Purpose of the test patterns
The purpose of our test patterns is to speed-up process development and reliability evaluation, which will save you time, therefore money.
There are many interacting factors to consider during the fabrication fo WLPdevices and they all have a definite impact in making the final product and insuring its reliability. Therefore, a statistical method of experimental design is mandatory to quickly optimize the process. Using DOE and our many years of experience with photolitho test patterns, we have designed patterns to provide quick numerical answers with minimal instrumentation. The various test patterns are evenly distributed over the surface of the wafer to obtain data on uniformity. The number of test points is sufficiently large to obtain statistically valid data.
Photolithography is at the root of patterning the metals and dielectrics in packaging. Indeed, the quality of WLP strongly depends on photo-imageable dielectrics, photoresists, wet and plasma etching control. Our patterns are designed to optimize process parameters, such as exposure time and development that ultimately control the optimal resolution of a material. Since processes are equipment dependent, the material manufacturers of photosensitive data can only provide a starting point for your process. We simplify the evaluation of edge distortion and profile, step coverage or planarization of the materials. Other factors, such as line space control measurements are estimated with the help of an optical microscope that greatly reduces the need to evaluate the quality of the photolithography in an SEM.
Etching is dependent on the quality of the photoresist, etching procedures, and materials. We designed some patterns for quick electrical measurements, preferably using a probe station, estimating the amount of under or over cut and the effective linewidth resulting from etching procedures. Other electrical measurements evaluate the cleanliness of vias and the amount of scumming, if any. Some patterns allow to evaluate the impact of bubbles that are forming and often clinging during etching. Bubbles are a frequent problem with aluminum because of evolved gas trapping.
Many laboratories have four-point probes to estimate sheet resistivity and mapping the results. Patterns that provide accurate measurement of the actual sheet resistivity can be distributed around the wafer. Test points are provided to measure the effective resistivity of interconnect lines.
Reliability test patterns
The majority of the cells on a test wafer are used to evaluate the reliability of the final product and the influence of physical design choices, such as overlap of the metallization over the dielectric or UBM layout. We incorporated test points, accessible from our test PCBs, to make it easier to locate defects or failures caused by cracking of the dielectric or metallization during temperature cycling. Other patterns allow measurement of the effective dielectric constant and the effect of moisture absorption on a polymer. Other important measurements are dielectric current leakage through moisture absorption or surface contamination, electromigration in conductor lines, and metallurgical stability of the solder and its UBM subjected to high current.