U.S. patent application number 11/647659 was filed with the patent office on 2007-05-17 for method for producing micromechanical structures and a micromechanical structure.
Invention is credited to Christoph Duenn, Wilhelm Frey.
Application Number | 20070111360 11/647659 |
Document ID | / |
Family ID | 32980949 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070111360 |
Kind Code |
A1 |
Frey; Wilhelm ; et
al. |
May 17, 2007 |
Method for producing micromechanical structures and a
micromechanical structure
Abstract
A method for producing micromechanical structures, in which a
functional layer is deposited onto a sacrificial layer, and the
sacrificial layer is removed again for the production of at least
one mechanical functional element, which is characterized by a
surface barrier layer, with which the functional layer begins on
the sacrificial layer, and which has a different state from the
remaining functional layer, is also removed at least to a
considerable part, or, on the functional layer, one layer or a
plurality of layers having at least approximately the same
properties with respect to stress in the layer or layers such as
the surface barrier layer is(are) applied. Additionally, a
micromechanical structure having a functional layer in which the
functional layer is constructed in such a way that the stresses are
neutralized or no stress gradient appears.
Inventors: |
Frey; Wilhelm; (Palo Alto,
CA) ; Duenn; Christoph; (Tuebingen, DE) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
32980949 |
Appl. No.: |
11/647659 |
Filed: |
December 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10817346 |
Apr 2, 2004 |
7176540 |
|
|
11647659 |
Dec 28, 2006 |
|
|
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Current U.S.
Class: |
438/48 |
Current CPC
Class: |
B81B 3/0072
20130101 |
Class at
Publication: |
438/048 |
International
Class: |
H01L 21/00 20060101
H01L021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 2, 2003 |
DE |
10314989.9 |
Claims
1. A method for producing micromechanical structures comprising:
depositing a functional layer onto a sacrificial layer; removing
the sacrificial layer again, for the production of at least one
mechanical functional element; and removing, at least to a
considerable part, a surface barrier layer, with which the
functional layer on the sacrificial layer begins, and which has a
different state from a remaining functional layer.
2. The method according to claim 1, further comprising: applying
the functional layer in a composition of the surface barrier layer
as seen in a direction of the remaining functional layer; and
etching at least one part of the surface barrier layer at least
initially selective to at least one of the surface barrier layer
and the remaining functional layer.
3. The method according to claim 2, further comprising: depositing
the functional layer, in at least one partial area of the surface
barrier layer, all the way into the remaining functional layer,
made of SiGe having a higher proportion of germanium than 65% to
70%, but less than 100%; and reducing the germanium proportion
during the depositing for the remaining functional layer, to a
proportion of less than 65% to 70%.
4. The method according to claim 1, further comprising applying the
functional layer completely in a composition which demonstrates no,
or only a weak selectivity for the sacrificial layer.
5. The method according to claim 1, further comprising applying to
the functional layer at least one layer having at least
approximately the same properties with respect to a stress in the
at least one layer, with which the functional layer begins on the
sacrificial layer and which has a different state from the
remaining functional layer.
6. The method according to claim 5, wherein the at least one layer
includes the surface barrier layer.
Description
RELATED APPLICATION
[0001] This application is a divisional of U.S. Ser. No. 10/817,346
filed on Apr. 2, 2004, which claims priority to German patent
application No. 10314989.9 filed on Apr. 2, 2003, which is hereby
incorporated by reference in its entirety.
BACKGROUND INFORMATION
[0002] Micromechanical systems may be produced based on multiple
material combinations. The production may take place especially
using the materials SiGe and Ge. In the case of integrated
micromechanical systems, on a regular basis a combination is
involved of electronic and mechanical components (e.g. resonators,
acceleration sensors and yaw rate sensors). Onto the electronic
components, as a rule, a printed circuit trace interconnect layer
is applied to connect the mechanical and electronic components.
Above that, there is a sacrificial layer, which, in SiGe technology
is preferably made of Ge. On top of the sacrificial layer an SiGe
functional layer is situated, of which the mechanical components
are implemented.
[0003] At present, the SiGe functional layer is mostly deposited
using the LPCVD method (LPCVD standing for low pressure chemical
vapor deposition). Other methods, which are also used, are
epitaxial growth or PECVD (plasma enhanced chemical vapor
deposition). However, these methods have not yet advanced
sufficiently. The advantages of LPCVD deposition are the low
deposition temperature for applying polycrystalline layers, which
are particularly important for a "backend integration". By backend
integration is meant the construction of the mechanical components
on the electronic components. In addition, the LPCVD method is
particularly good for batch processing. One disadvantage of the
LPCVD layers is that they have a comparatively high stress
gradient. Because of this stress gradient, for example, the
individual digits of a comb structure no longer overlap over the
entire thickness of the layer or the whole length of the digits, so
that the latter's function may be impaired. In the extreme case, at
great stress gradients, even movable elements on a substrate are
abrasively trimmed.
[0004] At this time it is not possible sufficiently to reduce a
disadvantageous layer stress gradient during the LPCVD SiGe
depositing without exceeding depositing parameter boundaries, such
as CMOS-compatible deposit temperatures, or having to use extremely
tight process windows which, at the present time, can hardly be
kept stable over an entire wafer surface. However, with the current
high stress gradients it is not possible to construct acceleration
sensors or yaw rate sensors having a large sensor surface and thus
also a high resolution. But it is just this area that one would
like to open up using the integration of micromechanics and
microelectronics. In SiGe technology, the high stress gradient of
functional layers is currently the main reason why there is not yet
a large number of integrated micromechanical systems, although the
integration with SiGe is a key technology for a large number of new
fields of application.
SUMMARY OF THE INVENTION
[0005] The present invention is based on the object of improving
the functionality of mechanical components in micromechanical
systems, such as micromechanical systems especially in the use of
SiGe technology, so that even producing comparatively large sensor
surfaces, having a high resolution, is possible.
[0006] This object is achieved through the features of the present
invention.
[0007] First of all, the present invention starts out from a method
for producing micromechanical structures in which a functional
layer is deposited onto a sacrificial layer, and, for the
production of at least one mechanical functional element, the
sacrificial layer is removed again. A first important aspect of the
present invention is that a surface barrier layer, which is the
beginning of the functional layer on the sacrificial layer, and
which has a different state from the remainder of the functional
layer, is also removed at least to a considerable part. This
procedure is based on the realization that the stress gradient in
known structures is essentially caused by the fact that the layer
stress at the under side of the layer is different from that at the
upper side of the layer, so that free (SiGe, for example) bars arch
upwards by a multiple of the layer thickness, and in addition, the
difference in the layer stress is caused more or less exclusively
by a comparatively thin surface barrier layer on the under side of
the functional layer, which, upon the production of the functional
layer differs from the remaining functional layer. As a rule, at
the under side of the functional layer a compressive stress
appears, while at the upper side a low compressive stress or a
tensile stress prevails. Thereby is caused, based on this stress
difference, a deflection of, for example, bars fixed on one side or
a bending of structures that are lying free in an upward direction,
that is, for example, away from a carrier in the direction of the
layer surface.
[0008] The reason for the behavior of the surface barrier layer in
the functional layer, which results in the stress gradient having
the described effects, is that the crystal structure changes during
the depositing. In a manner relatively independent of depositing
parameters, mostly an amorphous or very fine crystalline surface
layer barrier forms first of all, which has a thickness of between
a few 10 nm and a few 100 nm. Along with increasing depositing, the
crystallites in the layer keep on becoming bigger, the change in
the crystal growth after a certain layer thickness (surface barrier
layer), however, not being so great, or changes no longer take
place. Thus, the section of the layer in this area of the
functional layer is relatively homogeneous with respect to crystal
structure. Since the mechanical stress (as well as the thermal
coefficient of expansion) of a layer depends very greatly on the
crystal structure, the fine-crystalline or amorphous surface
barrier layer shows a different stress than the subsequent
polycrystalline main layer of the functional layer. That explains
why mainly the area of the surface barrier layer is responsible for
having the entire functional layer bending, such as upwards, based
on a stress gradient. According to the present invention, the
surface barrier layer is now removed in the lower region of the
functional layer, whereby the cause of the stress gradient is also
eliminated. Such a method may be used for all micromechanical
structures, and does not necessarily have to be limited to
integrated micromechanical systems. The removal of the surface
barrier layer may be performed in different ways. What is common to
all of them is that the sacrificial layer must really be removed
totally or at least partially, since otherwise the surface barrier
layer is not freely accessible, and then cannot be removed.
[0009] The sacrificial layer is normally made of a material which
is able to be etched to become the functional layer while having a
greater or lesser selectivity. That has the advantage that when the
sacrificial layer is removed, the functional layer is attacked
almost not at all, or to an inconsiderable extent. Say the
sacrificial layer is made of Ge, and the functional layer of SiGe,
the germanium proportion being less than 65% to 70%, in order to
achieve sufficient selectivity during sacrificial layer etching
between the sacrificial layer and the functional layer. Besides
H.sub.2O.sub.2, almost every other oxidizing fluid is suitable as
the sacrificial layer etching agent. The sacrificial layer does not
necessarily have to be made of Ge, it may also be constructed of
SiGe having a high proportion of germanium. Besides that, other
materials, such as silicon dioxide, come into consideration. In
that case, hydrofluoric acid (HF) may be used for etching the
sacrificial layer. In the case of a comparatively low germanium
proportion (less than 65% to 70%) in the functional layer, the
surface barrier layer may be removed wet-chemically, using etching
agents which are used for etching silicon, such as TMAH
(tetramethylammonium hydroxide), after the sacrificial layer has
been removed. But chemically removing the starting layer using
gaseous substances such as CIF.sub.3, XeF.sub.2 or even in a plasma
having an isotropic etching effect is also conceivable. For plasma
etching, SF.sub.6, for example, is suitable. Using such a gas phase
method, the etching of the sacrificial layer itself may be carried
out, if prior to that, the actual functional elements are suitably
passivated.
[0010] If the germanium proportion is greater than 65% to 70%, then
during the sacrificial layer etching of, for example, a pure
germanium sacrificial layer using, for instance, H.sub.2O.sub.2,
the functional layer is also attacked. This may be used
purposefully in order to remove the surface barrier layer already
during the etching of the sacrificial layer. In this case, then, no
additional step is necessary for reducing the stress gradient. The
selectivity between the sacrificial layer and the functional layer
may be set exactly by the germanium content (e.g. germanium content
gradient) of the functional layer.
[0011] Let the sacrificial layer be made of germanium and the
functional layer be made of SiGe having a germanium proportion of
greater than 65%, which is constant over the entire thickness of
the layer. The depositing conditions for the germanium sacrificial
layer, which may be, for example, made of two layers, may, for
example, for an LPCVD oven be 400.degree. C., 400 mTorr process
pressure, 190 sccm Si.sub.2H.sub.6 gas flow for a duration of ten
minutes for a germanium starting layer, and 400.degree. C., 400
mTorr pressure and 190 sccm GeH.sub.4 gas flow for the germanium
sacrificial layer itself. As parameters for the functional layer,
for example, 425.degree. C., 400 mTorr pressure, and for the gas
flows 100 sccm SiH.sub.4, 70 sccm GeH.sub.4 and 60 sccm
B.sub.2H.sub.6 come into consideration. If this sacrificial layer
is etched with H.sub.2O.sub.2, one automatically removes the lower
layer of the functional layer and thereby also removes the stress
gradient. The selectivity between such a combination of sacrificial
layer and functional layer is of an order of magnitude of 1:50. The
selectivity and etching time of the sacrificial layer here
determines the thickness of the layer removed from the functional
layer. Using the present method, for example, 3.5 .mu.m-thick and
500 .mu.m-long bars, made of an appropriately produced functional
layer, bend through by less than 1.5 .mu.m with respect to a plane.
In this context, the bars having the greatest evenness may
demonstrate deflections of less than 0.4 .mu.m per 500 .mu.m bar
length. These values are achieved in various boot positions even on
wafers, since the method is relatively robust with respect to
changes in depositing parameters. However, the deflection of bars
whose lower layer was not removed is typically more than ten times
as great. In addition, comparatively large divergences may appear
from wafer to wafer.
[0012] In one further preferred embodiment of the present
invention, the functional layer is applied in a composition of the
surface barrier layer as seen in the direction of the remaining
functional layer, so that at least a part of the surface barrier
layer is able to be etched at least initially selective to the
remaining surface barrier layer and/or to the remaining functional
layer. By this procedure, the surface barrier layer, for example,
may be developed into a sacrificial layer with a relatively small,
or no selectivity, whereby, during sacrificial layer etching, the
surface barrier layer is removed at the same time, but the
functional layer is hardly removed, or not at all.
[0013] In particular, it is preferred if the functional layer, in
at least one partial area of the surface barrier layer, is
deposited, into the remaining functional layer, made of SiGe having
a higher proportion of germanium than 65% to 70% but less than
100%, and during the depositing for the remaining functional layer
the germanium proportion is reduced, preferably to a proportion of
less than 65% to 70%. For example, in one and the same oven
deposit, directly one after another, without stabilization layers
lying in between, the surface barrier layer and the remaining
functional layer are deposited. To do this, for example, during the
depositing, the flow of germanium is simply lowered after a
specified time. In one preferred embodiment of the present
invention, a layer having a germanium content of more than 65% to
70%, but clearly less than 100% may be deposited in approximately
the thickness of the surface barrier layer, which is responsible
for the stress gradient.
[0014] Directly upon this, then, the functional layer may be
deposited, having a lower germanium content for improved
selectivity during sacrificial layer etching. Consequently, during
the etching of the sacrificial layer, only the surface barrier
layer having a high germanium proportion is removed, but no, or
essentially no areas of the functional layer. Consequently, right
on target, only the layer of the functional layer is removed which
causes the stress gradient, without one having to adhere to exact
time windows in the etching, in this context.
[0015] Starting from a method for producing micromechanical
structures in which a functional layer is deposited onto a
sacrificial layer, and for the production of at least one
mechanical functional element the sacrificial layer is removed
again, the object is also attained in that one layer or several
layers, having at least closely the same properties with respect to
layer stress in the layer or the layers as seen together as a
surface barrier layer, with which the functional layer begins on
the sacrificial layer and which has a different condition from the
remaining functional layer, is/are applied to the functional layer.
Hereby are produced layers having the same stress, on opposite
surface areas of the functional layer, whereby, using a uniform
layer between these layers, a stress neutralization takes place so
that no undesired twisting appears. The application of the
additional layer or the layers onto the functional layer may, for
example, be made so that the depositing process is simply stopped
and started again. Thereby is created naturally on the functional
layer a layer which at least approximately has the same properties
as the surface barrier layer has to the sacrificial layer, since
the surface barrier layer was also generated by starting up the
process having predefined depositing parameters. In response to
each start-up after a stop, a layer will at first be formed which
corresponds to the surface barrier layer of the functional layer,
if the initial conditions of the two layers are compatible. This
can be achieved, for instance, by a starting layer made of
amorphous silicon.
[0016] However, it is also possible to apply another layer onto the
functional layer if the former only has stress behavior that
corresponds to the surface barrier layer. Thereby the desired
stress neutralization in the functional layer is then achieved.
[0017] The production methods described have the advantage that
thereby functional layers may be generated having comparatively low
stress gradients under CMOS-compatible production conditions. The
methods are comparatively robust with respect to fluctuations in
the depositing parameters, since the stress gradient is not set via
the germanium content, the pressure, the temperature of the doping
concentration or even a gradient of these parameters. This makes
carrying out the process extraordinarily simplified, particularly
for batch processes, such as in an LPCVD oven depositing. At the
same time, the depositing parameters may be chosen in such a way
that the layers are deposited at a comparatively high rate, which
is of advantage with regard to economical efficiency. Besides that,
fewer test runs are required in establishing a manufacturing
method, since manufacturing methods according to the present
invention "function" in a large process window.
[0018] Also, starting from a micromechanical structure having a
functional layer made of SiGe, which, in a lateral region, is
especially completely separated from the remainder of the
structure, the object is attained in that the functional layer,
viewed over its thickness, has such a structure that stresses in
the layer are neutralized for the most part, or no stress gradient
makes an appearance. That is based on the above-described
realization that stress gradients in the functional layer are
regularly caused only by a thin layer whose effect may be offset by
a layer having appropriate properties.
[0019] In the simplest case, the functional layer is preferably
constructed at least approximately symmetrical to a center plane
through the layer, as seen over its thickness. In a symmetrical
layer construction, one may largely assume that a stress-neutral
behavior of the layer sandwich appears, towards the outside.
[0020] In this connection, it is also advantageous if, with respect
to the method described, the functional layer has a surface barrier
layer whose effect causing stress gradients is neutralized by an
appropriate layer on the upper side.
[0021] In the simplest case, the functional layer, as viewed in
cross section, is homogeneous, at least for the most part. A layer
that is per se homogeneous is of necessity stress-neutral. In the
case of minor inhomogeneities, a residual stress will set in which,
however, frequently is acceptable.
BRIEF DESCRIPTION OF THE DRAWING
[0022] The FIGURE shows an exemplary embodiment of a layer
construction for illustrating the present invention.
DETAILED DESCRIPTION
[0023] In a schematic cross section, the FIGURE shows a typical
construction of an SiGe functional layer 1. Functional layer 1 has
on its lower side a surface barrier layer 2, which is made up of
amorphous or very small crystallites. Remaining functional layer 3
is next to surface barrier layer 2. Remaining functional layer 3 is
made up of comparatively large crystallites, which to a great
extent are uniform over the entire thickness of this layer.
Therefore, this layer, as seen over its thickness d, has an
essentially constant layer stress. The constant layer stress is
either a comparatively slight tensile stress or compressive stress,
which are symbolized by arrows 4 in layer 3.
[0024] By contrast, in surface barrier layer 2, a substantially
greater compressive stress prevails (symbolized by arrows 5), with
the result that the layer in the FIGURE bends up towards the
top.
[0025] According to the present invention, only two types of thing
can happen, then.
[0026] 1. Either surface barrier layer 2 is removed, whereby a
remaining functional layer 3 is obtained, which is per se largely
homogeneous, and, in view of that, shows no substantial stress
gradients, which would lead to an undesired deformation of the
layer.
[0027] 2. One or more additional layer(s) is/are applied to layer
3, which has(have) comparable stress properties to surface barrier
layer 5. Hereby the high compressive stresses in the surface
barrier layer are able to be compensated for by correspondingly
high compressive stresses on the upper side of the layer
construction.
* * * * *