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Chapter 9:
Antireflection Coatings and Reflectivity Control

As the exposure radiation propagates from the mask to the resist in the form of the aerial image, it encounters interfaces between the exposure medium (vacuum, air, or liquid in the case of immersion lithography) and the top resist surface, as well as between the bottom surface of the resist and the substrate. At each of the resist interfaces, transmission and reflection of the exposure radiation take place to varying degrees, depending largely on the absorbance of the resist. A major aspect of the image in the resist derives from the reflections off the substrate (or films coated on the substrate). The total energy in the resist is the sum of the downwardpropagating image and the upward-propagating reflected image. The interference of these two images is dependent on the thickness of the resist, as well as that of other films above or below it. Two consequences of interference include standing waves (a sinusoidal variation of dose through the thickness of the resist) and swing curves (a sinusoidal dependence of the energy coupled into the resist as a function of the resist thickness).

Both standing waves and swing curve effects pose significant problems that negatively impact critical dimension control and significantly degrade depth of focus and exposure latitude. These problems are particularly acute in lithographic regimes where the resolution targets are significantly smaller than the wavelength of the exposure radiation, requiring tight critical dimension control. They are also acute for high-contrast resists.

Materials that suppress reflectivity from the substrate-otherwise called antireflection coatings-were developed to mitigate these problems, in particular, swing curve effects (see the section on antireflection coatings below). In addition, these materials can help to reduce the effects of topographical variations; they also help to improve postexposure delay stability, and can help to mitigate missing contact problems, as well as scumming.

Furthermore, the adoption of optical proximity correction (OPC) across several layers, starting in the 180-nm technology node, greatly accelerated the adoption of antireflection coatings because OPC programs do not account for substrate reflectivity; it is implicitly assumed in these programs that substrate reflectivity is nonexistent. Still further, pattern collapse due to capillary forces during solvent development becomes acute in the regime of subwavelength resolution lithography. Often resulting from the loss of adhesion at the base of the feature on account of the high aspect ratio associated with features in these subwavelength lithographic regimes, these features literally tumble over. To mitigate this problem, antireflection coatings are therefore designed to have good adhesion to the resist; such antireflection coatings are interposed between the resist layer and the substrate.

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