The ultimate resolution of lithography can be improved by improving the optics of the exposure system. Various techniques are used to obtain better aerial image contrast, such as off-axis illumination, phase-shift masks, and optical proximity correction. The resolution can also be improved by using better resists and by optimizing the methods for processing them. These are the traditional ways to improve lithographic resolution according to the Rayleigh equation, which states that resolution cannot be better than k 1 Î»âNA .
In addition to the traditional approaches, various innovative techniques have been implemented to further boost resist pattern resolution. The techniques are all based on treatment of the resist patterns after development. These so called âshrink processesâ use special physical or chemical processes to convert optically derived resist patterns into ultrafine patterns that are beyond optical resolution limits. For example, a resist film printed with a contact-hole pattern can be baked slightly above the glass transition temperature (T g ) of the resist so that it gets soft and starts to flow, resulting in smaller contact holes. This process is called thermal reflow. Another method is to submerge the resist patterns into chemicals that react at the surface of the resist patterns, resulting in conformal âgrowthâ of the resist patterns.
Shrink processes are capable of boosting the resolution of a process by one or two technology nodes without the need to purchase new exposure tools. For example, an ArF scanner with 0.75NA can print contact holes with diameters of 120 nm. Thermal reflow can further shrink the diameter to below 100 nm, which is in the resolution regime of a 0.93NA ArF tool. Implementation of the shrink process at some critical levels can help reduce costs, leading to a gain in technological leadership. Because of its huge potential for cost savings, shrink technology has attracted attention beyond lithographers, having also been adopted in film deposition processes. Plasma-assisted shrink processes have been used to shrink contact holes prior to etch.
Shrink processes actually âshrinkâ the open areas of the pattern by expanding/growing the resist area. These processes are more applicable to the lithographic levels where the space CD is crucial, such as the contact level or the first metal level.
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