Patent Application
20060144778
1. Field of the Invention
The present invention generally relates to a semiconductor structure and its fabrication, and more particularly, a low stress, ultra-thin membrane.
2. Background Information
The use of electron-transparent membranes to serve as enclosures for evacuated equipment and/or specimen chambers is known. The ability of electrons to penetrate a certain thickness of a solid is measured by its “electron range.” T. E. Everhart and P. H. Hoff, “Determination of Kilovolt Electron Energy Dissipation vs Penetration Distance in Solid Materials,” J. Appl. Phys. 42, 5837 (1971). Electrons with energies in the kilo-electron volt range, traveling in a solid, are scattered inelastically in collisions with the electrons in the material. For low-Z materials, such as organic insulators, scattering from the valence electrons is the major loss mechanism for incident electron energies from 10 eV to 10 keV.
Maximum thicknesses for electron transparency are also known. Still, the practical problem of actually fabricating a viable membrane that was thinner than the maximum thickness remained. Currently known fabrication techniques and materials cannot provide sufficiently thin membranes for use with low energy electrons and similar particles, such as photons. These types of low energy particles are associated with low energy X-ray, soft X-ray, X-ray microscopes, Extreme Ultra Violet, and Vacuum Ultra Violet analyses. Consequently, there is a need in the art for ultra-thin membranes that will provide transparency to low energy electrons. There is an associated need for such a viable membrane that could be made using Chemical Vapor Deposition (CVD), which it would make such membranes easier to manufacture.
The present disclosure is directed, in part, to a method for fabricating a low-stress, ultra-thin membrane as well as the low-stress, ultra-thin membrane, itself. The method includes: layering a first layer on a semiconductor substrate; etching a hole in the first layer; layering a second layer on the membrane of the first layer and over the hole; and the substrate beginning from the bottom surface thereof, such that at least a portion of the substrate aligned with the hole in the first layer is removed. These holes are preferrably created by etching, which may take the form of reactive ion etching, plasma etching, wet etching, and various combinations thereof. The first and second layers are made of substantially the same material, which will usually be silicon nitride, however, it is contemplated that other dielectric materials could be used. Generally, low pressure chemical vapor deposition will be used to create at least the first and second layers. As a result of this basic method, the second layer has an ultra-thin thickness.
In a preferred approach, the second layer has an amorphous structure. With such a structure, the method can further include measuring the thickness of the second layer (or membrane) and thinning the membrane to a desired thickness. The amorphous structure minimizes concern that such thinning could create undesirable pinholes in the second layer.
It is preferred that the second layer be formed with a slightly bubble-shape (i.e. semi-spherical like an egg shell) to help deflect stresses on the second layer. It may also be preferable to remove the substrate from the first layer near the end of fabrication.
The present disclosure also teaches, in part, a semiconductor structure having an, ultra-thin low-stress membrane including a first layer having a hole etched therein; and a second layer layered on the first layer, the first layer and second layer being comprised of substantially the same material, which will usually be silicon nitride, however, it is contemplated that other dielectric materials could be used. Generally, low pressure chemical vapor deposition will be used to create at least the first and second layers. The second layer has an ultra-thin thickness. The second layer may also have an amorphous structure and have a slightly bubble-shape to help deflect stresses on the second layer.
The semiconductor is useful in, among other devices, a device for detecting physical characteristics of a sample bombarded with low-energy electrons. In such a device, the ultra-thin, low-stress membrane of the novel semiconductor structure is positioned adjacent a detector. The device may further include an evacuated chamber at least partially bounded by the ultra-thin low-stress membrane.
While the present invention may be embodied in many different forms, the drawings and discussion are presented with the understanding that the present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.
It is crucial to the present invention for the second layer 130 to have a uniform, ultra-thin thickness. In the context of the present application, ultra-thin means a thickness, α, of the selected material that is transparent to low energy (1 KeV) electrons. For silicon nitride at these energies, ultra-thin means, α, of no greater than approximately 40-50 Å. Using known equations, the thickness necessary for transparencies to various electron energies for various compounds can be readily determined.
It is also important that membrane be low-stress to minimize the potential for self-destruction. One of the main reasons the final membrane is surprisingly robust is the fact that it is composed of a thick layer and a thin layer composed of the same materials with largely the same physical properties. Another significant reasons the membranes is surprisingly strong is the fact that by the nature of the process disclosed herein, the little membranes are slightly bubbled in shape, so they are semi-spherical like an egg shell, which is a mechanically-strong shape. With this bubble shape it will be compliant and able to take up stresses that are applied to it. In addition, silicon nitride has also proven to be a wonderfully strong material.
In the preferred approach these results are achieved through a “re-growth” technique, which can be illustrated referring to a silicon nitride embodiment, as follows:
The deposition process used in the preferred approach is LPCVD (Low Pressure Chemical Vapor Deposition). The LPCVD material is much stronger and more uniform than other silicon nitride processes. For example, silicon nitride made via evaporation or PECVD (Plasma Enhanced Chemical Vapor Deposition) does not make membrane quality material. LPCVD also allows for a the material to be deposited evenly, as opposed to evaporation which deposits the material in a line-of-sight from the source. Also, there's the contribution of the silicon nitride itself. Silicon nitride a super hard, super strong material with a hardness on the order of 9 on the mohs scale. Of course, it should be understand by those of ordinary skill in the art having the present specification before them that the present technique will also work with other materials, such as those listed above.
The method also presents a relatively simple way to make many of these membranes in a tightly packed formation.
Membrane 100 can also be used higher energy electrons where particularly higher strength windows are desired. The present method makes a stronger silicon nitride window than could be made with a single thickness nitride process. So, for instance, electron energies of something like 20000 Volt electrons will be transparent where a of second layer is approximately 2000 Angstroms thick.
The membrane is useful in devices for detecting physical characteristics of a sample bombarded with low-energy electrons. In such a device, the ultra-thin, low-stress membrane is positioned adjacent a detector. The device may further include an evacuated chamber at least partially bounded by the ultra-thin low-stress membrane.
The detector can be used to separate an ultrahigh vacuum from a much rougher and cruder vacuum or for running experiments in water using tools that are not compatable with water (e.g. looking at a live cell with an electron or an X-ray microscope). As shown in
The foregoing description and drawings merely explain and illustrate the invention and the invention is not limited thereto. While the specification in this invention is described in relation to certain implementation or embodiments, many details are set forth for the purpose of illustration. Thus, the foregoing merely illustrates the principles of the invention. For example, the invention may have other specific forms without departing for its spirit or essential characteristic. The described arrangements are illustrative and not restrictive. To those skilled in the art, the invention is susceptible to additional implementations or embodiments and certain of these details described in this application may be varied considerably without departing from the basic principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and, thus, within its scope and spirit.
This patent claims the benefit of U.S. Provisional Application No. 60/593,028, filed Jul. 29, 2004.
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC §202) in which the Contractor has elected to retain title.
| Number | Date | Country | |
|---|---|---|---|
| 60593028 | Jul 2004 | US |
