ION BEAM MACHINING for Manufacturing Engineering

ION BEAM MACHINING for Manufacturing Engineering

1. ION BEAM MACHINING

2. INTRODUCTION OF ION BEAM MACHINING The ion beam manufacturing method is an important non-conventional engineering technique used for micro- and nanofabrication that uses electrically accelerated ions to remove atoms on a surface. IBM is an advanced machining technique . The process is highly precise and is primarily used in industries where extremely fine surface finishes , precise microstructures , and high accuracy are required , such as in microelectronics , aerospac e, and optics . Ion-by-Ion removal of material.

3. What are Ions in IBM? Ions : An ion is an atom or molecule that has lost or gained electrons, making it charged. In IBM, ions are created by ionizing a gas (usually an inert gas like argon ) through a process that strips electrons , resulting in positively charged particles . These ions are then accelerated to high speeds , allowing them to interact with and remove atoms from the target material surface . What is an Ion Beam in IBM? Ion Beam : An ion beam is essentially a stream of high-energy ions , concentrated and directed toward the target material . The ions are accelerated and focused into a narrow beam using electromagnetic lenses, similar to how electron beams are manipulated in electron microscopes. Characteristics of the Beam : The ion beam’s properties, such as energy , focus , and current density , can be controlled to adjust the material removal rate , the depth of penetration , and the precision of the machining process .

4. WORKING OF IBM Process : An ion source generates ions, usually argon or other inert gases, and accelerates them to high velocities. The ion beam is directed toward the workpiece, where ions collide with the material and cause atoms to dislodge, effectively " sputtering " the material away. IBM is a non-contact process, so there is minimal thermal impact on the material. Materials Machined : Ideal for hard materials like ceramics , metals , semiconductors , and optical glass . Used in applications where chemical reactions might alter the material’s properties.

5. Mechanism of Material Removal In Ion Beam Machining Particle beam consisting of ionized atoms i.e. ions . A stream of ions of an inert gas, such as argon or metal such as gallium is accelerated in a vacuum by high energies and directed toward a solid work-piece. Ion beam knocks off atoms from workpiece by transferring kinetic energy and momentum to atoms on the surface of the object, Sputtering off: knocking out atoms from the work-piece surface through the transfer of kinetic energy from the incident ion to the target atoms Removal of atoms occurs when the energy transferred exceeds the binding energy

6. When an ion strikes a cluster of atoms on the work-piece, it dislodges between 0.1 and 10 atoms from the work-piece. This process is called Sputtering process. Sputter yield depends on the energy of the incident ions, angle of incidence on the work-piece surface, masses of ions & target atoms, and binding energy. Sputter Yield Y = No. atoms removed / No. of striking Ions Mechanism of Material Removal In Ion Beam Machining

7. Sputtering Y i e ld Ion Incident angle dependence Generally increasing the incidence angle increases the sputter yield – Max around 80 degrees . As the angle of incidence increases from normal incidence, the possibility of the target atoms escaping from the surface during collision cascades, increases and eventually leads to increased sputter . After reaching a maximum the sputter yield decreases again as the ion approaches glancing incidence.

8. Schematic Diagram of a Focused Ion Beam System Ion Beam (Ga + 3-30 KeV) Spot size 7 nm Scan Generator for SEM SED/SID Monitor Scan Generator for FIB Sample (mounted on a p re ci si o n go n io m e t e r)

9. Mass separator is a setup that allows only the required amount of ions with a fixed mass-charge ratio to pass through. Below the mass separator there is a long and thin drift tube, which eliminates the ions that are not directed vertically. The lower objective lens helps in reducing the spot size of the beam and in improving focus. Finally there is the electrostatic beam deflector which controls the final landing location of the ions. Ion Column LMIS: liquid m e t a l i o n s o u r ce

10. Ion Sources Liquid metal ion source E xt r a c tio n Voltage I on s Ions L iquid metal L iqui d metal Extraction Voltage I on s T o liq . H e reservoir Gas field ion source Gas in H2 or He Type of ion source Ion species Virtual Source size (nm) Energy s p r e a d , Δ E (eV) Unn o r m ali zed brightness (A/cm 2 sr) Angular b r i g h t n e ss (µA/sr) Liquid metal Ga + 50 >4 3 x 10 6 50 Gas field ion (supertip) H+, H + ,He + , 2 Ne + _ _ _ 0.5 ~ 1 5 x 10 9 35 17

11. Emission of secondary ions and electrons FIB Imaging (Low ion current) Sputtering of substrate atoms FIB Milling (High ion current) Chemical interactions (Gas assisted) FIB Deposition Enhanced Etching Basic Operating Modes 12

12. FIB Milling For milling applications it is desirable that the incoming ions interact only with the atoms at the surface. If the ion energy (momentum) is adequate the collision s u ffi c i e n t can transfer e n er g y t o t h e surface atom to overcome its surface binding energy ( 3.8eV for Au and 4.7 eV for Si). Note: There are other variants of the process like Reactive Ion Etching (RIE) where chemical species are incorporated and the process proceeds chemically Interaction solely depends on momentum transfer to remove the atoms, sputtering is purely a physical process. 13 Typical material removal rate is about 1  m3 per second

13. SUBSTRATE Focused Ion Beam S can Nanoscale Deposition Gas Nozzle Volatile products produced by ion impact Precursor Gas Molecules Deposited film FIB Deposition 14 For FIB induced deposition, the necessary processes are Adsorption of the chemical precursor gas onto the sample surface. Decomposition of gas molecules into volatile and non volatile products by focused ion beam. Focused ion beam scanning is our hand which defines the deposition area. 3 dimensional nanostructures can be fabricated using layer by layer deposition. Precursor must have two properties, namely : Sufficient sticking probability to stick to a surface of interest in sufficient quantity. Decompose more rapidly than it is sputtered away by the ion beam.

14. AT VERY LARGE ANGLE OF INCIDENCE, SURFACE ROUGHNESS VALUE RAPIDLY DECREASES BECAUSE THE CONVEX PARTS OF SURFACE ASPERITIES ARE EASILY SPUTTERED BY THE OBLIQUELY INCIDENT IONS. E F F ECT OF A N G LE OF IN C IDEN C E OF ION BEAM

15. SURFACE FINISH M A T E RIAL RE M O V AL R A TE SURFACE TEXTURE P A R AM E TRIC A N A L YSIS 16

16. FACTORS AFFECTING MACHINING CHARACTERISTICS WORK-PIECE MATERIAL : SPUTTERING YIELD IS A FUNCTION OF ATOMIC NUMBER, BINDING ENERGY, GRAIN SIZE, NO. OF ELECTRONS SHELL, ETC. OF THE WORK-PIECE MATERIAL . ION ETCHING GAS: THE SPUTTERING YIELD IS KNOWN TO BE DEPENDENT ON THE ATOMIC WEIGHT OF THE INCIDENT ION. IONS. HAVING HIGH ATOMIC NUMBER WILL YIELD HIGH MRR .  SPUTTERING YIELD IS RELATED TO THE BINDING ENERGY OF THE ATOMS IN THE MATERIAL BEING ETCHED. IT IS POSSIBLE TO VARY ITS VALUE BY INTRODUCING REACTIVE GASES .  OXYGEN WILL BE ABSORBED ON THE FRESH SURFACES OF MATERIALS LIKE TITANIUM, SILICON, ALUMINIUM AND CHROMIUM DURING ION ETCHING  IT WILL FORM OXIDES AND WILL REDUCE ETCH RATE.

17. EFFECT OF OXYGEN / ARGON ON ETCH RATE (MILLER-SMITH,1976) WHEN THE MACHINING CHAMBER IS FULL OF AIR, IT HAS MINIMUM ETCH RATE . AS THE CONTENT OF INERT GAS (PURE ARGON) INCREASES IN THE M AC HI N I N G C H A MB E R , T HE ET C H R A T E A L SO I NCR E A SE S . ACTIVATED CHLORINE OR FLORINE CONTAINING SPECIES WILL REACT WITH THE ABOVE MATERIALS TO FORM LOOSELY BOUND OR EVEN VOLATILE COMPOUNDS AND THUS INCREASES ETCH RATE.

18. ANGLE OF INCIDENCE : SPUTTERING YIELD INCREASES GRADUALLY REACHES A MAXIMUM AT AN ION INCIDENCE ANGLE OF NEARLY 50 AND AFTER THAT DECREASES RAPIDLY . . A N G UL A R D EPE N D E NC E O F T H E SPE C I F IC SP U T T E RING A N G UL A R D EPE N D E NC E O F SP U TT E RING YIELD(MIYAMOTO,I,1987) AS THE ION INCIDENCE ANGLE INCREASES, MORE ATOMS OF THE WORK-PIECE CAN BE KNOCKED OUT OR SPUTTERED AWAY EASILY FROM T HE S UR F AC E OF W O R K- PI E C E WHEN THE ION INCIDENCE ANGLE IS VERY HIGH, THE MACHINING RATE BEGINS TO DECREASE BECAUSE THE ION CURRENT DENSITY DECREASES BY COS  AND THE NUMBER OF IONS REFLECTED FROM THE SURFACE OF THE WORK-PIECE WITHOUT SPUTTERING OFF A T OMS OF T HE W O R K- PI E C E I NCR E A SES .

19. THE NUMBER OF ATOMS KNOCKED OUT BY THE INCIDENT IONS FROM THE TWO OR THREE ATOMIC LAYERS INCREASES WITH THE INCREASE IN THE ENERGY OF THE INCIDENT IONS F i g . I o n e n e r g y d ep e nd e n c e o f t h e s p e c i f i c s pu t t er - machining rate [1981,Taniguchi] ION E N E R GY : THE SPECIFIC SPUTTER-MACHINING RATES INCREASE LINEARLY WITH THE AMOUNT OF ION ENERGY AT ANY ANGLE OF THE INCIDENT ION .

20. EFFECT OF CURRENT DENSITY ON SURFACE ROUGHNESS AT DIFFERENT ION ENERGIES (SHIMAT.,1990) SKD-1=>HIGH CHROMIUM HIGH C A R B O N STE E L F O R G A UG ES D EP T H I N C R E ASES W I T H CURRENT DENSITY : M A C H I N I NG INCREASE HOWEVER, I N CURR E NT D E N S I T Y . I T L ARG E L Y D O E S NOT DEPEND ON THE ION CURRENT DENSITY WITH SMALL ION ENERGY  THE INCIDENT IONS LOSS THEIR KINETIC ENERGY DUE TO COLLISION WITH THE SPUTTERED IONS, AND ITS PROBABILITY BECOMES LARGER W H E N T H E HIGH. THIS PHENOMENON CUR R E N T D E N S I T Y IS IS SUPERSEDED BY INCREASE OF THE INCIDENT ION VELOCITY OR ION ENERGY.

21. SURFACE FINISH IN ION BEAM MACHINING

22. F AC T O RS A F FECTING SUR F ACE FINISH Workpiece material : SUCCESS OF THE ION BEAM POLISHING DEPENDS CRUCIALLY ON THE GRAIN SIZE AND INITIAL MORPHOLOGY OF THE SURFACE. S U R F A C E R O U GH N E S S O F T H E W O R K - PIE C E I N CR E A S ES W ITH INCREASING GRAIN SIZE OF TUNGSTEN CARBIDE (WC). GR A IN S I Z E D E P E ND E NC E OF T HE S UR F A C E R O U G H N E S S (M I Y A M O TO .1 9 9 3 ) 65

23. WITH VERY SMALL GRAIN SIZE , THE MACHINING RATE OF EACH GRAIN WILL BE ALMOST THE SAME, AND THEREFORE UNIFORM MACHINING OVER THE SURFACE WILL TAKE PLACE. FOR LARGE GRAIN SIZE, THE DIFFERENCE BETWEEN THE MACHINING RATES OF THE GRAINS RESULTS IN THE INCREASE IN VALUE OF SURFACE ROUGHNESS . EFFECT OF GRAIN SIZE

24. ANGLE OF INCIDENCE : AFTER AN INITIAL INCREASE, AN INCREASE IN ANGLE OF INCIDENCE SURFACE ROUGHNESS , DUE TO INCREASE IN THE MATERIAL REMOVAL RATE. EFFECT OF ION BEAM INCIDENCE ANGLE ON SURFACE ROUGHNESS ( SHIMA,T.1990)

25. CURR E N T D E N S I TY AN D I ON E N E R G Y : . F OR L OW CURR E NT D E NS I T Y AND ENERGY, THE SMALLER VALUE OF SURFACE ROUGHNESS FOR THE SAME ENERGY IF THE CURRENT DENSITY I S HIG H S UR F AC E ROUGHNESS IS HIGH . PR O B A B I L I T Y OF COLLISION BETWEEN THE IO N S INCIDENT SP U TT E R E D BECOMES L AR G E R AND A T OMS WITH INCREASING ION CURRENT DENSITY THAT CAUSES IRREGULAR MACHINING ON THE SURFACE. EFFECT OF CURRENT DENSITY ON SURFACE ROUGHNESS AT DIFFERENT ION ENERGIES (SHIMAT.,1990)

26. C O NC LU S IO N S . ION BEAM MACHINING IS AN IDEAL PROCESS FOR NANO-FINISHING OF HIGH MELTING POINT HARD AND BRITTLE MATERIALS SUCH AS CERAMICS, SEMICONDUCTORS, DIAMOND ETC. AS THERE IS NO LOAD ON THE WORK-PIECE WHILE FINISHING , IT IS ALSO SUITABLE FOR FINISHING OF VERY THIN OBJECTS, OPTICS AND SOFT MATERIAL. SURFACE ROUGHNESS INCREASES WITH INCREASE IN SIZE OF THE GRAIN S T R UC T U R E , ION E N E R GY AN D CUR R E N T D E N S IT Y . SUR F A C E M O R PHOLOGY H A S S IG N IFI C AN T E FF E C T O N T HE FI NA L SUR F AC E FI N I S H. SURFACE ROUGHNESS INCREASES FOR INCIDENT ANGLE FROM TO 50 THEN DECREASES RAPIDLY. NON-HOMOGENEITY IN GRAIN STRUCTURE MAY RESULT IN ROUGHENING OF THE WORK-PIECE SURFACE BY ION BEAM MACHINING BUT THAT CAN BE OVERCOME BY CHANGING THE MACHINING CONDITIONS. VERY LESS AMOUNT OF MATERIAL REMOVAL NEEDED TO ACHIEVE THE FULL POLISHING.

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