Tuning gas cluster ion beam systems

1. A method for processing a substrate, the method comprising:

passing a gas cluster beam through an ionizer, the ionizer having a first operating state in which the ionizer is turned ON to ionize the passing gas cluster beam and a second operating state in which the ionizer is turned OFF, the gas cluster beam comprising a distribution of clusters of different sizes and monomers;

exposing a detector to the gas cluster beam;

measuring a beam current at the detector while changing an operating state of the ionizer;

obtaining time-of-flight information of the clusters and the monomers in the gas cluster beam based on the beam current and a timing of the changing of the operating state of the ionizer;

obtaining size information relating to a size distribution of the clusters and the monomers in the gas cluster beam based on the time-of-flight information;

adjusting a process parameter of the gas cluster beam based on the size information; and

exposing the substrate to the gas cluster beam with the adjusted process parameter.

2. The method of claim 1 , wherein the detector is another substrate.

3. The method of claim 1 , wherein changing the operating state of the ionizer comprises changing from the first operating state to the second operating state, and wherein obtaining the size information comprises measuring a decay in the beam current when the ionizer changes from the first operating state to the second operating state.

4. The method of claim 1 , wherein changing the operating state of the ionizer comprises changing from the second operating state to the first operating state, and wherein obtaining the size information comprises measuring an increase in the beam current when the ionizer changes from the second operating state to the first operating state.

5. The method of claim 1 , wherein obtaining the size information comprises:

calculating a time derivative of the beam current; and

calculating the size distribution of the clusters and the monomers based on the time derivative of the beam current.

6. The method of claim 1 , wherein adjusting the process parameter of the gas cluster beam comprises adjusting a process parameter for a nozzle generating the gas clusters, adjusting a process parameter of gases forming the gas clusters, adjusting a process parameter for the ionizer, adjusting a process parameter for accelerating ions in the gas cluster beam, or adjusting a process parameter for a tool moving the substrate relative to the gas cluster beam.

7. The method of claim 1 , wherein the size information comprises a ratio of the clusters to the monomers.

8. A method for processing a substrate, the method comprising:

applying, at an ionizer, a drive pulse train to an ion source to ionize a gas cluster beam and transfer the drive pulse train to the gas cluster beam;

measuring, at a detector exposed to the gas cluster beam, a beam current synchronously with the drive pulse train;

obtaining time-of-flight information of the clusters and the monomers in the gas cluster beam based on the beam current and the drive pulse train;

determining size information relating to a size distribution of clusters and monomers in the gas cluster ion beam based on the time-of-flight information;

adjusting a process parameter of the gas cluster beam based on the size information; and

exposing the substrate to the gas cluster beam with the adjusted process parameter.

9. The method of claim 8 ,

wherein the drive pulse train comprises a plurality of pulses, each of the plurality of pulses having a time duration, and

wherein determining the size information comprises

dividing the beam current into a plurality of beam current pulses, a duration of each of the plurality of beam current pulses is the time duration, and each of the plurality of beam current pulses is associated with one of the plurality of pulses,

translating the plurality of beam current pulses to overlap over a common time duration, and

adding the translated plurality of beam current pulses to obtain a cumulative beam current; and

determining the size distribution from the cumulative beam current.

10. The method of claim 8 , further comprising

measuring, at the ionizer, a voltage response to the drive pulse train, wherein determining the size information further comprises compensating for a response time of the ionizer based on the voltage response.

11. The method of claim 8 , wherein obtaining the size information comprises:

calculating a time derivative of the beam current; and

calculating the size distribution of the clusters and the monomers based on the time derivative of the beam current.

12. The method of claim 8 , wherein adjusting the process parameter of the gas cluster beam comprises adjusting a process parameter for the ionizer.

13. The method of claim 8 , wherein adjusting the process parameter of the gas cluster beam comprises adjusting a process parameter for a nozzle generating the gas clusters, adjusting a process parameter of gases forming the gas clusters, or adjusting a process parameter for accelerating ions in the gas cluster beam.

14. The method of claim 8 , wherein adjusting the process parameter of the gas cluster beam comprises adjusting a process parameter for a tool moving the substrate relative to the gas cluster beam.

15. An apparatus comprising:

a source chamber having a beam-line for forming a gas cluster beam;

an ionization/acceleration chamber comprising an ionizer configured to ionize the gas cluster beam and operate in a first operating state in which the ionizer is turned ON and a second operating state in which the ionizer is turned OFF, the gas cluster beam comprising a distribution of clusters of different sizes and monomers;

a process chamber comprising a holder for holding a detector on the beam-line and expose the detector to the gas cluster beam and a current sensor to measure a beam current generated by the gas cluster beam at the detector while changing an operating state of the ionizer;

a processor; and

a non-transitory memory storing a program to be executed in the processor, the program comprising instructions to

obtain size information relating to a size distribution of the clusters and the monomers in the gas cluster beam based on the beam current and a timing of the changing of the operating state of the ionizer, and

adjust a process parameter of the gas cluster beam based on the size information.

16. The apparatus of claim 15 , further comprising:

a pulse generator to output a drive pulse train to the ionizer, the ionizer being configured to receive the drive pulse train and switch between the first operating state and the second operating state based on the drive pulse train.

17. The apparatus of claim 15 , wherein the ionizer comprises an extraction plate, the apparatus further comprising:

a measurement circuit to measure, at the extraction plate, a voltage response to the drive pulse train, wherein instructions to obtain the size information further comprises instructions to compensate for a response time of the ionizer based on the voltage response.

18. The apparatus of claim 15 , wherein the detector is a Faraday cup.

19. The apparatus of claim 15 , further comprising a current sensor with an analog front-end circuit coupled to the detector to convert the beam current into a digital signal, wherein the digital signal is provided to the processor.

20. The apparatus of claim 15 , further comprising a control circuit to receive the process parameter and send a command signal to the source chamber, the command signal comprising information relating to a change a process recipe for the source chamber in forming the gas cluster beam.