Laser blasts make cheap memory

By Eric Smalley, Technology Research News

In today's factories, memory chips are made by shining lasers through patterned masks onto silicon wafers that sport light-sensitive coatings, and then using chemicals to etch away the exposed areas. This produces tiny cells that store the electrical charges that represent digital bits.

Researchers from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan have found a simpler way to make these memory chip cells.

They were able to dispense with the usual masks and chemicals and instead zap layers of material with a laser. "Simplicity is the important advantage in our technology," said Jooho Kim, a principal research scientist at the institute.

The researchers haven't yet made entire memory chips, but their process to make individual memory cells is the first step toward manufacturing memory in a considerably simpler and potentially cheaper way than those used in today's factories. The cost of making memory using the technique could be as low as 10 percent of the cost of today's technology, said Kim.

The new laser process could also be used to make semiconductor components like solar cells and the photodiodes used in light sensors, according to Kim.

To make the memory cells, the researchers sandwiched a 20-nanometer mix of terbium, iron and cobalt metals between 50- and 100-nanometer layers of mixed zinc sulfide and silicon dioxide insulators. A nanometer is one millionth of a millimeter.

They fired a laser at the materials to heat small sections to temperatures between 300 and 510 degrees Celsius. The heat melted the layers, causing sulfur and oxygen from the insulating materials to mix with the metals. This changed the electrical properties of the metals.

Depending on how much they heated the materials, the researchers were able to turn sections into semiconductors, which allow less electric current through than metals, or insulators, which block current entirely. Semiconductors are useful for controlling the flow of electricity; electronic devices generally consist of combinations of metals, semiconductors and insulators.

The researchers have formed 50-nanometer semiconductor and insulator dots in the layers this way, according to Kim. The dots could be used to make the cells in dynamic random access memory (DRAM) commonly used in today's computers or flash memory chips used in devices like digital cameras, he said.

The presence of a charge in a dot would represent a 1 and the absence a 0. The dots could be changed from 0 to 1 by a specific type of electric field, said Kim. When an electric current running through a metal or semiconductor encounters a magnetic field oriented perpendicularly to the current, the current and magnetic field spawn an electric field running through the junction of the two like an arrow passing through the middle of an 'X'. When the current and magnetic field intersect over a dot, the electric field enters the dot, making it a 1, said Kim.

The researchers plan to read the bits by reversing the direction of the electric current in order to detect the stored charge, Kim said. They would erase bits to turn a 1 into a 0 by applying a decreasing, alternating current, which drains the stored charge, he said.

The smaller the dots, the more information a memory chip can hold. It is possible to make dots as small as 5 nanometers by using electron beams rather than lasers to heat the chips, said Kim. This is because electrons have much smaller wavelengths than visible or ultraviolet light; magnetic fields can be used to focus beams of electrons to about one nanometer or the length of 10 carbon atoms.

Using 5-nanometer dots, it would be possible to make memory chips that contain 250 billion bits per square inch, according to Kim. This is 50 times more than today's RAM chips, which contain as many as 10 billion bits per square inch, and 1,000 times more than today's flash memory chips, which contain 250 million bits per square inch, he said.

Using regular lasers, the method could produce memory devices that hold 25 gigabits per square inch, or about a quarter the data today's memory chips hold and 10 times more than today's flash memory, said Kim.

The limits of laser light could be a problem, however. The ultraviolet lithography used to make today's memory chips is higher resolution than the researchers' laser reaction process, said Franz Himpsel, a physics professor at the University of Wisconsin. Because the AIST method depends on a laser, "its wavelength puts a limit on the... resolution," he said. Using electron beam lithography to increase the resolution may be impractical, he added. Current electron beam lithography equipment is costly and is not widely used in commercial chipmaking.

The laser reaction process could be used in practical applications in two to five years, according to Kim.

Kim's research colleagues were Hiro Akinaga, Nobufumi Atoda and Junji Tominaga of the National Institute of Advanced Industrial Science and Technology in Japan. They published the research in the April 15, 2002 issue of the journal Applied Physics Letters. The research was funded by the National Institute of Advanced Industrial Science and Technology in Japan and Samsung Electronics Co., Ltd.

Timeline:   2-5 years
Funding:   Government, Corporate
TRN Categories:   Integrated Circuits; Data Storage Technology
Story Type:   News
Related Elements:  Technical paper, "Nanoelectronic Devices with Reactively Fabricated Semiconductor," Applied physics Letters, April 15, 2002


September 4/11, 2002

Page One

Chip juggles droplets

Software turns reading into writing

Radio ID locks lost laptops

Quantum software gets the picture

Laser blasts make memory


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