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Bias Waveforms

Introduction

A bias voltage waveform can be applied to a wafer surface to control the ions reaching the wafer surface. The choice of bias waveform shape affects the ion energy distribution (IED) at the wafer surface, which in turn affects the etching rate, selectivity, isotropy, minimum critical dimensions, and damage to the wafer. Traditionally, radio-frequency generators have been used for wafer bias. Today, there are other options like unipolar pulsing and bipolar tailored waveforms that bring new advantages to the etch process.

RF Generators

For decades, RF generators have been used for wafer bias. The RF generator outputs a sinusoidal waveform that is rectified and shifted by the plasma sheath. The ions see a negative voltage that accelerates them towards the wafer surface. This voltage is not constant in time, so the IED has a broad distribution and can have multiple peaks.

RF Bias

 

There are a few approaches that can improve the etching performance of RF generators:

  • Increasing the RF power can increase the etching rate. There are limitations to this approach as this requires higher voltage matching networks, cabling, vacuum feedthroughs, and other equipment.
  • With pulse biasing, the RF is on for a short duration. The timing of the RF pulse can be adjusted relative to the plasma generation to produce different results on the wafer surface.
  • Multiple RF generators with different frequencies can be used on the wafer. This distorts the waveform. This approach has limited ability to alter the IED at the wafer surface.

Due to the limited ion energy control, the era of RF bias is ending!

Unipolar Waveforms

Unipolar bias waveforms can provide additional control over the IED compared to RF generators. The unipolar generator produces unipolar pulses that charge the system capacitance to a particular voltage. During the pulse generator off time, the capacitance has a negative potential so that ions are accelerated toward the wafer surface. This ion current slowly discharges the capacitance and the voltage waveform droops back towards zero. If the capacitance is large enough, the droop will not reach zero before the next unipolar pulse recharges the capacitor and the process repeats. The center of the IED produced with unipolar pulses can be easily controlled. The width of the IED will depend on how much droop is produced by the ion current.

Unipolar Bias

 

EHT Semi has two unipolar pulse generators: Hoplite™ and Spartan™. These systems have several advantages over other systems:

  • Short pulse widths result in long wafer voltage long times
  • Lower power draw for similar peak wafer voltages
  • No matching network required
  • Simple circuit topology that simplifies control

Unipolar pulse generators are better for low ion-current loads where wafer voltage droop compensation is not required (i.e., where a tight IED is guaranteed by chamber geometry, load impedance).

Bipolar Waveforms

Bipolar bias waveforms significantly can produce very narrow IEDs on the wafer surface. The positive portion of the waveform behaves very similarly to the unipolar pulse generators above. However, the addition of the asymmetric negative pulse, combined with active droop compensation (controlled dV/dt), holds the wafer voltage constant. The net result is that ions are accelerated towards the wafer by a near constant potential, which results in a very narrow ion energy distribution.

Bipolar Bias

 

EHT Semi's Perseus™ produces asymmetric bipolar waveforms to provide precision IED control. These systems have several key advantages over other wafer bias options:

  • Waveforms are impacted less by presence and size of stray capacitance, inductance on load
  • Increased modularity improves scalability to higher power levels
  • Active droop compensation allows Perseus™ to maintain a very narrow IED for low-impedance loads
  • Continuously-adjustable multistate allows for arbitrary IEDs

Bipolar pulse generators are the state-of-the-art wafer bias power systems. They produce very narrow IEDs that can be adjusted on very fast timescales to sweep out arbitrary time-averaged IEDs.