2. INTRODUCTION? ??????????????????????????????????????????????????
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:
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)
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
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
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.
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
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)
7. THE FUTURE- TRANSITION FROM MICROMACHINING TO NANOMACHINING
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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