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Comparison of direct-total-reflection X-ray fluorescence, sweeping-total-reflection X-ray fluorescence and vapor phase decomposition-total-reflection X-ray fluorescence applied to the characterization of metallic contamination on semiconductor wafers
Spectrochimica Acta Part B 63 (2008) 1375–1381
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Comparison of direct-total-reflection X-ray fluorescence, sweeping-total-reflection X-ray fluorescence and vapor phase decomposition-total-reflection X-ray fluorescence applied to the characterization of metallic contamination on semiconductor wafers☆ Adrien Danel a,⁎, Nicolas Cabuil b, Thierry Lardin a, Dominique Despois b, Marc Veillerot a, Charles Geoffroy c, Motoyuki Yamagami d, Hiroshi Kohno d a
CEA, LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble cedex 9, France STMicroelectronics, 850 rue J. Monnet, 38926 Crolles cedex, France c Elexience, 9 rue des Petits-Ruisseaux, BP61, 91371 Verrières-le-buisson cedex, France d Rigaku corporation, 14-8 Akaoji-cho, Takatsuki-shi, Osaka 569-1146, Japan b
a r t i c l e
i n f o
Article history: Received 2 October 2007 Accepted 4 October 2008 Available online 1 November 2008 Keywords: TXRF Silicon wafer Metallic contamination
a b s t r a c t The issues related to the matching between the 3 modes of Total-reflection X-Ray Fluorescence available on the latest generation of commercial equipment: Direct-Total-reflection X-Ray Fluorescence, Sweeping-Totalreflection X-Ray Fluorescence and Vapor Phase Decomposition-Total-reflection X-Ray Fluorescence, are discussed for quantitative analysis of metallic contamination on Si wafers. Direct-Total-reflection X-Ray Fluorescence and Sweeping-Total-reflection X-Ray Fluorescence agrees very well (+/−20% for light elements, transition metals and heavy metals), but due to a poor surface coverage with Direct-Total-reflection X-Ray Fluorescence, the matching is correct on a whole wafer only for uniform contaminations. Vapor Phase Decomposition-Total-reflection X-Ray Fluorescence might agree with other Total-reflection X-Ray Fluorescence modes only if the collection of contaminants following the oxide decomposition step is 100% completed. This is not achieved for 2 situations: noble metals which plate on bare Si, and solid particles partially digested during the Vapor Phase Decomposition and collection protocol. Furthermore, even if the collection of contaminants is well completed, quantification after Vapor Phase Decomposition depends on the shape of the dried residues and the Total-reflection X-Ray Fluorescence incident angle. With the incident angle selected to maximize the signal to noise ratio for ultra trace applications, i.e. about 0.5 times the Si critical angle, an increase of the quantification by a factor up to 10 is often seen after Vapor Phase Decomposition because of particle-like shape of the metals against film-like shape for the initial distribution. Taking into account advantages and drawbacks of each Total-reflection X-Ray Fluorescence mode, a proposal for the use of Total-reflection X-Ray Fluorescence in advanced Integrated Circuit manufacturing is given and illustrated by practical results from a R&D pilot line and a mass production plant. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Two main routes drive the needs for metallic contamination analysis in advanced IC manufacturing: the insurance of high yields and the introduction of new materials. The first requirement is related to the shrink of devices who are sensitive to traces of contaminants. Accordingly, metallic contamination for nodes 90 nm and beyond is specified by the ITRS [1] at 5 × 109 at/cm2 for critical metals at critical production steps. Thus, analytical methods should offer capabilities in Abbreviations: TXRF, Total-reflection X-Ray fluorescence; VPD, Vapor Phase Decomposition; IC, Integrated Circuit; LLD, Low Limit of Detection. ☆ This paper was presented at the 12th Conference on Total Reflection X-ray Fluorescence Analysis and Related Methods held in Trento (Italy), 18-22 June 2007, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +33 438 782 069; fax: +33 438 789 485. E-mail address: [email protected] (A. Danel). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.10.031
the 108 at/cm2 range. The second one is related to the recent and fast introduction of new materials used to target specific electrical, optical or mechanical properties in advanced microelectronic, including non volatile memories and above IC features. Consequently, IC production faces the need to share production and metrology equipments between Si and SiO2-based devices and “exotic” elements [2]. A list of metallic contaminants of interest is given in Table 1. Thus, equipments for the control of contamination should offer a wide range of detection and quantification in order to control possible cross contamination from new materials, but also “standard” elements in new films: Hf dissemination from high-k processes, and Cu contamination in Hf -based dielectric, respectively, as an example. Equipments fully automatic are available on the market today to answer this need: D-TXRF is widely used as a non invasive method with a multiple beams excitation in order to detect elements from Na to U on a large variety of substrates [3] and VPD-TXRF offers ultimate Low Limit of Detection (LLD), in the 107 at/cm2 range for Si/SiO2
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Table 1 Metallic contaminants related to new materials introduced in advanced IC manufacturing Applications
Metals
Targeted properties
Advanced IC
• Hf, Zr, Al, Ti, La, Ta, Y, Ba, Sr, Pr, Gd, Dy • Ni, Co, Ti, Pt, Ru, Ir, W, Re, Rh, Nb • Ge • Ti, Ta Ni, Mo, P, Co, W, Pd, Cu, Ru, Tl • Co, Fe, Ni, Ir, Mn, Pt, Ta, Cu
High-K (oxide, silicates, laminates)
Non-volatile memories
Above IC
• Ge, Sb, Te • Ag, In, Se, Ge, Sb, Te • In, Sn, Sr, Ti • Pb, Zr, Ti • Mo, Pt, Cr, Au, Sn, In, Cu, Pt
Low resistivity metal gates (salicides, metallization) High mobility substrates (SiGe alloy) Interconnects, barriers (metallization) FRAM: magnetic, antiferromagnetic (metals and alloys) MRAM: magnetic (metals and alloys) PCRAM: chalcogenide glass Transparent, conductive (oxide) Piezoelectric (alloys) Metallization
wafers [4]. Very recently, SP-TXRF offering fast and entire wafer mapping capabilities has been proposed [5]. With the goal of a pertinent use of TXRF in advanced IC manufacturing, this work discusses advantages and limitations of each mode and their possible matching. Finally, a strategy for the use of a TXRF equipment offering the 3 modes is presented. This work has been performed on an automatic VPD module and a “Fab300” TXRF, both from Rigaku. The TXRF uses a 10 kW Tungsten rotating anode with optics for 3 sources of illumination: Beam 1 (B1) is W–Mα 1.77 keV, B2 is W–Lβ 9.67 keV and B3 is a fraction of the emission spectrum at 24 keV. 2. Performances of the different TXRF modes 2.1. Direct-TXRF Latest equipments dedicated to the control of metallic contamination on semiconductor wafers offer the advantages of multiple sources to cover all elements of interest. Present detectors limit detection of light elements, thus, in practice, detection of traces starts with Z ≥ Na. Generally speaking, the challenge for TXRF-based methods is rather fluorescence overlaps between elements. Considering the elements of interest and the possible occurrence of 2 elements showing overlap, one important issue on Si wafers is: Br(Lα) → Al(Kα 1.49 keV), and in a less extend: As(Lα) → Mg(Kα 1.25 keV); Zn(Lα) → Na(Kα 1.04 keV); In(Lα) → K (Kα 3.31 keV). As an example, Al contamination might not be accurately analyzed on wafers coming from production area showing a strong amount of volatile Br (used during etching steps, for instance).
Table 2 Surface coverage and throughput of D-TXRF
5 points 9 points 17 points
200 mm wafer
300 mm wafer
300 s/point
3.2% 5.7% 10.8%
1.4% 2.5% 4.8%
1.6 W per h 1.0 W per h 0.6 W per h
A more critical issue related to overlaps is the detection of “standard” elements in/on new films. This is illustrated in Fig. 1 with an Hf-based dielectric film analyzed by a W–Lβ source. Detection and quantification of Mg, Co and Cu is almost impossible. Peak separation thanks to detector improvement and data processing, and the use of specific excitation sources are the answers to this overlap issue [6]. The Low Limits of Detection (LLD) by D-TXRF have been measured on prime Si wafers with one element for each excitation beam (Al, Ni and Mo) using the relationship (1): LLD =
3Ci Ii
rffiffiffiffiffiffiffiffi Ibgd t
ð1Þ
where Ci is the known concentration of element i on a calibration wafer (film like contamination of about 1012 at/cm2), Ii is the net intensity (counts per second), Ibgd is the intensity at the same energy with a clean wafer (counts per second) and t (in second) is the net counting time. For the anode operating at 35 kV and 255 mA, for t = 1000 s, the following LLD were obtained: ✓ B1: LLD (Al) = 1.3 × 1011 at/cm2, using incident angle of 0.5° ✓ B2: LLD (Ni) = 6.5 × 108 at/cm2, using incident angle of 0.08° ✓ B3: LLD (Mo) = 5.0 × 109 at/cm2, using incident angle of 0.05°. Except light elements and some heavy metals detected with their L lines, present LLD of D-TXRF are good enough to detect contamination above detrimental thresholds specified in the different manufacturing areas. Finally, the main drawbacks of D-TXRF are the small surface coverage and the poor throughput, regarding industrial needs in microelectronics. Table 2 gives the throughput (Wafer per hour) and the percentage of wafer surface analyzed for typical point patterns, considering single beam analysis and that each point is about 2 cm2. 2.2. SP-TXRF SP-TXRF proposed by Mori [5] combines the advantages of local and average information with good LLD, acceptable throughput and
Fig. 1. Overlaps of Hf fluorescence lines with “standard” metals as analyzed by B2.
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Fig. 2. Example of SP-TXRF results with the matching between defectivity map (a) and metallic contamination maps (b).
adjustable wafer surface coverage, 90% typically. Indeed, the method consists in wafer mapping (except edge exclusion of 7 mm typically) with a very short integration time for each point (2 to 5 s) and the calculation of the average contamination over the wafer using the sum of all individual spectra. Thus, for a given beam analysis on a 200 mm wafer using a 181 point pattern to insure a full coverage [7], with 5 s integration time per point, the throughput is 1.6 wafer per hour. Using
the same analytical setup as described in Section 2.1, the LLD of the average contamination, corresponding to a 905 s net counting time, is about the same as D-TXRF: ✓ B1: LLD(Al) = 1.5 × 1011 at/cm2 ✓ B2: LLD(Ni) = 7.4 × 108 at/cm2 ✓ B3: LLD(Mo) = 1.3 × 1010 at/cm2. According to Eq. (1), the LLD of each local point is about 15 times the LLD of D-TXRF at 1000 s. Similarly as D-TXRF, these performances can be good enough to detect contamination above detrimental thresholds specified in the different manufacturing area. The issue of detection and quantification of localized spots of contamination by SPTXRF, not in the scope of this paper, has been studied in details by Borde et al. [8]. The main advantage of SP-TXRF compared to D-TXRF is illustrated in Fig. 2 with a contamination pattern related to a cross contamination by contact with a chuck (control 200 mm Si wafer processed up side down and front face analyzed by B2). 2.3. VPD-TXRF
Fig. 3. Comparison between VPD-TXRF and VPD-ICPMS applied to the monitoring of wet clean processes, highlighting the importance of cleanliness control during the VPDdroplet collection step.
The key benefit of integrated VPD-TXRF is obviously the achievement of ultra low LLD. But in practice, this corresponds to the challenge of ultra cleanliness control during the decomposition, the droplet collection, the drying and the handling steps. This issue is illustrated in Fig. 3: ultimate LLD for Ti, Cr, Fe, Ni and Cu in the 108 at/ cm2 range are validated for the automatic and integrated VPD module from Rigaku, while the use of a manual VPD protocol with a cleanliness not perfectly controlled can lead to a strong limitation (see Inductively Coupled Plasma Mass Spectroscopy (ICPMS) results in Fig. 3). In practice, monitor wafers from wet clean processes were controlled by VPD-TXRF and VPD-ICPMS. Each wafer has been first
Table 3 Quantitative analysis of contamination on wafer in Fig. 2
Fig. 4. The various forms of metallic contamination, whatever localized or distributed over large area.
Average contamination (1010 at/cm2)
Ca
Cr
Fe
Ni
Direct TXRF (average value, 9 points) SP-TXRF (average value, 181 points) VPD ICPMS (average value, entire wafer)
10.7 20.2 8
82 72 28
371 346 127
32.5 30.5 14
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Fig. 5. Example of incomplete contamination collection during the VPD protocol.
contamination on semiconductor substrates: cations distributed over the surface, dried residue after VPD, solid particles, salts, metals plated, precipitates (alloys with the substrate) and metals diffused into the bulk, as illustrated in Fig. 4. First of all, D-TXRF might agree SP-TXRF or VPD-TXRF only for uniform contaminations over the substrate. Furthermore, considering average value for the whole surface, VPD-TXRF can agree SP-TXRF only if the VPD protocol collects 100% of the contamination, and does not bring additional contamination. And finally, since TXRF sensitivity depends on the incident angle and the shape of the sample (bulk, film or particle type [10]), quantitative results from the 3 modes represent an issue.
measured by VPD-TXRF and then by VPD-ICPMS. In most of cases, the metallic contamination left on the wafer surface after cleaning is extremely low. This contamination related to cations quite homogeneously distributed on/in the chemical SiO2 oxide is known to be well collected by the VPD procedure [9]. In this test, the second VPD and droplet collection step performed for ICPMS analyses, was verified to be better than 90% efficient using relationship (2). Thus, the matching between VPD-TXRF and VPD-ICPMS is good. For ultra low contamination levels, the mismatch between the 2 methods is attributed to the residual contamination left by the manual VPD protocol. Circles in Fig. 3 clearly show that the dispersion of VPDICPMS background is different for each element: high for Fe, medium for Cu and low for Ni, Ti and Cr. This result highlights that for VPD -based methods, the VPD steps can be the limiting factor for LLD. Thanks to very good hardware design and cleanliness, the VPD-TXRF results presented here are limited by TXRF sensitivity. With analytical conditions slightly different than D- or SP-TXRF (modified angle of incidence for B1 and use of dried droplets calibration wafers: Al for B1, Ni for B2 and B3) practical VPD-TXRF LLD for t = 1000 s and 200 mm wafers are:
3.1. Error related to localized contamination Table 3 gives an example of common mismatch between the 3 TXRF modes. The contamination related to a contact with a chuck presented in Fig. 2 has been analyzed by the 3 modes. Results are given for the 4 main elements found on the surface. If D-TXRF and SP-TXRF agree well for Fe, Cr and Ni, this is thanks to a specific contamination distribution (half of the surface is contaminated) along with a specific measurement pattern of the 2 modes. With a high single spot of contamination outside the D-TXRF point pattern, the mismatch between D-TXRF and SP-TXRF is obvious for Ca.
✓ B1: LLD(Al) = 6.0 × 108 at/cm2, using incident angle of 0.2° ✓ B2: LLD(Ni) = 6.6 × 106 at/cm2, using incident angle of 0.08° ✓ B3: LLD(Mo) = 8.2 × 107 at/cm2, using incident angle of 0.05° These LLD agree well D-TXRF LLD divided by the wafer surface, i.e. the pre-concentration factor won by the VPD protocol.
3.2. Error related to incomplete collection during VPD When metallic contamination is related to solid particles, VPD – based methods under estimates the contamination. This conclusion plotted in Table 3 with the VPD-ICPMS results is drawn assuming that the VPD protocol does not totally digest solid particles. This is likely the case for stainless steel particles of Fig. 2, because a few minutes of
3. Matching between D-TXRF, VPD-TXRF and SP-TXRF The users of integrated equipment offering the 3 modes of measurement expect a good matching between each feature. To discuss this point, one must consider the real nature of metallic
Table 4 Typical collection efficiency of the VPD protocol
CE(%)
Na
Al
Ca
Ti
Cr
Fe
Co
Ni
Cu
Zn
Ge
Sr
Mo
Ru
Ag
In
Sn
Hf
Ta
Ir
Au
N 95
N 95
N95
N 95
95
N95
N 95
N 95
50 to 70
N 95
N95
N 95
95
75
b5
N 95
N 95
N95
N 95
N95
b5
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Table 5 Matching between D-TXRF, SP-TXRF and VPD-TXRF for wafers intentionally contaminated (E10 at/cm2) Na Wafer #1 D-TXRF SP-TXRF VPD-TXRF Wafer #2 D-TXRF SP-TXRF VPD-TXRF Wafer #3 D-TXRF SP-TXRF VPD-TXRF
ND ND 91.4 299 302 30.4 278 306 433
Al
Ca
Fe
Co
Cu
Ge
Mo
Ag
Ta
ND ND 84.4 1460 1504 158 1386 1445 402
ND 2.0 5.4 4.2 6.2 9.7 15.3 19.7 10.3
3.1 4.6 3.7 3.4 4.0 8.9 9.4 12.6 12.5
3.8 3.5 3.1 3.1 3.6 8.0 9.8 11.9 9.9
3.2 3.7 1.8 2.0 2.4 1.2 7.5 9.7 6.2
39.1 36.6 39.7 8.5 12.0 30.9 15.9 30.1 29.6
27.3 25.6 35.1 8.8 10.0 18.9 14.6 22.1 17.9
46.2 42.6 0.3 – – – – – –
– – – 6.2 8.8 22.5 – – –
HF vapors do not totally attack the metals, and the collection performed with HF, H2O2 or HNO3 mixtures diluted in ultra pure water, wets each unit of surface during 1 s only. In Fig. 5, repetitive SP-TXRF measurements have been used to show that VPD protocol does not collect 100% of the contaminants when solid particles are involved. Here, the origin of the contamination is an accidental front side contact with a pipette. SP-TXRF maps show that only a part of the contamination is collected during VPD and gathered at the center of the wafer. It comes that quantitative results by VPDTXRF are lower than the true. Meanwhile, SP-TXRF gives similar quantification before and after VPD. The lower post VPD value for Fe could be due to a saturation effect at the center point (very high contamination: 5 × 1014 at/cm2). On the other hand, it is well known that VPD protocol is very efficient to collect metallic cations physisorbed on oxide surfaces [9]. Table 4 gives the Collection Efficiency (CE) for some of the elements
Fig. 7. TXRF incident angle dependence on two 10 ng VPD dried residues (see Fig. s6a and c).
listed in Table 1. CE is calculated using 2 repetitive decomposition + collection on each test wafer. The test wafers (Si with native oxide) have been intentionally contaminated at about 1012 at/cm2 using the spin drying method. The amounts of contaminant collected after the first and second VPD are labeled Q1 and Q2, and CE is defined as: Q2 : CEðkÞ = 100 1− Q1
ð2Þ
CE measurements were repeated several times and show a dispersion of about 5% for a given element. The collection mixture
Fig. 6. TXRF spectra, dried residues pictures and quantification of different 10 ng samples.
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Table 6 Example of TXRF use in advanced IC manufacturing Fab area
Specificities
Needs
Method of choice
Time
Additional
Wet clean
Uniform and low contamination from cleanings
VPD–TXRF B1 + B2
200 mm: 60 min 300 mm: 70 min
SP–TXRF B3
Thermal Treatment
Uniform and low contamination from furnaces or boats Front face: local or distributed Back side: particle-like
Ultra low LLD Na, Al, Ca, Fe, Cu, Zn + others vs process Ultra low LLD Na, Al, Ca, Fe, Ni, Cu, Zn + others vs process Transition metals + others upon process
VPD–TXRF B1 + B2
200 mm: 60 min 300 mm: 70 min
SP–TXRF B3
200 mm: 40 min 300 mm: 75 min
SP–TXRF B3
Etching
Front face: local or distributed Back side: particle-like
Transition metals + others vs process
200 mm: 40 min 300 mm: 75 min
SP–TXRF B3
Implant
Front face: local or distributed Back side: particle-like
Transition metals + others vs process
200 mm: 40 min 300 mm: 75 min
SP–TXRF B3
Litho
Back side: particle-like
Metrology
Back side: particle-like
Maintenance
Back side: particle-like
Transition metals + Al + others vs process Transition metals + Al + others vs process Transition metals + Al + Na, K, Ca + others vs process
D–TXRF (Front face) 9 or 17 points, 100 s, B1 9 or 17 points, 50 s, B2 D–TXRF (Front face) 9 or 17 points, 100 s, B1 9 or 17 points, 50 s, B2 D–TXRF (Front face) 9 or 17 points, 100 s, B1 9 or 17 points, 50 s, B2 SP–TXRF B1 +B2
200 mm: 80 min 300mm: 90 min 200 mm: 80 min 300 mm: 90 min 200 mm: 80 min 300 mm: 90 min
B3 VPD–TXRF B3 VPD–TXRF B3 VPD–TXRF
Deposition CMP
in this test was 2% HF + 2% H2O2 in ultra pure water, and the collection kinetic was 1 s/unit of surface. As one can see in Table 4, except for noble metals which can deposit into their metallic form by oxidizing the denuded silicon [11,12], CE is N95%. As a consequence, except noble metals, the matching between VPD-TXRF, SP-TXRF and D-TXRF could be good for wafers contaminated during wet processes and wafers intentionally contaminated by the spin drying method. In Table 5, results agree these hypotheses, except for Na and Al analyzed by B1 and some heavy elements analyzed by B3 (Ge, Mo, Ta). 3.3. Error related to the shape of the dried VPD residue The last item studied for the matching between the 3 modes is the impact of the shape of the dried VPD residue on the TXRF quantification. This could explain the quantification differences in Table 5, up to a factor 10 for light elements. Each excitation beam of the VPD-TXRF mode is calibrated using a dried residue with a known amount of metals. For B1, B2 and B3 excitations, a droplet containing 10 ng of Na, 10 ng of Mg, 10 ng of Al and 10 ng of Ni has been dried on a clean Si wafer. The corresponding average contaminations per cm2 on a 200 mm wafer, taking into account an edge exclusion of 5 mm during the collection have been taken as calibration values and are 9.2 × 1011 at/cm2 for Na, 8.7 × 1011 at/ cm2 for Mg, 7.9 × 1011 at/cm2 for Al and 3.6 × 1011 at/cm2 for Ni. TXRF spectrum (B1, set time is 500 s, incident angle is 0.2°) and picture of the dried residue of this calibration sample is given in Fig. 6a. The variation of peaks intensity versus incident angle is shown in Fig. 7. The result is typical of a particle -type contamination. Then, different shapes of dried residues were obtained modifying the drying step of the VPD protocol and the volume of the collection droplet, while keeping constant the amount of intentional Na + Mg + Al + Ni contamination (10 ng of each element). TXRF spectra (B1, set time is 500 s, incident angle is 0.2°), quantitative results (at/cm2) and pictures of dried residues are given in Fig. 6b, c and d. One example of incident angle dependence (sample #2) is given in Fig. 7 for B1: the behavior being less particle-like than the calibration sample, the quantification of light elements for a measurement at 0.2° is lower. The use of the isokinetic incident angle might help to solve this issue (0.71° on Si for B1). Finally, it appears that in practice, with wafers coming from different production steps, it is very difficult to obtain a repeatable VPD residue shape, similar to the one of the calibration sample(s). Accordingly, TXRF quantification might varies a lot (up to a factor 10,
SP–TXRF B1 +B2 SP–TXRF B1 +B2
higher or lower depending on the type of calibration sample) and a good matching between VPD-TXRF and D-TXRF or SP-TXRF is not expected. 4. Conclusion: strategy for the use of TXRF in IC manufacturing Taking into account key parameters requested for contamination control, advantages and drawbacks of each TXRF mode, users in IC manufacturing can define the “method of choice” for each production area. Mori et al. in reference [13] discuss this issue, and the example of LETI's R&D pilot lines (200 mm and 300 mm wafers) is given in Table 6. For monitoring application within a given production area (always the same type of wafers and the same TXRF mode used), small variations of quantitative analysis are relevant (+/−30% might be the limit). For diagnostics, with the use and the comparison of different TXRF modes, a factor 5 on quantitative results might not be relevant. The focus must be put on problem identification (which elements and which surface distribution) rather than precise quantification. Acknowledgement This work was performed within the frame of the European MEDEA+ “HYMNE” project and the Authors whish to thank the program partners (www.hymne.org). References [1] International Technology Roadmap for Semiconductors: http://public.itrs.net, revised FEOL Process (2004). [2] A. Danel, et al., Management of metallic contamination in advanced IC manufacturing, ECS Trans. 1-3 (2005) 3–10. [3] R. Klockenkämper, A. von Bohlen, Total-reflection X-ray fluorescence moving toward nanoanalysis: a survey, Spectrochim. Acta Part B 56 (2001) 2005–2018. [4] S. Pahlke, et al., Determination of ultra trace contaminants on silicon wafer surfaces using total-reflection X-ray fluorescence: TXRF state-of-the-art, Spectrochim. Acta Part B 56 (2001) 2261–2274. [5] Y. Mori, et al., Whole surface analysis of semiconductor wafers by accumulating short-time mapping data of total-reflection X-ray fluorescence spectrometry, Anal. Chem. 74 (2002) 1104–1110. [6] D. Hellin, et al., Trends in total reflection X-ray fluorescence spectrometry for metallic contamination control in semiconductor nanotechnology, Spectrochim. Acta Part B 61 (2006) 496–514. [7] A. Danel et al., “Mapping of metallic contamination using TXRF”, in Ultra Clean Processing of Semiconductor Surfaces VIII, Trans Tech Publications Ltd editor, Zurich, Switzerland (2007), pp 269-272. [8] Y. Borde, et al., Sweeping-TXRF optimisation to monitor the metallic contamination into IC manufacturing, Spectrochem. Acta Part B 63 (2008) 1370–1374 (this issue).
A. Danel et al. / Spectrochimica Acta Part B 63 (2008) 1375–1381 [9] H.-Y. Chung, et al., Collection efficiency of metallic contaminants on Si wafers by vapor-phase-decomposition droplet-collection, Anal. Sci. 17-5 (2001) 653–658. [10] R. Klockenkämper, Total-reflection X-ray fluorescence analysis, in: J.D. Winefordner (Ed.), Chemical Analysis, vol. 140, Wiley-Interscience publications, 1996. [11] S. De Gendt, et al., Vapor phase decomposition droplet collection: can we improve the collection efficiency for copper contamination? Solid State Phenom. 65-66 (1999) 93–96.
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[12] A. Danel, et al., Contribution for the optimization of the vapour phase decomposition technique, Mater. Sci. Eng. B102 (2003) 213–217. [13] Y. Mori, et al., Sweeping-TXRF: a non-destructive technique for the entire-surface characterization of metal contaminations on semiconductor wafers, IEEE Trans. Semicond. Manuf. 18 (2005) 569–574.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Comparison of direct-total-reflection X-ray fluorescence, sweeping-total-reflection X-ray fluorescence and vapor phase decomposition-total-reflection X-ray fluorescence applied to the characterization of metallic contamination on semiconductor wafers☆ Adrien Danel a,⁎, Nicolas Cabuil b, Thierry Lardin a, Dominique Despois b, Marc Veillerot a, Charles Geoffroy c, Motoyuki Yamagami d, Hiroshi Kohno d a
CEA, LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble cedex 9, France STMicroelectronics, 850 rue J. Monnet, 38926 Crolles cedex, France c Elexience, 9 rue des Petits-Ruisseaux, BP61, 91371 Verrières-le-buisson cedex, France d Rigaku corporation, 14-8 Akaoji-cho, Takatsuki-shi, Osaka 569-1146, Japan b
a r t i c l e
i n f o
Article history: Received 2 October 2007 Accepted 4 October 2008 Available online 1 November 2008 Keywords: TXRF Silicon wafer Metallic contamination
a b s t r a c t The issues related to the matching between the 3 modes of Total-reflection X-Ray Fluorescence available on the latest generation of commercial equipment: Direct-Total-reflection X-Ray Fluorescence, Sweeping-Totalreflection X-Ray Fluorescence and Vapor Phase Decomposition-Total-reflection X-Ray Fluorescence, are discussed for quantitative analysis of metallic contamination on Si wafers. Direct-Total-reflection X-Ray Fluorescence and Sweeping-Total-reflection X-Ray Fluorescence agrees very well (+/−20% for light elements, transition metals and heavy metals), but due to a poor surface coverage with Direct-Total-reflection X-Ray Fluorescence, the matching is correct on a whole wafer only for uniform contaminations. Vapor Phase Decomposition-Total-reflection X-Ray Fluorescence might agree with other Total-reflection X-Ray Fluorescence modes only if the collection of contaminants following the oxide decomposition step is 100% completed. This is not achieved for 2 situations: noble metals which plate on bare Si, and solid particles partially digested during the Vapor Phase Decomposition and collection protocol. Furthermore, even if the collection of contaminants is well completed, quantification after Vapor Phase Decomposition depends on the shape of the dried residues and the Total-reflection X-Ray Fluorescence incident angle. With the incident angle selected to maximize the signal to noise ratio for ultra trace applications, i.e. about 0.5 times the Si critical angle, an increase of the quantification by a factor up to 10 is often seen after Vapor Phase Decomposition because of particle-like shape of the metals against film-like shape for the initial distribution. Taking into account advantages and drawbacks of each Total-reflection X-Ray Fluorescence mode, a proposal for the use of Total-reflection X-Ray Fluorescence in advanced Integrated Circuit manufacturing is given and illustrated by practical results from a R&D pilot line and a mass production plant. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Two main routes drive the needs for metallic contamination analysis in advanced IC manufacturing: the insurance of high yields and the introduction of new materials. The first requirement is related to the shrink of devices who are sensitive to traces of contaminants. Accordingly, metallic contamination for nodes 90 nm and beyond is specified by the ITRS [1] at 5 × 109 at/cm2 for critical metals at critical production steps. Thus, analytical methods should offer capabilities in Abbreviations: TXRF, Total-reflection X-Ray fluorescence; VPD, Vapor Phase Decomposition; IC, Integrated Circuit; LLD, Low Limit of Detection. ☆ This paper was presented at the 12th Conference on Total Reflection X-ray Fluorescence Analysis and Related Methods held in Trento (Italy), 18-22 June 2007, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +33 438 782 069; fax: +33 438 789 485. E-mail address: [email protected] (A. Danel). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.10.031
the 108 at/cm2 range. The second one is related to the recent and fast introduction of new materials used to target specific electrical, optical or mechanical properties in advanced microelectronic, including non volatile memories and above IC features. Consequently, IC production faces the need to share production and metrology equipments between Si and SiO2-based devices and “exotic” elements [2]. A list of metallic contaminants of interest is given in Table 1. Thus, equipments for the control of contamination should offer a wide range of detection and quantification in order to control possible cross contamination from new materials, but also “standard” elements in new films: Hf dissemination from high-k processes, and Cu contamination in Hf -based dielectric, respectively, as an example. Equipments fully automatic are available on the market today to answer this need: D-TXRF is widely used as a non invasive method with a multiple beams excitation in order to detect elements from Na to U on a large variety of substrates [3] and VPD-TXRF offers ultimate Low Limit of Detection (LLD), in the 107 at/cm2 range for Si/SiO2
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Table 1 Metallic contaminants related to new materials introduced in advanced IC manufacturing Applications
Metals
Targeted properties
Advanced IC
• Hf, Zr, Al, Ti, La, Ta, Y, Ba, Sr, Pr, Gd, Dy • Ni, Co, Ti, Pt, Ru, Ir, W, Re, Rh, Nb • Ge • Ti, Ta Ni, Mo, P, Co, W, Pd, Cu, Ru, Tl • Co, Fe, Ni, Ir, Mn, Pt, Ta, Cu
High-K (oxide, silicates, laminates)
Non-volatile memories
Above IC
• Ge, Sb, Te • Ag, In, Se, Ge, Sb, Te • In, Sn, Sr, Ti • Pb, Zr, Ti • Mo, Pt, Cr, Au, Sn, In, Cu, Pt
Low resistivity metal gates (salicides, metallization) High mobility substrates (SiGe alloy) Interconnects, barriers (metallization) FRAM: magnetic, antiferromagnetic (metals and alloys) MRAM: magnetic (metals and alloys) PCRAM: chalcogenide glass Transparent, conductive (oxide) Piezoelectric (alloys) Metallization
wafers [4]. Very recently, SP-TXRF offering fast and entire wafer mapping capabilities has been proposed [5]. With the goal of a pertinent use of TXRF in advanced IC manufacturing, this work discusses advantages and limitations of each mode and their possible matching. Finally, a strategy for the use of a TXRF equipment offering the 3 modes is presented. This work has been performed on an automatic VPD module and a “Fab300” TXRF, both from Rigaku. The TXRF uses a 10 kW Tungsten rotating anode with optics for 3 sources of illumination: Beam 1 (B1) is W–Mα 1.77 keV, B2 is W–Lβ 9.67 keV and B3 is a fraction of the emission spectrum at 24 keV. 2. Performances of the different TXRF modes 2.1. Direct-TXRF Latest equipments dedicated to the control of metallic contamination on semiconductor wafers offer the advantages of multiple sources to cover all elements of interest. Present detectors limit detection of light elements, thus, in practice, detection of traces starts with Z ≥ Na. Generally speaking, the challenge for TXRF-based methods is rather fluorescence overlaps between elements. Considering the elements of interest and the possible occurrence of 2 elements showing overlap, one important issue on Si wafers is: Br(Lα) → Al(Kα 1.49 keV), and in a less extend: As(Lα) → Mg(Kα 1.25 keV); Zn(Lα) → Na(Kα 1.04 keV); In(Lα) → K (Kα 3.31 keV). As an example, Al contamination might not be accurately analyzed on wafers coming from production area showing a strong amount of volatile Br (used during etching steps, for instance).
Table 2 Surface coverage and throughput of D-TXRF
5 points 9 points 17 points
200 mm wafer
300 mm wafer
300 s/point
3.2% 5.7% 10.8%
1.4% 2.5% 4.8%
1.6 W per h 1.0 W per h 0.6 W per h
A more critical issue related to overlaps is the detection of “standard” elements in/on new films. This is illustrated in Fig. 1 with an Hf-based dielectric film analyzed by a W–Lβ source. Detection and quantification of Mg, Co and Cu is almost impossible. Peak separation thanks to detector improvement and data processing, and the use of specific excitation sources are the answers to this overlap issue [6]. The Low Limits of Detection (LLD) by D-TXRF have been measured on prime Si wafers with one element for each excitation beam (Al, Ni and Mo) using the relationship (1): LLD =
3Ci Ii
rffiffiffiffiffiffiffiffi Ibgd t
ð1Þ
where Ci is the known concentration of element i on a calibration wafer (film like contamination of about 1012 at/cm2), Ii is the net intensity (counts per second), Ibgd is the intensity at the same energy with a clean wafer (counts per second) and t (in second) is the net counting time. For the anode operating at 35 kV and 255 mA, for t = 1000 s, the following LLD were obtained: ✓ B1: LLD (Al) = 1.3 × 1011 at/cm2, using incident angle of 0.5° ✓ B2: LLD (Ni) = 6.5 × 108 at/cm2, using incident angle of 0.08° ✓ B3: LLD (Mo) = 5.0 × 109 at/cm2, using incident angle of 0.05°. Except light elements and some heavy metals detected with their L lines, present LLD of D-TXRF are good enough to detect contamination above detrimental thresholds specified in the different manufacturing areas. Finally, the main drawbacks of D-TXRF are the small surface coverage and the poor throughput, regarding industrial needs in microelectronics. Table 2 gives the throughput (Wafer per hour) and the percentage of wafer surface analyzed for typical point patterns, considering single beam analysis and that each point is about 2 cm2. 2.2. SP-TXRF SP-TXRF proposed by Mori [5] combines the advantages of local and average information with good LLD, acceptable throughput and
Fig. 1. Overlaps of Hf fluorescence lines with “standard” metals as analyzed by B2.
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Fig. 2. Example of SP-TXRF results with the matching between defectivity map (a) and metallic contamination maps (b).
adjustable wafer surface coverage, 90% typically. Indeed, the method consists in wafer mapping (except edge exclusion of 7 mm typically) with a very short integration time for each point (2 to 5 s) and the calculation of the average contamination over the wafer using the sum of all individual spectra. Thus, for a given beam analysis on a 200 mm wafer using a 181 point pattern to insure a full coverage [7], with 5 s integration time per point, the throughput is 1.6 wafer per hour. Using
the same analytical setup as described in Section 2.1, the LLD of the average contamination, corresponding to a 905 s net counting time, is about the same as D-TXRF: ✓ B1: LLD(Al) = 1.5 × 1011 at/cm2 ✓ B2: LLD(Ni) = 7.4 × 108 at/cm2 ✓ B3: LLD(Mo) = 1.3 × 1010 at/cm2. According to Eq. (1), the LLD of each local point is about 15 times the LLD of D-TXRF at 1000 s. Similarly as D-TXRF, these performances can be good enough to detect contamination above detrimental thresholds specified in the different manufacturing area. The issue of detection and quantification of localized spots of contamination by SPTXRF, not in the scope of this paper, has been studied in details by Borde et al. [8]. The main advantage of SP-TXRF compared to D-TXRF is illustrated in Fig. 2 with a contamination pattern related to a cross contamination by contact with a chuck (control 200 mm Si wafer processed up side down and front face analyzed by B2). 2.3. VPD-TXRF
Fig. 3. Comparison between VPD-TXRF and VPD-ICPMS applied to the monitoring of wet clean processes, highlighting the importance of cleanliness control during the VPDdroplet collection step.
The key benefit of integrated VPD-TXRF is obviously the achievement of ultra low LLD. But in practice, this corresponds to the challenge of ultra cleanliness control during the decomposition, the droplet collection, the drying and the handling steps. This issue is illustrated in Fig. 3: ultimate LLD for Ti, Cr, Fe, Ni and Cu in the 108 at/ cm2 range are validated for the automatic and integrated VPD module from Rigaku, while the use of a manual VPD protocol with a cleanliness not perfectly controlled can lead to a strong limitation (see Inductively Coupled Plasma Mass Spectroscopy (ICPMS) results in Fig. 3). In practice, monitor wafers from wet clean processes were controlled by VPD-TXRF and VPD-ICPMS. Each wafer has been first
Table 3 Quantitative analysis of contamination on wafer in Fig. 2
Fig. 4. The various forms of metallic contamination, whatever localized or distributed over large area.
Average contamination (1010 at/cm2)
Ca
Cr
Fe
Ni
Direct TXRF (average value, 9 points) SP-TXRF (average value, 181 points) VPD ICPMS (average value, entire wafer)
10.7 20.2 8
82 72 28
371 346 127
32.5 30.5 14
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Fig. 5. Example of incomplete contamination collection during the VPD protocol.
contamination on semiconductor substrates: cations distributed over the surface, dried residue after VPD, solid particles, salts, metals plated, precipitates (alloys with the substrate) and metals diffused into the bulk, as illustrated in Fig. 4. First of all, D-TXRF might agree SP-TXRF or VPD-TXRF only for uniform contaminations over the substrate. Furthermore, considering average value for the whole surface, VPD-TXRF can agree SP-TXRF only if the VPD protocol collects 100% of the contamination, and does not bring additional contamination. And finally, since TXRF sensitivity depends on the incident angle and the shape of the sample (bulk, film or particle type [10]), quantitative results from the 3 modes represent an issue.
measured by VPD-TXRF and then by VPD-ICPMS. In most of cases, the metallic contamination left on the wafer surface after cleaning is extremely low. This contamination related to cations quite homogeneously distributed on/in the chemical SiO2 oxide is known to be well collected by the VPD procedure [9]. In this test, the second VPD and droplet collection step performed for ICPMS analyses, was verified to be better than 90% efficient using relationship (2). Thus, the matching between VPD-TXRF and VPD-ICPMS is good. For ultra low contamination levels, the mismatch between the 2 methods is attributed to the residual contamination left by the manual VPD protocol. Circles in Fig. 3 clearly show that the dispersion of VPDICPMS background is different for each element: high for Fe, medium for Cu and low for Ni, Ti and Cr. This result highlights that for VPD -based methods, the VPD steps can be the limiting factor for LLD. Thanks to very good hardware design and cleanliness, the VPD-TXRF results presented here are limited by TXRF sensitivity. With analytical conditions slightly different than D- or SP-TXRF (modified angle of incidence for B1 and use of dried droplets calibration wafers: Al for B1, Ni for B2 and B3) practical VPD-TXRF LLD for t = 1000 s and 200 mm wafers are:
3.1. Error related to localized contamination Table 3 gives an example of common mismatch between the 3 TXRF modes. The contamination related to a contact with a chuck presented in Fig. 2 has been analyzed by the 3 modes. Results are given for the 4 main elements found on the surface. If D-TXRF and SP-TXRF agree well for Fe, Cr and Ni, this is thanks to a specific contamination distribution (half of the surface is contaminated) along with a specific measurement pattern of the 2 modes. With a high single spot of contamination outside the D-TXRF point pattern, the mismatch between D-TXRF and SP-TXRF is obvious for Ca.
✓ B1: LLD(Al) = 6.0 × 108 at/cm2, using incident angle of 0.2° ✓ B2: LLD(Ni) = 6.6 × 106 at/cm2, using incident angle of 0.08° ✓ B3: LLD(Mo) = 8.2 × 107 at/cm2, using incident angle of 0.05° These LLD agree well D-TXRF LLD divided by the wafer surface, i.e. the pre-concentration factor won by the VPD protocol.
3.2. Error related to incomplete collection during VPD When metallic contamination is related to solid particles, VPD – based methods under estimates the contamination. This conclusion plotted in Table 3 with the VPD-ICPMS results is drawn assuming that the VPD protocol does not totally digest solid particles. This is likely the case for stainless steel particles of Fig. 2, because a few minutes of
3. Matching between D-TXRF, VPD-TXRF and SP-TXRF The users of integrated equipment offering the 3 modes of measurement expect a good matching between each feature. To discuss this point, one must consider the real nature of metallic
Table 4 Typical collection efficiency of the VPD protocol
CE(%)
Na
Al
Ca
Ti
Cr
Fe
Co
Ni
Cu
Zn
Ge
Sr
Mo
Ru
Ag
In
Sn
Hf
Ta
Ir
Au
N 95
N 95
N95
N 95
95
N95
N 95
N 95
50 to 70
N 95
N95
N 95
95
75
b5
N 95
N 95
N95
N 95
N95
b5
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Table 5 Matching between D-TXRF, SP-TXRF and VPD-TXRF for wafers intentionally contaminated (E10 at/cm2) Na Wafer #1 D-TXRF SP-TXRF VPD-TXRF Wafer #2 D-TXRF SP-TXRF VPD-TXRF Wafer #3 D-TXRF SP-TXRF VPD-TXRF
ND ND 91.4 299 302 30.4 278 306 433
Al
Ca
Fe
Co
Cu
Ge
Mo
Ag
Ta
ND ND 84.4 1460 1504 158 1386 1445 402
ND 2.0 5.4 4.2 6.2 9.7 15.3 19.7 10.3
3.1 4.6 3.7 3.4 4.0 8.9 9.4 12.6 12.5
3.8 3.5 3.1 3.1 3.6 8.0 9.8 11.9 9.9
3.2 3.7 1.8 2.0 2.4 1.2 7.5 9.7 6.2
39.1 36.6 39.7 8.5 12.0 30.9 15.9 30.1 29.6
27.3 25.6 35.1 8.8 10.0 18.9 14.6 22.1 17.9
46.2 42.6 0.3 – – – – – –
– – – 6.2 8.8 22.5 – – –
HF vapors do not totally attack the metals, and the collection performed with HF, H2O2 or HNO3 mixtures diluted in ultra pure water, wets each unit of surface during 1 s only. In Fig. 5, repetitive SP-TXRF measurements have been used to show that VPD protocol does not collect 100% of the contaminants when solid particles are involved. Here, the origin of the contamination is an accidental front side contact with a pipette. SP-TXRF maps show that only a part of the contamination is collected during VPD and gathered at the center of the wafer. It comes that quantitative results by VPDTXRF are lower than the true. Meanwhile, SP-TXRF gives similar quantification before and after VPD. The lower post VPD value for Fe could be due to a saturation effect at the center point (very high contamination: 5 × 1014 at/cm2). On the other hand, it is well known that VPD protocol is very efficient to collect metallic cations physisorbed on oxide surfaces [9]. Table 4 gives the Collection Efficiency (CE) for some of the elements
Fig. 7. TXRF incident angle dependence on two 10 ng VPD dried residues (see Fig. s6a and c).
listed in Table 1. CE is calculated using 2 repetitive decomposition + collection on each test wafer. The test wafers (Si with native oxide) have been intentionally contaminated at about 1012 at/cm2 using the spin drying method. The amounts of contaminant collected after the first and second VPD are labeled Q1 and Q2, and CE is defined as: Q2 : CEðkÞ = 100 1− Q1
ð2Þ
CE measurements were repeated several times and show a dispersion of about 5% for a given element. The collection mixture
Fig. 6. TXRF spectra, dried residues pictures and quantification of different 10 ng samples.
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Table 6 Example of TXRF use in advanced IC manufacturing Fab area
Specificities
Needs
Method of choice
Time
Additional
Wet clean
Uniform and low contamination from cleanings
VPD–TXRF B1 + B2
200 mm: 60 min 300 mm: 70 min
SP–TXRF B3
Thermal Treatment
Uniform and low contamination from furnaces or boats Front face: local or distributed Back side: particle-like
Ultra low LLD Na, Al, Ca, Fe, Cu, Zn + others vs process Ultra low LLD Na, Al, Ca, Fe, Ni, Cu, Zn + others vs process Transition metals + others upon process
VPD–TXRF B1 + B2
200 mm: 60 min 300 mm: 70 min
SP–TXRF B3
200 mm: 40 min 300 mm: 75 min
SP–TXRF B3
Etching
Front face: local or distributed Back side: particle-like
Transition metals + others vs process
200 mm: 40 min 300 mm: 75 min
SP–TXRF B3
Implant
Front face: local or distributed Back side: particle-like
Transition metals + others vs process
200 mm: 40 min 300 mm: 75 min
SP–TXRF B3
Litho
Back side: particle-like
Metrology
Back side: particle-like
Maintenance
Back side: particle-like
Transition metals + Al + others vs process Transition metals + Al + others vs process Transition metals + Al + Na, K, Ca + others vs process
D–TXRF (Front face) 9 or 17 points, 100 s, B1 9 or 17 points, 50 s, B2 D–TXRF (Front face) 9 or 17 points, 100 s, B1 9 or 17 points, 50 s, B2 D–TXRF (Front face) 9 or 17 points, 100 s, B1 9 or 17 points, 50 s, B2 SP–TXRF B1 +B2
200 mm: 80 min 300mm: 90 min 200 mm: 80 min 300 mm: 90 min 200 mm: 80 min 300 mm: 90 min
B3 VPD–TXRF B3 VPD–TXRF B3 VPD–TXRF
Deposition CMP
in this test was 2% HF + 2% H2O2 in ultra pure water, and the collection kinetic was 1 s/unit of surface. As one can see in Table 4, except for noble metals which can deposit into their metallic form by oxidizing the denuded silicon [11,12], CE is N95%. As a consequence, except noble metals, the matching between VPD-TXRF, SP-TXRF and D-TXRF could be good for wafers contaminated during wet processes and wafers intentionally contaminated by the spin drying method. In Table 5, results agree these hypotheses, except for Na and Al analyzed by B1 and some heavy elements analyzed by B3 (Ge, Mo, Ta). 3.3. Error related to the shape of the dried VPD residue The last item studied for the matching between the 3 modes is the impact of the shape of the dried VPD residue on the TXRF quantification. This could explain the quantification differences in Table 5, up to a factor 10 for light elements. Each excitation beam of the VPD-TXRF mode is calibrated using a dried residue with a known amount of metals. For B1, B2 and B3 excitations, a droplet containing 10 ng of Na, 10 ng of Mg, 10 ng of Al and 10 ng of Ni has been dried on a clean Si wafer. The corresponding average contaminations per cm2 on a 200 mm wafer, taking into account an edge exclusion of 5 mm during the collection have been taken as calibration values and are 9.2 × 1011 at/cm2 for Na, 8.7 × 1011 at/ cm2 for Mg, 7.9 × 1011 at/cm2 for Al and 3.6 × 1011 at/cm2 for Ni. TXRF spectrum (B1, set time is 500 s, incident angle is 0.2°) and picture of the dried residue of this calibration sample is given in Fig. 6a. The variation of peaks intensity versus incident angle is shown in Fig. 7. The result is typical of a particle -type contamination. Then, different shapes of dried residues were obtained modifying the drying step of the VPD protocol and the volume of the collection droplet, while keeping constant the amount of intentional Na + Mg + Al + Ni contamination (10 ng of each element). TXRF spectra (B1, set time is 500 s, incident angle is 0.2°), quantitative results (at/cm2) and pictures of dried residues are given in Fig. 6b, c and d. One example of incident angle dependence (sample #2) is given in Fig. 7 for B1: the behavior being less particle-like than the calibration sample, the quantification of light elements for a measurement at 0.2° is lower. The use of the isokinetic incident angle might help to solve this issue (0.71° on Si for B1). Finally, it appears that in practice, with wafers coming from different production steps, it is very difficult to obtain a repeatable VPD residue shape, similar to the one of the calibration sample(s). Accordingly, TXRF quantification might varies a lot (up to a factor 10,
SP–TXRF B1 +B2 SP–TXRF B1 +B2
higher or lower depending on the type of calibration sample) and a good matching between VPD-TXRF and D-TXRF or SP-TXRF is not expected. 4. Conclusion: strategy for the use of TXRF in IC manufacturing Taking into account key parameters requested for contamination control, advantages and drawbacks of each TXRF mode, users in IC manufacturing can define the “method of choice” for each production area. Mori et al. in reference [13] discuss this issue, and the example of LETI's R&D pilot lines (200 mm and 300 mm wafers) is given in Table 6. For monitoring application within a given production area (always the same type of wafers and the same TXRF mode used), small variations of quantitative analysis are relevant (+/−30% might be the limit). For diagnostics, with the use and the comparison of different TXRF modes, a factor 5 on quantitative results might not be relevant. The focus must be put on problem identification (which elements and which surface distribution) rather than precise quantification. Acknowledgement This work was performed within the frame of the European MEDEA+ “HYMNE” project and the Authors whish to thank the program partners (www.hymne.org). References [1] International Technology Roadmap for Semiconductors: http://public.itrs.net, revised FEOL Process (2004). [2] A. Danel, et al., Management of metallic contamination in advanced IC manufacturing, ECS Trans. 1-3 (2005) 3–10. [3] R. Klockenkämper, A. von Bohlen, Total-reflection X-ray fluorescence moving toward nanoanalysis: a survey, Spectrochim. Acta Part B 56 (2001) 2005–2018. [4] S. Pahlke, et al., Determination of ultra trace contaminants on silicon wafer surfaces using total-reflection X-ray fluorescence: TXRF state-of-the-art, Spectrochim. Acta Part B 56 (2001) 2261–2274. [5] Y. Mori, et al., Whole surface analysis of semiconductor wafers by accumulating short-time mapping data of total-reflection X-ray fluorescence spectrometry, Anal. Chem. 74 (2002) 1104–1110. [6] D. Hellin, et al., Trends in total reflection X-ray fluorescence spectrometry for metallic contamination control in semiconductor nanotechnology, Spectrochim. Acta Part B 61 (2006) 496–514. [7] A. Danel et al., “Mapping of metallic contamination using TXRF”, in Ultra Clean Processing of Semiconductor Surfaces VIII, Trans Tech Publications Ltd editor, Zurich, Switzerland (2007), pp 269-272. [8] Y. Borde, et al., Sweeping-TXRF optimisation to monitor the metallic contamination into IC manufacturing, Spectrochem. Acta Part B 63 (2008) 1370–1374 (this issue).
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[12] A. Danel, et al., Contribution for the optimization of the vapour phase decomposition technique, Mater. Sci. Eng. B102 (2003) 213–217. [13] Y. Mori, et al., Sweeping-TXRF: a non-destructive technique for the entire-surface characterization of metal contaminations on semiconductor wafers, IEEE Trans. Semicond. Manuf. 18 (2005) 569–574.