Silicone microdomains

Fig. 8 Schematic representation of block copolymer nanolithography process, a Schematic cross-sectional view of a nanolithography template consisting of a uniform mono-layer of PB spherical microdomains on silicon nitride. PB wets the air and substrate interfaces, b Schematic of the processing flow when an ozonated copolymer film is used as a positive resist, which produces holes in silicon nitride, c Schematic of the processing flow when an osmium-stained copolymer film is used as a negative resist, which produces dots in silicon nitride, (taken from [44])...

The modulus and yield kinetic parameters of the block polymer B can be related to those of the homopolymer in terms of a microcomposite model in which the silicone domains are assumed capable of bearing no shear load. Following Nielsen (10) we successfully applied the Halpin-Tsai equations to calculate the ratio of moduli for the two materials. This ratio of 2 is the same as the ratio of the apparent activation volumes. Our interpretation is that the silicone microdomains introduce shear stress concentrations on the micro scale that cause the polycarbonate block continuum to yield at a macroscopic stress that is half as large as that for the homopolymer. The fact that the activation energies are the same however indicates that aside from this geometric effect the rubber domains have little influence on the yield mechanism. 

Though our research has focused on PS-PI and PS-PB block copolymer thin films to form useful masks for patterning, their poor etch resistance under CF4 RIE limits the aspect ratio of fabricated features to no greater than 1. Future work will include the exploration of other block copolymer systems, such as those embedded with etch resistant metal clusters(J7) or silicon-containing block copolymers(J2). Etch resistant polymers or microdomains would enable the fabrication of features with larger aspect ratios which could be advantageous for filtration devices and memory storage. 

The earliest work on imaging block copolymer microdomains relied heavily upon transmission electron microscopy (TEM), and it still proves to be a useful tool to this day [19]. Samples are either microtomed or solvent cast to produce thin (ca. 100 nm) sections. PS-PI or PS-PB samples can be stained with osmium tetroxide to increase contrast. Osmium tetroxide reacts selectively with unsaturated double bonds such as found in PI or PB microdomains so as to provide mass contrast [21]. Unfortunately, TEM requires that the samples be freestanding or transferred to a transparent support (e.g. carbon), a cumbersome and time-consuming process that is largely incompatible with silicon or GaAs wafers. While silicon nitride membranes can be employed for TEM, these expensive and delicate structures are not easily accessible to all researchers [22]. 

For the purposes of patterning media such as silicon wafers, differentiation of the mierodomains is often desirable. For many systems, both copolymer blocks are carbonaceous, leading to an etch resistance between the microdomains and the matrix that is less than desirable (see Section 9.6). However, this can be differentiated by for example, incorporating metal clusters into one block. In what follows we describe suitable modification schemes. 

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