Investigation of Apertureless NSOM for Measurement of Stress in Strained Silicon. Colin McDonough, Jacob Atesang, Yunfei Wang, and Robert E.

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1 / , The Electrochemical Society Investigation of Apertureless NSOM for Measurement of Stress in Strained Silicon Colin McDonough, Jacob Atesang, Yunfei Wang, and Robert E. Geer College of Nanoscale Science & Engineering, University at Albany, SUNY Albany, NY Strain in blanket and patterned silicon-on-insulator (SOI) structures have been investigated via apertureless near-field scanning Raman spectroscopy, specifically, to investigate the efficacy of so-called tip-enhanced Raman scattering (TERS) for Si strain characterization and metrology for integrated circuit (IC) devices. The current study compares TERS generation using Ag-coated W tips in a 45º incident beam, fixed-tip geometry and Ag-coated SiO 2 capillary tips in a normal incident, scanning-tip geometry. The latter demonstrates superior performance and is used in a differential scheme to investigate strain profiles in patterned SOI structures. Specifically, a blanket device layer with an engineered strain of 0.075%, is patterned to form arrays of isolated mesa structures (2 µm diameter). These mesa structures exhibit a stress relaxation of 117 MPa over a region extending ~ 200 nm from the island edge. Introduction Illumination of a nanometer scale metallic tip in the immediate vicinity of a Ramanactive surface exhibits enhancement of Raman scattered light similar to so-called surfaceenhanced Raman scattering. Such effects have given rise to the development of apertureless near-field scanning optical microscopy (a-nsom) for which a metallic or metal-coated nanoprobe is utilized to generate local (< 50nm) scattering enhancement at a surface (1-5). This approach has been investigated as a route to provide high spatial resolution profiling of stress in strained-si device structures. Stress metrology in Si-based device structures has become an increasing critical issue for integrated circuit (IC) manufacturers which utilize strained Si channels in metal-oxide semiconductor fieldeffect transistors (MOSFETs) (6). We have carried out investigations of tip-enhanced Raman scattering (TERS) to determine the efficacy of this approach. For these measurements Veeco Aurora-3 and Nanonics MV2000 NSOMs have been integrated with a Renishaw Raman spectrometer. In place of conventional coated optical fibers metallized tips have been used as apertureless probes. Probes are maintained within 2-5 nm of the surface via a resonant vibration feedback loop. TERS imaging has been applied to blanket and patterned SOI test structures to investigate spatial and spectroscopic resolution. Results are discussed in detail below. While preliminary, these studies demonstrate the attractive potential of TERS-based approaches for strain metrology in Si-based device structures. Experimental NSOM System Schematics The two a-nsom systems employed in this study differ, primarily, in two aspects. Firstly, the Aurora system employs a shear-force approach for a fixed-tip feedback in Downloaded on to IP address. Redistribution subject to ECS 235 terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

2 which the metallized tip is mounted on one leg of a vertically-oriented tuning fork that is mounted rigidly in the system (Fig. 1), i.e. the tip position is fixed with respect to the farfield illumination beam with the sample position being controlled by a conventional piezoelectric tube scanner. In contrast, the metallized tip in the MV2000 system is mounted to a horizontally oriented tuning fork which utilizes a normal-force feedback approach and is controlled by a separate scanner so that it may be positioned at various points with respect to the sample and far-field illumination beam. Sample positioning within the MV2000 likewise employs piezoelectric positioners. Secondly, the Aurora system geometry employs a 45 angle of incidence between the far-field beam and the sample normal. The MV2000 operates in normal incidence mode. Figure 1. (Upper panels) Component schematic of the Nanonics MV2000 NSOM system integrated with a Renishaw optical spectrometer. The holographic notch filter assembly, in addition to rejecting Rayleigh-scattered light also acts as a high-rejection beam splitter for laser illumination of the tip. (Lower panels) Component schematic of the Aurora 3 NSOM system integrated with a Renishaw optical spectrometer. The Renishaw spectrometer system (RM-100) integrated with the MV2000 and the Aurora systems is equipped with a Leica DM/LM optical microscope. A longworking distance 50X objective was used for our experiments in both systems. This lens focuses the incident laser (Ar-ion, 514 nm) down to spot sizes of 2 µm and 3 µm diameters, respectively for the MV2000 and Aurora systems. The collected Raman signal is detected by a charge coupled device (CCD) cooled by a Peltier cooling system. The spectrometer uses a 3000 lines/cm grating for spectra acquisition. A notch filter is used 236

3 for rejecting the unwanted Rayleigh scattering in addition to serving as an efficient beamsplitter. SOI Sample: Characterization and Test Structure Fabrication A schematic of the SOI stack (supplied by SOITEC, Inc.) utilized for this work is shown in the left panel of Fig. 2. Prior to investigation with apertureless NSOM-based Raman the SOI sample was characterized using x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and microraman (tip retracted). The XPS elemental analysis (not shown) confirms the absence of significant contamination or other compositional variations that may effect the Raman spectra. XRD measurements were carried out on blanket SOI samples to independently characterize the stress state. A diffuse diffraction spot at h = , l = (indexed to the bulk Si (004) peak) in the x-ray diffraction pattern represented the (004) peak corresponding to the strained Si layer (7). The displacement in the h-l plane results from the inherent strain of the SOI top layer and the slight misorientation inherent in the bonding process of the strained layer to the Si handle wafer. The position of this peak indicates a change in the lattice spacing, c/c, of of the device layer. Assuming biaxial strain and using the accepted elastic constants for Si, the in-plane strain is estimated at 0.746%. This matches well with the SOITEC specifications which list an in-plane strain of 0.75% for this sample. χ 2 = A ssi = 19435±414 Counts (x 10 3 ) 6 3 ssi Si A Si = 18772±442 w ssi = ±0.033 w Si = ±0.062 x ssi = ±0.025 x Si = ± Raman shift(cm -1 ) Figure 2. (Left) Schematic profile of the SOI stack used. (Right) Raman far-field or micro Raman spectra of blanket SOI sample. The data was fit to a double Lorentzian. The fit parameters are listed in the plot. A, w, and x refer to the Lorentzian amplitude, width, and center position, respectively. The subscripts ssi and Si denote the strained and bulk Si peaks, respectively. The SOITEC SOI sample was evaluated using the Veeco Aurora NSOM in micro- Raman mode for which the tip was retracted but the sample maintained in the focal plane of the illumination objective. Figure 2 shows a typical Raman spectrum from the blanket SOI wafer. As expected two clear peaks are resolved, one corresponding to the strained 237

4 Si device (top) layer, the second corresponding to the bulk. The Raman shift of 5.6 cm -1 corresponds to a biaxial strain (σ xx +σ yy ) of 2.55 GPa using the models described by De Wolf and coworkers and references therein (8-10). This yields an in-plane strain of ± and an out of plane strain of ±0.0004, in reasonable agreement with the XRD data. Measurements of TERS: 45º Incidence Geometry To investigate the presence of TERS from SOI samples a series of experiments was undertaken to investigate Ag-coated W tips in the Veeco Aurora NSOM (45º incident beam geometry). Electropolished W tips (tip radii nm) were sputter-coated with Ag (30s) (5). Due to the irregular morphology of the etched W tips it is nontrivial to characterize an average sputtered thickness of Ag. Also, it is not experimentally feasible to determine the exact point on a tip which provides enhancement and, therefore, difficult to determine a specific, optimal coating thickness. Tips were attached to a vertically mounted tuning fork and evaluated for TERS using the SOI sample. Raman spectra were collected with the tip in feedback and with the tip retracted. To avoid slight changes in Raman intensity associated with misalignment of the optical focal plane the Veeco Aurora NSOM stage position was fixed at the focal plane of the NSOM objective lens via direct control of the vertical piezo scanner of the sample. This ensures that the position of the SOI sample with respect to the NSOM focal plane is fixed during tip-evaluation. Figure 3 illustrates SOI Raman peaks acquired with the Aurora tip in feedback (left panel of Fig. 3.) and out of feedback (right panel of Fig. 3). A measurable increase in signal is present for the data acquired in feedback compared with the data acquired with the tip retracted. This increase (after background subtraction) represents a raw enhancement of approximately 12%. Although modest in absolute terms, this increase in measured intensity corresponds to an enhancement factor of approximately 10 3 assuming an effective tip radius of 50 nm. Although the complex morphology of the tips makes an accurate estimation of the effective tip-radius difficult the enhancement factor is in the range typically reported in the literature (1-5). Counts 8000 χ2 = A 6000 ssi = 19234±492 A Si = 15699±768 x ssi = ± x Si = ±0.04 w ssi = 3.254± w Si = 3.000±0.048 Counts 8000 χ 2 = A ssi = 17235± A Si = 16773±800 xc ssi = ±0.04 xc Si = ±0.04 w ssi = 3.200±0.024 w Si = 3.617± Raman Shift (cm -1 ) Raman Shift (cm -1 ) Figure 3. Raman spectra from SOI sample with tip in feedback (left) and tip out of feedback (right). Each data set was fit using a double Lorentz peak model. Fitting parameters are shown in the plot. 238

5 Figure 4 shows the strained Si Raman peak height both with the tip in and out of feedback as a function of time. Each signal is exceedingly stable. This stability is critical in that it underscores the substantial (although relatively modest) change in intensity with and without the Ag-coated W tip in feedback. Likewise, the Raman peak position is extremely stable over the measurement time. Figure 5 plots the peak positions of the strained Si and Si Raman peaks as a function of time (with a Ag-coated W tip in feedback). The peak positions are extremely stable. The variation (1 sigma) over the measurement time is approximately cm Peak intensity Strain Peak Intensity (Feedback) Peak intensity Strain Peak Intensity (Retracted) time/sec time/sec Figure 4. Peak intensity for the strained Si peak from the SOI sample with a Ag-coated tip in feedback (left) and out of feedback (right) as a function of time. Both signals exhibit excellent stability (< 0.007% variation) over an 11 minute observation time Strained Si Peak Position Bulk Si Peak Position Peak position Peak position time/sec Time/sec Figure 5. Strained (left) and bulk (right) peak positions from a blanket SOI sample with a Ag-coated tip in feedback as a function of time. Both signals exhibit excellent stability (< cm -1 error) over an 11 minute observation time. Counter intuitively, both strained Si and bulk Si Raman peaks as measured with the Ag-coated W tip exhibit enhancement. This is expected for the strained (surface) Si peak, but not for the bulk. In fact, for the data shown in Fig. 3 the bulk Si Raman peak 239

6 exhibits a raw TERS enhancement of 6%, half that for the strained Si Raman peak. (As shown in more detail below this behavior was not observed for other optical configurations). However, for all the SOI Raman measurements undertaken with the Veeco Aurora NSOM bulk Si Raman enhancement accompanied strained Si Raman enhancement. This observation mirrors recent reports by other research groups in this field (11). It is likely that these apparent inconsistencies arise from optical interference effects associated with the SOI structure itself. The presence of the tip and effects associated with optical shadowing may exacerbate this effect. This effect is not seen for the normal incident TERS data described below. Measurements of TERS: Normal Incidence Geometry As a complement to the work described in the preceding section, measurements were undertaken with Ag-coated bent glass fiber tips on a Nanonics MV2000 NSOM. Unpatterned sections of the SOI stack investigated with the Aurora NSOM system were evaluated using the MV2000 system. This data is shown in Fig. 6. Note the magnitude of the surface peak relative to the bulk has decreased significantly compared to data acquired from the Aurora system. This is a function of the optical illumination geometry. Preliminary examination has attributed this feature, in part, to an interferometric effect. A quantitative description of this effect is under development. Also note that the relative TERS enhancement documented in Fig. 6 (~ 20%) is substantially larger compared to the data of Fig. 3. And more notably, the bulk peak actually sees a 13% reduction in intensity compared to data acquired with the tip retracted. This reduction is due to the shadowing of the sample by the metallized tip. The shadowing effect has been confirmed by displacing the tip away from the center of the beam while in feedback. Counts 6000 Tip Retracted Tip in Feedback Tip Retracted χ 2 = x Si = ±0.014 w Si = 3.10±0.035 A Si = 25188±253 x ssi = ±0.023 w ssi = 3.37±0.057 A ssi = 15346±253 Tip in Feedback χ 2 = x Si = ±0.016 w Si = 3.20±0.039 A Si = 21879±248 x ssi = ±0.019 w ssi = 3.20±0.047 A ssi = 18380± Raman Shift (cm -1 ) Figure 6. Raman spectra in feedback and retract mode from the SOI sample on a MV2000 NSOM. Enhancement of the strained Si Raman peak relative to the bulk is clearly evident. 240

7 From the data presented above we conclude that TERS provides a measurable enhancement for SOI blanket samples for both the off-axis and normal incidence illumination geometries. Moreover the data show that a stable enhancement can be achieved over spectra acquisition times suitable for high-resolution spatial and spectral imaging. Ostensibly, the normal axis geometry provides the most promising results from the point of view of SOI Raman peak enhancement. This not unexpected since the tipscanning feature of the MV2000 permits optimal placement of the tip in the optical illumination beam. The Aurora system does not permit this and, as a consequence, there is no methodology to effectively optimize tip position/orientation for that system. Based on the results described above, the normal incidence geometry will be utilized to investigate patterned SOI test structures discussion in the following section. TERS investigation of patterned SOI test structure. The SOI test structures used to evaluate normal-incidence TERS scanning consist of isolated mesas of strained Si patterned on the buried oxide. A conventional via mask was combined with a contrast inversion photolithography process to pattern a thin (50 nm) layer of Cr islands across the SOI wafer. A TMAH wet etch step resulted in the removal of unprotected Si. The buried oxide layer constituted a highly selective etch stop. The Cr was removed via a wet process which resulted in a field of strained-si mesas on a buried oxide substrate. A SEM image of a patterned strained-si mesa is shown in Fig. 7. The octagonal shape results from the anisotropic TMAH wet etch used to fabricate the test structure. Each mesa is approximately 2 microns in diameter. The thickness of the SOI device layer is approximately 63 nm and the thickness of the buried oxide layer is approximately 140 nm. Buried oxide Strained Si layer Buried oxide 1 µm Figure 7. SEM micrograph of a patterned SOI test structure consisting of a circular (octatgonal) mesa of strained Si (approximately 63 nm in thickness). The surrounding material consists solely of the buried oxide layer. The SOI test structures have been used to evaluate normal incidence TERS using 241

8 the MV2000 NSOM. To separate the tip-specific component of the Raman-shifted light a differential approach has been employed to isolate the near-field generated Raman signal from the far-field generated Raman signal. In this mode a series of spatially-resolved Raman spectra are acquired at a 2D array of points on the substrate with the tip in feedback. The same measurements are repeated with the tip retracted and the differential is taken to isolate the surface-specific TERS signal. Using this approach, topographic and spectral Raman imaging of the patterned SOI test structure was undertaken for comparative evaluation. Topographic imaging agreed in detail with the SEM image shown in Fig. 7. Spectral imaging results are shown in Fig. 8. Specifically, the intensity of the Raman peak associated with the strained-si peak (see Fig. 6) is mapped. The grayscale map and vertical height corresponds to linear intensity variation. Treating the edge of the mesa as a vertical boundary and the corresponding broadening due to a Gaussian resolution function a lower limit of the spatial resolution is estimated at nm. Considering the tip used to obtain this image has a diameter of nearly 175 nm it is likely that the resolution can be reduced below 45 nm with sub-100nm diameter tips. Figure 8. Intensity map of the peak intensity of the strained-si Raman peak in the vicinity of the strained-si mesa. Although the maximum spatial gradient observed in the strained-si peak height corresponds to the topographically-determined mesa position, a Raman halo is present about the mesa, evident in Fig. 8. This halo consists of relatively low intensity strained- Si Raman signal extending away from the mesa over several hundred nanometers. This likely resulted from the conventional far-field Raman signal associated with the spot size of the illuminating laser. The presence of residual far-field Raman scattering in the differential signal implies modification of the far-field (or background) signals due to the presence of the tip. It is probable that shadowing effects, drift, and diffuse Raman signal generation from the glass capillary combine to produce the halo. Current efforts have centered on increased efficiency for far-field Raman signal rejection using modified scan geometries and polarization selection. The mesa test structure offered the opportunity to investigate edge-induced strain relaxation and the efficacy of measuring such relaxation via TERS. Shifts in the strained- 242

9 Si Raman peak position were investigated as a function of spatial position across the mesa along a line passing through the mesa center. Figure 9 illustrates this dependence by plotting the position of the strained-si Raman peak as a function of spatial position across the mesa. The mesa edge occurs at approximately the 1.0 µm point in the scan. At this edge the strained-si Raman peak position is ±0.06 cm -1. This compares to an average value of ±0.06 cm -1 over the center region of the mesa. This corresponds to a reduction in the biaxial tensile stress of 117±18 MPa (along a single inplane principal axis). This represents a 9% reduction in strain at the mesa edges. The profile in Fig. 9 is fit to a Gaussian-smeared step with a characteristic width of 228±38 nm for empirical purposes only. It is likely that the same halo-effect seen from the strained-si Raman peak intensity maps is contributing to the Gaussian width of the profile in Fig. 9. Investigations into this are continuing as well as finite-element modeling of the stress relaxation in the mesas. Preliminary analysis implies small amounts of residual compressive stress in the underlying Si associated with the mesa relaxation. Detailed studies of this effect are continuing. Raman Shift (cm -1 ) Edge-Induced Strain Relaxation σ = σ = 117 ± 18 MPa Edge of Strained Si Island Position (µm) Figure 9. Spatial profile of the strained-si Raman peak position across the center line of the mesa structure depicted in Fig. 7. A stress relaxation of approximately 117 MPa is evident. The solid line is an empirical fit to a Gaussian-smeared step and served to characterize the spatial variation of the profile and as a guide to the eye. Although preliminary, these results suggest great promise in the use of a-nsom Raman imaging as a tool for strain/stress characterization in Si devices. Although spatial resolution must still be improved the initial results from differential Raman mapping of the SOI test structures described above demonstrate the technique s potential. Conclusion Strain in blanket and patterned silicon-on-insulator (SOI) structures have been investigated via apertureless near-field scanning Raman spectroscopy. A normal-incident 243

10 illumination geometry demonstrated promising results for TERS-based stress characterization in SOI-based mesa structures. TERS-based stress imaging in the SOI mesa structures revealed a stress relaxation of 117 MPa over a region extending ~ 200 nm from the island edge. Acknowledgements Support is gratefully acknowledged from the Semiconductor Research Corporation and DARPA through the SRC FRCP Interconnect Focus Center, New York Center for Advanced Interconnect Science and Technology and the New York Office for Science, Technology, and Academic Research. It is a pleasure to acknowledge SOI test wafers and technical discussions with G. Cellar, J. Rinderknecht, M. Hecker, L. Zhu and E. Zschech. It is also a pleasure to acknowledge technical NSOM assistance from H. Taha, test structure fabrication from N. Tokranova, B. Altemus, A. Gracias, and L. Clow, and XRD measurements from R. Matyi. References 1. L. Novotny, E. J. Sanchez, and X. S. Xie, Phys. Rev. Lett., 82, 4014 (1999). 2. A. Hartschuh, E. Z. Sanchez, X. S. Xie, and L. Novotny, Phys. Rev. Lett., 90, 9 (2003). 3. H. Watanabe, Y. Ishida, N. Hayazawa, Y. Inouye, and S, Kawata, Phys. Rev. B, 69, (2004). 4. W. X. Sun, Z. X. Shen, Ultramicroscopy, 94, 237 (2003). 5. Jacob Atesang, Robert Geer Proceedings of SPIE, Robert E. Geer, Norbert Meyendorf, George Y. Baaklini, Bernd Michel, Editors, Vol. 5766, p. 134, SPIE Press, Bellingham, WA (2005). 6. K. Rim, Solid State Electronics, 43, 1133 (2003). 7. R. Matyi, unpublished. 8. I. De Wolf, Semicond. Sci. Technol. 11, 139 (1996). 9. I. De Wolf, H. E. Maes, and S. K. Jones, J. Appl. Phys. 79, 7148 (1996). 10. E. Anastassakis, A. Cantarero, and M. Cardona, Phys. Rev. B 41, 7529 (1990). 11. A. Sokolov, unpublished. 244