Magazine summer03 thinkingoutsidethebox

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Thinking Outside the Box for Improved Overlay Metrology Ivan Pollentier, Philippe Leray, and David Laidler, IMEC Mike Adel, Mark Ghinovker, Jorge Poplawski, Elyakim Kassel, and Pavel Izikson, KLA-Tencor Corporation

Overlay mark fidelity (OMF), as defined in this article, is a source of overlay metrology uncertainty, which is statistically independent of the standard error contributors, i.e. precision, TIS variability, and tool-to-tool matching. Current overlay metrology budgeting practices do not take this into consideration when calculating total measurement uncertainty (TMU).

It is proposed that this be reconsidered, given the tightness of overlay and overlay metrology budgets at the 70 nm design rule node and below.

Introduction

In order to meet ever-shrinking lithographic overlay control budgets, overlay metrology uncertainty must be quantified and minimized. One important contributor to this uncertainty is the effect of the patterning process on the overlay mark, and the stability of this effect over time and as process parameters are changed — both intentionally and unintentionally. This has been studied extensively and many modifications to the box-in-box configuration have been proposed in order to address these issues.1, 2, 3 Similar efforts have also yielded results on the improvement of alignment marks. 4 We propose a metric and methodology by which the impact of the process on the metrology uncertainty may be quantified.

Such a metric is significant in that it is necessary to include it when estimating the total measurement uncertainty as part of the overall overlay budget for advanced processes, where every nanometer counts.

The new metric, termed “Overlay Mark Fidelity” can be determined by a method which allows it to be considered statistically independent of the standard uncertainty contributors of precision, tool induced shift (TIS), tool induced shift variation (TIS 3 sigma) and tool-to-tool matching.

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The impact of mask errors on critical dimension metrology has been investigated and reported.5 In the process of this study it became evident that, by appropriate statistical analysis, the contribution of the reticle to overlay metrology uncertainty can also be quantified to within a small fraction of a nanometer. To our knowledge this has not been previously characterized or reported. As the relentless drive towards tighter and tighter overlay budgets progresses, we believe that the process and reticle contributors of this metric will become an integral part of the overlay metrology budget.

Definitions and methodology

OMF may be defined as three times the standard deviation of N overlay measurement results from an array of N nominally identical marks printed in close proximity. By design, it is expected that these N measurements will produce identical overlay results.

However, in reality, due to process and metrology effects, the result is a distribution of overlay readings.

In this context, “process effects” refer to the sequence of steps starting with reticle manufacture and ending in a topographically complex structure on the wafer in which the overlay mark contains information from two different process layers. The distribution, quantified

by OMF, is an important component of the overlay metrology error, which is independent of the traditional metrology uncertainty contributors, i.e. precision, TIS and TIS variability. The OMF is computed using the overlay results from the array, after compensating for

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the precision contribution. TIS does not contribute to the OMF calculation as TIS variability within the array is negligibly small. This condition was experimentally validated: TIS 3 sigma within the array was measured to be less than 0.2 nm for both box-in-box and grating targets.

Specifically, let i, j, k be the indices for N,F, and W adjacent targets, fields, and wafers, respectively. Let OVL_Xijk and OVL_Yijk be the overlay in the X and Y directions respectively for target i, in field j and on wafer k.

The OMF of the field j on wafer k is defined as:

(1)

We will assume that of the W wafers in the lot, all were processed with the same reticle set, but with arbitrary process parameters. For a given wafer k, the pooled OMF across all F fields is estimated by: (2) As will be seen in the data below, the dynamic precision S (measured in L=5 dynamic loops) of an individual measurement is typically five-fold smaller than the OMF, so that this correction is small. We have retained this small correction for the sake of mathematical rigor. The next step in the analysis is to extract the component of this statistical estimator (which is constant for all arrays on all fields and all wafers) from the component that varies from array to array. The array independent component is attributed to the mask error, defined as: (3) Where the mean target overlay is calculated as follows: (4) The reticle overlay mark fidelity can them be estimated by the statistic:

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(5)

Finally, we estimate the process contribution by calculating the “random� component on an array-by-array basis, after correcting for the mask error. Then corrected overlay is defined as: (6) Therefore, the random or process OMF contribution is estimated in the same way as field OMF, where OVL_X is replaced with OVL_X_Corrected in Equation 1. In this way it is possible to separate the process contribution from the reticle contribution of the overlay mark fidelity. As a final check, we calculate the reticle contribution on a wafer-by-wafer basis; that is, we replace ME i with MEik and sum only over fields.

This parameter should yield identical results on all wafers to within reasonable statistical limits.

Wafer and reticle processing

A test reticle set with OMF arrays of both standard box-in-box and grating marks was manufactured on an ALTA3500 laser-writing tool. Using the above test reticle set, 14 wafers were processed in two different short loops. In the first short loop, the first patterning step was an active layer, followed by STI processing and an oxide CMP step. This was followed by a gate oxide process and poly-silicon deposition. The second patterning step was on this poly layer. Some wafers were measured before etch (DI) and some were etched and then measured (FI).

The second short loop was based on the same sequence of patterning with a simplified process. Silicon was etched with the active pattern, followed by a layer of photoresist that was spun over etched silicon and patterned with the poly reticle (see Table 1).

Results

Extensive measurements were made on box-in-box and new grating type targets in the four corners of the scanner field. Table 2 displays examples of typical boxin-box and grating style overlay targets, including the layout of the OMF arrays. Initially, the dynamic precision for five dynamic loops was measured for box-inSummer 2003

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equation 2) for box-in-box and grating targets. Figure 3 displays reticle OMF, on a wafer-by-wafer basis (as calculated in equation 5) summing over fields only, for box-in-box and grating targets.

Figure 4 displays the random (process) OMF, on a wafer-bywafer basis, as calculated using equations 1, 2 and 6, for box-inbox and grating targets. Finally, Figure 5 displays reticle OMF at the four corners of the reticle (as calculated in equation 5) for box-in-box and grating targets.

Table 1. Lot process dat a.

Discussion

Tabl e 2. Imag es of t ypical box -in-box (left) and grati ng ma rks (ri ght) studied ; above — single targets, below — t arget a rr a y s .

box and grating targets on all wafers (see Figure 1).

Extensive measurements were then carried out (five dynamic loops) on four corners of all fields for both target types on all wafers. Statistical calculations (as described in the Definitions and Methodology section) were then performed as follows. Figure 2 displays results of total OMF on a wafer-by-wafer basis (as calculated in

Grating targets display a major improvement (60 to 70 percent) in dynamic precision compared to the box-in-box structures on all wafers. Two explanations for this performance improvement have been advanced, citing increased information content and algorithmic advantages due to the nature of the periodic signal.6 Further work is warranted to understand this. Regarding the total OMF, in Figure 2 it is observed that the grating target is superior to box-in-box, with the improvement varying by 20 to 50 percent from wafer-to-wafer. Here also, increased information content is cited to explain the improvement but, as will be discussed below, the improvement is broken down into two components.

The methodology described in the Definitions and Methodology section above allows the reticle contribution to the OMF to be independently estimated on a wafer-by-wafer basis. In Figure 3 it is observed that, within the bounds of statistical uncertainty (±0.4 nm), the reticle OMF is constant at 2.4 nm for the grating marks. For the box-in-box marks, the reticle OMF is constant at 3.3 nm (±0.3 nm). This statistical check is

F i g u re 1. Pooled precisi on on a waf er-by-wa fer ba sis, fo r b ox-in-box

F i g u re 2. Total OMF on a wafer-by -wafer basi s ( as c alculat ed in

and grating tar get s. Note: full scale 1 nm.

equati on 2) for box-in -box and g rating t argets. Not e: full scale 10 nm.

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±0.9 nm on the grating marks and ±1.7 nm on the box-in-box marks. Furthermore, the pooled process OMF from all wafers is 1.6 nm and 2.8 nm for grating and box-in-box targets respectively. These two observations are interpreted as improved process robustness of the grating mark over box-in-box, specifically in terms of reduced site-by-site variations and reduced wafer-towafer variations as process conditions change over time.

Conclusions F i g u re 3. Reticl e OMF check, that is on a wafer by wafer bas is (as calcula ted in equati on 5) summi ng over fields only, for box-i n-box and g rating targ e t s .

We have introduced a new metric for process robustness of overlay metrology in microelectronic manufacturing.

This quantitative metric — termed overlay mark fidelity — estimates process robustness of the overlay mark and the overlay metrology process. A method to calculate OMF has been developed and implemented on a batch of front-end wafers.

We have demonstrated that, not surprisingly, overlay metrology errors exist on the reticle prior to any wafer processing. We have also described a method by which this reticle contribution to mark fidelity can be statistically separated from the process contribution, to within an uncertainty of ±0.2 nm.

F i g u re 4. Ra ndom (p rocess) OMF, on a wafer-by- wafer basis (a s ca lculated using equations 1, 2 and 6) for box-in -box and grating targ e t s .

F i g u re 5. Reticle OMF at the four corners of the r etic le (as calcul ated in equation 5 ) for box-i n-box and grating tar g e t s .

a strong indication that this component is, in fact, wafer independent and can be attributed to the reticle. Based on the complete statistics from all 14 wafers, the reticle component of the OMF can be estimated to within ±0.2 nm for both target types. The process OMF contribution can now be reliably estimated as shown in Figure 4. Statistically significant wafer-to-wafer fluctuations are apparent in this figure, of the order of

Furthermore, grating overlay marks have been compared with box-in-box overlay marks. Both reticle OMF and process-induced OMF show that grating marks perform better than box-in-box marks on all wafers. Specifically, grating marks show improved process robustness over box-in-box, as expressed by reduced site-to-site variations within close proximity and reduced wafer-to-wafer variations as process conditions change over time.

It is important to note that OMF, as defined in this publication, is a source of overlay metrology uncertainty, which is statistically independent of the standard error contributors, i.e. precision, TIS variability, and tool-totool matching. Current metrology budget practices do not take this into consideration when calculating total measurement uncertainty (TMU). It is proposed that this be reconsidered, given the tightness of overlay and overlay metrology budgets at the 65 nm design rule node and below.

Acknowledgements

We are grateful to Carlo Reita and Steve Johnson of Photronics for insightful discussions on the reticle OMF data.

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This work was supported by the “MAGNET� program of the Chief Scientist Office at the Israeli Ministry of Industry and Trade, Consortium of Emerging Dielectric and Conductor Technologies for the Semiconductor Industry. A version of this article originally published in the 2003 SPIE Microlithography proceedings 5038, SPIE Microlithography Conference, February 2003, Santa Clara, California, USA. References 1 . Luci, Alberto; Ballarin, Eugenio G. Optimization of Overlay Markers to Limit measurement error Induced During Exp o s u re by lens Aberration Eff e c t s , M e t ro l o g y, Inspection, and process Control for Microlithography XVI, Proceedings of SPIE Vol. 4690 (2002), p.374.

2 . Schmidt, Sebastian; Charles, Alain B.; Ganz, Dietmar; H o rnig, Steffen R.; Hraschan, Guenther; Maltabes, John G.; Mautz, K arl E.; Met zdor f , Th omas; Otto, Ralf ;

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Scheurich, Jochen; Schedel, Thorsten; Schuster, Ralf., The effect of Non-linear Errors on 300 mm Wafer Overlay Perf o rm a n c e , Optical Microlithography XIII, Proceedings of SPIE Vol. 4000 (2000), p. 857.

3 . Yang, Jin-seok; Lee, Sang B.; Oh, Seung-Chul; Huh, Hoon; Han, Sang-Bum, Novel Design of WIS-free Overlay Measurement Mark, Metrology, Inspection, and process C o n t rol for Microlithography XIV, Proceedings of SPIE Vol. 3998 (2000), p.764.

4 . Hideki Ina et. al. Alignment Mark Optimization to Reduce Tool- and Wa f e r-Induced Shift for XRA-1000, Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 7065-7070.

5 . Will Conley et al., Understanding Mask error Factor for sub-0.18 (m Lithography, P roceedings of the Micro l i t h ography Symposium INTERFACE 2000, p. 69.

6 . M. Adel, M. Ghinovker, B. Golovanevsky, P. Izikson, E. Kassel, D. Ya ffe, F. Bruckstein, R. Goldenberg, Y. Rubner, M. Rudzsky, Optimized overlay metrology fiducials: theory and experiment, to be published in IEEE Transactions on Semiconductor Manufacturing.

KLA-Tencor Trade Show Calendar July 14-16

SEMICON West, San Francisco, California

August 18-22

Canadian Semi. Tech. Conference, Ontario, Canada

September 9-10

BACUS/Photomask 2003, Monterey, California

September 14-18

SIMS XIV, San Diego, California

September 15-17

SEMICON Taiwan, Taipei, Taiwan

September 30- October 1

SEMI Expo CIS, Moscow, Russia

October 9

FSA, San Jose, California

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