Semiconductor device manufacturing: process – Bonding of plural semiconductor substrates – Subsequent separation into plural bodies
Reexamination Certificate
1999-07-14
2001-11-13
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Bonding of plural semiconductor substrates
Subsequent separation into plural bodies
C438S586000, C438S455000
Reexamination Certificate
active
06316333
ABSTRACT:
TECHNICAL DOMAIN
This invention relates to a process for obtaining a thin film, comprising an area protected from ions, involving an ionic implantation step. In particular it relates to obtaining a semiconducting thin film in which active layers are created, for example to create MOS transistor channel areas.
STATE OF PRIOR ART
In the subject of semiconductors, it is sometimes necessary to make semiconducting thin films, for example to make “Silicon On Insulator” substrates. Other methods of making thin semiconducting films have been developed. One of the most recent methods is based on the fact that implantation of ions of a rare gas or hydrogen in a semiconducting material induces the formation of brittle zones at a depth close to the average ion penetration depth. Document FR-A-2 681 472 divulges a process that uses this property to obtain a thin film of semiconducting material. This process consists of applying the following steps to a platelet of the required semiconducting material and comprising a plane face:
a first implantation step by bombarding the plane face of the platelet using ions capable of creating a layer of “gaseous microbubbles” within the thickness of the platelet at a depth close to the average ion penetration depth, this layer separating the platelet into a lower region containing the mass of the substrate and an upper region forming the thin film, the chosen ions being either rare gas ions or hydrogen gas ions;
a second step in which the plane surface of the platelet is put into intimate contact with a support (or stiffener) composed of at least one layer of rigid material, this intimate contact possibly being made by means of an adhesive substance, by the effect of prior preparation of surfaces and heat and/or electrostatic treatment to encourage interatomic bond between the support and the platelet;
a third heat treatment step of the platelet and support assembly at a temperature greater than the temperature at which the implantation was carried out and sufficient to create a separation between the thin film and the substrate mass. This temperature is about 400° C. for silicon.
This document proposes the following explanation for the various phenomena observed by experience. Firstly, the first ionic implantation step is carried out by presenting a plane surface of a semiconducting material platelet to an ion beam, the plane of this plane surface being either approximately parallel to a principal crystallographic plane in the case in which the semiconducting material is perfectly monocrystalline, or more or less inclined with respect to a principal crystallographic plane with the same indexes for all grains in the case in which the material is polycrystalline. This implantation can create a layer of gaseous microbubbles that will create a fracture zone at the end of the heat treatment. This layer of microbubbles thus created within the thickness of the platelet is at a depth close to the average ion penetration depth and delimits two regions within the platelet separated by this layer; the first region will form the thin film and the second region will form the rest of the substrate. The expression “gaseous microbubbles” means any cavity or micro-cavity generated by the implantation of hydrogen gas or rare gas ions in the material. The cavities may be either in a very flattened form, in other words with low height, for example of the order of a few inter-atomic distances, or they may be in approximately hemispherical form or in any other form different from the two forms mentioned above. These cavities may or may not contain a gaseous phase. During the third step, the heat treatment is carried out at a sufficiently high temperature to create the fracture zone and separation between the two regions, as a result of crystalline rearrangement in the semiconducting material, for example such as by the effect of micro-cavity growth and/or pressure in the microbubbles.
Depending on the implantation conditions, the cavities or microbubbles may or may not be observable after the implantation of a gas, for example such as hydrogen, using transmission electronic microscopy. In the case of silicon, there may be micro-cavities with a size varying from a few nm to a few hundred nm. Thus in particular, when the implantation temperature is low, these cavities can only be observed during the heat treatment step during which nucleation is achieved for example by means of a fast increase in temperature in order to terminate the fracture between the thin film and the rest of the substrate at the end of the heat treatment.
Furthermore, it appears that this process may be applicable to all types of crystalline or non-crystalline solid materials. This process can be applied to dielectric, conducting, semi-insulating materials, and amorphous semiconducting materials.
It may be useful if the thin film obtained comprises some elements or specific features that were generated when this film still formed part of its initial substrate. It is thus possible to make a three-dimensional structure by superposition of thin films. In the micro-electronics domain, this means that platelets can be obtained made by stacking thin semiconducting films, comprising electronic components in the three dimensions of space. However, the implantation of ions through electrically active layers may create defects that modify the characteristics of the components, or make them unusable. This is the case particularly for channel areas and grid oxide layers in MOS transistors.
DESCRIPTION OF THE INVENTION
Thus, the process divulged by document FR-A-2 681 472, which is technically very interesting, had limitations in some of these applications. The inventors of this patent have found a solution to this problem. They have discovered that under some conditions, a masking technique can be used to protect areas sensitive to the transfer of ions, which implies an absence of micro-cavities in the areas corresponding to the masked areas, and nevertheless obtaining cleavage in the substrate so that a thin film can be detached, This is possible if the width of each masked area does not exceed a limiting dimension determined for the material making up the substrate. This principle can also be applied to structures in which elements have been made before implantation, these elements masking areas of the substrate that are not necessarily sensitive to the implantation. In this case, the purpose of the invention is to make these elements significantly narrower than or equal to the limiting dimension.
Therefore, the purpose of the invention is a process for obtaining a thin film starting from a substrate made of a determined material, this thin film being composed of a region of the substrate adjacent to one of its faces and separated from the rest of the substrate, at least one structure being created from the said region, the process comprising the following steps:
generation of the said structure until at least two superposed areas making up the structure are obtained, the upper area acting as a means of masking the lower area by defining a width of the masked area in the substrate not exceeding a determined limiting dimension for the material of the said substrate,
ionic implantation of the substrate through the said surface of the substrate, capable of creating a layer of micro-cavities within the thickness of the substrate and at a depth close to the average ion penetration depth, delimiting the said region within the substrate except for an area corresponding to the masked area,
heat treatment at a sufficiently high temperature to make a fracture line along this layer of micro-cavities, this fracture line being either continuous if the width of the masked area is sufficiently small compared with the said limiting dimension, or discontinuous if the width of the masked area is not sufficiently small with respect to the said limiting dimension,
separation of the thin film from the rest of the substrate either by simple separation if the fracture line is continuous, or by the application of mechanical forces betwe
Aspar Bernard
Bruel Michel
Bowers Charles
Burns Doane , Swecker, Mathis LLP
Commissariat A l'Energie Atomique
Schillinger Laura M
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