Optical: systems and elements – Lens – With support
Reexamination Certificate
2001-04-23
2004-10-19
Mack, Ricky (Department: 2873)
Optical: systems and elements
Lens
With support
C359S819000
Reexamination Certificate
active
06807022
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to optical systems, devices, tools, and instruments, and to other systems having optical components and, more particularly, to a method for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of optical devices, and, a corresponding device and system for implementing the method thereof.
In contrast to common recreational and educational uses of optical viewing or projection devices and systems, the field of semiconductor device fabrication requires the technology of design, manufacture, and implementation of such devices and systems to be pushed to the utmost limit. Here, semiconductor devices are fabricated on silicon wafers, where a single wafer, capable of containing multiple semiconductor devices, is made up of a multiple of overlaid layers, in sequence, one on top of the other. Photolithography is an initial stage in the process of repetitively producing a single layer, involving the use of a stepper machine for optically projecting a patterned slide or mask onto a light sensitive layer or coating known as photo-resist, previously deposited onto the silicon wafer. The exposed photo-resist layer is then developed, leaving a patterned layer of photo-resist on the wafer, matching the pattern of the mask. Following completion of each layer, photo-resist can again be deposited on the wafer for forming another layer, and so on.
The continuously increasing technological requirements of complexity and speed of operation of semiconductor devices imply that wafer patterns must contain extremely fine features, on the order of a fraction of a micron wide. The design rule, or width of the finest pattern on a wafer, has significantly declined, by about a factor of 10, during the past decade. Today's fastest devices typically feature a 0.25 micron design rule, however, new devices are currently being developed using half this size. This translates to requiring a stepper featuring an optical system to achieve a high level of optical quality such that geometrical distortion and resolution of an image are each significantly less than the design rule.
Another implication of current semiconductor technology is that features printed on one layer of a wafer must be well aligned with other features, existing underneath in preceding layers, so as to minimize alignment error between layers, commonly known as overlay or misregistration error. Maximum allowable overlay error, known as overlay budget, is about one-third of the design rule.
An overlay, or registration, metrology tool operates in conjunction with the stepper, by using a microscope for viewing patterns created by the stepper. By using the overlay metrology tool for viewing patterns of different layers and a computer for image analysis, one is able to measure the overlay error between layers. Such measurements are used for calibrating, testing, and adjusting the stepper in order to minimize the overlay error. An overlay metrology tool, however, inherently introduces its own error into the overlay measurement. This error has two components, known as the accuracy error, and the repeatability or reproducibility error. Accuracy error, also referred to as Tool Induced Shift (TIS), directly arises from distortions and aberrations in the optics of the overlay metrology tool. Repeatability error may arise from several factors, optics being one of them. Since the overlay metrology tool is used to monitor and control the stepper, tolerances placed on the total error are significantly tighter. The stepper must produce an overlay better than or within the overlay budget, however, the overlay metrology tool must in turn, produce a total error less than about one tenth of that. This tight margin of total error, especially the TIS component, translates to extremely strict requirements on the optical quality of an overlay metrology tool.
A critical dimension (CD) metrology tool is another metrology tool used for calibrating, testing, and adjusting the stepper, which is used for measuring the width of the finest lines produced by the stepper. Currently, line width tools typically feature an electron microscope, rather than conventional optical viewing systems, but the latter are still used. Other types of metrology tools featuring an optical system may also be used for making critical measurements of wafer fabrication processes.
Thus, the constant drive to increase complexity and speed of semiconductor devices invokes the tightest possible constraints and tolerances on the optics and quality of steppers and metrology tools used in wafer fabrication processes. These characteristics determine and limit the achievable complexity and speed of next generation semiconductor devices.
Another field where optical quality of optical viewing devices and systems is of extreme importance is aerial or satellite photography. Although features and objects, such as landscape and buildings, viewed by such optical equipment are relatively large, the large distances from which they are viewed result in minute details appearing in a viewed image, which can be thought of as scaling to similar conditions and dimensions involved in micro-lithography.
With respect to understanding the present invention, the following terminology and definitions are provided here, referred to and used hereinafter. An optical system refers to any system including at least one optical device, along with any number of other devices, mechanisms, units, and/or components enabling functional and cooperative operation of the optical device and the system. An optical device refers to a device, such as a tool, instrument, or piece of equipment, featuring at least one optical assembly, and at least one peripheral structure and/or at least one peripheral mechanism, positioned and/or functioning along an optical path of the at least one optical assembly for enabling viewing or projecting by the optical device.
An optical assembly features at least one optical element, and at least one peripheral structure and/or peripheral mechanism positioned and/or functioning for holding, moving, or changing the direction or orientation of the at least one optical element. An optical element is ordinarily considered as a piece of material, such as uncoated or coated glass or plastic, specially shaped to affect light rays in a specific way, including refraction, reflection, transmission, absorption, diffraction, and scattering.
Exemplary optical elements are a lens, a window or flat, a reflector or mirror, and a prism. Special types of optical elements, featuring a specially configured optical element or a combination of optical elements, include a curved mirror such as a parabolic mirror, a part-mirror, and a beam splitter, also known as a cube. A part-mirror functions by partly enabling reflection and transmission. A beam splitter features two prisms geometrically configured for splitting a beam. An optical assembly featuring at least one lens and/or at least one mirror, is commonly referred to as a lens assembly. An optical assembly, such as that featured in a particle beam microscope in general, and an electron microscope in particular, can also be of a non-conventional form such as an electric field, a magnetic field, or an electromagnetic field, serving as a lens for affecting not light in the classical form, but rather in the form of charged particles.
A peripheral structure refers to a structure peripherally positioned and functioning for holding, moving, and/or changing the direction or orientation of at least part of an optical device, such as a mount, frame, cell, tube, column, barrel, turret, eyepiece, and nosepiece. A peripheral mechanism refers to a mechanism peripherally positioned and functioning for enabling operation of at least part of an optical device, such as a source for providing electromagnetic radiation such as light or a particle beam for viewing or projecting an image. An optional peripheral mechanism is positioned and functioning for enabling optional
Friedman Mark M.
Mack Ricky
Symmetritech Ltd.
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