Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-09-29
2003-02-04
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG, C250S559050
Reexamination Certificate
active
06515272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to optical imaging characterization, and more particularly, to a method and apparatus for improving the signal to noise ratio of optical imaging for use in the characterization of imaging systems used in the semiconductor industry.
2. Discussion of the Related Art
FIG. 1
illustrates a prior art optical stepper lithography apparatus
20
. In typical semiconductor chip processing configuration, a pulsed-laser signal source
22
provides a signal made up of a plurality of successive pulses at a given pulse frequency or repetition rate R. The pulses are provided to a lithographic imaging system
24
including lenses
26
,
28
imaging a reticle positioned at the station
29
onto a wafer
32
on a stage
34
. Also mounted on the stage
34
is a slit plate
36
which contains several slits
38
, each oriented in a different direction, and a photodetector
40
beneath the slit plate
36
.
In the use of the apparatus
20
of
FIG. 1
, and also with reference to
FIG. 2
, the apparatus
20
is used for measuring optical performance of a stepper. The reticle is replaced with a test reticle
30
is positioned at the station
29
. The stage
34
is positioned as shown in
FIG. 1
with the test reticle
30
imaged on the slit plate
36
. The slit
38
is then moved or scanned across the image plane, the longitudinal axis of the slit
38
being held perpendicular to the scan direction. During the scanning operation, the intensity of the transmission through the slit
38
is monitored by means of photodetector
40
, which, with the slit plate
36
, makes up an aerial image monitor
42
. The intensity profile measured by the photodetector
40
is provided to a data output recorder
44
and is compared to a previously calculated profile expected from a “perfect” stepper or to a baseline measurement. From this comparison the performance characteristics of the apparatus
20
are determined.
As the dimensions of features being printed with lithography systems get smaller and smaller, the requirements on the precision of aerial image monitors become more and more severe, and the available source power tends to decrease, leading to smaller signal levels. High precision can only be achieved with a high signal to noise ratio, i.e., a high ratio of the pulsed-laser signal detected intensity to the intensity of the noise that is detected, so that small changes in the signal corresponding to small variations in the intensity of the aerial image can be recorded
Again referring to
FIG. 2
, as noted above, the signal
46
from the pulsed-laser source
22
is provided as a series of pulses
48
at a given repetition rate R. Because each pulse
48
tends to have a slightly different intensity, the laser output signal exhibits noise over a wide frequency range.
A typical general noise frequency spectrum (signal intensity I plotted against frequency) of the signal
46
provided by the pulsed-laser source
22
is shown in
FIG. 3
, indicated by the sloping, irregular line
50
. The vertical line
52
indicates the laser pulse rate (or frequency) R.
FIG.
4
(
a
) illustrates the aerial image
53
produced by the imaging system, and FIG.
4
(
b
) shows the signal
56
produced by the photodetector
40
, which is recorded by the data output recorder
44
. This signal from the photodetector
40
includes a substantial amount of noise, as shown in FIG.
4
(
b
), which is a graph of signal
56
intensity provided by the photodector
40
to the data output recorder
44
plotted against time.
FIG.
4
(
c
) shows an enlarged view of a short segment of the signal of FIG.
4
(
b
), where individual laser pulses can be distinguished.
With such a poor signal to noise ratio, high precision of the aerial image monitor
42
cannot be achieved.
Therefore, what is needed is a method and apparatus for improving the signal to noise ratio of aerial image monitors for lithography stepper tools used to produce semiconductor devices.
SUMMARY OF THE INVENTION
In the present method for monitoring the image of a stepper imaging system, a pulsed-laser signal is provided, and is modulated at a chosen frequency. The modulated signal it is then provided to an imaging system. An output signal is provided from an aerial image monitor in response to the modulated signal provided to the optical imaging system, and only the component of that signal at the chosen frequency is amplified.
The present image monitoring apparatus includes a signal modulating device in the form of an optical chopper for modulating a laser pulse signal at a chosen frequency. Image monitoring means receive the modulated signal and provide an output signal in response to the modulated signal. A lock-in amplifier amplifies the component of the output signal at the chosen frequency.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.
REFERENCES:
patent: 5491724 (1996-02-01), Altes
patent: 5631731 (1997-05-01), Sogard
patent: 5866935 (1999-02-01), Sogard
patent: RE36509 (2000-01-01), Shigihara
patent: 1211802 (1986-09-01), None
patent: 0119711 (1984-09-01), None
patent: 0119711 (1984-09-01), None
Stanford Research Catalogue (2000), Application Note#3, pp 169-179 (Month Unknown).
C.H. Fields, W.G. Oldham, A.K. Ray-Chaudhuri, K.D. Krenz, R.H. Stulen, Direct Aerial Image Measurements To Evaluate The Performance Of An Extreme Ultraviolet Projection Lithography System, J. Vac. Sci. Technol. B 14(6), Nov./Dec. 1996, American Vacuum Society, pp. 4000-4003.
Fontaine Bruno La
Levinson Harry
Advanced Micro Devices , Inc.
Le Que T.
LandOfFree
Method and apparatus for improving signal to noise ratio of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for improving signal to noise ratio of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for improving signal to noise ratio of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3147154