Roland A. - PVD for microelectronics (779636), страница 59
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Since thesurface area of a high aspect ratio collimator ( ~ 3500 cm 2 for an AR =1.5"1 hexagonal cell collimator) is much greater than either a 200-mmwafer ( ~ 315 cm 2) or DC magnetron target (A ~ 700-1000 cm2), we seethat the collimator can have a major influence on the consumption of N 2 inthe chamber. The overall situation is schematically illustrated in Fig. 9.24.The net result of these competing processes typically leads to experimental data such as that shown in Fig. 9.25, where the deposition rate andsheet resistance of reactive sputtering of Ti in Ar/N 2 is plotted as a function of N 2 mass flow. At low flows of nitrogen into the process chamber,the deposition rate is high and characteristic of sputtering from an elemental Ti target, and the deposited film is Ti-rich TiN x.
The N/Ti ratio in thefilm increases with nitrogen fraction in the Ar/N 2 admixture. As nitrogenflow continues to increase, the curve exhibits a sharp fall off in depositionrate, reflecting the greatly reduced sputter yield of the nitrided target andthe lower ionization cross section and sputter efficiency of N~ versus Ar §If DC magnetron power is increased, the onset of this abrupt fall off occursat a higher N2/Ar fraction because the additional Ar § bombardment of thetarget sputter etches away the TiN that is forming. The deposition rate318R.
POWELL AND S. M. ROSSNAGELFIG. 9.24 Schematic illustration of N~ generation and consumption in a PVD chamber during reactive PVD of TiN.finally stabilizes at the lower value characteristic of sputtering from TiN,a target condition that workers sometimes describe as "poisoned" in thatthe target sputter yield has been degraded by the nitride surface layer. Thisterminology is somewhat hypocritical though, since one rarely hears thatthe desired TiN film produced from the "poisoned" target is "toxic"! In anyevent, the overall behavior seen in Fig.
9.25 has been modeled by severalworkers based on mass balance considerations, with similar phenomenaobserved in reactive PVD ofTiO 2 in Ar/O 2 discharges [9.25-9.28]. We alsonote that good control of target temperature is desired for process repeatability, since the rate of target nitridation involves temperature-dependentsteps such as dissociative N 2 chemisorption. A large increase in target temperature could, for example, change the Ti target state from metallic to nitrided for a given Nz/Ar ratio [9.29].Deposition of PVD TiN with the target in the nitrided mode (NM) raisesconcerns about the deposition of sequential Ti/TiN bilayers since the nitrided target would contaminate with nitrogen the Ti film of the nextTi/TiN bilayer.
This can be avoided either by depositing the Ti and TiN inPVD MATERIALS AND PROCESSES3193025(/)20n-15.o.mO10o50I!i ......................... ..............................I~...............................ti............ ;..................1!..................010203O4OP e r c e n t N i t r o g e n F l o w in A r g o n(a),,1 O0 - -~......................................................,.80-.!!".iif)rR)60-.................LL....
. . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~0C(/).m(/)or"40-$ ..... . . . . .i. . . . . . . .~,~.~c-or)20-................................................................!i!;,i"s ..........................." ..........................:,iii'i10203040.....................................i0r ..................................P e r c e n t N i t r o g e n F l o w in A r g o n(b)FIG. 9.25R e p r e s e n t a t i v e data on deposition rate and sheet resistance o f reactive P V D o f Ti in A r / N 2(after ref.
9.27). R e p r o d u c e d by permission o f The E l e c t r o c h e m i c a l Society, Inc.320R. POWELL AND S. M. ROSSNAGELtwo separate chambers or by using a single chamber with a mechanicalshutter that allows the nitrided Ti target to be sputter-cleaned in pure Ar between successive wafers. It is sometimes possible to avoid the shutterwhen collimated Ti/TiN is deposited since N sputtered from the temporarily nitrided target is pumped by unreacted Ti on the collimator surface,leading to deposition of an acceptably clean Ti film [9.30]. Finally, we notethat it has been possible to operate in the high-deposition-rate, non-nitridedmode (NNM) in which the Ti target is not saturated with N 2 yet there is asufficient partial pressure of N 2 at the wafer to ensure that TiN x with x =1 is grown.
This has been done by exploiting the nitrogen-pumping actionof a high aspect ratio collimator and carefully controlling nitrogen flowand partial pressure to achieve a stable target nitridation state [9.30].An interesting aspect of collimated TiN PVD is that the chemical composition of the film can, under some process conditions, vary over topography. In particular, TiN films deposited into high aspect ratio contactholes have been observed to be substantially nitrogen-deficient (TIN0.75) atthe bottom relative to the stoichiometric TiN on the field [9.31].
This is aresult of the flux of Ti atoms coming through the collimator being highlydirectional whereas the nitrogen flux is diffuse and characteristic of molecular N 2 in the gas phase. Hence, deep enough in topography, conditionscan be reached where there is insufficient nitrogen to fully nitride the Ti.This is less of a problem for a nitrided target since in this case the relativeN flux is initially much higher at the wafer. Also, a postdeposition thermalanneal in N 2 (e.g.
30 min at 450~ has been found sufficient to restore thecomposition of the in-depth depleted films to near-stoichiometric TiN[9.31].Figure 9.26 summarizes selected PVD film properties for Ti, noncollimated, nitrided-mode TiN and collimated, nonnitrided-mode TiN. It isworth noting that the resistivity of 1.5"1 collimated TiN ( < 4 5 / . ~ - c m ) isconsiderably lower than that of the uncollimated TiN (80-200/~fl-cm).
Inpart this reflects the excess nitrogen incorporated in the noncollimatedTiNx= ~.2 films that were deposited in the nitrided mode. However, the elimination of low-angle Ti atoms by collimation reduces the TiN film's lateralgrowth, resulting in more densely packed columnar grains with a morebulk-like conductivity ~ although not as low as the bulk resistivity ofsingle-crystal, stoichiometric TiN, which has been reported to be ~ 1 5 / ~ cm at room temperature for either (111) or (110) orientation [9.22].
Themechanical film stress in PVD TiN films is generally compressive andmuch greater in magnitude than that of PVD Ti deposited under similarconditions. In addition, the stress depends on temperature of deposition,degree of collimation, and underlying substrate (Si, SiO2). A major concernPVD MATERIALS AND PROCESSESXRF thickness NU(M-m)/(M+m) (%)AllTitaniumStandardnon-collimated TIN(nitfided mode)collimated TiN(nitddedmode)1.5:1collimated TiN(non-nitrided mode)<5<5<5<5<10<10<10Sheet resistance NUF,)i:1321<10''Bulk resistivity(la.Q.,cm)< 6080to 200<60Density (g/cm3)4.404.754.905,103O00250025OOMechanical stress (MPa)300Absolute reflecttvitymeasured at 440 nm (%)55Grain size (rim)3312StoichiomeW/N/A12:1SilverBrown Go.ld.Film color......< 45N/AN/A, .,N/A(Light) Brown~LightGoldFIG.
9.26 Selected properties of uncollimated and collimated (i.e., cds) Ti films on 200-mm wafers(after ref. 9.30).about high TiN film stress relates to particle generation from films deposited on shields and other chamber fixtures. Fortunately, this is muchless of a concern for PVD Ti/TiN bilayers since the film stress of the composite structureparticularly for higher deposition t e m p e r a t u r e s - canbe quite low (see Fig. 9.27). In a similar way, when a PVD chamber is usedalmost exclusively for TiN deposition (e.g., a dedicated chamber for TiNARC layers), one can periodically sputter a layer of Ti onto the TiNcovered surfaces, which serves to reduce overall stress and additionallyfunctions as a paste to prevent the flaking of thick TiN layers.
On the otherhand, if this pasting is done too frequently (e.g., more than once per cassette of wafers), the cost per wafer will increase due to the unproductiveconsumption of the Ti target (see Section 11.8).9.5.3 ANTIREFLECTIONCOATING(ARC)High specular reflectivity is one indication of PVD A1 film quality (e.g.,R ~ 90% from 200-800 nm).
However, this high reflectivity can adverselyaffect subsequent photolithographic patterning and etching of the blanket322R. POWELL AND S. M. ROSSNAGEL500~.NNM on Si0~C/).-500-1ooo.........................NNM on SiO 2._-.~_:--/--0NM on Si'NM on SiOe-='--is~176__~,.,,:_,.~-200017_i-25000"100200300...400..500600Temperature (~FIG. 9.27 Stress of composite Ti(300/~)/TiN(500/~) bilayers on Si and SiO 2 as a function of deposition temperature. The TiN was deposited in either a nitrided mode (NM) or non-nitrided mode(NNM).PVD AI film into separate metal interconnect lines [9.32, 9.33]. In particular, as shown in Fig. 9.28, light reflections can degrade pattern resolutionby three mechanisms: (1) off-normal incidence light can reflect backthrough and expose regions of the resist that were intended to be masked;(2) thin film interference effects can produce linewidth variations in areaswhere the resist thickness varies; and (3) "reflective notching" can occurin which the resist pattern is undercut at the metal-resist interface due toextraneous backscattered reflection of light from nearby regions of topography.
This can lead to notching of the metal line after pattern definitionetching with resulting reliability problems. For example, each nanometerof gate width change in an advanced MOSFET can reduce chip speed by1 MHz. As a result, a chip designer might want to keep these dimensionalvariations in gate width across the chip below 1%, which can require keeping reflected light levels < 0.5% during lithographic patterning.To minimize these problems, a PVD TiN antireflection coating (ARC)layer is often deposited on top of the A1 prior to photoresist application[9.34, 9.35]. One typically thinks of an antireflection layer as a transparent thin film whose index of refraction and thickness are engineered togive phase-shift cancellation of specific reflected wavelengths. On theother hand, TiN strongly absorbs visible light.
It turns out that a sufficiently thin film of TiN transmits enough light to be used for the ARC application. For example, the absorption coefficient of TiN at the 436-nmwavelength used in g-line optical lithography is about 3 x 105 cm-~, lead-PVD MATERIALS AND PROCESSESInclchtrs rmms LightFIG. 9.28 Light scattering and reflection through a photoresist layer can degrade lithographic patterning and lead to artifacts such as metal line notching.ing to a transmission of about 30% for a 400-A-thick film. As shown inFig. 9.29a, the reflectivity of an AI-Si alloy at 436 nm could be reduced bynearly 90% by deposition of a 350-A film of TIN. Moreover, the reductionin reflectivity held over a very wide range of wavelength (Fig.
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