KR100439749B1 - Method for fabricating optical waveguide on fused silica substrates using inductively coupled plasma etcher - Google Patents

Abstract

A method of manufacturing an optical waveguide on a fused quartz substrate using an inductively coupled plasma etching apparatus is disclosed. The manufacturing method uses a fused quartz-based substrate having a similar coefficient of thermal expansion to the core layer and the cover layer, which are optical waveguides, so that there is no need to deposit a buffer layer or a lower lid layer between the fused quartz-based substrate and the core layer. Therefore, the manufacturing process is shortened and the productivity is improved. In addition, by forming a conductive layer made of metal in contact with the lower electrode provided in the inductively coupled plasma etching apparatus on the lower surface of the molten quartz substrate, the core layer is etched to form a pattern of an optical waveguide, thereby forming an etching uniformity and etching. Speed is improved, yield and productivity are improved.

Description

METHOD FOR FABRICATING OPTICAL WAVEGUIDE ON FUSED SILICA SUBSTRATES USING INDUCTIVELY COUPLED PLASMA ETCHER}

The present invention relates to a method for manufacturing an optical waveguide on a fused quartz substrate using an inductively coupled plasma etching apparatus.

Optical waveguides, which are essential elements for integrated optical devices, are widely used in optical splitters, optical couplers, modulators, switch interferometers, semiconductor array lasers, and wavelength division multiplexing devices. As described above, the characteristics of the optical waveguide used in various fields can be improved only by minimizing scattering loss and polarization loss.

A conventional method for manufacturing an optical waveguide will be described with reference to FIG. 1. 1 is a flowchart illustrating a conventional optical waveguide manufacturing method.

As shown, in step S10, silica fine particles are sequentially deposited on a silicon-based solid substrate by Flame Hydrolysis Deposition (FHD), an underclading layer and a core layer. To form. In the case of flame hydrolysis deposition, a high-density heat treatment process is performed to convert the lower cover layer and the core layer into a transparent glass film. In this case, the densification heat treatment process may be performed after depositing the lower cover layer and after depositing the core layer. When forming the core layer, germanium or phosphorus is additionally added to the silica fine particles. The refractive index of the core layer is about 0.25-1.25% higher than that of the lower cover layer and the upper cover layer, so that light is guided along the optical waveguide. To do this.

Thereafter, in step S20, an etching mask layer is deposited on the core layer, and in step S30, a pattern is formed on the mask layer with a photosensitive agent. Next, the optical waveguide pattern is implemented on the core layer through etching (S40), the mask layer is removed (S50), and an upper cladding layer is deposited to complete the optical waveguide (S60).

In the above-described optical waveguide manufacturing process, the formation of the optical waveguide pattern through etching requires a thick film silica etching process of 10 microns or more. The etching process requires a vertical cross section for forming a high selectivity ratio, a high etching rate, a low loss optical waveguide with an etching mask, and minimization of roughness of the optical waveguide side to reduce light scattering loss.

Conventional methods for etching optical waveguide patterns include reactive ion etching, or inductively coupled plasma (ICP) that generates and etches 10 ¹² or more high-density plasma using a CF _X- based gas. Plasma) method.

The reactive ion etching method has excellent etching characteristics, but not only requires a thick mask layer made of metal such as Cr or Al due to low selectivity, but also has a very low etching rate of about 300-500 m 3 / min. Therefore, there is a disadvantage in that it takes a long time to form an optical waveguide having a depth of 8-10 microns or more.

In addition, the inductively coupled plasma etching method using CF _x -based gas has a high etching speed, but due to the low selectivity between the core layer and the etching mask layer, a thick mask layer of 5000-8000 Å is used, so the resolution of the waveguide is reduced. This results in poor verticality of the pattern. Then, there is a problem that the verticality of the optical waveguide also worsens later. In order to solve this problem, a method of performing an etching process by mixing gases such as CHF ₃ , H _2, etc. is used, but this may increase the etching selectivity with the mask layer, but has a disadvantage in that excessive polymer is generated. Then, the surface contamination occurs after the etching process to form a partial bonding failure at the interface between the core layer and the top cover layer when forming the top cover layer. As a result, since defects such as pore generation are caused and the manufactured device must be discarded, there is a disadvantage in that productivity is lowered.

The present invention has been made to solve the above disadvantages of the prior art, an object of the present invention is to use a molten quartz-based substrate having a similar coefficient of thermal expansion as the core layer and cover layer as a substrate, thereby reducing the manufacturing process to improve productivity The present invention provides a method for manufacturing an optical waveguide on a fused quartz substrate using an inductively coupled plasma etching apparatus.

Another object of the present invention is to form a metal conductive layer on the lower surface of the molten quartz substrate in contact with the electrode provided in the inductively coupled plasma etching apparatus when the core layer is etched using the inductively coupled plasma etching apparatus. In addition, the present invention provides a method of manufacturing an optical waveguide on a molten quartz substrate using an inductively coupled plasma etching apparatus capable of improving etching uniformity and etching speed to improve yield and productivity.

1 is a flow chart showing a conventional method for manufacturing an optical waveguide.

2A and 2B are flowcharts illustrating a method of manufacturing an optical waveguide according to an embodiment of the present invention.

Figure 3 is a view showing the configuration of a schematic reaction mechanism of the inductively coupled plasma etching apparatus for etching the pattern of the optical waveguide in the manufacturing method according to an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

100: inductively coupled plasma etching apparatus

110 chamber 120 lower electrode

130: inductively coupled plasma coil 141,145: RF1, RF2

150: plasma 210: fused quartz substrate

220: core layer 230: mask layer

240: conductive layer 250: cover layer

In the method for manufacturing an optical waveguide on a fused quartz substrate using the inductively coupled plasma etching apparatus according to the present invention for achieving the above object, the core layer is deposited by sequentially depositing silica fine particles on the upper surface of the fused quartz substrate Doing; Depositing a metal etching mask layer on the core layer; Depositing a conductive layer made of metal on a lower surface of the fused quartz substrate; Coating and transferring a photoresist on the mask layer to form a pattern corresponding to the pattern of the optical waveguide to be formed, and then removing the mask layer on which the pattern of the mask layer is not formed; The conductive layer is formed on the lower electrode of the inductively coupled plasma etching apparatus having an inductively coupled plasma coil positioned on the upper side of the lower electrode so as to face the lower electrode and the lower electrode, and only C ₂ F ₆ is injected into the reaction gas. Forming a pattern of an optical waveguide by mounting a lower surface of the fused quartz substrate having a pattern of a mask layer formed on the upper surface of the substrate and etching the core layer in an area not covered with the pattern of the mask layer; Removing the mask layer and the conductive layer; And depositing a cover layer covering the pattern of the optical waveguide.

Hereinafter, a method of manufacturing an optical waveguide on a molten quartz substrate using an inductively coupled plasma etching apparatus according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

2A and 2B are flowcharts illustrating a method of manufacturing an optical waveguide according to an embodiment of the present invention, and FIG. 3 is an inductively coupled plasma for etching a pattern of an optical waveguide by a manufacturing method according to an embodiment of the present invention. Figure shows the structure of the reaction mechanism of the etching apparatus.

As shown in step S110, silica fine particles are sequentially deposited on the upper surface of the fused quartz substrate 210 by flame hydrolysis deposition to form a core layer 220, and then a high temperature of 1200 ° C. or higher. The high density heat treatment is performed to form a transparent silica core layer 220. When depositing silica fine particles to form the core layer 220, germanium (Ge) or phosphorus (P) is additionally added, which is used to change the refractive index of the core layer 220 to the fused quartz substrate 210 and to be described later. The refractive index of the layer 250 is about 0.25-2.5% higher, so that light can be guided along the core layer 220 which is an optical waveguide.

In the present embodiment, a fused quartz substrate 210 having a thermal expansion coefficient similar to that of the core layer 220 and the cover layer 250 is used. The reason is to minimize stress generated inside the optical waveguide and to thereby minimize scattering loss and polarization loss, thereby improving device characteristics. In other words, when a conventional solid silicon substrate is used, a large stress is generated inside the waveguide due to a large difference in coefficient of thermal expansion between the substrate and the silica deposition film, thereby increasing scattering loss and polarization loss. The characteristics of the device are deteriorated. However, the molten quartz substrate 210 according to the present embodiment uses a molten quartz substrate 210 having a thermal expansion coefficient similar to that of the core layer 220, thereby minimizing stress generated inside the waveguide. More specifically, the thermal expansion coefficient of the core layer 220 and the cover layer 250 made of silica fine particles is 0.35 to 1.0 x 10 ^-6 / ℃, the thermal expansion coefficient of the substrate according to the embodiment also 0.35 to 1.0 x 10 ^A molten quartz-based substrate 210 of ^-6 / ° C is used.

Thereafter, in step S120, an etching mask layer 230 is deposited on the upper surface of the core layer 220, and a metal conductive layer 240 is deposited on the lower surface of the core layer 220. The mask layer 230 is generally a metal layer such as chromium (Cr), titanium (Ti) and aluminum (Al), a dielectric layer of amorphous silicon, and a polymer layer such as a photoresist. However, in the present embodiment, chromium (Cr) having the best selection ratio is used as the mask layer 230. More specifically, the mask layer 230 made of chromium (Cr) is deposited to a thickness of about 1500-3000 GHz by applying RF power of 600 W to 1000 W using a sputtering system. At this time, the thickness of the mask layer 230 may be formed in consideration of the material and thickness of the core layer 220 to be formed, the selectivity during the etching process, etc., but it is preferable to form a thin metal mask layer. It is desirable to improve the resolution of the waveguide, shorten the process time, and improve the quality and productivity. After forming the mask layer 230, the conductive layer 240 is deposited on the bottom surface of the fused quartz substrate 210 in the same manner as the deposition of the mask layer 230 with a thickness of about 1000-4000 μm. The reason why the metal conductive layer 240 is deposited on the lower surface of the fused quartz substrate 210 will be described later.

In this embodiment, the core layer 220 is formed directly on the upper surface of the fused quartz substrate 210 by using the substrate as a fused quartz substrate. However, conventionally using a silicon-based solid substrate in order to reduce the stress caused by the difference in thermal expansion coefficient between the substrate and the optical waveguide core layer, the SiO ₂ layer and the SiO ₂ layer having a thickness of about 1-2 microns to act as a buffer A lower cover layer was formed thereon. Therefore, in the manufacturing method according to the present embodiment, compared to the conventional manufacturing method, the time required to manufacture the entire device is reduced, thereby improving productivity.

In operation S130, a photosensitive film is coated on the mask layer 230 by spin coating, and then a photo developing method and a wet etching method are performed on the photosensitive film and the mask layer 230 using an exposure mask. In the step S140, the mask layer 230 is etched to form a two-dimensional planar pattern 235 of the optical waveguide. When the thick mask layer 230 of 5000 to 8000 Å is etched by wet etching to form the pattern 235 for forming the optical waveguide on the mask layer 230, the side of the mask layer 230 is etched due to the property of isotropic etching. Due to excessive etching, line width decreases, resulting in poor verticalness. This is an obstacle to the side surface roughness control and the formation of a rectangular vertical optical waveguide during the etching of the core layer 220 which is the next process. At this time, the side roughness and shape deformation of the excessive optical waveguide are a direct cause of loss in optical transmission. Therefore, in order to increase the selectivity with the core layer 220 when etching the core layer 220, forming the mask layer 230 to a minimum thickness is a very important factor that can reduce the design process error. Therefore, in the present embodiment, the above-described wet etching method using the dry etching method using the reaction ion etching method has been largely solved. However, it is preferable to minimize the thickness of the mask layer 230 even if a dry etching method is used in consideration of a reduction in process time and line width.

In operation S150, the core layer 220 of the area not covered with the pattern 235 of the mask layer is etched by using an inductively coupled plasma etching apparatus 100 to pattern the rectangular optical waveguide. (225). As shown in FIG. 3, the inductively coupled plasma etching apparatus 100 includes a lower electrode facing a load lock (not shown), a main chamber 110, a lower electrode 120, and a lower electrode 120. RF1 (141) and RF2 (145) generating apparatus for applying RF power to the inductively coupled plasma coil 130, the lower electrode 120 and the inductively coupled plasma coil 130, respectively, installed above the 120. Have Placing the inductively coupled plasma coil 130 on the upper side opposite to the lower electrode 120, which is a cathode, and applying RF power to the inductively coupled plasma coil 130 separately from the lower electrode 120, thereby providing a high density of plasma. The energy of the ions incident on the fused quartz substrate 210 may be adjusted while forming a. This is a characteristic configuration that is different from the conventional reactive ion etching apparatus.

In this embodiment, the following process sequence and conditions are carried out. After the conductive layer 240 is deposited on the lower surface of the fused quartz substrate 210, the pattern 235 is formed on the mask layer 230 deposited on the core layer 220, and then the fused quartz substrate 210 is formed. Is mounted on the lower electrode 120 of the inductively coupled plasma etching apparatus 100. In addition, when the pattern 235 of the mask layer 230 is formed by the etching process prior to performing the main process using C ₂ F ₆ , oxygen plasma is removed to remove photoresist that may remain without being completely removed. The used polymer removal process is performed. At this time, 600-1000W is applied to the inductively coupled plasma coil 130 and 10-50W is applied to the lower electrode 120, and a high-density oxygen plasma is formed while maintaining a pressure of 10-30 mTorr for 5 to 10 minutes. To perform. This is to improve the yield by eliminating defects such as pores that may occur when the cover layer 250 to be described later due to the excessive polymer remaining. Therefore, the manufacturing method according to the present embodiment can establish conditions for the polymer removal process using an appropriate oxygen plasma before the main process without a separate Asher equipment. The above-described process using the oxygen plasma is also carried out after the main process, in order to improve the yield by the above-described effect by removing the CFx-based polymer layer generated after the main process.

A main process of forming the pattern 225 of the optical waveguide by etching the core layer 220 by the inductively coupled plasma etching apparatus 100 using C ₂ F ₆ will be described. The reaction gas of C ₂ F ₆ is injected into the chamber 110, and RF2 145 power is applied to the inductively coupled plasma coil 130. Then, plasma 150 is formed by collision of electrons accelerated by the induced electric field with C ₂ F ₆ molecules. In the plasma 150, a plurality of radicals and ions such as C, CF _x , F, F ⁺ , and CF _x ⁺ exist, and the radicals and ions move to the fused quartz substrate 210 disposed on the lower electrode 120. The etching proceeds by forming and discharging SiF ₄ , CF ₄ , C0, CO ₂ , COF _2, etc. by chemical reaction and physical collision.

In the present embodiment, the core layer 220 of the region not covered with the pattern 235 of the mask layer 230 is etched using the inductively coupled plasma etching apparatus 100 to form a rectangular optical waveguide pattern 225. The process sequence and conditions according to an example are demonstrated.

A conductive layer 240 is deposited on the lower surface, and the fused quartz substrate 210 having the pattern 235 of the mask layer 230 formed on the upper surface is mounted on the lower electrode 120 inside the chamber 110. In this case, the molten quartz is characterized in that the inductively coupled plasma etching apparatus 100 for etching by supplying helium gas to the lower surface of the molten quartz-based substrate 210 through the lower electrode 120 on which the molten quartz-based substrate 210 is mounted. When the electrical conductivity of the system substrate 210 is low, the production yield is lowered due to lowering of etching characteristics such as etching speed and etching uniformity, and in some cases, the molten quartz substrate 210 itself must be discarded. In detail, the molten quartz substrate 210 according to the present embodiment is a molten quartz substrate having low electrical conductivity. As a result, the conductive layer 240 having a high electrical conductivity in contact with the lower electrode 120 is deposited on the bottom surface of the molten quartz substrate 210 to increase the electrical conductivity of the molten quartz substrate 210. Therefore, the etching characteristics such as the etching uniformity and the etching rate of the molten quartz substrate 210 is improved to improve the yield. In addition, in the case of the polymer removal process described above, the uniformity of the oxygen ion impact energy is ensured due to the conductive layer 240, and thus, the polymer may be removed even in a narrow region of 1 micron or less, so that the cover layer 250 to be described later ) Defects such as pores generated during formation can be further eliminated. In addition, even without a separate Asher equipment, it is possible to collectively work in the inductively coupled plasma etching apparatus 100 immediately before the core layer 220 etching process.

The etching process of the pattern 225 of the optical waveguide is performed using only C ₂ F ₆ gas. At this time, the reaction gas C ₂ F ₆ is supplied to the chamber 110 under a flow condition of approximately 10-60 sccm (Square Cubic cm), and the internal pressure of the chamber is maintained at 3mTorr-25mTorr. In addition, RF power of 50-150W is applied to the lower electrode 120, and 100W-1500W is applied to the inductively coupled plasma coil 130. In the present embodiment, by etching the core layer 220 using the C ₂ F ₆ of the lower F / C ratio than the CF ₄ as an etching reaction gas, the core without using additional gases such as CHF ₃ , H _2, etc. Etching of the layer: The mask layer 230 can be etched by the pattern 225 of the optical waveguide having an etching depth of 8-10 microns or more even by forming a sufficiently thin mask layer 230 with a high selectivity ratio of 70: 1 or more. .

In step S160, after etching the pattern 225 of the optical waveguide, the pattern 235 of the metal layer 240 and the mask layer 230 is removed. Before the pattern 235 of the mask layer 230 is removed, an ashing process using oxygen plasma is performed again. This is performed by removing the CC and CFx-based polymer layers generated after forming the pattern 225 of the optical waveguide. To improve the quality.

In step S170, similar to the manufacturing process of the core layer 220, silica fine particles are sequentially deposited to deposit a cladding layer 250, and a densification heat treatment is performed to complete the optical waveguide.

As described above, a method of manufacturing an optical waveguide in a molten quartz substrate using the inductively coupled plasma etching apparatus according to the present invention uses a molten quartz system having a similar thermal expansion coefficient as a core layer and a cover layer as the substrate. Thus, there is no need to deposit a buffer layer or bottom cover layer between the molten quartz substrate and the core layer. Therefore, the manufacturing process is shortened and the productivity is improved.

In addition, by forming a conductive layer made of metal in contact with the lower electrode provided in the inductively coupled plasma etching apparatus on the lower surface of the molten quartz substrate, the core layer is etched to form a pattern of an optical waveguide, thereby forming an etching uniformity and etching. Speed is improved, yield and productivity are improved.

In addition, when the core layer is etched using an inductively coupled plasma etching device to form an optical waveguide pattern, a metal deposited on the core layer is formed by using a C ₂ F ₆ single gas having a low F / C ratio as an etching reaction gas. Even when the mask layer is sufficiently thin, it is possible to form a waveguide having a good side roughness and verticality of an etching depth of 10 microns or more. Therefore, manufacturing time is shortened and productivity is further improved.

In addition, before and after performing the main etching process using C ₂ F ₆ , it is possible to remove the photosensitizer, CC and CFx-based polymer layers that may remain without being completely removed in the inductively coupled plasma etching apparatus. After the pattern of the optical waveguide is formed, defects such as pores that may occur when the cover layer is formed may be removed in advance. Therefore, the yield is further increased.

Although the present invention has been described above according to preferred embodiments of the present invention, those skilled in the art will be able to make various modifications, changes, and additions within the spirit and scope of the present invention, and such modifications, changes, and additions may be made to the following claims. Should be seen as belonging to.

Claims (5)

Depositing a core layer by sequentially depositing silica fine particles on an upper surface of the fused quartz substrate; Depositing a metal etching mask layer on the core layer; Depositing a conductive layer made of metal on a lower surface of the fused quartz substrate; Coating and transferring a photoresist on the mask layer to form a pattern corresponding to the pattern of the optical waveguide to be formed, and then removing the mask layer on which the pattern of the mask layer is not formed; The conductive layer is formed on the lower electrode of the inductively coupled plasma etching apparatus having an inductively coupled plasma coil positioned on the upper side of the lower electrode so as to face the lower electrode and the lower electrode, and only C ₂ F ₆ is injected into the reaction gas. Forming a pattern of an optical waveguide by mounting a lower surface of the fused quartz substrate having a pattern of a mask layer formed on the upper surface of the substrate and etching the core layer in an area not covered with the pattern of the mask layer; Removing the mask layer and the conductive layer; And depositing a cover layer covering the pattern of the optical waveguide. The method of claim 1, The thermal expansion coefficient of the fused quartz-based substrate, the core layer and the cover layer is 0.35 to 1.0 x 10 ^-6 / ℃, characterized in that the optical waveguide manufacturing method. The method of claim 1, The metal etching mask layer is made of Cr, The Cr mask layer and the conductive layer are deposited by applying RF power of 600W-1000W from the sputtering apparatus. The method of claim 1, Separate RF power is applied to the lower electrode and the inductively coupled plasma coil, and the inductively coupled plasma etching apparatus maintains a pressure of 3-25 mTorr to generate a high density plasma of 10 ¹² or more. Manufacturing method. The method of claim 1, Photoresist generated when the pattern of the mask layer is formed by injecting oxygen into the inductively coupled plasma etching apparatus before and after forming the pattern of the optical waveguide in the inductively coupled plasma etching apparatus The method of manufacturing an optical waveguide, characterized in that for performing the step of removing the CC and CFx-based polymer layer generated when the pattern of the optical waveguide.