The W Structure

The ideal W structure of BaFeis027 and SrFcigOz contains S blocks twice as thick between each R block to give the stacking sequence RS2R SJ. There are therefore seven non-equivalent iron sites, and Mdssbauer spectra at 5 and 78 K were able to resolve all 

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Figure 9. The arrangement of the atoms in the / -W structure. There are eight atoms in the primitive cube. Two atoms are centered at (0, 0, 0) and (1/2, 1/2, 1/2). Each of these atoms in surrounded by twelve atoms at the corners of a slightly distorted icosahedron.

On the large structures, a W CMP process may leave a very rough W surface behind as a result of insufficient W filling before CMP. The CMP process simply did not reach down to these recessed areas. Normally, the W structures... 

On a typical product wafer, the tungsten density is usually low, that is, about 20% or less. The remaining surface is covered mainly by oxide or other dielectric materials. This leaves four times more chances to have the scratch on the oxide or dielectric material than on tungsten. Furthermore, the tungsten structures are usually very small and a scratch is typically less visible. On a very small structure, a scratch on the W will look more like a pit. In addition, W is one of the hardest metals that is more difficult to scratch than other softer materials such as copper or aluminum. For all these reasons, after W CMP, it is more likely to see scratches on the dielectric materials than on the W structures (Fig. 17.18). 

FIGURE 17.21 The W residue on this image is pattern dependent. The W structure is above the edge of some underneath arrays of structures. These arrays have different topography that still remains after PMD deposition and even after not 100% efficient PMD CMP. 

We shall now consider several replacement variants of the W-structure. 

These aspects of the V structure of liquid water are discussed in detail in F. Hirata and P. J. Rossky, A realization of the W structure" in liquid water, J. Chem. Phys. 74 6867 (1981). 

Dulmage W J and Lipscomb W N 1951 The crystal structures of hydrogen cyanide, HON Acta Crystallogr. 4 330... 

Figure Bl.8.4. Two of the crystal structures first solved by W L Bragg. On the left is the stnicture of zincblende, ZnS. Each sulphur atom (large grey spheres) is surrounded by four zinc atoms (small black spheres) at the vertices of a regular tetrahedron, and each zinc atom is surrounded by four sulphur atoms. On the right is tire stnicture of sodium chloride. Each chlorine atom (grey spheres) is sunounded by six sodium atoms (black spheres) at the vertices of a regular octahedron, and each sodium atom is sunounded by six chlorine atoms.

It can be seen from Table 2 that the intrinsic values of the pK s are close to the model compound value that we use for Cys(8.3), and that interactions with surrounding titratable residues are responsible for the final apparent values of the ionization constants. It can also be seen that the best agreement with the experimental value is obtained for the YPT structure suplemented with the 27 N-terminal amino acids, although both the original YPT structure and the one with the crystal water molecule give values close to the experimentally determined one. Minimization, however, makes the agreement worse, probably because it w s done without the presence of any solvent molecules, which are important for the residues on the surface of the protein. For the YTS structure, which refers to the protein crystallized with an SO4 ion, the results with and without the ion included in the calculations, arc far from the experimental value. This may indicate that con-... 

Eijck B P, W T M Mooij and J Kroon 1995. Attempted Prediction of the Crystal Structures of Six Monosaccharides. Acta Crystallographica B51 99-103. 

Cohen F E, M J E Sternberg and W R Taylor 1982 Analysis and Prediction of the Paclung oi. i-E a iinst a /3-Sheet in the Tertiary Structure of Globular Proteins. Journal of AdoljcuLir E 156 821-862. 

Noble M E M, R K Wierenga, A-M Lambeir, F R Opperdoes, W H Thunnissen, K H Kalk, H Groendijk and W G J Hoi 1991. The Adaptability of the Active Site of Trypanosomal Triosephosphate Isomerase as Observed in the Crystal Structures of Three Different Complexes. Proteins Structure, Function and Genetics 10 50-69. 

If has long been known that the enthalpy of hydrogenalion of benzene (49.8 kcal moU Conant and Kistiakowsky, 1937) is not the same as three times the enthalpy of hydrogenation of cyclohexene (3 x 28.6 kcal moU ). Evidently, the double bonds that w e write in the Kekule structure of benzene... 

Substituting (1.22), (1.23) into (1.21), one can see that the differential equations (1.21) of second order with respect to U have the same structure as those of the three-dimensional elasticity equations (1.1)- (1.3). The system (1.24)-(1.25) contains the fourth derivatives of w. 

It is now easy to calculate the force on the narrow structure. If the pillar has a width w = 10 m where it passes through the ice sheet (thickness f = 2 m), it presents a section of roughly 20 m on which ice presses. The maximum stress the ice can take is 6 MPa, so the maximum force it can exert on the structure is... 

The function W(X) is called the potential of mean force (PMF). The fundamental concept of the PMF was first introduced by Kirkwood [4] to describe the average structure of liquids. It is a simple matter to show that the gradient of W(X) in Cartesian coordinates is related to the average force. 

Cohen, F.E., Sternberg, M.J.E., Taylor, W.R, Analysis and prediction of the packing of a-helices against a p-sheet in the tertiary structure of globular proteins. 

Figure 8.3 The DNA-binding protein Cro from bacteriophage lambda contains 66 amino acid residues that fold into three a helices and three P strands, (a) A plot of the Ca positions of the first 62 residues of the polypeptide chain. The four C-terminal residues are not visible in the electron density map. (b) A schematic diagram of the subunit structure. a helices 2 and 3 that form the helix-turn-helix motif ate colored blue and red, respectively. The view is different from that in (a), [(a) Adapted from W.F. Anderson et al., Nature 290 754-758, 1981. (b) Adapted from D. Ohlendorf et al., /. Mol. Biol. 169 757-769, 1983.]...

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