# A Micro-Coordinate Measurement Machine (CMM) for Large-Scale Dimensional Measurement of Micro-Slits
## 1 Introduction  
Precisely machined micro-features have been utilized widely with the development of micro-fabrication technologies such as micro-cutting [1; 2], micro- electrical discharge machining (EDM) [3; 4], micro- and nano-lithography [5; 6], nanoimprint [7] and micro-molding [8]. In particular, more precision micro-features with high aspect ratios represented by deep micro-holes and micro-slits are utilized for precision tools and measuring instruments that require sub-micrometric or nanometric dimensional accuracies. The requirements for dimensional measurements of precision micro-features with high aspect ratios are thus increasing. The micro-slit of a slot die coater is a good example of such precision micro-features. The slot die is a precision tool for coating of functional liquid materials on the surface of a substrate such as a glass plate or a film. On the top of the slot die coater is a micro-slit from which the liquid material is extruded. The micro-slit has a high aspect ratio with a gap width in the order of 100 \(\upmu\)m and a depth of more than 10 mm. Since the gap width of the micro-slit as well as its uniformity along the length direction of the slot die coater is a key parameter for determining the coating quality, it is necessary to measure the micro-slit gap width and its uniformity accurately. For dimensional measurement of micro-features with high-aspect ratios, the micro-coordinate measurement machine (micro-CMM) with a micro-probing system is one of the prospective measuring instruments because the inside of a micrometric feature such as a micro-hole or a micro-slit can be detected by the micro-probe with a micro-tip ball. On the other hand, the accuracy and reliability of the micro-probing system are determined by the positioning accuracy of the probe qualification of the probe tip, measurement strategies, and so on. The plastic deformation caused by the contact force between the probe and the measured surface is also a main error source, which requires the probing system to have a high sensitivity for detecting the probe-surface contact so that a small contact force can be achieved for reduction of the plastic deformation [9]. The contact detection is typically carried out by detecting the elastic deformation of the springs for supporting the stylus of a micro-probe. Different kinds of sensors have been integrated into the micro-probe for this purpose. For example, three integrated strain gauges have been employed to detect the probe-surface contact [10]. Optical systems have also been integrated into the micro-probes for contact detection. [PERSON] _et al._ have proposed a three-dimensional fiber probe for the measurement of micro parts. [11]. [PERSON] _et al._ have proposed a fiber probe with an ellipsoidal tip for three-dimensional measurement on micro-features [12]. [PERSON] _et al._ have developed low-cost and high precision micro-CMM by utilizing the focusing probe [13]. [PERSON] _et al._ have developed an opto-tactile sensor for the contact detection of a micro-probe in two and three directions at Physikalisch-Technische Bundesanstalt (PTB) in Germany [14], where the micro-probe has a tip ball of 60 \(\upmu\)m in diameter. The probe displacement can be detected by using an optical beam focused on the probe tip ball. [PERSON] _et al._ have proposed a nano-probe system based on the laser-trapping technique [15,16]. A micro-sphere with a diameter of 8 \(\upmu\)m, which is held by the focused laser beam, is used as the probe tip ball with a piconewton measuring force. Such optical-based detection methods are more sensitive than the traditional strain gauges. However, the effective working distance of the probe is confined by the numerical aperture of the focusing lens and it is difficult to measure the inside of a high-aspect ratio micro-feature. Detection of electric contact has been applied for micro-probing systems for highly sensitive probe-surface contact detection.