Semiconductor Arrangement Having Specially Fashioned Bond Wires and Method for Fabricating Such an Arrangement
The invention relates to a semiconductor arrangement having at least two integrated circuits arranged in a housing and electrically connected to each other via bond conductors, according to the features of the preamble of Claim 1, and a method for fabricating such a semiconductor arrangement.
Bond conductors or bond wires have long been known in the fabrication of integrated circuits. On the one hand, such bond conductors are used in the case of an integrated circuit to connect the contacts visible outside the housing (also called pins) to the chip lying inside the housing. The chip has electrical contact regions called bond islands or pads for this purpose. The bond conductor provides an electrical connection between the integrated circuit proper and the wiring substrate of an electronic circuit. As a rule the bond conductor is drawn from the contact pads of the chip to the inner part of the contact and is mechanically and electrically contacted at both ends. Both thermosonic bonding and ultrasonic bonding are known. After bonding, the integrated circuits are hermetically potted into a housing, preferably a plastic housing. The bond conductor in microelectronic construction and connection technology is commonly made of gold or a gold alloy. Bond conductors made of aluminum are also known.
Electrically connecting a chip located in the housing with the external contacts on the housing is not, however, the only use for bond conductors. Bond conductors are also used for electrically connecting chips inside a housing to one another. To this end, the integrated circuits located in the housing each have contact pads that are electrically connected with such bond conductors. According to G.G. Harman, Wire Bonding in Microelectronics, McGraw-Hill, 1997, pages 1-10, pages 67 ff. and pages 203 ff., in the known ball-wedge technique the bond wire is bonded to the contact pad of a first integrated circuit using a flamed-off sphere also known as a free-air ball (FAB), and at

the other end is flattened into a wedge shape and connected to the contact pad of the second integrated circuit.
What is problematic in this ball-wedge technique is the compressive loading of the second integrated circuit, that is, the circuit at which the bond wire is flattened and pressed—usually with ultrasonic aid—onto the contact pad of the second integrated circuit. In order to reduce the compressive loading of the second integrated circuit, it is known to provide so-called spherical intermediate elements made of electrically conductive material, which sit on the contact pad of the second integrated circuit. The second end of the bond conductor, flattened into a wedge shape, is pressed onto this spherical intermediate element. This intermediate element serves to relieve the pressure.
Bond conductors are commonly made of gold, although gold exhibits poorer electrical and mechanical properties than copper and silver. The reason for this lies in tarnishing, that is, the oxidation of copper upon oxygen contact. This oxidation of copper causes the bond conductor made of copper to lose its bonding capacity. For this reason, when bond conductors made of copper are employed, bonding is often carried out under shielding gas in order to prevent the oxidation of the copper. Up to now, bond conductors made of gold have proved best for the chip-to-chip connection. What is problematic in the case of bond conductors made of gold, however, is the high price of gold. Because of the increasingly good properties of integrated circuits and the increasingly high packing density inside housings, a huge number of bond conductors are necessary in a semiconductor arrangement. Hundreds up to a thousand of such bond conductors may be needed. The cost issue becomes important when bond conductors made of gold are employed.
For this reason, attempts to replace costly bond conductors made of gold with more favorable materials have already become known.
According to US 2003/0113574 Al, gold-jacketed silver or palladium wires have been furnished in order to save on costs in comparison with gold wires.
The use of copper-gold alloys is also known from DE 10 2005 011028 Al. In comparison with gold, copper-gold alloys are marked by significantly greater hardness.

This hardness is, however, undesirable for bond wires because the integrated circuit can be damaged by the pressing of the bond wire or bond conductor onto the contact pad of the integrated circuit. Gold-copper alloys on copper wires can cause cracking in the silicon chip of the integrated circuit when pressed against the contact pad, possibly leading to failure of the integrated circuit.
DE 10 2005 011028 Al further proposes a bond conductor made of copper in whose surface a small quantity of gold is contained. The said publication proposes as especially preferable a bond conductor made of copper having gold enriched in the surface in a quantity that would correspond to a jacketing of at most 50, in particular at most 50 nm, if the gold were fashioned uniformly as a coating on the wire.
What is problematic with such a gold-coated copper wire as bond conductor is that the fabrication of such a gold-coated copper wire is relatively expensive, so that costs are once again increased.
It is therefore an object of the invention to furnish a semiconductor arrangement having a bond conductor that is significantly more favorable than prior bond conductors in terms of cost.
This object is achieved with a semiconductor arrangement having the features of Claim 1.
Developments of this semiconductor arrangement are the subject of the dependent claims.
A method for fabricating such a semiconductor arrangement is the subject of Claim 11. This method provides the following steps:
— furnishing a first integrated circuit (IC1) having at least one first contact pad (3),
— furnishing a second integrated circuit (1C2) having at least one second contact pad
(6),
— furnishing a bond conductor for electrically connecting the first contact pad (3) and the second contact pad (6),
— applying an electrically conductive intermediate element on the contact pad (6) of the second integrated circuit (IC2) in such fashion that the electrically conductive

intermediate element (20) is in electrically conductive connection with the contact pad (6) of the second integrated circuit (IC2),
— mechanically attaching a first end (12) of the bond conductor (10) to the contact pad (3) of the first integrated circuit (IC1),
— mechanically attaching a second, wedge-shaped end (14) of the bond conductor (10) to the intermediate element (20),
— the material of the intermediate element (20) being chosen softer than the material of the bond conductor (10).
Developments of this method claim are set forth in Claims 12 and 13.
The semiconductor arrangement according to the invention is distinguished in that bond conductors made of a unified material that is significantly more favorable than gold can be employed. The employment of bond conductors fabricated in complicated ways, and coated for example with gold, is rendered superfluous by the present invention. This is achieved in that the wedge-shaped end of the bond conductor, which is made for example of copper or a copper alloy, is not directly applied to an integrated-circuit contact pad made for example of aluminum or an aluminum alloy; instead, this wedge-shaped end of the bond conductor is applied and contacted to an intermediate element that sits on the contact pad of the integrated circuit. This intermediate element is preferably fashioned in spherical or approximately spherical shape and is made of a material softer than the bond conductor. The intermediate element will preferably be made of gold, a gold alloy or doped gold.
Because the intermediate element is placed between the wedge-shaped end of the bond conductor and the contact pad of the integrated circuit, the pressure when the wedge-shaped end of the bond conductor is pressed on is not conveyed directly into the semiconductor of the integrated circuit but is first intercepted by the intermediate element. The mechanical loading of the integrated circuit is thus significantly reduced in comparison with conventional methods wherein the wedge-shaped end of the bond conductor is pressed directly onto the contact pad of the integrated circuit.

It falls within the scope of the invention that the bond conductor contacts the contact pads of two integrated circuits that are installed in a common housing. The two integrated circuits can be arranged one beside the other or one above the other. The bond conductor preferably has a diameter of 20-30 um and a Vickers hardness < 70.
In what follows, the semiconductor arrangement according to the invention and the method for its fabrication are explained in greater detail in connection with exemplary embodiments and with reference to the Drawings, in which:
FIG. 1 depicts a first exemplary embodiment having two integrated circuits ICI, IC2 arranged one beside the other in a housing and connected by a bond conductor;
FIG. 2 depicts two integrated circuits arranged one above the other in a housing and likewise electrically contacted one with the other via a bond conductor; and
FIG. 3 is a scanning electron micrograph of the bond conductor of FIG. 1 or FIG. 2 in the region of a wedge-shaped end of the bond conductors of FIG. 1 and FIG. 2.
In the Drawings that follow, unless stated to the contrary, identical reference characters identify identical parts with identical meaning.
FIG. I illustrates a housing 1 of an integrated semiconductor arrangement. Two integrated circuits ICI, IC2 sit one beside the other in the housing. First integrated circuit ICI has a first semiconductor body 2 having contact pads 3, one of these contact pads 3 being discernible in FIG. 1. Semiconductor body 2 is made for example of silicon and has on its surface contact pads that can be made of aluminum or an aluminum alloy. Second integrated circuit IC2 has a second semiconductor body 5 on whose upper surface a contact pad 6 is likewise discernible. Contact pads 3 and 6 of integrated circuits ICI, IC2 are electrically connected to each other via bond conductors 10 in accordance with the electrical requirements of the semiconductor arrangement. One of these bond conductors 10 is illustrated in FIG. 1. Bond conductor 10 is made of a material that is significantly more favorable than gold in terms of cost. Copper or a copper alloy, for example, is suitable as a material for bond conductor 10.
Bond conductor 10 is contacted with first integrated circuit ICI by the known ball-wedge method (ball-wedge bonding). Other contacting methods are also possible. In ball-

wedge bonding, a suitable tool is used to guide bond conductor 10 inside a capillary made of sintered metal or ceramic. The end of bond conductor 10 protruding from the bottom of the capillary is melted by a flame or an electrical discharge so that a sphere forms as a result of surface tension. This sphere is bonded to contact pad 3 of semiconductor body 2 of first integrated circuit IC1 by pressure, heat and ultrasound. The sphere is spread out to some degree by the capillary, so that a sort of nailhead comes about. This spherical end of bond conductor 10 is identified by reference character 12 in FIG. 1.
Bond conductor 10 is then led to the second contact point, here in the exemplary embodiment of FIG. 1 to contact pad 6 of semiconductor body 5 of second integrated circuit IC2. In contrast to conventional techniques, however, bond conductor 10 is not pressed directly onto contact pad 6. Instead, an intermediate element 20, which is preferably spherical or at least approximately spherical in shape, has been placed on this contact pad 6 in a prior processing step. This intermediate element is made of a softer material than bond conductor 10, for example of a softer material than copper if copper is employed for bond conductor 10. Gold, a gold alloy or doped gold has proved a suitable material.
Thus bond conductor 10 is led to contact pad 6 having intermediate element 20 placed thereon. Bond conductor 10 is set on and tightly bonded to intermediate element 20 by ultrasound, heat and pressure. A wire clamp mounted above the capillary of the bonding tool prevents bond conductor 10 being pulled along when the tool is lifted; instead, bond conductor 10 is broken in the region of intermediate element 20. What is produced is a wedge-shaped end 14 of bond conductor 10, which is electrically connected to intermediate element 20.
This spherical intermediate element 20 can be placed on contact pad 6 of semiconductor body 5 of second integrated circuit IC2 with, for example, a second tool. The second tool in turn can be a capillary made of sintered metal or ceramic. As in the shaping of spherical end 12 of bond conductor 10, a gold wire, for example, is led through the capillary of this second tool and the gold wire end protruding from the bottom of the capillary of the second tool can be melted with a flame or an electrical

discharge so that a sphere or ball is formed as a result of surface tension. This sphere or ball of the gold wire is placed on contact pad 6 of semiconductor body 5 of second integrated circuit IC2 by pressure, heat and ultrasound. The gold wire is then broken by the tool so that all that remains on contact pad 6 is spherical intermediate element 20 having a pin-shaped extension.
FIG. 2 depicts a semiconductor arrangement similar to that of FIG. 1. The difference, however, is that second integrated circuit IC2 sits with its semiconductor body 5 directly on semiconductor body 2 of first integrated circuit IC1. Contact pad 6 of second integrated circuit IC2 is connected to contact pad 3 of first integrated circuit IC1 via a bond conductor 10 made of copper. Spherical end 12 of bond conductor 10 sits on contact pad 3 of first integrated circuit IC1, while wedge-shaped end 14 of bond conductor 10 in turn sits on contact pad 6 of second integrated circuit IC2 via intermediate element 20.
Even though FIGS. 1 and 2 illustrate only a single bond conductor 10 for electrically connecting first integrated circuit IC1 and second integrated circuit IC2, it is understood that a multiplicity of such bond conductors must be provided for electrically connecting integrated circuits inside a housing 1. There may for example be a plurality of hundreds of such bond conductors 10.
FIG. 3 depicts three such bond conductors 10 in the region of their wedge-shaped ends 14 as these are electrically contacted, via intermediate elements 20, to contact pads 6 of an integrated circuit. Rectangular contact pads 6 are clearly discernible in FIG. 3. Sitting on contact pads 6 is intermediate element 20, which preferably is made of gold. This intermediate element 20 can no longer be discerned as spherical in the scanning electron micrograph, but it appears that there is, directly on the contact pad, a dish-shaped part 20a having a pin-shaped extension 20b located thereabove. Dish-shaped part 20a of intermediate element 20 and pin-shaped extension 20b are integrally connected to each other, and only in the scanning electron micrograph does it appear as if there are two elements. This is not the case, however. Dish-shaped part 20a having pin-shaped extension 20b comes about in that the capillary of the tool somewhat compresses

intermediate element 20, which was furnished in spherical shape, and a small part remains on this spherical element 20 when the gold wire is broken. This small part, which remains standing out from spherical part 20a, is pin-shaped extension 20b in FIG. 3.
List of Reference Characters
1 Housing
2 Semiconductor body
3 Contact pad

5 Semiconductor body
6 Contact pad
10 Bond conductor
12 Spherical end
14 Wedge-shaped end
20 Spherical intermediate element
IC1 First integrated circuit IC2 Second integrated circuit

Claims
1. A semiconductor arrangement having at least two integrated circuits (IC1, IC2) arranged in a housing and electrically connected to each other via bond conductors (10), at least one of the bond conductors (10) having one end (12) electrically connected to a first contact pad (3) of the first integrated circuit (IC1) and having a second end (14), tapering in wedge shape, electrically connected to an electrically conductive intermediate element (20) sitting on a contact pad (6) of the second integrated circuit (IC2), wherein the bond conductor (10) is made of a different material from the intermediate element (20) and wherein the material of the intermediate element (20) is softer than the material of the bond conductor (10).
2. The semiconductor arrangement of Claim 1 wherein the material of the bond conductor (10) is copper or a copper alloy.
3. The semiconductor arrangement of Claim 1 or 2 wherein the intermediate element (20) is made of gold, a gold alloy or doped gold.
4. The semiconductor arrangement of one of Claims 1 to 3 wherein the intermediate element (20) is furnished in spherical or approximately spherical shape.
5. The semiconductor arrangement of one of Claims 1 to 4 wherein the first end of the bond conductor (10) has a spherical or approximately spherical shape.
6. The semiconductor arrangement of one of Claims 1 to 5 wherein the bond conductor (10) has a diameter of approximately 20-30 µm.

7. The semiconductor arrangement of one of Claims 1 to 6 wherein the material of the bond conductor (10) has a Vickers hardness < 70.
8. The semiconductor arrangement of one of Claims 1 to 7 wherein the first integrated circuit (ICl) and the second integrated circuit (IC2) are arranged one beside the other.
9. The semiconductor arrangement of one of Claims 1 to 7 wherein the first integrated circuit (ICl) and the second integrated circuit (IC2) are arranged one above the other and the second integrated circuit (IC2) sits on the first integrated circuit (ICl).
10. A method for fabricating a semiconductor arrangement according to one of
Claims 1 to 9 having the following steps:
— furnishing a first integrated circuit (ICl) having at least one first contact pad (3),
— furnishing a second integrated circuit (IC2) having at least one second contact pad
(6),
— furnishing a bond conductor for electrically connecting the first contact pad (3) and the second contact pad (6),
— applying an electrically conductive intermediate element on the contact pad (6) of the second integrated circuit (IC2) in such fashion that the electrically conductive intermediate element (20) is in electrically conductive connection with the contact pad (6) of the second integrated circuit (IC2),
— mechanically attaching a first end (12) of the bond conductor (10) to the contact pad (3) of the first integrated circuit (ICl),
— mechanically attaching a second, wedge-shaped end (14) of the bond conductor (10) to the intermediate element (20),
— the material of the intermediate element (20) being chosen softer than the material of the bond conductor (10).

11. The semiconductor arrangement of one of Claims 1 to 10 wherein the contact pads (6) are formed from aluminum or an aluminum alloy.
12. The method of Claim 11 wherein the intermediate element (20) is applied to the contact pad (6) of the second integrated circuit (IC2) as a spherical or approximately spherical element and1
13. The method of Claim 11 or 12 wherein the bond conductor (10) is attached by ball-wedge bonding to the first integrated circuit (ICl) and the second integrated circuit (IC2) in that the first end (12) of the bond conductor (10) is melted as a sphere onto the contact pad (3) of the first integrated circuit (ICl) and the wedge-shaped second end (14) of the bond conductor is fashioned as a wedge bond.