The main advantages of the injection locking method are stable performance and low running cost.
First, as described above, with the injection locking method, it is relatively easy to time the start of discharge in the amplifier chamber. This is because the laser beam is reflected multiple times, by the mirrors arranged on both sides of the amplifier chamber, to lengthen the time that the laser beam stays in the amplifier chamber. Unlike the MOPA method, this feature allows the final output performance to be easily stabilized.
Figure 2 shows a comparison of the E95 stability data used as an index for spectrum performance between the injection locking and MOPA methods. The horizontal axis indicates the repetition rate; the vertical axis indicates the shift (jitter) in the emission timing between the line-narrowing chamber and the amplifier chamber. Each of these graphs shows the fields in which the same E95 performance is obtained by colors. In this figure, the injection locking method causes slightly less change in the E95 than the MOPA method as the repetition rate or jitter changes.
Figure 2. Comparison of E95 Stability for Injection Locking and MOPA Methods
On the other hand, the MOPA method causes a greater change in the E 95. This means that control of the jitter, etc., is a critical technology factor for the MOPA method. To minimize the jitter, the MOPA method supplies power to both the line-narrowing and amplifier chambers from a single power supply. However, several problems still remain, since the excimer laser requires adjustment of the supplied power to stabilize the energy amount. For example, it is difficult to simultaneously drive a new chamber and a deteriorated chamber.
In terms of running cost, the injection locking method features an amplifier chamber with a high emission efficiency. This makes a great contribution to lowering the running cost, since most of the laser running cost derives from its optical components, such as the chambers and line-narrowing module. For the injection locking method, the emission efficiency of the amplifier chamber is high so as to allow minimizing of the output from the line-narrowing chamber. That is, deterioration of the line-narrowing module and other peripheral optical components can be minimized. If the laser unit is running for a long period of time, our trial calculation shows that use of the injection locking method allows reduction of the running cost to approx. 40% of the running cost required for the MOPA method.
As described above, the injection locking method seems to be superior to the MOPA method. However, two big challenges to be cleared when it is used for photolithography: ASE and coherence.