Tamkang Journal of Science and Engineering, Vol. 7, No. 2, pp. 111−114 (2004)
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16×2 Thermally Actuating and Sensing Probe Array Ching Hsiang Tsai*, Chao Chiun Liang, Gen Wen Hsieh, Wei-Chih Lin and Yuh Wen Lee Electronics Research & Service Organization Industrial Technology Research Institute Jhudong, Taiwan 310, R.O.C. E-mail:
[email protected]
Abstract This paper developed an active scanning probe array. The probe array consists of 16×2 probes thermally actuating and sensing. The 32 probes are integrated with Ti/Pt temperature sensors and bimorph thermal actuators. Each probe is a Si3 N4 cantilever equipped with a sharp nano scale pyramid tip at its free end. When one of the probes is actuated, its neighboring probes can provide temperature sensing feedback. The temperature coefficient of resistance (TCR) of the embedded Ti/Pt sensor of each probe is 2100 ppm °C−1 , and the micro thermal actuator shows good reliability after 500 cycles over specification continuous actuating test. Key Words: Scanning Probe Microscope, Thermal Actuator Integrated Probe Array
1.
Introduction
Scanning probe microscope (SPM) is a powerful tool that is used to examine a variety of materials on atomic level, and has become an essential technique for surface science [1−3]. However, SPM generally takes several minutes to produce an image. It is because of its low scanning speed. Therefore the wide-spread application of SPM based technology such as lithography and data storage was limited [4−5]. In order to speed up the scan rate, using multiple probes in parallel is developed. It can achieve several times improvement in data access rate [6−8]. It has been demonstrated that an active probe with an actuator integrated can be speed up data rate 10 times improvement. The probe can be integrated with a piezo layer and then electrically actuated. It can offer a high frequency of actuation, but the displacement is too short [4]. In this research, a 16×2 probe array is designed to improve the scanning speed. And the probes are — — — — *Corresponding author
designed to be thermally actuated for large scale displacement. A Ti/Pt heater and temperature sensor is integrated on a Si3 N4 cantilever. When heating up a electrically, the cantilever beam was bended and the tip was actuated. When one of the probes is actuated, its neighboring probes can provide the desired temperature feedback.
2.
Experimental
The 16×2 active probe array is fabricated by silicon bulk machining. At the free end of the cantilever beam, there is a pyramidal tip with nano scale sharpness. The mic ro cantilever is capable of bending by the way of bimorph actuation result a controlled vertical displacement of the tip. The fabrication and actuation of the 16×2 probe array is investigated in the following sections. 2.1 16×2 Active Probe Array Figure 1 shows the fabrication process sequence of the 16×2 Si3 N4 based thermal actuating probe array. 4”, 400 µm thick, n-type, (100) oriented silicon wafer is used as a substrate.
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(a). Si wafer Ti/Pt RTD (b). Si tip cavity Ti/Pt heater (c). Si3 N4 deposition & pattern
(d). Ti/Pt heater deposition & lift off
(e). Parylene C front side protection
(f). Backside etching
(g). Parylene C removed Figure 2. OM & FE-SEM photos 16×2 thermal actuating probe array.
Figure 1. Fabrication process sequence of 16×2 therma l actuating probe array.
First, 3 kÅ hard mask of Si3 N4 was deposited on both side of silicon wafer. After lithography patterning, the Si3 N4 hard mask was opened by RIE, and then etched in KOH solution to form pyramidal pits. The residual Si3 N4 mask was removed by HF solution (steps a−b). Next, 1.5 µm low stress Si3 N4 was deposited on the silicon substrate (step c). A cantilever of 250 µm long and 50 µm wide was patterned by lithography and RIE Si3 N4 etching. 200 Å Ti and 6000 Å Pt bimorph heater layers were sputtered over the Si3 N4 and patterned by the lift off process (step d). Then the substrate was placed in the CVD chamber for Parylene C coating to protect the front side of the substrate (step e). The backside through wafer etching employed 35% KOH solution at 75 °C to release the cantilever (step f). Isopropyl alcohol (IPA) was added into the KOH solution to reduce the undercut. After through wafer etching, the residual Parylene C was removed by O2 plasma (step g).The OM & FE-SEM photos of 16×2 Si3 N4
Figure 3. A 16×2 thermal actuating probe array wire bonded on a PCB.
based thermal actuating probe array were shown in Figure 2. Finally, the probe array is mounted on a PCB (printed circuit board) and wire-bonded as shown in Figure 3. The functions of the probes were measured under an optical microscope.
16×2 Thermally Actuating and Sensing Probe Array
Command
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Driver Control Signal Controller
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Figure 4. Schematic diagram of a probe array controll system. 0
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Figure 6. Resistance of the Ti/Pt heater as a function of test temperature for TCR calculation.
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2.2 Actuation of the Probe Array As a compact design, the probe array is actuated and sensed by Ti/Pt-based thermistor devices of the same type. When one of the probes is actuated, its neighboring probes can provide the desired temperature feedback. The proposed design has the advantages of compact device size, simple process, low cost, and high reliability. This approach can be justified only when the actuating frequency is 10 times lower than the heat transfer rate. The requirements were achieved by designing small array size and selecting materials with high conductivity. The designed control system of the probe array is shown in Figure 4. The actuating and motion state of each probe can be represented by its temperature and sensed by individual embedded thermistor device. The tip motion of each probe can then actuated according to the command input and
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temperature feedback. In Figure 3, the feed forward actuator and feedback sensor are Ti/Pt based thermistor devices of the same type on neighboring probes.
3.
Results and Discussion
The important characteristics of the developed probe arrays to be measured are temperature vs. voltage relationship, resistance vs. temperature relationship, and reliability. With voltage vs. temperature relationship available, one can determine the applied voltage required to have a given temperature and hence the corresponding
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deflection. With resistance vs. temperature database available, one can determine the present temperature according to the measured resistance. 3.1 Characterizing Measurement for the Inte grated Probe Arrays The relationship between the applied voltage of one probe and the resistance of the neighboring probe was characterized in this paper. Using the measured result and the characteristics of Ti/Pt sensor and heater, the relationships of temperature versus voltage and resistance versus temperature are derived, as shown in Figure 5 and Figure 6, respectively. Figure 5 shows the relationship between the micro-heater operating temperature and the applied voltage. As can be seen from this figure, when the voltage was increased to 15 V, the probe was heated up to 450 °C. In this research, the temperature coefficient of resistance (TCR) of the embedded Ti/Pt temperature detector of the probe was 2100 ppm °C −1 , which was measured in the temperature range from room temperature to 450 °C. The resistance of Ti/Pt temperature sensor is linear with respect to the test temperature, as shown in Figure 6. The resistance of the sensor is a function of the test temperature, and shows a good characteristic of the RTD. Figure 7 shows the results of the reliability test of the on chip micro heater. The test was done by running 500 heating cycles continually; where in each cycle the sensor was heated to 450 °C for 5 seconds by the embedded micro-heater followed by a power-off cooling for 5 seconds. For simplicity, only the last 50 cycles were shown in Figure 7. The relia bility test is conducted by applying periodic on-off voltage waves to the actuator terminal pair of one probe, and measuring the resistance viewing into the terminal pair of the neighboring probe. If the actuated probe is burned down, the measured resistance will change dramatically. Therefore, by inspecting the stable resistance response in Figure 7, it is apparent the probe array developed in this paper is quite reliable .
4.
Conclusions
This paper had developed a process for fabrication of thermal actuating probe array integrating Ti/Pt sensor and heater on a Si3 N4 cantilever beam. When one of the probes is actuated, its neighboring probes can provide temperature feedback. The temperature coefficient of resistance (TCR) of the embedded Ti/Pt sensor of the probe was measured as 2100 ppm °C −1 , and the micro heater showed good reliability after 500 cycle continuous actuating test.
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Manuscript Received: Jan. 15, 2004 Accepted: Mar. 5, 2004
