Effective Depth Of Focus Research Articles

Study of the influence of the shape of projection lens focal plane on the focus control of advanced lithography

The high-precision focus control technology becomes more and more important, as the effective depth of focus (DOF) shortens dramatically in advanced lithography. In conventional focus control technology, the focal plane of projection lens is assumed as an ideal plane, but in fact it is a curved surface. The difference between the ideal plane assumed and its real shape results in partial defocus of wafer exposure field, thereby affecting the exposure quality. In our current work, the real shape of projection lens focal plane is brought into consideration for the focus control, and the simulation shows that this method can effectively improve the accuracy of focus control by 26%.

Compensation of phase aberration by using a virtual confocal scheme in digital holographic microscopy.

This work presents cost-effective, simple arbitrary phase-step digital holographic microscopy to suppress both zero-order and twin-image terms. A virtual confocal offset lens under in-line configuration is also used to compensate for the introduced quadratic phase by using a microscope objective lens. In addition to reducing the difficulties of physical confocal configurations, the proposed method significantly increases the magnification power, ultimately achieving the purposes of an optical zoom. An attempt is also made to reduce the noise interference of a high magnification system by developing a long focal lens to reduce light detection size, subsequently gaining an approximately plane wave light source to illuminate the object within the effective depth of focus. Experimental results indicate that the proposed high magnification system can be elevated with low noise interference, and image reconstruction without quadratic phase terms.

Optimized extended depth of focus simulated analysis for confocal microscopy

In imaging systems and especially in confocal microscopy systems there is a trade-off between the lateral resolution and the obtained depth of focus. The use of complex pupils to improve the lateral resolution by engineering the point spread function is a common approach; however the lateral improvement reduces the effective depth of focus and therefore the fluorescence efficiency. In this work we analytically develop an optimized approach for obtaining a complex pupil with an extended depth of focus. The proposed solution is numerically applied and tested in designing an improved focal depth in confocal microscope configuration.

Simplified dynamic focus method for time domain OCT

A new optical arrangement for performing dynamic focus in time domain optical coherence tomography (OCT) is demonstrated. Unlike previously reported schemes which require mechanical coupling of the object and reference arms, this method is confined to the object arm only and therefore does not impose design constraints on the OCT system layout. The scheme is tested on a high lateral resolution OCT system (NA = 0.13) and it is shown that the effective depth of focus is extended from 200 mum to better than 2 mm. The optimum correction is for media with a mean refractive index of 1.4.

Confocal Raman Microspectroscopy through a Planar Interface

We have developed a model to describe the effect of refraction through a planar interface on the collection efficiency and depth of focus when performing confocal Raman microspectroscopy. The planar interface introduces spherical aberration, which can substantially degrade the performance of the microscope, especially for large-numerical-aperture microscope objectives. This spherical aberration will increase the range of focal depths spanned by the paraxial and marginal rays of the illuminating laser beam within the sample. In the collection path, it will also distort the scattering volume defined by the confocal aperture; this results in a dramatic fall in the collected light intensity with increasing depth. We demonstrate that there is an optimum numerical aperture for collected light intensity at a given depth. The prediction of this theoretical model is compared to empirical results obtained by mapping the stress distribution within the diamond anvil of a high-pressure cell. Both the collected Raman intensity and the effective depth of focus are compared to the predictions from the theory.

Phase Defect Repair Method for Alternating Phase Shift Masks Conjugate Twin-Shifter Method

Phase shift masks (PSMs) enable current optical exposure systems to provide significantly higher resolution in effective depth of focus (DOF). Localized phase errors, or other transparent defects in the mask phase shifter elements, can cause loss of DOF and degradation of image contrast. Transparent defects of this type have prevented PSMs from becoming practical for large-scale production of integrated circuits containing deep submicrometer features. This paper describes a new technique that is useful for repairing Alternating PSMs containing transparent defects in the phase shifter elements.

Calculation and Measurement of Image Formation, Spatial Resolution, and Effective Depth of Focus for a Scanning Microscope with Confined Electromagnetic Waves

The resolution of scanning microscopes with confined waves is mainly determined by the diameter of the confinement region instead of by the wavelength. The ‘effective depth of focus’, i.e. the tolerable distance between the plane of minimum confinement of waves and the object plane is about one-quarter of the diameter of the confinement cross-section.

How to increase the effective depth of focus of the light microscope for photography.

Photography of objects whose dimensions exceed the depth of focus of a given objective may be effected by dividing the total necessary exposure between multiple underexposures on the same piece of film with the microscope focused at different levels.

Dynamic Focusing Techniques for Enhancing SEM Images

There are two major areas where Dynamic Focusing techniques can be used to enhance SEM image qualify and micrograph information content. When used in conjunction with the final condenser lens in the electron-optical column, it increases the effective depth of focus associated with an inclined specimen in the SEM. When used with an ultra-high resolution recording cathode ray tube it provides exceptional corner-to-corner sharpness on the micrograph.Most commercial scanning electron microscopes use a tilted specimen positioned close to the final condenser lens with the secondary electron collector system located in the tilting direction to facilitate efficient electron collection. electron beam pivoting through the center of the principle plane of this lens to minimize distortion and off-axis aberration problems. As a result, the final electron beam is truly focused only along a portion of the spherical surface with its center located at the pivoting point.

Power and Focusing Considerations for Recording with a Laser Beam in the TEM_00 Mode

Some of the aspects of recording and reading with a focused laser beam in the TEM(00) mode are analyzed. The effects of defocusing and laser power variations on the recorded spot size are presented for various peak-to-threshold intensity ratios. It is shown that by suitably choosing the f/number of the optical system minimum recording power is required. The distinction between optical writing and reading depth of focus is made. It is concluded that the effective depth of focus can greatly exceed the optical depth of focus, thus relaxing mechanical stability requirements in a practical laser recording system.