4. Improving E95 Stability

High-resolution Grating

Figure 5: High-resolution Grating

The biggest project conducted by Gigaphoton to stabilize the E95 is to enhance the performance of the line-narrowing module (LNM) that directly affects the E95. Figure 5 shows a conceptual diagram of the LNM. The module is mounted adjacent to the laser chamber and comprises the optical resonator with the front mirror. The LNM functions as a filter and narrows a wide E95 of several hundredths pm to several millionths pm.

The LNM consists of several prisms and diffraction gratings. Each prism is used to expand the laser beam, and the expanded laser beam is radiated upon the entire surface of the diffraction grating. The laser beam is reflected on the diffraction grating and returns to the laser chamber. At this time, the reflection angle slightly changes, depending on the wavelength. That is, wavelength distribution is converted into a special distribution. Slits are used to return only the light near the center to the laser chamber to narrow the spectral bandwidth. With the technology using this principle, use of a larger diffraction grating allows achievement of a narrower spectral bandwidth — that is, E95.

We at Gigaphoton have worked on a technology that allows development of the LNM mounted with the world’s largest high-resolution diffraction grating. Figure 5 shows the layout of a conventional diffraction grating and that of a high-resolution diffraction grating. The difference in size between the two gratings is 1.5 times or more. Because of this large size, the design for compact layout of the diffraction grating and the mounting method for achieving a uniform optical wavefront became the keys for developing the LNM.

Impact of F2 Gas on E95

Figure 6: Impact of F2 Gas on E95

Figure 6 shows the E95 data achieved by both conventional and high-resolution diffraction gratings. The vertical axis is the E95 while the horizontal axis is the pressure of the F2 gas in the laser chamber, which is a fluctuation factor of the E95. Use of the high-resolution grating allows reduction of the E95 to 0.7 pm at the F2 gas concentration of 300 Pa compared with 1.4 pm for the conventional grating. Similar reductions can be seen at other F2 gas concentrations.

In addition, fluctuation of the E95 for variation in the F2 concentration is also improved to 0.15 pm/100 hPa for the high-resolution grating from 0.40 pm/100 hPa for the conventional grating. This is an improvement of almost one-third. Use of the high-resolution grating provides a great improvement in fluctuation of the E95 caused by the first fluctuation factor of F2 partial gas pressure.

F2 Gas Control System

Figure 7: F2 Gas Control System

Having succeeded in development of the LNM with less fluctuation of the E95 for variation in the F2 concentration, we next developed the technology for stabilizing the F2 gas concentration in the laser chamber. This aims at ensuring the stability of the E95 in the long term by stabilizing the F2 gas concentration.

As previously described, the reason for changing of the F2 gas concentration (mainly increasing by refill) is to replace the F2 gas consumed by discharge and suppress reduction in the output power for a long period of time. However, excessive refill of the F2 gas too much causes the E95 to greatly fluctuate. Maintaining the F2 concentration in the laser chamber at a constant level is only one solution for stabilizing the E95 and suppressing a drop of the output power.

We developed a precision gas control technology that enables high-precision control of the F2 gas amount to be injected into the laser chamber via a mass flow controller. In addition, it enables reading of the amount of the F2 gas consumed in the chamber from each index. This allows maintenance of the F2 gas concentration at a constant level in the laser chamber. Also, to prevent excessive refill of the F2 gas, a feedback system is used to compensate for an expected shift of the F2 gas concentration by providing the measured spectral bandwidth of the monitor module as a feedback.

Development of these technologies allows the F2 gas concentration in the laser chamber to be controlled to within 300 to 350 hPa as a typical value. Combined use with the high-resolution diffraction grating enables suppression of the fluctuation of E95 to approx. 0.1 pm.

The above technologies are fully utilized for the 2-kHz KrF laser model G21K, 4-kHz KrF laser model G40K/G41K, and recent ArF laser models, making a great contribution to CD control at each user site.