Laser-Assisted Direct Imprint (LADI) Technology S. Y. Chou, C. Keimel, and J. Gu, Ultrafast and direct imprint of nanostructures in silicon, Nature, 417 (2002) Yingqi Jiang

Outline Fabrication process Experimental results Technology features – Quartz mould – Laser beam fluence – Surface monitoring Applications Summary

Fabrication process a) Quartz mould is brought into contact with the silicon substrate with external force. b) XeCl (308 nm wavelength) excimer laser pulse (20 ns pulse width) melts a thin surface layer of Si. c) Molten silicon is embossed in the liquid phase. d) Silicon rapidly solidifies. e) The mould and silicon substrate are separated, leaving a negative profile of the mould.

Experimental result (I)——Prototype Scanning electron microscope (SEM) images. a, The mould after the two LADI processes showing no visible damage. b, A uniform 300 nm period silicon grating patterned by LADI. The grating has 140 nm linewidth and is 110 nm deep. 2μm2μm500nm Quartz Mould Imprinted silicon substrate (a)(b)

Experimental results (II)——10nm resolution SEM image of the cross-section of samples patterned using LADI. a, A quartz mould. b, Imprinted patterns in silicon. The imprinted silicon grating is 140 nm wide, 110 nm deep and has a 300 nm period, an inverse of the mould. We note that the 10 nm wide and 15 nm tall silicon lines at each top corner of the silicon grating are the inverted replicas of the small notches on the mould (the notches were caused by the reactive ion etching trenching during mould fabrication). This indicates the sub-10-nm resolution of the LADI process.

Technology features Direct imprint – Only one step! No etching to generate the final structures Rapid process – The embossing time is less than 250 ns. High resolution – molten silicon has a viscosity of cm 2 s -1, which is one- third that of water (0.01 cm 2 s -1 ). This low viscosity enables the molten silicon to flow rapidly into all crevasses, filling them completely and conforming to the mould. – A variety of structures with resolution better than 10 nm have been imprinted

Features (cont.)——Quartz Mould Feasibility – Quartz has a melting temperature over 200 o C higher than Si (quartz is used as the crucible material in Si-melting ——Data and icture from wikipedia – Quartz does not absorb the laser energy because it has a bandgap larger than the photon energy (93% measured transmittance) – Quartz conducts heat much less well—two orders of magnitude more poorly—than Si. Pre-fabrication method – Fused quartz, 1.5mm×1.5mm, and 1mm thick – Conventional nanoimprinting + RIE Feasibility – Quartz has a melting temperature over 200 o C higher than Si (used in Si-melting furnaces)

Features (cont.)——Laser beam fluence Low fluence – a fluence lower than 0.8 J cm -2 does not melt the silicon surface High fluence – a fluence higher than 2 J cm -2, laser ablation of silicon will occur Final choice: – a laser pulse of 20 ns duration and 1.6 J cm -2 fluence melts a silicon surface sufficiently without ablation. earch/index.html

Features (cont.)——Monitoring of the melting of silicon surface Method – Measuring the time-resolved reflectivity of a HeNe laser beam (wavelength λ= 633 nm) from the silicon surface. solidifying melting saturating Principle – The silicon theoretically starts to melt immediately in less than picoseconds after the laser hit the surface. – When silicon melts, it changes from a semiconductor to a metal, hence its surface reflectivity to visible light increases by about a factor of two. Melting depth – On the basis of theoretical calculations and the melting depths experiments, the melting depth was estimated to be about 280 nm.

Applications LADI can be extended to large areas, other materials, and other processes. – Pattern areas as large as a whole wafer (4” or 8”), or a 1”- square die (that die can be used to cover an entire wafer by step and repeat), – Used for other materials beyond crystalline silicon and polysilicon (gates of MOSFETs). – Help to crystallize polysilicon further. – Well suited for three-dimensional patterning – Unique method to fill tiny holes in a dielectric (for example, SiO 2 ) with silicon and a unique means of flattening the surface of a semiconductor deposited on a dielectric.(Both are difficult issues in integrated circuit fabrication.) – …

Summary Demonstrate a rapid technique ——Laser-Assisted Direct Imprint (LADI) technology——for patterning nanostructures in silicon that does not require etching. Experimentally show the intriguing characteristics of LADI such as sub-10-nm resolution, sub-250-ns processing time, and excellent imprint of large isolated patterns. Analyze three features of LADI technology including quartz mould, laser influence, and surface monitoring.

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