Answer

Thin Film Fabrication Using Thermal Deposition

Thermal deposition of the thin film can be described as a vacuum technology through which pure material coatings are applied on object surfaces. The coatings which take the form of thin films have a thickness of between angstroms and microns. Besides, they can be made of a layered structure with multiple materials or a single material structure (Oluwatosin Abegunde, 2019). This paper seeks to examine the process of thermal deposition of thin films with a focus on the deposition of silver and gold on glass substrates.

Technique

Thermal evaporation entails solid material heating placed at the bottom of a high vacuum chamber, mostly in an upright crucible. According to Paul, Hossain, Muktadir, & Faisal (2017), the material is heated to a temperature at which some vapor pressure is produced. The vaporized content creates a vapor stream that flows from the chamber onto the substrate surface which is held by fixtures inverted on the top of the chamber (Oluwatosin Abegunde, 2019). Once the vapor hits the surface, it sticks to it and solidifies to form a film or coating on the substrate surface (Paul, Hossain, Muktadir, & Faisal, 2017).

The source/film material is heated using any of the two acknowledged means. The first method, known as filament evaporation uses an electrical filament or resistive heating element to generate the heat used in melting the source material (Oluwatosin Abegunde, 2019). Heat can also be acquired in form of an electron beam. The electron beam is directed into the crucible in which the source material is placed magnetically (Oluwatosin Abegunde, 2019).

How to Measure the Amount of Material Deposited

The amount of material deposited in form of a thin film is measured using the micro-balance method. This measurement method focuses on directly measuring the mass of the thin film deposited on the substrate (Lindner & Schmid, 2018). Several special requirements are attached to the microbalance to meet this purpose. One of these requirements is that the microbalance sensitivity should be able to take measurements to the order of 10^-8g/m^2 (Lindner & Schmid, 2018). It should also possess periodic damping and mechanically rigid characteristics while at the same time made of easily degas able material at elevated temperatures. The beam balance is fitted with a small mirror to measure a fraction of a monolayer thickness. The results will be the film mass per given area (Lindner & Schmid, 2018). When the area and mass are known, the thickness of the thin film can be established as in the graph below.

Figure: Measurement of Thin Film ((Lindner & Schmid, 2018).)
Gold on Glass Substrate

Application of gold on a glass substrate is challenging due to the issue of weak adhesion of gold to commonly used inert glass.in order to enhance the adhesiveness of gold thin film onto a glass substrate, an intermediate layer of oxidative metals is used. However, when an oxidative metal such as chromium is used as the intermediate layer, the electrical properties and morphology of the golden top layer are dramatically affected.

Figure: spectra of gold layer films of different thicknesses and deposition times.

Silver on a Glass Substrate

Like gold, silver has poor adhesion to glass substrates. To enhance adhesiveness, the substrate should be cleaned thoroughly then degassed. The substrate is then heated to more than 100 degrees Celsius to get rid of any water from the surface. This improves the adhesion of metals deposited on the glass surface. The thin films are prepared using ionic silsesquioxane as a cross-linking agent and silver stabilizer (Schneid et al., 2015). Once the stabilized silver nanoparticles are prepared, a layer of less than 10nm diameter is deposited on the glass substrate. This produces a high-quality silver film that is smooth and reflective (Schneid et al., 2015). Besides, the thin film remains stable up to a temperature of 200 degrees Celsius as shown in the graph below.

Figure: Stability of ionic silsesquioxane stabilized silver thin film (Schneid et al., 2015)