Robert H. Munnig Schmidt

High resolution heterodyne interferometer without detectable periodic nonlinearity

A high resolution heterodyne laser interferometer without periodic nonlinearity for linear displacement measurements is described. It uses two spatially separated beams with an offset frequency and an interferometer configuration which has no mixed states to prevent polarization mixing. In this research, a simple interferometer configuration for both retroreflector and plane mirror targets which are both applicable to industrial applications was developed. Experimental results show there is no detectable periodic nonlinearity for both of the retro-reflector interferometer and plane mirror interferometer to the noise level of 20 pm. Additionally, the optical configuration has the benefit of doubling the measurement resolution when compared to its respective traditional counterparts. Because of non-symmetry in the plane mirror interferometer, a differential plane mirror interferometer to reduce the thermal error is also discussed.

Simple heterodyne laser interferometer with subnanometer periodic errors

We describe a simple heterodyne laser interferometer that has subnanometer periodic errors and is applicable to industrial fields. Two spatially separated beams can reduce the periodic errors, and the use of a right-angle prism makes the optical configuration much simpler than previous interferometers. Moreover, the optical resolution can be enhanced by a factor of 2, because the phase change direction is opposite between reference and measurement signals. Experiments have demonstrated the periodic errors are less than 0.15 nm owing to the frequency mixing of the optical source. The improvements for reducing the frequency mixing of the optical system are also discussed.

A Novel 5 DOF Fully Parallel Robot Combining 3T1R Motion and Grasping

This paper presents an innovative 5 DOF robot that generates 3T1R motion (3 translations + 1 rotation) plus a linear grasping motion. To generate this type of grasping motion, a robot needs two end-effectors. Grasping motions are usually generated by adding a grasping device at the top of an already existing robot or by coordinating two distinct robots. We propose in this paper a new robot architecture which includes the grasping degree of freedom as a part of the mechanism itself. The two end-effectors of the mechanism are mounted on an articulated platform and can move together in a 3T1R motion and their distance to each other can be controlled to generate the grasping capability. The 5 actuators are located on the base and are connected with five identical legs to the two end-effectors using only mechanical links, forming a fully parallel robot with 5 DOF. The architecture of the robot will be presented in detail. Then, we will describe the kinematics needed for the control of the robot. Finally, geometric optimization results will be presented and discussed.© 2010 ASME

Design of a folded, multi-pass Fabry–Perot cavity for displacement metrology

We present a folded, multi-pass cavity design for displacement measuring Fabry–Perot interferometry. The cavity length is designed to be one-quarter of the physical length needed for a typical Fabry–Perot interferometer by using a quarter-wave plate and a retroreflector. This enhances the displacement sensitivity by a factor of four, allowing for higher resolution in viewing the effects caused by mechanical motions, refractive index changes and frequency fluctuations from the laser source. Furthermore, the geometrical error motions are minimized by using a retroreflector due to its tip–tilt insensitivity. In this note, a theoretical analysis of the folded, multi-pass Fabry–Perot cavity is described and analyzed with Jones matrices in ideal and non-ideal designs.

Planar Flat Product Transport Using Viscous Traction

In this paper a new contactless transport system for thin, flat products, such as glass substrates and silicon wafers, is introduced. The transport function is realized using viscous traction on the product surface. After an explanation of the operating principles, a mathematical model is presented. Preliminary computations indicate that the generated acceleration is approximately 2.5 m/s2 for a glass substrate with a surface area of 2 m2 and a thickness of 0.7 mm. Furthermore, the vertical bearing stiffness for the considered actuator geometry of the system is 4.2·106 N/m, depending on the properties of the inflow and outflow points of the actuator cells.Copyright

Validation of separated source frequency delivery for a fiber-coupled heterodyne displacement interferometer

The use of optical fibers presents several advantages with respect to free-space optical transport regarding source-frequency delivery to individual heterodyne interferometers. Unfortunately, fiber delivery to individual coaxial heterodyne interferometers leads to an increase of (periodic) nonlinearity in the measurement, because transporting coaxial frequencies through one optical fiber leads to frequency mixing. Coaxial beams thus require delivery via free-space transportation methods. In contrast, the heterodyne interferometer concept discussed in this Letter is based on separated source frequencies, which allow for fiber delivery without additional nonlinearity. This investigation analyzes the influence of external disturbances acting on the two fibers during delivery, causing asymmetry in phase between the two fibers (first-order effect), and irradiance fluctuations (second-order effect). Experiments using electro-optic phase modulation and acousto-optic irradiance modulation confirmed that the interferometer-concept can measure with sub-nanometer uncertainty using fiber delivered source frequencies, enabling fully fiber-coupled heterodyne displacement interferometers.

Autonomous Self-aligning and Self-calibrating Capacitive Sensor System

An autonomous capacitive sensor system for high accuracy and stability position measurement, such as required in high-precision industrial equipment, is presented. The system incorporates a self- alignment function based on a thermal stepping motor and a built-in capacitive reference, to guarantee that the relative position between the sensor electrodes is set to 10±0.1 μm. This is needed to achieve the performance specifications with the capacitive readout. In addition, an electronic zoom-in method is used to reach the 10 pm resolution with minimum power dissipation. Finally, periodic self-calibration of the electronic capacitance readout is realized using a very accurate and stable built-in resistive reference. The performance is evaluated experimentally and with simulations.

Balanced interferometric system for stability measurements

We describe two different, double-sided interferometer designs for measuring material stability. Both designs are balanced interferometers where the only optical path difference is the sample and the reference beams are located within the interferometer. One interferometer is a double-pass design, whereas the other is a single-pass system. Based on a tolerancing analysis, the single-pass system is less susceptible to initial component misalignment and motions during experiments. This single-pass interferometer was tested with an 86 nm thin-film silver sample for both short-term repeatability and long-term stability. In 66 repeatability tests of 30 min each, the mean measured drift rate was less than 1 pm/h rms. In two long-term tests (>9 h), the mean drift rate was less than 1.1 pm/h, which shows good agreement between the short- and long-term measurements. In these experiments, the mean measured length change was 2 nm rms.

Common zeros in synchronization of high-precision stage systems

In synchronization of high-precision motion systems, in particular the synchronization between a wafer stage system and a reticle stage system of a wafer scanner, a novel feedforward structure is studied. In this structure, the numerator of each plant model is described by an input shaping filter capturing the zeros of said model. The denominator is described by a feedforward filter capturing the poles. Ideally, this gives zero error tracking of both the reticle and wafer stage systems without the need for plant inversion. But in view of the different input shaping filter operations, appropriate synchronization behavior is not guaranteed. To obtain both appropriate tracking and synchronization behavior, we propose to augment the reticle stage filters with the zeros from the wafer stage plant model. Reversely, the wafer stage filters are augmented with the zeros from the reticle stage plant model. The feasibility of such an approach is confirmed by simulation results and, to some extend, by measurement results obtained from an industrial wafer scanner.

Sub-Nanometre Validation of a Deformable Mirror Concept

Abstract Highly accurate optical systems may suffer from heat induced optical aberrations, causing blurred images, or degraded sensing properties. Correcting these aberrations with nanometre precision requires a dedicated design of a deformable mirror and a measurement set-up that can validate the design. In this paper we focus on the quantitative validation of the measurement set-up and the deformable mirror. To obtain nanometre precise measurements vibration isolation, turbulence isolation, beam enhancement and static noise reduction are applied. This resulted in a repeatability of 0.35 [nm] and reproducibility of 2.0 [nm]. With these known characteristics, an uncertainty analyses is carried out on an actuated shape of the deformable mirror, which is 50X50X4 [mm]. The stability of a deformed shape is 0.46 [nm] RMS and the difference with the modelled deformation is 2.0[nm] RMS on an amplitude of 30 [nm]. These low numbers quantify the validity of the actuation concept.