Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – On insulating substrate or layer
Reexamination Certificate
2000-06-06
2002-04-02
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
On insulating substrate or layer
C438S478000, C148S033200
Reexamination Certificate
active
06365491
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of forming a regular network of semiconductor islands on an insulating substrate. Such semiconductor islands can be used for manufacturing quantal devices such as devices using a Coulomb blocking phenomenon.
The Coulomb blocking phenomenon occurs in conductive or semiconductor islands both insulated electrically from their environment and weakly coupled thereto by tunnel effect. Using this phenomenon at temperatures close to room temperatures requires the total capacitance of each island to be around 1 attofarad. The dimensions of the islands are generally around 1 nanometer.
The invention finds applications notably in the manufacture of logic circuits and memories with a very high integration density.
PRIOR ART
The accompanying
FIG. 1
depicts a highly schematic plan view of a device
10
using semiconductor islands.
References
12
and
14
designate first and second electron reservoirs of the quantal effect device
10
, whose functioning uses the Coulomb blocking phenomenon. These reservoirs are, for example, the drain and source of a structure of the field effect transistor type, or a microelectronic device such as a memory.
Between the two electron reservoirs
12
,
14
there is a region with a set of semiconductor islands
16
, or grains.
During the manufacture of such a region, it is found that the formation of the islands is random and irregular.
When the formation of the islands is obtained by a nucleation process, it complies with a statistical distribution law for the nucleation centres fixed by known thermodynamic laws and set out, for example, in the article “The Nucleation of CVD Silicon on SiO
2
and Si
3
N
4
Substrates” by W. Claassen, et al., Journal of the Electrochemical Society 128, No 6, pp. 1353-1359, (1981). It is known for example that a silicon nitride surface is more favorable to a high nucleation density than a silica (SiO
2
) surface since the mechanism for the deposition of silicon from silane is based on the formation of species of the SiH
2
type which diffuse rapidly on a surface with a high density of OH bonds, such as an SiO
2
surface.
It is also known that the nucleation density of silicon can be increased by specific treatments. In a treatment described in the article “Surface Treatment Effect on the Grain Size and Surface Roughness of as-Deposited LPCVD Polysilicon Films” by A. T. Voutsas, et al., Journal of the Electrochemical Society 140, No 1, pp. 282-288, (1993), the nucleation densities obtained after cleaning of the silica in a chemical bath (notably in a bath based on sulphuric acid and hydrogen peroxide), are greater than those obtained without treatment. This phenomenon is probably attributable to the presence of impurities left by the bath on the surface of the silica.
However, the nucleation, even when it is assisted by impurities, remains a statistical phenomenon, which does not make it possible to create regularly spaced islands of silicon.
The irregularity of the distribution of the nucleation kernels, and therefore of the islands of semiconductor material, is accompanied by a lack of homogeneity of the size of the islands. This phenomenon limits the quality and performance of the Coulomb blocking electronic devices, using such a structure.
DESCRIPTION OF THE INVENTION
The purpose of the present invention is to propose a method of forming a network of islands regularly spaced apart on an electrically insulating support.
One aim is also to propose such a method for obtaining islands of homogeneous size.
Another aim is to propose an electronic device of the Coulomb blocking type using a network of regularly spaced islands, obtained in accordance with the invention.
To achieve these aims, the object of the invention is more precisely a method of forming a network of islands of semiconductor material on a surface of an electrically insulating material. The method comprises:
a) the deposition of nucleation kernels on the surface of the electrically insulating material, by means of an auto-organisation process,
b) the formation of islands of semiconductor material on each of the nucleation kernels.
In accordance with the invention, the deposition of the nucleation kernels is effected using at least one layer, referred to as a distribution layer, made of a material having a substantially regular molecular structure, formed on the electrically insulating material surface, in order to distribute the nucleation kernels in a substantially regular fashion on the surface of the electrically insulating material by means of an auto-organisation process.
Semiconductor islands means grains of semiconductor material formed by growth on the nucleation kernels. These grains have small dimensions of between, for example, 1 and 10 nanometers. Their growth can be assisted or promoted by means of a heat treatment.
In addition, nucleation kernel means any impurity able to promote the local formation of a semiconductor grain, in particular a crystal. The impurity can cause the formation of the crystal either directly, or indirectly by causing in the surface of the electrically insulating material a local structure modification able to promote the formation of the semiconductor crystal. The kernels are, in particular, atoms.
The distribution layer can, in accordance with particular embodiments of the invention, be used either as a mask for the deposition of kernels, or directly as a kernel addition layer.
According to the first particular embodiment proposed, use is made of a distribution layer made of a material with a molecular structure having interstices spaced apart substantially regularly, and the nucleation kernels are deposited using the distribution layer as a deposition mask for uniformly distributing the nucleation kernels, the distribution layer being eliminated after the deposition of the nucleation kernels.
By way of example, the material used for the distribution layer can be an organic material of the phthalocyanine or porphyrin type (a molecule with a porphyry core). These molecules can be functionalised, that is to say can carry substituents, so as to initiate between them chemical bonds with fixed lengths. This auto-organisation process makes it possible to bring the kernels (in particular metallic centres) to a fixed and determined distance from each other.
The kernels are, for example, made of a metal such as, preferentially, Al, Mg, Ca, Se. These metals, in atomic form, are able to interact on the surface of the silica. Other metals such as Cu or Ni can also be used. These metals are however liable to diffuse, even at low temperature, in the insulating material, when the latter is made of silica.
The kernels are distributed on the distribution layer in order to fit in the interstices and be fixed on the insulating surface in the interstices. The fixing of the kernels on the insulating surface takes place notably by chemical sorption.
To allow an easy fitting of the kernels in the interstices of the distribution layer, this layer can, preferably, be produced in a monolayer form, that is to say in the form of a monomolecular layer. In this case, the molecules are functionalised so as to make them suitable for deposition in a monomolecular layer.
The formation of a monomolecular layer can take place, for example, according to a technique, known per se, referred to as the Langmuir-Blodgett technique. In this regard reference can be made to the French document FR-A-2 666 092 (Feb. 28, 1992.).
According to the second particular embodiment of the proposed invention, use is made of a distribution layer made of a material having a molecular support structure and nucleation kernels regularly distributed on the molecular support structure. After putting this layer on the surface of the insulating material, a treatment is carried out to separate the nucleation kernels, fixed on the surface of the insulating material, and the support structure, in order to eliminate this structure.
The material of the distribution layer is for example an organometallic material having metal sites, fo
Bourgoin Jean-Philippe
Martin François
Mouanda Brigitte
Nunzi Jean-Michel
Palacin Serge
Burns Doane , Swecker, Mathis LLP
Commissariat a l'Energie Atomique
Le Dung A
Nelms David
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