Figure 3 shows the configuration of the BCM in the laser unit.
The Bandwidth Control module (BCM) consists of two sub-modules: BCM Metrology, which allows high-precision measurement of E95, and BCM Control, which allows the spectrum to be made variable.
a. BCM Metrology sub-module
The BCM Metrology sub-module guides a part of the laser beam into the temperature-controlled high-finesse Etalon via an optical fiber and illumination optics to form a fringe. This fringe is guided into the low-noise VUV-detecting CCD sensor through high-resolution imaging optics, then converted into an electrical signal. This fringe signal is subject to a deconvolution process* to allow highly precise measurement of E95.
* The deconvolution process is a method for eliminating blur caused by equipment constants from the spectrum; it is indispensable for accurate spectrum measurement. We at Gigaphoton have performed the deconvolution process during measurement with a large-scale spectroscope. The BCM Metrology sub-module, however, allows the deconvolution process to be performed for measurement within the laser unit.
Table 1 shows the specifications of the BCM Metrology sub-module.
Table 1. BCM Metrology Sub-Module Specifications
|E95 measurement||Measurement Accuracy||±40 fm@GT40/60A
|Measurement Range||0.15 - 0.6 pm@GT40/60A
0.15 - 0.5 pm@GT61A
|Number of Accumulated Pulses||40 pulses|
b. BCM Control sub-module
the BCM Metrology sub-module. Gigaphoton originally developed the control method using optical components for the variable mechanism. Figure 4 shows the theory of the spectrum variable mechanism.
Optical components are arranged in a resonator to make the spectrum variable. Illustration (1) shows the case in which the spectrum control is not conducted. If parallel-plate optics are arranged in the resonator for laser, the laser beam incident to an optical component is transmitted through the optical component just as if it is a plane wave. The plane wave is incident to the grating in the resonator to diffract wavelength λ1. The diffracted beam is resonated to output a fine spectrum.
On the other hand, illustration (2) shows the case in which the optical components are arranged separately to transmit the laser beam. The laser beam changes from a plane wave to a spherical wave and is incident to the grating to diffract different wavelengths λ1, λ2, and λ3 to output a thicker spectrum. An interval between these two optical components can be adjusted to make the spectrum variable.
Spectrum control using optical components has the following three advantages:
High-speed Spectrum Control
The combination of optical components and high-speed-drive actuator allows control of the spectrum at a high speed.
Symmetric Spectrum Properties
In general, the lithography tool lens is designed based on the assumption that spectrum shape E95/FWHM is a constant ratio. If the spectrum is varied, it causes a shift from this assumption for designing the lithography tool lens unless the spectrum shape cannot maintain its symmetrical property. The BCM is able to maintain the symmetric property of the spectrum shape even if the spectrum is varied. Figure 5 shows the relationship between FWHM and E95 and the actual spectrum shape if the variable mechanism operates. It proves that E95/FWHM is almost constant, and there is no large deformation in the spectrum shape.
Impact upon Laser Performance
Figure 6 shows the behaviors of wavelength and energy stabilities if the spectrum changes to 0.3 pm and 0.5 pm. It confirms that neither the wavelength nor the energy stabilities have an impact if the spectrum performance E95 is changed.
The spectrum variation mechanism using optics, which is incorporated into the BCM Control sub-module, has such excellent features as described above. Thus, it is an ideal method for varying the laser spectrum.