Toshiharu Nakashima, Yasuhiro Ohmura, Taro Ogata, Yusaku Uehara, Tomoyuki Matsuyama, Nikon Corp., Kumagaya, Saitama, Japan Holly H. Magoon, Nikon Precision Inc., Essex Junction, Vermont

Leading-edge exposure tools must provide a means to control and compensate for projection lens thermal aberrations that result from today’s high powered laser sources and advanced off-axis illumination techniques.

Each new generation of lithography exposure tools requires increased productivity and enhanced resolution capabilities. Higher powered lasers coupled with off-axis illumination strategies are used to satisfy these critical needs. As a result, however, the increased power through the projection lens–and the light fluence locally concentrated in the lens pupil–can lead to problematic thermal aberrations. In order to maximize productivity and imaging capabilities, the scanner design must incorporate systems to control projection lens thermal aberrations in real-time, and enable pattern-specific compensation to optimize the performance of off-axis illumination [1, 2].

Thermal aberrations

To satisfy the demand for enhanced system throughput, today’s leading-edge exposure tools use excimer laser sources with higher power, sending more light through the scanner projection lens. That can potentially heat up the lens elements, inducing a change in the refractive index and shape of various optical elements, resulting in elevated aberration levels. In parallel, advanced off-axis illumination strategies are used to enhance resolution capabilities. This concentrates the light locally on elements within the lens, causing asymmetrical heating and introducing additional specific wavefront aberrations.

These two types of thermal aberrations are unlikely to be detected during system inspection due to minimal power loading; they are more likely to arise during high-volume manufacturing operations. However, thermal aberration issues do not correlate simply to usage levels. In reality, there is a significant dependency on the illumination conditions and the mask pattern used, as well as the specific interactions between them.

Illumination impacts on thermal aberrations

Typically, conventional circular central illumination and annular illumination strategies are used to image random mask pattern layouts. In such cases, the thermal distribution at the pupil is almost rotationally symmetrical. This effect induces thermal aberrations having mainly “0θ” Zernike terms, especially the lower order terms Z4 (defocus) and Z9 (spherical aberration). These angularly uniform types of aberration are easily compensated using a combination of the lens control (LC) system and the scanner’s auto-focus system located on the wafer stage. For example, lower-order spherical aberrations are easily controlled by shifting specific lens elements within the LC system along the optical axis.

However, in the case of dipole or cross-pole illumination, thermal aberration control is more complex. When dipole or cross-pole illumination is used to image small critical dimension (CD) dense patterns with features existing in one orientation (horizontal or vertical), only the 0th order and 1st orders of diffracted light from the mask enter the lens pupil. This, then, heats the lens locally in specific regions toward the edge of the pupil. This localized heating can induce significant wavefront aberrations, the primary type in this case being from the astigmatism family (2θ components in the Zernike polynomials). The astigmatism will appear effectively uniform across the image field.

Localized lens heating may also be observed with other illumination conditions and mask types, such as a Shibuya-Levenson type phase shift mask (PSM) having single orientation dense patterns, in combination with small-σ illumination. In this situation, the 0th orders of diffracted light do not appear, but both the +1st and -1st orders of light pass through toward the edge of the lens pupil. This causes localized lens heating with a thermal distribution very similar to that experienced when using dipole illumination. As a result, uniform astigmatism across the image field is induced as well.

Aberration control

Nikon utilizes two techniques to control low order aberrations and distortion. The first is an advanced lens control [3] (LC) system, which adjusts individual lens elements to control focus, distortion, spherical, coma, and other aberrations in real time. Several of the lens elements have movable supports driven by piezoelectric zirconate titanate (PZT) actuators, and they are moved up-and-down, and/or tilted individually as needed. Adjusting the positioning of these elements intentionally creates low order aberrations with minimal higher order side-effects. The second technique uses a proprietary infrared aberration control [4] (IAC) system (Fig. 1). The IAC employs an infrared laser and hollow optical fibers to selectively heat the lens pupil to correct for the challenging uniform astigmatism caused by dipole-like illumination, as well as situations where either vertical or horizontal line/space (L/S) patterns dominate and cause nonrotationally-symmetric pupil-fills.

Several key pieces of information are first required in order to control thermal aberration. These include the wavefront aberration rate associated with the motion (up-and-down, tilting) of controllable lens elements, the wavefront aberration rate for IAC IR laser power, and the aberration sensitivities of the aerial image sensor (AIS) mark, typically a 150nm isolated space pattern, for several illumination conditions [5].

The aberration changes as a function of exposure time are measured for several illumination conditions during system installation. Two parameters (time constant and saturation point) of the IAC and LC elements are then set to allow compensation of the focus and magnification behavior of the AIS mark for each illumination condition. Figure 2 shows an example of effective compensation using the IAC in a case where astigmatism is well controlled through both periods of lens heating (0?50 min), and cooling (>50 min) [6]. When the user selects a unique illumination condition, the necessary parameters are interpolated from the existing ones. Additionally, for actual device production, since the power through the lens depends on each individual reticle’s transmittance, the scanner must also measure the associated transmitted power using the monitor on the calibration stage. A power measurement is executed before exposure of each lot. The LC and IAC are then controlled on a shot-by-shot basis using the light power measurement and the calculated aberrations to achieve optimal control
during exposure of production lots.

Customized aberration compensation

The LC and IAC can also be customized to optimize the correction for a specific pattern on the product reticle, and a method for selecting this target pattern for correction will be discussed later. As described previously, the lens controller is typically adjusted based upon the measurement of the standard AIS pattern via the calibration stage. In the case of customized lens control, lens performance is optimized for the unique circuit pattern instead of the standard pattern.

When using cross-pole illumination to expose a pattern consisting of mostly dense vertical lines and spaces with a few horizontal patterns as well (where both the vertical and horizontal patterns differ from the AIS mark pattern), the projection lens pupil would initially encounter intense radiation corresponding to the diffraction caused by the vertical lines and weak radiation from the horizontal lines. As the pattern is repeatedly exposed and the lens begins to heat–without the application of customized lens control–some defocus of the vertical lines occurs (Fig. 3a, green line), while the horizontal lines experience minimal defocus (Fig. 3a, blue line).

The Nikon design enables two possible applications of customized lens control to compensate for this situation, depending upon the imaging objectives. If only the vertical features were critical or if there were no horizontally oriented features in the specific reticle, then the lens would be adjusted to maintain the focus of the vertical lines. Of course, typically both feature orientations are important, and here the customized LC alone would maintain the average optimal defocus over time (Fig. 3b, red line).

Ideally though, instead of using an average condition, the objective is to have both vertical and horizontal lines exposed at best focus. To achieve this, the IAC would be added to heat the lens in the less strongly illuminated areas corresponding to the horizontal features. This makes the lens heating and resulting thermal distribution more consistent, and reduces the level of uniform astigmatism. This in turn makes focus consistent across both the vertical and horizontal features (Fig. 3c).

Pattern-specific aberration simulations

Optimization for a unique circuit pattern using customized lens control is achieved by first simulating the lens wavefront aberrations thermally induced by the illumination condition and entire reticle pattern, and then correlating those to the imaging performance shown by the specific circuit pattern of interest. This enables correct lens adjustment without requiring direct circuit pattern measurements.

Clearly, accuracy of the simulation is vital. The simulated thermal wavefront aberrations (spherical and astigmatism) were matched to measurements obtained using the Nikon integrated projection optics tester (iPot) [7]. The iPot sensor exists on the calibration stage and is used to measure various aspects of optical performance, including wavefront aberration, apodization, illumination intensity and illumination polarization. Accurate simulations for multiple illumination conditions at varied locations in the exposure slit enable optimum adjustments by the customized lens controller.

Pattern sensitivities and determination of control target pattern

To minimize production impacts due to thermal aberrations, it is important to consider and understand the thermal aberration sensitivities of various patterns during process definition and development. A dipole illumination source is usually designed to be optimized for the minimum L/S pattern pitch and the orientation which is considered the “most critical.” As a result, the smallest pattern will have a very large depth-of-focus (DOF) and less sensitivity to spherical and astigmatism-family wavefront aberrations. However, in reality, other patterns may actually be far more sensitive, thereby encountering significant levels of thermal aberrations. For example, the edges of cell patterns, isolated lines between cells, and peripheral patterns are often more sensitive than the “most critical” CD, and similarly more susceptible to thermally induced aberrations.

Analysis of the optical proximity effect (OPE) curve is one method to determine the pattern most sensitive to thermal aberrations. Figure 4 is an example of an OPE-delta curve showing the difference between the CD as influenced by thermal aberrations, compared to the CD without aberration effects. This is shown for both vertical and horizontal features through pitch, as calculated from the imaging simulation for two positions in the static field (CE: field center, RI: right edge). Here, the horizontal features experience a significant decrease in CD around the 220nm and 450nm pitches. These pitches are then considered the weakest pattern with regards to thermal aberrations and may be selected for custom optimization of the IAC and LC, as in Fig. 3c.

Summary

Problematic thermal aberrations are induced by a combination of today’s high powered laser sources and the usage of advanced off-axis illumination RETs, and the lithographic effect of these thermal aberrations varies greatly for each combination of illumination condition and mask pattern. Challenges such as these make sophisticated thermal aberration control a key enabling technology for state-of-the art lithography systems.

Leading-edge Nikon scanners utilize proprietary LC and IAC systems to compensate for thermal aberration impacts and are fully effective for standard “generic” patterns used in manufacturing. In addition, customized LC and IAC compensation can be used successfully to optimize performance for the specific patterns that are particularly sensitive to thermal aberrations, as identified through methods such as OPE curve analysis. Enhanced thermal aberration control provided by systems such as these, is vital for the application of advanced RET methods in today’s high-volume manufacturing environment.

References

1. Andrew Hazelton et al., “Immersion Lithography in Mass Production: Latest Results for Nikon Immersion Exposure Tools,” SEMATECH Immersion Symposium, Keystone, CO, October 2007.
2. Toshiharu Nakashima et al., “Thermal Aberration Control in Projection Lens,” SPIE, Vol. 6924, paper 6924-66, 2008.
3. Tomoyuki Matsuyama et al., “High-NA and Low-residual-aberration Projection Lens for DUV Scanner,” SPIE, Vol. 4691, pp. 687-695, 2002.
4. Yusaku Uehara et al., “Thermal Aberration Control for low k₁ Lithography,” SPIE, Vol. 6520, 65202V, 2007.
5. Jacek Tyminski et al., “Aerial Image Sensor: In-situ Scanner Aberration Monitor,” SPIE, Vol. 6152, 61523D, 2006.
6. Tomoyuki Matsuyama et al., “Hyper-NA Catadioptric Projection Lens for 1.3NA Immersion Tool,” SEMATECH Immersion Symposium, October 2006.
7. Toru Fujii et al., “Integrated Projection Optics Tester for Inspection of Immersion ArF Scanner,” SPIE, Vol. 6152, paper 615237, 2006.

Toshiharu Nakashima received his BE and ME degrees in industrial mechanical engineering from the U. of Tokyo in 1992, before joining the IC Equipment Division of Nikon Corp in Tokyo. He is employed by the Strategic Technology Development Section Optical Design department and has worked on advanced imaging simulations for lithography processes since 1999.

Yasuhiro Ohmura received a BAa in physics from St. Paul’s U. (Japan) in 1992, prior to joining Nikon Corp. He works in the area of optical design for leading-edge microlithography systems.

Taro Ogata received his PhD in industrial physics from the U. of Tsukuba in 1996, before joining the IC Equipment Division of Nikon Corp. in Tokyo. He has worked on advanced imaging simulations for lithography processes since 2000.

Prior to joining the IC Equipment Division of Nikon Corp. in Tokyo, Yusaku Uehara received his ME degrees in electronics engineering from the U. of Hosei in 1999. Uehara focuses on development of lens control systems.

Tomoyuki Matsuyama received his degree in applied physics from the U. of Electro-Communications (Japan) in 1989, joining Nikon Corp. that same year. His main interests include aberration analysis methods, imaging system design, and optical design of lithographic lenses.

Holly H. Magoon received her BS in chemistry from St. Michael’s College. She is marketing manager for Nikon Precision Inc., focusing on development and launch of new products and technologies. Magoon joined NPI in 1995, and held applications engineering and supervisory positions prior to her marketing position. Nikon Precision Inc., 39 River Rd., Suite 5, Essex Junction, VT 05452; ph 802/879-5027; fax: 802/879-2734; e-mail hmagoon@nikon.com.