JP2021027114A - Semiconductor element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor element capable of reducing contact resistance. A semiconductor device (1) of the present invention comprises a semiconductor (10) having an ohmic electrode (12). The semiconductor is made of silicon carbide. Ohmic electrodes include molybdenum carbide, molybdenum silicide and nickel silicide. Molybdenum carbide is uniformly distributed in the ohmic electrode. The ohmic electrode according to the present invention may have a contact resistance of 1 × 10-4 Ω · cm2 or less. Such an ohmic electrode can be used, for example, by applying a pulse laser having an effective irradiation area of 2500 μm2 or more and an energy density of 2.35 J / cm2 or more in the effective irradiation area to a metal layer in which a Mo layer and a Ni layer are laminated. Obtained by irradiation. [Selection diagram] FIG. 4B

Description

The present invention relates to semiconductor devices and the like.

As the semiconductor material used for switching elements and the like, silicon carbide (SiC) having a wide forbidden band width (bandgap) has come to be used instead of conventional silicon (Si).

As a part of suppressing the loss of the semiconductor element made of SiC, it is required to reduce the contact resistance of the ohmic electrode. The ohmic electrode is usually formed by heat-treating (annealing) a metal layer provided on a semiconductor. If the semiconductor is Si, only low resistance metal silicide is formed. However, when the semiconductor becomes SiC, high-resistance graphite (C) can be deposited together with the formation of metal silicide. Proposals have been made for such low-resistance ohmic electrodes that suppress the precipitation of graphite, and for example, there are descriptions related to the following patent documents.

JP-A-2017-199807 Japanese Unexamined Patent Publication No. 2017-13050

In Patent Document 1, an ohmic electrode is formed by irradiating a Mo layer and a Ni layer laminated on the back surface of a SiC substrate with a laser. In this ohmic electrode, the diffusion and precipitation of high-resistance graphite are suppressed by the block-shaped Mo-carbide generated near the interface with SiC.

However, Mo-carbide itself is covalent and not low resistance. Therefore, in Patent Document 1, the block-shaped Mo-carbide is surrounded by a low-resistance metal-bonding Ni ceiling to secure a current path, and the resistance of the ohmic electrode is reduced. However, when further reducing the contact resistance, the presence of the block-shaped Mo carbide is not preferable.

In Patent Document 2, an alloy layer (mixed layer) of Nb and Ni is laser-annealed to form an ohmic electrode having an amorphous structure. As a result, precipitation of crystalline metal carbide formed when the laminated metal layer is annealed is avoided. As a result, it is described in Patent Document 2 that the contact resistance is 2.4 × 10 ^-4 Ωcm ² when the total amount of laser irradiation energy is 2.25 J / cm ^{2 ([0036], FIG.} 5).

By the way, as a comparative example, when a metal film in which Mo (thickness 50 nm) and Ni (thickness 50 nm) are laminated in order is laser-annealed, the contact resistance is 8 when the ^{total amount of laser irradiation energy is 2.25 J / cm 2.} There is also a description in Patent Document 2 that the amount is 0.0 × 10 ^-4 Ωcm ^{2 ([0045], FIG. 5).} However, there is no detailed description in Patent Document 2, and the specific structure of the ohmic electrode is unknown. If you dare to say it, since its contact resistance is large ( ^{close to about 10-3} Ωcm ² ), it is considered that a large amount of Mo carbide is generated near the interface with SiC.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an ohmic electrode having a new structure different from that of a conventional ohmic electrode.

As a result of diligent research to solve this problem, the present inventor has succeeded in forming an ohmic electrode in which molybdenum carbide is uniformly distributed. By developing this result, the present invention described below has been completed.

<< Semiconductor element >>
(1) The present invention is a semiconductor element made of a semiconductor having an ohmic electrode, the semiconductor made of silicon carbide, the ohmic electrode containing molybdenum carbide, molybdenum silicide and nickel silicide, and the molybdenum carbide is said to be the molybdenum carbide. It is a semiconductor element that is uniformly distributed in the ohmic electrode.

(2) In the semiconductor device of the present invention, molybdenum carbide (also referred to as "Mo carbide") is uniformly distributed in the ohmic electrode, not in a block shape. As a result, it is possible to form a current path that is substantially uniformly distributed in the ohmic electrode, and the contact resistance of the ohmic electrode can be reduced.

By the way, the ohmic electrode of the present invention contains molybdenum silicide (also referred to as "MoSiO") which is not found in the conventional ohmic electrode. Mo ceiling, like nickel silicide (also referred to as "NiSiO"), has a lower resistance than Mo carbide. Therefore, Mo ceiling contributes to the reduction of contact resistance in the same manner as Ni ceiling. In this way, the ohmic electrode of the present invention can stably realize a low contact resistance of ^{, for example, 1 × 10 -4} Ω · cm ^{2 or less regardless of the site.}

<< Manufacturing method of semiconductor elements >>
(1) The present invention is also understood as a method for manufacturing a semiconductor element as described above. For example, the present invention comprises a metal layer forming step of forming a metal layer in which a Mo layer and a Ni layer are laminated on at least one surface of a semiconductor, and irradiating the metal layer with a pulse laser to heat the metal layer. A method for manufacturing a semiconductor device including an annealing step. The pulse laser has an effective irradiation area of 2500 μm ² or more per pulse and an energy density of 2.35 J / cm ² or more in the effective irradiation area. It may be the manufacturing method of the semiconductor element which is.

(2) In the manufacturing method of the present invention, first, an ohmic electrode is formed by laser annealing of a metal layer, and the element portion (device structure) of the semiconductor element is less damaged during the formation.

By the way, the structure (structure) of the ohmic electrode differs greatly depending on the laser irradiation conditions. For example, even if the energy density is the same, if the beam profile of the laser is different, the balance between heating and cooling (heat dissipation by heat conduction) changes, which affects the temperature distribution of the irradiation area.

When a wide irradiation area is heated by a pulsed laser having a high energy density as in the production method of the present invention, the irradiation area is more likely to be maintained in a high temperature state than when a narrow irradiation area is heated. The reason for this can be considered as follows. Even if the local area (for example, the central part) is heated instantaneously, the temperature of the irradiation area does not rise so much due to the rapid cooling by the two-dimensional heat conduction. On the other hand, when the irradiation area is heated widely, heat conduction becomes one-dimensional, heat dissipation (cooling) becomes slow, and the irradiation area tends to be in a high temperature state.

When the metal layer is heated to a high temperature by laser irradiation, C and the like generated from SiC do not stay near the interface but diffuse widely. As a result, it is considered that an ohmic electrode in which Mo carbide is uniformly dispersed is formed. Further, it is considered that as a result of the high temperature of the metal layer, Mo ceiling, which was not generated by the conventional laser annealing, is newly generated.

Thus, according to the manufacturing method of the present invention, the high-resistance Mo carbide does not aggregate in the vicinity of the interface with the semiconductor (SiC), the low-resistance Mo silicide is also produced, and the ohmic electrode has a low contact resistance. Is considered to have come to be formed stably.

《Semiconductor device》
The present invention is also understood as a semiconductor device using the above-mentioned semiconductor element. For example, the present invention is a semiconductor device including a semiconductor element made of a semiconductor having an ohmic electrode, a metal body bonded to the semiconductor element, and a bonding portion for joining the ohmic electrode and the metal body. The semiconductor is made of silicon carbide, the ohmic electrode contains molybdenum carbide, molybdenum silicide and nickel silicide, and the molybdenum carbide may be a semiconductor device uniformly distributed in the ohmic electrode.

《Others》
(1) Whether or not the Mo-carbide referred to in the present specification is "uniformly" distributed is confirmed, for example, from an element map image. If you dare to say, "uniform" means that there are no Mo-carbide particles (nano-sized lumps) having a maximum length of more than 15 nm and even more than 10 nm. The element map image is generated by, for example, electron energy loss spectroscopy (EELS) or energy dispersive X-ray analysis (EDX) using a scanning transmission electron microscope (STEM).

(2) Unless otherwise specified, "x to y" in the present specification includes a lower limit value x and an upper limit value y. A range such as "ab" may be newly established with any numerical value included in the various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value. Unless otherwise specified, "x to ynm" as used herein means xnm to ynm. The same applies to other unit systems (μm, Ωcm ^{2, etc.).}

It is explanatory drawing which illustrates the manufacturing process of a semiconductor element. The beam profile of the irradiated laser. It is a schematic cross-sectional view which shows an example of a semiconductor element. It is an XRD profile about the ohmic electrode of sample 1. An XRD profile relating to the ohmic electrode of sample C1. It is a STEM image and an element map image concerning the ohmic electrode of a sample 1. It is a magnified STEM image which concerns on the ohmic electrode of sample 1. It is a STEM image and an element map image concerning the ohmic electrode of the sample C1. 9 is an enlarged STEM image of the ohmic electrode of sample C1. It is an AES profile of the ohmic electrode which concerns on a sample 1. It is an AES profile of the ohmic electrode which concerns on a sample C1. It is a 3DAP image and an element map image of the ohmic electrode which concerns on a sample 1.

One or more components arbitrarily selected from the present specification may be added to the components of the present invention. The contents described in the present specification may be applicable not only to the semiconductor device of the present invention but also to a manufacturing method thereof and the like. A component related to "method" can also be a component related to "thing".

《Ohmic electrode》
Ohmic electrodes include Mo-carbide, Mo silicide and Ni silicide. It is preferable that not only Mo carbide but also Mo ceiling and Ni silicide are uniformly distributed in the ohmic electrode. Furthermore, it is preferable that these compounds are distributed without forming a long-period crystal structure and are amorphous. These things are confirmed by the fact that the peaks appearing in the X-ray diffraction profile (pattern) are broad rather than sharp.

Although it is difficult to specify the ratio of each compound, it is preferable that Ni and Mo contained in the ohmic electrode are about the same or have more Ni than Mo. The atomic number ratio of Mo to Ni (Mo / Ni) is, for example, preferably 0.6 to 1 or even 0.7 to 0.9.

The thickness of the ohmic electrode is, for example, 30 to 300 nm and further 50 to 150 nm. The thickness may be, for example, the shortest distance (minimum value of the interinterface distance) of the image observed by an electron microscope (the same applies to other thicknesses). Excessive thickness leads to increased contact resistance. If the thickness is too small, the adhesion of the ohmic electrode may decrease.

<< Formation of ohmic electrode >>
As an example, the above-mentioned ohmic electrode is formed by the following laser annealing. That is, it is formed by a metal layer forming step of forming a metal layer in which a Mo layer and a Ni layer are laminated on at least one surface of a semiconductor, and an annealing step of irradiating the metal layer with a pulse laser to heat-treat the metal layer. Will be done.

(1) Metal layer forming step The Mo layer and Ni layer are formed by vapor deposition such as sputtering. The thickness of each layer is, for example, 20 to 200 nm and further 40 to 160 nm. The Mo layer and the Ni layer may have the same thickness, but the Mo layer may be thinner than the Ni layer by about 10 to 30 nm.

The Mo layer traps C released from SiC during heat treatment to form Mo carbide. As a result, precipitation of C (graphite or the like) in the ohmic electrode is suppressed, an increase in contact resistance due to C and adhesion of the ohmic electrode to the semiconductor (SiC) are ensured. Further, the Mo layer reacts with Si emitted from SiC in a high temperature region to generate low resistance Mo ceiling.

The Ni layer suppresses the oxidation of the Mo layer. In addition, the Ni layer has good laser absorption and efficiently converts the irradiated laser into heat energy. Then, the Ni layer reacts with Si emitted from SiC during the annealing step to generate low-resistance Ni silicide.

(2) Annealing Step As the laser (light source), a solid-state laser (semiconductor laser-excited solid-state laser, lamp-excited solid-state laser, etc.), an excimer laser, or the like can be used. The solid-state laser may YAG laser, YVO ₄ laser, or the like. Excimers (excited dimers) include F ₂ (wavelength 157 nm), ArF (wavelength 193 nm), KrF (wavelength 248 nm), XeCl (wavelength 308 nm), XeF (wavelength 351 nm) and the like.

As the laser, for example, a pulse laser having a pulse width of 10 to 100 nsec and further 30 to 70 nsec may be used. By adjusting the pulse width, the irradiation area can be heated efficiently.

Laser irradiation, for example, may an effective irradiation area per pulse 2500 [mu] m ² or more further to 3000 .mu.m ² or more. Further, the energy density (output density) of the effective irradiation area may be ^{2.3 to 3 J / cm 2 and} further 2.4 to 2.7 J / cm ^2. If the effective irradiation area or energy density is too small, the metal layer will not be sufficiently heated, and high-resistance Mo carbide will be intensively generated near the SiC interface. If the energy density becomes excessive, ablation will occur, leading to an increase in contact resistance and damage to SiC and the like. It is also effective irradiated area is large, but may from device constraints, say dare and 5000 .mu.m ² or less further to 4000 .mu.m ² or less.

The effective irradiation area referred to in the present specification is the area of the region corresponding to ^{1 / e 2} (13.5%) of the peak value of the laser intensity when the beam profile of the laser has a Gaussian shape. If the beam profile of the laser has a top hat shape, the area of the region corresponds to 50% of the peak value of the laser intensity.

Further, the energy density referred to in the present specification is determined by dividing the energy (integral value) for one passle in the effective irradiation area. The energy for one passle is obtained by dividing the laser output (W) by the laser frequency (Hz).

The ratio of superimposing the irradiation area of the pulse laser is adjusted by the oscillation frequency, scanning speed, size of the irradiation area (or the focal position of the pulse laser), and the like. Although it depends on the characteristics of the pulsed laser, it may be, for example, 20 to 95% or even 33 to 75%.

The oscillation frequency (20 kHz) of the pulse laser may be, for example, 10 kHz to 100 kHz. If the oscillation frequency is too low, the scanning speed will decrease. If the oscillation frequency is excessive, the energy density may decrease.

The irradiation range changes depending on the focal position of the laser. The focal position may be deviated from the outermost surface of the metal layer, but the closer to the outermost surface, the more stable the energy density can be annealed in each region. The laser irradiation is preferably performed in a vacuum atmosphere or an inert gas atmosphere in order to avoid the formation of unnecessary compounds on the surface of the metal layer (Mo layer or Ni layer).

<< Semiconductor element >>
Semiconductor elements are transistors and diodes made of SiC, and in particular, power devices that control large currents. Examples of the power transistor include MOSFET (metal oxide semiconductor field effect transistor), IGBT (insulated gate bipolar transistor) and the like. Examples of the power diode include SBD (Schottky barrier diode), FRD (fast recovery diode) and the like.

The ohmic electrode is, for example, a back surface electrode formed on the opposite surface side of the element portion (device structure). The back surface electrode is, for example, a cathode electrode for a power diode, a drain electrode or a collector electrode on the opposite surface side of the gate electrode for a power transistor, or the like.

<< Semiconductor element >>
(1) An example of a manufacturing process of a semiconductor element such as a power MOSFET or a power diode is shown in FIG. 1A. First, an n-type SiC wafer (simply referred to as a “SiC substrate”) doped with impurities is prepared. An element portion (device structure) is formed on one side (surface) of the polished SiC substrate by ion implantation or the like (device forming step). The opposite surface (back surface) side is ground and polished to a desired thickness (substrate thinning step).

Mo and Ni are sequentially vapor-deposited on the polished back surface by sputtering (metal layer forming step). Sputtering is performed in a vacuum atmosphere (the same applies to other sputterings). The Mo layer and Ni layer laminated on the back surface of the SiC substrate are irradiated with a pulse laser to perform laser annealing (annealing step). As a result, an ohmic electrode (layer) is formed on the back surface of the SiC substrate. The metal layer forming step and the annealing step are collectively called a back electrode forming step. After that, the SiC substrate on which the element portion and the ohmic electrode are formed is divided by blade dicing or the like (not shown) to obtain a chip (semiconductor element).

(2) In this embodiment, an evaluation chip 1 assuming a power diode was manufactured based on the above-mentioned manufacturing process (see FIG. 2). As shown in FIG. 2, the evaluation chip 1 has an ohmic electrode 12 (back surface electrode) formed on one surface (back surface) of the SiC substrate 10 (□ 4 mm × t0.28 mm).

The ohmic electrode 12 was formed by irradiating (laser annealing) the laminated Mo layer (thickness 70 nm) and Ni layer (thickness 100 nm) with a laser. Laser irradiation was performed using the two types of beam profiles shown in FIG. 1B (Sample 1, Sample C1).

First, a pulsed laser having a top hat shape (the central part of the effective irradiation area is flat) was irradiated with the beam profile. At this time, the type of laser: YVO ₄ , pulse width: <60 nsec, oscillation frequency: 20 kHz, effective irradiation area per pulse: 7800 μm ² , energy density of the effective irradiation area: 2.0 to 3.0 J / cm ² And said. The evaluation chips 1 thus obtained are collectively referred to as sample 1. The effective irradiation area and energy density were determined by the methods described above.

Next, laser annealing was performed by irradiating a pulse laser having a Gaussian-shaped (spier-shaped) beam profile as in the prior art (see JP-A-2017-199807). The laser irradiation conditions at this time were: laser type: YVO ₄ , pulse width: <60 nsec, oscillation frequency: 20 kHz, effective irradiation area: 31400 μm ² , energy density: 1.0 to 2.0 J / cm ² . The evaluation chip 1 thus obtained is referred to as sample C1.

《Contact resistance》
(1) Measurement The contact resistance of the ohmic electrode 12 of each sample was measured by the TLM (Transmission Line Model) method. An example of this is as follows.

In the case of sample 1, when the energy density at the time of forming the ohmic electrode was 2.6 J / cm ² , the contact resistance was 2 × 10 ^-5 Ω · cm ² . In the case of sample C1, when the energy density was 1.8 J / cm ² , the contact resistance was 2 × 10 ^-4 Ω · cm ² .

(2) Evaluation As described above, in the ohmic electrode of sample 1, when the energy density ^{exceeded 2.3 J / cm 2} , the contact resistance became stable ^{and became 1 × 10 -4} Ω · cm ² or less. On the other hand, the ohmic electrode of sample C1 had a contact resistance of more than 1 × 10 ^-4 Ω · cm ² .

"analysis"
In order to clarify the factors that the difference in ohmic electrodes (difference in laser annealing conditions) greatly affects the contact resistance, various analyzes (measurements / observations) shown below are performed on each ohmic electrode of sample 1 and sample C1. It was.

(1) XRD
The XRD profile obtained by analyzing the ohmic electrode of each sample with an X-ray diffractometer (manufactured by Rigaku Co., Ltd./X-ray used: Cu-Kα ray) is shown in FIGS. 3A and 3B (both figures are simply "combined". It is referred to as "Fig. 3").

In Sample 1, no peaks showing Mo or Ni were observed, and only peaks showing _{MoSi 2 (an example of Mo ceiling) were observed in addition to MoC (an example of Mo carbide) and Ni 2} Si (an example of Ni silicide). Was done. In addition, all peaks were broad. On the other hand, in sample C1, no _{peak indicating Mo ceiling (for example, MoSi 2} ) was observed, and a peak indicating Ni alone or Mo alone was observed. A sharp peak indicating MoC was also observed.

From these XRD profiles, it was clarified that the ohmic electrode according to the sample 1 and the ohmic electrode according to the conventional sample C1 have significantly different tissue structures and products.

(2) STEM-EDX
The cross section of each sample near the ohmic electrode was observed and analyzed with a field emission transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) and an energy dispersive X-ray analyzer attached thereto. The STEM image (bright field image) and element map image of Sample 1 (energy density: 2.6 J / cm ² ) are shown in FIG. 4A, and the enlarged STEM image is shown in FIG. 4B. Further, the STEM image (bright field image) and the element map image of the sample C1 are shown in FIG. 5A, and the enlarged STEM image is shown in FIG. 5B.

As is clear from FIGS. 4A and 4B (both figures are simply referred to as "FIG. 4"), Mo, Ni, C and Si are uniformly distributed in the ohmic electrode according to Sample 1, and the block. No compound or the like was found. The MoC layer observed near the junction interface of SiC was uniform and very thin with a thickness of less than 5 nm.

On the other hand, as is clear from FIGS. 5A and 5B (both figures are simply referred to as "FIG. 5"), the ohmic electrode according to sample C1 has a side of about 16 to 24 nm in the vicinity of the junction interface of SiC. Block-shaped Mo carbide was observed.

(3) AES
The results of Auger electron spectroscopy (AES) of the sample 1 (energy density: 2.5 J / cm ² ) and the ohmic electrode related to the sample C1 from the surface side thereof are shown in FIGS. 6A and 6B (both figures are simply combined). It is referred to as "FIG. 6").

As can be seen from FIG. 6A, it was clarified that the ohmic electrode according to Sample 1 has a structure in which Mo, Ni, C and Si are uniformly distributed. This tendency was similar near the junction interface of SiC.

On the other hand, as can be seen from FIG. 6B, in the ohmic electrode related to sample C1, Mo and C increased rapidly in the vicinity of the junction interface of SiC, and it was confirmed that a large amount of MoC was present. This result is also consistent with the presence of MoC in blocks, as shown in FIG.

(4) 3DAP
FIG. 7 shows a 3DAP image and an element map image obtained by analyzing the ohmic electrode of ^{sample 1 (energy density: 2.7 J / cm 2} ) using an atom probe field ion microscope (LEAP4000XSi manufactured by AMETEK). The element map image is intended for a region having a thickness of 5 nm from the junction interface of SiC.

As is clear from FIG. 7, it was found that in the ohmic electrode of Sample 1, Mo, Ni, C and Si were uniformly distributed even in the vicinity of the SiC junction interface. If the size of the Mo carbide on the XY plane (the Z direction is the thickness direction) is dared to be estimated, it can be said that it is 5 nm or less in the vertical direction and 15 nm or less in the horizontal direction. Therefore, it was clarified that the Mo carbide in the ohmic electrode according to Sample 1 was at most 15 nm or less (less than). By the way, in the case of the ohmic electrode according to the sample C1, the size of the Mo carbide was about 25 nm in the vertical direction and 16 to 30 nm in the horizontal direction.

1 chip (semiconductor element)
10 SiC substrate 12 ohmic electrode

Claims (8)

A semiconductor device made of a semiconductor having an ohmic electrode.
The semiconductor is made of silicon carbide
The ohmic electrode comprises molybdenum carbide, molybdenum silicide and nickel silicide.
The molybdenum carbide is a semiconductor element uniformly distributed in the ohmic electrode. The semiconductor element according to claim 1, wherein the contact resistance of the ohmic electrode is 1 × 10 ^-4 Ω · cm ^{2 or less.} The semiconductor element according to claim 1 or 2, wherein the ohmic electrode contains more Ni than Mo in terms of atomic number ratio. The semiconductor element according to any one of claims 1 to 3, wherein the ohmic electrode has a thickness of 30 to 300 nm. A metal layer forming step of forming a metal layer in which a Mo layer and a Ni layer are laminated on at least one surface of a semiconductor.
An annealing step of irradiating the metal layer with a pulse laser to heat-treat the metal layer, and
It is a manufacturing method of a semiconductor element provided with
The pulse laser is a method for manufacturing a semiconductor element having an effective irradiation area of 2500 μm ² or more per pulse and an energy density of 2.35 J / cm ^{2 or more in the effective irradiation area.} The method for manufacturing a semiconductor device according to claim 5, wherein the pulsed laser has a beam profile having a flat central portion of the effective irradiation area. The method for manufacturing a semiconductor device according to claim 5 or 6, wherein the pulse laser has a pulse width of 10 to 100 nsec and an oscillation frequency of 10 kHz to 100 kHz. The method for manufacturing a semiconductor device according to any one of claims 5 to 7, wherein the metal layer contains more Ni than Mo in terms of atomic number ratio.