Subtractive Patterning Research Articles

Molecular dynamics investigation of patterning via cold welding

We perform a series of molecular dynamics simulations of a subtractive cold-welding patterning process. The effects of film thickness and work of adhesion between the thin film and substrate are examined. For small works of adhesion, the film elastically debonds from the substrate before the onset of plastic deformation inside the film during stamp retraction. A simple model is proposed to describe the debonding and deformation of the film. The model provides an analytical framework that describes the playoff between adhesion, yield strength, and film thickness in determining the debond length of the film induced by stamp retraction.

The effect of pH on the electrochemical over-oxidation in PEDOT:PSS films

Chemical degradation of conjugated polymers is one cause of material failures in polymer-based (opto)electronic devices, but can also be used as a technique for subtractive patterning of polymer films. When a large anodic potential is applied to the conducting polymer blend poly(3,4-ethylenedioxythiophene)-poly(4styrenesulfonate), PEDOT:PSS, an over-oxidation reaction occurs, altering its electrical conductivity. Here, we have studied the effect of pH on the electrochemical over-oxidation process of PEDOT in PEDOT:PSS. High pH is associated with a decrease of over-oxidation potential and an increase of resistivity in the resulting film. Vibrational spectroscopy and photoelectron spectroscopy measurements on over-oxidized PEDOT:PSS films indicate that the decrease in conductivity results from cleavage of the conjugation pathway accompanied by the formation of sulfone, carbonyl and carboxylic groups in the polymer chain.

Nanoelectronics beyond silicon

The further scaling of silicon devices will hit some roadblocks in the future that may not be overcome by standard technology. The introduction of new materials into silicon technology may help for a while, but integration issues have to be solved in advance. Improvements can, however, also be achieved by new material configurations like nanocrystals that are not formed by subtractive patterning techniques but by self-organized bottom-up approaches. In this paper, we address the prospects for nanoelectronics to exploit the tremendous possibilities of carbon nanotubes (CNTs). A brief introduction of the CNT basics and growth methods will be given. A comparison of state-of-the-art silicon MOSFETs with CNT-FETs highlights the significantly better performance of CNT devices. However, the patterning of CNT devices is still an unsolved problem. Some of the most promising fabrication techniques are assessed with respect to integration into a silicon-like technology. Finally, a new device is presented that circumvents the selection and placement problems of CNTs for stand alone transistors.

A Lithographic Process for Integrated Organic Field-Effect Transistors

This paper reports a photolithographic process for fabricating organic field-effect transistors which provides two layers of metal with arbitrary via placement, and optionally allows for subtractive lithographic patterning of the transistor active layer. The demonstrated pentacene transistors have a field-effect mobility of 0.1/spl plusmn/0.05 cm/sup 2//(V/spl middot/s). Parylene-C is used both as the gate dielectric and an encapsulation layer which allows for subtractive lithographic patterning. Also demonstrated is a PMOS inverter without level shifting circuitry and level-restoring V/sub High/ and V/sub Low/. This work demonstrates a high definition, multilayer, integrated photolithographic process which creates organic field effect transistors suitable for use in integrated circuit applications such as a display backplanes.

Photolithographic patterning of organic electronic materials

The realization of organic electronic technologies requires the availability of patterning techniques that are compatible with chemically sensitive materials. We demonstrate an approach that allows the photolithographic patterning of organic films without their exposure to harmful solvents, and achieves micrometer resolution. Examples of additive and subtractive patterning of polymers as well as small molecules show this approach to be quite generic. The fabrication of a pentacene transistor with a 2 μm channel length and conducting polymer electrodes is demonstrated.

Effects of Atorvastatin and Simvastatin on Low-Density Lipoprotein Subtraction Profile, Low-Density Lipoprotein Oxidizability, and Antibodies to Oxidized Low-Density Lipoprotein in Relation to Carotid Intima Media Thickness in Familial Hypercholesterolemia

Background Little is known about the effects of statins on the quality of circulating low-density lipoprotein (LDL) in relation to atherosclerosis progression. Methods In a double-blind, randomized trial of 325 patients with familial hypercholesterolemia (FH), we assessed the effects of high-dose atorvastatin (80 mg) and conventional-dose simvastatin (40 mg) on LDL subtraction profile ( n = 289), LDL oxidizability ( n = 121), and circulating autoantibodies to oxidized LDL ( n = 220). Progression of atherosclerosis was measured by carotid intima media thickness (IMT) ( n = 325). Results At baseline, the patients showed an intermediate LDL subtraction profile composed of three LDL subtractions (LDL1, LDL2, LDL3), with LDL2 as the predominant subtraction. A strong negative correlation was found between plasma triglycerides and the LDL subtraction profile ( r = - .64, p = .000). Both plasma levels of triglycerides and small dense LDL3 correlated weakly with baseline IMT ( r = .11, p = .04 and r = .15, p = .01, respectively; n = 289). No association was found between baseline IMT and oxidation parameters or circulating antibodies to oxidized LDL. Atorvastatin reduced triglycerides, LDL cholesterol, and all LDL subtractions to a greater extent than did simvastatin and led to regression of carotid IMT. However, LDL subtraction pattern and plasma levels of autoantibodies to oxidized LDL remained unchanged in both treatment groups, and LDL oxidizability increased minimally to a similar extent in both groups. Significant treatment differences were found for the rate of in vitro oxidation of LDL and the amount of dienes formed during in vitro oxidation of LDL, which both decreased more following atorvastatin than after simvastatin. Conclusion Change of IMT after statin treatment was associated with baseline IMT ( r = .41), LDL cholesterol ( r = -.20), and the amount of dienes formed during in vitro oxidation of LDL ( r = .28) but not with plasma levels of antibodies to oxidized LDL, in vitro LDL oxidizability, and LDL subtraction profile.

Submicrometer-Scale Patterning of Ceramic Thin Films

The patterning of ceramic thin films is of great interest for use in MEMS and other applications. However, the complex chemistries of certain materials make the use of traditional photolithography techniques prohibitive. In this paper, a number of low-cost, high throughput techniques for the patterning of ceramic thin films derived from chemical solution precursors, such as sol-gels and ceramic slurries, are presented. A particular emphasis is placed on methods that are derived from soft lithographic methods using elastomer molds. Two categories of techniques are discussed: first, the focus is on methods that rely on the principles of confinement within the physical features of the mold to define the pattern on the substrate surface. Then, subtractive patterning techniques that rely on transferring a pattern to a spin-cast, large-area continuous thin film are described. While most techniques have been demonstrated with fidelities on the order of 100 nm, their inability to precisely register and align the patterns as part of a hierarchical fabrication scheme have thus far hindered their commercial implementation.

An expression pattern screen for genes involved in the induction of the posterior nervous system of zebrafish

The posterior nervous system, including the hindbrain and the spinal cord, has been shown to be formed by the transformation of neural plate of anterior character by signals derived from non-axial mesoderm. Although secreted factors, such as fibroblast growth factors (FGFs), Wnts, retinoic acid (RA) and Nodal, have been proposed to be the posteriorizing factors, the mechanism how neural tissue of posterior character is induced and subsequently specified along the anteroposterior axis remains elusive. To identify intercellular signaling molecules responsible for posteriorization of the neural plate as well as to find molecules induced intracellularly by the posteriorizing signal in the caudal neural plate, we screened by in situ hybridization for genes specifically expressed in posterior tissues, including the posterior neural plate and non-axial mesoderm when posteriorization of the neural plate takes place. From a subtracted library differentiating anterior versus posterior neural plate, 420 cDNA clones were tested, out of which 76 cDNA fragments showed expression restricted to the posterior tissue. These clones turned out to represent 32 different genes, including one novel secreted factor and one transmembrane protein. Seven genes were induced by non-axial mesodermal implants and bFGF beads, suggesting that these are among the early-response genes of the posteriorizing signal. Thus, our approach employing cDNA subtraction and subsequent expression pattern screening allows us to clone candidate genes involved in a novel signaling pathway contributing to the formation of the posterior nervous system.

Patterning pentacene organic thin film transistors

Organic semiconductors have demonstrated excellent electrical performance, in some cases rivaling inorganic systems in use today. These materials, which are processed at or near room temperature, have attracted considerable interest because they would enable the creation of active circuitry on organic substrates leading to a new generation of displays, rf identification systems, and smartcards. Many of these materials are intolerant to wet processing, however. This has led to a major processing limitation: the lack of a subtractive photolithographic patterning process to define active regions of the semiconductor. This article presents a process which uses a dry organic encapsulant (parylene) as a boundary layer between an organic semiconductor (pentacene) and photolithographic chemicals. Traditional photolithography may then be performed to use a dry etch to pattern the material stack. This process, which is fully subtractive, opens the path to the use of these materials in a wide range of applications.

Micropatterning of polymer thin films with pH-sensitive and cross-linkable hydrogen-bonded polyelectrolyte multilayers.

Polyelectrolyte multilayers of poly(acrylic acid) (PAA) and polyacrylamide (PAAm) were prepared via hydrogen-bonding interactions. These multilayers as assembled were stable at low pH but dissolved quickly in neutral pH water. We developed methods for stabilizing these multilayers to high pH through cross-linking by heating or UV-irradiation. Thermal treatment of the multilayers, which resulted in a partial imidization between carboxylic acid and amide groups, gave the multilayer good stability at high pH. In addition, we introduced photoreactive groups in the multilayer, which rendered the film insoluble after UV irradiation. Using these selective stabilization approaches, we have succeeded in micropatterning these films by ink-jet printing and photolithography to create subtractive patterns.

Nanolithography in the Evanescent Near Field

New applications in microscopy and nanofabrication have emerged from recent advances in the optical near field. This paper reviews the implications of using evanescently decaying components in the near field of a photomask as a lithography tool for the fabrication of nanoscale structures. Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using such evanescent near-field optical lithography (ENFOL) with broadband illumination (365–600 nm). Line widths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and both additive and subtractive pattern transfer processes have been developed. Full electromagnetic field simulations of the exposure process show that a high-contrast image is present within the resist layer, and that the exposure is dominated by one polarization for the grating structures studied. These reveal the potential of ENFOL in achieving feature sizes smaller than λ/20.

Organic Semiconductor Thin-Film Field-Effect Transistors

Pentacene is the best performing organic for thin-film fieldeffect transistor (TFT) applications and its performance rivals that of a-Si:H. We show how to fabricate pentacene TFT with source and drain electrodes lithographically patterned, before the deposition of the pentacene layer on them, and concurrently achieve performance equal to that of TFT fabricated with the reverse layering sequence. We also present a novel method for subtractive patterning of the pentacene layer that does not affect its transport properties. Earlier results on high performance organic TFT at low operating voltage are also presented.

Advanced Image-Processing Techniques for Automatic Nondestructive Evaluation of Adhesively-Bonded Structures Using Speckle Interferometry

In conventional optical nondestructive evaluation (NDE) of structures using shearography or electronic speckle pattern interferometry (ESPI), results are typically provided in the form of fringe patterns or deformation contour plots. However, in order to fully automate the process of defect detection, it is desirable to obtain simpler results which are easier to interpret. We present here one such optical system based on additive–subtractive shearography/ESPI. This system processes additive–subtractive fringe patterns and provides the sizes and locations of defects such as disbonds in adhesively-bonded composite structures. This is achieved by exciting the structure under inspection using an acoustic stressing mechanism which sweeps a range of vibration frequencies of the structure. Since the defective areas of the structure have different mechanical properties from their neighboring regions, varying and complex fringe patterns are obtained at different stressing frequencies. We propose an algorithm which enables the automatic identification and selection of relevant additive–subtractive fringe patterns that pertain only to localized deformations associated with defects, and which excludes images that pertain to any overall modes of the entire structure. The algorithm also includes a pixel-by-pixel adjustable thresholding scheme which compensates for intensity variations due to nonuniform reflectivity from unpainted and dirty test objects. Morphological processing is then performed to extract the shapes of the defect from the processed fringe clusters. Various structures, from simple aluminum specimens with simulated defects to a complex honeycomb-based aviation repair patch specimen, have been successfully evaluated using this system.

A Monolayer-Based Lift-Off Process for Patterning Chemical Vapor Deposition Copper Thin Films

We describe a non-lithographic monolayer based patterning process for depositing copper thin film microstructures by chemical vapor deposition (CVD). The technique combines the microcontact printing of octadecyltrichlorosilane (OTS) monolayers, nonselective copper CVD, and mild (abrasive-free) mechanical polishing to fabricate thin film microstructures on both planar and nonplanar substrates. This technique has been used successfully to deposit copper features with sizes ranging from 5 to 250 μm on variety of technologically important substrates including indium tin oxide (ITO), titanium nitride (TiN), thermal and plasma grown SiO2, Al2O3, and glass. Patterning is effected via adhesive failure in regions modified by microcontact printing (μCP); nucleation and growth are inhibited in these regions and the copper grains which eventually form on top of the OTS monolayer remain loosely adherent and, thus, are easily removed by mechanical means. The versatility and simplicity of the μCP process as a method for surface modification combined with chemical/physical routes of thin film deposition and patterning suggest a new, defect-tolerant method for fabricating thin film features with micrometer to centimeter dimensions with potential advantages over conventional subtractive patterning processes involving photolithography and chemical etching. Possible applications in the manufacturing of multichip modules (MCM-D) and more generally in microelectronic packaging and liquid crystal display (LCD) metalization are described.

Patterning of copper for multilevel metallization: reactive ion etching and chemical-mechanical polishing

Copper has been suggested as a desirable alternative for aluminum in future multilevel-metal ULSI circuits. The use of copper as an interconnect material will require, as one of the critical steps, the development of a highly accurate and manufacturable patterning method. Two possible approaches to this problem are the subtractive patterning of the metal by reactive ion etching and producing inlaid metal patterns by chemical-mechanical polishing. A general overview of issues involved with these two methods is presented. Recent work in the dry etching of copper is then reviewed, with particular reference to the use of halogen-based plasmas. The rather low volatility of the resulting etch products generally requires a substrate temperature above 200°C, but submicron-resolution patterning is feasible with a high-temperature-stable mask. Various approaches to lowering the etching temperature are examined. Additionally, recent work on the chemical-mechanical polishing of copper is discussed, and examples of the fabrication of inlaid metal structures are given. In conclusion, a brief comparison of the two patterning methods is made, with a view on their future application.

Reactive ion etching of high-aspect-ratio 100 nm linewidth features in tungsten

The subtractive patterning of fine-linewidth (sub-250-nm) tungsten absorbers for x-ray masks requires the ability to etch features with high aspect ratios and vertical sidewalls. In this letter, a reactive-ion-etching process which meets these requirements is described. The etch gases used are SF6 and H2. It is further shown that an intermittent etch process, whereby the sample is vented to atmosphere in between etches results in an etch profile with vertical tungsten sidewalls. This interrupted etching is compared with the results from continuous etching. The straighter sidewalls in the case of the interrupted etch suggests a passivation of the sidewalls which occurs during the venting process. Results are presented to show etching of 100 nm gratings and 200 nm dot arrays. An in situ endpoint detection method developed for the reactive-ion-etching system is presented.

X-ray masks with tungsten absorbers for use in the LIGA process

X-ray masks with tungsten absorbers have been made for the LIGA process by sputter deposition of tungsten and subtractive patterning through reactive ion etching with CF 4 and SF 6. Sputtering was optimised in order to minimise stress and the anisotropy of stress in tungsten layers. Absorbers with critical dimensions as small as 0.5 μm and thicknesses of 2.5 μm have been achieved. With tungsten absorbers, 2.2 μm in thickness, templates for electroplating, more than 20 μm in thickness, have been fabricated from PMMA.

30 nm resolution zero proximity lithography on high-Z substrates

In order to fabricate masks for x-ray lithography, there is growing interest in subtractive patterning of high atomic mass (high-Z) materials such as tungsten or gold. Favorable writing speeds and sub-50 nm resolution without proximity effects combine to make heavy ion focused ion beam lithography an ideal candidate for this area of nanofabrication. Using a 50 keV Ga+ beam with an 8 nm spot diameter, we have exposed a variety of proximity effect test patterns in 60 nm thick PMMA on 0.5 μm thick tungsten films. The results indicate that 30-nm resolution or better is possible at line/space pitches as small as 80 nm. The test patterns show no apparent proximity effects at these dimensions. An anomalous ‘‘inverse proximity effect’’ was observed, and was determined to be an artifact of the scanning electron microscope technique used to observe the PMMA resist.

Determination of basic grids for subtractive moire patterns

The simple formula derived in the paper establishes a direct relationship between moire beat patterns and basic grids for minute displacements. The possibilities of finding a basic grid for a desired moire pattern are pointed out. The analysis is illustrated by several examples of Fresnel moire zone plate patterns and concentric equidistant circular moire patterns obtained by changes of scale and rotation. Possible advantages of the practical use of this element are outlined.

Toward Optimizing the Detection of Atrial Depolarization with Floating Bipolar Electrodes

Some concepts derived from modern literature on the physics of cardiac muscle conduction regarding optimization of the design of electrode systems for detection of atrial depolarization and a preliminary clinical corroboration of the concepts are presented. The concepts of most import are 1) that the mechanical dimensions of sensing electrodes in all planes have to be as small as possible relative to the dimensions of the extracellular waveform dimensions associated with depolarization action potentials, and 2) that the distance between bipolar electrodes should be tuned to be greater than the dimensions of the extracellular waveform to avoid subtractive interference patterns associated with saltatory propagation in aging cardiac muscle. Additionally, in the practical application of chronic atrial sensing from a single lead for use in an implanted VDD pacing system, it is also important to consider electrode size and its effect on impedance level.