Some effects of temperature ramping on metal organic chemical vapordeposited Al film nucleation

D. Yanga, R. Jonnalagaddaa, V. Mahadeva, T.S. Calea,*, J.T. Hillmanb, R.F. Fosterb, B.R. Rogersc

aCenter for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287, USAbMaterials Research Corporation, 2120 W. Guadalupe Road, Gilbert, AZ 85233–2805, USA

cMaterials Characterization Lab, Motorola, Inc. 2200 W. Broadway Rd., M/D M360, Mesa, AZ 85202, USA

Abstract

We discuss observations of nucleation density and evolving film texture as a function of varying substrate temperature during the initialstages of tri-isobutyl aluminum (TIBA) sourced chemical vapor deposition of aluminum. Films were deposited onto TiNX coated siliconwafers. Nucleation density, as determined by field emission scanning electron microscopy (FESEM), and film orientation, determined by X-ray diffraction, are presented for deposition at a constant temperature of 300°C and depositions in which the substrate temperature wasramped up from 250°C, down from 400°C, or down from 350°C. Temperature ramp rates were−200°C/min for cooling and 100°C/min forheating. Depositions were stopped after 5, 15 and 30 s of TIBA exposure for each deposition condition, allowing ex situ analysis of films invarious stages of formation. Nuclei size distributions were determined by image analysis of the FESEM micrographs. The variousdeposition conditions resulted in significantly different nuclei size distributions and coverages. The ratio of aluminum (111) to (200)orientation also depends on deposition conditions. A 30 s deposition with a temperature ramp from 400 to 300°C resulted in a (111) to (200)ratio of 3.0 while a 30 s deposition at 300°C resulted in an aluminum (111) to (200) ratio of 1.0. A 5 s pulse while ramping down from400°C followed by a 25 s deposition at 300°C resulted in an aluminum (111) to (200) ratio of 2.0. However, the root mean square (rms)surface roughness, as determined by atomic force microscopy, was approximately half of the rms roughness of the film deposited whileramping from 400 to 300°C for 30 s. We conclude with a discussion of our efforts to elucidate the role of temperature ramping in thenucleation process and its possible extension in ‘microstructure engineering’ of the fully formed film. 1997 Elsevier Science S.A.

Keywords:Nucleation density; Aluminum films; Temperature ramping

1. Introduction

One of the thrust areas of the National Technology Road-map for Semiconductors (NTRS) [1] is the deposition oflow resistance, conformal metallic films for multilevelmetallization process flows. Chemical vapor deposition(CVD) of aluminum is being pursued as an alternative forCVD tungsten and physical vapor deposited aluminum-based alloy films for use in state-of-the-art microelectronicbackend processing because of its low resistivity and poten-tially high conformality.

Work done prior to 1991 was extensively reviewed bySimmonds and Gladfelter [2]. Deposition from tri-isobutylaluminum (TIBA) was extensively studied. Aluminumdeposition from TIBA was shown to occur through ab-hydride elimination mechanism. Deposited films had

rough surface morphologies, which were approximately15% of a film’s thickness. The rough morphology wasthought to be due to slow nucleation. Nishikawa et al. [3]showed that smoother films resulted from higher tempera-ture processes. Several studies focused on modifying thesurface prior to Al CVD in order to produce more uniformfilms. For example, in situ surface pre-treatments with TiCl4

increased film thickness uniformity and decreased surfaceroughness. Adsorbed TiClX species were thought to increasethe decomposition rate of TIBA.

For applications in multilevel metallization, Al (111) tex-tured films with ’tight’ or narrow grain size distributions arepreferred because they are apparently the most resistant toelectromigration [4–7]. It has also been reported that therate of Al deposition on an Al (111) surface is two to fivetimes faster than that on an Al (100) surface, thereby makingAl (111) textured films conducive to higher growth ratesthan non-textured films [2]. Kim et al. [3] reported that

Thin Solid Films 308–309 (1997) 615–620

0040-6090/97/$17.00 1997 Elsevier Science S.A. All rights reservedPII S0040-6090(97)00436-7

* Corresponding author.

increasing the ion energy and ion flux ratio in ‘primary iondeposition’ of Al on SiO2 led to preferential growth of the(111) texture relative to (011). This suggests that higherenergy reactant species in general may result in (111) tex-tured films. In the case of CVD this suggests that increasingthe substrate temperature might lead to higher degrees of(111) texturing. Higher substrate temperatures mightenhance surface diffusion of the Al atoms, allowing themto diffuse to energetically favorable sites. Since the (111)surface is the lowest energy surface in the face centeredcubic (fcc) structure [4] this may result in a more (111)textured film.

Increased substrate temperatures during deposition canalso create problems. Process integration issues such asdopant diffusion, outgassing and flow of inter-layer dielec-trics, drive the need to reduce temperature cycles in state-of-the-art microelectronic back-end processes. Also, in thecase of TIBA-sourced CVD Al, film resistivities havebeen shown to increase at higher deposition temperaturesdue to carbon incorporation in the films [8]. Therefore, thetime spent at elevated temperatures must be minimized.This could be accomplished through the use of an elevatedtemperature to initiate aluminum nucleation, followed by alower temperature growth stage. Reasonable depositionrates may still be attainable at reduced growth temperaturewhen an elevated temperature is used to initiate nucleation,since the activation energy for nucleation is typically higherthan that for film growth [9].

The concept of using pre-programmed temperature tra-jectories during CVD processes was first proposed by Caleet al. [10] to increase wafer throughput in single wafer reac-tors. By initiating a deposition at a higher temperature,when the features on a wafer have their minimum aspectratios, and decreasing the temperature to maintain conform-ality as the aspect ratios increase with fill, they showed thatwafer throughput could be substantially increased. Tracy etal. [11] proved the concept experimentally for the case oftungsten deposition from WF6 on silicon. In the case of AlCVD, temperature programming might lead to smootherfilms.

In this paper we discuss observations of nucleation den-sity and evolving film texture as functions of substrate tem-perature during the initial stages of TIBA-sourced

Fig. 1. Schematic of the Spectrum 202 CVD reactor used for aluminumdepositions.

Fig. 2. Field emission scanning electron micrographs of nuclei after 5 s depositions at: (A) ramp down from 400°C, (B) ramp down from 350°C, (C) 300°Cconstant temperature and (D) ramp up from 250°C. Ramp rates of 100°C/min and−200°C/min were used.

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aluminum chemical vapor deposition on TiNX coated siliconsubstrates. These investigations are motivated by the desireto control the nucleation stage of the CVD process. Controlof the initial stages of deposition may allow us to ‘set thestage’ for growth of smooth, highly (111) textured CVDaluminum films.

2. Experimental

A modified Spectrum 202, single wafer, cold wall lowpressure chemical vapor deposition (LPCVD) reactor wasused. Fig. 1 is a schematic of the experimental apparatus.

Semiconductor grade TIBA was used as the precursor.TIBA, a liquid at room temperature, was delivered to thereactor via a bubbler system using argon as the carrier gas.The bubbler temperature was maintained at 45°C to achievereasonable deposition rates. The gas delivery line was main-tained at 76°C in order to avoid condensation of the pre-cursor between the bubbler and the reactor chamber. Thewafers were heated by a lamp heater. The substrate tem-perature was measured using a thermocouple at the center ofthe wafer backside. The aluminum was deposited onto reac-tive-ion-sputtered (RIS) TiNX films. The wafers were notwet cleaned before aluminum deposition. However, parti-cles were blown from the surface using dry nitrogen.

Fig. 3. Nuclei size distribution histograms determined from micrographs of 5 s depositions at: (A) ramp down from 400°C, (B) ramp down from 350°C, (C)300°C constant temperature and (D) ramp up from 250°C. Ramp rates of 100°C/min and−200°C/min were used.

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All depositions reported here were performed using thefollowing process conditions: 1 Torr total pressure, 60 sccmcarrier gas flow, 76°C line temperature, and 45°C bubblertemperature. Based on previous work [12] we chose to workin the temperature range of 250–400°C, with our base tem-perature being 300°C. Four temperature trajectories, or pro-grams, were included in this investigation: 300°C constanttemperature deposition, ramp down from 400°C, rampdown from 350°C and ramp up from 250°C. Ramp ratesof −200°C/min and 100°C/min were used for cooling andheating, respectively.

The crystal structures of the films were analyzed by X-raydiffraction using Cu-Ka radiation. Field emission scanningelectron microscopy (FESEM) was used to image the result-ing films after 5, 15 and 30 s depositions. Image analysis ofplan view FESEM micrographs was used to determine thenuclei size distributions resulting from the various processconditions. Atomic force microscopy (AFM) was used inthe tapping mode to determine the rms roughness of thefilms.

3. Results and discussion

Fig. 2A–D shows a plan view of the FESEM micrographsafter 5 s TIBA exposure for the four temperature trajectoriesdescribed above. Notice that the deposition obtained byramping the temperature up from 250°C (Fig. 2D) is verysparse, which is to be expected for deposition in this tem-perature range [2]. The nuclei size distributions in thesemicrographs were determined using the public domainimage analysis software, NIH Image V. 1.60, availablefrom the National Institute of Health. The resulting histo-grams are presented in Fig. 3. These histograms plot thefraction of the total number of nuclei measured, binned bytop-down cross-sectional area. Note that the four tempera-ture trajectories result in significantly different nuclei sizedistributions.

Thicker films were deposited for XRD analysis, using 15

and 30 s total deposition times. Three temperature programswere used for the case of 15 s total deposition time; (I) 15 sdeposition time while ramping temperature from 400°C, (II)5 s deposition time while the temperature was ramped from400°C, the TIBA was shut off after 5 s until the waferreached 300°C, then the TIBA was opened for 10 s at300°C and (III) 5 s deposition time while the temperaturewas ramped from 350°C, the TIBA was shut off after 5 suntil the wafer reached 300°C, then the TIBA was openedfor 10 s at 300°C. Four temperature programs were used inthe case of 30 s total deposition time: (IV) 30 s depositiontime while ramping temperature from 400°C, (V) 5 sdeposition time while the temperature was ramped from400°C, the TIBA was shut off after 5 s until the waferreached 300°C, then the TIBA was opened for 25 s at300°C, (VI) 5 s deposition time while the temperature wasramped from 350°C, the TIBA was shut off after 5 s until thewafer reached 300°C, then the TIBA was opened for 25 s at300°C and (VII) 30 s deposition time at 300°C.

Table 1 presents the Al (111) and Al (200) peak intensi-ties normalized to the Si (111) substrate peak for each pro-cess condition. In all cases, both Al (111) and (200) werepresent in the deposited films. However, the normalizedintensity ratio of the Al (111) to (200) peaks varied withprocessing conditions. Films deposited for 15 or 30 s whilethe temperature was ramped from 400°C (temperature pro-grams I and IV) resulted in the highest Al (111) to (200)ratio for both. Temperature programs V and VI resulted insimilar Al (111) to (200) ratios. These results support ourhypothesis that depositing Al films at elevated temperaturesshould enhance the amount of (111) texture.

AFM images of the films after 30 s of growth are pre-sented in Fig. 4A–D. Notice that the 300°C deposition (tem-perature program VII, shown in Fig. 4D) results in larger‘grain’ sizes than either temperature program V or VI(shown in Fig. 4A,B, respectively). The films depositedusing temperature program VII also have the highest rmsroughnesses of the films deposited using temperature pro-grams IV–VII. Temperature program V resulted in the

Table 1

Intensity of Al (111) and Al (200) XRD peaks normalized by the Si (111) substrate peak for CVD Al films resulting from seven temperature programs. A− 200°C/min ramp rate was used

Program no. Program steps Intensity [Al (111)]a Intensity [Al (200)]a

I 15 s ramp from 400°C (TIBA on) 0.020 0.0067II 5 s ramp from 400°C (TIBA on), close TIBA,

cool wafer to 300°C, open TIBA, 10 s growth at 300°C0.0097 0.0057

II 5 s ramp from 350°C (TIBA on), close TIBA,cool wafer to 300°C, open TIBA, 10 s growth at 300°C

0.0094 0.0054

IV 30 s ramp from 400°C (TIBA on) 0.028 0.0092V 5 s ramp from 400°C (TIBA on), close TIBA,

cool wafer to 300°C, open TIBA, 25 s growth at 300°C0.016 0.012

VI 5 s ramp from 350°C (TIBA on), close TIBA,cool wafer to 300°C, open TIBA, 25 s growth at 300°C

0.016 0.0097

VII 30 s, 300°C (TIBA on) 0.020 0.016

aBased on 100% intensity [Si (111)].

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smoothest films, with rms roughnesses roughly half of theconstant 300°C growth (temperature program VII). The rmsroughnesses of the films deposited using temperature pro-grams IV and VI are similar. However, temperature pro-gram IV resulted in smaller average ‘grain’ sizes thantemperature program VI.

4. Conclusions

We have shown that varying the substrate temperatureduring the nucleation stage of TIBA-sourced chemicalvapor deposition of aluminum on TiNx significantly affectsthe nucleation density, surface roughness and microstruc-ture of the resulting aluminum film. Beginning the processwith a higher substrate temperature resulted in highernucleation density and films with a higher Al (111) to(200) ratio than films deposited at constant temperature.Also, turning off the TIBA flow after a short (5 s) elevated

temperature nucleation step followed by film growth at alower temperature resulted in smaller grains and smootherfilms than leaving the TIBA on while ramping the tempera-ture down to the lower temperature for growth.

Further studies are underway to better quantify theobserved trends. The goal of these investigations is to under-stand the mechanisms well enough to be able to predictivelyengineer the microstructure of a film by systematically andcontrollably varying process conditions during the deposi-tion process.

Acknowledgements

The authors gratefully acknowledge the SemiconductorResearch Corporation for funding this work. We also thankDr. A.M. Yates for her help with the XRD analyses and Dr.B.L. Ramakrishna for his help with the AFM analyses.

Fig. 4. Atomic force microscopy images of films resulting from: (A) 5 s ramp down from 400°C followed by 25 s at 300°C, (B) 5 s ramp down from 350°Cfollowed by 25 s at 300°C, (C) 30 s deposition while ramping down from 400°C, and (D) 30 s deposition at 300°C. A −200°C/min ramp rate was used.

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