Waveguide manufacturing method and optical structure

Abstract

The invention provides a waveguide manufacturing method and an optical structure. The optical structure includes a waveguide. The waveguide has a first portion, a second portion, and a transition portion having a first end connected to the first portion and a second end connected to the second portion. The first portion has a first thickness less than a second thickness of the second portion. The thickness of the transition portion gradually increases from a first thickness of a first end thereof to a second thickness of a second end thereof. In a manufacturing method employing chemical mechanical polishing (CMP), a first CMP control structure and a second CMP control structure are disposed on opposite sides of a waveguide. A cladding material is filled in a space between the waveguide and the CMP control structure. CMP is performed to reduce the thickness of the waveguide. The CMP control structure controls CMP of the waveguide to form a transition portion of the waveguide having a gradually increasing thickness.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to a waveguide manufacturing method and an optical structure. Background Art

[0002] The following relates to optical waveguides, methods for manufacturing optical waveguides, optical waveguide coupling, etc. Summary of the Invention

[0003] According to an embodiment of the present disclosure, a waveguide manufacturing method includes: forming a stacked layer including an etch stop layer disposed on a waveguide layer, the waveguide layer being disposed on a bottom cladding layer; patterning the etch stop layer and etching the waveguide layer after patterning to form a waveguide and a chemical mechanical polishing (CMP) control structure; filling a space between the waveguide and the CMP control structure with a cladding material; and performing CMP to reduce the thickness of the waveguide, wherein the CMP control structure controls the CMP of the waveguide to form a transition portion of the waveguide having a gradually changing thickness.

[0004] According to an embodiment of the present disclosure, an optical structure includes a waveguide. The waveguide has a first portion, a second portion, and a transition portion. The transition portion has a first end connected to the first portion and a second end connected to the second portion. The thickness of the first portion of the waveguide is less than the thickness of the second portion of the waveguide. The transition portion of the waveguide has a thickness that gradually increases from a first thickness at the first end of the transition portion of the waveguide to a second thickness at the second end of the transition portion of the waveguide.

[0005] According to an embodiment of the present disclosure, a method for manufacturing a waveguide having a variable thickness formed by chemical mechanical polishing (CMP) includes: depositing an etch stop layer on a silicon layer of a silicon-on-insulator (SOI) wafer; patterning the etch stop layer and etching the silicon layer after patterning to form a first CMP control structure, a second CMP control structure, and a silicon waveguide disposed between the first CMP control structure and the second CMP control structure; filling a space between the silicon waveguide and the first CMP control structure and the second CMP control structure with silicon dioxide using a shallow trench isolation (STI) process; and performing CMP to reduce the thickness of the silicon waveguide, wherein the first CMP control structure and the second CMP control structure control the CMP of the silicon waveguide to form a transition portion of the silicon waveguide having a gradually changing thickness. Brief Description of the Drawings

[0006] The present disclosure may be best understood from the following detailed description when read in conjunction with the accompanying drawings. It should be emphasized that, according to industry standard practice, various features are not drawn to scale and are for illustrative purposes only. In fact, for the sake of clarity in discussion, the dimensions of various features may be arbitrarily increased or decreased.

[0007] Figure 1 A top view schematically showing an optical structure including a waveguide;

[0008] Figure 2 Schematically shows a cross-sectional view of a waveguide of an optical structure taken along Figure 1 the cut line C0-C0 shown in Figure 1 ;

[0009] Figure 3 Schematically shows Figure 1 and Figure 2 a top view of an optical structure, in which certain dimensions are marked and cross-sections are shown for the cut line C1-C1, the cut line C2-C2, and the cut line C3-C3;

[0010] Figure 4 Schematically shows Figure 2 a cross-sectional view of a waveguide, in which certain dimensions are marked, and cross-sections are shown for Figure 3 the same cut line C1-C1, cut line C2-C2, and cut line C3-C3 in

[0011] Figure 5A , Figure 5B , Figure 5C , Figure 5D , Figure 5E , Figure 5F , Figure 5G and Figure 5H Schematically shows the cut line C3-C3 shown in Figure 3 and Figure 4 at successive steps of a waveguide manufacturing process;

[0012] Figure 6 Schematically shows two cross-sectional views taken along Figure 3 and Figure 4 the cut line C1-C1 shown in

[0013] Figure 7 where the left cross-section shows the structure before CMP and the right cross-sectional view shows the structure after CMP; Figure 3 and Figure 4 Schematically shows two cut views taken along

[0014] Figure 8 the cut C2-C2 shown in Figure 3 and Figure 4 where the left cut view shows the structure before CMP and the right cut view shows the structure after CMP;

[0015] Figure 9A cross-sectional view schematically showing a portion of an optical waveguide having a continuously varying waveguide height for delivering light to an optical detector. Detailed Description

[0016] The following disclosure provides many different embodiments or examples for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. Of course, these are merely examples and are not intended to be limiting. For example, forming a first feature above or over a second feature in the following description may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features such that the first and second features may not be in direct contact. Additionally, the present disclosure may repeat reference numerals and / or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and / or configurations discussed.

[0017] For convenience of description, spatially relative terms such as "beneath", "below", "lower", "above", "upper", etc. may be used herein to describe one component or feature's relationship to another component or feature as illustrated. In addition to the orientation depicted in the figures, the spatially relative terms are intended to encompass different orientations of the device in use or operation. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may be interpreted accordingly.

[0018] Silicon waveguides are used as silicon photon components to integrate electro-optical circuits, etc. Silicon waveguides typically use silicon as the core material with a higher refractive index for carrying light, and silicon dioxide as the cladding with a lower refractive index. Silicon waveguides are mainly made of silicon and silicon dioxide and are suitable for integration using existing silicon-based microelectronics and integrated circuit (IC) technologies.

[0019] In some applications, it may be useful for a silicon waveguide to have portions of different thicknesses (i.e., different heights). For example, as two non-limiting examples, a light input coupling waveguide portion may have a reduced height optimized to match the size of a light emitter, or a light detector coupling may have a larger height optimized to match the light detector. Waveguide portions with different heights increase manufacturing complexity because each different thickness (i.e., height) in a silicon waveguide requires an additional lithography patterning and etching process sequence.

[0020] Another problem is that each change in the height of a portion of the waveguide introduces a sharp step in the height of the waveguide. Ray reflection occurs at the height step, and the reflected rays constitute optical losses. Ray reflection also produces unwanted destructive optical interference. In the case of polychromatic light, the amount of reflection and constructive or destructive interference may be wavelength-dependent, which may introduce an undesirable wavelength dependence in the transmission efficiency of the waveguide.

[0021] Below, an improved waveguide is disclosed that advantageously provides a gradual transition between waveguide portions of different thicknesses (i.e., different heights) without sharp steps. The gradual transition substantially reduces or eliminates ray reflection at the transition. Also disclosed herein is a method of fabricating such an improved waveguide that advantageously achieves a gradual transition from one thickness to another without multiple lithography patterning and etching process sequences. The disclosed fabrication process utilizes a chemical mechanical polishing (CMP) process controlled by a CMP control structure to form a transition portion of the waveguide having a gradually varying thickness.

[0022] Reference Figure 1 and Figure 2 show, in Figure 1 , an optical structure including a waveguide 10 in a top view; and Figure 2 shows Figure 1 a cross-sectional view of the waveguide 10 of the optical structure taken along the cut line C0-C0 shown in Figure 1 . Without loss of generality, a Cartesian coordinate set of directions x, y, and z is shown in Figure 1 and 2 (and also shown in many other figures herein). In this system, the plane of the semiconductor wafer or substrate (not shown) on which the optical structure is fabricated is represented as the xy plane, and the direction z is perpendicular to the xy plane of the wafer. It should be noted that the depicted Cartesian coordinate set of directions x, y, z does not show any specific origin (i.e., does not show the origin where x = 0, y = 0, and z = 0); rather, it shows three mutually orthogonal directions x, y, and z. In this coordinate system, the layers disposed on the wafer or substrate have a thickness in the direction z. The thickness may also be referred to as the height, and the terms "thickness" and "height" may be used interchangeably herein and both refer to the direction perpendicular to the main surface of the wafer or substrate, represented as the direction z in the figures.

[0023] Specific reference is made to Figure 2, the exemplary waveguide 10 has a first portion 12, a second portion 14, and a transition portion 16. The transition portion 16 has a first end 20 connecting the first portion 12 and a second end 22 opposite the first end and connecting the second portion 14. The first portion 12 of the waveguide 10 has a smaller thickness (or height) than the second portion 14 of the waveguide 10, and when moving along the direction x from the first end 20 of the transition portion 16 to the second end 22 of the transition portion 16, the transition portion 16 of the waveguide 10 has a monotonically increasing thickness. It is noted that there is no steep step in the transition portion 16 of the waveguide 10.

[0024] Special return reference Figure 1 , as can be seen in a top view, the optical structure further includes (in addition to the waveguide 10) a first structure 30 disposed along a first side of the waveguide 10 and a second structure 32 disposed along a second side of the waveguide 10 opposite the first side. The first structure 30 and the second structure 32 are not optical operating members of the optical structure. Instead, as will be described later, the first structure 30 and the second structure 32 provide control for the chemical mechanical polishing (CMP) step of manufacturing the waveguide 10. This CMP control provides a gradual increase in the thickness of the transition portion 16 of the waveguide 10. Thus, the first structure 30 is also referred to herein as the first CMP control structure 30; similarly, the second structure 32 is also referred to herein as the second CMP control structure 32.

[0025] Figure 1 and Figure 2 also shown is a light emitter 34 disposed at the input end 36 of the first portion 12 of the waveguide 10. As Figure 2 shown, the input end 36 of the first portion 12 is remote from the transition portion 16 of the waveguide 10. The light emitter 34 can be, for example, the output end of an optical fiber or a laser, a light emitting diode (LED), etc. In operation, Figure 1 and Figure 2 the waveguide 10 of the exemplary optical structure receives light L output by the light emitter 34 into the input end 36 of the first portion of the waveguide 10. In a typical waveguide, there would be a steep step in height (i.e., a thickness step) between the first portion 12 of lower height and the second portion 14 of higher height; the steep step in height would introduce light reflection, thereby reducing the light transmission efficiency of the waveguide. However, in the exemplary waveguide 10, the gradual increase in the thickness of the transition portion 16 advantageously reduces or eliminates such reflections and generally improves the optical efficiency of the waveguide 10, and in particular the optical efficiency of light transmission from the first portion 12 through the transition portion 16 to the second portion 14.

[0026] The exemplary transition portion 16 of waveguide 10 has a varying thickness that linearly increases from the first portion 12 of waveguide 10 to the second portion 14 of waveguide 10 (or, in other words, linearly increases from the first end 20 of transition portion 16 to the second end 22 of transition portion 16). More generally, in order to limit or eliminate light reflection, the transition portion 16 should have a varying thickness that monotonically increases from the first portion 12 of waveguide 10 to the second portion 14 of waveguide 10 (or, in other words, the varying thickness monotonically increases from the first end 20 of transition portion 16 to the second end 22 of transition portion 16).

[0027] Now referring to Figures 3 to 5H , a method of forming a gradually varying thickness of the transition portion 16 of waveguide 10 by control of a chemical mechanical polishing (CMP) step is described. Figure 3 shows Figure 1 a top view thereof and indicates certain dimensions, while Figure 4 shows a C0 - C0 cross - section of waveguide 10 and indicates certain dimensions. Starting from Figure 4 , the first portion 12 of waveguide 10 has a thickness or height H1, the second portion 14 of waveguide 10 has a thickness or height H3 (where H3 > H1), and the transition portion 16 of waveguide 10 has a thickness that gradually increases from its first end 20 to its second end 22, and shows a selected thickness H2 (where H1 < H2 < H3) for the cross - line C2 - C2 at Figure 2 . Figure 4 also shows the cross - line C1 - C1 through the first portion 12 and the cross - line C3 - C3 through the second portion 14. Each of the cross - sections C1 - C1, C2 - C2, and C3 - C3 is a yz - plane using the xyz Cartesian directions shown in the figure. It should be noted that although Figure 4 shows a certain cross - line C1 - C1 through the first portion 12, any cross - line through the first portion 12 will have the same thickness H1 because the first portion 12 has a constant thickness along the x - direction; similarly, although Figure 4 shows a certain cross - line C3 - C3 through the second portion 14, any cross - line through the second portion 14 will have the same thickness H3 because the second portion 14 has a constant thickness along the x - direction. On the other hand, when the hypothetical cross - line through the transition portion 16 moves from the first end 20 to the second end 22, the height will increase from the height H1 at the first end 20 to the height H3 at the second end 22. Thus, the exemplary cross - section C2 - C2 with height H2 is an example.

[0028] Referring to Figure 3, the corresponding cross-sections C1, C2, and C3 passing through the first part 12, the transition part 16, and the second part 14 are shown again. At the cross-section C1-C1 passing through the first part 12, the first CMP control structure 30 is spaced apart from the waveguide 10 by a distance S1 (along the y direction); similarly, at the cross-section C1-C1, the second CMP control structure 32 is spaced apart from the waveguide 10 by the same distance S1. The first CMP control structure 30 has a uniform distance S1 along its entire length in the x direction.

[0029] At the cross-section C3-C3 passing through the second part 14, the first CMP control structure 30 is spaced apart from the waveguide 10 by a smaller distance S3; similarly, at the cross-section C3-C3, the second CMP control structure 32 is spaced apart from the waveguide 10 by the same distance S3. The second CMP control structure 32 has a uniform distance S3 along its entire length in the x direction.

[0030] On the other hand, when moving from the first end 20 to the second end 22 of the transition part 16 of the waveguide 10 along the x direction, the CMP controller structure 30 and the CMP controller structure 32 have a decreasing distance from the waveguide 10 along the transition part 16 of the waveguide 10. Figure 3 It is shown that each of the CMP control structure 30 and the CMP control structure 32 at the exemplary cross-section C2-C2 has a distance S2 from the waveguide 10 (where S1 < S2 < S3).

[0031] It should be noted that, for the sake of simplicity Figures 3 to 5H in the analysis depicted in Figure 3 the waveguide 10 shown in Figure 1 has a uniform width in the y direction, thus ignoring the slightly gradual increase in the width of the waveguide 10 in the y direction when it moves along the x direction as shown in

[0032] Now referring to Figures 5A to 5H , by the successive exemplary cross-sectional views taken along the cut line C3-C3 shown in Figure 3 and Figure 4 , the method of manufacturing an optical structure including the waveguide 10, the CMP controller structure 30, and the CMP controller structure 32 is shown in successive steps of the manufacturing process. As indicated by the Cartesian direction coordinates in each of Figures 5A to 5H , the cross-section of the cut line C3-C3 is a yz cross-section perpendicular to the x direction. In this illustrative example, the manufacturing process starts with a silicon-on-insulator (SOI) wafer 40, as shown in Figure 5AAs shown. SOI wafer 40 includes a silicon layer 42 disposed on a buried oxide (BOX) layer 44. SOI wafer 40 can be, for example, a commercial SOI wafer. As an illustrative example, buried oxide layer 44 is described herein as a silicon dioxide layer, but other suitable oxide materials can also be considered. The buried oxide layer 44 should have a refractive index less than the refractive index of silicon, so that the buried oxide layer 44 can be used as a lower refractive index cladding material below the waveguide 10 predetermined to be formed by the silicon layer 42. Although SOI wafer 40 is described as a base substrate for manufacturing and optical structures, it should be understood that other base substrates can be used, and the base substrate provides a silicon layer disposed on an oxide layer, wherein the refractive index of the oxide layer is less than the refractive index of the silicon wafer. As another non-limiting illustrative example, the structure can be a silicon substrate, on which a suitable oxide layer is deposited, followed by the deposition of a silicon layer. Furthermore, while the illustrative example employs silicon layer 42 to form silicon waveguide 10 (as well as silicon CMP control structures 30 and silicon CMP control structures 32), it is contemplated that another waveguide material may be employed as layer 42 in order to form waveguides of different materials using the fabrication methods described herein. Thus, more generally, layer 42 may be considered a waveguide layer 42, and layer 44 may be considered a bottom cladding layer 44.

[0033] refer to Figure 5B The manufacturing process begins with depositing an etch stop layer 46 on the silicon layer 42. In the illustrative example, an initial thin layer of silicon dioxide (SiO 2 ) layer 48, but the initial SiO 2 Layer 48 is optional. In the example herein, etch stop layer 46 is a silicon nitride (SiN) layer, but other etch stop layer materials are also contemplated. Etch stop layer 46 is suitably deposited by chemical vapor deposition (CVD), physical vapor deposition (PVD), sputtering, or another deposition modality.

[0034] refer to Figure 5C , the etch stop layer 46 is subjected to photolithographic patterning etching to remove the etch stop layer 46 except for the positions corresponding to the predetermined waveguide 10 and the CMP control structures 30 and 32. The photolithographic patterning etching requires that a photoresist is disposed on the etch stop layer 46, the photoresist is subjected to photolithographic exposure to form a latent image in the photoresist, the latent image is developed to remove the exposed area (if a positive photoresist is used) or to remove the unexposed area (if a negative photoresist is used) to form an opening in the photoresist, and then the etch stop layer 46 in the opening of the photoresist is removed by etching to form a corresponding opening 50 in the etch stop layer 46. Figure 5C In the example below, the SiO 2Layer 48 serves as the etch termination of etch stop layer 46; however, in other contemplated embodiments, if the etchant used to form opening 50 is selective for silicon nitride (or other material forming etch stop layer 46) relative to silicon layer 42 (or other waveguide material constituting layer 42), then SiO may be selectively and appropriately omitted. 2 Layer 48. In Figure 5C , since the etch forms opening 50, the etch stop layer is now labeled as patterned etch stop layer 46P to show the patterning of the layer. It should be noted that although not visible in the cross-sectional view along cut line C3-C3 shown in Figure 5C taken along the yz cross-section, if viewed in a top view (i.e., along direction z), the patterned etch stop layer 46P will have a pattern corresponding to the locations of waveguide 10 and CMP control structures 30 and CMP control structure 32, as shown in Figure 1 and Figure 3 .

[0035] Reference Figure 5D , silicon layer 42 is etched through opening 50 in patterned etch stop layer 46P to remove silicon layer 42 except for the area below patterned etch stop layer 46P. The etchant used is more selective for etching silicon (or other waveguide material constituting layer 42) than for silicon nitride (or etch termination material constituting other patterned etch stop layer 46P). The etchant used can also effectively remove optional SiO except for the area below patterned etch stop layer 46P. 2 Layer 48, so now it is patterned SiO 2 Layer 48P, as labeled in Figure 5D . After etching to form waveguide 10 and CMP control structures 30 and CMP control structure 32, the silicon remaining below patterned etch stop layer 46P. At the stage schematically depicted in Figure 5D , waveguide 10 and CMP control structures 30 and CMP control structure 32 have the layout shown in Figure 1 and Figure 3 , but waveguide 10 does not yet have the thickness (i.e., height) profile shown in Figure 2 and Figure 4 . Instead, at the stage schematically depicted in Figure 5D , waveguide 10 has a uniform height that is greater than (or at least equal to) the thickness H3 labeled for the second part 14 of completed waveguide 10 in Figure 4 . The etch leaves the optical structure at the stage schematically depicted in Figure 5D , where space 52 is located between waveguide 10 and CMP control structures 30 and CMP control structure 32.

[0036] Reference Figure 5E, the space 52 between the waveguide 10 and the CMP control structures 30 and 32 is filled with a cladding material 54. In the illustrative example, the cladding material 54 is silicon dioxide. Thus, after filling the cladding material 54, the illustrative silicon waveguide 10 is surrounded by a cladding material including silicon dioxide, i.e., enclosed. That is, the waveguide 10 is clad at its bottom by the buried oxide 44 of the initial SOI wafer 40 (see Figure 5A ), and is clad at its sides and top by the filled cladding material 54. Further, since in the illustrative embodiment, the buried oxide 44, the filled cladding material 54, and the patterned SiO 2 layer 48P are all silicon dioxide, at the manufacturing stage schematically shown in Figure 5E , these silicon dioxide regions form a continuum of silicon dioxide surrounding the waveguide 10, the CMP control structures 30, 32, and the patterned etch stop layer 46P. In some embodiments, the space 52 between the waveguide 10 and the CMP control structures 30 and 32 is filled with the silicon dioxide cladding material 54 by a shallow trench isolation (STI) process. In a suitable STI process, an initial conformal layer of silicon dioxide is selectively formed by thermal oxidation, and then silicon dioxide is deposited by CVD or another suitable deposition modality to a thickness sufficient to fill the space 52 and extend over the waveguide 10, the CMP control structures 30, 32, and the patterned etch stop layer 46P, as shown in Figure 5E .

[0037] The STI process typically includes subsequent planarization by chemical mechanical polishing (CMP). This is described next with reference to Figure 5F . CMP is sometimes referred to in the art as "chemical mechanical planarization". However, in the optical structure manufacturing method described herein, CMP does not produce a flat surface. This is because the structure to which CMP is applied has a laterally varying structure imposed by the waveguide 10 and the CMP control structures 30 and 32. As will be specifically explained later herein with reference to Figure 6 , this laterally varying structure causes the thickness of the silicon waveguide 10 to decrease by CMP, the amount of which varies according to the spacing between the waveguide 10 and the CMP control structures 30 and 32 (e.g., the spacings S1 or S2 or S3 labeled in Figure 3 ). The amount of thickness reduction can also depend on the width of the CMP control structure (i.e., the width along the direction y), since a larger width of the CMP control structure can increase the resistance to CMP. The thickness reduction can also depend on the width of the waveguide 10 (see, for example, Figure 1, showing that the waveguide width in the y-direction increases when moving in the x-direction. A wider waveguide presents greater resistance to CMP and will thus have less thickness reduction).

[0038] Reference Figure 5F , showing the effect of the CMP control structure 30 and the CMP control structure 32 on the cross-section C3-C3 through the second portion 14 of the waveguide 10. (In the C3-C3 cross-section, the CMP control structure 30 and the CMP control structure 32 are spaced from the waveguide 10 by a distance S3, as Figure 3 marked). The CMP control structure 30 and the CMP control structure 32 exhibit greater resistance to CMP compared to the silica cladding 54 disposed between the waveguide 10 and each of the CMP control structure 30 and the CMP control structure 32. This greater resistance is due to the fact that the silica of the cladding 54 has less resistance to CMP than the CMP control structure 30 and the CMP control structure 32, and due to the patterned etch stop 46P disposed on the CMP control structure 30 and the CMP control structure 32 generally having greater resistance to CMP than the silica of the cladding 54. As recognized herein, this results in the CMP not providing perfect planarization but rather producing a surface 56 that is bow-shaped or dish-shaped as schematically shown. The bow-shaped or dish-shaped surface 56 is produced by the CMP more effectively removing the material of the cladding 54 and the waveguide 10 compared to the CMP control structure 30 and the CMP control structure 32 that are more resistant to CMP processing. It is noted that the amount of curvature or depression of the surface 56 produced by the CMP will depend on the structure of the CMP control structure 30 and the CMP control structure 32, particularly the spacing of these structures from the waveguide 10. This will be discussed in more detail below with reference to Figure 6 this point.

[0039] In some embodiments, the method of manufacturing an optical structure including the waveguide 10 may terminate at Figure 5F the structure shown.

[0040] Reference Figure 5G , in other embodiments, an optional further process operation of depositing additional silica (or more generally, additional cladding material) 58 may be performed. For example, if CMP is performed as Figure 5F shown to produce the structure, the silica cladding is completely removed from the top surface of the waveguide 10 (see Figure 6Examples of cross-sections C1-C1 and C2-C2 as shown may be beneficial. In this case, the additional deposited silica 58 ensures that the waveguide 10 is fully embedded (i.e., surrounded) in the (illustrative silica) cladding material, the refractive index of the cladding material being lower than the (illustrative silicon) refractive index of the waveguide 10, such that light is guided through the waveguide 10 by total internal reflection (TIR).

[0041] Reference Figure 5H , in some embodiments, an optional second CMP process may be performed after depositing the additional silica 58 to provide a planarized surface 60 to the final optical structure. Since this second CMP operates only on a uniform layer of silica (or other cladding materials in other embodiments) and does not penetrate deeply enough to expose the CMP control structures 30 and 32, this second CMP is expected to produce a flatter final surface (as opposed to the bowed or dish-shaped surface 56 produced by the first CMP interacting with the CMP control structures 30 and 32).

[0042] Having referred Figures 5A to 5H to the manufacturing method,[[]]END]] Figures 5A to 5H Schematic cross-sectional views at cross-section C3-C3 through the second portion 14 of the waveguide 10 are shown. These views do not show how the thickness of the transition portion 16 of the waveguide 10 varies gradually along the direction x.[[]]END]]

[0043] Returning to reference Figure 3 and Figure 4 , and now further referring Figures 6 to 8 , it is explained how the CMP control structures 30 and 32 control the CMP to provide Figure 4 the height profile of the waveguide 10 as shown in Figure 6 and, in particular, how the thickness of the transition portion 16 from H1 to H3 gradually increases as it moves along the direction x.[[]]END]] Figure 7 Schematic cross-sectional view showing cross-section C1-C1;[[]]END]] Figure 8 Schematic cross-sectional view showing cross-section C2-C2;[[]]END]] Figure 6 、 Figure 7 and Figure 8 Each of Figure 5F shows two cross-sectional views: one presenting the structure before the CMP process (as previously described with reference Figure 6 、 7 and 8 each contain an arrow (→) labeled "CMP" to clarify the pre-cut / post-cut views.[[]]END]]Figure 6 The left side (before CMP) of the cut view of the cross-section C1-C1 also marks the spacing S1 between the first CMP control structure 30 and the second CMP control structure 32 at the cross-section C1-C1, which is also marked in Figure 3 . Figure 7 The left side (before CMP) of the cut view of the cross-section C2-C2 also marks the spacing S2 between the first CMP control structure 30 and the second CMP control structure 32 at the cross-section C2-C2, which is also marked in Figure 3 . Figure 8 The left side (before CMP) of the cut view of the cross-section C3-C3 also marks the spacing S3 between the first CMP control structure 30 and the second CMP control structure 32 at the cross-section C3-C3, which is also marked in Figure 3 . In addition, for simplicity, the silica cladding is generally marked as cladding 64 (as opposed to separately marking the cladding members (44, 48P, and 54) as done in the Figures 5A to 5H continuous cross-sectional views). As previously mentioned, S1 > S2 > S3, which can also be seen by comparing the left side (before CMP) views of Figure 6 , Figure 7 and Figure 8 .

[0044] As previously discussed when describing the CMP process in reference Figure 5F , the CMP control structure 30 and the CMP control structure 32 present a greater resistance to CMP than the silica cladding 54 disposed between the waveguide 10 and each of the CMP control structure 30 and the CMP control structure 32. This is because the silica cladding 54 presents a smaller resistance than the silicon of the CMP control structure 30 and the CMP control structure 32, and because the patterned etch stop 46P disposed on the CMP control structure 30 and the CMP control structure 32 is more resistant to CMP than the silica cladding 54. The magnitude of the resistance to CMP also depends on the width of the waveguide 10. In the example of Figure 1 , the waveguide 10 has a width that increases with the distance increasing along the x-direction (along the y-direction), such that the waveguide 10 itself also presents an increasing CMP resistance with the distance increasing along the x-direction. These effects result in a bowed or dish-shaped surface 56 after CMP, as shown in Figure 5F . Additionally, it should be noted that the amount of thickness reduction can also depend on the width of the CMP control structure (i.e., the width along the y-direction), because a larger width of the CMP control structure can increase the resistance to CMP.

[0045] It is expected that the amount of bending or depression will increase with the increase in the spacing between the waveguide 10 and the CMP control structure 30 and the CMP control structure 32. For the cross-section C1-C1 of Figure 6 , this spacing is S1; forFigure 7 transverse line C2-C2, this spacing being the smaller spacing S2; for Figure 8 transverse line C3-C3, this spacing being the even smaller spacing S3. That is, S1 > S2 > S3. Thus, for Figure 6 the maximum spacing S1, the bending or depression should be the greatest; for Figure 8 the minimum spacing S3, the bending or depression should be the least; for Figure 7 the intermediate spacing S2, the bending or depression should be intermediate. As seen in Figure 3 and Figure 4 , in the transition portion 16 of waveguide 10, the spacing between waveguide 10 and CMP control structures 30 and 32 decreases in a continuous manner from the maximum spacing S1 at the first end 20 of transition portion 16 to the minimum spacing S3 at the opposite second end 22 of transition portion 16, where the spacing S2 is at an intermediate point between the first end 20 and the second end 22. This decreasing spacing is expected to gradually reduce the bending or depression generated by CMP along direction x from the first end 20 to the second end 22 of transition portion 16, and thus the thickness reduction generated by CMP is expected to gradually reduce as it moves from the first end 20 to the second end 22. This results in a desired gradually increasing thickness (or height) of the transition portion 16 of waveguide 10. This effect is shown by the marked arcuate or dish-shaped surface 561 of the transverse line C1-C1 shown in Figure 6 , Figure 7 and Figure 8 ; it has a greater arc than the marked arcuate or dish-shaped surface 562 of the transverse line C2-C2 shown in Figure 6 ; it has a greater arc than the marked arcuate or dish-shaped surface 563 of the transverse line C3-C3 shown in Figure 7 . Figure 8

[0046] In addition to the variations in bending or depression due to the different spacings S1 > S2 > S3, the total amount of material removed is expected to depend on the total resistance exerted by waveguide 10 itself. This depends on the width of waveguide 10. In the Figure 1 example, the width of the transition portion 16 of waveguide 10 gradually increases from the first end 20 to the second end 22 of the transition portion 16 of waveguide 10.

[0047] Generally speaking, the slope or curvature of the thickness of the transition portion 16 of waveguide 10 will depend on the spacing between waveguide 10 and CMP control structures 30, 32, and may also depend on other factors, such as the width of waveguide 10 itself. In order to design a waveguide 10 with a transition portion 16 having a thickness variation with a specific desired slope or curvature along its length, different spacing variations can be used (e.g., using spacings corresponding to Figure 3different test structures of the layout but with different values of S1, S2, and / or S3 in each test structure) and optionally with different waveguide widths (e.g., as shown in Figure 1 ), a test matrix of test structures is fabricated, and these test structures can be CMP processed, and then techniques such as cross-sectional microscopy, profilometry, etc. are used to characterize the thickness slope or curvature obtained for the transition region 16 in each test case to determine the optimal geometry of the optical structure to obtain the desired thickness slope or curvature for the transition region 16 of the waveguide 10.

[0048] Return reference Figure 1 and Figure 2 , in this example, the transition portion 16 is used to minimize or eliminate the optical reflection of the light ray L input from the light emitter 34 to the input end 36 of the first portion 12, thereby increasing the optical efficiency of the waveguide 10. It should be understood that this is only one of many possible applications of a waveguide with an exemplary gradual transition that does not include any step in the thickness (i.e., height) of the light reflection.

[0049] Reference Figure 9 , showing another non-limiting illustrative application example. Figure 9 The example of Figure 9 employs the same waveguide 10, which includes a first portion 12, a second portion 14, and an inserted transition portion 16. The transition portion 16 has a first end 20 connected to the first portion 12 and a second end 22 connected to the second portion 14. However, Figure 9 the application described in Figure 1 and Figure 2 is different. In Figure 9 , the waveguide 10 transmits the light ray L to a photodetector 70, such as a photodiode, etc. In this case, the second portion 14 of the waveguide has a larger thickness (i.e., height) to accommodate a larger-sized photodetector 70. The transition portion 16 in the present application provides the transmission of the light ray L from the first portion 12 with a lower thickness to the second portion 14 with a larger thickness and reduces or eliminates light reflection. Again, it should be understood that

[0050] The light input coupling application shown in

[0051] and the light output coupling application shown in Figure 9 are non-limiting illustrative applications, and other applications can be envisioned, such as providing coupling and reducing or eliminating light reflection from a waveguide with a smaller height to a waveguide with a larger height.

[0050] Some embodiments are further described below.

[0051] In a non-limiting illustrative embodiment, a method of fabricating a waveguide is disclosed. The method includes: forming a stacked layer including an etch stop layer disposed on a waveguide layer, the waveguide layer being disposed on a bottom cladding layer; patterning the etch stop layer and etching the waveguide layer after patterning to form a waveguide and a chemical mechanical polishing (CMP) control structure; filling a space between the waveguide and the CMP control structure with a cladding material; and performing CMP to reduce the thickness of the waveguide, wherein the CMP control structure controls the CMP of the waveguide to form a transition portion of the waveguide having a tapered thickness. In some embodiments, the CMP control structure includes: a first CMP control structure disposed along a first side of the waveguide and spaced apart from the waveguide by a spacing that gradually varies at a portion of the first CMP control structure disposed along the transition portion of the waveguide; and a second CMP control structure disposed along a second side of the waveguide opposite the first side of the waveguide and spaced apart from the waveguide by a spacing that gradually varies at a portion of the second CMP control structure disposed along the transition portion of the waveguide; wherein the gradually varying spacing of the first CMP control structure disposed along the transition portion of the waveguide and the gradually varying spacing of the second CMP control structure disposed along the transition portion of the waveguide control the CMP of the waveguide to produce a tapered thickness of the transition portion of the waveguide. In some embodiments, the width of the waveguide gradually increases at the transition portion of the waveguide. In some embodiments, the waveguide layer includes silicon and the cladding material includes silicon dioxide. In some embodiments, the etch stop layer includes silicon nitride. In some embodiments, the space between the waveguide and the CMP control structure is filled with the cladding material using a shallow trench isolation (STI) process. In some embodiments, the CMP control structure controls the CMP of the waveguide to produce a transition portion of the waveguide having a changed thickness that monotonically increases from a first portion of the waveguide to a second portion of the waveguide, wherein after performing CMP, the thickness of the first portion of the waveguide is smaller than the thickness of the second portion of the waveguide. In some embodiments, the method further includes one of: disposing a light emitter at an input end of the first portion of the waveguide, wherein the input end is away from the transition portion of the waveguide; or disposing a light detector at an output end of the second portion of the waveguide, wherein the output end is away from the transition portion of the waveguide. In some embodiments, after performing CMP, the transition portion of the waveguide does not include an abrupt thickness step. In some embodiments, forming the stacked layer includes: depositing the etch stop layer on a silicon-on-insulator (SOI) wafer; wherein the waveguide layer of the stacked layer includes the silicon layer of the SOI wafer and the cladding of the stacked layer includes the buried oxide layer of the SOI wafer.

[0052] In a non - limiting illustrative embodiment, an optical structure includes a waveguide having a first portion, a second portion, and a transition portion, wherein the transition portion has a first end connecting the first portion and a second end connecting the second portion. The thickness of the first portion of the waveguide is less than the thickness of the second portion of the waveguide. The transition portion of the waveguide has a thickness that gradually increases from a first thickness at the first end of the transition portion of the waveguide to a second thickness at the second end of the transition portion of the waveguide. In some embodiments, the waveguide comprises silicon. In some embodiments, the optical structure further includes: a first structure, comprising silicon, disposed along a first side of the waveguide and spaced apart from the waveguide by a spacing that gradually varies at a portion where the first structure is disposed along the transition portion of the waveguide; and a second structure, comprising silicon, disposed along a second side of the waveguide opposite the first side of the waveguide and spaced apart from the waveguide by a spacing that gradually varies at a portion where the second structure is disposed along the transition portion of the waveguide. In some embodiments, the width of the transition portion of the waveguide gradually increases from the first end of the transition portion of the waveguide to the second end of the transition portion of the waveguide. In some embodiments, the optical structure further includes: a cladding, comprising silica, surrounding the waveguide at least on the bottom and sides of the waveguide. In some embodiments, the transition portion of the waveguide does not have any steep thickness steps.

[0053] In a non - limiting illustrative embodiment, a method of fabricating a waveguide having a variable thickness formed by chemical mechanical polishing (CMP) is disclosed. The method includes: depositing an etch stop layer on a silicon layer of a silicon - on - insulator (SOI) wafer; patterning the etch stop layer and etching the silicon layer after patterning to form a first CMP control structure, a second CMP control structure, and a silicon waveguide disposed between the first CMP control structure and the second CMP control structure; filling the space between the silicon waveguide and the first CMP control structure and the second CMP control structure with silica using a shallow trench isolation (STI) process; and performing CMP to reduce the thickness of the silicon waveguide, wherein the first CMP control structure and the second CMP control structure control the CMP of the silicon waveguide to form a transition portion of the silicon waveguide having a gradually varying thickness. In some embodiments, the spacing between the first CMP control structure and the silicon waveguide gradually varies at a portion where the first CMP control structure is disposed along the transition portion of the silicon waveguide; and the spacing between the second CMP control structure and the silicon waveguide gradually varies at a portion where the second CMP control structure is disposed along the transition portion of the silicon waveguide; wherein the gradually varying spacing of the first CMP control structure and the gradually varying spacing of the second CMP control structure control the CMP of the silicon waveguide to produce a gradually varying thickness of the transition portion of the silicon waveguide. In some embodiments, the etch stop layer comprises silicon nitride. In some embodiments, filling the space between the silicon waveguide and the first CMP control structure and the second CMP control structure includes performing a shallow trench isolation (STI) process to fill the space between the silicon waveguide and the first CMP control structure and the second CMP control structure.

[0054] In a non-limiting illustrative embodiment, a waveguide has a first portion, a second portion, and a transition portion, where the transition portion has a first end connecting the first portion and a second end connecting the second portion. A first thickness of the first portion is smaller than a second thickness of the second portion. The transition portion has a thickness that gradually increases from the first thickness at its first end to the second thickness at its second end. In a manufacturing method employing chemical mechanical polishing (CMP), a first CMP structure and a second CMP structure are disposed on opposite sides of the waveguide. The space between the waveguide and the CMP control structure is filled with a cladding material. CMP is performed to reduce the thickness of the waveguide. The CMP control structure controls the CMP of the waveguide to form a transition portion of the waveguide having a gradually increasing thickness.

[0055] The foregoing overview of the features and embodiments is provided to enable those skilled in the art to better understand aspects of the present invention. Those skilled in the art should understand that they can readily use the present disclosure as a basis for designing or modifying other processes and structures to achieve the same purposes and / or achieve the same advantages as the embodiments described herein. Those skilled in the art should also recognize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they can make various changes, substitutions, and alterations without departing from the spirit and scope of the present disclosure.

Claims

1. A waveguide manufacturing method, characterized in that: include: forming a stack of layers, the stack of layers comprising an etch stop layer disposed on a waveguide layer, the waveguide layer disposed on a bottom cladding layer; patterning the etch stop layer and etching the waveguide layer after the patterning to form a waveguide and a chemical mechanical polishing (CMP) control structure; filling a space between the waveguide and the CMP control structure with a cladding material; as well as CMP is performed to reduce a thickness of the waveguide, wherein the CMP control structure controls the CMP of the waveguide to form a transition portion of the waveguide having a graded thickness.

2. The waveguide manufacturing method according to claim 1, characterized in that: The CMP control structure includes: a first CMP control structure disposed along a first side of the waveguide and spaced apart from the waveguide by a spacing that gradually changes at a portion where the first CMP control structure is disposed along the transition portion of the waveguide; and a second CMP control structure disposed along a second side of the waveguide opposite the first side of the waveguide and spaced apart from the waveguide by a spacing that gradually changes at a portion where the second CMP control structure is disposed along the transition portion of the waveguide; The gradually varying pitch of the first CMP control structure disposed along the transition portion of the waveguide and the gradually varying pitch of the second CMP control structure disposed along the transition portion of the waveguide control the CMP of the waveguide to produce the gradually varying thickness of the transition portion of the waveguide.

3. The waveguide manufacturing method according to claim 1, characterized in that: The width of the waveguide gradually increases at the transition portion of the waveguide.

4. The waveguide manufacturing method according to claim 1, characterized in that: After performing the CMP, the transition portion of the waveguide does not include an abrupt thickness step.

5. An optical structure, characterized in that: include: a waveguide having a first portion, a second portion, and a transition portion, the transition portion having a first end connected to the first portion and a second end connected to the second portion; wherein a thickness of the first portion of the waveguide is less than a thickness of the second portion of the waveguide; and The transition portion of the waveguide has a thickness that gradually increases from a first thickness at the first end of the transition portion of the waveguide to a second thickness at the second end of the transition portion of the waveguide.

6. The optical structure according to claim 5, characterized in that: Also includes: a first structure comprising silicon disposed along a first side of the waveguide and spaced apart from the waveguide by a spacing that gradually changes at a portion of the first structure disposed along the transition portion of the waveguide; as well as A second structure, comprising silicon, is disposed along a second side of the waveguide opposite the first side of the waveguide and is spaced apart from the waveguide by a spacing that gradually changes at a portion of the second structure disposed along the transition portion of the waveguide.

7. The optical structure according to claim 5, characterized in that: A width of the transition portion of the waveguide gradually increases from the first end of the transition portion of the waveguide to the second end of the transition portion of the waveguide.

8. The optical structure according to claim 5, characterized in that: The transition portion of the waveguide does not have any steep thickness steps.

9. A method for manufacturing a waveguide having a variable thickness formed by chemical mechanical polishing (CMP), characterized in that: The waveguide manufacturing method comprises: depositing an etch stop layer on a silicon layer of a silicon-on-insulator (SOI) wafer; patterning the etch stop layer and etching the silicon layer after the patterning to form a first CMP control structure, a second CMP control structure, and a silicon waveguide disposed between the first CMP control structure and the second CMP control structure; filling a space between the silicon waveguide and the first and second CMP control structures with silicon dioxide using a shallow trench isolation (STI) process; and CMP is performed to reduce the thickness of the silicon waveguide, wherein the first CMP control structure and the second CMP control structure control the CMP of the silicon waveguide to form a transition portion of the silicon waveguide having a graded thickness.

10. The waveguide manufacturing method according to claim 9, characterized in that: The spacing between the first CMP control structure and the silicon waveguide gradually changes at a portion where the first CMP control structure is disposed along the transition portion of the silicon waveguide; as well as The spacing between the second CMP control structure and the silicon waveguide gradually changes at a portion where the second CMP control structure is disposed along the transition portion of the silicon waveguide; wherein the gradually varying pitch of the first CMP control structure and the gradually varying pitch of the second CMP control structure control the CMP of the silicon waveguide to produce the gradually varying thickness of the transition portion of the silicon waveguide.

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