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Wide Field Video Microscope Unit (WIDE VMU) and its Uses
Yasushi Fukumoto, Seiji Shimokawa, Koji Kubo, Yuko Hashizume
Product Development Div.., R&D Center, Mitsutoyo Corp.

1. Introduction
Our cylindrical-type microscope units and objective lenses are optical products that play the role of sensors in various types of equipment for inspection, measurement, and repair.
We entered the cylindrical-type microscope market with our microscope units VMU-1H/2H/3H for the first time in 1992. Since then we have been introducing a variety of new products, such as the standard unit VMU-V/H that responded to the demands for further miniaturization, higher quality, and higher performance; and the VMU-LB/L4B that supported a wider laser wavelength range for fine processing with a YAG laser, and met high rigidity specifications (1) responding to the increase in the speed of operating equipment; both of which were met with a widespread positive reception. However, in the recent semiconductor and FPD industries, the subjects to be inspected or measured have become finer and larger, and high throughput inspections have become common by driving tester machines by high accelerations. Consequently, there are growing demands for microscope units that are equipped with optics capable of handling these advanced conditions.
With the abovementioned background, we have developed the WIDE VMU Series with improved throughputs by introducing the wide-field observation that inspects a wide area all at once, and capable of dark-field observation that is suitable for appearance and scratch inspections. This edition describes the WIDE VMU Series and its features and applications.
Figure 1. WIDE VMU
2. Specifications
Table 1 shows an outline of the WIDE VMU specifications.
Table 1. WIDE VMU Specifications
3. Features
Two models of microscope heads are offered: the WIDE VMU-V/H for bright-field observation, and the WIDE VMU-BDV/BDH for bright-/dark-field observation. Their features are as follows:
Figure 2. Bright-field model
Figure 3. Bright-/Dark-field model
(1)Improved inspection efficiency through incorporating a wide-field image sensor
We successfully developed a ø30 image field that is capable of supporting large line sensors used for wide-field inspections and area sensors that are becoming larger and more precise in recent years.
Either the F-mount or the C-mount can be selected as the camera mount to meet the camera specifications as standard. Incorporating a 30 mm diagonal APS-C sensor provides an inspection field size about seven times larger than the conventional VMU Series (2/3” sensor), greatly assisting inspection throughput improvements.
Figure 4. Comparison of image field size
(2)Bulk inspections covering a wide area with high-density placement
The high-density placement of multiple units is just one example of an application that takes full advantage of the WIDE VMU’s wide-field observation capability.
For example, when installing the units in a piece of large FPD inspection equipment, the combination of two types of camera mounting orientations prevents interference between image sensors, as shown in Figure 5, and the user can choose their preferred layout of the illumination tubes to avoid interference between the illumination light guides and the image sensors. Thus, by making a high-density placement of wide-field microscope optical axes, the overall field of the multiple units is enlarged, and carrying out a stitching drive of these units on inspection equipment provides a significant improvement to inspection throughput.
Figure 5. High-density placement of multiple units
(3)Various observation functions according to purpose
The WIDE VMU is the first cylindrical-type microscope unit that has a standard dark-field observational function used for inspections of tiny scratches and defects, in addition to the bright-field observational function for general appearance inspections. This enables dark-field observation on installed equipment without needing to switch to a dedicated dark-field observation microscope unit.
Furthermore, the use of the optional polarizer unit enables polarized light observation that uses polarization properties of specimens.
Figure 6. Comparison of observation examples (Dark-field observation)
Figure 7. Comparison of observation examples (Polarized light observation)
(4)Unique ability to switch between bright- and dark-field observation (Bright-/Dark-field model)
The WIDE VMU Series is equipped with two illumination tubes for bright- and dark-field observation. Controlling the ON/OFF timing of the light source connected to each illumination tube enables rapid switching between bright- and dark-field observation. Also, since the lens tube does not have a bright-/dark-field switching mechanism or a driving mechanism, there is less concern over failures compared with devices that do have these mechanisms installed. This also prevents dust from getting inside when switching between modes. These features contribute to the improvement of the maintenance of the installed equipment.
Figure 8. Two illumination tubes