Fabrication of a Multilayered Low-Temperature CofiredCeramic Micro-Plasma-Generating Device

Amanda Baker* and Clive Randall

The Center for Dielectric Studies, Materials Research Institute, The Pennsylvania State University,University Park, Pennsylvania 16802

Randall Stewart and Richard Fantazier

RD Imagetech Inc., Lancaster, Pennsylvania 17602

Fred Wise

Wise Electronic Systems Inc., Windsor, Pennsylvania 17366

Plasma technology is currently being used in innumerable industrial applications. Some of the common uses of thistechnology include surface cleaning and treatment, sputtering and etching of semiconductor devices, excitation source forchemical analyses, cutting, environmental cleanup, sterilization, and phototherapy. The harsh conditions that these devicesmust endure require robust refractory materials systems for their fabrication and reliability. Low-temperature cofired ceramic(LTCC) material systems provide a durable and cost-effective platform for the manufacture of such devices, and allow forpossible integration into meso-scale microsystems. Our designs are based on RF microstriplines that capacitively couple andionize small gas discharge sites over the top electrode. In this paper, we have built several iterations of this micro-plasmagenerating device using LTCC material systems. The impact of electrode ink selection and processing, lamination methods,dielectric layer thickness, and electrode design has been investigated. Several micro-plasma-generating devices were thenevaluated for power requirements, output stability, and long-term reliability.

Introduction

It has been demonstrated time and again that low-temperature cofired ceramic (LTCC) material systems

make excellent platforms for meso-scale integrated mi-crosystems. Some excellent examples of these ceramicdevices that use integrated elements such as fluidic chan-nels, micro-mixers and heaters, thermistors, and inte-grated electronics include direct methanol fuel cells,1

DNA amplifiers,2 cell sorters,2 cell lysers,3 flow sensors,3

and various environmental sensors for water testing.4

To expand the pallet of meso-scale sized techno-logical elements available for microsystem integration,

Int. J. Appl. Ceram. Technol., 3 [6] 413–418 (2006)

Ceramic Product Development and Commercialization

This work was sponsored by a grant from the Pennsylvania Ben Franklin Technology

Partners.

*alb17@psu.edu

r 2006 Blackwell Publishing Ltd.

microdischarge devices have also been investigated. Suc-cessful attempts at the fabrication of LTCC versions ofthis device have been demonstrated in a collaborativeeffort between the University of Illinois and Motorola.5

In this paper, we take a somewhat different approach tobuild an array of RF micro-plasma-generating dischargesites using standard LTCC materials and processes. Ourdesigns are based on RF microstriplines that capacitivelycouple and ionize nitrogen gas in discharge sites over thetop electrode. The plasma is generated when a high-voltage, high-frequency field is applied to electrodes thatare separated by a thin dielectric layer. Pools of ions arecreated at the electrode crossover points.

Some industrial uses for plasma technology includesurface treatment and cleaning, semiconductor sputter-ing, etching, and deposition, toxic waste treatment,water treatment, excitation source for chemical analyses,sterilization, and phototherapy. By developing a micro-plasma-generating device that could be integrated intoan LTCC microsystem platform, it may be possible tobuild miniaturized versions of large-scale systems suchas a spectrophotometer.

There are many advantages of using LTCC for fab-ricating the micro-plasma generators. Ceramic materialis very stable and there are a variety of metallizationchoices and processing options available for devicefabrication. The meso-sized features necessary for theplasma generator are well within the tolerances ofLTCC technology. The material is inexpensive, theprocess is easily scaled up, and parallel processing oflayers results in cost reduction. It may be possible forthe device to be miniaturized by the inclusion of high-permittivity tapes.

Device Design and Experimental Overview

An RF plasma device with 15 discharge sites wasdesigned to be evaluated for performance in terms ofuniformity, reliability, and failure modes. On each de-vice, there were three silver RF lines and five fingerelectrodes, with the discharge sites located at the inter-section of the RF lines and the fingers (see Fig. 1).Commercial LTCC tapes and inks were selected to

2. RF Electrodes are screen printed with conductive ink

1. Bottom layers ofLTCC form a base for RF electrodes.

3. Finger electrodes are printed onto a sheet of green LTCC with conductive ink.

4. After printing, these layers are laminated

5. Optional top layer of green LTCC is mechanically punched with discharge holes corresponding to the intersection of the RF electrodes and the finger electrodes.

6. After punching, the top layer is laminated to the bottom using PEOX “glue”and reduced pressure. After firing, a nitric acid etchingstep forms the discharge holes in the fingers.

Fig. 1. Diagram of the general design and construction of the low-temperature cofired ceramic (LTCC) micro-plasma discharge device.

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make prototype devices, and because the finger electrodematerial was required to stand up to harsh ionizationconditions for the life of the device, various compatiblemetallization inks were evaluated for durability. Differ-ent approaches for imaging features in the electrodemetallization were explored to determine their impacton reliability and local field phenomena. These werecompared with conventionally processed-cofired metal-lization. Ionization holes were required to be uniform interms of dielectric thickness, dielectric constant, holediameter and shape, and edge uniformity. Hole uni-formity was critical because it had a direct relationshipwith the charge output of the device, and for our ap-plication, a discharge site uniformity of 72.5% wasnecessary. Below the RF electrodes, a dielectric layer wasrequired so that ionization occurred only at the dis-charge sites. Experiments with various LTCC layer for-mats were performed to enhance RF-input packaging.

In summary, fabrication of the devices used a com-bination of standard LTCC cofire and postfire methods.RF electrodes were screen printed onto base layers ofLTCC. An active dielectric layer was laminated on topof the electrodes. Next, RF fingers were printed on topof the dielectric layer by either a cofire or postfire pro-cess. The holes in the fingers were created either by me-chanical punching, conventional screen printing, or by amasking and etching processes. The goal of this pro-cedure was to optimize the uniformity of the apertureholes. A protective dielectric layer was added to thesamples to prevent arcing. Devices were characterized byoptical and/or scanning electron microscope to deter-mine feature tolerances and basic line resolution of theRF fingers before and after test firing. Recommenda-tions for new materials and/or changes in the fabricationprocess were made to improve plasma design and reli-ability. These different approaches enabled optimizationin the packaging of the interconnect micro-striplinesand also in the atmosphere exchange in the plasma.

Three basic approaches were undertaken for thefabrication of the micro-plasma-generating device. Ini-tially, the first two approaches focused on the machiningof the discharge holes. Variations in the layer thicknessand finger electrode ink selection were incorporated intothese designs and further studied for their effects onplasma stability and reliability. A third approach, whicheliminated the top dielectric cover and used a resistivescreen-printed ink for the finger electrodes, was under-taken as a result of problems that developed while evalu-ating the stability of the first series of samples.

Results

Mechanically Punched Discharge Hole Sample Type

Initially, we examined the mechanical punching ofdischarge holes in printed finger electrodes and the sup-porting LTCC layer. It was thought that by mechanic-ally punching the holes, their shape and size could becarefully controlled. After the printing and drying stepsof the finger electrodes were completed, the LTCC tapelayer was realigned on the punch, and 150 mm holeswere punched through the tape and metallized fingersto create the discharge sites. Metallic ‘‘smearing’’ thatoccurred from the punching step was cleaned up usinga post-fire nitric acid etching step. Although 27 of30 discharge sites were able to generate plasma, thisapproach was abandoned due to poor overall deviceperformance.

LTCC as Mask Sample Type

Next, we fabricated devices where the fired topLTCC layer was used as a mask for etching the dis-charge holes in solid silver finger electrodes below. Afterpunching and printing steps were performed, the bot-tom layers of this device were isostatically laminated atstandard temperatures and pressures. Poly(2-ethyloxazo-line) (PEOX)6 was then applied to the surface of thelaminated sample, and the top cover layer was thenlaminated to the base layers using reduced pressure(1000 psi/701C) to maintain punched discharge holesize and integrity. The use of this PEOX ‘‘glue’’ wasnecessary after several attempts at lamination undernormal conditions failed to give uniform discharge holesas required (see Fig. 2). In a post-fire process, discharge-hole sites in finger electrodes were created by a nitricacid etching of the silver finger electrodes through theholes in the top dielectric cover. Examples of thesedevices are shown in Fig. 3.

These samples were then powered up, and 60 of 60possible sites generated plasma. Test conditions were1250–1350 V at a frequency of 7400 kHz. A sampledevice was subjected to long-term testing at 1400 V, andthe test was successfully completed after reaching 100 hof continuous firing. A picture and description of thetest setup can be found in Fig. 4.

It was observed that during firing, some of the dis-charge sites had a slight ‘‘flicker’’ and this was causing anoutput amplitude modulation of greater than 50%.

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Most other sites were described as stable, with no fluc-tuation observed. Overall, the maximum charge outputremained constant, with all discharge holes very similarin output.

It was hypothesized that the ‘‘flickering’’ could becorrelated with the nitrogen environment. The samplewas placed in a sealed Teflon test jig, where nitrogen wasflowing. As the plasma fired, we believe that nitrogenwas being depleted locally in the discharge hole sites,and this caused the charge to weaken temporarily. Butthen we believe, as the nitrogen supply at the dischargeholes was replenished, the plasma output surged backup. This phenomenon may have been caused by thedischarge hole geometry, where the 96 mm depth of thehole did not allow for easy nitrogen replenishment. Tosolve the nitrogen depletion issue, it was proposed thatthe top layer dielectric thickness be decreased or entirelyeliminated.

No Top Cover Sample Type

A final group of devices were fabricated with no topdielectric cover. It was hoped that without the top cover,nitrogen would be freely available to the discharge sitesat all times, and this would help stabilize the plasma. Inthis new design, finger electrode width was increasedwhile electrode spacing as well as discharge hole size wasvaried within the test set. Silver finger electrodes wereimaged by a post-fire photolithographic process, fol-lowed by an etching step. Before testing, a dielectrictransformer varnish was applied to all spaces betweenfinger electrodes to eliminate arcing. The devices withthe largest diameter discharge holes gave the best outputstability ever observed for all previous samples.

Fig. 3. Multilayered low-temperature cofired ceramic micro-plasma devices with 15 discharge sites.

Fig. 4. Early test bench used for the initial evaluation of the testpieces. On the upper shelf from left to right; AC/DC power supply,digital oscilloscope, voltmeter RF power supply, and RF pulse widthmodulator. On the bench from left to right nitrogen supply,columnar nitrogen flow control, the test piece holder with test pieceinserted connected to RF supply and oscilloscope, capacitor test piecetuning device, computer used to control RF output.

80-100 µm 125 µm90-110 µm 90-100 µm

ab c

d

Fig. 2. Comparison of lamination techniques vs. hole quality. (a) Isostatic, 3000 psi/751C, no Shim (b) Isostatic, 3000 psi/651C, with shim(c) isostatic, 3000 psi/701C, with shim (d) platen press, 1000 psi/701C with poly(2-ethyloxazoline) ‘‘glue.’’ The goal was to create round holes,125 mm in diameter.

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Owing to the possibility of electrode ionization, wethought it was necessary to find materials other thansilver for the finger electrodes. A second set of sampleswere made using conventional screen printing methodsand several different conductor pastes, including palla-dium/silver and a ruthanate resistor ink for finger elec-trode fabrication. Different dielectric layer thicknessesbetween the RF electrodes and the finger electrodes werealso investigated.

It was determined that the resistive ruthanate fin-gers with the thinnest dielectric layer between the RFelectrodes and fingers provided the best stability of allsamples, and the relative charge output more thandoubled when compared with that of previous sample

sets. A picture of the device firing in the test fixture isshown in Fig. 5.

The new design seemed to enhance plasma stabilityand eliminate the ‘‘flickering’’ effects observed in earlierdevices. Tests showed that discharge uniformity wasgood, with a o2.5% variation in output. To measureoutput per RF burst, electrons that had been strippedfrom the nitrogen in the discharge site were collected bya probe located directly over the discharge site and con-verted to voltage by the amplifier located in the probe.This signal was then fed through a USB converter to acomputer as binary data and collected in a spread sheetfile for analysis. One thousand eight hundred outputsamples over 1 min that were recorded on hole #5 at3100 V peak to peak demonstrated excellent stability.These results are shown in Fig. 6. A discharge hole wassuccessfully fired for 21 continuous hours at 1600–1800 V peak to peak. Attempts to perform 100 h lifetest versus output will be undertaken to determineplasma stability and durability in these samples.

Conclusions

A series of LTCC micro-plasma-generating deviceswere fabricated using various electrode materials, dielec-tric layer thicknesses, electrode designs, and processingmethods. These devices were evaluated for uniformity,stability, and long-term reliability. Table I summarizesthe construction methods and test results of this study.

The ‘‘LTCC as mask’’ type samples had 96 mmthick dielectric top-cover layers and etched silver finger

Fig. 5. No top cover samples with ruthanate fingers shown with15 sites firing in the test jig. The charge output probe can be seeninserted through the nitrogen seal cover.

Discharge Hole Stability

600

700

800

900

1100

1200

600 800 1000 1200 1400

Sample #

Charge outputrelative tovoltage

1000

Fig. 6. Output readings for hole #5—no top cover samples. One thousand eight hundred samples were taken over a 1-min interval.

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electrodes. These samples demonstrated that the micro-plasma generators were robust, as they endured 100 h ofcontinuous firing. Problems with plasma stability wereobserved, however, and it was thought that depletion ofnitrogen at the discharge hole may have been causingthis instability. To solve the nitrogen depletion issue,samples with no top dielectric covers were constructed,and these showed less than a 2.5% variation in output,the best result of all previous samples. To enhance fingerelectrode durability, another set of samples were fabri-cated using a ruthanate resistive ink. These parts haveendured 21 h of continuous firing, and are currentlybeing evaluated for long-term reliability and stability.

From our results, it can be concluded that com-mercial LTCC material systems are a good fit for mak-ing devices such as these. We demonstrated two versionsof the micro-plasma generator, and all were robustenough to stand up to the harsh, high-temperature con-ditions caused by the plasma generation. LTCC can beeasily processed to provide the tolerances required forthese devices, and manufacturing and material costs arelow. Micro-plasma devices made from LTCC can beeasily integrated into ceramic Microsystems, and parallelprocessing allows for less waste on the manufacturingfloor.

Acknowledgments

The authors thank DuPont and Heraeus for gen-erously supplying materials and technical advice.

References

1. D. L. Wilcox Sr., ‘‘Ceramic Microsystems Technologies: An EmergingAddition to the MST Manufacturing Tool-kit,’’ Presented at The Pennsyl-vania State University’s Materials Day, April 15, 2004.

2. A. Baker, M. Lanagan, C. Randall, E. Semouchkina, G. Semouchkin, K. Z.Rajab, R. Mittra, R. Eitel, S. Rhee, P. Geggier, C. Duschl, and G. Fuhr,‘‘Integration Concepts for the Fabrication of LTCC Structures,’’ Int. J. Appl.Ceram. Technol., 2 [6] 514–520 (2005).

3. K. A. Peterson, K. D. Patel, C. K. Ho, S. B. Rohde, C. D. Norquist, C. A.Walker, B. D. Wroblewski, and M. Okandan, ‘‘Novel Microsystem Appli-cations with New Techniques in LTCC,’’ Proceedings of IMAPS/AcerS Inter-national Conference and Exhibition on Ceramic Interconnect and CeramicMicrosystems Technologies, April 10–13, 2005, Baltimore, MD.

4. M. R. G. Rubio, P. B. Verdonck, S. T. Kofuji, A. C. Seabra, E. D. M.Herandez, E. W. Simoes, and M. B. A. Fontes, ‘‘LTCC Sensors for Envi-ronmental Monitoring System,’’ Proceedings of IMAPS/AcerS InternationalConference and Exhibition on Ceramic Interconnect and Ceramic MicrosystemsTechnologies, April 10–13, 2005, Baltimore, MD.

5. B. A. Vojak, S. J. Park, C. J. Wagner, J. G. Eden, R. Koripella, J. Burdon, F.Zenhausem, and D. L. Wilcox, ‘‘Multistage, Monolithic Ceramic Microdis-charge Device Having an Active Length of B0.27 mm,’’ Appl. Phys. Lett., 78[10] 1340–1342 (2001).

6. D. L. Wilcox Sr. and M. Oliver, ‘‘LTCC, an Interconnect TechnologyMorphing into a Strategic Microsystem Integration Technology,’’ Proceedingsof the IMAPS Advanced Technology Workshop, Providence, RI, May 2–3,2002.

Table I. Summary of Construction Methods and Results of the Micro-Discharge Devices

Design TypeMethod of fingerhole fabrication

Finger electrodeink type Output stability

Long-term testcontinuous firing

Mechanicallypunched

Holes punched throughprinted electrodes

Silver Poor Poor

LTCC as mask Holes etched in solid electrodesthrough top cover

Silver Fair—nitrogen depletionat discharge sites

100 h

Wide fingers Screen printed, photo-imagedand etched

Silver Good n/a

Screen printed Ruthanate Resistor Good 21 h�

�Further studies on long-term testing are continuing.LTCC, low-temperature cofired ceramic.

418 International Journal of Applied Ceramic Technology—Baker, et al. Vol. 3, No. 6, 2006