5. Bias Caused by Changing the E95 Performance

In the previous lecture, we described how the E95 performance of the laser affects the CD performance of a lithography tool. In this lecture, we shall discuss the bias generated when the E95 fluctuates under the condition in which both isolated and dense patterns are mixed together.

Bossung Curve

Bossung Curves show the influence of the focus and dose upon the CD in a lithography tool. The shape of the Bossung Curves varies depending on the process pattern and the illumination conditions. Figure 8 shows typical samples of Bossung Curves.

Typical Samples of Bossung Curves

Figure 8: Typical Samples of Bossung Curves

Generally, the influence of the focus upon the CD becomes larger in isolated patterns than in dense patterns.

Resolving of Spectral Waveform

Although the spectral waveform of laser is given as a single waveform, it is actually formed by accumulating a variety of waveforms. Figure 9 shows a general illustration of this. The spectral waveform or E95 fluctuates depending on variations of (1) wavelength, (2) spectral shape, and (3) dose. The relationship between the E95 and bias is easily understood if it is considered as a combination of the variation of spectral shape and Bossung Curves.

Resolution of a Spectral Waveform

Figure 9: Resolution of a Spectral Waveform

Relationship between E95 and Bias

Figure 10 outlines the generation of bias caused by variation of the spectral shape. In this figure, item (1) indicates the variation of spectral shape. Item (2) indicates the relationship between laser wavelength ? and the focus position. Item (3) is obtained by converting the variation of laser wavelength into the variation of focus position; at this time, the spectral intensity becomes the dose intensity, since the intensity of spectral waveform indicates the dose distribution in the wavelength direction. Item (4) is the overlapping of Bossung Curves for isolated and dense patterns. From variation of the focus position or dose in item (3), you will notice that this affects the CD as shown in item (5).

Generation of Bias Caused by Variation of Spectral Shape

Figure 10: Generation of Bias Caused by Variation of Spectral Shape

Figure 11 is a magnified image of item (5) in figure 10. Item (A) indicates the CD in isolated patterns prior to variation of the wavelength, while item (B) indicates the CD after variation of the wavelength so that variation becomes larger. In addition, either item (A) or item (B) is a bias component caused by variation of the wavelength. In the dense patterns, it is noticed that the influence of variation of the wavelength upon the CD is smaller than in the isolated patterns. In consideration of a combination of isolated and dense patterns, the bias is generated as indicated by item (C) after variation of the wavelength. This is called “Iso dense bias (IDB).”

Generation of Bias

Figure 11: Generation of Bias

The above is the mechanism for generation of bias resulting from variation of the E95 performance.