Center for Materials for Information Technology an NSF Materials Science and Engineering Center Optical Lithography Lecture 13 G.J. Mankey

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Lithography The transfer of a pattern to a material or patterning of a material by a printing process. In optical lithography, specific areas of a layer of photosensitive resist or photoresist are illuminated through a mask. The illuminated resist either becomes soluble in developer (positive resist) or insoluble (negative resist). The patterns can then be transferred to the substrate using subtractive or additive processes. We perform mainly single-layer processes to pattern materials. For multiple layer processes, the successive layers must be aligned relative to one another using a contact mask aligner or projection stepper.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Positive Resist Processing The layer of resist is exposed in specific areas through a mask. Development washes away exposed resist. In the additive process, material is deposited through the holes in the resist. For a subtractive process, material is removed by ion milling through the holes in the resist. In the final step, resist is removed. substrate photoresist UV Light mask Exposure Developing Ion Milling Finished Product Additive Subtractive Deposition

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Negative Resist Processing substrate photoresist UV Light mask Exposure Baking 2nd Exposure Developing The layer of resist is exposed in specific areas through a mask. Baking helps to harden the exposed resist. A second exposure exposes the soft resist. Development washes away soft resist. Resist remains in the areas initially exposed through the mask.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Resolution Limit The minimum resolution of an optical element is given by the Raleigh criterion: k 1 is a constant that depends on the thickness and index of refraction of the photoresist (~0.6). is the wavelength of the radiation used to expose the resist. NA is the numerical aperture of the focusing optical elements (~0.5). The basic rule of thumb is that the minimum line width achievable is basically equal to the wave length of the incident radiation. We use the i-line from a Hg arc lamp which has a 365 nm wavelength. Shorter wavelengths are available, with the current limit below 200 nm.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Photoresist Coating Photoresist is coated onto the substrate using a spinner, which rotates the substrate up to 10,000 rpm while resist is placed on the surface using a dropper. We normally use Shipley positive resist. The thickness of resist is a function of concentration (viscosity) and rotation speed. A set of spin-curves should first be made to find the parameters for producing resist coating within the desired range of thickness and uniformity. Typical resist thickness is of the order of 100 nm, and the ellipsometer is the ideal tool for determining thickness. Resist is usually prebaked for a short period of time prior to exposure through a mask.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Resist Exposure The resist is exposed though the mask using the mask aligner. The vacuum contact method is best since it insures a good contact between mask and photoresist. The light intensity should first be checked using the photometer. Light intensity is controlled by adjusting the discharge current in the lamp. Exposure time is controlled by the aligner, and a series of exposures is necessary to optimize exposure time to achieve the desired resolution and aspect ratio. Array masks are available with lines and bars from tens of micrometers wide down to bars and dots 0.7 micrometers in size. Resist is then developed by immersing the substrate into developer solution. After a hard bake, the resulting resist pattern should be measured using the AFM. The optimized process can then be applied to additive or subtractive materials processing.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Process Parameters The process parameters should be optimized to produce the desired resist patterns: –Resist concentration. –Spinner speed and time. –Pre-exposure bake temperature and time. –Exposure intensity and time. –Developer concentration and time. –Post developing hard bake. Resulting pattern –Edge and corner definition. –Pattern replication. –Resolution / minimum feature size. –Aspect ratio.