surface is hydrophobic. After HMDS treatment the silicon surface oxide becomes silated, leaving a non-polar surface. The pristine silicon (100) face has a surface energy value of 56.9 mN/m, which is reduced to a value of 44.1 mN/m after HMDS treatment The molecular formula for hexamethyldisilazane is C6H19NSi2. Spin coating There are four basic parameters that are involved in spin coating: solution viscosity, solid content (density), angular speed, and spin time. A range of thicknesses can be achieved by spin coating. Most commonly the thicknesses range from 1-200 μm. The main properties that affect the thickness of the film are viscosity and spin speed. The more viscous the solvent the thicker the film will be and the faster the wafer is spun the thinner the film will be. By manipulating these two factors the different range of thicknesses is possible. • thickness: ρ - density μ - viscosity ω - angular velocity t - time The density and viscosity both relate to the actual properties of the photoresist. This parameter can be manipulated by diluting the photoresist and adding different components to it to change its properties. The angular velocity and time relate to the spin coater and how fast it is spinning and for how long. A common issue in spin coating is a "beading" up of solvent at the edge of the silicon wafer. A process known as backside washing is most commonly used to spin this bead off of the wafer. By programming multiple different spin speeds into the spin coating apparatus the thickness of the solvent can be made uniform without the "beading" up at the edges. Spin coating has its limitations, though. Currently engineers and scientist are working on figuring out a better way to apply photoresist to the substrate of a silicon wafer. Spin coating can result in issues with wafer topography such as non-round substrates, oversized substrates, fragile substrates and material consumption. One potential solution to this issue is spraying the photoresist onto the surface. By spraying the photoresist onto the surface of the wafer as opposed to spin coating the photoresist, a lot of photoresist is saved, and smaller and more precise parts can be made. Spray coating is still in its development phase and much more research needs to be done to make it cost effective and applicable. See also photolithography microfabrication lithography photomask HMDS photoresist spin coating References Lithography (microfabrication)