Changes in gas chemistry affect beam shape and quality. Changes in the beam profile is a function of the gas life. For example, the following is the laser beam profile of a new gas fill on a new gas head:
Beam profile of new gas fill on new laser head
After a new fill, the excimer laser beam profile is Gaussian along the short axis (with a dimension of approximately 7 to 8mm) and flat-topped along the long axis.
As the gas fill ages, changes in gas chemistry occur. These changes alter the electrical discharge properties of the laser gas, and cause the width of the discharge to increase, resulting in beam growth in the short axis.
Beam profile of mature gas fill on new laser
The beam profile along the short axis gradually changes from Gaussian to flat-topped. Along with beam growth, the divergence and incoherence of the laser beam increase, resulting in reduced peak pulse energy in the far field. Increased high voltage across the discharge also widens the beam. Because the laser stabilizes its output based on total pulse energy, as the beam area increases, the power density drops. This is a concern in applications that require tight power density on-target control.