Abnormal (100) grain growth has been characterized in predominantly (111)-textured Cu thin films as a function of deposition temperature, annealing temperature and the presence of a Ta or W underlayer. For films deposited at room temperature, bimodal grain size distributions are observed at annealing temperatures at or above 150 °C for Cu on Ta and 100 °C for Cu on W. Suppression of (100) abnormal grain growth was achieved by depositing Cu on either barrier layer at 150 °C. A bimodal grain size distribution was still observed for the film deposited on W at 150 °C but the large grains forming this distribution were found to be (111) oriented. These results are explained as the result of competition between strain energy minimization and surface and interface energy minimization. The (100) growth is shown to be driven by a reduction of the orientation-dependent strain energy that builds up due to the elastic anisotropy of Cu. Films deposited at higher temperatures have a lower yield stress which limits the achievable strain energy driving force, thereby suppressing the (100) growth. Surface energy minimization drives the (111) abnormal growth.
Biaxial stress and strain in (100) and (111) oriented grains have been measured as a function of annealing temperature for a Cu film on an oxidized Si substrate which exhibits abnormal (100) grain growth. The observed behavior indicates isostrain averaging, which is consistent with grain growth that is controlled by strain energy density minimization. In contrast, two films which do not exhibit (100) abnormal grain growth appear to follow isostress averaging. Strain energy density minimization in this situation favors (111) grain growth.
The lengths and spacings of twins in YBa2Cu3O7−δ thin films deposited onto MgO substrates have been measured by transmission electron microscopy as a function of film thickness t, for t ranging from 50 to 1400 nm. The twin length is linear in t, while the twin spacing follows a t1/2 dependence. This form for the twin spacing is consistent with the prediction of a simple free energy expression for the twinning transformation.
With a tunable ultra low dielectric constant, porous silica xerogel is an attractive dielectric material for ULSI interconnect applications and is potentially extendable to multiple future technology nodes. Porous silica xerogel films have been processed and integrated into device test structures as ultra low k intermetal dielectrics. A fully automated thin film deposition process is recently developed and gives high throughput and good repeatability. A surface modification technique is used to make the films hydrophobic. The film dielectric constant is measured to be less than 2, depending on porosity. Because of the small pore sizes, the films display high dielectric break down strength. With proper shrinkage control, porous silica xerogel shows excellent gapfill capabilities. Integration of the porous silica xerogel material into CMP planarized double level metal (DLM) test structures with both Al and W plugs in a gapfill scheme is successful. Porous silica xerogel structures provide 14% and 35% total capacitance reduction compared to structures with hydrogen silsesquioxane (HSQ) and high density plasma (HDP) oxide respectively. Reliability and current leakage data of porous silica xerogel are comparable to that of HSQ. Feasibility of integrating porous silica xerogel into Cu damascene structures is also demonstrated. Cu/xerogel damascene structures exhibit improvements in both resistance and capacitance compared with convention Al/Oxide gapfill structures.
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