Angle Resolved x-ray Photoelectron Spectroscopy, ARXPS

1
Angle Resolved x-ray Photoelectron Spectroscopy,
ARXPS Experience in the Wafer Processing
Industry so far

• C.R. Brundle, C.R. Brundle Associates, Soquel,
  CA
• G. Conti, Y. Uritsky DTCL, Applied Materials,
  Santa Clara, CA
• - J. Wolstenholme, Thermo Inc.

• Our practical experience using ARXPS for
  determining the following
• 1. Thickness for nominally single overlayer
  films (0-40Å) Characterization and
  Metrology
• 2. Composition depth distribution (0-40Å)
  Characterization
• Note Dose is a sub-set of composition
  (Metrology?)
• Taken as a given that XPS is a powerful
  technique for elemental and chemical state
  identification for 0-40Å films.

Acknowledgements - Charles Wang, Ghazal
Peydaye-Saheli
at Applied Materials
2
ARXPS Experience in the Wafer Processing
Industry

• Will use our experience over a 3-4 year period
  with 10-30Å Si/O/N gate oxide material, as
  produced in development by Applied Materials
  wafer processing tools and processes for
  Semiconductor Industry customers.
• Will refer to a few other necessary illustrative
  examples along the way.

3
What does this industry want?

• THICKNESS
• High Precision (better than 1 at 1s
  repeatability / reproducibility for a 10Å film).
• For Metrology, fast (seconds per point), 5/9
  point maps on 300mm wafers.
• Accuracy is of less concern. For metrology of no
  concern. Will be calibrated anyway, and a ?eff
  (effective attenuation length).
• Would like to be able to distinguish apparent
  thickness variations from what are really
  materials changes.
• ? ?eff changing with material change
• ? ?eff changing with t

4
What does this industry want?

• DOSE (e.g. N in Si/O/N As in Si(100))
• 1 precision at 10Å for 1x1015 atoms/cm2.
• Accuracy is again of less concern, BUT need to
  distinguish apparent dose changes from depth
  distribution changes.
• DEPTH DISTRIBUTION
• A crude distribution is OK (layer model
  approach?).
• BUT it needs to be reproducible and correct.
• Would like to be able to detect small variations
  in a given distribution (e.g. wafer to wafer or
  point to point on a wafer).

5
Other Issues

• For the Si/O/N work described here we assume flat
  (low roughness), laterally homogenous (over
  analysis area) films. We know this to be true.
• For Hf based high k work the above is not always
  true.
• All the work is done using the Theta 300 Thermo
  Inc tool.
• All the recipe development for converting ARXPS
  data to depth profiling is done by P. Mack at
  Thermo Inc. We are merely users, though we do
  have the freedom to vary some parameters.
• We often have to make correlations with data from
  the ReVera tool (Gate group at Applied
  Materials), which is a single angle only tool
  designed specifically for metrology (t, N dose)
  in Si/O/N

6
Theta Probe Parallel ARXPS (PARXPS)

• Theta Probe avoids the disadvantages by
  collecting all angles in parallel.

7
The Theta Probe ARXPS Solution
Two Dimensional Detector Measures Energy and
Angle Simultaneously
8
Collection Conditions

• Angular Range
• 20 to 80
• 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

9
How is Surface Sensitivity Achieved?

• Intensity as a function of depth
• 65 of signal from ltl
• 85 from lt2l
• 95 from lt3l
• Information depth greater than thickness of gate
  dielectric

l Inelastic Mean Free Path (0.4 - 4nm)
10
Typical XPS Full Spectrum For Si/N/O
11
ARXPS data for each element present
12
What is Angle Resolved XPS (ARXPS)?

• XPS as a function of the angle, q , (w.r.t. the
  surface normal) that the photoelectrons leave the
  surface

• A set of measurements over a range of q provides
  composition information over a range of depths.

13
Thickness Determination

• Based on the classical approach of determining
  the ratio of overlayer / substrate XPS
  intensities and using the Beer-Lambert equation
  and values for ?.
• For Si/O/N on Si(100) the overlayer signal is
  Si4 and the substrate is Si0.

?
?
?
14
Thickness Determination

• Many sources for ?Si , Si (and ? values in
  general see C. Powell publications)
• Classical approach ignores elastic scattering,
  ?e. We know (Powell et al) this can cause
  significant errors, so that a ?eff should be
  used, and that the errors vary with thickness, so
  that ?eff becomes a function of t.
• The effects of elastic scattering get greater at
  higher ? (more grazing angle), over representing
  the substrate, leading to a low estimate of t if
  a fit is made to equation 3 that includes data at
  high ? (see later).
• Our values of ? come from the Thermo Inc
  algorithm. They are calculated on the basis of
  formula, density, band gap, and KE.

4 4
sSi, SiO ?Si, SiO DSiO FSi ?Si, SiO
?SiO
8
R8
2
2
2
2
2
x



sSi, Si ?Si, Si DSi FSiO ?Si, Si
?Si
8
2
15
Thickness Measurement Testing Model Validity
9.0 nm
6.4 nm
4.3 nm
3.6 nm
2.3 nm
1.9 nm

• Silicon Dioxide on Silicon
• Plot ln1R/ R vs. 1/cos(q)
• Fitting Fit through the origin
• Gradient d/l
• NOMINAL THICKNESS VALUES FROM ELLIPSOMETRY

16
Comparison of XPS Results To Ellipsometry
Ellipsometry included C in layer thickness

• SiO2 on Si
• Excellent linearity
• Ellipsometry included C layer in thickness
• The offset will change as a function of time as
  more contamination is picked up

17
Thickness

• data considered for Si/O/N on Si(100)
• 1) 8 sample set with t 10-30Å
• N age 7-30
• - 4 from process A 4 from process B
• - Determine d, N dose, and Max. Ent. Derived
  depth profile
• - Only one set of experimental data, but
  evolving treatment over a 3 year period.
• Note very large t and N range
  not typical for metrology

18
Thickness

• Quality of Data?
• Manual Fits - Operator Influence?
• - Repeatability by single operator?
• Effect of changing composition (N age), which is
  large here?
• Effects of angular range used?
• - Depends on thickness, material
• - Consequence for single angle determination?
• Effect of composition variation with depth?
• ? Automated 3-layer model (p. Mack, Thermo)
• - No operator dependency
• - Completely reproducible
• - Iterative fit to 3-layer depth distribution
  model and t (i.e. value of N dose and its
  distribution effect, t)

19
Single Overlayer Model for Film
Thickness quality of data? manual fits?
A-11
There is ambiguity in assigning intensity between
the Si4 and Si0 peaks
20
XPS Measurements of SiO2 Thickness Effect of
angular range included?

• Comparison of ARXPS with fixed angle XPS
• Good agreement except at large thickness
• Single angle measurement samples large angular
  range.

• ARXPS measurements
• Effect of angular range upon measured thickness
• Minimum angle is 23 in all cases
• Highest usable maximum angle depends upon oxide
  thickness

J. Wolstenholme, Thermo, Inc.
21
Thickness

• 8 SAMPLE SET of Si/O/N One set of experimental
  data, but how it has been processed has changed
  from 2003 to 2007. Note Very large t and N range

Process A Process A Process A Process A Process B Process B Process B Process B
Slot No. Slot No. Slot No. 1 3 11 10 15 3 13 6
Nage Nage Nage 8.5 16.0 6.7 23.7 9.6 12.1 18.6 29.8
t(Å) June 2003 June 2003 14.1 16.3 19.8 20.1 10.4 11.2 14.2 21.1
t(Å)
t(Å) Jan 2007 1st Jan 2007 1st 14.3 15.9 18.9 19.4 10.7 11.6 14.0 20.4
t(Å) 2nd 2nd 19.3
t(Å) 3rd 3rd 19.0
Angle Restriction January 2007 Angle Restriction January 2007 76 1st 14.9 Underestimate Underestimate Underestimate Underestimate
Angle Restriction January 2007 Angle Restriction January 2007 2nd 14.9 Underestimate Underestimate Underestimate Underestimate
Angle Restriction January 2007 Angle Restriction January 2007 69 1st 18.3 Underestimate Underestimate Underestimate Underestimate
Angle Restriction January 2007 Angle Restriction January 2007 2nd 18.1 Underestimate Underestimate Underestimate Underestimate
Angle Restriction January 2007 Angle Restriction January 2007 61 1st 18.9 Identical, within statistics Identical, within statistics Identical, within statistics Identical, within statistics
Angle Restriction January 2007 Angle Restriction January 2007 2nd 18.8 Identical, within statistics Identical, within statistics Identical, within statistics Identical, within statistics
Angle Restriction January 2007 Angle Restriction January 2007 54 1st 18.8 Identical, within statistics Identical, within statistics Identical, within statistics Identical, within statistics
Angle Restriction January 2007 Angle Restriction January 2007 2nd 18.6 Identical, within statistics Identical, within statistics Identical, within statistics Identical, within statistics
3-layer model Jan 2007 3-layer model Jan 2007 1st 13.8 14.5 19.7 16.8 10.3 10.4 12.2 16.4
3-layer model Jan 2007 3-layer model Jan 2007 2nd 13.8 14.5 19.7 16.8 10.3 10.4 12.2 16.4
22
Single Overlayer Mod. for Film Thickness, slot 11
23
Thickness Conclusions

• Precision of data is no problem
• Validity of model should be tested (ie use
  angular data and fit to equation. ? not just a
  single angle determination)
• For Si4 (overlayer) / Si (substrate) fit to
  data, operator dependence for manual fit can be a
  problem
• Automated fit (3 layer model) can be completely
  reproducible
• Relative accuracy depends on validity of
  parameters input ?(f(t)?), density (f(Nage)),
  depth distribution (f(Nage)?)
• (e.g. 14.1Å for a 8.5N film going to 20.1Å for
  a 23.7N film, found using the
  manual non-iterative model, is a very different
  age change compared to 13.8Å going to 16.8A in
  the 3 layer model)

24
N Dose

• So far have been only listing Nage i.e. the
  usual XPS approach of peak intensities corrected
  for photoionization cross-section. This assumes
  homogenous composition.
• Dose is the total amount of N in the film.
• If uniform distribution Dose N.t.C
• If non-uniform, N.t.C becomes an Apparent Dose
  
• - The Apparent Dose can be greater or less
  than true dose, depending on depth distribution
• - ReVera single angle approach?
• Initially assumed a depth distribution???
• Now determines a depth distribution from a
  Tougaard background approach.
• Theta 300/Thermo N dose by integrating N depth
  profile distribution from (a) Full Max Ent
  approach or
• (b) 3-layer model (automated).

25
Effect of Distribution on Dose Calculation
CN N Concentration d depth

• True dose lt Apparent Dose

• True dose gtApparent Dose

• So, we need to know N distribution to get true N
  dose

26
N Dose (x e15 atoms/cm2) 8 sample set (from
integrating N depth distribution discussed later)
Process A Process A Process A Process A Process B Process B Process B Process B
Slot No. 1 3 11 10 15 3 13 6
Nage 8.5 16.0 6.7 23.7 9.6 12.1 18.6 29.8
t(Å) 14.1 16.3 19.8 20.1 10.4 11.2 14.2 21.1
June 2003 dose 7.61 17.0 8.57 31.9 6.33 8.41 17.2 40.5
June 2004 dose 7.25 17.8 7.35 32.3 6.00 8.50 19.0 47.8
3-layer model t(Å) 13.8 14.5 19.7 16.8 10.3 10.4 12.2 16.4
dose 7.6 16.0 8.2 30.1 6.7 8.6 16.6 35.3
(ratio to June 2003) 1.00 1.06 1.05 1.06 0.95 0.98 1.04 1.15


27
Nt VS N Dose
28
N Dose 3 layer Jan 2007 versus N Dose June 2003.
29
N Dose Conclusions

• NEED CALIBRATION/VERIFICATION BY MEIS!
• Striking agreement between 3-layer model and the
  June 2003 Max Ent results, except for very high N
  content (even though large differences in
  estimated t!).
• June 2003 About 8 spread from pure Nt
  approach.
• 3-layer About 15 spread from pure Nt
  approach, but linear
• Limiting angular range (66-55) produces up to
  10 variation (because Max Ent derived depth
  profile is different).
• Note very large dose variations are being
  considered here. Not usual for metrology.

30
Ultra-Thin Film Depth Profiling by ARXPS Status

• Because of the short mean free path lengths, ?,
  of the photoelectrons generated and used in XPS,
  non-destructive depth profiling is limited in the
  depth it can effectively go to
• 65 from lt 1 ? 85 from lt 2 ? 95 from 3 ?
• ? ranges from 0.5nm to 4nm (material and
  electron energy dependant)
• How limited depends on level of detail wanted
• ARXPS quite capable of detecting a substrate gt 3
  ? down, but not profiling the 3 ? overlayer or
  giving a precise thickness
• Detailed profiling possible up to 2 ?
  thickness
• Reliability of profile obtained by ARXPS?
• Relative Depth Plot, RDP - QUALITATIVE but
  simple, fast, model independent
• Maximum Entropy Method - QUANTITATIVE, but
  modelled and requires experience or a recipe

31
Processing the data RDP
A relative depth index can be calculated using
An indication of the layer order can then be
achieved by plotting out the relative depth index
for each species.
ln RDP ratio
Peak Area (Surface) Peak Area (Bulk)
32
RDP

• Construction
• Collect ARXPS spectra
• For each element, calculate

• Information
• Reveals the ordering of the chemical species

33
ALD TaN Film chemical state RDP Angle Resolved
Spectra from TaN Sample
TaOx
TaNt
34
Relative Depth Profile, RPD

• Advantages
• Fast
• Model independent, no assumptions
• Limitation
• No depth scale
• No concentration profile structures
• In my opinion an RDP is the most generally useful
  approach in ARXPS for characterization of unknown
  film structures seen during process development.

35
Max. Ent. Depth Profile Generation
Sample
Generate Random Profile
C
Al2O3
SiO2
Si
Calculate Expected ARXPS Data (Beer Lambert Law)
O
Si4
Tj(q) exp(-t/lcosq)
C
Sio
Al
Surface sensitive
More bulk sensitive
36
Depth Profile Generation (cont.)
Determine error between observed and calculated
data

• The MaxEnt solution is derived by minimising ?2
  while maximising the entropy
• Maximise the joint probability function
• Repeat process to obtain most likely profile

• Calculate the entropy associated with a
  particular profile (the probability of finding
  the sample in that particular state)
• cj,i is the concentration of element i in
  layer j

37
Reliability of Max Ent Modeling

• Simple model fit to the data can never be unique!
  The Max Ent approach (balance with Entropy) is a
  regularization approach. Detail of results are
  nearly always over-interpreted.
• Balance of ?2 and ? is operator (or recipe)
  chosen
• Requires experience with sample at that thickness
• Requires assumptions about unrealistic
  solutions
• e.g. Too spiky a distribution? ?2 weighting too
  high (or ? too small)
• e.g. Too smooth, substrate never reaches 100,
  film elements never go to 0? ? too big
• For a simple film of lt 2? with good statistics
  data a substrate with no species common to the
  film zero or small surface contamination
• Develop reliable recipe (?2, ?, verification?)
• Possible to obtain a reliable profile for system
  appropriate to that recipe (see examples
  following)
• Is it for Si/O/N with t, N dose variations?

38
HfSiON Reconstructed Profile
39
Comparison of ARXPS with MEIS
N
Hf
Si0
Total Si (MEIS)
O
Si4
40
PEO-thiol SAM on Silver
SAM -S-(CH2)11-(O-CH2-CH2-)3-OH
Depth Profile
Relative Depth Plot
Ag 3d
C 1s (H/C)
C 1s (Ether)
O 1s
S 2p
41
Example of Max Ent Derived Depth Profile on an
Ultra-Thin Si/O/N Film

• Reliability?
• Need high quality angular data good S/N
• Need constraints and a recipe for ? term

42
Effect of Depth Distribution on Peak Intensity
Ratios Extreme Example answer qualitatively
obvious from raw data or RDP, but cannot know
whether detailed Max Ent distribution is valid
without verification/calibration by some other
method.
43
Repeatability of ARXPS Concentration Profiles

• Three ARXPS datasets acquired dynamically from
  point on a Si oxynitride sample (sample
  repositioned each time).
• Concentration profiles reconstructed from each
  dataset
• Good reproducibility of reconstructed profiles.

44
Relative Depth Plot for 8 sample set process A
Set A
45
Relative Depth Plot for 8 sample set process B
Set B
46
June 2003. Max. Ent. a2e-4
Process A
Process 11A t 19.8Å N 6.7 N Dose 8.57 x
1014 atoms/cm2
Process 1A t 14.1Å N 8.5 N Dose 7.61 x
1014 atoms/cm2
Process 3A t 16.3Å N 16 N Dose 1.70 x
1015 atoms/cm2
Process B
Process 3B t 11.2Å N 12.1 N Dose 8.41 x
1014 atoms/cm2
Process 15B t 10.4Å N 9.6 N Dose 6.33 x
1014 atoms/cm2
Process 13B t 14.2Å N 18.6 N Dose 1.72 x
1015 atoms/cm2
47
Example of Chemical Depth Profiling, June 2003
Distinction of Si-O Using Si Chemical Shifts
SiO4
SiO3N
Film is actually more like this
Post Oxidation?
SET 6B
Graded Region
SiO2

• Different Si 2p binding energy for Si4 in SiO2
  and Si/O/N allows separation in profile
• t 21.1 Å N 29.8

Si/O/N
Si (100)
SET 10A t 20.1Å N 23.7
48
Normalized Overlays of N Distribution, June 2003

• Set A and set B are very similar (not expected)
• N distribution does not change much with N
  total dose
• Hard to get more than 10 N absolute at surface
  (air oxidation and HC pickup will reduce N
  content)
• No evidence for a nitrogen spike at the
  surface, cf.TOF SIMS.
• (this was the original reason for studying
  these sets of samples)

Set 1
Set 2
49
0ctober 2003. Max. Ent. a5e-007. Set A
50
0ctober 2003. Max. Ent. a5e-007. Set B
51
June 2004. Max. Ent. a5e-07. Process A
52
June 2004. Max. Ent. a5e-07. Process B
53
3 Layer Model for Silicon Oxynitride

• Assume 2 layers of SiO2, 1 layer SiOxNy,
  substrate
• Total d value is fixed from Si2p spectrum
• Adjust d1, d3 and N concentration to get best fit
  to ARXPS data
• Advantages
• Fast
• Only needs to fit 3 parameters (by least squares
  fitting)
• Easily automated
• Accurate
• Attenuation lengths can be calculated for each
  layer
• Precise
• Only needs to fit 3 parameters

54
Silicon Oxynitride
Automated N distribution correction
Si0
Si0
Maximum Entropy Results
O
O
N
Sin
Sin
N
Si0
Si0
Automated N distribution
O
O
Sin
Sin
N
N
55
3-layer model Jan 2007 (Cant sell this to a
process engineer!)
56
3-layer model Jan 2007
57
Effect of varying angular range. Jan 2007.
a5e007
58
Effect of varying angular range. Jan 2007. a5e007
59
Effect of varying angular range. Jan 2007.
a5e007
60
Effect of varying angular range. Jan 2007. a5e007
61
Comparison of 3-layer model to full Max. Ent June
2004
62
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63
Comparison of 3-layer model to full Max. Ent June
2004
64
Comparison of 3-layer model to full Max. Ent June
2004
65
Nitrogen Dose and Thickness

• 300 mm wafer
• Single measurement
• 49-point maps (after initial depth distribution
  determination at the wafer center)

Thickness
Dose
66
conclusions

• Thickness can be obtained to the required
  precision for 10 to 40A homogeneous composition
  (lateral and in depth) films. The accuracy, or
  even relative accuracy depends on how much
  effort is put into calibration and what range of
  thickness or materials changes are occurring. For
  inhomogeneous films (lateral or depth) errors
  will occur, which will depend on the specifics.
  OK for thickness metrology. Comparison to
  ellipsometry?
• At one extreme, for a first time analysis of a
  new film composition, with little or no
  constraints on what could be the situation, do
  not go beyond a dimensionless qualitative
  Relative Depth Profile approach (which can,
  nevertheless be extremely useful)
• At the other extreme, where a very constrained
  system is involved (ie you either already nearly
  know the answer, or the depth distribution is so
  extreme it is basically obvious from the raw
  data), ARXPS, plus appropriate data modeling, can
  give depth distributions to some degree, but
  never, in real situations, a unique highly
  precise profile.
• The quality of the data needed and the
  intellectual effort required to write (and
  verify) a recipe for fitting/modeling the data,
  which then only works within a narrow confine of
  constraints and, even then, only provides
  imprecise and not highly depth resolved
  information, means, in our opinion, that though
  ARXPS has its uses for characterization within
  the wafer industry, it is not suitable for rapid
  metrology intended to provide detailed
  information on depth distributions and related
  parameters which may rely on knowing the depth
  distribution (like dose for instance).
• May be OK for dose metrology for a small dose
  change range, using automated 3 layer model. Will
  be precise and reproducible, but only as accurate
  as the 3 layer model is accurate