# A Micro-Coordinate Measurement Machine (CMM) for Large-Scale Dimensional Measurement of Micro-Slits
## 1 Introduction
[PERSON] _et al._ have introduced a micro-probe, in which the probe-surface contact is detected based on the electrical contact. Since the a micro-probe tip is attached on the end of a long and sharp stylus shaft, the probe can be used to measure the inside of micro-holes [17,18]. The micro-probe tip has a pyramidal edge and it can detect the contact in one direction. As for the multi-axis detection of contact, [PERSON] _et al._ have developed a probing technique based on the method of scanning tunneling microscopy (STM) [19], in which a spherical probe tip is employed. However, the applications of such methods are only effective for measurement of conductive materials. Improvement of sensitivity for contact detection can also be achieved by applying mechanical vibrations to the micro-probe. [PERSON] _et al._ have proposed a vibrating shank for measurement of the inside of a micro-hole [20,21]. A fiber with a diameter of 7 \(\upmu\)m is vibrated in one direction by a crystal resonator, which is used as a virtual micro-probe for contact detection in one axis. The effective diameter of the vibrating fiber probe is approximately 30 \(\upmu\)m. With respect to a probe with multi-axis detection, [PERSON] _et al._ have developed a three-dimensional vibrating probe composed of PZT sensors and actuators [22,23]. The probe tip of the three-dimensional vibrating probe has a spherical tungsten tip with a diameter of approximately 70 \(\upmu\)m. [PERSON] _et al._ have presented an optical fiber probe which has a glass tip ball with a diameter of 5-50 \(\upmu\)m [24,25]. Since the vibrating probe allows the use of a probe stylus with a lower stiffness, it results in a lower probe-surface contact force. In addition, it is an effective means for the vibrating probe tip to reduce the influence of adhesion force of the measured surface against the probe. However, the measurement is influenced by the water layers on both the measured surface and the probe surface [9,22,25]. In responding to this problem, the authors have developed a shear-mode micro-probe, which can reduce the influence of the water layers through detecting the interaction force between the probe and the measured surface over the water layers [26]. It has been demonstrated that the shear-mode detection probe can achieve nanometric resolution and probing repeatability. The motivation of this research is to apply the shear-mode micro-probe for measurement of the gap width of the micro-slit on a precision slot die coater. The precision slot die is composed of two precision flat plates [27] made of stainless steel with a length of 200 mm. The two precision flat plates are separated with a gap with a nominal gap width of 85 \(\upmu\)m to form the micro-slit on the coater edge. It is required to make gap width measurement of the micro-slit with the expanded uncertainty of less than 100 nm [28]. Traditionally, mechanical feeler gauges have been used for the measurement of micro-slit gap width. However, the mechanical feeler gauge often induces damages on the edge areas of the micro-slit due to the large contact force. Although a CCD camera based optical system [28] has been proposed for non-contact measurement of the micro-slit gap width, it is difficult for the optical system to achieve sub-micrometer resolutions because of the diffraction limit of the optical system. As for the detection of single edge position, it can be achieved by using the optical system. However, with regard to the precision measurement of the slit gap width, it is necessary to detect the positions of two edges quantitatively. Therefore, the resolution in lateral direction is one of the important requirement for the gap width measurement. In addition, it cannot be used for measuring the gap width inside the micro-slit. Recently, a non-contact measurement system with a higher resolution by using a capacitive displacement sensor has been developed [27]. However, the measurable minimum gap width of the electric gap sensor is limited to 150 \(\upmu\)m, due to the sensor thickness.