JP4080712B2 - High repetition rate gas discharge laser with accurate pulse timing control - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is serial number 09 / 748,316 filed in the United States on December 22, 2000, serial number 684,629 filed on October 6, 2000, and filed on August 9, 1999. Serial No. 09 / 370,739 and Serial No. 09 / 118,773 filed on June 18, 1998, now US Pat. No. 6,151,346, now US Pat. This is a partially pending application of 5,936,988 and serial number 09 / 608,543 filed on June 30, 2000.
[0002]
The present invention relates to a gas discharge laser, and more particularly to a high repetition rate gas discharge laser.
[0003]
[Prior art]
Discharge gas laser
Discharge gas lasers are well known and have been used since the 1960s when the laser was invented. A high voltage discharge between the two electrodes excites the gaseous gain medium. The resonant cavity containing the gain medium then causes inductive amplification of the light extracted from the cavity as the shape of the laser beam. Many of these discharge gas lasers operate in a pulse mode.
[0004]
Excimer laser
Excimer lasers are a specific type of discharge gas laser and have been known since the mid 1970s. Excimer lasers useful for integrated circuit lithography are described in US Pat. No. 5,023,884, entitled “Compact Excimer Laser” and issued on June 11, 1991, and “Reliable, Modular, Production Quality Narrow-Band KrF”. US Pat. No. 5,991,324 entitled “Excimer Laser” and issued on November 23, 1999. Both of these patents are assigned to the applicant's user and are incorporated herein by reference. The excimer laser described in the above patent is a high repetition rate pulse laser. In FIG. 1, the main elements of the laser 10 are disclosed (FIG. 1 corresponds to FIG. 1 of the '884 patent). Discharge 22 is between two long (about 23 inches) electrodes 18 and 20 that are spaced about 5/8 inch apart. As people disclosed, the repetition rate of prior art lasers was typically in the range of about 100 to 200 pulses / second. These high repetition rate lasers were usually provided with a gas circulation system. The laser described above is made with a long Riscago type fan 46 having blades 48 as shown in FIG. 1 and FIG. 2 of FIG. 7 of the '884 patent. The fan blade structure is slightly longer than the electrodes 18 and 20 and provides sufficient circulation so that at a pulse frequency of 100-200 Hz, the discharge turbulent gas between the electrodes is clean during the pulse. Is done. The gas used for the laser contains highly reactive fluorine. The fan rotor that drives the fan shaft 130 is sealed within the same environmental system provided by the housing structural members 12 and 14 by a seal member 136 as described in column 9 line 45 of the '884 patent. The stator 140 is outside the seal member 136 and is thus protected from the corrosive action of fluorine gas. The heat of the gas generated by the rapid circulation and discharge of the gas is removed by the finned water-cooled heat exchanger 58. An important use of these lasers is as a light source for integrated circuit lithography. The nominal output wavelength of these lasers is determined by gas mixing. The KrF excimer laser operates at about 248 nm, the ArF excimer laser operates at about 193 nm, and F ₂ The excimer laser operates at about 157 nm.
[0005]
Pulse power
A discharge gas laser of the type described in US Pat. No. 5,023,884 utilizes the electrical pulse power system shown in FIG. 3 to create a discharge between two electrodes. In such prior art systems, the DC power source 22 is connected to a “charging capacitor” or “C ₀ The capacitor bank called "42" is charged to a predetermined and controlled voltage called "charging voltage" for each pulse. The magnitude of this charge voltage may range from about 500 to 1000 volts. C ₀ Are charged to a predetermined voltage, the solid state switch 46 is coupled to capacitor banks 52, 62 and 82 and inductors 48, to create a high voltage potential in the range of about 16,000 volts across the electrodes creating a discharge that lasts about 50 ns. Very fast via a series magnetic compression circuit having 54 and 64 and a voltage transformer 56, C ₀ The electrical energy stored in is closed to ring.
[0006]
In prior art lithographic laser systems, the time between closing the solid state switch and discharging is in the range of about 5 microseconds, however, an accurate C for the preselected voltage. ₀ In the past, about 400 microseconds, which can be said to be sufficiently fast with respect to the number of pulse repetitions smaller than about 2000 Hz, has been required. Reader C ₀ Since the voltage level control is the only practical control in these systems and the laser operator in turn has a discharge voltage that will have the main determination of the laser pulse energy, an accurate C ₀ It should be understood that the charge is very important. With respect to laser light sources utilized in integrated circuit manufacturing, for both stepper machines and scanner machines, the target area of the wafer is illuminated with multiple pulses, such as about 20 to 40 pulses, over a few millisecond interval. The exact timing of the pulses was not critical.
[0007]
Reticle
The reticle used in conjunction with integrated circuit lithography includes a pattern that is applied to a silicon wafer as part of a process for creating an integrated circuit. The reticle pattern is typically 3 or 4 times larger than the corresponding image on the wafer. Nevertheless, the dimensions of the reticle are very small, i.e. hundreds of nanometers. In the past, typically these patterns of reticles have been made with an electron beam, and both the reticle and the wafer have typically been viewed with a visible light microscope.
[0008]
There is a need for an excimer laser system that is optimized for reticle fabrication and inspection of both the reticle and the wafer.
[0009]
[Means for Solving the Problems]
The present invention provides a high repetition rate, compact, modular gas discharge, ultraviolet laser. Lasers are useful as light sources for very fast inspection of wafers in integrated circuit manufacturing processes. It is also useful for reticle drawing at very fast rates. In the preferred embodiment, it operates at a pulse repetition rate of 1000 to 4000 Hz and is designed for day and night production line operation. This preferred embodiment has a pulse control unit that controls the timing of the pulses to an accuracy of less than 4 nanoseconds. The preferred embodiment of this gas discharge laser operates with a KrF gas mixture, ArF gas mixture, or F2 gas mixture each containing a buffer gas to produce a 248 nm, 197 nm or 157 nm ultraviolet pulse.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
First preferred embodiment
A first preferred embodiment of the present invention is shown in block diagram form in FIG. The laser system 2 is modular and the main modules are assembled in the laser cabinet 4 so that they can be easily replaced and are shown in FIG. These modules are a laser chamber 6, a high voltage power supply 8, a solid magnetic switch module 10, a cooling water module 12, a gas module 14, a control module 16, and an energy monitor module 18. The details of the laser system are as follows.
[0011]
Laser system
This laser system is designed for production line operation. These lasers are practically continuous with pulse frequencies in the range of 1000 Hz to 4000 Hz or more, 24 hours a day, 7 days a week, 52 weeks a year, with minimal dine time for maintenance. Is expected to work. The planned use is 20 billion pulses per year. The laser must have minimal downtime for maintenance. Although designed in many ways, such as a production line lithography laser system as described in US Pat. No. 5,991,324, there are many important differences in this laser. The main difference is that this system does not require line narrowing and the pulse energy is somewhat lower than that required for lithography. However, individual pulse energy specifications, including pulse timing (at least when used for mask writing) are more stringent than those for lithographic lasers. Also, the laser beam coherence in this laser is substantially reduced compared to prior art lithography lasers. The laser system is smaller and half the size of a typical prior art high repetition rate lithography laser.
[0012]
Main uses
It is currently expected that the laser system of the present invention can be used primarily for reticle writing and wafer inspection.
[0013]
Reticle drawing
The present invention is particularly useful in the manufacture of reticles used in integrated circuit lithography. A preferred technique for reticle drawing using the present invention is shown in FIG. This technique utilizes light from the laser system of the present invention to print a pattern on a chromium film deposited on a transparent substrate. The pattern is created with laser UV light, either by evaporation of chromium by light or by utilizing UV light and a resist cured by subsequent acid treatment to remove unprotected chromium. These techniques are detailed in “High Performance Laser Pattern Generation Using Spacial Light Modulator (SLM) and Deep UV Radiation” (SPIE 2001 Microlithography Conference in Santa Clara, California) by Torbjorn, Sandstrom et al. It is also described in the outline as follows by the reference of FIG. In this mask writer system 20, the light from the excimer laser 2 of the present invention is reflected by the beam splitter 22 to a spatial optical modulator 24 that spatially adjusts the light at a very fast frequency. This is a million mirror array (1000 × 1000) that is computer controlled to reflect the desired pattern through beam splitter 22, lens 26, aperture 28 and lens 30 to produce the desired pattern of reticle 32. Is made with a higher order array of very small mirror surfaces. The position of the reticle 32 is accurately controlled by an interferometer controlled positioning system 34.
[0014]
Mask inspection
Another major use of the laser of the present invention is wafer inspection. In this application, a laser beam from an individually described laser is used to illuminate a small portion of the wafer (about 100 μm square to about 1 mm square), and the reflected light is imaged onto a CCD camera pixel array. The The CCD camera pattern is then compared to a predetermined pattern by the processor to determine if the wafer has been processed correctly. In this inspection, the wafer is automatically advanced to a position for inspection, automatically scanned during inspection, and then advanced to the next step after the inspection is completed. Expected in the department. A typical wafer is about 100 cm. ² With an integrated circuit surface area of Therefore, when a 100 μm view site is used, a total of about 1 million digital photographs are obtained from a fully inspected wafer. If one picture is taken during each pulse and the laser is operating at 3000 Hz, the test takes about 333 seconds or about 6 minutes.
[0015]
Laser system details
Laser chamber
A cross-sectional view showing important features of the laser chamber module 6 of the preferred embodiment of the present invention is shown in FIG. An enlarged view of the discharge region is shown in FIG. The chamber is enclosed in a pressure vessel 40 consisting of an extruded diameter aluminum tube 41 with an aluminum end flange. The wall thickness of the tube is 20 mm or 30 mm with a design pressure between 400 kPa and 600 kPa. Electrodes 42 and 44 are 40 cm in length and are separated by about 6 mm, creating a discharge width of about 4 mm. The electrode 42, called the anode, is electrically connected by five feedthrough rods 46 to a high voltage lens 48 activated by the peak capacitor bank 50. The electrode 44 is called an anode and is electrically grounded to the ground by the anode support bar 52. In this embodiment, the 20 current return vane provides a low inductance to return to the ground side of the peak capacitor bank 50. A single pre-ionizer tube 52, similar to that used in prior art lithography lasers, consists of a hollow cylinder of alumina with a ground rod in the center, running in the middle length and spring loaded. An electrically conductive flange 54 held against the tube causes the plasma to be entirely on the surface at the start of each discharge on the outer surface of the preionizer to create preionization in the discharge region between the two electrodes. Apply the high voltage potential of the peak capacitor. The laser gas is circulated in the chamber 40 by a 3 inch diameter fan 56 driven by a single DC water cooled motor. The rotor of the motor is sealed in the laser gas environment using a thin can sealing unit so that the motor starter is not exposed to the laser gas environment. This motor design is described in detail in US Patent Application Serial No. 09 / 748,316, which is incorporated herein by reference. The fan shaft bearing is a ceramic bearing as described in US Pat. No. 6,208,675. (If water cooling is not required, the motor described in the '675 patent is not required.)
The laser gas is maintained in the desired temperature range of about 30 ° C. to 50 ° C. in one or more finned water cooled heat exchangers 57 of the type described in US Pat. No. 6,128,323.
[0016]
Pulse power
The pulse power supply of this preferred embodiment is an improved version of the pulse power supply described in US Pat. No. 5,142,166.
[0017]
FIG. 7 is an electrical diagram of a preferred pulsed power supply for providing the electrical pulses necessary to create a discharge between electrodes 42 and 44. This figure does not show a charge unit utilizing a preferred embodiment type resonant charger as described in US Pat. No. 6,028,872. This type of resonant charge is up to 1500 volts at a rate of 4000 charges / second. ₀ Can be charged.
[0018]
C ₀ The charge stored in is controlled by the pulse energy controller with an accuracy of about 0.1 percent. C ₀ Since the charge of the battery determines the discharge energy, C ₀ The accuracy of the charge is important. The switch TR1 is closed by a command from the control module 16, and the diode D1 saturable inductor 60 and the inductor L ₁ Through C ₀ To discharge capacitor bank C ₁ Charge up. Capacitor C ₁ Is resonantly discharged through the saturable inductor MSC1 to provide an approximately 1500 volt pulse on the first side of the pulse transformer 62. The pulse transformer 62 is a general type 1/12 to 1 pulse transformer described in US Pat. No. 5,142,166. The pulse transformer amplifies the pulse voltage by a factor of 12, for example, from 1500 volts to 18,000 volts, or from about 2000 volts to 24,000 volts. This transformer has six hollow spool-shaped mandrels, each wound with Metglass to form six cores, each with a single primary so that it is in equilibrium with the primary coil of the other core It has a coil. The second transformer consists of two concentric conductors that are insulated from each other in the radial direction but are connected in series via the centers of the six cores.
[0019]
The output voltage of the transformer 62 is the capacitor bank C _p-1 Which discharges through the saturable inductor MSC2 to charge the peak capacitor Cp which provides purely the voltage to create the described discharge across the electrodes 42 and 44. In this preferred embodiment, the preferred values of capacitance and inductance for the components of the pulse delivery system of FIG. 7 are as follows:
C ₀ Capacitance = 1.2μF
Saturable inductor 60 Saturation inductance = 500nH
Inductor L ₁ Saturation inductance = 700nH
C ₁ Saturation inductance = 1.27μF
Saturable inductor MSC1 saturation inductance = 14nH
C _p-1 Saturation inductance = 8.8nF
Saturable inductor MSC2 saturation inductance = 80nH
C _p Saturation inductance = 8.8nF
[0020]
This circuit provides energy recovery using the technique described in US Pat. No. 5,729,562. The bias circuit indicated by 64 and 66 is a capacitor C ₀ A saturable inductor is biased in the circuit to initially counter the electrical energy flow from to the electrode. However, after closing the switch TR1, it saturates after a time that allows energy to flow towards the electrodes. Due to the mismatch, the electrons first flow from the cathode 42 to the anode 44 and then immediately back flow across the discharge region from the anode 44 toward the cathode 42 for the opposite charge Cp. Since all the saturable inductors remain in the conduction direction for a few microseconds, the pulse then starts. The pulse energy that is not depleted is C as opposed to that applied by the resonant charger. ₀ C to charge ₀ In order to return to the circuit in all ways, a ring is formed. Such charging is reversed immediately by charging the inverting circuit 66, all of which is described in US Pat. No. 5,729,562.
[0021]
Embodiments of the present invention operate with substantial improvements to efficiently reduce the induction of a portion of the pulse power circuit, including Cp and electrodes 42 and 44, shown at 70 in FIG. This reduction acts as a current return to return the current from the anode 44 to the ground plate of the Cp capacitor bank with five feedthrough copper rods to connect the high voltage bus of the Cp capacitor bank to the anode. This was accomplished by utilizing 20 curved nickel plated stainless steel plates. These changes are the result of a reduction in the discharge induction of this circuit that is less than 5 nH. The effect of this reduction in both pulse energy and energy stability is illustrated in the graph of FIG. 10, where the actual induction is comparable to 30% and a 300% increase in induction. These higher values are typical induction values for this part of the pulse power circuit of the prior art gas discharge laser.
[0022]
Pulse monitor
A preferred embodiment of the present invention includes a pulse energy monitor 18 as shown in FIG. A preferred monitor is a very high speed photodiode similar to that described in US Pat. No. 6,128,323. The output of this photodiode is sent to a microcontroller 16 that uses the output to select a voltage change for the next pulse if correction is required for jitter as shown below. The energy and jitter of each pulse is monitored and the results are compared to specification limits in real time. If any parameter falls outside the desired limits, appropriate action is taken immediately.
[0023]
When the laser system is used for inspection automatic control, the apparatus is preferably programmed to repeat the inspection when the specified pulse energy or jitter output is measured. If the output frequency or magnitude specification generation exceeds a predetermined level, correction maintenance is invoked. When the system is used for reticle drawing, the output of the specified pulse energy or jitter destroys the reticle. Thus, in some embodiments, control over the laser system indicates a warning when output of specification information is detected. This information is useful for requesting a specific post-operation check of the reticle.
[0024]
The pulse energy monitor shown at 18 in FIG. 5 may also incorporate a pulse energy correction loop as described in US Pat. No. 5,852,621 “Pulse Laser with Pulse Energy Trimmer”. This technique measures the pulse energy, pushes the output beam into the delay loop, and then trims a portion of the energy from the pulse to achieve the desired output.
[0025]
In other embodiments, the general technique described in the '621 patent may be used that reflects to remove any bad pulses for output of either specified pulse energy or jitter. Thereby, reticle drawing is backed up, and a defective pulse or a portion represented by a pulse can be drawn again.
[0026]
Jitter compression
As shown in FIG. 4 and the description of the reticle drawing apparatus described above, the pattern created by the mirror array 24 is drawn on the target 32 because the target 32 is positioned by the positioning device 34. The positioning of the mirror array is very fast, the mirror segments are repositioned in place at the start of each pulse, and the pulses are produced at a rate of, for example, 3000 pulses / second. Typically, however, the target 32 moves at a continuous rate of, for example, 100 mm / second (ie, 0.1 nm / ns). Therefore, the microcontroller must control the timing of the pulses so that the pulse energy is deposited at the desired location on the target. Because the reticle feature dimensions are in the range of about 100 nm to 200 nm or less, the timing of the pulse is accurate enough that the pulse energy can be deposited in the range of about 10-20 nm at the desired location. This is very important. Since the target moves at a rate of about 0.1 nm / ns and can move at a faster rate in the future, the timing of the pulses needs to be accurate within the range of 100 to 200 ns and within the range of 10 to 20 ns. It is preferably accurate. The pulse timing error is called "jitter".
[0027]
For the above reasons, the timing of pulses in reticle writing is more than the timing of pulses for integrated circuit lithography, where a portion of the wafer is exposed to about 15-50 pulses and the actual timing of the pulses requires an accuracy of a few microseconds. More critical.
[0028]
This embodiment of the present invention provides a jitter control technique called Jitter Compression Device (JCD) that ensures that the pulse timing is accurate within the range of about 10-20 ns. The optical delay from input jitter depends on the voltage of the solid state switch pulse power module. The higher the charge voltage, the shorter the delay between the trigger and the laser pulse. The delay between the trigger and the light is also in the pulse power system Magnetic material temperature Depends on. Thus, the delay between trigger and light is a function of voltage and temperature. The higher the temperature of the magnetic device of the pulse power system, the shorter the delay between the trigger and the pulse. If the voltage and temperature are fixed, the natural pulse-to-pulse change of the light pulse with the input trigger is small, about ± 5 ns.
[0029]
Thus, in a preferred embodiment, after triggering from the user, the jitter control is Magnetic material temperature And delaying the transmission of the trigger to the solid state pulse power of the laser by an amount corresponding to the operating voltage of the laser, so that the timing of the resulting pulse is accurate within about 20 ns.
[0030]
In other embodiments, the pulse controller adjusts the timing of the pulses for changes in the change voltage based on a specific change voltage for the next pulse. In this embodiment, no direct correction for temperature changes is provided. However, the correction is timed based on the timing error measured from the previous pulse. Since the temperature of the magnetic component is very slow, this pulse timing feedback technique substantially compensates for slow temperature changes and at the same time provides compensation for other more appropriate effects.
[0031]
This particular embodiment creates a feedback correction using a technique that provides sufficient 100% correction for large timing errors such as greater than 20 ns, but for timing errors smaller than 20 ns, for example, only 25% correction is achieved. Provided.
[0032]
This smaller percentage correction avoids oscillating around zero error.
[0033]
This particular embodiment also employs a trick to achieve 1 nanosecond resolution with wide dynamic range utilizing a digital counter with 40 MHz crystal oscillation. The 40 MHz oscillator provides clock signals at 25 ns intervals, which are used to charge an approximately linear analog capacitance charge circuit. The capacitor voltage is then read to determine a time accuracy of about 1.0 ns.
[0034]
Resonance capacity
The preferred embodiment of the present invention utilizes a curved-curved resonant cavity configuration. This configuration is shown in FIG. As shown in FIG. 9, the resonant cavity includes a plurality of reflecting mirrors and R for the output coupler. _max And a mirror. Both mirrors have a concave curvature of about 10 meters and have the effect of creating a highly incoherent excimer laser beam that is substantially divergent. R _max Has a reflectivity of about 100% and the output coupler has a reflectivity of about 4-30% on the inside of the output coupler and an anti-reflective coating on the outside.
[0035]
Various modifications can be made outside the scope of the present invention. Those skilled in the art will recognize other possible variations. For example, operation at very high repetition rates and very high loading rates may require further cooling of the laser gas and other components of the laser, including optical components, pulsed power components and fan motors.
[0036]
Therefore, the above description should not be limited and the scope of the present invention should be determined by the appended claims and equivalents thereof.
[Brief description of the drawings]
FIG. 1 shows a cross-sectional view of a prior art laser chamber.
FIG. 2 shows a cross-sectional view of a prior art laser chamber.
FIG. 3 shows a circuit diagram of a prior art pulse power system.
FIG. 4 shows a system for using the present invention as a light source for reticle drawing.
FIG. 5 is a block diagram of a preferred embodiment of the present invention.
FIG. 6 is a cross-sectional view of a preferred embodiment laser chamber.
7 is a cross-sectional view of a portion of the laser chamber of FIG.
FIG. 8 is a wiring circuit diagram of a pulse power system according to a preferred embodiment.
FIG. 9 illustrates a preferred resonant cavity.
FIG. 10 shows the effect of reduced laser head guidance.
FIG. 11 shows a technique related to jitter control.

Claims (20)

A) 1) Laser gas,
2) a pair of elongated electrodes constituting the discharge region;
3) a fan for recirculating the laser gas between the electrodes;
4) a heat exchanger for removing heat from the laser gas;
A laser chamber comprising:
B) a pulse power system for providing high voltage electrical pulses to create a discharge across the electrodes at a repetition rate of 1000 Hz or higher;
C) a controller configured to control the timing of the next pulse, processing a timing error measured for the previous pulse, and processing a specific pulse voltage for the next pulse; Outputting a compensation trigger delay for the next pulse; and
A gas discharge laser system having pulse timing control .   The laser system of claim 1, wherein the controller is configured to control the timing with an accuracy within a range of 0.1 microseconds or less.   The laser system according to claim 1, wherein the controller is configured to control the timing with an accuracy within a range of 100 to 200 ns or less.   The laser system according to claim 1, wherein the controller is configured to control the timing with an accuracy within a range of 10 to 20 ns or less.   The laser system of claim 1, wherein the laser chamber includes an easily replaceable module.   6. The laser system of claim 5, wherein most of the pulse power system includes an easily replaceable module.   7. The laser system of claim 6, wherein all or almost all components of the laser system include an easily replaceable module.   The laser system according to claim 1, wherein the laser system is configured to function as a light source in a reticle drawing system.   The laser system according to claim 1, wherein the laser system is configured to function as a light source in a reticle inspection system.   The laser system according to claim 1, wherein the laser system is configured to function as a light source in a wafer inspection system.   The laser system according to claim 1, wherein the pulse power system has a sub-circuit including a peak capacitor bank and two electrodes, and the sub-circuit has an inductance smaller than 5 nH.   The laser system according to claim 1, wherein the number of repetitions is about 2000 Hz or more.   The laser system according to claim 1, wherein the number of repetitions is about 3000 Hz or more.   The laser system according to claim 1, wherein the repetition rate is about 4000 Hz or more.   The laser system of claim 1, wherein the laser gas comprises krypton, fluorine, and a buffer gas, and the system is configured to produce laser light at a wavelength of about 248 nm.   The laser system of claim 1, wherein the laser gas comprises argon, fluorine, and a buffer gas, and the system is configured to produce laser light at a wavelength of about 193 nm.   The laser system of claim 1, wherein the laser gas comprises fluorine and a buffer gas, and the system is configured to produce laser light at a wavelength of about 157 nm.   The system has two mirrors forming a resonant cavity, one of the mirrors has a high reflectivity at the wavelength of the laser beam produced by the laser system, the other one of which is a partial The laser system according to claim 1, wherein the laser system is a reflection mirror and is configured to function as an output coupler, and both mirrors are concave mirrors.   19. The laser system according to claim 18, wherein the partial reflection mirror has reflectivity at the wavelength in the range of 4% to 30%.   The laser system of claim 18, wherein each of the mirrors has a concave curvature of about 10 meters.

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