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Photolithography Improved Without UV Light

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by Rebecca C. Jernigan

COLLEGE PARK, Md. – As computer chips become smaller and more powerful, it is important that their features remain precise. The fact that smaller light wavelengths create smaller features has demanded that developers and manufacturers use expensive and difficult to work with ultraviolet light as the basis for such photolithographic nanofabrication – until now.

RAPID1.jpg
All images are schematic depictions of RAPID lithography, the technique developed by University of Maryland professor John Fourkas and colleagues which enables the creation of features 2500 times smaller than the width of a human hair. (Images courtesy John T. Fourkas, professor of chemistry and biochemistry, University of Maryland)

Researchers at the University of Maryland College of Chemical and Life Sciences, in the pursuit of improving the resolution of multiphoton absorption polymerization, have developed a tabletop technique called RAPID (Resolution Augmentation through Photo-Induced Deactivation) photolithography that removes ultraviolet light from the equation. Though inadvertent, the discovery could have applications in many areas, including electronics, optics and biomedical devices.

The investigators used 800-nm light from two tunable titanium:sapphire lasers to both harden a substance and to prevent it from hardening, which enabled them to sculpt features 2500 times smaller than the width of a human hair.

Multiphoton absorption of a pulsed 800-nm light is used to initiate crosslinking in a polymer photoresist, hardening it in place in a way that is similar to that used by dentists when filling a cavity, according to John T. Fourkas, one of the researchers. At the same time, one-photon absorption of continuous-wave 800-nm light prevents the photoresist from hardening.

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RAPID2.jpgThe researchers used spatial phase shaping of the latter beam to control the area that was affected, in a manner similar to that used in stimulated emission depletion (STED) fluorescence microscopy. The use of continuous-wave light for shaping simplified the process, as users would not need to develop precise timing between the exitation and deactivation beams like they would if both beams were pulsed.

In traditional photolithography, the resolution is limited to around one-fourth of the wavelength, due to diffraction. In the university team's approach, it is possible to make patterns that are one-twentieth the size (along the optical axis) of the wavelength used, enabling the creation of much smaller features with larger light wavelengths. This was achieved using a specific phase mask to manipulate the deactivation beam. The scientists believe that by choosing a different phase mask, the same size feature could be achieved along the transverse axis, and by using two phase-masked beams the measurement could be achieved in all dimensions.

RAPID.gifIn principle, the technique could enable users to create 3-D nanostructures with features ranging from tens of microns down to 40 nm. Fourkas, who led the research, believes that with further developments and refinements RAPID could shrink the feature resolution to as low as 20 nm, which could make nanopatterning useful in a variety of new applications.

Though there is still room for improved materials and better two-dimensional spatial light modulators could make the technique easier to implement, Fourkas is pleased with the team's discovery. "In the way that we have implemented it, RAPID is far easier than we had ever imagined it could be."

Rebecca C. Jernigan
[email protected]

Published: April 2009
Glossary
absorption
Absorption is the process by which a material takes in energy from electromagnetic radiation (such as light, heat, or sound) and converts it to other forms of energy, typically internal energy (such as heat). This process occurs when the energy of the incident radiation is transferred to the atoms or molecules of the absorbing material, causing them to increase in vibrational, rotational, or electronic energy levels. In different contexts, absorption can refer to: Physics and optics:...
beam
1. A bundle of light rays that may be parallel, converging or diverging. 2. A concentrated, unidirectional stream of particles. 3. A concentrated, unidirectional flow of electromagnetic waves.
electronics
That branch of science involved in the study and utilization of the motion, emissions and behaviors of currents of electrical energy flowing through gases, vacuums, semiconductors and conductors, not to be confused with electrics, which deals primarily with the conduction of large currents of electricity through metals.
fluorescence
Fluorescence is a type of luminescence, which is the emission of light by a substance that has absorbed light or other electromagnetic radiation. Specifically, fluorescence involves the absorption of light at one wavelength and the subsequent re-emission of light at a longer wavelength. The emitted light occurs almost instantaneously and ceases when the excitation light source is removed. Key characteristics of fluorescence include: Excitation and emission wavelengths: Fluorescent materials...
phase
In optics and photonics, "phase" refers to a property of electromagnetic waves, such as light, that describes the position of a wave at a given point in time within its oscillation cycle. More specifically, it indicates the position of a wave relative to a reference point, typically the starting point of a cycle. When discussing phase in optics, it's often described in terms of the phase difference between two waves or the phase of a single wave. The phase difference between two waves is the...
photolithography
Photolithography is a key process in the manufacturing of semiconductor devices, integrated circuits, and microelectromechanical systems (MEMS). It is a photomechanical process used to transfer geometric patterns from a photomask or reticle to a photosensitive chemical photoresist on a substrate, typically a silicon wafer. The basic steps of photolithography include: Cleaning the substrate: The substrate, often a silicon wafer, is cleaned to remove any contaminants from its surface. ...
photon
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
photoresist
Photoresist is a light-sensitive material used in photolithography processes, particularly in the fabrication of semiconductor devices, integrated circuits, and microelectromechanical systems (MEMS). It is a crucial component in the patterning of semiconductor wafers during the manufacturing process. The primary function of photoresist is to undergo a chemical or physical change when exposed to light, making it selectively soluble or insoluble in a subsequent development step. The general...
polymerization
Process of synthesizing long molecular chain materials (polymers) by reaction of many small molecules (usually thousands) called monomers.
resolution
1. In optics, the ability of a lens system to reproduce the points, lines and surfaces in an object as separate entities in the image. 2. The minimum adjustment increment effectively achievable by a positioning mechanism. 3. In image processing, the accuracy with which brightness, spatial parameters and frame rate are divided into discrete levels.
ultraviolet
That invisible region of the spectrum just beyond the violet end of the visible region. Wavelengths range from 1 to 400 nm.
wavelength
Electromagnetic energy is transmitted in the form of a sinusoidal wave. The wavelength is the physical distance covered by one cycle of this wave; it is inversely proportional to frequency.
absorptionBasic SciencebeambiomedicalelectronicsFeaturesfluorescenceindustrialJohn FourkasMicroscopymultiphotonnanofabricationnanopatterningnanostructuresNews & FeaturesOpticsphasephotolithographicphotolithographyphotonphotoresistpolymerizationRAPIDresolutionSTEDultravioletUVwavelength

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