Laser-LIGA for Serpentine Ni Microstructures

Hengyi Jin 1, Erol C. Harvey, Jason P. Hayes, Muralidhar K. Ghantasala, Andrew Dowling,Matthew Solomon, and Sam T. Davies#

Industrial Research Institute Swinburne, Swinburne University of Technology,533-545 Burwood Road, P.O.Box 218, Hawthorn, Melbourne, Victoria 3122, AUSTRALIA

# Centre for Nanotechnology and Microengineering,University of Warwick, COVENTRY CV4 7AL, UK

ABSTRACT

A pulsed excimer laser (248 nm) based LIGA-like process is presented for the fabrication of Ni serpentinemicrostructures, such as those that might be used for micro-heaters. The structures were made on both Cu (60 µm) cladPCB and on Cu/Ti (up to 4 µm/15 nm) sputtered Si (100) substrates. The substrates were laminated with Laminar AXdry film (35 µm) photopolymer, which was then patterned by laser ablation to produce the mould for Ni electroforming.

The optimal ablation conditions were identified for laser patterning to prepare the micro polymer mould. Beam fluence(~ 1J/cm2) and number of shots (~ 60 pulses) for 50 µm wide features on this photoresist were established, and it wasobserved that an increased number of shots and increased fluence was needed for features less than 20 µm.Additionally, the Cu layer surface was cleaned by the use of 5 -10 laser pulses at the same fluence.

Ni electroforming has been carried out using standard Ni sulfamate bath at a current density of ~ 10mA/cm2. After Nielectroforming, both the Laminar AX dry film and the Cu layers around the electroformed Ni patterns were removedusing a combination of acetone, laser and Cu selective etching. Finally, a series of Ni microstructures were fabricatedconsisting of up to 50 µm wide and 35 µm thick serpentine tracks. The devices were measured using a confocal laserscanning microscope and it was found that using the excimer laser to remove the remaining dry film laminate alsosmoothed the electroplated Ni surfaces from an pre-laser treated Ra of 1.20 µm to 0.19 µm. Laser ablation also releasedthe finest features from the substrate.

Keywords: Excimer laser, LIGA-like process, Laminar AX dry film, Cu seed layer, laser cleaning, Ni microelectroforming, and laser confocal microscopy

1. INTRODUCTION

Integration of heater elements in MEMS systems enables a whole range of sensors and actuators applications1. Someexamples include absolute-humidity measurement2, gas sensor3 and biomedical applications4. These devices are mainlybased on silicon, polysilicon or ceramics to create their structures, which incorporate some heating function5-6.Candidate metals for micrometers have been identified7, however most devices have been made using either surfacemicromachining (thin film) or its combination with silicon bulk micromachining. It is generally difficult to use theseprocesses to realise durable thick metal features.

Recently there has been growing interest in LIGA processes where the X-ray lithography step is replaced by a cheaper,lower resolution alternative such as UV photolithography ("poor man's LIGA") or laser photoablation8. This paperinvestigates the processes involved in fabricating a thick Ni microstructure, that could be used for a microheater usingexcimer laser ablation of dry film photoresist on a thin copper seed layer. Laminar AX dry film has been chosen as aphotoablation material due to its low cost, good uniformity for thick samples, ease of handling, and good adhesionproperties on smooth, cleaned Cu surfaces. Excimer ablation of polymers generally occurs at fluences below thethreshold for ablation of metals, enabling the patterning of the dry film laminate down to the conductive seed layerwithout damaging the conductive layer itself. In the excimer laser ablation process, the photoactivity of the resist is not 1 Contact details: E-mail : hjin@swin.edu.au ; Tel : +61-3-9214 4333; Fax : +61-3-9214 5050

used, and the material was handled in room lighting at all times. After Ni micro electroforming, the excimer laser wasagain employed to facilitate removal of dry film photoresist between fine Ni tracks. Although fluences were again usedthat were below the ablation threshold for Ni, we observed significant smoothing of the electroformed Ni as theremaining dry film resist was cleaned away. For the smaller line width Ni features we also found that the laser cleaningprocess released the fine Ni structure from the substrate, while the larger pads at the end of the serpentine Ni structureremained anchored.

2. EXPERIMENTAL

2.1 Samples preparation and excimer laser setup

The detailed sample preparation and excimer laser system have been described in previous work9. Two substrates wereused; 900 µm copper clad Printed Circuit Board (PCB) and 4 µm Cu on 15 nm Ti on (100) Si wafers. Each substratewas then coated using 35 µm thick Laminar AX dry film photopolymer (Dynachem Inc., USA). Lamination was carriedout using a Dynachem laminator. Micromachining was performed using an excimer laser projection system (ExitechSeries 8000) equipped with a Lambda Physik LPX210i laser source, which was operated at 248 nm (KrF) in pulsedmode. Here, the laser repetition rate was kept constant at 20 Hz. The projection lens used had a 1:10 demagnificationfactor, NA of 0.3, a 1.5 mm diameter field and an optical resolution of 0.8 µm. The set up of the homogeniser and theprojection lens produced a partial coherence factor of 0.2. The laser pulse output energy was set to 300 mJ, and theincident fluence onto sample surface was adjusted using an external, CNC controlled attenuator. The fluence at theworkpiece was calibrated using a pyroelectric energy monitor (Molectron Type JD25). The CNC (ComputerNumerically Controlled) workpiece stage, incorporating a flat vacuum chuck, was able to move in the orthogonal X, Yand Z axes, and able to be rotated in a plane perpendicular to the vertical Z-axis. Likewise, the open frame mask stagecould be translated in the horizontal X and Y directions enabling one or more mask patterns to be brought, sequentially,into the static beam projection system.

2.2 Laser patterning

A chrome-on-quartz mask was prepared in advance. The mask included two circular patterns, to be used for theelectrical contact pads, and three serpentine shape patterns to be selected to create a heating element. Each of the threeserpentine sets included the left lead-in part, the right lead-out part, and a central repeating block of serpentines. Thisarrangement allows the designer to readily change the total length of the serpentine track between the pads simply byrewriting the laser CNC program to repeat the central block of serpentines without the need to use a new mask. Thismeans that the maximum width of the serpentine structure is limited to the maximum translation distance of theworkpiece stages, 200 mm in our case. When imaged onto the workpiece using the 1:10 lens the mask produced contactpads 900 µm in diameter, and serpentine tracks that were 50 µm, 20 µm or 10 µm wide depending upon the mask setused.

To prepare the polymer moulds for micro structures, the laser fluence and the number-of-laser-pulses needed to bedetermined. This was simply done using a CNC program that arrayed complete structures (2 pads + serpentines) on thesubstrate in a 2D array. The number of laser pulses was varied along one axis of the array, and the laser fluence variedalong the second axis. In this way both shot number and fluence were varied, while all other experimental and substratevariables remained unchanged. The criteria to identify the proper laser parameters were: (1) the laser ablation justpenetrated the laminated dry film without noticeable seed layer damage as viewed using an optical microscope; (2) theablated wall angle was sufficient to resolve the finest features. Criteria (1) drives one to use the lowest possible fluence,while criteria (2) forces one to use the highest possible fluence.10 After the optimal laser conditions were determined,laser patterning was carried out to produce the polymer mould for the whole micro structure pattern.

2.3 Ni micro electroforming

The above polymer moulds were used to electroform Nickel microstructures using a standard Nickel sulfamate bath at acurrent density of ~ 10 mA/cm2. The bath was maintained at 47oC and ~ pH 3, and agitated by stirring within thebeaker. The total Ni thickness was controlled by timing the electrodeposition, having established that the average Nigrowth rate was about 10 µm/hr in this bath.

2.4 Dry film removal and Cu selective etching

Acetone, 10% NaOH and excimer laser ablation were each used separately, or sequentially, to remove the dry filmphotoresist after Ni micro electroforming. The dry film photoresist had been exposed to a varying extent since nospecial lighting precautions were used when handling and ablating the layer. Thus the usual NaOH chemical removaltechnique was a less reliable stripping process than it would normally have been.

The Cu seed layer should also be removed since it short circuits the electroformed structure if allowed to remain. Toselectively remove Cu in the presence of fine Ni features, a controlled Cu etching was achieved after dry film removal.The etchant contained ammonium persulfate (2 wt %) and sulphuric acid (5 %). Using this etchant, the Cu layer overmost of the exposed area was removed, and the Ni serpentine features were released. The copper seed layer under thebigger circular Nickel contact pads was only partially etched, enabling the electrode pads to remain attached to thesubstrate.

The resulting micro electroformed structures were observed using an optical microscope (Olympus BX 60), a ScanningElectron microscope (Joel JSM – 35), and for 3D measurement and surface analysis, a confocal laser scanningmicroscope (Olympus OLS 1100).

3. RESULTS AND DISCUSSION

The 248 nm excimer laser ablation characteristic has been presented in previous work9, and is typically between 0.5 µmand 1.2 µm per pulse at fluences between 0.5 and 2.0 J/cm2 for features that are 400 µm x 400 µm using the opticalsystem described. Also, as reported previously10, the ablated wall angle varied with incident fluence, being closer toperpendicular (0º) at higher fluences. It follows therefore that for a given thickness of laminate, higher fluences arerequired to ablate higher aspect ratio moulds. However, since the thin, underlying seed layer must also survive the laserablation, a trade-off has to be made.

The optimal laser parameters and resultant polymer moulds have been summarised in Table 1:

Laser parameters Polymer mouldsTypical polymer moulds achieved onLaminar AX dry film Fluence (J/cm2) Number of pulses Aspect ratio Side-wall angle

900 µm circular shape (contact pads) 1.5 40 + 5 <0.039 <9°50 µm wide serpentine shape tracks 1.5 40 + 10 0.7 9-10°20 µm wide serpentine shape tracks 1.2 90 + 10 1.75 12°10 µm wide serpentine shape tracks 1.2 140 + 10 3.5 13°

Table 1: Optimal excimer ablation conditions. Number of pulses are for laminate ablation + additional Cu seed layer cleaning pulses.

Electrodeposition was performed on the completed ablated structures. Three example structures are shown in Figure 1– Figure 3 below.

Fig.1: Microheater with 50 µm wide Ni track Fig.2: Microheater with 10 µm wide Ni track

Figure 3b is an enlarged image of the 20 µm wide serpentine shaped Ni feature, as seen in the middle of Figure 3a.Each of the structures demonstrate good uniformity of electro deposition, indicating good exposure, and minimaldamage, of the electroplating seed layer by the laser ablation process.

Figures 4 and Figure 5 show the typical results of dry film removal after Ni electroplating. Figure 5 represents thetrials using either acetone or 10% NaOH, or a sequence of both chemicals. Even though such chemicals are reported tobe suitable for the removal of these types of Laminar dry film, it has been found that it’s difficult to remove those dryfilm segments between the fine electroformed Ni tracks. Three different thicknesses of electrodeposited Ni wereprepared (8 µm, 16 µm, and 25 µm) and we observed that the thicker the electroformed Ni feature, the more dry filmremained between the features. Possible reasons for this include photoresist exposure by side scattered laser light or byroom light during handling.

To solve this problem, the excimer laser has again been used for the removal of these dry film segments. The mask wasremoved from the laser beam line, and the workpiece held stationary during ablation. Using up to 35 pulses at about 1.8J/cm2, the remaining dry film segments started to “jump” out of 20 µm wide gaps between 16 µm thick Ni tracks. Aftera further 5 shots for full removal, the cleaned structures seen in Figure 6 were obtained.

During these “maskless” laser trials, we observed a modification of the surface of the electroformed Ni pattern, asshown in Figure 7 (compared with Figure 6). Using a scanning laser confocal microscope, we measured the Ni surfaceroughness Ra to be 1.20 µm for the original electroformed Ni surface, but 0.193 µm for the same surface after the aboveexcimer laser exposure.

Fig.3a: Serpentine structure with 20 µm wide Ni track Fig.3b: Electroformed Ni in polymer

Fig.5: Dry film removed by excimer laser Fig.4: Dry film remaining between the Ni

60 µm

60 µm 60 µm

900 µm

For the smallest Ni structures, we found that this laserexposure also resulted in releasing them from thesubstrate. Figure 8 shows a SEM image for 20 µmwide and 16 µm thick Ni serpentine shaped featurereleased from the substrate by the addition of a fewmore excimer laser pulses during the stripping process.This is likely to be a result of the pulsed momentum exerted on the exposed surface shocking the metal structure andreleasing it. It is notable that the shock, while releasing the Ni, is not sufficient to damage the Ni structure itself.

However, no releasing has been found for 20 µm wide and 25 µm thick Ni features with direct laser exposure of up to250 pulses at the same fluence as the above trials. The SEM image in Figure 9 also shows a small amount of dry film

Fig.8: Ni fine feature released by excimer laser

Fig.6: Original electroformed Ni surface Fig.7: After irradiation by excimer laser

40 µm 40 µm

900 µm

900 µm

Fig.9: No releasing of Ni fine features, but a small amount of dry film remaining near the edge of Ni features due to taper effect

remaining along the edge of the Ni patterns. This is a consequence of the sidewall angle ablated in the dry film laminate- the Ni mould has a 10º taper in one direction, and the stripped dry film has a 10º taper in the opposite direction.Although this leaves a resist buttress at the side of the Ni structure, the Cu selective etch still underetches the resist andreleases the Ni. The 900 µm diameter pads are not sufficiently underetched to be released and remain anchored to thesubstrate.

All of the above processes are compatible with traditional surface or bulk micromachining techniques. Work is ongoingto investigate the utility of such structures as microheaters.

4. CONCLUSION

The results in this work have demonstrated that laser – LIGA, based on excimer laser micromachining, can be used tofabricate a composite structure using Laminar AX dry film photoresist and electrodeposition on an exposed copper seedlayer. The excimer laser additionally provides an effective method for removing unwanted photoresist whilesimultaneously improving the Ni surface roughness. For Ni features 20 µm wide and 16 µm thick we found that theexcimer laser resist stripping process also released the structures from the substrate without damage. Since the processesdescribed are compatible with traditional surface or bulk micromachining, they can be used in combination with thesetechniques to realise thick electroformed LIGA-like structures.

5. ACKNOWLEDGEMENTS

HJ would like to express his gratitude to the CRC for Micro Technology of Australia for the financial support, and MrAlex Buick for his encouragement. Thanks to Mr Martin Lloyd-Diviny, who helped with the Olympus confocal laserscanning microscope. Many thanks to the Excimer laser group in IRIS, Swinburne University of Technology, inparticular, to Mr Brian Dempster for his great help.

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