Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1998-10-07
2001-04-10
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C430S302000
Reexamination Certificate
active
06214494
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention generally relates to methods for designing serif masks for photolithography.
Photolithography is the technology of reproducing patterns using light. As presently used within semiconductor technology, a mask pattern for a desired circuit is transferred to a wafer through light exposure, development, etch, and resist strip, etc. As the feature size on a circuit becomes smaller and smaller, the circuit shape on the wafer differs from the original mask pattern more and more. This effect is due to local and far-range proximity effects, including diffraction, exposure, baking, dissolution and etching factors. In particular, corner rounding, line end foreshortening and width changes of lines are typically observed, resulting in, for example, nested lines printing differently from isolated ones.
A key reason for optical proximity effects is due to light diffraction. Optical proximity effects coming from light diffraction can be overcome partly if one has the choice of using a shorter wavelength source of light, with a projection system possessing a larger numerical aperture. In practice, the wavelength of an optical light source is typically fixed (365 nm, 248 nm, 193 nm, etc.) and there is a practical upper limit on numerical aperture. So other resolution enhancement methods, including the use of phase-shifting masks and masks with serifs, have been developed to correct optical proximity effects.
SUMMARY OF THE INVENTION
An object of this invention is to provide a new method for arriving at a serif design that may be used to correct corner rounding and line end foreshortening, as well as more general undesirable two-dimensional shape distortions introduced into the lithographic printing process due to proximity effects. Another object of this invention is to provide specific new serif designs for correcting both corner rounding and line end shortening.
These and other objectives are attained with,a method for designing a mask for photolithography. The general technique described will be illustrated here in particular for the case of a rectilinear lithographic mask used in an optical projection system to create an image of the mask onto another structure, such as in the photoresist on a semiconductor wafer. (The general method that will be described, however, applies to general mask patterns, rather than just rectilinear ones, and it holds for including other physical effects of the other critical steps in the lithographic process (e.g., baking, dissolution, etching), being limited only in the accuracy of the convolution function for modeling the entire lithographic process.) Due to physical limitations of the optical projection system (e.g., numerical aperture less than unity and large wavelength of the optical source of light in relation to the pattern feature size), the image of the mask can have considerable distortion with respect to the mask pattern itself. As will be described, in accordance with this invention, a serif design can be added to each of the corners of a rectilinear mask pattern to compensate for the distortions of the lithographic printing process. For outside corners, such as in the corner of a rectangle, each serif design will be shown to lie completely outside the rectilinear mask and to be connected to the mask only at one of the corners. Similarly, for inside corners, such as in an “L” shaped structure, the optimal serif structure will be shown to lie inside the rectilinear pattern. The method described here applies to far more general conditions, but these are important specific structures that help to illustrate the method, and which can provide key leverage in improving printability in current microlithographic semiconductor technology.
Examples of specific serif designs that aid in the printability of rectilinear reticle structures are: a square, a rectangle, a quarter circle, a triangle and a polygon. For outside corners on a rectilinear structure, each serif design will normally be connected to, and outside the extension lines of, a respective one of the corners of the rectilinear structure. (If two outside corners are close enough together, then this may not be true, in which case the general method described here would need to be worked through to determine the optimum serif design.)
The method that will be described here can be proven to work exactly in the case of perfectly coherent light illumination, or the other extreme case of incoherent light illumination, combined with resist and etch developed properties that arise from a process that is entirely a convolution based mechanism. This method can be shown to hold approximately in the case of partially coherent light illumination and for resist and etch processes that involve more than a simple convolution based mechanism.
The advantage with these methods is that totally nonintuitive serif structures can be generated that significantly aid printability for the above ideal cases and less so, but still significantly, for the other situations. A very difficult aspect of making use of serif structures, as well as phase shift mask supporting structures, is in constructing the basis for the designs. This method significantly helps to solve this problem.
The size, shape, and placement of these serifs will be a function of the desired printed pattern, the allowed transmission values in the mask making process (e.g. whether chrome on glass is used, so only 1's and 0's are allowed, or more generally for PSM masks where transmission values ranging between −1 and +1 are allowed), and the convolution function, particularly its range, that characterizes the optical image and exposure process, the resist development process, and subsequent etching processes. By saying it this way, things like the numerical aperture, the wavelength, the partial coherence, the diffusion steps in the baking of the resist (temperature and baking duration), the dissolution of the resist, and physical factors governing the etching of underlying semiconductor structures, all in a sense become factors that govern the determination of the serifs, at least within the limitations of this convolution modeling process. More specifically, the convolution function does not separately identify each of these factors, but rather they all influence the type of function and its size parameters, as the convolution function becomes a phenomenological model, empirically determined, that describes the entire printing process. Again, we know that this model works perfectly in certain idealized cases, as described in the earlier sections, and according to our simulation work, it will provide considerable benefit under more general circumstances.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
REFERENCES:
patent: 5242770 (1993-09-01), Chen et al.
patent: 5340700 (1994-08-01), Chen et al.
patent: 5663893 (1997-09-01), Wampler et al.
patent: 5879844 (1999-03-01), Yamamoto et al.
patent: 8-101491 (1996-04-01), None
Bula Orest
Cole Daniel
Conrad Edward
Lu Ning
International Business Machines - Corporation
Sabo, Esq. William D.
Scully Scott Murphy & Presser
Young Christopher G.
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