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or image reversal photoresist Lithography= Producing of a desirable image in a photoresist layer Two types of the lithography: (i) mask-based - fast (ii) scanning - slow, however necessary for the mask production Photolithography with masks is widely used both in research laboratories (diffraction limited resolution R~λ/2) and production of modern integrated circuits (ICs) (R~λ/10). For usual photolithography based on single mask process the minimal size of an element is ~ 1 pm In the IC production, elements as small as 10 nm can be produced. One of the main tendencis is the use of sources with smaller λ

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or image reversal photoresist

Lithography= Producing of a desirable image in a photoresist layer

Two types of the lithography: (i) mask-based - fast (ii) scanning - slow, however necessary for the mask production

Photolithography with masks is widely used both in research laboratories (diffraction limited resolution R~λ/2) and production of modern integrated circuits (ICs) (R~λ/10). For usual photolithography based on single mask process the minimal size of an element is ~ 1 pm In the IC production, elements as small as 10 nm can be produced. One of the main tendencis is the use of sources with smaller λ

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Production of a standard photomask 1. Producing a mask design file (Autocad, etc.) 2. Deposition of non-transparent material (Cr) on a transparent substrate (quartz). 3. Coating by a photo (electron) resist layer.

4. Creating a image in accordance with the mask design file using a scanning system

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Production of a standard photomask

5. Development of the exposed photoresist.

6. Etching of the chromium film.

7. Removal of the resist

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Exposition methods in optical lithography

Contact/proximity optical lithography. Mask aligner MJB4 Three basic methods of wafer exposure

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UV Lithography

UV lithography is perhaps the most commonly used photolithography technique in operation today. As the name implies, the crux of UV lithography centers around the properties and attributes of UV (ultraviolet) light. The overall concept of UV lithography is quite simple. UV light is shined through

a mask onto a photoresist covered wafer. As the diagram shows, the mask stops some of the light from proceeding onto the resist covered surface.

Overall Process The overall process of UV lithography contains about 8 steps: 1. Surface Preparation 2. Resist Coating 3. Pre-Bake 4. Mask Alignment 5. Exposure 6. Development 7. Post-Bake 8. Photoresist removal/Processing

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Two methods of processing thin films deposited on a substrate.

Main limitation. Material of the substrate should be insensitive to the etching

Important limitation. The film thickness should should not exceed the thickness of the photoresist

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Etching (-) Chemical etching occurs also under a photoresist. It results in an image slightly different from that of in a photoresist. This makes chemical etching inappropriate for production of small elements.

(+) Very chip method.

Reactive ion etching (RIE) occur in chemically active plasma of high frequency electric discharge at a pressure of the order of 10-2 Torr which is controlled by the gas flow. Particular gas depends on an etched material and should produce a volatile chemical compound with the material. (++) RIE is able nearly perfectly reproduce the photoresist image. (-) Rather expansive systems. (-) Thick enough photoresist to withstand the ion etching. Otherwise additional masks (e.g., of Al film) should be produced. (-) RIE may be accompanied by a charge accumulation on the wafer (substrate).

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Gases frequently used in RIE

Fluorine species SF6, CF4, CHF3, C4F8 Used mainly for etching silicon based materials and a few metals Chlorine and

bromine species Cl2, BCl3, HBr Used for etching polysilicon, compound semiconductors, reactive metals Oxygen O2

is used as a reactive species for etching polymers

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Lift off

(+) Lift-off is a cheap method especially essential in cases where a direct etching of a material would have undesirable effects on the layer below. (-) The main problem of the lift off method is possible retention of a material in undesired places due to sticking to the material to be remained on the wafer or due to poor access of a solvent to the photoresist. Undercut Carrying out of the lift off process in the ultrasound bath partially eliminate this problem.

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Photoresist

Image reversal photoresist

Undercut or Lift-off Profile

Positive photoresist Overcut

Profile

There are three basic types of a photoresist: positive, negative and image reversal (either positive or negative depending on the treatment after the first exposure).

Image reversal photoresist is especially appropriate for the lift-off process

A wafer is covered with a uniform photoresist layer with the use of a spinner. The layer thickness is determined by the photoresist viscosity and the velocity of the spinner rotation. The desired thickness is determined by the following process: chemical or ion etching, material deposition, etc.

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Photoresist

Photoresist AZ 5214 E is normally used in ISSP RAN. It is also sensitive to the white light, but insensitive to the yellow light.

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Scanning electron microscope JEOL 7000 adapted for EBL and working in the vector scan mode.

Electron beam lithography (EBL) Basic parts of an EBL system Pattern generator sets an electron beam in a desired point and sends commands to the blanker

Beam blanker interrupts and restores an exposure by deflecting the electron beam.

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Raster scan Vector scan

f ii"; Ш i jsjg

i Step size-2 pixel size Vector scanning mode

Pixel size = field size / max. number of points = minimal step size. There no relation between the pixel size and a size of the electron spot. For practical reasons the latter should be larger.

Scanning modes

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Increasing of exposed field

Increasing of the exposed field is usually with the use of precisely moving stages (usually based on piezo drives and controlled by the laser beam interferometry. The stitching accuracy ~20 nm can be obtained.

Scanning fields I, II, III, and IV are stitched by moving the precise stage. Stitching of two subfields is shown in field I. Division into smaller subfields eliminates effects related to the beam astigmatism.

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CABL-9000C(50kV)series Electron Beam Lithography System

Specifications: Electron Emitter/

Acceleration voltage TFE(ZrO/W)/25~130kV

Min. beam diameter/

Min. line width 1.6nm/ 7.0nm

Scan method Vector scan(x,y)(Standard)

Vector scan(r,θ), Raster scan,Spot scan(Optional)

Advanced lithography

functions(Optional)

Field size modulation lithography,axial symmetry pattern

lithography

Field size 30μm□,60μm□,120μm□,300μm□,600μm□(Standard)

1200μm□,2400μm□(Optional)

Number of pixels

20,000×20,000dot,60,000×60,000dot,96,000×96,000dot,

240,000×240,000dot@Vector scan(Standard)

10,000×10,000dot@Raster scan(Optional)

Min. address size 10nm@600μm□field,2nm@120μm□field(Standard)

0.0012nm@600μm□field (Optional)

Work piece size 4,6,8inchΦ(work pieces of other sizes and other shapes can be

mounted with our flexible contrivances)

Stitching accuracy 50nm(3σ)@600μm□,20nm(2σ)@60μm□

Overlay accuracy 50nm(3σ)@600μm□

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pofwara

Вс I tiering

Resist

Backscattermg Substrate

20 kV

50 KV

CTi

Sigma For Forward scattered Distribution

3 pm—и Sigma For BackscatteredDistribution (20 kV)

Proximity effect in EBL - the main limitation of the resolution/half- pitch

Proximity effect is a result of the resist exposure by scattered and secondary electrons Produced in both, a resist and substrate. It strongly affects resolution of EBL, especially in the case of high density of elements.

Simulated resist profiles for 20 keV electron beam exposure of 1 pm PMMA resist layer on silicon substrate, the shaded area represents the ideal resist profile and the contours represent the actual resist profiles at exposure doses from 80 to 120 pC/cm2 with 10 pC/cm2 increment

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Reduction of the proximity effect in EBL

1. The use of high accelerating voltage ~ 100 kV. 2. The use of a thin photoresist layer. 3. Correction of the exposed area in comparison with desired one. 4. Variation of the exposure dose along the image in accordance

with appropriate software.

A Hall bar structure made of gold films on oxidized Si wafer. EBL in the machine with a spot size~30x50 nm2

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Reduction of the proximity effect in EBL

An ultra-high resolution obtained on 10 nm resist placed on 10-nm-thick Si3N4 membrane with the use of 100 kV scanning transmission electron microscope.

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A bridge contact in an electronic Mach-Zehnder interferometer

A role of the dose

An Au bridge with (i) completely exposed resist area at the bridge supports, (ii) Incompletely exposed resist at the bridge deck, (iii) Unexposed resist elsewhere

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Fil

m T

hic

kn

ess

(A)

Fil

m

Th

ick

nes

s

(A)

495PMMA C Resists

Solids: 2% - 6% in Chlorobenzene

10000 9000

6000

7000

6000

5000

4000

3000 n -----

2000 ------- --------------------------------------------- 1000 " ’C2

------------------------------------------------------------------------------------------------------------------------------------

0 ----------- . ----------- . ------------ . ------------ T ---------- . ------------ . ------------ 1 -------------

500 1000 1500 2000 2500 3000 3500 4000 4500

Spin Speed (rpm)

950PMMA C Resists Solids: 9% -10% in Chlorobenzene 950PMMA C Resists

Solids: 2% - 7% in Chlorobenzene

Spin Speed (rpm)

55000

50000

45000

40000

35000

30000

25000

20000 15000 10000

Spin Speed (rpm)

Resists for EBL The main positive resist is polymethyl methacrylate (PMMA) of different molecular weight (chain length) from a diapason 50x103 - 2x106 . Usually used are 495x103 and 950x103 resists. Resins of different concentration (2-12 %) in chlorbenzene or anisole are used. Typical doses are 50 - 500 pC/cm2

Negative resist Hydrogen silsesquioxane (HSQ) with the chemical formula [SiO3/2]n allows for as thin as 10 nm resist layer.

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The use of a double-layer EBL resists

The copolymer is an electron beam resist which needs much smaller dose than the main resist layer

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The Niemeyer–Dolan technique (Dolan technique or the shadow evaporation technique)

This technique uses an evaporation mask that is suspended above the substrate (see figure). The evaporation mask can be formed from two layers of resist. Depending on the evaporation angle, the shadow image of the mask is projected onto different positions on the substrate. By carefully choosing the angle for each material to be deposited, adjacent openings in the mask can be projected on the same spot, creating an overlay of two thin films with a well-defined geometry. The Niemeyer–Dolan technique is used to create thin-film electronic nanostructures such as quantum dots and tunnel junctions.

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Bridge-free technique (BFT)

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Quality control

• Scanning electron microscopy (SEM) - images of a sample by scanning the surface with a focused beam of electrons (+attachments).

• Transmission electron microscopy (TEM) – information about crystal structure, the quality of the crystal, the grain size and the orientation of the crystal axes, natural oxide.

• Scanning Probe Microscopy – topology and local electronic characteristics. • Atomic force microscopy (AFM) – is a very-high-resolution type of scanning probe

microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit.

• Scanning tunneling microscope (STM) – is an instrument for imaging surfaces at the atomic level, based on the concept of quantum tunneling

• Optical microscopy.