Multiple layer sputtering

In secondary ion mass spectrometry (SIMS) the sample surface is sputtered by an ion beam and the emitted secondary ions are analyzed by a mass spectrometer (review Ref. [360]). Due to the sputtering process, SIMS is a destructive method. Depending on the sputtering rate we discriminate static and dynamic SIMS. In static SIMS the primary ion dosis is kept below 1012 ions/cm2 to ensure that, on average, every ion hits a fresh surface that has not yet been damaged by the impact of another ion. In dynamic SIMS, multiple layers of molecules are removed at typical sputter rates 0.5 to 5 nm/s. This implies a fast removal of the topmost layers of material but allows quantitative analysis of the elemental composition. 

For many years, nearly all the substrate materials used in the electronics industry were ceramics produced by tape casting (Mistier, 2000). These substrates consist of multiple layers of tape-cast material acting as the insulative carrier, with layers of metal deposited by sputtering, CVD, or evaporation processes. Holes punched into the tapes fill with metal and act as vias or pathways between layers when the tapes are stacked. Stacked layers of tape-cast blanks are laminated together and then sintered to produce a monolithic substrate. This process is detailed in many handbooks on electronics packaging (e g., Harper, 2000). One of the prime benefits is that the layers of the final part can be different different hole patterns, different metallization patterns, even different dimensions on each layer. 

Based on eqs (4) and (5), the time axis (5) of Fig. 8.20D can be converted to depth (nm) for the entire sampling depth (150 nm). Therefore, the final form of the data now reveals concentrations as a function of depth. Figure 8.20E shows the quantified result of the Cu layer on a steel substrate shown previously in Fig. 8.20C as a time-intensity plot. The experimental results obtained using the quantitative method predict a depth of 54 2.1 nm (1 s at the 95% confidence interval), consistent with the nominal depth of Cu reported for the sample. In addition, this plot provides more valuable information than do the time-intensity plots as it allows the concentration of the multiple layers to be monitored as the sample is being sputtered. 

Single or multiple layers of TiAlOx, AI2O3, TiA10x/Al203, and T1O2/AI2O3 were synthesized by magnetron sputter-deposition techniques, and more recently by atomic layer (ALD) deposition (see details of oxide layer synthesis in the ehapter Microchip-Embedded Capacitors for Implantable Neural Stimulators). 

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. 

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