product review
The AFM goes mainstream Already a mature technology in the semiconductor industry, the instruments are becoming established in the life and materials sciences.
Rajendrani Mukhopadhyay
S
ince its first description by Gerd Binnig, Calvin Quate, and Christoph Gerber two decades ago (1), the atomic force microscope (AFM) has entered a variety of research and industrial niches. The AFM belongs to a larger family of instruments called scanning probe microscopes (SPMs). The unifying characteristic of SPMs is a miniaturized probe that closely tracks a physical property of a surface and produces a spatially resolved image of the physical property. A scanning tunneling microscope (STM) monitors the electron tunneling current between the probe and a conductive sample; a sharp tip on an AFM cantilever traces the topography of the surface. When Analytical Chemistry last reviewed the instruments (2001, 73, 627 A–635 A), the annual worth of the market was estimated to be in the hundreds of millions of U.S. dollars. Now the market value is expected to grow in the next few years, thanks to a push by the research funding agencies toward nanotechnology. Some experts point out that nanoscience owes its existence to SPMs. “That nanotechnology has become a recognized field in the last 20 years has more to do with the fact that we have eyes, ears, and even fingers, if you like, for manipulating at the nanometer scale, thanks to scanning probe microscopy,” says Stuart Lindsay of Arizona State University. “Even though there are aspects of nanoscience these days that don’t owe their existence directly to scanning probe [microscopy], nonetheless, the assumption is that scanning probe is there to analyze what is being done.” Two major types of AFMs are on the market: ultrahighvacuum (UHV) and ambient-pressure systems. The UHV AFMs are used predominantly in basic physics research, in which investigators aim to understand the fundamental properties of materials. The ambient-pressure AFMs are more widespread in various basic research and industrial applications and are standard in the semiconductor industry. Table 1 lists several commercially available ambient-pressure instruments; Table 2 has examples of UHV systems. The tables are meant to be representative, not comprehensive; vendors may offer similar products not listed. © 2005 AMERICAN CHEMICAL SOCIETY
Development of the AFM Atomic force microscopy grew out of the Nobel Prize-winning invention of scanning tunneling microscopy. A major limitation of an STM is that it can only interrogate an electrically conductive surface. “The idea of [an] AFM literally came about because the STM, its predecessor, simply relied on the tunneling current. You required conductive surfaces,” says Gerber at the University of Basel (Switzerland). “We developed the AFM to get atomic or molecular resolution on nonconductive surfaces.” In the first AFMs, the cantilever’s movement as it followed the contours of a surface was detected by an STM tip placed above it. However, the method wasn’t practical. Experts say that the optical lever detection technique was a breakthrough. In this method, a laser is shone onto the back of the cantilever and reflected onto a photodiode. The motion of the cantilever is proportional to the output from the photodiode. Paul Hansma at the University of California, Santa Barbara, recounts how the optical lever detection method was developed. “I had a retired researcher who just wanted space in my lab to work on gravitational experiments,” he says. “He used the optical lever for his gravitational experiments. The gravitational experiments weren’t working out too well, and our STM-based AFMs weren’t working out well. So I apD E C E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
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product review
Table 1. Selected ambient-pressure AFM instruments.1 Product
Company
Cost (U.S.D.) Operating modes
Maximum x–y scan range
Maximum z range
MFP-3D Atomic Force Microscope System
Asylum Research 6310 Hollister Ave. Santa Barbara, CA 93117 888-472-2795 www.AsylumResearch.com
Contact vendor
All standard operating modes as well as conductive AFM, Q-control, and piezo response
90 μm in closed loop
>15 μm sensored travel in closed loop; 28-μm option available
JSPM-5200
JEOL USA, Inc. 11 Dearborn Rd. Peabody, MA 01960 978-535-5900 www.jeolusa.com
Contact vendor
Contact, noncontact, STM, scanning KFM, force, contact current, scanning probe spectroscopy mapping, lithography, EFM, MFM
100 × 100 μm
10 μm
NanoWizard AFM
JPK Instruments AG Bouchéstrasse 12 Haus 2, Aufgang C 12435 Berlin, Germany +49-30-5331-12542 www.jpk.com
Contact vendor
All major operating modes
100 × 100 μm with 15 μm; optionclosed-loop control by al 100 μm with capacitive sensors CellHesion add-on
PicoPlus system
Molecular Imaging 4666 S. Ash Ave. Tempe, AZ 85282 480-753-4311 www.molec.com
Contact vendor
STM, contact, acoustic noncontact, MAC 100 μm open- and mode, EFM, MFM, KFM, dynamic lateral force, closed-loop scanner current sensing, impedance mapping, affinity recognition mapping (PicoTREC), volume spectroscopy mapping, scanning electrochemical AFM/STM
MultiView 3000 MultiProbe
Nanonics Imaging Ltd. Manhat Technology Park Malcha Jerusalem, Israel 91487 1-866-220-6828 www.nanonics.co.il
Contact vendor
AFM: contact, noncontact, intermittent con200 μm fine scanning 200 μm tact, phase, lateral force, MFM, KFM, EFM, and with 6 mm rough scanall electrical imaging modes; NSOM: reflection, ning transmission, collection, fluorescence, electrically induced photoluminescence, apertureless, and photon tunneling with evanescent field sample holder
easyScan 2 AFM/ STM
Nanoscience Instruments, Inc. 9831 South 51st St., Ste. C119 Phoenix, AZ 85044 888-777-5573 www.nanoscience.com
AFM: >$29,900; STM: $14,900
STM: constant current, constant height; AFM: AFM: 110 μm contact, dynamic, phase contrast, force modulation, spreading resistance, EFM, MFM
NTegra
NanoTech America/NT-MDT $110,000– 313 S. Jupiter Rd., Ste. 105 580,000 Allen, TX 75002 972-954-8014 www.nanotech-america.com
Contact: topography, lateral force, spreading 100 × 100 μm in con15 μm resistance imaging, error-feedback mode, ventional mode; 200 × MFM, force modulation; nanolithograhy and 200 μm with DualScan nanomanipulation; semicontact/noncontact: topography, phase imaging, error-feedback mode, EFM, MFM, scanning capacitance microscopy, scanning KFM; atomic force acoustical microscopy; STM; NSOM: transmission, reflection, collection, fluorescence
Novascan ESPM 3D
Novascan Technologies, Inc. $ 95,000– 131 Main St. 250,000 Ames, IA 50010 515-233-5400 www.novascan.com
Contact, lateral force, ac, phase, force plane Tip-scanning models: 2, 4.5, 8, 12 μm imaging, force spectroscopy, MFM, EFM, nano- 10, 20, 40, 80, 200 μm; available lithography open- and closed-loop available; add-on bottom scanner also available
Nano-R and Nano-I
Pacific Nanotechnology 3350 Scott Blvd. #29 Santa Clara, CA 95054 1-800-246-3704 www.pacificnanotech.com
Standard: contact, noncontact (vibrating), lateral force, phase imaging; optional: MFM, electric force, SHARK for electrical measurements, environmental, heating; light lever sensor, crystal force sensor
1
$70,000– 100,000
80 × 80 μm
Some companies may offer similar products not listed here. Contact the vendors for full product lines. EFM, electrostatic force microscopy; KFM, Kelvin force microscopy; MAC mode, magnetic ac mode using magnetic field to precisely control the vibration of the cantilever; MFM, magnetic force microscopy; NSOM, near-field scanning optical microscopy.
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8 μm
22 μm
8 μm
product review
Optical access
Software
Other features
Top-, bottom-, or dual-view optical access
Windows XP; software based in IGOR Pro (Wavemetrics) is user-programmable; advanced 3-D rendering capabilities are built-in
Sensored, closed-loop operation in all three axes using the Nanopositioning system; built-in nanolithography and manipulation, all-digital controller; models available are stand-alone and inverted optical for use on commercially available inverted optical microscopes for combined AFM and optical techniques (i.e., confocal, epifluorescence, total internal reflectance fluorescence, and phase contrast); environmental controls, including BioHeater and Closed Fluid Cell
On-axis and oblique
JEOL WinSPM
JSPM-5200 operates in ambient air, controlled atmosphere, fluid, or –6 vacuum; vacuum to 10 torr, heating to 500 °C; cooling to 130 K; very high stability (lattice resolution guaranteed on highly ordered graphite)
Top-view optics or inverted optical microscope with transmission illumination with differential interference contrast or phase contrast and epifluorescence, total internal reflectance microscopy
JPK SPM control and Image Process- Tip-scanning design for easy, flexible, and safe in-liquid operation in any ing software liquid container or fluid cell; BioCell, coverslip-based temperature-controlled fluid cell (15–60 °C) with perfusion and gas flow for living cell and single-molecule imaging and spectroscopy
Resolution $ 89,720
Contact, lateral force, digital pulsed force, noncontact, MFM, nanolithography, nanomanipulation
100 × 100 μm (200 × 200 μm optional)
20 μm
SPA400
>$100,000 SII NanoTechnology, Inc. Shintomi 2-15-5 Chuo-ku Tokyo 104-0041 Japan +81-3-6280-0066 www.seikoinstruments.com
Contact, dynamic AFM, multifunctional
150 μm
15 μm
1
Some companies may offer similar products not listed here. Contact the vendors for full product lines. EFM, electrostatic force microscopy; KFM, Kelvin force microscopy; MAC mode, magnetic ac mode using magnetic field to precisely control the vibration of the cantilever; MFM, magnetic force microscopy; NSOM, near-field scanning optical microscopy.
proached him with the idea, ‘Well, why don’t we try using your very sensitive optical lever to detect the deflection of the cantilevers?’ We combined two things that weren’t working very well and came up with something that worked very well!” The optical lever detection method was also independently developed by Meyer and Amer (2), and most commercial AFMs now use the technique to monitor the cantilever. The instrument is currently being developed in two ways. One push is to combine atomic force microscopy with other techniques. In particular, Raj Lartius of Novascan says, “There are a whole host of optical techniques combined with [atomic force microscopy] that are becoming very popular,” citing confocal, Raman, and total internal reflectance microscopies as examples. But experts say that caution is needed in combining AFMs with other instruments. “The development of new, multipurpose instruments has exciting scientific possibilities and makes sense from a marketing standpoint,” says Daniel Fletcher of the University of California, Berkeley. “[But] I think some of the performance measures can be compromised when moving away from the simple, compact designs. Limiting noise in the instrument is always a challenge, especially if you’re building one that can have various attachments and can sit on microscopes.” Another push is to make the cantilever scan faster over surfaces. At the moment, the time taken to acquire an image largely depends on the resonant frequency of the cantilever. Experts say that one way to increase the speed of the micro472 A
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scope is to use cantilevers with smaller dimensions. “If you go to small cantilevers, you push up the resonant frequency [of the cantilever],” says Jan Hoh of the Johns Hopkins University School of Medicine. “If you push up the resonant frequency, then you can scan faster and the noise also goes down.” Another approach to faster scanning is to use cantilevers actively modulated by piezos so that the resonance of the cantilever can be carefully controlled.
Operation of an AFM The cantilever is the business end of the instrument. When imaging, the cantilever can be raster-scanned over the surface in a number of “modes”. In contact mode, the probe runs continuously over the surface, rather like a fingertip running along a shelf. In noncontact modes, the cantilever doesn’t keep constant contact with the surface. The cantilever can be vibrated so that it taps along a surface, like a blind person using a stick to tap the ground while walking. The cantilever can also measure a range of forces at the piconewton level, such as van der Waals, electrostatic, and adhesive forces, between the probe and sample. By equipping the instrument with different types of probes, scientists can obtain information about various properties on a surface, such as capacitance and magnetism. Nevertheless, a caveat remains. “People need to be aware [that] with atomic force microscopy, you can measure topography very routinely now because there are reasonably good tips and the instru-
product review
Optical access
Software
Other features
Built-in 200× video microscope
ScanAtomic, Q-Analysis enhanced analysis and reporting software
Automatic tip approach; closed loop; lithography; sample heater; motorized x–y translation stage; sample size up to 6 × 6 in.; access to hardware and source code for customization; acoustic and vibration isolation chamber
Integrated
Windows XP-based NanoScope
Closed-loop hybrid technology x, y, z scanner; dedicated software for nanomanipulation in and out of plane of sample, and for nanolithography
Integrated high-resolution scientificgrade optical microscope; AFM objective with video camera system for direct and simultaneous sample and cantilever viewing
WITec ScanControl Plus Software Package for measurement adjustment, control, and image processing with a wide variety of image-processing, data-evaluation functions
Linear scan stage with feedback-controlled capacitive sensors to compensate for hysteresis, creep, and nonlinearity; digital pulsed force mode for imaging of surface properties such as adhesion, stiffness, and viscosity; modular design to upgrade to confocal/Raman and NSOM in one instrument
On-axis
Product is marketed only in Asia
ments are fairly well understood and characterized. However, when you get on a lot of these modes, like capacitance microscopy or electric force microscopy or magnetic force microscopy, they’re not as reliable, and they require a lot more knowledge and understanding of the technology,” says Paul West of Pacific Nanotechnology.
interesting to explore dynamic movements of cells,” such as the analyses of mechanical signal transduction and the binding of multiple receptors at a focal adhesion. However, Fletcher adds that the AFM must be used judiciously in force measurements. “While [atomic force microscopy] experiments have revealed fascinating behavior of molecules and cells, there can be a temptation to use the AFM where it may not be the best way to answer the biological problem,” he says. Researchers need to be aware that other techniques, like optical trapping and magnetic tweezers, also provide force measurements. The correct technique must be chosen for the type of measurement at hand.
Applications Vendors identify three broad markets for AFMs: the semiconductor industry, materials science, and the life sciences. Most companies choose to focus on a particular market. Information provided by an AFM about biological systems has generated a considerable amount of excitement in recent years. Because experiments can be performed in liquid, biological components can be studied under physiological conditions. Although the instrument has produced images of DNA, proteins, and cells at micrometer- to nanometer-scale resolution, it has also given insight into the types of forces that act both at the single-molecule level and in molecular ensembles (e.g., 3, 4). Hermann Gaub at Ludwig-Maximilians-Universität (Germany) points out, as an example, that ligand–receptor pairs can be studied at the single-molecule level because the probe can be modified to interrogate intermolecular forces. The instrument could also open up novel avenues for investigation. Fletcher notes, “I think there are many new areas in cell biology [for studying] larger forces, above the singlemolecule level, where [atomic force microscopy] can be very
Software Software can be considered the deal breaker for an AFM system. Chuck Mooney at JEOL says, “It’s why different instruments are preferred by different users. The software on one instrument might make what you’re trying to do easier than the software on another microscope, even though the physical configuration of the instruments will allow you to do the same thing and get good data from each.” The purpose of software in an AFM is twofold. The first part runs the instrument and acquires the data. The second part processes the data. Some debate exists about the extent to which the code of the software should be accessible to the user. Some companies argue that the software packages sold with the instruments must be closed-source so that the package can be D E C E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
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Table 2. Selected UHV AFM instruments.1
have a laboratory or a production environment where things have to be meaProduct Ultrahigh Vacuum (UHV) Variable Temperature UHV 350 and 750 Series AFM/STM sured [will] sooner or later need some AFM/STM (model VT AFM) type of AFM,” predicts Michael Serry Company Omicron NanoTechnology USA RHK Technology of Veeco Instruments. North American Headquarters 1050 East Maple Rd. Some hurdles remain, however. Ex14850 Scenic Heights Rd., Ste. 140 Troy, MI 48083 Eden Prairie, MN 55344 248-577-5426 perts say that researchers must first un952-345-5240 www.rhk-tech.com derstand exactly what an AFM can do www.omicron.de and determine its limitations and potenCost (U.S.D.) Contact vendor >$200,000 tial. Once the instrument’s capabilities Operating Contact/noncontact AFM, STM, EFM, MFM, AFM/STM, force, lateral force, conare recognized, researchers then must modes scanning KFM, and other “novel” modes sup- tact and noncontact, MFM, KFM, learn to use it effectively. ported and user-configurable modes A bigger obstacle is the wider acceptMaximum x–y 10 × 10 μm scan range (tip coarse-motion 8 μm (coarse x and y translation of ance of the technique. Although experts scan range range of 10 × 10 mm in x and y) 6.4 mm with 200-Å steps) recognize the possibilities that the inMaximum z 1.2 μm (tip coarse-motion range of 10 mm in z) 1 μm (coarse z position 0.5 mm with strument offers, convincing researchers range 4-Å steps; lift head 3" off sample for outside the field to use AFMs remains a deposition or surface modification) challenge. “I think the bottom line is Optical access Line-of-sight tip/sample junction access stan- Direct optical access to the sample/ that, while the potential is fantastic, the dard, multiple chamber viewports for easy tip interface through several viewtip/sample exchange ports and the CCD camera package instrument is still perceived to be a Ph.D. that comes standard physicist’s instrument,” Lindsay says. “BiSoftware Omicron proprietary Matrix software for data XPMPro user-programmable SPM ologists are not about to rush off like acquisition, data analysis by commercial data acquisition, analysis, image lemmings to try [it] because of the promScanning Probe Image Processor package processing, and display software; ise that starry-eyed people with physics (Image Metrology, Inc.) free download for image processing backgrounds tell them lies out there.” from company website Fletcher thinks the same acceptance Other features Integral vibration isolation (spring suspenHigh-performance modular design sion/eddy current damping combination); provides easy customization with dilemma applies to chemists. “For those remote-controlled piezo coarse motion for tip a built-in upgrade path; vibration who think in terms of reactions alone or positioning and coarse tip/sample approach; isolation, internal spring suspension reactions as a dominant feature of any in situ AFM detector (photodiode) and curand/or external air leg; complete kind of system, [they should] consider rent/voltage converter for STM; in situ tip/ nonmagnetic microscope options; sample storage (12 positions); probe/sample internal tip and sample storage; that [atomic force microscopy] can give exchange without having to break vacuum; external high-bandwidth, low-curinsight into not just the geometry of mode switching (AFM to STM and vice versa) rent preamplifiers (>5 kHz at
