Overlay Metrology for Double Patterning Processes

Overlay Metrology for Double Patterning Processes Philippe Leray, Shaunee Cheng, David Laidler, IMEC, Leuven, Belgium Daniel Kandel, Mike Adel, Berta Dinu, Marco Polli, KLA-Tencor Corporation, 4 Haticshoret St., Migdal Haemek 23100, Israel Mauro Vasconi, ST Microelectronics, Agrate, Italy Bartlomiej Salski, QWED, Warsaw, Poland ABSTRACT The double patterning (DPT) process is foreseen by the industry to be the main solution for the 32 nm technology node and even beyond. Meanwhile process compatibility has to be maintained and the performance of overlay metrology has to improve. To achieve this for Image Based Overlay (IBO), usually the optics of overlay tools are improved. It was also demonstrated that these requirements are achievable with a Diffraction Based Overlay (DBO) technique named SCOLTM [1]. In addition, we believe that overlay measurements with respect to a reference grid are required to achieve the required overlay control [2]. This induces at least a three-fold increase in the number of measurements (2 for double patterned layers to the reference grid and 1 between the double patterned layers). The requirements of process compatibility, enhanced performance and large number of measurements make the choice of overlay metrology for DPT very challenging. In this work we use different flavors of the standard overlay metrology technique (IBO) as well as the new technique (SCOL) to address these three requirements. The compatibility of the corresponding overlay targets with double patterning processes (Litho-Etch-Litho-Etch (LELE); Litho-Freeze-Litho-Etch (LFLE), Spacer defined) is tested. The process impact on different target types is discussed (CD bias LELE, Contrast for LFLE). We compare the standard imaging overlay metrology with non-standard imaging techniques dedicated to double patterning processes (multilayer imaging targets allowing one overlay target instead of three, very small imaging targets). In addition to standard designs already discussed [1], we investigate SCOL target designs specific to double patterning processes. The feedback to the scanner is determined using the different techniques. The final overlay results obtained are compared accordingly. We conclude with the pros and cons of each technique and suggest the optimal metrology strategy for overlay control in double patterning processes. Keywords: Overlay, Double Patterning, Scatterometry, Precision, Tool induced shift, Correctables, LELE, LFLE, Spacer defined.

1. INTRODUCTION In recent years, overlay metrology performance has been ahead of requirements. As we start to introduce double patterning (DPT) for future technology nodes, this is no longer the case. How should we now deal with the issues of precision, TIS and tool matching and what about accuracy, is zero overlay error really zero. According to the ITRS roadmap, the overlay metrology budget for the 32nm node is 0.57nm [1]. For the most challenging layers, double patterning lithography will use two lithography steps for the patterning of each layer. Moreover, in some double patterning technologies an overlay error results in a CD error. Consequently, the overlay measurement uncertainty for such layers may have to be reduced further below 0.4nm. Considering the fact that the currently available imaging overlay metrology has a measurement uncertainty of around 1nm (see Figure 1), overlay metrology performance is at risk for the 32nm node and beyond [2]. Optical imaging can in principle achieve the required performance, since there is no theoretical limit on the repeatability (precision) and accuracy of this technology. But TIS determination has always been a concern. The strategy of TIS feedback (mean or site by site) as well as the stability of TIS through the batch is a risk for the accuracy of translation control. Moreover, new optical imaging targets are emerging, reducing the size or increasing the amount of layers in the same field of view. Despite improvements in the quality of optics in recent years, the question of accuracy of the determined translation remains. In order to mitigate the risk associated with overlay metrology performance, we have already reported preliminary results on diffraction based overlay technology [3]. Metrology, Inspection, and Process Control for Microlithography XXIII, edited by John A. Allgair, Christopher J. Raymond Proc. of SPIE Vol. 7272, 72720G · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.814182

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Diffraction based overlay has two major advantages. First, the signal is not affected by optical aberrations, a major error contributor to TIS (tool induced shift) in imaging overlay metrology. It should therefore be relatively easy to achieve excellent TIS performance. Secondly, it uses periodic grating targets and detects the intensity of diffraction order(s). This intensity has a very weak sensitivity to focus and to the position of the illumination spot relative to the grating center (the residual sensitivity resulting from the finite size of the spot). Such sensitivities contribute significantly to tool matching errors. Thus, we expect this technique to have very good tool matching performance. 10

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In this paper, we present the overlay performance obtained with the listed technologies in various double patterning schemes and because of the TIS uncertainty and target size, we propose an alternative strategy to use a combination of Diffraction Based Overlay (DBO) targets and Image Based Overlay (IBO) targets.

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HARDWARE DESCRIPTION SCOL Sensor

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Figure 2. Schematic of Archer 200. The hardware used for this research is an Archer 200. This tool combines two sensors: an imaging sensor and a spectroscopic ellipsometer (figure 2). The broadband light source has a wavelength range from the UV to the IR. Light from the source goes through a polarizer and is then focused on the wafer by reflective optical components. The light specularly reflected from the wafer is directed through another polarizer (the analyzer) and collected with a spectrometer (figure 3). The considerations leading to this choice of hardware architecture are as follows; first, the broadband nature of the light source guarantees process robustness and sensitivity to overlay of a variety of target designs with different values of pitch and CD. Secondly, the polarization control enabled by the polarizer and analyzer improves precision and allows for precision optimization during recipe setup.

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3. TARGET DESIGN 3.1 Image Based Overlay Beyond the old but still impressive Bar in Bar (or Box in Box), some new target designs were developed in recent years to increase the redundancy of information (AIM). These targets use 10 bars per layers instead of 2. This redundancy increased the overlay performance on many stacks where the signal to noise was an issue. Recently, the need for smaller targets emerged. These smaller targets would allow the density of information in the field to be increased. Moreover, new designs including multiple layers in the same field of view allow overlay measurements in agreement with alignment schemes (i.e. Poly and Gate to Active) [4]. These various flavors of IBO are integrated in the IMEC test mask in a cell repeated 35 times in the field: a BiB, an AIM and 2 small IBO (10x10 um and 5x5 um). Each cell contains all the IBO and the DBO close to each other. 3.2 Diffraction based overlay Diffraction based overlay principles have already been reported in our previous paper [3]. The technique assumes a linear response of differential signals of 2 gratings on top of each other. Such stacked gratings form a cell. In each of the cells the top grating is intentionally shifted with respect to the bottom grating by design. The cells in a target are identical by design except for the intentional offsets which differ from cell to cell. The total offset between the top and bottom gratings in a given cell is the sum of the intentional offset of that cell and the overlay. The number of cells in a target and the values of the intentional offsets are chosen in order to allow determination of the overlay from the scattered intensities (from the different cells) using appropriate algorithms. The theoretical limits are the range of overlay measurable (roughly +/- 20 nm for a target with gratings at Pitch400 and self-calibration offset of 16 nm). This range can be extended by increasing the offsets but at the cost of moving outside the linearity approximation region (figure 4a). The precision and the range again are driven by the pitch of the gratings in the target. A large pitch will increase the measurable range at the cost of the precision (Figure 4b). A small pitch will show a very good precision at the cost of the range (Figure 4b). The choice of these 2 parameters is a trade off. We choose a pitch of 400 nm and a self-calibration offset of 16 nm for the best compromise for poly stacks. The Figure 5 shows the correlation of various SCOL (reference, large pitch, large calibration offset) targets versus AIM on reference stack (Resist on etch Si). The 0.92 linearity observed is linked to the algorithm inaccuracy already reported [3]. This accuracy is predictable (function of overlay but 0 when overlay is 0) but stacks and target dependants. In this case the slope shows up to 0.8 nm for 10 nm overlay. This includes algorithm inaccuracy, repeatability and possible process effects.

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4. REFERENCE STACK Usually, the best overlay precision is achieved with the simplest stack. In the case of IBO, for the optimum case, this would be resist to resist. However, for DBO, this stack is not possible. The simplest stack achievable is a first layer etched in silicon with a second layer exposed in resist on top of a planarizing material (BARC). Wafers were prepared with the first layer etched in silicon and the second layer in resist; silicon depth 60 nm, BARC thickness 95 nm and resist thickness 120 nm).

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Two wafers were exposed every day for a number of days on the XT:1900Gi scanner using both chucks. The wafers were measured using both DBO and IBO techniques for comparison. Figure 6 shows the residual signature over four days for one exposure chuck based on both IBO and DBO measurements. At the top of the figure, the results with IBO are very comparable to the results at the bottom using DBO. Some minor differences can be observed due to the algorithm inaccuracy already discussed, mainly at the edge of the wafer. The overall agreement between the two techniques is excellent; the correlation plot shown in figure 5a being extracted from this data. 0.7 0.6 0.5

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Figure 7. Dynamic repeatability (X direction) of 5 overlay target types over time on Archer 200. One of the wafers was measured every day, 10 dynamic runs for 9 sites per day. Figure 7 shows the dynamic repeatability of DBO compared to different flavors of IBO and the impressive precision of DBO (0.05nm 3σ) is clear. This impressive precision could already be used for scanner monitoring, where target size is of less importance, to detect earlier any drift in residual overlay fingerprint. A summary of the performance (average repeatability X and Y in dark colors, and TIS 3σ average X and Y in light colors) on this reference stack is shown in figure 8. BiB and AIM are shown as reference and are compared to DBO and two examples of small IBO targets, 10x10 um and 5x5 um), are also shown.

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5. DOUBLE PATTERNING STACKS In the front end scheme, two double patterning options at poly level are explored. The Litho Etch Litho Etch (LELE) implies two overlay measurement checks. For the first one after 1st poly litho, the stack (Resist/BARC/TEOS/Poly/Gate oxide/Si) is very comparable to the standard poly stack without double patterning (figure 9.a.). The second one occurs after poly 2 litho (figure 9.b.). Once the 1st poly is etched in the hardmask, the critical parameter is the overlay results of the 2nd poly to the 1st poly. The nature of the hardmask might make this measurement difficult (oxide hardmask in figure 9.b.). The challenge is twofold, how can we measure accurately (stack issues) and how can we measure efficiently (multiple measurements required).

Figure 9. DPT stack schemes 9a. LELE Poly to Active, 9b. LELE Poly 2 to Poly 1, 9c. SDDP Poly to Active. The second option is shown schematically in figure 9.c (Resist/BARC/SiO2/APF/Poly/Gate oxide/Si). Spacer Defined Double Patterning (SDDP) does not require multiple measurements because only one poly exposure is required. But this stack is interesting from the metrology point of view, because of the opacity of the stack (carbon containing layer). Figure 10 summarizes the performance of the different targets described in 3.1 and 3.2. The top two graphs show the results of poly to STI on the standard poly stack (LELE) on the left. Meanwhile top right shows the equivalent results on the SDDP stack. The performance is very different from target to target. The bottom left graph shows the overlay performance for 2nd poly to 1st poly. BiB could not be measured due to the lack of contrast. The bottom right graph shows the performance of IBO (AIM target) versus DBO in the LFLE context. The DBO target used in this case had a different design than the other DBO targets mentioned in the three previous graphs. In all cases, the performance of DBO is superior and the impressive precision of these targets should be stressed.

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Figure 10. Performance on DPT stack 10a. (top left) LELE Poly to Active, 10b. (bottom left) LELE Poly 2 to Poly 1. 10c. (top right) SDDP Poly to Active. 10d. (bottom right) LFLE Poly2 to Poly 1(average repeatability X and Y in dark colors, and TIS 3σ average X and Y in light colors). Beyond the impressive precision, another advantage is in the training of DBO targets. Once the parameters are defined (pitch, auto-calibration offset and geometry), the target does not require any further optimization. Meanwhile TIS determination and optimization is needed for IBO, the set-up of a DBO recipe is completely operator independent. 2

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Figure 11. Translation offset to BiB compared to different IBO and DBO with TIS corrected site by site. The TIS offset uncertainty, which at least in part contains Wafer Induced Shift (WIS), makes it difficult to establish the translation accurately and provide accurate feedback to the scanner. In figure 11, the translation offset to BiB for various techniques is shown. The translation values were extracted from data where the TIS was corrected site by site (3000 pts). The maximum offset is 0.5nm which is significant when as already discussed we need to achieve 2 to 3nm within layer overlay for DPT. The determination of TIS for IBO is operator dependent and the TIS stability through batch or batch to batch is not necessarily stable (figure 12). Actual variation of translation through batch is an important issue for overlay

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control and one that needs to be controlled. The typical XT:1900Gi performance for translation through batch is ±1nm, so the accuracy of translation determination is a key issue. How much if any of that ±1nm through batch is due to TIS instabiliy of the metrology technique used? DBO is not susceptible to TIS, but suffers of some algorithm predictable inaccuracy [3]. This inaccuracy is less than 0.2 nm on the LELE stack. 0.5

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Figure 13. WIS compared for IBO and DBO. DBO does not suffer from TIS/WIS (figure 13) which potentially makes it more accurate if designed properly, but the targets are very large compared to IBO targets. On the other hand, IBO is a well known and well understood technique and the targets can be as small as 5x5 um, but it suffers from TIS and TIS variation. Do we necessarily need to choose between the two techniques or could we not combine the advantages of both?

6. Combining IBO and DBO As DBO is believed to be more accurate, then it is ideal for extracting translation as well as other grid parameters. To do so, only one point per field (in the center) needs to be measured. The intrafield information is much less TIS or WIS sensitive and therefore small IBO can be used Any of the techniques can provide the same and accurate correctables, with the exception of translation and here for reasons already discussed, the advantage lies with DBO.

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To accurately describe the grid, the DBO target must be in the center of the field (this could be an issue due to the size of DBO). As the information from only one DBO per field might not be enough, the other IBO in the field must be combined to extract an accurate set of field by field correctables. The offset of IBO to DBO targets must be corrected for. An attempt at such a combined strategy is shown in figure 14. A wafer was measured (Poly to Active) using three target types; DBO, IBO and small IBO, which are next to each other. Figure 14a shows the simulated residuals using the 10 parameter model on IBO and DBO measured data. For the combined strategy (DBO + small IBO), we plot the simulated residual data of DBO measured data using grid correction from DBO and intrafield correction from small IBO. Figure 14b shows the simulated residuals using field by field correction on IBO and DBO residuals determined previously. For the combined strategy (DBO + small IBO), we show the simulated residual data of DBO measured data using field by field correction from DBO for translation and a small IBO for the other 4 intrafield parameters. As shown, the achievable residuals with combo strategy are close to the performance of SCOL only strategy with a minimized use of space. IBO only

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This strategy is compatible with the advanced corrections now available on the scanners: Field by field correction (FFC), grid high order correction (GHOC) and intrafield high order correction (IFHOC) [6, 7]. 7. CONCLUSIONS In this paper we have shown the impressive precision achieved with DBO. For tool monitoring, the size of these targets is not an issue and we suggest using DBO for monitoring the scanner immersion fingerprint. In a production reticle, the size of DBO targets is a significant issue. But the performance required for double patterning is a challenge for IBO. We proposed a combined strategy that offers the strengths of both DBO and IBO techniques. DBO is used to remove the TIS sensitive parameter (translation) as well as other grid terms. Small IBO (down to 5x5 um) is used to remove TIS insensitive parameters (intrafield).

ACKNOWLEDGMENTS The SOCOT Consortium is sponsored by the European Commission under the IST 6th Framework Program, Contract 016403. The authors would like to thank Rudi De Ruyter for the design of the mask, Koen D’have for Matlab scripts support and IMEC Pline for the support to produce the wafers measured in this work.

REFERENCES 1. 2. 3. 4. 5. 6.

International Technology Roadmap for semiconductors 2007 Metrology (www.itrs.net) Adel M. and Cheng S., “Double Metrology for Double Patterning”, Estrategies (2007) Leray P., Cheng S., Kandel D., Dinu B., Vasconi M. “Diffraction Based Overlay Metrology: Accuracy and Performance on Front end Stack” SPIE 2008 Vol 6922, 0O C.P. Ausschnit, W. Chu, D. Kolor, J. Morillo, J.L. Morningstar, W. Muth, C. Thomison, R.J. Yerdon, L.A. Binns, P. Dasari, H. Fink, N.P. Smith, G. Ananew “Blossom Overlay Metrology Implementation” SPIE 2007 Vol 6518, 0G K. D’have, D. Laidler, S. Cheng. “Immersion Specific Error Contribution to Overlay Control” SPIE 2009 To be published D.Choi, C. Lee, D. Cho, M. Gil, P. Izikson, S. Yoon, D. Lee. “Optimization of high order control including overlay, alignment and sampling” SPIE 2008 Vol 6922, 0P

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