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Chemical-mechanical polishing (CMP) Electrochemical machining (ECM) Electrochemical grinding (ECG)
ELID grinding Electric discharge machining (EDM) Abrasive water jet machining Ultrasonic grinding
Electron beam machining Laser beam machining Ion beam machining . . . . .
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Chemical-Mechanical Polishing (CMP)
A method of controlling the planarity of themultiple metal and dielectric layers
A process of physically removing material from
places of high topography to flatten and levelthe wafer surface (IC wafer planarization)
A wafer surface planarization technology
applied in the manufacturing of sub-0.35 umsemiconductor devices.
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Planarization
Non-planar and planar interconnect layers
[Borst, Grill and Gutmann 2002, pg. 46 -CMP of low dielectic constant polymersand organosilicate glasses, CL Borst, W. Gill, R.J. Gutmann]
Planarity across the die is important for photolithography processes,which projects a pattern of light onto the wafer surface
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Principle of CMP Process
[Borst, Grill and Gutmann 2002, pg. 48]
A slurry consisting of chemical regents and abrasive particles isdispensed to create a lubricating layer between the pad surface andthe wafer. The slurry contains chemicals that react with the wafersurface, and abrasive particles that impact the wafer surface toachieve mechanical removal.
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CMP Process Parameters
The wide array of parameters and the competing interactionof effects makes CMP a difficult process to model andpredict
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CMP Abrasives
The choice of slurry abrasive particle (vary in size, shape andhardness) is vital for achieving the desired removal rate andsurface roughness of a material.
CeO2, ZrO2 and SiO2 in high pH solutions are commonly used topolish silicon oxide, and Al2O3 is most commonly used for metal
(Cu W, AL) CMP.
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CMP- material removal
Chemical-mechanical planarization chemical reactionandmechanical energycombined to achieve material removal fromhigh regions on the wafer surface, leaving the low regionsrelatively untouched
[Borst, Grill and Gutmann 2002, pg. 49]
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CMP Process Models
Fluid mechanics-based:
2-step removal process=
chemical modification offilm surface layerfollowed by abrasion ofthe modified layer
Contact mechanics-based:
30-40mm thickliquid slurry film
Combination of pressure and velocity
Lubricatinglayer
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CMP - Process
Surface roughness nano-finish achievable
AFM scan before polished
AFM scan after polished
Nanometer-scale scratches observed
High roughness and scratchingcaused by large mechanicalabrasion. Low roughness andscratching suggest the presenceof a protective layer
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CMP - SPCR
Solid phase chemical reaction (SPCR)
[Chen, Shu and Lee 2003] J. of Matl Proc Techn, 140 (2003) 373-78
Chemical passivation layer generated: process ofinducing and removing the chemical passivation layerthru force of action
Silicon wafer as substrateand BaCo3 as abrasive
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CMP Machines
- self-leveling upper head with both rotational and linear(both vertical for loading and unloading and horizontalfor oscillating) motions, which holds a wafer or wafercoupon of any shape from 0.25 to 4,
- mechanically applied servo-controlled normal loadprogrammable from 5 to 500 N thus producing contactpressures from 0.05 to 500 psi,
- self-leveling spring-loaded upper holder with passiverotation, which holds a conditioner or another specimenfrom 0.5 to 4.25,
- either rotational or orbital lower platen for a polishingpad from 1 to 9. - slurry feeding and draining.
Example:
(source: http://www.cetr.com/Brochures)
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Electrochemical Machining (ECM)
A controlled anodic electrochemical dissolutionprocess of the workpiece (anode) with the tool(cathode) in an electrolytic cell, during an electrolysis
process
(source: http://www.unl.edu/nmrc/ecm1/ecm1.htm)
Scheme of electrochemical machining (ECM) process
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Principle of ECM (1)
An electrochemical anodic dissolution process inwhich a direct current with high density and lowvoltage is passed between a workpiece and a pre-shaped tool (the cathode).
At the anodic workpiece surface, metal is dissolvedinto metallic ions by the deplating reaction, and thusthe tool shape is copied into the workpiece.
A relatively new and important method of removingmetal by anodic dissolution and offers a number ofadvantages over other machining methods.
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Principle of ECM (2)
Example:electrochemical reactions during ECM of ironin sodium chloride (NaCl) electrolyte
At the anode (+):
At the cathode (-) :
electrolysis has involved thedissolution of iron from theanode, and the generation ofhydrogen at the cathode. Noother actions take place atthe electrodes.
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Principle of ECM (3)
Metal removal is effected by a suitably shaped toolelectrode, and the parts thus produced have thespecified shape, dimensions, and surface finish.
ECM forming is carried out so that the shape of thetool electrode is transferred onto, or duplicated in, the
workpiece.
High accuracy in shape duplication and high rates of
metal removal, effected at very high current densities ofthe order 10 100 A/cm2, at relative low voltage from 8to 30 V, while maintaining a very narrow machining gap(of the order of 0.1 mm) by feeding the tool electrode inthe direction of metal removal from the work surface,with feed rate from 0.1 to 20 mm/min.
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Common Uses for ECM
Duplicating, drilling and sinking operations in themanufacture of dies, press and glass-making moulds,turbine and compressor blades for gas-turbine engine,the generation of passages, cavities, holes and slots in
parts, and the like
Electrochemical sinking operation
NC electrochemical contouring usingsimple-universal tool-electrode
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ECM Machining System
the machine itself the power supply
the electrolyte circulation system the control system
ECM die sinking machine tool (courtesy AEG-Elotherm-Germany)
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Operating Parameters of ECM
Working voltage between the tool electrode(cathode) and workpiece (anode)
Machining feed rate
Inlet and outlet pressure of electrolyte (orflow rate)
Inlet temperature of electrolyte
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Typical parameters and conditions of ECM
Power supply -
Type: Direct Current
Voltage: 5 to 30 V (continue or pulse)
Current: 50 to 40,000 A
Current Density: 10 to 500 A/cm2 [ 65 to 3200 A/in2]
Electrolyte -Type and Concentration
Most used: NaCl at 60 to 240 g/l [ to 2 lb/gal]
Frequently used: NaNO3
at 120 to 480 g/l [1 to 4 lb/gal ]
Less Frequently used: Proprietary Mixture
Temperature : 20 to 50o C [68 to 122oF]
Flow rate: 1 l/min/100A [0.264 gal/min/100A]
Velocity : 1500 to 3000 m/min [5000 to 10,000 fpm]
Inlet Pressure: 0.15 to 3 MPa [22 to 436 psi]
Outlet Pressure: 0.1 to 0.3 MPa [15 to 43.6
Frontal Working Gap : 0.05 to 0.3mm [0.002 to 0.012 in]Feed rate: 0.1 to 20mm/min [0.004 to 0.7 in/min
Electrode material: Brass, Copper, Bronze
Tolerance -2-dimensional shapes: 0.05-0.2 mm
[0.002- 0.008 in]
3-dimensioanl shapes: 0.1mm [0.004 in]
Surface Roughness (Ra) 0.1 to 2.5 mm
[4 to 100 microinches]
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ECMed Parts
Examples of machined parts byECM (AEG-Elotherm-Germany)
Examples of machine parts after deburring(AEG-Elotherm-Germany)
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Summary of ECM
The rate of material machining depend on workpiecematerial, is equal from 1,200 to 2,500 mm3 for each1,000A of power supply
The accuracy of ECM depend on shape and dimensions of
machining workpiece and approximately from 0.05 mm to0.3 mm at using continuous current, and from 0.02 mm to0.05 mm at using pulse ECM;
The surface roughness of machined surface is decreasingwith increasing machining rate (for typical materials),approximately from Ra=0.1 mm to Ra= 2.5 mm;
ECM generates no residual stress into material ofworkpiece; and there is no tool wear.
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Nano Machining:
Micro Machining:
A work material removed process by cutting tools under
micro scales. That is the cutting parameters used are in
micrometer scales: 1 ~ 999 mm depth of cut or 1 ~ 999 mm
undeformed chip thickness.
A work material removed process by cutting tools under
nano scales. That is the cutting parameters used are in
nanometer scales: 1 ~ 999 nm depth of cut or 1 ~ 999 nm
undeformed chip thickness.
Non-conventional Precision Machining
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Precision Finishing (1)
Precision grinding involved a maximumprecision to about 1.0 mm and expected toreach 100nm
SPDT (single-point diamond turning), UPDG(ultra-precision diamond grinding), ELIDgrinding (electrolytic in-process dressing), etc.
Applications to optical and electronicindustries
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Precision Finishing (2)
SPDT, UPDG processes are similar in thatchips of usually small size
Capable of producing surfaces with mirrorfinishing w/o polishing
Using specially designed machine tools of
high rigidity with air bearing spindles
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SPDT vs. UPDG
Major problems is the appearance ofsubsurface defects in the form ofmicrocracks
SPDT is performed on very soft ductilemetals, e.g. pure copper, while UPDGusually performed on very hard brittle
materials, e.g. glasses and ceramics.
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Ductile Mode Machining
Especially for brittle materials, is a machining processthat work materials are removed by dislocation orplastic deformation rather than cracks propagation.That is, the cutting process is dominated bydislocation rather than flaw extension.
Comparison with brittle fracture:
Easy controlling of machining process; Free of cracks;
Smoother surface.
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High hardness
High strength
Good fracture toughness High wear resistance
Good chemical stability
Good thermal stability
Characteristics ofBrittle Materials
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A transition in the chip formation from brittle to ductile
as the depth of cut decreases to very small
Grinding brittle materials in a ductile manner early in
1954 by King and Tabor The first systematic studies of grinding ductility in 1979
by Swain
Other experiments of single grit abrasion tests on many
brittle materials including semiconductors, glasses andadvanced ceramics
Grinding of Brittle Materials
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Form/Dimensional
Accuracy < 0.1um
Surface Roughness
< 10 nm
Nano Precision Machining
Toshiba ULG 100C Diamond Turning Machine
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Silicon (111) Wafer:
3 inches Diameter
0.5 mm Thickness
Single Crystal Diamond Tool (orSPDT):
Rake Angle 0
Nose Radius 0.3 mm
Cutting Edge Radius 40 nm
Cutting Conditions:
Spindle Rotation Speed 1000 rpm
Nanomachining experimental setup
on an ultra-precision machine tool
Nano Surface Machining by SPDT
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(photos of silicon wafer surfaces)
(a)Diamond turning surface (b) Original polished surface
Mirror Surface Finishing
Mirror surface finish of hard and brittle material can be possible
when material removal taking place thru plastic deformationratherthan fracture
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Aspheric glass lens
Spherical mirror Mirror finish by diamond turning
Machining of aspheric glass mould
Mirror Finished Products
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ELID Grinding (1)
Schematic illustration of ELID grinding [Lim et al, 2002]
The wheel is continuously dressed while the part is machined The difference between ELID and conv. Grinding is the applicationof a current during grinding. Applications incl. grinding of silicon wafer, nano surface finishingon difficult-to-machine materials, e.g. glasses, ceramics, etc.
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ELID Grinding (2)
Set-up of the continuous ELID of a metal-bonded diamond wheel[Shaw 1996]
The bond is depleted continuously by a pulsed d.c. power
supply, enabling optimum protrusion at all timesProcess applicable to grinding of either electrical conducting ornon-conducting work materials but only with metal-bondedwheels
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ELID Grinding (3)
Schematic diagram of the experimental set-up
[Lim et al, 2002]
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NUS Experimental Setup
Machine tool: Deckel Maho DMU50V 5 Axis
Grinding wheel: #325,#1200,#4000 @3000rpm
Feed Rate : 100~600mm/min
Dressing Current : Duty ratio 10~60% @90V
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Principle of ELID Grinding
Cast-iron bonding material for holding the diamond particles is removed
by the electrolysis during in-process dressing and fresh diamond particles
protrude out for grinding.
Super fine diamond grit (grit size up to #150,000)
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Without ELID(0.3795mm) With ELID (Ra 0.1491mm)
Effect of ELID
Grinding conditions: Feedrate: 500mm/min
Spindle: 3000rpm
Electric power: 0%, 30%@90V
Wheel : CIB-D wheel #1200
Silicon afe plana i ation b
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Workpiece
Tool
dynamometer
Wheel
Electrode
Highly efficient due to high removal rate
Uniform ground surface across the wafer
Relatively low cost involved in this process
Silicon wafer planarization byELID Grinding
Ra 3nm (10nm required)
Multi-process Miniature Machine Tool
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Multi-process Miniature Machine Toolfor-machining
Multi purpose machine toolfor micro machining
(-turning, -milling, -
drilling, -EDM, -ECM)
Working area :
200100100mm
(Resolution 0.1 m)
DI water /Oil for EDM
medium
Design of motion controller
6.5 micron Hole1.5 mm length shaft
Contact: Dr. A.S. Kumar,[email protected], MicroTool
Interchangeable
spindle unit
Micro WEDG
MicroEDMMicro Milling
Micro Turning
Micro WEDM
On MachineMeasurement
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References
1. C.L. Borst, W.N. Gill and R.J. Gutmann, Chemical-Mechanical Polishing of Low Dielectric ConstantPolymers and Organosilicate Glasses, KluwerAcademic Publishers,, Boston,2002
2. M. C. Shaw, Principles of Abrasive Processing,Clarendon Press, Oxford, 1996
3. C.C. Chen, L.S. Shu and S.R. Lee, J. of MaterialsProcessing Techn, 140(2003), pp.373-78
4. ECM, http://www.unl.edu/nmrc/ecm1/ecm1.htm5. H.S. Lim, K. Fathima, et al., Intl J. of Machine Tool &Manufacture, 42 (2003) pp935-43.
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