U.S. patent application number 11/440127 was filed with the patent office on 2006-12-21 for laser processing.
Invention is credited to James Cordingley, William Lauer, Michael Plotkin, Donald V. Smart, Pierre Trepagnier.
Application Number | 20060283845 11/440127 |
Document ID | / |
Family ID | 22793200 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060283845 |
Kind Code |
A1 |
Lauer; William ; et
al. |
December 21, 2006 |
Laser processing
Abstract
The invention provides a system and method for vaporizing a
target structure on a substrate. According to the invention, a
calculation is performed, as a function of wavelength, of an
incident beam energy necessary to deposit unit energy in the target
structure. Then, for the incident beam energy, the energy expected
to be deposited in the substrate as a function of wavelength is
calculated. A wavelength is identified that corresponds to a
relatively low value of the energy expected to be deposited in the
substrate, the low value being substantially less than a value of
the energy expected to be deposited in the substrate at a higher
wavelength. A laser system is provided configured to produce a
laser output at the wavelength corresponding to the relatively low
value of the energy expected to be deposited in the substrate. The
laser output is directed at the target structure on the substrate
at the wavelength corresponding to the relatively low value of the
energy expected to be deposited in the substrate, in order to
vaporize the target structure.
Inventors: |
Lauer; William; (Westford,
MA) ; Trepagnier; Pierre; (Medford, MA) ;
Smart; Donald V.; (Boston, MA) ; Cordingley;
James; (Littleton, MA) ; Plotkin; Michael;
(Newton, MA) |
Correspondence
Address: |
GAUTHIER & CONNORS, LLP
225 FRANKLIN STREET
SUITE 2300
BOSTON
MA
02110
US
|
Family ID: |
22793200 |
Appl. No.: |
11/440127 |
Filed: |
May 24, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11130232 |
May 17, 2005 |
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11440127 |
May 24, 2006 |
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10428938 |
May 5, 2003 |
6911622 |
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11130232 |
May 17, 2005 |
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09968541 |
Oct 2, 2001 |
6559412 |
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10428938 |
May 5, 2003 |
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09212974 |
Dec 16, 1998 |
6300590 |
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09968541 |
Oct 2, 2001 |
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Current U.S.
Class: |
219/121.69 ;
219/121.68; 257/E23.15 |
Current CPC
Class: |
B23K 26/042 20151001;
B23K 26/082 20151001; B23K 2103/172 20180801; B23K 26/361 20151001;
B23K 2103/12 20180801; B23K 2103/50 20180801; B23K 26/10 20130101;
H01L 21/76888 20130101; B23K 26/0853 20130101; H01L 2924/0002
20130101; B23K 2101/40 20180801; H01L 2924/00 20130101; H01L
23/5258 20130101; B23K 2103/10 20180801; B23K 26/364 20151001; H01L
2924/0002 20130101; B23K 26/40 20130101; B23K 2103/08 20180801;
B23K 26/0622 20151001 |
Class at
Publication: |
219/121.69 ;
219/121.68 |
International
Class: |
B23K 26/40 20060101
B23K026/40 |
Claims
1-25. (canceled)
26. A system for vaporizing a target structure on a silicon
substrate, comprising: a laser pumping source; a laser resonator
cavity configured to be pumped by the laser pumping source; a laser
output system configured to produce a pulsed laser output beam from
energy stored in the laser resonator cavity and to direct the
pulsed laser output beam at the target structure on the silicon
substrate in order to vaporize the target structure, at a
wavelength below an absorption edge of the silicon substrate and
between about 0.30 microns and about 0.55 microns, the silicon
substrate being positioned beneath the target structure with
respect to the laser output, the laser output system being
configured to produce the pulsed laser output beam at an incident
beam energy; a computer programmed to generate computer-controlled
timing signals synchronized with the position of the pulsed laser
beam relative to the target structure; and an optical switch that
is controllably switchable based on the timing signals so as to
cause output pulses of the pulsed laser beam to be transmitted to
the target structure and to permit selective reduction of the
incident beam energy; wherein the incident beam energy at which the
target structure is vaporized is reduced relative to an incident
beam energy necessary to deposit unit energy in the target
structure sufficient to vaporize the target structure at a higher
wavelength below the absorption edge of the silicon substrate.
27. The system of claim 26 wherein the laser output system
comprises a wavelength shifter.
28. The system of claim 26 wherein the laser resonator cavity
produces laser radiation at the wavelength corresponding to the
relatively low value of energy expected to be deposited in the
substrate.
29. The system of claim 26 wherein the target structure comprises a
metal having a conductivity greater than that of aluminum.
30. The system of claim 29 wherein the metal comprises copper.
31. The system of claim 29 wherein the metal comprises gold.
32. The system of claim 28 wherein the target structure on the
substrate comprises a link of a semiconductor device.
33. The system of claim 32 wherein the semiconductor device
comprises an integrated circuit.
34. The system of claim 32 wherein the semiconductor device
comprises a memory device.
35. The system of claim 28 wherein the energy expected to be
deposited in the substrate is substantially proportional to the
incident beam energy necessary to deposit unit energy in the target
structure minus the energy deposited in the target structure,
multiplied by absorption of the substrate.
36. The system of claim 26 wherein the identified wavelength
corresponding to a relatively low value of the energy expected to
be deposited in the silicon substrate is within a visible region of
spectrum.
37. The system of claim 36 wherein the identified wavelength
corresponding to a relatively low value of the energy expected to
be deposited in the silicon substrate is within a green region of
spectrum.
38. The system of claim 26 wherein the laser output at the incident
beam energy comprises short pulses.
39. The system of claim 26 wherein the laser resonator cavity is a
neodymium vanadate laser resonator cavity.
40. A system for vaporizing a target structure on a silicon
substrate, comprising: a laser pumping source; a laser resonator
cavity configured to be pumped by the laser pumping source; a laser
output system configured to produce a pulsed laser output beam from
energy stored in the laser resonator cavity and to direct the
pulsed laser output beam at the target structure on the silicon
substrate in order to vaporize the target structure, at a
wavelength below an absorption edge of the silicon substrate and
between about 0.30 microns and about 0.55 microns, the silicon
substrate being positioned beneath the target structure with
respect to the laser output, the laser output system being
configured to produce the pulsed laser output beam at an incident
beam energy, the laser output comprising short pulses; a computer
programmed to generate computer-controlled timing signals
synchronized with the position of the pulsed laser beam relative to
the target structure; an optical switch that is controllably
switchable based on the timing signals so as to cause output pulses
of the pulsed laser beam to be transmitted to the target structure;
and an optical system including at least one focusing element for
focusing the incident beam energy onto the target structure with a
small spot size that is proportionally smaller according to
wavelength than a larger spot size that would be focused onto the
target structure by an optical system having an f number at a
wavelength of at least about 1.047 microns.
41. The system of claim 40 wherein the laser resonator cavity is a
neodymium vanadate laser resonator cavity.
42. A system for vaporizing a target structure on a silicon
substrate, comprising: a laser pumping source; a laser resonator
cavity configured to be pumped by the laser pumping source; a laser
output system configured to produce a pulsed laser output beam from
energy stored in the laser resonator cavity and to direct the
pulsed laser output beam at the target structure on the silicon
substrate in order to vaporize the target structure, at a
wavelength below an absorption edge of the silicon substrate and
between about 0.30 microns and about 0.55 microns, the silicon
substrate being positioned beneath the target structure with
respect to the laser output, the laser output system being
configured to produce the pulsed laser output beam at an incident
beam energy; a computer programmed to generate computer-controlled
timing signals synchronized with the position of the pulsed laser
beam relative to the target structure; an optical switch that is
controllably switchable based on the timing signals so as to cause
output pulses of the pulsed laser beam to be transmitted to the
target structure to permit selective reduction of the incident beam
energy; and an optical system including at least one focusing
element for focusing the incident beam energy onto the target
structure with a small spot size that is proportionally smaller
according to wavelength than a larger spot size that would be
focused onto the target structure by an optical system having an f
number at a wavelength of at least about 1.047 microns; wherein the
incident beam energy at which the target structure is vaporized is
reducible relative to an incident beam energy necessary to deposit
unit energy in the target structure sufficient to vaporize the
target structure at a higher wavelength below the absorption edge
of the silicon substrate.
43. A method of vaporizing a target structure on a silicon
substrate, comprising the steps of: providing a laser system
configured to produce a pulsed laser output beam at a wavelength
below an absorption edge of the silicon substrate and between about
0.30 microns and about 0.55 microns; and directing the pulsed laser
output beam at the target structure on the silicon substrate at the
wavelength and at an incident beam energy, in order to vaporize the
target structure, the silicon substrate being positioned beneath
the target structure with respect to the laser output; generating
computer-controlled timing signals synchronized with the position
of the pulsed laser beam relative to the target structure; focusing
the incident beam energy onto the target structure with a small
spot size that is proportionally smaller according to wavelength
than a larger spot size that would be focused onto the target
structure by an optical system having an f number at a wavelength
of at least about 1.047 microns; controllably switching an optical
switch based on the timing signals so as to cause output pulses of
the pulsed laser beam to be transmitted to the target structure;
wherein the incident beam energy at which the target structure is
vaporized is reduced relative to an incident beam energy necessary
to deposit unit energy in the target structure sufficient to
vaporize the target structure at the higher wavelength below the
absorption edge of the silicon substrate.
44. The method of claim 43 wherein the identified wavelength
corresponding to a relatively low value of the energy expected to
be deposited in the silicon substrate is within a visible region of
spectrum.
45. The method of claim 44 wherein the identified wavelength
corresponding to a relatively low value of the energy expected to
be deposited in the silicon substrate is within a green region of
spectrum.
46. The method of claim 43 wherein the laser output at the incident
beam energy comprises short pulses.
47. The method of claim 43 wherein the laser system comprises a
neodymium vanadate laser.
48. A method of vaporizing a target structure on a silicon
substrate, comprising the steps of: providing a laser system
configured to produce a laser output at a wavelength below an
absorption edge of the silicon substrate and between about 0.30
microns and about 0.55 microns; and directing the laser output at
the target structure on the silicon substrate at the wavelength and
at an incident beam energy, in order to vaporize the target
structure, the silicon substrate being positioned beneath the
target structure with respect to the laser output, wherein the
laser output at the incident beam energy comprises short pulses;
generating computer-controlled timing signals synchronized with the
position of the pulsed laser beam relative to the target structure;
and controllably switching an optical switch based on the timing
signals so as to cause output pulses of the pulsed laser beam to be
transmitted to the target structure to permit selective reduction
of the incident beam energy.
49. The method of claim 38 wherein the laser system comprises a
neodymium vanadate laser.
50. A method of vaporizing a target structure on a silicon
substrate, comprising the steps of: providing a laser system
configured to produce a laser output at a wavelength below an
absorption edge of the silicon substrate and between about 0.30
microns and about 0.55 microns; directing the laser output at the
target structure on the silicon substrate at the wavelength and at
an incident beam energy, in order to vaporize the target structure,
the silicon substrate being positioned beneath the target structure
with respect to the laser output; generating computer-controlled
timing signals synchronized with the position of the pulsed laser
beam relative to the target structure; focusing the incident beam
energy onto the target structure with a small spot size that is
proportionally smaller according to wavelength than a larger spot
size that would be focused onto the target structure by an optical
system having an f number at a wavelength of at least about 1.047
microns; and controllably switching an optical switch based on the
timing signals so as to cause output pulses of the pulsed laser
beam to be transmitted to the target structure and to permit
selective reduction of the incident beam energy; wherein the
incident beam energy at which the target structure is vaporized is
reducible relative to an incident beam energy necessary to deposit
unit energy in the target structure sufficient to vaporize the
target structure at a higher wavelength below the absorption edge
of the silicon substrate below the absorption edge of the silicon
substrate.
51. The system of claim 26, wherein the optical switch permits
selective reduction of the incident beam energy to about 10% of a
gross laser output 52. The system of claim 26 wherein, at the
wavelength below the absorption edge of the silicon substrate and
between about 0.30 microns and about 0.55 microns, a spot size of
the laser output at the target structure is proportionally smaller
according to wavelength than a larger spot size that would be
focused onto the target structure by an optical system having an f
number at a wavelength of at least about 1.047 microns.
53. The system of claim 40, wherein the optical switch permits
selective reduction of the incident beam energy.
54. The method as claimed in claim 43, wherein said step of
controllably switching the optical switch permits selective
reduction of the incident beam energy.
55. The method as claimed in claim 48, wherein at the wavelength
below the absorption edge of the silicon substrate and between
about 0.30 microns and about 0.55 microns, a spot size of the laser
output at the target structure is proportionally smaller according
to wavelength than a larger spot size that would be focused onto
the target structure by an optical system having an f number at a
wavelength of at least about 1.047 microns.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to laser processing systems and
methods, including systems and methods for removing, with high
yield, closely-spaced metal link structures or "fuses" on a silicon
substrate of an integrated circuit or memory device.
[0002] Laser systems can be employed to remove fuse structures
("blow links") in integrated circuits and memory devices such as
ASICs, DRAMs, and SRAMs, for purposes such as removing defective
elements and replacing them with redundant elements provided for
this purpose ("redundant memory repair"), or programming of logic
devices. Link processing laser systems include the M320 and M325
systems manufactured by General Scanning, Inc, which produce laser
outputs over a variety of wavelengths, including 1.047 .mu.m, 1.064
.mu.m, and 1.32 .mu.m.
[0003] Economic imperatives have led to the development of smaller,
more complex, higher-density semiconductor structures. These
smaller structures can have the advantage of operation at
relatively high speed. Also, because the semiconductor device part
can be smaller, a greater number of parts can be included in a
single wafer. Because the cost of processing a single wafer in a
semiconductor fabrication plant can be almost independent of the
number of parts on the wafer, the greater number of parts per wafer
can translate into lower cost per part.
[0004] In the 1980s, semiconductor device parts often included
polysilicon or silicide interconnects. Although poly-based
interconnects are relatively poor conductors, they were easily
fabricated using processes available at the time, and were
well-suited to the wavelengths generated by the Nd:YAG lasers
commonly available at the time. As geometries shrank, however, the
poor conductivity of polysilicon interconnects and link structures
became problematic, and some semiconductor manufacturers switched
to aluminum. It was found that certain conventional lasers did not
cut the aluminum links as well as they had cut polysilicon links,
and in particular that damage to the silicon substrate could occur.
This situation could be explained by the fact that the reflection
in aluminum is very high and the absorption is low. Therefore,
increased energy must be used to overcome this low absorption. The
higher energy can tend to damage the substrate when too much energy
is used.
[0005] Sun et al., U.S. Pat. No. 5,265,114 advances an "absorption
contrast" model for selecting an appropriate laser wavelength to
cut aluminum and other metals such as nickel, tungsten, and
platinum. In particular, this patent describes selecting a
wavelength range in which silicon is almost transparent and in
which the optical absorption behavior of the metal link material is
sufficient for the link to be processed. The patent states that the
1.2 to 2.0 .mu.m wavelength range provides a high absorption
contrast between a silicon substrate and high-conductivity link
structures, as compared with laser wavelengths of 1.064 .mu.m and
0.532 .mu.m.
SUMMARY OF THE INVENTION
[0006] The invention provides a system and method for vaporizing a
target structure on a substrate. According to the invention, a
calculation is performed, as a function of wavelength, of an
incident beam energy necessary to deposit unit energy in the target
structure. Then, for the incident beam energy, the energy expected
to be deposited in the substrate as a function of wavelength is
calculated. A wavelength is identified that corresponds to a
relatively low value of the energy expected to be deposited in the
substrate, the low value being substantially less than a value of
the energy expected to be deposited in the substrate at a higher
wavelength. A laser system is provided configured to produce a
laser output at the wavelength corresponding to the relatively low
value of the energy expected to be deposited in the substrate. The
laser output is directed at the target structure on the substrate
at the wavelength corresponding to the relatively low value of the
energy expected to be deposited in the substrate, in order to
vaporize the target structure.
[0007] Certain applications of the invention involve selection of a
wavelength appropriate for cutting a metal link without producing
unacceptable damage to a silicon substrate, where the wavelength is
less than, rather than greater than, the conventional wavelengths
of 1.047 .mu.m and 1.064 .mu.m. This method of wavelength selection
is advantageous because the use of shorter wavelengths can result
in smaller laser spots, other things being equal, and hence greater
ease in hitting only the desired link with the laser spot. In
particular, other things being equal, laser spot size is directly
proportional to wavelength according to the formula: spot size is
proportional to .lamda.f, where .lamda. is the laser wavelength and
f is the f-number of the optical system.
[0008] Moreover, certain applications of the invention involve
selection of a wavelength at which a substrate has low absorption
but an interconnect material has higher absorption than at
conventional wavelengths of 1.047 .mu.m and 1.064 .mu.m or
higher-than-conventional wavelengths. Because of the reduced
reflectivity of the interconnect material, the incident laser
energy can be reduced while the interconnect material nevertheless
absorbs sufficient energy for the interconnect to be blown without
multiple laser pulses (which can impact throughput) or substantial
collateral damage due to the laser beam.
[0009] The invention can effect high-quality laser link cuts on
high-conductivity interconnect materials such as copper, gold, and
the like, arranged in closely-spaced patterns, with only a single
laser pulse, and without damaging the substrate. The invention can
further allow a smaller laser spot size than would be obtainable at
wavelengths of 1.047 .mu.m, 1.064 .mu.m, or higher, while still
providing acceptable link cuts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a block diagram of a laser system according to the
invention for removing a link of a semiconductor device, where the
link is manufactured of a material such as copper or gold.
[0011] FIG. 2 is a perspective diagrammatic view of a link on a
substrate of a semiconductor device.
[0012] FIG. 3 is plot of absorption of copper, gold, aluminum, and
silicon as a function of wavelength.
[0013] FIG. 4 is a plot of an substrate absorption function
according to the invention, for copper, gold, and aluminum links on
a silicon substrate, as a function of wavelength.
[0014] FIG. 5 is a plot of the function L-S for copper, gold, and
aluminum links on a silicon substrate, where L is the absorption in
the link and S is the absorption in the substrate.
[0015] FIG. 6 is a plot of the function (L-S)/(L+S) for copper,
gold, and aluminum links on a silicon substrate, where L is the
absorption in the link and S is the absorption in the
substrate.
DETAILED DESCRIPTION
[0016] In the block diagram of FIG. 1, a system for removing a link
of a semiconductor device is shown. Laser 10 is constructed to
operate at a conventional wavelength such as 1.047 .mu.m. It is
aligned to a laser output system that includes a wavelength shifter
12, such as a frequency doubler or an optical parametric oscillator
(OPO), constructed to shift to a wavelength less than 0.55 .mu.m,
in the "green" region of the wavelength spectrum. As is explained
in more detail below, the beam is then passed through the remainder
of the laser output system, including a controlled
electro-acousto-optic attenuator 13, a telescope 14 that expands
the beam, and, a scanning head 15, that scans the beam over a
focusing lens 16 by means of two scanner galvanometers, 18 and 20.
The spot is focused onto wafer 22 for removing links 24, under
control of computer 33.
[0017] The laser 10 is mounted on a stable platform 11 relative to
the galvanometers and the work piece. It is controlled from outside
of the laser itself by computer 33 to transmit its beam to the
scanner head comprising the accurate X and Y galvanometers 18 and
20. It is very important, in removing links that the beam be
positioned with accuracy of less than 3/10 of a micron. The timing
of the laser pulse to correlate with the position of the
continually moving galvanometers is important. The system computer
33 asks for a laser pulse on demand.
[0018] A step and repeat table 34 moves the wafer into position to
treat each semiconductor device.
[0019] In one embodiment, the laser 10 is a neodymium vanadate
laser, with an overall length L of about 6 inches, and a short
cavity length.
[0020] The shifter 12 of this preferred embodiment is external to
the cavity, and is about another 4 inches long. In alternative
embodiments, laser 10 can be configured to produce a laser output
having an appropriate wavelength, so that no shifter would be
required.
[0021] The laser is a Q-switched diode pumped laser, of sufficient
length and construction to enable external control of pulse rate
with high accuracy by computer 33.
[0022] The cavity of the laser includes a partially transmissive
mirror 7, optimized at the wavelength at which the lasing rod 6 of
neodymium vanadate is pumped by the diode. The partially
transmissive output mirror 9 is also optimized at this
wavelength.
[0023] The pumping diode 4 produces between about one and two watts
depending on the design. It focuses onto the rear of the laser rod
6. As mentioned, the laser rod is coated, on its pumped end, with a
mirror 7 appropriate for the standard laser wavelength of 1.064
.mu.m or 1.047 .mu.m. The other end of the rod is coated with a
dichroic coating. Within the laser cavity is an optical Q-switch 8
in the form of an acousto-optic modulator. It is used as the
shutter for establishing the operating frequency of the laser.
Beyond the Q-switch is the output mirror 9. The two mirrors, 7 on
the pumped end of the laser rod and 9 beyond the acoustic optical
Q-switch, comprise the laser cavity.
[0024] A system optical switch 13 in the form of a further
acousto-optic attenuator is positioned beyond the laser cavity, in
the laser output beam. Under control of computer 33, it serves both
to prevent the beam from reaching the galvanometers except when
desired, and, when the beam is desired at the galvanometers, to
controllably reduce the power of the laser beam to the desired
power level. During vaporization procedures this power level may be
as little as 10 percent of the gross laser output, depending upon
operating parameters of the system and process. The power level may
be about 0.1 percent of the gross laser output during alignment
procedures in which the laser output beam is aligned with the
target structure prior to a vaporization procedure.
[0025] In operation, the positions of the X, Y galvanometers 10 and
12 are controlled by the computer 33 by galvanometer control G.
Typically the galvanometers move at constant speed over the
semiconductor device on the silicon wafer. The laser is controlled
by timing signals based on the timing signals that control the
galvanometers. The laser operates at a constant repetition rate and
is synchronized to the galvanometers by the system optical switch
13.
[0026] In the system block diagram of FIG. 1 the laser beam is
shown focused upon the wafer. In the magnified view of FIG. 2, the
laser beam is seen being focused on a link element 25 of a
semiconductor device.
[0027] The metal link is supported on the silicon substrate 30 by
silicon dioxide insulator layer 32, which may be, e.g., 0.3-0.5
microns thick. Over the link is another layer of silicon dioxide
(not shown). In the link blowing technique the laser beam impinges
on the link and heats it to the melting point. During the heating
the metal is prevented from vaporizing by the confining effect of
the overlying layer of oxide. During the duration of the short
pulse, the laser beam progressively heats the metal, until the
metal so expands that the insulator material ruptures. At this
point, the molten material is under such high pressure that it
instantly vaporizes and blows clearly out through the rupture
hole.
[0028] The wavelength produced by wavelength shifter 12 is arrived
at by considering on an equal footing the values of both the
interconnect or link to be processed and the substrate, in such a
way as to trade-off energy deposition in the substrate, which is
undesirable, against energy deposition in the link structure, which
is necessary to sever the link. Thus, the criteria for selecting
the wavelength do not require the substrate to be very transparent,
which is especially important if the wavelength regime in which the
substrate is very transparent is much less than optimal for energy
deposition in the link structure.
[0029] The criteria for selection of the appropriate wavelength are
as follows:.
[0030] 1) Calculate the relative incident laser beam energy
required to deposit unit energy in the link structure This relative
incident laser beam energy is proportional to the inverse of the
absorption of the link structure. For example, if the link
structure has an absorption of 0.333, it will require three times
as much incident laser energy to deposit as much energy in the link
structure as it would if the structure had an absorption of 1. FIG.
3 illustrates absorption of copper, gold, aluminum, and silicon as
a function of wavelength (copper, gold, and aluminum being possible
link structure materials and silicon being a substrate
material).
[0031] 2) Using the incident beam energy computed in step (1),
calculate the energy deposited in the substrate. For a well-matched
laser spot, this energy will be proportional to the incident energy
calculated in step (1), less the energy absorbed by the link
structure, multiplied by the absorption of the substrate. In other
words, the energy absorbed in the substrate is proportional to
(1/L-1).times.S (herein, "the substrate absorption function"),
where L is the absorption in the link and S is the absorption in
the substrate.
[0032] 3) Look for low values of the substrate absorption function
defined in step (2) as a function of laser, wavelength.
[0033] FIG. 4 illustrates the substrate absorption function for
copper, gold, and aluminum links on a silicon substrate, as a
function of wavelength in the range of 0.3 to 1.4 .mu.m. The values
of the substrate absorption function can be derived from the
absorption curves illustrated in FIG. 3, using a proportionality
constant (see step (2) above) arbitrarily chosen as 0.5 for the
sake of specificity (this constant merely changes the vertical
scale of FIG. 4, and does not alter any conclusions drawn from
it).
[0034] It can be seen from FIG. 4 that for structures of gold and
copper (but not for aluminum) there is a region of wavelength less
than roughly 0.55 .mu.m in which the substrate absorption function
is comparable to that in the region of wavelength greater than 1.2
.mu.m.
[0035] It will also be noted that this function is quite different
than the ones presented in FIGS. 5 and 6, which illustrate two
possible functions representing simple absorption contrast. More
specifically, FIG. 5 illustrates the function L-S, expressed as
percentage, and FIG. 6 illustrates the function (L-S)/(L+S). In
either case, the less-than-0.55 .mu.m wavelength region is not
found desirable according to FIGS. 5 and 6, even for gold or copper
link structures, because the function shown in these figures is
less than zero in this region. This negative value reflects the
fact that the substrate is more absorptive than the link structure
in this wavelength regime, and so, according to these models, this
wavelength regime should not be selected.
* * * * *