Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
1993-12-30
2001-01-23
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S745000, C216S094000, C216S096000
Reexamination Certificate
active
06177358
ABSTRACT:
The following coassigned patent applications are hereby incorporated herein by reference:
Ser. No.
Filing Date
TI Case No.
07/871,862
04/20/92
TI-15930
FIELD OF THE INVENTION
This invention generally relates to methods for etching CaF
2
.
BACKGROUND OF THE INVENTION
Without limiting the scope of the invention, its background is described in connection with the etching of CaF
2
epitaxial layers for resonant tunneling devices, as an example.
Heretofore, in this field, resonant tunneling devices have been made using GaAs/AlGaAs structures. However, if a practical resonant tunneling technology for the silicon analog could be devised, such a technology could leverage the existing mature silicon processing technologies and device product lines.
A silicon-based resonant tunneling technology would require a material that can be epitaxially interposed between two silicon layers. The interposed, or sandwiched, epitaxially formed material must exhibit a bandgap larger than silicon. It is possible to utilize the CaF
2
/Si interface to form resonant tunneling devices because CaF
2
has an energy bandgap of 12.5 eV. Moreover, CaF
2
has a room temperature lattice constant that is nearly identical to that of Si, and also has the same crystal space group as Si. Recent studies have indicated that the conduction band of Si is raised by 2.5 Ev, suggesting a CaF
2
/Si heterostructure is a viable material for resonant tunneling.
FIG. 1
shows the effect of the 2.5 Ev offset on the bandgap of CaF
2
.
CaF
2
must be patterned along with Si to form the above described resonant tunneling devices, however no acceptable method of etching CaF
2
has yet been developed. There are three methods at present to etch CaF
2
: water, dilute nitric acid (HNO
3
) and dry etching with various energy sources. The water etch is very slow (the solubility of CaF
2
in water is quite low −0.002 grams in 100 grams of water at 25 C) as well as unreliable—it is difficult to determine when etching begins, when it finishes, or even when it is ongoing. Additionally, when water is used to etch CaF
2
, it cracks the CaF
2
film, creating “ice floe” regions, and ruining the sample. The nitric acid approach is not much better than water. Undiluted, it will not etch CaF
2
. Too much dilution results in the same outcome observed with the water etch, presumably associated with the higher water content. A 3:1 or 2:1 water:nitric acid dilution etches CaF
2
very rapidly −2-3 seconds for a 1000-2000 Angstrom film. However, the reproducibility of this etch is very poor; the etch rate cannot be consistently reproduced in large volume production. Moreover, severe undercut of the film is a significant problem with a rapid, isotropic wet etch. This is particularly important in the use of CaF
2
in resonant tunneling devices, which will require nanometer size structures.
A dry etch method driven by any of a variety of energy sources such as RF, microwave, ECR, etc. would introduce severe damage into the delicate CaF
2
crystalline structure, destroying any possibility of forming nanometer scale resonant tunneling devices.
A typical problem experienced with the above methods is that the CaF
2
will resist etching for a period of time and then start to etch along preferred regions very rapidly, causing the CaF
2
to lift off in flakes. This results in a residual surface that is well textured. Two types of texturing are most prevalent with the prior art etch methods: undulations on the surface reflective of thickness variations and cracking of the film. Undulation-type texturing is a disaster, since small variations in the epitaxial film thickness cause large changes in the material/device behavior which are peculiar to resonant tunneling components. Cracking of the film is equally unacceptable, since leakage through the film destroys any type of device performance. Also, resistivity is an important parameter of the material that must be carefully controlled—a crack in the film would undermine the fabricated value. Accordingly, improvements which overcome any or all of these problems are presently desirable.
SUMMARY OF THE INVENTION
It is herein recognized that a need exists for a method to etch CaF
2
which overcomes any or all of the problems discussed above. The present invention is directed towards meeting those needs.
Generally, and in one form of the invention, a method is presented for the photo-stimulated etching of a CaF
2
surface, comprising the steps of exposing the CaF
2
surface to an ambient species, exciting the CaF
2
surface and/or the ambient species by photo-stimulation sufficiently to allow reaction of the CaF
2
surface with the ambient species to form CaF
2
/ambient species products, and removing the ambient species and the CaF
2
/ambient species products from the CaF
2
surface.
In another form of the invention, a method is presented for the photo-stimulated etching of a CaF
2
surface, comprising the steps of covering the CaF
2
surface with water, irradiating the CaF
2
surface and the water with visible and/or ultraviolet light, and draining the water from the semiconductor surface.
In yet another form of the invention, a method is presented for the photo-stimulated etching of a CaF
2
surface, comprising the steps of placing the CaF
2
surface into a vacuum environment, lowering the temperature of the CaF
2
surface to the dew point, introducing water vapor into the vacuum environment, irradiating the CaF
2
surface and any condensed water with visible and/or ultraviolet light and returning the environment to a vacuum.
An advantage of the invention is that it allows for the etching of CaF
2
in a controlled manner without the texturing problems produced by prior art methods.
REFERENCES:
patent: 4028135 (1977-06-01), Vig et al.
patent: 4478677 (1984-10-01), Chen et al.
patent: 5068040 (1991-11-01), Jackson
patent: 516142 (1992-12-01), None
patent: 0516142A2 (1992-12-01), None
patent: 119191 (1984-06-01), None
S. Wolf et al.,Silicon Processing for the VLSI ERA vol. 1:Process Technology, Lattice Press, Sunset Beach, CA (1989), pp. 520-523.
Suemasu et al., “Room temperature negative differential resistance of Metal (CoSi2)/Insulator (CaF2) resonant tunneling diode”,Electronics Letters, vol. 28, No. 15, Jul. 16, 1992, pp. 1432-1434.
Smith et al., “The electronic properties of epitaxial calcium fluoride-silicon structures”,Materials Research Soceity Symposia Proceedings, vol. 54: Thin Films-Interfaces and Phenomena, Boston, USA, Dec. 2-6, 1985, pp. 295-305.
Toyama, et al., “Crack-Free PZT Thin Films Micropatterned on Silicon Substrate for Integrated Circuits”, (abstract) Seiko Instruments Inc.
Watton, et al., “Materials and Technology Research in Uncooled Ferroelectric IR Detector Arrays”,Royal Signals and Radar Establishment, U.K.., pp. 69-77.
Kauffman, et al., “Studies of Reactions of Atomic and Diatomic Cr, Mn, Fe, Co, Ni, Cu, and Zn with Molecular Water at 15 K”,Journal of Physical Chemistry, vol. 89, No. 16, pp. 3541-3547, 1985.
Park, et al., “Reactions and Photochemistry of Atmoic and Diatomic Nickel with Water at 15 k”,High Temperature Science, pp. 1-15, vol. 25, 1988.
Hauge, et al., “Matrix Isolation Studies of the Reaction of Metal Atoms with Water”,High Temperature Science Inc., pp. 338-339, 1981.
Kauffman, et al., “Infrared Matric Isolation Studies of the Interactions of Mg, Ca, Sr, and Ba atoms and Small CLusters with Water”,High Temperature Science, pp. 97-118, vol. 18, 1984.
Douglas, et al., “Matrix Isolation Studies by Electronic Spectroscopy of Group IIIA Metal-Water Photochemistry”,J. Chem. Soc., Faraday Trans. 1, pp. 1533-1553, 79, 1983.
Douglas, et al., “Electronic Matrix Isolation Spectroscopic Studies of the Group IIA Metal-Water Photochemistry”,High Temperature Science, pp. 201-235, vol. 17, 1984.
Bowers Charles
Brady III Wade James
Kielin Erik
Telecky, Jr. Fred J.
Texas Instruments Incorporated
LandOfFree
Photo-stimulated etching of CaF2 does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photo-stimulated etching of CaF2, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photo-stimulated etching of CaF2 will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2494092