Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures -- November 1992 -- Volume 10, Issue 6, pp. 2877-2881

Electron-beam lithography with the scanning tunneling microscope

Christie R. K. Marrian,Elizabeth A. Dobisz, John A. Dagata

The scanning tunneling microscope (STM), operated in vacuum in the field emission mode, has been used in lithographic studies of the resist SAL-601 from Shipley. Patterns have been written by raising the tip–sample voltage above –12 V while operating the STM in the constant current mode. Resist films, 50 nm thick, have been patterned and the pattern transferred into the GaAs substrate by reactive ion etching. The variation of feature size with applied dose and tip–sample bias voltage has been studied. Comparisons have been made to lithography with a 10 nm, 50 kV electron e-beam in a JEOL JBX-5DII in the same resist thickness films. In all cases the resist films were processed in the standard fashion before and after exposure. The STM can write smaller minimum features sizes and has a greater process latitude. Proximity effects are absent due to the reduced scattering range of the low energy primary electrons. However, the writing speed is slower, being limited by the response of the piezoelectric scanner. Advances have been made recently in the construction of fast STMs which scan at video rates making the STM comparable in speed to the JEOL for nanolithography. The development of ultralow voltage e-beam lithography based on STM technology is discussed.

Some groups have reported that 7V is enough to expose 20 nm thick PMMA. Of course in plasmas, the resist is exposed to electrons with at least this much energy. That's why no one develops resist after plasma treatment.

I may have to rethink the claim of "no proximity effects".

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures -- November 1997 -- Volume 15, Issue 6, pp. 2877-2881

Influence of secondary electrons in proximal probe lithography

B. Volkel, A. Golzhauser, H. U. Muller, C. David, M. Grunze

This article describes the limitations of proximal probe lithography due to electrons that are mirrored by the electric field between the tip and the surface. The incident beam generates two kinds of electrons at the sample surface: primary electrons which are elastically backscattered and secondary electrons which are produced in the resist/substrate system. The electric field confines the electrons emanating from the surface. The electron trajectories are bent in such a way that the electrons impinge on the sample surface in the vicinity of their origin. These reflected electrons contribute to the exposure of the resist and therefore, limit the resolution. For hexadecanethiol monolayers on gold substrates, we have measured the energy distribution of the mirrored electrons and the secondary electron yield as a function of the primary energy. With near edge x-ray absorption fine structure spectroscopy, we have investigated the relevance of low energy electrons in the exposure of hexadecanethiol films. Simulations of secondary electron trajectories can explain the occurrence of triple line structures observed in field emission proximal probe lithography.

One problem with low-energy electrons is their high reflectivity (elastic scattering) from the surface. This is the basis of the LEEM technique.

Isn't that just backscattering, applied to low-energy electrons?