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User Stories "Semiconductor industry"

The production of semiconductors can be characterised by increasing miniaturisation of components followed by high demands on the in-line metrology and failure analysis of semiconductors component inspection by by optical methods. Reflective metallic surfaces, dull plastic surfaces and ever smaller structures push most optical measuring systems beyond their limits. Optical sensors from Precitec Optronik measure transparent coating and paint thicknesses; they determine and monitor mechanical and chemical etching processes in real time. Modular sensors support fast data acquisition, allowing the capture of both thickness and surface structure from one side. New line sensors, not characterised by a measuring point but through a measuring line, gather broad topographies on wafer surfaces in the shortest possible time.

NONCONTACT MEASUREMENT OF SEMICONDUCTOR CHIPS

FASTEST LINE SENSORS MEASURING TINIEST STRUCTURES

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.

 

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
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

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.

 

 

.. 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.

https://www.youtube.com/watch?v=3EOOPXnMO6U&feature=youtu.be

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