GB2327280A

GB2327280A - Making planar optical waveguide with polishing steps

Info

Publication number: GB2327280A
Application number: GB9815126A
Authority: GB
Inventors: Sun-Tae Jung
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1997-07-15
Filing date: 1998-07-14
Publication date: 1999-01-20
Anticipated expiration: 2018-07-14
Also published as: KR19990010190A; JPH1172637A; CN1105927C; GB2327280B; JP2902640B2; CN1205444A; GB9815126D0; KR100509510B1

Abstract

A uniform planar optical waveguide is made by (a) depositing a lower cladding layer 402 on a substrate 400 and polishing the deposited surface; (b) depositing a core layer on the resultant structure of the step (a) and polishing the deposited surface; (c) patterning the core layer whose surface is polished in the step (b), to generate an optical waveguide; and (d) depositing an upper cladding layer on the optical waveguide formed through the patterning of the step (c). In the case of an arrayed waveguide demultiplexer (AWG DEMUX), the phase difference at each channel matches the intended value, thereby decreasing crosstalk.

Description

FABRICATION METHOD FOR UNIFORM PLANAR OPTICAL WAVEGUIDE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for fabricating a uniform planar optical waveguide, and more particularly, to a method for fabricating a uniform planar optical waveguide by polishing after deposition.

2. Description of the Related Art A planar lightwave circuit has been developed for mass production as well as for making up for weak points in a macro-optic method for manufacturing an optical communications device. FIGs. 1A through 1C illustrate a conventional method for fabricating a planar optical waveguide. FIG. 1A illustrates a step of depositing a lower cladding layer 102 and a core layer 104 on a substrate 100.

FIG. 1B illustrates a step of patterning the core layer 104 of FIG. 1A and then forming waveguides 106. FIG. 1C illustrates a step of depositing an upper cladding layer 108 on the waveguides of FIG. 1B.

FIG. 1D is a flowchart illustrating the fabrication method illustrated in FlGs.

1A through 1C in detail. First, in step 112, the lower cladding layer and the core layer are deposited. For the deposition, an organic material such as polymer is deposited by a spin coating method while an inorganic material is deposited by a chemical vapor deposition (CVD), a modified CVD or a flame hydrolysis deposition (FHD) method. Here, according to the deposition method and conditions therefor, thickness of the layers is slightly different. According to the spin coating method, after synthesizing an organic material, the concentration and viscosity of the organic material are adjusted by adding a predetermined solvent, the mixture is injected into a spin coater and then an organic film having a thickness of several micrometers is formed by rotating the spin coater at high speed. In the CVD method, source gas as a material for a layer to be deposited is injected into a reaction furnace and energy is provided to the reaction furnace to form the layer on the substrate. The modified CVD method includes a low-pressure CVD (LPCVD), atmosphere pressure CVD (APCVD), and plasma-enhanced CVD (PECVD). In the FHD method, a reaction gas is synthesized with hydrogen and oxygen flames to form fine soot, and the fine soot is deposited on the substrate.

For each deposition, a silicon substrate is mainly used as the substrate. However, substrates made of quartz, aluminum oxide (Al2O3), gallium arsenide (GaAs), indium phosphide (InP) or semiconductor compounds of elements belonging to Group Ill and V of the periodic table may be used.

The pattern is manufactured in a clean room. A wafer onto which the layer has been deposited is washed and dried, and photo resist (PR) spin coating is performed in step 116. Here, between the steps of 112 and 116, a metal mask may be deposited according to the etching conditions in step 114. After the PR spin coating, the resultant structure is baked to harden the PR pattern in step 118, and a designed pattern is transcribed onto the wafer using a mask aligner and ultraviolet (UV) rays is irradiated thereon in step 120. After the pattern is formed by irradiating the UV rays, the unreacted PR is removed using a developing solution in step 122, and then exposed core layer is dry-etched in step 124. Here, the etching is performed by an inductively coupled plasma method or a reactive ion beam etching method. After the etching, the material (PR or metal film) used as the pattern mask is removed in step 126 (called lift-off etching), and postannealing is performed in step 128. Then, the upper cladding layer is formed through deposition in step 130. After the above steps performed in wafer units are completed, the wafer is cut into device units and complete devices are obtained through a packaging step.

As described above, in the conventional method for manufacturing the planar optical waveguide, basically, film deposition is repeated three times.

However, three or more depositions are required for manufacturing a multi-layered device. Here, even though the film deposition condition is optimized, the thickness of the layer is uniform to within 2~3% thickness. If the thickness of the film is not uniform, the thickness of the optical waveguide formed from the layer is not uniform, thereby causing non-uniform device characteristics. FIG. 2A is a vertical section view of an optical waveguide having a non-uniform thickness, and FIG. 2B is a side view of the optical waveguide shown in FIG. 2A having a non-uniform thickness. Here, reference numeral 200 represents a substrate, reference numeral 202 represents a core layer, reference numeral 204 is a cladding layer, d represents the thickness of the optical waveguide, w represents the width of the optical waveguide, and / represents the length of the optical waveguide. The above non-uniform thickness of the device provides the following effects. For example, an arrayed waveguide demultiplexer (AWG DEMUX) separates mixed wavelengths of an input optical signal and distributes the independent wavelength among channel. Here, the phase difference A of each channel should be determined to a predetermined interval, and assuming that AL is the path difference and ss is a propagation index of a waveguide, the phase difference can be expressed by the equation A = ss. The propagation index ss of the waveguide is expressed by the equation ss = kO dsin0, where k0 is a wave factor, d is the thickness of the waveguide and 6 is the incident angle. Here, if the waveguide is not homogeneous, the thickness d varies during the propagation of light, so that the light signal is not separated into an intended specific wavelength at each channel end, increasing cross talk, which causes serious problems in manufacturing.

Such problem may occur in a device adopting an optical waveguide, as well as in the AWG DEMUX. If an error caused by such problem is within an allowable range, the device can be used. However, if in a case of a device having a multiple layered structure, which requires more accurate controlling of the optical signal, a more accurate optical waveguide is required.

SUMM'ARY OF THE INVENTION To solve the above problems, it is an objective of the present invention to provide a method of fabricating a planar optical waveguide, which comprises a surface planarizing step for removing unevenness in the thickness of upper and lower cladding layers and a core layer, having a deviation in thickness of 2~3%, thereby minimizing the difference in the thickness of layers and increasing the degree of evenness in the surface.

Accordingly, to achieve the above objective, there is provided a method of fabricating a uniform planar optical waveguide comprising the steps of: (a) depositing a lower cladding layer on a substrate and polishing the deposited surface; (b) depositing a core layer on the resultant structure of the step (a) and polishing the deposited surface; (c) patterning the core layer whose surface is polished in the step (b), to generate an optical waveguide; and (d) depositing an upper cladding layer on the optical waveguide formed through the patterning of the step (c).

BRIEF DESCRIPTION OF THE DRAWINGS The above objective and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: FIGs. 1A through 1C are vertical section views illustrating a conventional method of fabricating a planar optical waveguide; FIG. 1D is a flowchart illustrating in detail the conventional method of fabricating a planar optical waveguide; FIG. 2A is a vertical section view of an optical waveguide having an uneven thickness; FIG. 2B is a side view of the optical waveguide shown in FIG. 2A having an uneven thickness; FIG. 3 is a flowchart illustrating a method of fabricating a planar optical waveguide having an even thickness according to the present invention; and FIGs. 4A through 4C are section views illustrating a surface planarizing step.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 3, a method of fabricating a planar optical waveguide includes a step 300 of depositing a lower cladding layer, a step 310 of polishing a first surface, a step 320 of depositing a core layer, a step 330 of polishing a second surface, a step 340 of patterning, a step 350 of depositing an upper cladding layer, and a step 360 of polishing a third surface.

In the steps 330, 320 and 350 of depositing an upper cladding layer, a core layer and a lower cladding layer, a spin coating, chemical vapor deposition (CVD), low pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), atmosphere pressure CVD (APCVD) or flame hydrolysis deposition (FHD) method is used.

Also, a substrate is formed of silicon, quartz, aluminum oxide (AI2O3), gallium arsenide (GaAs), indium phosphide (InP) or semiconductor compounds of elements belonging to Group III and V of the periodic table.

The first, second and third surface polishing steps 310, 330 and 360 are for planarizing the surface. The surface polishing method includes a mechanical polishing method and a chemical polishing method. In the mechanical polishing method, the surface is physically scraped off by a material having a greater hardness than that of the surface material to be polished. In the chemical polishing method, the surface is solubilized little by little using chemicals reacting with the surface to be polished. Also, a chemical mechanical polishing method, which is a combination of the above two methods, can be used. In the chemical mechanical polishing method, mechanical polishing efficiency is improved by changing the properties of the surface through a chemical reaction at the surface to be polished with chemicals. This method is mainly used for mass production using a wafer and in a semiconductor manufacturing process where fine surface polishing is possible. FIGs. 4A through 4C illustrate the above-described mechanical chemical polishing method. In detail, FIG. 4A shows an unpolished deposited surface, where reference numeral 400 represents a substrate and reference numeral 402 represents a deposited layer. FIG. 4B illustrates a chemical mechanical polishing method using a polishing tool 404, a polishing agent represented by "o" and chemicals represented by "". FIG. 4C shows the uniformly planarized surface after polishing.

For example, in a case of a silica optical waveguide, a target to be polished is formed of boro-phospho-silica'glass (BPSG) containing silicon oxide (SiO2) as the major component. Accordingly, the surface properties of the target change when a polishing agent such as SiO2 particles and ceramic particles containing alkali (-OH) such as KOH is used. The surface properties of the layer change through a chemical formula, thereby increasing the mechanical polishing efficiency.

In the method of fabricating the uniform planar optical waveguide according to the present invention, a lower cladding layer is deposited in step 300, and then the surface polishing is performed by the above-described method in step 310.

Then, a core layer is deposited thereon in step 320, and the surface polishing is performed again in step 330. After the deposition of the core layer and the surface polishing are completed, patterning is performed in step 340.

The patterning step 340 will be described in detail. After washing a wafer, photo resist (PR) spin coating is performed in step 342. Before the step 342, a metal mask may be deposited according to the etching conditions in step 341.

After PR spin coating, baking is performed to harden the PR pattern in step 343, and then a designed pattern is transcribed onto the PR by aligning a mask and ultraviolet (UV) rays are irradiated thereon in step 344. After the pattern is formed by irradiation of UV rays, it is developed by soaking in a predetermined solution in step 345, and then exposed core layer is dry-etched in step 346 by a plasma etching method, for example, inductively coupled plasma method or a reactive ion beam etching method. After the etching is completed, the material (PR or metal film) used as the pattern mask is removed in step 347, and then post-annealed in step 348, thereby completing the patterning step.

After patterning, an upper cladding layer is formed through deposition in step 350, and the surface polishing is also performed in step 360.

After the above steps are completed, a device having a multiple layered structure can be obtained by repeating the above steps.

In the case of a single mode silica waveguide which has undergone the above polishing, deviation in the thickness of the optical waveguide is reduced to a range within 500Â. Because the single mode silica optical waveguide has a core layer having a thickness of approximately 8pom, deviation in the thickness is about 0.6%, which is a 3-5 fold improvement compared to the conventional deviation of 2~3%. In the case of a multimode waveguide, the size of the optical waveguide increases and the thickness deviation is not changed, thereby further decreasing the deviation ratio. The fabrication method of an optical waveguide can be applied to a device relating to a plurality of wavelengths, a device adopting light with a long propagation distance or a device adopting a multiple layered optical waveguide.

In the method of fabricating a uniform planar optical waveguide according to the present invention, the surface planarizing steps are further included, thereby increasing evenness in the thickness of the optical waveguide. As a result, the effective refractive index within the optical waveguide becomes uniform and more accurate optical devices can be manufactured. In particular, in the case of the AWG DEMUX, the phase difference at each channel matches the intended value, thereby decreasing crosstalk.

Claims

What is claimed is:

1. A method of fabricating a uniform planar optical waveguide comprising the steps of: (a) depositing a lower cladding layer on a substrate and polishing the deposited surface; (b) depositing a core layer on the resultant structure of the step (a) and polishing the deposited surface; (c) patterning the core layer whose surface is polished in the step (b), to generate an optical waveguide; and (d) depositing an upper cladding layer on the optical waveguide formed through the patterning of the step (c).

2. The method of claim 1, wherein the substrate is formed of a material selected from the group consisting of silicon, quartz, aluminum oxide (Al2O3), gallium arsenide (GaAs), indium phosphide (InP) and semiconductor compounds of elements belonging to Group lli and V of the periodic table.

3. The method of claim 1, wherein the step (a) of depositing the lower cladding layer is performed by a method selected from the group consisting of spin coating, chemical vapor deposition, plasma-enhanced chemical vapor deposition, low-pressure chemical vapor deposition, atmosphere pressure chemical vapor deposition and flame hydrolysis deposition methods.

4. The method of claim 1, wherein the polishing is performed by a method selected from the group consisting of mechanical polishing, chemical polishing, and chemical mechanical polishing methods.

5. The method of claim 1, wherein the step (b) of depositing the core layer is performed by a method selected from the group consisting of spin coating, chemical vapor deposition, plasma-enhanced chemical vapor deposition, lowpressure chemical vapor deposition, atmosphere pressure chemical vapor deposition and flame hydrolysis deposition methods.

6. The method of claim 1, wherein the step (d) of depositing the upper cladding layer is performed by a method selected from the group consisting of spin coating, chemical vapor deposition, plasma-enhanced chemical vapor deposition, low-pressure chemical vapor deposition, atmosphere pressure chemical vapor deposition and flame hydrolysis deposition methods.

7. The method of claim 1, wherein the patterning step (c) comprises the sub-steps of: (cl) performing photo resist spin-coating on the surface-polished core layer; (c2) baking the resultant of the step (cl) to harden the photo resist pattern; (c3) transcribing a designed pattern onto the photo resist by aligning a mask and irradiating ultraviolet rays thereonto; (c4) developing the photo resist pattern by soaking in a predetermined solution; (c5) etching the resultant of the step (c4) according to the designed pattern, and removing the used pattern mask; and (c6) post-annealing the resultant of the step (c5).

8. The method of claim 7, wherein the patterning step (c) further comprises a sub-step of depositing a metal mask on the surface-polished coating layer before the step (cl).

9. The method of claim 7, wherein the etching step (c5) is performed by a plasma etching method.

10. A method of fabricating a uniform optical waveguide substantially as described with reference to figures 2 to 4 of the accompanying drawings.