Fabrication of micropatterns on the sidewalls of a thermal shape memory polystyrene block

Substrate sidewalls in the current microsystems have not been well used in building devices. On the other hand, the patterns generated on these sidewalls could serve as vertical interconnects or electronic components in 3D circuits. Since existing lithographic approaches could only generate patterns of limited shapes on the sidewalls, a new approach was developed in this work to fabricate various sidewall patterns via strain-recovery deformations of a polystyrene film. The fabrication procedure consisted of three basic steps: first strain recovery, pressing and second strain recovery. The deformations that the polystyrene experienced during the fabrication were first investigated with the help of mm-scaled lines marked on the samples. We found that pressing, instead of lateral stretching, transferred part of side surface of a polystyrene block into part of the top surface, and that the heating after the pressing was capable of transferring the same part of the top surface to the sidewall. The effects of temperature and recovery times on the deformations of the polystyrene were then investigated. We determined how the dimensions of a polystyrene block changed with the temperature. We also examined how many pressing-recovery cycles a polystyrene block could properly experience at 120 °C and 160 °C, respectively. Finally, the developed approach has been applied to generate Ag 50 × 50 µm2 dots, 150 µm wide lines and serpentine-shaped microresistors on both top and side surfaces of polystyrene blocks, followed by the characterization of these products, including shrinking and pressing deformations, surface roughness, wrinkling deformations and resistance changes. We found: (i) the shrinking deformations could be roughly considered as the recovery of the pressing deformations, although the two types of deformations were not identical in magnitudes due to different configurations (i.e. the shrinking deformations were affected by surface patterns, while the pressing deformations were not), and (ii) wrinkles might arise in surface patterns when the polystyrene blocks had large shrinking deformations, which might affect the functionality of the surface patterns and cause, for example, the decrease in their resistances.