Sputtering materials factor

One final factor which can influence stoichiometry has been described by Stupp . He reported that fresh molybdenum disulphide compacts initially emit sulphur faster than molybdenum. As a result the target surface becomes molybdenum-rich. As sputtering continues, sulphur diffuses to the surface at a rate which balances the removal of sputtered atoms, so that an equilibrium is established and the composition of the sputtered material remains constant. The problem of the initial variation in composition is overcome by a preliminary sputtering of the target before the specimen substrate is exposed to the sputtered particles. 

GDMS is slowly replacing SSMS because of its increased quantitative accuracy and improved detection limits. Like SNMS and SALI, GDMS is semiquantitative without standards ( a factor of 3) and quantitative with standards ( 20%) because sputtering and ionizadon are decoupled. GDMS is often used to measure impuri-des in metals and other materials which are eventually used to form thin films in other materials applications. 

In the field of microelectronics, there is continuing research in developing new materials to be used in semiconductor fabrication. They must be formed as thin films in a controlled, reproducible and uniform manner to be useful in semiconductor manufacturing applications. Depth profiling by AES is used to assess the properties of such films. The samples are sputtered with an argon ion beam and analysis performed using standard sensitivity factors, and it is possible to demonstrate that such films are uniform throughout a depth of, say, 250 nm. 

The time necessary for removing one monolayer during a SIMS experiment depends not only on the sputter yield, but also on the type of sample under study. We will make an estimate for two extremes. First, the surface of a metal contains about 1015 atoms/cm2. If we use an ion beam with a current density of 1 nA/cm2, then we need some 150 000 s - about 40 h - to remove one monolayer if the sputter yield is 1, and 4 h if the sputter rate is 10. However, if we are working with polymers we need significantly lower ion doses to remove a monolayer. It is believed [4] that one impact of a primary ion affects an area of about 10 nm2, which is equivalent to a circle of about 3.5 nm diameter. Hence if the sample consists, for example, of a monolayer film of polymer material, a dose of 10n ions/cm2 could in principle be sufficient to remove or alter all material on the surface. With a current density of 1 nA this takes about 1500 s or 25 min only. For adsorbates such as CO adsorbed on a metal surface, we estimate that the monolayer lifetime is at least a factor of 10 higher than that for polymer samples. Thus for static SIMS, one needs primary ion current densities on the order of 1 nA/cm2 or less, and one should be able to collect all spectra of one sample within a quarter of an hour. 

The fact that LEIS provides quantitative information on the outer layer composition of multi-component materials makes this technique an extremely powerful tool for the characterization of catalysts. Figure 4.19 shows the LEIS spectrum of an alumina-supported copper catalyst, taken with an incident beam of 3 keV 4He+ ions. Peaks due to Cu, A1 and O and a fluorine impurity are readily recognized. The high intensity between about 40 and 250 eV is due to secondary (sputtered) ions. The fact that this peak starts at about 40 eV indicates that the sample has charged positively. Of course, the energy scale needs to be corrected for this charge shift before kinematic factors Ef/E-, are determined. 

The model samples were synthesized and characterized in the Analytical Chemistry Dept of the Universite Libre de Bruxelles under the direction of Prof. C. Buess F.R. is grateful to P. Kons and E. Silberberg for the preparation of the samples. AES sputter profiles and factor analysis was performed at the Vrije Universiteit Brussel, Dept, of Metallurgy, Electrochemistry and Materials Science. Many thanks to Prof. Vereecken, Hubin and Terryn for the discussions concerning the results and to N. Roose and O. Steenhaut for the Auger sputter profiles. The technical collaboration of L. Binst (ULB) is greatly appreciated. 

Ion milling is a more widely applicable etching technique in that all materials may be sputtered away anisotropically. The ion milling rate typically does not vary more than a factor of 100. Redeposition of material from the substrate or surrounding fixtures can lead to undesirable cross-contamination. Hosaka et al. (39) used SIMS to show that redeposition of ionic impurities can occur during ion etching of SiO layers on Si when using a... 

An excellent way to create standards is ion implantation of the elements of interest into the matrix. This works exceptionally well for semiconductors since one can usually start with high-purity single-crystal materials that represent the matrix of interest. Also the use of Eq. (4.8) is well suited for this purpose since ion implanters usually quote doses in atoms per square centimeter. However, Eq. (4.5) serves just as well by converting the matrix concentration to atoms per cubic centimeter. In this procedure, the implant profile is sputtered through, the implant element secondary ions and the matrix element secondary ions are each summed, and the depth of the sputter profile is determined, usually by using a stylus profilome-ter. The sensitivity factor is then calculated from... 

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