Imaging at the Atomic Level

The characteristics of nanomaterials can be very different from those of large-scale versions of the same substance. Some of this behavior relates to unique structures, such as carbon nanotubes that exhibit very low resistance and semiconductor effects. In other cases, the large surface area-to-volume ratio of a nanomaterial makes the action of atomic-level forces more pronounced. An example is the greatly increased activity of nanocatalysts used to facilitate chemical reactions.

One thing all nanomaterials have in common is size: At least one dimension corresponds to only one or, at most, a few atoms. Measurements at the atomic scale have been possible for many years by using a transmission-mode electron microscope (TEM). However, TEMs are very expensive both to buy and operate and sensitive to ambient vibration and electrical noise and require conductive samples.

Subsequently, the scanning electron microscope (SEM) was developed, which detects secondary electrons emitted from the sample as it is illuminated by a raster-scanned electron beam. Developing an image from secondary rather than transmitted electrons loses some resolution but makes the SEM more versatile and lower cost than the TEM. Both the TEM and SEM work best with conductive samples and must operate in a vacuum.

In the mid ’80s, the scanning tunneling microscope (STM) was invented. In this technique, a very sharp stylus is brought within a few nanometers of a conductive sample surface and a voltage difference applied across the sample-to-stylus gap. Operating in a vacuum, an STM measures the miniscule current that flows because of quantum tunneling caused by the large electric field at the stylus tip. The STM uses a precision positioning mechanism based on piezoelectric materials that later was applied to other types of scanning microscopes.

It was a short step from the STM to the mechanically similar scanning probe microscope (SPM), although the principle of operation is very different. Initially, scanning force microscopes (SFM), also called atomic force microscopes (AFM), scanned a sample with the stylus tip in contact. The stylus is mounted at one end of a cantilevered arm, and it is the position of the arm that is measured, usually by reflecting laser light from the back of the arm. In constant-force mode, a feedback loop maintains the same degree of cantilever curvature, which is proportional to force, by adjusting the Z-axis position as the stylus scans.

Dragging the tip across a specimen yields a number of important measurements. For harder materials, the surface texture can be determined. In addition, some materials exert more attractive force on the stylus that shows up as drag. With softer samples, the tip may scratch the surface, and this action again causes a force that opposes tip motion. Of course, softer specimens as well as the tip itself can be damaged by scanning in contact, and this is a major limitation of contact SPM.

Nevertheless, SPMs with many specialized capabilities have been developed because of the technology’s inherent benefits. SPMs don’t require a vacuum. Nor do specimens have to be conductive. In fact, SPMs not only operate in air but also through a layer of water that may be unavoidable when examining biological specimens.

In addition to contact mode, a noncontact mode and an intermittent-contact mode, referred to as tapping mode, are used. In noncontact operation, the cantilever arm oscillates near its resonant frequency with a small amplitude and at a correspondingly small distance above the sample surface.

The atomic-level forces experienced as the stylus scans the surface change the resonant frequency. The output signal representing surface height is derived from the feedback correction necessary to maintain a constant frequency. Some noncontact SPMs achieve similar results using the principle of constant amplitude oscillation rather than constant frequency.

Tapping-mode operation is similar but differs because the oscillation amplitude is much larger and the stylus tip touches the sample surface on each cycle. This means, for example, that if a layer of water is present, the actual surface of the specimen is measured rather than the top of the water layer as in noncontact mode. Delicate samples can be scanned without damage, and elastic surfaces are not distorted as with contact scanning.

In addition to developing an image related to oscillation amplitude, tapping-mode SPMs can measure the phase change at each tip-to-sample contact. This information relates to the amount of energy lost during contact and helps quantify properties such as friction and elasticity. As shown in Figures 1a and 1b, there often is a large difference in appearance between height and phase images developed from the same scan.

Practical Examples

Speed
Several aspects of SPM use are being improved. For example, a major area attracting attention is scanning speed. It can take minutes to produce an image although a number of available techniques significantly reduce the required time. Using tuning-fork structures, a rate of more than 15 images/s is claimed, supporting interactive experimentation in real time.

In the VideoAFMâ„¢ video rate AFM attachment developed by infinitesima, a resonant fast-axis scanner drives the cantilever at up to a 10-kHz line rate while the sample stage simultaneously moves in a synchronized slow scan to produce a high-speed raster. The VideoAFM is used with a conventional AFM to provide real-time large-area search capability and zoom magnification. Once an area of interest is located, a slow-scan high-resolution image can be acquired.

Also in an effort to increase speed, Karma Technology has developed the Hydra concept in which a single AFM platform can carry as many as five separate scanning-probe modules. According to Tom Voehl, the company’s vice president of marketing and sales, “The Hydra approach can simultaneously scan five sites of 5 microns x 1 micron. This results in a throughput rate of up to 150 wafers, photo masks, or other substrates processed per hour.”

Pacific Nanotechnology AFMs are available with Dual Scan Technology (DST), which is similar to Hydra because it provides two scanners. But in this case, they do different things. One scanner is optimized for accurate metrology and the other for making high-speed scans.

Ease of Use
In an AFM, the probe is a replaceable part. Eventually, the tip will become damaged, or you must change it to run a different type of experiment. Replacing a probe can be very time-consuming because of the manual procedures required to align the detector and laser.

The laser and detector are prealigned in the Nanosurf machines available from Nanoscience Instruments. Alignment grooves etched into each probe chip match similar grooves in the probe mount. The result is probe seating within ±2 µm without further adjustment.

In addition, the Nanosurf Nanite instrument has been simplified through use of an electromagnetic scanner. Compared to traditional piezoelectric scanners, electromagnetic devices are inherently more linear and operate at far lower voltages. The design of the scanner allows reduced size and power requirements as well as lower cost.

Agilent Technologies recently added AFM systems to its catalog through the acquisition of Molecular Imaging. Within the Agilent AFM range, a number of features stand out. For example, easily interchangeable nose-cone cantilever modules allow you to quickly change from one type of AFM mode to another simply by plugging in a different module.

Access
Sample access is yet another area that has been difficult with most SPMs. According to Karen Gertz at Veeco Instruments, “Electrical and electrochemical characterization often implies a need for good physical sample access for insertion of extra leads or electrodes. However, an open-stage architecture usually runs counter to mechanical design criteria imposed by the need for good AFM performance.

“The Innovaâ„¢ AFM combines an open stage with a high-resolution platform,” she explained. “It provides physical and optical sample access with the capability to achieve atomic resolution, even with a 90-µm scanner.”

Ms. Gertz added that an AFM is not limited to periodic structures as are diffraction techniques but rather generates realspace images. An example of automation with regard to AFMs is their use in semiconductor manufacturing. She noted that changing the sample and probes, engaging the probe on the surface, analyzing data, and generating reports have been fully automated in product versions targeting the semiconductor and disk drive industries.

The Agilent AFM scanners feature a top-down configuration that isolates the scanning elements and electronics from the imaging environment. This allows samples to be imaged at temperatures up to 250°C over time periods as long as 10 hours. In addition, the optical microscope can directly view the cantilever and the area being scanned.

Portability relates to access in the sense that sometimes the AFM must go to the sample, and when it does, you don’t want to compromise image quality or measurement capabilities. The Nanosurf Mobile S Cordless Edition runs for about five hours on an internal rechargeable Li-Ion battery.

Two video cameras aid initial probe positioning in this 4.7-kg transportable instrument. The automatic approach function has a 4.5-mm range and a 0.12-mm/s rate of approach. But if you can position the probe within 1 or 2 mm of the sample by using the cameras, you will save considerable time.

The Mobile S has five parallel measurement channels from which you configure the SPM. Producing a conventional force vs. distance plot is one possibility, but so too are graphs of particular signals with or without averaging.

A Family of SPMs
Many variations of the basic SPM exist, such as Kelvin probe force microscopy that is sensitive to the local work function of the sample and the voltage applied to the probe tip. Atomic force acoustic microscopy (AFAM) and piezoresponse force microscopy are two additional techniques used to study local mechanical and electromechanical properties.

None of these approaches, however, can distinguish energy dissipation from other effects. In a new method developed by Asylum Research and Oak Ridge National Laboratory, the probe is driven by a complex band of frequencies. This type of band excitation is significantly different from traditional single-frequency AFM operation at or near resonance. By performing an inverse FFT on the recovered response signal, researchers have determined the dissipation associated with each image pixel–an important factor in the behavior of some materials.

Dual ACâ„¢ Mode, a related approach available on Asylum Research’s MFP-3Dâ„¢ Systems, was described by Terry Mehr, the company’s director of marketing: “In this mode, the cantilever is driven at or near two of its resonant frequencies. The motion is then analyzed by two quadrature lockin amplifiers shown in Figure 2. The outputs can be displayed, saved, combined with other signals, and used in ­user-selected feedback loops.”

By simultaneously exciting the probe at two resonances, previously unseen effects have been revealed. For example, Figure 3 is a 3-µm image of a silica-filled epoxied natural rubber (ENR) and polybutadiene rubber (BR) compound. Adding second resonance phase information differentiated the various constituents: black dots are silica, yellow and orange areas are ENR, and the dark patches are BR. The three substances have different elasticity, which is not sensed by a single-frequency AFM.

The NTEGRA Platform made by NT-MDT can perform conventional AFM as well as magnetic force microscopy (MFM), nanoscale sclerometry, and AFAM. These are all indirect means of investigating different aspects of the local structure of a sample, as is AFM. MFM measures the effect of an external magnetic field and is possible because the NTEGRA scanner uses no magnetic parts.

In hardness testing at the nanoscale level, tiny pits or scratches are made in a material. Comparing the applied force with the size of the resulting indentation gives a value for hardness. The NTEGRA datasheet states, “In some cases, the results obtained (from scratches) can provide more information than that obtained by nanoindentation because the width of a scratch, as a result of the elastic recovery, modifies less than its depth.”

Unlike the cantilever of common silicon AFM probes, the piezoceramic console of the probe for the NTEGRA machine has a greater hardness. This makes the degree of force applied to a sample much greater than in usual AFM systems. Nevertheless, because of its high resonant frequency, the same probe can be used to map hardness and elasticity properties of the sample.

Tips and Accessories
As you might expect, the nature of the probe tip influences the test results. Simply because of its geometry, a large radius tip cannot respond to small surface details, so a low-resolution image results. The typical radius for etched silicon tips is 10 nm. For example, NT-MDT markets the Etalon AFM Probe with a tip length of 5 to 10 µm mounted at the end of a polysilicon cantilever 32­?µm wide, 87? or 117?µm long, and 1.75?µm thick with a resonant frequency of 200 or 120 kHz depending on length.

Carbon nanotube tips combine the strength of a larger multiwalled tube for most of their length with a smaller radius tapered end. Removing the outer carbon layers near the end of the tube results in a tip with less than a 5?nm radius. In addition to their hardness and strength, nanotube probe tips provide a very high aspect ratio of length to diameter.

Different tip materials are required for sclerometry depending on the sample being tested. For example, the NTEGRA datasheet mentions the three-sided pyramidal nanoindenting diamond Berkovich tips and semiconductor diamond tips. In addition, C₆₀ fullerite tips are suitable for testing extremely hard surfaces because the tip material is used to scratch diamond.

Probe and tip characteristics can be complex. An example is the Akiyama probe that Nanosurf has adapted for use with the company’s AFM instruments. It comprises a tuning-fork section with its own resonant frequency and a cantilever-mounted tip that resonates at a different frequency. The effect of this dual spring-constant mechanical system is similar to that of the Asylum Research Dual AC Mode to the extent that an AFM with the Akiyama probe and special controller can measure dissipation.

Tips also may be coated with selected chemicals to investigate a sample’s interaction. Further, sample behavior in the presence of an electric field also might be important, so if a silicon tip were used, the tip and cantilever have to be conductively plated before the chemical coating is applied.

An optional low-coherence light source improves the sensitivity of low-force measurements for Agilent scanners. It delivers better performance because the low-coherence light source effectively eliminates laser interference, resulting in far more sensitive force detection.

Summary

If you’re working with nanomaterials, chances are that you’ve already run into an SPM. These versatile instruments are used to investigate nanoscale material properties of all kinds. Paul West, vice president of products and chief technical officer at Pacific Nanotechnology, said, “New types of nanosensors will rely on nanometer-sized objects such as carbon nanotubes and quantum dots. The AFM is ideal for visualizing and measuring these things.”

What kind of performance is possible? The Pacific Nanotechnology Nano-DSTâ„¢ is a research-targeted instrument built with a solid granite base and gantry and suitable for a wide range of AFM operating modes. The X-Y noise is specified <0.01 nm open loop with <0.06?nm Z-axis noise. Suitable vibration isolation can improve the Z-axis performance. To help put these numbers into perspective, the diameter of a hydrogen atom is approximately 0.1 nm.

This is an example of what can be achieved through very sturdy construction and with careful installation and operation. At the opposite end of the use model are portable machines such as the Nanosurf Mobile S and Veeco Caliberâ„¢.

The Nanosurf instrument specifies 0.07?nm maximum Z?axis noise and 0.15?nm X-Y resolution in its high-resolution mode with 10-µm range. This AFM uses 16-b control, measures up to five signals, and provides images as large as 2,048 x 2,048 points. No X?Y noise specification is given.

Over a 90?µm scanner range, the Veeco Caliberâ„¢ System claims <3?nm X-Y closed-loop noise and <0.1?nm Z-axis noise with vibration isolation. It uses 24?b control for each of the axes and features eight 16?b measurement channels. Both the Caliber and Mobile S instruments appear to be mechanically robust and easy to use.

Whether you need to measure surface features at the angstrom level or 10x larger nanometer level makes a big difference to which kind of instrument you need and the price you will pay.

Working With Incredibly Small Signals

As you can imagine, the forces and signals involved in scanning force microscopy are tiny. Nevertheless, because the oscillating frequency is known in noncontact and tapping modes, phase-sensitive detection (PSD) can be applied to improve the signal-to-noise ratio.

PSD is the principle behind lock-in amplifiers. This type of synchronous detector multiplies the input signal, which may have a phase offset ?, by the known oscillation frequency and low-pass filters the result. Because the two signals have the same frequency, it can be shown that the output of the multiplication comprises a term proportional to the signal amplitudes A and B and the phase difference ? as well as a term at twice the original frequency. This relationship is shown as

V_PSD = Acos?t x Bcos(?t + ?) = ½ABcos? + ½ABcos (2?t + ?)

The high-frequency second term is removed by the low-pass filter as is most nonsynchronous noise. Because there is no frequency drift between the two signals, a very small bandwidth is associated with the signal-to-noise ratio. Compared to a normal bandpass filter that might achieve a Q of 50, a lock-in amplifier provides a Q in excess of 100,000.

A refinement of the technique adds a second channel in quadrature that allows phase and amplitude changes to be detected separately. The amplifier’s extremely high sensitivity to phase or amplitude change and the high noise rejection make it possible to track SPM tip movement.¹

Reference

December 2007