Optics: measuring and testing – By particle light scattering
Reexamination Certificate
1999-12-07
2001-01-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By particle light scattering
C356S340000
Reexamination Certificate
active
06177993
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface particle detection, and more specifically, it relates to the detection of defects on lithographic mask blanks.
2. Description of Related Art
The proposed 1999 SIA Technology Roadmap for Semiconductors is accelerating the reduction in dense line critical dimensions (CDs) to 23 nm by the year 2011. This will put a tremendous burden on mask fabrication, particularly in the area of defect detection and reduction. Mask defects as small as one-eighth the equivalent CD are printable and may cause chip failure. Table 1 shows the maximum permissible defect size for each lithography generation out to the year 2011.
TABLE 1
Year of first
1999
2002
2005
2008
2011
2014
shipment
Generation (nm)
180
130
100
70
50
35
Maximum mask
90
65
50
35
25
18
defect size (nm)
(assuming a lithography tool magnification of 0.25)
A new infrastructure for mask inspection will be required to keep pace with this aggressive roadmap. Depending on the specific lithography used for a particular generation, mask inspection specifics may change, but the methodology will essentially remain the same. Mask blanks will have to undergo 100% area inspection for defects larger than a maximum acceptable size. Since masks are becoming a significant cost factor in the cost of ownership of lithography tools, this is a critical step—patterning defective mask blanks would be an economic disaster.
Inspecting mask blanks can be approached differently than patterned masks. Inspection does not necessarily have to be done at-wavelength since defects at the mask blank level will interact with visible light. Techniques using visible light are appealing because they are familiar to the user, relatively straightforward to manufacture and, if designed properly, extendable over many generations.
Current wafer inspection tools could play this role if silicon wafers are used as the mask blanks, but this is unlikely due to unfavorable thermal properties. Even then, detection of defects smaller than 100 nm has not been demonstrated with these tools. Wafer inspection tools operate by measuring the intensity of the light scattered by a surface defect.
FIG. 1
shows a typical optical system used for commercial wafer inspection. However, scatter decreases as the sixth power of the defect size, so as the critical defect size decreases, defects become extremely difficult to detect, i.e., as the defect size decreases, the scatter decreases by the fractional decrease in defect size to the 6
th
power. Additionally, this scattered intensity inspection technique cannot distinguish surface defects from internal defects in transparent substrates such as ULE, a prime candidate for future mask blanks. As shown in
FIG. 1
, incident light beam
10
having S and P polarizations, is directed onto a mask blank
12
at the site of a defect
14
. Scattered light
16
, scattered from defect
14
is collected with collector mirrors
18
and
19
and directed to detector
20
. This technique cannot distinguish whether the defect is located on the surface or within the transparent substrate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide new techniques for detecting a surface particle on a lithographic mask blank.
Defect detection, based on optical scattering, is a viable approach when there is sufficient signal-to-noise. Given a fixed amount of energy for illuminating the surface of the mask blank, typical instruments direct all of this light onto a given location on the mask blank and collect as much scattered light as possible (FIG.
1
). If the fractional amplitude of the scattered light in a particular direction is 1/s, the intensity is proportional to 1/s
2
. Since the detected intensity falls off as the sixth power of the defect size, defects much smaller than the wavelength scatter very little light. For example, a 10 nm defect illuminated with a 1 mw beam focussed to a 10 um spot will scatter only 10
−12
mw into a 0.1 numerical aperture.
The detected signal can be significantly amplified by making the following modifications:
(a) use part of the energy to illuminate the mask blank to scatter from a defect and use the remaining energy as a probe beam to coherently interfere with the scattered light (FIG.
5
);
(b) frequency shift the probe beam (~10-1000 MHz) so that heterodyne detection can be used (FIG.
6
); and
(c) the incident angle and polarization of the illumination and probe beams are chosen to maximize the particle scatter and to minimize the noise from the background scatter in the direction of the specularly reflected probe beam.
REFERENCES:
patent: 4541719 (1985-09-01), Wyatt
patent: 4571081 (1986-02-01), Ford, Jr.
patent: 4764013 (1988-08-01), Johnston
patent: 5343290 (1994-08-01), Batchelder et al.
patent: 5486919 (1996-01-01), Tsuji et al.
patent: 5502561 (1996-03-01), Hutchins et al.
Font Frank G.
Nguyen Sang H.
The Regents of the University of California
Thompson Alan H.
Wooldridge John P.
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