Using Non-Contact 3D Optical Profiling for Comprehensive Wafer Inspection

3D optical profiling provides many benefits over other measurement methods used for non-contact inspection of semiconductor packaging front-end process research and control. These benefits range from custom analysis and rapid measurement speeds to fully automated measurements and non-destructive inspection.

The most advanced optical profilers available today provide rapid and accurate surface measurements to quantify a range of properties about surfaces under inspection. Globally, these systems are used in research, engineering, and production process control in numerous markets, including medical, microelectronics, precision machining, semiconductor, MEMS, data-storage, solar, aerospace, automotive, and material science.

Measurement Advantages of White Light Interferometry

3D optical profiling allows industry-leading measurement speed while also maintaining the same nanometer Z accuracy at all optical magnifications. This combination enables a variety of surface parameters to be measured, such as curvature, pitch, step heights, surface roughness, lateral displacement and waviness, all in a single measurement and on almost all surfaces.

The coherence scanning interferometry measurement technique, also called white light interferometry (WLI) (as detailed in Figure 1), has the potential to quickly determine 3D surface shape and surface finish over large lateral areas, up to 8 mm in a single measurement, and vertical heights up to 10 mm.

A stitching algorithm can be used to measure even larger lateral surface areas as this allows multiple lateral images to be taken and stitched into a single image for analysis. These capabilities have paved the way for many metrology applications to meet the evolving wafer manufacturing requirements.

Figure 1. Basic white light interferometry with self-calibrating HeNe laser assembly.

3D Optical Metrology Analyses for Wafer Manufacturing

Typically, wafer fabrication consists of sequential process steps to construct components onto a silicon wafer, which ultimately ends up as a fully functional device for a broad range of end products, from memory chips or computer to LEDs.

With the size of consumer electronics components continuously decreasing, there is a corresponding demand for increased wafer metrology to refine and control the development of these intricate devices.

A description of a few common applications where 3D optical profiling is optimizing the manufacture and performance of wafers is given below. All examples were carried out using a ContourGT^® 3D Optical Profiler utilizing Vision64^® software (Bruker, San Jose, CA).

Figure 2. 3D optical profiling enables custom analyses of textured surfaces.

Trace Analysis

Traces are utilized by most component designs for device etching or electrical connectivity on the solid state device itself. Optical profiling can detect both horizontal and vertical lines and traces, as shown in Figure 3, where these measurements are performed by a Trace Analysis module. The parameters, heights, and widths for each trace were then reported by the trace analysis, including the surface finish of traces and the spaces between them.

Figure 3. Trace analysis.

Multiple Region Analysis

Solder bumps, copper pillar, and through silicon via (TSV) are extremely critical manufacturing electrical and mechanical connections. A Multiple Region Analysis will automatically detect levels, valleys, or peaks from the terms removal reference plane (Figure 4). After the regions of interest are automatically detected, varied parameters of these features, such as surface finish, pitch, width, depth, height, volume, and area, can be logged and controlled.

Figure 4. Multiple region analysis.

SureVision Analysis

Similar to multiple region analysis, a SureVision Analysis has the added ability to pattern match to a feature in the field of view, align that feature or features to a template, then carry out image masking, region modification, and analysis of up to 100 distinct regions in a single image.

This analysis option is considered to be extremely useful for under-bump metallization (UBM), which is commonly used for mechanical or electrical connection from the silicon die to a solder bump (Figure 5).

Figure 5. UBM measurement utilizing SureVision analysis.

Subtract Images

Profilers with improved software capability can subtract stitched or single images from one wafer to another or from within a wafer. In this analysis, a stitched image or reference image is initially captured, and then subtracted from sequential measurements of a similar area.

Pre-image waviness and form can be removed by the subtraction software, which can also align images run to run, and apply filtering as required to the pre- and post-subtraction image. This is extremely helpful for monitoring height deviations from wafer to lapping or growing steps (as seen in Figure 6).

Figure 6. Left: reference image (Z scaled ±70 nm), and Right: subtracted image (Z scaled ±0.8 nm).

Thick and Thin Film Analysis

Coatings and films are essential for isolation or insulation of key components within a device. Metallic dielectrical films and coating material can be automatically analyzed by Thick and Thin Film software. It is also possible to measure film thickness based on its index of refraction.

The film software can detect modulation peaks on top and bottom of the coating in order to measure the thickness of that film, as shown in the theory example in Figure 7A.

Figure 7A. Film theory.

Figure 7B. Film measurement analysis incorporating top and bottom surfaces.

After the data is captured, the algorithm reports maximum and minimum thickness along with the surface finish of the bottom or top surfaces, as illustrated in Figure 7B. In this example, the uncoated pad height is calculated by the film analysis software in reference to the base material under the coating.

Via Analyses

Custom software algorithms are provided by advanced 3D optical systems for unique process analysis. Via Analysis detects the via and calculates a range of statistics, including bottom, top and depth diameters, and roughness of the anchor and via regions.

Using a special measurement mode algorithm, the via top and bottom diameter and depth, including the height of the glass fiber reinforcement layer, are calculated by a Glass Via Analysis. A Solder Resist Analysis detects a hole in the solder resist and calculates the thickness of the solder resist layer, bottom diameter, top opening diameter, and tail diameter and depth (Figure 8).

Figure 8. Via and glass via analysis.

Overlay (Registration) Analysis

Advanced and basic Overlay (Registration) Analysis is used to analyze and characterize “feature-in-feature” geometries in order to track any relative shift of one surface with respect to the other surface, as seen for basic registry features analysis in Figure 9. It is also possible to analyze more advanced registry and laser-inscribed features by selecting the suitable checkbox needed for the analysis.

Figure 9. Basic registration features and analysis results.

Through Silicon Via (TSV) Measurements

For TSV measurements that are more challenging, certain measurement objective and field-of-view combinations can be used in order to measure the aspect ratios of “depth to width” of around 10 to 1. These TSV measurements are extremely important for wafer level packaging to enhance areal density for stacked components (Figure 10).

Figure 10. TSV measurement and objective schematic.

Measurement Automation and Wafer Handling

Advanced automation software is what often separates whether a system is truly useful for wafer manufacturing or not. Over the years, Bruker has developed multiple stage-automation modes by collaborating with semi industry leaders.

XY Scatter mode allows the user to randomly place multiple single-point measurement locations throughout the wafer measurement area. XY Grid mode automatically generates a grid of known die size in a given XY pattern of rows and columns. Multiple measurements can be made within each measurement grid die location (Figure 11).

Finally, XY MultiGrid mode is similar to XY Grid, but the measurement grid die locations can be randomly placed around the wafer. Additionally, XY MultiGrid can add fiducial alignment points for every single measurement grid location while permitting each grid location to carry out unique measurements from the other.

For each of the stage automations, a separate Vision recipe can be configured for all measurement locations, including stitching multiple images together. In addition, each of these measurement locations can include a unique Z location that the profiler will automatically move to, considerably reducing measurement times.

All automation types can also include alignment points that can be completely automated using Cognex pattern-matching capability. In addition to stage automation, Vision64 permits full automation of Autofocus, Auto Tip/Tilt, and Auto Intensity.

Figure 11. Example of XY Grid wafer automation.

Automatic feature centering in the field of view before measurement using pattern matching enhances measurement repeatability and robustness.

Wafers can either be loaded manually onto the profiler stage for analysis, or can be configured with an environmental enclosure and wafer handler (Figure 12). This enables unattended measurement of multiple wafers with integrated FOUP and also allows achieving class 2 type mini environments to meet all wafer manufacturing factory requirements.

Figure 12. Bruker's ContourGT-X ARM for wafer manufacturing.

Conclusion

For wafer manufacturing, such as TSV technology, flip-chip packaging, or wafer-level packaging, an advanced 3D optical profiler can provide quality control professionals, researchers, process designers, and engineers with a significantly enhanced method of characterizing features for overall functionality, surface finish, and shape.

3D optical profilers are well established across a variety of industries, ranging from aerospace components to medical implants, and have been shown be better than other measurement techniques in overall resolution, speed, accuracy, and repeatability. Industry-leading analysis and automation software is used by Bruker’s 3D optical profilers.

These profilers are production-floor ready, providing a preferred metrology option for research, process control, and quality inspection. The systems can make surface measurements with any reflectivity with the help of sophisticated algorithms and dual-LED light sources.

Correlation to stylus measurement systems can be obtained with upfront knowledge of the setup of the stylus tool and the surfaces being measured. The degree to which surface analysis can uniquely characterize surface shape and functionality is greatly extended with the addition of the 3D surface S parameters. The most advanced improvement in measured data for the wafer industry is offered by Bruker’s 3D optical profilers.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.