When it comes to the notion of "embedded," the microprocessor may have to renounce its sovereignty and share its scepter with the flat-panel display. The reason is simply that embedded may soon connote displays as well as microprocessors. When a product contains a display, which is the case in more applications, designers will be pushing to embed as many functions as possible onto the display substrate itself, assuming that the volumes warrant nonrecurring engineering costs.
So, which functions will migrate to the display next? Some display drivers, phase-locked loops (PLLs), and analog-to-digital converters (ADCs) have already been integrated. It's likely that many functions of a product may eventually wind up on the display—be it a cell phone, a PDA, or an entire computer.
Putting a computer on a display, however, probably is an application for the distant future. But it does seem like a natural progression in an integration sequence, following closely in the footsteps of the CPU, which migrated all of the attendant functions onto a single chip and took the name "microprocessor."
Whatever migrates, though, low-temperature polysilicon (LTPS) will undoubtedly be the enabling technology to move functions onto the display. Today, this manufacturing technology is being applied to active-matrix, thin-film-transistor (TFT) LCDs, as well as organic-light-emitting-diode (OLED) displays just now starting to trickle into products delivered to the market place.
"We see the growing adoption of LTPS as a natural evolution of the amorphous-silicon version of the active-matrix TFT LCD," says Vincent F. Sollitto Jr., Photon Dynamics' CEO and president. The company provides yield-management solutions for flat-panel displays, pc-board assembly, and semiconductor packaging. Sollitto points out that virtually every major display manufacturer has some research going on in LTPS.
The distinction in quality between LTPS and amorphous silicon is like night and day with respect to mobility and the ability to fabricate good circuits, with LTPS winning hands down. The most appealing benefit of LTPS display technology is its ability to integrate row and column driver circuitry right on the glass substrate, eliminating the need for separate driver devices. Its high carrier mobility, which ranges somewhere between 100 and 300 cm2/Vs, makes this possible.
By comparison, the carrier mobility of amorphous silicon, today's predominant display fabrication technology, is a paltry two orders of magnitude lower—from 0.5 to 1 cm2/Vs. Because of their far higher carrier mobility, LTPS circuits are much faster. So unlike amorphous-silicon circuits, they can handle high-definition images.
With LTPS, while you're fabricating the transistors for the display, you might as well print the circuitry for the row and column drivers. But why stop there? Adding the multiplexers, the shift registers, and more is a simple, logical step. "Now you're moving toward the computer-on-glass, or system-on-glass," notes Sollitto.
Systems-on-glass could be mobile, like an electronic book, viewer, DVD, car navigation, wireless telecommunications, or fully integrated LCDs on an entire "sheet" computer with a 32-bit CPU and 64-Mbyte memory. But fabricating an entire computer on glass means overcoming the major hurdle of mixing different geometries on a glass plate.
Processor circuitry and display circuitry have quite different resolution requirements. Today's processor geometries are down to nearly 0.1 µm, whereas design rules for displays are approximately 1 µm. This large discrepancy is important because maintaining uniformity and particulation across a display panel is extremely difficult due to a panel's huge size relative to that of a chip.
Integrating just the display's peripheral circuits offers great benefits. Consider the interconnection burden for a super video graphics array (SVGA) of 800 by 600 pixels made with TFT LCD technology. With drivers off the glass, amorphous silicon requires some 4000 connections to the row and column matrix on the display. With drivers on the glass, the interconnection count plummets to just 200—and that's for a slightly higher-resolution display, such as an extended graphics array (XGA) of 1024 by 768 pixels. What's more, no tape-automated-bonding/chip-on-glass (TAB/COG) ICs are required. Yields rise too because of fewer rejects due to fewer interconnections.
"Make no mistake about it, everything we think is coming in the brave, new world will become display-centric," says Sollitto. "And five years from now, if you're not a player in the display business, you won't be a player in the electronic equipment business either because there will be fewer and fewer products that don't employ a display."