Method for the manufacture of micro-mechanical components

Details Method for the manufacture of micro-mechanical components Method for the manufacture of micro-mechanical components

2001-12-20

2003-08-12

Pham, Long (Department: 2814)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Physical stress responsive

Reexamination Certificate

active

06605487

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention is directed to a method for the manufacture of micro-mechanical components with undercut layer elements.
Micro-mechanical components are being increasingly employed in pressure sensors, microphones, acceleration sensors, switches, micro-pipetting units, electrical biological protection and other apparatus. The micro-mechanical manufacturing methods enable a high degree of miniaturization, so that the advantages of a monolithic integration with a micro-electronically manufactured read-out or control electronics can be simultaneously utilized. In this way, micro-mechanical components can be manufactured in a large number of pieces in a compact, reliable and cost-beneficial way.
The manufacture of undercut layer elements plays a particular part in the generation of micro-mechanical components. What are understood by undercut layer elements are the regions of layers that do not lie on any foreign layer. Examples of undercut layer elements are micro-mechanical membranes, bridges, webs and the like. The manufacture of undercut layer elements for micro-mechanical components is usually implemented as follows (see FIGS.
1
A-
1
D): a layer stack
1
-
0
having a substrate
1
-
1
of silicon, what is referred to as a sacrificial layer
1
-
2
applied thereon and composed, for example, of silicon oxide, and, applied thereon, a layer
1
-
3
, which is to be undercut, is provided or generated (FIG.
1
A). The layer
1
-
3
, which is to be undercut, is often composed of polysilicon or of epitaxially grown silicon. In order to be able to undercut parts of the layer
1
-
3
, the layer
1
-
3
, which is to be undercut, must be opened or exposed, for example with the assistance of photolithographic methods. In
FIG. 1B
, the opening ensues by a structuring of the layer
1
-
3
, which is to be undercut, for example to form three bars that are connected at one side. When the sacrificial layer
1
-
2
is then isotropically etched, the sacrificial layer
1
-
2
is also etched under the three bars, so that the three bars develop into undercut layer elements
1
-
4
given an adequately long etching time. Hollow regions
1
-
7
in the region between undercut layer elements
1
-
4
and substrate
1
-
1
are thereby simultaneously generated.
FIG. 1B
also shows that the sacrificial layer
1
-
2
, due to the limited etching time, has remained only in the connecting region of the three bars
1
-
4
. The remaining region of the sacrificial layer
1
-
2
in this embodiment serves as a support for the three undercut bars
1
-
4
.
This method, however, has disadvantages. First, it is not possible with this method to manufacture membranes that lie on a sacrificial layer hermetically tight on all sides since, in this case, the layer, which is to be undercut, is not structured and, thus, no openings are present through which the sacrificial layer can be etched. Second, there is the problem for generating large-area, undercut layer elements because the necessary undercutting by the isotropic etching step can last for a very long time. For example, the etching time may last up to two hours in order to etch cavities having an average volume of 400 &mgr;m
3
with 10% hydrofluoric acid and a sacrificial layer composed of silicon oxide. Given these long etching times, moreover, the reproducibility for a planarly exact structuring of the sacrificial layer is established to only a limited extent. This long etching time can also result in that parts of the sacrificial layer, which, for example, should remain in place for supporting purposes, are occasionally likewise removed.
For solving this problem, small openings, what are referred to as etch holes, are additionally generated or formed in the layer, which is to be undercut, and these holes have the task of producing an optimally large attack area on the sacrificial layer lying therebelow for the isotropically etching medium. The etch holes can thereby comprise round shapes but can also comprise non-round shapes such as, for example, slot-shaped forms and can also be potentially adapted to the desired shape of the undercut layer elements to be generated.
It is advantageous for many applications that the etch holes are in turn closed after the isotropic etching. First, the undercut layer elements can thus largely regain their mechanical or electrical stability and, second, it may be necessary that the undercut layer elements cover structures on the substrate or, as in the case of pressure sensors, should form a hermetically tight closure.
So that the etch holes can be unproblemmatically closed with a cover material, the etch holes should have optimally small diameters.
FIG. 1C
schematically shows the micro-mechanical component of
FIG. 1B
with the three undercut bars
1
-
4
, with each bar being provided with an additional etch hole
1
-
5
. The etching time can be shortened in this way, so that the supporting region of the sacrificial layer
1
-
2
was enlarged in comparison to the device in FIG.
1
B. In this drawing, the etch holes are far larger compared to the bar structure size then in reality for reasons of presentation.
The materials of the layer stack
1
-
0
in this method are selected, so that the structuring of the layer
1
-
3
, which is to be undercut, can occur by etching selectively relative to the sacrificial layer
1
-
2
lying therebelow. In addition, the materials should allow the sacrificial layer
1
-
2
to be removed with wet-chemical methods without the layer
1
-
3
being destroyed. For example, the sacrificial layer
1
-
2
is composed of a silicon oxide and the layer
1
-
3
is composed of episilicon or polysilicon. The episilicon or polysilicon layer can be wet-chemically etched with a base such as KOH (large structures) or, when the micro-mechanical dimensions lie on the order of magnitude of a few &mgr;m or smaller, can be dry-chemically etched with gases in a plasma (for example, Cl
2
, Hbr or SF
6
gases or mixtures of gases).
The sacrificial layer of silicon oxide
1
-
2
is removed, for example, with a solution containing hydrofluoric acid (diluted HF or diluted NF3/HF) that does not attack the layer
1
-
3
of silicon, which is to be undercut and lies thereabove. The attack of the etching chemicals on the sacrificial layer
1
-
2
ensues via the open regions of the layer
1
-
3
which include the etch holes
1
-
5
. Typical dimensions of the etch hole diameter, which is dependent on the geometry of the micro-mechanical elements, lie in the range of a few 10 nm through a number of micrometers.
The closure of the etch holes is usually achieved by a deposition of a cover material. As a result of diffusion, the cover material, however, also proceeds into the hollow regions
1
-
7
and deposits thereat.
FIG. 1D
shows a cross-section through the micro-mechanical component shown in
FIG. 1C
after the deposition of the cover material
1
-
6
. The layer thickness of the cover material
1
-
6
required for closing the etch holes, however, also generates a layer in the hollow region
1
-
7
that increases the layer thickness of the undercut layer elements and modifies the mechanical or electrical behavior.
The size of the etch holes is dependent on the overall process execution with which the undercut layer elements
1
-
4
are manufactured. Further, their size is dependent on the planned final thickness of the layer elements
1
-
4
to be undercut. The diameter of the etch holes
1
-
5
must be all the smaller for the undercut layer elements with a thinner ultimate thickness. Etch holes having a diameter greater than 50 nm and smaller than 5 &mgr;m are thus currently being used.
A method of covering or closing etch holes with a small diameter is disclosed in European Patent Application EP 0 783 108 A1. Therein, the etch holes are closed by the deposition of a flowable BPSG layer (Boro-Phosphorous Silicate Glass). One disadvantage of this method, however, is that the undercut layer elements or, respectively, membrane layers are then no longer composed only of episilicon or polysilicon but of vario

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