Patent DE-19935035-A1 (DE)
Title: Laser system, useful for deep ultraviolet microlithography of silicon substrates, uses absolute wavelength calibration of constricted laser emission tuned to an absorption line of a photoabsorber mixed with the laser gas
Inventor: LEINHOS UWE
IPC: H01S3/139
Filed: 1999-07-26
Abstract: A laser system, with a detector (22) for absolute wavelength calibration of constricted laser emission tuned to the absorption line(s) of a photoabsorber mixed with the laser-active gas, is new. A laser system comprises: (a) a discharge chamber (2) filled with a gas mixture of a laser-active material and a photoabsorber with one or more photoabsorption lines within the laser emission spectrum; (b) a wavelength selector unit (6); and (c) a detector (22) capable of calibrating an absolute wavelength of a constricted laser emission tuned to at least one of the absorption lines. Independent claims are also included for the following: (i) a narrow band excimer laser comprising components for wideband spectral emission using a laser-active gas, for constricting the spectral emission, for tuning the constricted emission within the wideband emission spectrum, for detecting the constricted emission, for inducing photoabsorption within the wideband emission spectrum in the presence of the laser-active gas and for calibrating an absolute wavelength of the constricted emission when the constricted emission is tuned within the wideband emission spectrum and photoabsorption is induced; (ii) a method of determining an absolute wavelength of an excimer laser spectral emission by providing a laser-active gas and a photoabsorber with one or more photoabsorption lines within the wideband laser emission spectrum inside a gas discharge chamber, detecting a constricted spectral emission of the laser-active gas when the constricted emission is tuned within the wideband emission spectrum and determining the absolute wavelength of the constricted emission on the basis of the relative spectral positions of the constricted emission and the photoabsorption line(s) of the photoabsorber; (iii) an excimer laser comprising a discharge chamber containing a laser gas mixture, a device for exciting the gas mixture for laser beam production, a wavelength selector module for constricting and tuning the laser output band, a detector for monitoring the laser beam power and a processor for controlling the wavelength selector module operation and for monitoring the detected power, the discharge chamber also containing a trace amount of a gas with a known photoabsorption line within the laser tuning range and the wavelength selector module being tuned by the processor in accordance with the monitored absorption induced by the trace gas; and (iv) a method of calibrating an absolute wavelength of a laser by providing a laser-active material and a photoabsorber with one or more photoabsorption lines within a wideband emission spectrum of the laser, generating and constricting wideband spectral emission from the laser-active material, tuning the constricted emission within the wideband emission spectrum to obtain the photoabsorption line(s), detecting the constricted emission and calibrating the absolute wavelength by detection of reduced emission caused by the photoabsorption line(s). Preferred Features: The photoabsorber comprises atomic carbon, supplied by interaction of organic molecules in the gas mixture, or molecular oxygen, supplied by interaction of combined oxygen atoms or oxide molecules (CO, O2 or especially CO2 molecules) in the gas mixture. The photoabsorption lines are 193.0905, 193.114, 193.273, 193.292 and/or 193.493 nm lines when the laser is an ArF excimer laser