METROLOGY TOOLS PROVIDE SOLUTIONS
By Thomas Fries, Paul Flynn, And Paul Lamm, FRT of America
Manufacturing a micro- or nano-device must be done with sub-nm precision; the range of the radius of a gold atom. However, it is nearly impossible to produce this accuracy, which is required to enable integration and packaging. Parts must be selected by metrology.
Instruments to measure the precision of micro devices and the alignment should have nanometer resolution. Such ultra-precise quality control systems like atomic force microscopy (AFM) or an optical, 3-D profiler must be used near, or integrated in the production.
Integration and assembly by nanorobotics can be precise, but without related sensors and automated process, may not be possible. Metrology tools provide possible solutions. These systems use AFM, optical sensors, or a combination of the two; and may be integrated into the production line or set up as stand-alone instruments in the metrology lab. They enable the characterization of roughness, profiles, shape, topography, lateral dimensions, film thickness, bow and warp, total thickness variation (TTV), or mechanical properties of surfaces. It is possible to cover a range of nine orders of magnitude for measurements, reaching from nanometers to meters.
Motivation
Investigation of structure and roughness of surfaces is gaining technological relevance. Demands on components and finished surfaces have increased. Quality assurance and global competition aggravate the situation, especially as nanotechnology becomes a complete new industry. Assembly and integration is a growing field. There is also the challenge of finished product size, which leads to a surface roughness in microelectronics on the atomic scale.
To establish real systems, a perfect supplier industry is needed. To get a perfect supplier industry, standards in dimensions and instruments to measure these dimensions are needed. Topography, roughness, and contour must be measured precisely and non-destructively. Existing ISO standards do not meet these needs, because existing procedures do not fit to relevant dimensions. There is a need for new characteristic data sets specially defined for nano-applications.
Instruments like tactile profilometers, confocal microscopes, or 3-D interferometers do not fulfill the combination of high resolution with significant, high dynamical ranges in 3-D and non-destructive, metrological measurement. Problems include: low resolution, destructive measuring, bad aspect ratio, restriction to 2-D evaluation for tactile profilometers, bad performance at edges, small range of view, missing metrology for 3-D interferometers, poor height, and X-Y measuring range of confocal microscopes.
Bridging the Dimension Gap
Even with the high performance of AFM, it can’t bridge the gap of dimensions from state-of-the-art industrial metrology and the nano range; restricting the application of AFM to research labs and some high-tech companies.
One company’s* tools are equipped with a chromatic optical sensor and an AFM, both mounted permanently. Without changing the measuring device, the chromatic optical sensor measures complete component parts for industrial purposes, while the AFM allows the investigation of the surface at specific positions with nanometer resolution. The optical sensor allows rapid and accurate topography measurements on samples ranging from 200 × 200 µm up to 600 × 600 mm using precision X-Y scanning stages. The X-Y resolution of this sensor is 1 µm. The Z range may be chosen from 300 µm to 25 mm, giving a maximum vertical resolution of 3 nm.
 Figure 1. Metrology tool equipped with chromatic optical sensor and AFM. |
The optical sensor is complemented by a positioning camera, which makes it possible to define the scanning area (Figure 1). The optical sensor is mounted on a motorized linear axis to facilitate an automatic approach into the measuring range of the sensor. It uses the physical effect of chromatic aberration to create a chromatic-coded measuring head, which allows accurate height measurements without any moving parts within the sensor.
Optical Principle
Light from a white-light source is transferred via a fiber to the measuring head. One focus point along the optical axis corresponds with a color wavelength, indicating one specific height value in the Z direction. On the surface, there is always the spot of one single wavelength in focus. For this wavelength, the spectrum of the reflected light shows a significant maximum. Spectral evaluation investigates the maximum wavelength, and from this, obtains the height of the measured sample at the respective X-Y position. Fast profiles are taken by scanning along one direction in the X-Y plane. 3-D topography is gained by scanning in the X and Y directions by the use of a passive, small, and light measuring head. There are no moving components within the sensor. The measurements are independent of the materials properties; in each measurement, the complete measuring range is taken. Steps do not lead to artifacts. This results in a robust measuring system for production with the specifications of a lab instrument. Working distances range from 4.5 to 20 mm.
Atomic-force Microscope
The AFM has scanning ranges between 20 × 20 µm and 80 × 80 µm. The Z range is from 2 to 6 µm. The AFM resolution is better then 1 nm. It is mounted on a separate axis to allow independent approach and retraction of the AFM head. The sample is positioned under the AFM head with the X-Y scanning stage, using either the positioning camera, or a previously recorded topography image with the optical sensor.
The AFM may be equipped with various modes like magnetic force, lateral force, force modulation, phase shift, Kelvin probe, liquid compatibility, and atomic force acoustical microscopy (AFAM). AFAM allows for investigating mechanical properties and elastic modulus of surfaces.
During AFM measurements, the scanning stage is blocked and seated on the granite base. Both sensors may be mounted on a rigid, manually adjustable slide to cover a large range in specimen thickness.
Due to the scanning range, the calibration of the multi-sensor metrology tools is important. Both sensors have to be calibrated separately. To calibrate the AFM, known AFM standards are used. These standards include a grid coordinate system, with 10-µm spacing used to calibrate the AFM X-Y scanner to determine the offset between AFM scanning area and the spot of the optical sensor.
The chromatic sensor is calibrated by an interferometer. The calibration is performed once at the factory, and can be checked using a gauge block or a height calibration standard.
The X-Y staging of the tools is controlled by a closed-loop feedback system using Heidenhain glass scales. The original height deviation of the stage for the whole range is better than 1 µm. By measuring a reference plane with the instrument on an optical flat, and by deducting this “zero plane,” reproducibility in height is achieved. This high-precision stage allows metrological measurements for the whole range of movement.
Solder Bumps
Figure 2 shows a solder bump array; a complete, 25 × 25-mm device. The 3-D topographic measurement gives the overall structures, containing all necessary information, to characterize this device. So a high-resolution topography may be extracted, and details of the top of the bumps may be evaluated. To estimate the height and diameter of the solder bumps, 2-D profiles are taken. These give exact data of all features within the surface.
 Figure 2. Solder bumps application. |
Another important point is warp of the substrate. Here, the bumps are eliminated by the software, resulting in the bare substrate surface topography. The Z-scale of the warpage is around 3 µm, while the bump heights are about 37 µm. The information of the coplanarity of the bumps, in combination with the warpage of the substrate, allows for a complete functionality check of the device.
Packaging
Another field of application is thick-film technology. Printed circuits are an essential regime of electronics applications. All substrate materials are relevant, especially ceramic substrates. One of the main advantages of metrology tools is the ability to measure ceramics, bright or dark surfaces, or transparent surfaces.
The key features of printed circuits include step height, width of the structures, and the topography of junctions. The roughness of the substrates and the identification of defects play a role. All this can be done manually or fully automated.
High Dynamics in Range
Most instruments for surface metrology can’t measure both roughness and flatness, and are not equipped to do variable modes of one kind. A new instrument** has been developed that is able to perform local, high-resolution 3-D measurements, or high-resolution single profiles across the complete sample. If the local resolution doesn’t meet the application requirements, the AFM is used within the same system, allowing all available modes of operation with nanometer resolution. The sample positioning for AFM measurement is performed using either the camera or a previous optical sensor measurement. The point of investigation for the AFM is always found quickly and precisely.
Transparent Films on Wafers
Hybrid polymers are deposited as thin, transparent films on wafers. As the film has been partially removed, the film thickness could be determined from a profile across the wafer surface and film upper surface. Thickness would equal height difference. Conventional contact stylus measuring systems are unsuitable for this application, as they use mechanical contact techniques and scratch the soft surfaces being measured. Non-contact, optical measuring systems using confocal, auto focus, or triangulation sensors fail for the profile measurement on thin, transparent films, because the reflected light from the film’s upper and lower surfaces cannot be individually evaluated.
The sensor used measures the light reflected from each of the film surfaces, and determines the interference of the two light beams for each wavelength. In the multi-sensor metrology tool, the interferometric measurement enables highly resolved 3-D mapping of a layer thickness. This combination measures both film thickness and surface topography with highest local thickness resolution.
PCB, PGA, Die, and Bonding Pad
The whole back-end process, up to packaging and integration, brings the same measuring tasks. For bumps, conductive layers, and coatings, the central question is for step height. Coplanarity, bow, and warpage data are always deducted from planarity. An instrument that measures large areas, high-resolution profiles, and 3-D scans, with software options to evaluate the gathered data under various aspects, can cover all those applications necessary in PCB, PGA, die, bonding pads, etc. (Figure 3).
 Figure 3. Microelectronics application. |
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Conclusion
The main advantages of optical measurement are non-destructive measurement, high measuring speed, and long working distance. These instruments are used from workers self-testing to lab applications, or integrated in the line for fully automated operation. Large dynamics in range, combined with good resolution, especially in the Z direction, are achieved. With nanometer and sub-nanometer resolution, the range is limited. Here, the combination of AFM technologies allows for maximum resolution. Combining the various tools achieves the best performance for respective applications.
*FRT of America
**FRT MicroProf
THOMAS FRIES, president, FRT GmbH; PAUL FLYNN, director, FRT of America; and PAUL LAMM, regional sales manager, may be contacted at FRT of America, 48 South Road, #1, Somers, CT 06071; 866/378-7763; E-mail: [email protected], [email protected], and [email protected].