Texture and surface roughness of PRCVD aluminum films

Texture and surface roughness of PRCVD aluminum ®lms

D. Yanga, R. Jonnalagaddaa, B.R. Rogersb, J.T. Hillmanc, R.F. Fosterc, T.S. Caled,*

aCenter for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287, USAbMaterials Characterization Lab, Motorola, 2200 W. Broadway Rd, M/D M360, Mesa, AZ 85202, USA

cTokyo Electron Arizona, 2120 W. Guadalupe Rd., Gilbert, AZ 85233-2805, USAdDepartment of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA

Abstract

The effects of temperature, substrate type and diluent gas ¯ow trajectories on the ®lm growth rate, surface roughness, crystal orientation,

average grain size, and grain size uniformity of tri-isobutyl aluminum (TIBA) sourced CVD aluminum ®lms were studied. Films were

deposited in a lamp heated, single wafer, cold wall reactor using several process sequences which included variations on temperature

ramping, precursor ¯ow rate, and diluent gas ¯ow. For 30-s depositions on TiN coated Si(111) substrates, pulsing the precursor ¯ow for 5 s at

the start of temperature ramp down from 673 K followed by deposition for 25 s at 573 K resulted in ®lms with higher nucleation densities and

larger fractions of Al(111) texturing, in addition to lower surface roughnesses and smaller grain sizes, when compared to ®lms deposited at

573 K constant temperature. For 10-min depositions using four different temperature protocols, the type of substrate had a signi®cant effect

on the fraction of Al(111) texturing. The ®lms deposited during temperature ramp down from 673 to 573 K with 10-s pulses at the start of

deposition had higher Al(111) texturing and lower surface roughness than ®lms deposited during the other process trajectories for all four

different substrates. The estimated relative median time between electromigration failure (MTBF) is two to three times higher for the ®lms

deposited using a 10-s pulse during ramp down from either 673 or 623 K on PVD TiN coated Si(100) substrates as compared to other process

protocols on the different substrates. During the programmed substrate temperature ramp down from 673 to 573 K, introducing 25 sccm

diluent gas into the reactor at 573 K resulted in ®lms with higher growth rates, larger average grain sizes and higher Al(111) fractions when

compared to ®lms deposited without diluent gas. However, introducing 75 or 100 sccm diluent gas resulted in ®lms with lower growth rates,

smaller average grain sizes, lower grain size variances and higher Al(111) fractions when compared to ®lms deposited without diluent gas.

q 1998 Elsevier Science S.A. All rights reserved.

Keywords: Texture; Surface roughness; Programmed rate chemical vapor deposition; Aluminum; Substrate type; Diluent gas; TIBA

1. Introduction

Aluminum based alloys continue to be the most widely

used materials for metallization in ULSI circuits, and sput-

tering is the most common process used for their deposition.

However, as the device dimensions in microelectronic

circuits continue to shrink, the conformality and uniformity

of ®lms used to ®ll high aspect ratio features become critical

issues. Chemical vapor deposited (CVD) Al has been

pursued for several years as an alternative to physical

vapor deposited (PVD) aluminum alloy ®lms, because of

its potentially high conformality [1]. However, rough

surface morphology is still one of the problems with some

CVD aluminum ®lms. Higher temperatures during the early

stages of the deposition could enhance the surface diffusion

of Al adatoms, enabling them to form a higher fraction of

(111) clusters and smoother ®lms. For applications in multi-

level metallization, Al(111) oriented aluminum ®lms with

the lowest mosaic spread are preferred because they are the

most resistant to electromigration failures [2±5]. It has also

been reported that the rate of the surface limiting reaction

step for the deposition reaction on Al(111) is two to ®ve

times faster than on Al(100), thereby making Al(111)

morphology conducive to higher growth rates [6].

Tri-isobutyl aluminum (TIBA) is one precursor that has

received considerable attention for Al CVD [6±10]. The

activation energy for nucleation (3.2 eV) has been reported

to be higher than the activation energy for growth (1.1 eV)

for this process [7]. Therefore, using a constant substrate

temperature during deposition processes may not result in

optimal nucleation and growth. Cale et al. have reported that

a substantial decrease in processing time can be achieved for

a given step coverage by varying the deposition rate in a

prescribed manner by decreasing temperature during tung-

sten deposition [11,12]. Nisikawa et al. [9] observed that

Thin Solid Films 332 (1998) 312±318

0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved.

PII S0040-6090(98)01034-7

* Corresponding author Tel.:100 1 518 276 6059; fax:100 1 518 276

4030; e-mail: [email protected].

smoother ®lms resulted at higher temperatures. However,

depositions using TIBA at substrate temperatures above

603 K reportedly show carbon incorporation [13].

Recently, smoother ®lms with increased Al(111) orienta-

tion were deposited using pulsed precursor ¯ow along with

substrate temperature ramping [14,15]. We refer to CVD

protocols in which one or more process setpoints are varied

in a planned way as programmed rate CVD (PRCVD).

Higher temperatures during the early stages of deposition

could increase the nucleation density [14], and could

reasonably enhance surface diffusion of Al adatoms.

Increased nucleation density could reduce the surface

roughness of the ®nal ®lm [15], and increased surface diffu-

sion of the Al adatoms could increase the formation of low

energy (111) clusters [2].

Based on the results from our earlier work on temperature

ramped process protocols during TIBA sourced PRCVD of

aluminum on TiN coated Si substrates, we designed experi-

ments to investigate the effects of substrate type and dilute

gas ¯ows on the ®lm properties. Introducing diluent gas

¯ow in the system can change the incoming precursor ¯ux

to the substrate surface, which may enhance the effects of

adatom surface diffusion. Changes in the surface diffusion

length could affect the average grain size, and the ratio of

Al(111)/Al(200). Vaidya et al. [16] proposed a relationship

between the median time between electromigration failure

(MTBF) of ®ne Al lines and the average grain size, the grain

size distribution, and the fraction of Al(111) grains of the

lines. The proposed relation is �s=s2� £ log�IAl�111�=IAl�200��3,

in which the parameters are grain size, its uniformity and the

texture, respectively. This relationship suggests that a

higher fraction of Al(111) grains and tighter grain size

distribution with a larger average grain size will increase

MTBF.

2. Experimental details

Depositions were performed in a modi®ed SPECTRUM

202, single wafer, cold wall, lamp heated LPCVD reactor.

The experimental set-up has been discussed in detail

previously [14,15]. A single 4-inch wafer was heated by

backside halogen lamps, and its temperature was measured

using two thermocouples at the center of the backside of the

wafer. TIBA, the aluminum precursor, was delivered to the

reactor chamber by a bubbler type evaporator system using

D. Yang et al. / Thin Solid Films 332 (1998) 312±318 313

Table 1

Conditions used for early stage deposition experiments

Sample

ID

Substrate type Deposition time

during temperature

ramp (s)

Deposition time

at 573 K (s)

Initial

deposition

temperature

(K)

Final

deposition

temperature

(K)

TiN1 PVD TiN on Si(100) 5 25 673 573

TiN2 PVD TiN on Si(100) 30 0 673 573

TiN3 PVD TiN on Si(100) 5 25 623 573

TiN4 PVD TiN on Si(100) N/A 30 573 573

Table 2

Conditions used for substrate type dependent experiments

Sample

ID


during temperature

ramp (s)

Deposition time

at 573 K (s)

Initial

deposition

temperature

(K)

Final

deposition

temperature

(K)

Process

protocol IDa

Si1 Si(100) N/A 600 573 573 PP1

Si2 Si(100) 10 600 623 573 PP2

Si3 Si(100) 30 570 673 573 PP3

Si4 Si(100) 10 600 673 573 PP4

TiN5 CVD TiN on Si(100) N/A 600 573 573 PP1

TiN6 CVD TiN on Si(100) 10 600 623 573 PP2



TiN9 PVD TiN on Si(100) N/A 1200 573 573 PP1

TiN10 PVD TiN on Si(100) 10 600 623 573 PP2



Ti1 PVD Ti on Si(100) N/A 600 573 573 PP1

Ti2 PVD Ti on Si(100) 10 600 623 573 PP2

Ti3 PVD Ti on Si(100) 30 570 673 573 PP3

Ti4 PVD Ti on Si(100) 10 600 673 573 PP4

preheated argon as the carrier gas. The following process

conditions were common to all the experiments: 1 Torr total

pressure, 60 sccm carrier gas ¯ow during deposition, 318 K

bubbler temperature, 349 K bubbler outlet line and diluent

gas line temperatures, and 2200 K/min substrate tempera-

ture ramp down rate.

Details of the experiments with temperature trajectories,

deposition times and all other process conditions are given

in Table 1±3 and are shown schematically in Fig. 1. The

process sequences for temperature ramp down experiments

started by stabilizing the substrate temperature at the desired

initial value. To study the effects of different substrates, the

same four types of process protocols (see Fig. 1) were used

on the four selected substrate types: Si(100), CVD TiN

coated Si(100), PVD TiN coated Si(100), and PVD Ti

coated Si(100). Details of these experimental conditions

are summarized in Table 2.

Our previous results suggested that pulsing the precursor

¯ow for 10 s at the start of temperature ramp down from 673

to 573 K (PP4) resulted in Al ®lms with lower surface

roughnesses and increased (111)/(200) ratios [15]. Hence,

we adopted that process protocol (PP4) to study the effects

of diluent gas ¯ow trajectories. For the diluent gas ¯ow

trajectory PRCVD experiments, depositions were initiated

using 60 sccm carrier gas ¯ow through the liquid precursor.

The temperature ramp down from 673 to 573 K was also

started. The precursor ¯ow was stopped (the carrier gas was

bypassed) 10 s after the ramp down started, at a temperature

of 640 K. The precursor ¯ow was restarted and the diluent

argon ¯ow (25, 50, 75, or 100 sccm) was started 10 s after

the substrate temperature reached 573 K. After depositing

for 10 min at 573 K, the substrate heating, diluent gas ¯ow

and precursor ¯ow were stopped.

Weight gain measurements and scanning electron micro-

D. Yang et al. / Thin Solid Films 332 (1998) 312±318314

Table 3

Conditions used for diluent gas ¯ow dependent experiment (using PP4 protocol)

Sample

ID


during temperature

ramp (s)

Deposition time

at 573 K (s)

Initial

deposition

temperature

(K)

Final

deposition

temperature

(K)

Diluent gas ¯ow

rate (sccm)

TiN13 TiN on Si(100) 10 600 673 573 0

TiN14 TiN on Si(100) 10 600 673 573 25

TiN15 TiN on Si(100) 10 600 673 573 50

TiN16 TiN on Si(100) 10 600 673 573 75

TiN17 TiN on Si(100) 10 600 673 573 100

Fig. 1. Process protocols used for study of the dependence on substrate type and diluent gas ¯ow: 10 min at 573 K (PP1), 10 s ramping from 623 K/10 min at

573 K (PP2), 30 s ramping from 673 K/9 min 30 s at 573 K (PP3) and 10 s ramping from 673 K/10 min at 573 K (PP4).

scopy (SEM) were used to determine the ®lm thicknesses

and deposition rates. The crystal orientations of the ®lms

were determined by X-ray diffraction (XRD) using Cu Karadiation. Atomic force microscopy (AFM) was used to

observe the surface morphologies, and measure the ®lm

roughnesses.

3. Results and discussion

3.1. Initial 30-s deposition experiments

Fig. 2 presents the AFM images for the ®lms deposited

during the ®rst 30 s using four substrate temperature proto-

cols. The island sizes vary signi®cantly for various deposi-

tion conditions. For these samples, the deposition using a 5-s

pulse starting at 673 K (TiN1) resulted in smaller nucleation

sizes compared to the deposition (TiN3) using a 5-s pulse

starting at 623 K. Fig. 3 presents the ratio of Al(111) grains

to Al(200) grains, as determined by XRD, for the four ®lms

discussed above. Note that the two ®lms (TiN1 and TiN2)

deposited using 673 K as the initial substrate temperature

have higher ratios of Al(111) grains to Al(200) grains,

compared to the continuous 573 K deposited ®lms (TiN4).

The pulsed deposition (TiN1) resulted in an approximately

53% higher ratio of Al(111) to Al(200) grains, compared to

the continuously ramped deposition (TiN2). The pulsed

deposition initiated at 623 K (TiN3) resulted in a ®lm

with only marginally higher Al(111) to Al(200) ratio than

the continuous deposition at 573 K (TiN4).


Fig. 2. AFM images for Al ®lms deposited on PVD TiN on Si(100) substrate during initial 30 s using different process protocols: 5 s ramping from 673 K/25 s

at 573 K (TiN1), 30 s ramping from 673 K (TiN2), 5 s ramping from 623 K/25 s at 573 K (TiN3) and 30 s at 573 K (TiN4).

3.2. Effects of substrate type

Depositions on four substrate types were performed

according to the four process protocols shown in Fig. 1

(see also Table 2). Table 4 presents deposition rate, ®lm

thickness, the intensity ratio of Al(111) to Al(200), surface

roughness, normalized roughness (surface roughness/®lm

thickness), average grain size, and their standard deviations.

The trends of deposition rate among the designed process

protocols were very similar for all substrates. Except for

depositions on PVD TiN coated Si(100) substrates, ramping

down the substrate temperature and pulsing precursor ¯ow

resulted in lower deposition rates than depositions at

constant teperatures and continuous precursor ¯ows. On

PVD TiN, CVD TiN and PVD Ti coated substrates, the

PP4 protocol resulted in ®lms with higher fractions of

Al(111) orientated grains compared to the other protocols.

Films deposited using the PP4 protocol on PVD TiN coated

Si(100) substrates had signi®cantly higher (111)/(200)

ratios compared to the ratios of ®lms deposited on the

other substrates. In general, the PP4 protocol resulted in

®lms with reduced surface roughness. Deposition on PVD

TiN coated substrates resulted in ®lms with lower normal-

ized roughness as compared to the ®lms deposited on other

substrates.

The island sizes for these ®lms were determined from

AFM images using UTHSCSA Image Tool, an image analy-

sis software provided by the University of Texas at San

Antonio [17]. In general, the PP4 protocol resulted in the

smallest average grain sizes of ®lms deposited on TiN and

Ti coated Si(100) substrates. The average grain sizes of

®lms which were deposited on PVD TiN coated Si(100)

substrates were much bigger than those deposited on the

other three substrates. The trends involving the standard


Fig. 3. Al (111) to Al (200) ratios determined by XRD of Al ®lms deposited

on PVD TiN on Si(100) substrate during initial 30 s using different process

protocols: 5 s ramping from 673 K/25 s at 573 K (TiN1), 30 s ramping from

673 K (TiN2), 5 s ramping from 623 K/25 s at 573 K (TiN3) and 30 s at 573

K (TiN4).

Table 4

Results from substrate type and diluent gas ¯ow dependent experiments

Sample

ID

Deposition

rate

(nm/min)

Average ®lm

thickness

(nm)

IAl(111)/IAl(200) RMS

roughness

(nm)

Normalized

roughness

(%)

Average

grain size

(mm2)

Standard

deviation

(mm2)

Si1 42.63 426 2.71 301 70.7 0.51 0.95

Si2 38.99 396 2.68 209 52.8 0.77 0.72

Si3 35.86 359 2.91 185 51.5 0.76 0.65

Si4 35.47 361 2.64 175 48.5 0.71 0.56

TiN5 49.34 493 1.68 225 45.6 1.99 1.61

TiN6 40.92 416 1.47 197 47.4 1.27 1.06

TiN7 40.37 404 2.73 265 65.6 0.97 0.93

TiN8 37.69 383 2.81 197 51.4 0.74 0.65

TiN9 23.49 470 1.31 126 26.8 11.36 3.50

TiN10 51.39 523 5.05 115 22.0 4.49 2.54

TiN11 20.63 413 2.02 120 29.1 2.34 2.40

TiN12 43.06 438 118.8 107 24.4 2.66 1.39

Ti1 51.64 516 2.44 254 49.2 1.01 1.04

Ti2 45.64 464 2.48 239 51.5 0.71 0.70

Ti3 40.57 406 2.19 222 54.7 0.62 0.61

Ti4 36.82 374 2.78 177 47.3 0.53 0.49

Fig. 4. Estimated relative MTBF for ®lms deposited on different substrate

types using different process protocols: 10 min at 573 K (PP1), 10 s ramping

from 623 K/10 min at 573 K (PP2), 30 s ramping from 673 K/9 min 30 s at

573 K (PP3) and 10 s ramping from 673 K/10 min at 573 K (PP4).

deviation of grain sizes were very similar to the trends for

the average grain sizes. The PP4 protocol resulted in ®lms

with the highest grain size uniformities for Si(100) and TiN

coated Si(100) substrates. Using the proposed relation by

Vaidya et al. [16], Fig. 4 presents the estimated relative

MTBF (compared to the randomly orientated ®lm having

same value of average grain size and their standard devia-

tion) for ®lms deposited on different substrate types. Note

that the relative MTBF are two to three times higher for the

®lms deposited using 10-s pulses from either 673 or 623 K

on PVD TiN coated Si(100) substrate.

3.3. Effects of diluent gas ¯ow rate

Fig. 5a shows the dependence of the deposition rate on

the diluent gas ¯ow rate. The deposition time was the same

for all experiments. These data suggest that the Al growth

rate decreases with increasing diluent gas ¯ow, at least

above 25 sccm. Aluminum deposition from TIBA has

been reported to follow ®rst-order kinetics [13]. Increasing

the diluent gas ¯ow reduces the partial pressure of TIBA in

the reactor, which in ®rst order systems would decrease the

reaction rate. A 25 sccm diluent gas ¯ow rate corresponds to

the apparent maxima in the grain size and fraction of

Al(111) (see Fig. 5b,c). This is in agreement with the higher

growth rates on Al(111) surfaces (compared to growth rates

on Al(200) surfaces) reported by Bent et al. [13] and Yang et

al. [15].

The average grain sizes and standard deviations were

computed from AFM images using the UTHSCSA Image

Tool. Fig. 5b shows the dependence of average grain size

and grain size uniformity (standard deviation) of the depos-

ited ®lms on diluent gas ¯ow rates. The average grain size

and their uniformity (standard deviation) of the ®lms show

the apparent maxima upon increasing diluent gas ¯ow. The

apparent maximum average grain size at about 25 sccm

diluent gas ¯ow rate corresponds to the apparent maxima

in the ®lm deposition rates and Al(111) fractions. The larger

grain sizes may be a result of increased lateral growth rate of

Al islands on (111) surfaces [9]. The data also suggest that

depositions using about 75 or 100 sccm diluent gas ¯ow

result in ®lms with lower average ®lm thicknesses, smaller

average grain sizes and lower grain size standard deviations

when compared to ®lms deposited without diluent gas.

Smaller grain sizes at lower ®lm thicknesses have also

been reported by Nishikawa et al. [9].

Fig. 5c shows the IAl(111)/IAl(200) ratios of the deposited

®lms at different diluent gas ¯ow rates. The data suggest

that the Al(111) fractions of the ®lms deposited using dilu-

ent gas are somewhat higher when compared to the ®lm

deposited without diluent gas, with an apparent maximum

at about 25 sccm of diluent gas.

The root-mean-square (RMS) surface roughness for the

®lm deposited without diluent gas was 212 nm, at approxi-

mately 828 nm of average ®lm thickness. This is roughly

26% surface roughness when normalized with respect to its

average ®lm thickness. In Fig. 5d, the normalized rough-

nesses of ®lms deposited using diluent gas were higher by

12±33% than those of the ®lms deposited without diluent

gas.

4. Conclusions

For deposition of thin ®lms, pulsing the TIBA ¯ow for 5 s

during the elevated temperature nucleation step (673 K)

followed by ®lm growth at a lower temperature (573 K)

for 25 s resulted in reduced surface roughness, smaller

grains, and higher Al(111) texturing as compared to 573

K continuous deposition. In general, ®lms which were

deposited in the pulsed ¯ow experiments using the tempera-

ture trajectory of 673±573 K resulted in higher Al(111)

texturing and lower surface roughness than ®lms deposited


Fig. 5. The effects of diluent gas ¯ow rate on Al ®lms deposited on PVD

TiN by 10 s ramping from 673 K followed by 10 min at 573 K; (a) deposi-

tion rates, (b) average grain sizes and standard deviations, (c) ®lm textures

and (d) normalized roughness.

using the other trajectories for all four different substrates.

Our results show that depositions for a short time at higher

substrate temperatures, followed by depositions at lower

constant substrate temperatures result in enhanced nuclea-

tion density, higher uniformity, and increased fraction of

(111) orientation than continuous 573 K depositions.

Diluent gas ¯ow rates during PRCVD of TIBA sourced

Al ®lms affect deposition rates, grain sizes, uniformities,

and surface roughnesses. During the substrate temperature

ramp down process (from 673 to 573 K), introducing 25

sccm diluent gas ¯ow at 573 K resulted in ®lms with larger

average grain sizes, higher deposition rates, and larger

Al(111) fractions, when compared to ®lms deposited with-

out diluent gas. From the results, we may conclude that

deposition rate of TIBA sourced Al ®lm in general is

proportional to the combination of the partial pressure of

TIBA and the fraction of Al(111) of the ®lm.

Acknowledgements

We gratefully acknowledge support for this project from

the Semiconductor Research Corporation. We thank Dr.

A.M. Yates and Dr. B.L. Ramakrishna for their help with

XRD and AFM analyses.

References

[1] S. Sivaram, Chemical Vapor Deposition, VNR, New York, 1995.

[2] Y.W. Kim, I. Petrov, J.E. Greene, J. Vac. Sci. Technol. A 14 (2)

(1996) 346.

[3] D.B. Knorr, D.P. Tracy, K.P. Rodbell, Appl. Phys. Lett. 59 (1991)

3241.

[4] D.B. Knorr, T.-M. Lu, Appl. Phys. Lett. 54 (1989) 2210.

[5] S. Vaidya, A.K. Sinha, Thin Solid Films 75 (1981) 253.

[6] M.G. Simmonds, W.L. Gladfelter, in: T. Kodas, M. Hampden-Smith

(Eds.), The Chemistry of Metal CVD, VCH, Weinheim, 1994, Chap-

ter 2.

[7] K.-I. Lee, Y.-S. Kim, S.-K. Joo, J. Electrochem. Soc. 139 (1992)

3578.

[8] T. Kobayashi, A. Sekiguchi, N. Akiyama, N. Hosokawa, T. Asamaki,

J. Vac. Sci. Technol. A10 (3) (1992) 525.

[9] S. Nishikawa, K. Tani, T. Yamaji, J. Mater. Res. 7 (2) (1992) 345.

[10] S.S. Doad, G.J. Leusink, B. Jin, T.S. Cale, J.T. Hillman, R.F. Foster,

Conference Proceedings ULSI XI 1996, Materials Research Society,

1996, p. 627.

[11] T.S. Cale, M.K. Jain, G.B. Raupp, J. Electrochem. Soc. 137 (1990)

1526.

[12] K.M. Tracy, S. Bolnedi, G.J. Leusink, T.S. Cale, in: R.C. Ellwanger,

S.Q. Wang (Eds.), Advanced Metals and Interconnect Systems for

ULSI Applications in 1995, Mater. Res. Soc. Proc., Pittsburgh, PA

1995, p. 563.

[13] B.E. Bent, R.G. Nuzzo, L.H. Dubois, J. Am. Chem. Soc. 111 (1989)

1634.

[14] D. Yang, R. Jonnalagadda, V. Mahadev, T.S. Cale, J.T. Hillman, R.F.

Foster, B.R. Rogers, Thin Solid Films 308±309 (1998) 615.

[15] D. Yang, R. Jonnalagadda, V. Mahadev, T.S. Cale, J.T. Hillman, R.F.

Foster, Mater. Res. Soc. Symp. Proc. 472 (1997) 337.

[16] S. Vaidya, D.B. Fraser, A.K. Sinha, Proc. 18th Annu. Reliability

Physics Symp., IEEE, New York, 1980, p. 165.

[17] D. Wilcox, B. Dove, D. McDavid, D. Greer, UTHSCSA Image Tool

for Windows (Version 2.00), University of Texas Health Science

Center in San Antonio, San Antonio, TX.


Documents

Texture and surface roughness of PRCVD aluminum films