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Nanofabrication Overview 

Lee Chow

Department of Physics

University of Central Florida

Lecture #7

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Outline

•Why nanofabrication?

•Current technique —Photolithography (DUV)

•Future technique ---Extreme UV (EUV)

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The need for nanofabricationFirst we have to talk about “Moore’s law”

Intel co-founder Gordon Moore made a prediction more than 40 years ago, popularly known as “Moore’s Law”, that the number of transistors on a chip will double every two years. So far the semiconductor industry have managed to keep that pace for more than 40 years. This provides more functions on a computer chip at a much lower price per function.

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Number of transistors per microprocessor over the year

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The Intel 4004 micro-processor, running at 108 kHz and 0.6 MIPS.

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Inside the Intel 4004 micro-processor, it has 2,250 transistors.

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The first IBM PC was introduced in 1981

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Micro-processor----- 4.77 MHz Intel 8088

Memory----------- 16 kB

Storage------------ 160 kB of floppy disk

Price -------------- $1,565 (1981)

The IBM PC specifications

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Number of transistors‐‐‐‐ 100 millions

Clock frequency‐‐‐‐‐‐‐‐‐‐‐ 3.4 GHz

Speed ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 100,000 MIPS

Linewidth ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 65 nm

SiO2 gate thickness ‐‐‐‐‐‐ 1.2 nm

In 2000, Pentium Duo Core microprocessor specification

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Intel Core i7-990x Processor

Number of transistors---- > 1,000 millionsCloak cycle ----- 3.46 GHzNumber of processor ----- 6 coresLinewidth ----------------- 32 nmGate oxide ----------------- Hafnium oxide

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MIPS of microprocessor from 1970-2005 and beyond

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Critical dimension of microprocessor over the last 30 years

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As we can see from previous slide, in the late 1980 when the feature size of semiconductor devices started to break 1-m barrier, people start to worry physical limitation of the conventional photolithography. New light sources are needed and new photo-resist needs to be developed.

In the last two decades, many new techniques have been developed, such that conventional photolithographic technique still being used and the industry still push the “Moore’s Law” further down to even smaller and smaller dimension. Here we will briefly review the conventional photolithographic technique.

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Photolithographic technique

Photolithography is the process used in micro-fabrication to selectively remove part of a thin film or the bulk of a substrate. It use light to transfer a geometric pattern from a photomask to a light sensitive photoresist on a substrate. A series of chemical treatments then can be applied to engrave or to deposit new material in the desired pattern.

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Basic procedures

1. Thin Film deposition2. Photoresist application –Spin coating.3. Exposure4. Development5. Etching6. Photoresist removal

These basic procedures can be repeated more than 50 times for a typical VLSI circuit.

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The transfer of pattern (exposure) is usually done through (a) contact lithography, or (b) projection lithography.

The conceptual view of projection lithography is shown in the figure to the right.

Here we introduce two concepts:(a) Numerical aperture, NA

NA = nsinθ(b) Resolution, R

NAkR

1

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NAkR

1

R ----- Resolutionk1 ---- a factor contains process

parameters, ~ 1 - 0.4.λ ----- wavelengthNA --- numerical aperture

So from the above, we can see that in order to reduce the feature size, we need to reduce the wavelength of the light or to increase the numerical aperture, NA of the optics system. The coefficient k1 contains many processing parameters and will be discussed later.

Rayleigh Equation

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There is another parameter that we also need to take into account is the “Depth of Focus”, which is defined as:

22 )(NAkDF

where k2 is another coefficient that depends on process parameters. The depth of focus restricts the thickness of the photoresist and the depth of topographic on the wafer.

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Light source used in photolithography

Wavelength(nm)

Feature size(nm)

Time frame Notes

MercuryG‐line

436 7000‐1000 1970‐1985

Mercuryi‐line

365 1000‐350 1985‐1996

Excimer laserKrF DUV

248 350‐180 1996‐2000

Excimer laserArF DUV

193 180‐22 2000‐2011

Excimer laserF2 DUV

157 ??? ??? Need CaF lens

EUV 13 Need  vacuum

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The choice of wavelength used in lithography is significantly limited by air absorption edge at 185 nm.

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The numerical aperture, NA represents the ability of the optics system to collect light. Over the year, the NA for air-gap system has improved over time.

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During the last decade, several techniques were developed to obtain the feature size that is much smaller than the wavelength. In the following we will briefly discuss two of these techniques.

1. Immersion lithography

2. Phase shift mask

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Immersion microscopy

http://www.microscopyU.com/tutorials/java/objectives/immersion/

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Immersion lithography

Immersion lithography is a resolution enhancement technique for lithography. It replaces the air gap between the lens and the wafer with a liquid medium that has a higher refractive index. The basic principle is exactly the same as in the immersion microscopy in previous slide. The concept is extremely simple, but the implementation is not so obvious.

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Currently, ASML, Nikon and Canon are the only manufacturers of immersion lithographic equipments.Many manufaIBM, TI, and Toshiba started to use immersion technologyfor their 45nm node. Intel only started for their 32 nm node.

ASML XT: 1900i immersion lithography tool

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Phase shift mask technique

Phase shift masks are photomasks that take advantage of the interference effect generated by phase difference of the wavefront to improve the image resolution in photolithography. There exist alternating and attenuating phase shift mask.

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The basic principle of PSM is the destructive interference between the waves that came from adjacent patterns. Since there is a phase change of 180°, which implies at the midpoint there is a destructive interference to force the intensity to be zero

Alternating Phase Shift Mask

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Comparison of images of conventional, alternating, and attenuated PSMs

Even though the alternating phase shows better results, the attenuated PSM has gained more attention due to several critical issues of Alternating phase PSM.

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Three critical issues of Alternating PSMs1. Alternating PSMs are only good for pattern with high

periodicity, but it is very difficult to apply them to random pattern.

2. The second issue is the phase conflict. For every 180°phase-shifting region, there is a boundary. So it maybe possible that a 180° phase shift region right next to a zero phase region. This phase conflict will cause a dark line along the boundary.

3. The third issue is the mask manufacturing complexity.

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The improvement of k1

The k1 factor contains many process parameters, including phase shift mask, off-axis illumination, optical proximity correction, and etc.Over the years, it has improved gradually. Today k1 is approaching 0.25, the theoretical value.

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Double processing and double exposure

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2009 International Technology Roadmap for SemiconductorLithography Exposure Tool Potential Solutions

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Next generation technologyExtreme Ultraviolet (EUV)

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What’s happened to the 157 nm line from F2 Excimer laser?

There have been a lot of delays at the 157 nm technology node. At present, most major companies believe 193 nm source can carry us to the 11 nm node at 2016. But eventually, EUV has to take over.

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EUV has been designed as the next generation technology by the International Technology Roadmap for Semiconductor for more than ten years. At wavelength of 13 nm, no material is transparent, (not even air) so there is no lens we can use.

1. In vacuum2. Reflective optics3. Incoherent plasma sources4. Reflective mask

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193 nm Excimer Laser Source

Computer Console

Exposure Column(Lens)

Wafer

Reticle (Mask)

193 nm technology

• Operated in air.• Many lenses are used• Coherent laser source• 100 wafer per hour

throughput

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EUV technology

• The system is under vacuum• All reflective mirrors• All reflective masks• Incoherent plasma source

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Challenges of EUV Lithography

1. Maintain stability during exposure.2. Severe absorption at 13.5 nm.3. Design of high efficiency mirror at

13.5 nm.4. Manufacturing and handling of the

EUV mask.5. High power EUV light source.

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EUV light sources

Tin

Xe

• Synchrotron radiation• Plasma

-- Discharge-produced plasma-- Laser-produced plasma

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Gas Discharged Z-pinch EUV source

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The physics of Z-pinch plasma

The current due to the discharge between two electrodes generates an azimuthal magnetic field that results an inward pressure on the plasma column. The inward magnetic pressure is balanced out by the outward plasma pressure.

02

22

r

BBp

dr

d

oo

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EUV from laser-produced plasma

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The temperature of the plasma

The plasma temperature is governed by the Planck’s formula of blackbody radiation,

)(

2898)(max KelvinT

m or

)(

250)(max eVT

nm

At 13.5 nm, the temperature of the plasma is about 220,000 K.

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EUV Optics

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Plasma is intrinsically not compatible with ultrahigh vacuum. Particular for the laser-produced plasma, therefore there is a need for a debris collector.

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EUV mask

EUV mask is very similar to EUV mirror. It needs to be reflective. The patterned absorber of Cr is fabricated on the multilayer Mo-Si layers

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Making EUV mask

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EUV resists

Resists for EUV lithography are very much like those for DUV lithography, but required to have higher sensitivity and higher resolution capability, and low line edge roughness (LER). High sensitivity is required because of the low power level of EUV source.

Best Positive Resist

2.3mJ/cm2 LER=7.2nm

Best Negative Resist

3.2mJ/cm2 LER=7.6nm

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EUV resists

Gwyn C.W., and P. J. Silverman, Proc. SPIE 2003. 5446 pp990‐1004

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Requirements for EUV lithographic source