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EFFECTS OF MECHANICAL STRESS ON MOSSTRUCTURES WITH TiSi2 GATES

P. Reuters, J. Giese, M. Offenberg, W. Richter, S. Ewert, P. Balk

To cite this version:P. Reuters, J. Giese, M. Offenberg, W. Richter, S. Ewert, et al.. EFFECTS OF MECHANICALSTRESS ON MOS STRUCTURES WITH TiSi2 GATES. Journal de Physique Colloques, 1988, 49(C4), pp.C4-45-C4-48. �10.1051/jphyscol:1988408�. �jpa-00227830�

JOURNAL DE PHYSIQUE Colloque C4, suppl6ment au n09, Tome 49, septembre 1988

EFFECTS OF MECHANICAL STRESS ON MOS STRUCTURES WITH TiSi, GATES

P.J. REUTERS, J. GIESE, M. OFFENBERG, W. RICHTER, S. EWERT and P. BALK

Inst. of Semiconductor Electronics and Inst. of Physics, Technical University Aachen, 0-5100 Aachen, F.R.G.

Resume - Un piegeage d'electrons et une generation densite d-etats a l'interface par l'injection d'electrons ainsi qu-une deterioration du comportement de rupture ont CtC observe dans des capacites MOS ayant une electrode de grille en TiSi2 . Ces effets sont plus prononces pour des epaisseurs d-electrode plus importantes. 11s semblent Ctre causees par des contraintes mechaniques provoquees par l'electrode de grille de ce systeme MOS.

Abstract - MOS capacitors with TiSi2 gate electrode show electron trapping, interface state generation upon electron injection and reduction of breakdown strength: these effects become more pronounced for electrodes with increasing thickness. Evidence is presented that this degradation of the insulator is due to mechanical stress in the MOS system caused by the gate.

1 - Introduction

TiSi2 is an interesting material for interconnection, contact and MOSFET gate application because of its low resistivity and its ability to reduce native oxide films. For this reason this silicide finds application in the VLSI technology.

MOS capacitors with TiSi2 gate were found to show properties typical for highly defective insulator films. The contain electron traps with capture cross-sections of 10-15 to 10-16cms. Similar traps were found in oxides implanted with Ti and electroded with Al. This suggests that the traps in the first case could also be due to Ti. The above explanation would fit in with the observed diffusion behavior of Ti implanted into SiO2. However, the silicide gate leads to the occurrence of heavy compressive stress a t the Si-Si02 interface. It will be shown that the compressive stress rather than indiffusion of Ti into the oxide is responsible for the degradation in MOS properties.

The samples used for electrical mesurements were MOS capacitors fabricated on p-type (100) Si substrates. After growing oxide films of 50 and 90 nm in 0 2 silicide gates were prepared by e-gun coevaporation of Ti and Si. Polycide structures were fabricated by growing a 250 nm LPCVD poly-Si layer, doping it with As by ion implantation and finally depositing a Ti/Si film. Silicide formation was carried out a t 8000C in Ar. Capacitors were defined by optical lithography and dry etching. Avalanche injection, Fowler-Nordheim injection, quasi static C-V and I-V measurements were performed to characterize the trapping behavior, the interface state generation and the breakdown of the structures.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988408

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The samples for stress determination were fully TiSi2 coated oxidized Si wafers in the case of interferometeric measurement of the radius of curvature. For determining the shift of the Si TO phonon by Raman spectroscopy partially coated samples were used. Both measurement techniques yield information on the stress a t the Si-Si02 interface.

3 - Results and discussion

Results on the electron trapping in a sample with 90 nm oxide and 200 nm TiSi2 are shown in fig.1. This figure gives the shift in flatband voltage in dependence on the density of injected electrons. It follows from this lot that the oxide contains traps with cross-sections in the 10-15 to IO-P6 cm-2 region. However, removal of the silicide and replacing it with a n evaporated A1 electrode leads to the disappearance of most of the traps. This suggests that mechanical stress is dominating the trapping behavior. In contrast to chemical interaction between the silicide and the SiO2. stress effects should exhibit a dependence on the thickness of the silicide film. Fig. 2 shows electron trapping data for MOS capacitors with TiSi2 gates of different thickness. The

0.8 Silicide removed

0.4

0 4 0.8 N: . /cmJ X I O

17 '"J

Fig. 1: Effect of substituting TiSi2 by Fig. 2: Effect of TiSi2 thickness on A1 on electron trapping behavior electron trapping behavior

SiO2 film in these samples was 50 nm. For comparison, data on samples with polycide and with ,poly-Si gates are also shown. The low trapping density in the latter case is apparent. The trapping in the silicide gate capacitors indeed increases with the thickness of the gate electrode.

Similarly, the generation of the interface states upon Fowler-Nordheim injection of electrons is highest for the sample with the thickest TiSi2 electrode (fig.3). The injecting field had to be lowered in order to avoid the breakdown of this sample. Already at a low density of injected electrons a large density of interface traps is observed. Before injection all samples exhibited interface trap densities around 1010cm-2. In fig. 4 typical I-V curves showing the breakdown behavior of samples with different gates are presented. Polycide gate samples show a breakdown field comparable to that found with poly-Si gates. The values for the polycide gates are distinctly lower and decrease with electrode thickness. These observations indicate that the poly-Si interlayer in the polycide samples lowers the stress level near the Si-Si02 interface.

The optical interference method provides average stress values parallel to the interface over relatively large areas. In all silicide and polycide gate samples

this stress is compressive. Only in the poly-Si gate samples the stress was tensile but very small. Increasing nSip thickness leads to higher stress levels at the Si-St02 interface (fig. 5). The stress in polycide gate samples was distinctly lower which indicates the role of the poly-Si film in reducing the overall stress.

Fig. 3: Effect of TiSi2 thickness on Fig. 4: Effect of TiSi2 thickness on interface state generation breakdown characteristics

On the other hand, Raman measurement of the phonon frequency presents information on the deformation of the lattice perpendicular to the interface/ l / In this case the observed reduction of the phonon frequency indicates a stretching of the Si-Si bonds, corresponding to tensile stress in this direction. In contrast to the interferometric approach the Raman measurement provides information on stresses localized in relatively small areas. These are determined by the beam diameter, which in our case amounted to approximately 20 pm. It was ascertained by working with different beam intensities that the observed effects were not affected by local sample heating. The derivation of absolute values for the stress is somewhat problematic for this method. The data in fig. 6 show the change of the stress in the vicinity of the

T i S i 2 Thickness (nrn)

-120 -80 -40 0 40 80 120 Step Position/pm

Fig. 5: Stress at the Si-Si02 interface Fig. 6: Stress in Si at TiSi2 edge from form curvature measurements Rarnan study

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edge of a silicide gate. Also in this technique an increase in the stress (which is tensile in this case) for thicker TiSi2 films is observed. When passing the edge to the silicide the stress increases monotonically, i.e. without passing through a peak value. This fmding may be contrasted with reports of stress concentration (i.e. a maximum value of this quantity) at such topographical features /2.3/. However, it should be remembered that our technique presents information on the stress perpendicular to the surface, whereas the data in /2,3/ refer to the parallel direction.

From the data presented in the present study it may be concluded that chemical effects do not play a determining role in the degradation of the MOS properties of silicide gate capacitors. Stress effects alone are responsible for these effects.

/1/ Richter, W.: in. Springer Tracts in modem Physics Vol. 78, ed. G. HBhler. Z.A. Niekisch, Springer 1976

/2/ Zekeriya, V., Wong, A., and Ma, T.-P., Appl. Phys. Lett. 46 (1985) 80

/3/ Serebrinsky. J.H., Sol. St. Electr. 13 (1970) 1435