WO2012113799A1

WO2012113799A1 - Apparatus and method for direct wafer bonding

Info

Publication number: WO2012113799A1
Application number: PCT/EP2012/052950
Authority: WO
Inventors: Marcel Broekaart; Arnaud Castex
Original assignee: Soitec SA
Current assignee: Soitec SA
Priority date: 2011-02-24
Filing date: 2012-02-21
Publication date: 2012-08-30
Anticipated expiration: 2013-08-24
Also published as: FR2972078A1; TW201241955A

Abstract

An apparatus (200) for direct wafer bonding between two wafers (20, 30) comprises at least one wafer carrier device (210) comprising a support (211) constituted a support element (2110) for receiving one of the two wafers, and aligning elements (220, 230, 240) placed around said chuck. The support element (2110) of the chuck (211) has an overall contact surface area that is smaller than the surface area of the wafer (20) to be supported by the support element (2110).

Description

APPARATUS AND A METHOD FOR DIRECT WAFER BONDING

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. The Applicant has observed that the appearance of at least some of said non-uniform deformation is linked to the application of non-uniform loads to the wafers during manipulation thereof prior to bonding.

In particular with 3D integration, the non-uniform deformation resulting from low pressure direct wafer bonding then leads to a phenomenon of misalignment of the microcomponents of the various layers . That phenomenon of misalignment, also known as "overlay", described below with reference to Figure 1, is exhibited in the form of defects of the order of 50 run [nanometer] , which are substantially smaller than the precision in the alignment of the wafers 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 using a mask to define zones for the formation of patterns corresponding to the

microcomponents to be formed, and a second wafer or final substrate 520. The initial substrate 510 has been thinned after bonding 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.

However, even when 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 An , 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 (translation, 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.

For direct wafer bonding, bonding apparatus is used comprising a substrate carrier device or wafer carrier with a support platen (also termed a "chuck") on which there rests a first of the two wafers that are to be bonded together, the second wafer being placed on the first wafer. Before initiating propagation of a bonding wave between the two wafers, one or more operations of aligning the wafers is/are carried out, consisting in pushing the two wafers against abutment elements using a pusher. The contacts between the wafers and said aligning elements directly or indirectly result in mechanical stresses in the wafers that, if they are not relaxed, result in non-uniform deformation in the wafers compared with the same wafers that have not been

subjected to external contacts and mechanical forces. As a result, it is important to allow such stresses in the wafers to relax before proceeding to direct wafer bonding in order to avoid the appearance of the non-uniform deformation described above . Summary of the invention

The aim of the invention is to provide a solution that can relax the stresses arising in the wafers while they are being aligned and then to carry out direct wafer bonding with wafers that have little non-uniform

deformation and to thereby minimize the phenomenon of overlay in the resulting structure.

To this end, the present invention proposes an apparatus for direct wafer bonding between at least two wafers, the apparatus comprising at least one wafer carrier device comprising a chuck constituted by one or more support elements for receiving one of the two wafers, and aligning elements placed around said chuck; the apparatus being characterized in that the support element or elements of the chuck have an overall contact surface area that is not greater than 85% of the surface area of the wafer that is to be supported by said support element or elements.

As explained below in detail, the Applicant has observed that the non-uniform deformation resulting from direct wafer bonding that results in the largest overlays is located in a region in the vicinity of the side of the wafers. The Applicant has also determined that the wafer in contact with the chuck of the wafer carrier device cannot always change position freely relative to said chuck, in particular in the vicinity of the aligning elements due to an attractive force between the wafer and the chuck.

The apparatus of the invention includes a chuck that, in contrast to prior art chucks, has an overall contact surface area with the wafer that is to be supported thereby that is smaller than the total surface area of the wafer, which means that contactless zones can be provided between the wafer and the chuck, allowing the wafer to change position freely in these zones and to relieve stresses. The overall contact surface area of the support element or elements of the chuck may be less than 60%, or even 50% of the surface area of the wafer that is to be supported by said support element or elements .

In one possible variation, the support element or elements of the chuck are located at a predetermined distance from the aligning elements, which means that the wafer in contact with the chuck can change position freely in the vertical direction at least in the vicinity of said aligning elements, and so stresses arising at these elements during the alignment operations can be relaxed, or mechanical and/or electrostatic adhesion interactions linked to contact of the lower surface of the wafer with the chuck do not occur. This thus avoids the risk of non-uniform deformation in the wafers appearing after bonding. The distance between the support elements of the chuck and the aligning elements is preferably at least 5 mm [millimeter] .

In particular with 3D integration, the risk of overlays occurring during the subsequent formation of additional layers of microcomponents or when bonding two wafers, each including microcomponents intended to be mutually aligned, is greatly reduced.

In one embodiment of the bonding apparatus of the invention, the chuck comprises a support element in the form of a crenellated disk including recessed regions at its periphery respectively placed facing the aligning elements .

In another embodiment of the bonding apparatus of the invention, the chuck comprises a circular support element with a diameter smaller than the diameter of the circular zone corresponding to the diameter of the wafer to be placed on the chuck. The wafer carrier device may further include an annular region at the circular zone, said annular region projecting upwards to a height that differs from the height to which the chuck projects. In another embodiment of the bonding apparatus of the invention, the chuck includes a plurality of support elements, such as studs or pins, distributed within a circular zone corresponding to the diameter of the wafer that is to be supported by the chuck. Said support elements may have the same height or different heights in order to define a chuck surface with a convex or concave shape. In accordance with a particular aspect of the invention, the aligning elements are constituted by a pusher and two abutment elements. The two abutment elements may respectively correspond to a retaining pin and to a positioning pin intended to cooperate with alignment notches provided in the wafers.

In a particular possible embodiment of the

invention, the bonding apparatus further comprises movable spacer elements placed around the chuck to keep the wafers that are to be bonded together facing each other temporarily without contact.

The invention also provides a method of direct wafer bonding between at least a first wafer and a second wafer, said method being carried out with a bonding apparatus in accordance with the invention and comprising at least:

• a step of placing the first wafer on the chuck of the wafer carrier device of said bonding apparatus;

• a step of placing the second wafer on the first wafer ;

• one or more steps of aligning the two wafers carried out by contact between the wafers and the aligning elements; and

• a step of initiating propagation of a bonding wave .

In accordance with a particular implementation of the method of the invention, the method is carried out with a bonding apparatus in accordance with the invention comprising a pusher, two abutment elements, and at least three spacer elements placed around the chuck, the method being further characterized in that during the step of placing the two wafers on the chuck of the wafer carrier device of the bonding apparatus, the first wafer is placed in contact with the chuck of the wafer carrier device while the second wafer is placed facing the first wafer, interposing the spacer elements between the two wafers so as to maintain a space between the two wafers, the method further comprising, before the step of initiating propagation of a bonding wave:

· retracting one of the spacer elements;

• applying a first lateral force to the wafers by means of the pusher of said bonding apparatus in order to align the two wafers relative to each other, the wafers being retained by the abutment elements of said bonding apparatus;

• retracting the other spacer elements;

• retracting the pusher;

• applying a second lateral force to the two wafers by means of the pusher; and

· retracting the pusher.

In one aspect of the invention, the step of

initiating a bonding wave comprises mechanically applying a point of pressure to one of the two wafers.

Using the method of direct wafer bonding of the present invention means that during transfer of a layer of microcomponents , the phenomenon of overlay can be eliminated or limited, and very high quality multilayer semiconductor wafers can be produced. The layer of microcomponents may in particular comprise imaging sensors.

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;

• Figure 2 is a diagrammatic perspective view of a prior art bonding apparatus;

• Figure 3 is a diagrammatic perspective view of a bonding apparatus in accordance with one embodiment of the invention;

• Figures 4A and 4B are respectively perspective and sectional diagrammatic views of a bonding apparatus in accordance with another embodiment of the invention;

• Figure 5 is a diagrammatic perspective view of a bonding apparatus in accordance with another embodiment of the invention;

· Figure 6 is a flow diagram of the steps in a method of direct wafer bonding of the invention

illustrated in Figures 7A to 7J;

• Figures 7A to 7J are diagrammatic views of a method of direct wafer bonding in accordance with one implementation of the invention; and

• Figures 8A and 8B are respectively perspective and sectional diagrammatic views of a bonding apparatus in accordance with another 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 applying mechanical pressure to the exposed surface of one of the two wafers.

As indicated above, before initiating propagation of a bonding wave, one or more operations for aligning the wafers are carried out. Figure 2 shows a prior art bonding apparatus 100 that comprises a wafer carrier device 110 provided with a chuck 111 for receiving one of the two wafers that are to be bonded together. A pusher 120 and two abutment elements respectively constituted by a retaining pin 130 and by a positioning pin 140 are disposed around the chuck 111. The diameter of the chuck 111 is almost the same as that of the wafers intended to be disposed thereon so that the pusher, the retaining pin 130 and the positioning pin 140 are very close to the side of the chuck 111. Three spacer elements 150 to 152 for temporarily preventing contact between the two wafers that are to be bonded together are also present around the chuck 111. As is explained below in detail, a first wafer is placed on the chuck 111 while a second wafer is initially placed facing the first wafer on the three spacer elements 150 to 152. The second wafer is

progressively brought into contact with the first wafer by progressively retracting the spacer elements. After each operation of retracting one or more of the spacer elements, an alignment operation is generally carried out by actuating the pusher 120 against one or both wafers that are retained on the opposite side by the retaining pin 130 and the positioning pin 140.

During the alignment operations, mechanical stresses are exerted in the wafers at those portions of the wafers that are in contact with the pusher 120 and the retaining and positioning pins 130 and 140.

The Applicant has carried out tests for aligning a wafer with a bonding apparatus similar to the bonding apparatus 100 of Figure 2 in order to determine the source or sources of not-relieved stresses that result in non-uniform deformation in the wafer. The Applicant has established that, after one or more alignment operations consisting in placing the wafer against the retaining pin and positioning the pusher by actuation, there remain stresses and, as a consequence, deformation at the side of the wafer and that the deformation is greater in a defined zone in the vicinity of the aligning elements.

Said localization of deformation in the vicinity of the aligning elements is essentially due to the facts both that the intensity of the external contact forces applied against the wafer is at a maximum at the aligning elements (pusher and abutment elements), and also that the wafer cannot change position freely on the support in the vicinity of said elements.

When a contact force is exerted on the wafer by one of the aligning elements (pusher and/or retaining pins and positioning pins) , friction occurs between the contacting surfaces of the wafer and the chuck, thereby creating a tribological effect. Electrostatic charges are thus created and maintained on the chuck at the zones where the friction is greatest, i.e. in the vicinity of the aligning elements. These charges produce local electrical fields and attractive forces that retain the wafer on the chuck, which causes the appearance of local stresses in the wafer and constitutes a first source of non-uniform deformation.

Furthermore, the mechanical stresses caused thereby in the wafer cannot be relaxed because the wafer cannot change position freely on the support in the vicinity of the aligning elements.

To this end, the present invention proposes a bonding apparatus that comprises a chuck that has an overall contact surface area that is 85% smaller, preferably 60% smaller, than the surface area of the wafer that is to be supported. The reduction or absence of contact surface area with the wafer is preferably in the vicinity of the aligning elements. As is described below, the overall contact surface area of the chuck may be constituted by one or more support elements on which a wafer that is intended to be bonded with another wafer rests freely under gravity alone, i.e. without using a system for holding the wafer on the support element or elements of the chuck such as an electrostatic system or a vacuum device.

Figure 3 shows a bonding apparatus 200 in accordance with one embodiment of the invention. The bonding apparatus 200 comprises a wafer carrier device 210 provided with a chuck 211 constituted by a support element 2110 in the form of a crenellated disk and intended to receive one of the two wafers that are to be bonded together. A pusher 220 and two abutment elements respectively constituted by a retaining pin 230 and by a positioning pin 240 are disposed around the chuck 211 at a circular zone Z_C2 corresponding to the diameter of the wafer that is to be supported by the chuck. Three spacer elements 250 to 252 for temporarily preventing contact between the two wafers that are to be bonded together are also present around the chuck 111.

The support element 2110 includes three recessed regions 2111, 2112 and 2113 disposed respectively in the vicinity of the pusher 220, of the retaining pin 230, and of the positioning pin 240. Because of the presence of the recessed regions in the vicinity of these three aligning elements, the chuck 211 does not have a contact surface over a predetermined distance around these elements. Thus, during alignment operations involving applying contact forces between the wafer and the

aligning elements, there is no friction between the wafer and the chuck 211 over the zones corresponding to the positions of the recessed regions 2111, 2112 and 2113. This therefore prevents electrostatic charges from forming between the wafer and the support element at these non-contact zones, which means that the wafer in contact with the support element 2110 can change position freely in the vicinity of the aligning elements and relax the stresses caused therein by the aligning elements.

In the embodiment presented here, the chuck 211 further includes three recessed regions 2114, 2115 and 2116 that can produce diametrically opposite, i.e.

symmetrical actions/reactions. However, the support element 2110 of the chuck 211 of the bonding apparatus of the invention may include recessed regions only around the aligning elements such as the regions 2110 to 2112.

Figure 4A represents a bonding apparatus 300 in accordance with another embodiment of the invention that comprises a wafer carrier device 310 provided with a chuck 311 formed by a circular support element 3110 for receiving one of the two wafers that are to be bonded together. A pusher 320 and two abutment elements respectively constituted by a retaining pin 330 and by a positioning pin 340 are disposed around the chuck 311 at a circular zone Z_c3. Three spacer elements 350 to 352 for temporarily preventing contact between the two wafers that are to be bonded together are also present around the chuck 311.

In this embodiment, the element of the chuck 3110 has a diameter D₃u that is several times smaller than the diameter D_ZC3 of the circular zone Z_C3 corresponding to the diameter of the wafer that is to be supported by the chuck .

The chuck 311 further comprises a second support element constituted by an annular region 3111 that extends substantially at the circular zone Z_C3. As illustrated in Figure 4B, the annular region 360 has a width ί 360 for example of a few millimeters, in order to minimize the contact surface area with the wafer in this zone, and a height h3m that differs from the height h₃₁i of the chuck 311 in order to be able to adapt to the bow of the wafer.

The Applicant has observed that when the bow of the wafer that is to be rested on the chuck is accentuated, the appearance of bonding defects between the two wafers (non-transferred zones (NTZ) , blisters, voids, etc.) is reduced. As a consequence, if the wafer that is to be rested on the chuck has a concave bow, the height h₃m will be greater than the height h₃n. In contrast, if the wafer that is to be rested on the chuck 311 has a convex bow, the height ₃m will be smaller than the height h₃n as illustrated in Figure 4B. The support element 311 and/or the annular region 3111 may be mounted on a piston in order to adjust the height between these two elements as a function of the shape of the wafer.

In this embodiment of the bonding apparatus of the invention, the annular region has a width of only a few millimeters so as to present a reduced contact surface area with the wafer in the vicinity of the aligning elements and to allow it to change position freely in the vicinity of the aligning elements and relax the stresses caused by the aligning elements. Figure 5 represents a bonding apparatus 400 in accordance with another embodiment of the invention that comprises a wafer carrier device 410 provided with a chuck 411 constituted by a circular support element 4110 intended to receive one of the two wafers that are to be bonded together. A pusher 420 and two abutment elements respectively constituted by a retaining pin 430 and by a positioning pin 440 are disposed at a circular zone Z_C4 around the chuck 411. Three spacer elements 450 to 452 for temporarily preventing contact between the two wafers that are to be bonded together are also present around the chuck 411.

In this embodiment, the support element 4110 has a diameter D₄u that is smaller than the diameter D_ZC4 of the circular zone Z_C4 corresponding to the diameter of the wafer that is to be supported by the chuck. The diameter D₄ii may, for example, be 8 cm [centimeter] shorter than the diameter D_ZC4 of the circular zone Z_C4 such that the support element 4110 is at a distance of 4 cm from the pusher 420 and the abutment elements 430 and 440.

In this embodiment of the bonding apparatus of the invention, the chuck is sufficiently far from the aligning elements to allow the wafer held thereon to change position freely locally in the vicinity of the aligning elements and to relax the stresses caused by the aligning elements.

Further, since the support element 4110 has a smaller diameter than the diameter of the circular zone corresponding to the diameter of the wafer that is to be supported by the chuck, a wafer with a convex bow is preferably placed thereon so as to accentuate its bow and reduce the appearance of bonding defects between the two wafers .

Figure 8A shows a bonding apparatus 500 in

accordance with another embodiment of the invention that comprises a wafer carrier device 510 provided with a chuck 511 formed by a plurality of support elements 5110 for receiving one of the two wafers that are to be bonded together. At its end, each of the support elements 5110 has an individual contact surface Ssuo, for example with an area in the range 1 mm² [square millimeter] to 4 mm², and these surfaces are distributed within a circular zone z_c5 corresponding to the diameter of the wafer that is to be supported by the chuck.

A pusher 520 and two abutment elements respectively- constituted by a retaining pin 530 and by a positioning pin 540 are disposed around the chuck 511 at the circular zone Z_C5. Three spacer elements 550 to 552 for

temporarily preventing contact between the two wafers that are to be bonded together are also present around the chuck 511.

The height of the support elements 5110 may be the same for all of the elements, or it may be different in order to provide the chuck constituted by said elements with a convex or concave shape that can be used to accentuate the bow of the wafer and reduce the appearance of bonding defects. In the example described here and as illustrated in Figure 8B, the height of the support elements 5110 decreases from the center of the circular zone Z_C5 to the periphery of this zone, thereby producing a chuck with a convex shape.

In this embodiment of the bonding apparatus of the invention, the sum of the individual surface areas S₅no of the support elements corresponds to an overall contact surface area that is not greater than 85% of the surface area of the wafer that is to be supported.

The skilled person can readily envisage other shapes and/or dimensions for the chuck for a bonding apparatus of the invention that can be used to prevent contact between the wafer and the aligning element over a

predetermined distance around said aligning elements.

The dimensions of the bonding apparatus of the invention and in particular the dimensions of the

circular zone at which the aligning elements are disposed are adapted as a function of the diameter of the wafers that are to be bonded together; in particular, they may have diameters of 100 mm, 150 mm, 200 mm, 300 mm, or 450 mm.

With reference to Figures 6 and 7A to 7J, there follows a description of an example of direct wafer bonding between two wafers as carried out with the bonding apparatus of Figure 3 and in accordance with one implementation of a bonding method of the invention. The bonding apparatus 200, and more precisely the wafer carrier device 210 comprising the chuck 211 constituted by the support element 2110, the pusher 220, the

retaining pin 230 and the positioning pin 240, and the spacer elements 250 to 252, is placed in a sealed chamber (not shown in Figures 7A to 7J) , in which the pressure and the temperature may be controlled.

In Figures 7A and 7B, a first wafer or substrate 20 is placed on the support element 2110 of the chuck 211 of the wafer carrier device 210 of the bonding apparatus 200 (step SI) . The wafer 20 rests freely on the support element 2110, i.e. under gravity alone and without using active means for maintaining the wafer such as, for example, an electrostatic system or a vacuum device associated with the chuck.

Once the wafer 20 is held on the support element

2110 (Figure 7B) , the three spacer elements 250 to 252 for temporarily preventing contact between the two wafers are placed in position (step S2) . The pusher 220 of the bonding apparatus 221 includes a head 241. The pusher 220 is moved between a retracted position in which the head 221 is at a distance from the side of the wafers and does not exert any force on the wafers (as illustrated in Figure 7A) , and a mechanically aligned position in which the head 221 abuts against the sides of the wafers 20 and 30 and exerts an alignment force, principally in the radial direction, on the two wafers that are retained from the opposite side by the two abutment pins 230 and 240, the positioning pin 240 being for cooperating with the respective notches 21 and 31 in the wafers 20 and 30 (as illustrated in Figure 7D) . In its alignment

position, the head 221 of the pusher 220 exerts a force against the wafers that means that they can abut against the abutment pins 230 and 240 and ensure their alignment.

The wafer 30 is then deposited on the spacer elements 250 to 252 so as to position the lower surface or face 32 of the wafer 30 to face the upper surface 22 of the wafer 20 (Figure 7C, step S3) .

Once the wafer 30 has been put into place, the pusher 220 is placed in its mechanical alignment position and exerts a holding force on the wafers against the pins 230 and 240 so as to align the two wafers 20 and 30 for a first time (Figure 7D, step S4) .

In known manner, the respective surfaces 22 and 32 of the wafers 20 and 30 for bonding have been prepared (polishing, cleaning, hydrophobic/hydrophilic treatment, etc.) in order to allow direct bonding.

During the next operation, the spacer element 252 is withdrawn, and then the pusher 220 is placed in its retracted position (Figure 7E, step S5) , which causes the region of the wafer 30 at the location of the spacer element 252 and the holding pin 240 to drop onto the wafer 20.

The pusher 220 is again placed in its alignment position in order to keep the wafers aligned (Figure 7F, step S6) while the spacer elements still present between the two wafers, namely here the spacer elements 250 and 251, are withdrawn, the wafers 20 and 30 being subjected to compressive loads at this time (Figure 7F, step S7 ) .

The pusher 220 is then placed in its retracted position in order to free the wafer 30 from being held against the pins 230 and 240 and to leave the lower face 32 thereof resting entirely on the upper face 22 of the wafer 20 (Figure 7G, step S8) . The pusher 220 is once again placed in its

mechanically aligned position in order to ensure that the wafers 20 and 30 are properly aligned before initiating propagation of a bonding wave (Figure 7H, step S9) . The pusher is then placed in its retracted position

(Figure 71, step S10) .

In accordance with the invention, the above- described alignment steps S4, S6, and S9 are carried out with the chuck 211 that comprises the support element 2110 including recessed regions 2111, 2112, and 2113, and that can relax the stresses exerted by the aligning elements during these operations.

After the steps of mechanically aligning the wafers and of putting them into contact, direct wafer bonding is carried out (Figure 7J, step S10) . As illustrated in

Figure 7J, propagation of a bonding wave may be initiated by means of a tool 50 provided with a stylus 51 that can be used to 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 is controlled in order to limit deformations at the contact point. As illustrated in a highly diagrammatic manner in Figure 7J, the tool 50 may include a

dynamometer 53. The stylus 51 is connected to the dynamometer 53 and comprises 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 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) . Thus, by limiting the pressure applied to one of the two substrates during initiation of a point of contact in this manner, non-uniform deformation caused in the wafer is reduced, while carrying out direct wafer bonding over the whole of the contacting surfaces of the two wafers. 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 for coming 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 imprecise contact surface, and as a result produce a loss of accuracy in the applied pressure.

Further, too rigid a material could result in the

formation of defects (indentations) in 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 to a very low value, typically less than about 10 mbar [millibar] .

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 that are to be assembled together 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 surfaces or only in 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 an SOI type structure. The microcomponents are formed by photolithography by means of a mask that can be used to define zones for forming patterns corresponding to the microcomponents to be produced.

The face of the initial substrate including the microcomponents is then positioned facing and in contact with a face of a final wafer or substrate with a view to direct wafer bonding. In accordance with the invention, the steps of alignment between the initial substrate 100 and the final substrate, such as steps S4, S6, and S9 described above, are carried out with a bonding apparatus in accordance with the invention in order to allow the wafers to move freely relative to the chuck in the vicinity of the aligning elements.

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-uniform deformation or at least with a reduction in deformation such that significant offsets of the microcomponents are no longer observed before and after transfer of the initial substrate onto the final substrate. Thus, residual offsets can be limited to values of less than 50 nm over the entire surface of the wafer. The microcomponents of a second layer, even those of very small dimensions (for example < 1 urn) , 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 the 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 produce very high quality multilayer semiconductor wafers.

Claims

1. Apparatus (200) for direct wafer bonding between at least two wafers (20, 30), the apparatus comprising at least one wafer carrier device (210) comprising a chuck (211) constituted by one or more support elements (2110) for receiving one of the two wafers, and aligning

elements (220, 230, 240) placed around said chuck;

the apparatus being characterized in that the support element or elements (2110) of the chuck (211) have an overall contact surface area that is not greater than 85% of the surface area of the wafer (20) that is to be supported by said support element or elements .

2. Apparatus according to claim 1, characterized in that the aligning elements (220, 230, 240) are placed around said chuck on or in the vicinity of a circular zone (Z_C2 ) corresponding to the diameter of the wafer (20) to be supported by the chuck (211), and in that the support element or elements (2110) of the chuck (211) are placed at a predetermined distance from said aligning elements

(220, 230, 240) .

3. Apparatus according to claim 2, characterized in that the distance between the support element or elements (2110) of the chuck (211) and the aligning elements (220, 230, 240) is at least 5 mm.

4. Apparatus according to any one of claims 1 to 3 , characterized in that the chuck comprises a support element in the form of a crenellated disk (211) having recessed regions (2111, 2112, 2113) at its periphery respectively positioned facing the aligning elements (220, 230, 240) .

5. Apparatus according to any one of claims 1 to 3 , characterized in that the chuck (311; 411) comprises a support element (3110; 4110) that is circular in shape and has a diameter that is smaller than the diameter of a circular zone (Z_C3; Z_c4) corresponding to the diameter of the wafer that is to be supported by the chuck (311;

411) .

6. Apparatus according to claim 5, characterized in that the chuck further comprises an annular region (3111) at the circular zone (Z_c3) , said annular region projecting upwards to a height (h₃₆o) that differs from the height (h₃u) to which the chuck (311) projects.

7. Apparatus according to any one of claims 1 to 3 , characterized in that the chuck (511) includes a

plurality of support elements (5110) distributed within a circular zone (Z_Cs) corresponding to the diameter of the wafer that is to be supported by the chuck (511) .

8. Apparatus according to claim 7, characterized in that the support elements (5110) have different heights so as to define a chuck surface with a convex or concave shape.

9. Apparatus according to any one of claims 1 to 8, characterized in that it includes a pusher (220) and two abutment elements (230, 240).

10. Apparatus according to any one of claims 1 to 9 , characterized in that it further comprises spacer elements (250, 251, 252) placed around the chuck (211).

11. A method of direct wafer bonding between at least a first wafer (20) and a second wafer (30), said method being carried out with a bonding apparatus (200)

according to any one of claims 1 to 8 and comprising at least :

· a step of placing the first wafer (20) on the chuck (211) of the wafer carrier device (210) of said bonding apparatus (200) ; • a step of placing the second wafer (30) on the first wafer (20) ;

• one or more steps of aligning the two wafers (20, 30) carried out by contact between the wafers and the aligning elements (220, 230, 240); and

• a step of initiating propagation of a bonding wave .

12. A method according to claim 11, characterized in that the bonding apparatus comprises a pusher (220), two abutment elements (230, 240) and at least three spacer elements (250, 251, 252) placed around the chuck and in that during the step of placing the two wafers (20, 30) on the chuck (211) of the wafer carrier device (210) of the bonding apparatus (200) , the first wafer (20) is placed in contact with the chuck (211) of the wafer carrier device (210) while the second wafer (30) is placed facing the first wafer (20), interposing said at least three spacer elements (250, 251, 252) between the two wafers so as to maintain a space between the two wafers, and in that the method further comprises, before the step of initiating propagation of a bonding wave:

• retracting one (252) of the spacer elements;

• applying a first lateral force to the wafers (20, 30) by means of the pusher (220) of said bonding

apparatus (200) in order to align the two wafers relative to each other, the wafers being retained by the abutment elements (230, 240) of said bonding apparatus;

• retracting the other spacer elements (250, 251); · retracting the pusher (220) ;

• applying a second lateral force to the two wafers (20, 30) by means of the pusher (220); and

• retracting the pusher (220) .

13. A method according to claim 11 or claim 12,

characterized in that the step of initiating a bonding wave comprises mechanically applying a point of pressure to one of the two wafers .

14. A method according to claim 13, characterized in that the mechanical pressure applied to one of the two wafers by the mechanical pressure point is in the range 1 MPa to 33.3 MPa .