1. Field of the Invention
This invention is related to laser systems. In particular, this invention deals with adjusting alignment of laser beams in laser systems.
2. Description of the Related Art
When fabricating memory circuits, a laser repair system can be used to selectively sever conductive links, effectively removing faulty memory cells from the circuit.
As the size and spacing of link elements decreases, laser repair systems have had to increase in accuracy in order to perform their intended function. The complexity of a laser repair system capable of such accurate operation is significant. Multiple mirrors and other optical elements are used to generate and position a laser beam spot for severing a conductive link. Like the circuit fabrication process itself, laser repair systems are subject to many complex factors. For example, thermal expansion may lead to changes in the orientation or position of optical elements in the path of a laser beam. These changes to the elements that affect the laser beam can cause the laser beam spot to drift away from its intended location and can cause errors when trying to repair a circuit. Although the beam spot position is aligned with reference to wafer alignment markers with every new wafer processed, a misaligned laser beam path that deviates from a normal orientation to the work surface can still produce beam spots of unintended location, shape and/or size which adversely affect operation of the repair system.
U.S. Pat. No. 6,483,071 (hereinafter referred to as the '071 patent) entitled “Method and system for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site” is assigned the assignee of the present invention. The disclosure of the '071 patent is hereby expressly incorporated by reference in its entirety. The '071 patent discloses many features of a laser based system for memory repair, and is particularly related to accurate (sub-micron) and high-speed positioning of a laser beam waist relative to a link or similar target structure. In the '071 patent, an air-bearing based assembly was disclosed for positioning of optical components (e.g: an objective lens) along the optical (Z) axis. In addition to noise and reliability issues (ie: wearing mechanical parts) it was recognized that X,Y displacement errors during Z axis motion are much better controlled or eliminated with an air bearing system. Such displacements, even if a fraction of a micron, can lead to link severing results which are incomplete (e.g. contamination) or possibly cause damage to surrounding structures. Hence, a displacement of a laser beam from a target location by a fraction of one-micron, corresponding to a fraction of one spot diameter, may generally lead to reduced yield.
Traditionally, laser repair systems have undergone periodic, manual adjustment to correct problems with alignment. For example, every month, a trained technician may have to manually adjust optical elements in order to correct alignment problems that have developed since the last adjustment. In the M430 laser link blowing machine from GSI, coarse adjustments to laser beam alignment were made by manually adjusting the laser beam orientation while viewing the laser beam spot with a “thru-lens viewing system” (TTLV). The TTLV is essentially a camera and TV monitor arrangement coupled to the laser beam path. The spot position was determined relative to a crosshair. The beam was first aligned to be centered in the lens aperture. Then the beam was aligned for zero spot translation during zoom expansion. Zoom adjustments corresponded to a range of spot sizes. If the beam was properly aligned along the Z-axis, the beam would appear stationary on the monitor for all zoom settings. Finer beam alignment was carried out by adjusting the spot size to a minimum, placing a calibration grid on the work surface, and performing iterative manual adjustments of turning mirrors to align the optical system and reduce any lateral (X-Y) displacement to within a specified tolerance.
This traditional approach to adjusting the alignment of a laser beam has several drawbacks. For example, the means used by the technician to determine beam alignment may itself be subject to error. Alignments based on erroneous alignment data may augment alignment problems in the system. Other problems may include the significant time expense involved in manual adjustment. Delays arising from manual alignment can represent a serious cost for businesses operating laser repair systems. For these reasons and others, automated methods of static laser beam alignment have been developed. Such methods are described for example in U.S. Pat. Nos. 5,011,282 to Ream, et al., 5,315,111 to Burns, et al., 5,923,418 to Clark et al., and 6,448,999 to Utterback et al. Of these prior patents, Burns, Clark, and Utterback split off portions of the laser beam to optical detectors placed adjacent to the laser beam path. Alignment of the beam with respect to the detectors is used to deduce alignment of the beam to the workpiece. In the '282 patent to Ream, changes in laser beam spot position on a target are used to determine a laser beam deviation angle, which can then be used to correct the laser beam path alignment.