Maan M. Alkaisi

Sub-diffraction-limited patterning using evanescent near-field optical lithography

Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using evanescent near-field optical lithography with broadband illumination (365–600 nm). Linewidths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and a reactive ion etching process using SF6 has been developed to transfer the patterns to a depth of more than 100 nm into silicon. 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.

Metal Schottky diodes on Zn-polar and O-polar bulk ZnO

Planar Pd, Pt, Au, and Ag Schottky diodes with low ideality factors were fabricated on the Zn-polar (0001) and O-polar (0001¯) faces of bulk, single crystal ZnO wafers. The diodes were characterized by current-voltage and capacitance-voltage measurements. A polarity effect was observed for Pt and Pd diodes with higher quality barriers achieved on the O-polar face. No significant polarity effect was observed for Au or Ag diodes. The highest barriers were achieved with Ag as the Schottky metal with barrier heights varying between 0.77 and 1.02eV. This is possibly due to varying degrees of oxidation of the Ag contacts.

Surface texturing for silicon solar cells using reactive ion etching technique

Surface texturing is an effective and more lasting technique in reducing reflections and improving light trapping compared to antireflection coatings. A surface texturing technique using reactive ion etching (RIE) method suitable for crystalline and multi crystalline solar cells, which resulted in surfaces with negligible reflection in the visible band is described. Different texturing structures (pillars, holes and black silicon) have been studied and compared in the wavelength range from 250nm-2500nm. It is found that the reflectance of the textured column structures were less than 0.4% at wavelengths from 500 nm to 1000 nm and showed a minimum of 0.29% at 1000 nm while the reflectivity from black silicon is around 1% and hole structures is around 6.8% in the same wavelength range.

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.

Low temperature nanoimprint lithography using silicon nitride molds

Abstract Nanoimprint lithography has been performed at low temperatures using low stress chemical vapour deposited silicon nitride (Si x N y ) as a mold material, with no sticking problems and hence no need for a release agent or surfactant. Imprint temperature, and the effect of quenching have been investigated and it is found that temperatures as low as 50°C can be used successfully for imprint. Imprint profile due to resist overflow at these temperatures is discussed. Low temperature imprint is important for patterning substrates or polymer-based materials that are intolerant to high temperatures. A low temperature process could also be an advantage for alignment when the mold and substrate have different thermal expansion coefficients.

Nanolithography using optical contact exposure in the evanescent near field

Nanolithography has been performed using optical contact exposure in the evanescent near field of an amplitude mask. Low stress silicon nitride membranes are used as conformable masks, patterned using electron beam lithography and lift-off metallisation. By using thin resist layers (60 nm) features as small as 120 nm on a 400 nm period have been formed by broadband exposure from a mercury arc lamp source. This resolution is below the conventional diffraction limit for projection optical lithography and illustrates the promise for extending optical lithography into the sub-100nm realm.

70 nm Features on 140 nm period using evanescent near field optical lithography

We have investigated the resolution limits for Evanescent Near Field Optical Lithography (ENFOL) both experimentally and computationally. Feature sizes as small as 70 nm on a 140 nm period have been achieved using broadband illumination (365-600 nm). This resolution is well below the diffraction limit associated with projection lithography. Line widths down to 50 nm have been achieved for larger period gratings. Simulations of the exposure process show that feature sizes smaller than @l/20 can be resolved using this technique.

Effects of reactive ion etching on the electrical characteristics of GaN

We present an investigation of the electrical characteristics of plasma exposed GaN. The specific contact resistance of ohmic contacts fabricated on GaN after argon plasma bombardment for 2.5 min at 0.03 W/cm2 are measured to decrease by a factor of 4 compared to the unetched surface. Gold has been found to be the best material for GaN Schottky diode. A study of the electrical performance of diodes fabricated on plasma exposed GaN has been undertaken. To compare the effect of the chemical versus physical factors, as well as the role played by the ion mass of the etchant species during the etching process on diode behavior, GaN surfaces have been exposed to Ar, N2, as well as SF6+N2 plasmas before diode fabrication. Our data indicate that a plasma with low ion mass etchant species or a dominant chemical mechanism of etching with a high etch rate creates less surface damage. The use of a SF6+N2 plasma should be possible for GaN transistor gate recessing.

Comparison of DC and RF Sputtered Zinc Oxide Films with Post-Annealing and Dry Etching and Effect on Crystal Composition

Zinc oxide (ZnO) thin films were deposited on Silicon substrates by DC and RF sputtering deposition. Thermal annealing was performed at up to 900°C in N2 for 30 min. The samples were dry etched for 30 min using CHF3 plasma. The effect of different sputtering techniques, annealing and reactive ion etching (RIE) were investigated using X-ray diffraction, Rutherford backscattering (RBS), photoluminescence (PL) spectra, atomic force microscopy (AFM), scanning electron microscopy (SEM), and piezoelectric measurements. The PL response improved considerably during annealing and was further enhanced after RIE process. RBS traced a C rich surface layer on all etched samples, which is possibly caused by the etching gas.

NANOSCALE OPTICAL PATTERNING USING EVANESCENT FIELDS AND SURFACE PLASMONS

Patterning with sub-diffraction-limited resolution has been demonstrated using a simple photolithography technique. Evanescent fields and surface plasmons are critical to the image formation, which is investigated here using computer simulations and experiments. A regime exists in which surface plasmons are resonantly excited, which we have named Evanescent Interferometric Lithography (EIL); period halving and reduced exposure times characterize this exposure mode. Two other exposure modes have been investigated in which surface plasmons on a planar metallic film beneath the mask are used to improve pattern formation. In the first, Planar Lens Lithography (PLL), a planar silver layer excited near its plasma frequency is used to form a projected near-field image. For a 40-nm thick silver layer, we predict that resolution down to 40 nm should be possible. However, the image is affected by the loss in the silver layer, the mask period, duty cycle and surrounding refractive index. Experimental verification of PLL is presented for 1-micron period structures imaged through 120 nm of silver. Finally, simulations are used to show that surface plasmons on an underlying silver layer can be used to improve process latitude and depth of field. We have named this mode Surface Plasmon Enhanced Contact Lithography (SPECL).