Vapor phase treatment–total reflection X-ray fluorescence for trace elemental analysis of silicon wafer surface

Spectrochimica Acta Part B 90 (2013) 72–82

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Vapor phase treatment–total reflection X-ray fluorescence for trace elemental analysis of silicon wafer surface Hikari Takahara a,⁎, Yoshihiro Mori b, Harumi Shibata c, Ayako Shimazaki d, Mohammad B. Shabani e, Motoyuki Yamagami a, Norikuni Yabumoto f, Kazuo Nishihagi b, Yohichi Gohshi g a

Rigaku Corp., 14-8 Akaoji-cho, Takatsuki, Osaka 569-1146, Japan Horiba Ltd., 2 Miyanohigashi, Kisshoin, Minami-ku, Kyoto 601-8510, Japan SUMCO Corporation, Seavance North, 1-2-1 Shibaura, Minato-ku, Tokyo 105-8634, Japan d Toshiba Corporation, 8, Shinsugita-cho, Isogo-ku, Yokohama 235-8522, Japan e Mitsubishi Material Corporation, 1-297, Kitabukuro-cho, Omiya-ku, Saitama 330-8508, Japan f Analysis Atelier Co., 4-36-4, Yoyogi, Shibuya-ku, Tokyo 151-0053, Japan g Tsukuba University, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8571, Japan b c

a r t i c l e

i n f o

Article history: Received 8 March 2013 Accepted 24 October 2013 Available online 5 November 2013 Keywords: TXRF VPT Trace elemental analysis Silicon wafer

a b s t r a c t Vapor phase treatment (VPT) was under investigation by the International Organization for Standardization/ Technical Committee 201/Working Group 2 (ISO/TC201/WG2) to improve the detection limit of total reflection X-ray fluorescence spectroscopy (TXRF) for trace metal analysis of silicon wafers. Round robin test results have confirmed that TXRF intensity increased by VPT for intentional contamination with 5 × 109 and 5 × 1010 atoms/ cm2 Fe and Ni. The magnification of intensity enhancement varied greatly (1.2–4.7 in VPT factor) among the participating laboratories, though reproducible results could be obtained for average of mapping measurement. SEM observation results showed that various features, sizes, and surface densities of particles formed on the wafer after VPT. The particle morphology seems to have some impact on the VPT efficiency. High resolution SEM observation revealed that a certain number of dots with SiO2, silicate and/or carbon gathered to form a particle and heavy metals, Ni and Fe in this study were segregated on it. The amount and shape of the residue should be important to control VPT factor. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Total reflection-X-ray fluorescence (TXRF) spectroscopy is a kind of energy-dispersive XRF with a low glancing angle of around 0.1 deg. When a primary X-ray is directed to a mirror-polished substrate with a glancing angle of less than the total reflection critical angle, the primary beam is reflected completely by the sample surface. The scattering X-ray background is suppressed and X-ray fluorescence from materials on the surface is observed selectively and sensitively [1]. Therefore TXRF is suited for trace elemental analysis. It has been used in various fields such as environmental and biological studies and in the semiconductor industry [2]. Among them, semiconductor process diagnosis has been a particularly important application of TXRF in the last quarter of a century. Metallic impurities on semiconductor wafers are critical for device characteristics and must be

⁎ Corresponding author at: X-ray Analysis Division, Rigaku Corporation, 14-8 Akaojicho, Takatsuki, Osaka 569-1146, Japan. Tel.: +81 72 693 6813. E-mail address: [email protected] (H. Takahara). 0584-8547/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.10.006

controlled. Thanks to unique benefits of TXRF, i.e., non-destructive mapping capability and automatic operation, which contrast against characteristics of vapor phase decomposition–inductively coupled plasma mass spectrometry (VPD–ICP-MS), TXRF is now commonly accepted as an important analytical tool used in the semiconductor industry. [3] In the early days of semiconductor application of TXRF, the consistency of measurement values among different organizations was a salient issue that demanded standardization. In this context, the International Organization for Standardization/Technical Committee/Working Group 2 (ISO/TC201/WG2) was established in 1993 to standardize the TXRF measurement protocol. The first work was on the method of elemental contamination analysis of silicon wafers by direct-TXRF. It is now standardized as ISO 14706 [3]. In this international standard, it was concluded that TXRF is applicable down to 1 × 1010 atoms/cm2 of surface metal contamination. Although the tool should be calibrated with Ni or Fe reference material, no protocol to quantify Ni or Fe of the reference material was specified in ISO 14706, so that ISO/TC201/WG2 has moved to the next phase to standardize the calibration protocol. After many experimental studies, vapor phase decomposition (VPD)–TXRF was standardized as ISO 17331 [3] to quantify the reference materials described in

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ISO 14706. The Ni and Fe on reference materials are collected with the scanning solution and quantified with microdroplet standards. This test method applies down to 6 × 109 atoms/cm2, which is lower than the scope of ISO 14706. However, mapping capability, a unique benefit of TXRF, is lost in VPD–TXRF [4]. A method to achieve better sensitivity than direct-TXRF as well as maintaining mapping capability has been sought. Vapor phase treatment (VPT), a sample pretreatment method for TXRF, meets the requirements stated above [5–7]. The difference of VPT from VPD is that no surface scanning by a droplet solution is applied: after the decomposition of the surface oxide by HF vapor, the sample wafer is immediately dried without droplet scanning. VPT maintains the spatial information of surface metallic contamination and simultaneously brings about higher sensitivity of TXRF by changing the morphology of contaminants from film-type to particle-type. Moreover, the consistency of contamination morphology to particulate type, regardless of the morphology of starting contamination, could improve the accuracy of quantification for TXRF. Therefore VPT–TXRF is regarded as a promising method to expand the application field of TXRF in the semiconductor industry. When ISO/TC201/WG2 started the investigation of VPT–TXRF from 2003, the target was to achieve a detection limit of some 109 atoms/cm2. This detection limit is very difficult to achieve with direct-TXRF. This paper presents a summary of systematic study results of VPT– TXRF using ISO/TC201/WG2, which includes several round robin tests conducted between 2004 and 2008. Target metals are Fe and Ni on silicon wafer with concentrations between 5 × 109 and 5 × 1010 atoms/cm2. Participants conducted VPT pretreatment with their own VPD chambers and recorded the TXRF signal intensity before and after VPT. The efficiency of VPT was investigated from viewpoints such as the type of VPT method (chamber), spatial uniformity, repeatability, reproducibility, aging time, and type of sample preparation. Previous reports have described detailed experiments conducted using a fuming VPD chamber [7]. This study specifically examined other types of chambers. Finally, the sample surface after VPT was analyzed by using scanning electron microscopy (SEM). Based on these results, the characteristics and mechanisms of VPT were discussed.

2. Experimental Samples were 200-mm silicon wafers that were intentionally contaminated with 5 × 109 atoms/cm2 or 5 × 1010 atoms/cm2 of Fe and Ni together, with spincoat or immersion in alkaline hydrogen peroxide (IAP) method [8]. Concentrations were determined with VPD–ICP-MS. Round robin tests were held among several organizations with their own VPD boxes and TXRF spectrometers in the clean rooms with class 2–4. The employed VPD boxes were batch type chambers (VRC300T, VRC310, or VRC310S of S. E. S. Corp.), fuming type chambers (TXRFV300; Rigaku Corp. or TVD-910; Technos Co., Ltd.), and a manual type vessel [7]. Hydrofluoric acid (HF) of ca. 50% was used. The samples were exposed to HF vapor in each chamber or vessel for a predetermined time (15 s–30 min) at room temperature and were then dried in ambient atmosphere (except TVD-910 on which automated dry process with reduced pressure of N2 gas was used). TXRF measurements were performed before and after VPT with TXRF spectrometers (TXRF3750 or TXRF300 of Rigaku Corp. or TREX610, TREX630, or TREX6000 of Technos Co. Ltd.). The X-ray source was W-Lβ1 except for one organization with Au-Lβ1. Angle scanning from 0.00 to 0.15 deg by 0.01 deg steps and five-point mapping measurements at two different angles (0.05 or 0.1 deg, typically) were run. The integration time was set to 500 s. The VPT-treated samples in the round robin tests were collected and observed with SEM in a laboratory. Standard SEM was taken with 15 kV accelerating voltage (SEMVision G2; Applied Materials, Inc.) and surface-sensitive high-resolution SEM was taken at 0.3 and 1.0 kV (Ultra55; SII NanoTechnology Inc.).

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3. Results and discussion 3.1. Principle of VPT The well-known pretreatment technique to analyze surface contamination on a silicon wafer in a semiconductor field is VPD [10]. The native (or grown) silicon oxide on the wafer is decomposed by the reaction with hydrofluoric acid (HF) vapor, as expressed by the chemical reaction formulas (1) and (2). SiO2 þ 6HF→H2 SiF6 þ 2H2 O

ð1Þ

H2 SiF6 →SiF4 ð↑Þ þ 2HFð↑Þ:

ð2Þ

The wafer surface becomes hydrophobic by the complete removal of oxide layer. The wafer surface is then scanned with a droplet typically containing 1 mol/l hydrofluoric acid and 0.7 mol/l hydrogen peroxide to collect the metallic contamination. The recovered droplet is then dried on a Si wafer surface for TXRF measurement, or collected to a vial for ICP-MS measurement. The schematic diagram is presented in Fig. 1. VPD–TXRF can achieve a detection limit of as low as 1 × 108 atoms/cm2, whereas direct-TXRF attain only to 1 × 10 10 atoms/cm 2 . This improvement in the detection limit is brought about by the concentrating factor of VPD, which is roughly determined by the ratio of the wafer surface area to the size of the analysis spot as determined by the TXRF spectrometer detector. A general TXRF analysis spot is a circle of around 1 cm2 (10 mm diameter). Typical concentrating factor of VPT is ca. ×300 for 200 mm wafers that have ca. 300 cm2 surface area. Vapor phase treatment (VPT) is the decomposition reaction of SiO2 using HF vapor, described as chemical reactions (1) and (2) with subsequent drying. No droplet scanning is applied (Fig. 1). Reportedly, VPT enhances the fluorescence intensity of X-rays originating from surface contamination in TXRF. VPT was performed for 1 × 1010 to 1 × 1013 atoms/cm2 of Fe, Ni, and Zn intentionally contaminated on a silicon wafer [5]. The VPT process increased the elemental signal intensities by 4–7 times. SEM characterization of 1 × 1013 atoms/cm2 Ni contaminated sample revealed the formation of sub-micrometer particles on the wafer surface after VPT. The particles were comprised of Si, O, and transition metal elements according to Auger electron spectroscopy (AES) analysis. The increase of TXRF signal intensity by VPT pretreatment is explained by the change in contamination morphology from film type to particulate type. In film-type adsorption, the TXRF signal intensity increases along with the glancing angle that determines the penetration X-ray intensity in the film. In contrast, the signal intensity is ideally constant for particulate type contamination because the particles are excited completely irrespective of the glancing angle, as presented in Fig. 2 [11]. Because of the transformation of the surface contamination morphology from film type to particulate type by VPT, the TXRF signal increases at commonly used glancing angles (typically 1/4 to 3/4 of the critical angle). 3.2. Round robin test results Round robin tests were held by seven organizations twice with a half-year interval. The samples were intentionally contaminated wafers with 5 × 109 or 5 × 1010 atoms/cm2 of Fe and Ni together. All were prepared using the spincoat method at one time by one supplier, and were distributed to the participants. VPT was performed using VPD boxes owned by the respective laboratories. The VPT method was classified into three types based on the structure of VPD boxes: single-wafer manual, batch box, and fuming chamber [7]. In the batch-box type, hydrofluoric acid (HF) solution is placed at the bottom of the closed box. The vapor diffuses all over the box to expose the surfaces of the wafers settled in the slots. In the single-wafer manual type, we manually expose a wafer's surface to HF vapor, which naturally comes up from

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Direct-TXRF

VPD (Vapor Phase Decomposition)-TXRF HF vapor

HF VPT (Vapor Phase Treatment)-TXRF HF vapor

HF

. . . .. . . . . .. . . .... . . . .

Fig. 1. Schematic diagrams of direct-TXRF, VPD–TXRF and VPT–TXRF.

HF solution in the vessel. The fuming-chamber type is often mounted in the automated VPD tool. HF vapor is introduced into a single-wafer reaction chamber from an HF solution bottle into which N2 gas is bubbled. Four organizations of A–D employed the batch-box type, one organization, E, used the single-wafer manual type, and two organizations, F and G, used the fuming chamber type, respectively. Fig. 3 shows angle-scan profiles collected for sample wafers with 5 × 1010 atoms/cm2 Fe, before and after VPT for all organizations A–G. Angle scan profiles before VPT indicate film type adsorption of Fe contamination in all laboratories. After VPT, in the organizations A, B, D and E, the Fe intensity increased significantly over the entire angle and the enhancement factor of the intensity is larger at lower glancing angle. These facts suggest that the morphology of surface contamination changed partially from film to particulate by VPT pretreatment. In contrast, in C, F, and G, the increase of Fe intensity is less significant than in A, B, D and E. The angle scan profiles showed a very small change after VPT. A similar trend was also obtained for Ni samples for each organization (not shown). In order to evaluate the gain of VPT, we defined “VPT factor”, which is the net intensity ratio before and after the VPT. Table 1(a) and (b) lists TXRF signal intensities of Fe and Ni before and after the VPT treatment and the VPT factors in the round robin tests. The glancing angles were 0.05 and 0.1 deg for W-Lβ1 excitation source (critical angle is 0.18 deg), except organization D who used 0.08 deg instead of 0.10 deg because the Au-Lb1 excitation source was used (critical angle is 0.15 deg). In the round robin tests, all organizations obtained VPT

X-ray intensity

(a)

Φc

factors higher than 1.0. This confirms the benefit of VPT to enhance TXRF signals. It was also considered that we can expect a VPT factor of greater than 1.5 at 0.1 deg, with one exception for the second attempt of organization F. However, a large variation of VPT factor at 0.1 deg, from 1.2 to 4.7, was observed. This variation in VPT factor among the organizations correlates with the variation of the angle scan profiles in Fig. 3. The reasons for the interlaboratory variation were discussed among the participants and the difference of the three VPT methods was considered at first. Even in the batch box group, A to D, different trend was found out; C showed a small change in the angle scan profile and smaller VPT factor in both the tests, whereas in A, B, and D showed remarkable change in the angle scan profile and large VPT factors. This indicated that the efficiency of VPT was not simply determined by the VPT method. Then, we compared several experimental conditions (HF concentration, or time and temperature for VPT process, or drying atmosphere and humidity of the clean room might change the efficiency of VPT) that might vary the VPT factor as shown in Table 1(c). However, we could not find any parameters that made organization C as an exception. For example, organizations B and C ran the test with almost same conditions but C showed much lower VPT factors. Note that the VPT process time dependence was separately tested and we found no difference between 10 min and 30 min; process time is not an influence factor. Besides lab-to-lab difference, the second round robin test resulted in rather smaller VPT factor than the first test in each laboratory. And we suspected the difference of sample preparation between the two round robins. However, all samples in the both tests were prepared by

particle type contamination substrate

VPT

(b)

film type contamination substrate

Incident angle Fig. 2. Typical TXRF angle scan profiles for (a) particulate type and (b) film type of contamination. Φc is the critical angle.

H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

(a) A

(b) B

after

0.5

0.0

2.0

after

1.5 1.0 0.5 0.0

0.05

0.10

0.15

0.00

Glancing angle (deg.)

Fe-Ka intensity (cps)

Fe-Ka intensity (cps)

0.10

0.15

0.5

0.00

before after

0.5

0.0

2.5 2.0

Glancing angle (deg.)

0.15

3.0 before after

1.5 1.0 0.5

0.00

0.10

(f) F

0.0 0.15

0.05

Glancing angle (deg.)

3.0

0.10

after

0.0 0.05

(e) E

1.0

0.05

before

Glancing angle (deg.)

(d) D

0.00

before

Fe-Ka intensity (cps)

0.00

1.0

Fe-Ka intensity (cps)

before

1.0

(c) C

2.5

Fe-Ka intensity (cps)

Fe-Ka intensity (cps)

1.5

75

2.5

before

2.0

after

1.5 1.0 0.5 0.0

0.05

0.10

Glancing angle (deg.)

0.15

0.00

0.05

0.10

0.15

Glancing angle (deg.)

(g) G Fe-Ka intensity (cps)

4.5 4.0 3.5

before

3.0

after

2.5 2.0 1.5 1.0 0.5 0.0 0.00

0.05

0.10

0.15

Glancing angle (deg.) Fig. 3. Angle scan profiles obtained for sample wafers with 5 × 1010 atoms/cm2 Fe before and after VPT for A–G samples in the second round robin test except D in the first round robin test.

spin method at a single laboratory and supplied to all participants, and the preparation process was controlled as same as possible between the tests. Therefore, the overall difference of VPT factors among the two round robin tests must be little brought by the variation of sample properties. In order to find out the potential factors that influence to the variation of VPT factors, we added surface observation, mapping analysis and reproducibility, and discussed the possible mechanism of actual VPT reaction. 3.3. SEM observation For microscopic characterization of VPT residues, samples after the round robin test were collected to one laboratory for SEM observation. Fig. 4 shows SEM images for the VPT samples from organizations A–G in the second round robin test. Panels (a) and (b) of Fig. 4 are examples of wide view observations (organizations B and D). Particles were formed on the wafer surfaces after VPT, and the size and the surface density (10 4–106 particles/cm2 ) differed among laboratories. The expanded micrographs revealed that the

particles showed not only various sizes but also very different morphologies (Fig. 4(c)–(h)). In organizations A and B (Fig. 4(c) and (d)), the particles comprise a distinct solid core with 0.2–1 μm in diameter and the surrounding circle with a few micrometers in diameter. A larger core with 1–2 μm in diameter is observed for organization D (Fig. 4(f)). In contrast, others C, F, and G showed unclear or even missing cores reported rather unclear cores or even sometimes missing cores. Organizations A, B, and D, which reported large and distinct cores, showed particulate characteristics in the angle scan profiles and therefore larger VPT factors. In contrast, C, F, and G, which has vague cores, showed no significant change in angle scan profiles after VPT and had smaller VPT factors. However, neither the size of the core nor the whole shape showed any clear correlation to the magnitude of VPT factor. Furthermore, VPT factor did not necessarily depend on the number of particles that were estimated from the wide view pictures (Fig. 4(a) as an example). In summary, the VPT efficiency can be subject to the morphology of the particles formed on the surface by the VPT treatment, but it is not simply explained by the size or number of particles.

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Table 1 First (a) and second (b) round robin test results for Fe and Ni intentional contamination with 5 × 1010 atoms/cm2. VPT methods, TXRF conditions, and TXRF intensities before and after VPT; and VPT factors are summarized for participating organizations from A to G. The experimental conditions in batch type method for A to D (c). G participated in the second test only. Sample organization

VPT method

TXRF

Fe

Ni

X-ray source

Angle (deg)

Int. (cps) before

Int. (cps) after

VPT factor

Int. (cps) before

Int. (cps) after

VPT factor

0.05 0.1 0.05 0.1 0.05 0.1 0.05 0.08 0.05 0.1 0.05 0.1

0.09 0.37 0.13 1.10 0.47 – 0.13 0.19 0.25 0.82 0.50 1.50

0.25 1.61 0.92 2.97 0.92 – 0.27 0.43 1.40 2.23 2.41 3.22

2.8 4.4 7.1 2.7 2.0 – 2.1 2.3 5.5 2.7 4.9 2.1 4.1 2.8

0.14 0.62 0.11 1.08 0.70 – 0.09 0.31 0.38 1.16 1.15 2.97

0.43 2.58 1.27 5.11 1.16 – 0.45 0.71 1.93 2.86 3.52 4.32

3.2 4.2 11.5 4.7 1.7 – 5.0 2.3 5.0 2.5 3.1 1.5 4.9 3.0

0.05 0.1 0.05 0.1 0.05 0.1 0.05 0.08 0.05 0.1 0.05 0.1 0.05 0.1

0.05 0.49 0.15 1.06 0.13 0.26 – 0.14 0.29 0.84 0.44 1.41 0.34 1.35

0.23 1.18 0.70 1.96 0.16 0.46 – 0.27 1.57 2.51 0.67 1.67 0.82 2.43

4.8 2.4 4.7 1.8 1.2 1.7 – 1.9 5.5 3.0 1.5 1.2 2.4 1.8 3.4 2.0

0.09 0.56 0.23 1.09 0.18 0.27 – 0.20 0.45 1.02 0.66 1.85 0.50 1.93

0.30 1.67 1.32 3.43 0.15 0.50 – 0.40 1.64 2.48 0.85 2.36 1.08 3.18

3.3 3.0 5.7 3.1 0.9 1.8 – 2.0 3.7 2.4 1.3 1.3 2.2 1.6 2.9 2.2

(a) A

Batch

W-Lβ1

B

Batch

W-Lβ1

C

Batch

W-Lβ1

D

Batch

Au-Lβ1

E

Manual

W-Lβ1

F

Fuming

W-Lβ1

(b) A

Batch

W-Lβ1

B

Batch

W-Lβ1

C

Batch

W-Lβ1

D

Batch

Au-Lβ1

E

Manual

W-Lβ1

F

Fuming

W-Lβ1

G

Fuming

W-Lβ1

Average

Average

(c) Organization

Method

HF conc. (%)

Time (min)

Temp (°C)

Dry

CR humidity (%)

A B C D

Batch Batch Batch Batch

50 b50 50 b50

30 10 10 30

RT RT RT RT

Ambient Ambient Ambient Ambient

55 40 40 40

3.4. Detection limit of VPT–TXRF Table 2(a) gives an example of detailed VPT–TXRF data for A in the second round robin test: net and background intensities before and after VPT, and VPT factors for 5 × 1010 atoms/cm2 Fe and Ni samples. Each datum was averaged for five measurement positions. The detection limits, LLD, are also estimated by using Eq. (3).

intensity is too low to obtain statistically reliable data. Fig. 5 shows SEM micrograph for 5 × 10 9 atoms/cm 2 Fe and Ni samples after VPT for organizations B and D. The features of particles were similar to those of 5 × 1010 atoms/cm2 samples in the laboratory ((d) and (f) in Fig. 4). We believe that similar change occurred by VPT for 5 × 109 atoms/cm2 samples as well. 3.5. VPT with batch type box

rffiffiffiffiffiffiffiffiffi C Iback LLD ¼ 3 : Inet T

ð3Þ

Therein, C, Inet, Iback, and T denote the concentration, net intensity, background intensity, and integration time respectively. The background intensity increased slightly after VPT, probably because of surface roughening. The increase of net intensity is greater than 2-fold. Therefore, the improvement of the LLD by VPT was roughly consistent with VPT factor. In this result, the detection limit is improved from 1 × 1010 atoms/cm2 with direct-TXRF to 5 × 109 atoms/cm2 with VPT–TXRF. We also had evaluated the lower concentration samples of 5 × 109 atoms/cm2. An example of the result is presented in Table 2(b). In this result, net intensities were 0 cps before VPT, but they clearly increased to detectable intensities after VPT. Unfortunately, it was difficult to characterize angle scan profile and quantify VPT factor for 5 × 10 9 atoms/cm 2 samples because the net signal

The batch box type method uses a closed VPD (VPT) chamber that typically has more than 10 horizontal slots inside. The wafer surface on each slot is exposed by the vapor diffusing from the HF solution placed at the bottom of the box. Because this method can treat multiple wafers at once, unlike the other two methods treating one wafer at a time, it was first investigated whether VPT results are affected by the sample wafer slot position. Additionally, spatial uniformity, reproducibility, and process time dependence were also evaluated to characterize the basic properties of batch-type VPT. Fig. 6(a) depicts the slots in which the sample wafers with 5 × 1010 atoms/cm2 of Fe and Ni and dummy wafers were placed for three tests. The VPD box used had 27 slots. The number of the slot position is counted from top to bottom. The sample wafers were placed at slot positions of 3 and 5 each time, and one extra sample wafer was placed at slot number 1 only in the second test (named L1-3, L1-5, L2-

H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

(a) B

77

(b) D

20µm

20µm

(c) A

(d) B

(e) C

1µm

1µm

(f) D

1µm

(g) F

(h) G

1µm

1µm

2µm

Fig. 4. SEM images of VPT samples of 5 × 1010 atoms/cm2 Fe and Ni for A–G in the second round robin test. Wide area scan (Secondary Electron Image) for (a) B and (b) D, and expanded views (Backscattered Electron Image) for (c) A to (h) G.

Table 2 TXRF net intensities (Inet), background intensities (Iback), lower limit of detection (LLD) at 0.1 deg before and after VPT for Fe and Ni with 5 × 1010 atoms/cm2 (a) and 5 × 109 atoms/cm2 (b) in organization A in the second round robin test as an example. Net and background intensities were averaged from five-point mapping. The standard deviations for the five points are also given below the average. VPT factors are calculated from the net intensities before and after VPT. Inet (cps)

After

(b) Before After

LLD (atoms/cm2)

VPT factor

Fe 5 × 1010 atoms/cm2

(a) Before

Iback (cps)

Avg. Stdv Avg. Stdv

0.38 0.04 1.07 0.18

0.47 0.12 0.58 0.14

Avg. Stdv Avg. Stdv

Fe 5 × 109 atoms/cm2 0.00 0.45 0.00 0.10 0.10 0.46 0.10 0.10

Inet (cps)

Iback (cps)

LLD (atoms/cm2)

VPT factor

Ni 5 × 1010 atoms/cm2 10

0.57 0.10 1.37 0.23

0.74 0.15 0.92 0.21

1.0 × 1010

Ni 5 × 109 atoms/cm2 0.00 0.00 – 0.19 0.05

0.70 0.14 0.65 0.15

–

1.2 × 10

4.8 × 109

– 9

4.6 × 10

2.82

4.7 × 109

2.8 × 109

2.42

–

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H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

Fig. 5. SEM images (Backscattered Electron Image) of VPT samples of 5 × 109 atoms/cm2 Fe and Ni for organizations B and D in the second round robin test.

1, L2-3, L2-5, L3-3, L3-5, as presented in Fig. 6(a)). Remaining slots were filled with dummy wafers or were left empty (marked with dummy or none). Fig. 6(b) and (c) presents angle scan profiles of Fe before and after VPT at a position of L2-1 and L2-5 for example. The intensity increased more in L2-1 than in L2-5, indicating that the efficiency of VPT is subject to the slot positions. Fig. 7 shows the VPT factors at 25 points for L2-1, L2-3 and L2-5 (with using standard mapping measurement including angle alignment at each position, not with sweeping measurement excluding that). L2-1 showed mostly high VPT factors throughout the mapping positions. This might be because the slot 1 has different environment from other the lower slots. For example, it is known that the etching at the top slot starts earlier than the other slots in a VPT box; reactive gas arrives at the top slot more quickly than the lower slots. As for the spatial variation of VPT factor, no clear mapping position dependence was found in the magnitude through the samples. The averaged intensities of Fe and Ni before and after VPT, the averaged VPT factors and the CV are summarized in Table 3 for all samples. We can confirm that the top slot shows better VPT factor. For slots 3 and 5, average VPT factors were 2.5–2.8 in the three time repetitions. This suggests that reproducible VPT factor can be obtained, at least as average for mapping, except the top slot. Additionally, the influence of storage time to change of VPT sample was investigated, in order to check whether any surface chemical reaction is progressing even after the VPT. The sample wafers after VPT, stored 4 months in a clean box, were measured again. The angle scan profiles after 4 months are added for the corresponding samples in Fig. 6(b) and (c). The intensities slightly increased but each of the

profiles was not remarkably changed, suggesting that the formation or growth of particles does not occur during storage mostly. The VPT factors after the 4-month storage are also depicted for 25-mapping position in Fig. 7, and the average VPT factors after are given in Table 3. The VPT factors had slightly increasing trend. However, overall magnitude trend for 25 points was maintained after the 4-month storage, indicating that the surface change brought by VPT is mostly maintained for long-term storage. 3.6. VPT with single manual method The “single wafer manual” method is a simple mode of VPT; the wafer surface is just exposed to HF vapor from an open vessel for a short period of time (15 s in this study). One organization, E, used this method in the two round robin tests above and reported consistently good VPT factors of 2.4–3.0, as given in Table 1. We ran third round robin test that focuses on evaluating this method by four organizations other than E. Table 4 presents a summary of VPT factors with the single manual method. Most organizations yielded VPT factors above 1.5 at 0.1 deg, which is significantly lower than that of organization E. It can be concluded that there is no specific advantage in single wafer manual method, at least under the test conditions here. As for the lab-to-lab variation of VPT factor, the range was from 1.2 to 3.2 at 0.10 deg measurements among organizations. This magnitude of the variation is similar to that with batch type method. Same apply for the five-point mapping variations. In conclusion, VPT reaction of single wafer manual method might be similar to batch type method.

H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

(a)

(a)

Top

Down

1st Dummy Dummy L1-3 Dummy L1-5 Dummy

2nd L2-1 Dummy L2-3 Dummy L2-5 None

3rd Dummy Dummy L3-3 Dummy L3-5 None

6

right after VPT after 4 month storage

5

VPT Factor

Slot 1 2 3 4 5 6-25

79

4 3 2 1

(b)

5

before right after VPT after 4 months storage

1

3

5

7

9

11

13

15

17

19

21

23

25

mapping position

(b)

3

6

right after VPT after 4 month storage

5

2

1

0 0

0.05

0.1

0.15

0.2

VPT Factor

Intensity / cps

4

0

4 3 2

Glancing angle / degree 1

(c)

5

before

1

3

5

7

9

11

13

15

17

19

21

23

25

19

21

23

25

mapping position

after 4 months storage

(c)

3

6

right after VPT after 4 month storage

5

2

1

0 0

0.05

0.1

0.15

0.2

VPT Factor

Intensity / cps

0

right after VPT

4

4 3 2

Glancing angle / degree Fig. 6. (a) Sample slot configuration in VPD batch box for three repeated tests. Angle scan profiles for (b) L2-1 and (c) L2-5 before, right after VPT, and after 4-month storage.

1 0

1

3

5

For the studies described in the sections above, the silicon wafer samples with about 5 × 1010 atoms/cm2 of Fe and Ni were prepared with spin method. Because the VPT factor is the ratio of TXRF intensity after/before the treatment, the morphology of metals on the starting material must influence the value of VPT factor. The morphology might be subject to the concentration and sample preparation method. To assess the influence of these factors, two types of intentionally contaminated samples, spin and immersion in alkaline hydrogen peroxide solution method (IAP) [8], were compared. It is known that IAP provides stable film-type property than other methods. [9]. Fig. 8 presents the comparison of angle-scan profiles before and after VPT for spincoat and IAP samples with different concentrations. In spincoating, the lowest concentration sample (6 × 1010 atoms/cm2) showed small change in angle scan, which means that the transformation from film-type to particle-type is insufficient. Higher concentration samples showed clearer transformation to particle-type. In IAP, on the other hand, all samples showed very clear change from film-type to particle-type by VPT treatment. The essential difference of spincoat and

7

9

11

13

15

17

mapping position

3.7. Sample type dependence of VPT result

Fig. 7. VPT factor of L1–L3 for 25-point mapping measurements. The data were taken right after VPT and after 4-month storage.

IAP is that the former is physisorption while the latter is chemisorption. The stable transformation from film type to particle type for IAP wafer may be due to the fact that metal inclusion in surface chemical oxide film is very uniform and stable. Physisorption, i.e. for spincoat wafers, may not be stable or reproducible since the method basically relies on natural evaporation of liquid film on the surface. Due to the above reasons, IAP wafer would have been preferable for the estimation of VPT factors. Future studies should consider this point in the experimental design. 3.8. The VPT mechanism We confirmed the effect of VPT pretreatment to enhance TXRF intensity through the round robin tests and extensive studies. We have also found, however, that the VPT factor varied among laboratories

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H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

Table 3 Average VPT factors and the coefficients of variation, CV, for 25-point mapping results for 5 × 1010 atoms/cm2 of Fe and Ni by spincoat method. TXRF data were collected at 0.1 deg of glancing angle with W-Lβ1 source. The data was taken right after VPT reaction and after 4 months of storage. Sample

Slot

Fe

Ni

Right after VPT

L2-1 L1-3 L2-3 L3-3 L1-5 L2-5 L3-5

1 3

5

After 4 months of storage

Right after VPT

After 4 months of storage

VPT Factor

CV (%)

VPT factor

CV (%)

VPT factor

CV (%)

VPT factor

CV (%)

3.7 2.7 2.8 2.5 2.7 2.8 2.7

19.9 23.9 38.5 27.5 19.7 29.0 27.6

4.2 3.2 3.1 2.4 3.3 3.0 2.8

20.9 26.8 35.3 31.5 21.9 26.6 27.5

3.6 2.6 2.7 2.2 2.4 2.2 2.1

12.4 28.7 33.2 19.7 25.4 27.8 16.5

3.8 2.9 2.8 2.1 2.8 2.3 2.1

10.3 30.9 35.6 31.0 25.4 27.6 21.2

Table 4 Third round robin test results with single manual method for Fe and Ni intentional contamination with 5 × 1010 atoms/cm2. TXRF conditions, TXRF intensities before and after VPT, and VPT factor are summarized for the participating organizations from H to K. Sample organization

VPT method

TXRF

Fe

Ni

X-ray source

Angle (deg)

Int. (cps) before

Int. (cps) after

VPT factor

Int. (cps) before

Int. (cps) after

VPT factor

H

Manual

W-Lβ1

I

Manual

W-Lβ1

J

Manual

W-Lβ1

K

Manual

W-Lβ1

0.05 0.1 0.05 0.1 0.05 0.1 0.05 0.1

0.25 0.78 0.03 0.30 0.25 0.59 0.30 0.92

1.15 2.29 0.12 0.65 0.21 0.98 0.72 1.45

0.32 0.85 0.06 0.36 0.33 0.89 0.46 1.37

1.39 2.70 0.11 0.60 0.23 1.10 1.22 2.19

Manual

W-Lβ1

4.6 2.9 4.6 2.2 0.9 1.7 2.4 1.6 3.1 2.1 5.5 3.0

4.3 3.2 1.9 1.6 0.7 1.2 2.7 1.6 2.4 1.9 3.7 2.4

Average E in round robin test 2

(a) Spin 6x1010

(b) Spin 8x1011

0.4 0.3 0.2 before

0.1

after

0.05

0.10

4.0 3.0 2.0 before

1.0

after

0.0 0.00

0.15

60

Fe-Ka intensity (cps)

0.5

0.0 0.00

Glancing angle (deg.)

0.05

0.10

0.5 before after

0.10

20 after

0.15

0.05

0.10

0.15

250

15 10 before

5

after

0 0.00

before

10

(f) IAP 3x1013 Fe-Ka intensity (cps)

Fe-Ka intensity (cps)

1.0

0.05

30

Glancing angle (deg.)

20

Glancing angle (deg.)

40

0 0.00

0.15

(e) IAP 3x1012

1.5

0.0 0.00

50

Glancing angle (deg.)

(d) IAP 2x1011 Fe-Ka intensity (cps)

(c) Spin 7x1012

5.0

Fe-Ka intensity (cps)

Fe-Ka intensity (cps)

0.6

0.05

0.10

Glancing angle (deg.)

0.15

200 150 100 before

50

after

0 0.00

0.05

0.10

Glancing angle (deg.)

Fig. 8. Angle scan profiles obtained before and after VPT for samples with 6 × 1010–3 × 1013 atoms/cm2 prepared with spincoat and IAP method.

0.15

H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

81

Table 5 VPT methods, TXRF conditions, VPT factors, and SEM results for the samples M and N with 5 × 1010 atoms/cm2 Fe and Ni. Sample

VPT method

TXRF

VPT factor Angle (deg)

Fe

Ni

Particle size (μm)

Number of particles (cm−2)

0.05 0.08 0.05 0.1

1.1 1.2 3.3 2.3

1.0 1.1 3.5 2.9

0.4–1.3

3 × 105

0.3

5 × 106

M

Batch

Au-Lβ1

N

Fuming

W-Lβ1

both for batch and manual methods. The factor also varied for mapping around 30% in CV, though reproducible results were obtained for average of mapping measurement. To find the keys to control the VPT factor, more detailed mechanism of particle formation must be clarified. SEM observation with low accelerating voltages (0.3 and 1.0 kV) was attempted for two samples with low and high VPT factors. Table 5 presents the summary of VPT factors and standard SEM observation results for samples M and N. In the standard SEM observation, sample M had a smaller density of particles (3 × 105/cm2) and large variation of particle size from 0.4 to 1.3 μm. In contrast, sample N had a larger density of particles (5 × 106/cm2) with uniformly small size of 0.3 μm, which gave much larger VPT factor than sample M. Figs. 9 and 10 show highresolution SEM micrographs for samples M and N, respectively. For sample M, many small dots were observed in a single residue at 1.0 kV (Fig. 9(b)). The dots were more remarkable at 1.0 kV than at 0.3 kV (Fig. 9(a)), suggesting that the dots exist in the residue, not on top of the residue. The dots apparently gathered to form the residue because no dot was observed outside of the residue. In in-lens energysensitive backscattered electron micrographs (EsB), higher backscatter was observed at 0.3 kV (Fig. 9(c)) than at 1.0 kV (Fig. 9(d)), indicating that dots existing inside of the residue were composed primarily of lighter elements such as C and O: not of heavier elements. The heavier metals might be segregated on the outer skin of the residue. For sample N, smaller residue size and smaller numbers of dots were found in the residue compared with sample M, as shown in Fig. 10. Similarly to

100nm

100nm

SEM result

X-ray source

(a) In Lens0.3kV

(b) In Lens 1.0kV

sample M, many dots were formed in the residue (Fig. 10(a) and (b)) and were composed of lighter elements (Fig. 10(d)). In the drying process of VPT expressed by Eq. (2), Si might not be vaporized completely as SiF4, but some portion might remain as SiO2 and silicate [5]. Carbon, which might be from the surface molecular contamination on the wafer surface, can also be a component of the residue. The amount of SiO2, silicate and/or C in the residues on the surface can influence the VPT factor because the TXRF signal intensity depends on the distance of the analyte from the total reflection plane (i.e. the particle height), as are explainable from standing wave theory. In previous studies, moisture enhances VPT factors [5,7]. Higher humidity might affect to VPT factors, probably because evaporation of water molecule is deaccelerated by the moisture and thus evaporation of SiO2 is suppressed to generate larger Si-containing residues. The particles were dented from the top surface level in AFM images [7], which suggested that the condensed residues might etch the sample surface. We should consider how the particles are formed from the residue in the dry process. 4. Conclusion For trace elemental analysis without changing the spatial distribution, VPT is a simple and effective technique with improved sensitivity than direct-TXRF. Round robin test results show that a VPT factor of greater than 1.5 for 5 × 1010 atoms/cm2 of Fe and Ni on Si

100nm

100nm

(c) EsB 0.3kV

(d) EsB 1.0kV

dot

Fig. 9. High-resolution SEM micrographs obtained for sample M after VPT. (a) 0.3 kV InLens-SE, (b) 1.0 kV InLens-SE, (c) 0.3 kV EsB, and (d) 1.0 kV EsB.

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H. Takahara et al. / Spectrochimica Acta Part B 90 (2013) 72–82

(a) In Lens0.3kV

100nm

(c) SE2 1.0kV

100nm

(b) In Lens 1.0kV

100nm

dot

(d) EsB 1.0kV

100nm

Fig. 10. High-resolution SEM micrographs obtained for sample N after VPT: (a) 0.3 kV InLens-SE, (b) 1.0 kV InLens-SE, (c) 0.3 kV SE2, and (d) 1.0 kV EsB.

wafer samples can be expected. However, a large variation from 1.2 to 4.7 in VPT factor was found among the laboratories. Mapping variation of VPT factor was rather large (30% in CV), but about reproducible result was obtained for the average in mapping measurement. In SEM observations, various features, size and quantities of residues formed on the wafer were revealed. High resolution SEM observation showed large number of dots with SiO2, silicate and/or C that constitutes residues. Heavy metals, Ni and Fe in this study, might have segregated on the residue during drying. The amount and shape, especially the height of residue on the wafer surface should influence to the VPT factor. In future studies, control of these factors are the keys to establish stable VPT effect.

Acknowledgments The authors are grateful for helpful discussions with all members of ISO/TC201/WG2, K. Araki (Shin-Etsu Handotai Co., Ltd.), A. Urano (Sumitomo Electric Industries, Ltd.), Dr. T. Tanaka, H. Tanaka (NTT Advanced Technology Corp.), S. Taniike (Covalent Materials Corp.), T. Nakama (Munich Metrology), H. Horie (SUMCO Corp.), and Dr. K. Yakushiji (Showa Denko K.K.).

References [1] R. Klockenkaemper, Total-Reflection X-Ray Fluorescence Analysis, John Wiley & Sons, New York, 1997. 1–3. [2] A. Bohlen, Total reflection X-ray fluorescence and grazing incidence X-ray spectrometry — tools for micro- and surface analysis. A review, Spectrochim. Acta Part B 64 (2009) 821–832. [3] http://www.iso.org/iso/. [4] M. Funahashi, M. Matsuo, N. Kawada, M. Yamagami, R. Wilson, Enhanced analysis of particles and vapor phase decomposition droplets by total-reflection X-ray fluorescence, Spectrochim. Acta Part B 54 (1999) 1409–1426. [5] A. Shimazaki, K. Miyazaki, T. Matsumura, S. Ito, Analysis of low metallic contamination on silicon wafer surface by vapor phase treatment and total reflection X-ray fluorescence analysis, Jpn. J. Appl. Phys. 45 (2006) 9037–9043. [6] A. Shimazaki, S. Ito, K. Miyazaki, T. Matsumura, Proc. Int. Symp. Semiconductor Manufacturing, San Jose, 2005, IEEE, Piscataway, 2005, p. 46. [7] H. Takahara, Y. Mori, A. Shimazaki, Y. Gohshi, Method and mechanism of vapor phase treatment–total reflection X-ray fluorescence (VPT–TXRF) for trace element analysis on silicon wafer surface, Spectrochim. Acta Part B 65 (2010) 1022–1028. [8] Y. Mori, K. Shimanoe, T. Sakon, A standard sample preparation method for the determination of metal impurities on a silicon wafer by total reflection X-ray fluorescence spectrometry, Anal. Sci. 11 (1995) 499–504. [9] Y. Mori, K. Uemura, Total-reflection X-ray fluorescence analysis for semiconductor process characterization, Spectrochim. Acta Part B 58 (2003) 2085–2092. [10] A. Shimazaki, Defects in Silicon II/1991, in: M. Bullis, U. Gosele, F. Shimura (Eds.), Electrochemical Society, Pennington, 1990, p. 47. [11] R. Klockenkaemper, Total-Reflection X-ray Fluorescence Analysis, John Wiley & Sons, New York, 1997. 159–169.