James L. Wilbur

Microlithography by using neutral metastable atoms and self-assembled monolayers.

Lithography can be performed with beams of neutral atoms in metastable excited states to pattern self-assembled monolayers (SAMs) of alkanethiolates on gold. An estimated exposure of a SAM of dodecanethiolate (DDT) to 15 to 20 metastable argon atoms per DDT molecule damaged the SAM sufficiently to allow penetration of an aqueous solution of ferricyanide to the surface of the gold. This solution etched the gold and transformed the patterns in the SAMs into structures of gold; these structures had edge resolution of less than 100 nanometers. Regions of SAMs as large as 2 square centimeters were patterned by exposure to a beam of metastable argon atoms. These observations suggest that this system may be useful in new forms of micro- and nanolithography.

Pattern transfer to silicon by microcontact printing and RIE

Microcontact printing techniques employing self-assembled alkanethiol monolayers in the production of metal masks have been combined with reactive ion etch for subsequent pattern transfer to silicon. Silicon feature sizes of about 300 nm have been demonstrated. Some inadequacies in the self-assembled monolayers (SAMs)-formed metal masks have been characterized by electron microscopy. Particularly, nickel etch control and metal feature edge definition remain problems to be solved if the process is to be employed in submicron feature production. Nickel patterns produced in the process and used as masks without the gold overlayer were successful as masks in the reactive ion etching (RIE) process. They also appear to give a somewhat improved edge definition over processes in which the gold layer remains.

Self-assembled monolayers exposed by metastable argon and metastable helium for neutral atom lithography and atomic beam imaging

We used a beam of noble gas atoms in a metastable excited state to expose a thin (1.5 nm) self-assembled monolayer resist applied over a gold-coated silicon wafer. We determined exposure damage as a function of dose of metastable atoms by processing the samples in a wet-chemical etch to remove the gold from unprotected regions and then measuring the reflectivity with a laser and observing the microstructure with an atomic force microscope. We found that the minimum dose required to damage the resist substantially was 1.7(3)×1015 atoms/cm2 for metastable helium, and 25(7)×1015 atoms/cm2 for metastable argon.

Self-Assembly and Self-Assembled Monolayers in Micro- and Nanofabrication

Despite the extraordinary success of current techniques for microfabrication, new techniques are needed. One reason is scale: optically based lithography is reaching the lower limits for the size of features it can produce (˜100 nm). Another is efficiency: methods such as electron beam lithography are presently linear processes and will require significant development if they are to be used for large scale, high volume processing. Other considerations such as capital and processing costs, waste management, environmental concerns, and the degree of perfection of the final structures may also force the development of new methods for microfabrication.