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M I C R O S E N S O R & A C T U A T O R

Telemetry Pressure Sensor using LC Circuit

Micro Systems Lab. in AJOU UNIVERSITY

Fig. 1. The structure of the telemetry pressure sensor

Telemetry Silicon Pressure Sensor of LC Resonance Type
S. Y. Kim, H. J. Kim, J. S. Park, and S. S. Yang

ABSTRACT
  This paper presents an implantable telemetry LC resonance-type pressure sensor to measure the ventricle pressure. The sensor consists of an inductor and a capacitor. The LC resonant circuit is coupled magnetically with an external antenna coil. The resonance frequency of the circuit increases as the applied pressure decreases the capacitance of the sensor. The sensor is designed in consideration of the biocompatibility and long lifetime for continuous monitoring of the ventricle pressure.
  Another type of telemetric pressure monitoring system that includes integrated circuit need to receive power and transmit the pressure signal and has complex fabrication process and assembly of many parts such as pressure sensor, rechargeable battery, transmission system, coils, and signal processor, etc. In the case of LC resonance type, the sensor is simple and small. Another advantages of this type are the high resolution and the simplicity in the design for wide range of pressure and temperature. The previous papers show the satisfactory experimental results but theory and expression for simulation was insufficient. We focus on the different structure of the capacitance and making a smaller overall size. Also, the deflection of the diaphragm, the variation of the capacitance and the resonance frequency are analyzed and calculated. Detection range of the differential pressure applied to the ventricle pressure is 0 ~ 7 kPa.
  The telemetry pressure sensor is mainly fabricated by micromachining. The sensor is composed of two substrates. One is silicon wafer for p+ diaphragm, and the other is glass substrate for micro coil. The first step is The fabricated sensor assembled by anodic bonding. Total size is 7 mm ´ 6.5 mm. Coil has 24 turns, 10 mm thick, 50 mm wide and the side length of 4900 mm. Distance between coil and diaphragm is 10mm. Thickness of diaphragm is about 1.5 mm and size of that is 3 mm ´ 3 mm. The diaphragm has corrugation to remove residual stress and to increase the sensitivity. The sensor sealed hermetically by anodic bonding. There is the air vent to make the atmospheric pressure in the cavity of sensor. After anodic bonding the vent is filled with epoxy.
  The method of measurement is using inductive coupling. The sensor electroplated Cu coil on the glass substrate has self-inductance. The capacitance between the coil and Au plate on the p+ diaphragm depends on the deflection of diaphragm by pressure. Joining of the self-inductance and the capacitance generates the resonance frequency of the sensor. VCO applies frequency to the external loop antenna coil. When the magnetic field is matched the resonance frequency of the sensor, the impedance of the external coil changes. Pressure is calculated from measuring the peak of amplitude or the amplitude at same frequency or phase. Especially the phase of external coil decrease abruptly from 90 degree at resonance frequency. The first results of the measurement are shown in figure 3 and table 1. The inductance of sensor coil measured by LCR meter is 3.2mH at 10kHz. Figure 3 is the graph of amplitude of impedance vs. frequency measured by impedance analyzer HP4914A. The peak frequency shift from the first resonant frequency of external loop coil is about 1.5 MHz when sensor is on the coil. The shift of the frequency at zero phase is 1.4MHz. These experimental results show that the precision detection is available.

Fig. 2. The schematic diaphragm of the measurement system.


Fig. 3. Bottom substrate.	Fig. 3. The photograph of the pressure sensor.

KOREAN ABSTRACT

Micro Systems Lab. in AJOU UNIVERSITY

인체에 사용하는 압력센서에는 혈압, 뇌압, 안압 센서 등이 있는데, 인체에 삽입하기 위해서는 센서의 크기가 작아야하고, 생체 적합한 물질로 만들어져야하며, 원격측정이 가능해야한다. 본 논문에서는 뇌압을 측정하기 위한 목적으로 인체삽입용 압력센서를 설계하고 제작하였다. 압력신호를 처리하고 그 데이터를 전송하는 방식의 원격측정용 압력센서의 경우에는 센서와 송수신회로, 충전지 등이 집적화 되어야 한다[1]. 복잡한 회로를 집적하지 않고 공진을 이용하여 근거리에서 무선으로 측정하는 경우에는 삽입하는 센서부의 크기가 훨씬 작아진다[2]. LC 공진을 이용하여 고온, 고압에서 측정이 가능함을 보인 센서도 있다[3]. 본 논문에서는 압력에 따른 박막의 변위와 전기용량, 공진 주파수 등을 계산하고, 센서와 측정회로를 설계하고 제작하였다.
본 논문에서는 센서에 회로를 내장하지 않고, 원격으로 압력을 측정하는 이론과 센서를 실제 제작하였다. 제작된 센서는 9 ㎛의 박막 간격을 갖고, 내부저항은 10.7 Ω이며, 코일의 두께는 10㎛로 측정되었다. 시뮬레이션을 통하여 유도결합과 공진 주파수를 이용하여 원격측정을 할 수 있음을 확인하였다. 커패시턴스의 변화 즉 압력의 변화에 따른 공진 주파수의 변화와 외부 코일에서의 측정을 계산을 통하여 제작된 센서의 특성을 보았다. 공진 주파수를 이용한 압력의 측정은 높은 감도를 가지고 있다. 제작 공정 상에서 코일의 선폭을 줄이고 커패시터 박막간의 간격을 줄이면 센서의 크기를 더욱 줄일 수 있다.