Copper surface analysis with ToF-SIMS: spectra interpretation and
stability issues
C. Trouiller1, S. Petitdidier1, X. Ravanel1, L. Broussous1, M. Juhel1,
L.F.Tz. Kwakman2, C. Wyon3
1
2
STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France
Philips Semiconductors, 860 rue Jean Monnet, 38920 Crolles Cedex, France
3
CEA-LETI, 850 rue Jean Monnet, 38926 Crolles Cedex, France
chantal.trouiller@st.com
Keywords: ToF-SIMS, Copper, Surface contamination.
Introduction
In integrated circuit manufacturing, surface cleanliness is mandatory to combine high production
and device yield. In this domain, Time-Of-Flight Mass Spectroscopy (ToF-SIMS) technique offers
information both on elemental and molecular species from any surface and is appropriated for its
chemical sensitivity associated with sub-micrometer range spatial resolution and short acquisition
time. The benefits of this technique in the development or in the control of surfaces after post
copper chemical mechanical polishing (Cu-CMP) cleaning [1, 2] and after post via etch cleaning
have already been reported.
In this study, we focus on ToF-SIMS spectra interpretation and surface / measurements stability
issues.
The evolution of ToF-SIMS spectra of Cu surfaces as a function of delay after their PVD deposition
is investigated and correlated with surface Auger analysis. This study is of particular interest to
interpret the presence and variations of detected ions peaks intensity: especially CuxOyHz and
carbonate ions, and to determine the critical time delay between process steps, by analyzing the
surface evolution with time. These results are also used to evaluate the effective low detection limit
of compounds used on Cu cleaning processes.
ToF-SIMS acquisitions are usually conducted in the so-called static mode meaning that the pulsed
primary ion beam has damaged only a small amount of the surface at the end of the acquisition.
However, when the analyzed area is limited (e.g. metrology boxes on product wafers), the primary
ion dose density (PIDD) has to be increased for sufficient counting statistics. We have evidenced the
effect of PIDD on measurements reproducibility.
Experimental section
ToF-SIMS experiments are conducted on a ION-TOF.5 300 equipment using a Bi+ bunched primary
ion beam operating at 25 kV with a pulsed current of ~1pA. For static surface analyses, PIDD is
kept below 5E11 ions/cm². For investigating the effect of PIDD, intensity profiles of secondary ions
peaks are reconstructed from raw data files acquired with PIDD > 1E14 ions/cm².
Auger surface measurements are performed on a PHI670 tool using a 10kV / 10nA primary electron
beam. The sample is tilted by 30° with respect to normal incidence. Results are presented in
estimated surface content in at% calculated with peak-to-peak intensities from derivative spectra
and using tabulated sensitivity factors (C1: 0.076; O1: 0.212; Cu1: 0.269).
A bare PVD Cu surface deposited on a Ta/TaN barrier of a 300mm wafer is analyzed after
deposition with a minimum waiting time. Samples are cleaved close to the wafer’s centre. The
cleaved wafer’s halves are stored in a Front-Opening Shipping Box (FOSB) – some experiments are
also conducted on samples left on an Al foil in the lab. -. The Cu surface is progressively oxidized
and contaminated in ambient air. New samples are extracted from the wafer for each new analysis.
Results and discussions
Auger surface analysis: Cu, O and C elements are detected on the PVD Cu surfaces. Chemical
surface composition is plotted as a function of time after deposition (Figure 1). On the sample
analyzed after 90 min, unexpectedly high C content is found which is reduced by 34% one hour
later. Then a slight increase is detected from four days after deposition, most likely due to airborne
molecular contamination (AMC). The ratio of O/Cu contents increases continuously during the first
four days and then stabilizes close to 1. Assuming that O signal primarily comes from the copper
oxide, this result suggests that surface oxidation keeps going on at least within the first few days and
that the probed oxidized layer stoechiometry is close to CuO.
60
1
C1
16
Estimated at%
Estimated at%
18
14
12
10
0.9
50
0.8
40
O1
Cu1
O1/Cu1
30
1
10
100
1000
Time after PVD Cu deposition (hours)
1
10
100
0.7
0.6
1000
Time after PVD Cu deposition (hours)
Figure 1: Estimated PVD Cu surface (stored in FOSB) composition as a function of time from Auger surface spectra .
CO3- and CuOxHy- secondary ions peaks: The ToF-SIMS analysis performed 50 min after PVD
Cu deposition shows an intense carbonate (CO3-) ion peak. The normalized intensity of this peak
quickly decreases with time as the surface is oxidized / contaminated (Figure 2). The 30% decrease
between the first two samples is in quite good agreement with the Auger observation and suggests
that, on the first analyzed sample, CO3- is the major contributor to C content. This ion is likely to be
adsorbed from air on the highly reactive bare Cu surface. The evolution of CO3- in time is exactly
reproduced on a cleaned then He/H2 plasma treated PVD Cu surface (Figure 2) suggesting that this
peak might be a reproducible indicator of waiting time on a bare Cu surface.
2500
Ix *100 / I Cu
2000
PVD Cu
1500
CO3CuOCuO2CuO2H** CO3-
1000
Figure 2: Negative ion peaks intensity normalized to substrate peak as
a function of delay after surface preparation (stored in FOSB).
CO3-, CuO-, CuO2- and CuO2H-: measured on as-deposited PVD Cu
** CO3-: measured on a cleaned then He/H2 plasma treated PVD Cu
500
0
0.1
1
10
100
Time after deposition (hours)
1000
The continuous intensity decrease of CO3- peak is perfectly correlated to that of CuO-, CuO2- and
CuO2H- peaks (Figure 2). This reveals that these ions are not related to the amount of surface copper
oxide but, formed above the surface by the combination of Cu+ ion and Ox0 neutrals, they rather
indicate the amount of O present at the Cu surface. The apparent incoherence between this decrease
and the Auger O/Cu ratio increase could be due to ToF-SIMS probing “extreme” surface
contamination as opposed to Auger averaging information from deeper in the copper oxide.
AMC and effective detection sensitivity: When analysing processed Cu surfaces, AMC may
interfere with ion peaks considered as signature of cleaning chemistries compounds and limit their
effective detection limit. In the below examples, PVD Cu surface is used as a low level indicator:
- Various carboxylic acids, difficult to isolate with ToF-SIMS due to their common fragments, can
moreover be mistaken for CxHyO2- ion peaks brought by AMC (Figure 3).
10000
1000
PVD Cu
FOSB
100
10
1
1
Ix *100 / I Cu
Ix *100 / I Cu
10000
CHO2
C2H3O2
C3H5O2
C6H11O2
C8H15O2
C9H17O2
C12H23O2
PVD Cu
Lab
1000
100
10
1
1 1000
10
100
10
100
Time after deposition (hours)
Time after deposition (hours)
Figure 3: CxHyO2peak intensities
normalized to
substrate peak as a
function of delay after
PVD Cu deposition.
1000
- C2HO3- ion peak is 4 times more intense if Cu surface is cleaned with glycolic acid than with
oxalic acid or HF (Figure 4a). But, being also added with AMC, this tracer for glycolic acid is only
valid on freshly prepared surfaces compared to a clean reference having the same history (storage).
Ix *100 / I Cu
60
40
PVD Cu
in FOSB
PVD Cu
in Lab.
CMP Rinse Scrubber
C2HO3-
Wet Clean
+ He/H2 plasma
Wet Cleans
a)
20
Ix *100 / I Cu
100
0.8
2.33
8
25
100
293
2.33
8
25
100
293
48
48
48
48
48
48
48
48
48
48
48
48
336
336
432
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5
5
168
168
168
168
2.5
2.5
2.5
2.5
2.5
2.5
2.5
168
168
168
168
0
1000
PVD Cu
in FOSB
PVD Cu
in Lab.
C6H4N3CMP Rinse Scrubber Wet Clean
C H N CuDelay between surface preparation
and 2surface
+ He/H
plasmaanalysis (hrs) 6 5 3
Wet Cleans
b)
10
1
0.8
2.33
8
25
100
293
2.33
8
25
100
293
48
48
48
48
48
48
48
48
48
48
48
48
336
336
432
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5
5
168
168
168
168
2.5
2.5
2.5
2.5
2.5
2.5
2.5
168
168
168
168
PVD Cu
in FOSB
PVD Cu
in Lab.
C2H2N3Wet Clean
Delay between surface preparation
and surface
analysis (hrs) C2H3N3Cu
+ He/H
2 plasma
CMP Rinse Scrubber
Wet Cleans
c)
0.8
2.33
8
25
100
293
2.33
8
25
100
293
48
48
48
48
48
48
48
48
48
48
48
48
336
336
432
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5
5
168
168
168
168
2.5
2.5
2.5
2.5
2.5
2.5
2.5
168
168
168
168
Ix *100 / I Cu
0.1
10000
1000
100
10
1
0.1
Delay between surface preparation and surface analysis (hrs)
Figure 4: Peak intensity normalized to substrate peak for different Cu surfaces vs. delay after surface preparation.
Arrows = use of the compound of interest on these samples. a) C2HO3- (Glycolic acid). b) C6H4N3- and C6H5N3Cu(BTA). c) C2H2N3- and C2H3N3Cu- (TA).
Benzotriazole (BTA) and triazole (TA), used as corrosion inhibitor in Cu-CMP, are detected via
their de-protonated molecules (M-H)- and their complexes with Cu: MCu-, MCuO- and (M-H)2Cu-.
- C6H4N3- relative peak intensity after CMP or after chemical rinse is 10 to 100 times above the
baseline level induced by AMC (C6H4N3- or C8H6O-). After a scrubber clean, the BTA level is
below effective detection limit which is about 10 times above instrument detection limit (Figure 4b)
- C2H2N3- relative peak intensity after chemical rinse is 1000 times above the baseline level induced
by AMC. It is reduced by a factor 100 after scrubber clean and is still 10 times above effective
detection limit (Figure 4c).
Impact of PIDD on measured relative intensities: The TA related peaks intensity variation with
PIDD on a Cu surface is obtained with the profile reconstruction from a raw acquisition (Figure 5).
With increasing PIDD the larger fragments tends to disappear and the amount of the smaller ones
tends to increase. Indeed a progressive fragmentation of clusters present on the Cu surface leads to
increase the signal of their fragments. This may explain the maximum observed for M-H- signal for
a PIDD of ~3E13 ions/cm².
From this type of acquisition it is possible to determine the damage cross-section [3] from which the
PIDD for static conditions can be calculated. The result of this calculation on CxFyOz fragments of a
PerFluoroPolyEther (PFPE) contamination on Cu is presented (Figure 6). For the heaviest fragments
2 to 3E11 ions/cm² is already above the limit PIDD for static conditions.
1.E+12
Figure 5: Normalized negative ion peak intensities as a
function of primary ion dose density for TA fragments,
de- protonated molecule and clusters with Cu as
measured on post Cu-CMP wafer after scrubber clean.
C7F15O2
C10F21O3
C6F13O2
C5F11O2
PIDD (ions/cm²)
C4F9O
1.E+14
C4F9O2
1.E+13
C2F5O
1.E+11
C2F5
0.0
1.E+12
10% degradation
CF
0.5
20% degradation
CF3O
CN
CNO
C2H2N3
C2H2N3CuH
C2H2N3CuOH
(C2H2N3)2Cu
CF3
1.0
PIDD static (ions/cm²)
(Ix / ICu) / (Ix max/ICu)
1.E+13
Figure 6: PIDD limit for static conditions as function of
fragment of PFPE compound as detected on Copper
surface.
When analyzing product wafers, the available area may be limited by the dimensions of the
metrology boxes. For sufficient counting statistic on e.g. 50x50µm² area, PIDD up to 10 times
higher than the static limit has to be applied. In such case, measured relative intensities will differ
by 20 to 50% from those obtained on an equivalent surface analyzed in static conditions.
Conclusion
The surface of a PVD Cu was analyzed with ToF-SIMS as it is aging. An intense CO3- ion is
evidenced which tends to decrease in time. This ion is expected to be a reproducible indicator of the
waiting time on bare Cu surfaces and is likely to take part in the surface oxidation. It is
demonstrated that CuO, CuO2 and CuO2H peaks, as measured in static conditions, are not direct
indicators of surface oxidation but rather of oxygen surface content. These measurements also
allowed defining the effective detection limits for the compounds encountered in Cu cleaning
chemistries. Finally, the impact of PIDD on measurement reproducibility is illustrated.
Other primary ions may be mass separated from Bismuth source (Bi3+, Bi5+, Bi32+…) that could be
tested on theses surfaces. They are expected to generate less fragmentation of molecules.
References
[1] A. Nishi, M. Sado, T. Miki and Y. Fukui: Appl. Surf. Sci. 203-204 (2003), p. 470-472.
[2] S. Petitdidier et al., SEMICON Korea 2005 Conference Proc., Contamination-Free Manufacturing Technology
Seminar, Feb 2005, Seoul..
[3] F.Koetter, A.Benninghoven, Appl.Surf. 113 (1998) 47