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Reticle intensity based Critical Dimension Uniformity to improve efficiency for DOMA correction in a foundry 1

Kin Wai Tang, 1Teng Hwee Ng, 1Lei Huang, 1Susan Ng, 1Thomas Ku, 2Wee Teck Chia, 2 Lin Chua, 2William Li, 2Aaron Chin, 3Aditya Dayal, 3Tom Vavul, 3Trent Hutchinson, 1

GLOBALFOUNDRIES Singapore Pte Ltd., 60 Woodlands Industrial Park D Street 2, Singapore, 528830 2

KLA-Tencor Singapore, No.4 Serangoon North Avenue 5, Singapore, 554532

3

KLA-Tencor Corporation, One Technology Drive, Milpitas, CA 95035, USA

Abstract As transistor dimensions shrinks, the requirement for wafer critical dimensions control is becoming increasingly challenging. The intra-field critical dimension uniformity (CDU) of the features on the reticle is one of the many sources of wafer CD variation. In this paper, we study how the CDU on the reticle can be obtained by using the intensity information collected during reticle inspection (iCDUTM) on the KLA-Tencor TeraScan reticle inspection tool. The collected CDU information of the reticle is then applied as an intra-field dose correction function to improve wafer intra-field CD uniformity.

Using this method of extracting the reticle CDU from the intensity information allows for simple integration into a high-volume production environment and an improved capability for intra-field CDU correction without the need to expose any wafers for CD measurement nor any GDS design information. The ability to apply iCDU on prototype devices on first pass run can also accelerate device development.

Keywords: iCDU, intra-field CDU, transistor performance, yield

Introduction Intra-field wafer CD uniformity has been reported as a significant contributor to yield loss in advanced technology nodes. Hence, intra-field reticle CD uniformities are often characterized and studied so that they can be compensated for, to reduce intra-field CD variation on the wafer. The typical process involves exposing a wafer in advance and performing wafer CD SEM measurement on many locations of a similar pitch within a reticle field, to obtain the intra-field CDU information. This information is then fed back to the scanner for intra-field dose correction (see Fig. 1). The drawbacks of this method is that the wafer level CDU information obtained is a combination of reticle, process and scanner factors and the selection of the measured featured and their pitch can influence the accuracy of the results. Isolated features would be more sensitive to focus variations than dense pitches and would thus become highly influenced by the condition of the scanner at the time the data was collected. Moreover, this method is extremely time-consuming and represents an inefficient use of engineering resources, as CD SEM recipes have to be set up for hundreds of measurement locations.

Metrology, Inspection, and Process Control for Microlithography XXVI, edited by Alexander Starikov, Proc. of SPIE Vol. 8324, 83242W 路 漏 2012 SPIE 路 CCC code: 0277-786X/12/$18 路 doi: 10.1117/12.916335

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Figure 1 – schematic flow chart of conventional dose compensation by feeding wafer CD-SEM measurement data to DoseMapper to control the CDU of printed wafers

A novel method of reticle CDU measurement, called iCDU, utilizes the information collected during reticle inspection on the TeraScan or Teron inspection tool to give the full field CD uniformity information. Reflected intensity information is collected during a normal STARlightTM contamination inspection and converted into calibrated reticle CDU information (Figure 2). This calibrated reticle CDU data is subsequently used for intra-field dose correction (Figure 3). This information is collected concurrently during the reticle inspection without any throughput loss or additional engineering effort. iCDU was originally designed to work on memory reticles but has been adapted at GLOBALFOUNDRIES Fab 7for use on multi-die logic reticles with repeating identical cells.

Figure 2 – single reticle measurement giving both defect inspection results and iCDU results simultaneously without any throughput loss

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Figure 3 – illustrated flow chart of how TeraScan iCDU data is fed into DoseMapper for dose compensation to control the final wafer CDU

Experiments

3.1 Inspection Flow The experiment flow is described in Figure 4 below. Intensity information from TeraScan is converted to CD data using current scanner dose and CD sensitivity in a similar way that SEM CD data is used to input DoseMapper directly (Figure 5). The coordinate transformation from reticle to wafer is done by using the reference points on the reticle, wafer and wafer layout.

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Fig 4 - The experimental work flow

3.2 Conversion from intensity to CD As mentioned earlier in the paper, iCDU uses intensity information from the high resolution image collected by TeraScan™ reticle inspection tool to measure CD uniformity across the reticle. CD changes in repeating cells across the reticle result in proportional changes in the average intensity map, and these changes are detected and mapped with a high degree of precision. The relationship between normalized CD change and normalized intensity change can be expressed simply as:

ΔI = k ΔCD I CD where k is a scale factor that depends upon the optical parameters of the inspection tool as well as the pattern pitch and polarity. We empirically calculate the scale factor and provide the scaled intensity data to adjust the scanner dose for CD uniformity correction. Additionally we apply a coordinate transformation and 1/4X scaling to the reticle CDU map to convert it into wafer field coordinates.

Figure 5 – image explains how intensity uniformity is transformed from reticle to CD data in nm, wafer dimension get shrunk 4x after rotation from reticle level

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3.3 Dose Compensation By taking the CD calibrated iCDU data, DoseMapper is able to generate an intra-field dose correction recipe to compensate for intra-field CD nonuniformity (Fig. 6).

Figure 6 – Using CD calibrated iCDU data to generate Dose compensation Map

Results & Discussion

To test the performance of the iCDU compensation, the conventional, CD-SEM-based method of intra-field dose correction is used as the control. In both cases, multiple wafers are used to test iCDU performance against the conventional method. A production reticle was inspected on TeraScan in STARlight mode. The data extracted from the inspection result using TeraScan Review server is fed to scanner through DoseMapper compensation. All other process parameters are kept constant.

The CD 3 sigma (3s) is used to represent the intra-field uniformity. To calculate the improvement on intra-field uniformity of one method over another, the equation used is (3 sigma of iCDU – 3 sigma of conventional method)/3 sigma of conventional method. The same holds true for yield improvement: (yield of iCDU – yield of conventional method)/yield of conventional method.

As indicated in Fig 7, 25% 3 sigma improvements is observed, which leads to a 5% yield improvement over the conventional method, which was measured by final testing on all 3 wafers (Fig. 7).

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Figure 7 – Comparison of conventional DOMA and iCDU DOMA on intra-field basis in terms of 3 sigma and yield

To further validate iCDU performance, more devices were tested in comparison against the conventional method.

Overall, a significant CDU improvement was observed, ranging from 9% to 25%, with corresponding yield improvement from 0.8% to 3.5% (Fig. 8).

Figure 8 – CD uniformity and yield improvement with 5 devices

Besides the better yield performance, the iCDU method is a faster approach to compensate intra-field CD uniformity, as shown in figure 9 below.

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Figure 9 – iCDU feedback for DOMA is 3 times faster than conventional method

Summary

We have tested the iCDU method of correcting for reticle CD uniformity on numerous devices and have observed improvements in both wafer level CD uniformity and product yield.

The iCDU methodology has been successfully implemented in a wafer fab to feed forward to scanner and improve intra-field CD uniformity for 65nm technology node multi-die logic reticles. Compared against conventional method

by SEM measurement, iCDU provides obvious advantages in terms of productivity as well as capability. iCDU is implemented without having to expose a wafer for CD SEM measurements. This is extremely valuable in highvolume manufacturing with a large number of prototype devices. The ability to improve intra-field CDU during the first pass production run will help to accelerate device development.

Acknowledgements

The author would like to thank Eric Goh, Brian Tan, Jason Liu from KLA-Tencor Singapore, Ghee Soon Chong, and Gek Soon Chua, Shijie Shen, Poh Ling Lua from GLOBALFOUNDRIES Fab 7 Singapore for their contribution to this paper.

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References

[1] Wylie, M, Hutchinson, T, Pan, Gang, Vavul, T, Miller, J, Dayal, A, Hess, C, Green, M, Hedges, S, Chalom, D, Rudzinski, M, Wood, C and McMurran, J, "Critical dimension uniformity using reticle inspection tool", Proc. SPIE 7488, 74881O (2009).

[2] Jeewakhan, N, et al “Application of DoseMapper for 65nm Gate CD Control Strategies and Results,” Proc. Of SPIE vol. 6349 63490G-11 [3] Vellanki ,V, et al “Reticle Inspection-based Critical Dimension Uniformity,” Proc. Of SPIE Vol. 7379 73790D-8

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