Molecular Vapour Deposition
What is Molecular Vapour Deposition and how does it differ to other thin film deposition methods? MVD is the gas-phase reaction between surface reactive chemicals, known as Pre-Cursors (e.g. TMA – Tri-Methyl Aluminium) and an appropriately receptive substrate, (typically Silicon wafer) covered with appropriately receptive/reactive groups (e.g. hydroxyl groups). Repeated exposure to the precursors, will result in a build-up of multiple depositions, creating a thin film of a known thickness. Precursors react and/or decompose on substrate surfaces producing desired applications. These include; Semiconductor Devices, Conformal Films, Photovoltaic Devices and Metal-Organic Frameworks.
Thin films consisting of various precursors are deposited onto Silicon wafers in a tool known as an MVD chamber, such as the SPTS MVD300, which is installed and commissioned at the Sêr SAM (Sustainable Advanced Materials) project in Swansea University. The instrument is the First European R&D MVD system of its kind, allowing advanced research into this unique deposition method. The tool allows
precise chemical delivery, controlling film layer thicknesses down to the Angstrom level. The tool operates in three growth modes (SAM – Self-Assembly Monolayer, ALD - Atomic Layer Deposition & Continues Growth), all producing highly conformal & uniform layers.
The advantages of MVD over the afore mentioned deposition methods is that it can operate at Low Deposition Temperatures (35oC – 150oC), has a proven capability at high aspect ratios (5,000:1) and the reaction times are extremely suitable for the production of high quality films at very high throughput. MVD differs from both CVD and ALD since neat chemicals are employed when injecting into a mechanically evacuated chamber (no carrier gas). Molar dosing based on the ideal gas law, relates pressure in the expansion volume to the amount of moles of precursor. MVD employs expansion volumes to collect vapour prior to delivery to the main chamber, allowing processes to run at lower operating temperatures and depositing onto high aspect ratio substrates.
The tool is currently employed in a number of high-profile projects and is primarily utilized for the Advanced Functional Coatings for Integrative Semiconductor Materials and Devices project, which is investigating a series of precursor variants on the MVD system. The following case studies highlight the different modes the MVD tool has successfully been utilized.
MVD is mainly used in the semiconductor industry to manufacture a range of coatings and thin films including the following: anti-stiction, hydrophobic/philic, oleophobic, biocompatible, protective, capacitive and chemically reactive Nano-films.
Case Study 1
The deposition of FDTS (Perfluorodecyltrichlorosilane) & Water in CVD Mode to create a Self-Assembled Monolayer (SAM). Here the reaction between FDTS & Water involves the pre-conditioning and hydroxylation of the surface of the substrate, Hydrolyze Silane, before reacting the Silane molecule with the substrate.
FDTS is used because of its hydrophobic properties for semiconductor applications. Contact angle measurements of the deposited layers were measured to be 115°and 110°, which confirms the quality of the deposited films. 120° is considered a super hydrophobic surface.
Case Study 2
Deposition of Alumina (Al2O3) and Zinc Oxide (ZnO) have been conducted utilising the MVD instrument in ALD Mode.
Al2O3 Reaction: TMA (Tri-Methyl Aluminium) & Water to deposit a thin film of Al2O3:
ZnO Reaction: DEZ ( Di-Ethyl Zinc) & Water to deposit a thin film of ZnO:
Reaction occurs on the surface of the wafer by a series of self-limiting reactions.
Alumina - polymorphic material utilised as thin films in a variety of applications such as microelectronics devices & optoelectronics.
Zinc Oxide - wide bandgap semiconductor.
Post Layer Deposition Characterisation
Following deposition onto multiple substrates including silicon, metals, glass and acrylics, deposited layers are fully characterised and analysed in-depth utilising a multitude of platforms. Film thicknesses are typically measured employing a range of techniques such as Ellipsometry (which provide full mapping and film thickness uniformity) and Scanning Electron Microscopy (SEM).
This work is supported by the Avenues of Commercialisation for Nano and Micro technologies Project (ACNM) which is funded by the European Regional Development Fund (ERDF) and managed by The Welsh European Funding Office (WEFO). Project support is also provided by: SPTS, the Centre for Nano Health (CNH) at Swansea University and Pegasus Chemicals.