Space classification for resolution enhancement techniques

Details Space classification for resolution enhancement techniques Space classification for resolution enhancement techniques

2001-07-10

2003-07-29

Ho, Hoai (Department: 2818)

Computer-aided design and analysis of circuits and semiconductor

Nanotechnology related integrated circuit design

C716S030000, C716S030000, C716S030000, C716S030000

Reexamination Certificate

active

06601231

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The invention relates to lithography, and in particular, to optical and process correction techniques for lithography masks.
BACKGROUND OF THE INVENTION
The fabrication of integrated circuits on a semiconductor substrate typically includes multiple photolithography steps. A photolithography process begins by applying a thin layer of a photoresist material to the substrate surface. The photoresist is then exposed through a photolithography exposure tool to a radiation source that changes the solubility of the photoresist at areas exposed to the radiation. The photolithography exposure tool typically includes transparent regions that do not interact with the exposing radiation and a patterned material or materials that do interact with the exposing radiation, either to block it or to shift its phase.
Areas of the photoresist that are not exposed to the radiation do not change in solubility, so those unexposed areas (if “negative photoresist” is used), or the exposed areas (if “positive photoresist” is used) can be washed away by a developer, leaving patterned photoresist on the substrate. The pattern on the photolithography exposure tool is transferred or printed onto the photo resist. The patterned photoresist is then used as a protective layer during a subsequent fabrication step, such as etching an underlying layer or diffusing atoms into unmasked areas of the substrate.
“Masks” and “reticles” are types of lithography exposure tools, that is, tools that alter radiation to print an image on the exposed surface. The term “mask” is sometimes reserved for photolithography exposure tools that print an entire wafer in one exposure, and the term “reticle” is sometimes reserved for a photolithography exposure tool that projects a demagnified image and prints less than the entire wafer during each exposure. The term “mask” is more commonly used generically, however, to refer to any type of lithography exposure tool. The term “mask” is used herein in its broadest sense to mean any type of lithography exposure tool, regardless of the magnification, the type of exposing radiation, the fraction of the wafer that is printed in each exposure, or the method, such as reflection, refraction, or absorption, used to alter the incoming radiation.
A photolithography mask typically comprises a quartz substrate with a patterned layer of opaque chromium that corresponds to the circuit pattern to be transferred to the substrate. A mask can also include a material, such as silicon nitride, that shifts the phase of the exposing radiation. A reduced image of the mask is typically projected onto the substrate, the image being stepped across the substrate in overlapping steps to repeat the pattern.
As each successive generation of integrated circuits crowd more circuit elements onto the semiconductor substrate, it is necessary to reduce the size of the features, that is, the lines and spaces that make up the circuit elements. The minimum feature size that can be accurately produced on a substrate is limited by the ability of the fabrication process to form an undistorted optical image of the mask pattern onto the substrate, the chemical and physical interaction of the photoresist with the developer, and the uniformity of the subsequent process, e.g., etching or diffusion, that uses the patterned photoresist.
When a photolithography system attempts to print circuit elements having sizes near the wavelength of the exposing radiation, the shape of printed circuit elements becomes significantly different from the pattern on the mask. For example, line-widths of circuit elements vary depending on the proximity of other lines. The inconsistent line widths can then cause circuit components that should be identical to operate at different speeds, thereby creating problems with the overall operation of the integrated circuit. As another example, lines tend to shorten, that is, the line ends “pull back.” The small amount of shortening becomes more significant as the lines themselves are made smaller. Pulling back of the line ends can cause connections to be missed or to be weakened and prone to failure.
Because of the wave nature of light, even a perfectly straight, opaque edge will not produce a shadow that is absolutely dark in the shadowed areas. A phenomenon known as diffraction causes the light to bend around an edge to produce a pattern of alternating light and dark areas. The width of the alternating areas is on the order of the wavelength of the exposing light and the diffraction pattern intensity falls off rapidly in the shadowed zone. When integrated circuits were fabricated using conductor widths greater than one micron, the effect of diffraction was small and the differences between the pattern on the mask and the pattern produced on the substrate could be ignored. In modern circuits, with conductors widths well under a micron and even under two tenths of a micron, diffraction and other optical phenomena produce effects that are significant in relation to the size of features being produced by photolithography, and such effects can no longer be ignored.
Because the size of the diffraction effects is related to the wavelength of light used, one way to reduce diffraction effects is to use light of a shorter wavelength. The wavelengths used in new photolithography systems have decreased over the years from visible light to ultraviolet to deep ultraviolet. Systems using extreme ultra-violet or soft x-rays are currently being developed. It is desirable, however, to improve the resolution of existing photolithography systems because of the high cost of new systems and because it takes many years for a new generation of photolithography systems to become stable production tools. Moreover, the rate at which shorter wavelength systems are being developed is expected to be insufficient to keep up with the expected reduction in circuit feature size. Thus, it will likely be necessary to overcome diffraction effects, regardless of the wavelength used.
Because it can be determined in many instances how the pattern projected onto the substrate will vary from the mask pattern, the mask pattern can be altered to pre-compensate for the distortion. The printed pattern, rather than the mask pattern itself, then portrays the desired circuit. Techniques for pre-compensating the mask are examples of resolution-enhancing corrections or resolution enhancement techniques. A mask is typically altered by moving features of the mask or adding “sub-resolution” assist features, that is, features that are too small to be imaged individually on the substrate, but that scatter or bend light to alter the image of other, larger features on the mask. These “predistortions” cancel the distortions inherent in the lithography process, resulting in a layout that has improved fidelity to the intended design, improved manufacturing yield, and better circuit performance.
For example, it is known that diffraction effects tend to round off square corners and shorten lines.
FIG. 1A
shows a pattern
100
on a portion of a mask
102
, and
FIG. 1B
shows the pattern
104
printed by mask
102
onto a substrate
106
. Printed pattern
104
is shorter than mask pattern
100
and printed pattern
104
has rounded corners.
FIG. 1C
shows a modified mask pattern
108
having “serifs”
110
added.
FIG. 1D
shows the pattern
116
projected onto a substrate
118
by mask pattern
108
using serifs
110
. Printed pattern
116
is not as shortened as pattern
104
in FIG.
1
B and the corners are not as rounded. The use of serifs was described as early as 1981 by B. E. A. Saleh and S. Sayegh in “Reduction of Error of Microphotographic Reproductions by Optimal Correction of Original Masks,”
Opt. Eng
., vol. 20, p. 781, and is described more recently, for example, in U.S. Pat. No 5,707,765 to Chen for “Photolithography Mask Using Serifs and Method Thereof.”
It is also known that the diffraction patterns of closely spaced mask pattern features interact. For example,
FIG. 2A
shows a group of closely spaced parallel lines
20

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