We will present a complete example that demonstrates daily CD monitor for good CDSEM control, including sampling plan, monitoring procedure, and monitoring and matching data for multiple CDSEM. In addition, we also investigate two methods to address the carbon contamination problem. In the first method, carryover trends on three different film stacks, poly, metal, and multi-layer metal, before and after plasma clean are compared in search of ways to minimize carryover. The second method applies statistical treatment to remove the effect of carryover while maintaining sensitivity over small fluctuations in line CD monitor results. Both linear regression and exponentially weighed moving average calculated from daily monitor data are used to model the baseline carryover trend for the purpose of isolating tru tool variability. Using this method, we can easily quantify the long-term stability of each CDSEM, and with that, we are able to calculate the true long-term process variation Cp by subtracting the CDSEM variation component from the observed total Cp.
We studied the effect of ArF resist shrinkage under electron bombardment during ebeam metrology and also the effect of resist shrinkage on the after etch CD. The traditional approach is to reduce the electron energy and dose to minimize resist shrinkage, often at the cost of reduced precision and image quality. We found that resist trimming by high-density plasma etcher (ion density about 1012cm-3) can improve the stability of resist under ebeam. Exposed to beams of 600V and 300V accelerating voltage, fresh photoresist CD shrinkage was reduced by ~70% and ~50% after resist trimming in the etcher. The effect of resist trimming is similar to that of e-beam curing. More interestingly, after etch and clean of the wafer, no difference in average CD value was found between area exposed to ebeam measurement and area that were not measured. This suggests that the resist trimming step in the normal etching process may overwhelm resist shrinkage effect caused by ebeam metrology. The implication is that the key selection criteria for stable ebeam metrology on ArF resist is a beam that produces consistent shrinkage, not minimum average shrinkage.
This paper describes the design and implementation of a system for monitoring the performance of several major subsystems of a critical dimension measurement scanning electron microscope (CD-SEM). Experiments were performed for tests involving diagnosis of the vacuum system and column stability by monitoring of the following subsystems and associated functional parameters. These include: 1) Vacuum system with pressure as a function of time being recorded for the electron-optical column (gun chamber), the specimen chamber, and the sample-loading unit. 2) The action of several components of the wafer handling system can be timed. 3) The electron gun emission currents and other signals to monitor the characteristics of the condenser and objective lenses may be used to correlate with image quality. 4) Image sharpness, electron beam current, signal-to-noise ratio, etc. can be evaluated.
This paper presents results obtained using a low-voltage critical dimension scanning electron microscope (CD SEM) for the imaging and measurement of features patterned on quartz photomasks. The SEM system used was designed for handling silicon wafer substrates and has been adapted to accommodate 6', 250 mil photomask substrates. The scope for this initial characterization is limited to the two most common reticle metrology applications on current technology photomasks; patterned PBS photoresist features on chrome, and patterned chrome features on quartz. Images, data, and analysis of the characterization results are presented, and the unique difficulties of imaging and measurement of each of these two types of samples is discussed.
Fully automated, multi-mode CD-SEM metrology, utilizing both backscattered electron (BSE) and secondary electron (SE) detection, has been benchmarked to 180 nm critical dimensions using patterns generated by deep-UV lithography. Comparison of pure BSE with conventional SE SEM data used in a study of across-chip linewidth variation (ACLV) revealed that heterogeneous system matching depends on feature orientation as well as an offset between BSE and SE intensity profiles. The corresponding AFM data show that the BSE measurements are more accurate and less sensitive to feature orientation and sample charging. Using the multi-mode system, we found that SE profiles had a higher signal-to-noise ratio while the BSE profiles gave a better representation of the actual line shape. Static and dynamic measurement precision below 2 nm has been achieved with BSE on etched polysilicon. Move-acquire- measure (MAM) times at this precision were under 10 seconds per site. Models for orientation-independent measurement, generic wafer throughput, and overall equipment effectiveness were used to address the issues of system matching, tool productivity, and factory integration, respectively.