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Atomic Layer Annealing of AlN to Template The Growth of High Thermal Conductivity Heat Spreader Films

Scott Ueda‚ Aaron McLeod (University of California‚ San Diego); Michelle Chen‚ Chris Perez‚ Eric Pop (Stanford University); Dan Alvarez (RASIRC); Andrew Kummel (University of California‚ San Diego)

The low-temperature (<400 oC) deposition of polycrystalline AlN films is demonstrated by atomic layer annealing (ALA) which is a variant of ALD that utilizes a third pulse of ions in addition to the usual metal and co-reactant pulses [1]. As transistor size continues to decrease‚ it becomes a significant challenge to remove the heat generated during the operation of microelectronic circuits. The use of electrically conductive heat spreaders complicates 3D integration in VLSI CMOS or stack memory devices and induces parasitic losses in RF MMICs. Crystalline AlN has a thermal conductivity close to that of Cu and is a good electrical insulator making it an ideal heat spreader material.

Using trimethyl aluminum (TMA) and the highly reactive nitrogen-containing precursor hydrazine (N2H4)‚ AlN can be deposited at 200 oC [2]; however‚ these films are amorphous and would have low thermal conductivity due to phonon scattering. Using tris(dimethylamido) aluminum (TDMAA) or tris(diethylamido) aluminum (TDEAA) with N2H4 or NH3 at temperatures >350 oC‚ polycrystalline films can be deposited in a purely thermal process; however‚ the reported grain sizes are small (<5 nm) or there is a mixture of polycrystalline and amorphous phases [3-4]. ALA has been used to deposit crystalline films such as AlN [1‚5] and GaN [6] at low temperature‚ but a nitrogen-containing plasma was used.

In the present study of AlN ALA‚ two metal precursors (TMA and TDMAA) were compared using anhydrous N2H4 as a co-reactant and argon ions with tuned energy for the third pulse. High-quality AlN films are deposited with large grain size and low C/O contamination which can then be used as a templating layer for further high-speed AlN film growth.

The deposition of high quality‚ stress controlled AlN films deposited by ALA are successfully used as templates for thicker heat spreading layers deposited via sputtering and polycrystalline sputtered AlN films with near-record thermal conductivities of ~70 Wm-1K-1 were obtained for sub-0.5 micron thick films.

[1] H-Y. Shih et al‚ Scientific Reports 7:39717.

[2] M. Mizuta et al‚ Japanese J. Appl. Phys. 25(12)‚ L945-L948 (1986).

[3] R. G. Gordon‚ U. Riaz‚ and D. M. Hoffman‚ J. Mater. Res.‚ 7(7) (1992).

[4] A. I. Abdulagatov et al Russian Microelec. 47(2)‚ 118-130 (2018).

[5] W-C. Kao et al‚ RSC Adv. 9‚ 12226-12231 (2019).

[6] W-H. Lee et al ACS Sustainable Chem. Eng. 7‚1‚ 487-495 (2019).

Research was sponsored by ASCENT-JUMP-SRC-DARPA and the TDMAA was donated by EMD.

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