W<Si> and W1-xNx , where x= 15- 22 at%, thin films were grown using the ALD (Atomic Layer Deposition) principle. Growth rate of W<Si> films is about 4- 5 monolayers/ cycle at 300- 350 ºC. Growth rate of W1-xNx is 0.5 monolayer/cycle at 325- 350 ºC. Standard Deviation (STDV) of thickness is about 2% for 20nm layers. Specific resistivity is 180 mW cm for W<Si> and as low as 220-340 mWcm for W1-xNx 20nm films. 4- point probe sheet resistivity test is applied to Cu/ Barrier/ SiO2 stacks with ramping temperature. No changes of normalized resistance reflecting Cu-Barrier interaction was measured after 500 ºC annealing cycl

Aarnink, A.A.I.

Bystrova, S.

Holleman, J.

Wolters, R.A.M.

W&lt;Si&gt; and W1-xNx , where x=15-22 at%, thin films were grown using the ALD (Atomic Layer Deposition) principle. Growth rate of W&lt;Si&gt; films is about 4-5 monolayers/cycle at 300-350 ºC. Growth rate of W1-xNx is 0.5 monolayer/cycle at 325-350 ºC. Standard Deviation (STDV) of thickness is about 2% for 20nm layers. Specific resistivity is 180 mW μΩ for W&lt;Si&gt; and as low as 220-340 mWμΩ for W1-xNx 20nm films. 4-point probe sheet resistivity test is applied to Cu/Barrier/SiO2 stacks with ramping temperature. No changes of normalized resistance reflecting Cu-Barrier interaction was measured after 500 ºC annealing cycl

University of Twente Research Information

   Abstract—W<Si> and W1-xNx , where x= 15- 22 at%, thin films were grown using the ALD (Atomic Layer Deposition) principle.  Growth rate of W<Si> films is about 4- 5 monolayers/ cycle at 300- 350 ºC.  Growth rate of W1-xNx is 0.5 monolayer/cycle at 325- 350 ºC.  Standard Deviation (STDV) of thickness is about 2% for 20nm layers. Specific resistivity is 180 µΩ cm for W<Si> and as low as  220-340 µΩ cm for  W1-xNx  20nm films.  4- point probe sheet resistivity test is applied to Cu/ Barrier/ SiO2 stacks with ramping temperature. No changes of normalized resistance reflecting Cu-Barrier interaction was measured after 500 ºC annealing cycle.  Keywords— Atomic Layer Deposition; tungsten; copper metallization; diffusion barrier; tungsten nitride.  I. INTRODUCTION Diffusion barriers are required in silicon IC’s with Cu interconnects to protect Si and dielectrics against Cu- diffusion and Cu against diffusion of Si [1]. There are some major geometrical and physical requirements for barriers. First of all, the thickness of the diffusion barrier should be 10 nm in 2004 and 5 nm in 2010, according to International Technology Roadmap. The conventional ionised Physical Vapour Deposition (iPVD) is unable to deposit such a thin layer conformally over features with high aspect ratio. The Atomic Layer Deposition (ALD) method, based on the chemisorption of gas- precursors, allows to decrease the growth rate down to =1 monolayer/ cycle and to deposit films with 100% step coverage. References in the literature about ALD of W- based films on dielectrics are scarce, although WxNyC1-x-y films grown by ALD have been reported to be promising   barriers   referring   to   low  resistance  and   morphology [2].  Clearly, more research should be done in this field for understanding of the growth mechanism of the layers.    II. EXPERIMENTAL PROCEDURE A. ALD equipment Deposition experiments are carried out in ALD- system. The Si-wafer is placed on a Mo- susceptor in a load- lock. When the load- lock is pumped down to 1*10-5 mbar, the Mo- susceptor is transferred into a hot reactor and placed on the bottom heater. The bottom heater goes up forming a closed hot- wall reactor. The reactor volume is 24 ml. The base pressure in the reactor is 2.7* 10-7 mbar.    B. Wafer preparation and deposition experiments Prior the deposition, wafers with 700nm thermal oxide grown in water vapor were cleaned with a standard cleaning in fuming nitric acid and boiling 69% HNO3, rinsed with demi- water. Wafers were dipped into 0.3 %  HF for 5min., respectively, and  rinsed in the demi- water to form -OH groups on the surface of SiO2. These hydroxyl groups serve as reactive sites to initiate the film growth. When the wafer was loaded into the reactor, a Si- nucleation layer was grown by decomposition of Si2H6.  W- films were deposited with a sequence of Si2H6/WF6 pulses, N2 was used as a purging gas. A sequence of WF6/ NH3/ C2H4 /SiH4/ NH3 was applied to deposit WxN1-x films. Experiments were carried out Atomic Layer Deposition of W- based layers on SiO2  S. Bystrova1, J. Holleman1, R. A. M. Wolters2, A. A. I. Aarnink1 University of Twente1, Faculty of Fac. of Electrical Engineering, Mathematics & Computer Science (EEMCS), Semiconductor Devices Group (ICE), P.O.Box 217, 7500 AE Enschede, The Netherlands Philips Research2, The Netherlands Phone: +31 (0)53-489 27 29 Fax: +31 (0)53-489 10 34 E-mail:S.Bystrova@utwente.nl 730  at 1 mbar. Variable parameters were temperature, pulse time, number of cycles. The temperature range was 250- 350 ºC. N2 purging time was 2sec. The cycle time was 7.5- 11 sec for W and  14- 22 sec for WxN growth. The amount of reactive gas was controlled by mass- flow controller for  WF6, Si2H6, NH3, SiH4 and C2H4   at 5 sccm with a total flow in the reactor of 45- 80 sccm.  The composition of the films grown was measured with XPS (X- Ray Photoelectron Spectroscopy) using standard sensitivity factors. Surface roughness was measured with AFM (Atomic Force Microscopy). The thickness was calculated from the increase of the weight using  the  standard density for  W of 19.3 and for W2N  of  17.2 g/cm3.   C. Details of the resistivity test A 4- point probe sheet resistivity test was performed in vacuum at a base pressure of 4*10-6 mbar at elevated temperatures. The measurements are done with a ramping up and down step of 0.01degree/ sec. The test includes the heating up step to 500 ºC for 30 minutes step at constant temperature and cooling down to room temperature. Stacks of Cu(100nm)/ Barrier/ SiO2 and Cu(200nm)/ Barrier/ SiO2 grown on Si- substrate are used for the test. Cu was deposited on wafers by rf- sputtering. III. RESULTS AND DISCUSSIONS A. W growth in the Si2H6/ WF6 system W deposition was studied at the temperature range 250- 350 ºC. No temperature window for the deposition of W in ALD mode was found. Results of the experiments are shown in figure 1.  The growth of W films was about 3 to 5.7 monolayers/ cycle. Obviously, Si2H6 decomposes on the fresh W- surface, which is a well- known  catalyst, with the formation of more than 1 monolayer of Si/cycle. All Si formed is consumed by the following WF6 pulse. The adsorption of Si2H6 and high reactivity of WF6 with Si are determining factors of a high growth rate. The role of the Si- layer in the growth process was investigated at 325 ºC. The thinner the nucleation layer is, the slower the growth is on initial stages and the incubation time is longer, see figure 2. When the surface is converted into W, the growth rate for processes with a varying nucleation step is almost the same. The growth rate calculated from the slope of the growth curves is 4.5 monolayer/cycle for the Si2H6/ WF6 ratio equal to 4/3 and 2/1.5. The growth rate is 3.6 monolayer/ cycle for the Si2H6/ WF6 ratio equal to 1.4/2. This means that the nucleation layer helps to decrease the incubation time, but will not influence the final growth rate. The smaller incubation time of the growth the smoother the growing film is. AFM test showed roughness as high as 2.7 nm for 10 nm layer without nucleation and 0.52 nm for the nucleation at 2.3*107 L.   Figure 1. Temperature dependance of the W<Si> growth with a ratio of Si2H6 / WF6 = 4/3 after 20 cycles, Si- nucleation layer was grown at exposure to 4.5*109 L (Langmuir, 1L= 10-6 Torr*sec) of Si2H6. Growth rate was calculated from the total weight increase.  010203040500 20 40 60cyclesthickness, nm Figure 2. Thickness of W versus number of cycles at 325 C.  In the legend: ¢  is for Si2H6/WF6= 4/3, nucleation at 4.5*109L of Si2H6; n  is for Si2H6 / WF6= 4/3 and nucleation at 2.3*107 L; p  is for Si2H6 / WF6= 2/1.5 and nucleation at 2.7*105 L; ¬  is for Si2H6 / WF6= 1.4/2 and nucleation at 2.7*105 L. 0123456200 250 300 350 400T, 0Cgrowth rate, monolayer/cycle731  The resistivity of the grown films was in the range 160- 200 µΩcm for 25 nm films. The resistivity does not differ much with varying WF6/ Si2H6 ratios, however the uniformity of the films does change. The best results based on STDV of resistivity were obtained for the WF6/ Si2H6 ratio equal to 3/ 4 with a STDV of about 2 %, see figure 3.  01234560,0 0,3 0,5 0,8 1,0 1,3 1,5 WF6/ Si2H6 ratioSTDV of R, % Figure 3.  STDV of sheet  resistance versus the ratio of WF6 to Si2H6 for films with a thickness about 15 nm.  The nucleation step is 4.5*109 L.  The deposited layers contain about 12 at% of Si according to XPS.   B. Growth of WxN films in the WF6/ NH3/ C2H4/ SiH4 system W1-xNx is reported to have a high resistivity of about 4500 µΩcm [3]. WxCyN1-x-y layers possess a resistivity of 300- 400 µΩcm with STDV of 3.5 % for 25nm films[2]. The C2H4 was chosen as a cheap carbon precursor and as an alternative to triethylborane (TEB).  No continuous films were deposited with pulse sequences of WF6/ NH3/ C2H4, WF6/ C2H4/NH3. An extra step with a SiH4 pulse (a sequence of WF6/ NH3/ C2H4/ SiH4) resulted in roughness of 1.7 nm for a 20 nm film, due to direct interaction of Si with WF6. That is not acceptable for a barrier application. A pulse sequence of WF6/ NH3/ C2H4/ SiH4/ NH3 was applied for the growth of W1-xNx layers to prevent a direct interaction of Si with tungsten fluoride. No carbon and Si are incorporated however in the growing films. C2H4 and SiH4 play a sacrificial role and all products formed with carbon and Si are removed. The effect of the C2H4 exposure on the films growth is not clear yet, Table 1, N 5-6.  Table 1. Summary table of the final growth rate and incubation time at different conditions *). Process after a nucleation step with a large exposure to Si2H6 equal to 4.5e9 L  The temperature window for the W1-xNx growth was limited to 325- 350 ºC. It is not possible to go to lower temperature because of the formation of a WFx.NH3 adduct [4]. On the other hand, 350  ºC is the maximum possible temperature in the back-end for Cu/SiLK combination. The growth rate was about 0.5- 0.6 monolayer/ cycle for a varying pulse time of C2H4 and WF6, figure 4. The growth rate was calculated from the slope of thickness- cycles dependence, see figure 5.  Figure 4.  Growth rate of W1-xNx versus temperature with varying pulse sequence of WF6/ NH3/ C2H4/ SiH4/ NH3.  Time of reactive pulses is in the legend.  The thickness of initial Si film was the same for both temperatures (1.2 nm for 325 ºC and 1.5nm for 350 ºC) to equalize the starting conditions. Some experiments were performed after large initial exposure to Si2H6 4.5*109 L, figure 5. The thickness of the nucleation 0,00,10,20,30,40,50,60,70,8320 330 340 350 360T, oCgrowth rate, monolayer/cycle1/2/1/2/21/2/3/2/23/2/3/2/21/2/1/2/22/2/1/2/2WF6 / NH3 / C2H4/SiH4/NH3N WF6/NH3/ C2H4/SiH4/NH3 T, 0Cincubation time , cycles growth rate, monolayer/cycle1 1/ 2/ 1/ 2/ 2 325 0,7 0,472 2/ 2/ 1/ 2/ 2 325 5,3 0,553 1/ 2/ 1/ 2/ 2            *4,5e9 L 325 no 0,474 1/ 2/ 3/ 2/ 2            *4,5e9 L 350 no 0,575 1/ 2/ 3/ 2/ 2 350 no 0,516 1/ 2/ 1/ 2/ 2 350 no 0,507 3/ 2/ 3/ 2/ 2 350 3,9 0,68732  layer influences how fast the growing film becomes continuous, but not the final growth rate. Incubation time of the growth, calculated from the growth rate, see Table 1, was very short or, in some cases, the growth started almost immediately. The short incubation time resulted in a roughness as low as 0.43- 0.76 nm for 16 nm W1-xNx films.  The films possess extremely low resistivity up to 340 µΩcm  (STDV 2- 5%) for 10 nm films, compared to reported values [3].  The deposited films are W1-xNx compounds with x= 15- 22at%. Nitrogen suffers from the preferential sputtering during XPS profile measurements [5], therefore the initial part of the obtained profile is more useful to estimate a real nitrogen concentration. No Si peak was measured at the Barrier/ SiO2 interface.  Figure 5. Thickness  of the W1-xNx films versus cycles at 325 ºC. Ratio of gas reactants with a sequence of WF6/ NH3/ C2H4/ SiH4/ NH3 is in the legend.  C. Resistivity test Stacks of Barriers on SiO2/ Si with 200 nm Cu layer show no change in normalized resistance after  heating up to 500 ºC and cooling down to room temperature. Resistivity of Cu was 2.1 µΩcm. In contrary, samples with as deposited 100 nm copper film on SiO2 are characterized by a resistivity of 2.53 µΩcm. Such an increase of specific resistivity is caused by the amount of defects in the Cu layer.  The resistance of each sample is decreased after the test, Table 2 and figure 6. This lowering of resistivity corresponds to annealing of the defects in the thin Cu- layer. The resistivity approached 2.1 µΩcm and  the TCR became 0.0028 deg-1 for each sample up to 100 ºC. Thus no reaction between Cu and Barriers occurred and the Cu/Barrier combination survived the 500 °C annealing test.  Table 2.  Samples for the 4- point probe resistivity test with 100 nm Cu layer  Figure 6. Normalized resistance change versus temperature during the measurements. IV. CONCLUSIONS No ALD conditions for W growth are found in the WF6/ Si2H6 system. Deposition rate is about 3-5 monolayers/ cycle at 250- 350 ºC. The Content of Si in these W films was about 11- 12at%. Resistivity was up to 200 µΩcm with STDV about 2% for 25 nm film The growth rate in the WF6/ NH3/ C2H4/ SiH4 system was about 0.5- 0.6 monolayer/cycle. Grown films are W1-xNx  with x= 15- 22at % Resistivity of the deposited films was about 340 µΩcm with STDV of resistivity 2- 5 % for 10 nm films. 200 nm Cu metallization does not show any changes in resistance after the 500 ºC test. Annealed 100 nm Cu layer has the same specific resistivity as 200nm 2.1 01020300 50 100 150 200cyclesthickness, nm2/2/1/2/2_2.0e9 L1/2/1/2/2_4.5e9 L1/2/1/2/2_2.0e9 LNbarrier thick, nmcomposition by XPSRbefore, mΩ/sqRafter, mΩ/sqRafter/ Rbefore1 Cu/SiO2 253 206 0,812 25,8 W83N17 450 225 0,503 20,5 W83,5N16,5 335 214 0,644 17 W78N22 365 235 0,645 26,3 W84,5N15,5 372 220 0,596 14,5 W83.1Si11.2O5.8 310 206 0,6601230 200 400 600T, oCnormRW84.5N15.5W78N22Cu/SiO2733  µΩcm. No interaction of Barrier with Cu occurred during the annealing test up to 500 °C.  REFERENCES [1] S. Bystrova, J. Holleman, P. H. Woerlee, R. A. M. Wolters, SAFE 2000. [2]  Wei- Min Li et al., IEEE 2002, Electronic Components and Technology Conference. [3]  K.- E. Elers et al., Thin Solid Films, 434 p. 94- 99, 2003. [4]  J.W. Klaus et al., Appl. Surf. Sci. 162- 163, p. 479- 491, 2000. [5] J. L. Alay, et al., Surface and Interface Analysis, Vol. 17, p.373- 382, 1991  734

English

Atomic Layer Deposition of W-based layers on SiO<sub>2</sub>

https://ris.utwente.nl/ws/files/210264348/Bystrova2003atomic.pdf

Atomic layer deposition of W - based layers on SiO2

Abstract

Similar works

Full text

Available Versions

University of Twente Research Information