JP4645641B2 - Method for manufacturing SiC Schottky diode - Google Patents

Description

  The present invention relates to a method for manufacturing a SiC Schottky diode that uses a rectifying action by a barrier at an interface between silicon carbide (hereinafter referred to as SiC) and a metal.

For the purpose of controlling high frequency and high power, power semiconductor elements (hereinafter referred to as power devices) using silicon (hereinafter referred to as Si) have been improved in performance by various devices. However, the power device may be used in the presence of high temperature or radiation, and the Si power device may not be used under such conditions. In addition, application of new materials is being studied in response to the demand for higher performance power devices than Si power devices. SiC of the present invention has a wide forbidden band width (3.26 eV for the 4H type and 3.02 eV for the 6H type), and therefore has excellent controllability of electrical conductivity at high temperatures and radiation resistance, and is about 1 less than Si. Since it has a dielectric breakdown voltage that is an order of magnitude higher, it can be applied to high voltage devices. Furthermore, since SiC has an electron saturation drift velocity approximately twice that of Si, it is suitable for high-frequency and high-power control.
As one of the withstand voltage elements, there is a Schottky diode that utilizes a rectifying action by a barrier at the interface between a semiconductor and a metal.
Leakage current and reverse voltage drop (hereinafter referred to as ON voltage) when a reverse voltage is applied are important indicators characterizing a Schottky diode.

It is desirable that the leakage current when the reverse voltage is applied is sufficiently small until the applied voltage reaches the specified breakdown voltage of the diode. As a method for reducing the leakage current, there are selection of a Schottky electrode material, control of an interface between an electrode and a semiconductor, adoption of a withstand voltage structure, and the like. Control of barrier height is the key for electrode material selection, impurity contamination and reaction products are controlled for interface control, and electric field concentration is the key to reducing current by countermeasures based on the structure.
In order to reduce the forward ON voltage, which is listed as the second index, there is a method of using a metal having a small work function as a Schottky electrode. There have been reports of Schottky diodes using various metals. For example, D. Alok et al. Are Schottky diodes using titanium (hereinafter referred to as Ti), and have an on-voltage of about 1.5 V at 100 Acm ^-2. [D. Alok et al., “Effect of Surface Preparation and Thermal Anneal on Electrical Characteristics of 4H-SiC Schottky Barrier Diodes”, Materials Science Forum vols. 264-268, pp. 929-932, 1998]. They reduce the leakage current by a method of heat treating Ti / SiC. When heat treatment is performed, mutual diffusion occurs at the Ti / SiC interface, and it is expected that the adhesion is improved and the barrier height is changed by the reaction product. Another factor that affects the forward ON voltage is the contact resistance of the ohmic electrode.

The impurity concentration of a commercially available SiC substrate is at most 1.1 × 10 ¹⁹ cm ^{−3 for} the n-type and 1.9 × 10 ¹⁸ cm ⁻³ for the p-type. It is not possible to obtain an ohmic electrode having a Schottky property or having a small contact resistance only by forming a metal film.
When obtaining an ohmic electrode in a conventional SiC device, a method of forming a thin film of Ni, Ti or the like at the beginning of the process and performing a heat treatment at a high temperature of about 1000 ° C. [for example, Crofton, J., et al. , Porter, LM, and Williams, JR, Phys. Stat. Sol. Vol. (B) 202, No. 1, (1997) pp. 581-603, see]. However, if such a metal layer is formed at an early stage, the degree of freedom in subsequent processes is greatly impaired, such as limiting the surface treatment method immediately before forming the Schottky electrode.
Therefore, if a metal layer is formed in the final step like Si and an ohmic electrode can be obtained without heat treatment, it seems that it can contribute to simplification of device fabrication and improvement of characteristics.
As a means for realizing this, there is a method of increasing the impurity concentration of the ohmic electrode forming portion. In SiC, donor impurities are nitrogen, phosphorus, etc., which are group V elements of the periodic table. Several examples are known in which ohmic electrodes are formed on the (0001) Si surface by ion implantation of these elements. [For example, see Alok, Dev, Baliga, BJ and Malarty, PK, IEDM Technical Digest, (1993) pp. 691-694 etc.].

As another method for forming a high-concentration layer, a method using diffusion or liquid layer epitaxial growth has also been tried. However, in SiC, the impurity diffusion coefficient is small, and it is difficult to control the concentration only at an arbitrary place by the epitaxial growth method. Therefore, it is not suitable as a device process. In addition, there is an example in which the surface treatment before forming the metal film is devised and the ohmic electrode is formed without heat treatment, but it is still difficult to use in an actual process.
In view of such circumstances, an object of the present invention is to provide a SiC Schottky diode having good diode characteristics and a low forward ON voltage, and a method for manufacturing the same.

In order to solve the above problems, the SiC Schottky diode of the present invention is a SiC Schottky in which a Schottky electrode that generates a Schottky barrier on the (0001) Si surface of semiconductor SiC and an ohmic electrode provided on the (000-1) carbon surface. In the diode manufacturing method, it is assumed that the (0001) Si surface is subjected to hydrogen termination treatment before the Schottky electrode is formed, and the Schottky electrode is formed immediately after the heat treatment. In particular, the heat treatment conditions after the hydrogen termination treatment are preferably performed at a temperature of 700 to 800 ° C. under a reduced pressure of 1 × 10 ⁻⁴ Pa or less or in an inert gas atmosphere at normal pressure.
It has been found that when doing so, the reverse leakage current is small and good diode characteristics can be obtained. It is considered that a clean surface is obtained by the hydrogen termination treatment, and a Schottky electrode can be formed immediately after hydrogen is desorbed by the heat treatment.
As a method for forming the ohmic electrode, phosphorus ions are preferably implanted into the (000-1) C plane and activated by heat treatment at 1400 to 1600 ° C. to form a high concentration impurity layer.
By doing so, a layer having a higher impurity concentration than that of a normal epitaxial wafer substrate can be formed. When the temperature is lower than 1400 ° C., activation of the implanted phosphorus is insufficient, and when it is set to 1700 ° C., the surface starts to become rough.

Furthermore, if a high impurity concentration layer is formed on the (000-1) carbon surface with which the ohmic electrode contacts after the formation of the Schottky electrode , the contact resistance is reduced.
It is important that the impurity of the high impurity concentration layer is phosphorus and the average impurity concentration is in the range of 1 to 3 × 10 ²⁰ cm ⁻³ .
In SiC, the donor levels formed by phosphorus and nitrogen are approximately the same at 80 and 110 meV (6H-SiC), respectively [Troffer, T., Peppermuller, C., Pensl, G., Rottner, K. and Schoner , A., J. Appl. Phys. Vol. 80 (7), (1996) pp. 3739-3743]. As an element for forming the high concentration layer, phosphorus having an atomic radius close to that of Si is considered appropriate. If the concentration is less than 1 × 10 ²⁰ cm ⁻³ , the effect of reducing the contact resistance is small. On the other hand, even if the concentration exceeds 3 × 10 ²⁰ cm ⁻³ , the activation rate decreases and only the ineffective portion increases.

It is assumed that SiC is n-type and at least one of the Schottky electrode and the ohmic electrode has a three-layer structure of Ti / Ni / Au from the SiC side.
In general, when a semiconductor device is manufactured, it is necessary to connect a contact metal (Ti in the example of D. Alok described above) and a wiring metal such as gold (hereinafter referred to as Au) with good adhesion. For this reason, a metal thin film having a layer structure of contact metal / contact metal / wiring metal is used for the Schottky electrode or the ohmic electrode. In the Si device, there is an example in which nickel (hereinafter referred to as Ni) is used to connect Ti and Au, and the inventors' group applied this metal configuration to the SiC Schottky diode.
The Schottky barrier of Ti is smaller than that of conventionally used Ni and can reduce the on-voltage at the same current density. However, it is not suitable as a wiring electrode for electrical connection with the outside, and it is preferable to provide a wiring electrode with Ni in between.
Then, after laminating Ti, Ni, and Au on SiC, heat treatment was carried out under the same conditions as reported by D. Alok et al., And it was found that the leakage current was reduced compared to the case without heating.
In particular, when the thickness of the Ti layer is in the range of 500 to 1000 nm, the diffusion of Ni to the interface is suppressed and a uniform Schottky barrier is formed .

According to the present invention, at least one of the Schottky electrode and the ohmic electrode has a three-layer structure of Ti / Ni / Au from the SiC side, thereby reducing the on-voltage when the SiC Schottky diode is energized. It was shown that there was an appropriate value for the Ti film thickness.
Further, the on-voltage can be further reduced by forming a high impurity concentration layer by ion implantation of phosphorus or the like and providing a metal electrode. As a manufacturing method, the leakage current is particularly improved by forming a Schottky electrode immediately after the heat treatment at a temperature of 700 to 800 ° C. after the surface is hydrogen-terminated. Therefore, the present invention is an extremely effective invention for reducing the loss of a low breakdown voltage diode, and contributes to the spread and development of Schottky diodes.

Embodiments of the present invention will be described below with reference to the drawings.
[Example 1]
FIG. 2 is a cross-sectional view of the Schottky diode according to the first embodiment. The Schottky diode includes a SiC substrate 1, a Schottky electrode 2, and an ohmic electrode 3.
The production method and evaluation method of the Schottky diode of Example 1 are described below.
As the SiC substrate 1, an epitaxial wafer was used in which an n epitaxial layer 12 was grown on a 4H type SiC single crystal n ⁺ substrate 11 containing a high concentration of n type impurities. The thicknesses of the n ⁺ substrate 11 and the n epitaxial layer 12 are 300 μm and 10 μm, respectively, and the impurity concentrations are 1.1 × 10 ¹⁹ cm ⁻³ and 1 × 10 ¹⁶ cm ⁻³ , respectively. In the epitaxial wafer of this example, the n epitaxial layer 12 is grown on a plane inclined by 8 degrees in the <11-20> direction from the (0001) Si plane.
First, as a pretreatment of the SiC substrate 1 cut into 6 mm squares by a dicer, organic substances were removed with an organic solvent and an acid, and an incomplete surface layer was removed by thermal oxidation and hydrofluoric acid immersion. Thermal oxidation was performed at 1100 ° C. for 5 hours by a pyrogenic method. At this time, the oxide film thickness formed on the surface of the epitaxial layer is about 50 nm.

Next, the surface of this SiC substrate 1 is subjected to hydrogen termination treatment. The SiC substrate 1 was put in a horizontal reactor for CVD (chemical vapor deposition) and heated in a hydrogen atmosphere. Specifically, it was as follows. This is because hydrogen is bonded to dangling bonds on the SiC surface to prevent adhesion of other elements such as oxygen.
The SiC substrate 1 is placed on a carbon susceptor with the (0001) Si surface facing up, and is inserted into a quartz reaction tube with a water cooling jacket. The reaction tube is once evacuated to 1 × 10 ⁻⁴ Pa, and then high purity hydrogen is allowed to flow for 3 Lmin ⁻¹ . The SiC substrate 1 was heated by induction induction heating of the carbon susceptor. Heating was performed at 1000 ° C. for 30 minutes, and both the temperature increase and decrease rates were 100 ° C. min ⁻¹ . This treatment method has been reported by Tsuchida et al. [H. Tsuchida et al., "FTIR-ATR Analysis of SiC (0001) and SiC (000-1) Surfaces", Materials Science Forum vols. 264-268, pp. 351-354, 1998], the same method was adopted in this example.
Subsequently, the Schottky electrode 2 is formed on the (0001) Si surface of the SiC substrate 1 subjected to the above hydrogen termination treatment. First, prior to electrode formation, the SiC substrate 1 was heated in a vacuum in a sample heating chamber of a sputtering apparatus to desorb hydrogen atoms that terminated the surface. The degree of vacuum at the initial stage of heating was 1 × 10 ⁻⁴ Pa and the heating temperature was 600 to 900 ° C. The processing time is 30 minutes.

After that, the SiC substrate 1 was transported in a vacuum to a sputtering chamber, and Ni was deposited as the Schottky electrode 2. The thickness was 200 nm. After forming the thin film, the electrode was patterned by photolithography. Its size is 200 μm in diameter. Subsequently, in order to form the ohmic electrode 3, Ni was deposited in a thickness of 200 nm on the (000-1) C surface in the same chamber as described above.
Finally, in order to improve the adhesion between both electrodes and SiC, the SiC substrate 1 was heated in vacuum (1 × 10 ⁻⁴ Pa). The conditions are 200 ° C. and 5 minutes.
In order to investigate the effect of heating in the sputtering apparatus after hydrogen termination for the Schottky diode manufactured in this way, the current-voltage (IV) characteristic evaluation and the interface between the Schottky electrode 2 and the n epitaxial layer 12 were performed. Observation of the included cross section was performed.
FIG. 1 is a characteristic diagram showing the leakage current in the IV characteristic and the heat treatment temperature dependence of the n value described later. The horizontal axis represents the heat treatment temperature before Schottky electrode formation, and the vertical axis represents the n value calculated from the leakage current and forward characteristics when 500 V is applied in the reverse direction. The lines with the top and bottom stopped in the figure indicate the maximum and minimum values of the measured values.
The n value is one index indicating the degree of quality of the diode characteristics, and is obtained from the forward IV characteristics. Specifically, it is included in the following equation that expresses the relationship between the applied voltage V and the current density J normalized for the forward current I.

J = J _S [exp (qV / nkT) -1] (1)
When the diode characteristics are ideal, n = 1, and as the deviation from 1 increases (generally n> 1), the characteristics are considered to be worse. In the equation (1), J _s , q, k, and T are a saturation current density, an elementary charge amount, a Boltzmann constant, and an absolute temperature, respectively.
From FIG. 1, it can be seen that when the heat treatment temperature is 700 ° C. or higher, both the leakage current and the n value are less varied. Further, the n value is almost 1 at 700 and 800 ° C., and the leakage current is as small as μA order, which shows that good diode characteristics are obtained.
As a cross-sectional evaluation, TEM (transmission electron microscope) observation was performed, and it was found that the orientation of Ni with respect to the SiC substrate was improved in the sample heat-treated at 800 ° C.
Separately, the hydrogen-terminated SiC substrate was heated in the analyzer, and the hydrogen desorption state was measured. As a result, it was found that most of hydrogen was desorbed at 700 to 800 ° C.
Summing up the above results, it can be seen that ideal diode characteristics can be obtained by forming a Schottky electrode after hydrogen termination of the SiC surface and heating to 700 to 800 ° C. in vacuum. This is because the surface of SiC is flattened at the atomic level by hydrogen termination treatment, and becomes inactive against contamination of adhering atoms such as oxygen, and hydrogen is desorbed immediately before electrode formation, so that a steep metal-semiconductor interface is obtained. This is thought to be due to the fact that

As a method of controlling the interface, SiC is heated in hydrogen gas, and the method of terminating dangling bonds on the surface with hydrogen is known [Hara et al., "Metal / 6H-SiC interface formation by hydrogen gas treatment". , Proceedings of the 58th Japan Society of Applied Physics, No.1, pp.300, 1997]. However, there is no report of improving the characteristics of the Schottky diode using hydrogen gas treatment.
[Example 2]
FIG. 3 is a sectional view of a Schottky diode according to the second embodiment of the present invention.
The difference between this SBD and the Schottky diode of the first embodiment is that the Schottky electrode 4 is composed of a barrier metal 41, an adhesion metal 42, and a wiring metal 43.
Next, a method for manufacturing this diode will be described.
The processing up to the used SiC substrate, thermal oxidation, surface incomplete layer removal by hydrofluoric acid cleaning, hydrogen termination treatment, and subsequent heat treatment is the same as in the first embodiment. The heat treatment was performed at 700 ° C.
After the treatment, Schottky electrode 4 was formed on n epitaxial layer 12. Ti was used for the barrier metal 41, and its thickness was set to 300 to 600 nm. Subsequently, Ni was deposited to 300 nm as the adhesion metal 42 and Au was deposited to 2000 nm as the wiring metal 43. All film forming methods are sputtering methods.

After forming the Schottky electrode 4, patterning was performed by a photolithography method. The electrode diameter is 200 μm.
Next, ohmic electrode 3 was formed on the surface of SiC substrate 1 facing the Schottky electrode. For this, Ni having a thickness of 200 nm by sputtering was used.
Finally, the barrier metal 41 and the n epitaxial layer 12 were heated at 200 ° C. for 5 minutes under a reduced pressure of 1 × 10 ⁻⁴ Pa to cause a slight reaction.
In order to investigate the influence of the thickness of the Ti barrier metal 41, IV characteristic evaluation of the Schottky diode and elemental analysis in the thickness direction in the vicinity of the interface between the Schottky electrode 4 and the SiC substrate are performed by Auger electron spectroscopy (AES). It was done by law.
FIG. 4 is a characteristic diagram showing the leakage current in the IV characteristic and the dependence of the n value on the Ti film thickness. The horizontal axis represents the thickness of Ti as the barrier metal 41, and the vertical axis represents the leakage current and the n value when a reverse voltage of 500 V is applied.
When the thickness of Ti is 400 nm or less, not only the leakage current and the n value are large, but also the variation is large and good diode characteristics are not obtained. On the other hand, at 500 and 600 nm, the leakage current is small and the n value is almost 1, and the variation of both the leakage current and the n value is small, and the diode characteristics are improved.

5 and 6 are element profiles obtained by Auger electron spectroscopy (AES method). The horizontal axis is the depth from the surface of the Schottky electrode. FIG. 5 shows a case where the thickness of Ti is 300 nm, and FIG. 6 shows a case where the thickness is 500 nm.
In the analysis result of FIG. 5, Ni has diffused in the vicinity of the interface between the barrier metal 41 and the n epitaxial layer 12 in addition to Si, C, and Ti. On the other hand, in FIG. 6, Si, C, and Ti are near the Ti / SiC interface, and Ni has not diffused.
From the above results, it can be said that the variation in the diode characteristics seen in FIG. 4 is due to the diffusion of Ni to the interface between the barrier metal 41 and the n epitaxial layer 12. It can be seen that a good Ti / SiC interface can be obtained by setting the thickness of Ti to 500 nm or more.
Example 3
First, a preliminary experiment investigating contact resistance in the case where an ohmic electrode is formed by ion implantation of phosphorus into the (000-1) C face of the SiC substrate will be described. The preparation method of the sample for the preliminary experiment is as follows.
[Preliminary experiment 1]
As the SiC substrate, an n-type 4H-type SiC single crystal wafer was used. The impurity concentration of the wafer is 8 × 10 ¹⁸ cm ⁻³ and the thickness is 300 μm. This wafer was cut into 6 mm square SiC substrates by a dicer. As a pretreatment of the substrate, organic substances were removed with an organic solvent and an acid, and an incomplete surface layer was removed by thermal oxidation and hydrofluoric acid immersion. The wafer in this preliminary experiment is mirror-polished by tilting it by 8 degrees in the <11-20> direction from the (0001) Si surface.

Phosphorus ions were implanted into the (000-1) C face and (0001) Si face of this SiC substrate. The implantation conditions are multi-stage implantation with an acceleration voltage of 35 to 150 keV and a total dose of 3 × 10 ¹⁵ cm ⁻² . In order to activate the implanted ions, the substrate was heated at 1300-1700 ° C. for 30 minutes in an atmospheric pressure Ar atmosphere.
A Ti film was formed on the ion-implanted surface by sputtering, and a pattern for 4-terminal measurement was formed by photolithography. The diameter of each electrode was 200 μm, and the electrode interval was 800 μm.
FIG. 7 is a characteristic diagram showing changes in contact resistance due to activation heat treatment after ion implantation. Note that no heat treatment was performed after the metal film was formed.
When (000-1) ions were implanted into the C plane (marked with a circle), ohmic properties were exhibited by heat treatment at 1300 ° C., and a stable value of about 0.35 mΩcm ² was exhibited at a heat treatment temperature of 1500 ° C. or higher.
For example, if the contact resistance of 1 m [Omega] cm ^2, since the increase of 0.1V on-voltage 100Acm ^-2, the ON voltage during energization of large current greatly affected. When the temperature was less than 1400 ° C., activation of the implanted phosphorus was insufficient, and when activated at 1700 ° C., surface roughness of the substrate occurred. Therefore, 1400 to 1600 ° C. is appropriate as the heat treatment temperature for the implanted ions.

For comparison, the result of implantation into the (0001) Si surface is also shown (◇ mark). Even on the (0001) Si surface, at a heat treatment temperature of 1500 ° C. or higher, the same stable value as that of the (000-1) C surface was exhibited. When activated at 1700 ° C., surface roughness of the substrate occurred. This result is similar to the tendency reported in the following literature, although it cannot be simply compared because the sample preparation method and measurement method are different.
[Preliminary experiment 2]
Next, in order to investigate the relationship between the thickness of the contact metal Ti and the contact resistance, evaluation by a four-terminal method was performed.
This sample preparation method will be described. As the SiC substrate, the same 4H type SiC wafer as in the preliminary experiment 1 was used. The wafer was cut into 6 mm squares with a dicer. The steps from the removal of the organic substance by the organic solvent and acid of the substrate and the removal of the surface imperfect layer by thermal oxidation and hydrofluoric acid immersion to the heat treatment are the same as those in Example 1.
After this treatment, phosphorus ions were implanted into the (000-1) C face and the (0001) Si face of this SiC substrate. The implantation conditions are multi-stage implantation with an acceleration voltage of 35 to 150 keV and a total dose of 3 × 10 ¹⁵ cm ⁻² . The heat treatment was performed at 1500 ° C.
It was confirmed by SIMS analysis that the concentration was 2 × 10 ²⁰ cm ⁻³ and stepwise distributed to the depth of 0.2 μm. The phosphorus ions implanted by this heat treatment are about 25% activated.

After the activation heat treatment, the substrate was again thermally oxidized and washed with hydrofluoric acid for the purpose of removing the defective layer on the surface. The sample injected into the (000-1) C surface was heated at 1100 ° C. for 30 minutes in a wet atmosphere, and the sample injected into the (0001) Si surface was heated for 5 hours to form an oxide film. Under this oxidation condition, an oxide film of about 50 nm is formed in all cases.
The oxide film is removed with hydrofluoric acid, and an ohmic electrode is formed on the surface of the injection layer. Ti was used for the contact metal, and its thickness was set to 300 to 600 nm. The film forming method is a sputtering method. Subsequently, a Ni film of 300 nm was formed as an adhesion metal, and an Au film of 2000 nm was formed as a wiring metal. After these metal thin films were formed, the electrodes were patterned by photolithography. The electrode diameter was 200 μm, and the electrodes were arranged in a lattice pattern at intervals of 800 μm. Finally, the contact metal and the injection layer were heated at 200 ° C. for 5 minutes under a reduced pressure of 1 × 10 ⁻⁴ Pa in order to slightly react.
FIG. 8 is a characteristic diagram showing the relationship between the thickness of Ti used as the contact metal and the contact resistance. The horizontal axis is the thickness of Ti, and the vertical axis is the contact resistance. Moreover, the line | wire which stopped the upper and lower sides in a figure shows the maximum and minimum value of a measured value.
From this figure, it can be seen that when the film thickness of Ti is 500, 600 nm, the average value of the contact resistance is reduced by about an order of magnitude and the variation is also reduced. The contact resistance reduction of several mΩcm ² is 0. This means a reduction in on-voltage of several volts.

This result is considered to reflect the mutual diffusion of Ti of the contact metal 41 and Ni of the adhesion metal 42 described in the second embodiment. That is, when the film thickness of Ti is 400 nm or less, Ni reaches the Ti / SiC interface by heat treatment and increases the contact resistance. When the Ti film thickness is 500 nm or more, since Ni does not diffuse to the interface, a steep interface of Ti / SiC is realized, and the average value and variation of the contact resistance are reduced.
[Example]
FIG. 9 is a sectional view of a SiC Schottky diode according to a third embodiment of the present invention. The diode of this embodiment is provided with a Schottky electrode 4 composed of a SiC substrate 1, a Ti contact metal 41, a Ni adhesion metal 42, and an Au wiring metal 43, and phosphorus ions are formed on the back surface of the SiC base plate 11. An ohmic electrode 6 comprising a Ti contact metal 61, a Ni adhesion metal 62, and an Au wiring metal 63 is provided in contact with the surface thereof. Hereinafter, a method for manufacturing the SiC Schottky diode of Example 3 will be described.
As the SiC substrate 1, the same 4H type SiC single crystal epiwafer as in Example 1 was used. As a pretreatment of the substrate, organic substances were removed with an organic solvent and an acid, and an incomplete surface layer was removed by thermal oxidation and hydrofluoric acid immersion.

Phosphorus ions were implanted into the back surface of the SiC base plate 11. The implantation conditions and annealing are the same as in preliminary experiment 2.
After hydrofluoric acid treatment and RCA cleaning, oxidation was performed to remove the surface layer of the substrate. An oxide film was formed by heating at 1100 ° C. for 30 minutes in a wet atmosphere. The oxide film generated by this oxidation is about 50 nm on the (000-1) C plane and about 15 nm on the (0001) Si plane.
The oxide film was removed with hydrofluoric acid, and the Schottky electrode 4 was formed on the (0001) Si surface, that is, the epitaxial layer surface. The contact metal 41 is made of Ti and has a thickness of 500 nm. Subsequently, Ni of 200 nm was formed as the adhesion metal 42, and 2000 nm of Au was formed as the wiring metal 43. The film forming method is a sputtering method. After these metal thin films were formed, the electrodes were patterned by photolithography. The electrode diameter is 200 μm.
Thereafter, Ti, Ni, and Au were formed on the surface of the implantation layer 5 formed by phosphorous ion implantation so as to have thicknesses of 500, 200, and 150 nm, respectively, to form a three-layer ohmic electrode 6.
As a comparative example, an SiC Schottky diode manufactured by a conventional general manufacturing method was also manufactured. The SiC substrate cut out from the same epitaxial wafer was used. First, Ni was deposited on the (000-1) C surface to a thickness of 500 nm, and this was heated at 1000 ° C. for 10 minutes in an atmospheric pressure Ar atmosphere to form an ohmic electrode. . Thereafter, the substrate 1 was washed with hydrofluoric acid, a Ti thin film was similarly formed on the (0001) Si surface, and patterned to form a Schottky electrode. In this case, since the ohmic electrode is formed on the (000-1) C surface before patterning the Schottky electrode, there is no means for protecting the ohmic electrode from the etching solution when patterning the Schottky electrode. I have to take it.

In order to compare the two types of diodes thus produced, their current-voltage (IV) measurements were performed. FIG. 10 is a comparison diagram of current-voltage characteristics. The vertical axis represents the leakage current when a reverse voltage of 500 V is applied and the forward ON voltage at a current density of 100 Acm ⁻² . The lines with the top and bottom stopped in the figure indicate the maximum and minimum values of the measured values.
From this figure, it can be seen that the average value of the reverse leakage current is smaller by one digit and the variation is smaller in the diode of the third embodiment than in the Schottky diode of the comparative example. In the on-state voltage, the average value decreases from 1.35V to 1.30V by about 0.05V, and the variation is also small.
An example in which an ohmic electrode is formed by ion implantation of elements such as nitrogen and phosphorus is the one in which Dev Alok et al. Implanted nitrogen into the ohmic electrode formation part of a Schottky diode [Alok, Dev, Baliga, BJ and Malarty , PK, IEDM Technical Digest, (1993) pp.691-694] and Khemka, V. et al. Have produced pn diodes by implanting phosphorus ions into the Si surface of the p-type epitaxial layer [Khemka, V Patel, R., Ramungul, N., Chow, TP, Ghezzo, M. and Kretchmer, J., J. Electronic Materials, vol. 28 (3), (1999) pp. 167-174. In the former, ion implantation is performed so that the nitrogen concentration in the vicinity of the surface is 1 × 10 ²⁰ cm ⁻³ , heat treatment is performed at 1250 ° C., and activation is performed to obtain a contact resistance value of 2 × 10 ⁻⁵ Ωcm ⁻² . In the latter case, a contact resistance value of about 4 × 10 ⁻⁵ Ωcm ⁻² and a sheet resistance value of 160Ω / □ are obtained, but both are applied only to the (0001) Si surface, and (000-1) C Application to the surface has not been reported.

Example 4
After sacrificial oxidation at 1100 ° C. for 30 minutes in Example 3, a hydrogen termination treatment was further performed. The method was the same as in Example 1, and heating was performed at 1000 ° C. for 30 minutes. Further, a heat treatment was performed at 700 ° C. for 30 minutes under a vacuum of an initial vacuum of 1 × 10 ⁻⁴ Pa in a sample heating chamber of the sputtering apparatus to desorb hydrogen atoms terminating the surface.
After that, the SiC substrate was transported in a vacuum chamber to the sputtering chamber, and a high concentration layer surface in which a Ti / Ni / Au three-layer Schottky electrode was implanted on the (0001) Si surface and phosphorus ions were implanted in the same manner as in Example 3. In addition, an ohmic electrode having three layers of Ti / Ni / Au was formed. The thicknesses of Ti, Ni, and Au were 500, 200, 2000 nm, 500, 200, and 150 nm, respectively. The size of the patterned Schottky electrode was 200 μm in diameter, and 20 Schottky electrodes were formed per sample. Finally, the chip was heated in vacuum (1 × 10 ⁻⁴ Pa) in order to increase the adhesion between both electrodes and SiC. The conditions are 200 ° C. and 5 minutes.
The current-voltage characteristics of the Schottky diode of Example 4 produced in this way were measured. The results are also shown in FIG.

In the Schottky diode of the fourth embodiment, the reverse leakage current is two orders of magnitude smaller and the variation is smaller than that of the third embodiment as well as the comparative example. The on-voltage is substantially the same as in the third embodiment. As explained in the section of Example 1, this is because the SiC surface is flattened at the atomic level by hydrogen termination treatment, becomes inactive against contamination of attached atoms such as oxygen, and the hydrogen is removed immediately before the electrode formation. It is considered that a steep metal-semiconductor interface could be realized by separating them.
It was found that a SiC Schottky diode having excellent characteristics can be produced by such a simple method. In the above embodiment, an example in which the Schottky electrode is formed on the (0001) Si surface of 4H—SiC has been described. However, the method of the present invention can be applied to the (000-1) C surface of 4H—SiC or ( It was confirmed that the present invention can also be applied to the (0001) Si, (000-1) C plane.

Characteristic diagram showing leakage current of Schottky diode of Example 1 and dependence of n value on heat treatment temperature Sectional drawing of the Schottky diode of Example 1 Sectional drawing of the Schottky diode of Example 2 Characteristic diagram showing relationship between thickness of Ti film, leakage current, and n value of Schottky diode of Example 2 Element distribution diagram around Schottky electrode of Schottky diode of Example 2 (when contact metal thickness is 300 nm) Element distribution diagram around the Schottky electrode of the Schottky diode of Example 2 (when the contact metal thickness is 500 nm) The figure which shows the contact resistance of the ohmic electrode which is formed using phosphorus ion implantation Characteristic diagram showing the relationship between thickness of Ti film and contact resistance Sectional drawing of the Schottky diode of Example 3 Comparative chart of characteristics of Schottky diodes of Examples 3 and 4 and comparative examples

DESCRIPTION OF SYMBOLS 1 SiC substrate 11 n-type SiC base plate 12 n epitaxial layer 2 Schottky electrode 3 Ohmic electrode 4 Schottky electrode 41 Contact metal 42 Metal for adhesion 43 Metal for wiring 5 High concentration layer 6 Ohmic electrode 61 Contact metal 62 Metal for adhesion 63 Metal for wiring

Claims (3)

And shots hotkey electrodes in (0001) Si face of a semiconductor SiC produce a Schottky barrier, in the manufacturing method of (000-1) SiC Schottky diode ohmic electrodes are provided on carbon surface to form a Schottky electrode The (0001) Si surface was previously hydrogen-terminated, and the heat treatment conditions after the hydrogen-termination were as follows: the temperature was 700 to 800 ° C., and the pressure was 1 × 10 ⁻⁴ Pa or less or in an inert gas atmosphere at normal pressure. A method for producing a SiC Schottky diode, characterized in that a Schottky electrode is formed immediately after heat treatment to be performed . After forming the Schottky electrode, (000-1) Phosphorus ions are implanted into the carbon surface and activated by heat treatment at 1400 to 1600 ° C. to form a high concentration impurity layer, and an ohmic electrode is formed on the surface of the high concentration impurity layer. The method of manufacturing a SiC Schottky diode according to claim 1 . 2. The SiC Schottky diode according to claim 1, wherein the semiconductor SiC is n-type, and at least one of the Schottky electrode and the ohmic electrode has a three-layer structure of titanium / nickel / gold from the SiC side. Production method.

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