OMI describes a new approach to in-line strained SOI metrology.
Strained-SOI (sSOI) requires fast, accurate and non-contact strain metrology. To address new demands in the engineered substrates industry, several techniques compete: Photoreflectance spectroscopy, Raman spectroscopy, x-ray diffraction and optical birefringence. Photoreflectance appears as one of the most promising of these techniques.
Biaxial strain in sSOI modifies the electronic band structure of silicon. The resulting increase in electron mobility enables devices in strained silicon to run at higher clock frequencies with lower power requirements. But the modifications in the band structure can also be used as a strain probe by photoreflectance spectroscopy. Photoreflectance measures the energy of a particular transition (E1 transition) that gives rise to a strong absorption in the near ultraviolet for silicon. Biaxial strain results in a split of the valence band and subsequently of this photoreflectance signal. A pair of lineshapes emerges with strain, one of which shifts to lower energy. For example, an inplane tensile strain of 1% shifts this transition energy by 137.5 meV. This shift is easily and accurately measured by photoreflectance spectroscopy.
Traditional photoreflectance techniques require the spectra to be acquired in sequence resulting in long measurement times. The combination of a multi-channel demodulator with an array detector enables a full-range measurement of the spectrum simultaneously. Rapid photoreflectance spectroscopy enables acquisition time of the order of seconds, representing an important breakthrough in strain metrology. Optical Metrology Innovations (OMI), Inc. and Soitec are collaborating to develop this technique as in-line metrology for the silicon strain in sSOI.