Capacitive Micro Inclinometer with Scalloping-free and Footing-free Vertical Electrodes using Crystalline Etching of (110) Silicon Sung-Sik Yun, Dae-Hun Jeong, Jae-Yong An, Minho Jun, Jong-Hyun Lee
Chang-Han Je, Myung-Lae Lee, Gunn Hwang, Chang-Auk Choi
School of Information and Mechatronics Gwangju Institute of Science and Technology (GIST) Gwangju, Republic of Korea E-mail:
[email protected]
Microsystem Team Electronics & Telecommunications Research Institute (ETRI) Daejeon, Republic of Korea
defects such as footing, scalloping, parallel deviation and spherical deviation. These defects can degrade a performance of the micro devices which have the structure of high aspect ratio such as optical planes and capacitive sensing electrodes. Particularly, a capacitive sensor with vertical sensing electrodes is more sensitive to footing and scalloping defects because the overall area of the vertical sidewalls etched by DRIE should be used for sensing capacitance. In order to improve surface morphology of the sidewalls after DRIE process, the sensing electrodes can be improved by additional crystalline wet etching when the sensing electrodes are parallel to the (111) vertical plane of (110) silicon [7]. The vertical electrodes using additional crystalline wet etching of (110) silicon after DRIE process could enhance the resolution of a capacitive sensor.
Abstract — A micromachined capacitive inclinometer has been developed to detect inclination angles for a position sensing application. In order to enhance resolution, a (110) crystalline silicon-on-patterned-insulator (COPI) process has been proposed to remove the morphologic defects such as footing and scalloping which were formed from silicon deep reactive ion etching (DRIE) process. The sidewalls fabricated by the (110) COPI process remarkably became vertical and flat with few nanometer roughness. The micro inclinometer with flat and vertical sensing electrodes was evaluated in terms of capacitance change and detection limit. The capacitance change of the fabricated device is from -0.246 to 0.258 pF for the inclination angle (-90˚ to 90˚). The temporal deviation of the capacitance is as small as 0.2 fF, which leads to 0.3 or less resolution for ±70˚.
I.
In this paper, the capacitive micro inclinometer, which has a proof mass responded to gravitational force, has been demonstrated. The shift of the proof mass induces capacitance change between the sensing electrodes with respect to inclination angles. The capacitive micro inclinometer is fabricated using (110) crystalline silicon-onpatterned-insulator (COPI) process in order to enhance the surface morphology of the sensing electrodes. The crystalline wet etching of (110) silicon is employed to improve surface morphology such as scalloping of the electrodes after silicon DRIE process [7]. The footing effects can be eliminated by pattering of the insulator layer under the vertical structures.
INTRODUCTION
For many years, the inertia sensors based on microelectromechanical systems (MEMS) technology have been employed for the position sensing systems of the mechatronics devices, such as robot control systems, advanced surgical tools, navigation systems, and attitudecontrol systems, and portable electronic devices [1-2]. Recently, a capacitive inertia sensor has been paid attention as an important component of the position sensing systems through measuring an inclination angle [3]. Comparing with conventional tilt sensors using the sensing mechanism such as heat convection or electrolytes [4-5], the capacitive detection method for the inclinometer has been highlighted due to its low power consumption, long life time, simple fabrication, and insusceptibility to surrounding temperature.
II.
A capacitive micro inclinometer consists of a proof mass, springs, movable and fixed comb electrodes. Operational principle of the proposed micro inclinometer is shown in Fig. 1. The proof mass having an in-plane motion responds to a gravitational force with respect to the inclination angle.
The silicon DRIE process has been significantly remarked to fabricate a vertical structure of high aspect ratio for the optical devices and capacitive sensors [6]. However, the sidewalls etched by the DRIE process show several
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DESIGN
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IEEE SENSORS 2008 Conference
(a)
Figure 2. Fabrication sequence of the proposed micro inclinometer using (110) crystalline silicon-on-patterned-insulator (COPI).
III.
(b)
The capacitive micro inclinometer is fabricated by (110) COPI fabrication process as shown in Fig. 2. By using this fabrication method, we can obtain footing-free and scalloping-free vertical structures. The detailed fabrication conditions are as follows:
Figure 1. (a) Schematics of the proposed micro inclinometer and (b) The sensing principle of the inclinometer according to the gravitational force.
An in-plane component of the gravitational force is represented by a sinusoidal function and increases when the inclinometer is tilted. Simultaneously, the lateral motion of movable comb electrodes is induced by in-plane component of the gravitational force of the proof mass. The overlapped area of the movable and fixed electrodes is increased proportionally to the inclination angle. Therefore, the capacitance change (ΔC) with respect to the inclination angles is given by equation (1).
ΔC = 2 N ⋅ ε 0
te ⋅ M ⋅ g sin θ d ⋅k
FABRICATION
a) A (100) silicon wafer was prepared and thermally wetoxidized with 2 um in thickness. b) Thermal oxide layer was patterned with a photoresist (AZ 6612) and etched using a reactive ion etching of CF4/O2 gas. c) A cavity of the (100) silicon was generated with 15 umdepth using a crystalline wet etching of Tetramethylammonium hydroxide (TMAH) solution
(1)
d) A (110) silicon wafer of a devices layer was bonded to the prepared (100) silicon wafer. A thickness of the (110) silicon wafer was reduced down to 50 um in thickness using a chemical mechanical polishing (CMP).
where N, ε0, te, M, g, d, k, and θ are the number of comb fingers, permittivity of air, the thickness of the comb electrodes, mass of the movable structures, acceleration of gravity, gap between the movable and fixed comb electrodes, spring constant of the suspension, and the inclination angle, respectively. Note that the capacitance would be susceptible to fabrication errors because the footing effect reduces the thickness of comb electrodes and the surface defects by silicon DRIE varies in the gap distance between two comb electrodes. Therefore, the (110) COPI process has been proposed to remove the footing effect and to improve the surface defect such as surface roughness, parallel deviation and spherical deviation resulted from the silicon DRIE of a conventional SOI wafer.
e) For silicon DRIE process, the device patterns were created using a standard photolithography process. f) The capacitive inclinometer with vertical comb electrodes was fabricated without the footing effect using the silicon DRIE. g) The remaining photoresist was removed, and then the vertical comb electrodes were slightly etched using a KOH crystalline etching (45 wt%, 70°C) to improve the surface morphology of sidewalls.
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(a)
(b)
Figure 4. (a) the experimental setup for measuring the tilted angles, (b) the electrical arrangement for capacitance sensing.
Figure 3. SEM images of the fabricated micro inclinometer using (110) COPI process (a) bird’s eye view, (b) bottom substrate after removing the proof mass and moving electrodes, (c) the cross-section view of the fabricated inclinometer, (d) the fixed electrode with footing-free and scalloping-free vertical planes.
h) The proof mass and movable comb electrodes were released from the (100) silicon substrate using a hydrogen fluoride (HF) solution. The released inclinometer is sputtered with Cr/Au (50nm/200nm) for an electrical connection. The fabricated micro inclinometer with the vertical comb electrodes is shown in Fig. 3a. To investigate the etching characteristics along the A-A’ line, the proof mass is removed. The bottom substrate of (100) silicon wafer has a clear vestige of the DRIE process because the SiO2 layer under the vertical structures is already eliminated from the (100) bottom substrate as shown in Fig 3b and 3c. Accordingly, the vertical structure such as comb electrodes and suspension springs could be successfully fabricated without the footing effect as shown in Fig. 3d. Also, the additional KOH crystalline etching could reduce the surface defect of the sidewall of vertical comb electrodes. The pillar structures on the (100) bottom substrate is acted as supports to prevent joining the (110) silicon wafer to the (100) silicon wafer during the bonding and CMP process. The proof mass can be kept stationary with the pillar structures during DRIE and additional KOH crystalline etching. IV.
(a)
EXPERIMENTS
(b)
An experimental setup consists of the fabricated micro inclinometer, a 360˚ rotation stage, and a LCR meter (Agilent 4284A precision LCR meter) as shown in Fig. 4. The fabricated inclinometer, which is connected to the PCB using Au wiring and soldering, is attached to 360˚ rotation stage. The sensing electrodes are connected to the LCR meter to measure the capacitance change with respect to inclination angles.
Figure 5. Experimental results of the fabricated inclinometer: (a) capacitance change with respect to inclination angle and (b) resolution with respect to inclination angle.
The performance of the capacitive micro inclinometer was experimentally investigated in terms of the capacitance change and the resolution with respect to inclination angles. The capacitance is measured at every 10˚ of inclination angle from -90˚ to 90˚ as shown in Fig. 5.
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TABLE I.
SPECIFICATIONS OF THE FABRICATED INCLINOMETER. Parameters
electrodes were removed by additional crystalline etching of (110) silicon after silicon DRIE. The footing effects were also prevented by patterning the SiO2 layer under the vertical comb electrodes. The capacitance change was measured as 0.246 ~ 0.258 pF for the inclination angle of -90˚~90˚ and the resolution was estimated 0.3˚ or less for ±70˚. In further work, we will focus on integration with readout integrated circuit (ROIC) to reduce the parasitic capacitance variation and obtain a stable data acquisition of the micro inclinometer with high sensitivity.
Value
Gap between the movable and fixed comb electrodes
5.4 ㎛
Spring constant
0.685 N/m
Mass
405 ㎍
Measurement range
-90˚ ~ 90˚
Capacitance change
-0.246 ~ 0.258 pF
Resolution
0.3˚ or less for ±70˚
ACKNOWLEDGMENT This work was supported by the IT R&D program of MIC/IITA (2007-S054-01).
The resolution of the fabricated device is estimated using signal-to-noise ratio (SNR) based on empirical capacitance change and temporal deviation of the capacitance. The capacitance change is -0.246~0.258 pF for the -90˚~90˚ inclination angle. The measured capacitance change was a little different from the estimated capacitance change calculated from the fabricated dimensions due to the parasitical capacitance variation from electrical connections. The measured temporal deviation of the capacitance change was about 0.2 fF, which was ten times smaller than 2 fF of the temporal deviation for the non-crystalline etched device. The estimated resolution of the fabricated device is 0.3˚ or less for ±70˚. The specifications of the capacitive micro inclinometer are shown in Table I.
REFERENCES [1]
[2]
[3]
[4]
[5]
V.
CONCLUSIONS
A capacitive micro inclinometer for the position sensing systems has been developed in this paper. The scallopingfree and footing-free vertical electrodes were achieved by the (110) COPI fabrication process. In order to enhance the resolution, the morphologic defects on the sidewalls of the
[6] [7]
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Sergiusz Łuczak, Waldemar Oleksiuk, and Maciej Bodnicki, “Sensing Tilt With MEMS Accelerometers,” IEEE Sensors Journal, vol. 6, no. 6, pp. 1669~1675, 2004. Richard H. Dixon, and Jérémie Bouchaud, “Markets and applications for MEMS inertia sensors,” Proc. of SPIE, vol. 6113, pp. 6113061~611306-10. Joseph Bergeron, and Mark Looney “Making MEMS accelerometers work in motion control,” Electronic Engineering Times, Issue. 1487, pp. 35~36, Aug. 2007. S. Billat, H. Glosch, M. Kunze, F. Hedrich, J. Frech, J. Auber, H. Sandmaier, W. Wimmer, and W. Lang, “Micromachined inclinometer with high sensitivity and very good stability,” Sensors and Actuators A, vol. 97-98, pp. 125~130, 2002. Ho Jung, Chang Jin Kim, Seong Ho Kong, “An optimized MEMSbased electrolytic tilt sensor,” Sensors and Actuators A, vol. 139, pp. 23~30, 2007. J. Bhardwaj and H. Ashraf, “Advanced silicon etching using highdensity plasmas,” Proc, of SPIE, vol.2639, p.224-233, 1995. S.-S. Yun, S.-K. You, and J.-H. Lee, “Fabrication of vertical optical plane using DRIE and KOH crystalline etching of (110) silicon wafer,” Sensors and Actuators A, vol. 128, pp. 387~394, 2006.
