Tests resolve tension over silicon strain
By Matthew Miller, Editor-in-Chief, EDN.COM - January 8, 2009
In work that should help silicon-device makers optimize materials and processes, scientists at the NIST (National Institute of Standards and Technology) have demonstrated the ability to measure stress in regions of a semiconductor device as small as 10 nm in diameter.
The testing also helps to explain a longstanding discrepancy between two methods of measuring such strain: CRM (confocal-Raman microscopy), which measures frequency shifts in photons after they interact with atomic bonds in the crystal, and EBSD (electron-back-scattered diffusion), which analyzes the patterns of electrons bouncing back from crystal planes. Using refined forms of both types of instruments, the NIST researchers discovered the reason for the differing measurements: Whereas EBSD samples only the top 20 or 30 nm of a material, the photons in CRM penetrate 1 micron or more. By varying the wavelength and focus of a CRM system so that it measured the same region as EBSD, the researchers obtained results that agreed closely.
By in essence calibrating the two types of strain measurements, the work should help device makers control not only unwanted stress, which can, for example, shift the output color and reduce the lifetime of LEDs, but also the desirable stress that chip makers deliberately create to enhance transistor speed.