Laser thermal anneal boosts 3D memory performance

17th October 2013

IMEC

Nat Bowers

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imec and Excico have successfully demonstrated the application of laser thermal anneal in vertical polysilicon channel devices for 3D memory, resulting in an increased current. Compared to conventional polysilicon channel, the larger grain size of the laser recrystallized polycrystalline channel material offers up to 10 times higher read current and 2.5 times steeper sub-threshold slope. Providing a way to higher stacking, this enables higher bit density in 3D memory.

Prominent alternatives for many technologies including new generation nonvolatile memory applications, 3D vertical poly-Si channel devices are used both as memory cells and as string select transistors. Typically fabricated with a gate-first, channel-last approach, the formation of single-crystal silicon channel is complicated or even prohibitive. Therefore, electron conduction is dominated by scattering at grain boundaries and interface defects of the polycrystalline channel material. This significantly decreases the drive current needed for the read operation.

Grain size engineering is therefore necessary in order to to obtain larger grains and hence less grain boundaries. Channel formation with amorphous Si deposition followed by pulsed laser annealing (Excico LTA series, wavelength λ=308 nm, pulse duration < 200 ns) allowed researchers from Imec and Excico to achieve these larger grain sizes. Since low doses are not effective and high doses compromise device integrity, LTA dose needs proper adjustment in order to optimally crystallize the channel.

The larger grains obtained result in less grain boundaries, in turn leading to higher effective mobility and less temperature activation of the conduction mechanism.

Memory operation was evaluated in cells with oxide-nitride-oxide memory stack as well as in macaroni-type cells with a dielectric filler in the center. Proving that optimized LTA does not impact memory operation, key memory characteristics such as program/erase characteristics, endurance and room temperature retention on fresh and program/erase cycled devices were independent of crystallization thermal treatment. This is crucial for the successful fabrication of advanced vertical memory stacks using LTA.

These results were achieved in the framework of imec’s Industrial Affiliation Program on Advanced Memory Devices, with imec’s memory core partners including Intel, Micron, Samsung, SK Hynix, GLOBALFOUNDRIES, Panasonic, as well as Toshiba and SanDisk.