Microelectronics has one overall quality which distinguishes it from other electronics: size. Just as the first computers appeared as large, relatively clumsy devices, today's electronics will soon appear awkward and excessively large as well, with individual molecules replacing entire semiconductors.

Microelectronic technology is exceptionally small -  (i.e. components of modern integrated circuits are  fabricated from billions or trillions of atoms). Current electronic hardware and circuitry is exponentially larger than the nanoelectronic hardware currently being developed.

"Just as silicon transistors replaced old vacuum tube technology and enabled the electronic age, carbon nanotube devices could open a new era of electronics."    Margaret Blohm

MEMS (micro-electrical systems)

MEMS (Micro-Electro-Mechanical Systems) is used to describe the integration of electrical components at the micro scale while NEMS refers to nano-scale components and systems.  MEMS technology employs a range of tools and methodologies, which are used to create small structures (i.e. one millionth of a meter). MEMS can include  electrical components, mechanical devices, switches, sensors, actuators.  These microelectronic components are usually assembled on a silicon platform or substrate.

 In general, construction of microelectronics employs  microfabrication techniques. These methods often involve "etching" or carefully wearing away parts of a silicon chip or by carefully adding layers in what is termed "micromachining"

The importance of this type of micromachining to modern electronics cannot be exaggerated: the outcome will be complete circuits / systems on microscopic chips.  These micro-circuits will impact upon many aspects of electronics, in some respects providing many of the characteristics of a living system:

1. Micro-sensors - sight, pressure, light sensitivity - the ability to acquire information from the environment through measurement of physical,  mechanical, thermal, biological, chemical, optical, and magnetic parameters.
2. Smart-chips - circuits capable of limited decision-making and diagnostics  - electronics evaluate sensory information and determine an appropriate course of action.
3. Micro-tools and Articulators - microscopic tools will have the capability of holding, pumping, counting,  maneuvering and / or positioning microscopic objects to meet the requirements of the specific circumstance.

Despite the significant enhancements with respect to performance and size reduction, MEMS do share a number of aspects with traditional intergrated circuitry (IC).  Common aspects include:

	fabrication on chips or wafers of silicon
	fabrication as thin films or substrates
	photoithographic techniques produce structures

Fabrication

Overall, there are three fundamental stages in creating MEMS:

1. Deposition - depositing extraordinarily thin films or layers of material on a wafer, chip or substrate.
2. Photolithographic Imaging - the application of selective patterns, masks or films of materials to build up structures in precise ways.
3. Etching - the selective and precise removal of materials from the "mask" (overlay) to create three-dimensional structures.

Deposition

1. Chemical Deposition

   Chemical deposition creates solid materials from chemical reactions, starting with either a liquid or gas. This allows deposition of a thin even layer of material to a chosen surface / substrate. Example chemical deposition techniques can include:

    

   	CVD (Chemical Vapor Deposition)
   	Electro-deposition
   	Thermal oxidation
   	Epitaxy

    

2. Physical Deposition

Physical deposition  involves physically positioning  a material over and onto a substrate. Physical deposition is more of a location event since it does not involve a chemical change to the substrate itself.  Example physical depositions include:

1. Casting
2. PVD (Physical Vapor Deposition)

Photolithography

Lithography usually involves the transfer of a pattern. In "photolithography" the transferred material is photosensitive and can be selectively altered by the application of light or other controlled sources of electromagnetic energy.

http://bmrc.berkeley.edu/courseware/ICMfg92/images/gif/photolith.gif

By "masking" (protecting) parts of the photosensitive material to the incoming light, specific patterns (and over multiple successive layers) and shapes can be fabricated.

Etching

In order to shape and create microscopic structures for MEMS etching is used to "shave" off or removal materials from unwanted areas following the deposition process. This typically involves two different etching techniques:

1. Wet Etching  - this involves immersing the material in a "corrosive" chemical or biological solution to remove exposed layers of material.
2. Dry Etching - this involves applying highly reactive ions or vapors to the exposed areas of the material.

Transducers - the mechanical / electrical / optical Interface

The role of the transducer is to convert energy into other forms. This includes conversion of electrical energy into mechanical, or even optical signals or the reverse.

http://news.uns.purdue.edu/images/+2008/MEMS-varactor.jpg

In MEMS and NEMS systems transducers can accomplish more than  a simple electro-mechanical interface, being employed as sensors for chemical or mechanical events. If a transducer is set up to keep a mechanical component consistently vibrating, any variations or disruptions in the molecular environment can be observed as an electric oscillation / signal (i.e. a quantifiable nano-resonator). Sensory capabilities of transducers can include:

1. Chemical Concentration - changes in chemical concentration can be determined through alterations in mass which in turn affect transducer oscillation
2. Pressure Sensitivity - affecting the rate and height of oscillations
3. Temperature Measurement - affecting the molecular rigidity and oscillation rate

Nano-transducers have two types of response to stimulation:

1. Deflection proportional to the applied stimulation
2. Changes in frequency or amplitude of oscillation

Detection of transducer response to stimulation can employ a variety of systems, including piezoelectric systems,  magnetic apparatus, optics, etc.

The Molecular Switch

Then semiconductor switch is a cornerstone of modern electronics. When used in combinations these devices can be made to perform any number of computational and control functions. Nano-scale switches, however, completely re-define the capabilities of semiconductors.

Private companies as well as academic researchers have now created individual molecules capable of behaving as a switch. Some of these systems serve to invert the molecule with respect to its stereoisomeric forms: that is to say the molecule switches into a mirror image of itself. This means there are two identical, but left-right reversed forms of the molecular switch.

http://www.trnmag.com/Molecular%20Stator%20Rotor%20Full.jpg

Two clearly defined left-right or "on-off" signals are all that is required for binary code which is at the heart of modern computing and electronics. This nano-scale digital binary code is read by light or optical differences between the two (chiral) forms of the molecular switch.

Single Electron Devices

Tinier than the MEMS systems are single electron devices (SED's).  The concept of SED's is control the movement of electrons and thereby reducing the size of integrated circuits by a factor of 10³.

http://images.google.ca/images?hl=en&q=MEMS&gbv=2Current single electron

  devices employ two electrodes separated by a 1nm insulating layer that allows for the passage of only single electrons. Successful implementation of these devices will require technological advancement or a significant design change - the current models only work below 4º Kelvin (- 269.15º Celsius) which is nearly impossible to obtain.

Currently researchers are studying the possibility of a silicon-based  molecular gate system which shows considerable potential to function at room temperature. Below is aconceptual switch / gate system.

http://www.sciencedaily.com/images/2007/09/070904081912-large.jpg