By Pete Singer, Editor-in-Chief
Last year, Rudolph Technologies, Inc. announced the widespread adoption and success of its newest macro defect inspection tool, the NSX® 330 Series. The NSX 330 Series provides high-speed macro defect inspection with 2D\3D metrology for advanced packaging applications, which are being developed primarily to support mobility. The company said it had been “quickly and enthusiastically adopted,” garnering repeat orders from top foundries, integrated device manufacturers (IDMs) and outsourced assembly and test (OSAT) manufacturers.
The NSX 330 Series offers an array of metrology capabilities for both 2D and 3D metrology applications, including 100 percent bump height and coplanarity measurements. The NSX 330 series has now been further improved by incorporating a high speed bump laser triangulation sensor and the highly accurate VT-SS distance and thickness sensor. “We specifically offer these capabilities on a single platform because they improve total measurement accuracy on complex materials which have troubled the industry for some time,” said Scott Balak, director, inspection product management, Rudolph Technologies Inc. (Bloomington, MN).
Figure 1 illustrates the problem. The goal is to measure the actual bump height and overall coplanarity from bump top to polyimide (PI) surface. If one or more bumps are too high or too low, the other bumps won’t connect. A high-speed laser triangulation sensor attempts to see through the polyimide (PI) layer, which is typically 3-6 microns thick. “The problem is that polyimide isn’t completely transparent, so when the triangulation sensor attempts to detect the bottom of this PI layer, it is actually finding it somewhere in the middle. The current industry’s work around is to assume a PI thickness and apply a PI layer offset; however, PI thickness variation limits the accuracy of this approach,” Balak explained.
Inaccurate measurements create unnecessary review work. Because bumps may have acceptable coplanarity, but they are incorrectly flagged for further evaluation. “Customers use the review mode to determine if the bump is actually too big, or too small” Balak said.
Enter Rudolph’s Visible Thickness and Shape Sensor (VT-SS) sensor, which can concurrently measure the transparent layer thickness as well as the metal feature step height above the surface of the transparent layer. This is achieved through the integration of reflectometry and visible light interferometry principles. The direct reflection from the transparent layer provides direct thickness measurement of the transparent material, while the interferometry captures topography (distance from the sensor), allowing the system to measure the thickness of the opaque metals by scanning over the edge of the feature.
“Rudolph samples multiple bumps with both the laser triangulation and VT-SS sensors to accurately obtain a measurement average of wafer PI thickness while simultaneously calibrating the triangulation sensor with an accurate PI offset for the specific wafer being measured.,” Balak said. “The properly calibrated triangulation sensor then quickly and accurately measures millions of bumps per wafer correctly flagging bad bumps and eliminating the need to review good product. Wafer results are then sent to our Discover Analysis solution where customers can analyze correlations between defectivity and process metrology to improve the overall process. Whether it is understanding wafer and lot level trends or specific individual bumps; Discover provides the drill down capability required for root cause analysis.”