Nanofabrication Series: Metrology
Nanotechnology at its core is manipulating and understanding materials at the nanoscale, giving rise to unique properties or device performance. In talking about the nanoscale, to give you a better context, 1 nanometer makes up the size of about three atoms lined up side-by-side (see previous blog post on nanofabrication for more details). While several technologies and techniques have been developed to manipulate materials at this scale, being able to measure nanoscale features of a material or device is critical in developing reliable and powerful nanotechnology based devices. To meet this challenge, scientists and engineers have sought to measure and image what happens to a system at a nanoscale, using techniques grouped under the category of metrology or at this scale, nanometrology.
Nanometrology is the scientific study of measurement at a nanoscale. The word metrology, the science of measurements, derives from the Greek word “metron” which means to “measure, length or size.” Although the word has Greek roots, the invention of metrology dates back to the French revolution. In 1795, an effort to standardize units in France led to the establishment of the decimal-based metric system. Following suit with the enlightenment movement, several other nations later adopted the metric system. In 1875, when the Treaty of the Metre convened in Paris, the International Bureau of Weights and Measures was founded and under this organization the International System of Units (SI) was established, giving uniformity to many nations in the science of measurements.
As the semiconductor industry pushed to produce devices that are more complex in structure, smaller in size, and containing several different materials, measuring nanoscale structures for this purpose became fundamentally important. A saying that may be familiar to most engineers, “if you can’t measure it, you can’t build it” is also true in nanofabrication. Consequently, there are several different techniques and instruments used to understand the different nanoscale properties of a matter. While metrology in most cases refers to measuring a system or material’s physical dimensions, metrology techniques are used to evaluate every aspect of a system, including its electrical, optical, mechanical, chemical, and even atomic structural properties. In this post, we limit the scope of the discussion to the measurement of physical dimensions and reserve other metrology techniques for later discussions.
Measuring the topography of system is the most common form of metrology, as a typically nanofabrication process involves multiple patterning, adding (deposition), and removing (etching) material steps. Several specialized microscopes such as atomic force microscope (AFM), scanning electron microscope (SEM) and super-resolution optical microscopy are just a few of many metrology techniques that are utilized for this task.
AFM allows the generation of high-speed topographic images with sub-nanometer resolution. In AFM, a silicon cantilever tip with a radius of a few nanometers acts as a probe. The tip is scanned across a surface with an applied force (see diagram in above image). Changes in the surface topography cause the cantilever beam to flex as it moves across the features. These nanoscale movements are detected by measuring the deflection of an aligned laser off the end of the cantilever tip, as measured by an array of photodiodes. This is then translated in software to topological values, producing an image like the one shown above.
Just like AFM, SEM can be used to measure the shape, size and topography of nanoscale structures. Additionally, it can also characterize the chemical composition of the sample if configured with the appropriate detectors. SEM uses a focused beam of electrons to evaluate structures on the surface of the material. A high energy electron beam scans the surface of the sample, generating scattered and emitted energy at various levels as a result of this interaction. Various detectors spatially measure this energy, providing information about the surface topography and composition. Below is an example image produced by SEM, showing the microstructure of a silica gel.
Although optical microscopy has been around for centuries scientists have only recently discovered super-resolution methods that can allow optical methods to extend into nanoscale structures, including the use of advanced post processing algorithms. While still lagging in resolution when compared to AFM and SEM, optical microscopy allows for quick and larger area inspections, which are also critical to any fabrication process flow, as shown in the image below.
Another critical type of metrology is known as profilometry. AFM is a type of profilometry, which are techniques used to extract topographical or thickness data from a surface. While AFM is typically used to give 3D information, often, quick 2D measurements of etched or deposited films are needed, to confirm a target thickness has been achieved. Stylus profilometry is the most often used, again utilizing a cantilever tip that is rastered along a surface, measuring the tip deflection to quantify step height changes on a surface. Optical profilometry is also used for the same purpose, however this time utilizing optical reflection and interference effects to measure surface height differences.
Advances in metrology have enabled several decades of incredible nanoscale materials and device fabrication development. However, as devices continue to shrink, resolution, accuracy, and capabilities of current metrology instrumentation are being pushed to their limits. In particular, as new nanoscale device technologies move into commercial products, higher throughput and high reliable techniques will need to be developed.
Metrology is often an over looked process component in nanofabrication, but without it, all lithography, etching and deposition (see upcoming article) steps would be flying blind and would be impossible to maintain accurate and repeatable results. As the field moves forward, considerable research and development efforts will be needed to advanced nanometrology techniques, enabling new technologies and wide scale production.