Molybdenum mask

SEMs of the molybdenum mask (Type B) and the nickel mesh (Type C) used in the experiments. The Ni mesh was also used as a spacer of thickness 4.5pm and flexible enough to be folded over several times for spacer thickness to be increased in small increments. Mask Type A (not shown) was identical to Type B, except the aperture that had a cylindrical profile with straight walls, and not the biconical ones of Type A. 

Electrolysis of the molybdenum mask enables the lift-off layer to be removed much more quickly than with standard lift-off techniques. 

Another type of demasking involves formation of new complexes or other compounds that are more stable than the masked species. For example, boric acid is used to demask fluoride complexes of tin(IV) and molybdenum(VI). Formaldehyde is often used to remove the masking action of cyanide ions by converting the masking agent to a nonreacting species through the reaction ... 

As the result of the performed investigations was offered to make direct photometric determination of Nd microgram quantities in the presence of 500-fold and 1100-fold quantities of Mo and Pb correspondingly. The rare earth determination procedure involves sample dissolution in HCI, molybdenum reduction to Mo (V) by hydrazine and lead and Mo (V) masking by EDTA. The maximal colour development of Nd-arsenazo III complex was obtained at pH 2,7-2,8. The optimal condition of Nd determination that was established permit to estimate Nd without separation in solution after sample decomposition. Relative standard deviations at determination of 5-20 p.g of Nd from 0,1 g PbMoO are 0,1-0,03. The received data allow to use the offered procedure for solving of wide circle of analytical problems. 

Of experimental methods for studying the metal in enzymes, light absorption in the visible region from molybdenum chromophores is likely to be weak and frequently masked by stronger absorption from other enzyme constituents. Indeed only recently has a small molybdenum contribution to the absorption spectrum of even the most studied of these enzymes, xanthine oxidase, been demonstrated 33, see Section V F). 

In order to prevent the reduction between iron(II) and formaldoxime occurring, another iron complexing agent (potassium cyanide) was used in the presence of a reductant (ascorbic acid) that reduces iron(III) to iron(II). Aluminium, titanium, uranium, molybdenum and chromium also form light-coloured complexes that normally do not interfere in the determination of manganese in water or plant material by this method. If the aluminium or titanium concentrations are higher than 40 ppm an additional masking flow of tartrate is recommended [31]. 

The element is extractable from strong hydrochloric acid solutions into 4-methylpentan-2-one. This approach may be applied to the analysis of plant material, if the ash is extracted with the strong hydrochloric acid required.35 Kim et al.36 masked iron(m) by reduction to iron(n) with tin(n) chloride before extracting molybdenum as its thiocyanate complex with Aliquat 336 into chloroform. The latter was evaporated, and the residue extracted with 4-methylpentan-2-one prior to determination of molybdenum by AAS. The procedure was applied to soils, sediments, and natural waters. In fertilizer analysis, the thiocyanate complex of molybdenum has been extracted, after reduction of iron with tin(n) chloride, into 3-methylbutan-l-ol, and the latter extract analysed directly.37 In another thiocyanate-based procedure, total molybdenum from soils and geological materials was extracted into 4-methyl-pentan-2-one.38... 

The various heterocyclic ligands just described are easily cleaved from the respective molybdenum centers either by reaction with sulfur or through ligand exchange with l,2-bis(diphenylphosphino)ethane (Scheme 17).44a The Diels-Alder cycloaddition is successfully expanded to transient 1-phospha-1,3-diene complexes such as 25 and suitable dienophiles. As illustrated in Scheme 12, the isomeric 1,2-dihydrophosphetes (e.g., 26) are their masked stable precursors. Reaction of 26 with dimethyl acetylenedicarboxy-late or with benzaldehyde furnishes the Diels-Alder adducts of transient 25, compound 71, as a mixture of two isomers, and isomerically pure 72.44b (See Scheme 18.)... 

The coefficient of friction falls slightly with increasing temperature to a minimum at about 200°C and then rises as temperature increases further. With bonded films the effect of temperature on the binder will usually mask any effect on the molybdenum disulphide itself. 

All observed interferences, except those due to antimony, gold(III) and nitrite, were masked by the pumped addition of 0.02MEDTA (the pH adjusted to 3 with dilute hydrochloric acid) to the sample prior to the reduction step. In the presence of a 5000-fold excess of iron(III), however, depression of the dimethylarsenic signal cannot be completely restored by EDTA (Table 5). Extraction of the sample solution (at pH 2.0) with a 0.005 M solution of dithizone (diphenylthiocarbazone) in dichloromethane overcomes interferences due to silver(I), gold(III), chromium(VI), molybdenum(VI) and tin(II). 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

Tungsten reacts with dithiol in a similar way to molybdenum, hence it can be masked... 

Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe(ni), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37]. 

The chief interference is from Fe(lll), which forms a green complex with chromotropic acid. Before the determination of Ti, larger quantities of iron should be separated or smaller ones reduced with ascorbic acid or sulphite. Vanadium in quantities not exceeding those of titanium has no appreciable effect on the determination of Ti. Molybdenum at concentrations below 50 pg/ml does not interfere. Fluoride interferes by masking titanium, but can be removed by fuming with H2SO4. Oxidants (e.g. HNO3) must be absent because chromotropic acid is fairly easily oxidized. 

High salt concentrations, of the order of 500 fig of other metals per milliliter, produce multiple peaks during the atomization and mask the molybdenum peak. The peaks are most likely caused by scattered light from salt particles. The difficulty is avoided if the sample size and solution volume are selected to provide a maximum of 10 /xg/ml of other metals. 

Furthermore, because chromium has proven difficult to dry etch, there have been considerable efforts expended in finding alternative mask opaque absorber materials. Molybdenum silicide (MoSi) appears to be gaining traction in certain applications, particularly in attenuated phase-shifting mask (att-PSM) and EUV mask applications (see Section 14.3), and even for binary masks. ... 

---