microfabrication


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MEMS & Microfabrication
Written by: Khalid Tantawi

Introduction:

When conducting research in technologies in the  nano and micro scale, it is important to be in a clean  dust-free, temperature and humidity- controlled environment.  This is due to the fact that any dust particle may cause significant damage to your work, for example if you work on microfluidics with sub 50 µm diameters, a human hair may block a microfluidic channel and render it useless.  Another factor that is important to be controlled is the relative humidity, many processes require operating at very low pressures, for this purpose special pumps such as cryo and ion pumps are used, but the time required and efficiency of these pumps to reach the low pressure levels (typically on the scale of 10-3 to 10 -9 torr) strongly depends on the relative humidity present.  For this reason, microfabrication and Micro Electro Mechanical Systems (MEMS) labs are usually operated in humidity-and-dust controlled rooms, these special rooms are called Cleanrooms.  In the United States, these rooms are classified by the maximum number of particles and the size of the particles in µm permitted per square foot of air. The Nano and Micro Devices Center in University of Alabama in Huntsville operates a level 10,000 cleanroom, meaning that only 10,000 particles of size 0.5 µm are allowed per square foot.

Figure 1: Khalid In a class 10,000 cleanroom in the University of Alabama in Huntsville labs

Materials:

Silicon is the most commonly used material in microfabrication labs, this is due to many reasons, but mainly two reasons contributed to the domination of silicon over other materials in today's microfabrication labs:

  1. Its wide availability and low cost to obtain a single crystal wafer: silicon is one of the most commonly found elements on earth.
  2. The electrical properties of this semiconductor material are unique in that the electrical conductivity may be easily controlled by doping silicon, electrical isolation is also easily achieved by oxidizing the silicon, leading to the production of silicon on insulator wafers (SOI). This material also allows for its electrical properties to change by varying the voltage applied on it, allowing for the magic device, the transisitor to come to life. For this reason, silicon is the main element in electronics, it is no surprise to refer to the science park in San Francisco as the "Silicon Valley".

Another commonly used material as a substrate in MEMS is glass.  Glass has three advantages that made it stand out over metals and semiconductors:

  1. Thermal and electrical insulation.
  2. Ability to handle a wide variety of chemical compounds that silicon and metals cannot handle.
  3. Optical transparency

Tools and Processes:(Still under construction)  

I have tried to document all the main processes in microfabrication here with pictures. Some of these processes are:

1.  Lithographic Patterning: This process refers to transferring a geometric shape refered to as a pattern to a substrate.  The pattern is transfered from what is called a mask.  Some methods of lithographic patterning are:
  • Photolithography:  This is the most widely used process in microfabrication, it is usually the first step in microfabrication.  The trick here is to make use of the fact that some chemicals called photoresists change properties when they are exposed to ultraviolet (UV) light, such that these sticky polymers can be easily removed from the surface of a wafer (etched) if exposed to uv light at resolutions down to few microns, when the chemical is developed after being exposed,  this type of photoresists is called positive photoresist, if on the other hand the unexposed regions of the photoresist are etched during the developing, then it is called a negative photoresist.
The contact aligner is used in photolithography to expose substrates to UV light.
  • Electron-Beam Lithography: This method is much slower than photolithography and much more expensive, it is also much more accurate. Another advantage of E-beam lithography is that there is no physical mask, the pattern is a computer generated graphic that is loaded into the computer that drives the electron beam.

2.  Material Deposition:
many methodologies exist for deposting materials on the surface of another material, the best methodology depends on the application and purpose the deposition is needed for, and also on the type of materials being deposited, some of these methods are sputtering, thermal evaporation, and electroplating just to name a few. Each methodolgy utilizes a different mechanism for the deposition of the materials. Other techniques are used to deposit thin biological or chemical films on surfaces such as the Langmuir- Blodgett technique for depositing bilayer membranes.


The Discovery 18 sputtering tool by Denton Vacuum in the labs of Nano & Micro Devices Center in University of Alabama in Huntsville, the part on the left is the chamber with three cathodes, the part on the right is the control system,
Photo taken by Khalid tantawi, 2011.

Inside the sputterer chamber, two of the three cathodes are shown in the picture, the aluminum foil is used to protect the internal walls of the chamber from being deposited on them.
 

A Thermal Vapor Deposition system by Consolidated Vacuum Corporation in UAHuntsville Nano & Micro Devices Center

A Langmuir- Blodgett Trough while being used for deposition of a lipid bilayer membrane on a silicon surface.



3. Material removal processes: This process is called etching, many mechanisms exist, they are classified into:

- Wet Etching: such as anisotropically etching silicon in Potassium Hydroxide (KOH) or isotropically etching silicon in Hydrofluoric acid (HF) in an electrochemical cell.

- Dry Etching: here plasma is used to etch the material by three main mechanisms, chemically, physical bombardment, or a combination of both. One of these processes is called Reactive Ion Etching.


A PlasmaTherm Reactive Ion Etcher in the labs of Nano & Micro Devices Center in University of Alabama in Huntsville, photo taken by Khalid Tantawi, 2011.



4. Surface Analysis Tools:

There are numerous methods and techniques to analyze surfaces of nanostructures, some of the surface analysis requirements and tools are:

- Surface morphology and profile analysis: Atomic Force Microscopy (AFM) is the ultimate surface analysis tool due to its atomic level resolution. Other methods such as Scanning Electron Microscopy (SEM) and the conventional optical microscopy have their advantages and disadvantages.
The Atomic Force Mircoscope is the ultimate surface analysis tool

The Scanning Electron Microscope (SEM).

- Surface composition and Material Analysis: This is the hard part, it may be determined using spectroscopic techniques such as Raman and X ray spectroscopy, or a combination of them. Ellipsometry may also be used.


Raman Spectrometer in the lab of Dr. Emmanuel Waddel in Chemistry Dept. in UAHuntsville

- Surface wettability and hydrophobicity analysis: The most common way to analyze hydrophobicity is using the contact angle measurement method.

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