JP2012049347A - Silicon carbide schottky barrier diode and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an SiC Schottky barrier diode which can be formed by low temperature heat treatment and can have desired characteristics, and to provide a method of manufacturing the same.SOLUTION: The silicon carbide Schottky barrier diode has a Schottky electrode 5 on the surface of a first conductivity type silicon carbide layer 3, and a guard ring layer 4 formed on the periphery of the Schottky electrode 5 to overlap a part thereof. P type impurity ions are injected into a region of an N type silicon carbide layer 3 where the guard ring layer is formed. Thereafter, the ion implantation region is recrystallized and a part of injected impurity ions are heat treated in an activation temperature range thus forming the guard ring layer 4.

Description

  The present invention relates to a silicon carbide Schottky barrier diode and a manufacturing method thereof, and more particularly to a silicon carbide Schottky barrier diode including a guard ring layer and a manufacturing method thereof.

  Silicon carbide (SiC) has a dielectric breakdown electric field about 10 times that of silicon, thermal conductivity is about 3 times that of silicon, and electron saturation rate is about 2 times that of silicon, so it is compared with power devices using silicon. Thus, it has been attracting attention as a material that can easily increase the breakdown voltage of the device and can realize a low-loss power device by thinning and increasing the concentration of the device active layer. In particular, SiC Schottky barrier diodes were first put into practical use among SiC devices in the early 2000s, and when mounted in switching circuits, compared to silicon bipolar fast recovery diodes with the same breakdown voltage, Since the reverse recovery time and the recovery current are small, the device can realize a reduction in circuit loss.

  On the other hand, in order to realize a Schottky barrier diode exhibiting high withstand voltage characteristics by utilizing the physical properties of SiC, it is necessary to relax the electric field concentrated on the Schottky junction termination portion in the same way as a silicon power device during reverse bias operation. (Refer nonpatent literature 1).

FIG. 5 shows the structure of a conventional SiC Schottky barrier diode of this type. An n-type SiC epitaxial layer 2 having an impurity concentration lower than that of the n ⁺ -type SiC substrate on the n ⁺ -type SiC substrate 1 made of n-type hexagonal SiC having a high impurity concentration, and an impurity higher than that of the n-type SiC epitaxial layer 2 A low concentration n ⁻ -type SiC epitaxial layer 3 is formed. On n ⁻ type SiC epitaxial layer 3, Schottky electrode 5 that is in Schottky contact with the n ⁻ type SiC epitaxial layer is formed, and ohmic electrode 6 is formed on the back surface side of n ⁺ type SiC substrate 1. A guard ring layer 9 is formed so as to partially overlap the outer peripheral portion of the Schottky electrode 5. In addition, 7 is a pad electrode and 8 is a passivation film.

  In general, a SiC Schottky barrier diode has a guard ring structure using a P-type ion implantation layer (P-type guard ring layer) such as JTE (Junction Termination Extension). Such a guard ring structure is formed by ion-implanting boron or aluminum which is a P-type impurity to SiC at a high temperature of about 500 ° C., and then performing a heat treatment at 1500 ° C. or higher. It is general (refer nonpatent literature 2).

  As another example of the guard ring structure, a method of forming a high resistance guard ring structure by forming a crystal defect region (high resistance guard ring layer) by implanting argon ions is also known. Since argon does not become a P-type or N-type impurity with respect to SiC, the crystal structure is destroyed by ion implantation, and an acceptor trap is formed from the conduction band to a deep energy level within the SiC band gap. The electric field is relaxed. It is also known that such a high resistance guard ring structure has a problem that a leak current is large in a reverse operation (see Non-Patent Document 3).

  In the conventional P-type guard ring structure as described above, the P-type layer is depleted, so that the electric field concentrated on the outer peripheral edge of the guard ring structure is relaxed and a desired breakdown voltage is secured. Therefore, it is necessary to perform an activation process at a high temperature of 1500 ° C. or higher in order to sufficiently activate the implanted impurity ions.

  Also, in the high resistance guard ring structure, when an electric field is concentrated on the high resistance layer, an excessive electric field concentration does not occur by allowing a leakage current to flow through the high resistance layer at the Schottky electrode end and the outer peripheral end of the guard ring layer. I am doing so. Therefore, it is necessary to set the dimension long enough to obtain a desired breakdown voltage from the Schottky electrode end to the outer peripheral end of the guard ring layer.

B Jayant Baliga, “Silicon Carbide Power Devices” (USA), World Scientific, 2005, p. 38-70 Edited by Hiroyuki Matsunami, “Semiconductor SiC Technology and Applications”, Nikkan Kogyo Shimbun, 2003, p. 143-185 Dev Alok et al., IEEE ELECTRON DEVICE LETTERS, Vol.15, No.10, 1994, p. 394-395

  Of the conventionally proposed guard ring structures, the P-type guard ring structure needs to be heat-treated at 1500 ° C. or higher, and a method that can be formed at a low temperature is desired. On the other hand, in the high resistance guard ring structure, it is desired to reduce the reverse leakage resistance. Furthermore, in the high resistance guard ring structure, downsizing of the guard ring structure is desired.

  In view of the actual situation as described above, an object of the present invention is to provide a SiC Schottky barrier diode that can be formed by low-temperature heat treatment and that can obtain desired characteristics, and a method for manufacturing the same.

  In order to achieve the above object, the invention according to claim 1 of the present application is formed such that a Schottky electrode is provided on the surface of the first-conductivity-type silicon carbide layer, and a part thereof is overlapped around the Schottky electrode. In the silicon carbide Schottky barrier diode provided with a guard ring layer, the guard ring layer is recrystallized in a region where impurity ions of the second conductivity type are implanted into the silicon carbide layer of the first conductivity type. It is characterized in that it is a region which is made of a region and a part of the implanted impurity ions are activated to show the second conductivity type.

  The invention according to claim 2 of the present application is the silicon carbide Schottky barrier diode according to claim 1, wherein the guard ring layer is recrystallized in a region in which impurity ions to be P-type are implanted in the N-type silicon carbide layer. And a portion of the implanted impurity ions of the P type are activated to show a P type conductivity type.

  The invention according to claim 3 of the present application is a silicon including a Schottky electrode provided on the surface of a silicon carbide layer of the first conductivity type and a guard ring layer formed so as to partially overlap the periphery of the Schottky electrode. In the method for manufacturing a carbide Schottky barrier diode, impurity ions of the second conductivity type are formed in the region where the guard ring layer is to be formed on the surface of the silicon carbide layer of the first conductivity type, and crystal defects are present in at least the silicon carbide layer. A step of ion implantation under the implantation conditions to be generated; a step of recrystallizing the ion implantation region, and a heat treatment in a temperature range in which a part of the implanted impurity ions is activated to form the guard ring layer; A Schottky electrode in Schottky contact with the surface of the first conductivity type silicon carbide layer is provided. Wherein the periphery of and forming so as to overlap partially with the guard ring layer.

The invention according to claim 4 of the present application is the method for manufacturing a silicon carbide Schottky barrier diode according to claim 3, wherein the impurity ions are boron or aluminum, and the implantation amount is 5 × 10 ¹³ cm ^{−2 to} 5 ×. 10 ¹⁵ cm ⁻² , and the temperature range of the heat treatment is 900 ° C. to 1300 ° C.

  The SiC Schottky barrier diode formed by the manufacturing method of the present invention has a conventional high resistance guard ring structure and a guard ring layer having an intermediate characteristic between the P-type guard ring structure and the conventional high resistance guard ring structure. Compared to the resistance guard ring structure, the reverse leakage current is reduced, and the equivalent reverse leakage current characteristics are achieved despite the fact that the heat treatment is performed at a lower temperature than the conventional method requiring a high temperature treatment of about 1500 ° C. A SiC Schottky diode having the same can be obtained.

  Furthermore, the SiC Schottky barrier diode of the present invention has the advantage that the distance from the Schottky electrode end to the outer periphery of the guard ring need not be increased as in the conventional high resistance guard ring structure, and the semiconductor device can be miniaturized. .

  According to the manufacturing method of the present invention, it is sufficient that the processing is performed at a temperature lower by several hundred degrees or more than the conventional one. Therefore, it is only necessary to prepare a manufacturing apparatus used in a normal manufacturing process of a silicon semiconductor device. Can be manufactured at low cost.

It is sectional drawing of the SiC Schottky barrier diode of the Example of this invention. It is a figure which shows the reverse voltage-current characteristic of the SiC Schottky barrier diode using the guard ring layer of this invention, and the conventional high resistance guard ring layer. It is a figure which shows the reverse breakdown voltage of the SiC Schottky barrier diode using the guard ring structure of this invention and the conventional high resistance guard ring structure. It is a figure which shows the voltage-current characteristic of the guard ring layer of this invention. It is sectional drawing of the conventional SiC Schottky barrier diode.

  The guard ring layer of the SiC Schottky barrier diode of the present invention is an intermediate between a conventionally reported high resistance guard ring structure and a P type guard ring structure when ion-implanted P type impurities are sufficiently activated. Consists of layers that exhibit properties. That is, in the guard ring layer of the present invention, crystal defects caused by ion implantation are recovered (recrystallized) by heat treatment. Further, since the activation rate of the impurity ions implanted is about 1% or less, the impurity ion concentration is a very low region. Therefore, the guard ring layer according to the present invention has a low impurity ion concentration region formed by setting the implantation amount of impurity ions low and increasing the activation rate of the implanted impurity ions, that is, the ion implantation because the implantation amount is small. This is different from a region where activated ion species are present at a low concentration in a region where crystal defects are hardly generated by implantation.

  Further, in the method of manufacturing the SiC Schottky barrier diode of the present invention, after performing ion implantation for forming a normal P-type guard ring layer, crystal defects generated by this ion implantation are recrystallized. The impurity ions can be formed by heat treatment in a temperature range in which they are hardly activated. Examples of the present invention will be described below according to the manufacturing process.

FIG. 1 is a sectional view of a SiC Schottky barrier diode according to an embodiment of the present invention. An n-type SiC epitaxial layer 2 having an impurity concentration lower than that of the n ⁺ -type SiC substrate and an impurity concentration lower than that of the n-type SiC epitaxial layer 2 on the n ⁺ -type SiC substrate 1 made of n-type hexagonal SiC having a high impurity concentration An n ⁻ type SiC epitaxial layer 3 is formed. On n ⁻ type SiC epitaxial layer 3, Schottky electrode 5 that is in Schottky contact with the n ⁻ type SiC epitaxial layer is formed. The guard ring layer 4 of the present invention is formed so as to partially overlap the outer peripheral portion of the Schottky electrode 5. A pad electrode 7 is formed on the Schottky electrode 5, and a passivation layer 8 is formed so as to cover the exposed surface of the n ⁻ -type SiC epitaxial layer 3, the Schottky electrode 5, and the pad electrode 7. . The passivation layer 8 has an opening formed on the pad electrode 7 for wire bonding when the Schottky diode of the present invention is assembled into a package. An ohmic electrode 6 is formed on the back side of the n ⁺ type SiC substrate 1.

The guard ring layer 4 is selectively ion-implanted, for example, by patterning a mask such as a photoresist or a CVD oxide film into the n ⁻ -type SiC epitaxial layer 3 in accordance with a normal method for manufacturing a semiconductor device. When boron is used as an ion species to be implanted, ion implantation is performed with an implantation energy of 30 keV and an implantation amount of 1 × 10 ¹⁵ cm ⁻² at room temperature. Thereafter, in order to recover crystal defects caused by the ion implantation, heat treatment is performed at 1050 ° C. for about 90 minutes in an inert gas atmosphere such as nitrogen or argon. By this heat treatment, the portion where crystal defects are generated by ion implantation is recrystallized. On the other hand, in the heat treatment at about 1050 ° C., the activation rate of implanted ions is very low, generally said to be 1% or less, indicating a P-type conductivity type, but a region with a very low impurity concentration. Become.

The breakdown voltage of the SiC Schottky barrier diode having such a structure depends on the impurity concentration and thickness of the n ⁻ -type SiC epitaxial layer 3, and in the case of the structure shown in FIG. 1, the impurity concentration is from 5 × 10 ¹⁵ cm ^−3. By setting 2 × 10 ¹⁶ cm ⁻³ and the depth to be 5 μm or more, an operation with a withstand voltage of 1000 V or more is possible.

Further, when the impurity concentration was 5 × 10 ¹⁵ cm ⁻³ and the thickness was 10 μm, a reverse breakdown voltage of 1200 V or more could be obtained.

  FIG. 2 is a diagram comparing reverse current-voltage characteristics of the present invention and a conventional SiC Schottky barrier diode having a high resistance guard ring layer. As shown in FIG. 2, it can be seen that the leakage current in the reverse direction is smaller in the present invention. Such characteristics have been confirmed to be the same even when the ion species to be implanted is aluminum.

In order to form the guard ring layer of the present invention, the present invention is not limited to the above embodiment, and the following conditions may be set as appropriate within a range in which desired characteristics of the SiC Schottky barrier diode can be obtained. First, the ion species to be implanted is boron or aluminum, the implantation energy is set to 30 to 180 eV, and the implantation amount is set to the range of 5 × 10 ¹³ cm ^{−2 to} 5 × 10 ¹⁵ cm ⁻² . When the implantation energy is made constant and the implantation amount is increased, the degree of amorphization of the region into which the impurity ions are implanted becomes stronger, and the crystallinity recovery by the heat treatment tends to deteriorate. Therefore, in the present invention, the above implantation amount is set so that desired recrystallization occurs under the heat treatment conditions described below. The implantation energy is set to determine the implantation depth.

  Next, the heat treatment temperature range is set to a temperature range in which defects are generated in the crystal by ion implantation and then recrystallized, and the activation rate of the implanted impurity ions is low. The lower limit of the heat treatment temperature after ion implantation is 800 ° C., which is said to be recrystallized (for example, Materials Science Forum Vols. 389-393, 2002, p839-842 by T. Nakamura et al. Recrystallization is ensured by setting the temperature to 900 ° C., which is higher than that reported to be recrystallized at 800 ° C. or higher. Furthermore, the upper limit of the heat treatment temperature is 1500 ° C. (for example, J. Appl. Phys., Vol. 91, No. 7, 1 April 2002) where the activation rate at which the implanted impurity ions are activated as acceptors is about 1%. In year p.4242-4248, aluminum ions and boron ions are reported to be hardly activated at 1500 ° C.) By setting a lower 1300 ° C., most of the implanted impurity ions are activated as acceptors. Will not. Therefore, in this invention, the heat processing temperature is made into the temperature range of 900 degreeC-1300 degreeC.

  If the crystallinity is not sufficiently recovered, it becomes the same as the conventional high resistance guard ring structure, and the leakage current at the time of reverse bias increases, so the effect of the present invention cannot be obtained. As described above, according to the present invention, since the leakage current at the time of reverse bias is reduced, it can be seen that the guard ring structure of the present invention is different from the conventional high resistance guard ring structure.

  Next, it will be described that the guard ring layer of the present invention is different from the conventional P-type guard ring layer. First, the guard ring layer of the present invention and the conventional guard ring layer have different impurity concentrations. As described above, the conventional guard ring layer relaxes electric field concentration by depleting a P-type layer formed by ion implantation. Therefore, the P-type layer has a somewhat high impurity concentration. On the other hand, in the guard ring layer of the present invention, only a part of the implanted ions are activated. Therefore, it can be seen that the impurity concentration is low.

  By the way, when an ion implantation region having a low impurity concentration is formed, the ion implantation region can be formed by reducing the implantation amount and increasing the activation rate of the implanted impurity. When the ion implantation region is formed by increasing the activation rate in this way, the heat treatment for activation is naturally high. That is, it is necessary to perform a heat treatment at about 1500 ° C. As a result, in the ion implantation region, crystal defects generated by the ion implantation (since the amount of implantation is small, the crystal defects are small) are recovered and become an ion implantation region having a low impurity concentration. However, if a region with a low impurity concentration is used as a guard ring layer, the low-concentration P-type layer is not depleted, and a sufficient breakdown voltage cannot be obtained.

  In contrast, the guard ring layer of the present invention was able to obtain a sufficient withstand voltage while being a low concentration P-type layer. That is, it can be seen that the guard ring layer of the present invention has a completely different structure from the conventional P-type guard ring layer.

  Next, it will be described that the guard ring structure of the present invention is different from the conventional high resistance guard ring structure. As described above, in the high resistance guard ring structure, a minute current flows through the high resistance guard ring layer. Therefore, in order to ensure a sufficient breakdown voltage, it is necessary to set the dimension indicated by W3 in FIG. 5 longer. FIG. 3 is a diagram for explaining the relationship between the lateral dimension of the guard ring layer and the breakdown voltage. In the case of the conventional example, the dimension corresponding to W3 is the horizontal axis, and the dimension overlapping with the Schottky electrode is 20 μm. In the high resistance guard ring structure, excessive electric field concentration is prevented from occurring by passing a current through the high resistance guard ring layer. Therefore, a high breakdown voltage cannot be obtained unless the dimension of W3 is increased to some extent.

  On the other hand, it was confirmed that the guard ring structure of the present invention can obtain a sufficient breakdown voltage if the dimension of W2 shown in FIG. 1, that is, the dimension overlapping with the Schottky electrode is increased to some extent. FIG. 3 shows the relationship with the breakdown voltage with W1 = 20 μm and the dimension of W2 as the horizontal axis. As shown in FIG. 3, it was confirmed that with the guard ring structure of the present invention, a breakdown voltage of 800 V or more can be obtained when W2 = 5 μm.

  That is, it can be seen that if W1 + W2 = 25 μm, a sufficient breakdown voltage can be obtained. Further, it has been confirmed that the size can be reduced to about W1 = 10 μm, and the Schottky barrier diode can be downsized.

  In order to describe the guard ring layer of the present invention in more detail, the characteristics of the semiconductor region constituting the guard ring layer will be described below.

First, using the semiconductor substrate on which the SiC Schottky barrier diode described in FIG. 1 is formed, boron ions are implanted into the entire surface of the n ⁻ type SiC epitaxial layer 3 having an impurity concentration of 5 × 10 ¹⁵ cm ^{−3 at} an energy of 30 keV, Ion implantation is performed at an implantation amount of 1 × 10 ¹⁵ cm ⁻² and at room temperature. Thereafter, a heat treatment at 1050 ° C. is performed for activation. A titanium electrode was formed as a Schottky electrode on the surface of the ion implantation region thus formed, and the Schottky characteristics were examined. An ohmic electrode was formed on the back surface of the SiC substrate.

FIG. 4 shows voltage-current characteristics when the ohmic electrode 6 side is grounded and a positive bias and a negative bias are applied to the Schottky electrode side. The measurement temperature was 25 ° C, 100 ° C, and 175 ° C. As shown in FIG. 3, at the time of positive bias, when the measurement temperature is increased, the rising voltage of the current gradually decreases. Further, at the time of negative bias, when the measurement temperature is increased, the breakdown voltage is gradually increased. As shown in FIG. 4, it was confirmed that a PN junction was formed between the guard ring layer of the present invention and the n ⁻ -type SiC epitaxial layer 3.

  However, in the present invention, since the heat treatment temperature for activating the implanted impurity ions is set to be much lower than usual, the rate at which the implanted boron is activated as an acceptor is very low. In general, the rate at which boron is activated as an acceptor is less than 1% at 1500 ° C. Further, boron, which is a P-type impurity with respect to SiC, has a large ionization energy of about 0.3 eV. Therefore, the hole density at room temperature is further reduced.

1: n ⁺ type SiC substrate, 2: n type SiC epitaxial layer, 3: n ⁻ type SiC epitaxial layer, 4: guard ring layer, 5: Schottky electrode, 6: ohmic electrode, 7: pad electrode, 8: passivation Membrane, 9: Guard ring layer

Claims (4)

In a silicon carbide Schottky barrier diode comprising a Schottky electrode on the surface of a silicon carbide layer of the first conductivity type and a guard ring layer formed so as to partially overlap the periphery of the Schottky electrode,
The guard ring layer includes a region obtained by recrystallizing a region in which impurity ions of the second conductivity type are implanted into the silicon carbide layer of the first conductivity type, and a part of the implanted impurity ions. A silicon carbide Schottky barrier diode, wherein the region is activated and exhibits the second conductivity type. The silicon carbide Schottky barrier diode according to claim 1,
The guard ring layer is formed by recrystallizing a region in which impurity ions to be P-type are implanted into an N-type silicon carbide layer, and a part of the implanted impurity ions to be P-type is activated. A silicon carbide Schottky barrier diode, which is a region exhibiting a P-type conductivity. In a method for manufacturing a silicon carbide Schottky barrier diode comprising a Schottky electrode on the surface of a silicon carbide layer of the first conductivity type and a guard ring layer formed so as to partially overlap the periphery of the Schottky electrode ,
A step of ion-implanting impurity ions of the second conductivity type into the guard ring layer formation scheduled region on the surface of the silicon carbide layer of the first conductivity type under an implantation condition that causes crystal defects in at least the silicon carbide layer;
Recrystallizing the ion-implanted region, heat-treating in a temperature range that activates part of the implanted impurity ions, and forming the guard ring layer;
Forming a Schottky electrode in Schottky contact with the surface of the first conductivity type silicon carbide layer so that the periphery of the Schottky electrode partially overlaps the guard ring layer. A method for manufacturing a silicon carbide Schottky barrier diode. In the manufacturing method of the silicon carbide Schottky barrier diode according to claim 3,
The impurity ions are boron or aluminum;
The injection amount is 5 × 10 ¹³ cm ^{−2 to} 5 × 10 ¹⁵ cm ⁻² ,
The method of manufacturing a silicon carbide Schottky barrier diode, wherein a temperature range of the heat treatment is 900 ° C. to 1300 ° C.

Images