Depth and Time Dependence of Polystyrene Chain Diffusion Near the Polystyrene/Silicon Interface
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Fig. 1. Values of the diffusion coefficient, deduced in the study of Zheng, et al. [5], indicate that D* rises with roughly a 3/2 power depth dependence with distance from the silicon surface. The samples studied employed marker layer Mw = 90K.
Distance from the silicon surface, A
151 Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
Annealing times were selected so that the deuterated chains did not physically contact the SiO surface. It was concluded that the inhibition of diffusion could be due to entanglement effects near the attractive interface, or to an increase in the local glass transition temperature. It is logical to extend the study of Zheng, et al. to other molecular weights and temperatures, and to investigate whether the diffusion coefficient varies as a function of time, as might be expected when the as-deposited spin-cast film interacts with the attractive interface during the initial stages of annealing. In the present studies, a higher PS molecular weight is examined to allow the possibility of monitoring diffusion over a wide range of temperature. In order to fit the data produced in these studies, it is necessary to allow, and account for, significant segregation at the SiO and vacuum interfaces. The preliminary data reported here demonstrate two key effects: first, that the diffusion coefficient is again observed to vary superlinearly with the distance from the wall and is inhibited near the silicon surface; and second, that the diffusion coefficient for the chosen system displays a strong time dependence, decreasing with time near the silicon surface. RESULTS AND DISCUSSION We prepared 10 identical tri-layer polymer sandwiches. The bottom 280 A-thick 670K PS layer was spin-cast from a toluene solution onto an unstripped Si (100) wafer. The subsequent layers of 150A-thick marker 947K dPS and 860 A-thick matrix 670K PS were floated in DI water onto the sample surface. Nine samples were annealed separately in vacuum at 157 C for 2, 3, 6, 7.25, 10, 13.5, 22, 25, and 43 hours. All ten samples were run on an Atomika 3000-30 SIMS instrument using a 2 keV argon beam at 30' off-normal incidence. The method is described in references [6,7]. The obtained SIMS depth profiles are shown in Fig. 2. It is clear that along with a strong diffusion behavior, one can observe, developing in time, segregation of the deuterated component at the SiO interface, and later at the vacuum interface. As is well known, segregation occurs as a result of a difference between surface energies of PS and dPS [8].
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Fig. 2. The left plot shows the normalized SIMS concentration profiles for an
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