ASU scientists improve chip memory by stacking cells

December 20, 2009

Scientists at Arizona State University have developed an elegant method for significantly improving the memory capacity of electronic chips.

Led by Michael Kozicki, an ASU electrical engineering professor and director of the Center for Applied Nanoionics, the researchers have shown that they can build stackable memory based on “ionic memory technology,” which could make them ideal candidates for storage cells in high-density memory. Best of all, the new method uses well-known electronics materials. Download Full Image

“This opens the door to inexpensive, high-density data storage by ‘stacking’ memory layers on top one another inside a single chip,” Kozicki said. “This could lead to hard drive data storage capacity on a chip, which enables portable systems that are smaller, more rugged and able to go longer between battery charges.”

“This is a significant improvement on the technology we developed two years ago where we made a new type of memory that could replace Flash, using materials common to the semiconductor industry (copper-doped silicon dioxide). What we have done now is add some critical functionality to the memory cell merely by involving another common materia – silicon.”  

Kozicki outlined the new memory device in a technical presentation he made in November at the 2009 International Electron Devices and Materials Symposia in Taiwan. He worked with Sarath C. Puthen Thermadam, an ASU electrical engineering graduate student.

Kozicki said that given current technology, electronics researchers are fast reaching the physical limits of device memory. This fact has spurred research into new types of memory that can store more information into less and less physical space. One way of doing this is to stack memory cells.

The concept of stackable memory is akin to one’s ability to store boxes in a small room. You can store more boxes (each representing a memory cell) if you stack them and take advantage of three dimensions of the room, rather than only putting each box on the floor.

Kozicki said stacking memory cells has not been achieved before because the cells could not be isolated. Each memory cell has a storage element and an access device; the latter allowing you to read, write or erase each storage cell individually.

“Before, if you joined several memory cells together you wouldn’t be able to access one without accessing all of the others because they were all wired together,” Kozicki said. “What we did was put in an access, or isolation device, that electrically splits all of them into individual cells.”

Up until now, people built these access elements into the silicon substrate.

“But if you do that for one layer of memory and then you build another layer, where will you put the access device,” Kozicki asked. “You already used up the silicon on the first layer and it’s a single crystal, it is very difficult to have multiple layers of single crystal material.”

The new approach does use silicon, but not single crystal silicon, which can be deposited in layers as part of the three-dimensional memory fabrication process. Kozicki said his team was wrestling with how to find a way to build an electrical element, called a diode, into the memory cell. The diode would isolate the cells.

Kozicki said this idea usually involves several additional layers and processing steps when making the circuit, but his team found an elegant way of achieving diode capability by substituting one known material for another, in this case replacing a layer of metal with doped silicon.

“We can actually use a number of different types of silicon that can be layered,” he said. “We get away from using the substrate altogether for controlling the memory cells and put these access devices in the layers of memory above the silicon substrate.”

“Rather than having one transistor in the substrate controlling each memory cell, we have a memory cell with a built-in diode (access device) and since it is built into the cell, it will allow us to put in as many layers as we can squeeze in there,” Kozicki said. “We’ve shown that by replacing the bottom electrode with silicon it is feasible to go any number of layers above it.

With each layer applied, memory capacity significantly expands.

“Stackable memory is thought to be the only way of reaching the densities necessary for the type of solid state memory that can compete with hard drives on cost as well as information storage capacity,” Kozicki said. “If you had eight layers of memory in a single chip, this would give you almost eight times the density without increasing the area.”

Kozicki said the advance mimics an idea employed in early radios.

“We created a modern analog to the ‘cat’s whisker,’ where we are growing a nanowire, a copper nanowire, right onto the silicon to create a diode,” he said.

Cat’s whisker radios, a product of the 1930s, were simple devices that employed a small wire to scratch the surface of a semiconductor material. The connection between the semiconductor and the wire created a diode that they could use as part of a radio.

“It turns out to be a ridiculously simple idea, but it works,” Kozicki said of his stackable memory advance. “It works better than the complicated ideas work.”

“The key was the diodes, and making a diode that was simple and essentially integrated in with the memory cell. Once you do that, the rest is pretty straightforward.”

Associate Director, Media Relations & Strategic Communications


Men's Hoops hosts UCSB

December 21, 2009

ASU meets Gauchos
The Arizona State Sun Devils take on the UCSB Gauchos (4-3) at 6:30 Monday (KMVP 860 AM only/no television) at Wells Fargo Arena. ASU is 8-3 on the year after its 55-52 win over San Diego State on Saturday. ASU is now 34-9 (.791) at home in the past three seasons, including a 15-5 mark in 2007-08 that set the ASU mark for total home wins. ASU is 19-4 (.826) the past two seasons at home, including 7-1 this season. After going 8-22 (.267) in Coach Sendek's first year (2006-2007), the Sun Devils are 54-26 (.675) since and giving up just 60.7 points per game in his 111 games. The Sun Devils lead the Pac-10 in scoring defense (55.9 points per game), scoring margin (+15.5) and three-pointers made per game (8.0)

Glasser Notes
Derek">">D... Glasser is shooting 88.2 percent from the free throw line (45-of-51) this the past six games he has 40 assists and just 10 tied for first on the ASU career assist list with 454, a mark shared by the late Bobby Thompson, who had 454 from 1983-87...227-of-272 (.835) from the free throw line and has played 19 turnover free games in his 109-game career. The 83.5 percent is currently the top mark in ASU history...19-of-20 (.950) from the free throw line in the past five games. Download Full Image

Abbott notes
Ty">">Ty Abbott tied a season-high vs. San Diego State with 11 points (also vs. USF on Nov. 20)...When Abbott scores in double digits, ASU is 26-6 (.813).

Leftover notes from SDSU tilt
The Sun Devils are now 10-1 when Coach Sendek has at least five full days in between games the the past three seasons after the five-day layoff before the SDSU game...ASU is now 19-13 (.594) in the past three seasons in 10 points or less games after going 4-17 in Coach Sendek's first ASU's first season under Coach Sendek, it was 3-12 in two-possession games (six points or less), in the past three seasons it is 13-8 (7-4 past two seasons).

Random notes
Eric">">Eric Boateng leads the league in field goal percentage at 66.7 percent (40-of-60)...Derek Glaser leads the league in assist-to-turnover ratio at 3.1 (61 assists and 20 turnovers) and is second at 5.5 assists per game...ASU has held seven of its first 11 opponents to under 40 percent field goal shooting, as 24 of its past 46 opponents have shot under 40 percent...Herb Sendek is now 62-48 at ASU (.563).

Sun Devils lowest shooting percentage in ASU win under coach Sendek
ASU 53 (16-56/.286), @Arizona 47 (14-47/.298)--Jan. 21, 2009 ASU 55 (18-49/.367), San Diego State 52 (18-44/.409)--Dec. 19, 2009 ASU 49 (16-42/.381), @OSU 38 (15-46/.326)--Feb. 7, 2009 ASU 61 (21-53/.391), Pepperdine 40 (12-45/.267)--Nov. 23, 2008 ASU 64 (21-53/.396), Arizona 59 (21-54/.389)--Jan. 9, 2008 (OT).

Starting to score
After not posting a 20-point game in the first 96 games of his career, Derek">">D... Glasser has four 20-point games in his past 13 contests dating to last year. In that time he is shooting 46-100 (.460) from the floor, 20-of-45 (.444) from the three-point stripe and 52-of-60 (.867) from the foul line and is averaging 12.7 points. He also is averaging 13.0 points in his past 15 contests.

Glasser's top scoring games
24 vs. LSU (Nov. 27, 2009/NIT at NYC) 22 at BYU (Dec. 8, 2009) 22 vs. Temple (Mar. 20, 2009/NCAA) 20 vs. Texas State (Nov. 16, 2009) 18 vs. USC (Feb. 15, 2009) 17 @#13 Oregon (Feb. 8, 2007) 16 vs. USC (Mar. 14, 2009/P10 Champ.) 16 vs. #13 UW (Mar. 13, 2009/P10 Tourn.) 16 vs. North Carolina A&T (Dec. 21, 2006).