JPH0461331B2 - - Google Patents

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 (hereinafter referred to as "DFB") in the field of optical communication. It is used inside a laser (abbreviated as "laser").

Among these, DFB lasers, which have a diffraction grating in or near the light emitting region, have attracted attention as a light source for optical communications because they emit light in a single-axis mode.
There have been various proposals in the past. 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. The oscillation wavelength of such a DFB laser is determined by the period Λ 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. For example, a He-Cd laser beam 3 with a wavelength λ ₀ is split into two by a half mirror 4, and each split beam 3 is reflected by a mirror 5.
As shown in the figure, the synthesized wave of the split light 3 is irradiated onto the surface of a crystal coated with, for example, a positive type photoresist film 2 on the substrate 1, and an interference pattern is generated.The diffraction grating is then exposed by the interference pattern that is generated, and then developed and etched. can be formed. Here, the period Λ of the unevenness is determined by Λ=λ ₀ /2sinα (1), 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 exposes the portions corresponding to the grooves of the diffraction grating by sequentially scanning and irradiating them with an electron beam. This method can be applied when the period Λ of the diffraction grating is large, but when the period Λ of the asperities is When the period Λ is small (approximately 2000 Å), such as in a first-order diffraction grating, which is half the wavelength λ of light in the crystal, 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 finish scanning 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 above-mentioned two-beam interference exposure, and A and B
This is a method in which the regions are exposed separately using a metal mask. The figure shows a case where periodic unevenness is manufactured in region A, and at this time, region B has a thickness t (approximately
50 μm) is covered with a metal mask 6.
Note that a distance d (about several μm) is usually provided on the photoresist film 2. The interference pattern is most in area B.
As is clear from the figure, a region C is created where the laser beam 3 is not irradiated due to the influence of the thickness of the metal mask 6, that is, a region C where no unevenness is formed. Similarly, even if a metal mask 6 is applied to region A and two-beam interference exposure is performed to region B, a region C is formed in which no unevenness is formed, and as a whole, no unevenness is formed over twice as much as in region C.

For example, if the period Λ of the unevenness of the diffraction grating is 2400 Å, and the wavelength λ ₀ of the He-Cd laser is 3250 Å, then
The angle of incidence α is α=sin ^-1 (λ ₀ /2Λ)=sin ^-1 (3250/4800)43 [
If the mask thickness t is 50 μm and the gap is d, then the region C where no periodic unevenness is produced is C=(t+d)tanαt·tanα=47 μm.

Therefore, by performing the two-beam interference exposure twice, the area that is twice the area C where no unevenness is formed is 94 [μm], and since the total length of the light-emitting region is usually about several hundred [μm], the DFB The operating current of the 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, although FIGS. 3 and 4 show a conventional method in which a phase shifter plate 7 is used to expose areas A and B at a time by two-beam interference, 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 Λ of the diffraction grating. Here, in Figure 3,
Between the photoresist film 2 and the phase shifter plate 7〓d
In the example shown in FIG. 4, due to the diffraction effect of the light from the phase shifter plate 7 to the substrate 1,
Diffraction gratings are no longer formed near the boundaries of regions A and B, respectively.

As described above, it has been difficult to accurately manufacture a diffraction grating having a structure in which the phase of periodic asperities 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 the second diffraction grating is formed in the second region. It is about forming.

(Structure and operation of the invention) The present invention will be explained 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.

(b) Expose the second region 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.

(e) A diffraction grating pattern is formed on the substrate 1 by etching or the like.

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

(Second Step) (g) 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.

(i) A part of the first diffraction grating 9 is subjected to two-beam interference exposure using a metal mask 6 or the like. At this time, an interference pattern is formed on the diffraction grating, and the intensity of the reflected light 11 (dashed line) of the interference pattern varies greatly depending on whether the interference pattern matches the concave or convex portions of the unevenness. . Therefore,
The reflected light is monitored by a photodetector 10, and the position of the substrate 1 is finely adjusted using a piezo element or the like so that a predetermined phase shift occurs between the interference pattern and the first diffraction grating 9. Normally, the reflected light is strongest when the bright part of the interference pattern matches the convex part. If the surface of the substrate 1 is made of a material with high reflectance, such as metal, the effect will be significant. 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) (j) Remove metal mask, etc. Two-beam interference exposure 3 is performed in the state adjusted in step (i) above.

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

(l) The substrate 1 is etched by photolithography using the diffraction grating of the photoresist film 8 as a mask.

(m) By removing the photoresist film 8, a second diffraction grating 12 is formed in the second region.
Here, the first diffraction grating 9 and the second diffraction grating 1
In this case, there is a phase shift of 180° between the two.

(Effects of the Invention) As is clear from the above steps, in the present invention, a diffraction grating having an arbitrary phase shift in the first and second regions can be easily manufactured, and the boundary between the first region and the second region can be easily manufactured. The conventional drawback that unevenness is not formed in some areas can be overcome. Therefore, it is stable and has good characteristics.
It can be applied to DFB lasers, etc., 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 the drawing]

Figure 1 is a diagram explaining the principle of conventional two-beam interference exposure.
2, 3, and 4 are schematic diagrams of conventional examples in which a phase shift diffraction grating is manufactured by two-beam interference exposure, and FIG. 5 is for explaining the manufacturing process of a phase shift diffraction grating according to the present invention. FIG. 1...Substrate, 2...Positive photoresist film, 3...He-Cd laser beam, 4...Half mirror, 5...Mirror, 6...Metal mask, 7...
Phase shifter plate, 8... Negative photoresist film, 9... First diffraction grating, 10... Photodetector, 11... Reflected light of interference pattern, 12... Second diffraction grating.

Claims (1)

[Claims] 1. A negative type photoresist film is applied to the entire surface of the substrate, and a second region other than the first region in which the first diffraction grating is to be formed is exposed to light, and the first region and the second region are exposed to light. By performing two-beam interference exposure and developing, the diffraction grating of the photoresist film is formed in the first region, and the photoresist film is formed over the entire surface of the second region. a first step of forming the first diffraction grating in the first region by etching using the photoresist film formed in the first region and the second region; After applying a negative type photoresist film on the substrate and exposing the photoresist film in the first region, a desired two-beam interference pattern has a predetermined phase shift with respect to the first diffraction grating. a second step of adjusting the relative position between the substrate and the two-beam interference pattern; and a second step of adjusting the relative position of the substrate and the two-beam interference pattern; By exposing the photoresist film and performing development and etching on the second region, a second diffraction grating having a phase shift with respect to the first diffraction grating is formed on the second region.
a third step of forming a diffraction grating.