The Sputtering Process

There are many physical deposition (PD) processes which can be used to deposit lubricating films on surfaces, and several of them have been used, either separately or in combination, for depositing molybdenum disulphide. They include Ion Beam Enhanced (or Assisted) Deposition (IBED or IBAD), and Pulsed Laser Deposition (PLD), but the most important so far is sputtering, or more precisely sputter-coating. 

A negative potential of the order of 2 to 4 kV is then applied to the specimen which is to be coated. Positive argon ions from the plasma bombard the specimen 

DC sputtering cannot be used with targets which are electrically nonconducting, because impingement of the positive argon ions creates a positive charge on the surface of the target, which then ceases to attract the ions. It can be used for 

Further control of the ion paths can be obtained by introducing a positively charged screen or ring either between the target and the substrate or below the substrate . This can be used to apply a positive DC bias in the region of the substrate, and provides further control of the plasma and sputtering conditions. 

The introduction of unbalanced magnetron sputtering, in which part of the argon plasma can impinge on the substrate, allows ion bombardment of the coating as it forms. The use of ion-beam assisted deposition can be used concurrently or sequentially. This technique is described separately later in the chapter. 

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In the sputtering process, each surface atomic layer is removed consecutively. If there is no diffusion in the target, the composition of the vapor flux leaving the surface is the same as the composition of the bulk of the material being sputtered, even though the composition of the surface may be different from the bulk. This allows the sputter deposition of alloy compositions, which can not be thermally vaporized as the alloy because of the greatly differing vapor pressures of the alloy constituents. 

Alternative Thin-Film Fabrication Approaches. Thin films of electronic ceramic materials have also been prepared by sputtering, electron beam evaporation, laser ablation, chemical beam deposition, and chemical vapor deposition (CVD). In the sputtering process, targets may be metal... 

The detection limit of each element depends upon the electron affinity or ionization potential of the element itself, the chemical nature of the sample in which it is contained, and the type and intensity of the primary ion beam used in the sputtering process. 

Because SIMS can measure only ions created in the sputtering process and not neutral atoms or clusters, the detection limit of a particular element is affected by how efficiently it ionizes. The ionization efficiency of an element is referred to as its ion yield. The ion yield of a particular element A is simply the ratio of the number of A ions to the total number of A atoms sputtered from the mixing zone. For example, if element A has a 1 100 probability of being ionized in the sputtering process—that is, if 1 ion is formed from every 100 atoms of A sputtered from the sample—the ion yield of A would be 1/100. The higher the ion yield for a given element, the lower (better) the detection limit. 

The sputtering process is frequendy used in both the processing (e.g., ion etching) and characterization of materials. Many materials develop nonuniformities, such as cones and ridges, under ion bombardment. Polycrystalline materials, in particular, have grains and grain boundaries that can sputter at different rates. Impurities can also influence the formation of surface topography. ... 

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

The near-surface region is partially oxidized during OJ bombardment. During the sputter process the chemical bonding of the oxides is broken. Because the binding... 

Both positive and negative ions are produced during the sputtering process, and either can be recorded by an appropriate choice of instrumental parameters. Positive ions are the result of protonation, [M + H]", or cationiz-ation, [M +cation], whereas negative ions are preponderantly [M-H], but can also be formed by the addition of an anion, that is, [M+anion]". The type of pseudomolecular ion produced is governed by the chemical nature of the sample and by the composition of the matrix from which it is ionized. 

Modem trace analysis is interested in detailed information about the distribution of elements in microareas and their chemical binding forms (specia-tion). The limited sample mass implies methods with absolute detection limits as high as possible. Use of the sputtering process as a sampling technique localises the analytical zone at the outer layers of a solid, and allows analysis to progress into the interior. 

The Mg isotopic measurements were performed with a modified AEI IM-20 ion microprobe [13,14]. Secondary ions were generated by bombarding the sample with a focussed ion beam to excavate a small volume of the sample. A fraction of the sputtered material is ionized during the sputtering process and is drawn off into the mass spectrometer. A duoplasmatron ion source produces a... 

The generation of large numbers of complex, poly-atomic and multiply charged ions in the sputtering process [15,16,17] creates potentially severe problems for low resolution secondary ion isotopic analysis. To minimize the formation of hydride and... 

The sputtering process shown in Figure 19.1, utilizes the phenomena associated with a low-pressure gas discharge. The system comprises an anode and cathode generally the low-pressure chamber is earthed and forms an infinite area anode the small cathode surface is the target material from which gas ion-etching occurs, resulting in condensation of the material onto workpieces within the vacuum chamber. 

In secondary-ion mass spectrometery (SIMS) and its sister technique fast atom bombardment mass spectrometry (FARMS), a surface is bombarded with energetic particles, and the kinetic energy of the particles converts substrate and chemisorbed atoms and molecules to gas-phase species. The ejected (or sputtered) material is subsequently interrogated using various analytical tools, such as lasers and mass spectrometers, to indirectly deduce information about the initial surface. The relationships between sputtered material and the surface, however, are not always clear, and erroneous conclusions are easily made. Computer simulations have demonstrated that a fundamental understanding of the sputtering process is required to interpret experimental data fully ... 

Molecular dynamics simulations have yielded a great deal of information about the sputtering process. First, they have demonstrated that for primary ion energies of a few keV or less, the dynamics which lead to ejection occur on a very short timescale on the order of a few hundred femtoseconds. This timescale means that the ejection process is best described as a small number of direct collisions, and rules out models which rely on many collisions, atomic vibrations and other processes to reach any type of steady state . Within this same short-timescale picture, simulations have shown that ejected substrate atoms come from very near the surface, and not from subsurface regions. 

This technique yields a catalyst composed entirely of metal nanoparticles or nanocrystalline thin film, and it allows for control of size and distribution while eliminating the need for a dispersing and supporting medium. The obtained electrodes contained as little as 0.017 mg Pt/cm and performed as well as standard E-TEK electrodes (Pt loading 0.4 mg/cm ). The PLD technique may be of special interest as an alternative to the sputtering process in the production of micro fuel cells. 

Figure 4. The sputtering process—interaction of primary ions with a sample...

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