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8Applications of Focused Ion Beam and DualBeam forNanofabrication

Brandon Van Leer, Lucille A. Giannuzzi, and Paul Anzalone

225

1. Introduction

The use of focused ion beam (FIB) technology in the area of nanoprototyping andnanofabrication is becoming increasingly important as dimensions of emphasiscontinue to shrink from the micrometer to the nanometer level. The characteriza-tion of materials and devices using FIB and/or DualBeam technology (i.e., an FIBand a scanning electron beam on the same platform as shown in Fig. 8.1) hasproven to be critical for the research, development, and failure analysis researchlaboratory. FIB/SEM technology is also being utilized in novel ways to engineernanostructures and devices employing ion-and/or electron-beam deposition ofmetals, organic materials or insulators, and milling of materials with the ion beam.

The interaction of ions and electrons with target materials is a bit different. Thesignals generated from interaction of electrons and target materials have beenaddressed in Chapter 1. Ion–solid interactions produce secondary ions, x-rays,backsputtered ions, neutral atoms, secondary ions, and clusters from target mate-rials, and the penetration depth is only about 10–20 nm, quite different from thatof electrons as shown in Fig. 8.2.

The fabrication of micrometer scale structures with the FIB [1] has easilytransformed into nanoscale fabrication. For example, fabrication at the nanoscalewith FIB technology has been utilized to make sensors and electrical devices [2],to serve as nucleation sites for precise growth of either carbon nanotubes [3] orquantum dots [4], and in the fabrication of nanostructures such as photonic grat-ings [5]. The ability to ion beam deposit 3D free-standing structures [6] hasenabled a wide range of structures to be directly processed [7]. FIB nanofabrica-tion has also been performed in the processing of existing structures and materi-als such as probe tip modification for atomic force microscopy [8].

The use of FIB and DualBeam instrumentation for the nanometer precision oftransmission electron microscopy specimen preparation is well known [9,10].FIB based TEM specimen preparation techniques have been directly transferableto other analytical methods [11]. As recently summarized [12], the use ofFIB/DualBeam instrumentation as shown in Fig. 8.3 has become quite popular to

directly thin sample tips for field ion atom probe microscopy. Nanofabricationwith FIB and DualBeam technology has been enhanced with the addition ofeither onboard or external pattern generator engines and the use of scripting forindividual pixel control of beam parameters such as beam overlap and dwell time.In the section below, we will discuss these beam variables as they pertain to thenanofabrication of structures and devices, and show examples of DualBeam usefor nanofabrication.

2. Onboard Digital Patterning with the Ion Beam

FIB based processes to remove or deposit material are dependent on severalparameters that include the ion beam current, beam dwell time, raster refreshtime, and if using a chemical gas precursor, the gas flux. Historically, beam over-lap was fixed in either the x- or y-direction to ensure uniform exposure of the sur-face by the FIB [13]. Recently, leading FIB instrument manufacturers have begunproviding digital pattern generators, which allow for milling and deposition of

226 Brandon Van Leer et al.

Column

Atoms

Sample

Photons

Clusters

X-raysElectrons

100 – 200 Å

Ions�

Ions�

Ion� beam Column

Electrons

Sample

Photons

X-raysBS Electrons

1– 2µ

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FIGURE 8.2. Ion interaction (a) and electron interaction (b) with targeted materials, respec-tively.

(a) (b)

FIB:

FIB

SEM:Imaging

SEM

FIGURE 8.1. A FIB and a scanning electron beam on the same platform.

complex structures employing user defined inputs such as geometrical patternslike circles, rectangles, polygons and/or direct import of graphical bitmap files.For example, the flexibility of the FEI onboard pattern generator allows the userto vary up to 30 parameters to achieve structures for nanotechnology applications,and can be used for milling or in conjunction with either ion-beam assisted orelectron-beam assisted chemical vapor deposition (CVD) [14].

Critical beam parameters that control the time the beam resides in one spot(i.e., the dwell time) or the relative distance between beam position (e.g., definedeither by a percentage overlap or by an actual distance or “pitch”) can be key toachieving optimum results in machining or deposition. The beam overlap (OL) isdefined by the beam diameter and the step size of the beam movement as shownin Fig. 8.4.

Figure 8.5 shows schematic diagrams that define positive, zero, and negativebeam overlap conditions. A positive overlap is generally used for milling andimaging, while zero or a negative overlap is typically used when the ion beam isused with gases such as in CVD or enhanced etching.

Other important beam parameters in FIB applications are the beam dwell timeand raster refresh time. As previously stated, the dwell time is defined as the timethe beam rests in one position. These dwell time values can typically varybetween 100 ns and 4 ms. The number of pixels and the dwell time per pixeldetermines the time required to complete one raster across the pattern and is aptly

8. Applications of Focused Ion Beam and DualBeam 227

FIGURE 8.3. FEI FIB/SEM DualBeam instrumentation.

named raster refresh time. Figure 8.6 shows how the beam dwell time influencesthe effective FIB milled line width. A beam of 1 pA was used to FIB mill into Siusing by varying the dwell time from 250 ns to 10 µs. A constant 0% beam over-lap and the same number of passes were used for all lines. As shown in Fig. 8.6,as the dwell time decreases, the depth of the cut will decrease, and the effectiveFIB mill line width will also decrease. The actual FIB milled dimensions that canbe achieved will always be somewhat larger than the ultimate beam diameter andwill vary depending on the collision cascade defined by the ion–solid interactionsfor any given target. Note that with a FIB resolution of ~5 nm, 10 nm wide FIBmilled line is possible in Si (see Fig. 8.6), but that a factor of 3× larger line widthis observed if the beam conditions are not optimized.

One may use ion–solid interaction theory to alter the geometry and aspect ratioof FIB milled nanostructures. Figure 8.7 shows SEM images of FIB milled cross-sections of FIB milled lines performed at 52˚ incidence angle (left) and 0˚ inci-dence angle (i.e., with the ion beam perpendicular to the original sample surface,right). The lines were milled with all beam parameters identical. The only differ-ences in the milled lines were the angle of incidence of the beam with respect tothe sample surface. The differences in the aspect ratios of the FIB milled lines areevident in the SEM images in Fig. 8.7, where the 52˚ incidence angle cut showsa deeper cut with an overall improvement in the aspect ratio of the cut from ~2:1to 3:1. Since different materials exhibit different collision cascade characteristicswhich also vary with incidence angle [15], it is expected that different FIB milledaspect ratios will vary with material as well as incidence angle that are consistentwith ion–solid interaction theory. In addition, differences in FIB milled lines are


S = Beam step or pitch

d = Beam diameterd

d − sOL =

FIGURE 8.4. A schematic diagram defining beam overlap.

Positiveoverlap

Zero overlap

Negativeoverlap

FIGURE 8.5. A schematic diagram showing beam overlap conditions.


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FIGURE 8.6. Nanometer scale FIB milled lines in Si with varying dwell times.

(a) (b)

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FIGURE 8.7. SEM images of FIB milled lines milled at 52˚ (left) and 0˚ (right).

also observed when milled in a single beam pass down, up, or across, an inclinedslope, which is also consistent with ion–solid interaction theory [13].

3. FIB Milling or CVD Deposition with Bitmap Files

Pattern generation via milling or deposition in conjunction with graphic bitmapfiles allows the user to mill complex 3D structures [14]. The pixel spacing of eachbitmap defines the beam location and the overlap is then defined by controllingthe beam size. The color value of each pixel in the bitmap can be delineated todefine the beam dwell time and beam blanking. Figure 8.8 shows an example ofFIB milling a 3D structure from a bitmap image. The inset in Fig. 8.8 shows agrayscale bitmap image and the corresponding FIB milled SEM image in Fig. 8.8shows how the grayscale levels defined in the bitmap allows for a 3D structure tobe milled directly from the 2D bitmap by varying the dwell times for each pixel.Thus, pixels exhibiting a longer dwell time yield deeper FIB milled regions. Asshown in Fig. 8.8, pixels with a color value of 255 (white) provide the longestdwell time and pixels with a color value of 0 (black) have zero dwell time and aretherefore regions which are not FIB milled.

The deposition of 3D structures can also be achieved with bitmap files.Fundamentally, ion beam assisted CVD occurs when the precursor gas is crackedand decomposed directly by the ion beam as well as by the secondary electronsthat are generated as a result of ion–solid interactions. Electron beam CVD canalso be accomplished, where the size of the deposit is limited by the primarybeam as well as the electron–solid interactions. The precursor is emitted from a


FIGURE 8.8. FIB milling of a 3D structure from a 2D bitmap image (inset).

nozzle at a predefined height from the sample surface (typically ~50–200 µmdepending on the gas) with a predetermined flow rate and adsorbs onto the sam-ple surface. Next, the impinging beam and subsequent ion–solid interactions reactwith the adsorbed molecules decomposing the organic constituents. The volatilereaction products are removed by the vacuum system and the remaining materialdeposits onto the substrate surface. The schematic diagram in Fig. 8.9 depicts theCVD deposition process.

Three-dimensional features via bitmap FIB deposition can be achieved using asimilar process to FIB milling. Figure 8.10 shows how the subtlety in utilizing 2Dgrayscale control allows the user to precisely deposit 3D films with the ion beam.The bitmap image that was used to create the 3D Pt structure is inset in the SEMimage. Note that the SEM image was obtained with the sample surface tilted toemphasize the 3D nature of the deposited Pt film, and hence does not correspondto the exact geometrical dimensions as depicted in the bitmap image.

4. Onboard Digital Patterning with the Electron Beam

Changing variables with the onboard pattern generator can also be used todirectly fabricate structures [14]. As an example, Fig. 8.11 shows use of theonboard pattern generator applied to the electron beam of the SEM. Figures 8.11aand 8.11b show helical-shaped electron beam deposited nanoscale (<100 nm) Ptlines that were deposited by scanning with one pass of the beam and independ-ently changing the beam pitch and overall scan dimensions to form the differentfeatures shown.


Ion beam rastersacross surface

Precursormolecules

Volatile reactionproducts

Substrate Deposited film

Ion beam

FIGURE 8.9. A schematic diagram of the ion beam assisted CVD process. Electron beamCVD process is performed in a similar fashion.


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HFW5.12 µm

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1 µm

FIGURE 8.10. An SEM image of a 3D Pt structure fabricated using the 2D bitmap imageshown in the inset.

(a)

(b)

5 µm

5 µm

FIGURE 8.11. (a) and (b) are digitally patterned electron beam deposited Pt nanolines.

5. Automation for Nanometer Control

The use of scripting with DualBeam systems provides a powerful tool for automat-ing process steps for deposition and milling. It also provides precise control of theFIB parameters such as scan direction, beam position and a host of other variables.A script is just a set of software instructions that control the DualBeam instrument.Figure 8.12 is an SEM image showing 3D spiral growth of ion beam assisted CVDPt deposited performed by scripting. In this example, the speed and location of thebeam is managed with precise control of the dwell and overlap such that the Ptgrows continuously in a 3D free-standing spiral shape.

Scripting may also be used to control the DualBeam system to perform variousmilling, imaging and/or deposition tasks without user intervention. For example,automation is available for site-specific transmission electron microscopy speci-men preparation [16] as well as cross-section preparation to capture either a sin-gle SEM image, or for sectioning multiple images serially for subsequent 3Dreconstruction and tomography via AutoSlice and View™ [17,18]. Figure 8.13below demonstrates the ability to perform site specific TEM specimen prepara-tion (i.e., to within ~20 nm) where the TEM lamella thickness of 100 nm is eas-ily achievable.


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3 µm

FIGURE 8.12. SEM image of 3D free-standing Pt FIB deposited growth. (Image courtesyof S. Reyntjens.)

6. Direct Fabrication of Nanoscale Structures

Post- and fine-processing of structures and materials is a powerful FIB applica-tion for users in the nanoelectronics and nanoresearch communities. As alreadyshown, examples include device modification or edit, AFM tip or atom probe tipcreation, and TEM specimen preparation. Figure 8.14 is an SEM image of a probetip consisting of ion beam assisted CVD Pt deposited on silicon. Note that theeffective tip radius is smaller than 45 nm.

7. Summary

System advances in SEM/FIB dual platform instruments that allow precise controlof beam parameters like dwell time and overlap have allowed users to scale 3D pro-totyping from the micrometer to the nanometer scale. Just as opportunities exist forin situ nanocharacterization, the same can be said for nanoprototyping. As devel-opments in software applications, resolution, beam control and chemistry continue


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FIGURE 8.13. FIB prepared TEM specimen having a final thickness ~75 nm.

to grow with the use of FIBs, the user will have a larger tool box to draw from for3D nanofabrication and nanomanipulation. Additionally, postprocessing of existingmicro- and nanostructures with the SEM/FIB will be useful to repair damagedstructures or modify existing assemblies for novel purposes.

Acknowledgments. We thank Steve Reyntjens and Daniel Phifer for their con-tributions.

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mag25000x

HFW10.2 µm

tilt52� Molecular Foundry Nanolab

42 nm

100 nm

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4 µm

FIGURE 8.14. Nanoprobe tip creation of FIB deposited Pt on single crystal Si.

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