Atomic Force Microscopy Study of Conformai Sputtering in Strained Silicon Samples
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Atomic Force Microscopy Study of Conformal Sputtering in Strained Silicon Samples Kuo-Jen Chaoa) and Gary Goodmanb) Charles Evans & Associates, 810 Kifer Road, Sunnyvale, CA 94086 ABSTRACT Precise determination of the Ge outdiffusion into the Si cap in strained Si/SiGe structures and the depth distribution of dopants have been critical to the industry and a challenge to the SIMS community. Will the rough cross-hatch surface topography degrade the SIMS depth resolution, or alter the accuracy of the depth profile? Here we report on a novel AFM approach to show that the SIMS measurement process results in conformal sputtering. Therefore, maintaining excellent depth resolution and the depth distribution accuracy capabilities of SIMS for these material systems.
INTRODUCTION Outdiffusion of Ge into the Si cap in strained Si/SiGe is a key parameter to understand, both from an epitaxial growth rate standpoint and from a CMOS thermal budget standpoint. Similarly, the depth distribution of dopants in the Si/SiGe system is critical. SIMS is the principal analytical technique used to measure the outdiffusion of Ge and the depth profile of dopants. However, there is a question as to whether the cross-hatch pattern in strained Si/SiGe samples degrades the SIMS depth resolution, or alters the accuracy of the SIMS depth profile. The peak-to-valley of the cross-hatch pattern may range from 5-30nm in non-chemical mechanical polished (non-CMP) samples down to 1-3nm in CMP samples. Typically, rough surfaces have an adverse affect on the quality of the SIMS depth profile and severely degrade the overall depth resolution of the measurement. In this work, Atomic Force Microscopy (AFM) has been applied to show that the SIMS measurement process results in conformal sputtering where the surface topography is maintained throughout the SIMS sputtering process down to the crater bottom. Therefore, maintaining excellent depth resolution and depth distribution accuracy capabilities of SIMS for the strained Si/SiGe sample systems.
a)
email: [email protected]
b)
email: [email protected]
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EXPERIMENTAL In order to check the effects of the surface roughness on the SIMS measurement, two specially designed strained Si/SiGe samples were sputtered using the SIMS ion beam to form craters that could be studied by AFM for roughness and pattern comparisons. Sample A involved a chemical mechanical polishing (CMP) step during its growth and is very smooth and typical of a normal polished wafer. The low surface roughness of this sample should provide excellent SIMS data. Sample B did not undergo a CMP step and is therefore very rough with cross-hatch features on the scale of the Si cap thickness. The main question that will be answered is how does this extreme surface topography affect the SIMS data.
SUBSTRATE
Figure 1 shows the layer structure for these two CMP Step samples. A thick graded SiGe buffer layer was grown on top of the Si substrate followed by a thick SiGe layer or SiGe “substrate”. The surface of the relaxed SiGe substrate is r
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