This PDF file contains the editorial “Cost of Scaling” for JM3 Vol. 2 Issue 02

Microactuators are regarded as a key component in the field of microelectromechanical systems (MEMS). According to the motion of the actuator, it can be classified as an out-of-plane type or an in-plane type. Most of the existing out-of-plane thermal actuators are multi-layer structures. In this study, a novel electrothermal single-layer out-of-plane actuator is presented. The characteristics of this device are stated as follows: (1) This actuator consists of only a single thin film layer, therefore, it can prevent delaminating after a long-term operation. (2) The fabrication process is multi-user MEMS processes (MUMPs)-compatible, and it has the potential to integrate with many different micromachined components. (3) As demonstrated by the experiment, this device can be operated at a relatively low voltage. For the thermal actuator with beam length 275 μm, its deflection amplitude can reach 3.196 μm when driven at 5 V dc, and 5.316 μm when driven at 8 V dc. This structure offers the potential for application in adaptive optics systems and other optical systems. It also provides an interface to cooperate with integrated circuits and various optical elements to construct an embedded-control optical system.

The use of diffractive optical elements (DOEs) is increasing for several industrial applications. Most elements modulate the phase of incoming light or its amplitude, but not both. The phase modulation DOE is the most popular because it has a high diffraction efficiency. However, the phase-only limitation may reduce the freedom in the element design, increasing the design complexity for a desired optimal solution. To overcome this limitation, a novel, full complex-amplitude modulation DOE is presented. This element allows full control over both phase and amplitude modulation of any optical wave front. This flexibility introduces more freedom in the element design and improves the element's optical performance, even in a near-field operation regime. The phase grating of the element was fabricated in an amorphous hydrogenated carbon film. The amplitude modulation was obtained by patterning a reflective aluminum thin film, which was deposited on top of the phase grating. The apertures in the metal film determine the quantity of transmitted light. The use of a reflective layer in the fabrication decreases the risk of laser-induced damage since no absorption is involved in the process. With this device it is possible to obtain extremely efficient spatial filtering and reconstruct low noise images.

Recent development of high-precision aberration measurement techniques has enabled in situ characterization of the aberration response to wavelength offset. These measurements show that majority of the reconstructed Zernike terms exhibit some degree of sensitivity to wavelength. Although this dependence diminishes with the increasing order of Zernike polynomial, we consider the cumulative contribution of five Zernike terms, which have the strongest wavelength dependence (Z2, Z4, Z6, Z8, and Z11). The imaging impacts of KrF laser wavelength and spectral bandwidth are investigated using aerial image simulation; the behavior of the process window, mask error enhancement factor (MEEF), image placement, proximity effect, and sidelobe intensity is quantified. In this model, the chromatic aberrations are experimentally measured in a 0.68-NA KrF step-and-scan exposure system using the LITEL aberration test (InspecStep interferometer manufactured by LITEL Instruments, Inc., San Diego, California). The illumination spectrum input is characterized by spectroscopic measurement of a 2-KHz KrF laser source. In the lithography model, it is important to incorporate all of the wavelength-sensitive terms due to the additive contribution to the overall lens aberration balance. As shown previously, the longitudinal and lateral chromatic aberrations (image height and magnification) are the most sensitive to shift in center wavelength and have the strongest contribution to the aerial image modulation. Simulation results show several imaging changes for isolated lines and contact holes with changes in illumination spectrum. However, the rates of change are shown to decrease as bandwidth is reduced well into the subpicometer level. In the case of isolated contacts, the depth of focus (DOF) increases with the increase in bandwidth, however, at the expense of reduced exposure latitude. This suggests that engineering the spectral output of the laser can provide some process enhancement, although careful compromise is needed to utilize any DOF enhancement, since other image metrics including MEEF, side-lobe intensity, and image placement are also affected.

Based on exact symmetry considerations one can show that a cubic system is always optically isotropic. Nevertheless even a perfectly cubic crystal such as CaF₂ can show small optical anisotropy when interacting with light. Resolving this seeming contradiction leads to a phenomenon called spatial dispersion, which is an enhancement of optical anisotropy. While the initial tiny anisotropy is caused by the symmetry breaking of light, the enhancement that makes the effect observable is provided by the vicinity of a strong absorption. In semiconductors such an absorption is mainly given by the band gap but in an ionic crystal such as CaF₂ the bound electron-hole pair, a deep excitonic two-particle bound state, is an additional strong absorption causing response functions to diverge as ω-ω₀^–1 in its vicinity, where ω₀ is the bound state energy. We show that the exciton dispersion is able to explain in all details the optical anisotropy observed in CaF₂ including the spatial-dispersion-induced birefringence, the so-called "intrinsic birefringence." As opposed to normal birefringence, the effect in CaF₂ does not show up at large wavelengths and has seven optical axes instead of one.

We propose a framework for the analysis and characterization of the efficacy of any resolution enhancement technique (RET) in lithography. The method is based on extracting a distribution of the image log slope (ILS) for a given layout under a predefined set of optical conditions. This distribution is then taken as the optical signature for the image local contrast of the design. The optical signature can be created for an entire layout, or only for certain cells believed to be problematic. Comparisons can be made between the optical signatures generated using different illumination/RET strategies. We have used this method to evaluate and optimize two different RET approaches: subresolution assist features (SRAF) and double-exposure dipole illumination.

We have established effective dose metrology using a dose monitor mark named the effective dose meter, which has no focus response. By placing the effective dose meter onto the scribe line in a device reticle, in-line monitoring of the effective dose on a product has been realized. The effective dose meter is designed to monitor the effective dose as a resist line length whose dimension is detectable with an optical measurement tool. The design is considered to have no impact on both reticle fabrication and wafer processing. By monitoring with the effective dose meter, the contribution of effective dose error to critical dimension variation is obtained independently of focus error. Dose budget analysis from the in-line effective-dose monitor clarifies the current process ability on reticle linewidth variation and resist processing uniformity. This paper describes the mark design and the analysis result of the in-line effective dose monitor in device fabrication with KrF lithography.

Generation of extreme ultraviolet (EUV) radiation from solid targets is studied and a compact EUV source for small-scale lithographic applications and EUV metrology is developed. This source is based on a transfer of conventional x-ray tube technology into the EUV spectral range. As in an ordinary x-ray tube, electrons are generated by a tungsten filament and accelerated in a high-voltage electric field toward a solid target. In the demonstrated "EUV tube" beryllium and silicon targets are used to generate radiation at 11.4 and 13.5 nm, respectively. The absolute conversion efficiencies into EUV photons at 13.5 nm are measured. Prospects for a further power scaling of the EUV source are discussed.

In-plane displacement (IPD) of an extreme ultraviolet lithography (EUVL) mask in a flat state during the electrostatic chucking stage without friction is examined through simulations. For predicting IPD of an EUVL mask, a simulation model based on two-dimensional plane stress theory is developed. With regard to the absorber patterns both square and rectangle, film stress and absorber coverage dependency of IPD is investigated. Mitigation of IPD to the 1-nm level is possible by reducing absorber stress to ∓100 MPa. The change in surface height caused by absorber film stress of ∓500 MPa is less than 1 nm. The influence of change in surface height on image placement shift was found to be negligible because the image placement shift is 0.03 nm.

Minimizing mask-level distortions is critical to the success of electron projection lithography (EPL) in the sub-100-nm regime. A number of possibilities exist to reduce mask-fabrication and pattern-transfer distortion including subfield correction, "dummy" patterns, pattern splitting, and film stress control. Finite element modeling was used to illustrate the advantages and capabilities of these correction schemes for a 100-mm stencil mask with 1-mm×1-mm membrane windows. Static-random-access-memory-type circuit features, including both the interconnect and contact levels, were used, to simulate realistic circuit layouts with both cross-mask and intra-membrane pattern density gradients. With such correction techniques, it is possible to reduce the EPL mask-level distortions for "worst-case" mixed pattern types to less than 1.0 nm.