Análisis inferencial Prueba de normalidad

The Fenris system (diagrammed in Figure 11b)) combinatorially sputtered three facing guns (US Gun II, US Inc., San Jose, Ca) with 2” targets in a triangular off-axis arrangement around a 3” substrate holder. The distance from the center of each target to the center of the substrate was 2.5” in the plane of the substrate and ~2.2” perpendicular to this plane. The off-axis system allowed for less oxygen bombardment during reactive oxygen sputtering and perhaps Ar reflection as well. Cross-contamination would have been a problem at lower sputtering pressures, but since 30 millitorr was the standard operating regime, with a mean free path into the tenths of cm, this was not deemed a significant problem. The off-axis and high- pressure arrangement compared to the Gilgamesh sputtering system resulted in atoms impacting the substrate with much less kinetic energy and in a slightly different exponential decay model. However, at least according to XRD, the standard 500°C hot-sputtering seemed to minimize structural changes (such as higher texturing from off-axis deposition). Higher base pressures of 6 microtorrs may have resulted in greater oxygen and water incorporation into metallic films, but these were visually lacking, with oxide and partial-oxide XRD peaks in sputtered metallic films not noticed.

For underlayer and single-layer deposition, a top 4” US Gun II gun was employed. This 4” gun was pulled as far back from the substrate as possible when the other three 2” guns were in use due to the strong magnetic fields associated with its magnetron sputtering system influencing the plasma of the other guns, changing such variables as their dc biases and yields. In addition, when the 4” gun was in use, it was still pulled as far back as possible to give a reasonable deposition rate. Although the 4” target was larger than the 3” substrate, the majority of material sputtered still came

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from the typical magnetron ‘racetrack’ centered over the magnets buried in the gun. This ‘racetrack’, where the majority of secondary electrons that ionize the Ar responsible for sputtering were located, had a diameter slightly smaller than 3”. As such, placing the gun too close to the substrate would result in an unacceptable thickness variation. Also, reflected Ar and other ions coming from the gun could cause unwanted changes in the deposited film. A typical sputtering distance for depositing many types of metallic underlayers at 100 W in 10 millitorr Ar was ~16 cm, which seemed to produce a thickness variation of at most 10% between the center and edge of the substrate while leaving the deposited film with compressive ‘bulk-like’ properties.

Four rf power supplies, operating at a standard 13.6 MHz, were connected to each gun. An alternating power supply was not needed for the Gilgamesh system, since it mainly dealt with metallic targets. The metallic target could usually compensate for any positive charge from Ar+ impacts by e- compensation from the negatively charged base-plate electrically connected to this surface through the metal. In some cases of dc reactive sputtering (with a small amount of reactive gas) in which the author was involved, a thin non-metallic (i.e. oxide) layer would form on the surface. It would be thin enough to still allow a compensating charge from the base e-, but thick enough to show up as an increased voltage when under current control, requiring greater power to sputter. However, the Fenris system’s main emphasis was oxides. The higher oxygen reactive gas partial pressures produced surface oxides with thicknesses too high for charge compensation through the base plate. To provide this compensation, an rf power supply alternated the potential of the target between the standard dc negative voltage to attract Ar+ and a new positive voltage to attract e- from the plasma. These electrons were able to compensate for the positive charge from surface Ar instead of e- traveling through bulk target to the surface. In fact, since these

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e- in the plasma travel so much faster than the Ar+, a negative bias developed on the target inherent to the plasma conditions. For metallic sputtering, it stayed constant, but for oxide sputtering, this inherent bias could increase as the thickness increased. Since the applied potential is alternated, Ar+ ions were not always sputtering the target, resulting in reduced rates in comparison to dc systems.

To overcome this rate reduction dilemma when a target had to absolutely, positively be sputtered at higher rates in the Fenris system, a 5th power source used pulsed dc generation. It usually applied square waves that had a much longer duration at negative than positive voltages. Compensation from plasma electrons could still occur due to their much faster movement, but the larger negative potential would allow greater Ar+ bombardment for a higher sputtering rate. An added bonus was that any impedance mismatch at dc (even at the maximum 250 KHz operating regime of the pulse source) between the source and gun was much less than at rf, allowing a much higher power for sputtering without worrying as much about the resultant heat dissipation in the rf cable and target.

The substrate holder was connected to a rotating rod connected to a motor on the bottom of the chamber. Wafers could simply be held onto it by gravity, as opposed to the Gilgamesh slot arrangement. Only one film could be sputtered at a time on the single holder. This holder could be heated resistively up to ~500°C with the temperature monitored by type-K thermocouples. The lower temperature was indicative of both a more thermally connected substrate holder combined with a smaller more easily radiatively-heated than in Gilgamesh. Higher temperatures than 500°C could result in damage to the Viton O-ring seals, ruining the vacuum. A QCM switched in place of the substrate holder would determine rates in a prior pumpdown, with elements lighter than Si giving unreliable measurements, either from oxide formation, unreliable sticking coefficients to the crystal, or other effects [59].

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Similar to Gilgamesh, each of the triangular off-axis guns in this system could separately deposit a film whose rate exponentially decayed from the edge of the 3” substrate closest to it. If a 2-D axis on the film were defined with the origin at the target center, the x-axis perpendicular and the y-axis parallel to the gun, a typical equation for the deposition rate would be:

Rategun(x, y) ~ A*exp(-√[(x – xc)2 + c(y – yc)2]/d) (2.2)

where b and d were fudge-factors related to the rate and sputtering conditions (e.g. power, pressure, bias, etc.). An example for Pd at 30 W is seen in Figure 13. The main difference from on-axis sputtering was the ‘c’ factor, which related to the off-axis nature of the previously-described ‘sputter cone’ intersecting the substrate plane at an oblique angle. In most cases for metal targets, ‘c’ hovered around .8 while it was lower for oxides. This more elliptical pattern from the metal atoms may have been due to the higher rates and kinetic energies of target atoms translating to less scattering events before deposition.

With the conditions most often used for metallic films, the on-axis Gilgamesh sputtering system could reach ~60% of the composition space of a ternary alloy compared to ~40% for the Fenris off-axis system, as seen in the diagrams in Figure 14 for an A:B:C metallic alloy with a desired 1:1:1 ratio in the middle of the substrate. Since the hot-sputtered structures of the two systems were similar, this made the Gilgamesh system particularly appealing for covering a wide swath of composition space and the Fenris system interesting for zeroing in on a smaller portion. For instance, Gilgamesh could sputter compositions that atomistically changed by ~1%/mm while Fenris could decrease that to ~0.5%/mm.

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Figure 14. Phase diagram coverage at the sputtering conditions used in this study for a) Gilgamesh on-axis system and b) Fenris off-axis system when each gun is calibrated to produce a 1:1:1 atomic mixture in the middle of the substrate.