Patent Publication Number: US-2015069614-A1

Title: Semiconductor device and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-186709, filed Sep. 9, 2013 the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a semiconductor device and a manufacturing method thereof. 
     BACKGROUND 
     A power device (semiconductor device) is used in a wide range of fields such as industrial machinery, electric power generation and control, and consumer electronics. Among such power devices, an insulated gate bipolar transistor (IGBT) is widely used when a withstand (breakdown) voltage of 600 V or higher is required. In an IGBT, a trade-off curve between a saturation voltage and a switching loss is used as an index indicating IGBT characteristics. A saturation voltage can be reduced by reducing the thickness of a silicon portion. 
     On the other hand, in an IGBT for use with a high current density, a technique of providing a nickel layer on front and back surfaces of a chip in order to cool the device through both the front and back surfaces has been disclosed. However, when a nickel layer is incorporated into a chip, the chip may become warped. A chip easily warps when the thickness of a silicon portion is reduced for reduction of a saturation voltage. When a warp amount becomes large, it is hard to assemble devices because soldering the warped chips becomes difficult. Thus, by attempting to enhance a chip property (e.g., a reduction in saturation voltage) by reducing the thickness of a silicon portion, it becomes more difficult to assemble devices by increasing a chip warp amount. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment. 
         FIG. 2  is a flowchart illustrating a method of manufacturing a power semiconductor device according to the first embodiment. 
         FIGS. 3A and 3B  are X-ray diffraction curves of nickel layers in which the horizontal axis represents a value of diffraction angle (2θ) and the vertical axis represents an intensity of an X-ray diffraction. 
         FIG. 4A  is a diagram illustrating a semiconductor device according to an example embodiment. 
         FIG. 4B  is a diagram illustrating a semiconductor device according to a comparative example. 
         FIG. 5  is a graph depicting an effect of the thickness of a nickel layer on the back side on the warp amount of a chip, in which the horizontal axis represents the thickness of the nickel layer on the back side and the vertical axis represents the warp amount of the chip. 
         FIG. 6A  is a graph depicting an effect of the thickness of a silicon portion on the warp amount of a chip, in which the horizontal axis represents the thickness of the silicon portion and the vertical axis represents the warp amount of the chip. 
         FIG. 6B  is a graph depicting a relationship between the thickness of the silicon portion and a warp suppressing effect obtained by increasing the thickness of the nickel layer on the back side, in which the horizontal axis represents the thickness of the silicon portion and the vertical axis represents decrease in a warp amount. 
         FIG. 7  is a cross-sectional view illustrating a semiconductor device according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A semiconductor device of an embodiment includes a first metal layer disposed on a first surface of a semiconductor layer, or a portion thereof. The first metal layer comprises a first metal and at least a portion of the first metal layer is crystallized. A second metal layer is disposed on a second surface of the semiconductor layer. The second surface opposite the first surface, that is the first and second surfaces are on opposite sides of the semiconductor layer (e.g., front and back sides). The second metal layer also comprises the first metal and has at least a portion that is crystallized. 
     In general, according to one embodiment, there is provided a semiconductor device including: a semiconductor portion; a first metal layer that is provided on a top surface of the semiconductor portion and contains a first metal and of which at least a part is crystallized; and a second metal layer that is provided on a bottom surface of the semiconductor portion and contains the first metal and of which at least a part is crystallized. 
     According to another embodiment, there is provided a method of manufacturing a semiconductor device including: forming a first metal layer, which contains a first metal, on a top surface of a semiconductor portion; introducing dopants into a bottom surface of the semiconductor portion; performing a heat treatment to activate the dopants and to crystallize at least a part of the first metal layer; and forming a second metal layer, which contains the first metal, on the bottom surface of the semiconductor portion such that at least a part of the second metal layer is crystallized. 
     Hereinafter, exemplary embodiments will be described with reference to the drawings. 
       FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment. 
     As illustrated in  FIG. 1 , the semiconductor device  1  according to this first embodiment is an IGBT having a breakdown (withstand) voltage in a range of, for example, 600 V to 800 V. In addition, the external size of the semiconductor device  1  is a chip size having a length of one side of, for example, 10 mm (millimeter) to 15 mm. 
     In the semiconductor device  1  (hereinafter, also simply referred to as “device  1 ” or “chip  1 ”) of this example, a silicon portion  10  is provided as a semiconductor portion, a front electrode structure  20  is provided on a top surface of the silicon portion  10 , and a back electrode structure  30  is provided on a bottom surface of the silicon portion  10 . For example, a guard ring region (not specifically illustrated) including a field plate is provided around the chip as a termination region for improving a breakdown voltage. 
     In the silicon portion  10 , a p +  type collector layer  11 , an n +  type buffer layer  12 , an n −  type bulk layer  13 , a p type base layer  14 , and an n +  type emitter layer  15  are stacked in sequence. In addition, a trench gate electrode  16  is provided so as to pass through the n +  type emitter layer  15  and the p type base layer  14  from a top surface of the silicon portion  10  and reach the inside of the n −  type bulk layer  13 . The trench gate electrode  16  is a base electrode of the device  1 . A gate insulating film  17  including, for example, silicon oxide is provided around the trench gate electrode  16 . The silicon portion  10  is made of crystalline silicon (Si), and the thickness of the entire silicon portion  10  is, for example, 60 μm (micron) to 120 μm and is for example, 70 μm. The silicon portion  10  may be a single crystalline silicon. 
     The front electrode structure  20  comprises, in order from the silicon portion  10 , a titanium (Ti) layer  21  having a thickness of, for example, 30 nm (nanometer), a titanium nitride (TiN) layer  22  having a thickness of, for example, 150 nm (nanometer), an aluminum (Al) layer  23 , an aluminum-copper (AlCu) alloy layer  24 , a nickel layer  25  having a thickness of, for example, 5 μm, and a gold (Au) layer  26  having a thickness of, for example, 50 nm. The total thickness of the aluminum layer  23  and the aluminum-copper alloy layer  24  is, for example, 4 μm. The nickel layer  25  is formed using an electroless plating method, and includes nickel and phosphorus resulting in a nickel-phosphorus (Ni—P) compound. In the nickel layer  25 , a phosphorus content is, for example, 4 weight % to 10 weight %, and at least a portion of the nickel layer  25  is crystallized. In some embodiments, a large portion of the nickel layer  25  may be crystallized. The crystallized portion is not necessary to be placed uniformly. In the nickel layer, the crystallized portion can be localized, where a certain portion can be amorphous, the other portion can be strongly crystallized, and the other portion can be weakly crystallized. It is essential that the nickel layer contains at least a crystallized portion. 
     In this example, the front electrode structure  20  is configured to be an emitter electrode of the device  1 . The nickel layer  25  and the gold layer  26  are electrode pads which are soldered during packaging of the device  1 . In addition, the front electrode structure  20  may be provided with an interlayer dielectric film (not specifically illustrated). 
     In the back electrode structure  30  comprises, in order from the silicon portion  10 , an aluminum-silicon (AlSi) alloy layer  31  having a thickness of, for example, 200 nm, a titanium layer  32  having a thickness of, for example, 200 nm, a nickel layer  33  having a thickness of, for example, 1 μm, and a gold-silver (AuAg) alloy layer  34  having a thickness of, for example, 100 nm. The nickel layer  33  is formed using a sputtering method and is composed of substantially pure nickel, and at least a part of the nickel layer  33  is crystallized. In some embodiments, a large part of the nickel layer  33  can be crystallized. The crystallized portion is not necessary to be placed uniformly. In the nickel layer, the crystallized portion can be localized, where a certain portion can be amorphous, the other portion can be strongly crystallized, and the other portion can be weakly crystallized. In the embodiments, the nickel layer contains at least a crystallized portion. In this example, the back electrode structure  30  is a collector electrode of the device  1 . 
     It is desirable that the thickness of the nickel layer  33  of the back electrode structure  30  is greater than or equal to 15% of the thickness of the nickel layer  25  of the front electrode structure  20 . In the above-described example, since the thickness of the nickel layer  25  is 5 μm and the thickness of the nickel layer  33  is 1 μm, the thickness of the nickel layer  33  is 20% of the thickness of the nickel layer  25 . 
     Next, a method of manufacturing a semiconductor device will be described. 
       FIG. 2  is a flowchart illustrating the method of manufacturing a semiconductor device according to the first embodiment. Hereinafter, the description will be made with reference to  FIGS. 1 and 2 . 
     First, an n type silicon wafer is prepared as the silicon portion  10 . Hereinafter, for convenience of description, this silicon wafer will be referred to as “silicon portion  10 ”. In an initial state, the silicon portion  10  has two major surfaces which may be referred to as “a front surface” and “a back surface. 
     As illustrated in Step S 1 , various dopants are introduced from the front surface using an ion implantation method. After the ion implantation, in Step S 2 , a heat treatment of the semiconductor layer is performed to activate the dopants. As a result, the p type base layer  14  and the n +  type emitter layer  15  are formed inside the silicon portion  10 . 
     Next, as illustrated in Step S 3 , a trench structure is formed by a reactive ion etching method, the gate insulating film  17  is formed on the inside of the trench, and the trench gate electrode  16  is embedded into the trench. As a result, a trench gate structure is formed. 
     The other steps, including the ion implantation, the heat treatment, a wet etching process, a dry etching process, an insulating film formation, a conductive film formation and so on, are repeatedly performed to make a desired structure in the silicon portion  10  or on the silicon portion  10 . 
     Next, as illustrated in Step S 4 , the front electrode structure  20  on the front surface is formed on the silicon portion  10 . 
     Specifically in this example, using a sputtering method, the titanium layer  21  having a thickness of, for example, 30 nm is formed, the titanium nitride layer  22  having a thickness of, for example, 150 nm is formed, and the aluminum layer  23  and the aluminum-copper alloy layer  24  having a total thickness of, for example, 4 μm are formed. 
     Next, the nickel layer  25  having a thickness of, for example, 5 μm is formed by an electroless plating method using a plating solution containing phosphorus. Next, the gold layer  26  having a thickness, for example, 50 nm is formed by the electroless plating method. At this time, the nickel layer  25  is substantially amorphous. 
     Next, as illustrated in Step S 5 , a protective tape (not specifically illustrated) is attached onto a top surface of the front electrode structure  20  to protect the top surface thereof. 
     Next, as illustrated in Step S 6 , a back surface of the silicon portion  10  is ground and polished to decrease the thickness thereof to a predetermined value. Next, a portion of silicon portion  10  that may have been damaged by the grinding-and-polishing process is removed by wet etching. At this time, the thickness of the silicon portion  10  is, for example, 60 μm to 120 μm and is for example, 70 μm. Next, the protective tape can be peeled off at the end of this step, or can be peeled off after the other step. 
     Next, as illustrated in Step S 7 , various dopants are introduced from the back surface of the silicon portion  10  using an ion implantation method. As a result, the n +  type buffer layer  12  and the p + type collector layer  11  are formed inside the silicon portion  10 . 
     Next, as illustrated in Step S 8 , a heat treatment is performed to activate dopants that have been introduced into the silicon portion  10 . Before this heat treatment, the nickel layer is substantially amorphous. Due to this heat treatment, at least a part of the nickel layer  25  is crystallized. For example, a laser annealing method can be used as the heat treatment. A part of laser energy is carried by the silicon portion  10  and reach the nickel layer  25 . At this time, because the nickel layer  25  shrinks and the volume thereof is decreased, the nickel layer  25  applies a shrinking force to the top surface of the silicon portion  10 . This shrinking force acts such that the silicon wafer is warped to be downwardly convex. 
     Next, as illustrated in Step S 9 , the back electrode structure  30  is formed on the back surface of the silicon portion  10 . 
     Specifically in this example, by using a sputtering method, the aluminum-silicon alloy layer  31  having a thickness of, for example, 200 nm is formed, the titanium layer  32  having a thickness of, for example, 200 nm is formed, the nickel layer  33  having a thickness of, for example, 1 μm is formed, and the gold-silver alloy layer  34  having a thickness of, for example, 100 nm is formed. 
     Because the nickel layer  33  is formed using a sputtering method, at least a part (or a large part) of the nickel layer  33  is crystallized during the film formation. 
     When nickel is deposited on the titanium layer  32  and a part of it is crystallized, the deposited nickel shrinks in volume upon crystallization. Therefore, the nickel layer  33  applies a shrinking force to the back surface of the silicon portion  10 . This shrinking force acts such that the silicon wafer is warped to be upwardly convex. 
     In subsequent steps, the silicon wafer (silicon portion  10 ) with the front electrode structure  20  and the back electrode structure  30  can be divided into plural chips by dicing. As a result, the semiconductor device  1  according to this first embodiment is manufactured. 
     Next, the effects of this first embodiment will be described. 
     In a semiconductor device  1 , the thickness of the silicon portion  10  is, for example, 60 μm to 120 μm and is assumed as, for this specific example to be 70 μm. Since this thickness is relatively small for an IGBT having a breakdown voltage in a range of 600 V to 800 V, the balance between a saturation voltage and a switching loss is generally improved. For example, when the thickness of the silicon portion  10  is 80 μm, a saturation voltage is 2.0 V. However, when the thickness of the silicon portion  10  is 70 μm, a saturation voltage is decreased to 1.75 V. In this way, in an IGBT having a breakdown voltage in a range of 600 V to 800 V, a saturation voltage can be improved by 10% to 20% by reducing the thickness of the silicon portion  10  from 80 μm to 70 μm. 
     In addition, in the device  1 , the nickel layer  25  is formed above the silicon portion  10 , and electrode pads which are soldered during assembly are formed. The nickel layer  33  containing the same metal of nickel as that of the nickel layer  25  is formed below the silicon portion  10 . At least a part of each of the nickel layers  25  and  33  is crystallized. Therefore, the nickel layer  25  applies a shrinking force to the top surface of the silicon portion  10 , and the nickel layer  33  applies a shrinking force to the bottom surface of the silicon portion  10 . As a result, the shrinking force of the nickel layer  25  to warp the chip may be counteracted by the shrinking force of the nickel layer  33 , thereby suppressing the wrap amount of the chip. For example, in this example embodiment, when the length of one side of the chip is 10 mm, the warp amount of the chip is 80 μm. Typically, when a warp amount of the chip is less than or equal to 100 μm, assembly defects are not caused by chip warping, and a high yield of assembly can be obtained. 
     In addition, since at least a part of the nickel layers  25  and  33  is already crystallized, the crystallized portion of the nickel layers  25  and  33  are almost unchanged in subsequent soldering processes during assembly/packaging, and the warp amount of the chip is almost unchanged by the additional crystallization during soldering steps, or other subsequent steps. In this way, in the device  1 , the warp amount is small, and almost unchanged in the assembly process such as the soldering process. Therefore, an assembly process of the chip can be improved. 
     Accordingly, in the device  1 , even if the thickness of the silicon portion  10  is reduced to improve a trade-off between a saturation voltage and a switching loss, the warp amount of the chip may still be suppressed. That is, both chip characteristics and an assembly process can be simultaneously improved. 
     Furthermore, in this first embodiment, after the nickel layer  25  is formed using an electroless plating method in Step S 4  of  FIG. 2 , the heat treatment for activating the dopants is performed in Step S 8 . Therefore, a microstructure of the nickel layer  25  is substantially amorphous after the plating, but is crystallized by the heat treatment. In addition, in Step S 9 , the nickel layer  33  is formed using a sputtering method. Therefore, at least a part of the nickel layer  33  is crystallized after the film formation. In this way, according to this embodiment, the nickel layers  25  and  33  can be crystallized without performing a special crystallization treatment. 
     On the other hand, when the nickel layer  33  is formed using an electroless plating method and then a heat treatment is not performed thereon, the nickel layer  33  is substantially amorphous. In this case, the nickel layer  33  cannot generate a large shrinking force to act against the shrinking force of the nickel layer  25 , and the chip is warped to be downwardly convex. Therefore, for example, solder wettability deteriorates in the subsequent soldering process, and thus an assembly process deteriorates. 
     Whether the microstructure of the nickel layer is crystalline or amorphous can be determined by a 0-2θ method using X-ray diffraction (XRD). 
       FIGS. 3A and 3B  are diagrams illustrating X-ray analysis results of a nickel layer in which the horizontal axis represents a value of diffraction angle (2θ) and the vertical axis represents a diffraction intensity of an X-ray. 
     As illustrated in  FIG. 3A , when a nickel layer contains a crystallized portion, a peak of 2θ=44.45° indicating the (111) plane of nickel (Ni); and a peak of 2θ=51.88° indicating the (200) plane of nickel (Ni) are observed. The peak intensity becomes stronger with increasing the crystallized portion in the nickel layer. 
     On the other hand, as illustrated in  FIG. 3B , when a nickel layer is amorphous, an extremely broad peak having a weak intensity is observed around 2θ=40° to 50°, but a sharp peak indicating crystallinity is not observed. 
     Further, in this embodiment, the thickness of the nickel layer  33  on the back side is greater than or equal to 15% of the thickness of the nickel layer  25  on the front side. As a result, the warp amount of the chip may be more reliably suppressed. Hereinafter, this effect will be described using a test example. 
       FIG. 4A  is a diagram illustrating a semiconductor device according to an example, and  FIG. 4B  is a diagram illustrating a semiconductor device according to a reference example. 
     As illustrated in  FIG. 4A , the configuration of the device according to the example is the same as that of the device  1  illustrated in  FIG. 1 , and the thickness of the nickel layer  33  is 1 μm. In addition, as illustrated in  FIG. 4B , the configuration of the device according to the reference example is different from that of the device  1  illustrated in  FIG. 4A , in that the thickness of the nickel layer  33  is 700 nm. In this test example, samples illustrated in  FIGS. 4A and 4B ; and samples obtained by changing the thickness of each portion of the samples illustrated in  FIGS. 4A and 4B  were prepared, and the warp amount of each sample was measured. 
       FIG. 5  is a graph illustrating an effect of the thickness of a nickel layer (e.g., nickel layer  33 ) on the back side on the warp amount of the chip, in which the horizontal axis represents the thickness of the nickel layer on the back side and the vertical axis represents the warp amount of the chip. 
     As illustrated in  FIG. 5 , when the thickness of the silicon portion  10  was the same and the thickness of the nickel layer  33  on the back side was decreased, the warp amount of the chip was increased. In particular, when the thickness of nickel layer  33  was less than 750 nm, the warp amount of the chip was rapidly increased. 
     In the example illustrated in  FIG. 5 , the thickness of the nickel layer  25  on the front side was 5 μm. As illustrated in  FIG. 5 , when the thickness of the nickel layer  33  on the back side was greater than or equal to 15% of the thickness of the nickel layer  25  on the front side, for example, was greater than or equal to 750 nm, it is found that the warp amount of the chip was as small as less than or equal to 100 μm. As a result, an assembly process was improved. On the other hand, when the thickness of the nickel layer  33  was 700 nm, the warp amount of the chip was 120 μm. As a result, an assembly process was deteriorated. 
     On the other hand, when the thickness of the nickel layer  33  was greater than or equal to 1 μm, the chip warping suppressing effect was saturated—that is, chip warping was not significantly improved for thicknesses of nickel layer  33  greater than or equal to 1 μm. In addition, when the thickness of the nickel layer  33  was excessively great, nickel burrs were formed during dicing, and there were some cases where defects occurred on the chips as a result of the excess nickel layer thicknesses. Therefore, it is preferable that the thickness of the nickel layer  33  generally be less than or equal to 1.5 μm. But, even if the thickness of the nickel layer  33  is greater than or equal to 1.5 μm, the formation of burrs can be prevented by removing the nickel layer  33  from a dicing line area before dicing—although this increases the number of overall process steps in the manufacturing of the device  1 . 
     Based on the above results, it is preferable that the thickness of the nickel layer  33  on the back side be greater than or equal to 15% of the thickness of the nickel layer  25  on the front side and generally less than or equal to 1.5 μm overall. 
     Alternatively, if the thickness of the nickel layer  33  on the back side is fixed and the thickness of the nickel layer  25  on the front side is varied, the same effect can be obtained as the warp amount of the chip also depends on the thickness of the nickel layer  25  on the front side. 
     As described above, when the thickness of the nickel layer  25  was 5 μm, the warp amount of the chip was approximately 80 μm. When the thickness of the nickel layer  25  was 6 μm, the warp amount was increased to approximately 100 μm. On the other hand, when the thickness of the nickel layer  25  was 4 μm, the warp amount was decreased to approximately 60 μm. In this way, even when the thicknesses of the nickel layers  33  on the back side of various chips are the same, the warp amount of the chip  1  can be reduced by reducing the thickness of nickel layer  25 . 
     For packaging using the device  1 , the nickel layer  25  is generally soldered, but nickel is usually consumed by an alloying reaction with a solder. Therefore, if the nickel layer  25  is excessively thin, a solder reaches the aluminum-copper alloy layer  24  and the aluminum layer  23 . As a result, the reliability of the device  1  deteriorates. Therefore, in order to secure sufficient reliability, the thickness of the nickel layer  25  is preferably greater than or equal to 4 μm and more preferably greater than or equal to 5 μm. 
     In addition, since the nickel layer  25  is formed by an electroless plating method using a plating solution containing phosphorus, the nickel layer  25  contains approximately several percentages of phosphorus. On the other hand, since the nickel layer  33  is formed by a sputtering method, the purity of nickel is high. The shrinking force of the nickel layer  33  having a high purity of nickel is generally greater than that of the nickel layer  25  having a lower purity of nickel. Therefore, even if the nickel layer  33  is thinner than the nickel layer  25 , the nickel layer  33  can act against the shrinking force of the overall thicker nickel layer  25 . 
     Further, in this example embodiment, the thickness of the silicon portion  10  is 60 μm to 120 μm. As a result, by controlling a ratio of the thicknesses of the nickel layers  25  and  33 , the chip warping suppressing effect can be significantly large. 
       FIG. 6A  is a graph illustrating an effect of the thickness of a silicon portion  10  on the warp amount of a chip, in which the horizontal axis represents the thickness of the silicon portion  10  and the vertical axis represents the warp amount of the chip, and  FIG. 6B  is a graph illustrating a relationship between the thickness of the silicon portion  10  and a warping suppressing effect obtained by increasing the thickness of the nickel layer  33  on the back side, in which the horizontal axis represents the thickness of the silicon portion  10  and the vertical axis represents a decrease value in warp amount. “Decrease value in warp amount” is obtained from  FIG. 6A , and is a value obtained by subtracting the warp amount of the chip in which the thickness of the nickel layer  33  is 700 nm from the warp amount of the chip in which the thickness of the nickel layer  33  is 1 μm. 
     As illustrated in  FIGS. 6A and 6B , in a region A, the silicon portion  10  is thick, and the warp amount of the chip is already small. Therefore, the chip warping suppressing effect obtained by increasing the thickness of the nickel layer  33  on the back side is relatively small. 
     In a region B, the silicon portion  10  is thinner than that of the region A, and the warping of the chip occurs more readily. Therefore, the chip warping suppressing effect obtained by increasing the thickness of the nickel layer  33  on the back side is relatively large. 
     In a region C, the silicon portion is thinner, and the warping of the chip is extremely large. Therefore, the chip warping suppressing effect obtained by increasing the thickness of the nickel layer  33  on the back side is relatively small. Based on the above-described results, it is found that the chip warping suppressing effect obtained by increasing the thickness of the nickel layer  33  on the back side is relatively large in the region B. 
     As illustrated in  FIG. 6A , when the thickness of the silicon portion  10  is greater than or equal to 60 μm, the warp amount of the chip can be controlled to be less than or equal to 100 μm, thereby reliably realizing an improved assembly process. On the other hand, as illustrated in  FIG. 6B , when the thickness of the silicon portion  10  is less than or equal to 120 μm, the chip warping suppressing effect obtained by increasing the thickness of the nickel layer  33  on the back side is relatively large. Accordingly, when the thickness of the silicon portion is 60 μm to 120 μm, the effect of this embodiment is relatively large. 
       FIG. 7  is a cross-sectional view illustrating a semiconductor device according to a second embodiment. 
     As illustrated in  FIG. 7 , a semiconductor device  2  according to this second embodiment is a fast recovery diode (FRD). 
     In the device  2 , a silicon portion  40  is provided as a semiconductor portion, the front electrode structure  20  is provided above the silicon portion  40 , and the back electrode structure  30  is provided below the silicon portion  40 . An insulating film  50  is provided around the front electrode structure  20 . The layer structures of the front electrode structure  20  and the back electrode structure  30  are the same as those of the first embodiment. 
     The silicon portion  40  includes a high-concentration n type cathode layer  41  having a relatively high donor concentration and a low-concentration n type layer  42  having a relatively low donor concentration. Layer  41  and layer  42  disposed on each other. In addition, in a top surface of the low-concentration n type layer  42 , high-concentration p type anode layers  43  having a relatively high acceptor concentration and low-concentration p type anode layers  44  having a relatively low acceptor concentration are alternately arranged along a direction parallel to the top surface. 
     In this embodiment, similarly to the first embodiment, the warping of the chip can be suppressed by crystallizing at least a part of the nickel layer  25  on the front side and at least a part of the nickel layer  33  on the back side. In addition, this effect can be more reliably obtained by controlling the thickness of the nickel layer  33  to be greater than or equal to 15% of the thickness of the nickel layer  25 . The configuration, manufacturing method, operation, and effects of this second embodiment, other than the above-described features, are similar to those of the first embodiment. 
     In the above-described exemplary embodiments, the example in which the nickel layer is provided on both the front electrode structure  20  and the back electrode structure  30  is described. However, the metal layer to be provided on both the front and back sides is not limited to the nickel layer. For example, even when another metal layer such as an aluminum layer or a copper layer is provided, the above-described effect can also be obtained. If an aluminum layer is provided on the front electrode structure  20  instead of the nickel layer  25 , a pure aluminum layer may be provided on the back electrode structure  30  instead of the nickel layer  33 . However, an aluminum-silicon (AlSi) alloy layer or an aluminum-copper (AlCu) alloy layer may be provided on the back electrode structure  30 . This is because the hardness of AlSi and AlCu which are alloys is higher than that of a high-purity aluminum, and thus AlSi and AlCu can act against the shrinking force of the aluminum layer on the front side. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.