Patent Application
20250191916
The present disclosure relates to a semiconductor apparatus and a method for producing the same and, more particularly, to a semiconductor apparatus suitable for use in a power converter, such as an inverter, and a method for producing the same.
As a power semiconductor apparatus for use in, e.g., an inverter, an apparatus having a structure such that a front electrode of a semiconductor device is joined to an external electrode with solder has been known. If a semiconductor apparatus having such a structure is subjected to heat stress from a power cycle, a thermal cycle, or the like, thermal stress arising from a difference in coefficient of linear expansion between materials is generated. As a result, a material interface peels off at an outermost peripheral portion of a front electrode. When the peeling develops, the problem of, e.g., a front electrode crack in a semiconductor device occurs.
PTL 1 and PTL 2 disclose a technique for forming a covering film made of polyimide at a material boundary portion on a front electrode outermost periphery as an approach for preventing a front electrode crack in a semiconductor device.
However, formation of a polyimide film on a front electrode complicates production steps, causing a problem of an increase in production costs. The formation also causes problems which reduce ease of assembly, such as an increase in the amount of chip warpage due to polyimide film stress and reduction in solder joinability due to polyimide hardening heat treatment.
To solve the above-described problems, the present disclosure modifies an outermost surface of a front electrode by laser irradiation. An object of the present disclosure is to provide a semiconductor apparatus and a method for producing the same capable of achieving reduction in semiconductor apparatus costs and enhancement in ease of assembly.
The first aspect of the present disclosure is preferably a semiconductor apparatus comprising a front electrode solder-joined portion, solder, a resin sealing material, and a semiconductor device which is mounted underneath the front electrode solder-joined portion, wherein the front electrode solder-joined portion has a first laser-modified portion whose outermost surface is oxidized and roughened, and a first solder-joined region which is joined to the solder, and the laser-modified portion is covered with the resin sealing material.
The second aspect of the present disclosure is preferably a method for producing a semiconductor apparatus including a front electrode solder-joined portion, solder, and a resin sealing material, comprising a step of forming a laser-modified portion by pulse irradiation of a part of the front electrode solder-joined portion with laser light, a step of performing solder joining targeted at the front electrode solder-joined portion, and a step of performing resin sealing using the resin sealing material so as to cover the laser-modified portion.
According to the aspects of the present disclosure, it is possible to provide a semiconductor apparatus and a method for producing the same capable of achieving reduction in semiconductor apparatus costs and enhancement in ease of assembly by modifying an outermost surface of a front electrode by laser irradiation.
The semiconductor device 2 has a front electrode solder-joined portion 6 on the front. The front electrode solder-joined portion 6 is formed of a metal film. The front electrode solder-joined portion 6 is an electrode through which a principal current for the semiconductor device 2 is to be passed. The front electrode solder-joined portion 6 is connected to an external electrode 10 which is formed of a metal, such as Cu, using the conductive joining material 5 that is solder.
The semiconductor device 2 has a front electrode wire pad portion 8 on the front. The front electrode wire pad portion 8 is formed of a metal film. The front electrode wire pad portion 8 is used to drive and control the semiconductor device 2. The front electrode wire pad portion 8 is connected to an external electrode 14 which is electrically separated from the front electrode solder-joined portion 6, using a conductive wiring material 12, such as an aluminum wire.
The whole of the semiconductor apparatus 100 is sealed with a sealing material 16 to ensure electrical insulation, environment resistance, and the like. The sealing material 16 is formed of an insulating material which is made of, e.g., epoxy resin.
If a semiconductor apparatus having the above-described structure is subjected to heat stress from a power cycle, a thermal cycle, or the like, thermal stress arising from a difference in coefficient of linear expansion between materials is generated at an end of the front electrode solder-joined portion 6 of the semiconductor device. Upon the generation, interfacial peeling occurs at a front electrode, the conductive joining material, and the sealing material. When the interfacial peeling develops due to repeated thermal stress, a crack appears in a metal film, of which the front electrode of the semiconductor device is formed. To prevent such a metal film crack, a technique for forming a covering film made of polyimide at a front electrode end of a semiconductor device is disclosed in PTL 1 and PTL 2. A metal film crack is suppressed by forming a stress relaxation layer or enhancing adhesion to an underlying metal film by the technique.
However, formation a polyimide film causes problems in terms of production costs and ease of assembly. For this reason, in the semiconductor apparatus 100 according to the present embodiment, an outermost surface of a front electrode is irradiated with laser instead of forming a polyimide film. With this irradiation, a laser-modified portion 18 is formed. This enhances the degree of adhesion at an interface between the front electrode and the sealing material and suppresses peeling at each material interface.
A production process for the semiconductor apparatus 100 will be described.
A laser-irradiated portion 25 refers to a region to be irradiated with laser light 26 at an outermost layer of the front electrode solder-joined portion 6. A machining region 27 refers to a circular region which can be machined in one pulse irradiation operation with the laser light 26. A covering film 28 is formed at a position in contact with ends of the second metal film 22 and the third metal film 24 on the first metal film 20.
A semiconductor substrate 30 is arranged underneath the front electrode solder-joined portion 6. The semiconductor substrate 30 includes trenches 32 filled with poly-Si. Oxide films 34 are formed on the trenches 32.
A laser-modified portion 36 refers to a region which is irradiated with the laser light 26. In the laser-modified portion 36, thermal energy from the laser light 26 causes solid diffusion of Au and Ni. That is, Au contained in the third metal film 24 diffuses into the second metal film 22, thereby forming a Ni oxide layer at a superficial layer.
In the laser-modified portion 36, the superficial layer is concentrically scraped off by irradiation with pulses of the laser light 26. A region which is to be machined in one pulse irradiation operation is the machining region 27 described earlier. The processing is repeated until the whole of the laser-modified portion 36 is machined. As a result, a surface is roughened. The roughening increases an area of contact between the solder and the front electrode to increase surface tension at an interface between the solder and the front electrode. Thus, wetting with the solder is further inhibited.
At the time of laser machining, a laser species is selected in consideration of characteristics of a material to be irradiated with laser. In the present disclosure, a laser species suitable for Au machining is used. In this case, a size which can be machined in one pulse irradiation operation is a diameter of about several tens of μm because of a laser wavelength. However, if an area of a Ni oxide layer formed by laser irradiation is small, the Ni oxide layer is reduced by use of a solder material high in reduction power for Ni oxidation and a solder joining process. As a result, solder may spread.
In the present embodiment, the machining region 27 are formed to be multi-ringed, thereby performing machining such that the machining region 27 surround an outermost periphery of the front electrode. Since a given or larger area can be secured for the Ni oxide layer, a margin for a reduction action on the Ni oxide layer is increased. As a result, in a solder joining step, the solder does not spread to the laser-modified portion 36. After the solder joining, a Ni oxide layer of a given or larger size remains at the laser-modified portion 36 in an exposed state.
As described above, the present embodiment allows prevention of a metal film crack without formation of a polyimide film. Production steps, such as photoengraving, dispensation and application, and heat treatment, which are needed in the conventional art to form a polyimide film can be omitted. As a result, the costs for production of semiconductor apparatuses can be reduced. Since stress from a polyimide film and polyimide film hardening heat treatment are unnecessary, the prospect of reduction in the amount of chip warpage and enhancement in solder wettability is offered. As a result, enhancement in ease of assembly and improvement in quality can be expected. Additionally, surface roughness and a machining width of the laser-modified portion 36 can be adjusted by laser irradiation conditions. That is, a solder anti-wetting action and adhesion strength to the sealing material can be purposefully controlled, which enhances flexibility in semiconductor apparatus design.
A second embodiment is different from the first embodiment in that machining region 27 are arranged at fixed intervals. If the machining region 27 are arbitrarily arranged in the semiconductor apparatus according to the first embodiment, an unirradiated portion appears at a part in the laser-modified portion 36. In the unirradiated portion, a surface of the third metal film 24 remains exposed. For this reason, at the time of sealing with the resin sealing material 42, an interface between the third metal film 24 and the resin sealing material 42 is formed in each of some regions to reduce adhesion strength.
Under the above-described circumstances, in the present embodiment, the machining region 27 are arranged at fixed intervals so as to overlap, thereby preventing appearance of an unirradiated portion. As a result, an interface between a laser-modified portion 36 and a resin sealing material 42 is uniformly formed. This enhances adhesion strength and allows achievement of increased reliability of a semiconductor apparatus.
A third embodiment is different from the previous embodiments in that a width of a laser-modified portion 36 is set to at least 100 μm. In the semiconductor apparatus according to the first embodiment, the Ni oxide layer at the surface of the laser-modified portion 36 may be reduced in the solder joining step. In this case, solder spreads across the laser-modified portion 36 to narrow a width of the laser-modified portion 36. If the width narrows by a given or larger amount, a region where the laser-modified portion 36 and the resin sealing material 42 are in contact narrows after sealing with the resin sealing material 42. This becomes a factor in reducing adhesion strength.
Generally, if reduction power for Ni oxidation in a solder joining step is high, a phenomenon is confirmed in which Ni oxidation is reduced in a region measuring several tens of μm of a laser-modified portion in a solder major joined portion of a front electrode and solder spreads. For this reason, formation is performed such that a width of a laser-modified portion is set to at least 100 μm. Since a region where the laser-modified portion and a sealing material are in contact can be stably secured, adhesion strength is enhanced, and increased reliability of a semiconductor apparatus can be achieved.
In the semiconductor apparatus according to the first embodiment, energy of laser to be applied needs to be increased to enhance a laser modification effect. Upon the increase, influence of the energy reaches a non-laser-irradiated region, and the second metal film 22 is deposited. As a result, a surface may be oxidized to reduce solder wettability.
Under the above-described circumstances, in the present embodiment, an energy density of laser to be applied is reduced only in a region in contact with a front electrode solder-joined portion 6. That is, an energy density of laser to be applied to a laser-modified portion 36a is set smaller than that for a laser-modified portion 36b. This makes it possible to minimize influence of reduction in solder wettability of a non-laser-irradiated region. Even if the energy density is reduced, a surface of the laser-modified portion 36a becomes an Ni oxide layer. Thus, solder does not spread across the laser-modified portion 36a in a solder joining step. Adhesiveness of an interface between the laser-modified portion 36b and a resin sealing material 42 is not impaired by maintaining the energy density for laser to be applied to the laser-modified portion 36b.
A fifth embodiment is different from the previous embodiments in that laser light absorptance is set different for each metal film. Consider a case where laser irradiation energy is too high or laser light absorptance of the first metal film 20 or the second metal film 22 is higher than that of the third metal film 24 in the semiconductor apparatus according to the first embodiment. In this case, since a metal film melts into a semiconductor substrate bulk due to heat generated by laser irradiation, an abnormality occurs in electrical characteristics of a semiconductor device.
Under the above-described circumstances, in the present embodiment, a front electrode is formed such that laser light absorptance of a third metal film 24 is higher than those of a first metal film 20 and a second metal film 22. As a result, when an outermost surface of the third metal film 24 is irradiated with laser, heat generated in the first metal film 20 and the second metal film 22 that are underlayers reduces. This can reduce the risk of a metal film melting with heat from laser irradiation, damage to a semiconductor device can be lowered.
A sixth embodiment is different from the previous embodiments in that the sum of thicknesses of a first metal film 20 and a second metal film 22 is set to at least 1 μm and that a thickness of a third metal film 24 is set not more than 0.2 μm.
In the semiconductor apparatus according to the first embodiment, an abnormality occurs in electrical characteristics of the semiconductor device due to heat generated by laser irradiation, as described earlier. Under the circumstances, in the present embodiment, the sum of the thicknesses of the first metal film 20 and the second metal film 22 is set to at least 1 μm, and the thickness of the third metal film 24 is set not more than 0.2 μm. Since this setting makes a film to be subjected to laser machining thin and a film not to be subjected to laser machining thick, a Ni oxide layer can be formed with low power. As a result, damage to a semiconductor device from heat at the time of laser machining can be lowered.
In the semiconductor apparatus according to the first embodiment, an abnormality occurs in electrical characteristics of the semiconductor device due to heat generated by laser irradiation, as described earlier. Under the circumstances, in the present embodiment, the invalid cell region 44 is arranged immediately below the laser-modified portion 36. In the invalid cell region 44, trenches 32 and oxide films 34 which are needed to function as a semiconductor device are not mounted. Since the numbers of trenches 32 and oxide films 34 that are influenced by heat at the time of laser machining reduce, damage to a semiconductor device function in the whole semiconductor apparatus can be lowered.
In the semiconductor apparatus according to the first embodiment, an abnormality occurs in electrical characteristics of the semiconductor device due to heat generated by laser irradiation, as described earlier. Under the circumstances, in the present embodiment, the barrier metal layer 46 is provided at the boundary between the first metal film 20 and the oxide film 34. Since Al, of which the first metal film 20 is formed, can be prevented from solid-diffusing into a semiconductor substrate interface, damage to a semiconductor device from heat at the time of laser machining can be lowered.
A ninth embodiment is different from the previous embodiments in that a covering film 28 is formed so as to increase reflectance for laser light 26. In the semiconductor apparatus according to the first embodiment, the covering film 28 is arranged at an outermost peripheral portion of the front electrode solder-joined portion 6. An oxide film, an insulating film, or the like may be arranged underneath the covering film 28. In this case, a surface of the oxide film or the insulating film may be damaged by irradiation with the laser light 26.
Under the above-described circumstances, in the present embodiment, the covering film 28 is formed so as to increase the reflectance for the laser light 26. A polyimide film with a low refractive index can be given as an example of a film with a high reflectance for the laser light 26. Since damage to an oxide film or an insulating film from the laser light 26 can be minimized, improvement in quality of the semiconductor apparatus can be expected.
Since the whole region of the laser-modified portion 36 is irradiated with the laser light 26 in the semiconductor apparatus according to the first embodiment, a laser irradiation time lengthens. Under the circumstances, in the present embodiment, the semiconductor device information formation area 48 is provided at a part in the laser-modified portion 36. Only a portion where semiconductor device information is to be printed is irradiated with laser in the semiconductor device information formation area 48, unlike in the laser-modified portion 36. Since the number of irradiation pulses of laser reduces, a laser irradiation time can be shortened.
Besides semiconductor device information, for example, a mark indicating position information of the semiconductor device can be printed. In this case, in an assembly step of a semiconductor apparatus, the position information of the semiconductor device can be recognized on the apparatus side. Thus, visibility of the semiconductor device is enhanced, and enhancement of ease of assembly of the semiconductor apparatus can be expected.
Since the front electrode solder-joined portion 6 is continuous in the semiconductor apparatus according to the first embodiment, thermal stress may be generated at a solder joining interface. Under the circumstances, in the present embodiment, a solder joining interface is divided into a plurality of pieces by applying laser in a striped pattern, as in a laser-modified portion 36c. This allows dispersion of thermal stress generated in a solder joining interface at the time of a power cycle test.
Since the front electrode solder-joined portion 6 is continuous in the semiconductor apparatus according to the first embodiment, thermal stress may be generated at a solder joining interface. Under the circumstances, in the present embodiment, a solder joining interface is divided into a plurality of pieces by applying laser in a grid pattern, as in a laser-modified portion 36d. This allows dispersion of thermal stress generated in a solder joining interface at the time of a power cycle test.
In the semiconductor apparatus according to the first embodiment, when solder joining is performed in a step of assembling the semiconductor device 2, solder may spatter on the front electrode wire pad portion 8. When bonding is subsequently performed with, e.g., an aluminum wire, poor aluminum wire joining may occur.
Under the above-described circumstances, in the present embodiment, laser is applied to a surface of the front electrode wire pad portion 8 to form a laser-modified portion 36e. Since wetting of the front electrode wire pad portion 8 with solder can be suppressed in the case of a solder spatter, yield enhancement in an assembly step can be expected.
A semiconductor device according to the fourteenth embodiment includes a back electrode 52. The back electrode 52 has a fourth metal film 54 and a fifth metal film 56. The back electrode 52 is joined to a radiator plate which is formed of a conductive material, such as Cu, with solder in an assembly step. As a result, a solder-joined region is formed to follow a shape of the back electrode 52.
Consider a case where a thermal cycle test which applies thermal stress is conducted on the above-described semiconductor apparatus. In this case, thermal stress concentrates on the back electrode and the solder-joined region due to a difference in coefficient of linear expansion between materials, and a corner portion of the solder-joined region peels off. Since the peeling causes a crack in the solder, the semiconductor apparatus may deteriorate. Under the circumstances, in the present embodiment, laser is also applied to the back electrode 52.
When laser is applied to a part on the back electrode side, a laser-modified portion 36f is formed at a superficial layer of the fourth metal film 54. Although solder joining is subsequently performed, solder 38 does not spread on the laser-modified portion 36f. Since the solder 38 spreads over a portion other than the laser-modified portion 36f, solder joining is performed. At this time, an intermetallic compound layer 58 is formed. When resin sealing is subsequently performed, the laser-modified portion 36f is covered with a resin sealing material 42.
Note that although an example in which laser irradiation is performed at a back electrode such that each corner portion of an unirradiated portion is rounded is illustrated here, laser irradiation may be performed in another region such that each corner portion of an unirradiated portion is rounded.
A fifteenth embodiment is different from the previous embodiments in that an outermost peripheral portion of a semiconductor substrate 30 is irradiated with laser. In the semiconductor apparatus according to the first embodiment, shearing stress from the resin sealing material 42 is applied to an outermost periphery of the semiconductor device 2 when thermal stress, such as a thermal cycle, is applied. Upon the application, an interface between the resin sealing material 42 and the semiconductor device 2 may peel off.
Under the above-described circumstances, in the present embodiment, the outermost peripheral portion of the semiconductor substrate 30 is irradiated with laser. With this irradiation, interface adhesion strength between the resin sealing material 42 and the semiconductor substrate 30 is enhanced. This allows achievement of increased reliability of a semiconductor apparatus.
A laser-modified portion 36g is formed by laser irradiation at the outermost peripheral portion of the semiconductor substrate 30. The covering film 28 does not cover an outermost peripheral portion of a semiconductor device 2. For this reason, in a subsequent resin sealing step, the interface adhesion strength between the resin sealing material 42 and the semiconductor substrate 30 is enhanced by effects of the laser-modified portion 36g.
A seventeenth embodiment is different from the previous embodiments in that a semiconductor substrate 30 is formed of silicon carbide.
A currently most widely available material for a semiconductor substrate is silicon. Silicon carbide is 2.6 times larger in modulus of longitudinal elasticity and 1.5 times larger in coefficient of linear expansion than a silicon substrate, and adhesion strength to a resin sealing material is ⅔ or less that of the silicon substrate. For this reason, if a power cycle test is conducted on a conventional semiconductor apparatus using a silicon carbide substrate, interfacial peeling between a sealing material and a semiconductor device surface is likely to occur. That is, reliability of the semiconductor apparatus may reduce.
However, in a semiconductor apparatus according to the present disclosure, adhesiveness between a sealing material and a semiconductor device surface is expected to be enhanced by roughening by laser modification. For this reason, even if a silicon carbide substrate is used, increased reliability of the semiconductor apparatus can be expected.
In a conventional semiconductor apparatus, a covering film, such as a polyimide film, is used as a stress buffer layer. For this reason, a width of the stress buffer layer needs to be determined with consideration of not only material-induced features, such as viscosity and photosensitive characteristics but also coater-induced variation in accuracy, such as an application position, an air pressure, and an application amount.
However, in the semiconductor apparatus according to the first embodiment, dimensions of a machining region can be determined only with consideration of laser machining accuracy. That is, machining variation can be reduced compared with a conventional semiconductor apparatus. For this reason, reduction in semiconductor apparatus costs can be expected more than ever before by use of expensive silicon carbide for a semiconductor substrate of the present disclosure.
Note that although a case where a semiconductor substrate is formed of silicon carbide is illustrated in the present embodiment, the semiconductor substrate may be formed of a wide-bandgap semiconductor with a wide bandgap, like silicon carbide. An example of the wide-bandgap semiconductor is a GaN-based material or diamond.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/023813 | 6/14/2022 | WO |
