The exposure system of a mask aligner generates light and forms it into a large beam with uniform intensity, typically within 2% across a 6″ x 6″ square. The intensity of the light is calibrated and controlled to provide repeatable exposures.
Ultraviolet (UV) Light
A mercury arc lamp is used to generate UV light for photolithography. The lamp radiates various colors of light such as ultraviolet (365nm), violet (405nm), blue (435nm), green (546nm), and yellow-orange (578nm). Unwanted colors from the lamp is removed by optical filters. The Omega PL-360LP filter is installed to remove the deep UV light that is known to degrade the performance of SU8, a negative photoresist. The light from a typical mask aligner exposure system will have a spectrum of colors (Filtered Light) as shown in the graph below. This general purpose configuration allow different types of photoresist to be exposed using the same equipment.
The UV light is measured using a light intensity meter. The light intensity meter contains a light transducer, light filters and electronics that are designed to measure a particular color band. Two light intensity meter is installed inside the exposure system to continously measure the light intensity during an exposure. One is configured for ultraviolet (365nm) and the other is configured for violet (405nm). These intensity meters are calibrated using a portable light intensity meter that is calibrated at a factory following a standard that is traceable to NIST. Although the recommended calibration period is 1 year, in practice, the calibration period varies depending on many factors such as custodian knowledge, resources and performance demands.
A lamp controller is a power supply that provides many useful features. It starts a cold lamp by igniting an electric arc through the argon gas which generates enough heat to vaporize liquid mercury. The energized mercury gas emits light that looks white to the naked eye, but it is actually a spectrum of light with distinct colors. Once the lamp is on, the lamp controller keeps it on at idle power to ensure a long lifetime. During an exposure, the lamp controller adjusts the brightness of the lamp to maintain a constant intensity measured by the light sensor. This allows an operator to calculate an exposure time based on a calibrated light intensity. For example, to expose a photoresist to 80 [mJ/cm2] of 405nm light, an operator will set the lamp controller to use the 405nm light sensor configured to 25 [mW/cm2] and calculate an exposure time of 3.2 seconds.
The light emitted from a mercury arc lamp does not produce a uniform intensity across a 6″x6″ square area. This is achieved by passing the light through a series of optical elements which includes an ellipsoidal mirror, cold light mirror, shutter, fly’s eye, condenser lens and filter plates. When a new lamp is installed, the custodian will make adjustments to the position of the ellipsoidal mirror to produce a uniform intensity.
Light from the exposure system must travel through a photomask before reaching the photoresist. Some of the light will be absorbed by the photomask substrate. Each photomask substrate has its own transmission spectrum depending on the material, manufacturer and thickness, see the graph below. The photomask film, sodalime, quartz, and borosilicate are all suitable substrates for photolithography at 365nm and above.