# Patent

## Title
Fabrication method of semiconductor device

## Abstract

A fabrication method of a semiconductor device includes: forming a gate insulating film and a gate electrode on an N type well; forming first source/drain regions by implanting a first element in regions of the N type well on both sides of the gate electrode, the first element being larger than silicon and exhibiting P type conductivity; forming second source/drain regions by implanting a second element in the regions of the N type well on the both sides of the gate electrode, the second element being smaller than silicon and exhibiting P type conductivity; and forming a metal silicide layer on the source/drain regions.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority from Japanese Patent Application JP2008-018856 filed on Jan. 30, 2008, the disclosure of which application is hereby incorporated by reference into this application in its entirety for all purposes.
BACKGROUND OF THE INVENTION
A technology disclosed in this specification relates to a fabrication method of a semiconductor device, specifically to a method of forming a source/drain of a microfabricated P channel type MIS transistor in which nickel silicide is used.
In recent years, the structure in which impurity diffusion layers constituting source/drain regions are formed as thinly as possible has been adopted to suppress short-channel effects associated with the microfabrication of transistors. However, the reduction in depth of the impurity diffusion layers results in increase in sheet resistance of the impurity diffusion layers. As a result, it becomes difficult to achieve high-speed operation and low power consumption. Considering this, recent semiconductor devices adopt a silicide formation process in which a metal film is formed on a source/drain region and subjected to a heat treatment to make the metal and silicon react with each other and thereby form a silicide film. Many of the semiconductor devices whose gate length is 100 nm or less particularly adopt a nickel silicide formation process, which has the following advantages: (1) a nickel silicide film can be formed in a low temperature process (<600° C.) which suppresses deactivation of activated impurities and (2) nickel lends itself to formation of shallow junction because less silicon is consumed when nickel is used to react with silicon than other metals for forming silicide under the same sheet resistance.
A semiconductor device fabrication method utilizing a nickel silicide formation process is hereinafter described, with reference to FIG. 5. FIG. 5A to FIG. 5E are cross sections showing a conventional fabrication method of a semiconductor device.
First, as shown in FIG. 5A, an isolation region 1102 is formed in a silicon substrate 1101. Then, an N type well 1103a and a P type well 1103b are formed by photolithography and ion implantation.
Next, as shown in FIG. 5B, a gate oxide film is formed on the upper surface of the silicon substrate 1101 by thermal oxidation, and a non-doped polysilicon film is further formed on the gate oxide film. After that, photolithography and ion implantation are carried out to introduce boron in the non-doped polysilicon film above the N type well 1103a and phosphorus in the non-doped polysilicon film above the P type well 1103b. Then, photolithography and dry etching are carried out to form a gate oxide film 1104a and a P type gate electrode 1105a on the N type well 1103a, and a gate oxide film 1104b and an N type gate electrode 1105b on the P type well 1103b. After that, photolithography and ion implantation are carried out to introduce boron difluoride (BF2) in regions of the N type well 1103a on both sides of the P type gate electrode 1105a, thereby forming P type extension regions 1106a, and introduce phosphorus, thereby forming N type pocket regions 1107a below the P type extension regions 1106a. Similarly, arsenic is introduced in regions of the P type well 1103b on both sides of the N type gate electrode 1105b to form N type extension regions 1106b, and boron is introduced to form P type pocket regions 1107b below the N type extension regions 1106b. 
Then, as shown in FIG. 5C, a TEOS film and a silicon nitride film are formed on the silicon substrate 1101 in order. Etch back is then carried out by anisotropic etch to form a side wall 1110a composed of a TEOS film 1108a and a silicon nitride film 1109a on side surfaces of the gate oxide film 1104a and the P type gate electrode 1105a and form a side wall 1110b composed of a TEOS film 1108b and a silicon nitride film 1109b on side surfaces of the gate oxide film 1104b and the N type gate electrode 1105b. 
Then, as shown in FIG. 5D, photolithography and ion implantation are carried out to introduce boron difluoride (BF2) and boron in the N type well 1103a using a photoresist (not shown), the P type gate electrode 1105a and the side wall 1110a as a mask, thereby forming P type source/drain regions 1111a, and introduce arsenic and phosphorus in the P type well 1103b using a photoresist (not shown), the N type gate electrode 1105b and the side wall 1110b as a mask, thereby forming N type source/drain regions 1111b. 
Then, as shown in FIG. 5E, the impurities introduced in the P type gate electrode 1105a, N type gate electrode 1105b, N type well 1103a and P type well 1103b are activated by a high-temperature, high-speed heat treatment. After that, the upper surface of the silicon substrate 1101 is cleaned with a hydrofluoric acid solution, thereby removing a natural oxide film on the surface of the substrate. Then, a nickel metal film and a titanium nitride metal film are formed on the silicon substrate 1101. The silicon substrate 1101 is subjected to a heat treatment in a nitrogen atmosphere to make the nickel and silicon react with each other. The nickel metal film and the titanium nitride metal film which have not reacted with silicon are removed with a solution in which sulfuric acid and hydrogen peroxide are mixed. The silicon substrate 1101 is again subjected to a heat treatment in a nitrogen atmosphere to form nickel silicide 1112 on the P type gate electrode 1105a, N type gate electrode 1105b, P type source/drain regions 1111a, and N type source/drain regions 1111b. A semiconductor device including a P channel type MOS transistor 1201 and an N channel type MOS transistor 1202 each having nickel silicide is obtained in this way.
However, the P channel type MOS transistor using the nickel silicide formed by the above method has a problem that a junction leakage current flowing from a P type source/drain region to a well increases. The cause was investigated and found to be the nickel silicide formed on the P type source/drain region and having an inverted pyramid shape that invades the P type source/drain region. FIG. 6 is an example of a cross sectional shape of the nickel silicide formed on the P type source/drain region (Reference: The 63rd Annual Meeting of the Japanese Society of Microscopy MG20-E-1430). This kind of abnormal growth of nickel silicide occurs only in a P channel type MOS transistor. Although it has not yet been determined what causes this phenomenon, the following two points are considered to be the cause of the phenomenon: (1) remnant crystal defects in the P type source/drain region before formation of silicide and (2) deformed silicon crystal which is deformed because the P type source/drain region includes a large amount of boron whose atomic radius is smaller than the atomic radius of silicon, and as a result, abnormal diffusion of nickel is apt to occur.
A method for suppressing the abnormal growth of silicide in the P channel type MOS transistor is disclosed in Japanese Patent No. 3033528, for example. The method disclosed is a semiconductor device fabrication method including a step of forming titanium silicide on a silicon crystal containing an impurity element whose atomic radius is smaller than the atomic radius of silicon. In this method, the above impurity element is introduced in a silicon crystal containing an element whose atomic radius is greater than the atomic radius of silicon and which does not cause chemical reaction with silicon and does not generate a level in the silicon. By doing so, distortion of the silicon crystal is corrected. A titanium film is then formed on the silicon crystal and the silicon crystal with the titanium film is annealed to form titanium silicide.
SUMMARY OF THE INVENTION
However, when argon is selected as an element whose atomic radius is greater than that of silicon and which does not cause chemical reaction with silicon and does not generate a level in the silicon for forming nickel silicide by the above method, no improvement was found in the junction leakage current flowing from the P type source/drain region to the well in the P channel type transistor, and no effect was found in the nickel silicide formation process.
Another solution for the above problem may be to form the P type source/drain region deeper than the depth of the abnormally-grown nickel silicide. However, as mentioned earlier, it is difficult to increase the depth of a source/drain of a microfabricated transistor because of the need to reduce the depth of an impurity diffusion layer, which constitutes the source/drain and the like, to suppress short-channel effects.
The present invention was made to solve the above problem. An object of the present invention is to provide a fabrication method of a semiconductor device in which abnormal growth of silicide during formation of the silicide is suppressed in a P channel type MOS transistor, thereby suppressing increase injunction leakage current flowing from a source/drain region to a well, while maintaining shallow junction of the source/drain region.
A semiconductor device fabrication method of the present invention is a fabrication method of a semiconductor device which has a P channel type MIS transistor including a gate insulating film, a gate electrode, source/drain regions composed of first source/drain regions and second source/drain regions, and a metal silicide layer, the fabrication method including: (a) preparing a semiconductor substrate including an N type well surrounded by an isolation region; (b) forming the gate electrode on the N type well, with the gate insulating film interposed therebetween; (c) forming the first source/drain regions by implanting a first element in regions of the N type well on both sides of the gate electrode, the first element being larger than silicon and exhibiting P type conductivity; (d) after (c) forming the second source/drain regions by implanting a second element in the regions of the N type well on the both sides of the gate electrode, the second element being smaller than silicon and exhibiting P type conductivity; and (e) forming the metal silicide layer on the source/drain regions.
According to this method, a silicon crystal is once made amorphous by implanting the first element, which is larger than silicon, during formation of the source/drain regions and then recrystallized by a subsequent heat treatment. Crystal defects remaining in the source/drain regions can be corrected by this process. Moreover, speed of diffusion at which the metal diffuses in the source/drain regions can be slowed down because a large amount of elements larger than silicon exists in the source/drain regions. As a result, it is possible to reduce a thickness of the nickel silicide film and suppress junction leakage current.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A to FIG. 1E are cross sections showing a fabrication method of a semiconductor device according to the first embodiment of the present invention.
FIG. 2A is a cross sectional picture of a nickel silicide film formed on a P type source/drain region by a conventional fabrication method. FIG. 2B is a cross sectional picture of a nickel silicide film formed on a P type source/drain region by a fabrication method of the present invention.
FIG. 3 shows normal probability distributions of a junction leakage current flowing from a source/drain region to a well in the respective P channel type MOS transistor arrays formed by a conventional fabrication method (line “a”) and a fabrication method of the present invention (line “b”).
FIG. 4 shows short-channel characteristics of the respective P channel type MOS transistors formed by a conventional fabrication method (line “a”) and a fabrication method of the present invention (line “b”).
FIG. 5A to FIG. 5E are cross sections showing a conventional fabrication method of a semiconductor device.
FIG. 6 is a cross sectional picture of a silicide layer of a P channel type MOS transistor formed by a conventional method.



DESCRIPTION OF PREFERRED EMBODIMENT
Embodiment
An example of a semiconductor device fabrication method of the present invention is hereinafter described, with reference to the drawings. Although a P channel type MIS transistor and an N channel type MIS transistor are formed alternately in the transistor fabrication process, descriptions of a fabrication process of an N channel type MIS transistor is omitted here because the present invention relates to a fabrication method of a P channel type MIS transistor.
FIG. 1A to FIG. 1E are cross sections showing a fabrication method of a semiconductor device according to the first embodiment of the present invention. This semiconductor device includes a P channel type MOS transistor on an N type well 103a. 
First, as shown in FIG. 1A, an isolation region 102 composed of a buried insulating film or the like is formed in a semiconductor substrate 101 of silicon or the like. After that, the N type well 103a is formed by photolithography and ion implantation.
Then, as shown in FIG. 1B, a gate insulating film is formed on the upper surface of the semiconductor substrate 101 by thermal oxidation, and a non-doped polysilicon film is then formed on the gate insulating film. After that, photolithography and ion implantation are carried out to introduce boron in the non-doped polysilicon film above the N type well 103a. Photolithography and dry etching are then carried out, thereby forming on the N type well 103a a gate insulating film 104 and a P type gate electrode 105a which have a given width. Next, boron difluoride (BF2) is introduced in regions of the N type well 103a on both sides of the P type gate electrode 105a, thereby forming P type extension regions 106a. Phosphorus is introduced in the P type extension regions 106a, thereby forming N type pocket regions 107a below the P type extension regions 106a. 
Then, as shown in FIG. 1C, a TEOS film and a silicon nitride film are formed on the silicon substrate 101 in order. Etch back is then carried out by anisotropic etch to form a side wall 110 composed of a TEOS film 108a and a silicon nitride film 109 on side surfaces of the gate insulating film 104 and the P type gate electrode 105a. The steps above are the same as those of the conventional fabrication method shown in FIG. 5A to FIG. 5C.
Next, as shown in FIG. 1D, photolithography and ion implantation are carried out to introduce indium in the N type well 103a using a photoresist (not shown), the P type gate electrode 105a and the side wall 110 as a mask, thereby forming first P type source/drain regions 111c. Boron is then implanted to form second P type source/drain regions 111d. The implantation conditions at this time are, for example, an acceleration energy of 30 keV and a dose of 1×1015 cm−2 for indium and an acceleration energy of 1.5 keV and a dose of 4×1015 cm−2 for boron.
In the FIG. 1D, the first P type source/drain regions 111c and the second P type source/drain regions 111d are together referred to as P type source/drain regions 111a. 
If the implantation depth of the boron is deeper than that of the indium, the boron outside the indium is diffused in the semiconductor substrate 101 by a subsequent heat treatment. As a result, the junction between the semiconductor substrate 101 and the P type source/drain regions 111a is formed at a deeper portion and short-channel characteristics of the P channel type MOS transistor become deteriorated. To avoid this, boron is implanted such that the implantation depth of the boron is shallower than that of indium.
Next, as shown in FIG. 1E, the impurities introduced in the P type gate electrode 105a and the N type well 103a are activated by spike annealing. The process temperature at this time is 1050° C., for example. After that, the surface of the semiconductor substrate 101 is cleaned with a hydrofluoric acid solution, thereby removing a natural oxide film on the surface of the substrate. Then, a nickel metal film having a thickness of 9 nm, for example, and a titanium nitride (TiN) metal film having a thickness of 20 nm are formed on the semiconductor substrate 101 in order. The silicon substrate 101 is subjected to a heat treatment in a nitrogen atmosphere under the conditions of, for example, 320° C. for 30 seconds to make the nickel and silicon react with each other. The nickel metal film and the titanium nitride metal film which have not reacted with silicon are removed with a solution in which sulfuric acid and hydrogen peroxide are mixed. The silicon substrate 101 is again subjected to a heat treatment in a nitrogen atmosphere under the conditions of, for example, 550° C. for 30 seconds to form nickel silicide 112 on the P type gate electrode 105a and the P type source/drain regions 111a. 
FIG. 2A is a cross sectional picture of a nickel silicide film formed on each of P type source/drain regions by a conventional fabrication method. FIG. 2B is a cross sectional picture of a nickel silicide film formed on each of P type source/drain regions by a fabrication method of the present invention. It is apparent from comparison between these pictures that the thickness of the nickel silicide film formed by the fabrication method of the present embodiment is thinner than the thickness of the nickel silicide film formed by a conventional fabrication method. According to the fabrication method of the present embodiment, a silicon crystal is once made amorphous and then recrystallized by a subsequent heat treatment. Crystal defects remaining in the source/drain regions can be corrected by this process. Moreover, it is considered that speed of diffusion at which the metal diffuses in the source/drain regions is slowed down because a large amount of elements larger than silicon exists in the source/drain regions. It is considered that abnormal diffusion of Ni is suppressed for this reason and that the thickness of the nickel silicide film is reduced as a result.
FIG. 3 shows normal probability distributions of a junction leakage current flowing from a source/drain region to a well in the respective P channel type MOS transistor arrays formed by a conventional fabrication method (line “a”) and a fabrication method of the present invention (line “b”). It is apparent from the drawing that the junction leakage current is reduced more in the P channel type MOS transistor array formed by the fabrication method of the present embodiment than the P channel type MOS transistor array formed by the conventional fabrication method.
The above results show that junction leakage current is reduced because the reduction in thickness of the nickel silicide film results in increase in the distance between the bottom surface of the nickel silicide film and the junction interface. This is an effect of the fabrication method of the present embodiment.
FIG. 4 shows short-channel characteristics of the respective P channel type MOS transistors formed by a conventional fabrication method (line “a”) and a fabrication method of the present invention (line “b”). It is apparent from the drawing that the short-channel characteristics do not differ between the two fabrication methods and that shallow junction of the source/drain regions is also maintained by the fabrication method of the present invention. Thus, the fabrication method of the present embodiment is capable of suppressing abnormal growth of the silicide film and reducing leakage current, while keeping shallow junction depth of the source/drain regions.
Implantation conditions of indium are given in the above description. However, the above effect can also be obtained as long as an acceleration energy is in a range of 19 keV and 40 keV and a dose is in a range of 1×1014 cm−2 to 1×1016 cm−2. Although indium is used to form the first P type source/drain regions in the above description, the indium may be replaced with gallium which also exhibits P type conductivity in silicon and whose atomic radius is grater than that of silicon as with indium.
Further, forming nickel silicide by using a nickel metal film to which at least one or more of platinum, aluminum, copper, erbium and palladium is added does not only provide the above effect, but also improve heat resistance of the nickel silicide.
Although a gate length of the gate electrode is not specified in the fabrication method of the present embodiment, the gate length is preferably 100 nm or less because problems of a silicide film become noticeable if the gate length exceeds the figure.
As explained in the above, the method of the present invention is useful as a fabrication method of various semiconductor devices which include, for example, a P channel type MIS transistor.
The description of the embodiment of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiment described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. It is therefore intended that the following claims cover all modifications and changes which fall within the true spirit and scope of the invention.

## Claims

1. A fabrication method of a semiconductor device which has a P channel type MIS transistor including a gate insulating film, a gate electrode, source/drain regions composed of first source/drain regions and second source/drain regions, and a metal silicide layer, the fabrication method comprising:
(a) preparing a semiconductor substrate including an N type well surrounded by an isolation region;
(b) forming the gate electrode on the N type well, with the gate insulating film interposed therebetween;
(c) forming the first source/drain regions by implanting a first element in regions of the N type well on both sides of the gate electrode, the first element being larger than silicon and exhibiting P type conductivity;
(d) forming the second source/drain regions by implanting a second element in the regions of the N type well on the both sides of the gate electrode, the second element being smaller than silicon and exhibiting P type conductivity; and
(e) forming the metal silicide layer on the source/drain regions.

2. The method of claim 1, wherein a gate length of the gate electrode formed at (b) is 100 nm or less.

3. The method of claim 1, wherein
the first element is indium or gallium, and
the second element is boron.

4. The method of claim 1, wherein the metal silicide layer is composed of nickel silicide.

5. The method of claim 1, further comprising:
(f) forming extension regions in the regions of the N type well on the both sides of the gate electrode by implanting a P type impurity using the gate electrode as a mask, the extension regions having a P type impurity concentration lower than a P type impurity concentration of the source/drain regions; and
(g) forming a side wall on side faces of the gate insulating film and the gate electrode,
wherein at (c) and (d) the ion implantation is carried out using the gate electrode and the side wall as a mask.

6. The method of claim 1, wherein
at (c) the first element is implanted with an acceleration energy of from 19 keV to 40 keV, both inclusive, and a dose of from 1×1014 cm−2 to 1×1016 cm−2, both inclusive.

7. The method of claim 1, wherein
the first and second source/drain regions are formed such that a depth of the first source/drain regions is deeper than a depth of the second source/drain regions.

8. The method of claim 1, wherein
the metal silicide layer formed at (e) is composed of nickel silicide to which at least one of platinum, aluminum, copper, erbium, and palladium is added.