U.S. patent application number 10/004145 was filed with the patent office on 2003-04-24 for method of anodically bonding a multilayer device with a free mass.
Invention is credited to Sawyer, William David.
Application Number | 20030077876 10/004145 |
Document ID | / |
Family ID | 21709387 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030077876 |
Kind Code |
A1 |
Sawyer, William David |
April 24, 2003 |
METHOD OF ANODICALLY BONDING A MULTILAYER DEVICE WITH A FREE
MASS
Abstract
A method of anodically bonding a multilayer device with a free
mass includes positioning a support layer on either side of a free
mass structure including a free mass with an electrode on each
layer proximate the free mass; connecting both electrodes and the
free mass to a node at a floating potential and applying a voltage
across the layers and free mass structure to bond at least one of
the layers to the free mass structure.
Inventors: |
Sawyer, William David;
(Lexington, MA) |
Correspondence
Address: |
Iandiorio & Teska
260 Bear Hill Road
Waltham
MA
02451-1018
US
|
Family ID: |
21709387 |
Appl. No.: |
10/004145 |
Filed: |
October 23, 2001 |
Current U.S.
Class: |
438/455 |
Current CPC
Class: |
G01C 19/5719 20130101;
H01L 21/6835 20130101; B81C 1/00976 20130101; B81C 1/0092 20130101;
B81C 2203/031 20130101; B81C 3/001 20130101 |
Class at
Publication: |
438/455 |
International
Class: |
H01L 021/30 |
Claims
What is claimed is:
1. A method of anodically bonding a multilayer device with a free
mass comprising: positioning a support layer on either side of a
free mass structure including a free mass with an electrode on each
layer proximate the free mass; connecting both electrodes and the
free mass to a node at a floating potential; and applying a voltage
across the layers and free mass structure to bond at least one of
the layers to the free mass structure.
2. The method of anodically bonding a multilayer device with a free
mass of claim 1 in which said free mass is a proof mass.
3. The method of anodically bonding a multilayer device with a free
mass of claim 1 in which said layers and free mass include
different materials.
4. The method of anodic ally bonding a multilayer device with a
free mass of claim 3 in which said layers include glass and said
free mass includes silicon.
5. The method of anodically bonding a multilayer device with a free
mass of claim 1 in which said node is c ontiguous with said
multilayer device.
6. The method of anodically bonding a multilayer device with a free
mass of claim 1 in which said multilayer device is one of a
plurality formed in a wafer structure.
7. The method of anodically bonding a multilayer device with a free
mass of claim 6 in which each said multilayer device has a node
associated with it.
8. The method of anodically bonding a multilayer device with a free
mass of claim 7 in which said wafer structure is diced into
individual multilayer devices and the dicing disconnects the layers
and free mass from said floating potential node.
9. The method of anodic ally bonding a multilayer device with a
free mass of claim 6 in which said wafer structure includes two
wafers which form the support layers with a third wafer between
them forming the free mass structure.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method for anodically bonding a
multilayer device with a free proof mass or other mass.
BACKGROUND OF THE INVENTION
[0002] MEMS devices with free proof masses such as accelerometers
and gyroscopes typically mount the free proof mass structure to the
base layer using anodic bonding. The base layer typically includes
a sense plate or electrode to sense the movement of the proof mass.
Anodic bonding applies a high voltage e.g. 1000 volts across the
free proof mass structure and base layer to effect the bond. In
order to prevent the high voltage from attracting and bonding the
free proof mass to the base layer, the free proof mass is kept in
its bulk form until after the anodic bonding: the strength of the
unfinished free proof mass structure is sufficient to prevent it
from being flexed or drawn into contact with the base layer. Then,
after bonding, the bulk of the free proof mass structure is removed
leaving the suspended free proof mass. In order to improve the
signal sensed from the motion of the proof mass a second, top,
layer with another sense plate or electrode is mounted on the other
side of the free proof mass opposite the base layer. Now any motion
of the proof mass is sensed by both sense plates effectively
doubling the signal strength. However, anodic bonding of this
second layer presents a problem because now the proof mass is
indeed free and applying the anodic bonding voltage across the
layers will cause the free proof mass to be attracted and bonded to
one of the layers. To combat this problem it has been suggested to
ground the sense plates and the proof mass to prevent the
attraction and bonding of the proof mass during anodic bonding. N.
Ito, K. Yamada, H. Okada, M. Nishimura, T. Kuriyama, A Rapid and
Selective Anodic Bonding Method, International Conference on
Solid-State Sensors and Actuators, And Eurosensors IX, Proceedings
V1, pg. 227-280, 1995. However, this solution introduces a
substantial increase in complexity in the manufacturing process.
Both the base and top sense plates must be connected to ground
potential. In order to do this holes or vias must be made in the
base and top layers, typically glass, which support the sense
plates. These vias must be filled with metal or some conductor in
order to establish an electrical connection between the sense
plates and ground. In addition the proof mass, usually made of
silicon must also be electrically connected to ground. All three of
these electrical connections are difficult to effect and require
several processing steps which add cost to the device. All of these
vias and connections must be provided for each chip on a wafer
which contains hundreds or even thousands of said chips. Further,
after fabrication these ground connections must be removed to
ensure reliable operation.
BRIEF SUMMARY OF THE INVENTION
[0003] It is therefore an object of this invention to provide an
improved method of anodically bonding a multilayer device with a
free mass.
[0004] It is a further object of this invention to provide such an
improved method of anodically bonding a multilayer device with a
free mass which requires no additional, external vias or
connections.
[0005] It is a further object of this invention to provide such an
improved method of anodically bonding a multilayer device with a
free mass which dramatically reduces complexity and increases
yield.
[0006] It is a further object of this invention to provide such an
improved method of anodically bonding a multilayer device with a
free mass which requires no extra step in processing to undo
connections.
[0007] This invention results from the realization that the free
mass can be prevented from unwanted attachment during anodic
bonding not just by grounding the effected parts which requires
complex and potentially problematic connections and added
processing steps but by simply electrically connecting the effected
parts together internally to a floating potential which is easily
cut when the wafer is diced into chips and more particularly that a
multilayer device with a free mass can be more easily and simply
anodically bonded by positioning a support layer on either side of
the free mass with an electrode on each layer proximate the free
mass, connecting both electrodes and the free mass, to a node at a
floating potential and applying a voltage across the layers and
more to bond at least one of the layers to the silicon layer.
[0008] This invention features a method of anodically bonding a
multilayer device with a free mass including positioning a support
layer on either side of a free mass structure which includes a free
mass. There is an electrode on each layer proximate the free mass.
Both electrodes and the free mass are connected to a node at a
floating potential. A voltage is applied across the layers and the
free mass structure to bond at least one of the layers to the free
mass structure.
[0009] In a preferred embodiment the free mass may be a proof mass.
The layers and free mass may include different materials. The
layers may include glass and the free mass may include silicon. The
node may be contiguous with the multilayer device. The multilayer
device may be one of a plurality formed in a wafer structure. Each
multilayer device may have a node associated with it. The wafer
structure may be diced into individual multilayer devices and the
dicing may disconnect the layers and free mass from the floating
potential node. The wafer structure may include two wafers which
form the support layers with a third wafer between them forming the
free mass structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Other objects, features and advantages will occur to those
skilled in the art from the following description of a preferred
embodiment and the accompanying drawings, in which:
[0011] FIGS. 1, 2, and 3 illustrate the basic steps in anodic
bonding of a multilayer device having a free mass;
[0012] FIG. 4 is a schematic top plan view of a prior art device
for preventing the free mass from inadvertently bonding during the
bonding of the layers;
[0013] FIG. 5 is a schematic side elevational view of the prior art
device of FIG. 4;
[0014] FIGS. 6 and 7 are views similar to FIGS. 4 and 5 of a new
improved technique according to this invention for anodic bonding
of a multilayer device with a free mass;
[0015] FIG. 8 is a schematic top plan view of a wafer structure
including a plurality of multilayer devices with a free mass
according to this invention as shown in FIGS. 6 and 7; and
[0016] FIG. 9 is a block diagram showing the method of anodically
bonding a multilayer device with a free mass according to this
invention.
PREFERRED EMBODIMENT
[0017] There is shown in FIGS. 1, 2, and 3 the broad steps in
making an anodically bonded multilayer device 10 with a free mass.
As shown in FIG. 1 there is a lower support layer 12 and free mass
bulk plate 14 having substantial bulk. Typically lower support
plate 12 is made of glass and contains sense plate or electrode 16
for sensing the electric field associated with the movement of free
mass 18 shown still imbedded as a part of the free mass bulk plate
14. Free mass bulk plate 14 is typically silicon and provides
scribe line 20 which forms the outer boundary of device 10. Also
made of silicon are posts 22 and 24 and anchors 26 and 28 which
will eventually forma part of the free mass structure 17 along with
free mass 18. The anodic bonding is accomplished by applying a high
voltage such as at terminal 30 to layer 12 and grounding such as at
terminal 32 free mass bulk plate 14. The sheer bulk of plate 14
prevents the unfinished free mass 18 from being drawn down towards
sense plate 16 and layer 12 where it too would become bonded,
damaged and probably require the entire device 10 to be discarded
lowering the yield.
[0018] After the bonding takes place, an etching is done so that
all of the free mass bulk plate 14, typically made of silicon, is
removed above the dashed line indicated at 34 leaving just the free
mass structure 17. All of the material that is above the cross
hatch would then be removed. The resultant device 10a is shown in
FIG. 2. In a multilayer device a second upper support layer 40,
FIG. 3 is added which rests on scribe line 20 and posts 22 and 24.
Layer 40 also may be made of glass and includes another electrode
or sense plate 42.
[0019] Anodic bonding may now be applied again through a voltage,
typically 1000 volts applied at 30' at support layer 40 while
support layer 12 is grounded as at 32'. While in the description of
FIGS. 1, 2, and 3 anodic bonding takes place first as shown at FIG.
1 with respect to layer 12 and then a second time as shown in FIG.
3 with respect to layer 40. This is not a necessary limitation of
the invention. For example the anodic bonding in FIG. 1 of layer 12
can be delayed, and both layers 12 and 40 can be anodically bonded
at the same time, for example in the condition shown in FIG. 3. It
should also be noted that while a single multilayer device 10, 10a
and 10b is described here, in fact, a plurality of them, typically
as many as a hundred or more are made simultaneously in which case
layers 12 and 14 are actually constituted by large glass wafers and
the silicon layer in between is a third large wafer so that in fact
there is a triple stack wafer on which many of these devices 10 are
made simultaneously.
[0020] A problem arises because when the anodic bonding voltage is
applied, as shown in FIG. 3, free mass 18 in free mass structure 17
is truly free and can swing in either direction toward layer 12 or
layer 14 where it too will become bonded and damaged and destroy
the usability of the device 10. In order to prevent this in the
prior art each of the layers, 12 and 40, and free mass 18 are
connected to ground. This adds additional complexity and processing
steps to the fabrication process.
[0021] Such a technique is shown in FIGS. 4 and 5. In this case a
single multilayer device 10c, FIGS. 4 and 5 includes scribe line
20c which is actually the position for the saw cut or kerf when the
chips are sawed or diced from the wafer. A seal ring is also
present for the purposes of supporting and sealing each device
individually. The seal ring is not shown here for purpose of
clarity, but it is well understood by those skilled in the art.
Included in free mass structure 17 is free mass 18c supported on
four anchors, 26c, 26cc, 28c and 28cc; also included are silicon
posts 22c and 24c. The electrode or sense plate 16c can be seen
beneath free mass 18c. In order to make the connections to ground,
this prior art technique requires that each of the electrode or
sense plates 16c and 42c and the free mass 18c must be connected to
ground. Electrode or sense plate 16c is connected to ground by
connecting it through conductor 46c in via 48c, which then connects
to contact pad 50 and then to ground 52c. Similarly electrode or
sense plate 42c is connected to conductor 54c in via 56c, to
contact pad 58c and then to ground at 60c. And free mass 18c is
connected to conductor 62c in via 64c, to contact pad 66c and then
to ground at 68c. Thus, for each of the three connections
processing steps must be added in order to create the vias, fill
the vias, connect them to the outside contacts and then connect the
outside contacts to ground. These outside or external contacts are
off-wafer so that each of these grounds shown at 52c, 60c and 68c
are not proximate the device or individual chip but must be brought
across the wafer somehow with all the similar triple ground
connections from each of the other multilayer devices to some
ground point off the entire wafer. This makes for an extremely
complex and drawn out fabrication process and since there are so
many additional steps the reliability suffers and the yield is
naturally lower. In addition, a separate step or steps must be
taken after the processing is complete to sever these connections
from ground so that the system can operate as intentioned.
[0022] In accordance with this invention the upper and lower
layers, through their respective electrode or sense plates, are
connected not to ground through a complex process, but to a
floating point potential. All that is necessary is that the three
parts which are in proximity to each other during the anodic
bonding be at the same potential so that they are not attracted to
one another thereby distorting or deforming the free mass into
contact and bonding with one of the layers. The potential that they
are connected to for this purpose need not be ground and in fact it
is found in accordance with this invention, that it is far more
advantageous to not use ground but to simply use a floating point
potential or node to connect all three of these layers together.
Thus, referring to FIGS. 6 and 7 it can be seen that sense plate
42d is connected through conductor 70d to post 22d and then by
conductor 72d to contact 74d. Contact 74d is connected by conductor
76d to scribe line 20d which is conductive. Free mass 18d is
connected to all four of its anchors 26d, 26dd, 28d and 28dd. They
in turn through conductor 78d are connected to contact pad 80d
which in turn is connected to conductor 82d to the same conductive
scribe line 20d. Electrode or sense plate 16d is connected through
conductor 84d to contact pad 86d which through conductor 88d
connects to scribe line 20d. The scribe line 20d is actually the
scribe line for saw-cut along which the individual multilayer
device chips are cut to dice the wafer. Holes 90 and 92 may be cut
in the top wafer to provide access to terminals 74d, 80d and 86d so
that these terminals can function as four connections to the
outside world. Thus, it can be seen that all three of the effected
parts, electrode or sense plates 16d and 42d as well as free mass
18d are all connected to the same potential node, scribe line 20d,
during the anodic bonding, without any need for additional
processing steps or materials, in order to prevent free mass 18d
from being drawn to either electrode 16d or electrode 42d during
the anodic bonding process. And after the anodic bonding process is
completed a saw cut whose kerf follows 20d by design, cuts all of
the connections so that those three parts are no longer
electrically connected together or to the same node. The result is
that when a wafer such as the triple stack wafer 100, FIG. 8
according to the specific embodiment is diced the cuts along scribe
line 20 not only dices the multilayer devices 10 into individual
chips but it also severs the electrical connection of the upper and
lower support layers and the free mass from each other and the
floating potential node constituted by those scribe lines. Triple
stack wafer 100 actually includes in the construction of FIG. 8,
three individual wafers: one containing layers 12d, one containing
layers 40d and one containing free proof mass structure 17d.
[0023] In simple terms the invention according to this invention
requires that the upper and lower layers be positioned with their
sense electrodes and the free mass structure 17 with the free mass
18 between them, step 110, FIG. 9. Then an electrical connection is
made between each electrode or sense plate and/or its layer and the
free mass to a node at some floating point potential, not ground,
as in step 112. The anodic bonding voltage is then applied across
the layers and the free mass structure and free mass in step 114 to
anodically bond the layers and free mass anchors after which, if,
as is typically but not necessarily the case, a plurality of these
multilayer devices were made at once on a wafer, the wafer is diced
to separate the devices into separate chips and cut the connections
to the floating potential node in step 116.
[0024] Although specific features of the invention are shown in
some drawings and not in others, this is for convenience only as
each feature may be combined with any or all of the other features
in accordance with the invention. The words "including",
"comprising", "having", and "with" as used herein are to be
interpreted broadly and comprehensively and are not limited to any
physical interconnection. Moreover, any embodiments disclosed in
the subject application are not to be taken as the only possible
embodiments.
[0025] Other embodiments will occur to those skilled in the art and
are within the following claims.
* * * * *