Design and Testing of a Micro-thermal Sensor Probe for Nondestructive Detection of Defects on a Flat Surface

Transcription

1 ().,- volv)( ().,-volv) Nanomanufacturing and Metrology (2018) 1: (012 ORIGINAL ARTICLES Design and Testing of a Micro-thermal Sensor Probe for Nondestructive Detection of Defects on a Flat Surface uki Shimizu 1 uki Matsuno 1 uan-liu Chen 1 Hiraku Matsukuma 1 Wei Gao 1 Received: 29 November 2017 / Revised: 29 January 2018 / Accepted: 7 February 2018 / Published online: 29 March 2018 Ó International Society for Nanomanufacturing and Tianjin University and Springer Nature 2018 Abstract This paper presents a design and testing of a micro-thermal sensor probe for realizing the proposed concept of non-contact surface defect detection, in which a change in heat flow across the gap between the tip of the micro-thermal sensor probe and a target surface is utilized to recognize the existences of surface defects. In the proposed concept of surface defect detection, the probe tip, where the micro-thermal sensor is embedded, is required to be placed as close as possible to the target surface for highly sensitive surface defect detection. For the purpose, an optical tilt detection assembly based on the laser autocollimation is designed to detect relative tilt of the micro-thermal sensor with respect to a target surface. A sensor probe assembly, in which the fabricated micro-thermal sensor is embedded to the tip of a hollow-shaped piezoelectric actuator by using a homemade sensor mount, is also designed in such a way that the sensor element will face to a target surface. A prototype micro-thermal sensor probe is constructed by integrating the optical tilt detection assembly and the sensor probe assembly, and some experiments have been carried out to verify the basic performances of the prototype micro-thermal sensor probe. Keywords Micro-thermal sensor Heat flow Surface defect Optical angle sensor 1 Introduction Industrial components having smoothly finished surface with nanoscale surface roughness such as magnetic disks [1], semiconductor wafers [2] or solar wafers [3] are of great importance in state-of-the-art technologies, and strong demands on products employing such components can be found in many industrial fields [4]. Since the quality and fabrication yields of such products often depend strongly on the surface quality, surface inspection is one of the most important processes in product manufacturing [5]. & uki Shimizu yuki.shimizu@nano.mech.tohoku.ac.jp uki Matsuno mailmatsuno@nano.mech.tohoku.ac.jp uan-liu Chen yuanliuchen@nano.mech.tohoku.ac.jp Wei Gao gaowei@cc.mech.tohoku.ac.jp 1 Nano-Metrology and Control Laboratory, Department of Finemechanics, Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi , Japan The surface inspection consists of two major processes; the first is referred to as the surface defect detection, in which the existences of surface defects on a target of interest are recognized in a relatively high-throughput. The second is referred to as the defect classification, in which a precise evaluation of surface defect is carried out by using instruments such as scanning electron microscopes (SEMs) or atomic force microscopes (AFMs). Nowadays, these instruments to be used in the defect classification process have high resolution enough to carry out precise defect analysis. In addition, measurement throughputs of these measuring instruments are getting higher and higher [6, 7]. Meanwhile, the fields of view of such measuring instruments are basically limited, and therefore, the positions of surface defects are required to be recognized in the process of surface defect detection, in advance of precise analyses by these high-resolution instruments. For the surface defect detection, an optical method referred to as the laser scattering method has often been employed [8, 9]. In the method, surface defect detection is carried out by detecting light scattering patterns generated by the irradiation of a laser beam on an area of surface having a single or multiple surface defects. However, due

2 46 Nanomanufacturing and Metrology (2018) 1:45 57 to the Rayleigh scattering limitation [10], it becomes more difficult to carry out defect detection when the size of a defect gets smaller. In addition, the laser scattering method could strongly be affected by the roughness of a surface under test. To overcome the shortcoming of the conventional laser scattering method and to achieve further better defect detection performances, new techniques based on different principles have been studied so far [11, 12]. In responding to the above-mentioned background, a concept of the surface defect detection by using a microthermal sensor has been proposed [13]. In the proposed method, surface defect detection is carried out by monitoring the change in temperature of the micro-thermal sensor associated with heating at a collision between the sensor and a surface defect. Since the volume of microthermal sensor is quite small, even small heat input can generate a detectable change in temperature of the sensor. Feasibility of the proposed concept of the surface defect detection has successfully been demonstrated in experiments, in which a surface defect on a rotating magnetic disk surface has been detected by using the micro-thermal sensor embedded to a magnetic head slider flying over the disk surface [14]. However, this method has several drawbacks: for example, the collision could give damages to both the surface defect and the micro-thermal sensor. To overcome the above-mentioned drawbacks of defect detection method with a micro-thermal sensor relying on the detection of frictional heat, a new method for surface defect detection, in which heat flow across the gap between a micro-thermal sensor surface and a measuring surface is utilized for detect surface defects, has been proposed [15]. Differing from the scanning thermal microscopes [16, 17] having a limited field of view typically smaller than 100 lm lm and are mainly designed to investigate thermal property of a measurement target, the proposed method focuses on the surface defect detection, in which the existences of surface defects on a target surface are required to be detected in a relatively high-throughput. It should be noted that an application of the spiral scanning with the employment of a rotary stage and a linear slider for moving a measurement target and the micro-thermal sensor probe, respectively, is expected to achieve a wider field of view in the proposed method with proper feedback controls of the relative tilt and a gap between the measurement target surface and the tip of the micro-thermal sensor probe. Although similar thermal sensors being used as displacement sensors can be found in literatures [18, 19], the micro-thermal sensor employed in the proposed method is different in its usage and apparatus fabricated in a micrometric size on a flat substrate. In the proposed method, the micro-thermal sensor is preferred to be placed as close as possible to a measurement surface for achieving higher defect detection sensitivity. Based on the principle of proposed surface defect detection method, a possible resolution of the defect detection is at first estimated in analytical calculations. Since the micro-thermal sensor is fabricated on a flat substrate, attentions should be paid to achieve a high degree of parallelism between the sensor and a target surface. In this paper, a prototype micro-thermal sensor probe composed of a tilt detection assembly and a multi-degreeof-freedom (DOF) sensor probe assembly, which are expected to achieve a small gap at the interface between the micro-thermal sensor and a target surface, is therefore designed and fabricated. By using an experimental setup including the developed prototype micro-thermal sensor probe, basic experiments are also carried out to verify the feasibility of the proposed non-contact defect detection method. It should be noted that the targets of interest to be evaluated by the proposed method are limited to the ones such as a bare wafer or a bare glass substrate having a surface without any pattern structures. 2 Principle of the Proposed Surface Defect Detection Method A schematic of the proposed defect detection method is shown in Fig. 1. A micro-thermal sensor composed of a pair of thin-film electrodes and a sensor element fabricated on a glass substrate is utilized in the method to detect surface defects. The sensor element is a rectangular-shaped thin chromium film having micrometric dimensions and acts as a temperature-sensitive resistor. Electrical resistance of the micro-thermal sensor therefore changes with the change in its temperature. The change in electrical resistance of the sensor can be detected by applying a bias voltage to the sensor through a bridge circuit. Meanwhile, due to the Joule heating induced by the bias current flowing through the sensor element, the temperature of microthermal sensor becomes higher than that of the surroundings. Therefore, placing the micro-thermal sensor with respect to a target surface with a certain amount of gap will generate heat flow across the interface between the sensor surface and the target surface. In the proposed method, relative in-plane motion will be applied to the target surface with respect to the micro-thermal sensor for detecting surface defects. The system will be kept in thermodynamic equilibrium, and the temperature of micro-thermal sensor will also be kept constant as long as the amount of gap is kept constant (Fig. 1a). However, when the micro-thermal sensor passes over a surface defect, the system is no longer in thermodynamic equilibrium since the heat flow across the gap will be affected by the existence of defect, resulting in the change in sensor temperature (Fig. 1b, c). By

3 Nanomanufacturing and Metrology (2018) 1: Fig. 1 A schematic of the proposed surface defect detection method by using a micro-thermal sensor, a in thermodynamic equilibrium, b detection of a concave defect, c detection of a convex defect Bridge circuit Actuator Micro thermal sensor V OUT V OUT h t V OUT V OUT t h+δh V OUT V OUT t h-δh Target surface (a) (b) (c) utilizing the change in temperature, the surface defects are expected to be detected. On the assumption that the micro-thermal sensor surface and a target surface have uniform temperature distributions with the temperatures of T 1 and T 2, respectively, the energy equation at the interface between the sensor surface and the target surface with the gap h and relative velocity U can be expressed as follows [20]: qc P u ot ox þ qc Pv ot op op u v oy ox oy ¼ k o2 T oz 2 þ l ( ou 2 þ ov ) 2 ox oy ð1þ where k, q, p, C P, k, l, and T are a mean free path, a density, pressure, the specific heat at the pressure, thermal conductivity, viscosity, and temperature of the air, respectively. The sensor velocity components u and v along the X- and -directions, respectively, with respect to the target surface can be given as follows: u ¼ U 1 z þ ak h þ 2ak ; v ¼ 0 ð2þ where a is a constant referred to as the momentum accommodation coefficient [20]. By applying these boundary conditions to Eq. (1), an equation describing a rate of heat flow Q can be acquired as follows: Q ¼ Aq ¼ ka T 2 T 1 ð3þ h þ 2bk where q is heat flow across the gap h, and A is the effective area on the micro-thermal sensor surface. In the equation, the parameter b can be expressed as follows: b ¼ 2ð2 r TÞc ð4þ r T ðc þ 1ÞPr where c, r T, and Pr are a ratio of specific heat, a thermal accommodation coefficient, and the Prandtl number, respectively [19]. As can be seen in Eq. (3), the rate of heat flow Q increases inversely proportional to the decrease of gap h. Since the micro-thermal sensor detects the change in its temperature as a result of the change in Q induced by the existence of surface defect at the interface, a reduction of the interface gap h contributes to improve a defect detection sensitivity of the proposed method. Based on Eq. (3), a possible resolution of the defect detection by the proposed method is estimated. Now we assume a simple defect model, a schematic of which is shown in Fig. 2a. In the model, a sphere-shaped defect with a radius R is placed on a target surface, while the microthermal sensor is placed with a gap h with respect to the target surface. Regarding the geometric relationship, a rate of heat flow Q D-S between the defect surface and the sensor surface can be expressed as follows: The change in rate of heat flow ΔQ μw Sensor 0.01 Defect X Target surface (a) h=1000 nm 1 h=500 nm h=200 nm 0.1 h=100 nm R Defect radius R μm (b) Temperature: T 1 h Temperature: T 2 h=2000 nm Detectable by the sensor with a size of 10 μm 10 μm (fabricated on silicon substrate) Estimated to be detectable by the sensor with a size of 1 μm 1 μm Fig. 2 Estimation of the resolution of defect detection by the proposed method, a a model of defect on a target surface, b estimated DQ with the defect having a radius R

4 48 Nanomanufacturing and Metrology (2018) 1:45 57 kðt 2 T 1 Þ Q D S ¼ p h R ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R 2 ðx 2 þ y 2 Þ þ 2bk dxdy ð5þ The above equation can be rewritten in the polar coordinate system and can be solved as follows: B Q D S ¼ p C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dxdy R 2 ðx 2 þ y 2 Þ 2p " # R B ¼ 0 0 C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C 2 p rdrdh ¼ pb Cln 2R R 2 r 2 2 ðc RÞ ð6þ where B ¼ kðt 2 T 1 Þ; C ¼ h R þ 2bk ð7þ Meanwhile, from Eq. (3), a rate of heat flow Q T-S between the target surface and the sensor surface with the gap h over the circular area with a radius R can be obtained as follows: Q T S ¼ kðt 2 T 1 Þ pr 2 ¼ pr2 B ð8þ h þ 2bk C þ R From Eqs. (6) to(8), the change in rate of heat flow DQ due to the existence of the defect can be calculated as follows: " # C 2 DQ ¼ Q D S Q T S ¼ pb Cln 2R R2 2 ðc RÞ C þ R ð9þ Based on Eq. (9), DQ is estimated. Figure 2b shows the calculated variations of DQ due to the change in R for the cases of h = 100, 200, 500, 1 lm and 2 lm. For the calculations, the parameters summarized in Table 1 are employed. According to the previous study by the authors [13], the micro-thermal sensor with an effective area size of 10 lm 9 10 lm fabricated on a silicon wafer, whose sensitivity is lower than that fabricated on a glass substrate [15], can detect DQ of 10 lw. Judging from the calculation results shown in Fig. 2b, it is difficult for the sensor having the size to detect a sub-micrometric surface defect. Meanwhile, on the assumption that the detectable DQ is proportional to an effective area size of the sensor, a microthermal sensor having an effective area size of 1 lm 9 1 lm is expected to distinguish DQ of 0.1 lw, which is enough to detect a sub-micrometric surface defect under the condition where h is set to be lower than 200 nm. It should be noted that the above-mentioned analytical calculations are carried out to estimate the resolution of surface defect detection by the proposed method based on a simple defect model, and detailed investigation will be required for further precise estimation of the defect detection resolution. Figure 3 shows a typical voltage output waveform from a micro-thermal sensor through a bridge circuit when the sensor is made to scan over line pattern structures. As can be seen in the figure, deviation of sensor output corresponding to a profile of the line grating structures can be observed. Meanwhile, a linear component due to angular misalignments among the micro-thermal sensor, the target surface and the direction of relative motion in-between them can also be observed in the sensor output. To eliminate the linear component in the sensor output, angular positions of both the micro-thermal sensor and the target are required to be verified in advance of the surface defect detection. 3 Design and Construction of a Microthermal Sensor Probe 3.1 Fabrication Process of the Micro-thermal Sensor Figure 4 shows a flowchart of the fabrication process designed for the micro-thermal sensor. A glass plate having beveled edges is employed as the substrate of micro-thermal sensor. At first, a pair of electrode patterns is fabricated by photolithography process. It should be noted that masking process by adhesive tape patterns is introduced in advance of the patterning process so that electrodes on the beveled edges, which allow wiring points on the electrodes to be recessed from the top surface and thus allow the micro-thermal sensor to approach a target surface as close as possible, can successfully be fabricated. After the fabrication of electrode patterns, patterning of the sensor Table 1 Parameters employed in the analytical calculations Parameter Symbol Value Unit Temperature of the target surface and defect T K Temperature of the micro-thermal sensor T K Thermal accommodation coefficient r T 0.80 Prandtl number Pr Ratio of specific heat c 1.4 Thermal conductivity k Wm -1 K -1 Mean free path k m

5 Nanomanufacturing and Metrology (2018) 1: Sensor output 10 mv/div. 15 μm 15 μm Displacement of the line pattern structures 10 μm/div. Fig. 3 Typical voltage output of the micro-thermal sensor when scanning over a line pattern structures with a pattern period of 15 lm Glass substrate Tape Au sputtering Masking Resist coating Pattern exposure Mask Development, etching and resist removal Cr sputtering Resist coating Pattern exposure Mask Development, etching and resist removal Fig. 4 Fabrication process of the micro-thermal sensor element is carried out. Due to the restrictions come from the manufacturing equipments employed in the sensor fabrication, an effective area size of the micro-thermal sensor, which corresponds to the area in between the pair of electrodes, is set to be 15 lm 9 5 lm. It should be noted that a micro-thermal sensor having a smaller size is preferred to achieve better sensor sensitivity and a higher defect detection resolution. In addition, due to the limitations of employed manufacturing equipments, in this paper, the micro-thermal sensor is fabricated on the glass plate with a size of 20 mm 9 20 mm having a flatness of k/4. The dimensions of the electrodes are determined regarding the above-mentioned restrictions on the sizes of sensor and glass substrate. 3.2 Design of an Optical Tilt Detection Assembly In the proposed defect detection method with the microthermal sensor, the sensor tilt with respect to a target surface and its scanning direction should be adjusted so that the sensor can be placed as close as possible to the target surface for achieving highly sensitive defect detection. A micro-thermal sensor probe is therefore required to be equipped with a function of detecting angular positions of both the micro-thermal sensor and a measurement target. Noble methods for controlling such a gap can be found in literatures. For example, in Ref. [21], a method referred to as the interferometric-spatial phase imaging has been utilized to achieve a parallelism within 0.03 mrad (6.19 arcseconds). However, regarding the substrate size of microthermal sensor (20 mm 9 20 mm), a parallelism of better than 2 arc-seconds is at least required between the microthermal sensor surface and a target surface. Therefore, in this paper, an optical tilt detection assembly based on the laser autocollimation [22] is designed to detect tilts of both the micro-thermal sensor and a target surface. A schematic of the sensor tilt measurement by using the optical tilt detection assembly based on the laser autocollimation is shown in Fig. 5a. The optical tilt detection assembly consists of a laser diode (LD) with a collimating lens (CL), a polarized beam splitter (PBS), a quarter wave plate (QWP), a collimator objective (CO) with a focal length f and a quadrant photodiode (QPD). A collimated laser beam generated by the LD and CL is made incident to the substrate of micro-thermal sensor, and the laser beam reflected back from a reflective metal film on the sensor substrate surface is detected by the QPD through the CO. By employing the QPD as a position detector, two-degreeof-freedom (DOF) tilt measurement can be realized. Now displacements of the focused beam Dd V_Sensor and Dd H_Sensor on the QPD along the V- and H-axis in the local coordinate system of QPD, respectively, can be expressed by the following equations [23]: Dd V Sensor ¼ f tan 2Dh X Sensor ð10þ Dd H Sensor ¼ f tan 2Dh Sensor ð11þ where Dh X_Sensor and Dh _Sensor are the angular displacements of micro-thermal sensor about the X- and -axis, respectively. By detecting Dd V_Sensor and Dd H_Sensor from the photocurrent outputs of QPD, the angular displacements of sensor can be measured based on Eqs. (10) and (11). As can be seen in the equations, a distance between the optical tilt detection assembly and the micro-thermal sensor substrate will not affect the tilt measurement, which is one of the features of the optical tilt detection assembly based on the laser autocollimation.

6 50 Nanomanufacturing and Metrology (2018) 1:45 57 Δd H_Sensor Δd H_Target V H Δd V_Sensor V H Δd V_Target θ X θ X LD CL QPD CO Δd H_Sensor f PBS θ X θ X LD CL QPD CO Δd H_Target f PBS Sensor substrate Target QWP Δθ _Sensor Sensor substrate Target Δθ _Target QWP n 0 n 1 n 0 2Δθ _Target θ θ 2Δθ _Target (a) (b) Fig. 5 Tilt measurement by using the optical tilt detection assembly designed based on the laser autocollimation, a tilt of the micro-thermal sensor, b tilt of a measurement target In the same manner, angular displacements of the target surface can also be detected by the same optical tilt detection assembly when the collimated laser beam is made incident to the target surface through the glass substrate of micro-thermal sensor, as shown in Fig. 5b. The employment of glass plate as the substrate of micro-thermal sensor allows the optical tilt detection unit to measure the tilt angle of a target surface through the micro-thermal sensor. Tilt angles of the target surface Dh X_Target and Dh _Target can be measured by detecting displacements Dd V_Target and Dd H_Target of the laser beam focused on the QPD. The relationship in between them can be expressed as follows: Dd V Target ¼ f tan 2Dh X Target ð12þ Dd H Target ¼ f tan 2Dh Target ð13þ It should be noted that the laser beam reflected from the target surface will come out of the glass substrate of microthermal sensor at the same angles as it enters, as long as the top and bottom surfaces of the micro-thermal sensor substrate are parallel with each other; namely, the existence of micro-thermal sensor in between the optical tilt detection assembly and the target surface will not affect measurement of the target tilt, in principle, as long as the parallelism of the sensor substrate is assured. In practical case, attentions should be paid for parallelism of a sensor substrate, as well as its flatness. Figure 6 shows a schematic of the designed microthermal sensor probe. A sensor probe assembly, which is composed of a hollow-shaped piezoelectric (PT) actuator and the fabricated micro-thermal sensor, is integrated with the optical tilt detection assembly. In the sensor probe assembly, the fabricated micro-thermal sensor is embedded to the tip of the hollow-shaped PT actuator by using a homemade sensor mount in such a way that the sensor element will face to a target surface. The shape of the PT actuator allows the measurement laser beam from the optical tilt detection assembly and a laser beam reflected back from the micro-thermal sensor or a target surface to go through it. Since the PT actuator employed in this paper is capable of giving a translational motion along the -axis and rotational motions about the X- and -axis, the developed system can carry out the -directional positioning of the micro-thermal sensor as well as the adjustment of sensor tilts about the X- and -axis with respect to the target surface. Specifications of the optical components employed in the optical tilt detection assembly are summarized in Table 2. It should be noted that, in this paper, the optical setup for the tilt detection assembly is designed to have a single measurement laser beam for the sake of simplicity of the setup. Adding another measurement laser beam and a photodetector in the tilt detection assembly unit is expected to achieve simultaneous measurement of both the micro-thermal sensor and a measurement target.

7 Nanomanufacturing and Metrology (2018) 1: Fig. 6 A schematic of the designed micro-thermal sensor probe with the optical tilt detection assembly and the sensor probe assembly Optical tilt detection assembly X LD CL AP PD CO PBS QWP Optical angle sensor assembly Sensor probe assembly Hollow-shaped three-dof PT actuator Sensor mount Sensor probe assembly (on a glass substrate) Target on a three-axis PT stage Table 2 Specifications of the optical components employed in the optical tilt detection assembly Laser Wavelength 683 nm Diameter of measurement laser beam Focal length of the collimator objective Insensitive gap in the quadrant photodiode 4 Basic Experiments 0.8 mm 25 mm 10 lm 4.1 Evaluation of the Optical Tilt Detection Assembly Firstly, characteristics of the developed optical tilt detection assembly were evaluated. Figure 7 shows a schematic of the experimental setup. The developed optical tilt detection assembly capable of carrying out two-dof angular displacement measurement was placed above a two-axis PT tilt stage, while aligning the local coordinate system of the optical tilt detection assembly to that of the PT tilt stage. It should be noted that the PT tilt stage employed in this paper had internal tilt sensors capable of measuring its tilts about the X- and -axis. A silicon wafer was mounted on the tilt stage as shown in Fig. 7a, and angular displacements about the X- and -directions were applied to the wafer while measuring the angular displacements by the developed optical tilt detection assembly. Figure 8 shows voltage outputs from the developed optical tilt detection assembly when angular displacement about the X-axis was applied to the silicon wafer by the PT tilt stage. As can be seen in the figures, the voltage output of the optical tilt detection assembly about the X- axis increased in proportion to the change in angular displacement of the silicon wafer about the X-axis. Experiments were also carried out for the case, a schematic of which is shown in Fig. 7b, having a glass substrate in the Fig. 7 A schematic of the experimental setup for evaluation of the optical tilt detection assembly, a a silicon wafer as the measurement target, b a silicon wafer as the measurement target through a glass substrate, c a microthermal sensor as the measurement target Photodiode Collimating lens Si wafer Two-axis PT tilt stage Trans-impedance circuit Data acquisition unit Polarized beam splitter λ/4 plate θ Glass substrate θ X θ (a) (b) (c) X Glass substrate with the micro thermal sensor

8 52 Nanomanufacturing and Metrology (2018) 1:45 57 θ X output 5%/div. Sensitivity: %/arc-second Applied θ X 5 arc-second/div. (a) optical path of a measurement laser beam from the optical tilt assembly. In addition, angular displacements of the micro-thermal sensor mounted on the PT tilt stage were also measured by the optical tilt detection assembly, a schematic setup of which is shown in Fig. 7c. In the setup, the sensor was placed in such a way that the surface having the sensor structures was facing to the PT stage plate, while the laser beam from the tilt detection assembly was made irradiated to the sensor structures made of thin metal films having high reflectivities. It should be noted that the local coordinate system of the hollow-shaped three-dof PT actuator was also aligned to that of the optical tilt detection assembly in advance of the experiments. Table 3 summarizes the results. As can be seen in the table, sensitivities of the optical tilt detection assembly evaluated in the above three cases were almost the same; these results mean that both the angular displacements of a measurement target and the micro-thermal sensor can be detected by using the developed optical tilt detection assembly. Figure 9a shows voltage output waveform from the optical tilt detection assembly acquired by measuring cyclic angular motion of the silicon wafer under the setup shown in Fig. 7b. As can be seen in the figure, an angular motion with amplitude of 0.04 arc-second was successfully detected by the developed optical tilt detection assembly. In the same manner, the angular motion of the microthermal sensor under the setup shown in Fig. 7c was successfully measured as shown in Fig. 9b. From these results, the developed optical tilt detection assembly has been confirmed to have a resolution of better than 0.04 arcsecond, at least, which is enough for the micro-thermal sensor probe to carry out the tilt compensation of the micro-thermal sensor with respect to the measurement θ output 5%/div. Applied θ X 5 arc-second/div. (b) Fig. 8 Outputs from the tilt detection assembly when an angular displacement about the X-axis was applied to the measurement target (silicon wafer), a h X output, b h output surface. It should be noted that the influence of laser beam reflected from the backside of sensor substrate is small since the reflectivity of the backside of sensor substrate is low, and thus, the intensity of laser beam reflected from the backside is small. 4.2 Verification of the Effect of Tilt Compensation In the proposed concept of defect detection by using the micro-thermal sensor, the tip of micro-thermal sensor probe is preferred to approach a measurement surface as close as possible to achieve higher defect detection sensitivity. For the reduction of the gap at the interface between the sensor tip and the measurement surface, a tilt compensation of the micro-thermal sensor with respect to the measurement surface is required. In addition, an initial gap at the interface is required to be assured to avoid a collision in between the micro-thermal sensor and the measurement surface during the defect detection process. Figure 10 shows one of the possible procedures of setting the initial gap at the interface. At first, detect the tilt of target surface h Target with respect to the optical axis of tilt detection assembly. After that, detect the tilt of thermal sensor substrate h Sensor with respect to the optical axis of tilt detection assembly; this will compensate the tilt of micro-thermal sensor with respect to the target surface. Then, detect and adjust the thermal sensor tilt with respect to the target surface by giving relative in-plane motion between the micro-thermal sensor and the measurement surface. After the above-mentioned tilt adjustments, make the probe tip approach the target surface, while monitoring both the micro-thermal sensor output and the tilt detection assembly output. A vertical position of the micro-thermal sensor at which the contact is detected will be defined as a datum to determine the gap h at the interface between the microthermal sensor and the measurement surface. Since the micro-thermal sensor is mounted on a multi-dof PT stage having internal feedback displacement sensors, the gap h can be set as appropriate value. It should be noted that the contact between the micro-thermal sensor and the measurement surface is expected to be detected without giving any damage to both the sensor surface and the target surface until the target surface is kept stationary in-plane direction and the sensor is made to approach the target surface carefully. Following the adjustment of initial gap by the above-mentioned procedure, defect detection will be Table 3 Sensitivity of the optical tilt detection assembly evaluated in each case shown in Fig. 6 Target Silicon wafer (through a glass plate) Micro-thermal sensor h x sensitivity%/arc-second h y sensitivity%/arc-second

9 Nanomanufacturing and Metrology (2018) 1: Optical tilt detection assesmbly θ output 0.1 arc-second/div. Voltage applied to PT stage Tilt detection assembly Voltage applied to PT stage 2 mv/div. Time 1 s/div. (a) Optical tilt detection assesmbly θ output 0.1 arc-second/div. Voltage applied to PT stage Voltage applied to PT stage 2 mv/div. Tilt detection assembly Time 1 s/div. (b) Fig. 9 Comparison of the measured tilt angles, a micro-thermal sensor as the measurement target, b silicon wafer as the measurement target Detect the tilt of target surface θ Target with respect to the optical axis of tilt detection assembly Detect the tilt of micro thermal sensor θ Sensor with respect to optical axis of tilt detection assembly Adjust the thermal sensor tilt with respect to the target surface Make the probe tip approach the target surface Detect contact between the micro thermal sensor and the target surface (Define zero-gap) Pull-back the probe tip (Set the initial gap) Start scanning the target surface by the sensor probe Fig. 10 A procedure of how to set the initial gap between the microthermal sensor and a target surface carried out by making the sensor probe scan over the target surface. An effect of the tilt compensation was verified in experiments. In the experiment, a glass substrate with line pattern structures having a pattern period and height of 20 lm and 2.2 lm, respectively, was employed as a measurement target and was mounted on a precision stage system placed under the micro-thermal sensor probe shown in Fig. 6. The precision stage system was capable of generating translational motion along the X-, - and -directions and rotational motion about the X- and -axis. Experimental condition is summarized in Table 4. Figure 11a shows a part of the profile of the line pattern structures employed in this paper. A tip of the prototype micro-thermal sensor probe was placed with respect to the target surface with a gap of approximately 2 lm and was then held stationary in the setup. The glass substrate with the line pattern structures were then moved in the horizontal direction in the setup to give relative in-plane motion between the micro-thermal sensor and the target surface. At first, the experiment was carried out without the tilt compensation. Figure 11b shows voltage output waveform from the micro-thermal sensor. As can be seen in the figure, it was found that the output waveform contained a linear component, which was considered to be due to the tilts of both the micro-thermal sensor and the target surface with respect to the direction of motion axis of the target surface. Those tilts were then compensated through the above-mentioned tilt compensation procedure, and the experiment was carried out again. The result is shown in Fig. 11c. As can be seen in the figure, a variation of the voltage output with a period of 20 lm, which well agreed with the period of line pattern structures, was clearly observed, while eliminating the linear component. From

10 54 Nanomanufacturing and Metrology (2018) 1:45 57 Table 4 Experimental conditions Item Gap between the tip and the surface Scanning speed of the glass substrate Nominal height of the line pattern structure Period of the line pattern structure Material of the line pattern structure Value Approx. 2 lm 100 lm/s 2.2 lm 15 lm, 20 lm Photoresist (OFRP-800LB 200CP) Height 1 μm/div. output 10 mv/div. output 10 mv/div. -position 10 μm/div. (a) -position 10 μm/div. (b) -position 10 μm/div. (c) Fig. 11 A waveform of the voltage output from the micro-thermal sensor when relative motion was given in between the target surface with line pattern structures and the micro-thermal sensor, a profile of the line pattern structures measured by a commercial profilometer, b a waveform of the voltage output from the micro-thermal sensor without the tilt compensation, c a waveform of the voltage output from the micro-thermal sensor with the tilt compensation these results, the effect of tilt compensation on the reduction of linear component in the sensor output has successfully been verified. It should be noted that, due to the limitation on the manufacturing equipments employed for the sensor fabrication, the sizes of line pattern structures employed in the experiments were large compared with the target defect size. Further detailed investigation on the resolution of defect detection is remained to be addressed, and will be carried out in future work as well as the establishment of fabrication method of the micro-thermal sensor having a further smaller effective area size. 4.3 Detection of a Contact Between the Microthermal Sensor and a Target Surface The purpose of the above-mentioned tilt compensation is to achieve a small amount of gap at the interface between the micro-thermal sensor and a target surface for improving the detection sensitivity of the micro-thermal sensor. Figure 12 shows a variation of micro-thermal sensor output due to the change in gap h at the interface. As can be seen in the figure, the amplitude of sensor output increased when the tip of the micro-thermal sensor probe was brought closer to the line pattern structures; these results indicate that the reduction of the gap h has a possibility of improving the sensitivity of micro-thermal sensor. For the reduction of h, regarding the procedures of setting the initial gap shown in output 10 mv/div. output 10 mv/div. output 10 mv/div. Gap: h 0-5 μm -position 10 μm/div. Gap: h 0-4 μm -position 10 μm/div. Gap: h 0 (> 5 μm) -position 10 μm/div. Fig. 12 Variation of the voltage output waveform from the microthermal sensor due to the change in gap at the interface when the sensor is made to scan over the line pattern structures

11 Nanomanufacturing and Metrology (2018) 1: Fig. 10, precise verification of the zero-position of the tip of micro-thermal sensor probe is important. The detection of contact in between the micro-thermal sensor and a target surface is therefore important task as well as the tilt compensation mentioned above. In this paper, by using the developed experimental setup, an experiment was carried out after the tilt compensation to verify the possibility of detecting a contact between the tip of the micro-thermal sensor and a target surface. In the experiment, a bare silicon wafer was employed as the target and was mounted on a PT stage. The bare silicon wafer was made to approach the tip of the micro-thermal sensor probe in steps of 100 nm, while the sensor probe was kept stationary in the setup. During the approach of bare silicon wafer, both the micro-thermal sensor output and the optical tilt detection assembly output were monitored. Figure 13a shows the output voltage from the micro-thermal sensor and the sensor tilt measured by the optical tilt detection assembly. As can be seen in the figure, the micro-thermal sensor output continued to increase with the approaching of the silicon wafer surface to the tip of the micro-thermal sensor, while the optical tilt detection assembly output was almost unchanged. After that, the increase of the output voltage from the micro-thermal sensor stopped at a certain -position, while the optical tilt detection assembly output started to change in one angular direction. Figure 13b shows the closed-up of points of inflection of both the micro-thermal sensor output and the optical tilt detection assembly output. As can be seen in the figure, the -positions of both the points of inflection well agreed with each other. From these results, it can be concluded that the contact between the micro-thermal sensor and a target surface was detected by monitoring the change in the micro-thermal sensor output or detecting the variation of optical tilt detection assembly. In addition, as can be seen in Fig. 13, the voltage output from the micro-thermal sensor can be employed to detect the change in gap at the interface; this result implies the possibility of controlling the gap at the interface by using the voltage output from the micro-thermal sensor, while scanning the probe over a target surface. It should be noted that the tip of the sensor probe needs to follow a target surface during surface defect detection in a practical case, where not only the motion of measurement target but also the spatial frequency component in the profile of the measurement target should be taken into consideration. Further detailed investigation on the verification of achievable gap between a target surface and the micro-thermal sensor fabricated on a glass substrate having better flatness, and a demonstration of surface defect detection with feedback controls of both the tilt and the gap of micro-thermal sensor with respect to a measurement target remain to be addressed and will be carried out in future work. 5 Summary A micro-thermal sensor probe has been designed and developed for realizing the proposed concept of non-contact and nondestructive surface defect detection, in which a change in heat flow at the gap between the tip of the microthermal sensor probe and a target surface is utilized to recognize the existences of surface defects. To achieve highly sensitive surface defect detection, the micro-thermal sensor is required to be placed as close as possible to the target surface. A relative tilt of the micro-thermal sensor, which is fabricated as a thin metal film structure on a flat glass substrate by using photolithography process, should therefore be detected and compensated precisely with respect to the target surface. In addition, the micro-thermal sensor probe should have a function of detecting a contact at the interface between the probe tip and the target surface so that an initial gap at the interface can be assured. Regarding the above-mentioned requirements, in this paper, efforts have been made to develop the micro-thermal sensor probe and to verify its basic performances. For Estimated contact position output 100 mv/div. Micro thermal sensor Sensor tilt Sensor tilt 5 arc-second/div. output 100 mv/div. Micro thermal sensor Sensor tilt Sensor tilt 5 arc-second/div. Gap at the interface 5 μm/div. Gap at the interface 1 μm/div. Fig. 13 Variations of the micro-thermal sensor output and the optical tilt detection assembly output

12 56 Nanomanufacturing and Metrology (2018) 1:45 57 the purpose, an optical tilt detection assembly, which has been designed based on the laser autocollimation capable of measuring two-degree-of-freedom angular displacement of a measurement target by employing a two-dimensional photodetector, has been integrated into the micro-thermal sensor probe. In addition, a sensor probe assembly composed of the fabricated micro-thermal sensor and a hollowshaped piezoelectric actuator capable of giving translational motion along the -axis and rotational motions about the X- and -axis has been integrated in the micro-thermal sensor probe. Results of basic experiments have revealed that the developed optical tilt detection assembly has a measurement resolution of better than 0.04 arc-second, which is enough for the tilt adjustment of micro-thermal sensor probe. In addition, it has been revealed that the developed micro-thermal sensor is capable of detecting line pattern structures with a period of 20 lm fabricated on a silicon wafer surface, while the relative tilt of the microthermal sensor with respect to the target surface is being compensated by the developed micro-thermal sensor probe. Furthermore, it has also been demonstrated in experiments that a contact between the probe tip and the target surface can successfully be detected by using the micro-thermal sensor output or the optical tilt detection assembly output. It should be noted that attentions have been paid in this paper to design and develop the micro-thermal sensor probe for the proposed concept of the non-contact surface defect detection by utilizing the change in heat flow across the gap between the probe tip and a target surface. Verification tests of the developed micro-thermal sensor probe for the detection of surface defects while assuring the amount of gap at the interface will be carried out as future work, as well as the establishment of fabrication method of the micro-thermal sensor having a further smaller effective area size, and the evaluation of its resolution of defect detection. Acknowledgements This research is supported by Japan Society for the Promotion of Science (JSPS). References 1. Marchon B, Olson T (2009) Magnetic spacing trends: from LMR to PMR and beyond. IEEE Trans Magn 45: May GS, Sze SM (2004) Fundamentals of semiconductor fabrication. Wiley, New ork 3. Tsai DM, Luo J (2011) Mean shift-based defect detection in multicrystalline solar wafer surfaces. IEEE Trans Industr Inf 7(1): Brinksmeier E, Mutlugünes, Klocke F, Aurich JC, Shore P, Ohmori H (2010) Ultra-precision grinding. CIRP Ann Manuf Technol 59(2): International Roadmap for Devices and Systems 2016 Edition. 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13 Nanomanufacturing and Metrology (2018) 1: uki Shimizu received his Bachelor of precision engineering from Tohoku University, Japan, in 2000, followed by MS in precision engineering from Tohoku University, Japan, in He had spent his career at Hitachi Ltd. from 2002 to 2011, and had been involved in the research on head-disk interface technology for hard disk drive. He received his Ph.D degree from Nagoya University, Japan, in He is currently an associate professor in the Department of Finemechanics, Tohoku University, Japan. His research interest includes precision dimensional metrology and optical metrology. He is a member of JSPE, JSME, JAST and JSAT. uki Matsuno received her BS in mechanical engineering from Tohoku University in 2016 and MS degrees in mechanical engineering from Tohoku University in In Tohoku University, she has been dedicated to develop the method for surface defect detection. Hiraku Matsukuma graduated from school of engineering science at Kyoto University in He received his MS, and PhD degrees of mechanical engineering and science at Kyoto University in 2010 and 2013, respectively. He is currently working as an assistant professor of Tohoku University. His research interest includes optical engineering, spectroscopy and optical precision metrology. He is a member of JSPE. Wei Gao is the Director of Research Center for Precision Nanosystems of Tohoku University, Japan. He serves as the Chairman of The Scientific Technical Committee Precision Engineering and Metrology of CIRP and also served as a Vice President of Japan Society for Precision Engineering (JSPE) in He is the author of the book Precision Nanometrology (Springer). He received his Bachelor of Precision Instrumentation from Shanghai Jiao Tong University, China, and MSc and Ph.D degrees from Tohoku University, Japan. He is a fellow of the International Academy for Production Engineering (CIRP). uan-liu Chen received his BS, MS and PhD degrees in mechanical engineering from hejiang University in 2009, 2011, and 2014, respectively. He is currently an associate professor in the Department of Finemechanics, Tohoku University, Japan. His research interest includes precision metrology and precision nanofabrication. He is a member of JSPE.