Design And Fabrication Techniques Of Surface Acoustic Wave Sensor For Intracranial Pressure Measurement

Hydrocephalus is a common disease among the infants and the old people,which is usually caused by excessive generation of cerebrospinal fluid(CSF)or(and)obstruction in circulation and absorption.As a result,it could lead to symptoms such as increase in head size,headaches,or intellectual disability.Currently,the common method for the treatment of hydrocephalus is to insert a ventriculoperitoneal shunt in the ventricle,which could divert the redundant CSF to the abdominal cavity,where they could be absorbed completely to keep the intracranial pressure(ICP)at a normal range.However,the system now has a high risk of malfunction,such as blockage,overflow or inadequate drainage,threatening the patients' lives.At present,CT or MRI is adopted to help adjust the valve of shunt system,which has low accuracy,discontinuous measurement as well as high cost.In order to improve these drawbacks,this dissertation focuses on the research of a surface acoustic wave intracranial pressure sensor,including the key technologies of temperature compensation,structure design and fabrication techniques through the process of developing of a mathematical model and FE method in order to implement a cheap and continuous ICP measurement.This dissertation designs a differential type structure of ICP sensor based on ST cut quartz to overcome the temperature drift of the pressure sensor.The ST cut quartz is used as the piezoelectric substrate due to its temperature stability.The Aluminium electrodes are deposited on the top surface to form IDT and Reflectors;thus a resonator type sensor is designed.A fast and efficient simulation method is also proposed by combining the empirical formula and coupling of mode model(COM)together to improve the efficiency of sensor design.First,an empirical formula is used for preliminary design and the structure parameters are determined within a rough range.Then,COM model is established for precise simulation.The parameters like electrode pairs,coating thickness,etc.are designed targeting at obtaining the maximum amplitude of S11,so that the device with high Q value is acquired.Finally,the dual channel resonator type sensor is designed for temperature compensation and the temperature stability of the sensor is improved with the methods adopted.The precision of the sensor is measured around 3.21% by relevant experiments,which can satisfy the design target.As the coupling coefficient of ST cut quartz is small,a novel multilayered structure is designed by the use of two different kinds of materials with opposite temperature coefficient.The Si O2 film is coated on 128°YX cut Li Nb O3 substrate to form this multilayered structure having good temperature stability and large coupling coefficient.First,FEM analysis is used to help design the sensor.The eigenfrequency analysis is conducted to obtain the phase velocity of SAW,thus the curve of coupling coefficient is calculated with different Si O2 thickness.Second,the temperature field is then coupled into the model and the phase velocity under different temperature is achieved,so the temperature coefficients are obtained accordingly.As a result,the multilayered structure corresponding to zero temperature coefficient is simulated and the Si O2 film thickness together with coupling coefficient is also derived.Finally,the frequency domain analysis is appled to simulate S parameter of the sensor.The prototype is fabricated and tested for verification.The measured coupling coefficient of designed multilayered structure is much larger than that of quartz.The sensor prototype is fabricated through MEMS fabrication technique.The photolithography technique is adopted to fabricate the electrode structures of IDT and Reflector.The PECVD manufacturing technique is used to coat Si O2 film.Due to thick Si O2 film,the coat becomes cracked easily because of thermal stress incurred during PECVD process;so the PECVD process is improved by dividing it into three periods equally to release the stress and the problem is solved consequently.Two kinds of sensor packages are designed and compared.A method by using the alloyed membrane is proposed to reduce the package size.However,the repeatability is not good because of fabrication techniques.As a result,a sandwich structure is proposed to effectively improve the repeatability of the sensor.The testing system is established to calibrate the sensor,which includes a silicon resonant pressure sensor with 0.01% accuracy,a Mensor gas controller and a thermotank.The silicon sensor is used to measure the true pressure value and the gas controller and thermotank are used to control the desired pressure and temperature output respectively.The temperature and pressure performance of the sensor is studied and the feasibility of temperature compensation method is verified.The ICP sensor proposed in this dissertation can be used to realize the basic function of a small range pressure measurement,which could solve the inaccuracy and discontinuity of the current ICP measurement methods.Moreover,it can function wirelessly and passively by connecting the antenna.This research provides significant values in clinical applications and it can be extended to measure other physiological signals(like blood pressure)by adjusting the measurement ranges.