TMAH vs. KOH Silicon Etch for MEMS

TMAH and KOH are the two primary anisotropic wet etchants used to bulk micromachine crystalline silicon during MEMS fabrication, sensor manufacturing, and semiconductor wafer processing. In MEMS, anisotropic silicon etching shapes the membranes, cavities, V-grooves, and bulk micromachined structures at the heart of many devices. Both are hot alkaline solutions that etch silicon along its crystal planes, removing material from the (100) and (110) planes far faster than from the (111) planes. That selectivity produces the characteristic angled sidewalls and shapes used for membranes, cavities, and V-grooves. The two etchants reach the same geometry, but they differ in etch rate, anisotropy, surface finish, mask and metal compatibility, and CMOS compatibility, which determine where each one belongs within a MEMS fabrication flow.

Choosing the right silicon etchant is a practical decision that affects masking strategy, metal and device compatibility, etch time, and surface quality. This guide explains how anisotropic silicon etching works, compares the two chemistries, and provides practical guidance for selecting the appropriate etchant for MEMS and related microfabrication applications.

Same Geometry, Two Chemistries

Both etchants are anisotropic alkaline chemistries, so on (100) silicon they both create sidewalls along the (111) planes at the characteristic angle of 54.74 degrees, forming V-grooves, pyramidal pits, trapezoidal cavities, and thin membranes. The difference lies in the chemistry. KOH is an alkali metal hydroxide that carries potassium, while TMAH is a purely organic quaternary ammonium hydroxide that contains no metal ions. That single distinction drives the most important practical difference between them, which is compatibility with CMOS devices.

How Anisotropic Silicon Etching Works

Anisotropic wet etching relies on the crystal structure of silicon. The densely packed (111) planes etch far more slowly than the (100) and (110) planes, so the etch front stalls on the (111) planes and exposes them as smooth, well defined sidewalls. On a (100) wafer this produces sidewalls inclined at 54.74 degrees, which yields V-grooves, pyramidal pits, and trapezoidal cavities depending on the mask opening, and thin suspended membranes when the etch is stopped at a defined depth.

Etch depth is controlled by time and temperature or by an etch stop. Heavily boron doped silicon etches extremely slowly and serves as an etch stop, as does the buried oxide of a silicon on insulator (SOI) wafer, and an electrochemical etch stop can be used to define precise membrane thicknesses. Both etchants are typically masked with a patterned hard mask, with silicon nitride and silicon dioxide the common choices.

How KOH Etching Works

KOH, or potassium hydroxide, is the most established anisotropic silicon etchant. It is usually run at roughly 20 to 45 percent concentration and 70 to 80 degrees Celsius, where it provides the highest silicon etch rate, the highest anisotropy, and the smoothest etched surfaces, all with a simple, economical setup. These qualities make KOH an excellent choice for fast, deep bulk micromachining of V-grooves, cavities, and membranes.

KOH has two important limitations. It etches silicon dioxide relatively quickly, so a thin oxide is not a durable mask for long etches and silicon nitride is preferred. More significantly, KOH contains potassium, an alkali metal ion that can contaminate gate oxides and degrade MOS devices, so KOH is not CMOS compatible and is kept away from CMOS wafers and shared CMOS lines. KOH also attacks exposed aluminum quickly.

How TMAH Etching Works

TMAH, or tetramethylammonium hydroxide, is a purely organic etchant that contains no metal ions. It is typically run at roughly 20 to 25 percent concentration and 80 to 90 degrees Celsius. Because it introduces no mobile metal contamination, TMAH is CMOS compatible and can be used on wafers that carry active devices. It also has very high selectivity to silicon dioxide, so a patterned oxide can serve as the etch mask, and it works well with silicon nitride masks too.

TMAH etches silicon more slowly than KOH and gives slightly lower anisotropy, and its etched surface can develop hillocks and roughness unless the concentration and additives are optimized. A key advantage is metal compatibility: by dissolving silicon and adding a suitable buffering agent, TMAH can be tuned so that exposed aluminum and other metals are essentially untouched during the etch, which makes it well suited to post-CMOS micromachining. Because TMAH is sensitive to native oxide, a brief dip to remove it is common before etching.

How the Two Etchants Differ

Etch Geometry and Etch Stops

Because both etchants follow the same crystal planes, they reach the same fundamental geometry: 54.74 degree (111) sidewalls on (100) silicon, forming V-grooves, trapezoidal cavities, and pyramidal pits. Precise depths and membranes are defined with etch stops. A heavily boron doped layer, the buried oxide of an SOI wafer, and electrochemical etch stops all allow engineers to set membrane thickness reliably. The choice between KOH and TMAH does not change the shape; it changes the etch rate, the mask, and the compatibility with surrounding materials.

CMOS and Post-Processing Compatibility

Compatibility with active devices is often the deciding factor. When silicon must be micromachined on a wafer that already carries CMOS circuitry, or anywhere mobile metal ion contamination cannot be tolerated, TMAH is the appropriate choice because it introduces no metal ions and can be buffered to leave aluminum bond pads and interconnects intact. KOH, with its potassium content and aggressive attack on aluminum, is reserved for dedicated micromachining where no CMOS devices or exposed metals are present.

Etch Masks and How They Affect the Choice

Both KOH and TMAH are hot alkaline baths, so photoresist alone cannot survive the etch and a patterned hard mask is required. The two practical choices are silicon nitride and silicon dioxide, and which one works well depends heavily on the etchant. That makes the mask a real factor in choosing between KOH and TMAH, not just an afterthought.

The mask interacts with the process choice in several important ways. Because KOH etches silicon dioxide relatively quickly, a long or deep KOH etch usually calls for a silicon nitride mask, which means an LPCVD nitride deposition at high temperature. TMAH, by contrast, has very high selectivity to silicon dioxide, so an oxide already on the wafer can serve as the mask with no added nitride step. It is also worth budgeting for mask loss: in KOH an oxide mask is gradually consumed and must be thick enough to survive the full etch, whereas in TMAH the same oxide is barely touched.

This reinforces the compatibility picture above. The same wafers that rule out KOH for contamination reasons, those already carrying CMOS circuitry or completed metal, often also cannot tolerate a high temperature LPCVD nitride deposition, which leaves an existing oxide as the practical mask. Because TMAH treats that oxide as a durable mask while KOH consumes it, the masking requirement points the same way as device compatibility: toward TMAH for post-metal and CMOS integrated flows. When nitride masking can be added early, before metal, either etchant works and the decision returns to etch rate, surface finish, and cost.

Process Integration

Both etchants fit naturally into MEMS flows alongside masking, deposition, and dry etch steps. A typical flow deposits and patterns a nitride or oxide mask, performs the anisotropic etch to form membranes, cavities, or grooves, and may combine the result with deep reactive ion etching where vertical sidewalls are needed. Selecting the etchant by device compatibility, etch rate, and mask strategy lets engineers match the process to the wafer rather than forcing a single chemistry across every design.

TMAH vs. KOH Silicon Etch at a Glance

Rogue Valley Microdevices Silicon Etch Options

• Anisotropic wet silicon etch in both KOH and TMAH, run on a dedicated wet etch bench in a class 100 cleanroom.
• Available on 100mm, 150mm, and 200mm substrates.
• LPCVD silicon nitride and thermal oxide masks, matched to the chosen etchant.
• Front to back alignment for membranes, cavities, and through wafer structures.
• TMAH for CMOS compatible and post-metal micromachining; KOH for fast, smooth, economical bulk micromachining.
• Combined with dry etch, including plasma etch and DRIE, where vertical sidewalls are required.

Typical Applications

Application Examples

Choosing the Right Etchant

Choose TMAH when the wafer carries CMOS devices or must stay free of metal ion contamination, when exposed aluminum or other metals are present during the etch, or when a silicon dioxide mask is preferred. Choose KOH when the priority is the fastest etch rate, the smoothest surfaces, and the lowest cost, and when no CMOS devices or exposed metals are present.

A Simple Selection Workflow

1. Does the wafer carry CMOS devices or require freedom from metal ion contamination? Choose TMAH.
2. Is exposed aluminum or other metal present during the etch? Choose buffered TMAH.
3. Do you need the fastest etch rate, smoothest surfaces, and lowest cost? Choose KOH.
4. Do you want to use a silicon dioxide mask? TMAH offers the higher oxide selectivity.
5. For the most robust mask in a long KOH etch, use LPCVD silicon nitride.
6. Both etchants reach the same (111) geometry, so the choice is usually driven by compatibility, rate, and masking rather than shape.

Frequently Asked Questions