JP5551575B2 - Manufacturing method of semiconductor element - Google Patents

Description

  The present invention relates to a method for manufacturing a semiconductor element, and more particularly to a method for manufacturing a semiconductor element in which shapes having different etching depths are processed in the same plane.

  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, a plurality of etching processes are usually 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 semiconductor optical device fabrication, when etching with different depths is required, a plurality of mask formation, etching, and mask removal processes 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).

  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.

  In particular, when a hydrocarbon-based gas such as methane or ethane is used as the gas in dry etching using a plasma state gas for semiconductor etching, the gas is decomposed into a hydrocarbon group and hydrogen in the plasma state to be ionized or chemically respectively. 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. 4A and 4B show a phenomenon that occurs in a region without a mask when etching using methane plasma is performed, and FIGS. 5A to 5C 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. 4A). When the concentration of the uniformly distributed hydrogen plasma is low, there is insufficient hydrogen for etching, so a polymer is generated on the sample surface and the sample surface is covered, so that etching does not proceed (FIG. 4B). 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. 5 (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 (5 (b)). Therefore, the etching proceeds because the concentration of hydrogen plasma at the opening reaches a concentration sufficient to cause etching.

  Thus, 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

  In general, when a hydrocarbon gas such as methane or ethane is used as the gas in dry etching using a plasma gas for semiconductor etching, a plasma hydrogen gas is supplied together with the plasma hydrocarbon gas. By doing so, the reaction between hydrogen and a group V atom (P in InP, As in GaAs, etc.) in the III-V group compound semiconductor can be promoted to proceed the etching. Here, if the supply of hydrogen gas is reduced, a polymer is generated based on hydrocarbon groups, and the progress of etching can be stopped.

  In the selective dry etching described above, a region to be etched and a region in which a polymer is generated can be controlled by the supply amount of hydrogen plasma. For example, if the supply amount of hydrogen plasma is increased, the region in which hydrogen plasma diffuses into the opening is expanded, so that the etching region is expanded.

  However, when excessive hydrogen plasma reacts with the semiconductor, group V atoms (P in InP, As in GaAs, etc.) are desorbed excessively on the semiconductor surface, leading to induction of defects (group V atomic vacancy) or surface Morphological degradation occurs.

  If such a defect behaves as a recombination center, the device characteristics deteriorate, which is a problem. In addition, the deterioration of the surface morphology causes uneven portions on the semiconductor surface, so that when light is guided through the waveguide layer laminated on the semiconductor, the light is scattered by the uneven portions, which causes a problem.

  The present invention has been made in view of such problems, and the object of the present invention is not to induce defects (group V atomic vacancies) or deterioration of surface morphology due to desorption of constituent atoms of the semiconductor. Another object of the present invention is to provide a method for manufacturing a semiconductor element that can easily process shapes having different etching depths in the same plane.

In order to solve the above-described problem, 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 oxygen plasma. A method of manufacturing a semiconductor device that etches the semiconductor surface to a plurality of different depths, and etches the surface of the semiconductor for each region having a different opening width with respect to the hydrocarbon-based plasma and the oxygen plasma at a predetermined flow rate. A mask having an opening width set on the semiconductor surface so that only one of the first state in which the process proceeds or the second state in which a polymer is uniformly generated on the semiconductor surface appears. And forming the first state in a region where the opening width of the mask is narrower than a predetermined width and forming the first step in a region where the opening width of the mask is wider than the predetermined width. 2 The hydrocarbon-based plasma and the oxygen plasma flow state is expressed, a second step of irradiating the semiconductor surface with the mask, the polymer deposited on the semiconductor surface in the second step, the oxygen a third step of removing by plasma irradiation with, have a, to widen the predetermined width, such that the first state at least a part of a region where the second state was expressed is expressed The second and third steps are repeated by changing the flow rate of the oxygen plasma .

According to a second aspect of the invention, a method for manufacturing a semiconductor device according to claim 1, wherein the mask includes a first mask portion having a first pattern formed on the semiconductor surface, said first And a second mask part having a second pattern which is thicker than the mask thickness of the first mask part and defines an opening width of the first pattern. And

  INDUSTRIAL APPLICABILITY The present invention has the effect of facilitating processing of shapes having different etching depths in the same plane without inducing defects (group V atomic vacancies) or deteriorating surface morphology due to desorption of constituent atoms of the semiconductor. Play. Thereby, a high-speed semiconductor optical device can be provided.

(A)-(f) is a figure which shows the etching process in the case of performing the etching of different depth by the conventional method. (A)-(c) is a figure which shows a basic etching process. (A)-(c) is a figure which shows the etching process at the time of using the mask which does not react with an etching seed | species. (A), (b) is a figure which shows the phenomenon which arises in the area | region without a mask, when etching using methane plasma is given. (A)-(c) is a figure which shows the phenomenon which arises in the area | region enclosed by the mask, when etching using methane plasma is given. It is sectional drawing of the waveguide layer from which layer thickness changes in the same surface produced in Embodiment 1 of this invention. It is a figure which shows the mask used in order to produce the waveguide layer from which layer thickness changes in the same surface produced in Embodiment 1 of this invention. (A)-(e) is a figure which shows the preparation process of the waveguide layer from which layer thickness changes in the same surface produced in Embodiment 1 of this invention. FIG. 8A is a top view of FIG. 8B, and FIGS. 8B to 8D are cross-sectional views for each opening having different opening widths in FIG. (A) is a three-dimensional view of a semiconductor laser device using a waveguide structure with a varying layer thickness according to Embodiment 2 of the present invention for a spot size conversion unit, and (b) is a cross-sectional view thereof. It is a figure which shows the mask for producing the diffraction grating which concerns on Embodiment 3 of this invention. (A)-(e) is a figure which shows the preparation processes of the diffraction grating which concerns on Embodiment 3 of this invention. It is a figure which shows the structure of the DBR semiconductor laser using the diffraction grating which concerns on Embodiment 3 of this invention. (A) is a figure which shows the relationship between the distance from the end of the diffraction grating in the diffraction grating which concerns on Embodiment 3, and coupling coefficient ３, (b) When the diffraction grating which concerns on Embodiment 3 is used, and a coupling coefficient are shown. It is a figure which shows the reflection spectrum at the time of using 80 nm ^-1 constant diffraction grating. It is a figure which shows the behavior of the etching seed | species on the mask of FIG. (A) is a figure which shows the behavior of the etching seed | species when a thin film thickness mask is used, (b) is a figure which shows the behavior of the etching seed | species when a thick film mask is used. FIG. 10 is a diagram showing a SiN _x / SiO ₂ mask formed on the sample surface in the fourth embodiment. (A)-(d) 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 Embodiment 4. FIG. (A)-(e) is a figure which shows the preparation process of the diffraction grating in the preparation method which concerns on Embodiment 4. 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.

  The present invention is characterized in that oxygen is supplied in place of hydrogen in order to advance etching in dry etching using a plasma state gas. Oxygen does not cause detachment of semiconductor constituent atoms (such as P and As described above) unlike hydrogen, so that there is no induction of defects (group V atomic vacancies) or deterioration of surface morphology. Therefore, problems such as degradation of device characteristics due to desorption of constituent atoms of the semiconductor, and scattering of guided light due to it do not occur.

Details of the reaction will be described below.
When supplying conventional hydrogen plasma
Etching occurs based on the reaction of CH ₄ + H ₂ + InP → In organic compound + P hydride, and when hydrogen is insufficient, a polymer is generated based on excess hydrocarbon groups and the etching stops. In selective etching, hydrogen is insufficient on the InP surface without a mask, polymer is generated, and etching does not proceed. On the InP surface of the opening of the mask, hydrogen plasma on the mask diffuses into the opening. The hydrogen plasma increases and etching occurs.

When oxygen plasma is supplied instead of hydrogen plasma, CH ₄ + O ₂ + InP → In organic compound + P hydride + carbon oxide (CO, CO ₂ etc.)
Based on the above reaction, in a normal etching on a wide InP surface without a mask, if there is a sufficient amount of oxygen, excess hydrocarbon groups combine with oxygen to form oxygen carbide, so no polymer is produced, On the other hand, a small amount of hydrogen plasma is generated from the plasma decomposition of CH ₄ , but etching hardly occurs because the hydrogen plasma density is insufficient to etch a large area InP surface.

On the other hand, in the selective etching, a large InP surface without a mask has a shortage of O ₂ , so that carbon oxide (CO, CO _2, etc.) is not generated and a hydrocarbon group remains excessively, resulting in a polymer. On the other hand, oxygen plasma (mainly ions) is attracted to the mask charged up by plasma irradiation on the InP surface at the opening of the mask, and this oxygen plasma diffuses into the opening and reacts with hydrocarbon groups to react with carbon oxide (CO ₂ etc.) is generated. Therefore, hydrocarbon groups are reduced by the amount of carbon oxide that is generated by reaction with oxygen plasma, so that polymer formation is suppressed at the opening. Further, since a small amount of hydrogen plasma generated from the plasma decomposition of CH ₄ diffuses into the opening, the hydrogen plasma density in the narrow opening increases, so that InP etching occurs in the opening.

  When the oxygen flow rate is increased here, on the wide InP surface without a mask, excess hydrocarbon groups combine with oxygen to form oxygen carbide, so that no polymer is generated. On the other hand, on the InP surface with a narrow opening, as in the case before increasing the oxygen flow rate, the generation of polymer is suppressed, and InP etching is caused by a small amount of hydrogen plasma generated by plasma decomposition.

  When the oxygen flow rate is further increased, on the wide InP surface without a mask, excess hydrocarbon groups combine with oxygen to become oxygen carbide, so that no polymer is produced. On the other hand, on the InP surface with a narrow opening, the generation of polymer is suppressed, but a small amount of hydrogen plasma generated from plasma decomposition is also consumed by reacting with oxygen plasma, so that etching may not occur.

(Embodiment 1)
FIG. 6 is a cross-sectional view of a waveguide layer whose layer thickness varies in the same plane manufactured in Embodiment 1 of the present invention. Reference numeral 1800 denotes n-type InP, 1801 denotes an InGaAsP (composition wavelength: 1.2 μm) waveguide layer, and 1802 denotes an InP clad layer. A manufacturing method of this embodiment will be described below.

  FIG. 7 shows a mask used for manufacturing a waveguide layer whose layer thickness varies in the same plane manufactured in Embodiment 1 of the present invention. First, a mask in which the opening width 1900 changes is formed on the surface of InGaAsP (composition wavelength: 1.2 μm) 1801 laminated on the n-type InP substrate 1800. 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.

FIGS. 8A to 8E show a waveguide layer manufacturing process in which the layer thickness changes in the same plane manufactured in the first embodiment of the present invention. Reference numeral 1800 denotes n-type InP, and 1801 denotes an InGaAsP (composition wavelength: 1.2 μm) waveguide layer. The above-described SiO ₂ mask is formed on the surface (FIG. 8A). When this sample was first subjected to reactive ion etching (RIE) using a mixed gas of methane and oxygen at a methane flow rate of 40 sccm, an oxygen flow rate of 5 sccm, a discharge power of 100 W, and a gas pressure of 40 Pa, the width was 1.8 μm. Etching progresses in the first opening, and a polymer is generated on the surfaces of the second opening (width: 3.7 μm) and the third opening (width: 7.5 μm), and unevenness hardly occurs (see FIG. 8 (b)). 9A to 9D show cross sections of the openings having different opening widths from the top view of FIG. 8B. The polymer is deposited in most of the regions in the second opening and the third opening, but etching proceeds in the vicinity 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. 8C).

  Next, when the oxygen flow rate is 10 sccm with the methane flow rate, the discharge power, and the gas pressure kept constant, the etching proceeds in the first and second openings having a width of 3.7 μm or less, and the third opening On the surface (width: 7.5 μm), a polymer is generated and unevenness is hardly generated (FIG. 8D). 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. Therefore, in the first opening having a width of 1.8 μm, the etching by the second RIE further proceeds after the etching by the first RIE, so that it becomes deeper than the depth of the second opening.

  Next, when the methane flow rate, discharge power, and gas pressure are kept constant and the oxygen flow rate is 20 sccm, etching proceeds in the first, second, and third openings having a width of 7.5 μm or less (FIG. 8). (E)). As a result, the etching depth increases in the order of the first, second, and third openings. Furthermore, there are almost no irregularities on the etched surface.

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

  In the first embodiment, the etching in which the depth using the three opening widths is changed in three steps has been described. However, using a larger number of opening widths, the methane / By performing oxygen RIE, it is possible to perform etching in which the depth changes in more stages.

(Embodiment 2)
FIGS. 10A and 10B are a three-dimensional view and a cross-sectional view of a semiconductor laser device using a waveguide structure with a varying layer thickness 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 emitted light beam in order to improve the efficiency (coupling efficiency) of the light emitted from the element to enter 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.

2800 is an n-type InP substrate, 2801 is an n-type InP buffer layer, 2802 is an active layer, 2803 is an InGaAsP (composition wavelength: 1.2 μm) guide layer for spot size conversion, and 2804 is an InGaAsP (composition wavelength: 1.1 μm) guide. 2805 is a p-type InP buried layer, 2806 is an n-type InP buried layer, 2807 is a p-type InP clad layer, 2808 is a p-type InGaAs (composition wavelength: 1.65 μm) contact layer, 2809 is an SiO ₂ layer, and 2810 is An n-type ohmic electrode 2811 is a p-type ohmic electrode.

  The active layer 2802 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 this laser structure will be described. Similar to the first embodiment, an SiO ₂ mask having a plurality of openings with different opening widths is formed on the surface of the spot size conversion InGaAsP (composition wavelength: 1.2 μm) guide layer 2803 adjacent to the active layer 2802. This mask has a shape in which the opening width on the boundary side with the active layer 2802 is wide and the opening width on the emission end side is narrow. At the same time, the entire surface of the active layer is covered with a mask.

  When dry etching shown in Embodiment 1 is performed on this sample, the etching depth is shallow on the boundary side of the active layer and deep on the emission end side. Next, after laminating the InP clad layer, a spot size conversion portion is formed by forming a mesa forming stripe mask and processing it into a mesa structure. With respect to this structure, the structure of the semiconductor laser device of the second embodiment is fabricated by forming electrodes after the growth of the pn structure buried layer and the growth of the cladding layer and the contact layer.

  As described above, by using the present invention, a waveguide layer (spot size conversion section) whose layer thickness can be easily changed can be manufactured. The fabricated device exhibits 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.

(Embodiment 3)
FIG. 11 shows a mask for producing a diffraction grating according to Embodiment 3 of the present invention. The opening width (diffraction grating portion) 2010 of the mask is set in the same manner as in the first embodiment. The width of the diffraction grating outside 2011 is constant. By using this mask and performing etching while changing the oxygen flow rate in the same manner as in the first embodiment, a diffraction grating having a varying depth can be manufactured.

  12A to 12E show a manufacturing process of the diffraction grating according to Embodiment 3 of the present invention. RIE using a mixed gas of methane and oxygen is performed on the mask shown in FIG. 11 at a methane flow rate of 40 sccm, an oxygen flow rate of 5 sccm, a gas pressure of 40 Pa, and a discharge power of 100 W (FIG. 12A). Then, in the opening portion 2001 having a width of 1.8 μm, etching proceeds without depositing polymer. On the other hand, in the openings 2002 and 2003 having a width wider than 1.8 μm, the polymer is deposited and the etching does not proceed (FIG. 12B). 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. 12C).

  Next, when RIE is performed under conditions where the oxygen flow rate is changed (oxygen flow rate is 10 sccm), etching proceeds without depositing polymer in the openings 2001 and 2002 having a width of 3.7 μm or less. On the other hand, in the opening 2003 having a width of 7.5 μm, the polymer 2041 is deposited and the etching does not proceed (FIG. 12D). Accordingly, in the opening 2001 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 second opening 2002.

  Subsequently, after removal of the polymer by irradiation with oxygen plasma, RIE and oxygen plasma irradiation are alternately repeated while changing the pressure, whereby diffraction gratings having different depths can be formed (FIG. 12E). At this time, after the mask removal and mesa structure processing, the element structure shown in FIG. 13 is manufactured by stacking on the diffraction grating by metal organic vapor phase epitaxy (MOVPE).

  In the third 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, methane / By performing oxygen RIE, it is possible to perform etching in which the depth changes in more stages.

FIG. 13 shows the structure of a DBR semiconductor laser using a diffraction grating according to Embodiment 3 of the present invention. After removing the SiO ₂ mask from the sample surface having the diffraction gratings having different depths produced by the method of the third embodiment, a layer is deposited on the diffraction gratings by metal organic vapor phase epitaxy (MOVPE). A DBR semiconductor laser structure using a diffraction grating of varying depth as shown is fabricated.

  150 is an n-type InP substrate, 151 is an n-type InP buffer layer, 152 is a diffraction grating, 153 is an InGaAsP (composition wavelength: 1.1 μm) guide layer, 154 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), 155 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), 158 is an InGaAsP (composition wavelength: 1.3 μm) guide layer, 157 is a p-type InP cladding layer, and 158 is a p-type InGaAs ( Composition wavelength: 1.85 μm) A contact layer, 1591 is an n-type ohmic electrode, and 1592 is a p-type ohmic electrode.

  The oscillation wavelength of the element is 1.55 μm, and the length of the element 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.

FIGS. 14A and 14B show reflection characteristics obtained by the diffraction grating according to the third embodiment in the laser shown in FIG. FIG. 14A shows the relationship between the distance from one end of the diffraction grating and the coupling coefficient Κ in 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.

The reflection spectrum when this diffraction grating is used is indicated by α in FIG. For comparison, a reflection spectrum in the case where a diffraction grating having a constant coupling coefficient of 80 nm ⁻¹ is used for β in FIG. 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, by using this 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.

(Embodiment 4)
As a fourth embodiment, a method of manufacturing a diffraction grating whose depth changes in the plane will be described. The fourth embodiment is characterized in that the thickness of the lattice mask is different from the thickness of the opening width adjustment mask that determines the opening width in the longitudinal direction of the opening.

  FIG. 15 shows the movement of the etching species on the mask of FIG. When the diffraction grating is manufactured using the mask used in the third embodiment, not only the oxygen 84 flying on the opening width adjustment mask 82 but also the oxygen 83 flying on the lattice mask 81 as shown in FIG. Affects etching. Therefore, it becomes difficult to control the etching shape of the diffraction grating.

  Next, a case will be described in which dry etching is performed by forming on the semiconductor surface a mask having a different mask thickness that determines the thickness of the lattice mask according to the fourth embodiment and the opening width in the longitudinal direction of the opening. FIGS. 16A and 16B show the behavior of oxygen plasma when a thin film mask is used and when a thick film mask is used.

  When the thin mask 1011 is used (FIG. 16A), the amount of oxygen plasma 1012 attracted to the mask is small because the amount of charge-up of the mask during plasma irradiation is small. Therefore, the amount of carbon oxide produced by the reaction between oxygen plasma and hydrocarbon groups is small, and the number of hydrocarbon groups does not decrease, so the production of polymer is not suppressed.

  On the other hand, when a thick mask 1021 is used (FIG. 16B), the amount of charge up of the mask during plasma irradiation increases by the thickness of the film, so that the oxygen plasma attracted by the mask also increases. . As a result, the oxygen plasma reacts with the hydrocarbon groups to generate carbon oxide and the hydrocarbon groups are reduced, so that the production of the polymer is suppressed. Therefore, etching is caused by a small amount of hydrogen plasma decomposed from hydrocarbon groups.

  Therefore, if the thickness of the opening width adjustment mask 82 is made larger than the thickness of the lattice mask 81, the contribution of oxygen from the opening width adjustment mask 82 having a large mask thickness is large, and the lattice having a thin mask thickness is used. The contribution of oxygen from above the mask 81 can be reduced. Therefore, in a region where the opening width is narrow, oxygen plasma is hardly supplied from the thin lattice mask 81 to the opening, but diffuses from above the thick mask opening width adjustment 82 and diffuses to the opening. The supplied oxygen plasma reaches the center of the opening and contributes to the suppression of the production of the polymer, so that the etching rate is increased over the entire opening including the vicinity of the center of the opening.

  On the other hand, in the region where the opening width is wide, there is almost no supply of oxygen plasma from the thin lattice mask 81 to the opening, and the diffusion is supplied from the thick opening width adjustment mask 82 to the opening. Since the oxygen plasma does not reach the center of the opening, the etching rate is slow near the center of the opening. That is, with respect to the etching depth, the opening width adjustment mask 82 is dominant.

  For this reason, if such a mask is used, the influence of the supply of oxygen plasma from the thin lattice mask 81 to the opening is suppressed, and the etching depth near the center of the opening is reduced to the opening. It can be deep in the narrow region and shallow in the wide opening region.

FIG. 17 shows a SiN _x / SiO ₂ mask formed on the sample surface in the fourth 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 fourth 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 _8, etc.) using the resist pattern as a mask, the resist pattern is converted 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. By processing the SiN _x film by RIE using sulfur fluoride gas (SF ₈ or the like) using the resist pattern 1331 as a mask, the resist pattern is transferred to the SiN _x film 1341 (FIG. 18D). At this time, the diffraction grating portion SiO ₂ mask remains unetched because it is resistant to sulfur fluoride gas (SF ₈ 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 fourth 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 1.8, 3.7, and 7.5 μm, respectively. In a series of etching processes described below, the contribution of methane / oxygen plasma from above the opening width adjustment mask having a large mask thickness is large, and the contribution of methane / oxygen plasma from the lattice mask having a thin mask thickness is small.

  First, when RIE using a mixed gas of methane and oxygen is performed at a methane flow rate of 40 sccm, an oxygen flow rate of 2 sccm, a discharge power of 100 W, and a gas pressure of 40 Pa, the opening 2001 having a width of 1.8 μm has oxygen from above the mask. Is sufficiently supplied, the polymer deposition is suppressed and the etching proceeds. On the other hand, in the openings 2002 and 2003 having a width larger than 1.8 μm, the supply of oxygen is insufficient from above the mask, 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 under the conditions in which the oxygen flow rate is increased (methane flow rate 40 sccm, oxygen flow rate 5 ccm, gas pressure 10 Pa, discharge power 100 W), in the openings 2001 and 2002 having a width of 3.7 μm or less, Since the supply of oxygen increases, etching proceeds without polymer deposition.

  On the other hand, in the opening 2003 having a width of 7.5 μm, since the supply of oxygen is insufficient from above the mask, the polymer 2041 is deposited and the etching does not proceed (FIG. 19D). Accordingly, in the opening 2001 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 2002. Subsequently, a diffraction grating having different depths can be formed by alternately repeating the removal of the polymer by the oxygen plasma irradiation, the subsequent RIE with the changed pressure, and the oxygen plasma irradiation (FIG. 19E). ). Thereafter, the mask is removed and mesa structure processing is performed, and then the element structure shown in FIG. 13 is manufactured by stacking on this 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, but methane is used under a plasma condition corresponding to the opening width using a larger number of opening widths. / By performing oxygen RIE, it is possible to perform etching in which the depth changes in more stages. The diffraction grating produced in this way has the same characteristics as in the third embodiment.

  As described above, by using 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, oxygen plasma that did not contribute to the reaction on the mask flows into both ends of the opening of the diffraction grating. The generation of polymer is suppressed by the amount of oxygen plasma that has flowed in, and etching proceeds. Therefore, the etching proceeds only at a part of both ends in the opening where the polymer is deposited. 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 if the oxygen plasma diffused in the opening has a negligible distance (width) on the mask. There is no need. For example, the shape shown in FIG.

In the present invention, the depth of the diffraction grating is set to 5 to 30 nm. However, the depth is not limited to this depth, and the same effect can be obtained if the depth of the diffraction grating is not constant and varies within the element. . 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 present invention, 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 also be used. Further, the wavelength of the laser beam corresponding to the apparatus using the diffraction grating according to the present invention is 1.55 μm. It can handle long wavelength bands from 0 μm to 1.7 μm. By using other materials (InGaAlN, AlGaInP, AlGaSb, etc.) for the active layer, the wavelength is shorter than 1.0 μm or longer than 1.7 μm. It can also handle wavelength bands. 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, hydrogen, nitrogen, or argon may be used together with oxygen gas as well as oxygen 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.

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 158 InGaAsP (composition wavelength: 1.3 μm) guide layer 157 p-type 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 semiconductor 412, 512 etching species 1010 semiconductor 1011, 1021 mask 1012 etching species 1110 sample 513, 1120 mask 1310 InP substrate 1311 InP cladding layer 1321, 1341 silicon nitride (SiN _x ) film 1331 Resist pattern 1800, 2800 n-type InP substrate 1801 InGaAsP (composition wavelength: 1.2 μm) waveguide layer 1802 InP cladding layer 1900, 1913, 20 0 opening width 1901, 1915, 2011 mask width 1902 waveguide length 1911 mask 1912 diffraction grating length 1914 pitch 2001, 2002, 2003 opening 2801 n-type InP buffer layer 2802 active layer 2803 spot size conversion InGaAsP (composition wavelength) : 1.2 μm) Guide layer 2804 InGaAsP (composition wavelength: 1.1 μm) Guide layer 2805 p-type InP buried layer 2806 n-type InP buried layer 2807 p-type InP cladding layer 2808 p-type InGaAs (composition wavelength: 1.65 μm) contact Layer 2809 SiO ₂ layer 2810 n-type ohmic electrode 2811 p-type ohmic electrode

Claims (2)

A method for manufacturing a semiconductor device, comprising irradiating a hydrocarbon surface and an oxygen plasma on a semiconductor surface on which a mask having an opening whose opening width changes is formed, and etching the semiconductor surface to a plurality of different depths,
With respect to the hydrocarbon-based plasma and the oxygen plasma at a predetermined flow rate, the polymer is uniformly applied to the semiconductor surface in the first state where 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;
The flow rate at which the first state appears in a region where the opening width of the mask is narrower than a predetermined width and the second state appears in a region where the opening width of the mask is wider than the predetermined width . A second step of irradiating the surface of the semiconductor having the mask with the hydrocarbon-based plasma and the oxygen plasma ;
A third step of removing the polymer deposited on the semiconductor surface in the second step by plasma irradiation having oxygen;
Have a, to widen the predetermined width, the second and changing the flow rate of the oxygen plasma as the first state at least in part of a region where the second state was expressed is expressed A method for manufacturing a semiconductor element, wherein the third step is repeated . 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 1 .

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