Method of fabricating a semiconductor acceleration sensor

A P-type impurity is diffused into an N-type epitaxial layer formed on a P-type silicon substrate. A gauge resistor for measuring deformation is formed on this epitaxial layer, with an aluminum wiring provided between the gauge resistor and a pad. Then, bottom-surface etching is performed on the resultant structure to remove a groove portion, thus forming a cantilever, weight portion and rim portion. The groove portion between the weight portion and rim portion is formed to penetrate through the P-type diffusion layer and become narrower toward the top surface from the bottom surface, thus preventing dust or the like from entering the groove portion. The etching from the bottom can reduce the number of required etching steps, ensuring lower fabrication cost.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor acceleration sensor and a 
method of fabricating the same. More particularly, this invention is 
directed to a semiconductor acceleration sensor which has its silicon 
substrate subjected to three-dimensional processing to fabricate the 
formation of a weight portion or the like, and a method of fabricating the 
same. 
2. Description of the Related Art 
Conventionally, a semiconductor acceleration sensor of this type employs a 
wafer having an epitaxial layer formed on a silicon substrate, and has a 
weight portion and a cantilever formed by etching both surfaces of the 
wafer. In the method of fabricating the semiconductor acceleration sensor, 
first the bottom surface of the epitaxial layer is etched to the depth of 
the epitaxial layer to form half of a groove and the cantilever. Then the 
resultant structure is etched from the top surface only to form a groove 
region to permit penetration thereto, thus providing a weight portion. 
FIG. 1 presents cross-sectional views exemplifying a step-by-step 
fabrication of a conventional semiconductor acceleration sensor. As shown 
in FIG. 1A, a conventional semiconductor diffusion process is performed on 
an epitaxial layer 2 epitaxially grown on a P-type silicon substrate 1. A 
gauge resistor 3 which detects acceleration, a pad 8 to be provided in a 
rim region 6A, and an aluminum wiring 11 which connects the gauge resistor 
3 and the pad 8 are formed in a cantilever region 4A. Reference numeral 
"9" shows an oxide film and "7A" is a groove region. After an oxide film 
13 is formed on the bottom surface of the P-type silicon substrate 1, 
etching is performed by an anodizing process to form an oxide film on a 
cantilever 4 and a groove portion 7, as shown in FIG. 1B. Since the 
silicon substrate 1 and the epitaxial layer 2 differ in a conductive type, 
the etching naturally stops at the bottom surface of the epitaxial layer 
2. Further, as shown in FIG. 1C, etching restarts from the top surface of 
the N-type epitaxial layer 2 to complete a weight portion 5, a rim portion 
6 around the weight portion 5, and the groove portion 7, thus providing a 
semiconductor acceleration sensor. 
The above-described conventional semiconductor acceleration sensor involves 
problems in the structure of its weight portion or groove portion, so that 
dust or the like can easily enter that portion, deteriorating the 
sensitivity characteristics. 
Further, according to the above conventional fabrication method, the 
overall process becomes complicated due to the following steps required in 
the etching process of the weight portion, groove portion or cantilever: 
(1) Exposing and developing procedures have to be conducted twice in 
association with the bottom-surface and top-surface etching procedures. 
(2) A jig and tools used in the top-surface etching procedure should be 
replaced when the bottom-surface etching is performed. 
(3) The bottom surface of the silicon substrate should be covered with a 
protective film such as silicon oxide when the top-surface etching is 
performed after the bottom-surface etching. 
The complicated process is likely to cause dust to enter the weight portion 
or groove portion during etching, resulting in accidents or yielding 
defects, and will increase the fabrication cost by the increased number of 
steps. 
SUMMARY OF THE INVENTION 
It is therefore a first object of the present invention to provide a 
semiconductor acceleration sensor which prevents dust or the like from 
entering a groove portion, and has improved sensitivity characteristics. 
It is a second object of the present invention to provide a method of 
fabricating a semiconductor acceleration sensor which has a reduced number 
of etching and exposing steps, simplifying the fabricating process. 
It is a third object of the present invention to provide a method of 
fabricating a semiconductor acceleration sensor which can avoid accidents 
during an etching process, and has an inexpensive fabrication cost. 
The semiconductor acceleration sensor according to the first aspect of the 
present invention comprises a silicon substrate of one conductivity type; 
an epitaxial layer of the opposite conductivity type formed on the 
substrate; a diffusion layer of the one conductivity type diffused in the 
epitaxial layer; a gauge resistor formed on the epitaxial layer; a thin 
film cantilever formed by etching a bottom surface of that portion of 
resultant structure which corresponds to the gauge resistor; a square rim 
portion having a top surface provided with a pad connected to the gauge 
resistor, and coupled to the cantilever; and a weight portion isolated 
from the rim portion by a groove portion which becomes wider toward a 
bottom surface from the top surface by the bottom-surface etching, and 
coupled to the rim portion only through the cantilever. 
Meanwhile, the method of fabricating a semiconductor acceleration sensor 
according to the second aspect of the present invention comprises the 
steps of forming an epitaxial layer on a silicon substrate of one 
conductivity type, the epitaxial layer having the opposite conductivity 
type; diffusing impurities of the one conductivity type into the epitaxial 
layer deeper than the depth of the epitaxial layer to form a diffusion 
layer of the one conductivity type; forming a gauge resistor on the 
epitaxial layer and wiring the gauge resistor to a pad; and etching the 
silicon substrate from a bottom surface thereof to remove the silicon 
substrate to the depth of a top surface thereof at that portion where the 
diffusion layer is present to thereby form a groove portion, the etching 
automatically stopping at the epitaxial layer at that portion where the 
gauge resistor is present to thereby form an anodized film to provide a 
cantilever. 
Further, the method of fabricating a semiconductor acceleration sensor 
according to the third aspect of the present invention comprises the steps 
of forming an epitaxial layer on a silicon substrate of one conductivity 
type, the epitaxial layer having the opposite conductivity type; diffusing 
impurities of the one conductivity type into the epitaxial layer deeper 
than the depth of the epitaxial layer to form a diffusion layer of the one 
conductivity type and making a bottom surface of the diffusion layer 
smaller than a groove region; forming a gauge resistor on the epitaxial 
layer and wiring the gauge resistor to a pad; and etching the silicon 
substrate from a bottom surface thereof to remove the silicon substrate to 
the depth of a top surface thereof at a center of that portion where the 
diffusion layer is present and automatically stopping the etching at the 
epitaxial layer at a peripheral portion to form an anodized film, thereby 
forming a groove portion having a step, the etching automatically stopping 
at that portion where the gauge resistor is present to thereby form an 
anodized film to provide a cantilever.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will now be described 
referring to the accompanying drawings. 
FIG. 2 is a partially cutaway perspective view illustrating a semiconductor 
acceleration sensor according to one embodiment of the present invention. 
As shown in FIG. 2, the semiconductor acceleration sensor of the present 
invention comprises a P-type silicon substrate 1, an N-type epitaxial 
layer 2 formed on the substrate 1, a P-type diffusion layer (not shown) 
diffused in the epitaxial layer 2, a gauge resistor 3 formed on the 
epitaxial layer 2, a thin film cantilever 4 formed by etching the bottom 
surface of the substrate at the position of the gauge resistor 3, a rim 
portion 6 having a pad 8 formed on its top surface, which is connected to 
the gauge resistor, and a weight portion 5 isolated from the rim portion 6 
by a groove portion 7 which is formed to become wider toward the bottom 
surface from the top surface by the bottom-surface etching. The rim 
portion 6 is formed to be a square shape around the weight portion 5 and 
coupled to the weight portion 5 only via the cantilever 4. In this 
embodiment, an N/P-type silicon wafer formed on the P-type silicon 
substrate 1 is used for the N-type epitaxial layer 2, and the gauge 
resistor 3 is diffused in the N-type epitaxial layer 2 below which the 
cantilever 4 lies. The cantilever is the only portion which connects the 
weight portion 5 or the center portion of the semiconductor acceleration 
sensor to the rim portion 6 or the peripheral portion of the sensor. In 
the other portion than the cantilever 4, i.e., the groove portion 7, 
neither the epitaxial layer 2 nor the P-type silicon substrate 1 is 
present, thus separating the weight portion 5 from the rim portion 6. 
When acceleration or vibration is applied to this semiconductor 
acceleration sensor, the weight portion 5 would be deformed in the 
direction of the acceleration. The strain caused by that deformation of 
the weight portion 5 affords a piezoelectric resistant effect in the gauge 
resistor 3 above the cantilever 4, changing the resistance of the gauge 
resistor 3. The acceleration or vibration is detected as an electric 
signal by detecting this change in resistance through the pad 8. Since the 
groove portion 7 defined between the weight portion 5 and the rim portion 
6 is formed wider toward the bottom surface from the top surface, it is 
possible to prevent dust or the like from entering the sensor, thus 
enhancing the sensitivity of the sensor. 
FIG. 3 presents cross-sectional views of an acceleration sensor in FIG. 1 
shown by each step for explaining a method of fabricating the same 
according to one embodiment of the present invention. As shown in FIG. 3A, 
after the N-type epitaxial layer 2 and an oxide film 9 are formed on the 
P-type silicon substrate 1, the oxide film 9 on a groove region 7A is 
removed, followed by diffusing a P-type impurity such as boron to form a 
P-type diffusion layer 10. The groove region 7A serves as a groove to 
separate the weight portion 5 from the rim portion 6. The impurity is to 
be diffused as deep as the depth of the N-type epitaxial layer 2 or 
deeper, so that the impurity reaches the P-type silicon substrate 1. 
Further, the impurity is to be diffused wider than the width of the groove 
portion 7 which is formed by etching. As shown in FIG. 3B, the oxide film 
9 on a cantilever region 4A is removed and impurity diffusion or 
ion-injection is performed on the epitaxial layer 2 to form the gauge 
resistor 3. Further, the pad 8 is adhered on the oxide film 9 in a rim 
region 6A and is connected to the gauge resistor 3 by an aluminum wiring 
11, as shown in FIG. 3C. The resultant structure is therefore connectable 
to the outside through the pad 8. At this time the P-type diffusion layer 
10 is covered with the oxide film 9. As shown in FIG. 3D, the cantilever 
region 4A and the groove region 7A are removed from an oxide film 13 
formed on the bottom surface of the P-type silicon substrate 1, and 
bottom-surface etching is performed on the resultant structure, thus 
yielding the cantilever 4, the weight portion 5, the rim portion 6 and the 
groove portion 7. The etching method to be used is an anodizing method 
(ANOX method), which will be described more specifically below. According 
to the anodizing method, a voltage is applied between the P-type silicon 
substrate 1 and the N-type epitaxial layer 2 in the reverse bias 
direction, and an electrochemical etching is performed in this condition. 
When the P-type silicon substrate 1 is etched and that part of the N-type 
epitaxial layer 2 which becomes the cantilever 4 is exposed to an etchant, 
a current flows from the N-type epitaxial layer 2 constituting the 
cantilever 4 to the etchant. This is because a PN junction between the 
P-type silicon substrate 1 and the N-type epitaxial layer 2 is etched out, 
thus facilitating the current flow. The current causes an oxidization 
reaction in the surface layer portion of the N-type epitaxial layer 2 
which has been exposed to the etchant, thus forming an anodized film 12 on 
the surface. Consequently, etching to the N-type epitaxial layer 2 will 
stop. Since a P-type impurity such as boron has been diffused in the other 
portion than what becomes the cantilever 4 or the groove portion 7, there 
is no PN junction which is a condition for causing anodization. Etching of 
the groove portion 7 will not therefore stop and will proceed until it 
penetrates the substrate to reach the wafer surface. In other words, 
etching of the portion which becomes the cantilever 4 will automatically 
stop due to the anodization, while etching of the portion which becomes 
the groove portion 7 will proceed to separate the weight portion 5 from 
the rim portion 6, thus providing the acceleration sensor in the final 
form. 
FIG. 4 presents cross-sectional views for explaining a semiconductor 
acceleration sensor by each step and a method of fabricating the same 
according to another embodiment of the present invention. The impurity 
diffusion region is made relatively narrow according to this embodiment, 
as shown in FIG. 4A, whereas, according to the previous embodiment, the 
top portion of the groove portion 7 is made relatively wide and the P-type 
impurity diffusion layer 10 is made wider than the groove portion 7 which 
is formed by the bottom-surface etching. The design of this embodiment 
makes the top portion of the formed groove portion 7 considerably narrower 
than that of the previous embodiment, making it difficult for dust or the 
like to enter that portion. 
According to the method of fabricating the semiconductor acceleration 
sensor, as shown in FIG. 4A, a P-type impurity, such as boron, is diffused 
in the groove region 7A. The impurities may be diffused as deep as the 
depth of the N-type epitaxial layer 2, or deeper so as to reach the P-type 
silicon substrate 1. The width of the diffusion region is however to be 
made narrower than that formed in the previous embodiment. Then, as shown 
in FIG. 4B, the gauge resistor 3, pad 8 and aluminum wiring 11 are formed 
as per the previous embodiment. Further, as shown in FIG. 4C, the 
cantilever region 4A and the groove region 7A are etched to form the 
cantilever 4 and groove portion 7 as well as the weight portion 5 and rim 
portion 6. The etching is done using an anodizing method, as in the 
previous embodiment. According to the anodizing method, when etching of 
the P-type silicon substrate 1 proceeds and the N-type epitaxial layer 2 
is exposed to an etchant, forming the anodized film 12 on that portion. As 
the anodized film 12 is formed on the entire surface of the cantilever 4, 
etching will automatically stop. With regard to the groove portion 7, the 
anodized film 12 is formed only on that portion where the N-type epitaxial 
layer 2 is exposed, and etching will not automatically stop in that 
portion in the P-type diffusion layer 10 where the P-type impurity has 
been diffused. Consequently, the groove portion 7 narrower than the 
opening formed in the oxide film 13 at the bottom will be formed on the 
top surface of the resultant structure. 
Although two embodiments of a semiconductor acceleration sensor and a 
method of fabricating the same have been described, the present invention 
may be realized even if the conductivity types, i.e. P-type and N-type are 
reversed. 
As described above, in the semiconductor acceleration sensor according to 
the first aspect of the present invention, impurities of the same 
conductivity type as that of the silicon substrate are diffused in the 
epitaxial layer on the substrate, and that portion is etched from the 
bottom to form a groove portion which becomes narrower toward the top 
surface, thus preventing dust or the like from entering the groove portion 
and improving the sensitivity. 
The method of fabricating the semiconductor acceleration sensor according 
to the second aspect of the present invention needs a single etching 
process from the bottom to form a cantilever, multibeam or the like, thus 
reducing the number of etching steps and exposure steps and lowering the 
fabrication cost. 
According to the method of fabricating the semiconductor acceleration 
sensor according to the third aspect of the present invention, the groove 
portion of the epitaxial layer can be made narrower and the opening in the 
oxide film at the bottom of the silicon substrate can be made wider by 
adjusting the width of the impurity diffusion layer to be formed on the 
epitaxial layer. This design can improve the etching speed and reduce the 
possibility of dust or the like from entering the groove portion during 
etching to affect the etching.