Raman Characterization of Poly-Si Channel Materials for 3D Flash Memory Device Applications

Three dimensional (3D) vertical NAND (V-NAND) flash technology has been actively investigated. [1] 3D NAND architectures offer one of the best combinations of device performance, memory density, power consumption and manufacturing cost. All major memory device manufacturers are actively pursuing development of commercially viable devices. Development of bit cost scalable (BiCS) NAND technology is very important from both technology and commercialization points of view.

Device scaling was achieved by vertical staking of thin film transistors (TFTs) in contrast to the conventional planar device scaling techniques relying on advances in lithography. Poly-Si is used as channel materials in 3D V-NAND flash memory devices. Control of both poly-Si grain size and grain size distribution is very important for tight control of device yield and performance variations.

For poly-Si channel fabrication, amorphous Si (a-Si) film is deposited in the channel region and then converted to poly-Si by thermal annealing. To achieve desired properties of the poly-Si channel, grain size, resistivity, mobility and interface quality must be carefully monitored and controlled. One of the basic parameters of the poly-Si channel is grain size. Smaller grains lower in channel carrier mobility while larger grains increase device performance variations. The grain size and its distribution have to be optimized for the best device performance. Development of proper in-line characterization techniques for poly-Si grain size, after thermal annealing, is strongly desired.

In this paper, Raman spectroscopy was used for characterizing poly-Si after thermal annealing of chemical vapor deposited (CVD) thin a-Si films on SiO₂/Si wafers. The a-Si films on SiO₂/Si wafers were annealed in a single wafer rapid thermal furnace (SRTF-300 system [4, 5]) in the temperature range of 600 ~ 1050^oC under various ambient gases.

Systematic change of Raman spectra, indicating crystallization of a-Si films and grain growth of poly-Si with annealing temperature increase was measured (Fig. 1.) using an MRS-300 system. [6] As deposited a-Si/SiO₂/Si showed one sharp peak from the Si wafer at ~ 520.3cm^-1 and one broad peak centered at ~ 480cm^-1 corresponding to a-Si. After thermal annealing, the a-Si peak decreased in intensity and transformed into new peaks at ~ 495 and ~ 515 cm^-1, corresponding to poly-Si. The poly-Si peaks increased and sharpened as the annealing temperature increased. The change of Raman spectra indicates the degree of polycrystallization and grain growth under different annealing conditions. Figures 2 and 3 showed the Raman spectra from the annealed samples and their deconvolution results. Detailed Raman characterization, as a function of annealing temperature and type of ambient gas, will be reported at the conference.

References

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