RU2280320C1 - Thermally excited microresonator - Google Patents

Abstract

FIELD: silicon resonators.

SUBSTANCE: proposed thermally excited microresonator has supporting frame and oscillatory body in the form of silicon beam incorporating resistive heater. Silicon beam is 5 - 10 mm in length, its ends are fixed integral with carrying frame but electrically insulated from the latter; it has sections of reduced cross-sectional area, 30 - 50 μm in width and 5 - 15 μm in thickness, at ends and in center that function as articulated joints, central of them functioning also as thermistor and piezoresistor. Silicon beam body is non-uniformly alloyed so that resistivity of its resistive part is 100 - 1000 times higher than that of its remaining part functioning as current leads with contact pads disposed on opposite ends of frame; two parts of beam having higher cross-sectional area are used as inertial masses which can be enlarged by attaching additional weight.

EFFECT: enhanced efficiency, enlarged frequency and wave amplitude range, enhanced sensitivity and precision of microresonator.

1 cl, 1 dwg

Description

The invention relates to measuring equipment, and more particularly to converters of various physical parameters to electrical.

The use of resonators as universal converters is based on the fact that the frequency of natural vibrations depends not only on the shape, size and material of the beam or other resonating body, but also on many external conditions: pressure, composition, ambient temperature, linear and angular accelerations, state surfaces etc. The measurement of the resonant frequency can therefore serve as a measure of the listed and many other influences.

The excitation of resonator vibrations is possible due to various types of energy, but the most consistent with the microelectronic execution of the device is thermal, because its generation requires only a microresistor.

A known design of a silicon single crystal resonator with thermal excitation, in which the oscillations are created due to uneven thickness of the one-sided heating of the resonating beam using a resistor deposited on one of its sides, resulting in its pure bending [1]. In this design, the oscillating body - the silicon cantilever, made at the same time with the supporting frame, has a thickness of not less than 200 microns, because only in this case it is possible to create the temperature gradient necessary for deformation, i.e. the heated body is “thermally thick”.

Also known is the design of the resonator with optical excitation of vibrations, which we adopted as a prototype, but in it the radiant energy is converted into heat, and the deformation occurs precisely for the same reason as in the purely thermal version [2]. A characteristic feature of this design is that the oscillating body has a large (by the standards of micromechanics) thickness, and the deformation is a pure bend.

This device has the following disadvantages: the temperature of the heated body cannot be measured by modern means, and the lack of control of this important parameter of the device requires the use of an external temperature control system, which is associated with a sharp increase in inertia, energy consumption and mass;

the large thickness of the resonator body, due to heat transfer inside its volume, leads to the excitation of high-frequency oscillations of small amplitude, so the output signal is small, which explains the low sensitivity and accuracy of the sensor;

Creating a temperature gradient requires a waste of heat, since heating the volume only reduces deflection. The device is energy-intensive, and this complicates its use in a portable version.

The aim of the invention is to increase efficiency, expand the frequency range and amplitude of oscillations, and thereby increase the sensitivity and accuracy of the microresonator.

This goal is achieved by the fact that the silicon beam is made 5-10 mm long with pinched ends at the same time with the supporting frame, but isolated from it electrically, has at the edges and in the center sections of the reduced cross section with a width of 30-50 and a thickness of 5-15 microns, performing the role of hinges, the central one also serving as a thermo- and piezoresistor, and the body of the silicon beam is alloyed nonuniformly so that the resistivity of the resistive part is 100-1000 times higher than the rest of it, which is current leads with contact pads, p located on opposite sides of the frame, and two parts of the beam having an increased cross section serve as inertial masses, which can be increased due to the additional fixed load.

The device diagram is presented in the drawing, where:

1 - thermo and piezoresizer;

2 - inertial mass;

3 - pseudo-hinge;

4 - contact pad;

5 - supporting frame.

The novelty of the claimed invention lies in the fact that the current flows over the entire cross section of the beam, causing heating and elongation, leading to longitudinal bending of large amplitude, since the deformation stress is large and the moment of inertia of the cross section is extremely small (10 ^-21 -10 ^-24 m ⁴ ). The specified limits are due to: the lower - the mechanical strength of the beam; upper - the limit value of elastic deformation σ _pr

The microcavity having the dimensions indicated above is characterized by the giant heat transfer and thermophoresis effects discovered by the authors [3], which results in extremely low inertia, which is usually unusual for thermal processes, and the heating-cooling cycle can be as little as 10 ^-3 s, which allows pulse power to change the frequency of oscillations in the entire kilohertz range and tune the system to resonance in accordance with the inertial mass.

In the range of carrier concentrations in silicon 10 ¹⁴ -10 ¹⁷ cm ³ in the resistive section of the heater, its current-voltage characteristic clearly reveals the extreme temperature corresponding to the transition from impurity to intrinsic conductivity. Using an electronic circuit that uses this point as a reference, the temperature can be stabilized in the range of 140-380 ° C, regardless of the ambient temperature.

According to the authors, the present invention meets the criterion of "inventive step".

The microresonator operates as follows.

When a current pulse is applied to the resistor contacts and heated due to Joule heat, longitudinal compressive stresses arise, which, combined with the thermophoresis force directed toward the cold case, lead to longitudinal-transverse bending. This type of deformation provides an increase in efficiency compared to the prototype, because the oscillatory body contains mechanical stresses of only one sign, while in the prototype one side is stretched, and the other is compressed. A significant - 10-15 times - decrease in the thickness of the resonating body in comparison with the prototype provides a decrease in the resonant frequency and amplitude by 100-200 times and approximately the same increase in sensitivity

Example.

The resonator in the form of a frame with a transverse jumper — an oscillating body having a cross section in narrowed areas of 15 × 15 μm ² — was made of single-crystal silicon with a specific resistance of 7 Ω cm with a transition temperature from impurity to intrinsic conductivity of 180 ° С (reference point). The inertial mass was 300 mg.

Heating was carried out by current pulses of 1 ms, and the power was 50 mW.

The resonance frequency determined using a tensometer is 500 Hz, the amplitude of the oscillations is 200 μm. A comparison of the parameters is given in the table.

Prototype device Claimed device Frequency range, kHz 100 ... 200 0.5 ... 1 Amplitude, microns 0.1 Up to 200 Power consumption, mW 1000 fifty Stability (leaving,% within 100 hours) There is no data one

Information sources

1. T.S. J. Lammerink, M. Elsenspoek. "Performance of Thermally exited Resonators." Sensors and Actuators, v. A 21-23, 1990, 352-356.

2. L. M. Zhang, D. Uttamchandani, B. Culshaw. Excitation of silicon microresonators using short optical pulses. Sensors and Actuators A-21-A-23, 1990, 391-393. - prototype.

3. Reports of the Academy of Sciences of the Russian Federation, 1999, No. 2, p. 361

Claims (1)

A microcavity with thermal excitation, containing a support frame and an oscillating body in the form of a silicon beam with a resistive heater, characterized in that the silicon beam is made 5-10 mm long with pinched ends integral with the carrier frame, but electrically isolated from it, has edges and in the center there are sections of a reduced cross section with a width of 30-50 and a thickness of 5-15 microns that act as hinges, the central one also serving as a thermo- and piezoresistor, and the body of the silicon beam is not uniformly doped so that the specific e the resistance of the resistive part is 100-1000 times higher than the rest of it, which is current leads with contact pads located on opposite sides of the frame, and the two parts of the beam having an increased cross section serve as inertial masses that can be increased due to additional fixed cargo.