Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-11-02
2004-07-27
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
Reexamination Certificate
active
06768538
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to photolithography systems for semiconductor processing, and more particularly to such a system for increasing overlay accuracy.
BACKGROUND OF THE INVENTION
Patterning is one of the basic steps performed in semiconductor processing. It also referred to as photolithography, masking, oxide or metal removal, and microlithography. Patterning enables the selective removal of material deposited on a semiconductor substrate, or wafer, as a result of a deposition process. For example, as shown in
FIG. 1A
, a layer
104
has been deposited on a substrate
102
. After the photolithography process is performed, as shown in
FIG. 1B
, some parts of the layer
104
have been selectively removed, such that gaps
106
a
and
106
b
are present within the layer
104
. A photomask, or pattern, is used (not shown in
FIG. 1B
) so that only the material from the gaps
106
a
and
106
b
are removed, and not the other portions of the layer
104
. The process of adding layers and removing selective parts of them, in conjunction with other processes, permits the fabrication of semiconductor devices.
Alignment is critical in photolithography and deposition, as well as in other semiconductor processes. If layers are not deposited properly, or if they are not selectively removed properly, the resulting semiconductor devices may not function, relegating them to scrap, which can be costly. Such misalignment, or overlay shift, is shown in FIG.
2
. The layer
204
may or may not be deposited in a properly aligned configuration on the substrate
202
, whereas subsequent deposition layers
206
a,
206
b
, . . . ,
206
n
are misaligned. This is indicated by the reference marks
210
a,
210
b
, . . . ,
210
n,
which are shown in
FIG. 2
for illustrative clarity only. The reference marks
210
a,
210
b
, . . . ,
210
n,
should substantially align over the alignment marks
208
of the substrate
202
, but they do not.
In comparison to
FIG. 2
, correctly aligned layers are shown in FIG.
3
. The semiconductor wafer
202
has alignment marks
208
. The layer
204
is aligned thereupon. Similarly, the layers
206
a,
206
b
, . . . ,
206
n
are deposited upon the layer
204
, without any, or with minimal, overlay shift. This is indicated by the reference marks
210
a,
210
b
, . . . ,
210
n
aligning with the alignment marks
208
of the wafer
202
.
Alignment errors such as overlay shift are also referred to as overlay error or misalignment. A common error is a simple misplacement of a layer in the x and/or y directions. Another overlay error is rotational, where one side of the wafer is aligned, but patterns become increasingly misaligned across the wafer. Other misalignment problems associated with masks and stepper aligners are run-out and run-in. These problems arise when the chip patterns are not formed on the mask on constant centers, or are placed on the chip off center. The result is that only a portion of the mask chip patterns can be properly aligned to the wafer patterns, such that the pattern becomes progressively misaligned across the wafer.
A common rule of thumb is that circuits with micron or sub-micron feature sizes must meet registration tolerances of one-third the minimum critical feature size. An overlay budget is therefore determined for the total circuit. The overlay budget is the allowable accumulated alignment error for the entire mask set. For example, for a 0.35-micron product, the allowable overlay budget is usually about 0.1 micron.
Overlay error can thus be caused by limitations in scanner or stepper stage accuracy, limitations in magnification accuracy, lens distortion, lens aberrations, such as focus, spherical, coma, and astigmatism aberrations, as well as other limitations, errors, distortions, and aberrations. Scanners and steppers are semiconductor exposure equipment, or tools, used to align a photomask over a semiconductor wafer, and then expose the wafer through the photomask. Such equipment usually has a tolerance range or otherwise has a limited accuracy, such that overlay error can result. Limitations in magnification accuracy result from the masks have a greater size than the resulting exposed image on the wafer, resulting in overlay error. Distortions and aberrations in the lenses of scanners and steppers also cause overlay error.
Conventional approaches to correct overlay error include adjusting the stage, magnification, and/or field rotation and translation to compensate for overlay errors between different layers being deposited and/or exposed. That is, between the deposition and/or exposure of different layers on the semiconductor wafer, scanner and stepper parameters such as the stage, magnification, and field rotation and translation may be varied to account and compensate for and correct overlay error. However, typically there is residual overlay error that cannot be corrected. For instance, the use of different exposure tools and equipment for different layers of the semiconductor device can be responsible for residual overlay error. Different scanners and steppers have different types of lens, with specific signatures of distortion and aberrations, causing pattern placement error.
Matching different exposure tools to one another is a difficult process, because different scanners and steppers have different overlay specifications. As an example, one scanner may have an overlay specification of 35 nanometers (nm), whereas another scanner may have an overlay specification of 55 nm. Even if these two scanners can be theoretically matched, tolerances in their overlay accuracy may further impede matching in actuality. For instance, the former scanner may actually have an overlay specification of 20 nm, whereas the latter scanner may actually have a specification of 45 nm. Matching the scanners in theory, in other words, still causes overlay error in actuality.
FIG. 4
shows an example of such a conventional two-exposure tool photolithography system
400
. The system includes a pre-exposure track
402
, an exposure tool
404
, and a post-exposure track
406
for a front-end wafer
412
A, and a pre- and post-exposure track
408
and an exposure tool
410
for a back-end wafer
412
B. The back-end wafer
412
B can be the front-end wafer
412
A after it has been processed by the pre-exposure track
402
, the exposure tool
404
, and the post-exposure track
406
. The tracks
402
,
406
, and
408
are tracks in that a continual number of wafers can be moved through them in an automated line approach. The exposure tool
404
typically has different overlay specifications than that of the exposure tool
410
, such that use of the two tools
404
and
410
results in residual overlay error.
The pre-exposure track
402
handles processes that affect the photoresist prior to exposure by the exposure tool
404
. These include applying the photoresist, and soft baking the photoresist. Photoresist application is usually by spinning photoresist onto the semiconductor wafer. Spinning may be static, dynamically dispensed, dispensed by a moving arm, manually spun, and/or automatically spun, among other approaches. Furthermore, a backside coating may be applied, which is coating the back of the wafer with photoresist. Soft baking is a heating operation to evaporate a portion of the solvents in the photoresist. The resist film is still soft after the soft bake, as opposed to being baked to a varnish-like finish. Soft baking can be accomplished by convection ovens, manual hot plates, in-line single-wafer hot plates, moving-belt hot plates, moving-belt infrared ovens, microwave baking, and/or vacuum baking, among other approaches.
The exposure tool
404
usually performs both alignment and exposure. Alignment is the position of the required image on the wafer surface, whereas exposure is the encoding of the image in the photoresist layer by exposing light or another radiation source. The exposure tool
404
may be or include a stepper or a scanner. Different types of aligners include optical aligners, such as contact, proxi
Gau Tsai-Sheng
Yen Anthony
Esplin D. Ben
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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