WO2012126752A1

WO2012126752A1 - Apparatus and a method for direct wafer bonding, minimizing local deformation

Info

Publication number: WO2012126752A1
Application number: PCT/EP2012/054205
Authority: WO
Inventors: Gweltaz Gaudin; Alexandre BARTHELEMY
Original assignee: Soitec
Priority date: 2011-03-18
Filing date: 2012-03-12
Publication date: 2012-09-27
Also published as: TW201243979A; FR2972848A1

Abstract

Apparatus is described for direct wafer bonding (200) together at least first and second wafers of circular shape, said second wafer having an initial bow. The apparatus includes annular support means (211) for receiving at least the second wafer. The annular suppor means (211) define a central recess (2111) in which the second wafer is free to deform under its own weight.

Description

APPARATUS AND A METHOD FOR DIRECT WAFER BONDING, MINIMIZING LOCAL DEFORMATION

Technical field and prior art

The present invention relates to direct wafer bonding carried out between two wafers used, for example, to produce multilayer semiconductor wafers e.g. for 3D integration technology that requires the transfer of one or more layers of microcomponents onto a final support substrate, but also for circuit transfer or in the fabrication of back-lit imaging devices. The transferred layer or layers include microcomponents (electronic, optoelectronic, etc) produced at least in part on the initial substrate, said layers then being stacked onto a final substrate that may itself include components.

Primarily because of the very small size and large number of microcomponents present on the same layer, each transferred layer must be positioned on the final

substrate with great accuracy for successful, very strict alignment with the subjacent layer. Further, it may be necessary to carry out treatments on the layer after it has been transferred, for example in order to form other microcomponents, to expose the microcomponents on the surface, to produce interconnections, etc.

However, the Applicant has observed that after transfer, it is sometimes very difficult if not

impossible to form additional microcomponents that are aligned with the microcomponents that were formed before the transfer because of the appearance of non-uniform deformation in the wafers after bonding.

In particular with. 3D integration, the non-uniform deformation resulting from direct wafer bonding then lead to a phenomenon of misalignment of the microcomponents of the various layers. That phenomenon of misalignment, also known as "overlay", and described below with

reference to Figure 1, appears in the form of defects, for example of the order of 20 nm [nanometer] , which are substantially smaller than the accuracy with which the wafers are aligned at the time of direct bonding.

Figure 1 illustrates a three-dimensional structure 500 obtained by low pressure direct wafer bonding between a first wafer or initial substrate 510, on which a first series of microcomponents 511 to 519 is formed by

photolithography, which consists mainly in irradiating a substrate that has been rendered photosensitive (for example by applying a photoresin to the substrate) at predetermined zones corresponding to the positions where the microcomponents are to be formed, and a second wafer or final substrate 520. The initial substrate 510 has been thinned after bondinj in order to remove a portion of the material present above the layer of

microcomponents 511 to 519 and a second layer of

microcomponents 521 to 529 has been formed at the exposed surface of the initial substrate 510.

The substrate is typically irradiated with selective irradiation equipment, commonly known as a stepper, that acts during an irradiation operation, and in contrast to general irradiation equipment, to irradiate only a portion or "field" of the substrate through a mask made up of opaque and transparent zones that can be used to define the motif that is to be reproduced on the

substrate. One "field" covers a set of individual components (chips) and thus it is not possible (nor is it desirable for productivity reasons) to optimize and compensate for the alignment defects of each component.

The stepper is generally able to compensate for certain types of alignment defects such as those of the offset (or shift) type or of the rotation and radial type (also known as run-out, corresponding to radial

deformation that increases linearly with the radius of the substrate) by using a compensating algorithm.

However, even when such positioning tools are used, offsets occur between some of the microcomponents 511 to 519 relative to some of the components 521 to 529, such as the offsets Δ_1Χ, Δ₂₂, Δ₃₃, Δ₄₄ indicated in Figure 1 (respectively corresponding} to the offsets observed between the microcomponent pairs 511/521, 512/522,

513/523 and 514/524) .

Said offsets do not result from elementary

transformations ( ranslation, rotation, or combinations thereof) that could originate from inaccurate assembly of the substrates. Said offsets result from non-uniform deformation that causes local, non-uniform movements at certain microcomponents 511 to 519. In addition, some of the microcomponents 521 to 529 formed on the exposed surface of the substrate after transfer exhibit

variations in position relative to said microcomponents 511 to 519 that may be of the order of a few hundred nanometers, or even a micrometer.

That phenomenon of misalignment (also known as overlay) between the two layers of microcomponents may give rise to short-circuits, to distortions in the stack, or to connection faults between the microcomponents of the two layers. Thus, when the transferred

microcomponents are imaging devices made up of pixels, and when the post- transfer treatment steps are intended to form color filters on each of those pixels, a loss of the colorizing function has been observed on some of those pixels.

That phenomenon of overlay thus results in a

reduction in the quality and value of manufactured multilayer semiconductor wafers. The impact of that phenomenon is becoming more and more critical because of the constant demand for increasing the miniaturization of microcomponents and for increasing their density of integration per layer.

As illustrated in Figure 2A, for direct wafer bonding, bonding equipment 100 is used comprising a substrate carrier device or wafer carrier 120 with a support platen 121 (also termed a "chuck") on which there rests a first planar wafer 60 to be bonded with a second wafer 70 that has an initial curvature, also termed

"bow", due primarily to microcomponents being formed on it. The second wafer 70 is placed on the first wafer 60 for direct wafer bonding.

Once placed on the first wafer 60 held on the chuck

121, for example by means of an electrostatic system or by suction, the initial curvature of the second wafer 70 is modified because it rests on the first wafer solely via a limited region located substantially at its center. More precisely, the forces applied to the first wafer are not uniform depending on whether or not portions thereof are in contact with the first wafer. In fact, it the region of the second wafer 70 that is in contact with the first wafer 60, the gravitational force of attraction F_g exerted on the second wafer is compensated for by the reaction force R exerted by the first wafer (F_g + R = 0) . In contrast, the force of attraction exerted over the whole portion of the second wafer 70 that is not in contact with the first wafer 60 is not compensated for and so that portion of the second wafer deforms under the effect of its own weight, the deformation force exerted on the wafer being higher in the vicinity of its side (cantilever effect) . In other words, the deformation of the second wafer at the center of the wafer is not identical to that at its side.

The interaction between the bow of the second wafer 70 and the gravity exerted thereon contributes, inter alia, to the appearance of the overlay type misalignment phenomenon .

Figure 3 represents a model of the bow of the second wafer 70 and of the structure resulting after bonding the second wafer 70 to the first wafer 60, i.e. after

initiating propagation of a bonding wave between said two wafers, while taking or not taking into account the force of attraction exerted on the second wafer, as explained above . The curve A shows the bow exhibited by the second wafer 70 in its position before bonding as shown in

Figure 2A. It should be observed that under the effect of gravity, the amplitude of the bow of the first wafer is reduced to 18 μτη [micrometer] (sagging of side of wafer) .

Curve B shows the amplitude and shape of the bow exhibited by the structure resulting from bonding between the wafers 60 and 70, taking into account the effect of gravity exerted on the second wafer 70 as in Figure 2A. It can be seen that the shape of the structure is not quadratic, i.e. does not form a hyperbola and the

amplitude of the bow is only 5 μπι.

In contrast, if the effect of gravity is removed from the modeling computations, curve C is obtained, which shows that the structure resulting after bonding does indeed have a quadratic bow, here specifically the shape of a hyperbola, and a larger amplitude of the order of 19 μπ

The non- linear deformation exhibited by the

resulting structure having the bow that is illustrated by curve B originates from the lack of uniformity of the attraction and reaction forces exerted on the second wafer. However, in order to obtain a reliable correction in the overlay, the deformation of the wafers must be linear or pseudo- linear relative to the radius of the wafer. As indicated in Figure 4, when the curvature varies with the radius of the wafers, a significant positional error still exists at the side of the wafer despite the correction supplied by the stepper. The stepper is incapable of correcting the type of overlay induced by the absence of uniformity between the forces of attraction and reaction exerted on the second wafer before bonding.

Summary of the invention

The aim of the invention is to provide a solution that means that two wafers can be supported and bonded together by direct bonding, eliminating the lack of uniformity of the attraction and reaction forces on at least one of the two wafers, and thus minimizing the phenomenon of overlay induced in the resulting structure.

To this end, the present invention proposes an apparatus for bonding together at least first and second wafers of circular shape, the second wafer having an initial bow, the apparatus comprising annular support means for receiving at least the second wafer, the annular support means defining a central recess in which the second wafer is free to deform under the effect of its own weight at the moment of initiating propagation of a bonding wave between the first and second wafers.

Thus, by allowing the wafer having an initial bow to deform freely under its own weight, the bonding apparatus of the invention can be used to give said wafer a

constant bow immediately before bonding, i.e. at the moment of initiating propagation of the bonding wave between the two wafers. The wafer that deforms freely has no sagging at its side as it would in the presence of a non-uniform reaction force when using a prior art chuck as described above. The deformation or deformations of the wafer as well as the structure resulting from bonding are thus linear along the radius of the wafers. Any offsets or alignment defects may then by corrected by the positioning tools.

In particular with 3D integration, this greatly reduces the risks of misalignment or overlay at the side of the wafer during the subsequent formation of

additional layers of microcomponents or when bonding together two wafers, each including microcomponents that are intended to be in alignment.

In one embodiment of the invention, the bonding apparatus includes an annular support for receiving at least the second wafer. In another embodiment, the bonding apparatus

includes at least three support elements for receiving at least the second wafer, the support elements being uniformly distributed over an annular zone.

In one embodiment of the invention, the bonding apparatus further includes a chuck for holding the first wafer, the chuck being placed below the annular support means. The apparatus may then include means for moving the annular support means and the chuck vertically relative to each other.

In one aspect of the invention, the bonding

apparatus includes a means for mechanically applying a point of pressure to one of the two wafers .

In another aspect of the invention, the apparatus includes means for reducing the pressure between the two wafers .

The invention also provides a method of direct wafer bonding between at least a first wafer and a second wafer of circular shape, the second wafer having an initial bow, the method comprising the following steps:

• holding the second wafer under gravity in the vicinity of its annular side at a predetermined height such that it can deform freely under the effect of its own weight;

· bringing said second wafer into contact with the first wafer; and

• initiating propagation of a bonding wave between the two wafers while the second wafer is deformed under the effect of its own weight .

In one implementation of the invention, the first wafer is held on a chuck while the second wafer is placed on annular support means that may in particular be constituted by an annular support or holding pins with a central recess, said annular support means being held at a predetermined height from the chuck before bringing the wafers into contact so as to allow the second wafer to deform freely under its own weight in said central recess .

In another implementation of the invention, the first and second wafers are placed on annular support means including a central recess, said annular support means including a base having a predetermined height so as to allow the first and second wafers to deform freely under their own weight in said central recess.

In one aspect of the invention, a point of pressure is applied mechanically to one of the two wafers to initiate propagation of a bonding wave between the two wafers .

In another aspect of the invention, the pressure between the two wafers is reduced to initiate propagation of a bonding wave between the two wafers.

Brief description of the figures

Further characteristics and advantages of the invention become apparent from the following description of particular embodiments of the invention, given by way of non- limiting example and made with reference to the accompanying drawings in which:

• Figure 1 is a diagrammatic view showing a three- dimensional structure after direct wafer bonding in accordance with the prior art;

• Figures 2A and 2B are section views of prior art bonding apparatus;

• Figure 3 shows the modeling of the bow of a structure obtained, by bonding two wafers with and without the effect of gravity;

• Figure 4 shows an example of a measurement of the overlay when two bonded wafers have a non- linear

variation in bow;

• Figure 5 is a perspective view of bonding

apparatus in accordance with one embodiment of the invention; • Figures 6A to 6F are diagrammatic views of a method of direct wafer bonding carried out with the apparatus of Figure 5 in accordance with one

implementation of the invention;

· Figure 7 is a flow chart of the steps in a method of direct wafer bonding of the invention illustrated in Figures 6A to 6E;

• Figures 8A to 8C are diagrammatic views of a method of direct wafer bonding in accordance with another implementation of the invention;

• Figure 9 is a flow chart of the steps in a method of direct wafer bonding of the invention illustrated in Figures 8A to 8C; and

• Figure 10 is a perspective view of bonding

apparatus in accordance with a variant embodiment of the invention .

Detailed description of embodiments of the invention

The present invention is generally applicable to the production of composite structures comprising at least direct wafer bonding of a first substrate or wafer onto a second substrate or wafer.

Direct wafer bonding is a technique that is well known per se . It should be recalled that the principle of direct wafer bonding is based on bringing two surfaces into direct contact, i.e. without using a specific material (adhesive, wax, solder, etc) . Such an operation requires the surfaces that are to be bonded together to be sufficiently smooth, and free of particles or

contamination, and requires them to be brought

sufficiently close together to allow contact to be initiated, typically to a distance of less than a few nanometers apart. When this occurs, the attractive forces between the two surfaces are high enough to cause direct bonding (bonding induced by the set of attractive forces (Van der Waals forces) of electronic interaction between atoms or molecules of the two surfaces to be bonded) .

Direct bonding is carried out by initiating at least one contact point on a wafer in intimate contact with another wafer in order to trigger propagation of a bonding wave from that point of contact. The term

"bonding wave" is applied here to the bonding or direct bonding front that propagates from the initiation point and that corresponds to diffusion of the attractive forces (Van der Waals forces) from the point of contact over the whole intimate contact surface between the two wafers (bonding interface) . The point of contact may typically be initiated by application of mechanical pressure on the exposed surface of one of the two wafers.

As indicated above, bonding is carried out by placing a first wafer on a chuck of a wafer carrier device and placing a second wafer on that first wafer. However, because of the bow exhibited by the second wafer, only a portion of the wafer, generally located at the center thereof, rests on the first wafer. If

gravity, or more precisely the attractive gravitational force, is exerted in the same manner over the whole of the second wafer, it is compensated by the reaction force of the chuck only in the portion of the second wafer that is resting on the first wafer. As a consequence, the gravity that is exerted on the portion of the second wafer that does not rest on the first wafer is not compensated, and that portion is subjected to an

attractive force that results in local deformation in that portion of the wafer, modifying the overall bow of the wafer in a non- linear manner. In other words, the second wafer is deformed between its center and its periphery, but not in a uniform manner.

In order to overcome this disadvantage, the present invention proposes bonding apparatus and an associated bonding method in which at least the whole of the second wafer intended to be placed on the first wafer is capable of deforming freely under its own weight at the moment propagation of a bonding wave is initiated. To this end, the present invention proposes using support means that can support the second wafer only in the vicinity of its side, to allow it to deform under the effect of its own weight and to maintain it in free deformation under its own weight at the moment at which propagation of a bonding wave between the first and second wafers is initiated .

Figure 5 represents a bonding apparatus 200 in accordance with a first embodiment of the invention. The bonding apparatus 200 comprises a first wafer carrier device 210 provided with annular wafer support means formed by an annular support 211. In the example

described here, the annular support 211 has in its upper portion an annular contact surface 2110 intended to support a wafer of circular shape by its portion located in the vicinity of its side. The inside diameter Di_nt of the annular support 211 from which the contact surface 2110 extends is less than the diameter of the wafer intended to be placed on it. By way of example, for wafers with a diameter of 200 mm [millimeter] , 300 mm or more (for example 450 mm) , the inside diameter Di_nt of the annular support 211 is selected such that the side of the supported wafer overlaps an annular zone on the contact surface 2110 by a width in the range 1 mm to 50 mm, preferably in the range 2 mm to 10 mm, and more

preferably 3 mm.

The outside diameter D_ext of the annular support may optionally be larger than that of the wafer. In the example described here, the annular support 211 comprises an annular wall 2112 that extends from the outside diameter D_ext above the contact surface 2110.

The dimensions of the annular support of the

invention are adapted as a function of the diameter of the wafers to be bonded, which may in particular have diameters of 100 mm, 150 mm, 200 mm, 300 mm, and 450 mm. The annular support has a central recess 2111 in which the wafer can deform under its own weight. In the example described here, the annular element 211 has a height H₂n that is less than the distance over which the wafer deforms in the recess in order subsequently to be able to hold it in its position of free deformation under its own weight when making contact with another wafer with a view to direct wafer bonding as described below.

In order initially to allow the wafer to deform freely under its own weight, the annular support 211 is mounted on vertical movement means, here pistons 212, which mean that the annular support can be moved away from or closer to a chuck 220 intended to hold the other wafer with a view to performing direct wafer bonding.

Further, in order to allow the annular support 211 to be retracted, both to allow a first wafer to be placed on the chuck 220 and also to allow the bonding interface between the two wafers that are to be bonded together to be closed as described below, the annular support 211 in this example is formed by four independent sectors 2114 to 2117, each integral with a respective piston 212.

Each piston 212 is mounted inside the chuck 220 on a linear actuator (not shown in Figure 5) that can move the sectors 2114 to 2117 away from one another in the

directions indicated by the arrows in Figure 5.

With reference to Figures 6A to 6F and 7, there follows a description of an example of a method of direct wafer bonding between first and second wafers 20 and 30 carried out with the bonding apparatus of Figure 5 in accordance with one implementation of a bonding method in accordance with the invention. In known manner, the surfaces 21 and 31 respectively of wafers 20 and 30 for bonding have been prepared (by polishing, cleaning, hydrophobic/hydrophilic treatment, etc) to allow direct bonding.

The bonding apparatus 200, and more precisely the wafer carrier device 210 and the chuck 220, are placed in a sealed chamber (not shown in Figures 6A to 6E) in which the pressure and the temperature can be controlled.

In Figure 6A, the sectors 2114 to 2117 of the annular support 211 are moved away from one another to allow a first wafer or substrate 20 of planar shape to be placed on the chuck 220 (step SI) . The chuck 220 has flatness defects that are preferably less than

15 micrometers. The chuck 220 holds the first wafer 20, for example using an electrostatic system or suction system associated with the chuck or simply under gravity, with a view to assembling it with the second wafer 30 by direct bonding. The associated systems for holding the wafer (electrostatic or by suction) are used providing it has been checked that they do not deform the wafer, so as to avoid increasing any problems with overlay.

Once the first wafer 20 is in position on the chuck 220, the sectors 2114 to 2117 are moved together into the position illustrated in Figure 5. The second wafer 30 that has an initial bow is placed on the annular support 211 of the wafer carrier device 210 of the bonding apparatus 200 that is positioned on the pistons 212 (step S2 in Figure 6B) . In this step of the method, the annular support 2,11 is held by the pistons 212 at a height H_def from the chuck 220 so as to allow the second wafer 30 to deform freely under its own weight in the central recess 2111 without coming into contact with the first wafer 20 (step S3, Figure 6C) .

The second wafer 30 is simply placed on the contact surface 2110 of the annular support 211 without using an associated holding system such as an electrostatic or suction system so as to allow the wafer to deform freely without being held by its contact portion with the contact surface 2110 of the annular support 211. The shape of the inner side 2113 of the annular support 211 may be adapted, for example chamfered, in order to avoid any damage to the wafer during its deformation. The pistons 212 are then actuated in order to lower the annular support 211 to a height H_cont from the chuck 220, which means that a portion of the lower face 31 of the second wafer 30 can be placed in contact with the upper face 21 of the first wafer 20 (step S , Figure 6D) . The bonding apparatus 200 includes measurement means (not shown in Figure 6D) , for example optical means, that can adjust the height H_cont such that the portion of the second wafer 30 closest to the first wafer 20 is positioned closest to the face 21 of the wafer 20 without the wafer 30 resting completely on the wafer 20. In other words, the height H_cont must be adjusted to allow initiation of a bonding wave between the portions of the two wafers in contact while the first wafer is still in its free deformation position under the effect of its own weight.

After the step of bringing the wafers into contact, direct wafer bonding is carried out (Figure 6E, step S5) . As illustrated in Figure 6E, propagation of a bonding wave may be initiated by means of a tool 50 equipped with a stylus 51 that can mechanically apply a point of contact to the wafer 30. Advantageously, but not

necessarily, the mechanical pressure exerted by the stylus 51 on the wafer 30 may be controlled in order to limit deformation at the contact point. As illustrated in Figure 6E in a highly diagrammatic manner, the tool 50 may comprise a dynamometer 53. The stylus 51 is

connected to the dynamometer 53 and has a free end 52 with which mechanical pressure is exerted on the wafer 30 in order to initiate a point of contact between the two wafers 20 and 30. By knowing the value of the contact surface area 52a of the tool 50 with the wafer 30, it is possible to apply a mechanical pressure in the range 1 MPa [megapascal] to 33.3 MPa by controlling the load F exerted by the tool on the wafer (load = mechanical pressure x area of load) . By limiting the pressure applied to one of the two substrates during initiation of a point of contact in this manner, the amount of non- uniform deformation caused in the wafer is reduced while carrying out direct wafer bonding over the whole surface of the two wafers in contact . The load exerted by the end 52 on the wafer 30 is controlled by means of the dynamometer 53.

The loading element, and more particularly its end intended to come into contact with the wafer, may be produced from or covered in a material such as Teflon^®, silicone, or a polymer. In general, the end of the load element is produced from or covered in a material that is sufficiently rigid to be able to apply pressure in a controlled manner. Too flexible a material could deform and produce an inaccurate contact surface area and as a result a loss of accuracy in the applied pressure.

Further, too rigid a material could result in the

formation of defects (impressions) on the wafer surface.

Propagation of the bonding wave may also be

initiated spontaneously between the wafers 20 and 30 by reducing the pressure in the chamber of the bonding apparatus to a very low value, typically less than about 10 mbar [millibar] .

Once propagation of a bonding wave has been

initiated, and preferably as late as possible when the bonding wave is temporarily stopped by the side support for the second wafer 30 on the annular support 211, the second wafer 30 is released from the annular support 211 by moving the sectors 2114 to 2117 apart from each other in order to completely close the bonding interface between the wafers 20 and 30 (step S6, Figure 6F) .

Figure 8A illustrates a variation of a bonding apparatus of the invention. The bonding apparatus 300 differs from the bonding apparatus 200 described above in that it comprises a fixed annular support on which the two wafers to be bonded are placed. More precisely, the bonding apparatus 300 in this example comprises a wafer carrier device 31.0 provided with an annular support 311 resting on a base 320 that, like the support 211 described above, has an annular contact surface 3110 in its upper portion for supporting a wafer of circular shape via its region located in the vicinity of its side. The inside diameter Di_nt of the annular support 311 from which the contact surface 3110 extends is smaller than the diameter of the wafer intended to be placed over it . The outside diameter D_ext of the annular support may optionally be greater than that of the wafer. In the example described here, the annular support 311 comprises an annular wall 3112 that extends from the outside diameter D_ext above the contact surface 3110.

The lower portion of the annular support 311 is extended by an annular base 3114 that is used to define a central recess 3111 of height H₃₁₁ that is greater than the distance over which the wafer or wafers deform. In other words, the contact surface 3110 of the annular support 311 is held at a sufficient distance from the base 320 to allow the wafer or wafers to deform freely in the central recess 3111 without coming into contact with the base 320. The shape of the side 3113 of the annular support 311 on the side of the inside diameter Di_nt may be adapted, for example chamfered, in order to prevent possible damage to the wafer during deformation thereof.

The bonding apparatus 300, and more precisely the wafer carrier device 310 comprising the annular support 311, is placed in a sealed chamber (not shown in

Figures 8A to 8C) in which the pressure and temperature can be controlled.

An example of the method of direct wafer bonding between a first and a second wafer 80 and 90 carried out with the bonding apparatus 300 is described below with reference to Figures 8A to 8C, and Figure 9 that

describes an implementation of a bonding method of the invention .

In Figure 8A, the second wafer 90 is placed on the first wafer 80 with its side resting on the contact surface 3110 of the annular support 311 by gravity alone, i.e. without using a holding system such as suction or an associated electrostatic system (step S10) .

Once the second wafer 90 has been placed on the first wafer 80, the two wafers deform freely in the central recess 3111 under their own weight (step S20, Figure 8B) .

After the step of bringing the faces 81 and 91 of the respective wafers 80 and 90 into contact, direct wafer bonding is carried out (Figure 8C, step S30) by initiating propagation of a bonding wave between the faces 81 and 91 that may be carried out using a tool 150 equipped with a stylus 151 that can be used to

mechanically apply a point of contact to the wafer 90 in the same manner as described above with the stylus 50.

Propagation of the bonding wave may also be

initiated spontaneously between the wafers 80 and 90 by reducing the pressure in the chamber of the bonding apparatus to a very low value, typically less than about 10 mbar .

Figure 10 shows another variant embodiment of the bonding apparatus of the invention that differs from the bonding apparatus of Figure 5 in that the bonding

apparatus 400 comprises annular support means constituted by a plurality of independent holding pins 411 to 414 disposed uniformly over an annular zone Z_A having an inside diameter D_int that is less than the diameter of the wafer or wafers intended to be placed on the pins 411 to 414. Each pin 411 to 414 includes a respective contact surface 411a to 414a at its free end for supporting a wafer of circular shape in its region located in the vicinity of its side. The; bonding apparatus 400

comprises a chuck 420 for receiving a wafer to be bonded with the wafer held on the pins 411 to 414.

The pins 411 to 414 define a central recess 4111 in which the wafer they support can deform under its own weight. In order to allow the wafer initially to deform freely under its own weight, the holding pins 411 to 414 are mounted on vertical movement means (not shown in Figure 10) that can move the wafer supported by the pins towards or away from the chuck 420 for holding the other wafer with a view to direct wafer bonding.

Further, in order to allow the pins 411 to 414 to be retracted both to allow a first wafer to be placed on the chuck 420 and also to allow the bonding interface between the two wafers that are to be bonded together to be closed after initiating propagation of a bonding wave as described above with reference to Figures 6E and 6F, each of the pins 411 to 414 is mounted inside the chuck 420 on a respective linear actuator (not shown in Figure 10) allowing the pins to be separated from one another in the directions indicated by the arrows in Figure 10.

When supporting two wafers on each other to allow them to deform before bonding, as described above with reference to Figures 8A to 8C, the holding pins are maintained at a height sufficient to allow the wafers to deform in the recess 4111 without coming into contact with the chuck 420 or a base on which the pins are fixed.

The number of holding pins lies in the range 3 to 50, and is typically 10.

The bonding method of the invention is applicable to assembling any type of material that is compatible with direct bonding, in particular semiconductor materials such as silicon, germanium, glass, quartz, sapphire, etc. The wafers to be assembled may in particular have a diameter of 100 mm, 150 mm, 200 mm, 300 mm, or 450 mm. The wafers may also include microcomponents over the majority of their surface or in only a limited zone.

One particular, but non-exclusive, field for the bonding method of the present invention is that of producing three-dimensional structures by forming a first series of microcomponents on the surface of a wafer or initial substrate, the microcomponents possibly being whole components and/or only portions of components, and the initial substrate possibly being a monolayer structure, for example a layer of silicon, or a

multilayer structure such as a SOI type structure. The microcomponents are formed by photolithography by means of a mask that can be used to define zones for the forming motifs corresponding to the microcomponents to be produced .

After bonding, i.e. after propagation of a bonding wave between the two wafers, a second layer of

microcomponents is formed at the exposed surface of the initial substrate that might have been thinned. The microcomponents of the second layer may correspond to complementary portions of microcomponents of the first layer in order to form a finished component and/or to distinct components intended to function with the

microcomponents of the first layer. In order to form the microcomponents of the second layer in alignment with the buried microcomponents of the first layer, use is made of a photolithography mask similar to that used to form the microcomponents .

In a variation, the three-dimensional structure is formed by a stack of layers, each layer being transferred by the assembly method of the present invention, and each layer being in alignment with the directly adjacent layers. In yet another variation, the final substrate itself also includes microcomponents.

Because of the method of direct wafer bonding of the invention, it is possible to bond the initial substrate to the final substrate without non- linear deformation or at least with a reduction in such deformation so that major misalignment at the side of the wafer before and after transfer of the initial substrate onto the final substrate is no longer observed. The microcomponents of a second layer, even those of very small dimensions (for example < 1 μπι) may thus be formed easily in alignment with the microcomponents of a first layer, even after transfer of the initial substrate. This can, for example, be used to interconnect the microcomponents present in two layers, or on two distinct faces of the same layer, via metal connections, thereby minimizing th risks of poor interconnections.

As a result, the bonding method of the present invention can be employed to limit phenomena of nonuniform deformation of wafers during direct bonding thereof. Finally, when both of the wafers include microcomponents , the method can limit the phenomenon of overlay during transfer of a circuit layer onto another layer or onto a support substrate, and can produce very high quality multilayer semiconductor wafers.

Claims

1. Apparatus (200) for direct wafer bonding together at least first and second wafers (20, 30) of circular shape, said second wafer having an initial bow, the apparatus being characterized in that it comprises annular support means (211; 311; 411-414) for receiving at least the second wafer (30) , said annular support means defining a central recess (2111; 3111; 4111) in which the second wafer is free to deform under the effect of its own weight at the moment of initiating propagation of a bonding wave between the first and second wafers.

2. Apparatus according to claim 1, characterized in that it includes an annular support (211) for receiving at least the second wafer (30) .

3. Apparatus according to claim 1, characterized in that it includes at least three support elements (411-413) for receiving at least the second wafer (30) , said support elements being uniformly distributed over an annular zone .

4. Apparatus according to any one of claims 1 to 3 , characterized in that it further includes a chuck (220; 420) for holding the first wafer (20), said chuck being placed below the annular support means .

5. Apparatus according to claim 4, characterized in that it includes means (212) for moving the annular support means and the chuck vertically relative to each other.

6. Apparatus according to any one of claims 1 to 5 , characterized in that it includes means (50) for

mechanically applying a point of pressure to one of the two wafers .

7. Apparatus according to any one of claims 1 to 5 , characterized in that it includes means for reducing the pressure between the two wafers .

8. A method of direct wafer bonding between at least a first wafer and a second wafer (20, 30) of circular shape, said second wafer (30) having an initial bow, said method comprising the following steps:

• holding the second wafer (30) under gravity in the vicinity of its annular side at a predetermined height such that it can deform freely under the effect of its own weight;

• bringing said second wafer (30) into contact with the first wafer (20) ; and

9. A method according to claim 8, characterized in that the first wafer (20) is held on a chuck (220; 420) and in that the second wafer (30) is placed on annular support means (211; 411-414) having a central recess (2111;

4111) , said annular support means being held at a

predetermined height from the chuck (220; 320; 420) before bringing the wafers into contact so as to allow the second wafer to deform freely under its own weight in said central recess.

10. A method according to claim 8, characterized in that the first and second wafers (20, 30) are placed on annular support means (311) including a central recess (3111), said annular support means including a base (3114) having a predetermined height so as to allow said first and second wafers (20, 30) to deform freely under their own weight in said central recess (3111) .

11. A method according to any one of claims 8 to 10, characterized in that a point of pressure is applied mechanically to one (30) of the two wafers (20, 30) to initiate propagation of a bonding wave between the two wafers .

12. A method according to any one of claims 8 to 10, characterized in that the pressure between the two wafers is reduced to initiate propagation of a bonding wave between the two wafers.