Method for fabricating a micromechanical component

Details Method for fabricating a micromechanical component Method for fabricating a micromechanical component

1999-04-29

2001-07-31

Chaudhuri, Olik (Department: 2823)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Physical stress responsive

C438S052000, C257S415000, C361S280000, C073S514210, C073S514320

Reexamination Certificate

active

06268232

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the fabrication of a micromechanical component, in particular a surface-micromechanical acceleration sensor, according to a method that involves preparing a substrate and providing an insulation layer on the substrate in which a patterned circuit trace layer is buried. A conductive layer is provided on the insulation layer, in which the conductive layer includes a first region and a second region. A movable element is configured in the first region by forming first trenches and by directing an etching agent through the first trenches to remove at least one portion of the insulation layer from underneath the conductive layer. A contact element is formed and electrically connected to the circuit trace layer, in the second region by configuring second trenches. The resultant movable element is encapsulated in the first region.
BACKGROUND INFORMATION
German Published Patent Application No. 195 37 814 A1 describes a known method for fabricating a micromechanical acceleration sensor.
Although the present invention as well as the problem definition underlying the present invention are applicable in principle to any micromechanical components, they will be elucidated with reference to this known surface-micromechanical acceleration sensor.
FIG. 2
shows a schematic, cross-sectional representation of the known micromechanical acceleration sensor, fabricated using known methods.
FIG. 3
is a schematic plan view of the subject matter of FIG.
2
.
FIG. 4
is an enlarged schematic representation of the contact region of the known micromechanical acceleration sensor according to
FIG. 2
to elucidate the problem definition underlying the present invention.
In
FIGS. 2 through 4
, reference symbol
100
generally denotes a micromechanical acceleration sensor. The micromechanical sensor includes a substrate
10
, a bottom oxide
1
, and a top oxide
2
. Sensor
100
also includes a printed circuit trace of LPCVD polysilicon
3
buried between the two oxides
1
,
2
, and top oxide
2
includes contact holes
4
. A layer of epitaxial polysilicon
6
(i.e., polysilicon deposited in an epitaxial reactor to achieve a higher deposition rate) is also provided in sensor
100
. Further included in sensor
100
are a bonding pad of aluminum
7
, a solder glass layer
8
, first trenches
9
, second trenches
9
′, and a bonding pad base (socket)
20
of epitaxial polysilicon, also referred to as a contact element. Other elements arranged as elements of the sensor
100
include a frame structure of epitaxial polysilicon
21
, a movable element
25
having an anchored region
22
and a free-standing region
23
, a polysilicon contact plug
60
as a part of layer
6
of polysilicon, an Si protective cap or wafer cap
13
, contact regions B
1
-B
5
, circuit trace regions L
1
-L
5
, a sensor core region I, a capping (encapsulation) edge area II, a bonding pad region III. Reference character S identifies a dirt particle.
When the known technique is used to fabricate this acceleration sensor, the thicknesses of the various layers are typically as follows:
aluminum bonding pad 7:
1.35
&mgr;m
layer 6 of polysilicon:
10.30
&mgr;m
second oxide 2:
1.60
&mgr;m
buried circuit trace layer 3:
0.45
&mgr;m
first oxide 1:
2.50
&mgr;m
When the customary technique is performed, patterns are exposed in the 10 &mgr;m thick layer
6
of polysilicon by forming trenches and by removing the underlying sacrificial layer (oxide
1
,
2
).
To obtain freely movable sensor elements in region I, the undercut-type etching is not only necessary, but also desired. On the other hand, in region III, an undercut-type etching is not at all desired, and would be detrimental for reasons elucidated in the following. Region II is completely covered with polysilicon and is used to hermetically encapsulate the sensor
100
with the aid of the Si protective cap
13
.
As is apparent from the enlarged representation of region III in
FIG. 4
, the 10 &mgr;m thick bonding pad base
20
of polysilicon, which has trenches formed in it in the same process step in which the movable structure is formed in sensor core region I, bears aluminum bonding pad
7
and is connected via contacting plug
60
to the underlying, thin LPCVD polysilicon circuit trace
3
. Prior to the etching of the sacrificial layer, LPCVD polysilicon circuit trace
3
is embedded between the two oxide layers
1
,
2
.
When each of the first and second oxide layer
1
,
2
is etched in that region which is simultaneously etched as a sacrificial layer for movable element
25
of the sensor
100
, top oxide
2
is completely removed, and lower oxide
1
is partially removed. The result is the undercut-type etching of bonding pad base
20
and of circuit trace layer
3
, as described.
At these locations, conductive dirt particles S, produced, in particular, during sawing (slicing) operations as sludge from the saw, can accumulate and lead to electrical shunting between bonding pad base
20
and substrate
10
, or between bonding pad base
20
and circuit trace layer
3
.
In addition, breaks can occur in the buried circuit trace layer
3
or in the undercut (laterally etched) epitaxial polysilicon edges of bonding pad base
20
, particularly during the high-pressure scrubbing performed subsequently to the sawing operation. The consequences are possible shunts, an increased resistance, and dirty break edges.
Generally it is a drawback that, subsequent to the etching of the sacrificial layer, circuit trace layer
3
is no longer covered with a dielectric material; this is in violation of standard IC- design safety regulations.
Generally, therefore, it would be advantageous to implement a method that easily avoids shunts of this kind.
A customary approach used to avoid undercutting the bonding pads provides for covering second trenches
9
′ around the bonding pads with a negative resist.
In terms of process technique, the disadvantage of this approach is the fact that difficulties arise when working with step heights of more than about 10 &mgr;m, and the fact that this approach is not even practicable for all sensor structures. Also, the negative resist must be removed immediately following the HF (high frequency) gas-phase etching of the sacrificial layer, since the HF causes it to become saturated, so that it can no longer prevent the undercut-type etching. Completely removing the resist in the trenches turns out to be complicated in this context, and even small amounts of residue can cause the comb-type structures to become cemented.
A second approach provides, at least temporarily, an additional protective layer in a designated region underneath and around the bonding pad base on the sacrificial layer to prevent the sacrificial layer from being completely undercut underneath the bonding pad base. This makes the manufacturing process more costly.
SUMMARY OF THE INVENTION
Compared to the known approach for solving the problem definition, the advantage of the method of the present invention is the implementation of an etching step in the manufacturing process that prevents any more undercutting (lateral etching) of the sacrificial layer underneath the bonding pad base, and eliminates the danger of shunts to the circuit traces laid bare around the contact region or of other shunts, for example to the substrate. Also, breaks no longer occur during high-pressure scrubbing operations. Thus, the reliability and proper functioning of the micromechanical component are significantly enhanced.
According to the present invention, the insulation layer in the second region remains protected by the conductive layer when the movable element is formed in the first region.
In other words, the conductive layer is used as a protective layer in that trenches are formed in it only in region I, but not in region III, during etching of the sacrificial layer. Thus, during etching of the sacrificial layer, only the sacrificial layer in the sensor's core region I is removed to produce the movable element. Trenches are expediently formed a seco

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