Muspel and Surtr : CVD system and control program for WF6 chemistry

CVD (Chemical Vapour Deposition) is an advanced technique for depositing a coating on a substrate. CVD implies that a solid phase is deposited on a normally heated substrate surface using a reactive, gaseous mixture. The reaction gas mixture must be carefully chosen to prevent homogeneous nucleation in the gas phase. As the solid phase is formed, gaseous by-products are formed and they must be removed from the CVD system. The thermally activated CVD process requires a deposition system which can regulate the total pressure and mass flows of the separate gas components as well as maintain a sufficiently high temperature to initiate a chemical reaction on the substrate surface. In this thesis a new CVD system was constructed to meet these challenges. Initially it will be used to deposit hard, wear resistant coatings but by changing the gases, it is possible to explore other chemical systems. The CVD system functions well up to a deposition temperature of 1100 ºC as long as the CVD processes are thermally activated. Apart from manual operation, a LabView control interface was implemented that can automate process steps by reading recipe files as csv (comma-separated variables). In this way complex coating architectures can be deposited. The aim of this thesis is to give a detailed description of the hardware set-up and of the software developed for it. Provided in this work are also a few examples of W and WN (tungsten nitride) coatings, including a multi-layered structure to show the potential of complex structures. Since the system also contains a titanium precursor, a TiN (titanium nitride) coating is presented to conceptually show the flexibility of the equipment.CVD är en avancerad teknik för att lägga en tunn film runt ett substrat. CVD innebär att en fast fas bildas på den normalt uppvärmda substratytan från en reaktiv gasblandning. Gasblandningen är väl vald att inte förorsaka homogen kärnbildning i gasfasen. När den fasta fasen bildas så bildas också gasformiga biprodukter som måste pumpas ut ur systemet. Den termiskt aktiverade CVD processen kräver ett system som kan styra total trycket och massflödet av de individuella gaskomponenterna samt hålla en tillräcklig temperatur för att initiera kemiska reaktioner på substratytan. I denna avhandling presenteras ett CVD-system byggt för att möta dessa utmaningar. Initialt kommer systemet att deponera hårda, slittåliga skikt men genom gasbyte byte av gas kan andra materialsystem utforskas. CVD-systemet kan deponera andra typer av filmer upp till en deponeringstemperatur på 1100°C så länge som CVD-processerna är termiskt aktiverade. Utöver manuell styrning har ett styrprogram i LabView implementerats för att medge automatisering av processtegen genom att läsa av receptfiler i csv-format. På det här sättet kan mer komplicerade skiktarkitekturer deponeras. Målet med detta arbete är att ge en detaljerad beskrivning av uppställningen samt mjukvaran som framställts. Ett antal exempel på W- (volfram) och WN-skikt (volframnitrid) presenteras tillsammans med en multiskiktslösning för att visa potentialen för komplicerade strukturer. Eftersom systemet även har tillgång till en titankälla presenteras ett TiN-skikt (titannitrid) för att konceptuellt demonstrera utrustningens flexibilitet

Kinetics of the low-pressure chemical vapor deposited tungsten nitride process using tungsten hexafluoride and ammonia precursors

Tungsten nitride (WNx) is a hard refractory material with low electrical resistance that can be deposited using multiple methods. This study focuses on the microstructrual development of low pressure chemical vapor deposition grown WNx coatings. Also, the growth kinetics is studied and discussed in terms of the resulting microstructures. Samples of WNx were deposited using WF6, NH3, and Ar at 592-887 K in a hot-wall reactor with variable gas mixture compositions (NH3:WF6 = 0.5-25). The coatings were nitrogen-rich (x similar to 1.65) and oxygen-free as determined by time-of-flight-elastic recoil detection analysis. X-ray diffraction showed that the coatings transformed from being amorphous to crystallizing as beta-W2N at 641-690 K. The morphologies changed with deposition temperature. Being very fine grained and nodular at deposition temperatures 740 K and below, increasing the deposition temperature to 789 K while employing a NH3:WF6 molar ratio of 1, large disc-shaped protrusions were formed. When increasing the NH3:WF6 molar ratio to 25, striped facets became increasingly dominant. Investigating the latter by transmission electron microscopy, a microstructure of smaller ridges formed by twinning, oriented as in the out-of-plane direction, was revealed across the facet surfaces. Transmission Kikuchi diffraction confirmed that was the texture of these coatings. The partial reaction order of WF6 and NH3 at 740 K was determined to be close to 1/6 and 1/2, respectively. The apparent activation energy ranged from 82 to 12 kJ/mol corresponding to deposition temperatures from 592 to 887 K