Kateri E. Paul

The controlled formation of ordered, sinusoidal structures by plasma oxidation of an elastomeric polymer

This letter describes a technique for generating waves on polydimethylsiloxane (PDMS) patterned in bas-relief. The PDMS is heated, and its surface oxidized in an oxygen plasma; this oxidation forms a thin, stiff silicate layer on the surface. When the PDMS cools, it contracts and places the silicate layer under compressive stress. This stress is relieved by buckling to form patterns of waves with wavelengths from 0.5 to 10 μm. The waves are locally ordered near a step or edge in the PDMS. The wavelength, amplitude, and pattern of the waves can be controlled by controlling the temperature of the PDMS and the duration of the oxidation. The mechanism for the formation and order of the waves is described.

Using an elastomeric phase mask for sub-100 nm photolithography in the optical near field

Bringing an elastomeric phase mask into conformal contact with a layer of photoresist makes it possible to perform photolithography in the near field of the mask. This technique provides an especially simple method for forming features with sizes of 90–100 nm in photoresist: straight lines, curved lines, and posts, on both curved and planar surfaces. It combines experimental convenience, new optical characteristics, and applicability to nonplanar substrates into a new approach to fabrication. Nanowire polarizers for visible light illustrate one application for this technique.

Quantifying distortions in soft lithography

This article describes a moire technique for determining distortions in soft lithography. We use the technique to investigate distortions when soft lithography is performed in a variety of configurations; a method is identified for limiting maximum distortions to less than 1 μm over areas ∼1 cm2. We also suggest an approach for actively controlling these distortions, and we demonstrate in a simple way its feasibility.

Fabrication of Micrometer‐Scale, Patterned Polyhedra by Self‐Assembly

We recently proposed and demonstrated a strategy for fabricating self-assembling, three-dimensional (3D) electrical networks. In this demonstration, we used millimeter scale building blocks (polyhedra) whose faces were patterned with copper connectors and devices (light-emitting diodes). One significant hurdle to implementing self-assembly in practical systems is that of miniaturizing the assemblies. To do so would require us to construct building blocks similar to those of ~1 mm scale, but on the micrometer scale. The building blocks must have the following characteristics: a) polyhedral structures, b) faces patterned with arbitrary patterns that would serve as connectors, and c) microelectronic devices attached to the faces of the polyhedron. It is difficult to fabricate micrometer scale polyhedral structures. Structures with these dimensions are usually fabricated by projection lithography, and this technique is inherently planar. Most methods of fabrication in 3D utilize processes such as surface micromachining that are precise and versatile, but also expensive and limited in the range of materials that can be used and the types of structures that can be generated. It is also difficult to generate arbitrarily patterned structures in 3D or on curved surfaces. Techniques for patterning have been limited to microcontact printing, projection lithography on spherical substrates using elaborate optics, and shell plating onto die-cast mandrills. Fabricating devices on 3D objects is extremely difficult; this is because processes (e.g., ion implantation) used to build silicon-based microdevices are inherently planar techniques. This paper describes the fabrication of patterned polyhedra, having 100±300 lm sides, by the spontaneous folding of two-dimensional (2D) structures under the influence of the surface tension of liquid solder. Our examination of this approach was stimulated by the early work of Pister and Shimoyama on micromachined hinges and by the extensive research of Syms and others on the use of capillary forces in liquid solder and similar methods for directly shrinking polymer joints for the assembly of non-planar microstructures. The structures we describe can be patterned and processed in 2D using conventional techniquesÐphotolithography, evaporation, electrodeposition, etchingÐthat have been extensively developed by the semiconductor industry. In the past, auto-folding has been used primarily to actuate micrometer scale components in microelectromechanical systems (MEMS) devices. In our work, we demonstrate that the self-assembling process of auto-folding can be used as a strategy for fabricating patterned 3D components from 2D precursors. We have also demonstrated that it is possible to build 3D polyhedra whose faces contain single crystal silicon chipsÐthe most primitive electronic device, i.e., a resistor. The approach we demonstrate has four steps: 1) The desired structures are designed in planar form as a series of unconnected but adjacent faces. 2) The faces are fabricated in 2D on a sacrificial layer using a combination of photolithography, evaporation, etching, and electrodeposition. 3) The ensemble of faces is covered with a thin film of liquid solder by dip coating. 4) The structure is released from the substrate by dissolving the sacrificial layer, and allowed to fold under the influence of the surface tension of the molten solder. This strategy is sketched in Figure 1. We experimented with many different materials, structures, and processes. Figure 2 shows scanning electron microscopy (SEM) images of folded metallic polyhedra and the 2D precursors of these structures. The metallic faces of the polyhedra contained either holes (the trigonal pyramid in Fig. 2) or solid faces (as seen in the tetragonal pyramid, cube, and hexagonal prism). The faces ranged in size between 100±300 lm (on a side). The 2D precursors contained faces that were not hinged; the faces were aligned as close to each other as possible (given the mask and photolithographic capabilities). For 200±300 lm faces, spacings between 8 and 15 lm worked well; for 100 lm faces, a spacing of 8 to 10 lm was required. When the 2D structures were dipped in solder, the solder bridged the faces and formed a continuous layer. The 2D precursors were released from the wafer by dissolving a sacrificial layer on which they were built. The precursors were heated above the melting point of the solder. The liquid solder tried to minimize its surface area (capillarity); this process drew the faces together to form a compact 3D polyhedron. The equilibrated 3D polyhedron was, at this point, filled with solder; the folding thus worked best when the volume of the solder present was equal to the volume of the polyhedron. Since the volume of solder present was equal to that deposited on the 2D precursor, the critical step controlling the yield of the process was the deposition of solder. We controlled the amount of solder deposited by changing the surface tension of the liquid solder, as well as by changing the solder±copper interfacial energy. The surface tension of liquids decreases approximately linearly with increasing temperatures; as a result, when the solder dip-coating was carried out at elevated temperatures (100 C for a solder with melting point, m.p., 47 C), a smaller volume of solder was deposited. The solder± copper interfacial energy was also controlled using fluxes and acids that aid in cleaning organic contaminants and dissolving oxide layers at the solder and copper surfaces. When the con-

Generating ∼90 nanometer features using near-field contact-mode photolithography with an elastomeric phase mask

This article describes a near-field photolithographic method that uses an elastomeric phase mask in conformal contact with photoresist. The method is capable of generating ∼90 nm lines in commercially available photoresist, using broadband, incoherent light with wavelengths between 330 and 460 nm. Transfer of these patterns into silicon dioxide and gold demonstrates the integrity of the patterned resist.

Fabrication of palladium-based microelectronic devices by microcontact printing

This letter demonstrates the patterning of thin films of metallic palladium by microcontact printing (μCP) of octadecanethiol, and the use of the patterned films in the fabrication of a functional sensor. This technique was also used to prepare templates of palladium for the electroless deposition of copper. The resistivity of the palladium and copper microstructures was 13.8 and 2.8 μΩ cm, respectively; these values are approximately 40% larger than the values for the pure bulk metals. Palladium patterned into serpentine wires using μCP functioned as a hydrogen sensor with sensitivity of 0.03 vol % H2 in N2, and a response time of ∼10 s (at room temperature).

Imaging the irradiance distribution in the optical near field

This letter describes the use of a sensitive photoresist for direct imaging of optical intensity profiles in near-field photolithographic experiments. A comparison between experimental patterns in exposed, developed photoresist and calculated profiles of intensity shows that this procedure provides a reliable semiquantitative image of the irradiance distribution in the near field; experiment and theory correlate adequately. A potential use of the superficial diffraction contrast recorded in photoresist as the basis for a new method of the fabrication of nanostructures is discussed.

MASKLESS PHOTOLITHOGRAPHY : EMBOSSED PHOTORESIST AS ITS OWN OPTICAL ELEMENT

This letter demonstrates that features embossed on the surface of a layer of photoresist can direct UV light in the photoresist layer. These topographical features act as optical elements: they focus/disperse and phase shift incident light in the optical near field, inside the resist layer. A number of different surface topographies have been examined, which give 50–250 nm features after exposure and development. This method gives patterns of complex features over large areas, in a parallel process, that can then be transferred into silicon or metal. It provides a method for controlling the intensity of light inside a thin film of photoresist.

Imaging profiles of light intensity in the near field: applications to phase-shift photolithography

We describe a method of imaging the intensity profiles of light in near-field lithographic experiments directly by using a sensitive photoresist. This technique was applied to a detailed study of the irradiance distribution in the optical near field with contact-mode photolithography carried out by use of elastomeric phase masks. The experimental patterns in the photoresist determined by scanning electron microscopy and atomic force microscopy were compared with the corresponding theoretical profiles of intensity calculated by use of a simple scalar analysis; the two correlate well. This comparison makes it possible to improve the theoretical models of irradiance distribution in the near field. Analysis of the images highlights issues in the experimental design, provides a means for the optimization of this technique, and extends its application to the successful fabrication of test structures with linewidths of ~50 nm.

Patterning flood illumination with microlens arrays

We describe a convenient lithographic technique that can produce simple, repetitive micropatterns over large areas (several square centimeters). The technique uses an illuminated array of micrometer-scale lenses to generate an array of optical patterns in an image plane located within micrometer distances from the lens array. A layer of photoresist, placed in the image plane, records the patterns. Microlenses with different sizes, profiles, composition, and indices of refraction produce corresponding patterns in exposed and developed photoresist. Both spherical and nonspherical microlenses were examined. Several types of optical element containing arrays of microlenses were fabricated and used to demonstrate that this technique can generate uniform micropatterns over large areas (>4 cm2) in a single exposure. The smallest features produced had dimensions of approximately 100 nm.