Schottky barrier diode and method for manufacturing the same

A Schottky barrier diode provided herein includes: a semiconductor substrate; and an anode electrode being in contact with the semiconductor substrate. The semiconductor substrate includes: p-type contact regions being in contact with the anode electrode; and an n-type drift region being in contact with the anode electrode by Schottky contact in a range where the p-type contact regions are not provided The p-type contact regions includes: a plurality of circular regions located so that the circular regions are arranged at intervals between an outer side and an inner side at a contact surface between the semiconductor substrate and the anode electrode; and an internal region located in an inner portion of the circular region located on an innermost side at the contact surface and connected to the circular region located on the innermost side at the contact surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2014-259316 filed on Dec. 22, 2014, the contents of which are hereby incorporated by reference into the present application.

TECHNICAL FIELD

A technology disclosed herein relates to a Schottky barrier diode and a method for manufacturing the same.

DESCRIPTION OF RELATED ART

Japanese Patent Application Publication No. 2009-94433 (hereinafter referred to as “patent document 1”) discloses a Schottky barrier diode (hereinafter referred to as “SBD”). The SBD includes: a semiconductor substrate and an anode electrode being in contact with the semiconductor substrate. The semiconductor substrate includes: p-type contact regions being in contact with the anode electrode; and an n-type drift region being in contact with the anode electrode by Schottky contact in a range where the p-type contact regions are not provided. The p-type contact regions include: a single ring region extending along an outer circumference of the anode electrode; and a plurality of stripe patterned regions located in an inner portion of the ring region. Each of the stripe patterned regions is connected to the ring region. Further provided around the anode electrode (i.e. in an outer portion of the ring region) are p-type FLRs (field limiting rings) extending so as to surround the anode electrode. The FLRs are not in contact with the anode electrode. Application of a forward voltage to the SBD causes electrons to flow from the n-type drift region to the anode electrode through a Schottky interface between the anode electrode and the n-type drift region. This causes the SBD to be turned on. On the other hand, application of a backward voltage to the SBD stops the aforementioned flow of electrons, thus causing the SBD to be turned off. Further, when the SBD is turned off, depletion layer spread from the p-type contact regions to a portion of the n-type drift region that is located around the p-type contact regions. The depletion layer spreads so as to cover the Schottky interface. This prevents generation of a high electric field in an area in vicinity of the Schottky interface.

BRIEF SUMMARY

In the SBD of the patent document 1, the ring region is provided at an outer edge of a contact surface between the semiconductor substrate and the anode electrode. Therefore, when the SBD is turned off, a depletion layer extends from the ring region into a region located outer than the anode electrode (i.e. a region that is not covered with the anode electrode; hereinafter referred to as “peripheral region”). Meanwhile, the stripe patterned regions are connected to the ring region. Areas in vicinity of connected portions of the ring region to which the stripe patterned regions are connected have a higher p-type region ratio than areas in vicinity of non-connected portions of the ring region to which the stripe patterned regions are not connected. Therefore, the depletion layer extends more easily around the connected portions than around the non-connected portions. For this reason, the depletion layer extends from the ring region into the peripheral region faster in the areas in vicinity of the connected portions than in the areas in vicinity of the non-connected portions. Therefore, as shown inFIG. 6, in a process during which a depletion layer120extends from a ring region100into a peripheral region110, the depletion layer120becomes wider in areas in vicinity of connected portions102of the ring region100than in areas in vicinity of non-connected portions104of the ring region100. As a result, the depletion layer120has its outer edge in a wavy shape. For this reason, the SBD of the patent document 1 has such a problem that electric field easily concentrates in the peripheral region110and the peripheral region110is low in withstand voltage. The SBD of the patent document 1 is a type of SBD in which no holes flow from the p-type contact regions into the n-type drift region when the SBD is turned on. This type of SBD is called a JBSD (Junction Barrier Schottky Diode). On the other hand, there is a type of SBD in which holes flow from the p-type contact regions into the n-type drift region when the SBD is turned on (i.e. a type of SBD in which both electrons and holes contribute to current). This type of SBD is called an MPSD (Merged PIN Schottky Diode). When the MPSD is turned off, the MPSD operates in a same manner as the aforementioned JBSD does. Therefore, the MPSD has a problem which is similar to that of the JBSD. Therefore, the present specification provides a technology for improving withstand voltage in an SBD including p-type contact regions and an n-type drift region at a contact surface between a semiconductor substrate and an anode electrode (i.e. in an SBD encompassing a JBSD and an MPSD).

A Schottky barrier diode disclosed herein comprises: a semiconductor substrate; and an anode electrode being in contact with the semiconductor substrate. The semiconductor substrate comprises: p-type contact regions being in contact with the anode electrode; and an n-type drift region being in contact with the anode electrode by Schottky contact in a range where the p-type contact regions are not provided. The p-type contact regions comprise: a plurality of ring regions located so that the ring regions are arranged at intervals from an outer side to an inner side at a contact surface between the semiconductor substrate and the anode electrode; and an internal region located in an inner portion of the ring region located on an innermost side at the contact surface and connected to the ring region located on the innermost side at the contact surface.

The term “inner side” as used herein means a side with respect to a ring region on which a region surrounded by the ring region is located, and the term “outer side” as used herein means a side with respect to a ring region that is outside a region surrounded by the ring region. Further, the term “inner portion” as used herein means a region surrounded by a ring region, and the term “outer portion” as used herein means a region that is outside a region surrounded by a ring region.

In this SBD, the internal region is connected to the ring region (hereinafter referred to as “first ring region”) located on the innermost side. For this reason, a depletion layer extends more easily around a connected portion of the first ring region to which the internal region is connected than around a non-connected portion of the first ring region to which the internal region is not connected. For this reason, the depletion layer, which extends from the first ring region into an outer portion of the first ring region, has its outer edge in a wavy shape. In this SBD, however, another ring region (hereinafter referred to as “second ring region”) is provided in the contact surface so as to surround the first circular region. As with the first ring region, the second ring region is in contact with the anode electrode. Therefore, a depletion layer extends from the second ring region at substantially a same time as the depletion layer extends from the first ring region. A portion of the n-type drift region that is located between the first ring region and the second ring region is pinched off by the depletion layers extending from the first ring region and the second ring region. That is, the wavy-edged depletion layer extending from the first ring region and the depletion layer extending from the second ring region are combined to make a single depletion layer. For this reason, electric field concentration due to the influence of the wavy-edged depletion layer does not occur in the pinched-off region. Further, a depletion layer extends from the ring region located on the outermost side into an outer portion (i.e. a peripheral region) of the ring region. The internal region is not connected to the ring region located on the outermost side. This allows the depletion layer to uniformly extend from this ring region over into the peripheral region. Therefore, the electric field concentration is suppressed also in the peripheral region. Therefore, this SBD is high in withstand voltage.

A method for manufacturing a Schottky barrier diode is disclosed herein. This method comprises formation of p-type regions and formation of an anode electrode. In the formation of p-type regions, p-type impurities are implanted into a semiconductor substrate being of n-type to form p-type regions exposed on a surface of the semiconductor substrate. The p-type regions comprise a plurality of ring regions and an internal region. The plurality of ring regions is arranged with intervals from an outer side to an inner side, and includes a first plurality of ring regions located on the inner side and a second plurality of ring regions located on the outer side. The internal region is located in an inner portion of the ring region located on an innermost side at the contact surface. The internal region is connected to the ring region located on the innermost side at the contact surface.

In this method, the first plurality of ring regions that are in contact with the anode electrode serves as ring regions, and the second plurality of ring regions that are not in contact with the anode electrode serves as FLRs. This method allows the internal region and the ring regions, which are in contact with the anode electrode, to be formed at the same time as the FLRs.

DETAILED DESCRIPTION

Embodiments

FIGS. 1 and 2show an SBD10according to an embodiment. The SBD10includes a semiconductor substrate12. InFIG. 2, p-type regions are indicated by diagonal hatching. The semiconductor substrate12is made of SiC. The semiconductor substrate12has an upper surface12aon which an anode electrode14and an insulating film30are provided. A dotted line14inFIG. 2indicates a range in which the anode electrode14is provided (i.e. a contact surface15where the semiconductor substrate12and the anode electrode14are in contact with each other). The anode electrode14is provided in a central part of the upper surface12aof the semiconductor substrate12. A region of the upper surface12athat is not covered with the anode electrode14(i.e. a region on an outer side of the dotted line14; hereinafter referred to as a “peripheral region13”) is covered with the insulating film30. The semiconductor substrate12has a lower surface12bon which a cathode electrode16is provided.

Provided in an inner part of the semiconductor substrate12are stripe patterned p-type contact regions20, ring p-type contact regions22ato22e, FLRs24, a drift region26, and a cathode region28. It should be noted that the following description may refer to the ring p-type contact regions22ato22ecollectively as “ring p-type contact regions22”.

The stripe patterned p-type contact regions20, the ring p-type contact regions22, and the FLRs24are provided in a range exposed on the upper surface12aof the semiconductor substrate12. As shown inFIG. 2, the stripe patterned p-type contact regions20and the ring p-type contact regions22are provided in a portion of the upper surface12aof the semiconductor substrate12that is located within the contact surface15. The stripe patterned p-type contact regions20and the ring p-type contact regions22are in contact with the anode electrode14by a Schottky contact. The FLRs24are provided in a portion of the upper surface12aof the semiconductor substrate12that is located outside of the contact surface15. That is, the FLRs24are provided within the peripheral region13. The FLRs24have their upper surfaces covered with the insulating film30. As shown inFIG. 1, the stripe patterned p-type contact regions20, the ring p-type contact regions22, and the FLRs24are provided only in a surface part in vicinity of the upper surface12aof the semiconductor substrate12.

As shown inFIG. 2, each of the ring p-type contact regions22extends in a circle shape within the contact surface15. Each of the ring p-type contact regions22has a quadrangular shape having corners chamfered in a form of circular arcs. Each of the ring p-type contact regions22has linear portions each shaped in a straight line and corner portions each shaped in a circular arc. The ring p-type contact regions22are located so that the ring p-type contact regions22ato22eare arranged at intervals from an outer side to an inner side. That is, the ring p-type contact region22dis located in an inner portion of the ring p-type contact region22e, the ring p-type contact region22cis located in an inner portion of the ring p-type contact region22d, the circular p type contact region22bis located in an inner portion of the ring p-type contact region22c, and the ring p-type contact region22ais located in an inner portion of the ring p-type contact region22b. The ring p-type contact region22elocated on an outermost side has a greater width than any of the other ring p-type contact regions22. A portion of the ring p-type contact region22ethat is on the inner side than a center of the width is positioned within the contact surface15, whereas a portion of the ring p-type contact region22ethat is on the outer side than the center of the width protrudes outside the contact surface15. The ring p-type contact regions22ato22dare entirely positioned within the contact surface15.

As shown inFIG. 3, the corner portions of the ring p-type contact regions22ato22eare shaped in concentric circular arcs. More specifically, boundary lines of the corner portions (i.e. boundary lines between each of the p-type regions and the n-type region) are shaped in concentric circular arcs. For this reason, radiuses of the corner portions of the ring p-type contact regions22ato22eincrease as the ring p-type contact regions22ato22eare located closer to the outer side and decrease as the ring p-type contact regions22ato22eare located closer to the inner side. Because of such a structure, the ring p-type contact regions22ato22ehave widths that are substantially constant from the linear portions to the corner portions. Further, as shown inFIG. 3, an interval Lr between the ring p-type contact region22alocated on an innermost side and the ring p-type contact region22elocated on the outermost side is larger than a radius Rmin of the corner portion located on the innermost side.

As shown inFIGS. 1 and 2, the stripe patterned p-type contact regions20are located in an inner portion of the ring p-type contact region22alocated on the innermost side. The stripe patterned p-type contact regions20linearly extend parallel to each other. Each of the stripe patterned p-type contact regions20is connected to the ring p-type contact region22aat both ends of the stripe patterned p-type contact regions20.

The FLRs24are p-type semiconductor regions. Each of the FLRs24is provided in the peripheral region13, and extends circularly so as to surround the anode electrode14. The FLRs24are located so that the FLRs24are arranged at intervals from the outer side to the inner side. Each of the FLRs24has a quadrangular shape having corners chamfered in circular arcs. Each of the FLRs24has linear portions each shaped in a straight line and corner portions each shaped in a circular arc. As shown inFIG. 3, the corner portions of the FLRs24are shaped in concentric circular arcs with the corner portions of the ring p-type contact regions22. Because of such a structure, the FLRs24have widths that are substantially constant from the linear portions to the corner portions.

As shown inFIGS. 1 and 2, the SBD10includes the five ring p-type contact regions22ato22eand the three FLRs24. That is, a number of the ring p-type contact regions22is larger than a number of the FLRs24.

The drift region26is an n-type semiconductor region. The drift region26has a low density of n-type impurities. In the present embodiment, the drift region26has a density of n-type impurities of 9.5×1015atoms/cm3or lower. The drift region26is located under the stripe patterned p-type contact regions20, the ring p-type contact regions22, and the FLRs24. Further, in a position where the stripe patterned p-type contact regions20and the ring p-type contact regions22are not provided, the drift region26extends up to the upper surface12a(i.e. the contact surface15) of the semiconductor substrate12and is in contact with the anode electrode14by Schottky contact. Further, in a position in the peripheral region13where the FLRs24are not provided, the drift region26extends up to the upper surface12aof the semiconductor substrate12. As mentioned above, the stripe patterned p-type contact regions20are connected to the ring p-type contact region22located on the innermost side. Except for these connected portions, the drift region26is provided in regions between the p-type regions20,22, and24. The ring p-type contact regions22and the FLRs24are separated from each other by the drift region26.

The cathode region28is an n-type semiconductor region. The cathode region28has a density of n-type impurities higher than that of the drift region26. In the present embodiment, the cathode region28has a density of n-type impurities of 3.0×1018atoms/cm3or higher. Further, the cathode region28has a resistivity of 15 to 25 mΩ·cm. The cathode region28is located under the drift region26. The cathode region28is provided in a range exposed on the lower surface12bof the semiconductor substrate12. The cathode region28is in contact with the cathode electrode16by ohmic contact. The cathode region28is separated from the stripe patterned p-type contact regions20, the ring p-type contact regions22, and the FLRs24by the drift region26. An interval W is provided between each of the p-type regions20,22, and24and the cathode region28in a thickness direction of the semiconductor substrate12.

The following describes how the SBD10operates. Application of a forward voltage (i.e. a voltage that causes the anode electrode14to be higher in potential than the cathode electrode16) to the SBD10causes electrons to flow from the drift region26to the anode electrode14through a Schottky interface between the anode electrode14and the drift region26. That is, electrons flow from the cathode electrode16to the anode electrode14via the cathode region28and the drift region26. This causes the SBD10to be turned on. Further, in the SBD10, no current flows through the p-type regions20and22. That is, the SBD10is a JBSD.

Thereafter, application of a backward voltage to the SBD10stops the flow of electrons, thus causing the SBD10to be turned off. Further, when the SBD10is turned off, depletion layer spreads from the p-type contact regions20and22to portions of the drift region26that are located around the p-type contact regions20and22.

The application of a backward voltage causes the drift region26exposed on the contact surface15to be pinched off by the depletion layers spreading from the p-type regions20and22on both sides of the drift region26. This prevents application of a high voltage to the Schottky interface between the anode electrode14and the drift region26. InFIG. 4, a dotted line38shows a shape of a depletion layer in a process of extending from the ring p-type contact region22alocated on the innermost side to an outer portion of the ring p-type contact region22a. As noted above, the plurality of stripe patterned p-type contact regions20are connected to the ring p-type contact region22a. For this reason, areas in vicinity of connected portions40at which the ring p-type contact region22aand the stripe patterned p-type contact regions20are connected to each other have a high p-type region ratio. Therefore, the depletion layer easily extends into a portion of the drift region26that is at an outer portion of the connected portions40. Contrary to this, areas in vicinity of portions42(hereinafter referred to as “non-connected portions42”) other than the connected portions40have a low p-type region ratio. Therefore, it is harder for the depletion layer to extend into a portion of the drift region26that is an outer portion of the non-connected portions42. As a result, the depletion layer is greater in width in the areas in vicinity of the connected portions40than in the areas in vicinity of the non-connected portions42. That is, the depletion layer has its edge in a wavy shape. However, even when the depletion layer has its edge in such a wavy shape, a portion of the drift region26that is located between the ring p-type contact region22aand the ring p-type contact region22bis quickly pinched off by the depletion layer extending from the circular p type contact region22aand the depletion layer extending from the ring p-type contact region22b. Therefore, the wavy-edged depletion layer has little influence, and generation of a high electric field in the portion of the drift region26that is located between the circular p type contact region22aand the ring p-type contact region22bis suppressed.

Further, as mentioned above, the radiuses of the corner portions of the ring p-type contact regions22ato22edecrease as the ring p-type contact regions22ato22eare located closer to the inner side. Therefore, the corner portions have substantially constant widths. This allows depletion layers to evenly spread from the corner portions to areas near the corner portions, respectively. This suppresses the generation of a high electric field in the areas in vicinity of the corner portions.

Further, in the peripheral region13, a depletion layer spreads from the ring p-type contact region22elocated on the outermost side to an outer portion of the ring p-type contact region22e. The depletion layer spreads over to a further outer side via the plurality of FLRs24. This causes the peripheral region13to be depleted. In this way, the depletion layer extends from the inner side toward the outer side in the peripheral region13. If the depletion layer extending from the ring p-type contact region22eto the outer portion of the ring p-type contact region22ehas its outer edge in a wavy shape, the depletion layer spreads into the peripheral region13while keeping its wavy outer edge to some extent. For this reason, there is a risk that a high electric field may be generated locally in the peripheral region13. In the present embodiment, however, no other p-type region is connected to the ring p-type contact region22elocated on the outermost side. That is, unlike the ring p-type contact region22a, the ring circular p type contact region22edoes not have a connected portion40. Therefore, the depletion layer extending from the ring circular p type contact region22eto the outer portion of the ring circular p type contact region22ehas its outer edge not in a wavy shape, but in a flat shape. This prevents the generation of a high electric field in the peripheral region13.

Further, the waviness of the depletion layer extending from the ring p-type contact region22alocated on the innermost side may influence electric field distribution in the peripheral region13. In the SBD10according to the present embodiment, however, an interval Lr between the ring p-type contact region22aand the ring p-type contact region22eis larger than the radius Rmin of the corner portion located on the innermost side. The interval Lr is thus secured sufficiently wide. This curbs the influence on the peripheral region13by the wavy-edged depletion layer extending from the ring p-type contact region22a. Further, in the SBD10according to the present embodiment, the number of the ring p-type contact regions22is larger than the number of the FLRs24. That is, the many ring p-type contact regions22bto22dare provided between the ring p-type contact region22aand the ring p-type contact region22e. This also curbs the influence on the peripheral region13by the wavy-edged depletion layer extending from the ring p-type contact region22a. Therefore, in the SBD10, it is harder for electric field concentration to occur in the peripheral region13.

By the depletion layer spreading as described above, the drift region26is depleted in a range indicated by a dotted line50inFIG. 1. By the drift region26being thus depleted, the SBD10achieves high withstand voltage.

Once the depletion layer reaches the cathode region28(i.e. the region having a high density of n-type impurities), downward spreading of the depletion layer stops. Therefore, the interval W inFIG. 1is a width of the depletion layer in the thickness direction in a case where the depletion layer extends most widely. As mentioned above, the radius Rmax of the corner portion of the ring p-type contact region22elocated on the outermost side is greater than the interval W. By thus making the radius Rmax larger, electric field concentration in an area in vicinity of the corner portion can be further suppressed.

In manufacturing an SBD10, stripe patterned p-type contact regions20, ring p-type contact regions22, and FLRs24can be formed by a single implantation of p-type impurities. In this case, a mask having openings corresponding in shape to the p-type regions20,22, and24is formed on a surface of an n-type semiconductor substrate. Next, p-type impurities are implanted into the n-type semiconductor substrate via the mask. This causes the stripe patterned p-type contact regions20and a plurality of ring p-type regions (i.e. the ring p-type contact regions22and FLRs24) to be formed. Next, an anode electrode14is formed so as to be in contact with the stripe patterned p-type contact regions20and a first plurality of the ring p-type regions located on an inner side and so as not to be in contact with a second plurality of the ring p-type regions located on an outer side. This causes the anode electrode14to be in contact by Schottky contact with the p-type and n-type regions located under the anode electrode14. The first plurality of the ring p-type regions which are in contact with the anode electrode14by Schottky contact serve as the ring p-type contact regions22. Further, the second plurality of the ring p-type regions which are not in contact with the anode electrode14serve as the FLRs24. In this way, the single implantation of p-type impurities can form the stripe patterned p-type contact regions20, the ring p-type contact regions22, and the FLRs24.

Although, in the above embodiment, a JBSD has been described, a similar structure may alternatively be employed in an MPSD. Further, although, in the above embodiment, the p-type contact regions20and22are in contact with the anode electrode14by Schottky contact, the p-type contact regions20and22may alternatively be in contact with the anode electrode14by ohmic contact.

Further, in the above embodiment, the stripe patterned p-type contact regions20are provided in an inner portion of the ring p-type contact region22alocated on the innermost side. Alternatively, a grid like p-type contact region20shown inFIG. 5or a p-type contact region of another shape may be provided in the inner portion of the ring p-type contact region22alocated on the innermost side. That is, a p-type contact region provided in the inner portion of the ring p-type contact region22alocated on the innermost side may be of any shape, as long as the p-type contact region is connected to the ring p-type contact region22a.

A relationship between the components of above embodiment and the components of the claims will be described. The “stripe patterned p-type contact regions20” and the “ring p-type contact regions22” of the embodiment are an example of the “p-type contact regions” of the claims. The “ring p-type contact regions22” of the embodiment are an example of the “ring regions” of the claims. The “stripe patterned p-type contact regions20” of the embodiment are an example of the “internal region” of the claims. The “interval W” of the embodiment is an example of the “interval between the p-type contact region and the high density n-type region” of the claims. The “radius Rmax” of the embodiment is an example of the “radius of the corner portion located on an outermost side”. The “interval Lr” of the embodiment is an example of the “interval between the ring region located on the innermost side and the ring region located on an outermost side”. The “radius Rmin” of the embodiment is an example of the “radius of the corner portion located on the innermost side”.

The following enumerates technical elements disclosed herein. It should be noted that the following technical elements are each independently useful.

In a configuration disclosed herein as an example, each of the ring regions may comprise a corner portion shaped in a circular arc. Radiuses of the circular arcs may decrease as the ring regions are located closer to an inner side.

This configuration makes it possible to better uniform the widths of the corner portions.

In a configuration disclosed herein as an example, an interval between the ring region located on the innermost side and the ring region located on an outermost side may be larger than a radius of the corner portion located on the innermost side.

In a configuration disclosed herein as an example, the semiconductor substrate may comprise a plurality of Field Limiting Rings (FLRs) each of which extends so as to surround the anode electrode. A number of the ring regions may be larger than a number of the FLRs.

In a configuration disclosed herein as an example, the semiconductor substrate may comprise a high density n-type region located under the n-type drift region and having a density of n-type impurities higher than that of the drift region. The p-type contact region may be separated from the high density n-type region by the n-type drift region. A radius of the corner portion located on an outermost side may be larger than an interval between the p-type contact region and the high density n-type region.

The embodiments have been described in detail in the above. However, these are only examples and do not limit the claims. The technology described in the claims includes various modifications and changes of the concrete examples represented above. The technical elements explained in the present description or drawings exert technical utility independently or in combination of some of them, and the combination is not limited to one described in the claims as filed. Moreover, the technology exemplified in the present description or drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of such objects.