Jianming Zhou

Evanescent wave imaging in optical lithography

New applications of evanescent imaging for microlithography are introduced. The use of evanescent wave lithography (EWL) has been employed for 26nm resolution at 1.85NA using a 193nm ArF excimer laser wavelength to record images in a photoresist with a refractive index of 1.71. Additionally, a photomask enhancement effect is described using evanescent wave assist features (EWAF) to take advantage of the coupling of the evanescent energy bound at the substrate-absorber surface, enhancing the transmission of a mask opening through coupled interference.

Inorganic immersion fluids for ultrahigh numerical aperture 193 nm lithography

Immersion lithography has become attractive since it can reduce critical dimensions by increasing numerical aperture (NA) beyond unity. Among all the candidates for immersion fluids, those with higher refractive indices and low absorbance are desired. Characterization of the refractive indices and absorbance of various inorganic fluid candidates has been performed. To measure the refractive indices of these fluids, a prism deviation angle method was developed. Several candidates have been identified for 193 nm application with refractive indices near 1.55, which is approximately 0.1 higher than that of water at this wavelength. Cauchy parameters of these fluids were generated and approaches were investigated to tailor the fluid absorption edges to be close to 193 nm. The effects of these fluids on photoresist performance were also examined with 193 nm immersion lithography exposure at various NAs. Half-pitch 32 nm lines were obtained with phosphoric acid as the immersion medium at 1.5 NA. These fluids are potential candidates for immersion lithography technology.

ILSim: A Compact Simulation Tool for Interferometric Lithography

Interference imaging systems are being used more extensively for R&D applications where NA manipulation, polarization control, relative beam attenuation, and other parameters are explored and projection imaging approaches may not exist. To facilitate interferometric lithography research, we have developed a compact simulation tool, ILSim, for studying multi-beam interferometric imaging, including fluid immersion lithography. The simulator is based on full-vector interference theory, which allows for application at extremely high NA values, such as those projected for use with immersion lithography. In this paper, ILSim is demonstrated for use with two-beam and four-beam interferometric immersion lithography. The simulation tool was written with Matlab, where the thin film assembly (ambient, top coat, resist layer, BARC layers, and substrate) and illumination conditions (wavelength, polarization state, interference angle, demodulation, NA) can be defined. The light intensity distributions within the resist film for 1 exposure or 2-pass exposure are displayed in the graph window. It also can optimize BARC layer thickness and top coat thickness.

Snell or Fresnel : The influence of material index on hyper NA lithography

As immersion lithography is extended to ever increasing resolution, the resulting propagation angles in the materials involved become closer to grazing than to normal incidence. Classical laws of refraction and reflection cannot be used with either assumption however, as a collection of angles may exist across the entire range. Fresnel reflection at these angles becomes large enough that small disparities in refractive indices at material interfaces may lead to adverse effects. As an example, when water is used at numerical apertures approaching its refractive index, reflection effects are greater than the constraints imposed by refraction or absorption. This will limit the maximum NA value allowed by any given material to values sufficiently lower than its refractive index. Additionally, we have grown accustom to expanding the application of the Snell-Descartes Law to materials with low absorption, assuming that the contribution of the imaginary component of the refractive index is negligible. This is not the case for photoresists, fluids, or glasses, which can not strictly be considered as non-absorbing media. We have expanded the Snell-Descartes Law for absorbing media, with some interesting consequences. We will show that there is no real limit on the numerical aperture into a material, so long as its extinction coefficient is not zero. The relationship that lithographers have been using recently where NA< min[nglass, nfluid, nresist] will be shown to be inadequate and imaging at numerical apertures up to 1.85 will be demonstrated using materials with significantly lower (real) refractive index values.

Hyper NA water immersion lithography at 193nm and 248nm

Immersion lithography can allow for theoretical imaging to λ∕4n (where n is the refractive index of imaging fluid). As 193nm and 248nm technology is pushed toward this limit, experimental data becomes increasingly important. This paper describes research carried out to explore the limitations of water immersion lithography and its extension to higher numerical aperture values using modifications to the imaging fluid. Resist imaging to 38nm is demonstrated using water as an imaging fluid. Several alternative fluids are presented including phosphates, sulfates, and halides, which are shown to increase the refractive index of water.

Mask Enhancement Using an Evanescent Wave Effect

State of the art lithography is continually driven to resolve increasingly smaller features, forcing k1 values for lithography processes ever lower. In order to image these difficult features with reliable fidelity, lithographers must increasingly use Resolution Enhancement Techniques (RETs). One such technique that is proposed in this paper uses small, sub-wavelength grooves placed in close proximity to an aperture. These sub-wavelength grooves create evanescent fields bound to the surface between the absorber and the mask substrate, decaying exponentially in lateral directions. In this work we demonstrate the ability to use such Evanescent Wave Assist Features (EWAFs) to enhance the propagating near and far field energy within openings such as slits and contacts. Using a Finite Difference Time Domain model, the effects of these evanescent wave assist features are explored in both the near and far field regions. Several cases of absorber material, feature type, spacing, and illumination will be presented.

Outlook for potential third-generation immersion fluids

In a search for alkane candidates for 193 nm immersion fluids, several alkanes and cycloalkanes were synthesized, purified and screened to ascertain their absorption at 193 nm, refractive index and temperature dispersion coefficient in the context of the actual application. In general, cycloalkanes, and more specifically polycycloalkanes, possess a higher refractive index than do linear alkanes. Decalin, cyclodecane, perhydrophenanthreme (PHP), perhydrofluorene (PHF) and perhydropyrene (PHPY) are examined as potential second and third generation immersion fluids. The use of perhydropyrene, which possesses a high refractive index of 1.7014 at 193 nm, may be limited as an immersion fluid because of high absorption at 193 nm. Mixtures of cycloalkanes can lead to a higher enhancement of the refractive index together with a decrease on the viscosity. Exhaustive purification of the fluids is a critical step in determining the real absorption of the different fluids at 193 nm. Two simple purification processes of these cycloalkanes were developed that led to low absorption fluids in the VUV region. The possibility of forming the oxygen complex in aerated fluids was reduced by purging samples with argon or nitrogen. This easy elimination of the oxygen complex shows the weak bonding nature of this complex.

Synthetic defocus in interferometric lithography

Interference lithography has been widely utilized as a tool for the evaluation of photoresist materials, as well as emerging resolution enhancement techniques such as immersion lithography. The interferometric approach is both simple and inexpensive to implement, however it is limited in its ability to examine the impact of defocus due to the inherently large DOF (Depth-of-Focus) in two-beam interference. Alternatively, the demodulation of the aerial image that occurs as a result of defocus in a projection system may be synthesized using a two pass exposure with the interferometric method. The simulated aerial image modulation for defocused projection systems has been used to calculate the single beam exposure required to reproduce the same level of modulation in an interferometric system through the use of a “Modulation Transfer Curve”. The two methods have been theoretically correlated, by way of modulation for projection illumination configurations, including quadrupole and annular. An interferometric exposure system was used to experimentally synthesize defocus for modulations of 0.3, 0.5, 0.7 and 1.0. Feature sizes of 90nm were evaluated across dose and synthetic focus.

Applications of TM polarized illumination

The use of transverse electric (TE) polarization has dominated illumination schemes as selective polarization is used for high-NA patterning. The benefits of TE polarization are clear - the interference of diffracted beams remains absolute at oblique angles. Transverse magnetic (TM) polarization is usually considered less desirable as imaging modulation from interference at large angle falls off rapidly as the 1/cosθ. Significant potential remains, however, for the use of TM polarization at large angles when its reflection component is utilized. By controlling the resist/substrate interface reflectivity, high modulation for TM polarization can be maintained for angles up to 90° in the resist. This can potentially impact the design of illumination away from most recent TE-only schemes for oblique imaging angles (high NA). We demonstrate several cases of TM illumination combined with tuned substrate reflectivity for 0.93NA, 1.20NA, and 1.35NA and compare results to TE and unpolarized cases. The goal is to achieve a flat response through polarization at large imaging angles. An additional application of TM illumination is its potential use for double patterning. As double patterning and double exposure approaches are sought in order to meet the needs of 32nm device generations and beyond, materials and process engineering challenges become prohibitive. We have devised a method for frequency doubling in a single exposure using an unconventional means of polarization selection and by making use of the reflective component produced at the photoresist/substrate interface. In doing so, patterns can be deposited into a photoresist film with double density. As an example, using a projection system numerical aperture of 1.20, with water as an immersion fluid, and a conventional polyacrylate 193nm photoresist, pattern resolution at 20nm half-pitch are obtainable (which is 0.125lambda/NA). The process to transfer this geometry into a hardmask layer uses conventional materials, including the photoresist layer and thin film silicon oxide based materials.

Enhancement in hyper-numerical-aperture imaging through selective TM polarization

The degradation of projected images using TM polarization is not intrinsic because losses in image contrast can be recoverable. By controlling the photoresist/substrate interface reflectivity, high modulation for TM polarization can be maintained for angles approaching 90° in a photoresist. Although there is calculated loss of image contrast with increasing polarization angle using suppressed reflection (i.e., with an antireflection coating), the loss is not nearly as large when imaging over a reflective substrate. These results can potentially impact the design of illumination, possibly away from most recent TE-only schemes for oblique imaging angles and high numerical apertures (NA). Several cases of TM illumination are presented combined with tuned substrate reflectivity for 0.93NA, 1.20NA, and 1.35NA and compared to results using TE illumination. Additionally, a scheme for frequency doubling with a single TM polarized exposure is presented. Using a single exposure and selective TM polarization, the re...