Small-level recession

FTM and atom-probe studies of thin films of Ni, Au, Pt, a-Ge H, a-Si H and WO3, etc., on various substrates were reported by Krishna-swamy et a/.81 First, field ion tips each with a field evaporated surface were prepared. They are placed in an MRC model 8502 r.f. sputtering system. Tips were mounted on a recessed and shielded structure behind the sputtering surface which is bored with small holes about 1 to 2 mm in diameter. The very end of the tips came out of the holes to approximately the same level of the sputtering surface. Films were sputtered at about 20 mTorr Ar at an r.f. power of about 50 W. Thickness of a deposited thin film was controlled by both the r.f. power and the deposition time. Film thickness in the range of a few hundred to a few thousand A were studied. These tips were then imaged with Ne in the field ion microscope, or analyzed in the flight-time-focused ToF atom-probe. 

If the depth of the crevices, vias, or similar on the cathode is small (about 10 /zm to 1 cm), the distribution of current and thus that of the deposit should be uniform. In most cases, however, one observes that deposit is thicker over micro peaks (bumps) than, say, micro valleys. Such state of affairs is referred to as bad micro throw. When the opposite is true, one talks about true leveling. From simple geometric considerations it follows that given a V-shaped recess, even in the case of uniform metal distribution, still at the bottom the deposit would be expected to be thicker than at the top. However, pronounced leveling is obtainable by using suitable additives. These kinds of additives are known as levelers. ... 

The first recognized human metabolic defect in the biosynthesis of cholesterol and isoprenoids was mevalonic aciduria [10]. Mevalonic aciduria is an autosomal recessive disorder that is quite rare, with only 30 known patients (D. Haas, 2006). In normal individuals, a small amount of mevalonic acid diffuses from cells into the plasma at levels proportional to the rate of cellular cholesterol formation. Patients with the severe, classical form of mevalonic aciduria excrete 10,000-200,000 times the normal amount of mevalonic acid because they have severely reduced amounts of mevalonate kinase activity. Their clinical features include failure to thrive, anemia, gastroenteropathy, hepatosplenomegaly, psychomotor retardation, hypotonia, ataxia, cataracts, and dysmorphic features [10]. Surprisingly, patients with severe deficiencies in mevalonate kinase show normal plasma cholesterol levels and cultured mevalonic aciduria fibroblasts have a rate of cholesterol synthesis that is half that of normal cells. Close examination of cholesterogenic enzymes in mevalonic aciduria fibroblasts has revealed a 6-fold increase in HMG-CoA reductase activity, which is postulated to compensate for the low mevalonate kinase activity. Thus, mevalonate is overproduced. 

In both cases, the substrate is assumed to be much larger than the tip radius, a. The approach curves are also functions of the tip shape and can therefore provide information about tip geometry. Thus spherical or conical tips show different approach curves than disk-shaped ones. The approach curve over a conductor can also indicate when the conductive part of the tip is recessed inside the insulating sheath, which often happens with very small tips. In this case, only a small amount of positive feedback is observed before the insulating portion contacts the substrate and the /x-value levels off. Since characterization of small tips, such as by electron microscopy, is difficult, SECM is a useful way of learning about the tip size and configuration (19). 

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