Defect detection algorithm for wafer inspection based on laserscanning

A defect detection algorithm for wafer inspection based on laser scanning is presented. Microscopic anomalies, contaminants, and process induced pattern defects result in a two-dimensional (2-D) laser scattering signature, which closely resembles the coherent point-spread-function of the scanning laser beam. This point-spread-function is a 2-D Gaussian in the majority of cases and can be characterized by four parameters. The algorithm fits Gaussian surfaces to sampled data points. Events are accepted or rejected on the basis of how similar the Gaussian parameters are to that of the point-spread-function, known a priori. It is shown that the algorithm achieves a 95% capture for submicron particles and pattern defects on typical logic and array wafer regions. Results demonstrating the algorithm's performance relative to mechanical and electronic noise and to signal resolution are presented     

High-resolution ultrasound transverse flow measurement

A method for measuring the transverse component of flow velocity with high spatial resolution is reported. This technique is based on recording and cross-correlation of the signals scattered from two adjacent foci, produced by a newly developed two-beam acoustic lens. Experiments at 4.5 MHz are described and illustrated by results obtained from flow in the range 50-100 cm/s     

Laser-induced fluorescence spectroscopy and imaging ofsemiconductor wafers

Fluorescence spectra of selected films used in microelectronic fabrication have been recorded. We have used a 0.125-m focal length spectrophotometer and a 400-line/mm grating resulting in 4.2-nm spectral resolution. The optical setup employs a laser at 364 nm for excitation and a dark-field collection configuration-a geometry that we routinely use for laser scanning for inspection purposes. A simple, though thorough, analysis and methodology for the removal of the system spectral response is presented. Results show that films used in microelectronic fabrication, in general, yield a broadband fluorescence spectrum under 364-nm excitation. Further, a scanning system that bases the image contrast on laser-induced fluorescence from the wafer surface is described and demonstrated. It is shown that this is a particularly useful inspection/review modality when the wafer is at poly/metal process level and the contaminant is a fall-on or residue of an organic material     

Focusing ultrasound in biological media

The authors present a formulation for calculating the field distributions for an ultrasonic imaging system when focusing in biological media. A biological medium is characterized in terms of its complex and frequency-dependent compressibility that, in turn, depends on the medium relaxation time and specific heat ratio. The analysis is based on a spatial/temporal transfer function formalism and can predict the effects of dispersion, frequency-dependent attenuation, aperture apodization and pulse shape. These effects can be observed on a ;snapshot' of the field in time and space. A number of simulations for a 3-MHz broadband system are presented. The results indicate that, aside from geometrical considerations (F/number), the pulse-width and the dispersive processes can play a major role in imaging performance.     

Pulse-echo single frequency acoustic nonlinearity parameter (B /A) measurement

A pulse-echo single-frequency method for B/A measurement is reported. The technique is based on excitation of the medium at a progressively increasing amplitude level and the computing of B/A from the deviation in linearity of the transducer input-output relationship. Starting from the nonlinear wave equation, a three-dimensional analytical formulation that predicts the approximate field distribution for finite-amplitude waves is derived. This provides the computation for the transducer output voltage for progressively increasing signal levels. It is shown that, by a suitable normalization, the effects of the diffraction can be excluded. From a nonlinear least-squares fit of the experimental data to the analytical equation, the B/A is computed. A number of simulations for the model together with preliminary experiments that were carried out at 3.5 GHz are presented     

Pulse compression acoustic microscopy at 750 MHz

A pulse compression acoustic microscope working at a centre frequency of 750 MHz is reported. The pulse expansion and compression is achieved by SAW dispersive delay lines. The transmitted chirp has a bandwidth of 150 MHz and a dispersion of 1 MHz/ns. A processing gain in excess of 10 dB has been achieved.     

Pulse-compression subsurface acoustic microscopy

A pulse-compression, reflection acoustic microscope is described. This system has been used to image the interior planes of solid objects. We have demonstrated the advantages of this system over the conventional RF pulse acoustic microscopy in terms of detection sensitivity and signal/noise ratio. Subsurface micrographs of solid objects taken at 60 MHz are presented.     

Rayleigh wave suppression in reflection acoustic microscopy

Leaky Rayleigh waves, generated by an acoustic lens, may give rise to image artifacts in the reflection acoustic microscope. A new acoustic lens:transducer to suppress the Rayleigh wave excitation is proposed and demonstrated in experiments at 4 MHz.     

Transform techniques for analysis of thermal wave propagation in anisotropic composite materials

A general formulation for solving the three-dimensional thermal diffusion equation in anisotropic media is presented. The method is based on two-dimensional Fourier transform techniques and can provide a physical insight into the problem. The analysis can easily be adapted to take into account arbitrary spatial variations of the excitation beam (i.e., a laser or an electron beam). Results obtained from propagation of thermal waves in composites are presented and followed by simulations for cases where the source function is defined. Propagation through an anisotropic slab is formulated and applied to specific cases.     

Differential phase contrast acoustic microscopy using tilted transducers

A differential phase contrast reflection acoustic microscope is reported. In this system the lens is illuminated by two beams, tilted with respect to the lens axis. This leads to two adjacent off-axis foci at the focal plane of the lens. The phase difference between the signals received from these two foci provides the image contrast. Experiments at 10MHz are described and illustrated by preliminary results.