JPH098076A - Bonding wire - Google Patents

Abstract

PURPOSE: To enable making a high frequency current nicely flow not only on the outer peripheral surface of a wire but also inside the wire, regarding a bonding wire which is used, e.g. when a semiconductor chip of a semiconductor device and an outer electrode are connected, and especially a bonding wire which is used for a semiconductor device or the like driven by a high frequency current. CONSTITUTION: A skin part for making a high frequency current flow by the skin effect is formed inside conductor 1 constituting a bonding wire W. As the most typical constitution example, e.g. the conductor is formed in a hollow tube As another case, insulator is interposed inside the conductor, and the inner peripheral surface of the hollow tube conductor or the interface with the insulator is made a skin part for making a high frequency current flow by using the skin effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bonding wire used for connecting a semiconductor chip and an external electrode in a semiconductor device, in particular, an MPU (central processing integrated circuit) or a digital signal processor driven by a high frequency current. Bonding wire used for.

[0002]

2. Description of the Related Art As the bonding wire as described above, a linear conductor made of gold, aluminum, copper or the like and having a substantially circular cross section is generally used. When a high-frequency current is passed through such a bonding wire, the current concentrates on the surface of the conductor due to the so-called skin effect, and a phenomenon occurs in which almost or no current flows in the central portion.

As described above, the frequency f of the current at which no current flows in the center of the conductor varies depending on the material and radius of the bonding wire, but in general,

[0004]

[Equation 1]

Can be defined as However, in the above formula, r is the radius of the bonding wire, ρ is the specific resistance, and μ is the magnetic permeability.

For example, a bonding wire having a diameter of 30
μmφ gold wire (ρ = 2.35 × 10 ⁻⁸ Ωm, μ≈μ ₀ = 4π ×
10 ^-7 H / m),

[0007]

[Equation 2]

When a current having a frequency of f> 26.5 MHz is passed, the current flows only in the peripheral portion of the bonding wire, and the current does not flow in the central portion, that is, the central portion does not function as a conductor.

The thickness (hereinafter referred to as the skin thickness) δ of the conductor skin portion through which the current flows due to the above-mentioned skin effect is μ of the permeability of the conductor, σ of the electrical conductivity, and ω of the angular frequency of the current.
Can be calculated as δ = (2 / μσω) ^1/2 . When the conductor cross section is circular with a radius r, the cross sectional area of the conductor skin portion through which the high frequency current flows (hereinafter,
Since the effective area S has a ring shape with a thickness δ inward from the outer peripheral surface of the conductor, S = πr ² −π (r−δ) ² = 2πrδ−πδ ² .

In this case, δ is smaller than r as the frequency of the current flowing through the bonding wire is higher.
Ignoring the term δ ² in the above equation, the effective area S ₁ = 2πrδ
Can be approximated by On the other hand, in the case of a low frequency direct current, a current flows over the entire cross section of the bonding wire, and the effective area S ₂ is S ₂ = πr ² , so there is a difference of S ₂ −S ₁ for the high frequency current. Yes (πr ² -2πrδ)
That is, the area corresponding to does not function as a conductor.

As described above, in the bonding wire through which the high frequency current is passed, there is a problem that the effective area through which the current flows is reduced due to the skin effect, which increases the effective resistance value. To prevent this, the effective area through which the high-frequency current flows may be increased. For that purpose, for example, the area of the conductor skin portion may be increased.

Therefore, conventionally, as a bonding wire for high-frequency current, a wide ribbon-shaped conductor wire is used, or the number of bonding wires is increased. However, the method using a ribbon-shaped wide conductive wire has a limit to be applied to a semiconductor device or the like in which the pad pitch is narrowed with the recent increase in density of integrated circuits,
In addition, the method of increasing the number of bonding has a problem that it is difficult to secure the number of pads on the semiconductor chip and the productivity is lowered.

Further, for example, a method of using a litz wire (stranded wire) as a bonding wire as in Japanese Patent Laid-Open No. 6-302640 or a corrugated surface cross-sectional shape of the bonding wire as in Japanese Patent Laid-Open No. 6-283565. Although a method and the like have been proposed, the former one using a litz wire has a diameter of 12.5 μm even if a bonding litz wire having a diameter of 25 μm is manufactured with two twisted wires.
Since an extremely thin single wire of m must be produced, it is difficult to manufacture, and even if an extremely thin single wire can be manufactured, there is a problem that it is very difficult to operate and handle the stranded wire. Further, in the case of forming a corrugated surface like the latter, the angle of the wave is scraped by a capillary or the like during bonding and the surface becomes flat, and the effect of increasing the surface area is reduced or lost. There is a problem such as.

[0014]

As described above, all of the above conventional methods have been proposed with the intention of increasing the area of the outer peripheral surface of the wire through which the high frequency current flows due to the skin effect. It wasn't enough.

The present invention has been proposed in view of the above conventional problems, and an object of the present invention is to provide a bonding wire capable of favorably passing a high frequency current not only on the outer peripheral surface of the wire but also inside the wire.

[0016]

In order to achieve the above object, the bonding wire according to the present invention has the following constitution. That is, it is characterized in that a skin portion for passing a high frequency current by the skin effect is also provided inside the conductor forming the bonding wire.

As the most typical example of the structure in which the skin portion is provided inside the conductor forming the wire as described above, for example, the conductor is formed into a hollow tube, or an insulating material is interposed inside the conductor, and the hollow tubular conductor is formed. The inner peripheral surface or the boundary surface with the insulating material may be a skin portion for passing a high frequency current by the skin effect.

[0018]

As described above, by providing a skin portion for passing a high frequency current due to the skin effect inside the conductor which constitutes the bonding wire as well, the electric current flows inside the wire which has not conventionally functioned as a conductor. Therefore, it is possible to increase the effective area through which the high-frequency current flows without increasing the outer dimension of the wire.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The bonding wire according to the present invention will be specifically described below with reference to the embodiments shown in the drawings.

FIG. 1 is a cross sectional view showing an embodiment of a bonding wire according to the present invention. The bonding wire W of the present embodiment is formed by forming the conductor 1 made of gold or the like into a hollow tubular shape, and has a hollow hole 1a formed in the central portion. The inner peripheral surface of the hollow hole 1a functions as a skin portion for flowing a high frequency current by the skin effect, and the skin thickness δ through which the high frequency current flows on the inner peripheral surface of the hollow hole 1a is equal to the outer peripheral surface of the conductor 1. It is equal to the flowing skin depth δ.

In the above structure, when the outer diameter (radius) of the conductor 1 is r ₁ , the radius of the hollow hole 1a is r ₂ , and the skin thickness through which the high-frequency current flows is δ, r ₁ ≧ r ₂ + 2δ Is desirable.

Since the conductor 1 forming the bonding wire W is formed in a hollow tubular shape as described above, the radius of the bonding wire W becomes larger in a high frequency range where the skin depth δ is much smaller than the conductor radius r _1. The outer skin of the conductor 1 of r ₁ and the inner skin of the hollow hole 1a of radius r ₂ have a skin portion through which a high frequency current flows due to the skin effect. Assuming that the skin thickness is δ, the effective area of the skin portion through which the high-frequency current flows is radius r in the conventional bonding wire.
_Although the conductor peripheral surface of ₁ had only an effective area of 2πr ₁ δ, in the present embodiment, the effective area of 2πr ₂ δ can be increased on the inner peripheral surface of the hollow hole 1a in addition to the effective area of 2πr ₁ δ.

The manufacturing method and the like in the case of manufacturing the bonding wire W in which the conductor 1 is formed into a hollow tubular shape as described above are appropriate. For example, the hollow tubular conductor 1 having a predetermined size is formed by an extrusion method or the like. Alternatively, a hollow tubular conductor formed to have a diameter larger than a predetermined size in advance can be reduced in diameter to a predetermined thickness by, for example, drawing using a die.

Further, for example, a long ribbon-shaped material plate is inserted into a forming hole or the like formed successively smaller than the ribbon width, or is gradually curved into an arcuate cross section by roll forming or the like, and both widthwise end portions thereof are formed. As shown in FIG. 2, a single hollow tubular conductor 1 is formed by butting. Alternatively, as shown in FIG. 3, the material plate is spirally wound in multiple layers to form the conductor 1. Further, for example, the material plate as described above can be spirally wound as shown in FIG. 4 to form the hollow tubular conductor 1.

A plurality of hollow holes 1a of the conductor 1 having a hollow tubular shape as described above may be provided if necessary. FIG. 5 shows an example in which three hollow holes 1a having a circular cross section are formed in the conductor 1. Show. The number, size and sectional shape of the hollow holes 1a are appropriate. Further, the manufacturing method and the like are also appropriate, and for example, it can be manufactured by an extrusion method or a drawing process as in the case of the embodiment of FIG. Alternatively, as shown in FIG. 6, a part of the peripheral surface of the hollow tubular conductor 1 may be integrally and continuously bent inward into a substantially Ω shape to form three hollow holes 1a having a substantially circular cross section. As a manufacturing method thereof, for example, it can be manufactured by crushing a conductor formed in advance in a hollow tube into a substantially flat shape and then rolling it into a hollow tube again.

A hollow hole 1a is formed in the conductor 1 as in the above embodiment.
By providing a plurality of the above, the effective area can be increased according to the number thereof and the diameter of the hollow hole 1a.
For example, in each of the embodiments shown in FIGS. 5 and 6, when the radius of the outer peripheral surface of the conductor 1 is r ₁ and the radii of the three hollow holes 1 a are r ₂ , the conventional wire is attached to the outer peripheral surface of the conductor 1. In addition to the same effective area of 2πr ₁ δ as described above, the effective area of 3 × 2πr ₂ δ can be increased by the three hollow holes 1a. Generally, if n hollow holes 1a having a radius r ₂ are provided, the effective area of 2nπr ₂ δ can be increased as compared with the conventional wire.

In each of the above embodiments, a part or the whole of the surface of the conductor 1 including the hollow hole 1a may be filled or covered with an insulating material such as an epoxy resin if necessary. In this case, particularly in the case where there is a portion where the conductor 1 overlaps as in the embodiment of FIGS. 3 and 6,
If the insulating material 2 is interposed or adhered via the insulating material 2 to a part or all of the overlapping portion as shown in FIGS. 7 and 8, the entire surface of the conductor which contacts the insulating material 2 is covered with the skin. Due to the effect, it becomes possible to function as a skin portion through which a high-frequency current flows, and the effective area can be further increased. However, the same effect as the above can be obtained by simply forming a gap in a part or all of the portion where the conductor 1 is overlapped.

In the case of forming a hollow tubular conductor by spirally winding a ribbon-shaped material plate as in the embodiment shown in FIG. 4, the edges of the material plates adjacent in the axial direction are made of synthetic resin. It can be formed into a hollow tube by sequentially bonding and fixing with an insulating material such as. In that case, the material plates adjacent to each other in the axial direction are insulated by the above-mentioned insulating material, and an effective area equivalent to the case where the ribbon-shaped material plate is used as it is as the bonding wire can be ensured, and the external dimensions can be made by making it spiral. It can be made smaller than the ribbon width and can be used at a narrow pitch.

Further, in each of the above-mentioned embodiments, the hollow hole 1a is formed inside the conductor 1. However, it is not always necessary to form such a hollow hole. For example, as shown in FIG. Alternatively, they may be integrally fixed (adhered) via an insulating material 2 such as a system resin. If you do that,
The entire outer peripheral surface including the contact surface of each material plate with the insulating material 2 can function as a skin portion through which a high-frequency current flows due to the skin effect.

[Experimental Example] In the embodiment shown in FIG. 1, a conductor 1 having a circular cross-section made of gold and having an outer diameter of 30 μmφ and 35 μmφ was formed with hollow holes 1 a each having a diameter of 10 μmφ. A bonding wire having the same diameter and the same material as that of the prior art, which does not have the hollow hole 1a, was prepared, and the resistance value and the like were measured to examine the conductive property. The results are shown in Table 1 below. The effective area in the table is obtained by calculation, and the resistance R is a measured value per unit length (2 mm).

[0031]

[Table 1]

As is clear from the above table, when the operating frequency is higher than the high frequency range of each bonding wire, the resistance value is lower than that of the comparative example (conventional product) and the conductive characteristics are improved. I understood.

[0033]

As described above, in the bonding wire W according to the present invention, since the skin portion for passing the high frequency current by the skin effect is also provided inside the conductor 1 forming the wire W, the high frequency current is not generated. It is possible to make the effective area to flow larger than that of the conventional one, and it is possible to flow a larger amount of current as compared with the conventional wire having the same wire diameter.
On the contrary, since it is possible to flow current inside the wire that did not function as a conductor in the past, the external dimensions can be made smaller than the conventional wire while securing the same effective area, and the bonding pitch can be reduced. The present invention can be sufficiently and satisfactorily applied to recent semiconductor devices and the like which are required to be narrowed. When the conductor 1 forming the bonding wire W is a ferromagnetic material such as iron, nickel or cobalt, the magnetic permeability μ of the conductor 1
However, since it becomes extremely larger than a weak magnetic material such as gold or copper, the effective area is significantly reduced by the skin effect in the conventional product, but in the bonding wire according to the present invention, when the conductor is made of a ferromagnetic material, It also has the effect of suppressing the reduction of the effective area compared to conventional products.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a bonding wire according to the present invention.

FIG. 2 is a cross-sectional view showing an example of a method of manufacturing a bonding wire according to the above embodiment.

FIG. 3 is a transverse cross-sectional view showing another example of the method for manufacturing the bonding wire according to the above embodiment.

FIG. 4 is a transverse cross-sectional view showing another example of the method for manufacturing the bonding wire according to the above embodiment.

FIG. 5 is a cross-sectional view showing an example of a bonding wire having a plurality of hollow holes.

FIG. 6 is a cross-sectional view showing another embodiment of a bonding wire having a plurality of hollow holes.

FIG. 7 is a cross-sectional view showing an example of a bonding wire using an insulating material.

FIG. 8 is a cross-sectional view showing another embodiment of a bonding wire using an insulating material.

FIG. 9 is a cross-sectional view showing another embodiment of a bonding wire using an insulating material.

[Explanation of symbols]

 W Bonding wire 1 Conductor 1a Hollow hole 2 Insulation material

Claims (5)

[Claims] 1. A bonding wire characterized in that a skin portion for allowing a high-frequency current to flow due to a skin effect is also provided inside a conductor constituting the bonding wire. 2. The bonding wire according to claim 1, wherein the conductor is formed in a hollow tubular shape, and an inner peripheral surface of the hollow hole serves as a skin portion for passing a high frequency current by the skin effect. 3. The bonding wire according to claim 2, wherein a plurality of hollow holes forming the skin portion are provided. 4. The bonding wire according to claim 1, 2 or 3, wherein the conductor is formed of a ribbon-shaped material plate and the outer peripheral contour of the entire cross section is formed into a substantially circular shape. 5. A conductor skin including the hollow hole and the material plate is filled or covered with an insulating material.
The bonding wire according to 3 or 4.