Micro Machining

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ABSTRACT

  • The miniaturization of products and their manufacturing processes is considered one of the key trends in technology development.
  • Micromechanical parts tend to be rugged, respond rapidly, use little power, occupy a small volume, and are often much less expensive than conventional macro parts, added to that they have high thermal, chemical, and mechanical stability.
  • Micro machining technologies make devices ranging in size from a dozen millimeters to a dozen microns. 
  • These techniques combined with wafer bonding and boron diffusion allows complex mechanical devices to be fabricated.  The Lithographie, Galvanoformung, und Abformung (LIGA) technology makes miniature parts with spectacular accuracy. 
  • The report also deals with conventional methods such as laser machining.

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2. INTRODUCTION? ??????????????????????????????????????????????????

  • A micro- system is an intelligent miniaturized system comprising sensing, processing and/or actuating functions.
  • Micro engineering refers to the technologies and practice of making three dimensional structures and devices with dimensions in the order of micrometers.
  • Micromachining is the basic technology used for the production of miniaturized parts and components. Micromachining resulted in the fabrication of a broad class of devices whose defining characteristics were micrometer-scale feature size and electromechanical functionality, and these systems are collectively called as MEMS (MICRO-ELECTRO ?MECHANICAL-SYSTEMS).

 

2.1 MICROMANUFACTURING
Micro manufacturing is the set of design and fabrication tools that precisely machine and form structures and elements at a scale below the limits of our human perceptive faculties.
The methods commonly used for the fabrication of micro elements are almost as varied as their applications, but generally fall within two distinct categories:

Bulk micromachining.
Surface micromachining.

3. BULK MICROMACHINING

  • Bulk micromachining is a subtractive process, the term is applied to a variety of etching procedures, where the required structures are developed using the etching agents that selectively remove material.
  • Etching produces concave, pyramidal or other faceted holes, depending on which face of the crystal is exposed to the chemicals.
  • This technique has come to be known as bulk micromachining because the chemical that pits deeply into the silicon produces structures that use the entire mass of the chip.

Isotropic wet Etching
Most commonly used isotropic etching agents for Silicon are:

Etches dope extrinsic regions i.e. n+ and p+, more than intrinsic regions. In etching prior to the expose of substrate to the etching agent, a mask is used which is placed over the substrate material to obtain proper etched regions.
Most commonly used masks against this isotropic etch are:
? Nitrides (Si3N4)
? Noble metals
? SiO2 (if HF ratio is low and for short etches)

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Anisotropic Wet Etching

In this type of etching, the etching rate is orientation dependent in the crystal.
A. Inorganic alkaline solutions (KOH, LiOH, NaOH)
B. Organic alkaline solutions: (Ethylene diamine, pyrocatechol and water: EDP)

A completely anisotropic etchant will etch in one direction only. An isotropic etchant will etch in all directions at the same rate.

3.1 LIMITATIONS OF BULK MICROMACHINING

  • Bulk Micromachining has the disadvantage that it uses alkaline chemicals, foreign to conventional chip process.
  • Consequently, fabricating multiple, interconnected micromechanical structures of free-form geometry is often difficult or impossible.

4. SURFACE MICROMACHINING
It is called surface micromachining because it deposits a film of silicon a few microns thick, from which beams and other edifices can be built

  • Surface micromachining relies on encasing the structural parts of the device in layers of a sacrificial material during the fabrication process.
  • The sacrificial material (also called spacer material) is then dissolved away in a chemical etchant that does not attack the structural parts.
  • The final stage of dissolving the sacrificial layer is called "release".

 

In other words, there are two primary components in a surface micromachining process:
>> Structural layers -- of which the final microstructures are made;
>> Sacrificial layers -- which separate the structural layers and are dissolved in the final stage of device fabrication.

Surface micromachining involves depositing, removing, and patterning thin films on a substrate.

4.1 LIMITATIONS OF SURFACE MICROMACHINING

  • Indeed, surface micromachining has an important limitation. It is inherently a planar fabrication process, and is, therefore, limiting for mechanical design.
  • Hence, surface micromachining produces planar structures (i.e., low aspect ratio devices) with little in the way of Z topology

 

5. LIGA (LITHOGRAPHIE, GALVANOFORMUNG, ABFORMUNG)
A technique that allows overcoming the two-dimensionality of surface micromachining is the LIGA process. The technology was developed in Germany
It is the acronym for Lithographie (lithography), Galvanoformung (electroplating), Abformung(molding). It displaces the achievable height of the microstructure from few to hundreds microns and like bulk and surface micromachining relies on lithographic patterning.
Lithography is the technique by which the pattern on a mask is transferred to a film or substrate surface via a radiation-sensitive material. The radiation may be optical, x-ray, electron beam, or ion beam
Pattern generation begins with mask design and layout using computer-aided design (CAD) software, from which a mask set is manufactured. A typical mask consists of a glass plate coated with a patterned chromium (Cr) film.
Metal is then plated into the structure; This metal piece can become the final part or can be used as an injection mold.
Injection molding of microscopic parts can be carried out with a process. The process can be used for the manufacture of high-aspect-ratio

6. LASER MICROMACHINING
Over many years such processes have become well established as production techniques with improvements limited mostly to enhancements in laser drive technology rather than changes to the basic mask projection, beam handling and motion control techniques.
Pulsed laser micromachining techniques using mask projection methods are now widely used for the creation of miniature structures in both massive and thin substrates.
6.1 SYNCHRONISED IMAGE SCANNING (SIS)
In SIS the substrate moves continuously during pulsed laser triggering such that, simultaneously with each laser pulse,
The image projected onto the substrate has moved by exactly one pitch

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6.2 BOW TIE SCANNING
The laser scans in a straight line at high speeds across a section of the substrate by a galvanometer-driven mirror deflection,
While the substrate is moved on a linear stage at constant speed in the orthogonal direction.
After each transverse scan the galvanometer mirror decelerates, reverses and performs a scan in the opposite direction

 

6.3 MASK PROJECTION TECHNIQUES (MPT)


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7. THE FUTURE- TRANSITION FROM MICROMACHINING TO NANOMACHINING

  • Nano is the buzzword of the moment, which demands refinement in micromachining resulting in evolution of nanomachining.
  • Thus the transition from Micro-Electro-Mechanical-Systems (MEMS) to Nano-Electro-Mechanical-Systems (NEMS) will take place.
  • The smallest features that have been fabricated using lithography are only a few tenths of a nanometer in dimension.
  • Nanomachining has been used by a number of groups to fabricate quantum devices such as single-electron transistors (SETs) and metal-oxide junctions.

8. CONCLUSION
Micro engineering not only provides a new manufacturing route for existing products, but also, importantly, allows the creation of completely new products and new markets providing large volumes of low cost sensors to the automotive industry, and low volume high performance, small and light weight sensors to aerospace and defense.
The predominant technology at present state is surface micromachining, and current developments show that this trend will continue in the future. However the LIGA process will grow in importance, as it is the only method for producing true three-dimensional objects..

 

 

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