engineering atom chips michael kraft nano-scale systems integration group school of electronics and...

Engineering Atom Chips

Michael Kraft

Nano-Scale Systems Integration GroupSchool of Electronics and Computer Science

Southampton University

Michael Kraft Engineering Atom Chips 2

Overview

What are Atom Chips?Building Blocks of Atom Chips

WiresCavitiesActuators

Atom Chips ExamplesConclusions


Control electrons using wires

Control photons using optical fibres

How do you control atoms?ATOM CHIPS! • Using electromagnetic fields and light to interact with

clouds or single atoms• Atoms (or clouds) can be trapped in magnetic fields and

hover a few um above a chips surface

What are Atom Chip?


Devices for trapping and manipulation of atoms on integrated microchips.

Quantum laboratories on chip.

Fundamental research • Quantum behaviour• Low dimensional physics• Entanglement and coupling

Atom Chips

New devices – precise sensors• Atom interferometers• Atomic clocks• Accelerometers/Gyroscopes• Quantum information processing• Quantum computers


High c

urrent d

ensi

ty g

old w

ires

Electrostatic xy comb driveElectrostatic z parallel plate

Fibre gold coated at the tip

SiliconBose-Einstein atom

cloud

Tuneable optical cavity

Atom Chip


Multi-Domain MEM System• Electrostatics

– 3D Actuator for optical cavity alignment & tuning

• Electromagnetic– Confinement field for atom clouds

• Optical MEMS– Optical cavity for single atom detection

→ INTEGRATION is a key issue!


Processing Challenges• Wet and Dry Etching of Silicon

– Smooth cavities– DRIE for high aspect ratios

• Electroplating and/or etching of Gold– High current density, smooth edged gold wires

• Assembly– Multi-level wafer bonding with good alignment

• Ultra high vacuum compatible→ Considerable process development

necessary

→ Applicable to other MEMS devices


• High current density wires allow the creation, trapping and manipulation of cold atoms and BEC’s.

• Neutral atoms in a magnetic field feel a potential due their magnetic moment.

V - potential, µ - magnetic moment of the atom B - magnetic field.

• It is this potential that is used to trap and manipulate the atoms. Atoms accumulate in areas of minimum potential.

BVMag

Atom Guides - Wires


Splitting Atom Clouds

Minimum coalesces

Minimum splits


Cold Atoms and Bose-Einstein Condensate

High temperature

Solid balls

Low temperature

Wave packets

T = T(crit) =170nK for 87Rb

Bose-Einstein condensation

Matter waves overlap

T<T(crit)

Pure BEC, Single matterwave


Laser Cooling

Setup three counter propagating laser beams and a magnetic field

Du, PhD thesis, U. of Colorado, 2005

MOT on chip: use 3 lasers and a mirror


Wire Fabrication: Electroplating

Silicon substrate with 100nm of oxide deposited100nm of gold is depositedThe Cr/Au layer is patterned using a wet etchAn electroplating mould is created using photoresist5µm of gold is electroplating into the mouldThe resist is removed creating the finished chip

Silicon Silicon oxide

Chromium Gold

Photoresist


Four Wire Trap


Problems With Electroplating•Resist reflow


Problems With Electroplating•Mushrooming


Problems With Electroplating•Current density


Fabrication: Ion Beam Milling

Silicon Silicon oxide

Chromium Gold

Photoresist

Silicon substrate with 100nm of oxide deposited5µm of Gold is sputteredPhotoresist is spun and patterned The Gold is ion beam milled or wet etchedThe resist is removed creating the finished chip


Problems With Ion Beam Milling

Variable etch rate across the wafer, leading to over etching


Fabrication Challenges

Electrochemical deposition into a mould

Ion beam milling

Gold and chromium wet etch

Corrugation in these wires causes fluctuations in the magnetic field that leads to fragmentation in the atom cloud.


Gold Wires

2cm

Gold mirror

Gold mirror

Contact pads

Trapping wires

‘Atom Chip’Layout

Wire Atom Chip Under Test

23 mm

3.5 microns of gold

67 m

Atom Interferometer on a Chip

spectacular sensitivity too EM fieldso gravityo other feeble forces


Current cooling techniques• Atoms are cooled in a macroscopic magneto-

optical trap (MOT).• Clouds are then transferred from the

macroscopic MOT cloud to the microscopic Atom Chip.

Inverted Pyramid: MOT on a chip• Pyramids on chip can be used to act as a MOT• Simpler system, automatic alignment, arrays of

MOTs possible.

Pyramidal Micro-cavities


Pyramidal Micro-cavities

http://www.ic.ac.uk/research/ccm/research/micropyramids.htm

KOH etched inverted pyramids with current carrying wires


Pyramidal Cavities


Biggest pyramid in the mask design = 1.2 mm

Atomically smooth side walls

SEM


Electrophoritic resist is deposited in the pyramids and patterned

The gold and chromium is wet etched

The resist is removed leaving the flower patterned pyramids

Pyramid created by process shown previously leaving it with a gold coating

Pyramid PatterningReflected gold coating needs to be removed

at the edges to avoid disturbing light reflections


Patterned Pyramid With Wires


(100) Silicon wafer, 170nm of oxide is deposited followed by 50 nm of Nitride and the alignments etched into the backThe alignment marks are etched into the oxideOpenings are etched into the fronts for the pyramidal etch The back alignments are protected with a PECVD nitride layer The pyramids are etched in KOHThe front nitride and oxide are stripped170nm of TEOS oxide is deposited along with 50 nm of chromium and 100nm of goldThe chromium/gold layer is patternedAn electroplating mould is created from AZ9260 resistThe gold wires are electroplated, the resist removed and the chips completed

Silicon

Silicon oxide

Chromium

Gold

Photoresist

Silicon nitride

Pyramid Atom Chip Fabrication


Patterned Pyramid Atom Chip


Pyramid Atom Chip in the Lab

http://www.ic.ac.uk/research/ccm/research/micropyramids.htm


• Spherical microfabricated cavities are ideal for making high finesse optical resonators.

• The aim is to achieve single atom – photon interaction.

• Light couples directly in and out of the resonator through an optical fibre.

Spherical Micro-cavities



Focal spots clearly visible under microscope


A silicon substrate with 100nm of oxide deposited and patterned100nm of silicon nitride is deposited and patternedThe silicon is etched using a HF based solutionThe silicon nitride is stripped using orthophosphoric acidThe silicon is etched using an ASE isotropic etchA 50nm Chromium and 100nm Gold layer is sputteredPhotoresist is spun and patterned 3µm of Gold is sputteredPhotoresist is spun and patterned and the gold is ion beam milledThe resist is removed creating the finished chip

Silicon

Silicon oxide

Chromium

Gold

Photoresist

Silicon nitride

Spherical Cavities Fabrication


optic

al fi

bre

0.9999 bragg stack

dielectric coated micro-mirror 0.9999

reflection

finesse = 5200

74 pm

390 nm

100 m

High Finesse Optical Cavity



• Various etch rates can be used to make any radius of curvature

• Longer etch rates gives smoother mirrors


Atom Chip

s il ic o ns u b s t ra t e

g o ld w ire s

s il ic o ns u b s t ra t e

g la s ss u b s t ra t e


Actuation Design Guidelines

• xy actuation– Alignment of optical cavity– Misalignment between

fibre and spherical mirror during fabrication

– xy translation of 5-10 m– xy actuation accuracy of

0.5-1 m

• z actuation– Stable and tunable

optical cavity– z translation of 4-5 m

(coarse tuning)– z actuation accuracy

of a few nm (fine tuning)


Translation in +xy direction(xy-actuator)

At 117 V a maximum coverage area of 17.5 by

17.5 m is achieved.


Resonance Frequencies

Mode 1: Resonance frequency (fzres=581Hz) in z motion Mode 2: Resonance frequency (fxres

=820Hz) in x motion

Mode 3: Resonance frequency (fyres=820Hz) in y motion


Actuator Chip Prototype


Fabrication Process XY Actuator

Silicon substrate (380 um)1st dry-etch (320 um)Glass substrate (500 um)Anodic bonding2nd dry-etch (60 um)


Electrostatic x comb drive

Tuneable optical cavity(spherical cavity and plane mirror )

Bose Einstein atom cloud

Fibre with spherical gold coated cavity tip fitted in v-groove

High current density gold wires

Silicon

In Plane Atom Chip Design


• Microfabrication is a very suitable approach for manipulating clouds of or single atoms

• Established a modular ‘toolbox’ for atom chips, including wires, optical cavities and actuators

The near future• Atom arrays• Control over single atoms

Further forward• Miniaturised atom devices and sensors

Far future• Quantum computing with neutral atoms?

Conclusion


Imperial College London

E.A.Hinds

Pyramids• Jonathan Ashmore• Fernando Ramirez Martinez• Sam Pollock• Athanasios Laliotis

Cavities• Michael Trupke• Jon Goldwin• Joanna Khunner• Athanasios Laliotis Atom guides• Stefan Eriksson• Rob Sewel• Joss Dingjan

University of Southampton

Michael Kraft

• Gareth Lewis• Zak Moktadir• Carsten Gollasch

Nanoscale SystemsIntegration Group

People Involved

engineering atom chips michael kraft nano-scale systems integration group school of electronics and...

Documents

atom guides wires

atom clouds minimum

atom b magnetic field

single atoms atoms

manipulation of atoms

neutral atoms

minimum splits slide

manipulation of cold