JP2000002610A - Semiconductor pressure sensor and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a semiconductor pressure sensor to be manufactured at a low cost while abolishing a glass seating in the constitution of forming a reference pressure chamber in a single-crystal silicon substrate in a sealed state. SOLUTION: After a small opening part 19 is formed at a predetermined location in an insulating film 18 formed on the back surface of an Si substrate 12, an alkali anisotropic etching is performed. As etching speed on a (111) surface is extremely slow in the alkali anisotropic etching, small cavities in a shape surrounding the small opening part 19 along the (111) surface are formed in a connected state, and an edge part appears, and etching the edge part on a (311) surface, small cavities with the (111) surface as a wall surface are formed again. Finally, a cavity 17 corresponding to an envelope surrounding the overall small opening part 19 with the (111) surface is formed, and an Si diaphragm 13 for detecting pressure is formed on the opposite side. Then by sealing the small opening part 19 with a thin film 20, a reference pressure chamber 21 is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor pressure sensor in which a reference pressure chamber is formed by sealing a cavity formed in a semiconductor substrate, and a method of manufacturing the same.

[0002]

2. Description of the Related Art A conventional general semiconductor pressure sensor has a structure in which a cavity is formed by locally performing alkali anisotropic etching with an alkaline solution from the back surface of a single crystal silicon substrate (hereinafter, Si substrate). Thereby, a thin silicon diaphragm is formed, and a pressure sensitive element such as a piezoresistive element is arranged in a stress concentration portion of the diaphragm,
The rear surface side is anodically bonded to the glass pedestal, and the cavity is sealed in vacuum to form a pressure reference chamber.

[0003]

FIG. 14 schematically shows a cross-sectional structure of an example of this type of pressure sensor. That is, a cavity 2 is formed on the back surface of the Si substrate 1 by alkali anisotropic etching, a piezoresistive element 4 is formed by diffusion in a diaphragm 3 formed on the front surface, and the piezoresistive element 4 is formed through an opening 5 a of the insulating layer 5. Is formed. A reference pressure chamber 8 is formed by anodically bonding such a Si substrate 1 with a glass having a thickness of about 0.5 to 3 mm as a pedestal 7 and then separated as a sensor chip 9 by dicing cut. ing.

However, the glass pedestal 7 is generally made of Pyrex glass, has a high surface flatness for performing anodic bonding with the Si substrate 1, and has a very high degree of cleanliness while being mirror-polished. Therefore, it is required to minimize foreign substances on the surface. Therefore, there is a problem that the cost is increased. Also, after the anodic bonding, when cutting into chips by a dicing cut device, the overall processing cost is increased, such as poor workability and high processing cost.

As described above, the glass pedestal 7 occupies a large proportion of the cost of the sensor chip, and it is important to eliminate the glass pedestal 7 in order to reduce costs.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a structure in which a reference pressure chamber is formed in a single crystal silicon substrate in a hermetically sealed state, thereby eliminating the glass pedestal and manufacturing at low cost. It is an object of the present invention to provide a semiconductor pressure sensor and a manufacturing method thereof.

[0007]

According to the first to third aspects of the present invention, when pressure is applied to the diaphragm, the diaphragm bends and deforms in accordance with the difference between the applied pressure and the pressure in the reference pressure chamber. The applied pressure can be detected based on the deformation of the diaphragm.

Here, the reference pressure chamber seals a cavity formed by etching through a small opening of the insulating film provided on the back surface of the single crystal silicon substrate, and seals the small opening with a thin film formed on the insulating film. Since it is formed by stopping, it can be realized without using a glass pedestal.

In this case, since the diaphragm is formed from the single crystal silicon substrate itself and is homogeneous, even when a temperature change occurs, no thermal stress is generated in the diaphragm, and distortion is generated in the diaphragm. There is no.
Thus, the pressure applied to the diaphragm can be accurately detected.

According to the fourth aspect of the present invention, after forming an insulating film on the back surface of the single crystal silicon substrate having a surface orientation of (100), a small opening is formed in a predetermined region of the insulating film. An alkali anisotropic etching is performed through the small opening. At this time, an edge portion appears, and the edge portion is etched on a surface having a high etching rate. In this case, since the etching rate of the (111) plane is extremely slow in the single crystal silicon substrate, a cavity having the (111) plane as a wall surface can be finally formed.

In the depth direction of the etching, (10
The etching may be terminated when the etching proceeds on the 0) surface and the Si thickness at the bottom becomes a desired diaphragm thickness. If an SOI substrate is used as the Si substrate, etching proceeds from the back surface and stops when the buried oxide film appears, so that the shape of the cavity is automatically completed without managing the etching time. Or S
If electrochemical stop etching is used with an epitaxial substrate as the i-substrate, the etching is automatically stopped at the epi layer on the back surface, so that it is not necessary to manage the etching time. Then, the reference pressure chamber can be formed in a closed state by sealing the small opening and sealing the cavity with the thin film formed on the insulating film.

According to the fifth aspect of the present invention, a plurality of small openings are formed in a predetermined region, and a small cavity surrounded by the (111) plane is formed by injecting an etching solution through the small openings. Then, the small cavities communicate with each other. At this time, since an edge portion appears at a portion where the small cavities communicate with each other, the edge portion is etched on a surface having a high etching rate. As a result of the progress of the etching as described above, the small cavities gradually increase in size, and finally cavities corresponding to the envelope region surrounding the entire small opening in the direction along the (111) plane are formed. Will be.

According to the sixth aspect of the present invention, the small opening is formed in a continuous slit shape having an edge, and a continuous slit-shaped small cavity is formed by injecting the etching solution through the small opening. Is done. At this time, since an edge portion appears, the edge portion is etched on a surface having a high etching rate, and finally, the entire small opening becomes an envelope region surrounding the entire small opening in a direction along the (111) plane. A corresponding cavity will be formed.

According to the seventh aspect of the present invention, since the thin film is formed in a vacuum atmosphere, the reference pressure chamber can be hermetically sealed at a necessary and sufficient degree of vacuum.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment in which the present invention is applied to a pressure sensor will be described below with reference to FIGS. FIG. 1 schematically shows a cross-sectional structure of a sensor chip of a semiconductor pressure sensor. This figure 1
In the semiconductor pressure sensor 11, a diaphragm 13 is provided on a surface of a single crystal silicon substrate (hereinafter, Si substrate) 12 in a semiconductor pressure sensor 11, and a piezoresistive element 14 (see FIG. 2) is diffused into a stress concentration portion of the diaphragm 13. Is formed. The surface of the Si substrate 12 is covered with an oxide film 15 and an AL electrode 16 (see FIG.
The AL electrode 16 is electrically connected to the piezoresistive element 14 through the opening 15a formed in the oxide film 15.

A cavity 17 for forming the diaphragm 13 is formed on the back surface of the Si substrate 12 by etching, and the cavity 17 is covered with an insulating film 18 and formed in the insulating film 18. The reference pressure chamber 21 is formed in a sealed state by sealing the opening 19 with the thin film 20.

FIG. 3 shows a semiconductor pressure sensor 11 having the above configuration.
Is shown, and will be described in order below. (A) First, an Si substrate 12 (wafer state) is prepared. In this case, the plane orientation of the Si substrate 12 is (100) and the surface structure is an epitaxial substrate on which an epitaxial layer is grown when electrochemical stop etching is performed later.
Alternatively, if an SOI (Silicon On Insulator) substrate is used, a diaphragm with high thickness accuracy can be formed by performing stop etching with a buried oxide film.

(B) First, an oxide film 15 is grown on the surface of the Si substrate 12 by thermal oxidation. Thereafter, by performing photoetching, ion implantation and annealing, p
The piezoresistive element 14 is formed by a type impurity (boron) diffusion layer.

(C) An opening 15a for contact with the AL electrode 16 is formed in the oxide film 15 so that the AL electrode 16
Is formed by film growth by sputtering and photoetching. Thereafter, the back surface is mirror-polished to a desired thickness (generally 300 to 400 μm) by back polishing.

(D) An insulating film 18 serving as a mask for alkali etching is formed on the back surface of the Si substrate 12. The insulating film 18 uses a nitride film or an oxide film formed by plasma CVD, and needs to have a low stress and a thickness of about 1 to 10 μm so that the film is not cracked during subsequent etching.

(E) Next, a pattern for forming the diaphragm 13 is formed on the insulating film 18 by photoetching. This pattern is formed by arranging small openings 19 having a size of several μm square in an X shape as shown in FIG.
In this case, the small opening 19 is formed so as to be located on a diagonal line of the cavity 17.

(F) Next, anisotropic etching is performed through the small opening 19 with an alkaline solution. Here, the alkali anisotropic etching will be described. In the anisotropic etching of a Si single crystal using an alkali solution, the etching rate of the (111) plane is extremely slow, which is several tens or less of that of the (100) plane, and extremely extremely compared with other crystal planes. slow.

Therefore, when the Si substrate 12 is etched through the small opening 19 as shown in FIG. 5, the small cavity 2 having the (111) plane as a wall surface in the state where the etching has progressed.
2 (shown by a dotted line in FIG. 5) is formed. In this case, when the small cavities 22 corresponding to the small openings 19 are formed, the small cavities 22 communicate with each other. As shown in FIG. An edge portion having a wall surface appears. here,
When the alkali anisotropic etching proceeds on the edge portion having the (111) plane as the wall surface, a plane having the highest etching rate (here, the (311) plane) appears as shown in FIG. 311) The plane proceeds.

When the (311) plane is etched to the (111) plane which forms the wall surface of the small cavity 22, the etching rate becomes extremely slow, and the etching does not proceed any more (actually, the etching progresses slowly). But can be considered stopped). As a result, a larger small cavity is formed corresponding to the position of the small cavity 22, and the edge portion appears again.

Therefore, as a result of the etching stepwise progressing as described above, if the small openings 19 are arranged in an X shape as shown in FIG.
11) The cavity 17 (see FIG. 8) corresponding to the envelope region A (the region indicated by the two-dot chain line in FIG. 4) surrounded in the direction along the plane.
Is formed.

When the alkali anisotropic etching is completed as described above, the Si on the opposite surface (surface) of the Si substrate 12 is
Is the diaphragm 13. When an epitaxial substrate is used, a diaphragm having a thickness corresponding to the epitaxial thickness can be formed by electrochemical stop etching. When an SOI substrate is used, the etching stops at the buried oxide film, and
A diaphragm of OI thickness can be formed.

(G) After the etching is completed as described above, the Si substrate 12 is sufficiently washed and dried, and then the small opening 19 is filled by forming a thin film 20 on the back surface. The thin film 20 is made of a nitride film, an oxide film, a film of amorphous Si or the like by the same plasma CVD film as the base, polysilicon by low pressure CVD,
It is also possible to use amorphous Si or a thin film formed by sputtering or vapor deposition. The film thickness of the thin film 20 may be a thickness that can completely fill the small opening 19 and the mechanical strength of the thin film 20 may be ensured. As a result, the interior of the cavity 17 is sealed with the degree of vacuum during plasma CVD. The pressure during film formation by general plasma CVD or low pressure CVD is 50
0 Pa or less, and a sufficient degree of vacuum can be secured as a reference pressure chamber.

According to the above-described manufacturing method, the insulating film 1 formed on the back surface of the Si substrate 12 whose front surface is surrounded by the (100) plane
8, a plurality of small openings 19 are formed, and the entire small openings 19 are formed so that an envelope region surrounding the entire small opening 19 in a direction along the (111) plane corresponds to the formation position of the cavity 17. Since the alkali anisotropic etching is performed through the small opening 19, the cavity 17 having the (111) plane as the wall surface can be formed, and the small opening 1 can be formed.
By sealing 9 with thin film 20, reference pressure chamber 21 can be formed in a sealed state. Therefore, there is no need to use a glass pedestal as in the prior art, so that the pressure sensor can be manufactured at low cost.

In this case, it is conceivable to use the insulating film 18 as a diaphragm for the structure of the semiconductor pressure sensor. However, the small opening 19 formed in the insulating film 18 is filled with a foreign substance of the thin film 20. That is, the diaphragm is distorted due to the temperature change, and the characteristics of the semiconductor acceleration sensor are expected to deteriorate.

On the other hand, according to the above embodiment,
Since the diaphragm 13 is formed from the Si substrate itself and is homogeneous, even when a temperature change occurs, no thermal stress occurs in the diaphragm 13 and no distortion occurs in the diaphragm 13. Thereby, the pressure applied to the diaphragm 13 can be accurately detected.

The thin film 20 is formed by plasma CVD or reduced pressure C.
Since the cavity 17 can be sealed in an extremely low-pressure atmosphere by being formed by VD or the like, the reference pressure chamber 21 can be sealed with a necessary and sufficient degree of vacuum, thereby simplifying the entire process. Can be.

The small openings 19 may be formed so as to be uniformly scattered over the entire area where the cavities 17 are formed as shown in FIG. In this case, the small opening 19 is
Are formed in a scattered manner over the entire region corresponding to the above, so that the formation time of the cavity 17 can be shortened. in short,
An envelope area A (a two-dot chain line area shown in FIG. 9) surrounding the entire small opening 19 in a direction along the (111) plane is a cavity 17.
It is only necessary to correspond to the formation position.

(Second Embodiment) FIG. 10 shows a second embodiment of the present invention. The second embodiment is characterized in that the small opening is formed in a continuous slit shape having an edge portion. That is, the small opening 23 is formed in an L-shaped slit shape corresponding to a predetermined area corresponding to the cavity 17. When the alkali anisotropic etching is performed through the small opening 23 having such a shape, a slit-shaped small cavity (not shown) surrounded by the (111) plane is formed corresponding to the slit shape. In this case, in the small cavity, an edge portion having the (111) plane as a wall surface appears. When the etching proceeds to the edge portion, the etching proceeds on the (311) plane, and the (111) plane is etched.
Etching stops when it reaches the surface. Thus, the cavity 17 having the (111) plane as the wall surface can be formed, and the reference pressure chamber 21 can be formed in a sealed state by sealing the small opening 23 with the thin film 20.

According to the second embodiment, by forming the small openings 23 in a continuous slit shape, the cavities 17 can be formed in a short time by alkali anisotropic etching, and plasma CVD can be performed. Thin film 20
Since the reference pressure chamber 21 can be formed by the film formation, the pressure sensor can be manufactured at low cost as in the first embodiment.

The slit shape of the small opening 23 may be a U shape as shown in FIG. 11, a cross shape as shown in FIG. 12, or an X shape as shown in FIG. In short, it is only necessary that the envelope region A surrounding the entire small opening 23 along the (111) plane corresponds to the position where the cavity 17 is formed.

The present invention is not limited to the above embodiment, but can be modified or expanded as follows. If there is a layer serving as a stopper at the bottom of the etching, the bottom becomes a planar diaphragm, and finally an inverted trapezoidal cavity is formed.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a cross section of a sensor chip according to a first embodiment of the present invention.

FIG. 2 is a plan view of a sensor chip.

FIG. 3 is a diagram showing a procedure for manufacturing a sensor chip.

FIG. 4 is a rear view of the Si substrate showing a small opening.

FIG. 5 is an enlarged view of a Si substrate showing a state in which a small cavity corresponding to a small opening is formed.

FIG. 6 is a perspective view showing an edge portion of a cavity having a (111) plane as a wall surface.

FIG. 7 is a view corresponding to FIG. 6, showing a state where etching progresses on the (311) plane.

FIG. 8 is a sectional view of a sensor chip.

FIG. 9 is a view corresponding to FIG. 4, showing an embodiment of a modification;

FIG. 10 is a rear view of a Si substrate showing a second modification of the present invention.

FIG. 11 is a diagram corresponding to FIG. 10 showing an embodiment of a modification;

FIG. 12 is a diagram corresponding to FIG. 10 showing an embodiment of a modification;

FIG. 13 is a diagram corresponding to FIG. 10 showing an embodiment of a modification;

FIG. 14 is a schematic diagram showing a cross section of a conventional Si chip.

[Explanation of symbols]

11 is a semiconductor pressure sensor, 12 is a single crystal silicon substrate,
13 is a diaphragm, 17 is a cavity, 18 is an insulating film, 19
Is a small opening, 20 is a thin film, 21 is a reference pressure chamber, 22 is a small cavity, and 23 is a small opening.

Claims (7)

[Claims] 1. A single crystal silicon substrate having a (100) plane orientation and a (100) surface provided on the surface of the single crystal silicon substrate.
A diaphragm made of a surface, an insulating film provided on the back surface of the single crystal silicon substrate, and a cavity having a (111) plane as a wall surface formed in a depression on the back surface of the single crystal silicon substrate corresponding to the diaphragm And a small opening for injecting an etchant provided for forming the cavity in the insulating film by etching; and sealing the small opening provided on the insulating film to seal the cavity. And a thin film forming a reference pressure chamber according to (1). 2. The small opening is formed in a plurality in a region corresponding to the cavity, and the entire small opening is (1).
11) The semiconductor pressure sensor according to claim 1, wherein an envelope region surrounding in a direction along the plane is set corresponding to a region where the cavity is formed. 3. The small opening is formed in a continuous slit shape, and an envelope region surrounding the entire small opening in a direction along a (111) plane corresponds to a formation region of the cavity. The semiconductor pressure sensor according to claim 1, wherein the pressure is set. 4. A step of forming an insulating film on the back surface of a single crystal silicon substrate having a surface orientation of (100), forming a small opening in a predetermined region of the insulating film, and forming the small opening through the small opening. A step of forming a cavity having a (111) plane as a wall surface by etching an edge portion generated by performing alkali anisotropic etching on a surface having a high etching rate; and forming a thin film on the insulating film. Forming a reference pressure chamber by sealing the small opening and sealing the cavity. 5. A plurality of the small openings are formed in the predetermined region, and an envelope region surrounding the entire small opening in a direction along a (111) plane corresponds to a formation region of the cavity. An edge portion of a small cavity formed by communicating small cavities formed with the (111) plane as a wall surface by alkali anisotropic etching through the small opening is etched at a surface having a high etching rate. 5. The manufacturing of the semiconductor pressure sensor according to claim 4, wherein a cavity having a (111) plane as a wall surface and a diaphragm having a (100) plane are finally formed on the surface of the single crystal silicon substrate by repeating the above steps. Method. 6. The small opening portion is formed in a continuous slit shape having an edge portion, and the envelope region surrounding the entire small opening portion in a direction along a (111) plane is the cavity forming region. The edge of a continuous slit-shaped small cavity formed by alkali anisotropic etching through the small opening is etched at a surface having a high etching rate, and finally the (111) surface is formed. 5. The method for manufacturing a semiconductor pressure sensor according to claim 4, wherein a diaphragm having a (100) plane is formed on a cavity having a wall as a wall and a surface of the single crystal silicon substrate. 7. The method according to claim 4, wherein said thin film is formed in a vacuum atmosphere environment.