Pre-bias mask and wet etch process flow for smooth sidewalls in silicon nitride waveguides

Abstract

Aspects of the present disclosure relate to a process flow for fabricating a waveguide structure having a silicon nitride core with atomic-scale smooth sidewalls achieved by wet etch (rather than traditional dry etch treatment). The mask is pre-biased to account for lateral etching during the wet etching step.

Description

Technical Field

[0001] The present disclosure relates to various structures and manufacturing methods of integrated photon-based optical gyroscopes that utilize silicon nitride waveguides with smooth sidewalls. Background Art

[0002] A gyroscope (also simply referred to as "gyro") is a device capable of sensing angular velocity. Applications of gyroscopes include, but are not limited to, military, aircraft navigation, robotics, autonomous vehicles, virtual reality, augmented reality, gaming, etc. Gyroscopes can be mechanical or optical and can vary in terms of accuracy, performance, cost, and size. Since optical gyroscopes do not have any moving parts, they have advantages over mechanical gyroscopes because they are more resistant to the effects of shock, vibration, and temperature changes than mechanical gyroscopes with moving parts. The most common optical gyroscope is the fiber optic gyroscope (FOG), which operates based on the interference measurement of optical phase shifts caused by the Sagnac effect (a phenomenon caused by rotation encountered in interferometry). The construction of a FOG generally involves a coil that contains several turns of polarization-maintaining (PM) optical fiber. Laser light is injected into both ends of the PM optical fiber coil, causing two light beams to travel in opposite directions. If the optical fiber coil is moving, the light beams traveling in opposite directions will experience different optical path lengths. By setting up an interference system, we can measure the small path length difference, which is proportional to the loop area enclosed by the several turns of the optical fiber coil and the angular velocity of the rotating optical fiber coil. This path length difference is expressed as the phase difference (referred to as the "phase signal") between the two counter-rotating light beams.

[0003] The phase signal of an optical gyroscope is proportional to the Sagnac effect multiplied by the angular rotation speed, as shown in the following equation:

[0004] Δφ=(8πNA / λc)Ω

[0005] where N = the number of turns of the gyroscope,

[0006] A = the enclosed area

[0007] Ω = the angular rotation speed

[0008] Δφ = the optical phase difference signal

[0009] λ = the optical wavelength

[0010] c = the speed of light

[0011] Fiber-based gyroscopes can provide very high precision, but they are larger in footprint, very expensive, and difficult to assemble at this time because the device is built based on discrete optical components that require precise alignment. Usually, manual alignment is involved, which is difficult to scale up to mass production.

[0012] The inventors of this case propose to use waveguide-based integrated photon components to replace optical fibers for cost-saving and easy integration on a semiconductor platform, which is more promising for the mass production of gyroscopes. This application describes various structures, including a silicon nitride (SiN) waveguide core fabricated on a silicon platform, as detailed below. The SiN waveguide core disclosed herein can have smooth sidewalls, which is due to wet etching rather than traditional dry etching methods. Traditional dry etching methods usually result in sidewall roughness in the micron or nanometer range, which may be harmful to the optical performance of the gyroscope. Summary of the Invention

[0013] The following is a simplified summary of the disclosure of this case to provide a basic understanding of some aspects of the disclosure of this case. This summary is not an exhaustive overview of the disclosure of this case. It is neither intended to identify the key or critical elements of the disclosure of this case nor to outline any scope of a particular embodiment of the disclosure of this case or any scope of the claims. Its sole purpose is to present some concepts of the disclosure of this case in a simplified form as a prelude to the more detailed description presented later.

[0014] Aspects of the disclosure of this case relate to a processing flow for manufacturing a waveguide structure having a silicon nitride core with atomically smooth sidewalls achieved by wet etching (rather than traditional dry etching processing).

[0015] More specifically, a method is disclosed in which a waveguide structure is fabricated by: forming a silicon nitride (SiN) layer on top of a substrate having an oxide layer, where the oxide layer serves as the lower cladding (or bottom cladding) of the waveguide and the SiN layer acts as the core of the waveguide when patterned; forming a cap layer on top of the SiN layer; patterning the cap layer through a first wet etching step to form a patterned cap layer that includes a cap over the SiN; and performing a second wet etching step to form the SiN layer under the patterned cap layer to produce the core of the waveguide.

[0016] Pre-bias the mask to form the patterned cap layer of the correct size to form a waveguide core of appropriate width. After forming a waveguide core with ultra-smooth sidewalls (due to wet etching), deposit an upper cladding (or top cladding) layer.

[0017] The waveguide structure can be used as a rotation sensing component in an integrated photonics optical gyroscope. The rotation sensing component can be in the form of a waveguide coil. The waveguide coil can be distributed between multiple vertical layers, where light is evanescently coupled between the multiple vertical layers of the waveguide coil. Alternatively, the rotation sensing component can be in the form of a waveguide-based microresonator ring, which can be formed in one or more layers and evanescently coupled in the vertically distributed layers.

[0018] Although only the manufacturing process is detailed in this particular disclosure for brevity, the applicant incorporates by reference earlier filed patent applications that describe single-layer and multi-layer waveguide structures for optical gyroscopes, see U.S. Patent No. 10,969,548, issued April 6, 2021. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present disclosure will be more fully understood from the following detailed description and the accompanying drawings of various embodiments of the present disclosure.

[0020] Figure 1 is a schematic cross-sectional view showing a silicon nitride (SiN) waveguide core layer deposited on an oxide cladding according to an embodiment of the present disclosure.

[0021] Figure 2 is a schematic cross-sectional view showing a silica (SiO2) capping layer deposited on the SiN waveguide core layer according to an embodiment of the present disclosure.

[0022] Figure 3 is a schematic cross-sectional view showing the SiO2 capping layer patterned to an appropriate width to form the SiN waveguide core according to an embodiment of the present disclosure.

[0023] Figure 4 is a schematic cross-sectional view showing a SiN waveguide core fabricated by wet etching according to an embodiment of the present disclosure.

[0024] Figure 5 is an exploded schematic cross-sectional view of a SiN waveguide core fabricated by wet etching according to an embodiment of the present disclosure, showing smooth sidewalls.

[0025] Figure 6 is a scanning electron micrograph of a SiN waveguide core (with a hard mask on top) fabricated by wet etching according to an embodiment of the present disclosure, showing smooth sidewalls.

[0026] Figure 7Is a schematic cross-sectional view showing the patterning of a SiO2 capping layer using wet etching according to an embodiment of the present disclosure.

[0027] Figure 8 Is a schematic cross-sectional view showing the patterning of a SiN waveguide core using wet etching while pre-biasing a mask to obtain an appropriate width of the SiN waveguide core according to an embodiment of the present disclosure.

[0028] Figure 9 Is an exploded schematic cross-sectional view of a SiN waveguide core for smooth sidewalls fabricated by wet etching both a capping layer and a waveguide layer according to an embodiment of the present disclosure.

[0029] Figure 10 Is a scanning electron micrograph of a SiN waveguide core fabricated by wet etching (with a top cladding deposited on top of the core) according to an embodiment of the present disclosure, showing smooth sidewalls resulting in no voids between the waveguide core and the cladding. Detailed Description

[0030] Aspects of the present disclosure relate to methods for fabricating compact ultra-low-loss integrated photon-based waveguide cores with smooth sidewalls, which can be accomplished in large-scale manufacturing. These waveguides can be used as optical components on planar photonic integrated circuits (PICs) such as in photonic integrated optical gyroscopes. As discussed in the background paragraph, the key to the high performance of fiber-based optical gyroscopes is the long length of high-quality low-loss fibers for measuring the Sagnac effect. The inventors of the present case recognized that with the advent of integrated silicon photonics applicable to wafer processing, there is an opportunity to replace FOGs with smaller integrated photon chip solutions without sacrificing performance. Photon-based optical gyroscopes have reduced size, weight, power, and cost, but can also be mass-produced, are vibration-resistant, and have the potential to provide performance comparable to FOGs. When an integrated optical gyroscope is fabricated on a silicon platform, it is abbreviated as SiPhOG TM (Silicon Photonic Optical Gyroscope).

[0031] A key element of such integrated photon solutions is the production of ultra-low-loss waveguide cores made of silicon nitride (Si3N4) surrounded by an oxide or fused silica cladding. The entire waveguide structure (including the core and the cladding) is sometimes abbreviated as a SiN waveguide. The propagation loss in a SiN waveguide can be far lower than 0.1 db / m. This represents a huge improvement over current state-of-the-art SiN processes (with propagation losses in the range of 0.1 db / cm).

[0032] Figure 1 Shows the first step in fabricating a SiN waveguide on a conventional silicon substrate. Specifically, Figure 1A display substrate 102, which can be a silicon substrate. The substrate 102 can have the thickness "H" of a standard wafer. For example, the thickness can be 725 μm. Note that the thicknesses of different material layers are not drawn to scale. However, to convey the concept that the substrate 102 is much thicker than the remaining material layers shown in the figure, a discontinuity 101 is introduced in the middle of layer 102 only for visual effect. Layers 104 and 116 can have a thickness "h1" in the range of 15 μm on both sides of the substrate 102. Layer 104 serves as the lower cladding of the waveguide core layer 110. When the waveguide core layer 110 is patterned to the correct size (as Figures 4 to 5 shown), it can be considered one turn of a waveguide coil. The waveguide core layer 110 can have a thickness "h", and when patterned, has a width "w". Non-limiting exemplary dimensions of "h" can be 60 - 100 nm, and "w" can be 2 - 3 μm. The waveguide core layer 110 is made of silicon nitride (SiN). Note that when layers 104 and 110 are formed on one side of the substrate 102, the corresponding layers 116 and 118 are also formed on the other side of the substrate 102, even if these layers may not be used for waveguide purposes. Alternatively, if necessary, these layers can create waveguides in different layers. The upper cladding layer 114 with a thickness "h2" in the range of 2 - 3 μm can also be part of the structure. Both layers 114 and 116 can have the same material 120.

[0033] Figure 2 Shows the second step of manufacturing the SiN waveguide core. The SiO2 capping layer 106 is deposited on top of the waveguide core SiN layer 110.

[0034] Figure 3 Shows the third step of manufacturing the SiN waveguide core, where the SiO2 capping layer 106 is patterned to the appropriate width "w" (e.g., 2 - 3 μm) by etching. The SiO2 capping layer serves as a hard mask. A photoresist can be used as a mask for dry etching the SiO2 capping layer. Experiments show that wet etching the hard mask followed by wet etching the SiN layer results in the best sidewall roughness because when the resist is used as a mask and subjected to dry etching, the sidewall roughness on the resist is also "replicated" onto the underlying SiN layer. Refer to Figures 7 to 9 for a more detailed description of the first and second wet etchings.

[0035] Figure 4 Shows the fourth step of manufacturing the SiN waveguide core, where the waveguide core layer 110 is patterned to the appropriate width under the patterned SiO2 capping layer 106 with width "w" (e.g., 2 - 3 μm) by wet etching, as shown within the elliptical dashed line 400. The wet etching of SiN can be done with, for example, hot phosphoric acid. The hard mask should be selectively resistant to the wet etchant.

[0036] Figure 5 Displays an exploded view of a SiN waveguide core 110 fabricated by wet etching, showing smooth sidewalls 510 and 512. The dimension "x" shows the recesses below layer 106 due to possible over-etching (the "x" is typically in the range of 20 - 25 nm per side). The smooth sidewalls achieved by wet etching help reduce optical losses during propagation within the gyroscope waveguide coil. The sidewall roughness achieved by wet etching is at the atomic level, while the sidewall roughness achieved by dry etching is in the micron or nanometer range, i.e., much rougher than atomic-level smoothness. Depending on the longitudinal dimension of the waveguide core (e.g., thickness "h"), this smoothness can be an important factor in determining propagation loss and optical mode confinement, especially near waveguide bends.

[0037] Figure 6 Is a scanning electron micrograph of a SiN waveguide core (with a hard mask on top) fabricated by wet etching according to an embodiment of the present disclosure, showing smooth sidewalls.

[0038] Figure 7 Shows a photoresist layer 150 on top of the capping layer 106, which is patterned using a first wet etching process. This step can be considered Figure 1 and Figure 2 The step after the steps shown in. The width of the patterned capping layer can be greater than the target width of the waveguide core. This is achieved by pre-biasing the mask used to pattern the capping layer 106, i.e., writing features on the mask that are larger than the actual features on the wafer. The pre-bias helps compensate for lateral etching during wet etching of the capping layer and subsequent wet etching of the SiN core layer. The amount of lateral etching depends on the chemical conditions of the wet etching process. The lateral etching can be as small as 20 - 25 nm per side or as large as 500 nm. Prior knowledge of the amount of lateral etching helps design the size of the pre-biased mask.

[0039] Figure 8 Shows a second wet etching step to form the SiN waveguide core 110. As previously mentioned, the ideal width of the SiN waveguide core is 2 - 3 μm after etching. The width of the patterned capping layer 106 (after the first wet etching step) is closer to the desired target width of the waveguide core, but may still be slightly larger to account for lateral etching. The second wet etching can be done using hot phosphoric acid, which typically results in 20 - 25 nm of lateral etching. The resulting sidewalls (within the dashed line 800) are shown in Figure 9 The sidewalls 916 and 914 of the capping layer are also smooth and slightly curved, which would not be the case if dry etching were used. The sidewalls 912 and 910 of the SiN core 110 are smooth to the atomic level at the sub-nanometer scale.

[0040] After the second wet etching, an overclad (or top cladding) is deposited on top of the remaining hard mask over the SiN core. The remaining hard mask can be part of the overclad and ensure a high level of integrity and strength at the interface between the overclad and the core layer to maintain tight confinement of the optical mode. This can be shown in the SEM photograph of Figure 10 . The dashed line shows the outline of the waveguide core with a curved sidewall. There is no gap between the waveguide core and the overclad.

[0041] In the foregoing specification, embodiments of the present disclosure have been described with reference to specific exemplary embodiments of the present disclosure. It is evident that various modifications can be made without departing from the broader spirit and scope of the embodiments of the present disclosure as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense, rather than a restrictive sense. In addition, directional terms such as "top", "bottom", etc. do not limit the scope of the present disclosure to any fixed orientation, but cover various arrangements and combinations of orientations.

Claims

1. A method of manufacturing a waveguide structure, the method comprising: forming a silicon nitride (SiN) layer on top of a substrate having an oxide layer, the oxide layer serving as a lower cladding for the waveguide and the SiN layer serving as a core of the waveguide when patterned; forming a cap layer on top of the SiN layer; patterning the cap layer by a first wet etching step to form a patterned cap layer, the patterned cap layer including a cap over the SiN; and performing a second wet etching step to form the SiN layer under the patterned cap layer to produce the core of the waveguide.

2. The method according to claim 1, further comprising: Pre-biasing a mask, the mask being used to form the patterned cap layer and the waveguide core.

3. The method according to claim 2, wherein the pre-biasing comprises: writing the mask such that a width of a feature on the mask for manufacturing the core of the waveguide on the wafer is greater than a target width of the core of the waveguide.

4. The method according to claim 3, wherein a difference between the width of the feature on the mask and the target width of the core of the waveguide is predetermined based on an expected lateral etching during the first wet etching step and the second wet etching step.

5. The method according to claim 4, wherein the expected lateral etching depends on wet chemical conditions.

6. The method according to claim 1, wherein the target width of the waveguide core is 2 to 3 micrometers.

7. The method according to claim 1, wherein a thickness of the SiN layer is in the range of 60 to 100 nm.

8. The method according to claim 1, wherein a material of the cap layer is silicon dioxide.

9. The method according to claim 1, wherein the cap layer serves as a hard mask during the second wet etching.

10. The method according to claim 1, wherein a selectivity between the cap layer and the SiN during the second wet etching controls a size of a plurality of lateral recesses under the cap layer formed in the patterned SiN layer.

11. The method according to claim 1, wherein a sidewall roughness of the SiN waveguide core achieved by the second wet etching is atomic level.

12. The method according to claim 11, wherein the atomic level sidewall roughness is substantially less than the nanometer range.

13. The method according to claim 1, wherein the second wet etching is performed by hot phosphoric acid.

14. The method according to claim 1, wherein the waveguide structure is used as a rotation sensing component in an optical gyroscope.

15. The method according to claim 14, wherein the rotation sensing component is in the form of a waveguide coil or a waveguide-based microresonator ring.

16. The method according to claim 15, wherein the rotation sensing component is distributed between a plurality of vertical layers.

17. The method according to claim 16, wherein light is evanescently coupled between the plurality of vertical layers of the rotation sensing component.

Images