Capacitive pressure sensor and method for fabricating the same

A method for producing a capacitive pressure sensor is comprised steps of: etching front surface and depositing etching stopper on a rear surface of a silicon substrate; depositing an upper electrode on etching stopper; depositing insulating surface on the upper electrode surface; depositing a lower electrode on the insulating surface to form a capacitor; forming a metal pad electrode on the front glass substrate; hybrid-bonding lower electrode of the capacitor of silicon substrate to the metal pad electrode of glass substrate using an anodic bonding method; etching rear surface of silicon substrate bonded to the glass substrate to selectively expose the etching stopper to form a membrane; and removing the exposed region of etching stopper by wet etching. Thus, the output of the capacitive pressure sensor is linearly varied according to the input, which is used hafnium oxide having a high dielectric constant as an insulating material to provide high capacitance and improve the sensitivity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a capacitive pressure sensor, more particularly to a capacitive pressure sensor that has an area that changes according to a variation of the external pressure so that the output changes linearly according to the input, and uses hafnium oxide, which has excellent insulating properties and a high dielectric constant, as an insulating material of a capacitor so as to have high sensitivity, and a method for fabricating said capacitive pressure sensor.

2. Description of the Related Art

Research in semiconductor pressure sensors using a micromachining technique employing a semiconductor fabricating process has been going on for quite some time. Semiconductor pressure sensors obtained by the micromachining technique are widely applied to vehicle systems, industrial control, environmental monitoring, and biomedical diagnosis fields.

Pressure sensors are elements for measuring an absolute pressure or a relative pressure, and are divided into strain gauge-type metal pressure sensors, piezoresistive pressure sensors, piezoelectric pressure sensors, MOSFET pressure sensors, piezojunction pressure sensors, optical fiber pressure sensors, and capacitive pressure sensors, according to sensing principles. The above pressure sensors, except for the metal pressure sensors, are fabricated by the micromachining technique using a semiconductor substance, i.e., silicon.

Capacitive pressure sensors use a principle in which the capacitance of a parallel plate capacitor between a silicon thin film diaphragm (membrane) and a support changes according to a variation of a gap between two electrodes due to the deflection of the silicon thin film diaphragm (i.e., the deformation of the membrane) according to a variation of pressure supplied from the outside.

The above capacitive pressure sensors are classified into absolute pressure sensors and relative pressure sensors. An absolute pressure sensor is a capacitive pressure sensor that is fabricated with an enclosed diaphragm, and thus senses an external pressure on the basis of a designated pressure in a reference pressure chamber. The absolute pressure sensor is completely isolated from the outside, thus it is not influenced by the surrounding environment. However, air in the reference pressure chamber is compressed by the deflection of the diaphragm, generating nonlinearity, and is expanded by an increase in temperature, generating undesirable offset and temperature drift. A relative pressure sensor is a capacitive pressure sensor that has a reference pressure open to the environment through a perforated glass portion, and thus senses the relative pressures of two portions, i.e., a difference of pressures between the upper and lower portions of a diaphragm. The relative pressure sensor is open to the outside, thus being easily influenced by surrounding substances. However, since the reference pressure chamber is not closed, the relative pressure sensor is not influenced by the compression or expansion of the air in the reference pressure chamber.

The capacitive pressure sensors, which detect a capacitance between two electrodes using the deflection of a diaphragm (the deformation of a membrane), have a sensitivity several hundred times as high as piezoresistive pressure sensors, and have higher stability (lower temperature coefficient and stronger structure) and lower power consumption than piezoresistive pressure sensors.

FIG. 1-1Ato1F illustrate a process for fabricating a conventional capacitive pressure S sensor. Since the variation in capacitance according to a variation in pressure is on the order of picofarads, in order to remove a parasitic capacitance between the two electrodes, a lower electrode14is formed on a glass substrate13, an upper electrode12is formed on a silicon substrate11, and the two substrates13and11are hybrid-bonded using an electric (anodic) bonding method.

First,FIG. 1-1Aillustrates the silicon substrate11used as an upper substrate. As shown inFIG. 1-1B, the front surface of the silicon substrate11is etched to a designated depth by dry etching, and the upper electrode12of a capacitor, which is made of a metal, is formed on the etched region.

FIG. 1-1Cillustrates the glass substrate13used as a lower substrate. As shown inFIG. 3-3D, the lower electrode14is formed on the glass substrate13.

As shown inFIG. 1-1E, the two substrates11and13provided with the upper and lower electrodes12and14are hybrid-bonded using the electric (anodic) bonding method.

Finally, as shown inFIG. 1-1F, the rear surface of the silicon substrate11is etched by anisotropic chemical etching, thus forming a membrane. Thereby, the fabrication of a capacitive pressure sensor is completed.

The above capacitive pressure sensor electrically reads the change of distance between two electrodes of the capacitor through the movement of the membrane according to a variation of pressure.

The above conventional capacitive pressure sensor has excellent sensitivity and temperature characteristics (i.e., has higher sensitivity and is less sensitive to a variation of temperature) compared to a piezoresistive pressure sensor, thus being applicable to many fields. However, since the capacitance is in inverse proportion to the distance between two electrodes and the membrane does not maintain its planar shape while it moves downward but is bent when external pressure is applied thereto, the variation of capacitance according to a variation of pressure cannot exhibit linearity. That is, the linearity of output according to input is poor, and thus, it is difficult to achieve sensor compensation. Thereby, since the usable pressure range is narrow, the substantial use of the conventional capacitive pressure sensor is restricted.

Accordingly, in order to reduce the nonlinearity of the capacitive pressure sensor, various researches in the structural deformation of the capacitive pressure sensor has been carried out.

Various approaches for deforming the structure of the capacitive pressure sensor have been taken, as follows.

Korean Patent Publication No. 96-006113 discloses a semiconductor capacitive pressure sensor in which a capacitor electrode of a silicon diaphragm is deformed into a cross shape to improve nonlinear response characteristics. Further, Korean Patent Registration No. 10-0404904 discloses a differential capacitive pressure sensor and a method for fabricating the same, in which two sensing capacitors are disposed at both sides and a single-layered sacrificial layer is used to improve nonlinear characteristics of the pressure sensor. The intervals between the upper and lower electrodes of the two sensing capacitors become equal, using the single-layered sacrificial layer to simplify a fabricating process, and variations of capacitance of the sensing capacitors due to displacement become equal to increase the linearity of the pressure sensor.

In a general capacitive pressure sensor, when a membrane is thin in order to increase the sensitivity of the capacitive pressure sensor, the linearity of the capacitive pressure sensor is deteriorated. However, Aziz Ettouhami et al. solve the above nonlinearity problem using a series capacitor, thus improving the sensitivity and the linearity of a capacitive pressure sensor.

Further, see Satoshi Yamamoto et al. (2003). “Touch mode capacitive pressure sensor for passive tire monitoring system”. Japan. IEEJ Transactions on Sensors and Micro machines. pp. 9-15.

Satoshi Yamamoto et al. disclose a capacitive pressure sensor having high linearity, in which the distance between two electrodes of a capacitor is changed according to a variation of pressure. Such a capacitive pressure sensor has a principle in that, when pressure of a designated value or more is applied to the capacitive pressure sensor, the upper electrode, i.e. the membrane, contacts an insulating material deposited on the lower electrode.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a capacitive pressure sensor, in which the distance between two electrodes of a capacitor is not changed but the area of the pressure sensor is changed according to a variation of pressure, so that output changes linearly according to input, and hafnium oxide, having excellent insulating properties and a high dielectric constant, is used as an insulating material of the capacitor to increase the capacitance and thus to increase the sensitivity. It is another object of the present invention to provide a method for fabricating said capacitive pressure sensor.

In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a method for fabricating a capacitive pressure sensor comprising: etching the front surface of a silicon substrate to a designated depth; depositing an etching stopper, for stopping etching of the rear surface of the silicon substrate, on the surface of the silicon substrate; depositing an upper electrode of a capacitor on the surface of the etching stopper; depositing an insulating surface of the capacitor on the surface of the upper electrode; depositing a lower electrode of the capacitor on the surface of the insulating surface to form the capacitor; forming a metal pad electrode on the front surface of a glass substrate; hybrid-bonding the lower electrode of the capacitor of the silicon substrate to the metal pad electrode of the glass substrate using an electric (anodic) bonding method; etching the rear surface of the silicon substrate bonded to the glass substrate to selectively expose the etching stopper and thus to form a membrane; and removing the exposed region of the etching stopper by wet etching.

During the etching of the front surface of the silicon substrate, anisotropic chemical etching using potassium hydroxide (KOH) or ethylene diamine pyrocatechol (EDP) may be performed and silicon nitride (SixNy), silicon dioxide (SiO2) or a metal, selected from the group consisting of gold (Au) and chromium (Cr), may be used as a mask material.

Alternatively, in the etching of the front surface of the silicon substrate, dry etching may be performed and photoresist or a metal, selected from the group consisting of gold (Au), chromium (Cr) and aluminum (Al), may be used as a mask material.

The etching stopper, for stopping the etching of the rear surface of the silicon substrate by anisotropic chemical etching using potassium hydroxide (KOH) or ethylene diamine pyrocatechol (EDP), may be made of silicon dioxide (SiO2), during the depositing of the etching stopper.

The upper electrode may be made of doped polysilicon, during the depositing of the upper electrode of the capacitor.

The insulating layer may be made of hafnium oxide, during the depositing of the insulating layer of the capacitor.

The lower electrode may be made of doped polysilicon, during the depositing of the lower electrode.

The rear surface of the silicon substrate may be etched by anisotropic chemical etching, during the etching of the rear surface of the silicon substrate, and thus the capacitor may be formed in the shape of the membrane having a three-layer structure moving according to a variation of pressure, during the removing of the exposed region of the etching stopper.

In accordance with another aspect of the present invention, there is provided a capacitive pressure sensor fabricated by the above method.

The above capacitive pressure sensor of the present invention is characterized in that the capacitor is formed in the shape of the membrane having a three-layer structure, the area of the capacitor is changed according to a variation of external pressure, and hafnium oxide, having excellent insulating properties and a high dielectric constant, is used as the insulating layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a method for fabricating a capacitive pressure sensor having high linearity in accordance with the present invention will be described with reference toFIG. 2-2Ato2K.

First, as shown inFIG. 2-2Ato2K, a silicon substrate20is used to form a membrane30, and a glass substrate25is used to form a bonding metal pad electrode26.

As shown inFIG. 2-2A, the silicon substrate20is prepared. As shown inFIG. 2-2B, the front surface of the silicon substrate20is etched to a designated depth in a first step (S1). Here, when anisotropic chemical etching using potassium hydroxide (KOH) or ethylene diamine pyrocatechol (EDP) is performed, silicon nitride (SixNy), silicon dioxide (SiO2), or a metal, such as gold (Au) or chromium (Cr), is used as a mask material, or when dry etching is performed, photoresist, or a metal, such as gold (Au), chromium (Cr), or aluminum (Al), is used as a mask material.

As shown inFIG. 2-2C, silicon dioxide (SiO2) is deposited on the etched surface of the silicon substrate20in a second step (S2), thus forming an etching stopper21for stopping anisotropic chemical etching using potassium hydroxide (KOH) or ethylene diamine pyrocatechol (EDP), which will be later described in an eighth step (S8).

As shown inFIG. 2-2D, doped polysilicon is deposited on the surface of the etching stopper21, thus forming an upper electrode22of a capacitor, in a third step (S3). Thereafter, as shown inFIG. 2-2E, in order to increase the sensitivity of the pressure sensor, hafnium oxide having excellent insulating properties and a high dielectric constant is deposited on the surface of the upper electrode22, thus forming an insulating layer23, in a fourth step (S4). Since the hafnium oxide has excellent insulating properties, the use of an insulating layer23with a small thickness increases the capacitance of the capacitor29.

As shown inFIG. 2-2F, doped polysilicon is deposited on the insulating layer23, thus forming a lower electrode24of the capacitor29, in a fifth step (S5). Thereby, the formation of the capacitor29is completed.

As shown inFIG. 2-2G, a metal pad electrode26is formed on the front surface of the glass substrate25in a sixth step (S6). Thereafter, as shown inFIG. 2-2H, the lower electrode24of the capacitor29of the silicon substrate20and the metal pad electrode26of the glass substrate25are hybrid-bonded using an electric (anodic) bonding method in a seventh step (S7).

As shown inFIGS. 2-2Iand2J, in order to form the capacitor29, of the silicon substrate20bonded to the glass substrate25, in the shape of the membrane30, the rear surface of the silicon substrate20is etched by anisotropic chemical etching using potassium hydroxide (KOH) or ethylene diamine pyrocatechol (EDP), thus selectively exposing the etching stopper21, in the eighth step (S8). Here, silicon nitride (SixNy), silicon dioxide (SiO2), or a metal, such as gold (Au) or chromium (Cr), is used as a mask material, and silicon dioxide (SiO2) is used as the etching stopper21.

Finally, in a ninth step (S9) shown inFIG. 2-2K, the portion of the etching stopper21that was exposed from the surface of the capacitor29by the anisotropic chemical etching in the eighth step (S8), is removed by wet etching, forming the membrane30having a three-layer structure, which moves according to a variation of pressure. Through the above steps, the fabrication of the capacitive pressure sensor of the present invention is completed.

The membrane30having the above three-layer structure includes the upper electrode22, the insulating layer23, and the lower electrode24.

FIGS. 3-3Aand3B are schematic sectional views illustrating the operation of the capacitive pressure sensor of the present invention, which is fabricated by the above method.

When external pressure is not applied to the membrane30, the membrane30maintains its level state, as shown inFIG. 3-3A, and when external pressure is applied to the membrane30, the membrane30is concavely bent into an arched state and thus has an increased area, as shown inFIG. 3-3B. Since the area of the membrane30means the area of the capacitor, the capacitance of the capacitor is changed.

The capacitance is in proportion to the area of the membrane30, and when external pressure is applied to the membrane30, the membrane30is bent from the planar state into a concave arch. Accordingly, the variation of capacitance according to the variation of pressure exhibits linearity.

As apparent from the above description, the present invention provides a method for fabricating a capacitive pressure sensor that has an area changed according to a variation of external pressure so that output changes linearly according to input, and uses hafnium oxide, having a high dielectric constant, as an insulating material to increase the capacitance and thus to increase the sensitivity, and a capacitive pressure sensor fabricated thereby, thus allowing the capacitive pressure sensor to have high linearity and sensitivity.