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Page 51


Shadow Moiré Measurement Techniques for Discontinuous and Semi-Reflective Surfaces


By Neil Hubble, Director of Engineering, Akrometrix F


lip chip packages are seldom perfectly co-pla- nar; instead, there exists an angle between the die and package substrate, which


becomes a critical factor in maintaining product yield and reliability. Point-to-point height measur- ing quality checks for die tilt can be time-consum- ing, and may also create reproducibility issues from one user to another. Shadow moiré technology is a full-field optical


inspection technique commonly used for flatness characterization in the semiconductor industry, particularly at elevated temperatures. However, there are two limitations to shadow moiré. First, shadow moiré requires a diffuse reflective surface for measurement. Second, shadow moiré is unable to measure sudden step heights. Here we look at case study data


from samples with two or more sepa- rated planes. Using shadow moiré for this measurement can reduce the time it takes while improving the consis- tency of the measurements from user to user. As shadow moiré tools are often used during temperature flat- ness measurements, this added appli- cation can reduce the number of dif- ferent measurement tools needed in quality assurance labs


Using Shadow Moiré Shadow moiré uses directional


light, a camera, a vertical motion sys- tem, and a piece of glass with a pat- tern of opaque lines — a diffraction grating. Traditionally, shadow moiré measurements have been used to define warpage or flatness of a contin- uous surface having no ability to measure surfaces where height changes occur suddenly, by either exceeding a maximum slope of 0.2 or a maximum step height between two adjacent points no greater than 100 µm, depending on the grating pitch. Early shadow moiré, prior to any phase stepping technology, was based


As a general rule, changes greater than one fifth of the measurement fringe value between two adjacent pixels can start to introduce


issues. This translates to a maximum measurable slope of the surface of approxi- mately 0.2, which is equiva- lent to an angle of 11.3°.


on counting transitions of dark and light fringes. The geometry of shadow moiré allows calculation of a specific height associated with each fringe, known as “fringe value.” Inherent to this approach is the limitation that transitions between each fringe must be counted along a path. Therefore, very steep slopes or


sudden changes in height lose the count or order of the fringe. As a gen- eral rule, changes greater than one fifth of the measurement fringe value between two adjacent pixels can start to introduce issues. This translates to a maximum measurable slope of the surface of approximately 0.2, which is equivalent to an angle of 11.3°. Akrometrix previously studied


die tilt by hand, with more theoretical implementation. Software develop- ment has since been underway, and die tilt angle measurement now exists as a gauge in Akrometrix’s Surface Analysis processing software.


www.nordsondage.com I globalsales@nordsondage.com See at SEMICON West, Booth 5644


Flip chip with exposed die, measured with digital fringe projection.


Measurement Workflow Confirm if the optics of the die surface allows


for shadow moiré measurement. This is done by taking a physical shadow moiré measurement and confirming the phase amplitude threshold max/min. Then, identify multiple sample locations in the shadow moiré field of view. This step only needs to be done once per sample type and layout. Identify the die and reference surfaces in the


software GUI. These locations are reused for each surface, so this step is also done only once per sam- ple type. Repeat the process of identifying surfaces and die surfaces if multiple die locations exist per


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