Energy scaling transitions in machining of silicon by diamond

Tribology

UTTERWORTH EINEMANN

International

Vol. 28, No. 6, pp. 349-355, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All riehts reserved 0301-679X/95/$10.00 +O.OO

0301-679X(95)00019-4

ergy scaling transitions machining of silicon by K. E. Puttick”,

L. C. Whitmore”,

P. Zhdan”,

in

A. E. Gee+ and C. L. Chaos

The existence of an energy scaling brittle-ductile transition in the machining of ceramics and glasses is now well established. We have examined the surface layers of silicon crystals machined in the ductile regime by two methods: (i) a dedicated highly stiff single point diamond turning facility (SPDT), using cut depths of the order of 100 nm (ii) high precision cup grinding using a nominal cut depth of 500 nm. Surfaces were profiled by AFM and subsurface damage was characterized by Rutherford ion backscattering and cross section TEM. The main results to date are: [a) The SPDT specimens possessed a surface quality corresponding to that achieved by optical polishing, the R, = 0.6 nm and Rmax (P-V) = 6 nm. (b) The R, values of the ground specimens ranged between 7 nm and 20 nm, with 64 < R,,, < 148 nm. There were significant differences between wheels bonded by resin and by cast iron. (c) The mean depths of surface damage were in the range 200-400 nm in all cases. (d) In the SPDT crystals the subsurface damage consisted of dislocation loops intersecting the surface. In any one region of the specimen these represented slip on a single close packed system. (e) In ground slices the permanent damage comprised sub-micron length cracks as well as a high density of dislocations on various systems. Local regions of amorphous silicon were also present. (f) In SPDT ductile machining diamond tool wear is effectively homogeneous and much less variable than in normal machining. Keywords:

energy

scaling

transitions,

silicon,

diamond

Introduction Griffith’s1 criterion for the propagation of brittle fracture involves the transfer of energy from a volume to a surface. One important consequence is a special type of scaling law, since if any given situation is expanded or contracted in a geometrically similar way the energy sources and energy sinks scale at different

* Physks Department, University of Surrey, Guildford, Surrey GlJ2 5XH, UK ? School of Industrial and Manufacturing Science, Cranfield University, Crm~eld, Bedford MK43 OAL, UK * Chung-Chen Institute of Technology, Taiwan

Tribology

International

machining

rates. The choice of appropriate scaling length depends on the particular problem: it is usual to take the length of the crack itself, but in many situations of practical importance it is more convenient to consider instead the size of the strain energy field2,3. For example, in plastic-elastic indentation of brittle materials the length scale is determined by the dimension of the indentation itself4, while in the cognate problem of machining the usual convention is to consider the depth of cut as the relevant length scale. This energy scaling approach predicts a threshold size, d, of the form ER

d=cr-

i 6

Volume

(1)

1

28 Number

6 September

1995

349

Energy

scaling

transitions:

K.E. Puttick

et al.

where E denotes Young’s modulus, R surface work of fracture and Us the yield stress of the material in uniaxial compression. The factor in brackets is thus a length characteristic of the material, which it is proposed to call the Gurney length in honour of Charles Gurney who first recognized its importance5,6. (Y is a numerical constant for a given experimental configuration3. Thus energy scaling of the machining process leads to a critical depth of cut, given by Equation (l), below which no cracks should accompany the removal of material. This therefore defines a brittleductile transition for machining of brittle materials such as glasses and semiconductors.

recent publications (e.g. Wattsll). The topography of the machined surfaces, previously surveyed by conventional prolifometry12 was examined in detail by atomic force microscopy using a Nanoscope II (Digital Instruments) device with a 50-60” conical silicon nitride stylus of 15-20 nm radius tip. The subsurface damage was characterized by two methods: (i) Rutherford backscattering (RBS) of a 1.5 MeV ion beam in the SERC Beam Facility at the University of Surrey.

For such solids the Gurney length is about 1O-7 m, while cr in the case of machining is of order 10 (Puttick et uZ.~), so that within the limits of accuracy of present data the critical cut depth should be roughly 1 pm. For the purpose of testing this hypothesis we developed in collaboration with the National Physical Laboratory a highly stiff machine for single point diamond tuning8,9 in which the specimen was situated on the end of the spindle of a vertically mounted air bearing and turned by a single diamond tool controlled by a piezoelectric actuator to give downfeed to an accuracy of some 10 nm. Later the machine was modified by Gee et al.1° by the addition of an interferometer to monitor workpiece surface orientation and a brushless DC spindle drive for high speed operation. With this facility we were able to verify the validity of Equation (1) as a criterion for ductile regime machining of optical glass; more recently we have investigated the machining of single crystal silicon. Silicon is naturally of intrinsic importance as an industrial material, and considerable effort has been devoted to precise machining of the surface of substrates for device technology. It is also for our purpose an incomparable model material, being the purest, most perfect and most highly characterized of all crystals.

51

after

diamond

e~_~ti:ny

i 1-E-J)

RFt-1

Experimental We have studied silicon single crystal surfaces nanomachined in the ostensibly ductile regime in two ways. The first method employed the Surrey-NPL single point turning facility already described, using dry turning at speeds of about 3000 rpm (a linear velocity of -3 m s-l in our case) and a depth of cut of about 120 nm. In the second set of experiments silicon wafers were ground on the seven-axis grinding machine made by Cranfield Precision Engineering Ltd (CPE) at Cranfield University. Here the workpiece was machined by a cup grinding wheel at speeds of 3280 rpm in deionized water as a coolant. Wheels consisted of 3-6 pm diamond abrasive bonded either by resin or by cast iron, the former being dressed mechanically and the latter by electrolytic in-process dressing (ELID). The nominal depth of cut during grinding was 0.5 pm. In addition, the chemical composition of the surfaces was investigated by the method of X-ray photoelectron spectroscopy (XPS) with the kind collaboration of Dr J. F. Watts. This technique, involving an energy dispersive analaysis of photoelectrons liberated from a solid surface by X-rays, is fully described in many 350

Tribology

International

Volume

28 Number

SI

after

l;utt,Iny

Cl-E-7’1

AFI.1

Fig 1 (a) AFM profile of silicon ground with metal bonded wheel; (b) AFM profile of silicon ground with resin bonded wheel: (c) AFM profile of single point diamond turned silicon 6 September

1995

Energy Table ll Roughness Single

R, Rmax

of nanomachined

point

silicon

(nm)

(ii)

scaling

transitions:

K.E. Puttick

et al.

Transmission electron microscopy (TEM) of thin foil cross sections of the machined surfaces13.

Ground Resin bond

Metal bond

7.4 64

21 148

0.6 5.8

Results

Atomic force microscope surface profiles are shown in Fig 1. Figure l(a) represents a surface ground by a wheel bearing diamond abrasive bonded by cast iron, dressed by electrolytic in-process dressing (ELID). Figure l(b) is a scan of a surface ground by a wheel of similar geometry with diamond embedded in resin and mechanically dressed. Figure l(c) is a profile on the same scale of a silicon surface machined by single point diamond turning. Analysis of these figures is summarized in Table 1 in terms of the mean surface roughness (R,) and the maximum peak-tovaliey dimensions (R,,,). lFigure 2 is a photomicrograph of a ground surface displaying the characteristic spacing of marked grooves some 3.6 iurn apart, to be compared with Fig 3(a) of a turned surface observed by scanning electron microscopy. The state of the removed material on t surface is noteworthy. Figure 3(b) emphasizes the regularity of the machining marks with a separation of about 70 nm.

Fig 2 Optical micrograph with metal bonded wheel

of silicon

surface

ground

Fig 3 (a) Silicon crystal surface turned with roof-edge material; (b) Detail of turned grooves in (a) Tribology

It is clear that the single point turning has achieved fully ductile removal of material by plastic flow alone. Concomitant with this transition is a change in

diamond

International

tool, showing Volume

machining

28 Number

grooves

6 September

and removed 1995

351

Energy

scaling

transitions:

K.E. Puttick

et al.

tribological wear of the diamond tool which is of some technological importance. Whereas in the case of ‘normal’ machining of hard, brittle solids diamonds show irregular wear governed by local conchoidal fracture, the sharp edge tool used in our nanomachining experiments exhibited a well defined wear facet (Fig 4(a)) with a surface finish approaching that of polishing 0% 4(b)). In the technique of channelled Rutherford backscattering a collimated ion beam is directed at the sample along a low index crystal lattice direction, so that for a perfect crystal few ions are scattered. Crystal defects dechannel the beam and increase the scattering probability, with ions scattered from greater depths losing correspondingly more energy (e.g. Chu et al. 14). RBS spectra for the initial polished and for the turned silicon surface are compared in Fig 5, in which can be seen the low level of scattering from the starting material and the wide peak of intense scattering correspondmg to subsurface damage in the turned material. The horizontal axis in this figure represents a depth scale from right to left, so that the width of this peak represents a mean depth of damage of about 180 nm*. This of course represents an average value over the width of the ion beam of about 1 mm.

substrate

100

150

200

Channel

250

300

Number

Fig 5 Rutherford backscattering spectra (1.5 MeV He+ ions) from original polished and from turned surfaces of silicon disc

Similar mean values for the depth of permanent subsurface damage were obtained for the nanoground specimens. However, the nature of this damage is very different in turned and ground material, as is illustrated in Figs 6 to 8, showing cross-sections of the surface layers viewed by TEM. Figure 6 is a section of a succession of the deep grooves detected in Fig 2: they appear to be 100-200 nm deep, below which there is a region of intense lattice disorder in the form of a dense array of dislocations on a number of different slip systems. Closer inspection of these regions reveals in addition the presence of microcracks, of which an example is seen in Fig 7 displaying moire fringes formed by the misorientation of the lattices on

Fig 4 Worn facet on sharp edged diamond turning tool; (b) detail of (a) * It will be noticed that in Fig 5 the backscattered ion yield from the apparently undamaged region below the dislocated layer is much higher than that from the virgin material. The detailed interpretation of such depth spectra from an ion beam suffering both dechannelling and scattering is still a matter of some controversy. It is currently the subject of research by Dr C. Jeynes (to whom I am indebted for

352

Tribology

International

Fig 6 TEM cross-section of ground silicon crystal, showing spacing of deep grooves this comment) and Dr K. Gaertner of the University of Jena: a preliminary model suggests that backscattering from the region between 100 nm and 500 nm may be in part due to the presence of point defects.

Volume 28 Number 6 September

1995

Energy

Fig 7 Crack sectiofi )

beneath ground

surface of silicon showing

moire fringes.

either side of the crack. The groove itself is filled with a solid which does not exhibit diffraction contrast. The subsurface damage in nanoturned silicon possesses a remarkably simple structure exemplified in Fig 8, again a cross-section of a machined surface taken perper.dicular to the machining direction. The deformatior, consists of groups of dislocations spaced 50-100 nm apart., and detailed study of such features leads to the car-1c1us;onthat they consist of bundles of elongated loops imerriecting the surface, all on the same slip system with a single Burgers vector. We ficd that on the nanoturned crystals there is also a layer: of material providing no electron diffraction contrast, in these cases with a thickness of about 200 nar . This seemed to us to be of potential relevance

Fig 8 Cross-section of turned silicon crystal, dislocations viewed in multibeam condition Tribology

showing

International

scaling

Amorphous

transitions:

K.E. Puttick

material in groove (TEM

et a/.

cross-

to the machining process, and we analysed it with the aid of local electron diffraction and microprobe analysis, the results of which examination are summarized in Fig 9. The diffraction pattern of Fig 9(a) consists of diffuse rings typical of amorphous material, while Fig 9(b) shows that the microprobe spectrum is predominantly of silicon alone, with very small peaks of oxygen and carbon. We conclude that the structureless material is in fact mainly amorphous silicon. Nevertheless, this does not exclude the possibility of a thin superficial film of oxide, and we therefore used the technique of X-ray photoelectron spectroscopy (XPS) to analyse the first few nanometres of the samples. The relevant spectra from as-received silicon wafers and from nanoturned silicon are compared in Fig 10. Figure 10(a) from the virgin wafer reveals the presence of a significant amount of oxygen in the form of Si02: application of the Beer-Lambert lawll suggests that this is some 0.5 nm thick, (1-2 monolayers). The spectrum of Fig lO( b) from the nanoturned material is found on similar analysis to correspond to an oxide layer 1.5 nm deep, together with a significant amount of carbon transferred from the diamond tool. Discussion It is clear from the TEM observations that the single point diamond turned surfaces have indeed been machined by purely plastic removal of material, although the geometry of the deformation appears to be markedly different from that of recent models based on molecular dynamic simulation (e.g. Voter and Kress15). The bunching of dislocation loop generation is almost certainly related to the cross-feed motion of the tool as it generates a spiral cut, and is therefore presumably a function of machine design. The depth of damage must be influenced by the frictionally generated temperature field in which dislocation glide Volume

28 Number

6 September

1995

353

Energy

scaling

transitions:

K.E. Puttick

et al.

b)

Fig 9 (a) Electron material

diffraction

pattern from

amorphous

region;

(b) electron micropbe

analysis of amorphous

occurs: this is not yet realistically modelled computer simulations of ~a~omach~~ing~

I 0

100

200

300

400

500

blndlng

s,l,con

Single

wafer

point

as

diamond

received

600

700

600

-900

1000

energyiev

(0011

blnding

energy

IeV

turned

Sillcon

wafer

Fig 10 (a) X-ray photoelectron spectrum (XPS) of as received silicon crystal; (b) XPS analysis of single point diamond turned silicon surface 354

Tribology

International

Volume

28 Number

by current

While the cut depth in the turning experiments must be well within the limit prescribed by Equation (I), it has manifestly just exceeded the critical value during grinding. The nominal depth of 0.5 pm recorded by the machine agrees (perhaps fortuitously) with the experimental level of 0.5-0.8 pm observed by Chao16 during experiments on silicon with a ruling machine. It is important to note that the sub-surface cracks initiated during grinding do not usually intersect the surface, which accordingly exhibits the appearance of ductile machining. It follows that claims of ductile grinding based on surface appearances alone are invalid: we think that this is the first unambiguous evidence of the reality of a brittle-ductile transition in continuous machining. The amorphous layers detected in these experiments are of intrinsic interest in two respects. First, they establish beyond reasonable doubt the reality of the Beilby layer, at least in these circumstances, and provide quantitative information on its extent. Second, they indicate a new method of generating amorphous silicon, a material of considerable technological interest in the electronic field”. Further characterization of this surface phase is of considerable importance also for the manufacture of optical components, since surfaces produced by mechanical means must now be seen to possess a barrier layer of different refractive index from that of the substrate. 6 September

1995

Energy

6. Gurney, C., Mai, A340, 213

Acknowledgements We wish to thank Dr J. F. Watts and Mr S. Greavns of the Department of Materials Science and Engineering, University of Surrey for Figs 10(a) and (b), and Mr P. Shore of Cranfield University for the grount.. samples. We are also grateful to the Editor of Semiconductor Science and Technology and Dr C. Jeynes for permission to reproduce Fig 5 and to the Editor of The Philosphical Magazine for permission to reproduce Figs 2,3 and 6-9. Finally, we acknowledge particularly the help of Mr M. R. Rudman in many of the experiments described here and of Mrs S. Rudman in the production of this paper.

A.A.

Phil.

Trans. Phys.

2. Roeder

F.C.

Proc.

3. Puttick

K.E.

J. Phys.

4. Puttick K.E. 13, 375 5. Gur:tey

transitions:

Y.-W.

and Owen,

7. Puttick K.E., Shahid M.A. Appl. Phys. 1979, 12, 195

Sot.

D: Appl.

and Hnsseini

C. and Hunt

Roy.

M.M.

J. Proc.

Sot.

1920,

1956,

B69,

Phys.

Sot.

163.

9. Puttick K.E., Rudman Roy. Sot. 1989, A426, 10. Gee A.E., 1015, 74

McCandBsh

13, 2249

D: Appl. 1967,

Phys.

A299,

Tribology

1980,

International

R.C.

Proc.

Hosseini

Smith

S. and Puttick to Electron

M.M.

K.J. K.J.

Roy.

J. Phys.

D:

Report

to

Annual and Franks

K.E.

Proc.

Spectroscopy

Chao

C.L.

and

Mayer J.W. and Nicolet M.A. Academic Press, New York, 1978

1.5. Voter

A.F.

and Kress

16. Chao

C.L.

PhD

Volume

L.C.,

J.D.

Thesis,

A.S.P.E.

Cranfield

28 Number

SPIE

1988

University and Chao Phil.

Backscattering

Spring

Meeting, 1991

6 September

A. Proc.

Gee A.E.

University,

H. in Properties of Amorphous IEE, London, 1989

et al.

Sot. 1974,

12. Puttick K.E., Jeynes C., Rudman M.R., Gee A.E. C.L. Semicond. Sci. Technol. 1992, 7, 255

17. Fritzsche, INSPEC,

508

K.E. Puttick

Smith

M.R., 19

11. Watts J.F. An introduction Press, Oxford, 1990

14. Chu W.K., Spectrometry

981

1980,

J. Phys. Roy.

A221,

and

8. Puttick K.E., Rudman M.R., NPL (RA 143/013) 1984

13. Puttick K.E., Whitmore Mag. I994, 69, 91

References 1. GritSth

scaling

Silicon,

1995

1993

2nd

ed.,

355