High Spatial Frequency Roughness Research Articles

Surface microtopography evolution of monocrystalline silicon in chemical mechanical polishing

Chemical mechanical polishing (CMP) is a widely utilized technique for achieving ultra-smooth surfaces, yet a comprehensive understanding of the surface microtopography evolution remains elusive due to the intricate chemical and mechanical interactions inherent in chemical mechanical polishing. Accordingly, this study explores the evolution law of different spatial frequency features of surface microtopography of monocrystalline silicon by analyzing the changes in the power spectral density (PSD) curve. More importantly, the evolution mechanism of the chemical mechanical polishing surface micromorphology is revealed through the analysis of the power spectral density under different mechanical and chemical conditions. The results emphasize the noticeable impact of both mechanical and chemical actions on the evolution of different spatial frequency features in surface microtopography. The mechanical action of a large particle, less likely to contact the valley region with a small curvature radius, enhances the smoothing speed of high-spatial frequency roughness. Moreover, the mechanical action of polishing pad demonstrates a greater efficiency in smoothing mid-spatial frequency roughness when utilizing a hard pad. This is attributed to the limited impact of pad hardness on the removal depth of single particles within the reaction layer, rendering it less significant for high-spatial frequency roughness. Upon elevating the chemical reaction rate, a notable acceleration in the reduction of roughness in both mid- and high-spatial frequency regions is observed. Furthermore, insights derived from molecular dynamics (MD) simulations of monocrystalline silicon and quartz glass show that the manner of surface atom removal, whether through single atom removal or cluster removal, exerts a significant effect on the final high-spatial frequency components of the power spectral density. This study contributes valuable insights into the intricate formation mechanism of a smooth surface in chemical mechanical polishing.

Portable angle-resolved scattering system to measure high spatial frequency roughness for ultra-smooth surfaces

Ultra-smooth surfaces require stringent quality control of their high spatial frequency roughness (HSFR). Angle-resolved scattering (ARS) allows for fast, non-contact roughness measurement that is insensitive to environmental disturbances. However, there is a tradeoff between the roughness measurement bandwidth and the measurement speed for the current ARS methods. We propose a rapid ARS method for HSFR measurement. A prototype instrument is developed using 405 nm laser illumination and a CMOS sensor to capture the scattering distribution. By solving the inverse scattering problem under the Kirchhoff approximation, the power spectral density function can be obtained, from which the root mean square roughness is calculated. The method allows measurement of HSFR from 1 μm−1 to 2.5 μm−1 over a 2 mm area in one minute. Experiments on polished metal flats showed excellent agreement between our method and atomic force microscopy.

Ultra-precision optical surface fabricated by hydrodynamic effect polishing combined with magnetorheological finishing

In the traditional machining method, the surface quality will get worse during the surface figuring process while the surface figure will be destroyed after ultra-smooth polishing process. To obtain an ultra-precision optical surface, these two processes are conducted iteratively which is time-consuming and unstable. A new fabrication method combined with the magnetorheological finishing (MRF) and hydrodynamic effect polishing (HEP) technology was presented. The surface figure error can be quickly depressed by MRF due to its high deterministic figuring ability, while the surface quality will get worse due to the introduction of MRF polishing marks. The material removal of HEP depends on the chemical interaction between the nanoparticles and workpiece in elastic mode, and an ultrasmooth surface with sub-nanometer level can been easily achieved with the surface figure well maintained. The experiment results show that the MRF polishing marks are removed clearly and the surface figure is well maintained after HEP process. Combined with MRF, a quartz glass with initial figure accuracy of 0.415λ RMS (λ=632.8nm), mid-spatial frequency and high spatial frequency roughness of 0.407nm RMS and 0.525nm RMS has been improved to figure accuracy of 0.05λ RMS, mid-spatial frequency and high spatial frequency roughness of 0.268nm RMS and 0.163nm RMS. Power spectral density (PSD) analysis indicated that the surface spatial frequency error has all been greatly depressed. It demonstrates HEP is an effective ultra-smooth polishing method with good ability to maintain the initial surface figure.

Ultrasmooth Polishing with Sub-Angstrom Roughness on Fused Silica Surface

In order to superpolish fused silica surface, a non-contact polishing method is applied, which is called as computer numerical-controlled (CNC) micro-jet polishing (MJP) technology developed on the base of the hydrodynamic mechanism. In this paper, both the theory and the techniques about this new polishing technology are introduced in detail, and the curved surface of fused silica was polished. The results demonstrate that no new scratches were produced when the nanoparticles removed the atoms away from the surface. The roughness decreased monotonously with the removal of subsurface damage layer. And ideal Ultrasmooth surface without scratches was achieved by MJP with waveness less than 0.2nmrmsRMS while the high-spatial frequency roughness less than 0.1nmrmsRMS (sub-angstrom).

A soft X-ray beam-splitting multilayer optic for the NASA GEMS Bragg Reflection Polarimeter

A soft X-ray, beam-splitting, multilayer optic has been developed for the Bragg Reflection Polarimeter (BRP) on the NASA Gravity and Extreme Magnetism Small Explorer Mission (GEMS). The optic is designed to reflect 0.5 keV X-rays through a 90 degree angle to the BRP detector, and transmit 2-10 keV X-rays to the primary polarimeter. The transmission requirement prevents the use of a thick substrate, so a 2 micron thick polyimide membrane was used. Atomic force microscopy has shown the membrane to possess high spatial frequency roughness less than 0.2 nm rms, permitting adequate X-ray reflectance. A multilayer thin film was especially developed and deposited via magnetron sputtering with reflectance and transmission properties that satisfy the BRP requirements and with near-zero stress. Reflectance and transmission measurements of BRP prototype elements closely match theoretical predictions, both before and after rigorous environmental testing.

Low Energy Xe+ Ion Beam Machining of the Ultralow Expansion Glass Substrates for Aspherical Extreme Ultraviolet Lithography Projection Optics

Aspherical substrates for extreme ultraviolet lithography (EUVL) optics require an ultrahigh shape accuracy of less than 0.15 nm rms and a high-spatial frequency roughness (HSFR; spatial wavelength: less than 1 µm) of 0.12 nm rms. Generally, the ultra low expansion glass (ULE®) substrate with HSFR of 0.06–0.08 nm rms can be produced by mechanical machining methods. However, it is difficult to obtain the shape accuracy of less than 0.12 nm rms using mechanical machining methods. Therefore, ion beam figuring (IBF) may be adapted to final shape correction of the substrates for the projection optics of EUVL tools. In this study, we investigated the HSFR and machining rate of the ULE® substrate machined by a 0.3–1.0 keV Xe+ ion beam at off normal ion incidence angles and obtained the following results: the HSFRs of the ULE® substrate machined by a 1.0 keV Xe+ ion beam at a ion incidence angle of lower than 30° and a 0.3–0.5 keV Xe+ ion beam at an ion incidence angle of 0–45° are below 0.12 nm rms, which is smaller than the required HSFR specification of EUVL projection optics. From our experimental result and discussion, we concluded that the scan fine beam and tilt target mode smoothing for processing of the ULE® substrate meets the required specification of the HSFR (0.12 nm rms) of hemispherical ULE® substrates of EUVL projection optics.

Low energy Xe + ion beam machining of ULE® substrates for EUVL projection optics – Evaluation of high-spatial frequency roughness

Ion beam figuring (IBF) is a suitable technology for the final shape correction of substrates used in the projection optics of EUVL tools. In order to achieve HSFR below 0.10 nm rms, we have conducted our research on ion beam machining of the ULE® substrate by Xe + ion beam with energy less than 2 keV. The HSFR of the unprocessed ULE® surface was 0.06 nm rms, whereas the HSFR of the ULE® substrate machined by under 0.7 keV Xe + ion beam was less than 0.08 nm rms. This HSFR (0.08 nm rms) is lower than that (0.10 nm rms) of the ULE® substrate machined with Ar + ion beam. Therefore, Xe + ion beam with energy under 0.7 keV can be used for figure error correction of the ULE® substrates for projection optics used in commercially available EUVL exposure tools.

Two stage ion beam figuring and smoothening method for shape error correction of ULE® substrates of extreme ultraviolet lithography projection optics: Evaluation of high-spatial frequency roughness

The ULE® substrates used in projection optics of extreme ultraviolet lithography (EUVL) tools are mechanically prefinished with shape accuracy of several nanometer rms (specification: under 0.15 nm rms) with high-spatial frequency roughness (HSFR) (spatial wavelength: under 1 μm) of 0.06 nm rms. The ion beam figuring is used for the final shape error correction of the substrates at low spatial wavelength of greater than 1 mm using high energy (5–10 keV) Ar+ ion beam with 1 mm beam diameter. However, the surface roughness values of the ULE® substrates when machined by 5 and 10 keV Ar+ ion beams result in 0.15 and 0.17 nm rms, respectively; also it is to be noted that these values are greater than the required 0.15 nm rms specification of HFSR. Therefore, the authors developed a method where low energy (0.3 and 0.5 keV) ion beams are used for smoothening the surface of ULE® substrates, previously treated with high energy (5 and 10 keV) ion beams for figuring. By this two-stage operation of ion beam figuring and followed by smoothening, ULE® substrates with HSFR of 0.1 nm rms were obtained. Therefore, the authors conclude that the method presented here is one of the most effective methods for the figure error correction of ULE® substrate and will be useful in EUVL projection optics.

Low energy ion beam machining of ULE ® substrates: Evaluation of surface roughness

In order to apply ion beam figuring (IBF) to final shape correction of the substrates of projection optics for EUVL, ion beam machining characteristics such as high-spatial frequency roughness (HSFR) and mid-spatial frequency surface roughness (MSFR) of ULE ® substrate were investigated. Our previous research confirmed that the surface roughness of the ULE ® machined by Ar + ion beam with energy of 3–10 keV decreases with decreasing the ion beam energy. Therefore, we have conducted our research on ion beam machining of ULE ® substrate by Ar + ion beam with energy from 0.2 to 2 keV. The HSFR and MSFR of the mechanically pre-finished ULE ® substrate were 0.06 and 0.07 nm rms, respectively; whereas, the HSFR and MSFR of the substrate irradiated by Ar + ion beam at energy of 0.3 keV were less than 0.10 and 0.08 nm rms, respectively. The HSFR is the best result among our previous and other current research.

Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics

Cr/Sc and Ni/V multilayers, intended as normal incidence soft x-ray mirrors and Brewster angle polarizers, have been synthesized by employing a novel modulated low-energy and high-flux ion assistance as a means of engineering the interfaces between the subnanometer layers on an atomic scale during magnetron sputter deposition. To reduce both roughness and intermixing, the ion energy was modulated within each layer. The flat and abrupt interfaces yielded soft x-ray mirrors with near-normal incidence reflectances of R = 20.7% at the Sc 2p absorption edge and R = 2.7% at the V 2p absorption edge. Multilayers optimized for the Brewster angle showed a reflectance of R = 26.7% and an extinction ratio of R(s)/R(p)=5450 for Cr/Sc and R = 10% and R(s)/R(p)=4190 for Ni/V. Transmission electron microscopy investigations showed an amorphous Cr/Sc structure with an accumulating high spatial frequency roughness. For Ni/V the initial growth mode is amorphous and then turns crystalline after approximately 1/3 of the total thickness, with an accumulating low spatial frequency roughness as a consequence. Elastic recoil detection analyses showed that N was the major impurity in both Cr/Sc and Ni/V with concentrations of 15 at. % and 9 at. %, respectively, but also O (3 at. % and 1.3 at. %) and C (0.5 at. % and 1.9 at. %) were present. Simulations of the possible normal incidence reflective properties in the soft x-ray range of 100-600 eV are given, predicting that reflectivities of more than 31% for Cr/Sc and 5.8% for Ni/V can be achieved if better control of the impurities and the deposition process is employed. The simulations also show that Cr/Sc is a good candidate for mirrors for the photon energies between the absorption edges of B (E = 188 eV) and Sc (E = 398.8 eV).

Ion beam machining of Si layer deposited on Zerodur® substrate

Ion beam figuring is suitable for the final correction of the surface figure error of aspherical substrates using an extreme ultraviolet lithography tool. In ion beam figuring, however, the machined surfaces of substrates become rougher than the unprocessed surfaces. Moreover, the surface is positively charged due to the positive charges of impinging ions. In this experiment, a Si layer was deposited by ion beam sputtering on a Zerodur® substrate with a depth of ∼300nm; then, this was machined by an Ar+ ion beam with energies in the range of 3–10keV. The mid-spatial-frequency roughness of the surface machined to a depth of less than 50nm was comparable to that of an unprocessed surface. The high-spatial-frequency roughness (HSFR) of the unprocessed surface was 0.21nm rms, whereas the average HSFRs of the surface machined up to a depth of 50nm were 0.25, 0.33, 0.39, and 0.59nm root mean square at energies of 3, 5, 7, and 10keV, respectively. The HSFR of the machined surface increased with the ion beam energy.

Evaluation of surface roughness of ULE® substrates machined by Ar+ ion beam

Ion beam figuring is suitable for the final correction of the surface figure error of aspherical substrates in an extreme ultraviolet lithography optics. This paper reports the investigation of the surface roughness of ULE® substrate machined by an Ar+ ion beam of 3–10keV. For an Ar+ ion beam in the range of 3–10keV, the mid-spatial frequency roughness of the surfaces machined to a depth of 50nm was comparable to the surface roughness of an unprocessed one. The high-spatial frequency roughness of the unprocessed surface was 0.08nm rms; whereas, the HSFRs of the surface of the substrate at Ar+ ion beam of 3, 5, 7, and 10keV were 0.10, 0.14, 0.16, and 0.17nm rms, respectively.

Evaluation of surface roughness of Zerodur® substrates machined by Ar+ ion beam with energy of 3–10keV

Ion beam figuring is suitable for the final correction of the surface figure error of aspherical substrates in an extreme ultraviolet lithography apparatus. In ion beam processing, however, the surfaces are considered to become rougher. This paper reports the investigation of the surface roughness of Zerodur® machined with an Ar+ ion beam having an energy of 3–10keV. For an Ar+ ion beam with energies in the range of 3–10keV, the mid-spatial frequency roughness of the surfaces machined to a depth of less than 50nm was comparable to the surface roughness of an unprocessed one. The high-spatial frequency roughness of the unprocessed surface was 0.26nm rms; whereas the high-spatial frequency roughness of the surface machined with energies of 3, 5, 7, and 10keV was 0.46, 0.54, 0.63, and 0.65nm rms, respectively. The HSFR of the machined surface increases with an increase in the energy of the ion beam.

Atomic force microscopy characterization of Zerodur mirror substrates for the extreme ultraviolet telescopes aboard NASA's Solar Dynamics Observatory

The high-spatial frequency roughness of a mirror operating at extreme ultraviolet (EUV) wavelengths is crucial for the reflective performance and is subject to very stringent specifications. To understand and predict mirror performance, precision metrology is required for measuring the surface roughness. Zerodur mirror substrates made by two different polishing vendors for a suite of EUV telescopes for solar physics were characterized by atomic force microscopy (AFM). The AFM measurements revealed features in the topography of each substrate that are associated with specific polishing techniques. Theoretical predictions of the mirror performance based on the AFM-measured high-spatial-frequency roughness are in good agreement with EUV reflectance measurements of the mirrors after multilayer coating.

A Silicon-Based, Sequential Coat-and-Etch Process to Fabricate Nearly Perfect Substrate Surfaces

For many thin-film applications substrate imperfections such as particles, pits, scratches, and general roughness, can nucleate film defects which can severely detract from the coating's performance. Previously we developed a coat-and-etch process, termed the ion beam thin film planarization process, to planarize substrate particles up to approximately 70 nm in diameter. The process relied on normal incidence etching; however, such a process induces defects nucleated by substrate pits to grow much larger. We have since developed a coat-and-etch process to planarize approximately 70 nm deep by 70 nm wide substrate pits; it relies on etching at an off-normal incidence angle, i.e., an angle of approximately 470 degrees from the substrate normal. However, a disadvantage of this pit smoothing process is that it induces defects nucleated by substrate particles to grow larger. Combining elements from both processes we have been able to develop a silicon-based, coat-and-etch process to successfully planarize approximately 70 nm substrate particles and pits simultaneously to at or below 1 nm in height; this value is important for applications such as extreme ultraviolet lithography (EUVL) masks. The coat-and-etch process has an added ability to significantly reduce high-spatial frequency roughness, rendering a nearly perfect substrate surface.

Effect of surface quality on transmission performance for (111) CaF 2

With the advent of 193 nm optical lithography, calcium fluoride (CaF 2) has become the material of choice for excimer laser components and for chromatic aberration correction of stepper lens systems. Paralleling the need to maximize transmission performance is the challenge of processing CaF 2 to surface qualities equal to if not superior than for conventional fused silica optics. In this report, (111) calcium fluoride crystals were processed to two different surface qualities and tested for transmittance from 185 to 400 nm. Results show that surface quality in terms of mid- and high-spatial frequency roughness and subsurface damage strongly affect transmittance with a greater increase in transmittance with improved surface quality at lower wavelengths. Specifically for 193 nm, transmittance increased from 90.30% for a 1.4 nm RMS surface to 91.98% for a 0.8 nm RMS surface ( Δ=1.68%).

An ion-assisted Mo-Si deposition process for planarizing reticle substrates for extreme ultraviolet lithography

Substrate particles are a serious concern in the fabrication of reticles for extreme ultraviolet lithography (EUVL) because they nucleate defects in the reflective multilayer films that can print in the lithographic image. We have developed a strategy for planarizing reticle substrates with smoothing-layers and, in this letter, we investigate the smoothing properties of an ion-assisted Mo-Si deposition process. We have observed that ion-assistance can significantly improve the particle-smoothing properties of Mo-Si multilayer films and can do so without a significant increase in the high-spatial frequency roughness of the multilayer film. An ion-assisted Mo-Si smoothing-layer approach to reticle substrate planarization, therefore, shows significant promise for defect mitigation in EUVL reticles.

Smoothing Thin Films with Gas-Cluster Ion Beams

ABSTRACTThin-film magnetic sensor and memory devices in future generations may benefit from a processing tool for final-step etching and smoothing of surfaces to nearly an atomic scale. Gas-cluster ion-beam (GCIB) systems make possible improved surface sputtering and processing for many types of materials. We propose application of GCIB processing as a key smoothing step in thin-film magnetic-materials technology, especially spin-valve GMR. Results of argon GCIB etching and smoothing of surfaces of alumina, silicon, permalloy and tantalum films are reported. No accumulating roughness or damage is observed. The distinct scratches and tracks seen in atomic-force microscopy of CMP-processed surfaces, are removed almost entirely by subsequent GCIB processing. The technique primarily reduces high spatial-frequency roughness and renders the topographic surface elevations more nearly gaussian (randomly distributed).

Analysis of interface roughness’s effect on metal–oxide–semiconductor Fowler-Nordheim tunneling behavior using atomic force microscope images

For the first time, metal–oxide–semiconductor interface atomic force microscope images were used to directly calculate the Fowler-Nordheim tunneling behavior for thin gate oxides with different interface roughness. Assuming that the roughness is conformal on both interfaces, surface curvature and electric field distribution were used to calculate the tunneling current density caused by low spatial frequency roughness. On the other hand, a three-dimensional Laplace solver and a quasi-one-dimensional Fowler-Nordheim equation were used to calculate the tunneling current contributed by high spatial frequency roughness. Considering contributions due to both high and low spatial frequency roughnesses, we found good agreement between the theoretical calculation and experimentally measured tunneling current–voltage curves. This simulation indicates that oxide thinning is probably not the main reason for the increase of tunneling current in the thin oxide (∼6 nm) range. The higher surface electric field due to roughness curvature itself is most likely responsible for this increase.

Characterization of the low-pressure chemical vapor deposition grown rugged polysilicon surface using atomic force microscopy

In this work, rugged polysilicon films are deposited in a state-of-the-art low-pressure chemical vapor deposition reactor to produce the plates of dynamic random access memory charge storage capacitors with higher capacitance per unit area. The films are deposited at temperatures in the vicinity of 575 °C. The 〈311〉 textured, in situ crystallized films have high surface roughness as compared to the usual rather smooth, 〈110〉 textured, as deposited polysilicon. Atomic force microscopy measurement of the roughness of these films posed a challenge since the rugged polysilicon films are made up of a very hard material with a large amplitude of high spatial frequency roughness. A solution was found which involves using probes on which a carbon film has been applied to the standard Si probes. Combining these probes with carefully selected measurement parameters and proper data reduction techniques has produced good results. Using this solution has allowed us to measure the roughness of the rugged polysilicon films as a function of the critically controlled growth temperature and pressure. Substantial differences were found with small changes in temperature, with less difference coming from pressure variations. 570 °C and 200 mTorr produced the best results for large batch processing.