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Invited Paper

PROCESSING OF DIAMOND BY LASER BEAM IRRADIATION

Masanori Yoshikawa a and Atsushi Hirata '

a Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of TechnologyNagatsuta-cho, Midori-ku, Yokohama 227, Japan

bDepartment of Control and Systems Engineering, Tokyo Institute of Technology

O-okayama, Meguro-ku, Tokyo 1 52, Japan

Abstract

YAG and ArF excirner laser beams. of which wavelengths are 1 .06 un and 193 nm respectively, have been applied toprocessing of a variety of diamonds. Cutting and smoothing of natural. CVD and sintered diamonds have been performed.C\TD diamond films were prepared by arc discharge plasma jet CVD and microwave plasma CVD. and sintered diamondscontain metallic or ceramic binder have been used. Fundamental removal processes of diamond with YAG and ArF excimerlaser have been investigated using natural single crystal and CVD diamonds in various atmospheres changing laserirradiation conditions such as average power. energy density and pulse repetition rates. Cutting of natural and CVDdiamonds with YAG laser proceeds at higher peak power that occurs at lower pulse repetition rates. Smooth surfaces areobtained by excirner laser irradiation at the incident angle of 800. In the cases ofthe processing with YAG laser. the effect oflocal heating by laser beam irradiation mainly assists the diamond processing. and diamond appears to be removed aftergrapl1ltization and oxidization following vaporization in the atmosphere contains oxygen. The temperature measurement wascarried out at backside of irradiation surface. and increase of temperature when YAG laser beam was irradiated was largerthan that when excimer laser was irradiated. On the contrary. the detection of C. C2, C'. O- and CO from the emission at theirradiation area with ArF excimer laser beam suggests that processing partly proceeds by the separation of carbon atoms fromthe surface of diamond after braking bonds between carbon atoms caused by laser beam. Cutting of sintered diamond withmetallic binder was difficult because metallic binder remains in the groove while ceramic binder was easily removed.Processing technique using laser beams has been applied to surface planing, chip preparation and edge formation of CVDdiamond and curved surface formation on sintered diamond. Surface planing was carried out by directing the YAG laserbeam parallel to the surface of diamond films and diamond chips were prepared by the combination of cutting and surfaceplaning techniques. A sharp edge was formed betwcen the surfaces which cut by laser beam and inechanicall\ polished.Round nose was formed by gradually rotating the siniered diamond following YAG laser beam irradiation.

Keywords: diamond, YAG laser. ArF excimer laser. CYD diamond. sintered diamond. cutting. smoothing. planing. edgeformation

1. Introduction

Since the successful synthesis of diamond by CVD. CVD diamond has attracted much attention for its high potentialtowards many applications such as heat management. optics, electronic devices and micro-mechanical parts. To producethese devices and parts, pretreatment of diamond films is required. Mechanical processing such as grinding and polishingusing diamond abrasive has been applied to process natural and sintered diamonds. However. this method consumes extremelong time. requires large load because of high hardness of diamond. and its efficiency is depends on the orientation of crystalface. With these drawbacks of mechanical processing. as CVD diamond films are thin and polvcrvstalline stRicture, theapplication of conventional mechanical processing is more difficult. Thus, new types of diamond processing have beenrequired and several types of processing method using laser beam', ion beam, reaction with hot metal3 and plasma4 have

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been proposed. Among these new method. the use of laser is believed to be one of promising method because it is a non-contact aiid remote process and has large potential to various processing such as cutting. smoothing'. planing andpattcrnlng°. This paper focuses on diamoiid processing using lAG and ArF excimer laser and describes its fundamentalniechaiiisni and applications to produce several contiguiit ions.

2. Mechanism of laser processing

To date. ariet\ of lasers emitting in a broad spectral range from UV to hR have been operated and applied to processing of\ aiious iiiaterials. Processing such as cutting and sniootliiiig of diamond results in the removal of carbon atoms that composediamond stnteture in all cases of each laser operated. Thus. the removal mechanism of carbon atoms from diamond surfacew lien laser beam is directed onto diamond surface is important issue for us to apply the laser processing effectively. Thissection focuses on the fundamental processes of removal of several types of diamond by Nd:YAG and ArF excimcr lasersoperated at I .00 ttni and 193 iini. respectivel

2.1 RcrnoaI ol diamond bs YAG laser beamNatural single enstal diamond and sintered dmaiiiond were used to investigate their removal process when YAG laser

beam \as directed. These diamonds were linished with a #4(0) diamond wheel to equalize the laser incidence conditions.Laser beaiii operated with a Nd:YAG laser installed w i0i Q switch as introduced into a vacuum chamber through a silicaglass and directed onto the surface of diamonds mounted on a N-I table.

Figure 1(a) shows the surface of natural diamond afier lAG laser beam at high peak power of 23 kW was directed at thelow pulse repetition rate of 1 kHz and scanned in air. A groove was formed around which black deposit was obsen'ed. Thegroove like this was not formed with the continuous wave laser beam at the average power of up to 12 W. This deposit can beeliminated by etching with mixed acid (HNO : H2SO = I 9. 120°C) as shown in Fig. 1(b). From this result and theanalysis w itli X-ray diffraction pattern. the deposits identified graphite. The depth of the groove depended on the atmospherewhere diamond was irradiated, and deeper grooves were obtained at higher oxygen concentrations.

Fig. I A groove formed on a natural single en stat dianiond. (a) after YAG laser beani irradiation and (b) after

following acid etching.

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(a) (b)

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\\ hen "i'AG laser was directed onto the surface of sinicred diamond, cobalt binder deposited and remained with graphiteinside the groo\ c as shown in Fig. 2(a). so that ii was difficult to cut sintered diamond. This cobalt adhered so tightly on thesurface of groove that could not removed ith nuxcd acid. On the other hand, laser cutting of [lie diamond sintered withceraiiiic can be vell—conducted as ell as that of natural and CVD diamonds because binder materials arc renio ed b laserirradiation unliLe the laser cutting of diamond sinicrcd v. itli nietal binder.

From above our experimental results, it is found that YAG laser processing of diamonds consists of three processes such ashe temperature rise at local area. (lie grapliitii.aiioii 01' diamond and the removal of graphite and other materials such as

cobalt binder. The graphitization of diamond occurs b the rapid and local heating of the diamond surface with the shortpulses ol' 'm AG laser emission. After graphitiz.ation ot'cliamond. graphite is easily removed b oxidization and vaporization inthe lornis of CO and CO: in the atmosphere contains O\\ geil. \Vlien graphite and oiher niaterials iii the groove are difficult toremove b laser irradiation, progress of the cutillig of diainoiid becomes slow and of little practical use.

2.2 Removal of diamond b ArF excimer laserFigure 3 shows the groove formed after direction of ArF excinier laser beaiii onto au arc discharge plasma jet CVD

diamond film. Like the processing using YAG laser. graphite is observed around the groove of which amount has decreasediii higher oxygen atmosphere. This indicates that the fundaiiiental removal process of diamond is the same in both laserprocessing. however removal of diamond progress even iii the ox\ gen—less atmosphere with less graphite on the irradiatedarea hen ArF excimer laser is used.

lii order to investigate the mechanism of ArF excmnier laser processing. temperature variations of diamond hen laserbeam is directed were measured. A single crystal HP/HT diamond of -[.5 mm 3.0 mm . 0.4 mm was used. and thetenlperatiire mncasurenient at the opposite side of irradiating position as carried out with a thermocouple when a laser beamat average po\er of 3.0W was applied for it) seconds The temperature rises ere 160°C amid 'ln°C when using YAG laserand ArF excimer laser. respcctivel . This suggest that the clierg\ of ArF excimer laser beam transferred to heat is less than

Fig. 2 Cross sections of grooves formed oii diamond sintered with (a) metallic and (b) ceramic binder.

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Fig. 3 Grooves formed on an arc discharge plasma jet CVD diamond using ArF excimer laser in the atmosphere of (a) air(b) oxygen and (c) argon.

that of 'iAG laser beam. Figure 4 shows the result of spcclroscpic analysis during laser processing in air. From the spectrapattern of ArF ecxuner laser processing. C. C:. C Co arc detected On the other hand, these elements were not detected inY AG laser processing. These results indicate that exciiiier laser processing mainly proceeds vitli vaporazation of carbonatoms alter cracking the C—C bonds among carbon atoiii IbIlow ing the reaction with oxygen.

(a) (b) (c)

200 (urn

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S

Laser irradiation

200 400 600 800

Wavelength non

Fig. 4 Spectral pattern of the emission from laser incidence point during ArF excirner laser beam irradiation

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3. Application of laser processing

3.1 Surface planing and chip prel)arationOne of the most basic methods for making the growth surface of CVDdiamond flat is the removal of surface Protrusions

sc1cctivel In order to obtain smooth surface b appI ing this imiethod using YAG laser processing technique. it is iieccssarvto scan the focus point parallel to the macroscopic surface of the diamond films with irradiating the laser beam parallel todiamond surface.

We have tried surface planing of dianiond films according to the sequence of following procedure: firstl, the focus pointof the laser beam directed parallel to the surface of a diamond film is set at 10 tim lower position from the top of thediamondfilm surface. secondly, the focus point is scanned along over the surface of the diamond film: thirdl. the focus point of laserbeani is moved at 30 im lower position from the lop of the new surface and scanned over whole surface of the diamond film:lastly, these procedure is repeated until protrusions arc completely removed. Figure 5 shows the series of diamond filmplaning b the YAG laser. The surface roughness of time diamond film planed by laser is 3 .wi (peak-to-valley). Figire 6shows the diamond chip cut and planed by laser. It took 7 minutes to process 3 mm 2 nun x 0.4 mm size and Figure 7shows the exaumple of the application of this chip for a diamond cutting tool.

Fig. 5 Series of surface planing of CVD diamond by YAG laser

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Fig. 6 A CVD diamond clup prepared by laser Fig. 7 CVD diamond cutting toolcutting and planing

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Surface planing of diamond films is also achieved vt ith ArF excimer laser. Like YAG laser processing. large incidentangle of tip to approximately 80° leads to low surface roughness value. SEM photos of irradiated surfaces are shown in Fig. 8.However. valle of diamond surface was renio ed smmiimiltaiieotmslv with the top of the surface unlike YAG laser planing.Surface roimm.liness of microwave CVD diamond lilnis has been reduced from 3 ni to 0.3 wu (peak—to—valley) by 500 lasershots at time energy density of 9.8 J/cm and repetition rate of 1 kHz. The final surface roughness seems to depend oii thegmaiii site in the case of excimer laser processing technique.

1:o 8 Surface planing of microwave CVD diamond t'ilm by ArF excimer laser. (a) as-deposited and (b) surface planed

3.2 Cun'ed surface formationYAG laser beam has been applied to form a cur ed smooth surface by the same plaiiing technique described before.

Smntered diamonds contain metallic or ceramic binder have been used. Generall the rake face consists of a curved surface atthe w mdtli of a few millimeters is formed b incchanicaIl grimiding when siiitered diamonds arc applied to cutting tools

A curved surface was formed by repeating the procedure that consists of the removal of diamond surface and the followingrotation of the diamoiid. Figure 9 shows the method for forming the curved surface at 1.5 mm nose radius with rotating thediamond at 5° steps. An example of the formation of a curved surface b this method is shown in Fig. 10. The curved surfacewas formed on a diamond smntered with ceramic binder of which dimensions are 3 mm 3mm 3mm and, its surfaceroughness was 3() tm (peak-to—valley). It was required 15 minutes for making a curved surface of 1 mm width.

Fig. 9 Method of fornumig curved surface on diamond by YAG laser

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Though complete curved surface cannot Formed h\ this method because the curved surhice coilsists of a series of flatsurliccs. the surface roughness of I .4 gin at the rotation angle of 5U geometricalh calculated is ignored b' the surfaceroughness result in laser processing.

3.3 Edge lormationAn edge is formed at a boundary between two planes A plane of CVD diamond films with smooth roughness is obtained

by several methods such as mechanical polishing. laser processing. ion beam sputtering and thermo—cliemical polishing.Among these iiiethods. mechanical polishing is nov the most practical method to obtain a plane with lower surfaceroughness. however, it is not suitable for making sharp edge on CVD diamond films because chipping occurs at the edgeowing to polvcrvstalhine structure of CVD diamond films. Thus. we have investigated to form a edge between a surfacemnechianicalI polished and a surface irradiated by laser bcaiim.

Diamond films were synthesized by arc discharge plasnia jet CVD of which dimensions were 100 gm thick and 15 gniroughness (peak—to—valley). Growth side of this CVD diamond film was mechanically polished before laser irradiation. YAGand ArF excinmer laser bcanis were directed perpendicularly onto the surface of CVD diamond films, and a edge formedbetween mechanical polished surface and a cross section produced by laser irradiation was observed by SEM. Figure 11shows the edges formed when the mechaimicall polished CVD diamond surface was placed at entn and exit side of ArFexcinier laser beam. It can be seen that a shaper edge occurs between the polished surface of beam exit side and the crosssection, The radius of this edge was 0.5 gum measured with electron probe surface roughness anahzer as shown in Fig. 12.Shaper edges were also occurred between beani exit polished side and a cross section when YAG laser beam was directed.The energ distribution of laser beam aiid the concentration of heat are believed to cause the round edge at the laser entryside

Fig. 1(1 Curved surface formed on sintered diamond by YAG laser

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Fig. I? Surface profile of the edge formed bcteen the polished surface at laser exit and the cross section.

127

(a) (b)

Fig. I I Edges formed between growth surhice polished and cross section irradiated by ArF excimer laser.

Edges at (a) laser entry side and (h) laser exit side

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4. Summary

Fundamental mechanism and applications of YAG and ArF excimer laser processing of various diamond materials havebeen described. Removal of diamond by laser irradiation has been investigated using natural. CVD and sintered diamonds.Diamond was effectively removed when YAG laser beam at high peak power that occurs at low pulse repetition was directedin the atmosphere contains rich oxygen in a sequence process such as local heating. graphitization. oxidazation and

vaporazation. SpectrOsopic analysis has suggested that ArF excimer laser processing mainly progresses by the separation ofcarbon atoms from the surface of diamond after braking bonds between carbon atoms. Removal techniques using laser beams

have been applied to cutting, surface planing, chip preparation and edge formation of CVD diamond and forming of curvedsurface of sintered. diamond. Sharp edge was also formed between the cross section occurred by cutting with laser beam andmechanically polished surface.

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