Author: Dr. Dilek Isik Akcakaya

TMAH Etch Depth, Top- and Bottom-width Calculation
Etch rate of SiO2 hard mask in TMAH in a temperature range between 65- 95 °C and at different TMAH concentrations between 2-25%(Figure adapted from Ref [6]).  


TMAH Etch Depth, Top- and Bottom-width Calculation:

To obtain the etch rates at different temperatures, etch rates are provided for a 10% (no stirring) TMAH Tank

Etch Rate 60 oC: 11.5 µm/hr

Etch Rate 90 oC: 50 µm/hr

The following formula is useful to determine feature sizes of the final etch, or what is required when designing a mask to achieve a final feature size (for <100> orientation):


WTop = WBot + 2 x h x cot 54.7

The bottom of the cavity can be calculated using:

  • WTop = size of feature on mask
  • WBot = desired feature size at the bottom
  • h = bulk Si etch depth
  • Cot 54.7 = 0.708039 

For the calculation of etch times

Example Calculation top width, bottom width, etch depth for silicon wafer TMAH etch (Figure 1): 

WTop = 24 + 2 x 70 x 0.708039 = ≈124 µm

Time (60 °C) = 70/11.5 = 6 hr

Time (90 °C) = 70/50 = 1.4 hr (results in higher roughness)

Patterning of Si Wafers

Anisotropic wet chemical etching is used in the patterning of Si wafers. This process allows the creation of deep 3D MEMS structures (Figure 1). Silicon wet chemical etching can be performed using Ethylene Diamine Pycocatechol (EDP), Potassium Hydroxide (KOH) [1], [2], and Tetramethylammonium Hydroxide (TMAH). Tetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt commonly used in the anisotropic etching of Silicon wafers. 

Tuning the percent etchant, etch temperature, and additives the etch rate of crystal planes, and surface roughness can be controlled, precisely. For example, Triton [3] and Isopropyl alcohol (IPA) are common additives used with TMAH solutions, and they have been reported to be used in the anisotropic etching of silicon wafers. IPA additive provides very low surface roughness, as low as Ra = 1 nm, and using Triton-less convex-corner undercutting was achieved [4]. 

TMAH Silicon Wafer Etch
Figure 1 Illustration of anisotropic wet chemical etching of Si (100) wafers with the sidewalls being <111>. Scanning electron micrographs of approximately 124 μm wide and 70 μm deep cavities etched using TMAH anisotropic wet etch.

Bond strengths of neighboring Silicon atoms

The anisotropic wet chemical etch rate of silicon is controlled by the bond strengths of neighboring Silicon atoms. {100}> {110}> {311}> {221} planes are etched faster than {111} planes[3]. {111} planes are therefore the limiting planes in the wet chemical etching of Silicon. 

During TMAH etching of Silicon wafers sidewalls of the etch follow the <111> planes, the <100> plane is etched at a 54.7o angle relative to the <111> planes. In a (100) wafer with <110>, flat anisotropic etching will yield inverted pyramid shapes etched into the Si wafer while a (110) wafer will result in perpendicular trenches. 

An interesting approach for the use of (110) wafers is the Michelangelo step, which makes use of the 35.26° <111> planes to remove the scallops created during the DRIE Bosch process [5]. 

scanning electron micrographs fabricated
Figure 2 Scanning electron micrographs of 5 μm wide and 40 μm deep holes fabricated without (a) and with (b) the Michelangelo step [5]. 

TMAH etch rate is dependent on variables such as: 

  • Si dopant type
  • Temperature 
  • Time 
  • Bath Chemistry (Etchant, additives)
  • Stirring, Non-Stirring bath

The effect of the type of silicon substrate (n- or p-type), solution temperature, and concentration was experimentally studied by researchers (Figure 3). The silicon wet chemical etch rate increases with the solution temperature, decreases with the TMAH concentration for concentrations larger than 8 wt.% [6]. 

The effect of dopant type at 70, 80, and 90 °C`s is also shown in Figure 3 [6]. The dopant type does not have a large impact on the etch rate (Figure 3, Bottom). 

Etch rate Si (100) surface vs TMAH percent concentration dependency
Figure 3 (left) Etch rate Si (100) surface vs TMAH percent concentration dependency studied by different groups.
(right) Effect of dopant type to the etch rate at different TMAH concentrations. (Figure adapted from Ref [6]) 

Patterning and Etching a Silicon Wafer Using Anisotropic Wet Chemical Etching

Silicon wafers can be patterned with wet chemical anisotropic etching. For this purpose, a process design and flow have been created as follows (Figure 4):

process flow for TMAH or KOH etching
Figure 4 Process Flow for TMAH or KOH anisotropic wet chemical etching of Silicon Wafers. 

Silicon Wafer Surface Cleaning Procedures:

MEMS manufacturing comes with stringent requirements on Silicon Wafer surface cleaning procedures. All wafers are cleaned from organics, metals, and particles before thin film growth/ deposition, or other process steps. The dimensions of microstructures require a clean surface for precision. 

Thermal Oxide Hard Masks:

Wet or Dry thermal oxides, provided by Rogue Valley Microdevices are high quality and possess a high selectivity in TMAH baths allowing for hours long etching of the Si wafers. For patterning purposes, oxide masks are used. Thermal oxidation provides the growth of oxide film on both sides of the Silicon wafer at once, saving time and money. 

Lithography for Oxide Patterning on Silicon Wafers: 

The pattern to be transferred to the Silicon wafers is first created using basic lithography methods. A photoresist is coated onto the wafer and is exposed using a photomask, later the exposed photoresist is developed to reveal the transferred pattern. At Rogue Valley Microdevices, photoresists are either spin-coated or spray-coated depending on the requirements of the process.

Also, a variety of high contrast photoresists allows for the creation of deep, shallow, high, large, small features with ease. Spray coating is usually preferred for already patterned wafers to provide a conformal coating of the edges, corners. 

Oxide Film Etch:

Silicon dioxide (wet thermal oxide, dry thermal oxide, PECVD) can be patterned using dry or wet chemical methods. At Rogue Valley Microdevices, both RIE – Reactive Ion Etching and BOE – Buffered Oxide Etchant are used to pattern the hard oxide masks for TMAH / KOH anisotropic etch.

Anisotropic Wet Chemical Etching of Silicon:

After oxide layer pattern etch, the oxide layer acts as a highly selective hard mask for Si etching (Figure. Both backside and frontside of the wafer where the etch is not desired are protected with the SiO2 hard mask.

silicon wafers

Silicon Wafer Inquiries

For Silicon Wafer inquiries, search the Rogue Valley Microdevices Silicon Wafer Inventory for Silicon Wafer Diameters, Wafer Grades, Wafer Types, Dopants, Orientation, Wafer thickness, Surface Polish, Wafer Resistivity, Wafer TTV (please specify), Bow/Warp (please specify).

For custom requirements or questions regarding our process capabilities for the anisotropic wet chemical etching of Silicon Wafers, please contact us

References

1. H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgärtel, “Anisotropic Etching of Crystalline Silicon in Alkaline Solutions: I . Orientation Dependence and Behavior of Passivation Layers,” J. Electrochem. Soc., vol. 137, no. 11, pp. 3612–3626, Nov. 1990, doi: 10.1149/1.2086277.

2. U. Abidin, B. Majlis, and J. Yunas, “Fabrication of pyramidal cavity structure with micron-sized tip using anisotropic KOH etching of silicon (100),” J. Teknol., vol. 74, pp. 137–148, Jun. 2015, doi: 10.11113/jt.v74.4846.

3.K. Rola, “Anisotropic etching of silicon in KOH + Triton X-100 for 45° micromirror applications,” Microsyst. Technol., vol. 23, May 2017, doi: 10.1007/s00542-016-3103-0.

4. “Silicon anisotropic etching in Triton‐mixed and isopropyl alcohol‐mixed tetramethylammonium hydroxide solution”, doi: 10.1049/mnl.2015.0104.

5. S. Frasca et al., “The Michelangelo step: removing scalloping and tapering effects in high aspect ratio through silicon vias,” Sci. Rep., vol. 11, no. 1, p. 3997, Dec. 2021, doi: 10.1038/s41598-021-83546-w.

6. P.-H. Chen, H.-Y. Peng, C.-M. Hsieh, and M. K. Chyu, “The characteristic behavior of TMAH water solution for anisotropic etching on both Silicon substrate and SiO2 layer,” Sens. Actuators Phys., vol. 93, no. 2, pp. 132–137, Sep. 2001, doi: 10.1016/S0924-4247(01)00639-2.