JP5446161B2 - Schottky barrier diode and manufacturing method thereof - Google Patents

Description

  The present invention relates to a Schottky barrier diode and a manufacturing method thereof, and more particularly, to a Schottky barrier diode having an improved reverse withstand voltage and a manufacturing method thereof.

  Gallium nitride (GaN) has various excellent characteristics such as a band gap that is about three times that of silicon (Si), a breakdown electric field strength that is about ten times higher, and a larger saturation electron velocity. Since GaN can be expected to achieve both high breakdown voltage, which is difficult with conventional Si power devices, and low loss, that is, low on-resistance, it is expected to be applied to power devices (power semiconductor elements).

  Conventionally, semiconductor elements such as a Schottky barrier diode (SBD), a pn junction diode, and a MIS (metal-insulator-semiconductor) transistor using a GaN substrate have been proposed (see, for example, Patent Document 1). Conventionally, as a GaN material used for a power device, a GaN epitaxial layer formed on a heterogeneous substrate such as sapphire or SiC (silicon carbide) has been generally used. In contrast, a GaN epitaxial layer formed on a GaN substrate has a lower impurity concentration and a lower dislocation density than a GaN epitaxial layer formed on a heterogeneous substrate. Therefore, it is disclosed that a power device with a high breakdown voltage and a low on-resistance can be realized by performing GaN epitaxial growth on a GaN substrate (see, for example, Non-Patent Document 1).

In addition, a field plate (FP) structure is disclosed as a structure for suppressing electric field concentration at the electrode end portion of the power device and increasing the breakdown voltage (see, for example, Non-Patent Document 2).
JP 2006-100801 A Tatsuya Tanabe et al. “GaN epitaxial growth on GaN substrates and application to power devices”, SEI Technical Review No. 170, January 2007, p34-p39 Kenji Takada et al. “AlGaN / GaN HEMT Power Device”, Toshiba Review Vol. 59, No. 7, July 2004, p35-p38

  The inventor of the present invention has made a study on increasing the breakdown voltage of a Schottky barrier diode (SBD) using a GaN substrate. As a result, even when the field plate (FP) structure is applied to the SBD fabricated using the GaN epitaxial layer formed on the GaN substrate, the present inventor is based on the electric field concentration relaxation at the Schottky electrode end by the field plate structure. It was revealed for the first time that there was a problem that the effect of increasing the reverse withstand voltage was suppressed. That is, when an SBD is fabricated using a GaN epitaxial layer formed on a GaN substrate, the reverse leakage current based on the electric field concentration relaxation at the Schottky electrode end by the FP structure is applied even if the FP structure is applied to the SBD. In some cases, the effect of decreasing and increasing the reverse withstand voltage is suppressed.

  Therefore, a main object of the present invention is to provide a Schottky barrier diode having an improved reverse withstand voltage without suppressing the effect of increasing the reverse withstand voltage based on the electric field concentration relaxation of the Schottky electrode end by the field plate structure. Is to provide. Another object of the present invention is to provide a method for manufacturing a Schottky barrier diode with improved reverse withstand voltage.

  Even if the present inventor applied the FP structure to the SBD fabricated using the GaN epitaxial layer formed on the GaN substrate, the effect of increasing the reverse withstand voltage due to the relaxation of the electric field concentration at the Schottky electrode end by the FP structure is obtained. The reason for the suppression was examined. As a result, the following reason was estimated.

  When the nitride insulating film constituting the FP structure is formed by plasma CVD (Chemical Vapor Deposition), ammonia is usually used as a source gas. Therefore, ammonia molecules are dissociated during film formation, generating hydrogen radicals, hydrogen ions, and the like. In this specification, “hydrogen species” is used as a term including hydrogen radicals and hydrogen ions. Hydrogen is taken into the semiconductor layer due to generation of hydrogen species, and as a result, the hydrogen concentration in the nitride insulating film also increases. In the SBD having the FP structure, when hydrogen is taken into the semiconductor layer, it has an adverse effect.

  That is, the present inventor estimated that the hydrogen incorporated into the semiconductor layer suppresses the effect of increasing the reverse withstand voltage based on the electric field concentration relaxation at the Schottky electrode end by the FP structure. Therefore, the present invention has the following configuration.

The Schottky barrier diode according to the present invention includes a semiconductor layer having a main surface. A first nitride insulating film formed on the main surface and having an opening is provided. In addition, a Schottky electrode formed to be in contact with the main surface is provided inside the opening. The Schottky barrier diode further includes a second nitride insulating film formed on the first nitride insulating film. In addition, a field plate electrode is provided which is connected to the Schottky electrode and is formed to overlap the first nitride insulating film and the second nitride insulating film . The hydrogen concentration in the first nitride insulating film is less than 3.8 × 10 ²² cm ⁻³ . The hydrogen concentration in the second nitride insulating film is higher than the hydrogen concentration in the first nitride insulating film.

That lowering the hydrogen concentration of the first nitride insulating film is, that is that of reducing the amount of hydrogen species generated in forming the first nitride insulating film, thereby to hydrogen of the semiconductor layer Adverse effects can be eliminated. Therefore, the hydrogen concentration in the first nitride insulating film is less than 3.8 × 10 ²² cm ⁻³ , preferably less than 2.0 × 10 ²² cm ⁻³ , more preferably 1.6 × 10 ²² cm ⁻³ or less. By defining, it is possible to eliminate the adverse effect of hydrogen being taken into the semiconductor layer as compared with the case where a nitride insulating film having a high hydrogen concentration is used. Therefore, the effect of increasing the reverse withstand voltage based on the relaxation of the electric field concentration on the Schottky electrode end by the field plate structure is not suppressed. That is, a large electric field relaxation effect can be obtained and the reverse withstand voltage can be increased. The lower the hydrogen concentration in the first nitride insulating film, the better. For example, the hydrogen concentration in the first nitride insulating film is more preferably less than 2.0 × 10 ²² cm ⁻³ . Currently, the detection limit of the hydrogen concentration is 1.0 × 10 ¹⁷ cm ⁻³ . The Schottky barrier diode further includes a second nitride insulating film formed on the first nitride insulating film. In this case, the first nitride insulating film and the second nitride insulating film have a laminated structure. At this time, if the first nitride insulating film having a low hydrogen concentration is formed at the interface with the semiconductor layer, the reverse leakage current is reduced and the electric field is relaxed regardless of the hydrogen concentration in the second nitride insulating film. be able to. For example, SiN _x , SiON (silicon oxynitride film) or the like can be used as the material of the second nitride insulating film . In the case where the second nitride insulating film is stacked on the first nitride insulating film, the second nitride insulating film can bear a withstand voltage, and thus the thickness of the first nitride insulating film is the hydrogen at the interface with the semiconductor layer. The thickness may be 0.5 nm or more, which is a necessary thickness for reducing the concentration and obtaining the effect of the field plate structure. That is, the thickness of the first nitride insulating film can be not less than 0.5 nm and not more than 5 μm.

Here, the field plate structure is a structure including a nitride insulating film and a field plate electrode formed on the nitride insulating film. The field plate electrode is electrically connected to the Schottky electrode, and the Schottky electrode and the field plate electrode have the same potential. The field plate structure alleviates electric field concentration at the end of the Schottky electrode during operation, which causes device destruction, and enables the Schottky barrier diode to have a high breakdown voltage and a high output. The nitride insulating film refers to an insulating nitride-containing film such as SiN _x (silicon nitride film) or AlN (aluminum nitride).

  In the Schottky barrier diode, the semiconductor layer preferably includes a gallium nitride substrate. In this case, by applying a gallium nitride substrate to the semiconductor layer, the reverse leakage current is reduced, and the effect of electric field relaxation becomes more prominent.

Preferably, the dislocation density of the gallium nitride substrate is 1 × 10 ⁸ cm ⁻² or less. In this case, by applying a gallium nitride substrate having a low dislocation density, the reverse leakage current is further reduced, and the electric field relaxation effect becomes more prominent. The lower the dislocation density of the gallium nitride substrate, the better. For example, it is more preferable that the dislocation density of the gallium nitride substrate is 1 × 10 ⁶ cm ⁻² or less. Currently, the lower limit of the dislocation density of the gallium nitride substrate is about 1 × 10 ³ cm ⁻² .

Preferably, the dislocation density of the region in contact with the Schottky electrode in the semiconductor layer is 1 × 10 ⁸ cm ⁻² or less. In this case, the reverse leakage current is further reduced, and the effect of electric field relaxation appears more remarkably. The lower the dislocation density in the region in contact with the Schottky electrode in the semiconductor layer, the better. For example, it is more preferable that the dislocation density in the region in contact with the Schottky electrode in the semiconductor layer is 1 × 10 ⁶ cm ⁻² or less. Currently, the lower limit of the dislocation density in the region in contact with the Schottky electrode in the semiconductor layer is about 1 × 10 ³ cm ⁻² .

  Preferably, the material of the Schottky electrode is gold (Au), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), copper (Cu), silver (Ag), tungsten (W). And at least one substance selected from the group consisting of titanium (Ti). In this case, by using gold or the like as the material of the Schottky electrode, a low leakage current Schottky electrode can be realized, so that electric field relaxation occurs due to the field plate structure. As a result, the reverse leakage current decreases and the reverse withstand voltage increases.

Preferably, the thickness of the first nitride insulating film is not less than 10 nm and not more than 5 μm. If the thickness of the first nitride insulating film is less than 10 nm, the breakdown voltage of the first nitride insulating film is low, and the first nitride insulating film is destroyed first, and the effect of the field plate structure cannot be obtained. Moreover, if the thickness of the first nitride insulating film exceeds 5 μm, the electric field relaxation itself by the field plate structure cannot be obtained. The thickness of the first nitride insulating film is more preferably 0.2 μm or more and 2 μm or less, for example, in a design with a withstand voltage of 1 kV.

Preferably, the refractive index of the first nitride insulating film is not less than 1.7 and not more than 2.2. When the refractive index is 2.2 or less, it is possible to suppress a decrease in insulation in the first nitride insulating film, and thus it is possible to suppress an increase in conductivity, and as a result, it is possible to suppress a reverse leakage current. When the refractive index is 1.7 or more, a decrease in film density can be suppressed, so that the breakdown voltage of the first nitride insulating film can be improved.

Preferably, the length of the field plate electrode overlapping the first nitride insulating film is not less than 1 μm and not more than 1 mm. If the said length is less than 1 micrometer, control will become difficult and the effect of a field plate structure will not be acquired stably. If the length exceeds 1 mm, the electric field relaxation itself by the field plate structure cannot be obtained. For example, in the design with a withstand voltage of 1 kV, the length is more preferably 5 μm or more and 40 μm or less because the depletion layer width extends from 2 μm to 20 μm.

Preferably, a region in contact with the first nitride insulating film in the semiconductor layer is a gallium nitride compound. In this case, by applying a gallium nitride-based compound to a region in contact with the first nitride insulating film in the semiconductor layer, the reverse leakage current is reduced, and the effect of electric field relaxation becomes more prominent. Note that the gallium nitride-based compound is a compound represented by, for example, Al _x Ga _y In _(1-xy) N (0 ≦ x <1, 0 <y ≦ 1).

The manufacturing method of the Schottky barrier diode according to the present invention includes a step of forming a semiconductor layer. In addition, the method includes forming a first nitride insulating film having an opening on the semiconductor layer. After the step of forming the first nitride insulating film, prior to the step of forming the Schottky electrode, the method further includes a step of forming a second nitride insulating film so as to be stacked on the first nitride insulating film. Further, a step of forming a Schottky electrode inside the opening so as to be in contact with the semiconductor layer is provided. In addition, the method includes a step of forming a field plate electrode so as to connect to the Schottky electrode and overlap the first nitride insulating film and the second nitride insulating film . In the step of forming the first nitride insulating film, a source gas not containing ammonia is used as a main component. The hydrogen concentration in the second nitride insulating film is higher than the hydrogen concentration in the first nitride insulating film.

In this case, as a source gas for forming the first nitride insulating film by vapor deposition, without using the ammonia is the main cause of the hydrogen species generation, a first nitride insulating film. Thereby, the hydrogen concentration in the first nitride insulating film can be reduced. That is, since the influence of the hydrogen species at the time of forming the first nitride insulating film can be reduced, it can be excluded that hydrogen is taken into the semiconductor layer and has an adverse effect. Therefore, a large electric field relaxation effect can be obtained by the field plate structure, and the reverse withstand voltage can be increased. As this manufacturing method, a method of forming a first nitride insulating film by depositing SiN _x from SiH ₄ (monosilane) and N ₂ by using plasma CVD is preferable. After the step of forming the first nitride insulating film, prior to the step of forming the Schottky electrode, a step of forming a second nitride insulating film so as to be stacked on the first nitride insulating film is The nitride insulating film and the second nitride insulating film are formed in a stacked structure. At this time, if the first nitride insulating film having a low hydrogen concentration is formed at the interface with the semiconductor layer, the reverse leakage current is reduced and the electric field is relaxed regardless of the hydrogen concentration in the second nitride insulating film. be able to.

  Here, the source gas may be any gas that does not contain ammonia as a main component. Even if the raw material gas inevitably contains a low concentration of ammonia, the generation of hydrogen species is negligible, so there is virtually no effect. Therefore, unavoidably a gas containing ammonia is also included in the raw material gas.

Another method for manufacturing a Schottky barrier diode according to the present invention includes a step of forming a semiconductor layer. In addition, the method includes forming a first nitride insulating film having an opening on the semiconductor layer. After the step of forming the first nitride insulating film, prior to the step of forming the Schottky electrode, the method further includes a step of forming a second nitride insulating film so as to be stacked on the first nitride insulating film. Further, a step of forming a Schottky electrode inside the opening so as to be in contact with the semiconductor layer is provided. In addition, the method includes a step of forming a field plate electrode so as to connect to the Schottky electrode and overlap the first nitride insulating film and the second nitride insulating film . Then, in the step of forming a first nitride insulating film, a first nitride insulating film is formed using a physical vapor deposition method. The hydrogen concentration in the second nitride insulating film is higher than the hydrogen concentration in the first nitride insulating film.

In this case, vacuum deposition, ion plating, because it forms a first nitride insulating film by using a physical vapor deposition method such as sputtering, without using ammonia is the main cause of the hydrogen species generation, the first nitride insulating film Can be formed. Thereby, the hydrogen concentration in the first nitride insulating film can be reduced. In other words, since there is no influence of the hydrogen species at the time of forming the first nitride insulating film, it can be excluded that hydrogen is taken into the semiconductor layer and has an adverse effect. Therefore, a large electric field relaxation effect can be obtained by the field plate structure, and the reverse withstand voltage can be increased. After the step of forming the first nitride insulating film, prior to the step of forming the Schottky electrode, a step of forming a second nitride insulating film so as to be stacked on the first nitride insulating film is The nitride insulating film and the second nitride insulating film are formed in a stacked structure. At this time, if the first nitride insulating film having a low hydrogen concentration is formed at the interface with the semiconductor layer, the reverse leakage current is reduced and the electric field is relaxed regardless of the hydrogen concentration in the second nitride insulating film. be able to.

  In the Schottky barrier diode of the present invention, the reverse withstand voltage is improved without suppressing the effect of increasing the reverse withstand voltage due to the electric field concentration relaxation at the Schottky electrode end by the field plate structure. Further, according to the method for manufacturing a Schottky barrier diode of the present invention, a Schottky barrier diode having an improved reverse withstand voltage can be easily manufactured.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

(Embodiment 1)
FIG. 1 is a sectional view of a Schottky barrier diode of the present invention. FIG. 2 is a perspective view of the Schottky barrier diode shown in FIG. As shown in FIGS. 1 and 2, a Schottky barrier diode (SBD) 1 includes a GaN free-standing substrate 2 and a GaN epitaxial layer 3 that constitute a semiconductor layer. The GaN epitaxial layer 3 is formed on the surface 2 a of the GaN free-standing substrate 2. The SBD 1 also includes an insulating layer 4 as a nitride insulating film. The insulating layer 4 is formed on the surface 3a of the GaN epitaxial layer 3 as the main surface.

  The SBD 1 further includes an electrode 5 formed so as to be in contact with the surface 3 a of the GaN epitaxial layer 3 and overlap the insulating layer 4, and an electrode 6 formed on the back surface 2 b side of the GaN free-standing substrate 2. An opening is formed in the insulating layer 4, and the electrode 5 is formed inside the opening of the insulating layer 4. The electrode 5 is formed, for example, so that the planar shape is circular.

  The electrode 5 includes a Schottky electrode that is a portion in contact with the surface 3 a of the GaN epitaxial layer 3 inside the opening of the insulating layer 4 and a field plate electrode that is a portion overlapping the insulating layer 4. The field plate electrode and the insulating layer 4 form a field plate structure. The Schottky electrode forms a Schottky junction with the GaN epitaxial layer 3. On the other hand, the electrode 6 is an ohmic electrode that forms an ohmic junction with the GaN free-standing substrate 2.

The insulating layer 4 can be formed of a silicon nitride film (SiN _x ). The hydrogen concentration in the insulating layer 4 is less than 3.8 × 10 ²² cm ⁻³ , preferably less than 2.0 × 10 ²² cm ⁻³ , more preferably 1.6 × 10 ²² cm ⁻³ or less. Can do. Thus, SiN _x having a low hydrogen concentration in the film can be applied as an insulating film for forming a field plate structure. In this case, compared to the case where an insulating layer having a high hydrogen concentration is used, it is possible to suppress the generation of hydrogen species during the formation of the insulating layer and adversely affecting the GaN epitaxial layer 3. Therefore, the reverse withstand voltage increase effect based on the relaxation of the electric field concentration at the Schottky electrode end by the field plate structure is not suppressed. That is, with SBD1, a large electric field relaxation effect can be obtained, and the reverse withstand voltage can be increased.

The refractive index of the insulating layer 4 is preferably 1.7 or more and 2.2 or less, more preferably 1.8 or more and 2.1 or less. SiN _x having such a refractive index can be applied as an insulating film forming a field plate structure. In this case, when the refractive index increases, the film becomes rich in Si. The fact that the film is rich in Si means that the film has a composition close to that of Si, and therefore has higher conductivity than SiN _x that is an insulator. For this reason, if the refractive index is too high, reverse leakage current due to the insulating layer 4 is generated. From this point of view, the present inventor pays attention to the refractive index of the insulating layer 4, and as a result of earnestly studying the refractive index that suppresses the occurrence of reverse leakage current, it is effective when the refractive index is 2.2 or less. I found out. When the refractive index is 2.1 or less, the generation of reverse leakage current can be more effectively suppressed.

On the other hand, the inventor pays attention to the fact that the film density of the insulating layer 4 and the film withstand voltage have a correlation. If the film density is low, the material constituting the film is not densely packed and the refractive index is low. The inventors have found that a sufficient film breakdown voltage cannot be obtained. From this point of view, as a result of intensive studies by the present inventors, the finding that the film withstand voltage when the refractive index is 1.7 is 1 MV / cm, and the film withstand voltage when the refractive index is 1.85 is 9 MV / cm. Got. From this knowledge, it is preferable that the refractive index is 1.7 or more because the film withstand voltage of 1 MV / cm can be maintained. When the refractive index is 1.8 or more, the film density can be further improved, so that the film breakdown voltage can be further improved. As described above, when the insulating layer 4 having a high refractive index and a film density of 1.7 or more is formed by, for example, parallel plate type plasma CVD, a high plasma power density exceeding 200 mW / cm ² is required. In this case, dissociation of the reaction gas is promoted, and the bond between Si and N is promoted. As a result, the insulating layer 4 having a refractive index of 1.7 or higher and a high film density can be realized.

  The above-mentioned “refractive index” is a value measured by using a spectroscopic ellipsometry (polarization analysis method) for a wavelength of 633 nm, for example, with a spectroscopic ellipsometer.

The GaN free-standing substrate 2 and the GaN epitaxial layer 3 constitute a semiconductor layer. That is, the semiconductor layer includes a GaN substrate. The dislocation density of the GaN free-standing substrate 2 is desirably 1 × 10 ⁸ cm ⁻² or less. As a result, the dislocation density of the GaN epitaxial layer 3 is also equal to or less than 1 × 10 ⁸ cm ^−2, which is equivalent to that of the GaN free-standing substrate 2, the reverse leakage current in the SBD 1 is reduced, and the effect of electric field relaxation appears more remarkably. The dislocation density can be measured by, for example, counting the number of pits formed by etching in molten KOH and dividing by the unit area.

The semiconductor layer includes a GaN epitaxial layer 3. The dislocation density of the region 3c in contact with the Schottky electrode in the electrode 5 in the GaN epitaxial layer 3 is preferably 1 × 10 ⁸ cm ⁻² or less, and more preferably 1 × 10 ⁶ cm ⁻² or less. Thereby, as mentioned above, the effect of electric field relaxation appears more remarkably.

  Further, a region in contact with the nitride insulating film in the semiconductor layer is a gallium nitride compound. In the present embodiment, since the GaN epitaxial layer 3 is in contact with the insulating layer 4 in the semiconductor layer, GaN is used as the gallium nitride compound.

  The material of the electrode 5 desirably includes at least one substance selected from the group consisting of gold, platinum, nickel, palladium, cobalt, copper, silver, tungsten, and titanium. Since the portion of the electrode 5 that contacts the surface 3a of the GaN epitaxial layer 3 is a Schottky electrode, the above material is used as the Schottky electrode material. Thereby, a low leakage current can be realized.

  FIG. 1 shows the thickness of the insulating layer 4 as a dimension t. The thickness (dimension t) of the insulating layer 4 is desirably 10 nm or more and 5 μm or less. If the thickness (dimension t) of the insulating layer 4 is less than 10 nm, the withstand voltage of the insulating layer 4 is low, and the insulating layer 4 is destroyed first, so that the effect of the field plate structure cannot be obtained. Further, if the thickness of the insulating layer 4 exceeds 5 μm, the electric field relaxation itself by the field plate structure cannot be obtained.

  A dimension L shown in FIG. 1 indicates a field plate length. The field plate length is a length in which the field plate electrode overlaps the insulating layer 4. In the case of the present embodiment, the field plate length is a length in which the field plate electrode overlaps the insulating layer 4 in the cross section passing through the center of the planar circular electrode 5 of the SBD 1 as shown in FIG. is there. That is, when the planar shape of the opening of the insulating layer 4 is circular and the planar shape of the Schottky electrode which is a part of the electrode 5 is circular, the field plate length is the radial direction of the Schottky electrode. The length of the field plate electrode overlaps with the insulating layer 4.

  In other words, the field plate length means that the field plate electrode overlaps the insulating layer in the direction of a straight line that connects the center of gravity with respect to the planar shape of the Schottky electrode and a certain point on the outer periphery of the planar shape. Say length. Such a field plate length is desirably 1 μm or more and 1 mm or less. If the field plate length is less than 1 μm, the control becomes difficult and the effect of the field plate structure cannot be obtained stably. Further, if the field plate length exceeds 1 mm, the electric field relaxation itself by the field plate structure cannot be obtained.

  Next, the manufacturing method of SBD1 is demonstrated. FIG. 3 is a flowchart showing each step of the Schottky barrier diode manufacturing method. With reference to FIGS. 1-3, the manufacturing method of the Schottky barrier diode of this invention is demonstrated.

First, in the step (S10) shown in FIG. 3, the GaN free-standing substrate 2 is formed. Specifically, an n conductivity type (0001) plane GaN free-standing substrate 2 prepared by HVPE (Hydride Vapor Phase Epitaxy) is prepared. The carrier concentration of the GaN free-standing substrate 2 is, for example, 3 × 10 ¹⁸ cm ⁻³ , the thickness is, for example, 400 μm, and the average dislocation density is, for example, 1 × 10 ⁶ cm ⁻² . Next, in step (S20), an epitaxial film is formed. Specifically, an n-conductivity type epitaxial film having a carrier density of, for example, 5 × 10 ¹⁵ cm ⁻³ and a thickness of, for example, 7 μm is formed on the GaN free-standing substrate 2 by using an OMVPE (Organo-Metallic Vapor Phase Epitaxy) The GaN epitaxial layer 3 is produced by growing by the (phase growth) method. By this step (S10) and step (S20), a semiconductor layer including the GaN free-standing substrate 2 and the GaN epitaxial layer 3 can be formed.

Next, in step (S30), a nitride insulating film is formed. Specifically, SiN _x is formed as an insulating layer 4 on the GaN epitaxial layer 3 from SiH ₄ and N ₂ (nitrogen) by plasma CVD without using NH ₃ (ammonia). That is, the insulating layer 4 is formed using a source gas that does not contain NH ₃ as a main component. The film thickness (dimension t) of the insulating layer 4 is, for example, about 100 nm. Next, in step (S40), an ohmic electrode is formed. Specifically, after the back surface 2b of the GaN free-standing substrate 2 is subjected to organic cleaning and hydrochloric acid cleaning, Ti / Al / Ti / Au (20 nm / 100 nm / 20 nm / 200 nm) is formed on the back surface using an EB (Electron Beam) evaporation method. 2b is formed entirely. Thereafter, heating is performed at 600 ° C. for about 2 minutes in a nitrogen atmosphere, and alloying is performed to form an electrode 6 as an ohmic electrode.

  Next, in the step (S50), the nitride insulating film is etched. Specifically, patterning is performed on the insulating layer 4 by photolithography. Thereafter, the insulating layer 4 is wet-etched with BHF (Buffered Hydrogen Fluoride). Thereafter, the resist is removed by organic cleaning. In this way, the insulating layer 4 is etched, and an opening is formed in the insulating layer 4. At this point, the GaN epitaxial layer 3 is exposed in the opening. For example, the opening can be formed such that the side surface thereof has a truncated cone shape having a maximum diameter of 200 μm.

  Next, in step (S60), a Schottky electrode and a field plate electrode are formed. Specifically, patterning is performed by photolithography. Subsequently, after surface treatment of the GaN epitaxial layer 3 by hydrochloric acid cleaning is performed for 3 minutes at room temperature, Ni / Au (80 nm / 300 nm) is formed as an electrode material by EB vapor deposition and resistance heating vapor deposition. Thereafter, when the resist is removed, the electrode material formed on the resist is simultaneously removed (lifted off), and the electrode 5 is formed. The shape of the electrode 5 can be a shape having a larger diameter than the opening formed in the insulating layer 4. For example, the electrode 5 can be formed in a circular shape having a diameter of 220 μm.

  Thereby, a Schottky electrode which is a part in contact with the surface 3a of the GaN epitaxial layer 3 inside the opening of the insulating layer 4, a field plate electrode which is a part connected to the Schottky electrode and overlapping the insulating layer 4, Is formed. That is, since the diameter of the electrode 5 is larger than the diameter of the opening formed in the insulating layer 4, a part of the electrode 5 overlaps with the insulating layer 4 to form a field plate electrode.

  The SBD 1 shown in FIGS. 1 and 2 can be manufactured by the above manufacturing method. In this SBD 1 manufacturing method, in the step of forming the insulating layer 4 (S30), the insulating layer 4 is formed without using ammonia, which is the main cause of hydrogen species generation. Thereby, the hydrogen concentration in the insulating layer 4 can be reduced. That is, since the influence of the hydrogen species on the GaN epitaxial layer 3 at the time of forming the insulating layer 4 can be reduced, it can be excluded that hydrogen is taken into the GaN epitaxial layer 3 and has an adverse effect. Therefore, a large electric field relaxation effect can be obtained by the field plate structure, and the reverse withstand voltage can be increased.

  Here, in the step of forming the nitride insulating film (S30), the insulating layer 4 may be formed using a physical vapor deposition (PVD) method. As the PVD, for example, evaporation PVD such as vacuum deposition or ion plating, sputtering, or the like can be used. In PVD, since a thin film of a material that forms the insulating layer 4 is deposited on the surface of the GaN epitaxial layer 3 in a gas phase by a physical method, the insulating layer 4 is formed without using ammonia, which is a main cause of hydrogen species generation. Can be formed. Accordingly, as described above, hydrogen can be excluded from being taken into the GaN epitaxial layer 3 and adversely affected, so that a large electric field relaxation effect can be obtained by the field plate structure, and the reverse breakdown voltage can be increased. can get.

  In the above SBD manufacturing method, the example in which the Schottky electrode and the field plate electrode are simultaneously formed in the step (S60) has been described. However, the step of forming the field plate electrode after the step of forming the Schottky electrode is performed. It may be provided. That is, referring to FIGS. 1 and 2, a Schottky electrode is formed in the opening formed in insulating layer 4 so as to be in contact with GaN epitaxial layer 3, and subsequently connected to the Schottky electrode. At the same time, a field plate electrode may be formed so as to overlap the insulating layer 4. In this case, the field plate electrode may be formed of the same material as the Schottky electrode. Alternatively, the field plate electrode may be formed using a material different from the material of the Schottky electrode, such as a material having good adhesion to the insulating layer 4.

  In the present embodiment, the structure including the GaN free-standing substrate 2 and the GaN epitaxial layer 3 as the semiconductor layer has been described as an example. However, the present invention is not particularly limited to this. The semiconductor layer may not include the GaN free-standing substrate 2, and another substrate may be used instead of the GaN free-standing substrate 2.

(Embodiment 2)
FIG. 4 is a cross-sectional view of the Schottky barrier diode of the second embodiment. The SBD 11 according to the second embodiment and the SBD 1 according to the first embodiment described above basically have the same configuration. However, the second embodiment is different from the first embodiment in that the configuration of the insulating layers 14 and 17 is as shown in FIG.

  Specifically, as shown in FIG. 4, the insulating layer 14 as a nitride insulating film is formed on the surface 3 a of the GaN epitaxial layer 3. The insulating layer 17 as the second insulating film is formed on the insulating layer 14 and below the field plate electrode. That is, the insulating layer 14 and the insulating layer 17 have a laminated structure. The field plate electrode is formed on the insulating layer 14 as a nitride insulating film with the insulating layer 17 interposed therebetween, and constitutes a field plate structure together with the insulating layer 14.

The insulating layer 14 is SiN _{x formed} from SiH ₄ and N ₂ by plasma CVD without using NH ₃ , similarly to the insulating layer 4 of the first embodiment. That is, the insulating layer 14 is formed using a source gas that does not contain NH ₃ as a main component. The film thickness of the insulating layer 14 can be not less than 0.5 nm and not more than 5 μm, for example, about 100 nm. On the other hand, the insulating layer 17 is SiN _{x formed} by plasma CVD using NH ₃ . The film thickness of the insulating layer 17 is about 200 nm, for example.

According to the configuration of the second embodiment, although the hydrogen concentration in the insulating layer 17 is relatively high, the hydrogen concentration in the insulating layer 14 formed using the source gas not containing NH ₃ as the main component is 3 .8 × 10 ²² cm ⁻³ and lower. That is, in the SBD 11 shown in FIG. 4, the insulating layer 14 having a low hydrogen concentration is formed at the interface with the GaN epitaxial layer 3. Regardless of the hydrogen concentration in the insulating layer 17, since the insulating layer 14 is formed when the insulating layer 17 is formed, hydrogen can be prevented from entering the GaN epitaxial layer 3. As a result, the SBD 11 can obtain the effect of reducing the reverse leakage current and causing electric field relaxation. Since the other configuration of SBD 11 is as described in SBD 1 of the first embodiment, description thereof will not be repeated.

(Embodiment 3)
FIG. 5 is a cross-sectional view of a Schottky barrier diode according to Embodiment 3 of the present invention. The SBD 21 of the third embodiment and the SBD 1 of the first embodiment described above have basically the same configuration. However, the third embodiment is different from the first embodiment in that the configuration of the semiconductor layer is as shown in FIG.

Specifically, as shown in FIG. 5, the semiconductor layer of the SBD 21 includes a support substrate 23, a GaN foundation layer 22, and a GaN epitaxial layer 3, and does not include a GaN free-standing substrate. The dislocation density of the region 3c in contact with the Schottky electrode in the GaN epitaxial layer 3 is preferably 1 × 10 ⁸ cm ⁻² or less, more preferably 1 × 10 ⁶ cm ⁻² or less.

  The support substrate 23 is a conductive substrate. A GaN foundation layer 22 is formed on the support substrate 23. A GaN epitaxial layer 3 is formed on the GaN foundation layer 22. The support substrate 23 and the GaN foundation layer 22 are in ohmic contact. Moreover, when the support substrate 23 is a metal, the ohmic electrode 6 may be omitted. Since other configurations are the same as those in the second embodiment, description thereof will not be repeated.

  Note that the dislocation density of the GaN epitaxial layer 3 may be the same as or different from that of the region 3c. Since other configurations are the same as those in the first embodiment, description thereof will not be repeated.

  Next, a method for manufacturing the SBD 21 will be described. FIG. 6 is a flowchart showing each process of the Schottky barrier diode manufacturing method according to Embodiment 3 of the present invention. The manufacturing method of the SBD 21 in the present embodiment basically has the same configuration as the manufacturing method of the SBD 1 in the first embodiment, but further includes a step for forming a bonded substrate. Different.

  Specifically, the GaN free-standing substrate 2 is prepared as in the step (S10) of the first embodiment. Next, impurities are ion-implanted from the front surface or the back surface of the GaN free-standing substrate 2 in a step (S71). Thereby, a layer containing a large amount of impurities is formed near the front surface or the back surface of the GaN free-standing substrate 2. Next, in the step (S72), the ion-implanted surface and the support substrate 23 are bonded together. Next, in step (S73), heat treatment is performed in a state where the GaN free-standing substrate 2 and the support substrate 23 are bonded together. Thereby, the GaN free-standing substrate 2 is divided with a region containing a large amount of impurities as a boundary. As a result, a bonded substrate in which the support substrate 23 and a GaN base layer 22 thinner than the GaN free-standing substrate 2 are formed on the support substrate 23 can be formed.

Next, in step (S <b> 20), the GaN epitaxial layer 3 is formed on the GaN foundation layer 22. In the GaN epitaxial layer 3, the dislocation density in a region in contact with a Schottky electrode described later is preferably 1 × 10 ⁸ cm ⁻² or less.

  Next, as in the first embodiment, the insulating layer forming step (S30), the ohmic electrode forming step (S40), the insulating layer etching step (S50), and the Schottky electrode and field plate electrode forming step (S60) are performed. .

  By performing the above steps (S10 to S73), the Schottky barrier diode 21 shown in FIG. 5 can be manufactured.

  In the present embodiment, the GaN base layer 22 is formed using the GaN free-standing substrate 2, and the GaN epitaxial layer 3 is formed using the GaN base layer 22. However, the present invention is not limited to this.

  According to the configuration of the third embodiment, the semiconductor layer does not include a GaN free-standing substrate. That is, only a part of the expensive GaN free-standing substrate 2 is used. For this reason, since the remainder of the GaN free-standing substrate 2 can be reused, the manufacturing cost can be reduced.

  Examples of the present invention will be described below. The SBDs 1 and 11 described in the first embodiment and the second embodiment were manufactured, and an experiment was performed to measure the reverse withstand voltage. The specific manufacturing method and characteristics such as size of SBD 1 are as described in the first embodiment.

On the other hand, in the specific manufacturing method of the SBD 11, in the step (S30) of forming the nitride insulating film shown in FIG. 3, SiN _x is deposited to 100 nm by plasma CVD from SiH ₄ and N ₂ without using NH ₃ and insulated. Layer 14 was formed. Subsequently, an SiN _x film having a thickness of 200 nm was formed from SiH ₄ , NH ₃ , and H ₂ by plasma CVD to form an insulating layer 17. That is, the insulating layer 17 was formed to be laminated on the insulating layer 14 prior to the step (S60) of forming the Schottky electrode after the step of forming the nitride insulating film (S30). In the etching step (S50), both the insulating layer 14 and the insulating layer 17 were etched, and an opening was formed so that the GaN epitaxial layer 3 was exposed. Other manufacturing processes and size characteristics of the SBD 11 are the same as those of the SBD 1.

As a comparative example, an SBD as a comparative example was manufactured by plasma CVD using conventional NH ₃ . In a specific method of manufacturing SBD as a comparative example, in the step (S30) of forming an insulating layer shown in FIG. 3, with _{NH 3 SiH 4, NH 3,} H 2 from the SiN _x 100 nm formed by plasma CVD A film was formed to form an insulating layer. Other manufacturing processes and characteristics such as size of SBD as a comparative example are the same as SBD1.

  The reverse withstand voltage was measured for each SBD. Moreover, the hydrogen concentration in the insulating layer of each SBD was also measured. As a method for measuring the hydrogen concentration, SIMS (Secondary Ion Mass Spectrometry) was used.

  As a result, the reverse withstand voltage was 400 V for both SBD1 and SBD11, which are examples of the present invention, and 200 V for SBD as a comparative example.

Further, the hydrogen concentration in the insulating layer was 1.6 × 10 ²² cm ⁻³ in SBD1. In SBD11, the first insulating layer was 1.6 × 10 ²² cm ⁻³ , and the second insulating layer was 3.8 × 10 ²² cm ⁻³ . In SBD as a comparative example, it was 3.8 × 10 ²² cm ⁻³ .

  As described above, since the hydrogen concentration in the nitride insulating film is low in the SBD of the present invention, the reverse withstand voltage is doubled and greatly increased compared to the SBD as a comparative example using a nitride insulating film having a high hydrogen concentration. Was. Therefore, in the SBD of the present invention, since the hydrogen concentration in the nitride insulating film is reduced, that is, the adverse effect of hydrogen being taken into the semiconductor layer when the nitride insulating film is formed is eliminated, the field plate structure has a larger electric field relaxation. It was shown that an effect was obtained and the reverse withstand voltage could be increased. Further, it has been clarified that when the nitride insulating film and the second insulating film have a laminated structure, a similar effect can be obtained if a nitride insulating film having a low hydrogen concentration is formed at the interface with the semiconductor layer.

  The embodiments and examples disclosed herein are illustrative in all respects and should not be construed as being restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is sectional drawing of the Schottky barrier diode of this invention. It is a perspective view of the Schottky barrier diode shown in FIG. It is a flowchart which shows each process of the manufacturing method of a Schottky barrier diode. 6 is a cross-sectional view of a Schottky barrier diode according to a second embodiment. FIG. 6 is a cross-sectional view of a Schottky barrier diode in a third embodiment. FIG. 12 is a flowchart showing each step of a method for manufacturing a Schottky barrier diode in the third embodiment.

Explanation of symbols

  1,11,21 Schottky barrier diode, 2 GaN free-standing substrate, 2a surface, 2b back surface, 3 GaN epitaxial layer, 3a surface, 3c region, 4, 14, 17 insulating layer, 5, 6 electrode, 22 base layer, 23 Support substrate.

Claims (12)

A semiconductor layer having a main surface;
A first nitride insulating film formed on the main surface and having an opening;
A second nitride insulating film formed on the first nitride insulating film;
Inside the opening, a Schottky electrode formed so as to contact the main surface;
A field plate electrode connected to the Schottky electrode and formed to overlap the first nitride insulating film and the second nitride insulating film ,
The hydrogen concentration in the first nitride insulating film is less than 3.8 × 10 ²² cm ⁻³ ;
The Schottky barrier diode , wherein a hydrogen concentration in the second nitride insulating film is higher than a hydrogen concentration in the first nitride insulating film .   The Schottky barrier diode according to claim 1, wherein the semiconductor layer includes a gallium nitride substrate. The Schottky barrier diode according to claim 2, wherein the dislocation density of the gallium nitride substrate is 1 × 10 ⁸ cm ⁻² or less. 4. The Schottky barrier diode according to claim 1, wherein a dislocation density of a region in contact with the Schottky electrode in the semiconductor layer is 1 × 10 ⁸ cm ⁻² or less.   The material of the Schottky electrode includes at least one substance selected from the group consisting of gold, platinum, nickel, palladium, cobalt, copper, silver, tungsten, and titanium. The Schottky barrier diode described in 1. 6. The Schottky barrier diode according to claim 1, wherein a thickness of the first nitride insulating film is not less than 0.5 nm and not more than 5 μm. The Schottky barrier diode according to claim 6 , wherein the first nitride insulating film has a thickness of 10 nm to 5 μm. The refractive index of the first nitride insulating film is 1.7 to 2.2, the Schottky barrier diode according to any one of claims 1 to 7. The field plate electrode, the length overlapping the first nitride insulating film is 1μm to 1mm, the Schottky barrier diode according to any one of claims 1 to 8. The area in contact with the first nitride insulating film in the semiconductor layer is a compound of gallium nitride, the Schottky barrier diode according to any one of claims 1 to 9. Forming a semiconductor layer;
Forming a first nitride insulating film having an opening on the semiconductor layer;
Forming a Schottky electrode in contact with the semiconductor layer inside the opening; and
After the step of forming the first nitride insulating film, prior to the step of forming the Schottky electrode, forming a second nitride insulating film so as to be laminated on the first nitride insulating film;
Forming a field plate electrode so as to be connected to the Schottky electrode and overlap the first nitride insulating film and the second nitride insulating film ,
In the first step of forming a nitride insulating film, you have use the raw material gas not containing ammonia as the main component,
The method for manufacturing a Schottky barrier diode , wherein a hydrogen concentration in the second nitride insulating film is higher than a hydrogen concentration in the first nitride insulating film . Forming a semiconductor layer;
Forming a first nitride insulating film having an opening on the semiconductor layer;
Forming a Schottky electrode in contact with the semiconductor layer inside the opening; and
After the step of forming the first nitride insulating film, prior to the step of forming the Schottky electrode, forming a second nitride insulating film so as to be laminated on the first nitride insulating film;
Forming a field plate electrode so as to be connected to the Schottky electrode and overlap the first nitride insulating film and the second nitride insulating film ,
In the step of forming the first nitride insulating film, said first nitride insulating film, formed using a physical vapor deposition method,
The method for manufacturing a Schottky barrier diode , wherein a hydrogen concentration in the second nitride insulating film is higher than a hydrogen concentration in the first nitride insulating film .

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