Contact-Type Micro Thermal Sensor for Surface Defect Detection
Surface defect detection, which is carried out in advance of defect review process during surface defect inspection of products having smoothly finished surfaces such as bare semiconductor wafers, magnetic disks, and optical components, is important process to assure the quality of products. In this chapter, a surface defect detection method, in which defect detection is carried out in such a way that the existence of a surface defect on a target of interest is verified by detecting frictional heat induced by a collision between a micro thermal sensor and a surface defect, is described. Although the frictional heat to be generated by a collision between the micro thermal sensor and a surface defect is expected to be small since surface defects required to be verified during the inspection are quite small, the micro thermal sensor designed to have a micrometric sensor element is expected to carry out highly sensitive detection of the frictional heat and thus realize high-resolution surface defect detection. A principle of the surface defect detection method based on the micro thermal sensor is at first described. After that, design and fabrication of the micro thermal sensor based on photolithography process are presented. In addition, by using the developed micro thermal sensor, some experiments have been carried out to demonstrate the feasibility of the defect detection method. An example of the application of micro thermal sensor for surface defect detection in the hard disk drive industry is also introduced.
KeywordsSurface defect detection Micro thermal sensor
Aiming to realize next-generation defect detection, several methods have been developed so far (Attota and Silver 2011; Takahashi et al. 2011). A method employing a micro thermal sensor is one of these methods. In this chapter, the principle of surface defect detection method based on the micro thermal sensor is at first described. After that, design and fabrication of the micro thermal sensor are presented. In addition, by using the developed micro thermal sensor, some experiments have been carried out to demonstrate the feasibility of the defect detection method. An example of the application of micro thermal sensor for surface defect detection in the hard disk drive industry is also introduced.
Concept of Surface Defect Detection by Using a Micro Thermal Sensor
The purpose of the above mentioned method is to verify the existences of surface defects on a target surface. A setup shown in Fig. 3 is one of the examples of how to arrange the micro thermal sensor and a target of interest. Surface defect detection can be carried out by rotating the target with a precision spindle while moving the micro thermal sensor along the in-plane direction. By synchronizing the acquisitions of output signals from the micro thermal sensor, the rotary encoder and the linear encoder, exact positions of detected surface defects on a target surface can be determined. The information of defect positions can be utilized to carry out further detailed investigation on the detected surface defects in the review process, which will be carried out after the defect detection process.
A resolution of the surface defect detection, which is one of the important performances to be improved in next-generation surface defect inspection, by the abovementioned method will be affected by the sensitivity of micro thermal sensor to be employed in the system, as well as the amount of frictional heat to be generated by a collision between the micro thermal sensor and a surface defect. In the next section, theoretical analysis is carried out to estimate the amount of frictional heat by the collision based on a simple collision model between the micro thermal sensor and a surface defect.
Estimation of the Frictional Heat
Parameters for estimating frictional heat to be generated at a collision between the micro thermal sensor and a surface defect
Young’s modulus of the sphere
Young’s modulus of the flat surface
Poisson’s ratio of the sphere
Poisson’s ratio of the flat surface
Tip radius of the sphere (asperity)
5 × 10−9
Heat partition ratio
Simulation on the Change in Sensor Temperature Due to the Frictional Heat
Material properties used in the FEM simulation (Shimizu et al. 2014a)
4.0 × 103
3.0 × 101
Specific heat capacity
8.5 × 102
2.5 × 101
8.5 × 103
4.3 × 101
Specific heat capacity
4.6 × 102
Heat transfer coefficient at surroundings
1.0 × 103
Heat transfer coefficient at top surface of the base
A Design and Fabrication of the Micro Thermal Sensor
Evaluation of the Basic Characteristics of the Fabricated Micro Thermal Sensor
Sensitivity of the Micro Thermal Sensor
In the laser focusing optical system, a laser diode having a wavelength of 683 nm, whose emitting laser power can be controlled by using a laser driver, was employed as the light source. A laser beam from the LD was collimated by using a collimating lens, and the collimated laser beam was then focused on the micro thermal sensor by using an objective lens having a focal length of 50 mm. Regarding the diameter of the collimated laser beam (5 mm), a diameter of the focused laser beam was estimated to be less than 7 μm, which was small enough to heat the micro thermal sensor. In the following experiments, approximately 40% of the laser power was assumed to be absorbed by the micro thermal sensor, since the reflectivity of the sensor element was estimated to be approximately 60%.
Ability of Contact Detection
An interesting thing was that the temperature of the micro thermal senor was found to decrease by the collision with the glass ball probe. Since the temperature of the micro thermal sensor became higher than that of the surroundings due to the Joule heating by the applied bias current, the heat flow from the micro thermal sensor to the glass ball probe is relatively larger than the frictional heat generated by the collision between the sensor surface and the glass ball probe, resulting in the decrease of sensor temperature.
Application of the Micro Thermal Sensor
Summary and Outlook
In this chapter, design and development of the micro thermal sensor for surface defect detection has been described. Throughout some basic experiments, feasibility of the developed micro thermal sensor as a thermal sensing device and a contact detection device has successfully been verified. In addition, an industrial application of the micro thermal sensor for a magnetic head slider has also demonstrated that the surface defect detection by the micro thermal sensor could be a powerful tool for evaluation of the surface quality of magnetic disks. Since the micro thermal sensor can be fabricated in a simple manner, the sensor is expected to be employed in various industrial applications which require components having smoothly finished surfaces. Meanwhile, one of the disadvantages of the surface defect detection by using the micro thermal sensor is that at the moment of collision the surface defect detection could give damages on a surface defect, a precise investigation of which will be carried out in the review process followed by the defect detection. An establishment of a nondestructive method for the surface defect detection with the micro thermal sensor is therefore desired, which will be carried out in future work.
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