Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Reexamination Certificate
1999-10-20
2001-08-28
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C205S789000, C205S776500, C205S789500, C702S025000, C204S289000
Reexamination Certificate
active
06280602
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the determination of additives in metal plating baths, and more specifically to a method and apparatus for determination of organic suppressor and accelerator additives in semiconductor copper electrolysis plating baths.
2. Background of the Invention
Traditionally, aluminum (Al) has been used as the material of choice for metalization in forming interconnect layers in the manufacture of semiconductor microelectronic integrated circuits. Al is commonly deposited on semiconductor structures by chemical vapor deposition (CVD), which allows for precise control and highly uniform deposition of the product metal-containing film.
Despite the prior ubiquity of Al as a metalization medium, performance demands associated with increasing signal speeds and decreasing feature geometries of microelectronics have exceeded the capabilities of Al metal. Copper (Cu) therefore is increasingly being utilized as a semiconductor interconnect metal. The properties of Cu are not amenable to conventional CVD metalization approaches, due in part to the lack of suitable copper source reagents, and in consequence Cu is typically deposited on the microelectronic device structure via electroplating.
Electroplating of copper, however, has various associated problems.
Generally, Cu is plated onto a substrate by electrolysis in an etch solution, which may for example comprise copper sulfate, sulfuric acid, and hydrochloric acid. The plating process with an unaugmented etch solution of such type normally proceeds too rapidly. The result of such plating rapidity is that previously formed vias, i.e., passages to lower-level structures, e.g., electrodes or other conductors or semiconductor regions in the microelectronic device structure, are bridged over, and not filled with Cu. Accordingly, the desired electrical path to the underlying structure is not formed, and the semiconductor device structure must be reworked or discarded.
In order to combat such plating rapidity, the Cu plating process must be retarded. Additionally, the copper plating process requires acceleration in some aspects, to achieve desired coverage and leveling properties of the deposited metal. To achieve these concurrent opposing goals, organic additives are introduced into the copper electroplating bath to both slow down the plating process (suppressor additives) and to speed it up (accelerator additives). The speed of deposition of Cu on the substrate, and the quality and resulting electrical and mechanical properties of the metalization, are critically dependant on the concentration of these organic additives in the copper electroplating bath. However, the concentration of these additives is not constant, due to either “drag-out” by the wafers or by electrochemical reaction and loss during the electroplating. Accurate, real-time measurement of these electroplating bath additive concentrations, necessary for quality control, has been problematic.
The respective suppressor and accelerator organic compounds in the copper electroplating bath are usually present at very low, e.g., part-per-million by volume (ppmv) concentrations. This circumstance makes normal analytical procedures difficult to effectively apply, due to the masking effect of the high concentration of inorganic bath components (copper, acid, etc.). The most effective way of determining these organic compounds is by measuring their effect on the amount of Cu deposited.
Methods of measuring the effect of the concentration of the electroplating suppressors and accelerators are known in the art. U.S. Pat. No. 5,192,403, issued to Chang et al. on Mar. 9, 1993, describes one such method, comprising the steps of:
a) preparing a basis solution which contains all of the components of the plating solution to be measured (the “sample”), except the component of interest;
b) preparing a calibration solution which contains the component of interest in a known concentration near that which would be expected in the sample;
c) adding measured amounts of the calibration solution to a first defined volume of the basis solution, and plotting the copper plating (cathodic) charge in cyclic voltammetry in the mixed solution against the added volume of the calibration solution;
d) adding measured amounts of the sample solution to a second volume of the basis solution, and plotting the copper plating (cathodic) charge in cyclic voltammetry in the mixed solution against the added volume of the sample; and
e) comparing the slopes of the calibration standard curve and the sample mixture curve to determine the concentration of the component of interest in the sample solution.
Variations of this technique are employed in the art to measure the concentrations of organic suppressor and accelerator additives in Cu electroplating baths for semiconductor manufacturing. These techniques variously measure the plating charge or stripping (de-plating) charge, e.g., for electroplate deposition of Cu directly onto a test electrode via current supplied to a counting electrode in a plating step, and removal of previously plated copper in a stripping step. The charge is generally obtained by measuring the plating or stripping current while holding the voltage constant, and integrating to obtain the charge. Typically, an electrode is cyclically plated and de-plated (stripped of the previously deposited Cu) multiple times for each quantity measured. Each plating/measurement cycle comprises the following steps:
Clean—the test electrode surface is thoroughly cleaned electrochemically or chemically using acid bath, followed by flushing with water or acid bath,
Equilibrate (optional)—the test electrode and a reference electrode are exposed to the plating electrolyte and allowed to reach an equilibrium state.
Plate—Cu is electroplated onto the test electrode either at constant potential or during a potential sweep and the current between the test and counter electrodes is monitored and recorded, and
Strip—the Cu deposition is removed (e.g., by reversal of the plating current flow and/or exposure to an acid bath) by suitably changing the potential between the test and counter electrodes stepwise or in a sweep in the reverse direction and the current between the test and the counter electrode is monitored and recorded (and integrated to determine the “stripping charge”).
These four steps must be repeated for each plating/measurement cycle; each sample measurement is typically repeated several times to eliminate random errors introduced by variations in process conditions, e.g., composition, temperature, etc. Hence, an entire concentration determination sequence can require a considerable period of time to complete. To be useful as a quality control tool in copper metalization in semiconductor manufacturing, the concentration determination must be completed in a very short time frame so that significant depletion of the organic additives in the plating bath does not occur. Any significant depletion of organic additives during the determination will render the analytical method useless.
It would therefore be a significant advance in the art, and is accordingly an object of the present invention, to significantly reduce the time required for the concentration determination sequence to be completed, relative to the present state of the art.
To allow for fine control of the plating process, it is also desirable that concentration of organic additives be determined to a high degree of accuracy. Is therefore is a further objective of the present invention to determine the organic additive concentrations to a high degree of precision, preferably less than 10 percent of indicated value, and more preferably less than about five percent of indicated value.
It is another object of the invention to provide an improved system for determination of organic additive concentration in a copper electroplating bath, that is simple in operation, economic in capital cost and operating expense, and efficient in characterization of the electroplating medium.
Advanced Technology & Materials Inc.
Hultquist Steven J.
McLauchlan Robert A.
Noguerola Alex
Tung T.
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