Excimer light sources are a key enabler of advanced lithography. They provide deep UV illumination with narrow spectral bandwidth to enable critical patterning, and are scalable in energy and power to enable high scanner productivity. Their physics and chemistry are well understood, and thanks to major technical developments, excimer light source performance has kept pace with semiconductor industry requirements.

The term ‘excimer’ is a contraction of “excited dimer”, a class of molecules that only exist (for a very short period of time) in an excited state, but do not exist in a stable, non-excited state. As a consequence, forming excimer molecules automatically generates the population inversion (more molecules in an excited state than in a lower-energy state) and corresponding optical amplification necessary to initiate laser action.

Essential Elements of a Light Source
Let's now examine the three key elements that are all required to construct a light source.

Gain Generator
The gain generator is the active medium of the light source. In the case of an excimer light source, the gain generator is a high pressure mixture of either Krypton or Argon with Fluorine gas and Neon gas. The purpose of the gain generator is to provide optical amplification, taking a small optical signal and boosting it as it travels back and forth through the gain generator.

Excitation Source
An energy source is required to make the gain generator active. We call that energy source an excitation source. In the case of lithography excimer laser light sources, the excitation source is a fast pulsed electrical discharge that provides the energy to the gain generator.

Optical Resonator
Finally, some means of capturing the radiation within the gain generator for multiple passes is required to produce laser oscillation - we call this structure an optical resonator. The simplest form of optical resonator is formed by a fully reflective mirror at one end of the gain generator, and a partially reflective mirror at the other end. Laser radiation that 'leaks' through the partially reflective mirror becomes the usable output of the laser lightsource. In the case of a lithography excimer laser light source, the fully reflective mirror is highly spectrally selective to provide the narrow output spectrum (very small wavelength range) required for the lithography application.

When these three elements are together, the light that is spontaneously generated inside the gain generator, after it is excited, builds up by bouncing back and forth inside the optical resonator. Some of the light then leaks through the partially reflective mirror becoming the output laser beam.

Single Chamber Light Source System

In its most basic form, the single chamber lithography light source system puts these three elements together. The optical resonator incorporates a Line Narrowing Module (LNM) that selectively reflects light at specific wavelengths back into the gain generator.

With metrology and control systems in place, the lithography light source then can communicate with the lithography tool (the scanner or stepper) to control beam parameters.
 

Dual Chamber Light Source System

With a single chamber light source system there is a trade off between the power required for lithography applications and the narrowness of the spectral bandwidth that can be achieved - narrower bandwidth = less output power.

To break this trade-off, Cymer added a second gain generator to the system as a power amplifier, redirecting light from the oscillator system through this second gain generator to provide pure power amplification. In this way, Cymer was able to boost the power output without compromising bandwidth.

Our Dual Chamber light sources consist of a Master Oscillator (MO) and a second chamber, the Power Amplifier (PA). The MO chamber produces low per-pulse energy, but narrow bandwidth. The PA chamber boosts the injected signal level without compromising bandwidth, giving the system power scalability. In our XLA products, we use a double-pass amplification scheme, where the light is passed two times through the amplifier gain generator.
 

Power Ring Amplifier Architecture

In a further evolution of the dual chamber architecture, we developed the Power Ring Amplifier Architecture featured in our XLR products. In this architecture, we direct some of the output from the double pass amplification back through the amplification stage. As a consequence, we effectively increase the number of passes that the light interacts with the amplifier gain medium. This has the positive effect of driving the amplification more heavily into the saturated regime, increasing efficiency and further improving per-pulse stability of many key light source metrics (including energy and spatial beam parameters).


This short video shows the evolution from a dual chamber, to a dual chamber Recirculating Ring architecture.