Virage Logic readies self-repairing embedded memory

Virage Logic readies self-repairing embedded memory
By Anthony Cataldo, EE Times
March 7, 2001 (9:57 a.m. EST)
URL: http://www.eetimes.com/story/OEG20010306S0084

SAN MATEO, Calif. — Embedded-memory provider Virage Logic Corp. (Fremont, Calif.) plans to deliver a technology that takes built-in self-test and diagnostics a step further by enabling a chip to repair its own bad memory bits, company officials told EE Times.

The self-repair algorithm, geared for system-on-chip devices that incorporate big, monolithic blocks and widely distributed chunks of embedded SRAM, is intended to replace defect analysis and repair equipment now used for these chips. Virage Logic officials said they expect the technology will be available in some form within six months.

Adding self-repair processing circuits is the third leg in the company's effort to improve the yields of devices that use on-chip memory. So far, Virage has been leaning on the use of redundant bits and on-chip repair analysis, which includes built-in self-test. If a chip company wants to do bit repair, however, it has to turn to external softwa re and test equipment.

Bit repair is considered crucial to some chip manufacturers because without it, the yields of devices with large amounts of on-chip memory suffer. "Right now, you can't integrate large amounts of SRAM without solving the yield problem," said Virage Logic president and chief executive officer Adam Kablanian. "Anything over 2-Mbit is the break point where you start to see the impact on yield."

The problem is going to have to be addressed soon, Kablanian said, because embedded memory will eventually take up most of the real estate on high-gate-count system-on-chip devices. SRAM is commonly used for increasing on-chip performance and is less power-hungry than logic transistors. At the 0.13-micron generation, the company expects as much as 16 Mbytes of SRAM can be embedded cost-effectively.

The down side, however, is that memory is two times more prone to defects than logic, because it packs so many transistors into a smaller space. "Defect density is a function of how much mem ory you have," Kablanian said. "If you don't do some kind of repair you may in some situations have only 10 percent yield."

Today, devices that are manufactured with an unacceptable number of memory defects can be repaired by using extra bits designed into the memory array. These so-called redundant bits can be tapped by opening a fuse with a laser, bypassing the faulty bits.

While redundant bits and diagnostics have been incorporated into its memory compilers, Virage Logic will next fold in a "soft-repair" algorithm. Using this self-repair mechanism should reduce the die area penalty, improve yield and discard the need for the extra repair step during test, Kablanian said.

When the device powers up, the hard-wired algorithm automatically accesses the redundant bits when it detects bad bits. The sequence is stored in a register and executed with every power-up, Kablanian said.

Much of the self-repair research was conducted by a group of 12 researchers, including chief scientist Yervant Zorian , who joined Virage Logic last June. An IEEE fellow, Zorian was chief technical adviser of LogicVision Inc. and a researcher with Bell Laboratories before joining Virage Logic.

Zorian said key to the self-repair approach is that it integrates more yield-enhancing intelligence into the chip itself. "In general, the repair itself is a known thing to stand-alone memory manufacturers, but you always do it by external means. Now you don't need any additional equipment," he said.

The repair processor will take up "a few thousand gates," Zorian said. That's larger than a BIST module, which takes around 500 gates. But the required die area will be negligible when considering the size of the memories being implemented. It also eliminates the need for fuses, he said.

Zorian said he was uncertain whether the self-repair algorithm will be bundled into an existing compiler or offered as a separate product.

As Virage Logic's researchers find ways to make embedded memories smarter, its business managers hav e been forging relationships with a number of IP suppliers and design services companies that are making use of its embedded memory technology.

Most recently, 3DSP Corp., Lexra Inc., Tensilica Inc., Philips' TriMedia and Virtual IP Group Inc. are among those that have incorporated Virage Logic's embedded memories into their own products. Meanwhile, Cadence's Tality Corp., NurLogic Design Inc. and Sarnoff Corp. are the first design services companies authorized to provide the embedded memories to their customers.