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Multistate

Introduction

The multistate functionality of EHT Semi's pulse generators allows users to quickly change the bias voltage at the wafer to control the ion energy distribution (IED) during semiconductor processing. Pulse generators from other manufacturers only allow for a small, fixed number of states. EHT Semi pulse generators are continuously adjustable over the full voltage range, which means any arbitrary IED can be achieved. While multistate is available for all EHT Semi pulse generators, this tutorial will focus on the bipolar waveforms of Perseus™. Multistate operates similarly on unipolar pulse generators like Spartan™ and Hoplite™.

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.

Perseus Waveform Illustration

Perseus™ has been operated on multiple chambers and a wide range of plasma conditions. The waveforms on the lower left show the output voltage from Perseus™ (blue). The active droop compensation circuit produces a flat waveform on the wafer (green). Note that the actual wafer waveforms will depend on chamber geometry and plasma conditions.

Perseus Waveform Real

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.

Perseus Multistate - two states

Bias power systems from other vendors have a fixed and small number of states that can be accessed. EHT Semi pulse 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.

Continuously adjustable Multistate

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.

Multistate burst and IED V2s

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 RF generator or pulse generators from other manufacturers.

Learn more about EHT Semi's pulse generators: