WO2010138999A1 - Plasma etching of chalcogenides - Google Patents

Abstract

A process for etching a thin glass film comprising at least one chalcogen, the process comprising the step of etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching. In particular arrangements, the plasma is derived from a hydrogen rich mixture. Prior to etching the thin glass film, the process may further comprise the steps of: forming the thin glass film on a substrate; forming a mask layer on the thin glass film; mounting the substrate on a substrate plate in a reaction chamber of a plasma system, wherein the mounting provides thermal contact between the thin glass film and the substrate plate; heating or cooling the thin glass film to a desired etching temperature; evacuating the reaction chamber to a desired etching pressure; injecting one or more gaseous substances into the reaction chamber and striking a plasma in the plasma system, whereby the injected gases are disassociated by the plasma, such that substantial amounts of free hydrogen sufficient to effect etching of the thin glass film are formed in the reaction chamber. The thin film may be a chalcogenide glass film, or alternatively, the thin glass film may a tellurite glass film.

Description

PLASMA ETCHING OF CHALCOGENIDES TECHNICAL FIELD

[ 0001 ] The present invention relates to a method for etching chalcogen-containing materials and in particular to method for etching chalcogen-containing films using a plasma etch process.

[ 0002 ] The invention has been developed primarily for use as a method for plasma etching of thin films containing a significant proportion of one or more chalcogen elements using hydrogen-based plasmas and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND

[ 0003 ] Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.

[ 0004 ] Chalcogen-containing glasses are materials falling typically into one of two classes, namely tellurite or chalcogenide glasses. Tellurite glasses are materials largely comprising TeO₂ often doped with other materials including but not limited to elements such as Na, Zn, W, Nb, Th, Ge, etc for enhancement of specific properties of the basic TeO₂ glass. Other dopants, for example Er, Yb, Ho, Tm, Dy, Pr, etc may additionally be used to generate optical gain in such materials. Chalcogenides contain one or more of the chalcogen elements (Te, S, Se) in combination with other glass forming materials, including but not limited to Ge, Si, As, Sb. In addition these materials can be doped with Halide elements (I₂, Cl, F, Br) and/or oxygen to form chalcohalide or chalco-oxyhalide glasses with similar properties. Other dopants may also be incorporated in small concentrations (typically <10%) to modify particular properties of the glass, for example rare earth metals such as Er, Yb, Ho, Tm, Dy, Pr, Tb, etc may be doped into the material for various applications (e.g. optical amplifiers, laser materials etc).

[ 0005 ] These glasses have recently been a subject of increased interest because of their useful optical properties such as high linear and non-linear refractive index, broad range of optical transmission, low absorption losses at wavelengths beyond 1.3 microns, rapidly reversible stable phase transitions etc. Examples of common chalcogenide-containing glasses include tellurite TeO₂, As₂S₃, As₂Se₃, GeS₂, the As-S-Se family, the Ge-As-Se family (e.g. AMTIR-I Ge₃₃As₁₂Se₅₅), the Ge-As-S-Se family and the Ge-Sb-Te family. High quality thin films made of such glasses and the planar integrated optics and optoelectronic devices made of such films, are of significant scientific and commercial interest for near-infrared (NER.) and mid-infrared (MIR) applications, in data storage and memory for example phase change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM and C-RAM), telecommunications, sensing, medical, defense, national security and astronomical applications. A particular chalcogenide that is recognized as an excellent candidate for PRAM application is a germanium antimony telluride (Ge₂Sb₂Te₅) chalcogenide material, also known as "GST". [ 0006 ] Building planar integrated optics circuits from these materials requires patterning and etching of a thin film into three dimensional microstructures designed to interact with propagating light. For example, a periodical or otherwise appropriately spaced sequence of microstructures distributed along the axis of light propagation in an integrated optics device, can be used to define a Bragg grating. For data storage devices, the chalcogenide may need to be patterned into small islands defining storage cells. Finding a reliable method for etching such chalcogenide materials that fulfils the variety of requirements associated with creating stable low loss devices has become increasingly important.

[ 0007 ] Laser-writing has been used to produce chalcogenide waveguides, utilizing the photosensitivity of these materials to light near their band edges. The induced photo-induced refractive index change, however, is not stable in the long term due to relaxation of the structure, and is, therefore, unsuitable for practical applications. This issue can be addressed by finding a way to physically define the waveguides.

[ 0008 ] Wet-etching has already been applied with NH₄OH as an etchant (Jean-Francois Viens et al., "Fabrication and characterization of integrated optical waveguides in sulfide chalcogenide glasses," J. Lightwave Technol., vol. 17, no. 7, pp. 1184-1191 , 1999). However, serious undercutting of the pattern occurs during this process due to the isotropic chemical process. Such undercutting makes the control of waveguide dimensions difficult. To overcome this problem, dry etching has been applied in forming AS₂S₃ waveguides (W. Li, Y. Ruan, B. Luther-Davies, A. Rode, and R. Boswell, "Dry-etch Of As₂S₃ thin films for optical waveguide fabrication," J. Vac. Sci. Technol. A 23, pp. 1626- 1632, 2005). The process of dry etching usually involves forming masking pattern on the film to be etched.

[ 0009 ] There are further problems related to the fabrication of waveguides fabricated by chalcogenide and other similar types of glasses, related to finding suitable plasma chemistry for the etching of the glasses. For the envisaged applications of these glasses, it is important that the formed structures have smooth and nearly vertical sidewalls. In general this requires an anisotropic etching process. The type of glasses mentioned above generally contains a range of different phases characterized by different bond structures. Dry (plasma) etching processes usually use a combination of physical etching (sputtering) and chemical (reactive) etching. Chemical etching generally proceeds at different rates for the different phases and results in highly undesirable roughening of the etched surface. Physical etching, on the other hand, creates problems with the mask layer since for efficient pattern transfer, a large difference is required between the etch rate of the mask (protective layer) and the etch rate of the feature layer. The mask layer used to define the features of the desired structure should etch much slower than the layer into which the features are to be etched otherwise dimensional loss will occur.

[ 0010 ] Thus, for successful dry (plasma) etching it is required to identify plasma chemistry and etching conditions that produce anisotropic etching, large difference between the etch rates of the mask and the feature layer, and limited differential etching between the different glass phases. [ 0011 ] Previous attempts have been made for etching the chalcogenide As₂S₃ with CF₄-O₂, which have shown a high pattern dimension loss (-0.4 μm per edge for 1 μm etch depth) due to poor etch selectivity to the photoresist and its preferential edge erosion. This decreases the spatial resolution of the etch, which is important for the fabrication of waveguides.

[ 0012 ] For Tellurite glasses, there are no established etching procedures to fabricate structured devices of suitable quality. Tellurite glasses are a large family of glasses which have attractive properties for nonlinear optics compared with the widely investigated silica and other, non-tellurite- based, oxide glasses. The tellurite glass family has been extensively studied for many varied applications and with a large number of compositional variations, including use as a host for rare earth dopants, for example for laser applications, optical fibres, and optical amplifiers. The particular properties of tellurite glasses which make them attractive for optical or photonic applications include high acousto-optic figure of merit; high nonlinearity (up to 100 times that of silica); a large Raman shift (up to 2.5 times the Raman shift observed in silica); and good chemical and thermal stability. Tellurite glasses also have very wide transmission window from the UV at ~300 nm up to mid- infrared at ~7μm. Thin films of tellurites have been produced by chemical and sol-gel processes; thermal evaporation; reactive radio frequency (rf) sputtering using pure tellurium; and pulsed laser deposition.

[ 0013 ] Initial etching attempts to utilise standard oxide glass based etching using fluorine based active species results in the deposition of a layer of tellurium fluoride on all surfaces not exposed to ion bombardment (i.e. the sidewall structures), an example of which is shown in the scanning electron microscope (SEM) images of Figures IA and IB. Methane-Hydrogen etching is also well known in the H-V and II-VI semiconductor world but has not been previously applied to glass etching. Very recently, Pietrolunga et al. [S. Pietralunga, M. Lanata, M. Fere, D. Piccinin, G. Cusmai, M. Torregiani, M. Martinelli, "High-contrast waveguides in sputtered pure TeO₂ glass thin films", Optics Express, vol. 16, No. 26, pp. 21662-21670, 2008] has disclosed Argon sputter etching of tellurite glass films, but with only limited success.

[ 0014 ] Dry etching has also previously been disclosed for various other chalcogenide containing glasses, for example, chlorine-based etching of the chalcogenide Ge₂Sb₂Te₅ (GST) (for example, International patent application WO/2005/011011 to Unaxis Inc and United States Patent 7,256,130 to ST Microelectronics), reactive-ion etching Of Ge₂Sb₂Te₅ in CF₄/ Ar plasma [G. Feng, B. Liu, Z. Song, S. Feng, B. Chen, Microelectronic Engineering, Volume 85, Issue 8, Pages 1699-1704, 2008] and etching of GeSe chalcogenide glasses using hydrogen containing gas mixes [US Patent 6,831 ,019 to Li et al.].

[ 0015 ] Therefore, a need exists for a process for an anisotropic etching process for forming structures from chalcogenide-containing glasses, particular tellurite glasses suitable for industrial and commercial application. SUMMARY

[ 0016 ] According to a first aspect, there is provided a process for etching a thin glass film comprising at least one chalcogen. The process may comprise the step of etching the thin glass film with a plasma. The plasma may comprise substantial amounts of free hydrogen. The substantial amount of free hydrogen may be sufficient to effect etching. The substantial amount of free hydrogen may comprise more than 3 atomic% of disassociated hydrogen in the plasma and may comprise between about 3 atomic% to about 50 atomic% of disassociated hydrogen in the plasma. In alternative arrangements, the substantial amount of free hydrogen may comprise more than 5 atomic% of H₂, more than 10 atomic% Of H₂, more than 15 atomic% Of H₂, more than 20 atomic% of H₂, or more than 25 atomic% of hydrogen gas. The plasma may be hydrogen rich. The plasma may be derived from a hydrogen rich gas. The plasma may be derived from a hydrogen rich gas containing elements that may be such that disassociated species of the hydrogen rich mixture do not promote polymerisation of reaction materials onto the glass film during etching. In an exemplary arrangement, the hydrogen rich gas may be ammonia Alternatively, the plasma may be derived from a hydrogen rich mixture. The plasma may be derived from a hydrogen rich mixture containing elements that may be such that disassociated species of the hydrogen rich mixture do not promote polymerisation of reaction products onto the glass film during etching.

[ 0017 ] In an arrangement of the first aspect, there is provided a process for etching a thin glass film comprising at least one chalcdgen^ comprising the step of etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching.

[ 0018 ] The tern "hydrogen rich" in the context of the present specification may be taken to relate to compounds or substances comprising substantial amounts of free hydrogen when disassociated in a plasma, where the substantial amount of free hydrogen may be sufficient to effect etching. The term "free hydrogen" in the context of the present specification may be taken to refer to disassociated hydrogen atoms or alternatively to hydrogen-containing free radicals. The hydrogen-containing free radical may comprise a spare hydrogen bond and may be adsorbed onto the surface of the glass film and subsequently promote bond breaking of the glass film, for example, via ionic impact of RF activity to cause a reaction with the constituents of the glass film and promote liberation thereof from the surface of the glass film to effect etching of the glass film. In an exemplary arrangement, the plasma may be derived from a gas comprising ammonia (NH₄) which may promote formation of hydrogen-containing free radicals such as NH₃ which may react with the surface of the glass film to either effect or enhance etching. The hydrogen-containing free radicals my assist etching of the glass film.

[ 0019 ] The term "sufficient to effect etching" in the context of the present specification may be taken to relate to a minimum rate of etching of the thin glass film. The minimum etch rate may be in the range of between about 5 to 20 nm per minute or more. The etch rate may be in the range of between about 15 to 20 nm per minute up to about 100 nm per minute or up to about 200 nm per minute, or up to about 300 nm per minute or more. [ 0020 ] The free hydrogen may be achieved by injection of hydrogen gas (Ky into a plasma system where the H₂ molecules are disassociated by the plasma to form reactive ionic hydrogen species which are accelerated towards the glass to effect etching thereof. The etching of the thin glass film may be anisotropic.

[ 0021 ] Prior to etching the thin glass film, the process may further comprise the step of forming the thin glass film on a substrate. The process may further comprise the step of mounting the substrate on a substrate plate in a reaction chamber of a plasma system. The mounting may provide thermal contact between the thin glass film.and the substrate plate. The process may further comprise the step of heating or cooling the thin glass film to a desired etching temperature. The process may further comprise the step of evacuating the reaction chamber to a desired etching pressure. The process may further comprise the step of injecting one or more gaseous compounds or substances into the reaction chamber and striking a plasma in the plasma system. The injected gases may be disassociated by the plasma, such that substantial amounts of free hydrogen sufficient to effect etching of the thin glass film are formed in the reaction chamber.

[ 0022 ] In an exemplary arrangement, prior to etching the thin glass film, the process may further comprise the steps of forming the thin glass film on a substrate; forming a mask layer on the thin glass film, mounting the substrate on a substrate plate in a reaction chamber of a plasma system, wherein the mounting provides thermal contact between the thin glass film and the substrate plate; heating or cooling the thin glass film to a desired etching temperature; evacuating the reaction chamber to a desired etching pressure; injecting one or more gaseous compounds or substances into the reaction chamber and striking a plasma in the plasma system, whereby the injected gases are disassociated by the plasma, such that substantial amounts of free hydrogen sufficient to effect etching of the thin glass film are formed in the reaction chamber.

[ 0023 ] The injected gaseous compounds or substances may be hydrogen rich. The injected gases may comprise hydrogen gas and/or one or more hydrocarbon-containing gases, where "gas" in the present context also includes vapours. The one or more hydrocarbon-containing gases may comprise an alkane, preferably a linear alkane for example methane (CH₄), ethane (C₂H₆), propane (CjH₈), or butane (C₄H₁₀), which may be injected into the reaction chamber. In other arrangements, aromatic hydrocarbons, or branched and/or cyclic alkanes may be injected into the reaction chamber. In further arrangements still alkyne compounds or substances (linear-, branched- and/or cyclo-alkynes) or alkene compounds or substances (linear-, branched- and/or cyclo-alkenes) may be used. In other arrangements, alternative hydrogen rich compounds or substances may be envisaged, for example H₂O.

[ 0024 ] The injected gases may be halogen free. The injected gases preferably may be such that it does not comprise a halogen constituent for example one or more of the group comprising fluorine, chlorine, bromine or iodine. The injected gases may be free of halogen elements. The injected gases may be free of halogen compounds,. The injected gases may be free of both halogen elements and halogen compounds. [ 0025 ] The process may comprise the addition of a passivation means for promoting passivation of the glass film during etching. The passivation means may comprise may comprise a compound or substance which when disassociated promotes sidewall passivation of the thin glass film whilst etching of said film is occurring. The passivation means may be an additional gas added to the reaction chamber to promote sidewall passivation of the etched glass film. The passivation means may assist with the etching process. The passivation means may be adapted to produce sufficient reactive ionic species with sufficient directionality to effect anisotropic etching of the glass film. The passivation means may be a passivation promoting compound or substance and may be selected from a hydrocarbon-containing gas. The passivation promoting compound or substance may promote sidewall polymerisation. The passivation promoting compound or substance may comprise one or more hydrocarbon-containing gases such an alkanes, preferably a linear alkane. The passivation means may be selected from the group comprising methane (CVU), ethane (C₂He), propane (C₃H₈), or butane (C₄H₁0). In exemplary arrangements, the passivation promoting compound or substance may comprise methane. In other arrangements, aromatic hydrocarbons, or branched and/or cyclic alkanes may also be used. In further arrangements still, alkyne compound or substances (linear-, branched- and/or cyclo-alkynes) or alkene compound or substances (linear-, branched- and/or cyclo-alkenes) may be used. In other arrangements, the passivation promoting compound or substance may comprise either a fluorocarbon or a fluoro-hydrocarbon compound that promotes sidewall polymerisation and may be selected from the group of a fluoro-alkane, fluoro-alkene, fluoro-alkyne, fluoro-cyclo-alkane, fluoro-cyclo-alkene, fluoro-cyclo-alkyne, per-fluoro-alkane, per-fluoro-alkene, per-fluoro-alkyne, per- fluoro-cyclo-alkane, per-fluoro- cyclo-alkene, or a per-fluoro-cyclo-alkyne. An example fluorocarbon may be C₄F₈. In further arrangements still, the passivation means may be a compound comprising a halogen selected from the group comprising chlorine, fluorine and bromine; or a halogen-containing compound, for example TeF, ClF, hydrogen bromide or hydrogen iodide among many others.

[ 0026 ] The injected gaseous compounds or substances may further comprise an inert gaseous compound or substance which does not react with the glass when disassociated. The inert gaseous compound or substance may promote cleaning of reaction by-products formed on the film during the etching of said film. The inert gas may form additional ionised species in the reaction chamber during etching. The additional ionised species may be useful for sputtering of the surface of the etched thin glass film. The additional ionised species may act to keep the surface clean of etch reaction byproducts of the etching process. The additional ionised species may act to clean the surface of the thin glass film of solid etch reaction by-products of the etching process. The inert gaseous compound or substance may be a noble gas and may be selected from the group of helium, neon, argon, krypton, xenon, or radon. In an exemplary arrangement, the inert gaseous substance may be argon.

[ 0027 ] The injected gaseous compounds or substances may comprise an etch gas, and either: a passivation promoting gas; an inert gas; or both a passivation promoting gas and an inert gas. The etch gas may be hydrogen gas. In an exemplary arrangement, the flow rate of the hydrogen etch gas may be in the range of between 5 to 100 seem. It will be appreciated by the skilled addressee that the flow rate may be varied according to requirements. The flow rates may also vary based on operating conditions of a selected plasma system, the area of glass to be etched and other factors as would be appreciated by the skilled addressee.

[ 0028 ] The injected gaseous compounds or substances may comprise an etch gas, a passivation promoting gas and an inert gas. In an exemplary arrangement, the etch gas may be hydrogen (H₂), the passivation promoting gas may be methane (CH₄) and the inert gas may be argon. In this particular arrangement, the flow rate of hydrogen into the reaction chamber may be in the range of 5 to 100 seem; the flow rate of methane into the reaction chamber may be in the range of 1 to 30 seem; and the flow rate of argon into the reaction chamber may be in the range of 0 to 100 seem. In a further exemplary arrangement, the flow rate of hydrogen into the reaction chamber may be in the range of about 10 to 50 seem or may be about 30 seem; the flow rate of methane into the reaction chamber may be in the range of about 1 to 10 seem or may be about 5 seem; and the flow rate of argon into the reaction chamber may be in the range of about 10 to 20 seem or may be about 15 seem. It will be appreciated by the skilled addressee that these values may be varied according to requirements, and may also vary based on operating conditions or equipment selection.

[ 0029 ] The at least one chalcogen may be selected from one or more of the group of sulphur, selenium, and tellurium. The film may be a chalcogenide glass film and may comprise greater than 25 atomic % of at least one chalcogen. The at least one chalcogen may comprise tellurium. The glass film may be a tellurite glass film. The glass film may be a tellurite glass film and may comprise greater than 20 atomic% OfTeO₂ or between 20 atomic% and 100 atomic% of TeO₂.

[ 0030 ] The thin glass film comprises at least two constituent components, at least one component being a chalcogen selected from the group of sulphur, selenium, and tellurium. The thin glass film may comprise at least three constituent components, at least one component being a chalcogenide selected from the group of sulphur, selenium, and tellurium. The thin glass film is a chalcogenide glass film. The thin glass film may be a chalcogenide glass film comprising the chalcogen tellurium in a chemical compound also comprising germanium and antimony.

[ 0031 ] The plasma system may be selected from the group of: a parallel plate Reactive Ion Etch (RIE) plasma system; Inductively Coupled Plasma (ICP) system or alternatively an Electron Cyclotron Resonance (ECR) plasma etching system, Magnetically Enhanced Reactive Ion (MERI) plasma etching system, Helicon plasma etching system a remote plasma system or a high density plasma etching system. The plasma system may comprise a reaction chamber within which the thin glass film is supported during etching. The plasma system may form disassociated species of injected gasses within the reaction chamber to effect etching of the thin glass film supported therein. The reaction chamber may be maintained at a low pressure during the etching process. The pressure of the reaction chamber during etching may be in the range of between 1 mTorr and several hundred mTorr and may be in the range of between 1 and 500, 1 and 400, 1 and 300, or 1 and 200 mTorr. In particular exemplary arrangements, the reaction chamber may be at a pressure of about 5 mTorr to about 100 mTorr. [ 0032 ] According to a second aspect, there is provided a chalcogen-containing thin glass film etched by the process of the first aspect.

[ 0033 ] The thin glass film may be a chalcogenide glass film. The at least one chalcogen may be selected from one or more of the group of sulphur, selenium, and tellurium. The thin film may comprise greater than 25% of at least one chalcogen. The at least one chalcogen may comprise tellurium. The thin glass film may contain greater than 20 atomic % by composition of Teθ₂. The thin glass film may be a thin tellurite glass film.

[ 0034 ] The thin glass film comprises at least two constituent components, at least one component being a chalcogen selected from the group of sulphur, selenium, and tellurium. The thin glass film may comprise at least three constituent components, at least one component being a chalcogenide selected from the group of sulphur, selenium, and tellurium. The thin glass film is a chalcogenide glass film. The thin glass film may be a chalcogenide glass film comprising the chalcogen tellurium in a chemical compound also comprising germanium and/or antimony.

[ 0035 ] The etched thin glass film may have a surface roughness of less than 20 nm, alternatively less than 10 nm rms, or further alternatively less than 5 nm rms.

[ 0036 ] The etched thin glass film may be suitable for device purposes. The etched thin glass film may be suitable for an optical device, and may be suitable for a low-loss optical device for example an optical waveguide. In other arrangements, the etched thin glass film may be suitable for memory devices for example phase-change memory devices.

[ 0037 ] According to a third aspect there is provided a method for fabrication of a device. The method may comprise forming a thin glass film of a compound or substance comprising at least one chalcogen suitable for device purposes. The method may further comprise the step of applying an etch mask to the thin glass film to define the device structure. The method may further comprise the step of etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching to form the device according to the desired device structure.

[ 0038 ] According to an arrangement of the third aspect there is provided a method for fabrication of a device, the method comprising: forming a thin glass film of a compound or substance comprising at least one chalcogen suitable for device purposes; applying an etch mask to the thin glass film to define the device structure; and etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching to form the device according to the desired device structure.

[ 0039 ] The thin glass film may be etched using the process of any one of the arrangements of the first aspect. The device may be an optical device and may be a low loss optical device. The device may be a waveguide device for example a planar waveguide. The device may alternatively be a memory device. The device may comprise isolated mesa structures of glass. The thin glass film may be formed on a suitable substrate and the thin glass film may be etched all the way through to the substrate by the etching process to form the isolated mesa structures. [ 0040 ] According to a fourth aspect, there is provided a system for etching a thin glass film. The system may comprise a plasma system comprising a reaction chamber. The system may further comprise injection means for injecting a hydrogen rich compound or substance in to the reaction chamber. When in operation, the injected hydrogen rich substance may be disassociated by a plasma formed by the plasma system to provide substantial amounts of free hydrogen in the reaction chamber. The system may further comprise support means for mounting a substrate in the reaction chamber, the substrate comprising a thin glass film comprising at least one chalcogen formed thereon such that the thin glass film is etched by free hydrogen in the reaction chamber.

[ 0041 ] According to an arrangement of the fourth aspect, there is provided a system for etching a thin glass film, comprising: a plasma system comprising a reaction chamber; means for injecting a hydrogen rich gaseous compound or substance in to the reaction chamber which, when in operation, may be disassociated by a plasma to provide substantial amounts of free hydrogen in the reaction chamber; means for mounting a substrate in the reaction chamber, the substrate comprising a thin glass film comprising at least one chalcogen formed thereon such that the thin glass film is etched by free hydrogen in the reaction chamber.

[ 0042 ] The system may be adapted to form energetic free hydrogen radical species and/or highly directional ionic hydrogen species in the reaction chamber such that the thin glass film is etched by the energetic free hydrogen radical species and/or highly directional ionic hydrogen species. In any of the aspects or arrangements described herein, the apparatus, system or method may also comprise one or more of any of the following either taken alone or in any suitable combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 0043 ] Arrangements of the processes will now be described, by way of an example only, with reference to the accompanying drawings wherein:

[ 0044 ] Figures IA and IB show scanning electron microscope (SEM) images of a TeO₂ chalcogenide etched with a prior art process using CHF₃ showing growth and sputtering of tellurium fluoride onto the sidewalls of the etched structure and exposed resist;

[ 0045 ] Figure 2 shows a method 200 for forming a structured glass film according to the presently described processes;

[ 0046 ] Figures 3A to 3 J depict the steps of method 200 of Figure 2 schematically;

[ 0047 ] Figures 4A and 4B show SEM images of a Teθ₂ tellurite film etched using the hydrogen plasma etching method disclosed herein with an CH₄+H₂+Ar mixture showing clean etched surfaces and sidewalls;

[ 0048 ] Figure 5A is a plot of the refractive index dependence Teθ₂ films based on the O/Te composition ratios;

[ 0049 ] Figure 5B is a plot of the propagation loss for TeO₂ thin films at various O/Te ratios etched using the presently disclosed methods; [ 0050 ] Figure 5C is a plot of the a plot of the total optical losses of a TeO₂ waveguide formed using the process 200 including the hydrogen etch method disclosed herein.

[ 0051 ] Figures 6A and 6B show SEM images of a Gen sAs₂i ₅Se₆₇ chalcogenide film etched using the hydrogen plasma etching method disclosed herein with an CH₄+H₂+Ar mixture showing clean etched surfaces and sidewalls;

[ 0052 ] Figures 6A and 6B are SEM images of a Ge_H sAs_{2I 5}Se₆₇ chalcogenide glass film in an Ar/H2/CH4 mix, also showing highly anisotropic etching with smooth vertical sidewalls; and

[ 0053 ] Figures 6A and 6B are SEM images of a As₂S₃ chalcogenide etched using a H₂/CH₄/Ar mixture under different conditions.

DETAILED DESCRIPTION

[ 0054 ] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. For the purposes of the present invention, the following terms are defined below.

[ 0055 ] The articles "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" refers to one element or more than one element.

[ 0056 ] The term "about" is used herein to refer to quantities that vary by as much as 30%, preferably by as much as 20%, and more preferably by as much as 10% to a reference quantity. In addition, numerical parameters recited herein, such as in the examples, are also intended to refer to approximate terms which the skilled addressee would realise would refer to terms having a degree of experimental uncertainty, which may be about 10% of the stated quantity or as much as about 20% of the stated quantity. For example, a gas flow rate of 15 seem Ar, would refer to a gas flow rate of about 15 seem ± about 10%, e.g. between about 13.5 seem to about 16.5 seem.

[ 0057 ] Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements.

[ 0058 ] Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. It will be appreciated that the methods, apparatus and systems described herein may be implemented in a variety of ways and for a variety of purposes. The description here is by way of example only.

[ 0059 ] A "glass" as used herein is defined as an inorganic material comprised of several elements that form a material with disordered atomic configuration which may be amorphous or nanoscale phase separated. The material may exhibit polyamorphism between different amorphous states or possess a reversible phase transition from amorphous/nanoscale phase separated to a polycrystalline state upon application of suitable heating/cooling cycles or other means of energy input/removal. [ 0060 ] A "chalcogen" element is an element residing in Group 16 of the periodic table, with the exclusion of oxygen, and thus comprises the group of sulphur, selenium, tellurium, polonium and ununhexium; and a "chalcogenide" is chemical compound consisting of at least one chalcogen and at least one more electropositive element.

[ 0061 ] Referring to the drawings, it has been realised by the inventors that high quality anisotropic etching of at least some chalcogenide-containing glasses, in particular tellurite glasses, is possible with a dry plasma etching process comprising etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching.

[ 0062 ] In one particular arrangement, plasma etching using gas mixtures comprising substantial amounts of free hydrogen, for example, a plasma formed from a mixture comprising argon, hydrogen, and methane gases, results in anisotropic etching with smooth sidewalls. An example of a tellurite film etched with a CH₄+H₂+Ar mixture showing clean etched surfaces and sidewalls as shown in the SEM images of Figures 4A and 4B. Using this method, the inventors have, to the best of their knowledge, fabricated the first low loss waveguides ever made in tellurium dioxide, as discussed in the example below, which clearly demonstrates the veracity of the process disclosed herein which is capable of forming waveguide structures immediately suitable for commercial application.

[ 0063 ] As will be appreciated, the methods disclosed herein provide a direct route to the fabrication of high quality device structures, for example low loss planar optical devices, particularly using tellurite glass which is widely recognised as one of the most suitable materials for this application, due to their very desirable MIR transparency, large fast non-linear optical effects, high acousto-optic figures of merit, and a range of other properties making them almost uniquely suitable for optical system on-a-chip devices spanning the visible to MIR regions. The application space for these is therefore extremely wide ranging, including for example photonic applications in telecommunications for example for systems of all-optical processing of high bit rate data streams, optical sensing, and mid-IR spectroscopy applications for example, in astronomy or molecular fingerprinting for biological, chemical, or explosive agents. Additional application of the method may be in the fabrication of high quality memory devices for example using the a germanium antimony telluride (Ge₂Sb₂Te₅ or "GST") glass that is recognized as an excellent candidate for phase-change (e.g. PRAM) memory devices. Additionally it is clear that the technology works well on at least some other chalcogenide glass materials, for applications in the emerging non-volatile phase change memory market where thin films of chalcogenide glass need to be patterned and reliably etched to give devices with well-defined structural features and very low surface roughness.

[ 0064 ] One clear advantage 'of the^ etch method disclosed herein is that it is halogen free, that is, the etch gas does not comprise any halogen constituents such as fluorine, chlorine, bromine or iodine. A halogen-free method as disclosed herein become increasingly important as the electronics industry moves to halogen free processes on account of ozone hole and global warming concerns.

[ 0065 ] Additional advantages of the present methods include lower costs for etch gases, being non halogen based for compatibility with halogen free processes currently being sought in the electronics industry, and also that most known hydrides of common glass formers or rare earth dopants are volatile in vacuum, that is, the present methods are also directly applicable to doped or multi- component glasses which will etch cleanly to give etched structural features with smooth, well-defined vertical walls. The present method is also particularly applicable to etching of chalcogen-containing glasses comprising tellurium (e.g. tellurite glasses having TeO₂ as a significant component thereof) , for which the inventors are not aware of an alternative efficient etching process.

Plasma Etching Process

[ 0066 ] The etching process described herein is based on well-known plasma etching methodologies, for example the process is demonstrated herein using parallel plate reactive ion etch (RIE) plasma system. In other arrangements, however, the plasma etching system maybe chosen from one of: Inductively Coupled Plasma (ICP) system or alternatively an Electron Cyclotron Resonance (ECR) plasma etching system, Magnetically Enhanced Reactive Ion (MERI) plasma etching system, Helicon plasma etching system or high density plasma etching system as would be appreciated by the skilled addressee.

[ 0067 ] The present plasma etching process is a process for etching glass comprising at least one chalcogen, wherein the process comprises the step of etching the glass with a plasma comprising substantial amounts of free hydrogen. The amount of free hydrogen in the plasma is sufficient to directly affect etching of the glass. The free hydrogen may preferentially be achieved by injection of hydrogen (H₂) where it is disassociated by the plasma to form reactive ionic hydrogen species which are accelerated towards the glass to effect etching thereof. The chalcogen-containing glass may be a thin glass film.

[ 0068 ] The glass film is typically formed on an appropriate substrate, which in turn is supported in the reaction chamber of the plasma system, typically on a substrate plate using common techniques (e.g. electrostatic chuck or backside cooling methods) so as to provide good thermal contact to the substrate plate, which is also used to control the temperature of the glass film during the process. The temperature of the glass film may be maintained at room temperature (e.g. between about 2O⁰C and 3O⁰C, typically about 25⁰C) and the example below demonstrates etching at this temperature, however, the film temperature may be heated or cooled in accordance with requirements as would be appreciated by the skilled addressee. For instance, it may be particularly advantageous to heat or cool the glass film depending on the components of the glass (e.g. dopants etc) and the particular reaction by-products formed by the etching process. For example, for Erbium-doped tellurite glass, heating the glass film to a temperature of about 25O⁰C may have significant advantages for hydrogen-based plasma etching since a reaction by-product of the etching process, Erbium-hydride, becomes volatile in the plasma and may be readily driven off the glass film.

[ 0069 ] The reaction chamber is typically maintained under relatively low pressure during the etching process. Typical pressures may lie in the range of between 1 mTorr and several hundred mTorr depending on the particular plasma system (ICP, ECR, helicon etc), for example between about 1 mTorr and about 500 mTorr or more. In particular exemplary arrangements, the pressure may be maintained in the range of about 5 mTorr to about 100 mTorr. As would be appreciated by the skilled addressee, in conditions of very low chamber pressure (e.g. ~1 mTorr), it may be required to strike the plasma at a higher initial pressure or remotely from the reaction chamber, wherein the etching method would be a remote plasma etching method. In this case, additional gases may also be required in the reaction chamber to maintain suitable conditions as would be appreciated by the skilled addressee.

[ 0070 ] The etching rate of the glass films in the present process, as would be appreciated, depend on a large number of parameters, for example the size of the etching sample, composition of the films, the particular plasma source used (ICP, ECR etc) or the exact composition of the gases, but primarily on the radio frequency (rf) power of the plasma source the concentration of the free hydrogen in the reaction chamber. Typical values of the rf power will of course depend on the particular plasma system used, for example rf powers of between about 200W and about 300W may be applicable for a ICP RIE plasma system.

[ 0071 ] Typical etch rates for chalcogen-containing glass films, e.g. tellurite glass films, is in the range of about 20 nm per minute to about 100 nm per minute. For other chalcogenide materials, the etch rate may be up to about 200 to 300 nm per minute. Of course, it will be appreciated that the smoothness of the resultant etched material may depend on the etch rate, therefore, often the rate is adjusted by altering the operating conditions such that the etched glass film has the required smoothness for the particular application. The surface roughness of the resulting etched structures will typically determine the quality of the final structure, which of course depends on the application for the etched structures. For example, for optical applications, the surface roughness of the etched glass film will typically be required to be less than 20 nm RMS, however, this value will also depend on the desired operating wavelength for a device formed from the glass' film, therefore the surface roughness may need to be much less than 20 nm RMS (e.g. less than 10 or even less than 5 nm RMS). In other applications, a larger roughness may be tolerated, for example in memory applications (particularly where the film is etched through to the substrate to form isolated mesa structures of glass).

[ 0072 ] The etching of the glass ^■ film is generally governed by one of two processes (or both processes simultaneously) which lead to anisotropic etching and smooth sidewalls as desired for industrial and commercial application of the etched films.

[ 0073 ] The first is through free radicals or highly directional ionic species or involatile etch products generated by the plasma which impact the glass film and cause etching thereof. These free radicals or ionic species maybe very chemically reactive with the material being etched, may possess large amounts of kinetic energy, and are typically charged ionic species which may be accelerated towards the glass film. Etching can proceed by chemical attack, physical sputtering of the material by charged species, or by a combination of both processes. The high directionality of these accelerated ionic species leads to highly anisotropic etching of the film. It has been found by the inventors that ionised hydrogen species are particularly advantageous in etching chalcogenide-containing films as shown in the example below. Other etching gases may also be used, for example halogen-based gases (e.g. CHF₃, CF₄, C₄Fg, CH₃F, ClF, and many others), however these are generally many orders of magnitude more expensive than hydrogen and particular care is required regarding the reaction byproducts, hence hydrogen-based plasma etching is preferred. Also, halogen based processes are being phased out due to environmental concerns.

[ 0074 ] The second beneficial process in plasma etching of chalcogenide-containing films is the formation of reaction by-products of the etching process which result in side-wall passivation of the glass film during etching. The passivated sidewall acts to protect the sidewall from being chemically etched causing undercutting of the as-formed mesa structure of glass film, thereby resulting in highly anisotropic etching of the glass film and formation of etched structures with smooth sidewalls. In particular arrangements, depending on the particular components in the etching gas and/or the components of the glass film, the plasma etching method, or the particular operational conditions in the process, the anisotropic etching of the film may be effected by either the high directionality of the ionic species, or the sidewall passivation of the etched structure, or a combination of both. In particular arrangements, the additional gas may also aid the directionality of the main etch gas to increase the anisotropy of the etching process.

[ 0075 ] In particular arrangements, whilst not required under all conditions, the process may also comprise the addition of a passivation means to promote sidewall passivation of the glass film during the etching process. For instance, the additional of a passivation means may assist in an ICP RIE plasma system, however, other plasma systems may be able to produce sufficient reactive ionic species with sufficient directionality to effect anisotropic etching of the glass film alone. The passivation means may be an additional gas may be added to the reaction chamber to promote sidewall passivation and assist with the etching process. For example, in a hydrogen-based plasma etching system such as described herein, an additional gas such as hydrocarbon-containing compound or substances such as alkanes, preferably linear alkanes for example methane (CH₄), ethane (C₂H₆), propane (C₃H₈), or butane (C₄H₁₀), may be injected into the reaction chamber. In other arrangements, branched and/or cyclical alkanes may also be used. In further arrangements still, alkyne compounds or substances (linear-, branched- and/or cyclo-alkynes) or alkene compounds or substances (linear-, branched- and/or cyclo-alkenes) may be used. In other arrangements, the passivation promoting substance may comprise either a fluorocarbon or a fluoro-hydrocarbon compound or substance that promotes sidewall polymerisation and may be selected from the group of a fiuoro-alkane, fluoro- alkene, fiuoro-alkyne, fluoro-cyclo-alkane, fluoro- cyclo-alkene, fluoro- cyclo-alkyne, per-fluoro- alkane, per-fluoro-alkene, per-fluoro-alkyne, per-fluoro-cyclo-alkane, per-fluoro- cyclo-alkene, or a per-fluoro- cyclo-alkyne. An example fluorocarbon may be C₄F₈. In further arrangements still, the passivation means may be a compound comprising a halogen selected from the group comprising chlorine, fluorine and bromine; or a halogen-containing compound, for example TeF, ClF, hydrogen bromide or hydrogen iodide among many others.

[ 0076 ] It has been found by the inventors that the addition of the further gas does not significantly effect the etching conditions, for example the etch rate based on a particular arrangement, however the passivation of the sidewalls significantly increases the anisotropy of the etching and forms smooth walled structures. When disassociated the CH₄ (or other suitable gas) would produce additional free hydrogen in the reaction chamber to assist the etching process and the free CH_x species in the reaction chamber would promote passivation of the sidewalls through hydrocarbon based polymer formation. The ratio of the additional passivation gas means (e.g. CH₄) to the primary etching gas (e.g. H₂) may be quite small and may be in the ration of 1 :3 to 1 :6 or more, depending on the amount of passivation required for the particular glass film composition and/or the operating conditions of the plasma system. The relative concentration in the reaction chamber of the additional gas is kept at such low levels since high concentrations may lead to the uncontrolled formation of unwanted polymer species and a reduction of the etching rate of the glass film of polymer particulates inducing roughness in the etched surfaces. Conversely, if the concentration of the additional gas is too low, then passivation of the sidewalls may not occur and undercutting of the etched structure may result.

[ 0077 ] In further arrangements, again possibly not required under all operating conditions, an inert gas (e.g. a noble gas) may also be injected into the reaction chamber. Typically, argon is used in most cases although other inert gases may be suitable under different conditions, in general, any gas which does not react with the glass film when in decomposed form may be suitable as would be appreciated by the skilled addressee. The addition of the inert gas forms additional ionised species which may be useful for purely physical sputtering of the surface, which acts to keep the surface clean of particular reaction by-products. This may be particularly beneficial where the reaction by-products are solids (e.g. erbium hydroxides formed by hydrogen plasma etching of erbium-doped tellurite glass films).

Example 1 - Plasma Etching of Tellurite Glass

[ 0078 ] As discussed above, tellurite glasses are a large family of glasses containing tellurium oxide (TeO₂) as the main component, which have attractive properties for nonlinear optics compared with the widely investigated silica and other, non-tellurite-based, chalcogenide glasses. It is found that the disclosed etching process works well for all stoichiometric conditions such as Te rich, O rich or stoichiometric TeO₂ and it is expected that the etch process is applicable to most if not all compositional variants of chalcogen-containing glass films, particularly tellurite glass films.

[ 0079 ] Obtaining stoichiometric, low loss tellurite (TeO₂) thin glass films and waveguide is crucial for any future application in integrated photonics. The present example describes a process 200 as depicted in Figure 2 for the formation of tellurite thin glass films using reactive RF sputtering that results in films with the required properties (steps 101 to 111) and the fabrication of rib waveguides with very low loss using the above described hydrogen-based plasma etching process (step 113).

[ 0080 ] Figure 3A depicts the deposition (step 101) of the optical film 201 on a suitable substrate 203. There are several methods for thin film deposition including: thermal evaporation, pulse laser deposition, sputtering etc. The deposited film 201 usually has thickness of few hundreds nanometres to few micrometers depending on applications.

[ 0081 ] In the present example, TeO_x thin films were produced by reactive RF sputtering from a pure (99.95%) Te powder pressed target in an O₂/ Ar gas mixture. The sputtering parameters were chamber pressure (ranging from between about 2.0 mTorr to about 20 mTorr), RF power (between about 120W to about 360W), and percentage of O₂ flow (between about 20% to about 80%). The total flow of O₂ and Ar in the present example was maintained at 15 seem. The films were deposited on 4 inch thermal oxide substrates and were up to about 2 μm thick. The films were characterised using EDXA for composition; by a dual angle spectroscopic reflectometry method for thickness and refractive index; and by prism coupling for optical losses via image processing of the light streaks observed using an IR camera.

[ 0082 ] Figure 3B depicts depositing of a thin polymer layer 205, for example using a spin coating method, to protect the film 201 and help the exposure process (step 103). This layer 205 is typically a couple of hundreds nanometres thick and serves as a protected layer for the chemical process to follow and reduce the standing wave during the exposure to narrow line ultraviolet light.

[ 0083 ] Figure 3C depicts depositing (step 105) a layer of photoresist layer 207 over the protective polymer layer 205, for example using a spin coating method. The thickness of the photoresist layer 207 is typically of the order of few hundreds nanometres to several micrometers. A soft bake might also be recommended prior to further processing.

[ 0084 ] While only one protective layer 205 is described, it is envisaged that a plurality of intermediate layers may be located between the film 201 and the photoresist layer 207, only some of which may have protective functions. The remaining layers can, for example, be used to improve desired mechanical, optical or other properties of the resultant microstructure. In addition, the protective function of the protective layer 205 can be distributed among more than one layer. For example, the protection against the organic solvents used in the photoresist layer 207 and against the alkaline solution used for developing the photoresist layer 207, may be offered by different intermediate layers.

[ 0085 ] Figure 3D depicts exposure of the film (step 107) to ultraviolet light 209 under a mask 208 to form pattern on the photoresist layer. The exposure can be in contact mode or non-contact mode. A post exposure bake may also be performed.

[ 0086 ] The exposed photoresist layer 207 is then developed (step 109) in a suitable solution to remove unwanted photoresist areas leaving only a desired photoresist pattern 207a on the film 201 as depicted in Figure 3E. A bake might also be recommended. At this point the thin protective layer 205 is till in place.

[ 0087 ] The protective layer is then etched (step 111) to remove the protective layer 205 in regions not protected by the photoresist pattern 207a and to expose the optical film 201 as depicted in Figure 3F. The process is generally performed in an oxygen plasma 211. The film 201 is now ready for plasma etching (step 113) using the plasma etching process depicted in Figure 3G and described above to form the desired structures such as, for example, a waveguide 220 as depicted in Figure 3H.

[ 0088 ] In the present example, an parallel plate Reactive Ion Etch (RIE) plasma system (Oxford Series 80 RIE) with a gas mix that consisted of argon (Ar), hydrogen (H₂) and methane (CH₄) was used to form the etch plasma 215., In one exemplary arrangement, the gas flow of each of the gases were set at 15 seem, 30 seem and 5 seem (standard centimetre cubic per minute), respectively. The rf power of the plasma source was about 200 W giving an etch rate of approximately 66 ran per minute.

[ 0089 ] In other arrangements, the flow rates may be modified whilst still maintaining effective etch conditions. For the present example, the possible flow rates may typically range between about 0 to 100 seem for Ar, about 5 to 100 seem for H₂ and about 1 to 30 seem for CH₄. In the system used, a small amount of methane was seen to be beneficial to the quality of the etched film due to sidewall passivation as the directionality of the ionic hydrogen etching species was sub-optimal. Indeed, the addition of the methane also improved the directionality and the etching process directly, most likely due to the increased concentration of active radicals in the reaction chamber.

[ 0090 ] Once the plasma etching process in completed, further optional steps may be performed to prepare the etched structure for the desired application. For example, the remaining photoresist and protective area may be removed (step 115, and Figure 31), which may be performed by exposing the films to an oxygen plasma 211 or other suitable method. A light (perhaps preferentially isotropic) sputter etching (step 116) of the etched film may also be performed to remove any remaining byproducts of the etching process, for example when etching tellurite films, a thin hydroxyl layer may form on the glass surface. If the etched tellurite glass film is intended to be used for optical application, this hydroxyl layer can increase the optical loss of the structure at some wavelengths due to broad hydrogen absorption bands, particularly at approximately 1550 nm and around 2.9 μm. Other methods of removing the hydroxyl layer may also be used as would be known to the skilled addressee, for example annealing the sample at high temperature in oxygen or chlorine. For other applications, however, the effect of any remaining the reaction by-products may be negligible, therefore removal may not be required.

[ 0091 ] Furthermore, the etched films may optionally be coated (step 117, Figure 3J) with an upper cladding layer, for example, to protect it from dusts and other elements. The top layer can also act as an upper cladding for optical wave guiding applications. The waveguides of the present example were coated with an inorganic polymer glass (for example a polymer glass film with refractive index n = 1.509, available from Redfern Polymer Optics Pty Ltd of Canberra, Australia) to provide a protective top cladding.

[ 0092 ] A wide range of sputtering parameters were studied using the process 200 described above and the resulting films had different O/Te ratios. The summary of refractive indices at 1550nm against ratio of O/Te is shown in Figure 5A). The as-deposited amorphous TeO_x films can be classified into three types: Tellurium rich (x < 2); close to stoichiometric (x = 2); and Oxygen rich (x > 2). Tellurium rich films have excessive levels of unbonded Te atoms and are highly absorbing. These films have an effective band edge high up in the red or near infrared and refractive indices at 1550 nm greater than 2.1. Oxygen rich films have lower density, hence, lower refractive index as can be seen in Figure 5A The refractive index also strongly depends on the sputtering conditions; the dots with error bars are data points; the large dot represents bulk (stoichiometric) TeO₂ glass; and the solid line is a 3rd order polynomial fitting curve for eye guiding. [ 0093 ] The propagation losses of the films at 1550 nm were determined from observation of the light streaks for a set of as-deposited TeO_x thin films on silica substrates with film thickness of around 1.5 μm and O/Te composition ratios of 1.6 to 2.4 and the loss in dB/cm is plotted in Figure 5B. As can be seen, the minimum loss observed was less than 0.1 dB/cm (limited by sensitivity of the measurement equipment) at 1550nm, which is the first time such low losses have been observed. An excessive level of Te in the films, x < 2, is expected to produce high losses and this is indeed observed. However, the loss curve dips to a minimum right at the stoichiometric point before gradually increasing as the Oxygen content increases. The losses of oxygen rich films would be expected to be lower when they are annealed.

[ 0094 ] After the etching process was completed, the fabricated rib waveguides had dimensions of 4μm width, 1.8μm in total thickness and 0.8μm in ridge height, (as shown in the inset of Figure 5C) and 7cm long. The waveguide was characterized by white light loss measurement using an arc lamp source and optical spectrum analyser. Two fibre taper lenses, which provided beam waists of 2.5μm in diameter, were used to couple light in and out of the waveguide to measure the insertion loss spectrum from 600-1700 nm and the total loss of the waveguide is plotted in Figure 5C. The total losses includes coupling losses (~0.7 dB per end or ~1.4 dB total coupling loss), reflection (1.0 dB) averaged over the chip's Fabry-Perot response and propagation loss a minimum possible loss of 2.4 dB, as indicated by the solid horizontal line.

[ 0095 ] The minimum loss for the waveguide is found to be around 2.5 dB corresponding to propagation loss of less than 0.05 dB/cm, limited by the accuracy of the measurement. This is comparable with best achieved loss in similar non-linear optical waveguides and approaches the best results obtained in the current industry standard materials for passive circuitry, i.e. germanosilicate.

Example 2 - Plasma Etching of Chalcogenide Glass

[ 0096 ] The hydrogen-based plasma etching process disclosed herein has also been applied to chalcogenide glass films. For example, Figures 6A and 6B show SEM images of the resultant etched structures of a Gen 5As21 ₅Se_0? chalcogenide film etched using the hydrogen plasma etching method disclosed herein with an CH₄+H₂+AJ mixture in a RIE plasma system with similar condition to those described in Example 1 above (200W, 30mTorr, 15 seem Ar, 30sccm H₂, 5sccm CH₄). As can clearly be seen, the hydrogen plasma etching process results in highly anisotropic etching with smooth vertical sidewalls.

Comparative Example

[ 0097 ] Contrary to the above examples, Figures 7A and 7B show SEM images of hydrogen plasma etching of As₂S₃, probably the most widely studied chalcogenide glass, etched using the hydrogen plasma etching method disclosed herein with an CH₄+H₂+Ar mixture in a RIE plasma system with similar condition to those described in Example 1 above (200W, 30 mTorr, 15 seem Ar, 30 seem H₂, 5 seem CH₄). Clearly the etching accomplished here is not acceptable due to the enormous surface roughness generated, and indicating that Hydrogen based etching is potentially not universally applicable to chalcogenide glasses. It is suspected that rapid preferential removal of sulphur in the film as H₂S and/or the formation of c-H-S based polymers may be, at least partially responsible for the poor result obtained in this case, but the inventors to date have not studied the mechanisms in this example in detail. Improved etch quality may be achieved by substituting one or more of the injected gasses with, for example ammonia or butane instead of methane.

Summary

[ 0098 ] It will be appreciated that the processes and examples described/illustrated above at least substantially provide a process for plasma etching of glass films comprising a chalcogen-containing glass using a hydrogen-based plasma etching method, particularly for tellurite glasses.

[ 0099 ] The methods described herein, and/or shown in the drawings, are presented by way of example only and are not limiting as to the scope of the invention. Unless otherwise specifically stated, individual aspects and steps/conditions of the method may be modified, or may have been substituted therefore known equivalents, or as yet unknown substitutes such as may be developed in the future or such as may be found to be acceptable substitutes in the future. The methods may also be modified for a variety of applications while remaining within the scope and spirit of the claimed invention, since the range of potential applications is great, and since it is intended that the present methods be adaptable to many such variations.

Claims

CLAIMS:

1. A process for etching a thin glass film comprising at least one chalcogen, the process comprising the step of: etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching.

2. A process as claimed in claim 1 wherein the plasma is derived from a hydrogen rich mixture.

3. A process as claimed in claim 1, wherein prior to etching the thin glass film, the process further comprises the steps of: forming the thin glass film on a substrate; forming a mask layer on the thin glass film; mounting the substrate on a substrate plate in a reaction chamber of a plasma system, wherein the mounting provides thermal contact between the thin glass film and the substrate plate; heating or cooling the thin glass film to a desired etching temperature; evacuating the reaction chamber to a desired etching pressure; injecting one or more gaseous substances into the reaction chamber and striking a plasma in the plasma system, whereby the injected gases are disassociated by the plasma, such that substantial amounts of free hydrogen sufficient to effect etching of the thin glass film are formed in the reaction chamber.

4. A process as claimed in claim 3 wherein the injected gaseous substances are hydrogen rich.

5. A process as claimed in claim 3 wherein the process comprises the addition of a passivation means for promoting passivation of the glass film during etching.

6. A process as claimed in any one of claims 3 to 5 wherein the injected gaseous substances comprise a substance which when disassociated promotes sidewall passivation of the thin glass film whilst etching of said film is occurring.

7. A process as claimed in claim 5 wherein the passivation promoting substance is selected from a hydrocarbon-containing gas.

8. A process as claimed in either claim 5 or claim 6 wherein the passivation promoting substance is selected from the group comprising: methane (CH₄), ethane (CaH₆), propane (CjH₈), and butane (C₄H io).

9. A process as claimed in any one of claims 3 to 8 wherein the injected gaseous substances comprise an inert gaseous substance which does not react with the glass when disassociated.

10. A process as claimed in claim 9 wherein the inert gaseous substance promotes cleaning of reaction by-products formed on the film during the etching of said film.

11. A process as claimed in either claim 9 or claim 10 wherein the inert gaseous substance is argon.

12. A process as claimed in claim 3 wherein the injected gaseous substances comprise an etch gas, and either: a passivation promoting gas; an inert gas; or both a passivation promoting gas and an inert gas.

13. A process as claimed in claim 12 injected gaseous substances comprise an etch gas, a passivation promoting gas and an inert gas.

14. A process as claimed in claim 13 wherein the etch gas if hydrogen (H₂), the passivation promoting gas is methane (CH₄) and the inert gas is argon.

15. A process as claimed in claim 14 wherein: the flow rate of hydrogen into the reaction chamber is in the range of 5 to 100 seem; the flow rate of methane into the reaction chamber is in the range of 1 to 30 seem; and the flow rate of argon into the reaction chamber is in the range of 0 to 100 seem.

16. A process as claimed in claim 15 wherein: the flow rate of hydrogen into the reaction chamber is about 30 seem; the flow rate of methane into the reaction chamber is about 5 seem; and the flow rate of argon into the reaction chamber is about 15 seem.

17. A process a claimed in claim 16 wherein the thin glass film is tellurite glass.

18. A process as claimed in any one of claims 1 to 16 wherein the thin film is a chalcogenide glass film and comprises greater than 25 atomic% of at least one chalcogen.

19. A process as claimed in any one of claims 1 to 17 wherein the thin film is a tellurite glass film and comprises greater than 20 atomic% of TeCV

20. A process as claimed in any one of claims 1 to 16 wherein the at least one chalcogen comprises sulphur, selenium, or tellurium.

21. A process as claimed in any one of claims 1 to 17 wherein the at least one chalcogen comprises tellurium and the thin glass film is a thin tellurite glass film.

22. A process as claimed in any one of claims 1 to 17 wherein the thin glass film is a tellurite glass film.

23. A process as claimed in claim 22 wherein the thin glass film contains greater than 20% by composition OfTeO₂.

24. A process as claimed in any one of claims 1 to 16 wherein the thin glass film comprises at least two constituent components, at least one component being a chalcogen selected from the group of sulphur, selenium, and tellurium.

25. A process as claimed in any one of claims 1 to 16 wherein the thin glass film comprises at least three constituent components, at least one component being a chalcogenide selected from the group of sulphur, selenium, and tellurium.

26. A process as claimed in any one of claims 1 to 16 wherein the thin glass film is a chalcogenide glass film.

27. A process as claimed in any one of claims 1 to 16 wherein the thin glass film is a chalcogenide glass film comprising the chalcogen tellurium in a chemical compound also comprising germanium and antimony.

28. A chalcogen-containing thin glass film etched by the process of any one of the preceding claims.

29. A method for fabrication of a device, the method comprising: forming a thin glass film of a substance comprising at least one chalcogen suitable for device purposes; applying an etch mask to the thin glass film to define the device structure and etching the thin glass film with a plasma comprising substantial amounts of free hydrogen sufficient to effect etching to form the device according to the desired device structure

30. A system for etching a thin glass film, comprising: a plasma system comprising a reaction chamber; means for injecting a hydrogen rich substance in to the reaction chamber which, when in operation, is disassociated by a plasma to provide substantial amounts of free hydrogen in the reaction chamber; means for mounting a substrate in the reaction chamber, the substrate comprising a thin glass film comprising at least one chalcogen formed thereon such that the thin glass film is etched by free hydrogen in the reaction chamber.