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M. ROSSNAGEL36FIG. 2.11Emission distributions for sputtered atoms.The angle of incidence of the incident ion can have an effect on theemission dynamics. This was shown early on by Wehner and Rosenberg,who compared the emission distributions on Mo for smooth and rough surfaces (Fig. 2.13) [2.15, 2.16]. The rough surface showed no preferential direction, perhaps due to the intrinsic recapture of emitted atoms by thesteep, rough surface. However, the smooth surface showed forward emission, consistent with a fairly shallow, low number of collisions, which retains some of the incident direction of the bombarding particle. Recentwork by Doughty et al.
has confirmed this work and extended it to Cu[2.17].Forward sputtering is, of course, relevant to ion beam sputtering inwhich the incident ion's direction can be determined by design. However,in a plasma experiment, ions always impact the substrate surface at normalincidence, due to the planar electric field present over the sample surface.However, if the surface contains small features (perhaps on the micronscale), the incident ions (at 90 ~ may impact a slanting surface, resulting inthe potential for forward sputter emission down into a feature or onto anearby surface.
This will become relevant in Chapter 8, which discussesionized deposition.PHYSICS OF SPUTTERINGFIG. 2.1237E m i s s i o n m e a s u r e m e n t s as a function of ion e n e r g y [2.14].Another general departure from a cosine emission distribution occurs forthe case of single-crystal or oriented targets. First observed 40 years agoby Wehner, and described to this day as Wehner spots, the emission distributions have specific, preferred angles related to the underlying crystalstructure [2.18]. This effect has been incorporated into target design in effect by at least one manufacturer as a means of developing a more-normalincidence ejection profile [2.19]. While this last case may or may not bepractical, the existence of preferred directions in the emission profile dependent on crystalline orientation indicates that at least some aspect of theR.
POWELL AND S. M. ROSSNAGELFIG. 2.13 (a) Emission angular distribution for 250-eV Ar + onto Mo at about 20 ~ for smooth andrough surfaces [2.15], (b) emission distribution for various cases [2.121.original lattice structure withstands the rather violent sputtering event onthe target surface. This is further evidence of the lack of fully developedcascades in knock-on sputtering, which would lose any memory of theiroriginal structure or orientation.PHYSICS OF SPUTTERING392.3 Other Energetic Processes during SputteringThere are two additional aspects of sputtering that may lead to significanteffects on film deposition: reflected, energetic neutrals and negative ions.Both of these terms are slightly misleading but are in common usage.Reflected, energetic neutrals are the result of energetic ion bombardment of the target.
If the mass of the incident ion is equal to or less thanthe target atom mass, there is some probability of an elastic reflection ofthe ion from the surface. Since the ion is neutralized shortly before it impacts the surface, the reflected particle remains neutral and is unaffectedby local potentials or sheaths. The reflected neutral can carry significantkinetic energy, often 20-40% of the incident ion energy. The angular distribution of these reflected atoms varies, but again to first order it might beconsidered roughly a cosine distribution.The intrinsic problem with reflected neutrals is that they are very difficult to measure experimentally in the deposition system because they areuncharged.
They can deposit considerable energy to the film surface andhave long been thought to alter such physical properties as the film microstructure and stress. A long sequence of experiments by Dave Hoffmanand John Thornton explored this situation and has been summarized byHoffman [2.20].The flux of energetic, reflected neutrals is strongly dependent on theion-to-target mass ratio.
If this number is very small, such as in the case ofsputtering refractory materials like W or Ta, the reflected fluxes can approach the deposition rate, resulting in significant energy deposition alongwith the film atoms. For example, even though the kinetic energy of a sputtered Ta atom might be in the range of 25 eV [2.21 ], the average energy deposited during Ar § sputtering of Ta can approach 100 eV/Ta atom, resulting in significant sample heating and potential problems with stress andfilm microstructure.This can also be inferred from a classic experiment by H.
Winters [2.22].In this experiment, a thin, carefully calibrated calorimeter was bombardedby a well-defined ion beam. The function of the calorimeter was simply tomeasure the temperature of the sample, from which the deposited energycould be calculated. Winters then compared the deposited kinetic energy asa fraction of the incident kinetic energy for ion energies of a few tens ofeV up to 5 keV for various ion-sample combinations (Fig. 2.14). The datashows that for cases where the incident ion weighed much less than the target film, a sizable portion (20%) of the incident energy was not depositedbut presumably was removed in the form of energetic, reflected neutrals.As the ion mass was increased to an amount to exceed the target mass, theR.
POWELL AND S. M. ROSSNAGEL40I1.0car}O0.9. . . .I. . . .cni,_c:w0.7OXe-_0.4-0.3o0.1mLI_~~I-Ar/-0.50.2K... . . .. . . .I.._.___..--/ / ~ ~ ~He-/-0.6c0I....-/0.0_/..,..CE. . . .fca. 0.8 aI/_/-///_~- ~I,10- ~~ ~ I[10 ~Il,~110 ~,~~1J~10 2,~110 3,~~,i10 4Kinetic Energy (eV)FIG. 2.14 Deposited kinetic energy fraction as a function of ion energy for He, Ar, and Xe onto Auas a function of ion energy up to 10 keV [2.221.deposited energy moved closer to 100% of the incident energy. At thispoint, reflection was no longer present, although some energy was removed in the form of the kinetic energy of the sputtered atoms.Negative ions can occur during the sputtering of materials that havecomponents with high electronegitivity.
A common example is oxygen. Inmany solid compounds containing oxygen, one component may be fromthe far left side of the periodic table, such as Ba, Y, Zr, Ti, and so on. Thesespecies readily give up an electron, which can then be attached to the oxygen atom, forming a negative ion. This negative O ion is then acceleratedby the target sheath (to be discussed in Chapter 3) and enters the plasma atthe target potential, which is typically many hundreds of volts.
The negative O ion is then stripped of its electron in the plasma and continues on asa several-hundred-eV neutral [2.23].Unfortunately, this neutral is moving directly toward the film locationand can readily sputter the growing film. This resputtering effect may beminor, leading to small changes in film structure or composition. In casesPHYSICS OF SPUTTERING41of high levels of negative ion bombardment, the film structure or composition can be radically altered and the erosion rate can actually exceed thedeposition rate, resulting in a etched substrate rather than a deposited film.Negative ion effects are generally present when working with oxygen,although for cases such as A1, Ti, and Si, the effects are small.
For casessuch as ferroelectrics or pzieoelectrics (PZT, PLT, BST, etc.), the effect isquite strong, and it is extremely difficult to attain the correct film composition without a significant change (typically an increase in the level of thehighest-sputter yield components) in the target composition.2.4 Transport of Sputtered AtomsSputtered atoms must typically travel some distance (cms or more) beforethey can impact a sample surface to form a deposited film.
The operatingpressure for most sputtering applications ranges from 10 -5 to 10 -~ Torr,over which the mean free path for gas atoms varies from 500 cm down to5 mm. This complicates the issue of atom transport. At low pressures, typically 1 mTorr or less, the sputtered atoms travel with few if any gas-phasecollisions prior to deposition. This can be described as ballistic transport,or collision-free transport.
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