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CVD Integrates Inorganics.

There's a viable solution to the need for titanium- and tantalum-based liner films that work in emerging semiconductor ULSI manufacturing process flows.

An inorganic approach to chemical vapor deposition (CVD) that employs titanium and tantalum heavy halide compounds in an innovative low-temperature strategy for making device-quality films has been jointly developed by a long-standing, business-research center partnership. The developers are Gelest Inc., Tullytown, Pa., and the Univ. at Albany-SUNY (UAlbany), Albany, N.Y. The CVD process resulting from the collaboration helps to maintain device stability in ultra-large-scale integration (ULSI) microelectronics fabrication, and promises to finesse problems inherent in atom layer deposition (ALD) techniques, which come into play as structure dimensions shrink to smaller than 100 nm.

Gelest researchers, working in partnership with the UAlbany Institute for Materials (UAIM), evaluated integration and performance issues with aluminum and copper interconnect wires in high-performance integrated circuit applications. In geometries less than 0.25 [micro]m, for instance, the high diffusion rate of copper limits device stability. Copper is highly reactive and a fast diffuser in silicon. It forms deep trap levels, which ruin device performance. Alternatively, high current densities within small device topographies cause significant aluminum migration, a phenomenon known as electromigration. Electromigration leads to device failure from openings in interconnect wires due to the complete rearrangement of aluminum atoms.

Titanium-titanium nitride (Ti/TiN) films act as an adhesion promoter (glue layer) and diffusion barrier for aluminum. (Barrier and adhesion layers in such interconnect structures are called liners.) Tantalum-tantalum nitride (Ta/TaN) bilayered films provide a similar role in the case of copper, with titanium-tantalum nitride (Ti/TaN) potentially acting as a more viable copper liner in sub-100 nm device generations. No previous technology has allowed in situ bilayer deposition, in this case, Ti/TiN, Ta/TaN, or Ti/TaN deposition in a single chamber at process conditions that maintained device integrity.

"Titanium metal adheres well to substrates but is not a great barrier layer," says Barry Arkles, president of Gelest. "Tantalum nitride is a great barrier layer but does not stick to much except to titanium. So a bilayer deposition is preferred. The preferred stoichiometry for tantalum-based films is somewhere between tantalum metal and tantalum nitride to get the best barrier layers. So in the case of titanium, you want a true bilayer structure, and in the case of tantalum, you want the ability to dial it between stoichiometries."

What Gelest and UAIM developed was a low-temperature CVD and ALD technology that allows smooth transitions from metal to nitride deposition in a single process step.

Gelest, which has been in this business since the days of SEMATECH, a consortium charged with ramping up semiconductor research in the US, has a long-standing background in chemical expertise and organo-metallics. "We knew the technology from a chemical perspective," says Arkles. "And UAIM and SEMATECH had the device or microelectronics perspective."

"Combining the advantageous material properties of titanium- and tantalum-based compounds with the desirable process characteristics of ALD and CVD creates an exemplary solution for sub-130-nm device nodes," says Alain Kaloyeros, executive director of UAIM. "The resulting solution coats extremely narrow structures with conformal bilayered liners, leading to enhanced mechanical stability and improved electrical performance."

UAIM was founded by UAlbany as the umbrella organization that oversees, coordinates, and promotes the university's six research, development, education, technology transfer, workforce training, and economic outreach centers in the area of advanced materials.

"Our joint venture with Gelest represents a vertically integrated partnership that covers the entire spectrum of skills and resources necessary to develop and deploy advanced semiconductor technologies, from chemical source development to device prototyping and testing," says Kaloyeros.

Tantalum and its binary and ternary nitrides are highly refractory materials and are stable to extremely high temperatures; they are non-reactive with copper. Copper-tantalum contacts are stable up to 550 [degrees] C and the motion of copper is extremely slow through tantalum at temperatures used in microelectronic device fabrication. And needs for ever-thinner liners make tantalum-based alloys more desirable than titanium counterparts. Also, the tantalum alloys provide diffusion barrier performance required in ULSI structures.

The substrates intended for ULSI circuitry are patterned with holes, trenches, and other features with diameters that are currently around 130 nm, soon to be reduced below 100 nm, and even 70 nm.

To prepare these films, the Gelest-UAIM team dismissed metal-organic CVD and physical vapor deposition (sputtering) methods and turned to an inorganic approach to CVD and ALD. This approach uses titanium and tantalum heavy halide compounds capable of dissociating under mild thermal or plasma environments to form titanium or tantalum metal--or by including nitrogen or ammonia as reactant gases, to form the respective nitrides.

Previously titanium, tantalum, and its nitrides were grown by conventional sputtering techniques. However, these were not capable of conformal step coverage in aggressive trench and other structures, given the line-of-sight approach to metal deposition. Inorganic CVD and ALD solve this problem; these use the substrate surface as a catalyst for the deposition reaction. (Titanium provides a stable ohmic contact when deposited directly on silicon and alloys to it forming a titanium silicide.)

The Gelest-UAIM process uses halide-based complexes for the low-temperature growth of either the transition metal (Ti or Ta) or its nitrides and/or its silicide alloys, depending on the reactants used. Modifying the processing conditions provide certain flexibilities in situ. The titanium and tantalum nitride alloys are deposited using at least one carrier gas: hydrogen or hydrogen in combination with nitrogen or a variety of others. In one process (components in a chamber with a plasma that has a power density of about 0 to 10 W/[cm.sup.2] for an appropriate time), tantalum bromide is the preferred source precursor.

Alternating or sequentially deposited multilayers of pure titanium, tantalum and/or tantalum nitride alloy films can be deposited to the substrate while in a single reactor. Or multiple layers can be provided to the substrate when it is moved between two or more separate reactors, each used for deposition of the various titanium, tantalum, or nitride alloy layers depending on the application planned for the coated substrate. In this case, the reactors, preferably, are interconnected through leak-tight transfer arms and/or load locks.

The Gelest-UAIM CVD and ALD methods prepare pure (electronic grade) titanium, tantalum and tantalum nitride alloy films for use as diffusion barriers or as adhesion interlayers in integrated circuit fabrication and ULSI fabrication. Carefully selected precursors are placed in either a thermal or plasma-promoted CVD reactor. Titanium and tantalum-silicide nitride alloy films, and titanium and tantalum silicide alloy films can also be produced. These can be formed in bilayer and multilayer laminated structures as required.

The energy needed for bond cleavage is supplied entirely by thermal energy, in part by the high-energy electrons formed in a glow discharge or plasma with a power density of about 0 to 100 W/[cm.sup.2] as noted. Plasma-promoted CVD takes advantage of the high-energy electrons present in glow discharges to disassociate gaseous molecules. However, the low-power densities used in plasma-promoted CVD do not cause premature precursor decomposition in the gas phase, and therefore prevent undesirable film contamination and electrical damage to the film and substrate.

The plasma may be generated by various sources (direct current plasma, radio frequency plasma), and may be used for dual purposes. It may be used for in situ predeposition substrate cleaning and for actual deposition.

Alternatives, as to reactor, precursor, carrier gas, and energy sources, are greatly variable and depend on the type of film and purity required. The inventive titanium- and tantalum-based films, whether prepared by thermal or plasma-promoted CVD, typically have a nitrogen-to-tantalum ratio of about 0 to 3.

In addition to semiconductor substrates, other substrates that can be coated include metal, glass, plastic, or other polymers. Applications include hard protective coatings for aircraft components, automotive parts and engines, cutting tools, and even jewelry.

The in situ deposition of titanium, tantalum and tantalum-based bilayers and multilayers is convenient for preparation of ULSI devices. The method allows for formation of a bilayer or multilayer without the need to transfer the partially coated substrate between reaction chambers and risking exposure to air. That the bi- or multilayer can be made in a single reaction chamber greatly minimizes the risk of film contamination. Contamination is a particular problem for titanium, tantalum and tantalum-based films, because tantalum is typically reactive with oxygen--and even a slight amount could destroy the usefulness of the coatings in ULSI devices.
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Author:Poliski, Iris
Publication:R & D
Geographic Code:1USA
Date:Jul 1, 2001
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