Piezospectroscopy Measurements and Standards

Our objective is to develop accurate measurement methods for the nano-scale stress distributions and surface defects that control device performance and reliability (performance over service life) in microelectronic and micro- and nano-electromechanical systems (MEMS and NEMS). Such methods will enable manufacturing processes to be optimized for device performance and lifetime, and address a critical measurement need in the MEMS industry, i.e., 90 % of MEMS customers require a demonstration of device reliability, but only 50 % of vendors provide one.

We will develop piezospectroscopic methods that accurately map nano-scale stress distributions in Si and other materials, in real-work environments at video rates. Method development will focus on the use of super-resolution confocal Raman microscopy and coherent anti-Stokes Raman scattering to identify and measure the time evolution of defects that limit mechanical and electrical reliability. Calibration of the Raman piezospectroscopic coefficients will be performed using the known stress fields of special test structures, indentations, and cracks. Polarized Raman scattering mehtods will be developed in concert with special test structures to enable the entire stress tensor to be determined. The Raman-based measurements will be correlated with electron back scatter diffraction (EBSD) strain measurements on identical systems. A large-sample measurement system will be developed so that stress maps of 200mm wafers can be generated for comparison with commercial wafer curvature tool measurements. Standard reference materials for stress measurement will be developed.

Impact and Customers:

• The semiconductor microelectronics industry is a $250B worldwide market with 9% cumulative annual growth rate (CAGR) and 46 % US market share. The MEMS industry is a $50B worldwide market with 12 % CAGR and 41 % US share.

• Measurement of stress distributions at transistors in semiconductor devices will enable optimized processing of nano-scale engineered "stressors," which increase carrier mobility and thus device speed.

• Stress field measurements in MEMS and NEMS devices will enable lifetimelimiting defects to be identified and hence processing to be optimized to increase device reliability.

• NIST is working with semiconductor and MEMS manufacturers (e.g., Intel, Qualcomm), and deposition and measurement tool vendors (e.g., Novellus, Ultratech) to develop measurement methods and standards for nano-scale stress measurement.

Stress measurement by Raman scattering is based on measuring the shift of Raman phonon bands in materials under stress. A confocal microscopy-based Raman scattering system has been developed with state-of-the-art stress and spatial resolution. Automated peak fitting routines enable shifts in the 522 cm^-1 Raman peak in Si to be measured with approximately 0.02 cm^-1 uncertainty, corresponding to measurement-limited stress precision of about 10 MPa (strains of about 10^-4). Scans consisting of 128 x 128 hyperspectral arrays range from wide area, 150 mm x 150 mm, to small area, 10 mm x 10 mm, the latter giving rise to a pixel spacing of 80 nm. Each spectrum takes about 1 second, enabling a high resolution stress map to be generated in about 4 hr. Measurement development has focused principally on three aspects (using single-crystal Si test vehicles): (i) verification of the scalar (tensor-averaged) piezospectroscopic coefficients using the known stress distributions of indentation flaws and their associated cracks; (ii) identification and quantification of the stress "signatures" of controlled contact-induced defects in Si using nanoindentation techniques; and (iii) determination of the limits of the technique for measuring stress variations in engineered structures. An example of (i) is shown in the figure of a stress map of a Vickers indentation on a Si (001) surface. The indicated regions in the optical micrograph (a) are associated with stress maps of the entire indentation (b) and one of the four the crack tips (c). A Raman shift trace collinear with the crack in (a) is shown in (d). Using fracture mechanics formulations, the predicted and measured stress responses were found to be in excellent agreement.

Raman stress map of a Vickers indentation flaw in Si

Progress in (ii) has been marked by the ability to distinguish stress signatures for contact defects with plastic deformation, phase transformation, and fractures perpendicular and parallel to the surface. Arrays of indentations aligned along different crystallographic directions were used in (iii) and an example is shown in the next figure. Clear differences in the local stress fields of the spherical indentations are visible.

Stress measurement by EBSD is based on distortion of the diffraction pattern by strain in the crystalline lattice. A cross-correlation method is being developed that compares 

Raman stress map of a stress-engineered Si surface

diffraction patterns obtained from scanning electron microscope scans and produces stress maps with stress and spatial resolution comparable to those obtained with the Raman method. Use of the two techniques, based on different physical principles will enable the accuracy of stress maps and standards to be increased.