The market for CMOS image sensors and image signal processors (ISP) is one of the most exciting fields of the semiconductor market today. Driven largely by the phenomenal success of camera phones, the market has grown from less than $500 million in 2000, when the first camera phones went on sale in Japan, to $6.3 billion in 2007 and a growth of 6-10% per year in value is expected over the next few years.

In both 2005 and 2006, STMicroelectronics was the world’s leading supplier of CMOS image sensor modules for wireless phones, a remarkable achievement for a company that as recently as 1998 had no image sensing technology and no experience in producing optical devices. In fact, ST’s rise to the number one position in this highly demanding sector was the result of important strategic decisions by ST management and an implementation program that was a model of Execution Excellence.

Imaging technology involves three key aspects:

1. The silicon image-sensing technology itself;
2. Digital processing of the image;
3. Optical/mechanical assembly.

When ST decided in the late 1990s that it would target the embryonic imaging market, the Company made two important early decisions. The first was that it would master the entire imaging chain, from the silicon light-sensing technology and the complex signal processing required for functions such as noise reduction and error correction, through packaging and assembly of complete optical modules, to the end user display controllers. The second was that it would base its technology on CMOS image sensors.

At that time, CMOS image sensing was not considered suitable for high-performance applications such as high-end Digital Still Cameras and the silicon image-sensing market was dominated by Charge Coupled Device (CCD) technology. Although delivering high-quality images, this technology suffers from the twin disadvantages of being relatively expensive and incompatible with the CMOS process that is used for System-on-Chip solutions. However, ST had good reasons to believe that the performance of CMOS-based image sensors could be significantly improved, the result of which would allow the image sensor and the image processing circuitry to operate on the same silicon chip.

ST therefore decided to acquire a suitable CMOS-based image-sensing technology and it found the ideal candidate in VLSI Vision Ltd, a fabless company based in Edinburgh, Scotland, that had pioneered CMOS sensors and was enjoying commercial success in applications such as toys and videoconferencing equipment. In April 1999, ST completed the acquisition of the mother company, Vision Group, forming a new Vision and Imaging Business Unit within the former Consumer and Micro Group, now part of the expanded Home, Personal, and Communications (HPC) Group. One of the first priorities of the new Business Unit was to set up a task force comprising technical experts from Vision and from ST’s Front-end Manufacturing and R&D organizations to develop a new variant of its mainstream CMOS technology specifically optimised for image sensing and to port Vision’s IP to the new process. ST’s Advanced System Technology (AST) group also began developing and patenting advanced algorithms required for functions such as noise reduction and colour correction.

At the same time, ST established another Task Force, this time involving packaging experts from the Back-end Manufacturing organization, to develop optical packages. The most significant challenge here was to develop a complete module for mobile phones. In applications such as Digital Still Cameras, the image sensor can be housed in a package derived from the traditional transparent-lid ceramic package used for UV-EEPROMs. This package is unsuitable for the volume and height restrictions of the mobile phone. As a result, the Task Force decided to develop a new package for the entire camera module, including the image sensor and its carrier (e.g. a ceramic or plastic cavity), an IR filter, a lens holder, the lens barrel containing typically 2 -4 lenses, digital signal processing and power supply circuitry, and a flexible connector. ST’s packaging team developed its first complete solution within only 18 months.

Developing the optical module was a challenge, but manufacturing it in volume brought additional challenges. Because tiny particles of dust no bigger than a few microns that settle on the silicon surface can prevent an electronic circuit from working properly, all silicon chips are manufactured in ultra-clean environments (“clean rooms”) where the air is so pure that there is no more than one speck of dust larger than 0.5 microns in each cubic foot of air. However, once the chips are manufactured and protected by a tough protection layer they are normally assembled in their packages and tested in facilities where the air does not need to be filtered.

However, in the case of complete camera modules, the tiny lenses are also susceptible to contamination from dust particles. Eliminating dust at later stages of manufacturing provided a new challenge for ST. For the first time, it was necessary to develop a back-end manufacturing environment that included “clean room” specifications and to implement this in two different low-cost locations in order to guarantee the volume deliveries required by the mobile phone market. The “Vision Program” became a global effort, with teams in Singapore and Shenzhen, China, contributing their manufacturing expertise. The result was a vertically-integrated supply chain that allows ST, through complete control of the sensor, lens, module, and processor, to design and manufacture camera modules that are optimized for the ST sensor silicon, leading to the best possible quality while meeting the aggressive cost targets of the mobile phone market.

ST has been extremely successful in this market due to best-in-class pixel performance, sensor and module development capabilities, and full ownership of the manufacturing flow. Moreover the Company’s wide portfolio of products, from SMIA (Standard Mobile Imaging Architecture)-compliant camera modules, with a resolution of CIF (Common Intermediate Format) up to 5 Megapixels, to System-On-Chip products that integrate ST’s imaging and digital signal-processing technologies on a single chip, allows customers to easily upgrade their systems while preserving physical and logical interfaces.

According to independent market analyst Prismark Partners LLC, over 70% of the 1.2 billion mobile phones forecast to be sold in 2008 will have an on-board camera and this penetration is expected to grow as much as 80% by 2011. Moreover, many new mobile phones now contain a second camera, for conferencing applications. In addition to mobile phones, camera modules can be used in a wide range of portable devices such as PDAs, webcams, Digital Still Cameras (DSCs), and wireless security cameras. New applications are also being developed in, for example, the automobile sector where automatic visioning instruments promise to increase active and passive security, and in medical diagnostic equipment.

Notes on the Market
According to several market analysts, STMicroelectronics was the world leader in market camera modules for mobile phones in 2005 and 2006.

Mobile-phone camera modules are a fast-growing industry segment that Prismark predicts will grow by 70% to 1.3 billion units a year by 2011, representing a 80% penetration of primary cameras in phones worldwide, and over 23% of secondary (video) camera penetration in camera phones.

ST has shipped over 220 million modules to date.

November 2007

In July 2004, STMicroelectronics and Nokia released the Standard Mobile Imaging Architecture (SMIA): a comprehensive specification for camera modules aimed at standardizing all aspects of the modules, from electrical, mechanical and functional interfaces to their characterization, optical performance and reliability. The SMIA specification is offered for free to the mobile imaging industry and is available at www.smia-forum.org .

Glossary
Back-end refers to the second main stage of the manufacturing cycle of a silicon chip, where the individual silicon chip, usually containing millions of microscopic transistors, is assembled in a package that provides mechanical protection and interconnection to the outside world. The assembled package is then tested, marked and shipped.

CIF (Common Intermediate Format) refers to an image sensor with a resolution of 320 x 240 pixels.

CMOS (Complementary Metal Oxide Semiconductor) is the generic term for a family of semiconductor technologies used to make the vast majority of integrated circuits. CMOS technology allows millions of transistors to be built on a small silicon chip and interconnected to form complex electronic circuits.

CCD (Charge-Coupled Device) is a specialized semiconductor technology mainly used for image sensing applications. It’s main disadvantages are that it requires multiple, non-standard power supplies and cannot be integrated into the standard CMOS manufacturing flow, which means that CCD sensors and CMOS image processing circuitry cannot be integrated onto a single chip.

Image Sensors are semiconductor devices that produce an electrical signal that varies according to the amount of light incident on their surface.

Front-end refers to the initial stage in the production of a silicon chip, in which many identical devices are simultaneously produced on a single wafer of crystalline silicon, usually 200mm or 300mm in diameter, via a complex sequence of steps involving actions such as etching unwanted material, implanting carefully controlled amounts of materials that modify the local electrical behavior of the silicon, and so on, These manufacturing operations are carried out in an ultra-clean environment to eliminate contamination by dust particles and ensure the maximum degree of reproducibility. When all the wafer processing steps are completed, the wafer is cut to separate the individual devices, which are then assembled into their packages in the back-end stage.

UV-EEPROM is type of memory device where the contents of the entire memory array can be erased by exposing the surface of the chip to UV light. For this reason, the silicon chips are housed in a package that has a transparent lid, allowing UV light to reach the chip surface. This type of package is suitable for use in DSC and many other applications but is too big to be used in mobile phones.