JP2013232557A - Silicon carbide semiconductor element manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an ohmic electrode which can ensure adhesion of the electrode by inhibiting release of carbon while ensuring an ohmic property.SOLUTION: A silicon carbide semiconductor element manufacturing method comprises a process of forming a contact electrode on a silicon carbide substrate 1. In the process of forming the contact electrode, metal (pure metal or alloy which includes any one or more among titanium, zirconium and hafnium) which easily generates carbide and also easily generates nitride is used as an electrode chief material of the contact electrode and thermal annealing is performed in an atmosphere consisting primarily of nitrogen thereby to convert in thermal annealing, the metal in the electrode material of the contact electrode in a region of a contact hole 9 where the silicon carbide substrate 1 and the metal in the contact electrode material directly contact each other to a mixed layer 12 which includes both of carbide and nitride.

Description

  The present invention relates to a method for manufacturing a silicon carbide semiconductor device using a single crystal silicon carbide substrate, and more particularly to a method for manufacturing a silicon carbide semiconductor device manufactured as a high breakdown voltage vertical device.

  Single crystal silicon carbide (SiC) has a band gap and breakdown electric field strength that are significantly higher than single crystal silicon (Si), and is expected to be able to realize an ultrahigh breakdown voltage semiconductor device exceeding a breakdown voltage of 10 kV by itself. Yes. In the manufacturing process of a silicon carbide semiconductor element, it is most convenient to deposit a metal thin film and perform thermal annealing at about 1000 ° C. in an inert gas such as argon when forming an ohmic electrode. Currently, nickel (Ni) is generally used as the metal material (for example, see Non-Patent Document 1 below).

  During thermal annealing for ohmic electrode formation, only nickel silicide is produced from the combination of nickel and silicon carbide, so excess carbon precipitates in the interior and surface of the electrode in the form of graphite, increasing contact resistance and electrode adhesion. It is known to cause problems such as deterioration of sex. As one of the solutions, a technique has been developed in which a metal that easily generates carbides such as titanium is deposited together with nickel and reacted with graphite (see, for example, Patent Documents 1 and 2 below). .

JP 2000-208438 A JP 2006-344688 A

Saji Manabu, Yasuda Kazuto, Hayakawa Toshitaka: Material of the Institute of Electrical Engineers of Japan, EFM-90, No. 17-23, pp. 31-33, 1990

  However, in a general semiconductor device manufacturing process, a deposited film (interlayer insulating film) mainly composed of silicon dioxide, which is used for electrical isolation between wirings, and the metal such as titanium and nickel are in direct contact with each other. If there is a portion, the metal may diffuse while decomposing the interlayer insulating film depending on the thermal annealing conditions, and may reach the underlying substrate (referred to as a spike phenomenon). If the metal is left only in the opening provided in the interlayer insulating film and unnecessary portions are removed by chemical etching or dry etching, such a problem does not occur. However, in these methods, the alignment accuracy of the photolithography process is improved. As a result, there arises a problem that the effective metal line width decreases with etching, so that it is impossible to achieve fine process rules.

  In view of the above-described problems, an object of the present invention is to provide a method for manufacturing a silicon carbide semiconductor element capable of forming an ohmic electrode capable of suppressing the liberation of carbon and ensuring the adhesion of the electrode while ensuring ohmic characteristics. .

  In order to achieve the above object, a method for manufacturing a silicon carbide semiconductor device of the present invention includes a step of forming a contact electrode on the silicon carbide substrate in a method for manufacturing a silicon carbide semiconductor device using a single crystal silicon carbide substrate. The step of forming the contact electrode uses, as the contact electrode, a metal that easily generates carbides and nitrides as an electrode main material, and performs thermal annealing in an atmosphere containing nitrogen as a main component, In the region of the contact hole where the silicon carbide substrate and the metal of the contact electrode material are in direct contact, the metal of the contact electrode material is converted into a mixture containing both carbide and nitride during the thermal annealing. Features.

  The metal of the contact electrode material is a pure metal or alloy containing any one or more of titanium, zirconium, and hafnium.

  According to the above configuration, in the contact hole region where the silicon carbide substrate and the metal of the contact electrode are in direct contact, the metal of the contact electrode is converted into a mixture containing both carbide and nitride during thermal annealing. To do. As a result, a practically low contact resistance can be ensured without using nickel, and no spike is caused in the interlayer insulating film.

  According to the present invention, it is possible to obtain an ohmic electrode that can ensure the adhesion of the electrode by suppressing the liberation of carbon while ensuring the ohmic characteristics.

It is sectional drawing which shows the manufacturing process of the electrode of the silicon carbide semiconductor element in embodiment of this invention (the 1). It is sectional drawing which shows the manufacturing process of the electrode of the silicon carbide semiconductor element in embodiment of this invention (the 2). FIG. 7 is a cross sectional view showing a step for manufacturing an electrode for a silicon carbide semiconductor device in the embodiment of the present invention (No. 3). It is a graph which shows the current-voltage characteristic of the contact electrode in a comparative example. It is a graph which shows the current-voltage characteristic of the contact electrode in an Example.

  Exemplary embodiments of a method for manufacturing a silicon carbide semiconductor device according to the present invention will be explained below in detail with reference to the accompanying drawings.

  As a result of repeated studies to solve the above problems, the inventor does not contain nickel (Ni), and is easy to generate carbides such as titanium (Ti), zirconium (Zr), hafnium (Hf), and the ohmic electrode. Using a pure metal or alloy containing at least one of the electrode main materials as a metal that easily generates nitrides, and performing thermal annealing in an atmosphere containing nitrogen (N) as a main component, the carbide and nitride It was decided to convert to a mixture containing both. As a result, the present invention has been completed, in which low contact resistance that is practical enough can be secured without using nickel, and no spike is caused in the interlayer insulating film.

(Embodiment)
Hereinafter, preferred embodiments of a method for manufacturing a silicon carbide semiconductor device according to the present invention will be described in detail with reference to the drawings. Here, titanium is used as a metal that easily generates carbides and also easily generates nitrides.

  1 to 3 are cross-sectional views showing steps for manufacturing an electrode of a silicon carbide semiconductor element in the embodiment of the present invention. It is the schematic diagram which looked at the cross section of the ohmic electrode formed on the n-type silicon carbide substrate 1 from the cross-sectional direction in each manufacturing process. The following procedure 1 to procedure 3 correspond to the numbers in FIGS. Hereinafter, an example in which the ohmic electrode is formed using titanium without including nickel will be described.

[Procedure 1]
As shown in FIG. 1, a necessary device structure is formed on a single crystal n-type silicon carbide substrate (silicon carbide substrate) 1 cleaned by a chemical cleaning method or a plasma etching method. Seal. In this embodiment, a part of the MOSFET structure is shown as an example of the device structure. When the MOSFET is formed, the p-type well layer 3, the n-type drift layer 4, the n-type ion implantation region 5, the gate oxide film 6, the gate electrode 7, and the element isolation mesa 8 are provided.

[Procedure 2]
Next, as shown in FIG. 2, using a technique such as dry etching, a part of the interlayer insulating film 2 and the gate oxide film 6 is opened to form a contact hole 9, and the interlayer insulating film is used using a metal sputtering apparatus. A titanium layer 10 is deposited covering the 2 and n-type ion implantation regions 5. Thus, the titanium layer 10 is deposited in Procedure 2 and not in the form of titanium nitride in the initial Procedure 1. This is because, since titanium nitride has high chemical stability, when a titanium nitride layer is formed as an electrode, the titanium nitride layer cannot be patterned by chemical etching, and a patterning step (described later) is formed on the titanium layer 10. This is because it is limited to dry etching. The titanium layer 10 is preferably a titanium layer that can be patterned by either chemical etching or dry etching.

  If the thickness of the titanium layer 10 is at least about 50 nm, an ohmic junction can be formed. However, when used as a back electrode of a vertical device, the back surface of the silicon carbide substrate 1 is finished by mechanical polishing such as diamond slurry. In many cases, titanium must be penetrated deeper than the polishing damage layer. From this viewpoint, it is preferable that the thickness of the titanium layer 10 be at least about 100 nm.

  When it is necessary to form an electrode pattern, a desired electrode pattern is baked onto the photoresist coated on the titanium layer 10 by a general photolithography process (not shown), and a resist mask on which the electrode pattern is formed is applied. The titanium layer 10 is patterned by chemical etching or dry etching as a mask. In the case of chemical etching, a chemical solution containing ammonia (for example, a mixed solution of ammonia water and hydrogen peroxide) may be used as an etchant, and in the case of dry etching, chlorine-based plasma may be used as an etchant. In implementation, the etchant is not necessarily limited to the above chemical solution or plasma.

[Procedure 3]
Next, as shown in FIG. 3, silicon carbide substrate 1 is introduced into an annealing furnace, and thermal annealing is performed in a high-purity nitrogen atmosphere. The purity of nitrogen may be 6N (99.9999%) generally used in the semiconductor device manufacturing process. Due to the nature of the present invention, the partial pressure of nitrogen needs to be set considerably higher than when titanium nitride is deposited by reactive sputtering (around 1 Pa). It has been confirmed that this happens.

  In the contact hole 9 portion, the titanium layer 10 changes to the titanium nitride layer 11 when the outermost surface of about 30 to 40 nm reacts only with nitrogen. Further, at a deeper portion of the contact hole 9, that is, on the silicon carbide substrate 1 side, it reacts with nitrogen and silicon carbide and changes to a mixed layer 12 of titanium nitride and titanium carbide.

  On the other hand, on the interlayer insulating film 2, the outermost surface of the titanium layer 10 of about 30 to 40 nm reacts only with nitrogen and changes to the titanium nitride layer 11, and the rest reacts with silicon dioxide to form titanium nitride and titanium oxide. It changes to the mixed layer 13 of titanium silicide. In an atmosphere containing nitrogen as a main component, the reaction between silicon dioxide and titanium proceeds almost uniformly, so that a spike phenomenon does not occur as in a reaction in a vacuum or a rare gas.

(Examples and Comparative Examples)
Conventional thermal annealing in vacuum as a comparative example and thermal annealing in a nitrogen atmosphere of the above procedure were performed as examples. At this time, a TLM electrode pattern for measurement based on a transmission line model is formed on an n-type bare substrate (nitrogen-doped, 1 × 10 ¹⁸ cm ⁻³ , surface is mechanically polished with a 0.5 μm diamond slurry). The difference in contact resistance was compared.

  The film thickness of the metal before thermal annealing is a laminated structure of nickel 100 nm and titanium 80 nm (nickel closer to the substrate) in the comparative example, and titanium 100 nm in the example. The gap length between the electrodes was 10 μm to 100 μm and the width was 10 μm. The peak temperatures for thermal annealing were all set to 1000 ° C.

FIG. 4 is a chart showing current-voltage characteristics of contact electrodes in a comparative example. The horizontal axis is voltage, and the vertical axis is current. As shown in FIG. 4, the contact electrode containing nickel and titanium in the comparative example has a shape in which a Schottky junction component is superimposed on the current-voltage characteristics, and the contact resistance defined within a range that can be regarded as a substantially straight line is 5 ×. It was as high as 10 ⁻³ Ωcm ² . In the measurement through the interlayer insulating film 2 having a thickness of 500 nm, conduction was confirmed when an AC voltage of less than 1 V was applied between the contact electrode and the underlying silicon carbide substrate 1, and a spike phenomenon occurred. It was shown that

FIG. 5 is a chart showing current-voltage characteristics of the contact electrodes in the example. The horizontal axis is voltage, and the vertical axis is current. The contact electrode of this example is a contact electrode that contains only titanium and does not contain nickel. As shown in FIG. 5, even if nickel is not used, the current-voltage characteristics are not subjected to the Schottky junction component, and the contact resistance is reduced. A clear improvement of 1 × 10 ⁻⁴ Ωcm ² was observed. Further, in the measurement through the interlayer insulating film 2 having a thickness of 500 nm, the contact electrode and the underlying silicon carbide substrate 1 are not affected even when an AC voltage of 80 V assuming a gate-source durability test of the vertical power MOSFET is applied. No conduction was confirmed between the two, indicating that insulation was maintained.

  As described above, according to the embodiment of the present invention, a metal such as titanium (Ti), zirconium (Zr), hafnium (Hf), which easily generates carbides and nitrides, is used for the ohmic electrode. It is used as an electrode main material and is thermally annealed in an atmosphere containing nitrogen (N) as a main component to convert it into a mixture containing both carbide and nitride. As a result, a practically low contact resistance can be ensured without using nickel, and no spike is caused in the interlayer insulating film.

  As described above, the method for manufacturing a silicon carbide semiconductor element according to the present invention is useful for power semiconductor devices such as power devices, and power semiconductor devices used for industrial motor control and engine control.

1 Single crystal n-type silicon carbide substrate (silicon carbide substrate)
2 Interlayer insulating film 3 p-type well layer 4 n-type drift layer 5 n-type ion implantation region 6 gate oxide film 7 gate electrode 8 element isolation mesa 9 contact hole 10 titanium layer 11 titanium nitride layer 12 mixed layer of titanium nitride and titanium carbide 13 Mixed layer of titanium nitride, titanium oxide and titanium silicide

Claims (2)

In a method for manufacturing a silicon carbide semiconductor element using a single crystal silicon carbide substrate,
Forming a contact electrode on the silicon carbide substrate;
The step of forming the contact electrode includes:
As the contact electrode, a metal that easily generates carbides and nitrides is used as an electrode main material,
In the contact hole region where the silicon carbide substrate and the metal of the contact electrode are in direct contact with each other, thermal annealing is performed in an atmosphere containing nitrogen as a main component. Is converted to a mixture containing both a carbide and a nitride.   2. The method for manufacturing a silicon carbide semiconductor element according to claim 1, wherein the metal of the material of the contact electrode is a pure metal or an alloy containing any one or more of titanium, zirconium, and hafnium.

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