Structure of a three-electrode capacitive pressure sensor

[0002] This application is a division of Ser. No. 09/166,791 filed Oct. 6, 1998, now abandoned.

[0003] This invention is a capacitive pressure sensor. It is a three-electrode capacitive pressure sensor with structure and producing method for of especially integrating and producing both sensor capacitor and reference capacitor into the same pressure sensor cavity for eliminating environment interference of the same mode.

[0004] Two parallel electrode plates compose general capacitive pressure sensor. In micro sensor, these two electrode plates generally are a silicon film and a metal film on glass insulation material. Therefore, a dimensional connection of the two upper and lower electrode plates is usually seen in the producing procedure of capacitive pressure sensor. This procedure requires contact base of the two upper and lower electrode plates with optical masking that increases more producing procedure and causes cost enhancement.

[0005] For avoiding dimensional connection of upper and lower electrode plates for a single capacitor, one electrode plate can be separated into two parts and then form a planar connection by series of capacitors through another electrode plate. However, this method will reduce its pressure sensitivity because the original capacitor is divided into two smaller capacitors, which then are connected by series method.

[0006] The capacitor pressure sensor composed by the dual capacitors, one is sensor capacitor and the other is reference capacitor, can cancel off environment change under same mode through differentiated measurement. Such capacitive structure and design can enlarge at least one time more in sizes in comparison with that of single capacitive. Besides, an additional connection shall be made between electrode plates of such dual capacitors.

[0007] In order to solve these difficulties, a three-electrode capacitive pressure sensor structure and producing method with only a four-course optical masking is therefore developed which can integrate and produce dual capacitors into a pressure sensor cavity without increasing sensor size and complete dual capacitors requirement of differentiated measurement circuitry. Meanwhile, existence of the third electrode not only achieves planar connection production but also enhance its pressure sensitivity.

[0008] The drawings disclose an illustrative embodiment of the present invention which serves to exemplify the various advantages and objects hereof, and are as follows:

[0009] Drawing 1: Structure & Producing Structure Drawing of the invention (a three-electrode capacitive pressure sensor)

[0010] Drawing 2˜13: Production Flow & Illustrative Chart

[0011] Drawing 14: Characteristic Curve Graph of Exterior Pressure & Capacitor Values of Structure & Its Producing of a Three-Electrode Capacitive Pressure Sensor

[0012] Table 1: Environment Interference Testing Table

[0013] Please refer to Drawing 1. Drawing 1 shows structure drawing of its production and structure of the invention, a three electrode capacitive pressure sensor. In the drawing, p represents upper layer silicon electrode; a, b and c represent three metal film electrode on lower layer glass.

[0014] Square silicon film zone represents pressure-sensing cavity. According to documentation, the most sensitive zone of transformation of silicon film caused by pressure to change capacitor value occupies 60% to 70% of side length center of such film. Thus, 49% area of pressure sensing cavity center can be a sensing capacitor (C_ap), and the remaining area will be a reference capacitor (C_bp) by ring structure. So, dual capacitors can be placed in the single pressure-sensing cavity as differentiated measurement. However, these dual capacitors C_apand C_bpare of upper and lower dimensional electrode connection. Planar connection of avoiding dimensional electrode connection can be done by increasing a third electrode plate in the periphery of pressure sensing cavity. C_pcis composed by silicon electrode p and metal electrode c of larger area clipped with a film of glass electricity interface. Thus, C_pccapacitor value is at least 20 times greater than that of C_apand C_bp. C_acis series-wound by a small capacitor C_apand a large capacitor C_pcthrough silicon film electrode. Its capacitor value is similar to that of C_ap. Similarly, C_bcis series-wound by a small capacitor C_bpand a large capacitor C_pcthrough a film electrode with a capacitor value approximate to that of C_bp. So, C_acand C_bcare a dual capacitor structure with planar connection of original C_apand C_bpcapacitor value.

[0015] In planar connection of general capacitive pressure sensor, one electrode plate is divided into two parts and series-wound by capacitors through another electrode, i.e. please refer to Drawing that C_abis series-wound by C_apand C_bpthrough silicon film electrode. Capacitor value of C_abis half of that of C_apor C_bp, and pressure sensitivity of C_ab(ΔCab/ΔP) is ¼ of C_appressure sensitivity (ΔCap/ΔP). C_ap≈C_bpand ΔC_bp/ΔP≈0 (reference capacitor C_bpchanges very little with pressure).

[0016] C_ab=C_apC_bp/(C_ap+C_bp)=½C_ap

[0017] $\Delta C_{\mathrm{ab}}/\Delta P=\frac{C_{\mathrm{ap}}/\Delta P\left[C_{\mathrm{bp}}\left(C_{\mathrm{ap}}+C_{\mathrm{bp}}\right)-C_{\mathrm{ap}}{C}_{\mathrm{bp}}\right]}{{\left(C_{\mathrm{ap}}+C_{\mathrm{bp}}\right)}^2}=\frac{1}{4}\Delta C_{\mathrm{ap}}/\Delta P$

[0018] Capacitor C_acis approximate to capacitor C_ap, making pressure sensitivity of C_aca 4 times of that of C_ab, i.e. pressure sensitivity of producing method and structure of a three electrode capacitive pressure sensor of planar connection is 4 times of that of capacitive pressure sensor of general planar connection.

[0019] (A) Silicon Chip Production Flow:

[0020] Step 1: Growing oxide layer of silicon dioxide (Drawing 2); creating masking layer.

[0021] Step 2: Yellow light photographing (Drawing 3); determining scope of pressure cavity.

[0022] Step 3: Potassium hydroxide engraving (Drawing 4); determining depth of pressure cavity.

[0023] Step 4: Boric diffusion (Drawing 5); determining thickness of silicon film.

[0024] Step 5: Yellow light photographing II (Drawing 6); determining scope of reverse side etching.

[0025] (B) Glass Chip Production Flow:

[0026] Step 6: Yellow light photographing M (Drawing 7); determining metal electrode mold.

[0027] Step 7: Metal splash plated with chromium/platinum/chromium (Drawing 8)

[0028] Step 8: Getting rid of optical resistance (Drawing 9); forming metal electrode

[0029] Step 9: Glass splash plated (Drawing 10); forming metal electrode insulation layer

[0030] Step 10: Yellow light photographing IV (Drawing 11): determining exterior connection socket of metal electrode.

[0031] (C) Combination Production Flow:

[0032] Step 11: Anode combination (Drawing 12); combining silicon and glass chips.

[0033] Step 12: Etching of silicon reserve side (Drawing 13); forming silicon film.

[0034] The invention has following effects:

[0035] 1. Pressure Sensitivity Testing:

[0036] Drawing 14 shows capacitor values of structure and producing method of a three-electrode capacitive pressure sensor illustrated in Drawing 1 together with characteristics curve of exterior pressure. The result of experiment confirms that three electrode capacitive structure design has consisted with its expected effects:

[0037] (a) C_acand C_bccapacitor values are approximate to that of C_apand C_bp.

[0038] (b) Sensor capacitor C_acand C_apare more sensitive than that of reference capacitor C_bcand C_bpas pressure changes.

[0039] (c) Changes of sensor capacitor C_acof planar connection is consisted with sensor capacitor C_apof dimensional connection as pressure changes; i.e. the pressure sensor, designed by a three electrode capacitive structure, will not reduce pressure sensitivity of sensor.

[0040] (d) C_abis a planar connection of series-wound C_apand C_bp(as described in previous technology). Pressure sensitivity (0.05 pF/psi) of C_abreduces to 25% of pressure sensitivity of C_acor C_ap(0.21 pF/psi).

[0041] 2. Environment Interference Testing

[0042] A differentiated measured capacitor/voltage converter achieves this test. Test result of Table 1 shows experimental result of changing environment temperature under a fixed exterior pressure condition. V_ab-crepresents voltage output value of differentiated measurement (C_ac−C_bc) of sensor capacitor C_acand reference capacitor C_bc, and V_acrepresents output voltage value of differentiated measurement of sensor capacitor C_acand a fixed exterior-added reference capacitor C_O.

[0043] The test result shows C_acand C_bcwill increase values as temperature raises up. Therefore, V_abcdifferentiated measurement can cancel off environment change (ΔV_ab-c=3 mV) of same mode through differentiation method. V_acmeasurement has different changes of capacitor values due to different location and material between C_Oand C_acas temperature changes. As a result, V_acmeasurement can not cancel off environment change (ΔV_ac=29 mV) under different mode.

[0044] Many changes and modifications in the above-described embodiment of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, to promote the progress in science and the useful arts, the invention is disclosed and is intended to be limited only by the scope of the appended claims.