Keynote-Papers

Current Trends in Non-Conventional Material Removal Processes

Prof. Dr. ir. R . Snoeys (1); ir. F. Staelens; ir. W. Dekeyser

P r o f . dr. ir. R . SNOEYS ( 1 ) . ir. F. STAELENS, ir. Y. DEKEYSER

SUMMARY

The so-called non-conventional machining methods can no longer be called %on-traditional", since they found a wide range of applications. Moreover, these electm-physical and electro-chemical material removal methods often do compete with the more traditional machining techniques. Those non-conventional machining methods allow the machining of complex shapes, not at least because of the use of advanced CNC-technology. techniques. e.g. when hard steel alloys and special composite materials are concerned. Evidence is given of the growing economical importance and the enlarged scope of applications covered by these non-conventional manufacturing methods. This is illustrated by a number of specific examples Some recent developments and new trends are highlighted as well. Some industrial examples illustrate how some of these techniques can be competitive with classical ma- nufacturing methods. In many other cases they are the only efficient solution for realising specific industrial products. In combination with conventional machining methods improved performances may be achieved.

The cutability of material often suggests the use of these

INTRODUCTION. 1. Machinability

The title non-conventional material removal methods became partially demoded: electro-chemical and electro-physical material removal processes are indeed more end more deployed in the metal working industry today. The various techniques msy be conveniently classified according to the appearance of the applied energy ( fig. 1 ) . These non-conventional machining techniques may come into the picture as possible alternative machining methods for s number of reasons, the main ones being:

1. machinability of workpiece material.

2. workpiece shape complexity

3. automation of data communication.

4. surface integrity and precision requirements.

5. miniaturization requirements.

In modern msnufacturing practice a more frequent use of harder, tougher or stronger workpiece materials is noticed: Materials, in other words, which are much more difficult to machine with traditional methods. Reference is made to all kinds of high strength thermal resistant alloys, to various kinds of carbides, fiber reinforced composite materials, stellites, ceramic materials, various modern composite tool materials etc. The introduction of new ways of machining is stimulated because of the high force levels observed. In Some particular cases, those levels may simply not be sustained by the workpiece. Therefore, more attention is directed towards machining processes in which mechanical properties of the workpiece (mechanical strength, hardness, toughness etc ... ) are not imposing any limits. In elactro-physical processes the "cutsbility" limits are indeed more associated with material properties such as thermal conductivity, melting temperature, electrical resistivity, atomic valence etc...

YICUNICAL PROCESSES CrnICAI A ?ROCZSS

f

ZLECTRO-MEHICAL PROCESS

ELECRO-THEM PROCESSES A

ELEm DISCINRE MCHINIffi (EM) LASER BEAU MACHININC (LM) ELECTRON BEAH IVICIBININC (EM) ION BEAU MCIINING (IM) P W E M U MUMINING (M)

fig.1 : Models Shoving Various Non-Conventional Machining Methods.

Annals of the CIRP Vol. 35/2/1986 467

2. Shape complexity

Geometrical restrictions, design requirements, problems related to accessibility during machining or what could be conveniently defined as 'shape complexity', states another group of reasons for an increased interest in using one of the more recent material removal processes. To give a rather simple example : it is quite easy to drill a circular hole with conventional techniques, however, to drill a square hole or just any other shape would be impossible. For Electric Discharge Machining or Electro Chemical Machining on the contrary, the cross sectional shape of the hole is of little concern. To cut some pattern of grooves with a depth of a few microns would be a difficult task in conventional machining. A chemical machining operation using some kind of masking procedure could yield a simple solution.

3. Automated data transmission

In mechanical production the automation of communication is crucial. If the information flow is more automated, a coneiderable reduction of the throughput time can be achieved, yielding decreased production cost, reduced inventory etc. This aspect has been one of the reasons of the considerable succes of the introduction of NC- machines and later of CAD/CAM and CIM. Those techniques may in some cases be integrated much easier with some non conventional machining methods. EDM wire cutting is an obvious example. Also NC controlled LASER or electron beam cutting are applied partially because of the improved automation in data

The main elemente of a typical USN equipment construction is shown in fig.2. A high frequency generator (20 ... 100kHz: 0.1 ..40kW) is applied to a magnetostrictive or piezoelectric transducer, to obtain a mechanical vibration at the same frequency. The tool or sohotrode is attached to a concentrator, applied to enhance the energy density. A slurry with abrasive particles is fed between tool and workpiece. Concentration, grain size and hardness of the abrasive slurry affect stock removal. surface roughness and surface quality 121. Other crucial parameters are the frequency and amplitude of vibration. The two main hypotheses for desintegration are direct impact and cavitation erosion. A valuable model for describing the relative importance of those parameters has been proposed by Shaw [3].

transmission. There are many other types of applications in which the use of non conventional machinina methods drastically reduced the number of &ccessive elementary machine jobs. A die plate made of carbides for example, could be machined out of one piece using spark erosion; the claselcal way would require at least two pieces fitted together and produced eeperately on a profile grinder.

4. Precision requirements

The trend of precision requirement as indicated by Machining Equipment. Taniguchi [I], refers to nano-machining in tomorrow's ultra high precision machining. This kind of The process is used for manufacturing high quality precision may be obtained by removing atoms or graphite electrodes for EDM [ 4 ] , fig.3, even in the molecules, rather than chips. Machining operations cage of intricate shapes (cross-sectional area < 1 like sputtering ion beam machining etc. would be dm ) . possible candidates. The dietorsion of the surface layer due to mechanical or thermal action may be another reaeon to call upon some of the same non conventional machining techniques.

F%l-2 : Main Blements of a Typical Ultrasonic

5. Miniaturisation

Trends toward reducing the workpiece dimensions already exist for some time. Ultra small diameter holes (lO..lOOum) would not be possible to drill with conventional techniques. Electro Discharge Machining, Laser Beam Machining, Electron Beam Machining or even Micro Electro Chemical Machining techniques are now frequently applied for such purposes. Micromachining has recently become an important issue, further reducing possible attainable workpiece dimensions. Various techniques develloped for the production of micro electronic circuitry may be used for manufacturing extremely small items. Especially in the area of sensors, an integration of mechanical parts with the electronic circuitry may become a new possibility bringing the design and production of various sensors on the verge of drastic cost reductions.

1. PaCiUNICAL TECHNIQUES

1.1. Ultrasonic Machining (USM)

Hard and brittle materials, whether conducting or not (for example glass, quartz, diamond, carbides, semi- conducting materials, graphite etc.), can be machined with USM. The enhanced interest for ceramics in a wide area of industrial applications and some new developments for the production or reshaping of graphite EDU electrodes, contributed to a renewed attention for ultrasonic machining. Outside the machining area, ultrasonic machining technique- are applied in non-destructive testing, surface cleaning and welding.

Fig.3 : Graphite Electrode Produced by Ultrasonic Machining [ 4 ] .

Another interesting application area is the cutting of thennoplastic or combined natural-thennoplastic tissues (fig.4). Clean cuts are obtained because the ultrasonic vibrations melt the material, at the same time "welding" the particular cut fiber ends together. This technique can be applied in direct combination with weaving looms. Cutting speeds of 5...BOm/min can be obtained, depending on the target material. A rather recent application in this context is the cutting of KgVLAR [ 5 ] and rubber.

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revolving couplings are still yielding some problems for the supply of water at high pressure. Although the water jet reactive forces are quite limited (<50N), pressure waves due to the action Of the valves may cause vibrations of the robot joints.

Pig.4 : Border of a Tissue. Cut with USn-Equipment f51.

1.2. Water-Jet Machining (WJMY

In Water-Jet machining (WJM), the workpiece material is removed by the mechanical impact of a high velocity water jet. In fig.5, the basic diagram of the WJH equipment is shown. One of the most crucial elements is the nozzle, in which the water pressure (300..400~~a) is transformed into the high velocity water jet (600.. -900 m/s) - The construction and geometry of the nozzle seems to be critical, because of its influence on the jet profile, and the nozzle wear. A conventional type sapphire nozzle and a newly developed metallic nozzle are compared in fig.6. The new design yields smaller gaps with almost vertical side-walls. Water-Jet machining is applied in various industrial areas, ranging from cleaning, deburring and removing metal chips, to cutting of titanium and composite materials in aerospace industries. Presintered ceramics, paper. vinyl etc... are possible Workpiece materials. Water Jet cutting is a widely used technique for the machining of fiber reinforced plastics (FRP) [71, [81. Pump pressure, nozzle diameter, the number and

11 hydraulic drive

(optianat: additives additives) (optlonat)

I t o r k p i c c e water inlet

Pig.5 : Basic Scheme of a Water Jet Machining Equipent .

type of laminate layers and the fibre orientation are parameters influencing the cutting speed and surface quality. Applications are known in shipbuilding, aerospace, automotive, machine tool and sporting good industries. Thermal damage of the workpiece is almost completely avoided usina WJM. However, because of the limited transverse sfability of the -jet, grooves may result at the cut surface, or jet deflection may cause the FRP fibers to be jumped over. WJM also opens perspectives for construction engineering and under water machining [91. By adding abrasive additives to the water jet, the cutting efficiency of the jet is increased. The above mentioned behaviour of the jet to "jump" over hard material zones, is remarkably reduced and cut surfaces are smoother. This technique is also used for jet sharpening of grinding wheels [lo]. In the next paragraph, Abrasive Jet Machining is discussed in more detail. Complex contours can be shaped by mounting the water- Jet nozzle to a robot arm. Flexible tubes and

Pig.6 : Conventional Type Sapphire Nozzle ( l e f t ) in CGntpariSOn with a N W l y DeVdOp9d Metallic Nozzle fright1 f61.

1.3. Abrasive Jet Machining (AJM)

In Abrasive Jet Machining ( A m ) the workpiece material is removed by mechanical impact of a high velocity air jet with abrasive particles. Fig.? shows a schematic representation of the process. AJH is a rather slow process (...lo mg/min ... ): however. it is quite cheap, forces on the workpiece are limited and no thermal problems occur because of the cooling effect of the expanding air. Most often, aluminium oxide or silicon carbide powders are used as abrasive medium. Close control of the abrasive jet QKOC~SS is poseible. Nozzles are made of tungsten carbide or syrrthetic Sapphire, with circular openings from 0.15 to 2 mm diameter. The main parameters influencing the material removal rate and surface quality are air pressure. size of the abrasive particles (...6Oum...), spray angle, tool to work distance (2..15mm) and feed rate ( O . . I m / s ) [ll]. An other kind of AJM is Abrasive Plow Machining (Am). It is a finish machining process in which a special abraeive-filled semi-solid plastic medium is flowed through and across workpiece areas to produce a range of edge and surface conditioning effects [12]. An example is shown in fig. 8. Typical applications of AJM sre the machining of shallow and often intricate holes required in electronic industry (resistor paths in ineulstors. patterns in semiconductors). The pracess is also used for engraving registration numbers on motor car toughened-glass windows 1111. Further. many applications for deburring (surgical needles, hydraulic valves, plastic components etc...) are known.

compressor

air inlet

convergent-divergen t inlet nozzle mixing chamber

convergent exit nozzle

1 1 1 mechanical irnoact

workpiece

Pig.7 : Schematic Representation of the Abrasive Jet Machining Process.

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fig.8 : The Leading Edge Profile of a B l i s k Blade after Initial Rough Machining and after APM Micro-Machining which Produces a Precise 12:l Ellipse (top). Cut-away V i e w of Special AFM Fixturing of Blisk and the Abrasive Media Flow Path [arrows). Additional Restrictors Located at Each Blade's Leading Edge for Directing the Abrasive Flow. are Indicated as well lbelw) C121.

2. CHEMICAL TECHNIQUES

2.1. Chemical machining (CHM)

The principle of the chemical machining (CHM) is illustrated in fiq.1. The material removal is based on a chemical reaction between workpiece and etching fluid. On areas where a mask covers the surface no chemical reaction and thus no material removal

layers ( max. a few mm). Sometimes CHM may be in competition with ECM: CHM may be preferred for manufacturing small or middle sized series. For very large series the relative masking costs may become too important.CHM may also be preferred when very large workpieces have to be machined because of the large electric currents involved in ECM. For ultra small parts, the electric connection also causes problems in ECM; sometimes a floating workpiece may be used in such cases. Although the process is less complex as ECM, machining results still depend on experimetal work in the mask-making: the composition of the etching reagent is another item requiring practical experience.

occurs. The process is used for the removal of thin

wling] constant

pask

Recent technological CHM developments are the use of laser for cutting "cut and peel masks", for the etching of airplane wings (fig.9) [131. It yields lower through put times and avoids surface damage caused by manual cutting: damage which could even be amplified during the etching phase. While the processing technology of silicon integrated circuits consists of a sequence of planar processes, the fabrication of most micrOmechanica1 components relies on a 3D-structuring of the substrate. For this purpose anisotropic and selective etching as well as lithography techniques are suited. The techniques of silicon microfabrication allow the manufacturing of extremely small mechanical parts. A further integration of mechanical structures with electric circuitry may open a complete new area of applications. Reference is made especially to the manufacturing of sensors for fluid flow, accelerometers, force transducers, pressure devices, biomedical body implants, sensors for industrial process control including some chemical sensors and for automotive, marine and aviation diagnostics, etc. [J.41~[151,[161*

Anisotropic etching

There are several etch systems that exhibit a strong dependence of their etch rate on the crystallographic orientation of single crystal sylicon. The anisotropic etchants are generally alkaline solutions employed at an elevated temperature. Crystalline silicon shares with diamond the crystal structure called interlocking face-centered cubic [151.

A crystalline direction is designated by three coordinates called Miller indexes which are integer multiples of the length of one edge of a unit cube. Most anisotropic etchants progress rapidly in the crystal direction perpendicular to the (110) plane (fig.10) and less rapidly in the direction perpendicular to the [lo01 plane. The direction perpendicular to the (111) plane etches very slowly, if at all.

Anisotropic etchants create faceted holes, composed of the crystal planes that are etched slowest, unlike isotropic etchants which produce a gently rounded hole (fig lla) [15].

fig.10: Etching Directions for Crystalline Silicon.

fig.11: Etched Geometries in Crystalline silicon

The shape of an anisotropically etched hole is determined by the crystalline orientation of the silicon wafer surface, the shape and orientation of the openings in the mask on that surface and the orientation dependence of the etchant itself. A few examples of possible etched shapes on a <loo> wafer are presented in fig.1lb.c.d. A square opening on the mask oriented along the <110> directions of a <loo> yields a pyramidal pit with (111) side walls (fig.llb). With a larger mask opening and when stopping the etching before the intersection of the 1111) planes is reached, one obtains a flat-bottemed pit (fig.llc). Fig.lle refers to a <110> wafer orientation.

Application examples include ink jets, optical components, accelerometers etc. In fig.12 a piezoresistive accelerometer matrix array can be seen with 16 cantilevers operating at their resonance frequency covering a frequency-range from 4.3 kHz to 6.4 kHz. The thickness of the silicon

fig.9 : Set-up for Laser Cutting of Masks on Large cantilevers is 4 urn. The conversion of the Surf aces. mechanical deformation of the cantilevers into an

470

electric signal is obtained by means of polycrystalline piezoresistors. The major advantage of these arrays are the high sensitivity of the individual cantilevers operating at their natural frequencies and the possibility of obtaining a Fourier-transformed frequency spectrum without the necessity of further electronic conversion.

fig.12: Silicon Vibration Sensor Array [141

To illustrate the fast growing interest in anisotropic etching techniques, fig.13 shows the number of patents filed in world patent index [16].

100

7 5

50

25

Japan USA Europe ROW

fig.13: Number of Patents for Anisotropic Etching Filed in World Patent Index 1161

Selective etching

In case of highly boron doped silicon, a dopant concentration effect called "selectivity" is observed 1141. Up to a "critical" boron concentration (2.5 10'' atoms/cm3) the etch rate is practically independent of th doping level. With a concentration of 10% atoms/cm3 the etch rate is reduced with a factor 1000. This can be effectively employed for a verticel structuring of the silicon wafers. Highly boron doped layers can be used as an etch stop layer and exploited for example for the fabrication of silicon membranes, springs or masks for X-ray, electron or ion beam lithography.

A demonstration of the elastic properties of structured silicon membranes is exhibited in fig.14, where a spiral-like membrane is shown in an undeflected and deflected form. The thickness of the spiral is 2 . 5 um and the depth of the deflection is 300 um.

scale lOOumw

fig.14: Silicon spiral like membrane i n undeflected fleftl and deflected fright) state. Cl4l

3. ELECTRO-CHEMICAL TECHNIQUES

3.1 Electro-Chemical Machining (ECM)

The set up for the Electro Chemical Machining (ECM) and especially the ECM die sinking is shown in fig. 15. The material removal is based on the anodic dissolution during electrolysis [171. The tool electrode is used as a cathode while the workpiece is the anode. The electrolysis process takes place in a salt solution. The fundamentals of the ECM process a r e shown in fiq.16. The feed rate v f (fig.15) corresponding to the material removal rate ranges between 0.5 to 10 mm/min when applying a current density of 10 to 1000 A/cm . The major advantages are the absence of tool wear as well as any thermal load on the workpiece. Additionally, ECM accuracy is improved when applying higher material removal rates.

cleaning of electrolyte

1

1 va = dielectric flow

10..50 UIS rate

vf = feed rate = 0 . 5 . . 10

gap = 20 .. 2M)o P

specific rm 1 rate : 1 .. 2.5 mh.un

fig.15.- Set-up for the Electro-Chemical Machining t561

The major problem is the overcut (fig.17). i.e the undesired and uncontrolled electrochemical overdissolving of the workpiece. The working gap is indeed governed by a complex set of parameters. of which the electrolyte flow is one of the most crucial. It is important to have small working gap values, because the variatons in the gap width due to unwanted variations of process parameters, are proportional with the gap width.

47 1

3 Removed Volune: (pip 1

4 : a t d c number n : valence

[ I x t ) N 1 V~-X-X--X 'I

96500 n P

specific r m l rate:, (m3/A.dn)

9.--I--I- X ' I n p 96500

n i 1

p : density '1 : process efficiency I : current j : current density t : tile

feedrate vf = j . r [mldn)

fig.16 : Fundamentals of Electro-Chemical Machining.

overcut insulation lager

gap fig.17 : Illustration of the Effect of Passivation

( b ) and Insulation IcJ on the Accuracy in Die-Sinking ECM Compared with the Use of a Blank Tool Electrode fa)

3.1.1 Recent evolutions.

Recent evolutions concentrate on increased dimensional accuracy. ECM is implemented for mass production of small high-precision components (accuracy better than 10 um) [la]:

1.T-1 insulation: When using specific electrolytes the overcut is reduced by the anodic or passive layer, which is an oxide or hydroxide layer coating the anode. It yields s reduced metal removal efficiency, especially where the current density is smaller. New techniques have been developed yielding extremely thin but mechanically quite strong insulation layers. Good results have been reported for tungsten and molybdenium tools coated with a thin layer of silicon carbide (thickness 10-15 um) in which the insulation is been provided by an intermediate layer of silicon nitride (thickness 0.4 um) [19].

fig.18 : Effect of Pulsed Current in ECM Die-Sinking.

2.Pulsed current ECM: ECM with a pulsed current allow8 an intermediate electrolyte renewal, yielding smaller working gap values. It also results in a smaller overcut i fig. 18) 1201.

3.Model development: The gap dimensions are traditionally to be determined more or less experimentally. Hence, the importance of recent research in development of process models describing the influence of the dielectric flow conditions on the working gap dimension [ 2 1 ] .

4.High-precision machines together with an accurate temperature control of the electrolyte fluid are essential to obtain the required accuracy.

fig.19: Precision Machining Example using BCM. [561

3.1.2 Main applications.

1 ECM die-sinking. The tool shape is being projected into the workpiece. Fig.19 is illustrative for the new precision possibilities offered by ECM. 2 ECM drilling. In view of the very thin gap when drilling small holes, H2SO4 has to be used to hold some of the removed metal in solution. Several ECM drilling techniques are used especially in the aircraft industry for the machining of turbine blades. Table 1 compares the ECM drilling to the conventional drilling while Table 2 shows some characteristics of various ECM techniques. ESD (Electro Stream Drilling) yields the finest holes while the smallest inter-hole distances are obtained with ECFD (Electro Chemical Fine Drilling). Electro Jet Drilling does not need any tool feed. It basically uses an electrolyte-jet under high pressure, yielding small holes which are slightly conical and not deeper then 1 nun.

Hi61 Co 9 . 5 M4

L-- I40 x MI / ( ! I 0.51 3

160 : I

S 65

$ 1

J 0.04 I : 150

table 1: Comparison of ECM Drilling with Conventional Drilling f221

472

I I

table 2: Characteristics of Various ECM Drilling Techniques f221

3. Electrochemical Deburring and polishing. The process is basically the same as the ECM sinking, but without a tool feed. A preferential anodic dissolution is taking place on burrs due to the locally increased current density. The surface area to volume ratio is also important in this respect. The deburring process can easily be automized for instance for stamped metal sheet products. When no burrs are present, electro polishing takes place, removing a thin layer ( 10 urn) as well as the embedded impurities and subsurface corrosion. It yields higher fatigue live and corrosion resistance. Due to an improved insight in the process, elentropolishing has recently become suitable for materials such as copper alloys and molybdenium and is also implemented for larger workpiece sizes.

4. EC Grinding. ECG (fig.20) is mainly applied for the sharpening of carbide tools. In this field however, Electro Discharge Wire Cutting has recently been successfully applied. especially for NC-shapening of intricate profiles of various cutting tool tips. The rather law accuracy due to the overcut can be improved by adding some mechanical cutting action using metal bond CEN electrode wheels.

fig.20: set-up for Electro-Chemical Grinding.

ECM may also be used for in-process dressing of metallic bound grinding wheels 1231. This may be implemented for keeping grinding forces and cutting energy below some critical level. Such measures may be of interest when grinding high strength ceramics, especially silicon nitride, showing very poor grindablility. Fig.21 shows a scheme of the in- process dressing device. The rate of dressing is kept optimal by adjusting the current for stable grinding. The system has proved to be quite effective in reducing the cutting force (up to 90 % ) and improving the productivity for grinding high-strength ceramics. Surface roughness and accuracy are also improved.

Some combined techniques of ECM with Electro Discharge Machining (EDM) have been tried out. Electrochemical Arc Machining (ECAM) [24] superimposes an ECM supply with EDM pulses often combined with a vibration of the workpiece. This technique aims at higher ECM material removal rates while reducing passivation effects as a result of the EDH process. An increase of the specific material

Electro Conchctive

Current Supply Metal Jhin

Table Feed

fig.21:Scheme of an in-process dressing device for grinding Wheels [231. Grinding wheel Characteristics: Synthesized Diamond in Metal Bond. Wheel Thickness: lmm, Diameter: 127mm. Wheelspeed: 1200 m/min. Table Feed: 20 mm/min. Workpiece Material : S i 3 N 4 .

removal rate with 300 Z was reported compared to EM. Difficulties of process control and the rather complex equipment are considered to be serious drawbacks. Wire-ECM [25 ] uses a wire as the ECM tool electrode. Due to the small material removal rate and the observed roundness of the edges, it can not be considered as an alternative for Wire-EDM but it may be applied for finishing after NC Wire EDM. Finishing Wire-ECM yields a surface without thermally affected layer combined with an improved roughness and smoothness. AS s result of recent developments in EDM. yielding high surface quality and improved material removal rates, these combined techniques may probably not come out of the experimental stage.

4. ELECTRO-THERMAL TECHNIQUES

4.1. Electro Discharge Machining.

Electro Discharge Machining (EDM) is an electro- thermal process where the material removal is being achieved by electric discharges occuring between an anode (mostly tool electrode) and a cathode (workpiece), submerged in a fluid dielectric (fig 22). que t9 the very high power concentration ( lo5 W/mm w/mm2) a minute volume of workpiece material may be removed by melting and evaporation. Part of the total energy is also absorbed by the tool electrode yielding some tool wear. This wear can be reduced to 1 % or less of the material removal with an adequate selection of tool and workpiece material and appropriate generator settings [26]. Important problems in EDM are indeed the tool wear and the machining accuracy; the low material removal rate typical for Enn is urging for more sutonomous working machines.

- 10

fig.22 : Scheme of an EDM Die Sinking Machine.

EDM can be classified into die-sinking and wire- cutting; it is extensively used for the precision machining in die making and for the manufacturing of prototypes.

473

As an example of the growing importance of EDM, the Japanese market evolution is shown in fig.23 12.71 stressing especially the increasing importance of numerical controlled EDM systems.

I:?) ? M,l l , .mrn

\ ! CBtrml / L M i l l l a h l k )

%I-\.

fig.23: Japanese Market Evolution for EDM [271

Following trends in the field of EDM may be reported:

4-1.1 Die-Sinking EDM.

1. The introduction of planetary EDM in which the tool electrode is subjected to an orbital motion with a radius from a few tens to some hundreds of microns. Possibilities and advantages of the planetary EOM are illustrated in fig.24 [27]. 2 . Due to the introduction of Numerical Control, the accuracy of the die-sinking machines could be enhanced. The production of workpieces with an allowance of 5 um became possible in die sinking. Moreover, various kinds of relative movement aetween tool electrode and workpiece are introduced (fig.25). As a result EDM, die-sinking is evolving towards EDM- milling.

mnvmtional EDM I Planatnrymotion E.D.M. two s l a t r o b om rlcctrodc

- - ,

depth motion

working l ime m -

111

Xlom roiqhncn roughosu alone L X W & c

w a v i n u

fig.24: Advantages of Planetary EDM [271 The introduction of automatic high precision tool changers with a repeatability error of 3 um enables to split up complicated tool electrodes in a number of simple shaped electrodes. This turns out to be quite useful1 for the machining of plastic moulding dies expected to become at the same time increasing1.y

2. a u t e a t i c 3 . siaultaneous l i n e a r six di rec t ions enlargement two a x i s machining 1. X:Y.X! ...

j, s i d t a n e o o s l i n e a r and m t a t i o n a l ICI tw axis iach in ing

4 . machining i n three axis d i rec t ions

fig.25: V a r i o u s possibilities of NC-Die sinking EDM. [ 281

complicated in shape and requiring a higher accuracy- TO meet the latter requirement, automatic compensation for the machining depth is introduced (fig.26).

Fig. 26: Principle of Automatic Tool Wear Compensation [271

3. The introduction of Adaptive Control and Optimization techniques [29] together with the numerical control features, yields an increased process efficiency and allows enhanced unmanned machining (up to 48 hours and more). The implementation of tool wear sensors and optimization strategies [30,31] leads to improved accuracy.

4. Fine die-sinking EDM la now available with EOM polishing; even brilliant surface finishing is possible. These polished surfaces may be required for aesthetic purposes but also for a number of functional reasons as well. The workpiece is subjected to a planetary motion while applying very small amount of discharge energy (peak current 1-2 A, current pulse duration 1-5 us). The mirror like surfaces can only be obtained on those steel alloys that do not develop cracks in the recast layer of the EDM machined workpiece (i.e. alloys with low concentration of carbon and/or chromium). When the machining surface area is large, stray capacity influences the obtainable surface finish. To solve this problem the use of silicon plate back- up'ed with copper is proposed. The high resistivity of the silicon divides the discharge into several ChaMelS yielding a smaller crater on the surface. The surface roughness of Rmax = 1 is reported to be obtained on the area of 275 cm with a single electrade of the same area [321.

AS a recently developed special application of EDM die-sinking, the high speed fine hole drilling has to be mentioned also. These dedicated NC-EDM machines are suited for drilling holes of 0.03 to 3 mm, at high feed rates of 60 mm/min. They may be applied in drilling start holes for Wire-EDH and NC-drilling of a pattern of holes.

4.1.2 Wire LOR!. Wire cutting EDM (Wire-EDM) basically is a numerically controlled technique whereas the workpiece geometry is generated by a NC-controlled traveling wire (fig.27). Recent Wire-EDM machines are equiped with to 6 gxis control. The tapering of the workpiece up to 30 is fully integrated. It yields a fast and flexible machining method especially for intricate three dimensional geometries described by a set of straight lines (fig.28).

The main evolutions in wire cutting can be classified under the following headings:

a 4

474

4 M C - C O W T R O L II

I wire 1 0.039 tunnsten I

2 . Prolonged unmanned machining times. Increased automation is largely due to various developments such as automatic wire insertion, integrated start hole drilling and the integration of ACO strategies to avoid wire rupture [33]: 80 hours stand alone machining is claimed by some Wire-EDM manufacturers . 3. Machining of ultra hard materials. EDM (wire as well as die sinking EDM) recently is being used for the machining of electro-conductive ceramics as SIC, SI3Nd-TiN. ZrB2 etc. [36]. Fig.30 shows a micro hole on TIBz-B~C ceramic, with diameter 4s um and thickness 33 um. Other applications are the cutting of Cubic Boron Nitride for grinding wheel dressing and the cutting of Polycristaline Diamond (PCD) and even natural diamond for wire drawing dies and cutting tools 1371. Cutting speeds of about half the cutting speed on tool steel can be obtained on tungsten carbide covered with a PCD layer [381.

I

module 0.15

fig.27 : Wire-EDM Basically is a Numerical controlled Technique

I

fig.30: Micro-hole in TIBz-B~C Machined by EDM. Diameter 45 um, Thickness 33 um

module

fig.28: Turbine Blades Machined with Wire-EDM.

1. Increased materiel removal rate and accuracy. Wire cutting speeds of 200 mm2/min are being reported in a 60 mm thick tool steel specimen, with a surface roughness Rmax = 24 um. Among other reasons, this results from the uee of a transitorized power supply instead of a capacitor power supply; the former being able of generating high frequency (20-40 M z ) pulses with large peak currents (200-400 A ) . The application of special coated wires has also contributed to an increase of the machining speed. Control strategies to prevent wire breakage to occur 1331, may also be quite helpful1 as they make higher average cutting speeds possible. Workpieces up to 350 nun can be machined now with a linearity error on the straightness of roughly 20 um. Surface roughnesses of Ra = 1.4 um are possible after finishing (341. Shape accuracy by Wire-EDM in a w rking environment with temperature variation about 3 C can be obtained within 4 um. If temperature control assures +/- 1'C. the accuracy obtainable is about lum. Fig.29 illustrates the precision capability of wire-EDM by means of a machining example [22].

8

0.15 thicknees 5 m m I Work piece sintered tungsten

I carbide alloy lennth of motile 1 10.2mm

minimum Radius I machined time I

fig-29: Precision Machining Example for Wire-EDM 1271

4. Micromachining. To illustrate the high precision as well as the sometimes unexpected type of Wire-EDM applications, the Wire Electro Discharge Grinding can be cited C391 ( fig. 31

r i r s mid

a ) n o r m a l r r i n d i n l b) r r l n d l n r b o l l o r e n d

flg.31: Set-up for the Wire Electro Discharge Grinding 1391

This manufacturing method is applied for the micro machining of fine rods or needles needed in tool making in the area of micro-electronics cicuitry manufacturing. Fig.31 shows the set-up for this Wire EDG. Precision of the diameter along the axis of 1 urn for a rod of 30 um diameter is obtained. This corresponds with the precision of the NC-system. Tool wear in this case does not influence the obtainable precision. Fig.32 shows some pictures of machining results.

flg.32: Examples of Workpiaces Machined by wire Electro-Discharge Grinding. Workplece Material: Tungsten.

475

Three dimensional Wire-EDM is proposed by Masuzawa. A two dimensional template is used as a wire guide. The ridge of the template which is used as the electrode will be subjected to high wear rates. However the ridge is formed by the wire and is replaced continuously like in normal Wire-EDM. EDU 0.Olmm

4.2. Laser Beam Machining (LBM)

LASER is M abbreviation of "Light Amplification by Stimulated Emission of Radiation". A highly collimated, monochromatic and coherent light beam is generated and focused to a small spotZ High power densities can thus be obtained (lo6 W/mm ). A large variety of lasers is available on the market. Some typical continuous and pulsed laser types are represented in table 3, together with some characteristic values.

694 pulsed. 5W

solid I i:::AG

semi- GaAs 800-900 pulsed, c.w., 2..10mW conductor

1064 pulsed, c.w., 1..8OOW

Nd-glass 1064 pulsed, c.w., 2mW

molecular CO2 10.6~11 pulsed, c.w.(<15kW)

laser type typical perfomance

lion { Ar+ I 330-530 1 pulsed, C.W. 1W...5k

Excimer 200-500 I pulsed I I I I neutral He-Ne I 633 1 C.W. 20mW

I

table 3 : Typical Performances of PulSed and Continuous Wave (c.u.) Lasers.

It is possible to control laser-beams accurately within broad ranges. Therefore. lasers are used in manufacturing as very precise and multi-task tools. Metal-working remains an important segment of laser applications : cutting, drilling and surface heat treatment. Materials such as advanced composites, plastics, ceramics and exotic alloys can be machined. Difficult to machine materiels such as tungsten, titanium, alumina, tantalum and natural diamond are another growing applications area. Other application fields are non-destructive testing, measuring systems (compact disc), telecommunication with fiber optics C431, medical applications, military applications, printing and graphics devices etc... The market scale of the laser industry in Japan was about S 1 billion in 1983 [401. Sales for lasers, laser services and support equipment in the U.S.A. are expected to reach S 5 billion by the end of the decade [41].

electric

fig.33: Coaxial COz Gas Laser source t451.

4.2.1 Cutting and Drilling Applications

In metal-working industries, high power C02 lasers (fig.33) and pulsed Nd-YAG lasers (0.1-1.0 ms - power peaks 30 ... 50kW) are most often used for surface heat treatment, drilling, cutting and welding purposes. Heat treatment is often used for improving locally material properties (gear boxes, valves...).

I attainable machining precision spaad

mechsn. dri 11 ing I >O.O2mm

LASER 0.06d-0.OZd

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (ECM drilling) .......... of hole

....... ....... ....... ....... ....... ....... ....... ....... ....... ....... 100 . . . . . . . EDM I

10

1.0

0.1

hole diameter D

Table 4: Comparison of Various Techniques to Produce h a l l Holes C441.

More recently developed laser sources with optimized structure (cross beam, cross feed...) with high power output (several kw) enable higher cutting speeds. laser cutting applications are, however, still limited to thickness of roughly 10 mm for steel alloys [41].

Especially for drilling small holes, the laser process is becoming an important machining technipue- Table 4 represents a comparison of various techniques to produce small holes: a quite large application area is covered by Laser drilling.. The machining Speed with lasers may be smaller than with EBH, but there is no need for vacuum. Cutting of 3-0 contours and shaped plates with LBM has been developped during the last few years. To avoid the influence of the polarization on the cutting speed. circular instead of linear polarization is applied (fig.34). using a A/4 element.

linearly polarized beam

f l circularly polarized * beam 1

fig. 34: Circular vs. Linear Polarization in LEN.

To obtain constant machining results, the tool should remain perpendicular to the workpiece. CNC-machines or robots with at least 5 degrees of freedom are used. The distance between tool and workpiece is detected and corrected. The cutting of edges may be improved by changing from continuous to pulsed operation at the corners. C02 laser cutting can also be applied for cutting glass and aramid fiber reinforced plastics 171. Thermal damage of the material may create some problems.

Using a 1/4 Element t44).

476

4.2.2 Peripheral Devices and Special A p p l i c a e s

Laser peripheral devices have rapidly developed as well. Developments to supervise the process by means of adaptive control are mentioned in 1443. Due to enlarged flexibility. laser processing can be used for prototype or small-batch manufacturing. Transport of laser beams to a robot arm with fiber optics is available for distances up to 2% (500 W) [441. Lasers may ba used for machining hard materials (white iron casts, inconel ... ) in combination with a traditional machining process. The laser is directed onto a spot in front of the tool (cfr. Plasma Assisted Machining) [44].

Lasers with short wavelength (200 ... 4OOnm). such as excimer lasers (inert gas (Ar, Xe) and halogen gas (P Cl)) are used for IC mask patterning. Ar4 (1-488nm. 5 4 5 m ) and He-Ne (1-633nm) lasers can also be used for photon reactlve exposing of photo resist film [45]. For in-situ design and repair of IC's requiring a process that can be controlled and altered from chip to chip, laser direct writing seems to be the tool of the near future [42]. Direct write processing offers flexibility end accuracy. Following techniques can be used : photolytic, pyrolytic and photo-thermal, all three involving gas-phase or ad-layer decomposition of organometallic +compounds (dimethylcadmium, diethylzinc ... ). Ar lasers (2571~~. 5 1 4 m ) are used [421.

4.3. Electron Beam Machining (EBM)

Electron beam processing has been used in industry since the 1960's. initially in nuclear and aerospace welding applications. Small hole drilling, cutting, engraving and heat treatment represent a set of modern applications often used in semiconductor manufacturing, as well as in various micromachining areas.

4.3.1 Micromachining applicatione

The main advantages of the process are high degree of automation, high productivity, possibility to machine almost any material and attainable high precision. A typical equipment for EBM I s shown in fig.35.

work piece

fig.35: Electron Beam Processing Equipment C451.

Electrons are released from a heated tungsten or tantalum filament (30-150 kV). A Wehnelt-electrode controls the convergence and the intensity of the beam. A series of electromagnetic lenses focus the beam on the target in a vacuum chamber (.1 Pa).

According to recent investigations [45] stock removal by EBM is performed by splashing bubbles produced in a subsurface layer, by electrons penetrating to a depth of a few microns under the target surface.

A typical application of EBH refers to multi-small hole drilling. Very small diameters (<50 um) can be drilled at high rates. An example of a perforated part (a few thousand holes) of a jet engine produced with a four axes manipulator to drill under varying angles, is mentioned in 1461 (fig.36).

fig 36: EBH-Perforated Part of a Jet Engine. Total Time Required for Drilling a Pew Thousand H O ~ S in Different angles to the Surface. almut 20 min., Including Pump Down and Positioning Time C461.

is

fig 37: Hybrid Circuit Engraved with 40 um Traces: Machining Speed > 5m/sec.

A n example of an electron beam micromachining- application is shown in fig.37. It refers to the engraving of a hybrid circuit with 40 um wide traces.

4.3.2 Electron beam lithograg3

There are certain geometrical constraints using anisotropic etching techniques. Additionally, there are technically important materials which do not show the anisotropic etching properties so that this patterning process would not be viable at all [ M I . The problem can be partially relieved by using an appropriate lithography technique combined with selective plating.

Photolythography is limited by some important bottlenecks : each print requires an individual mask: this becomes un-economic when logic circuits are to be personalized for different individual logic functions. Another drawback relates to the mask making process. When chips with over 200.000 structural details have to be engraved the opto-mechanical pattern generator needs several hours to accomplish this task. Besides, there is a limit with regard to the minimum dimensions of the pattern. This is due to pattern resolution degradation at every step in the photolithographic process, thermal expansion and bending of the mask and wafer, and the relatively long wavelength of visible light [471.

Among the contending technologies X-ray and Electron Beam Lithography can be situated. Electron beam technology with its very short radiation wavelength and its capability of large frequency and beam deflecting offers both high through-put for volume production with electron beam projection systems and flexibility in pattern generation with electron beam scanning systems. The through-put of the former is facilitated by sacrificing somewhat in flexibility and, instead, projecting a fixed mask pattern to the wafer in one blow.

X-Ray lithography is particularly applicable for the fabrication of structures with lateral to vertical aspect ratio's of up to 1/100.

477

An impressive example for the application of this technique is the "deflecting nozzle system" employed for the seperation of uranium isotopes 235 and 238 as developed by the Kernforschungszentrum Karlsruhe (FRG) (fig.38). A high pressure mixture of UFg and a carrier gas H2 flows with high speed along CUNed walls. A partial separation of uranium isotopes is generated under the effect of centrifugal forces.

fig.38: Scanning Electron Microscope Picture of Plated Seperation Nozzle Structure C l 4 l

Typical equipment for a vector-scan variable shaped electron beam system is shown in fig.39 [48].

Host often, a lanthanum-hexaboride (La861 field emission cathode is used. In order to shape the beam, one square aperture is imaged onto another square aperture. The effective beam shape is finally determined by the overlappinq parts of the two apertures. Accurate positioning of the electron beam to the desired addresses is based on measuring the wafer position by laser interferometers attached to the x-y table. Any position errors are corrected by means of beam deflection. For the high speed vector scan system reported in C481 (fig.39). VLSI chip pattern data is called from a disk storage medium: the data is passed through high-speed RAM-buffers, and transferred to a high speed data-controller, varying the positioning of the beam at a high settling speed. The mentioned vector-scan variable shape system has an output of about three (-inch diameter wafers per hour, including overhead time. The beam size can be varied in the .5 to 5 um range in .1 um increments. The beam position can be sat at 50 nm intervals in a deflection field of 2.5 x 2.5 mm.

4 . 4 Ion beam machining (IBM).

Ion beam processing has developed very rapidly during the last decade. The process has applications in many areas, including fine patterning using masks, texturing surfaces, surface cleaning and smoothing etc. An application for shaping lenses is reported by Taniguchi 1451. Adjusting the thickness of thin films and membranes without affecting the surface finish represents another application 1491. Ion Beam Milling through masks is a technique used for example for the accurate production of shallow grooves. Similarly, microsieves may be formed for the production of ion source grid components 1491. Ion Beam texturing is applied for enhanced bonding of surfaces, especially for fluoropolymers, and also for increasing the surface area of capacitors 1501.

More recently, ion beam machining techniques such as Plasma Etching, Reactive Ion Etching (RIE), Reactive Ion Beam Etching (RIBE) etc., have become industrially available. They are of special interest for the semiconductor industry for machining patterns, even in the submicrometer field. Ion beams offer a number of substantial advantages compared to the classical wet chemical etching. Indeed, it is possible to machine almost any material and the material removal can be controlled accurately (dimensions<lOO nm). Further, the angle of incidence of the ion beam is fully controllable, resulting in good wall characteristics and surface finish.

4.4.1 Ion beam sputter machining.

If a surface in vacuum is bombarded with energetic ions, atoms of the target surface are removed by momentum transfer. The sputtering rate is proportional to the ion current density and the sputtering yield, defined as the ratio of the number of sputtered atoms to incident ions. Typical values for the ion energy are 300 ... 500 eV. The usual source of ions is argon, because of its yield of 0.1 to 10 atoms/incident ion. This depends on the ratio of the masses of ions and target atoms C5ll. The yield depends on the incident angle of the ion beam, e.g. the sputtering depth for SiOz with Ar ions becomes maximum at 60' 1491.

a) Plasma etching parallel-plate reactor (fig.40)

The reactor consists of two opposite electrodes .The wafers to be etched are positioned on the lower electrode. Argon gas is fed to the previously evacuated (..lo Pa..) recipient. The application of a high frequency (..lo MHz..) electric potential causes the gas to ignite. The basic requirement for etching is the formation of a volatile reaction product that can be removed with a vacuum pump. Due to the directed motion of the ions, the masked wafers can be anisotropically etched.

fig.39: Variable Shaped Electron Beam System blockdiagram r481 .

478

I , Workpiece (Insulator) Capacitor

HF oscillator (13.5 MHz)

fig.40: High Frequency Plasma Type Ion Beam Sputter Machining Equipment [IS].

b) Kaufman type ion shower (fig.41)

In the discharge chamber (..lPa..), argon gas is ionized by the bombardment of electrons emitted from a heated filament cathode. A solenoid coil provides a magnetic field for the confinement of the electrons. The argon ions are extracted into the machining chamber by a grid set at the front of the source. The practical limitation of both mentioned types is the heat input from the ion beam that the masking will tollerate. For applications with photoresist masking, a beam current density of 0.70 mA/cm and an energy of 600 eV would be typical. Under these conditions, etching rates of common semi-conductor materials (0.9. polysilicon, Si02. Si3N4) would be in the range of 0.5 to 0.7 nm/s [521.

4.4.2 Reactive beam etching

In reactive ion beam etching (RIBE), a flux of reactive particles is directed at the specimen, instead of a flux of inert argon ions. Typically, the source is fed with a gas such as CF4. CH4 or 02. resylting+in a fange of ionised particles such as CF4 , CF3 and F . For effective reactive machining, the selected free radical will not react with the mask material, but with the target surface to form products which are either volatile or easily machined by the kinetic energy of the ions. RIBE is successfully used in various semiconductor manufacturing processes. In etching a Si02 pattern with a polysilicon mask for example, the reactive process will increase the etch rate from .7 nm/s to 1.5 nm/s or higher. The ratio of the etch rate of the patterned layer to the etch rate of the substrate material is called the selectivity and reaches values larger than 10 [52].

A"& Cathode(hnted filament)

fig.41: Kaufman Type Ion Shower (451.

4.4.3 Other Ion Beam Processing Techniques

Ion Beam Implantation Process (IBIP) is a technique using a high energy ion beam of several hundred keV for the implantation of doping elements into a semiconductor wafer (451. Highly boron doped layers for example can be used as an etch stop layer during the fabrication of silicon membranes, springs, and masks for lithography. A layer formed by deposition of atoms, sputtered with Ion Beam Sputter Deposition ( I B S D ) , is attacnea firmly in the target surface. This process is used in micro-electronic element manufacturing.

4.5 Plasma beam machining.

A plasma is an electrical conductive gas state: it can be obtained by an electric discharge between an anode and cathode yieldingo an ionised gas at high temperatures up to 20.000 C and more. A typical plasma torch is constructed (fig.42) in such.a way that the plasma is constricted in a narrow column with a diameter of the order of 1 mu. Plasma Arc Beam (fig.42a) and Plasma Jet Beam (fig.42b) are applied respectively for electrical conductive and non-conductive materials. The parameters are the gas flow rate, the nozzle diameter and the Dower sunnlied to the arc (current

The gases used are 5-1000 A, voltage - 100-250 iri Argon. Helium or Nitrogen.

fig.42: Plasma Beem Sources (451. (a) plasma arc beam (b) plasma jet beam

The main welding area are:

applications of the plasma beam outside the

4.5.1. Plasma Beam Cutting. An increased interest of the contour beveling of thick Dlates usina nlasma is observed. Weld preparakion bevels on steel and aluminium plate can be cut on contoured shapes at high rates. For example, a 25 mm thick stainless steel is Cut at 1300 mm/min using 575 A arc current. Additionally, the edqe sauareness of modern plasma systems is quite gwd. (+/- 0.5 mm for 32 mm thick Al-plate, using a water-injected plasma torch, using similar work conditions) 1531.

4.5.2. Plasma Beam Coating. The plasma beam is used for the high quality coating of workpieces with metallic or ceramic materials, having high melting point temperatures (>ZOO0 C). The layers have a thickness of 0.1 nun and grant the workpiece a higher corrosion, wear and/or temperature resistance C541.

4.5.3. Plasma beam supported machining. As shown in fig.43, the material is heated up locally by a plasma beam before it is removed. When machining highly alloyed steel materials. this method may yield a considerable increase of tool life time ( > 400 % ) and of material removal rate ( > 60 % ) . [ 5 5 1

fig.43: Set-up for the Plasma Beam Supported Turning f 551

479

Acknowledgement: The author wishes to thank following ColleagueS, whose information was on the basis of this text :

Dr. D.F. Dauw. Charmilles Technologies, (Switzerland) Prof. G. Declerck, IMEC, K.U.Leuven (Belgium) Dr. M. Degrauws, CSEM (Switzerland) Ir. C. de Regt, Philips (Netherlands) Dr. K.G. Guenther, Siemens (FRG) Dr. K. Iliev, INRA (Bulgaria) Prof. w. Koenig. TH Aachen (FRG) Prof. J.A. HcGemugh, University of Aberdeen (U.K.) Prof. T. Masuzawa , University of Tokio (Japan) Prof. A. Moisan. Ensam (France) Prof. em. G. Pahlitzsch, TH Braunschweig (FRG) Dr. B.M. Schumacher, Agie (Switzerland) Dr. N. Taniguchi, University of Tokio (Japan) Prof. H.R. Toenshoff, University of Hannover (FRG) Mr. Tomita T., Honda RLD (Japan) Mr. T. Watanabe, Okuma (Japan) Ing. J. Wijers, Philips (Netherlands)

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