Grain Structure Key to Preventing Cu Diffusion
Systematic yield problems can result from design problems that were not adequately addressed in the development stage.
Peter Singer, Editor-in-Chief -- Semiconductor International, 7/1/1998
Fujitsu researchers have investigated the relationship between the grain structure of diffusion barriers deposited by various means (i.e., CVD and sputtering) and their effectiveness as a barrier against copper diffusion. Although they focused on TiN barriers, their results should be of interest to those working with other types of diffusion barriers, such as tantalum and tantalum nitride.
In a recent paper (Journal of the Electrochemical Society, June 1998), the researchers -- Takahiro Kouno, Hideo Niwa and Masao Yamada, who work in Fujitsu's Advanced Materials and Process Development Dept. in Kawasaki -- report that although many investigations on the diffusion barrier property in Cu metalization have been reported, most of them have focused on the case in which Cu atoms diffuse into silicon substrates or SiO2 films from the bottom of the copper interconnections. Their own research, however, focused on the diffusion from the sides of the interconnections because "the thickness of the barrier metal layer on the sides of the trenches must be thinner than that on the bottoms, as long as the barrier is deposited by a sputtering method."
Using TEM samples, combined with EDX and leakage current measurements, the researchers analyzed the properties of diffusion barriers deposited by CVD, conventional sputtering and long-throw sputtering. The CVD-TiN was deposited from a TiCl4, monomethylhydrazine and NH4 mixture at a temperature of 530°C.
The findings of the most interest were how the thickness of the films on the bottom and sides of the trenches varied and how the differences in the microstructure of the films impacted their effectiveness as diffusion barriers. Although the TiN thickness on the bottom of the trenches deposited by the three methods is almost the same at about 40 nm, the thickness on the side is very different. In the case of CVD, the thickness on the side is about the same as it is on the bottom. In the case of conventional and long-throw sputtering, the thickness on the side is thinner; with long-throw, it is only about half that of the bottom layer.
In terms of the microstructure, the researchers studied the tilt angle of the TiN grain boundaries and the voids between grains on the sides of the trenches. For the CVD TiN, the grain boundaries are normal to the sidewall on the side of the trench, with few voids between grains. The grain boundaries of the conventionally sputtered layer tilt upward by about 13° from the normal direction to the sidewall, with some voids observed. The layers deposited by long-throw sputtering have grain boundaries that tilt upward by about 27° and have larger voids compared to the conventionally sputtered films.
Both CVD and conventionally sputtered TiN films proved to be effective against copper diffusion: No Cu was detected in the SiO2 films, either outside the sidewall or underneath the bottom of the trench. With the long-throw approach, no copper was detected in the SiO2 films underneath the bottom of the trench, but some copper was detected outside the sidewall. This was true even when the TiN thickness on the side of the trench was equal to or greater than that of films deposited by the other two approaches.
The main conclusion of the researchers was that the grain structure of barrier metal layers is one of the most important factors in preventing diffusion in copper metalization.
Ferroelectric Etch Focus of Tegal Seminar
Tegal Corp.'s 24th annual Plasma Etch Technology Seminar will be held July 13 in San Francisco, Calif., in conjunction with SEMICON West 98. This year's forum focuses on the industry's transition to new materials such as platinum, iridium, BST and PZT. Qualified industry members are invited to R.S.V.P. to this event by contacting Tegal's Eileen Feeny via phone at (707) 765-5656 or (800) 828-3425 Ex. 5656 or fax at (707) 765-9311. The program, along with lunch and a copy of the proceedings, is free of charge.
Company News
Applied Materials (Santa Clara, Calif.) announced that it is participating on a project with SEMATECH (Austin, Texas) to characterize CMP processes for copper metalized wafers using the company's Mirra CMP system. It also began the second phase of a separate program with SEMATECH to develop processes that can be used to etch emerging low-k materials.
Dover Instrument completed a 30,000 ft2 building expansion at its headquarters in Westboro, Mass., which doubles the amount of space available for manufacturing.
Eaton Thermal Processing Systems (Peabody, Mass.) announced the first sales of its Summit Rapid Thermal Processing (RTP) system, which are capable of implant anneal, silicidation, oxide growth, BPSG reflow and other thermal processes. Eaton also launched a new web site at www.semiconductor.eaton.com.
POCO Graphite (Decatur, Texas) and Leybold Materials Inc. (Morgan Hill, Calif.) agreed to partner on the production of carbon sputtering targets for thin film material applications.
Schumacher, a unit of Air Products and Chemicals Inc. (Calrsbad, Calif.), and Sandia National Labs (Albuquerque, N.M.) will jointly research low dielectric constant thin films as part of a 12-month CRADA focused on methods of applying and measuring the performance of porous inorganic films.
Siemens AG and Motorola's Semiconductor 300 joint venture facility in Dresden purchased an Aspen III ICP strip system from Mattson Technology (Fremont, Calif.).
Soitec (Grenoble, France) and Motorola (Tempe, Ariz.) announced a joint R&D program to develop thin film silicon carbide on silicon (SiC on Si) technology, aimed at applications in high-temperature, high-power and/or high-radiation conditions.
