2. Definition and Impact of E95

What is E95

What is E95

Figure 1: What is E95

Here, we will explain about E95 in detail. Figure 1 shows a typical profile of a laser spectrum. As you know, for semiconductor manufacture, the laser is used to emit UV to form fine circuit patterns. The wavelength of a KrF laser is 248 nm while that of an ArF laser is 193 nm. Each of these wavelengths is determined by the center value of the spectrum. That is, the spectrum is the width broadening from this center value.

It has been conventionally known among semiconductor equipment manufacturers that the laser spectrum affects the critical dimensions (CD). However, there has not been much information on which part of the spectrum likely affects the CD. For this reason, as the simplest method, the bandwidth of 50% peak intensity is defined as the width of the spectrum; this is the definition called “full width, half maximum” (FWHM). Fluctuation of the spectrum caused by disturbance, etc., is simulated fairly well by the FWHM. In the past, the effect on the CD could be suppressed by controlling the FWHM.

In recent years, however, a new definition of the spectrum has been created. This is called E95, or “spectrum purity.” The definition of spectrum width for E95 is that in which 95% of the entire spectral energy is concentrated. The reason for creating this definition is that the FWHM alone is not enough to simulate variation of the CD as the numerical aperture (NA) increases while k1 decreases. By using a tool for simulating the lens characteristics, various spectrum profiles and lens contrasts are calculated to define E95 as the factor that most affects the contrast.

It has been said that E95 is the aspect of laser performance that is most difficult to control. The reason is that it is difficult to measure the E95 before controlling it. In the profile of the laser spectrum shown in figure 1, a foot part of the spectrum with an extremely low intensity needs to be measured. Therefore, this measurement requires a spectrometer with incomparably high precision. (Measurement of E95 will be discussed later in this article.)

Fluctuation Factors of E95

Fluctuation Factors of E95

Figure 2: Fluctuation Factors of E95

Figure 2 shows the conditions that likely cause fluctuation of E95. The horizontal axis indicates the pulse consumption while the vertical axis indicates E95.

The first factor of fluctuation is the laser gas (Factor 1). As you know, a mixture of Kr, Ne, and F2 gases is used to emit a KrF laser while a mixture of Ar, Ne, and F2 gases is used to emit an ArF laser. The laser performance varies greatly depending on the mixture ratio of the above gases. For this reason, gas control is one of the important technologies for excimer lasers. In general, the method for controlling the gases is to stabilize the output power. In the chamber in which laser gas is filled, pulse discharge is repeated thousands of times per second between the two electrodes. Repetition of this discharge causes the laser gas to gradually deteriorate. In particular, F2 gas is highly reactive, and its concentration tends to be lowered to drop the output power. This greatly affects the throughput of the lithography tool.

Therefore, F2 gas needs to be refilled to increase the output power to a level higher than its original level. However, a recent survey proved that this F2 concentration affects E95. In order to prevent a decrease of the laser output power, F2 gas is refilled more and more. But, as the concentration of F2 gas increases, the E95 value increases. The laser gas is refilled once a week. Because the gas mixture ratio is reasonable immediately after refilling the laser gas, the E95 value is small so that the CD becomes equivalent to its design value. On the other hand, under the gas control conducted mainly by stabilizing the output power, the concentration of F2 gradually increases to cause the E95 value to increase. As a frequent case, the CD varies immediately before and after refill of laser gas. Because the F2 concentration is high immediately before refill of the laser gas, the E95 value becomes larger to cause the CD to have a bias on the positive (+) side. Therefore, it can be said that the end of laser gas life (the time at which deterioration of the laser gas becomes significant) is a critical timing.

The second factor of fluctuation is deterioration of the optics in the long term (Factor 2). A line-narrowing module (LNM) is used to achieve a narrow E95 in the laser. The LNM incorporates multiple optical parts for wavelength dispersion, such as a prism and grating, to allow a wide E95 to become narrower when the laser passes through this LNM module. In the same manner as the illumination system for a lithography tool, the LNM is exposed to strong UV light for a long period of time. Therefore, distortion or a drop of transmittance causes the performance of the LNM to be gradually lowered. In this process, the E95 value tends to gradually broaden. One of the things that cause the CD to vary for a long period of time is fluctuation of the E95 value. As the result, it may become necessary to review the illumination conditions of the lithography tool. At present, the LNM performance has been enhanced so that the life of the LNM is about to exceed 20 billion pulses. That is, general users who consume 10 billion pulses per year will be affected by fluctuation of the E95 value in two years.

The third factor of fluctuation is replacement of a module (Factor 3). That is, the E95 value greatly fluctuates immediately after replacement of a module of the laser unit. Unlike the above two factors of fluctuation, this fluctuation is abrupt and large, and thus it is more critical. It occurs when the broadened E95 caused by a deteriorated module returns to its design value at replacement of this module with a new one. Most users have experienced variation of the CD caused by a module of the laser unit, but they cannot identify the cause and recover this trouble by changing the conditions of their lithography tools. From a different viewpoint, this means that the lithography tool is so adjusted to meet the broadened E95, thus sacrificing some important performance parameters such as resolution.