By Bruce Flickinger
Researchers from the Georgia Institute of Technology and Stanford University have developed a highly sensitive atomic force microscopy (AFM) technology capable of high-speed imaging that could be useful in measuring microelectronic devices and observing fast molecular interactions in real time.
Called Force sensing Integrated Readout and Active Tip, or FIRATTM, the technique utilizes a built-in optical interferometer probe, as opposed to standard AFM optics based on the beam-bounce method. The probe incorporates a membrane with a sharp tip that, in much the same way a microphone diaphragm picks up sound vibrations, begins taking sensory readings of a sample before touching it. When the tip touches the sample surface, the elasticity and stiffness of the surface determines how hard the material pushes back against the tip. So more than capturing a topography scan of the sample, FIRAT can pick up a wide variety of other material properties, such as adhesion, stiffness, elasticity, and viscosity.
“Many of the details of the technique, such as sensitivity, array fabrication, etc., have been already developed and published,” says Dr. Levent Degertekin, head of the project and an associate professor in the Woodruff School of Mechanical Engineering at Georgia Tech. “We expect the adaptation of the probe and techniques by others to be quite fast.” Funded by the National Science Foundation (NSF) and the National Institutes of Health (NIH), the research first appeared in the February issue of Review of Scientific Instruments.
FIRAT simultaneously captures numerous material properties from just one touch, including (from upper left to right) topography, adhesion energy, contact time, and stiffness. Photos courtesy of Georgia Tech.
This work shows that FIRAT enables much more detailed measurement and analysis of nanostructures. Georgia Tech researchers have been able to use FIRAT with a commercial AFM system to produce clear scans of nanoscale features at speeds as high as 60 Hz (or 60 lines per second). The same system was used to simultaneously image the topography as well as elastic and adhesive properties of carbon nanotube bundles.
Degertekin and his team are seeing strong interest from both AFM instrumentation and semiconductor processing equipment manufacturers. Initial applications include fast dimensional metrology and material property measurement for semiconductor manufacturing. For life sciences, the initial application would be parallel molecular force spectroscopy, where the bonding strength and bond kinetics are measured between single molecules.
The FIRAT probe incorporates principles of optical detection of membrane vibration and the use of electrostatic actuation for calibration signal generation. Photo courtesy of Georgia Tech.
“These interactions are used to understand the efficacy of a certain drug based on these molecules, as well as to understand basic biological processes,” Degertekin says. “This will help researchers better comprehend the behavior of molecules in more native environments.”
One area of particular interest is use of the FIRAT technique to manufacture smaller semiconductor devices than is currently possible. While the use of sharp probes to directly pattern features on the scale of 10 nm has been demonstrated, this method has not been transferred to a production environment because of the slowness of the probe and the lack of arrays. FIRAT has the potential to overcome these limitations because of its increased speed and capability to form two-dimensional arrays. “If this is successful, the current limitations of optical lithography due to wavelength can be overcome,” Degertekin says. “One can also use the probes to manipulate molecules in large numbers, which could help develop molecular electronics.”
In terms of the technique’s commercial viability, “the device is beyond just an experimental tool, and closer to a prototype,” Degertekin says. “With a small adapter, the FIRAT probe is compatible with several commercial AFM systems. We have used seven to eight different devices, each resulting in comparable data. Necessary environmental conditions are similar to the cleanroom requirements for AFMs in semiconductor processing environments. We think it can find real-world applications within a year.”