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

Summary

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.

Abstract

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.

Introduction

The exponential growth in computational power, as quantified by Moore's law^1,2 (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:

Here, λ and NA are the light wavelength....

Protocol

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 perform a deionized water wash.
   2. Follow a similar protocol for SC2 (10 min at 70 °C, followed by a wash).
      NOTE: The cleaning procedure removes....

Results

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.......

Discussion

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:

where A and r are the area and the size .......

Materials

List of materials used in this article

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