The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

The theory of image formation is formulated in terms of the coherence function in the object plane, the diffraction distribution function of the image-forming system and a function describing the structure of the object. There results a four-fold integral involving these functions, and the complex conjugate functions of the latter two. This integral is evaluated in terms of the Fourier transforms of the coherence function, the diffraction distribution function and its complex conjugate. In fact, these transforms are respectively the distribution of intensity in an 'effective source', and the complex transmission of the optical system-they are the data initially known and are generally of simple form. A generalized 'transmission factor' is found which reduces to the known results in the simple cases of perfect coherence and complete incoherence. The procedure may be varied in a manner more suited to non-periodic objects. The theory is applied to study inter alia the influence of the method of illumination on the images of simple periodic structures and of an isolated line.

On the diffraction theory of optical images

Combined effects of amplitude and phase variations on the irradiance in the image of a periodic complex object, which has amplitude and phase distributions, have been investigated for an optical system with partially coherent object illumination. A general expression to obtain the illuminance in the image and the image contrast of such an object is derived by use of the concept of the effective source specifying the coherence condition. The irradiance in the image for the diffraction-limited aberration-free optical system illuminated with bounded effective sources that have uniform and nonuniform radiance, and also with annular illumination, have been calculated. Numerical calculations have been made to obtain the image, in which harmonics up to the thirteenth order are taken into account. One of the results is that abrupt amplitude and phase variations are extremely influential for the appearance of the image of a periodic, low-contrast, complex object for any mode of illumination. On the other hand, less abrupt changes of the amplitude and phase in a complex object produce less variation of the appearance and contrast of the image for any illumination mode except annular illumination. Images of pure amplitude and phase objects have also been obtained by use of the general treatment, and their characteristics are evaluated. It is suggested that a technique to manipulate the radiance distribution in the effective source is valuable in improving the fidelity of the image of a periodic complex object formed by an optical system under partially coherent illumination.

Image of a periodic complex object in an optical system under partially coherent illumination

A method for analyzing an integrated circuit (IC). At least a portion of a layer of the IC is scanned using high magnification, to obtain images of the IC. The images are registered to create a mosaicked image. An IC layout database is created in the form of a set of polygons from the mosaicked image, where the step of creating the IC layout database is performed after, or pipelined with, the registering step. The process is repeated for plural IC layers, as necessary. Polygon sets from each layer are vertically registered into alignment with minimal distortion. A netlist or schematic diagram is generated to represent the scanned IC portion based on the registered set(s) of polygons.

Method to extract circuit information

A method for extracting design information from a semiconductor integrated circuit (IC) or at least a portion thereof comprising the steps of: (a) imaging at least a portion of one or more IC layers to obtain stored images of said portions of the IC; (b) using manual or automatic registration techniques to mosaic images; (c) using an IC layout package possessing a feature of allowing images to be displayed and moved and polygons to be created to allow the recreation of the IC layout in the form of polygons; (d) exporting or storing of a polygon database in a standard IC layout format; (e) creating a table of transistor connections (netlist); (f) organizing circuit netlist into functional blocks of increasing complexity; and (g) generating a schematic diagram.

Computer-assisted design analysis method for extracting device and interconnect information

Defocused image formation is treated theoretically from the viewpoint of coherence theory. Results are given for amplitude and phase bar objects and for periodic objects. By contrast to the defocused imaging of pure amplitude objects, the intensity distribution of pure phase objects depends on the defocus direction. We have found an asymmetrical behaviour of ¦grad I(δ)¦max (I(δ) is the intensity distribution at the image plane) which is strongly influenced by the coherence parameter S and the phase part of the object.

G. Nitzsche

Papers

Partially coherent defocused imaging of bar and periodic objects

Einfache Modelle zur Simulation der Abbildung 3-dimensionaler Strukturen

Optical preprocessing as inspection tool

Influence of schlieron on imaging properties of an optical system. II, Modulation transfer function (MTF)

High focal depth imaging by beam shaping optical elements