Excimer laser and method of operating an excimer laser

Details Excimer laser and method of operating an excimer laser Excimer laser and method of operating an excimer laser

2001-08-28

2004-03-30

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular active media

Gas

C372S038100, C372S086000, C372S038020, C372S038070

Reexamination Certificate

active

06714576

ABSTRACT:

The present invention relates to an excimer laser comprising a charging circuit including a parallel connection of a storage capacitor with a series connection of a reoscillation inductance, a charging inductance and a firing device for firing the excimer laser, wherein the storage capacitor has at least a terminal for connecting a power supply, a peaking capacitor connected in parallel with the charging inductance, a discharge path including two opposed electrodes, connected in parallel with the peaking capacitor, and a driving device for driving the firing device. Furthermore, it relates to a method of operating such an excimer laser, in which first the storage capacitor is charged from the power supply, and subsequently the firing device is driven such that this one changes to a conducting state to fire the excimer laser.
Excimer lasers as well as methods of operating excimer lasers known from the prior art. The assiciated circuit diagrams are shown in FIG.
1
and
FIG. 2
in schematic view.
First, referring to FIG.
1
: The excimer laser, the circuit diagram of which is shown in
FIG. 1
, comprises a storage capacitor C
B
, preferably realized as a capacitor bank including a plurality of single capacitors, as well as a reoscillation inductance L
B
in serial arrangement thereto. A charging circuit results from the both mentioned devices C
B
and L
B
as well as from a charging inductance L
L
arranged in series thereto, wherein the storage capacitor C
B
is charged from a current source I
L
. A peaking capacitor C
P
as well as a discharge path having two opposed electrodes E
1
and E
2
are arranged in parallel with the charging inductance L
L
. A thyratron having a control input A is arranged in parallel with the current source I
L
as a firing device Z. In operation of the excimer laser according to
FIG. 1
, for a single pumping operation of the laser, first the storage capacitor C
B
is charged from the current source IL, wherein the charging current flows through the storage capacitor CB, the reoscillation inductance L
B
as well as the charging inductance L
L
. Subsequently, by appropriately driving the input A of the thyratron Z the laser is fired, whereby the charge stored on the storage capacitor C
B
is transferred to the peaking capacitor C
P
to a high degree. This reoscillation results in a voltage arising between the two electrodes E
1
and E
2
, which is high enough to initiate the pumping process of the laser. The reoscillation inductance L
B
is composed of parasitic inductances of the single capacitors of the storage capacitor C
B
as well as an optional, additional discrete inductance which, in view of the resonant circuit resulting from the storage capacitor C
B
, the peaking capacitor C
P
and the reoscillation inductance L
B
, is to be dimensioned such that the reoscillation, i.e. the charge of the peaking capacitor C
P
, is effected within about 100 ns. A slower reoscillation would result in reduction of the efficiency, a faster reoscillation would result in an unnecessarily high stress of the devices as a result of the higher currents flowing.
Due to technology, in firing only a maximum of 80% of the charge stored on the storage capacitor C
B
can be transferred to the peaking capacitor C
P
. In cooperation with the thyratron Z as the firing device the following disadvantages result: first, it has to be pointed out that the fall time, i.e. the time until a thyratron changes from the non-conducting to the conducting state after appropriately driving via the input A, is about 10 ns. However, inherently a thyratron cannot defined be switched to a non-conducting state by appropriately driving. Rather, the residual energy stored on the storage capacitor C
B
oscillates back to the firing device, and there generates a current I
Z
whose time behavior is shown in FIG.
4
. This current oscillates as useless energy into the laser head, i.e. it flows onto the electrodes E
1
and E
2
, respectively, but there it does not result in laser radiation any longer, since the gas is already degenerated at this time. Rather, thereby burn-off and wear at the electrodes E
1
and E
2
are caused, resulting in frequent exchange of the electrodes—approximate lifetime about one milliard of pulse operations. Namely, since the conditions for a volume discharge are not given any longer, the energy explodes in discrete localized sparks. Because of the long recombination time inherent to a thyratron, repetition rates of the pulse operation can only be achieved on the order of about 300 Hz. A further disadvantage is the high standby power dissipation of a thyratron, which typically is 200 W.
In the further excimer laser known from the prior art, the circuit diagram of which is schematically shown in
FIG. 2
, the devices corresponding to devices of
FIG. 1
are designated by the same reference symbols and are not described again. In the excimer laser according to
FIG. 2
the storage capacitor C
B
is arranged in parallel with a current source I
L
to which the reoscillation inductance L
B
and the firing device Z are connect in series. Again, a peaking capacitor C
P
is arranged in parallel with this. The peaking capacitor C
P
is followed by three pulse compression stages, each comprising an inductance (L
1
, L
2
, L
3
) and a capacitor (C
1
, C
2
, C
3
), after which the discharge path with the electrodes E
1
and E
2
and the charging inductance L
L
, arranged in parallel with this, follow. In the circuit diagram shown in
FIG. 2
the firing device is realized by a thyristor or an IGBT (insulated gate bipolar transistor). The fall time of a thyristor is higher than 500 ns, the opening time, i.e. the time it takes for a thyristor to change from the conducting to the non-conducting state, is more than 20 &mgr;s. Therefore, a reoscillation operation typically takes between three and ten &mgr;s. This reoscillation would be too slow to initiate a pumping operation, since in the meantime the charge carriers would already recombine again in the preionization of the laser. Therefore, in using a thyristor or IGBT as a firing device, it is necessary to compress the firing pulse, i.e. to shrink it in time. Referring to
FIG. 5
, in which the time behavior of the current I
Z
is shown (solid line), it is apparent that per compression stage a compression with the factor of 4 is realizable (dashed lines). Due to the shorter recombination time of a thyristor or IGBT, repetition rates of the pulse operation up to 5 kHz can be realized. However, on the one hand the inclusion of pulse compression stages implies a considerable effort. On the other hand, due to many transfers of charge for the pumping process, at the end only about 50% of the energy originally stored on the storage capacitor C
B
are available for the pumping process.
Both excimer lasers described in conjunction with FIG.
1
and
FIG. 2
have furthermore the disadvantage that the capacitor C
B
has to be charged from an expensive current source. The cause for this is that after firing and the subsequent recombination of the firing device, the storage capacitor C
B
is substantially discharged, and connecting the storage capacitor C
B
to a voltage source, which would be cheaper, would result in destruction of the storage capacitor C
B
.
Therefore, the object of the present invention is to develop an excimer laser of the type mentioned at the beginning and to develop the method mentioned at the beginning, respectively, such that the storage capacitor can be charged via a cheap voltage source.
This object is achieved in that in the generic excimer laser the firing device is formed as a MOSFET array. In the method according to the invention the object is achieved in that in a further step the firing device is driven such that this one changes to a non-conducting state.
The invention is based on the idea that in using a MOSFET array the firing device can actively be shut off again, i.e. that it is not required to wait for all charge carriers not used in the discharge to have recombined, until a new charging and firing operation

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