JP5955183B2 - Die bond bonding structure of semiconductor element and die bond bonding method of semiconductor element - Google Patents

Description

  The present invention relates to a bonding structure applied when a semiconductor element is bonded to a substrate. Further, the present invention relates to a method for die-bonding a semiconductor element to a substrate.

  In die bonding bonding of various semiconductor elements to a substrate, a bonding material is bonded to either the semiconductor element or wiring on the substrate, the semiconductor element is placed on the substrate, and then heated to a bonding temperature according to the bonding material. We are joining. As this joining member, one using a brazing material such as an AuSn brazing material has been widely known. Also, in recent years, it has been required to reduce the bonding temperature in order to prevent fluctuations in the characteristics of semiconductor elements due to thermal stress generated during cooling after bonding. A die-bonding method using a metal paste in which a metal powder is dispersed in an organic solvent is known (Patent Documents 1 and 2). Metal paste has been widely used in recent years because it is easy to handle and can handle fine wiring.

JP-A-11-26480 JP 2002-158390 A

  The die-bonding method using the above brazing material or metal paste as the bonding material can obtain sufficient bonding strength by optimizing processing conditions such as appropriate selection of the bonding material and heating temperature setting, and also has good durability. It is supposed to be.

  However, as the number of application examples of die bonding to semiconductor device manufacturing processes increases, the problem of poor bonding is gradually pointed out. According to the study by the present inventors, there is a tendency that a junction defect is particularly found in a semiconductor device that receives a high load under a high temperature environment, such as a semiconductor device for power conversion / control called a so-called power device. . This bonding defect is a damage such as a crack in the bonded portion (bonding material), peeling of the bonded portion, and the like, and is not seen immediately after bonding, but is a change that occurs over time in the process of using the device.

  The present invention is based on the above-described problems, and provides a die bond bonding structure in which a bonding defect hardly occurs in a semiconductor device, particularly, a device used in a high temperature environment such as a power device. Also provided is a die bonding method for forming this bonded structure.

  In order to solve the above-mentioned problems, the present inventors first examined the cause of occurrence of a bonding defect that occurs in a conventional die bond bonding. As a cause of this occurrence, attention was paid to the diffusion of copper (Cu) constituting the wiring on the substrate in a high temperature environment. When copper diffuses into the bonding material, when the bonding material is a brazing material (for example, Au—Sn brazing material), the copper mainly spreads through the grain boundary of the brazing material alloy as a diffusion path. And the intensity | strength fall in a grain boundary arises and it becomes easy to produce a crack even if it is low stress. In addition, due to copper diffusion, the composition of the bonding material varies, and even the separation at the bonding surface between the wiring and the bonding material occurs.

  Further, when a metal paste is used as the bonding material, the influence of copper diffusion becomes more serious. This is because the material structure of the joint formed by the metal paste is a sintered structure of the metal powder, and is a porous material having a small space where the surface of the metal powder is exposed in the joint. In such a porous sintered structure, since the bonded metal powders are firmly bonded to each other, there is no problem in the original mechanical strength and there is no risk of defects. However, when copper diffusion can occur, copper diffusion also occurs on the metal powder surface in addition to the grain boundaries. At this time, copper on the surface of the metal powder is oxidized to form an oxide. A joint having a sintered structure formed of a metal paste may cause a bond between metal powders even after bonding. However, if an oxide is formed on the surface of the metal powder, this bond is inhibited and causes voids. .

  Thus, in preventing defects in the joint, it is necessary to suppress the diffusion of copper, which is a constituent metal of the wiring on the substrate, into the joint. Therefore, the present inventors have examined an effective means for suppressing copper diffusion, and as a result, have applied a bonding structure including a metal thin film serving as a barrier layer between the wiring and the bonding material. As a most effective barrier layer, a two-layered barrier layer made of ruthenium (Ru) and tantalum (Ta) was conceived.

  That is, the present invention relates to a die bond bonding structure formed by bonding a copper or copper alloy wiring formed on a substrate and a semiconductor element by die bonding through a bonding material, and between the wiring and the bonding material. And a first barrier layer made of ruthenium formed on the wiring side and a second barrier layer made of tantalum formed on the first barrier layer. is there.

  Hereinafter, the present invention will be described in detail. A basic form of the present invention is a bonding structure in which two barrier layers are formed between a wiring on a substrate and a bonding material, as shown in FIG. Accordingly, the same substrate as that used in the related art is applied to the substrate, its wiring, and the bonding material and semiconductor element.

  The reason why ruthenium and tantalum are used as the first and second barrier layers is that the combination of these metal thin films is the most effective for suppressing the diffusion of copper from the study by the inventors. For example, even when platinum or palladium classified as the same noble metal as ruthenium is applied, there is no such copper diffusion preventing action. Further, the barrier layer needs to have a two-layer structure, and a single layer of only a ruthenium film or only a tantalum film does not exhibit a sufficient effect. Further, when the configurations of the first and second barrier layers are reversed, that is, even if a tantalum film is formed on the wiring side and a ruthenium film is formed thereon, there is no sufficient effect.

  Regarding the thickness of the first and second barrier layers, the thickness of the first barrier layer is preferably 5 to 200 nm, and the thickness of the second barrier layer is preferably 5 to 200 nm. If it is too thin, the copper diffusion suppressing effect as a barrier layer will not be exhibited. On the other hand, if it is too thick, the barrier layer formation cost increases, which is not preferable. The more preferable film thickness of each layer is such that the thickness of the first barrier layer is 10 to 100 nm and the thickness of the second barrier layer is 10 to 100 nm. The relationship between the thickness of the first barrier layer and the thickness of the second barrier layer is that the thickness of the second barrier layer is 0.5 to 1.5 times the thickness of the first barrier layer. preferable.

  As described above, the bonding structure according to the present invention is characterized in that a barrier layer is provided between the wiring and the bonding material, and the other structures are the same as those of the conventional structure.

  As for the substrate and its wiring, the substrate is not particularly limited, and may be a ceramic substrate such as SiC, which is used as a power device substrate, in addition to silicon normally used in semiconductor devices. In addition, since the present invention aims to suppress the diffusion of copper constituting the wiring, it is assumed that the wiring is made of copper or a copper alloy. However, the shape, dimensions, thickness, etc. are not limited. As the wiring material, in addition to pure copper and oxygen-free copper, alloys such as Al—Cu and Al—Si—Cu can be applied as copper alloys.

  When forming wiring on the substrate, a metal such as titanium (Ti), tungsten (W), molybdenum (Mo), or zinc (Zn) is used as an underlayer between the substrate and the wiring for the purpose of obtaining stable bonding strength. A thin film may be formed, but such a base layer may be provided (FIG. 1B). In addition, a natural oxide film may be formed on the wiring surface before the barrier layer is formed. However, a natural oxide film may be present between the wiring and the first barrier layer. Furthermore, a nickel film (electroless nickel film) may be formed on the surface of the wiring. This nickel film is formed for corrosion prevention and the like, and an effect as a barrier layer cannot be expected. Therefore, the bonding structure of the present invention is also effective for a substrate on which a wiring having such a nickel film is formed (FIG. 1C). The underlayer and the electroless nickel film may be present at the same time.

  A structure similar to the conventional one is basically applied to the bonding material for bonding the wiring on the substrate and the semiconductor element. Further, the shape, size, and thickness of the bonding material are not particularly limited, and are set according to the semiconductor element to be die-bonded. In addition to Au—Sn brazing material, brazing materials such as Au—Si brazing material and Au—Ge brazing material may be applied as the joining material. The present invention is used to prevent defects in these joining materials. Is useful.

  However, the present invention particularly exhibits the result when it is formed of a metal paste as a bonding material. As described above, the influence of the copper diffusion from the wiring is larger in the bonding material formed of the metal paste. Here, the bonding material formed of the metal paste has a sintered structure of gold powder and silver powder having a purity of 99.9% by mass or more, which constitutes the metal paste. This sintered structure exhibits a porous structure formed by point contact or necking of metal powders, and has a density of 0.45 to 0.95 times the density of the bulk material of the constituent metal.

  Next, a die bonding method for a semiconductor element using the bonding structure according to the present invention will be described. This die bonding method is a method in which a semiconductor element is die-bonded to a wiring made of copper or a copper alloy formed on a substrate, and a first barrier layer made of ruthenium on the wiring side on the substrate, and the first barrier layer A second barrier layer made of tantalum is formed on the substrate, the semiconductor element is placed on the substrate via a bonding material, and is heated and bonded.

  As described above, the substrate and the wiring are the same as those used in the prior art, and there is no particular requirement in the present invention. In addition, the present invention is also useful in the case where an underlayer is formed on a substrate or an electroless nickel film is formed on a wiring, but the presence or absence of the formation process and conditions are not particularly limited.

  As a method for forming each barrier layer, a general thin film forming technique can be applied. Examples of the physical vapor deposition method include a vacuum vapor deposition method and a sputtering method. As the chemical vapor deposition method, a CVD method or the like can be applied. In these methods, the film thickness can be easily adjusted, and a barrier layer having a required film thickness can be formed. In order to maintain the adhesion of the barrier layer, the oxygen concentration in the atmosphere is preferably 50 ppm or less in the barrier layer film forming step. It is also effective to heat the substrate to about 100 ° C. to 200 ° C. at the same time.

  The steps after the formation of the first and second barrier layers are the same as in the conventional die bonding method, and the semiconductor element is placed on the substrate via the bonding material and heated to be bonded. At this time, when a brazing material such as an Au—Sn brazing material is used as the bonding material, an appropriately shaped brazing material is fused and fixed to the substrate or the semiconductor element, and then the semiconductor element is placed and heated. Join. The bonding temperature can be set in consideration of the melting point of the brazing material.

  Moreover, a metal paste is mentioned as a bonding material useful for die bonding. This metal paste is obtained by dispersing metal particles made of either silver powder or gold powder having a purity of 99.9% by mass or more and an average particle diameter of 0.01 μm to 1.0 μm in an organic solvent. The metal powder constituting this metal paste requires a high purity of 99.9% by mass or more because the hardness of the powder increases when the purity is low, and plastic deformation occurs during the formation of the joint during die bonding. This is because it becomes difficult, and further, the impurities form a surface oxide layer and inhibit the bonding between the powders. In addition, with regard to the average particle size of the metal powder, it is difficult for a metal powder having a particle size exceeding 1.0 μm to exhibit a preferable proximity state between particles when rearrangement occurs during die bonding. On the other hand, the lower limit of 0.01 μm is taken into consideration that when the particle size is less than this particle size, it tends to agglomerate when used as a paste, making it difficult to handle. The constituent metal of the metal powder is either gold or silver because these metals have good conductivity and are soft.

The metal paste is formed by dispersing the above metal powder in an organic solvent. As the organic solvent, ester alcohol, terpineol, pine oil, butyl carbitol acetate, butyl carbitol and carbitol are preferable. For example, 2,2,4-trimethyl-3-hydroxypentaisobutyrate (C ₁₂ H ₂₄ O ₃ ) can be cited as a preferred ester alcohol-based organic solvent. This is because the present solvent can be dried at a relatively low temperature.

  The content of the metal powder in the metal paste is preferably 70 to 99% by mass. If it is less than 70% by mass, the metal necessary for bonding is insufficient and a dense bonded part cannot be formed. On the other hand, if it exceeds 99 mass%, the viscosity of the paste becomes so high that the handling property is hindered.

  In addition to this organic solvent, this metal paste may contain one or more selected from acrylic resins, cellulose resins, and alkyd resins. When these resins and the like are further added, the metal powder in the metal paste is prevented from agglomerating and becomes more homogeneous, and a joint can be formed. Examples of acrylic resins include methyl methacrylate polymers, examples of cellulose resins include ethyl cellulose, and examples of alkyd resins include phthalic anhydride resins. Of these, ethyl cellulose is particularly preferable.

  The metal paste application process is not particularly limited, and various methods may be used in accordance with the bonding portion of the wiring or semiconductor element, such as a spin coating method, a screen printing method, a metal mask printing method, and an ink jet method. Can do.

  After applying the metal paste, the semiconductor element is placed on the substrate and bonded by heating and pressing. By heating and pressurizing, the metal particles in the paste and between the bonded surfaces of the members to be bonded and the metal particles are in close contact with each other, and the shape as a bonded portion is stabilized. The heating temperature is preferably 80 to 300 ° C. This is because point contact does not occur below 80 ° C. On the other hand, if the temperature exceeds 300 ° C., damage to the structural members may occur, such as deformation of the substrate or thermal effects during heating. And it is preferable that the pressurization at the time of joining shall be 0.5 Mpa-50 Mpa. This is because the metal paste cannot be brought into close contact with the entire surface to be joined in a region lower than 0.5 MPa, and no further improvement in the joining state is observed in a region higher than 50 MPa.

  Moreover, in the die-bonding method using a metal paste, an ultrasonic wave may be applied in addition to heating. By heating or a combination of heating and ultrasonic waves, plastic deformation and bonding of the metal powder can be promoted, and a stronger joint can be formed. When applying ultrasonic waves, the conditions are preferably an amplitude of 0.5 to 5 μm and an application time of 0.5 to 3 seconds. This is because application of excessive ultrasonic waves damages the joining member. The heating and ultrasonic wave application in the die bonding step may be performed on at least the bonding portion for the purpose, but may be performed on the entire bonding member. As a heating method, it is easy to use heat transfer from a tool when pressurizing the joining member. Similarly, it is easy to apply ultrasonic waves from a tool that pressurizes the joining member, but it is possible to remove distortion caused by pressing by heating at 80 to 300 ° C. for a certain time after ultrasonic joining. It is also possible to stabilize the crystal structure.

  As described above, by applying the die bond bonding structure according to the present invention, the diffusion of copper from the wiring to the bonding material is suppressed, and the occurrence of defects in the bonding material can be prevented. Since this joint is stable even at high temperatures, the present invention is particularly useful for die bonding of power devices and the like.

The figure explaining the aspect of the junction structure which concerns on this invention. No. 1 of the first embodiment. The AES analysis result performed after heating the test piece of 5 at high temperature. No. 1 of the first embodiment. The AES analysis result performed after heating 11 test pieces at high temperature. No. 1 of the first embodiment. The AES analysis result performed after heating 15 test pieces at high temperature.

First Embodiment : Here, a preliminary test for confirming the copper diffusion suppression effect by the two barrier layers was performed. In this preliminary test, metal thin films serving as various barrier layers are formed on a copper thin film formed on a substrate, and this is heated to check whether copper diffusion occurs on the surface of the metal thin film.

  Prepared are test pieces (dimensions 2 mm square, thickness 0.5 mm) each formed by sputtering on a Si substrate with a Ti thin film (50 nm) and a Cu thin film (100 nm), and various types of Ni, Pd, Pt, Ru, Ta A single layer or multiple layers of metal were formed. Then, an Au thin film or an Ag thin film (200 nm) was formed thereon. Each metal thin film was formed by a sputtering method (room temperature). And this test piece was heated at 200 degreeC and 300 degreeC for 300 hours, the presence or absence of the color change of Au surface which is the outermost surface was examined, and when the color change arose, it was judged that Cu spreading | diffusion had arisen. The evaluation results are shown in Table 1.

  From the results of this test, it can be said that the combination having an effect as a barrier layer is only a combination of ruthenium (first barrier layer) and tantalum (second barrier layer). It can be said that other metals (Ni, Pd, Pt) have almost no effect of suppressing copper diffusion at 300 ° C. In addition, the barrier layer of the ruthenium single layer and the tantalum single layer is effective to some extent because it was not discolored by heating at 200 ° C., but it was not effective at all by heating at 300 ° C. It can be said that it is necessary to adopt a barrier layer. Furthermore, no. From the comparison with the result of 19, it was confirmed that even if two types of ruthenium and tantalum were used, it was necessary to arrange ruthenium on the wiring side and tantalum on the bonding material side, and vice versa.

  Moreover, Auger electron spectroscopy analysis (AES) was performed about the test piece (No.5, No.11, No.15) after the said preliminary test after 300 degreeC heating. In AES analysis, analysis was performed while sputtering from the outermost surface, and the element distribution in the depth direction was investigated. The results are shown in FIGS. In each figure, only the spectrum of the metal element is described. No. in which discoloration was observed. 5 (using Ni and Pd as a barrier layer), No. 5 No. 11 (which uses only Ru as a barrier layer) was confirmed to be diffused because Cu was detected from the outermost surface. On the other hand, No. 2 having a Ru / Ta two-layer barrier layer. For No. 15, no diffusion of Cu was observed, and the spectrum of the constituent metal could be clearly confirmed for each layer. This analysis result also confirmed the effect of the two-layer barrier layer.

  In FIG. From the result of AES for 5, it was confirmed that when a Ni film is formed on the wiring surface, Ni diffuses into the bonding material together with Cu. Although it is not certain whether the diffusion of Ni adversely affects the bonding strength, at least an effect as a barrier layer cannot be expected. In this regard, no. As can be seen from the result of No. 18, even if Ni is present on the wiring, it is understood that the diffusion can be suppressed by forming the Ru / Ta two-layer barrier layer.

Second Embodiment : Here, assuming die bonding for actual device manufacturing, die bonding of Si chips is performed using a metal paste (gold powder) on copper wiring on a ceramic substrate, and bonding after high-temperature heating is performed. The strength was confirmed.

The metal paste used was a gold powder (average particle size: 0.3 μm) having a purity of 99.9% by mass produced by a wet reduction method, ester alcohol (2,2,4-trimethyl-3-hydroxy) as an organic solvent. What was mixed with pentaisobutyrate (C ₁₂ H ₂₄ O ₃ )) (metal powder content 90 mass%) was used.

  On the other hand, a 0.6 mm thick DBC substrate (Direct Bonding Copper substrate) in which a copper foil (0.15 mm) is bonded on a SiC ceramic substrate is prepared. Then, two barrier layers of Ru (100 nm) and Ta (100 nm) were formed on the surface of the copper foil on the substrate by sputtering. Moreover, Si chip (2 mm square) was prepared as a member to be joined. On the surface of the Si chip, a sputtered film of Ti (50 nm), Pt (50 nm), and Au (200 nm) is formed in advance.

After applying the above metal paste onto the copper foil of this DBC substrate, the Si chip was placed, and when pressurized at 5 MPa, the temperature was raised to 150 ° C. at 1 ° C./minute, held for 10 minutes, and then increased to 20 MPa. The temperature was raised to 300 ° C. for 10 minutes and held for 10 minutes for bonding. The density of the bonded part after bonding was measured and found to be 17.5 g / cm ³ . This is 0.91 times the gold density (19.3 g / cm ³ ) of the bulk material.

Comparative Example 1 For the above embodiment, a metal paste was applied in a state where a barrier layer was not formed on the copper foil of the substrate, and die-bonded in the same manner.

Comparative Examples 2 and 3 : Compared to the second embodiment, a ruthenium single layer (Comparative Example 2) and a two-layer film of nickel and platinum (Comparative Example 3) are formed on a substrate, and then a metal paste is applied. Joined.

  And about the DBC board | substrate with Si chip die-bonded in both embodiment and the comparative example, it heat-processed at 300 degreeC for 200 hours, and measured the shear strength of the junction part using the bonding tester about the test piece after a process. The measurement results are shown in Table 2.

  In this strength evaluation, all peeling occurred inside the bonding material in the vicinity of the bonding interface between the bonding material and the barrier layer. From Table 2, it was confirmed that the bonding structure including a suitable barrier layer has a bonding strength that is at least twice that of the bonding structure without the barrier layer.

By applying the die bond bonding structure having the ruthenium / tantalum two-layer barrier layer according to the present invention, it is possible to suppress the diffusion of copper in the wiring. Thereby, the generation | occurrence | production of the defect of a joining material can be prevented. The present invention can be applied to die bonding of semiconductor chips and the like, flip chip bonding, and the like, and is particularly useful for bonding of power devices used in a high temperature environment.

Claims (4)

In a die bond bonding structure formed by bonding a copper or copper alloy formed on a substrate and a semiconductor element by die bonding via a bonding material,
A first barrier layer having a thickness of 5 to 200 nm made of ruthenium formed on the wiring side between the wiring and the bonding material;
A second barrier layer having a thickness of 5 to 200 nm made of tantalum formed on the first barrier layer ,
The bonding material is an Au—Sn brazing material, an Au—Si brazing material, an Au—Ge brazing brazing material, or a sintering with a silver powder or a gold powder having a purity of 99.9% by mass or more. A die-bonding junction structure for a semiconductor element, characterized in that it has any one of a structure and a density 0.45 to 0.95 times that of a bulk material . In a method of die-bonding a semiconductor element to a wiring made of copper or a copper alloy formed on a substrate,
Forming a first barrier layer made of ruthenium having a thickness of 5 to 200 nm on the wiring side on the substrate, and a second barrier layer having a thickness of 5 to 200 nm made of tantalum on the first barrier layer;
It is a method of placing the semiconductor element on a substrate via a bonding material and heating and bonding ,
As the bonding material, any one of Au—Sn brazing material, Au—Si brazing material, Au—Ge brazing material is used,
A die bonding bonding method for a semiconductor element, in which the brazing material is fused and fixed to a substrate or a semiconductor element, and the semiconductor element is placed and heated and bonded. In a method of die-bonding a semiconductor element to a wiring made of copper or a copper alloy formed on a substrate,
Forming a first barrier layer made of ruthenium having a thickness of 5 to 200 nm on the wiring side on the substrate, and a second barrier layer having a thickness of 5 to 200 nm made of tantalum on the first barrier layer;
It is a method of placing the semiconductor element on a substrate via a bonding material and heating and bonding ,
As the bonding material, a metal paste in which metal particles made of either silver powder or gold powder having a purity of 99.9% by mass or more and an average particle diameter of 0.01 μm to 1.0 μm are dispersed in an organic solvent,
A method of die bonding a semiconductor element, in which the metal paste is applied to either a wiring or a semiconductor element on the substrate, and then the semiconductor element is placed on the substrate and heated and bonded while being pressed in one direction or both directions. The method for die bonding a semiconductor element according to claim 3 , wherein the heating temperature at the time of bonding is 80 to 300 ° C.

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