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Aluminum has been used as the interconnect material for the past 40 years since the beginning of integrated circuit processing. As the semiconductor technology is scaled below 0.2um, the limitations of Al interconnect became more pronounced. At 0.18um technology node, the industry began putting serious efforts into adopting Cu interconnect in place of Al. The interconnect-intensive logic technology leads the way to migration to Cu interconnects and memory technology follows.

As chip dimensions shrink, aluminum interconnects face serious challenges in processing capability, chip performance and reliability. Filling a narrow gap between Al lines with dielectric becomes a difficult process task. RC time delay due to Al interconnects becomes increasingly unbearable for high speed ICs. Electro-migration of Al interconnects poses a serious reliability concern with shrinking Al interconnect geometry and increasing current density. Thus the stage has been set for an alternative material to be used as an interconnect material in IC processing.

Copper is the best conductor of electricity. Electro-migration performance of Cu is significantly better than Al. These properties made Cu an ideal replacement for Al. However, implementation of Cu in silicon processing was not easy. One of the problems with copper is that it is a fast diffuser in the oxide. Unless it is encapsulated with a reliable diffusion barrier material, Cu can poison the oxide and destroy the MOS transistors underneath it.

The first company that brought Cu interconnects to commercially viable process is IBM. In the late 1980s, IBM made a breakthrough in Cu research, finding one metal that could be used as a diffusion barrier. After a total of nearly 15 years of research, IBM announced in September 1997 development of Cu process.

IBM began using Cu process in its sub-0.25um high performance logic technologies, delivering PowerPC chips and logic chips for its supercomputer. By March 2000, IBM shipped two million chips processed with Cu. By October 2001, IBM shipped more than 25 million so-called “copper chips”. By then, copper interconnect has already become a standard process for high performance logic technology at IBM.

Motorola followed closely the heels of IBM in pursuit of Cu interconnect. Motorola announced at the end of September 1997 that it also has developed copper interconnect technology and unveiled its Cu interconnect development results in December 1997 at International Electron Devices Meeting (IEDM), the same meeting where IBM disclosed its Cu research results.

Motorola began using Cu process at the 0.2um technology. It licensed its Cu interconnect process and high performance logic technology (HiPerMOS) to AMD as part of AMD’s alliance with Motorola for logic technology development. AMD manufactures its high-profile flagship products, Athlon and Opteron microprocessors using 0.18um HiPerMOS6 and 0.13um HiPerMOS7 technology. AMD’s effort brought Cu interconnect to a stable volume manufacturing process, as was described in its paper delivered at IEDM 2003.

A firm believer of transistor performance as most critical to its microprocessor performance and the leader of transistor performance in the industry, Intel took a less aggressive approach in its migration to Cu interconnect. Intel made Al-to-Cu conversion for its logic technology at 0.13um node.

Migration of interconnect technology from Al to Cu also took place in the silicon foundry houses starting at the 0.18um generation. For its 0.18um technology, TSMC offered customers the option of Cu interconnects for the top two layers, where the benefits of Cu interconnect could be maximized. For 0.13um and beyond, TSMC offers Cu process for all interconnect levels.

Although Cu interconnect is already being used in the mainstream volume manufacturing process, it is still a work in progress. The full potential of Cu interconnects and the great promise it brings to semiconductor industry in terms of performance, reliability, ease of processing, cost reduction, etc, has yet to be fully realized. Until then, Cu remains a great promise and also a great challenge as well.

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