The challenging metrology application for scatterometry and CD-SEM is to accurately measure both CD and profile. To apply this metrology specifically to dual-damascene hole structures is critical for the back-end processing, in order to control both the CD and the process overall. This paper discusses applications of Optical Digital Profilometry-based scatterometry to the advanced 90nm node dual-damascene process. The application includes contact ADI, via AEI, via etch, and via fill. The results show that scatterometry can measure CD, as well as provide sidewall angle and profile information that is unavailable by CD-SEM. Correlations to CD-SEM and cross-sectional SEM are also presented. For future applications, scatterometry is a viable solution for 3D structures, and provides higher precision, and more metrology information than current metrology methods for critical dual-damascene processes.
Contact hole lithography presents a variety of challenges for process development. Measurement of the bottom of the hole presents the most difficulty, and metrology error has traditionally been much larger for contact (3D) metrology than for line/space structures. In light of process windows being significantly smaller for contact holes than for line/space structures, it is difficult to maintain good Contact CD characterization of novel methods requires CD correlation to existing metrology tools including CD linearity across a range of pitches and target CDs. In this paper, we will present contact CD linearity results as characterized by integrated ODP scatterometry, where measurements of hole CD and profile have been made following the lithography process, in a method nondestructive to the 193nm resist pattern on the wafer. The CD linearity is characterized for a 90nm technology device film stack of patterned photoresist (PR), bottom anti-reflective coating (BARC), oxide, and SiC on top of a silicon (Si) substrate. The pattern densities range from dense to semi-dense to isolated, and the grating structures include circular holes aligned in an orthogonal pattern on the wafer. Measurement stability results are also shown, and correlation to CD-SEM and cross-section SEM is provided as a reference metrology. The results of the experiment show that ODP can be used successfully to not only characterize contact CD linearity, but to also monitor film thickness and profile variation, providing a valuable solution for contact hole process development.
The accurate measurement of CD (critical dimension) and its application to inline process control are key challenges for high yield and OEE (overall equipment efficiency) in semiconductor production. CD-SEM metrology, although providing the resolution necessary for CD evaluation, suffers from the well-known effect of resist shrinkage, making accuracy and stability of the measurements an issue. For sub-100 nm in-line process control, where accuracy and stability as well as speed are required, CD-SEM metrology faces serious limitations. In contrast, scatterometry, using broadband optical spectra taken from grating structures, does not suffer from such limitations. This technology is non-destructive and, in addition to CD, provides profile information and film thickness in a single measurement. Using Timbre's Optical Digital Profililometry (ODP) technology, we characterized the Process Window, using a iODP101 integrated optical CD metrology into a TEL Clean Track at IMEC. We demonstrate the Optical CD's high sensitivity to process change and its insensitivity to measurement noise. We demonstrate the validity of ODP modeling by showing its accurate response to known process changes built into the evaluation and its excellent correlation to CD-SEM. We will further discuss the intrinsic Optical CD metrology factors that affect the tool precision, accuracy and its correlation to CD-SEM.
Timbre's Optical Digital Profilometry (ODP) system is a scatterometry-based metrology. In lithography applications, the critical dimension (CD) is often patterned photoresist (PR) on an anti-reflective coating (ARC). When a patterned PR is exposed to the broadband light of the optical metrology tool, a change in reflectance may occur. For "sensitive" film stacks, the changing optical signals then produce changing ODP CD, sidewall angle, and film thickness measurements. This report summarizes the results of several resist and ARC stacks subjected to the repeated broadband light exposure of a Therma-Wave CCD-i reflectometry system. The purpose is to determine which resist-ARC stacks are significantly affected by repeated measurement exposure, and to quantify these effects. Our analysis shows that very little metrology exposure-induced change occurs for ArF resists. For KrF resists, the change is closely related to the type of KrF resist used; acetal-types incur large spectral changes upon repeated exposure, whereas ESCAP (Environmentally Stable Chemically Amplified Photoreist) resists measurements are very stable. Significant reduction of metrology induced spectral and CD change as achieved by incorporating a long-pass filter into the system. The changes due to a single measurement are negligible, however, they can be substantial for a sensitive material when characterizing metrology repeatability. Thus, it is recommended to use stable materials, such as oxide gratings, for metrology characterization.
Scatterometry has been most commonly applied to CD metrology of line/space grating structures. However, for the process development and control of 3D structures for contact hole lithography applications, the current metrology methods of CD-SEM, electrical CD (ECD) and/or cross-sectional SEM (X-SEM) produce the desired information either (a) as an incomplete solution, (b) too late in process flow, or (c) in a destructive manner. In this paper, we will present use cases for the application of scatterometry to 3D structures, i.e., post-lithography hole/space patterns, where measurements of CD, profile, and film thickness can be made immediately following the lithography process, in a method nondestructive to the wafer. These use cases demonstrate the capability of 3D metrology integrated onto a TEL Clean Track platform, where a Therma-Wave reflectometer was used to generate spectra that were then processed via Timbre ODP, for a film stack of patterned photoresist (PR), anti-reflective coating (ARC), and oxide on top of a silicon (Si) substrate. Focus-Exposure Matrix (FEM) wafers have also been produced in order to characterize the contact hole profile and CD variation as a result of changing focus and exposure conditions. The results of the experiment show that ODP can be used successfully to monitor CD, film thickness, and profile variation, providing a valuable solution to contact hole lithography. Tool precision and matching results are also shown, which indicate the stability of the measurement process, and correlation to CD-SEM is also provided as a reference metrology. These results suggest that integrated 3D scatterometry is a viable production metrology solution, enabling the progression toward Advanced Process Control (APC).
In the continuous drive for smaller feature sizes, process monitoring becomes increasingly important to compensate for the smaller lithography process window and to assure that Critical Dimensions (CD) remain within the required specifications. Moreover, the higher level of automation in manufacturing enables almost real-time correction of lithography cluster machine parameters, resulting in a more efficient and controlled use of the tools. Therefore, fast and precise in-line lithography metrology using Advanced Process Control (APC) rules are becoming crucial, in order to guarantee that critical dimensions stay correctly targeted.
In this paper, the feasibility of improving the CD control of a 193nm lithography cluster has been investigated by using integrated scatterometry. The target of the work was to identify if a dose correction on field and wafer level, based on precise in-line measurements, could improve the overall CD control. Firstly, the integrated metrology has been evaluated extensively towards precision and sensitivity in order to prove its benefits for this kind of control. Having a long-term repeatability of significantly better than 0.75nm 3σ, this was very promising towards the requirements for sub-nanometer CD correction. Moreover, based on an extensive evaluation of the process window on the lithography cluster, it has been shown that the focus variation is minimal and that CD control can be improved using dose correction only. In addition, systematic variations in across-wafer uniformity and across-lot uniformity have been determined during this monitoring period, in order to identify correctable fingerprints. Finally, the dose correction model has been applied to compensate for these systematic CD variations and improved CD control was demonstrated. Using a simple dose correction rule, a forty percent improvement in CD control was obtained.
ArF resist lines are tested using scatterometry to study the CD correlation with CDSEM, profile variation caused by baking temperature and pattern environment, as well as the evaluation of optical proximity effect (OPE). Results show reasonable profiles variation predicted by scatterometry spectra from different baking temperatures. Other good matches are the predicted resist line profiles from dark-field and clear-field pattern environment and various line-pitch ratios. They are found to be very similar with the images from the cross-section SEM. On the other hand, the CD linearity and OPE are also found with good matches between scatterometry CD and SEM CD. However, the maximum pitch size tested for OPE is 0.6 μm. More sparse patterns aer believed to have lower sensitivity caused by the weak characteristics spectrum detected. The spectrum sensitivity is another important topic in this paper. The CD and pitch information is contained across the entire spectrum while small profile variations, like t-top and footing, are predicted in the shorter wavelength region. To predict accurate resist profile for small CD, the usage of the shorter wavelength spectrum is inevitable.
SC831: Introduction to Scatterometry Metrology: Theory and Application
This course provides an introduction to the theory of scatterometry metrology, as well as its application for process control in semiconductor manufacturing. Both the software and the hardware portions of the scatterometry system are covered. Scatterometry is an optical method to measure profiles (CD, sidewall angle, height) features on semiconductor wafers and photomasks. Scatterometry is becoming widely adopted for production-worthy metrology as an alternative and complementary tool to CD-SEM and AFM.