JP2009094302A - Bonding wire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bonding wire for reducing as much as possible fluctuation in heights of connected wire loops. <P>SOLUTION: The bonding wire is used to assure conductivity among fine electrodes within an electronic element. This bonding wire has breakdown distortion in a range of 0.0009 to 0.0051 of wire material characteristic expressed with 2-linear approximation. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to wire bonding for ensuring electrical conduction between microelectrodes in an electronic device, and more particularly to a bonding wire that can suppress variations in the height of connected wire loops.

In general, a wire bonding method is used to ensure conduction between microelectrodes in an electronic element. The bonding wire used for the wire bonding method is required to have high electrical conductivity, excellent bondability with the terminal metal, and appropriate tensile strength and elasticity.
Conventionally, a pure gold wire having a wire diameter of several tens of μm and a purity of 99.99% or more has been used as a bonding wire. This pure gold wire has a breaking strength of about 0.014 kgf / mm ^{2 to} 0.016 kgf / mm ² , a Young's modulus of about 6,000 kgf / mm ² to 7, 200 kgf / mm ² , and a loop with a pitch of 40 to 70 μm can be formed. Is possible.

As the density of electronic elements increases, wire bonding technology is becoming increasingly loops and pitches are narrowed. Wires come loose and contact each other due to deformation. The so-called wire flow that is likely to occur is increasing.
In order to prevent the occurrence of wire flow, a technique is disclosed that uses a bonding wire in which a metal wire having a high Young's modulus is coated with a metal having excellent bonding properties (see, for example, Patent Document 1).
Analyzing the factors that govern the wire flow, it can be seen that there are factors at the time of joining, such as the joining shape, and factors of the wire material itself. It has been found that the Young's modulus has the greatest influence on the wire material, and the higher the Young's modulus, the smaller the wire flow. The bonding wire disclosed in Patent Document 1 uses tungsten (W), molybdenum (Mo) or rhodium (Rh) having a Young's modulus of about 10,000 kgf / mm ² to 13,700 kgf / mm ² as a core wire, A bonding wire in which about 10 μm is coated with a highly conductive metal such as gold (Au), copper (Cu) or palladium (Pd). By using this bonding wire, it is said that the generation rate of the flow when forming the same loop is reduced to 1/3 to 1/4.

In recent years, along with the downsizing of electronic devices, semiconductor packages are also becoming smaller and lighter. Along with this, the required quality of the materials used has become increasingly severe. Specifically, the semiconductor device can be miniaturized by reducing the external dimensions of the semiconductor package, the lead width for external connection can be reduced, and the like.
Electrodes are provided on the semiconductor element to connect the bonding wires. Conventionally, the interval between adjacent electrodes (hereinafter referred to as bond pitch) was about 0.070 mm to 0.15 mm. In recent years, the bond pitch has become finer, and semiconductor elements having a minimum bond pitch of 0.045 mm have been mass-produced.
Further, as the semiconductor element is miniaturized, the distance between the external connection lead and the electrode on the semiconductor element tends to be longer, and a semiconductor package in which the length of the bonding wire exceeds 5 mm has appeared.

When bonding at a fine pitch, since the bond pitch is narrowed, the capillary (leader) may come into contact with the adjacent bonding wire that has been bonded first, which may cause abnormalities such as loop collapse. For this reason, a bottleneck type capillary is used. This capillary is designed so that bonding can be performed without making contact with the adjacent bonding wire by reducing the shape of the tip. However, since the outer dimension of the capillary is reduced, it is necessary to reduce the inner diameter of the cylindrical hole provided in the capillary.
In general, when the clearance between the inner surface of the cylindrical hole provided in the capillary and the bonding wire is narrow, a loop shape abnormality or the like occurs, and thus it is necessary to sufficiently maintain the clearance between the inner surface of the cylindrical hole and the wire. For this reason, even in the case of a wide bond pitch of 0.070 mm or more, the bonding wire used has a wire diameter of 0.020 mm to 0.025 mm.

On the other hand, as described above, technological advances related to semiconductors are remarkable, and further fine pitches are expected in the future. Specifically, practical mass production with a bond pitch of 0.030 mm to 0.040 mm is planned. In order to promote finer bond pitches, it is necessary to reduce the wire diameter, and it is planned to use ultrafine wires with a bonding wire diameter of 0.012 mm to 0.020 mm. ing.
Such an ultra-fine bonding wire may be deformed, which is a major impediment to fine pitch.

Bonding on a semiconductor element that can handle fine pitches of 0.030 mm to 0.040 mm, ultra fine wires with a wire diameter of 0.012 mm to 0.020 mm, high Young's modulus, and less bending As a wire, a gold alloy containing Au as a main component and containing trace amounts of Sn, Zn, and Ca is disclosed (for example, see Patent Document 2).
Gold alloy disclosed in Patent Document 2, since the Young's modulus in bonding wire having a diameter of 0.012mm may be increased from 8,000kgf / mm ² up to 8,720kgf / mm ^2, hardly wire deformation, It is said that it is possible to provide a fine pitch bonding wire with less bending.
JP-A-11-274215 JP 2006-128560 A

  A bonding wire having a diameter of about several tens of microns used in an electronic element or the like is set in a bonding machine in a wound state, and used as a connection material for ensuring conduction between microelectrodes in the electronic element. In the bonding machine, there is a hollow part called a capillary that determines the path of the bonding wire, such as a sewing needle, and the relative position between the capillary and the microelectrode substrate is set in advance with the bonding wire passed through the hollow hole. First bonding and second bonding by devising a motion pattern that combines the movement status, direction, angle, and path in order and the bonding wire supply state, such as feeding or clipping the bonding wire from the capillary. Threading is performed so that a desired loop shape connecting the two can be obtained.

  Even when bonding wires of the same material are used, the material of the bonding wire, which is the final product, varies from lot to lot depending on the material variation between the ingots, which is the original material before drawing the bonding wire, or the material distribution inside the ingot. Alternatively, the height of the loop formed by bonding may vary due to partial differences within the same lot. For this reason, when forming multiple loops, which is a new bonding technology that stacks and arranges loops with different heights in the vertical direction of the microelectrode substrate, the loop height does not match the design due to the material variations described above. There is a risk of shorting in contact with other loops formed in the direction.

Therefore, when bonding wires of the same material are used and bonding is performed with the same motion pattern, it is possible to obtain a stable loop shape with very little height variation, which improves the yield of semiconductor devices using this. Is very important to make.
An object of the present invention is to provide a bonding wire that can suppress variations in the height of connected wire loops as low as possible.

As a result of earnestly investigating the relationship with the loop shape by paying attention to the Young's modulus, yield stress, and tangential coefficient, which are the material constants of the bonding wire expressed by a two-line approximation, the present inventor It was found that the variation in loop height can be reduced by keeping the yield strain having the greatest influence on the height within a predetermined range, and the present invention has been achieved.
That is, the present invention is a bonding wire for ensuring electrical conduction between microelectrodes in an electronic device, and a bonding wire having a yield strain of 0.0009 to 0.0051 as a wire material characteristic expressed by a two-line approximation. is there.

In the bonding wire of the present invention, it is preferable that the ratio of the tangent coefficient and the Young's modulus of the wire material characteristic expressed by two-line approximation is 0.0033 to 0.0067.
The Young's modulus is preferably 3,900 kgf / mm ² to 8,670 kgf / mm ² and the tangential coefficient is preferably 26 to 57.
Furthermore, it is preferable that the wire diameter is 0.012 mm to 0.020 mm, and the breaking strength is 0.005 kgf / mm ² to 0.007 kgf / mm ² .

  The bonding wire of the present invention contains gold as a main component and at least one element selected from Be, Mg, Ca, Ge, Sn, Ga, In, Y and rare earth elements as an alloying element is 0.001 to 0.00. By using an alloy containing 0.0005 to 2.0% by mass of 01% by mass, or in addition, at least one element selected from Cu, Ag, Ru, Rh, Pd, Os, Ir and Pt Achieved.

According to the present invention, the connected wire loop can be formed with a height _HL of 178.9 microns to 182.8 microns, that is, a height distribution width kept within ± 2 microns.
This makes it possible to cope with finer bond pitches and longer wire loops, which can greatly contribute to downsizing of electronic devices.

As a result of earnestly investigating the relationship with the loop shape by paying attention to the Young's modulus, yield stress, and tangent coefficient, which are the material constants of the bonding wire expressed by two-line approximation, the present inventor has found the following properties.
When an external force is applied to an object, the shape and volume change, which is called distortion. If the stress is within the elastic limit, an elastic strain is generated, and if the external force is removed, the strain disappears. When the stress exceeds the elastic limit, plastic deformation occurs and plastic strain remains. That is, the total strain ε of the bonding wire in one dimension is composed of an elastic strain ε _el and a plastic strain ε _pl . When the total strain ε is decomposed into an elastic strain ε _el and a plastic strain ε _pl , the following equations 1 to 4 are obtained. Become. From these equations, the total strain ε can be expressed by D / E and yield strain ε _Y using Young's modulus E and tangential coefficient D.

Overall strain ε: ε = elastic strain ε _el + plastic strain ε _pl = σ / E + ε _pl (Formula 1)
Stress at that time (one-dimensional) σ: σ = σ _Y + D (ε−ε _Y ) (Formula 2)
Here, E is Young's modulus and D is a tangent coefficient.
Further, from Equation 1, plastic strain ε _pl : ε _pl = ε-σ / E = ε-σ _Y / ED−E (ε-ε _Y )
= (Ε−ε _Y ) (1-D / E) (Formula 3)
Elastic strain ε _el : ε _el = σ / E = σ _Y / E + D / E (ε−ε _Y )
= Ε _Y + D / E (ε−ε _Y ) (Formula 4)
Here, σ _Y used in the above equation is the yield stress.

FIG. 1 shows an external view of a wire loop in general wire bonding, and defines a loop height _HL . FIG. 1 schematically shows a case where the surface of the microelectrode 7 on the semiconductor chip 3 in the first bonding portion 1 and the surface of the microelectrode 8 in the second bonding portion 4 are connected by the bonding wire 6. . In the figure, 5 is a capillary.
The loop height H _L is defined by the distance from the microelectrode 7 in the first bonding portion 1 to the highest point of the neck portion 2 of the bonding wire 6 in the drawing. It is an object of the present invention to minimize the variation in the loop height _HL .

FIG. 2 schematically shows the relationship between strain and stress in the wire bonding described above. As shown in the figure, the bonding wire is subjected to the stress of σ along the line segment OA-AB, and the total strain of ε is generated. Up to point A is elastic deformation, and from A to B is plastic deformation.
The stress at the yield point A is the yield stress σ _Y , and the strain is the yield strain ε _Y.
Total strain: ε is divided into elastic strain ε _el and plastic strain ε _pl (see Equation 1). Up to the yield point A in the figure, the stress and strain are proportional to each other according to the Young's modulus E of the material. That is, there is a relationship of σ _Y = E · ε _Y (ε _Y = σ _Y / E).
When the stress increases after the yield point A, the strain increases according to the polygonal line coefficient D.
In FIG. 2, the polygonal line coefficient D is the gradient of the line segment AB, so D = (σ−σ _Y ) / (ε−ε _Y ), and (Expression 2) is derived.

The meaning of each of the above formulas is that if D / E and yield strain ε _Y are the same, even if materials having different Young's modulus E, tangential coefficient D, and yield stress σ _Y are different from each other, elastic strain ε _el The ratio of the plastic strain ε _pl is the same, and the formed loop shape is the same. Based on the obtained knowledge, the dependency of the loop height H _L on D / E and ε _Y when a loop was formed with a general motion pattern as shown in FIG. 3 was investigated. The result is shown in FIG.
FIG. 3 is a diagram showing a motion pattern, and shows a trajectory for sequentially moving the capillary in the horizontal direction and the vertical direction. The scale is an arbitrary unit. The loop shown in FIG. 1 is formed with the capillary motion pattern shown in FIG.
FIG. 4 is a plot of the height _HL of the wire loop formed as described above against the ratio σ _Y / E of the yield stress σ _Y and Young's modulus E of each material.
D / E and εY of a general bonding wire actually used are in a range of (D / E) <0.01 and ε _Y <0.005, respectively. As can be seen from FIG. 4, when ε _Y <0.0011, the ε _Y dependency of the formed loop height H _L becomes very large (see curves c and d). Therefore, it can be easily expected that the bonding wire having ε _Y in this region will have a large change in loop height with only a slight change in the material ε _Y.

The present invention devised on the basis of the above knowledge is a bonding wire used for ensuring electrical conduction between microelectrodes in an electronic element. When a general motion pattern is used, two linear lines are used. If the D / E, which is the ratio of the material properties of the bonding wire expressed in approximation, is 0.0033 to 0.0067 and the yield strain ε _Y is within the range of 0.0009 to 0.0051, It can be seen from FIG. 4 that the height H _L of the connected wire loop can be formed from 178.9 to 182.8 microns, that is, the height distribution width ± 2 microns.

In the bonding wire of the present invention having such material characteristics, at least one element selected from Be, Mg, Ca, Ge, Sn, Ga, In, Y and rare earth elements as an alloy element is 0.001. It is preferable that it is -0.01 mass%. If it is less than 0.001% by mass, the strength in the case of a thin wire is insufficient, the Young's modulus is lowered, and the target value cannot be obtained, which is not preferable. On the other hand, if it exceeds 0.01 mass%, the material becomes brittle and thin wire processing becomes difficult.
In the bonding wire of the present invention, in addition to the above alloy elements, at least one element selected from Cu, Ag, Ru, Rh, Pd, Os, Ir, and Pt contains 0.0005 to 2.0 mass%. It is preferable that it is included. If it is less than 0.0005% by mass, the effect of improving the breaking strength is poor, and if it exceeds 2.0% by mass, the electric resistance increases, which is not preferable.
Such a gold alloy is subjected to a step of final annealing after combining forging and annealing to obtain a predetermined wire diameter, and after wire casting, direct wire drawing without rolling is performed, and wire drawing before final annealing. It is obtained by performing at least one intermediate annealing process during the process.
This gold alloy ultrafine wire has a Young's modulus of 3,900 kgf / mm ² to 8,670 kgf / mm ² , a fold line coefficient of 26 to 57, and a breaking strength of 0.005 kgf / mm ^{2 to} 0 at a wire diameter of 0.012 mm. An ultrafine wire of .007 kgf / mm ² is obtained.

Embodiment the Young's modulus E is 4484kgf / mm ² which is a material property of the bonding wire, expressed in 2 linear approximation as an example of the present invention below, the tangent modulus D 26, such as yield stress epsilon _Y is at 10 kgf / mm ² 2 A case where a gold wire bonding wire that can be expressed by linear approximation is prepared and bonded will be described.
The gold wire bonding wire is made of a gold alloy containing 0.005% by weight of Be as an alloy element.
D / E of the material is 0.0058, the center value of epsilon _Y is 0.0022, the variation width of the epsilon _Y was ± 0.01 microns.
Using such a gold wire bonding wire, bonding was performed with the general motion pattern shown in FIG. 3 to form about 500 identical loops. As a result of evaluating the variation of the loop height HL, it is 180.8 microns to 182.3 microns, and the height distribution range is ± 0.08 microns. Obtained.

  By adopting a thin electronic element with a high bonding density in the height direction, it is possible to provide a thin element with high reliability and thus can be differentiated from other products.

It is an external view of the wire loop in general wire bonding. It is a figure which shows the relationship between the distortion and stress in wire bonding. It is a figure which shows a general motion pattern. It is a figure explaining the dependence to D / E and (epsilon) _Y of loop height _HL .

Explanation of symbols

1 First bonding portion 2 Neck portion 3 Semiconductor chip 4 Second bonding portion 5 Capillary 6 Bonding wire 7, 8 Microelectrode H _L loop height

Claims (5)

  A bonding wire for securing electrical conduction between microelectrodes in an electronic element, wherein the yield strain of the wire material characteristic expressed by a two-line approximation is 0.0009 to 0.0051.   2. The bonding wire according to claim 1, wherein the ratio of the tangent coefficient and the Young's modulus of the wire material characteristic expressed by two-line approximation is 0.0033 to 0.0067. 3. The bonding wire according to claim 1, wherein the Young's modulus is 3,900 kgf / mm ² to 8,670 kgf / mm ² , and the tangential coefficient is 26 to 57. 4. The wire diameter is 0.012 mm to 0.020 mm, and the breaking strength is 0.005 kgf / mm ² to 0.007 kgf / mm ^{2. 5.} Bonding wire.   0.001 to 0.01% by mass of at least one element selected from Be, Mg, Ca, Ge, Sn, Ga, In, Y and rare earth elements as an alloy element, or in addition thereto 5 to 2.0% by mass of at least one element selected from Cu, Ag, Ru, Rh, Pd, Os, Ir, and Pt. The bonding wire of any one of Claims.