Method of fabricating bonded wafer

A method of fabricating a bonded wafer which is capable of reducing the concentrations of impurities, and more particularly the boron concentration, at the interface of bonding in the bonded wafer, wherein first and second wafers to be bonded are finish-cleaned, then the wafers are temporarily stored in a closed box so as to isolate the wafers from clean-room air, thereafter the first and second wafers are superposed in a clean atmosphere which is held out of direct contact with clean-room air, and finally the superposed first and second wafers are bonded together by a heat-treatment.

What is claimed is:

1. A method of fabricating a bonded wafer, comprising the steps of:

(a) finish-cleaning first and second wafers;

(b) superposing the finish-cleaned first and second wafers in a clean atmosphere which is held out of direct contact with clean-room air in a superposing apparatus comprising an enclosed box, means for supplying fluid to the inside of the enclosed box, and a turntable positioned inside the enclosed box, said clean atmosphere consisting essentially of ultrapure N.sub.2 gas, thereby preventing boron in the clean-room air from depositing on the wafers; and

(c) heat-treating the superposed first and second wafers to bond them together at an interface between said first and second wafers,

thereby obtaining a bonded wafer in which said interface has a boron concentration which is lower than if said superposing were conducted in said clean-room air.

2. A method of fabricating a bonded wafer, comprising the steps of:

(a) finish-cleaning first and second wafers;

(b) temporarily storing the finish-cleaned first and second wafers in a closed box so as to isolate the first and second wafers from clean-room air, thereby preventing boron in the clean-room air from depositing on the wafers;

(c) thereafter, superposing the first and second wafers in a clean atmosphere which is held out of direct contact with clean-room air in a superposing apparatus comprising an enclosed box, means for supplying fluid to the inside of the enclosed box, and a turntable positioned inside the enclosed box, said clean atmosphere consisting essentially of ultrapure N.sub.2 gas; and

(d) heat-treating the superposed first and second wafers to bond them together at an interface between said first and second wafers,

thereby obtaining a bonded wafer in which said interface has a boron concentration which is lower than if said superposing were conducted in said clean-room air.

3. A method of fabricating a bonded wafer, comprising the steps of:

(a) finish-cleaning first and second wafers;

(b) temporarily storing the finish-cleaned first and second wafers in a closed box so as to isolate the first and second wafers from clean-room air, thereby preventing boron in the clean-room air from depositing on the wafers;

(c) thereafter, rinsing the first and second wafers;

(d) superposing the rinsed first and second wafers in a clean atmosphere which is held out of direct contact with clean-room air in a superposing apparatus comprising an enclosed box, means for supplying fluid to the inside of the enclosed box, and a turntable positioned inside the enclosed box, said clean atmosphere consisting essentially of ultrapure N.sub.2 gas; and

(e) heat-treating the superposed first and second wafers to bond them together at an interface between said first and second wafers,

thereby obtaining a bonded wafer in which said interface has a boron concentration which is lower than if said superposing were conducted in said clean-room air.

4. A method of fabricating a bonded wafer, comprising the steps of:

(a) finish-cleaning first and second wafers;

(b) submerging the finish-cleaned first and second wafers in water so as to isolate the finish-cleaned first and second wafers from clean-room air, thereby preventing boron in the clean-room air from depositing on the wafers;

(c) drying the stored first and second wafers in a clean atmosphere which is held out of direct contact with clean-room air, said clean atmosphere consisting essentially of ultrapure N.sub.2 gas;

(d) immediately after the drying, superposing the first and second wafers in said clean atmosphere; and

(e) heat-treating the superposed first and second wafers to bond them together at an interface between said first and second wafers,

thereby obtaining a bonded wafer in which said interface has a boron concentration which is lower than if said superposing were conducted in said clean-room air.

5. A method of fabricating a bonded wafer as recited in claim 4, wherein said superposing the first and second wafers in step (d) is carried out in a superposing apparatus comprising an enclosed box, means for supplying fluid to the inside of the enclosed box, and a turntable positioned inside the enclosed box.

6. A method as recited in claim 1, wherein said interface has a boron concentration less than 10.sup.16 atoms/cm.sup.3.

7. A method as recited in claim 1, wherein said interface has a boron concentration less than 5.times.10.sup.14 atoms/cm.sup.3.

8. A method as recited in claim 1, wherein said interface has a boron concentration of about 2.times.10.sup.15 atoms/cm.sup.3 or less.

9. A method as recited in claim 1, wherein said interface has a boron concentration of about 10.sup.15 atoms/cm.sup.3 or less.

10. A method as recited in claim 1, wherein said wafers are wet when they are being placed in said superposing apparatus.

11. A method as recited in claim 2, wherein said wafers are wet when they are being placed in said superposing apparatus.

12. A method as recited in claim 3, wherein said wafers are wet when they are being placed in said superposing apparatus.

13. A method as recited in claim 2, wherein said interface has a boron concentration less than 10.sup.16 atoms/cm.sup.3.

14. A method as recited in claim 3, wherein said interface has a boron concentration less than 10.sup.16 atoms/cm.sup.3.

15. A method as recited in claim 4, wherein said interface has a boron concentration less than 10.sup.16 atoms/cm.sup.3.

16. A method as recited in claim 4, wherein said interface has a boron concentration less than 5.times.10.sup.14 atoms/cm.sup.3.

17. A method as recited in claim 4, wherein said interface has a boron concentration of about 2.times.10.sup.15 atoms/cm.sup.3 or less.

18. A method as recited in claim 2, wherein said interface has a boron concentration of about 10.sup.15 atoms/cm.sup.3 or less.

19. A method as recited in claim 3, wherein said interface has a boron concentration of about 10.sup.15 atoms/cm.sup.3 or less.

20. A method as recited in claim 4, wherein said interface has a boron concentration of about 10.sup.15 atoms/cm.sup.3 or less.