![]() ![]() We use this hard polymer as a template to etch the substrate. The soft polymer is washed away leaving a pattern of hard polymer on top of the substrate.Depending on the type of polymer used, it either softens or hardens where it is exposed to light. We expose the polymer with a pattern of light.the substrate) is covered with a layer of photo-sensitive polymer (a photoresist). A silicon wafer or a thin film of chrome on a photomask(i.e.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. Photolithography is essentially a patterning technology using light as the transfer medium. Absorbance, a closely related term, considers only absorption within the optical component but not scattering. ![]() 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. Technically, optical density is the amount of attenuation - or gradual intensity loss - that occurs when light passes through an optical component. 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. 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. 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. If you hold a chrome mask up to a light, you can vaguely see the light shining through the chrome, much like the light leaking through a solar eclipse filter or welding goggles. ![]()
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