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Expériences client "Semi-conducteur"

Expériences client "Semi-conducteur"

La course à la miniaturisation dans l’industrie du semi-conducteur entraîne une forte demande pour du contrôle en ligne de haute précision et de l’analyse des défauts des composants à l’aide de méthodes optiques. La dynamique de mesure des capteurs optiques Precitec permettent de mesurer sans difficulté des surfaces réfléchissantes (contacts, pistes) ou absorbantes/diffusantes (packaging des puces). En complément, la très grande précision permet d’analyser des structures complexes avec une très grande finesse telles que des TSV ou «trench». La possibilité de mesurer l’épaisseur directement à travers un wafer silicium permet une inspection en temps réel lors du processus de polissage avec des capteurs directement intégrés dans les machines de production. La disponibilité du nouveau capteur ligne Precitec décuple les possibilités déjà importantes du capteur point. Sans perdre en qualité ou précision, l’accès à des temps de cycles de quelques secondes permet de mesurer des topographies de dimensions importantes voir la totalité de la surface du Wafer en un temps reccord.



Semiconductor technology is celebrating a very special global jubilee: the law that is only valid for semiconductors turns 50 - Moore’s Law - it states that the complexity (sometimes also the level of integration) of integrated circuits doubles, depending on the source, every 18 to 24 months. In parallel and almost as impetuously as the digital revolution, which triggered this law, is the ongoing development of optical measurement techniques for semiconductors. Line sensors based on chromatic-confocal technology are nowadays state of the art. This report describes the technology and the results of this measurement technique for the smallest structures in the micrometer range.


No touching allowed: noncontact measurements


For the surface of a semiconductor chip (wafer) to be measured or analysed, 3D-data has to be collected of sufficient resolution to allow the examination of structures or geometries of the circuits on its surface. Today’s chip technologies require nanoscale resolution in the axial direction and a few micrometers in lateral direction. It goes without saying that the fragile parts should not be touched during the measurement and so the only option is to measure in a noncontact mode. More often than not semiconductor profiling comes down to the need to deliver high density 3D data from a large surface area that contains the finest structures. With a conventional chromatic confocal sensor this is likely to be a very time consuming business akin to painting an area of 1 square kilometer with a conventional paintbrush. With new range of chromatic confocal line sensors 192 measuring points simultaneously profile the surface in 5.2x10-3 (1/192) of the time needed compared to conventional sensors, which use only one measuring point.


Despite high measurement speed of the line sensors focusing on representative sectors can save even more time. These regions are usually defined by the manufacturing process; but nonetheless, it is most important to receive precise and descriptive results of those areas.



Figure1: 3D-Wafer-Topography, measured with a line sensor. The structures are 9 μm high compared to the inner circle area (yellow). The displayed area was scanned in less than a second. © Precitec Optronik

The Chromatic-confocal Measurement Principle


In their operation chromatic-confocal sensors exploit an optical aberration, not focusing the white light to a single point, but focusing the different wavelengths along the optical axis. Blue focuses closest to the optic, red furthest away with a continuum of visible wavelengths in between. As long as the surface remains within this working range a chromatic-confocal sensor will not need any optical axis movement to profile it. This method is especially suitable for polished and mostly specular wafer surfaces but is equally useful on ground and rough surfaces too.


Line Sensors


Line sensors are the latest chromatic-confocal measuring devices for semiconductor chips. These sensors measure numerous points close to each other so that the optical probe can cover a much larger area in a given time compared to a point sensor. The current generation of line sensors from Precitec Optronik operates with 192 measuring points, which measure, depending on the probe, in a line from 1 mm up to 5 mm in length.



Figure 2: A CHRocodile CLS Line sensor from Precitec Optronik with its different optical probes for measuring ranges between 200 μm and 4 mm. Resolution in the axial direction ranges from 20 nm up to 320 nm. © Precitec Optronik

Depending on the measuring area, three interchangeable optical probes provide accuracy ranging from 80 nm to 1,2 μm


3D-Data within the shortest time


The raw data delivered by the sensor can be processed in software to detect periodic patterns and structures. Most frequently used are coded height views to detect faults by observing the height, radii, diameters and gaps in structures. Another display possibility is the height profile, showing a cut through the scanned structure, which can be manipulated in software after the measurement has been made.



Figure 3: 2D-Height-view measured by a line sensor with an enlarged section shown to the right. The outer diameter of the circular embankment-like structures (bumps on an LED-Chip) is 230 μm. © Precitec Optronik

.. and it still goes on


Additional applications for the line sensors will emerge as chromatic-confocal optical probes with bigger measuring ranges and higher apertures are developed. The dream of higher scanning velocities requires additional adjustment options: The measuring frequency can be increased but this results in a decrease in measuring range by the same factor as the frequency. The current line sensors from Precitec Optronik operate with a measuring frequency of 2 kHz. The wafer topology measurements for this report were acquired at an enhanced frequency of over one million points per second. The decreased measuring range does not appear too severe because wafer structures are relatively flat. Line sensor systems are currently the fastest way of acquiring three dimensional semiconductor topographies. It is repeatedly reported that Moore’s Law will soon expire, but this has had to be revised several times (it is now said to end around 2030). This could also be said to apply to our line sensors, because, as with Moore's Law there appears to be no end in sight for the rapid advancement of either.

Figure1: 3D-Wafer-Topography, measured with a line sensor. The structures are 9 μm high compared to the inner circle area (yellow). The displayed area was scanned in less than a second. © Precitec Optronik
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    Figure1: 3D-Wafer-Topography, measured with a line sensor. The structures are 9 μm high compared to the inner circle area (yellow). The displayed area was scanned in less than a second. © Precitec Optronik

    3D-Wafer-Topography, measured with a line sensor. With new range of chromatic confocal line sensors 192 measuring points simultaneously profile the surface in 5.2x10-3 (1/192) of the time needed compared to conventional sensors, which use only one measuring point. In this application example height and form of bumps have been measured for contacting of semiconductors. 

    Chromatic confocal sensors utilize the property of an optical system not focusing white light at one point, but separated by wavelength at different distances along the optical axis. Chromatic confocal CHRocodile sensors from Precitec Optronik therefore use sophisticated technology for signal evaluation, demonstrating their particular strength to strongly scattering or semi-transparent surfaces where other well-known 3D measurement methods may fail. The current generation of line sensors operates with 192 measuring points, which are arranged depending on the measuring head on a line from 1 mm to approximately 5 mm. Three interchangeable probes offering accuracies from 80 nm to 1.2 microns depending on the measuring range.








    Grey rectangle in the center of the components: This is the dam of a viscous sealing compound. The height of the dam must be measured.

    A good example of inline 3D metrology is the measurement of 3D microstructures on printed circuit boards by optical sensors using chromatic confocal measurement technologies. Their ability to measure vertical profiles with high resolutions directly on the chip led to developing of dispensing machines which apply protective coatings and encapsulations to sensors. The term liquid encapsulation is understood to mean different methods to encapsulate electronic components with a liquid substance at room temperature without the aid of product-specific tools. After casting components are then protected against mechanical stress and environmental influences. Then the dispenser applies a material with lowest possible viscosity (thinner) to the inside surfaces of the dam which envelops fine structures and wires completely particularly well and without bubbles, until the part is completely covered.

    Referred to a case called "dam and fill" method a dam is first built on the PCB from a high viscosity (viscous) sealing compound, which completely surrounds the chip. Finally optical sensors measure the height of dams made of chip encapsulants with non-contact technology and accuracy down to a few micrometers. In this way it is ensured that the dam height for the subsequent casting of the inner area is sufficient and protective lacquer cannot inundate unprotected areas. The method is always used if there is only a limited area or height available for encapsulation. The rapidity of the measurement method enables 100% inspection of all components produced; the encapsulating of components is therefore also suitable for high-volume and fully automated production.

    Inline 3D metrology in dispensing machines for PCB production: Non-contact chromatic confocal sensors measure dam heights made of highly viscous chip encapsulants.

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