JP2011071282A - Method for manufacturing semiconductor photonic device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a semiconductor element, manufacturing a diffraction grating having its depth (coupling coefficient) easily changing to suppress the concavity and convexity on the surface of the diffraction grating of a manufactured element, and thereby decreasing an optical loss and showing characteristics of a side mode being suppressed. <P>SOLUTION: An SiO2 mask 1803 having the opening part changing its width is formed on an InGaAsP waveguide layer 1801. When RIE (reactive ion etching) is performed using a mixed gas of methane and hydrogen with a methane flow volume of 40 sccm, a hydrogen flow volume of 5 sccm, an electric discharge power of 100 W, and a gas pressure of 40 Pa, etching is promoted at a first opening part 1804 having a width of 1.8 μm to generates polymer on the surface of a second opening part 1805 and a third opening part 1806 without causing concavity and convexity (Fig. 7(b)). Successively, oxygen plasma is irradiated under an oxygen volume of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W, for one minute to remove the deposited polymer (Fig. 7(c)). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a semiconductor optical device, and more particularly to a method for manufacturing a semiconductor optical device in which dry etching is performed using a plasma state gas.

  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.

  FIG. 20 shows a conventional step of etching at a plurality of depths. In the case where the sample (for example, InP crystal) 1110 is etched at different depths, the portion not etched at the time of the first etching is covered with a mask (dielectric material) 1120 (FIG. 20A) (FIG. 20A) and then etched. FIG. 20 (b)). Next, after removing the mask for the first etching (FIG. 20C), the portion not etched again for the second etching is covered with a mask (FIG. 20D), and further etched (FIG. 20). 20 (e)-(f)) must be repeated.

  In the fabrication of semiconductor optical devices, when etching with different depths is required, a plurality of mask formation and etching steps are essential, leading 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). FIGS. 21A to 21C show the principle of etching using a conventional plasma state gas. 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. FIGS. 22A to 22C show etching characteristics when a mask is used in etching using a conventional plasma state gas. 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. It reaches the surface of the semiconductor not covered with (FIG. 22B). 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 etching of the semiconductor increases as the mask area increases. Therefore, according to this method, a simple groove structure whose depth changes can be easily manufactured.

In particular, 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 hydrocarbon groups and hydrogen in the plasma state, and each ionized. Alternatively, it is chemically activated (radicalized). 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, hydrocarbon plasma and hydrogen plasma do not react with the dielectric on the surface of the dielectric (such as SiO ₂ ) mask, so that they are deposited as a polymer.

  FIGS. 23A and 23B show a state in which hydrocarbon plasma and hydrogen plasma are used in a region without a mask. In the region without the mask, the plasma state hydrogen is uniformly distributed on the sample surface with respect to the plasma state methane (FIG. 23A). When the concentration of hydrogen in this uniformly distributed plasma state is low, there is insufficient hydrogen for etching, so that a polymer is generated on the sample surface and covers the sample surface (FIG. 23B), so that the etching does not proceed. This phenomenon also appears when the mask opening is wide.

FIGS. 24A to 24C show states when hydrocarbon plasma and hydrogen plasma are used in a region where a mask is present. In the region where the mask is present, particularly in the region where the opening width of the mask is narrow, hydrogen in the plasma state with respect to the methane plasma is uniformly distributed on the surface of the sample on the mask and on the surface of the opening (FIG. 24 ( a)). The hydrogen in the plasma state on the mask diffuses into the opening without reacting with the methane in the plasma state. As a result, the concentration of hydrogen in the plasma state at the opening increases (FIG. 24B). Therefore, the etching proceeds because the plasma concentration of hydrogen reaches a concentration sufficient to cause etching in the opening. On the other hand, since the mask material (SiO ₂ ) is not etched on the mask, the methane in the plasma state generates a polymer without contributing to the etching (FIG. 24C).

  As described above, in the region without the mask or the region where the opening width of the mask is wide, the polymer is generated and the etching does not proceed, and the etching proceeds in the region where the opening width is narrow. Therefore, shapes having different depths can be processed in one etching process.

JP 2004-247710 A

Kumagai, et al., “Physics and Applications of Vacuum”, Kubobo, 1970, p. 33, 44, 89

  However, in the boundary region between the region without the mask or the region where the opening of the mask is wide (the region where the polymer is generated and the etching does not proceed) and the region where the opening is narrow (the region where the etching proceeds), the formation of the polymer Etching occurs simultaneously. In this case, since a region where the polymer is generated and not etched and a region where the polymer is not generated and mixed are mixed, there is a problem that unevenness is generated on the semiconductor surface after plasma irradiation.

  FIGS. 25 (a) and 25 (b) show examples of unevenness caused by the polymer generated when the generation and etching of the polymer occur simultaneously. At this time, a polymer is partially generated on the surface, and etching proceeds using the polymer as a mask, resulting in unevenness (FIG. 25A). In addition, if the polymer is scattered on the surface and remains, etching proceeds using the remaining polymer as a mask, so that needle-like protrusions may be formed on the surface, resulting in significant unevenness (FIG. 25 (b)).

  FIG. 26 shows the dependence of the etching rate on the amount of hydrogen plasma when the amount of methane plasma is constant. When the amount of hydrogen plasma is from 0 to h0, the amount of hydrogen plasma is insufficient and a polymer is generated and etching is not performed. When h0 or more, etching starts simultaneously with the generation of the polymer. From h0 to h1, the formation of polymer is dominant and the etching rate is slow, so that the unevenness generated on the surface is not large. When it becomes h1 or more, the etching rate starts to increase and the unevenness of the surface becomes remarkable. When h2 or more, etching becomes dominant and the generation of polymer is reduced, so that the unevenness is also reduced. If it is 1 or more, the amount of plasma of hydrogen becomes sufficient, so that no polymer is generated, etching proceeds, and unevenness does not occur.

  Thus, in the region where the plasma amount of hydrogen is from h1 to h2, polymer generation and etching occur at the same time, and the surface unevenness becomes remarkable.

  A case where a sample having a mask on the surface is irradiated with methane / hydrogen plasma will be described. FIG. 27 shows the distribution of hydrogen diffusing from the mask edge. The vertical axis represents the hydrogen concentration Cx / Cs normalized by the hydrogen concentration Cs at the mask end. As the distance from the mask edge increases, the amount of hydrogen plasma decreases to h2 at a distance L2 and h1 at L1. That is, as described with reference to FIG. 26, the generation and etching of the polymer occur simultaneously in the region from h1 to h2, so that the surface unevenness becomes significant between the distances L2 and L1 from the mask end.

  As described above, there is a problem that unevenness is generated on the surface, and the quality of the crystal laminated on the surface is deteriorated. In addition, there is a problem that light is scattered by the concavo-convex portion when light is guided to the waveguide layer laminated on the surface.

  The present invention has been made in view of such problems, and the object of the present invention is to easily manufacture a diffraction grating whose depth (coupling coefficient) varies, and to manufacture the diffraction grating of the manufactured element. An object of the present invention is to provide a method for manufacturing a semiconductor element exhibiting characteristics in which surface unevenness is suppressed, light loss is reduced, and side modes are suppressed.

  In order to solve the above problems, the invention according to claim 1 is directed to irradiating a semiconductor surface on which a mask having an opening having a variable opening width is irradiated with hydrocarbon-based plasma and hydrogen plasma. A method of manufacturing a semiconductor device that etches the semiconductor surface to a plurality of different depths, wherein the semiconductor surface is formed for each region having a different opening width with respect to a predetermined diffusion distance and pressure of the hydrocarbon-based plasma and the hydrogen plasma. A mask whose opening width is set so that only one of the first state in which etching progresses or the second state in which a polymer is uniformly generated on the semiconductor surface appears. A first step of forming on the surface; and a second step of irradiating the surface of the semiconductor having the mask with the hydrocarbon plasma and the hydrogen plasma. .

  According to a second aspect of the present invention, in the method for manufacturing a semiconductor element according to the first aspect, the second step of removing the polymer deposited on the semiconductor surface in the first step by plasma irradiation having oxygen. It further has these.

  According to a third aspect of the present invention, in the method of manufacturing a semiconductor device according to the second aspect, the diffusion distance of the hydrogen plasma is changed by changing the pressure of the plasma conditions of the hydrocarbon plasma and the hydrogen plasma. The method further comprises a third step of changing the state of expression in at least one of the regions having different opening widths, and further performing a second step after the third step. To do.

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

  The present invention makes it possible to easily produce a diffraction grating with a varying depth (coupling coefficient), and suppresses irregularities on the surface of the produced grating, thereby reducing the optical loss of the produced semiconductor element. In addition, there is an effect of suppressing the side mode.

It is a figure which shows distribution of the hydrogen plasma from a mask edge in case a diffusion distance is 1 micrometer to 100 micrometers. It is a figure which shows distribution of the hydrogen plasma from the mask end in case diffusion distance is 10 micrometers and 100 micrometers. It is a figure which shows the relationship between a diffusion distance and a pressure. It is a figure which shows the cross section of the waveguide layer by which layer thickness changed in the same surface produced as Example 1 of this invention. It is a top view of the sample by which the mask which changed the width | variety of an opening part was formed on the InGaAsP (composition wavelength: 1.2 micrometer) waveguide layer concerning Example 1 of this invention. It is a figure which shows distribution of the hydrogen plasma from a mask end in case diffusion distance is 5, 10, 25 micrometers. It is a figure which shows the preparation processes of the waveguide layer which concerns on the present Example 1 of this invention. It is a figure which shows the structure of the semiconductor laser element which used the waveguide structure from which the layer thickness which concerns on Example 2 of this invention changes for the spot size conversion part. It is a top view of the sample in which the mask used in order to produce the diffraction grating which concerns on Example 3 of this invention was formed. It is a figure which shows the preparation processes of the diffraction grating which concerns on Example 3 of this invention. It is a figure which shows the structure of the DBR semiconductor laser which concerns on Example 2 of this invention. It is a figure which shows the coupling coefficient which the diffraction grating which concerns on Example 3 of this invention has. It is a figure which shows the reflection spectrum at the time of using the diffraction grating which concerns on Example 3 of this invention, and a reflection spectrum in case a coupling coefficient is constant 80nm- ¹ . It is a figure which shows a mode that an etching seed | species gathers in an opening part from the diffraction grating exterior and a diffraction grating part. (A) is a figure which shows the case where a thin film mask is used, and FIG.15 (b) is a figure which shows the case where a thick film mask is used. It is a figure which shows the SiNX / SiO2 mask formed in the sample surface in Example 4 of this invention. 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 which concerns on Example 4 of this invention. (A)-(e) is a figure which shows the preparation processes of the diffraction grating which concerns on Example 4 of this invention. It is a figure which shows the mask shape which can be used for this invention. (A)-(f) is a figure which shows the process of performing the etching of the conventional several depth. (A)-(c) is a figure which shows the principle of the etching using the gas of the conventional plasma state. (A)-(c) is a figure which shows the etching characteristic at the time of using a mask in the etching using the gas of the conventional plasma state. (A), (b) is a figure which shows the state at the time of using hydrocarbon plasma and hydrogen plasma in the area | region without a mask. (A)-(c) is a figure which shows the state at the time of using hydrocarbon plasma and hydrogen plasma for the area | region with a mask. (A), (b) is a figure which shows the example of the unevenness | corrugation produced by the polymer produced | generated when the production | generation and etching of a polymer occur simultaneously. It is a figure which shows the hydrogen plasma amount dependence of the etching rate when the amount of methane plasma is made constant. It is a figure which shows distribution of the hydrogen diffused from a mask end.

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

  FIG. 1 shows the distribution of hydrogen plasma from the mask edge when the diffusion distance is from 1 μm to 100 μm. The vertical axis represents the hydrogen concentration Cx / Cs normalized by the hydrogen concentration Cs at the mask end.

  FIG. 2 shows the distribution of hydrogen plasma from the mask edge when the diffusion distance is 10 μm and 100 μm. When the diffusion distance is 100 μm, h2 when the distance from the mask edge is 28 μm and h1 when the distance is 72 μm. Therefore, the unevenness becomes remarkable in the region where the distance from the mask edge is 28 μm to 72 μm. On the other hand, when the diffusion distance is 10 μm, h2 when the distance from the mask edge is 3 μm and h1 when the distance from the mask edge is 8 μm. Accordingly, the unevenness becomes remarkable in the region where the distance from the mask edge is 3 μm to 8 μm. As described above, by changing the diffusion distance of the hydrogen plasma, it is possible to change the region where the unevenness is remarkable.

  Next, the relationship between the diffusion distance and the pressure will be described. The average speed of the molecule

Where the Boltzmann constant is k, the temperature is T, the molecular radius is d, and the pressure is p, the diffusion constant D is

It is. Expression (1) is derived from, for example, Expressions (2.12), (2.46), and (2.140) in Non-Patent Document 1. The diffusion distance L _d is

(T is the diffusion time)

Is proportional to the pressure p.

FIG. 3 shows the relationship between the diffusion distance and the pressure. Here, the vertical axis represents the diffusion distance L _d / L _d0 normalized by the diffusion distance L _d0 when the pressure is 1 Pa. The diffusion distance decreases with increasing pressure, and the diffusion distance decreases by about 1/3 by increasing the pressure from 1 Pa to 10 Pa. When the pressure is further increased to 100 Pa, the diffusion distance is reduced by an order of magnitude compared to 1 Pa. Thus, the diffusion distance can be changed by changing the pressure. Therefore, by changing the pressure, the diffusion distance can be changed, and the area where unevenness is generated during plasma irradiation can be reduced.

Example 1
FIG. 4 shows a cross section of a waveguide layer whose layer thickness is changed in the same plane manufactured as Example 1 of the present invention. Reference numeral 1800 denotes an n-type InP substrate, 1801 denotes an InGaAsP (composition wavelength: 1.2 μm) waveguide layer, and 1802 denotes an InP clad layer. A manufacturing method of Example 1 will be described below.

  FIG. 5 shows a top view of a sample in which a mask in which the width of the opening is changed is formed on the InGaAsP (composition wavelength: 1.2 μm) waveguide layer according to Example 1 of the present invention. On the n-type InP1800 substrate, a mask in which the width of the InGaAsP (composition wavelength: 1.2 μm) 1801 and the opening 1900 is changed is formed. Here, the width of the opening is determined as follows. The mask width 1901 outside the opening is constant at 40 μm. The waveguide length 1902 is 500 μm.

  FIG. 6 shows the distribution of hydrogen plasma from the mask edge when the diffusion distance is 5, 10, and 25 μm. FIG. 6 is an enlarged view of a part of FIG. 1. Based on FIG. 6, etching is performed as described above when the relative amount (vertical axis) of hydrogen plasma is 0.3 to 0.6. It is assumed that the surface roughness is significantly increased due to simultaneous polymer formation.

  First, when the diffusion distance is 5 μm, the opening width is 1.8 μm or less, the relative amount of hydrogen plasma is 0.6 or more, and etching occurs predominantly. On the other hand, when the thickness is 3.7 μm or more, the relative amount of hydrogen plasma becomes 0.3 or less, and polymer formation occurs predominantly.

  Next, when the diffusion distance is 10 μm, the opening width is 3.7 μm or less, the relative amount of hydrogen plasma is 0.6 or more, and etching occurs predominantly. On the other hand, at 7.4 μm or more, the relative amount of hydrogen plasma becomes 0.3 or less, and polymer formation occurs predominantly.

  Next, when the diffusion distance is 25 μm, the opening width is 9.2 μm or less, the relative amount of hydrogen plasma is 0.6 or more, and etching occurs predominantly. On the other hand, at 18.4 μm or more, the relative amount of hydrogen plasma becomes 0.3 or less, and polymer formation occurs predominantly. Therefore, the first opening width is determined to be 1.8 μm, the second opening width is determined to be 3.7 μm, and the third opening width is determined to be 7.5 μm.

  First, when etching (methane / hydrogen plasma irradiation) is performed on the condition that the diffusion distance is 5 μm using a mask having such an opening width, etching occurs predominantly when the opening width is 1.8 μm or less. Therefore, only the first opening is etched. On the other hand, since the formation of polymer is dominant at 3.7 μm or more, the surface of the second opening (width: 3.7 μm) and the third opening (width: 7.5 μm) is polymerized and uneven. Hardly occurs.

  Next, when etching (methane / hydrogen plasma irradiation) is performed under the condition that the diffusion distance is 10 μm, the etching occurs predominantly when the opening width is 3.7 μm or less. Therefore, the first opening and the second opening The part is etched. On the other hand, since the generation of polymer predominantly occurs at 7.4 μm or more, a polymer is generated on the surface of the third opening (width: 7.5 μm), and unevenness hardly occurs.

  Next, when etching (methane / hydrogen plasma irradiation) is performed under the condition that the diffusion distance is 25 μm, the etching occurs predominantly when the opening width is 9.2 μm or less, so the first opening and the second opening And the third opening are etched.

  Here, in order to set the diffusion distance to 5 μm, 10 μm, and 25 μm, the pressure as the plasma condition is set to 40 Pa, 10 Pa, and 1.5 Pa, respectively.

  As described above, plasma conditions (diffusion distance or pressure) and mask shape (opening width) should be avoided so as to avoid a region of hydrogen plasma volume that causes simultaneous etching and polymer generation that significantly increase the surface roughness. By setting this, it is possible to process shapes having different etching depths in the same plane while suppressing surface irregularities.

  If the second opening width is set to 3.0 μm without using the present invention, the relative amount of hydrogen plasma at the opening width of 3 μm is about 0.4 when etching is performed under the condition that the diffusion distance is 5 μm. Therefore, the unevenness of the surface after etching increases remarkably. Since the uneven shape of the surface once generated in this way is maintained even after etching, unevenness remains on the surface of the waveguide layer in a region corresponding to the second opening in the manufactured waveguide layer. become. When such irregularities occur, the quality of the crystal laminated on the surface deteriorates, which causes a problem. In addition, when light is guided through the waveguide layer, the light is scattered by the concavo-convex portion, which is a problem.

  As described above, in the present invention, the plasma conditions (diffusion distance or pressure) and the mask shape (opening width) are set so as to avoid a region of hydrogen plasma amount in which etching and polymer generation occur simultaneously. As a result, it is possible to process shapes having different etching depths in the same plane while suppressing surface irregularities, so that the crystal quality of the clad layer laminated on the waveguide layer can be maintained well, Light can be guided while suppressing light loss without being scattered by the uneven portions in the waveguide layer.

7A to 7E show a manufacturing process of the waveguide layer according to the first embodiment of the present invention. Reference numeral 1800 denotes an n-type InP substrate, and 1801 denotes an InGaAsP (composition wavelength: 1.2 μm) waveguide layer. On the InGaAsP (composition wavelength: 1.2 μm) waveguide layer 1801 is formed an SiO ₂ mask 1803 in which the width of the opening is changed (FIG. 7A). When this sample was first subjected to RIE using a mixed gas of methane and hydrogen at a methane flow rate of 40 sccm, a hydrogen flow rate of 5 sccm, a discharge power of 100 W, and a gas pressure of 40 Pa, etching was performed at the first opening 1804 having a width of 1.8 μm. The surface of the second opening (width: 3.7 [mu] m) 1805 and the third opening (width: 7.5 [mu] m) 1806 is formed with a polymer, and there is almost no unevenness (FIG. 7 (b)). . However, although the polymer is deposited in most of the regions in the second opening 1805 and the third opening 1806, etching proceeds in a part of the boundary with the mask. Subsequently, when the oxygen plasma is irradiated at an oxygen flow rate of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W for 1 minute, the deposited polymer is removed (FIG. 7C).

  Next, when the methane flow rate, the hydrogen flow rate, and the discharge power are made constant and the gas pressure is applied at 10 Pa, the etching proceeds in the first and second openings 1804 and 1805 having a width of 3.7 μm or less. A polymer is generated on the surface of the opening (width: 7.5 μm) 1806, and unevenness hardly occurs (FIG. 7 (d)). Subsequently, when the oxygen plasma is irradiated at an oxygen flow rate of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W for 1 minute, the deposited polymer is removed. Accordingly, in the first opening 1804 having a width of 1.5 μm, the second RIE etching further proceeds after the first RIE etching, so that the first opening 1804 becomes deeper than the depth of the second opening 1805.

  Next, when the methane flow rate, the hydrogen flow rate, and the discharge power are made constant and the gas pressure is 1.5 Pa, etching proceeds in the first to third openings 1804 to 1806 having a width of 7.5 μm or less ( FIG. 7 (e)). As a result, the etching depth increases in the order of the first to third openings 1804 to 1806. Furthermore, there are almost no irregularities on the etched surface.

  Thereafter, an InP clad layer 1802 is laminated on the surface by metal organic vapor phase epitaxy (MOVPE), thereby producing a waveguide structure whose layer thickness varies within the same plane.

  In this embodiment, the etching using three opening widths in which the depth is changed in three stages has been described. However, a pressure (diffusion) corresponding to the opening width is used using a mask having a larger number of opening widths. By performing methane / hydrogen RIE under the condition of (distance), it is possible to perform etching with varying depth in more stages.

(Example 2)
FIG. 8 shows a structure of a semiconductor laser device using a waveguide structure with a layer thickness varying according to the second embodiment of the present invention for a spot size conversion unit. This laser element is characterized by having a spot size conversion portion in the outgoing light portion. The spot size conversion unit is a device that widens the diameter of the light-emitting beam in order to improve the efficiency (coupling efficiency) of the light emitted from the element incident on the optical fiber. In this spot size conversion section, the light distribution is oozed out into the cladding layer by gradually reducing the thickness of the waveguide layer, thereby increasing the spot size. 2000 is an n-type InP substrate, 2001 is an n-type InP buffer layer, 2002 is an active layer, 2003 is an InGaAsP (composition wavelength: 1.2 μm) guide layer for spot size conversion, and 2004 is an InGaAsP (composition wavelength: 1.1 μm) guide 2005, a p-type InP buried layer, 2006 an n-type InP buried layer, 2007 a p-type InP clad layer, 2008 a p-type InGaAs (composition wavelength: 1.65 μm) contact layer, 2009 a SiO ₂ layer, 2010 An n-type ohmic electrode, 2011 is a p-type ohmic electrode. The active layer 1802 includes an InGaAsP (composition wavelength: 1.1 μm) guide layer, six InGaAsP strained quantum well (strain amount: 0.8%) layers, and five InGaAsP (composition wavelength: 1.1 μm) barrier layers. It consists of an active layer (light emission wavelength: 1.3 μm) composed of a multiple quantum well layer and an InGaAsP (composition wavelength: 1.1 μm) guide layer.

A method for manufacturing the spot size conversion portion in the laser structure of the second embodiment will be described. A SiO ₂ mask similar to that of the first embodiment is formed on the surface of the spot size converting InGaAsP (composition wavelength: 1.2 μm) guide layer 2003 adjacent to the active layer 2002. This mask has a shape in which the mask width on the boundary side with the active layer 2002 is wide and the mask width on the emission end side is narrow. At the same time, the surface of the active layer 2002 is covered with the entire mask. When dry etching shown in Example 1 is performed on this sample, the etching depth is shallow on the boundary side of the active layer 2002 and deep on the emission end side. Next, after laminating the InP clad layer 2007, a spot size converting portion is formed by forming a mesa forming stripe mask and processing it into a mesa structure. In this structure, an electrode is formed after the growth of the pn structure buried layer 2005 and the growth of the cladding layer 2007 and the contact layer 2008, whereby the structure of the semiconductor laser device of the second embodiment is manufactured.

  As described above, if Example 2 is used, a waveguide layer (spot size conversion unit) whose layer thickness can be easily changed can be manufactured. The manufactured device has good characteristics such that the optical loss in the waveguide layer (spot size conversion portion) is reduced, the threshold current at room temperature is 6.7 mA, and the efficiency is 0.42 W / A.

(Example 3)
FIG. 9 shows a top view of a sample on which a mask used for manufacturing a diffraction grating according to Example 3 of the present invention is formed. FIGS. 10A to 10E show a manufacturing process of the diffraction grating according to Example 3 of the present invention. The opening width (diffraction grating portion) 3010 of the mask 3020 is set in the same manner as in the first embodiment. The width of the diffraction grating exterior 3011 is constant.

  Similar to the first embodiment, a diffraction grating with a varying depth can be manufactured by performing etching while changing the pressure using a mask 3020 in which the opening is changed.

  First, when RIE using a mixed gas of methane and hydrogen is performed at a methane flow rate of 40 sccm, a hydrogen flow rate of 5 sccm, a gas pressure of 40 Pa, and a discharge power of 100 W, the polymer is deposited in the opening 3001 having a width of 1.8 μm. Etching progresses without doing. On the other hand, in the openings 3002 and 3003 having a width larger than 1.8 μm, the polymer 3021 is deposited and the etching does not proceed (FIG. 10B). Subsequently, when the oxygen plasma is irradiated at an oxygen flow rate of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W for 1 minute, the deposited polymer is removed (FIG. 10C). Next, when RIE is performed under the condition where the pressure is changed (gas pressure is 10 Pa), etching proceeds without depositing polymer in the openings 3001 and 3002 having a width of 3.7 μm or less. On the other hand, in the opening 3003 having a width of 7.5 μm, the polymer 3021 is deposited and the etching does not proceed (FIG. 10D). Accordingly, in the opening 3001 having a width of 1.8 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 3002. Subsequently, after removing the polymer by irradiating with oxygen plasma, by changing the pressure and alternately repeating RIE and oxygen plasma irradiation, diffraction gratings having different depths can be formed (FIG. 10E). At this time, after the mask removal and mesa structure processing, the element structure shown in FIG. 11 is manufactured by stacking on the diffraction grating by metal organic vapor phase epitaxy (MOVPE).

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

FIG. 11 shows the structure of a DBR semiconductor laser according to Example 3 of the present invention. After removing the SiO ₂ mask from the sample surface having a diffraction grating with a different depth produced by the method of Example 2, the following layers are stacked on the diffraction grating by metal organic vapor phase epitaxy (MOVPE): A DBR semiconductor laser structure using a diffraction grating of varying depth shown in FIG. 11 is produced.

  3100 is an n-type InP substrate, 3101 is an n-type InP buffer layer, 3102 is a diffraction grating, 3103 is an InGaAsP (composition wavelength: 1.1 μm) guide layer, and 3104 is an eight-layer InGaAsP strained quantum well (strain: 1.0). %) And 5 layers of InGaAsP (composition wavelength: 1.3 μm) active layer consisting of multiple quantum well layers of barrier layers (emission wavelength: 1.55 μm, active layer length: 400 μm), 3105 is a DBR diffraction grating region InGaAsP ( Composition wavelength: 1.4 μm) Guide layer (diffraction grating length is 400 μm before and after the active layer), 3106 is an InGaAsP (composition wavelength: 1.3 μm) guide layer, 3107 is a p-type InP cladding layer, and 3108 is a p-type InGaAs ( Composition wavelength: 1.85 μm) A contact layer, 3191 is an n-type ohmic electrode, and 3192 is a p-type ohmic electrode. The oscillation wavelength of the element is 1.55 μm. The length of the element is 1500 μm. The pitch (period) of the diffraction grating 3102 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.

Here, the reflection characteristics obtained by the diffraction grating used in the DBR semiconductor laser according to Example 3 will be described. FIG. 12 shows the coupling coefficient of the diffraction grating according to the third embodiment. 5nm in depth element end portions of the diffraction grating, by varying such that the 30nm at the center, the coupling coefficient kappa 5 cm ^-1 at element end portions, changes 80 cm ^-1 at the central portion.

FIG. 13A shows a reflection spectrum when the diffraction grating of Example 3 is used, and FIG. 13B shows a reflection spectrum when the coupling coefficient is constant at 80 nm ⁻¹ for comparison. It can be seen that the side mode of the reflection spectrum is suppressed by changing the depth of the diffraction grating, that is, the coupling coefficient. This suggests that the side mode in the oscillation spectrum when the DBR laser using this diffraction grating is operated is suppressed.

  As described above, if Example 3 is used, a diffraction grating whose depth (coupling coefficient) changes easily can be manufactured. Since the fabricated element has suppressed surface irregularities in the diffraction grating, the optical loss is reduced and the side mode is suppressed as designed.

Example 4
Example 4 shows a method for manufacturing a diffraction grating whose depth changes in the plane. The fourth embodiment is characterized in that the thickness of the mask in the diffraction grating portion is different from the thickness of the mask outside the diffraction grating.

  First, the difference (action) in which the thickness of the mask 4001 at the diffraction grating portion is different from the thickness of the mask 4002 outside the diffraction grating will be described.

  When the diffraction grating is manufactured by using the method of the third embodiment, as shown in FIG. 14, not only the etching seed 4004 flying on the mask 4002 outside the diffraction grating but also the etching flying on the mask 4001 in the diffraction grating portion. Since the seed 4003 also affects the etching, it becomes difficult to control the etching shape of the diffraction grating.

  Therefore, in the fourth embodiment, etching is performed using a mask whose thickness is changed. FIG. 15A shows a case where a thin mask 4011 is used, and FIG. 15B shows a case where a thick mask 4021 is used.

  In the case where the mask 4011 having a small thickness is used, the etching species 4012 can diffuse from the surface of the semiconductor 4010 to the mask 4011 beyond the edge of the mask 4011, so that the etching species density on the surface of the semiconductor 4010 does not increase. Therefore, when the thin film mask 4011 is used, the etching rate of the semiconductor does not increase.

  On the other hand, in the case where the mask 4021 having a large thickness is used, the etching seed 4022 becomes an obstacle at the end of the mask 4021 and cannot exceed the end of the mask 4021, so that the etching seed 4022 is confined on the surface of the semiconductor 4020 and etched on the surface of the semiconductor 4020. Species density increases. Therefore, when the thick mask 4021 is used, the etching rate of the semiconductor is increased.

  Therefore, if the thickness of the mask 4002 outside the diffraction grating is made thicker than the thickness of the mask 4001 at the diffraction grating portion, the contribution of the etching species from the outside of the diffraction grating having a large mask thickness is large, and the diffraction grating having a thin mask thickness. The contribution of the etching species from the part can be reduced. Therefore, in a wide mask area of a mask having a large mask thickness, a large amount of etching species that do not react on the mask are diffused and confined in the opening, resulting in a high concentration of etching species in the opening and an etching rate. To increase. On the other hand, in the narrow mask area of the thick mask, the amount of etching species that does not react on the mask and diffuses into the opening is smaller than that in the wide mask area, and the etching rate is slow. Therefore, by using a mask having a large mask thickness, the characteristics that the etching depth is deep in a region having a large mask width and shallow in a region having a small mask width can be made remarkable.

FIG. 16 shows a SiN _x / SiO ₂ mask formed on the sample surface in Example 4 of the present invention. The mask thickness of the diffraction grating portion mask 4110 formed of SiN _x is 20 nm, the diffraction grating length 4112 is 500 μm, and the width 4113 is changed, and 1.8 to 3.7 from the center of the element toward both ends of the element. 7.5 μm. The pitch (period) 4114 is 240 nm (SiO ₂ part: 120 nm, window part: 120 nm). On the other hand, the mask thickness of the mask 4111 outside the diffraction grating formed of SiO ₂ is 1 μm, and the mask width 4115 is constant at 20 μm.

FIG. 17 shows a manufacturing process of masks having different thicknesses in the plane used for forming the diffraction grating according to the fourth embodiment of the present invention. First, a 30 nm thick silicon oxide (SiO ₂ ) film is formed on the surface of the InP clad layer 4210 on the InP substrate. 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 4211 for forming a diffraction grating portion is formed on the InP cladding layer 4210 (FIG. 17A).

Next, a 1 μm thick silicon nitride (SiN _x ) film 4212 is formed on the InP surface having the above-mentioned diffraction grating portion SiO ₂ mask (FIG. 17B). After applying a resist on the SiN _x film, a resist pattern 4213 for a mask outside the diffraction grating is produced (FIG. 17C). This resist pattern 4213 can be formed not only by electron beam exposure but also by resist exposure by ordinary photolithography. The resist pattern 4213 is transferred to the SiN _x film 4212 by processing the SiN _x film by RIE using sulfur fluoride gas (SF ₆ or the like) using the resist pattern 4213 as a mask. 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 4213, masks having different thicknesses are formed on the cladding layer 4210 at the diffraction grating portion and the outer portion of the diffraction grating (FIG. 17D).

  In this way, a fine pattern can be formed with a mask having a thin mask thickness, and a pattern for controlling the amount of etching species can be formed with a mask having a large mask thickness, thereby making it possible to manufacture a mask more easily. Become.

  18 (a) to 18 (e) show a manufacturing process of a diffraction grating according to Example 4 of the present invention. The above-described mask is formed on the surface of the InP clad 4300 (FIG. 18A). Here, the widths of the openings 4301, 4302, and 4303 are 1.8, 3.7, and 7.5 μm, respectively. In a series of etching processes described below, the contribution of methane / hydrogen plasma from the outside of the diffraction grating having a large mask thickness is large, and the contribution of methane / hydrogen plasma from the diffraction grating portion having a thin mask thickness is small.

  When RIE using a mixed gas of methane and hydrogen is first performed at a methane flow rate of 40 sccm, a hydrogen flow rate of 2 sccm, a discharge power of 100 W, and a gas pressure of 40 Pa, hydrogen is supplied from above the mask in the opening 4301 having a width of 1.8 μm. Therefore, etching proceeds without polymer deposition. On the other hand, in the openings 4302 and 4303 having a width wider than 1.8 μm, the supply of hydrogen is insufficient from above the mask, so that the polymer 4321 is deposited and the etching does not proceed (FIG. 18B). Subsequently, when the oxygen plasma is irradiated at an oxygen flow rate of 10 sccm, a gas pressure of 10 Pa, and a discharge power of 400 W for 1 minute, the deposited polymer is removed (FIG. 18C). Next, when RIE is performed under the condition where the hydrogen flow rate is increased (methane flow rate 40 sccm, hydrogen flow rate 5 sccm, gas pressure 10 Pa, discharge power 100 W), the openings 4301 and 4302 having a width of 3.7 μm or less are as follows: Since the supply of hydrogen is increased, etching proceeds without polymer deposition. On the other hand, in the opening 2003 having a width of 7.5 μm, since the supply of hydrogen is insufficient from above the mask, the polymer 4321 is deposited and the etching does not proceed (FIG. 18D). Therefore, in the opening 4301 having a width of 1.8 μm, the etching by the second RIE further proceeds after the first etching by RIE, so that the depth becomes deeper than the depth of the opening 4302. Subsequently, after removal of the polymer by irradiation with oxygen plasma, RIE with different pressures and oxygen plasma irradiation are alternately repeated, whereby diffraction gratings having different depths can be formed (FIG. 18E). Thereafter, after removing the mask and processing the mesa structure, the element structure shown in FIG. 11 is fabricated by stacking on the diffraction grating by metal organic vapor phase epitaxy (MOVPE).

  In the fourth embodiment, the etching in which the depth using three opening widths changes in three stages has been described. However, a larger number of opening widths are used and the pressure (diffusion distance) 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. The diffraction grating produced in this way has the same characteristics as in Example 3.

  As described above, by using the method of the fourth embodiment, it is possible to easily manufacture a diffraction grating whose depth (coupling coefficient) changes. Since the fabricated element has suppressed surface irregularities in the diffraction grating, the optical loss is reduced and the side mode is suppressed as designed.

  In the present invention, hydrogen (plasma) that did not contribute 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, etching proceeds 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 present invention, the mask width outside the opening in the mask is constant, but the mask width is constant as long as the distance that the hydrogen plasma diffusing into the opening can be disregarded on the mask is long (width). There is no need. For example, as shown in FIG. 19, a mask 4402 having a shape in which an opening is removed from what covers the entire sample 4401 may be used.

In this specification, the depth of the diffraction grating is set to 5 to 30 nm. However, the depth is not limited to this, and the same effect can be obtained if the depth of the diffraction grating is not constant and varies within the element. It is done. 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 this embodiment, 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 can be used. . 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 present invention, 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.

411, 511 Semiconductor 412, 512 Etching species 513 Mask 1110 InP crystal 1120 Mask 1800, 2000 n-type InP substrate 1801 InGaAsP waveguide layer 1802 InP clad layer 1803 Mask 1804 First opening 1805 Second opening 1806 Third Opening 2001 n-type InP buffer layer 2002 active layer 2003 InGaAsP guide layer for spot size conversion 2004 InGaAsP guide layer 2005 p-type InP buried layer 2006 n-type InP buried layer 2007 p-type InP clad layer 2008 p-type InGaAs contact layer 2009 SiO ₂ Layer 2010 n-type ohmic electrode 2011 p-type ohmic electrode 3001 first opening 3002 second opening 3003 third opening 3010, 30 1 mask opening width 3020 mask 3021 polymer 3100 n-type InP substrate 3101 n-type InP buffer layer 3102 diffraction grating 3103 InGaAsP guide layer 3104 active layer 3105 DBR diffraction grating region InGaAsP guide layer 3106 InGaAsP guide layer 3107 p-type InP cladding layer 3108 p-type InGaAs contact layer 3191 n-type ohmic electrode 3192 p-type ohmic electrode 4001 mask of diffraction grating portion 4002 mask outside diffraction grating 4003, 4004 etching species 4010, 4020 semiconductor 4011 thin masks 4012, 4022 etching species 4021 film thickness Thick mask 4110 Diffraction grating part mask 4111 Diffraction grating external mask 4112 Diffraction grating length 4113 Diffraction case Width 4114 pitch of the diffraction grating (period)
4115 Mask width 4210 InP cladding layer 4211 SiO ₂ mask 4212 Silicon nitride (SiN _x ) film 4213 Resist pattern 4301 First opening portion 4302 Second opening portion 4303 Third opening portion 4320 SiO ₂ mask 4321 Polymer 4401 Sample 4402 mask

Claims (4)

A method for manufacturing a semiconductor device, comprising: irradiating a semiconductor surface having a mask having an opening having a variable opening width with hydrocarbon-based plasma and hydrogen plasma; and etching the semiconductor surface to a plurality of different depths.
For the predetermined hydrocarbon-based plasma and the diffusion distance and pressure of the hydrogen plasma, the etching of the semiconductor surface proceeds for each region having a different opening width, or uniformly on the semiconductor surface. A first step of forming, on the semiconductor surface, a mask having the opening width set so that only one of the second states in which a polymer is produced is developed;
And a second step of irradiating the surface of the semiconductor having the mask with the hydrocarbon plasma and the hydrogen plasma.   The method for manufacturing a semiconductor device according to claim 1, further comprising a second step of removing the polymer deposited on the semiconductor surface in the first step by plasma irradiation having oxygen. The diffusion distance of the hydrogen plasma is changed by changing the pressure of the plasma conditions of the hydrocarbon-based plasma and the hydrogen plasma, and the state expressed in at least one of the regions having different opening widths is changed. And further comprising a third step of
The method for manufacturing a semiconductor element according to claim 2, wherein a second step is further performed after the third 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. 4. The method for manufacturing a semiconductor element according to claim 1, wherein

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