What is Anisotropic Wet Etching?

High-Density Pyramidal Texturing on Silicon Wafers: Optimizing Photovoltaic Output

A senior scientist requested a quote for the following.

I was wondering if you had silicon PV substrates with the highly textured surface used for enhancing performance.  They’re anisotropically etched to give a high density of pyramidal structures.  

UniversityWafer, Inc. Answered:

Prior to being polished, most Silicon wafers are alkaline etched. If they are one-side-polished then the back-side remains alkaline etched. The alkaline etch, is a anisotropic etch. It does result in a pattern of pyramids about 50 um high. The pyramids have characteristically square basis in case of [100] orientation wafers, and characteristically triangular basis in case of [111] wafers.

We have in stock a wide variety of wafers with one Alkaline etched side. And we can prepare for you wafers with both sides Alkaline etched.

For PV work one uses wafer that are 0.5 to 3.0 Ohmcm. Here are some such wafers that we have in stock:

Reference #116027  for specs and pricing.

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Substrates Used For Anisotropic Etching

A graduate research assistant requested a quote for the following.

We need 100 nm of PECVD or LPCVD silicon nitride on one side for use as Si etch mask, or stock equivalent suitable for anisotropic etching.

Reference #125657 for specs and pricing.

Quartz Wafers for Precise Anisotropic Etch

A principal investigator requested a quote for the following.

I'm looking into purchasing some 4" quartz wafers and have a couple of questions. One of my fabrication steps involves a fairly precise anisotropic etch. One of the 4" quartz wafers listed in the store (ID 1210) has a tolerance on the orientation; the others do not. Are the other items substantially less controlled? Do the different items come from different wafer manufacturers?

Reference #136454 for specs and pricing.

(112) SOI Wafers for Anisotropic KOH Etching and Sawtooth Pattern Fabrication

A graduate research assistant requested a quote for the following.

Would it be possible to get a SOI wafer with the (unusual) crystal orientation of {112}? I have a specific etch profile in mind. I want to perform a specific anisotropic KOH etch to create a specific structure (a sawtooth pattern). I included the paper I got the pattern from.

See Fig. 2d. I also need there to be an SiO2 layer underneath for the optical effect I want to test. I've seen {112} Si wafers for sale, but none for SOI. As far as the top Silicon wafer depth I'm flexible, at least for the initial test. A top layer thickness of ~1um would be good, as well as ~1um of oxide, but again these are flexible depending on what is available.

Reference #151221 for specs and pricing.

Prime vs. Test Grade 2″ & 4" Silicon Wafers <100> Wafers: Anisotropic Etching and Native Oxide Removal

A graduate research assistant requested a quote for the following.

The Cr adhesion is actually easier for us to process so lets go that way. As for quantity, can you give me a formal quote for 9 of the 100mm (4") sized prime grade silicon wafers. 

Would those wafers be appropriate for anisotropic etching in KOH).  Also is the sputter clean long enough to completely remove the native oxide?  Or put another way, can you sputter clean them long enough to completely remove the native oxide, because that would be one less thing we need to worry about.

Reference #155128 for specs and pricing.

Silicon Used for Anisotropic Wet Etching

Wet etching is an important step in semiconductor manufacturing and microfluidic micromachines that require microscale properties to optimize performance and generate laminar flow systems that are close - impossible to obtain on a macro scale.

Researchers have used the following wafer spec.

Si Item #2357
100mm P/B <110> 1-10 ohm-cm 500um SSP Prime Grade

Anisotropic Wet Etching

Wet etching is an important step in semiconductor manufacturing and microfluidic micromachines that require microscale properties to optimize performance and generate laminar flow systems that are close - impossible to obtain on a macro scale. This article focuses on a method that can deliver high etching rates by using a process that uses a series of separate etching and deposition steps. This is in contrast to conventional etching processes, which use a purely alkaline solution. In addition, the method is required for high-quality, cost-effective microscale features that can only be achieved through separate etching, separation and processing steps. [Sources: 3, 4, 6]
   
Depending on the task, wet etching can be connected to a number of wet erasers, including a movable shield to protect workers from etching splashes. [Sources: 0]
   
If the etching is used to create a cavity in the material, the depth of the cavity can be controlled by etching time and known etching rate. The size of this compensation geometry depends heavily on the etching depth and its characteristic etching. If there is a high undercut rate in a floating structure, then the high etching rate is sufficient to form cavity grooves that are less than half the etching time and vice versa. [Sources: 3, 7, 8]
   
Finally, we propose a wave-conductor lattice coupler that can be realized with anisotropic wet etching technology. Figure 2 outlines the manufacturing steps used in the realization of the mesh couplings using the isotropic dry etching process. [Sources: 5]
   
The etching age is significant because it affects the roughness and morphology of the etching surface as well as the etching rate shown in the previous section. In sharp edge etching, the convex edges are protected by a mask layer with a very thin discontinuous photoresist coating, which does not protect the desired area of the copper from ion bombardment. However, the roughness of the wall is significantly reduced during the wet etching process and is comparable to the roughness of the surfaces and surfaces. [Sources: 0, 3, 5, 7]
   
Etching takes place in the plane of the silicon crystal, and the etching rate in this direction means that the direction is etched perpendicular to the edge. The etching rate depends on the concentration of the etching agent and its solution temperature, the choice of etching type used, as well as the temperature and direction of the surface roughness and surface morphology. In the case of anisotropic wet etching, it strongly influences the etching rate at a fixed etching temperature, as the chemical activity of the etching solution is induced by the additives and echantics. [Sources: 2, 3, 7]
   
This effect can allow a very high anisotropic etching, where the 111 planes act as stop-etching. By applying a laminar flow limit on both sides of the desired etched channel, a certain layer is protected from etching. Due to the ability to change the Ori of chemicals and silicon crystals, many different channel shapes can be etched simultaneously by changing their Ori. [Sources: 5, 6]
   
The AP is most effective when the high etching rate is achieved on smooth etching surfaces with exposed Al. Some additives can be modified, while others alter one or two of the etched properties. [Sources: 3]
   
Since wet etching is a purely chemical process, waveguides are produced with very little scattering loss, while the waveguides "side walls are determined by the silicon crystal plane, which ideally has no irregularities at the atomic level. Although we stress here that the microstructure used in wet anisotropic etching can be determined by taking into account the lateral undercutting angle of the etched side walls, the surface of a silicon wafer mask may not be completely determined. This determines the correct etching direction and the ability to control the etching rate. The silicon wafer mask defines where the etching can take place, as well as the size and shape of each layer. [Sources: 2, 5, 7]
   
The use of the terms anisotropy and plasma etching should not be confused when it comes to Ori - dependent etchings. Etchings that occur at different speeds and in different directions are either isotropic (i.e. they may have a lateral undercutting rate when etched downwards) or isotropic (depending on the Ori of a substrate in the crystalline plane). Plasma etches can be either isotropic and have laterally below average rates at their downward velocity, or both and have aniisotropic properties. If the Ori of the substrate does not influence how the etching agent is removed from the material, there is no difference between the two etching types, which are the same in both directions, which is the case with wet and ionic etchers. [Sources: 6, 8]
   
However, an increase in the concentration of IPA in the etching solution increases the rate of the etching reaction, which in turn leads to an anisotropic effect. Since isotropic etching is difficult to control precisely and hydrofluoric acid difficult to handle, it is no faster than anisotropic etching. [Sources: 1, 2]
   
The loss of propagation (tm) of polarization is 1.08 dB / cm, which suggests that the proposed wet wetting process can be used to produce polarization structures - insensitive waveguide structures. A manufacturing process based on anisotropic wet etching technology is intended to achieve low propagation losses and at the same time maintain the insensitivity of the polarization. Wet etching, in which a chemical is used to remove a layer of the wafer, becomes bedridden, but the ability of the etching system to do so depends on the amount of hydrofluoric acid in the solution and the number of layers used. [Sources: 5, 6, 8]
   

Sources:

[0]: https://resources.pcb.cadence.com/blog/2020-isotropic-etching-to-anisotropic-etching-and-semiconductor-manufacturing

[1]: https://aip.scitation.org/doi/full/10.1063/1.5012125

[2]: https://www.modutek.com/isotropic-and-anisotropic-silicon-wet-etching-processes/

[3]: https://mnsl-journal.springeropen.com/articles/10.1186/s40486-021-00129-0

[4]: https://www.google.com/patents/US20040018734

[5]: https://www.frontiersin.org/articles/10.3389/fmats.2016.00010/full

[6]: https://openwetware.org/wiki/Wet_Etching_-_Eric_Ying

[7]: https://mnsl-journal.springeropen.com/articles/10.1186/s40486-015-0012-4

[8]: https://en.wikipedia.org/wiki/Etching_(microfabrication)