Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2002-10-28
2004-05-25
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514320, C073S504120
Reexamination Certificate
active
06739193
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a micromechanical structure, e.g., for an acceleration sensor or rotational speed sensor, including a substrate, which includes an anchoring device and a centrifugal mass, which is connected to the anchoring device via a flexible spring device, so that the centrifugal mass is elastically deflectable from its rest position. The present invention also provides a manufacturing method for said micromechanical structure.
Although it is usable on any micromechanical structure, the present invention and the problem on which it is based will be described in relation to a micromechanical rocker structure for an acceleration sensor or rotational speed sensor.
BACKGROUND INFORMATION
FIG. 6
shows a schematic top view of a known micromechanical structure in the form of a rocker structure for an acceleration sensor or rotational speed sensor;
FIG. 6
a
shows the micromechanical structure in top view and
FIG. 6
b
in a sectional view along line C-C′.
Micromechanical acceleration sensors and rotational speed sensors are conventional. The operation and configuration of an acceleration sensor are based on elastic vertical sensitivity, i.e., a direction of detection, to acceleration which is perpendicular to the chip plane.
A conventional micromechanical structure includes a substrate, which includes an anchoring device and a centrifugal mass in the form of a rocker including longitudinal and transverse bars and corresponding clearances, which is connected to anchoring device via a flexible spring device, so that centrifugal mass is elastically deflectable from its rest position.
This deflectability is implementable by etching a sacrificial layer
50
under centrifugal mass
30
. Sacrificial layer
50
is composed of a lower sacrificial sublayer
51
and an upper sacrificial sublayer
52
, between which electrode areas
60
are provided, which cooperate electrostatically with centrifugal mass
30
.
In this structure, sacrificial layer
50
is present in a first area situated under centrifugal mass
30
with a first etchable thickness d
1
, and in a second area situated under centrifugal mass
30
with a second etchable thickness d
1
+d
2
+d
3
, second thickness d
1
+d
2
+d
3
is greater than first thickness d
1
.
Thus, in this component, the movable components are situated in an upper electromechanically functional plane and are made of epitactical polysilicon. Under this plane, at a distance corresponding to sacrificial sublayer thickness d
1
, is a second electrically functional plane made of doped silicon, which acts as a capacitive counterelectrode to the upper functional layer.
This basic layer structure of such a vertically sensitive acceleration sensor is shown along line C-C′ in
FIG. 6
b
. Sacrificial layer
50
has been selectively removed according to
FIG. 6
b
. Sacrificial sublayer
52
has been left in place in the area of the anchoring device in order to connect the latter to underlying layers
60
,
51
, and thus to substrate
10
.
In order to manufacture this structure, sacrificial sublayer
51
including a layer thickness d
3
is deposited on underlying substrate
10
using a CVD method. The electrode layer made of doped silicon and including layer thickness d
2
is deposited on sacrificial sublayer
51
and structured to form electrode areas
60
. Subsequently sacrificial sublayer
52
including layer thickness d
1
is deposited on underlying electrode areas
60
, i.e., sacrificial sublayer
51
using the CVD method. Finally, centrifugal mass
30
is formed from an epitactical polysilicon layer, and sacrificial layer
50
is etched to provide deflectability.
When etching sacrificial layer
50
, i.e., sacrificial sublayers
51
,
52
, electrode areas
60
are not attacked, so that they act as etching depth stops. In contrast, etching between electrode areas
60
proceeds to substrate
10
.
The layer thickness of CVD sacrificial sublayers
51
,
52
is usually between 2.0 &mgr;m and 1.0 &mgr;m. They are usually made of TEOS (tetraethoxysilane) oxide, or of silane oxide.
TEOS is obtained via the following chemical reaction:
Si(OC
2
H
5
)
4
→SiO
2
+organic reaction products.
Silane oxide is obtained via the following reaction:
SiH
4
+4N
2
O→SiO
2
+4N
2
+2H
2
O
FIGS. 7
a
-
7
c
show enlarged details of the micromechanical structure according to
FIG. 6
underneath the movable bars during different phases of the sacrificial layer etching process.
The CVD deposition technique of sacrificial sublayers
51
,
52
causes contaminants
70
from the reaction products, such as for example organic components, to be incorporated in TEOS (schematically represented in
FIG. 7
for sacrificial sublayer
52
) or nitrogen inclusions to be incorporated in silane oxide.
This undesirable incorporation only slightly impairs the electrical insulating properties of sacrificial sublayers
51
,
52
, so that the microelectromechanical applications are unaffected.
However, contaminants
70
may cause a problem when they are not attacked during selective etching of sacrificial layer
50
or when they react with the etching medium forming non-soluble and/or non-volatile compounds as residues. During sacrificial layer etching, these residues become enriched from the already etched portions of the oxide on the SiO
2
etching front (see
FIG. 7
b
). They may cross-link, agglomerate to form larger structures, and remain on electrode areas
60
underneath movable centrifugal mass
30
as solid, non-conductive particles having a size of up to layer thickness d
1
of sacrificial sublayer
51
(
FIG. 7
c
). This creates the danger of these particles blocking the deflection of centrifugal mass
30
in the z direction or of producing an electrical short circuit therewith.
Intensive research has yielded the surprising finding that, due′ to the enrichment mechanism on the etching front (
FIGS. 7
a
-
7
c
), larger residue particles are formed in critical areas where during sacrificial layer etching multiple etching fronts come together shortly before the process end. In this case, only small particles having a small height, which are not critical for the functionality of z-sensitive centrifugal masses
30
, are formed when two etching fronts come together.
However, if three or more etching fronts come together, high particle structures may form, which greatly reduce the mechanical functional area.
FIGS. 8
a
-
8
c
schematically show critical areas of the micromechanical structure according to
FIGS. 6 and 7
.
Critical points in the micromechanical structure, i.e., the sensor configuration, where more than two etching fronts come together, are located
a) under end
36
of underetched, free-standing bar structures
35
(
FIG. 8
a
),
b) under flexion points
37
of underetched, free-standing bar structures
35
(
FIG. 8
b
),
c) under points of intersection
38
of underetched, free-standing bar structures
35
, as formed, for example, due to holes or clearances
39
for better underetching (
FIG. 8
c
).
For vertically sensitive acceleration sensors and rotational speed sensors, the above-named structure elements a) to c) are used to configure the mechanically functional plane. In the case of large-surface, horizontally arranged capacitance electrodes in the functional layer, the holes are arranged close together on the surface in order to achieve sufficient underetching. Previously, attention was only paid to optimum underetching or free etching of the mechanical structure, but not to possible etching residues.
The formation of etching residues at the above-mentioned points in the sensor configuration represents a problem in z-sensitive components in which sacrificial layer
50
is removed in a dry, isotropic etching step. Here they may not be detached or rinsed away by a liquid phase of the etching medium. They are critical for the functionality if they are located underneath the electromechanically functional structures such as, for example, the rocker structure of a z acceleration sensor, and t
Fischer Frank
Franz Jochen
Hein Peter
Henning Frank
Illing Matthias
Kenyon & Kenyon
Kwok Helen
Robert & Bosch GmbH
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