A waveguide structure and a method of forming a waveguide structure

Abstract

A waveguide structure and a method of forming a waveguide structure are provided, the waveguide structure comprises a substrate; a waveguide body comprising Germanium (Ge); two or more support members spaced apart from each other, the two or more support members being disposed between the substrate and a bottom surface of the waveguide body; wherein the two or more support members are arranged to support the waveguide body and to suspend the waveguide body away from the substrate; and further wherein the two or more support members each comprise Ge.

Description

[0001] A Waveguide Structure And A Method Of Forming A Waveguide Structure

[0002] TECHNICAL FIELD

[0003] The present disclosure relates broadly to a waveguide structure and to a method of forming a waveguide structure.

[0004] BACKGROUND

[0005] Non-dispersive infrared (NDIR) sensing is a method of analysing chemical or gas substances by analysing wavelength-specific absorption spectra of these substances. Waveguide-based NDIR allows an interaction of mid-infrared light with a chemical or gas substance in a controlled manner that may maximize a light path length over a compact interaction area.

[0006] Given a miniaturized form-factor, waveguide-based NDIR sensors may be deployed in a distributed network for applications that require monitoring and tracking of trace gases such as a source of leakage in a plant environment, or in a smart agricultural farm, or air quality control in densely populated hotspots, or for reaction residence time monitoring etc.

[0007] Since NDIR typically relies on a highly wavelength-specific nature of absorption by molecular bonds of a substance to be analysed, a broad input spectrum may create an absorption spectra signature that may allow for identification and quantification of chemical molecules of the substance to be analysed. Therefore, the inventors recognize that there is a need for a waveguide structure that is desirably broad in an operating wavelength range. On this note, the inventors recognize that an on-chip non-dispersive-infrared (NDIR) sensing may desirably have waveguide material with transparency in a fingerprinting range (e g. about a range of 2 to 20 pm).

[0008] The inventors recognize that Germanium (Ge) may be a candidate for such applications because of its compatibility with Complementary Metal-Oxide-Semiconductor (CMOS) processes and its transparency being in a range of about 2 to 14 pm. However, the inventors recognise that there currently is no CMOS-compatible cladding material having a similar transparency as Ge. The next suitable candidate appears to be Silicon (Si) which has transparency in a range of about 1 to 9 pm.Thus, it is recognized that Germanium-on-Silicon (Ge-on-Si) is a suitable candidate for Ge-on-Si mid-infrared (MIR) photonics structures with a transparency region of about 2 to 15 pm for Ge and about 1 to 9 pm for Si.

[0009] However, despite the 2 to 15pm transparency of Ge, the inventors recognize that an optical propagation loss of a Ge-on-Si waveguide is brought about by absorption in the Si substrate as cladding, which typically becomes significant at a wavelength A of more than 9pm.

[0010] In addition to the above intrinsic absorption, the inventors also recognize that the optical loss may also be caused by threading dislocation defects that originate from the Ge / Si interface, e.g. during Ge epitaxial growth due to an about 4% lattice mismatch between Ge and Si. These may lead to a relatively high threading dislocation density in the Ge layers.

[0011] The inventors recognize that one possible way to circumvent the abovementioned loss contributions is to create mechanically suspended Ge waveguide structures, i.e. remove cladding-related losses. However, the inventors recognize that other problems arise in that there is added cost and also complexities due to preparation of substrates / wafers e.g. for etching of patterns and formation of Ge structures (e.g. by epitaxially growing Ge) to create the mechanically suspended Ge waveguide structures.

[0012] Furthermore, the inventors recognize that there is an issue of mechanical robustness for such a mechanically suspended waveguide structure that requires additional consideration. In the context of liquid analytes or having a liquid substance for analysis, such a mechanical stability issue becomes more significant due to the presence of capillary forces during liquid sensing.

[0013] In view of the above, there exists a need for a waveguide structure and a method of forming a waveguide structure that may address at least one of the above problems.

[0014] SUMMARY

[0015] In accordance with an aspect of the present disclosure, there is provided a waveguide structure, the waveguide structure comprising, a substrate; a waveguide body comprising Germanium (Ge); two or more support members spaced apart from each other, the two or more support members being disposed between the substrate and a bottom surface of the waveguide body; wherein the two or more support members are arranged to support thewaveguide body and to suspend the waveguide body away from the substrate; and further wherein the two or more support members each comprise Ge.

[0016] The two or more support members may be each arranged in the form of one or more motifs.

[0017] The two or more support members may each comprise at least one dimension that is a sub-factor of an operating wavelength of the waveguide structure, the sub-factor being less than one.

[0018] The at least one dimension may comprise one of a height dimension, a feature width dimension and a pitch value.

[0019] A duty cycle of the two or more support members may be selected based on a base area of each of the two or more support members and based on the pitch value.

[0020] The duty cycle of the two or more support members may be based on DC = Amotif / p2, DC refers to the duty cycle, Amour refers to the base area of each of the two or more support members and p is the pitch value.

[0021] The height dimension of each of the two or more support members may be selected based on the feature width dimension of each of the two or more support members.

[0022] The height dimension of each of the two or more support members and the feature width dimension of each of the two or more support members may be selected based on a relationship Oxide thickness

[0023] tan 60° = - , where oxide thickness refers to the height dimension of each of Trench width

[0024] the two or more support members and trench width refers to the feature width dimension of each of the two or more support members.

[0025] In accordance with another aspect of the present disclosure, there is provided a method of forming a waveguide structure, the method comprising, providing a substrate; forming from the substrate two or more support members spaced apart from each other; forming a waveguide body comprising Germanium (Ge) wherein the two or more support members are disposed between the substrate and a bottom surface of the waveguide body; wherein the two or more support members are arranged to support the waveguide body and to suspend the waveguide body away from the substrate; and further wherein the two or more support members each comprise Ge.The method may further comprise forming the two or more support members using one or more motifs with each support member arranged in the form of the one or more motifs.

[0026] The method may further comprise forming the two or more support members to each comprise at least one dimension that is a sub-factor of an operating wavelength of the waveguide structure, the sub-factor being less than one.

[0027] The at least one dimension may comprise one of a height dimension, a feature width dimension and a pitch value.

[0028] The method may further comprise forming the two or more support members based on a duty cycle that is selected based on a base area of each of the two or more support members and based on the pitch value.

[0029] The method may further comprise forming the two or more support members with the height dimension of each of the two or more support members selected based on the feature width dimension of each of the two or more support members.

[0030] The method may further comprise forming the waveguide body, the two or more support members or both using an epitaxial growth technique and using a dielectric material.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Exemplary embodiments of the present disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

[0033] FIG. 1 is a schematic front view drawing of a waveguide structure of an exemplary embodiment.

[0034] FIGs. 2A to 2F are schematic drawings for illustrating possible example motifs for forming support members in other exemplary embodiments.

[0035] FIGs. 3A to 3G are schematic diagrams for illustrating a first method of forming a waveguide structure in an exemplary embodiment.FIGs. 3A to 3D and 3H to 3J are schematic diagrams for illustrating a second method of forming a waveguide structure in another exemplary embodiment.

[0036] FIG. 4 is a schematic perspective view drawing for illustrating a waveguide structure in another exemplary embodiment.

[0037] FIG. 5A is a series of five guided optical mode characteristics drawings based on different examples of exemplary embodiments.

[0038] FIG. 5B is a set of two drawings to illustrate optical propagation loss in a conventional Ge-on-Si waveguide structure and compared to a waveguide structure of an exemplary embodiment.

[0039] FIG. 6 is a schematic flowchart for illustrating a method of forming a waveguide structure in an exemplary embodiment.

[0040] DETAILED DESCRIPTION

[0041] Exemplary embodiments described herein may provide an optically-suspended Germanium waveguide structure comprising metamaterial cladding.

[0042] In the description herein, a motif refers to a recurring, distinctive, or dominant element, subject, shape, or figure in a design. A motif may comprise a repeated sequence or placement of two or more basic units or building blocks to create a larger pattern or design. A single design in a motif may be called a unit, a motif unit, a basic unit or a unit cell.

[0043] In the description herein, the terms “pillar”, “support member”, “pedestal” may be used interchangeably and these terms refer to the structure formed based on a motif.

[0044] In some exemplary embodiments, the phrase “one or more motifs” is used to envisage, if there are two motifs, a possible use of two different designs or two basic motif units each having a different design or different shape to form a repeated sequence or placement.FIG. 1 is a schematic front view drawing of a waveguide structure of an exemplary embodiment.

[0045] A waveguide structure 100 is provided. The waveguide structure 100 comprises a substrate 102. In the exemplary embodiment, the substrate may be a silicon (Si) substrate. The waveguide structure 100 comprises a waveguide body 104. The waveguide body 104 comprises Germanium (Ge). The waveguide structure 100 further comprises two or more support members e.g. 106, 108 that are spaced apart from each other. The two or more support members e.g. 106, 108 are disposed between the substrate 102 and a bottom surface 110 of the waveguide body 104. In the exemplary embodiment, the two or more support members e.g. 106, 108 are arranged to support the waveguide body 104 and to suspend (see numeral 112) the waveguide body 104 away from the substrate 102. Therefore, the waveguide body 104 is spaced apart from the substrate 102. In the exemplary embodiment, the two or more support members e.g. 106, 108 each comprise Ge.

[0046] In the exemplary embodiment, the waveguide body 104 and the two or more support members e.g. 106, 108 may be integrally formed.

[0047] As an example, the material for the waveguide body 104, the two or more support members e.g. 106, 108, or both, may be any possible alloyed or non-alloyed material that is able to be grown epitaxially. For example, such material may include, but are not limited to, SiGe and GeSn alloys, and / or Ge material that may be grown on a Si substrate.

[0048] In the exemplary embodiment, the two or more support members e.g. 106, 108 are arranged and provided to function as metamaterial cladding in relation to the waveguide body 104 for the waveguide structure 100. That is, the two or more support members e.g. 106, 108 may function as a lower cladding layer (see numeral 112) for the waveguide body 104. The two or more support members e.g. 106, 108 also function to optically suspend the waveguide body 104.

[0049] In the exemplary embodiment, the two or more support members e.g. 106, 108 are each arranged in one or more motifs, e.g. across the bottom surface 110 of the waveguide body 104. For example only, the one or more motifs may be in the form of an array and / or each is a repeated design for forming a pattern.

[0050] In the description, it will be appreciated that references to a motif and / or its basic unit include references to the support member(s) formed (e.g. grown) using the motif and / or itsbasic unit For example, where a motif basic unit is described to have subwavelength dimensions for its feature width, and / or for its pitch value to another motif unit cell, and / or for its height of its trench (e.g. etched to form the motif basic unit), and / or in relation to its trench aspect ratio, it will be appreciated that such dimensions are translated / imparted to the profile of a support member formed (e g. grown) based on the motif basic unit. For example, a formed support member (or in some cases, a formed pillar structure) may have, based on an etched trench / area of a motif basic unit, a subwavelength feature width, and / or a subwavelength pitch value to another support member (or pillar structure), and / or a subwavelength height, and / or having its height in relation to its feature width (with reference to the trench aspect ratio), with such profile being imparted / translated from the motif basic unit.

[0051] FIGs. 2A to 2F are schematic drawings for illustrating possible example motifs for forming support members in other exemplary embodiments.

[0052] In terms of the motifs, “periodicity” may generally mean the regularity of the repetition interval and “pitch” may generally mean the spacing or density of the motifs. Pitch and periodicity may be used interchangeably in the description. The pitch may refer to the physical distance from one point of a unit cell (of any motif) to a corresponding same point of an adjacent unit cell.

[0053] FIG. 2A shows a single cross motif design 202 viewed from a front elevated position. FIG. 2B shows a top view of an arrangement or an array of geometrical motifs e.g. 204, 206 based on FIG. 2A. The geometrical motifs e.g. 204, 206 are provided periodically as an ordered pattern. A unit cell 208 of the geometrical motifs e.g. 204, 206 is shown, i.e. bounded by the dotted lines.

[0054] FIG. 2C shows a single trifecta Y-shaped motif design 210 viewed from a front elevated position. FIG. 2D shows a top view of an arrangement or an array of geometrical motifs e.g. 214, 216 based on FIG. 2C. The geometrical motifs e.g. 214, 216 are provided periodically as an ordered pattern. A unit cell 218 of the geometrical motifs e.g. 214, 216 is shown, i.e. bounded by the dotted lines.

[0055] FIG. 2E shows a single circular motif design 220 viewed from a front elevated position. FIG. 2F shows a top view of an arrangement or an array of geometrical motifs e.g. 224, 226 based on FIG. 2E. The geometrical motifs e.g. 224, 226 are provided periodically as anordered pattern. A unit cell 228 of the geometrical motifs e.g. 224, 226 is shown, i.e. bounded by the dotted lines.

[0056] In FIGs. 2B, 2D and 2F, the arrangement of the motifs may be in the form of any type of grid arrangement. For example, in FIG. 2B, a four-sided square grid 208 is shown; in FIG.

[0057] 2D, a four-sided parallelogram grid 218 with dissimilar spacing with dimensions ai and a2 is shown, i.e. ai a2, and in FIG. 2F, a six-sided hexagonal grid 228 with similar spacing with dimensions ai and a2 is shown, i.e. ai = a2.

[0058] In the example motifs of FIGs. 2B, 2D and 2F, rotational symmetry of the motifs is shown. For example, the rotational symmetry may be about an axis that is perpendicular to a bottom surface of a waveguide body (compare bottom surface 110 of waveguide body 104 of FIG. 1).

[0059] In these exemplary embodiments, each motif design e.g. 202, 210, 220 comprises a trench e.g. 230, 232, 234, each formed for a later growth or deposition of a support member (compare support members e.g. 106, 108 of FIG. 1). Each trench e.g. 230, 232, 234 may be etched from dielectric material. Therefore, with reference to FIGs. 2B, 2D and 2F, there can be provided a lattice arrangement of subwavelength trenches according to the rotational symmetry of the motifs.

[0060] Each trench e.g. 230, 232, 234 has a height, denoted schematically as “h” (see e.g. numeral 242 of FIG. 2A).

[0061] In these exemplary embodiments, each trench e.g. 230, 232, 234 and therefore, each geometrical motif e.g. 204, 206, 214, 216, 224, 226 has a feature width, denoted schematically as “w” (see e.g. numerals 236, 238, 240). For the cross motif design 202, the feature width w is with reference to a width of one of the four limbs of the cross. For the trifecta Y-shaped motif design 210, the feature width w is with reference to a width of one of the three limbs of the trifecta Y-shape. For the circular motif design 220, the feature width w is with reference to a diameter of the circular shape.

[0062] In these exemplary embodiments, with reference to FIGs. 2B, 2D and 2F, the positions of the geometrical motifs e.g. 204, 206, 214, 216, 224, 226 are defined by a pitch (or a pitch value), i.e. a distance from one point of a motif to a corresponding same point of an adjacent motif. The pitch is denoted schematically as “a” (also ai, a2 in FIGs. 2D and 2F). See e.g. numerals 244, 246, 248.In these exemplary embodiments, the dimensions of the periodic arrangements of the geometrical motifs e g. 204, 206, 214, 216, 224, 226 may be determined by an operating wavelength (A) of interest. In one such exemplary embodiment, the operating wavelength (A) of interest may be between 2 to 14 pm and a sub-wavelength condition for such an exemplary embodiment is defined as A / 2 such that a target wavelength at the lower end of the spectrum may need comparatively smaller feature size / width (w) and pitch (a). Thus, the geometrical motifs e.g. 204, 206, 214, 216, 224, 226 may have sub-wavelength dimensions and the later growth of support members may result in support members that are each formed of a subwavelength dimension.

[0063] In these exemplary embodiments, as examples of subwavelength dimensions or dimension values, the geometrical motifs e.g. 204, 206, 214, 216, 224, 226 are formed with these conditions:

[0064] ■ Pitch (a) < A / 2, Condition (1)

[0065] ■ Width (w) < 2 / 4, Condition (2)

[0066] ■ Height ( / ?) > l / im, Condition (3)

[0067] In addition to the subwavelength dimensions, a duty cycle (DC) is also taken into account to optimize a later growth process of support members.

[0068] The duty cycle of the motifs may be viewed as being based on an area of a subwavelength trench and a pitch of the motifs. For example, an equation of the duty cycle of the motifs is given by:

[0069] DC = Amotif / p2, Equation (4)

[0070] where Amotif is the area of a subwavelength trench (and therefore, a base area, or area of its base, of a later formed support member) and p is the pitch or periodicity. The duty cycle equation may be applicable to various motifs of different shapes and sizes that may be used to define / form the later-formed support members.

[0071] In these exemplary embodiments, the trenches e g. 230, 232, 234 are formed and arranged to satisfy a trench aspect ratio. The trench aspect ratio is based on a ratio of dielectric thickness (or in some cases, sacrificial oxide thickness) to a trench width. The trench aspect ratio may be defined as:

[0072] Oxide thickness

[0073] tan 60° Equation (5)

[0074] Trench widthFor example, the trench width w can correspond to a width of at least one later formed support member. For example, the dielectric thickness or oxide thickness is the thickness or height h of a motif (or of at least one later formed support member), the height h being measured in a direction that is substantially perpendicular to a bottom surface of a waveguide body. For example, the trench width w is measured in a direction that is substantially parallel to the bottom surface of the waveguide body. For example, with the trench aspect ratio, an aspect ratio trapping (ART) technique may be utilised to later form the support members.

[0075] For example, in some exemplary embodiments, the dielectric used for patterning the one or more motifs e.g. 204, 206, 214, 216, 224, 226 may be other materials, apart from silicon oxide. For example, alternatives such as silicon nitride may be used as the trench definition material.

[0076] The inventors have recognised that threading dislocation defects at a Ge / Si growth interface may be usefully terminated at the oxide sidewall (or trench wall) by a selection of the trench aspect ratio, as provided as one example in Equation (5).

[0077] In exemplary embodiments that have an operating wavelength range of 2 to 14 pm, the maximum pitch, a, to obey the sub-wavelength Condition (1) ranges from 1 to 7 pm. The minimum feature size, e.g. width w, for any chosen shape or motif design may appreciably be dependent on the lower limit set by the lithography process.

[0078] As examples, for relatively simple polygonal shapes / patterns (e.g. circles, hexagons, squares, triangles etc.), the maximum feature size, e.g. width w, may range from about 0.5 pm (A=2pm, DC=50%) to about 4.9 pm (A=14pm, DC=70%). For relatively more complicated shapes, taking the “cross” motif of FIGs. 2A and 2B as example, the maximum feature size, e.g. width w, may range from about 0.2 pm (a lower limit for lithography, A=2pm, DC=60%) to about 1.6 pm (A=14pm, DC=70%).

[0079] In these exemplary embodiments, support members are formed (e.g. grown) using Ge based on the one or more motifs. The resultant sub-wavelength Ge pillars (or viewed as air gap metamaterial) form a bottom cladding for an optically suspended Ge waveguide (e.g. grown above the Ge support members and spaced away from a substrate). The inventors recognise that the refractive index of the support members collectively (or the metamaterial cladding) may usefully be engineered by selecting the duty cycle. From the perspective of waveguide propagation, the refractive index ranging from 2 to 3.4 is readily tunable (by selection of the duty cycle) to control the evanescent wave ratio used for sensing.In these exemplary embodiments, the duty cycle may be used to simplify the tuning of the refractive index, e g. a larger duty cycle may provide more material grown in the trenches (volumetrically), and hence, the refractive index may be larger, and vice versa. Equation (4) may therefore be used for tuning a total amount of material depending on the motif used.

[0080] In more detail, the inventors recognise that the pitch value p and the duty cycle (DC) [see Equation (4)] may be variable such that an effective refractive index of support members formed based on the one or more motifs, i.e. the two or more support members disposed below a waveguide body, is capable of being tuned. For example, an index contrast between the support members and the waveguide body may be selectable / tuned by a selection of the duty cycle. In an example, the pitch value / dimension is selected to be within a sub-wavelength condition (proportionally dependent on an operational wavelength of the product, i.e. the waveguide structure). For example, see Condition (1) where pitch value / dimension is selected as “pitch (a) < A / 2”. In such an example, the selection of a duty cycle can be dependent on the effective refractive index desired for the waveguide structure while simultaneously constrained within a range where the motif pattern (either trench or resulting pillars / supporting members) dimensions are within lithographic limits.

[0081] FIGs. 3A to 3G are schematic diagrams for illustrating a first method of forming a waveguide structure in an exemplary embodiment.

[0082] FIGs. 3A to 3D and 3H to 3J are schematic diagrams for illustrating a second method of forming a waveguide structure in another exemplary embodiment.

[0083] The above diagrams may illustrate respective fabrication flows to obtain optically suspended waveguide structures or waveguide bodies suspended and supported by two or more support members.

[0084] In FIG. 3A, a starting wafer of dielectric (such as SiO2 or SiNx) on Si substrate is provided. That is, a Si substrate 302 is provided. The substrate 302 is provided with a dielectric layer 304. The dielectric material of the dielectric layer 304 may be, for example but not limited to, SiO2. A photoresist layer 306 is provided with the dielectric layer 304, e g. by spin coating. Thus, FIG. 3A is for hardmask preparation.In FIG. 3B, hardmask patterning is performed on the photoresist layer 306. The patterning is to provide at least one pattern of two or more discontinuous geometrical motif basic units at sub-wavelength dimensions (e g. feature width, pitch value / dimension).

[0085] The thickness of the dielectric layer 304 is selected / determined to obey an aspect ratio, see the trench aspect ratio of Equation (5), i.e. based on a ratio of dielectric thickness to trench width. In these exemplary embodiments, the trench width is the feature width as patterned in FIG. 3B. The aspect ratio of at least 3 (or tan 60°) is selected as the inventors recognize that the propagation angle of dislocation lines in Ge is typically contained within 60°. Therefore, a purpose of the thickness determination of the dielectric layer 304 may usefully ensure that the threading dislocation(s) is terminated at the oxide (or dielectric) sidewall.

[0086] In FIG. 3C, the dielectric layer 304 is etched to obtain the patterning. For example, a combination of dry and wet etching may be used on the dielectric layer 304 to form window regions to expose the underlying Si substrate. For example, the patterns formed may be with reference to the examples shown in FIGs. 2B, 2D, 2F.

[0087] In FIG. 3D, Ge structures e.g. 308 are formed. For example, an epitaxial growth technique is used for a selective epitaxy growth of Ge. The Ge structures e.g. 308 are initially confined within the dielectric trenches formed in the dielectric layer 304. The Ge structures e.g. 308 filling the dielectric trenches may be formed as two or more support members.

[0088] Upon pillar emergence (i.e. subsequent to the filling of the dielectric trenches) and sustained growth of Ge, overgrowth in the lateral direction (e.g. over the dielectric layer 304) is allowed to continue until the Ge structures e.g. 308 (or the individual pockets of Ge) merge to form a continuous film 310. The film thickness is controlled by growth time and chemicalmechanical polishing (CMP) techniques may be optionally employed to flatten and smoothen a top surface of the continuous film 310.

[0089] At FIGs. 3E and 3H, another photoresist layer is provided on the film 310 and hardmask patterning is performed. The subsequent waveguide body of each of the first and second methods may be defined through lithography and etching. The respective patterned and resultant photoresist layers 312, 314 remain.For the first method, etching may be controlled to terminate at the surface of the dielectric layer 304. For the second method, etching may be controlled until Ge pillars not intended to be formed under a waveguide body are removed.

[0090] At FIG. 3F, for the first method, waveguide body patterning and etching is performed. A waveguide body e.g. 316 is formed.

[0091] At FIG. 3G, for the first method, the remaining dielectric material of the dielectric layer 304 is removed e.g. through wet removal methods. This may prevent unwanted absorption of the dielectric material into the resultant waveguide structure.

[0092] For the first method, additional Ge pillars e.g. 322 are provided, as etching has been controlled to terminate at the surface of the dielectric layer 304 at FIG. 3F, prior to removal of the dielectric material.

[0093] Therefore, a waveguide structure comprising the waveguide body e.g. 316 and two or more support members e.g. 318, 320 are formed on the substrate 302. The two or more support members e.g. 318, 320 are spaced apart from each other, and the two or more support members e.g. 318, 320 are disposed between the substrate 302 and a bottom surface of the waveguide body e.g. 316. The two or more support members e.g. 318, 320 are arranged to support the waveguide body e.g. 316 and to suspend the waveguide body e g. 316 away from the substrate 302. The waveguide body e.g. 316 and the two or more support members e.g.

[0094] 318, 320 each comprise Ge.

[0095] At FIG. 3I, for the second method, waveguide body patterning and etching is performed. A waveguide body e.g. 324 is formed.

[0096] At FIG. 3J, for the second method, the remaining dielectric material of the dielectric layer 304 is removed e.g. through wet removal methods. This may prevent unwanted absorption of the dielectric material into the resultant waveguide structure.

[0097] For the second method, etching has been controlled until Ge pillars not intended to be formed under a waveguide body are removed at FIG. 3I, prior to removal of the dielectric material.

[0098] Therefore, a waveguide structure comprising the waveguide body e.g. 324 and two or more support members e.g. 326, 328 are formed on the substrate 302. The two or moresupport members e.g. 326, 328 are spaced apart from each other, and the two or more support members e.g. 326, 328 are disposed between the substrate 302 and a bottom surface of the waveguide body e.g. 324. The two or more support members e.g. 326, 328 are arranged to support the waveguide body e.g. 324 and to suspend the waveguide body e.g. 324 away from the substrate 302. The waveguide body e.g. 324 and the two or more support members e.g.

[0099] 326, 328 each comprise Ge.

[0100] With reference to FIGs. 3G and 3J, in the dry etch process, a decision may be made to choose which layer the dry etch process is to stop at. Hence, the different decisions may result in the two end results shown in FIGs. 3G and 3J. The inventors recognise that both the first method and the second method described above can create the optically suspended waveguide structures.

[0101] Therefore, in the above exemplary embodiments, a waveguide structure may be formed comprising two or more support members that are each arranged in the form of one or more motifs. The two or more support members may each comprise at least one dimension that is a sub-factor of an operating wavelength of the waveguide structure, the sub-factor being less than one. For example, see Conditions (1), (2) and (3). The at least one dimension may comprise one of a height dimension, a feature width dimension and a pitch value. In the above exemplary embodiments, a duty cycle of the two or more support members may be selected based on a base area of each of the two or more support members and based on the pitch value. For example, see Equation (4). Further, in the above exemplary embodiments, the height dimension of each of the two or more support members may be selected based on the feature width dimension of each of the two or more support members. For example, see Equation (5).

[0102] FIG. 4 is a schematic perspective view drawing for illustrating a waveguide structure in another exemplary embodiment.

[0103] There is provided an optically suspended Ge-on-Si waveguide ring resonator 402, 404 that may be formed using the second method of FIG. 3J. The waveguide body 402 is supported and optically suspended from the Si substrate by two or more support members. FIG. 4 therefore shows an example of a photonic device which is the ring resonator 402, 404 that may be formed based on exemplary embodiments described herein.

[0104] Using the exemplary embodiments e.g. described with reference to FIGs. 2A to 2F, FIGs. 3A to 3J, the inventors set out to investigate the effectiveness of such exemplaryembodiments. Guided mode characteristics with different motif designs of Ge pillars and / or support members were simulated.

[0105] FIG. 5A is a series of five guided optical mode characteristics drawings based on different examples of exemplary embodiments.

[0106] The discussion here follows the drawings from the top left hand corner of FIG. 5A in clockwise direction.

[0107] From the first drawing (top left hand corner drawing of FIG. 5A), a waveguide body 502 is shown supported by two or more support members. The guided mode is shown confined in the waveguide body. There is no detection of a significant amount of propagation of light in the cladding material formed by the two or more support members. For the second drawing (clockwise direction from the first drawing), a waveguide body 504 is shown supported by two or more support members. There is a pillar spacing or pitch of 1 m. The guided mode is shown confined in the waveguide body. There is no detection of a significant amount of propagation of light in the cladding material formed by the two or more support members. For the third drawing (clockwise direction from the second drawing), a waveguide body 506 is shown supported by two or more support members. The guided mode is shown confined in the waveguide body. There is no detection of a significant amount of propagation of light in the cladding material formed by the two or more support members.

[0108] For the fourth drawing (bottom left hand corner drawing of FIG. 5A), a waveguide body 508 is supported by two or more support members. In this drawing, the support members are adjacent to each other (adjacent along the propagation direction, i.e. into the page). The feature width of each support member is 0.3pm and the height of each support member is 1pm. The guided mode is shown confined in the waveguide body. There is no detection of a significant amount of propagation of light in the cladding material formed by the two or more support members.

[0109] For the fifth drawing (anti-clockwise direction from the fourth drawing), a waveguide body 510 is supported by two or more support members. In this drawing, the support members are adjacent to each other (adjacent along the propagation direction i.e. into the page). The pillar aspect ratio (based on the trench aspect ratio of a motif used to form the support member) each support member is more than 5 (i.e. more than tan 60°), compare Equation (5). The guided mode is shown confined in the waveguide body. There is nodetection of a significant amount of propagation of light in the cladding material formed by the two or more support members.

[0110] As shown in FIG. 5A, the guided mode remains unperturbed / unaffected under different scenarios / situations with the subwavelength support members, where the optical mode (the waveguide body) remains ‘optically suspended’ above the silicon substrate. For the investigations, the pitch value for the support members / pedestals is selected to be <A / 2, with a pattern density that is optimized for selective epitaxial growth, i.e. also in consideration of a selected duty cycle or DC=Amotif / p2=50-70%. The drawings of FIG. 5A verify the effectiveness of exemplary embodiments to adopt subwavelength dimensions for the pillars or support members such that the support members as a metamaterial cladding appears as a uniform medium to the propagating light (shown confined in the waveguide body).

[0111] Moreover, for the investigations, the sizes / dimensions of the support members are smaller than a cut-off waveguide width, which prevents light localization inside the subwavelength pillars or support members, e g. a photon is unable to identify the support members as a guiding medium. In other words, a waveguide structure may support an optical mode if it is of a certain geometry (defined by its width and height). Since the pillars or support members are arranged with a pitch that is in a sub-wavelength region with a desired duty cycle, the pillars or support members are formed such that they are smaller than the waveguiding criteria (e.g. below a threshold to support propagation of light / wave). In other words, the cut-off waveguide width may refer to the structural width where the structure is able to support an optical mode and light propagation, and where a dimension that is smaller than such a cut-off width is anticipated or considered not to allow light propagation.

[0112] FIG. 5B is a set of two drawings to illustrate optical propagation loss in a conventional Ge-on-Si waveguide structure and compared to a waveguide structure of an exemplary embodiment. The top drawing shows a conventional waveguide structure 512. The bottom drawing shows a Ge waveguide body 514 supported by subwavelength Ge pillars / support members.

[0113] For FIG. 5B, an operating wavelength of A=9.5pm is selected to illustrate the optical loss originating from the silicon substrate. In the top drawing, the mode overlaps with the silicon substrate and such spatial overlap gives rise to an optical loss of about 3.43dB / cm. It is recognised that there is absorption in the silicon substrate. In contrast, in the bottom drawing, optically suspending the Ge waveguide body 514 leads to a reduction of propagation loss from 3.43dB / cm to about 0.36dB / cm, i.e. illustrating about two times smaller optical loss (about 3dBdifference in losses). The inventors also recognise that FIG. 5B shows that its result corresponds to the optical mode overlapping / confined substantially entirely within the Ge waveguide body 514 above the silicon substrate, in agreement with the Si absorption as the origin of loss in the top drawing of FIG. 5B. In the bottom drawing of FIG. 5B, there is a reduced spatial overlap with the silicon substrate, e g. with the optical suspension of the waveguide body.

[0114] FIG. 6 is a schematic flowchart of a method of forming a waveguide structure in an exemplary embodiment.

[0115] In the exemplary embodiment, at step 602, a substrate is provided. At step 604, two or more support members spaced apart from each other are formed from the substrate. For example, the formation may be based on an epitaxial growth of the two or more support members and based on an etched pattern. At step 606, a waveguide body comprising Germanium (Ge) is formed, wherein the two or more support members are disposed between the substrate and a bottom surface of the waveguide body. For example, the formation may be based on an epitaxial growth above the two or more support members.

[0116] Referring to step 604, the two or more support members are arranged to support the waveguide body and to suspend the waveguide body away from the substrate. Referring to step 604, the two or more support members each comprise Ge.

[0117] In the described exemplary embodiments, there can be provided an optically suspended Ge-on-Si waveguide body that may usefully address an issue of propagation loss contributed from inherent material properties and an issue of mechanical stability inherent in conventional (physically) suspended waveguides. In the described exemplary embodiments, optical suspension is provided, where a Ge waveguide body is supported by an array of subwavelength Ge support members (or pedestals / pillars) that may have dimensions that are smaller than the wavelength of light or an operating wavelength desired for the waveguide structure. The dimensions are a factor of the operating wavelength, with the factor being less than one. In the described exemplary embodiments, a Ge waveguide structure (comprising a body and support members) may be formed via selective epitaxy growth in periodically opened regions (or discontinuous regions) defined by dielectric material on a Si substrate. Ge pillars (or support members), grown in the periodic dielectric trenches with a pitch value in a sub- wavelength selection, may usefully be perceived as a continuous cladding material by the guided optical mode (compare e.g. FIG. 5A). In exemplary embodiments, the optical mode may usefully propagate in the waveguide region (the waveguide body) and may avoid a defective interfacial region (or region with dislocationdefects) that may be confined within the trench region and the absorptive Si substrate by controlling the Ge pillar / support member height. Further, in the exemplary embodiments, the Ge waveguide bodies are mechanically supported by the Ge pillars (plurality and in motif design) that may usefully address stability issues of other conventional suspended Ge waveguides.

[0118] The described embodiments may relate to a design of optically-suspended Mid-IR photonics. The described embodiments may relate to a method to realize an optically-suspended Germanium waveguide body. The described embodiments may relate to a selective growth of Ge via subwavelength trenches. The described embodiments may relate to a geometrical design of subwavelength motifs / trenches. The described embodiments may relate to subwavelength Ge motifs as both supporting structures and a transparent MIR bottom cladding.

[0119] In the described exemplary embodiments, Ge photonic structures may be formed supported by periodic Ge subwavelength pillars / support members. There may also be provided selective epitaxy growth of Ge through subwavelength SiO2 trenches, followed by lateral Ge growth for waveguide body formation. There may also be provided a photonic structure patterning, followed by SiO2 wet removal. In some described exemplary embodiments, SiO2 trench geometry may be non-trivially designed for mechanical stability during SiO2 wet removal.

[0120] In the described exemplary embodiments, a waveguide mode confinement may be usefully controlled by a Ge waveguide geometry and underlying sub-wavelength pedestal support structures. It has also been verified that by changing a pitch value and duty cycle, it is possible to obtain a singular material that is able to define an index contrast.

[0121] In the described exemplary embodiments, there can be provided a suspended Geon-air waveguide body enabled by loss-less Ge / air metamaterial as the lower cladding layer. Further, there is provided a use of aspect ratio trapping (ART) technique to grow a Ge film above trenches and an oxide / dielectric film for the formation of a waveguiding layer. The inventors recognize that ART may be used in other applications where long trenches are used as growth windows for high quality Ge film to be used in CMOS electronics or low-noise photodiodes. However, the inventors recognize that oxide film is not considered detrimental in these other applications, different from the exemplary embodiments where the oxide is substantially not present after usage of the ART technique. In the described exemplary embodiments, it may be provided that by selecting discontinuous motifs as a growth windowso that dielectric / oxide is removable after waveguide formation, a Ge / air metamaterial that is MIR (mid-IR) transparent may be usefully formed as the cladding layer. Furthermore, the cladding layer may also usefully serve as useful mechanical support for the Ge waveguiding layer.

[0122] In the description herein, the terms "coupled" or "connected" where used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

[0123] The terms “adjacent”, “previous”, “next” where used are intended to cover both directly adjacent or indirectly adjacent with one or more intermediate layers in between two layers / parts / components / objects and the like.

[0124] In the description herein, the terms “first”, “second”, third”, “fourth” and the like where used are used to refer to parts / components of the respective figures. These terms are not intended to be limiting or limited to a specific part / component. Rather, the terms should be read with the respective figures and the meanings ascribed accordingly.

[0125] The use of “a”, “an” or “the” is intended to mean “one or more” unless it is described specifically to the contrary.

[0126] The terms “configured to (perform a task / action)”, “configured for (performing a task / action)” and the like as used in this description include being programmable, programmed, connectable, wired or otherwise constructed to have the ability to perform the task / action when arranged or installed as described herein. The terms “configured to (perform a task / action)”, “configured for (performing a task / action)” and the like are intended to cover “when in use, the task / action is performed”, e.g. specifically to and / or specifically configured to and / or specifically arranged to and / or specifically adapted to do or perform a task / action.

[0127] The term "and / or", e.g., "X and / or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning. The use of “or” is intended to mean an “inclusive or,” and not an “exclusive or” unless it is described specifically to the contrary.

[0128] The terms "associated with", “related to” and the like used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to, a physical, a chemical or a biological relationship. For example, when elementA is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa.

[0129] The terms “exemplary embodiment”, “example embodiment”, “exemplary implementation”, “exemplarily” and the like used herein are intended to indicate an example of matters described in the present disclosure. Such an example may relate to one or more features defined in the claims and is not necessarily intended to emphasise a best example or any essentialness of any features.

[0130] Additionally, when describing some embodiments, the disclosure may have disclosed a method and / or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated that the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and / or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure.

[0131] Further, in the description herein, the word “substantially” whenever used is understood to include, but not restricted to, "entirely" or “completely” and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements / components recited after such terms, in addition to other components not explicitly recited. For an example, when “comprising” is used, reference to a “one” feature is also intended to be a reference to “at least one” of that feature. Terms such as “consisting”, “consist”, and the like, may, in the appropriate context, be considered as a subset of terms such as "comprising", "comprise", and the like. Therefore, in embodiments disclosed herein using the terms such as "comprising", "comprise", and the like, it will be appreciated that these embodiments provide teaching for corresponding embodiments using terms such as “consisting”, “consist”, and the like. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example, but not limited to, a variation of + / - 5% of the disclosed value, or a variance of 4% around / of the disclosed value, or a variance of 3% around / of the disclosed value, a variance of 2% around / of the disclosed value or a variance of 1% around / of the disclosed value.

[0132] Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1% to 5% is intended to have specifically disclosed sub-ranges 1% to 2%, 1% to 3%, 1% to 4%, 2% to 3% etc., as well as individually, values within that range such as 1%, 2%, 3%, 4% and 5%. It is to be appreciated that the individual numerical values within the range also include integers, fractions and decimals. Furthermore, whenever a range has been described, it is also intended that the range covers and teaches values of up to 2 additional decimal places or significant figures (where appropriate) from the shown numerical end points. For example, a description of a range of 1% to 5% is intended to have specifically disclosed the ranges 1.00% to 5.00% and also 1.0% to 5.0% and all their intermediate values (such as 1.01%, 1.02% ... 4.98%, 4.99%, 5.00% and 1.1%, 1.2% ... 4.8%, 4.9%, 5.0% etc.,) spanning the ranges. The intention of the above specific disclosure is applicable to any depth / breadth of a range.

[0133] It will be appreciated by a person skilled in the art that other variations and / or modifications may be made to the specific embodiments without departing from the scope of the claimed invention as broadly described. For example, in the description herein, features of different exemplary embodiments may be mixed, combined, interchanged, incorporated, adopted, modified, included etc. or the like across different exemplary embodiments. For example, exemplary embodiments are not necessarily mutually exclusive as some may be combined with one or more embodiments to form new exemplary embodiments. Furthermore, it will be appreciated that while the present disclosure provides embodiments having one or more of the features / characteristics discussed herein, one or more of these features / characteristics may also be disclaimed in other alternative embodiments and the present disclosure provides support for such disclaimers and these associated alternative embodiments. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

CLAIMS1. A waveguide structure, the waveguide structure comprising,a substrate;a waveguide body comprising Germanium (Ge);two or more support members spaced apart from each other, the two or more support members being disposed between the substrate and a bottom surface of the waveguide body;wherein the two or more support members are arranged to support the waveguide body and to suspend the waveguide body away from the substrate; andfurther wherein the two or more support members each comprise Ge.

2. The waveguide structure as claimed in claim 1 , further comprising the two or more support members that are each arranged in the form of one or more motifs.

3. The waveguide structure as claimed in claims 1 or 2, wherein the two or more support members each comprise at least one dimension that is a sub-factor of an operating wavelength of the waveguide structure, the sub-factor being less than one.

4. The waveguide structure as claimed in claim 3, wherein the at least one dimension comprises one of a height dimension, a feature width dimension and a pitch value.

5. The waveguide structure as claimed in claim 4, wherein a duty cycle of the two or more support members is selected based on a base area of each of the two or more support members and based on the pitch value.

6. The waveguide structure as claimed in claim 5, wherein the duty cycle of the two or more support members is based on DC = Amotif / p2, DC refers to the duty cycle, Amour refers to the base area of each of the two or more support members and p is the pitch value.

7. The waveguide structure as claimed in any one of claims 4 to 6, wherein the height dimension of each of the two or more support members is selected based on the feature width dimension of each of the two or more support members.

8. The waveguide structure as claimed in claim 7, wherein the height dimension of each of the two or more support members and the feature width dimension of each of the two or more Oxide thickn&ss support members are selected based on a relationship tan 60° = - , where Trench widthoxide thickness refers to the height dimension of each of the two or more support members and trench width refers to the feature width dimension of each of the two or more support members.

9. A method of forming a waveguide structure, the method comprising,providing a substrate;forming from the substrate two or more support members spaced apart from each other; forming a waveguide body comprising Germanium (Ge) wherein the two or more support members are disposed between the substrate and a bottom surface of the waveguide body; wherein the two or more support members are arranged to support the waveguide body and to suspend the waveguide body away from the substrate; andfurther wherein the two or more support members each comprise Ge.

10. The method as claimed in claim 9, further comprising forming the two or more support members using one or more motifs with each support member arranged in the form of the one or more motifs.

11. The method as claimed in claims 9 or 10, further comprising forming the two or more support members to each comprise at least one dimension that is a sub-factor of an operating wavelength of the waveguide structure, the sub-factor being less than one.

12. The method as claimed in claim 11, wherein the at least one dimension comprises one of a height dimension, a feature width dimension and a pitch value.

13. The method as claimed in claim 12, further comprising forming the two or more support members based on a duty cycle that is selected based on a base area of each of the two or more support members and based on the pitch value.

14. The method as claimed in any one of claims 12 to 13, further comprising forming the two or more support members with the height dimension of each of the two or more support members selected based on the feature width dimension of each of the two or more support members.

15. The method as claimed in any one of claims 9 to 14, further comprising forming the waveguide body, the two or more support members or both using an epitaxial growth technique and using a dielectric material.

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