JPH0587937B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a short arc type mercury vapor discharge lamp suitably used as a light source for exposing semiconductor wafers.

[Prior art and its problems]

For example, in a printing process in semiconductor manufacturing, it is necessary to irradiate a semiconductor wafer with perfectly parallel light of a predetermined wavelength in the ultraviolet region. For this reason, as a light source for exposing semiconductor wafers, an anode and a cathode mainly made of tungsten are placed facing each other in the light emitting space of the bulb, and mercury and rare gas are sealed in the light emitting space. Short arc type mercury vapor discharge lamps are often used in which a bright arc spot is formed. In order to increase the intensity of the light, it is necessary to make the bright spot placed at the focal point of the reflecting mirror as small as possible to make it clearer, so the cathode tip is made cone-shaped, and the anode tip has a trapezoidal cross-section. A flat portion is provided so that the arc spreads from the sharp tip of the cathode to the flat portion of the anode so that an arc bright spot is formed only at the tip of the cathode.

By the way, such discharge lamps cannot be turned on and off repeatedly in short cycles, so in the case of a step-and-repeat method, the discharge lamp is left on all the time, and the shutter is opened only when irradiating ultraviolet rays, and the rated power is turned on. A lighting method that alternates between high-input lighting that lights up and low-input lighting that lights up at less than the rated power when the shutter is closed (hereinafter referred to as
This uses a pulse lighting method (referred to as a pulse lighting method) to reduce power consumption and prevent overheating inside the shutter.

However, when such a pulse lighting method is adopted, there is a problem that thermal damage to the anode is more severe than in a continuous lighting method. This thermal damage to the anode can occur if the arc diameter at the tip of the anode is small and the current density increases locally, if the tungsten crystal cross section is exposed in a long state, or if the arc occurs in areas with minute irregularities on the surface. It is presumed that this is caused by evaporation of tungsten in that area when the tungsten is exposed to water. Here, the arc diameter at the anode tip is related to the size of the flat part of the anode, and when the flat part becomes large, the arc tip contracts and discharges in a narrow range, so the current density increases locally, and conversely, the flat part When the arc becomes smaller, the tip of the arc spreads and touches the sloped area around the flat area, but in the sloped area, the cross section of the tungsten crystal becomes longer and exposed, and minute irregularities are formed during cutting, which accelerates the evaporation of tungsten. be done. The evaporated tungsten then adheres to the inner wall of the bulb and turns black, shortening the service life of the discharge lamp.

[Purpose of the invention]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a short arc type mercury vapor discharge lamp which has a long service life because evaporation of tungsten from the anode is small even when the lamp is pulsed.

[Structure of the invention and its effects]

The structure of the present invention is that an anode with a flat tip and a cathode with a cone-shaped tip are arranged facing each other in the light-emitting space of the bulb, and the light-emitting space is filled with mercury and a rare gas, and the bulb is lit at rated power. In short-arc mercury vapor discharge lamps in which high-input lighting is repeated alternately with low-input lighting at less than the rated power, the arc is defined as a range of 5% or more of the brightness of the bright spot formed at the tip of the cathode. In this case, the flat part of the anode is characterized in that its size corresponds to the arc diameter at the tip of the anode during high-input lighting.

The outline of the arc generated by the discharge between the two electrodes is not necessarily clear, and the brightness decreases the further away from the center of the electrode, but the range of 5% or more of the brightness of the bright spot formed at the tip of the cathode is The discharge current density is also large and can be essentially considered as an arc.
Since the flat part of the anode has a size that matches the arc diameter at the anode tip, in other words, the arc at the anode tip spreads to its maximum size as long as it does not touch the slopes around the flat part, so the current density in the flat part becomes smaller and evaporation of tungsten is suppressed,
In addition, since tungsten does not evaporate from the slope where it easily evaporates, the bulb is less likely to blacken and the service life of the discharge lamp is extended.

〔Example〕

The present invention will be specifically described below based on embodiments shown in the drawings.

Figure 1 shows a short arc type mercury vapor discharge lamp with a rated power consumption of 1KW, which is used as a light source for exposing semiconductor wafers. A light emitting space 11 is located within a central bulge 12 of a bulb 1 made of quartz glass, and a predetermined amount of mercury and rare gas are sealed within this bulb 1. Sealing tube portions 13 extend from both sides of the central bulging portion 12, and metal foils 4 are embedded in each of the sealing tube portions 13. Then, in the light emitting space 11,
A cathode 2 and an anode 3 made of tungsten, each of whose ends are connected to a metal foil 4, are disposed facing each other. Here, the cathode 2 has a cone shape with a pointed tip, but the anode 3 has a tapered tip cut off to form a flat portion 31, and the periphery of the flat portion 31 is a sloped portion 3.
It is 2. The size of the flat portion 31 is set to match the diameter at the anode tip of the arc generated during high-input lighting. Further, the electrode distance between the tip of the cathode 2 and the flat portion 31 is 2.9 mm.
The discharge lamp is lit in a vertical position with the cathode 2 facing upward, and is pulsed alternately at a rated power consumption of 1 KW and a lower rated power consumption of 700 W, for example, in a 0.4 second cycle.

FIG. 2 shows the state of the arc A during high input lighting when the size of the flat portion 31 is changed. Here, the brightness of the bright spot K formed at the tip of the cathode 2 is 5
% or more is defined as arc A. In Figure 2 A, the diameter φ of the flat part 31 is 2 mm, and in B, the diameter φ is 3 mm.
mm and C indicate the case where the diameter φ is 4 mm, but as the diameter φ changes, the diameter of the arc A at the anode 3 also tends to change. Then, as shown in Figure 2 A,
When the diameter φ is 2 mm, the arc A reaches the inclined portion 32, and the evaporation of tungsten in the inclined portion 32 increases. In addition, as shown in Figure 2 C,
When the diameter φ is 4 mm, the arc A contracts and concentrates in the center of the flat portion 31, and the current density in that portion increases, so that the evaporation of tungsten in the flat portion 31 increases. On the other hand, when the diameter φ is 3 mm, as shown in FIG. For this reason, the current density in the flat portion 31 is reduced, suppressing the evaporation of tungsten, and tungsten does not evaporate from the inclined portion 32 where it is likely to evaporate. Therefore, if the size of the flat portion 31 is set to match the diameter of the arc A at the anode tip during high input lighting (diameter φ = 3 mm in this example),
Evaporation of tungsten is suppressed, the bulb is less likely to blacken, and the service life of the discharge lamp is extended.

Next, the size of the flat part 31 is 2 mmφ, 3 mmφ,
We conducted a lifespan test on three types of discharge lamps with a diameter of 4 mm.
The results are shown in FIG. The g-line intensity is at its initial value.
Judging that the life is over when the attenuation reaches 70%, the longest life is 1450 hours when the flat portion 31 has a size of 3 mmφ (the size corresponds to the diameter of the arc A during high input lighting). Ta. On the other hand, the case of 4 mmφ had the shortest life at 550 hours, and the case of 2 mmφ had an approximately intermediate life.

〔Effect of the invention〕

As explained above, in the short arc type mercury vapor discharge lamp of the present invention, when the arc has a range of 5% or more of the brightness of the bright spot formed at the tip of the cathode,
The size of the flat part of the anode is made to match the arc diameter at the tip of the anode during high-input lighting, so even when pulsed lighting is performed, tungsten evaporation from the anode is suppressed, and the bulb is less likely to blacken, extending its service life. It is possible to make a short arc type mercury vapor discharge lamp which can be suitably used as a light source for exposing semiconductor wafers.

[Brief explanation of the drawing]

FIG. 1 is an overall view of a short arc type mercury vapor discharge lamp, FIGS. 2A to 2C are explanatory views of the relationship between the anode tip and the arc, and FIG. 3 is an explanatory view of a life test. DESCRIPTION OF SYMBOLS 1...Bulb, 11...Light-emitting space, 12...Central bulging part, 13...Sealing tube part, 2...Cathode, 3...
...anode, 31...flat part, 32...slanted part, 4...
...metal foil, A...arc, K...bright spot.

Claims (1)

[Scope of Claims] 1. An anode having a flat portion at the tip and a cathode having a cone-shaped tip are disposed opposite to each other in the light emitting space of the bulb,
In a short arc type mercury vapor discharge lamp, in which mercury and rare gas are sealed in the light emitting space, and high-input lighting at rated power and low-input lighting at below rated power are repeated alternately, the tip of the cathode A short arc type mercury characterized in that the flat part of the anode has a size that corresponds to the arc diameter at the tip of the anode during high input lighting, when the arc is a range of 5% or more of the brightness of a bright spot formed in Steam discharge lamp.