Wire bonding process for copper-metallized integrated circuits

Abstract

A robust, reliable and low-cost metal structure and process enabling electrical wire / ribbon connections to the interconnecting copper metallization of integrated circuits. The structure comprises a layer of barrier metal that resists copper diffusion, deposited on the non-oxidized copper surface in a thickness such that the barrier layer reduces the diffusion of copper at 250° C. by more than 80% compared with the absence of the barrier metal. The structure further comprises an outermost bondable layer which reduces the diffusion of the barrier metal at 250° C. by more than 80% compared with the absence of the bondable metal. Finally, a metal wire is bonded to the outermost layer for metallurgical connection. The barrier metal is selected from a group consisting of nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. The outermost bondable metal layer is selected from a group consisting of gold, platinum, and silver.

Description

BACKGROUND OF THE INVENTION [0001] The present invention is related in general to the field of semiconductor devices and processes and more specifically to the process of wire bonding to bond pads of copper-metallized integrated circuits. DESCRIPTION OF THE RELATED ART [0002] In integrated circuits (IC) technology, pure or doped aluminum has been the metallization of choice for interconnection and bond pads for more than four decades. Main advantages of aluminum include easy of deposition and patterning. Further, the technology of bonding wires made of gold, copper, or aluminum to the aluminum bond pads has been developed to a high level of automation, miniaturization, and reliability. Examples of the high technical standard of wire bonding to aluminum can be found in U.S. Pat. No. 5,455,195, issued on Oct. 3, 1995 (Ramsey et al., “Method for Obtaining Metallurgical Stability in Integrated Circuit Conductive Bonds”); U.S. Pat. No. 5,244,140, issued on Sep. 14, 1993 (Ramsey et al., “...

AI Technical Summary

Benefits of technology

[0008] The present invention discloses a robust, reliable and low-cost metal structure and process enabling electrical wire connections to the interconnecting copper metallization of integrated circuits (IC). The structure comprises a layer of barrier metal that resists copper diffusion, deposited on the non-oxidized copper surface in a thickness such that the barrier layer reduces the diffusion of copper at 250° C. by more than 80% compared with the absence of the barrier metal. The structure further comprises an outermost layer reduces the diffusion of the barrier metal at 250° C. by more than 80% compared with the absence of the bondable metal. Finally, a metal wire is bonded to the outermost layer for metallurgical connection.

[0011] It is an aspect of the present invention to be applicable to bond pad area reduction and thus supports the shrinking of IC chips. Consequently, the invention helps to alleviate the space constraint of continually shrinking applications such as cellular communication, pagers, hard disk drives, laptop computers and medical instrumentation.

[0012] Another aspect of the invention is to fabricate the bond pad metal caps by the self-defining process of electroless deposition, thus avoiding costly photolithographic and alignment techniques.

[0014] Another aspect of the invention is to advance the process and reliability of wafer-level multi-probing by eliminating probe marks and subsequent bonding difficulties.

[0016] Another object of the invention is to use only designs and processes most commonly employed and accepted in the fabrication of IC devices, thus avoiding the cost of new capital investment and using the installed fabrication equipment base.

[0017] These objects have been achieved by the teachings of the invention concerning selection criteria and process flows suitable for mass production. The non-oxidized surface of the copper of the bond pad is seeded by a metal such as palladium and covered with a layer of a barrier metal such as nickel. The thickness of this barrier layer has to be such that it prevents excessive copper up-diffusion at the elevated temperatures of the bonding operation. The outermost layer is a bondable metal such as palladium or gold. The layer thickness has to be such that it prevents nickel up-diffusion to the surface, where it would oxidize and impede wire bonding. In mass production, the various metal layers are deposited by electroless plating, thus avoiding the need for expensive photolithographic definition steps.

Problems solved by technology

Consequently, the relatively high resistivity of the interconnecting aluminum now appears inferior to the lower resistivity of metals such as copper.

Further, the pronounced sensitivity of aluminum to electromigration is becoming a serious obstacle.

From the standpoint of the mature aluminum interconnection technology, however, this shift to copper is a significant technological challenge.

For bond pads made of copper, the formation of thin copper(I)oxide films during the manufacturing process flow has to be prevented, since these films severely inhibit reliable attachment of bonding wires, especially for conventional gold-wire ball bonding.

In contrast to aluminum oxide films overlying metallic aluminum, copper oxide films overlying metallic copper cannot easily be broken by a combination of thermocompression and ultrasonic energy applied in the bonding process.

As further difficulty, bare copper bond pads are susceptible to corrosion.

First, the fabrication cost of the aluminum cap is higher than desired, since the process requires additional steps for depositing metal, patterning, etching, and cleaning.

Second, the cap must be thick enough to prevent copper from diffusing through the cap metal and possibly poisoning the IC transistors.

Third, the aluminum used for the cap is soft and thus gets severely damaged by the markings of the multiprobe contacts in electrical testing.

This damage, in turn, becomes so dominant in the ever decreasing size of the bond pads that the subsequent ball bond attachment is no longer reliable.

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