ALD Barriers Show Promise, Not Perfection

At IEEE's latest International Interconnect Technology Conference (IITC) meeting, there were many hot topics including optical interconnects, copper/low-k integration and low-k packaging issues. But the burgeoning technology of atomic layer deposition (ALD or ALCVD) stole the show for many attendees.

ALD promises monolayer thickness control of ultrathin metal barrier and seed for copper interconnects. It is expected to take over when physical or chemical vapor deposition films can no longer meet conformality requirements of high-aspect-ratio vias and trenches — possibly as soon as the 65 nm node. In addition, ALD brings to bear new material options. For instance, WN_xC_y is a promising diffusion barrier that features higher growth rates than TiN, adhesion to a variety of low-k materials as well as copper, and relatively low process temperatures (300-350°C).

However, the ALD process is slow, so it can only be used for ultrathin films (<10 nm). Some ALD films, though they are showing compatibility with low-k films, will be difficult to integrate with next-generation, porous low-k dielectrics. According to Suvi Haukka of ASM International (Bilthoven, Netherlands), ALD films tend to deposit around open pores in a porous low-k dielectric and inside the pores of closed-pore structures. Together with colleagues from ASM Belgium (Leuven, Belgium), Haukka presented ALD grown mechanisms and explored the compatibility of a variety of barrier layers with low-k materials and copper.

Wei-Min Li of ASM Microchemistry Ltd. (Espoo, Finland) presented findings from the company's work with Philips Research (Eindhoven, Netherlands) and International SEMATECH (Austin, Texas) to characterize the ALD process for growing WN_xC_y (W:N:C @ 55:15:30). These films demonstrated compatibility with Cu, SiO₂, SiC, Si_xN_y, SiLK and Aurora films, and can be deposited at a rate of 0.8 Å/cycle.

One reaction cycle in an ALD reactor grows a monolayer of material. That cycle involves four steps — introduction of precursor and reaction by chemisorption with the substrate; N₂ purge; introduction and reaction of the second precursor (i.e. ligand removal agent); and N₂ purge. ASM evaluated several processes for ALD of TiN, WN_xC_y and Cu seed.

Haukka emphasized that ALD requires that the starting and growing surfaces provide reactive sites for depositing metal compounds, such as -OH and -NH_x on oxides and nitrides, respectively. For a non-metal compound the ligands of the absorbed metal complex serve as reactive sites.

The reaction temperature must be selected such that no uncontrolled CVD growth or condensation of the precursor occurs. During reaction, the metal precursor must react selectively with the reactive sites. Furthermore, a sufficient dose of precursor must be brought into contact with the surface to achieve saturation.

Prior to copper barrier/seed ALD, CuO at the via bottoms must be reduced to Cu. ASM uses a process involving organic compounds (alcohols, carboxyl acids, formic acid or aldehydes) at m400°C to form volatile organic byproducts.

Because copper does not have the proper reactive sites, copper ALD must be performed indirectly, through the reduction of CuO. CuO deposited from Cu(thd)₂ and O₃ adheres well to many substrates. It can then be reduced to copper in a subsequent process. The challenge with this approach may be barrier layer interaction with a strong oxidant, though the studies have shown no severe oxidation problem.

TiN and W_xN_y films can be grown at 350-400°C using TiCl₄/NH₃ and WF₆/NH₃, respectively. But severe copper pitting results, possibly due to the formation of volatile CuCl and CuF, according to ASM's studies. Pitting may be lessened by reducing the process temperature; however, >350°C is required to volatize the halides and to provide films with low levels of contaminants.

ASM has found that SiO₂ -based low-k materials containing carbon must be pretreated prior to barrier metal growth to remove the carbon for the topmost surface and to provide proper reactive sites. For organic low-k materials, TiN agglomerates on the surface. To solve this problem and others, a new process, ALD of WN_xC_y, was explored, which deposits uniformly on organics.

The ASM/Philips group deposited ultrathin (<5 nm) WN_xC_y films from WF₆, NH₃ and TEB (triethylboron) at temperatures of 250-400°C and found that typical ALD surface-controlled reactions take place at 300-350°C. Film concentration and sheet resistance are fairly constant in this temperature regime. The incorporation of carbon in the film reduces resistivity from typically 3000 µΩcm for WN_x to 300-400 µVcm for WN_xC_y.

Film impurity analysis from different laboratories confirmed ultralow levels of F, B and O in the films (<1 at%). Interestingly, the TEB presence seems to be instrumental in preventing copper pitting. In initial dual-damascene structures, a 3 nm WN_xC_y barrier demonstrated very low contact resistivity with low leakage (10^-13 A at 2 MV/cm, 200°C). Further characterization of Cu diffusion, compatibility and process optimization will follow this work.