Optical critical dimension (OCD) measurements for profile monitoring and control: Applications for mask inspection and fabrication

Optical Critical Dimension (OCD) measurements using Normal-Incidence Spectroscopic Ellipsometry (polarized reflectance) allow for the separation of transverse electric and transverse magnetic modes of light reflected from an anisotropic sample as found in a periodic grating structure. This can provide the means for determining linewidths and analyzing complex profiles for a variety of structures found in mask fabrication. The normal-incidence spectroscopic ellipsometer maintains much of the simplicity in mechanical design found in a standard reflectometer and the additional polarizing element has no effect on the footprint making the system amenable for integration, inline monitoring and advanced process control. The rigourous coupled wave analysis (RCWA) method provides an exact method for calculating the diffraction of electromagnetic waves by periodic grating structures. We have extended OCD technology to critical measurement points in the mask fabrication process: After development inspection (ADI), where OCD evaluates mask writer performance and after etch inspection (AEI) for monitoring and control of etched quartz structures for phase shift applications. The determination of important, critical dimensions via optical techniques is appealing for several reasons: the method is non-destructive to photoresist and the sample is not subject to charging effects; the technique is capable of measuring the critical dimensions of grating structures down to approximately 40 nm; minimal facilities are required for installation (no high vacuum, cooling or shielding, of electromagnetic fields). Results will be presented showing the capabilities of OCD metrology for ADI and AEI applications. Comparisons will be made with both CD-SEM and X-SEM and the application to monitoring/controlling the quartz etch process will be discussed.