Kramer Group Research Spotlight

Kinetics of Ordering of Block Copolymers on Patterned Substrates

 
 
Jakob Heier, 
Graduate Student, 
Materials Science & Engineering, 
Cornell University,  Ithaca, NY 14853, USA 
Current address:
Materials Department
UCSB
Santa Barbara, CA 93106
 
 
We investigate the kinetics of island formation in symmetric diblock copolymer thin films of poly(styrene2vinylpyridine) (PS-PVP) The substrates have patterns of self-assembled monolayers (SAMs) produced by microcontact printing HO-terminated (HO-) SAM stripes alternating with H3C-terminated (H3C-) SAM stripes. The PS-PVP lamellae over the H3C-SAM have a defect structure that attracts excess PS-PVP that would normally form islands on a uniform HO-SAM substrate. In the early stages of annealing waves of thickness develop from the H3C/HO-SAM boundary and propagate into the film over the HO-SAM. At any given time after annealing starts, these waves seem to have a fixed wavelength L. If the period H of the stripe pattern is large, the waves from each H3C/HO-SAM interface are damped; if H is smaller, we see a large amplitude pattern over the HO-SAM when H=L, 3L, and 5L. We also see destructive interference of the pattern in the center of the stripe when H=2L, 4L and 6L.
This behavior seems analogous to that seen for surface-directed spinodal composition waves in thin films of binary polymer mixtures. At long time islands one lamella high develop in the film on the HO-SAM and these dissolve by transport of copolymer to the H3C-SAM stripe. The well defined 2D geometry allows us to analyse the transport leading to this dissolution quantitatively. We can exploit the tendency for all the excess block copolymer to be transported to the film over the H3C-SAM to produce films with patterned height of block copolymer material. In the scanning force micrograph to the right, you can see the height pattern of amplitude 70 nm that results from the transport for a stripe pattern with a period H=6 micrometers. Such patterns may be useful for optical diffraction gratings and can be transferred into substrates using ion-etching methods.

This project started as a collaboration with Prof. Georg Krausch who is now at the University of Bayreuth, Germany. The SFM measurements were done in the Technical Operations Lab of the Cornell Center for Materials Research and in the Microscopy Facility of the UCSB Materials Research Lab, both of which are funded by the NSF-DMR-MRSEC Program. Funding was provided in part by NSF -DMR Polymers Program DMR 9803738 and a NATO Collaborative Research Grant (CRG no. 940599) .