3-D Laser Metrology: Supporting Micro-bump Technology
By Reza Asgari, Rudolph Technologies, Inc.
Emerging micro-bump wafers present unique challenges for measurement and inspection. At the most fundamental level, any viable technology must have the resolution and sensitivity required to measure critical dimensions of micro-bumps. Die with 25-µm (1 mil) bumps on a 50-µm (2 mils) pitch are in development, and smaller bumps with finer pitches are on the way. Additionally, with bump counts rising above 10,000 per die, wafer inspection systems must deal with an increasing number of bumps at both the die and wafer levels. Analysis software and computational hardware must have sufficient capacity to store and process location and dimensional data for millions of bumps per wafer.
The latest bump inspection tools use laser triangulation in conjunction with specially designed optics and analytical algorithms to provide high-quality measurements of micro-bump critical dimensions at full production speeds. This technology, already in use with gold bumps on a similar scale, is suited for current and emerging micro-bump applications.
Figure 1. Laser triangulation calculates bump height from the reflection of finely focused laser probe.
In laser triangulation, a finely focused laser scans the wafer surface and an optical system focuses the beam onto the detector (Figure 1). Benefits of applying 3-D laser triangulation to micro-bump metrology include precision, speed, and scalability. Alternative techniques are challenged by the small bump sizes and the spaces between them, as well as the millions of bumps on a fully populated wafer.
Because laser triangulation acquires substrate and bump top data during the probe’s inspection pass, which scans the bumps’ surface and surrounding substrate in a single pass, it eliminates the error and uncertainty inherent in measurements derived from multiple scans at different heights. The single-pass method yields measurements in the z-dimension with accuracy and precision that approaches the theoretical resolution of the receiver element — more than one order of magnitude better than bump dimensions.
Figure 2. A full wafer scan can acquire 40,000 data points/mm2.
In addition to gauge-capable accuracy and precision, laser triangulation provides production-capable speed, with data acquisition rates sufficient to support full-wafer inspection at production-level throughputs. In full-wafer mode, the system scans the entire wafer with a series of adjacent scans each covering a swath 600-µm wide at a typical data density of 40,000 data points/ mm2 (Figure 2). Variable sampling density allows for optimization of the measurement process to best meet the resolution, precision and throughput requirements of a particular application and will readily accommodate continuing reductions in bump diameter and pitch. Data density can be varied to increase or decrease number of data points based on feature size and throughput required. The system can perform full-wafer inspection at high throughputs. By implementing a sampling plan that measures only designated die, it is possible to further increase throughput while maintaining the statistical validity of the measurements.
Spacing between data points on the inspection system is user-selectable in both the X and Y directions. This inherent scalability permits the user to optimize the measurement process by tuning the sampling density to meet the speed, resolution, and precision requirements of a particular application. As bump sizes continue to decrease, the sampling density can be increased to maintain the optimal balance of measurement quality.
Line Scan Camera
Volume calculations are critical in characterizing and monitoring bump processes. For example, a bump exhibiting out-of-range volume suggests a possible upstream issue at the mask level. Accurate volume measurements require accurate height and diameter measurements. A time-delay integration line scan camera ensures the same high-level of performance for diameter measurements that laser triangulation provides for height measurements. Together, they permit volume measurements with the accuracy, precision, and speed required to control micro-bump processes in production applications.
Figure 3. The intuitive, visual presentation of bump measurements permits recognition of trends at both the die and wafer level.
Color contour plots show bump height, coplanarity, bump diameter, and volume for every bump on the wafer (Figure 3). The intuitive, visual presentation permits recognition of trends in the measurement data at both die and wafer level, facilitating rapid, accurate detection of process excursions, determination of root causes, and evaluation of corrective actions.
Primary defects to be detected by inspection are bridging defects where solder bridges the gap between bumps, and missing bumps.
Diameter and height are primary measurements. Volume measurements are calculated from them. Coplanarity (figure 4) is derived from bump heights in a single die and may be reported as the best fit plane, which minimizes the sum of the squares of the deviations of the bump heights from the plane, or the seating plane, based on the three bumps that determine the plane of first contact.
Figure 4. Coplanarity can be reported as the best fit plane or the seating plane.
One manufacturer verified the uniformity of their bump process, reporting standard deviations of approximately 0.5 µm for measured diameters and 1.5 µm for measured heights. They also found consistent mean values for diameter and height between wafers and good correlation between calculated bump volume and the estimated volume of the molds used to create the bumps. The chips in this process each contained approximately 11,000 bumps, resulting in a total bump count of nearly 9 million for a fully populated 200-mm wafer.
Demand for high-density micro -bump connection technologies continues to increase. Laser triangulation provides inspection and metrology capability required to control micro-bump processes, with resolution, precision, flexibility and speed sufficient to support developing geometry and decreasing sizes and pitches.
Reza Asgari, product marketing manager, wafer scanner may be contacted at Rudolph Technologies, Inc., One Rudolph Road, Flanders, NJ 07836 USA; 952/259-1647; Email: Reza.Asgari@rudolphtech.com.