Laser-Assisted Direct Imprint (LADI) Technology

1
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
2
Outline

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

3
Fabrication process

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

4
Experimental result (I)Prototype
Quartz Mould
Imprinted silicon substrate
(a)
(b)
2µm
500nm

• 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.

5
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.

6
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

7
Features (cont.)Quartz Mould

• Feasibility
• Quartz has a melting temperature over 200 oC
  higher than Si (used in Si-melting furnaces)

• 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 welltwo orders of
  magnitude more poorlythan Si.
• Pre-fabrication method
• Fused quartz, 1.5mm1.5mm, and 1mm thick
• Conventional nanoimprinting RIE

8
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/researc
h/index.html
9
Features (cont.)Monitoring of the melting of
silicon surface
solidifying
saturating

• Method
• Measuring the time-resolved reflectivity of a
  HeNe laser beam (wavelength ? 633 nm) from the
  silicon surface.

melting

• 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.

10
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.)

11
Summary

• Demonstrate a rapid technique Laser-Assisted
  Direct Imprint (LADI) technologyfor 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.

12
Thank you for your attention!