JP6640131B2 - Silicon carbide semiconductor device and method of manufacturing the same - Google Patents

Description

  The present invention relates to a silicon carbide semiconductor device having a Schottky junction, in which a part of a Schottky electrode forming a Schottky junction is in contact with a termination region such as a guard ring, and an outer peripheral end of the Schottky electrode is on an insulating film. The present invention relates to a silicon carbide semiconductor device.

  Since a Schottky barrier diode (SBD) is a unipolar device, switching loss can be reduced as compared with a normal bipolar diode. However, an SBD using silicon as a constituent material can practically obtain only a breakdown voltage of about 50 V or less. Therefore, it is not suitable for applications such as high-voltage inverters. On the other hand, since an SBD using silicon carbide as a constituent material can easily have a withstand voltage of about several kV, application to a power conversion circuit has recently attracted attention.

  As such an SBD using silicon carbide as a constituent material, for example, a device described in Patent Document 1 is known. In the SBD disclosed in Patent Document 1, in order to extend a depletion layer generated around a PN junction between a guard ring region and an n-type semiconductor layer when a static high voltage is applied, a Schottky electrode is provided. Have an overlay structure beyond the outer peripheral end of the guard ring area. By employing such a structure, the depletion layer easily extends into the n-type semiconductor layer, and the present SBD retains a higher voltage blocking capability.

JP 2005-286197 A (FIG. 1)

  However, when such an SBD is subjected to a high-frequency switching operation, electric field concentration occurs around the outer peripheral edge of the Schottky electrode, and there is a concern that the SBD may cause deterioration in withstand voltage.

  That is, at the time of the switching operation in which the transition is made from the conduction state to the inhibition state, a voltage that rises rapidly, that is, a high dv / dt voltage is applied to the SBD. At this time, in order to charge the PN junction between the guard ring region and the n-type semiconductor layer, a transition current proportional to the value of dv / dt flows through the guard ring region of the SBD. The guard ring region has a specific resistance value, and when a transition current proportional to the value of dv / dt flows, an electric field proportional to the transition current is generated in the guard ring region.

  In a conventional SBD, an overlay structure of a Schottky electrode is employed, but an etching residue is generated in a shape with a sharp tip at the outer peripheral end of the Schottky electrode when the Schottky electrode is formed by etching. When a high-frequency switching operation of the SBD is performed in such a situation where the residue of the Schottky electrode having a sharp tip exists, the electric field generated by the transition current tends to concentrate on the residue, and thereby the outer periphery of the Schottky electrode There is a concern that there is a possibility of causing trouble near the end.

  SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to provide a silicon carbide semiconductor device which does not cause deterioration in breakdown voltage even in a high-frequency switching operation.

In order to solve the above problems, in a silicon carbide semiconductor device according to the present invention, a drift layer of a first conductivity type, an annular guard ring region of a second conductivity type formed on a surface of the drift layer, and a surface of the drift layer A field insulating film formed so as to surround the guard ring region, and an outer peripheral end formed on the field insulating film to cover the surface of the drift layer inside the guard ring region, and between the field insulating film and the guard ring region. And a surface electrode pad formed on the Schottky electrode and in contact with the field insulating film beyond the outer peripheral end of the Schottky electrode. It has a high concentration region of the second conductivity type having a higher impurity concentration below the surface electrode pad, high concentration region of the guard ring region Forms an ohmic contact with the region and the Schottky electrode, except, the outer peripheral edge of the Schottky electrode are formed etching residue sharp shape of the tip, the surface electrode pad etch residues pointed shape of the tip It is formed so that it may cover.

According to the present invention, since the surface electrode pad is formed so as to cover the etching residue having a sharp tip, it is possible to suppress the deterioration of the breakdown voltage even in the high-frequency switching operation.

FIG. 2 is a plan view and a cross-sectional view schematically showing a configuration of a chip of the silicon carbide semiconductor device according to the first embodiment of the present invention. FIG. 13 is a cross sectional view schematically showing a configuration of a chip of a silicon carbide semiconductor device according to a second embodiment of the present invention. FIG. 13 is a cross sectional view schematically showing a configuration of a chip of a silicon carbide semiconductor device according to a third embodiment of the present invention. FIG. 4 is a cross sectional view showing the method for manufacturing the silicon carbide semiconductor device according to the present embodiment. FIG. 4 is a cross sectional view showing the method for manufacturing the silicon carbide semiconductor device according to the present embodiment. FIG. 11 is a cross-sectional view schematically showing a configuration of a modification of the chip of the silicon carbide semiconductor device according to the embodiment of the present invention.

<First Embodiment>
FIG. 1 is a plan view (a) and an AA cross-sectional view (b) schematically showing a configuration of a chip of a silicon carbide semiconductor device according to a first embodiment of the present invention. Here, an example is shown in which an SBD is provided as a semiconductor element on silicon carbide substrate 1. For convenience of explanation, FIG. 1A shows only components necessary for understanding the planar positional relationship of silicon carbide substrate 1, and therefore, FIG. 1B is referred for details. I want to.

  As shown in FIG. 1 (b), the silicon carbide semiconductor device according to the present embodiment has a first conductivity type (here, n-type) substrate layer 1a having a relatively high impurity concentration and epitaxial crystal growth on the upper surface thereof. It is formed using silicon carbide substrate 1 including first conductivity type drift layer 1b having a relatively low impurity concentration. Silicon carbide, which is a material, is a semiconductor material having a wider band gap than silicon, so that a semiconductor device using silicon carbide as a constituent material can operate at a higher temperature than a semiconductor device using silicon as a constituent material. It has become.

A guard ring region 2 of the second conductivity type (here, p-type) as a termination region is formed in an annular shape on one surface portion of the drift layer 1b. The drift layer 1b is exposed on one surface inside and outside the ring-shaped guard ring region 2. The guard ring region 2 forms a PN junction with the drift layer 1b. Field insulating film 3 is further formed on one surface of drift layer 1b so as to surround guard ring region 2. The field insulating film 3 has an opening at its center. In the present embodiment, the field insulating film 3 is a silicon oxide film, but may be a silicon nitride film.

  The Schottky electrode 4 is formed so as to cover the drift layer 1b exposed inside the guard ring region 2 formed in a ring shape, and forms a Schottky junction with the drift layer 1b. The Schottky electrode 4 extends so as to cover a part of the guard ring region 2 and forms an ohmic contact with the guard ring region 2. Further, the Schottky electrode 4 also extends on the field insulating film 3, and its outer peripheral end exists on the field insulating film 3. In the present embodiment, the metal constituting Schottky electrode 4 is titanium, and its thickness is about 200 nm. The metal forming Schottky electrode 4 may be a metal that forms a Schottky junction with a silicon carbide substrate, and may be a metal such as molybdenum, nickel, or gold.

  On the Schottky electrode 4, a surface electrode pad 5 for connection to an external terminal is laminated. The outer peripheral end of the surface electrode pad 5 is in contact with the field insulating film 3 beyond the outer peripheral end of the Schottky electrode 4. In the present embodiment, the metal constituting the surface electrode pad 5 is aluminum, and its thickness is about 4.8 μm. The metal constituting the surface electrode pad 5 may be a metal such as copper, molybdenum, nickel or the like, or may be an aluminum alloy such as Al-Si.

  One surface of silicon carbide substrate 1 is covered with protective film 6 so as to protect field insulating film 3, Schottky electrode 4 and surface electrode pad 5 formed thereon. The protective film 6 is desirably an organic resin film to reduce stress from the external environment. In addition, in the present embodiment, a polyimide resin film is used for the protective film 6 because it needs to withstand high-temperature processing. The protective film 6 has an opening on the surface electrode pad 5 for connection with an external terminal.

  Back surface electrode 7 is formed on the other surface of silicon carbide substrate 1. The back electrode 7 forms an ohmic contact with the substrate layer 1a. The back electrode 7 may be a metal that forms an ohmic contact with the substrate layer 1a, and is nickel in this embodiment, but may be aluminum or molybdenum.

  Next, a method for manufacturing the silicon carbide semiconductor device according to the present embodiment will be described with reference to FIGS. In each figure, the AA cross section of FIG. 1 is shown.

  A silicon carbide substrate 1 composed of a first conductivity type substrate layer 1a having a relatively high impurity concentration and a first conductivity type drift layer 1b having a relatively low impurity concentration grown epitaxially on the upper surface thereof is prepared. For example, the guard ring region 2 of the second conductivity type is formed on one surface of the drift layer 1b by selective ion implantation using a resist film 9 patterned into a predetermined shape by photolithography (FIG. 4A). ). Here, for example, aluminum ions or boron ions are implanted into the second conductivity type region as impurity ions, and the impurity ions are electrically activated by annealing at a high temperature of 1500 ° C. or more, so that the predetermined conductivity type is obtained. Region.

  Next, a silicon oxide film having a thickness of about 1 μm is deposited on one surface of the drift layer 1b by, for example, a CVD method, and thereafter the central silicon oxide film is removed by photolithography and etching. Is formed (FIG. 4B). The opening is formed inside guard ring region 2 including a part of guard ring region 2.

  Next, back electrode 7 is formed on the surface of silicon carbide substrate 1 where substrate layer 1a is exposed (FIG. 4c).

  Next, a metal film 8 serving as a Schottky electrode 4 is formed on the entire surface of one side of the drift layer 1b on which the field insulating film 3 is formed by a sputtering method, and a resist film having a predetermined shape is formed by photolithography. 9 is formed (FIG. 5a).

  Next, the metal film 8 is etched using the resist film 9 as a mask to form the Schottky electrode 4 having a desired shape (FIG. 5B). In etching the metal film 8, wet processing is desirable in order to reduce damage to the chip. In that case, the end of the formed Schottky electrode 4 tends to have a sharp pointed end. Here, the pointed portion is referred to as an etching residue 4a.

  Next, surface electrode pad 5 is laminated on Schottky electrode 4 so as to cover etching residue 4a, and protective film 6 is formed, whereby the silicon carbide semiconductor device according to the present embodiment is completed (FIG. 5c).

  Next, the operation of the silicon carbide semiconductor device according to the present embodiment will be described. In the silicon carbide semiconductor device according to the present embodiment, when a negative voltage is applied to back electrode 7 with respect to front electrode pad 5, a current flows from back electrode 7 to front electrode pad 5, and the device is brought into a conductive state. . Conversely, when a positive voltage is applied to the back electrode 7 with respect to the front electrode pad 5, the Schottky junction and the PN junction block current, and the device enters a blocking state.

  During a switching operation in which a transition is made from a conduction state to a blocking state, a rapidly rising voltage, that is, a high dv / dt voltage is applied to the device. At this time, an electric double layer called a depletion layer is generated around a PN junction between the guard ring region and the n-type semiconductor layer, and the depletion layer has a capacitance called a depletion layer capacitance. In order to charge the depletion layer capacitance, a transition current proportional to the value of dv / dt flows through the guard ring region 2 of the device from the PN junction toward the Schottky electrode 4. The guard ring region 2 has a unique resistance value, and when a transition current proportional to the value of dv / dt flows, an electric field proportional to the product of the transition current and the resistance value is generated in the guard ring region 2. Will be done.

  As in the conventional example, when the etching residue 4a having a sharp tip is left bare at the end of the Schottky electrode 4, the equipotential surface formed by the electric field generated in the guard ring region 2 due to the transfer current is etched. There was a possibility that the film might bend around the residue 4a and cause electric field concentration at this portion.

  In the silicon carbide semiconductor device according to the present embodiment, since the outer peripheral end of surface electrode pad 5 is configured to be in contact with field insulating film 3 beyond the outer peripheral end of Schottky electrode 4, the end of Schottky electrode 4 is formed. Since the etching residue 4a is covered with the conductive surface electrode pad 5, the equipotential surface formed by the electric field generated in the guard ring region 2 due to the transfer current does not curve around the etching residue 4a. There is no longer any possibility of causing electric field concentration in the part.

  Therefore, according to the present embodiment, there is an effect that a highly reliable silicon carbide semiconductor device can be obtained even in a high-frequency switching operation.

  Further, it is desirable that the outer peripheral edge of the surface electrode pad 5 exists above the guard ring region 2. When the outer peripheral edge of the surface electrode pad 5 has an overlay structure beyond the outer peripheral edge of the guard ring region 2 as in the conventional example, the equipotential surface formed by the electric field generated in the guard ring region due to the transition current becomes the guard ring region 2 This is because there is a possibility that the surface electrode pad 5 existing beyond the outer peripheral edge of the above-mentioned portion may be curved and cause a deterioration in withstand voltage at the portion. Therefore, such a configuration has an effect that a silicon carbide semiconductor device with higher reliability can be obtained even in a high-frequency switching operation.

<Embodiment 2>
FIG. 2 is a cross-sectional view schematically showing a configuration of a chip of a silicon carbide semiconductor device according to a second embodiment of the present invention. The plan view is the same as FIG. The present embodiment is characterized in that a p-type high-concentration region 2a having a higher impurity concentration than the guard ring region 2 is formed in the guard ring region 2 from its surface. Other configurations are the same as those of the first embodiment.

  With such a configuration, the depletion layer does not spread in the high-concentration region 2a, so that the intrinsic resistance value of the guard ring region 2 can be reduced, and the electric field generated in proportion to the resistance value is reduced. can do. Therefore, according to the present embodiment, there is obtained an effect that a silicon carbide semiconductor device with higher reliability can be obtained even in a high-frequency switching operation.

<Embodiment 3>
FIG. 3 is a cross sectional view schematically showing a configuration of a chip of a silicon carbide semiconductor device according to a third embodiment of the present invention. The plan view is the same as FIG. The present embodiment is characterized in that the opening of the field insulating film 3 has a tapered shape. Other configurations are the same as those of the first embodiment.

  Specifically, the opening of the field insulating film 3 has a tapered shape such that the thickness of the field insulating film 3 increases with increasing distance from the boundary with the opening to the outside. Since the opening of the field insulating film 3 has a tapered shape, the electric field at the end of the Schottky electrode 4 is further reduced, and the reliability of the chip is further improved. Therefore, according to the present embodiment, there is obtained an effect that a silicon carbide semiconductor device with higher reliability can be obtained even in a high-frequency switching operation.

  In the above description of the embodiment, a configuration is shown in which the semiconductor element is an SBD and the guard ring region 2 is provided in the termination region. However, the semiconductor element and the termination region of the silicon carbide semiconductor device according to the present invention are described. Is not limited to this. For example, the semiconductor element may be a junction barrier schottky diode (JBS) or a merged pn schottky diode (MPS) having a Schottky junction. Alternatively, a field limiting ring 10 may be provided in addition to the guard ring area 2 as shown in FIG. The first conductivity type is n-type and the second conductivity type is p-type. However, it is needless to say that the functions and effects of the present invention can be exerted even if the opposite is true.

  INDUSTRIAL APPLICABILITY The silicon carbide semiconductor device according to the present invention is applied to a device that performs power conversion such as AC to DC conversion, DC to AC conversion, or frequency conversion, thereby contributing to an improvement in the power conversion efficiency of the device. can do.

Reference Signs List 1 silicon carbide substrate 1a substrate layer 1b drift layer 2 guard ring region 2a high concentration region 3 field insulating film 4 Schottky electrode 4a etching residue 5 surface electrode pad 6 protective film 7 back electrode 8 metal film 9 resist film 10 field limiting ring

Claims (4)

A first conductivity type drift layer;
An annular second conductivity type guard ring region formed on the surface of the drift layer;
A field insulating film formed on the surface of the drift layer so as to surround the guard ring region;
A Schottky electrode formed so as to cover the surface of the drift layer inside the guard ring region, an outer peripheral edge is present on the field insulating film, and formed an ohmic contact with the guard ring region;
A surface electrode pad formed on the Schottky electrode and in contact with the field insulating film beyond the outer peripheral end of the Schottky electrode;
With
The guard ring region has a second conductivity type high concentration region having a higher impurity concentration than the guard ring region below the surface electrode pad,
Regions other than the high concentration region in the guard ring region form an ohmic contact with the Schottky electrode,
At the outer peripheral end of the Schottky electrode, a sharp-pointed etching residue is formed,
The silicon carbide semiconductor device, wherein the surface electrode pad is formed so as to cover the etching residue having a sharp tip.   2. The silicon carbide semiconductor device according to claim 1, wherein said guard ring region is formed so as to cover a periphery of said high concentration region.   3. The silicon carbide semiconductor device according to claim 1, wherein an opening of said field insulating film has a tapered shape. Preparing a silicon carbide substrate having a first conductivity type substrate layer and a first conductivity type drift layer formed on the substrate layer;
Forming a second conductivity type guard ring region on the surface of the drift layer;
Forming a second conductivity type high concentration region having a higher impurity concentration than the guard ring region in the guard ring region;
Forming a field insulating film to surround the Oite the guard ring region on a part of a surface of the drift layer,
After forming a metal film on the field insulating film and on the drift layer, the metal film is etched by a wet process to have a sharp-pointed etching residue at an outer peripheral end, and the guard ring region Forming a Schottky electrode having an ohmic contact with a region excluding the high concentration region ,
Forming a surface electrode pad on the Schottky electrode so as to be in contact with the field insulating film beyond the outer peripheral end of the Schottky electrode and to cover the etching residue having the sharp pointed end. A method for manufacturing a semiconductor device.

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