Thin optic constraint

The success of using thin substrates in various fields has urged researchers to further study the possibilities of improving the technology for future applications. For example, the high surface-area-to-weight ratio and strength of sheet glass allow flat-panel display technology to result in high-definition televisions that can be hung on walls like paintings. Sheet glass is also the prime candidate for grazing-incidence foil-optic X-ray telescopes, such as the segmented mirror approach considered for the NASA C onstellation X mission, where cost limitations necessitate lightweight substrates.The effects of different parameters present during the metrology of thin optics, such as gravity, frictional and thermal forces, are identified and analyzed. These forces alter the optic’s surface topography by tens of microns depending on how the optics are manipulated and constrained. This renders metrology and thus surface shaping process results inconclusive.A metrology truss utilizing monolithic flexures to kinematically constrain thin optics during metrology is designed. This device mitigates the effects of the forces mentioned above that are induced on the thin sheet while being mechanically constrained, thus significantly improving the repeatability of the optic surface map measurements.     

Design evaluation of a single-axis precision controlled positioning stage

This paper presents the design and control of a single-axis positioning stage with a total travel of 50 mm. The single-axis stage is comprised of a long-range slideway, running on ultra-high molecular weight polyethylene (UHMWPE) bearings, and a short-range positioning stage, comprised of a PZT driven flexure. Feedback errors associated with the long-range stage are reduced by the short-range stage, mounted on top of the long-range stage. Within the evaluation of the long-range stage, two alternative drives are assessed; a Roh’lix^® and a feedscrew drive. To determine the effects of dynamic interaction between the two drive systems, a further assessment of the single-axis stage was undertaken with the short-range stage operating at different bandwidths, ranging from 91 Hz to 2188 Hz. At the highest bandwidths, nanometer performance is demonstrated for a sinusoidal displacement demand corresponding to sinusoidal traverses of 1 mm and 5 mm with a maximum velocity of approximately 30 μm s⁻¹ and 150 μm s⁻¹, respectively. Furthermore, a 12 nm RMS controller error over a traverse of 25 mm at a maximum velocity of approximately 330 μm s⁻¹ was observed with the use of the feedscrew drive only.     

The long-range scanning stage: a novel platform for scanned-probe microscopy

This paper describes a magnetically suspended six degree-of-freedom precision motion control stage with a horizontal positioning noise of less than 0.6 nm three sigma. The vertical positioning noise is less than 2.2 nm three sigma. The stage utilizes four levitation linear motors to suspend and servo the moving element (platen) throughout its 25 mm × 25 mm × 0.1 mm range of travel. Position feedback is provided by three plane mirror interferometers and three capacitance probes. The suspended platen (12 kg mass) is floated in oil to enhance the stage’s disturbance rejection and to reduce power dissipation in the actuators. The stage has been designed to achieve a positioning accuracy of 10 nm and is used to position samples beneath a scanned probe microscope. The ultimate purpose of this measuring machine is to provide a means of measuring submicron-scale features with nanometer-scale accuracy. The technology can easily be scaled to larger travels, with accuracy limited primarily by the wavelength instability of the HeNe light source. This article gives an overview of the LORS project, emphasizing the system error terms, tolerancing, and experimental results.     

Design and analysis framework for linear permanent-magnet machines

This paper presents a design and analysis framework for the general class of permanent magnet electric machines. In the authors' analysis, surface-mounted linear motors consisting of permanent magnets and ironless current-carrying coils are treated in a uniform way via the magnetic vector potential. This analysis is developed to design novel linear magnetic levitators for driving precision motion control stages such as those used in wafer steppers. For one such motor structure, they give analytical formulae for its magnetic field, force, flux linkage, inductance of the winding, and back electromotive force. They provide experimental results with a six degree-of-freedom magnetic levitator. These results are in good agreement with analytical estimations. The levitator uses a permanent-magnet Halbach array in order to improve its power efficiency. By analogy, there also exists an electromagnetic dual of the Halbach array. One such dual utilizes a triangular winding pattern in order to achieve a primarily single-sided magnetic field     

High-precision magnetic levitation stage for photolithography

In this paper, we present a high-precision magnetic levitation (maglev) stage for photolithography in semiconductor manufacturing. This stage is the world’s first maglev stage that provides fine six-degree-of-freedom motion controls and realizes large (50 mm × 50 mm) planar motions with only a single magnetically levitated moving part. The key element of this stage is a linear motor capable of providing forces in both suspension and translation without contact. The advantage of such a stage is that the mechanical design is far simpler than competing conventional approaches and, thus, promises faster dynamic response and higher mechanical reliability. The stage operates with a positioning noise as low as 5 nm rms in x and y, and acceleration capabilities in excess of 1 g (10 m/s²). We demonstrate the utility of this stage for next-generation photolithography or in other high-precision motion control applications.     

Optimization and temperature mapping of an ultra-high thermal stability environmental enclosure

Precision metrology, lithography and machining systems will soon require sub-nanometer tolerances in order to meet the evolving needs of industry. This, in turn, requires thermal control of large environmental enclosures with sub-millidegree single-point stability and control of temperature gradients to several millidegrees. In order to optimize the system's thermal controls, it is essential to measure the open-loop transfer function. We report a technique that obtains the open-loop transfer function by utilizing a dynamic signal analyzer to perform a closed-loop frequency response measurement of the thermal system. Based on the transfer function, we designed a PI-lead compensation controller and achieved one-sigma air temperature stability of less than 1 m°C at a single point over 2 h. In order to rapidly map temperature gradients over large regions inside the 7 m³-volume enclosure, we have developed a measurement scheme that involves mechanically scanning a network of thermistors. Accurate cross calibration of the thermistors and a study of self-heating effects on temperature measurement in moving air have also been performed, which assures the relative accuracy of the thermistors is less than 1 m°C. Comparing temperature gradient maps taken before and after control improvements shows improved temperature stability over the mapped volumes.     

A high-bandwidth,high-precision,two-axis steering mirror with moving iron actuator

This paper focuses on the design, fabrication, assembly, and testing of high bandwidth two-axis mirror positioners for precision optical platforms, which we refer to as the Advanced Fast Steering Mirror (AFSM) and the small Advanced Fast Steering Mirror (sAFSM). These novel positioners consist of a mirror platform driven in two rotational axes by flux-steering electromagnetic actuators, and controlled via position feedback loops. The devices are designed for beam stabilization tasks in lithography, laser communication, lidar, and similar optical applications. We have experimentally demonstrated small-signal system bandwidth of 10 kHz using optical quad-cell feedback and a two-channel analog control architecture. The AFSM has a travel range of ±3.5 mrad, and a measured angular acceleration of 1 × 10⁵ rad/s².     

Modeling and vector control of planar magnetic levitator

The authors designed and implemented a magnetically levitated stage with large planar motion capability. This planar magnetic levitator employs four novel permanent-magnet linear motors. Each motor generates vertical force for suspension against gravity, as well as a horizontal force for drive. These linear levitation motors can be used as building blocks in the general class of multi-degree-of-freedom motion stages. In this paper, they discuss the electromechanical modeling and real-time vector control of such a permanent-magnet levitator. They describe the dynamics in a dq frame introduced to decouple the forces acting on the magnetically levitated moving part, namely, the platen. A transformation similar to the Blondel-Park transformation is derived for commutation of the stator phase currents. They provide test results on step responses of the magnetically levitated stage. They show 5 nm RMS positioning noise in x and y, which demonstrates the applicability of such stages in the next-generation photolithography in semiconductor manufacturing