Coherent light generators – Particular beam control device – Control of pulse characteristics
Reexamination Certificate
2001-01-09
2003-06-03
Lee, Eddie (Department: 2815)
Coherent light generators
Particular beam control device
Control of pulse characteristics
C372S010000, C372S011000, C372S012000, C372S030000, C372S018000, C219S121670, C219S121680, C219S121690
Reexamination Certificate
active
06574250
ABSTRACT:
TECHNICAL FIELD
The present invention relates to laser processing of memory or other IC links and, in particular, to a laser system and method employing a burst of laser pulses having ultrashort pulse widths to sever an IC link.
BACKGROUND OF THE INVENTION
Yields in 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 commonly 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 about 0.5-2 microns (&mgr;m) thick and are designed with conventional link widths
28
of about 0.8-2.5 &mgr;m, link lengths
30
, and element-to-element pitches (center-to-center spacings)
32
of about 2-8 &mgr;m 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, metal nitrides such as titanium or tantalum nitride, metal silicides such as tungsten silicide, or other metal-like materials.
Circuits
10
, circuit elements
14
, or cells
20
are tested for defects, the locations of which may be mapped into a database or program. Traditional 1.047 &mgr;m or 1.064 &mgr;m infrared (IR) laser wavelengths have been employed for more than 20 years to explosively remove circuit links
22
. Conventional memory link processing systems focus a single pulse of laser output having a pulse width of about 4 to 20 nanoseconds (ns) at each link
22
.
FIGS. 2A and 2B
show a laser spot
38
of spot size diameter
40
impinging a link structure
36
composed of a polysilicon or metal 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.
2
B), which is typically 2000-10,000 angstrom (Å) thick, and an underlying passivation layer
46
. Silicon substrate
42
absorbs a relatively small proportional quantity of IR radiation, and conventional passivation layers
44
and
46
such as silicon dioxide or silicon nitride are relatively transparent to IR radiation.
FIG. 2C
is a fragmentary cross-sectional side view of the link structure of
FIG. 2B
after the link
22
is removed by the prior art laser pulse.
To avoid damage to the substrate
42
while maintaining sufficient energy to process a metal or nonmetal link
22
, Sun et al. in U.S. Pat. No. 5,265,114 and U.S. Pat. No. 5,473,624 proposed using a single 9 to 25 ns pulse at a longer laser wavelength, such as 1.3 &mgr;m, to process memory links
22
on silicon wafers. At the 1.3 &mgr;m laser wavelength, the absorption contrast between the link material and silicon substrate
42
is much larger than that at the traditional 1 &mgr;m laser wavelengths. The much wider laser processing window and better processing quality afforded by this technique has been used in the industry for about three years with great success.
The 1.0 &mgr;m and 1.3 &mgr;m laser wavelengths have disadvantages however. The coupling efficiency of such IR laser beams into a highly electrically conductive metallic link
22
is relatively poor; and the practical achievable spot size
38
of an IR laser beam for link severing is relatively large and limits the critical dimensions of link width
28
, link length
30
between contacts
24
, and link pitch
32
. This conventional laser link processing relies on heating, melting, and evaporating link
22
, and creating a mechanical stress build-up to explosively open overlying passivation layer
44
. Such a conventional link processing laser pulse creates a large heat affected zone (HAZ) that deteriorates the quality of the device that includes the severed link.
The thermal-stress explosion behavior is also somewhat dependent on the width of link
22
. As the link width becomes narrower than about 1 &mgr;m, the explosion pattern of passivation layers
44
becomes irregular and results in an inconsistent link processing quality that is unacceptable and limits circuit density. Thus, the thermal-stress behavior limits the critical dimensions of links
22
and prevents greater circuit density.
U.S. Pat. No. 6,057,180 of Sun et al. and U.S. Pat. No. 6,025,256 of Swenson et al. more recently describe methods of using ultraviolet (UV) laser output to sever or expose links that “open” the overlying passivation by different material removal mechanisms and have the benefit of a smaller beam spot size. However, removal of the link itself by such a UV laser pulse requires the passivation material to be UV absorbing and is still a “thermal” process.
U.S. Pat. No. 5,656,186 of Mourou et al. discloses a general method of laser induced breakdown and ablation by high repetition rate ultrafast laser pulses.
U.S. Pat. No. 5,208,437 of Miyauchi et al. discloses a method of using a single pulse of a subnanosecond pulse width to process a link.
U.S. Pat. No. 5,742,634 of Rieger et al. discloses a simultaneously Q-switched and mode-locked neodymium (Nd) laser device with diode pumping. The laser emits a series of pulses each having a duration time of 60 to 300 picoseconds (ps), under an envelope of a time duration of 100 ns. Pulses having a duration time of 60 to 300 ps exhibit a “thermal” mechanism of material processing.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method or apparatus for improving the quality of laser processing of IC links.
Another object of the invention is to process links with bursts of ultrashort laser pulses that have a nonthermal interaction with the overlying passivation layer and the link material.
A further object of the invention is to employ the bursts of ultrashort laser pulses to process the links on-the-fly.
The present invention employs a burst ultrashort laser pulses to sever an IC link, instead of using a single multiple-nanosecond laser pulse of conventional link processing systems. The duration of the burst is preferably in the range of 10 to 500 ns; and the pulse width of each laser pulse within the burst is generally shorter than 25 ps, preferably shorter than or equal to 10 ps, and most preferably about 10 ps to 100 femtoseconds (fs). Because each laser pulse within the burst is ultrashort, its interaction with the target materials (passivation layers and metallic link) is not thermal. Each laser pulse breaks off a thin sublayer of about 100-2,000 Å of material, depending on the laser energy, laser wavelength, and type of material, until the link is severed. The number of ultrashort laser pulses in the burst is controlled such that the last pulse cleans off the bottom of the link leaving the underlying passivation layer and the substrate intact. Because the whole duration of the burst is in the range of 10 ns to 500 ns, the burst is considered to be a single “pulse” by a traditional link-severing laser positioning system. Thus, the laser system can still process links on-the-fly, i.e. the positioning system does not have to stop moving when the laser system fires a burst of laser pulses at each link.
In addition to the “nonthermal” and well-controllable nature of ultrashort-pulse laser processing, the most common ultrashort-pulse laser
Harris Richard S.
Sun Yunlong
Swenson Edward J.
Electro Scientific Industries Inc.
Landau Matthew C
Lee Eddie
Stoel Rives LLP
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