Residual pressure measurement system for Fabry-Perot cavity of optical MEMS pressure sensor and method thereof

Abstract

The present invention discloses a residual pressure measurement system for a MEMS pressure sensor with an F-P cavity and method thereof, the measurement system includes a low-coherence light source, a 3 dB coupler, a MEMS pressure sensor, an air pressure chamber, a thermostat, a pressure control system, a cavity length demodulator, an acquisition card and a computer. The measurement method comprises: performing cavity length measurement by using the reflecting light by the pressure control system at two temperatures, respectively, so as to calibrate the MEMS pressure sensor and establish a relationship between the absolute phase of a monochromatic frequency and the external pressure; performing linear fitting to the two measurement data to obtain all the external pressure when the cavity length of two measurement data are equal to each other, and substituting the theoretical equation for calculation to obtain the residual pressure under the flat condition of the diaphragm.

Claims

1. A residual pressure measurement system for a MEMS pressure sensor with an F-P cavity, including a low-coherence light source, a 3 dB coupler, a MEMS pressure sensor, an air pressure chamber, a thermostat, a pressure control system, a cavity length demodulator, an acquisition card and a computer; wherein the MEMS pressure sensor is arranged in the air pressure chamber, and the air pressure chamber is sealed; the pressure in the air pressure chamber is controlled by the pressure control system to scan an external pressure; the pressure control system includes a pressure controller, a vacuum pump and an air compressor, the pressure control system and the air pressure chamber, and the devices in the pressure control system are communicated with each other via pipelines; the MEMS pressure sensor comprises an F-P cavity composed of a substrate and a diaphragm, and residual pressure is sealed in the F-P cavity; light output from an optical fiber is partially reflected for the first time on a reflective coating to form a reflected reference light; and a rest light is transmitted to a diaphragm inner surface for a second reflection to form a reflected sensing light; the reflected reference light and the reflected sensing light form an interference signal having an optical path difference (OPD); a gas sealed in the F-P cavity expands or contracts with the change of the temperature, which has an effect on the diaphragm deflection; and the reflected reference light and the reflected sensing light pass through the 3 dB coupler and enter the cavity length demodulator, and the results after demodulation is input to the computer for further data processing via the acquisition card.

2. A residual pressure measurement method for a MEMS pressure sensor with an F-P cavity, comprising the following steps: Step 1: performing a pressure calibrating experiment at a first temperature T.sub.1, including: scanning an external pressure of the diaphragm by a pressure control system, a scanning first external pressure is P.sub.E1, and performing demodulation to obtain the relationship between a cavity length and the external pressure at the first temperature T.sub.1; Step 2: performing a pressure calibrating experiment at a second temperature T.sub.2, including: scanning the external pressure of the diaphragm by the pressure control system, a scanning second external pressure is P.sub.E2, and performing demodulation to obtain the relationship between the cavity length and the external pressure at the second temperature T.sub.2; Step 3: performing linear fitting to data obtained in the step 1 and step 2, the cavity length changes continuously within the pressure range of the scanning, and the value of each cavity length corresponds to a group of external pressure P.sub.E1 and P.sub.E2 at two temperature; Step 4: substituting each successive group of P.sub.E1 and P.sub.E2 into a equation $P_{R1}=\left(P_{E2}-P_{E1}\right)\frac{T_1}{T_2-T_1}$ to obtain a curve that the residual pressure P.sub.R1 varies with the external pressure P.sub.E1; the other curve describes the equation P.sub.R1=P.sub.E1, and obtaining a horizontal axis of the intersection which is the intersection of two curves; and Step 5: obtaining the residual pressure at an equilibrium state at the temperature T.sub.1 by subtracting an error VP.sub.R1 of the material of a substrate caused by the temperature expansion from the value of the horizontal axis of the intersection; where $V{P}_{R1}=\frac{h{\alpha }_g{T}_1}{S_2},$ h is the depth of the F-P cavity, α.sub.g, is the thermal expansion coefficient of the substrate, and S.sub.2 is a pressure sensitivity of the sensor obtained in the step 2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings illustrate one or more embodiments of the present disclosure and, together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.

(2) FIG. 1 is a structural diagram of a MEMS pressure sensor according to the present invention;

(3) FIG. 2 is a schematic diagram of a MEMS pressure sensor in an equilibrium state of internal and external pressure according to the present invention;

(4) FIG. 3 is a schematic diagram of the residual pressure measurement system for an F-P cavity of the MEMS pressure sensor according to the present invention;

(5) FIG. 4 is a diagram showing the results of the residual pressure measurement method for a F-P cavity of the MEMS pressure sensor according to the present invention, wherein

(6) FIG. 4(a) shows the cavity length results of the MEMS pressure sensor at two different temperatures measured according to the present invention;

(7) FIG. 4(b) is an illustrative graph showing the calculation process and calculation results of the residual pressure of the MEMS pressure sensor according to the present invention; and

(8) FIG. 5 is the diagram of the 50 times residual pressure measurement results of the MEMS pressure sensor according to the present invention.

(9) Wherein:

(10) TABLE-US-00001  1: substrate  2: Diaphragm  3: F-P cavity  4: external pressure  5: residual pressure  6: Reflective coating  7: diaphragm inner surface  8: Ferrule  9: Optical fiber 10: reflected reference light 11: reflected sensing light 12. Equilibrium state 13: low-coherence light source 14: 3 dB coupler 15: Thermostat 16: MEMS pressure sensor 17: Air pressure chamber 18: pipeline 19: Vacuum pump 20: Air compressor 21: Pressure controller 22: Computer 23: Acquisition card 24: Cavity length demodulator 25: measurement data intersection 26: Diaphragm flat area 27: Horizontal axis of the intersection

DETAILED DESCRIPTION OF THE EMBODIMENTS

(11) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.

(12) Embodiments of the invention are illustrated in detail hereinafter with reference to accompanying drawings. It should be understood that specific embodiments described herein are merely intended to explain the invention, but not intended to limit the invention.

(13) The present invention will be described in detail below with reference to the drawings in conjunction with the embodiments. The invention is not limited to the specific embodiment, and the description is not intended to limit the thereto.

(14) As shown in FIG. 1, the MEMS pressure sensor comprises an F-P cavity 3 composed of a substrate 1 provided on a ferrule 8 and a diaphragm 2, residual pressure 5 is sealed in the F-P cavity 3; and an optical fiber 9 is inserted into the ferrule 8. Light output from the optical fiber 9 is partially reflected for the first time on a reflective coating 6 to form a reflected reference light 10; and the rest light is transmitted to a diaphragm inner surface 7 for a second reflection to form a reflected sensing light 11. The reflected reference light 10 and the reflected sensing light 11 form an interference signal having an optical path difference (OPD) which is twice the corresponding cavity length. The gas sealed in the F-P cavity expands or contracts with the change of the temperature, which has an undesired effect on the diaphragm deflection. Moreover, the diaphragm deflection is also affected by factors such as thermal stress at the bonding interface between the substrate 1 and the diaphragm 2, and different elastic mechanical parameters of the diaphragm 2 at different temperatures. The diaphragm 2 is an edge clamped round diaphragm, the deflection variable of which under residual pressure 3, external pressure 4 and thermal stress is expressed as:

(15) $\begin{array}{cc}\omega =\frac{{\left(a^2-r^2\right)}^2\left(P_E-P_R\right)}{64D\left(1+\xi \right)},& \left(1\right)\end{array}$
where a is the radius of the F-P cavity; r is the radial distance from the center of the F-P cavity to the center of the optical fiber; D is the flexural rigidity of the diaphragm which is affected by the temperature; ξ is the compensation factor for the deflection caused by the thermal stress, which is affected by the temperature; P.sub.E is the external pressure, and P.sub.R is the residual pressure.

(16) The relationship between residual pressure and the temperature can be described by the ideal gas law as follows:
P.sub.RV=nRT  (2)
where n is the number of gas molecules, T is temperature of gas, which is in unit of K, and R is the ideal gas constant.

(17) Under the temperatures T.sub.1 and T.sub.2, and the diaphragm deflections equals to ω.sub.2 and equals to 0, the F-P cavity volume V.sub.1=V.sub.2, and the corresponding derived equation is as follows:

(18) $\begin{array}{cc}{P}_{R1}=\left(P_{E2}-P_{E1}\right)\frac{T_1}{T_2-T_1}& \left(3\right)\end{array}$

(19) Where P.sub.R1 is the residual pressure at the temperature T.sub.1, P.sub.E1 and P.sub.E2 are the external pressures when the diaphragm deflections are equal at the temperature T.sub.1 and T.sub.2, respectively.

(20) The formula (3) is established by:
P.sub.R1P.sub.E1  (4)

(21) which indicates that when the diaphragm deflection is zero, that is, under the equilibrium state 12 as shown in FIG. 2, the pressure on the internal and external sides of the diaphragm will be equal, and the residual pressure 5 is independent of flexural rigidity D and compensation factor ξ. In addition, the impact of thermal expansion of the substrate 1 on the cavity length has been ignored in the formulas (3) and (4), so the error ΔP.sub.R1 needs to be subtracted in calculation.

(22) $\begin{array}{cc}V{P}_{R1}=V\left(P_{E2}-P_{E1}\right)\frac{T_1}{T_2-T_1}=\frac{\mathrm{Vh}}{S_2}\frac{T_1}{T_2-T_1}=\frac{h{\alpha }_g{T}_1}{S_2}& \left(5\right)\end{array}$
Where h is the depth of the F-P cavity 3, α.sub.g is the thermal expansion coefficient of the substrate 1, and S.sub.2 is the pressure sensitivity of the sensor at the temperature T.sub.2.

(23) As shown in FIG. 2, when the diaphragm 2 is in the equilibrium state 12 of internal and external pressure, that is, under the flat condition, and diaphragm deflection is zero, the residual pressure 5 is obtained by equations (3), (4) and (5). FIG. 3 shows the residual pressure measurement system for a MEMS pressure sensor with a F-P cavity. The system of the present invention comprises a low-coherence light source 13, a 3 dB coupler 14, a MEMS pressure sensor 16, an air pressure chamber 17, a thermostat 15, a pressure control system, a cavity length demodulator 24, an acquisition card 23 and a computer 22.

(24) Wherein, the MEMS pressure sensor 16 is arranged in the air pressure chamber 17 and the air pressure chamber 17 is sealed. The pressure in the air pressure chamber 17 is controlled by the pressure control system to scan the external pressure 4. The pressure control system, which is operated by the computer 22, includes a pressure controller 21, a vacuum pump 19 and an air compressor 20, the devices in the pressure control system are communicated with each other via pipelines 18.

(25) The residual pressure P.sub.R1 at the temperature T.sub.1 is calculated by the following: measuring the relationship between the cavity length and external pressure through experiments, calibrating the sensor to obtain the external pressure P.sub.E1 and P.sub.E2 when the diaphragm deflections are equal to 0 at the temperature T.sub.1 and T.sub.2 respectively. The detailed steps are as follows:

(26) Step 1: a pressure calibrating experiment is performed at the temperature the temperature T.sub.1 of the thermostat 15 is set to 273K for 2 h to stabilize the temperature in the air pressure chamber 17, and the pressure in the air pressure chamber 17 is controlled by the pressure controller 21 within the range from 10 kPa to 50 kPa, the scanning interval is 1.0 kPa and the time interval of the change of the pressure is 2 min, and the demodulation is performed simultaneously with scanning. The cavity length demodulation is based on the principle of low-coherence interference; the light from the low-coherence source is coupled to the optical fiber 9, and then passes through the 3-dB coupler 14 to enter the MEMS pressure sensor 16. The reflected light signal from the optical fiber which includes a cavity length corresponding to the MEMS pressure sensor is re-coupled back to the optical fiber 9; the reflected light passes through the 3 dB coupler 14 to enter the cavity length demodulator 24, and the results after demodulation is input to the computer 22 for further data processing via the acquisition card 23, so that a relationship between the absolute phase of a monochromatic frequency and the external pressure P.sub.E1 at the temperature to be measured is established, as shown by the circle in FIG. 4(a).

(27) Step 2: a pressure calibrating experiment is performed at the temperature T.sub.2, the temperature T.sub.2 of the thermostat 15 is set to 323K for 2 h to stabilize the temperature in the air pressure chamber 17, and the pressure in the air pressure chamber 17 is controlled by the pressure controller 21 within the range from 10 kPa to 50 kPa, the scanning interval is 1.0 kPa and the time interval of the change of the pressure is 2 min, and the demodulation is performed simultaneously with scanning A relationship between the absolute phase of a monochromatic frequency and the external pressure P.sub.E2 at the temperature to be measured is established, as shown by the square shape in FIG. 4(a).

(28) Step 3: data obtained in the step 1 and step 2 perform linear fitting, as shown in FIG. 4(a), and the fitted linearity is 0.99996. The absolute phase changes continuously within the pressure range from 10 kPa to 50 kPa after the linear fitting, and the value of each phase corresponds to a group of the external pressure P.sub.E1 and P.sub.E2.

(29) Step 4: as shown in FIG. 4(b), each successive group of the external pressure P.sub.E1 and P.sub.E2 is substitute into equation (3)

(30) $P_{R1}=\left(P_{E2}-P_{E1}\right)\frac{T_1}{T_2-T_1}$
to obtain a curve that the residual pressure P.sub.R1 varies with the external pressure P.sub.E1; the other curve describes the equation (4) P.sub.R1=P.sub.E1, an intersection of the two curves form the measurement data intersection 25, that is, the diaphragm flat area 26, which has a horizontal axis 27 of 30.063 kPa.

(31) Step 5: the error VP.sub.R1 of the substrate 1 caused by the temperature expansion is calculated to obtain

(32) $V{P}_{R1}=\frac{h{\alpha }_g{T}_1}{S_2}=0.802kPa,$
wherein cavity depth h=26 μm, the pressure sensitivity of the MEMS sensor S.sub.2=28.578 nm/kPa at the temperature of 323K, the thermal expansion coefficient of the substrate 1 α.sub.g=3.23×10.sup.−6/K; and the residual pressure 5 at the equilibrium state 12 at the temperature 273K is obtained as 29.261 kPa by subtracting 0.802 kPa from the horizontal axis 27 of the intersection 30.063 kPa.

(33) The temperature is selected as follows: T.sub.1 is the temperature of the calculated residual pressure, T.sub.2 is the reference temperature, and the values of T.sub.1 and T.sub.2 should be selected to have a large temperature difference, which are in unit of K. The scanning range of the external pressure 4 is selected as follows: selecting the vicinity of the residual pressure of the MEMS pressure sensor, and performing fine scanning at small intervals. The pressure compensation equation is

(34) $V{P}_{R1}=\frac{h{\alpha }_g{T}_1}{S_2},$
where h is the depth of the F-P cavity, α.sub.g is the thermal expansion coefficient of the substrate 1, and S.sub.2 is the pressure sensitivity of the sensor obtained in the step 2.

(35) In order to further verify the stability of the measurement system of the present invention, experiments have been carried out for 50 times to collect 50 frames of interference signals, so that 50 residual pressure data are obtained and shown in FIG. 5. The average value of the 50 times experiments is 29.278 kPa and the standard deviation is 0.046 kPa, which shows that the measurement system of the present invention has high stability.

(36) Although the functions and working processes of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited thereto. The foregoing specific implementations are merely illustrative but not limiting. A person of ordinary skill in the art may make various forms under the teaching of the present invention without departing from the purpose of the present invention and the protection scope of the appended claims, and all the forms shall fall into the protection scope of the present invention.

(37) The foregoing description of the exemplary embodiments of the present disclosure has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

(38) The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present disclosure pertains without departing from its spirit and scope. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.

(39) The foregoing description of the exemplary embodiments of the present invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

(40) The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.