Surface preparation of the substrate is an important step for successful thin film deposition. Condensation rate and adhesion of the thin film will rely greatly upon the surface conditions when the sputtered species arrive at the substrate surface. There are many types of surface conditions which need to be catered for. Most prominent among them is the surface impurities. The substrates when received from the supplier will have many impurities. Grease is commonly left from fingerprints over substrates if handled without proper gloves. Dust particles can easily attach to the substrate surface from static charge, water vapour and hydrocarbons can condense over the substrate surface from the atmosphere. These impurities will affect deposited thin films in various ways. Presence of water vapour and grease can lead to poor adhesion or delamination of thin films, whereas presence of dust particles can act as a step point for agglomeration of high energy species leading to island formation instead of a continuous film layer. Presence of hydrocarbons and other reactive species could lead to chemical reaction with the arriving species, leading to change in properties at the interface. Surface irregularities are another type of defect which can affect the properties of the deposited thin film. Chips and internal cracks can lead to structural failure of the substrate and seriously affect the deposition process if not properly accounted for. Presence of scratches and bumps can act as points around which sputtered species agglomerate. These can also compromise the mechanical stability of deposited thin films. Certain applications require the surface to be not only cleaned but also properly conditioned to achieve the desired end result. Overall surface roughness is considered as a deposition parameter, as it affects the adhesion, wear rate and other mechanical properties of thin films. The surface can be conditioned in situ (inside the deposition chamber ) or ex situ (outside the chamber). The selection of surface cleaning and surface preparation technique is totally dependent on the nature of the substrate, type of impurity it attracts, and the surface conditions required to achieve the expected thin film property. Usable shape and size of substrate is usually decided by the design of the sample holder within deposition chamber and the characterization equipment.
For the development of superhard and biocompatible β-Ti3Au thin films, soda lime glass and Ti6Al4V sheets are selected as raw substrates. Soda lime glass is procured from Thermo-fisher scientific UK. Brindley metals, UK provided the medical grade Ti6Al4V to AMS 4911 specification. Ti substrates were received as rectangular pieces of 76mm by 26mm. They were first polished using SiC abrasive paper from P240, 320, 600, 2500 and 4000, with 3 minutes between each successive increment in grit size. A specialised jig was designed to spread the applied pressure equally over the substrate and a constant flow of coolant was maintained for all the pieces. After polishing, the substrates were measured for average surface roughness using an Alicona infinity focus surface measurement system. All the surfaces were polished to achieve an average roughness value better than 40nm across the surface. By achieving similar surface roughness for all substrates it can be eliminated as a variable. The polished substrates were cut in equal coupons of size 25mm by 19 mm. This size is defined by the design constraints of substrate holder within deposition chamber and extract preparation container used in the biocompatibility testing facility. After cutting in a guillotine, the edges of the coupons were again polished using a 180P grit paper to eliminate any possible variation among substrates.
The prepared Ti substrates along with the glass substrates, were taken into a clean room to perform surface cleaning, Each piece was rinsed with Deionised water for 2 successive runs. Each piece was then rinsed in 5:1 solution of Decon 90 cleaner and deionised water for further 2 minutes followed by a rinse in deionised water. This step ensures removal of grease and other oily residues from the surface. The substrates were then immersed in Deionised water and ultrasonically vibrated for further 10 minutes. Later the substrates were wiped successively with IPA and Acetone using lint free clothes. The cleaned substrates were again immersed in DI water for further 10 minutes of ultrasonic vibration. After removal, each piece was dried using a blast of nitrogen from a nitrogen gun. Cleaned substrates were stored inpre-cleaned substrate carrier boxes and were opened just prior to loading into the deposition chamber. Prior to loading into the chamber, a strip of high temperature kapton tape is applied across the glass substrate. During deposition this will act as a mask to the deposited film and create a step on the substrate when the kapton tape is peeled off. This step could be used to measure the film thickness straight after deposition using a Dekatak`s stylus profilometer. Two glass substrates and 5 Ti coupons were loaded in each run.
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