PART 2: OPTICAL STRAIN METROLOGY
Terence K.S. Wong
In this part, we describe three strain metrology techniques that make use of the interaction of light with a
sample to measure strain. The optical interactions are: (i) reflection, (ii) polarization change and (iii)
inelastic scattering. Although the interactions used are different for each technique, they share a common
principle that they are based on the so-called morphic effects of semiconductors. The morphic effects are
basically perturbation effects by external influences such as stress or pressure on the crystal structure and
the band structure of a semiconductor. By measuring the changes in the band structure, it is possible for
instance to infer the magnitude of the strain that causes the perturbation.
All three techniques make use of visible and ultraviolet light and are non-destructive and non-invasive.
They therefore involve little or no sample preparation and are well suited to the routine process monitoring
of strained semiconductor substrates such as strained silicon and strained silicon on insulator. However, the
techniques are mainly ex situ and with the exception of the Raman spectroscopy technique cannot be
applied to individual transistors on wafer. The tip-enhanced Raman technique with higher spatial resolution
is at present under development and is not yet ready for manufacturing.
Division of Microelectronics School of Electrical and Electronic Engineering Nanyang Technological University