Silicon Microfabrication Part 1

7/27/2019 Silicon Microfabrication Part 1

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Introduction to BioMEMS & Medical Microdevices

Sil icon Micro fabr icat ion Part 1

Companion lecture to the textbook: Fundamentals of BioMEMS and Medical Microdevices,

by Dr. Steven S. Saliterman www.tc.umn.edu/~drsteve

R012208



Steven S. Saliterman, MD, FACP

Micromachining vs. “Soft” Fabrication

Microfabrication is the process for the

production of devices in the submicron tomillimeter range.

Micromachining of silicon and other ceramics

is similar to integrated circuit fabrication. “Soft” fabrication techniques include molding,

embossing, stamping, casting, thick-filmapplication, self-assembled monolayers

(SAMs), and array patterning using polymersand biological substances.




Micromachining Materials MEMS devices are made from the same materials

used for microelectronics, including: Single crystal silicon wafers.

Deposited layers of polycrystalline silicon (polysilicon) forresistive elements.

Gold, aluminum, copper and titanium for conductors.

Silicon oxide for insulation and as a sacrificial layer (to allowrelease of moving parts).

Silicon nitride and titanium nitride for electrical insulation andpassivation.

The silicon materials have high strength at smallscales which allows higher strain levels and lesssusceptibility to damage and fracture.




“Soft” Fabrication Materials Polymers

Surface modification for improved functionality. Hydrogels

Environmentally induced changes in shape, sizeand other attributes.

Electroactive Polymers Electrically induced changes in shape, size and

other attributes.

Biological Materials DNA fragments, biotin labeled albumin, and

streptavidin coated polystyrene beads for example.




Microelectronics Revolution




From Molten Silicon to IC Chips




Electronic Grade Silicon (EGS)1. Quartzite is placed in a furnace with carbon releasing materials,

and reacts as shown, forming metallurgic grade silicon (MGS):

2Si0 (s) + 2C(s) Si(s) + 2CO(g)heat

→

2. MGS is then treated with hydrogen chloride to form

trichlorosilane:

3 2Si + 3HCl SiHCl (g) + H (g)→heat

3. Next fractional distillation reduction with hydrogen produceselectronic grade silicon (EGS):

3 2SiHCl (g) + H (g) Si(s) + 3HCl(g)heat

→




Czochralski Puller

Gardner JW et al, Microsensors, MEMS and Smart Devices, 2001




Cubic Crystal System

Sze 1985

(a) SC (b) BCC (c) FCC

Crystalline silicon forms a covalently bonded structure and coordinates

itself tetrahedrally (bottom). Silicon (and germanium) crystalize as two

interpenetrating FCC sublattices.




Miller Indices

Maluf 2005




Surface Micromachining Steps

Fatikow S & Rembold U, Microsystem Technology and Microrobot ics, 1997




Silicon Wafer Preparation




RCA Cleaning Bench




Thermal Silicon Oxide

SiO2 is a silicon atom surrounded

tetrahedrally by four oxygen atoms.

Structure may be crystalline (quartz) or

amorphous (thermal deposition).

Gardner JW et al, Microsensors, MEMS and Smart Devices, 2001




The chemical reaction thatoccurs is:

Dry oxidation at 900-1500°C in pure oxygenproduces a better oxide,with higher density thansteam oxidation.

Thermal silicon oxide isamorphous.

900 1200

2 2

2 2 2

Si (solid) + O (gas) SiO (solid)

and

Si (solid) + 2H O (gas) SiO (solid) + 2H (gas)

− °

→

→

C

Madou M, Fundamentals of Microfabrication, 2002

Thermal Silicon Oxide Methods




Spin-Casting Resist




Both “positive” and “negative” resists can

be chosen, depending on whether it isdesirable to have the opaque regions ofthe mask protect the resist, and hence the

substrate below, vs. having thetransparent regions protect the resistwhen exposed to UV.

Areas where the resist is removed willultimately be etched. Remember that“positive protects.”

Resist Types




Positive resists include poly(methyl

methacrylate) (PMMA), and a two partsystem, diazoquinone ester plus phenolicnovolak resin (DQN).

Negative resists include SU-8,bis(aryl)azide rubber and Kodak KTFR.

Critical Dimension - this is the smallestfeature size to be produced.

Resolution – smallest line width to beconsistently patterned.




Mask Creation




UV Exposure at 350-500 nm




Developing the UV Exposed Wafer




Etching Methods

Subtractive processes:

Dry etching (plasma), Glow discharge methods (diode setups):

Plasma etching (PE),

Reactive ion etching (RIE),

Physical sputtering (PS).

Ion beam methods (triode setups):

Ion beam milling (IBM),

Reactive ion beam etching (RIBE),

Chemical assisted ion beam etching (CAIBE).

Deep Reactive Ion Etching (DRIE).

Wet etching (chemical liquids).




Etching Profiles

Ziaie B et al, Hard and Soft Micromachining for BioMEMS, 2004

Isotropic Etching Anisotropic Etching




Plasma Etching occurs at relatively lowerenergy and higher pressure (less vacuum),and is isotropic, selective and less prone tocause damage.

Reactive Ion Etching is more middle ground in

terms of energy and pressure, with betterdirectionality.

Physical Sputtering and Ion Beam Milling relyon physical momentum transfer from higher

excitation energies and very low pressures,and result in poor selectivity with anisotropicetching and increased radiation damage.

Energy, Vacuum & Directionality




Plasma Etching (PE)

Madou M, Fundamentals of Microfabrication, 2002



Steven S. Saliterman, MD, FACP Madou M, Fundamentals of Microfabrication, 2002

Reactive Ion Etching (RIE)







Physical Sputtering Bombarding a surface with inert ions (e.g. argon) has

an effect related to the kinetic energy of the incomingparticles.

At energies < 3 eV (electron volts) particles are simplyreflected or absorbed.

At surface energies between 4-10 eV some surface

sputtering occurs. At surface energies of 10-5000 eV momentum transfer

causes bond breakage and ballistic material ejectionacross the reactor to the collecting surface. A low

pressure and long mean free path are necessary toprevent the material from redepositing.

Implantation (doping) occurs at 10,000-20,000 eV.




Sputter yield is the number of atoms removed

from the surface per incident ion. Sputter yield depends on the following:

Incident ion energy (max yield 5-50 keV).

Mass of the ion Mass of the substrate atom to be etched away.

Crystallinity and crystal orientation of the substrate.

Temperature of the substrate Partial pressure of oxygen in the residual gas.

Sputter Yield



Steven S. Saliterman, MD, FACP Madou M, Fundamentals o f Microfabrication, 2002 (left)

Ion Beam Milling (IBM)



Steven S. Saliterman, MD, FACP Madou M, Fundamentals of Microfabrication, 2002

Reactive Ion Beam Etching and

Chemical Assisted Ion Beam Etching

RIBE CAIBE




Resist Stripping




Profilometry




Profilometer Screen Display




Profilometry Graph




Dicing Chips







Wire Bonding




Summary

Microfabrication is the process for the

production of devices in the submicron tomillimeter range.

Micromachining of silicon and other ceramics

is similar to integrated circuit fabrication. Crystalline silicon forms a covalently bonded

structure and coordinates itself tetrahedrally

(bottom). Silicon (and germanium) crystalizeas two interpenetrating FCC sublattices.




Surface micromachining concepts discussed:

Mask creation, Silicon wafer preparation,

Thin-films deposition such as SiO2,

Resist (positive or negative) application, UV exposure and development,

Etching methods (substrative processes),

Resist stripping,

Inspection with profilometer .

Dicing and Wire Bonding

Documents

Silicon Microfabrication Part 1