WO2010138999A1 - Plasma etching of chalcogenides - Google Patents
Plasma etching of chalcogenides Download PDFInfo
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- WO2010138999A1 WO2010138999A1 PCT/AU2010/000665 AU2010000665W WO2010138999A1 WO 2010138999 A1 WO2010138999 A1 WO 2010138999A1 AU 2010000665 W AU2010000665 W AU 2010000665W WO 2010138999 A1 WO2010138999 A1 WO 2010138999A1
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- glass film
- etching
- thin glass
- plasma
- reaction chamber
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- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C15/00—Surface treatment of glass, not in the form of fibres or filaments, by etching
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N70/00—Solid-state devices without a potential-jump barrier or surface barrier, and specially adapted for rectifying, amplifying, oscillating or switching
- H10N70/011—Manufacture or treatment of multistable switching devices
- H10N70/061—Patterning of the switching material
- H10N70/063—Patterning of the switching material by etching of pre-deposited switching material layers, e.g. lithography
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N70/00—Solid-state devices without a potential-jump barrier or surface barrier, and specially adapted for rectifying, amplifying, oscillating or switching
- H10N70/20—Multistable switching devices, e.g. memristors
- H10N70/231—Multistable switching devices, e.g. memristors based on solid-state phase change, e.g. between amorphous and crystalline phases, Ovshinsky effect
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N70/00—Solid-state devices without a potential-jump barrier or surface barrier, and specially adapted for rectifying, amplifying, oscillating or switching
- H10N70/801—Constructional details of multistable switching devices
- H10N70/881—Switching materials
- H10N70/882—Compounds of sulfur, selenium or tellurium, e.g. chalcogenides
- H10N70/8828—Tellurides, e.g. GeSbTe
Definitions
- 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.
- 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.
- Chalcogen-containing glasses are materials falling typically into one of two classes, namely tellurite or chalcogenide glasses.
- Tellurite glasses are materials largely comprising TeO 2 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 TeO 2 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.
- these materials can be doped with Halide elements (I 2 , 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).
- 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 TeO 2 , As 2 S 3 , As 2 Se 3 , GeS 2 , the As-S-Se family, the Ge-As-Se family (e.g. AMTIR-I Ge 33 As 12 Se 55 ), 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.
- phase change memory also known as PCM, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM and C-RAM
- a particular chalcogenide that is recognized as an excellent candidate for PRAM application is a germanium antimony telluride (Ge 2 Sb 2 Te 5 ) chalcogenide material, also known as "GST".
- Boswell "Dry-etch Of As 2 S 3 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.
- 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.
- 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 ⁇ 7 ⁇ m. 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.
- Dry etching has also previously been disclosed for various other chalcogenide containing glasses, for example, chlorine-based etching of the chalcogenide Ge 2 Sb 2 Te 5 (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 Ge 2 Sb 2 Te 5 in CF 4 / 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.].
- 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.
- the substantial amount of free hydrogen may comprise more than 5 atomic% of H 2 , more than 10 atomic% Of H 2 , more than 15 atomic% Of H 2 , more than 20 atomic% of H 2 , 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.
- the hydrogen rich gas may be ammonia
- 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.
- 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.
- 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.
- the plasma may be derived from a gas comprising ammonia (NH 4 ) which may promote formation of hydrogen-containing free radicals such as NH 3 which may react with the surface of the glass film to either effect or enhance etching.
- NH 4 ammonia
- the hydrogen-containing free radicals my assist etching of the glass film.
- 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.
- the free hydrogen may be achieved by injection of hydrogen gas (Ky into a plasma system where the H 2 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.
- 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.
- 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.
- 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 (CH 4 ), ethane (C 2 H 6 ), propane (CjH 8 ), or butane (C 4 H 10 ), which may be injected into the reaction chamber.
- aromatic hydrocarbons, or branched and/or cyclic alkanes may be injected into the reaction chamber.
- alkyne compounds or substances linear-, branched- and/or cyclo-alkynes
- alkene compounds or substances linear-, branched- and/or cyclo-alkenes
- alternative hydrogen rich compounds or substances may be envisaged, for example H 2 O.
- 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.
- 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 (C 2 He), propane (C 3 H 8 ), or butane (C 4 H 1 0).
- the passivation promoting compound or substance may comprise methane.
- aromatic hydrocarbons, or branched and/or cyclic alkanes may also be used.
- alkyne compound or substances linear-, branched- and/or cyclo-alkynes
- alkene compound or substances linear-, branched- and/or cyclo-alkenes
- 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 C 4 F 8 .
- 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.
- 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.
- the inert gaseous substance may be argon.
- 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.
- 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.
- the injected gaseous compounds or substances may comprise an etch gas, a passivation promoting gas and an inert gas.
- the etch gas may be hydrogen (H 2 )
- the passivation promoting gas may be methane (CH 4 )
- the inert gas may be argon.
- 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.
- 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.
- 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% OfTeO 2 or between 20 atomic% and 100 atomic% of TeO 2 .
- 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.
- 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.
- the reaction chamber may be at a pressure of about 5 mTorr to about 100 mTorr.
- 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 Te ⁇ 2 .
- the thin glass film may be a thin tellurite glass film.
- 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.
- 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.
- 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.
- the etched thin glass film may be suitable for memory devices for example phase-change memory devices.
- a method for fabrication of a device 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.
- a method for fabrication of a device 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.
- 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.
- a system for etching a thin glass film 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.
- 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.
- 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.
- 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.
- the apparatus, system or method may also comprise one or more of any of the following either taken alone or in any suitable combination.
- Figures IA and IB show scanning electron microscope (SEM) images of a TeO 2 chalcogenide etched with a prior art process using CHF 3 showing growth and sputtering of tellurium fluoride onto the sidewalls of the etched structure and exposed resist;
- Figure 2 shows a method 200 for forming a structured glass film according to the presently described processes
- Figures 3A to 3 J depict the steps of method 200 of Figure 2 schematically;
- Figures 4A and 4B show SEM images of a Te ⁇ 2 tellurite film etched using the hydrogen plasma etching method disclosed herein with an CH 4 +H 2 +Ar mixture showing clean etched surfaces and sidewalls;
- Figure 5A is a plot of the refractive index dependence Te ⁇ 2 films based on the O/Te composition ratios
- Figure 5B is a plot of the propagation loss for TeO 2 thin films at various O/Te ratios etched using the presently disclosed methods;
- Figure 5C is a plot of the a plot of the total optical losses of a TeO 2 waveguide formed using the process 200 including the hydrogen etch method disclosed herein.
- Figures 6A and 6B show SEM images of a Gen sAs 2 i 5 Se 67 chalcogenide film etched using the hydrogen plasma etching method disclosed herein with an CH 4 +H 2 +Ar mixture showing clean etched surfaces and sidewalls;
- Figures 6A and 6B are SEM images of a Ge H sAs 2I 5 Se 67 chalcogenide glass film in an Ar/H2/CH4 mix, also showing highly anisotropic etching with smooth vertical sidewalls;
- Figures 6A and 6B are SEM images of a As 2 S 3 chalcogenide etched using a H 2 /CH 4 /Ar mixture under different conditions.
- 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.
- 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.
- plasma etching using gas mixtures comprising substantial amounts of free hydrogen results in anisotropic etching with smooth sidewalls.
- gas mixtures comprising substantial amounts of free hydrogen for example, a plasma formed from a mixture comprising argon, hydrogen, and methane gases.
- 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 CH 4 +H 2 +Ar mixture showing clean etched surfaces and sidewalls as shown in the SEM images of Figures 4A and 4B.
- 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.
- 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 (Ge 2 Sb 2 Te 5 or "GST”) glass that is recognized as an excellent candidate for phase-change (e.g. PRAM) memory devices.
- GST germanium antimony telluride
- 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.
- 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 TeO 2 as a significant component thereof) , for which the inventors are not aware of an alternative efficient etching process.
- 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.
- 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.
- 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 (H 2 ) 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.
- 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 2O 0 C and 3O 0 C, typically about 25 0 C) 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.
- heating the glass film to a temperature of about 25O 0 C 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.
- 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.
- the pressure may be maintained in the range of about 5 mTorr to about 100 mTorr.
- 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.
- additional gases may also be required in the reaction chamber to maintain suitable conditions as would be appreciated by the skilled addressee.
- the etching rate of the glass films in the present process 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.
- 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.
- Typical etch rates for chalcogen-containing glass films is in the range of about 20 nm per minute to about 100 nm per minute.
- the etch rate may be up to about 200 to 300 nm per minute.
- 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.
- 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).
- 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.
- 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.
- etching gases may also be used, for example halogen-based gases (e.g. CHF 3 , CF 4 , C 4 Fg, CH 3 F, 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.
- halogen-based gases e.g. CHF 3 , CF 4 , C 4 Fg, CH 3 F, ClF, and many others
- 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.
- 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.
- the additional gas may also aid the directionality of the main etch gas to increase the anisotropy of the etching process.
- the process may also comprise the addition of a passivation means to promote sidewall passivation of the glass film during the etching process.
- 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.
- an additional gas such as hydrocarbon-containing compound or substances such as alkanes, preferably linear alkanes for example methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), or butane (C 4 H 10 ), may be injected into the reaction chamber.
- alkanes preferably linear alkanes for example methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), or butane (C 4 H 10 .
- alkanes preferably linear alkanes for example methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), or butane (C 4 H 10 .
- alkyne compounds or substances linear-, branched- and/or cyclo-alkynes
- alkene compounds or substances linear-, branched- and/or cyclo-alkenes
- 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 C 4 F 8 .
- 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.
- H 2 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.
- an inert gas e.g. a noble gas
- 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).
- tellurite glasses are a large family of glasses containing tellurium oxide (TeO 2 ) 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 TeO 2 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.
- TeO 2 tellurium oxide
- stoichiometric, low loss tellurite (TeO 2 ) 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).
- Figure 3A depicts the deposition (step 101) of the optical film 201 on a suitable substrate 203.
- 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.
- TeO x thin films were produced by reactive RF sputtering from a pure (99.95%) Te powder pressed target in an O 2 / 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 O 2 flow (between about 20% to about 80%).
- the total flow of O 2 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 ⁇ m 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.
- 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.
- 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.
- protective layer 205 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.
- 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.
- 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.
- 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.
- 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.
- etch plasma 215. an parallel plate Reactive Ion Etch (RIE) plasma system (Oxford Series 80 RIE) with a gas mix that consisted of argon (Ar), hydrogen (H 2 ) and methane (CH 4 ) was used to form the etch plasma 215.
- RIE Reactive Ion Etch
- 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.
- the flow rates may be modified whilst still maintaining effective etch conditions.
- the possible flow rates may typically range between about 0 to 100 seem for Ar, about 5 to 100 seem for H 2 and about 1 to 30 seem for CH 4 .
- 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.
- step 115 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.
- 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 ⁇ m.
- 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.
- 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.
- 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) TeO 2 glass; and the solid line is a 3rd order polynomial fitting curve for eye guiding.
- 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 ⁇ m and O/Te composition ratios of 1.6 to 2.4 and the loss in dB/cm is plotted in Figure 5B.
- 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.
- 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.
- the fabricated rib waveguides had dimensions of 4 ⁇ m width, 1.8 ⁇ m in total thickness and 0.8 ⁇ m 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 2.5 ⁇ m 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.
- 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.
- FIG. 6A and 6B show SEM images of the resultant etched structures of a Gen 5As21 5 Se 0? chalcogenide film etched using the hydrogen plasma etching method disclosed herein with an CH 4 +H 2 +AJ mixture in a RIE plasma system with similar condition to those described in Example 1 above (200W, 30mTorr, 15 seem Ar, 30sccm H 2 , 5sccm CH 4 ).
- the hydrogen plasma etching process results in highly anisotropic etching with smooth vertical sidewalls.
- Figures 7A and 7B show SEM images of hydrogen plasma etching of As 2 S 3 , probably the most widely studied chalcogenide glass, etched using the hydrogen plasma etching method disclosed herein with an CH 4 +H 2 +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 H 2 , 5 seem CH 4 ).
- 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.
- etch quality may be achieved by substituting one or more of the injected gasses with, for example ammonia or butane instead of methane.
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