Adhesion Tester Ensures Robust CMP
-- Semiconductor International, 7/1/2001
Poor interfacial adhesion is a key failure mechanism for copper damascene structures, especially when low-k dielectrics are used. While companies have traditionally relied on micro-indentation tests and film hardness values to predict whether an interconnect stack can endure the CMP process, a new technique called four-point bending provides a direct correlation with CMP survivability.Intel researchers from Hillsboro, Ore., and Santa Clara, Calif., discussed the effectiveness of this method at the latest IEEE International Interconnect Technology Conference (IITC) in San Francisco. They determined a threshold adhesion energy of 5 J/m2 at the copper and low-k interface. Below this level, thin films delaminated and/or cracked. Packaging and reliability studies also confirmed the 5 J/m2 threshold.
Four-point bending provides a reproducible, quantitative method of measuring film adhesion energy. A sample of a multilayer thin-film stack on silicon is cleaved from the wafer and bonded face-to-face to a silicon backing such that the stack is sandwiched in the center of the sample beam. The sample is notched on one side to form a pre-crack and loaded in the four-point bending flexure tool. Then a load is applied until the crack propagates to the weakest interface in the stack. If any bulk film is weaker than the interface, cracking occurs cohesively within the film. The fracture interface is then identified using X-ray photoelectron spectroscopy (XPS). Interface adhesion energy is computed using the load/displacement output, according to:
In this equation, P is the applied load and E and v are the elastic modulus and Poisson's ratio of the silicon substrate. Intel measured film stacks of ECP Cu/seed/barrier/dielectric/Si substrate using a spin-on polymer (SOP), a CVD silicon oxycarbide (CDO) and two spin-on carbon-doped silicate films (referred to here as SOGs), along with PVD or CVD Ta, TaN and TiN barriers.
Three films with interface energies of <5 J/m2 exhibited delamination or cracking upon polishing, including the SOG1/Ta, SOG1/TiN and SOG2/TaN combinations. All three failed the four-point bending technique. The SOP/TiN and SOP/TaN films exhibited the best adhesion, and the CDO/TaN and CDO/Ta and SOG2/TiN structures also passed the test.
Once the correlation was established between the 5 J/m2 threshold and CMP survivability, Intel tested other systems, including CDO, SOP and CVD SiOF on PVD Ta, PVD TaN, CVD TaN, CVD TiSiN and CVD TiN. The equivalent PVD films adhered better than CVD films. The TaN CVD film exhibited the poorest adhesion, failing for CDO and SOP films. The addition of a barrier pretreatment step (Ar sputtering), commonly used to remove oxidized copper on underlying metal layers, reduced adhesion energy between the SOP and TiSiN film. The researchers hypothesized this was due to damage to the polymer structure from ion bombardment.
The four-point bending method is currently limited to unpatterned film stack testing.
— Laura Peters
