It’s all about what’s in the box…

Polaritek Systems offers proprietary hardware and software to determine defects and residual stresses in large-area, thin products for a variety of applications. We leverage our extensive industry knowledge to provide tools which generate valuable process control data for your specific manufacturing process. Customizations are available upon request.

According to the Solar Energy Industries Association (SEIA), “The greatest challenge the U.S. solar market faces is scaling up production and distribution of solar energy technology to drive the price down to be on par with traditional fossil fuel sources.” The Polaritek RSM-2 directly contributes to improved manufacturing throughput, process yield and long-term system reliability.

No such measurement system
has existed commercially… until now.

Crystalline Wafers

When it comes to solar energy, the keys are to harness, convert and distribute the light and power of the sun in the most efficient and cost-effective manner possible. The Polaritek RSM-2 measurement system (a.k.a. “the box”) shown in Figure 1 uses polarized infrared light transmitted through crystalline silicon wafers to measure the residual stresses in wafers of typical size (156mm x 156mm) to make certain that only the best wafers are processed into cells. It is an invaluable tool to help manufacturers inspect raw wafers, quantify sawing-induced damage, ensure effective damage removal and study the effects of wafer processing steps such as texturization and deposition of thin films. Polaritek offers the ability to generate full-field stress maps detailing local variation in stress which cannot be resolved with profile/curvature based calculations (Read more about Stoney Equation here »). Both maximum shear stress maps and, crucially, normal (principal) stress maps are generated for each wafer. Example stress maps are presented in Figure 2, which show that the vertical saw damage is not clearly revealed in the maximum stress map, but is readily apparent in the separated stress maps. Example wafer features revealed by Polaritek such as grain boundary stress, mechanical damage, surface damage, inclusions, and saw damage are shown in Figure 3-8.

 

Figure 1. RSM-2 Stress measurement system.

Figure 1. RSM-2 Stress measurement system.

Figure 2. Examples stress maps for a silicon wafer showing (a) maximum residual stress (b) normal principal stress in the x-direction (c) normal principal stress in the y-direction.

Figure 2. Example stress maps for a silicon wafer showing (a) maximum residual stress (b) normal principal stress in the x-direction (c) normal principal stress in the y-direction.

The Benefits

  • Critical enabling technology for fast, accurate inspection of silicon wafers for the PV industry
  • Reduced breakage and scrap during production and assembly processes
  • Reduced wafer, cell and system costs
  • Increased wafer, cell and system reliability
  • Reduced warranty liabilities by detecting defective silicon wafers during the manufacturing process

The Features

  • Full-field imaging of 156mm x 156mm wafer (or larger) in a matter of seconds
  • Non-contact, non-destructive, full-field transmissive IR measurement
  • +/- 1MPa resolution
  • 16um x 16um spatial resolution
  • Applications include crystal growth uniformity, post-production damage assessment and pre-cell fabrication substrate inspection
  • Substrate wafers and wafers with thin films applied
  • Multi and cZ Si, GaAs, epitaxial growth, others
  • Available as in-line or field-portable unit
  • Ready to be automated and inserted in-line
Figure 3. Example grain boundary stress in a multicrystalline wafer.

Figure 3. Example grain boundary stress in a multicrystalline wafer.

Figure 4. Influence of 4-point bending on grain boundary stress.

Figure 4. Influence of 4-point bending on grain boundary stress.

Figure 5. Simulated damage by Vickers indentation.

Figure 5. Simulated damage by Vickers indentation.

Figure 6. Residual stress around a hard inclusion.

Figure 6. Residual stress around a hard inclusion.

Figure 7. Maximum shear stress maps of saw damage.

Figure 7. Maximum shear stress maps of saw damage.

 Figure 8. Residual stress in an EFG wafer along twin boundaries.

Figure 8. Residual stress in an EFG wafer along twin boundaries.

Polymers

Residual stresses in polymers such as acrylic, polycarbonate, PET and others can be measured to provide crucial quality data and process health feedback. The “crossed polarizer” system for observing residual stresses has been used in the plastics industry for decades, but its use is complicated by subjective interpretation (Read more about Crossed Polarizers here »). A stress measurement instrument from Polaritek is now able to deliver this long sought after stress data in a meaningful way.

Polaritek Systems, Inc. proprietary hardware and software can extract stresses in plastic injection molded parts such as the preforms for plastic PET bottles. The injection molding process parameters on subsequent preform performance is often not well understood. Quantifying the residual stresses inside the molded part provides valuable new information. Adjusting parameters such as hold times, cooling times, packing, etc. can present interrelated and opposing effects of the resulting stress fields. Figure 1 shows the stresses in the side wall of an example PET preform. High stresses are observed at the base cap and also near the support ring. Figure 2 shows stresses and an instance of local crystallinity in the end cap near the gate. Polaritek can generate stress maps for this preform and identify the type of stress at different parts of the preform. In this example, bi-axial stress is observed near the support ring. The type and magnitude of stress at each location in the preform will have an effect on the finished bottle. Often, identifying and monitoring key features of the stress field can provide powerful insights into process health.

Figure 1. Stress in a plastic injection molded preform for PET bottle.

Figure 2. Stress in the end cap of a plastic injection molded preform for PET bottle.

In stretch blow molding, such as in production of PET bottles for carbonated soft drinks (CSD), proper optimization of process parameters are essential to achieve required product performance. Presently, such optimizations are often based more on art than science. Problems arise that cannot be easily traced back to a source. Residual stress maps from Polaritek offer a new and innovative method to ensure and improve product quality.

Industry wide light-weighting efforts are putting greater pressure on manufacturers to monitor and control their process. In recent years, automated wall thickness and dimensional measurement vision systems have provided new levels of process control. While these dimensionally-based inspection systems are valuable, Polaritek Systems can offer important new insights into material properties through stress measurements. Figure 3 shows the stress field in the base of an example CSD bottle. Identification and measurement of high stress regions and classification of stress fields offer a window in the stretching and orientation of PET in the base. Other features such as interruptions to the natural stress pattern and local crystallinity area detectable. Such features can be the source of product failure. In fact, in some cases, stresses in the bottle base can be correlated with performance in standard stress crack resistance testing. This gives manufacturers crucial process feedback in seconds, which otherwise would be unavailable for 30-60 minutes.

Figure 3. Stress field in the base of a stretch blow molded PET bottle for CSD with various features identified

Other

Polaritek has experience applying its stress measurement techniques to other application areas as well. Contact us with details regarding your specific application and our team of experts will provide a solution. Some example applications are presented here:

  • Semi-conductor wafers
  • Flexible Electronics Substrates
  • Composites
  • Glass
  • Sapphire
  • Fused silica