Etching a substrate: processes – Etching of semiconductor material to produce an article...
Patent
1995-11-27
1997-11-04
Powell, William
Etching a substrate: processes
Etching of semiconductor material to produce an article...
1566261, 1566461, 216 59, 216 79, B44C 122, H01L 2100
Patent
active
056835914
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for fabricating surface-micromechanical structures.
BACKGROUND INFORMATION
PCT International Application Ser. No. WO 92/03740 discloses a method for fabricating surface-micromechanical structures, which involves depositing, on a silicon substrate represented by a silicon wafer, a sacrificial layer of silicon oxide and patterning the layer in such a way that, inter alia, windows toward the silicon substrate are created as subsequent anchoring sites for free structures. Onto the sacrificial layer, a second layer of polysilicon is deposited and is likewise patterned in accordance with the lateral demarcations of the subsequent free structures.
Then, with the aid of aqueous hydrofluoric acid solution as an etching solution which has a high selectivity with respect to polycrystalline silicon and therefore with respect to the second layer, the sacrificial layer is removed. As a result, structures are formed which are free-standing above the silicon substrate at a distance equal to the thickness of the removed sacrificial layer and are anchored on the silicon substrate at the previously patterned window sites of the sacrificial layer.
After etching, the silicon wafer is rinsed, the rinse liquid then having to be removed again from the wafer and the micromechanical structures. A customarily employed drying procedure such as spinning or heating is not possible in this case, however, since the withdrawing liquids, owing to their surface tension, would exert forces on the microstructures exposed by etching. As a result, the flexible, exposed microstructures would be deformed, would come to lie against the silicon substrate and thus irreversibly adhere to the substrate surface. It is therefore known to replace the rinse liquid by a liquid which can be converted into the solid phase either by freezing (e.g. cyclohexane) or by solvent removal (photoresist/acetone mixture). Then, by means of sublimation or by chemical reaction, e.g. by the known photoresist/acetone ashing process, the transition from the solid into the gaseous state is effected, during which transition the forces due to surface tensions are minimal and as a result the micromechanical structures remain free.
SUMMARY OF THE INVENTION
The fabrication method according to the present invention does not, for the purpose of etching and exposing the micromechanical structures, involve dipping of the silicon wafer into an etching fluid nor does it, as a result, require laborious drying. Instead, the silicon wafer is exposed to the vapor phase of a mixture of anhydrous hydrofluoric acid and water in a vapor-phase etching process, suitable measures being required, e.g. heating of the wafer, to ensure that no condensate is formed on the wafer surface, which would cause the emerging micromechanical structures to adhere. While water is a product in the etching reaction, it is also a necessary initiator, since the removal of silicon oxide involves a reaction, via intermediate steps, according to the reaction equation:
This provides the advantage of being able, by adjusting the water content on the wafer surface, to control the reaction rate and check the etching operation. It is not only the water originating from the vapor phase, but also the amount of the water absorbed during etching or adsorbed prior to etching, which affects the etching rate. Short etching times of only a few hours are possible, and after a vapor-phase etching process, the micromechanical structures are free and dry. Time-consuming sublimation operations and rinsing taking up to several days, which typically arise in the traditional procedure, are unnecessary. Owing to the small forces acting on the structures during vapor-phase etching, even highly sensitive structures can be achieved. Vapor-phase etching can be carried out at various points of IC fabrication and thus facilitates the use of standard techniques and integrability of evaluation electronics. Moreover, the method can be implemented by simple means and be extended
REFERENCES:
patent: 4849071 (1989-07-01), Evans et al.
patent: 5174855 (1992-12-01), Tanaka
patent: 5470797 (1995-11-01), Mastrangelo
Lober and Howe; IEEE 1988--Surface-Micromachining Processes for Electrostatic Microactuator Fabrication; pp. 59-62.
Mastrangelo and Saloka; IEEE 1993--A Dry-Release Method Based Onpolymer Columns for Microstructure Fabrication; pp. 77-81.
Powell William
Robert & Bosch GmbH
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