A new algorithm for correcting misalignment between layers is introduced which is capable of compensating for interdependencies and arbitrary conventions of correctable factors. In this approach, optimal corrections are determined from solving a set of linear equations that exactly negate the effect of the observed misalignment. A series of calibration runs were performed by measuring the effect of a balanced set of forced input corrections on the resulting alignment in order to determine elements of a transformation matrix. This technique was able to calculate the average corrections required to reverse the input offsets within an average of 10nm for translation and 0.1ppm for magnification and rotation offsets. Estimated standard deviations between calculated and input offsets were smaller for y parameters than x, presumably because of better stage reproducibility in the vertical direction. The transformation matrix for Canon EX4 steppers highlighted that chip rotation is affected by inputting corrections to x axis wafer rotation. Calibration wafers with alignment sites that simulated wafer rotations of +/- 0.8585ppm were fabricated. These wafers verified that chip rotation occurs for EX4 steppers as a consequence of automated adjustment for x wafer rotation. The observed responses from these calibration wafers agreed within nanometers of the relevant element of the transformation matrix for that stepper family.
Optical end of line metrology, OELM, is a new method to measure relative line shortening effects using conventional optical overlay instruments. In this technique, a frame which has two adjacent sides that are constructed of lines and spaces is imaged onto a wafer. Since sub 0.5 micrometers gratings cannot be resolved using conventional optics,the alignment tool sees the sides compared of lines and spaces as solid edges. The purpose of this paper is to characterize errors implicit with this approach. First we show a general error analysis for determining best focus using OELM measurements. From this, we introduce the concept of local image quality as the inverse of the minimum lien shortening, and curvature of line shortening with focus.
Complementary alignment metrology (CALM) is a new metrology technique to visually measure stepper alignment correctable factors such as horizontal, vertical and rotation offsets as well as magnification errors. CALM is based on the concept that a line and space pattern exposed into resist will e completely cleared if, prior to development, it is exposed a second time by shifting the grating by exactly its half- pitch. We have used this principle to fabricate test wafers that visually indicate correctable factors. The estimated 3(sigma) accuracy of CALM readings compared to box-in-box measurements is 0.03 micrometers . Linearity between CALM readings and box-in-box measurements is maintained for misalignments of +/- 0.13 micrometers . Using such a technique allows baseline corrections to be performed on a more frequent basis.