INTERNAL DEFECT DETECTION IN THE VICINITY OF SI WAFER SURFACE USING EVANESCENT WAVE

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1 Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia 4 XVII IMEKO World Congress Metrolog in the 3rd Millennium June 22 27, 23, Dubrovnik, Croatia INTERNL DEFECT DETECTION IN THE VICINITY OF SI WFER SURFCE USING EVNESCENT WVE Rusuke NKJIM, Takashi MIYOSHI, Yasuhiro TKY and Satoru TKHSHI Dept. of Mechanical Engineering and Sstems, Osaka Universit, Osaka, Japan bstract In order to reduce and control ield loss in the fabrication process of net generation ULSI devices, nano-defects inspection technique for polished Silicon (Si) wafer surface becomes more essential. This paper discusses the new optical nano-defects detection method, which is applicable to the silicon wafer surface inspection technique for net-generation semiconductors. In our proposed method, the evanescent light is emerged on the wafer surface with total internal reflection (TIR) of infrared (IR) laser at the Si-air interface. nd b scanning the surface where is evanescent light emerging with ver shaped fibre probe, it enables to detect nanometre scale defects in the vicinit of Si wafer surface without diffraction limit to resolution. To verif the feasibilit of this method, both of the computer simulations based on Mawell s equations and the several fundamental eperiments are performed. FDTD simulation shows that the proposed method is effective to detect nano-defects eisting not onl on Si surface but also in the subsurface with high sensitivit. nd also the fundamental eperiments show the validit of this method b demonstrating nano-defects detections of subsurface as well as surface. Kewords: silicon wafer, internal defect, evanescent wave. 1. INTRODUCTION In modern semiconductor manufacturing processes, the design rule will shrink to under 1nm scale. In order to realie high productivit and reliabilit of the semiconductor fabrication, defects inspection techniques for polished Si wafer surface become more essential. Defects of Si wafer surface are classified into two tpes that mean eternal defects on the surface (surface defects) and internal defects in the subsurface (subsurface defects) b the state of defect. The eamples of surface defect are micro-scratch, COP (Crstal Originated Particle) [1] and so on. nd the subsurface defect means void like defect eisting in a few hundred nanometres deep. Reducing all of such defects is inevitable to reduce ield loss in the fabrication processes. The critical defect sie, which is required to detect in the 1nm design rule processes, become several ten nanometres. ut such small defects are undetectable b the conventional defects inspection techniques based on light scattering method, which reached phsical limits imposed b the wavelength of its own incident laser beam. nd there was no inspection technique to detect such a small subsurface defect. To overcome these serious problems, we propose the novel inspection technique that can detect such nano-defects in the vicinit of the polished Si wafer surface b appling IR evanescent light emerging on the surface. In this method, defects are evaluated not b using scattered light like conventional method, but b using evanescent light based on near-field optics. Therefore the resolution is independent of light wavelength. nd it is possible to detect the nanometre scale defects. This method has a potential to sensitivel detect subsurface defects as well as surface defects [2]. 2. PRINCIPLE Fig. 1 shows the concept of our proposed nano-defects inspection method. s shown in Fig. 1, IR laser light propagates in Si wafer that is transparent at IR wavelength. In the wafer, the laser light is reflected perfectl at the Si-air interface when the incident angle eceeds the critical angle. nd the IR evanescent wave is generated onto the near field region over wafer surface where the total-internal reflection (TIR) occurs. This IR evanescent field is disturbed b the interaction between the light propagating in the wafer and nano-defects eisting in the vicinit of the wafer surface. The scanning probe tip can detect the disturbed evanescent field when the tip is in near field on the wafer surface. In our method, we use a tip of l coated tapered optical fibre as a probe. nd the resolution isn t dependent on the light wavelength but on an aperture sie of tip of the probe. Therefore this method makes it possible to sensitivel detect the nano-defects, even if the eist in the subsurface [3]. Detection of intensit variation Detection Evanescent field Scanning Probe Defects inspection Surface defects (Micro-scratch, COP, Particulate contamination, etc.) Infrared laser Si wafer Subsurface defects (Void defect, etc.) Fig. 1. Concept of defects detection b using evanescent wave

2 Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia 4 3. FDTD SIMULTION In order to verif the feasibilit of this proposed method, we have developed a computer simulation tool that can analse the electromagnetic field in micro optical structure b solving Mawell s equation directl. The tool is based on the 3-dimensional FDTD (Finite-Difference Time-Domain) method [4] that has proven to be one of the most prominent tools for numerical analsis of electromagnetic field. nd we analsed the electromagnetic field of near field in the vicinit on the wafer surface using our developed FDTD simulator [] Computer model for simulation To confirm the validit of our concept, we modelled the scanning fibre probe tip that closes with the Si wafer (N=3.+.67i, N: comple refractive inde) surface as shown in Fig. 1. The diagram of this simulation model is depicted in Fig. 2. The unit grid length is 1nm in the unit simulation domain. The light source is located at the bottom of Si wafer (plane of =). nd the light source emits the incident light that is p-polaried plane wave at 164nm wavelength and the incident angle at to be TIR condition. The fibre probe core is made of SiO 2 (N=1.+.i) that is assumed to propagate the picked up light to detector. nd its tip is formed cone shape with the semi-vertical angle of 4. The probe tip is approached toward the surface with a gap of 1nm, and coated with aluminium (N= i) of 1nm thick. To pick up the variation of evanescent signal, the tip has an aperture that is 3nm in diameter. ir Si SiO 2 l coat beam (λ=164nm) 1.µm.7µm 3µm 2.96µm Fig. 2. Diagram of computer simulation model 3.2. Numerical results Fig. 3 shows the computational results for the case of eisting a surface defect. Here the pit like defect eisting on the surface is assumed as COP on the surface, and the sie has nm and the shape is concaved 3nm in depth. nd the surface defect is allocated under the probe tip. Fig. 3 is a gre-scale representation of time-average electric field intensit distribution, which was obtained b subtracting the reference intensit distribution of the model without a defect in order to etract the disturbance of the defect. It can be seen that the aperture of the fibre probe picks up the disturbance of evanescent wave resulting from the surface defect. This result suggests that our proposed method enables the detection for a defect with tens of nanometre sie. 4 3nm =1.µm =1.µm Fig. 3. Simulation result for the case of surface defect nm COP nm 1nm Subsurface defect nm nm nm Fig. 4. Simulation result for the case of internal defect Fig. 4 shows the results for the case of subsurface defects. The void defect eisting in the subsurface is modelled as a subsurface octahedral defect (the sie has nm; 1nm in depth). nd the subsurface defect is assumed to be vacuum (N=1.+.i) and also located under the probe tip. This figure shows that the distribution of scattering is different from that of surface defect. nd the aperture of the probe tip picks up the disturbance of evanescent wave resulting from the subsurface defect. This result suggests that even if the defect eists in the subsurface at the depth of 1nm, our proposed method enables the high sensitive detection for a defect with the sie of 1nm order. Position 1.µm of defect =1.µm =1.µm =1.µm Position 1.µm of defect =1.µm

3 Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia 4 4. EXPERIMENTS 4.1. Evanescent light measurement sstem In order to verif the feasibilit of our proposed method eperimentall, we also developed the measurement sstem to detect evanescent light and performed the several fundamental eperiments. Fig. shows a schematic diagram of the eperimental apparatus that is based on our proposed method. p-polaried beam of Nd: YG laser (λ=164nm; 1W) is used to generate the evanescent field on the surface of sample Si wafer that is mounted on the trapeiform prism. nd the incident laser beam illuminates the sample surface from its flip side with TIR condition b this prism. The prism is mounted on the -stage that enables probe scanning. To detect the intensit variation of the evanescent field, the fibre probe scans the near field of sample wafer surface. The probe tip, which altitude is controlled b - pieoelectric translator, is coated with metal to effectivel enhance the detected light with plasmon effect. The InGas photo diode connects with an optical fibre and detects the transmitted light from the fibre probe tip. To remove optical noise, the signal light can be lock-in detected b using light chopper and lock-in amplifier. 3nm 3.7µm 41nm (a) Section profile (b) ird view Fig. 8. FM image of micro-scratch like defect Micro-scratch Computer Moniter NC/RC IEEE 488 Lock in Optical fibre amplifier InGas photo diode Probe Nb:YG (λ=164nm) Silicon wafer Z-pieoelectric translator Microscope Trapeiform prism λ/2 plate Light chopper Fig.. Diagram of eperimental apparatus NC 4.2. Detection of surface defects t first, to confirm the capabilit of the above apparatus, we performed the eperiment to detect surface defects. In this fundamental eperiment, 2 tpes of surface defects that were fabricated on Si wafer using FI (Focused Ion eam) are used. One is assumed as COP, other one is assumed as micro-scratch. nd the FM images of these defects are shown in Fig. 7 and 8. Fig. 9. SIM image of the surface defects Scanning area Micro-scratch 1µm Scan line (4 lines) nm 1µm 2µm 31nm 36nm 4nm (a) Section profile (b) ird view Fig. 7. FM image of pit like defect Fig. 1. Scan area and defects location Fig. 7 shows the defect with 31nm 36nm in area and 4nm in depth, which imitates pit. nd Fig. 8 shows the defect with 3.7µm in length, 3nm in width and 41nm in depth, which imitates micro-scratch. Fig. 9 is the SIM (Scanning Ion Microscop) image of these surface defects. nd Fig. 1 indicates the diagram of the eperimental procedure that shows location of defects and scanning area of fibre probe. s shown in these figures, these two defects are separated for 1µm. nd to detect them, the fibre probe has scanned for 1µm 2µm in area. The scanning of

4 Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia 4 Output a.u. parrent effect b the surface defects Fig. 11. Eperimental result of surface defects detection Displacement µm Displacement µm 4 lines in -direction is performed at intervals of nm. The samplings are eecuted 3 times at intervals of nm in each scan line. Fig. 11 indicates the light intensit variations detected from the surface defects b the scanning fibre probe. This result shows these variations have an apparent peak (the arrows in Fig.11) corresponding to the position of both pit and micro-scratch. Characteristics of measured light intensit pattern reflect the orientation of the scratch. This result suggests that our proposed method enables inspection of surface defects with 1nm sie Detection of internal defect Second, to confirm the validit of our proposed method that enables to detect the internal defects, we performed the eperiment to detect the internal defects. In this eperiment, a defect was also fabricated using FI in the vicinit of Si wafer surface from its side face. The schematic diagram of this internal defect is shown in Fig. 12. The tunnel like internal defect was etched at about 3 nm in depth. The shape of internal defect is tapered. This results from FI process propert that enabled to estimate the length of the tunnel at ~6µm. The SIM image of the scanning area is shown in Fig. 13. nd Fig. 14 shows the diagram of eperimental procedure that shows the emplacement of internal defect and scanning area of fibre probe. s shown in these figures, there are 4 pit defects, which are same things in Fig. 7, as marker to indicate the emplacement of the internal defect. nd these pit defects are allocated at intervals of 1µm in -direction and 1µm in -direction to enclose the internal defect. nm s (markers) Fig. 13. SIM image of scan area Scanning area Scan line (3 lines) 1µm Si wafer 2µm Fig. 14. Scan area and internal defect emplacement To detect the internal defect with markers, the fibre probe has scanned for 2µm 1µm in area. The scanning of 3 lines in -direction is performed at intervals of nm. The samplings are eecuted 4 times at intervals of nm in each scan line. nd then, Fig. 1 shows the eperimental result that shows the light intensit variations detected from the internal defects and the markers defects b the scanning fibre probe. parrent effect b the defects 1µm 1µm 1µm.µm 4µm.3µm ~6µm 4µm Fig. 12. Diagram of internal defect Output a.u Displacement µm Displacement µm Fig. 1. Eperimental result of internal defect detection 2

5 Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia Proceedings, XVII IMEKO World Congress, June 22 27, 23, Dubrovnik, Croatia 4 s shown in Fig. 1, these variations have an apparent peak (the arrow in Fig.1) corresponding to the position of markers and the internal defect. 2nm in diameter and 3nm in depth Fig. 16. SIM image of internal defect (opened b FI machining) In Fig. 16, the SIM image of internal defect shows the diameter of the tunnel tapers off from about.µm to about 2nm. nd the depth of the defect is about 3nm. This eperimental fact suggests that our proposed wafer inspection technique enables to detect of internal defects with 1nm sie.. CONCLUSION.µm We proposed a novel Si wafer surface inspection technique for nano-defects in the vicinit of Si wafer surface, not b using scattered light but b using evanescent light based on near-field optics. In order to confirm the validit of our proposed method, we developed a computer simulation tool that was based on the 3-dimensional FDTD method and the measurement sstem to detect evanescent light. The results of FDTD simulation showed our proposed method enables detection the defects with 1nm sie not onl on the Si wafer surface, but also in the subsurface. Several fundamental eperiments are carried out to validate the capabilit of this developed sstem. nd also the eperimental results show that the detected light intensit variations indicate the defects position and configuration. Furthermore, the validit of this method is verified b demonstrating the detection of internal defect with 1nm sie. REFERENCES [1] T. Ruta, E. Morita, T. Tanaka, Y. Shimanuki, "Crstal- Originated Singularities on Si Wafer after SC1 Cleaning", J. ppl. Phs., vol. 29, No. 11, pp , 199. [2] S. Takahashi, T. Mioshi, Y. Takaa, R. Nakajima, "Nano- Defects Detection of Si Wafer Surface Using Evanescent Light Computer Simulation b Means of FDTD Method", Proceeding of the 2nd euspen Conf., vol. 1, pp. 12-1, 21. [3] R. Nakajima, T. Mioshi, Y. Takaa, S. Takahashi, M. Fujita "Novel Measurement Method for Nano-Defects of Si Wafer Surface Using Evanescent Wave", Proceeding of the 3rd euspen Conf., vol. 2, pp , 22. [4] K. S. Yee, "Numerical Solution of Initial oundar Value Problems Involving Mawell s Equations in Isotropic Media", IEEE Trans. ntennas Propagat., vol. 14, no. 4, pp , []. P. Zhao,. V. Räisänen, S. R. Cvetkovic, " Fast and Efficient FDTD lgorithm for the nalsis of Planar Microstrip Discontinuities b Using a Simple Source Ecitation Scheme", IEEE Microwave and Guided Wave Lett., vol., no. 1, pp , 199. uthors: Graduate student, R. Nakajima, Dept. of Mechanical Engineering and Sstems; Osaka Universit, 2-1, Yamadaoka, 6-871, Suita, Osaka, Japan, Tel: , Fa: , nakajima@optim.mech.eng.osaka-u.ac.jp Professor, T. Mioshi, Dept. of Mechanical Engineering and Sstems; Osaka Universit, 2-1, Yamadaoka, 6-871, Suita, Osaka, Japan, Tel: , Fa: , E- mail: mioshi@mech.eng.osaka-u.ac.jp ssociate professor, Y. Takaa, Dept. of Mechanical Engineering and Sstems; Osaka Universit, 2-1, Yamadaoka, 6-871, Suita, Osaka, Japan, Tel: , Fa: , E- mail: takaa@mech.eng.osaka-u.ac.jp ssociate professor, S. Takahashi, Dept. of Precision Engineering; The Universit of Toko, 7-3-1, Hongo, , unko-ku, Toko, Japan, Tel: , Fa: , E- mail: takahashi@nano.pe.u-toko.ac.jp