Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-01-20
2002-11-05
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S067000
Reexamination Certificate
active
06476905
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a photolithographic scanner that has an improved variable attenuator and more particularly, relates to a step and scan exposure system for a photolithographic process that is equipped with a plurality of attenuator blades for exposure control that is capable of controlling exposure at 10% increment in light transmission.
BACKGROUND OF THE INVENTION
Modern fabrication processes for producing semiconductor devices, such as integrated circuits, have long employed photolithography for transferring circuit patterns onto a semiconductor substrate, such as a wafer. In general, photolithography involves the performance of a sequence of process steps, including coating a semiconductor wafer with a resist layer, exposing the coated wafer to a patterned light source, developing the resist layer, processing the semiconductor wafer through the developed resist layer, and removing the resist layer. An optical photolithography scanner apparatus, sometimes referred to as a “step and scan” or “scanner”, is typically used to expose the resist layer. An image of each later of an IC die is formed on the stepper and a reduced image thereof is projected onto a portion of the resist layer covering the semiconductor wafer. Specifically, the reticle patterns are transferred to the wafer by scanning the patterns through a narrow illumination slit.
When numerous IC's are to be fabricated from a single wafer, a mask used in the fabrication of any one IC is also used in the fabrication of the other IC's from the wafer. This is accomplished by using the scanner to scan the wafer under an optical system which includes the mask or reticle. At each step, the photoresist is exposed to the optical system, typically with ultraviolet light, to form a serial image of the mask on the layer of photoresist. The wafer is then removed from the stepper and the image is developed. At that point, the wafer is etched to remove portions of the underlying film, following which the wafer is ready for the next stage of processing, which might include for example, ion implantation, deposition or other types of etching processes. At a later stage in the fabrication process, the wafer is returned to the scanner for exposure of the wafer to a different mask.
The reticle or mask is composed of a glass substrate, such as quartz, on which there is formed a circuit pattern composed of materials such as chromium which prevents ultraviolet light from transmitting therethrough. The reticle is set in the scanner in order to expose a semiconductor wafer to light, and the circuit pattern formed on the reticle is imaged by the scanner onto the semiconductor substrate.
Semiconductor manufacturing processes are aimed at achieving up to 0.25 micron resolution in a high speed production environment. This goal is being driven by the need to develop competitive device performance and lower manufacturing costs per device. In order to increase the field size and improve critical dimensional control below 0.25 micron resolution, improvements in step and scan technology plays a critical role. Improvements in the area of highly controllable, precise light sources, such as excimer lasers with appropriate dose control are important for solving illumination control problems and achieving exceptionally short exposure times.
In order to more precisely control the dose of light radiation projected on the wafer, the illumination system and scanner slit have, in the past, been provided with an adjustment that allows focusing of the image applied to the wafer. This adjustment system relies on movement and adjustment of mechanical elements, and particularly the displacement of the mechanical slit relative to the illumination source. As a result of the dependency on this mechanical adjustment, repeatable results are not always obtained from batch to batch since adjustment settings may change for a number of reasons. Moreover, the need to perform periodic preventive maintenance on equipment introduces the further possibility that adjustment settings may be inadvertently altered, thus making repeatable, precise dosage control impossible.
Referring initially to
FIG. 1
which depicts a conventional photolithography exposure system
10
and related dosage control technique. Radiation in the form of ultraviolet light is produced by a mercury lamp
20
and is focused by a reflector
24
onto a reflecting mirror
26
to produce a beam of light which is sequentially passed through a filter
28
, shutter
30
, attenuator
32
, zoom lens
34
, interference filters
36
, integrator lens
38
and field lens
40
onto an energy sensor
46
which measures the intensity of the beam. The remainder of the beam is reflected by the mirror
44
as a beam spot
48
which is directed through reticle masking blades
50
defining a slit
51
hence through a reticle or mask (not shown). Radiation in the beam passes through the slit
51
and circuit pattern defining reticle and is focused on a semiconductor wafer (not shown) that is supported on an exposure chuck (not shown) provided with a spot meter for measuring the intensity of the beam focused onto a portion of the wafer.
In the conventional photolithography exposure system
10
, the attenuator
32
utilized is generally of a variable type formed on a rotatable blade
12
, as shown in FIG.
2
. The rotatable attenuator blade
12
is mounted through a center aperture
14
to a shaft
16
which is in turn mounted on a housing member
18
for the photolithographic exposure system
10
. The housing member
18
further includes an optical port
22
for the ultraviolet light to go therethrough. In the conventional attenuator blade
12
shown in
FIG. 2
, the attenuator blade
12
is equipped with
3
attenuators
52
,
54
and
56
. The attenuators
52
,
54
and
56
are each individually formed and then mounted in the attenuator blade
12
. The attenuator
52
is formed with medium-sized apertures to allow 50% light transmission, the attenuator
54
is formed with large-sized apertures to allow 75% light transmission, while the attenuator
56
is formed with small-sized apertures to allow only 25% light transmission. By rotating the attenuator blade
12
, a suitable attenuator selected from
52
,
54
and
56
can be used to cover the optical port
22
for achieving an attenuated light beam.
In practice it has been found that the three stages of light transmission control at 25%, 50% and 75% is not adequate in providing desirable exposure control in a photolithographic process. Since a wafer is carried on an exposure chuck and moved on a stage in the step and scan exposure system, the stage speed must also be adjusted in order to compensate for the inadequate light submission control through the variable attenuator
32
. In a conventional step and scan exposure system
10
, there is a limit within which the stage speed can be adjusted. In order to keep a high throughput of the photolithographic process, the maximum stage speed is preferred. When the conventional variable attenuator blade
12
is utilized, since only three transmission levels are available, the stage speed must be slowed down in order to achieve the desirable exposure thus sacrificing the process throughput. The single attenuator blade design shown in FIG.
2
. is therefore inadequate for achieving a desirable exposure control while maintaining maximum stage speed, i.e., maximum throughput, at the same time.
For instance, as shown in
FIG. 4
, when the 25% transmission attenuator is utilized, in order to achieve a desirable exposure, the scan speed of the wafer stage needs to be slowed down from to 250 mm/sec to about 125 mm/sec. Similarly, when the 50% transmission attenuator is utilized, the scan speed needs to be decreased from 250 mm/sec to about 165 mm/sec. When the 75% transmission attenuator is used, the scan speed needs to be decreased from 250 mm/sec to about 185 mm/sec. As shown in
FIG. 4
, in the worst case of the 25% transmission attenuator, the scan speed decreases by almost 50% whi
Adams Russell
Nguyen Hung Henry
Taiwan Semiconductor Manufacturing Co. Ltd.
Tung & Associates
LandOfFree
Step and scan exposure system equipped with a plurality 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 Step and scan exposure system equipped with a plurality of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Step and scan exposure system equipped with a plurality of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2985745