The use of optical metrology techniques for process control is now widespread. These techniques are fast and nondestructive,
allowing higher throughputs than non-optical techniques like electron microscopies or AFM. We present
here new developments using complete Mueller polarimetry in the back focal plane of a microscope objective to
characterize overlay for microelectronic industry. Based on fundamental symmetries in the physics of periodic structures
and polarized light and redundancies in the angle-resolved Mueller images we define estimators which vary linearly with
the overlay. As a result, overlay measurement is sensitive to both the direction and sign of the overlay, and it does not
require any detailed modeling of the target structures, provided two independent targets with known overlay values are
available in close locations on the wafer. Realistic simulations on optimized structures suggest that accuracies in the
order of 1 or 2 nm or better should be achievable. Moreover, with high NA objectives the proposed technique can be
implemented with targets with lateral sizes as small as a few μm. Experimental results of both grating line profiles and
overlay determinations will be presented. The samples, elaborated at LETI, have been accurately characterized by optical
imaging AIM techniques and state-of-the-art AFM. The latest developments on the device itself as well as the
advantages, possibilities and limitations of this new metrology technique will be discussed.
Angle resolved Mueller polarimetry implemented as polarimetric imaging of a back focal plane of a high NA microscope objective has already demonstrated a good potential for CD metrology. Here we present the experimental and numerical results indicating that this technique may also be competitive for the measurements of overlay error δ. A series of samples of superimposed gratings with well controlled overlay errors have been manufactured and measured with the angle resolved Mueller polarimeter. The overlay targets were 20-μm wide. When the overlay error is δ is equal to 0, absolute values of elements of real 4×4 Mueller matrix M are invariant by matrix transposition. Otherwise this symmetry breaks down. Consequently, we define the following overlay estimator matrix as E = |M| − |M|t. The simulations show that matrix element E14 is the most sensitive to the overlay error. The scalar estimator of E14 was calculated by averaging the pixel values over a specifically chosen mask. This estimator is found to vary linearly with δ for overlay values up to 50 nm. Our technique allows entering small overlay marks (down to 5-μm wide). Only one target measurement is needed for each overlay direction. The actual overlay value can be determined without detailed simulation of the structure provided two calibrated overlay structures are available for each direction.
Angle resolved Mueller polarimetry implemented as polarimetric imaging of the back focal plane of a high NA
microscope objective has already demonstrated a good potential for CD metrology<sup>1</sup>. In this paper we present the
experimental and numerical results which indicate that this technique may also be competitive for measurements of the
overlay error δ between two gratings at different levels. Series of samples of superimposed gratings with well controlled
overlay errors have been manufactured and measured with the angle resolved Mueller polarimeter. The overlay targets
were 20 μm wide. When overlay error δ = 0 the absolute value of Mueller matrix elements is invariant by matrix
transposition. This symmetry breaks down when δ ≠ 0. As a result, we can define the following overlay estimator matrix:
Ε = |Μ | - |Μ |<sup>t</sup>. The simulations show that matrix element E<sub>14</sub> is the most sensitive to the overlay error. In the
experiments the scalar estimator of E<sub>14</sub> was defined by averaging the pixel values over specifically chosen mask. The
scalar estimator is found to vary essentially linearly with δ for the overlay values up to 50 nm. Our technique allows
entering quite small overlay marks (down to 5 μm wide). The only one target measurement is needed for each overlay
direction. The actual overlay value can be determined without detailed simulation of the structure provided the two
calibrated overlay structures are available for each direction.
We designed and built Matrix Distributed ECR (MDECR) PECVD reactor dedicated for dielectric filters deposition and equipped it with multiple sensors for process control. Planar matrix geometry of plasma source is based on electron cyclotron resonance effect at 2.45 GHz microwave frequency and provides scalability of the deposition on large area substrates. High (up to 5 nm/sec) deposition rate obtained due to high dissociation efficiency and careful design of the gas injection system. Optical emission spectroscopy, quadrupole mass-spectrometry and spectroscopic and multi-channel kinetic ellipsometry are installed for in-situ studies and control of the film deposition. We performed studies of the nature of high-density plasma discharge in silane, oxygen and nitrogen mixture and correlated its properties with optical and physical properties of deposited materials. To demonstrate the capabilities, a wide band gradient index antireflection coating on glass was realized by deposition of SiO<sub>x</sub>N<sub>y</sub> alloy thin films. The predefined variation of an index in a profile is obtained by changing the flows of precursors. Real-time control is performed with multi-channel kinetic ellipsometry.