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Microstructure and properties of AlSi-Ti multilayer structure
~~CROSTRUC~RE R.K. NAHAR
AND PROPERTIES OF AlSi-Ti MULTILAYER STRU~RE
and N.M. DEVASHRAYEE
Solid State Devices Division, Central Electronics Pilani, Rajasthan 333031, India Received
4 March
July 1986
MATERIALS LETTERS
Volume 4, number 5,6,7
Enpeering
Research Institute,
1986
AISi-Ti five layered structure is prepared by sputtering on an oxidized silicon substrate. The resistivity, surface smoothness and microstructure is studied. It is observed that the layered structure exhibits a smooth hillock-free surface compared with the conventional AlSi alloy film. The effect of thermal annealing between 400 and 500°C on the properties of the layered structure is also reported.
1. Introduction ~uminum is a widely used metal for contact and interconnections in integrated circuits owing to its low resistivity and Si compatibility. However, as device dimensions are scaled down the current density increases resulting in a decrease in interconnect reliability. The problems with pure Al are electromigration, spiking and hillocks formation [ 1). The spiking in the substrate silicon is overcome by using S&doped Al films, presatisfying the solubility requirement of Si and Al at the sintering temperature. After sintering the appearance of hillocks on Al and AlSi alloy thin films is well known. This is mainly due to a large thermal mismatch between the metal Al and the substrate Si or SiO,. Hillocks create both circuit yield and reliability problem. With the advancement in multilevel interconnect technology the pro’blem becomes more severe. In recent years owing to its technological importance elimination of hillocks in Al meta~~ation has been a subject of intensive research. Several different approaches such as doping with Cu [2], fast heat pulse annealing [3] and process modification [4] have been investigated to suppress the hillock growth. All these techniques have some drawbacks for practical application. More recently the work reported on AlSi-Ti alloy [S] and AlSi-Ti layered structures [6] shows that this alloy offers advantages over currently used materials for interconnections in ICs. In this pa0 167-.577x/861$03.50 0 Elsevier Science Publishers B.V. (~o~h-HoU~d Physics Publis~g Divisions
per we report the surface morphology, smoothness and resistivity of AlSi-Ti multilayered structure and compare with the conventions AlSi alloy film. The effect of annealing on the properties of the layered structure is also shown.
2. Experimental A schematic of the AlSi-Ti five layered structure prepared on an oxidized silicon substrate is shown in fig. 1. Polished Si wafers, after cleaning by standard procedures, were oxidized at 1100°C and *lo00 A SiOz was grown. The wafers were metallized in a MRC 8620 J rf sputter~g machine. Sputtering was
200 i 2300 i 200 A 2300 i\ 500 2\ 1000
I
i
Si SUBSTRATE
Fig. 1. Schematic diagram of AlSi-Ti multilayer structure. 265
Volume 4, number 5,6,7
MATERIALS LETTERS
July 1986
Table 1 Typical process parameters substrate SiOa thickness chamber base pressure sputtering gas argon gas pressure sputtering voltage substrate temperature total thickness of AlSi-Ti layer
oxidized Si 1000 A 8 X lo-’ Torr UHP argon 6 mTorr 1.5 kV 15OOC 5500 a
done following a standard procedure and taking all care to minimize the contents of moisture and oxygen in the sputtering chamber. High-purity Ti and Al 2% Si alloy targets were used and the films were deposited alternatively on the substrate to the desired thickness. Typical process parameters are given in table 1. The sheet resistance of the structure was measured using a four-point probe. The surface roughness was measured on a talystep. The structure and surface topography were characterized by SEM.
3. Results and discussion The scanning electron micrograph of the as-prepared AlSi-Ti structure is shown in fig. 2a. For comparison the SEM of the as-prepared AlSi ahoy film, prepared under similar growth conditions, is shown in fig. 2b, The difference is striking. The introduction of a thin Ti layer in the AlSi produces a significant improvement in the surface topography. The layered structure exhibits a smooth surface with tine grains compared to the AlSi alloy film without Ti layer. Notice that the hillocks seen on AlSi are suppressed on
Fig. 2. Scanning electron micrographs of as-prepared (a) AlSi-Ti layered structure, (b) AlSi alloy film.
the AlSi-Ti structure. This is also evident from the measured data on the surface roughness given in table 2. The resistivities in both cases is similar. The AlSiTi structure was annealed at 400,450 and .500°C for 30 min in forming gas. After annealing the surface
Table 2 Measured parameters on AlSi alloy and AlSi-Ti layered structure Parameter
AlSi alloy as-prepared
AlSi-Ti layered structure as-prepared
average surface roughness (A) average film resistivity (pa cm)
266
800 4.31
400 4.45
annealing temperature (“C) 400
450
500
400
350
450
5.13
4.95
6.61
Volume 4, number 5,6,7
MATERIALS LETTERS
July 1986
morphology of the as-deposited structure changes marginally. The scanning electron micrographs are shown in fig. 3. The average surface roughness and sheet resistivity of the structure measured after annealing, given in table 2, show noticeable variations. The best data are obtained after annealing at 450°C. Both the surface roughness and the film resistivity increase substantially after annealing at 5OO’C. The presence of Ti on the structure provides a better thermal match between substrate SiO, and AlSi. The thermal expansion coefficients for SiO2, Ti and Al are 0.5 X 10-6, 8 X lo@ and 25 X 10-6/“C respectively. This reduces mechanical stresses in the metal film and thus helps in suppressing hillocks growth. Further, Ti has the property of grain refining which favours the growth of homogeneous fine grain structure after annealing. This is a desirable property to improve the electromigration resistance. The increase in the structure resistivity is attributed to the formation of a high resistivity TiAl, layer due to the interaction between Al and Ti during annealing [6]. By optimizing the thickness of the Ti layer and the percentage of Si in the AlSi alloy film we believe that the overall resistivity of the layered structure can be reduced. Further work is being carried out to optimize the layered structure parameters.
References [ 1] D. Pramanik and A.N. Saxena, Solid State Technol.
Fi g. 3. Scanning electron micrographs of the AlSi-Ti layered structure annealed at (a) 4OO”C, (b) 450°C and (c) 500°C.
(1983) 127. [2] F.M. d’Heurle and R. Rosenberg, Physics of thin films, Vol. 7 (Academic Press, New York, 1973) p. 257. [3] T.J. Faith and C.P. Wu, Appl. Phys. Letters 45 (1984) 470. [4] K.C. Cadien and D.L. Losee, J. Vacuum Sci. Technol. 2 (1984) 82. [5] F. Fischer, Siemens Forsch. 13 (1984) 21. [6] D.S. Gardner, T.L. Michalka, K.C. Saraswat, T.W. Barbee, J.P. Movittic and J.D. Meindl, IEEE Trans. Plectron DevicesED(1985) 174.
267
AND PROPERTIES OF AlSi-Ti MULTILAYER STRU~RE
and N.M. DEVASHRAYEE
Solid State Devices Division, Central Electronics Pilani, Rajasthan 333031, India Received
4 March
July 1986
MATERIALS LETTERS
Volume 4, number 5,6,7
Enpeering
Research Institute,
1986
AISi-Ti five layered structure is prepared by sputtering on an oxidized silicon substrate. The resistivity, surface smoothness and microstructure is studied. It is observed that the layered structure exhibits a smooth hillock-free surface compared with the conventional AlSi alloy film. The effect of thermal annealing between 400 and 500°C on the properties of the layered structure is also reported.
1. Introduction ~uminum is a widely used metal for contact and interconnections in integrated circuits owing to its low resistivity and Si compatibility. However, as device dimensions are scaled down the current density increases resulting in a decrease in interconnect reliability. The problems with pure Al are electromigration, spiking and hillocks formation [ 1). The spiking in the substrate silicon is overcome by using S&doped Al films, presatisfying the solubility requirement of Si and Al at the sintering temperature. After sintering the appearance of hillocks on Al and AlSi alloy thin films is well known. This is mainly due to a large thermal mismatch between the metal Al and the substrate Si or SiO,. Hillocks create both circuit yield and reliability problem. With the advancement in multilevel interconnect technology the pro’blem becomes more severe. In recent years owing to its technological importance elimination of hillocks in Al meta~~ation has been a subject of intensive research. Several different approaches such as doping with Cu [2], fast heat pulse annealing [3] and process modification [4] have been investigated to suppress the hillock growth. All these techniques have some drawbacks for practical application. More recently the work reported on AlSi-Ti alloy [S] and AlSi-Ti layered structures [6] shows that this alloy offers advantages over currently used materials for interconnections in ICs. In this pa0 167-.577x/861$03.50 0 Elsevier Science Publishers B.V. (~o~h-HoU~d Physics Publis~g Divisions
per we report the surface morphology, smoothness and resistivity of AlSi-Ti multilayered structure and compare with the conventions AlSi alloy film. The effect of annealing on the properties of the layered structure is also shown.
2. Experimental A schematic of the AlSi-Ti five layered structure prepared on an oxidized silicon substrate is shown in fig. 1. Polished Si wafers, after cleaning by standard procedures, were oxidized at 1100°C and *lo00 A SiOz was grown. The wafers were metallized in a MRC 8620 J rf sputter~g machine. Sputtering was
200 i 2300 i 200 A 2300 i\ 500 2\ 1000
I
i
Si SUBSTRATE
Fig. 1. Schematic diagram of AlSi-Ti multilayer structure. 265
Volume 4, number 5,6,7
MATERIALS LETTERS
July 1986
Table 1 Typical process parameters substrate SiOa thickness chamber base pressure sputtering gas argon gas pressure sputtering voltage substrate temperature total thickness of AlSi-Ti layer
oxidized Si 1000 A 8 X lo-’ Torr UHP argon 6 mTorr 1.5 kV 15OOC 5500 a
done following a standard procedure and taking all care to minimize the contents of moisture and oxygen in the sputtering chamber. High-purity Ti and Al 2% Si alloy targets were used and the films were deposited alternatively on the substrate to the desired thickness. Typical process parameters are given in table 1. The sheet resistance of the structure was measured using a four-point probe. The surface roughness was measured on a talystep. The structure and surface topography were characterized by SEM.
3. Results and discussion The scanning electron micrograph of the as-prepared AlSi-Ti structure is shown in fig. 2a. For comparison the SEM of the as-prepared AlSi ahoy film, prepared under similar growth conditions, is shown in fig. 2b, The difference is striking. The introduction of a thin Ti layer in the AlSi produces a significant improvement in the surface topography. The layered structure exhibits a smooth surface with tine grains compared to the AlSi alloy film without Ti layer. Notice that the hillocks seen on AlSi are suppressed on
Fig. 2. Scanning electron micrographs of as-prepared (a) AlSi-Ti layered structure, (b) AlSi alloy film.
the AlSi-Ti structure. This is also evident from the measured data on the surface roughness given in table 2. The resistivities in both cases is similar. The AlSiTi structure was annealed at 400,450 and .500°C for 30 min in forming gas. After annealing the surface
Table 2 Measured parameters on AlSi alloy and AlSi-Ti layered structure Parameter
AlSi alloy as-prepared
AlSi-Ti layered structure as-prepared
average surface roughness (A) average film resistivity (pa cm)
266
800 4.31
400 4.45
annealing temperature (“C) 400
450
500
400
350
450
5.13
4.95
6.61
Volume 4, number 5,6,7
MATERIALS LETTERS
July 1986
morphology of the as-deposited structure changes marginally. The scanning electron micrographs are shown in fig. 3. The average surface roughness and sheet resistivity of the structure measured after annealing, given in table 2, show noticeable variations. The best data are obtained after annealing at 450°C. Both the surface roughness and the film resistivity increase substantially after annealing at 5OO’C. The presence of Ti on the structure provides a better thermal match between substrate SiO, and AlSi. The thermal expansion coefficients for SiO2, Ti and Al are 0.5 X 10-6, 8 X lo@ and 25 X 10-6/“C respectively. This reduces mechanical stresses in the metal film and thus helps in suppressing hillocks growth. Further, Ti has the property of grain refining which favours the growth of homogeneous fine grain structure after annealing. This is a desirable property to improve the electromigration resistance. The increase in the structure resistivity is attributed to the formation of a high resistivity TiAl, layer due to the interaction between Al and Ti during annealing [6]. By optimizing the thickness of the Ti layer and the percentage of Si in the AlSi alloy film we believe that the overall resistivity of the layered structure can be reduced. Further work is being carried out to optimize the layered structure parameters.
References [ 1] D. Pramanik and A.N. Saxena, Solid State Technol.
Fi g. 3. Scanning electron micrographs of the AlSi-Ti layered structure annealed at (a) 4OO”C, (b) 450°C and (c) 500°C.
(1983) 127. [2] F.M. d’Heurle and R. Rosenberg, Physics of thin films, Vol. 7 (Academic Press, New York, 1973) p. 257. [3] T.J. Faith and C.P. Wu, Appl. Phys. Letters 45 (1984) 470. [4] K.C. Cadien and D.L. Losee, J. Vacuum Sci. Technol. 2 (1984) 82. [5] F. Fischer, Siemens Forsch. 13 (1984) 21. [6] D.S. Gardner, T.L. Michalka, K.C. Saraswat, T.W. Barbee, J.P. Movittic and J.D. Meindl, IEEE Trans. Plectron DevicesED(1985) 174.
267