JP2015005635A - Manufacturing method for silicon carbide semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method for a silicon carbide semiconductor device, capable of suppressing cracking in a substrate during formation of a backside ohmic.SOLUTION: The present invention comprises: a step (a) for forming membrane structures (2, 3) in at least one of a front face and a rear face of a silicon carbide semiconductor substrate (1) and exposing an end of the silicon carbide semiconductor substrate (1); a step (b) for performing heat treatment at the rear face of the silicon carbide semiconductor substrate (1) and forming an ohmic junction.

Description

  The present invention relates to a method for manufacturing a silicon carbide semiconductor device.

  Silicon carbide (hereinafter also referred to as SiC) semiconductors have a larger breakdown electric field, band gap, and thermal conductivity than silicon semiconductors. Because of its large band gap and thermal conductivity, it has excellent heat resistance, enabling high temperature operation and simple cooling. Moreover, since the breakdown electric field is large, it is easy to reduce the thickness, and low loss and high temperature operation are possible.

  In the design of a SiC Schottky barrier diode (hereinafter referred to as SiC-SBD) or SiC-MOSFET (Metal Oxide Field Effect Transistor), the breakdown field of silicon is 0.3 MV / cm, whereas the breakdown field of SiC is Taking advantage of the feature of 2.8 MV / cm, the thickness and termination structure of the drift epitaxial layer as the active layer are determined.

  For SiC, which has a breakdown electric field about 10 times larger than that of silicon, the thickness of the drift epitaxial layer to be formed may be about 1/10 that of silicon. However, in order to maintain the mechanical strength of the substrate in the SiC substrate process, for example, an n-type 4H-SiC substrate having a thickness of about 350 μm is used in the case of a substrate diameter of 2 inches to 4 inches, and on top of that, An epitaxial layer having a thickness of several μm to several tens of μm is formed as an active layer.

  For example, in a k-class high breakdown voltage SiC-SBD, a Schottky electrode is formed on an n-type SiC epitaxial layer. In this structure, since the electric field is easily concentrated on the periphery of the junction surface between the epitaxial layer and the Schottky electrode, the p-type termination structure for reducing the electric field concentration is formed on the surface layer of the periphery of the junction surface (Schottky junction surface). Need to form.

  In order to form the p-type termination structure, generally, a p-type impurity such as Al (aluminum) or B (boron) is ion-implanted into the n-type epitaxial layer, and activation annealing is performed by high-temperature heat treatment at about 1500 ° C. or higher. Used.

  Subsequently, the back surface of the substrate is polished, and then a back surface ohmic is formed by a heat treatment at about 1000 ° C. using an ohmic material such as Ni. On the other hand, a Schottky junction is formed on the front surface of the substrate. Further, Al is generally formed with a thickness of about 5 μm on the front surface of the substrate as a pad for wire bonding.

  For activation annealing, which is a high-temperature heat treatment of about 1500 ° C. or higher, and backside ohmic formation, which is a heat treatment of about 1000 ° C., details of the heating program, matters concerning the atmospheric gas, temperature uniformity within the substrate surface, etc. It is important to prevent substrate cracking by optimizing the process details.

  At the time of activation annealing upstream of the process, the ion-implanted layer is ion-implanted, and no oxide film, metal film, or the like is formed. However, during the formation of the backside ohmic, which is a heat treatment at about 1000 ° C., an oxide film, a metal film, and the like are formed, which causes substrate warpage and internal stress. When the substrate warpage and internal stress increase and exceed the limit, the problem of substrate cracking occurs.

  Here, in order to suppress and prevent substrate cracking, it is important to pay attention to the edge of the SiC substrate so as not to cause cracking particularly in the thermal process. In addition, regarding the front surface and the back surface of the substrate, it is important to pay attention to reduce the substrate warpage and internal stress.

  As techniques related to these, for example, techniques described in Patent Document 1 and Patent Document 2 are known.

  Patent Document 1 describes a method of preventing chipping, cracking and cracking of a substrate by edge polishing (hereinafter, beveling) a nitride semiconductor substrate such as GaN.

  Patent Document 2 describes a method of patterning the back surface of a substrate in order to reduce substrate warpage. As the patterning shape, a lattice pattern and a dot pattern are shown, and a repetitive continuous pattern is assumed as a specific pattern shape.

JP 2004-319951 A Japanese Patent No. 4904688

  As described above, if the feature that the breakdown electric field of silicon is 0.3 MV / cm while the breakdown electric field of SiC is 2.8 MV / cm is utilized, the thickness of the drift epitaxial layer as the active layer is the same as that of silicon. About 1/10 is sufficient. At this time, the SiC substrate has a function of mechanically holding the epitaxial layer formed thereon.

  In the case of a silicon semiconductor substrate, the substrate quality is controlled in consideration of the formation of an epitaxial layer, the thinning process of the substrate, the formation of a film during or in the substrate process, and the reduction of the substrate thickness due to polishing. Specifically, the substrate quality is managed by managing the edge shape of the substrate before the substrate process cut out from the ingot, and further by managing the beveling.

  On the other hand, in the case of a SiC semiconductor substrate, such management is not sufficient, and quality can be managed at the same level as in the case of silicon in connection with the fact that operation at a higher temperature is possible. Often not easy. In particular, during the formation of the backside ohmic, the generation of substrate cracks could not be sufficiently suppressed.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a method for manufacturing a silicon carbide semiconductor device capable of suppressing substrate cracking when forming a backside ohmic. .

  A method for manufacturing a silicon carbide semiconductor device according to one aspect of the present invention includes: (a) forming a film structure on at least one of a front surface and a back surface of a silicon carbide semiconductor substrate, and forming an end portion of the silicon carbide semiconductor substrate And (b) performing a heat treatment on the back surface of the silicon carbide semiconductor substrate to form an ohmic junction.

  According to the said aspect of this invention, since the back surface ohmic can be formed in the state which exposed the edge part of the silicon carbide semiconductor substrate, the substrate crack at the time of forming a back surface ohmic can be suppressed.

It is a figure for demonstrating the manufacturing process of the silicon carbide semiconductor device regarding 1st Embodiment. It is a figure for demonstrating the manufacturing process of the silicon carbide semiconductor device regarding 2nd Embodiment. It is a figure for demonstrating the manufacturing process of the silicon carbide semiconductor device regarding 3rd Embodiment. It is a figure for demonstrating the manufacturing process of the silicon carbide semiconductor device regarding 5th Embodiment. It is the figure which showed the other example of patterning of the metal film regarding 5th Embodiment. It is sectional drawing which showed the structure regarding a base technology.

  Hereinafter, embodiments will be described with reference to the accompanying drawings.

  FIG. 6 is a cross-sectional view showing a structure related to the base technology, and is a view showing a general cross-sectional structure of the substrate immediately before the heat treatment for forming the back surface ohmic. In particular, at the edge (edge) of the SiC substrate, a protective insulating film 30 formed on the front surface of the n + substrate 1 and a metal film 20 formed on the back surface of the n + substrate 1 extend. Covers the edge of the substrate. Here, in order to suppress and prevent substrate cracking, it is important to pay attention to the edge of the SiC substrate so as not to cause cracking particularly in the thermal process. In addition, regarding the front surface and the back surface of the substrate, it is important to pay attention to reduce the substrate warpage and internal stress.

  Embodiment described below is related with the manufacturing method of the silicon carbide semiconductor device which solves the above problems.

<First Embodiment>
<Configuration>
Hereinafter, the outline of the silicon carbide semiconductor device and the manufacturing method thereof according to the present invention will be described using SiC-SBD as an example.

  For example, an n-type SiC layer is epitaxially grown by a CVD (Chemical Vapor Deposition) method on a silicon surface (0001) of an n-type 4H—SiC substrate having a diameter of 4 inches and an off angle of 4 °.

  The concentration and thickness of the n-type epitaxial layer are adjusted to achieve a desired breakdown voltage.

  After that, mark formation for alignment reference in the substrate required in the photoengraving process is performed, and thereafter, p-type termination structure formation, back surface polishing and back surface ohmic formation, Schottky junction formation on the front surface, wire bond The front surface pad for forming and the back surface metallization for die bonding on the back surface are sequentially performed. Here, the back surface polishing step, the back surface ohmic forming step, and the step that greatly affects the substrate cracking will be described in detail below.

  FIG. 1 is a diagram for illustrating a manufacturing process of the silicon carbide semiconductor device according to the first embodiment. Specifically, in the manufacturing process of SiC-SBD, it is a cross-sectional view just before the heat treatment at the time of backside ohmic formation.

  In FIG. 1, when heat treatment is performed to form the backside ohmic of the SiC substrate, at least a portion of the substrate edge that has been subjected to beveling processing is stripped of a metal or an oxide film, or no film is formed. The state where is exposed is shown. The substrate edge does not have to be beveled.

  FIG. 1 shows an n + substrate 1 having an n-type epitaxial layer formed on its upper surface, and a metal film 2 to be an ohmic electrode is formed on the back surface of the n + substrate 1. A protective insulating film 3 is formed on the surface. However, as long as SiC is exposed at least with respect to the edge of the SiC substrate, more precisely, the portion subjected to beveling, the structure and specifications of the front surface and the back surface of the substrate excluding the substrate edge are not limited.

Second Embodiment
<Configuration>
FIG. 2 is a diagram for describing a manufacturing process of the silicon carbide semiconductor device according to the second embodiment. Specifically, in the manufacturing process of SiC-SBD, it is a cross-sectional view just before the heat treatment at the time of backside ohmic formation.

  In the first embodiment, the substrate cross-sectional structure immediately before the heat treatment at the time of forming the back surface ohmic is explained by focusing on the fact that SiC is exposed at the edge of the SiC substrate. However, in FIG. The film 2 is not formed in a portion subjected to beveling processing, or once formed and then removed.

  This can reduce the substrate stress during the heat treatment and prevent the substrate from cracking.

  The second embodiment also assumes a manufacturing method in which a protective insulating film is not formed on the front surface of the substrate. For example, a dry process such as acid wet treatment or sputter etching is performed immediately before forming a Schottky junction by forming a Ti film. By carrying out the treatment, it is possible to clean the SiC front surface and to form a good bond.

<Third Embodiment>
<Configuration>
FIG. 3 is a diagram for describing a manufacturing process of the silicon carbide semiconductor device according to the third embodiment. Specifically, in the manufacturing process of SiC-SBD, it is a cross-sectional view just before the heat treatment at the time of backside ohmic formation.

  In the second embodiment, the substrate structure immediately before the heat treatment focusing on the metal film 2 formed on the back surface serving as the ohmic surface has been described. In FIG. 3, the structure of the front surface serving as the Schottky surface is illustrated. The substrate structure immediately before the heat treatment focusing on the above will be described.

  As shown in FIG. 3, the metal film for the ohmic electrode is not shown on the back surface of the substrate. As for the structure of the back surface of the substrate, a metal film 20 for ohmic electrodes may be formed up to the edge portion of the back surface of the substrate as shown in FIG. 6, or as shown in FIGS. The metal film for the back ohmic electrode may be patterned on the back surface.

  The reason why the protective insulating film 3 is formed on the front surface serving as the Schottky surface when performing the heat treatment for ohmic formation is that the Schottky metal film is formed after the heat treatment for ohmic formation. This is to prevent contamination of the SiC front surface, which is the Schottky formation planned surface, due to the influence of the previous process.

The protective insulating film 3, is formed by a CVD method, generally SiO ₂ oxide insulating film called deposition film or is formed by thermal oxidation, it is assumed that SiO ₂ oxide insulating film, commonly referred to as a sacrificial oxide film .

  When the film can be formed on only one side of the substrate, such as a coating film or atmospheric pressure CVD film, and the entire substrate including the front and back surfaces of the substrate, as well as the substrate edge, such as thermal oxidation or low pressure CVD. The part to be removed differs depending on whether the cover is completely covered. However, in any case, unnecessary portions are removed by a mechanical method such as wet or dry etching or grinding so that SiC is exposed at the edge of the substrate, more precisely, at the portion subjected to the beveling process.

  In this way, it is possible to protect the SiC Schottky surface from being contaminated even if the SiC Schottky surface is subjected to a heat treatment at about 1000 ° C. while minimizing the stress applied to the SiC substrate during the heat treatment for ohmic formation.

  A clean SiC surface can be formed with good reproducibility by removing the oxide film (protective insulating film 3) with hydrofluoric acid immediately after the backside ohmic is completed and immediately before forming a Schottky metal such as Ti.

<Fourth embodiment>
<Configuration>
As described in the first or third embodiment, when the protective insulating film 3 is formed using a sacrificial oxide film, the substrate edge is formed by performing at least one sacrificial oxide film formation and removal after the beveling. The mechanical damage layer on the outermost surface of the substrate edge by beveling can be removed. If the damaged layer is thick, it cannot be completely removed, but the fragile layer that has been mechanically damaged can be reduced just by reducing the thickness. Can be suppressed.

<Fifth Embodiment>
<Configuration>
4 and 5 are diagrams for illustrating a manufacturing process of the silicon carbide semiconductor device according to the fifth embodiment. FIG. 4 is a cross-sectional view immediately before the heat treatment at the time of forming the back surface ohmic in the manufacturing process of SiC-SBD. FIG. 5 is a diagram showing another patterning example of the metal film.

  In the first or second embodiment, regarding the substrate cross-sectional structure immediately before the heat treatment at the time of forming the back surface ohmic, it is effective for preventing the substrate from being cracked at the time of the heat treatment that the metal film is not formed on the portion subjected to the beveling process. Explained.

  Here, the metal film on the back surface is not formed on the entire back surface, but is arranged (patterned) in a ring shape (for example, concentric ring), so that the metal film is formed discontinuously from the center of the substrate toward the outer edge direction. The stress due to the electrodes can be effectively reduced (see the metal film 2a in FIG. 4). Therefore, substrate cracking can be effectively prevented.

  More preferably, as shown in FIG. 5, each annular patterning is not a continuous pattern of one round, but a cyclic pattern having a discontinuous portion (metal film 2 b), so that the metal film is also formed in the circumferential direction of the annular pattern. Is formed discontinuously, so that stress can be reduced more effectively. Therefore, substrate cracking can be effectively prevented.

  Details of dimensions and the like will be described below.

  Arranging the back metal in a concentric ring shape is effective in preventing substrate cracking. The distance between the metal films 2b arranged concentrically is preferably about 20 μm to 100 μm. When the opening width is narrower than 20 μm, the metal film 2b is not completely discontinuous due to the Ni silicide aggregation phenomenon during heating, and the effect of reducing internal stress and warping of the substrate is weakened. On the contrary, if the interval between the metal films 2b is 100 μm or more, it may cause a void in the die bonding process of the assembly.

  With regard to the width of the metal film 2b arranged in a concentric ring shape, the thinner (narrower) the effect is greater for the purpose of reducing internal stress and warping of the substrate. However, as in the case of the interval between the metal films 2b, about 1 mm or more is desirable from the viewpoint of the actual patterning process and the prevention of void generation in the die bonding step.

  Increasing the width of the metal film 2b arranged in a concentric ring shape means that the effect of reducing the internal stress and the warpage of the substrate is reduced. However, an actual device structure is used to form an ohmic junction. Since the internal stress and the amount of warpage of the substrate during steep heating (about 1000 ° C.) are different, the metal width or the number of metal films 2b arranged in a concentric ring shape may be determined as necessary.

  After the backside ohmic formation, backside metallization for die bonding is formed on the backside, but in this regard, heating at several hundred degrees C or higher is not necessary. Therefore, even if the backside metallization for die bonding is formed on the entire backside including the opening that is not ohmic-formed, no stress that causes substrate cracking occurs.

  Further, even if the ohmic electrode is formed to be extremely thin with respect to the opening in which ohmic formation is not performed in FIG. 5, it is effective in reducing internal stress. For example, according to selective film formation by a vapor deposition method using a metal mask corresponding to the pattern shown in FIG. Therefore, it is possible to reduce internal stress using an ultrathin ohmic electrode formed under the mask.

<Effect>
In order to prevent substrate cracking at the time of steep heating performed at about 1000 ° C. in which ohmic contact is formed, a remarkable effect can be expected by taking measures in all regions of the substrate front surface, substrate edge, and substrate back surface.

  As for the front surface, for example, a case where it is covered and protected by an insulating film such as a thermal oxide film is assumed. When the SiC substrate is thermally oxidized, the thickness of the film to be formed is about 20 times thicker on the (000_1) carbon surface on the back surface than on the (0001) silicon surface on the front surface. An oxide film is formed. Although it is not clear about the substrate edge, the film quality including the thickness of the oxide film is considered to vary greatly due to variations in the finished shape after beveling and back surface polishing.

  When abrupt heating at about 1000 ° C. is performed while being covered with such a thermal oxide film, the stress at the substrate edge due to the SiC substrate and the thermal oxide film becomes locally unstable, and the thermal oxide film is cracked. It is also expected to enter.

  In order to suppress this, it is desirable to heat in a state where the thermal oxide film formed on the substrate edge portion is removed. In the case of forming an oxide film by thermal oxidation, the oxide film is formed so as to cover all of the substrate front surface, substrate edge, and substrate back surface. The back surface is removed by a method such as etching or polishing if an oxide film or the like is formed before the metal for forming the ohmic junction is formed.

  Regarding the back surface, normally, for example, Ni is formed as an ohmic material on the entire surface by sputtering, and Ni silicide is formed by heat treatment to form an ohmic junction.

  In this case, it is considered that there is no extreme variation in thickness as in the thermal oxide film that is self-sacrificial oxidation, but the Ni silicidation reaction is similar to sacrificial oxidation in that the Si element of the SiC substrate is self-sacrificed. Since this reaction is combined with a metal such as Ni, the amount of Ni consumed and the degree of reaction vary during the Ni silicidation reaction in the periphery of the substrate, particularly in the beveled region.

  Therefore, it is desirable to heat at least a portion of the substrate edge that has been subjected to the beveling process in a Ni-removed state. In particular, when Ni silicide or local residual Ni formed during heating is generated due to variations in the finished shape after beveling and backside polishing, cracks or cracks are induced due to differences in the thermal expansion coefficient, and substrate cracking may occur. It is done.

  As described above, at least a portion subjected to beveling processing of the substrate edge is in a state in which all the coating films such as an insulating film such as an oxide film and a metal film serving as an ohmic material are removed and SiC is exposed. Is desirable.

  Furthermore, as for the backside ohmic, it is not essential to be formed on the entire surface due to required electrical characteristics. Therefore, an ohmic non-formed region is provided in order to reduce internal stress before and after heating by the backside metal and substrate warpage. This is also effective in preventing substrate cracking.

  In most cases, the warpage of the substrate is concave or convex, and in order to reduce internal stress and substrate warpage, it is effective that discontinuous backside metal is formed in the circumferential direction from the center of the substrate. Is. More desirably, by making each ring discontinuous rather than having a one-round continuous pattern, the stress that each ring causes on the SiC substrate can be further reduced.

  In the said embodiment, although the material of each component, material, the conditions of implementation, etc. are described, these are illustrations and are not restricted to what was described.

  In addition, within the scope of the present invention, the present invention can be freely combined with each embodiment, modified with any component in each embodiment, or omitted with any component in each embodiment.

  1 n + substrate, 2, 2a, 2b, 20 metal film, 3, 30 protective insulating film.

Claims (12)

(A) forming a film structure on at least one of the front surface and the back surface of the silicon carbide semiconductor substrate and exposing an end portion of the silicon carbide semiconductor substrate;
(B) performing a heat treatment on the back surface of the silicon carbide semiconductor substrate to form an ohmic junction,
A method for manufacturing a silicon carbide semiconductor device. The step (a) is a step of forming a metal film on the back surface of the silicon carbide semiconductor substrate and exposing an end portion of the silicon carbide semiconductor substrate,
The step (b) is a step of performing heat treatment using the metal film to form an ohmic junction,
A method for manufacturing a silicon carbide semiconductor device according to claim 1. The step (a) is a step of forming the annularly patterned metal film and exposing an end portion of the silicon carbide semiconductor substrate,
A method for manufacturing a silicon carbide semiconductor device according to claim 2. The step (a) is a step of forming the annularly patterned metal film having a discontinuous portion and exposing an end portion of the silicon carbide semiconductor substrate,
A method for manufacturing a silicon carbide semiconductor device according to claim 2. (C) before the step (b), further comprising the step of beveling the end of the silicon carbide semiconductor substrate,
The manufacturing method of the silicon carbide semiconductor device in any one of Claims 2-4. (D) A step of forming a sacrificial oxide film at an end portion of the silicon carbide semiconductor substrate and removing the sacrificial oxide film after the step (c) and before the step (b). And
A method for manufacturing a silicon carbide semiconductor device according to claim 5. The step (a) is a step of forming an insulating film on the front surface of the silicon carbide semiconductor substrate and exposing an end portion of the silicon carbide semiconductor substrate,
(C) After the step (a) and before the step (b), the method further comprises a step of forming a metal film on the back surface of the silicon carbide semiconductor substrate.
A method for manufacturing a silicon carbide semiconductor device according to claim 1. The step (c) is a step of forming the metal film patterned in an annular shape,
A method for manufacturing a silicon carbide semiconductor device according to claim 7. The step (c) is a step of forming the metal film patterned in an annular shape having a discontinuous portion,
A method for manufacturing a silicon carbide semiconductor device according to claim 7 or 8. (D) before the step (b), further comprising the step of beveling the end portion of the silicon carbide semiconductor substrate,
The manufacturing method of the silicon carbide semiconductor device in any one of Claims 7-9. The step (a) is a step of removing the film structure formed at the end portion of the silicon carbide semiconductor substrate to expose the end portion of the silicon carbide semiconductor substrate,
The manufacturing method of the silicon carbide semiconductor device in any one of Claims 1-10. The step (a) is a step of forming the film structure in a region excluding an end of the silicon carbide semiconductor substrate on at least one of a front surface and a back surface of the silicon carbide semiconductor substrate. ,
The manufacturing method of the silicon carbide semiconductor device in any one of Claims 1-10.

Images