The focus and overlay alignment of photolithographic exposure tools of the type wherein the location of the wafer is accurately tracked with respect to a baseline position, such as in step and repeat cameras, are evaluated by monitoring the output signal generated by a photodetector in response to the light radiated from one or more periodic test patterns carried by a re-useable calibration wafer while such a test pattern is being exposed to an aerial image of a matching calibration mask. Overlay alignment suitably is evaluated by stepping the pattern on the wafer from side-to-side and fore and aft of the aerial image while monitoring the photodetector for a peak output signal, whereby overlay alignment errors along the x-axis and y-axis of the exposure tool are determined by the displacement of the wafer positions at which such peak signals are detected from the positions at which such peak signals ae expected. Focus, on the other hand, suitably is evaluated by incrementally defocusing the imaging optics of the exposure tool while monitoring the rms width of the output signal generated by the photodetector as the wafer pattern is stepped through the aerial image of the calibration mask at each focal setting. The best focal setting minimizes the rms width of the output signal.

Focus and overlay characterization and optimization for photolithographic exposure

The use of moire fringe techniques in microfabrication equipment is reviewed with special emphasis on mask-to-wafer alignment. The alignment strategies and associated detection methods of a number of commercial wafer steppers are described and their limitations are discussed. Potential improvements to be gained by holographic detection of grating targets are pointed out and future applications in a dynamic exposure system are proposed.

Use of gratings and periodic structures as alignment targets on wafer steppers

This paper presents a new overlay metrology target design and scheme referred as MoireOVLTM. It utilizes Moire patterns of two overlapping gratings to amplify the kernel response to overlay misalignment and thereby has the potential to enhance kernel sensitivity, detectability and measurement accuracy. A Self-referenced (SR) MoireOVL design scheme, which enables MoireOVL to be measured with existing image-based overlay tools is proposed and evaluated on a contact layer. This paper demonstrated the feasibility of SR-MoireOVL on existing IBO tools. When comparing to the reference SEM-based overlay, a magnification factor of 6.9X with an R2 of 0.96 and a calibrated intercept of 0.34nm was observed on wafer. Comparison between MoireOVL and POR IBO on TIS, residuals, precision and TMU is presented. Lastly we present the idea of a Self-calibrated (SC) MoireOVL scheme to calibrate the magnification factor on the fly during measurement for enhanced usability.

Self-referenced and self-calibrated MoiréOVL target design and applications

Real-time focus and overlay measurement by the use of fluorescent markers (poster)

In lithography, overlay control is getting increasingly complex. Advanced Process Control (APC) is introduced to minimize excursions from the process window for the present exposure. APC uses metrology data of previously exposed wafers, hence, there is always a delay of tens of minutes before the required information is available. This paper proposes the combination of a patterned expose beam and a patterned fluorescent marker on a wafer to generate a fluorescent signal that carries real-time information of the focus and/or position error of the expose pattern with the pattern on the wafer. A practical realization requires some changes to the exposure process, stepper design and reticle lay-out. Firstly, a matched pair of markers on the wafer and reticle is required. Secondly, the generated fluorescent signal must be measured, for example with a (spectrally filtered) photon counter close to the expose area of the wafer. At last, the markers from the previous lithography step shall, after development, be filled with fluorescent material. This deposition requires an additional process step. Photon budget calculations suggest an overlay measurement accuracy of less than a tenth of a nm (real-time).

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Real-time focus and overlay measurement by the use of fluorescent markers

Overlay performance is usually determined by exposing wafers twice, processing the wafer, and then using vernier or electrical probe techniques to measure the overlay error. In this paper, we describe a very different technique where the overlay error is evaluated in situ on the lithographic exposure tool. The basis of this approach is the examination of moire fringes between the projected image of a grating test mask and a grating test wafer. This is a particularly attractive approach to lithographic metrology because the moire fringes contain information about both resolution and overlay error. A measurement instrument has been constructed which attaches to a MicralignTM projection printer. The fringe intensity is measured by a solid state detector array positioned on an image of the illuminated arc-shaped field. Since the overlay measurement can be done many times per second, we can directly observe vibrations between mask and wafer.

T. A. Brunner

Papers

Moire Technique For Overlay Metrology