U.S. patent application number 10/233921 was filed with the patent office on 2003-01-02 for apparatus and method for autofocus.
Invention is credited to Dey, Cassandra, Dorsel, Andreas N., Staton, Kenneth L., Tsai, George P..
Application Number | 20030001072 10/233921 |
Document ID | / |
Family ID | 23644703 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030001072 |
Kind Code |
A1 |
Dorsel, Andreas N. ; et
al. |
January 2, 2003 |
Apparatus and method for autofocus
Abstract
A laser generates a collimated laser beam which passes through a
lens off-axis. The beam is focused at a focal plane on a substrate
surface. A first position sensitive detector receives the laser
beam reflected from the substrate surface through the lens to
generate a first signal proportional to lateral beam offset. A beam
splitter may be provided to direct a portion of the laser beam
before passing through the lens toward a second position sensitive
detector to generate a second signal proportional to laser beam
pointing instability. Apparatus computes the difference between the
first and second signals, the difference being a defocused error
signal. It is preferred that the first position sensitive detector
be located at a distance from the lens that is at least twice the
lens focal length.
Inventors: |
Dorsel, Andreas N.; (Menlo
Park, CA) ; Staton, Kenneth L.; (San Carlos, CA)
; Dey, Cassandra; (New York, NY) ; Tsai, George
P.; (San Jose, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
Legal Department, DL429
Intellectual Property Administration
P. O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
23644703 |
Appl. No.: |
10/233921 |
Filed: |
September 3, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10233921 |
Sep 3, 2002 |
|
|
|
09415184 |
Oct 7, 1999 |
|
|
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Current U.S.
Class: |
250/201.5 ;
250/201.2 |
Current CPC
Class: |
G02B 7/32 20130101; G02B
7/28 20130101; G02B 26/128 20130101 |
Class at
Publication: |
250/201.5 ;
250/201.2 |
International
Class: |
G02B 007/04; G02B
027/40; G02B 027/64 |
Claims
What is claimed is:
1. Autofocus apparatus, comprising: a laser for generating a
collimated laser beam; a scan lens positioned to receive the laser
beam off-axis and to focus the beam at a focal plane on a substrate
surface; a position sensitive detector for receiving the laser beam
reflected from the substrate surface through the scan lens to
generate a defocus error signal proportional to lateral beam
offset; and means for reducing the defocus error signal to achieve
autofocus during scanning by varying the distance between the scan
lens and the position sensitive detector.
2. The autofocus apparatus of claim 1 wherein the position
sensitive detector is located at a distance from the scan lens that
is at least twice a focal length of the scan lens.
3. The apparatus of claim 1 wherein a proportionality factor
linking defocus and lateral displacement also varies.
4. The apparatus of claim 3 further including computational
circuitry to correct the varying proportionality factor.
5. Autofocus method, comprising: generating a collimated laser
beam; positioning a lens to receive the laser beam off-axis to
focus the beam at a focal plane on a substrate surface; detecting
the laser beam lateral offset after reflection from the substrate
surface through the lens to generate a lateral offset defocus error
signal; and reducing the defocus error signal to achieve autofocus
during scanning by varying the distance between the lens and the
position sensitive detector.
6. The method of claim 5 wherein a proportionality factor linking
defocus and lateral displacement also varies.
7. Autofocus apparatus, comprising: a laser for generating a
collimated laser beam; a lens positioned to receive the laser beam
off-axis and to focus the beam at a focal plane on a substrate
surface; a position sensitive detector for receiving the laser beam
reflected from the substrate surface through the lens to generate a
defocus error signal; an actuator adapted to move the substrate or
the lens to decrease the defocus error signal; and means for
reducing the defocus error signal to achieve autofocus during
scanning by varying the distance between the lens and the position
sensitive detector.
8. The apparatus of claim 7 wherein a proportionality factor
linking defocus and lateral displacement also varies.
9. The apparatus of claim 8 further including computational
circuitry to correct the varying proportionality factor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of, and claims
priority to, co-pending U.S. patent application Ser. No.
09/415,184, filed Oct. 7, 1999, the teachings of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an autofocus technique and more
particularly to an autofocus methodology for use with fluorescence
scanners.
[0003] In laser fluorescence scanning it is critical to eliminate
as much as possible fluorescence (and scattering) from materials
other than the surface-bound nucleic acids which have been tagged
with fluorescing molecules. One method for reducing such background
from sources outside the detection plane is the use of depth
discrimination. In such a system, signals outside a detection plane
are substantially minimized. The system, however, must make sure to
keep the detection plane at the peak of the sensitivity vs. depth
curve. If the system fails to keep the detection plane at the peak
of the sensitivity vs. depth curve there will be a non-uniformity
of the scanned images. Thus, in order to improve performance in
fluorescence scanning, a need exists for focusing with good
precision onto the plane that has a fixed (preferably zero)
distance from the surface of the substrate being scanned.
SUMMARY OF THE INVENTION
[0004] In one aspect, the invention is an autofocus apparatus
including a laser for generating a collimated laser beam. A lens is
positioned to receive the laser beam off-axis and to focus the beam
at a fixed focal plane at a substrate surface. A first position
sensitive detector receives the laser beam reflected from the
substrate surface through the lens to generate a first signal
proportional to lateral beam offset. A beam splitter may direct a
portion of the laser beam before passing through the lens toward a
second position sensitive detector to generate a second signal
proportional (except for possible offsets) to laser beam pointing
errors. In this case, additional apparatus may be used to compute
the difference of the positions from the first and second position
sensitive detectors, which will be (except for possible offsets)
proportional to the defocus error signal corrected for laser
pointing error. As will be appreciated by those skilled in the art,
this defocus error signal can be used in a control loop to reduce
this error signal to achieve autofocus. In one embodiment, an
offset is added so that the system will focus on a plane that
differs from the substrate surface. It is preferred that the first
position sensitive detector and the second position sensitive
detector be located approximately the same total distance from the
laser.
BRIEF DESCRIPTION OF THE DRAWING
[0005] FIG. 1 is a schematic diagram of the autofocus apparatus of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0006] With reference to FIG. 1 a laser 10 generates an essentially
collimated laser beam 12 which passes through a beam splitter 14
and enters a high numerical aperture scan lens 16 off-center with
respect to an optical axis 18 of the lens 16. When a sample surface
20 to be scanned is in the focal plane of the lens 16, a reflected
beam 22 will exit the lens 16 parallel to the incident beam 12 at
the same distance from the lens 16 optical axis 18 but on the
opposite side of the optical axis 18. The focal plane of the lens
16 is the plane with the highest signal on the depth discrimination
curve and may not coincide with the plane of smallest laser beam
diameter (focus shift of Gaussion beams). If the sample surface 20
moves out of the focal plane as shown by the dotted line 24, the
back-reflected beam will no longer be parallel to the incident beam
12 after passing through the lens 16 for the second time. The
surface 24 being out of the focal plane results in lateral beam
movement at larger distances from the lens. A dotted line 26
indicates such a lateral offset beam which falls upon a position
sensitive detector (PSD) 28. The PSD 28 generates an output signal
proportional to the lateral offset of, for example, the beam 26
with respect to the beam 22. The PSD 28 distance from the lens 16
is large with respect to the focal length of the lens 16. The
distance should be at least twice the focal length; a suitable
distance is up to 100 times the focal length. Those skilled in the
art will recognize that adding an offset to the output of the PSD
28 may be used for alignment (setting the system to the right
plane) as well as for intentionally focusing on a different
plane.
[0007] It is expected that the beam 12 from the laser 10 may
experience beam pointing instability (beam wander) and such
pointing instability can cause erroneous focus signals. That is, if
there is pointing instability in the beam 12 from the laser 10, the
PSD 28 could register a lateral offset signal even when the
scanning surface 20 is at the focal plane. To compensate for any
such beam wander, the beam splitter 14 directs a portion of the
beam 12 from the laser 10 onto a second position sensitive detector
30. As the beam 12 deviates the PSD 30 will generate a signal
proportional to the deviation. The outputs of the PSD 28 and PSD 30
are differenced to generate a defocus error signal 32. That is, the
difference of the two PSD signals is--except for possible
offsets--a measure of defocus. As those skilled in the art will
appreciate, the error signal 32 can be used in a closed loop system
to maintain the sample surface 20 at the focal plane of the lens 16
by using a position sensor and actuator 34 to move either the lens
or the sample.
[0008] The present invention offers a straightforward way to
maintain relative alignment of focal/detection plane and substrate
surface in the presence of beam wander. Unlike on-axis systems, the
present methodology eliminates the effects caused by interference
of light waves reflected from the front and back of transparent
substrates. It is noted that for systems in which the distance
between the scan lens 16 and the PSD 28 varies during scanning, the
proportionality factor linking defocus and lateral displacement on
the PSD varies also. In such a system the focusing planes are not
parallel to each other since only the true focus plane is flat; for
all non-zero offsets from the true focus plane, the system focuses
on hyperbolas. With suitable computational circuitry 40, however,
this can be compensated for so that focusing on an out-of-focus
plane is also easily accomplished by anyone skilled in the art.
[0009] It is recognized that modifications and variations of the
present invention will become apparent to those skilled in the art
and it is intended that all such modifications and variations be
included within the scope of the appended claims.
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