Low Temperature Polysilicon
 |
The early PC displays were passive matrix devices. As the expectations for contrast ratio, color and wide angle viewability went up, it became apparent that passive matrix displays would not survive. As the techniques and technology for making arrays of field effect transistors improved, technology known as active matrix displays became dominant.
Active matrix displays can be further divided into three categories:
|
Amorphous Silicon TFTs
Amorphous silicon semiconductor materials are deposited onto ordinary glass. In doing so, it is possible to build arrays of Field Effect Transistors (FETs) connected together with row and column buss bars. By applying a signal to the column, and applying a signal to a row, it possible to activate the intersecting transistor and apply a field to a liquid crystal material. By sequentially activating row by row as the signals to the columns are changed, it is possible to individually control each element in a rectangular display area. The drain pad of each transistor is connected to a LCD with a color filter that can switch red, green, and blue light. This comprises an amorphous silicon TFT.
But to drive those columns and rows, one must use a higher performance semiconductor material than you can put on the glass itself. If you look microscopically at the array on the glass, the deposited silicon material has very small crystalline domains that are randomly aligned in a lattice. (This is referred to as amorphous silicon). This material has relatively poor semiconductor performance, so you need an LCD driver that is typically built on a separate piece of silicon and is attached to transparent electrodes through a Tape Automated Bonding (TAB) circuit. The TAB circuit is attached to the transparent electrode (indium-tin oxide) through an anisotropic conductive film (ACF). The TAB bonding process provides the path to send the controlling data signals to the array on the glass.
Polysilicon TFTs
High Temperature Polysilicon
With polysilicon displays, the objective is to put the drive electronics – particularly the drivers and the shift registers – onto the same piece of glass. This can be done with high-temperature processes on a piece of fused silicon. With a single piece of quartz (fused silicon), a uniform lattice can be made with superior semiconductor characteristics. The methodology is to first deposit the silicon layer, and then heat it up to the point where it melts and slowly re-crystallizes, making semi-conductors with much higher performance than amorphous silicon TFTs.
This process can be used to make very high performance devices. It is used today in high performance video projectors.
It is not possible to make a large display using this high temperature process, because you can only make a quartz substrate up to about eight inches diagonal. To make reasonably priced larger diagonal displays, the industry needed a process to go onto ordinary glass.
Low Temperature Polysilicon (LTPS)
By replacing the high temperature annealing process with a laser annealing process (which is low temperature) you get domains larger than amorphous silicon, but not as large as the ordinary high temperature annealing process. This gives semiconductor performance (electron mobilities) that are 200 times faster than amorphous silicon TFTs. Now you can place the drivers and shift registers on the same substrate as the LCD cells. This approach allows very high resolution displays where the density of interconnects goes beyond that which is possible with TAB structures. For example, this process can produce VGA displays in a 4" format.
LTPS technology has evolved in two different forms. The first is known as n-channel version, but it has limitations due to electron mobilities. It is the most commonly available LTPS technology that has been in production the longest. You can produce quarter-VGA or smaller formats. Sanyo uses this for camcorder and digital still camera displays.
The second form of LTPS uses both n-channel and p-channel CMOS requiring many more layers (10-12 vs. 6-7) which is far more complex. This allows XGA resolutions in small formats. It is difficult to get this process to the cost point required for highly mobile applications such as PDAs and cell phones. This is the LTPS process adopted by Sony and Toshiba.
|