Microfabrication of gold wires for atom guides
Summary (2 min read)
1. Introduction
- In the last few years considerable interest in the design of microscopic atom traps for manipulating neutral atoms has developed.
- Miniature atom traps have potential applications in many fields including quantum computing and integrated atom optics.
- The overlap between the two ∗ Corresponding author.
- The purpose of this work is the microfabrication of wires capable of carrying high density currents necessary to create high magnetic field gradients used for atom trapping.
2. Atom trapping
- Neutral atoms with an unpaired electron have a magnetic dipole moment, and interact with magnetic fields.
- Creating these magnetic fields using microfabricated wires on a chip allows the atoms to be trapped and guided close to the surface, and the flexible geometry of the wires allows a wide variety of trapping potentials to be produced.
- These simulations were made for three different values of the current.
- Once the chip is loaded, the atoms can be cooled further using radiofrequency evaporative cooling.
- Fig. 3 shows absorption images of atom clouds; the sharp peaks are the signature of Bose–Einstein condensation on a chip.
3.1. Sputtering technique
- The challenge of the fabrication technique is to produce thick, high density current-carrying gold wires.
- The first fabrication process that was developed in this work is based upon sputtering and wet etching of a thick gold layer.
- The full fabrication process flow is described in the following: (1 0 0) silicon wafers (p-type, 17–33 cm resistivity, 100 mm in diameter) were cleaned in fuming nitric acid (FNA) for 10 min, followed by rinsing in de-ionised water and spin-dryed in a nitrogen environment.
- This was followed by a spin-coating step to deposit standard photoresist (S1818) using a hand spinner.
- The process flow is diagrammatically illustrated inFig.
3.2. Electroplating technique
- First, electrochemical deposition produces a high density of the deposited material in the holes of the template and leads to volume templating of the structure as opposed to templating of material.
- Various material properties, such as the grain size and surface roughness, can be controlled by the electroplating conditions[5,6].
- This was followed by spin-coating of thick positive photoresist SPR 220-7 (up to 14 m) which later was used as a mould for the gold electroplating[8,9].
- This was achieved by exposing the edges of the wafer for 700 s and developing for 2 min.
- Then, a thin Au–Cr layer (40, 300 nm) is evaporated and photolithographically patterned using a standard thin photoresist (SPRT 518) in order to define electroplating areas for gold wires, to provide electrical isolation between gold mirrors, gold wires, and contact pads.
4. Fabrication results
- 8 shows a top view of the atom guide prototype fabricated by the sputtering technique.
- Another requirement is to have a low surface roughness and a uniform grain size in the wire.
- These microstructural aspects were investigated using an atomic force microscope.
- Fig. 11 shows a SEM picture of gold 6.1 m thick electroplated wires after removal of the photoresist.
- The scan also shows that the sample has a finer grain structure of approximately 90 nm.
5. Conclusion
- The authors fabricated gold wires for atom chips using a sputtering and an electroplating technique.
- The authors have compared the microstructural properties of the wires fabricated by both techniques.
- Also, the surface roughness was smaller using the electroplating techniques.
- From these findings and considering the low-cost and simplicity of the electroplating technique, the authors decided to develop this process further to fabricate more complex atom guides for an atom interferometer.
- Currently, the fabricated wires are being tested by loading a Bose–Einstein condensate atom cloud and moving it in the magnetic confinement field along the wire.
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Citations
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Cites background from "Microfabrication of gold wires for ..."
...Furthermore, most microwires that are fabricated into microcoils are composed of non-magnetic metallic substances like gold [25, 26], copper [27], or aluminum [28] and carry currents to influence or sense the motion of nearby magnets....
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2 citations
1 citations
Cites background from "Microfabrication of gold wires for ..."
...The wires are micro-machined on a silicon substrate [10]....
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Cites background from "Microfabrication of gold wires for ..."
...With the advancement of micromachining technology, it is possible to realize such devices provided the processing challenges are appropriately addressed [5, 6]....
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References
1,212 citations
"Microfabrication of gold wires for ..." refers methods in this paper
...The next step was wet chemical etching of Au with aqueous KI 3 solution (4 g KI, 1g I2 in 40 ml H2O) and then Cr in a mixture of a ceric ammonium nitrate (H 8CeN8O18) with nitric acid (5 g H8CeN8O18, 4 ml HNO3 (70%) in 5 ml H2O) [4]....
[...]
...The next step was wet chemical etching of Au with aqueous KI3 solution (4 g KI, 1g I2 in 40 ml H2O) and then Cr in a mixture of a ceric ammonium nitrate (H8CeN8O18) with nitric acid (5 g H8CeN8O18, 4 ml HNO3 (70...
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...%) in 5 ml H2O) [4]....
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636 citations
"Microfabrication of gold wires for ..." refers methods in this paper
...Recently, Bose–Einstein condensed atom clouds have been produced using micrometer-sized atom traps fabricated on chips[1,2]....
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384 citations
"Microfabrication of gold wires for ..." refers methods in this paper
...Recently, Bose–Einstein condensed atom clouds have been produced using micrometer-sized atom traps fabricated on chips[1,2]....
[...]
203 citations
"Microfabrication of gold wires for ..." refers background in this paper
...Cold atoms trapped on atom chips have already been used as a highly sensitive probe of the magnetic field fluctuations above a conducting wire [3]....
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186 citations
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Frequently Asked Questions (14)
Q2. What is the r.m.s. surface roughness of gold wires?
The r.m.s. surface roughness of gold wires is about 16 nm, which is expected to be low enough to make it suitable for atom trapping applications.
Q3. What is the method for achieving the optical cooling of the atom clouds?
For their applications a very smooth mirror surface, which is crucial for realising the optics for the laser cooling of the atom clouds [11].
Q4. What was the gold substrate with the template?
The gold substrate with the template was the working electrode with a large area platinum gauze counter electrode and a custom-made saturated calomel reference electrode (SCE).
Q5. What is the problem with the surface of gold wires?
the surface of gold wires has to be very smooth since roughness of the surface may cause a non-homogeneous current distribution.
Q6. How many atom clouds were trapped above the central wires?
4. The four central wires, above which atom clouds can be trapped, were 50 and 100 m wide, 7 mm long and separated by a gap of 30 m.
Q7. What is the method for producing thin supported layers for photonic mirrors?
electrochemical deposition is ideal for the production of thin supported layers for applications such as photonic mirrors since the surface of the electrochemically deposited film can be very uniform [7].
Q8. What is the process used to define electroplating areas for gold wires?
a thin Au–Cr layer (40, 300 nm) is evaporated and photolithographically patterned using a standard thin photoresist (SPRT 518) in order to define electroplating areas for gold wires, to provide electrical isolation between gold mirrors, gold wires, and contact pads.
Q9. How many atoms are in the atom cloud?
Using a simple atom chip based on a single macroscopic wire, the authors have successfully produced Bose–Einstein condensates of 87Rb containing 5 × 104 atoms.
Q10. What was the process of electroplating Au?
Once the photoresist moulds were fabricated, electroplating of Auwas performed without a hardbaking the photoresist since this can cause SPR 220-7 to reflow.
Q11. What is the importance of uniform current distribution in the wire?
This is important for uniform current distribution which is required to obtain a high current density in the wire necessary for an atom guide.
Q12. What is the problem with the electroplating?
One problem with the described electroplating is that the achievable smoothness is not sufficient for the gold mirrors without further optimisation of the electroplating solution using special additives such as brighteners.
Q13. What was the thickness of the wafer?
This thickness was achieved with a single layer coating to get good contact between the mask and the wafer the authors had to remove the photoresist edge bead which was formed during the low-speed coating.
Q14. What is the difference between electrochemical deposition and templating of material?
electrochemical deposition produces a high density of the deposited material in the holes of the template (mould) and leads to volume templating of the structure as opposed to templating of material.