Electrode structures for discharge lamps

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%.

BACKGROUND

The present invention relates generally to electrode structures for discharge lamps.

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'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.

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.

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.

FIG. 1shows a conventional electrode structure for use in a ultra-high-pressure mercury lamp. Coil102is tightly wound around the electrode shaft portion104in one or more layers to form electrode head portion106. Front portion108is condensed by over-melting the ends of coil102. The electrode temperature is determined by the size of electrode100, which in turn is determined by the length of coil102, the number of coiled layers, and the diameter (or width) of the wires of coil102.

FIG. 2shows another conventional electrode structure for use in a ultra-high-pressure mercury lamp. Coil202is tightly wound around electrode head portion204. Head portion204, front portion206, and shaft portion208are formed by shaping a conventional massive electrode material such as tungsten with conventional machining techniques such as lathing or grinding. Electrode200has better emissivity than electrode100because of the shape of front portion206and coil202is wrapped around electrode head portion204, electrode head portion204being massive and can effectively conduct the heat generated in the front portion206to coil202.

As noted above, however, the amount an electrode size may be increased is limited in many applications for practical and/or commercial reasons.

SUMMARY

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.

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.

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%.

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%.

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%.

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%.

These and other features of the invention will be better understood when taken in view of the following drawings and a detailed description.

DESCRIPTION

As used herein, “width” may be the width of any shaped structure, including round wires. Thus, “diameter” may be substituted with “width”.

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.

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.

FIG. 3shows an electrode structure according to an embodiment. Electrode300includes single-layer coil302wound around electrode head portion304. Electrode head portion304is adjacent to electrode shaft portion306.

In some embodiments, coil302may be formed from tungsten wire. The emissivity of the electrode is increased by winding coil302at an optimized pitch around electrode head portion304. This increases the natural emissivity of electrode300by a factor of 65% above a flat surface and by 20% above a tightly wound coil (e.g., coil202ofFIG. 2). In some embodiments, the coil diameter or width of coil302is manufactured as small as possible in order to increase the heat conduction form the heat's origin at front portion308to the high emissive area of coil302. In some embodiments, a maximum preferred coil diameter is 0.2 mm.

The optimal pitch found in Finite Element Method simulations was about 140%, although other optimal pitches may be found depending on the coil material's emissivity. In general, significant improvements were found within a pitch range of
([(1.35∓0.15)×Wire Width]/Wire Width)×100.

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.

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.

FIG. 4is a graph showing the emissivity gain of electrode structures according to embodiments over a conventional electrode structure. As seen from graph400, 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.

FIG. 5is a bar graph showing electrode operating temperature measurements of a conventional electrode structure according to electrode200ofFIG. 2and an electrode structure according electrode300, with coil302wound at a pitch of 130%.

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.

Six lamps were investigated. Each of the lamps are designated in graph500by 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.

Graph500shows 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%.

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.

FIG. 6shows an alternative electrode structure according to an embodiment. Electrode600includes plurality of wires602attached to electrode head portion604in axial sections. Electrode head portion604is adjacent to electrode shaft portion606.

Plurality of wires602, if made of tungsten, is expected to have properties similar to coil302ofFIG. 3, and thus the optimized pitch of plurality of wires602would be around 140% with a groove width of approximately 0.2 mm.

FIG. 7shows an alternative electrode structure according to an embodiment. Electrode700includes raised groove features702formed as a result of grooving, carving, or etching electrode head portion704. Groove features702, if electrode head204is made of tungsten, is expected to have properties similar to coil302ofFIG. 3, and thus the optimized pitch of groove structure702would be around 140% with a groove width of approximately 0.2 mm.

It will be understood that the electrode structures shown inFIGS. 3,6, and7are only three possible electrode structures, and many more are within embodiments of the invention. For example, wire applied in a coil, as shown inFIG. 3, could also be applied in concentric sections. Similarly, groove structure702ofFIG. 7could 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.

FIG. 8is a flow chart for a method of manufacturing an electrode structure within an embodiment. At802, an electrode is provided. At804, a wire is attached to the front portion of the electrode. At806, the wire is coiled around the electrode head portion at an average pitch of at least 105%. At808, method800ends.