The semiconductor packaging industry continues to advance, with new designs adding more layers, finer features and more I/O channels to achieve faster connections, higher bandwidth and lower power consumption. As packaging technologies have evolved, manufacturers have adapted old processes and adopted new processes to connect chips to each other and to the outside world. Often these new processes use front-end-like tools and techniques to perform back-end tasks, blurring the traditionally sharp distinction between the two ends. This area of overlap has been called the middle-end and its growing importance has created an increasing need for specialized, high-precision measurement and inspection capabilities to detect defects and improve process control. The new NanoResolution MRS (multiple reflection suppression) sensor from Cyberoptics uses phase shift profilometry, an automated optical inspection technology, to address these needs.
Advanced packaging techniques are evolving rapidly and many different processes are used in different applications. In general, all seek to integrate more power and functionality in a single package that uses less space and has more numerous, shorter, faster connection paths. Most use some form of vertical integration, stacking chips on top of each other or on specially designed substrates. Vertical connections are frequently made using bumps or pillars that extend above the surface of the chip. Because these processes use known good die, the cost of failure is high. The cost of failure can be even greater when the health and safety of the user is at risk, as in automotive applications for assisted driving or self-driving cars. Fast accurate inspection and measurement of these and other similar structures is a critical requirement for improving yields and ensuring reliability.
Phase Shift Profilometry
Phase shift profilometry (PSP) uses structured light to measure three-dimensional (3D) objects. It projects a fringe pattern on the object and looks for shifts in the pattern that appear when surfaces at different heights are viewed from an angle to the projection direction. The intensity of the projected pattern varies sinusoidally across the fringes, permitting very precise measurements of shifts in the phase of the waveform.
Phase shift profilometry offers unparalleled speed and accuracy, with achievable data rates greater than 100 million 3D points per second and resolution scalable down to 1.5µm laterally and 25nm vertically. Importantly, for applications such as semiconductor packaging where production worthy throughput is critical, PSP can perform both 2D and 3D measurements in a single pass. It is widely used for 3D automated optical inspection (AOI) by electronics manufacturers assembling printed circuit boards (PCB) with surface mount technologies (SMT). It is also used for solder paste inspection (SPI) by PCB manufacturers and for dimensional measurements typically performed by coordinate measurement machines (CMM) in a variety of other industrial applications. PSP measurements are highly accurate and can be orders of magnitude faster than alternative methods.
However, conventional PSP measurements can be significantly challenged by certain aspects of semiconductor applications. Shiny mirror-like surfaces can generate glints that saturate the camera or direct light completely away from the camera and create ambiguity between the height and tilt of the surface. Multiple specular reflections between shiny surfaces can cause inaccurate height measurements. Densely packed components, especially short components near taller ones, can occlude visibility of adjacent areas. The variety of features and materials with widely ranging reflectivities requires special treatment in the analysis. Finally, the system must be fast enough to inspect 100% of the wafer surface at production speeds of 25 wafers per hour or more.
Multiple Reflection Suppression
The CyberOptics MRS sensor addresses these challenges. The MRS sensor uses a single vertically positioned fringe projector, a coaxial camera for 2D measurements, and multiple cameras arranged off-axis around the projector to capture images for 3D measurements from different perspectives. The digital fringe projector can project images over a range of frequencies and orientations. The sensor uses fringe patterns of different frequencies and sophisticated ‘phase unwrapping’ routines to achieve both fine resolution and extended range in vertical measurements. The use of a single projector and multiple cameras allows parallel data collection and unprecedented 3D measurement speed. Multiple views also ensure that none of the surface is hidden by adjacent tall features.
Multiple reflections among shiny surfaces can cause errors in PSP measurements. These spurious reflections will appear differently to different cameras and at different fringe frequencies. When the fringe spacing is of the same order as the physical spacing between the shiny features, the reflection can add coherently in the phase calculation. At higher fringe frequencies (more closely spaced fringes), the same reflection may reduce fringe contrast but will not affect the phase calculation. A key attribute of the MRS sensor is its ability to suppress these measurement errors by comparing information from images obtained from different perspectives and at different fringe frequencies.
The above is article is an extract from a white paper authored by Tim Skunes VP Technology and Business Deveopment at Cyberoptic. The fill paper is available for download
For more information: www.cyberoptics.com