Wednesday, December 30, 2009

Tokyo Electron Enhances Nano-Scale Wafer Inspection with Photonic Nanojet Metrology System



With the current drive towards smaller geometries of integrated circuit (IC) devices, measurement of IC device features is increasingly difficult as the features become smaller. Optical microscopy and spectroscopy technologies are well established. However, there are fundamental limitations of conventional optical microscopy.  Tokyo Electron's photonic metrology system overcomes these limitations with microsphere-generated nanojets which are unaffected by the diffraction of light which limits wafer inspection by conventional optical microscopy.  

In U.S. Patent 7,639,351 Tokyo Electron Limited (Tokyo, JP) inventors Zhigang Chen, Hanyou Chu, Shifang Li, and Manuel Madriaga  (San Jose, CA) detail the operations of a photonic nanojet metrology system which is able to detect nanoscale structures and defects on a wafer when manufacturing semiconductor devices with higher resolution than can be obtained using conventional methods.   

The inspected structure can be any isolated, nonperiodic, or periodic object formed on the semiconductor wafer, such as a gate, line, contact hole, via, drain, periodic structure, and the like. Additionally, the structure can be foreign matter, such as a contaminating particle.  

By determining the existence of the structure, the fabrication process can be evaluated. For example, if a structure is intended to be formed in a specific location on the wafer, the specific location can be examined to determine if the structure exists. If the structure does not exist, then a fault in the fabrication process can be detected.   

Alternatively, if a specific location on the wafer should be unpatterned, then the specific location can be examined to determine if a structure, including a contaminating particle, exists. If the structure exists, then a fault in the fabrication process or contamination of the fabrication process can be detected.  

A metrology system using a photonic nanojet can strongly interact with nanoscale particles and structures and cause several orders-of-magnitude enhancements in the backscattered signature from the nanoscale structures. Further computational investigation of the photonic nanojets has confirmed that photonic nanojets do greatly enhance the effective backscattering of light by nanometer-scale dielectric particles located within the nanojets.   

 This backscattering enhancement for nanoparticles exists for the nanojets generated by both microcylinders and microspheres. The only difference is that the order of magnitude of the enhancement is much higher in the case of microsphere-generated nanojets than in the case of microcylinder-generated nanojets.  

The inventors note that nanojet-inducing dielectric microsphere analysis differs significantly from the traditional microlens in terms of physical mechanisms.  A photonic nanojet system is a backscattering-detection system  as opposed to an imaging lens system. As a result, it is not affected by the usual diffraction limit. The effective backscattering of the nearby nanosphere is enhanced by the mutual interaction between the nano and microspheres.   

 The nanoparticle is first excited by the photonic nanojet emerging from the microsphere, and its scattering intensity is elevated by two orders of magnitudes, as determined by the intensity of the nanojet. The scattered fields generated by the nanojet-excited nanoparticle propagate into the microsphere, which leads to non-Rayleigh backscattering of light by the nanoparticle as part of the combined system. This interaction elevates the backscattered intensity from the nanojet-excited nanoparticle by four to nine additional orders of magnitude.  

The Tokyo Electron method of controlling a fabrication cluster using photonic nanojet optical metrology includes: performing a fabricating process on a wafer using a first fabrication cluster; generating a photonic nanojet, wherein the photonic nanojet is an optical intensity pattern induced at a shadow-side surface of a dielectric microsphere; scanning an inspection area on the wafer with the photonic nanojet; obtaining a measurement of retroreflected light from the dielectric microsphere as the inspection area is scanned with the photonic nanojet; determining the existence of a structure in the inspection area with the obtained measurement of the retroreflected light; and adjusting one or more process parameters of the first fabrication cluster based on the determination of the existence of the structure in the inspection area. 

A measurement is obtained of the retroreflected light from the dielectric microsphere as the photonic nanojet scans the inspection area. The existence of a structure in the inspection area is determined with the obtained measurement of the retroreflected light. One or more process parameters of the fabrication cluster may be adjusted based on the determination of the existence of the structure in the inspection area. 

In the case of imaging objects with optical fields propagating in the far-field zone, the fundamental constraint is the diffraction of light that limits conventional optical microscopy to a spatial resolution comparable to one-half wavelength, or about 200 nm for visible light. As problems of interest push further into the nanometric regime, the importance of imaging techniques that allow nanoscale resolution or sensitivity has been steadily increasing. 

FIG. 2 is an architectural diagram of a photonic nanojet metrology system.


FIGS. 3a-c illustrates the evolution of a photonic nanojet.



FIG. 11 is a flow diagram illustrating a Tokyo Electron process of controlling a fabrication cluster using photonic nanojet optical metrology.  








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