1 June 1990 Technique for the determination of best focus for ultratech 1X steppers in a production environment using aerial image analysis
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A new version of the Stepper Image Monitor (SIM) has been designed to evaluate best focus on Ultratech steppers. The SIM is a portable unit which will support a number of steppers in a fab. Each stepper has a permanently mounted detector assembly which uses a mirror to pick off the dark field image (except for a small portion required for Ultratech alignment) above the fold mirror. To run SIM, a chrome on glass amplitude diffraction grating with many 1 - 1.Sum wide by 1mm long windows on a Sum pitch is placed in the reticle position on the stepper. A SIM wafer with a similar number of 03 - 1.Oum wide by 1mm long bars on a Sum pitch is placed on the stage. The wafer is instructed to move by an external Run Mode 8 Ultratech stepper program, first in X and then in Y, across the Sum pitch in a number of discrete steps (typically 20). The intensity values measured by the SIM detector at each step are used to construct a discrete intensity profile that represents the aerial image of the grating. This procedure is repeated at several Z positions (typically 5). Each intensity profile is correlated to the appropriate diffraction limited intensity profile for the system being used. A parabolic fit is made from the correlation values at the various Z positions. The Z value for the maximum of the parabola is considered to be best focus. This method has several advantages over other methods currently in use for checking Ukratech focus: (1) More precise measurement; (2) Operator independent; (3) Faster; (4) No effects due to the resist or to processing; (5)SIM is a permanent artifact (i.e. no variation due to wafer differences). Results from beta site testing show that the method is very repeatable, with sigma =0.lSum being typical. SIM results also correlate very well to results obtained by conventional methods. It also tracks well to changes in Z offset dialed into the stepper. SIM has been shown to be an effective tool for quantifying the relationship between lens heating and focus shift on the Ultratech stepper. These improvements in speed and precision of focus measurements on the Ultratech stepper will lead to more usable stepper time and better stepper performance, which in turn translates directly into more throughput and higher yields. OAI would like to acknowledge the help of Suzanne Scullen and Synergy Semiconductor Corporation for their help and the use of their Ultratech stepper.
© (1990) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Victoria L. Rivera, Victoria L. Rivera, Joe Shiefman, Joe Shiefman, David Chui, David Chui, "Technique for the determination of best focus for ultratech 1X steppers in a production environment using aerial image analysis", Proc. SPIE 1261, Integrated Circuit Metrology, Inspection, and Process Control IV, (1 June 1990); doi: 10.1117/12.20080; https://doi.org/10.1117/12.20080

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