Cambridge University Engineering Department
Scientific Imaging Group
Three-dimensional surface imaging and height measurement using the Scanning Electron Microscope
Characterisation of surface microstructural topography is a significant requirement in many fields of current research, from catalysts to coatings, microelectronics to medicine.
Current investigative techniques include:
Surface profiling with a mechanical stylus
Light interference microscopy
Confocal laser microscopy
Stereographic mapping from micrographs
These methods either lack real time capability or fail to provide clear imaging of the surface under measurement.
A research project initiated in 1985, concluded with a stereo technique (Patented Worldwide and licensed to LEO Electron Microscopy) which permits both 3-D imaging and surface height measurement to be made within the SEM : this yields the two-fold benefits of simplified data interpretation and immediate verification or follow-up of findings.
Principle of Operation
An SEM uses an electron beam to illuminate the specimen surface. This beam is influenced by a series of electromagnetic fields for focusing onto the surface; scanning over the surface in a tv-style raster; and lateral shifting for both alignment purposes and field of view selection.
Our development makes further use of electromagnetic fields to pivot the angle of the beam about the specimen surface, presenting alternate ' left ' and ' right ' views of the surface without significantly impairing the quality of the image or the conventional capabilities of the microscope.
Currently there are three modes of operation:
1. Stereoscopic imaging
The alternate left and right views from a single specimen area are displayed on the SEM screen. For human assimilation, these must be coded such that each image is visible to only one eye; the brain then fuses the image to appear as one.( red/cyan 280 KByte gif image of a titanium sample ) An alternative method is to use a liquid crystal shutter and passive polaroid glasses. The system we use is circular polarizing. While viewing the specimen in the SEM, the field of view can be moved, the magnification changed and even the degree of perceived depth adjusted while observation is taking place. The images may be captured either as stills or in dynamic video mode.
2. Height measurement
To measure the difference in height between two surface features use is made of the following principle. A change in electromagnetic lens current causes the height at which left and right images are coincident to change, the relationship being a power law. Thus if convergence is achieved successively on each of the two or more features and the lens current difference is noted, this may be converted directly to a height change measurement. This operation can be performed manually or under computer control and allows differences less than 25nm to be detected.
3. Height contouring
As a logical extension to height measurement, a regular matrix of points may be selected for line profiles and contour mapping . Present development effort is centred upon techniques whereby this process can be carried out more quickly. Rather than refocusing at each point, the degree of mismatch between left and right scans is determined by the use of an image correlation algorithm. Automated measurement over the complete scanned area is then practical.
Most types of material can be imaged or measured in this way. The SEM itself normally requires a vacuum environment, but generally those materials which can be SEM-imaged can be quantified.
Some specific examples include:
Microelectronics development processes
These include step height, surface flatness and coating thickness measurements. A stylus can easily damage many materials, while other features may be too small for mechanical probing.
SEM's can incorporate, for example, tensile or temperature-regulated specimen stages which induce real-time changes in surface topography. Recording these events in 3-D, using video tape, allows - literally - an extra dimension of data to be acquired.
Surface attack studies
Chemical attack, wear or corrosion, can modify surfaces over a period of time. Pitting and scoring in anything from bones to bearings can be imaged and quantified.
The Cambridge University Engineering Department has a long and distinguished history in the development of the Scanning Electron Microscope. Prof Sir Charles Oatley initiated this research in 1948, with a series of innovative research students. There have been many developments since that time, for example, improved electron optical design, high brightness emitters, high quality detectors, a wide variety of applications and other advances and this process continues to the present day.
Publications & Articles
Laboratory News March 1994 included a section - Microscopy News which included an article detailing the current developments.
1. BC Breton, JTL Thong, WC Nixon. A dynamic real time 3-D measurement technique for IC inspection. Microelectronic Engineering 1986; 5 : pp 541 - 545.
2. BC Breton, JTL Thong, WC Nixon. Contactless 3-D measuring technique for IC Inspection. Proc. SPIE Integrated Circuit Metrology, Inspection and Process Control Conference, Santa, Clara CA Mar 4-6, 1987 Vol 775 : pp 109-117.
3. BC Breton, JTL Thong, WC Nixon. Advances in stereo SEM techniques. Inst. Phys. Conf. Series 1990 No.98; pp 617-620.
4. JTL Thong, BC Breton. A topography measurement instrument based on the scanning electron microscope. Rev. Sci. Inst. 1992; 63(1) : pp 131-138.
5. JTL Thong, BC Breton. In situ topography measurement in the SEM. Scanning 1992; 14 : pp 65-72.
6. K. Fuller, JTL Thong, TJ Chambers, BC Breton. Automated 3-D characterisation of osteoclast resorption lacunae by stereoscopic scanning electron microscopy. Journal of Bone and Mineral Metabolism, 1994
Updated 14 November 2009