Extraction of Depth Information from ARXPS Data

Transcription

1 The world leader in serving science Extraction of Depth Information from ARXPS Data John Wolstenholme

2 Theta Probe Features X-ray monochromator with spot size from 15 µm to 4 µm Real time angle resolved XPS analysis without sample tilting 2

3 Theta 3XT Parallel ARXPS from 3 mm wafers 3

4 Microfocusing Monochromator 15 µm 4 µm spot size 4

5 Geometry 5

6 Theta Probe 6

7 Collection Conditions Angular Range 2 to 8 Parallel collection Up to 96 channels in angle Generally, 16 angles are used giving an angular resolution of 3.75 Up to 112 channels in energy Parallel collection allows rapid snapshot acquisition Excellent for ARXPS maps Thickness maps Dose maps 7

8 Data Acquisition Binding energy Parallel (snapshot) or scanned Angle Always parallel Video of snapshot acquisition SiO 2 on Si Advantages of Parallel ARXPS Fast ARXPS from small features is possible Mapping possible Analysis area is constant ARXPS from large samples 8

9 Attenuation Length Each data point represents a different element or transition λ ~ E.5 M. P. Seah and W.A. Dench, Surface and Interface Analysis 1 (1979) 2 Intensity as a function of depth 65% of the signal from <λ 85% from <2λ 95% from <3λ 9

10 Relative Depth Plot (RDP) Select range of bulk sensitive angles (I bulk ) Select range of more surface sensitive angles (I surface ) Ln (I surface / I bulk ) Provides qualitative species depth distribution.6 HfO 2 SiO 2 Si Relative Norm. Area O1s Hf4f Si2p 3/2 Si2p(Ox) Angle ( ) 1

11 RDP HfO 2 Grown by ALD 3 Cycles ALD on thermal SiO 2 Hf4f O1s Si2pOx Si2p3 3 Cycles ALD on HF-last Si Hf4f O1s Si2pOx Si2p3 Advantages Excellent for showing the ordering of the layers Shows gross differences between samples Does not rely on mathematical models Does not require a knowledge of the material properties Disadvantages No quantitative data No means to interpret the differences between samples Subtle differences not apparent 11

12 Thickness Measurement Equations Photoemission from a thin film I = I [1-exp(-d/λcosθ)] Attenuation by a thin film I = I exp(-d/λcosθ) Where I = Emission from bulk material θ = Emission angle (with respect to sample normal) d = Film thickness λ = Attenuation length Silicon dioxide on silicon Plot: ln[1+r/ R ] vs. 1/cos(θ) Gradient d/λ ln(1+r/r ) nm 6.4 nm 4.3 nm 3.6 nm 2.3 nm 1.9 nm /cos(θ) 12

13 Thickness Measurements Single Layer Equations Photoemission from a thin film I = I [1-exp(-d/λcosθ)] Attenuation by a thin film I = I exp(-d/λcosθ) Where I = Emission from bulk material θ = Emission angle (with respect to sample normal) d = Film thickness λ = Attenuation length For thickness calculation ln[1+r/ R ] = d/(λ A cosθ) ARXPS Measurements (nm) SiO 2 on Si y = 1.77x Ellipsometry Measurements (nm) Beware of R For SiO 2 /Si experimental value is very different from calculated value 13

14 XPS Measurements of SiO 2 Thickness Thickness (nm) Maximum Angle ( ) ARXPS measurements Effect of angular range upon measured thickness Minimum angle is 23 in all cases Highest usable maximum angle depends upon oxide thickness Comparison of ARXPS with fixed angle XPS Good agreement except at large thickness Single angle measurement samples large angular range. Measured Thickness ARXPS Instrument 1 Single Angle Instrument 2 Single Angle Instrument 2, 2 Angles Linear (ARXPS) Nominal Thickness (nm) 14

15 Multiple Overlayers 1 2 n Substrate Choose XPS transitions representing the composition of each layer Measure signals as a function of angle Determine intensity ratios I j (θ)/i j+1 (θ) I j (θ) /I sub (θ) I j (θ) is the XPS signal from layer j as a function of angle Fit theoretical ratios to measured ratios using layer thickness as fitting parameter Best results are obtained by finding the best fit to all of the data simultaneously 15

16 Layer Thickness Calculation HfO 2 SiO 2 Layer Thickness (nm) Si 16

17 HfO 2 Growth on Thermal SiO 2 Thickness (nm) HfO 2 HfO 2 SiO 2 Si 2 4 ALD Cycles SiO 2 HfO2 Thickness (ARXPS) (nm) Thickness calculated from PARXPS data using multilayer thickness calculator Increasing growth rate Constant SiO 2 thickness Hf Coverage (RBS/1e15) 17

18 Thickness Measurement of SAMs Using PARXPS Measurements from 3 Alkanethiols deposited on gold C 9 H 19 SH C 11 H 23 SH C 16 H 33 SH C1s S2p Au4f 2.5 Acquisition Times C 1s 3 min Layer Thickness Au 4f 1 min S 2p 15 min Number of Carbon Atoms 18

19 Thickness Measured as Function of Time Conditions Angle integrated Acquisition time per point = 1.25 min Thickness/nm.6.3 Duration of PARXPS Measurement C 9 H 19 SAu Time/min Thickness/nm C 12 H 25 SAu Time/min Thickness/nm C 16 H 33 SAu Time/min ~2% Decrease in 7 minutes 19

20 Thickness Map Conditions Sample C 16 H 33 SAu Angle integrated Snapshot 2 x 2 pixels 1 µm spot size Time per point 3 seconds Clear evidence for X-ray damage caused during ARXPS measurement 2

21 Thickness Measurement Layer Thickness From Map From Static Point ARXPS can be applied to delicate samples by mapping the sample and summing the data. Only feasible with PARXPS Number of Carbon Atoms 21

22 Generation of Depth Profiles Summary of the Maximum Entropy method Start Generate Trial Profiles Use Genetic Algorithm 2 Generations Maximum Entropy Optimisation Generate output profile Average 5 or 2 profiles 3-point profiles generated Profiles generated from angular range <62 Average 5 cycles of 2, generations Powell optimisation End 22

23 Depth Profile Generation HfO 2 Al 2 O 3 SiO 2 Sample Si Relative Intensity (%) O1s Si2p Generate Random Profile Al2p Si2p(O) Angle ( ) Hf4f Atomic Concentration (%) O1s Hf4f Al2p Depth (nm) Calculate Expected ARXPS Data (Beer Lambert Law) T j (θ) = exp(-t/λcosθ) Si2p Si2p(O) 23

24 Depth Profile Generation (2) Determine error between observed and calculated data: Relative Intensity (%) χ Hf4f ( calc obs I ) k I = k O1s Si2p Al2p Si2p(O) Angle ( ) σ k 2 k 2 Calculate the entropy associated with a particular profile (the probability of finding the sample in that particular state) c j,i S = cj,i cj,i cj,ilog j i cj,i c j,i is the concentration of element i in layer j Maximise the joint probability function 2 Q = αs.5χ Repeat process to obtain most likely profile 24

25 Effect of Alpha on Generated, Unconstrained Profile HfO 2 SiO 2 Si Atomic Concentration (%) α= O Hf Si 4+ Si Atomic Concentration (%) α=3x Depth (nm) Depth (nm) Atomic Concentration (%) α=3x1-7 α=1-4 Atomic Concentration (%) Depth (nm) Depth (nm) 25

26 Add Constraints and Automate Need to apply constraints to prevent chemically unreasonable solutions (analogy to spectrum peak fitting) We know the composition of the substrate Assume mixtures of stoichiometric components ( Fit Units ) Examples HfO 2 and SiO 2 SiO x N y = (SiO 2 ) a + (Si 3 N 4 ) b Non-stoichiometric components / Free oxygen Examine data to obtain the optimum value for α (automated) Choose most appropriate angular range (automated) Depends upon layer thickness 26

27 Use Fit Units HfO 2 SiO 2 Si Atomic Concentration (%) O Hf Si 4+ Si Depth (nm) 27

28 Effect of Interlayer on 3 Cycle ALD Layer Grown on thin SiO 2 layer Grown on HF last surface Hf4f O1s Si2pOxSi2p3 Hf4f O1s Si2pOxSi2p3 Atomic Concentration (%) Hf4f O1s Si2pO Si2p Atomic Concentration (%) Hf4f O1s Si2pO Si2p 2 Depth (nm) Depth (nm) 28

29 SiON Chemical State Profile N 1s N (Low BE) 45 N (High BE) 42 N Si 4+ (Low BE) 399 Binding Energy (ev) O 396 N (High BE) 393 Atomic Concentration (%) N (Low BE) O Si 4+ N (High BE) Depth (nm) Si Si 29

30 Ti/W Alloy WO 3 W WO 2 Atomic Concentration (%) C1s O1s W4f (WO 3 ) Ti2p (TiO 2 ) W4f (WO 2 ) W4f Ti2p 41 W 5p 3/ Binding Energy (ev) Depth (nm) 3

31 Profiles from SAMs Nonanethiol 1-Mercapto-11-undecyl-tri(ethylene glycol) Concentration/% C 1s Au 4f S 2p Concentration/% C-O 1s O 1s C 1s Au 4f S 2p Depth/nm Depth/nm 31

32 Nitrogen Dose and Thickness 3 mm wafer Single measurement 49-point maps Thickness Dose 32

33 Conclusions Angle Resolved XPS is a powerful tool for the characterisation of ultra-thin films Layer ordering Multiple layer thickness determination Depth profiles Parallel ARXPS extends the capabilities of the technique Small features Large samples Mapping Larger number of angles 33

34 Acknowledgements Dr K Bonroy, IMEC, Belgium Dr T Conard, IMEC, Belgium Dr D Graham, Asemblon, USA Financial support of the EU-CUHKO project 34