Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
2001-01-17
2004-10-12
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C355S067000, C430S005000, C430S030000
Reexamination Certificate
active
06803995
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a manufacturing process requiring lithography and, in particular, to monitoring of lithographic process conditions used in manufacturing microelectronic components and providing improved focus control.
2. Description of Related Art
Control of a lithographic imaging process requires the optimization of exposure and focus conditions in lithographic processing of product substrates or wafers. Generally, because of the variations in focus, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable range. The importance of such monitoring increases considerably as the resolution limit, which is usually defined as minimum features size resolvable, of the lithographic process is approached. The patterns being developed in semiconductor technology are generally in the shape of lines both straight and with bends, having a length dimension equal to and multiple times the width dimension. The width dimension, which by definition is the smaller dimension, is of the order of 0.1 micron to greater than 1 micron in the current leading semiconductor technology. Because the width dimension is the minimum dimension of the patterns, it is the width dimension that challenges the resolution limits of the lithographic process. In this regard, because width is the minimum and most challenging dimension to develop, it is the width dimension that is conventionally monitored to assess performance of the lithographic process. The term “bias” is used to describe the change in a dimension of a feature from its nominal value. Usually the bias of interest is the change in the smallest of the dimensions of a given feature. Further, the term ‘bias’ is invariably used in conjunction with a process such as resist imaging, etching, developing etc. and described by terms such as image bias, etch bias, print bias, and the like.
Recent lithographic monitoring improvements have been in optical metrology which rely on human or machine-read visual measurement of targets which employ arrays of elements having line widths and spacing below the wavelength of the light used to make the measurements. Improvements in monitoring bias in lithographic and etch processes used in microelectronics manufacturing have been disclosed in U.S. Pat. Nos. 5,712,707; 5,731,877; 5,757,507; 5,805,290; 5,953,128; 5,965,309; 5,976,740; 6,004,706; 6,027,842; 6,128,089 and 6,130,750, the disclosures of which are hereby incorporated by reference. The targets and measurement methods of these applications rely on the increased sensitivity to focus variation provided by image shortening. Some of these types of targets use image shortening effects to make the visual measurements even though the individual array elements are not resolvable. Examples of such targets are disclosed in the aforementioned U.S. patents. Such targets permit visual monitoring of pattern features of arbitrary shape with dimensions on the order of less than 0.5 micron, and which is inexpensive to implement, fast in operation and simple to automate. These determine bias to enable in-line lithography/etch control using optical metrology, and wherein higher resolution metrology, such as SEM and/or AFM metrology, is required only for calibration purposes.
As described in particular in U.S. Pat. Nos. 5,953,128; 5,965,309; 5,976,740; 6,004,706; 6,027,842 and 6,128,089, the defocus of a lithographic image can be measured using dual-tone optical critical dimension (OCD) metrology. The problem remains, however, of providing a control system to feed back focus corrections to the lithography tool. The mere ability to determine that dose and/or focus is deviated from optimum is not sufficient in itself for closed-loop dose and focus control. For the most part, the prior art does not fully address 1) the need to determine both the sign and magnitude of a focus correction feedback to maintain an imaging system at optimum focus, 2) the need for adequate sensitivity to small defocus deviations from an optimum focus position, 3) the need to decouple and distinguish dose and focus variation, 4) the need for automated measurement and feedback. These requirements would be desirable for an automated focus control method and system.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved lithographic system for manufacturing microelectronic circuits.
It is another object of the present invention to provide improved focus control in lithographic processing.
A further object of the invention is to provide a focus control system for a lithography tool.
It is yet another object of the present invention to provide a lithographic focus control system which provides feedback to the lithography tool.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a process for controlling focus parameters in a lithographic process used in manufacture of microelectronic circuits. The process comprises initially providing a lithographic mask having a target mask portion containing a measurable dimension sensitive to defocus, projecting an energy beam through the target mask portion onto a first location of a substrate at a first focus setting, and lithographically forming a first target on the substrate corresponding to the first focus setting, the first target containing a measurable dimension sensitive to defocus. The process then includes projecting an energy beam through the target mask portion onto a second location of the substrate at a second focus setting, lithographically forming a second target on the substrate corresponding to the second focus setting, the second target containing a measurable dimension sensitive to defocus, and measuring the defocus sensitive dimension for each of the first and second targets on the substrate. The defocus sensitive dimension of the first and second targets are then compared and there is determined a desired focus setting of the energy beam based on the comparison of the dimensions of the first and second target. The process may be used to form focus setting targets on a semiconductor wafer for use in manufacture of microelectronic circuits.
Preferably, the targets comprise a plurality of spaced elements having essentially the same length and width and forming an array, ends of the individual elements being aligned to form first and second opposing array edges, the array elements having a predefined pitch. The defocus sensitive dimension measured and compared for each of the first and second targets on the substrate is the width of the array.
The target mask portion and the targets formed on the substrate may be characterized as each comprising a first area having a set of parallel array elements and a second, contrasting area having a set of contrasting parallel array elements parallel to the array elements on the first contrasting area, such that the target defocus sensitive dimension is measured by determining the distance between ends of the array elements on each of the first and second contrasting areas. More preferably, the targets comprise first and second complementary, tone reversed target portions. The first target portion comprises a plurality of spaced element shapes having essentially the same length and width and forming an array. The second target portion comprises a plurality of spaced element spaces having essentially the same length and width and forming an array, with the first target portion element shapes being of contrasting tone to the second target portion element spaces. Ends of the individual elements in each target portion are aligned to form first and second opposing array edges, with the array elements having a predefin
Adams Russell
C. Li Todd M.
DeLio & Peterson LLC
Esplin D. Ben
Peterson Peter W.
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