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.
An excimer or molecular fluorine (F2) 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.
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.
In a straightforward excimer laser there would be only one LDU, usually, however, there are two LDU'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.
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.
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.
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.
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.
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.
Referring now to the drawings, wherein like components are designated by like reference numerals,
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
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.
Each laser discharge unit is mounted on frame 18 via a resilient suspension. This suspension system is depicted without detail in
Wheels 20 are supported on a Y-axis support member 52, and wheels 50 are supported on a Y-axis support member 54.
As depicted in
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
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
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.
Referring again to
It should be noted that the many modules depicted in
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.
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.
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.
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.
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 24A of such a frame 24 is set forth below with reference to
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
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 24A 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.
Because in frame 24A the four feet, 84, 86, 88, and 90 are required to fix 3 DOFs, the torsional weakness in the frame about the X-axis (RX) is necessary to achieve a determined fixation of the frame, via the chassis, to the floor. The RX 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.
Chassis 12A 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
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 12A, 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 24A) and optical modules thereon from LDU vibrations that would be otherwise transmitted thereto, via the chassis, in the absence of a vibration isolating suspension.
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.
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.
Number | Date | Country | |
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60586569 | Jul 2004 | US |
Number | Date | Country | |
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Parent | 10645947 | Aug 2003 | US |
Child | 11176479 | Jul 2005 | US |