New Barrier/Etch Stop for Copper Demascene

-- Semiconductor International, 7/1/1999

Anew film based on silicon, carbon and hydrogen, called BLOk (barrier low k), has been developed by Applied Materials (Santa Clara, Calif.) and offers a promising alternative to the traditional nitride copper diffusion barrier/etch stop layer for damascene processes. With a dielectric constant of 4.5, the film reduces effective k (k_eff) of interlevel dielectric (ILD) stacks while offering twice the etch selectivity of silicon nitride to oxide-like films, better copper barrier properties than nitride and good adhesion to SiO₂, FSG (fluorosilicate glass) and carbon-based low-k dielectrics. 'One of the big challenges in using FSG is getting nitride to adhere to it,' said Kevin Fairbairn, general manager of Applied Materials' Plasma Dielectric CVD Business Unit. 'Our primary focus in development was using BLOk with our low-k Black Diamond film, but customers using FSG as part of their low-k scheme are also interested in BLOk because of the additional low-k and adhesion advantages it offers.' While the low-k barrier was specifically developed to replace the nitride barrier/etch stop layer, remarkably it also demonstrates the optical properties needed to apply it as a low-k anti-reflective coating in DUV lithography imaging.

Though BLOk is a SiC-based film, it is amorphous and exhibits a much lower k value than SiC, which is typically >8 (Fig. 1). The new film is deposited in a PECVD chamber using trimethylsilane supplied by Dow Corning (Midland, Mich.). With a deposition temperature of 350-400°C, the film does not require a subsequent anneal or curing. By reducing the k of the copper diffusion barrier/etch stop layer from ~7 to below 5, k_eff of the ILD stack can be reduced significan tly (Fig. 2). When combined with the Black Diamond film, for instance, k_eff can be cut below 3.0. The film's k value varies with CVD temperature, RF power and gas flow rates, and demonstrates a correlation with refractive index; k_eff depends on the thickness of the film, ILD used and device geometries. Fairbairn said the new film actually simplifies integration of some low-k dielectrics. 'Etching can be more of a challenge with low-k dielectrics that incorporate carbon. The higher etch selectivity of BLOk provides a much wider etch process window for profile control and selectivity,' he explained.

Fig. 2 Reducing the k value of the etch stop has a significant???

At IEEE's recent International Interconnect Technology Conference (IITC) in San Francisco, Applied revealed film properties and electrical integrity data for BLOk on capacitors formed using sputtered copper gate electrodes. The engineers applied simultaneous thermal and electrical stress (bias-temperature stress, BTS) in three cycles to analyze copper diffusion properties. Using a 500 Å BLOk film, breakdown field strength was >2 MV at 1 mA/cm². A 500 Å film demonstrated lifetime much greater than 50 hrs at 2757C and 1 MV/cm. Leakage current was 1 x 10^-10 A/cm² at 0.5 MV. The film is compressive with a stress of -50 to -150 MPa.

One of the greatest concerns with replacing nitride is maintaining the electrical integrity of the copper diffusion barrier. Based on a single comb Cu/damascene structure, with line-to-line capacitors at 0.25 mm spacing, BLOk demonstrated constant lower leakage up to 100 V, exceeding conventional Si₃N₄ performance.

Another concern, given the film's etch stop characteristics, is its etchability when making contact to the copper interconnect. Though Applied will not disclose the etch chemistry, Fairbairn said it is a soft etch process that does not sputter the copper onto via sidewalls. The company is entering its first installations of BLOk and Black Diamond processes. BLOk is available on the Centura platform and will be offered on the Producer platform later this year.