Electrical conductivity of high aspect ratio trenches in chemical-vapor ...

Electrical conductivity of high aspect ratio trenches in chemical-vapor deposition W technology Ivan P. Ivanov,a兲 Indradeep Sen, and Peter Keswick Cypress Semiconductor Corporation, 2401 East 86th Street, Bloomington, Minnesota 55425

共Received 7 October 2005; accepted 19 December 2005; published 8 February 2006兲 This article discusses the resistivity scaling challenges associated with the use of high aspect ratio trenches as W interconnects for the sub-130-nm semiconductor technology node. In this work, chemical-vapor deposition of W is employed to manufacture conductive trenches in a deposition sequence that includes a TiN barrier, a nucleation W, and a bulk W film. Composition, microstructure, resistivity, grain size, and surface roughness for these films are determined in the low thickness range. The results are used to examine the contribution of the electron-surface scattering and grain-boundary scattering to the overall increase in the electrical resistivity observed at film thickness comparable to the electron mean free path. Calculated and measured values for the film resistivity are matched by using a variable coefficient of elastic electron scattering at the grain boundaries. In first approximation, grain-boundary electron scattering is found to be the dominant mechanism and is almost entirely responsible for the resistivity increase in the thickness range studied. By using resistivity data obtained for each film and Kirchhoff’s rule for laminate structures, a simple physical model is used to predict the variation of the trench resistance as a function of geometrical factors such as film thickness and core 共seam兲 size. The agreement between the calculated and measured trench resistances is surprisingly good in view of the several simplification assumptions that are made and that no fitting parameters are used. The proposed model predicts reasonably well the sensitivity of the trench resistance with respect to the TiN film thickness. However, the impact of the nucleation W layer is overestimated, which suggests possible unaccounted interactions, related to structural or morphological changes in the bulk W. It is concluded that the trench conductivity is already significantly size limited for critical dimensions in the sub-130-nm range. The control of the film bulk resistivity and grain-boundary engineering of the conducting materials is therefore identified as the most important pathway for achieving desired electrical properties in conducting trenches filled by standard chemical-vapor deposition W technology. © 2006 American Vacuum Society. 关DOI: 10.1116/1.2166859兴

I. INTRODUCTION The “size effect” in a thin film can be defined as an abnormal increase in the film resistivity when the electron mean free path becomes comparable with some of its physical dimensions, e.g., film thickness, surface roughness, or grain size. As the interconnect linewidth in the semiconductor chips shrinks to deep-submicron dimensions, these effects are expected to play an important role in the electron transport and need to be critically evaluated. In the past few years, there has been increased interest in the study of size effects in thin films used in the development of the Cu interconnect technology, where Cu is usually deposited in trenches on TaN and Ta layers. The resistivity of a Cu line has been evaluated as a function of linewidth,1 and an unexpectedly large size effect has been found as the linewidth decreases from 1 to 0.02 ␮m. The observed extra resistivity increase has been attributed to small grain size in ultrathin Cu films and to Cu/ Ta interface roughness. With the transition from Al共Cu兲 to Cu metallurgy for interconnect applications, attention has been paid to these size effects in a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

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Ta, TaN, W, and WNx ultrathin films,2–4 which find application mainly as diffusion barriers and seed layers for Cu. In this role, the barrier film can affect the Cu film texture and thus have an impact on line’s resistivity and reliability. For such ultrathin films, phase composition, thickness, grain size, and surface roughness have been found to strongly influence electron scattering in size-limited regime of electron transport. Therefore, the electrical resistivity measurements have been the main method for the evaluation of size effects. These effects have also been studied for chemical-vapor deposition 共CVD兲 W,5 which is a material widely used in the semiconductor manufacturing for filling vias and contacts. In this article, the film resistivity has been measured as a function of thickness for W deposited directly on nonpatterned Si substrates from H2 reduction of WF6 at a temperature of 400 °C. The authors have found strong size effects in the film resistivity which are explained by electron scattering on grains with variable electron reflection coefficient R at grain boundaries resulting from segregated oxygen impurities. Further increase in the device density on the silicon wafers can be achieved by introducing a memory cell design that includes bifunctional features at the local interconnect level. For example, conductive trenches are being used to provide low contact resistance to Si for source and drain

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©2006 American Vacuum Society

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Ivanov, Sen, and Keswick: Electrical conductivity of high aspect ratio trenches

transistor’s electrodes as well as for “in-plane” briding connections 共local interconnect兲 over gate electrodes in some static random access memory 共SRAM兲 memory cells. Such a conductive trench will simultaneously carry electrical current in both vertical and horizontal 共in-plane兲 directions, as opposed to single-functional contacts to diffusion areas of the complementary metal-oxide semiconductor 共CMOS兲 transistors where the current flows strictly in the vertical direction. This cell design allows a dual damascene manufacturing process for contacts and interconnects that additionally reduces the cost per wafer. Due to its low bulk resistivity 共␳0 ⬃ 5.3 ␮⍀ cm兲 and excellent step coverage, CVD W is successfully utilized for filling trenches. This process normally requires a physicalvapor deposition 共PVD兲 or CVD barrier layer 共TiN or TiW兲 to be deposited first, followed by a thin W film obtained from SiH4 reduction of WF6 共called W nucleation layer兲 and a “bulk” W film from H2 reduction of WF6. In this W plug technology, films are deposited on the sidewalls as well as on the bottom forming a nanolaminate structure. Size effects are therefore expected to dominate the electron transport in such vertical nanolaminate structures to an even greater extent as compared to the in-plane multilayers. The objective of this work is to evaluate the size effects for in-plane thin films of TiN and of nucleation and bulk W and their impact on longitudinal conductivity of trenches filled by standard CVD W technology. In a series of experiments, the component of electrical resistivity due to electron scattering on grain boundaries and film surfaces is separately evaluated for each film obtained in deposition sequence intended to keep the film properties as close as possible to those in the CVD W process. The size effect is found to be significant in the films studied in the thickness range typically used in the CVD W technology, and electron scattering from grain boundaries is the most important mechanism limiting the electron conductivity. With the experimental data obtained, we propose and validate a physical model for longitudinal conductivity of a trench filled by the standard CVD W process. The model also takes into account the impact of a core 共seam兲 on the trench conductivity.

II. ELECTRON CONDUCTIVITY OF THIN FILMS SIZE EFFECTS The resistivity ␳ of a thin film can be determined as a sum of two components

␳ = ␳b + ␳s ,

共1兲

where ␳b is the intragranular resistivity of infinitely thick 共bulk兲 material and ␳s accounts for the so-called size effects. The bulk resistivity ␳b includes electron scattering on phonons and short-range structural imperfections such as defects and impurities, whereas ␳s is determined by electron scattering on film surfaces ␳共t兲, grain boundaries ␳共gb兲, and additional surface roughness ␳共r兲, i.e.,

␳s = ␳共t兲 + ␳共gb兲 + ␳共r兲. J. Vac. Sci. Technol. B, Vol. 24, No. 2, Mar/Apr 2006

共2兲

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The first component ␳共t兲 is determined from the FuchsSonderheimer theory,6,7 which is an adaptation of the classical Drude theory. The assumption that elastically scattered electrons will not alter the bulk resistivity leads to the equation 3ᐉ ␳共t兲 = 1 + 共1 − p兲, ␳b 8t

共3兲

where t is the film thickness, ᐉ denotes the electron mean free path, and p is the fraction of electrons specularly scattered at the film interfacial surfaces. For polycrystalline films, electron scattering on long-range structural imperfections must be considered. Mayadas and Shatzkes have studied theoretically8 the grain-boundary effects on the resistivity of thin polycrystalline films. The authors have derived a relationship between resistivity and grain size:

冉冊

3 1 ␳共gb兲 = 1 + ␣ − 3␣2 + 3␣3 ln 1 + 2 ␳b ␣ with

␣=

冉冊

R ᐉ , D 1−R

共4兲

共5兲

where R is the grain-boundary reflection coefficient and D is the average grain size. In Eq. 共4兲, the reflection coefficient R corresponds to Fuch’s parameter p and denotes the fraction of electrons reflected specularly at the inner grain boundary, while the fraction 1 − R travels undisturbed. For sufficiently large crystallites Eq. 共4兲 can be reduced to the equation 3ᐉ R ␳共gb兲 . =1+ 2D1−R ␳b

共6兲

Film surface roughness has an impact on the intensity of scattering from the film surfaces ␳共t兲. Although the exact solution to this problem has not been found, it is obvious that Eq. 共3兲 could be modified to accommodate the increased inelastic electron scattering as some function of film surface roughness. By using Monte Carlo simulations, Kuan et al.1 propose a modification of Eq. 共3兲 for thin Cu lines, which includes the normalized surface roughness S / t and the probability of specular electron scattering p as follows: 共1 − p兲S ␳共r兲 . = 1 + 0.75 ␳ t

共7兲

In the next sections we will determine the size effects in the resistivity of thin barrier TiN and of nucleation and bulk W films sequentially deposited by standard CVD W technology. As a second step, we will use the measured film resistivity to evaluate the longitudinal conductivity of a trench filled by the CVD W process for any combination of TiN and nucleation and bulk W thicknesses, including the impact of the so-called core 共seam兲 formed inside the trench. Based on the analysis of the resistivity components for each film, we will provide an estimate for the size-limited electron transport in the trenches used for in-plane interconnects in verylarge-scale integration 共VLSI兲 semiconductor technology.

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TABLE I. Film thickness and four-point probe sheet resistance for as-deposited TiN, nucleation W, and bulk W films. 共Ref. 18兲 CVD W 共Å兲

No.

Barrier 共Å兲 i-PVD TiN nucleation

W

Bulk W

Rs 共⍀ / sq兲

Rs unif 1␴ 共⍀ / sq兲

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

598 299 148 79 39.8 296 302 301 287 296 292 291 298 304 311 300 304

¯ ¯ ¯ ¯ ¯ 464 604 251 339 677 250 289 237 250 250 251 224

¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ 1145 2877 2404 1942 1486 4771 4498

11.70 24.56 66.90 150.10 346.89 35.23 31.96 170.45 61.41 29.51 1.67 0.46 0.59 0.79 1.14 0.21 0.23

0.839 1.690 4.783 12.493 36.344 0.586 0.403 21.415 3.851 0.278 0.070 0.020 0.024 0.032 0.038 0.014 0.016

In order to evaluate each component of the film resistivity ␳共t兲 , ␳共gb兲, and ␳共r兲 for TiN barrier and for nucleation and bulk W films, see Eqs. 共1兲–共6兲, the average electron mean free path ᐉ, grain size D, reflection coefficients p and R, and the film thickness and resistivity must be known. Whereas data for ᐉ , p, and R could be found in the literature for most films, variation of each film’s resistivity ␳ and grain size D as a function of film thickness t is found in experiments described below. III. THIN-FILM GROWTH AND CHARACTERIZATION TiN barrier films with varying thickness were deposited by reactive sputter deposition 关ionized PVD 共i-PVD兲兴 in an ENDURA5500 cluster tool on natively oxidized silicon wafers in Ar+ N2 gas mixture. A Ti target 99.999% pure was used in the experiments. TiN films ⬃40 to 600 Å thick were deposited on a rf-biased pedestal held at a standby temperature of 200 °C. The nucleation and bulk W films were deposited in an 8 in. Novellus Altus C2 low-pressure CVD 共LPCVD兲 tool at a temperature of 415 °C. Nucleation W films were obtained from SiH4 reduction of WF6 at a SiH4 / WF6 gas flow ratio of 0.19. Bulk W films were deposited from H2 reduction of WF6 at H2 / WF6 gas flow ratio of 0.1 in an Ar carrier gas resulting in a total process pressure of ⬃40 Torr. Nucleation W with thickness ranging between ⬃251 and 677 Å were deposited on ⬃300-Å-thick TiN films. Following the CVD W deposition sequence, a series of bulk W films in the thickness range between ⬃1145 and 4877 Å was deposited on 339-Å-thick nucleation W always using a 300 Å TiN film as glue layer to the natively oxidized Si substrate. The sheet resistance of the nucleation W and bulk W films was obtained in reference to wafer’s sheet resistance meaJVST B - Microelectronics and Nanometer Structures

sured before the respective film deposition by using the parallel resistors’ circuit. Results are summarized in Table I. The thickness of each film was estimated by using Hitachi S-4800 field-emission scanning transmission electron microscopy 共TEM兲 operated in scanning electron microscopy 共SEM兲 mode. Film surface morphology was studied on DI Dimension 9000 atomic force microscope 共AFM兲共Veeco Instruments兲 operating in tapping mode under conditions optimized for each film. Information about the surface morphology 关surface roughness rms Rq , Zmax, and average grain size D兴 was obtained by using the standard software NANOSCOPE III version 5.14 provided with this instrument. Film microstructure was studied by x-ray-diffraction 共XRD兲 method using a Bruker AXS D5005 diffractometer, equipped with a 2.2 kW sealed Cu source and a focusing graphite monochromator on the second side. X-raydiffraction method was used to determine the peak positions for diffracted x-ray beam for a TiN, nucleation W, and bulk W film stacks. Information about film elemental composition was obtained by secondary-ion-mass spectroscopy 共SIMS兲 carried out for selected samples. IV. FILM PROPERTIES A. Microstructure and composition

The polycrystalline nature of the W films is evident from the diffraction patterns in Fig. 1, obtained from a nucleation W 共dashed line兲 and bulk W 共solid line兲 film stacks. Although three different crystallographic phases of tungsten films are known, i.e., bcc ␣ phase 共lattice constant a = 3.1652 Å兲, pure cubic 共pc兲 ␤ phase 共a = 5.00–5.09 Å兲, and fcc ␥ phase 共a = 4.12–4.21 Å兲, the bcc ␣ phase is the only phase detected in this work. The phase composition of the W film is important for in-plane interconnect applications. For

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FIG. 1. X-ray-diffraction pattern from W / TiN film.

instance, the bulk resistivity of bcc ␣-phase W is typically in the range of 7–12 ␮⍀ cm, whereas ␤-phase W films are reported to have significantly higher resistivity 共150–350 ␮⍀ cm兲.9 The coexistence of these two phases has been found in the CVD W films and has been ascribed to an increased Si content in the W film.10 This poses a significant problem for controlling the electrical properties of W, especially in the integrated circuit 共IC兲 fabrication where W is used as in-plane interconnect. The diffraction patterns in Fig. 1 correspond to bcc ␣-W phase for both nucleation and bulk W. The diffraction peak at 40.455° 2⌰ is indexed as ␣-W 共110兲, whereas the diffraction peaks at 58.347° 2⌰ and 73.485° 2⌰ correspond to diffraction from ␣-W 共200兲 and ␣-W 共211兲 crystallographic planes, respectively. No other W phases are found in the CVD W films. From the angular peak position and Miller’s indices for diffraction planes, the lattice constant a for these films can be calculated. For the nucleation film, the calculated lattice contact a equals to 3.1575 Å, which is very close to the theoretical value of 3.1652 Å for ␣-W. This was expected, however, since the W thermal expansion coefficient is larger than Si, leading to the establishment of in-plane tensile differential thermal contraction upon cooling from the growth temperature. The presence of ␣-W in the nucleation film is also expected. The deposition of W films from SiH4 reduction of WF6 in our experiments takes place at very low SiH4 / WF6 ratio of 0.19. Under similar conditions, formation of two volatile molecules 共SiF4 or SiHF3兲 is a known concurrent process at the growing W surface resulting in the depletion of Si atoms from the film.11 Figure 2 shows a SIMS elemental profile obtained from the 677-Å-thick nucleation W film grown on a 300 Å TiN sublayer. The variation of O signal indicates the existence of very thin surface oxide at the top film surface, which could be native WO3, gradually decreases as a function of film thickness, and reaches a value of 1–2 at. % at a depth of ⬃120 Å. The accuracy of the thickness scale, however, may be questioned due to uneven sputtering rates from the grain and grain boundaries and the high surface roughness of these W films. After the surface oxygen is removed, very low levJ. Vac. Sci. Technol. B, Vol. 24, No. 2, Mar/Apr 2006

FIG. 2. SIMS depth elemental profiles for a 677 Å nucleation layer. It shows low Si content 共⬍0.1 at. % 兲 in the W film.

els of O at the interface with the TiN sublayer are detected. Low F and Si concentrations in the nucleation film are apparent in the SIMS elemental profile, both leveled at the instrument’s detection limit. The low Si content in the nucleation W is in agreement with the XRD data for the nucleation film in Fig. 1, where only the ␣-W phase is detected in this film. Growing of ␣-W can be stabilized at high deposition temperature and low impurity levels.

B. Surface morphology and grain size

Two AFM images taken from TiN films 共39.8 and 598 Å thick, see Table I兲 are shown in Figs. 3共a兲 and 3共b兲. The surface of the two samples is smooth with low rms Rq values of 5.7 and 12.1 Å, respectively. The normalized surface roughness Rq / t for TiN films decreases from 14% to 2% as the thickness varies from 39.8 to 598 Å. These films exhibit columnar microstructure as seen in Fig. 4, which is a crosssectional SEM 共XSEM兲 image for the thicker nucleation film shown in Fig. 3共b兲. The diffraction pattern for this TiN film has only one diffraction peak positioned at 42.528° 2⌰, which corresponds to TiN 共200兲 as the preferred orientation. Variation of the grain size D as a function of TiN film thickness t is shown in Fig. 5. As the film thickness increases, the grains enlarge predominantly in the direction of film growth, which results in a low proportionality constant 共0.164兲 between D and t. This is an indication for suppressed

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FIG. 4. XSEM image for TiN film shown in Fig. 1共b兲.

FIG. 3. AFM images of TiN films: 共a兲 39.8 Å and 共b兲 598 Å.

lateral grain growth and is typical for TiN deposited under conditions of insufficient low-energy ion bombardment and/or low homologous temperatures.12 The morphology of the next two films in the CVD W deposition sequence, the nucleation and bulk W layers, is illustrated in Figs. 6共a兲–6共c兲. For the 251-Å-thick nucleation W film, shown in Fig. 6共a兲, the grains have rounded top surface and become slightly elongated in the substrate plane as the thickness increases 共464 Å兲, see Fig. 6共b兲. Each grain in the nucleation W film originates from an underlying TiN grain, i.e., we observe some form of the “template” effect, which is seen in Fig. 7. Figure 6共c兲 shows an AFM image from the surface of a 1942-Å-thick bulk W film, which has a larger grain size, rough surface, and a single-mode grain-size distribution. Variation of the grain size D as a function of film thickness t for W films is shown in Fig. 8. Knowing this dependence is particularly important for films with electrical JVST B - Microelectronics and Nanometer Structures

conducting function in the semiconductor technology. The grain boundaries are potential barriers for conducting electrons, as discussed earlier, and their properties 共density and composition兲 have significant impact on the electrical conductivity and have to be evaluated. As seen in Fig. 8, the nucleation and bulk W exhibit two different proportionality constants of 0.72 and 0.32, respectively, on the common D -t curve. This illustrates possible differences in the surface reaction kinetics for SiH4 and H2 reduction of WF6 at the deposition temperature of 415 °C, discussed in Ref. 13. The high proportionality constant of 0.72 suggests a stronger correlation between D and t which is typical for polycrystalline metal films with no preferred orientation, where competitive three-dimensional 共3D兲 grain growth is randomized in all directions. This result is in agreement with the variation of deposition rate for each nucleation and bulk W film, as illustrated in Fig. 9. In this figure, films with thickness ranging between 251 and 677 Å are deposited from silane reduction of WF6, whereas films between 1144 and 2877 Å are obtained from H2 reduction of WF6. In the silane reduction process, deposition rate decreases rapidly from 63 to 42 Å / s in the thickness range of 251–677 Å, typically used in the CVD W process. The bulk W deposition takes place at a

FIG. 5. Variation of grain size D as a function of TiN thickness t.

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FIG. 7. XSEM image of nucleation W 共bright top layer兲 deposited on barrier TiN film.

lows an opposite trend. The normalized roughness is higher for the nucleation W films, decreases rapidly as function of nucleation film thickness, and is relatively constant for the bulk W. This ratio can be used in evaluating the component ␳共r兲 of the film resistivity ␳ due to increased surface roughness or change the probability p of specular electron scattering from interfacial surface, see Eqs. 共2兲 and 共3兲. As the feature size in the semiconductor technology decreases, the use of ultrathin films as interconnects is imminent and such effects have to be taken into consideration. V. FILM RESISTIVITY A plot of the film thickness as a function of reciprocal value of the sheet resistance 1/RS is an accurate measure of the intrinsic 共bulk兲 resistivity ␳b for a very thick film. From the slope of thickness-1/RS curve, values of 66 and 8 ␮⍀ cm were obtained for TiN and W films, respectively.

FIG. 6. AFM images for nucleation W 关共a兲 251 Å and 共b兲 464 Å兴 and bulk W 关共c兲 1942 Å兴 films.

lower deposition rate of 32–38 Å / s. As the W growth proceeds, the absolute value of the film surface roughness is found to increase. However, as shown in Fig. 10, the surface roughness normalized to the respective film thickness folJ. Vac. Sci. Technol. B, Vol. 24, No. 2, Mar/Apr 2006

FIG. 8. Variation of average grain size as a function of film thickness for W films.

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FIG. 11. Components of TiN film resistivity ␳ as a function of thickness.

FIG. 9. Variation of average deposition rate during CVD W process.

Variation of TiN resistivity ␳ as a function of film thickness t is shown in Fig. 11. The bulk resistivity ␳b ⬃ 66 ␮⍀ cm is represented in this figure by a solid horizontal line. The theoretical 共undistorted兲 bulk resistivity ␳0 for TiN is 25 ␮⍀ cm. Filled circles show measured total film resistivity ␳. The variation of ␳共t兲 as a function of thickness t, resulting from electron scattering from film interfacial surfaces, is estimated from Eq. 共3兲. For a given film thickness, this component is graphically equal to the difference between the open circles in Fig. 11 and the corresponding bulk resistivity ␳b 共solid line兲. In the calculations for ␳共t兲, we have used the average grain size D versus thickness t from Fig. 5. For the mean free path ᐉ and for the coefficient of specular electron scattering p, we have used values ᐉ = 49 Å and p = 0.3, reported in Ref. 14 for TiN films. The resistivity com-

FIG. 10. Variation of normalized surface roughness as a function of W thickness. JVST B - Microelectronics and Nanometer Structures

ponent ␳共gb兲 corresponds to grain boundary and is equal to the difference between the actual film resistivity ␳ and ␳b + ␳共t兲, see Fig. 11. The resistivity corresponding to the surface roughness ␳共r兲 is not evaluated independently; however, it is included in ␳共gb兲, which is calculated from Eqs. 共4兲 and 共5兲 by choosing an appropriate R value as a fitting parameter in the following way. For a given TiN thickness, the reflection coefficient R is varied between 0 共no grain-boundary scattering兲 and 1 共all electrons experience elastic scattering at grain boundaries兲 until the sum ␳b + ␳共t兲 + ␳共gb兲 equals the measured resistivity ␳. A similar approach is taken in the evaluation of ␳共t兲 and ␳共gb兲 for all films. Thus, the uncertainty in assumed R scales with the ratio of ␳共r兲 and ␳共gb兲. The results presented in Fig. 11 show that the size effect is insignificant for TiN films thicker than 300 Å. However, as the film thickness decreases, size effects gradually increase and nearly double the film resistivity ␳. For a 40-Å-thick TiN film, the electron scattering on the film interfacial surfaces contributes some 22% and the remaining part is due to scattering on grain boundaries. Figure 12 shows the assumed reflection coefficient R matching the measured and calculated film resistivities. The probability of specular refection

FIG. 12. Reflection coefficient as a function of film thickness for TiN films.

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FIG. 14. Reflection coefficient as a function of film thickness for W films. FIG. 13. Components of nucleation W and bulk W film resistivity ␳ as a function of thickness.

R is high in the area A 共0.5–0.65兲 in the low thickness range and decreases rapidly to 0.12–0.16 as the thickness increases above 250 Å 共zone B兲. Both time-dependant final deposition temperature of the TiN films in our experiments and the corresponding impact of the thermal budget on the film microstructure provide an explanation for this result. TiN deposition starts at relatively low heater temperature of 200 °C. During the i-PVD TiN deposition, Joule heat released from high-density plasmasubstrate interaction leads to an increasing substrate temperature. For instance, the substrate temperature of 460 °C is achieved for a 300-Å-thick film which requires a deposition time of ⬃80 s. For very thin film of 40 Å, however, the deposition time is shorter 共⬃10 s兲 which limits the final substrate temperature to below 300 °C. Such TiN films exhibit lower density, open grain boundaries, and higher resistivity.15 In turn, at high deposition temperature achieved for thicker films, the grain boundaries for a TiN are dense and this could contribute to the much lower reflection coefficient R. The value of R could be used as an indication for the quality of TiN as a diffusion barrier. In addition, our resistivity measurements are done ex situ and at room temperature. Such an air exposure is a part of the standard CVD W process, since TiN and CVD W are deposited on different tools with a vacuum break. Oxidation of open grain boundaries for the thinner TiN films is then possible, which will increase the coefficient of specular scattering R. A similar effect has been found for CVD W films where increasing the R values at low film thickness or high deposition temperature is correlated to a higher oxygen content at the film grain boundaries.5 Measured resistivity for nucleation and bulk W films 共filled circles兲 and its calculated components are shown in Fig. 13. Assumed reflection coefficient R for all films in the thickness range from 251 to 2877 Å is shown in Fig. 14. Nucleation W films are in the thickness range between 251 and 677 Å. All thicker films 共t ⬎ 1144 Å兲 are bulk W. The structural and compositional similarities found between the nucleation and bulk W films lead to one common curve for J. Vac. Sci. Technol. B, Vol. 24, No. 2, Mar/Apr 2006

the measured resistivity as a function of thickness in Fig. 13. This is not necessarily true for other silane reduced WF6 films, especially at a higher SiH4 / WF6 ratio. For comparison, we also show the resistivity data from Ref. 5 共open diamonds兲, measured for CVD W films deposited directly on silicon substrates from H2 reduction of WF6 at a temperature of 400 °C. For our bulk W films, the film resistivity ␳ increases from 10.1 to 19.1 ␮⍀ cm as the film thickness varies from 4771 to ⬃1144 Å. By using the plot of thickness versus 1/RS, we find that a value of 7.8–8 ␮⍀ cm is appropriate for the film bulk resistivity ␳b 共shown by solid line兲. For a given film thickness, the resistivity due to scattering at film interfacial surfaces is the difference between ␳b + ␳共t兲共shown with open circles兲 and ␳b. The resistivity ascribed to grainboundary scattering ␳共gb兲 is the difference between the measured film resistivity ␳ and ␳b + ␳共t兲. The resistivity ␳共gb兲 is equal to ⬃8 ␮⍀ cm at W film thickness of 1144 Å and is reduced essentially to zero for the 3958-Å-thick film. In the calculation of ␳共t兲 and ␳共gb兲, we have used data for electron mean free path in W equal to 413 Å, obtained elsewhere,16 and diffuse electron scattering 共p = 0兲 from interfacial film surfaces for W. Correspondingly, the reflection coefficient R matching the experimental and calculated resistivity data in Fig. 14 varies between 0.6 and 0.7 for bulk W. This means that the reflecting properties of the grain boundary remain near constant for a wide range of thickness variation for bulk CVD W obtained in our experiments. The reflection coefficient R is significantly higher for the nucleation W 共0.8– 0.95兲. This could be due to lower density of the grain boundaries and/or the film surface oxidation upon exposure to air, which is expected to have a higher effect on the resistivity of thinner W films. Since both nucleation and bulk films are pure ␣-phase W, it is reasonable to assume that the electrons’ mean free path will be the same in both films. Our resistivity measurements are done ex situ; therefore, possible contribution from oxygen segregation at the grain boundary cannot be ignored. However, such surface oxidation of the TiN and W films takes place in the CVD W deposition sequence due to the vacuum break required in this manufacturing process. By comparing the contribution of the thickness and the morphologic components of film resistivity ␳共t兲 and ␳共gb兲 in Fig. 3, we can conclude that in the low thickness range for

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CVD W films used to fill conducting submicron trenches, the electron scattering from grain boundaries is the dominant scattering mechanism. It is interesting to note that in the case of low silane-to-tungsten-hexafluoride ratio, the nucleation film matches the structural and compositional properties of the following bulk W. However, when using silane to initiate deposition, the nucleation W film growth proceeds with a higher proportionality constant 共0.72兲 in the grain-sizethickness dependence, as compared to the bulk W 共0.32兲. This means that in the silane reduction initiation process, a much faster rate of grain-size change is observed as a function of film thickness. If such a process is used to fill high aspect ratio trenches with critical dimension 共CD兲 ⬍1300 Å, the nucleation W process is almost entirely responsible for the trench longitudinal conductivity. VI. TRENCH CONDUCTIVITY Variation of film’s resistivity as a function of thickness for each film is used to calculate the resistance of the laminate formed inside the trench from the deposition of the barrier TiN and of the nucleation and bulk W films. The model includes geometrical effects only. However, changes in the thin-film properties affecting the electron transport could also be included in the calculations. For instance, the films on the trench sidewall could differ from those deposited on flat areas. The calculations for longitudinal trench resistance per 0.13 ␮m length 共which could be named trench “sheet resistance” RS for 0.13-␮m-wide trenches兲 are done for various TIN, nucleation W film thickness, and core size combinations. Following the Kirchhoff’s rule, the total trench resistor is then calculated from parallel resistors’ circuit, see Fig. 15共a兲. The core size is simulated as “missing bulk W” and is calculated as a difference between the top trench CD and the sum of the thicknesses of all sidewall films. For instance, for trench CD= 1300 Å, a sidewall thickness equal to 1300 Å means that core equals to 0 Å, at 1200 Å—core equals to 100 Å, etc. In all cases, the difference is due to bulk W film only. Other input parameters used are trench height and average CD. Additionally, information for the conformity of all films is required to define the resistors’ geometry. For TiN films, the sidewall and bottom coverages are 15% and 50%, respectively. For the nucleation W, known for its poor conformity, we used the experimentally determined difference between the top and bottom sidewall coverages, 70% and 55%, respectively. Other information needed to estimate the trench resistance is the film resistivity ␳ for each film. Measured values for ␳ as a function of t can be used in these calculations. However, in order to separately evaluate the contribution of film thickness and grain-boundary electron scattering 共the size effects兲, the calculated components from Eqs. 共3兲 and 共4兲 should be employed. For high aspect ratio trenches, the CVD W process will normally form identical films on each sidewall. However, due to microloading effects, the nucleation W deposition rate is higher at the trench entrance and decreases at the bottom. In our experiments we always observed some form of the JVST B - Microelectronics and Nanometer Structures

FIG. 15. 共a兲 Schematic diagram for resistance calculation. 1—TiN, 2—nucleation W, 3—bulk W. 共b兲 SEM image of a W trench with aspect ratio of 5:1 and top CD⬃ 1000 Å.

so-called bread-loafing effect, resulting from early closing of the top trench entrance. In this case, an additional resistor may form 关marked by a horizontal dashed line in Fig. 15共a兲兴, which can also be included in the calculations together with the sidewall resistors R1, R2, and R3 at the trench bottom. Figure 15共b兲 is a SEM image from a trench filled by CVD W deposition sequence described in this work. The formation of a seam, seen in this figure, increases the size effects in the electron conductivity, and our model can estimate this. In order to verify the model, the calculated values for RS are compared with experimental sheet resistance obtained from a 23 full factorial experimental design on patterned wafers. Three process variables 共TiN thickness, TiN bias power which controls the degree of ion bombardment during TiN deposition, and nucleation W film thickness兲 are used in this experiment. The trench sheet resistance is measured by using the Van der Pauw method.17 Figure 16 shows the sensitivity for each variable obtained experimentally 共solid lines兲 and from calculations 共open circles connected by dashed lines兲. The calculations are performed on trenches whose geometri-

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FIG. 16. Comparison between experimentally determined 共solid lines兲 and model-calculated 共dashed lines兲 main effects for TiN thickness 共Å兲, TiN bias power 共W兲, and nucleation W thickness 共Å兲. See the text for details.

cal dimensions are measured from silicon wafers. Input parameters are core size, varied between 50 and 100 Å, trench height of 4580 Å, an average trench CD varied between 1100 and 1300 Å, and TiN thickness from 100 to 200 Å. The film sidewall coverages of 15%, 50%, and 100% for TiN and for nucleation and bulk W are used, respectively. The impact of the nucleation film on the trench resistance is included through the core generated from the low sidewall and bottom coverage for this film. For instance, our measurements show that nucleation W sidewall coverages are ⬃70% and 55% at the trench top and bottom sidewall. Therefore, the core generated inside the trench will be approximately equal to 2共0.7− 0.55兲共454 Å兲 ⬃ 136 Å. Figure 17 shows the variation of core inside the trench as a function of the nucleation W thickness. From Fig. 16, we conclude that calculated and measured values for RS are in good agreement, as well as the response of RS to the variation of the TiN thickness is adequately predicted. The calculations overestimate the impact of the nucleation W thickness, predicting a higher RS sensitivity to this parameter. The model is based on several assumptions and does not take into account second-order effects, which

FIG. 18. Contour plot for normalized trench resistance RS size % as a function of core size and TiN thickness. Trench height= 4580 Å, CD = 1200 Å.

could explain the discrepancy observed. For instance, the grain size of the bulk W could be affected by the thickness of the nucleation W inside the trench, by the sidewall surface roughness, and by the impact of the TiN bias power, which are additional input data that may improve the accuracy of these calculations. The lower RS values obtained at the higher TiN bias power 共and the degree of ion bombardment兲 could be related to how changes in the TiN texture affect the properties 共bulk resistivity, grain size, microstructure, etc.兲 of both nucleation and bulk W. Figure 18 is a contour plot of the normalized calculated trench resistance “RS size %” as a function of both core sizes generated by changing the nucleation W thickness and TiN thickness. Size effect RS size % is calculated according to the equation RS size % =

FIG. 17. Core size resulting from nucleation W step coverage as a function of thickness. Trench height= 4580 Å, CD= 1200 Å. J. Vac. Sci. Technol. B, Vol. 24, No. 2, Mar/Apr 2006

RS ⫻ 100 % , RSbulk

共8兲

where RS is the calculated trench resistance from the actual film resistivity and RSbulk is the trench resistance under the assumption for zero size effects, i.e., by using bulk resistivity ␳b for TiN and W films. For trench CD= 1200 Å, the sheet resistance RSbulk equals to 0.2791 ⍀ / sq. The measured and calculated RS values in Fig. 16 are in the order of 0.8–1.1 ⍀ / sq. Figure 18 shows that the size effects are significant causing an increase of the RSbulk between ⬃300% and 400%. The formation of core as a result of poor sidewall coverage of the nucleation film as well as size effects thus has a significant impact on the trench sheet resistance. A more conformal deposition process for nucleation films is needed, which could extend the CVD W process for next semiconductor technology generations. Our results demonstrate the high sensitivity of trench resistivity to various process parameters. In fact, the input parameters—thickness, step coverage, and core size, all vary across the wafer and cannot be characterized by a single number. However, this

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model can be further improved by including second-order effects and interactions, determined from experiments, to be used for resistivity scaling applications as well as for a feedback for setting process parameters and their limits which is needed for a better process control.

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be helpful in choosing desired film properties, deposition sequence, and conditions needed to satisfy certain electrical design rule requirements in the semiconductor manufacturing. ACKNOWLEDGMENTS

VII. CONCLUSIONS For our experimental conditions and trench dimensions, the size effects lead to a three- to fourfold increase of the “bulk” trench resistance. Therefore, electron transport is size limited. In this regime, special attention to process control at any process step involved has to be paid. Our calculations show that the electron scattering on the grain boundaries dominates the size effects in TiN and in nucleation and bulk W films using the standard CVD plug/ trench fill process. Thicker CVD W films have low intragranular resistivity; however, the nucleation W layer has strongly correlated thickness and average grain size, which lead to rapid increase in film resistivity at low film thickness. This is ascribed to the increased contribution of the electron scattering on grain boundaries in the size-limited regime of electron transport, and the oxygen impurities segregated at grain boundaries could play significant role in it. The proposed simple calculation procedure uses measured film properties as input parameters to predict the absolute values, the general response, and the sensitivity of the longitudinal trench conductivity as a function of barrier TiN and nucleation W layer. Both model and experimental data suggest that the impact of the nucleation W layer is significant and not quite well understood. There might be second-order effects caused by different deposition time used for nucleation W film, such as changes in the nucleation density, step coverage, grain size, or resistivity of the bulk W film. This simple calculation procedure also offers an opportunity for studying physical properties inside the trench via their response to conducting electrical current. The calculations can

JVST B - Microelectronics and Nanometer Structures

The authors would like to acknowledge the help of John Hancin and Perry Hough from Novellus Systems for the assistance in the wafer depositions and Christopher Bonifas and Jerome Ohmann from Cypress Semiconductor for SEM and FIB measurements. 1

T. S. Kuan, C. K. Inoki, G. S. Oerhein, K. Rose, Y.-P. Zhao, G.-C. Wang, S. M Rossnagel, and C. Carbal, in Materials, Technology and Reliability for Advanced Interconnects and Low-K dielectrics, MRS Symposia Proceedings No. 612, edited by K. Maex, Y.-C. Joo, G. S. Oehrlein, S. Ogawa, and J. T. Wetzel 共Materials Research Society, Pittsburgh, 2000兲. 2 S. M. Rossnagel and T. S Kuan, J. Vac. Sci. Technol. B 22, 240 共2004兲. 3 S. M. Rossnagel, J. Vac. Sci. Technol. B 20, 2328 共2002兲. 4 S. M. Rossnagel, J. Vac. Sci. Technol. B 20, 2047 共2002兲. 5 A. Learn and D. Foster, J. Appl. Phys. 58, 2001 共1985兲. 6 K. Fuchs, Proc. Cambridge Philos. Soc. 34, 100 共1938兲. 7 E. Sondheimer, Adv. Phys. 1, 1 共1952兲. 8 A. F. Mayadas and M. Shatzkes, Phys. Rev. B 1, 1382 共1970兲. 9 P. Petroff, T. Sheng, A. Sinha, G. Rozgonyi, and F. Alexander, J. Appl. Phys. 74, 988 共1973兲. 10 J. Schmitz, M. Buinting, and R. Ellwanger, in Tungsten and Other Refractory Metals for VLSI Applications, edited by R. A. Blewer and C. M. McConica 共Materials Research Society, Pittsburgh, PA, 1989兲, p. 27. 11 J. Holleman, A. Hasper, and J. Middlehoek, in Chemical Vapor Deposition of Refractory Metals and Ceramics, MRS Symposia Proceedings No. 168, edited by T. M. Besmann and B. M. Gallois 共Materials Research Society, Pittsburgh, 1990兲, p. 107. 12 I. Petrov, L. Hultman, U. Helmersson, J.-E. Sundgren, and J. E. Greene, Thin Solid Films 169, 299 共1989兲; I. Petrov, P. Barna, L. Hultman, and J. E. Greene, J. Vac. Sci. Technol. A 21, S117 共2003兲. 13 M. Yu, K. Ahn, and R. Joshi, IBM J. Res. Dev. 34, 875 共1990兲. 14 N. Hirashita and J. E. Greene, J. Appl. Phys. 70, 4963 共1991兲. 15 Y. Tanaka, E. Kim, J. Forster, and Z. Xu, J. Vac. Sci. Technol. B 17, 416 共1999兲. 16 E. Fawcett and D. Griffits, J. Phys. Chem. Solids 23, 1631 共1962兲. 17 L. J. Van der Pauw, Phillips Res. Rep. B 13, 1 共1958兲. 18 I. Ivanov et al., J. Vac. Sci. Technol. B 共to be published兲.