Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-04-22
2002-01-01
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S830000, C029S846000, C204S192170, C427S096400, C427S097100, C427S099300, C438S637000, C438S644000, C438S648000, C438S660000, C438S675000, C438S688000
Reexamination Certificate
active
06334249
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to integrated circuit metallization structures and fabrication methods.
Background: Aluminum Metallization
In modern integrated circuit fabrication, it is increasingly necessary to fill vias and contact holes which have a high “aspect ratio”. This means a ratio of height to width which is 2:1 or more, and, as technology progresses, may be as high as 10:1 or more in future generations. Completely filling such holes with metal at an acceptably low temperature is very difficult, particularly for metals (such as aluminum and copper) which do not have a good low-temperature chemical vapor deposition (CVD) process. Moreover, even a good CVD process will not fill holes of infinitely high aspect ratios. The seamline within the cavity filled with CVD aluminum or copper will transform into a void and obstruct the current flow, resulting in a low electromigration lifetime.
Recently, contact and via filling with aluminum alloys has attracted a great deal of attention. Compared with contact/via filling with CVD tungsten, aluminum filling has the advantages of lower cost, higher yield, and potentially better electromigration resistance (since there is less flux divergence near the plug).
However, one concern with aluminum metallization is still electromigration: a pure aluminum line may gradually thin out, in service, in locations of high current density. However, the addition of copper greatly reduces this tendency. Longer electromigration (EM) lifetimes improve the product reliability. Thus, typical aluminum alloys use copper (typically one-half weight percent to one weight percent), alone or in combination with silicon (typically one-half weight percent to one weight percent), as an alloying agent. Efforts have been made to find other satisfactory aluminum alloy compositions; see e.g. Kikuta and Kikkawa, “Electromigration characteristics for Al—Ge—Cu,” 143 J. Electrochem. Soc. 1088 (1996), which is hereby incorporated by reference.
Background: Aluminum Plug Processes
As shown in prior art
FIG. 5
, a contact or via hole
502
has been etched through a dielectric layer
510
to expose an underlying layer
500
, followed by the filling of the cavities
502
with a layer of aluminum or aluminum alloy
520
and the etchback (or CMP) of the aluminum layer
520
on top of the dielectric
510
to form aluminum plugs
520
. As can be seen in
FIG. 5
, after the etchback of the aluminum layer
520
, the aluminum material
520
is typically recessed
530
from the surface of the dielectric layer
510
. This can undesirably result in a similar depression
550
forming in subsequently deposited metal layers
540
.
Aluminum plugs may be formed by a variety of methods, including sputter-reflow, blanket CVD, selective CVD, or high pressure extrusion fill followed by an isotropic etch step or a chemical mechanical polishing (CMP) process to remove any excess aluminum. Reflow methods apply a high temperature to help newly-arrived atoms to move around on the metal surface. Extrusion cavity filling methods (like the “Forcefill” (TM) process) apply physical pressure at high temperatures to force a soft layer of as-deposited material into the hole. The forcefill process is uniquely advantageous in filling contact or via holes with extremely high aspect ratios. Indeed, as of 1997, it appears that forcefill is the only known technique for filling holes with aspect ratios which are significantly greater than three to one.
A liner layer
505
(e.g. titanium silicide) is required for sputter-reflow, blanket CVD and high pressure extrusion fill. The liner layer
505
may also serve as a wetting layer which lowers the melting point and yield stress of the aluminum, as discussed in U.S. Provisional Patent Application Serial No. 60/037,123, filed Feb. 3, 1997, which is hereby incorporated by reference. In addition, various conductive coatings have been used on contact or via sidewalls in the prior art. For example, a barrier and adhesion layer (e.g. titanium nitride on titanium) is very commonly used. Such barrier, adhesion, and liner layers will typically be only about a few tens of nanometers thick.
In a typical CVD filling process, CVD has the disadvantage that a join
705
occurs in the middle of the cavity
720
when the cavity
720
is fully filled with CVD metal
700
, which is illustrated in prior art FIG.
7
A. After the metal
700
is heated, this join will become a bubble
710
, as shown in prior art
FIG. 7B
, which increases the net series resistance of the contact or via connection. CVD aluminum processes can achieve reasonably high rates of deposition (currently up to about
200
nanometers per minute), but are typically much more expensive than sputter deposition.
Background: Depression Formation in Aluminum Cavity-Filling Processes
In aluminum cavity-filling processes, the aluminum layer on top of the dielectric material and over the cavities is not etched back as in aluminum plug processes. As shown in prior art
FIGS. 3A and 3B
, the aluminum
320
is typically sputter deposited at a high temperature with a rapid deposition rate. This causes small cavities
310
to be readily bridged, with only a fairly small volume of metal
320
intruding into the cavity
310
(e.g. less than
10
percent of the volume of cavity
310
), as shown in FIG.
3
A. After the filling of the cavities
310
with an aluminum alloy
320
(e.g. by reflow or extrusion), a depression
330
typically forms over the cavity
310
(e.g. via, contact, or trench within a dielectric layer
300
). This depression
330
is a result of mass conservation, as the aluminum alloy
320
deposited on the surface, shown in
FIG. 3A
, is transferred into the cavity
310
, which is illustrated in FIG.
3
B. The volume of the depression
330
shown in
FIG. 3B
typically equals the volume of the cavity
310
.
A smooth surface can be achieved if the reflow or extrusion process is carried out at elevated temperature (e.g. greater than 450 degrees C.), or in an ultra-high vacuum (e.g. pressure less than 1E-8 Torr) to promote the surface diffusion of aluminum, which will smooth out the surface. However, at low temperatures (less than 450 degrees C., such as is required for use with low-k dielectrics) or in poor vacuum conditions (10
5
Torr or softer vacuum), the materials diffusion rate is too slow to smooth the surface, and thus a depression forms above the cavity.
These depressions undesirably are picked up as defects by defect detection tools, which increases the cycle time. A further problem with the formation of large depressions is that present lithography is unable to pattern small features above these depressions. These depressions are also undesirable for stacked via applications, because gap fill material, such as Hydrogen Silsesquioxane (HSQ) coated by spin-coating, becomes coated in the depression, making it difficult to perform subsequent via etching, since HSQ has a much slower etch rate than oxide dielectrics. Furthermore, as can be seen in prior art
FIG. 4
, these depressions
400
and
410
produce rough surfaces and increase the surface topography over dense cavity regions
420
and
430
, because the depressions
400
and
410
above the cavities
420
and
430
overlap and compete for materials needed to fill the cavities
420
and
430
, which results in incomplete filling of the cavities
420
and
430
.
Background: Graded-Temperature Aluminum Reflow
One conventional method of reducing the depression volume in aluminum cavity-filling processes uses a graded temperature aluminum deposition process, which is described in U.S. Pat. No. 5,108,951 to Chen et al. This process deposits a single aluminum layer, with temperature ramping, so that the aluminum is initially deposited at a low temperature, in order to reduce the likelihood of contact spiking and to begin deposition of aluminum into the cavity. Thereafter, the temperature is ramped up to a higher temperature to produce complete cavity filling and an allegedly smooth metal surface (at least for some aspec
Brady III W. James
Chang Rick Kiltae
Garner Jacqueline J.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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