Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1999-12-30
2002-09-24
Nguyen, Tuan H. (Department: 2813)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S622000, C438S627000, C438S629000, C438S625000, C438S660000, C438S672000
Reexamination Certificate
active
06455427
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to integrated circuit fabrication and, more particularly, to methods for forming metallization structures.
2. Description of the Related Art
The information described below is not admitted to be prior art by virtue of its inclusion in this Background section.
An integrated circuit includes numerous active and passive devices arranged upon and within a single substrate. In order to implement desired circuit functions, select devices or components of the integrated circuit must be interconnected. Metallization structures are often used to interconnect integrated circuit components. Metallization structures may be generally subdivided into two categories: laterally extending interconnect lines and vertically extending contacts or plugs. Interconnects are relatively thin lines of conductive material that largely extend parallel to the underlying devices. As the name implies, contacts are the metallization structures that actually contact the devices of the integrated circuit. Plugs mostly extend vertically between metallization levels.
Within each level of interconnect, metallization structures are separated from other structures on underlying or overlying levels, and from structures within the same metallization level, by dielectric materials. The dielectric materials prevent unwanted communication between separated metallization structures. In large part because of the difficulty in etching many metallization materials, metallization structures are often formed by first depositing the dielectric material which will separate the metallization structures and then patterning cavities in the dielectric material (i.e., metallization cavities) for the metallization structures. The metallization cavities patterned for interconnect structures are typically called trenches, and the metallization cavities patterned for plugs are typically called vias. Once the cavities are formed, metals can then be deposited in the cavities to form metallization structures. If necessary, the deposited metal can be planarized, a process that often involves chemical-mechanical polishing (CMP).
Despite the name, metallization structures are not required to actually be metals, but may instead be fabricated from any material sufficiently conductive to transmit an electrical signal (e.g., doped polysilicon, metal silicides, refractory metal nitrides). For metallization structures above the level of local interconnect, though, metals are the predominant metallization materials, and one of the most common metallization materials is aluminum. Aluminum is desirable as a metallization material because of, among other things, its relatively low resistance and good current-carrying density.
Aluminum is usually deposited using physical vapor deposition (PVD). PVD processing may also be known as sputter deposition, or sputtering. In general, sputter deposition may be considered to be any deposition process in which a material is deposited by sputtering the material from, e.g., a target composed of the material to be deposited. A typical method for sputtering a metal onto a substrate includes introducing an inert gas into a deposition chamber and forming a plasma that ionizes the inert gas by applying a potential between the substrate and the target. The ionized inert gas atoms are then attracted toward the target, and collide with the target with such force that atoms of the target are sputtered off. The sputtered atoms may then deposit on the substrate.
Sputtering can be used to deposit any variety of materials, including conductors, non-conductors, and high melting point compounds. Sputtering is advantageous because it may provide for good step coverage and accurate transfer of material composition from the target to the deposited metal. This last feature is particularly helpful when depositing alloys.
One process for forming a metallization structure incorporating aluminum involves first sputter depositing a titanium wetting layer into the cavity in which the metallization structure will be contained. The titanium wetting layer lines the sidewalls and base of the cavity. A bulk metal layer of aluminum is then sputter deposited onto the wetting layer to fill the cavity. The titanium wetting layer helps to minimize or avoid agglomeration of the aluminum bulk metal layer and provides for continuous metal coverage along the sidewalls and bottom of the cavity. The aluminum bulk metal layer serves as the primary conductive material of the resulting metallization structure.
Bulk metal layers composed of aluminum are typically deposited by either standard or collimated sputtering processes. Standard sputtering processes may be considered those sputtering processes that do not impart any significant degree of directionality to the sputtered atoms. Standard sputtering processes thus allow sputtered atoms to contact the deposition surface at a variety of angles, ranging from almost parallel through perpendicular. In contrast, collimated sputtering processes typically use a collimator arranged between the target and a deposition surface to block high impact angle atoms (e.g., those atoms having impact angles further from perpendicular) while allowing lower impact angle atoms (e.g., those atoms having impact angles closer to perpendicular) to pass through. Collimated sputtering processes, however, generally do not impart significant directionality to the sputtered atoms that are allowed to pass through the collimator.
Unfortunately, as the widths of metallization cavities continue to decrease and the aspect ratios of such cavities continue to increase, forming an adequate bulk metal layer becomes more difficult. One reason for this difficulty results from the buildup of deposited metal on the upper sidewalls of a metallization cavity. Because the majority of metal atoms deposited in a standard or even a collimated sputtering process will have impact angles deflected from perpendicular, the dielectric layer upper surface adjacent the cavity and on the upper sidewalls of the cavity may receive significantly more deposited metal atoms than the lower sidewalls and bottom of the cavity. This buildup on the dielectric layer upper surface adjacent the cavity and upper sidewalls of the cavity may result in metal overhanging, or shadowing, the lower cavity sidewalls and cavity floor. When this happens, deposited metal cannot reach the shadowed sidewall portions, and thus these areas may not receive sufficient metal coverage. This is particularly a problem for the lower portions of the cavity sidewalls, which are perhaps the portions most likely to be shadowed.
Furthermore, because the overhanging metal prevents deposited metal from reaching the shadowed portions of the cavity, metal may build up on the overhanging areas during deposition so much that the opening near the upper region of the cavity becomes closed. If the cavity was not filled before the opening was closed, a void (commonly known as a “keyhole void” because of its shape) can exist within the cavity. The presence of a void within a final metallization structure, of course, can be extremely detrimental to the performance of the structure.
In an attempt to resolve this problem, many processes have implemented hot sputter depositing of the bulk metal layer. Generally speaking, a metal can be either cold sputter deposited or hot sputter deposited. Cold sputter deposition processes deposit a metal at a temperature such that the deposited metal, upon deposition, cannot significantly reflow, and hot sputter deposition processes deposit a metal at a temperature such the deposited metal, upon deposition, can significantly reflow. Reflow of hot sputter deposited materials may result from solid phase and surface diffusion, possibly driven by capillary forces. Reflow may be aided by the thermal energy imparted by the impact of subsequently depositedatoms. Significant reflow preferably encompasses only those forms of bulk redistribution of a metal that occur at elevated temperatures. One benefit of hot sputter
Conley & Rose & Tayon P.C.
Cypress Semiconductor Corp.
Daffer Kevin L.
Nguyen Tuan H.
Pham T.
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