The majority of present day integrated circuits (ICs) are implemented by using a plurality of interconnected field effect transistors (FETs), also called metal oxide semiconductor field effect transistors (MOSFETs), or simply MOS transistors. An MOS transistor includes a gate electrode as a control electrode disposed overlying a semiconductor substrate and spaced apart source and drain regions disposed within the substrate and between which a current can flow. A control voltage applied to the gate electrode controls the flow of current through a channel within the substrate between the source and drain regions.
Epitaxially-grown silicon films often are used in MOS transistors to modify the performance of such transistors. For example, an epitaxially-grown silicon film can be used to increase the mobility of majority carriers through the channel of an MOS transistor by inducing stresses in the channel. The mobility of holes, the majority carrier in a P-channel MOS (PMOS) transistor can be increased by applying a compressive longitudinal stress to the channel, especially when the transistor is fabricated on a silicon wafer. It is well known that a compressive longitudinal stress can be applied to a silicon MOS transistor by embedding an epitaxially-grown material such as silicon germanium (SiGe) at the ends of the transistor channel. Similarly, the mobility of electrons, the majority carrier in an N-channel MOS (NMOS) transistor can be increased by applying a tensile longitudinal stress to the channel. Such a stress can be applied to a silicon MOS transistor by embedding a material such as epitaxially-grown silicon doped with carbon at the ends of the transistor channel. Such methods typically require the etching of trenches into the silicon substrate and the selective epitaxial deposition of silicon germanium and/or silicon carbon.
Ideally, the epitaxial growth of silicon films is conducted for a time sufficient to ensure that the film has grown to a predetermined thickness within all relevant trenches or on all relevant surfaces of all devices on a semiconductor wafer. Any undergrowth of the epitaxial silicon film can result in device-to-device variations and wafer-to-wafer variations that can reduce device yield. To ensure adequate growth of an epitaxial film across a semiconductor wafer, or from wafer to wafer, the epitaxial silicon film generally is grown for a period slightly longer than that typically required for the film to grow to a predetermined thickness. In other words, the film is slightly overgrown.
While overgrowth may be advantageous for certain purposes, it can be disadvantageous for others. If the overgrown epitaxial silicon film grows uniformly, that is, its leading edge has a relatively flat profile, and the overgrown epitaxial silicon film grows beyond surfaces of an adjacent structure, the epitaxial silicon film is said to have “popped” with respect to the adjacent structure. FIG. 1 illustrates an example of an overgrown epitaxial silicon film 20 that has popped relative to a silicon substrate 12. In this example, an MOS gate structure 10 is disposed on the silicon substrate 12 with dielectric spacers 14 that are formed about sidewalls 11 of the gate structure 10 and that extend beyond an upper surface 15 of the silicon substrate. MOS gate structure 10 comprises a gate insulator 24 on the silicon substrate 12 and a gate electrode 16 disposed on the gate insulator. Trenches 18 are etched into the silicon substrate and are filled with an epitaxially-grown silicon-comprising film 20. As evident in FIG. 1, the epitaxially-grown silicon-comprising film is uniformly overgrown such that the leading edge 17 of the epitaxial silicon-comprising film is relatively flat and the epitaxial silicon-comprising film 20 has grown along, not only side surfaces 19 of the trenches 18, but also along the side surfaces of gate insulator 24 and spacers 14. FIG. 2 illustrates another example of an overgrown epitaxial silicon-comprising film 62 that has popped relative to a shallow trench isolation (STI) structure 60 disposed on silicon substrate 12 and that extends from a surface 63 thereof. As evident in FIG. 2, the epitaxially-grown silicon-comprising film 62 is uniformly overgrown such that a leading edge 61 of the epitaxial silicon-comprising film 62 is relatively flat and the epitaxial silicon-comprising film 62 has grown along a side surface 64 of STI structure 60 and beyond the side surface. One problem with popping results because, typically following the growth of an epitaxial silicon-comprising film, the epitaxial silicon-comprising film is implanted with conductivity-type impurity ions to form source and drain regions of the MOS device. When implanting into the epitaxial layer, the energy of the implant must be adjusted to compensate for the thickness of the layer. For the “popped” case, the overgrown thickness follows the across-wafer, wafer-to-wafer, and lot-to-lot variations inherent in the epitaxy process. This can lead to significant device variability and even yield degradation.
In contrast, it is preferable that the epitaxially-grown silicon-comprising films are “pinned” to an adjacent structure. “Pinning” occurs when growth of the epitaxial silicon-comprising film along the surface of an adjacent structure terminates at the end of the surface, although growth towards the center of the epitaxial silicon-comprising film may continue. In this regard, excess epitaxial silicon growth is minimized particularly at the surface of the adjacent structure, which is most critical for controlling device characteristics, although overgrowth at the center of the film insures the overall growth of the epitaxial silicon-comprising film to a predetermined average thickness. Thus, the amount of additional epitaxial silicon-comprising film that the impurity ions must penetrate to form the source and drain regions or other doped regions is minimized. FIG. 3 illustrates an example of the overgrown epitaxial silicon-comprising film 20 of FIG. 1 that is pinned relative to the silicon substrate 12. As is evident in FIG. 3, growth of the epitaxial silicon-comprising film 20 along the side surfaces 19 of the trenches 18 terminated at an end of the side surfaces 19 while growth of the film 20 towards its center continued. Similarly, FIG. 4 illustrates the overgrown epitaxial silicon-comprising film 62 of FIG. 2 that is pinned relative to the STI structure 60. As evident in FIG. 4, growth of the epitaxial silicon-comprising film terminated at the end of the side surface 64 of STI structure 60 while growth of the film 62 towards its center continued. Thus, it is desirable to grow epitaxial silicon-comprising films that provide for overgrowth but also provide for pinning at surfaces of adjacent structures.
While it is desirable to have overgrowth and pinning that are uniform from device to device on a wafer and from wafer to wafer, such uniformity may be compromised when variations in the growth process arise due to, for example, equipment malfunction, change in environmental conditions, and the like. A change in the uniformity of overgrowth and pinning can result in implant shadowing and varied implant profiles, which in turn can result in significant differences in device characteristics from device to device, wafer to wafer, and lot to lot.
Accordingly, it is desirable to provide methods for calibrating a process for growing epitaxial silicon-comprising films that result in both overgrowth of the films and pinning of the films to adjacent structures wherein the overgrowth and pinning is uniform across a wafer. In addition, it is desirable to provide methods for growing epitaxial silicon-comprising films that result in overgrowth and pinning to adjacent structures wherein the overgrowth and pinning is uniform across a wafer. Further, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.