JPH0435726B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a manufacturing method for producing a diffraction grating with a fine period (~1 μm or less) on the surface of a transparent inorganic material such as SiO ₂ or glass using a holographic exposure method.

A brief explanation of an example of a conventional manufacturing method is as follows. First, as shown in FIG. 1, a photoresist 3 is applied to the surface of an oxide film 2 formed on a semiconductor substrate 1. As shown in FIG. Next, the photoresist 3 is periodically exposed using a holographic interference exposure method using two laser beams, so-called interference fringes, and developed to produce a photoresist lattice mask. Next, ion processing, plasma processing,
The surface of the oxide film is processed by chemical etching, etc., and finally the photoresist mask is removed to create a diffraction grating on the surface of the oxide film.

At this time, the exposure laser light is reflected at the boundaries of each layer, but since the reflection is particularly strong at the interface 4 between the semiconductor substrate 1 and the oxide film 2, interference between the incident light and the reflected light causes The light intensity distribution of the interference fringes occurs in the thickness direction of the oxide film 2 and the photoresist 3. In the light intensity distribution of this interference fringe, the interface 4 is almost a node,
The pitch is λ/2nicosφ, and the intensity ratio between the node and the antinode is 1-2｜ρ｜/1+｜ρ｜ ² /1+2｜ρ｜/1+｜ρ
｜ ² . Here, λ is the wavelength of the exposure laser, ni is the refractive index of the oxide film and photoresist, φ is the incident angle of the laser beam at the interface 4, and ρ is the reflectance at the interface 4. For example, for SiO ₂ formed on a Si surface, when the exposure laser wavelength λ = 4416 Å and the incident angle φ = 60°C, |ρ| 0.53, the pitch is approximately 2940 Å, and the intensity ratio is
It becomes 0.097. That is, near the nodes of the interference fringes, the light intensity is less than 10%, which tends to result in underexposure. In the manufacturing process of this diffraction grating, the exposure condition at the interface 5 between the photoresist layer 3 and the oxide film 2, which is the surface to be processed, is extremely important. Due to these interference fringes, the light intensity within the photoresist layer 3 changes in the thickness direction, but this state largely depends on the thickness of the oxide film 2, and the thickness of the oxide film varies within the plane. As a result, significant unevenness occurs in the exposure state at the boundary surface 5. This has been a major problem in the conventional method, degrading the quality of the diffraction grating.

The present invention is intended to solve these conventional problems. In order to prevent the exposure state of the photoresist from being significantly influenced by the thickness of the oxide film, the following method is used.

As shown in FIG. 2, a metal film 6 is first formed on the surface of an inorganic material such as SiO ₂ (oxide film) 2 formed on a Si substrate 1. As shown in FIG. Next, a photoresist is applied thereon, and then exposed and developed in the same manner as in the conventional method. Although a glass material or the like can be used directly as a substrate to which the present invention can be applied, here we will use a substrate with an SiO ₂ film formed on a Si substrate, which is useful for forming active devices in parallel. Give an example. In this way, since the exposure laser beam is reflected at the interface 7 between the metal film 6 and the photoresist 3, the position of the interference fringe node is fixed at the interface 7, and therefore the light inside the photoresist layer 3 is The distribution can be made the same at any position within the boundary surface serving as the surface to be processed, making it possible to achieve uniform exposure. For the metal film 6, Cr, Al, Ni, Pt, W, Mo, Au, Ag, etc. can be used. The thickness is generally
Select in the range of 0.01 to 0.5 μm. Post-process processing is
Either a method of processing the metal film and the oxide film sequentially (chemical etching, plasma etching) or a method of processing them simultaneously (ion processing) is possible, and the remaining photoresist and metal film are removed at the end.

In the example, Cr was vacuum-deposited to a thickness of 200 Å for the metal film 6, and exposed to photoresist using a well-known method.
After development, ion processing was performed. The processing conditions are, for example, an accelerating voltage of 500 V, a current density of 100 μA/cm ² , and a processing time of about 10 min. After ion processing, the photoresist and Cr deposited film were removed in this order to form a diffraction grating on the SiO ₂ film on Si.

Note that an aqueous ceric ammonium quenching solution was used to remove the Cr film. A uniform diffraction grating can be manufactured over almost the entire surface of the wafer with a high yield.

Furthermore, by using this metal film 6, it can also be used as a mask during reactive plasma and etching. This makes it possible to form high quality holographic gratings on glass substrates.

Figure 3 shows the process. 10 is an inorganic material such as a glass substrate on which the grating is to be formed. A metal film 6 of Al, Au, etc. is vacuum deposited to a thickness of 500 to 3000 Å. Photoresist 3 (Shipley
AZ-1350) to a thickness of 800 to 1000 Å. By irradiation with laser light 14 and development treatment,
The photoresist becomes grating-like (15 in FIG. 3b). During exposure, the laser beam 14
Since the light is reflected by the metal vapor deposited film 6, the light reflected from the bottom surface of the glass substrate does not cause a ghost on the photoresist.

Using the formed photoresist grating as a mask, ion processing (acceleration voltage 500V, current density 100μA/cm ² , time 5min) is performed to remove the exposed portion of the metal vapor deposited film. Figure 3c
The state will be as follows. Since Al, Au, etc. have an etching rate about one order of magnitude higher than that of photoresist, the photoresist grating pattern is transferred to the metal film grating 16 without deterioration. Next, using this metal film as a mask, reactive plasma etching is performed. Since reactive plasma etching has directivity in etching, the glass substrate after etching becomes a high quality grating 17 having a rectangular cross section as shown in FIG. 3d.
After etching, the metal film is removed to complete the grating 17 as shown in FIG. 3e. In addition,
In the case of Al as an etching solution for metal film removal,
Sodium hydroxide solution, and in the case of Au, iodine-based etching solution are recommended.

Using this method, a glass film (Corning, Inc., 7059) is deposited on top of the SiO ₂ oxide film by sputtering to create an asymmetric optical waveguide. Next, a grating with a pitch of 3000 Å and a depth of 1500 Å was created on the surface of the glass. Compared to gratings created using conventional methods, the diffraction efficiency was improved by one order of magnitude.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram for explaining the conventional method of manufacturing a diffraction grating, Figure 2 is an explanatory diagram for explaining the manufacturing method of the present invention, and Figure 3 is a process for manufacturing a diffraction grating on a glass substrate. FIG. DESCRIPTION OF SYMBOLS 1... Semiconductor substrate, 2... Oxide film, 3... Photoresist, 6... Metal film, 10... Inorganic material substrate.

Claims (1)

[Scope of Claims] 1. A method for producing a diffraction grating in which a diffraction grating with a pitch of 1 μm or less is formed on an inorganic material transparent to the laser beam by holographic exposure using laser beam interference, wherein the method comprises: A process of forming a metal thin film on a workpiece, a process of forming a photoresist film on this metal thin film, a process of exposing this photoresist film to the above-mentioned holographic light, and forming a diffraction grating mask on this photoresist film, a process of forming a diffraction grating on this photoresist film. a step of etching the metal thin film using a shaped mask; and a step of etching the workpiece using at least the metal thin film as a mask, and a film of interference fringes generated in the photoresist film during the holographic exposure. A method for producing a diffraction grating, characterized in that nodes of the light intensity distribution in the thickness direction are fixed at an interface between the metal thin film and the photoresist film. 2. The method of manufacturing a diffraction grating according to claim 1, further comprising the step of removing the metal thin film after the etching step of the workpiece. 3. The method of manufacturing a diffraction grating according to claim 2, wherein the diffraction grating is a component of an optical waveguide.