Photolithography is the process by which semiconductor circuitry is patterned on silicon wafers. The lithography light source provides the deep ultraviolet light needed to expose the photoresist on the wafer.

Deep ultraviolet refers to a range of light on the violet side of the color spectrum, far beyond what the human eye can see. Deep UV light for lithography is generated by excimer light sources. The light is passed through a Beam Delivery Unit (BDU), filtered through the reticle (or mask), and then projected onto the prepared silicon wafer. In this way it patterns a chip design onto a photoresist that is then etched, cleaned and the process repeats. After layer is built upon layer, the wafer yields the chips that power today’s most advanced electronic devices.

Light Source Requirements for Photolithography

A critical component of the process, the light source must be tightly controlled and transported. There are four key requirements for the light source;

• Wavelength
  Needed for optical (feature) resolution. 248nm (KrF) and 193nm (ArF) excimer lasers are currently used in development and volume manufacturing.
• Narrow, Tunable, Stable Spectrum
  Required to minimize the effects of chromatic aberrations in the imaging lens. Stable bandwidth enables stable imaging performance. The central wavelength must be tunable over a modest range, and must be highly stable to control focus
• Power
  High power is needed for high volume semiconductor manufacturing. Current lasers operate at powers between 40W and 90W.
• High Repetition Rate and Stable Energy
  Stable energy is required for accurate exposure dose of the resist to meet the required CD control. Current lasers run at 4000-6000 pulses/second and enable scanning lithography exposures. 

Advanced Photolithography Techniques

The demand for ever-shrinking feature size is high, and single exposure DUV lithography has quickly reached its limitations. With some advanced lithography techniques, DUV lithography is able to pattern features sizes as small as 32 nm and below. Immersion Lithography enables chipmakers to increase the resolution of chip features significantly by changing the imaging medium. Double Patterning techniques drive even smaller chip designs by exposing each layer twice to reduce pitch beyond those achieved with traditional single exposure.