Roger Posusta, senior marketing application specialist, Nano Surfaces and Metrology Division, Bruker
3D optical profiling provides many advantages over other measurement techniques for non-contact inspection of semiconductor packaging front-end process research and control. These advantages range from fully automated measurements and non-destructive inspection to custom analyses and fast measurement speeds. The most advanced optical profilers provide fast, accurate surface measurements to quantify a variety of surface properties. These systems are used around the world in an extremely wide range of markets, including aerospace, automotive, data storage, material science, medical, microelectromechanical systems (MEMS), microelectronics, precision engineering, semiconductor and solar.
Measurement advantages of white light interferometry
3D optical profilers are able to offer exceptional measurement speeds whilst maintaining the same nanometre Z accuracy at all optical magnifications. This enables the measurement of a very wide range of surface parameters, such as surface roughness, step heights, pitch, curvature, lateral displacement and waviness, all in a single step and on nearly any surface. The coherence scanning interferometry (CSI), also known as white light interferometry (WLI), measurement technique shown in figure 1 can quickly determine surface shape and finish over large lateral areas up to 8 mm and vertical heights up to 10 mm in a single measurement. To measure even larger lateral surface areas, a stitching algorithm can be applied that allows multiple lateral images to be taken and stitched into one image for analysis. These capabilities have led to many metrology applications to meet evolving wafer manufacturing needs.
3D optical metrology analyses for wafer manufacturing
Wafer fabrication typically consists of sequential process steps to build components onto a silicon wafer, which eventually ends up as a complete functional device for a wide range of end products, from computer or memory chips to LEDs. As consumer electronics component sizes continue to decrease, there is a corresponding demand for increased wafer metrology to refine and control the manufacturing of these complicated devices.
Outlined below are application examples where 3D optical profiling is improving the manufacture and performance of wafers. All examples focus on the use and capabilities of Bruker’s ContourGT-X 3D optical profiler, which utilises Vision64 analysis and automation software.
Trace analysis
Most component designs utilise traces for electrical connectivity or device etching on the solid-state device itself. Vision64’s trace analysis tool can automatically measure both horizontal and vertical lines, as shown in figure 2. It then reports the parameters, widths and heights for each trace, including for the surface finish of traces and the spaces between them.
Multiple region analysis
Copper pillar, solder bumps and through-silicon vias (TSVs) are very critical electrical and mechanical manufacturing connections. The software’s multiple region analysis (MRA) tool can automatically detect peaks, valleys or levels from the terms removal reference plane, as shown in figure 3. Once the regions of interest have been identified, different parameters of these features, such as surface finish, area, volume, width, height and pitch, can be logged and controlled.
SureVision analysis
Vision64’s SureVision analysis module is similar to MRA, except that it is also able to pattern match to a feature in the field of view (FOV), align that feature or features to a template and then perform image masking, region modification and analysis for up to 100 distinct regions in an image. This analysis option is very useful for under-bump metallisation (UBM), the creation of a thin film metal layer stack to provide an electrical or mechanical connection from the silicon die to a solder bump, as shown in figure 4.
Image subtraction analysis
The software’s image subtraction analysis tool subtracts single or stitched images from within a wafer or from wafer to wafer. A reference image or stitched image is captured and then subtracted from sequential measurements of a similar area. The software can remove pre-image waviness and form, align images run-to-run and apply filtering as needed to the pre- and post-subtraction image. This is very helpful for monitoring height deviations from wafer to wafer during growing or lapping steps, as shown in figure 5.
Thick and thin film analysis
Films and coatings are necessary for the insulation or isolation of key components within a device. Vision64’s thick and thin film analysis module automatically characterises metallic dielectrical films and coating material. The thickness of a film can also be measured, depending on its index of refraction.
The module has the ability to detect modulation peaks on the top and bottom of the coating to calculate the thickness of that film, as shown by the film theory example in figure 6a. Once the data is captured, the algorithm can report the minimum and maximum thicknesses of the top or bottom surfaces, as shown in Figure 6b. In this example, the film analysis software can also calculate the uncoated pad height in reference to the base material under the coating.
Via, glass via and solder resist analyses
Vision64 also provides custom algorithms for unique process analyses, for example, via analysis, glass via analysis and solder resist analysis. A via analysis calculates a range of statistics, including depth, top and bottom diameters, and roughness of the anchor and via regions.
A glass via analysis calculates the top and bottom diameter and depth of the via, including the height of the glass fibre reinforcement layer, using a special measurement mode algorithm, as shown in figure 7.
A solder resist analysis finds a hole in the solder resist and calculates the thickness of the solder resist layer’s top opening diameter, bottom diameter, and tail diameter and depth.
Overlay (registration) analysis
The software’s overlay (registration) analysis tool is used to analyse and characterise feature-in-feature geometries to track any relative shift of one surface with respect to another surface, as shown for basic registry features in figure 8. More advanced registry and laser-inscribed features can also be analysed by simply selecting the appropriate analysis checkbox.
Through-silicon via measurements
Certain measurement objective and FOV combinations can be used for challenging TSV measurements, allowing for depth-to-width aspect ratio measurements of around 10 to 1. These measurements are very important for wafer-level packaging (WLP) in terms of improving areal density for stacked components.
Measurement automation and wafer handling
Advanced automation software is what often decides whether a system is truly useful for wafer manufacturing. Over the years, Bruker has worked with customers in the semiconductor industry to develop the following three stage automation modes for Vision64:
- XY Scatter mode allows the user to place multiple single-point measurement locations randomly throughout the wafer measurement area.
- XY Grid mode automatically creates a grid of known die size in a given XY pattern of columns and rows. Multiple measurements can be made within each measurement grid die location, as shown in figure 9.
- XY MultiGrid mode is similar to XY Grid mode, but the measurement grid die locations can be randomly placed around the wafer. In addition, XY MultiGrid mode has the ability to add fiducial alignment points for each measurement grid location while allowing each grid location to perform measurements that are unique from the others.
Each of the stage automation modes allows a separate Vision recipe to be configured for each measurement location, including those that have multiple images stitched together. Furthermore, each of the measurement locations can have a unique Z location that the profiler will automatically move to, significantly reducing measurement times.
All modes can include alignment points that can be fully automated using the Cognex pattern matching tool.
As well as automation of stages, Vision64 provides full automation of autofocus, auto intensity and auto tip/tilt capabilities. Using pattern matching to achieve automatic feature centring in the FOV prior to measuring improves measurement robustness and repeatability.
The ContourGT-X can be manually loaded via its profiler stage, or another option is the fully automated InSight WLI 3D optical profiler and metrology system, which has an integrated wafer handler, as shown in figure 10. The latter allows for unattended measurement of wafers and meets class 1 type mini environment requirements.
The fully automated InSight WLI 3D optical profiler and metrology system.
Conclusion
For wafer manufacturing, such as wafer-level packaging,
flip-chip packaging or TSV technology, an advanced 3D optical profiler can give researchers, process designers, engineers and quality control professionals a significantly improved means of characterising features for shape, surface finish and overall functionality. Indeed, 3D optical profiling has been shown to outperform other measurement techniques in terms of overall resolution, repeatability, accuracy and speed.
Nano Surfaces and Metrology Division, Bruker