JP2012054367A - Semiconductor element, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor element including diffraction gratings which have a high effect of side mode suppression and excellent uniformity and reproducibility and whose depths are different each other, and a method of manufacturing the semiconductor element.SOLUTION: A mask with a plurality of openings having different opening widths is formed on a surface of a semiconductor comprising a plurality of layers with different compositions. Etching (methane/hydrogen plasma irradiation) is performed for each opening having the same opening width, in a condition where a hydrogen plasma concentration at a mask end becomes a predetermined concentration. By setting a plasma condition (a diffusion distance or a pressure) and a mask shape (the opening width) so as to avoid a region with a hydrogen plasma amount in which etching and polymer generation that drastically increase the unevenness occur simultaneously on its surface, a shape with different etching depths can be processed in the same plane while suppressing the unevenness on the surface. Etching is performed so that a layer to be etched among the layers with different compositions is different depending on the opening width.

Description

  The present invention relates to a semiconductor element and a manufacturing method thereof, and more particularly to a semiconductor element that processes shapes having different etching depths in the same plane and a manufacturing method thereof.

  In the research and development of high-performance and high-function devices, it is important to integrate the structure of complex devices, and device fabrication process technology such as processing (etching) and crystal regrowth technology is required. In particular, when a plurality of structures are integrated, usually a plurality of etching processes are required.

  FIGS. 1A to 1F show an etching process when etching is performed at different depths according to the conventional method. When etching is performed on the sample (for example, InP crystal) 1110 at different depths, a portion not etched at the time of the first etching is covered with a mask (dielectric material) 1120 (FIG. 1A) and then etched (FIG. 1). (B)). Next, after removing the mask for the first etching (FIG. 1C), the portion not etched again for the second etching is covered with a mask (FIG. 1D), and further etched (FIG. 1). 1 (e)) and the process of removing the mask (FIG. 1 (f)) must be repeated.

  In the fabrication of semiconductor optical devices, when etching with different depths is required, a plurality of mask formation, etching, and mask removal processes are essential, which leads to waste in terms of time and cost. . In consideration of such a problem, it has been substantially difficult to form fine and complicated structures having different depths by etching.

  Therefore, in order to solve the above-mentioned problem, when performing etching at different depths, it is possible to perform etching with different depths by performing etching once by using masks having different areas corresponding to the etching depths. Has been invented (see Patent Document 1).

  2A to 2C show a basic etching process. In the case of dry etching using a plasma state gas for etching a semiconductor, the etching proceeds by the reaction of the semiconductor 411 and the etching species (etching gas) 412 on the semiconductor surface in the etching process by the semiconductor.

  3A to 3C show an etching process when a mask that does not react with the etching species is used. When the surface of the semiconductor is covered with a material that does not react with the etching species 512 (for example, a dielectric such as silicon oxide or silicon nitride) using the mask 513, the etching species 512 on the mask is diffused and masked. The semiconductor surface that is not covered with is reached. As a result, the density of the etching species 512 increases on the semiconductor surface near the mask. This increase in the etching species 512 increases the etching rate of the semiconductor.

  As described above, the etching species flying on the mask diffuses on the semiconductor surface and promotes the etching of the semiconductor, so that the amount of etching of the semiconductor increases as the mask area increases. Therefore, according to this method, it is possible to easily produce a simple groove structure whose depth changes.

  A diffraction grating having a variable depth that can be manufactured using this method will be described. FIG. 4 shows oscillation spectra of a distributed reflection (DBR) laser having a diffraction grating with a constant depth and a distributed reflection (DBR) laser having a diffraction grating with a varying depth. It can be seen that the side mode is reduced when the depth of the diffraction grating is changed (91) and when the depth of the diffraction grating is not changed (92). This reduction in side mode provides wavelength stability to the laser characteristics. In order to obtain such an effect, it is important to increase the change in coupling coefficient by increasing the difference in depth that changes in the diffraction grating.

JP 2004-247710 A

  However, when forming diffraction gratings having different depths, the distribution of the etching gas tends to fluctuate at the boundary where the depth is changed. Therefore, if the difference in depth between adjacent diffraction gratings at this boundary is too large, the uniformity and reproducibility of the diffraction grating shape deteriorates. Further, in forming a diffraction grating having a large depth difference, a manufacturing error (a difference between an actually formed depth and a designed depth) increases. Furthermore, when a crystal is regrown on a diffraction grating with a large difference in depth, the crystal surface after the regrowth tends to be uneven due to the diffraction grating shape, and it is difficult to flatten the surface after the regrowth. It is.

  Therefore, it is important to reduce the difference in depth between adjacent diffraction gratings. On the other hand, if the difference in depth between adjacent diffraction gratings is too small, the amount of change in the coupling coefficient is also small, so that the effect of diffraction gratings having different depths (side mode suppression) is also small. Therefore, in order to reduce fabrication errors in the formation of diffraction gratings with different depths and to flatten the surface after regrowth, it is possible to use as large a gain coupling as possible using diffraction gratings with as small a depth difference as possible. It was necessary to make a difference.

  The present invention has been made in view of such problems, and the object of the present invention is to provide a semiconductor element including diffraction gratings having different depths that have a large effect of side mode suppression and are excellent in uniformity and reproducibility, and the same. It is to provide a manufacturing method.

  In order to solve the above-mentioned problems, an invention according to claim 1 is an optical semiconductor device having a diffraction grating in an optical waveguide, wherein the optical waveguide is a first perpendicular to the traveling direction of propagating light. A plurality of layers having different compositions stacked in the direction of the first, and the diffraction grating includes a plurality of regions having different depths of the diffraction grating in the first direction. The layer in which the diffraction grating exists is different in at least two of the regions.

  According to a second aspect of the present invention, an etching species is supplied to a surface of a semiconductor composed of a plurality of layers having different compositions, on which a mask having an opening having a variable opening width is formed. A method of manufacturing a semiconductor element to be etched includes the step of etching so that, among the plurality of layers having different compositions, the layer to be etched is different in at least two openings having different opening widths. Features.

  According to a third aspect of the present invention, in the method of manufacturing a semiconductor device according to the second aspect, the etching species are hydrocarbon plasma and hydrogen plasma, and the etching step includes the hydrocarbon system at a predetermined flow rate. For the plasma and the hydrogen plasma, whether the etching of the semiconductor surface proceeds in each region having a different opening width or the second state in which a polymer is uniformly generated on the semiconductor surface A first step of forming, on the semiconductor surface, a mask having an opening width set so that only one of them is developed; and the hydrocarbon-based plasma and the hydrogen plasma at the predetermined flow rates And supplying the polymer deposited on the semiconductor surface in the first step to a plasma supply containing oxygen. And a third step of removing the hydrogen plasma, and changing the distance that the hydrogen plasma reaches from the edge of the mask into the opening, and expressing the state in at least one of the regions having different opening widths. And a fourth step of changing the predetermined flow rate of the hydrogen plasma so as to change to either the state or the second state, and the second step is further performed after the fourth step. It is characterized by performing.

  According to a fourth aspect of the present invention, in the method for manufacturing a semiconductor element according to the second or third aspect, the mask includes a first mask portion having a first pattern formed on the semiconductor surface; And a second mask portion formed on the first mask portion and having a second pattern that is thicker than the mask thickness of the first mask portion and defines an opening width of the first pattern. It is characterized by.

  The present invention makes it possible to easily produce diffraction gratings with different depths that have a great effect of suppressing side modes and are excellent in uniformity and reproducibility. Thereby, a high-speed semiconductor optical device can be provided.

It is a figure which shows the etching process in the case of performing the etching of the different depth by a conventional method. It is a figure which shows a basic etching process. It is a figure which shows the etching process at the time of using the mask which does not react with an etching seed | species. It is a figure which shows the oscillation spectrum of the distributed reflection type (DBR) laser which has a diffraction grating of fixed depth, and the distributed reflection type (DBR) laser which has a diffraction grating from which depth changes. It is a figure which shows the laminated structure which produces the diffraction grating which concerns on this invention. It is a figure which shows the laminated structure with which the diffraction grating which concerns on this invention was produced. It is a figure which shows the relationship between the diffraction grating depth in a guide layer, and a coupling coefficient. It is a figure which shows the reflection spectrum in three types of diffraction gratings. It is a figure which shows the preparation methods of the diffraction grating part used for the DBR semiconductor laser which concerns on Embodiment 1 of this invention. 1 shows a structure of a DBR semiconductor laser according to Embodiment 1 of the present invention. It is a figure which shows the phenomenon which arises in the area | region without a mask when etching using methane plasma is given. It is a figure which shows the phenomenon which occurs in the area | region enclosed by the mask when etching using methane plasma is performed. It is a figure which shows the mask for producing the diffraction grating which concerns on Embodiment 2 of this invention. It is a figure which shows the preparation processes of the diffraction grating which concerns on Embodiment 2 of this invention. It is a figure which shows the behavior of the etching seed | species on a mask. It is a figure which shows the behavior of the etching seed | species in the case where a mask with a thin film thickness is used and a mask with a thick film thickness are used. In the present embodiment 3 is a diagram showing a SiN _X / SiO ₂ mask is formed on the sample surface. It is a figure which shows the preparation process of the mask from which thickness differs in the surface used for the diffraction grating formation in this Embodiment 3. FIG. It is a figure which shows the manufacturing process of the diffraction grating in the manufacturing method which concerns on this Embodiment 3. FIG. It is a figure which shows the mask shape which can be used for this invention.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 5 shows a laminated structure for producing the diffraction grating according to the present invention, and FIG. 6 shows a laminated structure produced with the diffraction grating according to the present invention. Invention of the present application. An InP buffer layer 151, a guide layer 153, an optical waveguide layer 155, a guide layer 156, and an InP cladding layer 157 are stacked on an n-type InP substrate 150. The optical waveguide layer 155 corresponds to the 1.4 μm wavelength band. The guide layer 153 is made of InGaAsP (1.1 μm wavelength composition, 20 nm thickness), and the guide layer 156 is made of InGaAsP (1.3 μm wavelength composition, 20 nm thickness) 156a and InGaAsP (1.1 μm wavelength composition, 20 nm thickness) 156b. A diffraction grating having two depths is formed on the guide layer 156 as shown in FIG. The depth is 15 nm and 25 nm, respectively.

  Here, the depth is a depth in a direction from the boundary between the guide layer 156b and the InP cladding layer to the guide layer 156a. Therefore, the diffraction grating is made of only InGaAsP (1.1 μm wavelength composition, 20 nm thickness) in the region of 15 nm depth, and InGaAsP (1.1 μm wavelength composition, 20 nm thickness) and InGaAsP (1.3 μm wavelength) in the 25 nm depth region. Composition, 20 nm thickness).

  FIG. 7 shows a relationship a33 between the diffraction grating depth and the coupling coefficient in this guide layer. For comparison, a31 when a diffraction grating is formed on a guide layer made of only InGaAsP (1.1 μm wavelength composition) and a32 when a diffraction grating is formed on a guide layer made only of InGaAsP (1.3 μm wavelength composition) are also shown. Thus, in each case, the coupling coefficient increases as the depth of the diffraction grating increases.

When a diffraction grating is formed on a guide layer made only of InGaAsP (1.1 μm wavelength composition), a31 is increased when a diffraction grating is formed on a guide layer made only of InGaAsP (1.3 μm wavelength composition), and the coupling coefficient is increased. The inclination (inclination) is substantially constant, and the inclination of InGaAsP (1.3 μm wavelength composition) a32 is larger. As described above, when a diffraction grating having a depth of 15 nm and 25 nm is formed, in the guide layer made of only InGaAsP (1.1 μm wavelength composition), the coupling coefficients for the depths of 15 nm and 25 nm are 25 cm ⁻¹ and 35 cm, respectively. ^-1 . Thus, the difference in coupling coefficient due to the difference in depth between 15 nm and 25 nm remains at about 10 cm ⁻¹ .

On the other hand, in the guide layer 156 composed of InGaAsP (1.1 μm wavelength composition, 20 nm thickness) 156b and InGaAsP (1.3 μm wavelength composition, 20 nm thickness) 156a of the present invention, the diffraction grating becomes InGaAsP ( In addition to InGaAsP (1.3 μm wavelength composition, 20 nm thickness) 156a as well as 1.1 μm wavelength composition, 20 nm thickness) 156b, the degree of increase (slope) in the coupling coefficient increases. In the case of forming a diffraction grating having a depth of 15 nm and 25 nm, the coupling coefficients for 15 nm and 25 nm are 20 cm ⁻¹ and 50 cm ⁻¹ , respectively. Thus, the difference in coupling coefficient due to the difference in depth between 15 nm and 25 nm increases to about 30 cm ⁻¹ .

Further, in the guide layer made of only InGaAsP (1.3 μm wavelength composition), the coupling coefficients for the depths of 15 nm and 25 nm are 55 cm ⁻¹ and 95 cm ⁻¹ , respectively. Thus, the difference in coupling coefficient due to the difference in depth between 15 nm and 25 nm is about 40 cm ⁻¹ . FIG. 8 shows reflection spectra in the above-described three types of diffraction gratings. In the case of a31 (a), the reflectance is as low as about 0.8 and the stop band is small. In the case of a32 (b), the reflectance is as high as 1.0, and the stopband is larger than that in the case of a31. However, the side mode reflectivity is as high as about 0.2.

  On the other hand, according to the present invention (c), the reflectance is high and the stop band is larger than in the case of a31, and the reflectance of the side mode can be reduced to less than 0.1. In an optical device having a diffraction grating such as a DBR laser, the wavelength variable range can be expanded if the stop band is large, and the mode jump (wavelength jump) during device operation can be suppressed if the side mode is small.

  In this way, by configuring the guide layer forming the diffraction grating from two compositions, the difference in depth change in the diffraction grating is made as small as possible and the change in the coupling coefficient is made as large as possible. By doing so, the stop band is large and the side mode can be reduced.

  Here, the depth of the portion of the diffraction grating that fits in the guide layer 156b and the depth of the portion of the diffraction grating that reaches the guide layer 156a are constant, but in the present invention, the depth of fitting the diffraction grating in the guide layer 156b. May be formed to have a plurality of different depths, or may be formed to have a plurality of different depths in the depth reaching the guide layer 156a.

(Embodiment 1)
FIG. 9 shows a method for manufacturing a diffraction grating portion used in the DBR semiconductor laser according to Embodiment 1 of the present invention. In the first embodiment, a case where reactive ion beam etching (RIBE) is used as a dry etching method will be described.

  The sample is composed of an n-type InP substrate 1210 and a laminated structure 1211 (FIG. 9A). The laminated structure 1211 includes an n-type InP buffer layer 1212, an InGaAsP (composition wavelength: 1.1 μm) guide layer 1213, a DBR diffraction grating region InGaAsP (composition wavelength: 1.4 μm) guide layer (diffraction grating length before and after the active layer, respectively) 400 μm) 1214 and an InGaAsP guide layer 1215. The InGaAsP guide layer 1215 is made of InGaAsP (1.1 μm wavelength composition, 20 nm thickness) and InGaAsP (1.3 μm wavelength composition, 20 nm thickness). This laminated structure corresponds to the laminated structure composed of 151, 153, 155, and 156 in FIG.

First, a silicon oxide (SiO ₂ ) film 1221 is formed on the surface of the laminated structure 1211 (FIG. 9B).

After applying a resist on the SiO ₂ film, a resist pattern 1231 for a diffraction grating fabrication mask is fabricated by electron beam exposure (FIG. 9C). By processing the SiO ₂ film by reactive dry etching using the resist pattern 1231 as a mask, the resist pattern is transferred to the SiO ₂ film 1221 (FIG. 9D). By removing the resist pattern 1231, a SiO ₂ mask for producing a diffraction grating is formed on InP (FIGS. 9E, 9F, and 9G).

The SiO ₂ mask shape 1250 has a diffraction grating length 1251 of 400 μm, a diffraction grating width 1252 of 3.0 μm, and a pitch (period) 1253 of 240 nm (SiO ₂ part: 120 nm, window part: 120 nm). The area of the mask is narrow at both ends of the element (mask width 1254 is 1 μm) and wide at the center of the element (mask width 1255 is 5 μm).

RIBE is applied to the sample on which the SiO ₂ mask is formed. The etching gas is chlorine, the gas flow rate is 4 sccm, the gas pressure is 10 Torr, the microwave discharge power is 300 W, the ion extraction voltage is 500 V, and the substrate temperature is 200 ° C. At the center of the diffraction grating with a wide mask width, a large amount of ionized or radicalized chlorine atoms (chlorine plasma) that do not react on the mask diffuses into the opening, resulting in a high concentration of the atom in the opening and an increase in the etching rate. To do. On the other hand, the chlorine plasma that diffuses into the opening without reacting on the mask at both ends of the diffraction grating with a narrow mask width is smaller than the region with a large mask width, and the etching rate is slow. Therefore, a shallow diffraction grating is formed with a depth of 25 nm at the center and a depth of 15 nm at both ends (FIG. 9H).

  In this diffraction grating, since the diffraction grating is made of only InGaAsP (1.1 μm wavelength composition, 20 nm thickness) in the region where the depth is less than 20 nm, the coupling coefficient is relatively small, and the diffraction grating is in the region where the depth is 20 nm or more. Since it is composed of InGaAsP (1.1 μm wavelength composition, 20 nm thickness) and InGaAsP (1.3 μm wavelength composition, 20 nm thickness), the coupling coefficient increases. Therefore, the change in the coupling coefficient in the diffraction grating is larger than when the guide layer is made only of InGaAsP (1.1 μm wavelength composition, 20 nm thickness).

After removal of the SiO ₂ mask (FIG. 9 (i)), the device structure shown in FIG. 6 is fabricated by stacking on this diffraction grating by metal organic vapor phase epitaxy (MOVPE). Here, since the diffraction grating depth is 25 nm or less, the unevenness of the surface of the stacked crystal is suppressed and flat.

FIG. 10 shows the structure of the DBR semiconductor laser according to Embodiment 1 of the present invention. After removing the SiO ₂ mask from the surface of the sample having the diffraction grating of different depth produced by the method of the first embodiment, the depth is obtained by stacking on this diffraction grating by metal organic vapor phase epitaxy (MOVPE). Thus, a DBR semiconductor laser structure using a diffraction grating with a varying diameter is fabricated.

  A DBR semiconductor laser structure using a diffraction grating having a variable depth includes an n-type InP substrate 150, an n-type InP buffer layer 151, a diffraction grating 152, an InGaAsP (composition wavelength: 1.1 μm) guide layer 153, and eight InGaAsP layers. Active layer 154 (emission wavelength: 1.55 μm, active layer length) composed of multiple quantum well layers of strained quantum well (strain amount: 1.0%) and five InGaAsP (composition wavelength: 1.3 μm) barrier layers 400 μm), DBR diffraction grating region InGaAsP (composition wavelength: 1.4 μm) guide layer 155 (diffraction grating length is 400 μm respectively before and after the active layer), InGaAsP (composition wavelength: 1.3 μm) and InGaAsP (composition wavelength: 1. 1 μm), p-type InP cladding layer 157, p-type InGaAs (composition wavelength: 1.85 μm) contact layer 1 8, n-type ohmic electrode 1591, a p-type ohmic electrode 1592. The oscillation wavelength of this element is 1.55 μm, and the element length is 1500 μm. The pitch (period) of the diffraction grating 152 is 240 nm (convex portion: 120 nm, concave portion: 120 nm), and the depth is 5 nm which is shallow at both ends of the element, and 30 nm which is deep at the center of the element.

  In the oscillation spectrum of this laser, the side mode is suppressed and the wavelength stability is excellent. In addition, the stopband is large and the wavelength tunable characteristics are excellent.

  As described above, when the first embodiment is used, the difference in the depth change in the diffraction grating can be made as small as possible, and the change in the coupling coefficient can be made as large as possible. It is possible to provide an optical semiconductor device having excellent wavelength stability by forming diffraction gratings having different depths with excellent properties.

(Embodiment 2)
As a second embodiment of the present invention, another method for manufacturing a diffraction grating in the DBR semiconductor laser of the first embodiment will be described. The second embodiment is different from the first embodiment in that a polymer generated in dry etching using a hydrocarbon-based gas is used.

  When a hydrocarbon gas such as methane or ethane is used as the gas in dry etching using a plasma gas for semiconductor etching, the gas is decomposed into a hydrocarbon group and hydrogen in the plasma state, and is ionized or chemically It is activated (radicalized). In the present specification, hereinafter, ionized or chemically activated (radicalized) hydrocarbon groups are referred to as hydrocarbon plasma, and similar hydrogen atoms are referred to as hydrogen plasma.

When the hydrocarbon plasma and the hydrogen plasma come into contact with the semiconductor, a process of etching the semiconductor and a process of forming and depositing a polymer on the semiconductor without etching the semiconductor occur. In general, when there is sufficient hydrogen plasma, the etching process is the main process, and when hydrogen plasma is insufficient, the process of polymer deposition is the main process. At the same time, on the surface of the dielectric mask (such as SiO ₂ ), the hydrocarbon plasma and the hydrogen plasma do not react with the dielectric, so that they are deposited as a polymer.

  Here, a case where the above-described dry etching is performed on a sample having a mask will be described. FIGS. 11A and 11B show a phenomenon that occurs in a region without a mask when etching using methane plasma is performed. FIGS. 12A to 12C show etching using methane plasma. This shows the phenomenon that occurs in the area surrounded by the mask when applied.

  In the region without the mask, the hydrogen plasma is uniformly distributed on the sample surface with respect to the methane plasma (FIG. 11A). When the concentration of the uniformly distributed hydrogen plasma is low, there is insufficient hydrogen for etching, so that a polymer is generated on the sample surface and covers the sample surface, so that the etching does not proceed (FIG. 11B). The polymer deposition due to the lack of hydrogen plasma also occurs in a region where the width of the opening of the mask is wide and the distance from the mask is sufficient.

On the other hand, in the region surrounded by the mask, particularly in the region where the opening width of the mask is narrow, first, hydrogen plasma is uniformly distributed on the sample surface on the mask as well as on the semiconductor surface of the opening (see FIG. FIG. 11 (a)). Since the mask material (SiO ₂ ) is not etched on the mask, the methane plasma does not contribute to the etching and a polymer is generated. The hydrogen plasma on the mask diffuses on the mask without reacting with the methane plasma and aggregates in the opening. As a result, the concentration of hydrogen plasma at the opening increases (FIG. 11B). Therefore, the etching proceeds because the concentration of hydrogen plasma at the opening reaches a concentration sufficient to cause etching.

  FIG. 13 shows a mask for producing a diffraction grating according to Embodiment 2 of the present invention. The opening width (diffraction grating portion) 1913 of the mask is set to 1.0 μm, 1.5 μm, and 2.0 μm. The width 1915 of the diffraction grating exterior 1911 is constant. By using this mask and performing etching while changing the hydrogen flow rate in the same manner as in the first embodiment, a diffraction grating with a varying depth can be produced.

  14A to 14E show a manufacturing process of the diffraction grating according to Embodiment 2 of the present invention. RIE using a mixed gas of methane and hydrogen is performed on the mask shown in FIG. 13 at a methane flow rate of 40 sccm, a hydrogen flow rate of 2.5 sccm, a gas pressure of 10 Pa, and a discharge power of 100 W for 20 seconds (FIG. 14A). Then, in the opening 2001 having a width of 1.0 μm, etching proceeds without depositing polymer (etching depth: 20 nm). On the other hand, in the openings 2002 and 2003 having a width wider than 1.0 μm, the polymer is deposited and the etching does not proceed (FIG. 14B). Subsequently, when the oxygen plasma is irradiated for 1 minute at an oxygen flow rate of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W, the deposited polymer is removed (FIG. 14C).

  Next, when RIE is performed for 20 seconds under a condition where the hydrogen flow rate is changed (5 sccm), etching proceeds without etching in the openings 2001 and 2002 having a width of 1.5 μm or less (etching depth). : 20 nm). On the other hand, in the opening 2003 having a width of 2.0 μm, the polymer is deposited and the etching does not proceed (FIG. 14D). Accordingly, in the opening 2001 having a width of 1.0 μm or less, the second RIE etching further proceeds after the first RIE etching, so that the depth becomes deeper than the depth of the second opening 2002 ( Total etching depth: 40 nm).

  Next, when RIE is performed for 10 seconds under a condition where the hydrogen flow rate is changed (10 sccm), etching proceeds without depositing polymer in all the openings 2001, 2002, and 2003 (etching depth: 10 nm). . Therefore, since the etching by the third RIE proceeds in the opening 2001 having a width of 1.0 μm or less, the depth is 50 nm in total, and the opening 2002 having a width of 1.5 μm is performed by the second RIE. Since the etching progresses, the depth becomes 30 nm in total, and the etching by the first RIE proceeds in the opening 2003 having a width of 2.0 μm, so the depth becomes 10 nm. Subsequently, by removing the polymer by irradiation with oxygen plasma, diffraction gratings having different depths can be formed (FIG. 14E).

  In this diffraction grating, since the diffraction grating is made of only InGaAsP (1.1 μm wavelength composition, 20 nm thickness) in the region where the depth is less than 20 nm, the coupling coefficient is relatively small, and the diffraction grating is in the region where the depth is 20 nm or more. Since InGaAsP (1.2 μm wavelength composition, 20 nm thickness) is included, the coupling coefficient increases. In the region where the depth is 40 nm or more, InGaAsP (1.3 μm wavelength composition, 20 nm thickness) further includes the coupling coefficient. Therefore, the change in the coupling coefficient in the diffraction grating is larger than when the guide layer is made only of InGaAsP (1.1 μm wavelength composition, 20 nm thickness).

After removing the SiO ₂ mask, the element structure shown in FIG. 10 is produced by stacking on this diffraction grating by metal organic vapor phase epitaxy (MOVPE). Here, since the diffraction grating depth is 50 nm or less, the unevenness of the surface of the stacked crystal is suppressed and flat.

  In the second embodiment, the etching in which the depth using three opening widths is changed in three steps has been described. However, a larger number of opening widths are used and the pressure (diffusion length) corresponding to the opening width is changed. By performing methane / hydrogen RIE under the conditions, it is possible to perform etching with depth changing in more stages.

  If the diffraction grating of the second embodiment is used, the same effect as that of the first embodiment can be obtained in the optical semiconductor element.

(Embodiment 3)
As Embodiment 3 of the present invention, a method of manufacturing a diffraction grating whose depth changes in the plane will be described. The third embodiment is characterized in that the thickness of the mask at the diffraction grating portion is different from the thickness of the mask outside the diffraction grating.

  First, the effect of the difference between the mask thickness at the diffraction grating portion and the mask thickness outside the diffraction grating will be described.

  When the diffraction grating is manufactured using the method of the third embodiment, as shown in FIG. 15, not only the hydrogen 84 that has come on the mask 82 in the outer part of the diffraction grating but also the hydrogen 83 that has come on the mask 81 in the diffraction grating part. Since this also affects the etching, it becomes difficult to control the etching shape of the diffraction grating.

  A case where masks having different thicknesses are used in dry etching of a semiconductor having a mask formed on the surface will be described.

  FIGS. 16A and 16B show the behavior of etching species when a thin film mask is used and when a thick film mask is used. When a thin mask is used (FIG. 16A), the amount of hydrogen plasma attracted to the mask is small because the amount of charge-up of the mask during plasma irradiation is small. Therefore, there is little hydrogen plasma that contributes to etching.

  On the other hand, when a thick mask is used (FIG. 16B), the amount of charge up of the mask during plasma irradiation increases as the film thickness increases, so that the hydrogen plasma attracted by the mask also increases. Accordingly, the hydrogen plasma contributing to etching increases and the etching depth also increases.

  Therefore, if the thickness of the mask 82 in the outer part of the diffraction grating shown in FIG. 15 is made larger than the thickness of the mask 81 in the diffraction grating part, the contribution of hydrogen from the outside of the diffraction grating having a larger mask thickness will be larger, and the mask thickness will be increased. The contribution of hydrogen from the thin diffraction grating portion can be reduced. Accordingly, in the region where the opening width is narrow, the etching species supplied from the thin mask 81 to the opening is hardly supplied, but the etching species supplied (diffused) from the mask 82 to the opening is Since it reaches the center of the opening and contributes to etching, the etching rate is increased as a whole including the vicinity of the center of the opening.

  On the other hand, in a region having a wide opening, there is almost no etching species supplied from the thin mask 81 to the opening, and the etching species supplied (diffused) from the mask 82 to the opening reaches the center of the opening. Since it does not reach, the etching rate becomes slow near the center of the opening. Therefore, if this mask is used, the influence of the supply of the etching species from the thin mask 81 to the opening is suppressed, and the etching depth near the center of the opening is deepened in the region where the opening width is narrow. It can be made shallow in a wide region.

FIG. 17 shows a SiN _x / SiO ₂ mask formed on the sample surface in the third embodiment. The thickness of the lattice-shaped mask 1910 made of SiN _x is 20 nm, the length of the diffraction grating 1912 is 500 μm, and the grating width 1913 is changed, and 1.8, 3.7, 3.7, 7.5 μm. The pitch (period) 1914 is 240 nm (SiO ₂ part: 120 nm, window part: 120 nm). On the other hand, the opening width adjustment mask 1911 formed of SiO ₂ has a constant thickness of 1 μm and a width 1915 of 20 μm.

18A to 18D show a process for manufacturing masks having different thicknesses in the plane used for forming the diffraction grating in the third embodiment. First, a 30 nm thick silicon oxide (SiO ₂ ) film is formed on the surface of the InP clad layer 1311 on the InP substrate 1310. After coating a resist on the SiO ₂ film, a resist pattern for a diffraction grating fabrication mask is fabricated by electron beam exposure. By processing the SiO ₂ film by reactive ion etching (RIE) using a fluorocarbon system (CF ₄ , C ₂ F _6, etc.) using the resist pattern as a mask, the resist pattern is turned into an SiO ₂ film. Transcript. By removing the resist pattern, a SiO ₂ mask 1311 for producing a diffraction grating portion is formed on InP (FIG. 18A).

Next, a 1 μm thick silicon nitride (SiN _x ) film 1321 is formed on the InP surface having the above-described diffraction grating portion SiO ₂ mask (FIG. 18B). After applying a resist on the SiN _x film, a resist pattern 1331 for an opening width adjustment mask is produced (FIG. 18C). This resist pattern can be formed not only by electron beam exposure but also by normal photolithography resist exposure. The resist pattern 1331 is transferred to the SiN _x film 1341 by processing the SiN _x film by RIE using sulfur fluoride gas (SF ₆ or the like) using the resist pattern 1331 as a mask (FIG. 18D). At this time, the diffraction grating portion SiO ₂ mask remains unetched because it is resistant to sulfur fluoride gas (SF ₆ or the like). As a result, by removing the resist pattern 1331, masks having different thicknesses are formed on the InP at the lattice-like portion and the opening width adjustment portion (FIG. 18D).

  19A to 19E show a diffraction grating manufacturing process in the manufacturing method according to the third embodiment. The above-described mask is formed on the surface of InP200 (FIG. 19A). Here, the widths of the openings 2001, 2002, and 2003 are 1.0, 1.5, and 2.0 μm, respectively. In a series of etching processes described below, the contribution of methane / hydrogen plasma from above the opening width adjustment mask having a large mask thickness is large, and the contribution of methane / hydrogen plasma from the lattice mask having a thin mask thickness is small.

  First, when RIE using a mixed gas of methane and hydrogen is performed for 20 seconds at a methane flow rate of 40 sccm, a hydrogen flow rate of 2.5 sccm, a gas pressure of 10 Pa, and a discharge power of 100 W, in the opening 2001 having a width of 1.0 μm, Since there is sufficient supply of hydrogen from above the mask, etching proceeds without polymer deposition. On the other hand, in the openings 2002 and 2003 having a width larger than 1.0 μm, supply of hydrogen from the mask is insufficient, so that the polymer 2021 is deposited and the etching does not proceed (FIG. 19B).

  Next, when the oxygen plasma is irradiated for 1 minute at an oxygen flow rate of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W, the deposited polymer is removed (FIG. 19C). Next, when RIE is performed for 20 seconds under the condition where the hydrogen flow rate is increased (5 sccm), the supply of hydrogen is increased in the openings 2001 and 2002 having a width of 1.5 μm or less, so that etching is performed without polymer deposition. Progresses (etching depth: 20 nm).

  On the other hand, in the opening 2003 having a width of 2.0 μm, the polymer is deposited and the etching does not proceed (FIG. 19D). Accordingly, in the opening 2001 having a width of 1.0 μm, the etching by the second RIE proceeds after the etching by the first RIE, so that the depth becomes deeper than the depth of the opening 2002 (the total etching depth). (40 nm).

  Next, when RIE is performed for 10 seconds under the condition where the hydrogen flow rate is increased (10 sccm), etching proceeds without depositing polymer in all the openings 2001, 2002, and 2003 (etching depth: 10 nm). . Therefore, since the etching by the third RIE proceeds in the opening 2001 having a width of 1.0 μm, the total depth is 50 nm, and the opening 2002 having a width of 1.5 μm is performed by the second RIE. Since the etching proceeds, the depth becomes 30 nm in total, and the etching by the first RIE proceeds in the opening 2003 having a width of 2.0 μm, so the depth becomes 10 nm. Subsequently, by removing the polymer by irradiation with oxygen plasma, diffraction gratings having different depths can be formed (FIG. 19E).

  In this diffraction grating, since the diffraction grating is made of only InGaAsP (1.1 μm wavelength composition, 20 nm thickness) in the region where the depth is less than 20 nm, the coupling coefficient is relatively small, and the diffraction grating is in the region where the depth is 20 nm or more. Since InGaAsP (1.2 μm wavelength composition, 20 nm thickness) is included, the coupling coefficient increases. In the region where the depth is 40 nm or more, InGaAsP (1.3 μm wavelength composition, 20 nm thickness) further includes the coupling coefficient. Therefore, the change in the coupling coefficient in the diffraction grating is larger than when the guide layer is made of only InGaAsP (1.1 μm wavelength composition, 20 nm thickness).

After removing the SiO ₂ mask, the element structure shown in FIG. 10 is produced by stacking on this diffraction grating by metal organic vapor phase epitaxy (MOVPE). Here, since the diffraction grating depth is 50 nm or less, the unevenness of the surface of the stacked crystal is suppressed and flat.

  In the third embodiment, the etching in which the depth using the three opening widths is changed in three stages has been described. However, using a larger number of opening widths, the pressure (diffusion length) corresponding to the opening widths is increased. By performing methane / hydrogen RIE under the conditions, it is possible to perform etching with depth changing in more stages.

  If the diffraction grating of Embodiment 3 is used, the same effect as that of the first example can be obtained in the optical semiconductor element.

In the first to third embodiments, the depth of the diffraction grating is set to 15-25 and 10-50 nm. However, the depth is not limited to this, and the depth of the diffraction grating is not constant and changes in the element. If the depth of the diffraction grating is set so as to reach a plurality of compositions in relation to the thickness of the guide layer having a plurality of compositions, the same effect can be obtained. At this time, the depth of the diffraction grating does not need to be symmetrical in the element length direction. The coupling coefficient that changes in accordance with this depth is also set to 5-80 cm ⁻¹ , but is not limited to this value.

  In the first to third embodiments, the case where the guide layer forming the diffraction grating is composed of two or three layers having different compositions has been described. However, the guide layer may be composed of more layers having different compositions. Moreover, although 1.1 μm, 1.2 μm, and 1.3 μm are used as the composition wavelength of the guide layer, the composition wavelength may be continuously changed as long as it is shorter than the composition wavelength of the optical waveguide layer. The composition wavelength of the guide layer was changed from the long wavelength to the short wavelength from the optical waveguide layer side toward the clad layer. Conversely, even if the wavelength was changed from the short wavelength to the long wavelength, 1.1 μm, 1.3 μm, 1 The effect can be obtained by changing in an irregular order such as .2 μm.

  In the first to third embodiments, the diffraction grating is formed on the guide layer on the clad side, but may be formed on the guide layer on the substrate side.

  In the first to third embodiments, the region to be etched at the opening end is changed by changing the flow rate of hydrogen and the amount of hydrogen plasma supplied from the mask end into the opening, but the power is changed. Thus, the amount of hydrogen plasma that contributes to etching may be changed. Alternatively, the region to be etched at the end of the opening may be changed by changing the diffusion length of the hydrogen plasma by changing the pressure.

  In the first to third embodiments, hydrogen (plasma) that has not contributed to the reaction on the mask flows into both ends of the opening of the diffraction grating. Etching proceeds with the reaction of methane by this amount of hydrogen plasma. Therefore, even in the opening where the polymer is deposited, the etching proceeds only at a part of both ends. Thus, there may be a problem that the depth is not uniform over the entire opening. However, since a mesa structure is adopted in an actual device, both ends of the opening are cut off when the mesa is formed, and this does not cause a problem in actual device fabrication.

  In the first to third embodiments, the mask width outside the opening in the mask is constant. However, the mask has a length (width) that allows the hydrogen plasma diffusing into the opening to be diffused over the mask. The width need not be constant. For example, the shape shown in FIG.

  In the first to third embodiments, a compound semiconductor InP crystal is used as the element semiconductor crystal. However, a compound semiconductor crystal such as GaAs or SiGe or a mixed crystal such as InGaAsP, AlGaInAs, InGaN, GaInNAs, or AlGaSb may be used. Is possible. Further, although 1.55 μm is used as the wavelength of the laser beam to which the apparatus according to the present invention is applicable, the wavelength is changed from 1.0 μm to 1.7 μm by changing the structure such as the composition of the InGaAsP crystal and the pitch (period) of the diffraction grating. Up to a long wavelength band up to 1, and by using other materials (InGaAlN, AlGaInP, AlGaSb, etc.) for the active layer, it also supports a short wavelength band of less than 1.0 μm and a long wavelength band of 1.7 μm or more. it can. The multi-quantum well structure in the active layer includes an 8 layer, 5 nm thick InGaAsP strained quantum well layer (strain amount: 1.0%), a 5 layer, 10 nm thick InGaAsP barrier layer (composition wavelength: 1.1 μm). However, the structural factors such as the amount of strain, the number of layers, the layer thickness, and the crystal composition may be changed.

  In the first to third embodiments, methane is used as an etching gas in dry etching for processing a semiconductor such as a diffraction grating, but other hydrocarbon gases such as ethane may be used. In addition, as a dilution gas when using a mixed gas as an etching gas, not only hydrogen gas but also nitrogen or argon may be used together with hydrogen gas.

  Further, although oxygen plasma is irradiated to remove the polymer, plasma containing oxygen may be used. For example, a mixed gas of oxygen and an inert gas such as nitrogen or argon, a mixed gas of oxygen and hydrogen, or the like may be used.

Further, although RIBE and RIE methods are used for the dry etching method, processing can also be performed using ion beam assist etching or the like. In addition to the electron beam exposure method, an interference exposure method can be used for producing the resist pattern. Although silicon oxide (SiO ₂ ) is used as a mask formed on the semiconductor surface during etching, a dielectric such as silicon nitride or titanium oxide, or a metal such as gold or titanium can also be used.

  In addition to dry etching using a plasma state gas, it is also possible to perform processing by gas etching using a non-plasma state gas or wet etching using an acid solution.

  In the first to third embodiments, the present invention is applied to the DBR laser, but the present invention can also be applied to other optical semiconductor elements such as an optical filter.

81, 82 Mask 83, 84 Etching species 150 n-type InP substrate 151 n-type InP buffer layer 152 diffraction grating
153 InGaAsP (composition wavelength: 1.1 μm) guide layer 154 active layer (emission wavelength: 1.55 μm, active layer length: 400 μm)
155 DBR diffraction grating region InGaAsP (composition wavelength: 1.4 μm) guide layer 156a InGaAsP (composition wavelength: 1.3 μm) guide layer 156b InGaAsP (composition wavelength: 1.1 μm) guide layer 157 InP cladding layer 158 p-type InGaAs (composition) (Wavelength: 1.85 μm) Contact layer 1591 n-type ohmic electrode 1592 p-type ohmic electrode 411, 511, 1010, 1020 semiconductor 513, 1011, 1021, 1120 mask 412, 512, 1012, 1022 Etching species 1210 n-type InP substrate 1211 lamination structure 1221 silicon oxide (SiO ₂₎ film 1231 resist pattern 1591 n-type ohmic electrode 1592 p-type ohmic electrode 1913,2010 opening width 1915,2011 mask width 19 1 mask 1912 the length of the diffraction grating 1914 pitch 2001,2002,2003 opening

Claims (4)

An optical semiconductor element having a diffraction grating in an optical waveguide,
The optical waveguide is composed of a plurality of layers having different compositions stacked in a first direction perpendicular to the traveling direction of propagating light,
The diffraction grating is composed of a plurality of regions having different depths of the diffraction grating in the first direction, and of the plurality of layers having different compositions, the layer where the diffraction grating exists is different in at least two of the regions. An optical semiconductor element characterized by the above. A method for manufacturing a semiconductor device, wherein an etching seed is supplied to a surface of a semiconductor composed of a plurality of layers having different compositions, on which a mask having an opening having a variable opening width is formed, and the semiconductor is etched to a plurality of different depths Because
A method for manufacturing a semiconductor device, comprising: etching a layer to be etched in at least two openings having different opening widths among the plurality of layers having different compositions. The etching species are hydrocarbon plasma and hydrogen plasma,
The etching step includes
With respect to the hydrocarbon-based plasma and the hydrogen plasma at a predetermined flow rate, the polymer is uniformly applied to the semiconductor surface in the first state where the etching of the semiconductor surface proceeds for each region having a different opening width. A first step of forming on the semiconductor surface a mask in which the opening width is set so that only one of the generated second states is manifested;
A second step of supplying the hydrocarbon-based plasma and the hydrogen plasma at the predetermined flow rate to the semiconductor surface having the mask;
A third step of removing the polymer deposited on the semiconductor surface in the first step by supplying a plasma having oxygen;
The state in which the hydrogen plasma reaches the inside of the opening from the edge of the mask is changed, and the state that appears in at least one of the regions having different opening widths is any of the first state and the second state. And a fourth step of changing the predetermined flow rate of the hydrogen plasma so as to change
The method for manufacturing a semiconductor element according to claim 2, wherein the second step is further performed after the fourth step. The mask is
A first mask portion having a first pattern formed on the semiconductor surface;
And a second mask portion formed on the first mask portion and having a second pattern that is thicker than a mask thickness of the first mask portion and defines an opening width of the first pattern. The method for manufacturing a semiconductor element according to claim 2, wherein: