Overlay accuracy calibration

Transcription

1 Overlay accuracy calibratin Eran Amit a*, Dana Klein a, Guy Chen a, Nuriel Amir a, Michael Har-Zvi a, Cindy Kat b, Hiryuki Kurita b a KLA-Tencr Crpratin, 1 Halavyan Street, Migdal Ha'Emek 23100, Israel b KLA-Tencr Japan Ltd., YBP East Twer, 134 Gd-ch, Hdgaya-ku, Ykhama, Kanagawa , Japan ABSTRACT In rder t fulfill the ever tightening requirements f advanced nde verlay budgets, verlay metrlgy is becming mre and mre sensitive t even the smallest imperfectins in the metrlgy target. Under certain circumstances, inaccuracy due t such target imperfectins can becme the dminant cntributin t the metrlgy uncertainty and cannt be quantified by the standard TMU cntributrs. In this paper we describe a calibratin methd that makes the verlay measurement rbust t target imperfectins withut diminishing its sensitivity t the target verlay. The basic assumptin f the methd is that verlay measurement result can be apprximated as the sum f tw terms: the accurate verlay and the measurement inaccuracy (independently f the cnventinal cntributrs). While the first term (the real verlay ) is rbust it is knwn that the verlay target inaccuracy depends n the measurement cnditins. This dependence n measurement cnditins is used t estimate quantitative inaccuracy by means f the verlay quality merit which was described in previus publicatins. This paper includes the theretical basis f the methd as well as experimental validatin. Keywrds: Overlay, Accuracy, TMU, Imaging 1. Intrductin Even thugh tremendus effrts are made t ptimize the chip fabricatin prcess the printed features are nt identical t thse pltted n the cmputer. The definitin whether r nt these variatins are negligible depends n the feature applicatin. In metrlgy the parameter that is directly affected by the target shape divergence is the measurement accuracy; it is defined as the amplified respnse f the metrlgy t the target shape imperfectins. Different metrlgies may respnd differently t the same target imperfectins; theretical analysis fr different verlay (OVL) techniques can be fund in Ref. 1. Cnventinal OVL errrs are well quantified using the Ttal Measurement Uncertainty (TMU). This includes the measurement precisin, Tl Induced Shift (TIS) variability and tl matching. In the ITRS anther term is mentin which includes additinal errrs such as inaccuracy 2. This term is nt quantified; furthermre, it is recmmended t use nly qualitative descriptin fr sme f its cntributrs 3. In the past the measurement inaccuracy cntributin culd be neglected r attributed t the prcess variatins. In the advanced ndes the inaccuracy term is n lnger negligible and cannt be ignred anymre. Because f its systematic nature it cannt be averaged ut using mre measurements and therefre if it is nt estimated and treated prperly it risks the manufactures yield. In rder t estimate it fabs use cmparisn f the OVL measurements t reference metrlgies such as TEM r electrical tests. The OVL mdel residuals and after-develp after-etch crrelatins can als be used under sme cnditins. In the past several years many attempts were made t estimate and crrect the measurement inaccuracy n-the-fly (sme examples can be fund in Ref. 4-6). Metrlgy, Inspectin, and Prcess Cntrl fr Micrlithgraphy XXVII, edited by Alexander Starikv, Jasn P. Cain, Prc. f SPIE Vl. 8681, 86811G 2013 SPIE CCC cde: X/13/$18 di: / Prc. f SPIE Vl G-1

2 KLA-Tencr addressed this challenge by intrducing a quality merit named Qmerit that identifies these measurement inaccuracies in imaging based verlay 7. The Qmerit is calculated n-the-fly withut measurement time penalty. Sme f its applicatins are utlier remval 7, setup ptimizatin and additinal prcess cntrl behind OVL 8. In this paper we take the accuracy treatment apprached t the next level and reprt calibratin that allws the Archer tl t reprt accurate OVL values even if the targets suffer frm sme imperfectins. 2. Archer Self Calibratin (ASC) thery When OVL target is measured the reprted values have tw cntributins: The accurate OVL and the measurement inaccuracy (the cnventinal TMU cntributrs are neglected). It can be described as: Eq.1: OVL measured =OVL accurate +Inaccuracy The accurate term is the term is the real OVL ; it is cmmn t all measurement techniques. The inaccuracy term depends n the target imperfectins cupling t the specific measurement setups. Specifically, it depends n the clr filter which is used t measure the targets. It was shwn bth in experiments and in simulatins that different illuminatins wavelength may respnd differently t target imperfectins 7. The inaccuracy term can be described as a functin f the quality merit mentined abve which gives the fllwing equatin: Eq.2: OVL measured =OVL accurate +f(qmerit). If we use different measurement setups t measure the same target we may get different OVL and Qmerit results, but the accurate OVL shuld be rbust t the measurement cnditins. The Qmerit functin describes the metrlgy respnse t the way the prcess affects the targets. It is cmmn t all sites within the wafer; mrever, it des nt change between wafers (unlike the Qmerit which is calculated each time an OVL measurement is perfrmed). As a cnsequence, nce the calibratin is fund it can be applied t all the fllwing wafers and lts. The calibrated OVL frmula is: Eq.3: OVL calib =OVL measured - f(qmerit). The accurate OVL rbustness prperty can be used t find the Qmerit functin that describes the target inaccuracy by measuring a wafer with different clr filters and ptimizing the OVL calib matching. It is imprtant t mentin that the calibrated OVL is nt a simple average f the different clr filter OVL values; it may be even utside f the clr filter OVL range. Once the Archer is calibrated the OVL values becme mre accurate which results in better crrelatin t OVL references, residuals, etch bias and all ther accuracy merits. 3. Experimental results Figure 1 shws discrepancy between the OVL mdels btained using tw different OVL target designs n the same wafer. It can be seen that the main deviance is in the field X magnificatin term which is big in bth targets and deviates by abut 70% between the targets. Prc. f SPIE Vl G-2

3 .inear field mdel terrris (white fil ) DO / Tr< f... c. r..' 1_t I i-j1.- Target B Target A AM Target B -. Figure 1: Field mdels term difference between tw OVL targets. There is a big difference f abut 70% in the X magnificatin term. In rder t understand which target design reprts the crrect OVL values Qmerit analysis was applied. Figure 2 shws the mean Target B Qmerit value fr each field lcatin acrss the wafer. It shws that Target B is damaged and suffers frm inaccuracy (big Qmerit values); the strng field signature (left lcatin vs. central and right lcatins) shws the symmetry f the damage rt cause. In additin the Qmerit allws us t pint ut which layer is prblematic; in this case it is the resist (current layer). This infrmatin allws better target selectin and in additin alerts that there is sme prblem with this symmetry in the prcess (we intentinally chse extreme case fr the demnstratin and nt prductin case in which inaccuracy mayy exists but in smaller magnitude). Mre detailed analysis can be fund elsewhere 8. Figure 2: The average Qmerit values acrss the wafer per lcatin The current layer Qmerit (blue) identifies strng field signature which affects mainly in X directin. This signature des nt exist in the previus layer (pink). Since Qmerit finds the inaccuracy the ASC can be applied t vercme it. Measurements in three different clr filters were used t find the Qmerit functin dminating targett B measurement inaccuracy. The results f the calibratin are presented in Fig. 3 and table 1. After the calibratin the matching between the different clrs significantly imprved. Prc. f SPIE Vl G-3

4 Target B ()VL X Yell w vs. WI lite ) ) L 1 mm y}ġ..1n g mr \ 1 l J\... N krhite (arb. units) Figure 3: Clr matching befre and after the calibratin The OVL values measured in Yellw filter are pltted as a functin f the White filter OVL. The dark and bright blue represents the un-calibrated and calibrated values, respectively. Theree is significant clr matching imprvement. Target B arb, unit s Ivry IVC > ry wi- iite 0.12 Yel lw 1.78 Iv( wi' >ry rite 0.05 YeI lw 0.32 Max md( d differeft OVL ASC White Yell E C ::. w Ivry i 0.05 ce White Yell w ( )7 0 0.( ) C ( )5 0 0.( ) C Table 1: Calibratin effect n clr matching The maximumm difference between the OVL mdel terms f different clrs are pltted fr X and Y (tp and bttm blcks, respectively) and befre and after the calibratin (left and right blcks, respectively). The majr deviatin and abut a factrr f 20 matching imprvement was bserved in the X Yellw matching t the ther clrs. Ivryy t white X matching was imprved by a factr f tw. Prc. f SPIE Vl G-4

5 Next, the calibrated OVL values f target B were cmparedd t target A. It is imprtant t mentin thatt the calibratin was dne n target B, independent f target A (which has small Qmeritt values and therefre was nt calibrated). Table 2 and Fig. 4 shw the huge target type matching imprvement after the calibratin. Unlike the raw OVL data, the calibrated OVL data f bth targets are cnsistent. Max mdel di ark X Y ffe re n ce -I OVL 'Sc ). units F A I3 A A R A B S I 0.2 cj between n targets Table 2: Target type matching f raw and calibrated OVL mdels After calibrating target B (withut using any infrmatin frm target A) the matching was imprved. The maximum difference between the OVLL mdel terms f different targets are pltted fr X and Y (tp and bttm blcks, respectively) and befre and after the calibratin (left and right blcks, respectively). There is a factr f 7 matching imprvement in the X matching after calibratin. B rn n U.L 0.1 r ield Magnificatin X t :erm calibri I.> 1 7 L.8 r Tarp at A O \ 'L ïsc!7 'target H Figure 4: X magnificatin term target type matching befre and after the calibratin The mdel based n the un-calibrated OVL shws abut 70% difference between the target types X magnificatin term. After calibrating targett B the magnificatin term f bth targets is cnsistent. The gd target type matching f the calibrated OVL values indicates that after the calibratin Target B can be used t reprt accurate OVL values. Ideally the target design and\r the prcesss shuld be imprved, but even withut any mdificatin the risk f yield lss due t wrng OVL reprts is significantly reduced. Prc. f SPIE Vl G-5

6 The Archer Self Calibratin uses the accurate OVL rbustness t study hw the prcess affects the targets and uses this infrmatin t make the OVL algrithm immune t the studied prcess effects. Once the tl is calibrated it reprts mre accurate OVL values withut affecting the measurement time. This algrithmic slutin cmbined with targets that are mre prcess cmpatible enable OVL reprts which are accurate enugh t meet the next ndes requirements. We wuld like t acknwledge Akihir Tbika, Haruki Suma and Hisashi Aidafrm frm SanDisk, and Ysuke Okamt and Kazutaka Ishig frm Tshiba fr prviding the experimental data. References: 4. Summary [1] Kandel, D., et al., "OVL accuracy fundamentals, Prc. SPIE8324, (2012). [2] ITRS 2011 Editin. [3] Taylr, et al., NIST technical nte 1297, 1994 Editin. [4] Slecky, E., Archie C. and Banke B., "New Cmprehensive Metrics and Methdlgy fr Metrlgy Tl Fleet Matching, Prc. SPIE 5752, (2005). [5] Ham, B., et al., New Analytical Algrithm fr Overlay Accuracy, Prc. SPIE 8324, 83240A1. (2012). [6] Chen, Y-L, et al., Quality Indicatrs f Image-Based Overlay Prc. SPIE 8324, 83241C-1 (2012). [7] Chen, G., et. al., "OVL Quality Metric, Prc. SPIE 8324, (2012). [8] Klein, D., et. al., Quality metric fr accurate OVL cntrl in <20nm ndes, Prc. SPIE (2013). Prc. f SPIE Vl G-6