Presented at UCPSS’06, Anvers, Belgium. Published in Solid State Phenomena, Trans Tech Publications Ltd, Switzerland, 2007
page 1
Mapping of metallic contamination using TXRF
A.Danel1,a, Y.Borde2,b, M.Veillerot1, N. Cabuil2, H. Kono3, M. Yamagami3
1
CEA-LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
2
3
STMicroelectronics, 850 rue J. Monnet, 38926 Crolles cedex, France
Rigaku corporation, 14-8 Akaoji-cho, Takatsuki-shi, Osaka 569-1146, Japan
a
adrien.danel@cea.fr, byannick.borde@cea.fr
Keywords: metallic contamination, silicon wafer, TXRF
Introduction
Two main routes drive the needs for metallic contamination analysis in Integrated Circuit (IC)
manufacturing: the insurance of high yields and the introduction of new materials used to target
specific electrical, optical or mechanical properties in advanced microelectronic, including non
volatile memories and above IC features. With metallic contamination specified by the ITRS at 5E9
at/cm2 for critical metals at critical production steps of nodes 90nm and beyond, analytical methods
should offer capabilities in the E8 at/cm2 range, this for a set of elements with Z ranging from Na to
Pb.
Among the methods available on the market today to answer this need, TXRF appears as a method
of choice [1]: it is non invasive and available through equipments compatible with industrial
requirements; the use of multiple excitation beams allows the detection of elements from Na to U
on a large variety of substrates; Vapor Phase Decomposition (VPD) TXRF offers ultimate Low
Limit of Detection (LLD) in the E7 at/cm2 range for Si/SiO2 wafers; and recent Sweeping (SP) TXRF mode has been proposed for fast and entire wafer mapping [2].
With the goal of a pertinent use of SP-TXRF in advanced IC manufacturing, this work follow
previous study [3] and discusses measurement relevance depending on the mapping parameters.
Surface coverage, throughput, LLD and noise are studied versus number of points per map and
integration time per point. Finally an optimal use of SP-TXRF is discussed.
Management of metallic contamination
Determination of acceptable levels of contamination in the different IC manufacturing areas and the
strategy for the processing of disruptive wafers must take into account monitoring capabilities of the
production line, especially the pertinence of quantitative analysis [4]. Thus, this work estimates how
a given contamination on wafer surface is seen and quantified using SP-TXRF: capture rate notion
is presented.
Mapping parameters and measurement relevance
If the mapping parameters of a SP-TXRF analysis are simple: number of points and integration time
per point, the relevance of measurement is not obvious and should be considered from exact
quantification and industrial requirements point of view. This work is performed on a “Fab300”
TXRF from Rigaku (W rotating anode with optics for 3 sources of illumination: Beam 1 (B1) is WM @ 1.77keV, B2 is W-L @ 9.67keV, B3 is High Energy @ 24keV) and results presented
hereafter for B2 can be generalized.
Surface coverage: The TXRF spot size must be considered for localized contamination, typically
less than a few mm2 large when originated from particles. Figure 1 shows the shape of the
quantification factor versus the positioning mismatch of the particle under the center of the detector
Presented at UCPSS’06, Anvers, Belgium. To be published in Solid State Phenomena
page 2
(1mm2 spot of Mn contamination used in this test). From this radial variation, one can define at 6.5
+/- 0.5mm a spot radius for which a single contamination spot is quantified at >= 50% (Rquantification),
and at 10.5 +/- 0.5mm for >= 10% (Rdetection).
800
Max
700
600
500
400
300
Rquantification
200
Rdetection
100
0
-40
-35
50%
[Mn] E10 at/cm 2
Then, in figure 2, taking into account these
quantification and detection radius of the
measurement spot, the surface coverage offered
by a mapping with points uniformly distributed
on the wafer has been calculated for a 200 mm
wafer taking into account a 15mm edge
exclusion (center of measurement spots is at best
at 15mm of wafer edge).
Throughput: Figure 3 gives for SP-TXRF
mode the measurement time of 200mm wafer
using different mapping conditions. It can be
seen that for integration time per point shorter
than 3sec. adjustment of the incident angle
becomes the dominating factor.
-30
10%
-25
x (mm): spot positioning
Figure 1: Quantification variation with detector
position above a contamination spot (1mm2 area of Mn
localized at x = -27mm; y = +52mm).
70
detection
detection
Measurement time
per wafer (min.)
Surface coverage (%)
80
120
120
100
100
80
80
60
60
40
40
20
20
quantification
quantification
60
50
2s
40
3s
30
Integration
time per point
20
5s
10s
10
00
0
00
50
50
100
100
150
150
200
200
250
250
0
100
150
200
250
Number of measurement points
Number of measurement points
Figure 2: 200 mm wafer surface coverage (without 15
mm edge exclusion) considering the detection and
quantification measurement spot radius.
50
Figure 3: Throughput of SP-TXRF, B2, 200mm wafer,
loading sequence included.
LLD and accuracy of quantitative measurements:
The local LLD of element i, point to point, is given by Eq. 1:
LLDi 
3.Ci I Bgd
.
Ii
t
(1)
where Ci is the concentration of element, Ii the X-ray intensity of element (cps), Ibgd is background
intensity at detection energy of element (cps), and t is the integration time (sec).
Practical measurement of LLD for Ni on prime Si wafers as been measured for the different TXRF
modes: 7.8E9 at/cm2 for a single point of a SP-TXRF map with t = 5sec.; 1.1E9 at/cm2 for the
integrated spectrum of the SP-TXRF map [2] with 65 points and t = 5sec.; and 2.0E9 at/cm2 for
Direct (D) -TXRF mode with t = 50sec. Corrected by the 1 / t factor, LLD data shows that
equipment performances are slightly degraded in SP mode (Ni LLD @ 1000sec. in at/cm2 is 4.4E8
for D-TXRF; 5.5E8 for SP-TXRF and 6.3E8 for the integrated spectrum of SP map). This could be
attributed to the non optimized method of incident beam control used in SP mode in order to
improve measurement throughput.
Presented at UCPSS’06, Anvers, Belgium. To be published in Solid State Phenomena
page 3
sigma (%)
Accuracy of quantitative analyses has been
160
D-TXRF point to point
estimated from repetitive measurements under
140
the different modes. Results are shown in Figure
SP-TXRF point to point
120
4 with the standard deviation (sigma %) versus
SP-TXRF integration
100
the level of contamination normalized to the
80
LLD value. In practice, with a Low Limit of
Quantification (LOQ) defined for a level of
60
contamination quantified with a standard
40
deviation of about 40%, the LOQ is roughly 3
20
times the LLD, whatever the TXRF mode.
0
Indeed, the impact of degraded incident angle
0.1
1
10
100
1000
adjustment in SP mode on the quantitative
contamination level (N x LLD)
measurement is low compared to the threshold
of LOQ considered at sigma = 40%. This can be Figure 4: Repeatability of quantitative measurements
estimated with the variation of Si signal across using different TXRF modes.
the wafer: sigma = 2.4% for D-TXRF and 12.6%
for SP-TXRF.
Capture rate: The issue of correct quantification of non uniform contamination must be
considered. This has been estimated for localized spot(s) of contamination, showing a surface
negligible compared to the measurement spot. This situation, representing practical situation of
contamination mainly related to particles, allows the calculation of the average quantification using
a given point pattern. In this work, the ratio of this value with the true level of contamination is
named the capture rate.
Considering a mapping pattern with two adjacent circular measurement points separated by a
distance 2R and a spot of contamination localized at r from the center of a measurement point with a
probability of 1/R (0 ≤ r ≤ R), the global quantification factor, i.e. the capture rate, of the
contamination spot is given by Eq. 2, with Q(r) expressed by Eq. 3 from the best fit of Figure 1.
Qglobal  0
R
1
1 R
dr . 2 0 Q( r )2rdr
R R
(2)
Q( r )  0.0014r 3  0.0223r 2  0.0046r  1
(3)
Figure 5a shows a D-TXRF analysis of a wafer contaminated with 10 local spots over the surface,
where a 10 points measurement pattern has been placed at the exact spot location. Figure 5b shows
SP-TXRF analysis by different mapping patterns with an increasing number of points (with a
uniform repartition of TXRF points over the surface for all maps).
21 pts 3s
93 pts 3s
a)
41 pts 3s
133 pts 3s
65 pts 3s
229 pts 3s
b)
500 pts 3s
Figure 5: TXRF mapping of a wafer contaminated with 10 Mn spots: a) 10 points D-TXRF
centred above spots, b) various SP-TXRF.
Presented at UCPSS’06, Anvers, Belgium. To be published in Solid State Phenomena
page 4
In Figure 6, the point to point quantifications of maps 5b normalized to the reference map 5a are
compared to calculation using Eq. 2 and 3. This result illustrates how one can estimates the capture
rate of a non uniform contamination by TXRF.
100
Quantification (%)
Equation 2
80
practical data
Local LLD Global LLD Throughput
Surface
coverage (%) (Ni at/cm2) (Ni at/cm2) (min. / wafer)
60
40
20
0
0
50
100
150
200
250
41pts, 3"
56.7
1.0E+10
1.8E+09
14
65pts, 3"
65pts, 3"
85.4
1.20E+10
1.0E+10
1.49E+09
1.4E+09
20
65pts, 5"
85.4
7.8E+09
1.1E+09
23
65pts, 10"
85.4
5.5E+09
7.8E+08
30
93pts, 3"
96.3
1.0E+10
1.2E+09
27
85.4
Number of points per map
Figure 6: Capture rate of a non uniform contamination
by SP-TXRF.
Table I: SP-TXRF mapping parameters and measurement
relevance.
Conclusion, optimum for the use of SP-TXRF
User’s expectations can be summarized as follow: high throughput, high surface coverage and high
capability (LOQ < contamination specifications). According to results presented in Figures 1, 2, 3
and 4, the best use of SP-TXRF in industrial environment is a compromise between these
expectations. Table I summarizes what could be the optimal use of SP-TXRF for B2 on 200mm
wafers. An 65 points – 3sec. pattern insures an 85% surface coverage, a point to point LOQ of a few
E10 at/cm2 and a global LOQ of a few E9 at/cm2, and a throughput of 3 wafers per hour. In this
table, throughput includes the entire wafer loading sequence, the surface coverage is considered
with a detection radius of 10.5mm, and the global LLD is the one of the integrated spectrum of the
SP-TXRF map [2].
Finally, the opportunity of full wafer mapping offered by the SP-TXRF mode suggests to specify the
levels of acceptable contamination in the different production areas as an average value per wafer,
but also as a maximum local value.
This work was performed within the frame of the European “HYMNE” project (Medea+ 2T102, www.medea.org).
References
[1] D. Hellin et al., “Trends in total reflection X-ray fluorescence spectrometry for metallic contamination control in
semiconductor nanotechnology”, Spectrochemica Acta part B, Vol. 61 (2006), pp 496-514.
[2] Y. Mori et al., “Whole surface analysis of semiconductor wafers by accumulating short-time mapping data of totalreflection X-ray fluorescence spectrometry”, Anal. Chem. 74 (2002), pp 1104-1110.
[3] A. Danel et al., “Comparison of D-TXRF, SP-TXRF and VPD-TXRF applied to the characterization of metallic
contamination on semiconductor wafers”, to be published in Spectrochemica Acta part B.
[4] A. Danel et al., “Management of metallic contamination in advanced IC manufacturing”, ECS Transactions, 1-3
(2005), pp 3-10.