TiN diffusion barriers for copper metallization

! Si;l Microelectronic Engineering 37/38 (1997) 221-228

ELSEVIER

TiN diffusion barriers for copper metallization J. B a u m a n n a'*, T. W e r n e r a, A. E h r l i c h b, M. R e n n a u a, Ch. K a u f m a n n a, T. G e s s n e r a

~Centre of Microtechnologies, Chemnitz University of Technology, 09107 Chemnitz, Germany hlnstitute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany

Abstract Selected properties concerning a possible Cu/TiN metallization were examined. TiNx films of 40 nm thickness with a N/Ti ratio between 0.59 and 1.01 were deposited on different substrates (HF cleaned Si, Si with native oxide and thermally grown Sit z). The investigated properties are adhesion, stress, microstructure, texture and resistivity, with respect to substrate and environmental effects. Adhesion, stress and texture of the Cu films deposited on TiNx were investigated. The barrier reliability of TiN films is tested. The result of a barrier test performed depends on environmental effects for the Cu/TiN/Si system. TiN is an effective diffusion barrier up to 650°C in hydrogen and 550°C in vacuum by electrical and analytical methods, whereas it fails after 450°C annealing in nitrogen.

Keywords: TiN; Cu; Barrier; Adhesion; Stress; Texture; Resistivity; Sputtering

1. Introduction New m a t e r i a l s - - c o p p e r and low k dielectrics--are being considered as candidates to meet the requirements within the interconnection scheme of future microelectronic devices [1,2]. The use of copper as a new metallization material will result not only in advantages but also in problems like e.g. enhanced corrosion sensitivity, the formation of deep levels in silicon, Cu3Si formation and poor adhesion [3-9]. For these reasons there is the demand to inhibit the interaction between copper and the adjoining system parts in both directions by use of a reliable barrier. Potential and applicability are successfully shown for a series of barriers [10]. TiN, a well known and widely used barrier compatible with semiconductor technology, is one candidate to fulfil these requirements. H o w e v e r according to Nicolet [11] beside reliability the requirements of a barrier will include its effect on the whole metallization system. Therefore this work deals not only with the barrier potential o f thin TiN films, but also with selected properties o f the whole metallization system.

2. Experimental The investigations were performed on four inch (100) p-type Si wafers as substrates. Samples with and without native oxide and thermally oxidized wafers were used. The oxide thicknesses were 2.3 nm and 106 nm, respectively, measured by ellipsometry ( A - - 6 3 2 . 8 nm). In addition n+p diodes *Corresponding author: Fax: + 49 371 531 3131; e-mail: [email protected] 0167-9317/97/$17.00

Copyright © 1997 Elsevier Science B.V. All rights reserved. PIIS0167-9317(97)00115-9

222

J. Baumann et al. / Microelectronic Engineering 37/38 (1997) 2 2 1 - 2 2 8

(xj = 400 nm) were used to characterize the barrier properties of TiN x. To ensure comparable results all samples of a chosen N/Ti ratio were coated simultaneously in one process. The native oxide was removed by an HF dip prior to the deposition process. The TiN films of about 40 nm thickness were deposited in the batch DC magnetron sputtering system HZSU-03 (Hochvakuum Dresden) by sputtering (P = 2 kW) titanium reactively in a Ar + N 2 mixture at a pressure of 0.2 Pa and a substrate temperature of 300°C. The base pressure of the process chamber was below 2 x 10 -5 Pa. After barrier deposition, the wafers were exposed to air and selected samples were loaded into a MRC 643 batch sputtering system. Starting at a base pressure below 1 × 10 -5 Pa Cu was then deposited by DC magnetron sputtering in Ar at a pressure of 2 Pa and a power of 3 kW. The specific electrical resistivity is about 1.95 ~lq cm for 500 nm thickness. Further details on the preparation are given in [12]. The film and barrier properties of T i N were investigated in the as deposited state and after step by step annealing in H 2 and N 2 atmosphere, respectively. The films were characterized by sheet resistance measurements (RS 50e, Prometrix Inc.), Rutherford Backscattering Spectroscopy (RBS), Scanning Electron Microscopy (SEM), Cross Sectional Transmission Electron Microscopy (XTEM), X-ray Diffractometry (XRD) and Atomic Force Microscopy (AFM). The Cu/TiN x stacks were characterized by XRD. Stress and adhesion measurements were performed on samples coated with TiN x, Cu and Cu/TiN (N/Ti = 1.01). The film stress was measured in a FLX 2900 (Tencor Inc.). The adhesion tests were performed according to [13], by varying the adhesion strength of the tape between 5 N/25 mm and 13 N/25 mm. The characterization of the barrier properties was performed by Electron Microprobe Analysis (HEPMA) during thermal annealing in vacuum (p ~ 1 0 - 3 Pa) [14] and by leakage current measurements. The diode characteristics were measured between - 1 V and 10 V after each annealing step on an automatic wafer prober equipped with a SMU 236 (Keithley Inc.).

3. Results and discussion

3.1. Characterization of the deposited films Numerous publications provide a fundamental database concerning TiN. However the above mentioned interaction between barrier and on-deposited copper requires the description of the whole metallization system in order to get information about compatibility and reliability. Therefore we will shortly characterize the prepared films. The stoichiometry of the films was between 0.59 and 1.01 as measured by RBS. The TiN x films consist of columnar grains with a grain size between 7 nm and 14 nm (Fig. la). The grain size is independent from the N/Ti ratio within the process parameters used. All investigated films show fine channels orientated vertically to the substrate. Moreover films deposited in the same equipment with a fourfold DC power increase and other equal conditions are of the same structure and grain sizes [15]. Since XTEM illustrates only a small cut out of the surface parameters, the samples were further investigated by AFM (Fig. lb, c). Neither roughness nor surface structure are dependent on the chosen substrate (Si or SiO2). The films are very smooth. The roughness R, is about 2 nm within a scanlength of 500 nm. The microstructure was investigated by Grazing Incidence Diffraction (GID) and Bragg-Brentano Diffraction (BB) mode (Fig. 2) by a PTS 3000 (Seifert FPM) and a XRD 7

J. Baumann et al. / Microelectronic Engineering 37/38 (1997) 2 2 1 - 2 2 8

223

Fig. 1. XTEM micrograph of a TiN (N/Ti = 1.01) film deposited on Si (a). Surface reliefs of TiN (N/Ti = 1.01) deposited on (b) silicon and (c) silicon dioxide. The film morphology is not affected by the substrate choice.

(Seifert FPM), respectively. The films exhibit the fcc-TiN phase. Beside a small increase in (111) texture with an increasing N/Ti ratio there is no influence of the stoichiometry using bare silicon substrates (with native oxide) or thermally oxidized substrates (Fig. 2a, b). Although the tendency is clear, it should be noted that the peak intensities are very small, because of the reduced effective diffracting volume of the film. The texture is clearly changed to {100) if TiN is deposited onto HF cleaned samples. An increased N/Ti ratio results in a higher intensity of the TiN (200) peak. It is not clear, if this is caused by a stronger texture and/or an improved crystalline quality of the film. Comparing the signal to noise ratios of all samples, we conclude that the cause is a lower degree of crystallization for the oxidized substrates since the film thickness was held constant. Fig. 3a illustrates the resistivity as a function of the N/Ti ratio, which is typical for TiN films. However the influence of annealing temperature and ambient atmosphere on the resistivity of TiN on thermally oxidized Si shows remarkable differences. Annealing in H 2 at temperatures up to 500°C results in small increased values especially for higher N/Ti ratios. A sharp increase is found after annealing at 900°C. In contrast to that behaviour the resistivity increase already starts after annealing at 450°C if the ambient is N 2, and exceeds the upper limit of measurement (500 kl)/sq.) after annealing at 900°C. The composition is now 67% oxygen and 33% titanium, independent of the former N/Ti ratio. The nitrogen content is below the detection limit, indicating that the former TiN, has completely reacted to TiO 2. We can exclude any interaction between TiN, and SiO 2 since the film compositions were measured on HF cleaned Si substrates. The oxidation of TiN is reported at 350°C-800°C even for as low as 0.1% 0 2 content [16-18]. Oxygen is also present in TiN films formed by RTP [19,20]. Therefore we conclude that the oxygen was taken from the annealing atmosphere. All films have a compressive stress in the as deposited state (Fig. 3b). This stress is increased after annealing at 450°C

224

J. Baumann et al. / Microelectronic Engineering 37/38 (1997) 221-228

TIN(111)

TIN(200)

~

1,ol

~

0,95

o,88 •

0,78

0,59 28

I

I

I

33

38

43

A)

2 theta / ~

-Jr . . . . . .

48

degree

~

1,01

~

0,95

o,88 4v ~-v w,.Jw

•

~---..~v

v~.~

0,78 0,59

B)

28

33

43

48

2 theta / degree Cu(lll) TIN(200) Cu(200)

r

.~

........

32

C)

38

37

0,9

42

47

52

57

62

2 theta/ degree

Fig. 2. X-ray diffraction patterns of TiNx films deposited on different substrates. (a) TiNx on 106 nm thermally grown SiO 2 (GID). (b) TiNx on Si with native oxide (2.3 nm) (GID). (c) Cu(100 nm)/TiN film stacks on HF cleaned Si (BB). Parameter is the N/Ti ratio.

in N 2. The complete oxidation after 900°C annealing in N 2 causes a turnover to a tensile stress. No formation o f pores or microcracks could be observed by SEM before and after annealing steps. The film adhesion is excellent on both Si and SiO 2. No peeling or removal takes place up to 13 N / 2 5 m m adhesion strength o f the tape. Copper films do not adhere on SiO 2. The failure is observed by the total removal beyond the area of the x-cut even if the adhesion strength of the tape is decreased to 5 N / 2 5 mm. This behaviour remains unchanged even if the substrates are heated up (200°C) in vacuum to desorb water molecules off the surface prior to deposition, as suggested in [3]. The adhesion is very good on HF cleaned Si substrates. A trace peeling or removal along incisions is found for 13 N / 2 5 m m adhesion strength of the tape. The tensile stress found is about 200 MPa in the as deposited state.

J. Baumann et al. / Microelectronic Engineering 37/38 (1997) 221-228 annealing at 900°C, 60 minutes, H 2 (~fJcm): 997.8 527.4 655.2 904.6

1275

225

2129.4

7'° T j

#

550~

..

450"C,60' N 21

/

- I - - 5000C,60' N,

/~

• 45o*c,~'i~ I

r/

3 5 0 <

/

I

150

A)

,o . o.,9

.

.

.

.

o,

.

.

. o.,,

.

.

o,,

.

1.o,

N / Ti

~1 12o~

--

'~ soo' ......

~ "

. i

. . . . . . . ,.. . . . . . . .

t-~-4so*c, 6o'N~t . -*-~'C,~O'N d

.

.

!

{

.

I / L

-ISOC .i..J]

0,59

:. . . . . . . . . . . . 0,78

i

~

0,9

........... 0,88

0,95

1,o1

N/Ti Fig. 3. Properties of TiN~ films as deposited and after thermal treatment in different ambients. (a) o-(N/Ti) characteristics (b) evolution of the film stress due to annealing in N2.

3.2. Cu/TiNx, metallization-resulting properties and barrier reliability The Cu films deposited on TiN x exhibit a preferred (111) orientation for nonoxidized and oxidized substrates, respectively. Even the strong (100} orientation of TiNx deposited on HF cleaned Si does not alter this behaviour (Fig. 2). However an increased intensity of the TiN (200) peak causes a higher intensity of the Cu (200), as well. An additionally performed annealing of C u / T i N / S i O 2 structures in vacuum (600°C, 60') causes no significant changes. The adhesion of Cu on TiN x is comparable to Cu on HF cleaned wafers. The Cu film stress calculated by taking an already T i N coated wafer as substrate is about 200 MPa. This is in agreement with the measured results obtained for Cu films on Si and SiO2/Si, respectively. After annealing at 450°C for 60 min in H 2 the film stress is increased by one order of magnitude. Furthermore the adhesion is slightly decreased. The annealing ambient affects the TiN x properties remarkably. Therefore the annealings of the samples for barrier testing were performed in N 2, in H 2 and under vacuum. Stoichiometric TiN films are stable diffusion barriers up to 650°C in H 2. Even if the thickness is reduced down to 20 nm, no

226

J. Baumann et al. / Microelectronic Engineering 37/38 (1997) 221-228

interaction is detectable by either a change of the diode leakage current or by SEM observation of the Si surface after removing the metallization. TiN x films containing excess Ti, including pure Ti films, fail at lower temperatures. Randomly distributed reaction spots appear on the Si surface and cause increased leakage currents of the diodes. These results are discussed in detail in [ 11 ]. The investigations under vacuum conditions were performed by monitoring the X-ray intensity of Cu, Ti and Si during thermal treatment of unpatterned HF cleaned wafers. Pure Ti (N/Ti = 0) fails clearly after about 30 min annealing at 450°C (Fig. 4a). The samples with TiN x inserted between Cu and Si remain stable up to 550°C, which is the maximum temperature in the measuring system (Fig. 4b). The annealing in N 2 atmosphere results in a wide spread of leakage current values (Fig. 5a). This behaviour is found for stoichiometric as well as for Ti rich films. The metallization begins to peel off from the insulator as well as from the Si. After removing the Cu the former TiN X film was not etchable in a H 2 0 2 / N H 4 O H / H a O solution [21]. Based on the above described results for single TiN x films we assume that TiO a was formed during annealing, which is known to be hardly etchable [22]. The layer was removable by performing a dry etch step in SF 6 followed again by the wet etching procedure. The Si surface is dotted with randomly distributed holes (Fig. 5b), indicating that a strong interaction took place and destroyed the structure.

°.__ zo 10 0

-

]

__

-----'--__-__---__-__-__)

mm

m

I

A)

o

20

80

,o ,o time / minutes

~>30,

°,.#

10-

\

B)

0

I

I

{

{

I

10

20

30

40

.~0

-cq • Tit 60

time / minutes

Fig. 4. Barrier testing by HEPMA during annealing in vacuum. The structure Si/Ti(40 nm)/Cu(100 nm) completely fails at 450°C after approximately 30 minutes (a), while the Cu(100 nm)/TiN(40 nm)/Si sample remains stable at 550°C (b).

J. Baumann et al. I Microelectronic Engineering 37138 (1997) 221-228

227

j A)

-1 0

1

2

3

4

S

6

7

|

9

10

v, aa~ lv~t] Fig. 5. A wide spread of the leakage current density values (as deposited values referring to 5 V: /z = 9.29 nA era-2; tr = 8.95 nA cm -z) occurs after annealing at 450°C in N2 for 60 minutes (a). The SEM micrograph (b) shows the Si surface of a diode after this treatment.

4. Conclusions Selected interaction problems within a Cu/TiN metallization were investigated. The results demonstrate TiN films to be a suitable barrier for copper interconnects. The poor adhesion of PVD-Cu o n S i O 2 is significantly enhanced by a TiN interlayer. The texture of TiN x films shows a clear dependence on substrate properties, which also results in a modified texture of the deposited PVD-Cu films. Annealing in different ambients results in modified resistivity and stress values, caused by film oxidation. Therefore, postulating the applicability of the stuffed barrier model [11] in the case of Cu, the decreased barrier thickness in future devices requires a well defined compromise between enhanced stability and the required film properties. The result of a barrier test performed depends on environmental effects. For annealing in H 2 or in vacuum our stability criterion is passed. No interaction is detectable up to 650°C and 550°C by electrical and analytical methods. In contrast with that behaviour, the structures fall after 450°C if annealed in N 2.

Acknowledgements The authors would like to thank W. H6sler (Siemens, Munich, ZFE, for RBS), U. Falke (XTEM), A. M611er and U. Rosseck (HEPMA) for sample characterization and for helpful discussions, as well as the team of the Centre of Microtechnologies at the Chemnitz University of Technology for general assistance. This work was supported by the Federal Department of Education and Research of the Federal Republic of Germany (BMBF project No. 01M2933A3).

228

J. Baumann et al. / Microelectronic Engineering 37/38 (1997) 2 2 1 - 2 2 8

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

Semiconductor Industry Association, The National Technology Roadmap For Semiconductors 1994. E Singer, Semiconductor International 19 (1995) 68. S.E Murarka, S.W. Hymes, Solid State Mater. Sci. 20 (1995) 87. M. O'Reilly, X. Jiang, J.T. Beechimoor, S. Lynch, C. Nf Dheasuna, J.C. Patterson, G.M. Crean, Appl. Surf. Sci. 91 (1995) 152. S.D. Brotherton, J.R. Ayres, A. Gill, H.W. van Kesteren, F.J.A.M. Greidanus, J. Appl. Phys. 62 (1987) 1826. H. Lemke, Phys. stat. sol. A 95 (1986) 665. K. Graft, Metal Impurities in Silicon-Device Fabrication, Springer-Verlag, Berlin, Heidelberg, 1995, p. 50. L. Stolt, A. CharM, F.M. D'Heurle, EM. Fryer, J.M.E. Harper, J. Vac. Sci. Technol. A 9 (1991) 1501. A. Cros, M.O. Aboelfotoh, J. Tu, J. Appl. Phys. 67 (1990) 3328. S.-Q. Wang, in: D.E Favreau, Y. Shacham-Diamond, Y. Horiike (Eds.), Advanced Metallization for ULSI in 1993, Materials Research Society, Pittsburgh, PA, 1994, p. 31. M.A. Nicolet, Thin Solid Films 52 (1987) 415. J. Baumann, Ch. Kaufmann, M. Rennau, Th. Wemer, T. Gessner, Microelectronic Engrg. 33 (1997) 283. ASTM D 3359-78 (x-cut tape test). E Gorfu, Ph.D. thesis, Dresden University of Technology (1992). C. Kaufmann, J. Baumann, T. Gessner, T. Raschke, M. Rennau, N. Zichner, Appl. Surf. Sci. 91 (1995) 291. H.G. Tompkins, J. Appl. Phys. 70 (1991) 3876. M. Wittmer, J. Noser, H. Melchior, J. Appl. Phys. 52 (1981) 6659. I. Suni, D. Sigurd, K.T. Hu, M.-A. Nicolet, J. Electrochem. Soc. 130 (1983) 1210. T.-S. Chang, W.-Z. Wang, F.-S. Huang, L.-E Wang, C. Hwang, Solid-State Electronics 38 (1995) 815. A.E. Morgan, E.K. Broadbent, A.H. Reader, in: Mat. Res. Soc. Symp. Proc. Vol. 52, Materials Research Society, Pittsburgh, 1986. S. Panjan, Thin Solid Films 175 (1989) 35. Brockhaus abc Chemie, Brockhaus Verlag Leipzig, 1987.