Stylus-based surface profiling is a standard technique for accurate, repeatable surface shape, topography and step height measurement in applications ranging from semiconductor R&D to solar cell QC. In recent years, the ability to map surfaces in 3D has greatly increased the capability of stylus profi lers; yet despite such recent advancements, it is not uncommon in cutting-edge fabs, solar cell companies, industrial manufacturing facilities, colleges, universities and various research institutes to see R&D, QC and process monitoring operations performed still using technologies developed over sixty years ago.
This application note describes the advantages of 3D measurement options available through a combination of Veeco's Dektak® Stylus Profiler and Vision® 3D analysis software.
3D Measurement with Dektak and Vision
The advantages of 3D measurements are easy to see. Using a simple 2D profile, as shown in figure 1, may not provide a complete picture of the sample surface. With 3D capabilities, an entire area can be mapped, as shown in figure 2. This enables visual inspection of defects and uniformity, as well as small surface pits and spikes that may have otherwise been missed.
Figure 1. Traditional 2D stylus profilers may not provide a complete picture of the sample surface. This profile from the same sample shown in figure 2, can provide accurate height, width and roughness data, but may miss defects or detailed surface features that can be measured by adding 3D analysis capabilities.
Figure 2. This image reveals how 3D measurements provide a wealth of data for analyzing surface features such as area roughness, volume and defect detection. (Image: 2 x 2mm scan of a nickel surface roughness scale generated on the Dektak 150 using Vision advanced analysis software.)
Dektak stylus profilers are designed to support both 2D and 3D surface profiling, with precision stages, wafer alignment pins, color video imaging, advanced parametric data analysis tools and other features for stable, and extremely repeatable, surface shape measurements.
The advanced resolution capabilities allow users to visually interpret defect roughness, symmetry and process resolution. The Dektak 150 enables 1um Y axis stepping accuracy for higher overall 3D characterization and resolution. This higher Y axis resolution, as shown in figure 2, reveals small defects and tooling marks and accurately measures are roughness.
Vision Analysis Software adds a range of analyses, filters, masking capabilities, databasing, statistics and import/export functionality to Dektak profilers. Chief among these features is the ability to automatically combine multiple traces into an accurate 3D map of precision surfaces and manipulate it using a very short, clear, user-friendly menu.
3D Surface Characterization Beyond 2D Ra
In many applications, 2D average roughness (Ra) is the sole parameter specified for monitoring surface texture. While Ra provides a quick gauge of general roughness, it provides little insight into the functional characteristics of the surface. Conversely, 3D metrology provides a clear picture of surface characterization over an entire area. Significantly more data is available than is possible with a single line profile. The risk of using 2D Ra as the only gauge is that a part can be well within the specifi cation across a single 2D profile (or even a sampling of 2D profiles), yet may still fail in actual function because the single 2D profile missed a defect or other surface features that would be readily evident in a 3D area map (see figure 3).
Figure 3. 3D image generated on the Dektak 150 of 340nm silica colloids on quartz. Note the small bumps on larger features with deep crevasses. This sample shows the diffi culty characterizing complex surfaces with 2D profi les as opposed to generating a 3D area map. (Sample provided by Tomika Velarde of the Wirth Research Group.)
A much more comprehensive interpretation of surface functionality can be derived using the 3D visualization, filtering and analysis options in Vision. Specific 3D parameters, such as the S parameter set, can be used as much more meaningful process control variables. As examples, 3D analysis can quantify the ability of a bearing surface to retain oil, the visual brightness of a brushed metal fi nish, or the tendency of a mating surface to chatter due to regularly spaced machining marks. Customized parameters can also be generated to track very specific functional aspects of surface texture.
Figures 4a and 4b show a profile of the same data shown in figure 3. The Vision software provides a variety of data filters, including programmable low pass, median, high pass and Fourier filters. Figure 4a shows a cross section of the unfiltered figure 3 data with a measured Ra of 833 nanometers. Figure 4b shows the same dataset after a high pass filter has been applied to filter out the low frequency larger peaks and valleys, revealing the smaller bumps on the surface. The data with the high pass filter applied exhibits much more accuracy, reducing the Ra by more than a factor of ten to nominally about 70 nanometers.
Figure 4a. Vision software provides a range of filters for manipulating data. Here, the unfi ltered data from figure 3 is shown. The cross-section of data has a measured Ra of 833nm.
Figure 4b. A high pass fi lter on the dataset in figure 4a filters out the low frequency larger peaks and valleys to reveal the smaller bumps on the surface and enable the roughness to be measured more accurately (note: Ra = 70nm after the high pass filter has been applied).
Flattening Filters and Terms Masking
A secondary method of correctly interpreting the heights of each trace is to use the Dektak "Flattening" features within the Vision software package. The leftmost image in figure 5 shows raw data of a 3D map with horizontal scan artifacts that can be caused by thermal drift or vibration. The image on the right shows the same data after the flattening algorithm has been applied.
Figure 5. The Dektak 150 Vision software has a special feature that can filter out scan artifacts caused by thermal drift or vibration during a 3D map operation.
Vision software also allows terms masks to be applied to remove features that may be present across some traces but not in others. Terms masks can enable the flattening algorithm to be applied to a selected area of data to remove scan artifacts. Together, these two methods provide excellent interpretation of the Z-heights for each trace, and thus enable excellent 3D mapping of the features.
In addition to data fi lters, the Vision software provides multiple color palettes that enable various surface features to be enhanced and highlighted (see figure 6). It even provides the ability to make the surface look "shiny" or change the angle and intensity of the light shading of the image.
Figure 6. Vision software can be used to accentuate the data by using different filters and color palettes to highlight and bring out various features of the image.
Flexibility for Setting up 3D Programs
The Dektak 150 software includes a number of features that allow a user to optimize scanning for best speed/best resolution. It can quickly and easily be used to set up and run 3D maps with a variety of different parameters to accommodate multiple applications. The Dektak 150 generates 3D maps by combining several individual profi le measurements or traces into a 3D image file. The user can visually determine the area that needs to be mapped by using the color video microscope. The operator simply uses the mouse to select the X and Y extent of the area of interest, and the software automatically calculates the length and width of the area to be measured, as well as the scan start location (see figure 7). Once the operator selects the area to be mapped, the resolution of the map can be determined by selecting how many individual traces are desired to map the area, as well as the resolution of each individual trace. Up to 500 traces can be used to create a map with a minimum spacing of 1 micron per trace.
Figure 7. This video image of the same sample in figures 3-6 shows how the programmable sample stage can be used to determine the X-Y extent of a 3D map.
3D Upgrade Capabilities
Stylus profilers are still largely used to obtain 2D profi le measurements rather than to generate 3D images. The primary reason for this is that 2D profilers are typically less expensive than 3D measurement tools. One major advantage of the Dektak 150 is that it provides a relatively low cost solution for 3D imaging and analysis. Another advantage (that no other stylus profi ler offers) is that the 2D model of the Dektak 150 can be upgraded to a 3D metrology system by upgrading from a manual sample stage to programmable sample positioning.
Accurate Z-Height Interpretation
In a stylus profiler, a 3D map is built up from a series of 2D traces. To accurately map the surface, it is necessary to correctly interpret the Z (vertical) height of each trace relative to the others. Other profilers with 3D capabilities make the assumption that each trace begins at the same Z height. This technique would make it impossible to accurately image and measure samples like the one in figure 3, where each scan begins at a different point in the Z axis. The Dektak 150 creates a 3D map by referencing all subsequent data points to the very first data point taken in the very first trace. This results in accurate measurements and 3D imaging of the surface in question.
Reliable Apex, Form and Slope Measurements
A similar challenge is addressed when measuring the height of spherical or aspherical surfaces, such as microlenses, lens molds, solder bumps, etc. With simple 2D profile measurements, it is very difficult to determine the apex of a spherical shape with a single scan. A variation of only a few hundred microns in the scan start location can produce quite a difference in the apex measurements. The deviation can be exponentially higher depending on the curvature of the lens. Using 3D mapping always captures the true apex, resulting in highly reliable height measurements.
During a scan, the stylus pivots and swings vertically in an arcing motion. This arc motion can produce errors in slope measurements as the stylus rides up one side of the slope and down the other. Figure 8 shows a pyramid configured calibration standard, which is a 2D profile generated from a 3D image. The dark line shows the affect of the arcing motion of the stylus to the data as the slope on the left side of the pyramid is not as steep as the trailing slope on the right side of the pyramid, giving the appearance that the standard is not spherical. The Vision software includes a special MicroForm filter to correct for the arcing motion of the stylus. Figure 8a in the gray exhibits the same data with the MicroForm fi lter applied to correct the slope angles and provide the true spherical shape of the lens. Figure 8b shows the 90 degree symmetrical grating, which confi rms the gray corrected image of the scan.
Figure 8. Vision software contains a "MicroForm" fi lter to provide more accurate slope and shape measurements by removing the shape of the stylus arc. The shape of a calibration standard (line) leaning to the right due to the motion of the stylus during the scan can be seen in 8a. The gray area is the actual surface. The 90 degree symmetrical grating is evident in 8b.
Furthermore, Vision software includes a Multi-Region Analysis that lets a user defi ne and compare multiple features within a dataset. Figure 9 shows a raw scan and map of a solder bump array. Using the multi region analysis function in Vision, the height of each bump, the diameter and the coplanarity can be readily determined. The data can also be exported as a .csv file, and stored in a customizable database for tracking and process control.
Figure 9. The Multi-region analysis feature of the Vision software automatically provides height and diameter measurements of multiple bumps within a 3D map.
Traditionally, stylus profi lers have been confi ned to 2D analysis, however, advances in hardware and software functionality have greatly expanded stylus profiling capabilities. Now, 3D measurement is possible, providing comprehensive visualization and quantification of precision surfaces for accurate assessment of process parameters and part functionality. From gauging nanometer-scale etch depth to measuring surface roughness on machined parts, the combination of Dektak Stylus Profilers and Vision analysis software offers the most accurate and repeatable method of 3D characterization available today.
This information has been sourced, reviewed and adapted from materials provided by Veeco.
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