Method of fabricating an NMOS transistor

A SiC region and a source/drain region are formed such that the SiC region includes a first portion overlapping the source/drain region and a second portion protruding from the source/drain region to a position beneath the LDD region. The concentration of crystalline SiC in the second portion is higher than the concentration of crystalline SiC in the first portion. The SiC region may be formed through a normal implantation before the second spacer is formed, or the SiC region may be formed through a tilt implantation or deposition epitaxially in a recess having a sigma-shape like sidewall after the second spacer is formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating an NMOS (n type metal-oxide-semiconductor) transistor.

2. Description of the Prior Art

Strained-Si scheme has become essential for 45 nm and beyond CMOS technology. For example, the embedded SiGe (eSiGe) is introduced into the S/D area to enhance PMOS performance, and, the embedded silicon carbide (eSiC) in the S/D area is for enhancing NMOS performance due to large tensile stress resulted from the smaller lattice constant of SiC. There are two different approaches to form the eSiC S/D. One is to recess S/D region and to grow SiC directly using selective epitaxy process. Another is to use carbon ion implantation to pre-amorphorize silicon in a source/drain region and solid phase epitaxy (SPE) anneal to form SiC in the S/D region. The resultant MOS structure is obtained as shown inFIG. 1. In which, the source/drain region12is formed through doping a dopant using a first spacer14, a second spacer16, and a gate structure18as a mask. SiC is formed after the source/drain region12is formed and accordingly mainly distributed within an upper portion20of the source/drain region12in a proximately uniform concentration as shown by a referral number22.

However, the great increase of the sheet resistance (Rs) in doped SPE SiC will offset the performance gain obtained from the strain effect. Moreover, the high temperature thermal processes will induce carbon atom precipitation out from substitutional sites and reduce the channel stress.

Therefore, there is still a need for a novel method to fabricate an NMOS transistor having relatively high SiC concentration for increasing electron mobility while without increasing sheet resistance (Rs).

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a method for fabricating an NMOS transistor having improved electron mobility while the Rs is not significantly increased.

In one aspect of the present invention, the method for fabricating an NMOS transistor includes providing a silicon substrate having a gate structure thereon; forming a first spacer on a sidewall of the gate structure; forming a lightly doped drain (LDD) region in the silicon substrate at each of two sides of the gate structure; forming a second spacer on the first spacer; and after forming the second spacer, forming a SiC region and an S/D region in the silicon substrate at each of two sides of gate structure, wherein the SiC region comprises a first portion overlapping the S/D region and a second portion protruding from the S/D region to a position beneath the LDD region, wherein a second concentration of crystalline SiC in the second portion is higher than a first concentration of crystalline SiC in the first portion.

In another aspect of the present invention, the method for fabricating an NMOS transistor includes providing a silicon substrate having a gate structure thereon; forming a first spacer on a sidewall of the gate structure; implanting carbon atoms at a normal angle into the silicon substrate for pre-amorphorizing the silicon of the silicon substrate in a predetermined S/D region and beneath a predetermined lightly doped drain (LDD) region at each of the two sides of the gate structure; after implanting the carbon atoms, implanting an n type dopant into the silicon substrate in the predetermined LDD regions; performing a rapid thermal process for forming the crystalline SiC and simultaneously forming the LDD regions; forming a second spacer on the first spacer; and forming an S/D region in the silicon substrate at each of the two sides of gate structure, wherein the SiC region comprises a first portion overlapping the S/D region and a second portion underneath the LDD region, and a second concentration of crystalline SiC in the second portion is higher than a first concentration of crystalline SiC in the first portion.

Alternatively, in the above embodiment, the n type dopant may be implanted in the predetermined LDD regions before the carbon atoms is implanted for pre-amorphorizing the silicon in the predetermined source/drain region and beneath the LDD region at each of the two sides of the gate structure.

Alternatively, in the above embodiment, the LDD region may be formed first and then followed by implanting carbon atoms, and thereafter, a rapid thermal process is performed to form SiC.

DETAILED DESCRIPTION

Please refer toFIG. 2showing a schematic cross-sectional view of a MOS transistor desired in the present invention. The MOS transistor30may be fabricated by a method according to an embodiment of the present invention, as shown by the flow chart inFIG. 3. Referring toFIGS. 2 and 3, Step101is carried out to provide a silicon substrate11having a gate structure18thereon. The silicon substrate may be a silicon wafer or a silicon-on-insulator (SOI) substrate. The gate structure18may include a gate dielectric32and a gate34. The gate dielectric32may include a high k (i.e. high dielectric constant) dielectric material and further an interlayer between the silicon substrate and the high k dielectric material. The gate34may include polysilicon or metal. A shallow trench isolation structure17may be formed to surround the active area of the MOS transistor30.

Next, Step103is carried out to form a first spacer14on a sidewall of the gate structure18. Thereafter, Step105is carried out to form a lightly doped drain (LDD) region24through doping an n type dopant, such as phosphorus or arsenic, of a light concentration in the silicon substrate11at each of two sides of the gate structure18, using the first spacer14and the gate structure18as a mask, and a thermal annealing. Thereafter, Step107is carried out to form a second spacer16on the first space14. The first and second spacers may each include silicon nitride, silicon oxide or one used conventionally.

Thereafter, Step109is carried out to form a SiC region26and an S/D region12in the silicon substrate at each of two sides of gate structure. The S/D region12is doped with an n type dopant, such as phosphorus or arsenic. In which, as shown inFIG. 2, the SiC region26(shown as the area surrounded by a dotted line) is formed in the substrate and includes a first portion27overlapping the S/D region12and a second portion28protruding from the S/D region12to a position beneath the LDD region24. The concentration of crystalline SiC in the second portion28is higher than the concentration of crystalline SiC in the first portion27. It may be also realized as that the average distribution density of crystalline SiC in the second region28protruding from the S/D region12to a position beneath the LDD region24is higher than that in the first region27overlapping the S/D region12.

Forming a SiC region26having the second portion28with a higher concentration of crystalline SiC and the first portion27with a lower concentration of crystalline SiC can be implemented in various ways. In one embodiment, as shown in the flow chart ofFIG. 4, Step109as shown inFIG. 3may include Steps111,113, and115. In detail, referring toFIG. 4, after the second spacer16is formed, Step111may be formed through an ion implantation40to implant carbon atoms at a normal angle and a tilt angle into the silicon substrate11for pre-amorphorizing the silicon at each of the two sides of the gate structure18, as denoted by42inFIG. 7, for example. In one embodiment, the implant may be allowed to reach into the silicon substrate at a depth in the range from 100 to 300 angstroms, for example, about 200 angstroms, which may depend on a desired technology node. Since the implantation angle is tilted, the carbon atoms can reach the position44under the LDD regions24. After the carbon atoms are implanted, an n type dopant (for forming S/D regions) is implanted into the silicon substrate11at each of the two sides of the gate structure18for doping. Thereafter, a rapid thermal process at a temperature for example in the range from 900 to 1100° C., such as about 1000° C., for about 5 to 6 seconds, and a laser spike annealing are carried out for forming the crystalline SiC and simultaneously forming the S/D regions12as shown inFIG. 2.

As shown inFIG. 4, a pocket dopant region may be formed by carrying out Step106to implant a p type dopant after the LDD regions are formed for inhibiting irregular punch-through phenomenon during the fabrication process. The formation of pocket dopant region can be carried out before or after LDD regions are formed, and the dopant is disposed at a position in front of the edge of the SiC region toward the channel region.

InFIG. 4, Step113is carried out after Step111; while, alternatively, Step113may be carried out before Step111. That is, the n type dopant may be implanted first, and then the carbon atoms are implanted at a normal angle and a tilt angle for pre-amorphorizing the silicon. Thereafter, a rapid thermal process and a laser spike annealing are performed.

In still another embodiment, as shown inFIG. 5, Step113is also carried out before Step111; while, Step113is followed by Step112to perform a rapid thermal process and optionally a laser spike annealing for forming the S/D regions, and Step111is followed by Step114to perform a laser spike annealing for forming the crystalline SiC.

In further another embodiment, the SiC regions may be formed by an SPE process with formation of S/D regions using epitaxial silicon simultaneously, as shown inFIG. 6showing a flow chart andFIG. 8showing a cross-sectional view. As shown inFIG. 6, after Step107to form a second spacer, the Step109of the embodiment illustrated byFIG. 2may include Steps201,203, and205. In detail, referring toFIG. 6andFIG. 8, after the second spacer16is formed, Step201may be carried out to form two recesses in the silicon substrate at the two sides of the gate structure, for example, by etching. The recesses are each formed to have a sigma-shape like sidewall extruding to a position beneath each of the LDD regions24. Then, Step203is carried out to perform a sequence of selective epitaxy processes50to fill epitaxial silicon incorporated with an n type dopant and carbon atoms within the recesses with a concentration of carbon atoms in the position beneath each of the LDD regions higher than the other position. Such concentration difference may be obtained by adjusting the source amount ratio of silicon, n type dopant and carbon during the epitaxy process according to the epitaxial position where the deposition of silicon, n type dopant, and carbon is going. For example, when the epitaxial deposition is taking place at the region54, the carbon amount ratio in the source may be more than when the epitaxy deposition is taking place at other place. It should be noted that the process conditions should be adjusted to allow carbon atoms to locate at interstitial sites of the silicon crystal lattices, not to form as crystalline SiC, when they are just deposited within the recesses. Thereafter, Step205is carried out to perform a thermal process for re-crystallization to form the crystalline SiC and simultaneously form the S/D regions.

In another embodiment variant to the flow chart shown inFIG. 6, the epitaxial silicon may be incorporated with carbon atoms only, and the n type dopant is doped later. In detail, a sequence of selective epitaxy processes are performed to fill epitaxial silicon incorporated with carbon atoms within the recesses with a concentration of carbon atoms in the position beneath each of the LDD regions higher than the other position, and then an n type dopant is implanted at a normal angle into the silicon substrate at each of the two sides of the gate structure for doping. Thereafter, a thermal process is performed for forming the crystalline SiC and simultaneously forming the S/D regions.

In other embodiments, the SiC regions may be formed before LDD regions are formed. As shown inFIG. 9, after Steps301and303to provide the substrate and form a first spacer, Step305may be optionally carried out to form pocket dopant regions. Step307is carried out to implant carbon atoms, generally just at a normal angle, into the silicon substrate for pre-amorphorizing the silicon of the silicon substrate in a predetermined S/D region. Because the implantation is performed before LDD formation, the implant can reach to a place beneath a predetermined LDD region at each of the two sides of the gate structure. After implanting the carbon atoms, Step308is carried out to implant an n type dopant into the silicon substrate in the predetermined LDD regions. Thereafter, Step309is carried out to perform a rapid thermal process for completing the formation of the crystalline SiC and the LDD regions simultaneously. Then, Step311and Step313are performed to form a second spacer on the first spacer and to form S/D regions in the silicon substrate at each of the two sides of gate structure.

Alternative to the above embodiment, Step308may be performed to implant an n type dopant into the silicon substrate in the predetermined LDD regions before Step307of implanting carbon atoms is performed.

In another embodiment of formation of SiC regions before formation of the second spacer, as shown inFIG. 10illustrating a flow chart, after Steps301and303to provide the substrate and form a first spacer, Step305is optionally carried out to form pocket dopant regions. Thereafter Step306is carried out to form an LDD region at each of the two sides of the gate structure by implanting an n type dopant and performing a rapid thermal process. Thereafter, Step307is carried out to implant carbon atoms, generally at a normal angle, into the silicon substrate for pre-amorphorizing the silicon of the silicon substrate in a predetermined S/D region and beneath the LDD region at each of the two sides of the gate structure. Thereafter, Step310is carried out to perform a thermal process to form two SiC regions. In one embodiment, the thermal process may be at about 600 to 700° C. for 60 minutes. A thermal process performed for forming a SiGe region in a PMOS transistor may be suitably utilized. Thereafter, Step311and Step313are carried out.

Carbon atoms (which may be presented as carbon atoms or in a carbon cluster or molecular moiety, as those conventionally used for carbon implantation) incorporated into the silicon in the present invention is preferably 1×1020to 1×1022atoms/cm3. The carbon atoms are allowed to be incorporated in the silicon at interstitial sites of the silicon crystal lattices. When a thermal process is carried out, carbon gets the substitutional site of the silicon crystal lattice and becomes crystalline SiC. It is known that the concentration of carbon obtaining substitutional sites of silicon crystal (i.e. the carbon of crystalline SiC) in crystalline silicon matrix is limited up to about 2%, which may be deemed as a solid solubility of carbon in crystalline silicon. However, an n type dopant for an S/D region will compete against carbon for the substitutional site. The n type dopant gets the substitutional site more easily than carbon. Accordingly, a higher concentration of n type dopant leads to a lower concentration of silicon carbon. Since a portion of carbon atoms are incorporated into a region underneath the LDD region which does not overlap the S/D region, the concentration of crystal SiC will be higher due to no n type dopant existing for competition. In other hand, the carbon atoms incorporated into the region overlapping the S/D region will form SiC in a lower concentration due to competition against the n type dopant.

In the present invention, as illustrated by the above embodiments, the SiC region is formed to have a first portion overlapping the SiC region and a second portion underneath the LDD region, and the concentration of crystalline SiC in the second portion is higher than the concentration of crystalline SiC in the first portion. According to the investigation by P. W. Liu, T. F. Kuo, C. I. Li, Y. R. Wang, R. M. Huang, C. H. Tsai, C. T. Tsai, and G. H. Ma, in “DOPANT AND THERMAL INTERACTION ON SPE FORMED SIC FOR NMOS PERFORMANCE ENHANCEMENT,” IEEE, 2009, especially the drawingsFIGS. 9 and 10, which is incorporated entirely herein by reference, since a relatively high concentration of SiC (for example region28inFIG. 2, region44inFIG. 7, and region54inFIG. 8) is formed beneath LDD regions and very close to the channel region, the channel region can be well strained to gain electron mobility. The SiC concentration in other portion, such as region27inFIG. 2, of the SiC region is lower, and much less in the surface portion of the S/D regions, both of which contribute the main current path of the MOS transistor, and thus the Rs is not significantly increased even though the SiC concentration is higher in the portion beneath the LDD regions.