A hermetically-sealed LC resonant structure has been fabricated to be implanted inside the body and transfers internal bio-pressure signals to the external detector. This structure consists of two parts: a pressure sensor which transfers internal pressure change into capacitance change; and an inductor coil which is connected with the pressure sensor in series. The basic idea of the device is that internal pressure sensor causes capacitance change according to pressure and this change leads to a resonant frequency change in the LC resonant structure. Externally-exited ac power is transfered to the device through inductive coupling, then the radical signal change at the exciting coil is detected at resonant frequency. Therefore a pressure information from resonant frequency variation can be detected wirelessly. This device can be applied for monitoring intraocular, cardiovascular, or brain pressures. In this work, a specific device structure has been designed for intraocular pressure measurement.
The fabrication of this device requires the development of thin film inductor coils, pressure sensor diaphragms, wafer to wafer anodic bonding technique and bulk micromachining technique. The pressure sensor consists of bulk micromachined silicon diaphragm and Cr/Au (300A/2000A) electrode on Corning 7740 glass. Diaphragm of the pressure sensor is made of heavily p+ doped silicon which is not selectively etched in an isotropic silicon etchant. Inductor coil is fabricated on glass by Cu-electroplating. It is required to reduce inductor resistance to achieve a high Q factor. Thick photoresistor patterning (>30um) for Cu-electroplating has been developed to achieve a Q factor close to 100. Inductance of the fabricated inductor is 220nH, resistance is 3Ω and the parasitic capacitance is 0.38pF. The self-resonant frequency of the inductor is over 500MHz which is much higher than the device operating LC resonant frequency of 200MHz. Finally, the glass wafer with the inductor coils and the silicon wafer with the p+ doped pressure sensor diaphragms are anodically bonded at 400℃ with 800V dc source to make a hermetic sealing for the pressure sensing. The complete-sealed structure will not expose nothing but silicon and glass to the body fluids ; therefore, it is safely used for biomedical applications for the direct and continuous wireless monitoring of bio-pressure signals. The size of this device is 2mm x 2mm. This device operates over the range of 0 ~ 100mmHg and the sensitivity is about 20000 ppm/mmHg.
