- Email: [email protected]
Room temperature anodic plasma oxidation of tantalum silicide: Oxide composition and electrical properties
Applied Surface Science 38 (1989) 125-132 North-Holland, Amsterdam
125
R O O M T E M P E R A T U R E ANODliC P L A S M A O X I D A T I O N OF TANTALUM SIUCIDE: OXIDE COMPOS~I'ION AND ELECr~CAL PROPERTIES A. C L I M E N T *, J. P E R R I E R E , A. L A U R E N T , B. L A V E R N H E Groupe de Physique des Solides, Universit~ Paris VII, Tour 23, 2 Place Jussieu, F-75251 Paris Cedex 05, France
R. P I ~ R E Z - C A S E R O a n d J.M. M A R T i N E Z - D U A R T Departamemo de Fisica Aplicada, C-12, e lnstituto de Ciencia de Materiales (B), CSIC, Universidad Aut6noma de Madria~ Cantoblanco, E-28049 Madria[ Spain
Received 19 March 1989; accepted for publication 17 April 1989 The room temperature anodic plasma oxidation of Ta sificide has been studied using ion backscattering (RBS), and nuclear reaction analysis (NRA). Thin layers of tantalun silicide were deposited on silicon substrates by cosputtering from Si and Ta targets and anodically oxidized in a multipolar plasma system. RBS spectra show that during anodization a mixed oxide of Ta2Os and SiO, is formed. The distribution of this oxide mixture is not uniform and two layers are clearly distinguished. An outer layer forms in which the oxide mixture is richer in Ta2Os than SiOe as compared to the starting silicide composition. The inner layer on the other hand, is richer in SiOe. The resulting oxide has been electrically characterized by measuring the I - V curves and the dielectric constant in the frequency range 0.1-100 kHz.
L Intreductlon Metal silicides are used in very large scale integrated (VLSI) devices as gate or interconnection materials because o f their g o o d electrical conductivity a n d high chemical stability. W h e n used as i n t e r c o n n e c t material it is necessary to cover the metal silicide with an insulating film such as SiO 2, to e n a b l e the use o f over-crossing c o n d u c t o r s in s u b s e q u e n t layers. T h u s a g o o d deal o f w o r k o n the thermal oxidation behaviour o f metal silJcides has b e e n d o n e over the p a s t few years [1-5]. As thermal oxidation requires t e m p e r a t u r e s o f a r o u n d 1000 ° C, u n w a n t e d p h e n o m e n a m a y occur such as interdiffusion a n d reactions w h i c h are not tolerated in the VLSI technologies. M o r e recently, in o r d e r to avoid high temperatures, low t e m p e r a t u r e oxidation processes o f the silicides, such as wet anodic [6-8], p l a s m a anodic [9-12] a n d p l a s m a at floating p o t e n t i a l [13], have b e e n undertaken. * Permanent address: Departamento de Fisica Aplicada, C-12, Universidad Aut6noma de Madrid, Cantoblanco, E-28049 Madrid, Spain. 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
126
A. Climent et al. / Room temperature anodic plasma oxidation
in the present work we investigate the electric field assisted plasma oxidation process (plasma anodization of cosputtered TaSi mixtures). The ox;de growth rate and mean electric field of formation are determined through the use of nuclear microanalysis (NRA) and Rutherford backscattering techniques (RBS). Tiffs latter technique is used to obtain the depth distribution of cations in the oxide.
2. Ex~efiment,'~l 2.1. S a m p l e preparation
Tantalum silicide thin films were prepared in a Varian S-magnetron sputter-deposition system, by simultaneous sputtering from tantalum and silicon targets onto silicon single crystal substrates. The samples were anodized in a multipolar plasma system described elsewhere [9]. During typical oxidations, the oxygen pressure was 5 × 10 -4 Torr. Successive anodization of the samples, first in 180 plasma up to 25 V and then with 160 plasma at several voltages up to 125 V was made in order to allow for the possibility of oxygen transport studies. 2.2. S a m p l e analysis
Nuclear microanalysis was used to determine the nature and dis "ribution of the various constituents of the films. The experiments were carried out using a 2.5 MeV Van de Graaff accelerator. The 160 and 180 contents of the films were measured respectively through the 160(d, p)170* and 180(p, a)lSN nuclear reactions [14]. Absolute amounts of Ta and Si in the silicide and in the oxide layers were obtained using 2 MeV 4He+ RBS. Samples were positioned normal to the primary beam, and backscattered ions were detected at an angle of 165 °. The ttfickness and composition of the sputtered tantalum silicide films were found to be - 1600 × 10 is a t o m s / c m 2 of TaSil.78. RBS was also employed to analyse the depth distribution of the Ta and Si cations in the oxide. Assuming lateral uniformity of the Ta and Si oxide mixture, we have carried out a computer analysis of the RBS spectra using the R U M P programme [15]. 2.3. Electrical characterization
Electrical contacts on the samples were made evaporating aluminum dots of 2.6 × 10 -3 cm 2 on the oxide layer and also covering the whole backside of the sample with aluminum to ensure a good electrical contact. An island structure was allowed in the back contact to check for the quality of the ohmic contact
A. Climent et aL / Room temperature anodic plasma oxidation
127
to the Si substrate. The equivalent parallel capacitance and loss factor (tan 3) was measured in the frequency range 0.1-100 kHz using an automatic capacitance bridge HP 4274A. I - V characteristics were automatically recorded using a plcoammeter with a built-in power supply HP 4041B mon/tored with a computer. Positive bias was applied to the front contact (aluminum dots).
3. Results and discussion 3.1. Nuclear microanalysis in fig. 1 we show the RBS spectra for Ta silicide samples, only oxidized at 25 V with 180 and the others at 25 V with tSO followed by oxidations of 30, 60 and 100 V with 160. The presence of oxygen incorporated during the oxidation process is clearly seen at the surface position, either 180 for the sample oxidized at 25 V with tsO or 160 for the other samples. The amount of incorporated oxygen increases as the final oxidation voltage increases. Due to the high mass resolution of RBS for light elements the 160 and 180 signals are very well separated, and for the sample oxid~.ed at 25 V 1SO plus 30 V 160 both signals are still separated. However, the 180 signal shifts towards lower energies and at the same time broadens indicating a redistribution of ~80 atoms while they are covered by t60 atoms. These shifts of the 180 atoms proceed as the reoxidation voltage with 160 increases. For the sample reoxidized at I00 V 160 both signals merge. The conclusion that can be &awn from these atomic movements will be the object of a future work. The tantalum and silicon signals axe also present at the surface positions, indicato.o
1 .o
! .2
1.6
12o 160
160
i !!
loo
i Ii
i!t
80 ~ N
I .o
"./~
"
,
.-" ! I !.q
Si
~1 ~!1
..."!! "!!
] Channel
Fig. 1. RBS spectra of T a silicide anodized samples, 25 V with teO ( ), 2.5 V t 8 0 + 30 V t 6 0 ( - - - ) , 25 V t S O + 6 0 V 160 ( . . . . . ) and 25 V t S O + 100 V t 6 0 ( . . . . . . ). The low energy portion of the spectra is magnified 3 times for clarity. Arrows indicate the surface sigmd of the elements.
128
A. Climent et aL / Room temperature anodic plasma oxidation
140
0;6
0;8
1;0
Energy (Me'g)
1;2
1;4
1.0
1;8
12°t11 160 180
-o S O ~
~ ~o
-
1~
~o
~6o
Channel
soo
Fig. 2. RBS spectrum of the sample anodized at 25 V 1so + 30 V 160-( + ) and simulation obtained with RUMP (solid line). i , g t h a t t h e o x i d e f o r m e d is a m i x t u r e o f m e t a l a n d silicon oxide. A detailed d i s t r i b u t i o n o f t h e c a t i o n s in t h e oxide h a s b e e n o b t a i n e d u s i n g t h e R U M P fitting procedure, a n d a s s u m i n g t h a t t h e o x i d e is a m i x t u r e of T a 2 0 5 a n d SiO 2 w h o s e relative a m o u n t varies in d e p t h . Kt is also e v i d e n t f r o m t h e s h a p e o f : h e s p e c t r a o f fig. 1 t h a t t h e o x i d e m i x t u r e is richer in T a at the o u t e r surface. I n this region the relative a m o u n t o f T a 2 0 5 is g r e a t e r t h a n t h a t expected f r o m a h o m o g e n e o u s o x i d a t i o n o f TaSil.78, while in t h e i n n e r o x i d e layer t h e a m o u n t is lower. I n fig. 2 we s h o w as a n e x a m p l e t h e e x p e r i m e n t a l R B S s p e c t r u m for t h e s a m p l e oxidized at 25 V t S o p l u s 30 V t 6 0 , t h e solid line s h o w s t h e s i m u l a t e d s p e c t r u m . S i m u l a t i o n s o f similar q u a l i t y were o b t a i n e d for all t h e s a m p l e s w h o s e R B S s p e c t r a are s h o w n in fig. 1, I n table I we s u m m a r i z e t h e layers a n d Table 1 Composition of the anodized sample 25 V lSO+100 V 160 Layer
Thickness (1015 atoms/cmz)
Composition
lSo amount (1015 atoms/cmz)
160 amount (1015atoms/cm2)
1 2 3 4 5 6 a)
75 175 200 500 600 150
13.1 25.5 23.4 106.7
51.9 109.7 112.1 317.8 302.9
7 8
980
Ta205 + 2 SiO2 TazO5 +2.2 SiO2 Ta20 s + 2.9 SiO2 Ta20 s +4.8 SiO2 Ta205 +4.6 SiO2 Ta205 + 2.26 SiO2 + TaSil.63 TaSil.63 Si substrate
27.4 -
59.9 -
a) Interface not well defined, possibly corrugated.
A. Climent et aL / Room temperature anodic plasma oxidation
129
Table 2 Overall amounts of 180 and 1°O of the anodized samples Sample
25 V laO 25 V laO + 30 V 160 25 V 180+60 V 160 25 V laO+ 100 V 160
180 amount (101S atoms/cm2) NRA RUMP 188 204.7 188.5 176.5 168 186.9 185 196.1
t60 amount (10is atoms/cm2) NRA RUMP 60 272 280.7 500 511.2 860 902.1
composition assumed to fit the spectrum of the sample reoxidized at 100 V ~60. This is the sample where more layers were needed in order to obtain a satisfactory fit. When fitting the results it was observed that the composition of the remMning Ta silicide layer changed gradually from TaSim8 for the unoxidized sample to TaSiL63 for the re,oxidized at 100 V 160. This fact could indicate either the possibility of a slight preference in the oxidation of Si atoms or inhomogeneties in the deposited silicide layer, although the depositions were made in a single run in a rotatory planetary type sample holder. The total contents of 160 and 180 was determined by N R A which is more precise than RBS. In table 2 we show these contents compared to the amounts determined with RUMP. The agreement using both procedures is reasonably good. 3.2. E l e c t r i c a l m e a s u r e m e n t s
Fig. 3 shows the equivalent parallel capacitance and loss factor, tan 8, measured in the frequency range 0.1-100 kHz for the set of samples studied. In all cases the capacitance shows a dispersion in this frequency range lower than 49~, being higher at lower frequencies and decreasing as the frequency increases. The loss factor varies between 0.01 and 0.003 and generally has a minimum between 10 and 40 kHz. An estimate of the thickness of the sample was made from the results obtained using the programme R U M P according to the oxide mixture composition of Ta 205 and SiO2 obtained previously for the different layers in each sample, and assuming that 10 ~ 5 0 atoms cm 2 in Ta205 is equivalent to a thickness of 1.8 A and 10 is O a t o m s / c m 2 in SiO 2 corresponds to 2.25 A. A theoretical value of the dielectric constant for each sample was computed considering the different layers as connected as series capacitors and assuming each layer as composed by an homogeneous mixture of Ta 205 and SiO 2 with relative dielectric constants of 28 and 3.9 respectively. In table 3 we summarize the experimental, evaluated at 1 kHz, and theoretical values obtained for the dielectric constant of the whole insulating
130
A. Climent et al. / Room temperature anodic plasma oxidation
10"
¢0
z, e
1.00 o ~ ~ d
,
,
o°
o :
0.98[
~
:
:
! 0.96[-
,
0.1
,
,
,
1
~
,
~
, 100
10
FREQUENCY I KHz
Fig. 3. Loss factor (tan 8) and capacitance of Ta silicide samples plasma anodized; CO refers to the capacitance value measured at 0.1 kHz. From thinner to thicker oxide Co is 442 pF (o), 212 pF (®), 125 pF (zx)and 78 pF (4). film. T h e a g r e e m e n t b e t w e e n e x p e r i m e n t a l a n d theoretical v a l u e s is v e r y g o o d for t h e t h i n n e r o r / d e s b u t s o m e w h a t w o r s e n s for t h e t h i c k e r ones. A s t h e o x i d e films b e c o m e thicker we n e e d to a d d m o r e o x i d e layers o f d i f f e r e n t relative c o m p o s i t i o n s o f T a 2 0 5 a n d SiO 2 so t h a t t h e m o d e l o f o u r s a m p l e r e p r o d u c e s t h e R B S spectra. W h i l e o n e layer is n e e d e d to fit t h e t h i n n e r one, six layers a r e n e e d e d for t h e thicker, h a v i n g , therefore, m o r e s o u r c e s o f error. W e c a n c o n c l u d e f r o m t h e s e results t h a t o u r a s s u m p t i o n o f t h e g r o w n o x i d e in t h e p l a s m a a n o d i z a t i o n p r o c e s s b e i n g a m i x t u r e o f T a 2 O s a n d SiO2 is q u i t e reasonable. I - V m e a s u r e m e n t s were p e r f o r m e d in o r d e r to o b t a i n t h e resistivity o f t h e oxide, the c o n d u c t i o n m e c h a n i s m s a n d t h e dielectric s t r e n g t h . All t h e r e s u l t s r e p o r t e d in this w o r k s t a n d for t h e case w h e r e t h e alurrfinum d o t electrode o n t h e oxide is positively b i a s e d w i t h r e s p e c t to t h e b a c k c o u n t e r e l e c t r o d e o n t h e Table 3 Experimental and theoretical values for the dielectric constant Sample
Oxide thickness (.~)
C=p (1 kHz)
Cth
25 V 180 25 V 180+30 V 160 25 V laO+60 V 160 25 V 180+ 100 V 160
429 946 1448 2378
8.1 8.5 7.7 7.8
8.0 8.7 8.6 8.8
A. Climent et al. / Room temperature anodicplasma oxidation
Table4 Summaryof electricalresults Resistivity(fl cm)
Dielectric strength(V cm-~)
7xlO14-5xlO1~
3X105-106
Vglueof highfrequency dielectricconstant ag.~,uminga Schottkytype co,aductionmechanism 2.5-5
silicon substrate. The procedure to elucidate the dominating conduction mechanism from the I - V data was done with the, aid of a computer which represented the data in different kinds of plots; log I versus log V for ohmic and space charge limited mechanisms (I(X V or Io: Vn, n > 2 ) [16], log I versus V 1/2 for dectrode limited (Schottky) mechanisms and log I / V versus V t/2 for Poole-Frenkel mechanisms [17]. The occurrence of stra/ght lines for the experimental I - V values would be indicative of the corresponding conduction mechanisms, and the value of the high frequency didectric constant obtained from the slope of the straight line when a Schottky or Poole-Frenkel mechanism is possible gives a further check for its plausibility. In general the results obtained were not very reprod~:ible, and it was difficult to assign one dominating conduction mechanism. The lack of reproducibility is assumed to be due to inhomogeneities in the th/ekness of the oxide and to a random distribution of points of low resistivity (pin holes). As a general feature it has been observed that for low voltages the conduction process is ohmic allowing the calculation of the DC resistivity and at higher voltages the conduction mechanism more often observed was of the Sehottky type. The results are summarized in table 4.
4. Conclusions We have studied the room temperature anod/e plasma oxidation of tantalum sil/cide layers deposited by cosputtering of tantalum and silicon targets. The growth oxide is a mixture of Ta2Os and SiO2 whose relative composition varies a.. a ~unction of depth with an enrichment of Ta2Os in the outer portion of the ox~J% The RBS measurements wh/ch were used to calculate the cation distribution, in the oxide were consistent with NRA measurements of oxygen concentration assuming that the oxide was a m/xture of Ta205 and SIO2. This assumption is further confh'med by the measurement of the d i d e c ~ c constant of the layered oxide film. I - V measurements have allowed us to obtain the resistivity and breakdown voltage of the oxide. The conduction mechanism most often observed is the one governed 3y the Schottky effect.
132
A. Climent et al. / Room temperature anodic plasma oxidation
Ac~mov~gemeu~s T h i s w o r k h a s b e e n s u p p o r t e d b y t h e N A T O ( f e l l o w s h i p to A. C l i m e n t ) , b y the C N R S ( G r e c o N o . 86) a n d b y the C I C Y T ( p r o j e c t N o . P M E 8 5 - 8 - C 0 3 ) .
References [11 S.P. Murarka, Silicides for VLSI Applications (Academic Press, New York, 1983). [2] M. Baxtur and M.A. Nicolet, J. Electrochem. Soc. 131 (1984) 371. [3] L.N. Lie, W.A. Tiller and K.G. Saraswat, J. Appl. Phys. 56 (1984) 2127. [41 F.M. d'Heurle, E.A. Irene and C.Y. Ting, Appl. Phys. Letters 42 (1983) 361. [51 S. Zirinsky, W. Hammer, F.M. d'Heurle and 3. Baglin, Appl. Phys. Letters 33 (1978) 76. [6] J.M. Martinez-Duart, M. FernAndez, E. Paule, A. Climent, J.M. Albella, J. Perriere and J. Siejka, Appl. Phys. Letters 47 (1985) 579. [7] A.J. Barcz, M. Batur, T. Bauwell and M.A. Nicolet, J. Electrochem. Soe. 132 (1985) 2312. [8] W.J. Strydour, J.C. Lombaard and R. Pretorius, Solid State Electron. 21 (1987) 947. [9] J. Perriere, J. Siejka, A. Laurent, J.P. Enard and F.M. d'Heurle, Mater. Res. Soc. Syrup. Proc. 38 (1985) 443. [10] B. Pelloie, J. Perriere, J. Siejka, J.P. Enard and A. Laurent, in: E-MRS Symp. Proe. Strasbourg (1986) p. 65. [11] E. Navarro, A. Climent, J.M. Martinez-Duart, B. Penoie, J. Perriere and .L Siejka, in: E-MRS Syrup. Proc., Strasbourg (1986) p. 59. [12] J. Perriere, J. Siejka, A. Climent, E. Navarro and J.M. Martlnez-Duart, J. Appl. Phys. 61 (1987) 2656. [13] A. Climent, J.P. Enard, B. Lavernhe, J. Perriere, A. Straboni, B. Vuillermoz and D. Levy, Appl. Surface Sei. 36 (1989) 185. [14] G. Amsel, J.P. Nadar, E. d'Artrmare, D. David, E. Girard and J. Moulin, Nuel. Instr. Methods 92 (1971) 481. [15] L.R. Doolittle, Nucl. Instr. Methods B 9 (1985) 344. [16] M.A. Lampert and P. Mark, Current Injection in Solids (Academic Press, New York, 1970). [17] D.R. Lamb, Electrical Conduction Mechanisms in Thin Insulating Films (Methuen, London, 1967).
125
R O O M T E M P E R A T U R E ANODliC P L A S M A O X I D A T I O N OF TANTALUM SIUCIDE: OXIDE COMPOS~I'ION AND ELECr~CAL PROPERTIES A. C L I M E N T *, J. P E R R I E R E , A. L A U R E N T , B. L A V E R N H E Groupe de Physique des Solides, Universit~ Paris VII, Tour 23, 2 Place Jussieu, F-75251 Paris Cedex 05, France
R. P I ~ R E Z - C A S E R O a n d J.M. M A R T i N E Z - D U A R T Departamemo de Fisica Aplicada, C-12, e lnstituto de Ciencia de Materiales (B), CSIC, Universidad Aut6noma de Madria~ Cantoblanco, E-28049 Madria[ Spain
Received 19 March 1989; accepted for publication 17 April 1989 The room temperature anodic plasma oxidation of Ta sificide has been studied using ion backscattering (RBS), and nuclear reaction analysis (NRA). Thin layers of tantalun silicide were deposited on silicon substrates by cosputtering from Si and Ta targets and anodically oxidized in a multipolar plasma system. RBS spectra show that during anodization a mixed oxide of Ta2Os and SiO, is formed. The distribution of this oxide mixture is not uniform and two layers are clearly distinguished. An outer layer forms in which the oxide mixture is richer in Ta2Os than SiOe as compared to the starting silicide composition. The inner layer on the other hand, is richer in SiOe. The resulting oxide has been electrically characterized by measuring the I - V curves and the dielectric constant in the frequency range 0.1-100 kHz.
L Intreductlon Metal silicides are used in very large scale integrated (VLSI) devices as gate or interconnection materials because o f their g o o d electrical conductivity a n d high chemical stability. W h e n used as i n t e r c o n n e c t material it is necessary to cover the metal silicide with an insulating film such as SiO 2, to e n a b l e the use o f over-crossing c o n d u c t o r s in s u b s e q u e n t layers. T h u s a g o o d deal o f w o r k o n the thermal oxidation behaviour o f metal silJcides has b e e n d o n e over the p a s t few years [1-5]. As thermal oxidation requires t e m p e r a t u r e s o f a r o u n d 1000 ° C, u n w a n t e d p h e n o m e n a m a y occur such as interdiffusion a n d reactions w h i c h are not tolerated in the VLSI technologies. M o r e recently, in o r d e r to avoid high temperatures, low t e m p e r a t u r e oxidation processes o f the silicides, such as wet anodic [6-8], p l a s m a anodic [9-12] a n d p l a s m a at floating p o t e n t i a l [13], have b e e n undertaken. * Permanent address: Departamento de Fisica Aplicada, C-12, Universidad Aut6noma de Madrid, Cantoblanco, E-28049 Madrid, Spain. 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
126
A. Climent et al. / Room temperature anodic plasma oxidation
in the present work we investigate the electric field assisted plasma oxidation process (plasma anodization of cosputtered TaSi mixtures). The ox;de growth rate and mean electric field of formation are determined through the use of nuclear microanalysis (NRA) and Rutherford backscattering techniques (RBS). Tiffs latter technique is used to obtain the depth distribution of cations in the oxide.
2. Ex~efiment,'~l 2.1. S a m p l e preparation
Tantalum silicide thin films were prepared in a Varian S-magnetron sputter-deposition system, by simultaneous sputtering from tantalum and silicon targets onto silicon single crystal substrates. The samples were anodized in a multipolar plasma system described elsewhere [9]. During typical oxidations, the oxygen pressure was 5 × 10 -4 Torr. Successive anodization of the samples, first in 180 plasma up to 25 V and then with 160 plasma at several voltages up to 125 V was made in order to allow for the possibility of oxygen transport studies. 2.2. S a m p l e analysis
Nuclear microanalysis was used to determine the nature and dis "ribution of the various constituents of the films. The experiments were carried out using a 2.5 MeV Van de Graaff accelerator. The 160 and 180 contents of the films were measured respectively through the 160(d, p)170* and 180(p, a)lSN nuclear reactions [14]. Absolute amounts of Ta and Si in the silicide and in the oxide layers were obtained using 2 MeV 4He+ RBS. Samples were positioned normal to the primary beam, and backscattered ions were detected at an angle of 165 °. The ttfickness and composition of the sputtered tantalum silicide films were found to be - 1600 × 10 is a t o m s / c m 2 of TaSil.78. RBS was also employed to analyse the depth distribution of the Ta and Si cations in the oxide. Assuming lateral uniformity of the Ta and Si oxide mixture, we have carried out a computer analysis of the RBS spectra using the R U M P programme [15]. 2.3. Electrical characterization
Electrical contacts on the samples were made evaporating aluminum dots of 2.6 × 10 -3 cm 2 on the oxide layer and also covering the whole backside of the sample with aluminum to ensure a good electrical contact. An island structure was allowed in the back contact to check for the quality of the ohmic contact
A. Climent et aL / Room temperature anodic plasma oxidation
127
to the Si substrate. The equivalent parallel capacitance and loss factor (tan 3) was measured in the frequency range 0.1-100 kHz using an automatic capacitance bridge HP 4274A. I - V characteristics were automatically recorded using a plcoammeter with a built-in power supply HP 4041B mon/tored with a computer. Positive bias was applied to the front contact (aluminum dots).
3. Results and discussion 3.1. Nuclear microanalysis in fig. 1 we show the RBS spectra for Ta silicide samples, only oxidized at 25 V with 180 and the others at 25 V with tSO followed by oxidations of 30, 60 and 100 V with 160. The presence of oxygen incorporated during the oxidation process is clearly seen at the surface position, either 180 for the sample oxidized at 25 V with tsO or 160 for the other samples. The amount of incorporated oxygen increases as the final oxidation voltage increases. Due to the high mass resolution of RBS for light elements the 160 and 180 signals are very well separated, and for the sample oxid~.ed at 25 V 1SO plus 30 V 160 both signals are still separated. However, the 180 signal shifts towards lower energies and at the same time broadens indicating a redistribution of ~80 atoms while they are covered by t60 atoms. These shifts of the 180 atoms proceed as the reoxidation voltage with 160 increases. For the sample reoxidized at I00 V 160 both signals merge. The conclusion that can be &awn from these atomic movements will be the object of a future work. The tantalum and silicon signals axe also present at the surface positions, indicato.o
1 .o
! .2
1.6
12o 160
160
i !!
loo
i Ii
i!t
80 ~ N
I .o
"./~
"
,
.-" ! I !.q
Si
~1 ~!1
..."!! "!!
] Channel
Fig. 1. RBS spectra of T a silicide anodized samples, 25 V with teO ( ), 2.5 V t 8 0 + 30 V t 6 0 ( - - - ) , 25 V t S O + 6 0 V 160 ( . . . . . ) and 25 V t S O + 100 V t 6 0 ( . . . . . . ). The low energy portion of the spectra is magnified 3 times for clarity. Arrows indicate the surface sigmd of the elements.
128
A. Climent et aL / Room temperature anodic plasma oxidation
140
0;6
0;8
1;0
Energy (Me'g)
1;2
1;4
1.0
1;8
12°t11 160 180
-o S O ~
~ ~o
-
1~
~o
~6o
Channel
soo
Fig. 2. RBS spectrum of the sample anodized at 25 V 1so + 30 V 160-( + ) and simulation obtained with RUMP (solid line). i , g t h a t t h e o x i d e f o r m e d is a m i x t u r e o f m e t a l a n d silicon oxide. A detailed d i s t r i b u t i o n o f t h e c a t i o n s in t h e oxide h a s b e e n o b t a i n e d u s i n g t h e R U M P fitting procedure, a n d a s s u m i n g t h a t t h e o x i d e is a m i x t u r e of T a 2 0 5 a n d SiO 2 w h o s e relative a m o u n t varies in d e p t h . Kt is also e v i d e n t f r o m t h e s h a p e o f : h e s p e c t r a o f fig. 1 t h a t t h e o x i d e m i x t u r e is richer in T a at the o u t e r surface. I n this region the relative a m o u n t o f T a 2 0 5 is g r e a t e r t h a n t h a t expected f r o m a h o m o g e n e o u s o x i d a t i o n o f TaSil.78, while in t h e i n n e r o x i d e layer t h e a m o u n t is lower. I n fig. 2 we s h o w as a n e x a m p l e t h e e x p e r i m e n t a l R B S s p e c t r u m for t h e s a m p l e oxidized at 25 V t S o p l u s 30 V t 6 0 , t h e solid line s h o w s t h e s i m u l a t e d s p e c t r u m . S i m u l a t i o n s o f similar q u a l i t y were o b t a i n e d for all t h e s a m p l e s w h o s e R B S s p e c t r a are s h o w n in fig. 1, I n table I we s u m m a r i z e t h e layers a n d Table 1 Composition of the anodized sample 25 V lSO+100 V 160 Layer
Thickness (1015 atoms/cmz)
Composition
lSo amount (1015 atoms/cmz)
160 amount (1015atoms/cm2)
1 2 3 4 5 6 a)
75 175 200 500 600 150
13.1 25.5 23.4 106.7
51.9 109.7 112.1 317.8 302.9
7 8
980
Ta205 + 2 SiO2 TazO5 +2.2 SiO2 Ta20 s + 2.9 SiO2 Ta20 s +4.8 SiO2 Ta205 +4.6 SiO2 Ta205 + 2.26 SiO2 + TaSil.63 TaSil.63 Si substrate
27.4 -
59.9 -
a) Interface not well defined, possibly corrugated.
A. Climent et aL / Room temperature anodic plasma oxidation
129
Table 2 Overall amounts of 180 and 1°O of the anodized samples Sample
25 V laO 25 V laO + 30 V 160 25 V 180+60 V 160 25 V laO+ 100 V 160
180 amount (101S atoms/cm2) NRA RUMP 188 204.7 188.5 176.5 168 186.9 185 196.1
t60 amount (10is atoms/cm2) NRA RUMP 60 272 280.7 500 511.2 860 902.1
composition assumed to fit the spectrum of the sample reoxidized at 100 V ~60. This is the sample where more layers were needed in order to obtain a satisfactory fit. When fitting the results it was observed that the composition of the remMning Ta silicide layer changed gradually from TaSim8 for the unoxidized sample to TaSiL63 for the re,oxidized at 100 V 160. This fact could indicate either the possibility of a slight preference in the oxidation of Si atoms or inhomogeneties in the deposited silicide layer, although the depositions were made in a single run in a rotatory planetary type sample holder. The total contents of 160 and 180 was determined by N R A which is more precise than RBS. In table 2 we show these contents compared to the amounts determined with RUMP. The agreement using both procedures is reasonably good. 3.2. E l e c t r i c a l m e a s u r e m e n t s
Fig. 3 shows the equivalent parallel capacitance and loss factor, tan 8, measured in the frequency range 0.1-100 kHz for the set of samples studied. In all cases the capacitance shows a dispersion in this frequency range lower than 49~, being higher at lower frequencies and decreasing as the frequency increases. The loss factor varies between 0.01 and 0.003 and generally has a minimum between 10 and 40 kHz. An estimate of the thickness of the sample was made from the results obtained using the programme R U M P according to the oxide mixture composition of Ta 205 and SiO2 obtained previously for the different layers in each sample, and assuming that 10 ~ 5 0 atoms cm 2 in Ta205 is equivalent to a thickness of 1.8 A and 10 is O a t o m s / c m 2 in SiO 2 corresponds to 2.25 A. A theoretical value of the dielectric constant for each sample was computed considering the different layers as connected as series capacitors and assuming each layer as composed by an homogeneous mixture of Ta 205 and SiO 2 with relative dielectric constants of 28 and 3.9 respectively. In table 3 we summarize the experimental, evaluated at 1 kHz, and theoretical values obtained for the dielectric constant of the whole insulating
130
A. Climent et al. / Room temperature anodic plasma oxidation
10"
¢0
z, e
1.00 o ~ ~ d
,
,
o°
o :
0.98[
~
:
:
! 0.96[-
,
0.1
,
,
,
1
~
,
~
, 100
10
FREQUENCY I KHz
Fig. 3. Loss factor (tan 8) and capacitance of Ta silicide samples plasma anodized; CO refers to the capacitance value measured at 0.1 kHz. From thinner to thicker oxide Co is 442 pF (o), 212 pF (®), 125 pF (zx)and 78 pF (4). film. T h e a g r e e m e n t b e t w e e n e x p e r i m e n t a l a n d theoretical v a l u e s is v e r y g o o d for t h e t h i n n e r o r / d e s b u t s o m e w h a t w o r s e n s for t h e t h i c k e r ones. A s t h e o x i d e films b e c o m e thicker we n e e d to a d d m o r e o x i d e layers o f d i f f e r e n t relative c o m p o s i t i o n s o f T a 2 0 5 a n d SiO 2 so t h a t t h e m o d e l o f o u r s a m p l e r e p r o d u c e s t h e R B S spectra. W h i l e o n e layer is n e e d e d to fit t h e t h i n n e r one, six layers a r e n e e d e d for t h e thicker, h a v i n g , therefore, m o r e s o u r c e s o f error. W e c a n c o n c l u d e f r o m t h e s e results t h a t o u r a s s u m p t i o n o f t h e g r o w n o x i d e in t h e p l a s m a a n o d i z a t i o n p r o c e s s b e i n g a m i x t u r e o f T a 2 O s a n d SiO2 is q u i t e reasonable. I - V m e a s u r e m e n t s were p e r f o r m e d in o r d e r to o b t a i n t h e resistivity o f t h e oxide, the c o n d u c t i o n m e c h a n i s m s a n d t h e dielectric s t r e n g t h . All t h e r e s u l t s r e p o r t e d in this w o r k s t a n d for t h e case w h e r e t h e alurrfinum d o t electrode o n t h e oxide is positively b i a s e d w i t h r e s p e c t to t h e b a c k c o u n t e r e l e c t r o d e o n t h e Table 3 Experimental and theoretical values for the dielectric constant Sample
Oxide thickness (.~)
C=p (1 kHz)
Cth
25 V 180 25 V 180+30 V 160 25 V laO+60 V 160 25 V 180+ 100 V 160
429 946 1448 2378
8.1 8.5 7.7 7.8
8.0 8.7 8.6 8.8
A. Climent et al. / Room temperature anodicplasma oxidation
Table4 Summaryof electricalresults Resistivity(fl cm)
Dielectric strength(V cm-~)
7xlO14-5xlO1~
3X105-106
Vglueof highfrequency dielectricconstant ag.~,uminga Schottkytype co,aductionmechanism 2.5-5
silicon substrate. The procedure to elucidate the dominating conduction mechanism from the I - V data was done with the, aid of a computer which represented the data in different kinds of plots; log I versus log V for ohmic and space charge limited mechanisms (I(X V or Io: Vn, n > 2 ) [16], log I versus V 1/2 for dectrode limited (Schottky) mechanisms and log I / V versus V t/2 for Poole-Frenkel mechanisms [17]. The occurrence of stra/ght lines for the experimental I - V values would be indicative of the corresponding conduction mechanisms, and the value of the high frequency didectric constant obtained from the slope of the straight line when a Schottky or Poole-Frenkel mechanism is possible gives a further check for its plausibility. In general the results obtained were not very reprod~:ible, and it was difficult to assign one dominating conduction mechanism. The lack of reproducibility is assumed to be due to inhomogeneities in the th/ekness of the oxide and to a random distribution of points of low resistivity (pin holes). As a general feature it has been observed that for low voltages the conduction process is ohmic allowing the calculation of the DC resistivity and at higher voltages the conduction mechanism more often observed was of the Sehottky type. The results are summarized in table 4.
4. Conclusions We have studied the room temperature anod/e plasma oxidation of tantalum sil/cide layers deposited by cosputtering of tantalum and silicon targets. The growth oxide is a mixture of Ta2Os and SiO2 whose relative composition varies a.. a ~unction of depth with an enrichment of Ta2Os in the outer portion of the ox~J% The RBS measurements wh/ch were used to calculate the cation distribution, in the oxide were consistent with NRA measurements of oxygen concentration assuming that the oxide was a m/xture of Ta205 and SIO2. This assumption is further confh'med by the measurement of the d i d e c ~ c constant of the layered oxide film. I - V measurements have allowed us to obtain the resistivity and breakdown voltage of the oxide. The conduction mechanism most often observed is the one governed 3y the Schottky effect.
132
A. Climent et al. / Room temperature anodic plasma oxidation
Ac~mov~gemeu~s T h i s w o r k h a s b e e n s u p p o r t e d b y t h e N A T O ( f e l l o w s h i p to A. C l i m e n t ) , b y the C N R S ( G r e c o N o . 86) a n d b y the C I C Y T ( p r o j e c t N o . P M E 8 5 - 8 - C 0 3 ) .
References [11 S.P. Murarka, Silicides for VLSI Applications (Academic Press, New York, 1983). [2] M. Baxtur and M.A. Nicolet, J. Electrochem. Soc. 131 (1984) 371. [3] L.N. Lie, W.A. Tiller and K.G. Saraswat, J. Appl. Phys. 56 (1984) 2127. [41 F.M. d'Heurle, E.A. Irene and C.Y. Ting, Appl. Phys. Letters 42 (1983) 361. [51 S. Zirinsky, W. Hammer, F.M. d'Heurle and 3. Baglin, Appl. Phys. Letters 33 (1978) 76. [6] J.M. Martinez-Duart, M. FernAndez, E. Paule, A. Climent, J.M. Albella, J. Perriere and J. Siejka, Appl. Phys. Letters 47 (1985) 579. [7] A.J. Barcz, M. Batur, T. Bauwell and M.A. Nicolet, J. Electrochem. Soe. 132 (1985) 2312. [8] W.J. Strydour, J.C. Lombaard and R. Pretorius, Solid State Electron. 21 (1987) 947. [9] J. Perriere, J. Siejka, A. Laurent, J.P. Enard and F.M. d'Heurle, Mater. Res. Soc. Syrup. Proc. 38 (1985) 443. [10] B. Pelloie, J. Perriere, J. Siejka, J.P. Enard and A. Laurent, in: E-MRS Symp. Proe. Strasbourg (1986) p. 65. [11] E. Navarro, A. Climent, J.M. Martinez-Duart, B. Penoie, J. Perriere and .L Siejka, in: E-MRS Syrup. Proc., Strasbourg (1986) p. 59. [12] J. Perriere, J. Siejka, A. Climent, E. Navarro and J.M. Martlnez-Duart, J. Appl. Phys. 61 (1987) 2656. [13] A. Climent, J.P. Enard, B. Lavernhe, J. Perriere, A. Straboni, B. Vuillermoz and D. Levy, Appl. Surface Sei. 36 (1989) 185. [14] G. Amsel, J.P. Nadar, E. d'Artrmare, D. David, E. Girard and J. Moulin, Nuel. Instr. Methods 92 (1971) 481. [15] L.R. Doolittle, Nucl. Instr. Methods B 9 (1985) 344. [16] M.A. Lampert and P. Mark, Current Injection in Solids (Academic Press, New York, 1970). [17] D.R. Lamb, Electrical Conduction Mechanisms in Thin Insulating Films (Methuen, London, 1967).