Atomic layer chemical vapour deposition of copper

Atomic layer chemical vapour deposition of copper

ARTICLE IN PRESS Materials Science in Semiconductor Processing 7 (2004) 343–347 Atomic layer chemical vapour deposition of copper Anil U. Mane, S.A...

376KB Sizes 0 Downloads 127 Views

ARTICLE IN PRESS

Materials Science in Semiconductor Processing 7 (2004) 343–347

Atomic layer chemical vapour deposition of copper Anil U. Mane, S.A. Shivashankar Materials Research Centre, Indian Institute of Science, Bangalore-560 012, India Available online 21 October 2004

Abstract Copper films with (1 1 1) texture are of crucial importance in integrated circuit interconnects. We have deposited strongly (1 1 1)-textured thin films of copper by atomic layer deposition (ALD) using [2,2,6,6-tetramethyl-3,5heptadionato] Cu(II), Cu(thd)2, as the precursor. The dependence of the microstructure of the films on ALD conditions, such as the number of ALD cycles and the deposition temperature was studied by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy. Analysis of (1 1 1)-textured films shows the presence of twin planes in the copper grains throughout the films. SEM shows a labyrinthine structure of highly connected, large grains developing as film thickness increases. This leads to low resistivity and suggests high resistance to electromigration. r 2004 Elsevier Ltd. All rights reserved. Keywords: Copper; (1 1 1) texture; ALD; Cu(thd)2; TEM; Morphology

1. Introduction Copper has been rapidly replacing aluminium-based alloys in the metallization of very large-scale integrated circuits (VLSI). This is due to its higher conductivity that results in reduced RC delays and to its greater electromigration resistance. The presently preferred process used for copper metallization is electrochemical, and employs seeding to initiate copper plating. It may be expected that, as circuit integration moves into the deep sub- micron regime, a vapour phase process, such as chemical vapour deposition (CVD), would be require to cover the high aspect ratio features in the circuits. It has been known that (1 1 1)-textured Cu films with large grains would be most desirable for device metallization, as this maximizes electromigration resistance [1]. Furthermore, the (1 1 1) surface of Cu also Corresponding author. IHP-Microelectronics, Im Technologie Park- 25, Frankfurt (Oder) 15236, Germany. Tel.: 0049 335 56 25 354; fax: 0049 335 56 25 681. E-mail address: [email protected] (A.U. Mane).

offers a higher oxidation resistance. Therefore, the development of (1 1 1)–texture in copper has been the focus of recent investigation. The texture of Cu thin films deposited by various physical and chemical processes has been studied [2–5]. Although there are a few reports on the atomic layer deposition (ALD) of copper films [6,7], the deposition of (1 1 1)-oriented films by ALD does not appear to have been reported to date. ALD offers certain advantages over CVD, especially for high-aspect-ratio via filling, due to the self-limiting surface adsorption that occurs in ALD, leading to highly conformal coverage. The ‘‘digital’’ control over film thickness that ALD provides is another advantage. We report here the ALD of Cu thin films on oxidized Si(1 0 0) surfaces using a Cu(II) b-diketonate complex as precursor. These films exhibit strong (1 1 1) texture, large grain size, as well as high conductivity.

2. Experimental Depositions were carried out in an automated ALD system designed and fabricated in house specifically for

1369-8001/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2004.09.094

ARTICLE IN PRESS A.U. Mane, S.A. Shivashankar / Materials Science in Semiconductor Processing 7 (2004) 343–347

344

use with subliming solid precursors [8]. The copper complex used as precursors was bis [2,2,6,6,-tetramethyl3,5-heptadionato] Cu(II), which was vaporized at 120–1401C. Ultra-high purity (UHP, 99.999%) argon was used as the carrier gas, UHP hydrogen as the reducing (reactant) gas, and UHP nitrogen as the purge gas in the ALD process. The substrates, viz., oxidized Si(1 0 0) and glass, were cut into 2 cm2 pieces, and cleaned by standard procedure. They were placed in the ALD reactor on a susceptor heated by a quartz halogen lamp. The pressure in the reactor was measured by a heated capacitance manometer, and maintained at 2660 Pa (20 Torr) through a motorized throttle control valve in a feedback loop with the capacitance manometer. The reactants and the purge gas were admitted in 5-s pulses through air actuated pneumatic valves capable of operating at temperatures up to 200 1C. The adhesion of films to the substrates was checked by the adhesive tape peel test, showing that films show good adhesion.

The powder X-ray diffraction patterns of the films deposited on glass at 350 1C, as a function of number of ALD cycles (film thickness), are shown in Fig. 2. It is clear that, even at a low thickness (40 nm) the films exhibit (1 1 1) texture, which grows stronger as film thickness increase. This is represented quantitatively in Fig. 3, where the ratio of the intensities of (1 1 1) and (200) reflections is plotted as a function of the number of ALD cycles. The extent of preferred orientation increases monotonically with film thickness. The textured growth of films on amorphous substrates may be interpreted in terms of minimization of the total surface energy of the film [9–11]. According to this hypothesis, that orientation of the film is preferred in which crystallographic plane with the highest atomic density is parallel to the growth surface. This happens to be (1 1 1) plane of FCC copper. The establishment of such preferred orientation is perhaps aided by the higher mobility of the nuclei at the rather elevated temperature of growth in a chemical process such as ALD. Such texturing is furthered as the number of ALD cycles increased. In effect, as the deposition continues, the

3. Results and discussion

Cu(111)

2000 1600 1200 Cu(200)

Intensity (a.u.)

800

(iv) 1000

400

(iii) 720 (ii) 450 (i) 300

0 30

150

50

70

80

90

Si(400)

Cu(200)

2000 1500

no. ALD cycles

1000 (iv) 1000 (iii) 720 (ii) 450 (i) 300

500

100

60

2θ (Deg.) 3000

Intensity (a.u.)

Thickness (nm)

200

40

(a)

2500

250

no. ALD cycles

Cu(111)

Film depositions were carried out on SiO2/Si (1 0 0) surfaces, as a function of various ALD parameters, i.e., substrate temperature, number of complete ALD cycles, and the relative duration of the pulses of the different reactants. Average film thickness was measured by the weight gain method and confirmed by cross-sectional SEM. The variation of film thickness, with the number of ALD cycles is shown in Fig. 1, for the films grown at 350 1C. It is seen that film thickness varies linearly with the number of ALD cycles, which is the signature of the ALD process. The average thickness of copper deposited per cycle (film deposited with 720 ALD cycles), as deduced from Fig. 1, is 150 nm, implying that the average thickness of copper deposited per cycle is about two monolayers.

0 40

50

(b) 200

400 600 800 Total no of ALD cycles

50

60

70

2θ (Deg.)

1000

Fig. 1. Thickness of copper thin films grown on SiO2/Si(1 0 0) substrate as a function of the number of ALD cycles. (Deposition temperature 350 1C, reactor pressure 2660 Pa).

Fig. 2. XRD patterns of Cu films grown on (a) Glass and (b) SiO2/Si(1 0 0) substrate simultaneously, as a function of total number of ALD cycles (Deposition temperature 3501C, reactor pressure=2660 Pa; number of ALD cycles is given in the inset of the figure).

ARTICLE IN PRESS A.U. Mane, S.A. Shivashankar / Materials Science in Semiconductor Processing 7 (2004) 343–347

underlying layers are annealed in situ, leading to strong preferred orientation. The (1 1 1) texture of the ALD-grown copper films was studied further by transmission electron microscopy (TEM). The Fig. 4(a) shows a bright field image of a film deposited at 350 1C, comprising grains measuring from 50 nm to 300 nm, the larger ones resulting from the coalescence of several grains. The presence of twins can be seen in several of the grains in this image. A

[Cu(111)/Cu(200)] Intensity ratio

20 glass Si(100)

15

10

5

400

800 Total no of ALD cycles

1200

Fig. 3. Intensity ratio of Cu(1 1 1) and Cu(200) reflections, in Cu films deposited simultaneously on glass and SiO2/Si(1 0 0) substrates, as a function of total number of ALD cycles (film thickness).

345

magnified image of one of the grains, which exhibits a high degree of twinning, is given in Fig. 4(b). Twinning is observed at about 45 1, where lies the lowest energy slip plane for (1 1 1)-oriented grains, meaning that the twins are generated from (1 1 1)-oriented grains. The generation of the twins could also be due to stress in the films, or to the continuous in situ annealing of underlying layers that takes place during a long deposition run, as referred to above. The presence of twins is also demonstrated by TEM diffraction pattern of the copper grain containing twin planes (Fig. 4(c)), indicating grain growth along the (1 1 1) direction. The grain from which this pattern is taken is almost spherical, located near the center of the films. The double bright spots in the patterns shown in Fig. 4(d) confirm that the grains contain strongly (1 1 1)–oriented twin planes and hence confirm the strong (1 1 1) texture of the copper film. The morphology of the films was examined by SEM as a function of film thickness, as shown in Fig. 5(a–d). It is seen that at small thickness, the growth is islandtype, with poor connectivity between the grains of fairly uniform size. As the film thickness increases connectivity develops between grain, leading to a labyrinthine pattern of grains. The connectivity is very high in the thickest films even though there are voids in the films. Such connectivity may be expected to lead to low resistivity in the thicker films. Indeed, the film grown at 350 1C for 720 ALD cycles has a room temperature resistivity of 1.7 mO cm, effectively the bulk value,

200 200 nm nm

(c)

(a)

100 nm

(b)

(d)

Fig. 4. TEM analysis of Cu film grown on SiO2/Si(1 0 0). (a) Bright field image at low magnification (b) bright field image at high magnification showing twin planes (c) electron diffraction pattern which shows the presence of (1 1 1) texture and (d) electron diffraction pattern which shows the twinning present in the Cu film, in agreement with Fig. 4(b). (Deposition temperature 350 1C, reactor pressure 2660 Pa).

ARTICLE IN PRESS 346

A.U. Mane, S.A. Shivashankar / Materials Science in Semiconductor Processing 7 (2004) 343–347

(a)

(b)

(c)

(d)

Fig. 5. SEM micrographs of Cu films grown on SiO2/Si(1 0 0) substrate using (a) 300 (b) 450 (c) 720 and (d) 1000 ALD cycles under identical conditions (Deposition temperature 350 1C, reactor pressure=2660 Pa).

indicating that the film is chemically pure (although no attempt was made to analyse the composition of the films). It is surmised that connectivity develops due to the considerable mobility of the copper atoms deposited, and that this mobility is enhanced by the heats of enthalpy of the chemical reactions leading to copper deposition. It must be noted that, although the connectivity among grains makes it difficult to specify a grain size, the average grain size is large in the thickest films, perhaps as much as 5–10 mm. This, together with the strong (1 1 1) texture, implies that the electromigration resistance of ALD–grown copper films would be quite high. Deposition of copper by ALD was also carried out simultaneously on two other substrates, i.e., glass and TiN-coated Si(1 0 0), which were placed along side the SiO2/Si(1 0 0) substrates. Strong (1 1 1) texture was observed in the Cu films grown on these other substrates as well. However, growth on the TiN surface was slow compared to growth on the other substrates. It is possible that the growth rate on TiN may be improved by choosing a different copper precursor.

4. Conclusion ALD using a Cu(II) b-diketonate complex as precursor has been carried out to obtain thin copper films on various substrates. The films grow with a strong (1 1 1) texture that gets stronger with increasing film

thickness. Although the films have a void structure, the connectivity among the grains is very high in the thicker films, leading to high conductivity and a large average grain size. These characteristics make ALD-grown Cu films very suitable for VLSI metallization. However, as growth rates in ALD process would be too low to be useful for metallization, it is proposed that ALD–grown Cu deposit may use seed layers for subsequent filling out of high aspect ratio features in VLSI circuits.

Acknowledgement The authors thank Dr. G.N. Subbanna for the TEM micrographs. One of the authors (AUM) thanks the University Grants Commission, New Delhi, for a research fellowship.

References [1] Vaidya S, Sinha AK. Thin Solid Films 1981;75:253. [2] Hurd JL, Rodbell KP, Gignac LM, Clevenger LA, Iggulden RC, Schnabel RF, Weber SJ, Schmidt NH. Appl Phys Lett 1998;72:326. [3] Tracy DP, Knorr DB, Rodbell KP. J Appl Phys 1994;76:2671. [4] Huang H, Gilmer GH, Diaz de la Rubia T. J Appl Phys 1998;84:3636. [5] Nucci JA, Keller RR, Sanchez Jr. JE, Diamand YS. Appl Phys Lett 1996;69:4017.

ARTICLE IN PRESS A.U. Mane, S.A. Shivashankar / Materials Science in Semiconductor Processing 7 (2004) 343–347 [6] Ma˚rtensson P, Carlsson J-O. J Electrochem Soc 1998; 145:2926. [7] Solanki R, Pathangey B. Electrochem Solid-State Lett 2000;3:479. [8] Mane AU. Atomic Layer Deposition (ALD) and Metalorganic Chemical Vapour Deposition (MOCVD) of Metals

347

and Metal Oxides, Ph.D. Thesis, Indian Institute of Science, Bangalore, India, 2003. [9] Smith HI, Flanders DC. Appl Phys Lett 1978;32:349. [10] Yoon JG, Oh HK, Lee SJ. Phy Rev 1999;B60:2839. [11] Mane AU, Shivashankar SA. J Cryst Growth 2003; 254:368.