Paul T. Konkola

Sub-100 nm metrology using interferometrically produced fiducials

Pattern-placement metrology plays a critical role in nanofabrication. Not far in the future, metrology standards approaching 0.2 nm in accuracy will be required to facilitate the production of 25 nm semiconductor devices. They will also be needed to support the manufacturing of high-density wavelength-division-multiplexed integrated optoelectronic devices. We are developing a new approach to metrology in the sub-100 nm domain that is based on using phase-coherent fiducial gratings and grids patterned by interference lithography. This approach is complementary to the traditional mark-detection, or “market plot” pattern-placement metrology. In this article we explore the limitations of laser-interferometer-based mark-detection metrology, and contrast this with ways that fiducial grids could be used to solve a variety of metrology problems. These include measuring process-induced distortions in substrates; measuring patterning distortions in pattern-mastering systems, such as laser and e-beam writers; and me...

Nanometer-level repeatable metrology using the Nanoruler

We report on the measurement of the fringe-to-substrate phase error in our Nanoruler system. This system utilizes scanning beam interference lithography to pattern and measure large-area, nanometer-accuracy gratings that are appropriate for semiconductor and integrated opto-electronic metrology. We present the Nanonruler’s metrology system that is based on digital frequency synthesizers, acousto-optics, and heterodyne phase sensing. It is used to assess the fringe-to-substrate placement stability and the accuracy of the feedback signals. The metrology system can perform measurements in real time, on the fly, and at arbitrary locations on the substrate. Experimental measurements are presented that demonstrate the nanometer-level repeatability of the system. Dominant error sources are highlighted.

Image metrology and system controls for scanning beam interference lithography

We are developing scanning beam interference lithography (SBIL) for writing and reading large gratings with nanometer level distortion. Our distortion goals require fringe locking to a moving substrate with subnanometer spatial phase error while measuring and controlling the fringe period to approximately one part per million. In this article, we describe the SBIL optical system design along with some major subsystems. The design incorporates measurements and controls of the parameters that limit the accuracy of our system. We describe in detail a novel image metrology scheme, which uses interferometry to measure in situ both the period and the phase of the grating image formed by the interference of two laser beams. For a grating period of approximately 2 μm, experiments demonstrate a period measurement repeatability of three parts per ten thousand, one sigma. Phase measurement indicates a slow fringe drift at 0.25 mrad/s. Both the repeatability error and the phase drift are expected to improve by about three orders of magnitude after several improvements including the installation of an environmental enclosure and thermally stable metrology frames.

Nanometer-accurate grating fabrication with scanning beam interference lithography

We are developing a Scanning Beam Interference Lithography (SBIL) system. SBIL represents a new paradigm in semiconductor metrology, capable of patterning large-area linear gratings and grids with nanometer overall phase accuracy. Realizing our accuracy goal is a major challenge because the interference fringes have to be locked to a moving substrate with nanometer spatial phase errors while the period of the fringes has to be stabilized to approximately one part per million. In this paper, we present a review of the SBIL design, and report recent progress towards prototyping the first-ever SBIL tool.

Digital heterodyne interference fringe control system

In traditional interference lithography, interference fringes are typically phase locked to a stationary substrate using analog homodyne photodiode signals that are fed back to control a phase-shifting device such as an electro-optic modulator or a piezoelectrically transduced mirror. Commercially available fringe-locking systems based on this approach often achieve stability of the interference fringes to within a small fraction of the fringe period p (typically ±p/20 peak-to-peak). We describe the performance of a heterodyne fringe control system utilizing acousto-optic phase shifters and digital controls that is designed to satisfy the much more stringent fringe control requirements for scanning beam interference lithography. We demonstrate locking to ±p/100, and expect further significant improvements. This versatile system can also be used to lock the phase of moving fringes in almost arbitrary fashion at fringe velocities up to 2.5×107 periods/s and to measure the phase of gratings.

Generalized scanning beam interference lithography system for patterning gratings with variable period progressions

We demonstrate a versatile interference lithography system that can continuously vary the pattern period and orientation during fabrication of general periodic structures in one or two dimensions. Initial experimental results, using closed-loop beam steering control and double exposures on a stationary substrate, are obtained in order to illustrate its principle of operation. A fringe-locking scheme for phase control is also demonstrated including discussion of issues related to future system developments.

Progress toward a general grating patterning technology using phase-locked scanning beams

The fabrication of large high-quality diffraction gratings remains one of the most challenging tasks in optical fabrication. Traditional direct-write methods, such as diamond ruling or electron-beam lithography, can be extremely slow and result in gratings with undesired phase errors. Holographic methods, while generally resulting in gratings with smoother phase, frequently require large aspheres and lengthy optical setup in order to achieve desired period chirps. In this paper we describe a novel interference lithography method called scanning-beam interference lithography (SBIL) that utilizes small phase-locked scanning beams to write general periodic patterns onto large substrates. Small mutually coherent beams are phase controlled by high-bandwidth electro-optic components and caused to overlap and interfere, generating a small grating image. The image is raster-scanned over the substrate by use of a high-precision interferometer-controlled air bearing stage, resulting in large grating patterns. We will describe a prototype system in our laboratory designed to write gratings with extremely low phase distortion. The system is being generalized to pattern gratings with arbitrary period progressions (chirps). This technology, with extensions, will allow the rapid, low cost patterning of high-fidelity periodic patterns of arbitrary geometry on large substrates that could be of great interest to astronomers.

Beam steering system and spatial filtering applied to interference lithography

Production of metrologically accurate interference patterns with subnanometer fidelity requires precise control of beam position and angle. We consider the beam stability requirements for the cases of interference by plane and spherical waves. Interferometers using beamsplitter cubes and diffraction gratings are among the analyzed topologies. The limitations of spatial filtering to remove angular variations are also discussed. We present a beam steering system that uses position sensing detectors, tip-tilt actuators, and digital control to lock the beam position and angle at the interference lithography system. We describe the prototype’s performance and limitations of this approach. This beam steering system allows us to locate the laser far (∼10 m) from the sensor assembly, thereby reducing the thermal and mechanical disturbances at the lithography station and allowing sharing of the laser between different lithography tools.

Beam alignment for scanning beam interference lithography

By interfering two small diameter Gaussian laser beams, scanning beam interference lithography (SBIL) is capable of patterning linear gratings and grids in resist while controlling their spatial phase distortions to the nanometer level. Our tool has a patterning area that is up to 300 mm in diameter. The motive for developing SBIL is to provide the semiconductor industry with a set of absolute metrology standards, but the technology is easily adaptable to other important applications such as the making of high precision optical encoders. In this article, we describe a system for carrying out automated beam alignment for SBIL. Our design goals require tight alignment tolerances, where beam position and angle alignment errors must be controlled to ∼10 μm and ∼10 μrad, respectively. We describe our system setup, and discuss the so-called iterative beam alignment principle, focusing specifically on deriving a mathematical formalism that can guide the development of similar systems in the future. Repeatability experiments demonstrate that our system fulfills the alignment requirements for nanometer-level SBIL writing.

High-accuracy x-ray foil optic assembly

Achieving arcsecond angular resolution in a grazing-incidence foil optic X-ray telescope, such as the segmented mirror approach being considered for the Constellation-X Spectroscopy X-Ray Telescope (SXT), requires accurate placement of individual foils. We have developed a method for mounting large numbers of nested, segmented foil optics with sub- micrometer accuracy using lithographically defined and etched silicon alignment micro-structures. A system of assembly tooling, incorporating the silicon micro-structures, is used to position the foils which are then bonded to a flight structure. The advantage of this procedure is that the flight structure has relaxed tolerance requirements while the high accuracy assembly tooling can be reused. A companion paper by Bergner et al. discusses how our process could be used for the SXT. We have built an assembly truss with a simplified rectilinear geometry designed to experimentally test this alignment and mounting technique. We report results of tests with this system that demonstrate its ability to provide sub- micrometer alignment of rigid test optics.