BASIC MOLECULE-ASSISTED DIRECT BONDING METHOD

Abstract

A method for manufacturing a multilayer structure by direct bonding between a first substrate and a second substrate includes the steps of: a) providing a first substrate and a second substrate respectively including a first bonding surface and a second bonding surface, b) bringing the first bonding surface and the second bonding surface into contact so as to create a direct bonding interface between the first substrate and the second substrate, c) disposing at least the direct bonding interface in a basic environment, and d) applying a thermal treatment at a temperature of between 20° C. and 350° C. so as to obtain the multilayer structure.

Claims

1. A method for manufacturing a multilayer structure by direct bonding between a first substrate and a second substrate, the method comprising the steps of: a) providing a first substrate and a second substrate respectively comprising a first bonding surface and a second bonding surface, b) bringing the first bonding surface and the second bonding surface into contact so as to create a direct bonding interface between the first substrate and the second substrate, c) disposing at least the direct bonding interface in a basic environment, and d) applying a thermal treatment at a temperature of between 20° C. and 1000° C. so as to obtain the multilayer structure.

2. The manufacturing method according to claim 1, wherein step c) of disposing the direct bonding interface in the basic environment is carried out for a duration of approximately 1 hour to 80 days.

3. The manufacturing method according to claim 1, wherein the first bonding surface and/or the second bonding surface are/is formed at least in part by a hydrophilic film made of a material chosen from a native oxide, a thermal or deposited silicon oxide, a silicon nitride, a copper oxide and a combination of these materials.

4. The manufacturing method according to claim 1, wherein the first bonding surface and the second bonding surface are completely flat.

5. The manufacturing method according to claim 1, wherein the basic environment is an aqueous basic solution.

6. The manufacturing method according to claim 5, wherein the aqueous basic solution is formed, by dissolving in deionized water, a basic compound chosen from NaOH, KOH, Na.sub.2CO.sub.3, NH.sub.4OH, an amino alcohol and an mixture of these basic compounds; the amino alcohol being selected from 2-ethanol DMAE, N,N-diethyl-2-amino-ethanol, monoethanolamine, N-methyldiethanolamine, aminomethanol, N-methylhydroxylamine, diethanolamine, dimethanolamine, triethanolamine, trimethanolamine and a mixture of these amino alcohols.

7. The manufacturing method according to claim 5, wherein the aqueous basic solution has a molar concentration of between 10.sup.−7 mol/l and 5 mol/l of basic compound.

8. The manufacturing method according to claim 1, wherein the basic environment is an atmosphere saturated with basic molecules in the vapor phase, phase, by evaporation in a hermetic enclosure of a basic stock solution comprising deionized water and a basic compound chosen from N,N-diethylethanolamine, dimethylaminoethanol, aminoethanol, N-methyldiethanolamine, aminomethanol, N-methylhydroxylamine, diethanolamine, dimethanolamine, triethanolamine, trimethanolamine, ethalonamine, diethyl-N—N-ethanol, ammonia and their combination.

9. The manufacturing method according to claim 1, wherein the first substrate and the second substrate are each formed by a material chosen from semiconductors, LNO, LTO and their combination.

10. The manufacturing method according to claim 1, wherein: the first substrate and the second substrate provided in step a) each comprise a silicon substrate having a diameter of between 25 mm and 300 mm, and in which the first bonding surface and the second bonding surface are each completely formed by a continuous hydrophilic film made of silicon oxide, step c) comprises disposing the direct bonding interface, obtained in step b), in the basic environment over a duration of between 21 and 40 days, the basic environment being an aqueous basic solution formed by dissolution of NaOH and having a molar concentration of between 10.sup.−7 mol/l mol/l, and step d) comprises applying a thermal treatment at approximately 300° C., so as to obtain a direct bonding between the first substrate and the second substrate having a bonding energy greater than 5 J/m.sup.2.

Description

[0067] Other aspects, objects and advantages of the present invention will appear better upon reading the following description of various variant embodiments thereof, given by way of non-limiting example and made with reference to the appended drawings. In the remainder of the description, for the sake of simplification, identical, similar or equivalent elements of the different embodiments bear the same reference numerals. The figures do not necessarily respect the scale of all the elements represented so as to improve their readability and in which:

[0068] FIG. 1 represents a schematic view of steps a) and b) of the method according to a first embodiment of the invention.

[0069] FIG. 2 represents a schematic view of step c) of the method according to the first embodiment of the invention.

[0070] FIG. 3 represents a schematic view of step d) of the method according to the first embodiment of the invention.

[0071] FIG. 4 represents a schematic view of steps a) and b) of the method according to a second embodiment of the invention.

[0072] FIG. 5 represents a schematic view of step c) of the method according to the second embodiment of the invention.

[0073] FIG. 6 represents a schematic view of step d) of the method according to the second embodiment of the invention.

[0074] FIG. 7 represents a schematic view of step c) of the method according to one variant embodiment of the invention.

[0075] As illustrated in FIGS. 1 to 3, the direct bonding method of the present invention comprises the steps of bringing a first substrate 1 and a second substrate 2 into contact (FIG. 1 steps a and b), a step of disposing in a basic environment, namely a basic aqueous solution 8 whose pH is strictly greater than 7.5 (FIG. 2 step c) and a bonding annealing thermal treatment step so as to obtain a multilayer structure 100 having a bonding energy greater than that obtained without step c) of disposing in a basic environment (FIG. 3 step d). The first and second substrates 1,2 are made of silicon and have a diameter of 200 mm and a thickness of 725 micrometers. These two substrates 1,2 respectively comprise a first bonding surface 3 made of a native oxide of silicon (not visible in the figures) and a second bonding surface 4 made of thermal oxide of silicon (hydrophilic film 5 of oxide of 145 nm thick). The first and second surfaces 3,4 are prepared before the contacting by a cleaning with ozonated water, SC1 (mixture of 30% ammonia, 30% hydrogen peroxide and deionized water in the respective volume proportions 1:1:5) followed by SC2 (mixture of 30% hydrochloric acid, 30% hydrogen peroxide and water in the respective volume proportions 1:1:5). These cleanings make it possible to remove the organic and particulate contamination which is very detrimental to the direct bonding. According to a variant not illustrated, the step of preparing the surfaces before the contacting comprises a conventional plasma treatment.

[0076] The first and second bonding surfaces 3, 4 are then brought into contact for a spontaneous direct bonding (step b). The direct bonding interface 6 of the multilayer assembly 7 thus obtained is disposed in a basic environment consisting of a basic aqueous solution 8 of NaOH in deionized water with a molar concentration of approximately 10.sup.−3 mol/l (step c).

[0077] The immersion of the multilayer assembly 7 in the basic aqueous solution 8 is maintained for 30 days at the end of which the multilayer assembly 7 is subjected to a bonding annealing thermal treatment at 300° C. (step d—the rise in temperature from the ambient temperature at 1° C./min to 300° C. for 2 hours). The measurement of the energy of bonding to the multilayer structure 100 obtained leads to the breakage of the silicon substrates 1,2 and not to the detachment of the substrates 1,2. This indicates that the obtained bonding energy is greater than 5 J/m2 which is the rupture energy of silicon. The same method carried out without step c) of immersion in a basic environment leads to a bonding energy of 4 J/m2 after annealing at 500° C.

[0078] According to an alternative not illustrated, one of the two substrates 1,2 provided in step a) has an embrittlement plane. The bonding annealing thermal treatment contributes to the thermal budget allowing a fracture at the embrittlement plane. The obtained multilayer structure 100 then comprises one of the two substrates 1,2 bonded to a transferred thin layer originating from the fracture and a negative of the other of the substrates 1,2.

[0079] According to a variant not illustrated, the immersion time of the direct bonding interface 6 in the basic environment is approximately 5 hours for 25 mm/l substrates (15 days for 200 mm diameter substrates). The duration of the immersion is also variable according to the nature of the substrates 1,2.

[0080] As illustrated in FIG. 4, a silicon oxide film 5 may also form the bonding surfaces 3,4 of the first and second substrates 1,2. FIG. 4 illustrates the two bonding surfaces 3,4 brought into contact so as to form the direct bonding interface 6. FIG. 5 illustrates step c) of the method which consists in disposing the multilayer assembly 7 obtained at the step b) in a basic environment formed by an aqueous basic solution 8 comprising the amino alcohol DMAE (acronym for 2-(dimethylamino)ethanol) having a molar concentration of 10.sup.−2 mol/I. The basic environment 8 covers the level of the direct bonding interface 6 and the immersion is maintained for 20 days. Then, the bonding reinforcement thermal treatment is applied at 200° C. to the multilayer assembly 7 for 3 hours according to step d) of the method.

[0081] These operations make it possible to obtain a reinforced bonding energy, in particular of more than 5 J/m2 at any point of the direct bonding interface 6 (value obtained by observing the rupture of the silicon during the application of the double lever method). When the same direct bonding method is carried out without step c), it is possible to separate the first and second substrates when applying the double lever method.

[0082] According to a variant embodiment illustrated in FIG. 7, the assembly 7 obtained in step b) is disposed in a basic environment formed by an atmosphere saturated with basic molecules in the vapor phase 8′. To this end, the method provides for the preparation of a hermetic enclosure 9 saturated with basic molecules in the vapor phase by evaporation of a basic stock solution 11 of ethalonamine having a concentration of 10.sup.−4 mol/l for one hour. Then the assembly is placed in the enclosure 9 for twice as long as with immersion (step c). After immersion of the direct bonding interface 6 in this atmosphere saturated with basic molecules in the vapor phase 8′, the assembly 7 is subjected to a thermal treatment at 300° C. for 2 hours (step d) to reinforce the bonding energy. At the end, the bonding energy measured by the double lever method is greater than 5 J/m.sup.2.

[0083] According to another possibility not illustrated, the two bonding surfaces 3,4 are plasma treated before the contacting according to step b) of the method and the thermal annealing according to step d) is applied at a temperature of between approximately 20 and 250° C., for example at 50° C., for a few hours.

[0084] According to a variant not illustrated in the figures, the first and/or second substrates 1 and 2 are/is made of a material chosen from Ge, InP, AsGa, SiC, GaN, which have a bonding surface made of a hydrophilic film such as a native oxide of the considered material, LNO and LTO which intrinsically have a hydrophilic bonding surface.

[0085] According to yet another variant not illustrated, the first substrate 1 provided in step a) is vignetted in several first vignettes, the exposed faces of which are first bonding surfaces 3. The first vignettes are bonded according to the method previously described on the second substrate 2 (full plate) according to a chip-plate bonding also known by the expression ‘chip to wafer’. According to yet another variant not illustrated, the second substrate 2 is also vignetted in several second vignettes and the method of the invention allows the direct bonding of the first vignettes and the second vignettes.

[0086] According to an alternative not shown, the first bonding surface 3 and the second bonding surface 4 are prepared so as to have copper-oxide bondable hybrid surfaces in direct bonding. These first and second hydrophilic bonding surfaces 3,4 are typically composed of copper pads with sides of 2.5 micrometers separated by 2.5 micrometers of SiO.sub.2. We then speak of a hybrid surface with a “pitch” of 5 micrometers. Then steps b) to d) of the method are reproduced as previously described.

[0087] Thus, the present invention proposes a method for manufacturing a multilayer structure 100 including a direct bonding between two substrates 1,2 having a high bonding energy, and making it possible to limit the temperature of the post-bonding thermal annealing. The preparation of a basic environment is inexpensive and the immersion step c) is applicable to many materials. It is in particular possible to bond substrates (or thick layers) of materials having a significant difference in thermal expansion coefficient. Moreover, when the materials of the first and second substrates 1,2 include devices, these are not damaged by the used temperatures.

[0088] It goes without saying that the invention is not limited to the variant embodiments described above by way of example but that it comprises all the technical equivalents and the variants of the means described as well as their combinations.