Demixing, pattern formation

In addition, patterns created by surface instabilities can be used to pattern polymer films with a lateral resolution down to 100 nm [7]. Here, I summarize various possible approaches that show how instabilities that may take place during the manufacture of thin films can be harnessed to replicate surface patterns in a controlled fashion. Two different approaches are reviewed, together with possible applications (a) patterns that are formed by the demixing of a multi-component blend and (b) pattern formation by capillary instabilities. 

While the demixing patterns in Fig. 1.2 are conceptually simple and exhibit only one characteristic length scale, more complex phase morphologies are obtained by the demixing of a multi-component blend [16]. With more than two polymers in a film, the pattern formation is (in addition to the factors discussed in the previous section) governed by the mutual wetting behavior of the components. Two different scenarios are shown in Fig. 1.4 [ 17]. While both films in Fig. 1.4(a) and (b) consist of the same three polymers, their mutual interaction was modulated by preparing the films under different humidity conditions [15],... 

In applications in the semiconductor industry, polymer structures are required on length scales down to individual molecules. A bottom-up approach is better than a top-down approach in order to achieve this. A lateral resolution less than 100 nm can be created by surface instabilities and pattern formation in polymer films. Steiner [6] discussed demixing of polymer blends and pattern formation by capillary instabilities for nanostructure formation. 

The formation and replication of patterns into polymer films using instabilities is a new contribution in the field of soft lithography, which typically requires the mechanical contact between a patterned master and the resist. Two classes of instabilities were discussed. The demixing of two incompatible polymers leads to a well known spinodal pattern. In thin films, this structure formation process can be guided by a pattern in surface energy. 

Fig. 4.27 Sketch of the demixing process observed in a sample containing a colloid polymer mixture at high polymer concentrations at q<03. The corresponding light scattering patterns are indicated as well a gel formation birth , b gel lifetime life , c gel collapse death , d macroscopic phase separation. Redrawn from Verhaegh et al. [107]...

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

---