The use of stimulants (methylphenidate and amphetamine) as cognitive enhancers by the general public is increasing and is controversial. It is still unclear how they work or why they improve performance in some individuals but impair it in others. To test the hypothesis that stimulants enhance signal to noise ratio of neuronal activity and thereby reduce cerebral activity by increasing efficiency, we measured the effects of methylphenidate on brain glucose utilization in healthy adults. We measured brain glucose metabolism (using Positron Emission Tomography and 2-deoxy-2[18F]fluoro-D-glucose) in 23 healthy adults who were tested at baseline and while performing an accuracy-controlled cognitive task (numerical calculations) given with and without methylphenidate (20 mg, oral). Sixteen subjects underwent a fourth scan with methylphenidate but without cognitive stimulation. Compared to placebo methylphenidate significantly reduced the amount of glucose utilized by the brain when performing the cognitive task but methylphenidate did not affect brain metabolism when given without cognitive stimulation. Whole brain metabolism when the cognitive task was given with placebo increased 21% whereas with methylphenidate it increased 11% (50% less). This reflected both a decrease in magnitude of activation and in the regions activated by the task. Methylphenidate's reduction of the metabolic increases in regions from the default network (implicated in mind-wandering) was associated with improvement in performance only in subjects who activated these regions when the cognitive task was given with placebo. These results corroborate prior findings that stimulant medications reduced the magnitude of regional activation to a task and in addition document a ''focusing'' of the activation. This effect may be beneficial when neuronal resources are diverted (i.e., mind-wandering) or impaired (i.e., attention deficit hyperactivity disorder), but it could be detrimental when brain activity is already optimally focused. This would explain why methylphenidate has beneficial effects in some individuals and contexts and detrimental effects in others.

A CVD-based low κ film was evaluated for inter-metal dielectric in < 0.18 µm generation devices. The film was deposited by conventional rf PECVD method using organosilane compound and oxygen. The measured dielectric constant of the film was 2.7~2.75. The κ value of the film was stable over several weeks and the moisture absorption was minimal. The chemical composition was in the form of SiO x C y H z , where the carbon content was less than 5 atomic %. Blanket film integration study was conducted to find out the manufacturing compatibility. The largest increase in κ value occurred during etching and ashing steps. However, SIMS compositional analysis revealed that the damage from these steps were limited to within top 300 Å, and the initial low κ value was recovered after the top damaged layer was removed by CMP. The final integrated dielectric constant was less than 3.0. The film density was measured as 1.4, compared to 2.3 g/cm 3 of conventional SiO 2 . The low density of the film resulted from the termination of SiO 2 network structures by Si-CH 3 and Si-H. INTRODUCTIONSilicon dioxide (SiO 2 ) has been dominantly applied as inter metal (IMD) and inter layer dielectric (ILD) throughout the history of microelectronics industry. The dominance is mainly attributed to its desirable electrical, thermomechanical characteristics as well as simplicity in integration with other materials. As the interconnection complexity required for high performance devices rapidly increases, however, several issues have arisen with traditional interconnection scheme, including enhanced RC delay effect, signal integrity due to cross talking and high power dissipation [1]. As part of the efforts to resolve these issues, new low dielectric constant material needs to be developed. Several approaches have been widely pursued in the past several years. Organic [2,3], inorganic [4,5] or hybrid[6,7,8] materials have been widely evaluated by either spin-on or chemical vapor deposition methods. Some of the issues with organic materials include thermal stability and reduced mechanical strength, which impose significant challenges on film integration. Inorganic low κ materials, in general, exhibit less integration issues, but κ stability still needs to be improved and there is difficulty in obtaining manufacturable materials with κ less than 3.0. Inorganic/organic hybrid material has received wide attention to obtain reliable low κ films while minimizing integration difficulties. In the development of hybrid low κ material, it is desirable to maintain low carbon content. High carbon content material has potential issues of via poisoning, etch and ash difficulties and thermomechanical instability. In this paper is presented the low carbon content low κ materials with carbon content less than 5%.

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