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 .....

Chemical-Mechanical Polishing (CMP)

A method of controlling the planarity of the multiple metal and dielectric layers A process of physically removing material from places of high topography to flatten and level the wafer surface (IC wafer planarization) A wafer surface planarization technology applied in the manufacturing of sub-0.35 um semiconductor devices.

Planarization
Planarity across the die is important for photolithography processes, which projects a pattern of light onto the wafer surface

Non-planar and planar interconnect layers

[Borst, Grill and Gutmann 2002, pg. 46 -CMP of low dielectic constant polymers and organosilicate glasses, CL Borst, W. Gill, R.J. Gutmann]

Principle of CMP Process

A slurry consisting of chemical regents and abrasive particles is

dispensed to create a lubricating layer between the pad surface and the wafer. The slurry contains chemicals that react with the wafer surface, and abrasive particles that impact the wafer surface to achieve mechanical removal.

[Borst, Grill and Gutmann 2002, pg. 48]

CMP Process Parameters

The wide array of parameters and the competing interaction of effects makes CMP a difficult process to model and predict

CMP Abrasives
The choice of slurry abrasive particle (vary in size, shape and hardness) is vital for achieving the desired removal rate and surface roughness of a material.

CeO2, ZrO2 and SiO2 in high pH solutions are commonly used to polish silicon oxide, and Al2O3 is most commonly used for metal (Cu W, AL) CMP.

CMP- material removal

Chemical-mechanical planarization chemical reaction and mechanical energy combined to achieve material removal from high regions on the wafer surface, leaving the low regions relatively untouched

[Borst, Grill and Gutmann 2002, pg. 49]

CMP Process Models

Combination of pressure and velocity

Contact mechanics-based:

Fluid mechanics-based: 2-step removal process= chemical modification of film surface layer followed by abrasion of the modified layer

Lubricating layer

30-40mm thick liquid slurry film

CMP - Process

Surface roughness nano-finish achievable

AFM scan before polished

Nanometer-scale scratches observed

High roughness and scratching caused by large mechanical abrasion. Low roughness and scratching suggest the presence of a protective layer

AFM scan after polished

CMP - SPCR
Solid phase chemical reaction (SPCR)
Chemical passivation layer generated: process of inducing and removing the chemical passivation layer thru force of action

Silicon wafer as substrate and BaCo3 as abrasive

[Chen, Shu and Lee 2003] J. of Matl Proc Techn, 140 (2003) 373-78

CMP Machines
Example:
- self-leveling upper head with both rotational and linear (both vertical for loading and unloading and horizontal for oscillating) motions, which holds a wafer or wafer coupon of any shape from 0.25 to 4, - mechanically applied servo-controlled normal load programmable from 5 to 500 N thus producing contact pressures from 0.05 to 500 psi,
- self-leveling spring-loaded upper holder with passive rotation, which holds a conditioner or another specimen from 0.5 to 4.25, - either rotational or orbital lower platen for a polishing pad from 1 to 9. - slurry feeding and draining.
(source: http://www.cetr.com/Brochures)

Electrochemical Machining (ECM)

A controlled anodic electrochemical dissolution process of the workpiece (anode) with the tool (cathode) in an electrolytic cell, during an electrolysis
process

Scheme of electrochemical machining (ECM) process

(source: http://www.unl.edu/nmrc/ecm1/ecm1.htm)

Principle of ECM (1)

An electrochemical anodic dissolution process in which a direct current with high density and low voltage is passed between a workpiece and a preshaped tool (the cathode).

At the anodic workpiece surface, metal is dissolved into metallic ions by the deplating reaction, and thus the tool shape is copied into the workpiece.
A relatively new and important method of removing metal by anodic dissolution and offers a number of advantages over other machining methods.

Principle of ECM (2)

At the anode (+):

At the cathode (-) :

electrolysis has involved the dissolution of iron from the anode, and the generation of hydrogen at the cathode. No

other actions take place at the electrodes. Example: electrochemical reactions during ECM of iron
in sodium chloride (NaCl) electrolyte

Principle of ECM (3)

Metal removal is effected by a suitably shaped tool electrode, and the parts thus produced have the specified shape, dimensions, and surface finish.

ECM forming is carried out so that the shape of the tool 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 of the order 10 100 A/cm2, at relative low voltage from 8 to 30 V, while maintaining a very narrow machining gap (of the order of 0.1 mm) by feeding the tool electrode in the direction of metal removal from the work surface, with feed rate from 0.1 to 20 mm/min.

Common Uses for ECM

Duplicating, drilling and sinking operations in the manufacture 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 using simple-universal tool-electrode

ECM Machining System

the the the the machine itself power supply electrolyte circulation system control system

ECM die sinking machine tool (courtesy AEG-ElothermGermany)

Operating Parameters of ECM

Working voltage between the tool electrode (cathode) and workpiece (anode) Machining feed rate Inlet and outlet pressure of electrolyte (or flow rate)
Inlet temperature of electrolyte

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] Frontal Working Gap : Feed rate: Electrode material: Electrolyte Type and Concentration Most used: Frequently used: Less Frequently used: Temperature : Flow rate: Velocity : Inlet Pressure: Outlet Pressure: Tolerance 2-dimensional shapes: 0.05-0.2 mm [0.002- 0.008 in] 0.1mm [0.004 in] 0.1 to 2.5 mm [4 to 100 microinches]

3-dimensioanl shapes: Surface Roughness (Ra)

0.05 to 0.3mm [0.002 to 0.012 in] 0.1 to 20mm/min [0.004 to 0.7 in/min Brass, Copper, Bronze

NaCl at 60 to 240 g/l [ to 2 lb/gal] NaNO3 at 120 to 480 g/l [1 to 4 lb/gal ] Proprietary Mixture 20 to 50o C [68 to 122oF] 1 l/min/100A [0.264 gal/min/100A] 1500 to 3000 m/min [5000 to 10,000 fpm] 0.15 to 3 MPa [22 to 436 psi] 0.1 to 0.3 MPa [15 to 43.6

ECMed Parts

Examples of machined parts by ECM (AEG-Elotherm-Germany)

Examples of machine parts after deburring (AEG-Elotherm-Germany)

Summary of ECM

The rate of material machining depend on workpiece material, is equal from 1,200 to 2,500 mm3 for each 1,000A of power supply The accuracy of ECM depend on shape and dimensions of machining workpiece and approximately from 0.05 mm to 0.3 mm at using continuous current, and from 0.02 mm to 0.05 mm at using pulse ECM; The surface roughness of machined surface is decreasing with increasing machining rate (for typical materials), approximately from Ra=0.1 mm to Ra= 2.5 mm; ECM generates no residual stress into material of workpiece; and there is no tool wear.

Non-conventional Precision 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.

Nano Machining:
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.

Precision Finishing (1)

Precision grinding involved a maximum precision to about 1.0 mm and expected to reach 100nm SPDT (single-point diamond turning), UPDG (ultra-precision diamond grinding), ELID grinding (electrolytic in-process dressing), etc. Applications to optical and electronic industries

Precision Finishing (2)

SPDT, UPDG processes are similar in that chips of usually small size
Capable of producing surfaces with mirror finishing w/o polishing Using specially designed machine tools of high rigidity with air bearing spindles

SPDT vs. UPDG

Major problems is the appearance of subsurface defects in the form of

microcracks

SPDT is performed on very soft ductile metals, e.g. pure copper, while UPDG usually performed on very hard brittle materials, e.g. glasses and ceramics.

Ductile Mode Machining

Especially for brittle materials, is a machining process that work materials are removed by dislocation or plastic deformation rather than cracks propagation. That is, the cutting process is dominated by dislocation rather than flaw extension.

Comparison with brittle fracture:

Easy controlling of machining process; Free of cracks; Smoother surface.

Characteristics of Brittle Materials

High hardness High strength Good fracture toughness High wear resistance Good chemical stability Good thermal stability

Grinding of Brittle Materials

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 and advanced ceramics

Nano Precision Machining

Form/Dimensional Accuracy < 0.1um Surface Roughness < 10 nm

Toshiba ULG 100C Diamond Turning Machine

Nano Surface Machining by SPDT

Silicon (111) Wafer:
3 inches Diameter 0.5 mm Thickness Single Crystal Diamond Tool (or SPDT): Rake Angle 0 Nose Radius 0.3 mm Cutting Edge Radius 40 nm Nanomachining experimental setup on an ultra-precision machine tool Cutting Conditions: Spindle Rotation Speed 1000 rpm

Mirror Surface Finishing

Mirror surface finish of hard and brittle material can be possible when material removal taking place thru plastic deformation rather than fracture

(a) Diamond turning surface

(b) Original polished surface

(photos of silicon wafer surfaces)

Mirror Finished Products

Spherical mirror

Mirror finish by diamond turning

Aspheric glass lens

Machining of aspheric glass mould

ELID Grinding (1)

The wheel is continuously dressed while the part is machined The difference between ELID and conv. Grinding is the application of a current during grinding. Applications incl. grinding of silicon wafer, nano surface finishing on difficult-to-machine materials, e.g. glasses, ceramics, etc.

Schematic illustration of ELID grinding [Lim et al, 2002]

ELID Grinding (2)

The bond is depleted continuously by a pulsed d.c. power supply, enabling optimum protrusion at all times Process applicable to grinding of either electrical conducting or non-conducting work materials but only with metal-bonded wheels

Set-up of the continuous ELID of a metal-bonded diamond wheel

[Shaw 1996]

ELID Grinding (3)

Schematic diagram of the experimental set-up

[Lim et al, 2002]

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

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)

Effect of ELID

Without ELID(0.3795mm)

With ELID (Ra 0.1491mm)

Grinding conditions: Feedrate: 500mm/min Spindle: 3000rpm Electric power: 0%, 30%@90V Wheel : CIB-D wheel #1200

Silicon wafer planarization by ELID Grinding

Highly efficient due to high removal rate Uniform ground surface across the wafer Relatively low cost involved in this process

Workpiece

Wheel

Electrode Tool dynamometer

Ra 3nm (10nm required)

Multi-process Miniature Machine Tool for -machining

Micro Turning Interchangeable Micro WEDG spindle unit

Multi purpose machine tool for micro machining (-turning, -milling, drilling, -EDM, -ECM) Working area : 200 100 100mm (Resolution 0.1 m) DI water /Oil for EDM medium Design of motion controller
Contact: Dr. A.S. Kumar, mpeask@nus.edu.sg, MicroTool 6.5 micron Hole
1.5 mm length shaft

Micro Milling

Micro EDM

Micro WEDM On Machine Measurement

References
1. C.L. Borst, W.N. Gill and R.J. Gutmann, ChemicalMechanical Polishing of Low Dielectric Constant Polymers and Organosilicate Glasses, Kluwer Academic 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 Materials Processing Techn, 140(2003), pp.373-78 4. ECM, http://www.unl.edu/nmrc/ecm1/ecm1.htm 5. H.S. Lim, K. Fathima, et al., Intl J. of Machine Tool & Manufacture, 42 (2003) pp935-43.