Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2002-05-07
2004-07-06
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S092000, C372S098000
Reexamination Certificate
active
06760358
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to an apparatus for adjusting the spectrum and bandwidth of a laser light source.
2. Description of Related Art
Excimer lasers are currently used as light sources for the integrated circuit lithography industry. These lasers produce a beam having a narrowband spectrum with a bandwidth of less than 1 pm at deep ultraviolet (DUV) wavelengths of 248 nm for a KrF laser or 193 nm for an ArF laser. Molecular fluorine lasers emit around 157 nm and will become more widely used for vacuum ultraviolet (VUV) lithography for producing even smaller structures on silicon wafers. There also exist semi-narrowband excimer lasers with bandwidths of more than 10 pm, for which the same principles hold. To produce extremely narrow-band UV light of low divergence and of a high spectral purity, a multitude of dispersive optical components may be utilized such as prisms, optical diffraction gratings and etalons or other interferometric devices. In general, adjustments to the wavelength and/or bandwidth of UV light emitted by these lasers may be made by using an electromechanical device (“EMD”), which in some way moves the position or changes the surface curvature of an optical component (“OC”) in the resonator of the excimer or molecular flourine laser. The EMD is coupled to a mechano-optical device (“MOD”) which transfers the motion of the EMD to the optical component, wherein the OC may be typically fixed to the MOD. Thus, when the optical component is moved, characteristics of the UV light output from the laser are changed.
In the normal operation mode of a DUV or VUV lithography laser system, it is desired to keep the laser wavelength substantially constant and the bandwidth (or another spectral property like full width at 1/e
2
, spectral purity, or an integral or differential quantity) below a specified value. These quantities can be monitored and controlled using a spectrometer such as an etalon spectrometer, grating spectrometer, prism spectrometer or other optical spectrometers in conjunction with a processor in a feedback loop including means for adjusting spectral parameters to desired values.
The typical temporal exposure pattern for the production of semiconductor chips is produced with pulse bursts of, e.g., 200 laser pulses and short breaks of 100 ms between them. A complete sequence may include 60 bursts and breaks, which is followed by a more or less long burst break of, e.g., 5 seconds. For lithography systems it is desired to keep the quality of the laser radiation under control for each pulse of the burst sequence. It is desired to have a lithography system wherein the wavelength of emission may be changed, particularly within a long burst break, over a wide range of up to 300 pm. This is to adapt the lithography system to environmental conditions like pressure and temperature. Furthermore, changes of wavelength in a small range of up to 0.6 pm without any laser pulses being emitted from the laser during that wavelength change are desired within short burst breaks, i.e. with an open feedback loop. This is to keep the quality of the lithography process under control, as changes in temperature of stepper optics can otherwise result in changes in exposure wavelength at an application process.
The desired tolerance limit of such componentry is extremely low in respect of hysteresis. Maintaining a stable and invariable position of optical elements and mechanical componentry within a defined range is, therefore, greatly desired.
It is further desired that the mounting of the OC be independent of environmental conditions, in particular of possible temperature gradients within the optical component. The demands set out above apply also to elements and componentry in motion during operation. These linear and rotary motions can be very small (e.g., <100 nm:x rad) and are essentially designed to fine tune the entire optical system to a desired wavelength of the UV light. This may result in high acceleration values and it is desired that such values be free of negative influence on the positional stability of the optical elements. Merest inaccuracies already prove undesirable during repeated starts at pre-defined set-points. These set-points can be reference coordinates at which the reference wavelength, for exampled 248.3271 nm, is found. To calculate various operating positions, it is desired that this value be recorded precisely and be maintained reliably. It is further desired that the hysteresis of such motional process be kept very small ensuring that the wavelength drift of the optical assembly is kept as minute as possible.
Spring mount contact pressure plates may be used for securely positioning OCs. A disadvantage is that there is a pointlike exerting of force into the substrates of OCs using this method. Consequently, this may lead to the development of partially irreversible strain birefringence. This causes severe wavefront deformation and striation in the beam profile. Optically acting gratings and etalons having adjustable orientations for controlling the wavelength and bandwidth of emission of the laser may be supported between high surface quality bearings which permit rotation of the OCs. Such systems are susceptible to hysteresis, and it is recognized herein that special consideration should be given to the design of bearing components. Important quantities are diameter of balls, surface quality of bearing components as well as sizing of pressure forces. In general small quantities of silicone-free lubricants are used for lubrication.
When designing components to support OC's, it is recognized herein that special consideration should be given for temperature gradient-dependent changes in length. Such assemblies may be generally very sensitive to temperature fluctuations reacting with play in bearings and thus producing hysteresis effects when adjusting positions and/or orientations of OC's. To a certain degree this is influenced by the breakaway friction of bearing components. This is the force necessary to leave the zone of elastic deformation of the bearing components and to proceed into a rotary, progressive motion.
SUMMARY OF THE INVENTION
The present invention relates to an apparatus for adjusting, with low hysteresis and high repeatability, the wavelength and/or bandwidth of a laser beam by moving an optical component.
In a particular embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes an electromechanical device comprising a drive element including a first contact surface, and a mechano-optical device for supporting the optical component including a second contact surface for contacting the first contact surface. The drive element permits adjustment of an orientation of the mechano-optical device by applying a force to the first contact surface, and thereby for adjusting an orientation of the optical component. The first and second contact surfaces are configured such that the drive element transmits a change of position to the mechano-optical device through a rolling contact between the first contact surface and the second contact surface.
In another embodiment, an apparatus for adjusting a position of an optical component within a laser resonator includes an electromechanical device comprising a drive element including a first contact surface, and a mechano-optical device for supporting the optical component including a second contact surface for contacting the first contact surface. The drive element permits adjustment of the position of the mechano-optical device by applying a force to the first contact surface, and thereby the mechano-optical device adjusts the position of the optical component. The apparatus also includes a controller for error correction of the position of the drive element which controls the electromechanical device, and a position measuring device which measures the position of the drive element. A signal feedback loop provides a signal indicative of the position of the d
Aab Konstantin
Kramer Matthias
Serwazi Marcus
Zimmermann Kay
Al-Nazer Leith
Ip Paul
Lambda Physik AG
Stallman & Pollock LLP
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