminium nitride (Al3N4) to a silicon substrate. Such a combination can be used in, for example, SAW transducers. According to the invention silicon nitride can be used as an interlayer material. Therefore if a target assembly is used carrying silicon and aluminium nitride sub-targets, a layer of silicon nitride can first be deposited which is graded into the aluminium nitride. It should be understood that a supply of oxygen or nitrogen would not be required from an assist gun in this method. Alternatively, if the targets are of silicon and aluminium the assist gun would supply nitrogen at a variable rate so as to enable a grading of silicon into silicon nitride into silicon aluminium nitride and finally into aluminium nitride. Furthermore it is also envisaged to use the method and/or apparatus of the invention to deposit materials in the fabrication of semiconductor circuits and devices and/or opto-electronic devices. In particular the invention is particularly suited to depositing new materials, hitherto considered impracticable to deposit due to their poor adhesion, such as for example novel metallisation materials.

It will be further appreciated that the invention is not restricted to use in a DIBS system and other deposition methods can be used such as, for example, evaporation or MOCVD to name two.

Claims

CLAIMS 1. A method of depositing a selected material onto a substrate, the selected material not readily adhering to said substrate, the method comprising: depositing a first material onto the substrate, said first material adhering to the substrate and then depositing the selected material onto the first material, characterised by simultaneously depositing the first and selected materials such as to produce a graded interface between the materials.
2. A method according to Claim 1 characterised by depositing the materials by directing a beam of particles onto a target to eject particles of the target onto the substrate.
3. A method according to Claim 2 and further characterised by providing a target comprising regions of different materials and effecting relative movement of the beam of particles to the target such as to deposit the first material, to then eject material from both regions of the target to form the graded interface and finally to deposit the selected material.
4. A method according to Claim 3 and further characterised by providing a rotatable target having adjacent faces, or regions, of said different materials and rotating the target between said adjacent faces, or regions, during deposition to deposit the first and selected materials.
5. A method according to any preceding Claim and further characterised by directing a beam of reactive particles onto said substrate during deposition to react with the, or each, deposited material to form a compound therewith.
6. A method according to any one of Claims 2 to 5 characterised by the or each beam of particles being derived from an ion beam source.
7. A method according to any preceding claim characterised by the first and selected materials comprising a metal oxynitride.
8. A method according to Claim 7 characterised by the first material comprising germanium oxynitride.
9. A method according to Claim 7 or Claim 8 characterised by the selected material comprising hafnium oxynitride or zirconium oxynitride.
10. A method according to any one of Claims 7 to 9 characterised by the substrate comprising zinc sulphide or zinc selenide.
11. A method according to any preceding claim characterised by deposited selected material comprising a protective coating to an environment and said coating being substantially transparent to visible and infrared radiation.
12. A method of depositing a selected material onto a substrate, the selected material not readily adhering to said substrate the method substantially as hereinbefore described or substantially illustrated with reference to Figure 1 or Figure 2 of the accompanying drawings.
13. A deposited layer comprising a first layer of first material which is graded into a layer of the selected material, said layers being deposited in accordance with the method of any preceding claim.
14. Deposition apparatus for depositing a selected material onto a substrate, the selected material not readily adhering to said substrate, the apparatus comprising: first deposition means for depositing a first material onto the substrate, said first material adhering to the substrate; second deposition means for depositing the selected material onto the first material, characterised in that said deposition means are capable of simultaneously depositing the first and selected materials such as to deposit a graded interface therebetween.
15. Deposition apparatus according to Claim 14 characterised by the first and second deposition means comprising ion beam sputter deposition apparatus.
16. Deposition apparatus according to Claim 15 wherein the ion beam sputter deposition apparatus comprises an ion beam source for generating a beam of particles, a respective target onto which said beam of particles is directed such that particles of the target are ejected towards and onto the substrate, characterised by the target having respective regions of different target materials and means for effecting relative movement of the beam of particles to the target such that the beam of particles is simultaneously incident on both regions such as to deposit a graded interface.
17. Deposition apparatus according to Claim 16 characterised by the target being rotatable and wherein adjacent faces, or regions, of the target comprise the different target materials.
18. Deposition apparatus according to any one of Claims 14 to 17 and characterised by further comprising a further ion beam source arranged to direct a beam of reactive particles onto the substrate during deposition to react with the, or each, deposited material to form a compound therewith.
19. Deposition apparatus for a selected depositing a material onto a substrate, the selected material not readily adhering to said substrate, said apparatus substantially as hereinbefore described or substantially as illustrated with reference to Figures 1 or 2 or the accompanying drawings.