Elimination of proximity effect in photoresist

Details Elimination of proximity effect in photoresist Elimination of proximity effect in photoresist

2000-01-11

2001-09-18

Young, Christopher G. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Including control feature responsive to a test or measurement

C430S322000, C430S330000

Reexamination Certificate

active

06291118

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the general field of photolithography with particular reference to the proximity effect and methods for its elimination.
BACKGROUND OF THE INVENTION
The patterns in an integrated are created by etching under a photoresist mask that was formed from a glass mask through a photolithographic process. The size of the minimum feature in an integrated circuit is usually referred to as the critical dimension (CD). As the CD approaches the wavelength of the light that was used to image the glass mask and expose the photoresist (between about 1830 and 3650 Angstroms for a Deep UV source), the patterns formed in the photoresist cease to reproduce the patterns on the glass mask with complete fidelity. Because the effect on any given feature is greatly influenced by the feature's surroundings, the phenomenon has been named ‘the proximity effect’.
FIGS. 1
a-c
illustrate three different manifestations of the proximity effect. In
FIG. 1
a
line
2
is isolated and has no immediate neighbours whereas lines such as
3
are crowded together, being separated by a space that is comparable to their width. Although lines
3
and line
2
had the same width on the glass mask from which they were imaged onto the photoresist, the proximity effect has caused lines
3
to be narrower than line
2
. In
FIG. 1
b
, line
4
on the glass mask had a length corresponding to dimension
5
but, in the photoresist image, it was considerably shortened, as shown. In
FIG. 1
c
, the rounding effect of a corner that was intended to be square is shown, photoresist being absent from the area marked as
6
.
Although the origins of the proximity effect are understood, calculating its magnitude for any given pattern can be very complicated and time consuming. Nevertheless, it is currently the general practice of the semiconductor industry to perform such calculations in order to generate an Optical Proximity Correction (OPC) which can be applied to the original glass mask pattern to compensate for the anticipated optical proximity effects.
The process of transferring the glass pattern to a photoresist image can be broadly summarised into four steps: 1) resist coating, 2) exposure, 3) post exposure bake (PEB) and 4) development. The surface on which the resist is coated may or may not be an anti-reflection coating (ARC). This is relevant as the proximity effect will be influenced by (among other things) the degree to which standing wave patterns are formed within the photoresist layer. However, it turns out that steps 2 and 3 are where proximity effects are introduced and, furthermore, by carefully controlling the conditions under which these two steps are implemented, the proximity effect can be eliminated, thereby removing the need for the OPC and associated costly calculations.
During a search for possible prior art, several references were found to be of interest. These include Itoo et al. (U.S. Pat. No. 5,436,114 July 1995), Ootaka et al. (U.S. Pat. No. 5,636,004 June 1997), and Gortych et al. (U.S. Pat. No. 5,680,588 October 1997) all of whom discuss the importance of numerical aperture and/or coherency. Liu et al (U.S. Pat. No. 4,988,284 January 1991) teach the need for a Post Exposure Bake but this is for electron beam resists and, furthermore, a temperature of at least 100° C. is specified.
SUMMARY OF THE INVENTION
It has been an object of the present invention to provide a method of removing any distortion in photoresist patterns that are due to the proximity effect.
A further object has been that said removal of distorting effects be through elimination of the proximity effect itself, as opposed to merely compensating for it.
Yet another object has been that said method not add to the cost relative to a photolithographic process in which no allowance has been made for the proximity effect.
These objects have been achieved by carefully controlling the values of three independent variables that are involved in the photolithographic process. These are the temperature at which Post Exposure Bake is performed, the numerical aperture of the exposure system and the partial coherence parameter. Specifically, the Post Exposure Bake temperature should be 20-25° C. lower than that recommended by the manufacturer, the numerical aperture should be around 0.5 and the partial coherence parameter around 0.8. If these guidelines are followed, no proximity effect is in evidence down to duty ratios less than 1.


REFERENCES:
patent: 4988284 (1991-01-01), Liu et al.
patent: 5436114 (1995-07-01), Itoo et al.
patent: 5636004 (1997-06-01), Ootaka et al.
patent: 5680588 (1997-10-01), Gortych et al.
patent: 6040119 (2000-03-01), Gau et al.

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