Probing Strained Semiconductor Structures with Nanoscale X-ray Diffraction

摘要

The tailoring of strain distributions within semiconductor features represents a key method to enhance performance in current and future generations of complementary metal-oxide semiconductor (CMOS) devices. Although the impact of strain on carrier mobility in semiconductor materials was first investigated over 50 years ago [1,2], its implementation within the inversion layer of the channels in CMOS device channels has only occurred within the past decade. This includes the deposition of liner materials that possess significant values of residual stress [3]. Eigenstrained structures, deposited epitaxially within recesses on either side of the Si channel, can be used to induce either compressive strain in the channel region, by using materials that possess a larger lattice parameter than Si (e.g., SiGe)[4], or tensile strain, by using materials with a smaller lattice parameter (e.g., SiC). Because these methods generate heterogeneous strain distributions within the composite structure, it is critical to experimentally determine the distribution of strain across the current-carrying paths of the device and the surrounding environment. Real-space x-ray microdiffraction measurements represent the optimal method to perform direct, in-situ characterization of strain within crystalline materials at a submicron length scale [5,6]. The diffraction facilities at the 2ID-D beamline at Argonne National Laboratory's Advanced Photon Source were used for the x-ray microdiffraction measurements [6] with a nominal beam size of approximately 0.25 μm. Samples under investigation included both CMOS devices possessing embedded stressor materials and silicon-on-insulator (SOI) structures with overlying stressor features. The embedded stressor devices, fabricated from 55 nm thick SOI layers, contained Si_(1-x)C_x with a C content, x, of 1.1% in the source and drain regions, approximately 1.85 μm in length. Because C has a smaller lattice parameter than that of Si, the e-SiC structures possess in-plane tensile stress, which is transferred into the adjacent, 60 nm long SOI channel. To obtain a reference value for the unrelaxed SiC strain, square pads 200 μm in length, also consisting of heteroepitaxially deposited SiC, were characterized. Another set of SOI structures was fabricated possessing overlying stressor features, where compressively stressed Si_3N_4 films of approximately 105 nm thickness were lithographically etched to produce a matrix of rectilinear features possessing lengths of 2048 μm and widths ranging from 1 μm to 2048 μm.