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A thickness model for the TiSi2TiN stack in the titanium salicide process module
ELSEVIER
Thin Solid Films 270 ( 1995) 589-595
A thickness model for the TiSi,/TiN stack in the titanium salicide process module Jiunn-Yann Tsai a,b,Pushkar Apte a aSemiconductor Processing and Device Center, Texas Instruments Inc. M/S 457, 13536 N. Central Expressway, Dallas, TX 75243, USA h Department
of Electrical and Computer Engineering North Carolina State University, Raleigh, NC 27695-7911.
USA
Abstract A reliable TiSi,/TiN stack thickness model is an essential component for modeling the titanium salicide process, and such a model is not well-developed in current process simulators and in the literature. To determine this model, a design of experiments was set up to examine five process variables, namely, as-deposited Ti thickness, reaction temperature, reaction time, As + implanted dose, and the reaction ambient pressure. Weight and sheet resistance measurements were used to evaluate the thickness and efficiency of reaction (%Ti converted to silicide). A good first-order linear model was obtained, with a residual standard deviation (variation of model from data) of N 30 A. The model establishes quantitatively, that the TiS& thickness is proportional to the as-deposited Ti thickness and reaction temperature, inversely proportional to the implanted As+ dose, and weakly proportional to the reaction time. Since TiN and TiSi2 are competing reactions, TiN exhibits inverse functional relationships with the variables, as compared with TiSi*. The efficiency of the reaction also has been quantified by the model. The ambient pressure has been found to have no impact on either the TiSi,/TiN stack thickness, or the reaction efficiency. The model has been validated by cross-sectional transmission electron microscopy, which agrees with the model prediction within experimental error. Keywords:
Titanium; Computer simulation
1. Introduction Self-aligned metal silicides (salicides) have been used widely in integrated circuits for reducing the resistance of polysilicon gates and source/drain diffusion regions [ 1,2]. Titanium disilicide (TiSi,) is the most popular metal silicide, due to its low resistivity [ 31. Typical Ti Salicide processing involves reaction in a nitrogen ambient to prevent lateral growth of TiSi, over the sidewall spacer [4]. This leads to two competing reactions, namely formation of TiSi, and TIN, and thus complicates reaction kinetics. We present here a model of the TiSi,/TiN stack thickness, that enables accurate determination of the final TiSiz thickness and the reaction efficiency. Lateral growth is mainly due to the fact that Si is the main moving species during the reaction [.5]. Lateral growth causes shorting between the gate and source/drain regions, and can result in low yield. This problem is controlled in the Ti Salicide process by the use of a two-step annealing process along with a nitrogen ambient [6]. The first anneal step is performed at a relatively low temperature (600 “C to 700 “C) to minimize the reaction of Ti with the sidewall dielectric and to reduce the Si diffusion rate. Since this results in TiSi, 0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDlOO40-6090(95)06709-4
formation in the high resistivity C49 phase, a second higher temperature anneal (750 “C to 900 “C) is necessary to transform the TiSi? into the low resistivity C54 phase [7]. The nitrogen ambient during first anneal causes formation of TiN and diffusion of N atoms to the grain boundaries of Ti, which reduces the silicide lateral growth dramatically [ 61. Two competing processes exist during reaction: the nitridation process at the top of the titanium film, and silicidation at the bottom. The final TiSi,/TiN stack thickness is determined by the relative growth rate of the two processes. It was identified by Iyer et al. [ 81 that there are three stages during the reaction process, namely, the initiation stage, the silicidation stage, and the quasi-steady-state stage. At the beginning of the reaction, the nitridation process has already begun, but the titanium has to consume the native oxide on the silicon substrate before silicidation can happen. The duration of the initiation stage strongly depends on the cleanness of the process, but can be well-controlled by hydrogen termination technique [ 9, lo] and limited delay before deposition. During the silicidation stage, both TiSiz and TiN begin to grow. The silicide growth rate is actually a strong function of the oxygen content in the system. The oxygen either can be incorporated into Ti film during deposition, or can be knocked into silicon
J.-Y. Tmi, I? Ape / Thin Solid Films 2 70 ( 1995) 5X9-595
590
increases the lateral growth probability, third stage should be minimized.
so the duration of the
2. Experimental To build a TiSi,/TiN stack thickness model, we used design-of-experiments (DOE) to create a response-surfacetype model for those process variables that might have an impact on the final TiSi,/TiN stack thickness. Fig. 1 illustrates the DOE function diagram. A screen-type DOE was generated using the ECHIP~~ program [ 121 with five process variables: initial as-deposited Ti thickness, reaction temperature, reaction time, As+ implanting dose, and the nitrogen ambient pressure, as shown in Table 1. Here 1.50 mm wafers are used in this experiment. After initial cleaning, 200 A of oxide was deposited in a low-pressure chemical vapor deposition (LPCVD) furnace at 800 “C before As+ implantation. The implanting As+ was implanted at 70 KeV, and then annealed at 900 “C for 5 min in an N2 ambient. The titanium film was deposited after a simulated spacer etch and a diluted HF dip for 30 s. The TiSi, reaction was performed in a-low pressure rapid thermal annealing (RTA) system. In order not to misinterpret the weight measurement results, a newly developed TIN etching solution with much higher selectivity was used [ 131. After TIN selective etch, a second thermal anneal was performed to convert the TiSi, from the C49 phase to a low-resistivity C54 phase. Since techniques such as cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering
Percentage
(1Torr- 10TOIT)
Fig. 1,
DOE model diagram
substrate from the screen oxide during source/drain implantation. The oxygen knock-on effect is especially pronounced when implanting with heavy ions like As+ [ 111. Since the solid solubility of oxygen in Ti silicide is low. the oxygen atoms incorporated into silicide tend to snow-plow into the interface of the unreacted Ti metal and Ti silicide [ 61. The presence of oxygen slows down the silicon diffusion into Ti metal for reaction. If sufficient oxygen is incorporated into silicide, more oxygen segregates at the metal/silicide interface, and in the extreme case, silicidation reaction can stop when a good diffusion barrier is formed between metal and silicide. However, in a system with good contaminant control, silicidation usually stops when the TiSi, and TIN fronts meet with each other, and the quasi-steady-state stage begins. From the thermodynamic point of view, TIN would grow at the expense of TiSi,, but the reaction is extremely slow at regular reaction temperatures [ 81. Excess thermal budget at this stage transforms the TiSi? phase from C49 to C54. but also Table 1 ECHIPscreen type design. Samples (3,5),
(7,8),
(9,10),
(6.11).
(4.14) are duplicate samples; sample 15 is the center point
Sample (trial)
Ti thickness (A)
As dose (cm-‘)
Reacttemp. (“C)
Reactiontime (s)
Ambient pressure (TOIT)
1 (7) 2 (10) 3 (3) 4 (1) 5 (5) 6 (3) 7 (2) 8 (2) 9 (4) 10 (4) 11 (3) 12 (9) 13 (6) 14 (1) 15 (11)
500 300 500 300 500 500 500 500 300 300 500 500 300 300 400 300
1 x lOIS I x lOIS 6x 10” 6X 10” 6X lOI 6X 10” 1 x lOIT I x lOI 1 x 10” 1 x lOI 6 X 10” 6X 10” 1 x 10’5 6 X 10” 3.5 x 10” 6x lo=
700 600 700 700 700 600 600 600 700 700 600 700 600 700 650 600
300 60 60 300 60 300 60 60 60 60 300 300 300 300 180 60
10
16 (8)
Ti Si
Si
W, & Rsl K&Q AfterAs’ implant AfterTi &Anneal Deposition
Tii
Ia
III
W,&Rs, After RfXCtiC%
W, & Rs, Aftu SelectiveEtch
W,&R.% After Final Anneal
Fig. 2. Five wafer stages for wafer weight and sheet resistance measurements.
10 10 10 10 10 1 1
10
I IO
5.5
I
spectrometry (RBS) are expensive and time-consuming, a non-destructive methodology was developed based on wafer weight measurement and sheet resistance measurement. These measurements were performed at five different wafer stages: after source/drain anneal, after Ti deposition, after reaction, after selective etch, and after final annealing, as shown in Fig. 2. From the weight measurement, the differ-
J.-Y. Tsui, P. Apte /Thin Solid Films 270 (1995) 589-595
Table 2 Weight measurement
results and calculated
U;-W, 1 2 3 4 sas3 6 7 8as7 9 lOas 11 as6 12 13 14as4 15 16
(mg)
3.5 2.1 3.65 2.15 3.45 3.55 3.45 3.4s 2.1 2.15 3.55 3.55 2 15 2.15 2.75 2.15
TiSi? and TiN thicknesses
W,- W2(mg)
Wz-W4 (mg)
TiSi, thickness
0.2 0.2 0.3 0.4 0.1 0.2 0.3 0.2 0.15 0.15 0.3 0.15 0.3 0.3 0.30 0.25
1 0.8 1.55 1 15.5 1.6 1.3 1.25 0.7 0.75 1.7 1.3 0.75 0.95 1.2 1.2
893 464 719 401 688 687 779 197 500 488 651 792 488 419 564 331
ence between W2 and Wl is the net weight of the deposited Ti film; the difference between W3 and W2 is the weight gain after nitrogen incorporation, and the difference between W2 and W4 is the weight for loss of unreacted Ti and Ti converted into Ti nitride and was etched off. In the course of processing, some oxygen will get incorporated and/or lost from the samples. However, the amount of this oxygen is not expected either to be detected by the weight measurement or to cause inaccuracy in the inferences made from the weight measurements. The thickness of the final TiSi, and TIN can be calculated from the percentage of Ti going into TiSi, or TIN. along with their reaction volume ratios, which are 2.5 and 1.1 for TiSi? and TiN, respectively. The reaction volume ratios can be calculated from the density and atomic mass of each of the materials, using the conservation of mass principle. The weight measurement results and the calculated TiSi, and TIN thickness are displayed in Table 2. It should be noted that the weight measurement was done in a micro-balance Table 3 Sheet resistance measurement
1 2 3 4 5as3 6 7 8as7 9 lOas 11 as6 12 13 14 as 4 15 16
139.7 151.8 75.8 76 76 76.5 140.6 142.5 140.8 142.5 76. I 75.9 143.4 75.8 84.4 75.8
591
(A)
TIN thickness 157 126 234 153 247 248 207 199 110 115 263 201 115 146 192 184
machine with resolution of 0.1 mg, and the accuracy of the weight measurement was 0.2 mg after the subtraction operation. This 0.2 mg accuracy is equivalent to about 30 A of Ti film, which can be converted to 75 A of TiS&, or 33 A of TIN. Therefore, three measurements were averaged to reduce this error. From Table 2 we can see the calculated TiSi, thickness0 differences between the duplicate samples range from 12 A to 36 A, with an average of 23 A. This deviation indeed is less than the estimated experimental error. The results of the sheet resistance measurement on five different stages are shown in Table 3. In order to ensure a complete C49-C54 phase transformation, the final annealing condition was raised to 800 “C for 60 s. Using the TIN thickness and TiSi, thickness calculated from the weight measurements as listed in Table 2, the resistivity can also be calculated. The resistivity of TiN ranges from 65 R cm to 140 fi cm, and the resistivity of C54 TiSi, ranges from 12 R cm to 16 0 cm, except sample 16. These values agree well
results from five different process stages
11.86 17.66 9.98 15.86 10.09 10.07 11.15 11.91 17.55 17.51 10.12 10.07 17.54 15.92 12.64 15.87
(A)
1.69 14.84 6.49 9.75 6.57 9.9 10.61 10.45 8.46 7.78 10.05 4.61 11.85 9.73 10.74 22.02
1.64 18.15 7.97 11.61 8.05 12.79 12.93 12.53 9.74 8.74 13.01 5.36 13.6 11.39 13.23 33.47
1.52 3.00 1.95 3.90 1.95 2.33 1.89 1.82 2.51 2.45 2.3 1.71 2.77 3.36 2.57 9.48
J.-Y. Tsui, P. Apte /Thin Solid Films 270
592
Table 4 Final inputs for
ECllIP
(I 995) 589-595
models and analysis
Sample (trial)
TiSiz thickness
Reaction efficiency
Si consumption
TiN thickness
Transform
I (7) 2 (10)
912 461 721 382 706 646 763 786 531 532 632 809 499 424
72.9 62.3 57.7 50.9 56.4 51.7 61.0 62.8 70.8 71.0 50.6 64.7 66.5 56.6 55.8 44.1
821 420 649 344 635 581 687 707 478 479 569 728 449 382 502 298
149 125 232 162 239 266 214 204 96 96 272 194 110 143
97.9 0 14.8 41.2 15.9 0 0 0 25.7 35.6 0 42.3 0 32.9 0 0
3 (5) 4(l) 5 (5) 6 (3) 7 (2) 8 (2) 9 (4) 10 (4) 11 (3) 12 (9) 13 (6) 14 (1) 15 (11)
558 331
16 (8)
Table 5 ECHIP significance/effects
195 184
%
table TiSi, thickness
Efficiency %
Si consumption
*** f..
I..
Ti thickness Reaction temp. Reaction time As dose Ambient pressure Constant Ti thickness Reaction temp. Reaction time As dose Ambient pressure
***
TiN thickness
Transform
%
. .**
..I
**I
.I.
f *.
***
1.1
599.34 332.10 92.60 36.80 - 144.15 - 24.65
60.00 3.25 9.05 3.31 - 15.23 -2.02
539.40 298.76 83.51 33.23 - 129.74 - 22.24
***
176.21 73.87 - 40.89 - 15.98 63.12 11.12
18.90 7.22 41.07 21.81 - 23.2 4.60
Table 6 ECHIP standard deviation table TiS& thickness Residual SD Replicate SD Fitting
30.0 16.5 LOF
(A)
Efficiency %
Si consumption
3.12% 1.97%
26.05 14.78 LOF
(A)
TiN thickness 12.83 7.39
(A)
Transform
%
16.26 4.1 LOF
LOF. lack of tit.
with the previously published results. The high resistivity for sample 16 is because of the therm4 agglomeration during annealing. The thinnest TiSi, (330 A) has the least thermal stability. The average C54 silicide resistivity of the other 15 samples is 14.1 0 cm. By assuming this value for all the samples, the thickness of the silicide again can be calculated. The final silicide thickness was determined by averaging the thickness calculated from weightmeasurement, and the thickness calculated from assuming a constant resistivity. The thickness deviations of the duplicate samples range from 1 A to 42 A with an average of 16 A. This deviation is smaller than those measured purely by weight or purely by sheet resistance. The transformation percentage was calculated
using the equation given by Georgiou et al. [ 141, by assuming the resistivity of the un-transformed Ti silicide (C49 phase) of 65 a cm. The Si consumption was about 90% of the silicide formed. The final results of TiS& thickness, reaction efficiency, TIN thickness, Si consumption, and phase transform percentage for DOE outputs are displayed in Table 4.
3. ECHIP model and analysis The DOE significance/effects table is shown in Table 5, where the number of asterisks implies the significance of the input variables. The significant process variables are initial
J.-Y. Tsai. P. Apte/Thin
TiSi2-thickness
Solid Films 270 (1995) 589-595
TiSi2-thickness
600
i 0
3
6-4
TI
593
Thickness
(b)
350
400
Ti
450
500
Thickness
Fig. 3. ECHIP TiSiz thickness linear contours. The reaction time is 180 s; the ambient pressure is 5.5 mTorr. The reaction temperature As+ doseis3SX 10LScm~‘for (b).
Eff
iciencq I
6
Eff
d
is 650 “C for (a), and the
iciencq
5-
4-
3-
2B 1
660
650
7do
sdo
Form-Tcmp
W
(b)
350 Tl
4do
450
5do
Thickness
Fig. 4. ECHIP reaction efficiency linear contours. The reaction time is 180 s: the ambient pressure is S.5 mTorr. The Ti thickness is 400 dose is 3.5 X lO”cm-’ for (b).
Si
Consumption
Si
600
(a) Fig. 5. ECHIP Si consumption
T1
linear contours. the As+ dose is 3.5 X 10” cm-’ for (b).
Thickness
and the As+
Consumption
-I (b)
Ti
Thickness
The reaction time is 180 s; the ambient pressure is 5.5 mTorr. The reaction temperature
Ti thickness, the As+ implant dose, and reaction temperature. There is only a weak dependence on the reaction time, which is expected, since the final TiN/TiSi, stack thickness is determined by the relative growth rate of silicidation and nitridation. If the duration for the reaction is long enough
as-deposited
A (a),
is 650 “C for (a), and
to reach the quasi-steady-state stage, the thickness change during this stage should be negligible. It is a little surprising that the nitrogen ambient pressure has little significance for all the parameters. The reaction efficiency almost has the same dependence as the TiS& thickness on the process vari-
594
J.-Y. Tsui, P. Ape / Thirl Solid Films 270 (1995) 589-595
TiN-Thickness
TiN-Thickness
(4
0))
TL
Thlckncss
Ti
Thickness
Fig. 6. ECHIP TiN thickness linear contours. The reaction time is 180 s; the ambient pressure is 5.5 mTorr. The reaction temperature As‘ dose is 3.5 X 10” crne2 for (b).
ables, except that it only depends weakly on the Ti thickness. Si consumption is linearly dependent on the TiSi, thickness, and therefore has identical significance for the process variables. The reaction time becomes a strong variable for phase transformation percentage because the phase transformation happens during the quasi-steady-state stage. If we examine each row of the effects table, all the parameters depend strongly on the reaction temperature and the As+ implant dose. All parameters except TiN thickness have a positive dependence on the reaction temperature, and a negative dependence on As + implant dose. The inverse dependence for the TIN thickness is because of the competing nature between silicidation and nitridation. Since this is only a simple linear model, some lack-of-fit on the TiSi, thickness, Si consumption, and phase transform percentage has been
is 650 “C for (a), and the
observed in Table 6. However, the residual standard deviation (SD), defined as the SD from model to the experimental data, of the TiS& thickness, is about 30 A. The replicate SD, defined as the SD of the duplicate samples, is 16.5 A which is close to our calculation of 16 A earlier. In summary, the linear model fits all the parameters reasonably except for phase transform percentage, which has a residual SD of more than 16%. Fig. 3 ( a) and 3 (b) show the TiSi, thickness linear contour plots. The silicide thickness decreases as the As+ dose increases, but increases with increasing reaction temperature. Since the final TiSi,/TiN stack thickness is determined by the competing reaction rate of silicidation and nitridation, this indicates a higher activation energy for the silicidation process than that of the nitridation process. This suggests the use
Fig. 7. XTEM picture showing the average thickness of TiSi, of about 815 A.
J.-Y. Tsai. P. Apre /Thin Solid Films 270 (1995) 589-595
of rapid thermal processing (RTP) over furnace processing to favor TiSi, formation. The reaction efficiency contour is shown in Fig. 4(a) with the only two strongly dependent process variables: As+ dose and reaction temperature. The reaction efficiency can be improved by decreasing the As+ dose with trade-off of a lower interface concentration, and a possible higher contact resistance and/or a higher junction leakage current. Reaction efficiency can also be increased by raising the reaction temperature with trade-off of possible excess thermal budget for silicide lateral growth. Fig. 4(b) shows the reaction efficiency contour with deposited Ti thickness dependence. Decreasing the deposited Ti thickness from 500 ,& to 300 A decreases the reaction efficiency only about 3%, but raises other issues in further scaling, such as poorer thermal stability for thinner silicide. Fig. 5 (a) and 5 (b) show the Si consumption contours. Since this parameter is linearly proportional to TiSi, thickness, we see identical trends here as in Fig. 3. This parameter is important for determining the silicide/silicon interface concentration, and thus crucial for contact resistance and junction leakage measurement and modeling. Fig. 6(a) and 6(b) show the TIN thickness contours. By comparing Fig. 3 and Fig. 6, it can be observed that the TiN thickness has an expected inverse dependence on As+ dose and reaction temperature relative to TiSi?. The TIN thickness can be used as a reference for generating the etching recipe and calculation of the silicide over etch. For most samples, we expect the TiSi? and TIN fronts to have met, and therefore we expected a quasi-steady-state to have been achieved. Finally, some of the samples were examined by XTEM in this work. We found that the measured thickness agreed with the prediction of the response-surface TiSi,/TiN thickness model within the residual standard deviation. Fig. 7 shows the XTEM picture of a TiSiz sample with an initial Ti thickness of 500 A, reaction temperature of 600 “C, reaction time of 120 s, and an equivalent As+ dose of 1 X lOI cm-’ (the implant was performed without a protecting oxide layer). The XTEM measured the average TiSi, thickness of about 8 15 A, while the model prediction is 795 A.
4. Conclusion A reliable TiSi,/TiN stack thickness model is an essential component for modeling the titanium salicide process. A well-predicted TiSi,/TiN stack thickness model provides insight into several aspects of silicide processing, such as the sheet resistance of the polysilicon gate, and the source/drain diffusion region, junction leakage and contact resistance increase due to Si consumption, thickness-dependent C49C54 phase transformation, thickness-dependent thermal agglomeration of the TiSi,, film, and optimal TiN strip process. An ECHIP~~ screen-type DOE (design of experiments) was created based on five process variables, i.e. initial as-
595
deposited Ti thickness, reaction temperature, reaction time, As+ implanted dose, and the reaction ambient pressure. A methodology for non-destructive evaluation of the TiSi,/TiN stack thickness has been developed by analyzing combined weight measurement and sheet resistance measurement. Using this methodology, five different outputs have been calculated, including TiSi, thickness, Ti nitride thickness, reaction efficiency, Si consumption, and phase transformation percentage. The experimental standard deviation was found to be only 16 A for the TiSi, thickness from the combined weight measurement and sheet resistance measurement. From the ECHIP=~ analysis, we found the TiS& thickness model obtained is reasonable for a linear model, with a model-to-data standard deviation of about 30 A. The model establishes quantitatively that the TiSi, thickness is strongly proportional to the as-deposited Ti thickness, and reaction temperature, but inversely proportional to the As+ implant dose, and weakly proportional to reaction time. The ambient pressure in the range of our experiment does not have a significant effect on the model. The efficiency of reaction also has been quantified by the model, which shows only a weak dependence on the deposited Ti thickness. The model has been validated by cross-sectional transmission electron microscopy (XTEM). The results agree with the model prediction within the experimental error.
Acknowledgements The authors would like to express their gratitude to Ajit Paranjpe, Doug Prinslow, Steve Huang, Scott Poarch for helpful discussions, Sean O’Brien for providing the TIN etching recipe, Jeff Large for the ellipsometry measurement discussion, Chuck Roth for performing rapid thermal annealer, Pete Chrissoverges for helping with micro-balance weight measurement, and Brent Jones for lot processing.
References [ 11 T. Shibata et al., JEDM Tech. Dig. ( 198 1) 647. [2] C.M. Osbum et al., Proc. Isr Inr. Symp. on VLSI Sri. and Technol., Electrochem. Sot., Vol. 82-l. 1982, p. 213. [ 31 S.P. Murarka. Sikcidesfor VLSI Applicafions, Academic Press, New York, 1983, p. 30. [4] C.K. Lau et al., lEDM Tech. Dig. (1982) 714. [5] W.K. Chu et al., Appl. Phys. Left., 25 ( 1974) 454. [6] L. Van den hove, Ph.D. Thesis, Katholieke Universiteit Leuven, June, 1988. [7] R. Beyers et al., J. Appl. Phys., 57 ( 1985) 5240. [8] S.S. Iyer et al., J. Elecrrochem. Sot.. 132 (1985) 2240. [9] J.C. Hensel et al., Appl. Phys. Letr., 44 (1984) 913. [lo] B.S. Meyerson et al., 50 (1987) 113. [ 111 H. Kotaki et al., Jpn. J. App.! Phys., 33 ( 1994) 532. [ 121 ECHIP version 6for PC, User’s Manual, 1994. [ 131 S. O’Brien, private communication, 1994. [ 141 G.E. Georgiou et al., J. Efecrrochem. Sot.. I41 ( 1994) 135 1.
Thin Solid Films 270 ( 1995) 589-595
A thickness model for the TiSi,/TiN stack in the titanium salicide process module Jiunn-Yann Tsai a,b,Pushkar Apte a aSemiconductor Processing and Device Center, Texas Instruments Inc. M/S 457, 13536 N. Central Expressway, Dallas, TX 75243, USA h Department
of Electrical and Computer Engineering North Carolina State University, Raleigh, NC 27695-7911.
USA
Abstract A reliable TiSi,/TiN stack thickness model is an essential component for modeling the titanium salicide process, and such a model is not well-developed in current process simulators and in the literature. To determine this model, a design of experiments was set up to examine five process variables, namely, as-deposited Ti thickness, reaction temperature, reaction time, As + implanted dose, and the reaction ambient pressure. Weight and sheet resistance measurements were used to evaluate the thickness and efficiency of reaction (%Ti converted to silicide). A good first-order linear model was obtained, with a residual standard deviation (variation of model from data) of N 30 A. The model establishes quantitatively, that the TiS& thickness is proportional to the as-deposited Ti thickness and reaction temperature, inversely proportional to the implanted As+ dose, and weakly proportional to the reaction time. Since TiN and TiSi2 are competing reactions, TiN exhibits inverse functional relationships with the variables, as compared with TiSi*. The efficiency of the reaction also has been quantified by the model. The ambient pressure has been found to have no impact on either the TiSi,/TiN stack thickness, or the reaction efficiency. The model has been validated by cross-sectional transmission electron microscopy, which agrees with the model prediction within experimental error. Keywords:
Titanium; Computer simulation
1. Introduction Self-aligned metal silicides (salicides) have been used widely in integrated circuits for reducing the resistance of polysilicon gates and source/drain diffusion regions [ 1,2]. Titanium disilicide (TiSi,) is the most popular metal silicide, due to its low resistivity [ 31. Typical Ti Salicide processing involves reaction in a nitrogen ambient to prevent lateral growth of TiSi, over the sidewall spacer [4]. This leads to two competing reactions, namely formation of TiSi, and TIN, and thus complicates reaction kinetics. We present here a model of the TiSi,/TiN stack thickness, that enables accurate determination of the final TiSiz thickness and the reaction efficiency. Lateral growth is mainly due to the fact that Si is the main moving species during the reaction [.5]. Lateral growth causes shorting between the gate and source/drain regions, and can result in low yield. This problem is controlled in the Ti Salicide process by the use of a two-step annealing process along with a nitrogen ambient [6]. The first anneal step is performed at a relatively low temperature (600 “C to 700 “C) to minimize the reaction of Ti with the sidewall dielectric and to reduce the Si diffusion rate. Since this results in TiSi, 0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDlOO40-6090(95)06709-4
formation in the high resistivity C49 phase, a second higher temperature anneal (750 “C to 900 “C) is necessary to transform the TiSi? into the low resistivity C54 phase [7]. The nitrogen ambient during first anneal causes formation of TiN and diffusion of N atoms to the grain boundaries of Ti, which reduces the silicide lateral growth dramatically [ 61. Two competing processes exist during reaction: the nitridation process at the top of the titanium film, and silicidation at the bottom. The final TiSi,/TiN stack thickness is determined by the relative growth rate of the two processes. It was identified by Iyer et al. [ 81 that there are three stages during the reaction process, namely, the initiation stage, the silicidation stage, and the quasi-steady-state stage. At the beginning of the reaction, the nitridation process has already begun, but the titanium has to consume the native oxide on the silicon substrate before silicidation can happen. The duration of the initiation stage strongly depends on the cleanness of the process, but can be well-controlled by hydrogen termination technique [ 9, lo] and limited delay before deposition. During the silicidation stage, both TiSiz and TiN begin to grow. The silicide growth rate is actually a strong function of the oxygen content in the system. The oxygen either can be incorporated into Ti film during deposition, or can be knocked into silicon
J.-Y. Tmi, I? Ape / Thin Solid Films 2 70 ( 1995) 5X9-595
590
increases the lateral growth probability, third stage should be minimized.
so the duration of the
2. Experimental To build a TiSi,/TiN stack thickness model, we used design-of-experiments (DOE) to create a response-surfacetype model for those process variables that might have an impact on the final TiSi,/TiN stack thickness. Fig. 1 illustrates the DOE function diagram. A screen-type DOE was generated using the ECHIP~~ program [ 121 with five process variables: initial as-deposited Ti thickness, reaction temperature, reaction time, As+ implanting dose, and the nitrogen ambient pressure, as shown in Table 1. Here 1.50 mm wafers are used in this experiment. After initial cleaning, 200 A of oxide was deposited in a low-pressure chemical vapor deposition (LPCVD) furnace at 800 “C before As+ implantation. The implanting As+ was implanted at 70 KeV, and then annealed at 900 “C for 5 min in an N2 ambient. The titanium film was deposited after a simulated spacer etch and a diluted HF dip for 30 s. The TiSi, reaction was performed in a-low pressure rapid thermal annealing (RTA) system. In order not to misinterpret the weight measurement results, a newly developed TIN etching solution with much higher selectivity was used [ 131. After TIN selective etch, a second thermal anneal was performed to convert the TiSi, from the C49 phase to a low-resistivity C54 phase. Since techniques such as cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering
Percentage
(1Torr- 10TOIT)
Fig. 1,
DOE model diagram
substrate from the screen oxide during source/drain implantation. The oxygen knock-on effect is especially pronounced when implanting with heavy ions like As+ [ 111. Since the solid solubility of oxygen in Ti silicide is low. the oxygen atoms incorporated into silicide tend to snow-plow into the interface of the unreacted Ti metal and Ti silicide [ 61. The presence of oxygen slows down the silicon diffusion into Ti metal for reaction. If sufficient oxygen is incorporated into silicide, more oxygen segregates at the metal/silicide interface, and in the extreme case, silicidation reaction can stop when a good diffusion barrier is formed between metal and silicide. However, in a system with good contaminant control, silicidation usually stops when the TiSi, and TIN fronts meet with each other, and the quasi-steady-state stage begins. From the thermodynamic point of view, TIN would grow at the expense of TiSi,, but the reaction is extremely slow at regular reaction temperatures [ 81. Excess thermal budget at this stage transforms the TiSi? phase from C49 to C54. but also Table 1 ECHIPscreen type design. Samples (3,5),
(7,8),
(9,10),
(6.11).
(4.14) are duplicate samples; sample 15 is the center point
Sample (trial)
Ti thickness (A)
As dose (cm-‘)
Reacttemp. (“C)
Reactiontime (s)
Ambient pressure (TOIT)
1 (7) 2 (10) 3 (3) 4 (1) 5 (5) 6 (3) 7 (2) 8 (2) 9 (4) 10 (4) 11 (3) 12 (9) 13 (6) 14 (1) 15 (11)
500 300 500 300 500 500 500 500 300 300 500 500 300 300 400 300
1 x lOIS I x lOIS 6x 10” 6X 10” 6X lOI 6X 10” 1 x lOIT I x lOI 1 x 10” 1 x lOI 6 X 10” 6X 10” 1 x 10’5 6 X 10” 3.5 x 10” 6x lo=
700 600 700 700 700 600 600 600 700 700 600 700 600 700 650 600
300 60 60 300 60 300 60 60 60 60 300 300 300 300 180 60
10
16 (8)
Ti Si
Si
W, & Rsl K&Q AfterAs’ implant AfterTi &Anneal Deposition
Tii
Ia
III
W,&Rs, After RfXCtiC%
W, & Rs, Aftu SelectiveEtch
W,&R.% After Final Anneal
Fig. 2. Five wafer stages for wafer weight and sheet resistance measurements.
10 10 10 10 10 1 1
10
I IO
5.5
I
spectrometry (RBS) are expensive and time-consuming, a non-destructive methodology was developed based on wafer weight measurement and sheet resistance measurement. These measurements were performed at five different wafer stages: after source/drain anneal, after Ti deposition, after reaction, after selective etch, and after final annealing, as shown in Fig. 2. From the weight measurement, the differ-
J.-Y. Tsui, P. Apte /Thin Solid Films 270 (1995) 589-595
Table 2 Weight measurement
results and calculated
U;-W, 1 2 3 4 sas3 6 7 8as7 9 lOas 11 as6 12 13 14as4 15 16
(mg)
3.5 2.1 3.65 2.15 3.45 3.55 3.45 3.4s 2.1 2.15 3.55 3.55 2 15 2.15 2.75 2.15
TiSi? and TiN thicknesses
W,- W2(mg)
Wz-W4 (mg)
TiSi, thickness
0.2 0.2 0.3 0.4 0.1 0.2 0.3 0.2 0.15 0.15 0.3 0.15 0.3 0.3 0.30 0.25
1 0.8 1.55 1 15.5 1.6 1.3 1.25 0.7 0.75 1.7 1.3 0.75 0.95 1.2 1.2
893 464 719 401 688 687 779 197 500 488 651 792 488 419 564 331
ence between W2 and Wl is the net weight of the deposited Ti film; the difference between W3 and W2 is the weight gain after nitrogen incorporation, and the difference between W2 and W4 is the weight for loss of unreacted Ti and Ti converted into Ti nitride and was etched off. In the course of processing, some oxygen will get incorporated and/or lost from the samples. However, the amount of this oxygen is not expected either to be detected by the weight measurement or to cause inaccuracy in the inferences made from the weight measurements. The thickness of the final TiSi, and TIN can be calculated from the percentage of Ti going into TiSi, or TIN. along with their reaction volume ratios, which are 2.5 and 1.1 for TiSi? and TiN, respectively. The reaction volume ratios can be calculated from the density and atomic mass of each of the materials, using the conservation of mass principle. The weight measurement results and the calculated TiSi, and TIN thickness are displayed in Table 2. It should be noted that the weight measurement was done in a micro-balance Table 3 Sheet resistance measurement
1 2 3 4 5as3 6 7 8as7 9 lOas 11 as6 12 13 14 as 4 15 16
139.7 151.8 75.8 76 76 76.5 140.6 142.5 140.8 142.5 76. I 75.9 143.4 75.8 84.4 75.8
591
(A)
TIN thickness 157 126 234 153 247 248 207 199 110 115 263 201 115 146 192 184
machine with resolution of 0.1 mg, and the accuracy of the weight measurement was 0.2 mg after the subtraction operation. This 0.2 mg accuracy is equivalent to about 30 A of Ti film, which can be converted to 75 A of TiS&, or 33 A of TIN. Therefore, three measurements were averaged to reduce this error. From Table 2 we can see the calculated TiSi, thickness0 differences between the duplicate samples range from 12 A to 36 A, with an average of 23 A. This deviation indeed is less than the estimated experimental error. The results of the sheet resistance measurement on five different stages are shown in Table 3. In order to ensure a complete C49-C54 phase transformation, the final annealing condition was raised to 800 “C for 60 s. Using the TIN thickness and TiSi, thickness calculated from the weight measurements as listed in Table 2, the resistivity can also be calculated. The resistivity of TiN ranges from 65 R cm to 140 fi cm, and the resistivity of C54 TiSi, ranges from 12 R cm to 16 0 cm, except sample 16. These values agree well
results from five different process stages
11.86 17.66 9.98 15.86 10.09 10.07 11.15 11.91 17.55 17.51 10.12 10.07 17.54 15.92 12.64 15.87
(A)
1.69 14.84 6.49 9.75 6.57 9.9 10.61 10.45 8.46 7.78 10.05 4.61 11.85 9.73 10.74 22.02
1.64 18.15 7.97 11.61 8.05 12.79 12.93 12.53 9.74 8.74 13.01 5.36 13.6 11.39 13.23 33.47
1.52 3.00 1.95 3.90 1.95 2.33 1.89 1.82 2.51 2.45 2.3 1.71 2.77 3.36 2.57 9.48
J.-Y. Tsui, P. Apte /Thin Solid Films 270
592
Table 4 Final inputs for
ECllIP
(I 995) 589-595
models and analysis
Sample (trial)
TiSiz thickness
Reaction efficiency
Si consumption
TiN thickness
Transform
I (7) 2 (10)
912 461 721 382 706 646 763 786 531 532 632 809 499 424
72.9 62.3 57.7 50.9 56.4 51.7 61.0 62.8 70.8 71.0 50.6 64.7 66.5 56.6 55.8 44.1
821 420 649 344 635 581 687 707 478 479 569 728 449 382 502 298
149 125 232 162 239 266 214 204 96 96 272 194 110 143
97.9 0 14.8 41.2 15.9 0 0 0 25.7 35.6 0 42.3 0 32.9 0 0
3 (5) 4(l) 5 (5) 6 (3) 7 (2) 8 (2) 9 (4) 10 (4) 11 (3) 12 (9) 13 (6) 14 (1) 15 (11)
558 331
16 (8)
Table 5 ECHIP significance/effects
195 184
%
table TiSi, thickness
Efficiency %
Si consumption
*** f..
I..
Ti thickness Reaction temp. Reaction time As dose Ambient pressure Constant Ti thickness Reaction temp. Reaction time As dose Ambient pressure
***
TiN thickness
Transform
%
. .**
..I
**I
.I.
f *.
***
1.1
599.34 332.10 92.60 36.80 - 144.15 - 24.65
60.00 3.25 9.05 3.31 - 15.23 -2.02
539.40 298.76 83.51 33.23 - 129.74 - 22.24
***
176.21 73.87 - 40.89 - 15.98 63.12 11.12
18.90 7.22 41.07 21.81 - 23.2 4.60
Table 6 ECHIP standard deviation table TiS& thickness Residual SD Replicate SD Fitting
30.0 16.5 LOF
(A)
Efficiency %
Si consumption
3.12% 1.97%
26.05 14.78 LOF
(A)
TiN thickness 12.83 7.39
(A)
Transform
%
16.26 4.1 LOF
LOF. lack of tit.
with the previously published results. The high resistivity for sample 16 is because of the therm4 agglomeration during annealing. The thinnest TiSi, (330 A) has the least thermal stability. The average C54 silicide resistivity of the other 15 samples is 14.1 0 cm. By assuming this value for all the samples, the thickness of the silicide again can be calculated. The final silicide thickness was determined by averaging the thickness calculated from weightmeasurement, and the thickness calculated from assuming a constant resistivity. The thickness deviations of the duplicate samples range from 1 A to 42 A with an average of 16 A. This deviation is smaller than those measured purely by weight or purely by sheet resistance. The transformation percentage was calculated
using the equation given by Georgiou et al. [ 141, by assuming the resistivity of the un-transformed Ti silicide (C49 phase) of 65 a cm. The Si consumption was about 90% of the silicide formed. The final results of TiS& thickness, reaction efficiency, TIN thickness, Si consumption, and phase transform percentage for DOE outputs are displayed in Table 4.
3. ECHIP model and analysis The DOE significance/effects table is shown in Table 5, where the number of asterisks implies the significance of the input variables. The significant process variables are initial
J.-Y. Tsai. P. Apte/Thin
TiSi2-thickness
Solid Films 270 (1995) 589-595
TiSi2-thickness
600
i 0
3
6-4
TI
593
Thickness
(b)
350
400
Ti
450
500
Thickness
Fig. 3. ECHIP TiSiz thickness linear contours. The reaction time is 180 s; the ambient pressure is 5.5 mTorr. The reaction temperature As+ doseis3SX 10LScm~‘for (b).
Eff
iciencq I
6
Eff
d
is 650 “C for (a), and the
iciencq
5-
4-
3-
2B 1
660
650
7do
sdo
Form-Tcmp
W
(b)
350 Tl
4do
450
5do
Thickness
Fig. 4. ECHIP reaction efficiency linear contours. The reaction time is 180 s: the ambient pressure is S.5 mTorr. The Ti thickness is 400 dose is 3.5 X lO”cm-’ for (b).
Si
Consumption
Si
600
(a) Fig. 5. ECHIP Si consumption
T1
linear contours. the As+ dose is 3.5 X 10” cm-’ for (b).
Thickness
and the As+
Consumption
-I (b)
Ti
Thickness
The reaction time is 180 s; the ambient pressure is 5.5 mTorr. The reaction temperature
Ti thickness, the As+ implant dose, and reaction temperature. There is only a weak dependence on the reaction time, which is expected, since the final TiN/TiSi, stack thickness is determined by the relative growth rate of silicidation and nitridation. If the duration for the reaction is long enough
as-deposited
A (a),
is 650 “C for (a), and
to reach the quasi-steady-state stage, the thickness change during this stage should be negligible. It is a little surprising that the nitrogen ambient pressure has little significance for all the parameters. The reaction efficiency almost has the same dependence as the TiS& thickness on the process vari-
594
J.-Y. Tsui, P. Ape / Thirl Solid Films 270 (1995) 589-595
TiN-Thickness
TiN-Thickness
(4
0))
TL
Thlckncss
Ti
Thickness
Fig. 6. ECHIP TiN thickness linear contours. The reaction time is 180 s; the ambient pressure is 5.5 mTorr. The reaction temperature As‘ dose is 3.5 X 10” crne2 for (b).
ables, except that it only depends weakly on the Ti thickness. Si consumption is linearly dependent on the TiSi, thickness, and therefore has identical significance for the process variables. The reaction time becomes a strong variable for phase transformation percentage because the phase transformation happens during the quasi-steady-state stage. If we examine each row of the effects table, all the parameters depend strongly on the reaction temperature and the As+ implant dose. All parameters except TiN thickness have a positive dependence on the reaction temperature, and a negative dependence on As + implant dose. The inverse dependence for the TIN thickness is because of the competing nature between silicidation and nitridation. Since this is only a simple linear model, some lack-of-fit on the TiSi, thickness, Si consumption, and phase transform percentage has been
is 650 “C for (a), and the
observed in Table 6. However, the residual standard deviation (SD), defined as the SD from model to the experimental data, of the TiS& thickness, is about 30 A. The replicate SD, defined as the SD of the duplicate samples, is 16.5 A which is close to our calculation of 16 A earlier. In summary, the linear model fits all the parameters reasonably except for phase transform percentage, which has a residual SD of more than 16%. Fig. 3 ( a) and 3 (b) show the TiSi, thickness linear contour plots. The silicide thickness decreases as the As+ dose increases, but increases with increasing reaction temperature. Since the final TiSi,/TiN stack thickness is determined by the competing reaction rate of silicidation and nitridation, this indicates a higher activation energy for the silicidation process than that of the nitridation process. This suggests the use
Fig. 7. XTEM picture showing the average thickness of TiSi, of about 815 A.
J.-Y. Tsai. P. Apre /Thin Solid Films 270 (1995) 589-595
of rapid thermal processing (RTP) over furnace processing to favor TiSi, formation. The reaction efficiency contour is shown in Fig. 4(a) with the only two strongly dependent process variables: As+ dose and reaction temperature. The reaction efficiency can be improved by decreasing the As+ dose with trade-off of a lower interface concentration, and a possible higher contact resistance and/or a higher junction leakage current. Reaction efficiency can also be increased by raising the reaction temperature with trade-off of possible excess thermal budget for silicide lateral growth. Fig. 4(b) shows the reaction efficiency contour with deposited Ti thickness dependence. Decreasing the deposited Ti thickness from 500 ,& to 300 A decreases the reaction efficiency only about 3%, but raises other issues in further scaling, such as poorer thermal stability for thinner silicide. Fig. 5 (a) and 5 (b) show the Si consumption contours. Since this parameter is linearly proportional to TiSi, thickness, we see identical trends here as in Fig. 3. This parameter is important for determining the silicide/silicon interface concentration, and thus crucial for contact resistance and junction leakage measurement and modeling. Fig. 6(a) and 6(b) show the TIN thickness contours. By comparing Fig. 3 and Fig. 6, it can be observed that the TiN thickness has an expected inverse dependence on As+ dose and reaction temperature relative to TiSi?. The TIN thickness can be used as a reference for generating the etching recipe and calculation of the silicide over etch. For most samples, we expect the TiSi? and TIN fronts to have met, and therefore we expected a quasi-steady-state to have been achieved. Finally, some of the samples were examined by XTEM in this work. We found that the measured thickness agreed with the prediction of the response-surface TiSi,/TiN thickness model within the residual standard deviation. Fig. 7 shows the XTEM picture of a TiSiz sample with an initial Ti thickness of 500 A, reaction temperature of 600 “C, reaction time of 120 s, and an equivalent As+ dose of 1 X lOI cm-’ (the implant was performed without a protecting oxide layer). The XTEM measured the average TiSi, thickness of about 8 15 A, while the model prediction is 795 A.
4. Conclusion A reliable TiSi,/TiN stack thickness model is an essential component for modeling the titanium salicide process. A well-predicted TiSi,/TiN stack thickness model provides insight into several aspects of silicide processing, such as the sheet resistance of the polysilicon gate, and the source/drain diffusion region, junction leakage and contact resistance increase due to Si consumption, thickness-dependent C49C54 phase transformation, thickness-dependent thermal agglomeration of the TiSi,, film, and optimal TiN strip process. An ECHIP~~ screen-type DOE (design of experiments) was created based on five process variables, i.e. initial as-
595
deposited Ti thickness, reaction temperature, reaction time, As+ implanted dose, and the reaction ambient pressure. A methodology for non-destructive evaluation of the TiSi,/TiN stack thickness has been developed by analyzing combined weight measurement and sheet resistance measurement. Using this methodology, five different outputs have been calculated, including TiSi, thickness, Ti nitride thickness, reaction efficiency, Si consumption, and phase transformation percentage. The experimental standard deviation was found to be only 16 A for the TiSi, thickness from the combined weight measurement and sheet resistance measurement. From the ECHIP=~ analysis, we found the TiS& thickness model obtained is reasonable for a linear model, with a model-to-data standard deviation of about 30 A. The model establishes quantitatively that the TiSi, thickness is strongly proportional to the as-deposited Ti thickness, and reaction temperature, but inversely proportional to the As+ implant dose, and weakly proportional to reaction time. The ambient pressure in the range of our experiment does not have a significant effect on the model. The efficiency of reaction also has been quantified by the model, which shows only a weak dependence on the deposited Ti thickness. The model has been validated by cross-sectional transmission electron microscopy (XTEM). The results agree with the model prediction within the experimental error.
Acknowledgements The authors would like to express their gratitude to Ajit Paranjpe, Doug Prinslow, Steve Huang, Scott Poarch for helpful discussions, Sean O’Brien for providing the TIN etching recipe, Jeff Large for the ellipsometry measurement discussion, Chuck Roth for performing rapid thermal annealer, Pete Chrissoverges for helping with micro-balance weight measurement, and Brent Jones for lot processing.
References [ 11 T. Shibata et al., JEDM Tech. Dig. ( 198 1) 647. [2] C.M. Osbum et al., Proc. Isr Inr. Symp. on VLSI Sri. and Technol., Electrochem. Sot., Vol. 82-l. 1982, p. 213. [ 31 S.P. Murarka. Sikcidesfor VLSI Applicafions, Academic Press, New York, 1983, p. 30. [4] C.K. Lau et al., lEDM Tech. Dig. (1982) 714. [5] W.K. Chu et al., Appl. Phys. Left., 25 ( 1974) 454. [6] L. Van den hove, Ph.D. Thesis, Katholieke Universiteit Leuven, June, 1988. [7] R. Beyers et al., J. Appl. Phys., 57 ( 1985) 5240. [8] S.S. Iyer et al., J. Elecrrochem. Sot.. 132 (1985) 2240. [9] J.C. Hensel et al., Appl. Phys. Letr., 44 (1984) 913. [lo] B.S. Meyerson et al., 50 (1987) 113. [ 111 H. Kotaki et al., Jpn. J. App.! Phys., 33 ( 1994) 532. [ 121 ECHIP version 6for PC, User’s Manual, 1994. [ 131 S. O’Brien, private communication, 1994. [ 141 G.E. Georgiou et al., J. Efecrrochem. Sot.. I41 ( 1994) 135 1.