A stable TiN reactive sputtering deposition process

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Applied Surface Science 38 (1989) 304-311 North-Holland. Amsterdam

A S T A B L E T~N R E A C F 1 V E S P U T Y E R I I N G D E P O S I T H O N P R O C E S S N. C I R C E L L I a n d G. Q U E I R O L O S T Mwroeh,ctrom~w, via Olivetti 2. 20041 Agrate Brianza. Italy

Received 20 March 1989: accepted for publication 17 April 1989

Titanium nitride is widely used in VLSI devices as diffusion barrier in AI-based metallization. TiN is usually obtained by reactive sputtering of Ti in argon/nitrogen atmosphere. The film properties strongly depend on the deposition conditions; with stoichiometry, in particular, being critically affected by both nitrogen partial pressure and Ti deposition rate. These two parameters influence each other and their optimization and control is an essential and non-straightforward prerequisite for a reliable process. In this work we report the results of characterization work perforrued using RGA, AES and XRD with the aim of finding the deposition conditions which ensure a stable process. An operational "' safe" area in the target applied power/nitrogen flow rate domain was identified where stoichiometric TiN films of good quality were always obtained. The "safe" operational area is a characteristic of the sputtering equipment used, but the method outlined in this work is of general validity.

R. ~ntroducfion T i t a n i u m nitride is u s e d in microelectronics as d i f f u s i o n barr4.er to p r e v e n t interaction b e t w e e n AI a n d silicon (or silicide) a n d to m a k e the c o n t a c t stable a n d reliaole [1]. F i l m properties like resistivity, stress, density, etc. d e p e n d o n s t o i c h i o m e t r y a n d m i c r o s t r u c t u r e a n d c a n v a r y over a wide r a n g e w h e n t h e d e p o s i t i n g c o n d i t i o n s are c h a n g e d . T h e d e p o s i t i o n p a r a m e t e r s m u s t t h e r e f o r e be carefully o p t i m i z e d in o r d e r to o b t a i n r e p r o d u c i b l y g o o d films. T i N is u s u a l l y d e p o s i t e d by reactive s p u t t e r i n g o f Ti in a r g o n / n i t r o g e n a t m o s p h e r e [2,3]. T i N f o r m s by the reaction o f t i t a n i u m with n i t r o g e n o n a n y s u r f a c e e x p o s e d to the i n c o m i n g Ti flux [4,5]. T h e m o s t critical p a r a m e t e r s in o b t a i n i n g a s t o i c h i o m e t r i c film are therefore local Ti arrival rate a n d n i t r o g e n partial pressulc. T h e s e two p a r a m e t e r s are n o t i n d e p e n d e n t as n i t r o g e n also reacts on the target s u r f a c e so p r o d u c i n g a r e d u c t i o n in Ti d e p o s i t i o n rate [6]. M a n y a u t h o r s h a v e s t u d i e d reactive s p u t t e r i n g to d e f i n e a m o d e l able to predict the b e h a v i o u r o f a very s i m p l e s p u t t e r i n g s y s t e m [7,8]. T h e y solved e q u a t i o n s c o n t a i n i n g m a n y p a r a m e t e r s , s u c h as t h e area covered in t h e c h a m b e r by Ti flux, t h e nitrided fraction o f the target surface, etc. T h e s e variables c a n n o t be d e t e r m i n e d in a m o r e c o m p l e x , p r o d u c t i o n o r i e n t e d system. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)

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In the present work we used a commercial system and we experimentally determined an operational area in the target applied power (or depositien rate)/nitrogen flow domain where stoichiometric, uniform TiN trims were obtained. The method used is rather simple and is mainly based on the residual gas analysis (RGA) measurements during the deposition. X-ray diffraction (XRD) and Auger electron spectroscopy (AES) were used to determine film structure and composition. This "safe" operational area is characteristic of the sputtering system used, but the method outlined can be applied to any equipment. The second phase of the process characterization was focused on the effects of the other deposition parameters (for instance substrate bias and temperature) to optimize film quality [9].

2. Experimental All TiN films studied in this work were deposited on (100) oriented, 150 mm, polished silicon wafers. A load-locked, D C magnetron sputtering system was used, operating in a batch mode (Electrotech MS6200), with a base pressure in the deposition chamber of less than 1 × 10 -7 Torr. The substrates were placed on a pallet which was rotating during deposition with a maximum speed of 10 rpm. They were heated up to 200 ° C by radiation, the temperature being measured by a thermocouple placed alongside the pallet. Sputtering was performed from a 99.99% pure Ti target at an operating power up to 6 kW, in a dynamic flow of 99.995% pure Ar and N 2. The two gases were fed into the chamber through mass flow controllers and their partial pressure was determined by RGA. The Ar flow was kept constant at 100 scorn, while the N 2 flow was varied up to 30 sccm and the c h a m b e r pressure varied accordingly from 6 × 10 -3 to 7 × 10 -3 Torr as measured by a Baratron gauge. The electrical characteristics of the target were continuoosly monitored during deposition. A negative bias up to - 165 V was applied to the substrates. The film sheet resistivity was measured by a Prometrix four point probe and film thickness was measured with a Dektak profilometer on etched steps. X-ray and Auger analysis were performed with a Siemens D-500 BraggBrentano X-ray diffractometer and a Varian scanning Auger spectrometer respectively.

3. Influence of nitrogen flow and deposition rate on TiN formation TiN is formed by reaction of nitrogen with Ti deposited on substrates, chamber walls and on the target surface [4-6]. Stoichiometric TiN can be

306

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obtained at any Ti arrival rate for a high enough nitrogen flow. In practice, however, due to process and equipment constraints, nitrogen availability can limit the nitridation process. In addition a change in the nitrided target surface can induce a variation in the Ti sputtering rate. To identify the set of parameters needed to produce uniform, stoichiometric TiN films, depositions were carried out with different target applied power and nitrogen flow rates. In fig. 1 we have plotted the target voltage as a function of different power levels for different flow rates. The change in target "status" (nitrided/metallic) is easily recognized by the sudden change in target voltage when Ti and N removal rates become higher than the nitridation rate. The nitrogen partial pressure in the chamber during deposition is reported in fig. 2 as a function of applied power for different flow rates. Three regions can be observed. When the target is completely nitrided (low power, high flow, see fig. 1) the nitrogen pressure slightly decreases with power. When the Ti and N removal rates become comparable with nitride formation, an increased nitrogen consumption is experienced. A sudden change is observed when the target surface is completely "cleaned" (high power, low flow) and all nitrogen

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Circelli, G. Queirolo / Stable TiN reactive sputtering process

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deposited under such conditions, with the pallet rotating at 1 rpm for 6 n'fin. The nitrogen and titanium concentrations are not uniform through the layer, but vary periodically with the instantaneous Ti arrival rate. This is higher when the substrate is directly under the target, and smaller (tending to vanish) when the samples enter or leave the deposition area. Stra/ght underneath the target the nitrogen partial pressure is not sufficient to form stoichiometric TiN, while anywhere else it is. This explanation is further supported by an AES or even a visual analysis of sample deposited with the pallet stationary. The borders of the deposition zone exhibit a golden coloured stoichiometric TiN film, whilst a metallic like, N deficient film is found ha the center. These results show that the variation in local Ti arrival rate must be taken into account when defining the operational conditions in any sputtering system.

4. B i ~ and eempemtm'e effect After having identified the region of target applied power/N 2 flow rate domain where stoichiometric TiN is obtained, it is possible to further opfinfize film quality (resistivity, density, etc.) by varying other deposition parameters. In particular, a negative bias applied to the substrate during deposition induces respnttering of impurities and loosely bound atoms which modify the microstructure giving" films of higher density and lower electrical resistivity [9-11]. Deposition was carried out applying a negative bias up to - 1 6 5 V. The film resistivity and compressive stress were measured and the results are reported in fig. 6. The trends are opposite; increase of the bias voltage corresponds to a reduction of the resistivity but to an increase of the stress.

310

N. Circelli, G. Queirolo / Stable TiN reactive sputtering process

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Therefore an adequate bias voltage must be chosen to provide both low resistivity and a stress low enough '~o avoid adhesion problems. On increasing the substrate temperature during deposition, the film resistivity decreases because of enhanced atom surface mobilhy. However, for temperatures up to 200°C, this effect was not found to be as pronounced as obtained using a bias, probably because the investigated temperatures are low with respect to the mat.erial melting temperature (Tin). It is known [12] that a clear effect of deposition temperature ( T ) is observable when T>_ 0.2 Tin, As 7],, of TiN is 2930 K, a deposition temperature of 200°C cannot produce strong effects on film structure. Heating the substrates during deposition can anyway improve film crystallization and impurity desorption.

5. Conclusions Titanium nitride properties are strongly dependent on deposition parameters, in particular on nitrogen partial pressure and Ti deposition rate. In this work we presented a simple method to define an operational "safe" area in the target applied power/nLtrogen flow rate domain where stoichiometric and uniform TiN films can be obtained. The actual values of both N 2 flow rate and power depend on the particular equipment used, but the method is of

N. Circelli, G. Queirolo / Stable TiN reactive sputtering process

311

general validity at,d can be employed to ensure tight control of any reactive sputtering deposition process. F u r t h e r i m p r o v e m e n t of the film quality can be obtained optimizing other deposition p a r a m e t e r s such as s u b s t r a t e bias a n d temperature.

References [1] M. Wittmer and M. Melchior, Thin Solid Films 93 (1982) 397. [2] J.E. Sundgren, B.O. Johansson and S.E. Karlsson, Thin Solid Films 105 (1983) 353. [3] J.P. Noel, D.C. Houghton, S. Este and F.R. Shepherd, J. Vacuum Sci. Technol. A 2 (1984) 284. [4] R.C. Ellwanger and J.M. Towner, Thin Solid Films 161 (1988) 289. [5] S. Berg, H.-O. Blom, T. Larsson and C. Nender, J. Vacuum Sci. Technol. A 5 (1987) 202. [6] J.E. Sundgren, B.O. Johansson and S.E. Karlsson, Surface Sci. 128 (1983) 265. [7] A.G. Spencer, R.P. Howson and R.W. Lewin, Thin Solid Films 158 (1988) 141. [8] S. Berg, T. Larsson, C. Nender and H.-O. Blom, J. Appl. Phys. 63 (1988) 887. [9] J.E. Sundgren, B.O. Johansson, H.T.G. Hentzell and S.E. Karlsson, Thin Solid Films 105 (1983) 385. [10] R.D. Bland, G.J. Kominiak and D.M. Mattox, J. Vacuum Sci. Technol. 11 (1974) 671. [11] J.M. Pointevin, G. Lempariere and J, Tardy, Thin Solid Films 97 (1982) 67. [12] J.A. Thornton, J. Vacuum Sci. Technol. 11 (1974) 666.