Application Note | Array Flatness
Wafer Array Flatness Measurement Using 3D Profilometry
How Far Must the Array Compress for All 100 Devices to Make Contact?
Research & Experimental Testing
Craig Leising
Visual Design & Editorial
Andrew Shore
Introduction
A critical factor that can dramatically affect the quality of wafer arrays is the flatness relative to adjoining components. The components of the wafer are typically intended to lie in an array pattern. If any component is not flat or planar relative to its neighboring components, it may become overly compressed or stressed, while neighboring components do not make proper contact.
Importance of 3D Non-Contact Profilometry for Wafer Array Study
Unlike other techniques such as stylus or interferometry, the 3D Non-Contact Profilometer, using axial chromatism, can measure nearly any surface; sample sizes can vary widely due to open staging and there is no sample preparation needed. Nano through macro range is obtained during surface profile measurement with zero influence from sample reflectivity or absorption, with advanced ability to measure high surface angles and no software manipulation of results. It measures any material: transparent, opaque, specular, diffusive, polished, rough, etc. The technique of the non-contact profilometer provides an ideal, broad and user friendly capability to maximize wafer array measurement, with the benefits of combined 2D and 3D capability.
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Measurement Objective
In this application, the NANOVEA ST400 Profilometer is used to measure a section of a wafer array. The area measured was selected at random and assumed large enough to be extrapolated to a much larger surface. Surface flatness, planarity and other surface parameters are used to analyze the surface.
NANOVEA ST400 Standard
Optical 3D Profilometer
Results
3D and 2D Profile
The 3D and 2D views of a randomly selected area show 100 components assembled in an array and a cross-sectional profile. Flatness variation can be seen clearly by color and height changes at the top of the devices.
3D Profile of Contact Tips
Using the data above, the top electrical contact pads were isolated to show flatness and planarity. The contact area at different depths highlights the deviation from flat over this small section of the wafer.
| Parameter | Lowest band (blue) | Middle band (green) | Highest band (red) |
|---|---|---|---|
| Projected area (%) | 12.220 | 65.362 | 22.418 |
| Volume of void (%) | 1.4835 | 52.123 | 92.373 |
| Volume of material (%) | 98.517 | 47.877 | 7.6265 |
| Volume of void (µm·mm²/mm²) | 0.17734 | 7.6812 | 16.178 |
| Volume of material (µm·mm²/mm²) | 11.777 | 7.0555 | 1.3357 |
| Mean thickness of void (µm) | 0.17734 | 7.6812 | 16.178 |
| Mean thickness of material (µm) | 11.777 | 7.0555 | 1.3357 |
Table 1: Contact area results by depth band (from Fig. 5; band limits 11.955 µm and 26.692 µm).
Conclusion
In this application, we have shown how the NANOVEA 3D Non-Contact Profilometer can precisely characterize the flatness and planarity of a wafer array. As seen above, the results accurately identify the contact area at different relative depths. With this information, it is identified that to make complete contact with all 100 devices shown, the wafer must compress by 44.205 µm from the tallest device. The planarity of the devices was also calculated, on average, to be around 0.61° for neighboring devices and 1.05° at maximum for devices with extreme differences. This planarity calculation is a result of both the overall flatness and individual device flatness.
Frequently Asked Questions About Wafer Array Flatness
Why does array flatness matter?
Array components are meant to lie in one plane; if one is not flat or planar relative to its neighbors it can be over-compressed or stressed while neighbors fail to make proper contact.
What was measured?
A section of a wafer array with 100 components, scanned in 3D, with the top electrical contact pads isolated to evaluate their flatness and planarity.
How much compression is needed for full contact?
The contact tips vary in height by 44.205 µm (ISO 12781 flatness), so the array must compress by that amount from the tallest device to contact all 100.
How planar were the devices?
On average about 0.61° between neighboring devices and 1.05° at most between devices with extreme differences, reflecting both overall and individual device flatness.
How is the height distribution shown?
The contact area is split into depth bands: 12.220% of the projected area lies in the lowest band, 65.362% in the middle and 22.418% in the highest.
Why non-contact for this?
It measures nano to macro heights without touching delicate contacts, with no sample preparation and no influence from reflectivity.
