Imaging at the Atomic Level

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 VideoAFM™ 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 Innova™ 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 AC™ Mode, a related approach available on Asylum Research's MFP-3D™ 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-DST™ 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 Caliber™.

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 Caliber™ 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.