Method for bonding by direct adhesion

Abstract

A process includes the successive steps of: a) providing first and second substrates, each including a first surface and an opposite, second surface, lateral edges connecting the first and second surfaces, b) bonding the first substrate to the second substrate by direct bonding with the first surfaces of the first and second substrates so as to form a bonding interface (IC), and making the lateral edges of the first and second substrates hydrophobic on either side of the bonding interface (IC).

Claims

1. A process for attaching a first substrate to a second substrate by direct bonding, the process comprising: a) providing the first and second substrates, each comprising: a first surface and an opposite, second surface, and lateral edges connecting the first and second surfaces; b) bonding the first substrate to the second substrate by direct bonding with the first surfaces of the first and second substrates so as to form a bonding interface (IC); and c) making, after the bonding, the lateral edges of the first and second substrates hydrophobic on either side of the bonding interface (IC).

2. The process according to claim 1, wherein c) is carried out by forming a layer on the lateral edges of the first and second substrates on either side of the bonding interface (IC), said layer being produced in a hydrophobic material.

3. The process according to claim 2, wherein the hydrophobic material is selected from paraffin wax or a fluorinated coating.

4. The process according to claim 1, wherein c) is carried out by applying a hydrophobic treatment to the lateral edges of the first and second substrates on either side of the bonding interface (IC).

5. The process according to claim 4, wherein the hydrophobic treatment is a chemical treatment applied by an acid solution or acid vapour.

6. The process according to claim 4, wherein the hydrophobic treatment is a surface treatment applied by a plasma.

7. The process according to claim 1, wherein b) is followed by applying a heat treatment to the bonded first and second substrates, the heat treatment being carried out at a temperature of between 100 C. and 300 C.

8. A process for attaching a first substrate to a second substrate by direct bonding, the process comprising: a) providing the first and second substrates, each comprising: a first surface and an opposite, second surface, and lateral edges connecting the first and second surfaces; b) bonding the first substrate to the second substrate by direct bonding with the first surfaces of the first and second substrates so as to form a bonding interface (IC), wherein b) is carried out at a temperature of between 20 C. and 30 C.; and c) making the lateral edges of the first and second substrates hydrophobic on either side of the bonding, interface (IC).

9. The process according to claim 1, wherein c) is carried out subsequent to b), after a waiting time of less than six hours.

10. The process according to claim 1, wherein the lateral edges of the first and second substrates provided in a) are produced in a material selected from Si, Ge, a silicon oxide, SiC, a III-V material, a II-VI material.

11. The process according to claim 4, wherein the hydrophobic treatment is a surface treatment applied by a fluorine-based plasma.

12. The process according to claim 1, wherein c) is carried out subsequent to b), after a waiting time of less than one hour.

13. The process according to claim 1, wherein c) is carried out subsequent to b), after a waiting time of less than 15 minutes.

14. The process according to claim 1, wherein the bonding in b) is hydrophilic bonding.

15. A process for attaching a first substrate to a second substrate by direct bonding, the first and second substrates each including a first surface and an opposite, second surface, and lateral edges connecting the first and second surfaces, the process comprising: bonding the first substrate to the second substrate by direct bonding with the first surfaces of the first and second substrates so as to form a bonding interface (IC); and making, after the bonding, the lateral edges of the first and second substrates hydrophobic on either side of the bonding interface (IC).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages will become apparent in the detailed description of various embodiments of the invention, the description being accompanied by examples and references to the appended drawings.

(2) FIG. 1 is a sectional schematic view along the normal to the first and second surfaces of the first and second substrates, illustrating direct bonding of two substrates with rounded edges.

(3) FIG. 2 is a sectional schematic view along the normal to the first and second surfaces of the first and second substrates, illustrating direct bonding of two substrates with rounded edges, and each covered with an oxide.

(4) FIG. 3 is a sectional schematic view along the normal to the first and second surfaces of the first and second substrates, illustrating the formation of a hydrophobic layer on the rounded lateral edges of the substrates.

(5) FIG. 4 is a sectional schematic view along the normal to the first and second surfaces of the first and second substrates, illustrating direct bonding of two substrates with sheer edges.

(6) FIG. 5 is a sectional schematic view along the normal to the first and second surfaces of the first and second substrates, illustrating the formation of a hydrophobic layer on the sheer lateral edges of the two substrates.

(7) FIGS. 6a and 6b are images obtained by acoustic microscopy, illustrating the effects of the presence and of the absence, respectively, of a hydrophobic layer on the quality of the bonding interface, the bonding interface being immersed for one week in liquid water after the formation of the hydrophobic layer.

(8) FIGS. 7a and 7b are images obtained by acoustic microscopy, illustrating the effects of the presence and of the absence, respectively, of a hydrophobic layer on the quality of the bonding interface, the bonding interface being subjected to storage for one week in humid air (45% relative humidity) after the formation of the hydrophobic layer.

(9) FIGS. 8a and 8b are images obtained by acoustic microscopy, illustrating the effects of the presence and of the absence, respectively, of a hydrophobic layer on the quality of the bonding interface, the bonding interface being immersed for two weeks in liquid water before the formation of the hydrophobic layer. After the formation of the hydrophobic layer, the bonded substrates are stored for two days.

(10) FIGS. 1 to 5 are not shown to scale in order to facilitate the understanding thereof.

DETAILED DESCRIPTION OF EMBODIMENTS

(11) Those elements that are identical or perform the same function will bear the same references for the various embodiments, for the sake of simplicity.

(12) A subject of the invention is a process for attaching a first substrate 1 to a second substrate 2 by direct bonding including the successive steps of: a) providing the first and second substrates 1, 2, each comprising: a first surface 10, 20 and an opposite, second surface 11, 21; lateral edges 12, 22 connecting the first and second surfaces 10, 20, 11, 21; b) bonding the first substrate 1 to the second substrate 2 by direct bonding with the first surfaces 10, 20 of the first and second substrates 1, 2 so as to form a bonding interface IC; c) making the lateral edges 12, 22 of the first and second substrates 1, 2 hydrophobic on either side of the bonding interface IC.
First and Second Substrates

(13) The first and/or second substrates 1, 2 may be produced in a semiconductor material. The first and/or second substrates 1, 2 may be covered with an oxide 3.

(14) The lateral edges 12, 22 of the first and second substrates 1, 2 provided in step a) may be produced in a material selected from Si, Ge, a silicon oxide, SiC, a III-V material, a II-VI material.

(15) As illustrated in FIGS. 1 to 3, the lateral edges 12, 22 of the first and second substrates 1, 2 may have rounded edges due to the presence of chamfers. As illustrated in FIGS. 4 and 5, the lateral edges 12, 22 of the first and second substrates 1, 2 may have sheer edges.

(16) Bonding Step b)

(17) Step b) is preferably carried out at ambient temperature, i.e. at a temperature of between 20 C. and 30 C. However, step b) may be carried out at a higher temperature, for example between 200 C. and 400 C.

(18) Step b) may be carried out at ambient pressure.

(19) Step b) is a step of hydrophilic bonding between the first and second substrates 1, 2. The term hydrophilic is understood to mean a surface that is characterized by a contact angle with a drop of water that is smaller than or equal to 40, preferably smaller than or equal to 10.

(20) Step c): Making the Lateral Edges Hydrophobic

(21) According to a first implementation, step c) is carried out by forming a layer 4 on the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC, said layer 4 being produced in a hydrophobic material. The hydrophobic material is advantageously selected from paraffin wax or a fluorinated coating. By way of nonlimiting examples, when the first and second substrates 1, 2 are produced in silicon or covered with a silicon oxide the fluorinated coating may be: a Novec 2702 coating, sold by 3M; a Novec 1700 coating, sold by 3M.

(22) According to a second implementation, step c) is carried out by applying a hydrophobic treatment to the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC.

(23) According to a first variant, the hydrophobic treatment is a chemical treatment, preferably applied by means of an acid solution or by means of acid vapour. When the first and second substrates 1, 2 are produced in silicon, the chemical treatment may be applied by means of an HF solution or by means of HF vapour. When the first and second substrates 1, 2 are produced in InP, the chemical treatment may be applied by means of an HCl solution or by means of HCl vapour. Such a chemical treatment may also make the second surfaces 11, 21 of the first and second substrates 1, 2 hydrophobic without, however, having any technical effect on the ingress of water molecules into the bonding interface IC.

(24) According to a second variant, the hydrophobic treatment is a surface treatment, preferably applied by means of a plasma, more preferably a fluorine-based plasma. The fluorine-based plasma may be a CF.sub.4 or a CHF.sub.3 plasma. When the first and second substrates 1, 2 are produced in silicon or covered with silicon dioxide, the surface treatment may be applied by means of a CF.sub.4 plasma. Such a surface treatment may also make the second surfaces 11, 21 of the first and second substrates 1, 2 hydrophobic without, however, having any technical effect on the ingress of water molecules into the bonding interface IC.

(25) Step c) is carried out subsequent to step b), advantageously after a wait of less than six hours, preferably less than one hour, more preferably less than 15 minutes. The wait between steps b) and c) advantageously takes place in a dry atmosphere, i.e. an atmosphere holding less than 1 ppm of water.

(26) Heat Treatment: Slowing the Kinetics of the Ingress of Water

(27) Step b) is advantageously followed by a step b.sub.1) consisting in applying a heat treatment to the bonded first and second substrates 1, 2, the heat treatment being carried out at a temperature of between 100 C. and 300 C. Step b.sub.1) is carried out before step c).

(28) Thermal Annealing: Strengthening the Bonding Interface

(29) The process advantageously includes a step d) consisting in applying a thermal annealing treatment to the bonded first and second substrates 1, 2, step d) being carried out after step c). The thermal annealing treatment applied in step d) is advantageously carried out at a temperature of more than or equal to 1000 C.

(30) The process advantageously includes a step, carried out immediately before step d), consisting in: removing the layer 4 formed in step c); or annihilating the hydrophobic treatment applied in step c).

Implementation Example No. 1

(31) The first and second substrates 1, 2 are two (001) silicon wafers of 200 mm in diameter and 725 m in thickness. The first and second substrates 1, 2 are cleaned and hydrolysed in baths of ozonated deionized water and in an APM (ammonia-peroxide mixture) solution at 70 C. so that the bonding surfaces are hydrophilic. The first and second substrates 1, 2 are direct-bonded at ambient temperature and at ambient pressure.

(32) Step c) is carried out by forming a fluorinated coating on the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC, as well as on the second surfaces 11, 21 of the first and second substrates 1, 2. In order to achieve this, step c) is carried out by turning the bonded first and second substrates 1, 2 in a bath of Novec 2702. Excess fluorinated coating may be removed using a solvent (e.g. Novec 7200) and by rinsing with isopropyl alcohol (IPA). This fluorinated coating may be removed later by means of an O.sub.2 plasma.

(33) In order to obtain a comparative study, implementation example No. 1 was repeated without step c).

(34) Results are illustrated in FIGS. 6a, 6b, after the following steps: the substrates 1, 2 are thermally annealed at 150 C. for 30 minutes; the substrates 1, 2 are immersed in liquid water for one week; the substrates 1, 2 are dried and thermally annealed at 300 C. for two hours.

(35) The absence of a ring C of defects once step c) has been carried out is observed.

(36) Similarly, results are illustrated in FIGS. 7a, 7b, after the following steps: the substrates 1, 2 are thermally annealed at 150 C. for 30 minutes; the substrates 1, 2 are stored in humid air (45% relative humidity) for one week; the substrates 1, 2 are dried and thermally annealed at 300 C. for two hours.

(37) The absence of a ring C of defects once step c) has been carried out is again observed.

(38) Furthermore, it was observed that carrying out step c) also prevents water from escaping from the bonding interface IC. In order to achieve this, implementation example No. 1 was modified by introducing a step consisting in immersing the substrates 1, 2 in liquid water for two weeks before carrying out step c). Additionally, as above, modified implementation example No. 1 was repeated without step c) in order to obtain a comparative study.

(39) Results are illustrated in FIGS. 8a, 8b, after the following steps: the substrates 1, 2 are thermally annealed at 150 C. for 30 minutes; the substrates 1, 2 are stored in a dry nitrogen atmosphere (less than 1 ppm of water) for two days; the substrates 1, 2 are thermally annealed at 300 C. for two hours.

(40) The presence of a ring C of defects up to the edge once step c) has been carried out is observed, which demonstrates that carrying out step c) prevents water from escaping from the bonding interface IC.

Implementation Example No. 2

(41) The first and second substrates 1, 2 are two (001) silicon wafers of 200 mm in diameter and 725 m in thickness. The first and second substrates 1, 2 are cleaned and hydrolysed in baths of ozonated deionized water and in an APM (ammonia-peroxide mixture) solution at 70 C. so that the bonding surfaces are hydrophilic. The first and second substrates 1, 2 are direct-bonded at ambient temperature and at ambient pressure.

(42) Step c) is carried out by forming a fluorinated coating on the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC, as well as on the second surfaces 11, 21 of the first and second substrates 1, 2. In order to achieve this, step c) is carried out while turning the bonded first and second substrates 1, 2 in a bath of Novec 1700. Excess fluorinated coating may be removed using a solvent (e.g. Novec 7200 or 7100) and by rinsing with isopropyl alcohol (IPA). This fluorinated coating may be removed later by means of an O.sub.2 plasma.

Implementation Example No. 3

(43) The first and second substrates 1, 2 are two (001) silicon wafers of 200 mm in diameter and 725 m in thickness. The first and second substrates 1, 2 are cleaned and hydrolysed in baths of ozonated deionized water and in an APM (ammonia-peroxide mixture) solution at 70 C. so that the bonding surfaces are hydrophilic. The first and second substrates 1, 2 are direct-bonded at ambient temperature and at ambient pressure.

(44) Step c) is carried out by adding paraffin wax to the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC, as well as to the second surfaces 11, 21 of the first and second substrates 1, 2. In order to achieve this, step c) is carried out by turning the bonded first and second substrates 1, 2 in a bath of molten paraffin wax. The paraffin wax may be removed later by means of a solvent such as acetone.

Implementation Example No. 4

(45) The first and second substrates 1, 2 are two (001) silicon wafers of 200 mm in diameter and 725 m in thickness. The first and second substrates 1, 2 are cleaned and hydrolysed in baths of ozonated deionized water and in an APM (ammonia-peroxide mixture) solution at 70 C. so that the bonding surfaces are hydrophilic. The first and second substrates 1, 2 are direct-bonded at ambient temperature and at ambient pressure.

(46) Step c) is carried out by means of a hydrophobic chemical treatment, applied to the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC, as well as to the second surfaces 11, 21 of the first and second substrates 1, 2. In order to achieve this, step c) is carried out by turning the bonded first and second substrates 1, 2 in a 10% HF bath. Such a chemical treatment may be annihilated later using an APM solution at 70 C., or using an O.sub.2 plasma.

Implementation Example No. 5

(47) The first and second substrates 1, 2 are two (001) silicon wafers of 200 mm in diameter and 725 m in thickness. The first and second substrates 1, 2 are cleaned and hydrolysed in baths of ozonated deionized water and in an APM (ammonia-peroxide mixture) solution at 70 C. so that the bonding surfaces are hydrophilic. The first and second substrates 1, 2 are direct-bonded at ambient temperature and at ambient pressure.

(48) Step c) is carried out by means of a hydrophobic surface treatment, applied to the lateral edges 12, 22 of the first and second substrates 1, 2 on either side of the bonding interface IC, as well as to the second surfaces 11, 21 of the first and second substrates 1, 2. In order to achieve this, step c) is carried out by subjecting the bonded first and second substrates 1, 2 to a CF.sub.4 plasma. Such a surface treatment may be annihilated later using an APM solution at 70 C., or using an O.sub.2 plasma.

(49) The invention is not limited to the described embodiments. A person skilled in the art is capable of considering all technically feasible combinations thereof and of substituting them with equivalents.