The purpose of the metallic coating on a photomask, such as chrome, is to attenuate the intensity of light enough that photoresist on the other side will not be activated. We want the coating to be thick enough to attenuate most of the light, but we also want it as thin as possible to get the best dimensional control on the mask since the thinner the chrome, the less etching is required. So decades ago, when the industry standardized on chrome as the absorber of choice, it was determined that an optical density of 3.0 was sufficiently opaque at common exposure wavelengths in the UV spectrum. Technically, optical density is the amount of attenuation - or gradual intensity loss - that occurs when light passes through an optical component. Mathematically, o ptical density is a logarithmic scale of how much light is transmitted through the absorber material: OD = Log (Power transmission factor).įor example, an optical density of 3 attenuates the light power by a factor of 10^3 (1,000). An optical density of 4 attenuates the light power by a factor of 10^4 (10,000). This optical attenuation may result from not only absorption of light but also from scattering of light. Absorbance, a closely related term, considers only absorption within the optical component but not scattering. Photolithography is essentially a patterning technology using light as the transfer medium. In basic terms, the process of photolithography involves: This is the fundamental process used to mass-produce the integrated circuits and electrical components used inside almost all of today's digital devices.A silicon wafer or a thin film of chrome on a photomask(i.e.the substrate) is covered with a layer of photo-sensitive polymer (a photoresist).We expose the polymer with a pattern of light.Depending on the type of polymer used, it either softens or hardens where it is exposed to light.The soft polymer is washed away leaving a pattern of hard polymer on top of the substrate.We use this hard polymer as a template to etch the substrate.We etch the exposed substrate (where the polymer was removed) with an acid (wet etch) or a plasma (dry etch).
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