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By Micheal Morales,2014-06-26 12:33
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    A Novel Capacitive Pressure Sensor

    A novel capacitive pressure sensor is presented,whose sensing structure is a solid-state capacitor consisting of three square membranes with Al/SiO2/n-type silicon.It was fabricated using pn junction self-stop etching combined with adhesive bonding,and only three masks were used during the process.Sensors with side lengths of 1000,1200,and 1400μm were fabricated,showing

    sensitivity of 1?8,2?3,and 3?6fF/hPa over the range of 410~1010hPa,respectively.The sensitivity of the sensor with a side length of 1500μm is 4?6fF/hPa,the nonlinearity is 6?4%,and the max hysteresis is 3?6%.The results show that permittivity change plays an important part in the capacitance change.

    1 Introduction

    Capacitive pressure sensors are widely used in many fields such as industrial control,biomedical instruments,and environmental monitoring.Compared with the piezoresistive sensor,this kind of sensor has advantages that include low power dissipation, low temperature drift,good hysteresis,and stability.According toC=εA/d, the capacitive sensor changes primarily

    due to variation in displacement,variation, and permittivity.A normal sensor based on displacement variation usually has severe nonlinearity since the capacitance is inversely proportional to the displacement between the electrodes as well as the nonlinearity due to large load-deflection bending[1]. In addition,feed through of the electrode in the sealed cavity leads to complex fabrication[2].A sensor based on area variation has also been reported[3].We have presented a new capacitive pressure sensor[4,5],a solidstate capacitor that consists of Au/Si3N4/SiO2/p++type silicon.Different from the above structures,the capacitance changes under pressure applied due to the variation of the permittivity,area,and displacement between the electrodes.The sensor was fabricated by heavily doped self-stop etching.However,heavy doping may induce large stress,and it is difficult to fabricate electronic devices on the p++-type layer.The mechanism leading to the variation of the capacitance also needs to be further analyzed.Thus,we redesign the sensor,which consists of three square layers of Al/SiO2/n-type silicon.This paper gives the design, fabrication steps,and test results of the sensor.The analysis results indicate that the capacitance changes mainly due to the permittivity variation under pressure applied.

    2 Design and fabrication

    A cross section of the proposed sensor is shown in Fig?1.This sensor consists of Al/SiO2/n-type silicon, where Al and n-type silicon are used as top and bottom electrodes,respectively,while SiO2is the dielectric layer.The membranes are deformed under pres-sure, causing a load-deflection bending, and hence leading to changes of the area and displacement between the electrodes. According to electrostriction effects[6,7],the permittivity also changes due to the induced stress under applied pressure. The sensor was fabricated by pn junction selfstop etching[8]combined with adhesive bonding.The pn junction self-stop etching was used to define the cavity and control the thickness of the membrane

    Fig.1 Cross-section of the capacitive pressure sensoraccurately,and adhesive bonding was used to form the sealed cavity. The detailed process flow shown in Fig?2,is listed as follows.

    (a) Diffuse phosphor on the p-type (100) oriented silicon to form an n-type layer,which is used not only as the bottom electrode of the sensor,but also the self-stop layer.The thickness of the n-type layer is about 3μm,and the square resistor is 8Ω/?.

    (b) Grow thermal oxide on the wafer by the dry method,which is used as a dielectric layer,and the thickness is about 0?2μm.

    (c) Deposit Si3N4(about 100nm)on the back side of the wafer using LPCVD (low pressure chemical vapor deposition) as a protective layer during bulk silicon etching.

    (d) Lithography contact.

    (e) Deposit and pattern Al (about 200nm)as electrodes.

    (f) Define the window in the sensing structure area on the back side by double-sided aligning and then open the window by RIE(reactive ion etch).

    (g) Setup the pn junction self-stop system.The wafer was mounted in a Teflon holder to protect the front side of the wafer from etching,and then immersed in 40wt% KOH, before etching began. The etching temperature is set to 80?and the responding etching rate is 1μm/min.

    (h) Form sealed cavity using epoxy by adhesive Fig.3 Top view of the capacitive pressure

    sensor bonding in the common pressure.The residual pressure in the cavity is about 1010hPa. Only three masks were used during the fabrication process (used to etch the contact and electrodes, and to open the windows on the backside during steps

    (d),(e) and (f)).Figure 3 gives the SEM photos of the sensor.

    3 Test results

    3.1 Membrane after the self-stop The sensitivity of the sensor is dependent on the membrane’s thickness of the sensor,and the characteristic uniformity of the batch sensors is also related to the thickness. As an important parameter, the thickness is measured.Five different samples are randomly chosen,whose thickness are about 3?0?1μm, showing good uniformity.

    Figure 4 shows the cross section of a membrane after self-stop,in which the membrane is 3μm

    thick,close to the thickness of ntype silicon (about 2?9μm thick by the pn junction coloration

    method).The test result indicates etching is effectively self-stopped at the interface of the pn junction.

Fig.3 Top view of the capacitive pressure sensor

    3.2 Pressure response

    Test results of the sensors with different side lengths are presented. As pressure applied is from 1010 to 410hPa(1hPa=100Pa)at room temperature, the initial capacitance and sensitivity corresponding to different side lengths are recorded in Table 1.A sensor with a large side length improves the sensitivity;On the other hand,it also increases the layout area and initial capacitance,and hence increases the power dissipation.It is necessary to optimize the parameters

    according to demand. A typical pressure response of the sensor with the side length of 1500μm

    is shown in Fig?5 (where the horizontal axis is differential pressure,defined as the difference of the

    pressure applied and the residual pressure in the cavity).The figure shows the sensitivity is 4?6fF/hPa and the nonlinearity is about 6?4% over the full range from 1010 to

    410hPa.Correspondingly,the horizontal axis is from 0 to 600hPa when the residual pressure in the cavity is 1010hPa.Figure 5 also shows the hysteresis characteristic of the sensor. The max hysteresis is about 3?6% and appears in the range from 200 to 400hPa.The sensor presents good linearity and hysteresis.

    4 Analyses

    The sensing capacitor is similar to a plate capacitor,and it is given By C =εA/d (1) where Cis

    the capacitance,εis the permittivity,and Aanddare the area and displacement of the

    electrodes,respectively. From Eq.(1),the relative variation of the capacitance is given by

    ΔC/C =-Δd/d +ΔA/A +Δε/ε(2)

    Fig.5 Curves of the capacitance as a function of pressure L=1500μm

    where the terms -Δd/dandΔA/Arepresent the influence of the geometric defromation,and the term

    Δε/εrepresents the influence of the permittivity variation.Δd/dandΔA/Acan be written as

    ΔA/A =?L/2-L/2?L/2-L/2(εx+εy)dxdy/L2(3)

    Δd/d =v(v -1)ΔA/A (4)

    Where Lis the side length of the square membrane εx and εy represnt the strain in

    thexandydirections, andvis the Poisson rate of the dielectric,which is 0?17 for SiO2.The complex membranes of the sensor deform and extend under applied pressure,and hence increase the area and decrease the displacement of the capacitor,both of which lead to increase in the capacitance.Table 2 gives the capacitance variationΔCdue to geometric deformation under different pressures. The general calculation is as follows:the entity model of the sensor is built in ANSYS,and mesh the model using Shell 181 element (50×50); Then, carry the boundary condition and pressure load. In the next step,calculate the strain at each node under different pressures.Finally,the calculation results were carried into Eqs.(3) and (4),and solved forΔCdue to

    geometric variation.The capacitance variation only due to permittivity change can be calculated by Eq.(2) combined with the value of Fig?5 and Table 2.Figure 6 shows that as the differential pressure varies from 0 to 600hPa,the permittivity change decreases the capacitance, and the total capacitance change due to the permittivity change is 3?23pF.Meanwhile the geometric deformation makes the capacitance increase,and the capacitance change due to the geometric deformation is 0?483pF. This result shows that the permittivity change has a reverse influence on the capacitance variation compared to the geometric change,and the permittivity change plays a major part in the capacitance variation.

    5 Conclusion

    A novel capacitive pressure sensor is presented; the sensing capacitor consists of complex membranes including metal/oxide/n-type silicon.The results show that permittivity variation plays an important role in the capacitance variation. The sensor introduced in Refs.[4,5] is incompatible with the CMOS process Fig.7 Conceptual schematic of the monolithic capacitive pressure sensor and difficult to integrate with its interface circuit since heavily doped self-stop etching technology was used during the fabrication.The improved structure presented in this paper overcomes these disadvantages.A conceptual cross section of the sensor integrated with its interface circuit based on CMOS process is shown in Fig?7,where the sensing capacitor consists of n well silicon/gate oxide/poly gate.The sensor can be fabricated in a standard CMOS process combined with some post processing,such as pn junction selfstop etching that is used to release the cavity and control

    the thickness accurately and anodic bonding that is used to seal the cavity. Table 3 gives the test results of the sensor with a side length of 1000μm compared to other kinds of capacitive pressure

    sensors. The results show that the proposed sensor has higher sensitivity and is easier to fabricate because it only needs three masks during fabrication.Further work should pay attention to the mechanism of the permittivity variation under applied pressure.

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