Patent Grant
7149234
Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of excimer lasers, useful for integrated circuit lithography, are described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact Excimer Laser” and U.S. Pat. No. 5,991,324 issued Nov. 23, 1999 entitled “Reliable, Modular, Production Quality Narrow-Band KrF Excimer Laser”. Both of these patents have been assigned to Applicants' employer, and these patents are hereby incorporated herein by reference. The excimer lasers described in the above patents are high repetition rate pulse lasers. In
Electric discharge gas lasers of the type described in U.S. Pat. No. 5,023,884 utilize an electric pulse power system shown in
In prior art lithography laser systems, the time between the closing of the solid state switch and the discharge is in the range of about 5 microseconds; however, the charging of C0 accurately to the pre-selected voltage has in the past required about 400 microseconds which was quick enough for pulse repetition rates of less than about 2,000 Hz. The reader should understand that accurate charging of C0 is very important since the control of the voltage level on C0 is in these systems the only practical control the laser operator has on the discharge voltage which in turn is the primary determiner of laser pulse energy. For laser light sources used for integrated circuit fabrication the precise timing of the pulses has not been critically important since for both stepper machines and scanning machines target areas on the wafer are illuminated with a number of pulses such as about 20 to 40 pulses during an interval of a few milliseconds.
Reticles used for integrated circuit lithography contain the patterns to be applied to the silicon wafer as a part of the process to create the integrated circuit. The pattern on the reticle is typically 3 or 4 times larger than the corresponding image on the wafer. Nevertheless, the dimension on the reticle are still very small, i.e., a few hundreds of nanometers. These patterns on the reticles typically in the past have been created with electron beams, and both reticles and wafers typically have been inspected with visible light microscopes.
What is needed are excimer laser systems optimized for reticle creation and inspection of both reticles and wafers.
The present invention provides a high repetition rate, compact, modular gas discharge, ultraviolet laser. The laser is useful as a light source for very rapid inspections of wafers in an integrated circuit fabrication process. It is also useful for reticle writing at very rapid rates. A preferred embodiment operates at pulse repetition rates of 1000 to 4000 Hz and is designed for round-the-clock production line operation. This preferred embodiment comprises a pulse control unit which controls the timing of pulses to an accuracy of less than 4 nanoseconds. Preferred embodiments of this gas discharge laser can be configured to operate with a KrF gas mixture, an ArF gas mixture or an F2 gas mixture, each with an approximate buffer gas, producing 248 nm, 197 nm or 157 nm ultraviolet light pulses.
A first preferred embodiment of the present invention is shown in block diagram in
This laser system is designed for production line operation. These lasers are expected to operate virtually continuously at pulse rates in the range of 1000 Hz to 4000 Hz or greater, 24 hours per day, seven days per week, 52 weeks per year with minimal downtime for maintenance. Estimated usage is 20 billion pulses per year. The laser must be built for minimum downtime for maintenance. It is designed in many ways like a production line lithography laser system such as the one described in U.S. Pat. No. 5,991,324 but there are many important differences in this laser. The principal differences are that the present system does not need to be line narrowed, and pulse energies are somewhat lower than those required for lithography. However, individual pulse energy specifications including the timing of the pulse (at least when used for mask writing) is much tighter than those for the lithography lasers. Also, coherence of the laser beam in this laser is substantially reduced as compared to the prior art lithography lasers. The laser system is much smaller, about one-half the size of typical prior art high repetition rate lithography lasers.
The laser system of the present invention is currently expected to be used primarily for reticle writing and wafer inspection.
The present invention is particularly useful for fabrication of reticles for use in integrated circuit lithography. A preferred technique for reticle writing using the present invention is shown in
The other principal use of the laser of the present invention is for wafer inspection. In this application, a laser beam from the laser described herein is used to illuminate a small portion (about 100 :m square to about 1 mm square) of a wafer and the reflected light is imaged onto a pixel array of a CCD camera. The CCD camera pattern is then compared by a processor to an expected pattern to determine if the wafer has been processed correctly. This inspection is expected to be part of an automated integrated circuit fabrication line, so that the wafer is automatically stepped into position for inspection, automatically scanned during the inspection and then stepped onto the next step after completion of inspection. A typical wafer comprises an integrated circuit surface area of about 100 cm2. Therefore, when 100 :m view sites are used, a total of about 1 million digital photographs must be taken of the wafer for a complete inspection. If one photograph is taken during each pulse and the laser is operating at 3000 Hz, the inspection will require about 333 seconds or about 6 minutes.
A cross-sectioned drawing showing important features of a laser chamber module 6 of a preferred embodiment of the present invention is shown in
The laser gas is maintained within desired temperature ranges of about 30° C. to 50° C. with one or more finned water cooled heat exchanger 57 of the type described in U.S. Pat. No. 6,128,323.
The pulse power supply for this preferred embodiment is an improved version of the pulse power supply described in U.S. Pat. No. 5,142,166.
The charge placed on C0 is controlled by a pulse energy controller to an accuracy of about 0.1 percent. The accuracy of the charge on C0 is important because the charge on co determines the discharge energy. Upon a command from control module 16 switch TR1 is closed discharging C0 through diodes D1 saturable inductor 60 and inductors L1 to charge up capacitor bank C1. Capacitor C1 is discharged resonantly through saturable inductor MSC1 to provide an approximately 1500 volt pulse at the primary side of pulse transformer 62. Pulse transformer 62 is a 1/12 to 1 pulse transformer of the general type described in U.S. Pat. No. 5,142,166. This pulse transformer amplifies the pulse voltage by a factor of about 12, for example, from 1500 volts to 18,000 volts or from about 2000 volts to 24,000 volts. This transformer is comprised of six hollow spool shaped mandrels, each wrapped with Metglass to form six cores, each of which has a single primary winding in parallel with the primary winding of the other cores. The secondary of the transformer is comprised of two-coaxial conductors radially insulated from each other but connected in series and passing through the center of six cores.
The output voltage of transformer 62 charges capacitor bank Cp-1 and it discharges through saturable inductor MSC2 to charge peaking capacitor Cp which in turn provides the electrical potential to produce the described discharge across electrodes 42 and 44. In this preferred embodiment, preferred values of capacitance and inductance for the components of the pulse supply system of
This circuit provides for energy recovery using the technique described in U.S. Pat. No. 5,729,562. Bias circuits shown at 64 and 66 bias the saturable inductors in the circuit to initially oppose electrical energy flow from charging capacitor C0 to the electrodes. However, after the closing of switch TR1 the inductors become saturated after a time to permit energy to flow forward to the electrodes. Due to a mismatch electrons flow across the discharge first from cathode 42 to anode 44 and then immediately backward across the discharge region from anode 44 to cathode 42 to oppositely charge Cp. Since all saturable inductors continue to be forward conducting for a few microseconds after the start of the pulse. The unspent pulse energy rings back through the circuit all the way to C0 to place a charge on C0 opposite to that applied by the resonant charger. That charge is immediately reversed by charge reversing circuit 66 all as described in U.S. Pat. No. 5,729,562.
Embodiments of the present invention operate with substantial improvements in efficiency due to reduction in the induction of the portion of the pulse power circuit including Cp and the electrodes 42 and 44, shown at 70 in
Preferred embodiments of the present invention include pulse energy monitor 18 as shown in
When the laser system is used for inspection automatic control equipment is preferably programmed to repeat the inspection when an out of specification pulse energy or jitter is measured. If the frequency or extent of out of specification occurrences exceeds a predetermined level, corrective maintenance is called for. When the system is used for reticle writing, out of specification pulse energy or jitter may ruin the reticle. Therefore, in one embodiment controls for the laser system indicates a warning when out of specification information is detected. This information may be utilized to require special post operation inspection of the reticle.
The pulse energy monitor shown at 18 in
Another alternative would be to use the general technique described in '621 to reflect away any bad pulse either due to out of specification pulse energy or jitter. This would permit the reticle writing system to back up and write again the portion represented by the bad pulse or pulses.
As indicated in
For the above reasons, the timing of pulses for reticle writing is much more critical than the timing of the pulses for integrated circuit lithography where sections of a wafer are exposed to about 15 to 50 pulses and the actual timing of the pulses needs to be accurate only to several microseconds.
This embodiment of the present invention provides a jitter control technique called a jitter compensation device (JCD) that assures that the pulse timing is accurate within about 10–20 ns. The light delay from an input trigger depends on voltage of solid state switched pulsed power module. The higher the charging voltage the shorter the delay between trigger and laser pulse. Delay between trigger to light also depends on the temperature of the magnetics inside the pulse power system. Thus, the delay between trigger to light is a function of voltage and temperature. The higher the temperature of the magnetic devices in the pulse power system, the shorter the delay between trigger and pulse. But fixed voltage and temperature, the natural pulse-to-pulse variation of the light pulse with the input trigger is small, about +/−5 ns.
Thus, in a preferred embodiment after a trigger from the user, the jitter control delays sending the trigger to the solid state pulse power of the laser by an amount corresponding to the laser's operating voltage and the temperature of the magnetics, so that the timing of the resulting pulse is accurate to within about 20 ns.
In another embodiment, the pulse controller adjusts the timing of the pulse for variations in charging voltage based on the specified charging voltage for the up coming pulse. In this embodiment, no direct correction for temperature variation is provided. However, a correction is made to the timing based on a timing error measured from the previous pulse. Since temperature of the magnetic components vary slowly, this pulse timing feedback technique in effect compensates for slowly varying temperature effects and at the same time provides compensation for other more suitable effects.
This particular embodiment makes the feedback correction using a technique which applies a full 100% correction for large timing errors such as greater than 20 ns but for smaller timing errors such as, for example, less than 20 ns a correction of only 25% is applied. This smaller percentage correction avoids oscillations about zero error.
This particular embodiment also employs a trick to achieve a 1 nanosecond resolution with a wide dynamic range using a digital counter with a 40 MHz crystal oscillator. The 40 MHz oscillator provides clock signals at 25 ns intervals but these signals are utilized to charge an approximately linear analog capacitive charging circuit. The voltage on the capacitor is then read to determine time accurate to about 1.0 ns.
Preferred embodiments of the present invention utilize a curved-curved resonant cavity configuration. This configuration is displayed in
Various modifications may be made to the present invention without altering its scope. Those skilled in the art will recognize many other possible variations. For example, operation at very high repetition rates and high duty factors may require additional cooling of the laser gas and other components of the laser including the optical components, the pulse power components and the fan motor.
Accordingly, the above disclosure is not intended to be limiting and the scope of the invention should be determined by the appended claims and their legal equivalents.
This application is a continuation of U.S. Ser. No 09/837,035 filed Apr. 8, 2001 and is a continuation-in-part of Ser. No. 09/748,316, filed Dec. 22, 2000, now U.S. Pat. No. 6,477, 193, Ser. No. 09/684,629 filed Oct. 6, 2000, now U.S. Pat. No 6,442,181 and Ser. No. 09/370,739, filed Aug. 9, 1999, now U.S. Pat. No. 6,151,346 which was a continuation-in-part of Ser. No. 09/118,773, filed Jul. 18, 1998 now U.S. Pat. No. 5,936,988 and Ser. No. 09/608,543, filed Jun. 30, 2000, all of which are incorporated by reference herein. This invention relates to gas discharge lasers and in particular to high repetition rate gas discharge lasers.
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| Number | Date | Country | |
|---|---|---|---|
| 20040047386 A1 | Mar 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09837035 | Apr 2001 | US |
| Child | 10650578 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09748316 | Dec 2000 | US |
| Child | 09837035 | US | |
| Parent | 09684629 | Oct 2000 | US |
| Child | 09748316 | US | |
| Parent | 09608543 | Jun 2000 | US |
| Child | 09684629 | US | |
| Parent | 09370739 | Aug 1999 | US |
| Child | 09608543 | US | |
| Parent | 09118773 | Jul 1998 | US |
| Child | 09370739 | US |
