1. Field of Invention
This invention relates to the fabrication of electronic chips and, more particularly, to a system and method for aligning a stencil mask with the bonding pads of an electronic chip.
2. Description of Prior Art
The screen printing of solder paste with very fine pitch dimensions for the assembly of electronic (Flip) chips to chip carrier substrates is becoming more common in the art. Screen printing entails depositing a circuit pattern corresponding to that of the stencil pattern onto a chip. A chip is placed onto a positionable stage and lifted to a point just below a screen having solder paste thereon. A squeegee then moves horizontally across the screen to push solder paste through the screen to produce a stencil pattern.
Chip attachment site fiducials are commonly used for alignment of the stencil mask to the actual chip carrier substrate to be printed. As the density of chips has increased dramatically, so have the chip carrier substrates, and the attachment site fiducials cannot be added to the design because they use up too much surface area on the substrate. Indeed, special technology is needed to increase the density of contacts on the surface of the chip, and space is at a premium.
Flip chip interconnects are one way in which the density of chips is maximized. Flip chip technology involves the electrical and metallurgical joining of a chip and a carrier to form a package. The chip, or die, has an array of die pads each having a solder bump positioned thereon. These solder bumps are commonly referred to as C4s, an acronym for controlled collapse chip connection. A chip having an array of die pads and C4 bumps may be flipped over so that the C4s can be die-bonded to the contacts on a chip carrier or substrate, thereby electrically interconnecting the chip and the carrier.
In order to properly align and print C4 solder bumps onto the corresponding die pads, conventional screen printers use a vision camera, such as the Vision Probe system available from Speedline Technologies of Franklin, Mass. (formerly MPM Corporation). Such cameras, however, are limited to approximately twenty microns minimum for feature size due to the resolution of the visual imaging systems. Therefore, pads whose sizes are 5 microns or less cannot be used as alignment targets. This problem is further complicated by the fact that the commonly used ceramic substrates present very little contrast with the pads. Therefore, screen printing is only of use in conjunction with high contrast substrates or large diameter pads.
In optical alignment systems, visible light is employed to perform measurements in alignment systems. However, when finer measurements need to be made, x-ray technology can be employed. Because of their smaller wavelengths, x-rays can provide resolution beyond the limits of conventional optical parameters.
In U.S. Pat. No. 4,016,416 issued to Shepherd et al for “Phase Compensated Zone Plate Photodetector,” a zone plate with a photodetector mounted on the opposite face is illustrated.
In U.S. Pat. No. 3,984,680 issued to Smith for “Soft X-Ray Mask Alignment System,” an x-ray mask alignment system is illustrated featuring x-ray fluorescence detectors mounted upon the mask. The x-ray detectors measure the x-ray fluorescent signal, which provides a low intensity output as compared with an electron flux.
In U.S. Pat. No. 4,614,433 issued to Feldman for “Mask-to-Wafer Alignment Utilizing Zone Plates,” a mask to-wafer alignment using zone plates illuminated by light during alignment is illustrated.
In U.S. Pat. No. 6,272,202 issued to Chiba et al on Aug. 7, 2001 for “Exposure Method and X-Ray Mask Structure for Use with the Same,” an exposure method for printing circuitry onto a silicon wafer is illustrated.
In U.S. Pat. No. 6,237,218 issued to Ogawa et al on May 29, 2001 for “Method and Apparatus for Manufacturing, Multilayered Wiring Board and Multi-Layered Wiring Board,” a method of using alignment marks during the lamination steps and a specialized x-ray vision and mechanical alignment machine are illustrated for fabricating printed wire boards.
In U.S. Pat. No. 5,168,513 issued to Maldonado et al on Dec. 1, 1992 for “X-Ray Metrology and Alignment Detection System,” a process for aligning an x-ray mask and a work piece with an alignment mark is depicted.
In Japanese Disclosure Document No. JP05-315215 issued to Koji in 1993 for “A Semiconductor Manufacturing Apparatus,” an alignment method using x-ray radiation through an aperture is shown.
In Japanese Disclosure Document No. JP62144325 issued to Shinichi on Jun. 27/1987 for “Positioning Method,” the alignment of a wafer is shown using a fluorescent screen and a metal shielding x-ray pattern.
Objects and Advantages
It is a principal object and advantage of the present invention to provide an improved method and apparatus for aligning a low contrast substrate with a fine pitch printing screen having apertures of less than 125 microns.
It is another object of this invention to provide an x-ray lithography system for aligning a work piece with a fine pitch mask.
Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter.