Scanning probe microscopy (SPM) is a method of sample surface observation that uses a physical probe to interrogate a specimen rather than light. This provides a wealth of information that cannot be obtained via light microscopy. The atomic resolution of SPM can routinely resolve sub-nanometer features, exceeding even advanced techniques like scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
This blog post will outline the working principles and applications of SPM in further detail.
At the heart of any scanning probe microscope (SPM) is this miniature physical probe with a very sharp nanoscale tip that interacts with the sample. There are many different types of SPMs and it is the nature of this tip-sample interaction that distinguishes between them.
Scanning tunnelling microscopy (STM), first developed in 1982, is considered the first form of SPM. In STM, the physical sensing probe is a fine wire that is cut or etched to form a very sharp tip. The piezoelectric scanner of the SPM raster scans this tip across the sample surface. An STM senses the surface by measuring the tunnelling current between the tip and the sample. This current measures from less than 1 picoamperes to several nanoamperes and varies exponentially with the tip-sample distance, which makes it a very responsive sensor. This interaction sets one of the principle disadvantages of STM however—the sample must be conductive.
This limitation was addressed in 1986 with the development of the atomic force microscope (AFM). The AFM replaces the wire probe of the STM with a micromachined probe, typically formed by photolithography and etching of silicon wafers.
An AFM probe consists of a millimetre-scale “chip”, which is attached to a cantilever, often rectangular with dimensions of below 200 micrometers long and a few tens of micrometers wide. At the very end of this cantilever, a very sharp point (the “tip”) extends down by a few micrometers and terminates at a fine point with a radius on the order of 10 nanometers. This cantilever forms a spring, which bends when the tip touches the sample surface. This bending can be detected by the AFM, most commonly by reflecting a laser off the back of the cantilever and measuring the motion of the reflected spot using a split photodiode detector. Though different from the principle used in STM, this cantilever-based AFM detector is also exquisitely sensitive to the tip-sample position.
At a very basic level the concept of SPM is similar to the way the stylus of a record player traces the grooves in a record. SPMs image samples by raster scanning the tip across the sample back and forth, line by line, while slowing scanning in an orthogonal direction down across the image area.
A typical image area for an SPM can be up to 100 micrometers or as small as a few nanometers. Conventional positioning technologies are subsequently unsuitable, as they produce motion on a scale may orders of magnitude larger than required. Instead, piezoelectric crystals are used for positioning samples for SPM. These ceramic materials expand when biased with an electric field, generating microscale motions.
One of the primary benefits of SPM are its myriad operating modes, because the different types of tip-sample interactions offer different information about the sample. The most common SPM modes are scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). AFM can be further subdivided into two major operating mode groups, contact mode techniques and dynamic mode techniques.
In contact mode, the tip is brought into proximity with the sample until it touches the sample, at which point the cantilever begins to bend up. The harder the tip is pushed against the sample, the more the cantilever bends. The position-sensitive photodetector continuously measures the cantilever deflection as a function of the reflected laser. An electronic feedback loop monitors this deflection and moves the tip up and down to keep the deflection (and therefore tip-sample interaction force) constant as the tip is raster scanned across the sample. This vertical motion is recorded as the topography of the sample surface. Because the tip and sample are scanned in constant contact, this force can dull the AFM tip and even damage the sample surface. Contact mode is subsequently used less often today except for a variety of related modes where the AFM tip is used as a nanoscale electrode to measure electrical properties while imaging.
Dynamic AFM modes were developed to reduce the potential for tip and sample damage. This class of modes is often called “AC mode” because the cantilever is vibrated near its resonance frequency, causing it to move the tip up and down in a sinusoidal motion. As the tip is brought into contact with a sample surface this motion is reduced by either attractive or repulsive interactions. Instead of measuring the quasistatic deflection of the cantilever, as in contact mode, AC mode measures the amplitude of this motion. Analogous to contact mode, this amplitude is tracked and monitored by a feedback loop to trace the sample topography. Thus, the tip intermittently contacts the sample as it scans—which leads to its other commonly used name, “tapping mode.” This AFM operating mode is the most commonly used mode by modern AFMs because it is gentler on both the tip and sample, while still capable of resolution as good or even better than contact mode.
Asylum Research is one of the leading authorities in AFM for academic research and industrial research and development. We offer an innovative family of atomic force microscopes (AFMs) with specialized systems for distinct areas of operation, including the Cypher, MFP-3D, and Jupiter AFMs which can measure electrical, mechanical, and functional properties all at the same nanoscale resolution.