Glenn Wedgbrow, business development manager, Micro-Epsilon UK
To make micro-processors and -chips more compact and to fit even more functionality into the smallest possible space, all production steps in semiconductor wafer manufacturing are subject to the most stringent requirements. This requires sensors that reliably monitor and control handling systems, machine parts, optics and other associated processes.
Semiconductor technology is the cornerstone of all modern technology. The progress of numerous industries depends on microprocessors and memory chips. The key drivers are smartphones, cloud computing and big data, plus megatrends such as artificial intelligence (AI), the Internet of Things (IoT) and autonomous driving.
At the same time, the quality requirements for semiconductor production are extremely high, meaning that all sensors and systems need to be put through complex, rigorous in-situ testing processes. Careful consideration must be given to the selection and placement of electronics, the mechanical manufacturing methods and the special process technologies. In addition, special manufacturing processes and application-specific sensor materials must be used to meet the highest quality requirements. This enables the manufacture of individual sensor solutions that prove themselves in precision mechanical engineering due to high performance and precision, as well as robust design.
The ensuing paragraphs detail how various sensors from Micro-Epsilon are used in the manufacture of semiconductor wafers.
Measuring tasks in wafer production
Ingot sawing requires high precision. One method uses an internal hole saw to cut the single crystal rod into thin wafer slices one at a time. The other uses multiple wire saws to cut several slices from the rod at the same time. Non-contact sensors are used to monitor the sawing processes.
Dimensional inspection of silicon ingots
Initially, laser profile sensors are used to measure the complete geometry of the silicon rod. To do this, the ingot passes a measuring station with several laser scanners. In a second step, the laser scanners check the dimensional accuracy of the orientation notches, which are later required for the exact alignment of the ingot. Blue laser scanners are used for this purpose because they generate a highly precise and interference-free measurement signal.
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Image 1A:Red laser profile scanners are used to detect the ingots‘ geometric properties (1a), and blue laser profile scanners check their notch profiles (1b).
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Image 1b: Red laser profile scanners are used to detect the ingots‘ geometric properties (1a), and blue laser profile scanners check their notch profiles (1b).
Monitoring the axial movement of internal hole saws
In the internal hole sawing process, wafers are cut from the silicon ingot. Here, eddy current displacement sensors are used to monitor the axial movement of the saw blades. They are insensitive to dust and dirt and so provide precise measurement results, even in adverse environmental conditions. The eddy current displacement sensors measure the distance to the saw blade support without contact and so are wear-free. The result is a homogeneous and uniform cut of the silicon wafers.
Eddy current displacement sensors are used to monitor the saw blade or the holder. Their high-frequency response and insensitivity to dust and contamination mean they are able to provide reliable measurements of the saw blade's axial deviation.
Monitoring the deflection of wire saws
In the wire sawing process, several wafers can be cut from the silicon ingot simultaneously. The thin wires have a diameter of just over 100 µm. The wire wears out and loses about 5 to 10 percent of its diameter due to abrasion. Therefore, wire wear must be monitored continuously. Here too, eddy current sensors are used, which record both the height of the wire and wire sag.
Non-contact eddy current displacement sensors are used to measure the wire height as well as the wire sag on the guide roller.
Colour measurement when cleaning wafers
Ozone is added to cleaning liquid to clean wafers. Colour sensors are used to permanently determine the concentration of the contained ozone. They monitor the colour of the liquid and from this detect the ozone concentration. Even the smallest colour deviations of the concentration are reliably detected.
Colour sensors are used to permanently determine the concentration of contained ozone in the cleaning of wafers.
3D shape measurement of wafers
The wafer slices created during sawing must be planar. Deflectometry systems are used to measure the flatness of the wafers with just one recording. 3D sensors project a striped pattern onto the wafer surface for this purpose. Integrated cameras record the reflected fringe pattern and the system software subsequently enables quick and easy evaluation. Distortions of the wafer cause an equally distorted, striped pattern. A highly accurate 3D representation enables the detection of such deviations at the micrometre level.
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5A:Deflectometry systems are used to measure the flatness or planarity of wafers in a single-image capture step. The sensor provides a 3D representation of the reflective surface, which can be used to determine the topology with micrometre precision.
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5B: Deflectometry systems are used to measure the flatness or planarity of wafers in a single-image capture step. The sensor provides a 3D representation of the reflective surface, which can be used to determine the topology with micrometre precision.
High-precision distance measurements in lithography
Photolithography is a central step in the wafer manufacturing process. In the exposure process, the image of a photomask is transferred to the wafer surface. A photosensitive photoresist layer is applied to the wafer beforehand. After this resist layer has been stabilised, the actual exposure begins.
Wafer lithography, especially extreme ultraviolet lithography (EUV) lithography, is one of the most complex industrial manufacturing processes. To deposit structures in the single-digit nanometre range on the wafer, the machine and optical elements must be positioned and aligned with correspondingly high precision. Sensors take on a decisive role here, namely nanometre-precise distance measurement in the positioning of the mask, the lens system and the wafer stages.
Positioning of the mask (reticle)
High-resolution and long-term, stable measurements of the machine movements must be made when feeding and positioning the lithography mask. Capacitive displacement sensors are used in this process to monitor mask alignment, meeting precision requirements in the nanometre range. Their modular design with multiple sensor channels and vacuum-compatible components mean they can be used in numerous measurement tasks.
Confocal chromatic sensors are used to check the parallel alignment of the mask. The gap between the glass and the substrate is detected using just one sensor. The rectangular design of the confocal chromatic sensor requires minimum installation space and is therefore suitable for confined areas.
Confocal chromatic sensors are used to monitor the gap between the mask and the glass. Their 90° design allows for exceptional space-saving integration.
White-light interferometers are used to achieve maximum measured value resolution. These generate absolute readings with sub-nanometre accuracy and therefore achieve accurate photomask alignment. Special evaluation algorithms and active temperature compensation ensure high signal stability. White-light interferometers are available in vacuum-compatible versions.
Alignment in the lens system
High-precision mirror optics are indispensable in lithography as they deliver the highest imaging accuracy. To ensure that they function properly, the mirrors, lenses and lens carriers must be positioned precisely. To achieve this, the horizontal and vertical movement of the individual optical elements must be monitored.
Highly dynamic displacement sensors are used to detect the position of lens elements and mirrors without contact so as to achieve the highest possible imaging accuracy. The sensors measure against the metallic mount and also directly onto the lens.
The alignment of the optics is measured using confocal chromatic sensors. This involves the nanometre-precise detection of the respective lens tilt. Confocal chromatic sensors and white-light interferometers are the ideal choice for use on glass.
The positioning of the lens carriers is monitored using capacitive displacement sensors. These check the tilt to nanometre accuracy. Several sensors measure the distance to the metallic carrier, which ensures exact positioning and therefore reproducible light projection onto the wafer.
Non-contact eddy current displacement sensors are also used to measure the position of lens elements. These displacement sensors are used to detect movement and position in up to 6 degrees of freedom (DoF), depending on the lens system. Even highly dynamic movements of the lens system can be monitored due to these sensors’ high cut-off frequency.
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8a: Four confocal chromatic sensors are used to detect the tilt of the lens (8a), and four capacitive displacement sensors measure the tilt of the lens carrier (8b).
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8b: Four confocal chromatic sensors are used to detect the tilt of the lens (8a), and four capacitive displacement sensors measure the tilt of the lens carrier (8b).
Position monitoring in the wafer stage
White-light interferometers and eddy current displacement sensors are used to monitor the position of the wafer stage or wafer platform. Here, they measure the X, Y, Z movements of the stage, where extremely high accelerations occur. White-light interferometers achieve sub-nanometre resolution and ensure that the wafer is optimally positioned for exposure. Vacuum-compatible measuring heads and insensitivity to strong magnetic fields enable them to be used in demanding environments.
Capacitive displacement sensors are used for fine positioning in the wafer stage. They measure the position of the stage at various points, which is necessary for fine alignment. Capacitive displacement sensors are insensitive to electromagnetic fields and achieve a resolution in the nanometer range thanks to their triaxial design. In addition, they achieve extremely high long-term stability. The non-contact sensors are suitable for both vacuum and ultra-high vacuum (UHV).
Inline wafer quality control
Sensors are used for quality control in various areas, for example, for testing coating thickness and for monitoring deflection and separation. In addition to high precision, a high measurement rate is also required to support high-speed manufacturing processes.
Single-sided thickness measurement of transparent coating layers
Confocal chromatic sensors are used for one-sided thickness measurements of layers. The confocal measuring principle enables the evaluation of several signal peaks, allowing the thicknesses of transparent materials to be determined. Even exceptionally thin layers are reliably detected due to their multi-peak measurement capability. The thicknesses of protective and paint layers can be detected with micrometre precision by just one sensor.
Monitoring of wafer bow and warp
The larger the wafer, the greater the risk of deflection. Confocal chromatic sensors are used to scan the wafer surface to detect bow, warp and distortion.
Confocal chromatic sensors are used to scan the wafer surface to detect bow, warp and distortion.
Total thickness variation (TTV)
Two confocal chromatic sensors are used to measure wafer thickness from both sides. The thickness profile of the entire wafer can also be used to determine the curvature and deflection of the wafer. High measuring rates enable thickness detection of the entire wafer in short cycle times.
Bumps
Confocal chromatic sensors are used to measure the height of bumps on shiny and textured wafer surfaces with micrometre accuracy. A high lateral resolution of 4 µm is achieved due to the focused light spot. In this measurement task, three sensors are moved above the wafer. Both the spherical shape and the height can be calculated precisely from the three tracks. This is possible because the controllers are connected via ethernet and trigger input, thus performing a synchronous measurement.
Checking the saw marks, cracks and break-offs
Confocal chromatic sensors are used to detect cracks, saw marks and other defects on the wafer. They reliably detect surfaces with varying reflection characteristics due to a high-speed surface compensation feature. An extremely small light spot and high resolution enable the reliable detection of the finest of anomalies on the wafer. The sensors ensure reliable detection of the saw marks and other depressions in the wafer.
Highly integrated actuator systems for demanding environmental conditions
A micromechatronic actuator system, comprising a fast-steering mirror (FSM) equipped with optimised non-contact displacement sensors, is used for applications in the semiconductor industry where it is necessary to monitor the rapid tilting of the mirror, for example, in beam stabilisation and laser dicing.
Precise monitoring in wafer handling
Wafers must be automatically removed and fed to the appropriate processing step throughout the manufacturing process. Accurate positioning is just as important as fast, reliable movement. Sensors are used to ensure exact alignment and safe transport in every process step. Clean room suitability of wear-free sensors is a basic requirement for their use.
Determining the position during wafer handling
Exact and repeatable positioning is crucial when handling wafers. Two laser micrometers are used to inspect the diameters of infeed wafers and thus determine their horizontal position. These micrometers afford high measuring accuracies and rates, thus providing reliable position information.
Two laser micrometers are used to check the diameters and horizontal positions of infeed wafers.
Tilt angle measurement of wafers
White-light interferometers are used to measure the horizontal tilt of wafers as they are fed. They provide absolute distance values at nanometre resolution, ensuring the greatest possible positional accuracy when picking up and removing wafers.
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