Fabrication-II. Electron Beam Lithography Pattern Design Thin Film Deposition

Transcription

1 Fabrication-II Electron Beam Lithography Pattern Design Thin Film Deposition By Charulata Barge, Graduate student, Prof. Zumbühl Group, Department of Physics, Universtity of Basel. Date:- 20th Oct. 2006

2 Outline Scanning Electron Microscopy Electron Beam Lithography Thin film deposition Lift off process

3 Outline Scanning Electron Microscopy Electron Beam Lithography Thin film deposition lift off process

4 What is SEM? Scanning Electron Microscopy Uses electrons rather than light to form an image large depth of field produces images of high resolution

5 Principle of SEM Microscopy by employing electron beams electron beam is also referred to as "probe Electrons are used instead of optical waves smaller wavelengths can be achieved. Wavelength depends on the accelerating voltage Sample information is obtained from interaction between electrons and sample Topographical, Compositional, Elemental, Structural images can be obtained

6 Operation of SEM

7 Components of SEM The source:- Electron Gun Two emission sources:- Thermionic emitter (TE) and field emitter (FE) TE:- electrical current heats up filament FE:- filament placed in a huge electrical potential gradient The electro-magnetic lenses Electron Optical Column Vacuum system

8 The electron gun Most comman filament (Cathode):- Tungsten (W) Heated until stream of electrons is produced Accelerated by positive potential down the column -ve potential to Wehnelt cap Electrons are repelled by Wehnelt cap towards optic axix Collection of electrons:- space charge Electrons move down the column Nearly perfect point source Monochromatic source Parallel to optic axis are allowed out of gun area

9 The electro-magnetic lenses acts like an optical lens a coil of copper wires inside the iron pole pieces Circular electro-magnets - circular magnetic field in a specified region weak in the center of the gap stronger close to the bore used to focus and steer electrons

10 Electron Optical Column Two to three magnetic lenses 1) Condenser lenses :- control beam current and spot size 2)Final lens (condenser lens):- Focus the beam of electrons Deflecting coils :- move the spot forth and back stigmator coils:- correct irregularities in the beam Detectors Secondary electron detectors Back scattered electron detector

11 Vacuum system Stability of beam Ultra High vacuum and high vacuum UHV: mbar for electron optical column HV:-10-6 mbar to 10-7 mbar for chamber Rotary pump (pre-vacuum pump) Ion getter pump Turbo pump

12 Specimen interactions

13 Outline Scanning Electron Microscopy Electron Beam Lithography Thin film deposition Lift off process

14 Electron Beam Lithography General Information Block diagram of EBL system Electron beam Lithography Resists Electron Scattering Factors affecting exposure

15 Lithography:- General information process used to transfer pattern from the mask/reticle to the layer of resist deposited on the surface of wafer Types of lithography photolithography (or optical lithography) uses UV radiation, X-ray lithography uses X-ray ion beam lithography uses ion beam. e-beam lithography uses electron bean

16 E-Beam Lithography E-beam lithography technique :- modified SEM system focused beam of electrons to expose the resist; Technique for creating extremely fine patterns (< 0.1μm) Process is very similar to what happens inside a television or CRT display. no mask is used as pattern is "written" directly into the resist; pattern transfer resolution below 100 nm; to manufacture high resolution masks for photolithography and X- ray lithography. Can work with a variety of resists Produces almost infinite number of patterns Slow when compared to optical lithography

17 Block Diagram on an EBL system

18 Beam Blanker for turning on / shutting off the e-beam with high precision during exposure Voltage applied to two parallel plates within beam path. Beam position moves as designated by the user.

19 Few EBL Terms Area dose= (beam current x area dwell time)/(area step size) 2 unit :- μc/cm 2 Line dose = (beam current x line dwell time)/(line step size) 2 unit :- μc/cm Dwell time :- The time the beam is still at each location. Step size : The length, the beam is moved between each dwell time, Beam current: - The e--current reaching the sample. Area dose : - The dose required for an area to fully develop that area, i.e. to remove resist. Line dose :- The dose required for a line to fully develop that line

20 Few EBL Terms Raster scan:- Vector scan Working area Write-field the electron beam is scanned across lines of pixels the wafer is shifted to the next line. an area of an individual chip is selected the beam draws out the features in that area one-by-one the part that is going to be written during exposure the area that can be covered by the e-beam with good precision For each magnification, of the design there is a certain write-field size

21 Few EBL Terms stiching If the design is larger than can be covered with one write-field, it is possible to put several write-fields adjacent to each other, with corresponding parts of the design exposed in each write-field Errors can be minimised with write field alignments The X-Y-Z stage (laser stage) high precision stage with laser-interferometric positioning system resolution 5 nm a combination of servo-motors and piezo-electric accutators

22 Pattern design The Design contains the pattern that is to be written can be made out of several "layers" Selectivity of writing The coordinates in the design called the UV coordinate system (U axis-horizontal, V-axis vertical) Alignment operations:-coordinate system is mapped on to the stage coordinate system Scaling for Area dose and line dose

23 E-Beam Resists Recording and transfer media for e-beam lithography. Polymers dissolved in a liquid solvent. Two types of e-beam resists: positive tone:- positive resists develop away exposed regions negative tone:- the developed region remains behind after development. Positive e-beam resists:-main-chain scission when exposed to e-beam Negative e-beam resists : -Formation of interchain linkages Positive e-beam resists: PMMA (Poly methyl methacrylate), EBR-9 (another acrylate based resist), PBS (Poly butene-1-sulphone), ZEP (a copolymer of a - chloromethacrylate and a -methylstyrene). Negative tone e-beam resists : COP ( an epoxy copolymer of glycidyl methacrylate and ethyl acrylate) and Shipley SAL (has 3 components, a base polymer, an acid generator, and a crosslinking agent).

24 Positive Resist Chemistry PMMA

25 Negative Resist Chemistry glycidylmethacrylate copolymers COP

26 E beam resists

27 Polymethylmethacrylate Acrylic PMMA Polymer Type:-Thermoplastic Transparent as glass and it allows 92% of the sunlight to pass Excellent clarity and UV resistance Soluble in Chlorobenzene and anisole Good abrasion resistance Hardness and stiffness. Low water absorption. Low smoke emission. Good track and arc resistance

28 PMMA thickness vs. spin speed curves

29 Electron Solid interactions Small angle forward scattering Large angle scattering events leading to backscattering. Additional exposure in the resist leading to what is called the electron beam proximity effect. Primary electrons slow down, much of their energy is dissipated in the form of secondary electrons with energies from 2 to 50 ev. Fast electrons :- secondary electrons having energy of the order of 1 kev..

30 Electron backscattering limits resolution Ref. D. Kyser and N.S. Viswanathan, J. Vac. Sc. Techn. 12, 1305(1975) Higher energy electrons have larger back-scattering range

31 Proximity Effect Collisions cause the striking electrons to 'scatter The scattering of electrons may be backward ( or back-scattering, wherein electrons 'bounce' back) scattering occurs as the electron beam interacts with the resist and substrate atoms. It broadens the diameter of the incident electron beam as it penetrates the resist and substrate It gives the resist unintended extra doses of electron exposure as back-scattered electrons from the substrate bounce back to the resist. Result in wider images Degrades the resolution of the EBL system. Closely-spaced adjacent lines can 'add' electron exposure to each other, a phenomenon known as 'proximity effect.

32 To minimize proximity effect Multilayer resist Thinner PMMA(150 nm- 200 nm) Beams of kev (can increase the backscattering) High contrast resist Proximity corrections of dose and spot size Low beam energies:- electron range is smaller than the minimum feature size

33 Developing the resist Chemicals used:- For PMMA 1:3 MIBK:IPA +1.3% MEK MIBK:- Methyl Isobutyl Ketone IPA:- Isopropyl alcohol MEK:- methyl ethyl ketone MEK gives better contrast The exposed part is removed Surface is cleaned with Isopropyl alcohol

34 Outline Scanning Electron Microscopy Electron Beam Lithography Thin film deposition Lift off process

35 Thin Film Deposition Depositing a thin film of material onto a substrate Types of deposition techniques Chemical deposition Plating Chemical vapor deposition (CVD) Plasma enhanced CVD Physical deposition Resistance thermal evaporator Electron beam evaporator Sputtering Pulsed laser deposition Other methods Reactive sputtering Molecular beam epitaxy Topotaxy,

36 Resistance Thermal evaporation Commonly used metal deposition technique. "indirect" thermal evaporation Vaporizing a solid material (pure metal, eutectic or compound) by heating a large current is passed through a filament container (usually in the shape of a basket, boat or crucible) which has a finite electrical resistance. The choice of this filament material is dictated by the evaporation temperature and its inertness to alloying/chemical reaction with the evaporant. Pressures lower than 1e-5 mbar are necessary Substrate-to-source distance of approximately 10 to 50 cm in a vacuum chamber. Good vacuum is a prerequisite

37 Electron Beam Evaporation Under vacuum, typically 10E-5 or deeper. (rotary and turbo pumps) A current (5 to 10 kv) is sent through a tungsten filament Thermionic emission of electrons takes place. The filament is located in an area outside the deposition zone, in order to avoid contamination. The electrons are focused and directed toward the evaporant The kinetic energy of ebeam motion is transformed to heat High energy is released often more than several million watts per square inch

38 Comparision Method Pro Con Metals and compunds E-Beam Evaporation 1. high temp materials 2. good for liftoff 1. alloys difficult 2. poor step coverage Al, Ti, Au, Ni, Ge, Cr, Cu, Pd, and Pt 3. highest purity Thermal Evaporation 1. simple to implement 2. good for liftoff 1. limited source material (no high temp) 2. alloys difficult 3. poor step coverage Al,Au,AuGe,In, Ni,Ti,Zn, SiO

39 Outline Scanning Electron Microscopy Electron Beam Lithography Thin film deposition Lift off process

40 Lift Off Process What is lift off? Ideal lift off process General procedure and Chemicals used for lift off

41 What is lift off? Process allowing definition of pattern on the wafer surface without etching To define geometry for metals which are hard to etch metals such as gold; Metal is lifted off in selected areas by dissolving underlying resist. Typically used for GaAs.

42 Ideal lift off process Substrate Spin Resist Pattern and develop resist Metal deposition Positive resist Remove resist Negative resist

43 PMMA undercut and metal deposition Substrate Spin Resist Pattern and develop resist Metal deposition PMMA Remove resist

44 General procedure and chemicals Chemicals used :- Acetone and Isopropanol Procedure:- Immerse sample in warm (50 0 C) acetone for min. Avoid contamination of the metal and resist on sample (use syring) Immerse sample in IPA for 5 min. Blow dry

45 Dose test samples Wafer:- Si Resist:- PMMA Spin speed: rpm D( PMMA):- 500nm Evaporant :- gold D(gold):- 60 nm Wafer:-Si Resist:- PMMA Spin speed: rpm D( PMMA):- 430nm Evaporant :-gold D(gold):- 60 nm

46 Thank you for your attention