Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2001-04-10
2004-09-21
Chen, Kin-Chan (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S719000, C438S723000, C438S745000
Reexamination Certificate
active
06794296
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for producing an aperture in a semiconductor material, for example (100)-oriented or polycrystalline silicon. Such apertures, whose size is in the micrometer range or below, are used, for example, as a component of probes for scanning near-field optical microscopes (SNOM). With this method, optical surface properties can be inspected with sub-wavelength resolution. As with any other scanning optical microscope, the resolution achievable by the scanning near-field optical microscope is limited by the geometry and the dimensions of the probe, which means in particular, the aperture and its distance from the surface of the sample piece. In order to achieve sub-wavelength resolution the light-emitting or detecting area of the probe must have lateral dimensions below 100 nm. In the prior art, there is no lack of attempts at producing such small dimensioned reproducible apertures in the 100 nm range or below. A method known from the prior art is shown schematically in FIG.
6
.
FIG. 6
a
reflects the cross-section through a semiconductor wafer
14
having an upper surface
16
and a lower surface
18
. The upper surface
16
comprises a plurality of cavities
20
, for example in the form of an inverse pyramid
30
, preferably produced by means of anisotropic etching. Thereafter, the lower surface
18
of the semiconductor wafer
14
, which consists of (100)-oriented silicon, for example, is etched back, particularly by means of anisotropic etching until the tips of the inverse pyramids are exposed, thus producing an aperture
10
, as shown schematically by the first and second illustrations in
FIG. 6
b
. The opening of the first aperture is too wide, the opening of the second aperture is ideal, while in the third example, the tip of the inverse pyramid has not yet opened at all.
This is due to the fact that the thickness of the semiconductor wafer varies highly. Even a variation in the thickness of only a few 10 nm can result in variations in the diameter or the cross-section of the aperture as shown in FIG.
6
. This example, which is based on prior art, demonstrates that as a result of the variation in the thickness of the semiconductor wafer
14
only very few tips of the inverse pyramids have a suitable aperture size. Also, the size of said aperture is subject to high scattering.
Furthermore, studies known from the prior art on the oxidation behavior of silicon (Markus et al., Journal of the Electrochemical Society, Solid State Science and Technology, pages 1278-1282, 1982, and Kao et al., IEE Transactions on Electronic Devices, Volume 34, No. 5, page 1008, 1987 and Volume 35, No. 1, page 25, 1988) revealed a high dependence on the orientation in the plane of the semiconductor wafer, on the temperature and the structure of the surface. It was found that at low oxidation temperatures of approx. 800° C. to 900° C. the thickness of the oxide layer on convex and concave edges of the structured surface, for example in trench cells, decreases relative to the thickness of the oxide layer on the surface. These findings have already been used for producing very sharp silicon tips (Marcus et al, Applied Physics letters, 54 (3), pages 236-238, 1990) where curvature radii in the range of approx. 1 nm were achieved. A similar method for producing very sharp silicon tips for the so-called cantilever probes used in scanning probe microscopy is specified in EP-A-0468071.
SUMMARY OF THE INVENTION
Based on a method for producing an aperture in a semiconductor material comprising the steps of preparing a semiconductor wafer, for example a (100)-oriented semiconductor wafer having an upper surface and a lower surface, and producing a cavity with a side wall in the upper surface of the semiconductor wafer by partially etching said upper surface, the aim of the invention is to provide a method for producing an aperture whose size is below approx. 1 micrometer, particularly at approx. 100 nm, where the size of the aperture is adjustable so as to be reproducible.
The problem is solved with the method having the above mentioned characteristic features substantially in that the cavity comprises a closed bottom area, which faces the lower surface and which preferably has, in particular, a convex or, in particular, a concave corner or edge or a curvature of this type, that an oxide layer is deposited on the semiconductor material, at least in the area of the cavity by means of oxidizing the semiconductor material, where the oxide layer comprises an inhomogeneity, at least in the bottom area, that the semiconductor material is selectively etched back on the lower surface of the semiconductor wafer until at least the oxide layer located in the bottom area is exposed, and that the exposed oxide layer is etched until it is at least severed.
The method offers a particular advantage in that the measurement of the size of the aperture is not dependent on the variations in the layer thickness of the semiconductor wafer. The result is that the apertures to be produced are highly reproducible and thus they are able to open up new areas of application and resolutions, for example when they are used in probes in scanning near-field optical microscopy.
A number of procedural steps are performed for producing apertures in semiconductor materials, for example in (100)-oriented mono-crystalline silicon or polycrystalline silicon.
First, particularly pyramidal or other cavities are produced which are tapering at the lower end and which are etched into the semiconductor material. For this purpose, masking layers are provided on the surface of the semiconductor wafer. By means of optical or electron beam lithography and subsequent chemical, electrochemical or plasma etching methods the required structures are applied to the masking layer. The cavities are etched by means of wet-chemical or plasma etching methods. Alternatively, the cavities can also be produced by means of a focused ion beam. In the next step, the semiconductor material is oxidized, where the resulting oxide layer varies depending on the crystal orientation, the oxidation temperature and the curvature of the respective local structure of the surface of the semiconductor wafer. When suitable oxidation temperatures are selected the oxide layer has a higher etching rate in the places with the highest curvature as a result of stress effects, which means that the oxide layer, for example in the case of a tapering cavity, has one or more “weak points” in the area of the tip as relates to the etching process. In the following step, the oxide layer, which may have developed during the oxidation process on the lower surface of the semiconductor wafer, is removed using methods known in the art. Thereafter, the semiconductor material on the lower surface is etched back by wet-chemical etching or plasma etching until finally the tip of the oxide layer located in the cavity is exposed. It is important that a selective etching method is used for this so as to fully or at least largely prevent that the oxide layer is also etched. The semiconductor material is etched back until the single or all of the oxide layers, for example of an array of cavities, are exposed. As a result of variations in the thickness of the semiconductor wafer it is quite possible that the multiple tips, if applicable, of the oxide layer will project to a greater or lesser degree from the lower surface of the semiconductor wafer.
This is not problematic for dimensioning the aperture size, however, insofar as the tips of the oxide layer which project to a greater or lesser degree all have substantially the same form, at least with regard to the thickness and form of the oxide layer, and particularly in the area of the tip, they each have one or more weak points. Thereafter, the oxide layer is thinned with an etching agent, which is selective with regard to the material of the oxide layer, until the oxide layer breaks through on the “weak points” of the oxide layer and the desired apertures are produced in the oxide stumps. The etching pr
Kassing Rainer
Mihalcea Christophe
Oesterschulze Egbert
Baker & Daniels
Chen Kin-Chan
Universitat Gesamthochschule Kassel
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