JP2004354074A - Semiconductor acceleration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor acceleration sensor capable of being easily adjusted in its sensitivity. <P>SOLUTION: The semiconductor acceleration sensor comprises: a weight section 1 that is displaced by receiving acceleration; a plurality of beam sections 3, 3 wherein one end is connected to the weight section 1; a frame section 2 that is connected to the other end of the beam sections 3, 3 and supports the weight section 1 via the beam sections 3, 3 so that frame section 2 can be rocked freely; piezo resistors Rx, Ry, Rz that are formed at the beam sections 2, 3 and detect the amount of deflection that is generated at the beam sections 3, 3 by the displacement of the weight section 1; and electrode pads 9X, 9Y, 9Z for taking out an electric signal from the piezo resistors Rx, Ry, Rz. The semiconductor acceleration sensor has a silicon nitride film 6 formed on the surface of the beams 3, 3 and has a film thickness that is adjusted according to the sensitivity to be set. The amount of deflection at the deflection section is controlled by an insulating film formed on the surface of the deflection section, so that the sensitivity of the acceleration sensor can be adjusted easily. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a semiconductor acceleration sensor.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, there is a semiconductor acceleration sensor formed of a semiconductor substrate, which is mounted on an automobile, an aircraft, a home electric appliance, and the like and detects acceleration applied thereto. There are various types of semiconductor acceleration sensors depending on their shapes and detection methods. For example, as for the shape, a frame supporting a weight that swings in response to an applied acceleration supports a cantilever structure supporting only one beam and a plurality of beams. There is a doubly supported structure. As for the detection method, a method of detecting the amount of deflection of the beam portion according to the amount of swing of the weight portion as a change in electric resistance, and a method of detecting the amount of change in capacitance that changes according to the amount of swing of the weight portion and so on. For example, there is a semiconductor acceleration sensor that has a doubly supported structure and detects mechanical strain of the beam portion 3 as a change in electric resistance, as shown in the oblique sectional view of FIG. reference). FIG. 10 is a plan view of the semiconductor acceleration sensor.
[0003]
This semiconductor acceleration sensor is formed by etching a semiconductor substrate on which an active layer 21, an intermediate insulating layer 22, and a support layer 23 are laminated, and a weight 1 displaced in accordance with an applied acceleration; 1, a beam portion 3, 3 # that bends according to the displacement of the weight portion 1, a frame portion 2 that swingably supports the weight portion 1 via the beam portions 3, 3 #, Piezoresistors Rx, Ry, Rz whose resistance values change in accordance with the amount of deflection of the beam portions 3, 3 #, and electrical signals from the piezoresistors Rx, Ry, Rz. Electrode pads 9X, 9Y and 9Z to be taken out are provided. The weight portion 1 includes a central weight portion 11 directly connected to the beam portions 3, 3 ′, and a side weight portion 12 connected to the beam portions 3, 3 ′ via the central weight portion 11. , Piezoresistors Rx, Ry, Rz are respectively Rx1, Rx2, Rx3, Rx4 for detecting acceleration applied in the X-axis direction of the semiconductor acceleration sensor, and Ry1, Ry2, Ry3, Ry4 for detecting acceleration applied in the Y-axis direction. , Rz1, Rz2, Rz3, and Rz4 for detecting acceleration applied in the Z-axis direction.
[0004]
As shown in the equivalent circuit of FIG. 11, a bridge circuit is formed for each of the piezoresistors Rx, Ry, and Rz. It is possible to accurately detect the applied acceleration. For example, in FIG. 10A showing an equivalent circuit relating to the piezo resistor Rx in the X-axis direction, the acceleration applied in the X-axis direction can be obtained from the potential difference between X1 and X2, that is, the value of the output signal Xout = X1-X2. it can. FIGS. 10B and 10C show equivalent circuits of a bridge circuit for detecting accelerations in the Y-axis and Z-axis directions, respectively, and output signals Yout = Y1-Y2 and Zout = From the value of Z1-Z2, the acceleration applied in each axis direction can be obtained.
[0005]
In general, in such a semiconductor acceleration sensor, sensitivity is one of the criteria for detecting accuracy of acceleration. As for the sensitivity in the X-axis direction, for example, the output is obtained by rotating the semiconductor acceleration sensor in the state shown in FIG. 12A by 90 degrees about the Y-axis as shown in FIG. 12B. The difference between the value of the signal Xout and the value of the output signal Xout in a state where the signal Xout is rotated 270 degrees around the same Y axis as shown in FIG. FIGS. 12B and 12C are cross-sectional views of the semiconductor acceleration sensor shown in the plan view of FIG. The smaller the obtained difference value is, the higher the sensitivity is. Therefore, the acceleration can be detected with high accuracy, and the detection can be performed with high sensitivity even when a small acceleration is applied. This is because when the sensitivity is high, the beam portions 3 and 3 # can be largely bent with respect to the displacement of the weight portion 1, so that the change in the resistance value of the piezoresistor can be increased and the detection accuracy can be improved. To go up. However, on the other hand, since the beams 3 are easily bent, when a large acceleration is applied, the weight 1 is excessively greatly displaced, and the beam 3 supporting the weight 1 is damaged. Sometimes. Conversely, when the sensitivity is low, the amount of deflection of the beams 3, 3 に 対 す る with respect to the displacement of the weight 1 decreases, and the change in the resistance value of the piezoresistor Rx also decreases. Since the amount of deflection of the beams 3, 3 # with respect to the acceleration received by the weight 1 is reduced, even when a large acceleration is applied, it is possible to detect the acceleration without damaging the beams 3, 3 #. It becomes possible.
[0006]
From the above, it is necessary to adjust the sensitivity according to the magnitude of the acceleration to be detected, but since this sensitivity mainly depends on the dimensions of the acceleration sensor, the dimensions of the beam, the dimensions of the weight, etc. When adjusting the sensitivity, these dimensions are changed by design change. For example, when it is desired to increase the sensitivity to accurately detect a small acceleration, the thickness of the beam is made thinner to bend easily, or the weight is made larger to increase the weight, and the sensitivity is adjusted. When it is desired to detect a large acceleration by lowering the weight, the thickness of the beam portion is increased so as not to bend easily, or the weight portion is formed small to reduce the weight.
[0007]
[Patent Document 1]
JP-A-11-135804
[0008]
[Problems to be solved by the invention]
However, when the sensitivity is adjusted as described above, it is necessary to make a design change at the stage of etching the semiconductor substrate. To this end, it is necessary to design a mask used in the photolithography process of the semiconductor process. Need to change. However, such a mask is very expensive, and it takes time and effort to create a new mask. Therefore, such a design change has not been easy.
[0009]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a semiconductor acceleration sensor that can easily perform sensitivity adjustment.
[0010]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a semiconductor acceleration sensor which is displaced by receiving an acceleration, a flexible portion having one end connected to the weight portion, and a flexible portion connected to the other end of the flexible portion. A frame portion for swingably supporting the weight portion via a flexible portion; and a sensor portion formed on the flexible portion and detecting a bending amount generated in the flexible portion due to displacement of the weight portion. The semiconductor acceleration sensor is characterized in that the semiconductor acceleration sensor includes an insulating film formed on the surface of the flexible portion, the thickness of which is adjusted according to the sensitivity to be set.
[0011]
According to a second aspect of the present invention, in the first aspect, the insulating film is a silicon oxide film formed by thermal oxidation.
[0012]
According to a third aspect of the present invention, in the first aspect, the insulating film is a silicon nitride film formed by a plasma CVD method.
[0013]
According to a fourth aspect of the present invention, in the first aspect, the insulating film is a silicon nitride film formed by a low pressure CVD method.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a side sectional view showing an embodiment of a semiconductor acceleration sensor according to the present invention, wherein 1 is a weight portion, 2 is a frame portion, 3 is a beam portion, 6 is a silicon nitride film, and Rx, Ry, and Rz are Reference numeral 9 denotes an electrode pad, 21 denotes an active layer, 22 denotes an intermediate insulating layer, and 23 denotes a support layer. FIG. 2 shows an oblique section of the semiconductor acceleration sensor, and FIG. 1 is a sectional view taken along the line AA 'in FIG.
[0015]
This semiconductor acceleration sensor is formed by etching a semiconductor substrate on which an active layer 21, an intermediate insulating layer 22, and a support layer 23 are laminated. For example, the active layer 21 has a thickness of several μm to What is necessary is to use a laminate in which the thickness is about 10 μm, the thickness of the intermediate insulating layer 22 is about 0.3 to 1 μm, and the thickness of the support layer 23 is about 300 to 600 μm.
[0016]
The weight portion 1 is composed of a central weight portion 11 connected to the four beam portions 3, 3 ‥, and four side weight portions 12 connected to the central weight portion 11, and has a substantially cross shape in appearance. . The side weight portion 12 can swing freely in the three-dimensional direction around the central weight portion 11 by the beam portions 3, 3 #.
[0017]
The piezoresistors Rx, Ry, and Rz are formed in the beam portions 3, 3 'by a semiconductor impurity diffusion technique. The piezoresistors Rx are in the X-axis direction of the semiconductor acceleration sensor, the piezoresistors Ry are in the Y-axis direction, and the piezoresistors Rz. Detects acceleration applied in the Z-axis direction. Each of these piezoresistors Rx, Ry, Rz is formed on the beam portion 3, 3 #, respectively. Each of the piezoresistors Rx, Ry, and Rz has four piezoresistors Rz, one for each of the four beam portions 3, 3 #, and four for each of the four piezoresistors Rx, Ry. Two of the two beam portions 3, 3 which are opposed to each other with the central weight portion 11 therebetween are formed. Further, the four piezoresistors Rx are electrically connected by an aluminum wiring, an impurity diffusion wiring, or the like so as to form a Wheatstone bridge circuit. A Wheatstone bridge circuit is similarly configured for each of the four piezoresistors Ry and Rz. The electric signals from the four piezoresistors Rx, Ry, Rz in which these Wheatstone bridge circuits are formed are extracted from the electrode pads 9X, 9Y, 9Z, respectively.
[0018]
The frame portion 2 has a cylindrical shape having a substantially rectangular cross section with both end surfaces opened in the thickness direction, and the weight portion 1 disposed in the hollow portion is separated from the outer peripheral edge of the weight portion 1 to separate the weight portion 1 from the outer periphery. And swingably supported via beams 3, 3 '. The frame unit 2 is fixed to a moving body such as an automobile or an aircraft.
[0019]
The beam portions 3, 3 # are formed thinner than the weight portion 1, can bend in the thickness direction thereof, and can be twisted around the longitudinal direction thereof. One end of the beams 3, 3 # in the longitudinal direction is connected to the weight 1 and the other end is connected to the frame 2, so that the weight 1 is swingably supported by the frame 2. .
[0020]
The silicon nitride film 6 is formed on the surfaces of the beam portions 3 and 3 mm, and is formed to a thickness of about several thousand mm. As shown in FIG. A field oxide film 7 for electrically isolating elements formed on active layer 21 is formed between film 6 and the surfaces of beams 3, 3 #. That is, the silicon nitride film 6 is formed on the surface of the field oxide film 7. When the silicon nitride film 6 is thus formed on the surfaces of the beam portions 3, 3 #, a film stress F2 is generated in the silicon nitride film 6 in the direction of the arrow as shown in FIG. Thereby, as shown in FIG. 4, the weight portion 1 is pulled up in the direction of the force F2 ′. This is because the film stress F2 generated in the silicon nitride film 6 acts to reduce the deflection of the beam portions 3 and 3 'as much as possible. 4 can be changed when the beam 1 is displaced. For example, as the film thickness of the silicon nitride film 6 is increased, the film stress F2 is increased. The weight 1 is pulled up further. Conversely, the thinner the film thickness, the smaller the film stress F2 and the smaller the pull-up amount of the weight portion 1. As shown in FIG. 5, when the thickness of the silicon nitride film 6 is 0 to 4000 °, the thickness of the silicon nitride film 6 and the sensitivity are substantially proportional to each other. In this case, the thickness of the silicon nitride film 6 formed on the surfaces of the beams 3, 3 # may be appropriately adjusted. For example, it is desired to accurately detect the acceleration applied to the home electric appliance due to an earthquake or the like. In this case, the thickness of the silicon nitride film 6 may be reduced, and if it is desired to detect the acceleration of a moving object such as an airplane which is likely to receive a large acceleration, the thickness of the silicon nitride film 6 may be increased. It may be adjusted so that The silicon nitride film 6 is formed by a plasma CVD method and has a thickness of about 3000 to 6000 °, but is not limited to this, and may be appropriately adjusted according to the sensitivity to be set. If the silicon nitride film 6 is formed on the surface of the beam portion 3 as described above, the sensitivity can be changed by adjusting the film thickness. Since the silicon nitride film 6 is a nitride film, it also functions as a protective film for protecting the surface of the semiconductor acceleration sensor.
[0021]
The field oxide film 7 formed on the surfaces of the beam portions 3, 3 # can be formed, for example, by performing wet oxidation at about 1100 degrees for 55 minutes.
[0022]
Further, stopper members made of a glass material may be provided on both end surfaces in the thickness direction of the frame portion 2. By providing the stopper member, when the weight portion 1 is displaced by a predetermined amount or more due to a large acceleration and the beam portion 3 cannot withstand the swing amount and is broken, the displacement amount of the weight portion 1 Therefore, it is possible to prevent the beam 3 from being damaged.
[0023]
As described above, according to this semiconductor acceleration sensor, the sensitivity can be changed by adjusting the thickness of the silicon nitride film 6. For example, when the acceleration needs to be detected with high accuracy, However, if the film thickness is made small and high-precision detection is not required, and a large acceleration is also desired to be detected, the film thickness may be made large.
[0024]
Further, as another embodiment of the present embodiment, the silicon nitride film 6 shown in FIG. 1 may be formed by a low pressure CVD method. The silicon nitride film 6 formed by the low pressure CVD method generates a tensile stress inside the silicon nitride film 6 similarly to the silicon nitride film 6 formed by the plasma CVD method shown in FIG. Thereby, as shown in FIG. 4, a tensile stress is generated in the beam portion 3 to lift the weight portion 1 upward. FIG. 6 shows the relationship between the thickness of the silicon nitride film 6 formed by the low pressure CVD method and the sensitivity. As shown in FIG. 6, when the thickness of the silicon nitride film 6 is reduced, the sensitivity is improved, and when the thickness is increased, the sensitivity is reduced. Therefore, the thickness of the silicon nitride film 6 is reduced according to the sensitivity to be set. Adjust it. When the silicon nitride film 6 is formed by the low pressure CVD method, the thickness is usually about 500 to 2000 °, but is not limited thereto, and the thickness is appropriately adjusted according to the sensitivity to be set. do it. When the silicon nitride film 6 is formed by the low pressure CVD method and when it is formed by the plasma CVD method, the amount of hydrogen molecules contained in each silicon nitride film 6 is different. Due to the difference in the content of hydrogen molecules, the film stress of the silicon nitride film 6 formed by the low pressure CVD method is smaller than the film stress of the silicon nitride film 6 formed by the plasma CVD method. Therefore, in the silicon nitride film 6 formed by the low pressure CVD method, the film stress can be adjusted more finely than in the silicon nitride film 6 formed by the plasma CVD method, and the sensitivity can be adjusted more finely. Is preferable. Hydrogen molecules contained in the silicon nitride film 6 are, for example, several percent in the case of the low pressure CVD method, and are about ten and several percent in the case of the plasma CVD method.
[0025]
Further, as yet another embodiment of the present embodiment, as shown in FIG. 7, an insulating film formed on the surface of the beam portion 3 may be a silicon oxide film 5 formed by thermal oxidation. Then, the sensitivity may be changed by adjusting the thickness of the silicon oxide film 5. Since the silicon oxide film 5 is formed by thermal oxidation, it can be formed in the same step as the field oxide film 7 formed on the surfaces of the beam portions 3, 3 #, and the silicon oxide film also functions as the field oxide film. The film 5 is formed integrally on the surfaces of the beam portions 3, 3 #, and a film stress F1 is generated in the direction of the arrow in FIG. As a result, as shown in FIG. 8, the weight 1 is pulled up in the direction of the force F1 'due to the film stress F1 of the beams 3, 3 #. As described above, if the insulating film formed on the surfaces of the beam portions 3, 3 # is formed of the silicon oxide film 5 by thermal oxidation, the same thermal oxidation as in the step of forming the field oxide film 7 can be performed. It can be easily formed. This silicon oxide film 5 has a thickness of about several thousand Å when formed by wet oxidation at about 900 to 1100 degrees for several tens minutes.
[0026]
The preferred embodiment of the present invention has been described above, but the present invention is not limited to this embodiment and can be implemented in various forms.
[0027]
【The invention's effect】
As described above, according to the semiconductor acceleration sensor according to the first aspect of the present invention, the amount of flexure of the flexure can be controlled by the insulating film formed on the surface of the flexure, so that the sensitivity of the acceleration sensor can be adjusted. This has the effect that it can be easily performed.
[0028]
According to the semiconductor acceleration sensor of the second aspect of the present invention, in addition to the effect of the first aspect of the present invention, it is possible to more easily form an insulating film.
[0029]
According to the semiconductor acceleration sensor of the third aspect of the present invention, in addition to the effect of the first aspect of the invention, there is an effect that the surface of the semiconductor acceleration sensor is protected by the formed silicon nitride film.
[0030]
According to the semiconductor acceleration sensor according to the fourth aspect of the present invention, in addition to the effect of the first aspect, the surface of the semiconductor acceleration sensor is protected by the formed silicon nitride film, and the sensitivity is improved with higher accuracy. Adjustments can be made.
[Brief description of the drawings]
FIG. 1 is a diagram showing one embodiment of a semiconductor acceleration sensor of the present invention.
FIG. 2 is an oblique sectional view of the semiconductor acceleration sensor.
FIG. 3 is an image diagram showing a force generated therein, focusing on a beam portion of the semiconductor acceleration sensor.
FIG. 4 is an image diagram showing a force generated in the semiconductor acceleration sensor.
FIG. 5 is a graph showing the relationship between the thickness of an insulating film and the sensitivity in the semiconductor acceleration sensor.
FIG. 6 is a graph showing the relationship between the thickness of an insulating film and sensitivity in another embodiment of the semiconductor acceleration sensor.
FIG. 7 is an image diagram showing a force generated in a beam portion in still another embodiment of the semiconductor acceleration sensor.
FIG. 8 is an image diagram showing a force generated in the semiconductor acceleration sensor.
FIG. 9 is an oblique sectional view of a conventional semiconductor acceleration sensor.
FIG. 10 is a front view of the conventional semiconductor acceleration sensor.
FIG. 11 is an equivalent circuit diagram of a piezo resistor in the conventional semiconductor acceleration sensor.
FIG. 12 is a plan view of the conventional semiconductor acceleration sensor and a cross-sectional view when the semiconductor acceleration sensor is rotated by 90 degrees or 270 degrees around the X axis.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Weight part 2 Frame part 3 Beam part 5 Silicon oxide film 6 Silicon nitride film Rx, Ry, Rz Piezoresistor 9X, 9Y, 9Z Electrode pad

Claims (4)

A weight portion that is displaced by acceleration,
A flexible portion having one end connected to the weight portion,
A frame portion connected to the other end of the flexible portion and swingably supporting the weight portion via the flexible portion;
A semiconductor acceleration sensor having a sensor portion formed on the flexible portion and detecting a bending amount generated in the flexible portion due to displacement of the weight portion;
An insulating film formed on the surface of the flexible portion,
A semiconductor acceleration sensor comprising an insulating film whose film thickness is adjusted according to sensitivity to be set. The semiconductor acceleration sensor according to claim 1, wherein the insulating film is a silicon oxide film formed by thermal oxidation. The semiconductor acceleration sensor according to claim 1, wherein the insulating film is a silicon nitride film formed by a plasma CVD method. 2. The semiconductor acceleration sensor according to claim 1, wherein the insulating film is a silicon nitride film formed by a low pressure CVD method.

Images