JPH02251904A - Diffraction grating and production thereof - Google Patents

Abstract

PURPOSE:To obtain high diffraction efficiency in a wide wavelength range by continuously changing the inclination angle (Blaze angle) of the side faces of the grating grooves having a sawtooth-shaped section in the direction intersecting with the grating grooves. CONSTITUTION:The diffraction grating patterns of a photoresist is formed on a substrate surface and are irradiated with an ion beam from a diagonal direction. Etching is thereby progressed as shown by (a) to (c) and the grating grooves having the sawtooth-shaped section are formed. The point or the line irradiated with the ion beam on the substrate moves and the incident angle of the ion beam on the substrate changes when the grating substrate is rotated. The diffraction grating with which the inclination angle of the groove side faces changes continuously is then obtd. The conditions of a Blaze diffraction is generated at any point of the grating surface at an arbitrary wavelength when such diffraction grating is used. The high diffraction efficiency in the range before and after this point is thus obtd. and the area part of the high diffraction efficiency moves on the grating surface in synchronization with the wavelength scanning of a spectroscope. The diffraction efficiency high in average over the wide wavelength range is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a diffraction element such as a diffraction grating or Fresnel zone plate in which the blaze wavelength varies depending on the location, and a method for manufacturing the same.

(Prior art) With an echelette-type reflection diffraction grating, when the direction of incidence of incident light and the direction of detection of diffracted light are determined, blazed diffraction is performed at one specific wavelength, and normally the wavelength range that is based on this blazed wavelength is spectrally analyzed. This is the measurement range of the instrument. However, the average diffraction efficiency in this range is low as indicated by the dotted line 1 in FIG. Furthermore, the wavelength range that can be covered by one diffraction grating is limited, and in order to obtain a spectrometer that can cover a wider wavelength range, it is necessary to prepare and switch between multiple diffraction gratings with different blaze wavelengths. The diffraction grating switching mechanism is complex and requires high precision, which increases the price of the spectrometer, and requires troublesome data processing to match measurement results before and after the diffraction grating switching wavelength.

For this reason, attempts have been made to expand the wavelength range that can be covered by one diffraction grating by using a diffraction grating in which the blaze wavelength differs depending on the location of the grating surface of the diffraction grating.

The conventional attempts mentioned above divide the lattice plane into several zones,
Each zone uses a diffraction grating that has a fixed blaze wavelength, which increases the average diffraction efficiency in the wavelength range and eliminates the need to switch diffraction gratings.
Wavelengths in the middle of adjacent blaze wavelengths have lower diffraction efficiency than light at the blaze wavelengths on both sides (this lowers the average diffraction efficiency over the entire target wavelength range).

Furthermore, it is extremely troublesome to create such a grid in which the blaze angles differ in stages.

(Problems to be Solved by the Invention) An object of the present invention is to provide a diffraction grating that eliminates changes in diffraction efficiency due to wavelength and provides high diffraction efficiency on average over a wide wavelength range.

(Means for Solving the Problem) In a diffraction grating having grating grooves with a sawtooth cross section, the inclination angle (blaze angle) of the side surface of the grating grooves is continuously adjusted in the direction intersecting the grating grooves as shown in FIG. Changed. In addition, in order to form such grating grooves with varying inclination angles, an ion etching method is used to form a grating pattern with a resist film on the grating substrate surface, and an ion beam is applied from an oblique direction to the grating substrate where the grating grooves are to be formed. The light was irradiated to one point or a negative line on the substrate surface, and the substrate was swung around an axis distant from the substrate surface.

(Function) In the lattice groove shown in FIG. 5, θ is the inclination angle (blaze angle) of the groove side surface, n is the normal line to the groove side surface, and N is the normal line to the lattice surface. Li is the incident light, Ld is the diffracted light, and the angle φ
is determined by the arrangement of the entrance slit and exit slit of the spectrometer. Since blazed diffraction is diffraction in which the incident light Li and the diffracted light Ld are in a specular reflection relationship on the grating groove surface, the blaze wavelength λ′ is λ = 2 dc, φsinθ·(1). On the other hand, wavelength scanning uses the direction of the lattice surface normal N as the incident light Li and the output light L.
This is done by rotating the angle to the right or left based on the direction that bisects the angle formed by d. Now, when the rotation angle from this reference direction is θ', the wavelength λ of the diffracted light in the Ld direction is expressed as λ=2 dstnφsin θ8. Therefore, when the rotation angle θ of the grating and the inclination angle θ of the side surface of the grating groove are made equal, blazed diffraction will occur in that part of the grating surface. Although it is not possible to make the inclination angle of the side surface of the grating groove variable, for example, if the inclination angle θ gradually changes from one end of the grating surface to the other, the conditions for blazed diffraction will be established somewhere on the grating surface at any wavelength. established,
High diffraction efficiency is obtained in the area in front and behind the lattice plane, and as the spectrometer scans the wavelength, this area with high diffraction efficiency moves on the lattice plane, and the average over a wide wavelength range is As a result, high diffraction efficiency can be obtained.

Next, when manufacturing the above-mentioned diffraction grating using the ion etching method and irradiating the sample surface with an ion beam obliquely, the grooves formed according to the resist forming the grating pattern will have an asymmetrical cross section. When the grating substrate is rotated by an axis located away from the grating substrate, the ion beam irradiation point or line on the substrate moves on the substrate surface, and the ion beam Since the angle of incidence on the substrate changes, a diffraction grating in which the inclination angle of the groove side surface changes continuously can be obtained.

(Example) A photoresist layer is formed on the surface of the substrate on which the diffraction grating is to be manufactured, and a two-beam interference pattern of laser light is exposed and developed.
A diffraction grating pattern of 7 photoresists is formed on the substrate surface. An enlarged view of the substrate surface on which the photoresist diffraction grating pattern has been formed is shown in Figure 2, and when ion etching is performed by irradiating it with an ion beam from an oblique direction, the results shown in A22B and C are shown in the same figure. As the etching progresses, a lattice groove with a serrated cross section is created. Here, the angle ε between the ion beam and the side surface of the groove to be formed is determined in advance by experiment.

FIG. 3 shows an outline of an ion etching apparatus according to an embodiment of the present invention. P is a diffraction grating substrate, which is fixed at right angles to the end of arm A, which is rotatable around the zero point. I is an ion source, and S is a slit, which limits the ion flux to form a thin plate-shaped ion beam. This ion beam B intersects a rotation locus circle C centered on the tip of the arm A, that is, the zero point at the center of the grating substrate, at a point Q (actually, a straight line passing through point Q and perpendicular to the plane of the drawing). The right end of the successful substrate P is the ion beam B.
Suppose that it is located at a position where it will be irradiated. The angle (incident angle) between the substrate surface and the ion beam at this time is defined as i. If arm A is rotated clockwise from this position and the ion beam incident angle is i゛ when the left end of grating substrate P is irradiated with ion beam B, (i'-t) is arm A. is the rotation angle of The change in the ion beam incident angle with respect to the minute rotation angle i of arm A is △i, and when ion etching is performed while arm A is oscillated within the ranges a and b in the figure, the side inclination angle of the grating groove is From the left end to the right end (i゛-ε) to (
i−ε), it decreases linearly. The length and rotation angle of arm A are determined as follows. Assuming that the minor wavelength of the measurement wavelength is λ1 and the longer wavelength is λ2, from the above equation (1), the grating groove side surface inclination angle θ is λ1, and the rotation angle of arm A is (θ2−θ1). The length A of the arm A is given by where X is the width of the grating (the length in the direction perpendicular to the groove). Also, at the position where the end of the grating with the smaller blaze angle is irradiated with the ion beam (when the substrate P is at the solid line position in Figure 1), the distance between the ion beam and the substrate surface is 01+ε
The center of rotation O of the arm A is positioned with respect to the ion beam B so as to form an angle of .

In the case of a concentric lattice pattern recorded by holography, the ion beam B is not shaped like a flat plate as shown in Figure 3, but is shaped like a thin beam to irradiate a minute area centered on one point on the substrate, as shown in Figure 4. A second arm A' is extended at right angles from the end to the arm, and the grating substrate P is rotated around a point W on the second arm A', and the center point of the concentric diffraction grating pattern is aligned with this W. The board P moves as the arm swings.
is rotated around W to give a constant groove side inclination angle to the curved groove.

In the above explanation, the diffraction grating is a planar type, but it is not necessary to change the side inclination angle of the grating groove in a strict functional relationship in the width direction of the grating (in short, it is necessary to change it continuously from θ1 to θ2 and at an average angle of - Since it is sufficient that fθdx is not much different from (θ2-θ1)/X, the apparatus shown in FIG. 3 or 4 can be used as is.

Furthermore, by using the apparatus shown in FIG. 4, even something like a Fresnel zone plate can be made with a blaze angle that varies continuously from the center toward the outer periphery.

(Effect of the invention) The wavelength characteristic of the diffraction efficiency of a diffraction grating with a constant blaze angle is
As shown in FIG. 1, the average efficiency between wavelength ranges λ1 and λ2 is shown by dotted line 1. Since the diffraction grating according to the present invention does not cause blazed diffraction on the entire surface for a single wavelength, the maximum efficiency is lower than that of a grating with a constant blaze angle (although the diffraction efficiency does not change depending on the wavelength, and as shown in Figure 6). As shown, it is possible to obtain high diffraction efficiency over a wide wavelength range as a whole.It is also easier to manufacture than a diffraction grating in which the blaze angle is changed stepwise for each part.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional conceptual diagram of the diffraction grating of the present invention, Fig. 2 is an explanatory diagram of the progress of ion etching, Fig. 3 is an apparatus configuration diagram of one embodiment of the invention, and Fig. 4 is an apparatus configuration of another embodiment. 5 is an explanatory diagram of blaze diffraction, and FIG. 6 is a graph of changes in diffraction efficiency of a diffraction grating depending on wavelength. P... Diffraction grating substrate, A... Rotating arm, ■... Ion source, S... Slit. Agent Patent Attorney Kosuke Agata

Claims (2)

[Claims] (1) In a diffraction element having grating grooves with a sawtooth cross section,
A diffraction grating characterized in that the side inclination angle of the grating grooves is continuously changed in the direction intersecting the grating grooves. (2) In order to form the grating grooves, an ion etching method is used to irradiate the grating substrate with an ion beam from an oblique direction to a point or line on the substrate surface, and the substrate is centered on an axis away from the substrate surface. A method for producing a diffraction grating characterized by making it oscillate.