Wet etching

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Wet etching
Cr etch photo.jpg
Technology Details
Other Names Wet Etch
Technology Wet etching
Equipment List of Wet Benches

Wet etching is the process of removing a material chemically with a liquid reactant. It may involve a chemical which dissolves the material to be etched, or may utilize a chemical mixture which first oxidizes the material and then dissolves the oxide.

Method of operation

Chemical etching is performed by immersing the sample in a container of the etchant mixture. Chemical etching is always performed in one of the wet benches in the lab. A variety of teflon sample holders and glass and plastic lab-ware is available at each wet bench.

Depending on sample size, type of etchant, and whether the etch is commonly performed in the LNF, etches may be done in a staff-maintained tank or may need to be mixed up each time by the researcher. Commercial etchant mixtures exist for a number of materials. The LNF stocks commercial Cr, Au and Al etchants, and buffered HF. Please check the specific wet bench and process pages for specific instructions for doing an etch in the LNF. Additional information on etch rates and selectivity can be found in the references cited below. If you need to etch a material for which the LNF does not have an established etch process, consult with LNF staff.

Examples of processing applications

In micro and nano-fabrication, the wet etching processes have many possible applications. In semiconductor device fabrication, some commonly used etchants are hydrofluoric acid for etching silicon dioxide, phosphoric acid for etching silicon nitride, and hydrogen peroxide-ammonia for etching GaAs. Wet etching can be used to completely remove a thin film (blanket etch), or, if samples are masked with a material that is not attacked by the etchant, it can be used to etch a pattern into a material.

While most chemical etching processes are isotropic, some etchants preferentially etch along certain crystal planes, e.g. KOH etching of silicon. These anisotropic etches can be used to create structures with angled geometries. Other materials will etch laterally faster than they etch vertically. Since wet etching is usually isotropic, meaning that it will etch laterally as much as vertically, it is useful for patterning larger features but cannot be used for small features or for features requiring straight side walls.

Figures of Merit

These are considerations when selecting a particular etch chemistry-

  • Etch rate in comparison with thickness of layer to be removed and feature size: If the etch rate is slow, and a thick layer needs to be etched away, the total etch time may be impractically long. If the etch rate is fast in comparison with the amount of material to be etched, uniformity and control may be poor. If a blanket etch is being done, a fast etch rate may be acceptable, but if a patterned etch is being done and the etchant is isotropic, then undercut, or the amount etched laterally beneath the mask layer, may be a problem. The aspect ratio of the feature to be etched should also be considered. If lateral dimensions are large compared with etch depth, then undercut is less of a problem. If lateral feature sizes are smaller than or comparable to etch depth, then undercut may be unacceptable.
  • Selectivity to the mask layer: The etchant and masking material should be chosen such that the masking layer can withstand the etchant for the duration of the etch. An etchant which significantly attacks the masking material will not work. In addition, feature size and how well the mask layer adheres can also affect the success of an etch. For instance, if the pattern to be etched has a lot of open area, and regions of photoresist between the open areas are small, then the photoresist may come off more easily during the etch than if large areas of the sample are covered. Ensuring good mask adhesion, by preparing the sample surface prior to spinning photoresist, is important when planning to wet etch.
  • Selectivity to underlying layers: The etchant should be chosen such that it does not attack other exposed materials on the sample. Ideally, the material underneath the layer being etched is not attacked at all, and acts as an etch stop.

Parameters

These parameters can affect wet etches-

  • Temperature: Etch rate is strongly temperature dependent. The chemical reactions controlling the etch typically follow an Arrhenius temperature dependence. Higher temperatures will result in faster etch rates. Therefore, for consistent etch rate, temperature must be held constant. Some etchants (e.g. phosphoric acid for etching silicon nitride) must be heated to give a sufficiently high etch rate.
  • Age/saturation of etchant bath: Etch rate can be affected by how long the etchant bath has been sitting, and/or how much material has been etched in the bath.
  • Chemical concentration: Etch rate and other properties of the etchant depend very strongly on the ratios and concentrations of the chemicals in the etchant. Therefore, it is important to measure chemicals accurately when mixing up an etchant.
  • Transport at sample surface: If etchant at the surface becomes saturated, and fresh etchant cannot reach the surface, then etching will slow down. Agitation can be used to bring fresh etchant to the surface and promote etching. Thus, for better run-to-run consistency, one should be consistent as to whether samples are agitated or not. Agitation can also improve uniformity.
  • Wetting of the mask layer: Hydrophobicity of the photoresist mask may inhibit wetting, and this can prevent etchant from getting to the surface of the sample. This can be a problem particularly when etching very small, deep features.

Because etch rate depends on many factors and can vary significantly, if the etch depth needs to be controlled precisely, then it is useful to measure etch rate and uniformity on a monitor wafer before etching actual samples.

Materials

The LNF has established methods for wet etching a wide variety of materials.

Processes

Equipment

Below is a general description of the wet etching equipment available in the LNF.

PFC Bench 01

Main article: PFC Bench 01

Cleaning and etching to prepare silicon for a furnace run.

Acid Bench 02

Main article: Acid Bench 02

CMOS-clean bench for cleaning and etching of furnace-compatible materials.

Acid Bench 12

Main article: Acid Bench 12

Acid Bench 12 is a Semiclean tool and is for etching some metals on 4" and 6" silicon and glass wafers.

Mask Bench 13

Main article: Mask Bench 13

This bench is strictly for etching, stripping, and cleaning of mask plates.

Acid Bench 23

Main article: Acid Bench 23

Acid bench 23 is a restricted materials bench for pieces. This is limited to III-V substrates, silicon and glass samples.

Acid Bench 72

Main article: Acid Bench 72

For processing sample sizes from small pieces to 6" wafers. An extensive list of materials can be processed in this bench.

Acid Bench 73

Main article: Acid Bench 73

For processing sample sizes from small pieces to 6" wafers. An extensive list of materials can be processed in this bench.

Acid Bench 82

Main article: Acid Bench 82

For processing sample sizes from small pieces to 6" wafers. An extensive list of materials can be processed in this bench.

Base Bench 91

Main article: Base Bench 91

KOH and TMAH etching are available in this bench.

Acid Bench 92

Main article: Acid Bench 92

General acid etchants and HF-Nitric etching are allowed in this bench.

EDP Bench 93

Main article: EDP Bench 93

Dedicated bench for EDP etching of silicon with a boron etch stop.

Further reading

The following sources are good references for a wide variety of different etches. If you are interested in doing any etch that is not currently supported by the LNF, consult with staff. Even if the LNF stocks the chemicals needed for an etch process, the process must first be authorized by staff before it can be performed.