JPH0513886A - Manufacture of semiconductor laser - Google Patents

Abstract

PURPOSE:To easily obtain a semiconductor laser having an excellent pattern accuracy by forming a pattern of a diffraction grating by using a contraction projecting exposure having an ultraviolet ray as a light source and a phase shift mask. CONSTITUTION:A pattern shape of a resist 21 of a predetermined diffraction grating is obtained on a semiconductor laser substrate 20 by 1/5 contraction exposure by using a phase shift mask C1 and an i-type beam stepper. Then, a connecting error of connecting parts of the respective patterns becomes 0.25% or less of a period of the grating. Since this process uses a projecting exposure with ultraviolet ray, its throughput is high, and no damage of the substrate and the mask is observed. The mask pattern forming process is faster as compared with a photomask method. Thus, when a submicron diffraction grating is manufactured by a contraction-exposure by using the ultraviolet ray exposure method and the phase shift mask and its periodic uniformity can be improved, and improvements in resolution and throughput can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor laser.

[0002]

2. Description of the Related Art Conventionally, the period is submicron, especially 500.
In the case of forming a diffraction grating of nm or less, a distributed feedback semiconductor laser (DFB-LD) or a distributed Bragg mirror semiconductor laser (DBR-LD) has insufficient resolution in ordinary photolithography. Interferometric exposure method (T. Matsuoka et al. Electro
n. Lett., vol.18, pp.27, 1982.) and photomask method (M.Okai et al. Appl.Phys. Lett., vol.55, pp.415,
1989.), EB exposure method (ASGozdz et al. Electron Le
tt., vo124, pp.123, 1988.), X-ray exposure method (T. Nishi
da et al. Japanese Journal of Applied Physics, vo
l.28, pp.2333, 1989.) has been used. However, these methods have the following problems. In such an optical element, since the emission spectrum width can be narrowed by the length of the diffraction grating having a uniform period, it is important to form a diffraction grating having a high period with high accuracy and high throughput.

[0003]

In the interference exposure method (FIG. 6) using two coherent light rays (two light fluxes), the exposure intensity distribution with respect to the position on the resist on the substrate has a sine function, so that the pattern edge portion The contrast was low. Therefore, when the pattern is transferred to the optical element substrate by the wet etching method, it becomes difficult to control the line and space (L / S) of the diffraction grating pattern. Such L / S heterogeneity is caused by the micro loading effect APCT89 (T. Nishida et al. In.
ternationalMeeting on Advanced Processing Characer
ization Technologies 1989. P-20.), it was difficult to obtain good device characteristics because the diffraction grating shape inside the optical element was non-uniform, and the diffraction efficiency inside the optical element was also non-uniform. .. Also, when transferring to the optical element substrate by the dry etching method, the resist pattern shape after development reflects the exposure intensity distribution and the resist film thickness decreases at the pattern edge portion (FIG. 7). The resist film disappeared due to the decrease in the resist film, and good pattern formation was impossible. Furthermore, DFB-L
In D, at the center of the diffraction grating pattern,
By introducing a phase shift of / 4 period (λ / 4 shift), that is, a phase shift of 1/2 of the period of the diffraction grating,
Although the probability of single longitudinal mode oscillation (SLM) of FB-LD can be dramatically improved (HAHaus et al. IEEE J Quantum El
ectonics QE-12, pp.532, 1976.), in order to realize such a phase shift of the diffraction grating by the interference exposure method, for example, in the literature (Utaka, et al. Electron. Lett., vol. 20, p
It was necessary to position the mask on the substrate as shown in p.1008, 1984.). However, since the interference exposure method has a sinusoidal exposure distribution, the margin of the proper exposure amount is small, and such a method has remarkably low reproducibility. Furthermore, when forming a plurality of diffraction grating arrays with slightly different diffraction grating periods, it is necessary to delicately change the positions of the two light beams to be interfered with each other. It was difficult to correct within the range.

In the photomask method, the resolution is improved as compared with the interference exposure method, but the diffraction grating pattern of the mask is
The diamond pen is mechanically controlled to scan and cut each line one by one, so that the mask manufacturing time is extremely long, and the contact exposure causes serious damage to the mask and a short mask life.

In the EB exposure, the resolution is very high and the degree of freedom of the pattern shape is large, and the exposure speed, which was a problem before, has been greatly improved in recent years. However, since the electron optical system has a larger aberration than the ordinary optical system, the phase error of the diffraction grating is 10% (for example, 1.55 μm DFB-LD).
In the case of the diffraction grating for use, the accuracy of pattern connection by the movement of the stage is insufficient to perform submicron writing while keeping the diffraction grating within 10 nm, and thus the possible diffraction grating region length remains around 1 mm.

Further, in the X-ray exposure method, if a highly parallel light source such as orbital radiation is used, a high resolution of 0.1 μm or less is possible, and the mask is patterned by EB exposure. Although the degree of freedom is also high, at present, since the 1: 1 equal-magnification exposure is performed, the limitation of the pattern area length in the EB exposure for mask drawing is similar.

0.2 μm in the normal phase shift mask method
Although it is difficult to form the following pattern, this is because the usual LSI process presupposes that the resist film thickness is 1 μm, and the resist film thickness is reduced in an optical element where the unevenness of the diffraction grating is shallow. The contrast can be improved. Therefore, the above problems can be solved by using the present invention as described later.

An object of the present invention is to provide a method for manufacturing a semiconductor laser which is easy and has excellent pattern accuracy.

[0009]

In order to achieve the above object, a method of manufacturing a semiconductor laser according to the present invention is a method having a diffraction grating, wherein the pattern of the diffraction grating is reduced projection using an ultraviolet ray as a light source. The feature is that it is formed using exposure and a phase shift mask.

[0010]

According to the method of manufacturing a semiconductor laser of the present invention,
A phase shift mask is used to improve the resolution with an ultraviolet light source having a short wavelength, and a diffraction grating resist pattern is formed.

[0011]

EXAMPLES Examples of the present invention described above are
InGaAa / InP 1.55 with secondary diffraction grating
A case of a μm distributed feedback semiconductor laser (hereinafter referred to as DFB-LD) will be described as an example.

Example 1 (Example of Application to Second-Order Diffraction Grating) FIG. 1 shows an example using a Levenson type (introduced article in "Nikkei Microdevice" 1991.5.p54) as a phase shift mask. First, a mask glass substrate 5 (with a Cr film) coated with an EB resist (CMS, manufactured by Tosoh) by an EB exposure method was used, and the period was 12.4 μm and L / S =
The 1/1 diffraction grating was repeated 74 times every 1 mm with an accuracy of 50 nm, that is, 0.4% with respect to the period, to connect the patterns to form a diffraction grating pattern having a total length of 75 mm. Further, a λ / 4 shift, that is, a phase shift 4 which is ½ of the diffraction grating period is provided in the central portion. The resist pattern thus formed by EB exposure is subjected to C by a Cr etching solution.
It was transferred to the r film to form a mask A31. The pattern of the mask A31 is transferred to the substrate C in which the Cr film is coated with the ultraviolet photoresist by the 1/5 reduction exposure using the g-line stepper, and the Cr pattern of the diffraction grating having a period of 2.48 μm and a diffraction grating region length of 15 mm. A mask C was obtained. Al ₂ O ₃ of 10 nm and SiO ₂ of 100 nm were formed on this mask C by a magnetron sputtering method, and then an ultraviolet photoresist was applied.

Then, by EB exposure, the Cr of the mask A is formed on the mask glass substrate 5 (with the Cr film and the EB resist).
Cycle 2 is placed at a position that overlaps the pattern space every 2 cycles.
A diffraction grating of 4.8 μm, L / S = 2.5 / 1.5,
Similar to the first exposure, the pattern was connected 74 times every 1 mm with an accuracy of 50 nm, that is, 0.8% with respect to the period, to form a diffraction grating pattern having a total length of 75 mm. Further, a λ / 4 shift, that is, a 1/8 phase shift 4 of the second-order diffraction grating period is provided in the central portion. The resist pattern thus formed by EB exposure was transferred to a Cr film with a Cr etching solution to form a mask B32. Mask B32
Was transferred to the mask C by a 1/5 reduction exposure using a g-line stepper, and a mask C in which the space of the Cr pattern and the space of the resist pattern overlap each other every two cycles of the Cr pattern of the mask C was obtained. The SiO ₂ film of the mask C on which the resist pattern has been formed by the EB exposure is etched with a buffered hydrofluoric acid solution (or a reactive ion etching method using C 2 F 6 gas may be used) to shift the phase shift mask C 1 Formed. At this time, the Al ₂ O ₃ film serves as an etching stopper for the SiO ₂ film.

As the semiconductor laser substrate 20, n-In
On a P (Sn-doped) <001> substrate, an InGaAsP active layer (λ = 1.55 μm, thickness 0.12 μm) and an InGaAsP guide layer (λ =
Using a substrate having a thickness of 1.3 μm and a thickness of 0.15 μm), an ultraviolet resist PFI-15 (manufactured by Sumitomo Chemical Co., Ltd.) was applied to a thickness of 100 nm.

This semiconductor laser substrate 20 was subjected to ⅕ reduction exposure using a phase shift mask C1 and an i-line stepper, and as shown in FIG. 2, a diffraction grating resist having a period of 496 nm and a diffraction grating region length of 3 mm. 21 pattern shapes were obtained, and the connection error of each pattern connection portion was 0.25% or less of the diffraction grating period. Moreover, since this process is projection exposure of ultraviolet rays, the throughput was high, and no damage to the substrate or the mask was observed. Also, the mask pattern forming process was much faster than the photomask method.

This InP substrate was treated with saturated bromine water: hydrobromic acid: water = 1: 10: 40 (T. Matsuoka and H. Nagai, “I.
nP Echant for Submicron Patterns "J.Electrochem.So
c, vol.133, 2485 (1986) etching solution) for 8 seconds to form a diffraction grating shape, and then the resist is peeled off to form a p-InP clad layer by MOCVD.
After forming the resonator stripe by wet etching, selective buried growth was performed by liquid phase growth to fabricate a buried hetero structure DFB-LD (FIG. 3). When a double-sided antireflection coating was applied with a cavity length of 3 mm and a cavity width of 1.5 μm, a threshold current Ith = 55 mA, an injection current 1.9 Ith, and an oscillation spectrum width of a single longitudinal mode 5
0 kHz was obtained.

Next, an embodiment of the present invention will be described by taking the case of an InGaAs / InP 1.55 μm distributed feedback semiconductor laser (hereinafter referred to as DFB-LD) having a first-order diffraction grating as an example.

Example 2 (Application Example to First-Order Diffraction Grating) First, a glass substrate for a mask (with a Cr film) coated with an EB resist (CMS, manufactured by Tosoh Co., Ltd.) by the EB exposure method was applied to a period 6 A diffraction grating having a total length of 75 mm was formed by repeating a diffraction grating of 0.2 μm and L / S = 1/1 74 times every 1 mm with an accuracy of 50 nm, that is, 0.8% with respect to the period. .. Further, a λ / 4 shift (that is, a phase shift of ½ of the diffraction grating period) is provided in the central portion. The resist pattern thus formed by EB exposure was transferred to a Cr film with a Cr etching solution to form a mask A. The pattern of the mask A is transferred to the substrate C having the Cr film coated with the ultraviolet photoresist by the 1/5 reduction exposure using the g-line stepper, and the period is 1.24 μ.
A mask C having a diffraction grating Cr pattern of m and a diffraction grating region length of 15 mm was obtained. Al ₂ O ₃ 10n is applied to this mask C.
m and SiO ₂ were formed by a 100 nm magnetron sputtering method, and then an ultraviolet photoresist was applied.

Next, by EB exposure, a period of 1 cycle is provided on the mask glass substrate (with a Cr film and an EB resist) so as to overlap with the space of the Cr pattern of the mask A every 2 cycles.
A diffraction grating of 2.4 μm, L / S = 2.5 / 1.5,
Similar to the first exposure, the pattern was connected 74 times every 1 mm with an accuracy of 50 nm, that is, 0.8% with respect to the period, to form a diffraction grating pattern having a total length of 75 mm. Further, a λ / 4 shift, that is, a ¼ phase shift of the diffraction grating period is provided in the central portion. The resist pattern thus formed by EB exposure was transferred to a Cr film with a Cr etching solution to form a mask B. The pattern of mask B is g
Transferring to the mask C by 1/5 reduction exposure using a line stepper, a mask C in which the space of the Cr pattern and the space of the resist pattern overlap every two cycles of the Cr pattern on the mask C was obtained. Thus, the SiO ₂ film of the mask C on which the resist pattern was formed by EB exposure was etched by a buffered hydrofluoric acid solution (or by a reactive ion etching method using C ₂ F ₆ gas) to form a phase shift mask C.

As the semiconductor laser substrate 20, n-In
On a P (Sn-doped) <001> substrate, an InGaAsP active layer (λ = 1.55 μm, thickness 0.12 μm) and an InGaAsP guide layer (λ =
1.3 μm, thickness 0.15 μm) is formed on the substrate, and an ultraviolet resist sal-601 having a thickness of 100 is used.
nm coating.

On this semiconductor laser substrate 20, a phase shift mask C and a KrF excimer laser light source are used.
5 When reduction exposure was performed, the use of an excimer laser light source with a wavelength shorter than that of an ordinary ultraviolet light source resulted in 2
As shown in FIG. 4, which has a short cycle of 48 nm, the cycle is 248 nm,
The pattern shape of the resist 21 of the diffraction grating having a diffraction grating region length of 3 mm was obtained, and the connection error of each pattern connection portion was 0.5% or less of the diffraction grating period. Moreover, since this process is projection exposure of ultraviolet rays, the throughput was high, and no damage to the substrate or the mask was observed. Also, the mask pattern forming process was much faster than the photomask method.

This InP substrate was treated with saturated bromine water: hydrobromic acid: water = 1: 10: 40 (T. Matsuoka and H. Nagai, “I.
nP Echant for Submicron Patterns "J.Electrochem.So
c, vol.133,2485 (1986)) to form a diffraction grating shape by etching for 2 seconds, then the resist is peeled off, a p-InP clad layer is formed by the MOCVD method,
After forming the resonator stripe by wet etching, selective buried growth was performed by liquid phase growth to fabricate a buried hetero structure DFB-LD (FIG. 5). When a double-sided antireflection coating was applied with a cavity length of 3 mm and a cavity width of 1.5 μm, a threshold current Ith = 40 mA and an injection current 1.9 Ith, and an oscillation spectrum width of a single longitudinal mode was 3
0 kHz was obtained.

In the present embodiment, the example in which the present invention is applied to the DFB-LD is taken up. However, a submicron period diffraction grating such as a DBR-LD forming a diffraction grating in an optical waveguide or other interference filters is used. It is obvious that the present invention can be applied to an optical element as a constituent element.

[0024]

As described above, when the ultraviolet exposure method and the phase shift mask are used for the production of the submicron diffraction grating, the X-ray exposure method using the one-to-one mask formed by electron beam exposure or electron beam exposure is used. In comparison with the case of using the interference exposure method or the photomask method, the resolution and the throughput can be improved while suppressing the high mask damage, as compared with the case of using the interference exposure method or the photomask method. It was clarified that the degree of freedom of the pattern seen in the introduction of / 4 shift can be realized. Further, it has been clarified that an optical communication light source having a narrow spectral line width can be realized when such a processing method is applied to DFB-LD fabrication. Further, in order to apply to a wavelength multiplex array in which the period of the diffraction grating is changed little by little with high precision, it is possible to make a new mask by changing the EB drawing pattern, but simply change the reduction ratio of projection exposure. This has the advantage that a multiple array pattern can be formed very easily.

In the above embodiment, the diffraction grating pattern is set to 1
After the mask A and the mask B were exposed, the Cr pattern (mask A) and the shifter pattern (mask B) were reduced and transferred to the phase mask substrate C. However, the accuracy of the field connection in the EB exposure was improved, and the pattern connection accuracy was improved. It goes without saying that the Cr pattern and the shifter pattern can be directly EB-exposed on the phase shift substrate C as long as the value is improved.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a process of forming a DFB-LD diffraction grating by a method for manufacturing a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a schematic view of a resist pattern formed according to Example 1 of the present invention.

FIG. 3 is a perspective sectional view schematically showing a DFB-LD manufactured by the method for manufacturing a semiconductor laser according to the first embodiment of the present invention.

FIG. 4 is a schematic view of a resist pattern formed according to a second embodiment of the present invention.

FIG. 5 is a perspective sectional view schematically showing a DFB-LD manufactured by the method for manufacturing a semiconductor laser according to the first embodiment of the present invention.

FIG. 6 is an example showing a forming method by a two-beam interference method.

FIG. 7 is a schematic view of a resist pattern formed by a two-beam interference method.

[Description of Reference Signs] 1 phase shift mask C 2 Cr film 3 phase shifter 4 λ / 4 shift 5 mask substrate glass 10 Ar laser 11 beam expander 12 half mirror 13 mirror 20 InP substrate (laser structure) 21 resist 25 laser beam 31 mask A 32 mask B

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location 7013-4M H01L 21/30 341 P

Claims (1)

1. A method of manufacturing a semiconductor laser having a diffraction grating, wherein the pattern of the diffraction grating is formed using reduction projection exposure using ultraviolet light as a light source and a phase shift mask. Manufacturing method of semiconductor laser.