Roland A. - PVD for microelectronics (779636), страница 8
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Since this energy regionis more amenable to statistical calculation, the theory is well developedand accepted. Good reviews of this field have been published [2.11, 2.12],but the topic will not be covered in this volume.4. Very High Energy (E > 50 keV). At these high energies, the incidention can penetrate down into the target lattice many layers before causing asignificant number of collisions. As a result, the affected volume is wellbelow the surface and few if any atoms can be emitted. At these high energies, the incident ion is effectively implanted into the bulk of the target.This may be quite important to the electrical properties of the materials,particularly for semiconductors, but it is mostly irrelevant for physicalsputtering.Since sputtering is mostly a momentum and energy transfer process between the incident particle and the target atoms, the particular species usedare very important.
As shown in Fig. 2.4, the yields are different for different target materials using the same ion species and energy. There aretwo reasons for these differences. First, the binding energy will be different for each target material, and this is the barrier that a target atom mustovercome to be emitted from the surface. There is a general trend towardPHYSICS OF SPUTTERING29TABLE 2.1SPUTTER YIELDS FOR SEMICONDUCTOR-RELATED MATERIALS FORNE, AR, AND KRAT 200, 500, AND 1000 EV [2.10].IonNeNeNeArArArKrKrKrKE (eV)BeCAISiTiNiCuZrNbMoAgTaWPtAu2000.145000.5410002000.145000.5110000.920.595000.570.131.170.510.571.62.60.710.650.913.40.620.641.52.610000.310.180.260.561.00.220.180.291.20.160.150.370.692000.170.040.470.220.250.751.20.310.260.411.60.300.300.681.10.320.110.160.491.00.200.190.341.30.320.360.701.21.010.530.511.42.50.620.590.953.50.931.02.03.31.42.30.470.430.602.20.340.350.771.371.62.40.622.41.93.11.23.83.62.33.71.44.8higher sputter yields for materials with lower binding energies, and thereis a general correlation between low melting point and low binding energy.This can be seen in Fig.
2.5, which plots the sputter yield for 1000 eV Ar §bombardment for a variety of materials as a function of the mass numberof the target. However, sputtering is not a thermal process, so these correlations should not be taken too strongly.A second reason for differences in yields is the efficiency of the momentum transfer process between the incident ion and the target atom. Byconservation of energy and momentum, the energy transferred is related tothe product of the masses of the two species divided by the square of thesum of the masses. This has a maximum value for two equal mass species,which implies that the highest sputter yields should be for cases of a targetbeing bombarded by an ion of the same species.
This situation is known asthe self-sputter yield. It suggests, though, that the sputtering process willbe rather inefficient and the yields relatively low for cases of a large mismatch between the incident and target masses.The sputter yields for various inert gases on Si over a wide range of ionenergies are shown in Fig. 2.6. In the high-energy regimes, there is a significant mass dependence to the yield. However, in the knock-on regime( < 1 keV), there is only a vague dependence of the yield on ion mass. ThisR.
POWELL AND S. M. ROSSNAGEL308I1III,I76o~5Agn4cu321PdAIC~~/tSiTiFe Zr NbI20ii40ooII60II80ztFIG. 2.5Sputter yields for 1000 eV Ar t bombardment as a function of target mass number [2.12].is particularly true for light-mass targets. It is routinely thought that goingfrom Ar to perhaps Kr or Xe will result in a higher sputter rate and, for deposition applications, a higher deposition rate. From a yield point of view,this is only true for relatively high-mass target species with a mass muchgreater than 40 (Ar).The angle of incidence of the bombarding ion can also have an effect onthe sputtering process. This is shown schematically in Fig.
2.7. The incident ion at normal incidence affects the target in a regime roughly characterized by the spherical volume outlined as a dotted circle. A small fractionof this circle intercepts the surface, and this defines the area from whichenergetic, sputtered atoms might be emitted. As the incident angle goes to45 ~ or so, the volume affected by the impact is moved closer to the surfaceand, as a result, more atoms near the surface can be emitted by the collision process. The sputter yield in this case can easily exceed the case of90 ~, normal incidence. However, as the incident angle becomes more grazing, eventually it is more likely that the incident ion will simply reflect offthe sample surface, resulting in little energy deposition and very little sputtering.
The angular dependence of the sputter yield, then, generally will bePHYSICS OF SPUTTERINGFIG. 2.631Sputter yields for Si as a function of ion energy for several inert gas ions [2.12].larger at angles near 45 ~ than at 90 ~ and then will fall rapidly as 0 ~ (grazing incidence) is approached (Fig. 2.8).The dependence in Figure 2.8 is often described as a cosine dependence.This can be a little confusing depending on the frame of reference.
If normal incidence (90 ~ in the prior discussion) is converted to 0 ~ and nearFIG. 2.7Schematic of ion bombardment at 90 ~ (normal incidence), 45 ~ and near 0 ~R. POWELL AND S. M. ROSSNAGELIIIII7-olkeVH++ 50KeVAr +x 1KeVAr +6-o1KeVAr +5-II;Ni-~Au~Ag;CCOS.......I1COS-20l,/o-//~"4-o~'-/~ "3-,///~176 / / ' / '-X~X~x~ __ ~ ~ _ _ + ~I0~120 ~\+110iFIG. 2.8II40 ~60 ~1\+-\I80 ~-"T h e sputter yield as a function of the angle o f i n c i d e n c e for the i m p a c t i n g ion [2.121.grazing incidence (0 ~ is converted to 90 ~ then the yield scales roughly asl/cosine of the angle from 0 ~ up to about 50 ~ This is the origin of the cosine dependence of the sputter yield.It is tempting at this point to infer that here is a way to increase the sputter emission rate from a target: If the surface were inclined at 4 5 - 5 0 ~ fromthe ion direction, the yield would be increased nearly 2 times.
However,there are two problems with this scenario. First, there is a differentiationbetween ions that come in the form of an ion beam and ions that come froma plasma. The ion-beam ions can be directed at will and the angle of incidence onto a surface is controllable simply by positioning the beam andsample. However, for plasma ion bombardment, which would be the casein an RF diode or a magnetron for example, the plasma sheath hugs thePHYSICS OF SPUTTERING33surface of the target and all ions are accelerated to 90 ~ (normal incidence)to the surface regardless of the overall macroscopic shape of the target.
Itwould be possible, though, to groove or texture the target surface in aplasma experiment such that the fine-scale surface is inclined at 45 ~ to thesurface normal. However, this requires that the grooves be much smallerthan the sheath thickness.Unfortunately, inclining the surface of the target to the incident ions byeither means results in a reduction of the ion current density to the surface.Switching back to the reference frame where the sputter yield scales as1/cos of the incident angle, the reduction in current density scales directlywith the cosine of the angle.
Therefore, these two terms cancel each otherand generally lead to no enhancement.2.2 Energy and Angular Distributions of Sputtered AtomsSputtering differs from evaporation in that the atoms are physically ejectedfrom the target surface and as such can have significantly more kinetic energy. An example of this is shown in Fig. 2.9, which compares the velocity distribution of evaporated Cu atoms to sputtered atoms.
Typically, thehigh-energy side of the sputtered-atom kinetic energy distribution followsa l/E2-dependence. The peak in the kinetic energy distribution differs for1.0----~ . . . . . . . . . . . . . . .~-:K ..................................,/"\,/t/}1E13../"6 0.5-\,,/"\,"~/Sputtered\NE~Jz/XEvaporated at 1500 K,,\.,\,.\!,024681012Particle Velocity (km/sec)FIG. 2.9at500 eV.The kinetic energy distribution for Cu atoms evaporated at1600 K andsputtered with Ar §R. POWELL AND S. M. ROSSNAGEL34each ion-target system and is also slightly dependent on the ion's kineticenergy.While the sputter emission of small clusters of atoms is relatively rare,such clusters should be expected to follow nominally similar emissioncharacteristics in their energy spectrum, perhaps adjusted for the larger effective mass.
Figure 2.10 shows work by H. Oechsner et al. measuring theenergy spectrum of emitted Mo single atoms as well as atom pairs [2.13].The kinetic energy of the Mo dimers is roughly one half that of the singleatoms.Perhaps more important than the exact distribution is the average kineticenergy of an emitted, sputtered atom. This will be a major component ofthe net energy arriving at the film surface during deposition.
A chart ofthese average kinetic energies is shown in Table 2.2. Other significantcomponents of energy that play a part in the deposition process come fromthe heat of s u b l i m a t i o n - which is essentially the binding energy of theatom and is an intrinsic part of any PVD deposition p r o c e s s - and fromother energetic processes related to the plasma. This can include photonsfrom the plasma itself as two energetic neutral processes (which will bediscussed below).The angular distribution of sputtered atoms is generally described as acosine distribution, which is accurate to first order for most situations.Traditionally, this is shown pictorially as in Fig. 2.11, which shows anI_1IIIII2 0 0 0 eV Ar + - - - M o1.0-lMo~" 0.5-0~~___0-I010IIII30E (eV)501-7O9FIG. 2.10 Kinetic energy of Mo and Mo dimers for 2000-eV Ar + ion bombardment.
The verticalscale has been normalized to show the comparative distributions. Typically, the emission of dimers isabout 0.01 the magnitude of the level of the single atoms [2.13].PHYSICS OF SPUTTERING35T A B L E 2.2THE AVERAGE KINETIC ENERGY FOR SPUTrERED-ATOM SPECIES COLLECTEDFROM SEVERAL SOURCES.Ave KETargetIon/Energy(eV)Peak eVReferenceBeKr/12008--2.5AIKr/1200Ar/900Kr/1200Kr/1200Ar/900Kr/1200Kr/1200Kr/1200Kr/1200Kr/1200Kr/1200Ar/500Ar/900Kr/1200Ar/900Kr/1200Kr/1200Kr/1200Ar/20(X)Kr/1200Kr/1200Kr/1200Kr/1200Ar/500Ar/2000Kr/1200Kr/1200Kr/! 200921013m2.52.312.52.52.312.52.52.52.52.52.52.252.312.52.312.52.52.52.132.52.52.52.52.212.132.52.52.5SiTiVCrMnFeCoCuNiGeZrMoRhPdAgTaWAuRe311131314121010--21741321212016933257m5342139impact point and an array of arrows at various angles.
These arrows represent the relative fluxes in each direction and can be rotated about a vertical axis. The length of each arrow is related to the length (i.e., yield) at normal incidence times the cosine of the angle from 90 ~. Departures fromcosine distributions occur as a function of incident ion energy. Generally,low energies change the distribution to a wider, less-normal-incidence distribution, described as under-cosine and higher energies have the oppositeeffect (over-cosine) (Fig. 2.12) [2.14]. These effects are fairly subtle, andthe range of ion energies available in most practical plasma experiments(e.g., magnetrons) produce very little variation in the emission profile.R. POWELL AND S.
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