With the constantly increasing capacity to fabricate dense structures at nanoscopic scales, the improvement of surface characterization techniques is of vital importance for the clear imaging and profiling of fabricated structures, as well as the properties associated with them. Surface characterization is the measurement of surface properties, such as surface area, morphology, optical and electrical properties, and mechanical characteristics.
The importance of surface characterization and measurement is due to the possible applications of modified surfaces in prominent industries, such as those of semiconductors, functional coatings, construction materials, and medicine.
There are a number of techniques for the characterization of various properties. Ideally, surface characterization will profile a surface up to a few nanometers of depth. Many advancements have been made to develop highly sensitive measurement methods to characterize different surfaces. In this article, we will discuss some of the most widely used characterization methods.
This is the most elementary technique for analyzing the surface of a material. It usually uses visible light and lenses in order to magnify the image of the material’s surface.
For scientific purposes, optical microscopes can be quite complex, as they need to work for various wavelengths and have many lenses and optical components. However, the underlying principle is always the same.
Optical microscopes primarily consist of an objective lens of very short focal length, which is used to make a highly magnified image of an object. This image is collected by a digital camera or an eyepiece with considerably longer focal length. The contrast is then modulated by illumination method control.
Depending on the requirements there are various illumination techniques for an optical microscope. The most popular among these are bright-field illumination (where contrast is due to the absorbance of light by the sample), and dark-field illumination (where contrast comes from light scattered by the sample).
Scanning Electron Microscope (SEM)
A scanning electron microscope (SEM) is an ultra-high resolution tool used to capture images of the surface of a material. It scans a focused electron beam over a surface, and the secondary electrons are collected at a detector to form an image. This is a highly efficient tool for gathering information about the surface topology and morphology. This tool combined with some other techniques can also provide information about the composition of the surface.
SEMs primarily consist of an electron source, a thermionic gun, a field emission gun, condensing lenses, a vacuum chamber with a sample chamber at its core, electron detectors, secondary electron detectors, an attached optical microscope, a backscatter detector, an x-ray detector, and a computer to process the information.
An electron gun shoots a beam of high energy electrons which are guided to a sample using magnetic condenser lenses. These condenser lenses are electromagnets which are necessarily coiled, wrapped tubes. By controlling the electronic flow in these coils the beam of electrons from the primary source can be manipulated. The prepared sample is then placed in a high vacuum chamber and the electrons are focused on the surface of interest. The sample is prepared by coating a very thin layer of conducting material on the top surface. When the incident electrons come into contact with the sample, energetic electrons are released from the surface of the sample. These electrons are located using a series of detectors.
A variety of detectors are employed for the identification of scattered electrons, secondary electrons, and in some cases x-rays. Diffracted backscattered electrons give important information about the topological and crystalline characteristics of a material’s surface. SEM is a very useful tool to form high-resolution images of fabricated nanostructures, surface contamination, and chemical composition.
Atomic Force Microscopy (AFM)
An atomic force microscope employs a sharp tip (or probe) near the surface of a material in order to scan it, using a feedback loop to adjust the parameters needed to capture an image of a surface. It uses a quantum mechanical tunneling effect and consequently does not need a conducting sample to operate. It can be used to measure van der Waals interactions, as well as the electrical, magnetic, and thermal characteristics of a surface.
Scanning Tunneling Microscopy (STM)
The scanning tunneling microscope was developed by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratories in Switzerland, for which they were awarded a Nobel Prize in physics in 1986. STM is capable of imaging in various environments such as in a vacuum, as well as in liquid and gaseous phases. Like AFM it is based on the quantum tunneling effect using a sharp probe, which is controlled by a biased voltage. It can provide a lateral resolution of 0.1 nm and a deep resolution of 0.01 nm.
X-Ray Fluorescence (XRF)
X-ray fluorescence (XRF) is a non-destructive surface analysis tool which can be used to determine the elemental composition of the surface of interest. A primary X-ray source is used to excite a sample. The excited sample then emits secondary X-rays by a process called fluorescence. These X-rays are then analyzed in order to determine the material composition.
A profilometer is used to measure the profile (or roughness) of a surface. Conventional profilometers measured static profiles of a surface. However, with the development of measurement techniques, time-resolved dynamic profiling has become a commonly used method in scientific research as well as industry.
There are two types of profilometer: ‘contact mode’ and ‘non-contact’ profilometers. In a contact mode surface profilometer, a diamond stylus is used with a contact force.Non-contact profilometers employ techniques such as interferometry, holography, and optical microscopy.
Thus, there are numerous techniques which can be used to characterize and measure the surface of a nanoscopic or microscopic material.We have reached a subnanometer resolution with extreme precision and clarity in image formation. These techniques are constantly improving and being modified according to the need of any specific industrial and research requirements. Two or more techniques are often coupled together to form a multi-analysis surface characterization tool.
The main challenge to higher resolution techniques is their cost of operation and maintenance. The development of non-invasive and non-destructive tools is also challenging, especially for biological samples which will be used for medical research. Various industries have coupled surface characterization tools with their fabrication facilities in a holistic approach, in order to reduce the possibility of errors and to correct the error-causing parameters in real time.