3. Challenges in the Injection Locking Method
The conventional injection locking method has two potential problems. One is that an increase in spatial coherence may cause the generation of speckles on a wafer surface. The other is deterioration of the resolution caused by amplified spontaneous emission (ASE). These two problems have so far prevented its implementation. We will explain here how our GigaTwin Platform has overcome these challenges.
Figure 3. Conventional Injection Locking Method (on the Amplifier Side)
Figure 4. Measurement Result of Spatial Coherence
Technology for Reducing Spatial Coherence
The spatial coherence of a laser defines the quality of phase within the beam surface. The laser beam of lower coherence is suited to photolithography because the lights with the same phase interfere with one another and cause speckles when the laser beam is shaped into the beam with a uniform irradiance distribution in the illumination system of the lithography tool. Speckles cause partial variation of the exposure amount so as to change the exposure pattern size (CD), because they appear around a wafer surface.
In the conventional injection locking method, the amplifying efficiency of the amplifier chamber is high, which allows minimizing of the line-narrowing chamber output and narrowing of the beam size to several millimeters. The narrowed seed light has an almost uniform spatial phase.
Figure 3 shows the method for introducing the beam into the amplifier chamber that is used for the conventional injection locking method. The narrowed seed light passes through a small hole, and it is gradually expanded in the amplifier chamber to increase the output. Therefore, the finally obtained beam inherits the uniform phase of the seed light, so that the spatial coherence becomes high.
In order to reduce the spatial coherence, special optics were developed for the GigaTwin platform, enabling it to achieve a high amplifying efficiency in the amplifier chamber without narrowing the seed light. As a result, compared with the conventional method, the output from the line-narrowing chamber is increased to optimize the beam size.
Figure 4 shows the measurement results of the spatial coherence measured for the conventional injection locking method, the method developed by Gigaphoton, and the single-chamber method already used in the field. The horizontal axis indicates the distance within the beam surface; the vertical axis indicates the visibility. As the visibility becomes larger, the spatial coherence becomes higher. These results show that the special optics developed by Gigaphoton achieve a spatial coherence that is several tens of times lower than that of the conventional injection locking system. It also improves the spatial coherence of the single-chamber method.
Figure 5. ASE Measurement Results
Technology for Reducing ASE
The amplified spontaneous emission (ASE) of a laser beam is generated simultaneously with the discharge in the chamber, and is the light with a weak and broad spectrum. It usually stands in the major narrow spectrums, and reaches a level that cannot be measured in the optical resonator in which light travels back and forth. In the lithography process, it is said that this weak and broad spectrum component affects the exposure pattern with the chromatic aberration of the lens.
The ASE is considered to be a problem, since a broad spectrum component may be generated if only the amplifier chamber operates with no seed light introduced. This is true not only for the injection locking method but also for the MOPA and single chamber methods.
For development of the GigaTwin platform, we succeeded in developing a method for measuring the actual ASE, as well as developing the ASE reduction technology based on the actual data. As a result, it became apparent that the amount of seed light is the dominant factor that influences the ASE amount.
Figure 5 shows the measurement of the ASE amount when the amount of seed light is changed while the time for introducing the ASE into the amplifier chamber is also changed. The red-line data (low injection) indicates the value when the amount of the seed light is minimized, and simulates the amount of seed light for the conventional injection locking method. The observed ASE amount was at the level of 1% of the entire output — an amount that may have an impact on the actual exposure pattern. On the other hand, the green-line (middle injection) and blue-line (high injection) data indicate that the ASE amount could be reduced to below 0.01% of the entire output. This is the same level as or below the ASE amount of the single chamber system, and never affects the exposure pattern.