Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-05-27
2002-01-29
Abraham, Fetsum (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S316000, C257S317000, C257S318000, C257S319000, C257S320000, C257S321000, C257S322000, C257S314000, C257S066000, C257S067000
Reexamination Certificate
active
06342716
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a dot element, a semiconductor device using the dot element and a method for fabricating the device. More particularly, the present invention relates to a method for forming a dot element out of an ultrafine particle of the size of several nanometers and functioning as a quantum dot element, a semiconductor device using the dot element and a method for fabricating the device.
Currently, a ULSI is formed by integrating a great number of MOS devices on a single chip. In general, as an MOS device is miniaturized, the performance thereof is enhanced correspondingly. However, if the gate length thereof is 0.1 &mgr;m or less, then the device can hardly operate normally as a transistor, because such a size is a physical limit for the device. A single-electron tunneling device, called a “coulomb blockade”, has attracted much attention, recently as a candidate for breaking through such a limit (Kenji Taniguchi et al., FED Journal, Vol. 6, No. 2, 1995). In principle, a single-electron tunneling device performs logical operations and storing operations by controlling the movement of individual electrons, and is very effective in reducing power consumption. However, in order to form a single-electron tunneling device, semiconductor or metal fine particles of the size of several nanometers, called “quantum dot elements”, are required. As disclosed in Japanese Laid-Open Publication No. 9-69630, for example, if a large number of Au dot elements are formed out of Au fine particles by sputtering or the like between metal electrodes formed on a substrate, then the Au dot elements form multiple bonds with each other, thereby realizing single-electron effects. In accordance with this method, however, it is very difficult to accurately control the positions where the Au dot elements are formed.
Thus, Sato et al. proposed another method for forming a dot element. In accordance with the method of Sato et al., 3-(2-aminoethylamino)propyltrimethoxy silane (APTS) is deposited on a substrate on which a PMMA resist pattern has been formed. Then, APTS on the PMMA resist is partially lifted off together with an unnecessary portion of the PMMA resist, thereby selectively leaving APTS at desired positions on the substrate. Thereafter, Au fine particles are attached onto only APTS, thereby forming Au dot elements.
Aside from the single-electron tunneling device, a different method for breaking through the limit of a device size using dot elements was also proposed. For example, S. Tiwari et al. disclosed in IEDEM Tech. Digest, 521 (1995) that an operating voltage would be lowered by using dot elements of silicon fine particles for the floating gate of a nonvolatile memory or the like. Tiwari et al. suggested that silicon dot elements could be formed directly on a substrate by performing a CVD process on accurately controlled conditions.
However, the methods of T. Sato et al. and Tiwari et al. have the following problems.
To control the positions of dot elements on a substrate by the method of T. Sato et al., the process steps of forming a PMMA resist pattern or the like on the substrate and then lifting off APTS with unnecessary portions of the PMMA resist pattern are required. Thus, the fabrication process is adversely complicated. In addition, in this method, the Au dot elements are formed onto APTS by utilizing the polarization of charges. Accordingly, if charges have been polarized at other sites on the semiconductor substrate, then Au fine particles are unintentionally attached to such sites. Therefore, it is not always possible to selectively form the Au dot elements only at desired sites.
On the other hand, in accordance with the method of Tiwari et al., silicon dot elements are directly formed on a substrate by a CVD technique. Thus, it is very difficult to control the sizes and positions of such dot elements on the substrate.
Because of these inconveniences, it is now hard to use dot elements, formed by the conventional methods, as a member of a semiconductor device or as quantum dot elements, in particular. That is to say, in accordance with the conventional methods, a semiconductor device, including dot elements formed with the sizes and positions thereof accurately controlled, is very much less likely to be realized.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for forming dot elements while accurately controlling the sizes and positions thereof by taking various measures to precisely control the positions and sizes of fine particles over a substrate. Another object of the present invention is to provide semiconductor device of various types, each including the dot elements functioning as quantum dot elements as a component.
A first method for forming a dot element according to the present invention includes the steps of: a) forming a first compound on a part of a substrate; b) attaching a second compound to the surface of a fine particle, the second compound having such a nature as to be combined with the first compound formed on the substrate; c) combining the first and second compounds together and selectively placing the fine particle only on the part of the substrate where the first compound has been formed, thereby forming a dot element out of the fine particle.
In accordance with the first method, the positional accuracy of the dot elements can be controlled based on the position of the first compound formed on the substrate. In addition, only by selecting fine particles of a desired uniform size from the beginning, the sizes of the dot elements can be easily controlled. Accordingly, the positions and sizes of dot elements can be accurately controlled by performing a simple process, without any need for complicated process steps. As a result, dot elements, functioning as quantum dot elements in a device, can be practically formed.
In one embodiment of the present invention, both the first and second compounds are preferably organic compounds.
In another embodiment of the present invention, one of the first and second compounds may be an antigen and the other may be an antibody of the antigen.
In such an embodiment, a dot element can be formed such that the fine particle is fixed at a desired position, not undesired position, with a lot more certainty by taking advantage of the high selectivity of an antigen-antibody reaction.
In still another embodiment, at least one of the first and second compounds may be a protein or an enzyme.
In such an embodiment, the above effects can be attained because a protein or an enzyme is generally likely to react with a particular material.
In still another embodiment, in the step a), an energy wave is preferably irradiated onto only a part of the substrate after the first compound has been formed on the substrate.
In such an embodiment, the first compound can be easily left only at a particular site on the substrate by appropriately selecting the first compound and the energy wave.
In still another embodiment, the energy wave may be selected from the group consisting of: light; X-rays; and electron beams.
In still another embodiment, the dot elements may be formed in matrix by using an interference pattern of the energy wave as the energy wave.
In such an embodiment, a matrix of regularly arranged dot elements can be provided as a component of a device.
In still another embodiment, an electron beam irradiated by an atomic force microscope or a scanning tunneling microscope may be used as the energy wave.
In still another embodiment, a gold fine particle may be used as the fine particle.
In such an embodiment, a dot element functioning as a quantum dot element can be formed particularly easily, because gold fine particles have already been practically used as ultrafine particles of the size in the range from 1 to 10 nm.
In still another embodiment, the first method may further include, posterior to the step c), the step of d) directly fixing the dot element onto the substrate by removing the first and second compounds.
In such an embodimen
Adachi Kazuyasu
Araki Kiyoshi
Endo Masayuki
Morimoto Kiyoshi
Morita Kiyoyuki
Nixon & Peabody LLP
Robinson Eric J.
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