Micro Nanofab Techniques

Micro/Nanofabrication Techniques

Prof. Bharat Bhushan Ohio Eminent Scholar and The Howard D. Winbigler

Professor and Director NLBB

[email protected]

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

© B. Bhushan 1 Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Outline • Top down fabrication – Micro/nanofabrication techniques

Planar lithographic fabrication processes Bulk micromachining Surface micromachining Sacrificial-layer lithography LIGA

Non lithographic processes E-beam writing, etc.

Soft lithography Microcontact printing, etc.

• Bottom up fabrication – Nanofabrication Nanochemistry, etc.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 2


• Planar Lithographic Processes Bulk machining Surface micromachining Sacrificial-layer lithography LIGA (based on X-ray lithography)

• Non-lithographic processes

E-beam writing Focused ion-beam (FIB) etching Laser machining Ultrasonic drilling Electrochemical discharge machining (EDM)

• Soft lithography

A mold is used to generate patterns Replica molding Nanoimprint lithography Microcontact printing

Some of these are high resolution but slow and serial, and hence expensive.

3

Top Down Fabrication - Microfabrication Techniques


Planar lithographic fabrication process Standard micromachining flow

Substrate Deposit thin films Lithography - apply mask - expose to pattern - develop Etch - dry etch (e.g., plasma etch, reactive ion beam etching) - wet etch Characterization - microscopy, electrical, adhesion, etc.

4


Photolithography Direct contact (no reduction) Projection (4x or 10x reduction) Positive/Negative tone

Positive resists develop in the exposed region and usually remain soluble for lift-off. Negative resists remain in the exposed region but are insoluble and not suitable for lift-off.

5

Positive and negative resist exposure and development


Deposit thin films

• Thin films – liquid source – solution spin coating – spray coating – sol gel processing – electroplating – self-assembled monolayers

• Thin films – gases

– Oxidation – doping, alloying

• Thin films – plasma

– radical polymerization – physical vapor deposition (sputtering, evaporation) – chemical vapor deposition – plasma enhanced chemical vapor deposition (PECVD)

6


Masks The stencil used to generate desired patterns on resist is called a mask. In a photomask, a flat glass (transparent to near UV) or quartz plate (transparent to deep UV) with a metal (e.g., an 80-nm thick Cr layer) absorber pattern is used. The absorber pattern is opaque to UV light, whereas glass or quartz is transparent. Plastic sheets with painted patterns are used for lower resolution lithography. A light field or dark field image (mask polarity) is then transferred to the photoresist.

Resist Before spinning on a resist, a thin oxide layer is grown on the Si wafer by heating it to between 900 to 1150 °C in humid environment or dry environment. The oxide can serve as a mask for a subsequent wet etch. Then a thin layer of an organic polymer, photoresist, sensitive to UV radiation is deposited using a spin coater.


Development

Development transforms the resist image formed during exposure into a relief image which will serve as a mask for further lithographic steps. During the development process, selective dissolving of the resist takes place. Wet development is commonly used.

Etching

Etching is used to remove the film underneath in the exposed areas. Both wet and dry etchants are used.


Commonly used light source in photolithography – Argon fluoride excimer laser with a wavelength of 193 nm (UV) used in

patterning 90-nm lines and spaces. For sub-100-nm patterning – deep-UV wavelengths, X-ray lithography, electron beam (e-beam) lithography, focused ion-beam lithography, maskless lithography, liquid immersion lithography, etc. E-beam – very powerful, high resolution, but slow and serial, and hence expensive.

9


Bulk Micromachining

Bulk micromachining employs anisotropic etching of single-crystal silicon. It is a proven, high-volume production process. Routinely used to fabricate accelerometers, pressure sensors and flow sensors, etc.

10


Surface micromachining is based on deposition and etching of sacrificial films to produce freestanding structures of LPCVD polysilicon films. It is used to produce sensors, actuators, microsensor arrays, motors, gears and grippers, etc.

11

Surface Micromachining


LIGA is used to produce high-aspect-ratio MEMS devices that are up to 1 mm in height and only few μm in width. It is based on X-ray lithography, electroplating and molding processes to produce 3-D devices with high aspect ratio

12

LIGA


The fabrication of nanostructures such as nanochannels with sub-10 nm resolution can be accomplished through several routes – e.g., e-beam lithography and sacrificial-layer lithography.

E-beam lithography

A finely focused electron beam is exposed over a resist surface. E-beam either breaks or joins the molecules in the resist. Further processes can either remove the exposed part (positive resist) or remove the unexposed part (negative resist). E-beam lithography can either be used to create photomasks for replication or create devices directly. Throughput of the e-beam lithography is very low since a single e-beam is used to create the entire pattern.


Sacrificial-layer lithography

(a) Growth of silicon nitride layer (etch stop) and base polysilicon deposition, (b) hole definition in base, (c) growth of thin sacrificial oxide and patterning of anchor points, (d) deposition of plug polysilicon, (e) planarization of plug layer, and (f) deposition and patterning of the protective, sacrificial and etch layers before final release of the structure in HF (Hansford et al., 2001). It is used for fabrication of nanostructures such as nanochannels with sub-10-nm resolution.

14

In this technique, the use of a sacrificial layer allows the direct control of nanochannel dimensions.


Soft Lithography It is a non-lithographic technique in which a master or mold is used to generate patterns, defined by the relief on its surface, on polymers by replica molding, embossing (nanoimprint lithography), or by microcontact printing. It is faster, less expensive and more suitable for most biological applications than glass or silicon micromachining. Polymer fabrication is about an order of magnitude cheaper than silicon fabrication.

15

Schematic illustration of procedure for fabricating PDMS stamps from a master having relief structure on its surface


Replica molding is the transfer of a topographic pattern by curing or solidifying a liquid precursor against the original patterned mold. The mold or stamp is generally made of a two-part polymer (elastomer and curing agent), called poly(dimethylsiloxane) (PDMS) from photolithographically generated photoresist master. Solvent-based embossing, or imprinting, uses a solvent to restructure a polymer film. Hot embossing, also called nanoimprint lithography, usually refers to the transfer of pattern from a micromachined quartz or metal master to a pliable plastic sheet. Heat and high pressure allow the plastic sheet to become imprinted. These sheets can then be bonded to various plastics such as polymethyl methacrylate (PMMA). Nanoimprint lithography can produce patterns on a surface having 10-nm resolution. Contact printing uses a patterned stamp to transfer ink (mostly self-assembled monolayers) only a surface in a pattern defined by the raised regions of a stamp. These techniques can be used to pattern line widths as small as 60 nm. Replica molding is commonly used for mass-produced disposable plastic micro/nanocomponents, for example micro/nanofluidic chips, generally made of PDMS and PDMA and is also more flexible in choice of materials for construction than conventional photolithography.

16


Microcontact Printing

Xia and Whitesides, Annu. Rev. Matls. Sci. 1998 17


Microfluidics fabrication

Voldman et al., Annu. Rev. Biomed. Eng. 1999 18

(a) Si substrate, (b) grow a thin layer of silicon dioxide, (c) pattern the channel using photolithography, (d) etch oxide, (e) after stripping photoresist, deposit silicon nitride, this layer cannot be etched by KOH and becomes a mask for subsequent etch, (f) perform another photolithography step on the backside to define inflow and outflow holes, (g) anisotropically etch the Si in KOH to form inflow and outflow holes, (h) remove Si3N4 , (i) finally bond a bare silicon wafer to the structure wafer to form the flow channels.

Bottom-up Fabrication (Nanofabrication)


The bottom up approach largely relies on nanochemistry. It includes

• Chemical synthesis • Spontaneous molecular self-assembly from simple reagents in solution

of biological molecules as building blocks to produce 3-D nanostructures • Quantum dots (nanocrystals) of arbitrary diameter (about 10 to 1 x 105

atoms) • Molecular-beam epitaxy (MBE) • Organometallic vapor-phase epitaxy (OMVPE) • Manipulation of individual atoms by STM or AFM • Variety of nonequilibrium plasma chemistry techniques • Nanotubes and nanoparticles

19

References


• B. Bhushan, Springer Handbook of Nanotechnology 3rd Edition, Springer, 2010.

• M. Madou, Fundamentals of Microfabrication, CRC Press, Boca Raton, FL, 1997

• C. Liu, Foundation of MEMS, Pierson Prentice Hall, Upper Saddle River, NJ, 2006

http://www.mecheng.osu.edu/nlbb/

20

Date post:	08-Apr-2016
Category:	Documents
Upload:	sgmartinspam15
View:	10 times
Download:	0 times

Micro Nanofab Techniques

Documents