Nanomagnets - Bottom Up and Top Down Processes

Nanomagnets RAPME Report

The practical use of nanomagnets and processes for creating

nanowires and nanomagnets from the “bottom-up” approach of

electrodeposition and the “top-down” approach of E-Beam

Lithography will be discussed. Results from these experiments will

also be shared.

2013

Miles Q. Braxton RAPME

VCU Nanotechnology Laboratories 7/24/2013

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Nanomagnets are essential to the future. They are currently being researched around the world to be used in the place of transistors in computers. Their purpose is to increase

computing power and decrease energy (essentially heat) used in a system. Nanomagnets would be ideal because they are so infinitesimal, that one could have a lot of them in even a

microscopic area. The fact that they are infinitesimal is also to their detriment because of the increasing entropy on a nanometer scale – quantum theory makes these magnets less

stable – a misplaced particle like an atom or electron can significantly change the properties of these magnets. The RAPME program brought in students from around the Richmond Metropolitan area to assist graduate students at Virginia Commonwealth University (VCU)

with their research on these nanomagnets in hope to kindle an interest in future STEM studies.

There are two approaches used to create nanomagnets: “bottom-up” and “top-

down”. The bottom-up approach starts with creating nano-pores. Essentially, the pores are

being built upon, creating layers upon layers. In the top-down approach, layers are first created, and by each step, components of the layers are being removed.

The RAPME program started with everybody suiting up in the clean room. Clean

room suits are used to protect the chemicals from human body - debris like hair, dead skin cells, and other organic matter on the body. The suits are needed when creating nanowires at the wet bench, as copious chemicals and solvents are handled. Other typical lab safety

materials are used in the clean room as well: gloves, goggles, hair caps, and shoe covers. These safety precautions are important to follow so the chemicals are protected from

excessive contamination and the scientists leave the clean room uncontaminated. Any experiments using solvents and chemicals must be carried out on the wet bench due to

safety protocols; therefore to produce nanowires, the wet bench has to be used.

Bottom-Up

Anodization

When creating nanowires, the first step is to attach a thin sample of aluminum to the electrochemical flat cell. This is done by holding the sample of aluminum over the side hole

on the flat cell with tweezers or simply a finger, and screwing the side of the cell completely so the hole is completely entrenched by the solid aluminum sample. It is imperative to make sure the hole is closed because liquid will be put in the cell during the next step; if the

hole in not completely covered, there will be a leak and acid would have been wasted. With the secure aluminum piece on the side of the flat cell, oxalic acid (H2C2O4) is poured in the

top opening of the cell using a funnel. The oxalic acid should fill the cell entirely, but not to an extent where the acid will overflow. Other acids can also be used for this process, like

sulfuric acid (H2SO4). Wires from a HP/Agilent Voltage Source are then distributed to the flat cell to give the cell a current to oxidize the piece of aluminum. The red wire coming from the voltage source is attached to the metal prong holding the aluminum in place on the

side of the flat cell. The black wire is attached to the opposite side the flat cell, to a discernible graphite prong sticking out of the cell. Once the wires are attached

appropriately, anodization is conducted at DC 40 V for 15 minutes. The 15 minute current

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is enough time to create about a micron thick nanofilm. This anodization step is done to oxidize the solid aluminum for electrodesposition – some samples nickel ion (Ni2+) and

some cobalt ion (Co2+). In order to retrieve the aluminum sample from the flat cell, vigorously and swiftly flip the flat cell up-side-down to drain the oxalic acid in a container.

Unscrew the sample holster while holding the aluminum piece with a pair of tweezers so it does not fall on the wet bench. There should a purplish-green tint in the aluminum sample

in a circular area; this represents the oxidation of the metal that took place and the micron thick nanofilm. This nanofilm contains thousands of nano-sized pores that cover the surface of the oxidized portion of the aluminum.

Figure 1.1

Making Solution

In order to complete the nanowires, a transition metal, such as nickel and cobalt, needs to be dispersed on the aluminum and into the pores. This is done using the flat cell again, but first solutions of both nickel and cobalt need to be made. Saumil discloses the

ingredients of the nickel solution are 250 mL Water (H2O), 6.57 g Nickel Sulfate (NiSO4), and 1.75 g Boric Acid (H3BO3). The nickel sulfate and boric acid are two powdery solids.

A scale is used to measure their two masses carefully after zeroing the scale to the small beakers used to put the powders in. This is a very riveting task in that all of the powder has

to be out of the beakers so the amount the solution calls for is exacting; the same applies for

the cobalt solution. Water is used to get powder sticking inside the beaker into the bigger beaker of solution. The cobalt solution consists of 250 mL Water, 7.02 g Cobalt Sulfate

(CoSO4), 1.75 g Boric Acid. Once the solutions are in separate beakers, the beakers are placed on heating magnetic stirrers overnight at 80˚C with the stirrers set at about 300 rpm.

The cobalt solution should be a red-pinkish color and the nickel solution should be a spearmint green color.

The aluminum square on the left is standard and has not been oxidized; the color of the

sample on the right resembles the color of the oxidized circle on the used aluminum samples.

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Figure 1.2

If there is an even number of oxidized aluminum samples, split them into cobalt and nickel samples. The aluminum samples are ready to go back on the cells. This is a very

arduous task because the circle of oxidation has to be aligned with the hole of the cell. At least ninety percent of the oxidized circle needs to cover the hole. Once the components of

the solutions are interspersed completely and the aluminum samples are properly aligned, the solutions can be poured in the clean flat cells, cobalt in one and nickel in the other.

Electrodeposition

Much experimentation occurred with which time intervals and voltage induced is needed for appropriate electrodeposition so the pores do not overflow with cobalt and nickel solution. This incident did arise during initial experimentation and readings were not as

accurate on the SEM. The disadvantage is that there will be a lower coercive field (Hc) reading, but the advantage is that its magnetic properties will be found easily. The proper

increments should be 15 V for 4 minutes for nickel samples and 12 V for 3 minutes with cobalt samples. The wire color placement does not matter now that the current is AC and

the polarity is irrelevant because the current oscillates from positive to negative. Therefore,

we specify the frequency to 250 Hz so the amount of times it oscillates back and forth can be regulated in a set period of time. There should be a grayish tint or faint black rim on the

sample around the oxidized circle on the aluminum sample when the time is up. If there is not, more time needs to be distributed for the samples to develop an electrodeposited layer.

As the samples finish, they are placed in boiling water for about 30 minutes, one beaker of boiling water for nickel, one for cobalt. This is done to prevent oxidation of cobalt and

nickel by closing the pores made by the anodization. We do not need oxygen in the pores, as oxygen molecules will oxidize the wires and nearly ruin the sample.

The solution of the left is Cobalt and has a red-pinkish coloring; the solution on the right is a Nickel

and has a rather spearmint or aqua-forest or dark teal coloring.

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Figure 1.3 Figure 1.4

SEM Imaging Nanowires have now been fabricated through the bottom-up approach of electrodeposition. In order to examine the images above and the nano-sized pores, a

Scanning Electron Microscope (SEM) is used. The SEM shoots a beam of concentrated high velocity electrons at a sample under the microscope to fabricate an image. The

electrodeposited samples are cut to fit on the SEM circular sample holding plate which is crewed on to the bottom half of the sample holder. This apparatus is then lined up under a

36 mm SEM measuring device used to make sure it will fit in the SEM appropriately; the space between the sample holder and the measuring device should be as close as possible without touching.

Before loading the samples into the SEM loading chamber, the loading chamber

needs to be flooded with air to remove the vacuum present. Once the SEM signifies the vacuum is gone with a beep, open the loading chamber. There are two prongs that receive

the two holes on the bottom of the sample holder; the sample holder is pushed on to the prongs and it is secure at the sound of a click. The black knob is then switched to “LOCK” so that the sample holder is fully secure in the SEM. The loading chamber is then tightly

closed and the button labeled “EVAC” is pressed to evacuate the air and recreate a vacuum. The chamber door inside of the loading chamber is then opened to place the sample holder

under the microscope by pressing the “OPEN” button. The rod that holds the sample is then pushed into the microscope and the sample holder is received by an interceptor under

the “eye” of the microscope. It is imperative to pull the rod out slowly at first as the sample holder might still be attached. The door is then closed and ready for viewing. Adjusting the SEM to get a clear picture of the sample is an onerous task and may take a while.

The SEM image on the left (Figure 1.3) is a near perfect example of a solution filled pores

and in essence a great nanowire. The SEM image on the right (Figure 1.4) is a perfect

example of an overfilled sample; the pores are not visible at all and the top is covered

either in nickel or cobalt excess.

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VSM Testing Whether creating nanomagnets from the bottom-up or top-down approach, a proper

MH Curve or Hysteresis Loop should be constructed. The Vibrating Sample Magnetometer (VSM) plots the saturation of a magnet at certain points by exposing a sample to increasing

and decreasing the magnetic field (H) and recording how the change in flux density (M) or magnetization. M is usually expressed in emu and H is usually expressed in oesterds. The

width of the gap in the MH Curve characterizes the metal. The x-axis of this graph is magnetic field and the y-axis is the magnification of flux density or magnetization. A significantly trimmed sample is loaded onto a special VSM sample holder and the sample

has to be about 35 mm from the end of the holder in the designated area for the sample. It should be under a tacky surface so that it does not fall off; if it falls off in the VSM, there is

no getting it out. When the sample is ready, flood the VSM with air so the sample can be loaded. Once the sample is loaded, recreate the vacuum; the cap above the sample should

be irremovable when vacuum is reestablished. 300 K is the standard temperature to use for

this experimentation. The temperature needs to be a control because its change can affect the saturation curve. A preset program on the VSM is used to measure the magnification of

the flux density from the sample at different points on the graph; positive and negative values of magnetic field are acted upon the sample.

On the left (Figure 1.5), Dr. Atul operates the SEM using the magnifying and focusing

knobs. The picture on the right (Figure 1.6) is a close-up of Vimal managing the loading

chamber and its components; the rod is sticking out of the chamber and the buttons are

above it.

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The picture on the top left (Figure 2.1) is showing how the small piece of nanomagnet sample

is aligned on the VSM sample holder. The picture on the top right (Figure 2.2) shows Vimal

putting the sample holder with the sample in the air-flooded VSM. The picture on the bottom

left (Figure 2.3) shows what a MH Curve looks like. Where the curve flattens out toward the

top is referred to as the saturation point or line. Temporary magnets like ferromagnets and

paramagnets have a residue of magnetization where there is still magnetism present when the

magnetic field goes back to zero. The distance between the x-intercepts is known as the

Coercive Force or Coercive Field. The larger the coercive field is, the stronger the

nanomagnet. The bottom right image (Figure 2.4) is an actual graph calculated by the VMS at

200 K.

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The distance between the x-intercepts is the coercive field (Hc). The coercive field represents the difficulty in changing a substance’s magnetization. The larger this field is, the

stronger the magnet is. If the Hc is too small, magnetism can be flipped in a different direction by the slightest fluctuation in transistors. The VSM was not operating correctly

because the sample was inserted vertically rather than horizontally giving inaccurate measurements of the MH Curve.

Bottom-Up After the steps of the bottom-up approach are completed, nanomagnets are then

created by the top-down approach. There are five main steps of the “top-down” approach of E-Beam Lithography: spin-coating, E-Beam Lithography, Developing, Deposition, and

Liftoff.

Spin-Coating

Silicon wafers are used during the E-Beam lithography process because they are impeccable semiconductors – they will not interfere with the viewing of nanomagnets on the

surface and can easily be coated by a 15 nanometer-thick layer metal. At about 50 nm think, the metal glaze would be visible on the silicon so the 15 nm layer will not be that

pronounced. A piece of silicon wafer is cut using a diamond-tipped pen. The diamond-tipped pen is used because diamond is the hardest material known in nature and it will

scratch without being scratched itself. This makes cutting the silicon wafer fairly easy. It is cut by scratching a line closer to the end of the wafer. Eventually a whole silicon strip will break off. The same scratching process can be done to break off several segments of the

strip; these will be the substrates. To initiate the spin-coating process, the small pieces need to be held on the vacuum securely without leak. This is not as simple as it sounds though.

There are small grooves in which air seeps out between the vacuum and the sample. The solution to this is to tape the grooves as well as possible and poke a hole where the vacuum

hole is with a stapler pin. There is a small disk that Vimal used to set on top the tape to rest the silicon piece on. When the disk is put on, the vacuum level should read 13 or 14. The goal is to get the pressure up to 19 or 20, once the silicon is put on the disk directly over the

vacuum. Once everything has settled and there is an adequate pressure reading the spin-coating can begin. Each piece of silicon is then spin-coated first in a layer of 495 Poly

Methyl Methacrylate (PMMA), or E-Beam resist, and then a layer of 950 PMMA. The two different numbers represent different molecular weights of the PMMA. Using a disposable

pipette, no less than 4 drops of 495 PMMA is spread on the silicon sample. The vacuum may suck some of the PMMA in, so it is important to have a hefty amount on the sample. The lid is closed and “Program J” is run. “Program J” entails that the spinner will run at

500 rpm for 5 seconds and 4000 rpm for 45 seconds. The 500 rpm for 5 seconds is to make sure the resist spreads evenly on the surface as at 4000 rpm most of it runs off. Once that

finishes, the same spinning process is done with the 950 PMMA. The bilayer coating is done to improve the shape of the nanomagnets post liftoff. The samples are then baked on a

120˚C hot plate for 5 minutes so the new E-Beam resist can make a packed layer on the silicon samples. The spin-coating step is now finished.

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E-Beam Lithography

The next step is to start making deigns using CAD software made especially to work with the SEM and for nano-etching. The program fills all polygons and Noel and Vimal

specifically asked for ellipses and quadrilaterals so they were made. My lab partner, Malcolm, and I also made our names in a bubble letter font hoping the images would come out well. Unfortunately, the designs did not come out well because of a blanker issue in the

SEM.

Figure 3.1 is a schematic of how the silicon substrate has a PMMA bilayer on top of it.

The top layer is the PMMA and the lower layer is 495. Figure 3.2 shows the top view

of the PMMA 950 on the spin-coated substrate. On the bottom left (Figure 3.3) is an

image of a silicon sample preparing to spin after the PMMA has been applied. The

picture on the bottom right (Figure 3.4) shows two E-Beam resist samples being

heated on the hot plate once the both PMMA dosages were applied.

Silicon Substrate

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The E-Beam resist samples are loaded onto the SEM sample holder and held on by clamps.

There are 3 pieces of material on the sample holder: 2 silicon substrates coated with E-Beam

resist and 1 silicon wafer piece with gold nano particles on it. The silicon with the gold

particles acts as a reference point. The size of the gold particles is of the order of 15 nm.

The gold particles are also ideal because they do not react with air, water, or most solvents.

The purpose of Gold standard is to get a good focus and resolution as the patterns designed

are of the same scale. It’s important to get a good resolution at the same scale to get patterns

with expected shapes. The substrates are then exposed to E-Beam at 30 KV. The areas

exposed to high energy E-beam on the PMMA on the silicon samples will loosen its

covalent bonds. Once the CAD files are created and saved properly, they are then loaded

onto the computer controlling the SEM. The nano-images do not take long to etch. The

designs are etched near the scratches made on the E-Beam resists so the locations of the

designs are easier to find with naked eye and under an optical microscope. The first scratch

is approximately at (-6.631 mm, 3.491mm) and the second near at (-5.466 mm, -0.284 mm).

The picture on the left (Figure 3.5) was taken after the liftoff process. The lines going

from pattern to pattern are quite visible. When the blanker has problems shutting

off the beam, the high velocity electrons drag across the design. This even interfered

with the pattern in the middle of the designs. What are supposed to be nano-sized

lines of shapes are now replaced by several microns of a dark bubble. On the right

(Figure 3.6) is a microscopic image of the girls’ sample. One out of the four designs

they made has been salvaged; this one is on the upper left. It has no colored circle in

the middle.

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The next step is the developing process. The etched samples are agitated in the

developer solution of 3:1 Methyl Isobutyl Ketone (MIBK) for 70 seconds. The samples are

then washed under Isopropyl Alcohol (IPA) for 20 seconds. This process washes away the

weakly bonded areas of the PMMA layers and creates holes.

In the SEM image on the left (Figure 3.7), the white dots represent gold particles

used to reference the scale to the other silicon samples. In the middle of that

picture, a relatively large gold particle measures about 39.6 nm x 51.5 nm. On the

right (Figure 3.8), the scratch on the silicon wafer through the PMMA and wafer solid

is very visible; the scratch is used as a reference point to remember where the CAD

designs were etched.

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Figure 4.3

Figure 4.4

These pictures show the steps of the developing process. The image on the top

left (Figure 4.1) shows Vimal preparing an etched sample for a 70 second

agitation in a wide beaker with the 3:1 MIBK solution. The photograph on the

top right (Figure 4.2) shows an etched sample being sprayed with the IPA

solution; it is then agitated for 20 seconds. The schematic on the bottom left

(Figure 4.3) shows the side view of the substrate after lithography and

development. Figure 4.4 shows the top view of the substrate with CAD designs

on top.

Silicon Substrate

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Deposition

The E-Beam Evaporator is first vented, letting air seep into the machine. Then the

levels of nickel and titanium targets are check. The samples are then loaded on the sample

holders. The vacuum chamber is opened and samples are put in. After the samples are put

in the Evaporator, the vacuum is reestablished, pumping air out of the machine to a pressure

of 2-5x10-7 Tor (1 atm = 760 Tor). This process can take about 8 hours to decrease the

pressure to this miniscule scale. 5 nm of titanium is deposited first. Titanium is deposited

because it improves cohesion between nickel and silicon substrates. It also clears any

impurities in vacuum chamber. Nickel is used in the deposition process to fill the holes

created in developing process. This creates a thin metallic coat of about 15 nm on the

etched silicon samples.



Silicon Substrate

Nickel

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Liftoff

The liftoff process is ready to begin. This process takes off the excess nickel and the

rest of the PMMA coating from the etched silicon substrate. All residues not in the holes or

pores will be cleaned off of the silicon wafer. First acetone (C3H6O) is heated on a hot plate

at 54˚C for 5 minutes. It is important not to heat it for too long or at too high a temperature

because acetone is flammable. The heated acetone helps improve the speed and efficiency

of the liftoff process. Before the etched samples are hampered by the acetone, there is a

slightly visible metal coating on the silicon; this is the nickel that was deposited. This

coating is 15 nm thick. The coating becomes very metallic looking at about 50 nm thick, so

15 nm thickness is not a very visibly significant coating. An etched sample is placed in the

54˚C acetone for 1 minute. Acetone is used because it dissolves most organic material with

ease, PMMA in this case. Some of the residue can actually be seen lifting off of the silicon

sample as microscopic bubbles float to the top of the acetone liquid. The same beaker that

contains the acetone and the etched silicon sample is placed on a vibrating table for initially

10 seconds to agitate the residue that is hard to remove. The shaker is submerged in water

so the vibration waves act upon all sides on the liquid in the beaker and not just the bottom;

it also helps to regulate temperature in the shaker. These steps are repeated in small

increments until the silver coating is all gone and the blue coating is left; this will signify the

E-Beam resist and the nickel have been removed. It is crucial to not over shake or have the

etched sample in acetone for an elongated period because the silicon metal coating will

eventually lift with the CAD designs that lay on that layer. The liftoff step and the “top-

down” approach of E-Beam Lithography is finished and ready to be viewed under a

microscope or the SEM. The microscope gives a general view of the micrometer patterns,

but the SEM gives an image magnified enough to indicate the nuances of the nanometer

CAD designs.

The picture on the top left (Figure 4.5) shows the sample holder used to hold the

silicon substrates in the E-Beam Evaporator. The photo on the top right (Figure 4.6)

shows Vimal giving a brief explanation of the E-Beam Evaporator on his right, with the

sample holder in his left hand. Figure 4.7 is a side view schematic of the nickel

deposition after the E-Beam Evaporation process and the metal has vaporized. Figure

4.8 is the top view of the nickel deposited represented by blue diagonal lines across.

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Figure 5.1

Figure 5.2


Figure 5.1 shows the schematic of the sample after all of the excess materials have been

stripped off the substrate, leaving the nickel that filled the holes done by the E-Beam

Lithography process. Figure 5.2 shows the top view of the substrate after liftoff. The

picture on the bottom left (Figure 5.3) shows an etched silicon E-Beam resist sample

sitting in the heated acetone for 1 minute. Residue can be seen slowly dissolving off of

the sample. On the bottom right (Figure 5.4) is a photograph of an image of the girls’ CAD

design captured by an optic microscope after the liftoff process. The microscope used was

on its highest magnification setting and the outside ellipses can only clearly be seen

because their size is microns; the 18 blurry blocks in the middle are in nanometers and the

SEM will do a much better job capturing a clearer, more focused, and more magnified

image.

Silicon Substrate

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SEM Imaging

The top picture represents the CAD

structure displayed by the SEM

before full magnification. The ellipses

on the outside are scaled in microns,

but between the large ellipses are

even smaller ellipses. These small

ones are not visible at this scale.

They are lined in rows and columns of

single, duos, and trios.

The bottom image is taken when the

SEM is fully magnified and focused on

the nano-ellipses. They are in pairs to

view how their positioning affects

their polarity in different

orientations. This can be done using

the Atomic Force Microscope (AFM).

The second nano-ellipse in the pair on

the top left measures 168 nm x 221

nm. The one that follows it,

downward diagonally measures 161

nm x 202 nm.

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Works Cited

Brain, Marshall. "How Semiconductors Work." HowStuffWorks. N.p., n.d. Web. 18 July

2013. <http://www.howstuffworks.com/diode.htm>.

"Coercive Field." Coercive Field Facts. Supermagnete, n.d. Web. 18 July 2013.

<http://www.supermagnete.de/eng/faq/What-does-coercive-field-strength-mean>.

Conway, James. "E-beam Lithography Resist Processing." Litho Processes: EBeam

Resists. N.p., n.d. Web. 18 July 2013.

<http://snf.stanford.edu/Process/Lithography/ebeamres.html>.

"Magnetic Hysteresis." Loop including the B-H Curve. Electronics Tutorials, n.d. Web.

19 July 2013. <http://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html>.

"New Method of Producing Nanomagnets for Information Technology." ScienceDaily.

Helmholtz Association of German Research Centres, 23 Jan. 2013. Web. 17 July 2013.

<http://www.sciencedaily.com/releases/2013/01/130123133618.htm>.

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