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Si by thermal annealing under NH3 of W on Si
Applied Surface Science 38 (1989) 139-147 North-Holland, Amsterdam
139
INFLUENCE OF THE PREIPARATION CONDITIONS O N T H E F O R M A T g O N O F W N x / W S I 2 / S i BY THER~.~AL A N N E A L I N G U N D E R N H 3 O F W O N Si A. D E N E U V I L L E , M. B E F ~ r A H Y A , M. B R U N E L *, J.C. O B E R L I N * *, J. T O R R E S * *, N. B O U R H I L A * *, J. P A L E A U * * a n d B. C A N U T * * * L E P E S CNRS, BP 166, 38042 Grenoble Cedex, France
Received 19 March 1989; accepted for publication 10 April 1989
The effect of annealing in NH 3 between 600 and II00°C of W films deposited on Si was studied by X-ray diffraction, RBS, nuclear reaction (N concentration) and I/R transmission (N bonding). WN~,/Si a~d WNx/WNSi/WSi2/Si structures are obtained for W thicknesses lower and higher than 100 A respectively. The WNSi layer contains Si-N bonds, but remains strongly metallic. Its thickness increases with Ta.
L [n~uefion
T h e c o n t i n u o u s decrease o f the M O S channels lengths leads to the use o f self aligned technics for the contacts o n t h e source a n d the drain o f the transitors [1]. Usually, a refractory metal silicide is f o r m e d preferentially o n Si b y thermal annealing u n d e r v a c u u m o r neutral a t m o s p h e r e o f a refractory metal d e p o s i t e d o n the whole chip. Recently, annealing o f the deposited Ti has also b e e n p e r f o r m e d in N H 3, resulting in crystalline T i N x / T i S i z / S i structures o n Si, a n d in crystalline T i N x o n $iO 2 w h i c h avoid the lateral f o r m a t i o n o f the silicid¢ as s h o w n b y Halperin et al. [2]. T o o u r p r e s e n t knowledge, n o such w o r k has b e e n dedicated to sLmilar f o r m a t i o n m e c h a n i s m s b y annealing in N H 3 o f o t h e r refractory metals d e p o s i t e d o n Si. F r o m the prel/minary w o r k o f Deneuville et al. [3], annealing o f 200 A thick W films d o e s n o t yield WSi 2 for 6 0 0 ° C < Ta < 1 1 0 0 ° C , while that o f 2000 A thick W films results in WSi 2 for 8 0 0 ° C < T~. F r o m T o r t e s et al. [4], this type o f annealing also avoid t h e lateral f o r m a t i o n o f WSi 2. * Permanent address: Laboratoire de Cfistallographie, LEPES CNRS, BP 166, 38042 Grenoble Cedex, France. * Permanent address: CNET CNS, BP 98, F-38243 Meylan Codex, France. * * * Permanent address: Department Physique de la Mati~re, Universit6 Claude Bernard, Bd. du 11 Nov 1918, Villeurbanne Cedex, France. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics PublLslfing Division)
t 40
A. Deneuville et aL / Effect of annealing under NH.~ of W on Si
We look here, at the W, Si and N concentrations profiles, and at the nature of N bonding and c..mpounds through the whole multilayer thickness as a function of the thickr; zss of the initial W film and of the temperature of anneafing.
2. 1FTepatafion a~d e×perimeutan set up The W layers are deposited on a Si substrate at 2 0 0 ° C by cathodic sputtering of a W target in a 20% H2/80% Ar reactive mixture of 5 × 10 -3 Torr, with a base pressure of 10 -7 Torr, after a careful outgassing of the substrate and of the deposition chamber. The samples are annealed in N H 3 in a classical Joule oven during 2 h, and at temperatures T~ ranging from 500 to 1100°C. The results will be reported mainly on the anneafing of the 200 and 2000 A thick W films, which will be called respectively "200 A W " and "2000 AW". The X-ray diffraction, excited with Cu K a radiation, was performed at a glancing angle of incidence of 0.5 o to increase the sensitivity. The Rutherford backscattering was done with impinging He ions of 2 MeV at normal or 50 o incidence. The results will be given as the variation of the n u m b e r of detected counts with the channel number, with an energy of appro~dmatdy 3.3 keV per channel. From the work of Amsel et al. [5], the total nitrogen content was measured from the nuclear reaction 14N(d, a'y)12C 2H +14 N ~ 1 2 C + 4 H e + 3~. The energy of the deuterium ions (d or 2H) decreases as they penetrate deeper and deeper in the film. Here, with an initial energy of 1.15 MeV, their energies always correspond to a flat part of the capture cross section for the nuclear reaction. We detect the variations of the n u m b e r of 4He particles enfitted in the vacuum with their energy. As the He ions are created at various depths, but always with the same energy, and loose energy to reach the surface before being emitted in the vacuum, the He energy scale is an image of the d e p t h scale, through the energy losses per unit length (which depends o n the local composition of the material). The measurements were calibrated with a L P C V D Si3N 4 film to derive numerical values for the N concentration. The results will be given here as the vmqation of the n u m b e r of counts vAth the channel number, with approximately 9 keV per channel. The infrared transmission was measured with a Pertdn-Elmer 683 double beam spectrophotometer. The thiclmesses of the various parts of the films were measured b y a Dektak o n steps formed after a preferential chemical etching of W and W N x.
A. Deneuville el al. / Effect of annealing under NHj of Won Si
141
3. Results and discussion
3.1.X-raydiffraction We extend here the previous results of Deneuville et al. [3]. For the "200 .~ W " films, there is formation of W N x o r mixture of WN x o n Si from Ta = 600 o C up to T~ = 1100°C. For the "1000 A W " f~ms, the WSi 2 peaks appears only at 1 1 0 0 ° C in addition to those of the tungsten nitrides. For the "2000 A W " films, the WSi 2 peaks already appear for T~ > 800°C.
3.2.Rutherfordbacksc~ttering For the "200 A W " f~lms, even at an angle of incidence of 50 °, we see only a decrease a n d a widening of the W signal, with a slight translation of the Si signal as Ta increases. After etching of the superficial metallic layer, only negligible W concentratlo• remains, which confirms the negligible W - S i interdiffusion. Fig. 1 shows the P ~ $ signals at normal incidence after annealing under N H 3 at 700, 800 a n d 1 0 0 0 ° C of the "2000 A W " f ~ n s . After annealing at 700 o C, some W losses are observed on the high energy edge of the W peak. As there is no Si at the surface, this has to originate from the formation of a W rfitrlde layer on the top of the film. The annealing at 800 o C introduces drastic changes. The W signal o n the high energy edge decreases o n a wider d e p t h indicating the formation of a wider and more N-rich W h i . d e layer o n the top of the film. Then, a step appears o n the low energy edge, at the interface between the W and the bulk Si,/rid/caring the formation of a layer with lower W concentration. A t the same time, a shonlder appears on the Si signal, indicating the formation of a layer with lower Si concentration. The analysis of
f/"\,\, ~ '
CIIANNEL
Fig. 1. RBS signals of the "2000 ,~ W" films (2000 A W/Si annealed 2 h in NH3).
142
A . Deneuville et aL / Effe'ct o f a n n e a l i n g u n d e r N H j o f W o n S i
l
-_=-: ,iii?iiii!i'
,~
/'~ :\
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i
:
,
l
i
*,~,
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~4. ('HANNI:I.
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Fig. 2. RBS signals of the "2000 .A W" films after W and WNx etching.
both the Si and w signals establishes the occurrence of a 820 .~ thick WSi 2 layer in contact with Si, while there is an intermediate layer of variable composition between this WSi 2 layer and a W-rich layer below the W nitride layer on the top of the film. Annealing at higher temperatures only emphasizes the previous trends. As an example, the RBS signal after annealing at 1000 ° C is shown in fig. 1. The W concentration on the surface yet decreases acros., a larger depth, which indicates a wider and more N-rich W nJtride layer on ~he top of the film. The mMn W signal is smaller and wider than after T~ = 800 o C, indicating now the occurrence of a W nitfide of lower N content below the N-rich rtitride layer on the top of the film. The low energy W step is widened, indicating a broader layer between the bulk Si and the nitride layers. A plateau on the Si edge appears clearly, confirming the occurrence of a wider layer with lower Si concentration than in the bulk Si. F r o m the analysis of both the Si and W signals, we deduce that a 1000 .~ thick WSi 2 layer is in contact with the Si, and with an intermediate layer in contact with the less N-rich nitride layer. To get more information about the intermediate layers, we record in fig. 2 the RBS signal of the samples of fig. 1 after etching of the W nitrides top layers. We have used an angle of incidence of 50 o for the sample annealed at 700 ° C in order to increase the depth sensitivity. A W signal is still observed, so that a beginning of interdiffusion between the Si and the W occurs already at this temperature. The Si and W edges are at the same energy as those of the samples annealed at 800 and 1 0 0 0 ° C which are recorded under normal incidence. The Si and W are thus both on the surface of the films. After annealing at 800 o C, the Si signal at the edge is smaller across a wider thickness, indicating the occurrence of a thicker intermediate layer of lower Si content than the S i - W interdiffusion layer. A t the same time, there is a peak
A. Deneuville et al, / Effect of annealing under N t t j of Ve"on Si
143
below the high energy edge in the W signal. There is thus an additional species at the beghming of this intermed/ate layer. The N signal starting at the surface and superimposed on the Si signal at lower energy shows that it is n/trogen. The intermediate layer aetuaUy contains W, Si, and N. The RB$ signal, after etching of the sample previously annealed at 1000 o C, only emphasizes the previous evolution. The W peak is larger, at lower energy and with a smaller concentration at the edge than before. This indicates a wider intermediate layer with a N higher concentration at the surface than for the sample annealed at T~ = 800 * C. The Si concentration exlfibRs a plateau and is lower than the bulk Si concentration on a wider energy range, confirming further a widening of the intermediate layer. 3. 3. N i t r o g e n c o n c e n t r a t i o n
Because the N signals are weak and superimposed on those from $i, Pd$S is not suited to study nitrogen in these films. This is done by the nuclear reaction of 2H (deuterium) with '-~N. This allows, in principle, the determination of the total N concentration (after calibration) and of the N profiles across the samples. However, even for "2000 A W" films, the energy resolution of the detector smoothes all the sharp N profiles near the sample surface and at the WSi 2 interface which are seen on the RB$ signals of figs. 1 and 2. Sharp structures near the surface and the interface of the "200 A W" films would thus not be seen. For the "200 A W" films, there is not significant concentration of chem/eal spec/es other than W and N in the top layer. We derive a mean N / W ratio at each annealing temperature from the N content measured by the nuclear reaction and the W content measured by RBS. This ratio increases from 0.37 to 1.85 for T~ from 500 to l l 0 0 * C (fig. 3). From the ASTM crystallograph/c tables, we deduce the W concentration per refit volume and the W / N ratio for each WN x crystal. The variations of their W concentration per cm3 with the
05 wf.~.~. [ ~-'~. ~W2Nofc 4,5[
=
~
t z'~.0
0.5
I I "Nw~-¢N,O)
I
~2N,,
I
1,0 N/W 1.5
2.0
2.5
Fig. 3. W concentrationversus N/W r.atiofor the WN~,crystals. N/W ratio ha the "200 A W" films(200A W/Si annealed2 h ha NHa).
144
A. Deneuoille et al. / Effect of annealing under N H 3 of W o n Si
5°01
/ ~o° .
400
300
° v
7e9o °
200
.
.
.
.
300 400 500 Calculated Thickness (A)
600
Fig. 4. Comparison of the measured and the calculated thickness of the "200 A W " films.
N / W ratio are plotted on fig. 3. Assuming then a rninture of the neighbouring crystals, we deduce for each of our annealed samples a mean W concentration per unit volume and a mean thickness from the total W concentration. This is compared to the measured thicknesses in fig. 4. For the W N x films obtained by annealing under NH3 of the 200 A, thick W films, a reasonable agreement is obtained between the measured thicknesses and those deduced from the RBS and nuclear reaction profiles, if we take into account the accuracy (50 A) of the Dektak in this thickness range. The N profiles measured by nuclear reaction for the "2000 A W " films of fig. 1 are shown on fig. 5. After annealing at 700 ° C, there is a peak in the N concentration at high energy, then a plateau with slightly decreasing concentrations as the depth increases, and then a small maximum at low energy. This agrees with the occurrence of a W nitride layer on top of the film as suggested by the RBS results. Moreover, it indicates the occurrence of another W nitride layer with lower N content underneath, across the whole film.
7~"C ....
8~C IOOO~C
/.
,,,
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,
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Fig. 5. N profiles of the "2000 ,~ W " films (nuclear reaction H i + N 14 --, C 12 -F He 4 + y).
A. Deneuuille et al. / Effect of annealing under N H j of Won Si
145
After annealing at 800°C, N peaks appear on both sides of a lower N concentration (similar to the previous one) in the central part of the film. The surface peak is higher and wider than that after annealing at Ta = 700 ° C, but the most striking effect is the higher value and the greater width of the low energy peak, which is more important than the surface peak. This stron~y confirms the RBS results on the same films, showing the occurrence of a W rdtride layer of higher N concentration and wider thickness than that formed at T, = 7 0 0 ° C (fig. 1) on top of the film, and that of an intermediate layer containing N between the WSi2 and the W rich layers (fig. 2). Moreover, from fig. 5, the N content in the intermediate layer is very important, and there is a lower N concentration similar to that obtained at Ta = 700 ° C in the central part of the " W " film. After annealing at 1000°C, there is only an emphasis of the previous evolutions. The surface peak slightly increases and broadens, while the interface peak remains at about the same maximum value, but is significantly widened. From these two evolutions, the central plateau seems to have disappeared on the 1000 ° C curve of fig. 5. This strongly confirms the RBS results on the same films which shows an increase of both the N concentration and the thickness of the W nitride layer on the top of the film, together with
700°
33
31
600
800 V(cm_l)1000
1200
Fig. 6. Infrared transmission o f the "2000 ,~ W " films after W and W N x etching.
146
A. Deneuville et al. / Effect of annealing under N H 3 of Won Si
an important widerfing in the same range of composition of the intermediate layer. 3.4. I n f r a r e d m e a s u r e m e n t s
For the "'200~) .& W " films annealed above 800°C, we have detected important N conc~5ntrations in intermediate layers containing also W and Si. The possibility of S i - N bonds has thus to be considered. As the important metallic infrared absorption hides any weak S i - N absorption signal in the samples obtained just after N H 3 annealing, the transmission measurements are done after etchi,ng of the top nitride layers. For the "200 A W " films, there are almost no S i - N absorption bands. For the initially 2000 .~ thick W films, the IR transmission spectra recorded after annealing at 700, 800, 900, and 1 0 0 0 ° C are shown in fig. 6. First, the transmission values are low and continuously decrease as the anr~ealing temperature increases. This means that the intermediate layer is metallic and confirms the RBS and nuclear results showing that its thickness increases as T~ increases. Then, there is an absorption b a n d around 850 cm-1 growing as T, increases. From Bustarret et al. [6], this can be attributed to a n increasing n u m b e r of the S i - N bonds in a local environment a-Si3N 4. So, the Infrared transmission shows the existence of a significant concentration of S i - N bonds in the intermediate layer, which retains however a strong metallic character.
4. Conc~mion We have studied the effect of 2 h annealing in N H 3 between 600 and 1 1 0 0 ° C on W films of various thicknesses sputtered on Si at 200°C. The composition of the restdting structures depends porimarily on the initial thicknesses of the W films. The border is around 1000 A where W N x / W S i J S i . For higher thicknesses, we get WN~/intermediate WNSi l a y e r / W S i 2 / S i . The nuclear measurement of the N profiles indicates the occurrence for all W thicknesses of a N-rich n i t r i d e layer on top of the film, whose N content increases with Ta. For the "200 A W " films the mean W / N ratio increases from 0.37 to 1.85 as Ta increases from 500 to 1100°C, in reasonable agreement with the measured thickness variation induced by annealing. For the "2000 ,~'" films, below the N-rich nitride layer on top of the film, there is another nitride layer with lower N content in contact with the intermediate layer. The intermediate layer contains S i - N bonds, but remains nevertheless strongly metallic. Its thickness increases with T~. The deternfination of the detailed bonding statistics in this intermediate layer is under progress.
A. Deneuoille et al. / Effect of annealing under NHj of Won Si
147
References [1] N.G. Einspruch, Ed., VLSI Electronics Microstructures Sciences (Academic Press, New York, 1982). [2] M. Halperin, T. Holloway, R. Ha.ken, C. Gosmeyer, R. Karnaughand and W. Parmentic, IEEE Trans. Electron Devices ED-32 (1985) 141. [3l A. Deneuvitle, M. Benyahya, M. Brunel and B. Canut, J. Phys. (Paris) 49 (1988) C4--499. [4] J. Tortes, J. Palleau, N. Bourhila, J.C. Gberlin, A. Deneucille and M. Benyahya, J. Phys. (Paris) 49 (1988) C4-183. [5] G. Amsel, J.P. Na'dai, E. d'Artemare, D. David E..L Girard and J. Moulin, Nucl. Instr. Methods 92 (1971) 481. [6] E. Bustarret, M. Bensouda, M.C. Habrard, J.C. Bruyd~re,S. Poulin and J.C. Gujrati, Phys. Rev. B 38 (1988) 8171.
139
INFLUENCE OF THE PREIPARATION CONDITIONS O N T H E F O R M A T g O N O F W N x / W S I 2 / S i BY THER~.~AL A N N E A L I N G U N D E R N H 3 O F W O N Si A. D E N E U V I L L E , M. B E F ~ r A H Y A , M. B R U N E L *, J.C. O B E R L I N * *, J. T O R R E S * *, N. B O U R H I L A * *, J. P A L E A U * * a n d B. C A N U T * * * L E P E S CNRS, BP 166, 38042 Grenoble Cedex, France
Received 19 March 1989; accepted for publication 10 April 1989
The effect of annealing in NH 3 between 600 and II00°C of W films deposited on Si was studied by X-ray diffraction, RBS, nuclear reaction (N concentration) and I/R transmission (N bonding). WN~,/Si a~d WNx/WNSi/WSi2/Si structures are obtained for W thicknesses lower and higher than 100 A respectively. The WNSi layer contains Si-N bonds, but remains strongly metallic. Its thickness increases with Ta.
L [n~uefion
T h e c o n t i n u o u s decrease o f the M O S channels lengths leads to the use o f self aligned technics for the contacts o n t h e source a n d the drain o f the transitors [1]. Usually, a refractory metal silicide is f o r m e d preferentially o n Si b y thermal annealing u n d e r v a c u u m o r neutral a t m o s p h e r e o f a refractory metal d e p o s i t e d o n the whole chip. Recently, annealing o f the deposited Ti has also b e e n p e r f o r m e d in N H 3, resulting in crystalline T i N x / T i S i z / S i structures o n Si, a n d in crystalline T i N x o n $iO 2 w h i c h avoid the lateral f o r m a t i o n o f the silicid¢ as s h o w n b y Halperin et al. [2]. T o o u r p r e s e n t knowledge, n o such w o r k has b e e n dedicated to sLmilar f o r m a t i o n m e c h a n i s m s b y annealing in N H 3 o f o t h e r refractory metals d e p o s i t e d o n Si. F r o m the prel/minary w o r k o f Deneuville et al. [3], annealing o f 200 A thick W films d o e s n o t yield WSi 2 for 6 0 0 ° C < Ta < 1 1 0 0 ° C , while that o f 2000 A thick W films results in WSi 2 for 8 0 0 ° C < T~. F r o m T o r t e s et al. [4], this type o f annealing also avoid t h e lateral f o r m a t i o n o f WSi 2. * Permanent address: Laboratoire de Cfistallographie, LEPES CNRS, BP 166, 38042 Grenoble Cedex, France. * Permanent address: CNET CNS, BP 98, F-38243 Meylan Codex, France. * * * Permanent address: Department Physique de la Mati~re, Universit6 Claude Bernard, Bd. du 11 Nov 1918, Villeurbanne Cedex, France. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics PublLslfing Division)
t 40
A. Deneuville et aL / Effect of annealing under NH.~ of W on Si
We look here, at the W, Si and N concentrations profiles, and at the nature of N bonding and c..mpounds through the whole multilayer thickness as a function of the thickr; zss of the initial W film and of the temperature of anneafing.
2. 1FTepatafion a~d e×perimeutan set up The W layers are deposited on a Si substrate at 2 0 0 ° C by cathodic sputtering of a W target in a 20% H2/80% Ar reactive mixture of 5 × 10 -3 Torr, with a base pressure of 10 -7 Torr, after a careful outgassing of the substrate and of the deposition chamber. The samples are annealed in N H 3 in a classical Joule oven during 2 h, and at temperatures T~ ranging from 500 to 1100°C. The results will be reported mainly on the anneafing of the 200 and 2000 A thick W films, which will be called respectively "200 A W " and "2000 AW". The X-ray diffraction, excited with Cu K a radiation, was performed at a glancing angle of incidence of 0.5 o to increase the sensitivity. The Rutherford backscattering was done with impinging He ions of 2 MeV at normal or 50 o incidence. The results will be given as the variation of the n u m b e r of detected counts with the channel number, with an energy of appro~dmatdy 3.3 keV per channel. From the work of Amsel et al. [5], the total nitrogen content was measured from the nuclear reaction 14N(d, a'y)12C 2H +14 N ~ 1 2 C + 4 H e + 3~. The energy of the deuterium ions (d or 2H) decreases as they penetrate deeper and deeper in the film. Here, with an initial energy of 1.15 MeV, their energies always correspond to a flat part of the capture cross section for the nuclear reaction. We detect the variations of the n u m b e r of 4He particles enfitted in the vacuum with their energy. As the He ions are created at various depths, but always with the same energy, and loose energy to reach the surface before being emitted in the vacuum, the He energy scale is an image of the d e p t h scale, through the energy losses per unit length (which depends o n the local composition of the material). The measurements were calibrated with a L P C V D Si3N 4 film to derive numerical values for the N concentration. The results will be given here as the vmqation of the n u m b e r of counts vAth the channel number, with approximately 9 keV per channel. The infrared transmission was measured with a Pertdn-Elmer 683 double beam spectrophotometer. The thiclmesses of the various parts of the films were measured b y a Dektak o n steps formed after a preferential chemical etching of W and W N x.
A. Deneuville el al. / Effect of annealing under NHj of Won Si
141
3. Results and discussion
3.1.X-raydiffraction We extend here the previous results of Deneuville et al. [3]. For the "200 .~ W " films, there is formation of W N x o r mixture of WN x o n Si from Ta = 600 o C up to T~ = 1100°C. For the "1000 A W " f~ms, the WSi 2 peaks appears only at 1 1 0 0 ° C in addition to those of the tungsten nitrides. For the "2000 A W " films, the WSi 2 peaks already appear for T~ > 800°C.
3.2.Rutherfordbacksc~ttering For the "200 A W " f~lms, even at an angle of incidence of 50 °, we see only a decrease a n d a widening of the W signal, with a slight translation of the Si signal as Ta increases. After etching of the superficial metallic layer, only negligible W concentratlo• remains, which confirms the negligible W - S i interdiffusion. Fig. 1 shows the P ~ $ signals at normal incidence after annealing under N H 3 at 700, 800 a n d 1 0 0 0 ° C of the "2000 A W " f ~ n s . After annealing at 700 o C, some W losses are observed on the high energy edge of the W peak. As there is no Si at the surface, this has to originate from the formation of a W rfitrlde layer on the top of the film. The annealing at 800 o C introduces drastic changes. The W signal o n the high energy edge decreases o n a wider d e p t h indicating the formation of a wider and more N-rich W h i . d e layer o n the top of the film. Then, a step appears o n the low energy edge, at the interface between the W and the bulk Si,/rid/caring the formation of a layer with lower W concentration. A t the same time, a shonlder appears on the Si signal, indicating the formation of a layer with lower Si concentration. The analysis of
f/"\,\, ~ '
CIIANNEL
Fig. 1. RBS signals of the "2000 ,~ W" films (2000 A W/Si annealed 2 h in NH3).
142
A . Deneuville et aL / Effe'ct o f a n n e a l i n g u n d e r N H j o f W o n S i
l
-_=-: ,iii?iiii!i'
,~
/'~ :\
/ '~ ',
i
:
,
l
i
*,~,
:~,,
2,.,
~4. ('HANNI:I.
42.
,f
1
~tl~
Fig. 2. RBS signals of the "2000 .A W" films after W and WNx etching.
both the Si and w signals establishes the occurrence of a 820 .~ thick WSi 2 layer in contact with Si, while there is an intermediate layer of variable composition between this WSi 2 layer and a W-rich layer below the W nitride layer on the top of the film. Annealing at higher temperatures only emphasizes the previous trends. As an example, the RBS signal after annealing at 1000 ° C is shown in fig. 1. The W concentration on the surface yet decreases acros., a larger depth, which indicates a wider and more N-rich W nJtride layer on ~he top of the film. The mMn W signal is smaller and wider than after T~ = 800 o C, indicating now the occurrence of a W nitfide of lower N content below the N-rich rtitride layer on the top of the film. The low energy W step is widened, indicating a broader layer between the bulk Si and the nitride layers. A plateau on the Si edge appears clearly, confirming the occurrence of a wider layer with lower Si concentration than in the bulk Si. F r o m the analysis of both the Si and W signals, we deduce that a 1000 .~ thick WSi 2 layer is in contact with the Si, and with an intermediate layer in contact with the less N-rich nitride layer. To get more information about the intermediate layers, we record in fig. 2 the RBS signal of the samples of fig. 1 after etching of the W nitrides top layers. We have used an angle of incidence of 50 o for the sample annealed at 700 ° C in order to increase the depth sensitivity. A W signal is still observed, so that a beginning of interdiffusion between the Si and the W occurs already at this temperature. The Si and W edges are at the same energy as those of the samples annealed at 800 and 1 0 0 0 ° C which are recorded under normal incidence. The Si and W are thus both on the surface of the films. After annealing at 800 o C, the Si signal at the edge is smaller across a wider thickness, indicating the occurrence of a thicker intermediate layer of lower Si content than the S i - W interdiffusion layer. A t the same time, there is a peak
A. Deneuville et al, / Effect of annealing under N t t j of Ve"on Si
143
below the high energy edge in the W signal. There is thus an additional species at the beghming of this intermed/ate layer. The N signal starting at the surface and superimposed on the Si signal at lower energy shows that it is n/trogen. The intermediate layer aetuaUy contains W, Si, and N. The RB$ signal, after etching of the sample previously annealed at 1000 o C, only emphasizes the previous evolution. The W peak is larger, at lower energy and with a smaller concentration at the edge than before. This indicates a wider intermediate layer with a N higher concentration at the surface than for the sample annealed at T~ = 800 * C. The Si concentration exlfibRs a plateau and is lower than the bulk Si concentration on a wider energy range, confirming further a widening of the intermediate layer. 3. 3. N i t r o g e n c o n c e n t r a t i o n
Because the N signals are weak and superimposed on those from $i, Pd$S is not suited to study nitrogen in these films. This is done by the nuclear reaction of 2H (deuterium) with '-~N. This allows, in principle, the determination of the total N concentration (after calibration) and of the N profiles across the samples. However, even for "2000 A W" films, the energy resolution of the detector smoothes all the sharp N profiles near the sample surface and at the WSi 2 interface which are seen on the RB$ signals of figs. 1 and 2. Sharp structures near the surface and the interface of the "200 A W" films would thus not be seen. For the "200 A W" films, there is not significant concentration of chem/eal spec/es other than W and N in the top layer. We derive a mean N / W ratio at each annealing temperature from the N content measured by the nuclear reaction and the W content measured by RBS. This ratio increases from 0.37 to 1.85 for T~ from 500 to l l 0 0 * C (fig. 3). From the ASTM crystallograph/c tables, we deduce the W concentration per refit volume and the W / N ratio for each WN x crystal. The variations of their W concentration per cm3 with the
05 wf.~.~. [ ~-'~. ~W2Nofc 4,5[
=
~
t z'~.0
0.5
I I "Nw~-¢N,O)
I
~2N,,
I
1,0 N/W 1.5
2.0
2.5
Fig. 3. W concentrationversus N/W r.atiofor the WN~,crystals. N/W ratio ha the "200 A W" films(200A W/Si annealed2 h ha NHa).
144
A. Deneuoille et al. / Effect of annealing under N H 3 of W o n Si
5°01
/ ~o° .
400
300
° v
7e9o °
200
.
.
.
.
300 400 500 Calculated Thickness (A)
600
Fig. 4. Comparison of the measured and the calculated thickness of the "200 A W " films.
N / W ratio are plotted on fig. 3. Assuming then a rninture of the neighbouring crystals, we deduce for each of our annealed samples a mean W concentration per unit volume and a mean thickness from the total W concentration. This is compared to the measured thicknesses in fig. 4. For the W N x films obtained by annealing under NH3 of the 200 A, thick W films, a reasonable agreement is obtained between the measured thicknesses and those deduced from the RBS and nuclear reaction profiles, if we take into account the accuracy (50 A) of the Dektak in this thickness range. The N profiles measured by nuclear reaction for the "2000 A W " films of fig. 1 are shown on fig. 5. After annealing at 700 ° C, there is a peak in the N concentration at high energy, then a plateau with slightly decreasing concentrations as the depth increases, and then a small maximum at low energy. This agrees with the occurrence of a W nitride layer on top of the film as suggested by the RBS results. Moreover, it indicates the occurrence of another W nitride layer with lower N content underneath, across the whole film.
7~"C ....
8~C IOOO~C
/.
,,,
/
, \ ,' ~,\,.
i
./
•
// 4211
4411
,.--\
,
,'.,/
iW
',
j /,"t, ~, ,'.-~, / 4f4i
'\. 4gll
CIIANNFL
Fig. 5. N profiles of the "2000 ,~ W " films (nuclear reaction H i + N 14 --, C 12 -F He 4 + y).
A. Deneuuille et al. / Effect of annealing under N H j of Won Si
145
After annealing at 800°C, N peaks appear on both sides of a lower N concentration (similar to the previous one) in the central part of the film. The surface peak is higher and wider than that after annealing at Ta = 700 ° C, but the most striking effect is the higher value and the greater width of the low energy peak, which is more important than the surface peak. This stron~y confirms the RBS results on the same films, showing the occurrence of a W rdtride layer of higher N concentration and wider thickness than that formed at T, = 7 0 0 ° C (fig. 1) on top of the film, and that of an intermediate layer containing N between the WSi2 and the W rich layers (fig. 2). Moreover, from fig. 5, the N content in the intermediate layer is very important, and there is a lower N concentration similar to that obtained at Ta = 700 ° C in the central part of the " W " film. After annealing at 1000°C, there is only an emphasis of the previous evolutions. The surface peak slightly increases and broadens, while the interface peak remains at about the same maximum value, but is significantly widened. From these two evolutions, the central plateau seems to have disappeared on the 1000 ° C curve of fig. 5. This strongly confirms the RBS results on the same films which shows an increase of both the N concentration and the thickness of the W nitride layer on the top of the film, together with
700°
33
31
600
800 V(cm_l)1000
1200
Fig. 6. Infrared transmission o f the "2000 ,~ W " films after W and W N x etching.
146
A. Deneuville et al. / Effect of annealing under N H 3 of Won Si
an important widerfing in the same range of composition of the intermediate layer. 3.4. I n f r a r e d m e a s u r e m e n t s
For the "'200~) .& W " films annealed above 800°C, we have detected important N conc~5ntrations in intermediate layers containing also W and Si. The possibility of S i - N bonds has thus to be considered. As the important metallic infrared absorption hides any weak S i - N absorption signal in the samples obtained just after N H 3 annealing, the transmission measurements are done after etchi,ng of the top nitride layers. For the "200 A W " films, there are almost no S i - N absorption bands. For the initially 2000 .~ thick W films, the IR transmission spectra recorded after annealing at 700, 800, 900, and 1 0 0 0 ° C are shown in fig. 6. First, the transmission values are low and continuously decrease as the anr~ealing temperature increases. This means that the intermediate layer is metallic and confirms the RBS and nuclear results showing that its thickness increases as T~ increases. Then, there is an absorption b a n d around 850 cm-1 growing as T, increases. From Bustarret et al. [6], this can be attributed to a n increasing n u m b e r of the S i - N bonds in a local environment a-Si3N 4. So, the Infrared transmission shows the existence of a significant concentration of S i - N bonds in the intermediate layer, which retains however a strong metallic character.
4. Conc~mion We have studied the effect of 2 h annealing in N H 3 between 600 and 1 1 0 0 ° C on W films of various thicknesses sputtered on Si at 200°C. The composition of the restdting structures depends porimarily on the initial thicknesses of the W films. The border is around 1000 A where W N x / W S i J S i . For higher thicknesses, we get WN~/intermediate WNSi l a y e r / W S i 2 / S i . The nuclear measurement of the N profiles indicates the occurrence for all W thicknesses of a N-rich n i t r i d e layer on top of the film, whose N content increases with Ta. For the "200 A W " films the mean W / N ratio increases from 0.37 to 1.85 as Ta increases from 500 to 1100°C, in reasonable agreement with the measured thickness variation induced by annealing. For the "2000 ,~'" films, below the N-rich nitride layer on top of the film, there is another nitride layer with lower N content in contact with the intermediate layer. The intermediate layer contains S i - N bonds, but remains nevertheless strongly metallic. Its thickness increases with T~. The deternfination of the detailed bonding statistics in this intermediate layer is under progress.
A. Deneuoille et al. / Effect of annealing under NHj of Won Si
147
References [1] N.G. Einspruch, Ed., VLSI Electronics Microstructures Sciences (Academic Press, New York, 1982). [2] M. Halperin, T. Holloway, R. Ha.ken, C. Gosmeyer, R. Karnaughand and W. Parmentic, IEEE Trans. Electron Devices ED-32 (1985) 141. [3l A. Deneuvitle, M. Benyahya, M. Brunel and B. Canut, J. Phys. (Paris) 49 (1988) C4--499. [4] J. Tortes, J. Palleau, N. Bourhila, J.C. Gberlin, A. Deneucille and M. Benyahya, J. Phys. (Paris) 49 (1988) C4-183. [5] G. Amsel, J.P. Na'dai, E. d'Artemare, D. David E..L Girard and J. Moulin, Nucl. Instr. Methods 92 (1971) 481. [6] E. Bustarret, M. Bensouda, M.C. Habrard, J.C. Bruyd~re,S. Poulin and J.C. Gujrati, Phys. Rev. B 38 (1988) 8171.