The assignee for this patent, patent number 8773637, is
Reporters obtained the following quote from the background information supplied by the inventors: "A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the 'scanning'-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
"Whichever type of apparatus is employed, the accurate placement of patterns on the substrate is a chief challenge for reducing the size of circuit components and other products that may be produced by lithography. In particular, the challenge of measuring accurately the features on a substrate which have already been laid down is a critical step in being able to position successive layers of features in superposition accurately enough to produce working devices with a high yield. So-called overlay must be achieved within a few tens of nanometers in today's sub-micron semiconductor devices, down to a few nanometers in the most critical layers.
"Consequently, modern lithography apparatuses involve extensive measurement operations prior to the step of actually exposing or otherwise patterning the substrate at a target location. These operations, being time-consuming, limit the throughput of the lithography apparatus, and consequently increase the unit cost of the semiconductor or other products. Various steps have been taken to mitigate these delays in the prior art. For example, an arrangement provides dual wafer tables, so that two wafers can be loaded in the machine simultaneously. While a first wafer is undergoing exposure in an exposure station, a second wafer is undergoing measurement processes to establish an accurate `wafer grid` and height map. The apparatus is designed so that the tables can be swapped without invalidating the measurement results, thereby reducing the overall cycle time per wafer. Other techniques to process measurement and exposure steps in parallel may be employed as well.
"One measurement task, which is typically used as a datum for interpreting many other measurements, is the image alignment measurement, by which a pattern projected by the patterning device itself is picked up by sensors coupled, directly or indirectly, to the substrate support. These sensors, in combination with sensors measuring relative positional movements of the substrate table in three dimensions, provide the datum levels by which other measurements can be used to place a desired portion of the substrate accurately in X and Y directions, and also in a Z (focus) direction. The accuracy and repeatability of these datum levels, whether individually or in statistical combinations of multiple measurements, is a limiting factor of the overall accuracy of patterning location and focus.
"Known image alignment sensors comprise sensors fixed in relation to the substrate table (fixed at least for a duration of the image alignment measurement). The positioning subsystem for the substrate table is used to move the table so as to scan the sensor in X, Y and Z directions through the projected radiation field. By interpreting the measured intensity of the sensor signals at various values of X, Y and Z, the actual position of the projected image can be derived in terms of the appropriate coordinate system of the positioning subsystem.
"In seeking to increase the accuracy of such a system, various obstacles are encountered. Firstly, the scanning motion for image alignment inevitably induces vibrations, and therefore inaccuracies in the image alignment result. To reduce the vibrations by reducing the speed of scanning would delay the measurement, and could impact overall productivity (throughput). Moreover, the positioning subsystem for the wafer table is not optimized for the image alignment scanning operation, but rather for the exposure operation. The resulting positioning inaccuracies during the image alignment scan can result in uneven sampling of the projected image.
"While these errors have been within tolerances for present generations, any source of error will become significant as manufacturers strive toward the goal of reaching ever-higher levels of positional accuracy. Speed of measurement is also key to improving throughput, while cost of the apparatus is also a factor."
In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "It is desirable therefore to mitigate further the measurement overhead and/or measurement and positioning errors in lithographic apparatus. A particular desire is to improve the performance and/or speed of the projected image measurement.
"According to an aspect of the invention, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, the apparatus comprising:
"a patterning subsystem for receiving said patterning device and projecting said pattern to a substrate held at a patterning location;
"a substrate support for holding the substrate while said pattern is applied;
"at least one positioning subsystem for moving said substrate support, said patterning subsystem and said patterning device relative to each other such that said pattern is applied at an accurately known location on the substrate; and
"a measuring subsystem for measuring the location of said substrate relative to the patterning location, and for supplying measurement results to said positioning subsystem,
"wherein said measuring subsystem includes at least one sensor for receiving radiation projected from an alignment mark, the sensor and alignment mark being associated one with the patterning device and the other with the substrate support, the processor, and a processor for receiving and processing signals from the sensor(s) to resolve spatial information in the projected alignment mark to establish a reference for measuring positional relationships between said substrate support and said patterning location, and wherein the sensor and the processor are operable to perform at least a final step in establishing the reference position while holding the substrate support and patterning device stationary with respect to one another.
"One option for implementing the apparatus according to this aspect of the invention is to provide separate actuators for moving the sensor relative to the substrate support or the patterning device.
"According to other embodiments, said sensor comprises an array of photodetector elements separated in at least one dimension, and a signal processor for calculating said reference position accurately in at least one dimension by combining signals representing radiation intensities measured by the individual elements of the array when the projected alignment mark falls on the array.
"According to some embodiments the processor is arranged to distinguish between different elements in accordance with respectively different optical path lengths from the alignment mark, thereby to calculate a reference position in a dimension (Z) parallel to an optical axis of the projection system. In the case of optical lithography, the alignment mark may be projected to the sensor (S) using the same projection system and the same illumination as projects the product pattern present on the patterning device. This approach, although strictly optional, is convenient and brings accuracy and simplicity to the measurement calculations, but other implementations are feasible.
"In the case of imprint lithography, the product pattern is applied more directly and is not projected optically. Nevertheless an optical projection of the alignment mark may still be deployed between the patterning device, or its supporting structure, and the substrate or its supporting structure. In principle, embodiments of the invention applied to imprint lithography may involve a sensor on the patterning device and marks projected from the substrate support or an associated element, to the sensor.
"While direct projection between alignment mark and the sensor is illustrated and described in the embodiments that follow, modifications are envisaged in which for some reason the projection of the alignment mark is reflected at one or other of the substrate support and the patterning device or its support, and the sensor and alignment mark are both at the same side of the projecting optical system.
"According to another aspect of the invention, there is provided a device manufacturing method comprising projecting a pattern from a patterning device onto a substrate, the method comprising:
"providing a patterning subsystem for receiving said patterning device and applying said pattern to a portion of said substrate held at a patterning location;
"holding the substrate on a substrate support;
"measuring the location of said substrate relative to the patterning location;
"operating said patterning subsystem while using results of said measuring step to position said substrate support, said patterning subsystem and said patterning device relative to each other in a sequence of movements such that said pattern is applied at a plurality of desired portions of the substrate; and
"processing said substrate to create product features in accordance with the applied pattern,
"wherein said measuring step includes a preliminary step of (i) receiving radiation projected from an alignment mark using a sensor and (ii) processing signals from the sensor to resolve spatial information in the projected alignment mark to establish a reference for measuring positional relationships between said substrate support and said patterning location in at least one dimension, and wherein the sensor and the measuring subsystem are arranged to perform at least a final step in establishing the said reference position while the substrate support and patterning device are held stationary with respect to one another.
"According to an aspect of the invention, there is provided a computer program product containing one or more sequences of machine-readable instructions for controlling a lithographic apparatus, the instructions being adapted for controlling the measurement and positioning steps of a method as set forth in any of the aspects of the invention above.
"These and other features and advantages of the invention will be understood by the skilled reader from a consideration of the exemplary embodiments discussed below."
For more information, see this patent: Van De Kerkhof,
Keywords for this news article include: Electronics, Semiconductor,
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC
Most Popular Stories
- U.S. Families 'Extraordinarily Vulnerable': Yellen
- Hillary Clinton to Address CHCI Conference
- Larry Ellison Steps Down as Oracle CEO
- Alibaba Prices IPO at $68 a Share
- Veterans to Get Training as Solar Panel Installers
- Apple Locks Itself Out of Devices
- Hispanics Doubt Marco Rubio's Chances
- Wildfires Rage in California
- John Cantlie Delivers ISIS Message to Save Life
- Alibaba: Today China, Tomorrow the World