On-the-fly beam path error correction for memory link...

Details On-the-fly beam path error correction for memory link... On-the-fly beam path error correction for memory link...

2002-02-15

2004-11-09

Chang, Audrey (Department: 2872)

Optical: systems and elements

Deflection using a moving element

By moving a reflective element

C359S199200, C219S121780, C219S121800

Reexamination Certificate

active

06816294

ABSTRACT:

TECHNICAL FIELD
This invention relates to laser processing of circuit links and, in particular, to a laser system and method employing a laser beam and substrate positioning system that incorporates a steering mirror to compensate for stage positioning errors and enhance link severing throughput.
BACKGROUND OF THE INVENTION
Yields in integrated circuit (“IC”) device fabrication processes often incur defects resulting from alignment variations of subsurface layers or patterns or particulate contaminants.
FIGS. 1
,
2
A, and
2
B show repetitive electronic circuits
10
of an IC device or workpiece
12
that are typically fabricated in rows or columns to include multiple iterations of redundant circuit elements
14
, such as spare rows
16
and columns
18
of memory cells
20
. With reference to
FIGS. 1
,
2
A, and
2
B, circuits
10
are also designed to include particular laser severable circuit links
22
between electrical contacts
24
that can be removed to disconnect a defective memory cell
20
, for example, and substitute a replacement redundant cell
26
in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links to program a logic product, gate arrays, or ASICs.
Links
22
are designed with conventional link widths
28
of about 2.5 microns, link lengths
30
, and element-to-element pitches (center-to-center spacings)
32
of about 8 microns from adjacent circuit structures or elements
34
, such as link structures
36
. Although the most prevalent link materials have been polysilicon and like compositions, memory manufacturers have more recently adopted a variety of more conductive metallic link materials that may include, but are not limited to, aluminum, copper, gold nickel, titanium, tungsten, platinum, as well as other metals, metal alloys such as nickel chromide, metal nitrides such as titanium or tantalum nitride, metal suicides such as tungsten silicide, or other metal-like materials.
Circuits
10
, circuit elements
14
, or cells
20
are tested for defects. The links to be severed for correcting the defects are determined from device test data, and the locations of these links are mapped into a database or program. Laser pulses have been employed for more than 20 years to sever circuit links
22
.
FIGS. 2A and 2B
show a laser spot
38
of spot size diameter
40
impinging a link structure
36
composed of a link
22
positioned above a silicon substrate
42
and between component layers of a passivation layer stack including an overlying passivation layer
44
(shown in
FIG. 2A
but not in
FIG. 2B
) and an underlying passivation layer
46
(shown in
FIG. 2B
but not in FIG.
2
A).
FIG. 2C
is a fragmentary cross-sectional side view of the link structure of
FIG. 2B
after the link
22
is removed by the laser pulse.
FIG. 3
is a plan view of a beam positioner travel path
50
performed by a traditional link processing positioning system. Because links
22
are typically arranged in rows
16
and columns
18
(representative ones shown in dashed lines), the beam position and hence the laser spots
38
are scanned over link positions along an axis in a first travel direction
52
, moved to a different row
16
or column
18
, and then scanned over link positions along an axis in a second travel direction
54
. Skilled persons will appreciate that scanning may include moving the workpiece
12
, moving the laser spot
38
, or moving the workpiece
12
and the laser spot
38
.
Traditional positioning systems are characterized by X-Y translation tables in which the workpiece
12
is secured to an upper stage that moves along a first axis and is supported by a lower stage that moves along a second axis that is perpendicular to the first axis. Such systems typically move the workpiece relative to a fixed beam position or laser spot
38
and are commonly referred to as stacked stage positioning systems because the lower stage supports the inertial mass of the upper stage which supports workpiece
12
. These positioning systems have excellent positioning accuracy because interferometers are typically used along each axis to determine the absolute position of each stage. This level of accuracy is preferred for link processing because the laser spot size
40
is typically only a little bigger than link width
28
, so even a small discrepancy between the position of laser spot
38
and link
22
can result in incomplete link severing. In addition, the high density of features on semiconductor wafers results in small positioning errors potentially causing laser damage to nearby structures. Stacked stage positioning systems are, however, relatively slow because the starting, stopping, and change of direction of the inertial mass of the stages increase the time required for the laser tool to process all the designated links
22
on workpiece
12
.
In split-axis positioning systems, the upper stage is not supported by, and moves independently from, the lower stage and the workpiece is carried on a first axis or stage while the tool, such as a fixed reflecting mirror and focusing lens, is carried on the second axis or stage. Split-axis positioning systems are becoming advantageous as the overall size and weight of workpieces
12
increase, utilizing longer and hence more massive stages.
More recently, planar positioning systems have been employed in which the workpiece is carried on a single stage that is movable by two or more actuators while the tool remains in a substantially fixed position. These systems translate the workpiece in two dimensions by coordinating the efforts of the actuators. Some planar positioning systems may also be capable of rotating the workpiece.
Semiconductor Link processing (“SLP”) systems built by Electro Scientific Industries, Inc. (“ESI”) of Portland, Oreg. employ on-the-fly (“OTF”) link processing to achieve both accuracy and high throughput. During OTF processing, the laser beam is pulsed as a linear stage beam positioner passes designated links
12
under the beam position. The stage typically moves along a single axis at a time and does not stop at each link position. The on-axis position of beam spot
38
in the direction travel
52
does not have to be accurately controlled; rather, its position is accurately sensed to trigger laser spot
38
to hit link
22
accurately.
In contrast and with reference again to
FIG. 3
, the position of beam spot
38
along cross-axes
56
or
58
is controlled within specified accuracy as the beam positioner passes over each link
22
. Due to the inertial mass of the stage or stages, a set-up move to start an OTF run produces ringing in the cross-axis position, and the first link
22
in an OTF run cannot be processed until the cross-axis position has settled properly. The settling delay or setting distance
60
reduces processing throughput. Without a settling delay (or, equivalently, a buffer zone of settling distance
60
) inserted before the first laser pulse, several links
22
would be processed with serious cross-axis errors.
Although OTF speed has been improved by accelerating over gaps in the link runs, one limiting factor on the effectiveness of this “gap profiling” is still the requirement for the cross axis to settle within its specified accuracy. At the same time, feature sizes, such as link length
30
and link pitch
32
, are continuing to decrease, causing the need for dimensional precision to increase. Efforts to further increase the performance of the stage or stages substantially increase the costs of the positioning system.
The traditional way to provide for two-axis deflection of a laser beam employs a high-speed short-movement positioner (“fast positioner”)
62
, such as a pair of galvanometer driven mirrors
64
and
66
shown in FIG.
4
.
FIG. 4
is a simplified depiction of a galvanometer-driven X-axis mirror
64
and a galvanometer-driven Y-axis mirror
66
positioned along an optical path
70
between a fixed mirror
72
and focusing optics
78
. Each galvanometer-driven mirror deflects the laser beam along a singl

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