Applications of Focused Ion Beam and DualBeam for Nanofabrication

Transcription

1 8 Applications of Focused Ion Beam and DualBeam for Nanofabrication Brandon Van Leer, Lucille A. Giannuzzi, and Paul Anzalone 1. Introduction The use of focused ion beam (FIB) technology in the area of nanoprototyping and nanofabrication is becoming increasingly important as dimensions of emphasis continue to shrink from the micrometer to the nanometer level. The characterization of materials and devices using FIB and/or DualBeam technology (i.e., an FIB and a scanning electron beam on the same platform as shown in Fig. 8.1) has proven to be critical for the research, development, and failure analysis research laboratory. FIB/SEM technology is also being utilized in novel ways to engineer nanostructures and devices employing ion-and/or electron-beam deposition of metals, 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. The signals generated from interaction of electrons and target materials have been addressed in Chapter 1. Ion solid interactions produce secondary ions, x-rays, backsputtered ions, neutral atoms, secondary ions, and clusters from target materials, and the penetration depth is only about nm, quite different from that of electrons as shown in Fig The fabrication of micrometer scale structures with the FIB [1] has easily transformed into nanoscale fabrication. For example, fabrication at the nanoscale with 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] or quantum dots [4], and in the fabrication of nanostructures such as photonic gratings [5]. The ability to ion beam deposit 3D free-standing structures [6] has enabled a wide range of structures to be directly processed [7]. FIB nanofabrication has also been performed in the processing of existing structures and materials such as probe tip modification for atomic force microscopy [8]. The use of FIB and DualBeam instrumentation for the nanometer precision of transmission electron microscopy specimen preparation is well known [9,10]. FIB based TEM specimen preparation techniques have been directly transferable to other analytical methods [11]. As recently summarized [12], the use of FIB/DualBeam instrumentation as shown in Fig. 8.3 has become quite popular to 225

2 226 Brandon Van Leer et al. (a) (b) SEM:Imaging FIB: SEM FIB FIGURE 8.1. A FIB and a scanning electron beam on the same platform. Column Ion beam Column Ion beam Ions Ions Atoms Electrons X-rays Clusters Photons Electrons BS Electrons X-rays Photons Sample Å Sample 1 2µ FIGURE 8.2. Ion interaction (a) and electron interaction (b) with targeted materials, respectively. directly thin sample tips for field ion atom probe microscopy. Nanofabrication with FIB and DualBeam technology has been enhanced with the addition of either onboard or external pattern generator engines and the use of scripting for individual 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 the nanofabrication of structures and devices, and show examples of DualBeam use for nanofabrication. 2. Onboard Digital Patterning with the Ion Beam FIB based processes to remove or deposit material are dependent on several parameters that include the ion beam current, beam dwell time, raster refresh time, and if using a chemical gas precursor, the gas flux. Historically, beam overlap was fixed in either the x- or y-direction to ensure uniform exposure of the surface by the FIB [13]. Recently, leading FIB instrument manufacturers have begun providing digital pattern generators, which allow for milling and deposition of

3 8. Applications of Focused Ion Beam and DualBeam 227 FIGURE 8.3. FEI FIB/SEM DualBeam instrumentation. complex structures employing user defined inputs such as geometrical patterns like circles, rectangles, polygons and/or direct import of graphical bitmap files. For example, the flexibility of the FEI onboard pattern generator allows the user to 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 or electron-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., defined either by a percentage overlap or by an actual distance or pitch ) can be key to achieving optimum results in machining or deposition. The beam overlap (OL) is defined by the beam diameter and the step size of the beam movement as shown in Fig Figure 8.5 shows schematic diagrams that define positive, zero, and negative beam overlap conditions. A positive overlap is generally used for milling and imaging, while zero or a negative overlap is typically used when the ion beam is used with gases such as in CVD or enhanced etching. Other important beam parameters in FIB applications are the beam dwell time and raster refresh time. As previously stated, the dwell time is defined as the time the beam rests in one position. These dwell time values can typically vary between 100 ns and 4 ms. The number of pixels and the dwell time per pixel determines the time required to complete one raster across the pattern and is aptly

4 228 Brandon Van Leer et al. OL = d s d d = Beam diameter S = Beam step or pitch FIGURE 8.4. A schematic diagram defining beam overlap. Positive overlap Zero overlap Negative overlap FIGURE 8.5. A schematic diagram showing beam overlap conditions. named raster refresh time. Figure 8.6 shows how the beam dwell time influences the effective FIB milled line width. A beam of 1 pa was used to FIB mill into Si using by varying the dwell time from 250 ns to 10 µs. A constant 0% beam overlap 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 effective FIB mill line width will also decrease. The actual FIB milled dimensions that can be achieved will always be somewhat larger than the ultimate beam diameter and will vary depending on the collision cascade defined by the ion solid interactions for any given target. Note that with a FIB resolution of ~5 nm, 10 nm wide FIB milled line is possible in Si (see Fig. 8.6), but that a factor of 3 larger line width is observed if the beam conditions are not optimized. One may use ion solid interaction theory to alter the geometry and aspect ratio of FIB milled nanostructures. Figure 8.7 shows SEM images of FIB milled crosssections of FIB milled lines performed at 52 incidence angle (left) and 0 incidence 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 differences in the milled lines were the angle of incidence of the beam with respect to the sample surface. The differences in the aspect ratios of the FIB milled lines are evident in the SEM images in Fig. 8.7, where the 52 incidence angle cut shows a deeper cut with an overall improvement in the aspect ratio of the cut from ~2:1 to 3:1. Since different materials exhibit different collision cascade characteristics which also vary with incidence angle [15], it is expected that different FIB milled aspect ratios will vary with material as well as incidence angle that are consistent with ion solid interaction theory. In addition, differences in FIB milled lines are

5 8. Applications of Focused Ion Beam and DualBeam 229 det TLD HV 5.00kV mag x tilt 0 3/22/ :22:59 PM 5.0 mm 500 nm FIGURE 8.6. Nanometer scale FIB milled lines in Si with varying dwell times. (a) (b) HV 15.00kV HFW 1.28 µm mag 100,000x det TLD 4.9 mm curr 0.58 na 11/19/2004 7:27:03 PM 200 nm HV HFW mag det 15.00kV 1.28 µm 100,000x TLD 4.9 mm curr 0.58 na 11/19/2004 4:34:36 PM 200 nm FIGURE 8.7. SEM images of FIB milled lines milled at 52 (left) and 0 (right).

6 230 Brandon Van Leer et al. also observed when milled in a single beam pass down, up, or across, an inclined slope, 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 bitmap files allows the user to mill complex 3D structures [14]. The pixel spacing of each bitmap defines the beam location and the overlap is then defined by controlling the beam size. The color value of each pixel in the bitmap can be delineated to define the beam dwell time and beam blanking. Figure 8.8 shows an example of FIB milling a 3D structure from a bitmap image. The inset in Fig. 8.8 shows a grayscale bitmap image and the corresponding FIB milled SEM image in Fig. 8.8 shows how the grayscale levels defined in the bitmap allows for a 3D structure to be 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. As shown in Fig. 8.8, pixels with a color value of 255 (white) provide the longest dwell time and pixels with a color value of 0 (black) have zero dwell time and are therefore 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 cracked and decomposed directly by the ion beam as well as by the secondary electrons that are generated as a result of ion solid interactions. Electron beam CVD can also be accomplished, where the size of the deposit is limited by the primary beam 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).

7 8. Applications of Focused Ion Beam and DualBeam 231 Ion beam Volatile reaction products Precursor molecules Ion beam rasters across surface Substrate Deposited film FIGURE 8.9. A schematic diagram of the ion beam assisted CVD process. Electron beam CVD process is performed in a similar fashion. nozzle at a predefined height from the sample surface (typically ~ µm depending on the gas) with a predetermined flow rate and adsorbs onto the sample surface. Next, the impinging beam and subsequent ion solid interactions react with the adsorbed molecules decomposing the organic constituents. The volatile reaction products are removed by the vacuum system and the remaining material deposits onto the substrate surface. The schematic diagram in Fig. 8.9 depicts the CVD deposition process. Three-dimensional features via bitmap FIB deposition can be achieved using a similar process to FIB milling. Figure 8.10 shows how the subtlety in utilizing 2D grayscale 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 SEM image. Note that the SEM image was obtained with the sample surface tilted to emphasize the 3D nature of the deposited Pt film, and hence does not correspond to 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 to directly fabricate structures [14]. As an example, Fig shows use of the onboard pattern generator applied to the electron beam of the SEM. Figures 8.11a and 8.11b show helical-shaped electron beam deposited nanoscale (<100 nm) Pt lines that were deposited by scanning with one pass of the beam and independently changing the beam pitch and overall scan dimensions to form the different features shown.

8 232 Brandon Van Leer et al. HV 5.00kV det TLD mag 50,000x HFW 5.12 µm 5.1 mm tilt 52 curr 1 µm 98 pa label FIGURE An SEM image of a 3D Pt structure fabricated using the 2D bitmap image shown in the inset. (a) (b) 5 µm 5 µm FIGURE (a) and (b) are digitally patterned electron beam deposited Pt nanolines.

9 8. Applications of Focused Ion Beam and DualBeam Automation for Nanometer Control The use of scripting with DualBeam systems provides a powerful tool for automating process steps for deposition and milling. It also provides precise control of the FIB 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 CVD Pt deposited performed by scripting. In this example, the speed and location of the beam is managed with precise control of the dwell and overlap such that the Pt grows continuously in a 3D free-standing spiral shape. Scripting may also be used to control the DualBeam system to perform various milling, imaging and/or deposition tasks without user intervention. For example, automation is available for site-specific transmission electron microscopy specimen preparation [16] as well as cross-section preparation to capture either a single SEM image, or for sectioning multiple images serially for subsequent 3D reconstruction and tomography via AutoSlice and View [17,18]. Figure 8.13 below demonstrates the ability to perform site specific TEM specimen preparation (i.e., to within ~20 nm) where the TEM lamella thickness of 100 nm is easily achievable. HV 15.00kV det TLD 5.1 mm HFW 10.2 µm mag 25,000x tilt 30 curr 3 µm 36 pa label FIGURE SEM image of 3D free-standing Pt FIB deposited growth. (Image courtesy of S. Reyntjens.)

10 234 Brandon Van Leer et al. det TLD HV 5.00 kv mag 12,500x tilt mm HFW 10.2 µm 4 µm Nova Nanolab FIGURE FIB prepared TEM specimen having a final thickness ~75 nm. 6. Direct Fabrication of Nanoscale Structures Post- and fine-processing of structures and materials is a powerful FIB application for users in the nanoelectronics and nanoresearch communities. As already shown, examples include device modification or edit, AFM tip or atom probe tip creation, and TEM specimen preparation. Figure 8.14 is an SEM image of a probe tip consisting of ion beam assisted CVD Pt deposited on silicon. Note that the effective tip radius is smaller than 45 nm. 7. Summary System advances in SEM/FIB dual platform instruments that allow precise control of beam parameters like dwell time and overlap have allowed users to scale 3D prototyping from the micrometer to the nanometer scale. Just as opportunities exist for in situ nanocharacterization, the same can be said for nanoprototyping. As developments in software applications, resolution, beam control and chemistry continue

11 8. Applications of Focused Ion Beam and DualBeam nm 100 nm det ETD HV 5.00 kv mag 25000x tilt mm HFW 10.2 µm 4 µm Molecular Foundry Nanolab FIGURE Nanoprobe tip creation of FIB deposited Pt on single crystal Si. to grow with the use of FIBs, the user will have a larger tool box to draw from for 3D nanofabrication and nanomanipulation. Additionally, postprocessing of existing micro- and nanostructures with the SEM/FIB will be useful to repair damaged structures or modify existing assemblies for novel purposes. Acknowledgments. We thank Steve Reyntjens and Daniel Phifer for their contributions. References 1. J. H. Daniel, D. F. Moore, and J. F. Walker, Eng. Sci. Educ. J., 7 (1998) G. Ben Assayag, J. Gierak, J. F. Hamet, C. Prouteau, S. Flament, C. Dolabdjian, F. Gire, E. Lesquey, G. Gunther, C. Dubuc, D. Bloyet, and D. Robbes, J. Vac. Sci. Technol., B13 (1995) L. Chow, D. Zhou, A. Hussain, S. Kleckley, K. Zollinger, A. Schulte, and H. Wang, Thin Solid Films, 368 (2000) A. J. Kubis, T. E. Vandervelde, J. C. Bean, D. N. Dunn, and R. Hull, Mat. Res. Soc. Symp. Proc., 818 (2004) M

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