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Vector Raptor 

Advanced Overlay

Simulation & Lot Correction Optimizer

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Simulation Video

Lot Correction Video

Allentown, PA (USA). Sept. 24, 2007 -- TEA Systems has announced the release of a new features and tools for Vector Raptor overlay and double patterning lithography (DPL) control.

The results from a calculation of the systematic variations in overlay modeled data can be used as a feedback mechanism to scanner positioning or as a highly accurate method of estimation of positioning errors in locations on the field, wafer or lot not actually measured. Two tabs appear after a modeled data analysis as shown in figure Sim1. These tabs provide utilities to assist in the evaluation of lot-correction techniques and in the visualization of performance through simulation.

Full-wafer overlay Simulation employs the systematic errors of the process to estimate distributions and is significantly more accurate than a corresponding statistical calculation that assumes a random distribution of the process errors. As a result this is the preferential method of advanced users for pass/fail or lot-gating metrics in the lithography sequence. The method however is rarely used for IntraField analyses and the full potential of the technique has therefore not be realized by the industry.
Lot Correction Optimization tools within Vector Raptor provide a method of examining the effects of various methods of correction averaging. Obviously the engineer cannot calculate lot pre-exposure corrections for every single field or wafer-quadrant on the lot. Corrections therefore need to be averaged and then inserted into the pre-lot recipe for exposure. The problem then arises on what level of averaging should be used?

Should corrections use the average of all wafers and fields measured in a calibration sequence or should more sophisticated alignment schemes employ finer levels of averaging?

Figure Sim-1: (Left) Raw data with overlay rotation                            (Right) Field-model fitted overlay meeting distortion by using only the Offset coefficient.

The following dataset illustrates the need for a Lot Correction Optimizer. The plot on the left side of Figure Sim-1 replicates a set of data on a wafer that contains only a wafer-rotation error.

If a simple field model of the form dx = A +BX –CY is applied to each field on the wafer, then the overlay distortion can be accurately calculated as shown in the figure on the right side of Sim-1. Notice that the model only needed to fit each individual field’s offset in order to successfully match the errors found on the wafer. The average field offset for the wafer is (X,Y) =(-0.586, -1.125) nm

If, as is common in production, the average field offset value were then used to correct for overlay error in the lot, the correction would be applied as shown in the left side of figure Sim-2. Unfortunately for this type of error, the wafer-rotation is not corrected and will show up in the residuals to the average field as is shown in the right side of figure Sim-2. This implies that any product lots exposed using the field-average correction on this tool/reticle combination will also result in an overlay distribution as shown in these residuals.

Figure Sim-2:  (Left) Average field offset correction applied to lot exposure                            (Right) Resulting residuals to average field-model correction.

Vector Raptor’s Lot Overlay Correction Optimizer provides the tools needed to discover the results that can be obtained through the use of lot-coefficient-averaging of corrections. Averaging schemes for lot, wafer and field based correction averaging can be combined with selective inclusion of individual coefficients to exposure the optimum strategy for minimization of errors due to processing, bake, lens/reticle heating, film stress and other highly systematic errors.

TEA Systems, a privately held corporation since 1988, specializes in advanced, intelligent and adaptive process modeling of the photomask, semiconductor process and it's toolsets. Products from TEA allow the user to decouple process, tool and random perturbations to enhance process setup, control and yield.

TEA Systems Corp. | Tel: +1 610 682 4146
65 Schlossburg St., Alburtis, PA USA

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