Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1998-05-13
2001-07-31
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S067000
Reexamination Certificate
active
06268907
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention relates to lithography and, in particular to a method for eliminating standard waves in a photoresist layer used for patterning another layer in semiconductor device manufacturing without changing the exposure source or photoresist material. The present invention also provides an optical lithographic system which is capable of overcoming the problem of standing waves in photoresists.
2. Background of the Invention
With the relentless advance in photolithography to smaller and smaller dimensions at a faster rate than the reduction of the exposing wavelength, new problems in imaging are arising that now need to be addressed. One of the problems in this area which has made itself known in recent years is the issue of standing waves in photoresists.
Standing waves in a developed photoresist image arise from the interference between the incoming light, and light reflected off of the bottom of the photoresisc film (reflected light). When this occurs, the light intensity in the photoresist consists of a series of constructive/destructive interference patterns oriented vertically in the photoresist profile. Upon exposure to light, a chemical reaction occurs in the photoresist such that when the photoresist is placed into a developer solution, the photoresist is either removed in areas wherein exposure occurs (positive tone resist) or the unexposed areas are removed (negative tone resist). The boundary between resist removal and resist remaining is typically set by a threshold dose of light.
If there were no standing waves, this threshold would propagate more or less vertically into the photoresist. With the addition of standing waves, however, the constant intensity profile appears as a sinusoidal curve propagating into the photoresist. The standing wave occurs because of reflection of the light off the bottom of the photoresist (or even films beneath the photoresist) sending light back towards the lens of the optical system creating a constructive, destructive interference pattern much like an interferometer. The standing wave effect of the phozoresist typically results in necking and/or bridging of adjacent pattern lines in the underlying layers of the structure being patterned.
In older semiconductor manufacturing technologies, standing waves were not a serious problem due to a number of factors which include: A broad-band illumination light source for exposure. The Micrascan II light source, for example, utilizes a mercury source with an exposing wavelength of from about 245 nm to about 255 nm as well as high diffusivity photoresists. In addition, after exposure, but before development, a post-exposure bake (PEB) step is typically employed. This PEB step among other things serves to diffuse the molecules of the photoresist smearing out standing waves in the photoresist.
The amount that the photoresist molecules are diffused is characterized by the diffusion length. As the requirements on what a lithography system is called on to do get progressively more tighter and stringent, the smearing due to the diffusion of the photoresist molecules can no longer be tolerated and so the diffusion length must be reduced. This in turn will bring back the standing waves which were smeared out by the diffusion.
The reduction in wavelength bandwidth is necessitated by the need to reduce chromatic aberrations in the lens. The stepper manufacture thus must strive to make the optical lithographic system as perfect as possible. If the wavelength remained broad, then, in the photoresist, there would exist an infinite number of separate standing waves, each with a wavelength equal to the wavelength of the exposing light divided by the index of refraction of the resist.
In the prior art, it is known to eliminate the standing waves by reducing the thickness of the photoresist by employing a PEB step or multiple exposure steps. Another technique currently employed in the prior art to eliminate standing waves is to utilize an anti-reflecting coating (ARC) material which is typically placed between the photoresist and the semiconductor structure that is be patterned. ARC is also used in the prior art to improve dimensional control. If the underlying films vary in thickness across the semiconductor wafer, then the amount of light reflecting off of these films will vary as well. This in turn will mean that the area being exposed will get a dose variation which leads to a variation in image size.
Elimination of the standing wave effect in photoresists by changing the thickness of the photoresist is disclosed, for example, in Japanese Patent Nos. 6-216068 (JP. '068) and 7-169676 (JP. '676). Specifically, JP. '068 eliminates standing waves by reducing the thickness of the photoresist by first exposing the photoresist to a first light source. Next, the exposed photoresist is thermally treated to reduce the thickness of the photoresist to a prescribed value. Thereafter, a second light exposure process is carried out. At this point of the process, the nodes and antinodes of light standing waves generated at the first and second light exposure process deviate from each other in position; therefore, the standing wave is canceled out.
In JP. '676, there is disclosed an exposure method whereby the thickness of the photoresist changes after each exposure step. When the exposure frequency and time between exposures is made optimum, the standing wave effect can be eliminated.
The use of ARC materials for eliminating standing waves i-n photoresists are disclosed, for example, in U.S. Pat. Nos. 4,377,339 and 5,580,701 as well as Japanese Patent Nos. 5-121285 and 8-45901.
Although each of the above processes are capable of eliminating the standing wave effect in photo-resists, they cause significant reduction in the processing window and thus this makes each of the prior art processes unsuitable for patterning smaller dimension semiconductor devices.
Additionally, extra processing steps as well as materials, e.g. ARC materials, are needed in the prior art to eliminate the standing wave effect in the photoresist which adds additional cost and time in manufacturing the semi-conductor devices.
In view of the drawbacks mentioned with prior art processes for eliminating standing waves in photoresists, there is a continued need to develop a new method and optical lithographic system which by itself is capable of eliminating the standing wave effect observed with today's current semiconductor manufacturing technologies.
SUBJECT OF THE INVENTION
One object of the present invention is to provide a method for eliminating standing waves in a photoresist which does not alter the overall thickness of the photoresist material or add additional processing steps to the overall patterning process.
Another object of the present invention is to provide a method of eliminating the standing waves in photoresists that does not require the use of an anti-reflective coating material, a special photoresist material or a change in the exposure source.
A further object of the present invention is to provide a method of eliminating the standing wave effect in photoresists whereby a phase shift in time, not space domain, is employed.
A still further object of the present invention is to provide a method of eliminating standing waves in photoresists such that no substantial loss in the processing window occurs.
These and other objects and advantages can be achieved in the present invention by changing the exposure light from a single dose at one phase, i.e. just one optical path length directed from the light source to the photoresist, to a plurality of doses at different phases so as to spread out the effects of the standing wave at each of those phases. The different phases are generated in the present invention by varying the optical path length through which the exposing light travels from the light source to the photoresist.
Specifically, the method of the present invention comprises the steps of:
(a) providing an optical lithographic system, wherein said optical l
Samuels Donald J.
Thomas Alan C.
International Business Machines - Corporation
Mathews Alan A.
Neff, Esq. Daryl K.
Scully Scott Murphy & Presser
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