Multistate
Introduction
The multistate functionality of EHT Semi's Direct RF and tailored waveform generators allows users to quickly change the bias voltage at the wafer to control the ion energy distribution (IED) during semiconductor processing. Generators from other manufacturers only allow for a small, fixed number of states. EHT Semi generators are continuously adjustable over the full output range, which means any arbitrary IED can be achieved. While multistate is available for all EHT Semi generators, this tutorial will focus on the tailored waveforms of Perseus™. Multistate operates similarly on unipolar pulse generator like Spartan™ and Orion.
Perseus Waveforms
A simplified illustration of a Perseus™ waveform is shown in the top left. This figure has a positive pulse that charges the system capacitance. After the positive pulse, the active droop compensation in Perseus™ generates a negative pulse with a negative going slope (-dV/dt) to produce a flat voltage profile on the wafer. These waveforms produce a very narrow IED.
Multistate Waveforms
In multistate, the states refer to the output voltage of the pulser and the corresponding voltage on the wafer. The example below focuses on the pulse generator output for simplicity and illustrates a burst pattern with two states. In the first state, the pulse generator outputs a burst at high voltage and then a second burst at low voltage. This will produce a high wafer voltage and then a lower wafer voltage. The burst pattern is repeated a second time.
Bias power systems from other vendors have a fixed and small number of states that can be accessed. EHT Semi generators offer continuously-adjustable multistate that allows users to dial in the exact voltage required at any given time during the process.
The figure below shows the waveform envelope for seven states with arbitrary voltage levels. A state's burst duration can be controlled down to a single pulse. The inset images show the pulse generator output waveform for the different states and the transitions between states. EHT Semi pulse generators can transition to different states within 1-2 pulses (~ 5 μs), including low to high voltage. This is significantly faster transitions than pulse generators from other manufacturers.
Multistate and Ion Energy Distributions
By controlling the time spent in each state, the time-averaged IED can be controlled. In the example below, the voltage envelope (left) shows a repeated burst with three states. The insets show the pulse generator waveform for each state.
The state duty cycle (SDC) is the fraction of the burst period in a particular state (see equation in the lower left). If the pulse generator spends equal time in all three states, then the time-averaged IED will have three peaks (top right). If SDC increases (SDC1 < SDC2 < SDC3), then the IED will have a higher peak associated with state 3 (middle right). If states 1 and 3 have the same duty cycle, then they will have the higher peaks (bottom right).
The continuously-adjustable multistates and control of the SDCs allow users to map out any arbitrary IED and control it in time.
Conclusion
The continuously-adjustable multistate feature allows manufacturers to optimize the IED for their specific etching process. EHT Semi pulse generators provide significantly more IED control than a traditional RF generator with matching network or pulse generators from other manufacturers.
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