Electrical characterization of reactively sputtered TiN diffusion barrier layers for copper metallization

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applied

surface science ELSEVIER

Applied Surface Science 91 (1995) 291-294

Electrical characterization of reactively sputtered TiN diffusion barrier layers for copper metallization C. Kaufmann *, J. Baumann, T. Gessner, T. Raschke, M. Rennau, N. Zichner Zentrum fiir Mikrotechnologien, Fakultiit fiir Elektrotechnik und lnformationstechnik, TU Chemnitz-Zwickau, 09107 Chemnitz, Germany

Received 21 March 1995; accepted for publication 21 April 1995

Abstract TiN films were characterized by sheet resistance measurements, Auger electron spectroscopy and cross-sectional transmission electron microscopy. The properties as diffusion barrier between copper and silicon were investigated by diode leakage current measurements on n+p diodes after aimealing at 350°C for 30 min and at 450, 500 and 550°C for 60 rain. Both Ti-rich and N-rich TiN films were deposited at a DC magnetron power of 8 kW. Furthermore, additional N-rich films were deposited at a DC magnetron power of 2 kW. The copper was then deposited by metalorganic low pressure chemical vapour deposition and by sputtering. Samples with and without a diffusion barrier were prepared. N-rich films deposited at DC magnetron powers of 2 and 8 kW are found to be an effective barrier up to an annealing at 500°C for 60 min in case of metallization with sputtered copper. On the other hand the Ti-rich barriers still fail after annealing at 450°C for 60 min. The barrier structures metallized with copper deposited by metalorganic low pressure chemical vapour deposition are almost broken even at lower temperatures.

1. Introduction C o p p e r has attracted m o r e and more attention as a new metallization material because o f its low electrical resistivity c o m p a r e d to conventional metallization materials and its expected good resistance to electromigration. Further advantages are the excellent step coverage and the possible selective deposition [1] which are achieved by use o f metalorganic low pressure chemical vapour deposition. H o w e v e r a diffusion barrier is necessary between the copper metallization and the silicon substrate to prevent deterioration o f devices. This is due to the high diffusity, the introduction o f deep levels in silicon [2]

* Corresponding author. Tel.: +49 371 531 3276; Fax: +49 371 531 3131.

and to the reaction with silicon at already 200°C resulting in formation o f Cu3Si crystallites [3-5]. TiN among other materials shows very interesting properties like low bulk resistivity and high thermal and thermodynamical stability making it compatible to semiconductor technology [6-12]. T i N films are already successfully used as diffusion barrier between aluminum and its alloys [6,9,10,12-16] and silicon. Furthermore, because o f the above-mentioned problems, the barrier properties o f TiN to prevent interaction between copper and silicon were studied by a lot o f groups using various methods [5,17-20]. This work will characterize the thermal stability o f TiN diffusion barriers to sputtered copper and copper deposited by metalorganic l o w pressure chemical vapour deposition by leakage current measurements.

0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00133-6

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2. Experimental All investigations were done on boron-doped Si(100) substrates with a resistivity of 12-20 I~- cm. For sheet resistance measurements, Auger electron spectroscopy and cross-sectional transmission electron microscopy a 100 nm SiO 2 layer was deposited prior to deposition of TiN. The TiN films were deposited by reactively sputtering Ti (target purity 99.9999%) in a mixture of Ar (99.9999%) and N 2 (99.9999%) at a constant total pressure of 2 × 10 -3 mbar. The used batch sputter system HZSU-03 (Hochvakuum Dresden) allows to process 12 wafers, situated on a rotating pallet, at the same time and it is equipped with a cryo-pump and a LN 2 Meissner trap near the pallet. Prior to the reactive process pure titanium was sputtered in a dulnmy process to improve the reproducibility of the experiments. Starting at a base pressure below 2 × 10 - 7 mbar the substrates were heated up to a temperature of 300°C, as measured by a thermocouple instrumented wafer. Now the power of the lamp heater was held constant and the deposition process was started at a pressure of below 3 × 10 - 7 mbar with the inlet of a prescribed nitrogen partial pressure and adding argon. For resistivity measurements, structural and analytical investigations on unpatterned samples the nitrogen partial pressure was varied between 4.6 × 10 - 4 and 6.5 × 10 -4 mbar at a DC magnetron power of 8 kW and between 1.5 × 10 - 4 and 6.5 × 10 -4 mbar at a DC magnetron power of 2 kW. The TiN films were of 100 nm thickness. The n+p diodes were prepared by implanting As + ions at 160 keV with a dose of 1 × 1014 c m - 2 through a Si3N4/SiO 2 (30 n m / 3 0 nm) double layer followed by an annealing at 900°C for 30 min in pure nitrogen atmosphere. The resulting junction depth was 0.2 /xm. The following leakage current measurements were done using diodes with a size of 8 0 0 / z m × 7 0 0 / z m at a reverse bias voltage of 5 V. The patterned samples were cleaned in a dilute HF solution (HF : H 2 0 = 1 : 100) for 3 min and rinsed in deionized water before loading in the HZSLI-03 load lock. In all cases the vacuum was broken after deposition of the TiN films. Copper films were fabricated in a Materials Research Corporation Series 643 sputter system (MZ grade target) with a DC magnetron power of 3 kW, Ar (99.9999%) pressure

2 × 10 -2 mbar and base pressure below 2 × 10 - 7 mbar. Alternatively copper was deposited by metalorganic low pressure chemical vapour deposition from the precursor Cu(hexafluoroacetylacetonate)trimethylvinylsilane (Cu(hfac)TMVS) at 200°C substrate temperature [21]. The copper films were about 500 nm. All annealing steps were done in a quartz tube furnace. Unpatterned samples were annealed for 60 min at temperatures of 450 and 700°C in a N2(80%):H2(20%) atmosphere. Patterned samples were annealed in pure hydrogen atmosphere for 30 min at 350°C and for 60 min at temperatures of 450, 500 and 550°C.

3. Experimental results The properties of TiN films deposited at a DC magnetron power of 8 kW and examined on T i N / S i O 2 structures can be summarized as follows. The N / T i ratios calculated from Auger electron spectroscopy depth profiles changed between 0.9 and 1.1 when the nitrogen partial pressure was varied in the range from 4.6 × 10 - 4 to 6.5 × 10 - 4 mbar. About 1 at% carbon is present in the films. The oxygen Content increases with increasing nitrogen content from 0.3 at% for Ti-rich to 4 at% for N-rich films. The specific electrical resistivity is a very sensitive function of the film stoichiometry [6,9,11,22,23] and varied in our case from 80 ~12. cm to 160 / z f l . c m . The expected minimum at a N / T i ratio equal to one, is found at a nitrogen partial pressure of 5.2 X 10 - 4 mbar. At this partial pressure also the colour of the TiN films changes from metallic silver to yellow gold and later (6.5 × 10 -4 mbar) to brown. The change of the specific electrical resistivity after annealing in N 2 : H 2 atmosphere depends on the former N / T i ratio. For Ti-rich films an annealing at 450°C results in a slightly decreased specific electrical resistivity, while N-rich films show a slightly increased value. The resistivity of the Ti-rich films after annealing at 700°C reaches almost the minimum value of as-deposited stoichiometric films accompanied by a colour change to gold. The cross-sectional transmission electron microscopy micrographs show that both Ti-rich and

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Fig. 1. Cross-sectional transmission electronic micrograph of a TiN film o n substrate surface (arrows and letters).

N-rich films have a columnar structure with grain sizes between 9 and 15 nm. This columnar growth starts at about 25 nm thickness if TiN deposited on SiO 2 (Fig. 1). All films show fine channels orientated vertically to the surface, similar to the " o p e n spaces" found in Refs. [11,17]. An interface layer between SiO 2 and TiN could not be found. Based on this results we have chosen the deposition parameters for the diode preparation. A DC magnetron power of 8 k W with a nitrogen partial

SiO2

as-deposited showing fine channels vertically to the

pressure of 5.1 × 10 -4 and 6.5 X 10 -4 mbar leads to 180 nm (Ti-rich) and 120 nm (N-rich) thick films, respectively. A DC magnetron power o f 2 kW at a nitrogen partial pressure o f 1.5 × 10 -4 mbar leads to a 90 nm (N-rich) thick film. The maximum leakage current densities for the different metallizations and annealing steps are shown in Fig. 2. It can be summarized as follows. Already for the as-deposited samples we find a different behaviour. While for Ti-rich films the leakage cur-

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il

Cu

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8

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Fig. 2. Maximum leakage current densities of Cu/(TiN/) n +p diodes after the sequential annealings.

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c. Kaufmann et al. / Applied Surface Science 91 (1995) 291-294

rent densities are in the range o f 10 -8 A / c m 2 the values for N-rich films are in the o f 10 - 9 A / c m 2. The Ti-rich barrier metallized by copper deposited by metalorganic low pressure chemical vapour deposition differs, but note that there is a thermal load for about 30 min during deposition. After annealing at 350°C for 30 min the copper metallized diodes without barrier layer are broken down. The leakage current densities o f all Ti-rich barrier structures are now in the same range o f 10 -8 A / c m 2. But in comparison with the N-rich ones they are still larger. After annealing at 450°C for 60 min the leakage current densities o f the by metalorganic low pressure chemical vapour deposition fabricated copper on Tirich TiN show significantly increased values indicating the failure o f these structures. After annealing at 500°C for 60 min, the N-rich barriers metallized by metalorganic low pressure chemical vapour deposited copper begin to break down with a current density increase o f about one magnitude o f order. But the structures metallized by sputtered copper are still stable. After annealing at 550°C for 60 min dramatically increased leakage current densities indicate that these remaining diodes are broken down now.

4. Conclusions A direct metallization o f silicon with copper resuits in the expected [ 2 - 5 ] breakdown o f the n ÷ p diodes after the first anneal at 350°C for 30 min. For deposition at 8 k W D C magnetron power the diodes with the Ti-rich barriers are broken even at lower temperatures c o m p a r e d to the N-rich barriers, although they were 180 and 120 nm thick, respectively. This improved barrier behaviour o f N-rich TiN caused by nitrogen and additionally oxygen stuffed at the defects resulting in blocked pathways was proposed by Nicolet [8]. It is found for copper [17,18] and aluminum [12,14,16,18] independent on the TiN deposition technologies. A n influence o f the DC magnetron power on the barrier properties of N-rich films is not clearly found but it is possibly overlapped by the effects resulting from the different film thicknesses [5]. A dependency o f the breakdown

on the copper metallization technology is found. Additional analytical investigations are needed to determine the failure mechanism.

Acknowledgements This work was supported by the Federal Department o f Research and Technology of the Federal R e p u b l i c o f G e r m a n y ( B M B F p r o j e c t No. 01M2933A).

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