MOCVD of WNx thin films using imido precursors

Details MOCVD of WNx thin films using imido precursors MOCVD of WNx thin films using imido precursors

2001-05-15

2003-07-22

Nazario-Gonzalez, Porfirio (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heavy metal containing

C556S057000, C546S002000, C549S206000, C427S255394, C427S593000, C106S001050, C106S001220, C106S001250

Reexamination Certificate

active

06596888

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to the field of semiconductors. More particularly, the invention relates to the use of single-source tungsten imido precursors in the formation of WN
x
based films on a substrate, and includes certain tungsten imido complexes and compounds (hereafter “TIC”) as precursors.
BACKGROUND OF THE INVENTION
In the manufacture of integrated circuits, barrier layers are often needed to prevent the diffusion of one material to an adjacent material. For instance, when aluminum contacts silicon surfaces, spiking can occur, and when aluminum comes into direct contact with tungsten, a highly resistive alloy can be formed.
Copper is of great interest for use in metallization of VLSI microelectronic devices and has begun replacing aluminum due to copper's lower resistivity, higher resistance to electromigration, low contact resistance to most materials, and ability to enhance device performance via reduction of RC time delays thereby producing faster microelectronic devices. Moreover, copper may allow a reduced number of metal levels to be necessary because it can generally be packed more tightly than aluminum.
Copper CVD processes which are suitable for large-scale manufacturing and the conformal filling of high aspect ratio inter-level vias in high density integrated circuits are regarded by many as extremely valuable to the electronics and optoelectronics industry, and are therefore being investigated in the art. Unfortunately Cu, like Al, is quite mobile in silicon. Upon direct contact with Si, Cu diffuses and reacts rapidly to form compounds such as Cu
3
Si. Formation of compounds such as Cu
3
Si can destroy shallow junctions and contacts during subsequent thermal annealing steps and can result in degraded device performance, or even device failure.
When copper is used as a conductive layer in a device having multi-layer metallization, long-term reliability can only occur if there is little to no interdiffusion between the copper and layers surrounding the copper layer. Copper interdiffusion can result in an increase in contact resistance, change the barrier height, result in leaky PN junctions, cause embrittlement of the contact layer, and destroy electrical connections to the chip. With the increased use of copper as an interconnect material to form high speed integrated circuits, an effective barrier against copper diffusion is required. Diffusion barriers are layers interposed between a material to be isolated (e.g. Cu) and the underlying circuit, and are commonly used in an attempt to prevent undesirable reactions involving the material to be isolated with one or more layers of the underlying circuit.
Research has indicated that nitride barriers are better candidates for reliable diffusion barriers from the interdiffusion standpoint and provide lower electrical resistivities than their pure metal counterparts. Titanium nitride (TiN) has been used as a diffusion barrier for aluminum, but its performance suffers when copper metallization is employed due to excessive copper interdiffusion therethrough. Tantalum nitride (TaN) has generally been the barrier material of choice to date for copper metallization. However, tantalum nitride requires a two-step chemical mechanical polishing (CMP) procedure that results in nearly a one order of magnitude increase in dishing of the copper surface and often results in scratching of the inter-layer dielectric (ILD). Dishing of the copper is caused by a substantially higher CMP etch rate of copper compared to TaN.
Unlike TaN, tungsten nitride (WN
x
) may be etched using a single CMP process and results in reduced copper dishing relative to copper dishing using TaN as a barrier material because the CMP etch rates of copper and WN
x
are essentially equal. Tungsten nitride is also known to be an effective diffusion barrier against copper penetration at temperatures of up to approximately 750° C. As used herein, WN
x
is understood to include the numerous tungsten nitride stoichiometries, such as WN, WN
2
, and W
2
N. However, there are many other WN
x
stoichiometries. It is known that WN has a hexagonal crystal structure, WN
2
is rhombohedral, and W
2
N has a face centered cubic structure. Each of the three stoichiometries listed above are thermodynamically stable, but W
2
N has the lowest resistivity of the three (50 &mgr;&OHgr;-cm bulk resistivity).
One application for tungsten nitride films is the formation of diffusion barriers between the tungsten of tungsten plugs and adjoining metallization layers on the surface of the wafer, such as copper. Such a diffusion barrier is shown in FIG.
1
.
FIG. 1
shows a tungsten plug
14
extending down to a silicon substrate
10
with an overlying copper layer
16
and an intervening diffusion barrier
12
. The tungsten plug structure is one example of an application where tungsten nitride has been found as a suitable replacement for titanium nitride, as it is easily formed over the tungsten plug
14
.
As feature sizes in integrated circuits have decreased to below 0.25 &mgr;m, the necessity for thermally stable, high conformity interface diffusion barriers and gate electrodes has become more important. Deposition of conformal and continuous barrier layers of tungsten nitride at relatively low temperatures on high-aspect-ratio structures is not possible with current processes. The two common methods for deposition of WN
x
include:
1) physical deposition techniques such as sputtering; and
2) chemical vapor deposition (CVD) from the reaction of tungsten halides and ammonia.
Each above method has associated difficulties. Conventional physical vapor deposition technology involves reactive sputtering from a tungsten target in an atmosphere of gaseous nitrogen with an argon carrier gas. Energized particle techniques, particularly sputtering, generally result in poor step coverage primarily due to shadowing. Applied to small features with high aspect ratios, poor step coverage of the barrier layer may result in areas of excessively thin or missing barrier material allowing copper diffusion into the underlying substrate. Moreover, sputter deposited layers are prone to generate high tensile stress to adjacent layers which may cause defects which may result in degraded device performance and yield loss in integrated circuits.
Current chemical vapor deposition processes for forming WN
x
generally involve the reduction of tungsten halides such as WF
6
and WCl
6
by NH
3
. Although some success has been achieved with halide reduction schemes, at least three major obstacles exist for their use in future generations of integrated circuits. First, the high deposition temperatures (>700° C.) that are required to dissociate tungsten halide molecules are incompatible with most future low dielectric constant materials and some metallization layers. Second, the reaction byproducts (such as HF and HCl) are extremely corrosive and can rapidly etch other exposed device layers (such as Si and SiO
2
) as well as decrease the operating lifetime of processing equipment. Finally, adduct formation, which can occur when using WF
6
and NH
3
, must be avoided, especially as feature sizes shrink below 0.18 &mgr;m. Adduct formation commonly results from gas phase nucleation, as opposed to the desired chemical reaction occurring at the wafer surface.
It is apparent from the above discussion that a need exists for a new process for forming high quality tungsten nitride films which overcomes the problems existing with conventional chemical vapor deposition and physical vapor deposition processes, and which can be used to form a suitable barrier against copper diffusion. The process should be operable at a sufficiently low temperature to avoid copper penetration, limit or avoid generation of corrosive reaction by-products and deposit highly conformal layers capable of filling high aspect ratio structures (e.g. vias).
It would be preferable to utilize a single-source metal organic precursor molecule for the MOCVD (or other suitable deposition technique) of tungsten nitride thin fi

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