Overview of Nanofabrication

• Material depostion methods– Thin films of materials– Thickness measurement

• Lithography– Pattern transformation on to planar suface– Direct write, or mask reproduction

• Imaging and Metrology methods– Electron Microscopy– Scanning probe microscopy

Thin film deposition techniques

• Vacuum deposition Methods– UHV (<10-8), HV– Sputtering– CVD– Laser Oblation– Thermal deposition

• Boat or crucible, E-gun

– Epitaxy, growth models

Sputtering

Ar, N2

Vacuum + 10-3 Torr Ar

RF Power

Ar

Target material

substrate

B

E

•RF plasma rectifies RF power, gives DC acceleration voltage

•Ions circle B field lines, increase colisson probability

E-beam evaporator

E-gunFilament

HV

Permanent Magnet

B

Thermal CVD system

http://www.iljinnanotech.co.kr/en/material/r-4-4.htm

Precurser GasFor growing Carbon Nanotubes

Carbon Nanotubes

http://www.iljinnanotech.co.kr/en/material/r-4-4.htm

MBE

"Molecular Beam Epitaxy is a versatile technique for growing thin epitaxial structures made of semiconductors, metals or insulators."

In a ultra-high vacuum, a beam of atoms or, more general, a beam of molecules is directed towards a crystalline substrate such that the atoms or molecules stick at the substrate’s surface forming a new layer of deposited material. But where is the difference between MBE and other material deposition methods as e.g. thermal vacuum evaporation?  

 

     

                      

http://www.wsi.tu-muenchen.de/E24/resources/facilities.htm

MBE and surface analysis chamber

The Knudsen Cell (effusion cell)

http://www.grc.nasa.gov/WWW/RT2002/5000/5160copland.html

Lithography

• Spin coat radiation sensitive polymer - Resist• Expose layer (through mask or direct write)• Develop• Etch away or deposit material

Positive and negative resist

Liftoff requires undercut

Resist Contrast Curve

D1 D2

100%

0%

Film

Ret

entio

n

1

1

2log :Contrast

D

D

D1 D2

100%

0%

Film

Ret

entio

n

Negative Resist Positive Resist

Logarithmic measure of slope of contrast curve

Positive Resist Chemistry

Molecular weight shift

Typical Positive Resist processEXAMPLE PROCESS: AZ5206 POSITIVE MASK PLATE

• Soak mask plate in acetone > 10 min to remove the original photoresist. Rinse in isopropanol, blow dry.

• Clean the plate with RIE in oxygen. Do not use a barrel etcher. RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 5 min. • Immediately spin AZ5206, 3 krpm. • Bake at 80 C for 30 min. • Expose with e-beam, 10 kV, 6 C/cm2, Make sure the plate is well grounded. (Other accelerating voltages may be used, but the dose will be different.) • Develop for 60 s in KLK PPD 401 developer. Rinse in water. • Descum - important Same as step 2 above, for only 5 seconds Or use a barrel etcher, 0.6 Torr oxygen, 150W, 1 min. • If this is a Cr plate, etch with Transene Cr etchant, ~1.5 min. If this is a MoSi plate, then RIE etch: 0.05 Torr total pressure, 0.05 W/cm2, 16 sccm SF6, 4.2 sccm CF4,1 min. • Plasma clean to remove resist: same as step 2 above, for 3 min.

Negative Resist Cemistry

Typical Negative resist process

EXAMPLE PROCESS: SAL NEGATIVE MASK PLATE•Soak mask plate in acetone > 10 min to remove photoresist. •Clean the plate with RIE in oxygen. Do not use a barrel etcher. RIE conditions: 30 sccm O2, 30 mTorr total pressure, 90 W (0.25 W/cm2), 5 min. •Immediately spin SAL-601, 4 krpm, 1 min. •Bake in 90 C oven for 10 min. This resist is not sensitive to room light. •Expose at 50 kV, 11 C/cm2. Be sure the plate is grounded. •Post-bake for 1 min on a large hotplate, 115 C. •Cool for > 6 min. •Develop for 6 min in Shipley MF312:water (1:1) Be sure to check for underdevelopment. •Descum 30 s with oxygen RIE: same as step 2, 10 s. •Etch with Transene or Cyantek Cr etchant, ~1.5 min. •Plasma clean to remove resist: Same as step 2, 5 min.

Photo Lithography

• Project UV light through Mask– Non contact with optical reduction (typical 4X)– Contact with one-to-one pattern transfer

– Mask – very flat SiO2 plate with Cr thin film

– Resolution limited by wave length (phase shift)– Optics hard for short wave lengths

Electron Beam Lithograpy

• Literature, Resources – Handbook of Microlithography Micromachining and

Microfabrication, ed. P. Rai-Choudhury, SPEI press (chapter two is on the web, linked from home page

– J C Nabity web site: http://www.jcnabity.com

Course material is posted on web site in restricted area:http://www.nanophys.kth.se education Intro. to e-beam Lithography

Link to restricted area (password protected)

Username: ebeamlecturePassword: lithogr

Some things you can do with EBL

Circuit of SQUIDs and Josephson Tunnel Junctions

1.5 mm

Contact “cage” to nano-circuit -- for rapid testing

Bonding Pads

ConnectingStrips

100 nm Al

Co

Circuit to measure spin injection from ferromagnet (Co) to normal metal (Al)

Ferromagnetic - Normal metal tunnel junctions

Innerdigitated Capacitor incoplanar waveguide

Cooper Pair Transistor

All these structure were made with

one layer of e-beam lithography and one vacuum deposition cycle!

Block Diagram on an EBL system

Electron Optics

detector

sample

Scanning the electron beam

Aperture

Lens

– convergence angle

Beam diameter2222dcsg ddddd

Electron scattering limits resolution

Higher energy electrons have larger back-scattering range

Double Gaussian profile

Overview of systems

• SEM conversion (NPGS)• SEM modification (Raith)• High end system

– SEM conversion limited in speed by slow beam deflection system (induction in magnet coils).

– Laser stage is big step in price, but necessary for accurate pattern writing and stitching.

– The more complex the system, the more service and higher user costs

– Industry Fab. machines not always well suited to research needs.

NPGS• Joe Nabity, one man company, good reputation, very helpful, good support• Works with many SEMS• Can do stage control, many SEMs come with micrometer, motor control

(accuracy)• Can do precision alignment in single field by scanning in reduced area to find

mark. Manual mark detection.

Good Web site: http://www.jcnabity.com list of references, pictures, ideas

Fabricated with NPGS

This picture shows part of a circular grating with a period of 0.15 microns. The lines appear almost straight, because they are near the outer edge of the grating where the radius is ~100 microns. The pattern was written in PMMA and has been coated with gold for viewing. The lithography was done at the Optical Sciences Center at the University of Arizona.

This image shows a pattern of radially placed dots in PMMA after development. The white bar at the bottom of the image is 1 micron long. The pattern was designed as radial lines, but the spacing of the exposure points was set 0.3 microns to produce discrete dots. Notice how the dot size and spacing is very consistent in all directions. The exposure was done with an SEM with no beam blanker and the image was taken with the NPGS digital imaging feature. The pattern was written by Dr. ChiiDong Chen at the Institute of Physics, Academia Sinica, Taiwan.

Proven resolution with our Raith 150 courtesy of Anders Holmberg

L= 80 nm 50 nm 30 nm 25 nm

20 nm 18 nm 16 nm 15 nm

L=Line width (pitch = 2L)

High End system, designed for Industry Fab.

Nanophys positive process for one-cycle tunnel junction fabrication

• Two layer resist, selective developers• Very large undercut – suspended bridge• Tunnel junction (top and base layer) in one layer

Top view of pattern

Exposed areas

Supporting resist

Undercut region

Next slides:Cut on this axis

Lithography and shadow

evaporation

ZEP 520

PMGI SF7

SiOx

Si

Lithography and shadow

evaporation

Irradiate with electron beam

Lithography and shadow

evaporation Develop the two layers selectively

Top layer: Bottom Layer:

Lithography and shadow

evaporation

Evaporate Al at an angle

Lithography and shadow

evaporation

Oxidize the first layer

Lithography and shadow

evaporation

Evaporate Al at opposite angle

Lithography and shadow

evaporation

Lift off the resist and excess metal

Tunnel junctions

Circuit of SQUIDs and Josephson Tunnel Junctions

Voilà

3D structuring using contrast curve

•Accurately measure thickness of film

•Do test pattern with dose profile to accurately measure contrast curve

Patterning in third dimensionth

ickn

ess

Dos

e

Desired structure:

Holography

Chalmers Group, S. Hård et al.Applied Optics vol. 33 p 1176, 1994

Positive electron resist SAL 110 Developer SAL 101

(Shipley)

Optical Kinoforms

Chalmers Group, S. Hård et al.Applied Optics vol. 33 p 1176, 1994Optical Comm. Vol. 88, p 37, 1992

Two basic types of pattern methods

• Direct Writing– Change pattern with each run– Slow, serial method of fabrication– Good for research and development– Low through-put, too costly for large scale production

• Lithography– pattern copying one process step– Fast, parallel method– High through-put makes low cost in large scale prod.– Not flexible enough for research and development.

Comparison of Lithographic methods

• Photo Lithography– UV, deep UV– Projection or contact

• Micro contact printing– Stamp formed from Soft material– Molecules (ink) is wet on to stamp, transferred

to surface

• Printing Press

Micro Contact printinghttp://www.research.philips.com/technologies/light_dev_microsys/softlitho/

Stamp fabrication

•Master made by direct writing methods (EBL on Si + etch)•Stamp gets dirty, wears out•Essentially old-style printing methods scaled to nm dimensions

SAM’s and molecular electronics

Optical Stepperhttp://www.sematech.org http://www.nanonet.go.jp/

For example: Nikon optical steppers

High through-put direct writing tool

http://www.micronic.se

Sigma 700 series from Micronic Laser systems, Täby Sweden

Spatial Light Modulator (SLM) chip10 6 electronically addressable mirrors

Alignment and overlay

• Alignment and overlay are more serious problems than actually making the small structure!

• Large area with fine detail requires “stitching” write fields together – laser interferometer stage, nm position and metrological accuracy!

• Overlay requires accurate alignment marks, mark detection, registration and extremely accurate pattern placement over large area (scaling accuracy 1 part 106).

3-layer process done in Albanova

Industry has MUCH more sophisticated circuits with 15-20 layers, 108 components, with very accurate overlay

Metrology and Imaging

• Laser interferometers on Stage– 5nm “resolution”– Reproducibility

• Thickness measurement– Profilometer, demonstration

• Scanning Probe microscipe– Vertrical resolution 1 Å level– Latteral resolution depends on tip sharpness

SPM system overview

Scanning Tunneling Microscopy (STM)Binnig and Rohrer 1981 (Nobel Prize in Physics 1986)

))(22

exp( 0 dEUmPI

Electric current proportional to quantummechanical probablility amplitude of ”tunneling”through the energy barrier

Wave´function decays eponentially in barrier region

z

)(zE(z)

E0 U

d

Single Atom imaging possible

•Sharp tip•Pristine surface •Ultra High Vacuum

The making of a Quanum CorralFe atoms on a Cu (111) surface

http://www.almaden.ibm.com/vis/stm/gallery.html

Check out this web page

Atomic Force Microscopy (AFM)

Two Basic AFM Modes:Contact mode (no vibrating tip)

Tapping mode (vibrating tip)

Many variations on Scanning Force Microscopy :Liquid AFM

Magnetic Force Microscopy (MFM)Latteral Force Microscopy (LFM)Intermitant and non-contact AFM

Force Modulation Microscopy (FMM)Electrostatic Force Microscopy (EFM)

Atomic Forces

Hard core repulsionContact region

Attractive force: van der WallsNon-contact retion

Force

zSeperation between tip and surface

Image molecular monolayers in liquid

S-layer protein monolayer on Si surfacein liquid environment, 500 nm x 500 nm

Zentrum für Ultrastrukturforschung - Universität für Bodenkultur. Austria

•Molecules must be immobilized on surface

•Local force measurements possible

Two basic scanning modes

1. Feedback off: Scan over surface with constant z0 (piezo voltage), control signal changes with tip-surface separation.

2. Feedback on: circuit regulates z piezo voltage to constant value of control signal (constantly changes tip-surface separation).

AFMContact mode

AFM tapping mode

Free space oscillation of cantileverresonance 10-100 kHz

Cantilever hits surfacesmaller amplitude of oscillation

Feedback loop tapping mode

Free oscillationLarge amplitude

Hitting surfacelower amplitude

Digital Insturments Multi-Mode

head, scanner and base

• Turn on the controller (the computer should be left on)

• Remove the scanner from under the microscope.