Process flow with pre-biased mask and wet etching for smooth sidewalls in silicon nitride waveguides

Abstract

Aspects of the present disclosure provide a process flow for fabricating waveguide structures with silicon nitride cores with atomically smooth sidewalls achieved by wet etching instead of the conventional dry etching process, where the mask is pre-biased to allow for lateral etching during the wet etching step.

Description

[Technical Field]

[0001] The present disclosure relates to various structures and fabrication methods for integrated optics-based optical gyroscopes that use silicon nitride waveguides with smooth sidewalls. [Background technology]

[0002] A gyroscope (also called "gyro" for short) is a device capable of detecting angular velocity. Applications of gyroscopes include, but are not limited to, military, air navigation, robotics, autonomous vehicles, virtual reality, augmented reality, and gaming. Gyroscopes can be mechanical or optical and vary in accuracy, performance, cost, and size. Because optical gyroscopes do not have any moving parts, they have advantages over mechanical gyroscopes because they can withstand the effects of shock, vibration, and temperature changes better than mechanical gyroscopes with moving parts. The most common optical gyroscope is the fiber optic gyroscope (FOG), which operates based on interferometric measurement of optical phase shift due to the Sagnac effect (a rotation-induced phenomenon encountered in interferometers). The structure of a FOG typically includes a coil containing several turns of polarization-maintaining (PM) fiber. Laser light is launched into both ends of the PM fiber coil, and two light beams travel in opposite directions. When the fiber coil is moving, the counter-rotating light beams experience different optical path lengths with respect to each other. By constructing an interferometer system, it is possible to measure a small path length difference that is proportional to the area of ​​the loop enclosed by the turns of the fiber coil and the angular velocity of the fiber coil's rotation. This path length difference is expressed as a phase difference (referred to as a "phase signal") between the two counter-rotating beams.

[0003] The phase signal of an optical gyro is proportional to the Sagnac effect times the angular rotation rate, as shown in the following equation: Δφ=(8πNA / λc)Ω where N = number of turns in the gyro, A = enclosed area, Ω = angular rotation rate, Δφ = optical phase difference signal, λ = wavelength of light, and c = speed of light.

[0004] Fiber-based gyroscopes can provide very high accuracy, but at the same time, they are larger in size, very expensive, and difficult to assemble, with devices built on separate optical components that need to be precisely aligned. Manual alignment is often required, making them difficult to scale up for mass production.

[0005] The inventors propose replacing fiber with waveguides based on integrated optical components for cost-effective and easy integration onto semiconductor platforms that are much more promising for mass production of gyroscopes. This application describes various structures including silicon nitride (SiN) waveguide cores fabricated on silicon platforms as detailed below. The SiN waveguide cores disclosed herein can have smooth sidewalls due to wet etching instead of conventional dry etching methods that often result in sidewall roughness in the micrometer or nanometer range that can be detrimental to the optical performance of gyroscopes. Summary of the Invention

[0006] The following is a simplified summary of the disclosure to provide a basic understanding of some aspects of the disclosure. This summary is not an extensive overview of the disclosure. It is not intended to identify key or critical elements of the disclosure, nor to delineate the scope of any particular implementation of the disclosure or any claims. Its sole purpose is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description that is presented later.

[0007] Aspects of the present disclosure provide a process flow for fabricating waveguide structures with silicon nitride cores with atomically smooth sidewalls achieved by wet etching instead of the traditional dry etching process.

[0008] More specifically, the waveguide structure is forming a silicon nitride (SiN) layer on top of a substrate having an oxide layer, the oxide layer serving as a lower cladding (or bottom cladding) of a waveguide, 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 process to form a patterned cap layer including a cap over the SiN layer; performing a second wet etching step to form a SiN layer below the patterned cap layer to create a waveguide core; A method for fabricating the same is disclosed.

[0009] The mask is pre-biased to form a patterned cap layer of the correct dimensions to form a waveguide core of the appropriate width. After wet etching forms the waveguide core with ultra-smooth sidewalls, an upper cladding (or top cladding) layer is deposited.

[0010] The waveguide structure can be used as a rotation detection element in an integrated photonics optical gyroscope. The rotation detection element can be in the form of a waveguide coil. The waveguide coil can be distributed among multiple vertical layers, with light evanescently coupled between the multiple vertical layers of the waveguide coil. Alternatively, the rotation detection element can be in the form of a waveguide-based microresonator ring, in the form of a single layer or multiple layers with evanescent coupling between the vertically distributed layers.

[0011] For the sake of brevity, only the fabrication process is detailed in this particular disclosure, and Applicant incorporates by reference a prior patent application, U.S. Pat. No. 10,969,548, filed April 6, 2021, which describes single-layer and multi-layer waveguide structures for optical gyroscopes.

[0012] The present disclosure will become more fully understood from the detailed description given below and the accompanying drawings of various implementations of the present disclosure. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating a silicon nitride (SiN) waveguide core layer deposited on an oxide cladding according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a schematic cross-sectional view illustrating a silicon dioxide (SiO 2 ) cap layer deposited on a SiN waveguide core layer according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a schematic cross-sectional view illustrating patterning of a SiO 2 cap layer of appropriate width to form a SiN waveguide core, according to an embodiment of the present disclosure. [Figure 4] FIG. 4 is a schematic cross-sectional view illustrating a SiN waveguide core fabricated by wet etching, according to an embodiment of the present disclosure. [Figure 5] FIG. 5 is a schematic exploded cross-sectional view of a SiN waveguide core fabricated by wet etching, showing smooth sidewalls, according to an embodiment of the present disclosure. [Figure 6] FIG. 6 is a scanning electron microscope image of a SiN waveguide core (with a hard mask on top) fabricated by wet etching, showing smooth sidewalls, according to an embodiment of the present disclosure. [Figure 7] FIG. 7 is a schematic cross-sectional view illustrating patterning of a SiO 2 cap layer with wet etching according to an embodiment of the present disclosure. [Figure 8] FIG. 8 is a schematic cross-sectional view illustrating patterning of a SiN waveguide core with wet etching during pre-biasing of a mask to obtain an appropriate width of the SiN waveguide core, according to an embodiment of the present disclosure. [Figure 9] FIG. 9 is a schematic exploded cross-sectional view of a SiN waveguide core fabricated by wet etching of both the cap layer and the waveguide layer for smooth sidewalls, according to an embodiment of the present disclosure. [Figure 10]FIG. 10 is a scanning electron microscope image of a SiN waveguide core (with a top cladding deposited on top of the core) fabricated by wet etching, showing smooth sidewalls resulting in no voids between the waveguide core and cladding, in accordance with an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0014] Aspects of the present disclosure provide a method for fabricating compact, ultra-low-loss, photonics-based integrated waveguide cores with smooth sidewalls that can be fabricated in large-scale manufacturing. These waveguides can be used as optical elements on planar photonic integrated circuits (PICs), for example, in photonics-integrated optical gyroscopes. As discussed in the background section, the key to the high performance of fiber-based optical gyroscopes is the long lengths of high-quality, low-loss optical fiber used to measure the Sagnac effect. The inventors recognize that with the advent of integrated silicon photonics suitable for wafer-scale processing, there is an opportunity to replace FOGs with smaller integrated photonic chip solutions without sacrificing performance. Photonics-based optical gyros have the potential to reduce size, weight, power, and cost, yet be mass-produced, vibration-insensitive, and demand performance comparable to FOGs. When an integrated optical gyroscope is fabricated on a silicon platform, it is abbreviated as SiPhOG™ (Silicon Photonics Optical Gyroscope).

[0015] One key element of this integrated photonic solution is the fabrication of a very low-loss waveguide core made of silicon nitride (Si3N4) surrounded by an oxide or fused silica cladding. The entire waveguide structure (including the core and cladding) is sometimes referred to as a SiN waveguide for simplicity. Propagation losses in SiN waveguides can be well below 0.1 dB / cm. This is a significant improvement over current state-of-the-art SiN processes, which have propagation losses in the 0.1 dB / cm range.

[0016] FIG. 1 illustrates a first step in the fabrication of a SiN waveguide on a conventional silicon substrate. Specifically, FIG. 1 illustrates substrate 102, which may be a silicon substrate. Substrate 102 may have a standard wafer thickness "H," e.g., 725 μm thick. Note that the thicknesses of the different material layers are not to scale. However, a cut 101 has been introduced in the middle of layer 102 for visualization purposes only to convey the idea that substrate 102 is significantly thicker than the rest of the material layers shown in the figure. Layers 104 and 116 may have thicknesses "h1" in the range of 15 μm on both sides of substrate 102. Layer 104 functions as a lower cladding for waveguide core layer 110. When patterned rightward (as shown in FIGS. 4-5), waveguide core layer 110 may assume the form of a one-turn waveguide coil. The waveguide core layer 110 may have a thickness "h" and, when patterned, a width "w." Non-limiting, exemplary dimensions for "h" may be 60-100 nm, and "w" may 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, corresponding layers 116 and 118 are also formed on the other side of the substrate 102, even if those layers are not used for waveguiding purposes. Alternatively, those layers may create waveguides in different layers, if desired. An upper cladding layer 114, having a thickness "h2" in the range of 2-3 μm, may also be part of the structure. Both layers 114 and 116 may have the same material 120.

[0017] 2 shows the second step in the fabrication of the SiN waveguide core: A SiO2 cap layer 106 is deposited on top of the waveguide core SiN layer 110.

[0018] FIG. 3 shows the third step in the fabrication of the SiN waveguide core, in which the SiO2 cap layer 106 is patterned by etching to an appropriate width "w" (e.g., 2-3 μm). The SiO2 cap layer functions as a hard mask. Photoresist can be used as a mask for dry etching of the SiO2 cap layer. It has been experimentally found that wet etching the hard mask followed by wet etching of the SiN layer gives the best sidewall roughness because when resist is used as a mask and dry etched, the sidewall roughness on the resist is "copied" to the SiN layer below. This first and second wet etch are described in more detail with reference to FIGS. 7-9.

[0019] 4 illustrates the fourth step in the fabrication of the SiN waveguide core, in which the waveguide core layer 110 is wet etched to an appropriate width, as shown within the dashed outline oval 400, below the patterned SiO2 cap layer 106 having a width "w" (e.g., 2-3 μm). For example, the SiN can be wet etched with hot phosphoric acid. The hard mask should be selectively resistant to the wet etchant.

[0020] FIG. 5 shows an exploded view of a SiN waveguide core 110 fabricated by wet etching, showing smooth sidewalls 510 and 512. The dimension "x" indicates the recession below layer 106 due to potential overetching ("x" is typically in the range of 20-25 nm on either side). The smooth sidewalls achieved by wet etching help reduce optical loss during propagation within the gyroscope waveguide coil. While the sidewall roughness achieved by dry etching is in the micrometer or nanometer range, i.e., much greater than atomic smoothness, the sidewall roughness achieved by wet etching is atomic. Depending on the longitudinal dimension (e.g., thickness "h") of the waveguide core, this smoothness can be an important factor in determining propagation loss and optical mode restriction, especially around waveguide bends.

[0021] FIG. 6 is a scanning electron microscope image of a SiN waveguide core (with a hard mask on top) fabricated by wet etching, showing smooth sidewalls, according to an embodiment of the present disclosure.

[0022] FIG. 7 shows a layer of photoresist 150 on top of the patterned cap layer 106 using a first wet etch process. This step can be considered a subsequent step to those shown in FIGS. 1 and 2. The width of the patterned cap layer can be larger than the target width of the waveguide core. This is achieved by pre-biasing the mask used to pattern the cap layer 106, i.e., by drawing features on the mask that are larger than the features on the actual wafer. The pre-bias assists in compensating for lateral etching during the wet etch of the cap layer and the subsequent wet etch of the SiN core layer. The amount of lateral etching depends on the chemistry of the wet etch process. The lateral etching can be as small as 20-25 nm on each side or as large as 500 nm. Prior knowledge of the amount of lateral etching assists in designing the dimensions on the pre-biased mask.

[0023] FIG. 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 cap layer 106 (after the first wet etching step) is close to the desired target width of the waveguide core, but may still be slightly larger to allow for lateral etching. A second wet etch using hot phosphoric acid can be performed, resulting in a lateral etching of approximately 20-25 nm. The resulting sidewalls (within dashed line 800) are shown in FIG. 9. The cap layer sidewalls 916 and 914 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 atomically smooth at the sub-nanometer scale.

[0024] After a second wet etch, an upper cladding (or top cladding) is deposited on top of the remaining hard mask above the SiN core. The remaining hard mask can function as part of the cladding, ensuring that the interface between the upper cladding and the core layer is highly defect-free and strong to maintain a tightly confined optical mode. This can be seen in the SEM photograph in Figure 10. The dashed line shows the outline of the waveguide core with its curved sidewalls. There is no air gap between the waveguide core and the upper cladding.

[0025] In the foregoing specification, implementations of the present disclosure have been described with reference to specific embodiments thereof. It will be apparent that various changes may be made thereto without departing from the broader spirit and scope of the present disclosure as specified in the following claims. Accordingly, the specification and drawings should be regarded in an illustrative rather than a restrictive sense. Furthermore, directional terms, such as "top," "bottom," etc., are intended to encompass various directional permutations and combinations rather than limiting the scope of the present disclosure to any fixed direction. [Explanation of symbols]

[0026] 102 Circuit Board 104 layers 106 Cap Layer 110 Waveguide core layer 114 Upper cladding layer 116 layers 118 layers 150 layers of photoresist 510 smooth sidewall 512 smooth sidewall 910 SiN core sidewall 912 SiN core sidewall 914 Sidewall of cap layer 916 Sidewall of cap layer

Claims

1. forming a silicon nitride (SiN) layer on top of a substrate having an oxide layer, the oxide layer serving as a lower cladding of a 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 process to form a patterned cap layer including a cap over the SiN layer; performing a second wet etching process to form the SiN layer below the patterned cap layer to create a waveguide core; 1. A method for fabricating a waveguide structure, comprising:

2. The method of claim 1 , further comprising pre-biasing a mask used to form the patterned cap layer and the waveguide core.

3. The pre-biasing step includes:

3. The method of claim 2, including imaging a mask used to fabricate the waveguide cores on a wafer such that the width of features on the mask is greater than the target width of the waveguide cores.

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

5. The method of claim 4 , wherein the predicted lateral etching is wet chemistry dependent.

6. The method of claim 1 , wherein the target width of the waveguide core is between 2 μm and 3 μm.

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

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

9. The method of claim 1 , wherein the cap layer acts as a hard mask during the second wet etch.

10. 10. The method of claim 1, wherein the selectivity between the cap layer and SiN during the second wet etch controls the dimension of a lateral recess below the cap layer created in the patterned SiN layer.

11. The method of claim 1 , wherein the sidewall roughness of the SiN waveguide core achieved by the second wet etch is within atomic levels.

12. The method of claim 11 , wherein the sidewall roughness at the atomic level is substantially in the sub-nanometer range.

13. 10. The method of claim 1, wherein the second wet etch is performed using hot phosphoric acid.

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

15. The method of claim 14 , wherein the rotational detection element is in the form of a waveguide coil or a waveguide-based microresonator ring.

16. The method of claim 15 , wherein the rotating sensing elements are distributed among a plurality of vertical layers.

17. The method of claim 16 , wherein light is evanescently coupled between the vertical layers of the rotating detection element.

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