A Design-of-Experiments Approach to Characterizing Beam-Induced Deposition in the Helium Ion Microscope
Larry Scipioni,1 * Colin Sanford,1 Emile van Veldhoven,2 and Diederik Maas2 1Carl Zeiss NTS, LLC, 1 Corporation Way, Peabody, MA 01960, USA 2TNO Science and Industry—Van Leeuwenhoek Laboratory, P.O. Box 155, 2600AD Delſt, Te Netherlands
*
l.scipioni@
nts.zeiss.com
Introduction Charged particle microscopes have been used extensively
for the creation of nanostructures. As a subset of the techniques for this, the process of beam-induced chemistry offers almost endless flexibility for both additive (by beam-driven precursor deposition) and subtractive (by beam-catalyzed etching) processing. A recent review article [1] makes it evident simply by its massive page count the number of materials that have been used to deposit a variety of conductive, insulating, magnetic, photonic, and other structures. To take advantage of these capabilities, though, the nanoarchitect must select the correct settings for a large number of parameters. One key figure of interest is the size of features that can be written by deposition of conducting material: how small can we go? Process development is complex, however: a standard beam-induced deposition process using the system described below calls for the control of sixteen different parameters! Several of these are used to set the flow of the reactant, several are required for defining the particle beam, and yet another set of parameters call out the routine by which the beam is scanned over the pattern of interest. In the end, the result provides just one recipe for one chemistry with one beam type. Te wide scatter of reported outcomes in the review article mentioned above indicates the complexity of the problem, where each result given can be considered just a snapshot of one small corner of the parameter space. Our goal here is to apply a quantitative optimization methodology for the determination of beam chemistry processes in the helium ion microscope (HIM). In this article, we discuss efforts toward finding the minimum obtainable line width and gap width between line pairs of deposited platinum lines and giving a predictive formulation of the same. Te HIM, first commercially available in 2007 [2], has
demonstrated imaging resolution better than 0.35 nm. Te small size of the probe, combined with the light mass of the ion and the specific beam-sample interactions, have also inspired experiments in nanofabrication. For example, milling [3] of 5-nm holes of high aspect ratio in a variety of materials has been demonstrated. Small, dense features have also been created with lithography and beam-induced deposition [4]. Over the past year, Carl Zeiss NTS, LLC (Peabody, MA) and TNO (Delſt, Te Netherlands) have been jointly pursuing characterization of beam-induced chemistry processes in HIM. Tere are several features of these processes to characterize: deposition size, deposition rate, chemical purity of deposits, and the electrical resistivity of deposits (both conductors and insulators). Most of this work to date has concentrated on the characterization of deposited conducting features from a common platinum
22
precursor. Te processes have to be recharacterized for each different performance dimension desired; for example,
the
smallest features might not also have the lowest resistivity. One quickly realizes that there is a large manifold of parameters to search over for these optima. A design-of-experiments (DOE) approach allows us to vary parameters thought to be most important for determining feature performance and then to cast quantitative relations between these process settings and the outcomes. Tis approach is explained in more detail in the next sections.
Materials and Methods All sample creation and inspection was carried out in
an Orion Plus® HIM (Carl Zeiss NTS, LLC) located at TNO Science and Industry in Delſt, Te Netherlands. Te tool was equipped with an OmniGIS (OmniProbe) gas injection system. Tis device houses reservoirs for three different reactive gases and has inputs for two carrier gases, to vary the flow and concentration of the active species. For all of the experiments reported here, deposition of platinum (bearing) deposits were generated from MeCpPt(IV)Me3 precursor (Colonial Metals). Te precursor was kept at a constant temperature of 30ºC. Te needle of the gas injection was placed at a distance of 100 mm from the target area. Te gas flow was kept constant to give a steady chamber pressure of 4 × 10–6 Torr. For maximum flexibility in defining writing strategies, the scan during deposition was controlled by an Elphy Plus (Raith GmbH) lithography pattern generator. A set of two lines with various pitches (48, 28, 24, 20, 16, 12, and 8 nm) was taken as the design. Te experiments were generated with the DOE approach. DOE PRO XL soſtware (
Sigmazone.com), which runs as an extension to Microsoſt Excel, was used for experimental setup and analysis of results. Four factors were initially considered as having influence on the width of the lines: primary ion beam current, ion beam dwell time per point, pixel step size per point, and writing direction (that is, lines oriented perpendicular to or parallel to beam scanning direction). A four-dimensional experiment would be quite extensive, so two of these parameters were fixed, based on existing experimental evidence. Previous work on pillar growth [5] showed that the narrowest and tallest pillars are obtained when the beam current is kept below 0.8 pA, so a primary beam current of 0.5 pA was used for the experiments herein. Te beam energy was 25 keV. Second, preliminary experiments determined that writing perpendicular to the line orientation provides consistently narrower lines. Tus only two parameters were chosen to be varied: step size and dwell time. Te parameter space was setup for a dwell time between 0.2 and 19.8 µs and a step
doi:10.1017/S1551929511000307
www.microscopy-today.com • 2011 May
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