Super-Resolution Photothermal Patterning in Conductive Polymers Enabled by Thermally Activated Solubility

Abstract

Doping-induced solubility control (DISC) patterning is a recently developed technique that uses the change in polymer solubility upon doping, along with an optical dedoping process, to achieve high-resolution optical patterning. DISC patterning can produce features smaller than predicted by the diffraction limit; however, no mechanism has been proposed to explain such high resolution. Here, we use diffraction to spatially modulate the light intensity and determine the dissolution rate, revealing a superlinear dependence on light intensity. This rate law is independent of wavelength, indicating that patterning resolution is not dominated by an optical dedoping reaction, as was previously proposed. Instead we show here that the optical patterning mechanism is primarily controlled by the thermal profile generated by the laser. To quantify this effect, the thermal profile and dissolution rate are modeled using a finite-element model and compared against patterned line cross sections as a function of wavelength, laser intensity, and dwell time. Our model reveals that although the laser-generated thermal profile is broadened considerably beyond the profile of the laser, the highly temperature dependent dissolution rate results in selective dissolution near the peak of the thermal profile. Therefore, the key factor in achieving super-resolution patterning is a strongly temperature dependent dissolution rate, a common feature of many polymers. In addition to suggesting several routes to improved resolution, our model also demonstrates that doping is not required for optical patterning of conjugated polymers, as was previously believed. Instead, we demonstrate that superlinear resolution optical patterning should be attainable in any conjugated polymer simply by tuning the solvent quality during patterning, thus extending the applicability of our method to a wide class of materials. We demonstrate the generality of photothermal patterning by writing sub-400 nm features into undoped PffBT4T-2OD.

Approaching Rapid, High‐Resolution, Large‐Area Patterning of Semiconducting Polymers Using Projection Photothermal Lithography

“…This result is unlike previous observations from our group, where we demonstrated super‐linear sub‐diffraction limited resolution, obtained using kW cm −2 laser irradiances with μs exposure times. [ 23 ] By contrast, in this publication, since we pattern using W cm −2 laser irradiances over longer exposure times (1–20 s) we observe feature shapes that conform to an equilibrium thermal gradient. Therefore the patterned features follow the same shape as the intensity profile of the light source.…”

“…Once the local temperature surpasses the dissolution temperature (DT), the polymer dissolves, leaving behind a negative patterned image. [23] Figure 1 depicts two different optical apparatuses that were designed and built specifically to study and characterize PPL of SPs. The first prototype consisted of an inexpensive 2.5 W 445 nm diode laser, a mirror to guide the laser, a photomask composed of a 1000 mesh transmission electron microscopy (TEM) grid sandwiched between two glass slides, and a 10× microscope objective lens to project the image onto a SP film immersed in a semi-poor solvent (Figure 1a).…”

“…Our group recently demonstrated that the primary factor controlling pattern feature resolution is a nonlinear relationship between polymer dissolution rate and temperature. [23] This nonlinear behavior can be exploited to produce sub-diffraction limited pattern features when patterning on small time scales (μs) with large laser irradiances (kW cm −2 ), with the diffraction limit defined as the Abbe linear diffraction condition. [26] The miscibility between the polymer and solvent influences the dissolution temperature (DT).…”

“…Once the local temperature surpasses the dissolution temperature (DT), the polymer dissolves, leaving behind a negative patterned image. [ 23 ]…”

“…[16,17,21,22] However, we recently demonstrated identical patterning rate laws for intrinsic SP films photothermally patterned in the presence of a marginal solvent. [23] While the direct-write system has demonstrated sub-diffraction limited pattern resolution, it is limited in overall throughput because individual pattern features are created sequentially. PPL significantly increases patterning throughput by allowing simultaneous formation of pattern features over large areas.Patterned semiconductors are essential for the fabrication of nearly all electronic devices.…”