Vijay Trimbak Chitnis

Automatic mask alignment without a microscope

In this paper we propose a two step approach for obtaining coarse and fine alignment without using a microscope. The new approach Is based on moire technique. The alignment marks on the mask and the wafer are in the form of coarse and fine gratings. The mask to wafer placement accuracy for the present system, is estimated to be /spl plusmn/40 /spl mu/m. This is well within the capture range of the coarse moire system of pitch 1000 pm. The coarse moire system gives automatic alignment within /spl plusmn/8 /spl mu/m, which is within the capture range of the fine moire system of pitch 25 /spl mu/m. The final alignment accuracy is better than /spl plusmn/50 nm. Thus very high alignment accuracy has been obtained automatically, in two steps, without using the microscope and avoiding the dependance on the expertise of the operator. >

Measurement of small angular displacement by a modified moiré technique

Small angle measurement using a modified moire interference technique is reported. The range of angle measurement for 25-μm pitch gratings is about ± 25 μrad and the accuracy of measurement is of the order of 0.2 μrad with a time constant of 0.1 s. A piezoelectric transducer (PZT) is used for angular displacement. The angular sensitivity under the present experimental conditions is found to be of the order of 21.8 mV/μrad.

Precision position control systems using moire signals

A positioning accuracy of a nanometer has been obtained by a precise positioning system using moire technique. Two precise position control techniques using moire signals have been investigated. One is a differential moire technique using two grating pairs in which the control signal is the differential signal between two moire signals. The other is a technique using one grating pair. The system using the latter technique is simpler in construction. However, in the former technique, since in-phase noises in the moire signals are cancelled and the effect of the drift of the laser intensity on the positioning accuracy is small, higher positioning accuracy is obtained than in the latter technique. The positioning accuracy of /spl plusmn/4 nm was obtained by optimization of the system.

Tracking and dynamic control of the angular alignment position in a photolithographic mask aligner by the moiré interference technique

An automatic tracking of the angular alignment position of mask with respect to the wafer in a photolithographic mask aligner is reported. It is demonstrated that automatic dynamic control with very high accuracy and stability of the angular alignment position is practically possible. The moire interference fringe method is employed for the present experiments. The alignment marks are used in the form of 25 μm pitch Ronchi ruling gratings. The best angular alignment accuracy obtained with the present experimental conditions is of the order of 2×10−7 rad. An alignment accuracy up to ±3×10−7 rad is reported for more than 30 min which enables the present technique to be used for the real photolithographic mask aligner.

Rough alignment system using moire gratings for lithography

Some types of rough alignment system for pre-alignment of precise alignment is proposed. One is a system using wide pitch gratings, with narrow pitch gratings for precise alignment. The other is a system using only narrow pitch gratings with small slit number, which is simple in construction. The operations are discussed on the basis of the results of computer simulation.

Precision alignment of pulse stage using moire signals

A simple and high accuracy optical intelligent alignment system using a position sensor of a diffraction grating is proposed. From a simulation of this system using a permeation type diffraction grating, we can conclude that it is possible to improve the wide position definition from /spl plusmn/0.25mm to /spl plusmn/12/spl mu/m with precision accuracy of /spl plusmn/4nm.

Photolithographic mask aligner based on modified moire technique

This paper presents an automatic, accurate mask aligner based on modified moire technique. In this technique the alignment marks are in the form of gratings. The high slope region of moire signal is used to obtain higher sensitivity and better position control accuracy. Automatic alignment is achieved by using difference of moire signal and its inverted signal obtained by computer. Accuracy for alignment, under the present experimental conditions, is of the order of +/- 0.06 micrometers . Our alignment technique by virtue of its higher alignment accuracy is suitable to X-ray lithography. However, due to nonavailability of a X- ray source the lithography was performed by UV source. Nevertheless, the effort is worth it so as to prove the workability of this technique. The overlay accuracy is estimated to be 0.8 micrometers which is limited by the diffraction effects of the exposure optics.

Mask alignment technique using phase-shifted moire signals

Moire technique with different variations has been successfully used for mask alignment, with very high accuracies. In this paper we report a new approach to computer controlled mask alignment using modified moire technique. In this technique alignment is controlled in the higher slope region of the moire signal using a single pair of grating alignment marks. In the present case a phase shifted signal is generated by the computer using the input moire signal. The point at which this phase shifted signal becomes equal to the moire signal is treated as the alignment point. The error signal for controlling alignment is obtained by computing the difference of instantaneous moire signal from the intensity of this point. Computer simulation studies as well as experimental studies were conducted on this approach. The results of these studies are presented.

Computer-controlled in-plane alignment using a modified moire technique

An automatic, accurate mask alignment method, based on a modified moire technique suitable for x-ray lithography is presented. In this technique, the alignment marks are in the form of gratings. The high slope region of the moire signal is used to obtain higher sensitivity and better position control accuracy. Automatic alignment is achieved by using the difference of the moire signal and its inverted signal obtained by computer. This difference signal is zero at a point in the higher slope region that is considered the correct alignment point. This difference signal is treated as an error signal, which is used for obtaining control signal by performing the proportional, integration and differential (PID) algorithm. The 12-bit analog-to-digital (A/D) and digital-to-analog (D/A) convertors are used to interface the piezoelectric-transducer- (PZT)-driven alignment system with the computer. Under the present experimental conditions, accuracy for alignment is of the order of ± 0.06 μm.

Computer Controlled Mask Aligner Using Modified Moire Technique

The operation of mask alignment is one of the most important steps in the process of fabrication of integrated circuits. In this paper we report a computer controlled mask alignment system using modified moire’ technique. In this technique the alignment is controlled in high slope region of the moire’ signal by using a single pair of grating alignment marks. Two methods have been proposed to obtain alignment. The error signal for alignment is taken as the difference of moire’ signal and a secondary signal computed from the moire’ signal. In one method this secondary signal is the inverted moire’ signal while in other it is the average value calculated from the maximal and minimal value of moire’ signal. Experiments were conducted using He-Ne laser of 0.633 μm wavelength and 25μm grating pitch. The alignment accuracy of 0.06μm has been achieved.