Capacitive type dynamic quantity sensor

In a capacitive type dynamic quantity sensor, a width of a beam in a beam portion extending in a direction that is perpendicular to a predetermined deformation direction and a gap disposed between a movable electrode and the fixed electrode in the predetermined deformation direction are approximately identical. Accordingly, manufacturing error is prevented from affecting the sensitivity of the capacitive type dynamic quantity sensor. For example, a manufacturing tolerance error of ±2.5% is allowed as a result of designing the width of the beam and the gap to be identical in length.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of Japanese Patent Application No. 2002-41503 filed on Feb. 19, 2002, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to dynamic quantity sensors, and specifically to a capacitive type dynamic quantity sensor that detects a dynamic quantity using a capacitance formed between a movable electrode and a fixed electrode.

BACKGROUND OF THE INVENTION

Conventionally, a capacitive type dynamic quantity sensor such as the one shown inFIG. 7is typically constructed by etching a substrate10such as a semiconductor substrate. The etching forms a trench in the substrate10to separate a movable portion including beam portions22and movable electrodes24from electrodes of fixed electrode groups32,42.

The beam portions22extend in a direction perpendicular to the Y direction in FIG.7and are spring-like in operation, as they deform in the Y direction with respect to a force applied thereon. The movable electrodes24also extend in a direction that is perpendicular to the Y direction and move in the Y direction along with the beam portions22. The movable electrodes24have, for example, a comb-shaped configuration.

The comb-shaped electrodes of the fixed electrode groups32,42are supported by and fixed on the substrate10to respectively face the movable electrodes24.

According to the above described capacitive type dynamic quantity sensor, a total capacitance CS1is formed in gaps D disposed between the movable electrodes24on the left side in FIG.7and the electrodes of the fixed electrode group32, and a total capacitance CS2is formed in gaps D disposed between the movable electrodes24on the right side in FIG.7and the electrodes of the fixed electrode groups42. When a physical quantity such as acceleration is applied to the capacitive type dynamic quantity sensor, the capacitances CS1, CS2vary with respect to an amount of the physical quantity. Therefore, the physical quantity is detected based on the variation of difference between the capacitances CS1, CS2.

In the above capacitive type dynamic quantity sensor, the fixed electrode groups32,42and the movable portion including the beam portions22and the movable electrodes24are formed at the same time by etching the trench in the substrate10. Therefore, a manufacturing error of width B is approximately the same relative to each of the beam portions22and the gaps D disposed between the movable electrodes24and the electrodes of the fixed electrode groups32,42. For example, as the widths B of the beam portions22increase in width, the gaps D disposed between the movable electrodes24and the electrodes of the fixed electrode groups32,42decrease in width.

Accordingly, the manufacturing error causes variations of the widths B and the gaps D, and therefore characteristic non-uniformity of the capacitive type dynamic quantity sensor becomes large.

Incidentally, the characteristic non-uniformity of the capacitive type dynamic quantity sensor can be minimized by enlarging the widths B and the gaps D. However, the capacitances CS1, CS2consequently decrease and sensor sensitivity also decreases.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a physical quantity sensor that is capable of obviating the above problem.

It is another object of the present invention to provide a physical quantity sensor that is capable of good sensitivity.

According to a capacitive type dynamic quantity sensor of the present invention, a width of a beam of a beam portion extending in a perpendicular direction relative to a predetermined deformation direction and a gap disposed between a movable electrode and a fixed electrode in the predetermined deformation direction are approximately identical.

Accordingly, the sensitivity of the capacitive type dynamic quantity sensor is not affected. For example, a manufacturing tolerance of ±2.5% in designing the width of the beam and the gap between the movable electrode and the fixed electrode is allowed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described further with reference to various embodiments shown in the drawings.

In the present embodiment, a differential capacitance type semiconductor acceleration sensor (acceleration sensor) S1, or, more generally, a capacitive type dynamic quantity sensor is shown.

FIG. 1shows a plan view of the acceleration sensor S1.FIGS. 2 and 3show cross sectional views of the acceleration sensor S1taken along lines II—II and III—III of FIG.1. The acceleration sensor S1is, for example, utilized as a vehicle acceleration sensor or a gyro sensor for controlling an airbag system, an Antilock Brake System (ABS), a side skid control system or in any other like system that requires sensing of a dynamic quantity.

The acceleration sensor S1is manufactured on a semiconductor substrate using micro-machine technology. Referring toFIGS. 2 and 3, an SOI substrate10is used for the semiconductor substrate. The SOI substrate10includes a first silicon substrate11, a second silicon substrate12and an oxide film13interposed between the first and second silicon substrates11,12. The first silicon substrate11corresponds to a first semiconductor layer, the second silicon substrate12corresponds to a second semiconductor layer, and the oxide film13corresponds to an isolation film.

Referring toFIGS. 1-3, the second substrate12has trenches14in which a configuration referred to collectively as a comb-shaped configuration of beams20-40including a movable portion20and fixed portions30,40is formed. The oxide film13includes an opening portion15in which the comb-shaped configuration of beams20-40is formed.

The movable portion20supported across the opening portion15includes a rectangular plumb portion21, beam portions22and anchor portions23a,23b. The rectangular plumb portion21, the beam portions22and the anchor portions23a,23bare integrated with each other, and the anchor portion23a,23bsupport the plumb portion21via the beam portions22. As shown inFIG. 3, the anchor portions23a,23bare formed at peripheral positions of the opening portion15of the oxide film13and are supported by first silicon substrate11. Therefore, the beam portions22and the plumb portion21are disposed above the opening portion15.

Each of the beam portions22has two beams, both of which extend in a parallel direction and join with each other at end portions thereof. Accordingly, the beam portions22form a rectangular frame and deform in a direction perpendicular to a longitudinal side of the beams. Specifically, according to the beam portions22, the plumb portion21moves in a Y direction (arrow direction inFIG. 1) when acceleration including a Y direction component is applied thereto, and returns to an initial position thereof when the acceleration decreases. That is, the movable portion20moves in a deformation direction (i.e., the Y direction) of the beam portions22above the opening portion15upon application of acceleration.

The movable portion20also includes movable electrode groups24that extend in a direction perpendicular to the Y direction from both sides of the plumb portion21. InFIG. 1, each side of the movable electrode groups24include four electrodes that protrude from right and left sides of the plumb portion21, respectively, and respective electrodes of the movable electrode groups24are positioned above the opening portion. Accordingly, the movable electrode groups24are integrated with the beam portions22and the plumb portion21and therefore move in the Y direction with the beam portions22and the plumb portion21.

The fixed portions30,40are supported on respective opposing peripheral sides of the opening portion15of the oxide film10, where the respective opposing peripheral sides are opposite the sides supporting the anchor portions23a,23b. The fixed portions30,40include a first fixed portion30on a left side ofFIG. 1 and asecond fixed portion40on right side thereof.

The fixed portions30,40include respective wiring portions31,41and a plurality of respective first and second fixed electrode groups32,42. The wiring portions31,41are fixed on the first silicon substrate11at the peripheral portion of the opening portion15of the oxide film10. InFIG. 1, each of the fixed electrode groups32,42is formed by four electrodes. Respective electrodes of the fixed electrode groups32,42are supported on the wiring portions31,41at end portions thereof and extend in parallel with, and oppose, respective electrodes of the movable electrode groups24so as to define respective predetermined gaps D therebetween. Hereinafter, the fixed electrode group32of the first fixed portion30will be referred to as a first fixed electrode group32, and the fixed electrode group42of the second fixed portion40will be referred to as a second fixed electrode group42.

Fixed electrode pads31a,41afor wire bonding are formed at predetermined positions of the wiring portions31,41of the first and second fixed portions30,40. A movable electrode wiring portion25is formed on the anchor23band has a movable electrode pad25aat a predetermined position thereof. The pads25a,31a,41aare, for example, made of aluminum.

The acceleration sensor S1is mounted on a package (not shown) at a reverse side of the first silicon substrate11corresponding to a side opposite the oxide film13via adhesive. An electrical detection circuit100(FIG. 4) is included in the package and is electrically connected to the electrode pads25a,31a,41avia wiring such as gold, aluminum or the like.

Manufacture of the acceleration sensor S1will now be described. A mask (not shown) corresponding to a shape of the comb-shaped configuration of beams20-40is formed on the second silicon substrate12of the SOI substrate10by photolithography. The trenches14are formed on the second substrate12by dry etching with CF4, F6or the like. Accordingly, the comb-shaped configuration of beams20-40is formed on the SOI substrate10. The oxide film13is then removed by sacrifice-etching with hydrofluoric acid or the like. Therefore, the comb-shaped configuration of beams20-40is supported by the first silicon substrate11.

According to the acceleration sensor S1, a total capacitance CS1is formed in gaps D defined in the Y direction between each of the movable electrodes24and corresponding ones of the fixed electrode group32, and a total capacitance CS2is formed in gaps D defined in the Y direction between each of the movable electrodes24and corresponding ones of the fixed electrode group42. When a physical quantity such as acceleration is applied to the capacitive type dynamic quantity sensor, the capacitances CS1, CS2vary with respect to an amount of the physical quantity. Therefore, the physical quantity is detected based on the variation between the capacitances CS1, CS2.

FIG. 4shows a schematic diagram of a detection electrical circuit100of the present acceleration sensor S1. The detection electrical circuit100includes a switched capacitor circuit (SC circuit)110having a capacitor111with a capacitance Cf, a switch112and a differential amplifier circuit113. The SC circuit110converts an input capacitance difference (CS1−CS2) between the capacitances CS1, CS2to a voltage.

According to the present acceleration sensor S1, for example, a carrier wave W1with an amplitude Vcc is applied to the fixed electrode pad31a, and a carrier wave W2with an amplitude Vcc that is inverted with respect to the carrier wave W1is applied to the fixed electrode pad41a. The switch112of the SC circuit110is opened and closed based on a predetermined timing. Therefore, an acceleration applied to the acceleration sensor S1is represented as an output voltage V0according to the following formula:
V0=(CS1−CS2)·Vcc/Cf(1)

Further, in the present acceleration sensor S1, the gaps D defined between each of the movable electrodes24and corresponding ones of the fixed electrodes32,42are defined as having the same width as the widths B of the beam portions22, with the widths B also being defined in the Y direction. Accordingly, it is possible to prevent the acceleration sensor from having decreased sensor characteristics due to manufacturing error without the need to enlarge the widths D, and therefore decrease the sensitivity of the acceleration sensor S1.

Generally, in a capacitive type dynamic quantity sensor, sensitivity varies linearly with capacitance. The variation of the capacitance corresponding to “ΔC” is expressed as follows, where a total capacitance formed between the movable electrodes24and the electrodes of fixed electrode groups32,42at which acceleration is zero corresponds to “Co”, mass of the movable portion20corresponds to “m”, and spring constant of the beam portions22corresponds to “k”. Incidentally, “D” corresponds to the gaps D as discussed above.
ΔC=(2·Co·m)/(k·D)  (2)

The sensitivity, i.e., the variation ΔC of the capacitance, varies based on manufacturing error such as etching error in forming the trenches14and sacrificial-etching error in removing the oxide film13. The manufacturing error is defined by two types of size non-uniformity, that is, size non-uniformity in a direction parallel to a plane surface of the SOI substrate10of the beam portions22, the movable electrodes24, and the electrodes of the fixed electrode groups32,42, and size non-uniformity in a direction parallel to the thickness of the SOI substrate10. The former corresponds to width non-uniformity ΔD, and the latter corresponds to thickness non-uniformity Δh.

The sensitivity ΔC expressed in formula (2) varies as follows, where a thickness of the beam portions22, the movable electrodes24, and the electrodes of the fixed electrode groups32,42corresponds to “h”, and widths of the movable electrodes24and of the electrodes of the fixed electrode groups32,42correspond to “W”. Incidentally, “D” corresponds to the gaps D and “B” corresponds to the widths B as discussed above.Δ⁢⁢C∝(h+Δ⁢⁢h)(D-Δ⁢⁢D)·(h+Δ⁢⁢h)·(W+Δ⁢⁢D)(h+Δ⁢⁢h)·(B+Δ⁢⁢D)3·(D-Δ⁢⁢D)(3)

The formula (3) transforms through the following formulae, resulting finally in formula (6).Δ⁢⁢C∝(h+Δ⁢⁢h)·(W+Δ⁢⁢D)(B+Δ⁢⁢D)3·(D-Δ⁢⁢D)2(4)Δ⁢⁢C∝(h+Δ⁢⁢h)(B+Δ⁢⁢D)2·(D-Δ⁢⁢D)2(5)Δ⁢⁢C∝(h+Δ⁢⁢h){B·D+(D-B)·Δ⁢⁢D-Δ⁢⁢D2)2(6)

Referring to formula (6), the denominator has a minimum value when the widths B are equal in size to the gaps D.

FIG. 5shows a relationship between the width non-uniformity ΔD and the variation of the capacitance ΔC. InFIG. 5, a solid line represents the relationship when the widths B are equal in size to the gaps D, and a dotted line represents the relationship when the widths B are larger in size than the gaps D. An inflection point of a quadratic curve illustrated by the solid line is a point of ΔD=0, and that of a quadratic carve illustrated by the dotted line is shifted from the point of ΔD=0.

In a manufacturing process, the variation of the capacitance ΔC is shifted from the center point, that is, 0 μm. For example, if ΔD is shifted in a range from −1 μm to +1 μm, non-uniformity ΔΔC1of the variation of the capacitance ΔC when the widths B equal the gaps D is smaller than non-uniformity ΔΔC2of the variation of the capacitance ΔC when the widths B are larger than the gaps D.

Also, it has been shown that the relationship is identical if non-uniformity of the variation of the capacitance ΔC when the widths B equal the gaps D is compared with non-uniformity of the variation of the capacitance ΔC when the widths B are smaller than the gaps D. The variation of the capacitance ΔC has a minimum value when the widths B equal the gaps D and an inflection point of a quadratic curve representing the relationship between the width non-uniformity ΔD and the variation of the capacitance ΔC is a point of ΔD=0. Accordingly, by designing the acceleration sensor S1to have identical widths B and gaps D, the sensitivity of the acceleration sensor S1is not affected due to manufacturing error.

Therefore, in the present embodiment, the size of the widths B are the same as the gaps D. This effect is preferably obtained with the acceleration sensor S1of which the beam portions22, the movable electrodes24and the electrodes of the fixed electrode groups32,42are simultaneously formed on the substrate10(the second substrate12) by forming the trenches14with etching.

Incidentally, an error tolerance of ±2.5% is acceptable in designing the widths B and the gaps D. This is because a manufacturing error of ±2.5% may be generated when a mask pattern corresponding to the comb-shaped configuration of beams20-40is manufactured.

In the first embodiment, the beam portions22can alternatively be adapted as a repeatedly turned-shaped pattern illustrated inFIG. 6A, or as an L-shaped pattern illustrated in FIG.6B. In these cases, widths B correspond to widths of beams extending in a direction that is perpendicular to the Y direction.

The opening portion15may alternatively be formed in the first silicon substrate11as well as in the oxide film13. In this case, after the comb-shaped configuration of beams20-40is formed in the second silicon substrate1, the first silicon substrate11is anisotropically etched and the oxide film13is further etched with hydrofluoric acid or the like.

In the above embodiments, an acceleration sensor is described; however, other capacitive type dynamic quantity sensors such as angular speed sensor may also be realized in a similar manner.

While the above description is of the preferred embodiments of the present invention, it should be appreciated that the invention may be modified, altered, or varied without deviating from the scope and fair meaning of the following claims.