2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)

The International Conference Centre Birmingham

20-23 August 20112, Birmingham, United Kingdom

Conductive AFM of transfer printed nano devices

Benedikt Weiler\ Mario BareiB\ Daniel Kalblein2, Ute Zschieschang2, Hagen Klauk2, Giuseppe Scarpal, Bernhard Fabd, Wolfgang Porod3, and Paolo Lugli'

IInstitute for Nanoelectronics, Technische Universitat Miinchen, TheresienstraJ3e 90, 80333 Miinchen, Germany 2Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

3Center for Nano Science and Technology, University of Notre Dame, 275 Fitzpatrick, IN 46556 USA

Abstract - Nano diodes show great potential for

applications in detectors, communications and energy

harvesting. However, to make them suitable for low-cost mass

production, these nano devices have to be fabricated reliably

over large areas while minimizing process time and costs.

Printing techniques are promising candidates to overcome

these economical drawbacks of conventional nanolithography

without a significant loss in structure quality. In this work, we

focus on nano transfer printing (nTP) to fabricate nm-scale

diodes over extensive areas. Using a temperature-enhanced

process, several millions of diodes were transfer-printed in one

single step. We show the reliable transfer of functioning

Schottky and MIM diodes of different sizes, which

demonstrates the versatility and usability of our approach

(nTP), paving the way to numerous applications in the fields of

e.g. infrared detection or energy harvesting. The nano devices

are characterized electrically by conductive Atomic Force

Microscopy (c-AFM) measurements. For these MIM

structures, quantum-mechanical tunneling was determined to

be the main conduction mechanism across the metal-ox ide­

metal junction.

Index Terms - nano transfer printing; ordered nanostructures; conductive AFM; metal-oxide-metal diodes

I. INTRODUCTION

Nano diodes are important devices for various electronic and optoelectronic applications, such as rectifiers for energy

harvesting [1, 2] or infrared detection [3], field-emission cathodes [4], and switching memories [5] . Key challenges

for large-scale manufacturing include increasing fabrication reliability and optimizing throughput, while minimizing process cost. Printing techniques play a crucial role in

efficient nanofabrication since they pave the way to large area patterning, while keeping overall process time shorter and costs lower than any other traditional nanolithography

technique. In this work, we concentrate on nano transfer printing (nTP) as a scalable, purely mechanical, fabrication technique to manufacture semiconductor devices suitable for various

applications [6] . More specifically, we improved

conventional protocols for nTP by a temperature-enhanced

process step to transfer highly-ordered large-scale arrays of

Au and metal-oxide-metal nanostructures on flat substrates, such as p-type Si and Si02 [7] . The device structures show

high quality and fidelity and protruding feature sizes below 100 nm over extensive substrate areas. Scanning electron

microscope (SEM) images showed that the transferred devices were structurally intact after transfer-printing, and the transfer yield was found to be ahnost 90% (defmed as

a) b)

Fig. I (a) SEM image of the nanostructures on the stamp as manufactured taken under a horizontally tilted angle. The height of the pillars is equal all over the stamp and amount to 80 nm. (b) Top view of the pillars showing their lateral dimensions of 50 nm diameter at a distance of 100 nm.

the amount of properly transferred structures divided by the overall amount of structures on the stamp prior transferring).

Moreover, we demonstrated that the transferred structures

were functional diodes using electrical characterization by c­

AFM measurements.

II. F ABRICA TION

The stamps consisted of flat Si wafers that were structured on the nm-scale to circular pillars in an e-beam lithography

process followed by a dry etching process performed by IMS

CHIPS, Stuttgart, Germany. The structured wafers are cut to the fmal stamp size of 1 cm x 1 cm. Before the transfer experiments, their surface morphology was checked in the

SEM. All the investigated pillars showed an equal height of

80 urn . In accordance with the feature sizes specified by the

manufacturer, the stamps comprised variously-sized pillars.

In our experiments diameters of 50 urn with a spacirlg of

100 urn between each pillar (referred to as 50 urn x 100 urn

in the following), pillars of 75 nm diameter at 75 urn spacing

and 100 nm diameter at 100 urn spacing were used. The transfer experiments were performed under clean room

conditions. A self-assembled-monolayer (SAM) was applied

on the surface of the stamp prior to metal evaporation as

depicted in Fig. 2. This anti-sticking SAM increases the hydrophobicity of the surface of the stamps and promotes

the delamination of the evaporated metal from the stamp to the substrate. The subsequent evaporation of stacks of different metal layers was accomplished usirlg either thermal

evaporation or e-beam evaporation. The pressures in the evaporation chambers were in the range of 10-6 mbar for the

thermal and 10-7 mbar for the e-beam evaporation in order to

ensure a high purity of the metal films. Basically two kinds

of stacks were evaporated onto structured stamps:

Stamp

. - . --SAM /'

Hydrophobic !

Stamp

.. - .. --

Plasma � � treatment

-....,

First layer: Au Last layer: Ti

Stamp

. - .

t t Fig. 2 Nanotransfer printing process: At first the stamp is covered by a hydrophobic self-assembled monolayer followed by the evaporation of the metal stack, in this case first an Au and then a Ti layer. Afterwards the surface of the stamp and the substrate is treated by an oxygen plasma to increase the hydrophobicity of the surfaces of stamp and substrate. This promotes the delamination of the metal stack from the stamp to the substrate in the final transfer step.

1. A 20 nm thick Au layer followed by a 4 nm Ti layer

which forms after the transfer process together with the Si substrate Au/Si-Schottky diodes and hence is referred to as

"gold diodes" later on;

2. A 20 nm thick layer of Au or AuPd was evaporated onto a stamp which served as a delamination layer since Au and

AuPd provide weak adhesion to the surface of the stamp. After this delamination layer, a 20 nm thick layer of Al was

deposited which represents the ftfst metal electrode of a MIM diode. After exposing the stamp to clean room

conditions, a 3.6 nm thick AIOx layer was grown on the Al in a plasma-induced process. In a second metal deposition process, the second metal electrode, namely Au or AuPd,

was thermally evaporated on the stamp. The two electrodes

comprising a thin AIOx dielectric in between represent a

MIM structure. In both cases a 4 nm thick Ti layer was evaporated as the last layer on the metal stacks on the stamp to promote good

adhesion of the metal stack from the stamp onto the target surface of the substrate. This thin Ti film is not a completely

closed film but shows an island-like structure thus an

influence of this adhesion layer onto subsequent electrical characterization (Section V) is not expected. Before

transferring, the hydrophilicity of both the metal stack on the stamp and the surface of the Si/Si02 substrate were

enhanced by a brief oxygen plasma treatment. The 0- ions

break chemical bonds on the oxygen saturated surface thus producing reactive OH-groups on the stamp and on the substrate. Two OH-groups on the stamp and on the substrate are supposed to form a covalently binding O-bridge between

the metal stack on the stamp and the substrate accompanied

by H20 evolution when brought into contact during the

transfer step [7] . In the subsequent transfer step, stamp and substrate were placed on top of each other in the pressure chamber of a NIL 2.5 Nanoimprinter (Obducat) and covered

with aluminum foils. This pressure chamber is gas-tight, heatable and coolable and allows pumping of nitrogen

against the foil/stamp/substrate sandwich stack. Besides

pressure, one can control and monitor temperature, and the

time of the single imprint steps by a PC. First a pressure of 30 bar was applied by the Nanoimprinter for I min. Then a temperature of 200°C was applied while the pressure was

kept constant at 30 bar for 4 min. At the end of the process the hot stamp-substrate stack is taken out of the

Nanoimprinter and the demolding step (the separation

between the stamp and the substrate) is performed before the

substrate has cooled down. Due to the hydrophilicity of the two plasma-activated surfaces the stamp and the substrate

usually stick together. Hence the stamp has to be separated vertically by from the substrate by carefully moving the

stamp and the substrate with respect to each other (Figure 2).

Since the temperature is raised during the transfer step this process is called temperature-enhanced nanotransfer printing. It is assumed that the increase in temperature during the process removes the H20 evolving during the

imprint step and this way supports the formation of 0-

bridges. This hard-to-hard single-step transfer printing technique can

be applied to any kind of stamp structure or evaporated metal stack demonstrating the large versatility of our

approach. Besides it is very fast and cost-efficient, does not need any organic adhesion promoters or flexible buffer

layers. Also chemical post-processing is not necessary and

most importantly the master template is preserved and available for further transfers after cleaning properly.

III. MORPHOLOGICAL CHARACTERIZATION OF TRANSFERRED

NANO DIODES AND GOLD PILLARS

The previously described fabrication process repetitively provided the transfer of more than 90% of the elevated pillar

structures on the stamp as determined by scanning electron

microscopy (SEM) investigations depicted in Figure 3. The

image shows the result of a transfer process of Au pillars

transferred by nTP on Si02/Si from a stamp with 75 nm

pillars at 75 nm spacing. The yield was determined by dividing the overall area of transferred nanopillars (defined

as structured area subtracted by the overall defective area)

by the structured area on the stamp. The defective area is

calculated as the sum of the black squares which mark defect

o

[]

o o o

a) b)

Fig. 3 (a) Overall amount of Au nanopillars on the substrate transferred from the structured area on the stamp (specifications: 75 nm x 75 nm). The structured area had a size of -300 11m x 300 11m. (b) The same image with defect sites marked by black squares. An estimation of the yield (area of nanopillars transferred to the substrate divided by structured area on the stamp) amounts to more than 90%.

sites. This investigation shows that the structures were successfully transferred approximately over the entire

structured area of 300 11m x 300 11m (Figure 3). By further morphological investigations of the transferred

nanopillars it could be seen that an almost identical copy of

the nanostructures on the master template was created. SEM images of the 75 nm diameter, 75 nm spacing stamp at very

high resolutions (Figure 4.a)) show that the only

recognizable difference is a morphological enlargement of the diameter of the pillars accompanied by a reduction of

their spacing. This observation is attributed to a conic growth of the deposited metal layers during their

evaporation on the stamps. Looking at a single pillar during

the evaporation of a specific layer sequence, new deposited metal atoms assemble also at the edge of the plateau of the pillars. Thus the diameter of the pillars should increase

laterally with each evaporated atomic layer by an estimated ratio of lateral to vertical growth speed of 1: 1 to 1 :2. This is

in good accordance with the results depicted in Figure 4.a). As the high resolution image shows, the transferred pillars have a diameter of about 90±5 nm at a distance of 65±5 nm.

Considering the evaporated metal thickness of 24±2 nm this

is in good accordance with the expectations.

a) b)

c)

Fig. 4 Morphological characterization of the transferred Au nanopillars (stamp specifications: 75 nm x 75 nm): a) Highest resolution, pillar

diameter was determined to be -90±5 nm at -65±5 nm spacing. This is in accordance with the stamp specifications, if measurement uncertainties and the conic growth of the pillars during evaporation are considered (see text). Despite of their lateral morphing quality and fidelity of the pillars is exactly the same as on the stamp. b) Intermediate resolution: The transferred gold diodes are highly regular like the master structure on the stamp. c) Low resolution: The resolution was set to the lowest reasonable value to check the nanopillars regularity and quality on large scales. Generally, there are only very few defect sites detectable.

(a)

· --•• a" •• -. •••••

... .. .. ....

... e •• •• •••

.... - .. .... � ••••

e .. &.� ·

............ -,. &a ••••••• 1:-·· • .... e .. . . . . .. . .

. . . . . . . . ...., . . . ••••• • • • ••• . . . . _ A .· ·�

.... &. ............. ' - - ... . .... . . '

.300nml • ••• � ••• •

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(b)

Fig. 6 (a) C-AFM of gold diodes (75 nm x 75 nm stamp) in contact mode taken by current measurements; (b) AFM measurements of gold diodes in contact mode taken by atomic force/deflection measurements. A defect rich area was chosen on the substrate to prove that only sites with gold diodes are electrically conductive (see text).

Fig. 7: Schematic of a c-AFM setup. The positive potential is applied to the Ti/Pt coated diamond tip which is in direct contact to the sample.

These investigations also show that our recent limit in the

fabrication of nanodiodes is at 70 nm as demonstrated in

Figure 4.a). For all stamp geometries used in our experiments, i.e.

50 nm x 100 nm, 75 nm x 75 nm and 100 nm x 100 nm, no asymmetric deformation of the pillars could be found. This

is true on typical resolutions of a few nm up to several 11m

(compare image sequence in Figure 4. a)-c)). This sequence also shows that, after transfer, the nanopillars regularity and

quality is almost perfectly preserved as fabricated on the template without any remarkable deterioration in fidelity. Even very small features were replicated faithfully. This

property of nTP, together with the low defect generation

(Figure 4.c)), makes this approach attractive for various

applications and demonstrates the large versatility of our

approach.

Also AFM images and c-AFM (sections IV and V) were taken of the transferred gold and MIM structures. The shape

of the pillars is identical before and after the transfer process which proves once more the reliability (Figure 6).

IV. ELECTRICAL CHARACTERIZATION OF N ANODIODES

The IV characteristic of the MIM device comprising the

oxide layer fabricated by the oxygen plasma was measured by conductive MFP-3D atomic force microscopy (Asylum

Research, California). The sample was clamped via the conductive substrate to an electrode, which was also

connected to the cantilever holder in the head. In this way, a closed electrical circuit was built (see Figure 7). The Si tips

of the conductive AFM setup were coated with a layer of

Ti/Pt (5/20) in order to provide physical strength and a low

106 .. •

........ 104 e • N

E 102

.. � U

� 10° � \ '(ij 10-2 .. nanoscal c Q)

10-4 '0 e diodes -c

10-6 Q) .... • microscale .... ::J 10-8

() • • diodes

10-10 0 2 4 6 8 10

Voltage (V)

Fig. 8: I-V-characteristics of previously transfer-printed microscale and nanoscale MIM diodes [6].

resistivity. In AC tapping mode, the topography of the sample was measured. Then, after a target structure, namely

a MIM diode, was identified, the AFM cantilever tip was brought into direct contact with the surface. After dithering the tip on the MIM top surface, it was held motionless as an

electrical bias was applied through a cyclic, triangle wave

pattern, to the sample through the gold electrodes on the

sample mount while the current was simultaneously measured at the tip. The resulting I-V -characteristic is shown

in Figure�. For identifying an asymmetric behavior around o V, both polarities are plotted on the positive axis. In the

voltage range between 0 V and 5 V, the slope for previously

fabricated MIM diodes comprising a micro scale area is

shown. In the range from 5 V to lO V, the slope for the nano

scale diodes is shown. Since they match perfectly, the reliability of this fabrication method is proven. Further, the

regime when direct tunneling and Fowler-Nordheim

tunneling occurs can be determined, as well as, static device

parameters can be extracted [6] .

V. ELECTRICAL CHARACTERIZATION OF TRANSFERRED GOLD

PILLARS

The Au nanopillars were characterized electrically by another conductive AFM device (Asylum Research,

California). In principle the setup and electrical circuit was the same as described in section IV. The diamond tips were

also coated with Ti/Pt (5/20). The topography of the samples was measured in AC tapping mode again. First the pillars

were imaged by the deflection of the tip due to the atomic forces between tip and sample surface in contact mode

(Figure 6.b)). Then the same region of the sample was imaged by measuring the current driven through the circuit, the tip and the gold pillars on the sample due to a voltage applied to the tip. In order to test the reliability of the

imaging method a defect rich region of the sample was chosen. Generally speaking, all Au nanopillars could be

imaged reliably in current mode whereas defect sites turned out to be not conductive at all. Though some pillars conduct better than others, this way the electrical functionality of the

Au pillars forming a Schottky junction with the underlying

Si substrate could be proven. This observation allows for

their further usage as nanojunctions in semiconductor devices (Figure 6).

VI. SUMMARY AND CONCLUSION

We could show that the fabrication of quantum devices

using the process method described here can be done over large areas. Morphological and electrical investigations

could prove the high fabrication yield when producing

millions of nanodevices and the high quality of individual diodes. We want to point out here, that nanotransfer-printing

is not only a suitable fabrication method for MIM diodes or

Schottky junctions but also more complex structures can be

manufactured.

The main advantage of the process described here with respect to e-beam evaporation is the reusability of the stamps after an acid treatment which is supposed to etch away the

relieved metal rests in the spacing between positive structure

features. This will save much process costs as the acid treatment is not only faster but also cheaper than the repetitive stamp fabrication by e-beam lithography.

In our future work, we will also focus on further reduction in size of the transferred diodes, and the implementation of the diodes into devices, such as antennas.

ACKNOWLEDGMENT

The authors acknowledge financial support from the German Research Funding (DFG), the International

Graduate School of Science and Engineering (IGSSE) and

the Institute for Advanced Studies (lAS), Focus group "Nanoimprint and Nanotransfer" and the German Excellence

Cluster 'Nanosystems Initiative Munich' (NIM). The authors

thank Dr. Edward M. Nelson and Prof. Gregory Timp for technical support concerning c-AFM measurements.

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978-1-4673-2200-3/12/$31.00 ©2012 IEEE