Image Sensor Improvements Change Landscape for High-End Applications

While complimentary metal-oxide semiconductor (CMOS) image sensors have made significant gains in applications like mobile phones, computers and low-cost cameras, the sensor's low-light sensitivity performance and image quality have been unattractive for high-end applications where charge-coupled device (CCD) sensors dominate. Despite refinements in pixel structure and low noise process technologies, obtaining high-image quality with low-light sensitivity in CMOS sensors has not been easy. Thus, making it difficult to capture the market in applications like digital still cameras (DSC) and video still cameras (VSC) where CCD-type image quality and sensitivity are desired.

To address the needs of some of these high-end applications, CMOS backers like Aptina have adopted shrinking geometries to cut the die size of electronic processing and control functions, thereby increasing pixel count and the associated light capture area. In addition, the company has further improved pixel structure and ensuing electronics to drastically improve the quantum efficiency (QE) of the sensor. With improved image quality and minimal crosstalk, latest CMOS sensors have been able to capture sharp images with lively color contrast – even in low-light conditions.

Frontside illumination

Aptina, in addition to others, has developed what is called frontside illumination or FSI technology. With this technique, light falls on the front side of the IC and passes through readout circuitry and interconnects before it is collected in the photo detector. Aptina's FSI pixel path incorporates microlenses with optimized shape, optimized color filters, recessed pixel arrays, lightguides, and anti-reflective coatings.

Called Aptina A-Pix technology, enhancements to the third generation Aptina FSI has resulted in substantial improvements in QE and crosstalk, resulting in 25 percent improvement in low-light sensitivity over second generation FSI technology. Exploiting these advances, the company has developed several products aimed at HD video in digital still and video cameras, PCs, and surveillance applications.

In fact, using FSI technology, Aptina has readied a portfolio of high quality, multi-mega pixel CMOS sensors that are improving the capabilities of CMOS cameras for high-end mobile phones, tablets, DVCs, DSCs, and other similar consumer gadgets.

According to Dave Amey, product manager for Aptina's Camera Business Unit, "The performance/cost proposition of FSI-based CMOS sensors is becoming attractive for high-end camera applications." These applications include those that require larger pixels, and where low light and overall imaging performance is more important than resolution.

An advance made possible by Aptina's A-Pix technology is the new 14-megapixel CMOS sensor, MT9F002, for DSC cameras with full high-definition (HD) - 1080p/60 frames per second (fps) capability.

Designed for DVC/DSC applications, the 5-megapixel MT9P001 delivers CCD image quality at lower cost and power consumption, in a small form factor and with short lead time for CMOS, according to the manufacturer. It features full-HD support at 60fps for max video performance with 20 percent additional imaging area for image stabilization.

With a master clock at 96 MHz, the sensor offers 15 fps at full resolution or 30 fps at 1080p with a dynamic range of 70 dB. The on-chip 12-bit analog-to-digital converter (ADC) facilitates high-resolution image capture with HDTV video formats. This single chip solution simplifies design in a small footprint.

Backside illumination

For those applications that require smaller pixel size (1.4 micron and below), manufacturers have adopted backside illumination (BSI). Two suppliers using this technology are Aptina and Omnivision Technologies. Higher-end consumer cameras are now using BSI-based CMOS sensors when cost is not most important.

The primary advantage of BSI is its ability to separate the optical elements from the electrical elements, allowing a photodiode with its electrical components on one side, and the optical path on the other. This means the optical path can be optimized independently of the electrical and vice-versa. As a result, it achieves a higher QE while minimizing crosstalk.

FSI and BSI basics, along with advantages and disadvantages of each of the technologies, are discussed in the white paper "An Objective Look at FSI and BSI" by Aptina. The paper shows that for 1.4 micron BSI pixels, QE is typically between 50 and 60 percent, with crosstalk between 15 and 20 percent. According to Aptina, the combination of BSI's high QE and somewhat degraded crosstalk at 1.4 µm results in a net overall image quality that is comparable to FSI. Another major advantage of BSI over traditional FSI image sensors is the pixel's shorter optical stack.

Meanwhile, 1.1-µm BSI pixels are in the early stage of development and expected to outperform comparable FSI pixels due to fabrication challenges in shrinking FSI pixels to 1.1 µm. Due to device structure, crosstalk continues to challenge BSI technology, requiring extra processing. This introduces additional cost and tolerances.

Leveraging Aptina A-Pix pixel technology, the manufacturer has demonstrated 1.1- and 1.4-µm BSI based 8-megapixel CMOS image sensors to address the needs of high-end consumer cameras.

Similarly, OmniVision Technologies released its first native HD video sensor last year, built on OmniBSI pixel technology for high-performance HD cameras in notebooks, netbooks, webcams, mobile phones, portable media players (PMPs) and other mobile entertainment devices. The compact 1/6-inch OV9726 delivers 720p HD video at 30 fps. Recently, OmniVision unveiled an advanced 8-megapixel image sensor (OV8830) to support enhanced, fast frame rate image capture and 1080p or 720p HD video recording for feature-rich smart phones.

Built on 300 mm wafers using a copper process with 65 nm design rules, the OmniBSI-2 sensor offers 20 percent improvement in peak quantum efficiency in all color channels, a 35 percent improvement in low-light sensitivity and a 45 percent increase in full-well capacity.

With ongoing improvements in image quality, crosstalk and low light sensitivity, CMOS image sensors are now well-suited for high-end cameras that previously depended on CCD technologies. Now CMOS sensors can deliver CCD-like performance to these applications and offer low power consumption and high speed at a low cost.

Reference

1. An Aptina Technology White Paper: An Objective Look at FSI and BSI