Micro structuring

To realize functional elements such as electrodes and conductive paths, bridges, ditches, or holes, membranes and other geometries we have various patterning methods for different materials. By a photo lithographic step, a protective layer (mask) is opened locally. The following micro structuring takes place only at the exposed areas.

Wet etching of metals:

By wet etching of metals the material is removed by a suitable etching solution. The structuring takes place isotropically, i.e. simultaneously in all directions.

Typical properties:

  • layer thickness that can be etched: a few nm – some 10 µm
  • Smallest feature: ~ 1µm
  • High selectivity against different materials

Metals that can be etched:

Al, Cr, Cu, Fe, Ni, Ti ...

Not etchable metals can be structured by lift-off instead.

patterned Chromium layer on glass
patterned Chromium layer on glass

Wet etching of Silicon:

Mono crystalline silicon can be etched anisotropically in suitable solutions.The etching in different directions of the crystal takes place at different speeds. It is typically used to expose the (111) crystal plane which is 54.7° inclined to the surface of (100) wafers.

Typical properties:

  • Etching depths: a few 10 nm – about 1mm
  • Smallest feature: some 100 nm
  • High selectivity against silicon dioxide, silicon nitride, silicon carbide and some metals

anisotropically etched pattern in silicon
anisotropically etched pattern in silicon

Wet etching of silicon dioxide:

Silicon dioxide as an insulator, optical transparent layer or masking layer, can also be etched wet chemically.

Typical properties:

  • Film thickness: a few 10 nm – some µm
  • Smallest feature: ~ 1 µm
  • High etch selectivity with respect to silicon and some metals

wet chemically etched silicon oxide
wet chemically etched silicon oxide

Reactive ion etching (RIE):

Reactive ion etching, or dry chemical etching, uses ionized gas molecules at low pressure. The resulting reactive ions turn into a chemical compound with the material that shall be ablated and convert these into the gas phase. By accelerating the ions to the substrate, also an anisotropic material removal can be achieved.

Typical properties:

  • Etching depth: a few nm – some µm
  • Smallest feature: a few 100 nm

Materials that can be patterned:

  • Silicon
  • Silicon dioxide
  • Silicon nitride
  • Silicon carbide

RIE of silicon nitride and silicon oxide
3 layers etched by RIE

Locally oxidized silicon (LOCOS):

Using the LOCOS method, only local areas of the silicon are oxidized. For this, first a silicon nitride layer is structured on a silicon wafer to expose the areas to be oxidized. In the next step, the exposed silicon surfaces is converted into silicon dioxide by thermal oxidation.
With LOCOS electrical insulating areas can be realized between conductive silicon regions. Also optically transparent areas in silicon membranes, which are nevertheless Gas and Liquid impermeable can be realized with this method.

Typical properties:

  • Smallest feature: ~ 1 µm
  • Thickness: a few 10nm – a few µm

Local oxidized silicon: Mono crystalline silicon is embedded in oxidized silicon
Local oxidized silicon:
Mono crystalline silicon is embedded
in oxidized silicon

Deep reactive ion etching of silicon (DEEP RIE):

Dry chemical deep etching of silicon uses a specially optimized etching process. Etch rates of a few microns / min into silicon are possible. As a result, silicon wafers can be etched through completely.

Typical properties:

  • Etch depth: a few 10 nm – 1 mm
  • Smallest feature: a few 100 nm

deep echted vertical trenches in silicon
deep echted vertical trenches in silicon
(cross sectional view)