controlled through exposure to light. Because Photoresists are non polar compounds, and silicon dioxide has a polar character, adhesion problems can manifest between the two materials. When photoresist does not adhere properly, features lose resolution. Photoresist Adhesion becomes very important as feature sizes get smaller. In order to reliably create small features, the surface of the silicon wafer must be made hydrophobic to promote photoresist adhesion. The adhesion characteristics can be observed and tested with a goniometer test. The surface energy characteristics of the silicon wafer can be measured by placing a drop of deionized water or ethylene glycol and measuring the contact angle of the droplet. Using the Young's relation and tabulated values for interfacial energy, we can estimate the surface energy of the solid. •Young's Relation: - Interfacial energy between solid and vapor - Interfacial energy between solid and liquid - Interfacial energy between liquid and vapor θ - Contact Angle Positive resist Positive photoresists are composed of a novolac resin, ethyl lactate solvent, and Diazonaphthaquinone (DQ) as the photoactive compound. Positive photoresist reacts with light to cause the polymer to break down and become soluble in a developer solution. Positive resist has better resistance to etchant than negative photoresist. Positive resists are better for producing small feature size, but does not adhere to silicon wafers as well as negative resist. When making small features it is critical to have good adhesion. Negative resist Negative photoresists are composed of a poly(cis-isoprene) matrix, xylene solvent, and bis-arylazide as the photoactive compound. Negative photoresists react to light by polymerizing. The unexposed portions can be removed by using a developer solution. negative resist has better adhesion and is great for features greater than 2 μm in size. HMDS A common method to increase adhesion of photoresist on the silicon wafer surface is to treat the wafer with Hexamethyldisilazane (HMDS). A new silicon wafer has a polar surface and has some adsorbed water on the surface. The wafer can undergo a dehydration bake to remove adsorbed water, and followed by the HMDS treatment also known as the priming stage. HMDS can be dispensed in liquid form onto the wafer using a syringe while the wafer is attached to a vacuum chuck in a spin coater. HMDS can also be applied in gas form in the process known as the vapor prime. The HMDS promotes good photoresist-to-wafer adhesion because it ensures the wafer