JPS627002A - Production of phase shifting diffraction grating - Google Patents

Abstract

PURPOSE:To invert recessed and projected phases periodically by forming the 1st diffraction grating on the 1st area of a substrate, covering the whole surface including the 1st diffraction grating with a negative type resist film, exposing the whole surface of the resist film so that the phase of a two light flux interference pattern is shifted by a prescribed value from the phase of the 1st grating, and then forming the 2nd grating on the 2nd area. CONSTITUTION:The negative type photoresist film 8 is applied to the whole surfaces of the 1st and 2nd areas on the surface of the substrate, normal exposure is applied to the 2nd area and two light flux interference exposure is applied to the whole surfaces by laser light 3. The exposed part is developed to generate the 1st diffraction grating 9 having a prescribed roughness, and while embedding the recessed part, the resist film 8 is applied to the whole surface again to fill the grating 9 part with the resist film 8 and the whole surface is exposed. Then, the 2nd area is covered with a metal mask, the two light flux interference exposure is applied to the whole surface again by laser light 3 and the reflected light 11 of an interference pattern obtained from the 1st area is detected by a photodetector 10. After removing the mask 6 and the detector 10, the normal 2nd grating is formed on the 2nd area adjacent to the 1st area.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a method of manufacturing a diffraction grating consisting of periodic irregularities using two-beam interference exposure. The present invention relates to a method of manufacturing a diffraction grating having a structure in which the phase of the concave and convex portions is shifted.

(Prior art and its problems) Diffraction gratings consisting of periodic concavities and convexities reflect or pass only light of a desired wavelength, so they are used as filters or distributed feedback semiconductor lasers (rDFBs) in the field of optical communications. It is used inside a laser (abbreviated as "laser").

Among them, the DFB laser, which has a diffraction grating in or near the light emitting region, emits light in a single axis mode.
It has been in the spotlight as a light source for optical communications, and various proposals have been made to date. Particularly recently, shifting the phase of the asperities near the center of the diffraction grating has attracted attention as a way to achieve more stable single mode operation. As described above, the oscillation wavelength of the FB laser is determined by the period A of the unevenness of the diffraction grating, and stable operation also depends on the manufacturing precision of the diffraction grating. Therefore,
The manufacturing precision of the diffraction grating influences the characteristics of the DFB laser.

Before describing a conventional manufacturing method of a diffraction grating having a structure in which the phase of the asperities is shifted, a method for manufacturing a diffraction grating having a structure in which the phase of the asperities is not shifted will be described first.

FIG. 1 is a diagram showing the principle of manufacturing a uniform diffraction grating by a conventional two-beam interference exposure method. Wavelength λ. For example, He-C
d The laser beam 3 is split into two by a half mirror 4, each separated beam 3 is reflected by a mirror 5, and the combined wave of the separated beams 3 is coated on a substrate 1 with, for example, a positive type photoresist film, as shown in the figure. A diffraction grating can be formed by exposing a crystal surface coated with 2 to an interference pattern generated by irradiation, developing and etching. Here, the period Δ of the unevenness can be obtained by the following equation, where α is the incident angle of the laser beam 3.

On the other hand, as a method for manufacturing a diffraction grating having a structure in which the phase of the diffraction grating is shifted at the center of the laser, there is electron beam scanning exposure using computer control. This method is
This method exposes parts corresponding to the grooves of a diffraction grating by sequentially scanning and irradiating them with an electron beam, but it can be applied when the period of the diffraction grating is large, but the period of the unevenness is affected by the light in the crystal. If the period of the first-order diffraction grating, which is half of the wavelength λ, is small (approximately 2,000 people), the resolution reaches its limit and manufacturing becomes substantially difficult.

Furthermore, since the electron beam exposure method involves individual sequential scanning, it takes a considerable amount of time to scan the entire surface of the diffraction grating pattern, making it difficult to apply this method to mass production processes.

Next, problems in manufacturing a diffraction grating having a structure in which the phases of concavities and convexities are mutually shifted in adjacent regions using two-beam interference exposure will be described.

Figure 2 is a schematic diagram of the case where a diffraction grating with a phase inversion, that is, a 180° phase shift, is manufactured by the two-beam interference exposure described above, and regions A and B are exposed separately using a metal mask. This is the way to do it. The same side is area A
2 shows a case where periodic unevenness is manufactured, and at this time, region B is covered with a metal mask 6 having a thickness t (approximately 50 μm).

Note that there is usually a gap d (about several μII+) on the photoresist film 2.
) has been established. The interference pattern shows the area closest to area B, but as is clear from the figure, laser beam 3
Due to the thickness of the metal mask 6, there is a portion that is not irradiated, that is, a region C where no unevenness is formed. Similarly, even if a metal mask 6 is applied to area A and two-beam interference exposure is performed to area B, an area C is created in which no unevenness is produced.
As a whole, no unevenness is formed over twice the area C.

For example, if the period of the unevenness of the diffraction grating is 2400,
e - Wavelength λ of the Cd laser. If there are 3,250 people, then the incident angle α is, and if the mask thickness t is 50μI and the gap is d, then the area C where no periodic unevenness is produced is C= (t+
d) tanazt-tanct = 47 (-μm
).

Therefore, by two times of two-beam interference exposure, the area twice the area C where no unevenness is formed becomes 94 [μI], and since the total length of the light-emitting region is usually about several hundred Cμm], D
The operating current of the FB laser increases, and single wavelength operation also becomes unstable. As a solution to this problem, a slight improvement can be made by reducing the thickness t of the metal mask 6 or by providing a slope at the upper surface edge of the inner end of the metal mask 6, but this still results in a region C where no unevenness is formed.

Furthermore, in FIGS. 3 and 4, A
This is a conventional method in which the eff region and the B eN region are exposed at the same time by two-beam interference exposure, but the flatness of the phase shifter plate 7.

Not only is the uniformity of the thickness a problem, but it is also necessary to adjust the step of the phase shifter plate 7 according to the desired period of the diffraction grating. Here, in FIG. 3, the photoresist film 2
Due to the gap d between the phase shifter plate 7 and the phase shifter plate 7, and due to the diffraction effect of the light from the phase shifter plate 7 until it reaches the substrate 1 in the example of FIG. no longer formed.

As described above, it has been difficult to accurately manufacture a diffraction grating having a structure in which the phase of periodic irregularities is shifted using a metal mask or a phase shifter as in the past.

(Objects and Features of the Invention) The present invention has been made to solve the above-mentioned drawbacks of the prior art. It is an object of the present invention to provide a method for manufacturing a diffraction grating that can realize a diffraction grating having a structure in which the phases of concave and convex portions are inverted.

The feature of the present invention is that a first diffraction grating is first formed in a first region on a substrate, a negative type photoresist film is applied on the substrate, and after exposing the first region, the required two-beam interference The position of the substrate is adjusted so that the phase of the pattern has a predetermined phase shift with respect to the first diffraction grating, exposure is performed using the adjusted interference pattern, and a second diffraction grating is applied to the second area.
The purpose is to form a diffraction grating.

(Structure and operation of the invention) The present invention will be described in detail below using the drawings.

FIG. 5 shows an embodiment of the present invention. - (First step) (a) Negative type photoresist film 8 on the substrate 1
Apply.

(Mountain) Expose the second area using a normal mask exposure method.

(C) Two-beam interference exposure is performed using the laser beam 3 over the entire surface of the photoresist film 8.

(d) By performing development, a diffraction grating of the photoresist film 8 is formed in the first region.

(11) Add a diffraction grating pattern to the substrate 1 by etching etc.
to form.

(f) The photoresist film 8 in the first region and the second region is removed to form the first diffraction grating 9 in the first region.

(Second Step) (Phantom) A negative type photoresist film 8 is applied onto the substrate on which the first diffraction grating 9 is formed.

(h) The photoresist film 8 in the first region is exposed using a normal mask exposure method.

(Using a metal mask 6 or the like, a part of the first diffraction grating 9 is subjected to two-beam interference exposure. At this time, an interference pattern is formed on the diffraction grating, and the interference pattern is between the concave part of the concave and convex part and the convex part of the convex part. The intensity of the reflected light 11 (dashed line) of the interference pattern differs greatly depending on which part of the interference pattern it matches.Therefore, the reflected light is monitored by the photodetector 10, and the intensity of the reflected light 11 (broken line) between the interference pattern and the first diffraction grating 9 is monitored. The position of the substrate 1 is finely adjusted using a piezo element or the like so that a predetermined phase shift occurs in the interference pattern. Normally, the reflected light is strongest when the bright part of the interference pattern matches the convex part. The effect is remarkable if the reflectance is high, such as.

For example, when manufacturing a diffraction grating in which the concave and convex portions are reversed in the first and second regions as described below, the bright portions of the interference pattern may match the concave portions.

(Third step) 0) Remove metal mask, etc. Two-beam interference exposure 3 is performed in the state adjusted in step (1) above.

(g)) By developing, a diffraction grating of the photoresist film 8 is formed.

V) Etching the substrate 1 by photolithography using the diffraction grating of the photoresist film 8 as a mask.

(ff+1 By removing the photoresist film 8, the second diffraction grating 12 is formed in the second region. In this case, between the first diffraction grating 9 and the second diffraction grating 12, A 180° phase shift has occurred.

(Effect of the invention) As is clear from the above steps, in the present invention, the first and second
A diffraction grating having an arbitrary phase shift in the region can be easily manufactured, and the conventional drawback that no unevenness is formed at the boundary between the first region and the second region can be solved. Therefore, it can be applied to a DFB laser etc. that is stable and has good characteristics, and its effects are extremely large. Further, although detailed specific explanations regarding mask exposure, post-exposure development steps, photoresist coating, etc. are omitted, ordinary photolithography techniques are used.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the principle of conventional two-beam interference exposure;
3 and 4 are schematic diagrams of conventional examples of manufacturing a phase shift diffraction grating by two-beam interference exposure, and FIG. 5 is a sectional view for explaining the manufacturing process of the phase shift diffraction grating according to the present invention. be. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Positive photoresist film, 3...He-Cd laser beam, 4...Half mirror-15...Mirror, 6...Metal mask, 7
...Phase shifter plate, 8...Negative type photoresist film, 9...First diffraction grating, 10...
Photodetector, 11...Reflected light of interference pattern, 12.
...Second diffraction grating. Voice 10 Y7JI520, 〒3 Ward〒40 Voice 50 -FtsuA Arrogant I/-1-Growing 2-Natsutsuki Judo 5 [%5Kuni]

Claims (1)

[Claims] A first step of forming a first diffraction grating in a first region on a substrate, applying a negative type photoresist film on the substrate, exposing the photoresist film in the first region, and then producing the required two luminous fluxes. a second step of adjusting the relative position of the substrate and the two-beam interference pattern so that the interference pattern has a predetermined phase shift with respect to the first diffraction grating; and the adjusted two-beam interference pattern. The photoresist film on at least a second area other than the first area on the substrate is exposed to light, and the second area is developed and etched. and a third step of forming a second diffraction grating that is phase-shifted with respect to the diffraction grating.