Semiconductor object pre-aligning method

DETAILED DESCRIPTION The present invention includes a prealignment apparatus for semiconductor objects which minimizes or obviates the above problems.
For one thing, the object engagement means or chuck holds the object in a non-slip manner, and motive means are included for adjusting the position of the engagement means and the object together as necessary to make the geometric center of the object precisely coincident with a chosen position.
In other words, the position adjustment of the wafer or other semiconductor object is made without disengaging the chuck from the wafer.
Thus, separation of the engagement means from the object during the pre-alignment is eliminated as a potential source of inaccuracies.
Most desirably, the engagement means is a chuck which is mounted on an X--Y table structure for movement in normally related directions in a plane orthogonal to its rotational axis.
The invention also includes an alignment method which eliminates inaccuracies which typically are provided in the mathematical determination of the location of a wafer center.
Most of such determinations rely on relatively complicated functions relating the distances between a wafer rotation center and points sensed on a wafer edge as the wafer rotates.
These functions are difficult to solve exactly, leading to inaccuracies.
For example, most calculations rely on a least square sine fit with a curve defined by a relatively complicated equation which relates the current distance between an edge point and the center of rotation to the rotation angle.
While this provides an approximate sine curve, making the use of a least square sine fit approach feasible, the fit is an approximation.
In the instant invention an elegant equation is obtained by eliminating parts of the original equations.
For example, an equation defining an exact sine wave is obtained with the preferred embodiment making the least square sine approach resolving the equation provide exact results.
This is accomplished by combining two measurements on opposite sides of the center of rotation in such a way that portions of the original equations are eliminated