# Patent

## Title
Method of fabricating a semiconductor structure

## Abstract

A method of fabricating a semiconductor structure, in which after an etching process is performed to form at least one recess within a semiconductor beside a gate structure, a thermal treatment is performed on the recess in a gas atmosphere including an inert gas before a silicon-containing epitaxial layer is formed in the recess through an epitaxy growth process.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor structure, and particularly to a method of fabricating a semiconductor structure during which rounding problem of an epitaxial layer can be avoided or reduced.
2. Description of the Prior Art
As the semiconductor processes advance to very deep sub micron era such as 65-nm node, even 28 nm or beyond, how to increase the driving current for MOS transistors has become a critical issue. In order to improve device performance, strained-silicon technology has been developed. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes MOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.
One of the methods to put a strain on the silicon lattice of the gate channel is that combining a selective epitaxial growth (SEG) technology. Recesses are formed in the substrate beside the gate structure and then an epitaxial layer, such as silicon germanium (SiGe) layer having a crystal lattice arrangement the same as that of the substrate is formed within the recesses through the epitaxial growth process to serve as a source/drain, so as to apply a stress to the crystal lattice of the gate channel.
FIG. 1 is a schematic, cross-sectional view illustrating a conventional semiconductor structure. As shown in FIG. 1, in the semiconductor structure 10, a gate structure 14 is formed on a substrate 12. The gate structure 14 includes a gate dielectric 14a, a gate electrode 14b, and a spacer 14c. Two recesses 16 (the dotted line) are formed in the substrate 12 adjacent to two sides of the gate structure 14. An epitaxial layer 18 is formed within the recesses 16 to serve as a source/drain region. Thereafter, a metal silicide layer 20 is formed on each up surface of the source/drain region and the gate structure 14. However, the resultant shape of the epitaxial layer 18 often becomes round without straight sides and it deviates very much from the original shape of the recess 16. In such result, the length of the gate channel 22 between the two recesses 16 under the gate structure 14 becomes longer than a predetermined one, and the stress effect of the epitaxial layer 18 to the gate channel 22 is reduced.
Therefore, there is still a need for a novel method of fabricating a semiconductor structure to solve the aforesaid issue.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a method of fabricating a semiconductor structure to avoid or reduce the rounding of the shape of the epitaxial layer.
The method of fabricating a semiconductor structure according to the present invention comprises steps as follows. First, a semiconductor substrate is provided. Next, a gate structure is formed on the semiconductor substrate. An etching process is performed to form at least one recess within the semiconductor substrate beside the gate structure. Thereafter, a thermal treatment process is performed on the recess in a gas atmosphere including an inert gas at a first temperature. After the thermal treatment process is performed, an epitaxy growth process is performed at a second temperature to form a silicon-containing epitaxial layer in the recess. During or after the epitaxy growth process, a dopant is incorporated into the silicon-containing epitaxial layer to form a source/drain region.
Without being bound to a theory, since, in the present invention, the recess is subject to a thermal treatment process in a gas atmosphere including an inert gas after it is formed, to alter or modify the bonding or adsorption relation of hydrogen (including atoms or ions) to the silicon of the semiconductor substrate, for example, to result in desorption of the hydrogen or alteration of the bonding type between the hydrogen and the silicon, such that the affect of the hydrogen during the subsequent epitaxy growth process can be alleviated or avoided. Accordingly, the resultant silicon-containing epitaxial layer grown in the recess may have a shape approximating to a designed or desired shape.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross sectional view illustrating a semiconductor structure obtained using a conventional method;
FIG. 2 is a presumed model;
FIG. 3 is a flowchart illustrating an embodiment of the method of fabricating a semiconductor structure according to the present invention;
FIGS. 4 and 5 are schematic cross sectional views for illustrating an embodiment of a method of fabricating a semiconductor structure according to the present invention; and
FIG. 6 is a flow chart illustrating another embodiment of the method of fabricating a semiconductor structure according to the present invention.



DETAILED DESCRIPTION
The inventors investigated the rounding issue occurred in the conventional method and discovered that it may be resulted from the affect of the hydrogen to silicon migration at the high temperature of the epitaxy growth process. As shown in the presumed model of FIG. 2, hydrogen atoms or ions, provided from ambient atmosphere, substance from previous process, or cleaning solution containing hydrogen gas (H2), hydrogen atoms, or hydrogen ions, are adsorbed on surface of the silicon substrate 13 or form chemical bonds with silicon atoms, especially at steps or corners of the silicon substrate 13. The hydrogen may trigger the migration of silicon atoms. Accordingly, when an epitaxy growth process is being carried out, silicon atoms located at these steps or in these corners migrate due to the existence of the hydrogen, such that the resultant epitaxial layer is significantly influenced to have a shape different from the shape of the recess originally formed. It may be noted that the word “hydrogen” is used sometimes herein to encompass hydrogen gas, hydrogen atom, and/or hydrogen ion for concise description.
For resolving the above-mentioned issue, a method of fabricating a semiconductor structure the present invention is provided and featured in that, after the recess is formed, a high-temperature thermal treatment is performed on the recess in a gas atmosphere including an inert gas to reduce or avoid silicon migration triggered by hydrogen during subsequent high-temperature epitaxy growth process. Presumably, high-temperature thermal treatment might alternate the adsorption or the bonding of the hydrogen within the recess (especially those hydrogen atoms located at the steps and in the corners) to the silicon. For example, the hydrogen is desorbed or the bonding type is altered, such that the hydrogen can not trigger the silicon migration anymore. In another presumption, the alternation of the adsorption or the bonding of the hydrogen might be resulted from a reaction with high-temperature oxygen. As a result, the method of the present invention may be advantageously utilized to fabricate MOS transistor structures, including pMOS and nMOS.
An embodiment of the method of fabricating a semiconductor structure according to the present invention is described hereinafter referring to FIG. 3 together with FIGS. 4 and 5. First, Step 101 is performed to provide a semiconductor substrate 12, such as silicon substrate. Next, Step 103 is performed to form a gate structure 14 on the substrate 12. The gate structure 14 may include a gate dielectric 14a, a gate electrode 14b, and a spacer 14c. The spacer 14c may be a single layer or a multilayer. Before the spacer 14c is formed, a lightly-doped source/drain (LDD) region 15 may be optionally formed through for example ion implantation.
Thereafter, Step 105 is performed to form recesses 17 in the substrate 12 beside the gate structure 14 through an etching process, as shown in FIG. 4. The shape of the recess in the present invention is not particularly limited and can be designed as required. For example, the cross section of the recess may have a shape of polygon, for example, a polygon having four or more sides, such as octagon, hexagon, diamond shape, or pentagon. The polygon may have at least one angle further in a shape of curved angle or chamfered angle as desired.
Thereafter, Step 107 is carried out to perform a thermal treatment process on the recesses 17 in a gas atmosphere comprising an inert gas at a first temperature. Amain objective of the thermal treatment process at high temperature is to destroy the adsorption or bonding of hydrogen to avoid or reduce influence of hydrogen to silicon, and thus it is not necessary for the thermal treatment to be long. For example, conventional rapid thermal process, spike thermal process, or spike laser thermal process may be utilized. The first temperature may be in a range from 850° C. to 1050° C., and preferably about 930° C. If the time for the thermal treatment is long, other elements on the substrate may be badly affected, and accordingly such situation should be avoided. The thermal treatment is performed in a gas atmosphere comprising an inert gas. The gas atmosphere may be one which is frequently utilized in a conventional semiconductor manufacturing process. For example, the gas atmosphere may be one, under normal pressure (1 atm), including an inert gas (which may be a single gas or a gas mixture) in a high concentration and an oxygen gas in an extremely low concentration. The inert gas may include nitrogen gas, helium, argon, and the like. The oxygen gas in an extremely low concentration is an amount of oxygen gas which typically and inevitably exists in the high-concentration inert gas prepared by the current technology. For example, the concentration of the oxygen gas may be less than 100 ppm. Therefore, the situation that an extremely low concentration of oxygen gas included in the gas atmosphere comprising an inert gas is not excluded from the present invention.
Furthermore, as shown in FIG. 6, in another embodiment of the present invention, after the thermal treatment process in Step 107 is performed, Step 108 may be performed optionally to wet-clean the recesses 17. Cleaning solution may include for example a diluted hydrogen fluoride solution, which is commonly utilized to remove native oxide layer in the industry.
Following Step 107 or Step 108 (if performed), Step 109 is carried out to perform an epitaxy growth process at a second temperature to grow a silicon-containing epitaxial layer 24 in each recess 17. The epitaxy growth process may be for example a selective epitaxy growth process. The silicon-containing epitaxial layer 24 may grow along the surface of the recess 17 and slightly protrude from the up surface of the substrate 12. The silicon-containing epitaxial layer may include for example those materials applicable to conventional strained silicon technology, such as silicon germanium (SiGe) or silicon carbide (SiC). The amount of germanium or carbon in the epitaxial layer may be as desired, for example, in a range from more than 0% to 50%, based on a total atom amount as 100%. The second temperature employed in the epitaxy growth process is preferably lower than the first temperature employed in the thermal treatment for the recess, but not limited thereto. The second temperature may be for example 600° C. to 900° C. For example, the substrate is pre-baked at 800° C., and the epitaxy growth is carried out in a range from 600° C. to 650° C.
Step 111 to form a source/drain may be performed during or after the epitaxy growth process of Step 109. The case that Step 111 and Step 109 are performed at the same time may be as follows for example. One is that the dopant of a desired concentration and the silicon-containing epitaxial material are allowed to epitaxially deposit within the recesses 17 together, to form source/drain regions. The concentration of the dopant may be varied by design according to the depth of the dopant in the epitaxial layer. Another is that the dopant may be from the substrate 12 (for example the source of the dopant may be implanted into the substrate 12 beside the gate structure 14 through ion implantation after the aforesaid spacer 14c is formed) and diffuse upon being heated during the epitaxial growth process to be incorporated into the silicon-containing epitaxial layer 24 to form source/drain regions. Alternatively, the case that Step 111 is performed following the epitaxy growth process of Step 109 may be for example that the dopant is doped into the silicon-containing epitaxial layer 24 through an ion implantation carried out on the silicon-containing epitaxial layer 24 to form source/drain regions.
Thereafter, a self-aligned metal silicide (salicide) process may be performed to form a metal silicide layer 20, such as a nickel silicide layer, on the silicon-containing epitaxial layer 24 and the gate electrode 14b (if it is polysilicon), giving a semiconductor structure 30 as shown in FIG. 5.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

## Claims

1. A method of fabricating a semiconductor structure, comprising:
providing a semiconductor substrate;
forming a gate structure on the semiconductor substrate;
performing an etching process to form at least one recess within the semiconductor substrate beside the gate structure;
performing a thermal treatment process on the recess in a gas atmosphere comprising an inert gas at a first temperature;
after performing the thermal treatment process, wet-cleaning the recess;
after wet-cleaning the recess, performing a pre-bake process on the recess at a second temperature lower than the first temperature;
after performing the pre-bake process, performing an epitaxy growth process at a third temperature lower than the first temperature to form a silicon-containing epitaxial layer in the recess; and
during or after the epitaxy growth process, incorporating a dopant into the silicon-containing epitaxial layer to form a source/drain region.

2. The method according to claim 1, wherein the thermal treatment process comprises a rapid thermal process, a spike thermal process, or a spike laser thermal process.

3. The method according to claim 1, wherein one cross section of the recess is substantially in a shape of polygon.

4. The method according to claim 3, wherein the polygon has at least one angle in a shape of curved angle or chamfered angle.

5. The method according to claim 1, wherein the epitaxy growth process comprises a selective epitaxy growth process.

6. The method according to claim 1, further comprising:
performing a self-aligned metal silicide process to form a metal silicide layer on the silicon-containing epitaxial layer.

7. The method according to claim 1, wherein the first temperature is in a range from 850° C. to 1050° C.

8. The method according to claim 1, wherein the second temperature and the third temperature are each in a range from 600° C. to 900° C.

9. The method according to claim 1, wherein the silicon-containing epitaxial layer comprises silicon-germanium (SiGe) or silicon-carbide (SiC).

10. The method according to claim 1, wherein, forming the gate structure comprises steps of forming a gate dielectric, a gate electrode, and a spacer.

11. The method according to claim 1, wherein wet-cleaning the recess includes utilizing a hydrogen fluoride solution to clean the recess.