Coherent light generators – Particular beam control device – Control of pulse characteristics
Reexamination Certificate
2001-05-14
2003-04-29
Ip, Paul (Department: 2878)
Coherent light generators
Particular beam control device
Control of pulse characteristics
C372S057000, C372S038020, C372S038070
Reexamination Certificate
active
06556600
ABSTRACT:
BACKGROUND OF THE INVENTION
Prior Art Lithography Lasers
KrF excimer lasers are the state of the art light source for integrated circuit lithography. Such lasers are described in U.S. Pat. No. 4,959,840, U.S. Pat. No. 5,991,324 and U.S. Pat. No. 6,128,323. The lasers operate at wavelengths of about 248 nm. With the KrF laser integrated circuits with dimensions as small as 180 nm can be produced. Finer dimensions can be provided with ArF lasers which operate at about 193 nm or F
2
lasers which operate at about 157 nm. These lasers, the KrF laser, the ArF laser and the F
2
lasers, are very similar, in fact the same basic equipment used to make a KrF laser can be used to produce an ArF laser or an F
2
laser merely by changing the gas concentration, increasing the discharge voltage and modifying the controls and instrumentation to accommodate the slightly different wavelength. A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in
FIGS. 1
,
1
A and
1
B. A cross section of the laser chamber of this prior art laser is shown in FIG.
1
B. As shown in
FIG. 1A
, pulse power system
2
powered by high voltage power supply
3
provides electrical pulses to electrodes
6
located in a discharge chamber
8
. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 to 2000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 to 2,000 cm per second. This is done with tangential blower
10
located in the laser discharge chamber. The laser gases are cooled with a heat exchanger
11
also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module
18
(sometimes referred to as a line narrowing package or LNP). Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
High voltage power supply module,
High voltage compression head module,
Commutator module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module,
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes
6
consist of cathode
6
A and anode
6
B. Anode
6
B is supported in this prior art embodiment by anode support bar
44
which is shown in cross section in FIG.
1
B. Flow is counter-clockwise in this view. One corner and one edge of anode support bar
44
serves as a guide vane to force air from blower
10
to flow between electrodes
6
A and
6
B. Other guide vanes in this prior art laser are shown at
46
,
48
and
50
. Perforated current return plate
52
helps ground anode
6
B to the metal structure of chamber
8
. The plate is perforated with large holes (not shown in
FIG. 3
) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor bank comprised of an array of individual capacitors
19
is charged prior to each pulse by pulse power system
2
. During the voltage buildup on the peaking capacitor, one or two preionizers
56
weakly ionize the lasing gas between electrodes
6
A and
6
B and as the charge on capacitors
19
reaches about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 to 2 cm per millisecond, created by blower
10
, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one half to one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of the subsequent pulse is close to the desired energy. These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance.
Injection Seeding
A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band “seed” beam into a gain medium. In one such system, a laser called the “seed laser” or “master oscillator” is designed to provide a very narrow laser band beam and that laser beam is used as a seed beam in a second laser. If the second laser functions as a power amplifier, the system is typically referred to as a master oscillator, power amplifier (MOPA) system. If the second laser itself has a resonance cavity, the system is usually referred to as an injection seeded oscillator (ISO) and the seed laser is usually called the master oscillator and the downstream laser is usually called the power oscillator.
Jitter Problems
In gas discharge lasers of the type referred to above, the duration of the electric discharge is very short duration, typically about 20 to 50 ns (20 to 50 billions of a second). Furthermore, the population inversion created by the discharge is very very rapidly depleted so that the population inversion effectively exists only during the discharge. In these two laser systems, the population in the downstream laser must be inverted when the beam from the upstream laser reaches the second laser. Therefore, the discharges of the two lasers must be appropriately synchronized for proper operation of the laser system. This can be a problem because within typical pulse power systems there are several potential causes of variation in the timing of the discharges. Two of the most important sources of timing variations are voltage variations and temperature variations in saturable inductors used in the pulse power circuits. It is known to monitor the pulse power charging voltage and inductor temperatures and to utilize the data from the measurements and a delay circuit to normalize timing of the discharge to desired values. One prior art example is described in U.S. Pat. No. 6,016,325 which is incorporated herein by reference. There in the prior art timing errors can be reduced but they could not be eliminated. These errors that ultimately result are referred to as “jitter”.
Laser Wavelength and Bandwidth
A typical KrF laser has a natural bandwidth of about 300 pm (FWHM) centered at about 248 nm and for lithography use, it is typically line narrowed to about 0.6 pm. ArF lasers have a natural bandwidth of about 500 centered at about 193 nm and is typically line narrowed to about 0.5 pm. These lasers can be relatively easily tuned over a large portion of their natural bandwidth using the line narrowing module 18 shown in FIG.
2
. Also for the KrF and ArF lasers, the absolute wavelength of the output beam can be determined accurately by comparing its spectrum to atomic reference lines during laser operation. F
2
lasers typically produce laser beams with most of its energy in two narrow lines centered at about 157.63 nm and 157.52 nm. Often, the less intense of these two lines (i.e., the 157.52 nm line) is suppressed and the laser is forced to operate at the 157.63 nm line. The natural bandwidth of the 157.63 nm line is pressure dependant and varies from about 0.6 to 1.2 pm. An F
2
laser with a bandwidth in this range can be used with lithography devices utilizing a catadiophic lens design utilizing both refractive and reflective optical elements, but for an all-refractive lens design the laser beam should have a bandwidth to produce desired results. It is also known that the centerline wavelength of the output beam will vary somewhat depending on condition in the discharge region.
Lasers for lithography equipment are very complicated and expensive. Further reduction in bandwidth could greatly simplify the lens design for lithography equ
Ershov Alexander I.
Ness Richard M.
Oh Choonghoon
Onkels Eckehard D.
Partlo William N.
Cymer Inc.
Ip Paul
Monbleau Davienne
Ross John R.
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