DE20005531U1 - Short arc lamp - Google Patents

Description

Patent Trust Company

for electric light bulbs mbH., Munich

Short arc lamp . -

Technical area

The invention is based on a short arc lamp according to the preamble of claim 1. These are high-wattage mercury high-pressure discharge lamps for microlithography. A particularly critical weak point in these lamps is the electrically conductive connection between molybdenum sealing foils and electrodes of mercury arc lamps, especially with high mercury densities.

State of the art

For the connection between the sealing foil and the electrode in short-arc lamps, the foil ends are normally connected to the electrode via an electrode shaft. The shaft can be an integral part of the electrode. In the case of high-wattage, high-load lamps, it is advantageous to also insert an auxiliary part made of metal (usually molybdenum) between the foil and the electrode shaft. Sheet metal disks with thicknesses of around 0.5 mm (DE-A 196 18 967) up to a thickness of between 2 and 20 mm (EP-A 479 087) are known as auxiliary parts. The outer edge of the disks can also be bent into a cup shape.

The auxiliary part offers a safe way to attach a large number of foils to an electrode. Auxiliary parts are required if the foil ends of more than two foils are to be reliably connected to an electrode shaft. The typical application area is for lamps with more than 1000 W power and 40 A current consumption.

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The relatively massive auxiliary part can be reliably connected to the electrode shaft by soldering with platinum (EP-A 479 089), welding or pressing. In contrast, a reliable connection of the thin foils (typical thickness 20-50 µm) to the auxiliary part (for example massive sheets or disks) is difficult.

To create this connection, welding of the foil, whereby the foil is connected to the auxiliary part at several points, has become established (DE-A 196 18 967).

In its simplest form, this connection is not very mechanically stable, since the film becomes locally brittle at the high welding temperatures and the weld becomes fragile.

However, a high mechanical stability of the connection is necessary for higher power lamps that have heavier electrodes (anode with a mass of 50 g or more, especially with a mass of 100 g or more), so that the mass forces emanating from the electrodes do not cause the connection to loosen during lamp manufacture or during subsequent transport of the lamp due to vibrations or shocks.

Electrodes with these masses are used, as a rough guide, in lamps from about 1000 W, especially from 2500 W.

A simple method to increase the strength of the connection is to use a very large number of welding points (approx. 20 to 40). This requires a solid metal auxiliary part and increases the manufacturing costs significantly.

A better and more permanent connection of the molybdenum part (melting point 2610 ⁰ C) has so far been achieved with the help of an additional foil ("welding aid") made of a lower melting metal (Nb with melting point 2468 ⁰ C or Pt with melting point 1769 ⁰ C) or ductile metal (Re with

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Melting point 3180 ⁰ C or Ta with melting point 2996 ⁰ C). This welding aid is used as an intermediate layer for spot welding between a foil and the auxiliary part. This leads to a connection that is more mechanically resilient. Platinum foil was originally used as a welding aid between an auxiliary part made of molybdenum and a molybdenum foil, since platinum is still sufficiently stable at the processing temperatures of melting in quartz glass, has a much lower melting point than molybdenum and bonds well with molybdenum.

Platinum is a very inert material that allows complete electrode systems to be cleaned in reducing gases in simple tube furnaces. Decomposition of the Pt welding aid by gaseous impurities released from the electrodes during lamp operation - as is to be feared when using more reactive metals - can be ruled out.

Since platinum is expensive and cannot withstand the higher processing temperatures required for pinch seals, alternative materials are used, such as metal powder made from molybdenum and rhenium (EP-A 495 588). However, sintered bodies made from metal powder are brittle and cannot be used on curved surfaces such as a round foil melt. Another suggestion is to add carbon to a molybdenum part (EP-A 657 912). However, carbon also has an embrittling effect, so this technique is difficult to control.

A disadvantage of platinum is its lack of resistance to mercury, which occurs under certain circumstances in mercury lamps.

Extensive investigations have shown that conditions can arise during operation of mercury short-arc lamps for which the mercury

The resistance of platinum is no longer sufficient. In these cases, the welded joint dissolves due to the formation of a platinum amalgam, which leads to the failure of the lamp. The lamp goes out or shatters due to overheating in the area of the failed contact points.

An extensive analysis has shown that the platinum welding aid works reliably for lamps with a mercury filling of less than 10 mg/cm ³ Hg.

However, with increasing mercury concentrations, an increasing failure rate can be observed. A high mercury content of more than 10 mg/cm ³ , as disclosed for example in US 5,670,844, apparently promotes lamp failure due to failure of the electrical connection.

The number of failures due to the electrical failure described above is shown in the table below.

Table 1

Hg concentration Failure frequency in ppm Anode performance

[arbitrary units] weight [in g] [in W]

< 10 mg/ cm ³ < 5 (not significant) 71 2000

10-20 mg/ ^cm3 30 ±20 59 1750

20-30 mg/ ^cm3 60 ±20 73 1750

30 - 40 mg/cm ³ 50 ±10 204 2000

40 -50 mg/cm ³ 110 ±30 65 2000

The statistical analysis carried out includes lamps with more than 1000 W electrical input power and an anode weight of at least 50 g, where a welding aid is required. Table 1 shows that the higher the Hg concentration, the more the failure frequency increased. The high

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Anode mass generally requires the use of a welding aid due to the required strength. The power is correlated with the operating current.

Description of the invention

It is an object of the present invention to provide a short arc lamp according to the preamble of claim 1, which has an electrical connection with good mechanical strength, wherein the electrical connection has sufficient temperature resistance and additionally has high mercury resistance.

This object is achieved by the characterizing features of claim 1. Particularly advantageous embodiments can be found in the dependent claims.

Mercury short-arc lamps are used for photolithographic processes for the production of integrated circuits, flat screens and circuit boards. Since the investment in such manufacturing facilities is extremely high, the lamps used have to meet very high requirements in terms of explosion safety and premature failure.

As the productivity of manufacturing facilities has increased, the demands on lamp performance have continuously increased. High lamp performance is crucial for short exposure times and thus high productivity.

At the same time, the lamps are operated with high mercury concentrations above 10 mg/cm ³ to achieve high lamp efficiency.

When manufacturing mercury short-arc lamps, an electrically conductive connection must be made between the molybdenum sealing foils embedded in quartz and the electrodes located inside the lamp, which meets the following requirements:

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- high mechanical strength, since with increasing lamp power heavier electrodes exert mass forces on the connection and can damage the connections.

- sufficient temperature resistance to withstand high processing temperatures of the quartz glass in the manufacturing process and also high operating temperatures of the lamp.

Intensive investigations have shown that failures of high-wattage lamps with a high mercury content can be caused by a lack of resistance of the electrical connection to the effects of mercury.

In particular, a short-arc lamp is recommended with a discharge vessel having two ends, with an anode and a cathode facing each other in the discharge volume. The discharge volume contains a specific amount of mercury of more than 10 mg/cm ³ Hg.

At least the anode (but possibly also the cathode, although the latter is usually lighter) has a shaft which is sealed in the associated end of the discharge vessel by means of a metallic auxiliary part, which in particular has a curved contact surface, and a set of more than two foils made of molybdenum, in particular with a curved contact surface, the auxiliary part providing the electrically conductive connection between the shaft and the foil set, and the connection between the auxiliary part and the foil set being made by a welded connection. The welded connection is achieved with the aid of a welding aid which forms an intermediate layer between part of the contact surface of the auxiliary part and the contact surface of the foil set (both are advantageously curved, as in the case of a round foil melt), the welding aid being made of tantalum, niobium or rhenium.

The auxiliary part is usually made of molybdenum. The auxiliary part is usually disc-shaped and is a plate or a perforated disc with a thickness between 0.3 and 20 mm. The outer edge of the auxiliary part can be bent.

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The welding aid can be arranged on the front of the auxiliary part or on the circumference if the auxiliary part is a disk. The circumference is then formed by the outer edge of the disk.

The weight of the anode is particularly critical in this context. The electrical connection according to the invention is particularly recommended when the anode, consisting of head part and shaft, has a mass of at least 50 g. The invention is particularly useful for masses of at least 100 g, since a welding aid appears almost indispensable for sufficient mechanical strength. Large anode masses are generally required for high power (from about 1000 W) and/or high currents (from about 40 A).

The welding aid according to the invention is preferably a film, in particular with a thickness of between 10 and 100 μm. Such films are easy to handle. Alternatively, the welding aid can also be a paste.

A preferred embodiment of the seal is provided by at least three films being distributed over the circumference of an insulating cylinder body (in particular an elongated solid or hollow quartz glass roll). The auxiliary part lies against the electrode-side end of the cylinder body. The auxiliary part and cylinder body should have approximately the same external diameter.

To determine a suitable welding aid, tests were carried out with various metal foils and pastes with a melting point > 1800 ⁰ C.

The selection of metals was limited to those that were more resistant to mercury and the spot welds were tested for good mechanical strength. The strength of the connection was determined by tensile tests on a weld point in the direction of the foil. A classification was made into three classes:

high strength: corresponding to a tear-off force of more than 20 N;
medium strength: corresponding to a tear-off force of 5 to 20 N;
low strength: corresponding to a tear-off force of less than 5 N.

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Table 2

Metal mechanical strength material thickness

Ta high 0.05 mm thick foil

Ta medium paste

Nb high 0.01 mm thick film

Re high 0.025 mm thick film

Zr g ^erm 8 Paste

MoB g ^erm g Paste

MoRu low paste
10

Good results have been achieved with tantalum, niobium and rhenium as welding aids, whereby the metals are preferably used as foil (with a thickness of between 10 and 100 µm), but can also be used as paste. Tantalum and niobium, both of which are significantly cheaper than platinum, must be processed in a vacuum, as both metals also act as getters. In contrast, rhenium, like Pt, can be processed in H ₂ . Both metals are similarly expensive.

Although the melting point of these metals is similar to that of molybdenum, the strength of the electrical connection achieved is surprisingly high. However, this property is explained by the high ductility of the welding aids tantalum, niobium and rhenium and the alloys MoTa, MoNb and MoRe formed from them in the weld zone.

&bull; · * · &diams; ta«

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In a particularly preferred embodiment with Ta or Nb, a getter is also added to the discharge volume. The background to this is as follows: the metals found to be mercury-resistant for the welding aid have the at first glance unpleasant property of acting as getters. Both tantalum and niobium are known to act as getters. The use of Ta and Nb therefore prohibits a cost-effective reducing cleaning annealing of the welded electrode systems, which is generally required for complex electrode systems. Instead, only a complex cleaning annealing in a high vacuum is permitted. This has previously hindered the use as a welding aid, since a getter reaction affects the stability of the electrical connection. However, it has surprisingly been found that if the operating temperature of the welding aid is carefully selected (below 1200 ^° C), the getter effect is negligible and no deterioration in the stability of the electrical connection is observed.

If the welding aid is operated at temperatures above about 1200° C, the following trick helps to ensure that the welding aid functions reliably over the long life of the lamp: at the same time as the welding aid, another getter material is introduced into the discharge volume. It is important that this getter is more reactive than the material of the welding aid. This property is ensured by either using the same material but at a higher temperature or by using a more reactive material (niobium or zirconium instead of tantalum).

In concrete terms, this means that if tantalum is used as a welding aid, either a tantalum getter is attached to the electrode shaft (since there is a higher operating temperature) or a Nb or Zr getter is placed elsewhere in the discharge volume.

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Alternatively, when using niobium as a welding aid, either a niobium getter is attached to the electrode shaft (since there is a higher operating temperature) or a Zr getter is placed elsewhere in the discharge volume.

This behavior can be summarized as follows: a getter is accommodated in the discharge volume, which is made either of the same material as the welding aid or of a more reactive material, whereby the getter is particularly attached at a location that reaches a higher operating temperature than the welding aid.

characters

The invention will be explained in more detail below using several exemplary embodiments. They show:

Figure 1 a mercury short arc lamp, in section

Figure 2 the foil-plate connection of Figure 1 in magnification as detail

Figure 3 shows another embodiment of a connection in detail

Figure 4 a third embodiment of a connection in detail

Description of the drawings

Figure 1 shows a cross-section of a mercury high-pressure discharge lamp 1 with an output of 1.5 kW. It has a bulb 2 made of quartz glass, which is elliptical, barrel-shaped or similar. Two ends 3 are connected to this on two opposite sides, which are designed as bulb necks 4 and each contain holding parts 8. The necks have a front conical part 4a, which contains a support roller 5 made of quartz glass as an essential component of the holding part, and a rear cylindrical part 4b, which forms the sealing seal. The front part 4a has an indentation 6 with a length of 5 mm. This is followed by the support roller 5 with a central bore, which is conical in shape.

ft* ·· ·■· ft« «·

►··· * ft ft ···*

** · * ft ft ft ft ft ft ft

* ft ft ft ft »«ft·

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A rod or shaft 10 of an anode 7 is guided axially in the bore of the first support roller 5, which extends into the discharge volume and carries a head part 11 there. The rod 10 is extended backwards beyond the support roller 5 and ends at a plate 12 made of molybdenum, to which a cylindrical quartz block 13 is connected. Behind this is a second plate 14 made of molybdenum, which holds an external power supply in the form of a molybdenum rod 15 in the middle. Four foils 16 made of molybdenum are guided along the outer surface of the quartz block in a manner known per se and are fused to the wall of the bulb neck.

In a similar manner, the cathode 17, consisting of a separate head part 18 and shaft 19, is held in the bore of the second support roller 5.

The discharge volume also contains a getter 20 made of niobium, which is housed on the shaft 10.

Figure 2 shows the electrical connection between the four foils 16 on the quartz block 13 and the plate 12 in detail. The molybdenum foils 16 with a thickness of 40 μm end at the front edge of the plate 12, which is 5 mm thick. The plate 12 is soldered to the shaft 10. A tantalum foil 20 with a width that corresponds to the thickness of the plate 12 is inserted between the plate 12 and the foils 16 as a welding aid. The tantalum foil 20 is 50 μm thick and as wide as the foil 16. The molybdenum foils 16 are reliably welded to the plate 12 (made of molybdenum) via the tantalum foil 20 by performing approximately five spot welds along the circumference. The total mass of the anode 7 (head 11 with shaft 10) was 75 g.

In Figure 3, another embodiment of the electrical connection between the four foils 16 on the quartz block 13 and the plate 12 is shown in detail. The molybdenum foils 16 with a thickness of 40 pm are bent over the front edge of the plate 12, whose thickness is 5 mm, and end approximately halfway up the radius of the plate. The plate 12 is again soldered to the shaft 10. Between the front 26 of the plate 12 and the bent ends 27 of the foils 16, a short piece of tantalum foil 28 is inserted as a welding aid (embodiment according to the upper half). In another embodiment (lower half-

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te), the tantalum foil 28' is guided a short distance along the shaft 10. The tantalum foil 28 or 28' is 50 pm thick. The molybdenum foils 16 are reliably welded to the plate 12 (made of molybdenum) via the tantalum foil 28 by carrying out a series of spot welds. The total mass of the anode 7 (head with shaft) was 200 g.

Figure 4 shows a further embodiment of the electrical connection between the four foils 16 on the quartz block 13 and the plate 22 in detail. The molybdenum foils 16 with a thickness of 22 μm end at the front edge of the plate 22. The latter is 0.5 mm thick, with the outer edge 21 of the plate being bent forwards, towards the shaft 10. The plate thus forms a perforated disk 22 in the shape of a cup, which is placed on the shaft 10 and soldered there in the area of the central hole. This cup shape of the plate is supported by a small perforated disk 25 made of quartz glass, which also sits on the shaft in front of the plate 22. A tantalum foil 29 with the width of the outer edge of the plate 22 is inserted between the outer edge 21 of the plate 22 and the foils 16 as a welding aid. The tantalum foil 29 is 20 pm thick and is placed around the entire circumference of the edge 21 of the plate 22. The molybdenum foils 16 are reliably welded to the plate 22 (made of molybdenum) via the tantalum foil 29 by performing a series of spot welds.

Claims (12)

1. Short arc lamp with a discharge vessel (2) which has two ends (3), an anode (7) and a cathode (17) facing each other in the discharge volume, and the discharge volume contains a specific amount of mercury which is more than 10 mg/cm ³ Hg, and at least the anode (7) has a shaft (10) which is sealed in the associated end (3) of the discharge vessel by means of a metallic auxiliary part (12) and a set of more than two foils (16) made of molybdenum, the auxiliary part (12) providing the electrically conductive connection between the shaft (10) and the foil set (16), and the connection between the auxiliary part (12) and the foil set (16) being made by a welded connection, characterized in that the welded connection is achieved with the assistance of a welding aid (20) which forms an intermediate layer between a part of the contact surface of the auxiliary part (12) and the contact surface of the foil set (16), wherein the welding aid (20) is a sheet, foil or paste consisting of tantalum, niobium or rhenium or a mixture of these three metals. 2. Short arc lamp according to claim 1, characterized in that the auxiliary part (12) consists of molybdenum. 3. Short arc lamp according to claim 1, characterized in that the auxiliary part has a disc shape and is a plate (12) or a perforated disc (22) with a thickness between 0.3 and 20 mm. 4. Short arc lamp according to claim 3, characterized in that the outer edge (21) of the auxiliary part is bent over. 5. Short arc lamp according to claim 3, characterized in that the welding aid (20) is arranged on the circumference or on the front side (26) of the disc. 6. Short arc lamp according to claim 3, characterized in that the welding aid is arranged on the circumference formed by the outer edge of the disc. 7. Short arc lamp according to claim 1, characterized in that the weight of the anode (7), consisting of head part and shaft, is at least 50 g. 8. Short arc lamp according to claim 1, characterized in that the contact surfaces of the auxiliary part (12) and the foils (16) are curved. 9. Short arc lamp according to claim 1, characterized in that the welding aid is a foil, in particular with a thickness between 10 and 100 µm. 10. Short arc lamp according to claim 1, characterized in that at least three foils (16) are distributed over the circumference of an insulating cylinder body (13), the auxiliary part (12) resting on the electrode-side end of the cylinder body (13). 11. Short arc lamp according to claim 1, characterized in that the current consumption of the lamp is at least 40 A, preferably 60 A. 12. Short arc lamp according to claim 1, characterized in that a getter (20) is accommodated in the discharge volume, which consists either of the same material as the welding aid or of a more reactive material, the getter (20) being attached in particular at a location which reaches a higher operating temperature than the welding aid.