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Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Published: February 12th, 2017



1Department of Chemistry, Portland State University, 2Logic Technology Department, Intel Corporation

Uniformly sized nanoparticles can remove fluctuations in contact hole dimensions patterned in poly(methyl methacrylate) (PMMA) photoresist films by electron beam (E-beam) lithography. The process involves electrostatic funneling to center and deposit nanoparticles in contact holes, followed by photoresist reflow and plasma- and wet-etching steps.

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

The exponential growth in computational power, as quantified by Moore's law1,2 (1), is a result of progressive advances in optical lithography. In this top-down patterning technique, the achievable resolution, R, is given by the well-known Raleigh theorem3:

Equation 1

Here, λ and NA are the light wavelength and numerical aperture, respectively. Note that NA = η·sinθ, where η is the refractive index of the me....

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1. Derivatize and Characterize the Surface of the Silicon Wafers

  1. Clean the surface of wafers using Radio Corporation of America (RCA) cleaning solutions SC1 and SC2.
  2. Prepare SC1 and SC2 by volumetrically mixing the following chemicals:
    SC1: H2O2:NH4OH:H2O = 1:1:5 v/v and SC2: H2O2:HCl:H2O = 1:1:5 v/v.
    1. Immerse the wafer in SC1 for 10 min at 70 °C, and then p.......

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Figure 2 shows an SEM image of 20-nm GNPs deposited in 80-nm diameter holes patterned in a 60-100 nm-thick PMMA film driven by electrostatic funneling. As observed by others22, the process resulted in about one particle per hole. The distribution of particles around the center of the holes was Gaussian (top right inset). Most holes (93%) contained one GNP, and 95% of these particles occurred within 20 nm of the center. Further optimization, discuss.......

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Shot-noise (SN) in lithography is a simple consequence of statistical fluctuations in the number of photons or particles (N) arriving in a given nano-region; it is inversely proportional to the square root of a number of photons/particles:

Equation 3

where A and r are the area and the size of the exposed region, respectively. For example, when using an ArF 193-nm (6.4-eV) excimer laser to pattern .......

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Intel Corporation funded this work through grant number 414305, and the Oregon Nanotechnology and Microtechnology Initiative (ONAMI) provided matching funds. We gratefully acknowledge the support and advice of Dr. James Blackwell in all phases of this work. Special thanks go to Drew Beasau and Chelsea Benedict for analyzing particle positioning statistics. We thank Professor Hall for a careful reading of the manuscript and Dr. Kurt Langworthy, at the University of Oregon, Eugene, OR, for his help with E-beam lithography.


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Name Company Catalog Number Comments
AATMS (95%) Gelest Inc. SIA0595.0 N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane 
Gold colloids (Ted Pella Inc.) Ted Pella 15705-20 Gold Naoparticles
hydrogen peroxide Fisher Scientific  H325-100 Analytical grade (Used to clean wafer)
hydrochloric acid Fisher Scientific  S25358 Analytical grade
Ammonium hydroxide Fisher Scientific  A669S-500SDS Analytical grade (Used to clean wafer)
hydrogen fluoride Fisher Scientific  AC277250250 Analytical grade(used to etch SiO2)
Toluene (anhydrous, 99.8 %)  Sigma Aldrich 244511 Analytical grade (solvent used in Self Assembly of AATMS
 Isopropyl alcohol (IPA) Sigma Aldrich W292907 Analytical grade (Used to make developer)
Methyl butyl ketone (MIBK) Sigma Aldrich 29261 Analytical grade(used to make developer)
1:3 MIBK:IPA developer Sigma Aldrich Analytical grade (Developer)
950 k poly(methyl methacylate (PMMA, 4 % in Anisole) Sigma Aldrich 182265 Photoresist for E-beam lithography
Purified Water : Barnstead Sybron Corporation water purification Unit, resistivity of 19.0 MΩcm Water for substrate cleaning
Gaertner ellipsometer  Gaertner Resist and SAM thickness measurements
XPS, ThermoScientifc ESCALAB 250 instrument Thermo-Scientific Surface composition
An FEI Siron XL30 Fei Corporation Characterize nanopatterns
Zeiss sigma VP FEG SEM Zeiss Corporation E-beam exposure and patterning
MDS 100  CCD camera Kodak Imaging drop shapes for contact angle measurements
Tegal Plasmod Tegal Oxygen plasma to etch photoresist
I2 Sigma Aldrich 451045 Components for gold etch solution
KI Sigma Aldrich 746428 Components for gold etch solution
Ellipsometer ( LSE Stokes model L116A); Gaertner L116A AATMS self assembled monolayer film thickness measurements

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