590 Surface and Coatings Technology; 54/55 (1992) 590-5t

Current Industrial Practices

Maskless laser etching of kerfs in ceramic materials for applications in magnetic recording heads slider fabrication

J a m a l K h a n * Digita! Eqldpment Corporation. Shrewsbllry, MA 01545 (USA)

Abstract

A maskless laser-etching process is described for defining kerfs in alumina films. Computer-controlled XeCI excimer (308 nm) and Q-switched neodymium-doped yttrium aluminum garnet (Nd : gAG) (1.064 gin) laser systems were used. Line patterns were geneJ'ated by scanning focused beam spots. The etching with the Nd:YAG laser was very efficient and fairly deep (up to 80 ~.tm) trenches were etched. The excimer laser system gave a cleaner etch, and kerr and part-offpatterns were generated without damaging the nearby pad structures. These maskless laser etching processes may have the potential to be developed as an alternative to the conventiom~l grinding process, for applications in the slider fabrication operations for magnetic recording thin film heads.

1. Introduction

Ceramic films and substrates are commonly used in the fabrication of magnetic recording heads where semi- conductor- type wafer processing coupled with mechan- ical slider fabrication operations are required [1]. Although planar heads have been demonstrated where mechanical slider fabrication operations are eliminated, this process is not commonly used [2].

In the conventional process, thin film pattern depos- itions are done on the relatively thick ceramic substrates. Wafers are transferred to the slider processing area after complet ion of all the required thin film processing steps, including deposition of alumina encapsulation layer. Generat ion of grooves (known as kerfs) in the combined alumina layers {about 45 gm thick) is usually done by a relatively gentle grinding process. These kerfs are used for guiding a gang of diamond wheels for a subsequent grinding operation for bar slicing from the thick (typi- cally about 0.4cm) ceramic substrate. These bars are further separated into individual slider units and air- bearing surface patterns are defined, usually by the mechanical processes of grinding and lapping.

The grinding of a relatively thick alumina film for defining kerfs may lead to chipping and cracking in the film because of its brittle nature and high compressive stress. Similarly, the mechanical processes may be unsuit- able for generating complex air-bearing surface patterns

*present address: 3 Boston Drive, Shrewsbury, MA 01545, USA.

on the slider material. Therefore i t is appa ren t that a alternative process is needed to c a r r y out these oper~ tions satisfactorily.

A maskless laser-etching process has been reporte for direct generation of patterns in a lumina films an ceramic substrates [3, 4]. In this process, compute l controlled laser systems are used. The da ta on last etching parameters, pattern d imens ions and locat ion eo are fed into the computer and the desi red structures ar directly etched in the workpiece, by a laser ablat io process. Applications of this a u t o m a t e d process in th fabrication of complex slider a i r -bear ing surface pattern has been demonstrated [4]. B o t h excimer and Q switched neodymium-doped y t t r i u m a luminum game (Nd:YAG) laser systems were used. We now repor t th results on defining kerfs in a l u m i n a encapsulat ion film by this maskless laser-etching process as an al ternat iv to the conventional grinding process .

2. Experimental set-up and results

Alumina films were sputter depos i ted on T i C - A I 2 0 : (30:70) ceramic wafers (about 0.4 c m in thickness). The surface was lapped off to leave a l u m i n a abou t 13 ~tn thick to act as the base layer for all substrates. Sample with pad structures were prepared by the convent iona thin film patterning processes o f sput ter ing ant electroplating through photoresist masks . Pat tern etch ing was tried with computer -cont ro l led Q-switchec

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J. Khan / Maskless laser etching q[kerf~ in ceramic materials 591

Nd : YAG and XeCI excimer pulsed-laser systems (emis- sion wavelengths of 1.064 pm and 308 nm respectively). Details of the maskless patterning process have been described previously i-3, 4]. A beam spot diameter of about 40 I.tm was obtained with the Nd : YAG laser. The excimer laser systen't, on the contrary, gives a vari- able rectangular spot size from 1 I.tm x 1 gm to 100 ~tm x 100 I.tm. Line patterns were generated by scan- ning the beam spots with the desired overlap. The sample is kept stationary in the Nd :YAG laser processing, moving the laser beam around at desired locations on the sample. This is in total contrast with the excimer laser set-up where the sample, placed on a movable stage, is scanned under a stationary beam spot.

Examination of etched structures was done using optical microscopy and scanning electron microscopy (SEM). Etch depth and surface topography were obtained with mechanical stylus profilometers.

Typical kerr patterns on a wafer are shown in Fig. 1. Only horizontal kerfs are usually defined by the grinding process to prepare the wafer for bar slice operation with diamond wheels. Vertical patterns, called part-off cuts, are made at the bar level directly with diamond wheels and all the way through the substrate.

(al

2.1. Etching with the neodymium-doped yttrium aluminum garnet laser system

The focused circular beam spot (about 40 pm in diame- ter) from a Q-switched Nd :YAG laser (1.64 nm emission wavelength) was scanned on a sample with sputtered alumina about 15 I-tin thick on a TiC-A1203 wafer. Figure 2 illustrates the results of etching a deep (about 80 gin) trench in a lumina and the ceramic wafer surface. Much powdery deposit is observed around the etch pattern. SEM examinat ion of the etched structure at a high magnification indicated that the side walls are quite jagged. When a t tempts were made to etch a line pattern

DEVICE BLOCK

S U B A r

pAWF-OFF

z

Fig. I. Schematic diagram of a kerf and part-off grid pattern in alumina fihn (typically about 45 gm in thickness) on a thick (about 0.4 cm) ceramic substrate after completion of wafer fabrication steps, Conventionally, only kerr patterns are defined on the wafer and part- offcuts are done later at the bar level and across the substrate material to separate individual sliders.

(b)

Fig. 2, SEM mierograph of a line pattern etched on a sample with the focused beam, about 40 ~m in diameter, from a Q-switched Nd:YAG laser (1.064 nm emission wavelength). The sample used had sputtered alumina approximately 15 .am thick on aTiC-AlzO 3 ceramic wafer approximately 0.4 em thick, (a) A deep (about 80 ;am) trench was etched through the wafer surface. (b) Note the jagged edges in the etch pattern at a high magnification together with the nearby powdery redeposits.

near metallic structures in a prototype sample, severe cracking and material delamination in tll e encapsulating alumina film was observed. Similar problems have been reported in the processing of alumina films to define slider air-bearing surface patterns 1-5"1.

It is apparent that the N d : Y A G laser system is unsuitable for defining kerr patterns in alumina films on real samples where a number of metallic structures such as alignment marks, lapping guides and pads are located near the kerr. However, the high throughput of the laser system may be used to etch deep trenches and ultimately in the machining of the harder ceramic substrate material which is easier to process without the complications of

592 J. Khan / Maskless laser etching of kerfs il, ceramic materials

cracking and chipping. The Nd : YAG laser system in its Q-switched mode may be operated at up to 3000 pulses s - t, resulting in a high materials ablation rate.

It should be noted that, although alumina has limited absorption in the N d : Y A G laser emission range (1.064 nm), the TiC-AlzO3 ceramic substrate material apparently enhances coupling between alumina surface and the laser radiation, making pattern etching possible [3, 4]. The substrate material itself has fairly high coupling with the Nd : YAG laser radiation and patterns can be etched quite efficiently [3-5].

2.2. Excimer laser etching

Vias and line pattern etching in alumina films and ceramic substrates have been successfully demonstrated with excimer laser systems [3]. This process was extended to explore its applications in defining kerfs using an XeCI laser (308 nm) at fluences of 10-20 J cm- 2. The samples were scanned under a rectangular beam spot. Figure 3 shows etching of kerf and part-off patterns in an alumina film about 15 txm thick on a prototype wafer. One of the kerf patterns is located close to the bonding pads. A magnified SEM view of the structure shows that alumina and part of the underlying ceramic surface are cleanly etched without causing any damage to the alumina film between the kerf and pad area (Fig. 4). Examination of the side walls and the etched structure at the kerr and part-off junction, at high magnifications, also showed fairly clean etching of the alumina film (Fig. 5).

These results indicate that XeC1 excimer laser etching shows good potential as a kerr and part-off etch process. This XeC1 laser etching is much smoother than that with the Nd : YAG laser system. The major drawback of

Fig. 3. Photograph showing etching of kerfs and part-off cuts on a prototype sample using a XeCI excimer laser (emission wavelength, 308 nm). The sample was scanned on a x-y stage under a rectangular beam spot.

Fig. 4. Magnified view of the XeCl-laser-etched kcff pattern near a bond pad. Note the clean etching of alumina film. No delamination or cracking of the alumina film between the pad and kerf structure was present.

the excimer systems, as was also no ted in the slider air- bearing surface etching applications I-4], is their rela- tively low pulse rate, typically a m a x i m u m of 100 Hz, which results in a low throughput.

3. Discussion and conclusions

The maskless laser-etching process descr ibed here may have applications in defining kerfs in thick a lumina fihns and in machining ceramic wafers. S m o o t h e r etching with the XeC1 excimer laser system makes it suitable for the kerf etch process where an adverse effect on the nearby device structures is to be avoided. The Q-switched Nd :YAG laser system, on the cont rary , m a y be more useful in machining operations where a high throughput is of utmost importance. It is conceivable tha t a combina- tion of these two laser systems may el iminate the need for the conventional grinding process in slider fabrication operation.

The excimer-laser-based kerf etch process may have several advantages over the convent iona l process of grinding. These include (a) reduced chipping and cracking, resulting in a high c o m p o n e n t yield, (b) accommodation of kerf design change wi thout the necessity of retooling, (c) elimination of the need for different tools for a product mix (with varying kerr dimensions), (d) automation and reproducibi l i ty of kerf location, thereby eliminating yield loss due to inaccurate placement of kerfs which is opera tor dependen t in the conventional grinding processes, (e) p lacement of kerfs close to the lapping guides, leading to reduced chipping problems in the laser process and reducing the lap time

J, Khan / Maskless laser etching of kerfs in ceramic materials 593

(a)

and (f) the ability to produce part-off cuts together with the kerfs simultaneously in the laser process.

These laser systems may also be used for other process- ing applications such as defining slider air-bearing sur- face patterns [2]. The need for expensive capital equipment for other processes may be thereby reduced. Wafers with kerr patterns in the thick alumina layer (which is electrically insulating) may also be used for electrical discharge machining (EDM) since the exposed slider material, TiC--A1203, is a conductor. This EDM process may be developed as an alternative to the grinding process for bar slicing from the ceramic sub- strate. Excimer laser etching is a convenient option for removing insulating films in order to expose the under- lying conductor surfaces in other EDM process applica- tions also.

Acknowledgments

I am grateful to Dr Hal Shukovsky and Dr Robert Rottmayer for their support for this work. Thanks are due to Agnes Johnson and Sileshi Woldemariam for technical assistance.

(b)

Fig. 5. Photograph showing (a) the junction of the kerr and part-off line patterns and (b) the side-wall profile of a structure etched with a XeCI excimer laser system. Note the clean etch even at these high magnifications. (Magnifications: (a) 500 x ; ( b ) I000 • .)

References

I J. J. H. Roche, Proc. Int. Syrup. on Microelectronics, 1984, 1984, p. 377.

2 J. P. Lazari and P. Dereux-Dauphin, IEEE Trans. Magn., 25 (1989) 3 t90.

3 J. Khan, in H. A. Atwater, F. A. Houle and D. H. Lowndes (eds.), Surface Chemistry and Beam-Solid Interactions, Materials Re.~earch Society Syrup. Proe., Vol. 201, Materials Research Society, Pitts- burgh, PA, 1991, p. 465.

4 J. Khan, in C, Ashby, J. H. Brannon and S. Pang (eds.), Photons and Low Energy Particles in Surface Processing, Materials Research Society. Syrup. Proc., Vol. 236, Materials Research Society, Pitts- burgh, PA, 1992, p. 33.

5 R. A. Strom, US Patent 4 785 161, 1988; US Patent 4802042, 1989.