Abstract:
A waveguide structure includes a bottom dielectric layer, a core layer disposed over the bottom dielectric layer, an etch stop layer disposed over the core layer, and a cladding layer or a buffer layer disposed over the etch stop layer. The waveguide structure is configured to guide a light signal through different geography, such as straight, taper, turning, grating and tight coupling sections.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to an integrated circuit and more particularly an optical waveguide structure. 
       BACKGROUND 
       [0002]    In some photonic integrated circuits, waveguides are formed in the form of a rib or a channel structure. However it is difficult to control different etching depth levels across a wafer and across different patterns for rib type waveguides, which affects performance and yield. An exposed waveguide core during multi-step etching also results in a rough surface with significant transmission loss. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0004]      FIG. 1A  is a schematic top view of an exemplary waveguide structure according to some embodiments; 
           [0005]      FIGS. 1B-1C  are cross sections of the exemplary waveguide structure in  FIG. 1A  according to some embodiments; 
           [0006]      FIGS. 2A-2C  are exemplary intermediate fabrication steps of the coupling section of the waveguide structure in  FIG. 1A  according to some embodiments; and 
           [0007]      FIGS. 3A-3D  are exemplary intermediate fabrication steps of the bending section of the waveguide structure in  FIG. 1A  according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use, and do not limit the scope of the disclosure. 
         [0009]    In addition, 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. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features. 
         [0010]      FIG. 1A  is a schematic top view of an exemplary waveguide structure  100  according to some embodiments. The waveguide structure  100  is configured to guide a light signal. The waveguide structure  100  includes core layers  110   a  and  110   b  with a coupling section  102  and a bending section  104  after the coupling section  102 . The cross section views of the coupling section  102  and the bending section  104  are shown in  FIG. 1B  and  FIG. 1C  respectively. 
         [0011]    A core layer portion  110   a  is arranged to have the incident light signal coming from a wider section  101 , and then the light signal is coupled to another core layer portion  110   b  at the coupling section  102 . The core layer  110   b  with a bending section  104  guides the light signal with a desired wavelength depending on a bending radius R. 
         [0012]    In some embodiments, the core layer  110   a  has a width W 1  ranging from 0.3 μm to 3 μm and W 2  from 5 μm to 15 μm at the wide end. In some embodiments, the core layer  110   b  has a width ranging from 0.3 μm to 3 μm. In some embodiments, the bending radius R ranges from 5 μm to 100 μm. In some embodiments, the spacing S between the core layers  110   a  and  110   b  ranges from 150 nm to 500 nm. 
         [0013]      FIGS. 1B-1C  are cross sections of the exemplary waveguide structure in  FIG. 1A  according to some embodiments.  FIG. 1B  is a cross section view of the coupling section  102  in  FIG. 1A  along a cross section line  103 . The substrate  106  comprises silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide (GaAs), an alloy of silicon and germanium, indium phosphide (InP), silicon on insulator (SOI), or any other suitable material. The substrate  106  is not shown in subsequent figures for simplicity. 
         [0014]    A bottom dielectric layer  108  is disposed over a substrate  106 . The bottom dielectric layer  108  has a thickness ranging from 0.2 μm to 1 μm in some embodiments. The bottom dielectric layer  108  has a thickness greater than 0.5 μm in some other embodiments. The bottom dielectric layer  108  has a refractive index (RI) ranging from 1.2 to 1.5 and comprises silicon dioxide or a low-k dielectric material in some embodiments. 
         [0015]    The core layer  110   a  and  110   b  is disposed over the bottom dielectric layer  108 . The core layer  110   a  and  110   b  has an RI ranging from 1.8 to 2.2, a thickness ranging from 100 nm to 500 nm, and comprises silicon nitride (Si x N y ) or a high-k dielectric material in some embodiments. 
         [0016]    An etch stop layer  112  is disposed over the core layer  110   a  and  110   b.  The etch stop layer  112  has an equal or higher refractive index (RI) compared to the cladding layer  116  and a lower RI compared to the core layer  110   a  and  110   b  in some embodiments. The etch stop layer  112  has an RI ranging from 1.2 to 1.6 in some embodiments. The etch stop layer  112  covers the core layer  110   a  and  110   b  conformally with no voids in some embodiments. The etch stop layer  112  has a thickness ranging from 150 Å to 300 Å and comprises silicon oxynitride (SiO x N x ) or a low-k dielectric material in some embodiments. 
         [0017]    A buffer layer  114  is disposed over the etch stop layer  112 . The buffer layer  114  between the etch stop layer  112  and the cladding layer  116  reduces scattering loss and increases optical coupling efficiency. The buffer layer  114  has a higher refractive index (RI) compared to the cladding layer  116  and the etch stop layer  112 , and a lower or equal RI compared to the core layer  110   a  and  110   b  in some embodiments. The buffer layer  114  has an RI ranging from 1.6 to 1.8 in some embodiments. The buffer layer  114  has a thickness ranging from 500 Å to 2500 Å and comprises silicon oxynitride (SiO x N x ) or a high-k dielectric material in some embodiments. 
         [0018]    A cladding layer  116  is disposed over the buffer layer  114 . The cladding layer  116  has an RI ranging from 1.2 to 1.5 in some embodiments. The cladding layer  116  has a thickness ranging from 0.2 μm to 1 μm in some embodiments, or greater than 0.5 μm in some other embodiments. The cladding layer  116  comprises silicon dioxide or a low-k dielectric material in some embodiments. 
         [0019]      FIG. 1C  is a cross section view of the bending section  102  in  FIG. 1A  along a cross section line  105 . The cross section view in  FIG. 1C  is similar to the cross section view in  FIG. 1B  except that the buffer layer  114  is not present. 
         [0020]    The waveguide structures  102  in  FIG. 1B and 104  in  FIG. 1C  include the etch stop layer  112  covering the core layer  110   a  and/or  110   b  that reduces the surface roughness of the core layer  110   a  and/or  110   b  from later etching processes. Also the buffer layer  114  in  FIG. 1B  between the etch stop layer  112  and the cladding layer  116  could reduce scattering loss and increase coupling efficiency. 
         [0021]      FIGS. 2A-2C  are exemplary intermediate fabrication steps of the coupling section  102  of the waveguide structure in  FIG. 1A  according to some embodiments. In  FIG. 2A , the core layer (such as  110   a  and  110   b ) is formed over the bottom dielectric layer  108  by plasma-enhanced chemical vapor deposition (PECVD) and dry etching, for example. The core layer  110   a  and  11   b  has a generally uniform thickness and there is no need for different etching depth control. 
         [0022]    The bottom dielectric layer  108  has a refractive index (RI) ranging from 1.2 to 1.5 and comprises silicon dioxide or a low-k dielectric material in some embodiments. The bottom dielectric layer  108  has a thickness ranging from 0.2 μm to 1 μm in some embodiments. The bottom dielectric layer  108  has a thickness greater than 0.5 μm in some other embodiments. The core layer  110   a  and  110   b  has an RI ranging from 1.8 to 2.2, a thickness ranging from 100 nm to 500 nm, and comprises silicon nitride (Si x N y ) in some embodiments. 
         [0023]    In  FIG. 2B , the etch stop layer  112  is formed over the core layer  110   a  and  110   b  by atomic layer deposition (ALD), physical vapor deposition (PVD), or CVD, for example. The buffer layer  114  is deposited over the etch stop layer  112  and the bottom dielectric layer  108  by PECVD, for example. 
         [0024]    The etch stop layer  112  has an RI ranging from 1.2 to 1.6 in some embodiments. The etch stop layer  112  has a thickness ranging from 150 Å to 300 Å and comprises silicon oxynitride (SiO x N x ) or a low-k dielectric material in some embodiments. The etch stop layer  112  covering the core layer  110   a  and  110   b  reduces the surface roughness of the core layer  110   a  and  110   b  from later etching processes. 
         [0025]    Also the buffer layer  114  could reduce scattering loss and increase coupling efficiency. The buffer layer  114  has an RI ranging from 1.6 to 1.8 in some embodiments. The buffer layer  114  has a thickness ranging from 500 Å to 2500 Å and comprises silicon oxynitride (SiO x N x ) or a high-k dielectric material in some embodiments. 
         [0026]    In  FIG. 2C , the cladding layer  116  is formed over the buffer layer  114  by PECVD, for example. The cladding layer  116  has an RI ranging from 1.2 to 1.5 in some embodiments. The cladding layer  116  has a thickness ranging from 0.2 μm to 1 μm in some embodiments. The cladding layer  116  has a thickness greater than 0.5 μm in some other embodiments. The cladding layer  116  comprises silicon dioxide or a low-k dielectric material in some embodiments. 
         [0027]      FIGS. 3A-3D  are exemplary intermediate fabrication steps of the bending section  104  of the waveguide structure in  FIG. 1A  according to some embodiments. In  FIG. 3A , the core layer  110   b  is formed over the bottom dielectric layer  108  by plasma-enhanced chemical vapor deposition (PECVD) and dry etching, for example. The formation of the core layer  110   b  has a generally uniform thickness and there is no need for different etch depth control. 
         [0028]    The bottom dielectric layer  108  has a refractive index (RI) ranging from 1.2 to 1.5 and comprises silicon dioxide or a low-k dielectric material in some embodiments. The bottom dielectric layer  108  has a thickness ranging from 0.2 μm to 1 μm in some embodiments. The bottom dielectric layer  108  has a thickness greater than 0.5 μm in some other embodiments. The core layer  110   b  has an RI ranging from 1.8 to 2.2 and a thickness ranging from 100 nm to 500 nm, and comprises silicon nitride (Si x N y ) or a high-k dielectric material in some embodiments. 
         [0029]    In  FIG. 3B , the etch stop layer  112  is formed over the core layer  110   b  by ALD, PVD, or CVD, for example. The buffer layer  114  is deposited over the etch stop layer  112  and the bottom dielectric layer  108  by PECVD, for example. 
         [0030]    The etch stop layer  112  has an RI ranging from 1.2 to 1.6 in some embodiments. The etch stop layer  112  has a thickness ranging from  150  A to  300  A and comprises silicon oxynitride (SiO x N x ) or a low-k dielectric material in some embodiments. The etch stop layer  112  covering the core layer  110   b  reduces the surface roughness of the core layer  110   b  from later etching processes. 
         [0031]    The buffer layer  114  has an RI ranging from 1.6 to 1.8 in some embodiments. The buffer layer  114  has a thickness ranging from 500 Å to 2500 Å and comprises silicon oxynitride (SiO x N x ) in some embodiments. 
         [0032]    In  FIG. 3C , the buffer layer  114  is etched by dry or wet etching in some embodiments. Even though the buffer layer  114  is etched in the bending section  104  in  FIG. 3C , the buffer layer  114  in the coupling section  102  in  FIG. 2C  remains as a part of the waveguide structure to reduce scattering loss and increase coupling. In other embodiments, bending section  104  could be covered with masking material (e.g., photoresist material or a hard mask material) during formation of buffer layer  114  over core layer  110   a  and  110   b  in coupling section  102 , in order to prevent formation of buffer layer  114  in bending section  104 . This avoids the need to remove buffer layer  114  from core layer  110   b  in bending section  104 . 
         [0033]    In  FIG. 3D , the cladding layer  116  is formed over the etch stop layer  112  and the bottom dielectric layer  108  by PECVD, for example. The cladding layer  116  has a thickness ranging from 0.2 μm to 1 μm in some embodiments. The cladding layer  116  has a thickness greater than 0.5 μm in some other embodiments. The cladding layer  116  comprises silicon dioxide or a low-k dielectric material in some embodiments. 
         [0034]    According to some embodiments, a waveguide structure includes a bottom dielectric layer, a core layer disposed over the bottom dielectric layer, an etch stop layer disposed over the core layer, and a cladding layer disposed over the etch stop layer. The waveguide structure is configured to guide a light signal. 
         [0035]    According to some embodiments, a method of fabricating a waveguide structure includes forming a core layer over a bottom dielectric layer. An etch stop layer is formed over the core layer. A cladding layer is formed over the etch stop layer. The core layer, the etch stop layer, and the cladding layer are arranged to guide a light signal. 
         [0036]    A skilled person in the art will appreciate that there can be many embodiment variations of this disclosure. Although the embodiments and their features have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosed embodiments, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. 
         [0037]    The above method embodiment shows exemplary steps, but they are not necessarily required to be performed in the order shown. Steps may be added, replaced, changed order, and/or eliminated as appropriate, in accordance with the spirit and scope of embodiment of the disclosure. Embodiments that combine different claims and/or different embodiments are within the scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure.