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A wave generation component during an off cycle when the on-off valve is closed to build pressure from the process gas in an accumulation volume. During an on cycle when the on-off valve is open the wave generation component releases the process gas according to a time constant. A flow restrictor in

A wave generation component during an off cycle when the on-off valve is closed to build pressure from the process gas in an accumulation volume. During an on cycle when the on-off valve is open the wave generation component releases the process gas according to a time constant. A flow restrictor installed downstream in a throat of the on-off valve, outputs the rapid square waves to the conduit at a predefined magnitude. The flow restrictor is selected to have an impedance that is high enough to significantly raise the time constant during the on cycle such that pressure decay in each square wave pulse over the on cycle decreases to within a tolerance, wherein the time constant is at least in part a function of the flow restrictor impedance.

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1. A gas delivery apparatus to output a process gas to a semiconductor process as rapid square waves by increasing a time constant to a desired time constant, comprising: an enclosure attached to a base with a conduit channeling through the base to receive a supply of the process gas and output the

1. A gas delivery apparatus to output a process gas to a semiconductor process as rapid square waves by increasing a time constant to a desired time constant, comprising: an enclosure attached to a base with a conduit channeling through the base to receive a supply of the process gas and output the process gas from the gas delivery apparatus to the semiconductor process;a wave generation component comprising a gas supply component and an on-off valve downstream from the gas supply component, coupled to receive the process gas in the conduit, the wave generation component during an off cycle when the on-off valve is closed to build pressure from the process gas in an accumulation volume, and during an on cycle when the on-off valve is open to release the process gas according to a time constant; anda flow restrictor installed in a throat of the on-off valve to output the rapid square waves to the conduit at a desired magnitude of flow, a size of the flow restrictor selected to have an impedance that is high enough to significantly raise the time constant during the on cycle such that massflow decay in each square wave pulse over the on cycle decreases to within a certain tolerance, wherein the time constant is at least in part a function of the flow restrictor impedance. 2. The gas delivery apparatus of claim 1, wherein the flow restrictor comprises at least one of: an orifice, a venturi, a laminar flow element (LFE), or a very small-bore diameter of an outlet flow conduit. 3. The gas delivery apparatus of claim 1, wherein the flow restrictor size is selected based on one or more of: a supply pressure, a maximum massflow target, and a percentage of time for the on cycle to a total time for an on-off cycle. 4. The gas delivery apparatus of claim 1, wherein a pressure in the accumulation volume throughout on and off cycles is substantially higher than a pressure without the flow restrictor. 5. The gas delivery apparatus of claim 1, further comprising: an accumulation chamber, disposed in the first conduit between the gas supply component and the on-off valve, the accumulation chamber increasing an accumulation volume to further increase the time constant, wherein the time constant at least in part a function of the flow restrictor impedance and the accumulation volume,wherein the accumulation volume is initially defined by a volume between a valve seat of the gas supply component and the valve seat of the on-off valve. 6. The gas delivery apparatus of claim 1, further comprising: a relief valve, disposed in the first conduit between the gas supply component and the flow node, to reduce bleed down time of transients between gas flows by venting at least a portion of process gas from the accumulation volume. 7. The gas delivery apparatus of claim 1, wherein the gas supply component comprises at least two mass flow controllers corresponding to at least two different types of process gas mixed in the accumulation volume. 8. The gas delivery apparatus of claim 1, wherein the gas supply component comprises a mass flow controller to deliver the process gas to the accumulation at a determined mass flow rate. 9. The gas delivery apparatus of claim 8, wherein the mass flow controller is initially given a set point higher than necessary to maintain steady-state in order to build up pressure in the accumulation volume more quickly, the set point being reduced to the desired flow once the flow out to the process approaches the desired flow. 10. The gas delivery apparatus of claim 1, further comprising: a pressure transducer; anda temperature sensor, wherein the gas supply component comprises an electronic regulator to pressurize the accumulation volume to a determined pressure. 11. The gas delivery apparatus of claim 10, further comprising: a printed circuit board (PCB), coupled to the wave generation component, the PCB containing electronics to detect and adjust flow of the process gas to the accumulation volume based on feedback from at least one of the pressure transducer and the temperature sensor. 12. The gas delivery apparatus of claim 1, wherein each of the substantially square wave pulses is characterized by a vertical ramp up time, a horizontal droop at a desired magnitude, and a vertical ramp down time, over the on cycle, within predefined tolerances. 13. A method for outputting a process gas as rapid square waves by increasing a time constant to a desired time constant, comprising the steps of: receiving a supply of the process gas;building pressure from the process gas in an accumulation volume during an off cycle, and releasing the process gas according to a time constant during an on cycle; andinstalling a flow restrictor selected to have an impedance that is high enough to significantly raise the time constant during the on cycle such that pressure decay in each square wave pulse over the on cycle decreases to within a tolerance, wherein the time constant is at least in part a function of the flow restrictor impedance. 14. The method of claim 13, wherein the flow restrictor comprises at least one of: an orifice, a venturi, a laminar flow element (LFE), or a very small-bore diameter of an outlet flow conduit. 15. The method of claim 13, wherein the flow restrictor size is selected based on one or more of: a supply pressure, a maximum massflow target, and a percentage of time for the on cycle to a total time for an on-off cycle. 16. The method of claim 13, further comprising: increasing an accumulation volume with an accumulation chamber to further increase the time constant, wherein the time constant at least in part a function of the flow restrictor impedance and the accumulation volume,wherein the accumulation volume is initially defined by a volume between a valve seat of the gas supply component and the valve seat of the on-off valve. 17. The method of claim 13, further comprising: reducing bleed down time of transients between gas flows by venting at least a portion of process gas from the accumulation volume. 18. The method of claim 13, wherein the supply of gas is provided by a mass flow controller that is initially given a set point higher than necessary to maintain a desired flow in order to build up pressure in the accumulation volume more quickly, the set point being reduced once the flow to process approaches the desired flow.