James Spallas

Modular MEMS design and fabrication for an 80 x 80 transparent optical cross-connect switch

The ability to transparently switch optical signals from one fiber to another without conversion to the electrical domain is a basic functionality that has a wide range of applications within the fiber optic industry. The so-called 3D-MEMS architecture has emerged as the preferred approach for building transparent, scalable systems with port-counts ranging from 16x16 to 1024x1024. The primary components of the 3D-MEMS architecture are fiber array, lens array, and MEMS mirror array. While a central theme in the MEMS industry is integration, we adopted a strategy of modularization. The key MEMS components, which include mirror array, ceramic substrate, and high-voltage drivers, were manufactured separately and then combined to yield a working product. Central to our modular approach was critical design parameter tolerancing to ensure manufacturability. Results from a large sampling of MEMS components and MEMS assemblies are presented to highlight manufacturability and performance.

On the limits of miniature electron column technology

Miniature columns or microcolumns are a relatively new class of electron beam columns fabricated entirely from silicon using advanced micromachining processes. The main characteristics of these columns are thermal field emission (TFE) sources, low voltage operation (typically <3keV), simple design (two lenses, no crossover), microfabricated lenses, and all electrostatic components. Current production versions of miniature columns achieve <10nm resolution at 1keV, and have demonstrated <6nm resolution at higher beam energies.1,2 While this performance is suitable for most applications, previous studies of the electron optics of miniature electrostatic lenses show better performance should be attainable under “ideal” conditions.3 In practice, achieving these conditions is challenging because, in addition to the manufacturing errors from the miniature optics, other subsystems can impose additional constraints. An understanding of the major contributors to column performance, whether optical or mechanical, is essential, and can provide a roadmap for further improvements in the existing technology.

Proposed architecture of a multicolumn electron-beam wafer inspection system for high-volume manufacturing

While optical patterned wafer inspection systems have been successfully used for many years to both improve and control yield during wafer fabrication, their sensitivity is no longer adequate to meet advanced device manufacturing requirements. An alternative is to use wafer inspection systems based on electron-beam technology, which have been commercially available for many years. However, the relatively low throughput of these single-beam systems has kept them from being used to support high-volume manufacturing. One possible way to improve the throughput would be to use multiple columns. However, it has proven difficult in the past to build columns small enough to fit many on a 300 mm wafer and also have the performance of a large single column. Recently, a commercially available scanning electron microscope that utilizes a miniature field-emission column was shown to have adequate resolution to locate defects in a test pattern provided by SEMATECH. The column is all electrostatic and the lenses and def...

High-voltage energy dispersive x-ray spectrometry using a low-energy primary beam

This paper proposes a way to do energy dispersive x-ray spectroscopy (EDS) at high accelerating voltages using a low-voltage field emission scanning electron microscope (FESEM). By biasing the sample to a positive voltage, the electron landing energy (also known as the x-ray excitation energy) is increased, effectively extending the EDS capabilities of the low-voltage SEM. This positive biasing reduces the signal-to-noise ratio of the FESEM image, but it also introduces scaling and offset errors. A one-time calibration procedure is shown to compensate for these distortions allowing the user to view the unbiased image, and then do a precise superposition of the low energy image, high energy image, and EDS elemental map. The proposed solution is implemented on a commercial low-voltage FESEM with an EDS energy resolution of 130 eV.

Miniature Electron Beam Columns: From the Lab to the Field

Scanning electron microscopes (SEMs) have been the workhorse of high-technology research and development for well over 50 years with applications ranging from the life sciences to forensic sciences. Improvements in SEM performance, operating conditions, ability to accommodate analytical instrumentation, portability, user interface, software, and automation, have progressed at a rapid rate bringing SEM into main stream usage. Today, innovations such as low-voltage imaging and compound or cathode lenses allow researchers to routinely image and analyze nanoscale structures at <2nm resolution [1]. However, the fundamental architecture of SEMs -a high-brightness macro-scale electron source coupled with precision-machined magnetic and electrostatic lens, deflection and correction elements -has also imposed, with few exceptions, the requirement that the sample be brought to the system rather than the system to the sample. Removing this constraint could create new types of field applications which would benefit from the SEM’s high resolution imaging and analytical capabilities. An example from space exploration is shown in Fig 1, where undetected fine-grained lunar dust can create potentially life threatening situations [2].

Ultralow voltage imaging using a miniature electron beam column

Miniature columns that are fabricated from silicon using advanced micromachining processes are a relatively new class of electron beam column. The defining characteristics of these columns are a thermal field emitter source, low voltage operation (typically <3 keV), simple design (two lenses, no crossover), microfabricated lenses, and all electrostatic components. Current production versions of miniature columns achieve <10 nm resolution at 1 keV, and have demonstrated <7 nm resolution at higher beam energies [Spallas et al., J. Vac. Sci. Technol., B 24, 2892 (2006)]. The resolution of miniature columns is limited by competing requirements to minimize optical aberrations, relax physical or geometric constraints (e.g., working distance and electrode–electrode distance), and maintain manufacturable mechanical tolerances of the column components (e.g., alignment, diameter, and placement). In this paper, the authors investigate imaging using a miniature electron beam column in a commercial field emission elec...

High-brightness miniature column for high-speed multicolumn wafer inspection

Using miniature columns in a multicolumn architecture in a high-speed wafer inspection system capable of detecting small defects at speeds compatible with high volume manufacturing is feasible if the columns can deliver enough current to the sample and if they can be scaled sufficiently to meet the throughput requirement. The authors show that miniature columns currently in production can meet the required beam current and spot size specifications by modifying an aperture. Simulations of the column show that, because of the length of the column, Coulomb interactions do not significantly limit the current that can be delivered to the sample. The authors discuss scaling and present reasons why the technologies used to manufacture the production columns are capable of producing columns with dimensions small enough to be arrayed in a multicolumn configuration for high volume manufacturing inspection.

Blanking characteristics of a miniature electron beam column

A commercial field emission scanning electron microscopy (SEM) based around a miniature electron beam column includes an integrated electrostatic blanker, making the system well-suited for lithography. A previous version of the miniature column design demonstrated direct-write lithography with 70 nm lines and spaces written into resist. That column also demonstrated an 85 MHz blanking speed and a 6 nA beam current using a condenser lens. This article presents a study of the field emission SEM’s integrated blanking system, including measurements and simulations of the electron beam optics. Rise and settle times are discussed, as well as beam extinction ratios under various operating conditions. The column’s conjugate blanking optics are described, and measurements are presented, which suggest that the column may be used in a conjugate blanking mode for lithography. Finally, this article discusses how these results will feedback into the system design in order to improve the SEM’s existing lithography capabilities.A commercial field emission scanning electron microscopy (SEM) based around a miniature electron beam column includes an integrated electrostatic blanker, making the system well-suited for lithography. A previous version of the miniature column design demonstrated direct-write lithography with 70 nm lines and spaces written into resist. That column also demonstrated an 85 MHz blanking speed and a 6 nA beam current using a condenser lens. This article presents a study of the field emission SEM’s integrated blanking system, including measurements and simulations of the electron beam optics. Rise and settle times are discussed, as well as beam extinction ratios under various operating conditions. The column’s conjugate blanking optics are described, and measurements are presented, which suggest that the column may be used in a conjugate blanking mode for lithography. Finally, this article discusses how these results will feedback into the system design in order to improve the SEM’s existing lithography capab...