IBM Reveals Key Copper/Low-k Integration Issues for Production
-- Semiconductor International, 8/1/2000
Considered by many to be the best interconnect conference yet addressing process integration issues, the recent IEEE International Interconnect Technology Conference (IITC) explored many of the issues device manufacturers face when integrating copper interconnects with low-k insulators. IBM (E. Fishkill, N.Y.) was perhaps the most forthcoming with its disclosure of details of both its copper/FSG (k = 3.5) and copper/SiLK (k = 2.65) low-k processes. IBM is using copper and FSG in its 0.18 µm processes and is in the process of moving copper and SiLKlow-k dielectric from Dow Chemical (Midland, Mich.) into production for 0.15 µm and 0.13 µm device production. Interestingly, IBM found no new failure modes with the copper/FSG or copper/SiLK process integration, indicating new types of failures appear more with the transition from aluminum to copper interconnects than with a change in interlevel dielectric.Edward Barth and coworkers at IBM (Hopewell Junction, N.Y.) showed how a front-end-of-line based on silicon-on-insulator technology is being combined with a copper/FSG back-end-of-line to enable 1 GHz high-performance logic operation on a variety of products. IBM uses a bilayer dielectric approach, filling 80% of the via level with undoped oxide (USG) and topping it with FSG (fluorinated SiO2) film. By extending FSG 20% below the line level, IBM dramatically reduces line-to-line capacitance beyond that resulting from an FSG level coincident with the interconnect line bottom. The company noted that all lithography, etch, metalization and barrier metal processes using FSG are common with its mainstream copper/USG processes.
Ron Goldblatt explained the challenges of integrating SiLK organic dielectric with copper, an IBM offering that yields an effective k of 3.0 and 37% better performance than aluminum/SiO2 in a 0.13 µm device. After integrating a variety of organic and inorganic low-k dielectrics, Goldblatt said IBM initially chose SiLK because it met critical thermal, mechanical and chemical properties. For instance, it maintains thermal and dimensional stability to 400°C, has a high glass transition temperature, low moisture absorption and a stable, isotropic dielectric constant. SiLK further meets mechanical needs including adhesion to various materials, crack resistance, and manageable film stress levels and modulus. Integration also requires chemical compatibility, etch selectivity and compatibility with CMP and wire bonding/C4 bonding processes. SiLK dielectric is used at the metal-2 through metal-5 layers with USG or FSG at the global interconnect layers in an eight-layer copper structure.
According to IBM, SiLK integration is less complex than other low-k alternatives, though proponents of CVD low-k dielectrics often cite complexity as a reason not to use spin-on low-k dielectrics. IBM uses a bilayer hardmask integration without an etch stop layer to facilitate etching and CMP without the capacitance compromise associated with a buried etch stop. The company developed with its equipment suppliers new selective etching and post-etch cleaning schemes to achieve acceptable profiles and prevent problems of feature bowing or microtrenching. The low modulus of SiLK required copper CMP adjustments, though liner/seed deposition and copper plating processes from the company's copper/USG process did not require modification. To survive wire bonding, IBM uses a terminal metal pad reinforcement process first proposed by Texas Instruments (Dallas) and laser fuses in the last levels of USG/FSG dielectric to provide mechanical and hermetic protection to the underlying levels. IBM plans to extend SiLK ILD processes to the 0.10 µm generation, with k = 2.2 porous versions of SiLK. The company is running SiLK/copper processes in its E. Fishkill facility now and will run them in its Burlington, Vt., fabs in the first quarter of 2001. It claims ~ 90% yields.
Many other companies addressed the integration issues of SiLK and copper throughout the IITC program. Dow Chemical, together with many suppliers of track systems, CMP, etching and cleaning equipment, showed the level to which SiLK has advanced in preparation for production implementation. Dow Chemical recently announced a new version of SiLK that does not require an adhesion layer for process simplification and cost reduction. With the IBM endorsement, SiLK integration appears to be leading the way to low-k use, with CVD contenders of carbon-doped oxide and spin-on alternatives such as FLARE and HOSP from Honeywell close behind.
- Laura Peters
