RELIABLE INTEGRATION OF PIEZOELECTRIC LEADZIRCONATE TITANATE WITH MEMS FABRICATION

PROCESSES

S.J. Gross1, Q.Q. Zhang1,2, S. Tadigadapa1, S. Trolier-McKinstry1, T.N. Jackson1 and F. Djuth2

1The Pennsylvania State University, University Park, PA 168022Geospace Research, Inc. El Segundo, CA 90245

ABSTRACT

The high piezoelectric effect of lead zirconate titanate (PZT) films enables improved performance inmicroelectromechanical systems (MEMS). The material’s reliable integration into current and mainstream MEMSmicrofabrication processes is then of great interest. In this paper we report on high reliability fabrication processesthat can be used for producing PZT based MEMS devices. Pattern definition and release of PZT, low stress siliconnitride, platinum, and/or zirconia structures via wet and dry chemical etching and ion beam etching, including theiraffects on the piezoelectric properties of PZT are reported. Ion beam etching results in appreciable imprint in thepolarization – electric field hysteresis loop of the PZT, which can be ameliorated by annealing in ambient air at450°C. PZT on silicon nitride cantilever structures were defined and released by dry xenon difluoride siliconsacrificial etching. The advantages and difficulties of wet release etching versus xenon difluoride are also presented.

1. INTRODUCTION

In many microelectromechanical systems such as integrated sensors and actuators, there is a need for thetransduction of electrical and mechanical signals. Examples of these MEMS applications include accelerometers,linear and rotary motors, pumps, switches, and acoustic devices. There are many phenomena currently employed toachieve this, including electrostatic, electromagnetic, piezoresistive and piezoelectric effects. In piezoelectricmaterials an applied stress generates a net polarization or surface charge, and conversely, an applied electric fieldinduces a strain. Piezoelectric PZT films have an advantage over many of the other phenomena in terms of theavailable energy density that can be stored in the material in order to perform work. In addition, it has high dielectricand piezoelectric constants, as well as the potential for high electromechanical coupling coefficients.1 SeveralMEMS devices using piezoelectric (PZT) thin films have been reported.2,3,4 However, the integration of PZT filmsinto MEMS devices is relatively immature and complex. Thus, this paper focuses on the reliable integration of PZTthin films in microfabrication processes.

The solid solution of lead zirconate and lead titanate is not only piezoelectric but ferroelectric as well; meaningthat the spontaneous polarization in a randomly poled ceramic can be reoriented to obtain a net polarization by theapplication of an electric field. Thus by the application of external electric field, PZT films can be either poled witha net polarization normal to the plane of the film or in the plane of the film. In the former case, the PZT film issandwiched between two conducting layers such as platinum, and in the later case interdigitated electrodes on top ofthe film are used while film itself is deposited on an insulating material such as ZrO2 (zirconia). Since both of thetwo modes of using PZT films are of interest, we will investigate fabrication processes capable of producing devicesin either mode of operation. The magnitude of the remanent polarization in a polarization – electric field hysteresisloop strongly affects the magnitude of the available piezoelectric coefficients. In this work we use remanentpolarization as the parameter to evaluate the effects of various fabrication processes on PZT thin films. Althougheffects of the fabrication processes on the properties of PZT films are the primary focus of this paper, we will alsoreport the effects of such processes on the structural materials commonly used in PZT-based MEMS devices.

2. EXPERIMENTAL PROCEDURE

2.1 Film DepositionIn the deposition of PZT films, the choice of bottom electrode or substrate stack is of critical importance to the

viability and reliability of any MEMS device that employs PZT. The most common conducting electrode is

Reliability, Testing, and Characterization of MEMS/MOEMS, Rajeshuni Ramesham, Editor,Proceedings of SPIE Vol. 4558 (2001) © 2001 SPIE · 0277-786X/01/$15.0072

platinum but its reactivity with Si at even moderate temperatures (400°C) necessitates a barrier layer, normally SiO2.Various oxides such as RuO2 and ZrO2, have been investigated due their ability to prevent the interdiffusion of Siand Pb and thus act as a suitable buffer layer.5 Samples for experiments were prepared using both platinum onsilicon nitride and zirconia on silicon oxide or silicon nitride (Figure 1). Since zirconia is non-conducting, the PZTis actuated using an interdigitated electrode scheme on top of the film. This results in the applied field being parallelto the substrate surface. When platinum is used, the field points along the surface normal.

In this work both the PZT (52/48) films and ZrO2 films were deposited via the sol-gel process.6,7 The PZTprocess is described as follows. Lead acetate trihydrate and 2-methoxyethanol (2MOE) were mixed and refluxed at115 °C for one hour in an argon atmosphere. The water of hydration was vacuum distilled at 115 °C under vacuum.Separately, zirconium n-propoxide and titanium iso-propoxide were dissolved in 2MOE at room temperature. Thezirconium and titanium precursors were then added to the lead acetate complex and refluxed for 2 hrs at 120 °C. Thesolution was again distilled and 2MOE was added to achieve the final desired concentration. The solution wasfiltered, dispensed onto a substrate and spun at 3000 rpm. The substrates were then placed on at hot plate at 300-450°C to evaporate the solvent and decompose the organic compounds. A rapid thermal anneal at 650°C for oneminute was performed to crystallize the layer. These steps produced PZT layers approximately 200 nm thick. Theprocess is repeated to achieve the desired thickness and then annealed in an oven at 700 °C for one hour to densifythe material. The zirconia films were prepared in a similar manner with film thicknesses of 50nm per coating.

Platinum films were sputter-deposited in a magnetron sputtering system. Typical process conditions were:base pressure 10-6 Torr, Ar pressure 10 mTorr, D.C. power 100 watts, substrate temperature 275°C. The elevatedtemperature during deposition was used to control the intrinsic stress in the platinum films.

Silicon nitride is often used in MEMS devices as a structural material due to its high modulus of elasticityand resistance to many wet etches. However, stoichiometric silicon nitride films have high intrinsic tensile stress(∼ 1Gpa) when deposited on silicon wafers. Recently, silicon-rich silicon nitride film deposition processes, with lowfilm stress <100 MPa, have been developed for MEMS applications.8 Prior studies have found the PZT sol-gel filmto possess a tensile stress of 100 Mpa when deposited on silicon/Silicon dioxide wafers. To minimize the overallstress in the stack, low stress LPCVD silicon nitride was employed as the structural material.

Figure 1: PZT stacks used in experiments.

2.2 Wet Pattern TransferIn a typical PZT process, all the stack layers are sequentially deposited over the whole wafer. After

deposition, the materials are etched in order to transfer a desired pattern to the films. Much research has beenconducted in the etching of thin layer ferroelectrics for use in non-volatile memory devices.9 Techniques include wetchemical etching,10 and dry etching methods such as, reactive ion etching (RIE),11 and reactive ion-beam etching(milling).12 Here we focus on the etching techniques for realizing PZT based MEMS structures made from PZT,zirconia and platinum thin films.

PZT can be wet etched in solutions containing hydrofluoric acid. Since the process is isotropic,undercutting of the mask occurs and is a problem when dimensions are small. The reaction produces insoluble metalfluoride byproducts that can be removed with HCl either during the etching or as a separate step. In many cases, it isdesirable to perform a subsequent clean-up etch in HCl, as this minimizes the overall undercut for the pattern. Wetetching is fast compared with dry methods, but the etch rate is not as easily controlled. Another problem is that HF

PZT

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silicon substrate

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silicon nitride

silicon substrate

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attacks the adhesion between the photoresist and PZT making long etches impractical. Our experimentsdemonstrated an etch rate of 120 nm/min using 10:1 buffered oxide etchant at room temperature.10 This approachcan be used to pattern thick (∼ 7 µm) PZT films with a 2:1 undercut.

Several acid and base solutions were used in an attempt at etching the zirconia buffer layer. Zirconiasamples were prepared on SiO2 substrates and placed in solution for various times; in some cases longer than anhour. The chemical and conditions used are listed in table 1. The results fell into two categories: either the zirconiadid not etch at all, or the etchant attacked the interface between the zirconia and substrate and the film cracked andpeeled off. In either case, no suitable etching solution was found for zirconia.

HF Room Temp. (RT) NAOH 70-80°CHCL RT, 70-80°C NH4OH RT, 70°C

H2C2O4 RT, 70-80°C H2O2:HCL:H2O RTH2SO4 RT, 70-80°C HCL:BOE:H2O RTHAc RT, 70°C HNO3;HF:H2O RT

HNO3 RT, 70°C

Table 1: Solutions tested to etch zirconia

Platinum films can be wet etched in aqua regia (HCL:HNO3) but the slow etch rate, 16 nm/min, combinedwith its attack of PZT makes this method unattractive. To our knowledge there are no other wet etch chemistries toetch platinum. It is evident from the above discussion that although wet etching can be used to pattern PZT films, itis not a suitable method for defining platinum and zirconium films. Thus, dry etching techniques were investigated.

2.3 Dry Pattern Transfer

PZT films, one micron thick, were prepared on Pt/SiO2/Si substrates to test the viability of ion beametching. Platinum films 500 nm thick and Zirconia films 500 nm thick were also prepared. All ion beam etchingwas performed in an Oxford series 300 dual beam system with a 15 cm etch source. Etching parameters such asbeam current, ion energy, and angle of incidence were varied to characterize the process. The observed etch ratesfor the films of interest are shown in Table 2. The reported values are for the following conditions: beam densitiesof 0.6 mA/cm2, energies of one keV, process pressure of 0.2 mTorr, and an angle of 45°.

PZT 22 nm/minZirconia 18 nm/minPlatinum 40 nm/min

Low stress Silicon Nitride 38 nm/min

Table 2: Ion beam etching rates

Figure 2 shows a PZT(0.8µm)/Pt(0.250)/Si3N4(0.5) stack etched by ion beam etching. A 2.7 µm thickpositive photoresist (Shipley 1800 series) hard baked at 150°C for ten minutes was used as a mask. After themilling, the resist was removed by an oxygen plasma etch. Figure 3 shows an enlarged view of the etching showingthat a resolution of a couple of microns is possible with this technique. We believe that using lower energies or aresist with better thermal stability this definition can be improved.

It is known that energetic ion bombardment of the type used in various etch process used here couldpossibly damage the ferroelectric films. A series of experiments was therefore performed to evaluate the affect ofion-beam etching on the ferroelectric and dielectric properties of the PZT films. Samples were prepared by spinning2 µm PZT films on Pt/SiO2/Si substrates. Circular platinum top electrodes were sputter deposited on one of thesamples using a shadow mask and the polarization versus electric field was measured using a commercial RT66A(Radiant Technology) ferroelectric test system. The ferroelectric hysteresis loop for this sample is shown in Figure4. The remnant polarization (Pr) and coercive field (Ec) were measured to be 24 µC/cm2 and 30 kV/cm respectively.To investigate the worst case scenario, samples without top Pt electrodes or any other masking material were ion-beam etched for 15 minutes with a beam current of 0.45 mA/cm2 and energies of one keV. The background pressure

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was 2x10-4 and the samples were held at 45 degrees to the beam while rotating. After etching, electrodes weresputtered on to the film, and the ferroelectric characteristics for these samples were measured (Figure 5). Thehysteresis loop is considerably distorted. A shift in the coercive field is observed in the ion-beam-etched samples.This shift is caused by an internal field Ei whose magnitude is given by,9

2−+ −

= cci

EEE .

The samples display an Ei of 4 kV/cm before etching and is increased to 84 kV/cm after the etch process. Theetched samples with electrode were then annealed at 450°C for fifteen minutes and the results are shown in Figure 6.After the annealing process the Ei is reduced to 8 kV/cm, demonstrating the recovery of the material.

Figure 2: Patterned PZT/Nitride cantilevers Figure 3: Etching resolution

Figure 4: Original PZT hysteresis loop prior to ion beam etching. Figure 5: PZT sample after ion beam etching.

Electric Field (kV/cm)

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Figure 6: PZT etched sample post anneal

During normal etching, a masking material is used to transfer the desired pattern to the PZT film. A testwas conducted to investigate the affect of photoresist as a mask during the ion-beam etching of PZT. Samples wereprepared by spinning 0.8 µm of PZT on Pt/SiO2/Si substrates and sputter depositing Pt top electrodes through ashadow mask. Positive photoresist (Shipley 1827) was spun on at 4000 rpm and baked at 155°C for 10 min. Thesamples were then exposed to an ion bombardment of 0.45 mA/cm2 and energies of 1100 eV for 30, 45, 60, 75, and90 minutes. The hard-baked resist was removed with oxygen plasma RIE for 60 min at 25 watts. The samples werethen annealed at 600°C for five minutes. Figure 7 shows the P-E loops before etching, after etching, and afterannealing. As is observed, there is little change in the ferroelectric behavior of the film, indicating that thephotoresist serves as an effective barrier to the energetic ions. The curves for the other etch doses are comparableand thus are not shown. To quantify the effect, the remanent polarization is plotted versus dose in Figure 8. Thedifference between the etched sample and the original is well within the measurement variance denoted by the errorbars. Figure 9 shows the dielectric constant for each etching time before and after etching and after annealing thesamples at 600°C for 5 min. Although there is a small decrease in the permittivity after etching, the value is fullyrecovered after annealing. Since the decrease in permittivity appears to be independent of etch time, it waspresumed that the effect originated from oxygen plasma process. To test this hypothesis the samples were exposedto the RIE plasma for an additional 30 and 60 minutes. No significant change was observed in the inducedpolarization, coercive field or permittivity. Since oxygen plasma etching does not seem to affect PZT films in anyappreciable way, it can be concluded that the reversible damage to PZT films is mostly arising from the ion millingprocess.

Ion milling offers a simple and single step technique for patterning the entire PZT stack. Although theetched edges showed ∼ 2 µm roughness, it can be improved by using better masking techniques. Any damage to thePZT arising from ion milling seems to be reversible and the film properties could be recovered back by a suitableanneal step. The major drawback of ion milling is its poor selectivity between the various materials in the PZTstack. Thus, control of etched depth will have to rely on etch time than on suitable etch stop layers. However, byappropriately designing the final release process, the need for an accurate etch stop can be reduced.

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Figure 7: Ferroelectric loops before, after etching, and after annealing.

Figure 8: Remanent polarization versus etching dose. Figure 9: Permittivity versus etching dose

2.3 Structure ReleaseA distinguishing feature of MEMS micromachining is the need to release three-dimensional structures by

removing sacrificial or spacer layers. Figure 2 shows a cantilever (PZT/Pt/Si3N4) that was released by etching the 2µm thick low temperature oxide spacer layer with an aqueous solution of HF:HCL:H2O (1:1:1). Since this acidmixture also attacks PZT quite easily and silicon nitride to a lesser degree, a double coat of photoresist was used asan encapsulation mask. Figure 10 shows an SEM of the free end of one of the 100 x 500 µm beams. Despite theencapsulation mask, the acid etchant attacks the adhesion of the resist causing a wicking effect that draws the acidand corrodes the PZT. Hard baking at elevated temperatures and vapor priming of an adhesion promoter showedlittle improvement. Other groups have used silicon nitride as an encapsulating material.13

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Figure 10: Damage cause to PZT during release Figure 11: XFl2 released cantilever

Stiction is another very serious problem associated with releasing structures in an aqueous environment. Itis a result of the capillary forces of the fluid that in some cases can cause catastrophic failure of devices or in lesssevere cases limit the yield. Many techniques have been devised to combat this problem and can be divided intothree lines of approach. The first aims at mechanically overcoming the surface tension of the fluid with bumps orstiffening ridges, the second makes use of low surface tension liquids while the third tries to avoid the liquid state alltogether. Examples of the latter include dry etching and critical point drying. We used drying in an IPAenvironment after the release to improve released cantilever yield when wet release process was used. However, asdiscussed, the wet release process damaged the PZT layer and instead a dry etch process was used. This methodcompletely avoided any issues with stiction.

It has been demonstrated that silicon can be chemically etched using vapor-phase xenon difluoride.14 Theetching takes place at room temperature with no plasma and is selective to a broad variety of materials, includingphotoresist and stoichiometric silicon nitride. The process is nearly isotropic with reported etch rates as high as 10µm/min. Since the etch is purely chemical, no ion induced damage to the piezoelectric film is to be expected, andsince the process occurs in the gas phase, stiction is almost completely mitigated. XeF2 etches in this work werepreformed on a commercial XeF2 etch tool (Xetch by Xactix Inc) operating in the pulse mode. In this mode ofoperation, XeF2 gas is introduced into the etching chamber and allowed to react for a specified time (60 min). Thechamber is then evacuated and purged, and the cycle starts over.

One hundred micron wide cantilevers of different lengths were patterned fromAu(200nm)/PZT(800nm)/Pt(200nm)/Si3N4(500 nm low stress) and Au(200nm)/PZT(8000nm)/ZrO2(300nm)/Si3N4(500nm)-layer stacks on silicon wafers. The cantilever structures were encapsulated withphotoresist and released by etching the silicon approximately 60 µm in depth. The total process time to achieve thiswas 240 minutes, however only about half of that corresponds to actual etching time. A XeF2 partial pressure of 2Torr and 10 Torr of nitrogen were maintained during the etching. It has been reported that the silicon-rich lowstress silicon nitride is consumed by XeF2 gas unless nitrogen is added to the process.15 A silicon nitride test samplewas used to determine the etch rate of the low-stress silicon nitride film used in this work under the etchingconditions used here. We found that 500nm of low stress silicon nitride were completely removed in 120 minutes ofetching despite introducing nitrogen gas during the etching cycle. Figure 11 shows one of the cantilevers releasedusing XeF2 release process. Although there is concern with the low-stress nitride being removed, similarly released

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cantilevers with PZT and zirconia layers on silicon nitride in the stack showed the proper piezoelectric responseattesting to the viability of the XeF2 process. Thus the XeF2 process provides a reliable way to release PZT basedMEMS structures without damaging the PZT and has the additional advantage of avoiding stiction related problems.

In Figure 11, the cantilever is seen to be bending upwards due to a stress differential in the stack. Since thefocus of our effort was into investigating high reliability fabrication processes for PZT based MEMS structures,particular attention was not paid to the individual stresses in the various films. A combination of stresscompensated stack layers with cantilever definition using ion milling and XeF2 release techniques will be the focusof our future effort.

3. SUMMARY AND CONCLUSIONS

PZT films have been etched in a number of acidic solutions. Although these chemicals etch PZT films, thezirconia buffer layer or platinum bottom electrodes are not etched in them. Ion beam etching was investigated topattern these materials. We found acceptable pattern resolution (2µm) using standard positive photoresist as a mask.To test the effect of ion bombardment on the ferroelectric properties of PZT, samples were prepared with andwithout a photoresist mask. The experiments revealed that the ion flux at energies of one keV does degrade theferroelectric properties of the material. However, this deterioration is fully recoverable by annealing at 450°C inambient air. The samples protected by resist show no loss in remnant polarization and only a moderate decrease inpermittivity, which was also recoverable by annealing.

Since MEMS structures often need to be released by etching a sacrificial spacer material, two methodswere investigated and are compared. The first is a common wet etching of silicon oxide using HF solution. Theproblems associated with this method are stiction and damage to the PZT. To avoid these, gas phase xenondifluoride etching of silicon was studied. Cantilever structures were fabricated with PZT on zirconia and PZT onplatinum. It was found that the xenon process etched the low-stress silicon nitride sample even with a partialpressure of nitrogen. Both types of cantilevers were observed to curl upwards signifying a stress mismatch in thefilms and the need to control the residual stress.

ACKNOWLEDGMENTS

This work was sponsored by a SBIR Phase I (AF01-099) Award and by Northrop Grumman. The authorswould like to thank the Electrical and Computer Engineering department at Carnegie Mellon University for use oftheir Xenon Difluoride etch tool.

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