Part V Fabrication of Microelectronic Devices and Micromanufacturing

Part V: Fabrication of Microelectronic Devices and Micromanufacturing Group 5 Josh Agenbroad Joy Best Iris Gallegos Keith Griego November 21, 2005

Fabrication of Microelectronic Devices l l The invention of the Transistor in 1947 was the catalyst for Microelectronics are now more abundant in our lives since Integrated Circuit (IC) technology became the foundation for calculators, robotics, space travel, weaponry, wrist watches, and automotive controls. ICs are very small, and low cost so they have allowed for manufacturers to make smaller and smaller products. The semiconducting material that the IC is made on is called a Chip, which typically hold from 10 million devices to 100 million devices, a huge improvement from 100 devices.

Clean Rooms l l l ICs are typically a few millimeters in length, and the smallest feature maybe as small as a few nanometers. Clean rooms are need to keep dust and other contaminates from coming into contact with the IC. Clean rooms are classified from 1 to 10. Class 10 has 10 or fewer particles per cubic foot. Hospitals= 10 K particles per cubic foot.

Semiconductors and Silicon l l Silicon has surpassed Germanium as the industry standard for several reasons: There is an abundance of silicon Silicon has a larger energy gap than germanium, which allows silicon to operate at high temperatures (270 F vs. 180 F) Silicon dioxide is a great electrical insulator, which is the bases for Metal-Oxide-Semiconductors used for memory devices, processors, the largest semiconductor material produced worldwide. Silicon has a diamond type crystallographic structure

Crystal Growing and Wafer Preparation l l Take Silica and corbon and heat together in a furnace = 95 -98% pure Polycrystalline silicon. Convert this into trichlorosilane, which is purified making electronicgrade silicon (EGS) EGS goes thru the CZ process, which produces a crystalline cylinder ingot This is then sliced into wafers

Wafers l The process is as follows to the right:

Film Deposition l l Films of many different types are used extensively in microelectronicdevice processing, particularly insulating and conducting films. Common deposited films include polysilicon, silicon nitride, silicon dioxide, tungsten, titanium, and aluminum. Epitaxy is defined as the growth of a vapor deposit, occurring when the crystal orientation of the deposit is related directly to the crystal orientation in the underlying crystalline substrate. Techniques for film deposits include evaporation, sputtering, chemical vapor deposition, lower chemical vapor deposition, vapor-phase epitaxy, liquid phase epitaxy, molecular beam epitaxy.

Oxidation l Like film deposition, Oxidation is a process of a growth of a layer of oxide layer. These layer will have less impurities in them than the deposition layers. l Dry oxidation Wet oxidation l

Lithography l l Lithography is the process by which the geometric patterns that define devices are transferred to the substance. Most common technique would be photolithography, however Electron beam and X-ray lithography are the most useful for miniaturization of ICs.

Lithography l l l Photolithography is limited to the wavelength of light being used. X-ray is superior to Photolithography due to the shorter wavelength, which allow for finer patterns. Electron beam and Ion beam are even better allowing for finer detail to the pattern.

Etching Is the process by which entire films or particular sections of films are removed, and it plays an important role in the fabrication sequence.

Wet Etching • Wet etching is the removal of material by immersing the wafer in a liquid bath of chemical etchant. There are two kinds of wet etching etchants, isotropic etchants and anisotropic etchants: - Wet etching works very well for etching thin films on substrates, and can also be used to etch the substrate itself. I Effective etching requires the following conditions: 1) Etchant transport to the surface. 2) A chemical reaction. 3) Transport of reaction products away from the surface. 4) Ability to stop the etching process rapidly in order to obtain superior pattern transfer(etch stop).

Isotropic and Anisotropic Etching Anisotropic etchants are used for: Isotrophic etchants are used for: • Terminating crystal planes with little undercut • Removal of damaged surfaces • Vertical etching • Rounding of sharp etched corners to avoid stress concentrations • Creating structures in single-crystals • Evaluating defects.

Dry Etching • What is dry etching? - Materials removed from reactions that occur in the gas phase. The dry etching process requires only small amounts of the reactant gases, whereas the solutions used in the wet-etching process have to be refreshed periodically. Types of dry etching - Sputter etching - Plasma based dry etching

Dry Etching Advantages • Eliminates handling of dangerous acids and solvents • Uses small amounts of chemicals • Isotropic or anisotropic etch profiles • Directional etching without using the crystal orientation of Si • Faithfully transfer lithographically defined photo resist patterns into underlying layers • High resolution and cleanliness • Less undercutting • No unintentional prolongation of etching • Better process control • Ease of automation (e. g. , cassette loading)

Chemical vs. chemical/physical etching Purely chemical etching (using only reactive neutral species) Isotropic etching - Makes the surface more reactive. - Clears the surface of reaction products and allows the chemically reactive species to access the cleared areas. - Also provides the energy to drive surface chemical reactions, however the neutral species do most the etching. Physical-chemical etching is anisotropic. This tecnique allows the generation or near vertical walls with very large aspect ratios. Since the ion bombardment does not remove material directly, masks can be used. Chemical + physical etching (using reactive neutral species and ionic species) Anisotropic etching

This simple device is known as junction diode. This is the fabrication sequence of the conductor. These are mostly semiconductor devices.

Metallization and Testing In the previous sections we talked about device fabrication. However generating a functional circuit the devices must be interconnected. Here is the process of metallization and testing. This really means the final product.

Wire Bonding and Packaging This is a schematic illustration of thermosonic welding of gold wires from package which leads to bonding pads.

Micromechanical Devices Small scale like microelectronic l 3 D instead of 2 D l Scale of Micrometers l

MEMS l l Micro-Electo-Mechanical Systems Include both mechanical features and electric circuits A gyroscopic sensor for use in automobiles. Sensors are a common application of MEMS.

MEMS l l l Manufactured using similar techniques to microelectrical devices. Do no have to be semiconductor based. High wear/adhesion problematic with silicon.

MEMS Microdevices generated from 3 -D CAD data

Bulk Micromachining l l l Traditional and simple technique for micromaching. For simple shapes 3 D

Bulk Micromachining l l l Diffuse dopent to desired pattern Deposit masking film Etching leaves elevated cantilever

Micro Surface Machining l l l More complicated designs Uses a sacrificed spacer layer along with bulk machining techniques Accelerometers, pressure sensors, micro pumps, micro motors, and actuators.

Micro Surface Machining l l l Deposition of spacer layer (phosiliate glass) Etching spacer layer Deposition of polysilicon Etching of polysilicon Selective wet etching removes spacer layer

Bulk & Surface Micomachining Example Micro Lamp

Stiction l l Traped liquid meniscus after spacer layer is removed due to surface tension at small scale. Causes deformation or damage

Folding Deployed folding mirror

Folding l l l Deposition of spacer layer followed by polysilicon hinge piece Deposition of second spacer layer Selective etching of spacer layer Deposition of polysilicon staple Selective etching of spacer to allow rotation Hinge can be set in folded position and will remain due to high adhesion of silicon at small scale

SCREAM single crystal silicon reactive etching and metalization l l 1 -3 Standard lithography & etching 10 -50 micrometers Vapor deposited silicon oxide Anisotropic etching removes bottom Dry plasma etching extends cavities

SIMPLE Silicon Micromachining by Single Step Plasma-Etching l l Single step process to create deep cavities like those created with SCREAM Uses plasma etching with layers of various doping

Etching Combined with Diffusion Bonding l Very complex shapes with large suspended sections.

The LIGA Microfabrication Process l LIGA is a German acronym for the combined process of x-ray lithography, electrodeposition, and molding.

LIGA Microfabrication Process

LIGA Process l Final product may consist of: - A free-standing metal structure resulting from the electrodeposition process. - A plastic injection-molded structure. - An investment-cast metal part using the injection-molded structure as a blank. - A slip-cast ceramic part produced with the injection-molded parts as the molds.

Multilayer X-Ray Lithography l Powerful technique in producing MEMS devices with large aspect rations and reproducible shapes.

HEXSIL l l This process combines hexagonal honeycomb structures, silicon micromachining, and thin-film deposition to produce high aspect-ratio, free-standing structures. HEXSIL can produce tall structures with a shape definition that rivals LIGA.

HEXSIL Process

Solid Free-Form Fabrication of Devices l l l Another term for rapid prototyping. Rapid prototyping automates the making of a prototype. It builds a prototype part from a 3 -D CAD drawing Automotive Medical Entertainment

Methods l Microstereolithography - Similar to stereolithography but laser is more highly focused (to a diameter as small as 1 µm), as compared to 10 -100 µm. - Layer thicknesses are around 10 µm, which is an order of magnitude smaller than in stereolithography. - Support structures are not needed since the smaller structures can be supported adequately by the fluid.

Electrochemical Fabrication (EFAB) l l l Is the solid free-form fabrication of MEMS devices using instant masking. The Instant Mask consists of an insulator patterned on an anode. Instant Masking patterns a substrate by pressing the mask against the substrate, electrodepositing material through apertures in the insulator and then removing the mask from the substrate.

EFAB Process (a). A blanket of material is rapidly deposited (b). The layer is planarized (c). The process is repeated to create multiple layers (d). A selective chemical etch removes sacrificial material.

Electrochemical Fabrication (EFAB) l l The result is a layer rapidly deposited and patterned in a single step. The process is significantly faster than photolithography and makes it possible to fabricate MEMS devices with dozens of patterned layers in a single day, compared with several weeks for a conventional 3 -layer MEMS device.

References l l l http: //www. devicelink. com/mem/archive/02/10/bang. html Kalpakjian, Serope, and Schmid, Steven R. Manufacturing Engineering and Technology. Prentice-Hall, Fifth Edition. Lindbeck, John R. Product Design and Manufacturing. Prentice-Hall, 1995 http: //geae. com http: //boeing. com