Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2001-02-21
2003-12-16
Bruce, David V. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201300
Reexamination Certificate
active
06664524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a focusing method for accurately adjusting a focus of an objective lens to a surface of a target object, for example, in a repairing apparatus of a semiconductor mask for use in exposure steps in the semiconductor manufacturing process, an optical inspecting apparatus, or a laser processing apparatus.
2. Description of the Related Art
It is known that optical systems for focus detection employs an astigmatism method, a knife edge method, a skew method and the like. A number of approaches have been proposed for preventing malfunctions or for offset of a focus in association with such systems (see, for example, Japanese Patent Laid-open Publication No.3335/1992 and Japanese Patent Laid-open Publication No.312510/1993).
Japanese Patent Laid-open Publication No.250406/1996 discloses an autofocus device for an exposure apparatus. With the autofocus device, a focus is detected by irradiating a target object with light for focus detection without the intervention of a projection lens and then detecting reflected light from the target object by a sensor to perform signal processing. Also, such focus detection is performed at a number of positions and the detected values are averaged to perform operations for obtaining the height of a step to which a focus is to be adjusted, and an offset amount is calculated.
As shown in an embodiment of Japanese Patent Laid-open Publication No.250406/1996, the averaging is effective in detecting a focus when a wafer has a pattern with projections and depressions. However, no consideration is given to errors caused by focus detection on a surface of a target object on which reflectances vary greatly with positions, for example at an edge of a pattern formed on a glass substrate in a semiconductor mask. Namely, it provides no improvement in the accuracy of focus detection itself.
In an automatic focus detecting method conventionally proposed, in the case where a pattern is formed on a glass substrate such as in a semiconductor mask, since the reflectance on the pattern is very different from that at other positions, inconsistency may occur in the focuses detected on each position. When focus detection is performed at a portion of the pattern smaller than the diameter of a luminous flux for focus detection or at an edge of the pattern, for example, the intensity distribution of reflected light returning to a sensor may not be uniform due to the influence of diffraction or the like, which cannot be distinguished from signal changes due to misfocusing and results in malfunctions.
In addition, when foreign matter such as dust is present on a target object, malfunctions may occur since a luminous flux for focus detection returning to the sensor is deviated from a predetermined light receiving position of the sensor. Specifically, scattering or diffraction of the luminous flux for focus detection due to dust or the like causes a nonuniform intensity distribution of the reflected light returning to the sensor similarly to the aforementioned case, resulting in confusion with signal changes due to misfocusing.
Especially when such a focus detecting apparatus is used in a laser processing apparatus, the occurrence of dust from the processing increases the probability of malfunctions. Also, when the peripheries of a processed area are not processed at uniform height, focus detection at those nonuniform heights of the processed area causes a nonuniform intensity distribution of the luminous flux for focus detection returning to the sensor, leading to malfunctions.
A device capable of solving the aforementioned problems is a focus detecting apparatus in Japanese Patent Laid-open Publication No.306554/1999. The focus detecting apparatus comprises light source
10
, optical device
11
, objective lens
12
, objective lens driving apparatus
13
, stage
14
, autofocus signal processing circuit
15
, and stage driving control circuit
16
, as shown in FIG.
1
.
Luminous flux
17
for focus detection enters from light source
10
on optical device
11
to a position deviated from optical axis
28
of objective lens
12
, and the luminous flux is polarized. Luminous flux
17
for focus detection is not aligned with optical axis
28
but is substantially parallel, and passes through polarization beam splitter
18
. The luminous flux through polarization beam splitter
18
passes through quarter-wave plate
19
for changing linearly polarized light into circularly polarized light, and then passes through relay lens
20
. Relay lens
20
consists of two groups and is formed to converge the light once. Relay lens
20
can change the diameter of luminous flux
17
as required. The luminous flux through relay lens
20
passes through dichroic mirror
27
and is converged by objective lens
12
near the surface of target object
21
, and then reflected by target object
21
and again enters objective lens
12
. Target object
21
is fixed on stage
14
which is movable by stage driving control circuit
16
.
The returning light from objective lens
12
again passes through quarter-wave plate
19
after transmission through dichroic mirror
27
and relay lens
20
. At this time, the action of quarter-wave plate
19
changes the returning light into linearly polarized light with its polarized direction 90 degrees different from the original polarized direction when it entered polarization beam splitter
18
. Thus, the returning light through quarter-wave plate
19
is efficiently reflected by polarization beam splitter
18
on which it enters next. The returning light reflected by polarization beam splitter
18
is branched to two directions by next beam splitter
22
. One of the branched returning lights is converged by condenser lens
23
, while the other is converged by the same condenser lens
24
as condenser lens
23
. The optical axes of condenser lenses
23
and
24
are arranged on optical axis
28
. The returning light through condenser lens
23
enters two-division sensor
25
disposed in front of a position A where condenser lens
23
converges the returning light when the focus of objective lens
12
is adjusted to the surface of target object
21
. On the other hand, the returning light through condenser lens
24
enters two-division sensor
26
disposed behind a position B where condenser lens
24
converges the returning light when the focus of objective lens
12
is adjusted to the surface of target object
21
. Signals obtained by respective sensors
25
and
26
are subjected to operational processing at autofocus signal processing circuit
15
. Two-division sensors
25
and
26
are disposed with the same optical inclination.
When this focus detecting apparatus is applied to an optical inspecting apparatus, a laser processing apparatus or the like, a luminous flux for inspecting or processing may enter objective lens
12
through dichroic mirror
27
.
Autofocus signal processing circuit
15
for performing the aforementioned operational processing detects values of a focus signal while it drives objective lens
12
with respect to target object
21
at rest, and stops objective lens
12
at the position where the value of the focus signal is equal to zero, and this position is set to a focus position. When stage
14
is moved by stage driving control circuit
16
, driving apparatus
13
for objective lens
12
is controlled such that a value F of the focus signal is always zero during the stage movement.
The value of the focus signal is obtained on the basis of the values of output signals from two-division sensors
25
and
26
as follows.
FIG. 2
shows autofocus signal processing circuit
15
for performing the operational processing of the output signals from two-division sensors
25
and
26
.
In
FIG. 2
, assuming that the values of output signals from light receiving sections A1 and A2 constituting two-division sensor
25
are a1 and a2, respectively, and the values of output signals from light receiving sections B1 and B2 constituting two-division sensor
26
are b1 a
Moribe Hideyuki
Morishige Yukio
Ogura Yukio
Bruce David V.
Dickstein Shapiro Morin & Oshinsky LLP.
NEC Corporation
Song Hoon
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