Hot-wire chemical vapour deposition at low substrate temperatures for optoelectronic applications

Abstract

The need for large quantities of rapidly and cheaply produced electronic devices has increased rapidly over the past decades. The transistors and diodes that are used to build these devices are predominantly made of crystalline silicon. Since crystalline silicon is very expensive to produce on a large scale and cannot be directly deposited on plastic substrates, much research is being done on thin film amorphous or nanocrystalline semiconductors and insulators. Hot-wire chemical vapour deposition (HWCVD) is a novel, low cost, and convenient way to deposit these materials. The process can be controlled in such a way that specific chemical reactions take place and unwanted side reactions are minimized. It can easily be scaled up to produce large-area thin film electronics. Conventionally, plasma enhanced chemical vapour deposition (PECVD) is used to deposit semiconductors and inorganic dielectrics. Recently, HWCVD has been explored for fast deposition of such materials. An adaptation of HWCVD, initiated chemical vapour deposition (iCVD), offers the unique possibility of producing organic materials and polymers in a vacuum reactor, without the use of solvents. This technique was originally proposed at the Massachusetts institute of technology (MIT) by Prof. Karen Gleason. The iCVD process involves the creation of radicals by dissociation of a peroxide (a molecule with a ~O-O~ bond) by a heated wire in a vacuum reactor. This radical initiates a polymerization reaction of a vinyl (a molecule with a double carbon-carbon bond, ~C=C~) monomer at a substrate held at room temperature. This thesis describes a dedicated iCVD reactor for polymer deposition, installed at Utrecht University, along with a reactor with a cooled substrate holder in an existing HWCVD multi-chamber setup for low-temperature silicon nitride (SiNx) depositions. The most important features of these reactors are described and the characterization techniques are explained. This thesis contains four new fields of HWCVD thin film research. Firstly, a technical description is given of the iCVD reactor that was installed at the start of the research. The important parameters of the depositions are described and a model is presented of the heat transfer within the reactor. Secondly, two HWCVD materials are investigated that can be combined to form non-permeable barrier coatings for the protection of flexible electronics, being poly(glycidyl methacrylate) (PGMA) made by iCVD and amorphous silicon nitride (a-SiNx) made by “traditional” HWCVD. Both materials are deposited at low substrate temperatures, which are ~25 C for PGMA and <230 C for a-SiNx. Thirdly, the possibility of creating semiconducting conjugated polymers from alkynes (molecules with a triple carbon carbon bond, ~C?C~) with iCVD is explored. This research has led to oligomers of phenyl acetylene, which are semiconducting after a reaction with iodine vapour. Finally, we have demonstrated the deposition of the polymer PMDEB, which is suitable for application as a “low-k” dielectric layer.