This application claims priority to Japanese Patent Application No. 2005-99247 filed Mar. 30, 2005, which is hereby expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention is related to a semiconductor sensor and a method of manufacturing a semiconductor sensor.
2. Description of the Prior Art
Conventionally, as one example of semiconductor sensors, a triaxial (3-axis) acceleration sensor is known that can detect acceleration in 3-axis directions, for example. The 3-axis acceleration sensor known in the art includes: a support frame of substantially rectangular shape made of silicon; a plurality of thin beams with flexibility provided inside the support frame; an actuation diaphragm (often including a weight member) oscillatably supported by the plurality of beams; and a plurality of bridge circuits constituted from piezo resistance elements respectively disposed on the beams and interconnected by metallic wiring lines.
In this type of 3-axis acceleration sensor, the actuation diaphragm oscillates as the 3-axis acceleration sensor is accelerated in a predetermined direction, in response to which the beams are subject to elastic deformation. This changes the resistance value of each of the piezo resistance elements, thereby varying the output voltages generated by the bridge circuits. Therefore, by sensing the variation of the output voltages, the 3-axis acceleration sensor can detect the acceleration applied.
As another example of the semiconductor sensors, a pressure sensor is also known that has the same configuration as the afore-mentioned 3-axis acceleration sensor but serves to detect the load applied to the actuation diaphragm.
In the meantime, Japanese Patent Laid-open Publication No. HEI. 7-234242 discloses a 3-axis acceleration sensor of the type having thin films made of oxide silicon or nitride silicon provided on the opposite surfaces of respective beams for the purpose of avoiding any warpage or distortion of the beams which would otherwise occur due to the heat applied to the 3-axis acceleration sensor in the manufacturing process thereof.
By the way, the 3-axis acceleration sensor taught in the '242 publication has metallic wiring lines, on one major surfaces of the respective beams, whose coefficient of thermal expansion drastically differs from that of the thin films made of oxide silicon or nitride silicon. For this reason, thermal stresses may be developed in the beams depending on the usage environmental temperature, thereby resulting in such an instance that the beams are deformed even when no load or acceleration is applied to the beams. In this case, in spite of the fact that no load or acceleration acts on the semiconductor sensor, the output voltages generated from a plurality of bridge circuits are unintentionally changed from a zero offset, which poses a problem in that acceleration or load cannot be detected in a precise and accurate manner.
Taking the above-mentioned and other problems into account, it is an object of the present invention to provide a semiconductor sensor and a method of manufacturing a semiconductor sensor that can absorb thermal stresses developed in beams due to the difference of coefficients of thermal expansion between the beams and metallic wiring lines, thereby preventing any inadvertent deformation of the beams.
In order to achieve the object, in one aspect of the present invention, the invention is directed to a semiconductor sensor. The semiconductor sensor of the present invention includes:
a frame having an opening;
an actuation diaphragm provided inside the frame in a spaced-apart relationship with respect to the frame;
a plurality of flexible beams provided to interconnect the frame and the actuation diaphragm, each of the flexible beams having piezo resistance elements thereon;
metallic wiring lines provided on one major surfaces of the respective flexible beams for connecting each of the piezo resistance elements to each other; and
a plurality of thermal stress absorbing portions provided on the other major surfaces of the respective flexible beams for absorbing thermal stresses developed in the beams due to the difference of coefficients of thermal expansion between the respective beams and the corresponding metallic wiring lines.
According to the semiconductor sensor having the configuration as described above, by providing the thermal stress absorbing portions on the other major surfaces of the respective flexible beams, the semiconductor sensor can prevent the beams from deforming attributable to the thermal stresses that may be developed in the beams due to the difference of coefficients of thermal expansion between the respective beams and the corresponding metallic wiring lines. This enables the semiconductor sensor to accurately detect physical quantities, such as acceleration, pressure and the like, which are measurable by the semiconductor sensor.
Further, in the semiconductor sensor of the present invention, it is preferable that each of the thermal stress absorbing portions comprises a film whose thickness is selected depending on a wiring pattern and a volume of the wiring lines in each of the beams.
By controlling the thickness of the film in this manner, it becomes possible to adapt the film to different wiring patterns of the metallic wiring lines and different kinds of metals and volumes of the metal used in forming the metallic wiring lines.
Moreover, in the semiconductor sensor of the present invention, it is preferable that the film is formed of a member selected from the group including metal, metal oxide and metal nitride.
This makes it possible to assure proper selection of a material for the film.
Furthermore, in the semiconductor sensor of the present invention, it is preferable that the semiconductor further includes a weight member bonded to one major surface of the actuation diaphragm, wherein, in the case where the semiconductor sensor is subject to acceleration, the actuation diaphragm and the weight member are displaced as a unit in response to the acceleration, and the semiconductor sensor is adapted to detect the acceleration based on the resistance values of the piezo resistance elements which vary with the amount of displacement of the actuation diaphragm and the weight member.
This makes it possible to precisely detect the acceleration given to the semiconductor sensor.
Further, in the semiconductor sensor of the present invention, it is preferable that, in the case where the actuation diaphragm receives a load, the actuation diaphragm is displaced in proportion to the magnitude of the load received, and the semiconductor sensor is adapted to detect the load based on the resistance values of the piezo resistance elements which vary with the amount of displacement of the actuation diaphragm.
This makes it possible to precisely detect the load exerted on the actuation diaphragm of the semiconductor sensor.
Further, in another aspect of the present invention, the invention is directed to a method of manufacturing a semiconductor sensor. The method of the present invention includes:
preparing a semiconductor substrate;
forming a plurality of piezo resistance elements on one major surface of the semiconductor substrate;
forming an actuation diaphragm by subjecting the semiconductor substrate to an etching process from the other major surface of the semiconductor substrate;
forming metallic wiring lines on the one major surface of the semiconductor substrate to connect each of the piezo resistance elements to each other;
removing a part of the semiconductor substrate to form a frame outside the actuation diaphragm in a spaced-apart relationship with respect to the actuation diaphragm and a plurality of flexible beams for interconnecting the actuation diaphragm and the frame, each of the flexible beams having a plurality of piezo resistance elements formed on one major surfaces of the corresponding flexible beam; and
forming thermal stress absorbing portions on the other major surfaces of the respective flexible beams, the thermal stress absorbing portions being adapted to absorb thermal stresses developed in the beams due to the difference of coefficients of thermal expansion between the respective beams and the corresponding metallic wiring lines.
According to the method of manufacturing a semiconductor sensor including the steps as described above, it is possible to manufacture a semiconductor sensor capable of effectively preventing deformation of the beams attributable to the thermal stresses that may be developed in the beams due to the difference of coefficients of thermal expansion between the respective beams and the corresponding metallic wiring lines. Therefore, it is possible to provide the method of manufacturing a semiconductor sensor that can accurately detect physical quantities, such as acceleration, pressure and the like, which are measurable by the semiconductor sensor.
Further, in the method of the present invention, it is preferable that the step of forming the thermal stress absorbing portions comprises the steps of:
forming a film on the other major surface of the semiconductor substrate in which the actuation diaphragm, the beams and the frame have been formed, the film being formed of a member selected from the group including metal, metal oxide and metal nitride; and
removing the film formed on the actuation diaphragm and the frame.
This makes it possible to form the film just on the other major surfaces of the beams.
Moreover, in the method of the present invention, it is preferable that the step of forming the film comprises one coating method selected from the group including a sputtering method, a vapor deposition method and a chemical vapor deposition method.
This helps to readily control the thickness of the film formed on the actuation diaphragm, the beams and the frame.
Furthermore, in the method of the present invention, it is preferable that the step of removing the film comprises an etching process.
This makes it possible to clearly remove the film formed on the other major surfaces of the actuation diaphragm and the frame with accuracy while the film leaving on the bottom major surfaces of the beams intact.
Further, in the method of the present invention, it is preferable that the method further includes the steps of:
bonding a glass substrate or a metal substrate to the other major surface of the semiconductor substrate after the step of forming the thermal stress absorbing portions; and
removing a part of the glass substrate or the metal substrate to form the frame and a weight member suspended from the plurality of flexible beams via the actuation diaphragm.
The semiconductor sensor manufactured by the method of the present invention allows the actuation diaphragm and the weight member to displace as a unit. This means that the weight (heaviness) of the weight member can be increased in case of using the semiconductor sensor thus manufactured as a 3-axis acceleration sensor. Accordingly, it becomes possible to manufacture a semiconductor sensor that can quite accurately detect the acceleration applied to the semiconductor sensor.
The foregoing and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment of the present invention given in conjunction with the accompanying drawings, in which:
Hereinafter, preferred embodiments of a semiconductor sensor and a method of manufacturing the same according to the invention will be described with reference to the appended drawings. First, the configuration of the semiconductor sensor according to the present invention will be described.
Referring to
The thermal stress absorbing portions 71 serve to absorb the thermal stresses that may be developed in the beams 19a, 19b due to the difference of coefficients of thermal expansion between the respective beams 19a, 19b and the corresponding aluminum wiring lines 33. According to the present invention, by providing the thermal stress absorbing portions 71 in the semiconductor sensor 1, it becomes possible to effectively prevent deformation of the beams 19a, 19b when no physical quantity (for example, load, acceleration or the like) targeted for detection is applied to the actuation diaphragm 16.
In this case, it is preferable that each of the thermal stress absorbing portions 71 is formed with a film whose thickness is selected depending on wiring patterns, kind of metal and volumes of the aluminum (metallic) wiring lines 33. As set forth later, it is preferable that the film is made of one of metal, metal oxide and metal nitride.
Now, description will be given for the configuration of the top major surface of the semiconductor sensor 1 in accordance with the present invention. As can be seen in
According to the semiconductor sensor 1 of the present invention, the piezo resistance elements 30a, 30c formed on the beams 19a are adapted to detect the forces applied to the actuation diaphragm 16 in X-axis and Z-axis directions in the rectangular coordinate system shown in
As set forth above, the piezo resistance elements 30a, 30b, 30c are connected to each other by means of the aluminum wiring lines 33. The aluminum wiring lines 33 are also connected to a plurality of aluminum pads 34 lying adjacent to the peripheral edges of the silicon frame 21. As best shown in
Next, operation of the semiconductor sensor 1 in accordance with the present invention will now be described in detail.
In the case where force is applied to the actuation diaphragm 16 of the semiconductor sensor 1, the beams 19a, 19b are elastically deformed and the resistance values of the piezo resistance elements 30a, 30b, 30c formed on the beams 19a, 19b vary with the displacement of the actuation diaphragm 16 of the semiconductor sensor 1. This results in variation of the output voltages of the three bridge circuits constituted from the piezo resistance elements 30a, 30b, 30c, the aluminum wiring lines 33 and the aluminum pads 34. The semiconductor sensor 1 of the present invention can detect the force applied to the actuation diaphragm 16 of the semiconductor sensor 1 by detecting such voltage variation.
In particular, the semiconductor sensor 1 of the present invention can play a role of a pressure sensor that detects a pressure at the time when the pressure (load) is applied to the actuation diaphragm 16. Further, by virtue of bonding a weight member 42 (see
Hereinafter, the semiconductor sensor 1 of the present invention will be described in detail with regard to a 3-axis acceleration sensor 2 which is an embodiment of the semiconductor sensor 1. In this regard, the following description will be centered on an instance that the weight member 42 is bonded to one major surface of the actuation diaphragm 16 in order to detect, with increased sensitivity, the acceleration applied to the 3-axis acceleration sensor 2. In this case, in view of the fact that the actuation diaphragm 16 serves to support the weight member 42, the actuation diaphragm 16 will be referred to as a weight member supporting portion 16 in the following description of the 3-axis acceleration sensor 2.
As shown in
The silicon substrate 10 includes: a silicon frame (a part of a frame) 21 having an opening; a weight member supporting portion 16 provided inside the silicon frame 21 in a spaced-apart relationship with respect to the silicon frame 21 for supporting a weight member 42 composed of a part of a glass substrate 40; four flexible beams 19a, 19b provided to interconnect the silicon frame 21 and the weight member supporting portion 16 in which the respective flexible beams 19a, 19b have piezo resistance elements 30a, 30b, 30c on their top major surfaces (one major surfaces); aluminum (metallic) wiring lines 33 (see
Further, in the present embodiment, the glass substrate 40 is formed of a Pyrex® glass. The glass substrate 40 includes a weight member 42, and eight glass frames (a part of the frame) 43 disposed around and outside the weight member 42 in a spaced-apart relationship with respect to the weight member 42.
Moreover, in the present embodiment, the base 50 is made of silicon. The base 50 includes a bonding portion 52 having a substantially rectangular ring-shape to be bonded to the bottom major surfaces of the eight glass frames 43 of the glass substrate 40, and a concave portion 51 having a substantially rectangular shape defined by the ring-shaped bonding portion 52. The concave portion 51 is so sized and arranged that it can prevent physical interference with the weight member 42 when components of the 3-axis acceleration sensor 2 are assembled together.
The top major surface of the silicon substrate 10 of the 3-axis acceleration sensor 2 has the same configuration as that of the semiconductor sensor 1 set forth above. Further, as with the semiconductor sensor 1, the thermal stress absorbing portions 71 described above are formed on the bottom major surfaces of the beams 19a, 19b of the silicon substrate 10. There is a possibility that the 3-axis acceleration sensor 2 may be broken if the weight member 42 makes contact with the bottom major surface of any of the beams 19a, 19b in the course of oscillating movement of the weight member 42 and the weight member supporting portion 16 as a unit. In order to avoid such a possibility, it is preferable that the weight member supporting portion 16 has a greater thickness than that of the actuation diaphragm 16 employed in the semiconductor sensor 1 described above. By bonding the weight member 42 to the bottom major surface of the weight member supporting portion 16 having such an increased thickness, the weight member 42 and the weight member supporting portion 16 can be displaced to an extent great enough to precisely detect the acceleration applied. In the following description, it should be understood that the weight member supporting portion 16 has a greater thickness than that of the actuation diaphragm 16 as described above.
According to the 3-axis acceleration sensor 2, the piezo resistance elements 30a, 30c formed on the beams 19a are adapted to detect the acceleration in X-axis and Z-axis directions in the rectangular coordinate system shown in
If acceleration is applied to the 3-axis acceleration sensor 2 having such a configuration, the weight member supporting portion 16 and the weight member 42 supported by the weight member supporting portion 16 oscillate as a unit. In response, the beams 19a, 19b are elastically deformed and the resistance values of the piezo resistance elements 30a, 30b, 30c formed on the beams 19a, 19b vary with the acceleration applied to the 3-axis acceleration sensor 2. This results in variation of the output voltages of the bridge circuits constituted from the piezo resistance elements 30a, 30b, 30c, the aluminum wiring lines 33 and the aluminum pads 34. Thus, the 3-axis acceleration sensor 2 can detect the acceleration applied to the 3-axis acceleration sensor 2 by detecting such voltage variation.
Next, description will now be given for one example of the method of manufacturing the 3-axis acceleration sensor 2 of the present embodiment using
First, a silicon substrate 10 is prepared as shown in
The silicon substrate 10 is heated in an oxidizing atmosphere so that the top and bottom major surfaces of the silicon substrate 10 can be oxidized as illustrated in
Next, impurities (boron in the present embodiment) are injected into the silicon substrate 10 from the side on which the silicon oxide layer 11 lies. Then, by subjecting the silicon substrate 10 to thermal treatment, the impurities are dispersed through the silicon layer 22. Thus, a plurality of first piezo resistance regions 31 are formed in the silicon layer 22 in the vicinity of an interfacial boundary of the silicon layer 22 and the silicon oxide layer 11, as illustrated in
Subsequently, parts of the silicon oxide layer 12 that correspond to the first piezo resistance regions 31 thus formed as described above are removed from the bottom major surface of the silicon substrate 10 by subjecting the silicon substrate 10 to an etching process. In addition, the parts of the silicon layer 22 from which the silicon oxide layer 12 has been removed are further etched to form concave portions 13 as illustrated in
Thereafter, impurities are once again injected into the silicon substrate 10 from the side on which the silicon oxide layer 11 lies. Then, by subjecting the silicon substrate 10 to thermal treatment, the impurities are dispersed through the silicon layer 22. In this case, at the parts adjoining the first piezo resistance regions 31, that is, at the outer sides of the first piezo resistance regions 31 in
Then, by subjecting the silicon substrate 10 to thermal treatment, silicon oxide layers 14 are formed on the partial areas of the concave portions 13. Thereafter, the silicon substrate 10 is subjected to an etching process so that the limited parts of the silicon layer 22 which has not been covered by the silicon oxide layers 12 or 14 are removed, thereby forming concave portions 15 of trapezoidal shape in section, as illustrated in
Subsequently, as illustrated in
Further, as illustrated in
Thereafter, an aluminum layer is vapor-deposited on the top major surface of the silicon oxide layer 11. By subjecting the aluminum layer to an etching process after carrying out patterning on the aluminum layer thus deposited, aluminum (metallic) wiring lines 33 and aluminum pads 34 (see
Subsequently, as can be seen in
As illustrated in
Subsequently, as illustrated in
Thereafter, the film 70 formed on bottom major surface of the weight member supporting portion 16, the silicon frame 21 and the recesses 13 are removed by means of an etching process. Thus, parts of the film 70 intact only on the bottom major surfaces of the beams 19a, 19b are left, as can be seen in
In addition, a glass substrate 40 made of Pyrex glass is prepared as illustrated in
Next, as illustrated in
Subsequently, as illustrated in
Next, abase 50 is prepared. The bottom major surfaces of the eight glass frames 43 are bonded to the bonding portion 52 of the base 50 (see
In this regard, the semiconductor sensor 1 shown in
Now, description will be given for the correlation of a temperature and a sensitivity ratio before and after application of the present invention.
More specifically,
The 3-axis acceleration sensor 2, which is one embodiment of the semiconductor sensor 1 of the present invention, can be advantageously used as: a sensor for detecting inclination or vibration of each of household electronic appliances and video appearances; a sensor for detecting the posture of potable game devices and game controllers; a security sensor mounted to a window or the like for detecting vibration thereof; a sensor for detecting the posture of robots; a sensor for detecting the posture of players to ascertain (or check) their forms in the field of sports such as golf; a sensor for detecting the dropping of electronic precision equipments; a counting sensor in step counters (that is, a pedometer) and the like.
As described in the foregoing, according to the semiconductor sensor 1 of the present invention, by forming the thermal stress absorbing portions 71 on the bottom major surfaces of the flexible beams 19a, 19b, it is possible to prevent deformation of the beams 19a, 19b attributable to the difference of coefficients of thermal expansion between the respective beams 19a, 19b and the corresponding aluminum (metallic) wiring lines 33.
This enables the semiconductor sensor 1 of the present invention to accurately detect physical quantities such as acceleration, pressure and the like, regardless of the usage environmental temperature.
Although the semiconductor sensor 1 and the method of manufacturing the semiconductor sensor 1 according to the invention has been descried with reference to the preferred embodiment shown in the drawings, the invention is not limited thereto.
For example, in the semiconductor sensor 1 of the present invention although it has been described that the thermal stress absorbing portions 71 are made of aluminum, it should be appreciated that the material for the thermal stress absorbing portions 71 is not limited to aluminum. For example, the material for the thermal stress absorbing portions 71 may include other metal, metal oxide and metal nitride, as long as they are capable of absorbing the thermal stresses developed in the beams 19a, 19b due to the difference of coefficients of thermal expansion between the respective beams 19a, 19b and the aluminum (metallic) wiring lines 33.
Further, in the semiconductor sensor 1 of the present invention, although it has been described that the film 70 is coated by means of a sputtering method or a vapor deposition method, the film coating method is not limited to the sputtering method or the vapor deposition method. For example, the film 70 may be formed by means of a chemical vapor deposition (CVD) method.
Moreover, in the semiconductor sensor 1 of the present invention, although it has been described that the weight member 42 is formed of glass by use of the glass substrate 40, the weight member 42 of the semiconductor sensor 1 is not limited to the one made of glass. For example, the weight member 42 of the semiconductor sensor 1 may be formed of a metal by use of a metal substrate.
Furthermore, in the semiconductor sensor 1 of the present invention, although it has been described that the film 70 formed on the bottom major surfaces of the beams 19a, 19b has a thickness of 0.15 μm, the thickness of the film 70 (that is, the thickness of each of the thermal stress absorbing portions 71) may be properly selected depending on wiring patterns, kind of metal and volumes of the aluminum (metallic) wiring lines 33 formed on the top major surfaces of the beams 19a, 19b.
Although a preferred embodiment of the present invention has been set forth in the foregoing, it will be apparent to those skilled in the art that various changes or modifications may be made thereto within the scope of the invention defined by the claims.
Number | Date | Country | Kind |
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2005-99247 | Mar 2005 | JP | national |