Feature Sidewall Research Articles

Sidewall profiles in thick resist with direct image lithography

Maskless lithography is an increasingly popular method of micro-patterning in research settings. This technical note presents methods for controlling the feature size and sidewall profile of thick-resist features, when exposed using maskless direct imaging systems. Maskless lithographic systems use focussed, uncollimated light, and therefore do not naturally produce the vertical sidewalls that may be required for soft lithography and other secondary processes. We explore exposure dose energy, placement of the focal plane, and the use of multiple focal planes to optimize features in thick (∼800 μm) SU-8 resist. We find that placing the image plane at the mid-height of the resist film produces structures with a good compromise of sidewall angle and feature sharpness. We also find that using multiple exposures at multiple heights can produce sidewall angles that are stable over a range of dose energies.

Optical design optimization of high contrast light guide plate for front light unit

This study addresses the optical design of LED edge-lit light guide plate (LGP) for front light unit panel of high illuminating contrast. Front light displays have excellent readability in outdoor environment with bright ambient light. A LED light guide is still required as an auxiliary lighting, referred to as a front light unit (FLU), in a dim environment. The ray tracing of the FLU light guide plate can be roughly classified into the noise rays refracted out of LGP directly and the image rays refracted downward to illuminate the reading media. Only the image rays is effective for FLU, and the noise ray will result in glare problems. This study defines the Effective Contrast Ratio (ECR) as the percentage of the image rays over the total rays within the ± 30° viewing angle. The layout of cylindrical microstructures are analyzed to determine a preferred configuration design from either concave or convex features deployed on the emitting surface or the bottom surface of LGP. Thought the ray tracing simulations of both the convex bottom and the concave bottom feature designs show excellent ECR over 90%, the concave design is preferred for better durability and assembly concerns at a cost of lower illuminating efficiency. The optimum parameters of the truncated cone microfeature with maximum ECR are identified as the base diameter of 70 (μm), the aspect ratio of 0.46, and the feature sidewall angle of 78° using a sequential optimization of Kriging Surrogate. A preliminary prototype of the LGP has been fabricated using Polydimethylsiloxane (PDMS) molding process from a SU8 master. A qualitative comparison of the photographic pictures of a sample text demonstrates the excellent transparency and uniform illumination of the LGP with the proposed feature design.

Ion Beam Etch for Patterning of Resistive RAM (ReRAM) Devices

ABSTRACTWe investigate the feasibility of inert ion beam etch (IBE) for subtractive patterning of ReRAM-type structures. We report on the role of the angle-dependent ion beam etch rates in device area control and the minimization of sidewall re-deposition. The etch rates of key ReRAM materials are presented versus incidence angle and ion beam energy. As the ion beam voltage is increased, we demonstrate a significant enhancement in the relative etch rate at glancing incidence (for example, by a factor of 2 for HfO2). Since the feature sidewall is typically exposed to glancing incidence, this energy-dependence plays a role in optimization of the feature shape and in sidewall re-deposition removal.We present results of SRIM simulations to estimate depth of ion-bombardment damage to the TMO sidewall. Damage is minimized by minimizing ion energy; its depth can be reduced by roughly a factor of 5 over typical IBE energy ranges. For example, ion energies of less than ∼250 eV are indicated to maintain damage below ∼1nm. Multi-angle and multi-energy etch schemes are proposed to maximize sidewall angle and minimize damage, while eliminating re-deposition across the TMO. We utilize 2-D geometry/3-D etch model to simulate IBE patterning of tight-pitched ReRAM features, and generate etched feature shapes.

The Application of Advanced Pulsed Plasma in Fin Etch Loading Improvement

With the background of general integrated circuit (IC) feature size shrinking designers to get high performance transistors, Self-Alignment-Double-Patterning (SADP) FinFETetch was widely used in 14/16 nm technologies node and beyond. Rigorous process loading control (CD, profile, depth) in reactive ion etch (RIE) becomes more critical and challenging to ensure precise patterning and to avoid performance degradation among different features.[1]However, it’s quite hard to tackle such notorious loading for its inherent high Te induced ionization in traditional continuous wave (CW) plasmas.Thus, significant improvements in controlling plasma properties in a dry etching reactor are essential to satisfy such stringent requirement. Pulsed plasmas have demonstrated several advantages compared to CW plasmas. By varying the pulse frequency and the duty cycle, pulsed plasma provide additional “control knobs” in controlling critical plasma parameters, such as ion and electron densities, electron temperature, flux and energy of the ions and radicals incident on surface. These critical factors affecting overall on-wafer performance during plasma etching processes at both within wafer and within die levels, etching and deposition, vertical and lateral etch rate, profile control, feature sidewall passivation and plasma damage.[2]In view of characters of pulsed plasma noted above, it appears to be a promising approach to improve RIE-lag loading in SADP fin etch. In this paper, we systematically investigated the influence of both bias pulsed plasma and synchronous pulsed plasma to fin CD, profile (sidewall angle) and depth loading performance among dense and semi-isolated lines. It was found that proper duty cycle and frequency of pulsed power could drastically impact the loading performance. Based on the above learning, we finally delivered the overall solution by introducing synchronous pulsed plasma etch in mandrel and shallow trench isolation (STI) to address the rigorous loading control between dense and isolated patterns. [1] Y. Wang, et al, CSTIC, “Process Loading Reduction on SADP FinFET Etch”, 2015 [2] S. Banna, et al, JVST A, “Pulsed High-density Plasmas for Advanced Dry Etching Process”, 2012

Image quality and pattern transfer in directed self assembly with block-selective atomic layer deposition

Self-assembled block copolymer patterns may render more robust masks for plasma etch transfer through block-selective infiltration with metal oxides, affording opportunities for improved high contrast, high fidelity pattern transfer for sub-15 nm lithography in wafer-scale processes. However, block selective infiltration alters the self-assembled block copolymer latent image by changing feature size, duty cycle, and sidewall profile. The authors systematically investigate the effects of aluminum oxide infiltration of 27 and 41 nm pitch line/space patterns formed using polystyrene-b-poly(methyl methacrylate) block copolymers and evaluate the process compatibility with directed self assembly. The degree of image distortion depends on the amount of infiltrated material, with smaller amounts resulting in complete mask hardening and larger amounts shifting and collapsing pattern features. An attractive feature of the resulting oxide mask is the relatively smooth line edge roughness of the final transferred features into Si with a 3σ = 2.9 nm line edge roughness.

Understanding the relationship between true and measured resist feature critical dimension and line edge roughness using a detailed scanning electron microscopy simulator

Top-down critical dimension scanning electron microscopy (SEM) is still the workhorse metrology tool used for nanoscale structure analysis, such as measurement of photoresist features, during integrated circuit manufacturing. However, the degree to which top-down SEM imaging can accurately be used to quantitatively determine the size, shape, and roughness characteristics of three-dimensional structures such as photoresist features has not been carefully characterized. A rigorous Monte Carlo simulation of scanning electron microscopy has been developed to probe the relationship between the roughness of a three-dimensional feature and the line edge roughness (LER) as measured by SEM. The model uses the differential Mott cross section to compute elastic scattering, while inelastic scattering and secondary electron generation are handled using dielectric function theory. The model can calculate the electron scattering for any arbitrary three-dimensional geometry. Experimental SEM measurements of photoresist nanostructures show good agreement with the simulation output. The critical dimension of the resist determined from SEM best matches the true resist feature width when the line edge is defined using a high image threshold because the roughness on the outer edge of the resist tends to cause an increase in SEM signal that is nonproportional to the amount of material on the outer edge of the feature. LER determined from SEM was found to be significantly smaller than the true resist feature sidewall roughness. The measured LER is typically greater than 50% smaller than the actual sidewall roughness.

Sidewall damage in plasma etching of Si/SiGe heterostructures

Plasma etching is a critical tool in the fabrication of Si/SiGe heterostructure quantum devices, but it also presents challenges, including damage to etched feature sidewalls that affects device performance. Chemical and structural changes in device feature sidewalls associated with plasma-surface interactions are considered damage, as they affect band structure and electrical conduction in the active region of the device. Here the authors report the results of experiments designed to better understand the mechanisms of plasma-induced sidewall damage in modulation-doped Si/SiGe heterostructures containing a two-dimensional electron gas. Damage to straight wires fabricated in the heterostructure using plasma etching was characterized both by measuring the width of the nonconductive “sidewall depletion” region at the device sidewall and by measuring the noise level factor γH/N determined from spectra of the low frequency noise. Observed increases in sidewall depletion width with increasing etch depth are tentatively attributed to the increase in total number of sidewall defects with increased plasma exposure time. Excess negative charge trapped on the feature sidewall could be another contributing factor. Defects at the bottom of etched features appear to contribute minimally. The noise level shows a minimum at an ion bombardment energy of ∼100 eV, while the sidewall depletion width is independent of bias voltage, within experimental uncertainty. A proposed explanation of the noise trend involves two competing effects as ion energy increases: the increase in damage caused by each bombarding ion and the reduction in total number of incident ions due to shorter etch times.

Quantitative characterizations of a nanopatterned bonded wafer: force determination fornanoimprint lithography stamp removal

The nanoimprint lithography process consists of two mechanical steps: molding and stampremoval. While many publications dealing with anti-sticking layer properties or theunderstanding of polymer flow during imprinting have recently been published, only a fewstudies have been carried out to deeply characterize the demolding step. Regarding thesmall amount of theoretical work dedicated to this issue, in this paper both experimentaland first theoretical approaches are proposed to characterize the demolding process in apeeling scheme. Full 200 mm stamp and imprinted wafers were used to identify theexperimental limitation of such a full wafer peeling demolding scheme. A rectangularstamp and substrate samples with or without nanoscale features combined with anaugmented beam theory are proposed to extract quantitative data for the requireddemolding force as well as the friction stress along the feature sidewall. Thereforeboth adhesion and friction forces were characterized on single stamp structures.

Submicron resolution of secondary radiation in LIGA polymethyl-methacrylate resist exposure

Secondary radiation during LIGA polymethyl-methacrylate (PMMA) resist exposure adversely affects feature definition, sidewall taper, and overall sidewall offset. Additionally, it can degrade the resist adjacent to the substrate, leading to the loss of free-standing features through undercutting during resist development or through mechanical failure of the degraded material. The source of this radiation includes photoelectrons, Auger electrons, fluorescence photons, etc. Sandia's Integrated Tiger Series (ITS), a coupled electron/photon Monte Carlo transport code, is used to compute dose profiles within 1 to 2 µm of the absorber edge and near the interface of the resist with a metallized substrate. The difficulty of submicron resolution requirement was overcome by solving a few local problems, having carefully designed micron-scale geometries. The results for a 10-keV x-ray photons source indicate a 2-µm dose transition region near the absorber edge, resulting from PMMA photoelectrons. This region leads to sidewall offset and to tapered sidewalls following resist development. The results also show a dose boundary layer of around 1 µm near the substrate interface due to electrons emitted from the substrate metallization layer. The maximum dose at the resist bottom under the absorber can be very high and can lead to feature loss during development. This model is also used to investigate resist doses resulting from multilayer substrate.

Model for a multiple-step deep Si etch process

A multiple-step deep Si etch process involving separate etching and polymerization steps is often employed for fabrication of microelectromechanical systems, microfluidics devices, and other assorted deep structures in Si. An integrated plasma equipment-feature evolution model for this multiple-step deep Si etch process is described in this article. In the two-dimensional plasma equipment model, the etching (SF6/O2) and polymerization [octafluorocyclobutane(c-C4F8)] chemistries are separately simulated assuming steady-state conditions. The outputs of the equipment simulations are combined in a string-based feature profile evolution model to simulate the multiple-step deep Si etch process. In the plasma equipment models, detailed gas phase plasma chemistries including electron impact processes, ion–molecule reactions, and neutral chemistry have been considered for both the etching and polymerization gas mixtures. The plasma–surface interaction mechanisms in the feature profile evolution model are based on qualitative information available in literature and the correlation of modeling results with experimental data. Under the relevant operating conditions, F is assumed to be the primary Si etchant, film deposition in c-C4F8 is due to sticking of C, CF2, and C2F4 under ion bombardment, and the polymer is etched by energetic ions through physical sputtering. It is demonstrated that predictions of the resulting model are in close agreement with experiments. The validated model is used to understand the dynamics of the multiple-step deep Si etch process and how etching characteristics can be controlled using a variety of process parameters. Etching characteristics have been found to be quite sensitive to gas pressure, coil power, bias power, and relative step time during both etching and polymerization processes. The Si etch rate and feature sidewall angle are coupled to each other over a wide range of operating conditions.

The influence of X-ray fluorescence on LIGA sidewall tolerances

Fluorescence radiation during LIGA X-ray exposure and the impact of the resulting secondary dose on feature sidewall tolerances are examined using numerical methods. The study addresses fluorescence emitted by the substrate of the PMMA resist and focuses on the tolerances obtained for resists patterned with a range of feature sizes. New models of the secondary dose and subsequent feature development are presented and discussed. These models are coupled such that the evolving feature geometry during development depends on the local total dose, as well as the development temperature and the transport of PMMA fragments away from the dissolution front. We find that sidewall tolerances for typical exposure and development conditions are less than 0.1 micron for a metallized silicon substrate, but exceed several microns for thick titanium, copper and nickel substrates. We also find that these tolerances are very sensitive to the feature geometry, development temperature and the magnitude of the primary dose.

High-density plasma patterning of low dielectric constant polymers: A comparison between polytetrafluoroethylene, parylene-N, and poly(arylene ether)

The pattern transfer of SiO2 hard masks into polytetrafluoroethylene, parylene-N, and poly(arylene ether) (PAE-2) has been characterized in an inductively coupled plasma source. Selected results obtained with blanket parylene-AF4 films are included in this work. These dielectrics offer a relatively low dielectric constant (k∼2–3) and are candidate materials for use as intra- and interlayer dielectrics for the next generations of high-speed electronic devices. Successful patterning conditions were identified for Ar/O2 and N2/O2 gas mixtures. It was found that the formation of straight sidewalls in Ar/O2 discharges relies on the redeposition of oxygen-deficient etch products on the feature sidewall. Furthermore, the etch rates of parylene-N, parylene-F, and PAE-2 for blanket and patterned films could be captured by a semiempirical surface coverage model, which balances the adsorption rate of oxygen and the ion-induced desorption rate of oxygenated etch products.

The influence of feature sidewall tolerance on minimum absorber thickness for LIGA x-ray masks

Minimizing mask absorber thickness is an important practical concern in producing very small features by the LIGA process. To assist in this minimization, we have developed coupled numerical models describing both the exposure and development of a thick polymethyl methacrylate (PMMA) resist. The exposure model addresses multi-wavelength, one-dimensional x-ray transmission through multiple beam filters, through the mask substrate and absorber, and the subsequent attenuation and photon absorption in the PMMA resist. The development model describes one-dimensional dissolution of a feature and its sidewalls, taking into account the variation in absorbed dose through the PMMA thickness. These exposure and development models are coupled in a single interactive code, permitting the automated adjustment of mask absorber thickness to yield a prescribed sidewall taper or dissolution distance. We have used this tool to compute the minimum required absorber thickness yielding a prescribed sidewall tolerance for exposures performed at the ALS, SSRL and NSLS synchrotron sources. Results are presented as a function of the absorbed dose for a range of the prescribed sidewall tolerance, feature size, PMMA thickness, mask substrate thickness and the development temperature.