Michael Crawford

Observation of superconductivity in (Y1−xCax)Ba2Cu3O6

Abstract Substitution of Ca 2+ for Y 3+ in YBa 2 Cu 3 O 6 converts this tetragonal semiconductor into a tetragonal superconductor with a T c of 50 K. Furthermore, the structure of (Y 0.8 Ca 0.2 )Ba 2 Cu 3 O 6 , determined by neutron powder diffraction, is fully consistent with partial oxidation of the Cu(2) O sheets, whereas the linear O Cu(1) O units do not undergo oxidation. Observation of a limited solid solubility for Ca 2+ in this structure is attributed to a maximum allowed oxidation state for Cu(2). This implies that in the superconductor YBa 2 Cu 3 O 7 the Cu O chains are responsible for the T c of 90 K, whereas the Cu O sheets alone produce superconductivity at 50 K in YBa 2 Cu 3 O 6.5 .

Tl2−xCdxBa2CuO6+δ: Varition of Tc with cadmium substitution

Abstract The compositions Tl2−xCdxBa2CuO6+δ have been prepared in the range of x=0 to 0.7 using either BaO or BaO2 as sources of barium and oxygen. The materials are superconducting with Tc varying smoothly as x increases. When the materials is prepared using BaO2 as the Ba source, Tc falls progressively from 92 K to 0 K as x proceeds from 0 to 0.5. When BaO is the Ba source, however, Tc rises from 65 K (x=0) to 90 K (x=0.2) before falling to 0 K at x=0.7. These behaviors indicate 1) that Tc is very sensitive to the average Cu oxidation state (hole concentration), 2) that the hole concentration can be adjusted either by oxygen excess (δ) or by cation doping (x). Metallic behavior is observed in the region x>62;0.6. High resolution electron microscopy, X-ray microanalysis and a structural refinement based on neutron powder diffraction data for Tl1.75Cd0.25Ba2CuO6+δ (14/mmm a=3.851(1) A, c=23.313(8) A) confirm Cd2+ substitutes exclusively for thallium. A comparison with the structure of Tl2Ba2CuO6+δ shows a decrease in the Cu-O bond length correlating with oxidation of the CuO2 sheet. An increase in the c-axis results from the substitution of the larger Cd2+ for Tl3+. X-ray absorption measurements suggest this substitution has no effect on the average oxidation state of the thallium in these materials.

Design of very transparent fluoropolymer resists for semiconductor manufacture at 157 nm

Photolithography at 157 nm requires development of new photoresists that are highly transparent at this wavelength. Transparent fluoropolymer platforms have been identified which also possess other materials properties required for chemically amplified imaging and aqueous development. Polymers of tetrafluoroethylene (TFE), a fluoroalcohol-substituted norbornene and an acid-labile acrylate ester show the best combination of properties. A solution, semibatch, free-radical polymerization process was developed allowing synthesis of the terpolymers on a multikilogram scale. Further property enhancements may arise from replacing the norbornene with functionalized tricyclononenes. Formulated resists have been imaged in a 157 nm microstepper.

Second generation fluids for 193nm immersion lithography

Water is the first generation immersion fluid for 193 nm immersion lithography. With a fluid refractive index of 1.436 and an optical absorbance of 0.01/cm at 193 nm, water immersion technology can enable optical lithography for the ITRS’ 65 nm half-pitch node. However, to achieve numerical apertures above 1.35 and to go beyond the 45 nm node, low absorbance fluids with indices higher than 1.6 are needed for the second generation of immersion lithography. We have developed a number of Gen. 2 high index fluids for immersion lithography at 193 nm. These highly transparent fluids have 193 nm indices up to 1.67. 32 nm 1:1 line space imaging has been demonstrated using two of our Gen 2 candidate fluids, representing major advance in optical lithography. To understand the behavior and performance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis, and Lorentz oscillator modeling.

Luminescence of materials based on LuTaO4

Abstract The photoluminescence and X-ray luminescence of samples of LuTaO4-A (A=Ti, Pr, Sm, Eu, Gd, Tb, Dy, Tm) are reported. The results can be interpreted using simple models. Many samples show pronounced thermoluminescence after X-ray irradiation at room temperature. Several of the materials possess great potential as X-ray phosphors.

Imaging of 32-nm 1:1 lines and spaces using 193-nm immersion interference lithography with second-generation immersion fluids to achieve a numerical aperture of 1.5 and a k 1 of 0.25

Water-based immersion lithography using ArF illumination is able to provide optical solutions as far as the 45-nm node, but is not able to achieve the 38- or 32-nm nodes as currently defined. Achieving these lithographic nodes will require new, higher refractive index fluids to re- place the water used in first-generation immersion systems. We have developed a number of such second-generation high-index fluids for im- mersion lithography at 193 nm. These highly transparent fluids have 193-nm indices up to 1.664. To understand the behavior and perfor- mance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis and Lorentz Oscillator modeling. Interference imaging printers have long been available, and they now have a new use: a rapid, cost- effective way to develop immersion lithography, particularly at extremely high resolutions. Although interference printers will never replace classi- cal lens-based lithography systems for semiconductor device production, they do offer a way to develop resist and fluid technology at a relatively low cost. Their simple image-forming format offers easy access to the basic physics of advanced imaging. Issues such as polarization of the image-forming light rays, fluid/resist interaction during exposure, topcoat film performance, and resist line edge roughness LER at extremely high resolutions, can all be readily studied. 32-nm 1:1 line/space L/S imaging is demonstrated using two of the second-generation fluids. These resolutions are well beyond current lens-based system capabili- ties. Results on the performance of various resists and topcoats are also reported for 32-nm L/S features.

Single crystal structure determination of double layered strontium iridium oxide [Sr3Ir2O7]

Single crystals of Sr3Ir2O7 were grown for the first time from a SrCl2 flux using SrO2 and IrO2 in a sealed Pt tube under ambient pressure. The compound crystallizes in a tetragonal unit cell [a = 3.896 (1) and c = 20.879 (3) A] with space group I4/mmm (No. 139) and Z = 2. The crystal structure was solved by means of single crystal X-ray diffraction and refined to R = 0.014. The structural refinement showed that the IrO6 octahedra are rotated about the c-axis by about 12 degrees, similar to the rotational distortion recently found in Sr2IrO4. However, unlike in Sr2IrO4, the IrO6 rotations in Sr3Ir2O7 are not correlated, hence no superlattice reflections appear.

Local structural anomaly near Tc observed by pulsed neutron scattering

Abstract Pulsed neutron scattering studies carried out with the atomic pair distribution method show that the actual atomic structure of superconducting oxides has significant local deviation from the crystallographic average structure, and the deviation shows an anomalous change through the superconducting transition. This anomaly is likely to provide information regarding the coupling mechanism.

Sr2rho4 and sr2iro4: structural and magnetic studies of 4d and 5d transition metal analogs of la2cuo4

Abstract X-ray and neutron diffraction studies of Sr2IrO4 and Sr2RhO4 reveal structural distortions from the ideal K2NiF4 structure, in which the IrO6 and RhO6 octahedra are rotated about the crystallographic c-axes by 11° or 9.7°, respectively. These rotations reduce the symmetry from I4 mmm to I4 1 acd . We find that Sr2IrO4 is an electrical insulator which exhibits weak ferromagnetism at 250 K, attributable to canted antiferromagnetism associated with long-range three-dimensional ordering of the Ir4+ moments. Although no ferromagnetic ordering is found in Sr2RhO4, susceptibility data shows a peak possibly associated with two-dimensional short-range antiferromagnetic order. Doping of electrons in Sr2RhO4 results in compositions which show weak ferromagnetism, perhaps arising from double-exchange-type magnetic interactions due to the presence of rhodium in 4+ and 3+ oxidation states.

High-pressure study of structural phase transitions and superconductivity in La 1.48Nd 0.4Sr 0.12CuO 4

We have determined the crystal structures and superconducting transition temperatures of La1.48Nd0.4Sr0.12CuO4 under nearly hydrostatic pressures in diamond anvil cells to 5.0 GPa and 19.0 GPa, respectively. Synchrotron x-ray powder diffraction measurements were used to establish the pressure-temperature structural phase diagram. Under pressure the superconducting transition temperature increases rapidly from Tc = 3 K to a maximum value of 22 K at 5 GPa, a pressure slightly greater than that required to stabilize the undistorted I4/mmm structure in the superconducting state. Increasing the pressure further to 19 GPa leads to a decrease in Tc to ~12 K. These results are discussed in relation to earlier high pressure measurements for similar materials.