JPH0643179A - Acceleration sensor and method of manufacturing the sensor - Google Patents

Abstract

PURPOSE: To detect an acceleration both upward and downward in directions, while avoiding the bending of the resonator of a press load by disposing a flexible spring having symmetric structure, provided with upper and lower resonators in the center of a silicon platelet. CONSTITUTION: Structural parts for resonators 10, 11 are fabricated on the upper and lower sides 5, 6 of a silicon platelet 4. The range of a frame 2 and an oscillatory mass 3 is then masked, and a flexible spring 1 is manufactured by etching the platelet 4 and undercutting the structural part. When acceleration is generated in the vertical direction, the mass 3 is displaced. Consequently, a tensile load is generated in one of the resonators 10, 11, while a pressing load is generated in the other resonator, and the resonance frequencies of the resonators 10, 11 are varied. In the case of a tensile load, the frequency is a measure of the displacement of the mass 3, and thereby a measure of acceleration, and the acceleration can be detected. However in the case of a pressing load, the frequency is not a measure of acceleration. Since a symmetric structure is employed, one resonator can detect the acceleration without fail, while preventing the other pressing resonator from bending.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon acceleration sensor having at least one flexure spring, at least one resonator and at least one oscillating mass suspended on a frame, the sensor comprising: An acceleration sensor and a method for manufacturing the sensor, wherein acceleration is detected based on a change in frequency of the resonator, and the bending spring, the frame, and the oscillating mass are created by structuring a silicon small plate. .

[0002]

2. Description of the Related Art The well-known publication "Satechel et.
al. : Sensor and Actuator
S., 17 (1989) pp. 241 to 245 ", the following acceleration sensor is known: an oscillating mass is suspended in a frame by a flexure spring and a resonator, the acceleration being based on a change in the frequency of the resonator. Accelerometers that are detected by the method are known: the oscillating mass and the frame are made by structuring the silicon platelets, the resonator is located above the silicon platelets and the flexure spring is below the silicon platelets. The excitation of the vibration of the resonator is caused by the thermal expansion of the heat-generating component and is detected by the deposited piezoelectric strain gauge, however, this sensor is used when the detection of the sensor signal is In other words, it is difficult to detect the sensor signal when acceleration is generated in a direction in which the resonator is loaded by pressing.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the drawbacks of conventional acceleration sensors and to provide an acceleration sensor having an improved sensor signal detection capability and a method of manufacturing the sensor.

[0004]

SUMMARY OF THE INVENTION According to the present invention, a flexible spring is arranged in the center of a silicon platelet, and at least one resonator is arranged above and below the silicon platelet. Configured and resolved.

Further, according to the present invention, according to the present invention, a structural portion for the resonator is formed on the upper side and the lower side of the silicon small plate, the range for the frame and the vibrating mass is covered by masking, and the bending spring is formed from the silicon small plate. The problem is solved by performing etching processing and undercutting the structural portion for the vibrator.

The advantage obtained by the acceleration sensor according to the characterizing part of claim 1 of the present invention is that the acceleration sensor is
It has a symmetrical structure with at least one flexure spring in the center of the silicon platelet and one resonator each on the upper side and the lower side of the silicon platelet. The symmetric structure achieves the following. That is, it is achieved that at least one of the plurality of resonators is subjected to a tensile load when acceleration occurs in a direction perpendicular to the upper side or the lower side. This is particularly advantageous. This is because bending of the pressure-loaded resonator is avoided thereby. Due to the symmetrical structure with the neutral axis passing through the center between the two resonators, lateral sensitivity is low. An advantage obtained by the method according to claim 7 of the present invention is that the acceleration sensor can be manufactured using conventional micro-configuration techniques.

Advantageous configurations and improvements of the acceleration sensor and the method of manufacturing the sensor according to the invention are described in the dependent claims. Aging of a resonator made of silicon is slight, and its material property data is very well known. Further, such a resonator can be manufactured with high precision. Resonators or flexure springs made of a dielectric material can be manufactured particularly precisely. The resonator is particularly simple as a flexure bar. Excitation of the resonator is achieved by a structured piezoelectric film deposited on the resonator. This excitation causes only a small amount of heat-induced strain. In this case, the vibration of the resonator is particularly easily excited by the following structure. That is, it is easily excited by a structure in which the piezoelectric film is a constituent element of the oscillator. By comparing the frequency of the upper resonator and the frequency of the lower resonator, the frequency shift due to heat is removed by the averaging effect. This is because this deviation occurs evenly in both resonators. The manufacture of the sensor is particularly simple if the thickness of the flexure spring is determined by the etching process time. This is because relatively few processing steps are required for manufacturing.
If the production is carried out by joining two silicon platelet halves, a highly precise adjustment of the thickness of the flexure spring can be achieved. In this case, the flexible spring is made of a silicon material or a piezoelectric material.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings.

In FIG. 1, a silicon platelet is shown at 4. The frame 2 and the vibrating mass 3 are created by structuring the silicon small plate 4. This vibrating mass 3 is
The flexible spring 1 in the center of the silicon small plate 4 and the resonators 10 and 11 provided on the upper side 5 and the lower side 6 are joined to the frame 2. A piezoelectric film 7 having a lead conductor 8 is arranged on the resonator. An insulating portion 9 is arranged between the lead conductor 8 and the upper side or the lower side of the silicon small plate 4.

FIG. 2 shows a plan view of the acceleration sensor. FIG. 1 corresponds to the cross-sectional view taken along the broken line I-I in FIG. From the plan view of FIG. 2, the oscillating mass 3 shows the bending spring 1
And that the resonator 10 on the upper side 5 is connected to the frame 2. The resonator 1 is provided below the bending spring 1.
Corresponding to 0, another resonator 11 for the lower side 6 is provided. For simplicity, the lead conductor 8 for the piezoelectric film 7 and the insulation for the upper surface 5 are not shown in the plan view.

The frame 2 is fixedly connected to the object. The acceleration of this object should be measured perpendicular to the upper side 5 or the lower side 6. The oscillating mass is the flexure spring 1
And is suspended only on the resonators 10 and 11. In this case, the rigidity of the flexible spring 1 is designed as follows. That is, the oscillating mass 3 is set so as to be displaced from the rest position of the oscillating mass 3 when acceleration occurs in a direction perpendicular to the upper side 5 to the lower side 6. This displacement causes mechanical stress in the two resonators 10, 11. At that time, a tensile load is generated on one of the resonators and a pressing load is generated on the other. This 2
The resonance frequency of the resonator changes based on one load. Mechanical vibrations are excited in the resonators 10, 11, and 12 by applying a voltage to the piezoelectric film 7. The vibration amplitude of this mechanical vibration is particularly large when the excitation frequency corresponds exactly to the natural vibration of the resonator. The corresponding feedback coupling excites the resonator as follows. That is, the resonator is excited to resonate, that is, to vibrate in the primary natural vibration of the bending bar. This frequency changes depending on the tensile load or pressure load. This frequency is a measure for the displacement of the oscillating mass, ie for acceleration. In the case of a tensile load, the change in frequency is a measure for the tensile stress up to the breaking point of the resonator. In the case of a pressing load, bending of the resonator occurs at a comparatively small pressing load, so that a non-linear rigidity change appears with respect to the pressing load. In this case, the change in frequency does not serve as a measure for the applied pressure load. This means the following: That is, it means that the signal when the resonator is tension-loaded can be evaluated over a much wider acceleration range than the signal when the resonator is pressure-loaded. It is therefore advantageous to arrange one resonator on each of the upper side 5 and the lower side 6 in order to ensure that one resonator is always under tensile load. Furthermore, due to the symmetrical structure, the bending of the second resonator (the change in the rigidity of this resonator becomes non-linear) which is pressed and loaded is reliably avoided.

1 and 2, the resonators 10 and 11 are constructed as a single flexure bar. However, instead of this single flexure bar, it is likewise possible to use multiple flexure bar components, for example the triple oscillator or the double oscillator described in the abovementioned Satchell. Further, when it is guaranteed that the bending spring (flexing bar) 1 has a pressing stress on one side and a tensile stress on the other side, other arrangements or configurations of the bending bar 1 are possible. .

The acceleration sensor is manufactured by forming a bending spring 1, a frame 2 and a vibrating mass 3 made of a silicon small plate 4. In this case, it is utilized that the etching of silicon when using a basic etching solution strongly depends on the crystal orientation. The sensor shown here comprises an oscillating mass 3 having a rectangular plane, for example 100-
Etching is performed from a silicon small plate having a crystal orientation. However, it is also possible to use silicon platelets with different crystallographic orientations. For the production of the sensor, a structured membrane for the resonator is deposited on the silicon platelet 4. This structured membrane is undercut in order to create the flexure spring 1, the frame 2 and the oscillating mass 3 from the silicon platelet 4 by structuring.

The surface of the silicon platelet 4 is shown in FIG. The structure 21 for the resonator 10 is attached to this surface. In this case, the resonator 10 must have a predetermined orientation with respect to the crystal structure of the silicon platelet 4. In the case of etching silicon with a basic etching solution, the crystal planes of silicon bounded by the etching form the 111-plane. This one
The 11-plane intersects the surface of the silicon platelet 4 at a predetermined angle. If the straight edge lying parallel to the 111-plane is provided on the silicon platelet by a non-etching material, this non-etching layer is not undercut. That is, this layer acts as an etching mask. Therefore, in order to be undercut during anisotropic etching, the resonator 10 must have a predetermined angle 18 with respect to the 111-direction of the silicon platelet 4. In this case, the size of the angle 18 depends on the length and width of the resonator 10. The material for the structure 21 for the resonator 10 should not be etched at all or only slightly by the basic etching solution for the silicon platelet 4. This compatible material is, for example, a dielectric film made of silicon oxide or silicon nitride. Furthermore, the very strongly doped silicon is compatible with the silicon forming the pn-junction to the rest of the silicon platelet 4. Heavily doped silicon is only slightly corroded by the basic etching solution. p
At the n-junction, etching is suppressed by applying a voltage.

The thickness of the flexure tongue can be adjusted by controlling the etching process time. At that time, if the etching is finished in a timely manner, a predetermined thickness can be left in the flexible spring 1. Another means of adjusting the thickness of the flexure spring 1 is shown in FIG.

FIG. 4 shows two silicon platelet halves at 22 and 23. The two silicon platelet halves 22, 23 are joined together as indicated by the arrow. An etching mask 25 is provided on the side facing each other. The two silicon plate halves 22, 23 are joined together as follows. That is, the structural portions 24 for the flexure spring 1 are joined so as to be adjusted and positioned relative to each other. However, it is likewise possible that only one of the two silicon platelet halves 22, 23 is provided with a structure 24 for the flexure spring 1. Two
The joining of the two silicon plate halves 22, 23 is carried out by the so-called bonding process. In this process, the silicon platelet halves 22 and 23 are chemically treated and overlaid on one another and subjected to a heat treatment at a temperature below 400 °. The structure 24 for the flexure spring 1 can also be produced by corresponding taping of the silicon platelet halves 22,23. During strong doping, the structure 24 for the flexure spring is stable against basic etching solutions without any further measures, and during weak doping the structure 24 is stable.
And pn between the rest of the silicon plate halves 22 and 23 again.
The junction must be constructed and a corresponding voltage applied. It is also conceivable that the structure 24 is made of a dielectric material such as silicon oxide or silicon nitride. These dielectric materials are stable to basic etching solutions without alternative means. The silicon platelet 4 formed by the combined construction of the two silicon platelet halves 22, 23 is etched by immersion in a basic etching solution. At that time, the etching mask 25 is oriented as follows with respect to the crystal structure. That is, the etching mask 25 is oriented so that it is undercut, at all or only slightly. If the silicon platelets 4 consist of silicon with an orientation of 100, the sidewalls of the oscillating mass are etched, as indicated here by the dashed lines.
The sidewall has an angle of about 55 ° with the surface.

FIG. 5 illustrates the evaluation of the sensor signal.
The flexure spring 1 is also shown in this figure. This flexure spring 1
The vibrating mass 3 is fixed to the frame 2. Upper 5
The resonator 10 is arranged on the lower side 6 and the resonator 1 is arranged on the lower side 6.
1 is arranged. Each of these resonators 10 and 11 has a piezoelectric film 7 and is a component that determines the frequency of the oscillator 15. Due to the corresponding feedback between the oscillator 15 and the resonators 10, 11, the oscillator 15 is
It vibrates at the natural frequency of the flexure spring 1 of 0 and 11. In this case, the desired vibration mode is selected by the corresponding bandpass filtering of the oscillator 15. The frequencies at which the two oscillators 15 oscillate are compared with each other in the comparator 16. The difference between the two frequencies of the oscillator 15 forms the output signal. Such a detection method of the output signal has the following advantages. That is, there is an advantage that the frequency shift of the resonators 10 and 11 depending on the temperature is suppressed. This is because this frequency shift has the same effect on the two resonators.

[0018]

The advantage obtained by the acceleration sensor according to the characterizing part of claim 1 of the invention is that the acceleration sensor has at least one flexing spring in the center of the silicon platelet and one resonator for each silicon. It has a symmetrical structure on the upper side and the lower side of the small plate. The symmetric structure achieves the following. That is, it is achieved that at least one of the plurality of resonators is subjected to a tensile load when acceleration occurs in a direction perpendicular to the upper side or the lower side. As a result, bending of the pressure-loaded resonator is avoided. The lateral sensitivity is low due to the symmetrical structure with the neutral axis passing through the center between the two resonators.

An advantage obtained by the method of the invention is also that the acceleration sensor can be manufactured using conventional micro-configuration techniques.

According to the invention, the temperature-dependent frequency deviation of the resonator is suppressed.

[Brief description of drawings]

1 is a cross-sectional view of an acceleration sensor according to the present invention.

FIG. 2 is a plan view of a resonator relative to a wafer.

FIG. 3 is a diagram showing an arrangement configuration of resonators relative to a wafer.

FIG. 4 is a diagram showing a configuration of two silicon small plate halves.

FIG. 5 is a diagram illustrating a procedure for sensor evaluation.

[Explanation of symbols]

 1 Bending Spring 2 Frame 3 Vibration Mass 4 Silicon Small Plate 5 Upper 6 Lower 7 Piezoelectric Film 8 Lead Conductor 9 Edge 10 Resonator 11 Resonator 15 Oscillator 16 Comparator 21 Structure 22 Silicon Small Plate Half Piece 23 Silicon Small Plate Half Piece 24 Structure 25 Etching mask

Claims (12)

[Claims] 1. An acceleration of silicon having at least one flexure spring (1), at least one resonator (10-12) and at least one oscillating mass (3) suspended on one frame (2). A sensor, in which acceleration is detected based on a change in the frequency of the resonator (10-12), and the bending spring (1), the frame (2), and the vibrating mass (3) are An acceleration sensor made by structuring the silicon platelets (4), wherein the flexure spring (1) is arranged in the center of the silicon platelets (4) and of at least one resonator (10-12). An acceleration sensor, characterized in that each is arranged on the upper side (5) and the lower side (6) of the silicon platelet (4). 2. The flexure spring (1) and / or the resonator (10-12) is made of silicon and comprises an oscillating mass (3).
The acceleration sensor of claim 1, wherein the acceleration sensor is differently doped. 3. The acceleration sensor according to claim 1, wherein the resonator (10-12) and / or the bending spring (1) is made of a dielectric material. 4. The resonator (10-12) according to claim 1, wherein the resonator (10-12) is configured as a flexure bar.
The acceleration sensor described. 5. Acceleration sensor according to claim 4, characterized in that a structured piezoelectric film (7) is deposited on the resonator (10-12). 6. A resonator (10) for the piezoelectric film (7).
~ 12) are components of the oscillator (15), the frequency of the upper (5) resonator (10-12) being compared with the frequency of the lower (6) resonator (10-12). Item 5
The acceleration sensor described. 7. The method of manufacturing an acceleration sensor according to claim 1, wherein the resonator (10-12) is provided on an upper side (5) and a lower side (6) of the silicon small plate (4). A structural part (21) for the frame (2) and the range for the vibrating mass (3) are covered with a masking (25), and the bending spring (1) is etched from the silicon small plate (4). A method of manufacturing an acceleration sensor, comprising undercutting a structural portion (21) for the vibrator (10 to 12). 8. Structures (21) for the resonators (10-12) are oriented at an angle (18) with respect to the plane of 111 of the silicon platelet (4), whereby anisotropic etching is performed. Method according to claim 7, characterized in that the structure (21) for the resonator (10-12) can be undercut. 9. The flexure spring (1) is made of silicon with the same doping properties as the oscillatory mass (3) and the thickness of the flexure spring (1) is defined by the etching period. The method described. 10. The silicon platelet (4) is manufactured by joining two silicon platelet halves (22, 23),
9. The method according to claim 7, wherein at least one of said silicon platelet halves (22, 23) has a structure (24) for the flexure spring (1) on the side facing each other. . 11. Method according to claim 10, characterized in that the structure (24) for the flexure spring (1) is made of silicon with a doping characteristic different from that of the silicon platelet halves (22, 23). 12. The method according to claim 10, wherein the flexure spring (1) is made of a dielectric material.