Patent Number: 
Section: claims

1. A gas contamination sensor, configured to determine a level of contaminant gas mixed with a primary gas, the gas contamination sensor comprising:an inlet, configured to receive a sample of a gas comprising the primary gas and the contaminant gas mixed with the primary gas to be tested by the gas contamination sensor;an ion source, configured to generate a beam of ions from the sample of gas to be tested;a field generator, configured to generate a field that deflects a path of ions within the beam of ions by an amount depending on a ratio of the mass to the charge of the ions;a first ion detector, positioned to receive ions deflected by the field by a first extent and generated from the primary gas in the sample; anda second ion detector, positioned to receive ions deflected by the field by a second extent, different from the first extent, and generated from the contaminant gas in the sample,wherein the primary gas is H2 gas having a pressure in a range of about 1×10−2 mbar to about 1×10−1 mbar. 2. A gas contamination sensor according to claim 1, wherein the second ion detector includesan aperture configured such that the second ion detector, in use, detects ions that have been deflected by the field by a range of deflections, corresponding to the ions having a range of mass-to-charge ratios. 3. A gas contamination sensor according to claim 2, wherein the aperture is configured such that the ions detected by the second ion detector, in use, are ions having a unified atomic mass unit of between about 100 Da and about 200 Da. 4. A gas contamination sensor according to claim 1, wherein the second ion detector is a secondary electron multiplier. 5. A gas contamination sensor according to claim 1, wherein the first ion detector is a Faraday cup. 6. A gas contamination sensor according to claim 1, wherein the field generator is configured to generate a magnetic field and/or an electrostatic field in order to deflect the path of ions in the beam of ions. 7. A gas contamination sensor according to claim 1, further comprising a controller including a processor and a memory, wherein calibration data is stored in the memory, and wherein the processor is configured to determine a pressure of the gas received at the inlet using a measurement signal from the first ion detector and the calibration data from the memory. 8. A gas contamination sensor according to claim 7, wherein the processor is configured to determine a partial pressure of the contaminant gas received at the inlet using a measurement signal from the second ion detector, the calibration data from the memory, and the pressure of the gas received at the inlet determined by the processor. 9. A gas contamination sensor according to claim 1, wherein the gas contamination sensor is configured such that the first and second ion detectors are operated simultaneously. 10. A gas contamination sensor according to claim 1, wherein the ion source is a closed ion source. 11. A gas contamination sensor according to claim 1, wherein the gas further comprises a second contaminant gas, and where the gas contamination sensor further comprises a third ion detector positioned to receive ions deflected by the field by a third extent, different from the first and second extents, and generated from the second contaminant gas that is mixed with the primary gas in the sample. 12. A lithographic apparatus comprising:a gas contamination sensor, configured to determine a level of contaminant gas mixed with a primary gas within a chamber within the lithographic apparatus, wherein the primary gas is H2 gas having a pressure in a range of about 1×10−2 mbar to about 1×10−1 mbar, the gas contamination sensor comprisingan inlet, configured to receive a sample of a gas comprising the primary gas and the contaminant gas mixed with the primary gas to be tested by the gas contamination sensor,an ion source, configured to generate a beam of ions from the sample of gas to be tested,a field generator, configured to generate a field that deflects a path of ions within the beam of ions by an amount depending on a ratio of the mass to the charge of the ions,a first ion detector, positioned such that it receives ions that are deflected by the field by a first extent and that are ions generated from the primary gas in the sample; anda second ion detector, positioned such that it receives ions that are deflected by the field by a second extent, different from the first extent, and that are ions generated from the contaminant gas in the sample;an illuminator configured to condition a beam of radiation;a support configured to support a patterning device, the patterning device being configured to pattern the beam of radiation; anda projection system configured to project the patterned beam of radiation onto a substrate. 13. A method of determining a level of contaminant gas mixed with a process gas, the method comprising:receiving a sample of gas comprising the process gas and the contaminant gas mixed with the process gas to be tested, wherein the process gas is H2 gas having a pressure in a range of about 1×10−2 mbar to about 1×10−1 mbar;generating a beam of ions from the sample of gas to be tested using an ion source;providing a field that deflects a path of ions within the beams of ions by an amount depending on a ratio of the mass to the charge of the ions;detecting ions generated from the process gas and deflected by the field by a first extent using a first ion detector; anddetecting ions generated from the contaminant gas and deflected by the field by a second extent, different from the first extent, using a second ion detector. 14. A device manufacturing method comprising:determining a level of contaminant gas mixed with a process gas in a lithographic apparatus, said determining comprisingreceiving a sample of gas comprising the process gas and the contaminant gas mixed with the process gas to be tested, wherein the process gas is H2 gas having a pressure in a range of about 1×10−2 mbar to about 1×10−1 mbar,generating a beam of ions from the sample of gas to be tested using an ion source,generating a field that deflects a path of ions within the beams of ions by an amount depending on a ratio of the mass to the charge of the ions,detecting ions deflected by the field by a first extent and that are ions generated from the process gas with a first ion detector, anddetecting ions deflected by the field by a second extent, different from the first extent, and that are ions generated from the contaminant gas with a second ion detector; andprojecting a patterned beam of radiation onto a substrate with the lithographic apparatus, wherein the determining is completed before, during, or after, or any combination of before, during, and after the projecting.