JP2022076737A - Manufacturing method of semiconductor device - Google Patents

Abstract

To provide a manufacturing method of a semiconductor device capable of reducing contact resistance and securing a deflective strength of an element in a compatible manner by means of laser anneal.SOLUTION: An ohmic electrode is formed by irradiating a silicon carbide substrate 1 comprising a metal thin film 110 on a surface with laser beams 50 of different energy densities twice or more. An alloy layer 113 is formed by irradiating the metal thin film 110 on the silicon carbide substrate 1 with a laser beam 50 of such an energy density that silicon carbide is not fused. The alloy layer 113, namely, the inside of a region which has been already irradiated with the laser beam 50, in the metal thin film 110 is irradiated with a following second laser beam 50 and the energy density thereof is higher than that of the first laser beam 50. Thus, an electrode ohmic-joined with the silicon carbide can be formed without applying excessive energy to the silicon carbide, and reduction of contact resistance and the deflective strength of an element can be made compatible.SELECTED DRAWING: Figure 2E

Description

The present invention relates to a method for manufacturing a semiconductor device that realizes both low contact resistance of an ohmic electrode and ensuring anti-folding strength in a semiconductor device made of a semiconductor material such as silicon carbide (hereinafter referred to as "SiC").

When forming a semiconductor element such as a vertical power device using a semiconductor substrate such as a SiC substrate, contact resistance is reduced when forming an electrode for connecting the device to an electric circuit or the like, particularly a drain electrode on the back surface side of the substrate. It is desired to form a freed ohmic electrode.

Ohmic contact in SiC requires the formation of an alloy layer of SiC and metal, such as metal silicide or metal carbide, which requires high temperature treatment. For example, when forming nickel silicide (NiSi), the SiC substrate is heated at a temperature of 900 ° C. or higher.

In the case of a SiC semiconductor device, the back electrode is formed after the device structure is formed on the front side, but if the entire wafer on which the device structure is formed is treated at high temperature in a high temperature furnace or the like, the device structure and characteristics on the front side will be affected. It ends up. Therefore, a method has been proposed in which the back surface electrode is locally heated by irradiating the back surface electrode with a laser beam (hereinafter referred to as “laser annealing”) to form an alloy layer on the back surface electrode without raising the temperature of the device region on the front surface side.

Laser annealing can reduce the contact resistance of the ohmic electrode of the back surface electrode and reduce the thermal effect on the device on the front side, but on the other hand, a part of the SiC substrate disappears and unevenness is formed on the SiC substrate, and stress is formed on the unevenness. The concentration of the material reduces the anti-folding strength.

In order to achieve both low contact resistance of the ohmic electrode and ensuring the bending strength of the element, the electrode is divided into a region for low contact resistance and a region for ensuring the bending strength, and the laser beam is emitted for each region. A method of changing the energy density of the above has been proposed (for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2020-27871

In recent years, in this type of semiconductor device, the miniaturization of semiconductor devices and devices has been promoted, and the semiconductor devices are required to have improved bending strength so as to withstand the stress generated in the thermal cycle during use. ing.

However, when the semiconductor element is further miniaturized, it becomes difficult to divide the area in the electrode and irradiate laser light having different energy densities for each area, and the above method cannot be adopted. Further, in the region of the electrode where the energy density of the irradiated laser beam is high, the contact resistance is lowered, but the bending strength is lowered, and in the region where the energy density is low, the bending strength can be secured, but the contact resistance is high. The reduction will be insufficient. Therefore, in the formation of the alloy layer of the electrode by laser irradiation, it has not been possible to form a region in the electrode that has both low contact resistance and anti-folding strength.

In view of the above points, the present invention is a semiconductor capable of achieving both low contact resistance of the electrode and securing of bending strength of the semiconductor element without dividing the region in the electrode of the semiconductor element in laser annealing. It is an object of the present invention to provide a method of manufacturing an apparatus.

In order to achieve the above object, the method for manufacturing a semiconductor device according to claim 1 is a silicon carbide semiconductor substrate (1) having a main surface (1a) and a back surface (1b), and a side of the main surface of the silicon carbide semiconductor substrate. A method for manufacturing a silicon carbide semiconductor device having an ohmic electrode (11) ohmically bonded to one surface of silicon carbide on at least one of the sides of the surface and the back surface, at least on top of the ohmic-bonded silicon carbide. The metal thin film (110) containing the metal material constituting the above is formed, and the metal thin film is irradiated with laser beams (50, 51) having different energy densities multiple times to react with the metal thin film and Si in silicon carbide. In the case of irradiating the metal silicide multiple times, including the production of the metal silicide, the second and subsequent irradiations of the laser light include at least within the range of the metal thin film irradiated with the laser light. It irradiates a laser beam with a higher energy density than the laser beam that has already been irradiated.

As a result, while increasing the energy density stepwise and irradiating the metal thin film on the SiC substrate with laser light multiple times, it is possible to suppress excessive heat from being applied to the SiC substrate, while the metal is contained in the metal thin film. Silicides can be formed. Therefore, it is possible to form an ohmic electrode while suppressing the disappearance of SiC and the resulting decrease in bending strength in the region of the SiC substrate irradiated with laser light, and it is possible to reduce contact resistance and secure bending strength. It is a method of manufacturing a semiconductor device that is compatible with each other.

The reference numerals in parentheses attached to each component or the like indicate an example of the correspondence between the component or the like and the specific component or the like described in the embodiment described later.

It is sectional drawing which shows an example of the SiC semiconductor device which concerns on embodiment. It is sectional drawing which shows the formation process of the drain electrode in the SiC semiconductor device shown in FIG. 1. It is sectional drawing which shows the forming process of the drain electrode following FIG. 2A. It is sectional drawing which shows the formation process of the drain electrode following FIG. 2B. It is sectional drawing which shows the forming process of the drain electrode following FIG. 2C. It is sectional drawing which shows the forming process of the drain electrode following FIG. 2D. It is sectional drawing which shows the forming process of the drain electrode following FIG. 2E. It is sectional drawing which shows the formation process of the ohmic electrode by the laser annealing of the comparative example. It is sectional drawing which shows the forming process of the ohmic electrode following FIG. 3A. It is a figure which shows the relationship between the surface roughness and the bending strength. It is a figure which shows the contact resistance of the ohmic electrode in the comparative example and the Example. It is a figure which shows the surface resistance and the bending strength of the ohmic electrode in the comparative example and the Example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each of the following embodiments, the parts that are the same or equal to each other will be described with the same reference numerals.

(Embodiment)
Before explaining the method of manufacturing the semiconductor device according to the embodiment, a SiC semiconductor device as an example of the semiconductor device will be described with reference to FIG.

In the present specification, for example, a SiC semiconductor device including a planar type vertical power MOSFET suitable for application to an inverter will be described as a typical example, but the present invention is not limited to this SiC semiconductor device.

[Semiconductor device]
For example, as shown in FIG. 1, the vertical power MOSFET is formed by using the n ⁺ type SiC substrate 1 as the silicon carbide semiconductor substrate. The n ⁺ type SiC substrate 1 has a main surface 1a on the upper surface and a back surface 1b on the lower surface opposite to the main surface 1a, and is made of single crystal SiC. For example, as the n ⁺ type SiC substrate 1, one having an impurity concentration of 1 × 10 ¹⁸ cm ^-3 or more can be used.

On the main surface 1a of the n ⁺ type SiC substrate 1, an n ^- type epitaxial layer (hereinafter referred to as n ^- type epi layer) 2 composed of SiC having a dopant concentration lower than that of the n ⁺ type SiC substrate 1 is formed. It is laminated.

In the predetermined region on the surface layer portion of the n ^- type epi layer 2, p ^- type base regions 3a and 3b having a predetermined depth are formed apart from each other. The p ^- type base regions 3a and 3b include deep base layers 30a and 30b that are partially thickened. The deep base layers 30a and 30b are formed in a portion that does not overlap with the n ⁺ type source regions 4a and 4b described later. Of the p ^- type base regions 3a and 3b, the thickened portion where the deep base layers 30a and 30b are formed has a higher impurity concentration than the thin portion where the deep base layers 30a and 30b are not formed. ing. By forming such deep base layers 30a and 30b, the electric field strength between the n ⁺ type SiC substrate 1 and the deep base layers 30a and 30b can be increased, and the avalanche breakdown is caused at this position. It can be facilitated.

An n ⁺ type source region 4a shallower than the p ^- type base region 3a is formed in a predetermined region on the surface layer of the p ^- type base region 3a. Further, in a predetermined region in the surface layer portion of the p ^- type base region 3b, an n ⁺ type source region 4b shallower than the p ^- type base region 3b is formed.

Further, recesses 6a and 6b are formed on the surface portions of the p ^- type base regions 3a and 3b and the n ⁺ type source regions 4a and 4b, and the deep base layer having a high p-type impurity concentration is formed at the bottoms of the recesses 6a and 6b. 30a and 30b are exposed.

Further, the surface portion of the p ^- type base region 3a and 3b between the n ^- type epi layer 2 and the n ⁺ type source region 4a and the n ⁺ type source region 4b is used as a channel region, and at least on this channel region. A gate insulating film 7 made of a silicon oxide film or the like is formed. The gate insulating film 7 is formed on the upper surfaces of the n ⁻ type epi layer 2 and the n ⁺ type source regions 4a and 4b including the channel region. Further, a gate electrode 8 is formed on the gate insulating film 7, and the gate electrode 8 is covered with an insulating film 9 made of a silicon oxide film or the like.

Then, on the main surface 1a side of the n ⁺ type SiC substrate 1, the source electrode 10 is formed so as to cover the insulating film 9, and is connected to the n ⁺ type source regions 4a and 4b and the p ^- type base regions 3a and 3b. Has been done.

Further, the drain electrode 11 is formed on the back surface 1b side of the n ⁺ type SiC substrate 1. The drain electrode 11 is an ohmic electrode, and an alloy layer containing at least a metal silicide is formed on the back surface 1b of the n ⁺ type SiC substrate 1 by laser laser annealing. As a result, the drain electrode 11 is ohmic-bonded to the n ⁺ type SiC substrate 1 via the alloy layer. Therefore, the drain electrode 11 is configured to contain at least a material that constitutes metal silicide by reacting with SiC. Further, the surface of the drain electrode 11 is covered with a bonding electrode 12, and the structure is such that electrical connection with a metal plate or a circuit board (not shown) can be made via the bonding electrode 12.

For example, as the metal constituting the drain electrode 11, nickel (Ni), molybdenum (Mo), titanium (Ti) or the like can be used. Ni, Mo, and Ti react with Si to form silicide, and Mo and Ti can be combined with C to form carbide. In addition to the metal forming silicide, the drain electrode 11 may contain, for example, a metal such as tungsten (W), niobium (Nb), tantalum (Ta), etc., which is combined with C to form a carbide. The metal constituting the drain electrode 11 does not have to be one kind of material, and may be a material obtained by combining a plurality of the materials mentioned here, or may be a material obtained by laminating a plurality of materials, for example. It is said to be Mo / Ni. Further, the material constituting the drain electrode 11 may contain impurities.

For example, the drain electrode 11 is an ohmic electrode having a metal silicide and a metal carbide by subjecting a metal thin film having a structure in which Mo and Ni are laminated from the n ⁺ type SiC substrate 1 side two or more times with laser annealing. .. The formation of the drain electrode 11 by laser annealing two or more times will be described in the method for manufacturing a SiC semiconductor device described later.

As the metal constituting the bonding electrode 12, any material suitable for bonding such as soldering may be used, and for example, Ti / Ni / Au or the like can be used.

The above is the basic configuration of the SiC semiconductor device. In a configuration in which a device region is provided on the front surface side and an electrode is provided on the back surface side as in this SiC semiconductor device, a laser is used to form an ohmic contact between the back surface electrode and the semiconductor substrate while reducing the thermal influence on the device region. Annealing is done.

〔Production method〕
Next, the manufacturing method of the vertical power MOSFET of FIG. 1 will be described. Since the basic manufacturing method of the vertical power MOSFET is the same as the conventional one, the method of forming the drain electrode 11 different from the conventional one is described in FIG. 2A. -The description will be mainly made with reference to FIG. 2F.

First, as shown in FIG. 2A, an n ⁺ type SiC substrate 1 having a thickness of, for example, 350 μm is prepared. The n ⁺ type SiC substrate 1 is manufactured, for example, by slicing a SiC ingot doped with an n-type impurity and then polishing it. Then, although not shown, a device forming step of forming at least a part of the components of the semiconductor element on the surface side of the n ⁺ type SiC substrate 1 is performed. That is, after epitaxially growing the n ^- type epi layer 2, the steps of forming the p ^- type base regions 3a and 3b and the deep base layers 30a and 30b by ion implantation using a mask (not shown), the n ⁺ type source region 4a, The forming step of 4b is performed. Further, by performing a gate insulating film 7 forming step, a gate electrode 8 forming step, an insulating film 9 forming step, a source electrode 10 forming step, and the like, a vertical power MOSFET is formed as a device structure on the side opposite to the back surface 1b. Form each component of.

After that, although not shown, a thinning step is performed in which a part of the back surface 1b side of the n ⁺ type SiC substrate 1 is removed by grinding and polishing, and the n ⁺ type SiC substrate 1 is thinned. For example, the back surface 1b side of the n ⁺ type SiC substrate 1 is turned to the front side, and one surface on the opposite side is attached to the glass substrate, and then CMP (abbreviation of Chemical Mechanical Polishing) is performed to perform the n ⁺ type SiC substrate 1 A part of the back surface 1b side of the above surface is removed. At this time, the surface roughness Ra of the back surface 1b after the thinning step is set to, for example, 5 nm or less. Then, through the steps shown in FIGS. 2B to 2D, a step of forming the drain electrode 11 on the back surface 1b of the n ⁺ type SiC substrate 1 after thinning is performed.

Specifically, as a step shown in FIG. 2B, a metal thin film 110 is formed on the back surface 1b of the n ⁺ type SiC substrate 1 after thinning. For example, the metal thin film 110 is composed of a laminated film of a first layer 111 made of Mo and a second layer 112 made of Ni. The metal thin film 110 is formed, for example, by surface-treating the back surface 1b of the n ⁺ type SiC substrate 1 to activate it, and then performing electroless plating. The thickness of the metal thin film 110 is, for example, 50 to 250 nm.

When the metal thin film 110 on the back surface 1b has a laminated structure of a first layer 111 made of Mo and a second layer 112 made of Ni, the molar ratio of Ni and Mo is, for example, 1 to 2: 1. In addition, it is preferable that Ni has a molar ratio of Mo or more. Further, the metal thin film 110 is not limited to the laminated structure of the Mo layer and the Ni layer, and may be a mixed metal of Ni and Mo.

Next, as a step shown in FIG. 2C, laser annealing is performed by irradiating the metal thin film 110 with the laser beam 50. For example, a solid-state laser such as an LD-pumped solid-state laser is used to scan the n ⁺ type SiC substrate 1 on which the metal thin film 110 is formed on the XY plane while scanning, and the laser beam 50 is emitted from the n ⁺ type SiC substrate 1. Irradiate the back surface 1b side of.

More specifically, a tophat beam forming element and a condenser lens are arranged at the irradiation port of the solid-state laser so that the tophat type laser irradiates the metal thin film 110. For example, a solid-state laser having a basic wavelength of 1064 nm is used, and a laser beam 50 converted to a wavelength of 355 nm, which is a triple wave, or 266 nm, which is a quadruple wave, is used as a laser beam 50. By using these wavelengths, the laser beam 50 can be prevented from passing through SiC. Then, while adjusting the energy density described later, for example, the spot diameter is 75 μm, the overlap rate at the time of spot irradiation, that is, the ratio of the overlapping length to the diameter of continuous spots when scanning the laser beam 50 is 50 to Make it 70%. At this time, the overlap rate may be the same in each scanning direction in the XY directions, or may be different, such as 50% in the X direction and 50% to 70% in the Y direction.

The laser beam 50 is irradiated to the metal thin film 110 at least twice or more, and the second and subsequent laser beam irradiations are performed within the region already irradiated with the laser beam 50 and the energy density of the laser beam 50. Adjust so that is gradually higher. For example, it is assumed that the first irradiation of the laser beam 50 is performed on a predetermined region of the SiC wafer with an energy density of 2.0 J / cm ² . In this case, for example, the second irradiation of the laser beam 50 is performed within the range of the first irradiation of the laser beam 50 while setting the energy density to 2.2 J / cm ² . If the third irradiation of the laser beam 50 is performed, the energy density is set to a value larger than, for example, 2.2 J / cm ² , and the irradiation is performed within the irradiation range of the second laser beam 50.

Regarding the multiple irradiations of the laser beam 50, for example, the energy density of the laser beam 50 is set to a value of a predetermined value or less at which SiC does not melt. This is to prevent the SiC from melting in the n ⁺ type SiC substrate 1 and partially disappearing, thereby reducing the bending strength of the semiconductor element. Hereinafter, the energy density of the laser beam 50 having a value below a predetermined value that does not melt SiC may be referred to as "low energy density" for convenience.

Further, the irradiation conditions such as the energy density of the laser beam 50 are set so that the amount of heat required for changing the metal thin film 110 to the alloy layer 113 described below can be obtained. For example, when the metal thin film 110 is Mo / Ni, the irradiation conditions of the laser beam 50 (for example, for example) are such that the temperature is equal to or higher than the temperature at which NiSi is formed (for example, 600 ° C.) and the temperature is lower than the temperature at which SiC does not melt (for example, 970 ° C.). Energy density) may be adjusted.

Further, the irradiation interval of the laser beam 50 may be such that an interval such that the silicide reaction is completed at the irradiation point of the laser beam 50 in the metal thin film 110 can be secured, for example, but is not limited to 1 second or more. It is said that.

When the metal thin film 110 is irradiated with the laser beam 50 for the first time, the region of the metal thin film 110 irradiated with the laser beam 50 is an alloy layer having a metal silicide and a metal carbide, as shown in FIG. 2D, for example. 113 is generated. When the first layer 111 is Mo and the second layer 112 is Ni, the alloy layer 113 is mainly composed of Ni _x S _y (x> 0, y> 0) and MoC. At this time, the alloy layer 113 is in a state where Ni _x Si _having various compositions are mixed, and the contact resistance with the n ⁺ type SiC substrate 1 is not yet sufficiently reduced.

Subsequently, for example, the first laser beam 50 is used as the first laser beam, the second laser beam 50 is used as the second laser beam, and the second laser beam has a higher energy density than the first laser beam. As shown in FIG. 2E, the laser beam of No. 1 is applied to the alloy layer 113 of the metal thin film 110. In other words, the region of the metal thin film 110 that has already been irradiated with the laser beam 50, that is, the alloy layer 113 is irradiated with the laser beam 50 from the second time onward. As a result, further energy is applied to the alloy layer 113, the composition of the metal _silicide Ni _x Si in the alloy layer 113 changes, and the proportion of Ni ₂ Si having a small contact resistance with the n ⁺ type SiC substrate 1 increases. .. As a result, for example, as shown in FIG. 2F, a drain electrode 11 (ohmic electrode) having a small contact resistance with the n ⁺ type SiC substrate 1 is formed.

Note that FIGS. 2C to 2F show an example of irradiating a part of the metal thin film 110 with the laser beam 50 for the sake of clarity, but the present invention is not limited to this, and the metal thin film 110 is not limited to this. The ohmic electrode may be formed by irradiating the entire area with the laser beam 50.

In this way, by performing local annealing such as laser annealing, the drain electrode 11 can be ohmic-bonded by a low-temperature process that can suppress the temperature rise of the region not irradiated with the laser. Therefore, it is possible to suppress the influence on the device formed on the main surface 1a side of the n ⁺ type SiC substrate 1. Further, since the energy density of the laser beam 50 to be irradiated is set to a value equal to or less than a predetermined value that does not melt the SiC, the partial disappearance of the SiC and the accompanying decrease in the bending strength of the semiconductor element are suppressed. Therefore, it is possible to achieve both low contact resistance and bending strength of the element.

After that, although not shown, the bonding electrode 12 can be formed by laminating Ti as a barrier metal, Ni as a eutectic material at the time of soldering, Au as an oxidation protective agent, and the like in this order. Then, a dicing tape is attached to the drain electrode 11 side and peeled off from the glass substrate, and then dicing is performed to divide the dicing tape into chip units to complete the SiC semiconductor device.

The above is the manufacturing method of the vertical power MOSFET. As described above, in the present embodiment, in order to form an ohmic electrode while achieving both low contact resistance and bending strength of the element, the laser beam 50 is irradiated at least twice and the energy density is gradually increased. Do it while doing.

[Effects of multiple laser irradiations]
Next, the effect of laser annealing of the embodiment will be described with reference to.

First, as a comparative example, FIGS. 3A and 3B show the formation of an ohmic electrode by laser annealing in which a laser beam 50 having an energy density for melting SiC (hereinafter, referred to as “high energy density” for convenience) is irradiated once. Will be described with reference to.

3A and 3B are enlarged views showing a spot-irradiated portion of one point of the laser beam 50 in the metal thin film 210 on the n ⁺ type SiC substrate 1.

In the formation of the ohmic electrode in the SiC semiconductor device, in the laser annealing of the comparative example, for example, as shown in FIG. 3A, the laser beam 50 is spot-irradiated on the metal thin film 210 formed on the n ⁺ type SiC substrate 1, and the laser beam is emitted. It is done by scanning 50. Like the metal thin film 110, the metal thin film 210 is made of a metal material capable of forming a metal silicide or a metal carbide, and is composed of a single layer or a laminate.

In the laser annealing of the comparative example, basically, the same region is not irradiated with the laser beam 50 more than once except for the overlapping portion with respect to the diameter of the continuous spots when the laser beam 50 is scanned. According to the laser annealing of the comparative example, the portion of the metal thin film 210 irradiated with the laser beam 50 changes to an alloy layer containing the metal silicide, and as shown in FIG. 3B, for example, ohmic contact with the n ⁺ type SiC substrate 1. Ohmic electrode 21 can be formed.

However, at this time, a part of the n ⁺ type SiC substrate 1 is melted, and the ohmic electrode 21 is formed in the melted part. In other words, a part of the n ⁺ type SiC substrate 1 disappears, unevenness is formed at the interface of the n ⁺ type SiC substrate 1 with the metal thin film 210, and the surface roughness increases. According to the study results of the present inventors, it has been found that when the surface roughness of the n ⁺ type SiC substrate 1 increases, the bending strength of the substrate tends to decrease.

Specifically, a metal thin film forming a metal silicide is formed on the SiC substrate, and a plurality of samples are prepared by laser annealing by changing the irradiation conditions of the laser beam 50 on the metal thin film, and the surface of the ohmic electrode is prepared for each sample. Roughness and bending strength of the sample were evaluated. As a result, for example, as shown in FIG. 4, as the surface roughness Ra (unit: nm) of the sample increased, the bending strength (unit: MPa) of the SiC substrate tended to decrease. It is considered that this is because the stress concentration on the surface unevenness increases as the surface unevenness of SiC generated by the laser annealing increases. The broken line in FIG. 4 is an approximate curve obtained by a method such as the least squares method based on each data.

That is, in the laser annealing of the comparative example, the ohmic electrode having a small contact resistance with the SiC substrate can be formed by irradiating the laser beam with high energy density, but the bending strength of the SiC substrate is lowered. It was difficult to achieve both. However, although the melting of SiC can be suppressed and the bending strength of the element can be maintained simply by reducing the energy density of the laser beam 50, the silicide reaction becomes insufficient and low contact resistance cannot be realized.

In order to solve this problem, a first region in the metal thin film to be irradiated with a laser beam having a low energy density, a region different from the first region, and a second region to be irradiated with a laser beam having a higher energy density than the first region. It has been proposed to divide the area into (for example, Patent Document 1).

However, in recent years, further miniaturization of semiconductor devices and semiconductor devices has been promoted, and it is difficult to divide them into a region for reducing contact resistance and a region for ensuring the bending strength of the device as described above. Become. Therefore, in the ohmic electrode, it is required to form a region that achieves both low contact resistance and ensuring the bending strength of the element.

As a result of diligent studies by the present inventors, a semiconductor device capable of achieving both low contact resistance and ensuring the bending strength of the device by performing laser beam irradiation with different energy densities in two or more steps in stages. I came up with the idea of a manufacturing method.

As shown in FIGS. 2C to 2F, the laser annealing according to the embodiment is performed by irradiating the laser beam 50 having a low energy density that does not melt SiC twice or more, and the energy of the second and subsequent irradiations of the laser beam 50. The density is made higher than the laser beam 50 already irradiated. Further, the second and subsequent irradiations of the laser beam 50 are performed at a low energy density and an energy density higher than that of the already irradiated laser beam 50, and are performed within the region already irradiated with the laser beam 50. According to this method, for example, as shown in FIG. 2F, the drain electrode 11 ohmic-bonded to the n ⁺ type SiC substrate 1 is formed while suppressing the melting of SiC in the n ⁺ type SiC substrate 1 under the metal thin film 110. be able to.

Next, FIG. 5 shows the contact resistance, surface roughness and bending strength of the samples obtained by laser annealing of Comparative Examples and Examples (hereinafter, simply referred to as “Samples of Comparative Examples” and “Samples of Examples”). , FIG. 6 will be described.

In FIG. 5, the contact resistance on the vertical axis is displayed on a logarithmic scale, and the difference between the upper limit value and the lower limit value on the vertical axis is one digit. In FIG. 6, the plot shows the surface roughness on the left vertical axis, and the bar graph shows the bending strength on the right vertical axis. Further, "comparative example" on the horizontal axis of FIGS. 5 and 6 indicates a sample of the comparative example, and "example" indicates a sample of the example.

The sample of the comparative example and the sample of the example are samples provided with an ohmic electrode formed by irradiating a metal thin film on a SiC substrate with a laser beam 50, except for the energy density of the laser beam 50 and the number of irradiations. The other conditions are the same.

First, regarding the contact resistance (unit: Ω), as shown in FIG. 5, for example, the sample of the example had the same resistance value as the sample of the comparative example. This is because even if the laser annealing is performed by the laser beam 50 having a low energy density, the same region is repeatedly irradiated with the laser beam 50 twice, so that the contact resistance is as low as that of the laser annealing by the laser beam 50 having a high energy density. It shows that the effect of conversion can be obtained.

Regarding the surface roughness, for example, as shown in FIG. 6, the sample of the example was smaller than the sample of the comparative example. This is because the low energy density laser beam 50 suppresses the melting of SiC during laser annealing and reduces the unevenness at the interface between the SiC substrate and the ohmic electrode, resulting in a surface roughness of the ohmic electrode surface. It suggests that it has been reduced. Hereinafter, for convenience of explanation, the unevenness at the interface between the SiC substrate and the ohmic electrode is referred to as “interface unevenness”.

Regarding the bending strength, for example, as shown in FIG. 6, the sample of the example was nearly twice as large as the sample of the comparative example. It is considered that this indicates that the melting of SiC at the time of laser annealing was suppressed and the interfacial unevenness was reduced, and as a result, the stress concentration on the SiC substrate was alleviated and the bending strength was improved.

The bending strength that can secure the element strength is 1000 MPa or more, and it is desirable that the bending strength is 1000 MPa or more in all the manufactured products. The sample of the comparative example had a bending strength of less than 1000 MPa, whereas the sample of the example had a bending strength of 1000 MPa or more.

As described above, by setting the laser beam 50 to a low energy density and performing laser annealing in at least two steps, it is possible to secure the bending strength while lowering the contact resistance at the same portion of the ohmic electrode. That is, the ohmic electrode is subjected to laser annealing including a first step of forming an alloy layer while suppressing melting of SiC and a second step of changing the composition of the metal silicide formed in the first step to lower the contact resistance. To form. As a result, it is possible to secure the bending strength of the semiconductor element while reducing the contact resistance in the same region of the electrode, and it is possible to manufacture a highly reliable semiconductor device.

According to the present embodiment, it is not necessary to irradiate the laser beam 50 having a high energy density or to divide the area of the laser beam irradiation, and it is possible to obtain a semiconductor device manufacturing method capable of achieving both low contact resistance and ensuring the bending strength of the device. Become. Further, since the laser beam 50 can be applied to the entire area of the metal thin film 110 on the back surface 1b of the n ⁺ type SiC substrate 1, it can be applied even when the semiconductor element is miniaturized.

(Other embodiments)
Although the present invention has been described in accordance with the examples, it is understood that the present invention is not limited to the examples and structures. The present invention also includes various modifications and variations within a uniform range. In addition, various combinations and forms, as well as other combinations and forms including only one element thereof, more or less, are also within the scope and scope of the invention.

For example, in the above embodiment, the SiC semiconductor device has been described as a typical example, but it is possible to form an ohmic electrode by laser annealing a plurality of times for a semiconductor device other than the SiC semiconductor device, and the semiconductor device is limited to the SiC semiconductor device. Not done. Further, the drain electrode 11 is mentioned as an example of the ohmic electrode by laser annealing, but the drain electrode 11 is not limited thereto.

1 Silicon carbide semiconductor substrate 1a Main surface 1b Back surface 11 Ohmic electrode 110 Metal thin film 50 Laser light

Claims (4)

A silicon carbide semiconductor substrate (1) having a main surface (1a) and a back surface (1b) is ohmic-bonded to one surface of silicon carbide on at least one of the main surface side and the back surface side of the silicon carbide semiconductor substrate. A method for manufacturing a silicon carbide semiconductor device having an ohmic electrode (11).
Forming a metal thin film (110) containing at least a metal material constituting silicide on the silicon carbide semiconductor substrate to be ohmic-bonded.
It comprises irradiating the metal thin film with laser light (50) having different energy densities multiple times and reacting the metal thin film with Si in the silicon carbide to form a metal silicide.
In irradiating the laser beam a plurality of times, for the second and subsequent irradiations of the laser beam, the energy density is higher than that of the laser beam already irradiated in at least the range of the metal thin film irradiated with the laser beam. A method for manufacturing a semiconductor device, which irradiates a high-grade laser beam. The method for manufacturing a semiconductor device according to claim 1, wherein the metal thin film is formed by laminating Mo and Ni from the side of the silicon carbide. The method for manufacturing a semiconductor device according to claim 1 or 2, wherein when the laser beam is irradiated a plurality of times, the energy density of the laser beam is set to a value equal to or less than a predetermined value that does not melt the silicon carbide. In irradiating the laser beam multiple times,
The first step of irradiating the metal thin film with the first laser beam (50) and reacting the metal thin film with Si or C in the silicon carbide to produce the metal silicide.
After the first step, a second laser beam having a higher energy density than the first laser beam is irradiated into the region of the metal thin film irradiated with the first laser beam to obtain the metal silicide. The method for manufacturing a semiconductor device according to any one of claims 1 to 3, further comprising a second step of changing the composition.

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