Galvanic Corrosion of Miniaturized Polysilicon Structures

Electrochemical and Solid-State Letters, 8 共9兲 G223-G226 共2005兲

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1099-0062/2005/8共9兲/G223/4/$7.00 © The Electrochemical Society, Inc.

Galvanic Corrosion of Miniaturized Polysilicon Structures Morphological, Electrical, and Mechanical Effects David C. Miller,z Ken Gall, and Conrad R. Stoldt Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427, USA The morphological, electrical, and mechanical effects generated by the galvanic corrosion of polysilicon immersed in a standard aqueous HF solution are described. Micromachined test structures consisting of phosphorus-doped polysilicon in contact with a gold metallization layer are examined. Corroded test structures demonstrate a heterogeneous cracking or porosity across the polysilicon surface, a greatly increased electrical resistance, and a decrease in the characteristic frequency of mechanical resonators. This first systematic study demonstrates the impact of corrosion on miniaturized structures, indicating a potential impact upon the material properties, design, performance, fatigue, tribology 共friction/ wear兲, and manufacture of micro- and nanoscale devices. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.1960009兴 All rights reserved. Manuscript submitted April 14, 2005; revised manuscript received May 5, 2005. Available electronically July 18, 2005.

The fabrication and assembly of micro- and nanoscale sensors and actuators relies on chemical processing steps for the realization of mechanically freestanding devices. Early studies in microelectromechanical systems 共MEMS兲 hinted at the possibility of adverse effects of processing on device performance. For example, with the immersion of polycrystalline silicon 共poly-Si兲 in aqueous hydrofluoric acid 共HF兲 solution, the modulus and fracture strength varied with acid concentration and exposure time.1 Another study demonstrated morphology changes similar to stress corrosion cracking, with delamination occurring between poly-Si layers for material immersed in HF.2 Others have noted a change in surface roughness, decrease in fracture strength, and alteration of fracture morphology for poly-Si immersed in HF for extended periods of time.3 Some of these specimens were studied using transmission electron microscopy 共TEM兲 and have demonstrated the formation of amorphous surface layers at the top and bottom of poly-Si, and with internal veins of porous poly-Si.4 Last, an initial study demonstrated that nonuniform etching occurred preferential to the grain boundaries of poly-Si immersed in HF, even when the acid was in vapor form.5 Many of the aforementioned studies make use of phosphorus-doped poly-Si,2,3,5 but mechanical property variations are reported in undoped material as well.1 The addition of an electrical bias is also shown to further enhance the corrosion process.3 For example, silicon wafers patterned with metallic contacts are etched autonomously during immersion in HF solution, according to a process known as galvanic corrosion.6 A galvanic cell develops when any two materials of different electrochemical potential are brought together in an electrolytic solution, resulting in a spontaneous oxidation-reduction reaction. For instance, consider a gold-contacted poly-Si device immersed in an aqueous HF solution. The oxidizing agent 共i.e., dissolved oxygen in solution兲 is reduced at the gold surface 共cathode兲, inducing electrical current flow that initiates oxidation of the poly-Si surface 共anode兲. Alternately, two differently doped poly-Si layers can generate a galvanic cell, because of their intrinsically different electrochemical potential. Corrosion is driven by the difference in electrochemical potential, and its extent by the relative exposed surface areas of each material layer. Unfortunately, researchers lack a systematic understanding of the effects of wet chemical processing on miniaturized devices. Some insight into these effects is gained from the study of porous silicon processing, where near-surface porosity is generated by applying an external electrical bias to single-crystal silicon wafers immersed in an HF solution. Since porous silicon finds application in photonic devices and sensors, its electrical and morphological properties have been studied in detail.7,8 While the chemistry of silicon corrosion remains unclear, the corrosion process in aqueous HF solution is believed to occur by the injection of holes 共hVB兲 into the valence band of the silicon. The overall reaction is described in Eq. 1 and 26

z

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Si + 2H+ + 2hVB → Si4+ + H2

关1兴

Si4+ + 6HF → SiF2− 6 + 6H

关2兴

In addition to single-crystal material, externally anodized poly-Si has been studied. This process yielded a nanoporous surface layer above a voided bulk material that exhibits etching with nanoporous regions along grain boundaries.9 If sufficient current is present, the corrosion process may alternately result in thick oxide formation in the electropolishing regime.10 In our research, we have observed thermal11,12 and electrostatic13 actuators rendered completely nonfunctional as a result of galvanic corrosion during aqueous HF chemical processing. Drastic changes in device performance were observed, even for structures fabricated without specific design considerations. The effects of corrosion at surfaces and grain boundaries may become even more prevalent as device miniaturization is continued, since the surface area to volume ratio increases rapidly with decreasing scale. The goal of this work is to better understand the morphological, electrical, and mechanical effects resulting from the galvanic corrosion process. Quick and thorough understanding are paramount, since the corrosion phenomenon has not yet been studied in detail, whereas microsystems technology is currently being commercialized. This Letter, therefore, describes new studies directed at determining the effects of galvanic corrosion on gold-contacted poly-Si microstructures after immersion in aqueous HF solution. A series of designated test structures were fabricated using the multiuser MEMS process 共MUMPS兲 provided by the MEMSCAP Corporation.14 Briefly, the process can be used to create structures having one fixed and two free layers of phosphorus-doped poly-Si, with material thickness in the micrometer range. Each poly-Si layer has a different material thickness and dopant concentration. In one of the final steps of the standardized fabrication procedure, a 0.5 ␮m thick layer of gold is adhered to the topmost poly-Si layer using a 20 nm thick chromium layer. All of the material layers, including the sacrificial phosphosilicate glass 共PSG兲 interlayers are patterned using photolithographic techniques. The finished poly-Si microstructures were mechanically released through removal of the PSG layers by immersion in 48% weight aqueous HF solution for the duration of 5, 10, 15, 20, 25, and 90 min. The first test structure utilized in this study was the laterally driven resonator 共or comb drive兲,13 shown in Fig. 1. Pairs of resonators were fabricated with and without gold on the wire bond pads used to connect to and actuate the devices. Using an HP 33120A waveform generator, a sinusoidal ac plus dc offset signal was supplied to actuate the resonators. Resonator evaluation was performed on a vacuum probe station 共MMR Technologies, Inc.兲 at pressures 艋15 mTorr, thereby enhancing the quality factor of the resonance. For the experimental setup, the nominal resonant frequency of 38 kHz could be visually determined to within ±0.06%, i.e., Q ⬎ 825. A series of traces was fabricated to investigate the electrical effects of the galvanic corrosion process, Fig. 1. The traces were ter-

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Figure 1. SEI micrograph of a micromachined poly-Si resonator 共a兲 and electrical trace 共b兲. The shuttle of the structure 共a兲 resonates, if an electrical bias is applied between the opposing wire bond pads used for electrical interconnection. The trace 共b兲 consists of a single wire between two wire bond pads.

minated at both ends by wire bond pads, the same as that used for the resonators, also shown in Fig. 1, however with the gold layer added. The width of the 0.5 ␮m thick traces was fixed at 4 ␮m, with the aspect ratio 共AR兲, defined as the ratio of length to width, being 5, 50, 510, or 4800. This corresponds to a surface area ratio 共SAR兲 of 1.17, 1.13, 0.80, and 0.21 共gold:poly-Si兲 for the traces of said geometry. That is, all traces have an equivalent area of metallization with different amounts of exposed poly-Si. Electrical measurements were performed using two HP 34401A multimeters to separately monitor the current and voltage supplied to an individual trace by the power supply. A Micro-Manipulator 4000 probe station was used to achieve electrical contact to the specimens. Measurements were

made using two 共single tip兲 probes, with the nominal 共probe to probe兲 resistance being 4.9 ± 1.0 ⍀ when both probes were contacted to the gold on a single wire bond pad. Secondary electron imaging 共SEI兲 and backscatter electron imaging 共BEI兲 were performed using a JEOL 5910 scanning electron microscope. An acceleration voltage of 10 keV at the working distance of 20 mm was used to limit the extent of beam penetration into the specimens, thereby increasing the spatial resolution and mitigating any sample charging. Images were taken with the specimen inclined at 60° relative to the electron beam. Morphological effects resulting from the galvanic corrosion process are shown in Fig. 2. SEI of the fixed poly-Si layer surrounding a bond pad, with and without a gold coating, are shown in Fig. 2a and b, respectively. When no metal is added, the surface topography consists of an irregular grain structure. When metal is added to the bond pads, SEI reveals localized cracks within the material and a less distinct surface texture. The BEI of the corroded poly-Si 共not shown兲 reveals nonuniform contrast indicating a roughened surface or more exposed grain boundaries not seen in the reference material. This is validated using optical microscopy, where the same localized poly-Si corrosion is observed as a darkening or staining of the material. The nonuniform poly-Si corrosion observed across the surface of test structures is likely due to the stochastic nature of the corrosion process as well as the proximity of the poly-Si to the metallization layer. The poly-Si trace structures were immersed in aqueous HF solution for varying times. Figures 3 and 4 summarize the electrical effects of galvanic corrosion on the poly-Si. Figure 3 shows the measured electrical resistance of the 510 AR trace as a function of actuation voltage for exposure times to aqueous HF solution including 0, 5, 10, 15, and 20 min. For most poly-Si traces examined, the measured resistance was found to slightly increase with increasing actuation voltage, which is thought to be associated with Joule heating of the traces occurring as a result of the test itself. Resistance increased much more substantially 共logarithmically兲, when traces on separate dice were exposed to aqueous HF solution for more prolonged periods of time. Figure 4 summarizes the electrical measurements for all traces as a function of HF immersion time. The results shown in Fig. 4, also in logarithmic scale, demonstrate a significant percentage increase in resistance relative to the reference sample. In general, the shortest traces 共smallest AR兲 were most affected by exposure to HF. Resistance increases greater than 100% were not uncommon for all AR tested. While not illustrated in Fig. 3 and Fig. 4, extended HF immersion led to the measured resistance exceeding 10 M⍀, which is the input impedance value for the test equipment. Mechanical testing was performed using the resonators of the type shown in Fig. 1, with measured results given in Table I. In general, the addition of a metal layer to the bond pads decreased the resonant frequency of the comb drives after HF release when compared to adjacent resonator devices with no added metal released under identical conditions. A decrease in resonant frequency of 4-6% was typical, with the average being 4.6 ± 2.3%. The results reported in Table I apply to the resonators constructed using the first free poly-Si layer. The mechanical results from this set of test structures are consistent with results from an alternate series of devices. For example, the resonant frequency of cantilever beams was seen to decrease by as much as 2.2%, when metal was added. During exposure to aqueous HF solution, the formation of a galvanic cell impacts the morphological, electrical, and mechanical properties of thin film poly-Si. First, the SEI shown in Fig. 2 indicates that cracking or voiding may occur in the material as the result of galvanic corrosion. The heterogeneous nature of the galvanic corrosion may result from the production of hydrogen gas during the etching process,15 which can prevent wetting of the HF solution to the oxidized silicon surface.15 In addition to the attack on material grain boundaries, the depletion of phosphorus dopant cannot be ruled out in the corrosion process. Dopant depletion could facilitate electrical current flow, Eq. 1. Note that while the dopant concentration has not been measured in test specimens, the observed morphol-


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Figure 3. Results of I-V characterization performed on 510 AR traces on separate dice immersed in aqueous HF solution for 0, 5, 10, 15, and 20 min.

modulus of the poly-Si. Porosity will also influence the fracture and fatigue characteristics of the material. Similarly, thick oxide formation may yield analogous effects. For example, the surface oxidation of poly-Si was determined to be significant enough to promote cycle-dependent failure in MEMS test structures.16 Change in modulus or device lifetime is certain to impact a precise analysis and representation of micro- and nanoscale devices. Therefore, material properties may have to be specified according to the chemical postprocessing procedure used. The impact of chemical corrosion on thermally and electrostatically driven devices promises to become more prevalent in the future. Device dimensions and complexity will continue to evolve with technological demands, resulting in a significant increase in the respective surface area vulnerable to chemical attack. In other cases, lengthy HF exposure will be required because etch-release holes are not viable in some device designs, or alternately, essential fluid circulation may not be possible due to some device packaging schemes.17 If the material’s surface is affected, corrosion occurring during mechanical release or even earlier, for example, during the surface oxide removal in advance of poly-Si deposition, may also impact tribological properties such as friction and wear. The study of galvanic corrosion effects on poly-Si micro- and nanostructural components is a new and necessary direction for

Figure 2. SEI micrographs of the poly-Si surface 共a兲 with and 共b兲 without a gold layer added to the wire bond pads. Both surfaces were imaged after being immersed in aqueous HF solution for more than 20 min.

ogy changes including attack and removal of poly-Si material from grain boundaries and the free surface validate measured changes in electrical and mechanical performance. The consequences of galvanic corrosion during MEMS postprocessing have serious implications for both design and device application. Based on a simple rule of mixtures argument, the increased porosity from corrosion will decrease the effective mechanical

Figure 4. Change in electrical performance for traces of AR 5, 50, 510, and 4800 immersed in aqueous HF solution for 5, 10, 15, 20, and 25 min.


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Table I. Change in resonant frequency for resonators constructed using the first free structural polysilicon layer as a function of immersion time in aqueous HF solution.

Time 共min兲 Change in resonant frequency 共%兲

5

10

15

20

25

Average

Standard deviation 共%兲

1.19

3.57

6.46

6.92

4.71

4.57

2.32

small-scale device design and fabrication. Study is needed not only because the previously unforeseen effects have been demonstrated to be significant, but also because the related technology is currently being commercialized as well as being integrated into many fields of study. Future work will include additional test structures to enable the characterization of residual stress and strain, as well as material modulus. Furthermore, we plan to investigate the other wet chemistries commonly utilized by the MEMS community for the postprocessing of micromachined poly-Si sensors and actuators. Acknowledgments The authors acknowledge support from the University of Colorado at Boulder for this work. D.C.M. also acknowledges support

from a Sandia National Laboratories ESSI fellowship, assistance from Nancy Yang, Miles Clift, and Jeff Chames, as well as microscopy performed at Sandia’s Livermore facilities. University of Colorado at Boulder assisted in meeting the publication costs of this article.

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