Atomically Traceable Nanostructure Fabrication

Summary

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Abstract

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

Introduction

As nanotechnology becomes more important in a wide variety of arenas, understanding the structures being formed gains importance, especially in fields of lithography and electronics. To emphasize the importance of metrology at the nanoscale, specifically at scales below 10 nm, it should be pointed out that a variation in feature size of only 1 nm indicates a fractional variation at least 10%. This variation can have significant implications for device performance and material character.^1,2–4 Using synthetic methods, very precisely formed individual features such as quantum dots or other complex molecules can be fabricated,^2,5,6

Protocol

1. Ex-Situ Sample Preparation

1. Prepare chips
   1. Design appropriate etch mask to put identifying markers in the Si(100) wafer. Using standard optical lithography and RIE, etch a grid of lines as fiducial marks into the wafer from which STM samples will be taken. The lines should be 10 μm wide, 1 μm deep, and at pitch of 500 μm. After etching, strip remaining photoresist from sample.
      Note: The fiducial marks must be identifiable in-situ for tip location on the sample as well as in AFM and SEM during metrology.
   2. Protect wafer surface by applying standard tack blue dicing tape, tacky side down.
   ....

Results

In the cases described here, HDL is performed using multi-mode lithography.²⁴ In FE mode, performed with 8 V sample bias, 1 nA, and 0.2 mC/cm (equivalent to 50 nm/sec tip speed), the tip moves over the surface either parallel or perpendicular to the Si lattice, producing lines of depassivation. While this lineshape is very tip dependent, in the case here, the completely depassivated portion of the lines was approximately 6 nm wide, with tails of partial depassivation extending another 2 nm on either side of th.......

Discussion

Performing metrology on the nanostructures described above requires the ability to bridge the tip positioning during HDL and pattern location using other tools such as AFM and SEM. In contrast to other well-developed patterning tools with high-resolution position encoding such as e-beam lithography, the HDL performed here was performed with an STM without well controlled coarse positioning, so extra position identification protocols were used, as shown in Figure 3. First, a long-focal-length microscope i.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Si Wafer	VA Semiconductor		P type (Boron) Si<100> ± 2 degrees, 280 mm ± 25 mm thick, 0.01-0.02 ohm-cm
Ta foil	Alfa Aesar	335	0.025 mm (0.001 in) thick, 99.997% (metals basis)
Methanol	Alfa Aesar	19393	Semiconductor Grade, 99.9%
2-Propanol	Alfa Aesar	19397	Semiconductor Grade, 99.5%
Acetone	Alfa Aesar	19392	Semiconductor Grade, 99.5%
Argon	Praxair		Ultra high purity (grade 5.0)
Deionized water	Millipore	Milli-Q Water Purification System	>18 MW resistance water produced on demand.
TiCl4	Sigma Aldrigh	254312	≥99.995% trace metals basis
O2	Matheson	G2182101	Research Grade
SF6	Matheson	G2658922	Ultra high purity (grade 4.7)
Blue Medium Tack Roll	Semiconductor Equipment Corporation	18074	Thickness 75 μm / 0.003”  Length 200 M / 660’