Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-04-16
2004-10-26
Ghyka, Alexander (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S407000, C438S440000, C438S475000, C438S766000
Reexamination Certificate
active
06808967
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing a layer of material embedded in another material. It is applicable in particular to the field of semiconductors and especially for producing substrates of the Silicon on Insulator type.
At present, substrates of the Silicon on Insulator or SOI type are of very great interest for microelectronic applications in the domain of low consumption. There are several methods for obtaining SOI substrates. Those used most these days are the SIMOX process (Separation by Implanted Oxygen) and methods based on bonding by molecular adhesion (wafer bonding). In order to obtain thin films of silicon on silica, these methods, using wafer bonding, are combined with thinning methods. As a thinning process, one can quote that revealed in document FR-A-2 681 472 where the cleavage of a substrate is obtained by coalescence, provoked by a thermal treatment of microcavities generated by ion implantation. One can also cite processes using epitaxied barrier layers and selective etching.
STATE OF PRIOR ART
It is known that implantation by bombardment of an inert gas or hydrogen in a semiconductor material (see FR-A-2 681 472), or in a solid material, crystalline or Anot (see FR-A-2 748 850), is capable of creating microbubbles (called platelets or nanoblisters) at a depth close to the average depth of ion penetration. The morphology (dimension, shape . . . ) of these defects can change during thermal treatments, in particular the size of these cavities can increase. Depending on the nature of the material and above all its mechanical properties, these cavities, present at the average depth of penetration of gas species can, depending on the thermal treatment conditions, induce surface deformations or blisters. The most important parameters to monitor to obtain these blisters are the gas dose introduced during implantation, the depth to which the gas species are implanted and the total thermal budget supplied to the material. In certain cases, the conditions of implantation are such that, after annealing, microcavities or microbubbles are present at the level of the average depth of ion implantation but their size and pressure inside these cavities are not sufficient to induce surface deformations. Then there is a continuous layer of defects embedded without any degradation of the surface. As an example, implantation of hydrogen in a silicon plate according to a dose of 3.10
16
H
+
/cm
3
and an energy of 25 keV creates a continuous embedded layer of microcavities of about 150 nm thickness at an average depth of about 300 nm. These microcavities have an elongated shape: their size is of the order of 6 nm in length and two atomic planes in thickness. If annealing is carried out at 600° C. for 30 minutes on this plate, the microcavities grow and their size changes from 6 nm to more than 50 nm in length and from a few atomic planes to 4-6 nm in thickness. On the other hand, no surface disturbance is observed.
The presence of microcavities can also be seen in the case of implantation by helium bombardment, at the level of the average implantation depth (Rp) in silicon. In this case, the cavities have a stable form which does not change during annealing. Reference can be made to the article “Radiation damage and implanted He atom interaction during void formation in silicon” by V. RAINERI and M. SAGGIO, Appl. Phys. Lett. 71 (12), 22 Sep. 1997.
Furthermore, it is known that defects present in materials are the preferential nucleation centres for the formation of a heterogeneous phase. As an example, concerning the formation of oxide precipitates, three types of nucleation are listed in the bibliography: in the homogeneous phase, in the homogeneous phase under stresses, in heterogeneous phases (see for example the article entitled “Oxygen Precipitation in Silicon” by A. BORHESI et al., J. Appl. Phys. 77(9), 1995, pages 4169-4244). This oxygen which precipitates is contained in the initial material. It comes, for example, from the formation stage of the material.
By nucleation is meant the formation of aggregates of several oxygen atoms in silicon to form nucleation centres called “nuclei” or “precipitate embryos”. More simply, nucleation can appear in crystalline sites corresponding to knots of the network where several interstitial oxygen atoms are close to each other (homogeneous nucleation) or on network defects (heterogeneous nucleation). It is known that these network defects can be point defects induced by the presence of an element exterior to the matrix (for example carbon in the silicon) or complexes such as, for example, oxygen-carbon complexes (see the article mentioned above by A. BORHESI et al.). For example, the point defects intrinsic to the material such as clusters of vacancies formed during silicon growth can also be nucleation centres for obtaining “nuclei”. Furthermore, as an example of defects induced by the presence of an external element, the case can be cited of carbon introduced into the substrate to create a continuous embedded layer rich in carbon which will act as a nucleation zone. The introduction of carbon can be obtained by implantation of carbon by bombardment.
After the formation phase of these nucleation centres, to obtain a precipitate of larger size, it is necessary to have a precipitation phase. Precipitation in a material is a phenomenon of aggregation of atoms to form small particles or precipitates.
The critical radius r
c
defining the minimum size of precipitates capable of existing is given, for a concentration of interstitial oxygen in the material, by the equation r
c
=(2&sgr;/&Dgr;H)(Ts/Ts−T) where
&sgr; is the surface energy,
&Dgr;H is the formation enthalpy,
T is the temperature expressed in Kelvins
Ts is the equilibrium temperature corresponding to the given quantity of oxygen,
(see the article “Oxygen Precipitation Czochralski Silicon” by R. CRAVEN, Elec. Chem. Soc., Proceedings of the 4th Int. Symp. on Silicon Materials, Science and Technology Vol. 81-5, 1981).
Starting from this equation, it can be clearly seen that the rise in temperature brings about growth of the precipitates.
To resume, defects create nucleation centres, which will serve to form precipitates which will then become bigger.
On the other hand, studies have demonstrated the possibility of reducing the number of discontinuities of the oxide layer embedded in the case of SOI substrates obtained by the “low dose” SIMOX process with the help of oxidation at high temperature (over 1350° C.) of the silicon film (see patent U.S. Pat. No. 5,589,407 and the article entitled “An Analysis of Buried-Oxide Growth in Low-dose SIMOX Wafers by High-Temperature Thermal Oxidation” vt S. MASUI et al., Proceedings 1995 IEEE International SOI Conference, October 1995). This process, named ITOX (Internal Oxidation), makes it possible to oxidise the oxide layer embedded by means of oxygen diffusion from the surface to the embedded oxide layer. Other authors demonstrate that the same phenomenon occurs at lower temperatures, of the order of 1200° C. (see article “Internal Oxidation of Low Dose Separation by Implanted Oxygen Wafers in Different Oxygen/Nitrogen Mixtures” by P. ERICSSON and S. BENGTSSON, accepted for publication in Appl. Phys. Lett.).
The latter results indicate that the introduction of oxygen into the material depends firstly on the time applied at high temperature and not on the quantity of
oxygen in the annealing atmosphere. Thus it seems that the introduction of oxygen may be limited by the solubility limit of oxygen in silicon. Thus, the higher the temperature, the faster the oxidation effect for the embedded oxide layer. An example of this phenomenon indicates that at 1200° C., if 5% oxygen is introduced into nitrogen, 8 hours of annealing allow the embedded oxide layer to grow in thickness from 860 Angstrom to 1330 Angstroms. This “internal” oxidation is of interest because it reduces the density of discontinuities of the embedded oxide.
From the abstract of the document JP-A-56 110 247 a method is known for
Aspar Bernard
Bruel Michel
Moriceau Hubert
Commissariat a l'Energie Atomique
Ghyka Alexander
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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