Feature Size Control Research Articles

Matrix-assisted dip-pen nanolithography and polymer pen lithography.

The controlled patterning of nanomaterials presents a major challenge to the field of nanolithography because of differences in size, shape and solubility of these materials. Matrix-assisted dip-pen nanolithography and polymer pen lithography provide a solution to this problem by utilizing a polymeric matrix that encapsulates the nanomaterials and delivers them to surfaces with precise control of feature size.

A simple and scalable route to wafer-size patterned graphene

Producing large-scale graphene films with controllable patterns is an essential component of graphene-based nanodevice fabrication. Current methods of graphene pattern preparation involve either high cost, low throughput patterning processes or sophisticated instruments, hindering their large-scale fabrication and practical applications. We report a simple, effective, and reproducible approach for patterning graphene films with controllable feature sizes and shapes. The patterns were generated using a versatile photocoupling chemistry. Features from micrometres to centimetres were fabricated using a conventional photolithography process. This method is simple, general, and applicable to a wide range of substrates including silicon wafers, glass slides, and metal films.

Shape and Feature Size Control of Colloidal Crystal-Based Patterns Using Stretched Polydimethylsiloxane Replica Molds

In this work, we fabricated various patterns using colloidal crystals as master molds via the soft lithography method. Even though colloidal crystals consist of spherical colloidal particles, nonspherical shaped patterns such as rectangular or elongated hexagonal shaped patterns can be fabricated using a stretched polydimethylsiloxane (PDMS) replica mold. The pattern shape and feature size can be easily controlled by changing the stretching axis and ratio of the PDMS replica mold. The deformations of the PDMS mold were simulated using the finite element method, and they are consistent with experimental results. The elongated patterns were used as templates to offer new types of colloidal crystal superlattice structures. A proposed pattern-control method will significantly expand the usefulness and diversity of micro/nanopatterning technology.

Redox-Activating Dip-Pen Nanolithography (RA-DPN)

Dip pen nanolithography (DPN) involves the direct transfer of an ink from a coated atomic force microscope (AFM) tip to a substrate of interest and uses as many as 55,000 pens to form arbitrary patterns of alkanethiols, oligonucleotides, proteins, and viruses. Two limitations of DPN are the difficulty in transporting high molecular weight inks and the need to optimize individually the transport rates and tip inking methods of each molecule. As an alternative strategy that circumvents these two challenges, a method termed redox activating DPN (RA-DPN) is reported. In this strategy, an electrochemically active, quinone functionalized surface is toggled from the reduced hydroquinone form to the oxidized benzoquinone form by the delivery of an oxidant by DPN. While the benzoquinone form is susceptible to nucleophilic attack in Michael-type additions, hydroquinone is not and acts as a passivating agent. Because both forms of the quinone are kinetically stable, the patterned surface can be immersed in a solution of a target containing any strong nucleophile, which will react only where the benzoquinone form persists on the surface. For proof-of-concept demonstrations, quinone surfaces were patterned by the delivery of the oxidant cerric ammonium nitrate and were immersed in solutions of AF549 labeled cholera toxin beta subunit or oligonucleotides modified at the 5' end with an amine and the 3' end with a fluorophore. Fluorescent patterns of both the proteins and oligonucleotides were observed by epifluorescence microscopy. Additionally, RA-DPN maintains the advantageous ability of DPN to control feature size by varying the dwell time of the tip on the surface, and variation of this parameter has resulted in feature sizes as small as 165 nm. With this resolution, patterns of 50,000 spots could be made in a 100 x 100 microm(2) grid.

Controllable Feature Sizes of Highly Conductive Poly(3,4-Ethylenedioxythiophene) Nanofilms Patterned on SiO2 Surface

In this paper, we report a novel patterning method for a poly(3,4-ethylenedioxythiophene) (PEDOT) nanofilm deposited on an OTS monolayer-coated silicon wafer substrate by the vapor phase polymerization method. To scrutinize the adhesion improvement, electrical conductivity and feature controllability, patterned PEDOT nanofilms were investigated with a Scotch tape peel test, I-V curve measurement, and optical and atomic force microscopes. The scrutinization strongly indicates that the adhesion improvement is most likely due to direct chemical bonds formed between ethylenedioxythiophene (EDOT) molecules and photo-oxidized OTS monolayer during a vapor phase polymerization reaction. The investigation also discovered that the feature size of the film can be chemically controlled by the reaction between OTS and reactive atomic oxygen gases, and the patterned films generally show a noticeably good electrical conductivity (approximately 500 S/cm at merely approximately 100 nm thick film). These results successfully demonstrate that the patterned PEDOT nanofilms are qualified enough to be employed as an electrode component of an OTFT device since the electrode materials must show an electrical conductivity of at least 50 S/cm or higher.

Polymer Pen Lithography

We report a low-cost, high-throughput scanning probe lithography method that uses a soft elastomeric tip array, rather than tips mounted on individual cantilevers, to deliver inks to a surface in a “direct write” manner. Polymer pen lithography merges the feature size control of dip-pen nanolithography with the large-area capability of contact printing. Because ink delivery is time and force dependent, features on the nanometer, micrometer, and macroscopic length scales can be formed with the same tip array. Arrays with as many as about 11 million pyramid-shaped pens can be brought into contact with substrates and readily leveled optically to ensure uniform pattern development.

The Integration Solution of Copper Barrier Deposition for Nanometer Interconnect Process

As the dimensions of devices are shrunk quickly, the requirements of metallization become more critical. For VIA barrier and seeding layer filling and deposition, the process was mostly applied with the copper physical vapor deposition methodology in the back-end of line flow of the interconnection metallization. The criteria for barrier and seeding layer deposition are the metal continuity inside the VIA feature and grain size and orientation control for film diffusion barrier and qualities. Besides, while the interconnection size shrunk to nano-scale, the barrier thickness would be very thinner to maintain the VIA resistance; however, it would face the film conformity and continuity consistence within the wafer and different features. The integration solution would be developed and studied with the re-sputter process step adding into the convectional physical vapor deposition process. The resputter process step could not only improve the film conformity and continuity in the VIA's sidewall; but also reduce the resistance of VIA feature over 20%. The improvement of the resputter method adding into the deposition process would be contributed to the standard barrier deposition in the nano-scale feature of the interconnect. Besides, we also discussed the effect of the film properties after the resputter process introduced into the barrier deposition.

A monolithic approach for topology optimization of electrostatically actuated devices

This paper presents a new topology optimization scheme for nonlinear electrostatic systems actuated by Coulomb’s forces. For successful optimization, computational issues such as (i) alternating governing equations with respect to spatially defined design variables, (ii) imposing interaction boundary conditions between insulator (air) and conductor (solid) and (iii) control of minimum geometrical feature sizes must be addressed. To address the first two issues, the paper presents a monolithic formulation based on continuum mechanics theory which simultaneously calculates the electric potential and structural displacements. To interpolate between insulator and conductor with continuous design variables, the monolithic approach distinguishes between the permittivity value of the electric Poisson’s equation and that of Maxwell’s stress tensor. For the optimization, standard material interpolations are used for Young’s modulus and permittivity values. Moreover, a recently developed morphology filter is applied to control electrode gaps and other geometrical features.

Biodegradable polymer tubes with lithographically controlled 3D micro- and nanotopography

We introduce a novel scaffold fabrication process for making tubular constructs for vascular tissue engineering. Every aspect of the scaffold has been engineered providing unprecedented control of feature size and placement over five orders of magnitude by a combination of electron beam and photolithography. A wide range of micro- and nanotopographies can be incorporated into the scaffold – these have been demonstrated to elicit specific cell responses, and utilising a specific topography can enhance the scaffold’s final performance.

Fabrication of a multiple-diameter branched network of microvascular channels with semi-circular cross-sections using xenon difluoride etching

The majority of microfluidic devices employ networks of channels that have rectangular cross-sections. At the microvascular scale of 30 to 300 microm in diameter, however, the distribution of fluid mechanical stresses and the induced shape of cultured cells will be quite different in a rectangular channel from the near-circular cross-sections seen in vivo. While round-cross-section channels have been produced before by wet etching, fine control of feature size has not been demonstrated, and prior work has only produced channels of a single diameter on a given device. In this work, the xenon difluoride process for isotropic etching of silicon was optimized for production of channels with semicircular cross-sections. This process was then used to produce a network of microvessel-scale semicylindrical channels on a silicon chip, the diameter of which was decreased with each level of branching. Additionally, it was demonstrated that endothelial cells will adhere to both the bottom and sides of these channels, indicating that such chips may be useful in the future for culturing in vitro models of the microvasculature.

Topographical evolution of sputtered chromium nitride thin films

The modification of the morphology, with special emphasis on the topographical evolution, of chromium nitride thin films has been studied by varying the sputter power, bias voltage, temperature, total pressure and Ar/N 2 ratio in an unbalanced magnetron sputtering process. Six different topography types (here designated pyramid, grain, crater, cone, ribbon and hillock) were identified. The growth conditions for each topography type are specified and summarized in a topography zone model showing the occurrence of each as function of temperature, Ar/N 2 ratio, deposition rate and bias voltage. Furthermore, the relationship between the size of the topographical features and the deposition parameters was investigated and is reported in detail. The control of topographical type and feature size appears sufficient to hold promise of generating topographies designed for specific functions.

Application of Mueller polarimetry in conical diffraction for critical dimension measurements in microelectronics

Fast and efficient metrology tools are required in microelectronics for control of ever-decreasing feature sizes. Optical techniques such as spectroscopic ellipsometry (SE) and normal incidence reflectometry are widely used for this task. In this work we investigate the potential of spectral Mueller polarimetry in conical diffraction for the characterization of 1D gratings, with particular emphasis on small critical dimensions (CDs). Mueller matrix spectra were taken in the visible (450-700 nm) wavelength range on a photoresist grating on a Si substrate with 70/240 nm CD/period nominal values, set at nine different azimuthal angles. These spectra were fitted with a rigorous coupled-wave analysis (RCWA) algorithm by using different models for the grating profile (rectangular and trapezoidal, with or without rounded corners). A detailed study of the stability and consistency of the optimal CD values, together with the variation of the merit function (the mean square deviation D2) around these values, clearly showed that for a given wavelength range, this technique can decouple some critical parameters (e.g., top and bottom CDs, left and right sidewall projections) much more efficiently than the usual SE.

Statistical variation analysis of sub-5-nm-sized electron-beam-induced deposits

We report on the statistical analysis of the variations in the size and position of sub-5nm tungsten-containing dots in regular arrays deposited by electron-beam-induced deposition. Full widths at half maximum of the dots are 4.2 and 2.0nm in average. It can be observed in the recorded annular dark-field images that there is a variation in intensity for these dots. We have analyzed these variations and it is found that the relative standard deviation for the mass per dot is 0.092 for the 4.2nm dots and 0.26 for the 2.0nm dots. Comparing this to a relative standard deviation in the estimated number of precursor molecules that are pinned down per dot of 0.041 for the 4.2nm dots and 0.11 for the 2.0nm dots, it appears that the dot-to-dot variation in mass for both dot sizes compares reasonably well with the values expected from Poisson statistics on the number of molecules per dot. It can be concluded that at these dimensions, the statistics on the number of pinned precursor molecules dominates the control of feature sizes.

Experimental and simulation comparison of electron-beam proximity correction

In electron-beam lithography, electron scattering in the resist, and underlying substrate limits the control over feature sizes [M. Parikh, J. Vac. Sci. Technol. 19, 1275 (1981)]. To correct for this effect, one applies the so-called “proximity correction” that adjusts the beam dose of each individual feature. The algorithms that are commonly used to calculate this correction model the electron scattering as a double-Gaussian function. However, a point-spread function (PSF) describes the scattering process more accurately. Therefore, we have investigated whether the use of the PSF in the proximity correction algorithm leads to an increase in feature size control; as compared to the double-Gaussian PSF. In order to compare the two PSFs directly, we have performed a proximity correction on a pattern of contact holes, using each of the approximations. Our measurements show that the algorithm using the PSF performs a substantially better proximity correction, leading to a more accurate control over the contact hole sizes. Moreover, we have developed a method to assess the proximity correction’s success using the SELID [M. Bohn et al., Proc. SPIE 5256, 695 (2003)] simulation program. This strongly reduces the need for costly and time-consuming experiments.

Dimensional metrology for nanometre-scale science and engineering: towards sub-nanometreaccurate encoders

Metrology is the science and engineering of measurement. It has played a crucial role inthe industrial revolution at the milli-inch length scale and in the semiconductorrevolution at the micrometre length scale. It is often proclaimed that we arestanding at the threshold of another industrial revolution, brought by the advent andmaturing of nanotechnology. We argue that for nanotechnology to have a similarlyrevolutionary effect a metrology infrastructure at and below the nanometre scale isinstrumental and has yet to be developed. This paper focuses on dimensionalmetrology, which concerns itself with the measurement of lengths and its applicationssuch as pattern placement and feature size control. We describe our efforts todevelop grating- and grid-based scales with sub-nanometre accuracy over 300 mmdimensions using the nanoruler—a scanning-beam interference lithography tool.

Impact of metallisation on resistor matching in a 0.35 μm analogue CMOS process

Feature size control in deep sub-micron CMOS processes requires attention to the local and global metal density. This can lead to the placement of metal on top of sensitive analogue components. In this study we report the first results on the impact of metal coverage on the matching of high and low ohmic polysilicon resistors in a 0.35 μm analogue CMOS technology. A significant degradation in matching is observed, especially when matched resistors have unequal metal coverage. In contrast to reported results in one earlier other technology, the mismatch degradation is not strongly influenced by process changes such as modified sinter temperature or the presence of titanium in the metallisation. Limited data show that the effect is also seen in bulk silicon resistors, with a larger magnitude. It is demonstrated that the most likely explanation for the effect is the impact of stress from the metal film on the silicon resistance. A consequence of the effect is that in analogue designs metal must be kept a sufficient distance (10 μm in this process) away from matched resistor pairs.

Arrays of Magnetic Nanoparticles Patterned via “Dip-Pen” Nanolithography

A versatile new method for generating magnetic nanostructures with dimensions ranging from several hundred nanometers to sub-100 nm is presented here. The method is based on dip-pen nanolithography and allows precise feature size control by tuning the tip–substrate contact time and the scan speed. The Figure shows an AFM topography image of an array of iron oxide nanodots on a gold substrate.

Silicon micromachining using a high-density plasma source

Dry etching of Si is critical in satisfying the demands of themicromachining industry. The micro-electro-mechanical systems (MEMS) communityrequires etches capable of high aspect ratios, vertical profiles, good featuresize control and etch uniformity along with high throughput to satisfyproduction requirements. Surface technology systems' (STS's) high-densityinductively coupled plasma (ICP) etch tool enables a wide range ofapplications to be realized whilst optimizing the above parameters.Components manufactured from Si using an STS ICP include accelerometers andgyroscopes for military, automotive and domestic applications. STS's advancedsilicon etch (ASETM) has also allowed the first generation ofMEMS-based optical switches and attenuators to reach the marketplace. Inaddition, a specialized application for fabricating the next generationphotolithography exposure masks has been optimized for 200 mm diameter wafers,to depths of ~750 µm.Where the profile is not critical, etch rates of greater than8 µm min-1 have been realized to replace previous methods such as wetetching. This is also the case for printer applications. Specializedapplications that require etching down to pyrex or oxide often result in theloss of feature size control at the interface; this is an industry wideproblem. STS have developed a technique to address this.The rapid progression of the industry has led to development of the STS ICP etch tool,as well as the process.

A New Family of Polymerizable Lyotropic Liquid Crystals: Control of Feature Size in Cross-Linked Inverted Hexagonal Assemblies via Monomer Structure

An efficient and versatile synthesis of a series of polymerizable amphiphilic mesogens that affords control over tail length and position of the polymerizable group is described. The synthesis employs a novel and facile method of preparing styrene ethers. The monomers are sodium salts of styrene ether-modified fatty acids that can be used to form cross-linkable inverted hexagonal (HII) lyotropic liquid crystal (LLC) phases at ambient temperature with controllable nanometer-scale dimensions. Examination of a series of regioisomers with the same alkyl chain length but with the styrene ether group at different locations along the chain revealed that the position of the styrene ether has a profound effect on the dimensions of the resulting HII phase at a fixed temperature and composition. Increasing overall monomer tail length also has a significant, although smaller, effect on the unit cell dimensions of the LLC phase. By controlling the structure of the LLC monomer in this manner, cross-linked HII phases wi...

Nanolithography using wet etched silicon nitride phase masks

A new technique for performing chromeless phase shift optical nanolithography is presented. Phase masks have been wet etched into silicon nitride membranes, using either hot H3PO4 or HF. Contact exposure through these masks results in developed photoresist features as small as 90 nm, even for masks with shallow sidewall slopes. The observed effect is attributed to the presence of an abrupt phase step at some point on the wet etch profile, although this does not need to correspond to a π phase shift in order to obtain good contrast exposures. Simulations have been performed for typical wet-etching profiles, and it is found that good contrast is expected for a very wide range of etch depths. In addition, the width of the null in the near-field intensity profile varies with etch depth, indicating that control of feature size is possible. This technique is simple and inexpensive, making it an attractive candidate for nanopatterning a wide variety of substrates.