Patent Application
20120015498
The present invention relates to the field of temporary substrates used in the semiconductor industry.
Semiconducting structures serve as the basis for the electronics industry. In order to improve performance, methods have been developed for increasing more and more the density of circuits per unit area. However, a physical limit is approached. It is for this reason that three-dimensional integration methods have appeared. Instead of always reducing the size of circuits, it is sufficient to stack them in 3D structures and to connect them by vertical interconnections.
The production of these types of structure requires the successive transfer of the layers of which they are made. These layers are in point of fact produced separately on specific substrates called “original substrates”. It is then necessary, in order to transfer them in the right direction onto their final substrate, to use a temporary substrate-also called a “sacrificial substrate”.
The steps of an example of such a transfer are shown in
Other applications exist for temporary substrate, like for instance, for mechanically supporting a semiconductor substrate while this substrate is processed (thinning operation, interconnection and/or vias formation, deposition of functional layers . . . ). Such temporary substrate and applications are described in US patent application US2004/222500.
One difficulty lies in the final detachment of the temporary substrate 100. A first technique called “laser lift off” requires the use of a transparent substrate and an adhesive material that is sensitive to laser excitation. Another technique, described in US patent application US2004/222500 mentioned above, proposes the use of a rough temporary support that may be detached by a final mechanical and/or chemical action.
These solutions give satisfaction but remain very slow to process: the larger the substrate, the longer the detachment time. Although for 100 mm diameter substrates the detachment time may be acceptable, this detachment time proves to be much too long for profitable industrial use for substrates having diameters of 200 mm, 300 mm or more.
European patent application EP1889887 discloses a temporary substrate with an adhesive layer incorporating thermally expandable microsphere. But the temporary substrate described in this document cannot be used for processing step that requires temperature greater than 140° C. Additionally, the nature of the adhesive layer and the uncontrolled disposition of the microsphere into the adhesive layer does not meet the requirements of providing a sufficiently high adhesion during processing and enabling the controlled detachment of the substrate after processing. This is particularly true if the processing steps are exerting high levels of stress on the attached assembly, as is the case with grinding or polishing operations.
Accordingly, improvements are needed over these prior art processes, and these are now provided by the present invention.
The present invention advantageously provides a temporary substrate facilitating final detachment, while eliminating the risk of premature detachment. This temporary substrate makes it possible to gain processing time by shortening the time necessary for detachment. This temporary substrate does not comprise adhesive material that would degrade if exposed to the required thermal treatment, and thus can sustain temperatures greater than 200° C.
In particular, the present invention relates, according to a first feature, to a temporary substrate for the processing of an original substrate, characterized in that it comprises a surface layer having a plurality of inserts therein, the inserts being made of a material having a coefficient of thermal expansion that is significantly different from that of the material constituting the rest of the surface layer.
By “processing” of an original substrate, it is meant in particular the formation of a thin layer from the original substrate, for example for transferring it to a final substrate.
The presence of these inserts with a different coefficient of thermal expansion means that, by heating the structure before detachment, cavities appear at the interface between the temporary substrate according to the invention and the original substrate. The area of attachment is thus reduced, which in turn brings about a considerable reduction in the overall adhesion and easy detachment of the thin layer.
In the temporary substrate according to the invention, a local debonding is thermally triggered that reduces the total surface energy, thus permitting a controlled detachment of the original substrate after processing. While the assembly is exposed to a temperature that is less than the trigger or full detachment temperature, the total surface energy can reach a value as high as 800 mJ/m2 or even 1000 mJ/m2, levels that are sufficient to withstand the most aggressive mechanical processing (e.g., back side grinding).
The invention also provides other advantageous non-limiting features:
According to a second feature, the invention relates to a method for processing an original substrate on a temporary substrate according to the first feature of the invention, comprising the steps of:
Other advantageous non-limiting features of the method includes:
According to a third feature, the invention relates to a method for producing a temporary substrate according to the first feature of the invention, comprising the steps of:
According to other advantageous non-limiting features:
Other features and advantages of the present invention will become apparent on reading the following description of a preferred embodiment. This description will be given with reference to the appended drawings in which:
In a preferred embodiment, the invention includes a temporary substrate having a bonding surface prepared for receiving an additional substrate that will transfer a thin layer. This temporary substrate comprises a principal part or support and a surface layer thereon with the surface layer having a plurality of inserts therein. These inserts are preferably made of a material having a coefficient of thermal expansion that is significantly different from that of the material constituting the surface layer.
Another preferred embodiment relates to a method for transferring a thin layer onto a temporary substrate. This method comprises preparing a bonding surface on a temporary substrate of the type described herein; bonding an original substrate to the temporary substrate at a bonding interface; removing part of the original substrate to provide a selected portion of the original substrate on the surface layer; heat treating the bonded substrates to form detachment zones but incomplete overall detachment at the bonding interface due at least in part to expansion of the inserts by the heat treating; and subsequently detaching remaining portions of the interface that are outside of the detachment zones to remove the selected portion of the original substrate from the temporary substrate.
This method advantageously includes strengthening the bonding of the substrates by applying a heat treatment at a temperature that is below that which produces the detachment zones. Also, the selected portion of the original substrate are generally attached to a final substrate prior to removing the selected portion from the temporary substrate.
Yet another preferred embodiment relates to a method for producing a temporary substrate. This method comprises forming a temporary substrate by depositing a first material on a principal part or support to provide a surface layer thereon; etching the surface layer to form cavities therein; depositing a layer of a second material into the cavities and onto the surface layer to form inserts of the second material deposited in the cavities; polishing the deposited layer of the second material until the surface layer is exposed; and depositing a thin layer of additional first material to cover the inserts. As disclosed herein, the second material has a coefficient of thermal expansion that is significantly different from that of the first material.
With reference to
The material or materials of the inserts 3 are different from the materials making up the rest of the surface layer 2 and are chosen so as to have significantly different coefficients of thermal expansion (typically, the material of the inserts 3 should have a greater coefficient of expansion than the material of the layer 2). The term “significantly different” means that the coefficient of thermal expansion of the inserts differs by at least 1.5, preferably by at least 2 and more preferably by at least 2.5 times that of the material of the surface layer.
Many couples of material may be envisaged, and in a particularly preferred manner a silicon oxide (especially tetraethoxysilane (TEOS) oxide or a silane) will be taken for the layer 2, and a metal such as copper for the inserts 3. Indeed, a material such as copper has a high thermal conductivity, a tendency to expand (its coefficient of linear expansion a, corresponding, assuming the material to be anisotropic, to the elongation factor of a part for an increase of 1° K, is 16.5×10−6, compared with the value 0.6×10−6 for silicon oxide) and good ductility. Copper is moreover easily electrodeposited. Metal compounds are generally preferably chosen for the inserts 3, but other types of materials may be envisaged such as Al2O3.
Advantageously, the surface layer 2 covers a principal part 1 or additional layer which ensures the rigidity of this substrate on account of its much greater thickness than that of the surface layer 2. Typically, the surface layer has a thickness of few thousands of Å to a few μm, while the support has a thickness of about 100 to 1000 microns. This principal part 1 may consist of all the materials normally used in substrates, notably based on silicon (Si, SiC, SiGe), glass, ceramic or a metal alloy. The choice of material could be made according to constitution of the layer 4 to be transferred, as known and selected by a person skilled in the art.
Preferred geometries for the arrangement of the inserts 3 in the surface layer 2 are described subsequently, it being possible for the inserts 3 to cover for example the entire surface of the temporary substrate 100.
A temporary substrate has the function of receiving, in a transient manner, an original substrate (comprising circuits for example in a thin upper layer 4) with a view to process it, for instance to transfer the thin upper layer 4 of the original substrate 5 to a final substrate.
The invention thus relates generally, according to a second feature, to a method of processing an original substrate 5 on a temporary substrate 100 such as previously described. The original substrate usually presents a thin upper layer 4 at its upper surface that may comprise micro components. The exposed surface of the upper layer 4 has been prepared for facilitating its attachment, as it will be described below. This preparation step may comprise the deposition and planarisation of a TEOS encapsulating layer.
As shown in
The attachment or bonding step may also preferably comprise a heat treatment to provide sufficient adhesion between the original substrate and the temporary substrate. The heat treatment can be between 50° C. to 400° C. from a 1 minute to 12 hours and preferably between 150° C. and 350° C. for 1 to 5 hours. For instance, in case of a surface layer 2 and an upper surface of the original substrate 4 in TEOS and copper inserts 3, the heat treatment can be of 250° C. for two hours, and this allow to reach a surface energy of about 600 mJ/m2.
The original substrate 5 is then processed. For instance, a part 50 of the original substrate thickness could be removed, by chemical or mechanical means (backside thinning, edge grinding, polishing. In addition or in replacement of the removal step other processing steps can be performed. It can be the formation of vias or contacts, deposition of functional layers on the exposed face of the original substrate 5.
In a particular embodiment of the invention, the structure obtained, shown in
A heat treatment is then performed for forming detachment zone between the upper face of the original substrate 5 and the temporary substrate. This heat treatment preferably takes the form of annealing with a temperature ramp that can reach several hundreds of degrees Celsius. For example, in the case of a layer 2 made of silicon oxide and copper inserts 3, the temperature to be reached during heat treatment will preferably lie between 350° C. and 420° C., preferably for at least two hours. This treatment brings about the expansion of the assembly, in particular the inserts 3 which will experience a substantial increase in thickness relative to that of the rest of the surface layer 2 due to the significantly different coefficient of thermal expansion of the inserts. The temperature of the thermal treatment is limited by the maximum temperature that can be applied to the original substrate, in particular if the original substrate comprises components. Typically, microcomponents cannot be exposed to temperature above 450° C. without risk of degradation. Also, the heat treatment must not be conducted under conditions that cause detachment of the thin layer. A suitable heat treatment includes a temperature of 300° C. to 425° C. for 2 to 4 hours.
Various types of interface between the substrate 100 and the layer 4 are present. At the level of zones A vertically above an insert 3, there is only a very small distance between this insert 3 and the interface. On the other hand, at the level of zones B, the substrate is only made of material of the surface layer 2 over all its thickness. At the level of the zones A, the thermal expansion of the inserts 3 pushes the substrate 1 and thus induces detachment at the level of the zones B with the creation of detachment 6 (cavities) zones that may be seen in
Then the temporary substrate 100 is withdrawn by a mechanical demounting action at the level of the weakened attachment interface so as to arrive at the final structure that may be seen in
In complement, or in substitution of the mechanical demounting action, an etching solution can be introduce at the level of the interface between the temporary substrate and the original substrate 5 (or its remaining part) to etch away part of the surface layer 2 and/or layer 7. The efficiency of the etching solution is facilitated by the presence of the voids thermally induced by the presence of inserts in the surface layer.
Advantageously, the inserts 3 are distributed in the surface layer 2 in a regular pattern, in particular in a checkered pattern, with the inserts 3 having a square section. The invention is not however in any way limited to this geometry and may take many other forms such as a triangular layout.
In the case of a checkered pattern, it is particularly preferred to separate inserts two-by-two by a distance equivalent to their width, this distance being normally between 250 and 500 μm. The advantages of such geometry will become clearly apparent in
In such a configuration, considering that the coefficient of expansion of silicon oxide is 0.6×10−6 and that of copper is 16.5×10−6, that the latter is sufficiently ductile so that all expansion occurs in direction of the attachment interface, and that the thickness of the inserts is 1 μm, detachment of a layer having a thickness of greater than 100 Å is obtained, for annealing at 400° C., this dimension corresponding to the height of the cavities 6. In an alternative embodiment, it is possible to perform the detachment heat treatment before the processing of the original substrate. As explained above, this treatment will decrease the adhesion of the original substrate to the temporary substrate, but this decreased adhesion could in certain instances be sufficient to withstand the processing of the temporary substrate.
The invention finally relates, according to a third feature, to a method for producing a temporary substrate 100 such as previously described.
The production method, starting from the main bare part 1, commences by a step of depositing the surface layer 2, which may advantageously be carried out by PECVD (plasma-enhanced chemical vapour deposition) if the material is a TEOS oxide or a silane. PECVD is a known method for depositing a thin layer on a substrate from a gaseous state and makes it possible to obtain small thicknesses equal to or even less than a micron that are necessary for the invention. The temporary substrate 100 being produced is then in the state shown in
The surface layer 2 is then etched to form cavities 10 that will shelter the inserts 3. Photolithography may be used for this purpose. A photosensitive resin 9 that may be seen in
Various etching techniques whether by a dry method (plasma) or a wet method (chemical attack, for example by hydrofluoric acid) are known to a person skilled in the art. Once the cavities 10 have been etched (
The cavities 10 are then filled with the material constituting the inserts 3. If this is copper, it is quite simply electrodeposited by electrolysis on the surface to slightly overfill the cavities 10 (
The excess material of the inserts 3 is then removed by mechano-chemical polishing until the material of the surface layer 2 is revealed. All that remains is to cover the inserts 3 at present in place, as may be seen in
| Number | Date | Country | Kind |
|---|---|---|---|
| FR 1055767 | Jul 2010 | FR | national |
This application is a continuation-in-part of application Ser. No. 12/897,409 filed Oct. 4, 2010, the entire content of which is expressly incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12897409 | Oct 2010 | US |
| Child | 13174363 | US |
