Issue



Development to manufacturing using integrated scatterometry


07/01/2005







Integrated scatterometry will be an essential part of the monitoring and control in product manufacturing at the 65nm node and beyond. Several integrated scatterometry applications are being developed and implemented, while many applications still remain to be explored. At the same time, it is becoming clear there is a need for integrated scatterometry during development to ease the transition into manufacturing. Using integrated scatterometry to support development can yield more robust processes and products, thereby reducing risks and speeding the transition into manufacturing.

Given the stringent requirements in the semiconductor industry for production at the 65nm node and beyond, integrated metrology (IM) will be a vital tool to increase productivity and reduce costs. Several forms of IM have been used for years, including macrodefect inspection for lithography tracks and unpatterned film thickness measurements for deposition. Integrated scatterometry is a specific type of IM and has grown as a natural extension of stand-alone scatterometry. As a fast optical measurement that can provide full profile information, scatterometry [1] is being increasingly implemented by semiconductor manufacturers. Further behind in implementation than stand-alone scatterometry, integrated scatterometry has been considered by many to have great potential, and is beginning to realize some of those expectations.

One of the first applications of integrated scatterometry by equipment suppliers and semiconductor manufacturers has been that of gate pattern monitoring and control. Because gate control is so critical, and improvements in such control can tie directly to yield improvements, this strategy has made sense. Other applications, such as shallow-trench isolation (STI) critical dimension (CD) and depth control, and back-end-of-line (BEOL) trough CD and depth control, are now under investigation. Some other applications are being examined using stand-alone scatterometry [2] but could also be achieved with integrated scatterometry, while others barely have been considered at all.

Integrated scatterometry is generally considered a manufacturing metrology; however, it can also be used to support process development. The ability of scatterometry to collect a large amount of profile information very quickly enables it to more easily detect some problems that are difficult to discover using other metrologies. Integration of these measurements onto process tools permits a smoother transition from development to integrated manufacturing. Before describing in more detail how integrated scatterometry can support development and why this is important, it is useful to understand how production can benefit from this method.

Integrated scatterometry for manufacturing

Stand-alone scatterometry has many applications in manufacturing; some of them, such as STI and gate, have already been implemented by major semiconductor manufacturers [3, 4]. Others are in earlier stages of evaluation and implementation. Similarly, integrated scatterometry has a wide range of applications, many of which overlap with those of stand-alone scatterometry. Organized by tool and process sector, the table lists many applications that, in principle, can be implemented in manufacturing using integrated scatterometry. Each of the tool sectors has the capability to use integration to feedback one or more parameters, such as CD, depth, or thickness, in order to control them. Etch is one tool sector that can also use integrated scatterometry to feedforward measurements to the process tool. In this case, CD can be controlled for many process sectors.

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Note that processes must have their own parameters, or “knobs,” with which to control these measurable parameters. Furthermore, to make feedforward and feedback control practical, the entire system should be fully automated to perform and process the measurements, calculate the new process conditions, and then process the wafer. The tool-level control should also be integrated into any existing factory-level automation for data collection and lot-level changes to process conditions.

Several applications in the table are well known and in various stages of implementation [5], such as the gate and BEOL trough applications on etchers; however, many others have not been widely considered within the semiconductor industry. For example, during the formation of the deep trench capacitor, integrated scatterometry can be used to control CD. This is an especially appealing application because the traditional CD-SEM is often challenged by the high aspect ratio of the photoresist and hard mask. Later in the formation of the deep trench capacitor, integrated scatterometry can be used to measure recess depth. In some cases, scatterometry may provide the only opportunity to measure these depths nondestructively.

Lithography and etch are not the only opportunities for integrated scatterometry; CMP and deposition can also be controlled using this technique. Controlling these processes by measuring large unpatterned regions is not adequate because such areas often behave differently than patterned regions due to loading and other effects. In the STI sector, liners are often deposited after the trench etch, and their thicknesses can be measured using integrated scatterometry. After the trench is filled, the oxide remaining after the CMP process can also be monitored with this method. Like the STI sector, BEOL processing has multiple applications for integrated scatterometry. After lithography and etch, liners can be controlled at deposition, and stop layers and copper line thicknesses can be controlled at CMP.

Only a few of these applications have been demonstrated enough to prove their return on investment. Others are technically feasible, but have not yet been proven to be economically practical because they are in the early stages of development. Integrated scatterometry, though, is still evolving, and with time, the industry will uncover more applications that are both technically feasible and economically prudent. These successful applications will: 1) enable tighter process control; 2) reduce the load on other, more expensive, metrology equipment; 3) reduce overall wafer processing time; and 4) reduce the mean time to detect process equipment problems.

WTW feedforward control of gate photoresist trim

To better appreciate the benefit of integrated scatterometry for production, it is helpful to understand a specific example of this technique. A well understood application for integrated scatterometry is wafer-to-wafer (WTW) feedforward control of the gate photoresist trim process. This process has several parameters that can be used to vary the amount of trim, enabling smaller CD values and, when combined with WTW feedforward control, tighter CD distributions than those printed during lithography. Here, integrated scatterometry is used to provide wafer-level CD information by sampling 100% of the incoming wafers for WTW control. This sampling also enables detection of WTW variations that cannot be seen with lot-level monitoring.

The wafer-level CD values are fed into an advanced process-control (APC) application and used as input for feedforward control. APC determines the delta between the measured CD values and the final target. Using a predetermined relationship called a trim curve, between this delta and the trim process-control parameter, the APC application outputs to the process tool the correct processing conditions for the wafer to eliminate the deviation of the final CD from the final target. Similarly, integrated scatterometry can be used to enable feedback process control by using post-etch measurements.

Because feedforward control is based on pre-etch measurements, accurate CD measurements are critical. To determine measurement accuracy, Total Measurement Uncertainty (TMU) analysis [6] can be used to correlate measurements from a system under test to a reference metrology system. TMU is a 3σ metric of the scatter caused by the system under test around the best-fit line for a data set, after removing the contributions from the reference measurement system. In this work [7], the CD-SEM was used as the reference system.


Figure 1. TMU of 0.21nm for ODP vs. CD-SEM on gate photoresist structures.
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Figure 1 is a TMU plot showing the correlation of Optical Digital Profilometry (ODP) to CD-SEM for a group of 25 wafers. ODP is a scatterometry solution that can be either implemented on stand-alone systems or integrated onto a wide range of process equipment. The TMU of 0.21nm for ODP indicates good correlation with CD-SEM. The offset of about 23nm between the systems has two sources. First, ODP measured the middle CD of the resist, which is slightly smaller than the bottom CD that the CD-SEM measured. Second, the systems measure using fundamentally different physical techniques; this creates an offset, which is common.


Figure 2. Wafer-to-wafer feedforward control of gate photoresist trim enabled a reduction in across-lot CD uniformity from 3.8nm (3σ) to 1.3nm (3σ).
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Using an etcher with ODP-based integrated scatterometry, we have been able to achieve a significant reduction in across-lot and lot-to-lot CD uniformity. Experimental lots were created with differing amounts of incoming CD variation, from standard production to artificially large variations. These lots were processed using WTW feedforward control; an example of one of these lots at pre- and post-etch is shown in Fig. 2. The measured post-etch across-lot CD uniformity of 1.3nm (3σ) represents a significant uniformity improvement for the 13 wafers. This improvement in uniformity, when coupled with the centering of the CD distribution around the post-etch target using feedback control, can significantly improve Cpk and yield.

Integrated scatterometry for development

Although integrated scatterometry can be widely applied to manufacturing, it can also benefit development. While scatterometry currently requires a large amount of setup time for model building and testing, the payoff is significant for development projects that use more than a handful of wafers/model, with fast measurements containing full profile information. Integrating the scatterometry onto process equipment further improves development efficiency by providing measurement results immediately after processing.

Traditional process development in lithography, etch, CMP, and deposition relies heavily on metrologies such as CD-SEM, atomic force microscopy (AFM), XSEM, optical unpatterned-film thickness measurements, and, less often, CD-AFM and transmission electron microscopy (TEM). Other methods are also used, but the capabilities of these metrologies overlap with those of scatterometry. Two of these, CD-SEM and unpatterned-film thickness measurements, are about as fast as scatterometry, but don’t provide as much information. Also, CD-SEM systems collect information over a much smaller area than scatterometers. Unless a very small and specific structure must be measured, collecting over a larger area is preferable; the averaging is more representative of the “true” average for that chip region.

Optical blanket-film thickness measurements cannot measure patterned regions, which may behave differently than unpatterned regions. Other systems, such as XSEM and TEM, are capable of providing detailed profile information, much like scatterometry. After the initial library setup, however, scatterometry is able to provide these results immediately, whereas other profile-generating metrologies often have turnaround times measured in hours or days.

With integrated scatterometry, the results from process experiments are available almost as soon as processing is completed. With this reduced time-to-results, scatterometry can significantly speed up learning, thereby reducing process development cycle times. Also, because scatterometry is not destructive, the wafers can be used for subsequent processing and experimentation. With the advent of 300mm wafers combined with the tight requirements of 65nm-node technology, detection of across-wafer variations during development has become essential. This cannot be easily accomplished with destructive metrology; however, scatterometry is well qualified to provide this across-wafer information for a multitude of parameters.

Process changes are much easier to implement early in a product’s development cycle. Processes that are characterized with limited metrology are at a greater risk of not being optimal, and thus are more likely to limit yield. To avoid this problem, processes need to be fully characterized early on [8], so that necessary changes can be easily implemented. Waiting to fully characterize a process can force IC manufacturers to make costly last-minute process changes if a critical process flaw is discovered.

Given the essential role that integrated scatterometry will have for manufacturing at the 65nm node, integrated scatterometry and APC are well suited for developing and characterizing individual processes. To take full advantage of integrated scatterometry, processes with precisely controlled and repeatable parameters that enable feedforward and feedback control need to be constructed early in development. If no such parameters are available, it may be necessary to change the process. Once again, such a change is likely to be difficult unless it is done early in development. For feedforward and feedback control to succeed, it is also necessary for the metrology to accurately and repeatably measure the quantity linked to the process control parameter. Thus, the metrology and APC for manufacturing should be co-developed along with the process itself.

Because scatterometry can measure so many parameters, it can also monitor some aspects of earlier processing. So, in a larger sense, integrated scatterometry can also support development of process integration. By developing the metrology, process control, processes, and integration together, overall product development time is reduced and the product’s performance is improved. For manufacturing processes that are best monitored and controlled with integrated scatterometry, the most seamless transition to this stage starts with integrated scatterometry early in development.

Conclusion

Integrated scatterometry is still in its infancy, and the industry still has much to learn about it. Of the applications described here, some are being implemented, while many others are only beginning to be explored. It is becoming evident that integrated scatterometry has a significant impact on process monitoring, control, and yield. Furthermore, process and product development can benefit from integrated scatterometry due to its unmatched ability to quickly and nondestructively collect profile and underlayer information. The use of integrated scatterometry early in the development cycle enables ease of transition from early development to full manufacturing.

Acknowledgments

TMU (Total Measurement Uniformity) is patent pending.

References

  1. X. Niu, N. Jakatdar, J. Bao, C. Spanos, “Specular Spectroscopic Scatterometry,” IEEE Trans. on Semi. Mfg., Vol. 14, No. 2, pp. 97-111, May 2001.
  2. M. Sendelbach, D. Shneyder, W. Lu, K. Boyd, “Comparison of Scatterometry, AFM, Dual Beam System, and XSEM to Measure Etched Via Depths,” Proc. SPIE Met., Inspec., & Process Control for Microlith. XIX, Vol. 5752, 2005.
  3. R. Dare, B. Swain, M. Laughery, “Logic Gate Scanner Focus Control in High-volume Manufacturing Using Scatterometry,” Proc. SPIE Metrology, Inspection, and Process Control for Microlithography XVIII, Vol. 5375, pp. 713-720, 2004.
  4. K. Lensing, R Markle, B. Stirton, M. Laughery, “STI Scatterometry Metrology in a High Volume Fab,” Proc. ISSM, pp. 195-198, 2001.
  5. M. Sendelbach, W. Natzle, C. Archie, B. Banke, D. Prager, et al., “Feedforward of Mask Open Measurements on an Integrated Scatterometer to Improve Gate Linewidth Control,” Proc. SPIE Metrology, Inspection, and Process Control for Microlithography XVIII, Vol. 5375, pp. 686-702, 2004.
  6. M. Sendelbach, C. Archie, “Scatterometry Measurement Precision and Accuracy below 70nm,” Proc. SPIE Metrology, Inspection, and Process Control for Microlithography XVII, Vol. 5038, pp. 224-238, 2003.
  7. H. Yue, A. Yamashita, M. Tomoyasu, H. Kambara, M. Sendelbach, et al., “Modeling of 193 Photoresist Trim Process for Advanced Process Control of Sub-50nm Gate CD,” Proc. ECS-ISTC, Sept. 2004.
  8. C. Ke, S. Yu, Y. Wang, Y. Chou, J. Chen, et al., “90nm Lithography Process Characterization using ODP Scatterometry Technology,” Proc. SPIE Metrology, Inspec., and Process Control for Microlithog. XVIII, Vol. 5375, pp. 597-604, 2004.

For more infomation, contact Matthew Sendelbach at IBM, Hopewell Junction, NY; e-mail [email protected].

Dan Prager, Timbre Technologies Inc., Hopewell Junction, New York