MEMS

What is MEMS?

MEMS is an abbreviation for Micro Electro Mechanical Systems, a general term for systems featuring a minute 3-dimensional structure for handling various input/output signals.
These are high value added devices that integrate micro-mechanical components, electrical circuits, sensors, and actuators on a single substrate utilizing microfabrication technology.

MEMS Process

The MEMS process is based on a general semiconductor process flow that includes film deposition, photolithography, and etching.

[Basic MEMS Process]

Below are some of the key technologies used in the MEMS process.

Wafers

SOI Wafers

SOI, short for Silicon On Insulator, is a silicon wafer in which a single silicon crystal layer is formed on an oxide film.This technology is widely adopted in power devices and MEMS. In MEMS the oxide layer can be used as a stopper layer for silicon etching, making it possible to form complex 3-dimensional structures.

[SOI Wafer]

TAIKO Grinding (TAIKO is a a trademark of Disco Corp.)

TAIKO grinding is a wafer grinding process developed by Disco Corp. that grinds only the inner portion, leaving the outer edge of the wafer intact.

[TAIKO Grinding] / [TAIKO Ground Wafer]

TAIKO grinding offers several advantages over the normal method, including less wafer warpage, greater wafer strength, easier handling, and improved consistency with other processes.

Wafer Attaching / Thermal Release Sheet Process

Support wafers and thermal release sheets facilitate handling of thin wafers.

[Wafer Attaching Example (Thin Wafer Processing)]

Wafer Bonding

Wafer bonding can broadly be divided into 2 types, direct and intermediate layer bonding.

[Wafer Bonding]
[Bonding Using an Intermediate Layer]

Direct bonding is performed, for example when creating SOI wafers by bonding wafers to each other using intermolecular force through heat treatment without the use of adhesives.
Intermediate layer bonding is a method for joining wafers to each other using adhesive.

Etching

Isotropic and Anisotropic Etching

The technique for etching using particles, such as ions discharged from plasma in a low vacuum, is called reactive ion etching.
In a plasma comprised of a mix of charged ions and neutral radicals, two etching actions are carried out, isotropic etching by radicals and anisotropic etching by ions.

[Reactive Ion Etching]

Silicon Deep Etching

The most common silicon deep etching technology utilizes the Bosch process which combines the advantages of anisotropic and isotropic etching.

[Bosch Process]
[Scallops]

Repeating the process of Si Etching ⇒ Polymer Deposition ⇒ Bottom Polymer Removal makes it possible to achieve nearly vertical deep etching
The irregularities on the side walls are called ‘scallops’, since they resemble the scallop shells.

Deposition

ALD (Atomic Layer Deposition)

[Overview of ALD Steps]

ALD, short for Atomic Layer Deposition, is a thin-film deposition method in which chemicals called precursors are sequentially inserted and purged to build up atomic layers through surface reaction with the substrate.
This method makes it possible to form thin films with nano-thickness control even on the side wall of a small hole or the bottom of a deep hole (provided there is a gap through which the film formation materials can pass), allowing a uniform film to be deposited through MEMS processing, such as protection film deposition after deep etching.

[ALD Film Deposition]

ROHM's thin film piezo MEMS foundry provides total support from prototyping, development to mass production to realize small, energy-saving and high-performance products by combining our advanced thin film piezo and LSI microfabrication technology and abundant mass production experience.

> Thin-Film Piezo MEMS Foundry (Services)

The next page contains a brief summary of MEMS-related terminology.