Lili Ji

Resolution improvement for a maskless microion beam reduction lithography system

A maskless ion projection lithography technique called maskless microion beam reduction lithography (MMRL) has been developed at Lawrence Berkeley National Laboratory. As a candidate for a next generation lithography system, MMRL is designed to achieve sub-70 nm resolution. By adding a 100 μm diam limiting aperture at the focal plane of the ion beam, an aperture angle of 0.56 mrad [corresponding to numerical aperture (NA)=5.6×10−4] at the image plane has been obtained. Small NA can decrease the geometrical and axial chromatic aberrations of the MMRL system. Thus resolution better than 150 nm has been obtained. Simulation results indicate that the resolution can be further improved by decreasing the aperture diameter and minimizing the limiting aperture induced aberrations. Meanwhile, the single-shot exposure time can still be as low as 193 ms at NA≈10−5 if the high brightness ion source design is employed in the MMRL system.

Combined electron- and ion-beam imprinter and its applications

A combined electron- and ion-beam system employing a double-chamber plasma source and a single accelerator column has been developed to provide focused electron and positive-ion beams simultaneously, with no need for a separate electron source or accelerating column for sample neutralization. The self-aligned ion and electron beams can be used to micromachine and image a variety of materials, both conducting and insulating. Together with an ion-beam imprinting scheme, the combined electron/ion beam system is compact and provides low-cost, high-throughput, and large-area micromachining.

Conformal metal thin-film coatings in high-aspect-ratio trenches using a self-sputtered rf-driven plasma source

A thin-film coating system has been developed for the deposition of both conductive and insulating materials. The system employs a radio-frequency (rf)-discharge plasma source with four straight rf antennas, which is made of or covered with the deposition material, thus serving simultaneously as a sputtering target. The average deposition rate of the copper thin film can be as high as 500nm∕min when operated under continuous-wave mode. Film properties under different operating conditions (gas pressure and rf power) have been investigated experimentally. Three thin-film coating schemes have been developed, one of which has been demonstrated to be suitable for conformal deep-trench coating. Conformal coating over trenches of high-aspect ratio (>6:1) has been demonstrated at both micron and submicron scales.

Development of a 27.12MHz radio frequency driven ion source with 3mTorr operation pressure for neutron generators

An inductively coupled rf plasma ion source has been developed for neutron generators. The ion source configuration has been optimized for low pressure operation. Both 13.56 and 27.12MHz rf powers have been used to generate hydrogen plasma. Experimental results show that 27.12MHz operation is more efficient than 13.56MHz in a low pressure region. The ion source can also be operated in pulsed mode. Current density higher than 30mA∕cm2 can be extracted from a 2-mm-diam aperture at 2kW rf input power and 3mTorr operation pressure.

Development of C60 plasma ion source for time-of-flight secondary ion mass spectrometry applicationsa)

Initial data from a multicusp ion source developed for buckminsterfullerene (C(60)) cluster ion production are reported in this article. A C(60)(+) beam current of 425 nA and a C(60)(-) beam current of 200 nA are obtainable in continuous mode. Compared to prior work using electron impact ionization, the multicusp ion source provides at least two orders of magnitude increase in the extractable C(60)(+) beam current. Mass spectra for both positive and negative bismuth cluster ions generated by the multicusp ion source are also included.

Combined electron and focused ion beam system for improvement of secondary ion yield in secondary ion mass spectrometry instrument

Using a combined electron and focused ion beam system to improve performance of secondary ion mass spectrometry instruments has been investigated experimentally. The secondary ion yield for an Al target has been enhanced to about one order of magnitude higher with the postionization induced by the low energy electrons in the combined beam. It can be further improved with the increase of electron beam current. When the combined beam is applied to insulating targets, sample charging is also eliminated. For Teflon targets, the secondary ion signal is increased by more than a factor of 20.

Formation of Nanopore-Arrays by Plasma-based Thin FilmDeposition

The ability to fabricate membranes with arrays of apertures only a few nanometers in diameter are important to many fields of research, including ion beam lithography, DNA sequencing, single ion implantations, and single molecule studies. Because even the state-of-the-art lithography tools are limited in their ability to produce nanoscale features, alternative methods of fabricating single pores of nanometer scale have been developed, using ion-beam sculpting and focused-ion-beam assisted deposition. However, these methods cannot simultaneously produce multiple holes of nanometer dimension. Here we report a means of forming arrays of nanopores simultaneously on a thin, solid-state membrane using plasma-based thin-film deposition. By depositing layers of metallic thin films, the aperture sizes of pores in a pre-fabricated membrane can be reduced from a couple of micrometers down to tens of nanometers and even smaller. The technique offers a way to reduce the sizes of aperture of any shape in a variety of substrate materials, both conducting and insulating. Such arrays of nanopores can serve as membrane channels for DNA sequencing, as masks in ion-beam imprinters, for the fabrication of quantum dots, and in other applications.