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Samsung builds 4Gbit NAND at 65nm

Posted: 18 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Geoff MacGillivray? Teardown? Samsung? 4Gbit NAND flash?

Samsung has established the No. 1 market position in the top two semiconductor memory markets. But even as the memory leader, the company is not known for coasting. It continues to scale memory products aggressively into advanced geometries, a technique it used to achieve its current position. The latest example of Samsung's scaling efforts is the 4Gbit NAND flash, the first standalone memory to be manufactured at the 65nm node. The device has the smallest 4Gbit die size seen by Semiconductor Insights to date. That gives Samsung a cost advantage in the NAND market.

Samsung holds the top position in both the NAND market (52.9 percent) and DRAM sector (32.1 percent), the two largest memory markets. In fact, Samsung is more than twice as large as its nearest competitor in NAND and nearly twice as large as the second-place manufacturer in the DRAM space. The NOR market is the only major memory segment in which Samsung does not hold the top spot; it was the fourth-ranked NOR manufacturer in 2005, according to iSuppli Corp.

The secret to Samsung's success is its aggressive strategy of moving its products into advanced process technologies as rapidly as possible. By moving quickly to advanced process technologies, Samsung is able to manufacture devices with smaller die sizes. Those small die sizes translate into more dice per wafer and, thus, lower manufacturing costsand ultimately, more price flexibility. The strategy has given Samsung a competitive advantage in the memory industry that other manufacturers have been unable to duplicate.

Recent examples of Samsung's technology scaling include the first 90nm DRAM, a device that was manufactured several months ahead of any other 90nm DRAM. Samsung will also likely be the first manufacturer to produce 80nm DRAMs and will be doing so while many competitors are still ramping volume in their 90nm process technologies. It is now very clear that NAND flash production will take place on much more advanced process nodes than DRAM. In the NAND market, Samsung was the first manufacturer to the 70-/73nm node and is now the first to the 65nm node. NAND competitors like Toshiba and Hynix are close behind, however, and have some technological advantages that help their position.

Cell size trends

The Samsung 65nm 4Gbit NAND flash (K9F4G08U0A) is a monolithic device that uses single-bit-per-cell technology. The device is fabricated using a triple-metal, double-poly, 65nm CMOS process on a 131mm2 die. The single-transistor flash cell measures approximately 0.126-by-0.13n, for a total cell area of only 0.016n2. The overall bit-efficiency rating is 31.3Mbits/mm2.

The 65-nm device is 15.9 percent smaller than the previous 4Gbit-generation device, which was manufactured with 73nm process geometry. However, the die efficiency on the 65nm device is 54 percent, down approximately 6 percent from the 73nm device. The loss in die efficiency is likely due to scaling differentials between the array and peripheral circuitry.

The 65nm and 73nm devices share several innovative process technologies that have been successfully scaled from 73nm geometries to 65nm. These features include a self-aligned floating-gate poly cell along with similar control-gate and floating-gate structures. However, the 65nm device includes some new process techniques to deal with floating-gate coupling, select transistor contacts and shallow trench-isolation depths.

The device uses single-bit-per-cell (SBC) technology, storing only 1bit per memory cell. SBC technology has been the standard flash technology for many years and is easier to implement than multilevel-cell (MLC) technologies. SBC technology is more reliable than MLC and requires less testing time. But it suffers some cost disadvantages compared with MLC implementations because of the lower density.

The 4Gbit NAND flash generation is becoming an extremely competitive area, with several vendors offering very similar devices. The Samsung K9F4G08U0A has the smallest 4Gbit die size that Semiconductor Insights has observed to date. It is 5 percent smaller than the next smallest 4Gbit device, an offering from Toshiba that is manufactured at the 90nm node. Samsung's early 4Gbit offerings were not as competitive as Toshiba's 4Gbit, 90nm MLC device, and it is likely that this part was expressly designed to compete with Toshiba's offering.

The die size advantage offered by Samsung's 65nm technology allows the company to manufacture approximately 469 gross dice per 300mm wafer. This is 6.4 percent more than the Toshiba 4Gbit offering and 11.4 percent more than Hynix/STMicroelectronics' 70nm product.

However, Samsung faces stiff competition from both of those companies. Toshiba is the second-ranked player in the NAND market, and its strategy is to use MLC technology to manufacture devices with very high bit-efficiency ratings in older process nodes. MLC flash stores 2bits of information at each memory cell site, allowing Toshiba to pack more bits into a given area of silicon and lowering the manufacturing cost for a given size of memory. The Toshiba 4Gbit offering is manufactured at the 90nm process node and has a bit-efficiency rating of 29 Mbits/mm2. Toshiba was the first manufacturer to produce a monolithic 4Gbit device and was able to do so with a very effective die size. This device is now eclipsed only by Samsung's 65nm 4Gbit device. Toshiba continues to use its MLC strategy and now has an 8Gbit, 70nm MLC device, the largest monolithic NAND flash device currently on the market.

Hynix and STMicroelectronics are relatively new entrants to the NAND flash market but have been able to jump quickly into the technology race. Their latest offering in the 4Gbit generation is a 70nm single-bit-per-cell device that has a die size of 144mm2 and a bit-efficiency rating of 28.4, a commendable achievement that positions this new entrant well in the NAND market.

Memory producers ranking

Die size has a direct effect on the manufacturing costs for the NAND flash devices shown in the table above. Calculations from a cost model from IC Knowledge LLC provide estimates of the gross number of dice that can be manufactured for each device. These rankings mirror die size statistics and show that Samsung can manufacture just under 30 more devices per wafer than the second-ranked device.

Currently, Semiconductor Insights has been unable to locate 4Gbit offerings from IM Flash and Renesas Technology. IM Flash will need competitive offerings in the 4Gbit generation in order to compete with the top three NAND manufacturers. Renesas is producing a 90nm 4Gbit MLC NAND but has announced it will not continue research and development efforts beyond that node.

Samsung has successfully moved its flash technology to the 65nm node by addressing several scaling challenges, including cell alignment, tunnel oxide thickness and flash cell coupling. Now the company can focus on expanding its product line at the 65nm node with an MLC device and on development at the 55nm node. Development of an MLC product will be necessary to compete with Toshiba's 70nm MLC device for design slots in cost-sensitive applications.

The NAND market space is definitely becoming more competitive, and it is necessary for manufactures to produce devices in leading process nodes. Samsung, Toshiba and Hynix/ST are all at or below the 70nm node, so it is likely that those manufactures will move quickly to 65- or 55nm process technologies. IM Flash will also need to move quickly to those nodes in order to be competitive. Therefore, Samsung cannot rest easily; it must move quickly to widen its product line and push to the 55nm node.

Devices at that geometry are currently in development and are forecast to be in production in late 2006 or early 2007. They will likely be 16Gbit MLC devices from Samsung, Toshiba and Hynix/ST. The 55nm node will likely feature cell sizes in the range of 0.00842 and initial die sizes between 140- and 160mm2. Those projections are derived from historical die and cell sizes. The 55nm devices will share some characteristics with their 65nm and 70nm predecessors but will need to address some critical scaling areas, such as cell alignment, tunnel oxide, interpoly dielectric, coupling between adjacent cells (crosstalk) and high-voltage transistor design.

Improved alignment between patterned layers is an obvious and constant concern as devices are scaled down. Smaller devices demand lower-voltage operation, which in turn drives the need for thinner tunnel dielectrics for transferring charge onto and off the floating gate. But thinner dielectrics are not as reliable. In advanced geometries, the active area for a floating gate to influence the cell transistor is smaller, but the control-to-floating-gate coupling ratio needs to remain constant. Therefore, a thinner interpoly dielectric is required. In the case of both dielectrics, a higher-k (dielectric-constant) material can reduce the effective electrical thickness while maintaining higher reliability with a greater physical thickness.

Nevertheless, introducing new materials brings its own challenges. Packing cells tighter increases the risk of charges on one floating gate influencing the operation of an adjacent memory cell. Finally, flash operation relies on relatively high voltages to program and erase cells. Control transistors for switching those voltages need to be kept small so that they don't reduce the memory cell efficiency for a given silicon area.

Samsung has fielded the first 65nm memory device in the market. The K9F4G08U0A is further proof that Samsung continues to push manufacturing into advanced process nodes to gain a competitive advantage over other memory manufacturers. However, process node leadership is not enough in the cost-competitive memory market, and circuit techniques such as MLC programming are necessary to compete in cost-sensitive memory applications.

Now that Samsung has a working 65nm part, it will likely focus on expanding its 65nm line to include an MLC offering and then push toward 55nm.

- Geoff MacGillivray
Technology Analyst for Memories, Semiconductor Insights

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