JP2006086318A - Electromechanical transducer and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the manufacture of a sensor using a piezoresistor results in higher costs. <P>SOLUTION: In the electromechanical transducer, a mechanically movable mover piece has an element for transducing mechanical fluctuations into electric fluctuations, an anchor having a neck, a beam for supporting the anchor; and a beam connecting base where the anchor contacts the beam. The manufacturing method unites the steps of forming the beam, and of forming the anchor to properly use three etching steps of different characteristics which are a second anisotropic etching step, a third isotropic etching step, and a fourth anisotropic etching step. It can further select materials to simultaneously form the beam and anchor in a small number of steps, and to simplify the manufacturing steps. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a structure that converts mechanical vibration into an electrical signal or a structure that converts an electrical signal into mechanical vibration, a structure that can improve the detection sensitivity of the structure, and a manufacturing method that can reduce the cost.

  In recent years, a wide variety of sensors have been developed and the size has been reduced. This sensor consists of electrically capturing the physical changes that the material itself has against environmental changes. There are many physical changes. Among them, a sensor that converts mechanical vibration into an electrical signal is widely known as a physical sensor.

  Various products using such sensors have been developed and put on the market. The challenge is how small, light and cheap these products can be. Therefore, it is natural to reduce the size of parts that are part of the product. However, even if the size is reduced, performance is not allowed to drop.

  Therefore, sensor sensitivity has been improved and sensors using different methods are being developed. Taking an acceleration sensor as an example, recent acceleration sensors have been made semiconductor. Typical methods include a capacitive type, a piezoelectric type, and a piezoresistive type.

  The capacitance type is a method for detecting a change in capacitance between an electrode that is fixed and does not move and an electrode that is deformed and operated when acceleration is applied. The structure for changing the capacitance uses a gap change between the electrodes. As an advantage, correction for temperature change is easy and sensitivity is high. The disadvantage is that since the capacitance change between a pair of electrodes is small, the amount of change must be increased by connecting them in parallel. In addition, since noise is likely to enter, it is necessary to devise measures that are not affected by stray capacitance as much as possible, such as placing a detection circuit near the sensor. For example, when the sensor and the detection circuit have separate structures, there is wire bonding as a method for connecting the sensor and the detection circuit. Since the metal wire used for this wire bonding also serves as an antenna, it is easily affected by noise. Further, since the wiring becomes long, it is easily affected by the stray capacitance generated between the wirings.

  The piezoelectric method is a method for detecting electric charges generated by strain applied to a piezoelectric body due to acceleration or the like. The advantages are small size and light weight. Disadvantages include high output impedance and inability to detect static acceleration.

  The piezoresistive method is a method in which the resistance value of an object changes due to stress applied to the object, and the resistance value change is detected as a voltage change by forming a bridge in a peripheral circuit. The advantage of this method is that it can be easily downsized and the detection circuit can be simple. The disadvantage is that since the piezoresistive element is a semiconductor, the change with respect to temperature is large and correction is required.

  Among these various methods, the piezoresistive type is widely used because it may be small, simple, and mass-produced. What is important in this piezoresistive equation is the sensitivity of the piezoresistor itself. In order to increase this sensitivity, various structures and manufacturing methods have been devised (for example, see Patent Document 1).

In the structure of the acceleration sensor shown in Patent Document 1, a silicon substrate and an epitaxial layer are combined, and a piezoresistive element is arranged in the epitaxial layer. The weight portion is formed by a dicing saw and wet etching. The constricted region at the junction between the beam and the weight is formed using the difference in etching rate due to the difference in the conductivity type of the diffusion layer formed in the silicon substrate. By forming this constricted region, a large stress is applied to the beam, and the sensitivity is improved.

Japanese Patent No. 3493858 (page 5, FIG. 1)

  In the prior art disclosed in Patent Document 1, the sensitivity as an acceleration sensor is improved by providing a constricted region in a part of the weight portion (joint portion between the beam and the weight). However, there are many manufacturing steps, and the manufacturing process becomes complicated.

  Current demands for accelerometers are focused on improving sensitivity as well as reducing costs. It is a big problem that there are many manufacturing processes and the manufacturing cost itself cannot be reduced.

  In the prior art disclosed in Patent Document 1, regions having different conductivity types are formed on a silicon substrate by a weight forming process, and a constricted portion of a joint portion between a beam and a weight is formed by utilizing the difference in the etching rate. is doing. Further, the piezoresistive element is formed in an epitaxial layer formed on the silicon substrate. This manufacturing method requires an ion implantation process for forming the constricted portion and an epitaxial layer forming process for forming the piezoresistive element.

  That is, in order to form the constricted portion, many manufacturing processes such as resist coating, exposure, ion implantation, and diffusion are required, leading to an increase in manufacturing cost. Therefore, the conventional technique shown in Patent Document 1 is not practical. Absent.

[Object of invention]
The object of the present invention is to solve the above-mentioned problems of the prior art and to secure sufficient sensitivity and manufacture the same in a simpler process, including an acceleration sensor using a piezoresistive element having a beam structure. It is to provide an electromechanical transducer.

  In order to achieve the above object, the following structure is adopted in the electromechanical transducer of the present invention.

In an electromechanical transducer comprising a mechanically movable movable piece, a conversion element that converts mechanical fluctuations into electrical fluctuations, a weight part for giving mechanical fluctuations, and a beam part that supports the weight part.
The beam portion has a conversion element and a weight connection base where the weight portion and the beam portion are in contact, the upper surface of the weight portion is in contact with the weight connection base, the lower surface is open, and the vicinity of the upper surface of the weight portion Has a constriction,
The upper surface of the weight portion has a smaller area than the lower surface.

  The weight portion has a bell shape, and its cross section gradually decreases from the lower surface to the upper surface, and has a shape that rapidly decreases at the constricted portion.

  The beam part and the weight connection base part are formed integrally.

  In order to achieve the above object, the following manufacturing method is adopted in the electromechanical transducer of the present invention.

A first nitride film forming step of providing a first silicon nitride film on the surface of the silicon semiconductor substrate;
A polycrystalline silicon film forming step of selectively providing a polycrystalline silicon film containing a low-concentration impurity on the first silicon nitride film;
An ion implantation step of selectively providing a high concentration impurity region in the polycrystalline silicon film by an ion implantation method;
A second nitride film forming step of providing a second silicon nitride film on the entire upper surface of the first silicon nitride film, the polycrystalline silicon film, and the high-concentration impurity region;
A first mask process for forming a first mask on the back surface of the silicon semiconductor substrate, and selectively etching anisotropically to a predetermined distance from the back surface to the surface of the silicon substrate using the first mask; A first etching step;
A second mask step of forming a second mask on the surface of the silicon semiconductor substrate, and selectively using the second mask to selectively form a part of the first silicon nitride film and the second silicon nitride film A second etching step of anisotropically etching to form a beam portion;
Using the second mask as it is, a third etching step of isotropically etching the silicon semiconductor substrate from the surface of the silicon semiconductor substrate to form a constricted portion of the weight portion below the weight connection base provided on the beam portion; ,
A fourth etching step of performing anisotropic etching so that the surface of the silicon semiconductor substrate reaches a predetermined distance removed by the first etching step;
A mask removing step of removing each of the first mask and the second mask.

  The third etching step is a dry etching method using sulfur hexafluoride as an etching gas and utilizing plasma.

  The third etching step is a wet etching method using a mixed solution of hydrofluoric acid and nitric acid.

The electromechanical transducer of the present invention includes a mechanically movable element that converts mechanical fluctuations into electrical fluctuations, a weight portion having a constricted portion, a beam portion that supports the weight portion, and a weight portion. A weight connection base that is in contact with the beam. The weight portion has a bell shape, and a larger mechanical variation can be transmitted to the beam portion via the weight connection base portion by the constricted portion.
In the manufacturing method, a silicon nitride film is used for the beam portion and silicon is used for the weight portion, so that the formation process can be integrated. Three processes having different characteristics, an anisotropic second etching process, an isotropic third etching process, and an anisotropic fourth anisotropic etching process are appropriately used, and the number of processes is small. Thus, the beam portion and the weight portion can be formed simultaneously, and the manufacturing process can be simplified.

  In the embodiments of the present invention described below, the movable piece has a shape having a fixed portion and a movable portion, and a beam portion connecting the fixed portion and the movable portion, and mechanical fluctuations are electrically applied to the beam portion. It demonstrates as a shape which provides the element converted into a fluctuation | variation. An element that converts mechanical variation into electrical variation is described as a piezoresistive element. Hereinafter, the electromechanical transducer of the present invention will be described with reference to the drawings.

[Description of structure: Fig. 1]
First, the structure of the electromechanical transducer of the present invention will be described with reference to the structural diagram of FIG.
FIG. 1 is a structural diagram of an electromechanical transducer of the present invention. FIG. 1A is a plan view. FIG. 1B is a cross-sectional view taken along the line AA ′ of FIG.
In FIG. 1, 10 is a silicon semiconductor substrate, 12 is a weight portion, 12a is an upper side surface of the weight portion, 12
b is a lower surface of the weight portion, 14 is a constricted portion, 16 is a first silicon nitride film, 18 is a second silicon nitride film, 20 is a piezoresistive element, 22 is a diffusion wiring, 24 is a metal wiring, and 26 is a beam. Reference numeral 28 denotes a weight connection base.
The first silicon nitride film 16 and the second silicon nitride film 18 constitute a beam portion 26. The weight connection base portion 28 is formed integrally with the beam portion 26, and the weight portion 12 is connected to the weight connection base portion 28.

  The actual uppermost surface is the second silicon nitride film 18, but in FIG. 1, four movable element beam portions 26 and four piezoresistive elements provided on the beam portion 26 are provided so that the structure can be easily understood. 20 and the weight part 12 are shown. As shown in FIGS. 1A and 1B, the characteristic structure of the electromechanical transducer according to the present invention is a piezoresistor that converts mechanical fluctuations into electrical fluctuations in a mechanically movable mover piece. The element 20 is provided with a weight part 12 for giving mechanical variation and a beam part 26 for supporting the weight part 12, and the beam part 26 is in contact with the piezoresistive element 20, the weight part 12 and the beam part 26. The upper surface 12a of the weight portion 12 is in contact with the weight connection base portion 28, the lower surface 12b is open, and the constricted portion 14 is provided in the vicinity of the upper surface 12a of the weight portion 12.

In the plan view shown in FIG. 1A, there are four beam portions 26 of the mover piece on the top, bottom, left, and right in the figure, and these are cross-shaped at the center of the drawing. Furthermore, the intersection portion of the cross shape becomes the weight connection base portion 28. Although the weight connection base portion 28 is described as a shape larger than one width of the beam portion 26, the shape is not limited to this.
As shown in FIG. 1B, the weight portion 12 is located below the weight connection base portion 28. The weight portion 12 is in contact with the weight connection base portion 28 at the upper side surface 12a of the weight portion 12, but the size of the weight portion 12 is determined in view of the shape and weight of the weight portion 12, the magnitude of acceleration to be detected, and the like. Since the selection is made, the size of the weight connection base 28 can be freely changed.

  Further, as shown in FIG. 1B, the upper side surface 12a of the weight portion 12 has a smaller area than the lower side surface 12b. In other words, the weight portion 12 has a bell shape, and the cross section gradually decreases from the lower side surface 12 b to the upper side surface 12 a and rapidly decreases at the constricted portion 14. By adopting such a shape, the beam portion 26 is more likely to be distorted than a shape that linearly decreases from the lower side surface 12b to the upper side surface 12a.

  Further, the beam portion 26 and the weight connection base portion 28 are integrally formed by the first silicon nitride film 16 and the second silicon nitride film 18 provided on the upper portion of the silicon semiconductor substrate 10. This is a feature of the present invention, and there is an advantage that the forming process can be simplified by using the same material for the beam portion 26 and the weight connection base portion 28.

A specific material structure in the present invention includes a first silicon nitride film 16 used as the beam portion 26, a piezoresistive element 20 and a diffusion wiring 22 formed on the first silicon nitride film 16, and a second It has a silicon nitride film 18 and a metal wiring 24 that is a wiring for connecting to an external circuit.
The piezoresistive element 20 works as a sensor by causing a resistance value change due to strain applied to the beam portion 26 via the weight connection base portion 28 by the force applied to the weight portion 12.

[Description of Manufacturing Method: FIGS. 5 to 13]
Next, a method for manufacturing the electromechanical transducer of the present invention shown in FIG. 1 will be described with reference to FIGS. 1B and 2 to 10.
As shown in FIG. 2, a first silicon nitride film 16 is formed on the silicon semiconductor substrate 10. As an example, it is formed with a thickness of 0.3 μm.

Next, as shown in FIG. 3, a polycrystalline silicon film 30 is formed on the first silicon nitride film 16. Boron (B +) is ion-implanted into the entire surface of the polycrystalline silicon film 30 at 25 KeV under the condition of 1.0 × 10 ¹⁵ (atoms / cm ² ). This polycrystalline silicon film 30 becomes a piezoresistive element 20 by processing it into a desired shape in a later step. Therefore, the thickness of the polycrystalline silicon film 30 can be arbitrarily selected according to the characteristics of the piezoresistive element 20. As an example, it is formed with a thickness of 0.2 μm.
The reason why boron (B +) is used as the impurity to make the resistance of the P-type conductivity type is because there is an advantage of high strain sensitivity.

Next, as shown in FIG. 4, after providing a resist 32 on the polycrystalline silicon film 30, the resist 32 is selectively patterned so as to open a region where the diffusion wiring 22 is to be formed. Thereafter, boron (B +) is ion-implanted at 30 KeV under the condition of 1.0 × 10 ¹⁶ (atoms / cm ² ). As a result, the diffusion wiring 22 is formed. Thereafter, the resist 32 is removed.

Next, as shown in FIG. 5, a resist (not shown) is formed again on the polycrystalline silicon film 30, and then the resist is patterned according to the shape in which the piezoresistive element 20 is to be formed. Thereafter, the polycrystalline silicon film 30 is selectively etched so as to have a desired shape by a dry etching method to form the piezoresistive element 20. Thereafter, the resist is removed.
Next, a second silicon nitride film 18 is formed on the entire surface. As an example, it is formed with a thickness of 0.4 μm.

Although not shown, the second silicon nitride film 18 at a desired position of the diffusion wiring 22 is opened to form a contact hole. This is for connecting the diffusion wiring 22 and the metal wiring 24.
As shown in FIG. 6, aluminum for forming the metal wiring 24 is formed. As an example, it is formed by sputtering with a thickness of 1.0 μm, a resist pattern (not shown) is selectively left to form a wiring pattern, and the metal wiring 24 is formed by dry etching.
Although not shown, a pad region for connecting the metal wiring 24 and an external circuit is also formed. Since the pad region is made of metal, it can be formed simultaneously with the step of forming the metal wiring 24.

Next, as shown in FIG. 7, resists 33a and 33b are formed on both the upper and lower sides of the silicon semiconductor substrate 10, respectively. These are the first mask and the second mask used for etching.
In order to form the weight portion 12, the resist 33b on the lower surface side of the silicon semiconductor substrate 10 is patterned into a desired shape.
Next, the silicon semiconductor substrate 10 is placed from the lower surface to a predetermined distance by a dry etching method that is a first etching step having anisotropy. Etching is performed (to the extent that the first silicon nitride film 16 is not reached). This dry etching condition is a method in which etching and deposition are alternately performed using SF ₆ (sulfur hexafluoride), which is a well-known etching gas, and C ₄ H ₈ (cyclobutane), which is a deposition gas. is there. Thereafter, the lower side resist 33b is removed.

Next, as shown in FIG. 8, the resist 33a is patterned so as to leave the cross-shaped beam portion 26 and the beam connection base portion 28 on the upper side surface. Next, etching is performed using this as a mask.
explain in detail. First, the second silicon nitride film 18 and the first silicon nitride film 16 which are the uppermost layers are etched by a dry etching method which is a second etching step having anisotropy. As an etching gas at this time, a mixed gas of CF ₄ (carbon tetrafluoride) and He (helium) is used. Further, in order to etch the silicon semiconductor substrate 10 below the beam portion 26 while leaving the resist 33a as it is, the etching conditions are changed to a third etching step having isotropic properties. As the etching gas at this time, SF ₆ (sulfur hexafluoride) that performs isotropic etching with respect to silicon is used alone.

  When the third etching step having this isotropic property is completed, the result of observation in the B-B ′ cross section is as shown in FIG. At this stage, since the etching of the final process has not been completed, the previous stage constricted part 15 which is the stage before finally becoming the constricted part 14 is starting to be formed.

  That is, after the second silicon nitride film 18 and the first silicon nitride film 16 are etched in the anisotropic second etching process, the silicon semiconductor substrate 10 is beamed in the isotropic third etching process. Etching is performed from the side of the portion 26.

  FIG. 9B shows a C-C ′ cross section of FIG. 8. Since the third etching process is isotropic etching conditions, the silicon semiconductor substrate 10 is etched at the same etching rate in both the depth direction and the lateral direction. Therefore, the etching in the lateral direction is more performed than the first etching condition used for etching from the lower surface. Therefore, it can be seen from the figure that the silicon semiconductor substrate 10 under the beam portion 26 has disappeared. By this third etching step, the beam portion 26 can be moved away from the silicon semiconductor substrate 10 as the beam portion 26.

Next, using the fourth etching step, etching is performed from the upper side to the depth (the depth etched to a predetermined distance) that etched the silicon substrate from the lower surface to the middle in the first etching step, To penetrate. This fourth etching condition is an anisotropic dry etching condition. The etching gas at this time is a mixed gas of SF ₆ (sulfur hexafluoride) and C ₄ H ₈ (cyclobutane) which is a deposition gas.
FIG. 10A shows a plan view with the resist 33a remaining after the fourth etching step is completed. FIG. 10B is a cross-sectional view of the cross section taken along the line DD ′ of FIG. The constricted portion 14 is formed by the third and fourth etching steps.

  The etching conditions are changed from the second etching condition to the fourth etching condition without breaking the vacuum of the etching apparatus. The beam portion 26 and the weight portion 12 are formed using these three different etching conditions. After the fourth etching step, the resist 33a is removed, and the electromechanical transducer of the present invention as shown in FIG.

  The third etching step uses dry etching, but can also be formed using a wet etching method. However, at this time, it is necessary to break the vacuum once between the second etching process and the fourth etching process. In other words, it must be taken out of the apparatus once, not in the same etching apparatus. For this reason, although the labor of the manufacturing process is somewhat increased, it may be preferable to perform wet etching in relation to other manufacturing processes (for example, the relationship of the use cycle of the drug). When the wet etching method is used, a mixed solution of hydrofluoric acid and nitric acid is used.

The manufacturing method of the electromechanical transducer of the present invention makes it possible to fuse the forming steps of the beam portion 26 and the weight portion 12 by using etching conditions having four different characteristics.
That is, the first etching step of anisotropic etching etches the silicon semiconductor substrate 10 from the lower side to a predetermined distance, and the second etching step of anisotropic etching causes the first and first steps from the upper side. 2 and the third etching step of isotropic etching, the beam portion 26 is connected to the silicon semiconductor substrate 10 from the upper surface.
The weight 12 is formed in a bell shape by advancing the etching from the lower surface to the depth etched in the first etching step by the step of moving away from the first step and the fourth etching step of anisotropic etching. .
Resist 33a and resist 33b are respectively formed on the upper side and lower side of the silicon semiconductor substrate 10 and used as a mask. In the first etching step, etching is performed from the lower side, so that the resist 33a protects the upper side. Used. The resist 33b is patterned and is removed at the end of the etching of the silicon semiconductor substrate 10 on the lower surface. When etching the upper surface, the resist 33a is patterned and removed at the end of the fourth etching step. By using this step, the beam portion 26 and the weight portion 12 can be formed simultaneously with a small number of steps, which has the effect of contributing to cost reduction in the manufacturing process.

  In the electromechanical transducer of the present invention, the beam portion is a first silicon nitride film and a second silicon nitride film, and the weight portion is silicon. In this way, by selecting the material according to the constituent parts, it is possible to fuse the forming steps of the beam portion and the weight portion by using etching conditions having four different characteristics.

  The electromechanical transducer of the present invention can be used for various sensors. For example, a pressure sensor, an acceleration sensor, or the like. Furthermore, since it can contribute to the cost reduction of a manufacturing process, it is suitable for sensors which require high sensor sensitivity while requiring cost reduction.

It is a figure which shows the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is a top view which shows the manufacturing process of the electromechanical converter of this invention. It is sectional drawing which shows the manufacturing process of the electromechanical converter of this invention. It is a figure which shows the manufacturing process of the electromechanical converter of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Silicon semiconductor substrate 12 Weight part 12a Upper weight part 12b Lower weight part 14 Constriction part 15 Preliminary constriction part 16 1st silicon nitride film 18 2nd silicon nitride film 20 Piezoresistive element 22 Diffusion wiring 24 Metal wiring 26 Beam Part
28 Weight connection base 30 Polycrystalline silicon film 32 Resist 33a Resist 33b Resist

Claims (6)

Electromechanical transducer comprising a mechanically movable movable piece, a conversion element for converting mechanical fluctuations into electrical fluctuations, a weight part for giving the mechanical fluctuations, and a beam part for supporting the weight parts In
The beam portion includes the conversion element, and a weight connection base where the weight portion and the beam portion are in contact with each other, the upper surface of the weight portion is in contact with the weight connection base, and the lower surface is open, In the vicinity of the upper surface of the weight portion has a constricted portion,
The electromechanical transducer characterized in that the upper side surface of the weight portion has a smaller area than the lower side surface.   2. The weight portion according to claim 1, wherein the weight portion has a bell shape, and a cross-section of the weight portion gradually decreases from the lower side surface toward the upper side surface and rapidly decreases at the constricted portion. The electromechanical transducer as described.   The electromechanical converter according to claim 1, wherein the beam portion and the weight connection base portion are formed integrally. A first nitride film forming step of providing a first silicon nitride film on the surface of the silicon semiconductor substrate;
A polycrystalline silicon film forming step of selectively providing a polycrystalline silicon film containing a low concentration impurity on the first silicon nitride film;
An ion implantation step of selectively providing a high concentration impurity region by ion implantation in the polycrystalline silicon film;
A second nitride film forming step of providing a second silicon nitride film on the entire upper surface of the first silicon nitride film, the polycrystalline silicon film, and the high-concentration impurity region;
A first mask step of forming a first mask on the back surface of the silicon semiconductor substrate, and selectively anisotropically using a first mask from the back surface to the surface of the silicon substrate up to a predetermined distance. A first etching step for performing etching,
A second mask step of forming a second mask on the surface of the silicon semiconductor substrate, and selectively using the second mask to selectively form the first silicon nitride film and the second silicon nitride film A second etching step of anisotropically etching a part of the beam to form a beam portion;
Using the second mask as it is, the silicon semiconductor substrate is isotropically etched from the surface of the silicon semiconductor substrate to form a constricted portion of a weight portion below the weight connection base provided on the beam portion. Etching process of
A fourth etching step of performing anisotropic etching so that the surface of the silicon semiconductor substrate reaches the predetermined distance removed by the first etching step;
A method of manufacturing an electromechanical transducer, comprising: a mask removing step of removing the first mask and the second mask, respectively.   5. The method of manufacturing an electromechanical transducer according to claim 4, wherein the third etching step is a dry etching method using sulfur hexafluoride as an etching gas and using plasma.   5. The method of manufacturing an electromechanical converter according to claim 4, wherein the third etching step is a wet etching method using a mixed solution of hydrofluoric acid and nitric acid.

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