Symposium on Microoptical Imaging and Projection (2)

Here is a quick overview of the second day, mainly devoted to the technology of the micro-optics.

Stephan Heimgartner (Heptagon) started the day with a talk about wafer-level micro-optics for computational imaging.  He highlighted the technology of Heptagon, ranging from wafer-level optics (= lenses on a glass wafer of 8”) to wafer level packaging of these lenses.  The most complex wafer level packaging technology does include 4 lenses (= 2 wafers with lenses on both sides), 2 spacer structures and an IR cut-off filter.  This stack of optical elements is used today on low-resolution sensors.  For the higher Mpix sensors this technology is not suitable.  The reason for this is the limitation that is defined by the tolerances of all the various materials and structures involved.

In the second half of the talk Stephan explained the Heptagon technology for multi-aperture cameras.  Remarkable is the location of the colour filters : on top of the micro-optical stack.  Also intriguing is the back focus adjustment of the structures that can be done after the lens stack is completed.  During the talk a prototype of a 2 x 2 multi-aperture camera was shown built on a 2M pixel sensor.

Zouhair Sbiaa (Nemotec) more or less confirmed what was already told by the previous speaker.  The optical modules built by means of the wafer-level technology are limited to two wafers due to tolerances.  Zouhair showed a proto-type of a micro-optical component on top of a HD 720p sensor.  This optical module was individually place on top of the sensor.

Although not indicated in the program, Steven Oliver (Lytro) gave a talk about their light field camera.  The talk started and ended with some marketing stuff, but in between there were some very interesting slides shown.  With the light field camera, more freedom can be generated in (after-)focusing of the image, but also some freedom in perspective view is possible.  Playing around with filters (in the software) can add more features to the image, also great was the demo with the movement of light and shadows.

More on the technical side : the 3.0 camera (intended for social media) has a high quality lens included (8x magnification, F2).  Just in front of the sensor, a micro-lens array is placed.  The latter had 330 x 330 micro-lenses, arranged in a hexagonal grid.  The pitch of the micro-lenses is 13.9 um, and every micro-lens is covering 10 x 10 pixels of the sensor.  The sensor itself is a 14M pixel device, finally cropped into 11M pixels.

Flavien Hirogoyen (ST Microelectronics) gave us literally a deep insight in the pixels by showing great results about his simulations of the optical characteristics of the CMOS pixels.  The pixels were 1.45 um in size, and it is remarkable how good the FTDT (Finite Difference Time Domain) simulations fit to the measured data of quantum efficiency.  The speaker showed results for monochromatic light as well as for white light of different colour temperatures.

Andreas Spickermann (IMS Fraunhofer, Duisburg) concluded the morning session with an overview of what his company is able to do in the field of CMOS image sensors.  Too much actually to list here, but worthwhile to mention : IMS Fraunhofer have their own 0.35 um opto process with pinned-photodiodes, colour filters and micro-lenses.  Besides many others, an interesting option is the use of a nitrogen-enriched Si3N4 layer for the passivation.  The layer has a relatively larger transmission in the region above 200 nm wavelength.

Palle Dinesen (Kaleido) kicked off in the afternoon with an all-glass solution approach for micro-optics.  He gave a nice overview of all issues involved with the making of the tools to mold the glass lenses.  Glass has the advantage over plastic to be much thinner, less sensitive to temperature variations .  The know-how of Kaleido is situated in the making of the tools (by a grinding method) as well as in the molding process.  What could be understood : the material is pre-heated before it enters in the molding chamber.  The molder chamber is kept at a relative high temperature, and after the molding the end-result gets a specific cooling-down treatment.  The formation of the lenses on two sides of the carrier is done in one step.  The technology was demonstrated by means of a proto-type on 2”, and now the technology is expanded to 4”.  Mass production will start in Q3/2013.

The final presentation came from Reinhard Voelkel (Suss Microoptics) who tried to give us an answer to the question : “How many channels do you need in an array ?”.  To get an answer on this question, the speaker checked what is present in nature.  Conclusion : or 2 (there were many examples in the room), or 8 (some spiders seem to have 8 eyes) or many (insects).  So draw your conclusion ….

One final word about the organization of the symposium : perfect !  The organization even distributed rain coats to the participants.

Albert, 28-11-2012.

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