Electric heating – Metal heating – By arc
Reexamination Certificate
2001-07-11
2003-07-15
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121690
Reexamination Certificate
active
06593542
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
TECHNICAL FIELD
The present invention relates to a laser-based system or method for severing integrated circuit (IC) device fuses, and, in particular, to such a system or method that employs a single UV laser pulse to sever an IC fuse.
BACKGROUND OF THE INVENTION
FIGS. 1
,
2
A, and
2
B show repetitive electronic circuits
10
of an IC device on a wafer 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 fuses or 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 logic products, 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.
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
. Before link processing is initiated, circuits
10
, circuit elements
14
, or cells
20
are tested for defects, the locations of which may be mapped into a database or program that determines locations of links
22
to be processed. Typically, the same IR laser beam used for processing the links is used, at reduced intensity, to locate the position of the focused spot of the IR laser beam with respect to reflective alignment marks, such as metal on oxide, positioned at the corners of the dies and/or wafers supporting the electronic components.
Conventional memory link processing systems focus a single pulse of IR 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 (A) 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. The quality of the crater formed in
FIG. 2C
is neither uniform nor predictable.
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 several years with great success.
The 1.0 &mgr;m and 1.3 &mgr;m laser wavelengths have disadvantages however. In general, the optical absorption of such IR laser beams
12
into a highly electrically conductive metallic link
22
is less than that of visible or UV beams; 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
.
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. Thus, the thermal-stress behavior limits the critical dimensions of links
22
and prevents greater circuit density.
U.S. Pat. No. 6,025,256 of Swenson et al. describes methods of using ultraviolet (UV) laser output to expose links that “open” the overlying passivation or resist material with low laser power through a different mechanism for material removal and provide the benefit of a smaller beam spot size. The links are subsequently etched.
U.S. Pat. No. 6,057,180 of Sun et al. describes methods of using UV laser output to remove links
22
positioned above a passivation layer of sufficient height to safeguard the underlying substrate from laser damage. This technique advocates modification of the target material and structure well in advance of laser processing.
Thus, improved link processing methods are still desirable.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a system or method that employs a single UV laser pulse to sever an IC fuse.
The present invention provides a Q-switched, diode-pumped, solid-state (DPSS) laser that employs harmonic generation through nonlinear crystals to generate green and/or IR and UV light. In a preferred embodiment, the type and geometry of the nonlinear crystals are selected to produce excellent beam quality suitable for subsequent beam shaping and focusing necessary to produce focused spot sizes that are advantageous for severing of IC fuses. The temperatures of the nonlinear crystals may also be precisely regulated using temperature feedback control loops to maintain advantageous phase matching conditions so as to produce uniform processing laser pulse characteristics. In addition, beam shape quality may also be enhanced by an imaged optics module capable of spatially filtering unwanted beam artifacts.
In a further preferred embodiment, because many standard alignment targets are difficult to detect with a UV laser beam, a fraction of the green or IR output may be utilized for the separate purpose of target alignment. The fractional green or IR target alignment beam follows a separate optical path with a separate set of optical elements and is attenuated to the proper power level. An imaged optics module for the fractional green or IR beam optimizes its shape for alignment scans. The green or IR alignment beam and the UV alignment beam pass through detection system modules and are separately aligned to a calibration target through a beam combiner common to both optical paths and their respective resulting reflected light is detected to calibrate the alignment beam with the UV link processing beam. The green or IR alignment beam can then be used to align the beam(s) to a given die, and the desired links on the die can be severed by the UV link processing beam without further calibration.
This invention provides the capability to produce high quality, focused spots that are smaller than con
Baird Brian W.
Lo Ho Wai
Nilsen Brady E.
Electro Scientific Industries Inc.
Evans Geoffrey S.
Stoel Rives LLP
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