Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2000-09-14
2003-04-01
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S372000, C382S149000
Reexamination Certificate
active
06541747
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P11-261177 filed Sep. 14, 1999, which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a focal point position control method and a focal point position control mechanism for auto-focussing an objective lens. This invention also relates to a method and apparatus for inspecting the appearance of a semiconductor wafer.
2. Description of Related Art
A semiconductor device is produced by forming a fine device pattern on a semiconductor wafer. If, in a manufacturing process for a semiconductor device, there occurs an attachment of contaminants, pattern defects or unusual dimensions, defects are produced in the device pattern. The semiconductor device, suffering from these defects, produces rejects to lower the yield in the production process.
For maintaining the yield in the manufacturing process at a high level, it is necessary to find defects ascribable to the contaminants, pattern defects or unusual size, at an earlier stage, to locate the causes and to take effective measures for the manufacturing process. If the cause of the defects is determined quickly to take proper measures in the manufacturing process to improve the yield, it is possible to start a new process quickly to obtain a high yield in the process.
If a defect is produced in a semiconductor device, the defect is detected, using a microscopic device for inspecting the semiconductor, to search the cause of the defect, and, from the result of that search, specify the equipment or the process responsible for the defect. This microscopic device for inspecting the semiconductor is a device, such as an optical microscope, that is able to enlarge the defect on the semiconductor wafer for inspection or to image the enlarged defect to demonstrate the image on a monitor. By using this microscope device for inspecting the semiconductor, it becomes possible to discriminate the sort of the defect produced on the reject device.
According to the current design rule for the semiconductor manufacture process, the patterns prevalently have a line width of 0.18 &mgr;m, which tends to be even finer, such as 0.15 &mgr;m or 0.13 &mgr;m. In keeping pace with the tendency to use a finer design rule in the semiconductor process, fine defects which could be discounted in the past now may raise problems, requiring smaller defects to be detected.
Therefore, in a microscopic device for semiconductor inspection, an objective lens with a higher multiplying factor is needed in order to allow for observation of these fine defects.
However, an objective lens with a high multiplying factor has an extremely short depth of focus. For example, if the numerical aperture (NA) is 0.9, and the multiplying factor 100, the depth of focus is ±0.5 &mgr;m or less. It is extremely difficult to adjust the focal point position with a short depth of focus by a manual operation each time the inspection is executed. Thus, with the microscopic device for semiconductor inspection, such a mechanism is required which effects auto-focussing accurately and speedily without using a manual operation.
The conventional microscopic device for semiconductor inspection is provided with a distance detection mechanism for detecting the distance between the semiconductor wafer and the objective lens by causing the laser light or the light from an LED to fall on the objective lens through a light probe for measurement and by detecting the reflected light. In the conventional microscopic device for semiconductor inspection, the distance between the objective lens and the semiconductor wafer is controlled, based on the distance information as detected by this distance detection mechanism, in order to bring the focal point position of the objective lens into coincidence with the surface of the semiconductor wafer to be observed to enable auto-focussing.
An auto-focussing mechanism
100
, used in such conventional microscopic device for semiconductor inspection, is shown in FIG.
15
.
The auto-focussing mechanism
100
includes a stage
102
, for supporting a semiconductor wafer
101
to be inspected, a laser diode
103
for radiating the laser light, an objective lens
104
for condensing the laser light radiated from the laser diode
103
to illuminate the semiconductor wafer
101
, and a photodetector
105
for receiving the laser light reflected by the semiconductor wafer
101
, as shown in FIG.
15
.
The conventional auto-focussing mechanism
100
also includes a halfmirror
106
for separating the optical path for the outgoing light from the laser diode
103
from that of the reflected light from the semiconductor wafer
101
, a knife edge
107
provided between the laser diode
103
and the halfmirror
106
and a collimator lens
108
provided between the half mirror
106
and the objective lens
104
.
This conventional auto-focussing mechanism
100
also includes a pre-amplifier
111
for generating a position detection signal from a detection current of the photodetector
105
, and a servo circuit
112
for driving the stage
102
based on the position detection signal from the pre-amplifier
111
.
On the stage
102
is placed a disc-shaped semiconductor wafer
101
to be inspected. This stage
102
causes the semiconductor wafer
101
, placed thereon, to be moved in the height-wise direction, that is in a direction towards and away from the objective lens
104
. The stage
102
is controlled in its movement according to a driving signal supplied from the servo circuit
112
.
The laser diode
103
radiates the laser light of a pre-set wavelength. The laser light radiated from the laser diode
103
has its spot shaped to a semicircular profile and is incident in this state on the half mirror
106
. The half mirror
106
reflects the laser light radiated from the laser diode
103
. The laser light, reflected by the half mirror
106
, is collimated by the collimator lens
108
into a parallel light beam which then falls on the objective lens
104
. The objective lens
104
converges the collimated laser light to illunminate the semiconductor wafer
101
.
The laser spot formed on the semiconductor wafer
101
has a semicircular shape because of the provision of the knife edge
107
.
The laser light, converged by the objective lens
104
, is reflected by the semiconductor wafer
101
, and is again passed through the objective lens
104
and the collimator lens
108
to fall again on the half mirror
106
. The half mirror
106
now transmits the reflected light from the semiconductor wafer
101
. The reflected light, transmitted through the half mirror
106
, is illuminated on the photodetector
105
.
The photodetector
105
is conjugated in its arraying position with respect to the laser diode
103
. The photodetector
105
receives the reflected light from the semiconductor wafer
101
and converts the reflected light into electrical signals which are routed to the pre-amplifier
111
.
The pre-amplifier
111
finds a difference signal, indicating the position of the center of gravity of a laser spot formed on the photodetector
105
, from the electrical signals supplied from the photodetector
105
. From the difference signal, the pre-amplifier
111
generates a distance detection signal indicating the distance between the semiconductor wafer
101
and the objective lens
104
.
The servo circuit
112
drives the stage
102
in the height-wise direction, so that the supplied distance detection signal is equal to a target value, to execute servo control. By setting the target value so as to be equal to the focal length of the objective lens
104
, the height-wise position of the semiconductor wafer
101
is brought into coincidence with the focal point position of the objective lens
104
.
The principle of the distance detection by this conventional auto-focussing mechanism
100
is hereinafter explained.
The laser spot formed on the semiconductor wafer
101
is of a semicircul
Kikuchi Hiroki
Nogami Asahiko
Pyo Kevin
Sohn Seung C.
Sonnenschein Nath & Rosenthal
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