RU2799390C1 - Sensitive element of capacitive micromechanical pressure sensor - Google Patents

Abstract

FIELD: sensors.

SUBSTANCE: invention relates to capacitive micromechanical sensors for absolute and gauge pressure of gases and liquids. A sensitive element of a capacitive micromechanical pressure sensor comprises a membrane that deforms under pressure and acts as a movable electrode, a base on which the membrane is fixed, and a cover on which a fixed electrode is located. The membrane is formed in a structural layer of single-crystal silicon with low resistivity and is completely isolated from the bulk of the material of the structural layer by means of an annular trench. The base and cover are made of high resistivity monocrystalline silicon. The stationary electrode is in the form of a thin metal film.

EFFECT: increasing the dynamic range of the pressure sensor by reducing the noise level of the processing circuit as a result of minimizing the parasitic capacitances of the sensing element.

1 cl, 5 dwg

Description

The invention relates to capacitive sensors for absolute and gauge pressure of gases and liquids, in particular, micromechanical sensors, and can be used in various fields of science and technology related to measuring pressure in a medium.

Known capacitive pressure sensors manufactured by surface technology, for example, in the patent of the Russian Federation No. 2324159, the sensitive element of the capacitive pressure sensor of liquid and gaseous media is considered, which is a fixed electrode located on the substrate, a layer of X-ray sensitive dielectric and a movable electrode. At the same time, cavities are formed in the X-ray sensitive dielectric layer by deep lithography, and the movable electrode in the form of a thin-film membrane is hermetically fixed on the dielectric. This makes it possible to simplify the design of the sensitive element of the pressure sensor and improve its manufacturability, as well as improve the accuracy of pressure measurement by increasing the area of the sensitive element while reducing parasitic capacitances.

The disadvantage of the proposed design is the relatively large value of the nominal capacitance of the sensor (about 10 pF, excluding parasitic capacitances). It is due to the fact that the reduction of parasitic capacitances is achieved by increasing the thickness of the dielectric layer. In this case, the gap between the electrodes also increases, which leads to a decrease in sensitivity. In order to maintain sensitivity, the authors have to increase the area of the electrodes. In addition, despite the claimed reduction in parasitic capacitances, their value remains at a high level also due to the large area of the sensitive element.

A large value of the sum of the nominal and parasitic capacitances leads to an increase in the noise level in the output signal processing circuit of the sensitive element, thereby limiting the dynamic range of the sensor, and also leads to a limitation in the speed of the circuit and an increase in power consumption.

US Pat. No. 1,0183,857 describes the construction of a capacitive sensing element in which the membrane can be formed both by surface technology and by volume. For the purpose of electrical isolation, ring trenches are formed around the electrodes.

The disadvantage of the proposed design is unoptimized parasitic capacitance between the structural layer and the base, as well as between the structural layer and the cover. The presence of large parasitic capacitances also leads to the following consequences:

- reducing the dynamic range of the sensor,

- limiting the speed of the circuit,

- increase in consumed current and power.

The technical problem to be solved is the creation of a design of the sensitive element of a capacitive micromechanical pressure sensor with minimized parasitic capacitances while maintaining the required value of the nominal capacitance.

The achieved technical result is an increase in the dynamic range of the sensor by reducing the noise level of the processing circuit as a result of minimizing the parasitic capacitances of the sensing element.

The solution of the problem in the present invention is achieved due to the fact that in the sensitive element of the capacitive micromechanical pressure sensor, the membrane deforming under pressure and acting as a movable electrode is formed in the structural layer of single-crystal silicon with low resistivity and is completely isolated from the bulk of the structural material. layer by means of an annular trench with an optimal width, while the base, on which the membrane is fixed, and the cover, on which the fixed electrode is located, formed in the form of a thin metal film, are made of single-crystal silicon with high resistivity, which minimizes parasitic capacitances between the membrane and the base, between the membrane and the cover, as well as between the membrane and the bulk of the structural layer.

The essence of the invention, its feasibility and the possibility of industrial application are illustrated in Fig.1-5, illustrating an example of the proposed sensor.

Figure 1 shows a block diagram of the design of the sensing element of the capacitive micromechanical pressure sensor in the section passing through the center of the membrane (contact output from the membrane is shown).

Figure 2 shows a top view of the design of the sensitive element of the capacitive micromechanical pressure sensor (the base, the structural layer and the output of the contact from the membrane are shown; the dark color shows the metallization on the cover, the light color shows on the structural layer).

Figure 3 shows a block diagram of the design of the sensing element capacitive micromechanical pressure sensor in the section passing through the center of the fixed electrode located on the cover (contact output from the electrode on the cover is shown).

Figure 4 shows a top view of the design of the sensitive element of a capacitive micromechanical pressure sensor (the base, the structural layer, the electrode and the output of the contact from the electrode on the cover are shown; the dark color shows the metallization on the cover, the light color shows on the structural layer).

Figure 5 shows a diagram of the capacitance-voltage converter on switched capacitors, taking into account the parasitic capacitance of the sensing element of the micromechanical pressure sensor.

Designations in Fig.1-5:

1 - cover

2 - structural layer

3 - base

4 - insulator layer between base 3 and structural layer 2

5 - external pad

6 - fixed electrode

7 - insulator layer between fixed electrode 7 and cover 1

8 - internal pads

9 - electrical leads from internal pads 8 to external

10 - membrane (movable electrode)

11 - sealing seam

12 - annular trench around the membrane 10

13 - hole in cover 1

14 - anchor (part of the membrane 10, fixed on the base 3)

15 - air gap between the membrane (movable electrode) 10 and the fixed electrode 6

P _inside - pressure of the medium inside the SE

P _external - pressure of the external environment

A-A - section passing through the center of the membrane

B-B - section passing through the center of the fixed electrode located on the cover

C ₁ - electric capacitance of the electrode structure, F

C ₀ - nominal electric capacitance of the electrode structure, F

C _p - parasitic electric capacitance, F

C ₂ - electric capacitance of the tuning capacitor, F

C ₁ - electric capacitance in the feedback of the operational amplifier, F

S - key in the feedback of the operational amplifier, Ф

U _out - output voltage of the capacitance-voltage converter, V

U _1.2 - anti-phase rectangular pulse signals supplied to the input, V

The claimed design of the sensitive element (SE) in

the considered example of execution (see figure 1) is made from:

- silicon-on-insulator (SOI) structures, the bearing part of which is the base 3, and the upper part is the structural layer 2;

- the plate from which the cover is made 1.

The material of the lower part of the SOI structure and the plate that forms the cover is single-crystal silicon with the following parameters: thickness - 450 μm, crystal lattice orientation - <100>, doping - p-boron, resistivity - 12000 Ohm⋅cm.

The material of the upper part of the SOI structure is single-crystal silicon with the following parameters: thickness varies depending on the upper limit of pressure measurements, doping - p-boron, resistivity - 0.01-0.02 Ohm⋅cm.

Splicing of the SOI structure and the cover is carried out by the method of eutectic bonding, which provides high strength of the connection. In this case, the sealing seam 11 is located along the SE perimeter.

The main element of the SE - a round membrane 10 - is formed in the structural layer 2. The fixing of the membrane 10 on the base 3 is carried out by the anchor 14, which is an annular area around the membrane.

To measure the membrane deflection in the SE, an electrode structure is implemented, which includes a movable and a fixed electrode. The fixed electrode 6 is formed on the cover 1 by sputtering a metal layer onto the insulator layer 7. The membrane 10 itself, made of low resistivity silicon, acts as the movable electrode. An air gap 15 is formed between the electrodes, the size of which, together with the area of the electrodes, determines the sensitivity of the SE.

In order to electrically isolate the membrane 10 from the main part of the structural layer 2 and, as a result, to minimize parasitic parameters (capacitances and resistances), an annular trench 12 is formed around the membrane by deep ion-reactive etching. The output of electrical contact from the membrane 10 (see Fig.1 and 2) is carried out through the cover, for which internal contact pads 8 are formed on both sides of the annular trench. In this case, in order to avoid mechanical stresses caused by thermal deformations, the contact pad is located away from the membrane 10, on the anchor 14.

The output of the contact from the fixed electrode 6, located on the cover 1, to the structural layer 2 is also carried out using the internal contact pad 8, located away from the contact pads of the membrane (see Fig.3 and 4).

The device works as follows:

On the one hand, the pressure of the environment inside the SE acts on the membrane, and on the other hand, the pressure of the external environment supplied through the hole in the base 3. The pressure inside the SE is formed when the SE is installed in the outer housing (not shown in the figure), creating the required level inside the housing pressure (atmospheric or vacuum) and the communication of the body medium with the internal cavity of the SE using the hole 13 in the cover 1. Thus, the SE can be used both as part of a relative pressure sensor and as part of an absolute pressure sensor.

When the pressures are equal on both sides of the membrane 10, the electric capacitance of the electrode structure C ₁ is formed by the sum of the nominal capacitance C ₀ and the parasitic capacitance C _p .

The nominal capacitance is formed between the membrane 10 and the fixed electrode 6, is determined by the following relationship [Mohamed Gad-el-Hak. The MEMS Handbook, 2002. p.926]:

Where

- электрическая постоянная;

- electrical constant;

e is the permittivity of the medium between the electrodes;

A - electrode overlap area, m ² ;

d - width of the air gap, m;

Parasitic capacitance, by analogy with formula (1), is determined by the following relationship:

Where

А ₁ , А ₂ , А ₃ , are overlapping areas between structural layer 2 and cover 1, between structural layer 2 and base 3 and between stationary electrode 6 and cover 1, respectively, m2;

d ₁ , d ₂ , d ₃ are the widths of the gaps between the structural layer 2 and the cover 1, between the structural layer 2 and the base 3 and between the fixed electrode 6 and the cover 1, respectively, m.

In the presence of a pressure difference acting on the membrane 10 from opposite sides, it begins to sag. The deflection of the membrane leads to a change in the size of the air gap 15, which in turn leads to a change in the electrical capacitance between the membrane 10 and the fixed electrode 6.

To convert the capacitance change into the output voltage U _out, the pressure sensor output signal processing circuit includes an integrated capacitance-voltage converter, which is usually built according to the switched capacitor circuit (Fig. 5). The circuit includes a tuning capacitor with a capacitance C ₂ ≈ C ₁ , which is connected in parallel with C ₁ , as well as an operational amplifier (not shown in Fig.), In the feedback of which there is a key S and a capacitor with a capacitance C _f . During the measurement process, two opposite-phase rectangular pulse signals U ₁ and U ₂ are fed to the input C ₁ and C ₂ . As a result, a charge proportional to the change in capacitance is formed at the input of the operational amplifier. When the switch S is opened in the feedback, the total charge flows to the capacitance in the feedback, the value of the output voltage in this case can be described as follows:

Where

ΔC ₁ - change in the nominal capacitance of the electrodes in the presence of a pressure difference acting on the membrane from opposite sides, F;

C _f ≈ ΔС ₁ - capacitance in the feedback of the operational amplifier, F;

U ₀ - amplitude of a rectangular pulse signal, V.

The dynamic range of a sensor is characterized by the signal-to-noise ratio. For a switched capacitor circuit, the noise level can be estimated using the following formula [Kaajakari V. Practical MEMS. Las Vegas: Small Gear Publishing, 2009. p.478]:

Where

- постоянная Больцмана;

- Boltzmann's constant;

- температура, К;

- temperature, K;

- электрическая емкость электродной структуры, Ф;

- electric capacitance of the electrode structure, F;

- электрическая емкость подстроечного конденсатора, Ф;

- electric capacitance of the tuning capacitor, F;

- электрическая емкость в обратной связи операционного усилителя, Ф;

- electric capacitance in the feedback of the operational amplifier, F;

- изменение номинальной емкости электродов при действии давления.

- change in the nominal capacitance of the electrodes under the action of pressure.

Based on formula (4), the larger the capacitance C ₁ , the greater the noise level of the transducer and the smaller the dynamic range of the pressure sensor. In this case, the value of capacitance C _f due to the required value of the gain of the circuit, and can not be much more than ΔС ₁ . The increase in ΔC ₁ is also limited, since it is possible either by increasing C ₁ , which will lead to an increase in the noise level and limiting the dynamic range, or by increasing the non-linearity, which will lead to an increase in the pressure measurement error and also to a decrease in the dynamic range of the sensor.

Thus, in order to achieve a technical result in terms of increasing the dynamic range of the sensor by reducing the noise level of the processing circuit, it is necessary to reduce the parasitic capacitance value while maintaining the required nominal capacitance value, which is possible using the proposed invention.

According to the results of an electrical calculation performed by means of finite element modeling of SE with overall dimensions of 4 × 4 mm and a nominal capacitance of electrodes of 6 pF, in the case of the formation of an annular trench around the membrane with an optimized width of 50 μm, the value of parasitic capacitance decreased from 18.2 pF to 7 .7 pF; in the case of subsequent use of silicon with a high resistivity of 12000 Ohm⋅cm as the material of the cover and base, up to 3.2 pF. Thus, the total parasitic capacitance is reduced by a factor of 5.7, and the noise level is reduced by a factor of 1.6, which corresponds to an increase in dynamic range by 4 dB.

Thus, the claimed technical result is achieved in full.

Claims (1)

A sensitive element of a capacitive micromechanical pressure sensor, containing a membrane that deforms under pressure and acts as a movable electrode, a base on which the membrane is fixed, and a cover on which a fixed electrode is located, characterized in that the membrane is formed in a structural layer of single-crystal silicon with low resistivity and is isolated from the bulk of the material of the structural layer by means of an annular trench, while the base and cover are made of single-crystal silicon with high resistivity, and the stationary electrode is formed in the form of a thin metal film.

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