Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

MEMS process integration – the technique by which processing steps are combined into an ordered sequence with the goal of creating a methodology that allows the production of functional MEMS devices. Following is the process flow diagram.

Some of the blocks of the process integration are as follows –

Wafer cleaning

• Contamination on a wafer consists of particles which must be removed by wafer cleaning.
• In order to remove metallic impurities, organic contaminants etc. wafers were subjected standard RCA cleaning procedure.
• The purpose of the RCA clean is to remove organic contaminants (such as dust particles, grease or silica gel) from the wafer surface; then remove any oxide layer that may have built up; and finally remove any ionic or heavy metal contaminants.

Thin film deposition:

Silicon wafer is placed in a deposition chamber, and the constituents of the film are delivered through the gas phase to the surface of the substrate where they form the film. There are two types of thin film deposition –

i) Physical Vapor Deposition (PVD) - In this case physical methods are used to produce the constituent atoms which pass through a low-pressure gas phase and then condense on the substrate. It has two types

a) Evaporation – It is nothing but heating of a solid or molten source until it vaporizes.

b) Sputtering – Bombarding a solid source with energetic ions formed in a plasma is called sputter deposition.

ii) Chemical Vapor Deposition (CVD) - In this case, reactant gases are introduced into the deposition chamber, and chemical reactions between the reactant gases on the substrate surface are used to produce the film. It further has three types

a) APCVD

b) LPCVD

c) PECVD

Photolithography

• Photolithography is the process step used to define and transfer a pattern to its respective Layer.
• The photolithography process occurs several times during the fabrication of a microsystems device as layers build upon layers. The linkage assembly would require "at least" six layers.
• In the photolithography process a light source is typically used to transfer an image from a patterned mask to a photosensitive layer (photoresist or resist) on a substrate or another thin film.

• This same pattern is later transferred into the substrate or thin film (layer to be etched) using a different process called etch.

  

Etching

Etching is a process by which patterns are transferred by selective removal of unmasked portion of a layer. There are two types of etching:

1) Wet etching –

• In this process the removal of unmasked layers is done by selective liquid etchant. It is a purely chemical process.
• The batch of wafers is dipped into highly concentrated pool of acid & the exposed areas of wafer are etched away.
• Wet etching gives isotropic profile i.e. it etches equally in all directions. Isotropic etching profile means removal of material equally in all directions which results in undercutting and uncontrolled etch features.
• HF is used as etchant for Si.

2) Dry etching –

• Dry etching involves removal of substrate material by gaseous etchants. It is a more physical process than chemical process.
• It may involve chemical reactions by gases radical species, ion bombardment and/ or combination of both.
• Dry etching gives anisotropic etching profile i.e. material is etched in preferred direction only.
• It is a crystallographic dependent etching, where etch rate varies according to type of crystal plane exposed to the etchant.

Wafer bonding

• Wafer bonding is a packaging technology on wafer-level for the fabrication of microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation.
• The wafers' diameter range from 100 mm to 200 mm (4 inch to 8 inch) for MEMS/NEMS and up to 300 mm (12 inch) for the production of microelectronic devices.
• In microelectromechanical systems (MEMS), the package protects the sensitive internal structures from environmental influences such as temperature, moisture, high pressure and oxidizing species.
• The long-term stability and reliability of the functional elements depend on the encapsulation process, as does the overall device cost.

Dicing

• Wafer dicing is the process by which die are separated from a wafer of semiconductor following the processing of the wafer.
• The dicing process can involve scribing and breaking, mechanical sawing (normally with a machine called a dicing saw) or laser cutting.
• All methods are typically automated to ensure precision and accuracy.