CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Chinese Patent Application No. 201910843158.8, filed Sep. 6, 2019, which is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method for fabricating an optical waveguide, in particular to a method for fabricating a spherical concave mirror in an optical waveguide, which is used in the technical filed of interconnections of optical backplanes.

BACKGROUND

In recent years, an optical backplane interconnection technology has become a hotspot in researching the information interconnection of high-end devices. It will be widely used in the fields of wideband communication, supercomputers, big data centers, etc. by virtue of its big bandwidth, low energy consumption, low cost, and other advantages.

As one of key technologies in the fields of optical backplane interconnections, an efficient vertical coupling for optical fibers and optical waveguides has the advantages of full use of device space, high design flexibility, easy integration of photoelectric devices, and the like. The efficiency of vertical coupling between the optical fibers and the optical waveguides is mainly improved by means of convergence of concave mirrors, in this way, light beams can be converged to improve the match of numerical apertures (NA) between coupled devices, and the mode field diameter of incident light can be decreased to improve the match of mode field coupling. Up to now, many methods for fabricating concave mirrors are put forward, such as a chemical etching method (Patent Publication No.: CN 1272182A), a fabrication method based on non-invasive water molecules (Patent Publication No.: CN103395739A), an annular laser etching method (Patent Application No.: 201710076388.7), and a deep proton writing (DPW) method (document: Design and fabrication of embedded micro-mirror inserts for out-of-plane coupling in PCB level optical interconnections). However, the chemical etching method has low accuracy, complex processes, difficulty in controls over concave shapes, and the like; the fabrication method based on non-invasive water molecules, which obtains concave surfaces by means of compactness of materials with a liquid drop as a mold, has complex processes, low accuracy, difficulty in obtaining the concave surfaces in optical backplanes; the annular laser etching method as a laser “cold” processing method, which obtains concave mirrors by means of laser etching after optical backplanes are fabricated, has high roughness of the obtained concave surfaces and certain influence on transmission performance of optical waveguides and needs to lower the roughness by means of postprocessing; and the DPW method which processes concave mirrors when optical waveguides are fabricated has extremely complex processes, and difficulty in accuracy controls or mass production. As shown inFIG. 1(a)toFIG. 1(f)inFIG. 1, a common process for fabricating optical waveguides by means of UV lithography mainly includes: a substrate1is cleaned; a lower cladding photoresist2is spin-coated; an interlayer photoresist3is spin-coated; UV exposure4is performed by means of a mask plate5; an optical waveguide core6is formed after developing; and an upper cladding photoresist7is spin-coated. Light penetrating through the mask plate5generally without grayscale distributions has the same intensity, and consequentially, the interlayer photoresist cannot form a concave structure. To obtain the concave structure, other supplementary means are needed, in this case, additional steps for fabricating concave mirrors are required, and the complexity of the process is increased. In view of this, it is urgent to provide a method which is simpler, more effective, feasible, and suitable for fabricating concave mirrors on a large scale.

SUMMARY

The objective of the present invention is to overcome the shortcomings of the prior art by providing a method for fabricating a spherical concave mirror in an optical waveguide based on ultraviolet (UV) Grayscale lithography, which significantly improves efficiency of vertical coupling between optical fibers and optical waveguides. The present invention has easy operation, desirable controllability, and optimal compatibility with an optical waveguide fabrication process.

To achieve the above objective, the present invention adopts the following concept:

A method for fabricating a spherical concave mirror in an optical waveguide based on UV grayscale lithography is mainly implemented by means of UV exposure performed by a lithography machine capable of generating a mask pattern. The mask pattern includes a semicircle with a grayscale distribution, and the grayscale distribution refers to that gray values in different areas of the mask pattern are changed in a stepped manner, so that light penetrating through different areas of the semicircle has different intensities. The lithography machine performs exposure according to the mask pattern, and the light penetrating through a semicircular area with the grayscale distribution is reduced in intensity from the center to the edge of a circle in the radial direction according to a change law of a decreasing function. Accordingly, an interlayer photoresist in the semicircular area is irradiated by the light having the intensity regularly distributed and is then developed to obtain a spherical concave surface.

The operating principle for directly fabricating a concave mirror in an optical waveguide through a semicircular mask pattern with a grayscale distribution is as follows:

The mask pattern actually includes a rectangle and a semicircle adjacent to the rectangle, where a rectangular area has no grayscale distribution, in this case, UV light penetrating through all portions of the rectangular area has the same intensity; and a semicircular area has the grayscale distribution, and the UV light penetrating through the semicircular area is reduced in intensity in the radius direction according to the change law of the decreasing function. When exposed under the UV light, the interlayer photoresist in the rectangular area is fully irradiated and is developed to form an optical waveguide core; only an upper portion of the interlayer photoresist in a partial semicircular area is fully irradiated according to a grayscale distribution law of the intensity of the UV light and is increased in thickness on the whole from the center to the edge of the circle in the radius direction according to the change law of the decreasing function, and the spherical concave surface is formed after developing. A grayscale distribution function is determined in a manner that the intensity of the UV light, which is required by lithography at all points, is determined by lithographic depths at all the points on the obtained spherical concave surface and has a rule that gray values are gradually decreased from the center to the edge of the circle in the radial direction. The obtained spherical concave surface is coated with a metal film or a dielectric high-reflective film, in this way, the spherical concave mirror is fabricated on the optical waveguide.

The technical solution adopted by the present invention according to the above concept is as follows:

A method for fabricating a spherical concave mirror in an optical waveguide based on UV grayscale lithography is mainly implemented by means of UV exposure through a lithography machine capable of generating a mask pattern. The mask pattern shows a semicircular mask with a grayscale distribution, and the grayscale distribution refers to that gray values in different areas of the mask pattern are stepped, so that light penetrating through different areas of the semicircular mask has different intensities and is finally gradually decreased in intensity in the radial direction of the semicircular mask. The lithography machine performs exposure according to the mask pattern, and the light penetrating through the semicircular mask with the grayscale distribution is reduced in intensity from the center to the edge of a circle in the radial direction according to a change law of a decreasing function. In a process for fabricating the optical waveguide by means of UV lithography, an interlayer photoresist in the semicircular mask is irradiated by the light having the intensity changed according to the grayscale distribution and is then developed to obtain a spherical concave surface or convex surface; and afterwards, the obtained spherical concave surface or convex surface is coated with a metal film or a dielectric high-reflective film and then spin-coated with a cladding photoresist, in this way, a spherical concave mirror is fabricated on the optical waveguide. To obtain different intensities of the light penetrating through different areas of the semicircular mask, the grayscale distribution in the present invention adopts a rule that gray values of images are changed in a stepped manner in the radial direction of the semicircular mask.

Preferably, the mask pattern includes a rectangle and a semicircle, where a rectangular area of a mask has no grayscale distribution, in this case, UV light penetrating through all areas of the mask has the same intensity; and a semicircular area having a radius longer than the width of the rectangular area has the grayscale distribution, and the UV light penetrating through the mask is reduced in intensity from the center to the edge of the circle in the radius direction of the semicircular area according to the change law of the decreasing function. The semicircular area having the radius longer than the width of the rectangular area has the grayscale distribution, and the UV light penetrating through the mask is regulated in intensity in the radius direction of the semicircular area according to the change law of the decreasing function.

Preferably, the rectangular area of the mask is completely opaque or completely transparent.

Preferably, if a spherical concave structure is fabricated on the optical waveguide by means of a positive interlayer photoresist, when the mask is exposed under the UV light, the interlayer photoresist in the rectangular area is fully irradiated; after being developed, the interlayer photoresist irradiated by the UV light is removed, and the interlayer photoresist unirradiated by the UV light is retained, so that an optical waveguide core is formed; and the interlayer photoresist in a partial semicircular area has an upper half fully irradiated according to a grayscale distribution law of the intensity of the UV light and increased in thickness from the center to the edge of the circle in the radius direction of the semicircular area according to a change law of an increasing function as well as an incompletely irradiated lower half retained after being developed, so that the concave surface is formed on a corresponding interlayer in the semicircular area of the mask. Afterwards, the obtained spherical concave surface is coated with the metal film or the dielectric high-reflective film and then spin-coated with the cladding photoresist, in this way, the spherical concave mirror is fabricated on the optical waveguide.

Preferably, if the spherical concave structure is fabricated on the optical waveguide by means of a negative interlayer photoresist, when the mask is exposed under the UV light, the negative interlayer photoresist in the rectangular area is fully irradiated; after being developed, the negative interlayer photoresist unirradiated by the UV light is removed, the negative interlayer photoresist irradiated by the UV light is retained, so that an optical waveguide core is formed; and an upper half of the negative interlayer photoresist in the partial semicircular area is fully irradiated according to the grayscale distribution law of the intensity of the UV light and is decreased in thickness in the radius direction of the semicircular area according to the change law of the decreasing function, and only the fully irradiated upper half of the negative interlayer photoresist is retained after being developed, so that a convex surface is formed on the corresponding interlayer in the semicircular area of the mask, and a concave surface is formed opposite the convex surface in the interlayer. Afterwards, the obtained spherical concave surface is coated with the metal film or the dielectric high-reflective film and then spin-coated with the cladding photoresist, in this way, the spherical concave mirror is fabricated on the optical waveguide.

Preferably, a grayscale distribution function of the corresponding light intensity in the semicircular area of the mask is controlled as follows:

at the same lithographic period, the intensity of the UV light, which is required by lithography at all points, is determined by lithographic depths at all the points on the obtained spherical concave surface to determine the grayscale distribution function of the light intensity in the semicircular area of the mask; and due to circular symmetry of the spherical concave surface, the grayscale distribution function of the light intensity in the semicircular area of the mask has a rule that gray values are gradually decreased from the center to the edge of the circle in the radius direction.

Compared with the prior art, the present invention has the following outstanding substantive features and significant advantages:

1. Due to technical limitations, most conventional methods for fabricating a concave mirror have complex processes, low accuracy, high roughness of concave surfaces, difficulty in transplanting, or low compatibility with an optical waveguide fabrication process. The method for fabricating a spherical concave mirror in an optical waveguide based on UV grayscale lithography of the present invention is mainly implemented by means of UV lithography through a semicircular mask pattern with the grayscale distribution, thus having simple process, high accuracy, low roughness of concave surfaces, compatibility with the process for fabricating the optical waveguide, and mass production.

2. The method of the present invention has the compatibility with the process for fabricating an optical waveguide and organically combines with a process for fabricating the concave surface during the UV exposure; and furthermore, the concave surface is mainly fabricated by means of a semicircular mask pattern with grayscale. Accordingly, the method of the present invention has good compatibility with a mature process for fabricating the optical waveguide by means of the UV lithography, low costs of device improvements, and easy implementations.

