Emission timing control apparatus for pulsed laser

Details Emission timing control apparatus for pulsed laser Emission timing control apparatus for pulsed laser

1999-03-31

2002-06-04

Leung, Quyen (Department: 2881)

Coherent light generators

Particular component circuitry

For driving or controlling laser

C372S038100, C372S038030, C372S038040, C372S038070, C372S025000, C372S037000, C372S086000, C372S029010, C372S029012, C372S029015

Reexamination Certificate

active

06400741

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an emission timing control apparatus for pulsed lasers, effecting pulsed laser emission by exciting a laser medium through pulsed discharge at a prescribed frequency of repetition using a magnetic pulse compression circuit, which improves the precision of synchronizing the emission timing of the pulsed laser with the control timing for the semiconductor exposure apparatus.
2. Description of the Related Art
Much attention is being given to the use of excimer lasers as exposure sources for reduced projection exposure apparatuses (referred to below as steppers) for semiconductor manufacturing. These are expected to provide a great many advantages: the possibility of extending the limits of the exposure light to below 0.5 &mgr;m with the short wavelengths of excimer lasers (the wavelength for KrF is 248.4 nm); deeper focal depth than the g lines and i lines of the mercury lamps, which are usually used and which have the same resolution; increasing the exposure area with a small numerical aperture (NA) lens; and achieving high power.
FIG. 9
shows the general constitution of the control system for an excimer laser
1
and a stepper
10
.
The excimer laser
1
comprises the following:
a laser chamber
2
housing discharge electrodes or the like;
a pulse power source apparatus
3
for applying the high frequency voltage, synchronized with the frequency of repetition of the pulsed discharge, between the discharge electrodes;
an energy monitor
4
to monitor the energy, wavelength, and the like of the laser beam output from the laser chamber
2
; and
a laser controller
5
to supply and control laser gas, control laser oscillation wavelength, and control the power source voltage of the pulse power source apparatus
3
, on the basis of the monitoring values from the energy monitor
4
and the energy command E from the stepper
10
.
The stepper
10
comprises a movable wafer table
12
whereon wafers are mounted and a stepper controller
11
to transfer the pulse oscillation synchronizing signal TR, which is the trigger signal for repeated pulse oscillation, and the target energy command E for laser oscillation to the excimer laser. The wafers on the wafer table
12
are exposed with a reduced projection system using the laser beam from the excimer laser.
In recent years, magnetic pulse compression circuits have come to be used as the pulsed power source apparatus
3
in
FIG. 9
; these improve the durability of the main switch of a cyclotron, GTO, or the like.
FIG. 10
shows an equivalent circuit for a capacitance switching, magnetic pulse compression discharge apparatus.
FIG. 11
shows a waveform diagram of the voltage and current in each portion of the circuit in FIG.
10
.
The discharge circuit in
FIG. 10
is a two-stage magnetic pulse compression circuit utilizing the saturation phenomena of three magnetic switches AL
0
-AL
2
comprising saturable reactors.
The energy command value E is input from the stepper
10
before the first laser oscillation trigger signal is received. The laser controller
5
therefore calculates the power source voltage necessary to output this energy and adjusts the voltage of the high voltage power source HV based on this calculated value. At this time, the capacitor C
0
is precharged with a charge from the high voltage power source HV by means of the magnetic switch AL
0
and coil L
1
.
Afterwards, the main switch SW is turned on when the first pulse oscillation synchronizing. signal (trigger signal) TR is received from the stepper
10
(
FIG. 11
, time t
0
). When the main switch SW is turned on, the potential VSW of the main switch abruptly drops to 0. After that, the time product (time integral value of voltage VC
0
) S
0
of the voltage difference VC
0
−VSW of the main switch SW and the capacitor C
0
reaches the limit value determined by the settings of the magnetic switch AL
0
. VC
0
and VSW are the voltage of both terminals of the magnetic switch AL
0
. At that time t
1
, the magnetic switch AL
0
becomes saturated and the current pulse i
0
flows through the loop formed by the capacitor C
0
, magnetic switch AL
0
, main switch SW, and capacitor C
1
.
The time &dgr;
0
, from when that current pulse i
0
starts to flow until it becomes zero (time t
2
), is determined by the inductance and capacitance of the capacitor C
0
, magnetic switch AL
0
, and capacitor C
1
, if loss due to the main switch SW or the like is ignored. More specifically, the charge transfer time
0
is the time necessary for charge to move completely from the capacitor C
0
to the capacitor C
1
.
Meanwhile, the time product S
1
of the voltage VC
1
of the capacitor C
1
reaches the limit value determined by the settings of the magnetic switch AL
1
. At this time t
3
, the magnetic switch AL
1
becomes saturated and has low inductance. As a result, the current pulse i
1
flows in the loop formed by the capacitor C
1
, capacitor C
2
, and magnetic switch AL
1
. This current pulse i
1
becomes zero at time t
4
once the prescribed transfer time
1
, determined by the inductance and capacitance of the magnetic switch AL
1
and capacitors C
1
, C
2
, has passed.
Also, the time product S
2
of the voltage VC
2
of the capacitor C
2
reaches the limit value determined by the settings of the magnetic switch AL
2
. At this time t
5
, the magnetic switch AL
2
becomes saturated, causing the current pulse i
2
to flow through the loop formed by the capacitor C
2
, peaking capacitor CP, and magnetic switch AL
2
.
The voltage VCP of the peaking capacitor CP rises throughout the charging process. At the time t
6
when this voltage VCP reaches the prescribed main discharge initiation voltage, the laser gas between the main electrodes
6
undergoes dielectric breakdown and the main discharge starts. The laser medium is excited by this main discharge and a laser beam is emitted after several nanoseconds.
This type of discharge action is performed repeatedly by the switching action of the main switch
5
synchronized with the trigger signal TR; as a result, pulsed laser oscillation is effected at the prescribed repetition frequency (pulse oscillation frequency).
The magnetic compression circuit shown in
FIG. 10
is set so that the inductance of each stage of the charge transfer circuit, composed of magnetic switches and capacitors, progressively decreases in farther stages. Pulse compression is carried out so that the peak values of the current pulses i
0
i
2
gradually increase and the current amplitude gradually narrows. As a result, a strong discharge is attained between the main electrodes
6
in a short period of time. Also, each magnetic switch AL
0
-AL
2
is reset at each pulse to the initial state with the reset circuit of a saturable reactor. The saturation point (action point) of each magnetic switch AL
0
-AL
2
is the same for the voltage and becomes uniform from pulse to pulse.
With the abovementioned magnetic compression circuit, however, the saturation time &agr;
0
(&dgr;
0
+&agr;
1
), (&dgr;
1
+&agr;
2
) of each magnetic switch AL
0
-AL
2
that is determined by the voltage time product changes when the initial charging voltage V
0
changes. Accordingly the time td (referred to below as emission delay time) changes as well. The time td is from the time t
0
when the trigger TR is input and the magnetic switch SW called up until the time t
6
when the laser beam :is actually emitted.
In an excimer laser, as discussed above, the power source voltage V
0
is one of control parameters for maintaining uniform laser output and can be varied during laser operation. Specifically, power source voltage V
0
is variably controlled with consideration given to various factors such as power lock control for controlling power source voltage taking into consideration the drop in laser output due to a decrease in halogen gas, and spike-killer control for controlling power source voltage in order to resolve the spiking phenomenon wherein laser output becomes high in the spike zone, includi

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