Electric heating – Metal heating – By arc
Reexamination Certificate
2001-10-25
2004-04-13
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121850
Reexamination Certificate
active
06720522
ABSTRACT:
This patent application is based upon and claims the benefit of the earlier filing dates of Japanese Patent Application Nos. 2000-326361 and 2001-213671 filed Oct. 26, 2000 and Jul. 13, 2001, respectively, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a technique of laser beam machining and a method for manufacturing semiconductor devices using the laser beam machining approach, and more substrates such as semiconductor wafers, glass substrates, or resin substrates, and thin films formed on these substrates.
2. Description of Related Art
Since laser beam machining is capable of delineating fine patterns of an order of a micron (&mgr;m) without requiring a lithography process, it has been attracting a great deal of attention as an approach to manufacturing semiconductor devices. In producing semiconductor devices, various types of layers, such as resist films, resin films, insulating films, metal films, etc. are formed and laminated on a wafer. Fine machining is needed not only for forming VIA holes, circuit patterns, and interconnections in the laminated layers, but also for selective removal of the laminated layers along the circumference of the wafer for the purpose of preventing dust from arising during wafer transfer, or revealing the manufacturer serial numbers formed in the wafers.
However, if laser beam machining is carried out in ordinary atmosphere, dust adheres to and accumulates on the processed areas. Adhesion of the dust causes poor exposure, short-circuit, and breakdown, which further causes the manufacture yield to drop.
Moreover, since laser beam machining makes use of ablation (i.e., removal of materials as a result of melting and evaporation), the laser beam that illuminates the substrate or the laminated layers of metals (e.g., aluminum alloy, copper, etc.), insulators (SiO
2
, Si
3
N
4
, etc.), resins, etc. often causes damage to the irradiated regions and the area around them.
FIG. 1
illustrates examples of the damage caused by the conventional technique of laser beam machining.
FIG. 1A
shows damage to a silicon substrate,
FIG. 1B
shows damage to a metal layer,
FIG. 1C
shows damage to a Si
3
N
4
film, and
FIG. 1D
shows damage to a photoresist.
As illustrated in
FIG. 1A
, a silicon single crystal wafer
1100
is machined in ordinary atmosphere using the fourth harmonic wave of a Q-switch Nd YAG laser, and the cross-sectional view of the machined area is observed by a transmission electron microscope (TEM). Polycrystalline silicon
1101
and void
1101
A are formed around the machined area (or the irradiated area
1100
), and many dislocation lines
1102
are observed.
Of these, it is thought that polycrystalline silicon
1101
and void
1101
A are produced when melted silicon that has been fused by laser-beam irradiation solidifies. Moreover, since a steep temperature gradient is produced around the irradiated region
1110
by irradiation of the laser beam, a large amount of thermal stress accumulates even in domains in which the silicon single crystal wafer
1100
is not fused, and as a result, dislocation
1102
arises. With the deepening of the depth from the surface of the silicon single crystal wafer
1100
, dislocation
1102
is apt to increase. At a depth of 200 &mgr;m, dislocation
1102
is observed over a wide area with a radius of about 100 micrometers from the center of the irradiated region
1110
.
In addition, swelling
1103
of fused silicon arise from the top face of the silicon wafer
1100
around the irradiated area
1110
, and silicon grains
1104
scattered by laser beam machining adhere to the swelling
1103
and around it.
This damage is observed even if the energy density of the laser beam is reduced to about 2.5 J/cm
2
, which is the lower limit of laser-beam processing. Similar damage is observed even if a KrF excimer laser or its analogues are used to process the silicon substrate in ordinary atmosphere. Although machining lasers with a pulse width of several nanoseconds or greater, such as Q-switch Nd YAG lasers and KrF excimer lasers, are comparatively inexpensive and reliable in operation, the damage accompanying the irradiation of the leaser beam can not be avoided.
It is reported that using a laser beam with a very narrow pulse width of 1 picosecond or less can to some extent prevent fusion and the resultant thermal stress caused in a silicon wafer. Titanium sapphire laser is known as such a narrow-pulse laser with a pulse width of 1 psec or less. However, since titanium sapphire lasers are expensive, they are not suitable for processing semiconductor devices.
Moreover, voids
1101
A and dislocations
1102
produced in the silicon single crystal substrate during laser beam machining lower the mechanical strength of the silicon wafer
1100
, and induce further damage to the circuit elements or interconnects formed on the silicon wafer
1100
. Swelling
1103
and scattered silicon grains
1104
will also induce degradation of the upper layers. These defects result in a reduced yield of semiconductor devices.
FIG. 1B
is a cross-sectional view of a laser-processed thin metal film (copper, aluminum alloy, etc.)
1130
formed on the silicon single crystal substrate
1100
via a silicon oxidation film
1120
. The thin metal film
1130
was machined in ordinary atmosphere using the fourth harmonic wave of a Q-switch Nd YAG layer. Similarly,
FIGS. 1C and 1D
illustrate a silicon nitride film
1150
and a photoresist film
1160
, respectively, processed by the fourth harmonic wave of the Q-switch Nd YAG laser in ordinary atmosphere.
Swelling
1133
arises around the laser irradiation area
1110
on the thin metal film
1110
, as in the silicon single crystal substrate
1100
shown in
FIG. 1A. A
large number of metal grains
1134
are scattered by the irradiation of the laser beam
1140
, and they adhere to the swelling
1133
and its surrounding area. The height of the swelling
1133
is about 2 &mgr;m to 5 &mgr;m, and the diameter of the metal grain
1134
reaches several micrometers. The swelling
1133
and the metal grains
134
deteriorate the reliability of the upper layers, and cause the yield of semiconductor devices to fall.
If the thin metal film
1130
is a cupper film, it is found by scanning micro-auger (&mgr;-AES) analysis that carbon (C) contamination
1135
has occurred around the laser-beam irradiation area
1110
. Such carbon contamination is conspicuous at the swelling
1133
, and the carbon contents reaches as much as a several tens percentage. Generally, the thin metal film
1130
is patterned into interconnections or electrodes. Carbon contamination
1135
partially increases the resistance of the interconnections and the electrodes, and designed circuit characteristics cannot be obtained. These defects also result in the decreased manufacture yield of semiconductor devices.
Swelling and scattered grains are also observed in the silicon nitride film
1150
and the photoresist film
1160
. After the silicon nitride film
1150
is laser-beam machined in ordinary atmosphere, swelling
1153
arises around the laser-beam irradiation area
1110
, and a large number of silicon nitride grain
1154
adhere to the swelling
1153
. Similarly, if laser beam machining is conducted to the photoresist film
1160
in ordinary atmosphere, swelling
1163
and a large number of photoresist grains
1164
that have adhered to the machined surface are observed.
Since the silicon nitride grains
1154
and the photoresist grains
1164
are small compared with the metal grains
1134
, these particles scatter over hundreds of micrometers around the laser-beam irradiation area
1110
. The widely spread silicon nitride grains
1154
adversely affect the upper thin films formed on the silicon nitride film
1160
. The scattered photoresist grains
1164
induce poor exposure and poor development in the photolithography process. In any cases, the manufacturing yield is reduced.
SUMMARY OF THE INVENTION
In one aspect of the invention,
Hayasaka Nobuo
Ikegami Hiroshi
Elve M. Alexandra
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
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