(491a) Engineering the Vapor Deposition of Organic Thin Films and Devices

Authors: 
Gleason, K. K., Massachusetts Institute of Technology

Two important families of chemical processes have been discovered which allow ultrathin organic layers of exceptional quality to be coated onto virtually any substrate. Chemical vapor deposition (CVD), as practiced by the semiconductor industry, typically utilizes high powers and high temperatures to drive non-selective chemistry. These aggressive conditions are compatible with inorganic materials, but destroy reactants having fragile organic functional groups. By marrying the all-dry microfabrication technology with the elegant design of selective chemical mechanisms, a wide range of functional, and responsive organic polymer films have been synthesized.  These revolutionary “gentle” processing platforms, initiated and oxidative CVD, vapor-phase organic monomers adsorb and react rapidly to form pure solid polymeric films directly onto near room temperature surface.  This new paradigm for surface modification and device fabrication is ideally suited for insoluble and infusible materials such as fluoropolymers, crosslinked organic networks, and conjugated polymers, representing a portfolio of >70 CVD homopolymers and copolymers.

The new CVD polymerization methods are a dramatic departure from past plasma enhanced CVD (PECVD) work, where plasma excitation creates a complex array of reactive species which limits chemical control, produce defects, reduces the retention of organic functional groups, and results in plasma heating of the surface. Eliminating the plasma excitation and instead, designing controlled reaction mechanisms, simultaneously improves the quality and deposition rate of the CVD polymer films. Utilizing selective chemistry and judicious choice of reactants allows deposition rates to be high, even when energy input is low.  Low-energy, low-temperature processing is essential for allowing delicate organic functionalities to be fully incorporated on the surface and to avoid the production of defects.

The principles underlying the CVD polymerization methods have been elucidated and experimentally verified through quantitative process models for deposition rate, conformality, number average molecular weight, polydispersity, and film properties, revealing the complex interplay between system chemistry, mass and energy transport, and reactor design. Monomer adsorption to the substrate is often the rate-limiting step and typically follows the Brunauer Emmett Teller adsorption isotherm.  Kinetic modeling of the fundamental gas phase and surface reactions reveal that initiated CVD polymerization is identical to the radical polymerization in a bulk liquid phase, albeit on a surface.  Indeed, the kinetic polymerization rate constants for many vinyl monomers at the gas-solid surface match those for standard bulk polymerization in solution. Using no adjustable parameters, the kinetic model further predicts the number average molecular weight as a function of processing conditions. Additionally, the initiated CVD reactivity ratios for the copolymers based on interfacial monomer concentrations agree with liquid phase measurements. The models further reveal that the degree of conformality over the micro- and nanoscale features is a function of the sticking probability of the volatile reactants. Using dimensionless analysis, the theory of a scale up for initiated CVD has resulted in an increase by 100x in area (>5 ft in width) and from batch to semicontinuous roll-to-roll processing.

CVD polymers find utility across a diverse array of fields including biotechnology, nanotechnology, optoelectronics, photonics, microfluidics, sensing, composites, and separations. One of the earliest applications was for  superhydrophobic, nanostructured, and ultrathin CVD fluoropolymers, a technology that has been commercialized by GVD Corporation (gvdcorp.com).  Among GVD’s products is a service in which a CVD fluoropolymer is robustly applied to the surface of a mold. This semi-permanent technology enables the rapid release of tires from their molds and eliminates the need for the frequent application of environmentally undesirable spray-on mold release agents. GVD has developed a suite of fully automated large scale reactors which capitalize on economies of scale to bring down the cost of manufacture. Additionally, GVD’s commercial lab-scale reactor system has allowed the CVD polymer technology to be adopted by academic, industrial, and government laboratories. Last year, a method to make these superhydrophobic CVD surfaces durable by grafting and crosslinking was demonstrated as an advance for maintaining dropwise steam condensation for improving the efficiency of steam power cycles and other industrial processes requiring a condensation step. This technology is being commercialized by DropWise Corporation (drop-wise.com). In addition to control over surface energy, CVD polymers enable unique approaches to controlling the morphology of surfaces. Ordered herringbone patterns have been demonstrated by the controlled release of two dimensional stretching of an elastomeric substrate with a hard grafted initiated CVD polymeric skin.  One example of an organic device is a resistive biosensor fabricated by conformal vapor deposition of a conductive copolymer with functional carboxylic acid groups onto a high surface area electrospun mat.  Additionally, lightweight, flexible, and foldable photovoltaic arrays were fabricated directly on ordinary paper substrates using CVD conducting polymers.