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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.
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