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July, 2017


www.us-tech.com Shadow Moiré Measurement Techniques Continued from page 51


sample. Die tilt angle can be shown from die to die or substrate to multi- ple dice.


Measure multiple groups or lots


of samples as desired. Measurement time is two seconds per group/lot, regardless of sample quantity, along with load and unload time. Finally, use the reference and die surface files created to batch process all results together. Die tilt angle can be calculated from the data sets within the batch processing interface.


Case Study Preparation In this study, 60 samples were


measured in two single shadow moiré measurements. The samples were measured inside JEDEC trays, as received. The chips came in a 4 x 11 layout JEDEC tray. The samples themselves were already production- quality devices that likely received outgoing quality checks. The focus of this case study was more to validate the concept and throughput of the solution.


Gauge results from the 60 sam-


ples were collected and considered these variables: height of the die from the substrate using a newly proposed technique to predict this height, given an initial estimate; die tilt angle with and without compen- sation for die warpage; and through- put time per batch/JEDEC tray. Traditionally, shadow moiré is


not used to measure surfaces with sudden changes in height. To under- stand this limitation and to find ways to circumvent it, we must look more closely at the shadow moiré phase stepping algorithm. The first step in a shadow moiré measure- ment is to determine the height associated with a single fringe peri- od, the fringe value. In practice, the fringe value is


calculated by comparison with a known step height calibration block, to make it more accurate. With a known height per fringe, the “phase” of each single pixel can be deter- mined by extracting the grayscale intensity value from each image, called intensity images, taken dur- ing the phase stepping process. A four-step phase stepping is used in this case. The sample and grating are moved apart the distance of the fringe value divided by four between each image, representing a 90° shift in the periodic fringe pattern. To this point, none of the calcu-


lations are limited by step heights. Thus, even across sudden step heights, the phase term is still valid. It is this concept that is critical to the new approach to measuring dis- continuing step heights, given an initial height estimate. The final equation converting


phase data to z-height information is where step heights pose a problem. Because phase occurs in a periodic manner, in order to translate phase data to z height, a counter integer must be tracked whenever a new phase begins. In order to determine a correct fringe order, a steady change in height is required.


A New Strategy A new strategy is proposed to


use an approximate height value input from the user and then use the available portion of the shadow moiré equation to determine a more accu- rate z height. This approach was test-


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ed on the same samples as the die tilt case study. To look more closely at the numbers involved with the prediction of height and measurement accuracy, the initial height prediction must be within the fringe value divided by 2, and the accuracy for the shadow moiré measurement is fringe value divided by 100. In this case a 100 line per inch (LPI) grating is used so actu- al values translate to a required pre- diction accuracy of 127 µm and a final z-height accuracy of 2.5 µm. Using digital fringe projection


(DFP), the step height of the sample studied is first determined to be used as our prediction. DFP is capable of measuring surface step heights, but has worse accuracy, a smaller field of


Continued on page 57


Page 55


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