Chris Maloney

Impact of pupil plane filtering on mask roughness transfer

Line edge roughness (LER) is a common problem to all lithography techniques and is seen as an increasingly important challenge for advanced technology nodes. Contributions to LER can come from the aerial image itself or the resist related processes. Mask roughness belongs to the former group, which can contribute to the low frequency roughness. This paper investigates the mitigating effect of pupil plane filtering on the mask roughness transfer. Experiments were performed using a mask with edge roughness programmed at different periods on 128 nm pitch vertical line/space patterns. A target phase filter was optimized for ArF illumination source and the roughness period of 200 nm. The filter introduces an orientation dependent defocus; hence, reducing the image fidelity in the direction of roughness features without significantly impacting the fidelity of vertical line and space features. Experimental results showed significant reduction in mask roughness transfer for the target roughness period.

Improving low, mid and high-spatial frequency errors on advanced aspherical and freeform optics with MRF

Advanced optical systems are more and more demanding in terms of resolution, imaging quality and speed of capture. Controlling mid-spatial frequencies and surface roughness on these optics is critical for mitigating scattering effects such as flare and energy loss. By improving these two frequency regimes, higher contrast images can be collected with improved efficiency and lower distortion. Classically, Magnetorheological Finishing (MRF) is implemented in production to correct low order errors generated by conventional polishing techniques on planos, spheres, on- and off-axis aspheres and freeform optics achieving figure errors as low as 1nm RMS while using careful metrology setups. MRF is also used routinely to turn a sphere into an asphere or freeform, or to print high resolution wavefront corrective patterns on optical surfaces to compensate for system errors or bulk material inhomogeneity. Recent advancements enable correction of mid-spatial wavelengths as small as ∼1mm and smoothing of surface roughness to ∼1Å RMS. Using these new developments combined with correction of low order form error have improved MRF performance to manufacture higher precision optics with respect to the mid- and high-spatial frequency regimes. Efficient mid-spatial frequency corrections utilize optimized process conditions; raster polishing with a small tool size. Furthermore, a novel MRF fluid, called C30, can finish surfaces to ultra-low roughness (ULR) and its low removal rate is optimal for fine figure correction of mid-spatial frequency errors. C30 MRF fluid is able to achieve <1.5Å RMS roughness on Silicon, CaF2, Fused Silica, glass and other materials. It is best utilized within a fine figure correction process to target mid-spatial frequency errors as well as smooth surface roughness ‘for free’ all in one step. These expanded capabilities of MRF technology are well suited for producing high precision optics to be used for industrial, medical or semiconductor optics.

Feasibility of compensating for EUV field edge effects through OPC

As EUV Lithography (EUVL) continues to evolve, it offers a possible solution to the problems of additional masks and lithography steps that drive up the cost and complexity of 193i multiple patterning. EUVL requires a non-telecentric reflective optical system for operation. This requirement causes EUV specific effects such as shadowing. The absorber physically shadows the reflective multilayer (ML) on an EUV reticle resulting in pattern fidelity degradation. To reduce this degradation, a thinner absorber may help. Yet, as the absorber thickness decreases, reflectivity increases in the ‘dark’ region around the image field, resulting in a loss of contrast. The region around the edge of the die on the mask of unpatterned absorber material deposited on top of ML, known as the image border, is also susceptible to undesirable reflections in an ideally dark region. For EUVL to be enabled for high-volume manufacturing (HVM), reticle masking (REMA) blades are used to shield light from the image border to allow for the printing of densely spaced die. When die are printed densely, the image border of each neighboring die will overlap with the edge of a given die resulting in an increase of dose that overexposes features at the edge of the field. This effect is convolved with a fingerprint from the edge of the REMA blades. This phenomenon will be referred to as a field edge effect. One such mitigation strategy that has been investigated to reduce the field edge effect is to fully remove the ML along the image border to ensure that no actinic-EUV radiation can be reflected onto neighboring die. This has proven to suppress the effect, but residual out-of-band radiation still provides additional dose to features near the image border, especially in the corners where three neighboring fields overlap. Measurements of dense contact holes (CHs) have been made along the image border with and without a ML-etched border at IMEC in collaboration with Micron using the ASML NXE:3100. The implementation of these measurements allow for further mitigation, i.e., compensation by OPC. Mentor Graphics’ Calibre software uses the scanner’s point spread function and convolves it with the mask layout to generate a flare map. It also has the capability to add additional dose to the image border which can be optimized to fit the experimental data. This includes the transition region between the image field and border that results in a linear rolloff of dose due to partial shadowing of the REMA blades. By applying this flaremap that accounts for neighboring die to the already calibrated optical and resist models, OPC can now be enabled to compensate for field edge effects. This study has two goals. First, we will show that OPC can be used to compensate both for field edge effects with and without a etched ML border. The second is to investigate the limitations that exist for OPC in the areas altered by neighboring die. This will predict when a process to mitigate the field edge effect is needed to enable EUV HVM.

High brightness limited area cathodes

In this paper the theoretical description of limited area thermionic cathodes for use in electron microscopes and related equipment is presented. With these cathodes the emitting area is set by the extent of the active cathode surface and not by a control electrode, as is the case for the conventional triode electron microscope gun. This allows the accelerating field at the cathode surface to be greatly increased and the deleterious effects of space charge eliminated. The field can also be increased to the point that the Schottky effect enhances emission without the need to use sharply pointed cathodes. For example, considerable Schottky enhancement can be realized for cathodes with tip radii around 50 μm. The overall effect is that brightness can be increased by more than an order of magnitude over the standard triode gun. Other advantages are that the total beam current can be much higher than it is with field‐emission and thermal field‐emission cathodes and that the Boersch effect is reduced because no...

Manufacturing aspheric mirrors made of zero thermal expansion cordierite ceramics using Magnetorheological Finishing (MRF)

NEXCERATM cordierite ceramics, which have ultra-low thermal expansion properties, are perfect candidate materials to be used for light-weight satellite mirrors that are used for geostationary earth observation and for mirrors used in ground-based astronomical metrology. To manufacture the high precision aspheric shapes required, the deterministic aspherization and figure correction capabilities of Magnetorheological Finishing (MRF) are tested. First, a material compatibility test is performed to determine the best method for achieving the lowest surface roughness of RMS ~0.8nm on plano surfaces made of NEXCERATM ceramics. Secondly, we will use MRF to perform high precision figure correction and to induce a hyperbolic shape into a conventionally polished 100mm diameter sphere.

Freeform metrology using subaperture stitching interferometry

As applications for freeform optics continue to grow, the need for high-precision metrology is becoming more of a necessity. Currently, coordinate measuring machines (CMM) that implement touch probes or optical probes can measure the widest ranges of shapes of freeform optics, but these measurement solutions often lack sufficient lateral resolution and accuracy. Subaperture stitching interferometry (SSI™) extends traditional Fizeau interferometry to provide accurate, high-resolution measurements of flats, spheres, and aspheres, and development is currently on-going to enable measurements of freeform surfaces. We will present recent freeform metrology results, including repeatability and cross-test data. We will also present MRF® polishing results where the stitched data was used as the input “hitmap” to the deterministic polishing process.

Fine figure correction and other applications using novel MRF fluid designed for ultra-low roughness

An increasing number of technologies require ultra-low roughness (ULR) surfaces. Magnetorheological Finishing (MRF) is one of the options for meeting the roughness specifications for high-energy laser, EUV and X-ray applications. A novel MRF fluid, called C30, has been developed to finish surfaces to ULR. This novel MRF fluid is able to achieve <1.5Å RMS roughness on fused silica and other materials, but has a lower material removal rate with respect to other MRF fluids. As a result of these properties, C30 can also be used for applications in addition to finishing ULR surfaces. These applications include fine figure correction, figure correcting extremely soft materials and removing cosmetic defects. The effectiveness of these new applications is explored through experimental data. The low removal rate of C30 gives MRF the capability to fine figure correct low amplitude errors that are usually difficult to correct with higher removal rate fluids. The ability to figure correct extremely soft materials opens up MRF to a new realm of materials that are difficult to polish. C30 also offers the ability to remove cosmetic defects that often lead to failure during visual quality inspections. These new applications for C30 expand the niche in which MRF is typically used for.

Longer wavelength EUV lithography (LW-EUVL)

Extreme UV Lithography (EUVL) is generally accepted as the leading candidate for next generation lithography. Several challenges remain for EUVL, especially as its insertion point is pushed to finer resolution. Although diffractive scaling may suggest a transition to shorter EUVL wavelengths, several issues arise that would make that difficult. Challenges involve issues such as flare, multilayer (ML) bandwidth, and reflector throughput which tend to worsen with decreasing wavelength. In this study, we have evaluated the tradeoff between flare scaling effects and diffractive scaling effects for EUVL, where flare induced image degradation is likely to dominate as sub-13.5 nm wavelengths are considered. With surface scatter effects scaling as 1/λ2, the idea of longer wavelength (LW-EUVL) becomes interesting. Since a working wavelength is driven by the selection of ML materials (which are molybdenum and silicon for 13.5 nm), the identification of suitable alternatives is an initial challenge. We have optimized aluminum and various refractory metals at 17.2 nm and present results. The optimized combination of aluminum with yttrium, zirconium, and other metals result in theoretical reflectivity values above 75%. We also describe possibilities for alternative LW-EUVL sources for 17.2 nm operation as well as the impact on resist absorption, especially through halogens of higher molar absorption (such as fluorine). The impact on mask absorber materials is also presented, which may also exhibit increased absorbance, leading to a lowering of film thickness requirements.

Hybridization of a sigma-delta-based CMOS hybrid detector

The Rochester Imaging Detector Laboratory, University of Rochester, Infotonics Technology Center, and Jet Process Corporation developed a hybrid silicon detector with an on-chip sigma-delta (ΣΔ) ADC. This paper describes the process and reports the results of developing a fabrication process to robustly produce high-quality bump bonds to hybridize a back-illuminated detector with its ΣΔ ADC. The design utilizes aluminum pads on both the readout circuit and the photodiode array with interconnecting indium bumps between them. The development of the bump bonding process is discussed, including specific material choices, interim process structures, and final functionality. Results include measurements of bond integrity, cross-wafer uniformity of indium bumps, and effects of process parameters on the final product. Future plans for improving the bump bonding process are summarized.

New high-precision deep concave optical surface manufacturing capability

This paper describes the manufacturing steps necessary to manufacture hemispherical concave aspheric mirrors for high- NA systems. The process chain is considered from generation to final figuring and includes metrology testing during the various manufacturing steps. Corning Incorporated has developed this process by taking advantage of recent advances in commercially available Satisloh and QED Technologies equipment. Results are presented on a 100 mm concave radius nearly hemispherical (NA = 0.94) fused silica sphere with a better than 5 nm RMS figure. Part interferometric metrology was obtained on a QED stitching interferometer. Final figure was made possible by the implementation of a high-NA rotational MRF mode recently developed by QED Technologies which is used at Corning Incorporated for production. We also present results from a 75 mm concave radius (NA = 0.88) Corning ULE sphere that was produced using sub-aperture tools from generation to final figuring. This part demonstrates the production chain from blank to finished optics for high-NA concave asphere.