Optical Microscopy and Imaging ( Part 2 )

Transcription

1 1 Optical Microscopy and Imaging ( Part 2 ) Chapter 7.1 : Semiconductor Science by Tudor E. Jenkins Saroj Kumar Patra, Department of Electronics and Telecommunication, Norwegian University of Science and Technology ( NTNU )

2 2 Objectives Thermography Nomarski Microscope Confocal Microscope

3 3 Thermography Thermography is based on the principle of infrared imaging. Infrared energy emitted by a semiconductor device is detected to built up an image of the temperature distribution of the operating device. The detected infrared radiation is presented as a digital temperature map of the circuit. The thermograph of a circuit is compared with that of a known, correctly functioning device, allowing failures in parts of the device to be analyzed. Thermography can be used to determine hotspots in circuits.

4 4 Nomarski Microscope It belongs to the class of phase contrast microscopes. A small change in thickness or slight change in refractive index changes the phase of the reflected or transmitted light. This phase change is used as the source of contrast to observe the micro defects and dislocations. Light passes through a polarizer, strikes a beam splitter and is reflected through a Nomarski prism. Nomarski prism consists of two separate birefringent quartz prisms cemented together whose optic axes are at right angle to each other. When positioned properly, light through the prism splits into two in-phase components. Separation between two beams is of the order of 0.2 micron. Thedivergenceofthebeamsmatchesthefocal length of the objective lens.

5 5 Nomarski Microscope ( continued ) Parallel beam of light strikes the sample surface. For perfectly smooth reflecting surface, beams are reflected in phase and produces plane polarized light after passing through the Nomarski prism. The analyzer is set such that this light is not received. When two beams strike the surface having different heights, phase difference will be introduced between two beams. Due to this phase difference, the transmitted light from the Nomarski prism will be elliptically polarized and hence some of the light will be transmitted by the analyzer. A typical micrograph from a Nomarski microscope is shown in the figure. The chemical surface is first etched. Due to different local etching rates, shallow etch pits are formed This shallow (3nm) etch pits can be clearly visible using the Nomarski microscope.

6 6 Confocal Microscope Confocal microscope illuminates the sample one point at a time through a pinhole. Light from a laser passes through a pinhole to the microscope objective, forming a diffraction limited spot at the surface. Light reflected back from the sample passes through the objective, then a pinhole to the detector. Since the microscope illuminates one point at a time, galvanometer mirrors or acousto-optic beam deflectors are used to scan the whole sample surface. Light intensity reflected from any point falls off rapidly to zero as it goes out of focus. Therefore images at different depths or images of surface relief can be produced, since features of one layer are not obscured by the glare from another

7 7 Confocal Microscope ( continued ) By storing images at different depths, it is possible to build up a 3D image of the sample which is also known as optical sectioning. The figure below shows a scanning confocal microscope image of an integrated circuit built-up from images taken with the focus at the substrate, 1.2 micron above and 2.4 micron above the substrate.

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