E L S E V I E R Thin Solid Films 292 (1997) I24-I29

.,d~.-,,g©dOd~

A comparative study on the properties of TiN films prepared by chemical vapor deposition enhanced by r.f. plasma and by electron cyclotron

resonance plasma

Jong-Seok Kim a, Byung-Hyuk Jun a, Eung-Jik Lee a, Chan-Yong Hwang b, Won-Jong Lee a ' *

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305 701, South Korea b Materials Evaluation Center, Korea Research Institute of Standard and Science, Taejon, South Korea

Received 21 February 1996; accepted 2 July 1996

Abstract

TiN thin films were prepared by both r.f. plasma enhanced chemical vapor deposition (r.f.-PECVD) and electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) using TiCI4, N2 and H2 as the reactants at various deposition temperatures. The effects of deposition temperature on the compositional ratio [N] / [Ti], impurity content, crystallinity, lattice parameter, grain size, deposition rate, resistivity and step coverage were studied. TiN films prepared by ECR-PECVD were highly crystalIized at a low temperature of 350 °C, while TiN films prepared by r.f.-PECVD began to show obvious crystallinity above 500 °C. TiN films deposited by ECR-PECVD at lower temperatures had lower impurity contents and lower resistivity than those deposited by r.f.-PECVD.

Keywords: Chemical vapour deposition (CVD) ; Metallization; Plasma processing and deposition; Titanium nitride

1. Introduction

TiN thin films have been used in the integrated circuit industry as a barrier to the interdiffusion of silicon and alu- minum. Until now, TiN has, most commonly, been deposited by the physical vapor deposition (PVD) method, sputtering. The sputtering technique, however, has a serious problem of poor step coverage and this becomes more serious as the contact size decreases and the aspect ratio of the structure increases. Therefore, there is substantial interest in replacing sputtering with chemical vapor deposition (CVD). Low pres- sure (LP) CVD of TiN using TIC14 and NH3 provides stoi- chiometric TiN films with excellent diffusion barrier properties, but in order to obtain films with low resistivity and low chlorine content, a high temperature of 600-700 °C is needed [ 1]. Metal-organic (MO) CVD processes using TDMAT and TDEAT have been applied to obtain films with good step coverage and reduced stress at low temperature, but they have the drawbacks of high impurity content and high resistivity [1]. Plasma enhanced CVD (PECVD) is very promising for obtaining high quality TiN films with good step coverage and low resistivity at lower deposition temper- atures, because the precursors can be sufficiently activated in

* Corresponding author.

the plasma. R.f. plasma enhanced CVD (r.f.-PECVD) of TiN has been widely used for the fabrication of hard coatings [2,3]. Recently, electron cyclotron resonance (ECR) PECVD has been tried to obtain TiN films with low resistivity and good bottom coverage at low deposition temperatures [4,5].

In the present work, we adopted both r.f. and ECR plasmas to prepare TiN films using TIC14, N2 and Ha as reactants, and analyzed comparatively the effect of the deposition temper- ature on the compositional ratio [N] / [Ti], impurity content, crystallinity, lattice parameter, grain size, deposition rate, resistivity and step coverage.

2. Experimental details

The schematic diagrams of the r.f.-PECVD system and the ECR-PECVD system used in this study are depicted in Fig. l(a) and (b).

The r.f.-PECVD system is a conventional planar capacitive type consisting of a reaction chamber, an r.f. power supply, a gas feeding system and a pumping system. TIC14 and Na gases were used as reactants and H2 gas as a reducing agent. TiN films were deposited at various deposition temperatures ranging from 400 °C to 620 °C. The deposition pressure, r.f.

0040-6090/97/$17.00 Copyright © 1997 Elsevier Science S.A. All rights reserved PHS0040-6090 (96) 09098-0

J.-S. Kim et al. /Thin Solid Films 292 (1997) 124-129 125

Rea~ton

N2, I-I2 Ar

(a)

13.56MHz

RF power generator

i . . [ Suseeptor

MFC

MFC 1 ~ 2 1 i _ _

......... i ~ i i ' ~ ....... N2, H2 ~ C n/~ ~' ~ [ ~ ~ m ~ ] Susieptor :'t::tt:t ;tttt Pumping Load-lock system TiCI + system Reactio 22Zaaam chamber l J

F ++ I l box Pumping (b) system Fig. 1. Schematic diagrams of (a) the r.f.-PECVD system and (b) the ECR-PECVD system for TiN film formation.

2.45 GHz Microwave

p [ ~ + j ~ Magnet system

power and gas flow rate were 2 Torr, 50 W and TiC14:Na:H2=1.5:45:70 (in standard cm3min -1) respec- tively, which was the optimum condition obtained from pre- vious experiments [6] .

The ECR-PECVD system consists of a discharge chamber, a reaction chamber, a gas feeding system and a pumping system. TIC14 and Nz gases were used as reactants and H2 gas as a reducing agent. The deposition temperature was changed from 350 °C to 500 °C. The deposition pressure, microwave power and gas flow rate were 3 X 10 -3 Torr, 1300 W and TiC14:Nz:H2=2;90:25 (in standard cm 3 min - I ) respec- tively. The deposition conditions are summarized in Table 1.

The thickness of the TiN film was measured with an a- step instrument from a step made using an etching solution (28%NH4OH:35%H2Oa:H20 = 1:2:6). The chemical com- position of the film was analyzed using Auger electron spec- troscopy (AES, Perkin-Elmer SAM 4300) with an incident electron energy of 5 keV and modulation voltage of 4 V. The samples were sputtered with an Ar ion beam acceleration voltage and current density of 3 keV and 300 b A cm -2

Table 1 Deposition conditions of TiN thin films

r.f.-PECVD ECR-PECVD

Pressure (Torr) 2 3 x 10 .3 Deposition temperature 400-620 350-500 (°c) Power (W) 50 (r.f. power) 1300 (microwave

power) TiCL flow rate 1.5 2 (standard cm 3 min - t ) N2 flow rate 45 90 ( s t andard c m 3 min - 1 ) H2 flow rate 70 25 (standard cm 3 min- t) Deposition time (min) 10-20 5-7 Film thickness (nm) 140-200 85-I20

respectively. The Ti + N overlapped peak around 383 eV in the AES spectra consists of the N peak at about 379 eV and the Ti peak at about 383 eV. To calculate the compositional ratio of Ti and N, the peak-to-peak height of N at about 383 eV was extracted from the Ti + N overlapped peak by the method proposed by Hofmann [7]. From the AES spectra of standard Ti and TiN samples, the relative sensitivity factor of Ti vs. N, Sri/SN, of 0.65 was calculated. For quantitative analysis of the CI content in the film, the CI peak at 181 eV was used and the relative sensitivity factor of C1 vs. N, Sct /

SN of 0.50, was calculated from Rutherford backscattering spectroscopy (RBS, NEC 5SDH-II) compositional analysis. The relative sensitivity factor of O vs. N, S o / S N of 1.82, was obtained from the AES handbook [8]. The crystallinity of the TiN films was investigated using an X-ray diffractometer (XRD, Rikagu Inc.) with Cu Ko~ radiation. The X-ray beam was fixed at a glancing angle of 2 ° from the specimen surface. The lattice parameter of the TiN films was calculated from the XRD (200) peak. The average grain size was determined from the half-width value of the XRD (200) peak of TiN films [9]. The resistivity of the film was derived from the thickness and the sheet resistance measured by a four-point probe method. The step coverage of the TiN film was observed using scanning electron microscopy (SEM, JEOL JSM 5200), after the TiN film had been deposited onto Si substrate patterned with steps 0.6 txm in width and 0.6 bm in depth.

3. Results and discussion The chemical composition of the TiN films deposited by

r.f.-PECVD and ECR-PECVD with respect to the deposition temperature is shown in Fig. 2(a) and (b) respectively. A large compositional change is observed for the r.f.-PECVD TiN films with temperature, while only a slight variation is

126 J.-S. Kim et al. / Thin Solid Films 292 (1997) 124-129

7O

6O

5o

g 4 0

s o e'~

20 o

1o

(a) RF-PECVD

400 450 500 550 600 Substrate Temperature (°C)

650

70

6O

50

o = 4 0

3 0 e.-,

Eo 2o O

lO

(b) ECR-PECVD

8 - - , ~ - - e - .-e

a ~ o 8ubstrate Temperature (°C)

Fig. 2. Chemical composition of TiN films deposited by (a) r.f.-PECVD and (b) ECR-PECVD as a function of deposition temperature.

found in the ECR-PECVD TiN films. The r.f.-PECVD TiN films deposited at 400 °C contained as much as 62.0 at.% chlorine. The chlorine content then reduced rapidly with tem- perature and was below 1.5 at.% at 620 °C. For the ECR- PECVD TiN films, the chlorine content was below the detection limit at all deposition temperatures. The Ti and N contents of the r.f.-PECVD TiN films increased from 23.0 at.% and 17.0 at.% to 48.2 at.% and 50.3 at.% respectively, forming more stoichiometric films as the deposition temper- ature increased from 400 °C to 620 °C. The ECR-PECVD TiN films deposited at a low deposition temperature of 350 °C showed a compositional ratio [Ti] / IN] close to stoichi- ometry, 48.4 at.% Ti and 50.8 at.% N, and the ratio approached unity with increasing temperature. In both cases, oxygen impurity, possibly from water vapor on the chamber wall, was observed at low temperature and not detected at high temperature. From the above, it is likely that in r.f.- PECVD a relatively high temperature is required to obtain TiN films with reduced chlorine content, and in ECR- PECVD, TiN films containing almost no chlorine are obtained successfully at a lower temperature. It is well known that dense films with lower impurity level can be deposited at a high temperature because the activity and the mobility of the reactants ,are increased and removal of the byproducts becomes more probable. This temperature effect is clear in the r.f.-PECVD TiN, In the ECR-PECVD process, however,

this temperature effect is not clear since the high density ECR plasma is so effective in decomposing, thus activating the reactants and removing the byproducts, finally allowing a low impurity content at a low temperature.

The AES depth profiles of TiN films deposited by r.f.- PECVD and ECR-PECVD are shown in Fig. 3(a) and (b) respectively. The oxygen intensity in Fig. 3(a) and (b) is exaggerated by a factor of 4 to emphasize the native oxide layer near the interface between the Si and TiN. A measurable amount of oxygen was detected at the TiN-Si interface for the r.f.-PECVD TiN, while the oxygen signal from the ECR- PECVD TiN was of the noise level. This result indicates that the ECR hydrogen plasma was very effective in removing the native oxide layer.

The XRD patterns of the TiN films prepared by r.f.- PECVD and ECR-PECVD are shown in Fig. 4(a) and (b) respectively. A strong (200) peak was observed only at a high temperature of 620 °C for the r.f.-PECVD TiN, while similar crystallinity was found for the ECR-PECVD TiN deposited even at 350 °C. The r.f.-PECVD TiN films depos- ited at low temperatures of 400 °C and 470 °C did not show strong crystatlinity. The films deposited at 400 °C were observed to contain TiNCI crystallites as shown in Fig. 4(a) and this is not surprising considering that about 62 at.% chlo- rine was incorporated in the film. This large amount of chlo- rine impurity also seems to impede TiN crystallization. Hedge et al. [ 10] reported that LPCVD TiN films deposited at 400 °C using TIC14 contained much chlorine and were poorly crystallized. Strong crystallinity was observed for the films deposited at 540 °C and 620 °C. In contrast to the r.f. case, the TiN films deposited by ECR-PECVD were highly crys- tallized TiN phase with strong (200) orientation at all tem- peratures above 350 °C. The low temperature at which highly crystallized TiN films can be obtained by ECR-PECVD can be attributed to the high energy and high density ECR plasma which helps the decomposition and reaction of the reactants greatly.

The lattice parameters of the TiN films deposited by r.f.- PECVD and ECR-PECVD are given in Fig. 5. The lattice

a) -P OVD

8

Depth of TiN film 100,

80 t (b) ECR-PECVD Si/]

8 o 4o I T, 1 o <

Depth of TiN film

Fig. 3. AES depth profiles of TiN films deposited by (a) r.f.-PECVD (at 620 °C) and (b) ECR-PECVD (at 500 °C).

J.-S. Kim et al. / Thin Solid Films 292 (1997) 124-129 127

. , . . l

¢ - .

=_

r -

t--

62ooc (a) RF-PECVD

t - . - -L , . ~ (200)

470°C

, 'r, ' ~ 4000C TiNCl (012)

~ ] i ~ ' ,, ~ TiNCI(212,114)

20 30 40 50 60 70 80 Diffraction Angle (20)

(200) 450 °C

(220)

20 30 40 50 60 70 80 D i f f r a c t i o n A n g l e (20)

Fig. 4. XRD patterns of TiN films deposited at various temperatures by (a) r.f.-PECVD and (b) ECR-PECVD.

0.430

E 0.428

0.426

I --0-- RF-PECVD - - I - - ECR-PECVD

(3. 0.424 ................ ¢ = ........... O .....

° ./ ...l

0.422

3;0 ' 4(~0 ' 450 ' 500 ' 5;0 ' 6 ;0 ' 650

Deposi t ion Temperature (°C) Fig. 5. Lattice parameter of TiN films deposited by r.f.-PECVD and ECR-

PECVD as a function of deposition temperature.

parameter of the r.f.-PECVD TiN film deposited at 470 °C was 0.426 nm owing to the high chlorine content; it decreased with increasing temperature, reaching 0.424 nm at 620 °C, which is the same as that of bulk TiN. For ECR-PECVD TiN films, the lattice parameters of the TiN films deposited at 350 °C and above 400 °C were 0.422 nm and 0.424 nm respectively.

The variation in the grain size of the TiN films deposited by r.f.-PECVD and ECR-PECVD with deposition tempera-

ture is shown in Fig. 6. For the r.f.-PECVD TiN films, the grain size increased rapidly from 8 nm to 27 nm as the dep- osition temperature increased from 470 °C to 620 °C. In contrast, the grain size of the ECR-PECVD TiN films was about 20 nm regardless of the deposition temperature. It is likely that grain growth does not occur for the r.f.-PECVD TiN films deposited at low temperature because the migration of the reactants is restricted by chlorine. For the ECR-PECVD TiN films, however, the surface mobility of the activated reactants is so high, even at lower temperatures, that grain growth is possible.

The deposition rates of the TiN films prepared by r.f.- PECVD and ECR-PECVD are shown in Fig. 7. The growth rate decreased greatly from 15.7 nm min - t to 7.0 nm min - as the deposition temperature increased from 400 °C to 620 °C. The large decrease in deposition rate for the r.f.-PECVD TiN films seems to be related to the large decrease in the chlorine content with increasing deposition temperature. Other researchers [2,4] also reported that the deposition rate of PECVD TiN films decreases with increasing deposition temperature. However, the ECR-PECVD TiN fi!ms, which contain almost no chlorine, showed a constant deposition rate

351 I

3o!

E 25

._N_. 20 09 ._ 15

¢..9 10

--O--RF'PECVD I ~ - - I - - ECR-PECVD 0

0

/ O

a;o '4;0 '410 's;o ' 5;0 6;0 650 Deposition Temperature (°C)

Fig. 6. Grain size of TiN films deposited by r.f.-PECVD and ECR-PECVD as a function of deposition temperature.

20

O _ _ ~ U 0 •

15

n,, lO e.- .o ~ o .m 1/1

Q.° 5 I --o-- RF-PECVD

I o - o - EoR Eovo

0 460 560 8S0

Deposit ion Temperature (°C)

Fig. 7. Deposition rate of TiN films deposited by E.f.-PECVD and ECR-

PECVD as a function of deposition temperature.

128 J.-S. Kim et al. /Thin Solid Films 292 (1997) 224-129

700

°•1 - o - RF-PECVD

600 - I - ECR-PECVD

o E 500

-=CL 400

-= 300 ._~ (/) ~a ty 200

100 ° ~ o

o 3 ;0 '460 '4;o '5;o ' 6;o' 6so

Deposition Temperature (°C) Fig. 8. Resistivity of TiN films deposited by r.f.-PECVD and ECR-PECVD as a function of deposition temperature.

of about 17.0 nm min - 1 in the range of deposition tempera- ture used in this study, which is in good accordance with the result of Akahori et al. [4].

Fig. 8 shows the variation in resistivity of the TiN films prepared by r.f.-PECVD and ECR-PECVD with deposition temperature. The resistivity of the r.f.-PECVD TiN films decreased from 670 ~I2 cm to 55 tx~2 cm as the deposition temperature increased from 400 °C to 620 *C. The resistivity of the ECR-PECVD TiN films decreased slightly from 60 ~f2 cm to 46 txO cm with increasing temperature. The resis- tivity of the TiN film is generally affected by such factors as the thickness, preferred orientation, grain size, stoichiometry and impurity content [ 11 ]. This decrease in the resistivity of the r.f.-PECVD TiN films is largely due to factors such as the reduced impurity content, enhanced stoichiometry and crys- tallinity and enlarged grain size. The slight change in the resistivity of the ECR-PECVD TiN films can be explained by the slight change in crystallinity, grain size, stoichiometry and impurity content with increasing temperature.

SEM cross-sectional views of TiN films deposited by r.f.- PECVD and ECR-PECVD onto Si substrate with step pat- terns of 0.6 ~m both in width and depth are given in Fig. 9 (a) and (b) respectively. The bottom coverage and the side cov- erage of the r.f.-PECVD TiN films were 60% and 50% respec- tively. The bottom coverage and the side coverage of the ECR-PECVD TiN films were 70% and 45% respectively. The conformality around the step edge was very good for both processes. The step coverage of the ECR-PECVD TiN films was worse on the side wall but better on the bottom, compared with the r.f.-PECVD TiN films, because of the anisotropy of the ECR plasma. It is considered that the thinner the TiN film on the side wall, the easier is contact hole filling in the following process.

Fig. 9. SEM cross-sectional view of TiN films deposited onto Si substrate patterned with steps of 0.6 p~m width and 0.6 ~m depth by (a) r.f.-PECVD (620 °C) and (b) ECR-PECVD (500 °C).

sition (ECR-PECVD) using TIC14, N2 and H2 as the reactants at various deposition temperatures. The effects of deposition temperature on the compositional ratio I N ] / [ T i ] , impurity content, crystallinity, lattice parameter, grain size, deposition rate, resistivity and step coverage were studied. TiN films deposited by ECR-PECVD even at lower temperature have better quality than those deposited by r.f.-PECVD, and the ECR-PECVD process lower deposition temperature to obtain high quality films. This is because the ECR plasma is a high density plasma at a lower pressure and effectively promotes the decomposition and reaction of the reactants.

Acknowledgements

This research was performed within the 'Studies on Advanced Basic Technology' for future microelectronics project of Korean Electronics and Telecommunications Research Institute.

4. Conclusion

TiN thin films were prepared by r.f. plasma enhanced chemical vapor deposition (rf-PECVD) and by electron cyclotron resonance plasma enhanced chemical vapor depo-

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