Optics - Free Lithography On Colloidal Nanocrystal AssembliesSantosh Shaw, Kyle J. Miller, Julien L. Colaux, Ludovico Cademartiri

Motivation• Long-term demands for high-density and faster devices, is pushing the size scale of device features to nanometers

• Optics-based lithographic technique is reaching their physical limits, whereas e-beam and x-ray based processes are proven to be impractical in production settings[1]

• These scenarios demand for novel patterning materials and technologies - to continue fueling the $200 billion/year semiconductor industry - to achieve 5 nm node by 2020[2]

Goal• To achieve features size of <5nm, with pitch <10nm

• Pattern transfer technique should not involve any wavelength based processes

• A bottom-up approach, where ideally, pattern features size could be defined independently from lithography

• Patterning technology should be compatible with the existing manufacturing infrastructure

• Single technology that can create a range of features size from mm to µm to nm

Our Approach• Lithography on colloidal nanoparticles[3] using Nanocrystal Plasma Polymerization (NPP)[4]

• Colloidal nanocrystal assemblies (CNAs) is used as resist, and potentially may be the active material

• Smallest feature size achievable is ideally defined by the size of the nanoparticles

• Plasma processing will change the dispersibility of nanoparticles in solvents

• Plasma will etch or cross-link the ligands capped on inorganic surface thereby changing its dispersibility

Conclusions• Demonstrated a plasma-based approach to transfer pattern into colloidal nanocrystal assemblies

• With duration of plasma exposure, control on cross-linking or partial etching or complete etching of ligands is possible

• Plasma etching in capacitive mode is anisotropic compared to inductive mode

• Rate of ligand etching can be controlled by plasma power

• Complete ligand etching is not necessary to change the dispersibility of particles

• Shorter exposure to plasma results in crosslinking of exposed CNAs, thereby making them impervious to redispersion

• Under-etching could be reduced by switching to soft masks

• LER of pattern has improved by an order of magnitude by switching to insulating mask

References[1] C. J. Hawker, T. P. Russell, MRS Bulletin 2005, 30, 952.[2] ITRS International Roadmap[3] S. Shaw, K. J. Miller, L. Cademartiri, in TECHCON 2015, SRC-P084502, Austin, Texas 2015.[4] L. Cademartiri, A. Ghadimi, G. A. Ozin, Accounts of Chemical Research 2008, 41, 1820.

AcknowledgementsWe thank S. Schlorholtz, and C. Pan for assistance with XRD measurements, and patterning experiments, respectively. The research has been supported by MSR-Intel program of SemiconductorResearch Corporation under Award Number 2015-IN-2582 and partially by LC start-up funds.

Chemical and Morphological Insights to Plasma Processing of Nanocrystal Assemblies

• Average crystallite size (in our case it is the diameter of nanoparticle) hardly changed after prolonged plasma exposure

• Under non-equilibrium plasma (“cold”), sample temperature remains <60ºC, hence grain growth or phase change is not possible in these processing conditions

• Following conclusions derived from Ion Beam Analysis (IBA): 1. Carbon conc. drops to nearly zero after 48hrs of O2 plasma 2. Homogenous etching of carbon 3. Film densifies as carbon is etched out 4. Amount of zirconium in the film remains nearly constant, which indicates hardly any ablation with plasma exposures

• Choice of cohesive ZrO2 inorganic phase is to avoid any ablation by plasma

• Particle diameter of 3.7nm is way below the target feature size of 5nm

TEM image of ZrO2 nps SEM image of unprocessedZrO2 CNA

SEM image of processedZrO2 CNA

crys

talli

te s

ize

[nm

]

before NPP after NPP

TEM image of processed ZrO2 CNA

crystallite size from Scherrer analysis

carbon profile from IBA analysis

Optics-Free Lithography• Plasma processing was performed in capacitive set-up, to reduce under-etching

• Plasma power was increased to 250W, to reduce processing time to 10mins

• Oxygen plasma was done which is commonly used plasma to etch organics

• Hard mask was used: 1. For rapid optimization of the process 2. Understanding the interaction of plasma and hard mask on pattern transfer

• Metallic and insulating masks was used to test their effect on pattern fidelity

plasma

substrate

200mesh Cu grid

after

developmentZrO2 CNAs

100μm

5μm

plasma

substrate

sapphire

after

developmentHfO2 CNAs

5μm

masked area

film

Insulating mask improved the LER from ~103nm to ~102nm