The invention relates to substrate transfer techniques, which are used in particular in microelectronics.
In this field, it is often aimed to use a temporary support, also known as a “handle”. On such a support may be temporarily fixed one or more components, which may then again be transferred onto another, definitive support. The support or the handle substrate may then be reused for another transfer.
The problem is often posed of being able to detach in a simple manner the component(s) that have been fixed temporarily onto the handle substrate.
Moreover, components tend to be smaller and smaller, and known transfer techniques, and thus the substrates commonly used as handles, are not always adapted to constantly decreasing dimensions.
The invention relates to a method of carrying out a transfer of one or more first components or of a first layer on a first substrate to a second substrate, comprising the following steps:
a) the placing in contact, against the first substrate, made of a ferroelectric material, of the first component(s) or the first layer, and maintaining them by electrostatic effect against this first substrate,
b) a placing in contact, direct or by molecular adhesion, of these first components or this first layer with the second substrate,
c) a separation or a dismantling of the first substrate, leaving at least one part of each of said components or of said layer on the second substrate.
Step a) implements the internal field of the substrate made of ferroelectric material, which is charged in an intrinsic manner.
Consequently, there are no leaks of charges and the polarisation may be maintained as is as long as there are no discharges by an exterior intervention. There is thus no time limit to the use of the assembly between the first substrate and the components or the first layer.
The dismantling of the first substrate may be assisted:
Advantageously, the ferroelectric material of the first substrate is LTO (LiTaO3) or a material of same structure as LTO, such as LiNbO3, BaTiO3, or SrTiO3, or LaAlO3, or LiAlO3, or any other ferroelectric material.
Advantageously, it is possible to carry out on the surface of the second substrate a preparation for the purpose of a molecular bonding, the components or the layer then being placed in contact with this prepared surface and assembled with it by molecular bonding.
One or more of said components, or said layer, may have undergone, before step a), or undergo, between step a) and step b), a treatment by etching, and/or ion implantation, and/or deposition and/or a thermal treatment.
In the case of a treatment by ion implantation before step a), or between step a) and step b), a fragilization area is formed in at least one of said components or said layer, along which a fracture is made during step c).
In the case of a treatment by ion implantation before step a), a fragilization area is formed in at least one of said components or said layer, along which a fracture is made between step a) and step b).
Depending on the depth of the implantation and the face of the components or the layer traversed by the implantation, the layer transferred during step c) may be more or less thick.
A layer may be deposited on the surface of the ferroelectric material, to promote the bonding, for example a layer of silicon oxide SiO2 or a polymer (PDMS, BCB) with weak or very weak strength or adhesion.
The transfer operation may be carried out in several stages, in particular by means of the same first ferroelectric substrate, a layer being thus transferred, or components being thus transferred, onto an already transferred layer, or onto already transferred components, which makes it possible to form stacks or stages of components on a same second substrate.
Thus the object of the invention is also a method of carrying out a transfer of at least two stages of components and/or layer onto a second substrate, comprising the following steps:
The transfer of a second stage may be carried out by:
a′) the application and the maintaining, by electrostatic effect, of said one or more second components or said second layer, on said first stage, by means of a substrate, made of a ferroelectric material, electrically charged,
b′) a placing in contact and a transfer of these components or this layer onto said first stage,
c′) a dismantling of said substrate made of a ferroelectric material, leaving the component(s) or said second layer on the first stage.
The transfer of the first stage may be carried out with the same substrate, made of a ferroelectric material, as the transfer of the second stage.
Whatever the embodiment, a method according to the invention enables a novel method of handling a film or stamps, while having a temporary support, by electrostatic bonding.
A first example of method according to the invention will be given with reference to
Components 8, 10, for example of the “stamp” type (hereafter, the term component or stamp are used indiscriminately) are deposited on this substrate 2 (
The reference 30 designates possible alignment marks on the substrate 2, which will make it possible to achieve later a good alignment on a second substrate 20, or transfer substrate, particularly if alignment marks are also provided on the second substrate 20.
These components 8, 10 may be of all sizes, they are for example chips. Each side of one of these components may measure between several micrometers and several millimeters or several centimeters, for example between 1 μm or 5 μm and 1 mm or 5 mm or 5 cm.
The dimensions, and in particular the thickness of each component, may be such that they cannot be handled individually. In particular, its thickness may be of the order of several nanometers to several tens of nanometers, for example between 5 nm or 10 nm and 50 nm. This is the case, in particular, of certain stamps or certain chips. A transfer technique according to the invention makes this stamp or this chip handleable.
The thickness of the components may also be higher, for example 750 μm or more, or even several millimeters.
The surfaces 8′, 10′ (
In an alternative, these surfaces 8′ and 10′ may not be at the same height h in relation to this same surface 2′ of the substrate 2. This is the case in particular if at least one of the components comprises at least one surface made of a material having an elasticity or compliant, for example a polymer. It has been shown by Jourdain et al. “BCB Collective Hybrid Bonding for 3D-Stacking” Conference on Wafer Bonding for MEMS Technologies and Wafer Level Integration, 2007 that a compliant polymer layer, for example made of BCB, can accept during transfers height differences of more than 3 mm.
Prior to the deposition, the components 8, 10 may have undergone a step of treatment for the purpose of promoting the contact or the adhesion with the surface 2′ of the first substrate 2, for example a treatment by polishing or by plasma activation. This type of treatment may reveal, at the interface between the stamps and the surface 2′ of the substrate 2, several monolayers of water.
Advantageously, it is possible to carry out the same type of preparation on the surface 20′ of the second substrate 20, for the purpose of a molecular bonding. But said bonding 20-20′ may also be achieved by means of adhesive.
The electrostatic adhesion of each component or stamp 8, 10 on the first substrate 2 is obtained by the natural electrical charges of this substrate 2. The spontaneous polarisation of the latter may be reinforced by the application of an electric field.
Once maintained on the substrate 2, the components 8, 10 may undergo one or more treatments, for example an etching, and/or an ion implantation, and/or a deposition, etc.; in the case of steps with elevation of temperature, the ramp(s) are chosen, during the thermal budget, in such a way as not to have a polarisation inversion phenomenon of the first substrate (ferroelectric); typically, one or more ramp(s) less than 5° C./minute are used. Moreover, the temperature is limited to a value less than the Curie temperature Tc of the ferroelectric material (for tantalate, this Curie temperature is equal to around 600° C.; for niobate it is around 1200° C.).
In an alternative, the components 8, 10 may be prepared for the purpose of a direct or molecular bonding with the surface 20′ of the second transfer substrate 20, if necessary after, or in combination with, one or more of the previous treatments.
The assembly obtained may thus also, as illustrated in
The transfer substrate 20 is for example made of silicon or another semi-conductor material, or any other material such as fused silica or quartz.
The surface 8′, 10′ of the components or stamps 8, 10 and the surface 20′ of the second substrate 20 (
In an alternative, the disbondment may be obtained by thermal effect, or instead a thermal effect may assist the mechanical effect to disbond the stamps from the substrate 2.
More specifically, a step of heating makes it possible to carry out the separation of the components 8, 10 from the substrate 2. This heating (to temperature of several hundreds of ° C., for example between 100° C. or 500° C. and 1200° C., for a time between several minutes and several hours, for example between 1 h or 4 h and 10 h or even 30 h) also makes it possible to reinforce the direct or molecular adhesion of the components 8, 10 on the second substrate 20, compared to the adhesion on the substrate 2.
Under the effect of temperature, differences between the thermal expansion coefficients of the materials of the substrates 2 and 20 and/or between the thermal expansion coefficients of the materials of the substrate or substrates 2 and/or 20 and the stamps 8, 10 may also promote a disbondment of the first substrate. This is in particular the case if the thermal expansion coefficient of the material of the first substrate is greater than that of the stamps 8, 10. This condition is met for LTO (just as for the other ferroelectric materials already envisaged for the substrate 2), which expands in general more than the stamps (mainly made of semi-conductor material). The latter thus do not move or barely move.
During the disbondment step, an increasing thermal ramp may be applied. In particular, under the effect of a thermal ramp greater than 5° C./min, an accumulation of charges in the material of the substrate 2 enables a discharge of the latter and thus enables or promotes the disbondment.
A disbondment of the substrate 2 is thus carried out by thermal effect or by combination of mechanical and thermal or electrical and thermal effects.
Another embodiment of the invention will be explained starting with a second substrate 20, identical or similar to that of
By the same transfer technique as that already described by means of the ferromagnetic substrate 2, the second stage of components 8″, 10″, etc. may be deposited on the first stage of components 8, 10. The bond between 8 and 8″, 10 and 10″ may be of the same nature as the previous bond between substrate 20 and components 8, 10, it may also be achieved by means of adhesive. This transfer of a second stage may be carried out by means of the same ferroelectric substrate 2 as the transfer of the first stage.
Once again, an appropriate marking made for example both on the substrate 2 and the substrate 20, makes it possible to achieve a good alignment and a good superposition of the components on top of each other. Marks 30, 30′ are identified in the substrates 2, 20 of
This alternative of the invention makes it possible to form stacks or stages of components on the second substrate 20.
In
What has been described for individual components or stamps applies to a single layer: as illustrated in
A second stage may be transferred onto the layer 18 of
This transfer of a second stage may be carried out by means of the same ferroelectric substrate 2 as the transfer of the first stage, and according to one of the methods described above.
Another embodiment implements a method of substrate fracture, such as the “Smart Cut™” method, for example described in the article of B. Aspar and A. J. Auberton—Hervé “Silicon Wafer Bonding Technology for VLSI and MEMS applications”, edited by S. S. Iyer and A. J. Auberton—Hervé, 2002, INSPEC, London, Chapter 3, pages 35-52, or instead in the documents already cited above.
An assembly of the implanted layer 18 is then carried out, by its face 18′ through which the implantation has been carried out, with the transfer substrate 20 (
For these operations, the techniques used are those already described above.
In an alternative (
A buried area 19′ is thus obtained constituting a fragilized area, which separates the layer 18 into two parts:
An assembly (
The structure of
An assembly of the layer 23′ is then carried out, by its free face 23′-1, with the transfer substrate 20: these are the steps described above with reference to
The assembly of stamps with a ferroelectric substrate 2 is of the type described above with reference to
The corresponding steps are similar to those described above for
After separation, a structure (
The implantation takes place this time before assembly with the ferroelectric substrate 2 (thus before the step represented in
The corresponding steps are similar to those described above for
After separation, a structure (
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08 52302 | Apr 2008 | FR | national |
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PCT/EP2009/054007 | 4/3/2009 | WO | 00 | 12/15/2010 |
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