RU2774181C1 - Microvacuummeter - Google Patents

Abstract

FIELD: vacuum measuring equipment.

SUBSTANCE: invention relates to vacuum measuring equipment for measuring the vacuum level in microcavities, microvolumes and housings of sensors of microsystem technology, in particular to microvacuum meters using the principle of resonance as the main mechanism of operation. In a microvacuum gauge with a sensitive element made on the base and consisting of a resonator oscillating along the plane of the base and connected to it by means of four suspensions; a comb control electrode forming an interdigital structure with the comb electrode of the resonator, and signal electrodes for signal reading. The base and the resonator are made of single-crystal silicon, the suspensions are movable along the base plane and contain two combined mutually perpendicular spring elements, the resonator contains a system of additional dampers.

EFFECT: increasing the range of pressure measurement towards high vacuum and the stability of the microelectromechanical vacuum gauge.

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Description

The invention relates to vacuum measuring equipment for measuring the vacuum level in microcavities, microvolumes and housings of sensors of microsystem technology, in particular to microvacuum meters using the principle of resonance as the main mechanism of operation.

The lack of control in measuring pressure in evacuated housings or microcavities after their manufacture complicates the analysis of the performance of microelectromechanical devices. Residual gases that may exceed the set threshold may render the sensor completely inoperable or cause incorrect output characteristics. Among the available options for measuring the vacuum level in microvolumes, there are microsystem vacuum gauges based on the use of a microelectromechanical resonator in a sensitive element (SE).

Micromechanical part of the vacuum gauge in the works: Squeeze-film damping in the free molecular regime: model validation and measurement on a MEMS / H. Sumali, J. Micromech. Microeng. Vol. 17 (2007) P. 2231-2240; Electrostatically driven vacuum encapsulated polysilicon resonators. Part I: Design and fabrication / R. Legtenberg and H.A.C. Tilmans, Sens. Actuators A, Vol. 45 (1994) P. 57-66; Damping of a microresonator torsion mirror in rarefied gas ambient / A. Minikes, I. Bucher. G. Avivi, J. Micromech. Microeng, Vol. 15 (2005) P. 1762-1769; Model-based design of MEMS resonant pressure sensors / Suijlen, M.A.G. Eindhoven: Technische Universiteit Eindhoven (2011) P. 136; USA 7047810 B2 (May, 2006); EP 1530036 B1 (April, 2007) is a structure consisting of a fixed base - a supporting silicon plate, usually covered with a layer of thin metal, and a movable membrane, fixed on elastic suspensions, overlapping the base plane and spaced at some distance above it. In the sources cited, this design is referred to as a plenary resonator. The disadvantage of this technical solution, on the one hand, is the use of heterogeneous materials in the manufacturing process: the upper part of the resonator - the movable membrane is made of thin metal, as in the works: Squeeze-film damping in the free molecular regime: model validation and measurement on a MEMS / H. Sumali, J. Micromech. Microeng. Vol. 17 (2007) P. 2231-2240 and EP 1530036 B1 (April, 2007), a single-crystal silicon base, which, with temperature changes, can adversely affect the resulting output characteristics, in particular, lead to drift, hysteresis, and oscillation stall. Others: Electrostatically driven vacuum encapsulated polysilicon resonators, Part I: Design and fabrication / R. Legtenberg and H.A.C. Tilmans, Sens. Actuators A, Vol. 45 (1994) P. 57-66; Model-based design of MEMS resonant pressure sensors / Suijlen, M.A.G. Eindhoven: Technische Universiteit Eindhoven (2011) P. 136 and USA 7047810 B2 (May, 2006) the use of polycrystalline silicon as the main structural material for the furnace membrane can adversely affect the strength characteristics of the resonator: initial deformations, cracks, imbalance and mixing are possible oscillation mode. There are devices described in Squeezed film damping measurements on a parallel-plate MEMS in the free molecule regime / L. Mol, L.A. Rocha, E. Cretu. R.F. Wolffenbuttel, J. Micromech. Microeng. (J. Mieromechanies and Microengineering) Vol. 19, Issue (7), (2009) 074021 P. (1-6); A study on wafer level vacuum packaging for MEMS devices / B. Lee, S. Seok. K. Chun, J. Micromech. Microeng. Vol. 13 (2003) P. 663-669 and Model-based design of MEMS resonant pressure sensors / Suijlen, M.A.G. Eindhoven: Technische Universiteit Eindhoven (2011) P. 136, in which the microelectromechanical resonator is presented in a vertical design, i.e. resonator oscillations are carried out along the plane of its base. The excessive rigidity of such structures reduces the sensitivity of the sensor, thereby reducing the vacuum measurement range. The lower limit of the measured values for the listed structures is 1-10 pascals.

The closest in technical essence to the claimed technical solution is a micropressure sensor for measuring vacuum, described in the article A study on wafer level vacuum packaging for MEMS devices / B. Lee. S. Seok, K. Chun. J. Micromech. Microeng. Vol. 13 (2003) P. 663-669 (prototype). The sensitive element of the vacuum gauge is a resonator, in which the movable inertial mass (IM) with the smallest height dimension is fixed on the side faces on four suspensions on both sides above the base, in pairs on each side. On the free faces of the MI, there is an interdigital structure of electrodes for setting the control and readout signals. Therefore, SE is a vertical microelectromechanical resonator. Oscillations are carried out along the plane of the base, where the greatest damping effect from the residual gas is carried out in the gaps of the interdigital structure with the maximum deviation from the equilibrium position at the time of resonance. The vacuum level is measured by measuring the quality factor of the system - Q. The disadvantage of this design is the low sensitivity to the vacuum level, the lower limit of the range of measured values is limited to a value of units of pascal, due to low damping between the interacting surfaces, their insufficient number, excessive rigidity in the places of the resonator termination.

The objective of the invention is to increase the sensitivity of the microvacuummeter, the design of which will allow measuring small pressure values in microcavities and microvolumes of microelectromechanical devices, increasing the stability of the microvacuummeter.

The technical result of the proposed solution is to increase the pressure measurement range towards high vacuum and the stability of the microvacuum gauge.

The technical result is achieved by the fact that in a microvacuum gauge with a sensitive element made on the base and consisting of a resonator oscillating along the plane of the base and connected to it by means of four suspensions; comb control electrode, which forms an interdigital structure with the comb electrode of the resonator, and signal electrodes for signal reading, the base and the resonator are made of single-crystal silicon, the suspensions are movable along the base plane and contain two mutually perpendicular spring elements, the resonator contains a system of additional dampers.

The proposed technical solution is illustrated by the following figures.

The figure 1 shows a schematic view of the sensitive element of the microvacuum gauge, where

1 - base of single-crystal silicon;

2 - resonator;

3 - comb control electrode;

4 - signal electrodes for signal reading (signal electrodes);

5 - contact pads.

The figure 2 shows the functional elements of the resonator of the sensitive element of the microvacuum gauge, where

6 - resonator suspensions;

6a - spring elements of suspensions;

7 - comb electrode of the resonator;

8 - damper system.

The figure 3 shows the plots of the quality factor on the vacuum level for the experimental sample, the proposed microvacuum gauge, where the solid line shows the calculated dependence, and the dots show the experimental values of the quality factor.

The microvacuum gauge contains a sensitive element made on the basis of single-crystal silicon 1, consisting of a resonator 2, a comb control electrode 3, signal electrodes for signal reading 4 and contact pads 5 (Fig. 1). Resonator 2 (Fig. 1), which oscillates along the base plane, consists of several functional elements: four suspensions with spring elements 6, a comb electrode 7, and a system of dampers 8 (Fig. 2). The comb control electrode 3 forms an interdigital structure with the comb electrode of the resonator 7.

The resonator 2 of the sensitive element of the microvacuum gauge and the base 1 are made of single-crystal silicon. The resonator 2 is connected to the base 1 by means of four suspensions with spring elements 6 (Fig. 2). Suspensions 6 contain two combined mutually perpendicular spring elements with two directions of oscillation in the same plane, thereby providing the necessary displacement of the resonator 2 to manifest the damping effect. The system of dampers 8 (Fig. 2) allows, when the resonator itself deviates from the equilibrium position, to achieve the minimum distance between the moving and fixed parts of the sensitive element, is designed to increase the maximum area of interaction of the resonator 2 elements with gas molecules and thus obtain the maximum value of the useful component signal. On the comb control 3 and signal 4 electrodes and the resonator 2 there are contact pads 5 for applying and reading the signal.

The device works as follows.

A control signal is applied to the comb control electrode 3. In this case, the resonator 2 deviates from the equilibrium position and oscillates. The resonator 2 and the fixed structural parts of the sensitive element at a certain point in time are at a fairly close distance from each other. Thus, the mechanism of viscous damping in thin compressed gas films is activated, due to compression and friction on the surface of the residual gas in the interdigital structure formed by the resonator comb electrode 7 and the comb control electrode 3, as well as between the system of dampers 8 and signal electrodes 4. Useful the signal is taken from the signal electrodes 4. The vacuum level is measured by calculating the quality factor of the system - Q using the oscillation amplitude damping method according to the formula:

where τ is the relaxation time during which the initial amplitude decreases by a factor of e, ƒ ₀ is the resonant frequency of oscillations.

Modeling and measurement of an experimental sample of a microvacuum gauge was carried out, plots of the quality factor versus vacuum level are shown in figure 3. The calculated values are indicated in figure 3 by a solid line, the experimental values are indicated by dots. During the experimental determination of the dependence of the quality factor on pressure, the correctness of the calculated model of the vacuum gauge was confirmed.

The microvacuummeter, due to the design of the sensitive element, has increased stability and allows you to measure the vacuum level with high sensitivity over a wide range of values.

Claims (1)

Microvacuum gauge with a sensitive element, made on the base and consisting of a resonator, oscillating along the plane of the base and connected to it by means of four suspensions; a comb control electrode, which forms an interdigital structure with the comb electrode of the resonator, and signal electrodes for reading the signal, characterized in that the base and the resonator are made of single-crystal silicon, the suspensions are movable along the base plane and contain two mutually perpendicular spring elements, the resonator contains a system of additional dampers.

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