Industry trend toward devices with higher bump densities is driving a need for advances in test equipment for measuring the uniformity of bump height across a device. As bump height decreases, accuracy requirements become tighter. As bump lateral size decreases, resolution required to inspect those bumps increases. Inspecting at higher resolution typically results in decreased throughput so the equipment must be faster. To address these needs, Rudolph Technologies, Inc. has developed a new generation laser triangulation 3D inspection sensor.
With laser triangulation, distance to one or more points on a wafer is measured by projecting structured laser light on the wafer at one angle and viewing the pattern of light reflected off the wafer from another angle. Bump height is determined by subtracting the distances to the bump top and surrounding wafer surface.
Rudolph’s new generation laser triangulation sensor uses a violet laser line projector positioned 45 degrees from the wafer normal to generate a very narrow line of light on the wafer. The line is viewed with a proprietary, custom-designed, state-of-the-art CMOS camera and high-end microscope optics positioned 45 degrees from the wafer normal opposite the projector. Distance to over 1000 points along the line are measured for each camera frame. Inspection rates three to five times faster than previous technology can be achieved with improved accuracy and repeatability. Tests show that a 300mm wafer with 16 million pillar bumps can be inspected in as little as 3 minutes at resolutions appropriate to achieve 0.15um 3sigma height repeatability. Bump heights ranging from 2um to 350um can be measured in a single pass.
In this paper, we will describe the new sensor in detail and present performance test results for a copper pillar bump wafer.