Roland A. - PVD for microelectronics (779636), страница 57
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At the lower pressure, both targets produce a thickness uniformity with a pronounced off-axis peak at4.5 mm that is associated with the annular erosion groove produced byR. POWELL AND S. M. ROSSNAGEL306Ii .......................i........... +....................+......................+.....................+......................i .................. i100-~+ ............. :~!!!-tli .................. ~ .................. 4 .................. ~ ......................
i ..................... ' ...................... '~..... ii"~l i-!'i ...........o.1oi::........i...::.,.:.........:,.,,..:..+.,.,.::..:...... ..._+::14 ..............i."........t ..............I+++ ......... +............. + ........"tii .............i,,,,,:'.:::..........:....:.+.............::.:..::t...:....:,.:.:-::..... +.........:::!:o++.................... +-i-4s0o.........'c .......... +4 ......................
.+."...................... + ....................... +...................... ;....................... +...................... +....................... +IIIIII1.101.151.201.a51000/T1.no1.351.401.45(K "1)FIG. 9.15 Heaters from different PVD vendors are compared using the Ti + 3A! = TiA! 3 reactionmethod described in section 9.3.5.
At the same nominal setpoint of 500~ wafer temperature differedby 45~(Reprinted with permission from R. Wilson et al, J. Vac. Sci. & Tech. BIS(1): 122-126(1997). Copyright 1997 American Institute of Physics.)the particular magnetron source used. In effect, the erosion profile of thetarget is imaged into the thin film. Although the curves are similar, theyare not identical but reflect differences in the angular emission distribution of Cu and AI.
At higher pressure, gas-phase scattering smears out thepeak and broadens each profile, although the effect is less pronounced forthe 64Cu, whose scattering angle with the 4~ is lower than that of 27A1.Thus, at higher pressure, the memory of the sputtering distribution at thetarget is lost through randomizing collisions with the sputtering gas.To the extent that the separate A1 and Cu sputter distributions can be superpositioned to describe a compound AI-Cu target, we would expect thecomposition of an AI-Cu film deposited from the magnetron of Fig.
9.16 todepend on sputter pressure and be relatively Cu-rich at the center versusthe edge, as shown in Fig. 9.17. In practice, such effects may be smallerthan predicted due to such things as the surface diffusion of Cu at elevateddeposition temperature. Also, it is worth noting that very little Cu is involved in an absolute sense. For example, if all of the Cu in the bulk of a8000-~ A1-0.5%Cu alloy film segregated to the surface, the thickness ofthe resulting Cu layer would only be about 15 A.PVD MATERIALS AND PROCESSES7001-,9~176176s~176/'iI4OOlt,9,307.........
, . . . .9 Experimentos,0r~i,-~ 3oo200100 I0.L0_-J ,5,--10q15r=-rrlCn"2025Radius (mm)200-,ITv-,-,-,9Experiment150 ~ I ~ I ~ T~-OSIMSPUD100s~t0/0...5...10O0.15~ 1 7 6 1 7 6 1oo@76 L2025Radius (mm)(a)FIG. 9.16E x p e r i m e n t a l a n d m e a s u r e d radial t h i c k n e s s protiles for e l e m e n t a l (a) Al and (b) Cu tar-gets s p u t t e r e d at 5 m T o r r and 20 m T o r r f r o m a 5 - c m - d i a m e t e r m a g n e t r o n w i t h a 6 - c m s o u r c e - t o - s u b strate s p a c i n g [9.19].9.4 Titanium9.4.1 METALLURGICALISSUES FOR P V DWhile PVD Ti is used for a variety of purposes in multilevel interconnectschemes (e.g., its role as a wetting layer to enhance hot A1 PVD processingis discussed in Chapter 7), its critical application is to reduce interfacialR.
POWELL AND S. M. ROSSNAGEL3081600 ...................+"'12009I=9.....-I9"I'"9ExperimentQ SlMSPUD~~E~I!~9 800~--~9re-~-9o400~ ~ O O i l ~ l r t a c t r t08005-,10..15..2025Radius (mm)'~i--"-""'-9Experimento SIMSPUD"-'E 6009 400._ot-t--I-"200009-'|5........Q~Oa~O~OOO~n--~-~r,~10152025Radius (mm)FIG. 9.16(b)oxide impurities and thereby improve adhesion and reduce contact resistance between a via plug and an interconnect line or between a contact plugand silicon. The key attribute of Ti that makes this possible is its ability toreduce native silicon oxide through the formation of TiO or TiO 2 (e.g.,SiO 2 -t- Ti = TiO 2 4- Si) as well as to reduce other insulating metal oxidessuch as A1203 whose formation cannot always be prevented and whose insitu removal by sputter etching can be problematic (see Section 5.3.3).The key challenge for PVD Ti is getting sufficient bottom coverage inhigh aspect ratio f e a t u r e s - in both an absolute and a percentage sense(see Fig.
9.18). Consider the case of a contact to Si. In this case, thereshould be enough Ti at the bottom to completely reduce the native sili-PVD MATERIALS AND PROCESSES1.20309' ', . . . . . . . . . . . . . . . . .1.15~ 1.00i...,.I~" 0.950.9051015Radius (mm)202530FIG. 9.17 Cu concentration variation expected for an AI-Cu alloy sputtered deposited under the conditions of Fig. 9.16 [9.19].con oxide. Even though the native oxide is ultrathin ( ~ 20-30 A), thePVD Ti initially forms discontinuous islands so that a rather thick Ti film( ~ 100 A) is required to reduce the native oxide over the entire surfacearea of the contact. Subsequent high-temperature annealing is often usedto convert the unreacted Ti into TiSi 2, which has relatively low resistivity and can further reduce contact resistance by consuming interfacialcontamination.
On the other hand, if too thick a Ti film is deposited, somuch of the underlying, active Si region may be consumed during silicidation as to compromise junction integrity.Even if an optimal absolute amount of Ti reaches the bottom of the contact (e.g., 100 ~), the percentage of bottom coverage needs to be highenough to avoid bread-loafing that could restrict the top of the hole. Also,it is desirable to prevent depositing very thick Ti films on the field regionssince this could lead to the formation of even thicker and more resistiveTiAI 3 films after PVD AI. Since the bottom coverage of conventional PVDTi in a 4:1 aspect ratio contact hole is < 5%, 100 A of Ti at the bottomtranslates into 2000 A or more on the field.
For this reason, advanced device applications of PVD Ti in contact or via holes involve some directional enhancement (such as low pressure) to reduce gas-phase scattering,physical collimation (variously called coherent sputtering, filtered sputtering, and controlled divergence sputtering or cds) and, more recently,310R.
POWELL AND S. M. ROSSNAGELFIG. 9.18 For PVD Ti and TiN barriers and liners, one generally desires a high-percentage bottom(B/A) and sidewall (D/A) coverage, with robust corner thickness (large C). A flat-bottomed profile(B ~ C) is also preferred for such applications as a Ti contact layer.ionized metal PVD. Deposition of two-dimensional Ti films for a planar,slab AI interconnect do not typically use such directional enhancementswhich may involve an unwanted trade-off of blanket uniformity againstbottom coverage.Finally, we note that a flat profile for the PVD Ti at the bottom of thehole is in general preferred over the domed shape that can result from applying PVD to high aspect ratio features.
A domed profile of Ti in a contact hole would lead to an unwanted variation in Ti-silicide thickness overthe contact area. Also, in a via hole, the thinning of Ti at the edge could bereplicated in a barrier overlayer (e.g., TiN) and compromise its ability toperform as intended.
Unfortunately, Ti is a refractory metal whose meltingpoint is sufficiently high ( T p ~ 1670~that hot PVD processes cannoteasily be exploited to flatten surface topography as with AI. Therefore,other methods of redistributing Ti mass at the bottom (such as resputtering) must be considered.PVD MATERIALSAND PROCESSES3119 . 4 . 2 P V D TI PROCESS RESULTSFigure 9.19 presents representative PVD Ti process conditions and filmproperties, and Fig. 9.20 shows the bottom and sidewall coverage of PVDTi films deposited with the directional enhancement of a 1.5:1 aspect ratiocollimator (1.5:1 cds Ti), which allows moderate coverage ( ~ 25%) inhigh aspect ratio topography.
Ironically, even though the percent coverageof PVD Ti films in steep structures is relatively low, the films can appearvery conformal. From Fig. 9.20 we see that the bottom and sidewall coverage of steeper features (AR > 4:1) are comparable, so we would expectcollimated Ti (and TiN) films to uniformly coat such structures. This isseen in the SEM micrographs in Fig.
9.21, where 1.5:1 cds TiN was deposited onto a very high aspect ratio (AR ~ 8:1) sub-0.25-/~m hole. WhileFIG. 9.19wafer).Representative PVD Ti process conditions and film properties (1.5:1 collimator, 200-mm312FIG. 9.20R. POWELL AND S.
M. ROSSNAGELBottom and sidewall coverage (i.e., step coverage) of PVD Ti film in a contact or via holeas a function of hole aspect ratio and collimation.collimation greatly improves bottom coverage, its use may degrade blanket uniformity somewhat.
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