Patent Document

CROSS REFERENCE TO PRIOR APPLICATIONS  
       [0001]     The instant application is a continuation-in-part of U.S. patent application Ser. No. 10/645,947, filed Aug. 22, 2003. The instant application also claims the priority of U.S. Provisional Application No. 60/586,569, filed Jul. 9, 2004. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates in general to the assembly of excimer laser systems including at least one laser discharge unit and optical modules for forming a resonator, and delivering an output beam for the laser. The invention relates in particular to an assembly in which the laser discharge unit and the optical modules are mechanically isolated from each other.  
       DISCUSSION OF BACKGROUND ART  
       [0003]     An excimer or molecular fluorine (F 2 ) laser system includes at least one laser discharge unit (LDU). The laser discharge unit includes a laser chamber including a laser gas mixture, discharge electrodes across which a discharge is repeatedly fired by application of a high potential across the electrodes, and a fan for circulating the laser gas mixture through a gap between the electrodes in which the discharge occurs. The LDU also includes pulse generating electronics, mounted on the laser chamber, for generating the high potential as a plurality of electrical pulses.  
         [0004]     The excimer laser system usually includes optical modules arranged in cooperation with the laser chamber of the LDU for forming an optical resonator; defining the exact operating wavelength of the laser within a range of wavelengths characteristic of the gas mixture; delivering a beam from the optical resonator; stretching optical pulses from the resonator; and providing beam diagnostics. The system includes power supplies for the discharge unit and for driving the fan, and other mechanical devices, for example, for cooling the LDU.  
         [0005]     In a straightforward excimer laser there would be only one LDU, usually, however, there are two LDU&#39;s, one forming part of a master oscillator, and the other forming part of an optical amplifier or a power oscillator for amplifying the output of the master oscillator. Such systems are usually referred to as MOPA (master oscillator, power amplifer) systems or MOPO (master oscillator, power oscillator) systems.  
         [0006]     Whatever the system, all of the above-discussed optical, mechanical, and electrical components are assembled onto a single main frame or chassis to form the system into an integrated unit. The chassis is usually covered with plates and the like to prevent accidental or unauthorized access to the components.  
         [0007]     Mechanical devices such as electric motors and fans inevitably provide a source of mechanical vibration of the system chassis. Vibration can also result from electrical pulsing devices. Any of this vibration that is transmitted the LDU and the optics modules can adversely effect the performance of the laser system. Such effects can include instability of the lasing wavelength or the spectral width of the laser output, and instability of pointing (the general propagation direction of the laser beam.  
         [0008]     In prior-art systems such vibration effects are reduced by mounting the LDU (or LDUs) and associated optics on sub-mount frame and assembling that frame onto the main frame or chassis via some kind of vibration-damping suspension such as rubber blocks or stiff metal springs. It has been determined however that certain applications of the output beam of such systems, such as optical lithography, laser micro-machining, or material processing, would benefit from even greater stability of beam parameters than is provided by prior-art system assembly arrangements.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention is directed to minimizing instability of parameters of a beam delivered by a laser system. In one aspect, a laser apparatus in accordance with the present invention comprises a laser chassis on which a laser system is to be assembled. A resilient suspension is connected to the chassis for supporting a laser discharge unit (LDU) of the laser. An optics frame is provided for supporting optical elements on the chassis, the optical elements being cooperative with the laser discharge unit for forming a laser resonator, and the optics frame being separate from the resilient suspension. The resilient suspension minimizes transmission of vibrations from the laser discharge unit to the chassis and accordingly to the optics frame supported on the chassis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.  
         [0011]      FIG. 1  schematically illustrates one preferred embodiment of a laser system in accordance with the present invention including a laser chassis, an optics frame including tables for optical modules, and supported on the chassis, and two LDUs both supported on an LDU frame, the LDU frame being supported on the chassis via a vibration-damping suspension.  
         [0012]      FIG. 1A  is a three-dimensional view schematically illustrating a Cartesian X, Y, and Z-axis system, and torsional degrees-of-freedom R X , R Y , and R Z  corresponding to the X, Y, and Z-axes, respectively.  
         [0013]      FIG. 2  is a three-dimensional view schematically illustrating one preferred example of a vibration-damping suspension for an LDU in the system of  FIG. 1 , including wheel assemblies attached to the LDU, a support structure for the LDU on which the wheel assemblies rest, and steel W-springs attached to the support structure and the chassis for vibration isolating the LDU from the chassis.  
         [0014]      FIG. 3A  is a plan view from above schematically illustrating details of the support structure and steel W-springs of  FIG. 2 .  
         [0015]      FIG. 3B  schematically illustrates details of a wheel in a wheel assembly of  FIG. 2  supported on a transverse rail of the support structure of  FIG. 2 .  
         [0016]      FIG. 3C  schematically illustrates details of a steel W-spring in the suspension of  FIGS. 2 and 3 A.  
         [0017]      FIGS. 3D and 3E  are three-dimensional views schematically illustrating details of the steel W-springs attached to an end-stop bracket in the suspension of  FIG. 2 .  
         [0018]      FIG. 3F  is cross-section view seen generally in the direction  3 F- 3 F of  FIG. 3C , schematically illustrating details of connecting a steel W-spring to the LDU support structure and chassis of  FIG. 2 .  
         [0019]      FIG. 4  is three-dimensional view schematically illustrating one example of a frame construction for the optics frame of  FIG. 1 .  
         [0020]      FIG. 5  is three-dimensional view illustrating one example of a chassis construction for laser system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment  10  of an excimer laser MOPA system in accordance with the present invention. Laser system  10  includes a laser chassis  12 , designated only fragmentarily in  FIG. 12  for convenience of illustration. Those skilled in the art will be aware that such a laser chassis is a single main frame, usually covered, on which all components of the laser system are assembled, one way or another. Here, chassis  12  rests on feet  14  in contact with a rigid floor  16 . Floor  16  preferably is a concrete floor of a building.  
         [0022]     In chassis  12  of MOPA  10 , components are assembled on, or supported on, either a frame  18  having upper and lower levels  20  and  22 , respectively, or a frame  24  having upper and lower levels  26  and  28 , respectively. Frames  18  and  24  are depicted in  FIG. 1  in a simplest form, for convenience of illustration. A practical example of such a frame is described in more detail further hereinbelow.  FIG. 1A  schematically illustrates a Cartesian axis system  30  of the MOPA to which reference is made frequently in the following description. Axis system  30  has the usual X, Y, and Z-axes. The Y-axis is a horizontal axis perpendicular to the plane of the drawing of  FIG. 1 . The X-axis and the Z-axis are respectively vertical and horizontal axes parallel to the plane of  FIG. 1 . A laser beam emitted by the system is emitted parallel to the X-axis. In the description of MOPA  10  set forth below reference is made to six degrees-of freedom (DOFs) of motion of chassis  12  that are variously controlled. Three of these DOFs are translations parallel to the X, Y, and Z-axes. The other three of these DOFs are torsions R X , R Y , and R z , about the X, Y, and Z-axes, respectively.  
         [0023]     MOPA  10  includes two laser discharge units (LDUs)  32  and  34 , each thereof having an electrical-pulse compressor  36  surmounting a laser chamber  38 . The pulse compressors receive a sequence of electrical pulses from a pulsed power supply (not shown), and electronically compress the pulses, thereby shortening the duration and increasing the peak power of the pulses. Laser chamber  38  includes a lasing gas, and electrodes (not shown) across which the sequence of compressed optical pulses are applied to strike a corresponding series of gas discharges in laser gas between the electrodes. Laser chamber  38  also contains a fan for circulating the laser gas between the electrodes.  
         [0024]     Each laser discharge unit is mounted on frame  18  via a resilient suspension. This suspension system is depicted without detail in  FIG. 4  by triangles  40 . The resilient suspension provides for vibration-isolating the laser discharge unit from frame  18  and, accordingly, the chassis. This in turn isolates frame  24  from vibrations produced by the LDU. The suspension system may be provided by something as simple as a rubber block mounting. It is preferable, however, that the suspension can be arranged to have different stiffness in different degrees of freedom. A brief description of one such suspension system is set forth below with reference to  FIG. 2  and FIGS.  3 A-F.  
         [0025]      FIG. 2  schematically illustrates details of laser discharge unit  34  of  FIG. 1  mounted on a preferred example of a suspension  40 . Here, laser power amplifier  38  of laser discharge unit  34  has an upper chamber  38 A, including discharge electrodes and pre-ionizing units (not shown). A lower chamber  38 B includes a fan assembly arranged to circulate laser gas from the lower chamber, through the upper chamber, and back to the lower chamber. Suspension  40  includes a wheel assembly  42  including a wheel  44 , and also a wheel assembly  48  including a wheel  50 . The wheel assemblies are attached to a side of lower chamber  38 B of the discharge unit via plates  46 . There are corresponding wheel assemblies (not visible in  FIG. 2 ) on the opposite side of the LDU providing a total of four wheel assemblies and four wheels.  
         [0026]     Wheels  20  are supported on a Y-axis support member  52 , and wheels  50  are supported on a Y-axis support member  54 .  FIG. 3B  depicts further detail of Y-axis support member  54  and wheel  50 , wherein it can be seen that the Y-axis support member includes an upper ridge  55  having a truncated V-shaped cross-section, and that wheel  50  has a corresponding, circumferential, truncated V-shaped groove. Y-axis support member  52  and  54  are connected by longitudinal tie members  56  the cross-members and are welded to steel W-springs  58 , which are configured to be attached to frame  18 .  
         [0027]      FIG. 3A  schematically depicts one preferred arrangement for attaching the Y-axis support member  52  and  54  to springs  58  and for attaching the springs to frame  18 . Further details of the springs and the cross members are depicted in FIGS.  3 C-F.  
         [0028]     As depicted in  FIG. 3A , frame  18  has transverse members  60  extending between longitudinal side-members  22  thereof. Transverse members  60  preferably have a channel section or the like to impart stiffness to the transverse members. Springs  58  are mounted under the Y-axis support members  52  and  54 , and above a surface of transverse members  60 . Springs  58  are attached to Y-axis support member  52  and to Y-axis support member  54 , here, by welding. Only a center arm  62  of each spring  58  is welded to the Y-axis support members. LDU  34  can move on Y-axis support members  52  and  54  (see  FIG. 2 ) via wheels  44  and  50 .  
         [0029]     Suspension  40  also includes end-stop brackets  64 . End-stop brackets  64  are also attached to center arm  62  of springs  58 . End stops  66  are coupled to the end stop brackets  64 . End-stop faces  68  of end-stop  66  contact side member  22  of frame  18 . The end stops are used for positioning the LDU  34  in the Y-direction. Differential screw assemblies  70  are used to adjust the LDU  34  positioning in the Y-direction. Differential screw assemblies  70  are fixed relative to the Y-axis support members  52  and  54  by holders  72 . Holders  72  operate to couple the Y-axis support members to the end-stop brackets  64  via attachments  74  on the end-stop brackets. The differential screw assemblies interface with wheel assemblies  42  and  48  (see  FIG. 2 ), such that as the differential screws are adjusted, the position of the wheel assembly (and the LDU) moves along the Y-axis support members.  
         [0030]     The position accuracy of the LDU  34  relative to optics frame  24  is important. The position in Y-direction of the LDU  34  in regard to the optics frame can be determined by adjustable end stops  66  and similar adjustable end stops could also be mounted on the LDU itself. The position of the LDU in Z-direction is determined by the level or height of Y-axis support member  52  and Y-axis support member  54 , and by the position of the diameter of the wheels  44  and  50  of wheel assemblies  42  and  48  (see  FIG. 2 ). The X-axis position is determined by the position of Y-axis support member  54  and the wheels  50  that are positioned on Y-axis support member  54  (typically these would be v-grooved wheels as depicted in  FIG. 3B ). Springs  58  have no flexibility in the X-axis. For laser system  10  the Y- and Z-directions are of greatest importance.  
         [0031]      FIGS. 3C and 3F  schematically illustrate a preferred arrangement for mounting springs  58  to frame  18 , here, via transverse member  60  of the frame  18 . In this arrangement, outer arms  63  of each spring  58  are attached to frame member  60  via screws (not shown) inserted through countersunk holes  65 . A relief slot  61  is provided in member  60  (see  FIG. 3F ) to allow travel of center arm  62  of spring  58  as indicated by double arrow A. Vibrational energy of components of the LDU  34  such as the fan, is dissipated by W-springs  58  so as to reduce vibrations imparted to the laser chassis via frame member  60 . In this way, the W-springs provide a resilient coupling between the laser chassis and the LDU, which dissipates vibrational energy in the system. While this example of suspension  40  is described with reference to mounting LDU  34 , the same suspension may be used for mounting LDU  32 .  
         [0032]     It should be noted here that the brief description of an example of suspension  40  is presented merely for completeness of description. Further details of this particular suspension are provided in published U.S. patent application No. 2004/0101018, the complete disclosure of which is hereby incorporated by reference. It should be further noted, however, that the present invention is not limited to this type of suspension. Those skilled in the art to which the present invention pertains may deploy a different type of suspension without departing from the spirit and scope of the present invention.  
         [0033]     Referring again to  FIGS. 1 and 1 A, in laser system  10  the optical beam path is defined by modules that are mounted to four separate optical tables. These are designated as table  70  (upper-right), table  72  (upper-left), table  74  (lower-right) and table  76  (lower-left). Supported on table  70  is a master-oscillator-rear-optics module (MO-ROM). Supported on table  72  are a master-oscillator-front-optics module (MO-FOM), a master-oscillator-monitor-optics module (MO-MOM), and a wavelength-control module (WCM). Supported on table  74  are a power-amplifier-front-optics module (PA-FOM), an energy-monitor module (EMO), a power-amplifier-monitor-optics module (PA-MOM), and a beam-measuring unit (BMU- 2 ). Supported on table  76  are a power-amplifier-rear-optics module (PA-ROM) and another beam-measuring unit (BMU- 1 ). Other modules including a power-meter module (PM) and a beam-shutter module (BS) are supported directly on optics frame  24 . An optical pulse expander (PEX) is supported directly on chassis  12 . The optical modules are interconnected by tubes (not specifically designated) through which the beam passes, as indicated by single and double arrows, the double arrows indicating beam-circulation in a resonator. The resonator for master oscillator LDU  36  is formed between a grating in module (MO-ROM) and a partially transmitting mirror in module (MO-FOM).  
         [0034]     It should be noted that the many modules depicted in  FIG. 1  are depicted and described herein merely for completeness of description. It is not necessary to include all such modules in a laser in accordance with the present invention. Usually, however, there would be at least one LDU and sufficient optics to form a resonator including that LDU.  
         [0035]     The optics modules, and accordingly the optical tables, must be aligned and fixed in both relative and absolute position. The optical tables define the exact positions of the optical modules and, with that, the absolute and relative positions of the laser beam. Frame  24  that supports the optics tables and the optical modules may be designated as “the optical resonator structure” (ORS), and such terminology is used herein as an alternative designation for frame  24 . The laser beam, as well as the position of the ORS, has to be referenced to any apparatus utilizing the laser beam, for example a laser wafer scanner (not shown). A beam delivery unit (not shown), which delivers the beam from the laser output to the wafer scanner, is referenced to the floor  16  in absolute position. The stiff and stable floor provides the reference for both laser system  10  and the wafer scanner. Following this concept, the ORS (frame  24 ) has to provide the stable connection of the optic tables to the floor.  
         [0036]     Three main error sources for a stable position of the optics tables can be distinguished. These are deflections caused by static loads, deflections caused by vibrations, and deflections caused by temperature gradients. An ORS that is able to fit the stability requirements of the present invention and can cope with the different error sources has been designed in a way that all six degrees of freedom (DOFs) are fixed only one time each. A high stiffness is important to meet the requirements. All mounts, structural elements or assemblies are designed such that all six degrees of freedom are singly constrained. This assures that movement will be prevented, while stress will not be introduced into the structure.  
         [0037]     Prior-art methods for fixing degrees of freedom in mounting of an optics platform are typically based on a kinematic (three-point support) mount for the platform. Such mounts may be variously configured with respect to the three support- points. By way of example, in a first configuration, three balls are provided engaging three V-shaped grooves. In this configuration, each ball fixes two DOFs. In a second configuration there is a first ball engaging a hole and fixing three DOFs; a second ball engaging a plane surface and fixing 1 DOF; and a third ball engaging a V-shaped groove and fixing 2 DOFs.  
         [0038]     For the first configuration, the thermal centre lies in the (virtual) heart of the V-shaped grooves. For the second configuration, the ball in the hole determines the thermal centre.  
         [0039]     For inventive laser system  10 , the above-described traditional kinematic mount appeared to be not suitable for supporting frame  24  on chassis  12  because of the geometry requirements of the frame. For a kinematic mount, a basic requirement is that the body being supported must be rigid. Given the geometry and size of the ORS, such a rigid body structure, while not impossible to design, could not be economically built because of a demanding combination of material and space requirements. For frame  24  (the ORS) a convenient way of achieving a stiff structure that is fixed to its specified DOFs is to use a combination of stiff and elastic elements connected (indirectly, via chassis  12 ) to the rigid floor. In this way the rigidity of the floor helps to achieve a stable positioning of the ORS and the optical tables attached thereto. Trying to achieve the same frame rigidity without help of the rigidity of the floor would require a lot more material, for example, approximately 5 to 10 times more. A description of the design of one preferred example  24 A of such a frame  24  is set forth below with reference to  FIG. 4 , wherein the axis system of  FIG. 1A  is included to facilitate the description.  
         [0040]     The basis for the optical resonator structure design is a frame that connects the four optics tables. On both ends, the two opposite optics tables, i.e., tables  70  and  72 , and tables  74  and  76 , are connected rigidly together in six DOFs, to effectively form two sub-assemblies  80  and  82 . The two sub-assemblies are connected rigidly to each other in five DOFs. Only torsion around the X-axis is kept weak in the connecting structure. This is necessary because the whole ORS is placed on four adjustable feet,  84 ,  86 ,  88 , and  90 , on the outer edges of the frame. The four feet are rigidly connected, via chassis  12  (not shown in  FIG. 4 ) to the floor. In this way, the four edges of the frame  24 A can be regarded as rigid in the Z-direction. By adjusting the feet in the Z-direction the frame can be leveled in the X and Y-axes. Because of the X-axis torsional weakness of frame  24 A around the X-axis, all four feet will remain in contact with the chassis while being adjusted.  
         [0041]     To cope with differential thermal expansion between the frame and the chassis, two feet  84  and  86  at the right hand end of the frame are made stiff in X-direction. The left hand end feet  88  and  90  can deflect easily in both X and Y. This provides that thermal centre of frame  24 A in the X-direction is at the right-hand-end feet position, and also provides that the DOF of the frame in the X-direction is fixed. In the Y-direction, all four feet can deflect. Here, are at each end, a plate  93  cut-out to leave “bow-tie” stiffening structures  95 , is used to connect the optics frame via the chassis to the floor. The plates are shaped in such a way that the middle of the LDUs on frame  18  is aligned to an optical axis defined by the optical modules. Because of this, the thermal centre in Y-direction will lie on this optical axis. The two plates fix the Y-direction of the optics frame and the rotation around the Z-axis. Since the four feet fix the Z-direction and the torsion around the Y-axis and X-axis, the whole frame position is fixed.  
         [0042]     Because in frame  24 A the four feet,  84 ,  86 ,  88 , and  90  are required to fix 3 DOFs, the torsional weakness in the frame about the X-axis (R X ) is necessary to achieve a determined fixation of the frame, via the chassis, to the floor. The R X  torsional weakness of the optics frame should be limited, however, to allow the frame to be transported separately and safely. Connections between the four optical tables have to provide, given the available space and accessibility constraints, as much as possible a high stiffness for rotational (torsional) movements of the tables. Translations are considered less critical. Frame deflections at Eigen-frequencies should cause mainly translational aberrations. This is achieved by connecting the optical tables in a kind of parallelogram. Opposite tables are made torsionally stiff by connecting the table together through a box section  92  on the back of the frame. Torsional stiffness of the tables is needed to keep the deflection within the system requirements. Main parts of frame  24  are preferably made from stainless steel with low expansion co-efficient, for example, INVAR or SUPER INVAR.  
         [0043]      FIG. 5  schematically illustrates one example  12 A of a chassis arrangement suitable for a laser in accordance with the invention. The terminology “laser chassis” as used by practitioners of the excimer laser art usually refers to an overall housing or structure for an excimer laser system. Typically, the laser chassis would hold one or more laser discharge units, support optics modules, power supplies, computer controllers and other elements which are necessary for the overall operation of the laser system. Covers are typically provided to prevent accidental or unauthorized access to components assembled on the chassis. Such covers are not shown in  FIG. 5  to allow details of the chassis construction to be seen.  
         [0044]     Chassis  12 A has a base  94  and a pedestal  96 . Bulkheads  98  connect the base and the pedestal to form a stiff stable platform for the two separated frames, i.e., frame  18  (see  FIG. 1 ) supporting LDUs  34  and  32 , which frame can be designated the pedestal frame, and optics frame  24 , referred to herein also as the ORS. Attached to this platform are left and right end-walls  100  and  102 , respectively. Spaces between bulkheads  98  are used to accommodate, power supplies, gas units, cooling units, purge units electronics, and the like. The chassis is supported on feet  14  corresponding to feet  14  of  FIG. 1 . It is to be noted that in chassis  12 A pedestal  96  would correspond to fraction portions  12  of the chassis of system  10  of  FIG. 1 .  
         [0045]     Those skilled in the art will recognize that while frame  18  is described above as being a unit separate from chassis  12  it is possible to make that frame an integral part of chassis  12  or  12 A, i.e., with the chassis itself being the frame on which the LDUs are supported. In accordance with the present invention, however, the LDUs must still be supported on the chassis via a vibration isolating suspension, which can be a steel W-spring type suspension as described above, or any other type of suspension. This is because such a suspension is required to isolate the separate ORS (frame  24  or  24 A) and optical modules thereon from LDU vibrations that would be otherwise transmitted thereto, via the chassis, in the absence of a vibration isolating suspension.  
         [0046]     The present invention is described above in terms of a preferred and other embodiments. From the description, those skilled in the art may devise, without undue experimentation, or without departing from the spirit or scope of the invention other embodiments. Such embodiments may include but not be limited to embodiments having a different chassis structure, with or without an integral frame for supporting one or more LDUs, a different separate optics frame, or a different vibration isolating suspension for the LDU or LDUs. All such embodiments or deviations should be construed to be within the scope of the claims appended hereto.

Technology Category: 5