U.S. patent application number 09/832896 was filed with the patent office on 2002-04-25 for acceleration sensor and method of manufacturing the same.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Fukaura, Teruya, Nakamura, Kunihiro, Yamaguchi, Yasuo, Yamasaki, Shiro.
Application Number | 20020048838 09/832896 |
Document ID | / |
Family ID | 18799320 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048838 |
Kind Code |
A1 |
Yamaguchi, Yasuo ; et
al. |
April 25, 2002 |
Acceleration sensor and method of manufacturing the same
Abstract
providing an acceleration sensor in which a base portion and a
cap portion are bonded to each other and a sensor portion is sealed
off between these two, and which has an improved bonding strength
between the base portion and the cap portion. A sensor portion and
a frame portion surrounding a periphery of the sensor portion are
disposed on a semiconductor substrate. A base portion is comprised,
where a diffusion preventing layer and a non-doped polycrystalline
silicon layer are stacked one atop the other on the frame portion.
A cap portion is comprised, where a nickel layer is formed on a
base unit. The non-doped polycrystalline silicon layer of the base
portion and the nickel layer of the cap portion are bonded to each
other by eutectic bonding.
Inventors: |
Yamaguchi, Yasuo; (Tokyo,
JP) ; Nakamura, Kunihiro; (Tokyo, JP) ;
Yamasaki, Shiro; (Tokyo, JP) ; Fukaura, Teruya;
(Hyogo, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
2-3, Marunouchi 2-chome
Chiyoda-ku
JP
100-0004
|
Family ID: |
18799320 |
Appl. No.: |
09/832896 |
Filed: |
April 12, 2001 |
Current U.S.
Class: |
438/50 ;
257/415 |
Current CPC
Class: |
B81B 7/0077 20130101;
B81C 2201/019 20130101; G01P 15/0802 20130101; G01P 1/023 20130101;
B81C 1/00357 20130101 |
Class at
Publication: |
438/50 ;
257/415 |
International
Class: |
H01L 021/00; H01L
029/82 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 20, 2000 |
JP |
2000-321140 |
Claims
What is claimed is:
1. An acceleration sensor in which a base portion and a cap portion
are bonded to each other and a sensor portion is sealed off between
said base portion and said cap portion, comprising: a) a base
portion comprising: a semiconductor substrate; a sensor portion and
a frame portion surrounding a periphery of said sensor portion,
which are formed by a polycrystalline silicon layer deposited on
said semiconductor substrate and doped with an impurity; and a
diffusion preventing layer and a non-doped polycrystalline silicon
layer which are stacked one atop the other on said frame portion;
and b) a cap portion comprising: a base unit; and a nickel layer
formed on said base unit so as to be in contact with said non-doped
polycrystalline silicon layer of said base portion, wherein said
non-doped polycrystalline silicon layer of said base portion and
said nickel layer of said cap portion are bonded to each other by
eutectic bonding, and said sensor portion is sealed off between
said base portion and said cap portion.
2. The acceleration sensor according to claim 1, wherein said
diffusion preventing layer is a layer selected from a group of a
silicon oxide layer and a silicon nitride layer.
3. The acceleration sensor according to claim 1, wherein the
impurity is phosphorus.
4. The acceleration sensor according to claim 1, wherein said cap
portion includes a titanium layer between said base unit and said
nickel layer.
5. An acceleration sensor in which a base portion and a cap portion
are bonded to each other and a sensor portion is sealed off between
said base portion and said cap portion, comprising: a) a base
portion comprising: a semiconductor substrate; a sensor portion and
a frame portion surrounding a periphery of said sensor portion,
which are formed by a polycrystalline silicon layer deposited on
said semiconductor substrate and doped with an impurity; and a
non-doped polycrystalline silicon layer with a predetermined film
thickness stacked on said frame portion; and b) a cap portion
comprising: a base unit; and a nickel layer formed on said base
unit so as to be in contact with said non-doped polycrystalline
silicon layer of said base portion, wherein said non-doped
polycrystalline silicon layer of said base portion and said nickel
layer of said cap portion are bonded to each other by eutectic
bonding, and said sensor portion is sealed off between said base
portion and said cap portion, and the film thickness of said
non-doped polycrystalline silicon layer is larger than a distance
over which the impurity contained in said frame portion diffuses in
said non-doped polycrystalline silicon layer.
6. The acceleration sensor according to claim 5, wherein the film
thickness of said non-doped polycrystalline silicon layer is within
a range of 1 .mu.m to 5 .mu.m.
7. The acceleration sensor according to claim 5, wherein the
impurity is phosphorus.
8. The acceleration sensor according to claim 5, wherein said cap
portion includes a titanium layer between said base unit and said
nickel layer.
9. A method of manufacturing an acceleration sensor in which a base
portion and a cap portion are bonded to each other and a sensor
portion is sealed off between said base portion and said cap
portion, comprising the steps of: depositing a polycrystalline
silicon layer doped with an impurity on a semiconductor substrate;
processing said polycrystalline silicon layer, and forming a sensor
portion and a frame portion surrounding a periphery of said sensor
portion; stacking a diffusion preventing layer and a non-doped
polycrystalline silicon layer one after another on said frame
portion to thereby form a base portion; preparing a cap portion of
a base unit comprising a nickel layer in a contacting area with
said non-doped polycrystalline silicon layer of said base portion;
placing said cap portion on said base portion so as to contact said
nickel layer and said non-doped polycrystalline silicon layer to
each other; and heating said base portion and said cap portion,
bonding said non-doped polycrystalline silicon layer and said
nickel layer to each other by eutectic bonding, and sealing off
said sensor portion between said base portion and said cap
portion.
10. The method of manufacturing according to claim 9, wherein said
diffusion preventing layer is formed by a film selected from a
group of a silicon oxide layer and a silicon nitride layer.
11. The method of manufacturing according to claim 9, wherein the
impurity is phosphorus.
12. The method of manufacturing according to claim 9, wherein said
cap portion includes a titanium layer between said base unit and
said nickel layer.
13. A method of manufacturing an acceleration sensor in which a
base portion and a cap portion are bonded to each other and a
sensor portion is sealed off between said base portion and said cap
portion, comprising the steps of: depositing a polycrystalline
silicon layer doped with an impurity on a semiconductor substrate;
processing said polycrystalline silicon layer, and forming a sensor
portion and a frame portion surrounding a periphery of said sensor
portion; stacking a non-doped polycrystalline silicon layer with a
predetermined film thickness on said frame portion to thereby form
a base portion; preparing a cap portion of a base unit comprising a
nickel layer in a contacting area with said non-doped
polycrystalline silicon layer of said base portion; placing said
cap portion on said base portion so as to contact said nickel layer
and said non-doped polycrystalline silicon layer to each other; and
heating said base portion and said cap portion, bonding said
non-doped polycrystalline silicon layer and said nickel layer to
each other by eutectic bonding, and sealing off said sensor portion
between said base portion and said cap portion, wherein the film
thickness of said non-doped polycrystalline silicon layer is larger
than a distance over which the impurity contained in said frame
portion diffuses in said non-doped polycrystalline silicon layer at
the step of eutectic bonding.
14. The method of manufacturing according to claim 13, wherein the
film thickness of said non-doped polycrystalline silicon layer is
within a range of 1 .mu.m to 5 .mu.m.
15. The method of manufacturing according to claim 13, wherein the
impurity is phosphorus.
16. The method of manufacturing according to claim 13, wherein said
cap portion includes a titanium layer between said base unit and
said nickel layer.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an acceleration sensor and
a method of manufacturing the same, and more particularly, a
capacitive acceleration sensor in which a sensor element is sealed
off and a method of manufacturing the same.
[0002] FIGS. 3, 4A and 4B show steps of manufacturing a
conventional capacitive acceleration sensor. First, as shown in
FIG. 3, a polycrystalline silicon layer doped with phosphorus is
deposited on a silicon semiconductor substrate 1. Next, the
polycrystalline silicon layer is selectively removed, thereby
forming a sensor portion 12 and a frame portion 2. In the sensor
portion 12, an acceleration sensor element (not shown) is formed
using a conventional method. At this step, a base portion 10 of the
acceleration sensor is completed.
[0003] Next, as shown in FIG. 4A, a cap portion 20 for covering the
sensor portion 12 of the base portion 10 is prepared. FIG. 4A is a
cross sectional view along lines A-A in FIG. 3. At the step of
preparing the cap portion 20, first, a concave portion is formed in
one surface of a silicon substrate, and a cap main unit 5 is
accordingly formed. Next, a metallic layer 6 is formed so as to
coat the surface of the cap main unit 5 including the concave
portion. The metallic layer 6 is obtained by vapor depositing a
titanium layer on the cap main unit 5 and thereafter vapor
depositing a nickel layer.
[0004] Next, as shown in FIG. 4B, the cap portion 20 is placed over
the base portion 10 so that the metallic layer 6 overlaps the frame
portion 2 of the base portion 10. This is heated as it is to
400.degree. C., for instance, to thereby react silicon of the frame
portion 2 with nickel of the metallic layer 6 and hence form an
eutectic alloy. As a result, the base portion 10 and the cap
portion 20 are bonded to each other such that the sensor portion 12
is sealed off between these two, and an acceleration sensor
generally denoted at 101 is formed.
[0005] Since a bonding failure is created easily between the frame
portion 2 and the metallic layer 6 in the acceleration sensor 101,
there is a limit to an improvement in yield rate for manufacturing.
In addition, such bonding failures deteriorate the reliability of
the acceleration sensor 101.
[0006] An endeavor to identify a cause of this has led to a fact
that phosphorus contained in polycrystalline silicon of the frame
portion 2 diffuses and reaches the metallic layer 6 during heating
and precipitates at a junction interface between silicon and
nickel, and the precipitation causes an incomplete junction and a
dorp in bonding strength.
SUMMARY OF THE INVENTION
[0007] Accordingly, the present invention aims at providing an
acceleration sensor in which a frame portion of a base portion is
favorably bonded to a metallic layer of a cap portion, and
providing a method of manufacturing the same.
[0008] The present invention is directed to an acceleration sensor
in which a base portion and a cap portion are bonded to each other
and a sensor portion is sealed off between said base portion and
said cap portion, comprising:
[0009] a) a base portion comprising: a semiconductor substrate; a
sensor portion and a frame portion surrounding a periphery of said
sensor portion, which are formed by a polycrystalline silicon layer
deposited on said semiconductor substrate and doped with an
impurity; and a diffusion preventing layer and a non-doped
polycrystalline silicon layer which are stacked one atop the other
on said frame portion; and
[0010] b) a cap portion comprising: a base unit; and a nickel layer
formed on said base unit so as to be in contact with said non-doped
polycrystalline silicon layer of said base portion,
[0011] wherein said non-doped polycrystalline silicon layer of said
base portion and said nickel layer of said cap portion are bonded
to each other by eutectic bonding, and said sensor portion is
sealed off between said base portion and said cap portion.
[0012] Thus, forming the diffusion preventing layer, it is possible
to prevent diffusion of the impurity from the frame portion to the
eutectic bonding surface. As a result, it is possible to achieve
excellent eutectic bonding, and hence, to improve a bonding
strength.
[0013] The diffusion preventing layer is preferably a layer
selected from a group of a silicon oxide layer and a silicon
nitride layer.
[0014] This is for effectively preventing diffusion of the impurity
into the non-doped polycrystalline silicon layer.
[0015] The present invention is also directed to an acceleration
sensor in which a base portion and a cap portion are bonded to each
other and a sensor portion is sealed off between said base portion
and said cap portion, comprising:
[0016] a) a base portion comprising: a semiconductor substrate; a
sensor portion and a frame portion surrounding a periphery of said
sensor portion, which are formed by a polycrystalline silicon layer
deposited on said semiconductor substrate and doped with an
impurity; and a non-doped polycrystalline silicon layer with a
predetermined film thickness stacked on said frame portion; and
[0017] b) a cap portion comprising: a base unit; and a nickel layer
formed on said base unit so as to be in contact with said non-doped
polycrystalline silicon layer of said base portion,
[0018] wherein said non-doped polycrystalline silicon layer of said
base portion and said nickel layer of said cap portion are bonded
to each other by eutectic bonding, and said sensor portion is
sealed off between said base portion and said cap portion, and the
film thickness of said non-doped polycrystalline silicon layer is
larger than a distance over which the impurity contained in said
frame portion diffuses in said non-doped polycrystalline silicon
layer.
[0019] Thus, since the non-doped polycrystalline silicon layer has
a predetermined film thickness, it is possible to prevent the
impurity diffused from the frame portion from reaching the eutectic
bonding surface. As a result, it is possible to enhance a bonding
strength at the eutectic bonding surface.
[0020] The film thickness of said non-doped polycrystalline silicon
layer is preferably within a range of 1 .mu.M to 5 .mu.M.
[0021] Such a film thickness makes it possible to effectively
prevent the impurity from reaching the junction interface at the
step of bonding the base portion and the cap portion.
[0022] It is preferable that the impurity is phosphorus.
[0023] The cap portion may include a titanium layer between said
base unit and said nickel layer.
[0024] The present invention is further directed to a method of
manufacturing an acceleration sensor in which a base portion and a
cap portion are bonded to each other and a sensor portion is sealed
off between said base portion and said cap portion, comprising the
steps of: depositing a polycrystalline silicon layer doped with an
impurity on a semiconductor substrate; processing said
polycrystalline silicon layer, and forming a sensor portion and a
frame portion surrounding a periphery of said sensor portion;
stacking a diffusion preventing layer and a non-doped
polycrystalline silicon layer one after another on said frame
portion to thereby form a base portion; preparing a cap portion of
a base unit comprising a nickel layer in a contacting area with
said non-doped polycrystalline silicon layer of said base portion;
placing said cap portion on said base portion so as to contact said
nickel layer and said non-doped polycrystalline silicon layer to
each other; and heating said base portion and said cap portion,
bonding said non-doped polycrystalline silicon layer and said
nickel layer to each other by eutectic bonding, and sealing off
said sensor portion between said base portion and said cap
portion.
[0025] Thus, forming the diffusion preventing layer, it is possible
to prevent diffusion of the impurity from the frame portion to the
eutectic bonding surface.
[0026] Preferably, said diffusion preventing layer is formed by a
film selected from a group of a silicon oxide layer and a silicon
nitride layer.
[0027] The present invention is still further directed to a method
of manufacturing an acceleration sensor in which a base portion and
a cap portion are bonded to each other and a sensor portion is
sealed off between said base portion and said cap portion,
comprising the steps of: depositing a polycrystalline silicon layer
doped with an impurity on a semiconductor substrate; processing
said polycrystalline silicon layer, and forming a sensor portion
and a frame portion surrounding a periphery of said sensor portion;
stacking a non-doped polycrystalline silicon layer with a
predetermined film thickness on said frame portion to thereby form
a base portion; preparing a cap portion of a base unit comprising a
nickel layer in a contacting area with said non-doped
polycrystalline silicon layer of said base portion; placing said
cap portion on said base portion so as to contact said nickel layer
and said non-doped polycrystalline silicon layer to each other; and
heating said base portion and said cap portion, bonding said
non-doped polycrystalline silicon layer and said nickel layer to
each other by eutectic bonding, and sealing off said sensor portion
between said base portion and said cap portion, wherein the film
thickness of said non-doped polycrystalline silicon layer is larger
than a distance over which the impurity contained in said frame
portion diffuses in said non-doped polycrystalline silicon layer at
the step of eutectic bonding.
[0028] Since the non-doped polycrystalline silicon layer has a
predetermined film thickness, it is possible to prevent the
impurity diffused from the frame portion from reaching the junction
interface.
[0029] The film thickness of said non-doped polycrystalline silicon
layer is preferably within a range of 1 .mu.m to 5 .mu.m.
[0030] Such a film thickness makes it possible to effectively
prevent the impurity from reaching the junction interface at the
step of bonding the base portion and the cap portion.
[0031] It is preferable that the impurity is phosphorus.
[0032] The cap portion may include a titanium layer between said
base unit and said nickel layer.
[0033] As described clearly above, it is possible to prevent the
impurity from diffusing and reaching the junction interface between
the base portion and the cap portion, and hence, to achieve
excellent eutectic bonding, in the acceleration sensors according
to the present invention.
[0034] In consequence, a yield rate for manufacturing the
acceleration sensors improves.
[0035] Further, it is possible to improve the reliability of the
acceleration sensors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIGS. 1A-1C are cross sectional views of the acceleration
sensor according to the first embodiment.
[0037] FIG. 2 is a cross sectional view of the acceleration sensor
according to the second embodiment.
[0038] FIG. 3 is a top view of the base portion in the conventional
acceleration sensor.
[0039] FIGS. 4A and 4B are cross sectional views of the
conventional acceleration sensor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] First Embodiment
[0041] FIGS. 1A-1C are cross sectional views showing steps of
manufacturing an acceleration sensor according to a first
embodiment.
[0042] At these production steps, first, as shown in FIG. 1A, a
polycrystalline silicon layer is deposited on a semiconductor
substrate 1 of silicon and selectively removed, thereby forming a
sensor portion 12 and a frame portion 2 surrounding the sensor
portion 12. In the sensor portion 12, an acceleration sensor
element (not shown) is formed. This step is the same as in
conventional production for manufacturing an acceleration
sensor.
[0043] Next, as shown in FIG. 1B, a diffusion preventing layer 3 is
deposited on the semiconductor substrate 1 so as to coat the sensor
portion 12 and the frame portion 2. As the diffusion preventing
layer 3, silicon oxide or silicon nitride may be used, or
alternatively, metal such as aluminum and gold may be used.
[0044] Next, a non-doped polycrystalline silicon layer 4 is formed
to cover the diffusion preventing layer 3.
[0045] Next, using a conventional lithographic technique and an
etching technique, the non-doped polycrystalline silicon layer 4
and the diffusion preventing layer 3 are removed selectively so as
to leave only these two in a top portion of the frame portion
2.
[0046] Next, as shown in FIG. 1C, a cap portion 20 is placed on the
base portion 10. The cap portion 20 is formed similarly to the
conventional step shown in FIG. 4A. Thus, using a sputtering
method, for instance, a titanium layer is formed on a surface
including a concave portion of a cap main unit (base unit) 5 of
silicon, and further, a nickel layer is formed on the titanium
layer. With the cap portion 20 placed over the base portion 10, the
non-doped polycrystalline silicon layer 4 formed on the frame
portion 2 contacts the nickel layer of the metallic layer 6.
[0047] Next, this is heated as it is in a heating furnace to an
eutectic temperature of silicon and nickel, preferably, a
temperature between about 350.degree. C. and about 450.degree. C.,
and more preferably, about 400.degree. C. A heating time is from
about scores of minutes to about a few hours. During heating, the
heating furnace may be kept vacuum, or alternatively, may contain
inert gas such as nitrogen.
[0048] In this manner, heating causes silicon contained in the
polycrystalline silicon layer 4 and nickel contained in the
metallic layer 6 to form an eutectic alloy, so that the base
portion 10 and the cap portion 20 are bonded to each other. This
completes an acceleration sensor 100 in which a sensor element is
sealed off.
[0049] According to the first embodiment, since the diffusion
preventing layer 3 is formed on the frame portion 2, at the heating
step described above, phosphorus is prevented from diffusing from
the frame portion 2 into the non-doped polycrystalline silicon
layer 4.
[0050] In consequence, excellent eutectic bonding is obtained
without allowing precipitation of phosphorus at the junction
interface between the non-doped polycrystalline silicon layer 4 and
the metallic layer 6.
[0051] Second Embodiment
[0052] FIG. 2 is a cross sectional view of an acceleration sensor
101 according to a second embodiment. In the acceleration sensor
101 shown in FIG. 2, a non-doped polycrystalline silicon layer 7
with a predetermined film thickness is disposed instead of the
diffusion preventing layer 3 and the non-doped polycrystalline
silicon layer 4 of the acceleration sensor 100 shown in FIG. 1C.
The film thickness of the non-doped polycrystalline silicon layer 7
is preferably about from 1 .mu.m to 5 .mu.m, for example. Other
structures are the same as in the acceleration sensor 100.
[0053] In the acceleration sensor 101 according to the second
embodiment, the film thickness of the non-doped polycrystalline
silicon layer 7 is a predetermined thickness. Hence, at the heating
step, even if phosphorus diffuses from the frame portion 2 into the
non-doped polycrystalline silicon layer 7, phosphorus does not
reach the junction interface between the non-doped polycrystalline
silicon layer 7 and the metallic layer 6.
[0054] Hence, it is possible to obtain excellent eutectic bonding
without allowing precipitation of phosphorus at the junction
interface between the non-doped polycrystalline silicon layer 4 and
the metallic layer 6.
[0055] It is confirmed, according to a result of experiments by the
inventors, that phosphorus does not reach the junction interface if
the film thickness is at least even 1 .mu.m thicker with the
heating condition above.
[0056] The film thickness of the non-doped polycrystalline silicon
layer 7 may be changed in accordance with a heating temperature and
a heating time at the heating step.
[0057] While the first and the second embodiments relate to an
example that phosphorus doped the frame portion 2 diffuses, the
present invention is also applicable to where other dopant, such as
boron, gallium and arsenic, is used. In such a case, in the
acceleration sensor 101 according to the second embodiment, the
film thickness of the non-doped polycrystalline silicon layer 7 is
adjusted depending on the type of the dopant.
* * * * *