Fixture and method for uniform electroless metal deposition on integrated circuit bond pads

Abstract

A method and an apparatus for uniform electroless plating of layers onto exposed metallizations in integrated circuits such as bond pads. The apparatus provides means for holding a plurality of wafers, and rotating each wafer at constant speed and synchronous within the plurality. Immersed in a plating solution flowing in substantially laminar motion and at constant speed, the method creates periodic superposition of directions and speeds of the motion of the wafers and the motion of the plating solution. The invention creates periodically changing wafer portions where the directions and speeds are additive and where the directions and speeds are opposed and subtractive. Consequently, highly uniformly layers are electrolessly plated onto the exposed metallizations of bond pads. If the plated layers are bondable metals, the process transforms otherwise unbondable pad metallization into bondable pads.

Description

FIELD OF THE INVENTION [0001] The present invention is related in general to the field of semiconductor devices and processes and more specifically to a fixture and process for electroless plating bondable metal caps onto bond pads of integrated circuits having copper interconnecting metallization. DESRCIPTION 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 ease 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. ...

AI Technical Summary

Benefits of technology

[0012] It is an aspect of the present invention to be applicable to bond pad area reduction and thus to be in support of 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.

[0013] Another aspect of the invention is to deposit the bond pad metal caps by the self-defining process of electroless plating, thus avoiding costly photolithographic and alignment techniques.

[0015] 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.

[0017] 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.

[0019] In the first embodiment of the invention, an apparatus is disclosed for uniform electroless plating of layers onto exposed metallizations in integrated circuits, such as bond pads, which are positioned on the active surface of semiconductor wafers. The apparatus is suitable for simultaneous processing of a plurality of wafers. It provides rotation at constant speed synchronously to the wafers and thus creates relative motion, between the wafers and the chemical solution of a plating bath.

[0021] In all preferred embodiments, 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.

The structure provides a metal layer plated onto the copper, which impedes the up-diffusion of copper.

This layer is topped by a bondable metal layer, which also impedes the up-diffusion of the barrier metal.

It is difficult, though, to plate these bond pad caps uniformly in electroless deposition systems, because electroless deposition is affected by local reactant concentrations and by the agitation velocities of the aqueous solution.

Increasing the agitation of the solution only exacerbates the deposition non-uniformity, which is influenced by the flow direction of the solution.

The problem is further complicated when a whole batch of wafers is to be plated simultaneously in order to reduce cost, since known control methods have been applied only to process single wafers under applied electrical bias.

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