Scatterometry is currently being used in lithography production as an inline metrology tool to monitor wafer processing
and detect excursions. One well-documented excursion is the process variation caused by differences in resist batches.
This paper describes the use of Tokyo Electron Limited's integrated Optical Digital Profilometry (iODPTM)
scatterometry system to detect process variations caused by resist batch changes. This system was able to detect a
significant shift in resist sidewall angle (SWA) on an incoming resist batch that was undetected by the primary
metrology in use at the time, a scanning electron microscope (CD-SEM). This SWA shift correlated to an undesirable
shift in post-etch CD created by the new resist batch. Experiments performed in conjunction with the resist supplier
confirmed that the normal batch-to-batch variation of a key resist component was enough to produce a change in SWA
after processing. This validation led to quality improvement controls by the resist vendor and Qimonda and resulted in
the use of iODP as the primary metrology for this process.
In this paper, three different types of spectral scatterometry hardware are compared using Timbre Technologies' Optical Digital Profiler (ODP) as a common software platform. The hardware under consideration includes a spectroscopic reflectometer (R), polarizing spectroscopic reflectometer (RP) and a spectroscopic ellipsometer (SE). Four advanced lithographic applications are evaluated-two from Spansion's 110-nm Flash memory technology line, and two from AMD's 90-nm logic process. ODP models are developed and optimized for each application and each type of hardware. Results include static and dynamic repeatability, throughput, correlation to incumbent metrology and correlation to cross-section. For each application, the authors also attempt to determine the level of model complexity supported by each hardware type, with special attention paid to the relative sensitivity of each system to changes in critical dimension (CD) and resist profile. The results generally indicate that the SE is the most sensitive hardware type while the R is the most stable. The RP occupies some form of middle ground on both counts. These generalizations are largely application dependent and clear differentiations do not always exist. Selecting the right spectral scatterometry hardware, therefore, is a function of one’s application complexity and control objectives.
Tool matching and optimal process control are critical requirements for success in semiconductor manufacturing. It is imperative that a tool’s operating conditions are understood and controlled in order to create a process that is repeatable and produces devices within specifications. Likewise, it is important where possible to match multiple systems using some methodology, so that regardless of which tool is used the process remains in control. Agere Systems is currently using Timbre Technologies’ Optical Digital Profilometry (ODP) scatterometry for controlling Nikon scanner focus at the most critical lithography layer; logic gate. By adjusting focus settings and verifying the resultant changes in resist profile shape using ODP, it becomes possible to actively control scanner focus to achieve a desired resist profile. Since many critical lithography processes are designed to produce slightly re-entrant resist profiles, this type of focus control is not possible via Critical Dimension Scanning Electron Microscopy (CDSEM) where reentrant profiles cannot be accurately determined. Additionally, the high throughput and non-destructive nature of this measurement technique saves both cycle time and wafer costs compared to cross-section SEM. By implementing an ODP daily process check and after any maintenance on a scanner, Agere successfully enabled focus drift control, i.e. making necessary focus or equipment changes in order to maintain a desired resist profile.
A series of experiments were performed to determine if the ThermaWave INTEGRA CCDi reflectometer combined with Timbre Technologies’ Optical Digital Profiler (ODP) software could meet the requirements for lithography cell integration and process control of critical 0.13-micron Flash memory applications. Shallow Trench Isolation (STI), First Poly Gate, Stacked Gate and Aluminum Interconnect layers were examined as a part of this study. ODP models were developed for each of these applications and their output was compared to Critical Dimension Scanning Electron Microscopy (CDSEM) and cross-section SEM to demonstrate adequate correlation to incumbent metrology techniques. ODP is shown herein to correlate to CDSEM while providing the throughput required to measure every wafer without creating a bottleneck for the lithography cell. Experimental results also suggest that, in many cases, ODP can deliver profile determination beyond the fundamental capability of standard in-line metrology techniques.
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