Michael W. Metzger, Nikon Instruments Inc.
Semiconductor companies currently produce a large assortment of three-dimensional architectures for semiconductor integration and packaging. End users rely on diverse stacking strategies to conserve PC board space. Even stacking die to wafer and wafer to wafer is under evaluation for more efficient 3-D integrated circuit (IC) production. As a result, Z-height inspection of bumps, wires and other key features and packaging components is becoming more critical to control and improve process yields.
The need to reduce the size of components across the IC for more density, input/output, etc., is driving a shift toward noncontact techniques for characterization of high- and low-density interconnects. Such measuring strategies range from optical coordinate measuring machines to optical profilers based on interferometry and confocal microscopy. Within the noncontact category of instruments, two fundamentally different metrology strategies are emerging — one designed to characterize spatial coordinates specifically within the field of view of the objective lens, and one that can concentrate spatially within an entire machine coordinate system.
Optical coordinate measuring machines, which are often called automatic noncontact vision systems or possibly video measuring systems, fall into this latter category. In semiconductor applications, the key strength of these instruments centers on their ability to generate sophisticated coordinate relationships for components such as wires, pads, posts and bumps. Unlike with optical profilers, end users can optimize the definition of these coordinate relationships by including data that is essentially gathered from a larger coordinate system. It also is possible to compare information in one field of view with that from another that might be several fields of view away.
Although optical profilers and confocal microscopes can measure in great detail within a specific field of view, they are not as strong at relating information to other fields of view. Such techniques can gather large amounts of 3-D data within the objective lens’s field of view, but that information is limited in scope. It must then be stitched together with neighboring boxes to create measurement relationships outside of any single field of view.
The accuracy of optical coordinate measuring machines, such as Nikon Instruments Inc.’s Nevix VMR H3030 with dual measurement heads, is ultimately a product of the resolution of the optics and reference scales, as well as the mechanical positioning capabilities of the stage and Z-axis travel.
Increasingly, options for optical coordinate measuring machines include high-numerical-aperture microscope lenses, dual optical systems, through-the-lens lasers for Z-axis height measurements, 0.01-μm precision staging and multiple types of advanced illumination. The value of employing differing kinds of illumination, such as oblique or on-axis options, lies in the ability to enhance contrast.
During inspection with an optical coordinate measuring machine, a precision positioning stage will move the object to be measured across the field of view. To measure Z height, the laser beam is directed through the lens to a pinpoint focus on the object. The beam, used like a stylus, will reflect off the object and back through the lens to a detector, with feedback analyzed for focus position. The computer system then tells the objective lens and laser to move either up or down until the signal received at the detector indicates a null reading. Once the null condition exists, the lens is in perfect focus at the object plane.
At this time, a precision Z-axis scale can record the exact Z-position at that point on the sample. In addition, the system will rely on video edge detection techniques to acquire X-Y coordinates anywhere in the machine’s measuring envelope. Object features are selected as points, edges or planes for measurement through part programming in the part or machine coordinate system.
One of the most critical benefits with this type of measurement scheme — at least for integrated circuit manufacturing — is its speed. Feature measurements can be relatively fast compared with other techniques because information is gathered only as needed to capture feature-to-feature relationships.
Meet the author
Michael W. Metzger is department manager for measuring instruments at Nikon Instruments Inc. in Melville, N.Y.; e-mail: firstname.lastname@example.org.