System Requirement Analysis of Laser Interference Nanolithography

Zuobin Wang and Jin Zhang Yury K. Verevkin Manufacturing Engineering Centre Institute of Applied Physics

Cardiff University Russian Academy of Sciences Cardiff, UK Nizhny Novgorod, Russia

{WangZ & ZhangJ8}@cf.ac.uk verevkin@appl.sci-nnov.ru Changsi Peng and Chunlei Tan Thierry Berthou and Stéphane Tisserand Optoelectronics Research Centre SILIOS Technologies

Tampere University of Technology Peynier, France Tampere, Finland {thierry.berthou & stephane.tisserand}@silios.fr

{Changsi.Peng & Chunlei.Tan}@tut.fi Santiago M. Olaizola Isabel Ayerdi and Ainara Rodriguez CEIT and Tecnun (University of Navarra)

CEIT and Tecnun (University of Navarra) San Sebastian, Spain San Sebastian, Spain Yolaizola@ceit.es

{iayerdi & airodriguez}@ceit.es Abstract - This paper presents a system requirement analysis of multi-beam laser interference nanolithography for nanoscale structuring of materials including seven sections: introduction, formation of multi-beam laser interference patterns, user requirements, system architecture, experiments, discussions and conclusions. Analytical expressions were obtained for the spatial distribution of radiation of the interfering beams as a function of their amplitudes, phases, angles of incidence on the sample, and polarization planes with computer simulation and experimental results. The environmental effect and technological potential were also discussed. Index Terms – interferometry, nano patterning, nano structuring, laser interference nanolithography.

I. INTRODUCTION

Laser interference lithography (LIL) is concerned with the use of interference patterns generated from two or several coherent beams of laser radiation for the structuring of materials. The interference patterns can be arrays or matrices of laser beam lines or dots. The intensity distribution of the interference patterns exposes materials with a pitch of sub-wavelength of the interfering light. When using such radiation to interact with materials, feature sizes down to a fraction of the laser wavelength can be created. This technology provides a way for nano patterning periodic and quasi-periodic patterns that are spatially coherent over large areas [1-3]. Fig. 1 illustrates a 4-beam laser interference configuration.

Today there are mainly three lithography technologies available for direct structuring of nano features in the range of 40 nm by means of a top-down strategy. One is the well established ion beam lithography (IBL) technology, the second is the electron beam lithography (EBL) technology, and the third is the scanning probe lithography (SPL) technology. The IBL technology is possibly the most

advanced nano-structuring technology on the market, with a leading position in milling and etching applications. IBL is targeted primarily at high ion flux rates in a single ion beam with ultra-high resolution, using a sequential writing strategy. For high resolution IBL applications liquid metal ion sources are used, because these sources allow localized field emission of ions which is necessary for focusing ion beams down to nano dimensions. IBL is convenient for small areas or processes that need only dilute pattern density and for inspection tasks that benefit from an in situ imaging possibility and can accept contamination. EBL is established for e-beam writing in resist materials. In some cases EBL is also used for reactive etching and deposition but the main application is the exposure of electron sensitive resists followed by the transfer of the pattern from the structured resist to the substrate via an etching process. SPL is based on cantilever driven probes facilitated for structuring. The probes can be used as functionalized AFMs (atomic force microscopes) and STMs (scanning tunnelling microscopes). IBL, EBL and SPL technologies are all using a time consuming sequential writing strategy, which requires high mechanical and electrical stability of the system. This indicates the significant drawback of the technologies compared to LIL.

LIL is highly innovative in nanolithography due to the facts that its high efficiency, large working areas and low cost in nano scale structuring of materials as compared to the IBL or EBL technology. With respect to the SPL technology the advantage of LIL is the non-contacting projection mode with a large working distance and extremely efficient fabrication which are two decisive advantages with respect to emerging nanotechnology production requirements. In general, LIL has the following advantages compared with other nanolithography technologies:

• Relative simplicity of the setup;

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Proceedings of the 2007 IEEEInternational Conference on Mechatronics and Automation

August 5 - 8, 2007, Harbin, China

• Very high efficiency; • Creation of structures on large areas up to hundreds

of millimetres in diameter; • Low cost.

LASER BEAM 1

LASER BEAM 2

LASER BEAM 3

LASE

R BE

AM 4

LASER BEAM 1

LASER BEAM 2

LASER BEAM 3

LASE

R BE

AM 4

Fig. 1 A 4-beam laser interference configuration.

The LIL technology is proposed for the processing of large areas simultaneously, which is suitable for efficient volume production, using laser interference beams. The LIL technology has been known for many years since the invention of the laser. Various experimental and commercial solutions with different exposure wavelengths have been built depending on the application demands. In principle, a laser beam is split into two or more beams. They are passed through optical components and then interfered at a specific angle on a light sensitive material (photoresist). LIL provides a method for fabricating periodic and quasi-periodic patterns that are spatially coherent over large areas. However, there had been little research on laser interference lithography for structuring materials in nanotechnology applications due to its low resolution. In recent years, the development of new laser sources has paved the way for nanoscale structuring of materials using the LIL technology. The most recent basic research has shown that, due to the interference of coherent beams, it is possible to realize the condition of localizing laser energy with feature sizes down to nanoscales. This means that when using such radiation to interact with the surface of materials, objects of sizes down to a fraction of the interfering light wavelength. Therefore, the short-wavelength lasers (such as 157nm 193nm, 266nm and 308nm lasers) are quite promising for implementing laser interference lithography with a feature size down to ~ 30nm. The fundamental advantage of the pulsed interference impact of the UV radiation on materials is the possibility of local heating of a huge number of nanoscale areas (more than 108 with modified parameters) to a temperature of several thousand degrees over several nanoseconds. These features are absent in other lithography technologies. This will ensure local crystallization and control of self-organization processes on the surface of monocrystals

and epitaxial films, etc. This paper presents a system requirement analysis of multi-beam laser interference nanolithography for nanoscale structuring of materials. The formation of multi-beam laser interference patterns, user requirements and system architecture were discussed. Analytical expressions were obtained for the spatial distribution of radiation of the interfering beams as a function of their amplitudes, phases, angles of incidence on the sample, and polarization planes with computer simulation and experimental results. The environmental effect and technological potential were also discussed.

II. FORMATION OF LASER INTERFERENCE PATTERNS

The interference patterns, generated from multiple coherent beams of laser radiation, can be arrays or matrices of laser beam lines or dots. The intensity distribution of the interference patterns exposes materials with a pitch of sub-wavelength of the interfering light. When using such radiation to interact with materials, feature sizes down to a fraction of the laser wavelength can be created. This technology provides a way for the nano patterning of periodic and quasi-periodic patterns that are spatially coherent over large areas. Considering a general case for N-beam laser interference, the electric field vectors of N laser beams 1E

�, 2E�

, 3E�

, …

and NE�

can be written as [4-6]

)2cos( 11111 φπ +±⋅= vtrnkpAE ����

)2cos( 22222 φπ +±⋅= vtrnkpAE ����

)2cos( 33333 φπ +±⋅= vtrnkpAE ���� (1)

…… )2cos( NNNNN vtrnkpAE φπ +±⋅= ����

where mA ( m =1, 2, 3, …, N) is the amplitude. mp� ( m =1, 2,

3, …, N) is the unit polarisation vector. λπ2=k is the

wave number ( λ = wavelength). mn� ( m =1, 2, 3, …, N) is the unit vector in the propagation direction. r� is the position vector and v is the frequency. mφ ( m =1, 2, 3, …, N) is the phase constant.

In the case of N-beam interference, the superposition of the N beams can be expressed as

( )��==

+±⋅==N

mmmmm

N

mm vtrnkpAEE

112cos φπ�����

(2)

It can be seen from (2) that the analytical expression is a function of amplitudes, phases, angles of incidence, polarization planes and the wavelength.

III. USER REQUIREMENTS

The user requirements for the LIL technology in nano

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patterning of materials can be considered as two parts: application requirements and system requirements, as outlined in the following sections.

A. Application Requirements The application requirements for laser interference nanolithography are shown in Table I.

TABLE I APPLICATION REQUIREMENTS FOR LASER INTERFERENCE NANOLITHOGRAPHY

Substrate dimensions

Circular Up to 300 mm ø

Thickness Up to 1.5 mm

Substrates Face to be processed

Plane

Circular 130 mm ø Pattern size

Rectangular 180 × 180 mm

Minimum 30 nm Feature size

Maximum 2 μm

Feature geometry Symmetrical features

Features

Feature description Matrix features

Type Resists, polymers and non transparent materials to laser wavelengths

Process Materials

Thickness 1 μm

Critical Dimensions 30 nm

Alignment accuracy 10 to 20 nm

Depth of Field 50 nm - 2 μm

Throughput 20 - 100 pieces/hour

Equipment

Equipment objective cost -

In the table, the pattern size is mainly related to the laser power, coherence length and optical components. The feature size is a fraction of the laser wavelength. Pattern and feature sizes are two basic requirements for many nanolithography applications. B. System Requirements Considering the various user application requirements, a modularized system could be a solution. In this case, the system is divided into modules and each module has separated functions. There are different combinations of components for users to choose in one module. The following are the basic functions and design requirements for a laser interference nanolithography system:

• Patterning 1D or 2D nanostructures; • Direct writing or through a series of processes; • Configurable pattern geometry and size; • Easy installation and maintenance;

• Modularized system.

IV. SYSTEM ARCHITECTURE

Fig. 2 shows a scheme of a multi-beam laser interference lithography system for formation of laser interference patterns. The system consists of nine parts:

• Laser radiation; • Beam shaping; • Beam splitting; • Phase control; • Interference control; • Polarization control; • Beam monitoring; • Sample positioning; • System control.

Fig. 2 Scheme of a multi-beam laser interference lithography system.

A. Laser Radiation The role of laser radiation is to supply coherent light with an appropriate wavelength, power and coherence length. It can be performed by a single laser or a laser with amplifiers. Solid-state lasers are common for laser ablation and can be a good choice for direct writing. Excimer lasers are also suitable for high instantaneous power applications, and ideal for direct writing. Users can choose lasers according to their requirements. The emission wavelength is often the first parameter considered for laser interference nanolithography system for the following reasons:

• It determines the minimum feature size and pitch of the pattern produced.

• The interaction between lasers and materials varies at different wavelengths. It is better to choose a wavelength that matches with the materials in which patterns will be produced.

LASER RADIATION

BEAM SHAPING

BEAM SPLITTING

PHASE CONTROL

SAMPLE POSITIONING

BEAM MONITORING

POLARIZATION CONTROL

SYSTEM CONTROL

INTERFERENCE CONTROL

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• The system cost is directly related to the wavelength employed.

Power is a crucial parameter of the laser, especially for the function of direct writing or modification of materials. It has the following impacts on the system:

• A pulsed laser with enough power can be used for direct writing. The threshold depends mainly on the recording material and the pattern size.

• A higher power is suitable for a larger pattern size. • A high power laser demands for optical components

with high damage thresholds. The duration of a pulse is important for a laser interference nanolithography system. A short pulse is necessary since a long duration of the pulse will induce extra heat which will damage the region around the feature or pattern. It is estimated that the pulse duration in nanoseconds is ideal to establish an effective direct-writing nanolithography system. The coherence length of a light source is crucial for an interference system. A coherence length of ~30cm is ideal for many applications and it can be calculated using the following equations [4]:

λλΔ

=2

cL (3)

or

νΔ= cLc (4)

where Lc, λ, Δλ, c, and Δν are the coherence length, wavelength, wavelength variation, velocity of light and frequency variation. B. Beam Shaping As for almost all lasers, the intensity of laser beam is Gaussian distribution. The intensity I changes with the deviation r from the centre of a laser beam as shown in Fig. 3.

Fig. 3 Intensity distribution.

If such laser beams are used for laser interference, there will be different intensity distributions of the pattern in different regions. Pattern uniformity could be a problem for many nanolithography applications. The Gaussian beams must be transformed into a flat-top distribution before interference so that a uniform pattern can be produced. C. Beam Splitting Beam splitting is needed to obtain several coherent beams for a laser interference nanolithography system. Two-beam interference produces linear fringes which are interesting for a

number of applications while a multi-beam system is required for producing two or three dimensional structures. It is desirable to be able to select the number of output beams to generate different patterns in recording materials. For a multi-beam interference system, the choice of beam splitting methods varies with the number of beams, pattern size, flexibility of the system, laser wavelength and power. D. Phase Control Phase control is significant for a laser interference nanolithography system, as it is related to pattern orientation or localization of a pattern. An interference pattern structure is a function of the phases of interfering beams, and the phase control is necessary for a laser interference nanolithography system to obtain the required pattern structures. The phase of a beam is in accordance with the optical path length difference from the laser to the sample and it can be shifted by changing the optical path length. Figs. 4 and 5 show that there is a phase shift between the two patterns in y direction.

Fig. 4 An interference pattern.

Fig. 5 An interference pattern with a phase shift in y direction.

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E. Interference Control Interference control is concerned with the arrangement of the coherent beams to form required interference patterns. It has the following two basic functions:

• Control of the parameters concerned including the feature size, period, pattern shape and size. The pattern period is a function of the incident angles [2].

• Optimisation of optical path length differences between interfering beams to form high quality interference patterns.

F. Polarization Control Polarization control is related to the arrangement of the polarization states of interfering beams to form required interference patterns. It can be seen clearly from (2) that an interference pattern is a function of the polarization states of interfering beams. Therefore, it is essential to be able to select the polarization states of interfering beams to produce required interference patterns. G. Beam Monitoring Beam monitoring is concerned with the monitoring of a multi-beam interference nanolithography process. The process requirements could include the following:

• Tuning of the interference pattern parameters (feature size, period, pattern shape and size) according to application targets before processing samples.

• Monitoring of the beam parameters (shape, intensity and polarization) to ensure the quality of the beams.

H. Sample Positioning The following are the issues to be considered for a motorized position control stage:

• Positioning of samples (x, y, z). • Repeated patterning of large samples (x, y). • Changing of the incident angles of the laser beams

and the periods to keep the sample in a position for patterning (z).

• Rotation of the angle between the sample and the incoming beams to have more flexibility to the shape of patterns that can be defined.

V. EXPERIMENTS

Considering the case of a four-beam laser interference that the beams k1 and k2 are located in the plane [YZ], and the beams k3 and k4 in the plane [XZ] as shown in Fig. 6, the electric fields can be written as [7, 8]

]2)cossin(cos[ 111111 φπθθ +±−= vtzykpAE �� (5)

]2)cossin(cos[ 222222 φπθθ +±−−= vtzykpAE �� (6)

]2)cossin(cos[ 333333 φπθθ +±−= vtzxkpAE �� (7)

]2)cossin(cos[ 444444 φπθθ +±−−= vtzxkpAE �� (8)

where 1θ , 2θ , 3θ and 4θ are the incident angles of the four

beams.

The interference intensity distribution can be determined by taking the square of the sum of the above electric field vectors and averaging it over time [8]. In the experiment, a 4-beam laser interference lithography system was used to form interference patterns and to create surface structures. Fig. 7 shows an AFM image of a silicon surface after structuring from the laser interference lithography system achieved by direct writing. Fig. 8 shows another LIL surface pattern with an area of 2700×2700 nm2. The two patterns have different types of feature structures.

Fig. 6 Two pairs of beams in [YZ] and [XZ] planes.

Fig. 7 An AFM image of a silicon surface after structuring from a laser

interference lithography system.

Fig. 8 An AFM image of an LIL surface pattern.

y

x

z

k1k2

k3k4

1θ2θ

3θ4θ

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Fig. 9 shows the result of computer simulation for formation of 4-beam laser interference patterns for the laser interference lithography system used in the experiment. It can be seen that the results from experimentation and simulation are in accordance with the physical principle.

Fig. 9 A computer-generated interference pattern.

VI. DISCUSSIONS

There are two issues related to the use and development of the laser interference nanolithography technology: Environmental effect on its performance and its technological potential. A. Environmental Effect The main environmental parameters including temperature, humidity, and composition of the atmosphere are still under investigation. At the current stage of laser interference nanolithography development, temperature variation in a room is often the most important parameter. Temperature could affect both the performance of the laser-radiation source and the result of the interference action. B. Technological Potential It has been reported that a half-pitch of n4λ ( n is the refractive index) could be achieved with the immersion technique [2, 9]. In contrast, the technological potential of the multi-beam laser interference nanolithography technology for direct writing or modification of materials is closely related the following three parameters:

• Pattern feature size – a feature size down to a fraction of the wavelength (~ 30 nm);

• Pattern period – a period of <100 nm; • Pattern size – up to hundreds of millimetres in

diameter. It is expected that the technological potential of the multi-beam laser interference lithography technology for direct writing or modification of materials could be fully implemented in a few years with the contribution from research scientists worldwide.

VII. CONCLUSIONS

A system requirement analysis of multi-beam laser interference nanolithography for nanoscale structuring of materials has been presented. The formation of multi-beam laser interference patterns, user requirements and system architecture have been discussed. Analytical expressions were obtained for the spatial distribution of radiation of the interfering beams as a function of their amplitudes, phases, angles of incidence on the sample, and polarization planes with computer simulation and experimental results. The computer simulation and experimental results are in accordance with the physical principle.

The laser interference nanolithography technology provides a way for the nano patterning of periodic and quasi-periodic patterns that are spatially coherent over large areas (up to hundreds of millimetres in diameter).

ACKNOWLEDGEMENTS

This work was part of the “Development of lithography technology for nanoscale structuring of materials using laser beam interference (DELILA)” project funded by the European Commission under the 6th Framework Programme (FP6) – Priority 2: Information Society Technologies (IST). The authors would like to thank the European Community for supporting the project. The Manufacturing Engineering Centre is also a participant of the EU FP6 NMP project “Improvement of Industrial Production Integrating Macro-, Micro- and Nanotechnologies (IPMMAN)”.

REFERENCES [1] Development of Lithography Technology for Nanoscale Structuring of

Materials Using Laser Beam Interference (DELILA), http://www.delila.cf.ac.uk/, 20 March 2007.

[2] S. R. J. Brueck, “Optical and interferometric lithography – nanotechnology enablers,” Proc. IEEE, vol.93, no.10, pp. 1704-1721, October 2005.

[3] H. H. Solak, C. David, J. Gobrecht, L. Wang, and F. Cerrina, “Four-wave EUV interference lithography,” Microelectronic Engineering, vol. 61-62, pp. 77-82, 2002.

[4] K. J. Gåsvik, Optical Metrology, John Wiley & Sons Ltd., Chichester, 1995.

[5] E. Hecht, Optics, 3nd Ed, Addison Wesley, 1998. [6] J. Sladkova, Interference of Light, Iliffe Books Ltd., London, 1968. [7] J. Zhang, B. Feng, Y. Guo, “Long focal depth photolithography for

obtaining nanometer array patterns with multi-beam interference,” Opto-Electronic Engineering, vol. 31, no. 3, pp. 8-11, March 2004.

[8] A. Fernandez and D. W. Phillion, “Effect of phase shifts on four-beam interference patterns,” Applied Optics, vol. 37, no. 3, pp. 473-478, 1998.

[9] M. LaPedus, “IBM sees immersion at 22nm, pushes out EUV,” EE Times, http://www.eetimes.eu/uk/197008463, 23 February 2007.

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