J.A. Liddle

Solid state quantum computer development in silicon with single ion implantation

Spawned by the finding of efficient quantum algorithms, the development of a scalable quantum computer has emerged as a premiere challenge for nanoscience and nanotechnology in the last years. Spins of electrons and nuclei in 31P atoms embedded in silicon are promising quantum bit (qubit) candidates. In this article we describe single atom doping strategies and the status of our development of single atom qubit arrays integrated with control gates and readout structures in a “top down” approach. We discuss requirements for 31P qubit array formation by single ion implantation, and integration with semiconductor processing.

Status of EUV micro-exposure capabilities at the ALS using the 0.3-NA MET optic

The success of recent static printing experiments at Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS) using the EUV LLC Engineering Test Stand (ETS) Set-2 optic has demonstrated the utility of synchrotron-based EUV exposure stations. Although not viable light sources for commercial lithography, synchrotrons provide clean, convenient, and extremely flexible sources for developmental microfield lithography. The great flexibility of synchrotron-based illumination arises from the fact that such sources facilitate active coherence reduction, thus enabling the coherence function, or pupil fill, to be actively sculpted in real time. As the commercialization of EUV progresses, the focus of developmental EUV lithography is shifting from low numerical aperture (NA) tools such as the 0.1-NA ETS to higher-NA tools such as the 0.3-NA Micro Exposure Tool (MET). To support printing with MET optics at the ALS, a new printing station has been developed, relying on a scanning illuminator to provide programmable coherence (pupil-fill) control. The illuminator is designed to operate up to a coherence factor (s) of 1 and support the full 200′600 design printed field of view. In addition to a new illuminator design, new focus sensing and dose-control systems have also been implemented. Here we describe the MET printing capabilities in detail and present preliminary printing results with the Sematech Set-2 MET optic.

A dual-mode actinic EUV mask inspection tool

To qualify the performance of non-actinic inspection tools, a novel EUV mask inspection system has been installed at the Advanced Light Source (ALS) synchrotron facility at Lawrence Berkeley National Laboratory. Similar to the older generation actinic mask inspection tool1, the new system can operate in scanning mode, when mask blanks are scanned for defects using 13.5-nm in-band radiation to identify and map all locations on the mask that scatter a significant amount of EUV light. By modifying and optimizing beamline optics (11.3.2 at ALS) and replacing K-B focusing mirrors with a high quality Schwarzschild illuminator, the new system achieves an order of magnitude improvement on in-band EUV flux density at the mask, enabling faster scanning speed and higher sensitivity to smaller defects. Moreover, the system can also operate in imaging mode, when it becomes a zone-plate-based full-field EUV microscope with spatial resolution better than 100 nm. The microscope utilizes an off-axis setup, making it possible to obtain bright field images over a field-of-view of 5x5 um2.

EUV interferometric testing and alignment of the 0.3-NA MET optic

Extreme ultraviolet (EUV) interferometry has been successfully performed for the first time at 0.3 numerical aperture (NA). Extensive EUV “at-wavelength” testing including alignment, was performed on a newly created Micro Exposure Tool (MET) optic designed for sub-50-nm EUV lithographic imaging experiments. The two-mirror, 0.3 NA MET is ar-guably the highest resolution light-projection lithography tool ever made. Using both lateral shearing and phase-shifting point-diffraction interferometry, the wavefront was measured across the field of view, and the alignment was optimized in preparation for imaging. The wavefront quality reached 0.55 nm RMS (lambda[EUV]/24.5) in a 37-term annular Zernike poly-nomial series, dominated by higher-order spherical aberration. Measurements included calibrations of the interferometer accuracy, assessment of repeatability, and cross-comparisons of visible and EUV interferometric measurements. The comparisons and the final, measured wavefront quality were affected by an apparent alignment drift, several tenths of a nm in magnitude. Significant unresolved differences between testing strategies shows that continued work is needed to improve the measurement accuracy to levels required for EUV lithography.

Ion Implantation with Scanning Probe Alignment

We describe a scanning probe instrument which integrates ion beams with the imaging and alignment function of a piezoresistive scanning probe in high vacuum. The beam passes through several apertures and is finally collimated by a hole in the cantilever of the scanning probe. The ion beam spot size is limited by the size of the last aperture. Highly charged ions are used to show hits of single ions in resist, and we discuss the issues for implantation of single ions.

Quantum Computer Development with Single Ion Implantation

AbstractSpins of single donor atoms are attractive candidates for large scale quantum information processing in silicon. Formation of devices with a few qubits is crucial for validation of basic ideas and development of a scalable architecture. We describe our development of a single ion implantation technique for placement of single atoms into device structures. Collimated highly charged ion beams are aligned with a scanning probe microscope. Enhanced secondary electron emission due tohigh ion charge states (e.g., 31P13+, or 126Te33+)allows efficient detection of single ion impacts. Studies of electrical activation of low dose, low energy implants of 31P in silicon show a drastic effect of dopant segregation to the SiO2/Si interface,while Si3N4/Si retards 31P segregation. We discuss resolution limiting factors in ion placement, and process challenges forintegration of single atom arrays with control gates and single electron transistors. PACS: 03.67.Lx, 34.50.Dy, 85.35.Gv, 73.23, 61.72, 86.40.py, 07.79.-v

Single ion implantation with scanning probe alignment

We present results from our development of a single ion implantation technique integrated with a scanning force microscope. Accurate alignment at the 5nm level is a crucial requirement for reliable single ion placement. We address this through integration of the ion beam with a scanning probe tip containing an aperture. Single ion registration is based on detection of secondary electron bursts from single, high charge state ions. We describe formation of scanning probe tips with holes and sensing poles by focused ion and electron beam processing (drilling and thin film deposition). Ion transport studies through apertures show stable transmission for >10h with 1nA scale beam intensities on precollimators.

Extreme ultraviolet binary phase gratings: Fabrication and application to diffractive optics

Diffractive optics play an important role in a variety of fields such as astronomy, microscopy, and lithography. In the extreme ultraviolet region of the spectrum they have been difficult to make due to the extremely precise control required of their surface structure. We have developed a robust fabrication technique that achieves the required topographic control through the deposition of a thin film of Si on a Cr etch stop. We have fabricated binary phase gratings using this approach that have an efficiency of 80% of the theoretical maximum. This technique could be applicable to similar binary phase structures requiring precise topography control.

Formation of 15nm scale Coulomb blockade structures in silicon by electron beam lithography with a bilayer resist process

We have formed Coulomb blockade structures with widths of 15–30nm in silicon-on-insulator (SOI) by electron beam lithography (EBL) in a bilayer resist process. The bilayer structure consisted of HSQ (hydrogen silsesquioxane) and AZ organic resist. The organic resist protects the buried oxide and allows removal of exposed HSQ features with hydrofluoric acid (HF). Measurements at 4.2K show pronounced Coulomb blockade signatures for 15nm wide wires. This bilayer resist process provides direct lithographic access to 15nm level features in SOI without the need for size reduction by oxidation.

Fabrication and performance of nanoscale ultrasmooth programed defects for extreme ultraviolet lithography

The authors have developed processes for producing nanoscale programed substrate defects that have applications in areas such as thin film growth, extreme ultraviolet lithography, and defect inspection. Particle, line, pit, and scratch defects on the substrates between 40 and 140nm wide, 50–90nm high have been successfully produced using e-beam lithography and plasma etching in both silicon and hydrogensilsesquioxane films. These programed defect substrates have several advantages over those produced previously using gold nanoparticles or polystyrene latex spheres—most notably, the ability to precisely locate features and produce recessed as well as bump-type features in ultrasmooth films. These programed defects were used to develop techniques for planarization of film defects and results are discussed.