Investigation Of Chemical Mechanical Polishing Of Novel Diffusion Barrier Co/TaN For Cu Interconnect

Co/TaN bilayer, as a novel diffusion barrier for copper interconnect, has good potential to be applied in the next generation VLSI technology. However, there is lack of study about chemical mechanical polishing (CMP) of Co based barrier. The chemical corrosion, galvanic corrosion and removal mechanism of chemical mechanical polishing of Co need to be studied. This work has significant meaning in both scientific research and potential application of novel CMP process of Co based barrier layer.We firstly investigated chemical mechanical polishing of Ta. The effects of pH value and H2O2on chemical mechanical polishing of Ta were studied. It is found only in the strong acid slurry containing low H2O2or strong alkaline slurry with high H2O2, Ta has high removal rate. However, theseslurries can easily cause damage to Cu and low-k dielectrics. Chemicals which can enhance the polishing rate of Ta in weak acid are needed. We found that guanidine salts can obviously enhance the polishing rate of Ta in the slurry with pH value at2-12. In the slurry with pH=6, it isfound that Guanidine sulfate can make silica abrasives more easily adsorbed on Ta surface, which can enhance the contact force between silica and Ta,thusincreasingthe mechanical removal of Ta. By measuring the OCP and corrosion current of Ta in the slurry with or without guanidine, it is found that guanidine can make Ta oxide more reactive and increase the corrosion rate of Ta, which then can enhance the Ta polishing rate by chemical reaction.Next we studied the thermal stability of Cu(15nm)/Co(5nm)/TaN(5nm) on the Si and low k substrates. The in-situXRD, SEM, AES and electrical measurement results demonstrate that the the Co/TaN stack can be thermal stable after annealing at450℃. It is found that, in comparison to Cu on Ta, Cu on cobalt surface has better crystallization and preferered (111) orientation, which is beneficialfor reducing the resistance of Cu lines and enhancing the resistance for Cu electromigration.Then we studied the role of pH value, H2O2, Glycine and2-MT and BTA on polishing of Co. It is found that the corrosion of Co in the acid slurry is much higher than that in the alkaline slurry, and H2O2can intensively enhance the corrosion rate and polishing rate of Co. In the acid slurry containing H2O2, CMP of Co is a chemical dominant process. We also find that glycine is a good complex agent for Co.In the acid slurry, glycine can greatly enhance the polishing rate of Co bysynergisticallyreacting with H2O2. Glycine can directly react with CoO and then inhibit Co oxide crystal growth which makes the Co surface smoother.We reported that2-MT and BTA are good inhibitors forCo corrosion in CMP slurries for the first time.2-MT has relatively better inhibiting efficiency then BTA. After using0.02mM2-MT, the corrosion inhibitionefficiency of2-MT can achieve to90%. In the slurry with pH=5, adding2-MT can inhibit the galvanic corrosion between Co and Cu. In the slurry without H2O2, BTA can adsorb on Co surface by physical and chemical adsorption, and obeys Langmuir adsorption isotherm. In the slurry with H2O2, BTA can serve either as a complex agent oras an inhibitor for Co: when the concentrations of BTA and Co2+arevery low, BTA can complex with Co2+ions which can accelerate Co corrosion, then BTA is a complex agent for Co;while when the concentrations of BTA and Co2+are high, the insoluble Co-BTA reaction product is thick enough to cover the whole surface of Co.Those Co-BTA reaction products can isolate the Co surface with the slurry, which can prevent further corrosion of Co, in this case BTA is an inhibitor for Co.We successfully made Cu/Co/TaN based patterned wafers in our lab. By polishing the pattered wafer, it is found that using Co-BTA reaction product to prevent the corrosion of Co on the sidewall of trench is hard to control. While compared with the commercial Ta/TaN barrier slurry, the self-made LHS-1slurry which contains2-MT shows much better polishing results. This verifies the research results of slurry component effects on Co CMP process.Last we investigated the effects of plasma treatment on low-k dielectrics and its role on preventing low-k to absorb moisture. In PECVD chamber, NH3, H2and He plasma treatment all can cause serious carbon depletion of low-k dielectrics and make low-k from hydrophobic to hydrophilic. By using CH4plasma treatment, it is found that the plasma treatment can deposit a carbon layer on the low-k surface which can keep low-k surface hydrophobic. We then dipped the original low k samples and the low k samples treated using different plasma into the commercial polishing slurries and compared their properties. It is found that the CH4plasma treated low k sample adsorb less moisture and has the lowest k value and leakage currentchange.So CH4plasma treatment on low-k dielectrics is good way for prevent low-k adsorbing moisture in CMP process.