Semiconductor device manufacturing: process – Thinning or removal of substrate
Reexamination Certificate
2002-03-20
2004-10-26
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Thinning or removal of substrate
C438S120000, C438S015000
Reexamination Certificate
active
06809044
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for making a thin film of solid material. In particular it relates to the production of a thin film made of a semi-conducting material, for example such as silicon.
STATE OF PRIOR ART
Document FR-A-2 681 472 (corresponding to U.S. Pat. No. 5,374,564) discloses a process for manufacturing thin films made of a semi-conducting material. This document divulges that implantation of a rare gas and/or hydrogen in a substrate made of a semi-conducting material could create a layer that could contain micro-cavities or micro-bubbles (or platelets) at a depth approximately equal to the average penetration depth of the implanted ions. The implanted face of this substrate is brought into intimate contact with a support acting as a stiffener. Furthermore, heat treatment can be applied at a sufficiently high temperature to induce an interaction (or coalescence) between the micro-cavities or micro-bubbles causing a separation of the semi-conducting substrate into two parts, namely a thin semi-conducting film bonding to the stiffener, and secondly the rest of the semi-conducting substrate. The separation takes place at the location at which the micro-cavities or micro-bubbles are present, in other words along the micro-cavities layer. The heat treatment is such that the interaction between the micro-bubbles or the micro-cavities created by implantation causes a separation between the thin film and the rest of the substrate. Therefore, there is a transfer of a thin film from an initial substrate as far as a stiffener that acts as a support for this thin film.
This process may also be applied to the manufacture of a thin film made of solid material other than a semi-conducting material (a conducting or dielectric material) that may or may not be crystalline. This film may be single layer or multi-layer.
Thus, the implantation of gaseous compounds can create in-depth cavities or micro-bubbles or micro-cracks that will form a weakened layer close to the depth at which the ions stop. The implanted zone is more or less fragile depending on the nature and implantation conditions. They are chosen such that the implanted surface of the substrate is not deformed in any way. If any deformations in this surface occur in the form of blisters, these deformations will cause excessive weakening of the implanted zone.
Document FR-A-2 681 472 describes how, in order to transfer a thin film onto a support, the implanted substrate and the support (or stiffener) have to be bonded together before causing separation of the thin film from its original substrate, this separation possibly being caused by a heat treatment and/or a mechanical treatment (as described in document FR-A-2 748 851). Bonding is achieved by putting the implanted substrate and the support into intimate contact by means of molecular bonding, or a glue or an intermediate compound (insulating layer, conducting layer, etc.). This bonding is only possible if there are no deformations on the implanted surface, and therefore if no blisters have occurred.
In some cases, it is impossible to bond the implanted substrate and the support acting as a stiffener, particularly due to different coefficients of thermal expansion. It is also possible that the bonding forces are not sufficient to cause the stiffening effect. Therefore a thin film, for example a mono-crystalline film, can be obtained on any support using a process derived from the process divulged by document FR-A-2 681 472, for example the process divulged by document FR-A-2 738 671 (corresponding to U.S. Pat. No. 5,714,395). According to this process, the implanted gaseous compounds rust be at a sufficient depth and/or a layer of a material able to make the structure sufficiently rigid to obtain separation at the implanted zone must be deposited after the implantation step, in order to separate the thin film from its original substrate. The film obtained is then self supporting.
For the two processes mentioned above, the surface roughness of the thin film after transfer is variable depending on the implantation and/or separation conditions (heat and/or mechanical treatment) used to obtain this separation. In this case it may be useful to further weaken the zone containing the cavities. Separation would then be easier than in the normal case, in other words separation would be possible by applying lower mechanical forces and and/or a smaller thermal budget. This is particularly useful for structures composed of materials with different coefficients of thermal expansion and for which there are limiting heating temperatures.
The various means of weakening the implanted zone include an increase in the dose of implanted gaseous compounds and/or carrying out a heat treatment that may correspond to the heat treatment divulged in document FR-A-2 681 472. However, as mentioned above, the implanted dose and/or the thermal budget need to be limited before the bonding step in order to prevent deformations of the implanted surface.
Thus, there is no acceptable means of further weakening the implanted zone before applying the separation step. The existence of such a means would make it possible Go reduce thermal budgets and/or the mechanical forces necessary for separation. Thus, thin films could be transferred onto supports that cannot resist high temperatures, by using the process divulged in document FR-A-2 681 472. It would also be possible to more easily separate thick films using the process described in document FR-A-2 738 671. These thick films could then be transferred onto any type of support, even supports for which it would be impossible to obtain high bonding forces between the film and the support. Furthermore, increased weakening of the implanted zone would make it possible to reduce the roughness of the free surface of the film after transfer, while encouraging fracture.
Therefore, the problem that arises is to further weaken the implanted zone without inducing any blisters on the implanted surface of the original substrate.
PRESENTATION OF THE INVENTION
The invention provides a solution to this problem. It is proposed to apply pressure on the implanted face of the substrate, at least during part of the coalescence of micro-cavities, in order to facilitate this coalescence and prevent implanted gaseous compounds from escaping from the substrate. The result is that weakening is increased.
Therefore, the purpose of the invention is a process for making a thin film from a substrate of a solid material with a plane face, comprising:
the implantation of gaseous compounds in the substrate to form a layer of micro-cavities located at a depth from the said plane face corresponding to the thickness of the required thin film, the gaseous compounds being implanted under conditions that can weaken the substrate at the layer of micro-cavities,
partial or complete separation of the thin film from the rest of the substrate, this separation comprising a step in which thermal energy is input and in which pressure is applied to the said plane face.
The “Mechanistic Studies of Silicon Wafer Bonding and Layer Exfoliation” document by M. K. WELDON et al., published in Electrochemical Society Proceedings, volume 97-36, specifies that application of a compression stress on a glued structure composed of an implanted substrate and a stiffener is a means of closing micro-cracks and preventing exfoliation, while a uniform external tension can cause exfoliation at a lower temperature. It also mentions that application of a uniform pressure at lower temperatures is a means of developing more uniform micro-cracks such that a more uniform exfoliation can be obtained when the pressure is released and heat is applied. In this document, the applied pressure is a means of obtaining uniform micro-cracks but does not provide any information about increased weakening of the implanted zone by an increase in the size of micro-cracks. Thus, in this document, exfoliation is achieved by releasing the pressure and applying heat at a temperature a priori greater than the temperature used
Aspar Bernard
Bruel Michel
Moriceau Hubert
Commissariat a L'Energie Atomique
Deschere, Esq. Linda M.
Jr. Carl Whitehead
Schillinger Laura M
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