Abstract:
An electrode structure configured to operate in a discharge lamp and a method to make such an electrode structure are described. The electrode structure includes an electrode head portion comprising a plurality of raised features arranged in a configuration such that an average pitch of the plurality of raised features is at least 105%. The method includes providing an electrode configured to operate in the discharge lamp and forming raised features on an electrode head portion of the electrode at an average pitch of at least 105%.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to electrode structures for discharge lamps. 
         [0002]    Electrodes in short-arc discharge lamps typically operate in a high-temperature environment. Reducing the operating temperature of the electrodes is desirable in order to reduce degradation from evaporation and extend the lifetime of the lamp. The electrode operating temperature is determined by the electrical power input, which heats electrodes, and Planck&#39;s radiation law (i.e., the electro-magnetic emission of an electrode, which results in the electrode cooling). Thus, increasing the emissivity of an electrode structure will increase the heat dissipation of the electrode. 
         [0003]    Because electrodes are routinely operated near the melting point of the electrode material (e.g., tungsten), the emissivity of an electrode structure is important parameter in discharge lamp design. For example, high-power DC lamps used in microlithography include massive anodes that are coated or microstructured to increase emissivity. Such anodes are expensive and not practical in lower-power, short-arc lamps. This technique also has the drawback that neither the coating or microstructure can be applied as close to a front portion of an electrode as desired because a non-tungsten coating will either melt or sublimate at temperatures approaching the tungsten melting point. Moreover, re-crystallization and surface diffusion will destroy tungsten microstructures over time. 
         [0004]    Massive anodes are also not practical in some lamps because electrode size restrictions of many discharge lamps. That is, many discharge lamps are designed to accommodate only electrodes with small diameters or widths. Thus it is not always possible to reduce the electrode operating temperature at a given electrical power input by greatly increasing the size of an electrode. 
         [0005]      FIG. 1  shows a conventional electrode structure for use in a ultra-high-pressure mercury lamp. Coil  102  is tightly wound around the electrode shaft portion  104  in one or more layers to form electrode head portion  106 . Front portion  108  is condensed by over-melting the ends of coil  102 . The electrode temperature is determined by the size of electrode  100 , which in turn is determined by the length of coil  102 , the number of coiled layers, and the diameter (or width) of the wires of coil  102 . 
         [0006]      FIG. 2  shows another conventional electrode structure for use in a ultra-high-pressure mercury lamp. Coil  202  is tightly wound around electrode head portion  204 . Head portion  204 , front portion  206 , and shaft portion  208  are formed by shaping a conventional massive electrode material such as tungsten with conventional machining techniques such as lathing or grinding. Electrode  200  has better emissivity than electrode  100  because of the shape of front portion  206  and coil  202  is wrapped around electrode head portion  204 , electrode head portion  204  being massive and can effectively conduct the heat generated in the front portion  206  to coil  202 . 
         [0007]    As noted above, however, the amount an electrode size may be increased is limited in many applications for practical and/or commercial reasons. 
       SUMMARY 
       [0008]    Embodiments provide apparatuses and methods for reducing the electrode operating temperature without increasing the size of the electrode and without adding significant costs to the electrode manufacturing process. 
         [0009]    Embodiments include electrode structures that may be implemented in a discharge lamp. Embodiments include electrode structures that may be implemented in AC and/or DC discharge lamps. 
         [0010]    Some embodiments include an electrode structure configured to operate in a discharge lamp, the electrode structure including an electrode head portion and a coil, wherein the coil is wrapped around the electrode head portion at an average pitch of at least 105%. 
         [0011]    Some embodiments include an electrode structure configured to operate in a discharge lamp, the electrode structure comprising an electrode head portion comprising a plurality of raised features arranged in a configuration such that an average pitch of the plurality of raised features is at least 105%. 
         [0012]    Some embodiments include a discharge lamp including two electrode structures, wherein at least one of the two electrode structure includes an electrode head portion and a coil. The coil is wrapped around the electrode head portion at an average pitch of at least 105%. 
         [0013]    Some embodiments include a method of manufacturing an electrode structure for a discharge lamp. The method includes providing an electrode configured to operate in the discharge lamp and forming raised features on an electrode head portion of the electrode at an average pitch of at least 105%. 
         [0014]    These and other features of the invention will be better understood when taken in view of the following drawings and a detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which: 
           [0016]      FIG. 1  shows a conventional electrode structure for use in a ultra-high-pressure mercury lamp; 
           [0017]      FIG. 2  shows another conventional electrode structure for use in a ultra-high-pressure mercury lamp; 
           [0018]      FIG. 3  shows an electrode structure according to an embodiment; 
           [0019]      FIG. 4  is a graph showing the emissivity gain of electrode structures according to embodiments over a conventional electrode structure; 
           [0020]      FIG. 5  is a bar graph showing electrode operating temperature measurements of a conventional electrode structure and an electrode structure according to an embodiment; 
           [0021]      FIG. 6  shows an alternative electrode structure according to an embodiment; 
           [0022]      FIG. 7  shows an alternative electrode structure according to an embodiment; and 
           [0023]      FIG. 8  is a flow chart for a method of manufacturing an electrode structure according to an embodiment. 
       
    
    
     DESCRIPTION 
       [0024]    The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. 
         [0025]    As used herein, “width” may be the width of any shaped structure, including round wires. Thus, “diameter” may be substituted with “width”. 
         [0026]    As used herein, “head portion” will be understood to mean the portion of an electrode that raised features are attached to or formed into for the purposes of increasing emissivity of an electrode. 
         [0027]    Raised features include, but are not limited to, coils, groove structures, formations formed from etching, and/or a round, oval, or polygon-shaped wire or plurality of wires. 
         [0028]      FIG. 3  shows an electrode structure according to an embodiment. Electrode  300  includes single-layer coil  302  wound around electrode head portion  304 . Electrode head portion  304  is adjacent to electrode shaft portion  306 . 
         [0029]    In some embodiments, coil  302  may be formed from tungsten wire. The emissivity of the electrode is increased by winding coil  302  at an optimized pitch around electrode head portion  304 . This increases the natural emissivity of electrode  300  by a factor of 65% above a flat surface and by 20% above a tightly wound coil (e.g., coil  202  of  FIG. 2 ). In some embodiments, the coil diameter or width of coil  302  is manufactured as small as possible in order to increase the heat conduction form the heat&#39;s origin at front portion  308  to the high emissive area of coil  302 . In some embodiments, a maximum preferred coil diameter is 0.2 mm. 
         [0030]    The optimal pitch found in Finite Element Method simulations was about 140%, although other optimal pitches may be found depending on the coil material&#39;s emissivity. In general, significant improvements were found within a pitch range of 
         [0000]      ([(1.35±0.15)×Wire Width]/Wire Width)×100.
 
         [0031]    As used herein the “pitch” is defined as the distance between two raised features (e.g., wire center to wire center) divided by the width of the raised features, expressed as a percentage. Thus, a pitch of 100% indicates that adjacent raised features are touching and a pitch of 200% indicates that consecutive raised features are spaced apart a distance equal to the width of the raised feature. 
         [0032]    The term “average pitch” will be understood to mean the sum of the distances between consecutive raised features divided by the number of pairs of raised features. For example, a coil wrapped around an electrode head portion three times will have two distances to sum and two pairs of raised features. Average pitch may also be calculated using other methods such as the median or mode. 
         [0033]      FIG. 4  is a graph showing the emissivity gain of electrode structures according to embodiments over a conventional electrode structure. As seen from graph  400 , the spacing of coils leads to a significantly reduced electrode temperature compared to a tightly-wound coil design. As the pitch increases beyond 140-150%, however, the emissivity gain begins to diminish. In a tungsten electrode embodiment for ultra-high pressure lamps that included a pitch of 130%, the operating temperature on the front area was reduced by 50° K compared to a tight winding electrode structure. The lower temperature resulted in a 50% reduced evaporation rate over a tight winding electrode structure. 
         [0034]      FIG. 5  is a bar graph showing electrode operating temperature measurements of a conventional electrode structure according to electrode  200  of  FIG. 2  and an electrode structure according electrode  300 , with coil  302  wound at a pitch of 130%. 
         [0035]    Ultra-high pressure mercury lamp test samples were produced with a conventional electrode structure as a first electrode and an embodiment electrode structure as second electrode in the same burner to ensure that both electrodes were operated under identical conditions. 
         [0036]    Six lamps were investigated. Each of the lamps are designated in graph  500  by unique hatching patterns, wherein the hatching patterns match for the two electrodes in each lamp. The temperatures on the electrode surface were measured with IR pyrometry, excluding areas on the electrode where the IR signal is superposed by plasma radiation. 
         [0037]    Graph  500  shows the electrode temperatures normalized to the average operating temperature of the conventional coil electrodes. The average operating temperature of the embodiment coils were reduced by more than 2%. Because the tungsten evaporation rate is exponentially related to temperature, the tungsten evaporation rate is halved with an average temperature reduction of approximately 2%. 
         [0038]    Thus lamps with an electrode structure according to an embodiment, will last longer at a given temperature or can be operated at higher temperatures over conventional electrode structures. Moreover, manufacturing electrode structures according to an embodiment will typically entail inexpensive modifications to existing electrode manufacturing equipment. 
         [0039]      FIG. 6  shows an alternative electrode structure according to an embodiment. Electrode  600  includes plurality of wires  602  attached to electrode head portion  604  in axial sections. Electrode head portion  604  is adjacent to electrode shaft portion  606 . 
         [0040]    Plurality of wires  602 , if made of tungsten, is expected to have properties similar to coil  302  of  FIG. 3 , and thus the optimized pitch of plurality of wires  602  would be around 140% with a groove width of approximately 0.2 mm. 
         [0041]      FIG. 7  shows an alternative electrode structure according to an embodiment. Electrode  700  includes raised groove features  702  formed as a result of grooving, carving, or etching electrode head portion  704 . Groove features  702 , if electrode head  204  is made of tungsten, is expected to have properties similar to coil  302  of  FIG. 3 , and thus the optimized pitch of groove structure  702  would be around 140% with a groove width of approximately 0.2 mm. 
         [0042]    It will be understood that the electrode structures shown in  FIGS. 3 ,  6 , and  7  are only three possible electrode structures, and many more are within embodiments of the invention. For example, wire applied in a coil, as shown in  FIG. 3 , could also be applied in concentric sections. Similarly, groove structure  702  of  FIG. 7  could also take the form of circumferential slots machined by micro-machining techniques at an optimized pitch, depth, and width. The slots could be applied near the tip and/or elsewhere. Other machined shape variations may include cork screw slots, axial slots, or hole patters. 
         [0043]      FIG. 8  is a flow chart for a method of manufacturing an electrode structure within an embodiment. At  802 , an electrode is provided. At  804 , a wire is attached to the front portion of the electrode. At  806 , the wire is coiled around the electrode head portion at an average pitch of at least 105%. At  808 , method  800  ends. 
         [0044]    While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.