Process for smoothing the surface of a semiconductor-on-insulator substrate

Abstract

A method of fabricating a semiconductor substrate includes the following activities: a) providing a donor substrate with a weakened zone inside the donor substrate, the weakened zone forming a border between a layer to be transferred and the rest of the donor substrate, b) attaching the donor substrate to a receiver substrate, the layer to be transferred being located at the interface between the donor substrate and the receiver substrate; c) detaching the receiver substrate along with the transferred layer from the rest of the donor substrate, at the weakened zone; and d) at least one step of smoothing the surface of the transferred layer, wherein the semiconductor substrate obtained from step c) is kept, at least from the moment of detachment until the end of the smoothing step, in a non-oxidizing inert atmosphere or in a mixture of non-oxidizing inert gases. Semiconductor substrates are fabricated using such a method.

Claims

1. A method of fabricating a semiconductor substrate including a transferred thin layer, the method comprising: providing a donor substrate with a weakened zone inside the donor substrate, the weakened zone forming a border between a thin layer to be transferred and the rest of the donor substrate; and attaching the donor substrate to a receiver substrate, the thin layer to be transferred being located at the interface between the donor substrate and the receiver substrate; and detaching the receiver substrate and the thin layer from the rest of the donor substrate, at the weakened zone, and forming a semiconductor substrate comprising the receiver substrate and a transferred thin layer; and at least one step of smoothing carried out on the surface of the transferred thin layer, in which the semiconductor substrate comprising the receiver substrate and a transferred thin layer is kept, at least from the moment of detachment until the end of the smoothing step, in a non-oxidizing inert atmosphere or in a mixture of non-oxidizing inert gases, wherein the at least one smoothing step is carried out at a temperature of 900° C. or less.

2. The method of claim 1, wherein the detaching of the receiver substrate and the thin layer from the rest of the donor substrate and the at least one step of smoothing are carried out in a non-oxidizing inert atmosphere or in a mixture of non-oxidizing inert gases.

3. The method of claim 2, wherein the non-oxidizing inert atmosphere or the mixture of non-oxidizing inert gases has a level of O.sub.2 lower than 10 ppm.

4. The method of claim 3, wherein the at least one step of smoothing is carried out directly after detaching the receiver substrate and the thin layer from the rest of the donor substrate.

5. The method of claim 4, wherein the detaching of the receiver substrate and the thin layer from the rest of the donor substrate and the at least one step of smoothing are carried out in one and the same device.

6. The method of claim 5, wherein the detaching of the receiver substrate and the thin layer from the rest of the donor substrate and the at least one step of smoothing are carried out in one and the same oven.

7. The method of claim 6, wherein the at least one step of smoothing is carried out for at least one hour.

8. The method of claim 7, wherein the at least one step of smoothing is carried out for from one to three hours.

9. The method of claim 8, wherein the detaching of the receiver substrate and the thin layer from the rest of the donor substrate is carried out by applying a thermal treatment to the donor substrate to cause fracture along the weakened zone.

10. The method of claim 9, wherein the at least one smoothing step is carried out at a temperature of from 650° C. to 900° C.

11. The method of claim 10, wherein the donor substrate comprises at least one material selected from among silicon, monocrystalline silicon, germanium, and SiGe.

12. The method of claim 11, wherein the receiver substrate comprises at least one material selected from among silicon and monocrystalline silicon.

13. The method of claim 12, wherein a RMS surface roughness of the transferred thin layer is lower than 0.3 nm after the at least one smoothing step.

14. The method of claim 1, wherein the at least one step of smoothing is carried out directly after detaching the receiver substrate and the thin layer from the rest of the donor substrate.

15. The method of claim 1, wherein the detaching of the receiver substrate and the thin layer from the rest of the donor substrate and the at least one step of smoothing are carried out in one and the same device.

16. The method of claim 1, wherein the at least one step of smoothing is carried out for at least one hour.

17. The method of claim 1, wherein the detaching of the receiver substrate and the thin layer from the rest of the donor substrate is carried out by applying a thermal treatment to the donor substrate to cause fracture along the weakened zone.

18. The method of claim 1, wherein a RMS surface roughness of the transferred thin layer is lower than 0.3 nm after the at least one smoothing step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure may be understood by referring to the following description together with the appended figures, in which numerical references identify the elements of the disclosure.

(2) FIG. 1 schematically shows the various steps of the fabrication process according to one embodiment of the disclosure.

(3) FIG. 2 shows the levels of roughness of substrates that have or have not undergone various types of smoothing operations after the detachment step.

DETAILED DESCRIPTION

(4) The fabrication process of the disclosure is described in detail by FIG. 1. It comprises the steps of formation, detachment and transfer of a thin layer from a donor substrate to a receiver substrate and a step of smoothing carried out on the surface of the transferred thin layer, in steps A to E.

(5) FIG. 1A shows a donor substrate 1, preferably a silicon or monocrystalline silicon substrate, but also a germanium or silicon-germanium (SiGe) substrate, with or without its surface oxide, with a surface 3.

(6) FIG. 1A shows a receiver substrate 5 with a surface 7. The receiver substrate 5 may be a silicon, a monocrystalline silicon, or else any other type of base substrate, in particular, a substrate that withstands temperatures higher than 700° C., with or without its surface oxide.

(7) Next, such as illustrated in FIG. 1B, the donor substrate 1 is subjected to an implantation of ionic or atomic species 9. This implantation process introduces the implanted atomic species 9 into the donor substrate 1 with a maximum concentration at a predetermined depth of the donor substrate 1 with respect to the bombarded surface 3, with a view to creating a weakened zone 11. The weakened zone 11 forms the border between a thin layer 13 and the rest 15 of the donor substrate 1.

(8) The implantation of ionic or atomic species 9 may be a single implantation operation, i.e., the implantation of a single atomic species, such as, for example, an operation of implanting hydrogen, helium or any other noble gas. The implantation may also be a co-implantation of ionic or atomic species 9, i.e., an operation of implanting at least two different species, such as, for example, the co-implantation of hydrogen and helium.

(9) FIG. 1C shows the step in which the receiver substrate 5 is brought into contact, via one of its faces 7, with the face 3 of the donor substrate 1. The bond between the two substrates 1 and 5 is formed by molecular adhesion and the bond interface bears the reference 17 in order to form a multilayer stack 19.

(10) FIG. 1D shows the step of detachment from the rest 15 of the donor substrate 1, along the weakened zone 11, so as to transfer the thin layer 13 onto the receiver substrate 5 in order to create the semiconductor substrate 21.

(11) Detachment may be carried out by means of a thermal or a mechanical treatment, or else a treatment involving both a thermal treatment and a mechanical treatment.

(12) By way of example, thermal detachment subjects the multilayer stack 19, illustrated in FIG. 1C, to thermal annealing, over the course of which a spontaneous detachment from the rest 15 of the donor substrate 1 takes place along the weakened zone 11. This thermal detachment operation is typically carried out in an oven 23 at temperatures between 100° C. and 600° C., preferably at about 500° C. In an alternative, a mechanical detachment operation may be achieved, for example, by using a blade on the weakened zone 11.

(13) The atmosphere of the oven 23 is non-oxidizing, with levels of O.sub.2 contamination of the order of 10 ppm or better. Thus, the surface 25 of the transferred thin layer 13 remains a surface free of surface oxide.

(14) FIG. 1E shows the smoothing step according to the disclosure in which the smoothing of the surface 25 of the transferred thin layer 13 of the semiconductor substrate 21 is carried out by means of a thermal treatment in an oven. Here, the oven used is the same oven 23 as that already used for the step illustrated in FIG. 1D.

(15) According to the disclosure, this thermal treatment is carried out at a temperature that is lower than 900° C., in particular, within a temperature range from 650° C. to 900° C., in a non-oxidizing inert atmosphere or a mixture of non-oxidizing inert gases, in particular, with a level of O.sub.2 that is lower than 10 ppm. The one or more inert gases may be, for example, hydrogen, argon or xenon. The thermal treatment is applied for at least one hour, preferably from one to three hours.

(16) The atmosphere between the steps must remain a non-oxidizing inert atmosphere in order to avoid the formation of a surface oxide.

(17) The semiconductor substrate 21 exhibits an RMS roughness obtained for the surface 25 of the transferred thin layer 13 that is lower than 1 nm over the surface of the semiconductor substrate. The roughness measurements are, for example, carried out using an atomic force microscope (AFM) or by means of a haze measurement.

(18) FIG. 2 shows the levels of haze roughness reached for substrates that have or have not undergone various types of smoothing operations after the detachment step of the fabrication process according to the disclosure.

(19) This haze value, measured in ppm, is the result of a method that uses the optical reflection properties of the surface to be characterized. A “HAZESUP” measurement corresponds to the maximum value of the haze on a wafer and is a very good indicator of the level of roughness of a substrate. The “HAZESUP” measurements illustrated in FIG. 2 have been made using an optical reflectometry instrument.

(20) FIG. 2 shows the results of the “HAZESUP” measurements for seven substrates 21a to 21g, all of which have undergone the formation and detachment steps of the fabrication process according to the disclosure.

(21) The substrate 21a exhibits a “HAZESUP” value of the order of 4000 ppm, having been measured directly after the detachment step, in a neutral atmosphere of argon alone with a level of O.sub.2 that is lower than 10 ppm, condition A in FIG. 2.

(22) The substrates 21b, 21c and 21d exhibit a “HAZESUP” value of between 280 ppm and 400 ppm, having been measured after a step of smoothing carried out at 650° C. for two hours in a neutral atmosphere of argon alone with a level of O.sub.2 that is lower than 10 ppm, condition B in FIG. 2.

(23) The substrate 21e exhibits a “HAZESUP” value of between 50 ppm and 60 ppm, having been measured after a step of smoothing carried out at 850° C. for two hours in a neutral atmosphere of argon alone with a level of O.sub.2 that is lower than 10 ppm, condition C in FIG. 2. This level is comparable to a 30 s RTA thermal treatment, carried out at a temperature of 1100° C.

(24) The substrates f and g exhibit a “HAZESUP” value of the order of 4000 ppm, having been measured directly after the detachment step, in ambient atmosphere, condition D in FIG. 2.

(25) As was to be expected from the prior art, the substrates (here a, f and g) having undergone the formation and detachment steps without subsequently having undergone a thermal smoothing treatment exhibit high “HAZESUP” values, of the order of 4000 ppm, regardless of whether the “HAZESUP” measurements are carried out in ambient or a neutral atmosphere, while the substrates that have undergone a thermal smoothing treatment according to the disclosure after detachment exhibit a substantial decrease in the “HAZESUP” value (substrates b to e).

(26) Thus, a smoothing operation carried out at 850° C. for 2 h (substrate e) gives a “HAZESUP” value of the same order as the “HAZESUP” value obtained after an RTA thermal treatment at 1100° C. for 30 s.

(27) Thus, a degree of smoothing that is equivalent to that obtained for an RTA thermal treatment is obtained using a thermal smoothing treatment at 850° C., hence at a much lower temperature, in a non-oxidizing atmosphere and directly after having carried out detachment.

(28) A certain number of embodiments of the invention have been described. However, it will be appreciated that various modifications and improvements may be made without departing from the scope of the following claims.