How to Conceal Image Lag

Many 4T pixel designers are forced to make a difficult trade-off between full well, dark current and image lag. The bigger full well, the lower dark current, the larger image lag grows. Small pixel size makes these trade-offs extremely painful, as virtually any combination sacrifices image quality in one way or another.

In that respect it's interesting to look on Micron's patent US7417677, published just a few days ago. Its main idea is to combine a regular readout with a full discharge of the photodiode - see the pictures below:




After the photodiode charge is read, a low voltage, for example, ground, is applied to the floating diffusion and the photodiode. Essentially, the photodiode is charged to its full well or so. Then the photodiode is reset again to high voltage by the usual readout sequence. So the history is mostly deleted, sans a slow trapped charge which needs long time to wipe out.

What we get in the end? All the conventional image lag measurements, such as one described in Nokia-ST SMIA standard, would show very small image lag, if any. This is great, everybody knows that image lag is a thing to avoid.

What we lose? We totally distort low-light linearity in comparison with "true" lag-free sensor. Best case, the small signal would be reduced to even smaller one, much out of proportion. Worst case, the small signal would virtually disappear. However, the small signal linearity is tricky to measure. And for some reason, SMIA allows not to measure linearity below 10% of full scale.

So, using this idea, one ends up with a great looking spec, but bad real world low-light performance. Probably, low-light linearity data should complement the image lag number to give a true understanding of sensor behavior.

Update: Just because there exists a patent does not mean it is used by Micron, or used in the manner described.

Image Sensors Reverse Engineering

Everybody looking for reverse engineering reports starts from Chipworks and Semiconductor Insights. Now it appears they have a competition from China. Sanguine Microelectronics (Shanghai) offers image sensor reports too. The sensors in these reports are not that recent as Chipworks and SI ones and choice of sensors is quite limited, but it's always good to see a healthy competition.

Ziptronix Targets BSI Wafers Manufacturers

I-Micronews published an article about Ziptronix, a provider of wafer bonding solutions. Recently Ziptronix targets BSI sensors to use its patented room temperature direct oxide bonding process to attach a sensor wafer to a carrier wafer.

Thanks to J.B. for letting me know about the new Ziptronix offering.

Omnivision Earnings Call

Omnivision reported lower than expected earnings but higher revenue. As expected, the company trimmed down the next quarter guidance. Seeking Alpha conference call transcript adds more information on Omnvision's business:

• VGA and below was 70% of revenue.
• Gross margin was 25.2% compared to 27.2% previous quarter. Excluding stock compensation, gross margin was 25.7%, down from the 27.7%.
• VisEra, the color filter and micro lens JV with TSMC is preparing for Omnivision's eventual migration to 300mm wafer production.
• In smart phone market Omnivision takes market share from competitors, which will drive higher volume shipments this quarter for 2MP, 3M and further 5MP sensors.

The call also has an interesting mention of lens on dye technology:

Doug Freedman - American Technology Research:

Can you talk a little bit about where the company stands on lens on dye technology? And where you see that fitting in?

Bruce Weyer, VP of Marketing:

Obviously, we stay on the cutting edge of technology that you seen with the OmniBSI architecture announcement we made. And certainly lens on dye technologies are something that everyone is monitoring closely and considering for the next generation in the market. So certainly we got our eye on that technology and we are doing internal assessments and if not yet this time announce any technology that into the marketplace but we continue to monitor that and we will discuss that at the appropriate time.

Doug Freedman:

Is this impact ownership that all liability at the industry was just switch to lens on dye type of product?

Peter Leigh, CFO:

I think the short answer is no. I mean first of all our ownership interest we then take is reasonably small and we are not going to allow our ownership at a small stage in a CSP supplier to have a major impact on our choices about technology. That’s just not possible.

No analyst asks questions on the status of BSI and TrueFocus technologies. After a wave of downgrades, it looks like nobody sees them as a meaningful boost for Omnivision.

Talking about the BSI status, the company only tells that "all of the tier-1 mobile phones suppliers have expressed strong interest in our BSI-based designs." and "Our 5 megapixel and 8 megapixel OmniBSI architectures are generating strong interest among the mobile phone makers. We already engaged in our first specific design activity as a Tier 1 customer. We look forward to sharing detailed information about these exciting products in the near future."

No explicit word was said about BSI product sampling promised first in June, then in July. To me this looks like a sign of BSI having some serious problems making it un-sampleable.

Update: Probably "lens on dye" is badly transcribed "lens on die". So Doug Friedman asks about wafer level cameras.

Tessera Presentation at OPTO Taiwan 2008

Tessera published its nice image sensor packaging presentation on OPTO Taiwan 2008. Naturally, it does not talk a lot about Tessera's package problems and disadvantages, but it's interesting read anyway.

Intertech-Pira 2008 Image Sensors Conference

Intertech-Pira holds 2008 Image Sensors Conference, this time in San Diego on Nov. 17-19. It's mostly marketing event and it has quite a few interesting presentations in the agenda:

Pre-conference seminar: Image processing challenges in the world of small pixels
Dr Frederic Guichard, Chief Scientist Officer, DXO LABS, France

Current status and future trends in imaging
Jason Hartlove, Imaging Industry Executive, GREENWAY CAPITAL AND MANAGEMENT, US

Market trends and prognosis for image sensors
Shri Sundaram, Manager, Business Development, TOSHIBA, US

Handsets: Mpixels, small pixels and image quality challenges
Arie Gavriely, Director of Marketing /Business Development ADVASENSE, Israel

Creating big results from small pixels in camera phones
Feisal Mosleh, CMOS WW Marketing and Business Development Director, EASTMAN KODAK, US

Reflow: Compatible wafer-level CMOS imaging lenses
Jyrki Saarinen, SVP of Marketing and Business Development, HEPTAGON, Finland

RICA architecture for camera phone image processing
Graham Townsend, CEO, SPIRAL GATEWAY, UK

Liquid lens: The best solution for camcorder phones
John Barber, VP Marketing, VARIOPTIC, France

Hardware accelerated capturing software
Sami Neimi, CTO, SCALADO, Sweden

Mobile imaging in emerging markets
Saku Hieta, Director, Camera Sourcing, NOKIA, Finland

Why will all optics be digital?
Frederic Guichard, Chief Scientist Officer, DXO, France

Benefits of xenon flash for today’s mobile phone cameras
Rudi Blondia, Director of Lighting Technology, PERKINELMER, US

Backside illumination technology
Michael Okincha, Business Development and Technical Marketing Manager, OMNIVISION, US

Development of a fully-depleted, thick back-illuminated CCD with high sensitivity to visible and near-infrared wavelengths
John Gilmore, Manager, Image Sensor Group, HAMAMATSU, US

Advances in CMOS image sensors
Homayoon Haddad, Director, Advanced Pixel Research Center, MAGNACHIP, US

Low-noise CMOS sensor for high-quality imaging
Hirofumi Sumi, Distinguished Researcher, SONY CORPORATION, JAPAN

Standard CMOS single chip time of flight sensors
Cyrus Bamji, CTO, CANESTA, US

Its time to go small
Mark Boomgarden, VP of Wafer-Level Camera, TESSERA, US

Application specific CMOS imagers for defense and space
John Tower, Technical Director, SARNOFF CORPORATION, US

Image sensor requirements for 2D barcode scanners
Bradley S Carlson, Senior Principal Engineer, MOTOROLA, US

Industrial/professional imaging differentiation
Gareth Powell, CMOS Marketing Manager, E2V, France

Image sensor requirements for retinal prosthesis
Kelly McClure, Sr Director of Externals Engineering, SECOND SIGHT, US

High dynamic range sensor requirements for vehicle imaging applications
Kenji Tashiro, R&D Engineer, JAI INC, US

CMOS imaging system-on-chip for HD video and beyond
Lester J Kozlowski, CTO and Founder, ALTASENS, US

Image sensing requirements in surveillance applications
Ricardo J Motta, CTO and Vice-President of Imaging, PIXIM, US

Tessera on Mobile Imaging Trends

Mobile Handset DesignLine: Tessera's R&D Director Giles Humpston started a four-part article taking a look at the trends and design challenges of image acquisition and processing on cell phones. The first part discusses camera modules.

Some of the most sought-after camera module characteristics include:

• Low light sensitivity, especially the ability to take photographs indoors without flash. Flash photography is very power hungry in a product where battery life is often a major consideration for the consumer. Flash photography can also introduce undesirable image artifacts, such as red-eye, that then need correction.
• Focus, image stabilization and optical zoom. Clearly, photographs need to be in focus. Image stabilization is, in many ways, analogous to focus because it is also image blurring, but in a lateral direction. Because the distance of the object to the camera varies, what is required is the means of either adjusting the focus to suit or extending the depth of field. Optical zoom allows the user to get closer to the subject of the photograph and details of the scene to be magnified. Digital zoom degrades image quality, so is undesirable if the captured quality is already low.
• Higher resolution. Although higher resolution does not directly translate into higher picture quality, acquisition of additional information by a higher resolution imager facilitates effective image enhancement by software.
• Size reduction. The height of camera modules is one factor limiting the thinness of cell phones where the current fashion is for extreme thinness. Camera modules are typically around 5mm high, but would ideally be less than 1.5mm tall.
• Cost reduction. Camera modules and the associated image processor are relatively expensive components and contribute to the overall handset price. This is especially true for cell phones that have two cameras, where the cost of the cameras is around 12 percent of the total handset bill of materials [Semiconductor Insights, 2008]. A long-term goal of the industry is the $1 VGA camera module.

Part 2 of this article series looks at "revolutionary new hardware technology" (Tessera's one?) and its impact on camera module design and size. Part 3 will discuss how software enhanced lenses can favorably boost camera phone performance. Part 4 considers how numerical image enhancement and face detection can be embedded in mobile devices to bolster the image taking and viewing experience.

Analysts Cautious on Omnivision

Barron's: Doug Freedman, an analyst at American Technology Research, says that both the low-end and the high-end of the image sensor market are being squeezed by both competition from Korean vendors and by Micron’s Aptina unit. “In an intensely competitive commodity business, OVTI must demonstrate the ability to deliver both gross margin improvement and increased revenue, either with more cost effective products or differentiated technology,” he writes - it looks like Doug does not believe in OmniBSI or TrueFocus as possible differentiators.

J.P. Morgan’s Paul Coster also wrote a cautious note on Omnivision.

More on Omnivision BSI Plans

Tech-On published an article based on conversation with Bruce Weyer, OmniVision's VP Marketing.

Bruce tells that Omnivision's 1.4um BSI pixel sensor can achieve performance equivalent to a 1.75um pixel sensor that uses conventional frontside illumination (FSI) technology. He also tells that BSI technology allows for greater zoom tolerance due to the wider chief ray angle (CRA), which enables shorter lens heights and in turn allows for thinner camera modules. In addition, BSI technology affords a much larger aperture size, facilitating lower f/stops.

According to Weyer, OminVision's first product using BSI technology will probably be an 8MP image sensor targeting the mobile phone markets. OmniVision started demonstrating sensors using BSI technology to its major customers in May. Sampling "was scheduled to start in July" - quite a strange statement for an article written in mid-August. Mass production is expected to start in the later part of 2008.

Aptina Extends 2.2um Pixel Life

Yahoo: Aptina announced MT9P014 5MP image sensor for camera-centric mobile phones. The new third generation 2.2um pixel, 1/2.5" optical format sensor is supposed to enhance low light sensitivity, reduce pixel noise, enhance color fidelity and provide dynamic high light performance. The sensor supports HD video 720p @ 60fps. The data output options include a high throughput 2-lane MIPI serial interface capable of 1500 Mbps or greater.

The sensor interfaces with Aptina’s newly announced MT9S311 stand-alone ISP.

MT9S311 and MT9P014 samples are presently available with mass production scheduled for September 2008.