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Modern aging biology aims to understand how health and morbidity evolve over an individual’s life. To conceptualize health and “healthspan” in different ways that may be useful for biological or sociological study, several common analyses quantify lifetime health using statistics that combine measures of health and lifespan. While such efforts appear reasonable, we argue that such statistics confuse what types of processes contribute to health and lifespan; and, in fact, they risk circular reasoning by treating as an assumption what is in fact a very specific hypothesis: that lifespan and healthspan are tightly correlated. While this hypothesis is not impossible a priori, it remains a matter of active debate. Such hypotheses must not be unwittingly smuggled into the literature as hidden assumptions of specific analytic techniques. Consequently, we caution against generally quantifying lifetime health by any metric that combines both healthspan and argue that investigators should not mix longevity statistics with health measurements.

Age-related physiological changes are most notable and best-studied late in life, while the nature of aging in early- or middle-aged individuals has not been explored as thoroughly. In C. elegans, many studies of movement vs. age generally focus on three distinct phases: sustained, youthful movement; onset of rapidly progressing impairment; and gross immobility. We investigated whether this first period of early-life adult movement is a sustained “healthy” level of high function followed by a discrete “movement catastrophe”—or whether there are early-life changes in movement that precede future physiological declines. To determine how movement varies during early adult life, we followed isolated individuals throughout life with a previously unachieved combination of duration and temporal resolution. By tracking individuals across the first six days of adulthood, we observed declines in movement starting as early as the first two days of adult life, as well as high interindividual variability in total daily movement. These findings suggest that movement is a highly dynamic behavior early in life, and that factors driving movement decline may begin acting as early as the first day of adulthood. Using simulation studies based on acquired data, we suggest that too-infrequent sampling in common movement assays limits observation of early-adult changes in motility, and we propose feasible strategies and a framework for designing assays with increased sensitivity for early movement declines.

Silicon etch rate with caustic is widely investigated due to its anisotropic etch behavior which is important for semiconductor device fabrication. In wafer manufacturing, caustic or acid etching is used to remove surface defects produced from prior processing steps. Prevention of metal deposition is an important consideration for use of caustic in silicon wafer manufacturing process (1). Deposition behavior of nickel on doped silicon wafer from caustic solution is investigated here. A number of process parameters like type of caustic, doping level, and process temperature on deposition behavior of nickel from KOH and NaOH solution are considered. The mechanism of nickel deposition is discussed along with prevention of deposition from caustic solution. Boron doped P-type silicon wafer of low and high resistivity and orientations is used for the investigation. The wafers were etched approx. 48%-50% by wt. caustic solution at temperatures and duration indicated for investigation. The level of nickel concentration in caustic solution is less than 100 ppb. Surface nickel is investigated following treatment in caustic solutions followed by rinsing with DI water. For bulk measurements of nickel, the wafers were subjected to standard polishing and cleaning procedures following the indicated caustic etch treatment indicated below. Both surface and bulk contamination of silicon wafer by caustic solutions is investigated. Surface adsorption of nickel from these two caustic solutions is shown in Figure 1. Contamination is found to be higher with NaOH compared with KOH. Bulk contamination of nickel from each of these caustic solutions as a function of resistivity is shown in Figure 2. The amount of deposited nickel tends to increase with increasing doping level for both the NaOH and KOH used here. The increased level of nickel adsorption from NaOH is likely due to lower stability of the nickel ion in NaOH compared to KOH based caustic solution. This is explained by increased availability of water molecule in KOH based caustic compared to NaOH based solution (2). The impact of doping level on deposition is explained by higher reduction potential of boron compared to silicon which enhances the oxidation by OH ion as substrate resistivity is reduced. Furthermore, deposition of nickel ion from alkaline solution also favors the deposition on substrate due to higher reduction potential for the metal compared to silicon in solution (3). The amount of deposited nickel from caustic solution is also found to increase with solution temperature due to increase in metal diffusivity with increasing solution. Finally, the prevention of nickel deposition from the caustic solution is found to be effective with the addition of chelating agent like TTHA and DTPA in caustic solution due to higher stability index of nickel ion with these agents in high pH caustic solution. Addition of peroxide in caustic is also found to be effective in preventing nickel deposition by enhancing the stability of nickel ions in solution. References G. B. Larrabee, J. Electrochemical Soc., 108, 1130 (1961). H. Freiser and Q. Fernando, Ionic Equilibria in Analytical Chemistry, John Wiley and Sons Inc., New York, 1963. Yoshihiro, M, K. Uemura, K. Shimanoe and T. Sakon, J. Electrochem. Soc., Vol. 142 (9), 3104, 1995.

Adverse affect of surface contamination, including metal, on silicon wafer resulted into continuous evolution of RCA cleaning process.1 Deposition behavior of nickel on silicon wafer from SC1 solution, based on TMAH, is investigated here. A number of process parameters like chemical concentration, process time and temperature on deposition behavior of nickel from SC1 solution are considered. The mechanism of nickel deposition is discussed along with possible formation of some nickel silicide on the surface. Increased level of nickel deposition, as a function of TMAH concentration, from TMAH based SC1 solution is shown in Fig. 1. Time dependency of nickel deposition is carried out by exposing wafer into a nickel-contaminated SC1 solution with incremental period of time. The level of deposited nickel is found to increase with time as shown in Fig. 2. In other words, the non-equilibrium nickel deposition is observed, for the nickel contamination level used here and the period of investigation. Additional nickel deposition study is performed by exposing wafer to TMAH based SC1 and ammonia based SC1 solution as a function of temperatures separately. Higher nickel deposition tendency, from SC1 bath, is observed in TMAH based SC1 solution compared to ammonia-based SC1 solution. Furthermore, the amount of deposited nickel from SC1 solution increases with temperature. The increased level of nickel deposition from TMAH based SC1 solution compared to ammonia- based SC1 solution is likely influenced by the increased stability of the nickel ion in ammonia with the formation of ammonia-nickel complex. 1 Characterization of surface nickel from nickel-contaminated TMAH-based SC1 solution is carried out by dippingsilcon wafers in SC1 that contains 1ppb Nickel for 4.5 min., 9 min and 18 mins. respectively. This is shown in Figure 3. Amount of deposited nickel in chemical oxide was first analyzed with VPD-ICPMS. Subsequently, non-removable surface nickel on the surface was analyzed by ICP/MS following etching with surface with mixture of hydrofluoric and nitric acid. The mechanism of nickel deposition can be considered to proceed by the reduction of Ni+2 ion via electrochemical oxidation of silicon in the alkaline solution.2 In other words the growth of chemical oxide on silicon is due to high redox potential of the SC1 solution containing peroxide. As indicated above the amount of metal oxide is expected to increase with treatment time observed in a similar study with copper.3 Furthermore, the observation of silicon oxide between the deposited nickel and substrate in alkaline solution is observed in an EDX analysis of the interface.4 The oxide layer is proposed to be produced by reaction of oxidized silicon with OH- ion promoted by Ni+2ion. Part of Ni adsorbed on wafer from SC1 (TMAH+H2O2) possibly form nickel silicide, which may be difficult to remove in traditional low pH metal cleaning step. Fig. 1 Amount of Nickel deposited on the wafer following SC1 traetment from SC1 solution with increasing ratio of TMAH to H2O2 concentration. Fig. 2. Nickel adsorption from SC1 solution contaminated with 1 ppb of nickel. Figure 3. Nickel adsorption from TMAH-based SC1 solution and NH3-based SC1 solution as a function of temperature is shown below. Elevated nickel deposition is observed in TMAH-based solution. Figure 4. Removal and non-removal nickel deposited on the wafer surface treated with dilute hydrochloric acid following depostion from from SC1 solution. Increased amount of non-removable nickel is observed with time. References 1.W. Kern and D. A. Puotinen, RCA Rev., vol. 31, 187 (1970). 2.P.M.M.C Bressers, S.A.S.P. Pagano, and J. J. Kelly, J. Electroanal. Chem., vol. 391, pg. 159 (1995). 3. J. S. Kim, H. Morita, J. D. Joo and T. Ohmi, J. Electrochem Soc. Vol. 144 (9), pg. 3275 (1997). 4. N. Takano, N. Hoseda, T. Yamada and T. Osaka, J. Electrochem Soc., vol 146 (4), pg. 1407 (1999). Figure 1

Recent increases in the demand for advanced discrete and power devices produced from low-resistivity heavily-doped wafer demand better understanding of the surface preparation for this type of silicon wafer. Production of these doped wafers with low surface defects require, among other things, etch behavior of the materials by the standard clean (SC1) solution of the wet cleaning process. Determination of the silicon etch rate of a number of N-type and P-type doped epitaxial silicon wafers with different crystal orientations, dopant types and dopant levels have been made. Furthermore, observed variation in the removal rate of the chemical mechanical polishing (CMP) process for these doped silicon wafers is correlated with the etch rates. The surface quality of these wafers following CMP and SC1 chemical treatment is found to impact the defect level and cleanliness of the post-CMP cleaning process. Information obtained thereof is useful to develop wafer manufacturing process starting from silicon crystal to the final clean substrate or epi wafer with low surface defect.

Trace metals in hydrogen peroxide and high-purity water, used in wet cleaning of wafers, is correlated to the metallic contamination of the wafer surface since metal contamination adversely impacts IC device yield. Hydrogen peroxide with different level of trace metals, ranging from 100 ppt level to 10 ppb level, is used in the experiment to determine the level of selected metal deposition on the silicon surface. Significant amounts of these trace metallic impurities can be removed by subsequent treatment of the silicon wafer by an acidified solution of water. Contamination of a wafer surface by sodium is also correlated to the level of sodium present in deionized water used in the final rinsing of the wafer. The threshold and mechanism for metal deposition for various metals on wafer surfaces are different. This information is useful in determining purity requirements for process chemicals and improving the wet cleaning process for the next generation of semiconductor devices.

Ultrapure water containing dissolved ozone is being increasingly considered for providing contamination-free environment required for advanced wet chemical processing of wafers. Fluoropolymers, including polyvinylidene fluoride, have become industry standard materials for the construction of piping and components used in the distribution of ozonated ultrahigh-purity water for the semiconductor industry. In order to determine the chemical stability of polyvinylidene fluoride in the presence of dissolved ozone, we used a number of complementary analytical techniques: scanning electron microscopy with energy dispersive spectroscopy, angle-resolved x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy, and ion sputtering for chemical characterization of the polyvinylidene fluoride pipe surface. Both qualitative and quantitative information obtained from these analyses revealed a loss of fluorine with minor surface oxidation. This was found to be limited to several monolayers near the surface of the ozone-treated polyvinyldiene fluoride pipe. The loss of fluorine is explained via a dehydrofluorination mechanism which normally initiates via free radicals produced by interaction of ozone with polyvinylidene fluoride. Implications of such surface reactions for wet chemical cleaning of wafers for the semiconductor industry are discussed.

Interaction between low-extractable polyvinyl chloride (PVC) pipe and high-purity water containing 50 ppb of dissolved ozone is investigated for a period of five weeks. Rinse water analysis from the pipe effluent showed an increased level of chloride ion following ozonation. A number of analytical investigations such as scanning electron microscopy, x-ray photoelectron spectroscopy, and static secondary ion mass spectroscopy revealed chemical interactions between the PVC pipe surface and the dissolved ozone. Organic solvent cement used for joining the pipe is also affected chemically by the action of ozone. Oxidative degradation of the PVC by dissolved ozone can be explained by dehydrochlorination mechanism.

Determination of the silicon etch rate of a number of n- and p-type doped epitaxial silicon wafers with different crystal orientations, dopant types, and dopant levels have been made. Oxidation behavior of the silicon in solution is found to be affected by the doping level of the arsenic doped wafer. Observed variation in the removal rate of the chemical mechanical polishing (CMP) process for these doped silicon wafers is correlated with the etch rates. The surface quality of these wafers following CMP and standard clean chemical treatment is found to impact the defect level and cleanliness of the post-CMP cleaning process. Resistance to oxidation and chemical dissolution of silicon is explained by the generation of surface charge, charge transfer mechanism, etching anisotropy, and activation energy for surface reaction caused by the dopant and crystal orientation. Information obtained thereof is useful to develop the wafer-manufacturing process, starting from silicon crystal to the final clean substrate or epi wafer with low surface defects.

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