Line narrowing unit with flexural grating mount

Light narrowing unit with flexible grating fixture

Technical field

The present invention relates to lasers, especially high-power gas discharge lasers with a line narro watt ing unit based on a grating. The present invention claims the priority of application No. 09/451,407 filed on November 30, 1999, which is filed on Application No. 09/390,579 on September 3, 1999, which is now U.S. Patent No. 6,212,217B1 Partially continued.

Background technique

Narrow-band gas-discharge lasers are generally used as light sources for integrated circuit lithography. A preferred prior art technique for narrowing light is to use a grating-based narrowing unit together with an output coupler to form a laser resonant cavity. The gain medium in the cavity is produced by discharging to a circulating laser gas such as krypton, fluorine and neon (for KrF laser); argon, fluorine and neon (for ArF laser); Or fluorine and helium and/or neon (for F2 lasers).

The narrowing light component is shown in FIG. 1 as a schematic diagram of such a prior art system, which is extracted from Japanese Patent No. 2,696,285. The system shown includes an output coupler (or front mirror) 4, a laser cavity 3, a cavity window 11, and a grating-based light narrowing unit 7. The light-narrowing unit 7 is usually provided as an easily replaceable unit in a lithographic laser system, and is sometimes referred to as "light-narrowing unit" or simply "LNP". The prior art unit includes two beam expanding prisms 27 and 29 and a grating 16 arranged in a Litro tile structure. The gratings used in these systems are extremely sensitive optical devices. A typical grating surface may have 10,000 grooves per inch in an aluminum layer or multiple aluminum layers on a thick glass substrate. These gratings and the technology used to manufacture them are described in U.S. Patent No. 5,999,318, the content of which is incorporated herein for reference. A prior art for avoiding distortion of the grating surface is to mount the grating on a metal grating fixing member made of a material with a small thermal expansion coefficient and very close to the thermal expansion coefficient of the grating glass substrate. In the presence of oxygen in standard air, the grating quickly deteriorates under ultraviolet radiation. For this reason, the optical components of the light-narrowing unit of the laser used for lithography are usually constantly cleaned during operation with nitrogen.

Fig. 2 is a prior art light narrowing unit manufactured by the applicants employer Cymer, Inc. as a part of a prior art light narrowing lithographic KrF laser system using the device. The unit includes three beam expanding prisms 8, 10 and 12, a rotating mirror 14 and a grating 16. It may be noted that the nitrogen cleaning agent from the bottle 44 enters the unit on the back side of the rotating mirror 46 to avoid the cleaning agent from flowing directly onto the surface of the grating. In this system, the wavelength of the laser beam 6 is controlled in a feedback structure. In the feedback structure, the monitor 22 measures the wavelength of the beam, and the computer controller 24 uses the wavelength information to adjust the angular position of the rotating mirror 14 to reduce the wavelength Control to a required value. A bandwidth control device 20 is used to mechanically bend the grating 16 such as to make it slightly recessed. The device is described in detail in U.S. Patent No. 5,095,492 assigned to Cymer. Using this device can reduce the bandwidth slightly, but when the laser is running at a high duty cycle, the resulting bandwidth still exceeds the specification.

For many years, designers of light-narrowing lasers believed that the distortion of the laser beam may be caused by airflow near the surface of the grating. Therefore, laser designers used to make special efforts to prevent the nitrogen used for cleaning from flowing directly onto the surface of the grating. Several examples of these efforts are described in Japanese Patent No. 2,696,285 cited above. In the example shown in the excerpt of FIG. 1, the flow of cleaning agent is directed from the N2 gas bottle 44 through the discharge port 46 toward the back side of the grating 16.

Larger repetition frequency The narrowed ultraviolet laser sources currently used in the integrated circuit industry typically produce about 10 millijoules per pulse at a repetition rate of about 1000 Hz and a duty cycle of about 20 percent. In the case of a higher repetition frequency and a larger duty factor, a larger integrated circuit output can be obtained. The applicants employer currently sells a 2000 Hz gas discharge lithography laser, and the applicant and their colleagues have designed a 4000 Hz gas discharge lithography laser. With these higher repetition frequencies and busyness, applicants have encountered difficulties in maintaining a consistently narrow bandwidth.

People need reliable devices and technologies for narrowing the light of gas discharge lasers with high repetition rate and high duty cycle.

Summary of the invention

The present invention provides a grating-based narrowing light device for a narrowing light laser for generating a high-energy laser beam. Technologies are provided to minimize the adverse effects of the heat generated by the laser beam in the narrowing device.

A flexible grating fixing member is provided, which truly eliminates the stress on the grating caused by the different thermal expansion between the grating and the LNP shell structure. In a preferred embodiment, a flexible grating fixture is used to attach a grating comprising a very thin aluminum veneer surface on a thick ultra-low expansion glass substrate to an aluminum housing structure . At least one flexure connection part is provided in the grating fixing member, which allows the aluminum housing to expand and contract without generating undesired mechanical stress in the glass substrate of the grating. In some embodiments, the fixing member includes a metal plate, and the flexure connection part is an H-shaped flexure connection part machined into the metal plate. In another embodiment, two H-shaped flexure connections are provided. In other embodiments, the flexure connecting portion is a dovetail connecting portion, allowing one end of the fixing member to slide relative to the other end.

In another preferred embodiment, the gas flow is directed across the surface of the grating. In other embodiments, helium is used as the cleaning agent gas to reduce the effect of the hot gas layer on the grating surface, and in other embodiments, the pressure of the cleaning agent gas is reduced to reduce the hot gas layer. Optical function.

Brief Description of the Drawings Fig. 1 shows a first prior art light narrowing laser system.

Figure 2 shows a second prior art laser system with light narrowing.

Figures 2A and 2B show a prior art grating installation technique.

Figure 3 shows the detrimental effect on the bandwidth of a hot gas layer on the surface of a narrowed light grating.

Figures 4A and 4B show a preferred embodiment of the present invention.

FIG. 5A shows the bandwidth trajectory of the cleaning using the prior art at various repetition frequencies.

Figure 5B shows the bandwidth trajectory of the cleaning according to the present invention at various repetition frequencies.

Figures 6A, 6B and 6C show alternative embodiments of the present invention.

Figures 7 and 8 show an LNP equipped for fast feedback control.

Figure 9 shows the heating of the gas layer on the grating surface.

Figure 10 shows a technique for reducing the pressure of the cleaning agent gas.

Figures 11A, B, C, and D show features of a preferred embodiment of the present invention.

Figures 12A, B and C show features of another preferred embodiment of the present invention.

Figures 13A, B, C, D and E show features of another preferred embodiment of the present invention.

detailed description

The preferred embodiments of the present invention can be described with reference to the accompanying drawings.

Laser performance at high average power. A prior art light-narrowed KrF excimer laser operating at a relatively low average power, usually less than 5 watts, usually produces a center at about 248 nanometers. , A laser beam with a bandwidth less than 0.6 picometers. As long as the average power is less than 5 watts, the laser can operate without problems at high repetition rates of up to 2000 Hz or higher. The pulse energy of a typical lithographic KrF excimer laser is 10 millijoules. Therefore, in order to avoid increasing the average power, the laser must operate with a lower duty cycle. For example, it can run at 2 kHz with a burst of 200 pulses with an interval between bursts of approximately 0.45 seconds. Such a working state will produce the following average power:As the average power increases, problems with bandwidth control begin to appear. For example, this happens when the delay between bursts decreases. For example, if the laser also operates in a 200-pulse burst, but the delay between the bursts is 0.1 seconds, the average power will be:At the maximum value, that is, when the laser works in a continuous mode of 200 Hz and 10 millijoule pulse energy, it is equivalent to an average power of 20 watts.

When the prior art laser system operates at a higher average power, the bandwidth gradually increases from an initial bandwidth of less than 0.6 picometers after a period of about 5 to 20 minutes, and basically remains greater than 0.6 picometers. Such an increase in bandwidth should be avoided in the production run of microlithography, because it will blur the pattern due to the color alternation of the projection lens. Another important application is the use of lasers to test the thermal performance of another lithographic printing component, such as the projection lens itself, under high duty cycles. In this application, it is assumed that the bandwidth and other parameters of the laser remain within the specified range for the duration of the test.

4000 Hz lasers are now being tested for production. Lithographic laser systems include lasers designed to operate at 4000 Hz. Maintaining the desired laser beam quality at these increased repetition rates is a challenge. Compared with a 2000 Hz system, the thermal effect is significantly increased.

Grating Distortion FIG. 2A is a side view showing a method of mounting the grating 16 to the bottom plate of the LNP housing in the related art. In this case, the thick glass substrate of the grating is attached to the fixing plate 16A with a very short epoxy resin pillar at each of the three locations. Figure 2B shows the approximate horizontal positions of the pillars at 17A, 17B, and 17C. The fixing plate is firmly screwed to the bottom plate of the LNP housing using two screws 16B and 16C. In the prior art design, the fixed plate is made of inexpandable steel, and its thermal expansion coefficient is close to zero, which is also close to that of the grating glass substrate of ultra-low thermal expansion glass. However, the chamber housing is made of aluminum, which has a thermal expansion coefficient significantly different from that of inexpandable steel and grating glass substrates. As a result, the temperature deviation in the LNP generates bending stress in the fixed plate that is tightly screwed to the bottom of the LNP housing, which in turn generates bending stress on the grating through the epoxy short post. As long as the temperature deviation is small, the short column is flexible enough to avoid obvious distortion; but under the high repetition frequency in the range of 2000 Hz to 4000 Hz and high duty cycle, the thermal distortion in the grating must be large. As a result, both the bandwidth and the stability of the wavelength center line adversely affect the laser beam quite severely.

Looking for the cause of this problem, people first found that when the beam quality decreased with the increase of beam energy, the cause of the cause was unclear, and various possible causes were studied. The surface heating effect of the cleaning agent gas on the grating surface is one of the causes, and by directing the cleaning agent gas flow on the grating surface (as described in the following section), the initiation can be largely corrected the reason. But this cannot completely eliminate the problem.

Finally, one of the applicants loosened the screw 16C as shown in FIG. 2A, and the quality of the beam was significantly improved. In this way, applicants have realized that different thermal expansions cause undesired bending of the grating, which in turn leads to a reduction in the quality of the grating. As a result, the applicant devised several modifications in LNP to solve this problem.

The dovetail design shows a solution to this problem in Figures 11A, B, C, and D. 11A and 11B are bottom views of a grating fixing member for minimizing thermal stress on the grating. The fixing member includes two main parts 100 and 102, and the part 102 slides into a dovetail groove 104 in the part 100. The part 102 is bolted to the bottom plate of the LNP by using the threaded hole 106, and the part 100 is bolted to the bottom plate of the LNP by using the threaded hole 104. Figure 11C shows the dovetail groove in the part 100, and Figure 11D is an end view of the part 102 showing how it is designed to fit snugly in the groove. FIG. 11E shows the part 102 located inside the groove 104 of the part 100. Two spring tension rollers 108 press the part 102 against the side surface 110 of the groove 104. The grating (not shown) is preferably installed on top of the part 100 using three short epoxy resin posts with a diameter of about 1 cm and a height of about 4 mils. This design allows the base plate of the LNP to expand and contract without applying mechanical stress to the grating.

The H-shaped flexure connecting portion can refer to FIGS. 12A, B and C to describe a second embodiment of the present invention. The three figures are respectively a top view, a side view and a bottom view of a grating fixture with a single H-shaped flexure connection. The fixing member is made of non-expandable steel, which has a very small coefficient of thermal expansion as shown above, and is roughly the same as the coefficient of thermal expansion of the grating glass substrate made of molten silicon. The fixing member is about 1/2 inch thick, and its length is approximately equal to the length of the thick glass substrate of the grating to be mounted on it. The holes 120 are cut out in the fixing part, mainly to reduce its weight. An H-shaped flexure connection part is machined in the base of the fixing member, as shown in Fig. 12A. Two dog-bone-shaped holes 124 are cut in the fixture to create an "H"-shaped flexure connection, and the four flexure legs 126 are each about 0.060 inches thick. The grating is preferably installed on the fixture at position 128 using three epoxy resin stubs about 4 mils high and 1 cm in diameter, as shown in FIG. 12A.

The fixing piece is firmly screwed to the bottom of the LNP housing using a threaded hole 130 as shown in FIG. 12C. The H-shaped flexure connection allows the bottom of the housing to expand and contract without transmitting any significant stress to the grating. The connecting portion produces only a very small resistance to small forces along the longitudinal direction of the grating. The coefficient of expansion in this direction is about 0.001 inches per pound, but it is very strong and resistant to forces in any other direction.

A third embodiment of the two H-shaped flexure connecting parts can be described with reference to FIGS. 13A, B, C, D, E, and F. FIG. 13A is a top view, FIG. 13B is a side view, and FIG. 13C is a bottom view. The fixture is similar to the fixture described above and shown in 12A. However, the fixing member has two H-shaped flexure connecting portions 134 and 133, as shown in enlarged form in FIGS. 13D and 13E, respectively. The fixing part is made of aluminum like the LNP shell. The fixing member is mounted on the housing as described in the previous embodiment. The grating is attached to the fixture at positions 132A, 132B, and 132C using three epoxy short posts as described above, as shown in FIGS. 13A, 13D, and 13E. The surface of the fixing part at the epoxy resin position (in this embodiment and in other embodiments) is preferably grit blasted with #40 dry coarse sand to produce a better epoxy resin surface. The legs of the H-shaped flexures 134 and 133 are approximately 0.030 inches wide. The flexure 133 allows the fixing member to expand in the long direction of the grating relative to the grating, and the flexure 134 allows the expansion in a short direction of the grating, as shown in the figure. The second H-shaped connecting portion flexure allows allowed to use aluminum as a material of the fixing member, because the aluminum is cheaper than invar, and ease of processing. Preferably, in this embodiment and other embodiments, when attaching the grating to the fixing member, a 4 mil shim is used to ensure that the epoxy resin stub has the correct thickness (that is, the height ).

The number of readers who match the coefficient of thermal expansion should note that the better design of the grating fixture should take into account the material used for the fixture. For example, in the examples in Figs. 11A-D and Figs. 12A-C, the fixing member uses an inexpandable steel whose thermal expansion coefficient is similar to that of a thick ULE glass grating substrate. Therefore, the grating is attached to the long part of the fixing member using three epoxy stubs at two far apart positions, and is not attached to the short part of the fixing member. In these two examples, one end of the long portion of the fixing piece is attached to the cavity, and the portion of the end of the fixing piece at the other end of the fixing piece is separately attached to the cavity. The flexure connections in both examples allow the fixture and the cavity (which have significantly different coefficients of thermal expansion) to expand and contract at different rates.

In the example shown in Figs. 13A-E, the long part of the fixing member is firmly attached to the housing at two locations. Since the fixing parts and the housing are made of aluminum, this does not produce significant stress. A grating substrate whose expansion coefficient is significantly different from that of aluminum is attached to each of the three separate parts of the fixture, which are free to move relative to each other due to the two flexure connections. Therefore, if the fixing member is made of aluminum in the examples in FIGS. 11A-D and 12A-C, then in each case, only the long part of the fixing member should be attached to the cavity bottom plate, and the grating should Attach to two parts. Similarly, in the example shown in Figs. 13A-E, if the fixing member is made of non-expandable steel, the grating should be attached to the long solid part, and the fixing member should be attached to the cavity at positions 132A, 132B and 132C.

The hot nitrogen blanket applicants have determined that the poor performance at high repetition rates as shown in Figure 3 is partly a result of the development of a hot nitrogen blanket that is on the grating 48 after a period of about 5 minutes. Built up on the surface. The hot gas is heated by the grating surface, which in turn is heated by absorbing part of the incident laser beam. Generally, as much as 15 to 20% of the incident laser light will be absorbed by the grating surface. The temperature of the grating surface may rise to 10 to 15°C. The temperature rise is uneven, high in the middle of the grating and low at the ends, as shown in Figure 9. Therefore, the air in front of the middle part of the grating is hotter than the air in front of the edges. Therefore, when the laser beam 80 is incident on the grating surface 86, it passes through the boundary layer 82. Because the air has the same pressure, the hotter the air, the lower its density. Therefore, the air near the center of the grating is thinner than the air near the edges. For this reason, the laser beam 80 will have a different phase shift when it reaches the middle part and the edges of the grating. Therefore, an incident light ray with a parallel wavefront 88 will have a curved wavefront 90 corresponding to the diverging light beam. This happens even if the grating 16 is completely flat.

Applicants have developed a better modified form of the unit for narrowing the light to substantially eliminate the hot nitrogen blanket.

The flow across the surface of the grating shows a first preferred embodiment of the invention in Figures 4A and 4B. In this case, about 2 liters of nitrogen cleaning agent per minute flows upwards through approximately 1 mm diameter holes, which are arranged at intervals of 1/4 inch in a gas manifold used as a cleaning agent with an inner diameter of 3 /8 inch, 10 inch long tube. The blocking plate 60 and the blocking cover 62 force most of the nitrogen cleaning agent to flow in the direction indicated by the arrow in FIG. 4B. Such a structure produces good results, as shown in Figure 5. In this case, an increase in the average output power from 0.1 watts to 20 watts results in a range of about 0.4 to 0.5 picometers. It is interesting to note that at an average power of 10 watts, the bandwidth is actually slightly smaller than the bandwidth at 0.1 watts.

It is important to carefully control the flow of cleaning agent across the surface of the grating to avoid flow-related distortions. Applicants tested various flows and determined that excessive flow would cause excessive harm. For example, a flow rate of 20 liters per minute gives very poor results. The recommended flow rate is in the range of about 0.5 liters per minute to about 10 liters per minute.

It is also important to note that the cleaning agent does not significantly reduce the temperature of the grating. The grating is still hot. What the cleaning agent does is move the air in front of the grating quite continuously so that it has no time to be heated by the grating. The very small flow and the resulting gas velocity prevent any distortion of the air caused by the airflow itself from affecting the operation of the laser.

Other cleaning agent structure arrangements have many possible structure arrangements that provide airflow across the surface of the grating to prevent the generation of problematic thermal layers, as shown in FIG. 3. For example, a slit of about 0.5 mm extending on the manifold section can be used instead of the small holes. It is also possible to use a slit nozzle such as the one shown in the cross-section of FIG. 6A to provide a more stable air flow, or to provide slit nozzles on both the top and bottom of the grating, as shown in FIG. 6B. Also, a very small fan that can be used in a semi-enclosed system provides airflow across the surface of the grating, as shown in Figure 6C. In this case, a conventional nitrogen cleaning agent can be provided, as in the prior art of FIG. 2. In the embodiment of FIG. 6C, the cavity between the grating and the blocking plate is not sealed, and the cleaning agent gas is allowed to flow in and out of the cavity, as shown at 64 and 66. The pipe 68 leading to and from the fan 70 is connected near the center of the slit pipes 72 and 74, which position is just above and below the hottest area on the grating 16.

Reduced gas pressure A second solution to the hot gas layer problem is to reduce the gas pressure in the narrowed light assembly.

Gas convection changes the gas density spatially, resulting in an uneven refractive index distribution, which in turn causes phase front abnormalities. The amount of any aberrations caused by gas density fluctuations (due to gas convection near the heated grating surface) varies roughly linearly with the nominal value of sensitivity or refractive index, and thus with the gas density.

The convective cooling effect of the surface of the grating or other optical element will not be significantly reduced, as long as the mean free path of the gas molecules is not less than the distance between the "hot" and "cold" surfaces in the LNP. If we assume that these distances are about 10 cm, then based on experience, we can say that the gas pressure should not be lowered below the pressure at the mean free path of about 10 cm. The pressure is in the range of about 1 to 10 millibars, so that the gas density in the LNP is about 0.1 to 1.0 percent of the density in the atmospheric state.

Figure 10 is a schematic diagram showing a system for maintaining the controlled pressure in the LNP at about 1 to 10 mbar. Nitrogen enters the sealed LNP7 through the orifice 90. A vacuum pump 92 is used to generate a vacuum in the LNP. The controller 94 uses a feedback signal from the pressure sensor 96 to control the needle valve 98 to maintain the required vacuum. Since the LNP is a sealed system and the pressure is approximately equalized, the sensor 96 can be thermocouple.

Cleaning with helium gas Another solution to reduce the thermal layer effect is to clean the LNP with helium gas. Helium has a smaller differential refractive index, so the thermal layer produces smaller distortions. In addition, compared with nitrogen, helium has much better heat transfer performance. Argon can also be used, which has the same advantages.

But helium is much more expensive than nitrogen.

Those skilled in the art will recognize that in addition to the specific embodiments of the present invention described above, there may be many other embodiments that may cope with distortions. For example, the fixing member can be made of materials other than inexpandable steel or aluminum. For the above reasons, these other materials should have a thermal expansion coefficient matching the rigid grating substrate or the cavity shell. Another technique used to deal with the hot gas layer is to provide active bandwidth control to correct the adverse effects of the hot gas layer. A technique for controlling several wavelength parameters substantially in real time is described in US Patent Application Serial No. 09/390,579 filed on September 3, 1999, and the content of this application is incorporated herein for reference. These technologies include fast feedback control of the position of the beam expanding prism, the curvature of the grating, and the position of the rotating mirror. It can also provide control of the position of the laser cavity. Fig. 7 is a schematic diagram of a combined block diagram of the entire laser system, and Fig. 8 is a diagram of an LNP with an added feedback control feature. In this embodiment, the curvature of the grating is controlled by the grating curvature stepping motor 30 to compensate for the distortion caused by the hot gas layer on the surface of the grating. Therefore, the protection scope of the present invention should be determined by the appended claims and their legal equivalents.