JP2018107378A - Silicon carbide semiconductor device and method of manufacturing the same, and method of forming oxide film of silicon carbide semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon carbide semiconductor device with an enhanced function of a passivation film, and to provide a method of manufacturing the same.SOLUTION: A silicon carbide semiconductor device 100 has: a silicon carbide substrate 101; a first passivation film 102A that covers a principal surface of the silicon carbide substrate 101; and a second passivation film 102B formed on the first passivation film 102A. The first passivation film 102A is a thermal oxide film having a thickness equal to or more than 0.1 μm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a silicon carbide semiconductor device, a method for manufacturing the same, and a method for forming an oxide film of a silicon carbide semiconductor.

  In general, a passivation film is formed on the surface of a semiconductor substrate constituting a silicon carbide semiconductor device (device) and a terminal portion of a junction structure formed and exposed on the surface for the purpose of protecting them. The passivation film is formed using various materials and manufacturing methods depending on the application. For example, Patent Document 1 describes an oxide film formed using a chemical vapor deposition (CVD) method as a protective film. Patent Document 2 describes a passivation film made of polyimide. Patent Document 3 describes a passivation film made of non-stoichiometric silicon nitride formed by sputtering on a thermal oxide film of a silicon carbide substrate.

11A to 11C are schematic cross-sectional views of silicon carbide semiconductor devices 200, 300, and 400 having conventional passivation films. As shown in FIGS. 11A and 11B, a conventional passivation film uses a CVD oxide film (SiO ₂ ) 202A and a CVD nitride film (SiN) 302A under the polyimide films 202B and 302B, respectively. There is what I did. However, since the CVD oxide film 202A has a large porosity, it has low moisture resistance and is not sufficiently reliable as a protective film for the device. Further, the CVD nitride film 302A has low adhesion and is easily peeled off. In the case where a conventional passivation film is used, a device for dealing with these problems is required.

  Although the thermal oxide film has high adhesion to SiC and has better moisture resistance than the CVD oxide film, it is generally considered that it is difficult to use it as a passivation film because it cannot be formed thick. In Patent Document 3, the thickness of the thermal oxide film is set to 100 to 500 angstroms. As shown in FIG. 11C, the thickness of the thermal oxide film is increased as a passivation film. A two-layer structure formed by forming the film 402C has been proposed. However, in this structure, due to the presence of the CVD oxide film 402C, the moisture resistance is lowered as described above, and the reliability as the protective film of the device is insufficient.

JP-A-6-283523 JP 2013-219150 A Japanese Patent No. 5254037

The present invention has been made in view of the above circumstances, and an object thereof is to provide a silicon carbide semiconductor device having an improved function of a passivation film and a method for manufacturing the same.
Another object of the present invention is to provide a method for forming an oxide film of a silicon carbide semiconductor having an improved function as a passivation film.

  The oxide film used for the gate portion of the MOSFET and the like is considered to be as thin as possible from the viewpoint of enhancing the electrical characteristics of the MOSFET, and there has been no motive for forming the oxide film as thick as about 0.2 μm. Therefore, the conventional thermal oxidation furnace has a structure that assumes processing at a temperature of 1200 ° C. or lower, and enables high-temperature processing exceeding 1200 ° C. necessary to form an oxide film with a thickness of 0.2 μm or more. There was no structure that did.

  Under such circumstances, as a result of intensive studies, the present inventors have found that the function as a passivation film is enhanced by combining a thick thermal oxide film and a film made of polyimide. The present invention provides the following means in order to solve the above problems.

(1) A silicon carbide semiconductor device according to one aspect of the present invention includes a silicon carbide substrate, a first passivation film covering a main surface of the silicon carbide substrate, and a second passivation formed on the first passivation film. And the first passivation film is a thermal oxide film having a thickness of 0.1 μm or more.
(2) In the silicon carbide semiconductor device according to (1), the second passivation film may be made of polyimide.
(3) In the silicon carbide semiconductor device according to any one of (1) and (2), the plane orientation of the silicon carbide substrate has a (0001) Si plane having an off angle or an off angle (000− 1) C plane may be sufficient.
(4) In the silicon carbide semiconductor device according to any one of (1) to (3), even if a Schottky barrier diode and / or a field effect transistor is formed on a main surface of the silicon carbide substrate. Good.
(5) A method for manufacturing a silicon carbide semiconductor device according to an aspect of the present invention includes a silicon carbide substrate, a first passivation film that covers a main surface of the silicon carbide substrate, and a first passivation film stacked on the first passivation film. A thermal oxidation step of thermally oxidizing the main surface of the silicon carbide substrate at a temperature of 1300 ° C. or higher to form the first passivation film; A POA process of annealing the first passivation film in an inert gas atmosphere containing nitrogen (Post Oxidation Annealing).
(6) In the method for manufacturing a silicon carbide semiconductor device according to (5), only nitrogen may be used as the atmospheric gas in the POA process.
(7) In the method for manufacturing a silicon carbide semiconductor device according to any one of (5) and (6), the processing temperature in the POA step may be the same as the processing temperature in the thermal oxidation step.
(8) In the method for manufacturing a silicon carbide semiconductor device according to any one of (5) to (7), the second passivation film is formed directly on the annealed first passivation film. And a polyimide may be used as a material for the second passivation film.
(9) A method for forming an oxide film of a silicon carbide semiconductor according to one aspect of the present invention is a method for forming an oxide film of a silicon carbide semiconductor in which a thermal oxide film is formed on a surface of a silicon carbide semiconductor substrate, and the thermal oxide film The annealing temperature (POA) is performed in an inert gas atmosphere containing nitrogen continuously after the formation of the thermal oxide film.
(10) In the method for forming an oxide film of a silicon carbide semiconductor according to (9), the temperature of the POA is the same as the temperature of forming the thermal oxide film.

  In the silicon carbide semiconductor device of the present invention, the passivation film has a two-layer structure including a thermal oxide film having a sufficient thickness of 0.1 μm or more and a polyimide film. Since the thermal oxide film has a dense and high-quality structure and is excellent in adhesion to the silicon carbide substrate and moisture resistance, the passivation film of the present invention provided with this is formed by CVD as in the prior art. Compared with the passivation film, it has a high function. Therefore, the reliability of the electrical characteristics can be improved by forming a silicon carbide semiconductor device using the passivation film of the present invention.

In the method for manufacturing a silicon carbide semiconductor device of the present invention, the oxide film constituting the passivation film is formed by thermal oxidation. Since this thermal oxidation is performed at a high temperature of 1300 ° C. or higher, the growth rate can be increased, an oxide film having a sufficient thickness can be formed, and thus a passivation film having a high function can be formed. Therefore, it is not necessary to further increase the film thickness by performing CVD or the like after thermal oxidation, and the process can be simplified.
In the method of forming an oxide film of a silicon carbide semiconductor according to the present invention, the thermal oxide film of silicon carbide is formed at a high temperature of 1300 ° C. or higher, and then the POA process is performed to increase the thickness and the interface characteristics. An excellent oxide film can be formed.

It is sectional drawing of the silicon carbide semiconductor device which concerns on one Embodiment of this invention. (A)-(e) It is sectional drawing in each manufacturing process of the silicon carbide semiconductor device of FIG. (A)-(f) It is sectional drawing in each manufacturing process of the conventional silicon carbide semiconductor device. (A), (b) It is a graph which shows the electrical property of the silicon carbide semiconductor device which concerns on Example 1 of this invention. (A), (b) It is a graph which shows the electrical property of the silicon carbide semiconductor device which concerns on the comparative example 1. FIG. (A), (b) It is a graph which shows the electrical property of the silicon carbide semiconductor device which concerns on the comparative example 2. FIG. It is a graph which shows the speed | rate in which the passivation film (oxide film) is formed in the manufacture process of the silicon carbide semiconductor device which concerns on the comparative experiment of an oxide film. It is a graph which shows the electric charge amount contained in the inside of the passivation film which comprises the silicon carbide semiconductor device which concerns on the comparative experiment of an oxide film, or an interface. It is a graph which shows the interface state density of the passivation film which comprises the silicon carbide semiconductor device which concerns on the comparative experiment of an oxide film. It is a graph which shows the breakdown electric field of the passivation film which comprises the silicon carbide semiconductor device which concerns on the comparative experiment of an oxide film. (A)-(c) is sectional drawing of the conventional silicon carbide semiconductor device.

  Hereinafter, the present invention will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, in order to make the features of the present invention easier to understand, the features may be enlarged for the sake of convenience, and the dimensional ratios of the components are different from the actual ones. There is. The materials, dimensions, and the like exemplified in the following description are merely examples, and the present invention is not limited to these, and can be implemented with appropriate modifications within the scope of the effects of the present invention.

[Configuration of silicon carbide semiconductor device]
FIG. 1 is a cross-sectional view of a silicon carbide semiconductor device 100 according to an embodiment of the present invention. Silicon carbide semiconductor device 100 includes a silicon carbide substrate (SiC substrate) 101, a first passivation film 102A covering one main surface 101a of the silicon carbide substrate, and a second passivation film formed on first passivation film 102A. And 102B as main components.

  An n-type silicon carbide epitaxial film (SiC epitaxial film) 101A is formed in a region having a predetermined depth from one main surface 101a of the silicon carbide substrate. Hereinafter, it is assumed that one main surface 101a of the silicon carbide substrate means the surface of silicon carbide epitaxial film 101A.

  FIG. 1 further shows a case where functional portion (functional element) 110 is formed on one main surface 101a of the silicon carbide substrate. Functional portion 110 is a portion directly responsible for the operation of silicon carbide semiconductor device 100 as an electronic device, such as a Schottky barrier diode (SBD) or a field effect transistor (FET). FIG. 1 illustrates the case where the functional unit 110 has a Schottky barrier diode structure. Specifically, Schottky electrode 103 and surface PAD electrode 104 are sequentially formed on one main surface 101a of the silicon carbide substrate. In addition, P-type ion implantation region 101B is formed in silicon carbide epitaxial film 101A.

  One main surface 101a of the silicon carbide substrate is preferably a (0001) Si surface having an off angle (offset angle) or a (000-1) C surface having an off angle. In particular, the (0001) Si surface is less affected by the C / Si ratio than that of the (000-1) C surface when doping, and the carrier concentration is easily maintained. Therefore, it is more preferable for operation.

  Examples of the material for the Schottky electrode 103 include Mo (molybdenum) and Ti (titanium). In the examples described later, the case where Mo is used is illustrated.

  The front surface PAD electrode 104 and the back surface PAD electrode 105 are metal films (laminated films) formed by laminating Ni films, Ti films, Al films, and the like. In this case, the thicknesses of the Ni film, Ti film, and Al film are preferably about 100 nm, about 150 nm, and about 4 nm, respectively.

The first passivation film 102A is a thermal oxide film (SiO ₂ film) having a thickness of 0.1 μm or more. This thermal oxide film is formed by bonding oxygen molecules diffused in the vicinity of a film formation surface (main surface 101a in this case) of silicon carbide epitaxial film 101A with silicon atoms existing there. Therefore, a good interface having a continuous crystal structure is formed between silicon carbide substrate 102 and first passivation film 102A. Moreover, since this thermal oxide film is formed at a high temperature of 1300 ° C. or higher as will be described later, it is a dense and high-quality film. Further, this thermal oxide film is superior to the CVD oxide film in chemical resistance.

Although a CVD oxide film can be formed at a lower temperature than thermal oxidation, the ratio of SiC and O ₂ is largely dependent on the film formation conditions, and there are many unbonded hands, and the film quality and film thickness accuracy (uniformity) are also high. It is inferior to the oxide film, and the breakdown voltage is also inferior to the thermal oxide film.

  The second passivation film 102B is made of polyimide (polyimide resin). Polyimide has advantages such as higher heat resistance than other organic materials and excellent electrical and mechanical properties. The thickness of the second passivation film 102B is preferably about 50 to 100 times that of the first passivation film 102A. The second passivation film 102B is preferably formed directly on the first passivation film 102A.

  The passivation film 102 according to the present embodiment has a two-layer structure including a thermal oxide film having a sufficient thickness of 0.1 μm or more and a polyimide film. Since the thermal oxide film has a dense and high-quality structure and is excellent in adhesion to the silicon carbide substrate 101 and moisture resistance, the passivation film 102 provided with this is formed by CVD as in the prior art. Compared to the passivation film, it has a higher function. Therefore, the reliability of the electrical characteristics can be improved by forming the silicon carbide semiconductor device 100 using the passivation film 102 according to the present embodiment.

[Method for Manufacturing Silicon Carbide Semiconductor Device]
Silicon carbide semiconductor device 100 according to the present embodiment can be manufactured mainly through the following steps A1 to A8.

<Process A1>
As shown in FIG. 2A, a silicon carbide substrate 101 having an n-type silicon carbide epitaxial film 101A formed on one main surface 101a is prepared.

<Process A2>
Using a photolithography method, ions of a p-type impurity such as aluminum (Al) are implanted into a predetermined region of the silicon carbide epitaxial film 101A to form a p-type ion implantation region 101B. The p-type ion implantation region 101B here is a region having a depth of about 0.8 μm from one main surface 101a of the silicon carbide substrate at a position at least partially overlapping with a Schottky electrode formed in a later step.

<Process A3 (thermal oxidation (dry oxidation) process)>
Silicon carbide substrate 101 on which silicon carbide epitaxial film 101A is formed is placed in a thermal oxidation furnace formed of an SiC tube, and this silicon carbide substrate 101 is heated (thermal oxidation, dry oxidation) using a cantal heater. Thereby, as shown in FIG. 2A, a thermal oxide film (first passivation film) 102A can be formed on one main surface 101a of the silicon carbide substrate.

  Normally, a thermal oxidation furnace composed of a quartz tube is used. In this type of thermal oxidation furnace, the heating temperature is 1200 ° C. at the maximum, and the thickness of the thermal oxide film formed in this case is 0. Only about 05μm. In contrast, in the type using the SiC tube and the Kanthal heater, heating at a high temperature of 1350 ° C. or higher is possible. The oxidation rate for heating to 1350 ° C. is about 8 times the oxidation rate for heating to 1200 ° C. at the same supply oxygen flow rate. Therefore, when heating to 1350 ° C., a 0.2 μm thick thermal oxide film can be formed in the same oxidation time as when heating to 1200 ° C.

  In order to set the thickness of the thermal oxide film to be formed to 0.1 μm or more, the heating temperature here is preferably 1300 ° C. or more, and the heating time is preferably 50 minutes or more.

<Process A4 (POA process)>
The object to be processed that has undergone the thermal oxidation process is placed in an annealing furnace, and the inside of the furnace is put into a nitriding gas (N ₂ or N ₂ O) atmosphere, and then the first passivation film 102A is annealed (POA: Post Oxidation). (Annual) processing. The heating temperature here is preferably 1300 ° C. or more and 1350 ° C. or less, and the heating time is preferably 30 minutes or more and 60 minutes or less. The POA treatment is preferably performed continuously after thermal oxidation. The temperature of the POA treatment is preferably the same as the temperature of thermal oxidation. In this case, there is no need to increase or decrease the temperature in the furnace between the thermal oxidation process and the POA process, and only the gas switching in the furnace (O ₂ → N ₂ ) needs to be performed. Processing between processes can be simplified.

  By this annealing, the first passivation film 102A is strengthened and the adhesion strength to the substrate can be increased. The POA process here is to harden the thermal oxide film. By performing a high temperature POA process in which a nitriding gas is introduced on a dense thermal oxide film formed by high temperature thermal oxidation, a hardened thermal oxide film can be obtained while maintaining adhesion. This thermal oxide film is excellent in moisture resistance and has an advantage as a passivation film.

<Process A5 (Back Electrode Formation Process)>
As shown in FIG. 2A, a back ohmic electrode 105 is formed on the other main surface 101b of the silicon carbide substrate. Specifically, first, a coater developer applies a resist to one main surface 101a to protect it, and then the thermal oxide film formed on the other main surface 101b in step A3 is removed by hydrofluoric acid treatment. Subsequently, a metal film made of Ni or the like is formed with a thickness of about 100 nm on the other main surface 101b of the exposed silicon carbide substrate by sputtering or vapor deposition. Further, by performing heat treatment at about 950 ° C. for about 5 minutes in an inert gas atmosphere or in vacuum, back electrode 105 that makes ohmic contact with the silicon carbide substrate is obtained.

<Process A6 (Schottky electrode formation process)>
As shown in FIG. 2B, etching with hydrofluoric acid is performed to remove the thermal oxide film 102A in the electrode formation region. Subsequently, as shown in FIG. 2C, a Schottky electrode 103 is formed on one main surface 101a of the silicon carbide substrate exposed by the etching process using a sputtering method or an evaporation method. Further, Schottky electrode annealing is performed by performing a heat treatment at about 650 ° C. for about 10 minutes in an inert gas atmosphere or vacuum.
<Process A7 (surface electrode formation process)>
As shown in FIG. 2C, a metal film formed by laminating a Ni film, a Ti film, and an Al film on the Schottky electrode 103 formed in step A6 by using a sputtering method or a vapor deposition method is about 4. 2 μm is formed, and the PAD electrode 104 is obtained.

<Process A8>
As shown in FIG. 2D, a photosensitive polyimide film (second passivation film) 102B is formed on the entire surface of the object to be processed after the electrode formation step by a coating method. The second passivation film 102B is directly formed on the first passivation film 102A. Subsequently, as shown in FIG. 2E, the silicon carbide semiconductor device 100 according to the present embodiment is obtained by performing an etching process to expose at least a part of the surface PAD electrode 104.

  Next, the method for manufacturing silicon carbide semiconductor device 100 according to the present embodiment will be compared with the method for manufacturing silicon carbide semiconductor device 400 having the conventional structure shown in FIG. Therefore, a method for manufacturing silicon carbide semiconductor device 400 will be described with reference to FIGS. Silicon carbide semiconductor device 400 can be manufactured mainly through the following steps B1 to B8.

<Process B1>
Similar to step A1, a silicon carbide substrate 401 having an n-type silicon carbide epitaxial film 401A formed on one main surface 401a is prepared.

<Process B2>
Similar to step A2, p-type ion implantation region 401B is formed in a predetermined region of silicon carbide epitaxial film 401A.

<Process B3 (thermal oxidation (dry oxidation) process)>
Silicon carbide substrate 401 on which silicon carbide epitaxial film 401A is formed is placed in a conventional thermal oxidation furnace composed of a quartz tube, and silicon carbide substrate 401 is heated. Thereby, as shown in FIG. 3A, thermal oxide film 402A can be formed on one main surface 401a of the silicon carbide substrate. Since the heating temperature is 1200 ° C. at the maximum, the thickness of the formed thermal oxide film 402A is about 500 mm at the maximum.

<Process B4 (CVD oxidation process)>
In order to form an oxide film having a thickness of about 2000 mm that functions as a passivation film, a CVD oxide film 402C having a thickness of about 1500 mm is formed on the thermal oxide film 402A as shown in FIG.

<Process B5 (Back Electrode Formation Process)>
Similarly to step A5, as shown in FIG. 3B, a back surface ohmic electrode 405 is formed on the other main surface 401b of the silicon carbide substrate.

<Process B6 (Schottky electrode formation process)>
Similarly to step A6, as shown in FIG. 3C, the thermal oxide film 402A in the electrode formation region is removed, and as shown in FIG. 3D, one main surface 401a of the silicon carbide substrate is removed. A Schottky electrode 403 is formed by a sputtering method or an evaporation method.
<Process B7 (surface electrode formation process)>
As in step A7, as shown in FIG. 3D, a Ni film, a Ti film, and an Al film are stacked on the Schottky electrode 403 formed in step B6 by sputtering or vapor deposition. A PAD electrode 404 is obtained by forming a metal film of about 4.2 μm.

<Process B8>
As shown in FIG. 3E, a photosensitive polyimide film (second passivation film) 402B is formed on the entire surface of the object to be processed after the electrode formation process by a coating method. Subsequently, as shown in FIG. 3F, an etching process is performed to expose at least part of the surface PAD electrode 404, whereby silicon carbide semiconductor device 400 having a conventional structure is obtained.

  In the manufacturing method of silicon carbide semiconductor device 400 having a conventional structure, the oxide film constituting the passivation film is formed through two steps of thermal oxidation and CVD oxidation, whereas silicon carbide semiconductor device 100 according to the present embodiment is formed. In the manufacturing method, it can be formed through only one step of thermal oxidation.

  Note that the CVD film 402C contains a large amount of porous and has a higher etching rate than the dense thermal oxide film 402A. Therefore, in step B6, when etching with hydrofluoric acid is performed, the amount of abrasion differs between oxide film 402A and CVD film 402C, and the wall surface (etching surface) is perpendicular to one main surface 101a of the silicon carbide substrate. Difficult to form. In this case, since the size of the electrode formed in the etched region changes, there is a possibility that predetermined electrical characteristics cannot be obtained.

  On the other hand, since the passivation film of the present embodiment is composed only of the thermal oxide film, the etching rate is uniform over the entire film. Therefore, in the method for manufacturing silicon carbide semiconductor device 400 according to the present embodiment, when the passivation film is etched, a vertical wall surface can be formed, and thus the above problem occurs. Absent.

  In the method for manufacturing silicon carbide semiconductor device 100 according to the present embodiment, the oxide film constituting passivation film 103 is formed by thermal oxidation. Since this thermal oxidation is performed at a high temperature of 1300 ° C. or higher, the growth rate can be increased, an oxide film having a sufficient thickness can be formed, and thus the passivation film 102 having a high function can be formed. Therefore, it is not necessary to further increase the film thickness by performing CVD or the like after thermal oxidation, and the process can be simplified.

  In this specification, an example is shown in which a second passivation film using polyimide is formed and used on an oxide film formed by performing a thermal oxidation film formation temperature of 1300 ° C. or higher and subsequently performing POA in a nitrogen atmosphere. However, this 0.1 μm thick thermal oxide film can also be used as a protective film of another structure.

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

Example 1
In silicon carbide semiconductor device 100 according to the above-described embodiment, the current-voltage characteristics of the Schottky barrier diode constituting functional unit 110 were examined. Various conditions for manufacturing silicon carbide semiconductor device 100 were set as follows.

  The heating temperature for forming the thermal oxide film 103A was 1350 ° C., the heating time was 100 minutes, and as a result, a thermal oxide film 103A having a thickness of 0.2 μm was formed on the silicon carbide substrate 101. The formed thermal oxide film 103A was subjected to POA treatment in a nitrogen atmosphere. At this time, the heating temperature was 1350 ° C. and the heating time was 30 minutes. Further, the polyimide raw material coating conditions were adjusted to form a 10 μm thick polyimide film on the thermal oxide film 103A. The thicknesses of the Schottky electrode 103, the front surface PAD electrode 104, and the back surface PAD electrode 105 were 0.1 μm, 4.2 μm, and 0.7 μm, respectively.

FIG. 4A is a graph showing current-voltage characteristics in the forward direction of the Schottky barrier diode at each position of the silicon carbide substrate 101. The horizontal and vertical axes of the graph indicate the forward voltage V _f [V] and the forward current I _f [A], respectively. From the graph of FIG. 4A, it can be seen that there is no current variation at each position, a predetermined current value is obtained, and the current-voltage characteristic is good in the forward direction.

FIG. 4B is a graph showing current-voltage characteristics in the reverse direction of the Schottky barrier diode at each position of the silicon carbide substrate 101. The horizontal and vertical axes of the graph indicate the reverse voltage V _r [V] and the reverse current I _r [A], respectively. From the graph of FIG. 4B, there is no current variation at each position, the reverse leakage current is suppressed to 10 μA at the withstand voltage design value (1200 V here), and the current-voltage characteristics are also observed in the reverse direction. It turns out that it is favorable.

(Comparative Example 1)
In the silicon carbide semiconductor device 200 having the conventional configuration shown in FIG. 11A, the current-voltage characteristics of the Schottky barrier diode constituting the functional unit were examined. When manufacturing the silicon carbide semiconductor device, an oxide film (SiO ₂ film) that forms a two-layer structure together with the polyimide film was a CVD oxide film having a thickness of 2000 mm by plasma CVD. Other configurations are the same as those in the first embodiment.

FIG. 5A is a graph showing current-voltage characteristics in the forward direction of the Schottky barrier diode at each position of the silicon carbide substrate 201. The horizontal and vertical axes of the graph indicate the forward voltage V _f [V] and the forward current I _f [A], respectively. From the graph of FIG. 5A, it can be seen that there is a large current variation at each position and a predetermined current value is not obtained, and the current-voltage characteristics are not good in the forward direction.

FIG. 5B is a graph showing current-voltage characteristics in the reverse direction of the Schottky barrier diode at each position of the silicon carbide substrate 201. The horizontal and vertical axes of the graph indicate the reverse voltage V _r [V] and the reverse current I _r [A], respectively. From the graph of FIG. 5B, the current variation at each position is large, and the reverse leakage current flows as much as 1 mA at the withstand voltage design value (1200 V here), and the current-voltage characteristics are not good even in the reverse direction. I understand that.

(Comparative Example 2)
In the silicon carbide semiconductor device 400 having the conventional configuration shown in FIG. 11C, the current-voltage characteristics of the Schottky barrier diode constituting the functional unit were examined. Of the oxide film (SiO ₂ film) that forms a three-layer structure together with the polyimide film when manufacturing the silicon carbide semiconductor device, the lower layer side is a thermal oxide film having a thickness of 500 mm, and the upper layer side is 1500 mm thick by plasma CVD. The CVD oxide film was used. Other configurations are the same as those in the first embodiment.

FIG. 6A is a graph showing current-voltage characteristics in the forward direction of the Schottky barrier diode at each position on the silicon carbide substrate 401. The horizontal and vertical axes of the graph indicate the forward voltage V _f [V] and the forward current I _f [A], respectively. From the graph of FIG. 6A, there is little current variation at each position, and a predetermined current value is obtained. It can be seen that the forward current-voltage characteristics are improved as compared with the case where the oxide film is formed only by CVD (Comparative Example 1).

FIG. 6B is a graph showing current-voltage characteristics in the reverse direction of the Schottky barrier diode at each position on the silicon carbide substrate 401. The horizontal and vertical axes of the graph indicate the reverse voltage V _r [V] and the reverse current I _r [A], respectively. From the graph of FIG. 5 (b), it can be seen that there is a large current variation at each position, and the breakdown voltage does not reach the design value (1200V here), and the current-voltage characteristics are not good in the reverse direction.

(Example 2)
<Comparison of oxide film formation methods>
The oxide film quality performance test was conducted with different oxide film formation methods.
For the case where thermal oxidation and POA treatment are performed under the following conditions [1] to [5], the oxidation rate (oxidation rate), the amount of charge at the inside / interface of the oxide film, the interface state density, and the oxide film breakdown electric field A capacitor (MOSCAP) device was created and examined. The structure of the MOSCAP element is such that an oxide film is formed on the epitaxial side surface of the silicon carbide epitaxial wafer, electrodes for characteristic measurement are formed on the upper and lower surfaces, and the elements are separated. The surface on which the oxide film is formed is a (0001) Si surface.

The following five types ([1] to [5]) were compared as conditions for forming the oxide film and POA. [1] is the condition of the present invention in which the formation temperature of the thermal oxide film is 1300 ° C. or higher and the POA process is continuously performed after the thermal oxidation process.
[1] Dry oxidation / oxidation species: O ₂ , N ₂ -POA / treatment temperature: 1350 ° C.
[2] Dry oxidation / oxidation species: O ₂ , N ₂ -POA / treatment temperature: 1200 ° C.
[3] Dry oxidation / oxidation species: O ₂ , N ₂ O—POA / treatment temperature: 1200 ° C.
[4] Dry oxidation / oxidation species: O ₂ , H ₂ —POA / treatment temperature: 1200 ° C.
[5] Wet oxidation (pyrogenic oxidation) / oxidized species: N ₂ / treatment temperature: 1200 ° C.

FIG. 7 is a graph showing the oxidation rate (nm / min) when the POA treatment is performed under the conditions [1] to [5]. On the horizontal axis, the conditions [1] to [5] are abbreviated in order from the left side. As shown in FIG. 7, the oxidation rate when the thermal oxidation and the POA treatment are performed under the condition [1] is about eight times as high as when performed under the conditions [2] to [4]. From this result, it can be seen that a combination of high-temperature dry oxidation (thermal oxidation) and POA treatment in which N ₂ gas is introduced is a more preferable condition for forming a thick oxide film.

  The wet oxidation process is more complicated than the dry oxidation process, and the film formed by wet oxidation is generally not stable in film quality. Therefore, it is preferable to form the thermal oxide film by dry oxidation. Therefore, the oxidation rate when the thermal oxidation and POA treatment is performed under the condition [5] is about 3.3 times that when the thermal oxidation and POA treatments are performed under the conditions [2] to [4]. In terms of performance, it is not a preferable condition.

  When the thermal oxidation rates of the Si surface and the C surface are compared, the thermal oxidation rate of the C surface is about three times the Si surface thermal oxidation rate under the same thermal oxidation conditions.

FIG. 8 is a graph showing the amount of charge Q _eff (C / cm ² ) generated in the oxide film / interface when the POA process is performed under the conditions [1] to [5]. As shown in FIG. 8, when the POA process is performed under the condition [1], the charge amount Q _eff is minimized, so that high-temperature dry oxidation with nitrogen gas introduced damages due to charging. It can be seen that this is a more preferable condition for avoidance. On the other hand, when the POA treatment is performed under the condition [4], since the charge amount Q _eff is particularly large, it is not sufficient to introduce hydrogen gas in order to avoid damage due to charging. It turns out that introduction of gas is effective.

FIG. 9 is a graph showing the interface state density D _it (eV ⁻¹ · cm ⁻² ) generated at the oxide film interface when the POA treatment is performed under the above conditions [1] to [5]. is there. As shown in FIG. 9, when the POA process is performed under the condition [1], the interface state density _Dit is minimized, so that high-temperature dry oxidation with introduction of nitrogen gas causes interface state density. It can be seen that this is also a more favorable condition for suppressing the occurrence of positions. In addition, when the POA process is performed under the condition [1], the interface state density _Dit is suppressed to 1/3 or less of that when the POA process is performed under the condition [2]. The effectiveness of increasing the thermal oxidation treatment temperature can be confirmed.

FIG. 10 is a graph showing the breakdown electric field (MV / cm) of the oxide film when the POA process is performed under the conditions [1] to [5]. It can be seen that the oxidation breakdown electric field has the same magnitude regardless of the POA treatment under any condition.
From the above results, it can be seen that the thermal oxide film forming method of the present invention is generally excellent.

(Example 3)
A high-temperature and high-humidity bias test was performed using a silicon carbide semiconductor device having a conventional structure and the silicon carbide semiconductor device of the present invention. Specifically, under the conditions of temperature = 85 ° C. and humidity = 85% using a thermostatic chamber, a specified value bias was applied to the device, and the electrical characteristics (I _R (reverse current) in 1000 hours of continuous test) , V _R (reverse voltage)) abnormality rate was determined as the defect rate. Those with a defect rate of 0% were judged as acceptable.

  As a silicon carbide semiconductor device having a conventional structure, a type having a two-layered oxide film (thermal oxide film + CVD oxide film) shown in FIG. 11C was used. The thickness of the thermal oxide film was 500 mm, and the thickness of the CVD oxide film was 1500 mm. In the silicon carbide semiconductor device of the present invention, the thickness of the thermal oxide film was 2000 mm.

  The silicon carbide semiconductor device having a conventional structure was satisfactory with a pass rate of 100% (failure rate of 0%) up to 700 hours of continuous test, but was completely destroyed after 700 hours or more, and a pass rate of 0% (failure rate of 100%). )

  In the silicon carbide semiconductor device of the present invention, the success rate was 100% (defective rate 0%) in 1000 hours of the continuous test, and the effect of the present invention was confirmed. In the 1350 ° C. dry oxidation (+ 1350 ° C. POA treatment) performed in the present invention, the oxidation rate is fast, so that an oxide film of 1000 mm or more can be formed without requiring a long time. In addition, since the charge amount in the oxide film and at the interface is low and the current path can be suppressed as the passivation, the effect of suppressing leakage and improving the breakdown voltage can be expected, and the reliability (life) as the passivation film can be sufficiently obtained.

  The silicon carbide semiconductor device and the manufacturing method thereof of the present invention can be widely used as a technique for forming a surface protective film of a silicon carbide power semiconductor device.

100, 200, 300, 400 ... Silicon carbide semiconductor devices 101, 201, 301, 401 ... Silicon carbide substrates 101a, 201a, 301a, 401a ... One main surface 101b, 101b, 101b of the silicon carbide substrate 101b, the other principal surfaces 101A, 201A, 301A, 401A of the silicon carbide substrate, silicon carbide epitaxial films 101B, 201B, 301B, 401B, P-type ion implantation regions 102, 202, 302, 402,. ..Passivation films 102A, 402A ... thermal oxide film (first passivation film)
102B, 202B, 302B, 402B ... polyimide film (second passivation film)
202A, 402C ... CVD oxide film 302A ... CVD nitride film 103, 203, 303, 403 ... Schottky electrodes 104, 204, 304, 404 ... Surface PAD electrodes 105, 205, 305, 405 ..Back side PAD electrodes 110, 210, 310, 410 ... functional unit

Claims (10)

A silicon carbide substrate;
A first passivation film covering the main surface of the silicon carbide substrate;
A second passivation film formed on the first passivation film,
The silicon carbide semiconductor device, wherein the first passivation film is a thermal oxide film having a thickness of 0.1 μm or more.   The silicon carbide semiconductor device according to claim 1, wherein the second passivation film is made of polyimide.   3. The carbonization according to claim 1, wherein the plane orientation of the silicon carbide substrate is a (0001) Si plane having an off angle or a (000-1) C plane having an off angle. Silicon semiconductor device.   The silicon carbide semiconductor device according to any one of claims 1 to 3, wherein a Schottky barrier diode and / or a field effect transistor is formed on a main surface of the silicon carbide substrate. A method for manufacturing a silicon carbide semiconductor device comprising: a silicon carbide substrate; a first passivation film covering a main surface of the silicon carbide substrate; and a second passivation film laminated on the first passivation film,
A thermal oxidation step of thermally oxidizing the main surface of the silicon carbide substrate at a temperature of 1300 ° C. or higher to form the first passivation film;
And a POA step of annealing the first passivation film in an inert gas atmosphere containing nitrogen (Post Oxidation Annealing). A method for manufacturing a silicon carbide semiconductor device, comprising:   6. The method for manufacturing a silicon carbide semiconductor device according to claim 5, wherein only nitrogen is used as an atmospheric gas in the POA process.   The method for manufacturing a silicon carbide semiconductor device according to claim 5, wherein a processing temperature in the POA step is the same as a processing temperature in the thermal oxidation step. Forming the second passivation film directly on the annealed first passivation film;
The method for manufacturing a silicon carbide semiconductor device according to claim 5, wherein polyimide is used as a material for the second passivation film. A silicon carbide semiconductor oxide film forming method for forming a thermal oxide film on a surface of a silicon carbide semiconductor substrate,
An oxide film of a silicon carbide semiconductor, characterized in that a thermal oxide film is formed at a temperature of 1300 ° C. or higher and annealing (POA) is performed in an inert gas atmosphere containing nitrogen continuously after the thermal oxide film is formed. Forming method.   The method for forming an oxide film of a silicon carbide semiconductor according to claim 9, wherein the temperature of the POA is the same as the formation temperature of the thermal oxide film.

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