Norio Uchida

A mask‐to‐wafer alignment and gap setting method for x‐ray lithography using gratings

A new optical‐heterodyne interferometry alignment and gap setting method for x‐ray lithography is developed, where the phases of beat signals are used for detection. The fact that the lateral displacement and the gap between mask and wafer can be detected independently is shown, based on the results of analysis and experiments. Two pairs of gratings are arranged as detection marks. One pair forms a window on the mask and a checkerboard grating on the wafer, while another pair forms two linear gratings at right angles to each other. Two He‐Ne laser beams with slightly different frequencies illuminate the gratings from the ±1st‐order diffraction light directions. The lateral displacement is detected by measuring the beat signal phase difference between the two (0,1)th‐order diffraction lights from two pairs of gratings. To detect the gap, the signal phase difference between the (0,1)th‐ and (1,1)th‐order diffraction lights from the pair of linear gratings is used. Better than 0.02 μm (3σ) alignment repeatability and 0.1 μm gap setting accuracy have been achieved.

A scheme for reducing vibration during precise positioning control

A scheme for reducing vibration that occurs during precise positioning control is discussed. The positioning drive mechanism used in ULSI manufacturing is composed of many mechanical parts: a rotational dc motor, a ballscrew, etc. Such a mechanism causes vibration while the positioning stage approaches a desired position. Therefore, this article proposes ways to diminish this vibration. The control system currently in use utilizes the output from a tachogenerator (TG) attached to the DC motor as a damping loop. However, the TG cannot detect the positioning stage velocity but only the dc motor velocity. The vibration reduction scheme proposed in this article is to use the damping loop of the directly measured stage velocity. The stage velocity is calculated from the stage positions which are detected by a laser interferometer. A new control scheme has been designed after modeling the positioning drive mechanism and the control system currently in use. The validity of the proposed control scheme has been co...

Damping of micro electrostatic torsion mirror caused by air-film viscosity

A method of analyzing the damping characteristics of electrostatically driven torsion mirror actuators which have deep grooves on their electrodes is described. The damping force is caused by viscous friction of the gas film between a moving mirror plate and the electrodes. The grooves decrease the damping force and enable the moving plate to be driven at high speed and low driving voltage. To calculate the damping force correctly, it is necessary to consider the viscous friction not only on the moving plate and electrodes, but also on the sidewalls of the grooves. For that purpose, the idea of hydraulic mean depth is introduced and is applied to the Reynolds equation. The calculated damping force shows good agreement with the measured damping force of the developed torsion mirror actuator for optical heads.

A Six Degrees of Freedom Fine Motion Mask Stage for X-Ray Aligner

Summary A fine motion mechanism has been developed for an x-ray aligner. The mechanism to align a mask and a wafer precisely has to move with a nanometer-order resolution for several hundred micrometers, which is impossible to achieve with piezoelectric actuators. The mechanism is driven by servomotors, and their displacements are reduced by wedges and ball screws. To obtain high resolution, these transmission parts are designed not to have hysteresis. A better than 20 nm resolution in feedback control with an encoder and a better than 2.5 nm resolution in feedback control using a capacitance micrometer have been achieved.

Dynamic motion of mask membrane in x-ray stepper

This article presents a new analysis method and experimental results for the deflection and vibration of x‐ray masks, which are caused by the squeeze effect of the gas film between a mask and a wafer in x‐ray steppers. The deflection and vibration occur when a mask‐to‐wafer gap setting is executed or if the wafer vibrates in the mask direction during stepping motion with a narrow gap. They are not only detrimental to the throughput, but may also damage the mask membrane. Lees’ difference approximation method is applied to a compressible Reynolds’ equation and the equation of motion for the mask membrane. The main results of calculations and experiments, which showed good agreement with each other, are outlined below. (1) When the wafer approaches the mask at a constant velocity, the mask deflection increases in proportion to the velocity, and the mask deflection is larger for a smaller mask tensile stress. (2) When the wafer vibrates in the mask direction, the amplitude of the mask vibration increases wit...

Evaluation of overlay accuracy for the x‐ray stepper TOXS‐1

This article presents the overlay accuracy for the newly developed prototype x‐ray stepper TOXS‐1. Checkerboard gratings on a mask and wafer were used for an optical‐heterodyne interferometry alignment system so that the alignment signals from the mask and wafer gratings can be detected independently without mutual interference. The alignment signal varied slightly with the mask‐to‐wafer gap due to multiple reflection of the alignment beams between the mask and wafer, which deteriorates the alignment accuracy. To reduce the multiple reflection, the mask grating and the mask alignment window were coated with opaque film and antireflecting film, respectively. A 0.025 μm (mean+3σ) overlay accuracy has been achieved for SiO2 processed wafers. The overlay accuracy was further measured for four different kinds of processed wafers, and a 0.035 μm (mean+3σ) accuracy has been obtained except for an aluminum processed wafer.

X‐ray stepper aiming at 0.2 μm synchrotron orbital radiation lithography

A vertical x‐ray stepper has been developed for 0.2 μm synchrotron orbital radiation lithography. The key features of this prototype stepper are a new gap setting algorithm, an optical heterodyne alignment system, and a newly developed fine motion mask stage. Gap setting is executed so as to make the mask and wafer parallel to the travel plane of the wafer x–y stage so that only one gap setting per wafer is required. The gap setting accuracy between 20 and 50 μm gaps is better than ±1.5 μm (3σ) for each exposure position. The optical heterodyne alignment signal obtained by detecting the diffraction beams from two checkerboard gratings has a detectable resolution of better than 0.01 μm and has only a small dependence of ±0.02 μm on gap variation. The alignment signals are fed back to the mask stage which can align the mask and wafer with a resolution of 5 nm. In exposure experiments, 0.15 μm lines and spaces were printed on a negative resist (SAL 601) and a 0.05 μm overlay accuracy has been obtained.

0.05 µm (3σ) Overlay Accuracy Through-the-lens Alignment in an Excimer Laser Lithography System

This paper presents a through-the-lens (TTL) alignment, called separated mark TTL alignment (SMART), applied to a KrF excimer laser lithography system. This SMART optical system does not require any complicated compensation mechanisms for longitudinal chromatic aberration. Several advanced methods for SMART optics to obtain higher overlay accuracy, such as optical heterodyne interferometry, real-time alignment system, and alignment signal processor, have been developed and adopted. The performance of such an improved SMART in an excimer laser lithography system was experimentally evaluated using dynamic random access memory (DRAM)-processed wafers. An overlay accuracy of 0.05 µm (3σ) has been achieved.

Monitoring of Structural Vibration by Using Distributed PVDF Film Sensors.

In recent years, the new vibration monitoring methods using distributed continuous sensors have been actively developed in the field of smart sensors and actuators. PVDF film sensors have been mainly used for such purposes because they are less expensive and easy to handle. A lot of techniques have been reported so far, but they are not available for the structures whose boundary conditions are unknown. The authors propose the new approach using the PVDF films with the shape of appropriate Fourier series, which is applicable to the structure of arbitrary boundary conditions. Identification of the mode shape is simulated in the case of cantilever beam, which shows the availability of the proposed method. Furthermore, the experiment was carried out with the cantilever-like plate. The good agreements were obtained when compared with the calculated results.

Vibration distribution reconstruction and control of a thin plate by using distributed PVDF sensors affixed to the plate

In this study, vibration distribution of a thin plate is detected using outputs from a number of long-span piezoelectric films that serve as distributed vibration sensors affixed on the plate. Using long-span piezoelectric films, only the integrated strains along the direction of the sensor span could be obtained. However, in the case that plural sensors were affixed and a kind of transformation was used, the vibration distribution could be obtained. We are considering applying the method for vibration control of space structures, namely antennas and/or solar battery paddles. In this study, basic technology for realizing the applications is presented. The strain distributions and the sensor outputs are predicted and calculated using NASTRAN (Finite Element Method). The strain distributions of a vibrating rectangular thin plate to which a number of long-span PVDF films were affixed are detected by experiment. These PVDF films are cut into several kinds of sinusoidal shapes and the outputs of the films are composed to the vibration distribution. This method is based on the theory that waves of any kind are composed by the sum of trigonometric function series. The usefulness of this approach is confirmed by comparing the calculational and experimental results.