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1 METHOD OF MAINTAINNG GAS QUALITY IN A LASER DISCHARGE CHAMBER BACKGROUND 5 [0001] Photolithography is a process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography radiation source provides DUV radiation (radiation having wavelengths in a range of about 100 nanometers (nm) to about 400 nm) used to expose a photoresist on the wafer. Often, the radiation source is a laser source and the radiation is a pulsed laser beam. The radiation beam is passed through a beam delivery unit, then through or reflected by a reticle 10 or a mask, and then projected onto a silicon wafer coated with photoresist. In this way, a chip design is patterned onto a photoresist that is then etched and cleaned. [0002] Many systems that generate a beam of DUV laser radiation do so by creating discharges in a discharge region between electrodes in a discharge chamber. More precisely, the DUV discharge chamber has a pair of electrodes (a pair being comprised of one anode and one cathode) arranged to 15 generate plasma discharges from a lasing gas (generally a gas mixture) when a high voltage is applied across the electrodes. The discharge is pulsed at a repetition rate of, for example, 6 kHz (6000 times per second), to produce a train or burst of DUV radiation emission pulses. [0003] During a discharge the gas in the discharge region between the electrodes becomes depleted of one or more components. The gas when depleted must be replaced to ensure that the 20 performance in the DUV laser radiation source remains within specifications. Thus a gas control system carries out a process of evacuation and refilling of gases referred to as a refill. Also, because in an excimer laser the concentration of the reactive gas, e.g., fluorine, must be maintained, the gas control system performs gas injections or simply “injects” during which a small amount of chamber gas is removed and replaced with a variable concentration of the different process gases. A gas injection 25 performed when the DUV laser radiation source is idle is referred to as a “passive inject,” while a gas injection performed when the DUV laser radiation source is firing is referred to as an “active inject.” [0004] In one aspect, the gas control system computes the required concentration of reactive, e.g., fluorine, gas and performs gas injections at prespecified intervals. One objective is to reduce contaminant levels and introduce the reactive gas at such a rate as to maintain certain Key Performance 30 Indicator (KPI) signals within specifications. Gas control systems have in general been successful in maintaining long intervals between refill operations. This is a benefit as it reduces the amount of gas consumed in refill operations overall. However, this controlled gas injection process can still be costly as it can still consume a significant amount of the gas mixture that is injected. [0005] Conventionally, a refill operation can be triggered from any one of several sources. For 35 example, a user can manually initiate a refill operation using a user interface. Also, an automated system can initiate an automatic refill request after a specified idle time. A refill operation request can also be generated when a number of gas injections exceeds a predetermined gas maintenance limit. 2 [0006] Also, the gas control system can be configured to perform a refill operation at set intervals, e.g., every eight hours, when the DUV laser radiation source is idle. The objective is to prevent the gas from retaining an excessive amount of contaminants when the DUV laser radiation source is called upon to expose wafers. This objective is achieved by having the system request a refill operation 5 when the number of pulses generated by the DUV laser radiation source since an immediately prior refill exceeds a predetermined limit. Another reason for performing refill operations when the DUV laser radiation source is idle is to extend the life of a chamber. This process can be costly as a significant amount of gas mixture is required to achieve this goal, especially when the DUV laser radiation source is idle for long periods of time. 10 [0007] It would be advantageous to improve the process of replenishing gas in the discharge chambers of a DUV laser radiation source in ...