Photonics at Work

Laser Machining

Fine Dialllonds With Laser Machining

UV tools 1/lake quick work of the lvorld:~ hardest Inaterial.

CQ)f all matel1als. diamond . is harder than any

known solid. while also exhibiting the highest values for elas~ tic modulus. atomic denSity. Debye temperature and thennal conduc­tivity at room temperature. Diamond is chemically inert. and Is highly transparent throughout a broad range of the optical spectrum. It Is a wide bandgap semiconductor that may be useful at high temperatures or high voltages.

These properties. along with the ease of growth of diamond Jl1ms by chemical vapor deposition. have made the material desirable for potential applications such as heat spreaders. optical windows. x-ray lithography masks. low-friction and wear-resis­tant sudaces. cutting-tool coatings and active electronic devices. I

Because of the hardness of dia ­mond-like materials. mechanical grinding. polishJng and cutting is lime-consuming and costly. Fabri­cation and shaping problems are compounded by the fact that many potential applications require a high degree of precision and spatial res­oluUon. The availability of simple and affordable tecbnJques for shap­ing and patterning on spatial scales in the micron to millimeter range is expected to sJgnJficantly enhance the utility of diamond as a structural. electroniC and optical material.

The old ways Most current diamond-fabrication

technology has evolved from tech­n iques used in fabrication of gem­stones and diamond tools from nat­urally occu rring diamond. For the

by C. Paul Christensen

Figure 1. Production of pat­terns by scanning the sub­strate during laser ablation.

most part these technJques are based on diamond-on-diamond abrasion processes Involving saw . polishing wheels and similar tools.

However. lasers have also been used in the dJamond lndustry for T0Ugh sha:ping of Industrial diamond and removal of flaws from gems. 2

Laser etching of a diamond surface involves a two-step process: local­ized graphitization of the illuminated surface area, followed by sublima­tion and ablation of the optically ab­sorbing but thermally insulating graphite film.

F u ndamental and second-har­mornc wavelengths ofNd:YAG lasers are currently used for many dia­mond-cutting and -drilling appl1ca­

tions. However. it has been demon­strated that pulsed ultraviolet lasers offer a number of advantages for pre­cise etching of diamond surfaces.s,u

Fine sculpture In compadson to infrared and vis~

Ible wavelengths. ultraviolet radia­tion 1s much more strongly absorbed by many ctiamond-Uke substrates so that the graphJtization step in abla­tion can be achieved with relatively low laser energy and a well-defined Interaction volume. This allows more control over the etching process. pro­duces higher spatial resolution and prevents damage to subsurfac ma­terial. It also enables use of smaller lasers.

Reprinted from the November 1993 issue of PHOTONICS SPECTRA © Laurin Publishing Co., Inc.

Photonics at Work

Laser Machining

into a diamond substrate withou t an intennediate mask-generation step.

During the raster motion, the laser output is gated on and off to produce the desired geometry. U e of a CAD/CAM program as a graphical in­terlace allows long sequences of mo­tion commands associated with com­plex shapes to be generated with a minimal amount of programming.

Since the time needed to generate a pattern is proportional to the laser pulse repetition rate, most practical tools will incorporate a high-repeti­tion -rate laser.

Ablation depth is cGntrolled by vary­ing the X and Y step size between laser pulses and by adjusting the laser flu­ence. As s uccessive laser pulses are overlapped on the worksurface during

Figure 2. Laser micromachining configuration. scarmlng. the net exposure of any area become proportional to the spatially

Small ultraviolet lasers can shape lion. The typical 50-}JJ output of a averaged fluence of the focused beam. diamond surfaces with very high pre­ wavegUide KrF laser focused by an Consequen tly. pulse overlap can be cision and spatial resolution. These optic with numerical aperture of 0.5 used to reduce the effect of beam "hot optical tools combine the small laser will produce a focal spot of 15 to 20- spots- and reduce or eliminate the sources with high-resolUtion imaging }Jm diameter and corresponding flu­ need for beam homogenization. and precision motion-control systems ence of 10 to 20 J/cm2 • Appropriate The equipment configuration used for sculpting. planarlzing. marking and beam-delivery optics ca n produce for diamond micromachining at patterning of diamond and dlamond­ smaller spots of the same fluence by Potomac PhotOniCS is shown in Figure like materials. Applications for tools of imaging of the apertured beam. 2 . A waveguide KeF laser producing this type are pervasive, extending from 50- l-hl pulses of 80-ns duration at production of three-dimensional mi­ Computer control pulse repetition rates eJ!..i.ending to 2 croparts to sculpting likenesses ofAunt Raster or serpentine scanning of kHz i used as the optical source. The Martha·s grandchildren on a facet of the substrate under the focused bean1 laser emission is shaped and coIli­her wedding ring. is a simple technique that allows ar­ mated by beam-delivery optics and

The tluence required for ablation bitrary shapes to be drawn using a focused onto a worksurface with a of diamond materials varies widely CAD/CAM program and then etched UV-transmilUng microscope objec­with the purity of the material and tile wavelength of the laser source. In general. fluences between 0.1 and 10 J/cm2 are required for ablation at ultraviolet wavelengths. Single­pulse ablation depth in diamond is typically a few tens of nanometers. This suggests that precise depth con­trol should be poSSible 10 UV-laser­ablated microstructures.

Laser-energy requirements are a function of the area of laser focal spot on the worksurface. Beam divergence is also an important parameter that influences the minimum focal spot size and achievable "uence. On mi­cron spatial scales. only a few micro­joules of UV laser energy are needed to produce the Ouence levels required for ablation of diamond materials.

Consequently. small UV sources. suchas waveguide excimer lasers. are desirable for microstructure fabrica-

Computer and Stage Controller

Video Camera and Viewing Optics

X-Y-Z Stages

Figure 3. Gear structure formed in the surface of a polished type Ib dia­mond substrate by 248-nm laser ablation.

Photonics at Work

tive. The objective also images the in­teraction region onto a high-resolu­tion video camera to allow the opera­tor to closely monitor the machining process.

The workpiece is mounted on pre­cision X-V stages driven by DC mo­tors, and stage position is monitored by encoders. Computer control of stage velocity is carried out using CAD/CAM software, and laser puls­ing is synchronized to stage motion.

Almost any shape Figure 3 shows an example of a di­

amond gear structure produced by the raster scanning technique. The gear is 300 pm in diameter and is fabricated on the surface of a 0.3 X

1.5-mm chip of type Ib diamond . Depth of the relief around the gear is approximately 50 pm.

The structure was formed using a laser focal spot diameter of 10 pm and a fluence of 5J/cm2 • The gear shown in Figure 3 is not released from the substrate as a separate component. However, a released structure could be formed by simply using a thinner substrate.

CAD/CAM software and the direct­Write approach to laser machining allows almost any shape to be gen­erated in a diamond surface. For ex­ample, Figure 4 shows a butterfly formed in relief in a polished dia­mond surface by the same scanning approach used in Figure 3. In this case, the shape of the butterfly was derived from clip art. saved in a DXF format, and imported into the ma­chining program.

Since the single-pulse ablation depth of diamond is only a few tens of nanometers, it should be possible to produce smooth. sloping surfaces with continuous height variation by UV laser ablation. Repetitive raster scan­ning of the sample work surface is one means of producing such three­dimensional structures. This tech­nique forms the part by removal of material layer by layer. Production of smooth, continuous-relief surfaces by raster scanning requires optimization of scanning rates in both transverse dimensions. The opportunity exists to adjust the beam focus between

. scans, so that the technique is com­patible with tightly focused beams that exhibit limited depth of field.

Laser Machining

Quick work Figure 5 shows a pyramid~like

structure fabricated with a wave­gUide KrF laser beam of 20-pm di­ameter. The number of scans used to expose each side of the structure varies linearly from the shallowest portion near the center to the deep­est portion near the outer edge. The resulting surface exhibits a contin­uous slope and has submicron roughness. The thin graphite layer produced by the laser has been re­moved by heating the substrate in an oxidizing atmosphere.

The continuous surface relief shown in Figure 5 demonstrates the feasi­bility of true three-dimensional mi­crofabrication in 'diamond. Structures ofthis type woUld be nearly impossi­ble to form with conventional me­chanical tools. For UV laser optical tools. it's a few minutes' work. 0

Acknowledgements This investigation was carried out with support from the National Science Foundation under SBIR contract III­9261361. The author acknowledges co­workers L. Rodriguez and M.T. Duignan for their assjstance In equipment f1;lbri­

. cation and computer programmjng.

References 1. Jin. S .. et al. (April 1990). APPL PHYS.

LE1T. 60: 16: 1948-1950. 2 . Gresser. H.D. (1976). Laser sawing of

Figure 5. Top view of pyramid struc­ture formed in relief in type la dia­mond. The substrate is tilted 15 0 from the viewing axis and its cross-sec­tional shape is shown in the inset. Width of the ablated region is 400 11m.

. Sloping sidewalls were produced by overlapping of raster scan patterns.

illamonds. Techriical Paper MR76-855. Soc. of Manuf. Eng. .

3. Rothschild. Moo et al. (Jan. 1986). J. VAC. SCI. 1ECHNOL B4:1 :310-316.

4. Malshe. A.P . . et al. (June 1991). MATERIALS LE1T. II :5: 175-179.

5. Konov. V.I .. et al. (Oct. 1991). SOV. J. QUANT. ELEC1RON. 21: 10: 1112-11 15. .

Meet the author C. Paul Christensen is founder and pres­ident of Potomac Photonics Inc .. a man­ufacturer of UV lasers and laser mjcro­machining systems. He holds a doctor­ate in electrical engineering from the University of Califorrua at Berkeley and has 25 years' experience in laser systems and laser/material interactions.