JEM-1384-R10 21/7/05 7:29 AM Page 1150
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`Journal of ELECTRONIC MATERIALS, Vol. 34, No. 8, 2005
`
`Regular Issue Paper
`
`Novel Multilayered Ti/TiN Diffusion Barrier for Al Metallization
`
`WEN-FA WU,1,4 KOU-CHIANG TSAI,1,2 CHUEN-GUANG CHAO,2
`JEN-CHUNG CHEN,2 and KENG-LIANG OU 3
`
`1.—National Nano Device Laboratories, Hsinchu 300, Taiwan, Republic of China. 2.—Department
`of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan,
`Republic of China. 3.—Graduate Institute of Oral Sciences, Taipei Medical University, Taipei,
`Taiwan, Republic of China. 4.—E-mail: wfwu@mail.ndl.org.tw
`
`A novel, multilayered Ti/TiN diffusion barrier is proposed and successfully
`applied for Al metallization. The multilayered Ti/TiN structure is effective
`in enhancing the barrier properties since the very thin Ti layer inserted into ti-
`tanium nitride (TiN) barrier can cause disruption of the TiN columnar growth
`and reduction of open grain boundaries resulting in retarded interdiffusion be-
`tween metal and silicon. Multilayered Ti/TiN films are deposited sequentially
`by sputtering without breaking vacuum. It is found that TiN grain boundaries
`are discontinuous when a Ti layer is inserted into TiN. Multilayered Ti/TiN
`has a better barrier performance than single-layer TiN in Al metallization.
`However, the barrier performance is related to the number and thickness
`of the inserted Ti layers, because increasing titanium will enhance chemical
`reactions between Al and barrier layers, and produce more titanium-
`aluminum compounds. The total thickness of introduced Ti layers should be
`reduced to improve barrier performance.
`
`Key words: Diffusion barrier, titanium nitride, multilayer, aluminum
`
`INTRODUCTION
`An interconnection system with a barrier metal
`layer is indispensable in submicron ultra-large-
`scale integrated devices to realize high reliability
`such as resistance against electromigration, hillocks,
`stress-induced voids, and Si precipitation at silicon/
`metal interfaces.1,2 Many diffusion-barrier materials
`have been studied for the Al-Si contact system. Reac-
`tively sputtered titanium nitride (TiN) film has been
`widely used as a diffusion barrier layer between the
`aluminum and the silicon substrate due to its high
`thermal and chemical stability, low electrical resis-
`tivity, and excellent mechanical properties.3–5 The
`stoichiometric TiN has a NaCl-type face-centered
`cubic structure with a lattice constant of 4.24 Å.
`The phase is stable over a broad composition range
`concerning the nitrogen concentration.6 In general,
`resistivity of TiN film depends on the deposition
`conditions, and minimum resistivity has been found
`at a composition corresponding to stoichiometric
`TiN.7
`
`(Received November 2, 2004; accepted February 28, 2005)
`
`1150
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`Devices with Al interconnects require some ther-
`mal processes to improve interface property and
`achieve low contact resistance. It is reported that
`sputtered TiN film has a columnar grain structure
`with both inter- and intracolumnar voids.8–10 The
`dominant failure of TiN diffusion barrier is attrib-
`uted to diffusion via fast diffusion paths in columnar
`grains.11,12 When TiN film is used as the diffusion
`barrier in Al metallization, Al and Si interdiffuse
`through the grain boundaries of the TiN film during
`annealing at elevated temperature, resulting in
`degradation of the electrical characteristics and de-
`vice failure. Barrier performance of TiN layer can be
`improved by in-situ plasma treatment and stuffing
`grain boundaries with a thin Al interlayer.1,13,14
`The purpose of this work is to demonstrate a novel,
`multilayered Ti/TiN diffusion barrier for Al metal-
`lization. Al-contacted systems with high thermal
`stability are obtained. It is found that fast diffusion
`paths in columnar TiN grains are destroyed when a
`thin Ti layer is inserted into TiN. Additionally, effects
`of number, thickness, and distribution of inserted Ti
`layer on barrier properties of multilayered Ti/TiN
`films are investigated.
`
`NVIDIA Corp.
`Exhibit 1109
`Page 001
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`Novel Multilayered Ti/TiN Diffusion Barrier for Al Metallization
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`1151
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`to investigate the Al diffusion after annealing. A
`four-point probe system was employed to measure
`sheet resistance. Contact resistance was analyzed
`using the four-terminal Kelvin structure. Leakage
`currents of diodes were measured at a reverse bias of
`5V by a HP 4156 semiconductor parameter analyzer
`(Hewlett-Packard, Palo Alto, CA).
`RESULTS AND DISCUSSION
`Figure 1 shows cross-sectional TEM micrographs
`of the typical TiN and multilayered Ti/TiN films.
`Typical columnar grain structure is observed for con-
`ventionally sputtered TiN barrier layer, as shown in
`Fig. 1a. A sawtooth profile is exhibited in a projected
`two-dimensional view as is expected for films with
`
`a
`
`b F
`
`ig. 1. Cross-sectional TEM images of (a) typical TiN film on the
`SiO2/Si substrate and (b) multilayered Ti/TiN (TiN-3) film on the Ti/Si
`substrate. The thickness of typical TiN and TiN-3 barriers is 100 nm.
`
`EXPERIMENTAL PROCEDURE
`
`The structure of the Al/barrier/Ti/n⫹-p junction
`diode was used to investigate the barrier capabilities
`of multilayered Ti/TiN films in this work. The starting
`materials were p-type (100)-oriented silicon wafers
`with resistivity of 15–25 Ω cm. The wafers were ad-
`ministered a local oxidation of silicon process to define
`active regions after RCA cleaning. The n⫹-p junctions
`were formed by As⫹ implantation at 60 keV with a
`dose of 5 ⫻ 1015 cm⫺2 and subsequent rapid thermal
`annealing in N2 ambient at 1,050°C for 30 sec. The Al
`alloy (Al-Si-Cu), Ti, and TiN films were deposited by
`sputtering in a multichamber cluster system without
`breaking vacuum. The wafers were dipped in a dilute
`HF solution (HF: H2O ⫽ 1:50) to remove native oxide
`prior to loading into the system. The Ti layer, 40-nm
`thick, was deposited onto the Si substrate first and
`employed as a conventional contact metal layer to im-
`prove contact property. Reactively sputtered TiN film,
`100-nm thick, was used as the standard (STD) barrier
`layer. Multilayered Ti/TiN films were prepared by
`inserting thin Ti layers into the TiN barrier. Various
`multilayered Ti/TiN films, as summarized in Table I,
`were employed to investigate the effects of number,
`thickness, and distribution of the inserted Ti layer on
`barrier properties. The total thickness of multilayered
`Ti/TiN barrier layer was 100 nm. Al-Si-Cu film was
`deposited on top of the barrier layer at 15 kW and
`200°C. All samples were alloyed in forming gas ambi-
`ent at 400°C for 30 min to improve contact properties.
`The samples were further subjected to a cumulative
`furnace annealing in forming gas ambient at 500°C
`for 30 min to investigate thermal stability and barrier
`performance.
`The microstructure and grain size of the film were
`examined using transmission electron microscopy
`(TEM). Structure and crystalline orientation of the
`as-deposited and annealed samples were analyzed
`by an x-ray diffractometer with Cu Kα radiation op-
`erated at 50 kV and 250 mA. Surface roughness and
`morphology of the film were observed using a Digital
`Instruments Nanoscope II model atomic force micro-
`scope (Vecco, Cleveland, OH) with a 0.5 Hz scanning
`speed in air ambient. The surface morphology was
`studied with a field emission scanning electron mi-
`croscope. Compositional depth profiles analyzed by
`secondary ion mass spectrometry (SIMS) were used
`
`Table I. Contact Systems with Various Multilayered Ti/TiN Barriers Used in the Study
`
`Barrier Type
`
`Thickness of Ti/TiN (nm)
`
`Thickness of Ti
`Contact Layer (nm)
`
`TiN-1
`TiN-2
`TiN-3
`TiN-4
`TiN-5
`TiN-6
`TiN-7
`TiN-8
`
`TiN (100)
`TiN (47.5)/Ti (5)/TiN (47.5)
`TiN (30)/Ti (5)/TiN (30)/Ti (5)/TiN (30)
`TiN (21.3)/Ti (5)/TiN (21.2)/Ti (5)/TiN (21.3)/Ti (5)/TiN (21.2)
`TiN (45)/Ti (10)/TiN (45)
`TiN (42.5)/Ti (15)/TiN (42.5)
`TiN (25)/Ti (5)/TiN (75)
`TiN (75)/Ti (5)/TiN (25)
`
`40
`40
`40
`40
`40
`40
`40
`40
`
`NVIDIA Corp.
`Exhibit 1109
`Page 002
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`JEM-1384-R10 21/7/05 7:29 AM Page 1152
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`1152
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`Wu, Tsai, Chao, Chen, and Ou
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`Fig. 2. Histograms showing statistical distributions of reversed-
`biased leakage currents of Al/TiN (100 nm)/Ti/Si and Al/TiN (47.5
`nm)/Ti (5 nm)/TiN (47.5 nm)/Ti/Si junction diodes after annealing at
`500°C for 30 min.
`
`voided grain boundaries for which growth proceeds
`in a three-dimensional mode. The trace of the
`surface facets is along the [022] direction that is con-
`sistent with the facets being {111} planes. Discontin-
`uous columnar grains are successfully formed as a
`very thin Ti layer is inserted into the TiN barrier, as
`shown in Fig. 1b. The thickness of the Ti layer in-
`serted into TiN to form multilayered Ti/TiN films is
`5 nm. The very thin Ti interlayer will cause disrup-
`tion of the TiN columnar growth. Figure 2 shows the
`statistical distributions of leakage currents of Al/
`barrier/Ti/Si junction diodes. Al-contacted junction
`
`Fig. 3. The SIMS depth profiles of Al/TiN-1/Si and Al/TiN-2/Si
`contact systems after annealing at 500°C for 30 min.
`
`diodes with multilayered Ti/TiN (TiN-2) barriers
`exhibit lower leakage currents than those with STD
`TiN (TiN-1) barriers after cumulative annealing at
`500°C for 30 min, indicating that multilayered
`Ti/TiN structure has successfully impeded the inter-
`diffusion of Al and Si. The SIMS process is further
`employed to investigate the barrier capability of
`multilayered Ti/TiN film against diffusion. Figure 3
`plots SIMS depth profiles of Al and Si for the Al/bar-
`rier/Si system after annealing at 500°C for 30 min.
`It is also found that Al is less diffused in the multi-
`layered Ti/TiN sample than that in the STD sample.
`Figure 4 schematically shows the possible brief
`reactions of the Al/Ti/Si, Al/TiN/Ti/Si, and Al/TiN/
`
`Fig. 4. Schematic illustrations of the possible brief reactions of the Al/Ti/Si, Al/TiN/Ti/Si, and Al/TiN/Ti/TiN/Ti/Si contact systems before and after
`high-temperature annealing.
`
`NVIDIA Corp.
`Exhibit 1109
`Page 003
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`Novel Multilayered Ti/TiN Diffusion Barrier for Al Metallization
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`1153
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`Ti/TiN/Ti/Si contact systems before and after high-
`temperature annealing. It is reported that the Ti
`layer between Al and Si behaves as a sacrificial
`barrier because it reacts with Al to form TiAl3 com-
`pounds at temperatures above 400°C. The Ti is a
`good diffusion barrier as long as the Ti is not com-
`pletely consumed. Once the Ti has completely re-
`acted to form TiAl3, its diffusion barrier properties
`are lost. It is expected that TiAl3, TiSix, and Al spik-
`ing will be formed for the Al/Ti/Si contact system
`after high-temperature annealing, as shown in
`Fig. 4a. The TiN is also an attractive barrier mater-
`ial because it is chemically and thermodynamically
`stable and behaves as a passive barrier. However, it
`is typical to form TiN films with columnar grains, as
`mentioned previously. It is expected that Al and Si
`would interdiffuse through the grain boundaries of
`the typical columnar TiN barrier during annealing,
`which results in spiking in the Si substrate, as
`shown in Fig. 4b. Discontinuous columnar grains
`are formed for the multilayered Ti/TiN barrier, as
`indicated in the TEM image in Fig. 1b. The diffusion
`has been relatively reduced, as shown in Fig. 3,
`because the thin Ti interlayer has successfully
`resulted in disruption of the TiN columnar growth
`and reduced open grain boundaries of TiN film. Fur-
`thermore, the inserting Ti interlayer creates a bind-
`ing spot for Al. Short-circuit diffusion paths through
`the TiN would be plugged by the Ti or Ti-Al com-
`pound. In order to further explore effects of very
`thin Ti interlayer on barrier capabilities and ther-
`mal stabilities, multilayered Ti/TiN films with vari-
`ous numbers, thicknesses, and distributions of Ti
`interlayer were prepared and used in this work.
`Effects of the Number of Ti Interlayer
`Figure 5 shows the x-ray diffraction (XRD) spec-
`tra of as-deposited STD TiN and multilayered
`Ti/TiN barriers with various numbers of the Ti
`interlayer on Ti/Si substrates. All the XRD patterns
`show three peaks of TiN (111), Ti (10–11), and Ti
`(0002). Ti (10–11) and Ti (0002) peaks are believed
`to result from 40-nm Ti contact layer on the Si sub-
`strate because it is much thicker than inserting Ti
`interlayer. The high (0002) orientation of underlying
`Ti layer will enhance the development of TiN (111)
`preferred orientation since they have similar atomic
`arrangement patterns. The (111) planes of TiN are
`built of alternate Ti and N layers and are the most
`closely packed planes.15 The relative intensity of the
`TiN (111) peak in the TiN-2 sample is lower and the
`peak shape becomes broader than that in the TiN-1
`(STD TiN) sample, indicating that grain size de-
`creases in the TiN-2 sample. Moreover, the relative
`intensity of the TiN (111) peak in the TiN-3 sample
`is much lower and the peak shape becomes broader
`than that in the TiN-2 sample. That is, the more
`the Ti-inserting layer, the lower the relative inten-
`sity of TiN (111) and the broader the peak shape.
`It is reported that the first 10-nm or 20-nm TiN
`films grow in an amorphous-like or nanograined
`
`Fig. 5. XRD spectra of as-deposited TiN-based barriers on Ti/Si
`substrates.
`
`structure followed by columnar grain growth. The
`columns in the TiN film increase in size with an
`increase in distance from the film/substrate inter-
`face.16 The nanocrystalline barrier is more effective
`than the polycrystalline barrier since the nano-
`crystalline film can slow interdiffusion.13,17–21 The
`nanocrystal TiN grains in the multilayered Ti/TiN
`barrier will enhance the effectiveness of the barrier.
`Furthermore, the Ti interlayer will turn into an alu-
`minide, causing a step change in Al composition,
`and thus a lower concentration gradient for Al below
`the Ti layer. The step change will reduce the gradi-
`ent of Al diffusion flux. The diffusion barrier perfor-
`mance of TiN film is significantly improved by
`inserting a thin Ti interlayer into TiN, as shown in
`Figs. 2 and 3.
`Figure 6 shows the variation percentage in sheet
`resistance of the Al/barrier/Ti/Si sample as a function
`of the annealing temperature for various multilay-
`ered Ti/TiN barrier layers. The samples are subjected
`to a cumulative furnace annealing in forming gas
`environment from 400°C to 550°C for 30 min. Sheet
`resistance increases with increasing annealing tem-
`perature. The increase in sheet resistance is attrib-
`uted to formation of the compounds. The increasing
`rate in sheet resistance of the STD-TiN (TiN-1)
`sample is higher than those in multilayered Ti/TiN
`(TiN-2, TiN-3, and TiN-4) samples, as indicated in
`Fig. 6. Low increasing rate for multilayered Ti/TiN
`
`NVIDIA Corp.
`Exhibit 1109
`Page 004
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`1154
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`Wu, Tsai, Chao, Chen, and Ou
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`Fig. 6. Variation percentage in sheet resistance of Al/barrier/Ti/Si
`contact system as a function of annealing temperature.
`
`samples is believed to result from discontinuous
`columnar grains in multilayered Ti/TiN films, which
`alleviate the interdiffusion and formation of the com-
`pound. It is also seen in Fig. 6 that the increasing
`rate in sheet resistance of the TiN-3 sample is higher
`than that in the TiN-2 sample, and the increasing
`rate in sheet resistance of the TiN-4 sample is higher
`than that in the TiN-3 sample. That is, variation per-
`centage in sheet resistance increases with increasing
`number of Ti interlayer. X-ray diffraction is employed
`to analyze annealed Al/barrier/Ti/Si samples to more
`distinctly display the differences among barrier lay-
`ers. It is found that the relative intensity of the TiAl3
`phase increases with increasing Ti interlayer. As
`mentioned previously, the Ti layer between Al and Si
`behaves as a sacrificial barrier because Al and Ti
`start to react with each other at 400°C and above.
`The limited efficiency is probably caused by the fact
`that increasing Ti interlayer reduces total effective
`thickness of TiN in the multilayered Ti/TiN barrier.
`Barrier performance is determined by a competition
`among the disruption effect of columnar growth,
`gradient of Al diffusion flux, and total effective thick-
`ness of TiN. A thin Ti interlayer will result in discon-
`tinuity or shift in columnar TiN grain and create a
`binding spot for Al, thus alleviating interdiffusion
`and improving barrier performance. However, more
`Ti interlayers will reduce the total effective thickness
`of TiN, causing barrier performance to be degraded.
`Effects of the Thickness of Ti Interlayer
`Effects of thickness of the inserting Ti interlayer
`on the barrier properties of multilayered Ti/TiN
`films are further investigated. Figure 7 shows the
`statistical distributions of reverse-biased leakage
`currents of the diodes with the TiN-1, TiN-2, TiN-5,
`and TiN-6 barriers. The distributions of reverse-
`biased leakage currents are measured at a reverse-
`biased voltage of 5 V after cumulative annealing at
`400°C and 500°C for 30 min. Diodes with Al/bar-
`rier/Ti/Si exhibit low leakage currents of the order of
`10⫺11–10⫺8 amperes after cumulative annealing at
`400°C and 500°C for 30 min. The diodes with TiN-2
`
`Fig. 7. Histograms showing statistical distributions of reversed-biased
`leakage currents of Al/barrier/Ti/Si junction diodes after annealing at
`500°C for 30 min.
`
`barriers show lower leakage currents than those
`with STD TiN (TiN-1) barriers, indicating that mul-
`tilayered Ti/TiN structure has successfully impeded
`the interdiffusion of Al and Si. However, barrier per-
`formance is degraded as the thickness of inserting
`Ti interlayer increases. The diodes with TiN-5 and
`TiN-6 barriers show higher leakage currents than
`those with TiN-1 (STD TiN) and TiN-2 barriers. It is
`felt that increasing the thickness of the Ti interlayer
`reduces the total effective thickness of TiN in multi-
`layered Ti/TiN barrier since the total thickness
`of the TiN-based barrier layer is 100 nm. The multi-
`layered Ti/TiN structure is effective in enhancing
`the barrier performance and retarding Al/Si inter-
`diffusion, but the thick inserting Ti interlayer in
`multilayered Ti/TiN barrier is not suggested due to
`resulting degradation in barrier performance.
`Figure 8 presents the contact resistance (Rc) of
`Al/barrier/Ti/Si contact systems, which include multi-
`layered Ti/TiN barriers with various thicknesses of
`inserting Ti interlayer. It can be seen that the contact
`resistance in the TiN-2 contact system is lower than
`that in the TiN-1 (STD TiN) contact system, and
`the contact resistance of the Al/barrier/Ti/Si contact
`system increases as the thickness of the Ti inter-
`layer increases. High contact resistance is found after
`
`Fig. 8. Contact resistance of Al/barrier/Ti/Si system as a function of
`thickness of inserting Ti interlayer.
`
`NVIDIA Corp.
`Exhibit 1109
`Page 005
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`Novel Multilayered Ti/TiN Diffusion Barrier for Al Metallization
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`1155
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`annealing at 500°C because high-temperature an-
`nealing will enhance interdiffusion of Al and Si via
`microscopic defects (i.e., grain boundaries) and for-
`mation of the compounds. Low contact resistance in
`the TiN-2 contact system is attributed to the fact
`that the additional Ti interlayer has successfully alle-
`viated the interdiffusion of Al and Si. It is important
`to note that the formation of TiAl3 is more likely to
`occur in the Al-Ti system than in the Al-TiN system
`after thermal processing, since thermal stability be-
`tween Al and Ti layers is lower than that between Al
`and TiN layers.22–24 Increased thickness of the insert-
`ing Ti interlayer will enhance the formation of TiAl3
`compounds, resulting in increased contact resistance.
`Effects of the Distribution of Ti Interlayer
`Figure 9 shows the statistical distributions of
`reverse-biased leakage currents of the diodes with
`TiN-1, TiN-2, TiN-7, and TiN-8 barriers. TiN-2, TiN-
`7, and TiN-8 barriers have different inserting posi-
`tions of Ti interlayers in TiN, as summarized in
`Table I, to explore effects of distribution of inserting
`Ti interlayer on barrier properties of multilayered
`Ti/TiN barriers. Superiority of multilayered Ti/TiN
`as a barrier can be gauged from the leakage current
`of the diode. Low leakage currents are obtained
`for multilayered Ti/TiN films compared to the TiN-1
`(STD-TiN) sample. Reduction in leakage current of
`the TiN-7 sample is apparent. Though the detailed
`mechanisms need to be further investigated, the
`gradient of Al diffusion flux is believed to be one of
`the key factors on TiN barrier performance. As men-
`tioned previously, all short-circuit diffusion paths
`through the TiN would be plugged by the Ti or Ti-Al
`compounds. If there is a fixed Al concentration on
`the substrate side of the Ti interlayer, it would be
`expected that pulling the Ti barrier further away
`from the Ti contact metal would reduce Al diffusion
`flux by reducing the gradient. Thin TiN film (25 nm)
`on the inserting Ti interlayer in the TiN-7 sample
`leads to better barrier capability due to the reduc-
`tion of Al diffusion flux.
`
`Fig. 9. Histograms showing statistical distributions of reversed-
`biased leakage currents of Al/barrier/Ti/Si junction diodes after
`annealing at 500°C for 30 min.
`
`CONCLUSIONS
`A novel method to improve TiN-based diffusion
`barriers using multilayered Ti/TiN structure has
`been proposed. Multilayered Ti/TiN barriers can
`alleviate interdiffusion via fast diffusion paths in
`columnar TiN barriers, because the thin Ti inter-
`layer results in disruption or shift in TiN columnar
`grains and creates a binding spot for Al. Less inter-
`diffusion is observed in multilayered Ti/TiN barriers
`from SIMS analyses. The sheet resistance of the
`Al/TiN/Ti/TiN/Ti/Si contact system is relatively
`stable compared to the Al/TiN/Ti/Si (STD) contact
`system. The Al/barrier/Ti/Si junction diodes with
`multilayered Ti/TiN barriers show relatively low
`leakage currents compared to those with STD TiN
`barriers. However, an increase in the number and
`thickness of the inserting Ti interlayer will reduce
`efficiency of the TiN-based barrier because Ti is a
`sacrificial barrier. More titanium layers or increased
`thickness in the multilayered Ti/TiN film enhances
`the chemical reactions between Ti and Al. This re-
`duces the total effective thickness of TiN, causing a
`degradation of barrier performance. As the thick-
`ness of the inserting Ti interlayer in TiN increases
`from 5 nm to 15 nm, degradation of the multilayered
`Ti/TiN barrier and enhanced formation of TiAl3 are
`found. The total effective thickness of the inserting
`Ti interlayer in TiN should be reduced to improve
`barrier performance. Barrier performance of the
`multilayered Ti/TiN barrier is also affected by the
`distribution of the inserting Ti interlayer in TiN.
`Thin TiN film on the inserting Ti interlayer shows
`better barrier performance against Al diffusion due
`to reduced Al diffusion flux.
`
`ACKNOWLEDGEMENTS
`This work was financially supported by the Na-
`tional Science Council of the Republic of China
`(Contract No. NSC 93-2215-E-492-005) and sup-
`ported, in part, by the Ministry of Economic Affairs
`of the Republic of China (Contract No. 91-EC-17-A-
`08-S1-0003). Technical support from the National
`Nano Device Laboratories is greatly acknowledged.
`
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`NVIDIA Corp.
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`NVIDIA Corp.
`Exhibit 1109
`Page 007