A capacitor (that will serve as the transient power supply to produce the discharge) is first charged to 12kV. Then it is discharged in a very short time (75 ns) across electrodes present in the laser cavity. The plasma discharge this creates produces Kr+ and F- ions, and these ions then combine to form the KrF* excimers (with the process that creates the excited population of dimers called pumping). The laser light is produced by stimulating this created population of excimers to decay within a very short time. Some of the excimers begin to decay spontaneously. The light they emit passes through the gas containing the excimer population, and this stimulates additional excimer decay (which creates more radiation).

This is the amplification process alluded to in the acronym term laser. The laser cavity also uses mirrors to reflect radiation back into the gas. Thus, the stimulated decay rate (and laser light emission) continues to increase, until almost all of the excimers have undergone stimulated decay. In fact, in a properly designed laser cavity, the stimulated decay process is completed very quickly after it is initiated.

Thus, the light of an excimer laser is emitted in the form of short pulses of energy (i.e., they do not emit laser light continuously, as do continuous-emission lasers). The laser light exits the laser cavity though one of its ends, at which a partially-reflecting surface is located. As soon as the capacitor can be charged again to a high-enough voltage (making it able to act as the excitation source for creating the discharge) the excitation, pumping, and laser emission cycle can be repeated.

One of the biggest problems in using excimer lasers as lithographic light sources, however, is the fact they emit their energy in such a pulsed mode. Excimer lasers with a power output that is useful for lithographic applications (2–20 W) typically produce pulses of laser energy at a rate of 200–2000 Hz, with each pulse being only 5–20 nsec in length and containing about 10 mJ of energy.

Since each pulse is so short, and there is a relatively long time between pulses, the peak power within each pulse is extremely high (even when the average power of the laser appears to be quite modest), and such peak powers can cause damage to the optical coatings and materials of the optical lithography system if it is concentrated in a small area (e.g., the stepper lens). Thus, design features must be employed to keep the power density low at all points within optical elements. Increasing the repetition rate of the laser is also beneficial, since this will produce the same average power with a lower power per pulse, and will improve exposure dose control.

Excimer lasers are attractive as DUV light sources for a number of reasons. First, KrF lasers can deliver much more usable power at 248 nm than can the mercury-arc lamp between 235 and 260 nm. The radiance of the laser is several orders of magnitude greater than that of the arc lamp because the laser emits a highly collimated beam of light, whereas the arc lamp emits light in all directions (isotropically). Excimer lasers also have a low degree of coherence compared to most other kinds of lasers, as well as a relatively broad linewidth (which results in low temporal coherence). As noted in Sect 13.2, low coherence is generally a desirable characteristic of a lithographic light source. On the other hand, the 0.300-nm (300-pm) spectral width of the 248-nm laser is not narrow enough to satisfy the bandwidth requirements of the refractive optical systems of step-and-repeat aligners.

The lenses of these steppers have no chromatic correction, and this forces the illumination source to have a spectral bandwidth of less than 0.001 nm (or <1 pm). It is therefore necessary to put dispersive optical elements (such as prisms and diffraction gratings) within the laser cavity to reduce the bandwidth of the KrF laser light to this value. These elements somewhat reduce the total power of the laser, but more importantly they also decrease the stability of the power level.

A feedback system must be employed to keep the center wavelength of the line-narrowed laser light beam constant to about the same level of accuracy as the bandwidth. This control is needed because if the laser wavelength drifts by a few thousandths of a nm, the chromatic aberrations of the lens will cause an excessive shift in the focus of the lens. For each shift of the wavelength by 1 pm (0.001 nm), the lens focus will shift by about 0.15 µm. By 1998, excimer laser wavelength stability of much less than 1 ppm was being achieved (i.e., ±0.25 pm).

Another issue related to pulse power is exposure control. The method for controlling exposure in a stepper illuminated with a pulsed laser is to integrate the pulsed energy until the required exposure dose is reached (at which point a signal is sent to stop the laser from pulsing). To achieve a 1% control of accuracy, at least 100 pulses must be used. Early excimer lasers had low pulse repetition rates, and thus had slow exposure times. Newer laser models have overcome this limitation (repetition rates are up to 2000 Hz), and by 1996 the exposure speeds of excimer laser steppers could match those of arc lamp illuminated systems.

In addition to the issue of pulsed power, excimer lasers have some other drawbacks. They are much larger than an arc-lamp illumination source, typically occupying a cleanroom footprint of 2x5 feet in the crowded vicinity of the stepper (Fig. 13-50). The excimer laser is also expensive ($500,000– $1,000,000), which in 1999 adds 10-20 percent to the cost of the stepper. Gas used by the laser is also costly and toxic, and the plasma cavity in which it is contained, is not sealed. Thus, it must be periodically refilled with this gas (about every 5 days). This requires a gas-handling system that can fulfill the rigorous industrial safety requirements for toxic gases. (The interval between laser gas refilling has been increased by more than a factor of 10 since the earliest lasers were used for lithography, and improvement is continuing.)