Effective Vibration Isolation Solutions to Eliminate AFM Noise
In industries such as bioengineering, materials science, and nanotechnology where highly sensitive equipment and instruments such as Scanning Probe Microscopes (SPMs) are used, even trace amounts of vibration noise can affect the quality of images and data.
There are a wide range of probes used in scanning probe microscopy, and the information gathered can take many forms, including topography, elemental composition, conductivity, and others. In order to record and measure data accurately, the instrument must maintain a uniform distance between the probe, stage, and detector.
Because these techniques are sensitive to environmental disturbance, SPMs generally require some form of vibration isolation. Depending on a number of factors, such as the environment’s level of noise (acoustic, vibration and electromagnetic noise), thermal fluctuations, field of application, and the type of equipment utilized, either passive or active vibration isolation systems are recommended to eliminate AFM noise.
SCANNING PROBE MICROSCOPY (SPM)
Scanning probe microscopes (SPM) operate by rastering a probe across the surface of a sample to obtain information. There are a wide range of probes used in SPM and the information gathered can take many forms – topography, elemental composition, conductivity, etc. – depending on the type of probe used.
SCANNING TUNNELING MICROSCOPY (STM)
Scanning tunneling microscopy (STM) was the first type of SPM to be developed. The first STM was developed by researchers at IBM in 1981. STM operates by bringing an atomically sharp tip (usually tungsten) into near contact with a sample, then applying a bias voltage to the tip which generates a tunneling current. As the tip is rastered across the surface, the level of current is compared to a reference level and a topography of the sample’s surface is generated. STM imaging can be performed in open air or in ultra-high vacuum chambers (UHV-STM).
STM enabled researchers to look at samples at resolution levels that were never before possible. They could now look at and manipulate individual atoms. It is not an exaggeration to say that the development of the STM revolutionized the field of nanotechnology research. Not only did it provide new capabilities itself, but, by establishing some of the basic concepts of SPM, STM proved to be the basis for a new field of microscopy.
Atomic force microscopy (AFM) is the most widely used SPM technique. AFM operates by dragging an ultra-fine mechanical probe, called a tip, across the surface of a sample. Instead of actually touching the sample, the tip comes near the surface of the sample and interacts with the atomic forces on the surface of the sample. The tip is attached to a cantilever, which is deflected as the tip rasters across the sample surface. A laser is reflected off of the back of the cantilever and into a detector, which collects information as the probe moves and produces an image of the sample. The resulting image provides an excellent view on the sample’s topography at an extremely high level of resolution. In addition to the original contact mode described above, a number of other modes of operation have been developed for the AFM, including non-contact mode, tapping mode, and force modulation.
The first commercial atomic force microscopes became available in 1988. Since that time, AFM has developed into one of the foremost tools enabling nanotechnology research. AFM has risen to prominence because of its ease of use and unparalleled ability to gather highly accurate topographical data on the nanometer scale. AFMs are now commonplace in university science departments and the research and development departments of large companies. Novel applications of AFM continue to be developed, such as using AFM techniques to diagnose and investigate cancer cells.
AFM Probe being imaged using interferometry. The tip is suspended in air in order to measure the amount of noise reaching the probe. This video is taken measuring the quality of application under the following conditions: without vibration isolation; using an optical table; and using a Herzan TS-150 active vibration isolation table.
You can observe that the noise reaching the probe has almost been completely eliminated.
This is from a third floor lab at the University of Wisconsin – Madison, Materials Science Center.
OTHER TYPES OF SPM
Since the development of the first scanning probe microscopes, the principles of SPM have been applied to an array of new techniques. Scanning Near-field Optical Microscopy (SNOM or NSOM) utilizes a tip with an aperture as an optical probe to provide images with excellent spatial resolution and spectroscopic information. Scanning ion conductance microscopy (SICM) measures topography and ion currents of samples using a charged pipette filled with electrolytes in a non-contact mode. SICM is especially useful for biological applications. Dip Pen Nanolithography (DPN) applies scanning probe techniques to lithography, using a cantilever and tip to deposit material onto a substrate.
Scanning probe microscopy techniques are notoriously sensitive to environmental disturbance. This sensitivity is a result of operating at extremely high levels of precision and the mechanical structure of the instruments themselves. To get accurate data, the instrument must maintain a uniform distance between the probe, stage, and detector. Even small AFM noise levels can frustrate the spatial relationship between the components and result in inaccurate data.
SPMs require some form of vibration isolation. SPMs are generally not massive instruments, so they are easily excited by even normal environmental vibrations. Additionally, because of their ease of use, SPMs’ are deployed in a range of environments, from highly controlled research labs to production environments. Some SPMs utilize passive vibration control mechanisms, such as bungee systems and air tables. The highest precision applications require active vibration control. Vibration measurement equipment should be used to determine the optimal location for the instrument prior to installation.
The same factors that make SPM sensitive to vibrations also leave them sensitive to acoustic noise and air currents. Using an SPM in open air is not advisable. Acoustic enclosures can be deployed at the level of the sample, in the design of the instrument itself, or around the entire testing set-up. Demanding SPM applications require high performance soundproof hoods designed around the requirements of the particular instrument.
Other environmental challenges include controlling thermal fluctuation. Materials behave differently depending on temperature levels, which can negatively affect the repeatability of measurements. Electronic controls can be another source of AFM noise. SPMs which measure conductance and electrical properties of materials require isolation from stray electromagnetic interference (EMI).
Herzan is the leading supplier of environmental solutions to the SPM community. Please contact us to discuss your application.