No abstract

A focused ion beam (FIB) is used to accurately sculpt predetermined micron-scale, curved shapes in a number of solids. Using a digitally scanned ion beam system, various features are sputtered including hemispheres and sine waves having dimensions from 1-50 µm. Ion sculpting is accomplished by changing pixel dwell time within individual boustrophedonic scans. The pixel dwell times used to sculpt a given shape are determined prior to milling and account for the materialspecific, angle-dependent sputter yield, Y(θ), as well as the amount of beam overlap in adjacent pixels. A number of target materials, including C, Au and Si, are accurately sculpted using this method. For several target materials, the curved feature shape closely matches the intended shape with milled feature depths within 5% of intended values. Results and DiscussionThere have been few attempts at 3-dimensional ion milling or sculpting for the purpose of controlling feature shape [1][2][3][4][5]. The method [1-3] we employ involves numerical determination of the dose required per pixel to attain a user-specified shape. Of note, the method accounts for the sputter yield variation with incidence angle (Y(θ)) when solving for the required dose. For sculpting, an initially planar sample is fixed with the angle of ion incidence set at 0 o (normal incidence), and an area is outlined in plan view. The dose required per pixel is then partitioned according to the number of user-specified scan repeats.The angular dependence of yield is determined experimentally prior to sculpting. We have completed this determination for a number of materials and find that the yield depends on angle similar to the formulation put forth by Yamamura [6]. In general, the yield depends on the incident energy, E, and angle of incidence, θ, as Y(E,θ) = Y(E) t f exp[-Σ(t-1) ] with t = 1/cos θwhere f and Σ are parameters determined from a fit of the experimental data. Figure 1 plots the experimentally-determined yield for Si versus angle. This data was obtained by tilting a silicon specimen to different angles of incidence prior to separate mill steps. The line shown in this plot is a fit using the Yamamura formulation with f = 2.43 and Σ = 0.43. For Si the yield at 0 o (normal incidence) is measured to be 1.94 + 0.06. Additional values of yield, f and Σ are listed for C and Au in Table 1.Using the values listed in Table 1 a number of curved shaped features were ion beam sculpted. Figure 2 shows an example of a sinusoidal wave and a hemisphere sputtered into carbon. The maximum depth of these features comes close to the targeted values although consistently 10-15% 806

Solid-state membranes having isolated, nanometer-scale pores are currently of interest for a variety of research activities including studies of near-field optics[1] and investigations of biomolecule-surface interactions [2]. For example, with regard to studies of DNA translocation through membranes, an isolated nanopore offers the ability to evaluate the behaviors and properties of a single molecule. This is of interest for sequencing DNA -central to genomics. To date, research involving solid-state membranes has largely relied upon fabricating pores into commercially-available, freestanding silicon nitride or silicon oxide membranes that are bonded at their edges to much thicker silicon substrates [3,4]. Recently, several studies have demanded a more diverse set of membrane materials and pore geometries.In this study, we demonstrate a method for fabricating nanopores in different membrane materials including polycrystalline metals. First, low stress films of the desired material are coated by vapor deposition onto commercially-available silicon nitride membranes (attached to silicon substrates for ease of handling). Focused ion beam methods are then used to locally remove an approximate 25 x 25 µm window of silicon nitride providing access to the deposited material. Once exposed, this new membrane is FIB sputtered at a single pixel to define a pore.For sputtering nanopores, we employ a dual lens, ion-pumped Magnum FIB column (FEI, Co.) mounted on a custom vacuum system. We also utilize a backside channelplate detector for endpointing in order to minimize pore diameter and overcome the challenges of point-to-point variation in sputter yield (a common challenge to working with polycrystalline metal membranes). The endpointing instrument includes an APD 3040MA 12/10/8 D backside microchannelplate detector (Burle Industries, Inc.) and external electronics (preamplifier, computer with software). With real time monitoring, we are able to identify the time of perforation and terminate sputtering soon thereafter. For these experiments, the detector front plate is biased to -1500 V and the back plate is grounded. See Fig. 1.Representative detector responses associated with nanopore fabrication are shown in Fig. 2. This includes data taken from FIB single-pixel sputtering of amorphous silicon nitride, silicon nitride with a capping layer of W, nickel and aluminum. In each set of experiments, we use a focused, 30 pA gallium beam. Membrane thicknesses are ~ 200 nm. Previous analysis of the silicon nitride 'drilling' experiments demonstrates that a through hole (pore) is completed at the time that corresponds to the onset of a maximum detector signal [4]. We refer to this as the time to perforation. These prior experiments also showed that hole diameter increases if the ion beam is kept 'on' for longer time. The benefit of the endpointing technique shown in Fig. 1 is clear. If a minimum pore diameter is desired, FIB sputtering can be terminated soon after the onset of a maximum detector signal. For metals this endpointing t...

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006