Method for producing composite structure with metal/metal bonding

Abstract

Method for producing a composite structure comprising the direct bonding of at least one first wafer with a second wafer, and comprising a step of initiating the propagation of a bonding wave, where the bonding interface between the first and second wafers after the propagation of the bonding wave has a bonding energy of less than or equal to 0.7 J/m.sup.2. The step of initiating the propagation of the bonding wave is performed under one or more of the following conditions: placement of the wafers in an environment at a pressure of less than 20 mbar and/or application to one of the two wafers of a mechanical pressure of between 0.1 MPa and 33.3 MPa. The method further comprises, after the step of initiating the propagation of a bonding wave, a step of determining the level of stress induced during bonding of the two wafers, the level of stress being determined on the basis of a stress parameter Ct calculated using the formula Ct=Rc/Ep, where: Rc corresponds to the radius of curvature (in km) of the two-wafer assembly and Ep corresponds to the thickness (in m) of the two-wafer assembly. The method further comprises a step of validating the bonding when the level of stress Ct determined is greater than or equal to 0.07.

Claims

1. A method for producing a composite structure, comprising: direct bonding of a first wafer and a second wafer by initiating propagation of a bonding wave, wherein a bonding interface between the first and second wafers after the propagation of the bonding wave has a bonding energy of less than or equal to 0.7 J/m.sup.2, wherein initiating the propagation of the bonding wave is performed under one or more of the following conditions: placement of the first and second wafers in an environment at a pressure of less than 20 mbar, application to one of the first and second wafers of a mechanical pressure of between 0.1 MPa and 33.3 MPa; after initiating the propagation of the bonding wave, determining a level of stress induced during bonding of the first and second wafers, the level of stress determined by a stress parameter Ct calculated by a formula:
Ct=Rc/Ep where: Rc corresponds to a radius of curvature (in km) of a two-wafer assembly comprising the first and second wafers, and Ep corresponds to a thickness (in m) of the two-wafer assembly; determining whether the stress parameter Ct is less than 0.07 or whether the stress parameter Ct is greater than or equal to 0.07; validating the bonding between the first and second wafers when the stress parameter Ct is determined to be greater than or equal to 0.07; and rejecting the first and second wafers when the stress parameter Ct is determined to be less than 0.07 such that the bonding between the first and second wafers is not validated.

2. The method according to claim 1, wherein at least one of the first and second wafers is held flattened on a support having deviations in planarity of less than or equal to 2 m while initiating the propagation of the bonding wave.

3. The method according to claim 2, wherein each of the first and second wafers comprises, on its bonding face, a material selected from the group consisting of tungsten, aluminium, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, silicon nitride and silicon.

4. The method according to claim 3, wherein at least one of the first and second wafers comprises a silicon substrate covered with a layer of material selected from the group consisting of tungsten, aluminium, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, silicon nitride and silicon, and wherein the layer of material forms the bonding face of at least one of the first and second wafers.

5. The method according to claim 4, wherein validating the bonding between the first and second wafers comprises at least one step of treating the two-wafer assembly that is selected from the group consisting of: a thermal treatment of the assembly, performed at a temperature of less than or equal to 500 C.; and a chemical or chemical-mechanical thinning of one of the first and second wafers.

6. The method according to claim 1, wherein each of the first and second wafers comprises, on its bonding face, a material selected from the group consisting of tungsten, aluminum, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, silicon nitride and silicon.

7. The method according to claim 1, wherein at least one of the first and second wafers comprises a silicon substrate covered with a layer of material selected from the group consisting of tungsten, aluminum, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, silicon nitride and silicon, and wherein the layer of material forms the bonding face of the at least one of the first and second wafers.

8. The method according to claim 2, wherein at least one of the first and second wafers comprises a silicon substrate covered with a layer of material selected from the group consisting of tungsten, aluminum, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, silicon nitride and silicon, and wherein the layer of material forms the bonding face of at least one of the first and second wafers.

9. The method according to claim 1, wherein validating the bonding between the first and second wafers comprises at least one step of treating the two-wafer assembly that is selected from the group consisting of: a thermal treatment of the assembly, performed at a temperature of less than or equal to 500 C., and a chemical or chemical-mechanical thinning of one of the first and second wafers.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other features and advantages of the disclosure will emerge from the following description of particular embodiments of the disclosure, which are given as non-limitative examples, with reference to the appended drawings, in which:

(2) FIGS. 1A and 1B are schematic views of a method of direct bonding in accordance with one embodiment of the disclosure;

(3) FIG. 2 is a schematic view of a method for direct bonding in accordance with another embodiment of the disclosure; and

(4) FIG. 3 shows the measurement of the parameters for calculating the level of stress at the bonding interface, in accordance with the disclosure.

DETAILED DESCRIPTION

(5) The disclosure applies, generally speaking, to the production of composite structures, comprising at least the direct bonding of a first substrate or wafer to a second substrate or wafer, where the resulting bonding interface has a limited bonding energy, namely, a bonding energy of less than or equal to 0.7 J/m.sup.2, even after a bonding reinforcement bake at 500 C. Such limitation of bonding energy following production of a direct bond comes about, in particular, in the following cases: when the bonding faces are made of metallic material or covered with a metallic material having a low self-diffusion coefficient D, i.e., of less than 10.sup.50 m.sup.2/s, such as tungsten, aluminium, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, etc.; when the bonding faces are made of silicon or covered with silicon and when hydrophobic bonding is performed, i.e., without using an oxide-type bonding layer; when the bonding faces are made of silicon nitride or covered with silicon nitride. The wafers for assembly may, in particular, have a diameter of 100 mm, 150 mm, 200 mm or 300 mm.

(6) Direct, or molecular-adhesion, bonding is a technique well known per se. To recall, the principle of direct bonding is based on the contacting of two surfaces directly, in other words, without the use of a specific material (adhesive, wax, braze, etc.). Such an operation requires that the surfaces to be bonded together should be sufficiently smooth, free of any particles or contamination, and that they be sufficiently close together to initiate a contact, typically at a distance of less than a few nanometers. In this case, the attractive forces between the two surfaces are quite high, so as to trigger molecular adhesion (bonding induced by the combination of attractive forces (van der Waals forces) of electronic interaction between atoms or molecules of the two surfaces to be bonded).

(7) The molecular adhesion is produced by initiation of at least one contact point on a wafer in intimate contact with another wafer, so as to trigger the propagation of a bonding wave from this contact point. The term bonding wave is used here to refer to the bonding or molecular adhesion front that propagates from the point of initiation and that corresponds to the spreading of the attractive forces (van der Waals forces) from the contact point over the entire surface of intimate contact between the two wafers (bonding interface). The contact point is typically initiated by applying a mechanical pressure to the exposed surface of one of the two wafers.

(8) In accordance with the disclosure and in order to reduce the stresses accumulated at the time of bonding, the initiation of the propagation of a bonding wave between two wafers to be assembled, having bonding faces made of a material that does not result in a bonding energy of greater than 0.7 J/m.sup.2, as described above, is accomplished by placing the wafers in an environment at a pressure of less than 20 mbar and/or by applying to one of the two wafers a mechanical pressure of between 0.1 MPa and 33.3 MPa.

(9) Where the propagation of a bonding wave is initiated by placing the wafers in a low-pressure environment, and as shown in FIG. 1A, a first wafer or substrate 20 is placed into a chamber 110 of a bonding machine 100, the chamber comprising a substrate carrier device 40. In the example described here, the first wafer 20 is composed of a silicon substrate 21 comprising a layer 22 of metallic material having a low self-diffusion coefficient and forming the bonding face 20a of the wafer 20. The layer 22 here is composed of tungsten. The substrate carrier device 40 comprises a support plate 40a, which exhibits deviations from planarity of preferably less than or equal to 2 microns. The support plate 40a holds the first wafer 20, by means, for example, of an electrostatic or suction system, in association with the support plate 40a, or by simple gravity, for the purpose of its assembly by molecular adhesion to a second wafer or substrate 30, composed of a silicon substrate 31 comprising a tungsten layer 32, forming the bonding face 30a of the wafer 30. The associated systems for holding the wafer (electrostatic or by suction) are used to the extent that it has been verified that they do not deform the wafer, so as not to give rise to any increase in the problems of accumulation of stresses at the bonding interface.

(10) As illustrated in FIG. 1B, the wafer 30 is then set down on the wafer 20, so as to bring the bonding faces 20a and 30a, respectively, of the wafers 20 and 30 opposite one another. Alignment operations may be performed, moreover, during the setting-down of the wafer 30 onto the wafer 20.

(11) The wafers 20 and 30 are placed into the hermetic chamber 110, which is equipped with partial vacuum means such as a vacuum pump or similar (not shown in FIG. 1B).

(12) When wafers 20 and 30 are ready to be bonded, the pressure in the chamber 110 is lowered to a pressure P1 of less than or equal to 20 mbar, preferably less than 5 mbar, so as to initiate the propagation of a bonding wave between the two wafers. The pressure of the environment into which the wafers are placed during the initiation of the propagation of a bonding wave is constant or not (i.e., potentially variable during the initiation step). The propagation of a bonding wave is initiated spontaneously here, in other words, without application of mechanical pressure on the wafers, thereby minimizing the stresses during bonding and reducing the level of stored stress at the bonding interface.

(13) Where the propagation of a bonding wave is initiated by application of a mechanical pressure, the disclosure proposes controlling the mechanical pressure applied at the contact point so as to limit the stresses in this zone, while allowing the initiation and propagation of a bonding wave between the two wafers in contact. In accordance with the disclosure, the pressure applied at the contact point is between 0.1 megapascal (MPa) and 33.3 MPa. The initiation point may be located anywhere on the wafer. It is preferably situated close to the center of the wafer. The surface area of the zone of application of this pressure is typically less than a few mm.sup.2, for example, 1 mm.sup.2. Larger application surface areas are possible but avoided because of the risk of too great a contact surface area (greater than 5 mm.sup.2, for example) leading to an increase in the deformations and in the level of stored stresses at the bonding interface. The application of a mechanical pressure of this kind is sufficient to initiate a contact point between two wafers and, consequently, to allow the propagation of a bonding wave over the entire surface of contact between the wafers, without giving rise to excessive stresses. Accordingly, by controlling the mechanical pressure applied to initiate the contact point, the deformations caused in the wafer are reduced. The pressure applied at the contact point is preferably less than 10 MPa, and still more preferably, this pressure is between 0.1 to 5 MPa.

(14) The duration for which the mechanical pressure is applied corresponds at least to the minimum duration allowing the bonding wave propagation phenomenon to be activated. This minimum duration corresponds substantially to the duration required by the bonding wave to propagate over the surface of contact between the wafers. The duration of application of the mechanical pressure varies generally between 1 and 10 seconds, typically 5 seconds, for assembly of wafers having a diameter of 200 mm.

(15) The controlled application of the mechanical pressure may be performed by means of a tool. In FIG. 2, a first wafer or substrate 120 is placed into a bonding machine comprising a support plate 140 that has deviations from planarity of preferably less than or equal to 2 microns. The support plate 140 holds the first wafer 120, by means, for example, of an electrostatic or suction system combined with the support plate 140, or by simple gravity, for the purpose of its assembly by molecular adhesion with a second wafer or substrate 130. As in the example described above, the wafer 120 or 130, respectively, is composed of a silicon substrate 121 or 131, respectively, covered with a tungsten layer 122 or 132, respectively, forming the bonding face 120a or 130a, respectively, of the wafer 120 or 130.

(16) The bonding faces 121a and 131a of the wafers 120 and 130 are brought into intimate contact with one another. The initiation of a contact point for the molecular adhesion is performed by means of a tool 50. As illustrated very schematically in FIG. 2, the tool 50 comprises a support element 51, such as a stylus, and a spring gauge 53. The support element 51 is connected to the spring gauge 53 and comprises a free end 52 by which a mechanical pressure is exerted on the wafer 130 so as to initiate a contact point between the two wafers 120 and 130. The end 52 has a contact surface area 52a of between 0.3 mm.sup.2 and 1 mm.sup.2. When the value of the contact surface area 52a of the tool 50 with the wafer 130 is known, it is possible to apply a mechanical pressure of between 0.1 MPa and 33.3 MPa by controlling the supporting force F exerted by the tool on the wafer (supporting force=mechanical pressuresupporting surface area). The supporting force exerted by the end 52 on the wafer 130 is controlled by means of the spring gauge 53. This force is between 0.1 newton (N) and 10 N.

(17) For example, when the desire is to use a tool whose end has a contact surface area of 1 mm.sup.2 to apply a mechanical pressure of 3.5 MPa (a pressure sufficient to initiate a contact point and, consequently, a bonding wave between the two wafers), a supporting force is exerted of 3.5 N.

(18) The support element and, more particularly, its end for contact with the wafer may be made of or covered with a material such as TEFLON, silicone or a polymer. Generally speaking, the end of the support element is made of or covered with a material that is sufficiently rigid to apply the pressure in a controlled way. The reason is that too flexible a material could deform and lead to an imprecise contact surface area and, consequently, to a lack of precision of the pressure applied. Moreover, too stiff a material could lead to the formation of defects (indentations) at the surface of the wafer.

(19) The method for initiation of molecular adhesion of the disclosure may be employed automatically in a bonding machine. In that case, the machine comprises a support element connected to an actuator (for example, a cylinder or a mechanical arm). The machine further comprises a force sensor (spring gauge, strain gauge, etc.) and a servo control for steering the actuator. The servo control steers the actuator in such a way as to control the mechanical pressure applied by the support element. More specifically, the servo control receives the data from the force sensor and compares it with a supporting force value predetermined as a function of the mechanical pressure that is desired to be applied and of the surface area of the support element end.

(20) The means for initiating the propagation of a bonding wave as described above may be combined, which means that a mechanical pressure point may be applied between the two wafers at a supporting pressure of between 0.1 MPa and 33.3 MPa, while at the same time, the two wafers are placed into an environment maintained at a pressure of less than 20 mbar.

(21) Before bonding is performed, the bonding faces of the wafers may be prepared, in particular, by means of chemical-mechanical polishing.

(22) Once the direct bonding has been performed, the method of the disclosure further comprises a step of measuring the level of stress at the bonding interface between the two wafers, and a control step of determining whether the level of stress is acceptable to allow subsequent treatments to be applied to the two-wafer assembly without risk of disbonding.

(23) For this purpose, the disclosure proposes evaluating the level of stress induced during bonding of the two wafers on the basis of a stress parameter Ct calculated using the following formula:
Ct=Rc/Ep(1)

(24) where: Rc corresponds to the radius of curvature, in km, of the two-wafer assembly, Ep corresponds to the thickness, in m, of the two-wafer assembly.

(25) FIG. 3 illustrates an assembly 200 produced by direct bonding of a first wafer 220, composed of a silicon substrate 221 covered on its bonding face with a tungsten layer 222, and a second wafer 230 made of tungsten. In order to determine the level of stress at the bonding interface between the wafers 220 and 230, a determination is made of the radius of curvature Rc presented by the assembly 200, by means of the following approximation formula:
Rc=D.sup.2/8B

(26) where: B corresponds to the curvature or bow, in m, of the assembly, D is the diameter, in mm, of the wafers in the assembly.

(27) A measurement is made, moreover, of the thickness Ep of the assembly, corresponding to the cumulative thickness of the bonded wafers 220 and 230. The radius of curvature Rc is measured in km, whereas the thickness of the assembly is measured in m. The stress parameter Ct is then calculated using the formula (1) given above.

(28) When the stress parameter has been calculated, it is compared with a reference value Cref, which is 0.07. If the calculated stress level Ct is greater than 0.07, preferably greater than 0.15, bonding is validated in the sense that the stress level present within the assembly is sufficiently low to prevent disbonding of the wafers in the course of subsequent treatments and, more particularly, during thermal, chemical or chemical-mechanical treatments.

(29) If the calculated stress parameter Ct is less than the reference level, bonding is not validated, allowing the defective batch to be removed from the fabrication process prior to the disbonding of the wafers during treatments carried out after bonding. The wafers in the rejected assembly may be disbonded and rebonded anew, to give a satisfactory level of stress.

(30) When bonding is validated in accordance with the disclosure, the production of the composite structure can continue by the performance of one or more of the following treatments on the assembly: thermal treatment performed at a temperature of less than or equal to 500 C., thinning of one of the two wafers by chemical etching or chemical-mechanical polishing of one of the two wafers, deposition or growth of further layers on the assembly.

(31) The table below shows results of simulation obtained in terms of level of stress for a plurality of batches, each corresponding to the assembly of two wafers by molecular adhesion.

(32) TABLE-US-00001 Radius of Wafer curvature, Thickness, Stress diameter Curvature assembly assembly parameter Batch (D) mm (B) m (Rc) km m (Ct) 1 100 8 156.25 1050 0.14881 2 100 16 78.125 1050 0.074405 3 100 50 25 1050 0.02381 4 100 100 12.5 1050 0.011905 5 100 1000 1.25 1050 0.00119 6 200 1 5000 1450 3.448276 7 200 23 217.391304 1450 0.149925 8 200 45 111.111111 1450 0.076628 9 200 100 50 1450 0.034483 10 200 1000 5 1450 0.003448 11 300 1 11250 1550 7.258065 12 300 10 1125 1550 0.725806 13 300 50 225 1550 0.145161 14 300 102 110.294118 1550 0.071157 15 300 1000 11.25 1550 0.007258

(33) For batches 3, 4, 5, 9, 10 and 15, it is found that the calculated stress parameter is less than the reference value of 0.07, whereas the other batches have a stress parameter greater than this reference value.