For intra-cavity high resolution spectroscopy of acetylene (C2H2), an extended cavity diode laser (ECDL) system emitting at 1.55 micrometers has been built using a 600 lines/mm diffraction grating mounted in Littrow configuration. In a single pass absorption cell the linear absorption of a number of rovibrational overtone transitions of C2H2 has been registered. An external FP cavity (finesse of 300, free spectral range FSR equals 520 MHz) was built and utilized for C2H2 intra-cavity spectroscopy as well as for ECDL frequency stabilization. The Pound-Drever-Hall technique was used to lock the diode laser frequency to the external cavity with the servo bandwidth of 50 kHz. The C2H2 intra-cavity linear absorption was recorded in transmission at a pressure of 1.3 Pa. By combining the reflected and transmitted signals the influence of residual frequency fluctuation of the ECDL with respect to the external cavity was reduced and the signal to noise ratio was improved to 34 dB for a single pass linear absorption of 0.2%.
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.
An automatic, accurate mask alignment method, based on a modified moiré 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 moiré signal is used to obtain higher sensitivity and better position control accuracy. Automatic alignment is achieved by using the difference of the moiré 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.
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.
An automatic and accurate technique
for angular positioning of mask with
respect to wafer is reported.. Alignment
marks are in the form of gratings and the
moire signal is obtained by the relative
displacement between the gratings. The
higher slope region of the moire signal
is used to obtain higher sensitivity and
better position control accuracy. The
experiments are performed with 25 micrometer pitch gratings and Piezo-
Electric Transducer is used for the
angular displacement. The angular
accuracy of the order of 2 x 10 radian is
reported with a time constant of 0.2 sec.
In the recent years, use of moire
interference technique for mask to wafer
alignment has attracted much attention
especially in VLSI fabrication technology
where a highly accurate linear and
angular positioning is involved. In this
technique the alignment marks are in the
form of gratings.
A laser beam is passed normally
through a mask grating and reflected from
the wafer grating. Intensity variation in
the reflection mode due to relative
angular rotation is detected by a
photodetector and converted into the
electrical signal, known as moire signal.