Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-02-08
2001-07-17
Bowers, Charles (Department: 2823)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S611000, C438S628000, C438S977000, C438S978000
Reexamination Certificate
active
06261943
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of materials fabrication, and in particular to a method for fabricating very thin metal films that are unsupported by any auxiliary substrate over a predetermined area.
BACKGROUND OF THE INVENTION
Thin metal films (also referred to as membranes) are useful in a broad array of devices, and especially in optical photonic devices, medical and chemical filters, x-ray lithography, and micro-electromechanical system devices (“MEMS,” also referred to as micromachines). Thin metal films are also useful in enhanced light transmission apparatus, such as those disclosed in U.S. Pat. No. 5,973,316 assigned to NEC Research Institute, Inc., for example.
Recently, a need has emerged for thin metal films that are unsupported by a substrate, referred to herein as “free-standing thin metal films.” Free-standing thin metal films are thin metal films that not supported by a substrate (any substantially solid material other than the thin metal film itself) over a predetermined area on either side of the metal film for mechanical stability or the like. The ability to make free-standing thin metal films opens a broad range of design possibilities in which the unique and substantial properties of metals (mechanical, structural, electrical, optical, thermal, etch resistance) may be exploited in new devices. However, due to the thinness of such metal films (typically on the order of several microns to less than a micron), manufacturing free-standing thin metal films is difficult. It is therefore desirable to provide practical and reliable methods for fabricating thin free-standing thin metal films.
SUMMARY OF THE INVENTION
Generally speaking, in accordance with the invention, methods for fabricating a freestanding thin metal film are provided. A first method comprises the steps of: (a) providing a sacrificial silicon nitride membrane structure comprising a silicon wafer having first and second surfaces, a first silicon nitride layer applied to the first surface of the silicon wafer and a second silicon nitride layer applied to the second surface of the silicon wafer, the second silicon nitride layer and the silicon wafer being etched to provide a window that exposes a predetermined area of the first silicon nitride layer, whereby the exposed predetermined area of the front silicon nitride layer comprises a sacrificial silicon nitride membrane unsupported by any auxiliary substrate over the predetermined area; (b) depositing a thin metal film on the first silicon nitride layer; and (c) removing the sacrificial silicon nitride membrane, whereby the portion of the thin metal film exposed by the removal of the silicon nitride membrane comprises the free-standing thin metal film.
A second method comprises the steps of: (a) providing a sacrificial silicon nitride membrane structure comprising a silicon wafer having first and second surfaces, a first silicon nitride layer applied to the first surface of the silicon wafer and a second silicon nitride layer applied to the second surface of the silicon wafer, the second silicon nitride layer and the silicon wafer being etched to provide a window that exposes a predetermined area of the first silicon nitride layer, whereby the exposed predetermined area of the front silicon nitride layer comprises a sacrificial silicon nitride membrane unsupported by any auxiliary substrate over the predetermined area; (b) applying a sacrificial photoresist layer to a surface of the first silicon nitride layer which is opposite that of the silicon wafer; (c) removing the sacrificial silicon nitride membrane, exposing a portion of the sacrificial photoresist layer through the etched window; (d) depositing a thin metal film on a surface of the sacrificial photoresist layer which is opposite that of the silicon wafer; and (e) removing the portion of the sacrificial photoresist layer exposed through the etched window, whereby the portion of the thin metal film exposed by the removed portion of the sacrificial photoresist layer comprises the free-standing thin metal film.
A third method comprises the steps of: (a) providing a sacrificial silicon nitride membrane structure comprising a silicon wafer having first and second surfaces, a first silicon nitride layer applied to the first surface of the silicon wafer and a second silicon nitride layer applied to the second surface of the silicon wafer, the second silicon nitride layer and the silicon wafer being etched to provide a window that exposes a predetermined area of the first silicon nitride layer, the window including side walls of the silicon wafer and the second silicon nitride layer; (b) applying a sacrificial photoresist layer to the second silicon nitride layer and the side walls of the etched window; (c) removing the first silicon nitride layer; (d) depositing a thin metal film on the surfaces of the silicon wafer and the sacrificial photoresist layer exposed by the removal of the first silicon nitride layer; and (e) removing the sacrificial photoresist layer, whereby the portion of the thin metal film within the etched window exposed by the removed sacrificial photoresist layer comprises the free-standing thin metal film.
The invention includes methods in which free-standing thin metal films may be fabricated by deposition on a variety of sacrificial membranes, the sacrificial membrane being removed after the metal deposition. Methods for fabricating the sacrificial membranes are also provided.
One goal of the present invention is to provide methods for fabricating free-standing thin metal films that avoid the use of strong acids (so-called “wet etches”) on the metal film which may otherwise etch the metal film or degrade the mechanical attachment of the free-standing thin metal film to its peripheral supporting structure.
The free-standing thin metal films fabricated according to the present invention may be used in optical photonic devices in which a photonic structure is fabricated in a further step on or of the free-standing thin metal film. The lack of an auxiliary substrate can provide highly desirable performance improvements in such devices. Liquids may also be introduced to come into contact with the free-standing thin metal films which may alter the optical properties in a useful manner. Further, optical devices may utilize the free-standing thin metal films in which the films may be elastically altered in shape by electric fields or hydrostatic pressure.
The free-standing thin metal films fabricated according to the present invention may also be used as filters for liquids (particularly useful, for example, in medical techniques such as cell separation) if a sufficient filter pattern is etched into the membrane. The thinness of the freestanding thin metal film is beneficial in that flow rate in such filters can be maintained even while pore size is decreased.
Another use for free-standing thin metal films fabricated according to the present invention is in X-ray lithography, in which the opacity of certain metals to X-rays may be utilized advantageously as X-ray lithographic masks. In this type of use, the free-standing thin metal films etched with the desired pattern. Small feature size is aided by the thinness of the freestanding thin metal film. The free-standing thin metal films fabricated according to the present invention advantageously provide unobstructed etched apertures which are an improvement over traditional silicon nitride-supported masks, in which the silicon nitride auxiliary substrate obstructs the apertures.
Furthermore, free-standing thin metal films fabricated according to the present invention are useful in many MEMS applications. Actuators and sensors similar to those fabricated in silicon and related materials may instead be made of metal using the free-standing thin metal films described herein. The properties of a free-standing thin metal film may be very useful in such applications. For example, the reflectivity of a free-standing thin metal film may be useful in fabricating adjustable micromirrors.
Accordingly, an object
Bowers Charles
Brewster William M.
Isztwan Andrew G.
NEC Research Institute Inc.
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