96 metal deposition of acoustic wave devices for mobile ... · intense consolidation (through...

The Internet platform and mobile devices haveushered in a new phase in human productivity andsocial life. This profound transition is manifested

by 2.8 billion Internet users and 5.2 billion mobilephone users in 2014 (www.kpcb.com/internet-trends).The next leap in connectivity is anticipated with theevolution in the Internet of Things (IoT). As many as50 billion connected devices are envisioned for 2020 andconsequently an increasingly high premium will be puton the maximization of spectrum utilization. To ensurehigh-quality communication within each of the boomingnumber of frequency bands, engineers deploy filters toreduce the interference among bands. With high per-formance, small size and low cost, bulk acoustic wave(BAW) and surface acoustic wave (SAW) filters win overthe market from the traditional ceramic and LC filters. The number of acoustic wave devices utilized in a cel-

lular phone has increased from 2 in a feature phone toabout 5 in a low-end smart-phone and more than 15 ina high-end smart-phone. The market size of acousticwave devices is projected to grow from $1.9bn in 2011to $4.9bn in 2016, with BAW growing more than twiceas fast as SAW (www.bccresearch.com). Although thelower insertion loss, steeper filter skirts and better out-of-band rejection of BAW enable it to perform better athigh frequency and for communication bands of closeproximity, recent innovation in temperature compensationand circuit design have expanded SAW into previous BAWterritory, especially in China’s latest-generation TD-LTE(time-division long-term evolution) communication marketdue to the Asian market’s sensitivity to cost. The battlebetween SAW and BAW manufacturers remains fierce. In light of the increasing competition from both the

well established (i. e. Qualcomm) and a number ofstartup CMOS companies in the mobile power amplifier(PA) marketplace, which is currently dominated bycompound semiconductor (CS) IC companies, thecompound semiconductor industry is seeking opportu-nities to grow their top line (revenue) meaningfully.Acoustic wave devices become the most natural busi-ness opportunity for the expansion of compound semi-conductor IC companies for the following reasons:

The total component price of acoustic wave devicesfor a high-end mobile device now far surpasses that ofPA devices. Acoustic wave devices therefore enable ahigh growth rate in future revenue. The similarity in processing technology between theacoustic wave and PA devices is such that the technicalbarrier of entry into the SAW/BAW business is not highfor compound semiconductor RF IC companies. Much of the processing equipment used in the com-pound semiconductor RF IC fab could be used just aswell for SAW/BAW processing. This enables flexibilityto maximize equipment and facility utilization. Nonetheless, in order to profitably serve the acoustic

devices market, compound semiconductor RF IC com-panies must still address the problem of many patentsbeing owned by other companies. After a few years ofintense consolidation (through merger, acquisition,partnership and alliance), many leading compoundsemiconductor companies (i.e. Avago, Qorvo, Skyworksetc.) are now in a good position to grow their acousticwave business aggressively. Since SAW and BAW are fabricated on different substrate materials thancompound semiconductor RF IC, the SAW/BAWs arenot integrated into the same chip with the PAs. How-ever, acoustic wave devices are incorporated into thesame module as PA devices in order to minimize thesize and to reduce the cost of mobile devices. This paper presents the many considerations involved

in the metal evaporation process for high-yield acoustic wave devices and the benefit of Ferrotec’s UEFC-5700/4900 evaporators for the production ofSAW and BAW devices (see Figure 1).

Device characteristics of acoustic wave filtersFBAR (film bulk acoustic resonator) and BAW-SMR(solidly mounted resonator) are popular implementationsof BAW devices. FBAR confines the acoustic energy viathe large difference in acoustic impedances betweenthe FBAR structure and air on both the top and bottomsides. On the other hand, BAW-SMR has an acousticreflector at the bottom. There should be a large mismatchin acoustic impedance between the electrode and the

Technical focus: Deposition

semiconductorTODAY Compounds&AdvancedSilicon • Vol. 10 • Issue 10 • December 2015/January 2016 www.semiconductor-today.com

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Kuohsiung Li PhD of Raisin Technologies and John Hubschmann discuss theapplications of SAW and BAW devices in wireless communication and IoT.

Metal deposition ofacoustic wave devices formobile & Internet of Things

reflector to ensure the confinement of acoustic wave.The superior confinement of acoustic energy by FBARand BAW-SMR makes these filters ideal for communi-cation at higher frequency compared with SAW filters.Consequently they are widely deployed in the newgeneration (4G) of smart-phones and 5GHz Wi-Fi. AlNis the piezoelectric material of choice for BAW filters dueto its higher acoustic velocity (10,400m/s) comparedwith ZnO (6330m/s) and less acoustic loss (5dB/usversus 8.3dB/us @1GHz). As a rule, the piezoelectricmono-crystal should also have a high degree of crystalorientation and be very smooth (~5Å rms) for the pro-duction of filters with high-Q (quality factor). Molybdenum (Mo) is a popular electrode material for

FBAR with high acoustic impedance and high powerhandling capability. W, Au, Ti, Al, Pt, V and Cu could alsobe used as electrodes. BAW is inherently less sensitiveto temperature drift than SAW over the typical operatingrange of –20°C to 85°C experienced by the end users.With boron (B)-doped SiO2 juxtaposed with one of theelectrodes, the temperature coefficient of frequency (TCF)could further be reduced to nearly zero (Avago, 2009,US Patent 7561009). The improvement in TCF enablesthe deployment of BAW for communication bands ofvery close proximity (for example, LTE Band 40 hasonly 1MHz of transition guard band with 2.4GHz Wi-Fi).Furthermore, FBAR and SAW-SMR have the advantageof decreasing size with increasing frequency.Al (Aluminum) is a popular electrode material for

SAW with very low electrical resistance and low density(2.7g/cm3) which minimizes the mass loading effect.Quartz, LiTaO3 and LiNbO3 are common piezoelectricmaterials. Typically, 2% Cu (by weight) is added to theAl to serve as ‘glue’ among Al grains, and hence toinhibit Al electromigration that moves grain boundariesat high temperature or at high power for better devicereliability. Although the addition of Cu improves thepower handling capabilityof SAW devices by mini-mizing hillock and voidformation, too much Cuwill increase the electrical resistance (see Figure 2). On theother hand, Cu concentra-tion of less than 1% ren-ders the Cu ineffective.There is an optimal rangeof Cu concentrations thatreduces electromigrationwithout unduly increasingelectrical resistance. Although SAW devices

typically have a higherTCF of about –45ppm/°C,over-coating of IDT

(inter-digital transducer) structures with a dielectriclayer (i.e. SiO2) that increases the acoustic stiffness athigh temperature can reduce the TCF to nearly zero.Furthermore, the insertion loss in the pass-band couldbe reduced as well (TriQuint, 2011, US Patent 8044553).

Considerations in metal evaporationprocess for acoustic wave devicesThe most critical processing step for the manufacturingof SAW/BAW devices is the deposition of IDT (inter-digital transducer) metallization, which determines thedevice performance and reliability under high tempera-


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Fig. 2: The resistivity of AlCu as a function of Cu (weight %) concentration(www.nist.gov/data/PDFfiles/jpcrd221.pdf).

Fig. 1: Ferrotec UF-5700 electron-beam evaporatorfor the manufacturing of SAW and BAW devices.

ture and high power. Electron-beam evaporation hasmany advantages for the production of IDT fingers.Chief among them is that the directionality of evapora-tion facilitates the lift-off process and consequentlyeliminates the need to selectively etch the blanket metalfor the formation of IDT fingers. Additionally, the largebatch process reduces the cost of ownership (CoO) andthe gentle deposition of evaporants drastically reducesthe surface damage on the wafers that would impactacoustic coupling. For SAW devices, the deposition of alloys such as AlCu

presents a challenge in the control of Cu concentration(typically 2% by weight) within the wafer, wafer-to-waferand run-to-run. When the Cu concentration errs on thelow side, the Al migration at subsequent high-temper-ature process is not arrested and thus results in cata-strophic failure. When the Cu concentration errs on thehigh side, the resistivity of AlCu film and the insertionloss of SAW devices increase. This challenge is due tothe difference in vaporization temperature between Al and Cu (2327°C and 2595°C, respectively) whichresults in a higher evaporation rate of Al than that ofCu. Ferrotec has developed a proprietary procedurethat efficiently prepares the AlCu source in the crucibleto ensure 2% Cu concentration in the deposited filmreliably. This recipe will be provided to Ferrotec’s cus-tomers to quickly bring the production process on line. Tight control of the electron-beam evaporation process

is required to produce an alloy of precise stoichiometry

and thickness. Controlof beam sweep andevaporation rate are themost critical. Ferrotec’s electron-beam sweepcontroller allows usersvirtually infinite controlover sweep patternsand beam dwell (sweepspeed) time. This —along with the deposi-tion rate controller, fea-turing programmabletemperature ramp ratesand dwell (soak) steps— gives users totalcontrol over the meltand evaporation condi-tions (see Figure 3).Patented planetarydomes are also incor-porated into Ferrotec’sUEFC-5700/4900 evapo-rators to further improvethe uniformity control onboth the Cu concentra-tion and film thickness.

It is essential to eliminate all surface contaminationprior to the evaporation of IDT metal to ensure effectiveacoustic coupling. A robust multi-stage cleaning processis widely adopted throughout the acoustic wave industry.For example, in a first-stage process to remove chemicalresidues from the CMP (chemical mechanical planariza-tion) polishing process used by the substrate suppliers,NH4OH/H2O2 solution is heated to 50–60°C to clean thesubstrates and then followed by a cascade rinse indeionized (DI) water until the DI water reaches14–16MΩ-cm resistivity. The second stage of surface cleaning removes

hydrocarbon contamination from either the substratesuppliers’ process or other natural sources. Thisrequires two heated (50–60°C) ultrasonic tanks. Thefirst tank is filled with acetone to completely immersethe substrates for 10 minutes of ultrasonic agitation.Substrates are then transferred to the second tank forisopropanol alcohol (IPA) immersion with 10 minutesof ultrasonic agitation. The substrates are then imme-diately dried in a heated ‘N2 only’ spin dryer.The last stage is oxygen plasma cleaning to remove

stubborn hydrocarbon residue. The oxygen plasmamust be energetic enough to exhibit a glow dischargefrom the ionization process. Oxygen should also have ahigh flow rate to maintain a partial pressure of 1 Torrfor approximately 10 minutes. In addition, some devicemanufacturers also use ultraviolet ozone cleaner (UVOC)for 10 minutes to deplete the surface of very aggressive,



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Fig. 3: Ferrotec UF-5700/4900 system control hardware.

chemically bonded OH molecules which cancause issues with subsequent processesthat are sensitive to OH contamination.The cleanliness of the surface will need to

be verified to ensure the efficacy of thecleaning process. A simple method is com-monly employed to measure changes inthe surface energy with a goniometer tomeasure the contact angle. Small angle θindicates high surface energy exhibitsfrom a clean surface (see Figure 4). Priorto the above-mentioned rigorous surfacecleaning procedure, quartz substrates typically have contact angles rangingbetween 20° to 40°, and 2° to 3° after amulti-stage cleaning. In SAW/BAW processing, the application

of photoresist is necessary to define vari-ous layers. Photoresist is a hydrocarboncontaminant and must be completelyremoved before subsequent processesand before final encapsulation of theacoustic wave devices. The contact angletest can also be used to verify that nophotoresist residue (scum) remains; otherwise contactangles of ≥20° are often observed. The presence ofany contamination dampens the wave velocity andadversely impacts the coupling of RF electrical energywith the piezoelectric surface and results in an outputfrequency higher than originally designed. Furthermore,the insertion loss will also be higher. Process engineersmay be tempted to increase the IDT metal thickness tobring the frequency back to specification, only to seethe next batch of wafers out of specification due to theunpredictability of the photoresist contamination. There-fore emphasis has to be placed on substrate surfacecleaning from the start. Proper cleaning ensures anysubsequent process steps will only need a short pre-cleanvia an O2 plasma de-scum to return the surface to aclean baseline. TheUEFC-5700/4900evaporators alsohave an option toprovide in-situcleaning with anion gun. Thisallows for removalof several atomicsurface layers toensure efficientacoustic couplingto the piezoelectricsubstrate.Care should

further be takenon the accuracy of

thickness measurement. The conventional methodused for thickness measurements of thin-film metalshas been the surface profilometer. This tool has severalinherent draw backs for measurement on films below800Å. First of all, the signal-to-noise ratio is insufficientfor ±1% accuracy. Secondly, the stylus contacts thesurface and has a downward force as it moves acrossthe measured features and may damage them. Thesolution is to incorporate numerous design verificationstructures across each wafer in sacrificial areas fornon-contact thickness measurement via white lightinterferometer (WLI). In addition, the accurate measurement of the thin-film

resistivity of IDT fingers is critically important as a keyindicator of film quality and Cu concentration in AlCu.



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Fig. 5: A Van der Pauw structure, known as a ‘Greek cross’ for voltage measurement withapplied current to extract the sheet resistance.

Fig. 4: Deionized water droplets showing contact angle as a functionof surface energy.

The resistivity measurement must be sensitive enoughto detect minute changes that reflect the variation incopper concentration. A Van der Pauw structure, knownas a ‘Greek cross’ in the semiconductor industry, iscommonly used (see Figure 5). Automatic test on theproduct wafers could thus be performed ‘in process’(before bond pad metal). This sensitive ‘in process’test allows for early screening of bad wafers to savemanufacturing cost.

Advantages of Ferrotec UEFC-5700/4900for SAW, FBAR & BAW-SMRFerrotec’s UEFC-5700 and UEFC-4900 incorporate twomain features to minimize the CoO of an electron-beam(E-beam) evaporator. The first is the patented magneticdrive HULA (High Uniformity Liftoff Assembly), which is

a set of planetary rotating wafer carrier domes thatchange the location of each wafer relative to the evaporants in the source crucible continuously via acontactless magnetic drive. Each wafer remains ortho-gonal to the direction of evaporation. This directionalityis critical to a high-yield lift-off process. The second isthe patented conic shape chamber design that minimizesboth the volume and surface area of the product chamber.This leads to much reduced pump-down time andhence better throughput (see Figure 6). The magneticHULA drive is contactless and therefore generates noparticles and thus results in better yield. The planetarydomes rotate around the central axis of the productchamber and each dome is also self-rotating to facilitateuniform depositions for all wafers. Furthermore, thedesign of UEFC-5700 is footprint efficient, such that

the batch size is increased by 40% forthe same footprint (as Ferrotec’s moretraditional ‘box coater’ FC-4400) to allowfor better utilization of cleanroom space. Ferrotec has previously demonstrated

the ability of UEFC-5700 and UEFC-4900for the precise control of eutectic com-position (Amirani I., 2015. The Resur-gence of Electron Beam Evaporation. Compound Semiconductor magazine).This allows for the control of eutecticmelting temperature within a narrowrange to ensure the bonding quality.Even without the utilization of a unifor-mity mask, UEFC-5700/4900 couldcontrol the 80%Au/20%Sn eutecticcomposition to 0.02% standard deviation(see Figure 7) within-wafer, and about0.3% standard deviation from wafer towafer. Care should be taken to controlthe batch-to-batch thickness repeata-bility as well.Ferrotec’s UEFC systems have good

repeatability on AlCu evaporated from a



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Fig. 6: Ferrotec’s HULA Planetary rotating domes (left) and conic chamber (right).

Fig. 7: Precise control of Au/Sn eutectic composition to 0.02% standarddeviation within-wafer in a Ferrotec HULA system, without mask.



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single-source crucible as well,for the manufacturing of SAWdevices. As discussed in theprevious section, 2% Cu (byweight) in Al inhibits the electromigration of Al at hightemperature and/or at highpower. The within-wafer thickness uniformity of AlCu(98%/2% by weight) is about1% standard deviation and therun-to-run uniformity is alsoabout 1% standard deviationeven with a refill of AlCu at thesource crucible for run number 8(see Figure 8). This test is con-ducted at a relatively high evap-oration rate of 10Å/s to simulatehigh-throughput production.The control of Cu concentration

from run to run is also evaluatedin a Ferrotec UEFC-4900 system.The AlCu was evaporatedrepeatedly from a single-source crucible and a sourcerefill was performed at the 11th run. The Cu concen-tration could be tightly controlled to 0.2% standarddeviation for run-to-run repeatability (see Figure 9).In addition to electrodes of acoustic wave devices,

Ferrotec’s evaporators are also ideal for the metal deposition of bond pads, such as NiCr/Au where NiCrserves as a diffusion barrier for Au, or Cr/Al where a verythin layer (<100A) of Cr strength-ens the bond pad attachment. In summary, the applications of

acoustic wave devices haveexploded with the current wideadoption of mobile devices andthe future expansion of IoT. Thebattles between SAW, FBAR andBAW-SMR are fierce, fueled byinnovation in device and circuitdesign. Mo, AlCu and other elec-trode materials can be uniformlyevaporated by Ferrotec’s UEFC-5700 and UEFC-4900 at low costof ownership. Ferrotec providescustomers with guidance on thecleaning of piezoelectric materials,preparation of AlCu source crucibleand the high-yield lift-off processof the evaporated metals from asingle crucible with precision control on thickness and composi-tion. This is accomplished via thepatented HULA planetary domesand conic product chamber.

Authors: Kuohsiung Li PhD, Raisin Technologies, and John Hubschmann J. Hubschmann was with Raytheon for 13 years and is a SAWprocess specialist.For more information about the Ferrotec products cited, visit www.temescal.net

Fig. 8: Within-wafer and run-to-run thickness uniformity of AlCu (98%/2% byweight) evaporated at 10Å/s in a Ferrotec UF-4900 with source refill at run #8.

Fig. 9: Run-to-run repeatability of Cu concentration in AlCu targeting 2% Cuwith source refill at run #11 in a Ferrotec UF-4900 evaporator.

96 metal deposition of acoustic wave devices for mobile ... · intense consolidation (through...

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