JP2009059912A - Schottky-barrier diode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Schottky-barrier diode having a field plate structure capable of stably providing an electric field concentration mitigating effect. <P>SOLUTION: This Schottky-barrier diode 1 is provided with: a GaN epitaxial layer 3 formed on a GaN self-standing substrate 2; a Schottky electrode 5, an insulation layer 4, and an electrode part 7. The Schottky electrode 5 is formed to contact a surface of the GaN epitaxial layer 3. The insulation layer 4 is formed to adjoin the Schottky electrode 5 on the surface of the GaN epitaxial layer 3. The electrode part 7 is connected to the Schottky electrode 5, extends in contact with the insulation layer 4, and is formed of a material different from the material constituting the Schottky electrode 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a Schottky barrier diode, and more particularly to a Schottky barrier diode having a field plate structure.

  Conventionally, semiconductor elements such as a Schottky barrier diode (SBD), a pn junction diode, and a MIS transistor using a GaN substrate have been proposed (see, for example, Patent Document 1). As a GaN material used for a power device, a GaN epitaxial layer formed on a different substrate such as sapphire or 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 known as a structure for suppressing electric field concentration on 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 present inventor has studied to apply the above-described field plate structure to the SBD in order to increase the breakdown voltage of the Schottky barrier diode (SBD) using the GaN substrate. Specifically, an insulating layer (field plate insulating layer) such as a silicon nitride film is formed adjacent to the SBD Schottky electrode, and the outer peripheral portion of the Schottky electrode extends on the insulating layer to form the field plate. The structure used as an electrode was examined. In this study, the present inventor has found that the adhesion between the constituent material of the Schottky electrode constituting the field plate electrode (for example, gold (Au)) and the field plate insulating layer is low, and the field plate insulating layer to the field plate We have found the problem that the electrode peels relatively easily. When the field plate electrode is peeled from the field plate insulating layer as described above, the electric field concentration relaxation effect by the field plate electrode cannot be stably obtained.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a Schottky barrier diode having a field plate structure capable of stably obtaining an electric field concentration relaxation effect. Is to provide.

  A Schottky barrier diode according to the present invention includes a semiconductor layer formed on a substrate, a Schottky electrode, an insulating layer, and an electrode portion. The Schottky electrode is formed in contact with the surface of the semiconductor layer. The insulating layer is formed adjacent to the Schottky electrode on the surface of the semiconductor layer. The electrode portion is connected to the Schottky electrode, extends in contact with the insulating layer, and is made of a material different from the material constituting the Schottky electrode.

  In this way, the electrode portion extending on the insulating layer can be made to act as a field plate electrode constituting the field plate structure. Furthermore, since a material different from the material constituting the Schottky electrode can be used as the material of the electrode portion, the electrode portion can be used even when the adhesion between the material constituting the Schottky electrode and the insulating layer is low. By appropriately selecting the material that constitutes, the adhesion between the electrode portion and the insulating layer can be improved. For this reason, it is possible to suppress the occurrence of a problem that the field plate electrode is peeled off from the insulating layer as in the case where the Schottky electrode is directly brought into contact with the insulating layer to form the field plate electrode. As a result, the electric field concentration relaxation effect by the field plate electrode can be stably obtained.

  Here, the field plate structure is a structure constituted by an insulating layer and a field plate electrode formed on the insulating layer. 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.

  In the above Schottky barrier diode, the electrode portion may include an extending portion that extends on the upper surface of the Schottky electrode. The electrode portion may be connected to the Schottky electrode at the extending portion.

  In this case, the extending portion of the electrode portion is laminated on the Schottky electrode. Therefore, after forming the Schottky electrode and the insulating layer, the electrode portion is formed so as to cover the Schottky electrode and extend to the upper surface of the insulating layer adjacent to the Schottky electrode. A key barrier diode can be easily manufactured.

  In the Schottky barrier diode, the Schottky electrode may include an electrode extension portion extending on the electrode portion. The Schottky electrode may be connected to the electrode portion at the electrode extension portion.

  In this case, since the electrode extension part of the Schottky electrode can be used as a member constituting the field plate electrode, the electrode part is interposed between the electrode extension part and the insulating layer, and the electrode part and the insulating layer What is necessary is just to form only the thickness required in order to ensure adhesiveness. For this reason, even if it is a case where a comparatively expensive material is used as a material of an electrode part, it can suppress that the manufacturing cost of a Schottky barrier diode increases.

  In the Schottky barrier diode, the substrate may be a gallium nitride (GaN) substrate. In this case, gallium nitride (GaN) has various excellent characteristics such as a band gap about 3 times that of silicon (Si), a breakdown electric field strength about 10 times higher, and a larger saturation electron velocity. Therefore, it is expected to achieve both high breakdown voltage and low loss, that is, low on-resistance, which is difficult with a conventional Schottky barrier diode using Si. The present invention is particularly effective when a field plate structure that further promotes such high breakdown voltage is employed.

  In the Schottky barrier diode, the material constituting the Schottky electrode is selected from the group consisting of gold (Au), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), and copper (Cu). It may contain at least one kind.

In this case, by using gold or the like as the material of the Schottky electrode, a Schottky electrode with a low leakage current can be realized, so that electric field relaxation by the field plate structure becomes obvious. As a result, the reverse leakage current decreases and the reverse withstand voltage increases.

  In the Schottky barrier diode, the material constituting the electrode portion is titanium (Ti), nickel (Ni), molybdenum (Mo), chromium (Cr), aluminum (Al), tungsten (W), and palladium (Pd). It may contain at least one selected from the group consisting of In this case, it is possible to form an electrode portion having good adhesion to the material constituting the insulating layer described later.

In the Schottky barrier diode, the material constituting the insulating layer is silicon nitride (SiNx), silicon oxide (SiO ₂ ), silicon oxynitride (SiON), hafnium oxide (HfO ₂ ), aluminum nitride (AlN), aluminum oxide It may contain at least one selected from the group consisting of (Al ₂ O ₃ ), gallium oxide (Ga ₂ O ₃ ), magnesium oxide (MgO), and scandium oxide (Sc ₂ O ₃ ). In this case, it is possible to secure sufficient insulating properties as the insulating layer and to obtain good adhesion with the material constituting the electrode portion described above.

  As described above, according to the present invention, since the field plate electrode including the electrode portion can be prevented from peeling from the insulating layer, the Schottky barrier having the field plate structure capable of stably obtaining the electric field concentration relaxation effect. A diode can be obtained.

  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 schematic cross-sectional view of a first embodiment of a Schottky barrier diode according to the present invention. FIG. 2 is a schematic perspective view of the Schottky barrier diode shown in FIG. FIG. 1 shows a cross section taken along line I-I in FIG. In addition, in FIG. 2, in order to show the cross-sectional structure of the Schottky barrier diode (SBD) 1, a quadrangular portion of the planar shape that is about a quarter of the Schottky barrier diode 1 is virtually removed. A cross section of a Schottky barrier diode is shown. A first embodiment of a Schottky barrier diode according to the present invention will be described with reference to FIG. 1 and FIG.

As shown in FIGS. 1 and 2, in the Schottky barrier diode 1, a GaN epitaxial layer 3 as a semiconductor layer is formed on the main surface of the GaN free-standing substrate 2. The GaN epitaxial layer 3 may include a plurality of epitaxial layers. For example, a stacked structure in which an n + GaN epitaxial layer formed on the main surface of the GaN free-standing substrate 2 is 2 μm and an n-GaN epitaxial layer as a drift layer formed on the n + GaN epitaxial layer is 7 μm may be formed. . At this time, the carrier concentration of the drift layer may be, for example, 1 × 10 ¹⁶ cm ⁻³ . In the GaN free-standing substrate 2, an ohmic electrode 6 is formed on the surface (back surface) opposite to the main surface on which the GaN epitaxial layer 3 is formed. An insulating layer 4 is formed on the GaN epitaxial layer 3. The thickness T of the insulating layer 4 can be set to an arbitrary thickness. In addition, an opening having a circular planar shape is formed at the center of the insulating layer 4.

In this opening, the upper surface of the GaN epitaxial layer 3 is exposed. A Schottky electrode 5 is formed inside the opening so as to be in contact with the upper surface of the GaN epitaxial layer 3. An electrode portion 7 is formed so as to extend from above Schottky electrode 5 to the upper surface of insulating layer 4. The planar shape of the electrode portion 7 is circular as can be seen from FIG. The planar shape of the Schottky electrode 5 described above is also the same circular shape as the planar shape of the opening formed in the insulating layer 4.

  An electrode 8 is formed from the electrode portion 7 and the Schottky electrode 5. In the outer peripheral portion of the electrode portion 7, the portion extending on the upper surface of the insulating layer 4 becomes a field plate (FP) electrode portion 9. The length L of the field plate electrode portion 9 (the distance from the side wall of the opening formed in the insulating layer 4 to the outer peripheral end of the field plate electrode portion 9) can be set to an arbitrary distance. L can be 1 μm or more and 1 mm or less. Moreover, the thickness T of the insulating layer 4 can be, for example, not less than 10 nm and not more than 5 μm.

  Thus, by adopting a laminated structure of the Schottky electrode 5 and the electrode portion 7 as the electrode 8, it is possible to form the GaN epitaxial layer 3 and the Schottky electrode as materials constituting the Schottky electrode 5. A material (for example, a material such as gold (Au), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), copper (Cu)) can be used. A material having excellent adhesion to the insulating layer 4 can be selected independently of the material of the Schottky electrode 5 as a material constituting the electrode portion 7 including the field plate electrode portion 9. For example, titanium (Ti), nickel (Ni), molybdenum (Mo), chromium (Cr), aluminum (Al), tungsten (W), palladium (Pd), or the like is used as a material constituting the electrode portion 7. it can. As a result, the Schottky characteristics of the electrode 8 can be maintained satisfactorily by the Schottky electrode 5 and the occurrence of the problem that the field plate electrode portion 9 is peeled off from the insulating layer 4 can be suppressed. As a result, the effect of relaxing the charge concentration by the field plate structure can be obtained stably.

  Next, a manufacturing process of the Schottky barrier diode shown in FIGS. 1 and 2 will be described. FIG. 3 is a flowchart for explaining a manufacturing process of the Schottky barrier diode shown in FIGS. 1 and 2. 4 to 12 are schematic cross-sectional views for explaining a manufacturing process of the Schottky barrier diode shown in FIG. A manufacturing process of the first embodiment of the Schottky barrier diode according to the present invention will be described with reference to FIGS.

As shown in FIG. 3, a substrate preparation step (S10) is first performed. In this step (S10), a gallium nitride (GaN) free-standing substrate 2 (see FIG. 4) is prepared. As the GaN free-standing substrate 2, a substrate formed by an arbitrary manufacturing method can be used. For example, a (0001) -plane GaN free-standing substrate 2 manufactured by the HVPE method is prepared. The average dislocation density in the GaN free-standing substrate 2 is preferably 1 × 10 ⁷ cm ⁻² or less. The dislocation density can be measured by, for example, a method of counting the number of pits formed by etching in molten KOH and dividing by the area of the etched substrate.

  Next, an epitaxial film forming step (S20) is performed. Specifically, the GaN epitaxial layer 3 is formed on the main surface of the GaN free-standing substrate 2 by an arbitrary method. As a result, a structure as shown in FIG. 4 is obtained. As a method for forming the GaN epitaxial layer 3, any method can be used. For example, an organic metal vapor phase epitaxy (OMVPE method) may be used.

Next, as shown in FIG. 3, an insulating layer forming step (S30) is performed. In this step (S30), the insulating layer 4 is formed on the GaN epitaxial layer 3 as shown in FIG. Any method can be used as a method of manufacturing the insulating layer 4. In addition, any material can be used as the material constituting the insulating layer 4, but for example, silicon nitride, silicon oxide, silicon oxynitride, hafnium oxide, aluminum nitride, aluminum oxide, gallium oxide, magnesium oxide, scandium oxide Etc. can be used. As a result, a structure as shown in FIG. 5 is obtained.

  Next, an ohmic electrode formation step (S40) is performed. In this step (S40), as shown in FIG. 6, the ohmic electrode 6 is formed on the back surface of the GaN free-standing substrate 2 (the surface opposite to the surface on which the GaN epitaxial layer 3 is formed in the GaN free-standing substrate 2). . Prior to the step of forming the ohmic electrode 6, it is preferable to clean at least the back surface of the GaN free-standing substrate 2. As this cleaning method, for example, organic cleaning and hydrochloric acid cleaning may be combined.

  As the ohmic electrode 6, an electrode made of a single material may be formed, but the ohmic electrode 6 having a stacked structure in which a plurality of conductors are stacked may be formed. For example, the ohmic electrode 6 may have a structure in which a metal layer as a conductor is laminated such as titanium / aluminum / titanium / gold from the GaN free-standing substrate 2 side. These metal layers can be formed using any method such as an electron beam evaporation method (EB evaporation method). As a result, a structure as shown in FIG. 6 is obtained.

  Next, a Schottky electrode and electrode part forming step (S50) is performed. In this step (S50), specifically, a resist film 10 having a pattern is formed on insulating layer 4 by using a photolithography method. As a result, a structure as shown in FIG. 7 is obtained. In the resist film 10 shown in FIG. 7, an opening pattern is formed on a region in the insulating layer 4 where the opening where the Schottky electrode 5 (see FIG. 1) is to be disposed is formed.

  Then, using the resist film 10 as a mask, the insulating layer 4 is partially removed by etching. As a result, an opening 11 is formed in the insulating layer 4 as shown in FIG. Here, as the etching for partially removing the insulating layer 4, dry etching may be used, or wet etching may be used.

  Next, as shown in FIG. 9, a metal film 12 to be the Schottky electrode 5 (see FIG. 1) is formed. As the metal film 12, a material having good Schottky characteristics such as gold described above can be used. As a method for forming the metal film 12, any method can be used, and for example, an EB vapor deposition method or the like can be used. Prior to the step of forming the metal film 12 described above, the surface of the GaN epitaxial layer 3 exposed in the opening 11 may be cleaned by a cleaning process using hydrochloric acid or the like.

  Next, by removing the resist film 10, the metal film 12 formed on the resist film 10 is simultaneously removed (lift-off). As a result, as shown in FIG. 10, the metal film remains inside the opening 11, and the Schottky electrode 5 made of the metal film is formed.

  Next, as shown in FIG. 11, a resist film 13 having a predetermined pattern is formed using a photolithography method. In the resist film 13, the Schottky electrode 5 is exposed and an opening pattern is formed in a region where the electrode portion 7 (see FIG. 1) is to be formed. As a result, a structure as shown in FIG. 11 is obtained.

  Next, as shown in FIG. 12, a metal film 14 constituting the electrode portion 7 (see FIG. 1) is formed by an arbitrary method. Thereafter, by removing the resist film 13, the metal film 14 formed on the resist film 13 is also removed (lift-off). As a result, an electrode portion 7 (see FIG. 1) laminated on the Schottky electrode 5 is formed.

Next, a post-processing step (S60) is performed. In this step (S60), a dividing step for dividing the GaN free-standing substrate 2 on which the GaN epitaxial layer 3, the electrode 8, the ohmic electrode 6 and the like are formed into individual chips is performed. In this way, the Schottky barrier diode 1 shown in FIG. 1 can be obtained.

  FIG. 13 is a schematic cross-sectional view showing a modification of the first embodiment of the Schottky barrier diode shown in FIGS. 1 and 2. A modification of the first embodiment of the Schottky barrier diode according to the present invention will be described with reference to FIG. FIG. 13 corresponds to FIG.

  The Schottky barrier diode 1 shown in FIG. 13 basically has the same structure as the Schottky barrier diode shown in FIGS. 1 and 2, but the opening of the insulating layer 4 in which the Schottky electrode 5 is disposed. The side wall shape of the Schottky barrier diode 1 shown in FIG. That is, in the Schottky barrier diode 1 shown in FIG. 13, the sidewall of the opening in the insulating layer 4 is inclined by the inclination angle θ with respect to the upper surface of the GaN epitaxial layer 3. The inclination angle θ (angle formed between the sidewall of the opening of the insulating layer 4 and the upper surface of the GaN epitaxial layer 3) is, for example, 0.1 ° to 60 °, more preferably 1 ° to 45 °. Can do. More preferably, the inclination angle θ may be 1 ° or more and 22 ° or less, and more preferably 1 ° or more and 11 ° or less. In this way, the angle of the corner of the electrode 8 at the end of the bottom wall of the opening in the insulating layer 4 can be made obtuse, so that the effect of alleviating electric field concentration can be further increased.

  The manufacturing method of the Schottky barrier diode 1 shown in FIG. 13 is basically the same as the manufacturing method of the Schottky barrier diode 1 shown in FIGS. 1 and 2, but the steps in the manufacturing process shown in FIG. The contents of (S50) are partially different. That is, the etching conditions in the step of forming the opening 11 in the insulating layer 4 shown in FIGS. 7 and 8 are different from the Schottky barrier diode manufacturing method shown in FIGS. Specifically, wet etching is used as the etching for forming the opening 11, and the side wall of the opening 11 is inclined with respect to the upper surface of the GaN epitaxial layer 3 by adjusting the conditions of the wet etching.

  As an example of the step of inclining the side wall of the opening 11 with respect to the upper surface of the GaN epitaxial layer 3, the following steps can be used, for example. First, steps (S10) to (S40) in FIG. 3 are performed to obtain a structure as shown in FIG. Thereafter, the upper surface of the insulating layer 4 is organically cleaned with acetone / isopropyl alcohol.

  Next, a dehydration baking process with a heating temperature of 120 ° C. and a heating time of 2 minutes is performed on the GaN free-standing substrate 2 on which the GaN epitaxial layer 3 is formed. Thereafter, a resist (positive resist) is applied to the surface of the GaN epitaxial layer 3. Further, a pre-bake process is performed on the resist at a heating temperature of 110 ° C. and a heating time of 2 minutes. Thereafter, an exposure / development process is performed to form a resist film 10 having an opening pattern as shown in FIG. Further, a post-bake process is performed on the resist film 10 under the conditions of a heating temperature of 130 ° C. and a heating time of 2 minutes. Thereafter, the insulating layer 4 is partially removed by wet etching using buffered hydrofluoric acid (BHF) using the resist film 10 as a mask. As this BHF, 10% BHF (BHF110) was used.

  In this way, an opening having an inclined sidewall can be formed in the insulating layer 4. The inclination angle θ of the side wall formed at this time is about 22 °, for example.

In the step of forming the opening described above, the sidewall inclination angle θ is further reduced by performing wet etching as it is without performing post-baking processing after development processing (for example, the inclination angle θ is set to about 11 °). can do. This is because by omitting the post-bake treatment, the adhesion between the resist film and the insulating layer is not improved by the post-bake, so that the etching of the opening side wall in the wet etching is further promoted.

  After forming the opening with the inclined sidewall as described above, the same process as that described with reference to FIGS. 9 to 12 is performed, whereby the Schottky barrier diode shown in FIG. 13 can be obtained.

(Embodiment 2)
FIG. 14 is a schematic sectional view showing a second embodiment of the Schottky barrier diode according to the present invention. A second embodiment of the Schottky barrier diode according to the present invention will be described with reference to FIG.

  Referring to FIG. 14, a Schottky barrier diode 1 according to the present invention basically has the same structure as the Schottky barrier diode shown in FIGS. 1 and 2, but the structure of electrode 8 is different. That is, in the electrode 8 of the Schottky barrier diode 1 shown in FIG. 14, a portion of the insulating layer 4 located on the upper surface of the insulating layer 4 outside the opening 11 is in contact with the upper surface of the insulating layer 4. An electrode portion 7 is formed on the substrate. The electrode portion 7 has a laminated structure including two layers of a lower layer electrode portion 21 and an upper layer electrode portion 22. As a material constituting the lower electrode portion 21, for example, titanium can be used. For example, gold can be used as the material constituting the upper electrode portion 22.

  Schottky electrode 5 is formed so as to extend from above electrode portion 7 to the upper surface of GaN epitaxial layer 3 exposed at opening 11. Even with such a structure, the same effects as those of the Schottky barrier diode shown in FIGS. 1 and 2 can be obtained. That is, the field plate electrode portion 9 has a laminated structure of the electrode portion 7 and the Schottky electrode 5 when viewed from the insulating layer 4 side. For this reason, by selecting a material having excellent adhesion to the insulating layer 4 as the electrode portion 7, it is possible to suppress the occurrence of a problem that the field plate electrode portion 9 is easily separated from the insulating layer 4.

  The manufacturing method of the Schottky barrier diode shown in FIG. 14 is basically the same as the manufacturing method of the Schottky barrier diode shown in FIG. 3, but the contents of the Schottky electrode and electrode portion forming step (S50) are the same. This is different from the manufacturing method of the first embodiment of the Schottky barrier diode according to the present invention. Specifically, in the insulating layer 4, the opening 11 is formed using a process similar to the process shown in FIGS. 7 and 8. Thereafter, a resist film having an annular groove pattern is formed so as to expose a region where the electrode portion 7 shown in FIG. 14 is to be formed (an annular portion outside the opening 11 and surrounding the opening 11). . And the metal film which should become the lower layer electrode part 21 and the upper layer electrode part 22 which comprise the electrode part 7 is formed by arbitrary methods. Thereafter, a portion other than the metal film to be the electrode portion 7 is removed by using a lift-off method. Specifically, the metal film formed on the resist film is removed together with the resist film.

  After that, the planar shape is circular in the region where the Schottky electrode 5 shown in FIG. 14 is to be formed (the annular portion surrounding the opening 11 where the electrode portion 7 is formed and the inside of the opening 11). A resist film having a shape opening pattern is formed again. Then, a metal film to be the Schottky electrode 5 is formed on the resist film and inside the opening pattern. Then, by removing the resist film, the metal film formed on the upper surface of the resist film is removed (lift-off). As a result, the Schottky electrode 5 (see FIG. 14) extending from the inside of the opening 11 to the upper surface of the electrode portion 7 can be obtained. After the electrode 8 is formed in this way, the Schottky barrier diode 1 shown in FIG. 14 can be obtained by performing the same process as the post-processing process (S60) in FIG.

  FIG. 15 is a schematic cross-sectional view showing a modification of the second embodiment of the Schottky barrier diode shown in FIG. A modification of the second embodiment of the Schottky barrier diode according to the present invention will be described with reference to FIG. The Schottky barrier diode 1 shown in FIG. 15 basically has the same structure as that of the Schottky barrier diode 1 shown in FIG. 14, but the side wall of the opening 11 formed in the insulating layer 4 is a GaN epitaxial layer. 3 is different in that it is inclined with respect to the upper surface. That is, the inclination angle θ, which is the angle formed between the extending direction of the side wall of the opening 11 of the insulating layer 4 and the upper surface of the GaN epitaxial layer 3, is an acute angle. This inclination angle θ can be set to 0.1 ° to 60 °, more preferably 1 ° to 45 °, similarly to θ shown in FIG. More preferably, the inclination angle θ may be 1 ° or more and 22 ° or less, and more preferably 1 ° or more and 11 ° or less. Even with such a structure, the same effect as that of the Schottky barrier diode 1 shown in FIG. 14 can be obtained, and the same effect as that of the Schottky barrier diode 1 shown in FIG. 13 can be obtained.

  The manufacturing method of the Schottky barrier diode shown in FIG. 15 is basically the same as the manufacturing method of the Schottky barrier diode shown in FIG. 14, but etching is performed when the opening 11 is formed in the insulating layer 4. The conditions are different. Specifically, wet etching is used as the etching for forming the opening 11, and the side wall of the opening 11 is inclined with respect to the upper surface of the GaN epitaxial layer 3 by adjusting the conditions of the wet etching.

  Although there is a part which overlaps with embodiment of this invention mentioned above, the characteristic structure of this invention is enumerated below.

  A Schottky barrier diode 1 (SBD1) according to the present invention includes a GaN epitaxial layer 3 as a semiconductor layer formed on a GaN free-standing substrate 2 as a substrate, a Schottky electrode 5, an insulating layer 4, and an electrode portion. 7. Schottky electrode 5 is formed in contact with the surface of GaN epitaxial layer 3. The insulating layer 4 is formed adjacent to the Schottky electrode 5 on the surface of the GaN epitaxial layer 3. The electrode portion 7 is connected to the Schottky electrode 5, extends in contact with the insulating layer 4, and is made of a material different from the material constituting the Schottky electrode 5.

  In this way, the electrode portion 7 extending on the insulating layer 4 can act as a field plate electrode constituting the field plate structure. Further, since a material different from the material constituting the Schottky electrode 5 can be used as the material of the electrode portion 7, the adhesion between the material constituting the Schottky electrode 5 and the insulating layer 4 is low. In addition, the adhesiveness between the electrode portion 7 and the insulating layer 4 can be improved by appropriately selecting the material constituting the electrode portion 7. Therefore, it is possible to suppress the occurrence of a problem that the field plate electrode is peeled off from the insulating layer 4 as in the case where the Schottky electrode 5 is brought into direct contact with the insulating layer 4 to form the field plate electrode. As a result, the electric field concentration relaxation effect by the field plate electrode can be stably obtained.

  In the Schottky barrier diode 1, the electrode portion 7 may include an extending portion (portion laminated on the Schottky electrode 5) extending on the upper surface of the Schottky electrode 5 as shown in FIG. 1. Good. The electrode portion 7 may be connected to the Schottky electrode 5 at the extending portion.

In this case, the extending portion of the electrode portion 7 is laminated on the Schottky electrode 5. Therefore, after the Schottky electrode 5 and the insulating layer 4 are formed, the electrode portion 7 is formed so as to cover the Schottky electrode 5 and extend to the upper surface of the insulating layer 4 adjacent to the Schottky electrode 5. Thus, the Schottky barrier diode 1 according to the present invention can be easily manufactured.

  In the Schottky barrier diode 1, as shown in FIG. 14, the Schottky electrode 5 may include an electrode extension portion (field plate electrode portion 9) extending on the electrode portion 7. Schottky electrode 5 may be connected to electrode portion 7 at the electrode extension portion (field plate electrode portion 9).

  In this case, since the field plate electrode portion 9 of the Schottky electrode 5 can be used as a member constituting the field plate electrode, the electrode portion 7 is interposed between the field plate electrode portion 9 and the insulating layer 4, and the electrode portion. It is only necessary to form a thickness necessary for ensuring the adhesion between the insulating layer 4 and the insulating layer 4. For this reason, even if it is a case where a comparatively expensive material is used as a material of the electrode part 7, it can suppress that the manufacturing cost of the Schottky barrier diode 1 increases.

  In the Schottky barrier diode, as described above, a gallium nitride (GaN) substrate (GaN free-standing substrate 2) is used as the substrate. In this case, gallium nitride (GaN) has various excellent characteristics such as a band gap about 3 times that of silicon (Si), a breakdown electric field strength about 10 times higher, and a larger saturation electron velocity. Therefore, it is expected to achieve both high breakdown voltage and low loss, that is, low on-resistance, which is difficult with a conventional Schottky barrier diode using Si. The present invention is particularly effective when a field plate structure that further promotes such high breakdown voltage is employed.

  The Schottky barrier diode 1 further includes an ohmic electrode 6 formed on the back surface of the GaN free-standing substrate 2 opposite to the surface on which the GaN epitaxial layer 3 is formed. In this way, the Schottky barrier diode 1 has a vertical structure in which current flows from one of the Schottky electrode 5 and the ohmic electrode 6 to the other. Generally, in the power device, the vertical structure can pass a larger current than the horizontal structure, and therefore the vertical structure is a structure more suitable for the power device. (Because sapphire is insulative, a power device using a sapphire substrate cannot have a vertical structure.) As described above, when the GaN free-standing substrate 2 is used as the substrate, the GaN free-standing substrate 2 is conductive. Therefore, a vertical structure in which the ohmic electrode 6 is formed on the back surface side is possible.

  In the Schottky barrier diode 1, the material constituting the Schottky electrode 5 is a group consisting of gold (Au), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), and copper (Cu). It may contain at least one selected from. For example, the material constituting the Schottky electrode 5 is any one of gold (Au), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), copper (Cu), or these metals. An alloy containing two or more of them may be used. Further, the Schottky electrode 5 may be a single layer made of any of the metals described above or an alloy including two or more of the metals, or a laminate including at least one of the metals. It may be a structure.

  In this case, by using gold or the like as the material of the Schottky electrode, a Schottky electrode with a low leakage current can be realized, so that electric field relaxation by the field plate structure becomes obvious. As a result, the reverse leakage current decreases and the reverse withstand voltage increases.

In the Schottky barrier diode, the material constituting the electrode portion 7 is titanium (Ti), nickel (Ni), molybdenum (Mo), chromium (Cr), aluminum (Al), tungsten (W), and palladium (Pd). At least one selected from the group consisting of: For example, the electrode portion 7 may be a single film made of at least one of the above metals, or may contain an alloy containing two or more of the above metals. The electrode portion 7 may have a laminated structure including at least one of the above metals. In the laminated structure, it is only necessary that a portion of the electrode portion 7 that contacts the insulating layer 4 contains the metal, and a layer that does not directly contact the insulating layer 4 in the laminated structure (a layer other than the surface layer that contacts the insulating layer 4). ) May be made of a material (conductor) other than the above metal. In this case, an electrode portion having good adhesion with a material constituting the insulating layer 4 described later can be formed.

In the Schottky barrier diode 1, the material constituting the insulating layer 4 is silicon nitride (SiNx), silicon oxide (SiO ₂ ), silicon oxynitride (SiON), hafnium oxide (HfO ₂ ), aluminum nitride (AlN), At least one selected from the group consisting of aluminum oxide (Al ₂ O ₃ ), gallium oxide (Ga ₂ O ₃ ), magnesium oxide (MgO), and scandium oxide (Sc ₂ O ₃ ) may be included. In this case, it is possible to ensure sufficient insulating properties as the insulating layer 4 and to obtain good adhesion with the material constituting the electrode portion 7 described above.

In the Schottky barrier diode 1, the dislocation density of the GaN free-standing substrate 2 may be 1 × 10 ⁸ cm ⁻² or less.

Here, the present inventor has found the following phenomenon in the course of studying a high breakdown voltage of a Schottky barrier diode using a GaN substrate. That is, when a Schottky barrier diode is produced using a GaN epitaxial layer (semiconductor layer) formed on a heterogeneous substrate such as a Si substrate or a sapphire substrate, which has been generally used as a GaN material for conventional power devices, Even when a field plate (FP) structure is applied to the Schottky barrier diode, the effect of reducing the reverse leakage current and increasing the reverse withstand voltage based on the electric field concentration relaxation at the Schottky electrode end due to the FP structure is remarkable. The present inventor has found a phenomenon that cannot be obtained. The present inventor examined the cause of the phenomenon described above. As a result, the dislocation density in the formed GaN epitaxial layer is often as high as 1 × 10 ⁸ cm ⁻² or more due to the difference in crystal structure between GaN and a heterogeneous substrate such as a Si substrate or a sapphire substrate. Presumed to be the cause. For this reason, by using the GaN free-standing substrate 2 having a low dislocation density of 1 × 10 ⁸ cm ⁻² or less as described above, dislocations in the semiconductor layer (GaN epitaxial layer 3) are reduced. As a result, in the Schottky barrier diode 1 having the field plate structure, electric field relaxation occurs due to the field plate structure under the condition that the reverse leakage current is reduced. Therefore, the reverse leakage current is further reduced, and the reverse withstand voltage can be increased. The lower the dislocation density of the GaN free-standing substrate 2, the better. For example, the dislocation density of the GaN free-standing substrate 2 is more preferably 1 × 10 ⁶ cm ⁻² or less. Currently, the lower limit of the dislocation density of the GaN free-standing substrate 2 is about 1 × 10 ³ cm ⁻² .

  In the Schottky barrier diode 1, the thickness t of the insulating layer 4 may be not less than 10 nm and not more than 5 μm. If the thickness t of the insulating layer is less than 10 nm, the withstand voltage of the insulating layer 4 is low, and the insulating layer 4 may be destroyed first and the effect of the field plate structure may not be obtained. Further, if the thickness t of the insulating layer 4 exceeds 5 μm, the electric field relaxation itself by the field plate structure cannot be obtained. The thickness t of the insulating layer 4 is more preferably 0.2 μm or more and 2 μm or less in, for example, a withstand voltage 1 kV design.

In the Schottky barrier diode 1, the length L (see FIG. 1) of the portion that acts as a field plate electrode including at least the electrode portion 7 and overlaps with the insulating layer 4 may be 1 μm or more and 1 mm or less. If the length L is less than 1 μm, it is difficult to control the size of the portion where the field plate electrode overlaps the insulating layer 4, and the effect of the field plate structure cannot be obtained stably. Further, if the length L 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 L is more preferably 5 μm or more and 40 μm or less because the depletion layer width extends from 2 μm to 20 μm.

  In the Schottky barrier diode 1, the end face (side wall) facing the Schottky electrode 5 of the insulating layer 4 is inclined with respect to the surface of the GaN epitaxial layer 3 as shown in FIG. 13 and FIG. The inclination angle θ, which is an angle formed between the end face and the surface of the GaN epitaxial layer 3, is preferably 0.1 ° or more and 60 ° or less. Moreover, it is preferable that the part of the electrode acting as a field plate electrode including the electrode part 7 is overlapped with the insulating layer 4 so as to adhere to the end face of the insulating layer 4. In this case, since the end face of the insulating layer 4 is inclined with respect to the surface of the GaN epitaxial layer 3, the effect of field relaxation by the field plate structure can be increased. Can be further improved.

  Here, the electric field relaxation effect by the field plate structure increases as the tilt angle decreases, and the breakdown voltage of the Schottky barrier diode 1 can be improved. However, if the tilt angle θ is less than 0.1 °, the angle reproducibility becomes difficult to obtain, which may cause a problem in manufacturing. Also, the field plate electrode through which no current flows is in contrast to the Schottky electrode 5. Since it becomes relatively large, extra raw materials are required, which is disadvantageous in production. On the other hand, if the inclination angle θ exceeds 60 °, the effect of electric field relaxation becomes small. The inclination angle θ is more preferably 1 ° or more and 30 ° or less. The inclination of the end face of the insulating layer 4 can also be formed by wet etching as described above, dry etching, or the like.

In the Schottky barrier diode 1, the insulating layer 4 may be a nitride insulating film, and the hydrogen concentration in the nitride insulating film may be less than 3.8 × 10 ²² cm ⁻³ . Note that the nitride insulating film refers to an insulating nitride-containing film such as SiNx (silicon nitride film) or AlN (aluminum nitride).

  Here, the present inventor found the following phenomenon while proceeding with the study on the high breakdown voltage of the SBD using the GaN substrate (GaN free-standing substrate 2). That is, even if the FP structure is applied to a Schottky barrier diode manufactured using a semiconductor layer such as the GaN epitaxial layer 3 formed on the GaN substrate, the reverse resistance based on the electric field concentration relaxation at the Schottky electrode end by the FP structure. The inventor has discovered a phenomenon that the effect of voltage rise is suppressed.

As a result of examining the cause of occurrence of such a phenomenon, the present inventor has estimated the cause of the occurrence as follows. That is, when the nitride insulating film constituting the FP structure is formed using the plasma CVD method, ammonia is usually used as a source gas. Therefore, hydrogen radicals, hydrogen ions, and the like are generated by the dissociation of ammonia molecules during film formation. In this specification, “hydrogen species” is used as a term including hydrogen radicals and hydrogen ions. As a result of the generation of hydrogen species, hydrogen is taken into the substrate, resulting in an increase in the hydrogen concentration in the nitride insulating film. And in SBD which has FP structure, when hydrogen is taken in into a board | substrate, it will have a bad influence. That is, the present inventor has estimated that the hydrogen incorporated into the substrate 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, reducing the hydrogen concentration in the nitride insulating film as described above means that the amount of hydrogen species generated when the nitride insulating film is formed is reduced. As a result, adverse effects of hydrogen on the GaN epitaxial layer 3 (semiconductor layer) can be eliminated. Accordingly, when the nitride concentration in the nitride insulating film is regulated to be less than 3.8 × 10 ²² cm ⁻³ , more preferably less than 2.0 × 10 ²² cm ⁻³ , the nitride insulating film having a high hydrogen concentration is used. As compared with, the adverse effect of hydrogen being taken into the substrate can be eliminated. Therefore, it is possible to reduce the possibility that the effect of increasing the reverse withstand voltage based on the relaxation of the electric field concentration on the end of the Schottky electrode 5 by the field plate structure is suppressed. That is, a large electric field relaxation effect can be obtained and the reverse withstand voltage can be increased. Here, the lower the hydrogen concentration in the nitride insulating film, the better. For example, the hydrogen concentration in the nitride insulating film is more preferably less than 2.0 × 10 ²² cm ⁻³ . At present, the hydrogen concentration detection limit is 1.0 × 10 ¹⁷ cm ⁻³ .

  The embodiment disclosed this time should be considered as illustrative in all points and not 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.

  The present invention is particularly advantageously applied to a Schottky barrier diode using a wide band gap semiconductor substrate such as GaN and having a field plate structure.

1 is a schematic cross-sectional view of a first embodiment of a Schottky barrier diode according to the present invention. FIG. 2 is a schematic perspective view of the Schottky barrier diode shown in FIG. 1. 3 is a flowchart for explaining a manufacturing process of the Schottky barrier diode shown in FIGS. 1 and 2. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 4 is a schematic cross-sectional view for explaining a manufacturing process for the Schottky barrier diode shown in FIG. 3. FIG. 6 is a schematic cross-sectional view showing a modification of the first embodiment of the Schottky barrier diode shown in FIGS. 1 and 2. It is a cross-sectional schematic diagram which shows Embodiment 2 of the Schottky barrier diode by this invention. FIG. 16 is a schematic cross-sectional view showing a modification of the second embodiment of the Schottky barrier diode shown in FIG. 14.

Explanation of symbols

1 Schottky barrier diode, 2 GaN free-standing substrate, 3 GaN epitaxial layer, 4 insulating layer, 5 Schottky electrode, 6 ohmic electrode, 7 electrode portion, 8 electrode, 9 field plate (FP) electrode portion, 10, 13 resist film , 11 opening,
12 metal film (for Schottky electrode), 14 metal film (for FP), 21 lower electrode part (Ti), 22 upper electrode part (Au).

Claims (7)

A semiconductor layer formed on a substrate;
A Schottky electrode formed in contact with the surface of the semiconductor layer;
An insulating layer formed on the surface of the semiconductor layer so as to be adjacent to the Schottky electrode;
A Schottky barrier diode comprising: an electrode portion connected to the Schottky electrode, extending in contact with the insulating layer and made of a material different from a material constituting the Schottky electrode. The electrode portion includes an extension extending on an upper surface of the Schottky electrode;
The Schottky barrier diode according to claim 1, wherein the electrode portion is connected to the Schottky electrode at the extending portion. The Schottky electrode includes an electrode extension extending on the electrode portion;
The Schottky barrier diode according to claim 1, wherein the Schottky electrode is connected to the electrode portion at the electrode extension portion.   The Schottky barrier diode according to claim 1, wherein the substrate is a gallium nitride substrate.   5. The Schottky barrier diode according to claim 1, wherein the material forming the Schottky electrode includes at least one selected from the group consisting of gold, platinum, nickel, palladium, cobalt, and copper. .   The material which comprises the said electrode part contains at least 1 type selected from the group which consists of titanium, nickel, molybdenum, chromium, aluminum, tungsten, and palladium, The Schottky of any one of Claims 1-5. Barrier diode.   The material constituting the insulating layer includes at least one selected from the group consisting of silicon nitride, silicon oxide, silicon oxynitride, hafnium oxide, aluminum nitride, aluminum oxide, gallium oxide, magnesium oxide, and scandium oxide. Item 7. The Schottky barrier diode according to any one of Items 1 to 6.

Images