WO2023107008A2 - Methods of fabricating barium titanate (bto) waveguides and devices thereof - Google Patents

Methods of fabricating barium titanate (bto) waveguides and devices thereof Download PDF

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Publication number
WO2023107008A2
WO2023107008A2 PCT/SG2022/050893 SG2022050893W WO2023107008A2 WO 2023107008 A2 WO2023107008 A2 WO 2023107008A2 SG 2022050893 W SG2022050893 W SG 2022050893W WO 2023107008 A2 WO2023107008 A2 WO 2023107008A2
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Prior art keywords
layer
bto
metal
proton beam
refractive index
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PCT/SG2022/050893
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French (fr)
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WO2023107008A3 (en
Inventor
Yu Cao
Aaron James DANNER
Haidong LIANG
Andrew Anthony BETTIOL
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National University Of Singapore
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Publication of WO2023107008A3 publication Critical patent/WO2023107008A3/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/13—Integrated optical circuits characterised by the manufacturing method
    • G02B6/134—Integrated optical circuits characterised by the manufacturing method by substitution by dopant atoms
    • G02B6/1347—Integrated optical circuits characterised by the manufacturing method by substitution by dopant atoms using ion implantation
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/13—Integrated optical circuits characterised by the manufacturing method
    • G02B6/134—Integrated optical circuits characterised by the manufacturing method by substitution by dopant atoms
    • G02B6/1342—Integrated optical circuits characterised by the manufacturing method by substitution by dopant atoms using diffusion
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12035—Materials
    • G02B2006/12047—Barium titanate (BaTiO3)
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/12142—Modulator

Definitions

  • the present invention relates to the fields of photonics and optoelectronics.
  • the invention relates to methods of fabricating optoelectronic waveguides made of a barium titanate material.
  • Electro-optic modulators play a central role in telecommunications where the data rate is directly influenced by the modulation speed. In these devices, the output from an external laser is coupled into an electro-optic modulator where it is modulated via the Pockels effect. Electro-optic modulators in the market are typically fabricated in lithium niobate, a crystal with a large Pockels coefficient (around 30 pm/V).
  • This value is directly proportional to the voltage needed to induce a n- phase shift and is thus directly related to both the speed of a modulator, size of a modulator, and the required driving voltage.
  • Large electro-optic effect with Pockels coefficient of 900 pm/V has been achieved in barium titanate (BaTiCh or BTO) thin films.
  • barium titanate has great potential in replacing lithium niobate in known modulators, because substantial improvements can result, such as, in compactness, speed of operation, and lowering of the driving voltage.
  • the fabrication methods with barium titanate involve complicated fabrication processes such as wafer bonding. Besides, both the materials of lithium niobate and barium titanate are difficult to etch and thus increase the difficulty of production of optical devices on chip.
  • titanium diffused waveguides are used, where titanium are diffused into lithium niobate bulk, but diffused titanium has low effect on the refractive index (increase ⁇ 0.01) locally to improve waveguide performance.
  • Other alternative fabrication methods such as by dry etch can result in undesirable rough sidewalls, because these perovskite materials are difficult to etch.
  • the present invention provides in-situ and ex-situ processing methods for forming regions of a perovskite layer with different refractive indices or selective local modifying of refractive index in a perovskite layer, such as BTO on insulator or BTO on SOI.
  • ‘local’ refers to dimensions of photonic and optoelectronics devices which is usually a few hundred nanometers to a few microns.
  • the design and fabrication methods provide an approach for photonic or waveguide device fabrication, and avoids the difficulty of direct etching of the perovskite material.
  • a method of fabrication of a BaTiO s (BTO) waveguide device comprising a BaTiOs (BTO) layer comprises: forming selected regions of the BaTiO s (BTO) layer with refractive index contrast by any one process from the following group: i) irradiating a proton beam on the selected regions; ii) growing the electro-optic layer at different regions with different process conditions to produce different crystallinities; and iii) diffusing a metal into a selected region to increase or decrease the refractive index; wherein the three processes do not require etching of the BaTiCh (BTO) layer.
  • irradiating the proton beam further comprises: patterning by lithography to define the selected region; and irradiating with the proton beam by flood exposure, or irradiating with the proton beam by direct proton beam writing.
  • the method uses either large area irradiation exposure combined with state-of-the-art lithography or selective local exposure with sub-micron accuracy with direct proton beam pattern writing.
  • the proton beam irradiation may be tuned by varying a parameter from the group of: a) dose, b) energy and c) type of ions, or any combinations thereof.
  • the energy of the proton beam irradiation is proportional to a thickness of the BaTiOs (BTO) layer.
  • BTO BaTiOs
  • the different process conditions to produce different crystallinities comprise a process temperature.
  • the refractive index modification is obtained for the layer in- situ by only controlling growth conditions, avoiding complex equipment and processes. It provides an easy approach of photonic device fabrication only by the combination of state- of-the-art thin film growth and lithography.
  • Growing the BaTiCh (BTO) layer may further comprise: depositing a chromium layer over a substrate layer; patterning the chromium layer by lithography and etching to define the selected region; depositing an isolation layer followed by conducting a lift-off process and etching the chromium; a first BaTiO s (BTO) layer over the insulating substrate layer and growing a second BaTiOs (BTO) layer over the isolation layer, so that the first and the second BaTiOs (BTO) layers have different crystallinities.
  • the BTO electro-optic layer grown at different temperatures have different crystallinities, varying from single crystal to polycrystal to amorphous nanocrystallites.
  • the refractive indices are different as the crystallinity change.
  • the isolation layer may be amorphous barium titanate (BaTiOa) or amorphous aluminum oxide (AI2O3) or amorphous magnesium oxide (MgO) or amorphous silicon dioxide (SiCh). Any other suitable material may be used as well.
  • diffusing the metal comprises: depositing the metal over the BaTiCh (BTO) layer; patterning the metal by lithography and etching to define the selected region; and diffusing the metal into the BaTiOs (BTO) layer by annealing at a temperature in a reducing atmosphere.
  • the metal for diffusion is Cu, Mo, Ta or Ti or a combination thereof. This method provides an approach for the refractive index modification of the BaTiOs (BTO) layer by post-deposition metal diffusion. It provides different levels of refractive index modification by diffusing different metals.
  • the layer is barium titanate (BaTiCh or BTO).
  • the layer may be grown on a silicon substrate layer or a silicon-on-insulator (SOI) substrate layer with the refractive index substantially lower than the refractive index of the BaTiCh (BTO) layer.
  • the BTO on insulator can stand high temperature metal diffusion and thus device fabrication by metal diffusion is applicable.
  • the integration of the BTO can improve electro-optic functions in silicon photonic devices.
  • the choice of the substrate layer can allow the grown BTO to have a dominant crystal orientation.
  • An oxide layer in SOI can provide vertical optical mode confinement. It can be fabricated into optical waveguide devices by etching a ridge structure, via lithography and etching like ion milling, inductively coupled plasma etching and the like. It can be fabricated into optical waveguide devices by forming a ridge structure, via lithography and deposition of a ridge (such as SiNx or a-Si or other transparent materials with similar refractive index).
  • an electro-optical waveguide device is provided that is obtained by any method as described above.
  • FIG.1 illustrates the refractive index of an embodiment of the waveguide device obtained by high-energy proton beam irradiation with different doses.
  • FIGs. 2 and 3 are schematic illustrations of fabrication of the waveguide device, obtained by high-energy proton beam irradiation, where FIG. 2 show fabrication using lithography for patterning followed by flood exposure with large area proton beam irradiation, and FIG. 3 shows fabrication by direct proton beam writing with a focused proton beam.
  • FIG. 4 illustrates an electromagnetic simulation of a single waveguide device fabricated by the method described in FIGs. 2 and 3, showing the optical field distribution.
  • FIG. 5 illustrates the refractive index of a single crystal BTO layer and amorphous BTO layer as achieved by a low index isolation method according to an embodiment.
  • FIG. 6 is a schematic illustration of an embodiment of the fabrication method for the waveguide device by low index isolation.
  • FIG. 7 illustrates an electromagnetic simulation of a waveguide fabricated by the method described in FIG. 6, showing the optical field distribution.
  • FIG. 8 illustrates increase or decrease refractive index change of the BTO layer after metal diffusion as measured by the prism coupling method at 532 nm.
  • FIG. 9 illustrates a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is etchable by a chromium etchant and the metal diffusion increases the refractive index of the BTO layer.
  • FIG. 10 is a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is etchable by a chromium etchant and the metal diffusion decreases the refractive index of the BTO electro-optic layer.
  • FIG. 11 is a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is not etchable by a chromium etchant and the metal diffusion increases the refractive index of the BTO electro-optic layer.
  • FIG. 12 is a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is not etchable by a chromium etchant and the metal diffusion decreases the refractive index of the BTO electro-optic layer.
  • FIG. 13 illustrates an X-ray diffraction 20-co scan of a BTO electro-optic layer grown on buffered silicon.
  • FIG. 14 and 15 illustrate fabricating processes with Si/SOI wafer technology for forming a ridge waveguide on BTOI substrate, from which the above BTO waveguides are fabricated.
  • FIG. 16 shows a cross-sectional electromagnetic simulation of a ridge waveguide device fabricated from the BTOI substrate illustrated in FIGs. 14-15.
  • a method for a waveguide device fabrication is described with BTO as the electro-optic layer on a substrate layer of silicon or silicon-on- insulator (SOI).
  • This invention provides a promising approach to fabricating active photonics or improving known silicon photonics.
  • the electro-optic material is not limited to BTO-on-low-index-insulator but may apply to other BTO thin film form on a substrate.
  • the substrate layer is a low refractive index insulator.
  • the BTO layer can also be a BTO thin film formed directly on the low refractive index insulator (BTOI) or a BTO layer on other substrate layers such as silicon with a low refractive index buffer layer or a BTO layer on SOI wafer or wafer bonded BTO on a supporting substrate layer, fabricated according to processes in the art for integrated circuits.
  • FIG.1 illustrates a first refractive index modification method of the present invention, which involves a high-energy proton beam irradiation of a BTO layer. At different doses, a substantially 1 pm thick BTO layer is grown on a commercially available LSAT substrate layer as an example to illustrate this fabrication method.
  • the BTO layer is irradiated by high- energy proton beam with different doses.
  • the refractive indices of the original as-grown BTO layer and irradiated BTO layer are measured by the prism coupling method.
  • the results in FIG. 1 indicate that both ordinary and extraordinary refractive indices decreased by -0.02.
  • the contrast in refractive index in the electro-optic layer can thus be used to design structures for light confinement.
  • the fabrication using high-energy proton beam irradiation can be exemplified by two methods, depending on whether proton beam irradiation is by large area exposure or by focused beam writing, as described below:
  • the first fabrication method uses lithography and etch to pattern a chromium layer 20 deposited over a crystalline BTO layer 15, which is supported on an insulating substrate layer 10.
  • the chromium layer 20 is deposited on the BTO layer 15.
  • a photoresist layer 25 is spin-coated over the chromium layer 20 followed by patterning via lithography and etching of the chromium layer 20 by a chromium etchant.
  • a high-energy proton beam irradiation 60 is used to irradiate the patterned BTO layer 15 by flood exposure.
  • the proton beam irradiation 60 irradiates the exposed areas of the BTO layer 15 whilst the surrounding area is blocked by the patterned chromium metal layer 20. Thereafter, the photoresist layer 25 is stripped off and the chromium layer 20 is etched off to produce a patterned proton beam treated BTO layer 30, which refractive index or crystallinity has been altered.
  • FIG. 3 illustrates an alternate embodiment, where the proton beam irradiation 60 is conducted by a direct proton beam writing with a focused proton beam.
  • the BTO layer 15 on the insulating substrate layer 10 is mounted on a sample stage 35.
  • the focused proton beam irradiation 60 is directly scanned on the BTO layer 15 as per a designed waveguide pattern while moving the sample stage 35.
  • the proton beam irradiation 60 thus modifies the refractive index of the irradiated BTO as evident from FIG.l.
  • FIG. 4 shows a cross-sectional electromagnetic simulation of a single waveguide device 100 fabricated by the method described in FIG. 2 or 3, wherein the optical field is well confined in the waveguide device 100.
  • dimension of the waveguide device 100 is about 2 pm in width and 1 pm in thickness.
  • the proton beam H + dose is about IxlO 15 and the beam energy is about 250 keV.
  • the beam energy can be varied for different thicknesses 80 of the BTO layer 15.
  • FIG. 6 shows a second fabrication method of the present invention, which involves growing electro-optic layers with different the refractive indices.
  • the refractive indices shown are for the fabricated single crystalline BTO layer 15 and the amorphous BTO layer 45.
  • the method relies on the fact that the BTO layer 15 grown at different temperatures have different crystallinities, varying from single crystal to polycrystal to amorphous nanocrystallites.
  • the refractive indices are different as the crystallinity change. This kind of refractive index change can be used to design structures of the waveguide 100.
  • FIG. 1 shows a second fabrication method of the present invention, which involves growing electro-optic layers with different the refractive indices.
  • the refractive indices shown are for the fabricated single crystalline BTO layer 15 and the amorphous BTO layer 45.
  • the method relies on the fact that the BTO layer 15 grown at different temperatures have different crystallinities, varying from single crystal to polycrystal to
  • FIG. 5 shows changes in refractive index according to crystallinity of the electro-optic layer, where c-BT stands for crystalline BTO layer 15 and the a-BT stands for amorphous BTO layer 45.
  • c-BT stands for crystalline BTO layer 15
  • a-BT stands for amorphous BTO layer 45.
  • the results shows that the amorphous BTO layer 45 has isotropic optical property, and n 0 and n e of crystalline BTO layer 15 are -0.04 and -0.02 larger than the refractive index of the amorphous BTO layer 45, respectively.
  • This example describes crystallinity control by, but not limited to, varying the growth temperature. Methods that can change the refractive index by controlling crystallinity should also apply.
  • the waveguide device 100 may be designed and fabricated as described hereafter: First, a chromium layer 20 is deposited on a low refractive index substrate layer 10 and then a photoresist layer 25 is spin coated, followed by photolithography or electron beam lithography and subsequent etching of the chromium layer 20 by a chromium etchant. Next, by a lift-off process, an amorphous isolation layer 40 with low refractive index is grown.
  • the amorphous isolation layer 40 may be an amorphous BTO, or an amorphous AI2O3 or an amorphous MgO or an amorphous SiO2 or any other suitable material in the art.
  • FIG. 7 shows a cross-sectional view of an electromagnetic simulation for a single waveguide device 100 fabricated by the method described in FIG. 6, showing the optical field well is confined in the waveguide device 100.
  • the channel waveguide device 100 is about 1 pm wide and about 1 pm thick, with the amorphous isolation layer 40 being about 500 nm thick.
  • a method of forming regions of an electro-optic layer with different refractive indices involves diffusing a metal into the electro-optic layer. Diffusing the metal into the BTO layer 15 can potentially modify the refractive index.
  • FIG. 8 shows the changes in refractive index for the BTO layer 15 after diffusing Cu, Mo, Ta, Ti into the BTO layer 15.
  • FIG. 8 shows that diffusion of some metals such as Ta, Ti cause very small changes to the BTO layer 15, while diffusions of metals like Cu and Mo induce substantial changes, and thus can be used to fabricate the waveguide 100. Other metals may also be explored.
  • the third method of fabrication of the present invention is described in the following four embodiments:
  • FIG. 9 describes a method when the metal 50 gets etched by the chromium etchant or any other metal etchant and diffusion of the metal 50 increases the refractive index of the BTO layer 15.
  • a selected region of the BTO layer 15 having a metal diffused BTO layer 55 with refractive index contrast is configured to fabricate a waveguide device 100.
  • a metal 50 is deposited on the BTO layer 15 over the insulating substrate layer 10.
  • a photoresist layer 25 is spin coated, followed by patterning via photolithography or electron beam lithography.
  • the metal 50 is etched by a chromium etchant or any other suitable metal etchant.
  • the photoresist layer 25 is stripped by acetone, IPA, and DI water successively.
  • the metal 50 is diffused into the BTO layer 15 by annealing at a high temperature of typically 700-1200°C in a reducing atmosphere such as nitrogen gas flow or argon gas flow or argon/hydrogen gas flow, to obtain the metal diffused BTO layer 55.
  • the refractive index of the metal diffused BTO layer 55 becomes higher.
  • FIG. 10 describes a method when the metal 50 gets etched by a chromium etchant or any other metal etchant and diffusion of the metal 50 decreases the refractive index of the BTO layer 15.
  • a selected region of a BTO layer 15 having a metal diffused BTO layer 55 is configured to fabricate a waveguide device 100.
  • a metal 50 is deposited on the BTO layer 15 over the insulating substrate layer 10.
  • a photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography.
  • the metal 50 is then etched by a chromium etchant or any other suitable metal etchant.
  • the photoresist layer 25 is stripped by acetone, IPA, and DI water successively.
  • the metal 50 is diffused into the BTO layer 15 underneath the metal 50 by annealing at high temperature of typically 700-1200°C in a reducing atmosphere such as nitrogen gas flow or argon gas flow or argon/hydrogen gas flow, to produce the metal diffused BTO layer 55.
  • the refractive index of the metal diffused BTO layer 55 becomes lower.
  • FIG. 11 describes an alternate method of FIG. 9 when the metal 50 does not get etched by a chromium etchant and diffusion of the metal 50 increases the refractive index of the BTO layer 15.
  • a selected region of the BTO layers 15 having a metal diffused BTO layer 55 is configured to fabricate a waveguide device 100.
  • a chromium layer 20 is deposited on a BTO layer 15 over an insulating substrate layer 10.
  • a photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography. Then, the chromium layer 20 is etched by the chromium etchant.
  • the metal 50 is deposited on the BTO layer 15.
  • the photoresist layer 25 is then stripped by acetone, IPA, and DI water successively, followed by removal of chromium by the chromium etchant. Finally, the metal 50 is diffused into the BTO layer 15 by annealing at high temperature of typically 700-1200°C, in a reducing atmosphere such as nitrogen gas flow, or argon gas flow or argon/hydrogen gas flow, to obtain the metal diffused BTO layer 55. In this embodiment, the refractive index of the metal diffused BTO layer 55 becomes higher.
  • FIG. 12 describes an alternate method of FIG. 10 when the metal 50 does not get etched by a chromium etchant and diffusion of the metal 50 decreases the refractive index of the BTO layer 15.
  • a selected region of the BTO layer 15 having a metal diffused BTO electro-optic layer 55 is configured to fabricate a waveguide device 100.
  • a chromium layer 20 is deposited on the BTO layer 15 over an insulating substrate layer 10.
  • a photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography.
  • the chromium layer 20 is then etched by the chromium etchant.
  • a metal 50 is deposited and the photoresist layer 25 is stripped by acetone, IPA, and DI water successively.
  • the chromium layer 20 is then etched by the chromium etchant.
  • the metal 50 is diffused into the BTO layer 15 by annealing at high temperature of typically 700-1200°C in reducing atmosphere such as nitrogen gas flow or argon gas flow or argon/hydrogen gas flow, to produce the metal diffused BTO layer 55.
  • the refractive index of the metal diffused BTO layer 55 becomes lower.
  • All of the above four 4 methods involves depositing metal onto the BTO layer use the lithography to write pattern on the waveguide device 100, and the chromium layer 20 or the metal 50 is deposited before spin coating the photoresist 25 because: (1) the BTO layer 15 is transparent and is not visible for the machine to adjust focus in photolithography (laser writer), while the chromium layer 20 or the metal 50 provides visibility for the machine. (2) for electron beam lithography, the BTO layer 15 is not conductive and the metal 50 is needed to eliminate the charging effect, which causes the electron beam to drift and thus affect the electron beam lithography.
  • FIG. 13 is an X-ray diffraction 20-co scan of the BTO layer 15 grown on the insulating substrate layer 10, which is a buffered silicon.
  • the BTO layer 15 may be grown on silicon or SOI with a buffer layer.
  • aluminum is sputtered on the Si or SOI substrate in high vacuum to improve the surface.
  • CeO2 is deposited in vacuum as a buffer layer (not shown) and finally the BTO layer 15 is deposited over the buffer layer, followed by annealing in an oxygen rich environment.
  • the grown BTO layer 15 has a dominant (110)/(101) orientation as indicated by the X-ray diffraction (XRD).
  • the BTO layer 15 can be grown on SOI substrate using the methods described above. SOI is chosen because its silicon dioxide layer can provide vertical mode confinement.
  • the BTO layer 15 can then be integrated with silicon or SOI wafer fabrication to form ridge waveguide devices, according to another embodiment of the present invention, with structures patterned by lithography, and followed by either depositing a ridge 90 (such as SiNx or a-Si or other transparent materials with similar refractive index as the BTO) on top the BTO layer 15 or dry etching (ion milling, inductively coupled plasma etching) the BTO layer 15 to form a ridge 90 on top of the BTO layer 15, to obtain a ridge BTOI substrate.
  • FIG. 14 illustrates the above ridge BTOI substrate is integrated with current silicon/SOI wafer processes.
  • a chromium layer 20 is deposited on the BTO electro-optic layer 15, with the BTO layer being formed over an insulating substrate layer 10.
  • a photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography.
  • the chromium layer 20 is then etched by a chromium etchant. Then dry etching is performed followed by a lift off process and Cr etching and thus a ridge 90 is formed on the BTO layer 15.
  • FIG. 15 illustrates the above ridge BTOI substrate is also integrated with current silicon/SOI water process.
  • a chromium layer 20 is deposited on the BTO layer 15, with the BTO layer being formed over an insulating substrate layer 10.
  • a photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography.
  • the chromium layer 20 is then etched by a chromium etchant. Then a ridge material is deposited followed by a lift off process and Cr etching, and thus a ridge 90 is formed over the BTO layer 15.
  • the above ridge BTOI substrates can then be used to fabricate BTO waveguide devices 100 according to the above 3 methods, ie. for fabricating the ridge BTOI substrates with refractive index contrasts or different refractive indices to obtain ridge BTO waveguides.
  • FIG. 16 shows an electromagnetic simulation of a cross-sectional ridge waveguide device fabricated from the BTO layer 15 grown on SOI substrate. This shows that the fabrication methods of the present invention can be integrated into or compatible with current wafer fabrication technology.
  • the resulting BTO based electro-optic waveguide or modulator obtained above is thus more compact, achieve higher speed, and with lower driving voltage than the conventional lithium niobate technology.
  • the issue of the difficulty of etching BTO by the state-of-the-art technologies is resolved for production of the optical devices on chip.
  • this invention can be used to fabricate high-speed modulators, which can be used in high-speed telecommunication market, such as for use on submarine fiber optic cables as well as in highspeed interconnect.

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Abstract

The present invention describes methods of fabrication BaTiO3 (or BTO) waveguide devices (100) comprising a BaTiO3 (BTO) layer (15). The methods comprise: forming selected regions of the BaTiO3 (BTO) layer (15) with contrasts in the refractive index. The refractive index contrast is produced by irradiating a proton beam (60) on one of the selected regions; growing the BaTiO3 (BTO) layer (15) at selected regions with different process conditions to produce different crystallinities; or diffusing a metal (50) into one of the selected regions to increase or decrease the refractive index. These processes do not require etching the BaTiO3 (BTO) layer (15).

Description

METHODS OF FABRICATING
BARIUM TITANATE (BTO) WAVEGUIDES AND DEVICES THEREOF
Related Application
[001] The present invention claims priority to Singapore patent application no. 10202113740S filed on 10 December 2021, the disclosure of which is incorporated in its entirety.
Field of Invention
[002] The present invention relates to the fields of photonics and optoelectronics. In particular, the invention relates to methods of fabricating optoelectronic waveguides made of a barium titanate material.
Background
[003] Silicon photonics has many well developed applications in passive photonics devices, while integration of electro-optic material onto the silicon photonics platform can extend its functionalties to produce active photonics devices. Electro-optic modulators play a central role in telecommunications where the data rate is directly influenced by the modulation speed. In these devices, the output from an external laser is coupled into an electro-optic modulator where it is modulated via the Pockels effect. Electro-optic modulators in the market are typically fabricated in lithium niobate, a crystal with a large Pockels coefficient (around 30 pm/V). This value is directly proportional to the voltage needed to induce a n- phase shift and is thus directly related to both the speed of a modulator, size of a modulator, and the required driving voltage. Large electro-optic effect with Pockels coefficient of 900 pm/V has been achieved in barium titanate (BaTiCh or BTO) thin films. Thus, barium titanate has great potential in replacing lithium niobate in known modulators, because substantial improvements can result, such as, in compactness, speed of operation, and lowering of the driving voltage. However, the fabrication methods with barium titanate involve complicated fabrication processes such as wafer bonding. Besides, both the materials of lithium niobate and barium titanate are difficult to etch and thus increase the difficulty of production of optical devices on chip. Therefore, in known lithium niobate modulators, titanium diffused waveguides are used, where titanium are diffused into lithium niobate bulk, but diffused titanium has low effect on the refractive index (increase <0.01) locally to improve waveguide performance. Other alternative fabrication methods such as by dry etch can result in undesirable rough sidewalls, because these perovskite materials are difficult to etch.
[004] It can thus be seen that there exists a need for a simple and robust method for fabricating the electro-optic thin films into photonics and optoelectronics devices, in particular BTO waveguides.
Summary
[005] The following presents a simplified summary to provide a basic understanding of the present invention. This summary is not an extensive overview of the present invention, and is not intended to identify key features of the invention. Rather, it is to present some of the inventive concepts of this invention in a generalized form as a prelude to the detailed description that is to follow.
[006] The present invention provides in-situ and ex-situ processing methods for forming regions of a perovskite layer with different refractive indices or selective local modifying of refractive index in a perovskite layer, such as BTO on insulator or BTO on SOI. ‘local’ refers to dimensions of photonic and optoelectronics devices which is usually a few hundred nanometers to a few microns. The design and fabrication methods provide an approach for photonic or waveguide device fabrication, and avoids the difficulty of direct etching of the perovskite material.
[007] According to an embodiment, a method of fabrication of a BaTiO s (BTO) waveguide device comprising a BaTiOs (BTO) layer is provided. The method comprises: forming selected regions of the BaTiO s (BTO) layer with refractive index contrast by any one process from the following group: i) irradiating a proton beam on the selected regions; ii) growing the electro-optic layer at different regions with different process conditions to produce different crystallinities; and iii) diffusing a metal into a selected region to increase or decrease the refractive index; wherein the three processes do not require etching of the BaTiCh (BTO) layer. [008] Preferably, irradiating the proton beam further comprises: patterning by lithography to define the selected region; and irradiating with the proton beam by flood exposure, or irradiating with the proton beam by direct proton beam writing. Thus, the method uses either large area irradiation exposure combined with state-of-the-art lithography or selective local exposure with sub-micron accuracy with direct proton beam pattern writing. The proton beam irradiation may be tuned by varying a parameter from the group of: a) dose, b) energy and c) type of ions, or any combinations thereof. The energy of the proton beam irradiation is proportional to a thickness of the BaTiOs (BTO) layer. The number of doses and type of ions determine the change in the refractive index.
[009] Preferably, the different process conditions to produce different crystallinities comprise a process temperature. Herein, the refractive index modification is obtained for the layer in- situ by only controlling growth conditions, avoiding complex equipment and processes. It provides an easy approach of photonic device fabrication only by the combination of state- of-the-art thin film growth and lithography. Growing the BaTiCh (BTO) layer may further comprise: depositing a chromium layer over a substrate layer; patterning the chromium layer by lithography and etching to define the selected region; depositing an isolation layer followed by conducting a lift-off process and etching the chromium; a first BaTiO s (BTO) layer over the insulating substrate layer and growing a second BaTiOs (BTO) layer over the isolation layer, so that the first and the second BaTiOs (BTO) layers have different crystallinities. The BTO electro-optic layer grown at different temperatures have different crystallinities, varying from single crystal to polycrystal to amorphous nanocrystallites. The refractive indices are different as the crystallinity change. The isolation layer may be amorphous barium titanate (BaTiOa) or amorphous aluminum oxide (AI2O3) or amorphous magnesium oxide (MgO) or amorphous silicon dioxide (SiCh). Any other suitable material may be used as well.
[0010] Preferably, diffusing the metal comprises: depositing the metal over the BaTiCh (BTO) layer; patterning the metal by lithography and etching to define the selected region; and diffusing the metal into the BaTiOs (BTO) layer by annealing at a temperature in a reducing atmosphere. Preferably, the metal for diffusion is Cu, Mo, Ta or Ti or a combination thereof. This method provides an approach for the refractive index modification of the BaTiOs (BTO) layer by post-deposition metal diffusion. It provides different levels of refractive index modification by diffusing different metals. [0011] Preferably, the layer is barium titanate (BaTiCh or BTO). The layer may be grown on a silicon substrate layer or a silicon-on-insulator (SOI) substrate layer with the refractive index substantially lower than the refractive index of the BaTiCh (BTO) layer. The BTO on insulator can stand high temperature metal diffusion and thus device fabrication by metal diffusion is applicable. The integration of the BTO can improve electro-optic functions in silicon photonic devices. The choice of the substrate layer can allow the grown BTO to have a dominant crystal orientation. An oxide layer in SOI can provide vertical optical mode confinement. It can be fabricated into optical waveguide devices by etching a ridge structure, via lithography and etching like ion milling, inductively coupled plasma etching and the like. It can be fabricated into optical waveguide devices by forming a ridge structure, via lithography and deposition of a ridge (such as SiNx or a-Si or other transparent materials with similar refractive index).
[0012] According to another embodiment, an electro-optical waveguide device is provided that is obtained by any method as described above.
Brief Description of the Drawings
[0013] This invention will be described by way of non-limiting embodiments of the present invention, with reference to the accompanying drawings, in which:
[0014] FIG.1 illustrates the refractive index of an embodiment of the waveguide device obtained by high-energy proton beam irradiation with different doses.
[0015] FIGs. 2 and 3 are schematic illustrations of fabrication of the waveguide device, obtained by high-energy proton beam irradiation, where FIG. 2 show fabrication using lithography for patterning followed by flood exposure with large area proton beam irradiation, and FIG. 3 shows fabrication by direct proton beam writing with a focused proton beam.
[0016] FIG. 4 illustrates an electromagnetic simulation of a single waveguide device fabricated by the method described in FIGs. 2 and 3, showing the optical field distribution. [0017] FIG. 5 illustrates the refractive index of a single crystal BTO layer and amorphous BTO layer as achieved by a low index isolation method according to an embodiment.
[0018] FIG. 6 is a schematic illustration of an embodiment of the fabrication method for the waveguide device by low index isolation.
[0019] FIG. 7 illustrates an electromagnetic simulation of a waveguide fabricated by the method described in FIG. 6, showing the optical field distribution.
[0020] FIG. 8 illustrates increase or decrease refractive index change of the BTO layer after metal diffusion as measured by the prism coupling method at 532 nm.
[0021 ] FIG. 9 illustrates a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is etchable by a chromium etchant and the metal diffusion increases the refractive index of the BTO layer.
[0022] FIG. 10 is a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is etchable by a chromium etchant and the metal diffusion decreases the refractive index of the BTO electro-optic layer.
[0023] FIG. 11 is a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is not etchable by a chromium etchant and the metal diffusion increases the refractive index of the BTO electro-optic layer.
[0024] FIG. 12 is a schematic illustration of an embodiment of the waveguide device fabrication by metal diffusion when the metal is not etchable by a chromium etchant and the metal diffusion decreases the refractive index of the BTO electro-optic layer.
[0025] FIG. 13 illustrates an X-ray diffraction 20-co scan of a BTO electro-optic layer grown on buffered silicon.
[0026] FIG. 14 and 15 illustrate fabricating processes with Si/SOI wafer technology for forming a ridge waveguide on BTOI substrate, from which the above BTO waveguides are fabricated. [0027] FIG. 16 shows a cross-sectional electromagnetic simulation of a ridge waveguide device fabricated from the BTOI substrate illustrated in FIGs. 14-15.
Detailed Description
[0028] One or more specific and alternative embodiments of the present invention will now be described with reference to the attached drawings. It shall be apparent to one skilled in the art, however, that this invention may be practised without such specific details. Some of the details may not be described at length so as not to obscure the present invention.
[0029] According to an embodiment, a method for a waveguide device fabrication is described with BTO as the electro-optic layer on a substrate layer of silicon or silicon-on- insulator (SOI). This invention provides a promising approach to fabricating active photonics or improving known silicon photonics.
[0030]The other methods disclosed herein are for fabricating an electro-optical waveguide device comprising a BTO layer wherein regions of the BTO layer have refractive index contrasts, without engaging any step of etching the BTO layer, which BTO layer is difficult to etch.
[0031] Here, three waveguide-based optical device fabrication methods are provided to form regions of the BTO layer with refractive index contrast or regions with different refractive indices. The electro-optic material is not limited to BTO-on-low-index-insulator but may apply to other BTO thin film form on a substrate. Preferably, the substrate layer is a low refractive index insulator. The BTO layer can also be a BTO thin film formed directly on the low refractive index insulator (BTOI) or a BTO layer on other substrate layers such as silicon with a low refractive index buffer layer or a BTO layer on SOI wafer or wafer bonded BTO on a supporting substrate layer, fabricated according to processes in the art for integrated circuits. Selected regions of the BTO layer are formed with refractive index contrasts or different refractive indices. These three fabrication methods, namely, irradiating a high- energy proton beam to modify the refractive index of the BTO layer, or controlling crystallinity growth of the BTO layer, or diffusing a metal to provide regions of the BTO layer with different refractive indices. [0032] FIG.1 illustrates a first refractive index modification method of the present invention, which involves a high-energy proton beam irradiation of a BTO layer. At different doses, a substantially 1 pm thick BTO layer is grown on a commercially available LSAT substrate layer as an example to illustrate this fabrication method. The BTO layer is irradiated by high- energy proton beam with different doses. The refractive indices of the original as-grown BTO layer and irradiated BTO layer are measured by the prism coupling method. The results in FIG. 1 indicate that both ordinary and extraordinary refractive indices decreased by -0.02. The contrast in refractive index in the electro-optic layer can thus be used to design structures for light confinement. The fabrication using high-energy proton beam irradiation can be exemplified by two methods, depending on whether proton beam irradiation is by large area exposure or by focused beam writing, as described below:
[0033] As shown in FIG. 2, the first fabrication method uses lithography and etch to pattern a chromium layer 20 deposited over a crystalline BTO layer 15, which is supported on an insulating substrate layer 10. Under this method, the chromium layer 20 is deposited on the BTO layer 15. A photoresist layer 25 is spin-coated over the chromium layer 20 followed by patterning via lithography and etching of the chromium layer 20 by a chromium etchant. Next, a high-energy proton beam irradiation 60 is used to irradiate the patterned BTO layer 15 by flood exposure. The proton beam irradiation 60 irradiates the exposed areas of the BTO layer 15 whilst the surrounding area is blocked by the patterned chromium metal layer 20. Thereafter, the photoresist layer 25 is stripped off and the chromium layer 20 is etched off to produce a patterned proton beam treated BTO layer 30, which refractive index or crystallinity has been altered.
[0034] FIG. 3 illustrates an alternate embodiment, where the proton beam irradiation 60 is conducted by a direct proton beam writing with a focused proton beam. Herein, the BTO layer 15 on the insulating substrate layer 10 is mounted on a sample stage 35. The focused proton beam irradiation 60 is directly scanned on the BTO layer 15 as per a designed waveguide pattern while moving the sample stage 35. The proton beam irradiation 60 thus modifies the refractive index of the irradiated BTO as evident from FIG.l.
[0035] FIG. 4 shows a cross-sectional electromagnetic simulation of a single waveguide device 100 fabricated by the method described in FIG. 2 or 3, wherein the optical field is well confined in the waveguide device 100. In this embodiment, dimension of the waveguide device 100 is about 2 pm in width and 1 pm in thickness. The proton beam H+ dose is about IxlO15 and the beam energy is about 250 keV. The beam energy can be varied for different thicknesses 80 of the BTO layer 15.
[0036] FIG. 6 shows a second fabrication method of the present invention, which involves growing electro-optic layers with different the refractive indices. The refractive indices shown are for the fabricated single crystalline BTO layer 15 and the amorphous BTO layer 45. The method relies on the fact that the BTO layer 15 grown at different temperatures have different crystallinities, varying from single crystal to polycrystal to amorphous nanocrystallites. The refractive indices are different as the crystallinity change. This kind of refractive index change can be used to design structures of the waveguide 100. FIG. 5 shows changes in refractive index according to crystallinity of the electro-optic layer, where c-BT stands for crystalline BTO layer 15 and the a-BT stands for amorphous BTO layer 45. The results shows that the amorphous BTO layer 45 has isotropic optical property, and n0 and ne of crystalline BTO layer 15 are -0.04 and -0.02 larger than the refractive index of the amorphous BTO layer 45, respectively. This example describes crystallinity control by, but not limited to, varying the growth temperature. Methods that can change the refractive index by controlling crystallinity should also apply.
[0037] As illustrated in FIG. 6, the waveguide device 100 may be designed and fabricated as described hereafter: First, a chromium layer 20 is deposited on a low refractive index substrate layer 10 and then a photoresist layer 25 is spin coated, followed by photolithography or electron beam lithography and subsequent etching of the chromium layer 20 by a chromium etchant. Next, by a lift-off process, an amorphous isolation layer 40 with low refractive index is grown. The amorphous isolation layer 40 may be an amorphous BTO, or an amorphous AI2O3 or an amorphous MgO or an amorphous SiO2 or any other suitable material in the art. The photoresist layer 25 is then stripped off and the chromium layer 20 is etched. Finally, a crystalline BTO layer 15 is then deposited or grown on the substrate layer 10, by suitably selecting the material of the substrate layer 10 and the temperature of the deposition or growth. An amorphous BTO layer 45 is then grown on top of the isolation layer 40, and a channel waveguide device 100 is thus obtained. FIG. 7 shows a cross-sectional view of an electromagnetic simulation for a single waveguide device 100 fabricated by the method described in FIG. 6, showing the optical field well is confined in the waveguide device 100. The channel waveguide device 100 is about 1 pm wide and about 1 pm thick, with the amorphous isolation layer 40 being about 500 nm thick.
[0038] According to a third method of the present invention, a method of forming regions of an electro-optic layer with different refractive indices involves diffusing a metal into the electro-optic layer. Diffusing the metal into the BTO layer 15 can potentially modify the refractive index. FIG. 8 shows the changes in refractive index for the BTO layer 15 after diffusing Cu, Mo, Ta, Ti into the BTO layer 15. FIG. 8 shows that diffusion of some metals such as Ta, Ti cause very small changes to the BTO layer 15, while diffusions of metals like Cu and Mo induce substantial changes, and thus can be used to fabricate the waveguide 100. Other metals may also be explored. Depending on whether the metal gets etched by a chromium etchant, and whether diffusion of the metal increases or decreases the refractive index, the third method of fabrication of the present invention is described in the following four embodiments:
[0039] (i): FIG. 9 describes a method when the metal 50 gets etched by the chromium etchant or any other metal etchant and diffusion of the metal 50 increases the refractive index of the BTO layer 15. A selected region of the BTO layer 15 having a metal diffused BTO layer 55 with refractive index contrast is configured to fabricate a waveguide device 100. As shown in FIG. 9, a metal 50 is deposited on the BTO layer 15 over the insulating substrate layer 10. Then, a photoresist layer 25 is spin coated, followed by patterning via photolithography or electron beam lithography. Then, the metal 50 is etched by a chromium etchant or any other suitable metal etchant. Next, the photoresist layer 25 is stripped by acetone, IPA, and DI water successively. Finally, the metal 50 is diffused into the BTO layer 15 by annealing at a high temperature of typically 700-1200°C in a reducing atmosphere such as nitrogen gas flow or argon gas flow or argon/hydrogen gas flow, to obtain the metal diffused BTO layer 55. In this embodiment, the refractive index of the metal diffused BTO layer 55 becomes higher.
[0040] (ii): FIG. 10 describes a method when the metal 50 gets etched by a chromium etchant or any other metal etchant and diffusion of the metal 50 decreases the refractive index of the BTO layer 15. A selected region of a BTO layer 15 having a metal diffused BTO layer 55 is configured to fabricate a waveguide device 100. As shown in FIG. 10, a metal 50 is deposited on the BTO layer 15 over the insulating substrate layer 10. A photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography. The metal 50 is then etched by a chromium etchant or any other suitable metal etchant. Next, the photoresist layer 25 is stripped by acetone, IPA, and DI water successively. Finally, the metal 50 is diffused into the BTO layer 15 underneath the metal 50 by annealing at high temperature of typically 700-1200°C in a reducing atmosphere such as nitrogen gas flow or argon gas flow or argon/hydrogen gas flow, to produce the metal diffused BTO layer 55. In this embodiment, the refractive index of the metal diffused BTO layer 55 becomes lower.
[0041 ] (iii): FIG. 11 describes an alternate method of FIG. 9 when the metal 50 does not get etched by a chromium etchant and diffusion of the metal 50 increases the refractive index of the BTO layer 15. A selected region of the BTO layers 15 having a metal diffused BTO layer 55 is configured to fabricate a waveguide device 100. As shown in FIG. 11 , a chromium layer 20 is deposited on a BTO layer 15 over an insulating substrate layer 10. A photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography. Then, the chromium layer 20 is etched by the chromium etchant. Next, the metal 50 is deposited on the BTO layer 15. The photoresist layer 25 is then stripped by acetone, IPA, and DI water successively, followed by removal of chromium by the chromium etchant. Finally, the metal 50 is diffused into the BTO layer 15 by annealing at high temperature of typically 700-1200°C, in a reducing atmosphere such as nitrogen gas flow, or argon gas flow or argon/hydrogen gas flow, to obtain the metal diffused BTO layer 55. In this embodiment, the refractive index of the metal diffused BTO layer 55 becomes higher.
[0042] (iv): FIG. 12 describes an alternate method of FIG. 10 when the metal 50 does not get etched by a chromium etchant and diffusion of the metal 50 decreases the refractive index of the BTO layer 15. A selected region of the BTO layer 15 having a metal diffused BTO electro-optic layer 55 is configured to fabricate a waveguide device 100. As shown in FIG. 12, a chromium layer 20 is deposited on the BTO layer 15 over an insulating substrate layer 10. A photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography. The chromium layer 20 is then etched by the chromium etchant. Next, a metal 50 is deposited and the photoresist layer 25 is stripped by acetone, IPA, and DI water successively. The chromium layer 20 is then etched by the chromium etchant. Finally, the metal 50 is diffused into the BTO layer 15 by annealing at high temperature of typically 700-1200°C in reducing atmosphere such as nitrogen gas flow or argon gas flow or argon/hydrogen gas flow, to produce the metal diffused BTO layer 55. In this embodiment, the refractive index of the metal diffused BTO layer 55 becomes lower.
[0043] All of the above four 4 methods involves depositing metal onto the BTO layer use the lithography to write pattern on the waveguide device 100, and the chromium layer 20 or the metal 50 is deposited before spin coating the photoresist 25 because: (1) the BTO layer 15 is transparent and is not visible for the machine to adjust focus in photolithography (laser writer), while the chromium layer 20 or the metal 50 provides visibility for the machine. (2) for electron beam lithography, the BTO layer 15 is not conductive and the metal 50 is needed to eliminate the charging effect, which causes the electron beam to drift and thus affect the electron beam lithography.
[0044] From here, crystalline BTO thin film integration/growth on silicon/SOI is described: FIG. 13 is an X-ray diffraction 20-co scan of the BTO layer 15 grown on the insulating substrate layer 10, which is a buffered silicon. The BTO layer 15 may be grown on silicon or SOI with a buffer layer. During fabrication, first, aluminum is sputtered on the Si or SOI substrate in high vacuum to improve the surface. Then, CeO2 is deposited in vacuum as a buffer layer (not shown) and finally the BTO layer 15 is deposited over the buffer layer, followed by annealing in an oxygen rich environment. The grown BTO layer 15 has a dominant (110)/(101) orientation as indicated by the X-ray diffraction (XRD). According to the crystal orientation of BTO (110)/(101) for the BTO layer 15, an optimized effect would be expected when an applied electric field is along [110]/[010] direction to access Pockels term r42 (rsi), while the applied electric field along the other orthogonal direction in plane access an effective Pockels effect of mixture of r42 (rsi) and other Pockels terms.
[0045] To fabricate the waveguide device 100, the BTO layer 15 can be grown on SOI substrate using the methods described above. SOI is chosen because its silicon dioxide layer can provide vertical mode confinement. The BTO layer 15 can then be integrated with silicon or SOI wafer fabrication to form ridge waveguide devices, according to another embodiment of the present invention, with structures patterned by lithography, and followed by either depositing a ridge 90 (such as SiNx or a-Si or other transparent materials with similar refractive index as the BTO) on top the BTO layer 15 or dry etching (ion milling, inductively coupled plasma etching) the BTO layer 15 to form a ridge 90 on top of the BTO layer 15, to obtain a ridge BTOI substrate. [0046] FIG. 14 illustrates the above ridge BTOI substrate is integrated with current silicon/SOI wafer processes. As shown in FIG. 14, a chromium layer 20 is deposited on the BTO electro-optic layer 15, with the BTO layer being formed over an insulating substrate layer 10. A photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography. The chromium layer 20 is then etched by a chromium etchant. Then dry etching is performed followed by a lift off process and Cr etching and thus a ridge 90 is formed on the BTO layer 15.
[0047] FIG. 15 illustrates the above ridge BTOI substrate is also integrated with current silicon/SOI water process. As shown in FIG. 15, a chromium layer 20 is deposited on the BTO layer 15, with the BTO layer being formed over an insulating substrate layer 10. A photoresist layer 25 is then spin coated, followed by patterning via photolithography or electron beam lithography. The chromium layer 20 is then etched by a chromium etchant. Then a ridge material is deposited followed by a lift off process and Cr etching, and thus a ridge 90 is formed over the BTO layer 15.
[0048] The above ridge BTOI substrates can then be used to fabricate BTO waveguide devices 100 according to the above 3 methods, ie. for fabricating the ridge BTOI substrates with refractive index contrasts or different refractive indices to obtain ridge BTO waveguides.
[0049] FIG. 16 shows an electromagnetic simulation of a cross-sectional ridge waveguide device fabricated from the BTO layer 15 grown on SOI substrate. This shows that the fabrication methods of the present invention can be integrated into or compatible with current wafer fabrication technology.
[0050] Advantages of the present invention: Given that BTO has high Pockels coefficient, the resulting BTO based electro-optic waveguide or modulator obtained above is thus more compact, achieve higher speed, and with lower driving voltage than the conventional lithium niobate technology. The issue of the difficulty of etching BTO by the state-of-the-art technologies is resolved for production of the optical devices on chip. In other words, this invention can be used to fabricate high-speed modulators, which can be used in high-speed telecommunication market, such as for use on submarine fiber optic cables as well as in highspeed interconnect.
[0051 ] While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations and combinations of variations disclosed in the text description and drawings thereof could be made to the present invention without departing from the scope of the present invention.

Claims

CLAIMS:
1. A method of fabrication of a BaTiCh (BTO) waveguide device (100) comprising a BaTiCh (BTO) layer (15), the method comprises: forming selected regions of the BaTiO s (BTO) layer (15) with refractive index contrasts or with different refractive indices by any one process of: i) irradiating a proton beam (60) on one of the selected regions; ii) growing the BaTiOs (BTO) layer (15) at the selected regions with different process conditions to produce different crystallinities; or iii) diffusing a metal (50) into one of the selected regions to increase or decrease the refractive index; wherein the three processes do not require etching the BaTiOs (BTO) layer (15).
2. The method according to claim 1 , wherein irradiating the proton beam (60) comprises: patterning by lithography to define the selected regions; and irradiating the proton beam irradiation (60) by flood exposure, or irradiating the proton beam irradiation (60) by direct proton beam writing.
3. The method according to claim 2, further comprises: tuning the proton beam irradiation (60) by varying a parameter from the following group: a) dose, b) energy, or c) type of ion; or any combinations thereof.
4. The method according to claim 3, wherein the energy of irradiating the proton beam (60) is configured proportional to a thickness (80) of the BaTiCh (BTO) layer (15).
5. The method according to claim 1, wherein growing the BaTiCh (BTO) layer (15) at the selected regions at different process conditions comprises altering a process temperature.
6. The method according to claim 5, wherein growing the BaTiCh (BTO) layer
(15) at the selected regions at different process conditions further comprises: depositing a chromium layer (20) over a substrate layer (10); patterning the chromium layer (20) by lithography and etching to define a first selected region; growing an isolation layer (40) followed by conducting a lift-off process and etching the chromium layer (20); growing a first BaTiOs (BTO) layer (15) over the insulating substrate layer (10) and growing a second electro-optic layer (45) over the isolation layer (40), so that the first and the second electro-optic layers have different crystallinities.
7. The method according to claim 6, wherein the isolation layer (40) comprises amorphous BaTiOs or amorphous AI2O3 or amorphous MgO or amorphous SiCK
8. The method according to claim 1, wherein diffusing the metal (50) comprises: depositing the metal (50) over the BaTiCh (BTO) layer (15); patterning the metal (50) by lithography and etching to define the selected regions; and annealing the metal (50) in a reducing atmosphere to diffuse the metal into the BaTiO3 (BTO) layer (15).
9. The method according to claim 1 or 8, wherein the metal (50) is selected from the group of: Cu, Mo, Ta, and Ti.
10. The method according to claim 2, 6 or 8, wherein the lithography comprises photolithography or electron beam lithography.
11. A method according to claim 6, wherein the substrate layer (10) is silicon or silicon-on-insulator (SOI).
12. A BaTiO s (BTO) waveguide device (100) obtained by the method according to any one preceding claim.
13. The BaTiOi (BTO) waveguide device (100) according to claim 12 is structured on any one of the following: a substrate, an insulator, SOI, SOI wafer, bonded wafer or a wafer.
14. The BaTiO3 (BTO) waveguide device (100) according to claim 12 is structured on a ridge BT-on-insulator substrate.
16
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