MEASURING THE OPTICAL PROPERTIES OF SUB-MICRON PARTICLES THROUGH THE OPTICAL THEOREM METHOD
Creators: name: SANVITO, TIZIANO
Publisher: name: Universit� degli Studi di Milano
Year: 2013

description: We describe two novel light scattering methods for the sizing of small particles in fluids. The two methods stem from the applications of the celebrated Optical Theorem OT for metrological purposes recently developed by the Optics Grup at the University of Milan.
Basing upon a novel insight into the fundamentals of the OT for the case of depolarized light scattering, we developed a novel Dynamic Depolarized Light Scattering technique (DDLS). It is based upon a tight confocal, zero scattering angle, heterodyne scheme. The method is almost immune from any multiple-scattering contributions, so that it can work with highly concentrated, non-index-matched colloidal suspensions. This overcomes the typical limitations of the traditional DDLS methods. This possibility has never been possible before, and opens a completely new approach in the study of colloidal suspensions. Operated with known samples, the method brings the expected results for the size as well as the polydispersity, determined through the cumulant methods modified to the aim. Results and characterizations of MFA spherical particles, PMMA core-shell particles and noncentrosymmetric BaTiO_3 nanocrystal particles are discussed. We then describe a method for calibration-free measurements of size and refractive index of single submicron particles through a self-reference interferometric scheme relying on the fundamentals of the OT. The development of this instrument follows a couple of patents owned by UNIMI. This method has been exploited within a FP7-ENIAC project IMPROVE, in collaboration with a SME from Italy and Micron Semiconductors Inc. We studied the feasibility of measuramentes on liquid suspensions of interest for nanoelectronic fabs, such as ultrapure water and slurries for chemical-mechanical polishing CMP of wafers.
This machine (named OTM) is capable of sizing and determining the properties of single particles by means of analysis of the intensity distribution resulting from interference between the forward wave scattered by a particle and the fraction of the incident wave which propagates undisturbed. Particles are driven with constant and uniform speed through the focal region of a laser light beam, and both the scattered and the transmitted light are collected by a sensor placed in the far field of the main beam.
We derived a semi-analytical model describing the OTM system without any free parameter, and compared experimental and analytical results for calibrate polystyrene spherical particles. To increse the size sensibility and resolution of the OTM, a dedicate custom FE electronics has been exploited. The same device has been used for an extended characterization of the intensity stability of different laser sources.
A dedicated C/LabView interface software has been developed for data acquisition and analysis. A proper Pulse Shape Analysis (PSA) as been developed and the structure of the data reduction is discussed in details.
The advantages of OTM with respect to current methods are: high size resolution; estimate of the particle refractive index; completely calibration free; very simple optical layout. Moreover, thanks to fundamental scattering properties, the forward scattered wavefronts show minimum dependence on the particle shape, thus limiting one of the typical sources of uncertainties of traditional light scattering techniques.
Two applications of the OTM have been develped so far. A collaboration with Micron Semiconductors Inc. has been started in order to characterize Cerium oxide slurries for the qualification of CMP processes. A FP7 project has been submitted in collaboration with SMEs and laser manufacturers for adpating the existing device to the clean room of Micron, and implementing the method to be exploited as a reference within the Advanced Process Control activities.
A portable version of OTM has also been developed to carry out the characterization of water particulate suspension in the size range 1um - 10 um. In collaboration with a group of geologists from Chambery University we studied the solid water suspensions in Miage lakes (Val Veny - AO) and in Pre de Bar glacier and its outcoming stream (Val Ferret - AO).