Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Involving measuring – analyzing – or testing
Reexamination Certificate
2002-11-06
2004-03-23
Koehler, Robert R. (Department: 1775)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Involving measuring, analyzing, or testing
C205S775000, C205S780500, C205S787000, C205S793500, C205S794000
Reexamination Certificate
active
06709561
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is concerned with analysis of reducing agents in electroless plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Plating baths are widely used by the electronics industry to deposit a variety of metals (copper, nickel and gold, for example) on various parts, including circuit boards, semiconductor chips, and device packages. Both electroplating baths and electroless plating baths are employed. For electroplating, the part and a counter electrode are brought into contact with the electroplating bath containing ions of an electrodepositable metal, and the metal is electrodeposited by applying a negative potential to the part relative to the counter electrode. For electroless plating, the bath also contains a reducing agent which, in the presence of a catalyst, chemically reduces the metal ions to form a deposit of the metal. Since the deposited metal itself may serve as the catalyst, the electroless deposition, once initiated, proceeds without the need for an externally applied potential.
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. Accelerated deposition may result from mass-transport-limited depletion of a suppressor additive species that is rapidly consumed in the electrodeposition process, or from accumulation of an accelerating species that is consumed with low efficiency. The most sensitive methods available for detecting leveling additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions, for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such potential cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically cycling the potential of the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
For CVS and CPVS analyses, the metal deposition rate may be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. A typical CVS/CPVS rate parameter is the stripping peak area (A
r
) for a predetermined electrode rotation rate. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate baths are employed in the “Damascene” process (e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.) to electrodeposit copper within fine trenches and vias in dielectric material on semiconductor chips. CVS methods for controlling the three organic additives in acid copper baths needed for plating ultra-fine Damascene features are described in U.S. patent application Ser. No. 09/968,202 to Chalyt et al. (filed Oct. 1, 2001), now U.S. Pat. No. 6,572,753. In the Damascene process, as currently practiced, vias and trenches are etched in the chip's dielectric material, which is typically silicon dioxide, although materials with lower dielectric constants are under development. A barrier layer, e.g., titanium nitride (TiN), tantalum nitride (TaN) or tungsten nitride (WN
X
), is deposited on the sidewalls and bottoms of the trenches and vias, typically by reactive sputtering, to prevent Cu migration into the dielectric material and degradation of the device performance. Over the barrier layer, a thin copper seed layer is deposited, typically by sputtering, to provide enhanced conductivity and good adhesion. Copper is then electrodeposited into the trenches and vias. Copper deposited on the outer surface, i.e., outside of the trenches and vias, is removed by chemical mechanical polishing (CMP). A capping or cladding layer (e.g., TiN, TaN or WN
X
) is applied to the exposed copper circuitry to suppress oxidation and migration of the copper. The “Dual Damascene” process involves deposition in both trenches and vias at the same time. In this document, the term “Damascene” also encompasses the “Dual Damascene” process.
Damascene barrier layers based on electrolessly deposited cobalt and nickel are currently under investigation [e.g., Kohn et al., Mater. Sci. Eng. A302, 18 (2001)]. Such metallic materials have higher electrical conductivities compared to metal nitride barrier materials, which enables copper to be electrodeposited directly on the barrier layer without the use of a copper seed layer. Higher barrier layer conductivity also reduces the overall resistance for circuit traces of a given cross-sectional area. In addition, electroless deposition provides, highly conformal coatings, even within ultra-fine trenches and vias, so that the overall coating thickness can be minimized. Electroless cobalt and nickel baths being investigated for Damascene barrier deposition typically also contain a refractory metal (e.g., tungsten, molybdenum or rhenium), which co-deposits with the cobalt or nickel and increases the maximum temperature at which effective barrier properties are retained.
For electroless cobalt and nickel baths, hypophosphite (H
2
PO
2
) is typically used as the reducing agent, which introduces phosphorus into the deposit. The codeposited phosphorus reduces the deposit grain size and crystallinity (compared to electrodeposits), which tends to improve the deposit barrier properties. Alternative reducing agents include the boranes, dimethylamineborane (DMAB), for example. Use of a borane reducing agent introduces boron into the deposit. A typical bath for electroless deposition of Damascene barrier layers comprises 0.1 M cobalt chloride or sulfate, 0.2 M sodium hypophosphite, 0.03 M sodium tungstate, 0.5 M sodium citrate, 0.5 M boric acid, and a small amount of a surfactant. Such Co(W,P) baths typically operate at about pH 9 and a temperature of 85°-95° C., and may also contain organic additives.
For electroless deposition of cobalt and nickel on dielectric materials, such as silicon oxide, or on metals that are not sufficiently catalytic for the electroless process, such as copper, a seed layer of a catalytic metal is generally employed. Typically, catalytic palladium is deposited by immersion of the part in an acidic activator solution containing palladium chloride and fluoride ion. The fluoride ion t
Bratin Peter
Chalyt Gene
Kogan Alex
Pavlov Michael
Perpich Michael James
ECI Technology Inc.
Koehler Robert R.
Tench D. Morgan
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