Geraldo F. Mendes

Analysis of an active stabilization system for a holographic setup

An active stabilization system for a holographic setup based on detection of phase shift between the interference pattern and a reference hologram is described. Its basic feature is the possibility of operating for 0, pi or +/-pi/2 at will, always in a null-detection mode. The reference hologram may be a previously recorded permanent hologram or a real-time (even reversible) one. The use of the open loop response of the stabilization system is developed for analyzing its performance, which allows closer insight into parameters limiting Its behavior. The effects of different noise sources are analyzed in detail. The real-time effect in a positive resist is successfully employed for operating the stabilization setup for recording an improved grating in this material.

Optical monitoring of the end point in thin film plasma etching

Abstract An in-process optical technique is described for accurately monitoring the end point in plasma etching processes. A grating pattern is lithographed somewhere in the film to be etched. The grating modulation decreases as the film is etched out and the process may be monitored by measuring the diffraction of a low power He-Ne laser beam aimed at the grating. The etching end point is accurately detected by the disappearance of all diffracted orders. The laser beam does not need to be directed at normal incidence and so any available plasma etching equipment is suitable. The detection is carried out with low cost photovoltaic detectors but simple visual inspection is satisfactory also. Comparative experimental results are presented.

Gratings for metrology and process control. 2: Thin film thickness measurement

A method of measuring thin film thickness is described, based on a previous paper where the authors analyzed a graphical means for grating modulation calculation. Metallic or dielectric films on any substrate may be measured, and precision was shown to be comparable with that achieved ellipsometrically at least in the 300–1500-A thickness range. The method is non-contact and destructive: the grating is recorded on the film. Measurements are simple, requiring a low-power He–Ne laser and a photodetector, and may be carried out at a distance from the sample. Experimental results are presented for three types of sample, including measurements by reflection and transmission.

Stabilized holographic recording using the residual real-time effect in a positive photoresist

The weak real-time effect in a positive-resist film was used for operating an actively stabilized holographic setup in order to carry out the entire recording process in a stabilized mode. Illustrative experimental results are reported. The successive recording in phase of spatially shifted holographic gratings is also shown.

Gratings for metrology and process control. 1: a simple parameter optimization problem; errata

A simple graphical method is developed for calculating modulation and groove-to-period ratio of a shallow lamellar grating from its diffraction spectra and for simultaneously checking its assumed shape. It applies to both reflection nd transmission and to the conducting or dielectric nature of both the grating and its substrate. The effect of noise and distortion is briefly discussed. The method is illustrated for the limiting case of a reflecting metallic grating. Uses will be presented in subsequent papers.

Plasma-etched depth measurement of films using the diffraction of a lamellar grating

Abstract An appropriately structured grating allows accurate measurement of the thickness of a thin film. We propose the use of this technique to measure the point-by-point evolution of plasma etching, and consequently the etching rate. We compare this real-time remote sensing technique with other available techniques. We note that this technique is suitable for measuring rapid fluctuations in plasma processes. We describe some experimental results concerning a photoresist film coated on a silicon wafer and predict the theoretical behaviour of other commonly occuring films.

Fourier synthesis for fabricating blazed gratings using real-time recording effects in a positive photoresist

A method is described which allows the relative phase-shift adjustment of two successively recorded holographic gratings. The residual real-time recording effect in a positive resist was used as a reference hologram for operating an actively stabilized holographic setup and for adjusting both the spatial frequency and the corresponding phase shift of the successively recorded Fourier components to produce a predetermined periodic profile structure in the film. Synthesis of a blazed holographic grating is reported here as an illustration of the possibilities of the process. The method may find use in optical and electrooptical device fabrication requiring accurate spatial frequency and phase adjustment.

Pinhole Imaging Holograms For Optical Interconnects

We suggest the use of a pinhole imaging hologram as a new approach for optical interconnects. It can avoid the formation of intermodulation gratings and has some other advantages. On recording, the object beam passes through a pinhole thus forming an image plane hologram, but when reconstructing the pinhole can be removed. The reconstructed image can be programmed and processed by using optical information processing elements placed in the position of the reconstructed pinhole or pinholes. We present theoretical considerations and experimental results for pinhole holograms produced for holographic optical interconnects.

Active Stabilization For Real-Time Holographic Recording

The two-wave mixing of the recording beams at a photorefractive crystal during real-time holographic recording is used for operating an active system for the stabilization of the holographic setup. A previously recorded permanent hologram does also perform two-wave mixing and may similarly operate the stabilization system. The performance of both stabilization methods are quantitatively characterized and compared.

Response curve for photosensitive films: a derivative method.

A derivative technique is developed for measuring the material response of photosensitive films. The method consists of recording a low modulated grating superimposed with different uniform preexposures. The modulation of the resulting shallow gratings recorded on the photosensitive film is measured by diffraction techniques and the derivative response is thus computed. Compared with other diffraction-based methods, this one is not affected by nonlinearities. Qualitative visual monitoring of these diffraction data may be used for optimizing materials and processes in photolithography and optical recording. Some experimental results concerning positive and negative photoresists are presented.