Method of manufacturing a diffraction grating

Abstract: A method for manufacturing a diffraction grating includes the steps of applying a resist (12,42), of which developing speed has the extreme value in regard to certain exposure inten­ sity, to a substrate (1,31) on which the diffraction grating is to be formed, performing interfe­ rence exposure with beam intensity in which the maximum and minimum values of exposure intensity are on both sides of intensity which makes the developing speed to be the extreme value, patterning the resist (12,42) by development, and etching the substrate (1,31) using the patterned resist (12,42) as a mask.

Description: FIELD OF THE INVENTION The present invention relates to a method for manufacturing a diffraction grating as claimed in the preamble of claim 1, having a period half of the wavelength of the illumination. BACKGROUND OF THE INVENTION Figure 7 is a view showing an interference exposure apparatus used for manufacturing a conventional diffraction grating. Referring to figure 7, a laser beam 17a is outputted from a laser beam source 4. The laser beam 17a is divided into laser beams 17b and 17c by a half mirror 5 and the laser beams 17b and 17c are reflected by mirrors 6a and 6b, respectively and then applied to a resist 2 on a substrate 1. Figure 8 are cross-sectional views showing conventional manufacturing steps of the diffraction grating by an interference exposure method and a diagram showing distribution of exposure intensity. Next, the conventional manufacturing steps of the diffraction grating will be described in reference to figure 8. First, referring to figure 8(a), a resist 92 is applied onto a substrate 91. Then, referring to figure 8(b), the resist 92 is periodically exposed by a two-beam interference exposure method. Referring to figure 8(c), exposure intensity of a laser beam applied to the resist 92 cyclically varies. Then, referring to figure 8(d), when the exposed resist 92 is developed, the resist 92 is patterned and a diffraction grating 93a is formed. Thereafter, referring to figure 8(e), the substrate 91 is etched away using the patterned resist 92 as a mask and then a diffraction grating 93b is formed. Then, a principle of the interference exposure method used for manufacturing the conventional diffraction grating will be described in reference to figure 7. According to the device shown in figure 7, the laser beam 17a outputted from the laser beam source 4 is divided into two directions by the half mirror 5 and they are reflected by the mirror 6 and meet again on the substrate 1. At this time, intensity of the beam on the substrate has the distribution of a cycle Λ because of interference of the laser beam. The cycle Λ is represented by: Λ = λ 2 sin ϑ where λ is a wavelength of the laser beam and ϑ is an incident angle of the laser beam to the substrate. The conventional diffraction grating is formed using the above principle, in which the resist applied to the substrate is exposed at a pitch of the cycle Λ and then the resist is developed and then the substrate is etched away by using the patterned resist as a mask. Figure 9 is a sectional view showing a conventional single wavelength semiconductor laser device disclosed in, for example Technical Magazine Optics Vol. 15, No 2, pp.115 - 121. In figure 9, an n type InGaAsP guide layer 107, an InGaAsP active layer 108 and a p type InP layer 109 are sequentially formed on an n type InP substrate 101 in which a diffraction grating 102 having a phase shift region in the center thereof is formed. An n side electrode 110 is provided on a back surface of the substrate 101 and a p side electrode III is provided on the p type InP layer 109. In addition, a nonreflective coating film 113 is provided on each end surface 114 of the laser. Next, operation thereof will be described. In the semiconductor laser device, electrons in the n type InP substrate 101 and holes in the p type InP layer 109 are both injected into the InGaAsP active layer 108 and then emission recombination occurs. In a distributed feedback (DFB) laser device having the diffraction grating 102 having the phase shift region in an active region, the light generated by the emission recombination is reflected by the diffraction grating 102 and goes and returns in an element, whereby a laser is oscillated. Since the diffraction grating 102 effectively reflects a beam having a wavelength λ where λ = 2 n eff Λ/n (n eff is equivalent refractive index, Λ is a pitch interval of the diffraction grating and n is an integer), an oscillation wavelength is a wavelength whose gain is largest in the active region among wavelengths represented by 2 n eff Λ/n. As for the oscillation wavelength λ, when n = 1, the diffraction grating is called primary diffraction grating and when n = 2, the diffraction grating is called secondary diffraction grating. According to the single wavelength oscillating semiconductor laser device having a structure shown in figure 9, since the beam is reflected only by the diffraction grating 102 in the active region and then confined in the element, beam density in the center of the elements is increased. As a result, linearity of a beam output in regard to an injected current and stability of single wavelength oscillation are reduced under an influence of hole burning or the like. As described above, according to the conventional method for manufacturing a diffraction grating, it is not possible to form a diffraction grating having a cycle which is a half of the wavelength λ of the laser beam source 4 or less. In addition, according to the conventional single wavelength oscillating semiconductor laser, linearity of the beam output in regard to an injected current and stability of the single wavelength oscillation is reduced under an influence of the hole burning or the like. The US patent US-A-4,847,857 discloses a single wavelength oscillating semiconductor device emitting with a uniform light intensity over the whole element region by providing a grating having a reduced coupling coefficient in the center. The reduced coupling is achieved by a grating having a phase shift region with a reduced amplitude in the center of the laser. W. Streifer et al. report on a higher order gratings coupled laser in an article having the title "Coupling Coefficients for Distributed Feedback Single- and Double-Heterostructure Diode Lasers", published in IEEE J. of Quantum Electronics, Vol. QE-11, No. 11, 867, 1975. In Applied Optics, Vol. 21, No. 11, 1892 (1982), L. F. Johnson reports in an article having the title "Optical Writing of Dense 1000 Å in Photoresist" on a method using the non-linearity of high-contrast photoresists to produce features considerably smaller than the wavelength of the exposing beam. SUMMARY OF THE INVENTION The present invention is directed to solve the above problems and has for its object to provide a method for manufacturing a diffraction grating, by which a diffraction grating with a short cycle can be formed. This object is solved by the method according to appended claim 1. According to a preferred embodiment of the present invention a method for manufacturing a diffraction grating is provided, by which diffraction gratings having two kinds of cycles can be formed on the same plane by one interference exposure at the same time. Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 are cross-sectional views showing manufacturing steps of a diffraction grating in accordance with a first embodiment of the present invention and a diagram showing distribution of exposure intensity; Figure 2 is a cross-sectional view showing a single wavelength oscillating semiconductor laser device manufactured in accordance with the present invention; Figure 3 are cross-sectional views showing manufacturing steps of a diffraction grating in accordance with a second embodiment of the present invention and a diagram showing distribution of exposure intensity; Figure 4 is a cross-sectional view showing a semiconductor laser having a primary diffraction grating in the neighborhood of resonator facets and a secondary diffraction grating in the center of a resonator; Figure 5 is a diagram showing the relation between exposure intensity and developing speed of an image reversible resist after it is baked at a temperature of 110 to 120°C; Figure 6 is a diagram showing distribution of exposure intensity by an interference exposure method; Figure 7 is a view showing an interference exposure apparatus; Figure 8 are cross-sectional views showing manufacturing steps of a diffraction grating by a conventional interference exposure method and a diagram showing distribution of exposure intensity; and Figure 9 is a sectional view showing a conventional single wavelength oscillating semiconductor laser device. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A method for manufacturing a diffraction grating in accordance with the present invention will be described in detail hereinafter in reference to the drawings. Figure 1 are cross-sectional views showing manufacturing steps of a diffraction grating in accordance with a first embodiment of the present invention and a diagram showing distribution of beam intensity. In figure 1, a resist 12 (image reversible resist), of which developing speed has the minimum value in regard to certain exposure intensity, is applied to a substrate 1. Next, the manufacturing steps of the diffraction grating in accordance with this embodiment will be described in detail hereinafter. First, referring to figure 1(a), the resist (image reversible resist) 12, which is baked at a temperature of 110 to 120°C before exposure so that the developing speed may have the minimum value in regard to certain exposure intensity, is applied to the substrate 1 where the diffraction grating is to be formed. Then, it is baked at a temperature of 110 to 120°C for five minutes. Then, referring to figure 1(b), the resist 12 is exposed by a two-beam interference exposure method with exposure intensity in which the maximum and minimum values are on both sides of intensity which makes the developing speed to be the minimum value. Figure 1(c) is a diagram showing distribution of the beam intensity at this time. Then, referring to figure 1(d), the resist 12 is developed so that the resist 12 at portions exposed with the beam intensity which makes the developing speed to be the minimum value may be left and patterned to form a diffraction grating 23a. Then, referring to figure 1(e), the substrate 1 is etched away with appropriate etchant such as HBr/HNO₃/H₂O mixed solution, HBr/HNO₃/CH₃OH mixed solution or Br₂/CH₃OH mixed solution using the patterned resist 12 as a mask to form a diffraction grating 23b. Next, operation of the resist (image reversible resist) in which the developing speed has the minimum value in regard to certain exposure intensity will be described hereinafter. It is known that when the image reversible resist is baked at a temperature of 110 to 120 °C before exposure, the developing speed has the minimum value in regard to exposure intensity of α as shown in figure 5 (referring to "Electron Material", June, 1986). Meanwhile, the exposure intensity by the interference exposure method has cyclic distribution as shown in figure 6. The cycle of the distribution is Λ in the above equation (1). Therefore, when the resist is exposed so that the maximum value of the exposure intensity by the interference exposure may be more than a point where the developing speed has the minimum value (a point of α in figure 5) and the minimum value of the exposure intensity may be less than the point of α, the resist can be developed so that the resist at portions exposed with the intensity which makes the developing speed to be the minimum value may be left. As a result, after etching, it is possible to form a diffraction grating having a cycle which is a half of the cycle of an interference fringe obtained by the interference exposure method. Although description was given of the resist in which developing speed had the minimum value in regard to certain exposure intensity in the above embodiment, the same effect can be attained even if the resist in which developing speed has the maximum value in regard to certain exposure intensity is used. Next, a single wavelength oscillating semiconductor laser device in accordance with the present invention will be described in detail in reference to the drawings. Figure 2 is a cross-sectional view showing a single wavelength oscillating semiconductor laser device manufactured in accordance with the present invention. In figure 2, an n type InGaAsP guide layer 53, an InGaAsP active layer 54 and a p type InP clad layer 55 are sequentially formed on an n type InP substrate 51 where a primary diffraction grating 61 and a secondary diffraction grating 62 are formed, the latter has a phase shift region in the center thereof. An n side electrode 59 is provided on a back surface of the substrate 51 and a p side electrode 60 is provided on the p type InP layer 55. A nonreflective coating film 58 is provided on both facets of laser. Next, operation of the signal wavelength oscillating semiconductor laser manufactured in accordance with this embodiment will be described. A beam is more effectively reflected by a low ordered diffraction grating. Therefore, as shown in figure 2, when the order of the diffraction grating 62 in the center of the element is higher than that of the diffraction grating 61 in the neighborhood of resonator facets, the reflection intensity of the beam in the center of the element is less than that in the neighborhood of the resonator facets. As a result, the beam generated at the active layer 54 is not confined only in the center of the element and most beams advance to the neighborhood of the resonator facets and are then reflected by the diffraction grating 61. Therefore, beam density is not high only in the center of the element and can be uniform over the whole element, with the result that hole burning is not likely to occur and then it is possible to provide the single wavelength oscillating semiconductor laser device having high linearity of a beam output in regard to an injected current and high stability of single wavelength oscillation. In addition, the laser beam can be outputted in the vertical direction to the substrate by the secondary diffraction grating 62 in this laser device. Meanwhile, according to the conventional method for manufacturing the diffraction grating shown in figure 8, since the diffraction grating having the same cycle is formed over the whole surface of the substrate, it is not possible to form the semiconductor laser having the primary and secondary diffraction gratings shown in figure 2. According to a method for manufacturing a diffraction grating in accordance with the present invention, the diffraction grating having two kinds of cycles can be formed on the same plane surface by one interference exposure at the same time. This method for manufacturing the diffraction grating in accordance with the second embodiment of the present invention will be described hereinafter in reference to the drawing. Figure 3 are cross-sectional views showing manufacturing steps of a diffraction grating in accordance with the second embodiment of the present invention and a diagram showing distribution of beam intensity. In figure 3, a resist (image reversible resist) 42, of which developing speed has the minimum value in regard to certain exposure intensity, is applied to a substrate 31 and then a semi-transparent mask 43 is partially arranged on the resist 42. The manufacturing steps of the diffraction grating in accordance with the embodiment will be described hereinafter. First, referring to figure 3(a), the resist (image reversible resist) 42, which is baked at a temperature of 110 to 120 °C before exposure so that developing speed may have the minimum value in regard to certain exposure intensity, is applied to the substrate 31 where the diffraction grating is to be formed. Then, it is baked at a temperature of 110 to 120 °C for five minutes. Then, referring to figure 3(b), the semi-transparent mask 43 is partially arranged on the resist 42 and then by a two-beam interference exposure method, the resist 42 is exposed with exposure intensity in which the maximum and minimum values of the exposure intensity are on both sides of intensity which makes the developing speed to be the minimum value in a portion which is not covered with the semi-transparent mask 43 and exposed with exposure intensity in which the maximum value of the exposure intensity is less than the intensity which makes the developing speed to be the minimum value in a portion covered with the semi-transparent mask 43. Figure 3(c) is a diagram showing distribution of the beam intensity at this time. Then, referring to figure 3(d), the resist 42 is developed so that the resist 42 at portions exposed with the beam intensity which makes the developing speed to be the minimum value may be left and patterned to form a diffraction grating 45a. Then, referring to figure 3(e), the substrate 31 is etched away with appropriate etchant such as HBr/HNO₃/H₂O mixed solution, HBr/HNO₃/CH₃OH mixed solution or Br₂/CH₃OH mixed solution using the patterned resist 42 as a mask to form a diffraction grating 45b. As described above, when the image reversible resist is exposed so that the maximum value of the exposure intensity by the interference exposure may be more than a point where the developing speed has the minimum value (a point of α in figure 5) and the minimum value of the exposure intensity may be less than the point of α, the resist can be developed so that the resist at portions exposed with the intensity which makes the developing speed to be the minimum value may be left. As a result, after etching, it is possible to form the diffraction grating having a cycle which is a half of the cycle of an interference fringe obtained by the interference exposure method. Meanwhile, when the maximum value of the exposure intensity by the interference exposure is less than the point of α or the minimum value of the exposure intensity is more than the point of α, the resist is left in a cycle of an interference fringe obtained by the interference exposure, whereby the diffraction grating having the cycle of the interference fringe is formed. Therefore, in a case where a part of the substrate is covered with the semi-transparent mask, when the maximum value of the exposure intensity in a portion which is not covered with the mask is more than the point of α and the minimum value thereof is less than the point of α and the maximum value of the exposure intensity in a portion covered with the mask is less than the point of α, it is possible to form the diffraction gratings having cycles of Λ and Λ/2 by one interference exposure at the same time. Next, a description will be given of manufacturing steps of a semiconductor laser device shown in figure 4, which has primary and secondary diffraction gratings like the semiconductor laser device shown in figure 2 by using the method for manufacturing the diffraction grating in accordance with this embodiment. Similar to the semiconductor laser shown in figure 2, the semiconductor laser shown in figure 4 comprises primary and secondary diffraction gratings 76 and 75 formed on a substrate 71, in which a phase shift region is not formed in the center of the secondary diffraction grating 76. Referring to figure 4, a p type InGaAsP guide layer 72, an InGaAsP active layer 73 and an n type InP clad layer 74 are sequentially formed on a p type InP substrate 71 where the primary and secondary diffraction gratings 76 and 75 are formed. In addition, electrodes and coating on facets are not shown. The diffraction gratings are manufactured on the p type InP substrate 71 instead of the substrate 31 shown in figure 3 in accordance with a flow shown in figure 3. More specifically, first, referring to figure 3(a), the resist (image reversible resist), which is baked at a temperature of 110 to 120°C before exposure so that developing speed may have the minimum value in regard to certain exposure intensity, is applied to the substrate. Then, it is baked at a temperature of 110 to 120 °C. Then, referring to figure 3(b), the substrate is partially covered with the semi-transparent mask 43 and then the resist is exposed by the interference exposure method with exposure intensity in which the maximum and mininum values of the exposure intensity are on both sides of intensity which makes the developing speed to be the minimum value in a portion which is not covered with the semi-transparent mask 43 and exposed with exposure intensity in which the maximum value of the exposure intensity is less than the intensity which makes the developing speed to be the minimum value in a portion covered with the semi-transparent mask 43. A width of the semi-transparent mask 43 and distance between them are approximately 300 µm. Thereafter, the substrate is etched away using appropriate etchant to form the diffraction gratings 75 and 76. Then, the InGaAsP guide layer 72, InGaAsP composition ratio of which permits the layer to have a band gap corresponding wavelength of 1.15 µm, the InGaAsP active layer 73, InGaAsP composition ratio of which permits the layer to have a band gap corresponding wavelength of 1.3 µm and the n type InP clad layer 74 are sequentially formed and then divided into chip in the center of the primary diffraction grating by cleavage. Thus, a DFB laser shown in figure 4 is completed. Although the semiconductor laser having no phase shift region was described in the above embodiment, when a phase shift plate is introduced into the interference exposure apparatus, the phase shift region can be formed in the center of the element in accordance with the manufacturing method of the diffraction gratings in accordance with the present invention. As a result, the semiconductor laser device shown in figure 2 can be manufactured. In addition, although the diffraction grating with the cycle of Λ is formed in a portion covered with the semi-transparent mask and the diffraction grating with the cycle of Λ/2 is formed in a portion not covered with the semi-transparent mask in the above embodiment, the same effect can be attained even if the whole surface of the resist on the substrate is exposed before or after the interference exposure so that the maximum value of the exposure intensity may be more than the point of α and the minimum value thereof may be less than the point of α in the portion covered with the semi-transparent mask and the minimum value of the exposure intensity may be more than the point of in the portion which is not covered with the mask. As described above, according to the present invention, the method for manufacturing the diffraction grating comprises the steps of applying a resist, of which developing speed has the extreme value in regard to certain exposure intensity, to a substrate where the diffraction grating is to be formed, performing interference exposure with beam intensity in which the maximum and minimum values of the exposure intensity may be on both sides of intensity which makes the developing speed to be the extreme value and etching the substrate using a pattern, which is obtained by developing the resist, as a mask. Thus, a diffraction grating pattern having a cycle which is a half of an interference fringe obtained by an interference exposure method can be formed. In addition, according to the present invention, since the order of the diffraction grating in the center of an element is higher than that of the diffraction grating in the neighborhood of resonator facets in a semiconductor laser device having the diffraction grating in an active region, beam density is prevented from being increased in the center of the element and very high linearity of a beam output in regard to an injected current and high stability of single wavelength oscillation can be obtained. In addition, according to the present invention, the method for manufacturing the diffraction gratings comprises the steps of applying a resist, of which developing speed has the extreme value in regard to certain exposure intensity, to a substrate where the diffraction grating is to be formed, partially covering the resist with a semi-transparent mask, performing an interference exposure with beam intensity in which the maximum and minimum values of the exposure intensity are on both sides of intensity which makes the developing speed to be the extreme value in either a portion covered with the semi-transparent mask or another portion and etching the substrate using a pattern, which is obtained by developing the resist, as a mask. As a result, two kinds of diffraction gratings having different cycles are formed by one interference exposure at the same time and the semiconductor laser having the primary and secondary diffraction gratings can be easily formed.

Claims: A method for manufacturing a diffraction grating by interference exposure, the grating having a period of half the wavelength of the illuminating radiation, said method comprising the steps of applying a resist (12) to a substrate (1) on which the diffraction grating (23b) is to be formed; performing interference exposure patterning said resist (12) by development; and etching said substrate (1) using said patterned resist (12) as a mask; said method being characterized in that a resist (12) is applied whose developing speed as a function of the exposure intensity has an extremum at a certain non-zero value of the exposure intensity, and in that said step of interference exposure is performed with beam intensity determined such that the maximum and minimum intensities at the resist (12) lie on both sides of the intensity value at which the resist developing speed reaches its extremum. A method for manufacturing a diffraction grating as claimed in claim 1, characterized in that said extremum of the developing speed of said resist (12) at a certain exposure intensity is the minimum value. A method for manufacturing a diffraction grating as claimd in claim 1, characterized in that said extremum of the developing speed of said resist (12) at a certain exposure intensity is the maximum value. A method for manufacturing a diffraction grating as claimed in claim 2, characterized in that said resist (12) is developed so that portions exposed with intensity of the extremum value are left. A method for manufacturing a diffraction grating as claimed in claim 3, characterized in that said resist (12) is developed so that portions exposed with intensity of the extremum value are removed. A method of manufacturing a diffraction grating as claimed in one of the preceding claims, characterized in that said resist (42) is partially covered with a semi-transparent mask (43) before performing said interference exposure.

Inventors: FUJIWARA MASATOSHI C O MITSUBI, OHKURA YUJI C O MITSUBISHI DEN, FUJIWARA, MASATOSHI, C/O MITSUBISHI DENKI K. K., OHKURA, YUJI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01L33
IPC codes: H01L21, H01S5, G03F7, G02B5