Design and analysis of a scanning beam interference lithography system for patterning gratings with nanometer-level distortions

Abstract

This thesis describes the design and analysis of a system for patterning large-area gratings with nanometer level phase distortions. The novel patterning method, termed scanning beam interference lithography (SBIL), uses the interference fringes between two coherent laser beams to define highly coherent gratings in photo resist. The substrate is step and scanned under the interference pattern to expose large gratings. Our experimental system, the "Nanoruler", employs interference lithography optics, an X-Y air bearing stage, column referencing displacement interferometry, refractometry, a grating length-scale reference, a beam alignment system, and acousto-optic fringe locking. Supporting systems also include an environmental enclosure, a beam steering system, and vibration isolation with feedforward. The system can pattern 300 mm diameter substrates. The errors are categorized and analyzed. The image-to-substrate motion during writing is comprised of "servo error", which is calculated from interferometric measurements, and unobservable error. The Nanoruler contains a built-in metrology capability where it can measure directly the image-to-substrate motions, which includes the unobservable error. In this special metrology mode, measurements can be performed at all substrate locations and on the fly - a capability possessed by no other patterning machine. This feature is used to assess the image-to-substrate motions. On-the-fly writing and metrology is further noted to be important because periodic errors in the interferometry can be eliminated. I control the fringe placement with a novel system of stage control and acousto-optic fringe locking. The experimentally verified system performance allows control of the servo error to the limits of quantization and latency.

(cont.) The impacts of stage controller performance and vibration isolation feedforward performance on unobservable errors are modeled and verified. Extremely high resonant frequency metrology frames were designed that provided unusual insensitivity to disturbances. The vibration errors are estimated to be sub angstrom (0 to 100 Hz). Based on my results and modeling, it is concluded that SBIL is capable of satisfying sub nanometer placement requirements. In my work I have demonstrated long term (1 hour) fringe placement stability of 1.4 nm, 3 (0 to 1.4 Hz). Also, the short term placement stability is less than 4 nm, 3 (O to 5 kHz). When considering the integrated intensity of the scanned image traveling at 100 mm/s, the dose placement stability is 2.1 nm, 3. The wafer mapping repeatability was shown to be 2.9 nm, 3a while measuring a 100 mm substrate. The repeatability is consistent with error models. The index of air uniformity and the thermal stability of assemblies currently limit the repeatability. An improved system of thermal control, enclosed beam paths, and lower coefficient of thermal expansion components is critical for achieving sub nanometer placement error.