Vacuum Packaging Technology Using Localized

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  556 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 11, NO. 5, OCTOBER 2002 Vacuum Packaging Technology Using LocalizedAluminum/Silicon-to-Glass Bonding Yu-T. Cheng, Wan-Tai Hsu  , Member, IEEE  , Khalil Najafi  , Fellow, IEEE  , Clark T.-C. Nguyen  , Student Member, IEEE  ,and Liwei Lin  Abstract— A glass vacuum package based on localizedaluminum/silicon-to-glass bonding has been successfully demon-strated. A constant heat flux model shows that heating can beconfined locally in the dielectric layer underneath a microheateras long as the width of the microheater and the thickness of siliconsubstrate are much smaller than the die size and a good heat sinkis placed underneath the silicon substrate. With 3.4 W heatingpower, 0.2 MPa applied contact pressure and 90 min wait timebefore bonding, vacuum encapsulation at 25 mtorr ( 3.33 Pa)can be achieved. Folded-beam comb drive -resonators areencapsulated and used as pressure monitors. Long-term testing of vacuum-packaged -resonators with a Quality Factor ( ) of 2500has demonstrated stable operation after 69 weeks. A -resonatorwith a factor of 9600 has been vacuum encapsulated andshown to be stable after 56 weeks. [686]  Index Terms— Localized heating, MEMS packaging, microres-onators, vacuum encapsulation, wafer level packaging. I. I NTRODUCTION V ACUUM and hermetic encapsulation of resonant devicesis required not only to protect them from damage andcontamination, but also to provide a controlled low-pressureor vacuum environment for low-loss (high -factor) operation.Contaminants,likemoistureanddust,cangreatlyaffectthesen-sitivity and resolution of -resonant devices. For example, be-cause the typical mass for a very high-frequency -resonatoris about 10 kg, even small amounts of mass-loading cancause significant resonance frequency shifts and induced phasenoise [1]. In addition, most micromachined resonant deviceshave very large surface-to-volume ratios and vibrate in a verytightspace.Forsuchdevices,viscousandsqueeze-filmdampingeffects can also reduce their factor [2], [3]. In microelectromechanical systems (MEMS) vacuum pack-aging, two major approaches have been demonstrated: theintegrated encapsulation approach [2]–[7] and the postprocess packaging approach [8]–[14]. A typical integrated encapsu- lation approach utilizes a 2 3- m thick phosphorus-dopedsilicon glass (PSG) or doped polysilicon layer on top of the micromachined mechanical component as a sacrificiallayer, followed by the deposition of several microns thick of  Manuscript received May 4, 2001; revised February 28, 2002. This work was supported in part by the DARPA MTO/MEMS Program under ContractF30602-97-2-0101. The work of L. Lin was supported in part by an NSF CA-REER Award (ECS-0096098). Subject Editor G. Stemme.Y.-T. Cheng, W.-T. Hsu, K. Najafi, and C. T.-C. Nguyen are with the Centerfor Wireless Integrated MicrosystemsDepartment of Electrical Engineering andComputer ScienceThe University of Michigan, Ann Arbor, MI 48109 USA.L. Lin is with the Department of Mechanical Engineering, University of Cal-ifornia at Berkeley, Berkeley, CA 94720 USA (e-mail: Object Identifier 10.1109/JMEMS.2002.802903. polysilicon or silicon nitride layer with permeable holes as aprotection shell. The permeable holes provide leakage pathsto buffered hydrofluoric acid (BHF) or silicon etchant for thereleasing process to free mechanical microstructures. The holesare sealed by growing another low-pressure chemical vapordeposition (LPCVD) layer of polysilicon after the releasingprocess. Integrated encapsulation can achieve low pressure andgood hermeticity in wafer level fabrication and provide lowmanufacturing cost. However, the lack of controllability of cavity pressure, which is determined by the deposition condi-tions of CVD materials during the sealing process and the hightemperature of CVD deposition are limitations of this approach[6]. Furthermore, this approach is process specific and notsuitable for a wide range of MEMS packaging applications.On the other hand, the postprocess packaging approach has thepotential to solve these problems and is chosen as the preferredmethod in this work.In the postprocess packaging approach, integrated microsys-tems and protection shells are fabricated on different substrates,either silicon or glass, at the same time. The two substrates arethen bonded together using silicon fusion, anodic, or low tem-perature solder bonding to achieve the final encapsulation. Lowpackaging cost can be obtained due to wafer-level processing.Low bonding temperature and short process time are both de-sirable process parameters in device fabrication to provide lowthermal stress and high throughput. However, most chemicalbonding reactions require aminimum and sufficient thermal en-ergytoovercomethereactionenergybarrier,normallycalledtheactivation energy, to initiate the reaction and to form a strongbond. As a result, high bonding temperature generally resultsin shorter processing time to reach the same bonding qualityat a lower bonding temperature. Since thermal effects to thesurrounding circuitry or MEMS devices of packaged microsys-tems are inevitable when bonding temperature is high, local-ized heating has been developed to provide to alleviate theseeffects for bonding-based package and assembly applications[15]–[17]. In a previous paper, a novel hermetic package using localizedaluminum/silicon-to-glass bonding with excellent bondingstrength and durability was reported by our group [18]. Thispaper presents a detailed analysis of the fundamental principleof localized heating, bonding and presents a glass packageused for vacuum encapsulation of surface micromachined-resonators. This post-process wafer-level packaging methodcan be applied to a variety of MEMS devices which requirecontrollability of the cavity pressure, low-temperature pro-cessing at the wafer-level, excellent bonding strength, lowfabrication cost and high reliability. 1057-7157/02$17.00 © 2002 IEEE  CHENG  et al. : VACUUM PACKAGING TECHNOLOGY USING LOCALIZED ALUMINUM/SILICON-TO-GLASS BONDING 557 II. P ACKAGE  D ESIGN AND  F ABRICATION  A. Thermal Analysis The localized heating and bonding concept is utilized forbonding two substrates as shown in Fig. 1(a). Resistive heatingby using microheaters on top of a device substrate is used toform a strong bond to a silicon or glass cap. According to theresults of two-dimensional (2-D) heat conduction finite elementanalysis [16], [19], the heating region of a 5- m-wide polysil- icon microheater covered with a Pyrex glass cap at steady statecan be confined locally as long as the temperature of the siliconsubstrate is maintained at ambient temperature.Thephysicsoflocalizedheatingcanbeunderstoodbysolvingthe governing heat conduction equations for a device structurewithout a cap. These equations are solved under a steady-statecondition with constant heat flux and adiabatic boundary con-ditions as illustrated in Fig. 1(b). Because the width of the mi-croheater (2 5 m) used in the bonding experiment is muchsmaller than its length ( m), the temperature distribu-tion around the heated region can be reasonably approximatedby applying a 2-D model instead of a three-dimensional (3-D)one. The governing equations and the boundary conditions are[13], [20] (1)(2)(3)where , , , , ,and are the temperature, thermal diffusivity, effective thick-ness and thermal conductivity of silicon substrate and electricalinsulation layer, is the bottom temperature of the siliconsubstrate, is the heat flux density at the die surface and ishalf of the length of a die. In most MEMS, silicon dioxide andnitride are used as electrical insulators. The effective thicknessandthermalconductivityoftheelectricalinsulationlayercanbecalculated from (3) where , , , and are the thicknessand thermal conductivity of silicon oxide and nitride, respec-tively. These boundary conditions are based on the followingassumptions:1) the system will be at steady state during bonding be-cause the time needed for the whole structure to reachthermal equilibrium is only several seconds [21] (this ismuch shorter than the bonding time of several minutes in Fig. 1. Schematic diagram of localized heating and bonding. (a) 3-D view. (b)2-D heat transfer model, geometry and boundary conditions (  B.C. ). theseexperimentsorhoursinothertestsdependingonthemethod of heating and bonding materials);2) the spreading of heat in polysilicon is negligible sincethe lateral thermal heating length in polysilicon is muchsmaller than the width of polysilicon [22];3) the heat transfered from the top surface of the die to theambientisnegligiblebecausebothnaturalconvectionandradiation are much smaller than heat conduction to thesubstrate at moderate temperature [21], [22]; and 4) the thermal resistance of the interface between the elec-trical insulation layer and the silicon substrate is neg-ligible due to the high quality of the interface betweensilicon oxide, silicon nitride and silicon.The analytical solutions and are solved as[22] (4)–(5) shown at the bottom of the next page. The temper-ature of the silicon substrate, , is a function of thermal con-ductivity, heater and die size and input power. The temperatureat point , right underneath the microheater and at the inter-face of the electrical insulator and silicon substrate ( , or) is(6)Since the thermal conductivity of silicon is about 100 times thatof silicon dioxide and 50 times that of silicon nitride, the tem-perature of can be approximated to the first order as:(7)  558 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 11, NO. 5, OCTOBER 2002 Here, is the total power input per unit length into the micro-heater. This analysisshows the geometryof the microheater, thedie size, the thermal conductivity difference between the elec-trical insulator and silicon and the thickness of the silicon sub-strateareallfactorstobecontrolledinthebondingexperiments.As long as the width of the microheater and the thickness of thesilicon substrate are much smaller than the die size and a goodheat sink is placed underneath the silicon substrate, heating canbe confined locally. The temperature of the silicon substrate canbe kept low or close to room temperature.The bonding temperature can be estimated from the temper-ature of the microheater since bonding occurs at the interfacebetween the heater and silicon or glass cap [16]. An elec-trothermal model of the line shaped microheater based onthe law of energy conservation and the linear dependence of resistivity with respect to temperature has been developed byLin  et al.  [21]. It has verified that it could provide a wayto control bonding temperature by knowing the geometry, theinput current and the temperature coefficient of resistivity of the microheater. According to this model, the total input powerof the microheater is a function of the thickness of the elec-trical ground layer ( ) as shown in (8)–(9) at the bottom of the page where , , , and , are the thickness, thermalconductivity, temperature coefficient of resistivity and excessflux shape factor of the microheater, respectively and isthe input current density. Fig. 2 shows the relationship of the thickness of the silicon oxide layer and total input powerdensity when the desired heater temperature for bonding isfixed at 800 Cand the thickness of the silicon nitride is 0 min this case. It is observed that the input power density canbe reduced if the thickness of the oxide layer is increased toprovide better thermal isolation. On the other hand, it is alsoimportant to calculate the spreading of localized heating (bycalculating which is the distance away from the micro-heater where the temperature drops to 100 C if the desiredheater temperature is kept at 800 C). This is achieved fromthe previous analytical solutions of the constant flux model.The surface temperature of silicon dioxide is alsoa function of the oxide thickness:(10)The simulation results show that decreases (better for lo-calized heating) with the decrease of oxide thickness but withthe increase of the input power density as shown in Fig. 2.For example, as indicated by the arrows in the Fig. 2, is7.85 m for an input power density of 1.55 10 Watt/cmon a 5- m-wide microheater that is on top of 7- m-thick (4)(5)Input Power(8)(9)  CHENG  et al. : VACUUM PACKAGING TECHNOLOGY USING LOCALIZED ALUMINUM/SILICON-TO-GLASS BONDING 559 Fig. 2. Simulated results of input power density versus the thicknessof silicondioxide and the distance of the heating region where temperature is 100 Cbased on a 1-D electrothermal model and a 2-D heat transfer model. Themicroheater is maintained at 800 C, nitride thickness    0    m,        Cand        m, in this case. silicon oxide layer. Although reducing the oxide thickness to1.1 m can effectively reduce to 4 m, the required inputpower density will rise to 1.1 10 W/cm in order to keepthe microheater at 800 C. In summary, optimized heater andinsulation parameters can be determined from these analyticalequations.Because of the linear temperature dependence of polysiliconresistance, a line shaped polysilicon is used as the temper-ature sensor for measuring the temperature surrounding themicroheater. Fig. 3 shows a polysilicon four-point resistancemeasurement structure as a temperature sensor 15 m awayfrom the 5- or 7- m-wide polysilicon microheater on top of m thick silicon dioxide. No drastic resistivity change fromthe temperature sensor is measured while the microheater isheated up over 1000 C as shown in Table I. It is estimatedthat every 1000 C temperature rise in the temperature sensorcorresponds to a resistivity change of up to 12%. As indicatedin Table I, these results further indicate that the heating regionis confined locally to within 15 m of the heating source.  B. Vacuum Encapsulation Processes The vacuum packaging approach presented here is basedon the hermetic packaging technology using localized alu-minum/silicon-to-glass solder bonding technique reportedpreviously [18]. Built-in folded-beam comb drive -resonatorsare used to monitor the pressure inside the package. Fig. 4shows the fabrication process of the package and resonators.Thermal oxide (2 m) and LPCVD Si N arefirst deposited on a silicon substrate for electrical insulationfollowed by the deposition of 3000 LPCVD polysilicon.This polysilicon is used as both the ground plane and theelectrical interconnect to the -resonators as shown in Fig. 4(a).Fig. 4(b) shows a 2- m LPCVD SiO layer that is depositedand patterned as a sacrificial layer for the fabrication of polysil-icon -resonators using a standard surface micromachiningprocess. A 2- m-thick phosphorus-doped polysilicon is usedfor both the structural layer of micro resonators and the on-chipmicroheaters. This layer is formed over the sacrificial oxide intwo steps to achieve a uniform doping profile. The resonators Fig. 3. Temperature measurement nearby the microheater using a polysiliconresistor. are separated from the heater by a short distance, 30 m, toeffectivelypreventtheir exposuretothe highheater temperatureas shown in Fig. 4(c).In order to prevent the current supplied to the microheaterfrom leaking into the aluminum solder during bonding, anLPCVD Si N SiO Si N sand-wich layer is grown and patterned on top of the microheateras shown in Fig. 4(d). Fig. 4(e) and (f) show that polysilicon(5000 ) and aluminum (2.5 m) bonding materials are de-posited and patterned. The sacrificial release is the final stepto form free-standing -resonators. Fig. 4(f) shows a thick AZ-9245 photoresist is applied over the aluminum area toprotect it against attack from concentrated hydrofluoric acid.After an 8-minute sacrificial release in concentrated HF, thesilicon substrate as shown in Fig. 4(g) is ready for vacuumpackaging. Fig. 5 shows SEM photos of a number of released-resonators surrounded by a 30- m-wide microheater withaluminum/silicon bonding layer on top. A Pyrex glass cap witha 10- m deep recess is then placed on top with an appliedpressure of 0.2 MPa under a 25 mtorr vacuum and the heateris heated using 3.4 W input power (exact amount depends onthe design of the microheaters) for 10 min to complete thevacuum packaging process as shown in Fig. 4(h).III. E XPERIMENTAL  R ESULTS To evaluate the integrity of the resonators packaged using lo-calized aluminum/silicon-to-glass solder bonding, the glass capis forcefully broken and removed from the substrate. It is ob-served that no damage is found on the -resonator and a partof the microheater is stripped away as shown in Fig. 6, demon-strating that a strong and uniform bond can be achieved withoutdetrimentaleffectsontheencapsulateddevice.Outgassingfromthe glass and gas resident inside the cavity are two major fac-tors that should be minimized in order to achieve a low pressureenvironment in all vacuum-based encapsulation processes.  A. Outgassing During the bonding and encapsulation process, outgassingfrom the glass capsule could degrade the vacuum inside thepackage [23], [24]. In this encapsulation process, the volume of  the cavity formed by the recessed Pyrex glass cap and the de-vice substrate as shown in Fig. 5 is about 1.2 10 cm . Anyoutgassing would result in a drastic increase of pressure in sucha small volume. Two possible outgassing mechanisms could
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