Scanning Beam Interference Lithography Research Articles

Precision fringe metrology using a Fresnel zone plate

In scanning beam interference lithography, a resist-coated substrate is scanned under phase-locked interfering beams to pattern large gratings or grids with nanometer level distortions. To achieve design goals, it is required to measure the spatial period, phase, and rotation of the interference fringes in real-time with high precision. In this article, we report the use of a Fresnel zone plate to characterize interference fringes. With properly controlled errors, it should enable us to achieve high precision measurement of fringe characteristics for a wide range of fringe periods. The mathematical model, details of the metrological procedure, and experimental results are presented.

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.

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.

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.

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 measuring field distortions and alignment errors in steppers and scanners. We describe a proposed standard for pattern-placement metrology, which includes both a fiducial grid and market-type marks. Finally, a number of methods through which phase-coherent periodic structures can be patterned are shown, including “traditional” interference lithography, achromatic interference lithography, near-field interference lithography, and scanning-beam interference lithography.