OPT - Trockenätzanlage (RIE/ICP, RIBE)/English

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Plasmaätzanlage

RIE/RIBE Oxford PlasmaLab 100

The Oxford PlasmaLab 100 is used for structuring material by dry etching processes.

Before considering RIE for a material not on the list below, please talk to the operators. Even if a process is generally feasible, contamination of the chamber walls can negatively influence standard processes (This is especially the case for nickel).

Alternatives


Operators

Operation Modes

  • Reactive ion etching
    • Sulfur hexafluoride (SF6)
    • N2
    • Argon
    • Oxygen
    • Chloride
    • Fluoroform (CHF3)
    • BCl3
    • HBr
  • Reactive ion beam etching
    • Argon
    • Cl2

The RIE chamber is equipped with a laser based endpoint detection.

Established Processes

RIE

  1. Titanium
  2. Chrome
  3. Silicon, anisotropic profile (Cryo Process)
  4. Silicon, isotropic profile (Underetching of Si sacrificial layers)
  5. Silicon nitride
  6. Silicon oxide/glass


Trained Users

Dry Etching Methods

Advantages of Dry Etching

Wet etching is an estalished method for microstructuring that provides fast results. However, if you want to have

  • an anisotropic etch profile
  • low mask undercut
  • no trouble with stiction due to capillary forces
  • high aspect ratio structures (esp. silicon),

then dry etching might be your method of choice. Of course, many materials can also be structured with isotropic profiles.


Functioning Principles

Reactive Ion Beam Etching (R)IBE

File:RIBE.png
Fig. 1: Schematic of a reactive ion beam etching chamber

Argon gas is let into the plasma chamber, where it is ionized. This can be done by accelerating electrons emitted from the cathode towards the anode, where they will start a collision chain reaction that ends with the loss of electrons of the argon atoms.

The extraction grid has a positive voltage applied in order to accelerate the argon ions towards the sample. Before they reach the sample, the neutralizer adds electrons to the ions which results in a beam which has neutral charge on average.

The accelerated ions/atoms attack the surface of the sample only with their kinetic force, no chemistry is involved. A chemical component can be added by letting additional gasses through valves in the sample holder. These will be ionized and accelerated by collisions with the argon atoms.

The sample stage can be tilted face down to an angle of 70°. Usually, the angle is 10-30°, and the sample plate rotates to prevent redeposition (see Fig. 2).

File:RIBE Etch Redeposition.png
Fig. 2: Redeposition during RIBE. Substrate residue removed from the bottom of the etch profile is more likely to redeposit on the profile's sidewalls with an increase of the aspect ratio h/w


Advantages and Disadvantages of RIBE

The main advantage of RIBE is that it can be used to structure virtually everything. The physical attack does not disciminate largely between materials, and the plasma chamber is separated from the etch chamber, thus preventing any contamination of the plasma with sample residue.

Disadvantages are low etch rates as well as low selectivity between the resist and the sample material. Because of the selectivity < 1 and redeposition, the maximum aspect ratio is 1:1.

Reactive Ion Etching

File:RIE ICP Chamber.png
Fig. 3: Schematic of a reactive ion etching chamber

A mixture of reactive gasses is let into the chamber, where a plasma is created by applying an AC power source between the wafer stage and another plate. As this is essentially a capacitator, this setup creates a so called capacitive coupled plasma (CCP). The capacitive coupling also accelerates the ions away from and towards the wafer, permitting some of the ions to collide with it and therby etching the wafer material.

After a plasma has been ignited, an AC powered coil can be used to increase the amount of ionization by the method of induction. This is termed the inductive coupled plasma (ICP).

While CCP controls both the physical attack mode by accelerating atoms as well as the chemical attack mode by creating ions, the ICP provides a separate control parameter for ionization only. Together with the chamber pressure, which controls the mean free path of the ions, a fine control of physical and chemical contribution to the etch process is possible.

Additionally, the sample holder can be cooled with helium and liquid nitrogen for temperatures down to -150 °C, which allows the use of the silicon cryo process to create high aspect ratio structures.

The silicon cryo process

TBA

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