Local mechanical stress in silicon structures is an important topic of concern in microelectronics technology. Any significant stress may trigger the formation of cracks into the chip areas.
Raman spectroscopy is a technique that has become increasingly popular for stress measurements in semiconductors, because it allows to carry out nondestructive measurements with high spatial resolution and high sensitivity.
In this work the measurement of stress in a micro-structured silicon sample by Raman spectroscopy is presented.
A commercially available Raman microscope Confotec™ NR500 (SOL instruments Ltd) with an Echelle grating has been used. The sample consists of 1, 1.5, 2 and 4 µm wide Si stripes separated by 4 µm shallow trenches.
Typical Raman spectra obtained in back scattering from the sample are shown in Fig.1. Any stress in the silicon structures leads to the Raman spectra modification. If compressive stress is exerted on the crystal lattice, the lattice constant decreases, and the Si peak will shift towards higher wavenumbers. When the compression is released, the peak will shift in the opposite direction to lower wavenumbers (Fig.1).
The next equation can be applied to the Si stress monitoring:
σ (MPa) = −435 • (ω – ω0) (cm−1) (1)
Where σ is the stress value, w0 = 520.5 cm-1 is the peak position of the stress-free state [1], ω is the Si peak position at the stressed state.
Equation (1) permits to understand the stress level in MPa. The Confotec™ NR500 instrument with an Echelle grating has got a spectral precision of 0.01 cm–1 (either the Si peak of the mass-center analysis or the Lorentzian fitting of the Si peak). It corresponds to the Silicon stress sensitivity of 4.35 MPa.
The line scan measurements across the Silicon stripes with 488 and 633 nm laser excitation are shown in Fig.2. The shift of the strained silicon peak from its stress free value is plotted as a function of the scan position. The stress in the trenches between the stripes is zero (Fig.2). The experimental results show a slight increase of the compressive stress from the 1 µm to 4 µm wide Si stripe (the signal shifts to higher wavenumbers). Variation of stress in Fig.2 is in the range between +7.7 MPa (tensile stress) and -108.7 MPa (compressive stress), approximately.
Two laser wavelengths with different penetration depths were used to study the depth distribution of the stress within the sample. From the data (Fig.2), the stress seems to be located at the surface as the lower stress values for the deep penetrating red laser light indicate.
Conclusions
Raman microscopy is a powerful nondestructive analysis technique for the study of local stress in silicon. The Confotec® NR500 instrument with an Echelle grating can be used to perform high-resolution stress mapping. The probed depth may be controlled by changing the wavelength of the exciting laser.
References
- V. Poborchii, T. Tada, and T. Kanayama, Appl. Phys. Lett. 91, 241902 (2007).
Fig.1. Raman spectroscopy application for Si stress analysis. Any stress in the silicon structure leads to the Raman spectrum modification. The shift of the Raman frequency from its stress-free value may be observed.
