1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. Specifically, the present invention relates to a field effect transistor having a configuration capable of suppressing formation of a slit in a stressor film to improve drivability of the transistor and to a method for fabricating the same.
2. Description of the Prior Art
As the design rule of semiconductor devices shrinks, the integration degree of circuits is rapidly improved. It is even possible to mount more than or equal to a hundred million transistors on a chip. In order to realize such a chip, progress in ultra-fine patterning technology such as lithography or etching requiring a processing accuracy of the order of several tens of nano meters is required. In addition to the progress, an increase in drivability of the transistor is highly required for ensuring an absolute magnitude of a current also in a case of forming a fine transistor.
As one of techniques to improve the drivability of the transistor, exerting stress to a channel section has been a subject of interest in recent years. This is a method for exerting stress on silicon serving as a substrate to change a band structure thereof for improving the carrier mobility. A conventional research has been proved that it is effective to exert a tensile stress on a channel section in the gate length direction to improve the mobility in an n-channel MIS (Metal Insulator Semiconductor) transistor (NMIS). Meanwhile, as to a p-channel MIS transistor (PMIS), it is effective to exert s compressive stress on the channel section in the gate length direction.
As a method for exerting stress on a channel section, using a stressor film has been proposed (for example, see Japanese Laid-Open Patent Publication No. 2003-60076).
FIGS. 10A through 10C are cross sectional views showing a main part of a conventional semiconductor device, with reference to which a method for fabricating the conventional semiconductor device is described in the order of steps.
First, as illustrated with FIG. 10A, an active region 500 is formed in a semiconductor substrate 501. Then, over the active region 500, a gate electrode 503 is formed through a gate dielectric film 502. Subsequently, in regions in the active region 500 at both sides of the gate electrode 503, n-type source/drain regions 504 having a shallow junction depth are formed.
Next, on side surfaces of the gate electrode 503 and the gate dielectric film 502 and over the active region 500, dielectric films 505 having L-shaped cross sections are formed. Then, on inner surfaces of the L-shaped dielectric films 505, side-walls 506 are formed. Each L-shaped dielectric film 505 and its corresponding side-wall 506 constitute a side-wall spacer 521. Subsequently, n-type source/drain regions 507 having a deep junction depth are formed in regions in the active region 500 outside the side-wall spacers 521. Then, on the gate electrode 503 and on the n-type source/drain regions 507, silicide layers 508 are formed.
Next, as illustrated with FIG. 10B, over the semiconductor substrate 501, a stressor film 509a of a silicon nitride film is formed to cover the gate electrode 503 and the side-wall spacers 521. The stressor film 509a immediately after the formation has a very small film stress. Therefore, in the step, almost no tensile stress is exerted on a channel region in the gate length direction.
Next, as illustrated with FIG. 10C, ultraviolet irradiation 510 is performed for causing contraction of the stressor film 509a to form a stressor film 509 for exerting a great tensile stress on the channel region in the gate length direction. Then, on the stressor film 509, an interlayer dielectric film is formed, although the interlayer dielectric film is not shown.
By forming a stressor film having a great film stress through the steps above, a tensile stress is exerted on the channel region of an NMIS transistor in the gate length direction, which improves the drivability of the NMIS transistor.
Moreover, it is disclosed in the document of K. Mistry et al., Symp. on VLSI Tech., Digest of Tech. Papers, pp. 50-51 (2004) that drivability of an NMIS transistor depends on the film thickness of a stressor film, and that increasing the film thickness of the stressor film formed of a nitride film to 80 nm improves the drivability of the NMIS transistor by 12%. That is, it can be said that drivability of the NMIS transistor can be effectively increased by forming a stressor film of a material having a great tensile stress and by increasing the thickness of the stressor film as much as possible.
However, it turned out that slits are formed in regions 10A of FIG. 10C in the stressor film 509 formed according to the above-mentioned, conventional method for fabricating a semiconductor device. The reason for this is assumed that in the step of performing the ultraviolet irradiation 510 for causing contraction of the stressor film of FIG. 10C, the contraction of the stressor film 509 occurs on the side-wall spacers 521 and on the silicide layers 508, and the contraction force becomes greater near interfaces between the side-wall spacers 521 and the silicide layers 508 over the semiconductor substrate 501, resulting in formation of the slits in the regions 10A. As in this case, if slits are formed in the stressor film 509, a problem arises that the drivability of the NMIS transistor does not increase even if the thickness of the stressor film is increased in an attempt to increase the tensile stress exerted on the channel region in the gate length direction.