Patent Publication Number: US-2004046121-A1

Title: Method and system for analyte determination in metal plating baths

Description:
RELATED APPLICATIONS  
     [0001] This application is a continuation in part of U.S. patent application Ser. No. 10/196,491 filed on Jul. 15, 2002 which claims the benefit of U.S. Provisional Applications Serial Nos. 60/305,650; 60/305,651; and 60/305,760, all filed on Jul. 15, 2001, the disclosures of which are hereby incorporated by reference in their entireties. This application is related to copending U.S. patent application Ser. No. 10/196,491, filed on Jul. 15, 2001 entitled “Method and System for the Determination and Remediation of Arsenic in Aqueous Media” (Attorney Docket No. A70452-1/MSS/MDV), the disclosure of which is incorporated herein by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally to the field of metal plating. More specifically, the present invention is related to a method and system for determining the presence of analytes in metal plating solutions using Raman spectroscopy.  
       BACKGROUND OF THE INVENTION  
       [0003] Metal plating is used in a large variety of industrial processes. Plating systems, in which an object is placed in a plating solution to apply a metallic coating to the object, are well known in the art. Metal plating is used to plate a variety of metals, such as for example copper, zinc, nickel and gold. Many metals are plated simply by immersion in a metal plating bath, or electroplated when electrodes are placed in the bath. Another method of plating that is commonly used is known as electroless plating, a technique that uses a chemical reducing agent instead of galvanic current. Recently, copper plating has received renewed interest due in part to its application to the semiconductor industry. Semiconductor fabrication includes the formation of different layers of material on substrates to form conductors and insulators to create integrated circuit patterns.  
       [0004] Copper wiring is increasingly being used in integrated circuits (ICs) because metal wiring resistance and capacitance effects have become a limiting factors in microprocessor speed. This effect is generally referred to as RC delay. Because the transistor switching speed is no longer the limiting factor, a great deal of attention has focused on the successful integration of lower resistance copper wiring and low-dielectric constant materials to reduce RC delay. Copper wiring has approximately 40% lower resistance than conventional aluminum conductors and is deposited by the electrolytic filling of copper into trenches etched in a dielectric material. The copper wiring is connected to other wiring levels by a “via” of either tungsten or copper metal. The process of inlaying copper as both the wire and via in a dielectric trench is called the “dual damascene process.” In copper damascene processing, after the deposition of barrier and copper seed layers, the inlay of copper is achieved by electrolytic copper plating, in which the silicon wafer, or substrate, serves as the cathode. A sacrificial copper anode completes the circuit. The key to this process is filling of the trench from the bottom up. Otherwise, opposing sides of a deep vertical trench or via tend to grow together and “pinch off,” forming voids which negatively affect device integrity. A more detailed review of IC metallization and semiconductor processing may be found in H. Xiao, Introduction of Semiconductor Manufacturing Technology Prentice Hall, N.J., 2001.  
       [0005] Special inorganic and organic based additives are typically added to the acidic electrolytic copper sulfate solution to facilitate filling of the trenches with copper from the bottom up. These additives have a profound effect on the efficacy and quality of the metal deposition process. These additives are commonly known as accelerators, brighteners, suppressors, and levelers, and are generally organic-based molecules or macromolecules. Chloride ions are also occasionally introduced to enhance adsorption of certain organic additives. Many of these additives and the bath formulations are proprietary formulations. However, in general, accelerators are small organic molecules containing sulfide or disulfide groups such as sulphopropyl sulfides or sulphopropyl disulfides.  
       [0006] In the electrolytic plating process, small accelerator molecules migrate into the trench and increase the rate of copper deposition in the trench from the bottom of the trench upward. Accelerators are chemically active molecules that coordinate with copper ions to mediate the transfer of electrons. Accelerator molecules are directly consumed during the plating process and decompose to a variety of byproducts. Brighteners are typically small molecules such as formaldehyde, or in some cases, sulfides or disulfides that affect the grain size of the plated copper. Grain size is important with regards to annealing and crystal structure which ultimately affects conductivity. Coarse grain sizes tend to diffract light, while smaller grain sizes are more reflective (thus the origin of the name brightener). Suppressors are usually low molecular weight macromolecules, such as, for example polyethers, with molecular weights in the approximate range of 2000 to 5000 grams per mole. They are used as grain size refinement aids or as mediators to regulate the reactivity of accelerators near the top of trenches that are being filled. This suppressing action, which typically occurs by surface adsorption, prevents metal from rapidly spilling over the side of a filled trench or overflowing out of the trench. Due to size constraints, suppressor macromolecules cannot get into the very small trench, but rather are thought to migrate and collect at the top corners of the trench or at the surface. Suppressor molecules are slowly consumed during the plating process via decomposition or chain cleavage. Chloride ions are typically introduced to aid the adsorption of the suppressor. Levelers are used to passivate the top surface on the outside of the trench in a dual damascene copper plating process for IC interconnects. Levelers are usually macromolecules, with molecular weights that may approach approximately 1 million or more grams per mole. Similarly to suppressors, levelers mediate the rate of metal deposition by blocking accelerators outside of a trench. However, levelers are active well outside of the trench due to molecular size constraints. Levelers ultimately make the surface more level and smooth, which improves the efficiency of post-processing steps, such as for example chemical-mechanical polishing. Levelers are typically more resistant to decomposition and chain cleavage than suppressors. Some manufacturers interchangeably use the terms accelerator, brightener, suppressor, and leveler which may cause confusion, depending on the manufacturer, the application, and the plating formulation. For the purposes of the present invention, the above simplified summary and description will suffice. More detailed information on bath formulations and their behaviors can be found in the following U.S. Pat. Nos. 4,347,108; 4,490,220; 4,786,746; 4,897,165; 5,252,196; and 5,730,854.  
       [0007] Electroless plating is a metal plating technique that has recently gained momentum in the fabrication of interconnect structures. Electroless plating involves the use of an in situ chemical reducing agent such as hypophosphite, dimethylamine borane, borohydride, formaldehyde and the like to reduce metals in solution, thus providing a conformal coating on the substrate. A variety of metals and metal alloys may be deposited directly from solution in this fashion. Electroless plating has proven to have value in the deposition of barrier or capping films to protect copper interconnect structures. The layers deposited by electroless deposition have significant advantages over SiN layers formed by physical vapor deposition (PVD), including a lower overall dielectric constant for the stack and decreased electromigration or voiding at the copper/barrier interface. Films that may be deposited in this manner include cobalt tungsten phosphide (CoWP), cobalt tungsten boride (CoWB), and nickel boride (NiB). Other potential applications for electroless deposition in integrated circuit structures include the deposition of copper seed, as well as copper fill. Additional information on electroless deposition in IC manufacturing may be found in G. Malloy and J. Hajdu, Electroless Plating Fundamentals and Applications, Reprint Edition, Noyes, N.Y., 1990 as well as in to U.S. Pat. Nos. 5,695,810, 6,323,128, and 6,287,968, and Lopatin, et. al., in Characterization of Cu, Co, Ni and Their Alloys for ULSI Metallization, Conference Proceedings ULSI XIII, Materials Research Society, 1998.  
       [0008] Electrolytic and electroless plating are wet processing techniques that are very sensitive to process variations. The metal plating bath additives have a profound effect on the quality of the metal deposited. In prior applications of electrolytic and electroless plating, the process has generally been rather loosely controlled. These prior techniques are not well suited to semiconductor fabrication which requires tightly controlled and high quality, reproducible processes. For semiconductor processing, maintaining the chemical purity of the plating bath is extremely important because the plating solution must be capable of producing a metal layer of high quality without impurities. Organic plating additives are typically very dilute in metal plating solutions and thus may be difficult to detect. For example, the concentration of the organic additives in an electrolytic copper sulfate plating solution may be in a range from less than approximately 100 ppm or even lower than approximately 1 ppm, depending on the formulation. In contrast, the copper sulfate content is typically on the order of tens of grams per liter of copper and sulfate, usually in massive excess. As the plating process progresses, accelerator, suppressor, and leveler additives are consumed to varying degrees and must be replenished. In prior art systems, replenishment is typically achieved either by complete dumping of the bath or by a bleed and feed protocol in which fresh additive solutions and/or a complete replacement bath is slowly fed into the active bath while the old bath solution is drained away as in a continuous flow stirred tank reactor (CFSTR). This is an expensive and wasteful method of ensuring that the additive concentrations remain within optimal parameters. Accordingly, there is a significant need for a method and system for accurately and quickly determining the presence of chemical species such as plating bath additives in metal plating solutions, and further the composition and/or concentration, and thus the purity, of such plating solutions. Additionally, there is a need for a feedback loop for the auto-dosing or replenishment of these chemicals to optimize process.  
       [0009] Because both electrolytic and electroless plating techniques are increasingly utilized in IC manufacturing, where process control and analysis must meet stringent specifications, there is a need for a real-time in situ analytical technique for monitoring the constituents of plating baths over time. Currently, cyclic voltammetric stripping (CVS), and polarography techniques are used to monitor both electroless and electrolytic plating bath constituents and performance. CVS measures the rate of metal plating, which is highly dependent on the additive concentration. Polarography measures the change in current versus changing voltage. With the CVS technique, the potential of the inert electrode is cycled at a constant rate in the bath, so that a small amount of metal is alternately plated and removed (stripping). The area under the stripping peak is proportional to the plating rate and thus the concentration of the additives and their ratio to one another. It is easy to see that this technique is an indirect measurement of additive concentration, and thus solely depends on the ratios and concentrations of these components, as well as their synergistic interactions (both positive and negative). The CVS method is thus highly empirical and demands significant input from a highly skilled and experienced operator. Polarography measures the current as a function of a voltage ramp. The current flow may be ascribed to the concentration of the analyte. Each component under study must be measured separately, and the solutions may require treatment to obtain an accurate measurement and to screen out interfering chemical species. Typically, a polarography or CVS measurement will take up to 1-hour including calibration and measurement of standards. Finally, CVS and related methods may require handling of bath chemistry for analysis in a chemistry lab and generate a waste stream that requires special handling and disposal. Nonetheless, due to a lack of alternatives, metal plating industries and semiconductor plating operations have adapted CVS and related methods for electrolytic plating bath analysis and process control despite its limitations and expense.  
       [0010] Because of the limitations of CVS and other available analytical methods for quantifying concentrations of metal plating bath additives, an improved analytical technique for measuring bath performance in relation to the additive concentration is highly desirable. Ideally, such a method would insure that the proper concentration of these materials are maintained, and that the process is stable. Spectroscopic methods are direct with results obtained in real-time and thus can be used for real-time process control with minimal lag time. A direct in situ spectroscopic method is preferable to an indirect method such as titration, liquid chromatography, CVS or similar electrochemical analyses. However, because metal plating baths use water as a solvent and contain dissolved metal ions and/or complexes, typical spectroscopic absorption techniques are of little to no utility. For example, UV-visible and infrared (IR) spectroscopic techniques have severe limitations in detecting dilute additives in metal plating solutions because metal plating solutions are highly absorbing in the IR and UV-visible range and have features that overlap the signal of the chemical constituents under study. As illustrated in FIG. 1 which shows an absorbance spectrum, a typical copper electroplating solution has significant absorption bands in the wavelength range of approximately 200 nm to 340 nm, and also in the range of approximately 550 nm to 800 nm and higher. Although there is a small window between approximately 340 nm and 600 nm in which no interfering bath absorption occurs, many plating bath additives do not absorb or fluoresce in this range. Infrared spectroscopy is not a useful technique because water has a very strong —OH vibrational band at about 3500 cm −1  that obscures most useful chemical information.  
       [0011] Raman spectroscopy is a spectroscopic technique that operates on the principle that light of a single wavelength striking a molecule is scattered by the molecule through a molecular vibration state transition. The resultant scattered light has wavelengths different than the incident or excitation light. The wavelengths present in the scattered light are characteristic of the structure of the molecule. The intensity and wavelength or “Raman Shift” of the scattered light is representative of the concentration of the molecules in the sample. Raman spectroscopic analysis interrogates polarizability changes in the molecule to determine the presence or absence of molecular bonding, and by inference, the chemical species. Approximately 1 part in 1 million of the incident light is scattered. When a photon of incident light interacts with a molecule, in most cases, this interaction leads to the molecule assuming a more excited (higher energy) vibrational state with the emission of a photon at a longer (less energetic) wavelength. Because a small fraction of molecules in any sample already exist in an excited vibrational state, some interactions between an incident photon and a molecule may lead to a decrease in the molecule&#39;s vibrational energy state with a concomitant emission of a photon at a shorter (more energetic) wavelength. These Raman effects, including resonance Raman spectroscopy (RRS), surface enhanced Raman spectroscopy (SERS) and surface enhanced resonance Raman spectroscopy (SERRS) are generally described in greater detail in Grasselli et al.,  Chemical Applications of Raman Spectroscopy , Wiley-Interscience, John Wiley and Sons, New York, 1981. In addition, a variety of Raman spectroscopy devices have been developed in the industry. For example, a fiber optic type device is described in Angle, S. M., Vess T. M., Myrick, M. L., Simultaneous multipoint fiber optic Raman sampling for chemical process control using diode lasers and a CCD detector,  SPIE  vol. 1587, p. 219-231 , Chemical, Biochemical, and Environmental Fiber Sensors III,  1992.  
       [0012] Many important molecular functional groups are inactive or weak in absorption processes, but show significant activity in Raman spectroscopy. These functional groups include, but are not limited to, carbon-carbon bonds; carbon-hydrogen bonds, phosphorous-hydrogen bonds, boron-hydrogen bonds, metal oxygen bonds (metal-oxo-anions); and main group oxyanions such as borate, sulfate, sulfite, phosphate, phosphite, hypophosphite, and nitrate. A preferred Raman sensitive functionality has the proper symmetry of chemical bonds so that a strong Raman response is obtained. Raman responses are typically characterized as being in the range from weak (lowest sensitivity) to very strong (highest sensitivity). For example, the Raman sensitive functionality may comprise a chemical group, such as for example a nitrile or a quaternerized amine, that has a strong scattering response in a wavelength range where water scattering does not occur. Other Raman sensitive groups may include, among others, carbonyls, ketones, hydrazones, saturated and unsaturated carbon, alcohols, organic acids, azo, cyanates, sulfides, sulfones, and sulfonyls. A great variety of organic and inorganic compounds yield useful Raman signals. A large variety of transition metal oxo-anions and complexes, as well as ions selected from the main-group elements also have a good Raman scattering response.  
       [0013] While Raman spectroscopy has been described, in current applications it suffers from many difficulties that limit its usefulness in commercial applications. One significant problem with Raman spectroscopy is the low intensity of the scattered light compared to the incident light. Isolating, amplifying and processing the scattered light signal typically requires elaborate and costly equipment. A further problem is interference with the Raman signal due to fluorescence, or emission of light due to electronic state transitions, from a solution or composition under analysis. Many compounds fluoresce or emit light when exposed to laser light in the visible region. Fluorescence bands are generally broad and featureless, and the Raman signal can often be obscured by the fluorescence. Again, complicated and costly sensors and signal processing equipment are needed to process the signal.  
       [0014] Additional problems with Raman spectroscopy include overlapping peaks of multiple compounds in a sample being analyzed and solution self-absorption. When a variety of compounds are present in a sample to be analyzed, all of the compounds contribute to the Raman signal. Determining and quantifying chemical analytes in metal plating solutions on a real time basis in an industrial setting requires a method and system capable of identifying the analytes despite spectral interference from one or more other compounds present in the plating bath. In solutions with strong absorbance at or near the wavelength of the incident light, the strength of the resultant Raman signal is decreased due to absorption of both the incident light and Raman scattered light by the solvent and solution components. Attenuation of the incident light degrades the intensity of the Raman interactions of irradiated molecules by decreasing the incident photon flux while absorption of the scattered light increases the difficulty of extracting useful species identification and quantification information from the background spectral noise. Thus, further developments in Raman spectroscopy systems and methods are needed  
       SUMMARY OF THE INVENTION  
       [0015] It is an object of the present invention to provide a system and method for determining the concentration of one or more analytes in a metal plating bath using Raman spectroscopy.  
       [0016] In one embodiment of the present invention, a Raman spectroscopy system is provided for quantifying concentrations of one or more metal plating bath additives in a metal plating bath sample. The system includes a laser light source providing a beam of incident monochromatic light to induce emission of a Raman scattered light spectrum from the sample. This spectrum generally contains one or more spectral peaks. The emitted spectrum is collected on a detector that quantifies the area under the peaks and/or the height of the peaks as a function of peak wavelength. The incident light beam passes through a lens that focuses the beam at a focal point. The focal point is disposed less than approximately 1 mm within the bulk of the sample. The detector provides output to a spectrum processor that is configured to determine concentrations of the one or more metal plating bath additives by deconvoluting the peaks in the bath emission spectrum of Raman scattered light based on one or more pre-calculated ratios of the areas under and/or heights of a plurality of peaks in a standard emission spectrum for each of the one or more metal plating bath additives.  
       [0017] In an alternative embodiment of the invention, a method is provided for determining the concentration of one or more metal plating bath additives in an electrolytic or electroless plating bath. The method includes the following steps. A standard Raman emission spectrum in response to monochromatic light at a chosen wavelength is collected individually for each of the one or more metal plating bath additives. A ratio of peak areas and/or peak heights between each of the resultant peaks in the standard emission spectra are identified and quantified. A beam of incident monochromatic light is provided at the chosen wavelength from a monochromatic light source to the metal plating bath. The beam is focused such that the focal point penetrates less than approximately 1 mm into the sample. Light emitted by Raman scattering from the sample is detected on a light detector. The detected light is converted into a bath emission spectrum which is analyzed to quantify the concentrations of the metal plating bath additives. This analysis is performed by creating a series of coupled linear equations in which the concentrations of the metal plating bath additives are unknowns and the peak area and/or peak height ratios are knowns. The resultant set of coupled linear equations is solved using linear algebra techniques.  
       [0018] In a further embodiment of the present invention, a chemical auto-dosing system is provided for controlling the concentration of one or more plating bath additives in a metal plating bath. The system includes a Raman spectroscopy analyzer subsystem for quantifying and analyzing a Raman spectrum emitted from the plating bath to determine real time concentrations of the plating bath additives. One or more additive reservoirs, each containing one of the plating bath additives are connected to metering pumps that control the flow of the plating bath additives from the reservoirs to the plating bath in response to control outputs from a processing subsystem controller that receives and processes concentration data from the analyzer subsystem.  
       [0019] In one mew embodiment of the present invention, a Raman spectroscopy system is provided for quantifying concentrations of one or more analytes in a metal plating bath sample in approximately real time. The system includes a light source that provides a beam of incident monochromatic light to induce emission of a Raman scattered light spectrum containing one or more peaks from the sample. A detector is also provided for quantifying the area under the peaks and/or the height of the peaks as a function of peak wavelength or wavenumber. The beam passes through a lens that focuses the beam at a focal point that is disposed less than approximately 1 cm within the bulk of the sample. A spectrum processor is configured to determine concentrations of each of the metal plating bath analytes from the peak heights and/or areas using one or more predictive models for Raman peak response as a function of wavelength. These predictive models are determined by analyzing one or more individual solutions of the analytes.  
       [0020] In another new embodiment of the present invention, a method is provided for quantifying concentrations of one or more metal plating bath additives in a metal plating bath sample in approximately real time. Raman emission spectra are collected for one or more standard solutions of each of the analytes of interest. These standard spectra quantifying the spectral response to monochromatic light at a chosen wavelength for each of the one or more analytes in the bath at varying concentrations of the analytes. The resulting spectral peak area and/or height data from these standard spectra are fitted to one or more predictive models. A beam of incident monochromatic light at the chosen wavelength is provided from a monochromatic light source to the metal plating bath containing one or more additives. The beam is focused such that the focal point penetrates less than approximately 1 cm into the sample. Light emitted by Raman scattering from the sample is detected on a light detector and then converted into a bath emission spectrum that is analyzed to quantify the concentrations of the one or more metal plating bath additives based on the predictive models.  
       [0021] In a further new embodiment of the present invention, a chemical auto-dosing system is provided for controlling the concentration of one or more plating bath additives in a metal plating bath. A Raman spectroscopy analyzer subsystem as described in one of the preceding embodiments quantifies and analyzes a Raman spectrum emitted from the plating bath to determine real time concentrations of the plating bath additives in the plating bath. One or more additive reservoirs, each of which contains one or more plating bath additives supply these additives to the bath at flow rates that are metered by one or more pumps. A processing subsystem controller receives and processes concentration data from the analyzer subsystem to provide control outputs to the metering pumps. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] Other objects and advantages of the present invention will become apparent upon reading the detailed description of the invention and the appended claims provided below, and upon reference to the drawings, in which:  
     [0023]FIG. 1 is a chart showing a sample absorbance spectrum for light wavelengths between 200 and 1400 nm for a representative metal plating bath.  
     [0024]FIG. 2 is a schematic diagram illustrating the Raman device of one embodiment of the present invention.  
     [0025]FIG. 3 is a schematic diagram showing a more detailed view of a sampling probe according to one embodiment of the present invention.  
     [0026]FIG. 4 is a schematic diagram showing a more detailed view of a flow cell according to one embodiment of the present invention.  
     [0027]FIG. 5 is a schematic diagram showing a detail of a probe head with a ball lens according to one embodiment of the present invention.  
     [0028]FIG. 6 is a schematic diagram showing a detail of a probe head with an adjustable focal length lens according to one embodiment of the present invention.  
     [0029]FIG. 7 is a graph showing an example of a Raman calibration curve of peak area vs. concentration for an additive in an electrolytic metal plating bath.  
     [0030]FIG. 8 is an example of a representative Raman spectrum for a solution containing a polyethylene glycol suppressor.  
     [0031]FIG. 9 is a flow chart showing the steps by which a spectrum of overlapping peaks is deconvoluted to calculate concentrations of multiple analytes.  
     [0032]FIG. 10 is a schematic diagram showing an integrated plating bath analyzer system according to one embodiment of the present invention.  
     [0033]FIG. 11 is an example of a representative Raman spectrum for an electroless cobalt phosphorous plating bath.  
     [0034]FIG. 12 is a graph showing an example of a Raman calibration curve of peak area vs. concentration for a reducing agent additive in an electroless metal plating bath.  
     [0035]FIG. 13 is a graph illustrating the detection of a suppressor additive at a concentration of 8 ppm in an electrolytic plating make up solution in accordance with one embodiment of the method and system of the present invention.  
     [0036]FIG. 14 is a graph illustrating the detection of an accelerator additive at a concentration of 2 ppm in an electrolytic plating make up solution in accordance with one embodiment of the method and system of the present invention.  
     [0037]FIG. 15 is a chart showing a spectrum obtained from analysis of a plating bath solution as detailed in experimental example 2.  
     [0038]FIG. 16 is a schematic diagram illustrating the Raman device of a new embodiment of the present invention.  
     [0039]FIG. 17 is a flow chart showing the steps of the method of the present invention according to a new embodiment for determining the concentrations of one or more analytes in a metal plating bath.  
     [0040]FIG. 18 is a graph showing an example of a Raman calibration curve of peak area vs. concentration for a reducing agent additive in an electroless metal plating bath. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0041] The present invention provides a method and system for identifying chemical analytes in metal plating solutions. More specifically, the present invention provides a method and system for determining the presence and/or concentration of inorganic and organic additives in metal plating solutions using Raman spectroscopy.  
     [0042] The present invention provides a rapid and real-time method and system of quantifying organic and inorganic additives and other species in metal plating solutions and, in a further embodiment, of automatically replenishing the concentrations of additives of interest in response to the measurements. Concentrations of these species may be in the ppm (part per million) range in electrolytic baths or in the grams per liter range in electroless baths. More specifically, the present invention provides a methodology and system for the quantification of inorganic or organic chemicals in plating baths over a broad concentration range. The present invention provides a method for detection and quantification that employs Raman spectroscopy in conjunction with inventive techniques that diminish or eliminate photon absorbance characteristics of the aqueous system that can interfere with accurate detection of analytes of interest. The Raman spectroscopy system and method of the present invention provides rapid and quantitative measurement of relatively dilute organic and inorganic species which are extremely difficult to quantify in real time using prior art methods.  
     [0043] First discovered by C. V. Raman in 1928 ( Nature , v. 121, 1928, p. 501), Raman spectroscopy has great potential as a novel and efficient method for real-time quantitative metal plating bath analysis. In general, Raman spectroscopy involves the scattering of incident light by molecules. While most of the incident radiation is scattered elastically, a small fraction of photons return with higher or lower energy, usually 1 in 1 million or so. A net loss of photon energy (increase in wavelength) results from the photon&#39;s induction of a molecular vibration in a molecule it encounters. In contrast, a gain in energy (decrease in wavelength) by the photon is a result of the absorption of a molecular vibration by the photon interacting with a previously excited molecule that drops to a less energetic vibrational state as a result of the interaction. Formally, the photon interactions are a result of a change of molecular bond polarizability (P) due to the interaction with a photon&#39;s electric field (E), as expressed in equation 1:  
       P=αE   (1)  
     [0044] The Raman effect increases in strength at shorter incident light wavelengths. Observed Raman peaks are typically shifted to lower energies than the incident radiation (Stokes shift). This is due to the higher probability of a change in polarizability, or vibrational transition at room temperature, because most of the molecules are at a lower energy vibrational state. However, the photon can interact with a small fraction of high energy vibrational states that are also populated, resulting in emission of a higher energy photon (anti-Stokes shifted). Raman response is also dependent on laser wavelength. Signal intensity I, is dependent on wavelength (λ), as expressed in equation 2:  
     I≈λ −4   (2)  
     [0045] According to equation 2, a 532 nm laser yields approximately 5 times greater Raman response intensity than a 785 nm laser. Therefore, the inventors have discovered that it is advantageous to chose a higher energy laser to promote greater signal to noise ratio and shorter spectrum acquisition times. However, shorter wavelength photons can also induce fluorescence emissions which may mask the Raman response in some samples. According to the present invention, a further consideration in Raman spectroscopy is taught that, though the intensity of Raman signal is linearly dependent on the power of the incident light and it may in some cases be advantageous to employ a higher powered light source, sheer brute force application of additional incident radiation power may not be advantageous due to the potential for inducing undesirable physical and chemical changes in the sampled bath under high power density conditions.  
     [0046] Raman spectroscopy has significant advantages over absorption techniques such as UV-visible, near infrared and mid infrared, especially in aqueous plating bath analysis. Water is a weak Raman scatterer in the range of approximately 300 to 800 nm. However, non-Raman spectroscopic techniques may be overwhelmed by absorption of incident photons by dissolved ions or water itself due to its presence in overwhelming excess. An effective normalized range for Raman signals in wavenumbers is typically from 200 cm −1  to 3000 cm −1 , which allows for the detection of a large variety of inorganic and organic species in aqueous media.  
     [0047] A metal plating bath containing aqueous copper sulfate may be first analyzed by UV-visible spectroscopy. As shown in FIG. 1 and discussed above, the solution is highly absorbing (high extinction coefficient), except in the wavelength range of approximately 300 to 680 nm. Absorbance is described by Beer&#39;s law:  
       A=εBC   (3)  
     [0048] Where A is absorbance, ε is the extinction coefficient, B is the path length, and C is the concentration. Identifying the window of relatively high transmission in the absorption spectrum of an electrolytic plating bath allows a choice of incident laser light, preferably a diode laser source, that transmits light with a wavelength in the range of approximately 300 to 680 nm. If a laser is chosen that transmits radiation at or near the absorption maxima of the solution, the Raman effect is greatly diminished as photons that would otherwise be available to stimulate Raman emissions from the molecules of interest are attenuated by absorbance within the bulk fluid. Moreover, substantial solution absorption at the laser wavelength results in an exponential relationship between intensity and concentration, which is a significant source of error in quantitative detection of the analyte or analytes of interest. Therefore, it is important to choose the correct laser incident wavelength. In this embodiment of the present invention, an 84 mW green Nd:YAG laser source that transmits at 532 nm is used. The power of the laser is not limited, however, a range of 5 to 200 mW is preferred for best signal generation. A 532 nm diode laser source is preferred for the analysis of copper sulfate solutions because it emits within the window of solution light transmission and is compact and efficient. Those skilled in the art can select the correct wavelength of incident monochromatic light for other applications based on the teaching of the present invention.  
     [0049] In this embodiment of the present invention, a method and system for sensing analytes in metal plating solutions is provided wherein a Raman spectroscopy sensor is utilized. While the exemplary embodiment described herein shows an electrolytic copper metal plating solution, the present invention may be used for a variety of metal plating solutions including electroless plating solutions. The Raman sensor generally includes a monochromatic light source to probe a metal plating solution containing one or more analytes. Generally, the plating solution is passed through a fluid path which intersects the light source. The monochromatic light source may be a diode laser, gas laser, filtered high intensity light source and the like. As the monochromatic light source probes the plating solution, light is scattered. Individual wavelengths of the scattered light are separated using a compact monochromator in either a static or scanning mode, with detection provided by a detector such as a high sensitivity CCD or diode array detector. Additionally, source photons may be carried to the solution utilizing a series of bundled fibers which return the light to the detector for subsequent evaluation.  
     [0050] A Raman spectroscopy sensor  100 , particularly suitable for detection of analytes in metal plating solutions, in accordance with this embodiment of the present invention is illustrated in FIG. 2. The sensor  100  generally includes a monochromatic light source  102 , a spectrograph  104 , a probe  120  that is coupled to the light source and spectrograph through an excitation fiber  130  and a collection fiber  132  respectively, for delivering incident light to and collecting scattered light from a sample  124 , a fiber input  106  and CCD array  110  coupled to the spectrograph  104 , and a personal computer data processor with interface electronics  112  for controlling the system and processing the output from the spectrograph  104 .  
     [0051] In the exemplary embodiment shown in FIG. 2, the monochromatic light source  102  is preferably comprised of a frequency doubled YAG diode laser, operating at 20 mW, 0.1 nM stability with 1.5 mrad beam divergence. The diode laser is powered by a power supply (not shown) which preferably is 120 V temperature stabilized. In one embodiment, the excitation light from the diode laser is focused onto a fiber end of the excitation fiber  130  which conducts the incident light to the probe  120  for focusing into a sample subvolume  124 . Preferably both the excitation fiber  130  and the collection fiber  132  are comprised of a poly-micro fiber optic cladded light guide.  
     [0052] A plating solution sample  124  to be analyzed enters the sample subvolume either through normal operating circulation of the bulk bath or via one or more pumps (not shown). The bath interacts with the excitation light delivered by the excitation fiber  130  to the probe  120  to yield Raman scattered light. Light scattered from the solution—the Raman radiation or signal—is collected by the probe  120  and delivered to the fiber input  106  via the collection fiber  132 . From the fiber input  106 , collected scattered light passes into the spectrograph  104  wherein it is analyzed to yield a spectrum which is quantified in real time via a CCD array  110 .  
     [0053] The Raman signal preferably passes through a filter  133  which is preferably a reject filter chosen to filter out light at the incident wavelength to prevent swamping of the CCD detector, and is coupled via a SMA connection to fiber optic borosilicate glass, prior to analysis in the spectrograph. Borosilicate fiber has a Raman shift of a well defined wavelength notch for baseline frequency calibration. Various spectrographs  104  may be used. In one embodiment, the spectrograph is a CS400 Micropac with Hamamatsu 256Q cooled array. A serial interface  114  may be provided for coupling the processed signal to a computer system and interface electronics  112  for display and/or analysis.  
     [0054] The spectrometer is optical and mechanical in nature. The Raman scattered light delivered via the collection fiber  132  from the sample is projected onto the CCD array  110 . A charge-coupled device (CCD) is a light sensitive integrated circuit that quantifies the intensity of the light by converting the light into an electrical charge. The CCD data or spectrum is then analyzed to calculate the concentration levels of additives and byproducts. The computer system  112  preferably consists of a computer, a CCD controller card that plugs into the computer mother board, communication PC cards such as a modem and an Ethernet card among others, and digital and analog input/output ports.  
     [0055]FIGS. 3 and 4 are schematic diagrams providing additional detail of an exemplary system according to one embodiment of the current invention. An immersible probe  120  that transmits the incident light  122  from a diode laser light source  102  to the analyte solution sample  124  and also receives the scattered signal  126  is used in this embodiment. Incident light  122  is transmitted from the monochromatic light source  102  to the probe via an excitation fiber optic cable  130 . Scattered light is collected by the probe and transmitted to a fiber input  106  to a spectrograph  104  by a collection fiber optic cable  132 . The focal point, or working distance  134  of the laser light  122  is adjusted so that its penetration depth into the solution sample  124  is preferably in the range of approximately 0.1 mm to 1 cm, with a range of approximately 0.1 to 5 mm most preferred. The working distance  134  is adjusted according to the turbidity of the solution as well as its self-absorption characteristics. The probe  120  is constructed of materials that resist the corrosive effects of an acidic aqueous environment or a caustic environment, such as, for example Monel alloy, Teflon, or other inert materials. A probe window or more preferably a lens  136  is provided through which incident and scattered light pass out of and into, respectively, the probe. This window or lens  136  is preferably constructed of either sapphire or quartz. The probe  120  is immersed into the plating bath or some other subvolume containing a sample such that ambient light is excluded. It is preferred that the probe  120  is immersed in a subvolume or region of the plating bath or test solution in which circulation past the probe is sufficient for continuous monitoring of a dynamic chemical environment that is representative of the bath as a whole. The probe  120  may be preferably placed in a pipe or some other custom built chamber with appropriate pumps to circulate the solution past the probe and prevent interference from ambient light.  
     [0056]FIG. 3 also shows additional details regarding a preferred embodiment of the probe. In a preferred embodiment of the present invention, an 84 mW green Nd:YAG laser source is provided that transmits at 532 nm in conjunction with a short path length quartz flow cell to reduce the absorbing characteristics of the solution. The sample  124  is housed in a pipe or chamber (not shown) that interfaces with the probe  120 . In this embodiment, light conducted to the probe by the first or excitation fiber optic fiber  130  enters the chamber and passes through a collimating lens  140  which collimates the light. The collimated light beam  142  then passes through a bandpass filter  144  and a dichroic filter  146  before exiting the probe via a focusing lens  136  that focuses the light beam  142  on the sample  124  at the desired working distance  134 . Light scattered from the sample  120  passes back through the focusing lens  136  into the probe  120  where the dichroic filter  146  diverts light that differs from the incident beam wavelength at a 90° angle to a mirror  150  angled at 45° to redirect the scattered light beam  152  parallel to the incident collimated beam  142 . The scattered light passes through a second focusing lens  154  that focuses it into the second, collection fiber optic fiber  132  for transmittal to the detector.  
     [0057] Because absorbency is proportional to path length, the path length is chosen to minimize absorbance of the incident laser by the solution under analysis. A path-length that is too long may result in the capture of both incident laser light and the emitted Raman signal by the inherent absorbancy of the sample. A flow cell with a fixed path length as shown in FIG. 4 may preferably be used for continuous monitoring of the dynamic plating bath environment. Sample solution  161  is circulated through the flow cell  160  via pressure or aspiration by mechanical and/or micromechanical pumps  162 . The flow cell path length may preferably be in the range of approximately 0.1 to 10 mm. More preferably, the flow cell path length through which incident light from the probe passes is in the range of approximately 0.1 to 1 mm. The cell preferably interfaces with a fiber optic probe of the same general design as shown in FIG. 3.  
     [0058] In another embodiment of the present invention, an immersible probe as shown in FIG. 3 is provided that includes a ball lens. Use of a ball lens provides the following advantages: the focal distance is always tangent to the ball lens surface and thus constant thereby providing a constant sample volume, the probe is always properly aligned when it is in contact with a sample, and there are no moving parts. A general schematic of an exemplary ball probe according to this embodiment is shown in FIG. 5 which includes a ball lens  170  having a focal point  172  on its surface  174 . The ball lens  170  is mounted in a probe head  120  that includes appropriate optics (not shown) to convey an excitation beam of monochromatic light  122  to the ball lens  170  and a beam of scattered light  126  away from the ball lens and to an appropriate detector or detectors. In general, the ball lens  170  is housed in a barrel-shaped probe that is preferably constructed of materials such as for instance Monel alloy, Teflon, or other inert, acid resistant materials. The ball lens is preferably constructed of sapphire or quartz or other materials that are resistant to acid or basic conditions, are and transparent to the incident and scattered light wavelengths. Because the ball lens probe has its focus at the surface of the sphere, constant sampling precision and repeatability is enhanced. It is preferable to position the probe in contact with the bath such that ambient light is excluded and where circulation of the bath past the probe is sufficient to allow for continuous monitoring of the dynamic chemical environment within the bulk of the bath. The probe is thus preferably placed in a pipe or chamber or other customized subvolume equipped with appropriate pumps to circulate a sample of the bath past the ball lens and exclude ambient light.  
     [0059] In a preferred embodiment of the present invention, an immersible probe  180  as illustrated in FIG. 6 is provided. The probe  180  includes an adjustable focal point  182  for incident light  122  provided by an excitation fiber  130  from a monochromatic light source  102  as shown in FIG. 2. The focal point  182  of the incident laser light is adjusted by moving an adjustable lens  184  within the probe body  186 . The focal point  182  is adjusted such that it is within the sample subvolume immediately outside of a sealed probe window  190  through which the focused beam is projected. The close proximity of the beam focal point to the window—it is preferably in the range of approximately 0.1 to 5 mm from the outer surface of the window  190 —mitigates potentially confounding effects of solution absorption and light scattering by particles on the collected Raman spectrum and subsequent analytical steps.  
     [0060] Spectral data collected via the aforementioned embodiments are preferably analyzed for features that can be ascribed to certain chemical species. The Raman shift of individual chemical species is preferably identified prior to analysis by separate measurement of individual components. Quantification of the individual components in a plating bath mixture is preferably achieved by determination of the peak area and/or height of the chemical species of interest, followed by comparison of these data to a straight-line calibration curve. The linear calibration curve is preferably generated by plotting peak area and/or height versus concentration of samples in which the concentration of the analyte of interest is known. Standard methods of statistical analysis including, but not limited to, linear regression may be applied to obtain a best fit straight line calibration curve. FIG. 7 shows an exemplary calibration curve generated by Raman analysis of known samples of a polyether suppressor macromolecular additive typically used in electrolytic plating baths. The best fit calibration line was determined by linear regression. FIG. 8 shows an exemplary Raman spectrum of a solution containing a polyethylene glycol suppressor additive. The spectrum reveals a variety of alkyl group signatures which are useful for quantitative analysis. Commercially available software packages for spectral analysis may be used in conjunction with the above described system and method. These include Unscrambler by CAMO Technologies (Woodbridge, N.J.), which is used to create calibration curves and goodness of fit metrics and to perform integration of peak areas and quantification of peak height. In addition, the software includes routines that eliminate extraneous effects that could have a negative impact on the area or peak height measurement, such as, for instance, fluorescence. Spectral software package for qualitative and quantitative analysis that include quantification of peak area and height are Unscrambler by CAMO and the GRAMS/AI package provided by Thermo Galactic, Salem, N.H. PLSplus/IQ, also provided by Thermo Galactic is used to perform partial least squares analyses on spectral data as is Unscrambler. Another useful software, MATLAB, is offered by Mathworks of Natick, Mass. Eigenvector Research, Inc., of Manson, Wash. also provides a useful software program for data analysis called PLS Toolbox.  
     [0061] In a preferred embodiment of the present invention, a method is provided for calculating concentrations of individual additives and other analytes in a plating bath solution based on a single Raman spectrum captured as described above in the previous embodiments. The method of this embodiment is outlined in the flow chart shown in FIG. 9. In general as shown in FIG. 9, the method  300  comprises preparing and analyzing spectra from standard solutions of expected analytes at step  310 . Primary and secondary peak height and/or area ratios for each analyte is calculated at step  320 . Next, the spectrum for the sample of interest is collected at step  330 . At step  340 , identifying and quantifying a first analyte in a non-overlapped region of the spectrum is performed. The peak height and/or area of overlapping analyte peaks is estimated using primary/secondary height and/or area rations to create a system of linear algebraic equations at step  350 , and the system of equations is solved at step  360 .  
     [0062] More specifically, the sample spectrum contains a plurality of peaks, some of which are attributable to Raman scattering by analytes of interest such as one or more plating bath additives. In general a spectrum of a solution containing multiple analytes has regions of the spectrum where peaks attributable to more than one analyte overlap. This embodiment of the present invention provides a method for deconvoluting a spectrum comprised of peaks from numerous analytes. Prior to analysis of a sample spectrum, standard spectra are prepared for each analyte expected to be found in the sample. A primary and one or more secondary peaks are identified for each standard. In general, the peak heights and/or areas of each of the primary and one or more secondary peaks vary linearly with the concentration of the analyte. As such, the ratios of the area and/or height of an individual secondary peak to the primary peak as well as to other secondary peaks in the spectrum of a single analyte are approximately constant and independent of the concentration of the analyte. This property is used in conjunction with standard spectra and peak ratios from the expected analytes to differentiate the concentrations of multiple overlapping analytes in a sample spectrum as follows. A region of the sample spectrum containing only a single primary or secondary peak from a first analyte is identified. The concentration of that analyte is determined based on a calibration curve like the one shown in FIG. 7 based on the area and/or height of that peak in the standard spectrum. If, for example, a secondary peak from the first analyte occurs in the same region of the sample spectrum as the primary peak of a second analyte, the total area and/or height observed on the sample spectrum in the wavelength region of the primary peak of the second analyte is reduced by the expected height and/or area under the first analyte&#39;s secondary peak based on the concentration of the first analyte known from the primary peak height and/or area of the first analyte, the calibration curve, and the known ratio of the height and/or area of the primary and secondary peaks of the first analyte. This process is repeated as necessary to quantify all of the analytes of interest in a sample spectrum. Overlapping of multiple peaks from multiple analytes in a single wavelength region of a sample spectrum requires construction of a matrix of linear algebraic equations. The resulting matrix can be readily solved to identify the concentrations of each of the analytes by one of skill in the art provided that at least one peak of one analyte occurs alone in a discrete region of the spectrum.  
     [0063] Bilinear projection methods, like PCA (Principal Components Analysis), PCR (Principal Components Regression), PLS (Partial Least Squares regression, or Projection to Latent Structures regression) extract systematic information from the combination of many measurement variables. They also offer great interpretation features, to visualize sample patterns and variable relationships in easily interpretable graphical pictures. The multivariate models can then be used for indirect measuring, data reduction, exploration, prediction or classification/identification. These methods are easy to use and handle most multivariate problems despite intercorrelations, noise, errors, missing data, or extreme data table dimensions. Sub-routines and algorithms such as those featured in the aforementioned commercially available software packages may also be used to streamline the data analysis process or for conversion of peak height or areas directly to additive concentrations.  
     [0064] In a further embodiment of the present invention, the Raman analysis and bath additive concentration system and method are integrated with a commercially available chemical auto-dosing system to ensure that bath additive concentrations are maintained within an acceptable range during an ongoing, dynamic process. In this embodiment, shown schematically in FIG. 10, an integrated plating bath analyzer system  200  maintains the proper concentrations of additives in a plating bath by providing a feedback signal from a Raman spectroscopy system to an autodosing system to control the rates at which selected additives are added to the bath. In this embodiment, an analyzer subsystem  202  interfaces with a process subsystem  204  to provide chemical concentration data as well as control capability.  
     [0065] In general, a plating bath  206  is used to deposit a film on a wafer or some other substrate is maintained such that the concentrations of one or more plating additives in the bath do not vary from within optimal performance ranges. As the plating bath  206  is used in the deposition process, the composition of the bath changes. Byproducts are generated from the composition of the bath during the process as certain components are depleted. Maintaining the concentration level of key components, such as for instance additives in the bath, is essential in controlling the process. Also, certain byproducts can have adverse effects on the process. Typically, the bath and the additive metering hardware are a part of the process subsystem. However, it is not a requirement. The plating bath  206  is pumped to a wafer surface in a controlled manner and a filtration step is typically incorporated. Temperature, pH, and other parameters are controlled in the bath. The bath is constantly recirculated and filtered to ensure complete mixing and to remove contaminants such as for instance particles.  
     [0066] The analyzer subsystem  202  includes a spectrograph  104  including a fiber input and CCD array (not shown in FIG. 10) as described above. The spectrograph is preferably connected to a personal computer based control system  112  with control electronics for processing the signals received and quantified by the spectrograph and CCD array. The computer system  112  preferably consists of a computer, a CCD controller card that plugs into the computer mother board, communication PC cards such as a modem and an Ethernet card among others, and digital and analog input/output ports.  
     [0067] One or more additives supplied from one or more additive reservoirs  210  are metered into the bath  206  via metering pumps  212  to maintain the required concentration levels. In this embodiment, the concentrations of additives and byproducts are monitored via Raman spectroscopy as outlined in the preceding embodiments. These data are used to safe guard against processing with the bath in a non-optimal condition. Additives are supplied to the bath via supply lines  214  from the additive reservoirs at rates metered by the metering pumps  212  based on feedback received from the analyzer subsystem  202 . In this closed-loop control scheme, the concentrations of key components of the bath are tightly controlled without dependence on empirical relationships or historical bath depletion data. As a component is added into the bath via any control and metering process, the concentration of that component is at its peak. This concentration gradually decreases over time during processing. The amplitude of the concentration variability can theoretically be minimized by supplying a continuous, uniform addition of additives to the bath  206 . However, because real process conditions are never ideal or constant, constant corrections of the addition rate are necessary. Analyzer subsytem  202  provides continuous feedback to a process subsytem controller  216  that in turn controls the metering pumps  212  to adjust the delivery rate of the additives from the reservoirs  210  to the bath  206 . The process subsystem controller  216  has built in algorithms and hardware inputs and outputs to directly control the additive metering pumps  212 .  
     [0068] The system and method provided by this embodiment is capable of directly controlling the metering pumps  212  or transmitting data on the concentrations of additives and byproducts. Table 1 lists several interface capabilities and methods of the analyzer subsystem that may be included to accommodate varying requirements of different process equipment and systems into which this system and method may be integrated.  
               TABLE 1                          Interface capabilities of an exemplary analyzer subsystem                         Item   Signal Type   Purpose and Description                                 1   Digital Input/Output   For controlling the metering the           (24 VDC typical)   metering pumps directly               For process alarm and               permissive (go or no go for the               process equipment).       2   Relay/Dry Contact   For hardware safety interlock           (varying voltage level)   circuit if required in addition to               item 1 functionalities.       3   Analog Input/Output   For transmitting concentration           (4 to 20 mA or   levels by scaling the signal from           0 to 5 VDC typical)   0 to 5 VDC to be 0 g/L to 50               g/L for example               For controlling metering pump               rate       4   Serial Communication   For communicating           (RS232, Universal Serial Bus,   concentration levels and other           Ethernet, and others typical   set up parameters. By varying           of Personal Computers)   communication drivers, wide               range of protocols can be               supported. Typical protocols               include custom ASCII, SECS,               TCP/IP, etc.                  
 
     [0069] The aforementioned embodiments of the system and method of the present invention are directed to analysis of plating bath additives in electrolytic plating systems. In another embodiment, the system and method of the present invention are applied to analysis of additives in electroless plating baths. As previously noted, electroless plating is a metal plating technique that has recently gained momentum in the fabrication of interconnect structures. Electroless plating involves the use of an in situ chemical reducing agent such as hypophosphite, dimethylamine borane, borohydride, formaldehyde and the like to reduce metals in solution, and thus provide a conformal coating on the substrate. A variety of metals and metal alloys can be deposited directly from solution in this fashion. Electroless plating has proven to have value in the deposition of barrier or capping films to protect copper interconnect structures. The layers deposited by electroless deposition have significant advantages over PVD SiN, such as a lower overall dielectric constant for the stack and decreased electromigration or voiding at the copper/barrier interface. Films deposited include cobalt tungsten phosphide (CoWP) and cobalt tungsten boride (CoWB), and nickel boride (NiB). Other potential applications for electroless deposition in integrated circuit structures include the deposition of copper seed, as well as copper fill. As noted previously, additional information on electroless deposition in IC manufacturing can be found in G. Malloy and J. Hajdu,  Electroless Plating Fundamentals and Applications , Reprint Edition, Noyes, N.Y., 1990 as well as in to U.S. Pat. Nos. 5,695,810, 6,323,128, and 6,287,968, and Lopatin, et. al., in Characterization of Cu, Co, Ni and Their Alloys for ULSI Metallization, Conference Proceedings ULSI XIII, Materials Research Society, 1998.  
     [0070] In order for a metal or metal alloy to be electrolessly deposited onto a surface, such as a metal layer, a barrier layer, or a dielectric, the surface of the material must be susceptible to the autocatalytic growth of copper. If the surface is not susceptible to such growth, then the surface must be activated for electroless growth of copper. Without such surface treatment, electroless growth of a metal or metal alloy will not occur on a non-catalytic surface, when the wafer is subjected to the electroless deposition solution.  
     [0071] As disclosed in U.S. Pat. No. 5,695,810 and in Lopatin et al (in Characterization of Cu, Co, Ni and Their Alloys for ULSI Metallization,  Conference Proceedings ULSI XIII, Materials Research Society,  1998, above), a representative electroless deposition solution is comprised of complexed cobalt, tungstate, and hypophosphite, among other constituents. A sample Raman spectrum of an electroless plating bath of this composition is shown in FIG. 11. Because of the different set of additives expected to be found in an electroless plating solution and the differing chemical and spectroscopic environment to be analyzed, it is preferable to apply the aforementioned system and method for analysis of an electrolytic plating solution to analysis of an electroless solution. A different wavelength of monochromatic light may produce a Raman spectrum of superior quality from an electroless solution. Identification of the preferred wavelength for analysis of an electroless plating bath is preferably achieved by careful consideration of the Raman behavior of the additives of interest and by examination of a UV-visible absorbance spectrum collected for the particular plating bath to be analyzed. Separate calibration curves are prepared for individual electroless plating bath additives as described above in the description of the exemplary embodiment of the electrolytic plating bath analysis method. These calibrations, such as the exemplary curves shown in FIG. 12 and FIG. 13, are preferentially used to predict a concentration for a sample of unknown concentration. FIG. 14 shows the results of such a prediction.  
     EXPERIMENTAL  
     [0072] A number of experiments were conducted according the method and system of the present invention. These experiments are intended for illustration purposes only, and are not intended to limit the scope of the present invention in any way.  
     Example 1  
     [0073] In one example, two plating solutions were tested; one containing only a make up solution of copper sulfate, and the other containing the make up solution with 8 ppm of a suppressor additive. FIG. 13 shows the Raman spectra for each of the solutions. Of significant advantage, the additive is clearly detected within the make up solution. In another example, again two plating solutions were tested with the method and system of the present invention; one solution containing the copper sulfate make up solution, and the other containing the copper sulfate make up solution with 2 ppm of an accelerator additive. FIG. 14 shows the Raman spectra for each of the solutions, and again the additive is clearly detected within the make up solution.  
     Example 2  
     [0074] In another experimental example, an electrolytic copper plating bath solution was analyzed for suppressor and accelerator content. The commercially available bath composition was as follows: 60 g L −1  copper sulfate, 120 mL L −1  of 98% sulfuric acid, 8 mL of accelerator, and 6 mL of suppressor. The system used to analyze the plating bath is as described above and depicted schematically in FIG. 4. A 532 nm, 84 mW green Nd:YAG laser was used in conjunction with a fixed probe head as described above. The system integrates an internal laser calibration system based on an internal neon discharge. This enables greater measurement precision and a discrete non-varying laser output. The result is greater repeatability and more consistent peak areas. A thermoelectrically cooled CCD detector of the dimensions 1024×128 was used. The spectral resolution is 4 cm −1 . The bandwidth of analysis was 400 to 3000 cm −1 . A commercially available personal computer running commercially available spectral analysis software packages (Unscrambler by CAMO Technologies and GRAMS/Al and PLSplus/IQ by Thermo Galactic) were used for data analysis and peak height and area determination. A 3 mL sample was withdrawn from the plating bath and a placed in a borosilicate glass vial. Acquisition times varied from approximately 1 to 10 minutes. A 5-minute acquisition period was used to obtain a spectrum as shown in FIG. 15. The peak height and peak area of alkyl groups for the suppressor and accelerator in the 2900 cm −1  region of the spectrum were detected and analyzed. Accelerator sulfonic acid groups are difficult to differentiate from sulfuric acid Raman signals. Alternatively, carbon-sulfur and sulfur-sulfur bonds are analyzed for the accelerator in the approximate wavenumber range of 400 to 800 cm −1 . Based on comparison of the bath emission spectrum to known controls and a standard calibration curve, it was determined that the data thus obtained was consistent with the additive concentration as provided by the bath manufacturer.  
     Example 3  
     [0075] In a further experimental example of the present invention, an electroless plating bath was analyzed using a 785 nm Raman system. The plating bath absorption maximum was 529 mm, so monochromatic light was provided at 785 mm. To compensate for the approximately fourfold reduction in sensitivity at this wavelength versus 532 nm as predicted by equation 2, the incident laser power was boosted to 150 mW. As noted above, Raman signal sensitivity is a linear function of power.  
     [0076] An electroless plating bath formulation containing complexed divalent cobalt, tungstate, and hypophosphite was analyzed using a quartz cell with a Renishaw Ramascope Raman System  1000  coupled to an Leica DMLM microscope. The system is equipped with diode laser excitation (785 nm., 150 mW of power), a entrance slit of 50 microns, an 1800 groves/mm high efficiency aluminized grating, and a high sensitivity thermoelectrically cooled CCD detector. The Raman spectra for reference areas were collected on adjacent clear field areas. Raman spectra are collected at 4 cm −1  resolution from 200 to 3600 cm −1 , on liquid samples ranging from 300 microliter to 1 liter volumes. Under these conditions, an acquisition time of one minute was sufficient to generate spectral data for calibration and unknown analysis with less than 1% error.  
     New Embodiment  
     [0077] One of the primary hindrances to the use of Raman spectroscopy to quantify analytes in a concentrated solution such as a metal plating bath is the high absorbance of the solution for the incident laser light. This concern is greater for electroless plating baths which may have analyte and other solution component concentrations that are several orders of magnitude larger than electrolytic baths. The previously described embodiments disclosed a method and system in which a wavelength for the incident beam of excitation light for generating a Raman spectrum is chosen to avoid overlap with peaks in the solution absorbance spectrum. In this new embodiment, the inventors describe a method and system for obtaining reproducible measurements of analyte concentrations in electrolytic and electroless plating baths without tuning the incident Raman excitation light to a wavelength at which the solution absorbance is minimized. This new embodiment of the invention is applicable to a range of plating bath samples with a greater range of concentration and solution absorbance conditions. Additionally, by permitting a broader choice of incident light wavelength, it is possible to choose a shorter wavelength light source with the accompanying increase in photon energy and Raman signal intensity as shown in equation 2.  
     [0078] In the new embodiment, the incident light beam is focused such that penetration into the bath sample is minimized, thereby minimizing the solution absorbance factor. In this manner, the incident light beam passes through only a short thickness of the solution prior to reaching the focal point, and the emitted light that travels back along the incident beam&#39;s axis to the detector travels a similarly short distance through the solution. In general, the Raman spectroscopy system  400  of this new embodiment, as shown in FIG. 16, includes a monochromatic light source  410 , a detector  412 , a lens  414 , and a spectrum processor  416 .  
     [0079] More specifically, with reference to FIG. 16, a sample of an electroless or an electrolytic plating bath  420  is illuminated by a light beam  422  from the monochromatic light source  410 . The light source  410  may be a diode or gas laser, a filtered high intensity light source, or other source providing a collimated, monochromatic beam of light. The beam  422  passes through a lens  414  which focuses the beam  422  at a focal point  424  very close to the periphery or surface or interface  426  of the plating bath sample  420 . In a preferred embodiment, the monochromatic beam  422  penetrates less than 1 cm into the bulk of the sample  420  before reaching the focal point  424 . More preferably, the beam  422  penetrates less than 1 mm into the sample  422  before reaching the focal point  424 . Even more preferably, the focal point  424  lies less than 50 μm inside the sample  422 . At the focal point  424 , the light beam  422  interacts with the sample  420  to produce an emission of Raman scattered light  430 . This scattered light is emitted in all directions from the focal point  424 . The light beam  422  may travel from the light source  410  to the lens  414  directly, as shown in FIG. 16, or alternatively it may be conveyed via one or more fiber optic cables (not shown). Similarly, scattered light  430  may be transmitted directly from the sample  420  to the detector  412 . Alternatively, the scattered light  430  may be focused onto one end of one or more fiber optic cables (not shown) such that it is conveyed to the detector  412 . In another alternative embodiment, one or more mirrors and/or beam splitters (not shown) may be employed to route the incident light beam  422  and/or the scattered light  430  between the light source  410  and the lens  414  and between the focal point  424  and the detector  412 , respectively. It is preferred that extraneous light is excluded from the sample to avoid interference with detection of scattered light from the sample  420 .  
     [0080] The detector  412  quantifies the intensity of the scattered light  430  as a function of wavelength and outputs this information to the spectrum processor  416 . It is preferred to use a compact monochromator in either static or scanning mode to separate the scattered light  430  into one or more spectral wavelengths prior to quantification on the detector. It is also preferred to pass the scattered light through a filter  434  to remove back-scattered or reflected light at the wavelength of the incident light beam  422 . The filter  434  is preferably a reject filter at the wavelength of the incident light beam  422 . In this manner, the scattered light intensity is not swamped by the several orders of magnitude more intense incident light. In one embodiment, the detector  412  is one or more CCD arrays. Alternatively, the detector  412  is one or more diode array detectors. The detector  412  quantifies the scattered light using the peak height and/or peak area of each wavelength peak in the spectrum and outputs this information to the spectrum processor  416 .  
     [0081] The spectrum processor  416  comprises one or more computer systems and interface electronics for coupling the computer system to the detector  412  and converting the output signals from the detector  412  into data that is readable by the computer system. Output data from the detector  412  is analyzed by the computer system using data from one or more calibration experiments to convert the peak heights and/or areas of the emitted light spectrum to a concentration value for one or more analytes in the plating bath sample  420 . In general, a series of standard solutions of varying concentration, each containing one of the analytes in a plating bath, is analyzed to determine the peak hight and/or area of one or more peaks in the standard spectrum for each analyte as a function of wavelength. Based on these predictive models, which may be linear calibration curves, or possibly more complex mathematical relationships between one or more peak metrics and the analyte concentration, the spectrum processor  416  converts measured peak data from the bath sample to concentration data for each of the plurality of analytes in the plating bath. For more complicated bath spectra in which more than one analyte peak overlap in the same region of the bath spectrum, an algorithm that is capable of deconvoluting a complicated spectrum of one or more overlapping peaks from one or more analytes in a sample may be run on the computer system to obtain concentration data for the one or more analytes in the plating bath. One algorithm by which this analysis may be performed is described above in the description of the previous embodiments.  
     [0082] The sample  420  may be contained in a transparent spectroscopy vial. Alternatively, the sample  420  may be contained within a subvolume of the bath or the sample may be the entire bath. In this embodiment, a window that is transparent to the incident light and Raman scattered light is provided through which the light beam  422  passes into the sample  420  and through which the scattered light  430  passes out of the sample to the detector  412 . In a preferred embodiment, the lens is in direct contact with the sample. In this embodiment, the lens may be a ball lens as described above. If the lens is in direct contact with the sample, its focal length is preferably such that the focal point  424  is as close as possible to the lens surface in contact with the sample  420 . As noted above, the focal point  424  preferably lies less than approximately 1 cm inside the sample  420 , more preferably less than 1 mm, and most preferably less than 50 μm inside the sample  420 . If the system includes a transmission window between the lens and the sample  420 , the focal length of the lens is chosen such that it is slightly longer than the distance between the lens and the inside wall of the window or the vial such that the penetration distance into the sample is minimized as described above.  
     [0083] Because the sample volume is typically substantially smaller than that of the plating bath and because the volume that is actually irradiated at the focal point by the focused incident light beam  422  is generally even smaller, it is preferred to include one or more pumps or other mechanisms for circulating the plating bath such that the sampled portion is in equilibrium with the remainder of the bath. The use of such a circulation mechanism or comparable system is preferred when the system of the present invention is used to calculate real time changes in the concentrations of one or more analytes in the plating bath. Plating baths may be extremely caustic and/or acidic. As such, it is preferable for the lens and/or transparent window as well as any other components of the Raman spectroscopy system that are in contact with the bath to be constructed of one or more materials that are resistant to acidic, caustic, or other chemical attack.  
     [0084] In an additional embodiment, the system of the present invention may be used in conjunction with an adjustable focal length ball lens probe assembly as described above and shown in FIG. 5. In this embodiment, the ball lens probe is operated such that the focal point  424  of the beam  422  is at the surface of the lens, which is also the lens-sample interface such that the incident light has an effectively negligible path length through the sample  420 . Incident light is transmitted to the lens via one or more optical fibers and scattered light is conveyed back to a detector via one or more other optical fibers. Alternatively a system of lens and/or mirrors may be used to direct light from the incident light source and back to the detector.  
     [0085] The method and system of the new embodiments described herein are preferably operated such that real time or near real time solution constituent concentration data may be obtained. The detector and spectrum processor are preferably set to collect and process spectral data into concentration data at a frequency of greater than one analysis per five minutes. More preferably, more than approximately one sample cycle occurs per minute. More frequently collected data, such as at the rate of multiple sampling cycles or more per minute is also envisioned and included as an equivalent embodiment of the present invention.  
     [0086] In another embodiment of the present invention, the Raman spectroscopy analysis system described above is incorporated into a chemical autodosing system such that feedback from the analysis subsystem to a process control subsystem is used to maintain the concentrations of one or more plating bath additives within a desired range. In this embodiment, one or more additive reservoirs are provided. Each reservoir contains a supply of one of the plating bath additives, either in solution, or in some other meterable form. A pump or other comparable supply metering device such as a metering valve or the like is coupled to each reservoir for supplying each additive to the plating bath at a controllable rate. The process control subsystem may be a computer or other programmable device executing one or more algorithms. These algorithms take as inputs the outputted concentration data from the analyzer subsystem. Based on the concentration data, the process control subsystem provides commands to the metering devices to supply additives to the bath as needed to maintain the desired concentrations for optimized bath performance. In this embodiment, it is preferred that the analyzer subsystem operates on a continuous basis. As such, a ball lens probe positioned within the bath is a preferred embodiment of the analyzer system. Alternatively, one or more sampling pumps may be used to mix the plating bath and to circulate a sample of the bath solution past a window through which the incident excitation light beam is focused as described above. Alternatively, a flow cell comprising a transparent vial into which the beam is focused may be used.  
     [0087] In a further embodiment of the present invention, a method is provided for determining analyte concentrations in a metal plating bath. As summarized in FIG. 17, the method  500  includes the following steps. Individual standard Raman emission spectra are collected for each of the analytes of interest in the plating bath, as well as for other components of the bath that are expected to be present  502 . These emission spectra are collected at the chosen wavelength of the incident light to be used in analysis of the bath. A standard spectrum is collected for each analyte at one or more concentrations. Based on the standards, one or more predictive models are developed  504 . The simplest predictive model is a linear calibration curve. However, more complex mathematical relationships relating one or more peak metrics such as height and/or area to the concentration of a given analyte may be applied. A beam of incident light at the chosen wavelength is used to excite a Raman emission spectrum from a sample of the plating bath  506 . The beam is focused such that its penetration into the bath sample is minimized  510 . It is preferred that the penetration depth of the focused beam is less than approximately 1 cm. More preferably, the focal point of the beam is less than approximately 1 mm and most preferably less than approximately 50 μm inside the bulk of the sample.  
     [0088] Raman scattered light emitted from the sample is detected using a detector which may be a CCD array or other diode array detector  512 . This detected light is converted into a bath emission spectrum  514  which is then analyzed using one or more of the predictive models discussed above to convert the observed peak heights and/or areas into analyte concentration values  516 . For more complicated bath spectra in which more than one analyte peak overlap in the same region of the bath spectrum, an algorithm that is capable of deconvoluting a complicated spectrum of one or more overlapping peaks from one or more analytes in a sample may be run on the computer system to obtain concentration data for the one or more analytes in the plating bath. One algorithm by which this analysis may be performed using the ratios of peak heights and/or areas obtained from standard spectra of individual analytes is described above in the description of the previous embodiments.  
     Experimental Example 4  
     [0089] An electroless plating bath formulation containing complexed divalent cobalt, tungstate, and hypophosphite was analyzed using a 532 nm Chromex Sentinel Raman System (Chromex, Inc., Albuquerque, N. Mex.). The system features self-calibration to a neon discharge and an Andor CCD detector. The 80 mW diode laser system utilized a submersible probe with adjustable focal point as supplied by Infotonics, Inc. (Norwood, Mass.).  
     [0090] Because the electroless plating bath absorption maximum was 529 nm—very close to the wavelength of the incident light beam—the focal point of the submersible adjustable probe was adjusted to minimize absorption effects and thus maximize signal intensity. The bath was then analyzed for hypophosphite concentration and a linear calibration curve was produced from a series of samples. A personal computer running commercially available spectral analysis software packages (Unscrambler by CAMO Technologies and GRAMS/AI and PLSplus/IQ by Thermo Galactic) were used for data analysis and peak height and area determination.  
     Experimental Example 5  
     [0091] An electroless plating bath formulation containing complexed divalent cobalt, tungstate, and hypophosphite was analyzed using a 532 nm Chromex Sentinel Raman System (Chromex, Inc., Albuquerque, N. Mex.). The system features self-calibration to a neon discharge and an Andor CCD detector. The 80 mW diode laser system utilized a submersible ball lens probe with a fixed focal point at the lens solution interface. The probe was a modified version of the submersible probe as supplied by Infotonics, Inc.  
     [0092] Because the electroless plating bath absorption maximum was 529 nm—very close to the wavelength of the incident light beam—a ball lens probe with minimal penetration of incident light into the analyte solution was used. It was found that the ball lens probe neutralized the absorption effects and thereby maximized the signal intensity. The bath was analyzed for hypophosphite concentration and a linear calibration curve was produced from a series of samples. A personal computer running commercially available spectral analysis software packages (Unscrambler by CAMO Technologies and GRAMS/AI and PLSplus/IQ by Thermo Galactic) was used for data analysis and peak height and area determination.  
     Experimental Example 6  
     [0093] An electroless plating bath formulation containing complexed divalent cobalt, tungstate, and hypophosphite was analyzed using a 532 nm Chromex Sentinel Raman System (Chromex, Inc., Albuquerque, N. Mex.). The system features self-calibration to a neon discharge and an Andor CCD detector. The 80 mW diode laser system utilized an external sample holder with integrated probe. Borosilicate glass vials were used to contain the bath samples. The external sample holder was adjusted so that the incident beam focal point was located just within the wall of the glass vial, at the solution interface. In this fashion, solution absorption effects are minimized and signal intensity is maximized. The bath was analyzed for hypophosphite concentration and linear calibration curve was produced from a series of samples. FIG. 18 shows the calibration curve. The results of prediction of hypophosphite concentration in unknown plating bath samples are shown in Table 2. A personal computer running commercially available spectral analysis software packages (Unscrambler by CAMO Technologies and GRAMS/AI and PLSplus/IQ by Thermo Galactic) was used for data analysis and peak height and area determination, as well as the prediction of concentrations.  
               TABLE 2                          Raman Prediction of Hypophosphite Concentration                             Unknown Sample   g/l   Raman Predicted g/l   Difference g/l                                      4   23.35   22.363   0.987        5   23.35   22.484   0.866        6   23.35   22.504   0.846        7   23.35   22.537   0.813        8   23.35   22.628   0.722       Ave   23.35   22.5032   0.8468       sd       0.095842058   0.095842058       % error           3.626552463        9   20.4   20.537   −0.137       10   20.4   20.45   −0.05       11   20.4   20.535   −0.135       12   20.4   20.593   −0.193       13   20.4   20.571   −0.171       Ave   20.4   20.5372   0.1372       sd       0.054462831   0.054462831       % error           0.67254902       14   20.49   20.547   −0.057       15   20.49   20.689   −0.199       16   20.49   20.68   −0.19       17   20.49   20.685   −0.195       18   20.49   20.713   −0.223       Ave   20.49   20.6628   0.1728       sd       0.065963626   0.065963626       % error           0.843338214       19   21.1   20.495   0.605       20   21.1   20.568   0.532       21   21.1   20.705   0.395       22   21.1   20.775   0.325       23   21.1   20.683   0.417       Ave   21.1   20.6452   0.4548       sd       0.112219428   0.112219428       % error           2.155450237       24   21.67   21.531   0.139       25   21.67   21.736   −0.066       26   21.67   21.707   −0.037       27   21.67   21.581   0.089       28   21.67   21.73   −0.06       Ave   21.67   21.657   0.013       sd       0.094501323   0.094501323       % error           0.059990771                  
 
     [0094] The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications, embodiments, and variations are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.