Metal fusion bonding – Process – With measuring – testing – indicating – inspecting – or...
Reexamination Certificate
2001-05-24
2002-08-20
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
With measuring, testing, indicating, inspecting, or...
C228S119000
Reexamination Certificate
active
06435398
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to the field of semiconductor devices and processes and more specifically to a method for chemically reworking metal layer misprocessed on bond pads of integrated circuits having copper metallization.
DESCRIPTION OF THE RELATED ART
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. Pat. No. 5,244,140, issued on Sep. 14, 1993 (Ramsey et al., “Ultrasonic Bonding Process Beyond 125 kHz”); U.S. Pat. No. 5,201,454, issued on Apr. 13, 1993 (Alfaro et al., “Process for Enhanced Intermetallic Growth in IC Interconnections”); and U.S. Pat No. 5,023,697, issued on Jun. 11, 1991 (Tsumura, “Semiconductor Device with Copper Wire Ball Bonding”).
In the continuing trend to miniaturize the ICs, the RC time constant of the interconnection between active circuit elements increasingly dominates the achievable IC speed-power product. 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. Consequently, there is now a strong drive in the semiconductor industry to employ copper as the preferred interconnecting metal, based on its higher electrical conductivity and lower electromigration sensitivity. From the standpoint of the mature aluminum interconnection technology, however, this shift to copper is a significant technological challenge.
Copper has to be shielded from diffusing into the silicon base material of the ICs in order to protect the circuits from the carrier lifetime-killing characteristic of copper atoms positioned in the silicon lattice. 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.
In order to overcome these problems, a process has been disclosed to cap the clean copper bond pad with a layer of aluminum and thus re-construct the traditional situation of an aluminum pad to be bonded by conventional gold-wire ball bonding. A suitable bonding process is described in U.S. Pat. No. 5,785,236, issued on Jul. 28, 1998 (Cheung et al., “Advanced Copper Interconnect System that is Compatible with Existing IC Wire Bonding Technology”). The described approach, however, has several shortcomings.
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.
A low-cost structure and method for capping the copper bond pads of copper-metallized ICs has been disclosed on U.S. Patent Application No. 60/183,405, filed on Feb. 18, 2000. The present invention is related to that application. The structure provides a metal layer electrolessly plated onto the copper, which impedes the up-diffusion of copper. Of several possibilities, nickel is a preferred choice. This layer is topped by a bondable metal layer, which also impedes the up-diffusion of the barrier metal. Of several possibilities, gold is a preferred choice. In a variation of this sequence of metal layers, a second barrier layer such as palladium is electrolessly deposited on the nickel, blocking its up-diffusion, and then followed by a bondable metal such as gold. Metallurgical connections to the stacked cap of layers can then be performed by conventional wire bonding.
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. Deposition depletes the reactants in areas around the bond pads. Increasing the agitation of the solution only exacerbates the deposition non-uniformity, which is influenced by the flow direction of the solution. Also, a few bond pads may not receive any metal deposition. 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. See, for example, U.S. Pat. No. 5,024,746, issued Jun. 18, 1991, and U.S. Pat. No. 4,931,149, issued Jun. 5, 1990 (Stierman et al., “Fixture and a Method for Plating Contact Bumps for Integrated Circuits”).
An urgent need has arisen for a reliable method of reworking metal caps over copper bond pads which combines minimum fabrication cost with maximum correction control of the IC structure. The reworking method should be flexible enough to be applied for different IC product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput, and without the need of expensive additional manufacturing equipment.
SUMMARY OF THE INVENTION
The present invention discloses a method for reworking integrated circuit (IC) wafers having copper-metallized bond pads covered by deposited layers of a barrier metal and a bondable metal. After identifying the wafers with off-spec metal layers, the wafers are chemically etched using selective etchants consecutively until the metal layers over the bond pads are removed without damaging the copper metallization. Replacement metal layers are finally deposited over the bond pads.
The present invention is related to high density and high speed ICs with copper interconnecting metallization, especially those having high numbers of metallized inputs/outputs, or “bond pads”. These circuits can be found in many device families such as processors, digital and analog devices, logic devices, high frequency and high power devices, and in both large and small area chip categories.
It is an aspect of the present invention to increase the process yield of wafer fabrication, since it provides a low-cost rework method for wafers misprocessed almost at the end of the process flow. It is towards the completion of the fabrication process when a high amount of work and time has already been invested and the wafers represent high value.
Another aspect of the invention is to support technologies applicable to bond pad area reduction and thus 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.
Another aspect of the invention is to support the fabrication of bond pad metal caps by the self-defining process of electroless deposition, which is preferable over costly photolithographi
Hartfield Cheryl
Moore Thomas M.
Dunn Tom
Honeycutt Gary C.
Johnson Jonathan
Navarro Arthur I.
Telecky Fred
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