1980s Ushio USH-2002MA

The USH-2002MA is a 2 kW high-brightness short-arc UV source intended for semiconductor microlithography, used specifically in the Canon MPA-500, 501, 600 Super, and 1500 projection mask aligners. This lamp is peculiar in the fact that it is designed for an operation in the vertical position with the cathode on top. Normally, when burned vertically, DC short-arc lamps have their cathode located at the bottom to ensure the most stable arc operation possible. This is achieved by having the same flow direction for the gas convection and the cathodic arc jet which arises from the current gradient in the arc from the point of plasma attachment to this electrode.

The fact that the electrode position is inverted here has to do with the kind of optical system used in Canon’s machines: the light-collecting mirror is placed on top, aligned with the lamp’s axis and oriented downward. The bulky anode is then placed at the bottom to mitigate light shadowing. Arc stability is achieved via a very short electrode gap length of 3 mm only, combined with a rounded anode to prevent the formation of turbulences. Moreover, to ensure a homogeneous temperature distribution of the lamp, the arc is offset towards the anode side, which is coated with an oxide material to conserve heat and ensure that all of the dosed mercury is evaporated during operation. Heat transfer to the cathode is so significant in that configuration that a radiator is added on its long rod.

With a nominal power dissipation of 2.0 kW (2.4 kW max), the arc power load reaches 4870 W/cm, which is higher than in UHP lamps. Such extreme condition, combined with a high mercury vapor pressure (50-75 bars), results in an arc temperature reaching about 10,000 K, which leads to an extremely high source brightness needed for an efficient control of the emitted light. Due to the nature of the lamp fill and the high arc temperature, most of the radiated light lies in the UV-A and -B domains of the spectrum.