JPH02196202A - Formation of phase shift type diffraction grating - Google Patents

Abstract

PURPOSE:To form the diffraction grating with excellent reproducibility with decreased disturbance in the gratings on a smooth substrate by providing steps consisting of a material which is different from the material of the substrate and is nontransparent to luminous fluxes on the substrate. CONSTITUTION:The step materials 2a, 2b which are different from the substrate material and are nontransparent to luminous fluxes, for example, SiNx and Al, are deposited on the substrate 1 and are subjected to photolithography to form prescribed stripes and spaces. A resist 3 is then applied thereon and is subjected to two beam interference exposing by the asymmetrical luminous fluxes 6, 7, thereafter, the resist 3, the Al, and the SiNx are successively removed by an etching liquid to form the diffraction plating having parts 4 periodically shifted in phase by lambda/4 on the smooth substrate. The phase shift type gratings are, therefore, formed on the smooth substrate and the disturbance in the gratings near the phase shift parts is decreased, in addition, the gratings are formed with the good reproducibility.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a distributed feedback semiconductor laser (hereinafter referred to as DFB-LD).
This invention relates to a method for forming a phase-shifted diffraction grating in ).

(Prior art) The DFB-LD achieves single-axis mode oscillation of up to about 10' bits/second even during high-speed modulation due to the strong wavelength selectivity of the diffraction grating built into the element. For uniaxial mode oscillation, the temperature fluctuation of the oscillation wavelength is about 1 person/℃, compared to about 5 people/℃ for Fabry-Perot semiconductor lasers.
It has been reported that it is extremely stable compared to (IEEE JOUR-NAL OF QUANTUM
ELECTRONICS, VOL, QE-18, NO
, 8゜AUGUS↑1982).

However, in a DFB-LD with a uniform diffraction grating, when the light waves traveling and receding in the active Bragg waveguide in the device match the Bragg wavelength of the diffraction grating,
Phase mismatching of optical electromagnetic waves occurs and progresses.

Laser oscillation does not occur because the waves are greatly attenuated.

When the optical structure of the active waveguide is symmetrical, laser oscillation occurs in two axial modes in which the longer wavelength side and the shorter wavelength side have the same oscillation threshold, separated by an interval called a stop band with the Bragg wavelength in between.

In addition, even when the waveguide symmetry is broken, the difference in dark values between both modes is not large enough, so it is theoretically possible that the oscillation wavelength changes from one mode to the other depending on the driving conditions of the element. and has been elucidated through experiments.

It is clear that such characteristics are inappropriate for the purpose of using semiconductor lasers for optical communications.

Therefore, as a promising method to match the optical transmission wavelength of this Bragg waveguide and the Bragg wavelength, DFB-L
The phase of the diffraction grating is set at the center of the waveguide at D by λ/4 (
λ−λ. /n, where λ. is the laser oscillation wavelength measured in vacuum, and n is the equivalent refractive index of the Bragg waveguide).
-LD).

In other words, the phase of a light wave that matches the Bragg wavelength changes by x/2 (-λ/4) due to reflection on the diffraction grating, but when the light passes through the phase shift part of the diffraction grating, that amount of phase change is absorbed. , it becomes possible to achieve phase matching between the advancing and receding light waves. In addition, this wavelength has the highest diffraction efficiency by the 11 diffraction grating, so the oscillation threshold current value as a semiconductor laser is small, and the threshold gain difference between the main and sub modes can be large, allowing single oscillation of the main mode. can be achieved stably.

A common method for forming the diffraction grating incorporated in the DFBLD is to use a three-beam interference optical system using, for example, a ll5-Cd laser as a light source, as schematically shown in Figure 5. When the holographic pattern is transferred to the resist coated on the substrate 1 and then chemically etched (hereinafter referred to as etching), fine irregularities are left on the substrate according to the pattern and a diffraction grating is formed. That is what it is.

The principle of forming a phase shift diffraction grating using this two-beam interference exposure method can be roughly divided into 1) negative/positive resist method, 2) phase plane projection method, 2) step shift method, and 2) phase mask contact method.

A conventional method using these principles will be briefly described.

(2) In the negative/positive resist method shown in FIG. 6, since both resists interfere with each other, pattern resolution deteriorates and reproducibility decreases.

Therefore, a method of depositing a dielectric film using a vapor phase method before forming a positive resist to prevent mutual interference may be considered, but it takes 100% to prevent thermal deterioration of a positive resist.
Since the dielectric film must be deposited at a temperature lower than 100%, it is difficult to obtain a dense dielectric film, and during the etching process of the dielectric film and substrate, non-uniformity occurs in lateral etching, making it difficult to reproducibly produce a uniform diffraction grating. There was a problem with not being able to get good results.

■ The quartz plate used in the phase plane projection method shown in Figure 7 is
Good homogeneity of the material and surface roughening must be achieved with high precision (error of approximately 200 nm or less).
The production of the optical phase modulation mask was very successful.

Furthermore, due to light scattering occurring at the boundaries of the unevenness of the optical phase modulation mask, the disturbance of the diffraction pattern spreads to several tens of micrometers or more centering on the phase shift part of the diffraction grating. The problem was that the losses could no longer be ignored.

■ The step shift method shown in Figure 8 shows that when InP and GnlnAsP are regrown on a diffraction grating on a non-smooth substrate,
Discontinuities in the crystal layer or non-uniform N thickness often occur, which not only deteriorates the coupling of the traveling and receding optical waves in the active waveguide but also significantly increases the waveguide loss, resulting in poor characteristics as a DFB-LD. There was a problem in that it was difficult to obtain a good reproduction.

■ The phase mask contact method shown in Figure 9 requires homogeneity of the phase mask and high precision in processing, and has the disadvantage that the substrate and the uneven phase mask made of quartz, for example, come into contact with each other, causing damage to the substrate. was there.

(Problems to be Solved by the Invention) In view of the above points, the present invention aims to form a phase shift type diffraction grating on a smooth substrate surface with less grating disturbance in the vicinity of the phase shift portion and with excellent reproducibility. The purpose is to provide a method.

(Means for Solving the Problems) In the case of a two-beam interference exposure method, the present invention provides two beams of light onto a substrate (hereinafter referred to as a substrate) provided with a stepped region, and a substrate on which a resist is coated. In the case of an electron beam exposure method in which the electron beam is incident at an asymmetrical angle to the normal line and scanned at equal intervals, a method of forming a diffraction grating on the substrate by making the electron beam incident from a direction that does not coincide with the normal line of the substrate. In this step, a step is formed above the substrate using a material different from that of the substrate and opaque to the light flux or particle beam serving as the exposure light beam.

(Function) A material different from the substrate is placed on a smooth substrate as a step material.
It can be formed while precisely controlling the layer thickness using a vacuum evaporation method or the like.

Therefore, by removing only the step substance by selective etching, it is possible to obtain a step with a desired height with good reproducibility. In addition, since the step material is non-transparent to the light beam for exposing the interference pattern corresponding to the diffraction grating onto the resist or to the particle beam such as an electron beam that directly draws the pattern of the diffraction grating, the step material and the substrate The pattern of the diffraction grating formed in the resist on the stepped material will not be disturbed by the light beam or particle beam reflected from the boundary of the step material.

(Example) FIG. 1 shows an example of the present invention, in which SiNx is deposited on a substrate 1 as step materials 2a and 2b for 200 ns+.
and 50 nm of Affi were deposited by chemical vapor deposition and vacuum evaporation, respectively (FIG. 1-A).

Next, a resist is applied by a spin code method, and stripes and spaces with a finger period of 300 μm are formed by ordinary photolithography (FIG. 1-B).

Next, the resist 3 diluted with thinner is applied again to a thickness of about 50 nm by the spin code method, and three-beam interference exposure and development are performed using asymmetrical light incidence (FIG. 1--).

Next, A! and SiNx are sequentially selectively etched using phosphoric acid-based and hydrofluoric acid-based etching solutions, respectively, according to the pattern of the resist 3 (FIG. 1-D).

Next, the substrate 1 is selectively etched with a saturated bromine aqueous etching solution (FIG. 1-E).

Finally, by sequentially removing the resist 3, AI1, and SiNx using acetone, phosphoric acid, and hydrofluoric acid etching solutions, a portion 4 having a periodic phase shift of λ/4 is formed on the smooth substrate. A λ/4 phase-shifted diffraction grating having a λ/4 phase shift type is formed (FIG. 1-F).

In addition, the step material 2a in this embodiment may be replaced with another dielectric, a semiconductor, or a metal such as Affi, or a structure in which the step materials 2a and 2b are stacked can be replaced with an exposure light or a particle beam such as an electron beam. It is clear that there is no problem in replacing the dielectric material with a non-transparent dielectric material.

In addition, in this example, the oscillation wavelength is in the 1.55μ band λ/
We have described the formation of a diffraction grating for a 4-phase shift type DFB-LD, but the step height (h), interference light intensity peak path incident angle (θ), and incident luminous flux half angle (ψ/2) must be selected appropriately. Therefore, DFB-
It goes without saying that any phase shift type diffraction grating can be formed by applying the present invention to LDs as well.

Figure 2 shows that two light beams 6.7 are incident on the normal line 5 of the substrate at asymmetric angles, φ and ψ, respectively, using the three-beam interference exposure system shown in Figure 5. When the height difference is 8
The principle that a phase shift 10 can be caused by moving the projected position of the interference light intensity peak course 9, which is caused by the interference of the light beams 6 and 7, on the top surface of the stepped object M2a and the surface of the substrate 1 due to the existence of the step M2a. is schematically shown.

The following four points can be cited as effects of the present invention using this principle.

1) Less disturbance of interference fringes on the step material.

2) A diffraction grating is formed on a smooth substrate surface.

3) The transition region where the interference fringes are disturbed by the step is extremely small.

4) Error in phase shift amount is extremely small.

These four points will be explained in sequence below.

Regarding the matter 1), when interference fringes are transferred to the resist 3 coated on the entire surface of the substrate by the exposure method shown in FIG. For example, as a light source with a wavelength that can produce interference fringes that are the same as the diffraction grating period of a certain DFB-LD, a light source with a wavelength of 325
When selecting the He-Cd laser beam which is r++s, for example, dielectrics or semiconductors with strong ultraviolet absorption such as SiO, Ti01, ZrO□, or A! A metal such as A1, Ti, etc. was deposited on a dielectric (or semiconductor) that can be deposited by sputtering or chemical vapor deposition, including metals such as , Ti, or the dielectric described above, taking into account the adhesion with the dielectric. If a material is selected as the step material, the He-Ccl laser beam will not pass through, so disturbance of interference fringes on the step material caused by multiple reflections occurring between the step material surface 11 and the substrate surface 12 can be suppressed. Good interference fringes are transferred to the resist 3.

Note that the light source is not limited to He-Cd laser light.

Regarding 2), the 1 formed by interference exposure
/JIS[・Following the pattern, step materials 2a and 1
/Using the cyst 3 as a mask, the substrate 1 is sequentially and selectively etched. Thereafter, by selectively etching and removing the stepped material again, a phase shift type diffraction grating with a constant depth can be obtained on the smooth substrate surface 12''.

Regarding matter 3), for example, as the phase shift amount, λ
/4, as shown in FIG. Assuming that the angle formed (half angle of incident light beam) is ψ/2, the condition for forming a λ74 phase shift type diffraction grating for a laser oscillation wavelength of 1.55 μ-, for example, when the light source is a GaInAsP/InP system, is given by the step height ( h
), θ and ψ/2 are calculated as the third
As shown in the figure.

In addition, FIG. 4 shows the transition region length of the transition region 13 where the interference fringes are distorted because the incident light beam is diffracted and reaches the resist by the step 8 corresponding to the transition region 13 shown in FIG.
d), the transition region length (d) is calculated as a function of the step height (11) under conditions that form a 774 phase shift.

From Figures 3 and 4, the step height (h) is 250 nm.
As mentioned above, if ψ/2 and θ are chosen to be about 50° and about 64°, respectively, the transition region length (d) will be about 1μ island,
The number of gratings included in this is about 47 in this case, whereas the number of gratings included in the semiconductor laser resonator with an in-shell cavity length of about 300 IJm is about 1200 in this example, so the transition Since the scattering loss in region 13 is relatively extremely small, the λ74 phase shift type DFB
-The effect on the characteristics of the LD is clearly negligible.

Regarding the issue 4), for example, if the step material is deposited on the substrate by the vacuum N deposition method while monitoring the step material with a film thickness meter, it is currently possible to obtain a step height (h) error of ±10 rim or less. For example, D F B −1, which oscillates at 1.55 μm,
, D, this value is expressed as the interference light intensity peak path incident angle (
θ), the error in the phase shift amount becomes na. When the oscillation wavelength is 1.55 μm, the error in the phase shift amount can be completely ignored since λ/4 is about 125 nm.

Furthermore, even if the thickness of the step material deviates from the set value, a λ/4 phase shift can be easily created by appropriately adjusting the angles θ and φ/2, as shown in Figure 3. It is clear that it is possible to do so.

In addition, in an exposure method using a particle beam such as an electron beam, if scanning is performed at an interval of 5 inches (θ) times the period of the diffraction grating from the direction corresponding to the interference light intensity peak path 9 in FIG.
Needless to say, the same result as in the two-beam interference exposure method in which the light beam is incident asymmetrically with respect to the normal to the substrate can be obtained.

In addition, the light source is set to 1.5577-wavelength band Ga1nAsP/
I have explained about Inp-based semiconductor lasers, but
It is clear from the structure of the invention that the wavelength band of the semiconductor laser and the substrate used are not limited to those of this embodiment.

(Effects of the Invention) As explained above, the present invention provides a phase shift type diffraction grating that is formed on a substrate according to a transfer pattern such as interference fringes, and then
Remove the material that formed the step. Therefore, this phase shift type diffraction grating can be formed on a smooth substrate, and can be formed using a dielectric material such as SiNx, 5iO1, etc., which has little anisotropy against etching as a step material, and AJ! Since metals such as , Ti, etc. are selected, the step shape becomes close to a step shape.

In addition, the section where the resist layer thickness changes at the step portion becomes smaller, and a phase shift type diffraction grating whose transition region is almost equal to the calculated value shown in Fig. 4 and has good reproducibility can be formed. .

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 is a diagram schematically showing the configuration of the present invention, and Fig. 3 is a diagram showing the incident luminous flux half angle (ψ/2) and the interference light intensity peak. Figure 4 shows the path incidence angle (θ) as a function of step height (h), Figure 4 shows the transition region length (d) as a function of step height (h), and Figure 5 shows the two-beam Figure 6 schematically shows the configuration of the optical system used in the interference exposure method. Figure 6 schematically shows the principle of the negative/positive resist method. Figure 7 shows the principle of the phase plane projection method. FIG. 8 is a diagram schematically showing the principle of the step shift method, and FIG. 9 is a diagram schematically showing the principle of the phase mask contact method. The correspondence with each number in the figure is as follows. 1 is the substrate, 2a and 2b are the step materials, 3 is the resist, 4 is the part where the phase is periodically shifted by 174, 5 is the normal to the substrate, 6 and 7 are the luminous flux, 8 is the step difference, 9 is the interference light intensity Peak path, 10 is phase shift formed on resist, 11
12 is the surface of the step material, 12 is the substrate surface, and 13 is the transition region.

Claims (1)

[Claims] A two-beam interference exposure method is used to make two light beams incident on a stepped substrate at an asymmetrical angle with respect to the normal direction of the substrate, or a particle beam exposure drawing method is used. A method for forming a phase-shift diffraction grating in which the particle beam is scanned and incident from a direction different from the normal direction of the substrate to form a fine striped pattern on the surface of the substrate, comprising: a material different from the substrate; A method for forming a phase shift diffraction grating, characterized in that a step is provided above the substrate using a material that is non-transparent to the light flux or the particle beam.