Patent Publication Number: US-2004040842-A1

Title: Electrochemical analytical apparatus and method of using the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of Invention  
       [0002] The present invention relates to an analytical apparatus for analyzing electrochemical deposition solutions.  
       [0003] 2. Related Art  
       [0004] One technique used extensively for plating metal onto a substrate is by electrochemical deposition process, which uses an electrochemical deposition solution (hereinafter “ECD solution”) containing metal components in ionic form.  
       [0005] The metallurgical properties of the metal deposit depend on the composition of the ECD solution. For example, the concentration of metal ions in the ECD solution affects the plating rate and the plating potential. Moreover, most ECD solutions contain one or more organic additives, which affect the plating performance of the ECD solutions, by improving leveling and throwing power. Throwing power refers to the ability to provide uniform plating in through holes for interconnections as well as on the top surface of multilayer printed circuit boards. Therefore, the concentration of organic additives in the ECD solutions are crucial in optimizing characteristics or properties of plated metallurgy, such as ductility, tensile strength, and solderability. In order to ensure consistent deposition of metal films, it is necessary to accurately determine the concentration of the metal ions and the organic additives in the ECD solution throughout the metal plating process, so as to maintain the concentration of such metal ions and organic additives within a specified range.  
       [0006] Various analytical tools have been designed in the past for monitoring compositions of ECD solutions. However, none of the conventional ECD analytical tools provides temperature monitoring and/or controlling mechanism for reducing temperature fluctuations during the composition analysis of the ECD solutions.  
       [0007] Such temperature fluctuations affect the measurement of component concentrations in the ECD solutions. For example, the determination of organic additive concentration, based on a standard addition method without temperature control, results in an error rate of 8%, when the temperature fluctuation is about ±3° C.  
       [0008] It is therefore an object of the present invention to provide precise temperature control during the analysis process of the ECD solutions, by using an electrochemical analytical apparatus with temperature management system for monitoring and/or controlling the measurement temperature during the ECD analysis.  
       [0009] Moreover, most commercially available ECD analytical tools use rotating disk electrodes (RDEs) as testing electrodes, which provide a high flux of ECD solution towards the electrode surface and result in stronger analytical signals (e.g., plating current or plating potential). Such RDEs rotate at an average rotating speed above 1000 rpm, and are therefore vulnerable to mechanical breakdowns.  
       [0010] It is therefore another object of the present invention to provide an electrochemical analytical apparatus comprising RDEs of enhanced mechanical strength and durability, which can be used for analyzing ECD solutions over an extended period of time so as to increase the reproducibility and reliability of the analytical data.  
       [0011] It is a further object of the present invention to provide an electrochemical analytical apparatus designed and constructed to minimize cross-contamination between various analytes during different measurement cycles.  
       [0012] Other objects and advantages will be more fully apparent from the ensuring disclosure and appended claims.  
       SUMMARY OF THE INVENTION  
       [0013] The present invention in one aspect relates to an electrochemical analytical apparatus for analyzing an electrochemical deposition solution, comprising a testing electrode, and a temperature detector attached thereto for monitoring temperature of the testing electrode.  
       [0014] The present invention in another aspect relates to an electrochemical analytical apparatus for analyzing an electrochemical deposition solution, comprising a rotating disk electrode, having at least one mercury contact switch for establishing electrical connection between such rotating disk electrode and other stationary components of the electrochemical analytical apparatus.  
       [0015] The present invention in a further aspect relates to an electrochemical analytical apparatus for analyzing an electrochemical deposition solution, comprising multiple analytical cells, wherein each analytical cell is used for analyzing one analyte contained in the electrochemical deposition solution.  
       [0016] The present invention in a still further aspect relates to an electrochemical analytical apparatus for analyzing an electrochemical deposition solution, comprising:  
       [0017] (a) an analytical cell comprising a liquid inlet, a sample solution holder, and a liquid outlet, wherein a sample electrochemical deposition solution is flew into such analytical cell via the liquid inlet and out of such analytical cell via the liquid outlet, and the sample solution holder comprises a front wall and a back wall placed in close proximity so as to hold the sample electrochemical deposition solution in form of a liquid film;  
       [0018] (b) an irradiation light source for irradiating light onto the liquid film held by the sample solution holder;  
       [0019] (c) a light detector for detecting light transmitted or reflected by the liquid film; and  
       [0020] (d) a computational device connected with the light detector, for determining concentration of at least one target species contained by the sample electrochemical deposition solution, based on absorbance of the irradiated light by the sample electrochemical deposition solution.  
       [0021] Additional aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022]FIG. 1 is a perspective view of a testing electrode with a temperature detector and a heating element attached thereon, and a microcontroller that receives signal from the temperature detector and uses the heating element to adjust the temperature of the testing electrode, based on such signal.  
     [0023]FIG. 2 is a perspective view of a testing electrode with a resistance temperature detector (“RTD”) attached thereon, while the RTD is connected to a remote computer, which corrects the concentration measurements based on the temperature measured by the RTD to eliminate the effect of temperature fluctuations.  
     [0024]FIG. 3A is a perspective view of a rotating disk electrode comprising a single rotating disk with a temperature detector of bifilar winding attached thereon.  
     [0025]FIG. 3B shows a cross-sectional view of the rotating disk electrode of FIG. 3A from line I-I.  
     [0026]FIG. 4A is a perspective view of a rotating disk electrode comprising two electrically connected rotating disks, with a temperature detector of bifilar winding attached to one of such rotating disks.  
     [0027]FIG. 4B shows a cross-sectional view of the rotating disk electrode of FIG. 4A from line II-II.  
     [0028]FIG. 5A is a perspective view of a rotating disk electrode assembly comprising multiple electrically isolated rotating disks, with a temperature detector of bifilar winding attached to one of such rotating disks.  
     [0029]FIG. 5B shows a cross-sectional view of the rotating disk electrode of FIG. 5A from line III-III.  
     [0030]FIG. 6 is a perspective view of a prior art rotating disk electrode with a conventional contact brush for electric connection thereto.  
     [0031]FIG. 7A is a perspective view of a rotating disk electrode according to one embodiment of the present invention, having a mercury contact switch for electric connection thereto.  
     [0032]FIG. 7B is a perspective view of a rotating disk electrode according to another embodiment of the present invention, having a self-sealing mercury contact switch for electric connection thereto.  
     [0033]FIG. 8A is a top view of an electrochemical solution analyzer with two measuring cells separated by a built-in dividing wall.  
     [0034]FIG. 8B is a cross-sectional view of the electrochemical solution analyzer of FIG. 8A from line IV-IV.  
     [0035]FIG. 9 is a perspective view of an Infra-Red (IR) light absorption-based electrochemical solution analyzer, which measures the absorption of IR light by a thin film of the ECD sample solution for determining concentration of organic additives therein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF  
     [0036] In order to reduce temperature fluctuations during the measurement of electrochemical deposition solutions, so as to enhance the reliability and reproducibility of the measurement results, the present invention provides an electrochemical analytical apparatus with a temperature management system, which limits the temperature fluctuations to below ±2° C.  
     [0037] In one embodiment of the present invention, the electrochemical analytical apparatus comprises a testing electrode that has a temperature detector attached thereto. In FIG. 1, the testing electrode  10  comprises a platinum electrode tip  12 , and a platinum temperature detector  14  integrated with the platinum electrode tip  12 . It is preferred that the platinum electrode tip  12  and the platinum temperature detector  14  are made of the same piece of platinum metal. Alternatively, the platinum temperature detector  14  and the platinum electrode tip  12  can be manufactured separately and subsequently integrated together, via welding, adhering, or other form of binding. The temperature at the surface of the electrode  10  is measured directly by the temperature detector  14 , due to the electrical and thermal connection between the platinum electrode tip  12  and the platinum temperature detector  14 .  
     [0038] Moreover, the testing electrode  10  of FIG. 1 further comprises a heating element  16 , which is controlled by a microcontroller  18 , for adjusting the temperature of the platinum electrode tip  12 . A microcontroller is an inexpensive single-chip computer, which is capable of storing and running a program, and the PIC® microcontrollers manufactured by Microchip Technology (Chandler, Ariz.) are preferably employed herein for the purpose of controlling the heating element  16  of the present invention. Specifically, the microcontroller is connected with the platinum temperature detector  14 , so as to receive temperature measurement results therefrom and to adjust the temperature of the platinum electrode tip  12  through the heating element  16 , based on such temperature measurement results.  
     [0039] In another embodiment of the present invention, as shown in FIG. 2, a testing electrode  20  comprises a metal electrode tip  22 , and a resistance temperature detector (“RTD”)  24  integrated therewith. The temperature detector  24  is communicatively connected with a computational device  18 , so as to send temperature measurement results to such computational device  18 . The computation device  18  then mathematically corrects the concentration measurements, based on the electrode temperature measured by the RTD, so as to eliminate the effect of temperature fluctuations on the concentration measurements.  
     [0040] The operating current and voltage of the temperature detector in the present invention float with those of the testing electrode. In such manner, there is not interference with the operating of the electrode current.  
     [0041] In FIGS. 1 and 2, both temperature detectors are electrically and thermally connected to the testing electrodes. Alternatively, the temperature detectors may be integrated with the testing electrodes through bifilar winding, in which the temperature detectors are electrically insulated from but are thermally connected to the testing electrodes by an inert, electrically non-conductive, and thermally conductive material, such as highly directionally oriented graphite, glass, ceramic, etc. The bifilar winding, formed preferably by platinum wire spirals around the testing electrode, reduces the effective enclosed area of the coil, minimizes magnetic (or noise) pickup, and therefore enhances the signal-to-noise ratio.  
     [0042]FIG. 3A shows a perspective view of a rotating disk electrode  30  with a single rotating disk  32  made of platinum, to which a temperature detector  34  with bifilar winding  36  is attached. Specifically, the temperature detector  34  is attached to the rotating disk  32  by epoxy potting material  38 , and the rotating disk  32  is structurally integrated with other parts of the rotating disk electrode  30  by a mechanically stable and inert material  37 . The material  37  preferably is a fluorocarbon-based polymer, more preferably is polychlorotrifluoroethylene (“PCTFE”), which is commonly referred to as Kel-F®.  
     [0043]FIG. 3B shows a cross-sectional view of the electrode surface  31  of the rotating disk electrode  30 , from the line I- 1 .  
     [0044]FIG. 4A shows a perspective view of a rotating disk electrode  40  with two rotating disks  42 A and  42 B, which are electrically connected together by the connector  42 . The two rotating disks  42 A and  42 B are both off center in relating to the rotating disk electrode  40 , providing better laminar flow of the electrochemical deposition solution. A temperature detector  44  is attached to one rotating disk  42 B, with bifilar winding  46 . The temperature detector  44  is likewise attached to the rotating disk  42 B by epoxy potting material  48 , and the rotating disks  42 A,  42 B, and their connector  42  are structurally integrated with other parts of the rotating disk electrode  40  by a mechanically stable and inert material  47 .  
     [0045]FIG. 4B shows a cross-sectional view of the electrode surface  41  of the rotating disk electrode  40 , from the line II-II.  
     [0046]FIG. 5A shows a perspective view of a rotating disk electrode  50  with multiple rotating disks  52  and  53 , which are electrically isolated to each other. A temperature detector  54  is attached to one rotating disk  53 , with bifilar winding  56 . The temperature detector  54  is likewise attached to the rotating disk  53  by epoxy potting material  58 , and the rotating disks  52  and  53  are structurally integrated with other parts of the rotating disk electrode  50  by a mechanically stable and inert material  57 .  
     [0047]FIG. 5B shows a cross-sectional view of the electrode surface  51  of the rotating disk electrode  50 , from the line III-III.  
     [0048] The rotating disk electrode  50  of FIG. 3 provides a device analogous to the rotating ring disk electrode, and can therefore be used for kinetic studies in place of the conventional rotating ring disk electrode.  
     [0049] The rotating disk electrodes as described hereinabove provides an intimate chemical bond between the epoxy potting material, the metal rotating disk, and the Kel-F® material. Such chemical bond enhances the electrode seal and avoids seepage of electrolytes into the rotating disk electrode, which is particularly important for maintaining a constant electrode tip surface area and a constant current density.  
     [0050] The electrochemical analytical apparatus of the present invention may use other temperature monitoring/controlling mechanisms than the temperature detector as described hereinabove, to achieve improved temperature control of the analytical process.  
     [0051] For example, such electrochemical analytical apparatus may be placed in a chamber equipped with adjustable heating elements, so that operating temperature within such chamber is strictly controlled within a predetermined range, with limited fluctuations.  
     [0052] Alternatively, metal blocks or metal tubes connected to an external heating element or thermo-controller can be used in analytical cells of such electrochemical analytical apparatus, for conveying thermal energy thereto so as to keep the temperature of the analytical cells within a predetermined range, with limited fluctuations.  
     [0053] Alternatively, the metal electrode of common ionic species can be directly connected to an external heating element or thermo-controller, and used concurrently as a heating element to supply thermal energy to the electrochemical deposition solutions analyzed by such electrochemical analytical apparatus, for the purpose of keeping the temperature of the analytical apparatus within a predetermined range, with limited fluctuations.  
     [0054] It is clear from experimentation that the measurement results obtained by electrochemical analytical apparatuses without any temperature control mechanism have an average error rate of 8%, due to temperature fluctuations of about ±3° C. That means that for each 1.5° C. fluctuation in temperature, there is a measurement error equivalent to 2 ml of organic species per liter of sample electrochemical deposition solution being measured.  
     [0055] By providing an electrochemical analytical apparatus with temperature management system, the present invention intends to limit temperature fluctuations within ±2° C., more preferably within ±1° C., and most preferably ±0.5° C., so as to reduced the error rate caused by such temperature fluctuations.  
     [0056] Conventional electrochemical analytical apparatus comprising rotating disk electrodes employs contact brushes to electrically connect the continuously rotating electrode with other stationary components of said analytical apparatus, by passing electrical current from the stationary components to the rotating disk electrode through the contact brushes.  
     [0057]FIG. 6 shows a contact brush  67  used in conventional electrochemical analytical apparatus. The shaft  64  of a rotating disk electrode is driven by a rotatory actuator  62 , so as to engage in a continuously rotating motion. The contact brush  67  is fixed at one end to a stationary component  66  of the electrochemical analytical apparatus, while the other end of such contact brush  67  directly contacts the rotating shaft  64 , so as to form an electrical connection therewith without hindering the rotating of the shaft  64 . Electrically current can be passed to the rotating shaft  64 , from an electrical source  68  through the stationary component  66  and the contact brush  67 .  
     [0058] However, since most of the rotating disk electrodes used for electrochemical analysis purposes are operated at very high rotating speeds, usually above 1000 rpm, said contact brush becomes shorter and shorter during operation, due to constant abrasion between the contact brush and the rotating disk electrode. The electrical connection established by the contact brush therefore becomes unreliable after an extended period of time and is vulnerable to disconnection, which forces the electrochemical analysis process to be stopped in order to allow replacement of the shortened contact brush. The analytical data so obtained is therefore irreproducible, and does not satisfy the strict reproducibility requirements generally imposed by the semiconductor industry.  
     [0059] The present invention therefore provides a novel rotating disk electrode assembly that is electrically and mechanically robust for continuously operation at high rotating speed.  
     [0060] Specifically, such rotating disk electrode assembly comprises a mercury contact switch, in place of the conventional contact brush, for establishing electrical connection between the rotating disk electrode and other stationary components of the analytical apparatus.  
     [0061]FIG. 7A shows one type of mercury contact switch useful for purpose of practicing the present invention, according to one preferred embodiment. The rotating shaft  74  of a rotating disk electrode is driven by a rotatory actuator  72 , so as to engage in a continuously rotating motion. A stationary component  76  of the electrochemical analytical apparatus comprises a contact extrusion  76 A, which is inserted into a recess  73  on the rotating shaft  74  of the rotating disk electrode. The contact extrusion  76 A does not directly contact the recess  73  or any other part of the rotating shaft  74 . Instead, the recess  73  contains a conductive metal liquid  75 , preferably mercury, which establishes an indirect electrical connection between the stationary component  76  and the rotating shaft  74 , so that electrical current from an electrical source  78  can be passed to the rotating shaft  74 , through the stationary component  76 , its contact extrusion  76 A, and the conductive metal liquid  75  that is in contact with the rotating shaft  74 . Since the contact extrusion  76 A of FIG. 7A does not directly contact the recess  73  and therefore does not provide any sealing therefor, sealing caps  77 A and  77 B are provided herein, for the purpose of sealing the recess  73  and preventing the conductive metal liquid  75  from spilling or contaminating other part of the electrochemical analytical apparatus.  
     [0062] Alternatively, the stationary component may comprise a contact extrusion that functions concurrently as a sealing for the recess on the rotating shaft, therefore avoiding use of separate sealing caps. FIG. 7B shows a stationary component  76 ′ that comprises a contact shaft  76 A′, which directly contacts the recess  73  peripherally, so as to seal the recess  73  and to prevent escape of the conductive metal liquid  75  therefrom. The contact shaft  76 A′ supports a contact extrusion  76 B′, which dips into the conductive metal liquid  75  for establishing an electrical connection between the stationary component  76 ′ and the rotating shaft  74 .  
     [0063] The contact shaft  76 A′ may be coated with an abrasion-resistant material, such as ceramic, to provide a tight sealing that is not subject to leakage, even after extended period of operation. The use of ceramic coating effectively reduces particle formation and allows the rotating assembly to rotate reliably without hinderage for extended period of time in harsh and corrosive environments.  
     [0064] Although the above description only shows one electrical connection established by a mercury contact switch between the rotating disk electrode and a stationary component, in general practice, the rotating disk electrode assembly comprises at least two rotating connections, one for the rotating disk electrode, and the other for the temperature detector as mentioned hereinabove. More preferably, the rotating disk electrode assembly comprises three rotating connections, one for the rotating disk electrode, and the other two for both ends of a resistance temperature detector (“RTD”).  
     [0065] Another inventive aspect of the present invention involves use of separate analytical cells for analysis of different analytes, so as to avoid cross-contamination of different analytes during different analytical cycles. For example, if the electrochemical analytical apparatus is used to analyze n analytes in a sample electrochemical deposition solution (“ECD solution”), such analytical tool comprises n analytical cells, each for analysis of one analyte, so as to conduct simultaneous analysis of the analytes free of the risk of cross-contamination.  
     [0066] For example, FIG. 8A shows a cross-sectional view of an illustrative electrochemical analytical apparatus  80  used for analyzing two organic analytes (for example, accelerator and leveler) in an ECD solution. The electrochemical analytical apparatus  80  comprises an oval cavity bounded by peripheral walls  82 , wherein such oval cavity is divided into a first analytical cell  84 A and a second analytical cell  84 B, by a dividing wall  86 . Each analytical cell may comprise a testing electrode, a reference electrode, and a current source electrode for independent analysis of one analyte contained by the ECD solution. Moreover, each analytical cell may comprise a temperature control element  88 , which can be a metal block (e.g., a copper block), for adjusting the operating temperature of such analytical cell, as shown in FIG. 8B. From draining holes  87 , the sample ECD solution can be drained after each analytical cycle.  
     [0067] The peripheral walls  82  and the dividing wall  86  can be formed of a polymeric material, preferably polyolefin, and more preferably a 4-methylpentene-1 based polyolefin, commercially available as TPX® from Mitsui &amp; Co. Ltd., Japan.  
     [0068] By using a separate cell for each analyte, the cross-contamination problem persistent in the conventional designs of electrochemical analytical apparatuses can be effectively solved. Moreover, the electrochemical analytical apparatus of the present invention, having multiple analytical cells, may be used to simultaneously carry out multiple analytical cycles for multiple analytes.  
     [0069] A further inventive aspect of the present invention relates to use of a light sensitive detector for detecting light absorbance of an ECD sample solution and determining composition of such ECD sample solution, based on characteristic absorbance of various analytes in the ECD solution.  
     [0070] Specifically, an electrochemical analytical apparatus according to one embodiment of the present invention comprising an analytical cell with a liquid inlet manifold  93 A, a sample solution holder  94 , and a liquid outlet manifold  93 B. The sample solution holder  94  receives the ECD sample solution from the liquid inlet manifold  93 A and discharges such into the liquid outlet manifold  93 B, while such sample solution holder has a front wall  94 A and a back wall  94 B in close proximity to each other, so as to hold the ECD sample solution in form of a sufficiently thin liquid film  95 . An irradiation light source  92  is provided for irradiating light onto the liquid thin film  95 . The irradiating light includes, but is not limited to, infrared light, ultraviolet light, visible light, etc. Such irradiating light preferably is infrared (IR) light. A photodiode  96  is provided for detecting light transmitted or reflected by the liquid thin film  95 , and preferably the photodiode  96  is IR-sensitive. The photodiode  96  is connected to a computational device  98 , so that characteristic absorbance data of specific species in the ECD solution can be collected and sent to such computational device  98  for determining the concentration of specific species in the ECD solution. Such absorbance-based concentration determination is quick, and can be used for continuous and non-intrusive measurement of the ECD solution, while measured sample solution can still be used for electrodeposition.  
     [0071] Although the invention has been variously disclosed herein with reference to illustrative embodiments and features, it will be appreciated that the embodiments and features described hereinabove are not intended to limit the scope of the invention, and that other variations, modifications and other embodiments will suggest themselves to those of ordinary skill in the art. The invention therefore is to be broadly construed, consistent with the claims hereafter set forth.