On chip Color Resist for Image Sensors

The image sensor is a semiconductor that converts the brightness of the light that enters the camera lens into electric signals. It has the same function of the retina in the human eye. By converting the light into electric signals, we are able to reproduce the light as images or videos.
There are mainly two types of image sensors, CMOS and CCD. In the past, the two types of sensors were used in different applications according to their characteristics. CCD sensors, which have excellent image quality and little distortion, were often used for digital single-lens reflex cameras. On the other hand, CMOS sensors which have smaller power consumption and are easy to miniaturize, were often used for mobile phones and compact digital cameras.
Nowadays, CMOS sensors that match the image quality of CCD sensors have been developed, so most cameras and smartphones now adopt the CMOS sensors.

On chip Color Resist for Image Sensors

• What is an Image Sensor?
• Structure of Image Sensors
• Types of On-Chip Color Filters
• Applications of Image Sensors
• Required Properties of On-Chip Color Resists
• On-Chip Color Resist of TVS

What is an Image Sensor?

Color filters are used not only for FPDs, but also for small chips (semiconductors) installed in smartphones or digital cameras. Especially, the image quality of a digital camera is highly influenced by the performance of a semiconductor called the “image sensor.”

The image sensor is a semiconductor that converts the brightness of the light that enters the camera lens into electric signals. It has the same function of the retina in the human eye. By converting the light into electric signals, we are able to reproduce the light as images or videos.

There are mainly two types of image sensors, CMOS and CCD. In the past, the two types of sensors were used in different applications according to their characteristics. CCD sensors, which have excellent image quality and little distortion, were often used for digital single-lens reflex cameras. On the other hand, CMOS sensors which have smaller power consumption and are easy to miniaturize, were often used for mobile phones and compact digital cameras. Nowadays, CMOS sensors that match the image quality of CCD sensors have been developed, so most cameras and smartphones now adopt the CMOS sensors.

Structure of an Image Sensor

Structure of Image Sensors

An image sensor consists of three layers, the on-chip lens, the color filter and the photo diode. An on-chip lens consists of many micro lens arranged to form a sheet. This arrangement increases the light-gathering power and enhances the sensitivity of the image sensor.

Each micro lens corresponds to a subpixel on the color filter and the corresponding color information (red, green or blue) will be given to the light transmitted through each part of the sensor. The light receiving element of the photo diode then detects the color and brightness, converts the information into electric signals, and finally records the scene as an image.

Other than the primary color filter (red, green, blue), there is also the complementary color filter (yellow, cyan, magenta, green) for on-chip color filters. These two types are used according to the purpose of the image sensor. The primary color filter is advantageous in expressing colors more vibrantly and is often used for digital still cameras. The complementary color filter has a higher resolution and is often used for video cameras.

Types of On-Chip Color Filters

Applications of Image Sensors

Image sensors are installed not only in home appliances such as digital cameras and smart phones, but also in cameras for special applications such as surveillance cameras, in-vehicle cameras, and endoscopes. The sensor technology we have today is able to detect light beyond the visible region such as infrared rays, making the images that were once invisible, visible.

Security Cameras

Images that were taken at night used to be in black and white, but by sensing infrared rays, they can now be displayed in vivid color just like images taken at daytime. This way, a security camera can record the shape of a person's face and the color of their clothes even in a dark environment.

In-Vehicle Cameras (Night Vision)

Night vision is a system developed in order to prevent car accidents during night-time. By detecting infrared rays emitted from the human body, we are able to see pedestrians and obstacles that are over 200 meters away (the maximum range of headlights).

Endoscope Cameras (Capsule Cameras)

The capsule camera is a tablet shaped camera that is only 25 mm long and can also be swallowed. It can film parts of the digestive system where non-swallow type endoscope cameras cannot reach, so there is no need for laparotomy to confirm the symptoms and give a diagnosis.

Required Properties of On-Chip Color Resists

As cameras become smaller and more sophisticated, the color filters in the image sensors are also required to become smaller and have higher resolutions. For example, in the case of a 55 inch full Hi-vision TV, the standard size per one pixel is about 200µm X 600µm, but in case of a smartphone, the size per one pixel is about 1µm X 1µm. This is about 1 to 100,000 in terms of area ratio. As the size of each pixel becomes smaller, the negative effect of foreign objects (such as pigment particles) on the image quality becomes greater. Therefore it is very important that the pigment particles in the on-chip color resist are made extremely small and are uniformly dispersed.

Since image sensors are being introduced for various applications, the improvement of color reproducibility is also required of the displays. Conventional color filters for FPDs could only sense colors in the visible region (400～700mm). However, since the applications of image sensors have spread to devices such as security cameras and medical cameras, filters that can sense a wider range of light (the infrared and ultraviolet region) are being developed. So the development of a new color resist that can sense these specific wavelengths is required.

On-Chip Color Resist of TVS

Through the development of printing inks and paints, we have realized the technology to disperse pigments at the nano-level. By using finer beads to crush the pigment and finer filters to remove any unwanted particles, we are able to conduct a finer and cleaner dispersion. We have also designed new dispersants that can stabilize the dispersed state even at high pigment concentrations. This makes it possible to make color filters with a smaller film thickness. With our advantages in synthesis and dispersion technology, we are able to keep up with the rapid multifunctionalization of color filters and offer new color materials to satisfy your needs.