Optical lithography is employed in the fabrication of advanced microelectronic circuits, as well as microfluidic, micro-optical and micromechanical devices. Lithography is typically used to generate patterns in such devices by employing thin films of resist, which change certain critical properties upon exposure to optical radiation. This change either enhances or hinders removal of portions of the resist in subsequent steps.
The ultimate resolution limit in optical lithography is determined by the wave nature of light. The wavelength of the exposure tool determines a minimum length scale over which the intensity pattern projected by the exposure tool into the resist can generate light and dark regions. Resists used in microelectronics are typically designed to respond such that resist retention occurs at a single threshold of time integrated intensity, or exposure dose, resulting in sharp features. For instance, for a sinusoidal intensity modulation in the resist, a single line/space pair is printed with sharp edges for each period of the aerial image. It is also straightforward to show that at the resolution limit of the exposure tool, the patterning resolution cannot simply be doubled by performing a second exposure, translated by half the spatial period, in the same layer of photoresist. The two exposures combine in such a fashion that all spatial information is lost at the resolution limit of the tool.
Because of this limitation, higher feature densities have been typically been achieved by using shorter wavelength exposure sources and/or employing immersion fluids between the final optic and resist (which has the effect of reducing the effective wavelength of the source). Certain advanced lithography schemes envision shrinking the radiation wavelength to less than 20 nm by employing either EUV or x-ray radiation.
Because adopting a shorter wavelength source has brought significant engineering challenges, techniques have been proposed to overcome the optics-imposed resolution limit at a specific wavelength. The simplest conceptually is to pattern and develop sequentially two separate sets of features, each shifted accordingly. For instance, to double the resolution of a grating, two separate gratings can be patterned, each shifted by one-half the period. Over-exposure or over-development techniques can be used to insure the feature size produced from each exposure is below the half-pitch. However, this technique requires separate coating, alignment, exposure, and processing steps.
There exists a need for alternative approaches for overcoming optics-imposed resolution limits. Methods and compositions that can provide better resolution without the need fro multiple coating, alignment or processing steps would satisfy a long felt need in the art.