TWI849803B - Process flow with pre-biased mask and wet etching for smooth sidewalls in silicon nitride waveguides - Google Patents

Abstract

Aspects of the present disclosure are directed to process flow to fabricate a waveguide structure with a silicon nitride core having atomic-level smooth sidewalls achieved by wet etching instead of the conventional dry etching process. A mask is pre-biased to account for lateral etching during the wet-etching steps.

Description

Processing flow for pre-biasing masking and wet etching of smooth sidewalls in silicon nitride waveguides

The present invention discloses various structures and fabrication methods for integrated photonic-based optical gyroscopes that utilize silicon nitride waveguides with smooth sidewalls.

A gyroscope (also referred to as a "gyro") is a device capable of sensing angular velocity. Applications of gyroscopes include, but are not limited to, the military, aircraft navigation, robotics, autonomous vehicles, virtual reality, augmented reality, gaming, etc. Gyroscopes can be mechanical or optical, and can vary in accuracy, performance, cost, and size. Since optical gyroscopes do not have any moving parts, they have an advantage 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 the interferometry of the optical phase shift caused by the Sagnac effect (a phenomenon induced by rotation encountered in interferometry). The construction of a FOG generally involves a coil that contains several turns of polarization-maintaining (PM) fiber. Laser light is injected into the two ends of the PM fiber coil, causing the two beams to travel in opposite directions. If the fiber coil is moving, the beams traveling in opposite directions experience different optical path lengths from each other. By setting up an interferometric system, it is possible to measure a small path length difference that is proportional to the loop area enclosed by the turns of the fiber coil and the angular velocity of the rotating fiber coil. This path length difference is expressed as the phase difference between the two counter-rotating beams (called the "phase signal").

The phase signal of an optical gyroscope is proportional to the Sagnac effect multiplied by the angular rotation speed, as shown in the following formula: Δφ=(8πNA/λc)Ω Where, N=number of turns of the gyroscope, A=enclosed area Ω=angular rotation speed Δφ=optical phase difference signal λ=light wavelength c=speed of light

Fiber-optic based gyroscopes can provide very high accuracy, but at this time they take up more space, are very expensive, and are difficult to assemble because the devices are built from discrete optical components that need to be precisely aligned. Typically, manual alignment is involved, which is difficult to scale up for mass production.

The inventors of this case propose to use waveguide-based integrated photonic components to replace optical fibers for cost-effective and simple integration on semiconductor platforms, which is more promising for high-volume production of gyroscopes. This application describes various structures, including silicon nitride (SiN) waveguide cores fabricated on silicon platforms, as described in detail below. The SiN waveguide cores disclosed herein are capable of having smooth sidewalls due to wet etching rather than conventional dry etching methods, which typically result in sidewall roughness in the micrometer or nanometer range, which can be detrimental to the optical performance of the gyroscope.

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 a detailed 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 specific implementation methods of the disclosure of this case or any scope of the scope of the patent application. Its only 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.

Aspects of the present disclosure relate to a process flow for fabricating a waveguide structure having a silicon nitride core with atomically smooth sidewalls achieved by wet etching rather than conventional dry etching processes.

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, the oxide layer serving as a lower cladding (or bottom cladding) of the waveguide, and the SiN layer serving as a core of the waveguide when patterned; forming a capping layer on top of the SiN layer; patterning the capping layer by a first wet etching step to form a patterned capping layer, the patterned capping layer comprising a cap above the SiN; and performing a second wet etching step to form the SiN layer below the patterned capping layer to produce the core of the waveguide.

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

The waveguide structure can be used as a rotation sensing element in an integrated photon optical gyroscope. The rotation sensing element can be in the form of a waveguide coil. The waveguide coil can be distributed between multiple vertical layers, wherein light is evanescently coupled between the multiple vertical layers of the waveguide coil. Alternatively, the rotation sensing element can be in the form of a waveguide-based micro-resonance ring, which can be formed in one or more layers and evanescently coupled in the vertically distributed layers.

Although for the sake of brevity only the manufacturing process is described in detail in this particular disclosure, the applicant incorporates by reference an earlier filed patent application describing single-layer and multi-layer waveguide structures for optical gyroscopes, see U.S. Patent No. 10,969,548, published on April 6, 2021.

Various aspects of the present disclosure relate to methods for fabricating compact ultra-low-loss integrated photonic-based waveguide cores with smooth sidewalls that can be accomplished in large-scale manufacturing. These waveguides can be used as optical components on planar photonic integrated circuits (PICs), for example, in photonic integrated optical gyroscopes. As discussed in the background section, the key to high performance of fiber-based optical gyroscopes is long lengths of high-quality, low-loss optical fibers for measuring the Sagnac effect. The inventors of the present invention recognized 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. Optical gyroscopes based on photons have reduced size, weight, power and cost, but are also mass-producible, vibration-resistant, and have the potential to provide comparable performance to FOGs. When an integrated optical gyroscope is fabricated on a silicon platform, it is referred to as SiPhOG ^TM (Silicon Photonics Optical Gyroscope).

A key element of this integrated photonic solution is the production of extremely low-loss waveguide cores made of silicon nitride (Si ₃ N ₄ ) surrounded by an oxide or fused silicon dioxide cladding. The entire waveguide structure (including core and cladding) is sometimes referred to as a SiN waveguide. The propagation losses in a SiN waveguide can be well below 0.1 db/meter. This is a huge improvement over the current state-of-the-art SiN processes, which have propagation losses in the 0.1 db/cm range.

FIG1 shows the first step in fabricating a SiN waveguide on a known silicon substrate. In detail, FIG1 shows a substrate 102, which may be a silicon substrate. The substrate 102 may have a thickness “H” of a standard wafer, for example, the thickness may be 725 μm. Note that the thicknesses of the different material layers are not drawn to scale. However, to convey the idea that the substrate 102 is much thicker than the rest of the material layers shown in the figure, a discontinuity 101 is introduced in the middle of the layer 102 for visual effect only. Layers 104 and 116 can have a thickness “h1” in the range of 15 microns on both sides of the substrate 102. Layer 104 serves as a lower cladding for the waveguide core layer 110. The waveguide core layer 110 can be considered as one turn of a waveguide coil when patterned to the correct dimensions (as shown in FIGS. 4-5 ). The waveguide core layer 110 can have a thickness “h” and, when patterned, 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, corresponding layers 116 and 118 are also formed on the other side of the substrate 102, even though these layers may not be used for waveguide purposes. Alternatively, these layers can create waveguides in different layers if necessary. 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 be of the same material 120.

Figure 2 shows the second step in fabricating the SiN waveguide core. A _SiO2 cap layer 106 is deposited on top of the waveguide core SiN layer 110.

FIG3 shows the third step of fabricating the SiN waveguide core, in which the _SiO2 capping layer 106 is patterned by etching to an appropriate width "w" (e.g., 2-3 μm). The _SiO2 capping layer acts as a hard mask. Photoresist can be used as a mask for dry etching the _SiO2 capping layer. Experiments have shown that wet etching the hard mask followed by wet etching the SiN layer achieves the best sidewall roughness because when the resist acts as a mask and is dry etched, the sidewall roughness on the resist is also "copied" down to the underlying SiN layer. The first and second wet etches are described in more detail with reference to FIGS. 7 to 9.

FIG4 shows the fourth step of fabricating the SiN waveguide core, wherein the waveguide core layer 110 is patterned by wet etching to a suitable width below the patterned _SiO2 cap layer 106 having a width "w" (e.g., 2-3 μm), as shown within the elliptical dashed line 400. Wet etching of SiN can be done by, for example, hot phosphoric acid. The hard mask should be selectively resistant to the wet etchant.

FIG5 shows an exploded view of a SiN waveguide core 110 fabricated by wet etching, showing smooth sidewalls 510 and 512. The dimension "x" shows the recess beneath layer 106 due to possible overetching ("x" is typically in the range of 20-25 nm per side). The smooth sidewalls achieved by wet etching help reduce light losses during propagation within the gyroscope waveguide coils. The sidewall roughness achieved by wet etching is at the atomic level, while the sidewall roughness achieved by dry etching is in the micrometer or nanometer range, i.e., a much higher roughness than atomic level smoothness. Depending on the waveguide core longitudinal dimensions (e.g., thickness "h"), this smoothness can be an important factor in determining propagation losses and optical mode confinement, especially near waveguide bends.

FIG. 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.

FIG7 shows a photoresist layer 150 on top of the cap layer 106 that is patterned using a first wet etch process. This step can be considered as a step following the steps 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., writing features on the mask that are larger than the actual features on the wafer. Pre-biasing helps to compensate for lateral etching during the cap layer wet etch and the subsequent SiN core layer wet etch. The amount of lateral etching depends on the chemistry 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 in designing the dimensions of the pre-biased mask.

FIG8 shows a second wet etch 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 etch step) is closer to the desired target width of the waveguide core, but may still be slightly larger to account for the lateral etching. The second wet etch can be done using hot phosphoric acid and typically produces a lateral etch of 20-25 nm. The resulting sidewalls (within dashed line 800) are shown in FIG9. The sidewalls 916 and 914 of the capping layer are also smooth and slightly curved, which would not be the case if dry etching was used. The sidewalls 912 and 910 of the SiN core 110 are smooth to the sub-nanometer atomic level.

After the 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 act as part of the cladding and ensure that the interface between the upper cladding and the core layer has high integrity and strength to maintain tight confinement of the optical mode. This can be shown in the SEM image of Figure 10. The dotted line shows the outline of the waveguide core with curved sidewalls. There is no gap between the waveguide core and the upper cladding.

In the foregoing specification, embodiments of the present disclosure have been described with reference to specific exemplary embodiments of the present disclosure. Obviously, various modifications may be made without departing from the broader spirit and scope of embodiments of the present disclosure as set forth in the attached claims. Therefore, the specification and drawings are to be regarded as illustrative rather than restrictive. 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.

101: Discontinuous part 102: Substrate 104: Layer 106: Cap layer 110: Waveguide core layer 114: Upper cladding layer 116: Layer 120: Material 150: Photoresist layer 400: Elliptical dotted line 510,512: Smooth sidewall 800: Dotted line 910,912,914,916: Sidewall

A more complete understanding of the present disclosure will be obtained from the detailed description set forth below and the accompanying drawings of various implementations of the present disclosure.

FIG. 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.

FIG. 2 is a schematic cross-sectional view showing a silicon dioxide (SiO ₂ ) capping layer deposited on a SiN waveguide core layer according to an embodiment of the present disclosure.

FIG. 3 is a schematic cross-sectional view showing a SiO ₂ capping layer patterned to an appropriate width to form a SiN waveguide core according to an embodiment of the present disclosure.

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

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

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.

FIG. 7 is a schematic cross-sectional view showing a _SiO2 capping layer patterned by wet etching according to an embodiment of the present disclosure.

FIG. 8 is a schematic cross-sectional view showing a method of patterning a SiN waveguide core using wet etching while pre-biasing a mask to obtain a suitable width of the SiN waveguide core according to an embodiment of the present disclosure.

9 is an exploded schematic cross-sectional view of a SiN waveguide core for smooth sidewalls fabricated by wet etching both a cap layer and a waveguide layer according to an embodiment of the present disclosure.

FIG. 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 gaps between the waveguide core and the cladding.

Domestic storage information (please note in the order of storage institution, date, and number) None Foreign storage information (please note in the order of storage country, institution, date, and number) None

104: layer 106: cap layer 110: waveguide core layer 800: dashed line 910,912,914,916: side wall

Claims (14)

A method for manufacturing a waveguide structure includes: forming a silicon nitride (SiN) layer on top of a substrate having an oxide layer, the oxide layer serving as a cladding of the waveguide structure, and the SiN layer serving as a core of the waveguide structure 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 comprising a cap on the SiN layer; and performing a second wet etching step to A patterned SiN layer is formed below the patterned capping layer to produce the core; wherein a mask is pre-biased to form the patterned capping layer and the core; wherein the pre-biasing includes: writing the mask in a manner such that a width of a feature on the mask used to fabricate the core on the substrate is greater than a target width of the core; wherein a difference between the width of the feature on the mask and the target width of the core is predetermined based on an expected lateral etch during the first wet etching step and the second wet etching step. A method as claimed in claim 1, wherein the expected lateral etching depends on wet chemistry. The method as claimed in claim 1, wherein the target width of the core is 2 to 3 microns. The method as described in claim 1, wherein the thickness of the SiN layer is in the range of 60 to 100 nm. A method as described in claim 1, wherein one of the materials of the capping layer is silicon dioxide. The method of claim 1, wherein the capping layer acts as a hard mask during the second wet etching step. The method of claim 1, wherein the selectivity between the cap layer and the SiN layer during the second wet etching step controls the size of a plurality of lateral recesses below the cap layer produced in the patterned SiN layer. The method as described in claim 1, wherein the sidewall roughness of the core achieved by the second wet etching step is at the atomic level. The method as described in claim 8, wherein the atomic-scale sidewall roughness is substantially smaller than the nanometer range. A method as claimed in claim 1, wherein the second wet etching step is performed using hot phosphoric acid. A method as described in claim 1, wherein the waveguide structure is used as a rotation sensing element in an optical gyroscope. A method as described in claim 11, wherein the rotation sensing element is in the form of a waveguide coil or a waveguide-based micro-resonance ring. A method as described in claim 12, wherein the rotation sensing element is distributed between multiple vertical layers. The method of claim 13, wherein light is evanescently coupled between the plurality of vertical layers of the rotation sensing element.