Etching a substrate: processes – Etching of semiconductor material to produce an article...
Reexamination Certificate
2001-12-18
2004-01-20
Alanko, Anita (Department: 1765)
Etching a substrate: processes
Etching of semiconductor material to produce an article...
C216S079000, C216S080000, C438S050000, C073S514320
Reexamination Certificate
active
06679995
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of micromechanical manufacturing of fixed and movable layer-like electrodes of a semiconductor element, for example, a capacitive acceleration sensor, which are exposed over a substrate over a certain area, a sacrificial layer arranged between the substrate and the fixed and movable electrodes being removed in an etching step in order to expose the electrodes with respect to the substrate, and an acceleration sensor thus manufactured.
BACKGROUND INFORMATION
It is believed that in manufacturing semiconductor elements using micromechanical technology, such as acceleration and rotational speed sensors, for example, a silicon dioxide sacrificial layer may be selectively etched in order to obtain the functional components made of silicon. This reactive etching of the sacrificial layer may take place in the gas phase via an azeotropic mixture of H
2
O and HF. See, for example, M. Offenberg et al., “Acceleration Sensor in Surface Micromachining for Airbag Applications with High Signal/Noise Ratio”; Sensors and Actuators, 1996, page 35, and German Published Patent Application No. 4 317 274.
In the event of insufficient progress during the process, oxide residues may remain underneath the functional component layer, which may result in interference and inaccurate response behavior in sensors, for example, acceleration sensors having non-movable electrode fingers fixed on one side. These residues may appear at technologically unfavorable points which are characterized by the fact that, after the sacrificial layer has been etched, the functional layer over them is arched upward from the etching front due to the intrinsic stress gradient, and in the process end state the distance between the bottom of the structure layer and the top of the sacrificial layer becomes greater. There are technologically favorable points where the distance between the surface of the sacrificial layer and the bottom of the structure layer underneath the components anchored on both sides, such as elastically suspended seismic masses of an acceleration sensor, may be reduced due to the intrinsic stress gradient. An increased etching rate may result, which may cause considerable inhomogeneity of the etching rate within a sensor structure, which may result in oxide residues underneath the fixed electrodes and leaves considerable underetching in other places.
The considerably inhomogeneous etching rate underneath the non-movable electrode fingers fixed on one side and the movable central mass of an acceleration sensor may result in a criterion for ending the etching process that is difficult to define when such acceleration sensors are mass produced. Considerable unwanted underetching may occur at several points of the component, for example, at the electric leads, while substantial oxide residues affecting the functionality of the sensor structure still exist in the area of the sensor core underneath the fixed electrodes.
SUMMARY OF THE INVENTION
An exemplary method of the present invention is directed to providing a method which allows micromechanical manufacture of components in view of the difficult of the microscopically inhomogeneous etching rate in the manufacturing of semiconductor elements, where layer-like electrode areas are exposed using sacrificial layer etching, as in the case of fixed, comb-like electrodes of an acceleration sensor.
Another exemplary method of the present invention is directed to providing a method which allows the micromechanical manufacture of components in which the etching rate in selective isotropic etching of silicon dioxide in an H
2
O/HF gas phase can be homogenized on a microscopic scale.
Another exemplary embodiment and/or exemplary method of the present invention is directed to reducing the thickness of the sacrificial oxide layer underneath the non-movable electrode fingers fixed on one side. Thus the etching rate may be substantially increased for kinetic reasons at these points and may approach the etching rate underneath the seismic mass. It is believed that reduction of the thickness of the sacrificial oxide layer underneath the fixed electrode fingers may be achieved by correspondingly increasing the thickness of the epitaxial polycrystalline silicon layer formed over the sacrificial layers which is the material of the subsequently etched bare electrode fingers.
In another exemplary embodiment and/or exemplary method of the present invention, the total time of the gas phase etching process (GPA) may be substantially reduced by adjusting the etching rate underneath the seismic mass and the fixed electrode fingers. This may also provide improved homogeneity of the oxide removal from the sacrificial layer or sacrificial layers within a semiconductor element, for example, an acceleration sensor structure, over the entire wafer.
Another exemplary method of the present invention is directed to providing a method of micromechanical manufacture of fixed and movable layer-like electrodes of a semiconductor element, for example, a capacitive acceleration sensor, which may be exposed over a substrate over a certain area, a sacrificial layer arranged between the substrate and the fixed and movable electrodes being removed in an etching step in order to expose the electrodes with respect to the substrate. The thickness of the sacrificial layer located in the area of the fixed electrodes may be less than the thickness of the sacrificial layer located in the area of the movable electrodes.
Another exemplary method of the present invention is directed to providing a method where the thickness of the fixed electrodes after sacrificial layer etching is greater than the thickness of the movable electrodes. The increase in the thickness of the fixed electrode fingers of a sensor structure of this kind may result in the following effects.
For example, if the functional structures had the same layer thickness, the effective capacitive electrode area may be reduced, since a fixed electrode finger may be bent upward due to the stress gradient. Since at the same time the seismic mass, fixed on both sides, may arch downward, the electrode area of the opposite electrodes may be effectively reduced.
For example, if the fixed electrodes have a greater thickness than the movable electrodes, the central mass itself may still be opposite the counterelectrode in the event of a high stress gradient.
In the proposed layer structure, the sacrificial layer underneath an area of the fixed electrodes may be composed of a first sacrificial layer, and the sacrificial layer underneath an area of the movable electrodes may be composed of the above-named first sacrificial layer and a second sacrificial layer located directly over it.
Another exemplary method of the present invention is directed to providing a method of manufacturing in the following consecutive steps:
a) applying the first sacrificial layer to the entire surface of the substrate so that it covers the areas where the fixed and movable electrodes are subsequently formed;
b) applying a first conductive layer over the first sacrificial layer so that it only covers the area where the fixed electrodes are subsequently formed;
c) applying the second sacrificial layer to the entire surface of the first sacrificial layer and the first conductive layer so that it covers the areas where the fixed and movable electrodes are subsequently formed;
d) opening the second sacrificial layer via the first conductive layer using a masked etching step so that the first conductive layer is exposed and only a small portion of the depth of the first sacrificial layer is etched away;
e) applying a relatively thick doped epitaxial layer to the entire surface of the second sacrificial layer and the conductive layer exposed in step d) to the thickness of the fixed and movable electrodes to be subsequently formed;
f) applying a mask structuring the fixed and movable electrodes to the surface of the epitaxial layer, and, using the mask, ditches are etched into the epitaxial layer to a depth delimited by the su
Banjac Branko
Bueche Dirk
Fischer Frank
Schielein Doris
Alanko Anita
Kenyon & Kenyon
Robert & Bosch GmbH
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