Meeting the Challenge
BY MICHAEL METZGER
With accelerated growth of wafer-level package (WLP) manufacturing comes new challenges in height measurement for existing inspection and measurement processes. Higher circuit density means complex packages are growing by leaps and bounds. Multi-layer chip measurement, and position and height of the wire and micro-bump connecting multiple layers are critical. Fusion of these technologies requires high accuracy and speed, and precise measurement for small 3-D structures composed of differing materials.
Solving measuring problems inherent in 3-D coordinate semiconductor packaging requires a resolution level that makes it possible to consistently measure 2-D+ height; processing cycle-time fast enough to enable high through-put measurements of the entire package; accuracy under 1 µ with a high-speed measurement capability; and simultaneous measurement capability for different reflect ratios from 1 to 100%.
Confocal Technology
Confocal microscopy offers several advantages over conventional optical microscopy, including controllable depth of field; elimination of image degrading from out-of-focus information; and the ability to collect serial “Z”-axis optical sections from samples. The key to the confocal approach is the use of spatial filtering to eliminate out-of-focus flare in samples thicker than the focal plane.
Confocal microscopy uses different imaging modes, but all rely on techniques needed to produce high-resolution images – or optical sections – in sequence through thick sections. For example, the optical section needs for biology require that data be collected from sections in single-, double-, triple-, or multiple-wavelength illumination modes for images collected on the detection device.
Researchers at one company* recently developed a confocal measuring head technology for industrial applications requiring fast and accurate height measurement (Figure 1). This measuring head** uses a Nipkow disk, which is a scanning disk with multiple, symmetrically placed spirals of pinhole apertures, through which illumination light is passed and split into multiple “mini-beams.” When the disk spins, the light scans the sample in a raster pattern. As the disk rotates, each beam emerging from a pinhole scans across a portion of the field. Light from the sample is projected onto a digital CCD camera.
 Figure 1. Confocal optics provide optimized imaging solutions. |
Sub-micron-level measuring technology addresses challenges such as lagging “Z” throughput because of small fields of view. The common way to achieve µ-level Z measures is by using high-optical magnification >500×. To assure µ-level Z measurement results, lenses with at least 0.3-µ depth of field must be used (Nyquist theory), and to get the shallower depth of field, higher magnification lenses are needed. Unfortunately, with ultra-high magnification, the field of view is diminished for X and Y to as small as 160 µ for a 500× optical system (50× objective lens). Therefore, when a large semiconductor device is examined, the wider field-of-view area measurement (as much as 8,000 µ for X and Y) of the confocal covers more real estate in each shot, increasing cycle times.
To obtain accurate Z measurements, it is necessary to control reflected light interpolation. Different reflection rates complicate measurement for different applications. When moving the objective lens along the optical path, the confocal optical plane also shifts to capture the image at different height positions. The position of the focal plane and the position of the sample surface is the point where the strongest reflected light captures the image. In other words, the image itself becomes a profile of the sample’s sectioning on the optical plane. The strongest contrast position of the X, Y, and Z coordinate of each pixel image is the point position of the 3-D coordinate. The Z-height measurement is captured by imaging all pixels in the field of view and processing multiple section images. The system differentiates between the high- and low-gain reflectivity and compensates for those differences, enabling measurements for the wide range (Figure 2).
 Figure 2. Confocal principles of measurement as applied in semiconductor metrology. |
This measuring technology stresses optical performance in the following key areas:
Low distortion – optimizes measuring accuracy of the whole field of view.
Telecentricity – the head shifts the objective lens while keeping the sample and CCD camera pixel image correlated between the focal plane and non-focal plane, assuring the accuracy of measurement and magnification.
Low aberration – the measuring head delivers low overall and spherical aberration. If spherical aberration exists, the position of the concentrated light is changed by light reflected through the objective lens. The Nipkow disk is arranged on horizontal axis, which follows the same direction as the optical axis.
Precise measuring of semiconductor packaging requires high accuracy and speed. The most efficient solution for obtaining high-speed measurement is to maximize a wide field of view. Three types of objective lenses (1.5, 3, and 7.5×) are available, allowing users to select a field of view that best fits a specific application.
A new approach to the Nipkow disk has been developed using a pinhole diameter and position, which allows the height-measurement datum plane to correspond with the Nipkow disk plane. This results in a disk plane featuring higher accuracy and flatness, and prevents errors resulting from rotational issues.
This technology deals with a wide range of reflectivity. When a confocal measuring system detects different reflection rate applications simultaneously, the illumination flare from the measuring head is minimized. When the reflection rate is <1%, the confocal head requires strong illumination to capture the image for height measurement. If the optical head experiences flare caused by strong illumination, it will overwhelm the CCD camera detection range and produce a poor-quality image. If the illumination is reduced too much, the sample will not be adequately measured because the reflection illumination will not be strong enough to cover the CCD camera detection range.
A correlation between focal plane scanning by the objective lens and the reflected light of the sample ensures a high degree of performance. For every scan by the objective lens, the pixels will be identical for each recorded movement. A constant scan speed of the objective lens, coupled with robust stage accuracy, ensures accurate focus sample scanning. To maintain a tight correlation between focal-plane scanning by the objective lens and the reflected light of the sample, the friction responsible for inconsistent scanning results and carefully balanced scan head motion is minimized using precisely developed mechanical parts, including a motor for moving the objective lens, a parallel spring mechanism, and a non-contact linear encoder. These parts enabled a 100% free friction and gravity motion shifted focus.
High-speed Image Processing
The high-speed height inspection system is based on an innovative confocal capability and operates according to the process shown in Figure 3.
 Figure 3. High-speed image processing ensures maximum uptime – minimal disruption. |
Parallel processing enables high-speed measurement, but the actual image capture and height measurement calculation are handled by pipeline processing. After the image is captured, the height measurement calculation finishes and the next multi-measurement process begins and outputs the results. As soon as the stage moves to next target position, the system prepares to capture the image.
Dual Optical Head Design
Measurement of the semiconductor package requires a high degree of accuracy for height measurement. The confocal system addresses those needs, but when measuring the features of an IC package, a conventional vision system is often required. A system was designed with a second optical path that has basic video measuring capabilities of edge detection, coordinate measuring, and laser scanning. Brightfield edge detection and laser auto focus (AF) is separate from confocal, and each optical system can be switched by users employing an easy-to-use GUI interface. The brightfield optical system is switched using a mechanical structure, and the laser AF is handled by autofocus power emissions. The objective lens is common for brightfield, laser AF, and confocal. With these capabilities, it is possible to measure 2-D and height measurement simultaneously. Users can select the best measuring solution depending on their measuring application.
Semiconductor companies are producing an assortment of 3-D architectures for semiconductor integration and packaging. “Z” height inspection for measurement of bumps, wires, and packaging components are critical to control and maximize process yields.
This new confocal measuring instrument is an integrated system combining a confocal optical measurement system with 2-D measurement brightfield optics to measure semiconductor packaging composed of different materials and developed on a fine scale. The technology handles complex 3-D structural applications at high speeds and accuracy rates (Figure 4), and is effective for hybrid applications where different materials exist.
 Figure 4. Outstanding results for fast, automated measurement. |
* Nikon
** Nexiv VMR-K3040ZC
MICHAEL METZGER, general manager, industrial department, may be contacted at Nikon Instruments Inc., 1300 Walt Whitman Rd., Melville, NY 11747; 631/547-8593; E-mail: [email protected].