JP3315541B2 - Method of forming electrode on SiC - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon carbide (Si) having an advantage that it can operate even in a high temperature environment.
C) The present invention relates to a method for forming an electrode used for manufacturing a semiconductor device.

[0002]

2. Description of the Related Art The latest information on silicon carbide (SiC) crystals, which are known as semiconductor materials, is described in "S.
This paper is described in a paper by Hiroyuki Matsunami entitled "iC Semiconductor Materials, Devices and Contact Materials"("Research Report on Ultrafine Structured Electrode Materials" edited and published in March 1994 by the Japan Electronic Industry Development Association) . According to this paper, silicon carbide crystals are now commonly used as materials for integrated circuits and have about three times the thermal conductivity (5 W / cm ^o K) and about twice that of silicon single crystal (Si). Have a saturated electron drift velocity. In addition, hexagonal 6H-SiC, which is usually used as a highly stable polytype,
The bandgap reaches a large value of 2.93 eV. Therefore, the dielectric breakdown voltage is about 10 times higher than that of Si, the operable temperature reaches 773 ^° K (500 ^° C.), and the conductivity type can be easily changed to either p-type or n-type. Can be controlled.

[0003] SiC has the advantages of various physical properties as described above, and is therefore expected as a material for semiconductor devices that are to be operated under severe environments, such as high-temperature operating devices, high-power devices, and radiation-resistant devices. It has been. Actually, since SiC is rather difficult to grow a large area and high quality crystal due to thermal and chemical stability, semiconductor devices made of SiC except for special devices such as varistors are used. Hardly ever used. However, with the remarkable progress of crystal growth technology in recent years, large wafers as large as 30 mm in diameter have become commercially available.

In realizing a semiconductor device using silicon carbide as a material, it is important to supply a crystal having a large area and to determine whether a stable ohmic electrode or Schottky electrode can be formed on such a crystal. On the point. Since silicon carbide has a wide band gap and chemical stability, it is necessary to alloy at a relatively high temperature to form an ohmic junction (contact) therewith. As a material for forming an alloy ohmic contact to a silicon carbide crystal, Ni, Ti, Mo, C
r, W, AuTa, TaSi _{2 and the} like, and for p-type crystals, Al, AlSi, AL / Ti, Al / TaS
such as i ₂ are known. The alloy formation temperature is the lowest Al
Over a fairly high temperature range, from 1173 ^° K for the highest W to 2073 ^° K for the highest W.

Further, Au, Pt, Ti, W, Pd and the like are used as materials for forming a Schottky junction with a silicon carbide crystal for an n-type crystal, and Au and Pt are used for a p-type crystal. Etc. are known. These metals can be used as deposited on silicon carbide crystals by a film forming technique such as sputtering, or after deposition from 873 ^o K, as in the case of Pt or Ti.
They are used after forming an alloy in a temperature range of about 773 ^o K.

According to the above-mentioned paper, when Ni is used as a metal, carbon of a graphite phase segregates at an interface between a metal layer and SiC due to heat treatment at a high temperature, and a silicidation (silicidation) reaction occurs. As a result, it is pointed out that nickel silicide (NiSi) is formed in the Ni layer.
According to the experimental data published in the same paper, it is also shown that nickel carbide (NiC) is formed in the Ni layer as a result of the carbonization reaction.

As described above, when a metal layer is formed on silicon carbide and subjected to a high-temperature heat treatment, both silicon and carbon, which are SiC compositions existing near the interface with the metal layer, are mixed with the metal layer. And metal silicides and carbides are formed. Carbides of metals, has a so-called interstitial structure that carbon atoms penetrates in closest-packed in a metal crystal, a carbide of formula MC and MC ₂ in accordance with the atomic radius of the metal (M). This metal carbide is chemically stable due to its high binding energy, has a high melting point and high hardness, and has electrical conductivity.It is suitable for silicon carbide intended for operation in a high temperature environment. It is very convenient as an electrode material.

[0008]

However, the conventional method for forming an electrode has the following problems. First, as in the case of Ni described in the above-mentioned article, since silicide is generated in a larger amount than carbide depending on the metal, the advantage as the above-described electrode as compared with the case where carbide is mainly generated is obtained. There is a problem that is reduced.

Next, since silicon and carbon atoms, which are compositions of SiC, penetrate into the metal layer at different ratios, a one-to-one ratio of Si and C near the interface between the metal layer and the SiC crystal. The element ratio collapses, and carbon segregates at the interface with the metal layer as in the case of Ni. Alternatively, for other metals, it is conceivable that Si segregates at the interface with the metal layer. In other words, it means that the interface with the metal layer that has the most significant effect on the electrical characteristics is in an abnormal state that can no longer be regarded as a crystal of SiC.

Further, although neither described nor suggested in the above-mentioned paper, various impurity elements mixed in a single crystal of SiC to provide controlled conductivity such as n-type and p-type are disclosed. It is also conceivable that they may penetrate into the metal electrode layer by itself or in the form of carbide or silicide. As a result, a high resistance layer deficient in an impurity element is formed near the interface with the electrode, and there is a concern that the performance as an electrode may be significantly reduced. Accordingly, an object of the present invention is to solve the above-mentioned various problems and to form a S-bond which can form a good and highly reliable joint.
An object of the present invention is to provide a method for forming an electrode on iC.

[0011]

According to the present invention, there is provided a method of forming an electrode on SiC, comprising the steps of forming a metal electrode layer on the surface of a silicon carbide crystal substrate; A carbonization step of converting into a metal carbide electrode layer by supplying carbon from the outside,
A heating step of heating the silicon carbide crystal substrate on which the metal carbide electrode layer is formed.

[0012]

The metal forming the electrode layer on the silicon carbide crystal substrate is carbonized to become a carbide suitable as an electrode material of SiC. This carbonization differs from the prior art in that SiC
Rather than by the intrusion of carbon from the crystals of
This is performed by supplying carbon to the electrode layer from outside the SiC crystal substrate using a CVD method or the like. This metal carbide has a so-called interstitial structure in which carbon atoms are inserted in the metal crystal at the closest density. Therefore, if the inside of the electrode layer is saturated with carbon by sufficiently performing this carbonization, even if a high-temperature heat treatment is performed thereafter for the purpose of alloying or the like, both carbon and silicon are no longer impurities from the SiC crystal. Also cannot enter the electrode layer.

As a result, SiC in the vicinity of the interface between the metal carbide and the electrode layer is kept in a state before the heat treatment, including impurities contained therein, and an extremely good and highly reliable Schottky junction or ohmic junction is formed. You. Since the electrode itself is formed of a metal carbide, it is chemically stable and suitable for operation in a high-temperature environment in which both the melting point and the hardness are high. Hereinafter, the present invention will be described in more detail with reference to Examples.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for forming an electrode on SiC according to an embodiment of the present invention will be described with reference to the sectional view of FIG. First, as shown in FIG.
A mask m is formed on the electrode forming region 11 on the surface of the substrate by using a known photolithography method (photolithography). By implanting oxygen ions into the entire surface in this state, a high-resistance layer 12 into which oxygen ions have been implanted is formed on the surface of the substrate 10 other than the electrode formation region 11.

Next, as shown in FIG. 1B, aluminum nitride (Al) is formed on the surface of the substrate 10 excluding the electrode formation region 11 by a combination of a conventional film forming technique and photolithography.
N) The insulating layer 13 is formed. Next, as shown in FIG. 1C, after forming a resist layer R on the insulating layer 13,
The metal layer M mainly composed of a high melting point metal is formed by using an appropriate film forming technique such as vapor deposition, sputter rig, CVD, and compound reduction.
Is deposited.

Subsequently, as shown in FIG. 1C, carbon is supplied onto the metal layer M by using a microwave plasma CVD method. This microwave plasma CVD is performed under the following conditions. Reaction gas: CH ₄ + H ₂ , reaction gas mixture ratio: 1.0 vol
%, Reaction pressure: 40 Torr, gas flow rate: 100 ccm, microwave power: 380 W, substrate temperature: 850 ^oC , supply time:
2 hours

The carbon supplied to the surface of the metal layer M penetrates from the surface of the metal layer M to form a so-called interstitial structure in which carbon atoms enter the metal crystal at the closest density. When the carbonization of the metal is sufficiently performed and the inside of the metal layer M is saturated with carbon, a diamond layer C is deposited on the surface of the metal carbide layer MC.

Next, a diamond layer C is formed by applying an ashing technique using oxygen plasma or oxygen ion beam.
Is removed, the resist layer R, the metal carbide layer MC and the diamond layer C formed thereon are dissolved and removed together with a chemical. As a result, as shown in FIG. 1E, an electrode 14 made of a metal carbide is formed on the surface of the SiC crystal substrate. Finally, the substrate is alloyed while maintaining the substrate at a heat treatment temperature determined according to the composition of the metal carbide electrode 14 and whether the electrode 14 is to be formed into an ohmic junction or a Schottky junction, thereby completing the process.

The refractory metal used to form the metal electrode 14 is determined whether the conductivity type of the surface of the substrate 10 of the SiC crystal is p-type or n-type, and whether the ohmic junction or the Schottky junction is to be formed. For example, the selection is made as follows depending on whether or not there is an error.

When forming a Schottky junction irrespective of the conductivity type of the SiC substrate surface, Au is used as the high melting point metal.
Or Pt, Ir, Os, Re, Ru, R belonging to the platinum group
At least one metal may be selected from the group of h and Pd. Further, when a Schottky junction is formed on the surface of an n-type SiC substrate, a group IVa, group Va, or
At least one metal belonging to Group VIa may be selected.

When an ohmic junction is formed on an n-type SiC crystal substrate, Ni, Ti, Mo, Cr, W, A
At least one metal may be selected from the group consisting of uTa and TaSi ₂ . When an ohmic junction is formed on a p-type SiC crystal substrate, Al, AlSi, Al / Ti,
At least one metal may be selected from the group of Al / TaSi ₂ .

It is more preferable to add a doped impurity element to the refractory metal in order to control the conductivity of the surface of the SiC crystal substrate. This is because when the entire substrate is heated to a high temperature in an attempt to alloy the metal electrode, impurities near the interface with the metal electrode layer enter the electrode layer and become depleted, resulting in a high resistance near the interface. This is because a layer may be formed and the performance as an electrode may be significantly reduced.

Therefore, by adding an impurity element to the high melting point metal in advance, the deficiency state of the impurity element in the vicinity of the interface is offset by the impurity element thermally diffused from the inside of the electrode layer, thereby forming a high resistance layer. Can be avoided. For the same reason, it is also possible to adopt a configuration in which Si is added to the refractory metal in advance to avoid deficiency of Si near the interface with the electrode.

Although the embodiment of the present invention has been described above, various embodiments and modifications different from the above can be adopted in detail. For example, if the metal carbide electrode is too hard to be suitable for bonding by thermocompression bonding, a modified example of forming a metal layer of appropriate hardness on the electrode may be adopted. Conversely, when temporarily increasing the hardness of the metal electrode in preparation for subsequent mechanical polishing, the supply of carbon is continued even after the carbonization of the electrode is completed, so that the metal A modified example of forming a diamond layer may be employed.

[0025]

As described in detail above, the method of forming an electrode on SiC according to the present invention includes a step of carbonizing a metal electrode layer. However, no carbon, silicon, or impurities can enter the electrode layer from the SiC crystal. As a result, Si near the interface with the metal carbide electrode layer
C is kept in a state before the heat treatment including the impurities contained therein, and a very good and highly reliable Schottky junction or ohmic junction is formed.

Further, since the electrode itself is formed from a metal carbide, it is chemically stable and suitable for operation in a high-temperature environment where both the melting point and the hardness are high.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining a method of forming an electrode according to one embodiment of the present invention.

[Explanation of symbols]

 10 SiC crystal substrate 11 Electrode formation area 12 High resistance layer 13 AlN insulating layer 14 Metal carbide electrode layer R Resist layer M Metal layer MC Metal carbide layer C Diamond layer

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 21/28 H01L 29/40

Claims (9)

(57) [Claims] A step of forming a metal electrode layer on the surface of a silicon carbide crystal substrate; and supplying carbon to the metal electrode layer from outside of the silicon carbide crystal substrate. And a heating step of heating the silicon carbide crystal substrate on which the metal carbide electrode layer has been formed. 2. The method according to claim 1, wherein the metal is a group of Pt, Ir, Os, Re, Ru, Rh, and Pd, and IVa.
A method for forming an electrode on SiC, comprising at least one metal belonging to Group Va, Group Va or Group VIa, or a metal thereof or an alloy of these metals. 3. The method according to claim 1, wherein the metal is a group of Ni, Ti, Mo, Cr, W, AuTa, TaSi ₂ or Al, AlSi, Al / Ti, Al / TaSi _2.
S comprising at least one member belonging to the group of
A method for forming an electrode on iC. 4. The method according to claim 1, wherein the metal includes Si. 5. The method according to claim 1, wherein the metal includes an impurity element contained in the silicon carbide crystal substrate. 6. The method according to claim 1, wherein the metal electrode layer is formed by vapor deposition, sputtering, CVD, or reduction of a compound. 7. The method according to claim 1, wherein the carbonizing step includes a step of decomposing the organic carbon compound by a microwave plasma method or a thermionic plasma method while supplying the organic carbon compound to the metal electrode layer. A method for forming an electrode on SiC. 8. The method for forming an electrode on SiC according to claim 1, further comprising a step of forming a metal electrode layer on the metal carbide electrode layer. 9. The method for forming an electrode on SiC according to claim 1, further comprising the step of forming a diamond layer on the metal carbide electrode layer.