3. Measurement Accuracy of Laser Scanning Microscopes
The use of laser scanning microscopes for measurement is largely divided into two categories: horizontal measurement using an intensity image at a high resolution, and three-dimensional measurement using a height image. While there may be many causes of errors that must be dealt with when attempting to perform these measurements with high accuracy, this section focuses on the components that are important in terms of accuracy.
3-1. Horizontal Measurement
During horizontal measurement using an intensity image, the most important determining factor of measurement accuracy is control of the oscillation angle of the scanning mechanism. Many products are periodically calibrated using a standard sample to ensure stable measurement for long periods of time. The galvano mirror often used in the scan optical system uses a coil for position detection, so it takes some time to stabilize. As the intensity dramatically changes around the focal position in the confocal optical system, focusing is one of the factors that affects the repeatability of a measurement result. When the line width of a pattern on a separated sample surface with a laser scanning microscope, it is desirable to use the relatively faster X axis direction which is not easily affected by vibration and other disturbances for measurement, especially because the X and Y axes have different speeds.
3-2. Vertical Measurement (Three-Dimensional Measurement)
Although the image is an important element of three-dimensional measurement as it is in line width measurement, the driving factor for accuracy is the Z drive mechanism, which can move the objective lens and sample relative to each other.. The method of driving the revolving nosepiece to which the objective lens is attached in the Z direction or the method of driving the XY stage on which the sample is positioned in the Z direction are the possible implementations of a Z drive mechanism. Because the mass of every sample is different and the XY stage has a somewhat larger mass, and it is the total of these two masses that must be driven in the Z direction with the latter method, the method of driving a revolving nosepiece equipped with an objective lens which has a constant, relatively small mass or a number of objective lenses to ensure stability when driving in the Z direction is often adopted. If a laser scanning microscope is used to measure the height, the highest intensity at each pixel must be found by moving in the Z direction as described above. Therefore, the travel mechanism must be accurately moved with a resolution of about 10 nm at the highest magnification. Although a detailed discussion on the drive mechanism is beyond the scope of this document, a highly accurate linear guide and feeding screw are used together and a pulse motor or other device is used in most cases to drive in increments of a few millimeters, usually with a resolution accuracy of a few nanometers. There is also a device that incorporates a measuring apparatus using a scale, laser interference, or other method for accuracy improvement. In addition, the I-Z curve in Figure 2 is determined based on the magnification of the objective lens in an actual measurement, and therefore, there is an appropriate Z axis travel step for each objective lens. If the I-Z curve is captured with coarse steps, an incorrect position is measured as the maximum intensity. However, if the steps are too fine, it takes more time than necessary and the measurement value may be affected by the device drift, causing errors. It is therefore desirable to clarify the I-Z curve before determining the Z-axis step or use the value recommended by the manufacturer. The confocal optical system and this highly accurate Z-axis driving mechanism currently allow the laser scanning microscope to be used with the highest magnification objective lens and to resolve data down to several nanometers in the Z direction. In addition to these items, a variety of factors deteriorate the accuracy of three-dimensional measurement (e.g. flatness of the optical system). A laser scanning microscope designed for high accuracy can minimize these factors during the design and manufacturing phases.
- 1. Basic Principles of Laser Scanning Microscopes
- 2. Resolving Power of Laser Scanning Microscopes
- 3. Measurement Accuracy of Laser Scanning Microscopes
- 4. Software for Laser Scanning Microscopes
- 5. Basic Features of Olympus Industrial Laser Scanning Microscopes
- 6. Application Examples
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