Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1998-05-06
2001-04-17
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298180, C204S298120, C204S298160
Reexamination Certificate
active
06217716
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of thin film deposition. More specifically, the present invention relates to improving uniform target erosion in a physical vapor deposition (PVD) hollow cathode magnetron sputter source.
2. The Background
The deposition of thin film layers is a common processing step in the fabrication of very large scale integrated (VLSI) circuits and ultra large scale integrated (ULSI) circuits on semiconductor substrates or wafers. A semiconductor wafer is the foundation from which is built a large quantity of discrete devices, commonly known in the art as integrated circuit chips. Metallic thin film layers are typically employed as device interconnects which are deposited on to a wafer by known physical or chemical vapor deposition techniques. In addition, it is also frequently required that small holes; referred to in the art as vias, or narrow grooves; referred to as trenches, be properly filled with metallization in order to provide electrical connection between device layers.
Recent innovations and cost constraints within the semiconductor industry have hastened the need to improve techniques used to dispose thin films on substrates. By example, wafer size has increased from 6-inch (150 mm) diameter to 8-inch (200 mm) diameter with a growing acceptance of even larger 12-inch (300 mm) diameter wafers. As wafer size increases, the ability to impart requisite directionality to thin films becomes increasingly difficult. Many current preferred methods of forming thin films, such as PVD sputtering, are only able to meet directionality and uniformity requirements by inducing a trade-off of slower processing rates. In today's highly competitive commercial semiconductor market improvements that carry with them increases in processing time are not viable economical alternatives. Similarly, the heightened complexity of current discrete devices have increased wafer densities and have led to vias and trenches with higher aspect ratios (depth of the via versus the width of the via) and smaller geometries. As the geometries of such vias and trenches decrease, it becomes increasingly more difficult to conformally deposit material throughout the entire depth of vias or grooves. Therefore, the need exists within the semiconductor industry to continue to strive for an adaptable, highly efficient means for thin film deposition.
Conventional PVD sputtering allows for the deposition of relatively pure thin films on substrates of various types and geometries. Standard sputtering is accomplished by creating at a relatively low pressure of a plasma forming gas a plasma comprising, typically, an inert gas, such as argon (Ar), in the vicinity of a target cathode which is made of the material to be deposited. Positively charged plasma atoms, known as ions, then strike the cathode target causing atoms of the target cathode to be ejected into the plasma. These target atoms then travel through the sputtering vacuum and are deposited onto the semiconductor substrate. Conventional diode PVD sputtering has shown to be both inefficient and, in some instances, incapable of providing required directionality to thin films when constructing VLSI and ULSI circuits. The plasma that is created with a standard PVD sputtering device lacks a sufficient amount of ionized target material atoms. The degree of ionization of a plasma is referred to in the art as the plasma intensity. The more intense the plasma, the greater the ability to steer and focus the plasma and, thus, impart an adequate amount of directionality to the ions in the plasma. By improving the ion directionality, it insures that the thin films being deposited have adequate coverage in vias and trenches. In addition, when the intensity of the plasma makes it more conducive to focusing operations overall processing times typically decrease and target material utilization is optimized.
As a means for overcoming the limitations of conventional PVD sputtering, use magnetic fields in magnetron sputtering devices have successfully been introduced into the process. These magnetron sputtering systems have seen wide-spread use in semiconductor manufacturing for the deposition of metallization layers, such as aluminum (Al), titanium (Ti), titanium nitride (TiN) and titanium tungsten (TiW) alloys. As with standard sputtering devices, the magnetron sputtering apparatus consists of a vacuum chamber which confines an inert support gas, commonly argon (Ar), at a relatively low pressure, typically 3-5 millitorr. An electrical field (E) is then created within the vacuum chamber by introducing a negative potential across the target cathode and creating an anode, typically, by means of grounding the overall sputter chamber or a using a self-biased floating anode. A magnetic field (B) is introduced into the vacuum chamber, typically in an orientation such that the field lines loop through the cathode for the purpose of creating and confining a plasma near the target cathode. As positive ions from the plasma strike the target cathode, atoms are ejected from the surface of the cathode. The magnetic field serves to attract an electron-rich portion of the plasma in the vicinity of the cathode. In addition, electrons trapped about the cathode allow for an increase in the collisions between the neutral atoms ejected from the surface of the target and the rapidly moving electrons. By increasing the quantity of collisions, the likelihood increases that a neutral ejected target atom will be struck by a sufficiently energetic particle within the plasma, thus causing the ejected target atom to lose one or more electrons and result in an ionized atom. By increasing the quantity of ionized target atoms within the plasma the overall effect is the increase in the plasma density, i.e., the number of particles in a given confined area. This increase in plasma density is also known in the art as an increase in the intensity of the plasma. As the plasma intensity increases so does the probability that further ionization of ejected target atoms will occur.
Magnetron sputtering devices have shown a wide variance of success in being able to deposit thin films efficiently and with the requisite step coverage and uniformity. A high percentage of such devices are limited in their ionization efficiency due in part to the fact that the vast majority of metal atoms ejected from the target remain neutral and the cathode configuration of such devices only result in a small volume of the plasma being retained in front of the target surface. Even with the use of magnetic fields to trap plasmas about the target cathode, the intensity of the plasma remains insufficient and, in certain embodiments, upwards of 98% or greater of the deposition material atoms remain un-ionized as they travel through the sputter chamber to the substrate. The general understanding is that atoms are ejected from the surface of the sputter target at random angles and that the mean-free path of travel between the target cathode and the substrate for these neutral metal atoms is reduced by random collisions with other target atoms or inert gas ions. When the predominately neutral atoms in these plasmas do come in contact with the substrate they characteristically do so over a wide range of angles, generally conforming to a cosine distribution. In particular, when atoms are disposed on substrate surfaces at angles less than normal it poses significant difficulty in uniformly filling trenches and interconnect vias. The emphasis on adequate step coverage of thin films is exasperated by the demands of the semiconductor industry. As the overall semiconductor geometries have shrunk and the chip densities have increased, so too have the demands on being able to impart required directionality to thin films in narrower and deeper vias and/or trenches.
The teachings found in U.S. Pat. No. 5,482,611 (the '611 or Helmer patent) entitled “Physical Vapor Deposition Employing Ion Extraction from a Plasma” have shown to be highly effective in providing a
Cantelmo Gregg
Nguyen Nam
Novellus Systems Inc.
Tso Roland
LandOfFree
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