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) 835-837.

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.

2μm500nm

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 0.003 cm2 s-1, which is one-

third that of water (0.01 cm2 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 oC 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 oC

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.

http://www.earthsci.unimelb.edu.au/isotope/research/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,

SiO2) 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.

Thank you for your attention!