SUPER OBLIQUE INCIDENCE INTERFEROMETER USING SWS PRISM

Transcription

1 SUPER OBLIQUE INCIDENCE INTERFEROMETER USING SWS PRISM Yukitosi OTANI 1,2), Yasuiro MIZUTANI 2), Noriiro UMEDA 2) 1) Optical Sciences Center, University of Arizona, Arizona ) Dept, of Mec. Sys. Engg., Tokyo University of Agriculture and Tecnology, Koganei, Tokyo, JAPAN Abstract A super oblique incidence interferometer is proposed by using an anti-reflection prism wit sub-wavelengt structure (SWS). Its sensitivity is lower tan an ordinary interferometer wit ig contrast. A fringe interval of its interferometer depends on te incident angle. Te SWS prism as a feature of increasing even if near te critical angle. Te rigorous coupled wave analysis (RCWA) is used to analyze te anti-reflection effects of SWS. An experimental result of of SWS prism agrees well wit te calculated result. Hig contrast fringe is acieved at 86º of te incident angle. A flatness measurement of a silicon wafer wit circuit patterns is succeeded by tis interferometer. Keywords anti-reflection prism oblique incidence interferometer, flatness measurement, SWS(sub-wavelengt structure), 1 Introduction A surface profile measurement of silicon wafer is still important in te field of te semiconductor industry. A size of wafer is larger tan 3 mm of te diameter and its tickness is tinner and tinner. Moreover, a fabrication process is more complicate for multilayer interconnection of te circuit patterns. Terefore, te flatness and/or waviness in tis large area become bigger and bigger. Meanwile, te requirement of te flatness in te micro area is more accurate using a process of te cemical mecanical polising. Tere are many reports to overcome contrariety requirements 1). Te fringe interval of a common interferometer wit vertical incidence is alf of wavelengt. If te flatness is muc waviness, we can not detect te surface profile because of 2π uncertainty and/or overlapped fringes. In general, an interferometer, especially a wite ligt interferometer, is powerful tool for tis purpose. An oblique interferometer is also possible to reduce te sensitivity of te fringe 2,3). Abramson proposed an oblique incidence interferometer by using a rigt-angled prism 2) but tere is a limitation of incident angle to increase te fringe interval because of a critical angle. To overcome tis problem, we propose a super oblique incidence interferometer using a sub-wavelengt structure (SWS) prism wit te anti-reflection effect. Recently, many papers related a SWS for anti-reflection effect ave been proposed 4-7). Te period of widt and eigt of SWS is almost equal to te wavelengt of te incident ligt. An anti-reflection effect of mot eye structure was first discovered by Bernard 4). A vector model for

2 SWS was first proposed by Moaram called te rigorous coupled wave analysis (RCWA) 5). It is powerful tool for accurate analysis of te diffraction of electromagnetic waves by periodic structure. We ave proposed a model error resulted at te numerical model divided SWS into several sections of model by te RCWA 7). Most of te anti-reflection studies ave been for vertical angle of te incident ligt. We study optical beaviors at SWS for oblique incidence. Experimental results are compared accurately wit simulation results of simulation model by using AFM. 2. Super oblique incidence interferometer by SWS prism 2.1 Experimental setup Figure 1 is an optical configuration for a super oblique incidence interferometer using SWS prism. Tis interferometer is based on Mac-Zender type wit two pair of SWS prism. A beam expander (L1, L2) and two alf mirror (HM1, HM2) are used for collimate and tilt of te ligt. Te incident beam is divided into a reference and a object beam by HM1 and HM2. One of te prisms is faced to te sample as an object beam and te oter is used as a reference to te M2 mirror. A ligt source is possible to use bot a wite ligt and a monocromatic ligt. Its sensitivity can be adjusted by canging te incident angle θ to te sample. Te eigt of a sample (x,y) is sown ( x, y) = φ, ( 1) ( x, y) 4 cosθ were is te wavelengt and φ(x,y) is te pase difference between te reference and te object beam. Te intensity I(x,y) of te interference fringe captured by te CCD camera is expressed as, ( xy ) I = a + b + 2abcos φ + δ. ( 2) 2 2 ( xy, ) (, ) L1 L2 M1 SWS prism ligt source HM1 SWS prism φ M2 ref. M3 HM2 object sample Fig.1 Super oblique incidence interferometer L3 CCD Here, a and b are te bias of te reference and te object beam. δ is te pase sift. Four step pase-sifting tecnique is employed to analyze tis fringe. In case of a wite ligt interferometer, we use te reference SWS prism to compensate wavelengt dispersion. However, we can remove it wen a monocromatic ligt source is utilized. fringe 2.2 Fabrication of SWS prism A SWS prism wit anti-reflection effect is illustrated in Fig.2. Figure 2 (a) is a detailed structure of SWS. Te requirement of tis structure is to fabricate a cycle of widt and eigt equal to te wavelengt. An oblique

3 4 Oblique incidence n 1 z x 2 µm eigt [nm] n 2 w W, (a) Structure of SWS and effective refractive index n 1 2 n index (b) 3D view by AFM x [nm] (C) Cross section and simulation model for RCWA Fig.2 Sub-wavelengt structure of anti reflection effect for oblique-incidence TE (SWS) TM (SWS) TE (flat) TM (flat) output angle [deg] Fig.3 RCWA simulated results of incidence wave transmitted from region I (refractive index: n 1 ) to region II (refractive index: n 2 ). Te refractive indices between two mediums along optical axis are canging slowly. Effective refractive index is obtained on te average of refractive indices of z axis in Figure 2(a). Tis means te Fresnel reflection at a boundary is also reduced. Figure 2(b) sows a surface profile of te manufactured SWS measured by a critical dimension (CD)-AFM. Its cross-section is sown in Fig.2(c). As tis result, a eigt of SWS is 35 nm, a widt is l97 nm and a period is 433 nm. (c) External result of te Fig.4 SWS prism A of SWS for te oblique incidence as been simulated by using RCWA. Te surface profile for tis simulation is measured by CD-AFM. Te equality orizontal lines in Fig.2(c) mean te simulation (a) Picture of SWS prism (BK7 n=1.52) 55.3mm anti-reflection (b) Detail of SWS prism He-Ne(633nm) 136mm 39.6 wit anti-reflection witout anti-reflection incident angle [deg] 41mm

4 row 1 14 row col 12 1 col unwrap _86_unwrap2 Table 1 Experimental result of surface profile measurement Incident angle[º] interferogram pase map surface profile ( =1.2µm) [mm] 1 [!m] [mm] 24 [!m] 86 ( =4.5µm) [mm] [mm] 5mm y [mm] 2. Measurement area x [mm] eigt [µm] (b) surface profile Fig.5 Surface profile of silicon wafer wit circuit pattern model for te SWS and te number of layers is 5 7). Figure 3 sows te along output angle compared te SWS prism wit te flat surface by RCWA. A wavelengt of incident wave is nm and polarizations of te incident wave are p and s-polarization. Te reflectance of TE mode is lower tan flat surface at all incident angles. Figure 4 slows an experimental result of te anti-reflection effect for te oblique incidence of SWS prism. Te SWS prism is made of BK7 glass and polymer film of te sub-wavelengt structure sown in Fig.4(a). Te

5 size of te prism is illustrated in Fig.4(b). Te two parts of SWS, as sown circled area, glued on te bottom of a prism SWS. Figure 4(c) is te experimental result. Te by SWS prism is muc iger tan by witout SWS. Te of 41º is succeeded to increase more tan 5%. 3. Experimental results of surface profile measurement We measured a piece of silicon wafer by te super oblique incidence interferometer sown in Fig.1. In tis experiment, we utilized a He-Ne laser at 632.8nm as a ligt source. A reference minor (M2) is moved for pase-sifting by a piezoelectric actuator (PZT, NEC-TOKIN). Table 1 sows two difference interferogram, pase map and surface profile given by different incident angles. Te silicon wafer is taped down on te glass substrate. An oblique incident angles are and 86º. A fringe space of 86º is wider tan tat of º. We can measure a smoot surface profile suc as a silicon wafer by tis metod. Te waviness of tis silicon is 47µm. Figure 5 sows te measured result of a silicon wafer wit multilayer interconnection of circuit patterns as Low-k + Cu -DD (Duel Damascene) wit CMP (Cemical Mecanical Polising). Its measurement area is 3x2 mm. We can succeed to measure wafer surface even if te large steps and waviness. 4. Conclusion We ave proposed a super oblique incidence interferometer by SWS prism. We succeeded to increase an efficient at te oblique incidence by te SWS prism under te condition of te vicinity or critical angle. A sape of te structure is a trapezoid wit eigt and widt of sub-wavelengt order periodically. Te experimental results of agree well wit te calculated results by RCWA. Te incident angle of te oblique interferometer could be acieved 86º. Te measurement of a silicon wafer wit multilayer interconnection of circuit patterns was succeeded to measure by tis interferometer. Te autors tank to Japan Veeco for elping te AFM measurement. References 1. D.Malacara : Optical sop testing (Wiley, 1992). 2. N.Abramson: Optik, 3, 56 (1967). 3. Y.Otani, N.Okuara, T. Yosizawa : Opt. Eng (1998). 4. C.G.Bermard : Endeavour, 26,79 (1967.) 5. M.G..Moaram, T.K.Gaylord : Applied Optics, 2, 24 (1981). 6. Y.Kanaori, M.Sasaki, K.Hane: Opt.Lett., 24, 2, 1422 (1999). 7. Y.Mizutani, K.Minato, Y.Otani, N.Umeda: ICO '4, 54l, (24).