Critical dimension adapted alignment for EBDW

Microelectronic Engineering 78–79 (2005) 16–21 www.elsevier.com/locate/mee

Critical dimension adapted alignment for EBDW U. Weidenmueller a,*, P. Hahmann a, L. Pain b, M. Jurdit b, D. Henry c, Y. Laplanche c, S. Manakli c, J. Todeschini d a

LEICA Microsystems Lithography GmbH, Goeschwitzer Str. 25, D-07745 Jena, Germany b CEA/ LETI, 17 Rue des Martyrs, F-38054 GRENOBLE Cedex 9, France c STMicroelectronics, 850 Rue Jean Monnet, F-38926 Crolles Cedex, France d PHILIPS Semiconductors, 860 Rue Jean Monnet 38926 Crolles Cedex, France Available online 19 January 2005

Abstract Electron beam direct write (EBDW) promises a good solution in lithography applications, where standard optical lithography is not suitable. With shrinking dimensions in semiconductor technologies the overlay capability for the lithography has to be enhanced. The overlay depends on the tool capability as well as on the process and technology requirements and conditions. The different effects contributing are presented. We investigated a new design for alignment marks in order to improve the overlay capability. A SB320-50DW was used for the experiments. The goals were to improve the overlay to the previous lithography step and at the same time to use marks which are compatible with the standard semiconductor process. We compared gratings with double crosses regarding the achievable accuracy. All marks had the same outer shape. The gratings were varied in the width (100–240 nm) and pitch (400–640 nm) of the lines. The influence of the new mark design on signal quality, detection accuracy and achievable overlay results is discussed.
2005 Published by Elsevier B.V. Keywords: E-beam direct write; E-beam lithography; Mix and match; Overlay; Alignment accuracy; Mark recognition; Mark gratings

1. Introduction Semiconductor technologies have entered dimensions way below 100 nm. Electron beam di* Corresponding author. Tel.: +49 0 3641 65 1930; fax: +49 0 3641 65 1922. E-mail address: [email protected] (U. Weidenmueller).

0167-9317/$ - see front matter
2005 Published by Elsevier B.V. doi:10.1016/j.mee.2004.12.088

rect write (EBDW) promises to be a suitable solution to extend the lithography capability [1]. Furthermore, with growing complexity to achieve high resolution lithography EBDW comes into view for some niche applications. The major drawback of EBDW is the sequential writing strategy and resulting long exposure times. A Mix ans Match technique where only the critical layers are exposed by electron beam lithography can con-

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tribute to reduce the total exposure time to an acceptable value. A critical point for the lithography process especially in a Mix ans Match process is beside the process compatibility between optical and electron beam lithography the overlay capability. The alignment marks have an important influence on the overlay. It is known form optical lithography as well as from metrology, that normal alignment or measurement structures with dimensions far above the targeted CD are effected by the processing steps [2,3] as etch or chemical mechanical polishing (CMP). Gratings were introduced instead of the conventional structures to improve the achievable accuracy. The signal from a grating contains more information regarding their position. This has to be weighted against the signal loss due to the interference between neighboring lines and the reduced signal efficiency for small lines. The effect becomes more prominent with a reduced pitch. However gratings promise to be capable of reaching the necessary overlay accuracy for the next design nodes which the ITRS Roadmap defines as 18 nm for the 45 nm node [5]. Fine gratings represent the same dimensions as the real structures on the wafer. This ensures that the marks used will be affected in the same way as the critical structures by all processing steps. An additional advantage of gratings is the neglect of point defects in the structure itself and the summarization over multiple position information thus reducing the influence of fluctuation. The design of such gratings for alignment purposes has to fulfill the special requirements for detection with an EBDW tool. The alignment technique has to be compatible with the normal process flow to avoid additional process steps. A critical part in this respect is the signal quality. For a metallization layer the signal quality is largely enough as the used high Z-materials allow a sufficient signal to noise ratio. However these materials are used mainly in back end applications. For the first layers a topographical mark has to be used. As with shrinking lateral dimensions the layer thickness decreases, the signal to noise ratio for the topographical marks decreases as well. Experiments show that for pure topographical marks a mark depth of 300 nm is suit-

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able to achieve the necessary contrast for detection with an electron beam at 50 kV [4]. Further tests show that even smaller step heights might be possible.

2. Overlay accuracy The achievable overlay accuracy depends on a number of factors. These generally can be divided into two groups: tool related effects and process/ technology related effects. The tool related effects are the general beam stability, the accuracy of the exposure field alignment, the wafer flatness and the position accuracy of the stage during stage movement. From the technology point of view there is the line edge roughness and line edge profile of the used marks, the mark homogeneity over the wafer and the mark profile itself including design and depth. Additionally, there are some effects which come from the combination of different lithography techniques. First there is the field and wafer matching to note, as in difference to the optical lithography where the patterns are exposed with certain projection errors constant from chip to chip EBDW should write every position on the whole wafer with the same accuracy. Matching strategies can correct for this problem and allow as well the adaptation to differences in wafer flatness and stage movement. Further the alignments structures for the optical lithography and the EBDW are not matched thus different structures are used for the alignment. Further, as in fact the electron beam exposes the resist at the alignment mark, it might not be possible to use this mark in a following lithography step.

3. Experiments In this study, we investigated only the overlay accuracy dependence on the design of the alignment marks. For this different gratings were tested in an array of 6 alignment marks. The used marks are shown in Fig. 1. We choose gratings with 100 nm/300 nm, 120 nm/360 nm, 160 nm/400 nm,

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Fig. 1. Alignment structures investigated. Top: double cross 1 lm, gratings 100, 120 nm; bottom: gratings 160, 200, 240 nm.

200 nm/400 nm and 240 nm/400 nm and as a reference a double cross with 1 lm/1.5 lm line/space. The gratings were chosen in order to achieve the best compromise between the number of features in the gratings and their suitability for detection with the used EBDW tool. The outer structure of a cross design represents the standard design but it is not limited to this. The gratings were exposed as alignment marks for a test chip in a standard mapping together with first level overlay test structures. In an etch step performed afterwards the marks were transferred into the silicon test wafer with a depth of about 600 nm. A top view on one line after the resist strip for the 100 nm grating is shown in Fig. 2. The slope width can be approximated to 25 nm with a top width of the line of about 134 nm. Figs. 3 and 4 show the 100 nm grating and the corresponding signal without resist detected with the electron beam of a shape beam tool. The beam had a size of 80 · 2000 nm2 at 17 A/cm2 for the detection. This is a compromise between the high resolution targeted and the measurement conditions used in a normal measurement regime. The detection sweep had 10 lm with 500 points in total. This calculates to about

4 · 104 electrons per point. The general positional accuracy of the beam is about 2 nm 3r for the tool used.

Fig. 2. 100 nm line (design) after etch – top 134 nm, bottom 94 nm, left 21 nm, right 19 nm.

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4. Results and discussion 4.1. Signal quality

Sweep(+X) 11x 100nm/300nm signal [arb.unit]

160 140 120 100 80 60 0

2000

4000 6000 Position [nm ]

8000

10000

Fig. 4. 500 nm deep grating detected by Leica SB320-50DW.

For reproducibility the different marks were measured in a separate run using the same measurement method as applied during the alignment measurement for an exposure. All these data are taken from an uncoated wafer. Test exposures were then performed using the 100 nm grating for alignment on one half of the wafer and the double cross as reference on the other half. The overlay measurements were performed on the whole wafer with a KLA Archer10 on 4 box in box structures located in the scribe line of the chips between the etched mark layer and the exposed resist. We evaluated the mark detection signal and accuracy as well as the final overlay results.

measured signal 1.10 normalized signal amplitude

Fig. 3. Etched grating 100 nm/300 nm line/space.

We measured the influence of the grating pitch on the signal amplitude. Fig. 4 shows a typical signal from such a grating. In Fig. 5, the signal amplitude vs. the grating line width is shown. The signal of the double cross is taken as a reference. At very low line width the signal amplitude decreases significantly as expected. The critical value corresponds to a pitch of 600 nm. The signal to noise ratio is 6.7 for 100 nm/300 nm grating with 600 nm mark depth, which is clearly sufficient for an accurate mark recognition. The prediction for a reduced mark depth below 240 nm indicates that the signal to noise ratio becomes critical. This is even worse if the marks are buried as it is the case in the normal process flow. The signal to noise ratio has to stay at minimum above 2 in order to achieve acceptable results for the detection accuracy. Fig. 6 shows the 1r deviation of the mark detection reproducibility over all measured marks vs. the alignment mark. One can see that the standard deviation can be reduced by approximately 20% for the gratings compared to the double cross. This is a result of the increased number of edges the alignment is performed on. In the investigated case the number of edges is increased from 4 up to 22. The increase of mark features overcompensates the decrease of contrast at low feature dimension for reflected electrons to some degree. The graph suggests as well a slight reduction of reproducibility

1.00 0.90 0.80 0.70 0.60 0.50 0

200

400 600 line width [ nm]

800

1000

Fig. 5. Measured signal amplitude for different line width for the gratings.

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mark detection reproducibility 1.3 1.2

Table 1 Comparison of overlay results for 2 exposures averaging over 23 chips and 4 measurements per chip

1.1

Exposure

Averaged standard deviation 3sigm X

Y

X

Y

X

Y

Double cross

1 2

20.3 20.1

15.2 15.9

29 30

20 22

21 24.5

19.1 21.8

Grating 100 nm

1 2

14.4 13.6

13.6 13.5

21 19

19 18

21.2 22.6

21.6 18.4

1 0.9 0.8 0.7 0.6

nm 20 0n m /4 00 nm 16 0n m /2 00 nm 12 0n m /3 60 nm 10 0n m /3 00 nm do ub le cr os s

0.5 24 0n m /4 00

standard deviation 1sigma [nm]

20

Full range

Maximum deviation

structure

Fig. 6. Mark detection reproducibility.

for the smallest gratings in our experiments. This might be a result from the reduced signal amplitude. A further increase of the number of features in the mark could improve the reproducibility further. However, this has to be carefully chosen as an increased number results as well in an increased mark size. The measurement resolution, the increased number of electrons used for the measurement and therefore stronger exposure of the resist around the mark has to be carefully compromised. 4.2. Overlay accuracy We applied the new mark version to a real overlay exposure. Many effects contribute to the overlay accuracy. Even if mark detection is not the dominating contribution, the tendency should show an improvement at least. In the overlay measurements an improved 3r value as well as an improved range for the distribution of the overlay values can be found (Table 1). The good results can be explained by the increased number of features in the used alignment mark as already mentioned above. Beside this we observed a reduction of the difference in the distribution for X and Y. An additional effect comes from the technological point of view. The smaller feature size has a positive impact on the mark profile distribution resulting in a higher homogeneity for X and Y. Together these effects could reduce the 3r value from

20 nm/15 nm down to an average of 14 nm and the range from 27 nm/22 nm to an average of 20 nm.

5. Conclusion The quality of the alignment marks contributes strongly to the final overlay result. Beside the physical effects improving the measurement accuracy by averaging over multiple structures it reduces the influence of process, technology and tool fluctuations. By using gratings instead of a double cross for the alignment measurement the reproducibility could be improved by approximately 20%. Down to 200 nm/400 nm line/space gratings the signal height is not significantly reduced. Smaller gratings can use a higher number of features but reduce as well the signal amplitude. This was no problem in this test, but it has to be confirmed with buried marks as used in the real process flow. The choice of measurement conditions together with the applied alignment mark design has to be carefully weighted against the signal to noise ratio in order to achieve the necessary detection accuracy. The overlay results on the real exposures could confirm the promising results even if the maximum deviation could not be reduced. An improved standard deviation as well as a reduced range had been achieved. The results show that the use of gratings as alignment marks can contribute to achieve the high requirements of the next design nodes for the overlay.

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References [1] L. Pain, et al., Proc. SPIE (2002) 4688. [2] M. Adel, et al., Proc. SPIE (2003) 453.

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[3] Ph. Lerag, et al., Proc. SPIE (2003) 49. [4] J. Yamamoto, et al., Jpn. J. Appl. Phys. 38 (1999) 7040– 7045. [5] ITRS Roadmap 2003, Lithography.