In some general aspects, a dose control apparatus for an extreme ultraviolet (EUV) light source includes a dose controller. The dose controller is configured to receive an EUV pulse energy set-point associated with a desired EUV pulse energy for exposing a substrate and a measurement of an EUV pulse energy created from an EUV interaction, the EUV interaction occurring between a target light pulse and a target. The dose controller includes: an interaction module configured to determine an interaction ratio based on the measured EUV pulse energy and to provide the interaction ratio to a target light source that produces the target light pulse, the interaction ratio defining a percentage of the average number of EUV interactions over a period of time; and an adjustment module. The adjustment module is configured to: calculate an adjusted EUV pulse energy set-point from the received EUV pulse energy set-point based on the determined interaction ratio; and provide the adjusted EUV pulse energy set-point to a target light source controller.Implementations can include one or more of the following features. By providing the adjusted EUV pulse energy set-point to the target light course controller, dose stability at the substrate that received the EUV pulse energy can be improved. For example, the interaction module can determine the interaction ratio by comparing the measured EUV pulse energy with the EUV pulse energy set-point associated with the desired EUV pulse energy for exposing the substrate. The interaction ratio can be provided to the target light source to thereby cause the target light source to produce the target light pulses in accordance with the interaction ratio such that the number of EUV interactions on average corresponds to the interaction ratio.The adjustment module can calculate the adjusted EUV pulse energy set-point by calculating the adjusted EUV pulse energy set-point when the interaction ratio is either inside an unacceptable range or is outside an acceptable range. The interaction ratio can be inside the unacceptable range whenever a dose is unstable and is outside the acceptable range whenever the dose is not within a range of stability. The adjustment module can determine whether the interaction ratio is inside the unacceptable range or is outside the acceptable range by analyzing whether a dose stability metric is within a range of a local maximum or within a range of a local minimum. The dose controller can further include memory configured to store one or more of the unacceptable ranges and acceptable ranges of the interaction ratio. The dose controller can be configured to access the memory. The adjustment module can include: an interaction ratio averaging module configured to average the values of the interaction ratio and output an average interaction ratio; and an update module. The update module can be configured to receive the average interaction ratio; to receive an EUV pulse energy set-point associated with a desired EUV pulse energy for exposing the substrate; and to calculate the adjusted EUV pulse energy set-point based on the average interaction ratio.The dose controller can include the target light source controller, and the target light source controller can be in communication with the adjustment module and the target light source. The dose controller can include an interaction averaging module configured to: receive the measured EUV pulse energy over a set number of target light pulses; isolate only target light pulses that interact with the target; and average the values of the measured EUV pulse energy for these isolated target light pulses to produce an average EUV target energy of EUV interactions. The target light source controller can be configured to receive the average EUV target energy from the interaction averaging module and the adjusted EUV pulse energy set-point from the adjustment module, and to determine energy in one or more target light pulses supplied to an interacting
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