All-in-one static and dynamic nanostencil atomic force ...

All-in-one static and dynamic nanostencil atomic force microscopy/scanning tunneling microscopy system Percy Zahl, Martin Bammerlin, Gerhard Meyer, and Reto R. Schlittler Citation: Review of Scientific Instruments 76, 023707 (2005); doi: 10.1063/1.1852925 View online: http://dx.doi.org/10.1063/1.1852925 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/76/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A variable-temperature nanostencil compatible with a low-temperature scanning tunneling microscope/atomic force microscope Rev. Sci. Instrum. 85, 023706 (2014); 10.1063/1.4864296 Multitip atomic force microscope lithography system for high throughput nanopatterning J. Vac. Sci. Technol. B 29, 06FD03 (2011); 10.1116/1.3662396 Development of UHV dynamic nanostencil for surface patterning Rev. Sci. Instrum. 79, 103904 (2008); 10.1063/1.2999547 A double lamellae dropoff etching procedure for tungsten tips attached to tuning fork atomic force microscopy/scanning tunneling microscopy sensors Rev. Sci. Instrum. 74, 1027 (2003); 10.1063/1.1532833 The use of a special work station for in situ measurements of highly reactive electrochemical systems by atomic force and scanning tunneling microscopes Rev. Sci. Instrum. 70, 4668 (1999); 10.1063/1.1150130

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REVIEW OF SCIENTIFIC INSTRUMENTS 76, 023707 共2005兲

All-in-one static and dynamic nanostencil atomic force microscopy/scanning tunneling microscopy system Percy Zahl,a兲 Martin Bammerlin,b兲 Gerhard Meyer, and Reto R. Schlittler IBM Zurich Research Laboratory, Säumerstrasse 4, 8803 Rüschlikon, Switzerland

共Received 24 September 2004; accepted 6 December 2004; published online 20 January 2005兲 The nanostencil is a tool for resistless lithography. It allows the direct patterning of complex nanometer-sized structures composed of a wide range of materials in an ultrahigh vacuum environment. This is combined with state-of-the-art scanning probe microscopy techniques 共atomic force microscopy, scanning tunneling microscopy兲 and an electronic four-point probe. Moreover, all these capabilities are in situ and autoaligned in the field of view. The direct patterning is based on the shadow-mask technique and allows multimask processes in a static and dynamic manner. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1852925兴

I. INTRODUCTION AND OVERVIEW

Scanning probe microscope 共SPM兲 lithography methods provide access to the 10 nm scale and below, and are currently the subject of intensive studies for use as future lithography techniques.1–4 Moreover there is a wide range of surface manipulation processes down to atomic scale using SPM techniques. In particular, there is a need for a high-resolution technique that can pattern a wide range of materials such as metals, ionic compounds and organic molecules directly onto surfaces. Direct patterning allows the fabrication of structures in an ultraclean environment, as is important for the study of nanosystems or molecular electronics. In that case a technique is required that combines stepby-step in situ construction 共patterning兲, microscopic analysis at the atomic scale 关atomic force microscopy 共AFM兲/ scanning tunneling microscopy 共STM兲兴 and optional electronic characterization. The nanostencil tool we present here is one approach to address these demands. It provides a tool for resistless lithography using the shadow-mask technique,5,6 thus allowing the direct patterning of larger areas 共up to several square millimeters兲 of complex nanometer-sized structures as small as 40 nm composed of a wide range of materials in an ultrahigh vacuum 共UHV兲 environment. The optional capability of using multiple aligned masks allows multimask processes in a static and dynamic manner. Finally it is combined with scanning probe microscopy7–9 techniques including state-ofthe-art STM, contact/noncontact AFM and an electronic four-point probe—all in situ and prealigned in the field of view. II. REALIZATION OF THE COMBINED STENCIL AND ANALYSIS SYSTEM

To realize the all-in-one nanostencil, positioning elements were incorporated into approved SPM/AFM construca兲

Electronic mail: [email protected] Present address: Concentris GmbH, Davidsbodenstr. 63, 4012 Basel, Switzerland.

b兲

0034-6748/2005/76共2兲/023707/5/$22.50

tion methods to yield a design, instead of modifying commercially available instruments 共Fig. 1兲. The stencil subsystem is based on a set of aligned and exchangeable mask carriers to allow maximal flexibility for patterning. The mask aperture can cover up to about 5 mm2 of the sample area, but structure sizes are only limited by the maskfabrication abilities, such as the focused ion beam 共FIB兲 treatment of silicon nitride membranes etched into a silicon chip carrier. This currently should allow feature sizes down to about 10 nm—depending on the FIB system. A precision carousel-like turntable with snap-in lock mechanism provides a set of highly precise 共⬍1 ␮m兲 repeated positions, and accepts the transferable mask carriers also using precision limit-stop slots.10 Similar to the mask carrier, a four-point-probe and the SPM detector, i.e., the STM tip or AFM cantilever can be placed and turned to the reference position. This key feature of our nanostencil ensures that even a single nanostructure can easily be found within the SPMs scan range. Moreover, AFM cantilever exchange can be performed within this precision. The carousel positioning type used here is superior to other positioning solutions, such as a high-precision and wide-range 共several centimeters兲 XY stage for this application, because of its simplicity and stability—it has only one turning part. It can be actuated simply by turning, and the snap-in lock precludes position sensing. Thus it is well suited for the UHV environment. The sample on a transferable holder is attached by means of a precise retainer to the tube scanner, which is mounted on a standard microslider XYZ stage11 above the carousel. The entire nanostencil is attached to a rigid base plate, which sits on a viton-stack damping stage within the UHV chamber. A very important design issue is visual sample access from multiple directions. This is crucial not only for SPM approach control, but also for the long-distance microscopy used for mask and sample viewing and alignment. Another

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FIG. 1. 共Color online兲 Design of the all-in-one nanostencil: sample above mask in the carousel. Left: Sketch of the evaporator-mask-sample geometry.

key feature is the direct perpendicular access to the sample for molecular-beam epitaxy, i.e., the deposition of 共multi兲material sources via the mask.

A. Nanostencil-embedded SPM design

Employing standard etched tungsten tips, STM can be performed using a low-noise and in-vacuum preamplifier 共double-JFET and feedback resistor兲. For the AFM detection unit, we use a standard optical light beam-deflection method.12 A fiber-coupled Superlum 10 mW light emitting diode 共LED兲 light source13 provides stable intensity and a fine focus on the cantilever 共⬍3 ␮m兲, i.e., only about 1 / 12 of the cantilever width. A positionsensitive four-quadrant photodiode detector 共PSD兲 with highspeed preamp 共3 MHz bandwidth, in UHV兲 is used for signal reception. Two two-axis UHV mirror adjustments allow optimal beam positioning on the detector. Perfect X / Y decoupling is achieved by fine adjustment of the PSD X / Y orientation with respect to the cantilever. The STM/AFM control system is fully digital: In noncontact AFM mode 关dynamic force microscopy 共DFM兲兴 a digital phase-locked loop and excitation control with 2 MHz bandwidth14 is used to detect the frequency shift. A DSPcontrolled feedback, scan control and data acquisition system15,16 not only provides standard imaging, but is also a tool for complex dynamic patterning. Compared to the size of the mechanical system, high stability 共Z noise ⬍1 Å兲 and atomic resolution in the AFM and STM modes were achieved using the currently mounted scanner featuring a scan range of up to 50 ␮m. Figure 2 demonstrates atomic step resolution of the nanostencil SPM system in the STM and AFM modes. The top images show three monolayers of C60 epitaxially grown on flame-annealed Au共111兲 on a mica substrate. For AFM tests the surface of a NaCl single crystal cleaved in UHV was imaged 共bottom image兲. Separate biasing of tip/cantilever and both ends of the sample 共split sample holder兲 is possible.

FIG. 2. 共Color online兲 Top: Three monolayers C60 on Au 共111兲 imaged in STM mode 共200⫻ 200 nm兲. Bottom: NaCl crystal imaged by AFM 共1000 ⫻ 1000 nm兲. Monoatomic steps are clearly resolved.

B. Core of the nanostencil system

The core mechanical system of the repeated positioning mechanism is based on a simple and rigid carousel principle using snap-in lock and limit-stop techniques. Designed to work properly in an UHV environment, it proved to be rigid enough to be used for positioning SPM elements 共probe/ substrate/etc.兲. The carousel is designed to hold five slots 共representing the repeated positions兲, which can be turned to the working position where a snapper mechanism automatically repeats the position within about 1 ␮m. Each slot holds transferable, precisely machined blocks locked using limitstop positioning within the slot. Each block can hold a stencil mask or the four-point probe. A special slot is used to preci-

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sion mount the SPM detector, which is a STM tip or AFM cantilever. The alignment of all slots to each other is adjusted once at instrument setup. Tiny remaining static errors can be compensated by using a reference block for each slot to align masks 共or other objects兲 later to compensate for the known static error. Combining SPM, multishadow-mask patterning and electronic probing 共four-probe兲 allows all in situ, contaminationless 共resistless兲 complex direct patterning, pattern inspection and testing. Thus simple and cheap prototyping of arbitrary nanostructures is possible. In particular the system design ensures that structures are aligned within the SPM scan range without searching and allows quick intermittent 共during the patterning process兲 structure inspection using STM or AFM. Using separate masks instead of the cantilever as described previously5 provides greater flexibility and larger areas for patterning or, in the case of multiparallel patterning, a much larger area of pattern coverage. The repeated positioning of masks allows multimask processes with optional in situ/in process/intermittent pattern verification without substrate transfer and structure search. For multimask processes it is important to mention that the repeated positioning does not 共yet兲 achieve the resolution of single-mask or dynamic-mask processes, which are used for high resolution, but is limited to the achieved mask prepositioning 共⬇10 ␮m兲 and the carousel repetition precision 共better than ⬇1 ␮m兲. However, the prepositioning error can be compensated by readjusting the sample position in between. Thus, there is an absolute position-alignment uncertainty of less than 1 ␮m for multimask processes. This limits us to specially designed patterns that which must be tolerant to shifts of up to 1 ␮m. An example is simple crossed line structures with a line length greater than 2 ␮m. Our current investigation of an integrated electronic analysis stage using the four-probe aims for quick in situ testing. If the mask共s兲 are designed to fit to the four-probe footprint, i.e., if we prepare a set of four 5 ␮m square-shaped contact pads, only the four-point tester needs to be turned in position and approached to the surface, using one of the cantilevers as distance sensor similar to AFM. C. Construction details

Refer to Fig. 1 for a sketched perspective system overview and to Fig. 3 for a mechanical embodiment: The basic embodiment of the nanostencil core 共see Fig. 3兲 is constructed on a base plate 共8兲 mounted on a damping stage, which holds the XYZ stage 共9兲. The scanner 共1兲 with substrate 共2兲 is attached to the movable portion of the XYZ stage 共9兲. The carousel mechanism 共4兲 for repeating positions is built into the base plate 共8兲. The repeated positions 共3兲 are defined via limit stop for all three dimensions of a block 共5兲 mounting mechanism within the carousel plate 共4兲. The radial and axial precision of a set of spring-loaded 共7兲 ball bearings 共6兲 keeps the carousel plate 共4兲 in position. The rotational/angular position is defined by a precision snap-in mechanism 共snapper兲 共10兲. The snapper is constructed by using an axial fix 共spring-loaded, linear ballbearing slider兲 and a ball bearing, which snaps between pairs

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FIG. 3. 共Color online兲 Construction details of the carousel mechanism using a snapper for repositioning 共sketch not to scale兲. 共1兲 scanner, 共2兲 substrate, 共3兲 stencil mask mounted on carrier block at repeated position, 共4兲 carousel, 共5兲 block in limit-stop mount, 共6兲 ball bearing, 共7兲 spring-loaded fixture, 共8兲 base plate on damping stage, 共9兲 XYZ stage with attached scanner, 共10兲 snapping mechanism, 共11兲 cantilever/tip.

of locking rods fixed on the carousel plate 共4兲 positioned at the desired repeated positions. The snapper itself is fixed to the base plate.

III. MASKS AND MBE STENCIL PROCESS

Masks were fabricated by FIB structuring of silicon nitride membranes. Membranes are etched into silicon carrier chips using standard lithography processes and are commercially available.17 We use membranes of 100 and 150 nm thickness and square-shaped apertures with a side length between 200 and 500 ␮m within the carrier chip. Before FIB treatment, masks were Piranha cleaned to remove all organic substances and coated with about 20 nm gold on both sides. Using our FIB,18 we can write structures down to 40 nm. After aligning the mask on the stencil mask holder and sample loading, the mask can be approached to the sample using optical distance control and final mask-sample resistance measurement. Parallel mask-sample surfaces 共⬍0.05° → ⬇ 2.5 ␮m separation, this means a maximal geometric feature broadening of 25 nm in our current system geometry; evaporation source-to-mask distance is 150 mm兲 are mechanically realized. By using special self-aligning mask holders, this small remaining error can be almost fully compensated and has now been proved to be below 5 nm. A single static stencil process involves depositing material from one of four e-beam metal sources 共Cu, Au, Fe, etc.兲 or one of three load-lock loadable molecule evaporators 共C60, etc.兲. The evaporation sources are mounted on an X / Y / Z manipulator and thus can be aligned to point directly to the mask/sample target. A crystal balance with rate monitor is installed for deposition rate adjustment.

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FIG. 4. 共Color online兲 CCD image of a 100 ␮m wide gap structure of Au on SiO2 共Au on top and bottom兲. The cantilever is centered above the gap on the right. Three smaller deposited Cu pads are just visible in the left area of the gap; much smaller structures in between are not resolved. The picture was imaged using a long-distance microscope from Questar.

IV. VIEWING AND POSITION CONTROL MICROSCOPES

For approach control a telephoto lens with a charge coupled device 共CCD兲 camera is placed at about 15° with respect to the sample surface to view the tip, lever, or mask. The stencil process, mask alignment and the scan field of view can be monitored directly from below 共looking along the surface normal兲 using a long-distance microscope from Questar19 and a retractable mirror. This allows a resolution of about 4 ␮m at a working distance of 150 mm. A highly sensitive CCD video camera20 allows the imaging of faint and low-contrast structures using only ambient and scattered light. Figure 4 displays a typical situation with sample, cantilever, and a large test structure: Within a 100 ␮m wide, prefabricated gap structure of Au on SiO2, three smaller Cu pads were placed using the nanostencil. These pads can be used for contacting final nanostructures placed across the smaller pads. The third one within the wide gap can be contacted using the cantilever itself. Using our optical setup we were able to detect contrast between bare SiO2 starting at about 10 nm thickness of the Cu pads.

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FIG. 5. Our crossed two-material structure of Cu and C60, image size: 12 ␮m ⫻ 20 ␮m. Left: topographic DFM image, right: damping signal.

the topographic image, but is actually about 5 nm in height. The image on the right shows the measured damping signal. Interestingly a marked contrast appears at the crossing areas. The width 关full width at half maximum 共FWHM兲兴 of the thinner Cu line is about 500 nm; the wider C60 sections are about 2000 nm in width. This demonstrates our ability of in situ and in-UHV creation and imaging of multimaterial structures combining metallic and organic substances. Working towards electronic characterization of thin and long nanowires and junctions of different metallic and organic materials, we created a test structure of crossed C60 and Cu wires on native silicon oxide 共cleaned by heating to 500 ° C in UHV兲 as shown in Fig. 6共a兲. The wire width varies from 150 nm 共not shown here兲 at the ends of the line to about 400 nm in the center of the line because of the slit opening in the mask due to residual strain in the membrane and because of the lower resolution of the FIB using greater scanning ranges 共40 ␮m long line兲. The horizontal wire is made of C60 and overlaps the larger Cu contact pads partially shown at the left and right. The vertical Cu wire is connected to a third pad on top 共not shown in the scan range here兲. A closer look at the junction area, Fig. 6共b兲, shows a smooth Cu wire about 6 nm in height overlaid by a C60 wire of about the same height. The C60 material appears smooth on a large

V. RESULTS

Tests were performed on SiO2 surfaces using Cu and C60 fullerenes. Fullerenes like C60 are particularly nice materials for patterning with the nanostencil tool, because they are chemically not very reactive and self-assemble into wellordered layers. Moreover, by endohedral or exohedral doping it is possible to tailor their properties electronically from insulating to metallic. However, these materials are very difficult to handle when conventional lithography techniques are used. Figure 5 demonstrates a two-step stencil process using two different aligned masks. The two wider wires are C60 crossing a thinner Cu line. This is a demonstration of in situ nanopatterning, not yet possible otherwise, using this kind of material combinations. Owing to a constant and not minimized sample-cantilever potential, which is necessary for stable C60-imaging conditions in DFM mode, some contrast inversion occurs. The Cu line appears as a depression in

FIG. 6. 共a兲 DFM image of two long, crossed wires of Cu and C60 on native silicon oxide. 共b兲 Magnification of the wire junction.

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FIG. 7. 共Color online兲 Test pattern on native silicon oxide. 共a兲 FIB image of the stencil mask used here. 共b兲 DFM image of the Cu structure. 共c兲 DFM image of the C60 structure. 共d兲 Line profile of the section marked in 共b兲.

scale but is composed of smaller grains about 10 nm in size. The grainy structure is less visible in the junction area, which appears at the expected double height of 2 ⫻ 6 nm. The set of images shown in Fig. 7 demonstrates the ability to create more complex sub-100 nm structures using one static nanostencil process. The stencil mask used was created by FIB and is shown in Fig. 7共a兲. Our FIB currently limits us to a linewidth of about 40 nm, as shown here. Figure 7共b兲 is the result of Cu deposition via this mask onto a native silicon-oxide surface, which was cleaned in an UHV environment by heating to about 500 ° C for about 1 h. Comparing Figs. 7共a兲–7共d兲 shows that we are able to transfer structures as small as 40 nm almost 1:1 to the substrate via the mask. The average FWHM of the lines is about 40 nm and the average feature height is 5 nm, see Fig. 7共d兲 for a height profile accross two lines as marked in Fig. 7共b兲. In Fig. 7共c兲 we deposited C60 using the same mask. The result looks smeared out; the broadening to about 110 nm is the result of the diffusion of C60 at room temperature. Noting the precise reproduction of the stencil mask pattern as shown in Fig. 7共b兲, it now appears reasonable to be able to transfer arbitrary patterns even smaller than 40 nm from a suitable mask to the sample using any material that is stable and immobile on the substrate surface. In conclusion, the nanostencil has been proved to work as expected. We are confident that we will be able to achieve arbitrary 10-nm-sized multimaterial structures in the future, depending almost entirely on mask fabrication abilities. ACKNOWLEDGMENTS

The authors acknowledge the ideas and work of Hans Peter Ott in the machine shop. The partial support of the Swiss Federal Office for Education and Science 共OFES兲 in the framework of the EC-funded projects “ATOMS” and

“NICE” is gratefully acknowledged. Financial support from the National Center for Competence in Research 共NCCR兲 “Nanoscale Science” and the Swiss National Science Foundation is gratefully acknowledged. The authors thank H.-R. Hidber, A. Tonin, and R. Maffiolini from University of Basel for the design of the in-UHV AFM signal detection unit and amplifier electronics. The authors thank R. Allenspach for discussions and for supporting the Nanostencil project. 1

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