Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
1997-07-15
2001-04-24
Williams, Alexander O. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S770000, C257S347000, C257S388000, C257S410000, C257S744000, C257S743000, C438S937000, C438S688000, C438S149000
Reexamination Certificate
active
06222271
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a method of sputter deposition of an aluminum-containing film onto a semiconductor substrate, such as a silicon wafer. More particularly, the invention relates to using hydrogen gas with argon during the deposition of aluminum or aluminum alloys to form an aluminum-containing film which is resistant to hillock formation.
2. State of the Art
Thin film structures are becoming prominent in the circuitry components used in integrated circuits (“ICs”) and in active matrix liquid crystal displays (“AMLCDs”). In many applications utilizing thin film structures, low resistivity of metal lines (gate lines and data lines) within those structures is important for high performance. For example with AMLCDs, low resistivity metal lines minimize RC delay which results in faster screen refresh rates. Refractory metals, such as chromium (Cr), molybdenum (Mo), tantalum (Ta), and tungsten (W), have resistances which are too high for use in high performance AMLCDs or ICs. Additionally, the cost of refractory metals is greater than non-refractory metals. From the standpoint of low resistance and cost, aluminum (Al) is a desirable metal. Furthermore, aluminum is advantageous because it forms an oxidized film on its outer surfaces which protects the aluminum from environmental attack, and aluminum has good adhesion to silicon and silicon compounds.
An aluminum film is usually applied to a semiconductor substrate using sputter deposition. Sputter deposition is generally performed inside the vacuum chamber where a solid slab (called the “target”) of the desired film material, such as aluminum, is mounted and a substrate is located. Argon gas is introduced into the vacuum chamber and an electrical field is applied between the target and the substrate which strikes a plasma. In the plasma, gases are ionized and accelerated, according to their charge and the applied electrical field, toward the target. As the argon atoms accelerate toward the target, they gain sufficient momentum to knock off or “sputter” atoms and/or molecules from the target's surface upon impact with the target. After sputtering the atoms and/or molecules from the target, the argon ions, the sputtered atoms/molecules, argon atoms and electrons generated by the sputtering process form a plasma region in front of the target before coming to rest on the semiconductor substrate, which is usually positioned below or parallel to the target within the vacuum chamber. However, the sputtered atoms and/or molecules may scatter within the vacuum chamber without contributing to the establishment of the plasma region and thus not deposit on the semiconductor substrate. This problem is at least partly resolved with a “magnetron sputtering system” which utilizes magnets behind and around the target. These magnets help confine the sputtered material in the plasma region. The magnetron sputtering system also has the advantage of requiring lower pressures in the vacuum chamber than other sputtering systems. Lower pressure within the vacuum chamber contributes to a cleaner deposited film. The magnetron sputtering system also results in a lower target temperature, which is conducive to sputtering of low melt temperature materials, such as aluminum and aluminum alloys.
Although aluminum films have great advantages for use in thin film structures, aluminum has an unfortunate tendency to form defects, called “hillocks”. Hillocks are projections that erupt in response to a state of compressive stress in a metal film and consequently protrude from the metal film surface.
There are two reasons why hillocks are an especially severe problem in aluminum thin films. First, the coefficient of thermal expansion of aluminum (approximately 23.5×10
−6
/°C.) is almost ten times as large as that of a typical silicon semiconductor substrate (approximately 2.5×10
−6
/°C.). When the semiconductor substrate is heated during different stages of processing of a semiconductor device, the thin aluminum film, which is strongly adhered to the semiconductor substrate, attempts to expand more than is allowed by the expansion of the semiconductor substrate. The inability of the aluminum film to expand results in the formation of the hillocks to relieve the expansion stresses. The second factor involves the low melting point of aluminum (approximately 660° C.), and the consequent high rate of vacancy diffusion in aluminum films. Hillock growth takes place as a result of a vacancy-diffusion mechanism. Vacancy diffusion occurs as a result of the vacancy-concentration gradient arising from the expansion stresses. Additionally, the rate of diffusion of the aluminum increases very rapidly with increasing temperature. Thus, hillock growth can thus be described as a mechanism that relieves the compressive stress in the aluminum film through the process of vacancy diffusion away from the hillock site, both through the aluminum grains and along grain boundaries. This mechanism often drives up resistance and may cause open circuits.
A hillock-related problem in thin film structure manufacturing occurs in multilevel thin film structures. In such structures, hillocks cause interlevel shorting when they penetrate or punch through a dielectric layer separating overlying metal lines. This interlevel shorting can result in a failure of the IC or the AMLCD. Such a shorted structure is illustrated in FIG.
11
.
FIG. 11
illustrates a hillock
202
in a thin film structure
200
. The thin film structure
200
comprises a semiconductor substrate
204
, such as a silicon wafer, with a patterned aluminum layer
206
thereon. A lower dielectric layer
208
, such as a layer of silicon dioxide or silicon nitride, is deposited over the semiconductor substrate
204
and the patterned aluminum layer
206
. The lower dielectric layer
208
acts as an insulative layer between the patterned aluminum layer
206
and an active layer
210
deposited over the lower dielectric layer
208
. A metal line
212
is patterned on the active layer
210
and an upper dielectric layer
214
is deposited over the metal line
212
and the active layer
210
. The hillock
202
is shown penetrating through the lower dielectric layer
208
and the active layer
210
to short with the metal line
212
.
Numerous techniques have been tried to alleviate the problem of hillock formation, including: adding elements, such as tantalum, cobalt, nickel, or the like, that have a limited solubility in aluminum (however, this generally only reduces but not eliminates hillock formation); depositing a layer of tungsten or titanium on top or below the aluminum film (however, this requires additional processing steps); layering the aluminum films with one or more titanium layers (however, this increases the resistivity of the film); and using hillock resistant refractory metal films such as tungsten or molybdenum, rather than aluminum (however, as previously mentioned, these refractory metals are not cost effective and have excessive resistivities for use in high performance ICs and AMLCDs).
In particular with AMLCDs, and more particularly with thin film transistor-liquid crystal displays (“TFT-LCDs”), consumer demand is requiring larger screens, higher resolution, and higher constrast. As TFT-LCDs are developed in response to these consumer demands, the need for metal lines which have low resistivity and high resistance to hillock formation becomes critical.
Therefore, it would be advantageous to develop an aluminum-containing material which is resistant to the formation of hillocks and a technique for forming an aluminum-containing film on a semiconductor substrate which is substantially free from hillocks while using inexpensive, commercially-available, widely-practiced semiconductor device fabrication techniques and apparatus and without requiring complex processing steps.
SUMMARY OF THE INVENTION
The present invention relates to a method of introducing hydrogen gas along with argon gas into a sputter deposition vacuum chamber d
Raina Kanwal K.
Wells David H.
Micro)n Technology, Inc.
Trask & Britt
Williams Alexander O.
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