JPH05157924A - Production of microoptical element - Google Patents

Abstract

PURPOSE:To obtain the process for production of the microoptical element which can make treatment at a low temp. to prevent the thermal deformation of a silicon substrate and can execute the treatment with inexpensive equipment and materials at the time of forming the microoptical element, such as optical waveguide, on the substrate. CONSTITUTION:The silicon substrate 1 is immersed into an H2SiF6 soln. produced by adding SiO2 powder to a hydrofluoric acid soln. and stirring the soln. As a result, an SiO2 film 4 having a uniform thickness is formed on the surface of the substrate 1. The surface of the film 4 is coated with a masking film 5 exclusive of the aperture of a prescribed optical element pattern by photolithography method, etc., and thereafter, the substrate is put into water and is heated to 95 deg.C. As a result, fluorine is eluted from the inside of the SiO2 film in the masking apertures and the refractive index of this part 7 is made higher than the refractive index in the peripheral part. This high refractive index region 7 functions as an optical element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for forming a micro optical element such as an optical waveguide on a silicon substrate.

[0002]

2. Description of the Related Art In recent years, with the advancement of optical communication systems,
An important device is a branching / combining element for branching / combining light, an optical switch, an optical demultiplexer / multiplexer for dividing or multiplexing an optical signal having a plurality of wavelengths for each wavelength.

Conventionally, such functions have been carried out by lenses, prisms, mirrors, filters, etc., but there is a demand for thinner and thinner devices, labor saving of combining individual parts, and input / output fibers. From the viewpoint of facilitating connection with the optical waveguide, attention has been paid to the optical waveguide.

Optical waveguides are usually obtained by forming high-refractive-index linear paths on a low-refractive-index transparent substrate. As a transparent substrate, generally, an amorphous material typified by quartz glass, aluminosilicate glass, or lithium niobate (LiN) is used.
Oxide dielectric crystals such as bO ₃ ), sapphire (Al ₂ O ₃ ) and quartz (SiO ₂ ) have been used.

Although not a light-transmitting material, in recent years, a semiconductor crystal substrate made of gallium arsenide (GaAs), indium phosphide (InP), silicon (Si) or the like has a transparent buffer layer having a low refractive index on its surface. After the formation, a structure in which a core layer having a high refractive index is laminated on the surface has come to be used for forming a waveguide.

Among them, the silica glass (SiO ₂ ) waveguide formed on a Si wafer (single crystal) is suitable for producing a functional element because it is easy to obtain a low loss one, and
Since silica-based optical fibers have almost the same refractive index, there are advantages such as minimizing the loss associated with splicing.

By the way, in many cases, as a method of forming a SiO ₂ waveguide on a Si wafer, after the Si wafer is thermally oxidized at a temperature of 900 ° C. or higher to be laminated on a buffer layer having a low refractive index, A method has been adopted in which a waveguide is left in a slit shape by selective etching by a dry etching method.

Recently, as a method of forming a waveguide of higher purity and low loss, SiCl ₄ , T on a Si wafer is formed.
A method of depositing SiO ₂ fine particles by a flame hydrolysis reaction of a raw material gas containing iCl ₄ , BCl ₃ or the like as a main component has also been tried. In this method, the deposited SiO ₂
The fine particle layer is made transparent by heating it to a high temperature of about 1200 ° C., but it can be made into a buffer layer and a core layer separately by selecting the source gas composition at the time of depositing the fine particle layer.

As described above, in the case of a Si wafer substrate, a buffer layer having a low refractive index is first formed, and then a waveguide is formed by laminating a core layer having a high refractive index. Generally, the buffer layer and the core layer are formed. If the refractive index difference between the two is 0.7% or more, the optical waveguide can be formed.

[0010]

However, in the method of vitrification by thermal oxidation of Si wafer or heating of SiO ₂ fine particle layer, in addition to the problem that the substrate is easily warped due to high temperature, miniaturization and integration of optical parts In response to the above requirement, the problem of using more expensive equipment and requiring more strict control cannot be ignored.

Further, in addition to the optical waveguide, a minute optical element such as a microlens array in which a large number of extremely minute lenses are two-dimensionally arranged is required.

[0012]

In the method of the present invention for solving the above problems, first, a Si wafer is brought into contact with an aqueous solution of hydrosilicofluoric acid (H ₂ SiF ₆ ) containing silicon dioxide in a supersaturated state. Allows the low refractive index S on the surface of the Si wafer.
An iO ₂ film is formed.

In this case, H ₂ containing silicon dioxide in supersaturation
An aqueous solution of SiF _{6 is} a solution obtained by saturating an aqueous solution of H ₂ SiF ₆ with silicon dioxide, boric acid (H ₃ BO ₃ ), aqueous ammonia (NH ₄ OH), a metal halide typified by aluminum chloride (AlCl ₃ ), or It is obtained by adding a metal having a higher ionization tendency than hydrogen. Alternatively,
It can also be obtained by saturating silicon dioxide in an aqueous solution of H ₂ SiF ₆ at a temperature of 15 ° C. or lower and then heating the temperature of this solution to 30 ° C. or higher (temperature difference method).

By any of these methods, the concentration of the H ₂ SiF ₆ aqueous solution containing silicon dioxide in the supersaturation can be determined by
Considering the denseness of the O ₂ film, a concentration of 1.5 mol / liter or more is suitable. The SiO ₂ film thus obtained has a refractive index of 1.42 to 1.43.

Then, in order to form an optical element such as an optical waveguide, a water-insoluble material is applied on the SiO ₂ film on the surface of the substrate, and well-known photolithography or the like is used.
An opening having a desired optical element pattern is formed and the outside of the element region is masked.

Then, the substrate is dipped in water to partially remove the fluorine contained in the SiO ₂ thin film, so that the SiO ₂ film exposed in the masking opening has a refractive index of 1.43 or more. To be enhanced.

[0017]

Function: Si whose refractive index is increased by the above fluorine removal treatment
The area outside the masking of the O ₂ film functions as an optical element such as an optical waveguide or a lens.

[0018]

EXAMPLES First, 1000 g of a commercially available hydrofluoric acid solution (having a content of 47% for the semiconductor industry) was added to SiO ₂ powder (having a purity of 99.999% for optoelectronics).
340 g, and the stirring speed is set to about 800 rpm and the mixture is stirred for 48 hours.

After stirring, in order to examine the saturation of the solution, a Si substrate already having an oxide film is dipped in this solution for 1 minute to test whether SiO ₂ is etched.

An etch test was conducted, and if SiO ₂ was not etched, this solution was regarded as SiO ₂ saturated, SiO ₂ particles were filtered from the solution, and then diluted with ultrapure water to obtain a H ₂ SiF ₆ solution having a predetermined concentration. create.

It should be noted here that when diluting the solution, H ₂ Si is slowly added while stirring ultrapure water.
The three points are that F ₆ is added, that stirring is continued for about 30 minutes after the completion of dilution, and that the temperature of the solution must be maintained at about room temperature. If the temperature rises at this time, the equilibrium changes, and before the temperature is raised or H ₃ BO _{3 is} added, SiO _{2 is} added.
₂ may be precipitated.

When H ₃ BO ₃ is added, 10 out of the ultrapure water added when the H ₂ SiF ₆ solution is diluted with ultrapure water are added.
After leaving about cc, a predetermined amount of H ₃ BO ₃ is dissolved therein, and then slowly added to the H ₂ SiF ₆ solution.

When a H ₂ SiF ₆ solution having a predetermined concentration is prepared,
As shown in FIG. 1, a silicon substrate 1 (p-type, 100 surface resistivity 3 to 5 Ω · c) that has been previously subjected to organic cleaning and hydrofluoric acid treatment.
m) is H ₂ Si containing SiO ₂ in supersaturation in the container ₂
It is immersed in the F ₆ solution 3 so as to lean against the inner wall of the container 2 with the surface facing downward. By doing this, SiO
₂ Particles do not adhere to the substrate surface and the film is deposited cleanly.
By the above processing, the SiO ₂ film 4 having a film thickness of 1 μm was formed on the silicon substrate 1. Then, the surface of the SiO ₂ film 4 is masked with a film 5 made of a water-insoluble material while leaving the opening 6 of the waveguide pattern, and the mask is placed in water and heated to expose the SiO ₂ film in the opening 6. Fluorine (F) elutes from the portion of No. 4, and the larger the amount of elution, the higher the refractive index of this portion.

In this way, after the underwater heat treatment, the region 7 having a relatively higher refractive index than the surroundings, that is, the optical waveguide is formed in the SiO ₂ film immediately below the masking opening 6.

The following experiment was conducted in order to investigate the relationship between the elution amount of fluorine from the SiO ₂ film 4 and the change in the refractive index. The Si wafer having the SiO ₂ film 4 of uniform thickness formed on the surface thereof under the above-mentioned processing conditions was cut into 8 pieces, and 7 pieces of these were soaked in pure water and heated to 95 ° C.
After treatment for 4, 6, 8, 24, 30 and 48 hours, the refractive index of the SiO ₂ film was measured by an ellipsometer. The results are shown in FIGS. 2, 3 and 4.

As is clear from these figures, the SiO ₂ film obtained by the method of the present invention has a refractive index increasing with the elution amount of fluorine, and can be treated in pure water at 95 ° C. for 24 hours or more. The refractive index is 1.435 or more, and a difference in refractive index of 0.7% or more is obtained, and the condition as an optical waveguide is provided.

Although the optical waveguide has been described above as an example, the method of the present invention is not limited to this and can be applied to the manufacture of various micro optical elements. For example, as shown in FIG. 5, a high refractive index portion 7 is formed in the SiO ₂ film 4 by the method of the present invention.
A microlens array is obtained by forming a large number of high density arrays.

That is, through the masking opening 6, Si
Since the phenomenon that fluorine is eluted from the O ₂ film 4 isotropically occurs, if the size of the opening 6 is sufficiently small, the outer edge shape of the high refractive index region 7 becomes substantially hemispherical and functions as a spherical lens.

[0029]

Since the method of the present invention can be carried out at a temperature as low as 100 ° C. or lower, thermal deformation such as warpage does not occur on the substrate, and therefore the problem of element displacement due to substrate deformation can be avoided. A highly accurate micro optical element can be obtained.

Furthermore, there is a production advantage that the equipment and raw materials for processing are very inexpensive.

[Brief description of drawings]

FIG. 1 is a sectional view showing stepwise a method of forming an optical waveguide using the method of the present invention.

FIG. 2 is a graph showing an example of the relationship between the processing time and the change in the refractive index of the Si film by the method of the present invention.

FIG. 3 is a graph showing an example of the relationship between the processing time and the elution amount of fluorine from the Si film by the method of the present invention.

FIG. 4 is a graph showing an example of the relationship between the elution amount of fluorine from the Si film and the refractive index according to the method of the present invention.

FIG. 5 is a cross-sectional view showing a microlens array as another example of the micro optical element obtained by the method of the present invention.

[Explanation of symbols]

 1 Silicon Substrate 2 Container 3 Hydrofluoric Acid Aqueous Solution Containing Silicon Dioxide in Supersaturation 4 Si Film 5 Masking Film 6 Opening 7 High Refractive Index Region (Micro Optical Element)

Claims (1)

[Claims] 1. A silicon dioxide thin film is formed on a silicon substrate by bringing an aqueous solution of hydrofluoric acid containing silicon dioxide into supersaturation into contact with the silicon substrate, and then a water-insoluble material is formed on the surface of the thin film. To form a mask while leaving a desired optical element pattern portion, and then to immerse this substrate in water to partially remove the fluorine contained in the film of the masking opening portion to increase the refractive index, A method of manufacturing a micro optical element, which comprises forming a micro optical element.