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Study of glass frit induced stiction using a micromirror array
Microelectronics Reliability 52 (2012) 2256–2260
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Study of glass frit induced stiction using a micromirror array F.Z. Ling a,b,⇑, J. De Coster a, A. Witvrouw a, J. Celis a, I. De Wolf a,b a b
IMEC, Leuven, Belgium Dept. Metallurgy and Materials Engineering, KULeuven, Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 3 June 2012 Accepted 9 June 2012 Available online 12 July 2012
a b s t r a c t Adhesion caused by various forces can lead to either permanent or temporary stiction. This remains one of the most critical reliability issues for micro-electro-mechanical system (MEMS). In this paper, we present a study using a poly-SiGe micromirror array as a ‘‘stiction sensor’’ to investigate the stiction induced by outgassing during the glass frit bonding process. By comparing the width of the ‘‘pull-in windows’’, the effect of the induced adhesion between the micromirror and its landing electrode can be identified and quantified. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Due to their large aspect ratio and micro-scale dimensions, microelectro-mechanical systems (MEMS) are highly sensitive to interfacial forces such as capillary, electrostatic, van der Waals forces and other kinds of chemical interactions or bonds [1–4]. When a MEMS device operates in an uncontrolled environment, these forces can be the cause of what is often referred to as ‘‘in-use stiction’’. In order to avoid this, hermetic packaging is required. Packaging is an essential technological step for encapsulating and protecting sensitive MEMS structures such as accelerometers, resonators, micromirrors and gyroscopes against atmospheric influences, especially moisture. In other words, most MEMS devices must operate in a hermetic package for achieving better performance and reliability [5–8]. In the past decades, glass frit bonding has been one of the most widely used bonding materials for wafer level vacuum packaging. However, the impact of outgassing during the glass frit bonding process on the reliability of the packaged MEMS, especially in terms of stiction, has not been reported yet. Furthermore, on-chip in-use stiction is also difficult to measure directly. In this study, a SiGe micromirror array is used as a ‘‘stiction sensor’’ to study the effect of outgassing during the glass frit bonding process. 2. Experimental details 2.1. Glass-frit outgassing procedures The glass frit used in this study is FX11–036 from Ferro Corp. To achieve a bonding temperature below 450 °C, the main composiAbbreviations: LDV, Laser-Doppler vibrometer; AR-XPS, Angle resolved X-ray photoelectron spectroscopy; CVD, Chemical vapor deposition; CMP, Chemical mechanical polishing. ⇑ Corresponding author. Present address: IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. Tel.: +32 16 28 19 77; fax: +32 16 28 85 00. E-mail address: [email protected] (F.Z. Ling). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.06.009
tion of the glass frit is low melting point lead or lead silicate glass. The bonding procedure includes thermal treatment and compressive bonding steps. Most of the solvent and organic additives can be removed during the thermal treatment step [9]. However, the compressive bonding process at a higher temperature could lead to further outgassing from the glass frit. The glass frit outgassing experiment was carried out and adapted according to the standard bonding procedures. Firstly, the glass frit was deposited on a carrier wafer and loaded onto a heating stage in a vacuum chamber. At a pressure of 10 mbar, the glass frit was heated up to around 120 °C for 10 min in order to diffuse out the solvents. Next the glass frit was heated up to an intermediate temperature around 325 °C for 15 min to get rid of the organic additives. Subsequently, the MEMS stiction sensor sample was loaded into the chamber while the glass frit was heated up to 425 °C for 10 min. Fig. 1 shows a schematic representation of the setup used for this experiment and the detailed temperature profile of the outgassing process. 2.2. Device structure, fabrication and operation The MEMS devices investigated in this study are electrostatically actuated micromirror arrays made of poly-SiGe (see Fig. 2). The fabrication of the samples starts from a silicon wafer with a silicon oxide layer to mimic the CMOS backend and a SiC passivation layer. The SiC layer is used to protect the oxide from the HF release step later on. On the SiC layer, a 400 nm SiGe electrode is deposited by Chemical Vapor Deposition (CVD). On top of SiGe, a 250 nm SiO2 hard mask is deposited and patterned to define the electrodes. Next, a second 200 nm thick SiGe layer (used for the 600 nm thick electrode) is patterned by using a resist mask, together with the first SiGe layer protected by the hard mask. To create flat mirrors, a 1200 nm SiO2 layer is deposited and planarized by chemical– mechanical polishing (CMP) down to the electrode level. Then, a 400 nm Si-oxide sacrificial layer is deposited and patterned to
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F.Z. Ling et al. / Microelectronics Reliability 52 (2012) 2256–2260 Table 1 Summary of the micromirror parameters. No.
L W (lm)
Hinge length (lm)
Electrodes thickness (nm)
A B C
88 20 20 20 20
1.5 3 4
600 400 400
Fig. 1. (a) Glass frit outgassing setup. (b) Temperature profile of the outgassing process.
Fig. 3. Typical displacement–voltage characteristic of an electrostatically actuated device.
the mechanical restoring torque acting on the mirror are equal. As the applied voltage increases, the micromirror is electrostatically attracted towards the electrode and at a certain value the electrostatic torque exceeds the mechanical restoring torque and the micromirror pulls in. The corresponding voltage value is called the pull-in voltage. After pull-in, one edge of the micromirror touches the landing electrode. Once the micromirror and the landing electrode come into contact, the adhesion force between both surfaces starts affecting the operational characteristic. As the applied voltage decreases, from a certain value the mechanical restoring torque again exceeds the combination of the electrostatic torque and the torque generated by the adhesion force, and thus the mirror releases. This occurs at a voltage which is called pullout voltage. A typical pull-in/pull-out characteristic is shown in Fig. 3. Fig. 2. Top-view SEM picture of (a) a single micromirror (20 20 lm), (b) a micromirror array and (c) schematic view of a single micromirror.
construct the anchors. A 330 nm optimized poly-SiGe structural layer is deposited by CVD at 450 °C followed by a CMP step, resulting in 300 nm thick flat mirrors. The micromirrors are laid out in arrays of either 40 40 (8 lm) or 16 16 (20 lm) devices with two different mirror sizes. The 300 nm thick micromirror is attached to two anchors by narrow torsion hinges. Under each micromirror there are two actuation and two landing electrodes with a height of 0.6 or 0.4 lm, resulting in a gap between the mirror and the electrodes of 0.4 and 0.6 lm, respectively. The length of the hinge is 1.5 lm, 3 lm or 4 lm. A schematic view of a single mirror is shown in Fig. 2. Detailed parameters are listed in Table 1. When the micromirror is electrostatically actuated by an applied voltage, it tilts and the angle of rotation depends on the actuation voltage applied between the micromirror and bottom electrodes. The mechanical restoring torque is a linear function of the tilt angle; the electrostatic force is a quadratic function of the same angle. In static equilibrium, the electrostatic torque and
2.3. Laser Doppler vibrometer The characterization of the micromirror array was performed in vacuum (1 mbar) at 125 °C. The micromirror array is actuated with a bipolar triangular waveform of frequency 1 kHz. The capacitance change of a single micromirror within an array cannot be measured by a regular electrical method. Hence the out-of-plane movements of individual mirrors were recorded by a Polytec laser-Doppler
Fig. 4. Experimental setup for tracking and recording device displacement.
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vibrometer (LDV). The laser is pointed at a spot near the edges of the mirrors, where the largest displacement occurs. From these optical measurements, the displacement-versus-voltage characteristic can be recorded. A schematic view of the measurement setup is shown in Fig. 4.
3. Results and discussion 3.1. Surface characterization The freshly released micromirror arrays were inspected by an optical microscope to check for any contamination or defects. Before testing, each device was also inspected for any curvature or buckling. Since it is not possible to measure the roughness of the bottom electrodes without destroying the micromirror, several micromirror arrays from the same wafer but different dies were removed in order to inspect the electrodes underneath as well as the backside of the micromirrors. Typical AFM topographic images (1 1 lm2) are presented in Fig. 5. The average root mean square (RMS) roughness of the reference thick and thin electrodes and the backside of micromirror are 2.16 nm, 6.53 nm and 0.39 nm, respectively. As described before, after formation of the thick and thin electrode, a 1200 nm
sacrificial oxide layer is deposited. Such a layer is then planarized by chemical–mechanical polishing (CMP). The CMP stops on top of the thick electrode. Because of this CMP step, the thick electrode exhibits a lower roughness than the thin electrode. Similar RMS values were measured on the arrays after they had been exposed to glass frit outgassing. This suggests that most of the solvent and organic additives in the glass frit were indeed burned out during the two pre-anneal steps. To further analyze the composition of the surfaces, samples were inspected by X-ray photoelectron spectroscopy (AR-XPS). The measurements were carried out in AR-mode using a Theta 300 system. Sixteen spectra were recorded at exit angles between 22° and 78° as measured from the perpendicular direction of the sample. The measurements were performed using a monochromated Al Ka X-ray source (1486.6 eV) and a spot size of 400 lm. Fig. 6 shows the C and Pb spectra of the sample surface before and after exposure to glass frit outgassing. The atomic concentration measured at 21.88° from the perpendicular direction is shown in Table 2. It is clearly shown that more C and some Pb contamination were found after exposure to glass frit outgassing. The increasing amount of C and Pb can be attributed to further outgassing of organic additives together with Pb from the glass frit at higher temperature. Thus these contaminations can land on the surface and possibly lead to unwanted attracting forces during
Fig. 5. Comparison of AFM topographic images (1 1 lm2) of the landing electrodes (a), (b) and the backside of micromirror, (c) before and after being exposed to glass frit outgassing. The average RMS roughness of (a)–(c) are 2.16 nm, 6.53 nm and 0.39 nm respectively.
Fig. 6. Overlay Pb4f (a), C1s (b) spectra of the outgassing and reference surfaces.
F.Z. Ling et al. / Microelectronics Reliability 52 (2012) 2256–2260 Table 2 Concentration measured at 21.88° from the perpendicular direction. A: sample after outgassing; B: reference sample. Sample
O1s
Si2p
Si2pO
C1s
Ge3d
Ge3d (GeO)
Ge3d (GeO2)
Pb4f
A B
22.19 37.32
1.89 7.19
5.37 9.24
48.82 13.12
8.88 19.08
2.23 3.63
8.29 10.41
2.32 0.00
contact which may increase the adhesion force. To check this, the pull-out behavior of the mirrors is investigated as discussed in the following paragraph.
3.2. Optical measurement As reported in our previous work, we analyze stiction of the mirrors by studying the hysteresis in the displacement-versusvoltage characteristics of electrostatically actuated micromirrors. The anomalous widening of the ‘‘pull-in window’’, caused by a stiction-induced delay of the pull-out, is used as an indication of the amount of mirror-to-surface stiction. The differences in the pullout voltage of mirrors within an array indicate varying levels of stiction from device to device. This stiction is mostly induced by the capillary forces that originate from moisture absorbed from the atmosphere. It can be significantly reduced by either applying a hydrophobic self-assembled-monolayer or annealing the devices in vacuum at an elevated temperature [10].
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In order to decouple the effect of moisture-induced stiction described above from the effect of glass frit induced stiction, the micromirror stiction sensor sample was first annealed in a vacuum chamber at 125 °C and measured in situ inside the chamber at 1 mbar. Eight to sixteen micromirrors within array A were tested in each measurement. As shown in Fig. 7a, the displacement of eight micromirrors from type A array are plotted as a function of the applied voltage. It was observed that all the micromirrors within this array consistently pulled in at 33.2 V and pulled out at 21.7 V with a relatively small spread. The displacements from the same micromirrors after glass frit outgassing are shown in Fig. 7b. Although the pull-in voltage (33.1 V on average) of all the micromirrors is equal to the value that was measured on the clean devices (i.e. before exposure of the sample to glass frit during the bonding step), the pull-out voltage shifts down by 2.5 V to 18.2 V, thus showing a widening the ‘‘pull-in window’’ as indicated by the arrows in Fig. 7. Since both measurements were performed in the vacuum chamber at an elevated temperature, the cause of the difference in the pull-out voltages cannot be due to capillary forces but only due to the effect of glass frit outgassing. Fig. 8 shows the results obtained from micromirrors (array C) that are much more sensitive to the adhesion forces due to their weaker torsional hinges and larger contact areas. The spread of the pull-out voltage increases from 0.18 V to 0.55 V before and after the outgassing experiment, respectively. The average pullout voltage shifts down by 0.5 V and some of the micromirrors become permanently stuck. Again, the significant decrease of the
Fig. 7. Measured displacement-versus-voltage characteristic from array A: (a) before glass frit outgassing and (b) after glass frit outgassing.
Fig. 8. Measured displacement-versus-voltage characteristic from array C: (a) before glass frit outgassing and (b) after glass frit outgassing.
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Table 3 Detailed results of tested samples and standard deviation (l is the mean value; r is the standard deviation, Cv is the coefficient of variation). No.
Pull-in (±) [V]
l
r
Pull-out (±) [V] Cv (%)
l
r
Cv (%)
0.54 0.59 0.80
21.65 5.10 3.65
0.12 0.23 0.18
0.54 4.50 5.05
0.56 0.62 0.95
18.19 3.55 3.1
0.11 0.34 0.55
0.62 9.54 17.74
Before glass frit outgassing A B C
33.15 7.75 5.05
0.18 0.05 0.04
After glass frit outgassing A B C
33.12 7.75 4.98
0.18 0.05 0.05
outgassing. This indicates that the adhesion forces do indeed increase, and may lead to stiction. AR-XPS analysis on the contacting surfaces after exposure to the glass frit bonding step shows an increased amount of C and Pb which can be the cause of the raising adhesion force. Acknowledgments The authors would like to thank Alain Moussa for doing the AFM measurements and the support the SBO project ‘GEMINI’ (‘Generic Electronics and Microsystems INtegration Initiative’), IWT-nr 60046, 2007–2011. The authors also would like to thank IMEC REMO and MEMS team for their kind help. References
average pull-out voltage is attributed to the increased adhesion forces between the backside of the micromirrors and the landing electrodes. The increased spread of the pull-out voltage is attributed to random distribution of outgassed contaminants, as well as to the random nature of the bottom electrode roughness and effective contact area between mirrors and bottom electrodes. A detailed summary of the results is shown in Table 3. 4. Conclusions Poly-SiGe micromirror arrays were used as a ‘‘stiction sensor’’ to study the effect of adhesion forces induced by glass frit bonding outgassing. The results show that thermal treatment of the glass frit before bonding can indeed remove most of the solvent and organic additives. Nonetheless, this does not guarantee that no extra elements will outgas during the compressive bonding step at a higher temperature. A clear widening of the ‘‘pull-in window’’ is observed on micromirrors that have been exposed to the glass frit
[1] Bhushan B. Adhesion and stiction: mechanisms, measurement techniques, and methods for reduction. J Vac Sci Technol B 2003;21:2262–96. [2] Maboudian R, Howe R. Critical review: adhesion in surface micromechanical structures. J Vac Sci Technol B 1997;15:1–20. [3] Van Spengen WM, Puers R, De Wolf I. On the physics of stiction and its impact on the reliability of microstructures. J Adhesion Sci Technol 2003;17:563–82. [4] de Boer M, Knapp J, Clews P. Effect of nanotexturing on interfacial adhesion in MEMS. Proc Int Conf Fracture, Hawaii 2001;16:1–6. [5] Sparks D, Trevino J, et al. An all-glass chip-scale MEMS package with variable cavity pressure. J Micromech Microeng 2006;16:2488–91. [6] Chang H et al. High hermetic performance of glass frit for MEMS package. In: Proceedings of microsystems packaging assembly and circuits technology conference (IMPACT), 5th international; 2010. [7] Knechtel R. Glass frit bonding: an universal technology for wafer level encapsulation and packaging. Microsyst Technol 2005;12:63–8. [8] Sparks D, Massoud-Ansari S, Najafi N. Reliable vacuum packaging using nanogetters TM and glass frit bonding. Proc SPIE 2004;5343:70–8. [9] Sparks D, Massoud-Ansari S, Najafi N. Long-term evaluation of hermetically glass frit sealed silicon to Pyrex wafers with feedthroughs. J Micromech Microeng 2005;15:1560–4 [Aug.]. [10] Ling FZ, De Coster J, Lin W-Y, Witvrouw A, Celis J-P, De Wolf I. Investigation of temporary stiction in poly-SiGe micromirror arrays. Sens Actuators, A, in press.
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Study of glass frit induced stiction using a micromirror array F.Z. Ling a,b,⇑, J. De Coster a, A. Witvrouw a, J. Celis a, I. De Wolf a,b a b
IMEC, Leuven, Belgium Dept. Metallurgy and Materials Engineering, KULeuven, Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 3 June 2012 Accepted 9 June 2012 Available online 12 July 2012
a b s t r a c t Adhesion caused by various forces can lead to either permanent or temporary stiction. This remains one of the most critical reliability issues for micro-electro-mechanical system (MEMS). In this paper, we present a study using a poly-SiGe micromirror array as a ‘‘stiction sensor’’ to investigate the stiction induced by outgassing during the glass frit bonding process. By comparing the width of the ‘‘pull-in windows’’, the effect of the induced adhesion between the micromirror and its landing electrode can be identified and quantified. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Due to their large aspect ratio and micro-scale dimensions, microelectro-mechanical systems (MEMS) are highly sensitive to interfacial forces such as capillary, electrostatic, van der Waals forces and other kinds of chemical interactions or bonds [1–4]. When a MEMS device operates in an uncontrolled environment, these forces can be the cause of what is often referred to as ‘‘in-use stiction’’. In order to avoid this, hermetic packaging is required. Packaging is an essential technological step for encapsulating and protecting sensitive MEMS structures such as accelerometers, resonators, micromirrors and gyroscopes against atmospheric influences, especially moisture. In other words, most MEMS devices must operate in a hermetic package for achieving better performance and reliability [5–8]. In the past decades, glass frit bonding has been one of the most widely used bonding materials for wafer level vacuum packaging. However, the impact of outgassing during the glass frit bonding process on the reliability of the packaged MEMS, especially in terms of stiction, has not been reported yet. Furthermore, on-chip in-use stiction is also difficult to measure directly. In this study, a SiGe micromirror array is used as a ‘‘stiction sensor’’ to study the effect of outgassing during the glass frit bonding process. 2. Experimental details 2.1. Glass-frit outgassing procedures The glass frit used in this study is FX11–036 from Ferro Corp. To achieve a bonding temperature below 450 °C, the main composiAbbreviations: LDV, Laser-Doppler vibrometer; AR-XPS, Angle resolved X-ray photoelectron spectroscopy; CVD, Chemical vapor deposition; CMP, Chemical mechanical polishing. ⇑ Corresponding author. Present address: IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. Tel.: +32 16 28 19 77; fax: +32 16 28 85 00. E-mail address: [email protected] (F.Z. Ling). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.06.009
tion of the glass frit is low melting point lead or lead silicate glass. The bonding procedure includes thermal treatment and compressive bonding steps. Most of the solvent and organic additives can be removed during the thermal treatment step [9]. However, the compressive bonding process at a higher temperature could lead to further outgassing from the glass frit. The glass frit outgassing experiment was carried out and adapted according to the standard bonding procedures. Firstly, the glass frit was deposited on a carrier wafer and loaded onto a heating stage in a vacuum chamber. At a pressure of 10 mbar, the glass frit was heated up to around 120 °C for 10 min in order to diffuse out the solvents. Next the glass frit was heated up to an intermediate temperature around 325 °C for 15 min to get rid of the organic additives. Subsequently, the MEMS stiction sensor sample was loaded into the chamber while the glass frit was heated up to 425 °C for 10 min. Fig. 1 shows a schematic representation of the setup used for this experiment and the detailed temperature profile of the outgassing process. 2.2. Device structure, fabrication and operation The MEMS devices investigated in this study are electrostatically actuated micromirror arrays made of poly-SiGe (see Fig. 2). The fabrication of the samples starts from a silicon wafer with a silicon oxide layer to mimic the CMOS backend and a SiC passivation layer. The SiC layer is used to protect the oxide from the HF release step later on. On the SiC layer, a 400 nm SiGe electrode is deposited by Chemical Vapor Deposition (CVD). On top of SiGe, a 250 nm SiO2 hard mask is deposited and patterned to define the electrodes. Next, a second 200 nm thick SiGe layer (used for the 600 nm thick electrode) is patterned by using a resist mask, together with the first SiGe layer protected by the hard mask. To create flat mirrors, a 1200 nm SiO2 layer is deposited and planarized by chemical– mechanical polishing (CMP) down to the electrode level. Then, a 400 nm Si-oxide sacrificial layer is deposited and patterned to
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F.Z. Ling et al. / Microelectronics Reliability 52 (2012) 2256–2260 Table 1 Summary of the micromirror parameters. No.
L W (lm)
Hinge length (lm)
Electrodes thickness (nm)
A B C
88 20 20 20 20
1.5 3 4
600 400 400
Fig. 1. (a) Glass frit outgassing setup. (b) Temperature profile of the outgassing process.
Fig. 3. Typical displacement–voltage characteristic of an electrostatically actuated device.
the mechanical restoring torque acting on the mirror are equal. As the applied voltage increases, the micromirror is electrostatically attracted towards the electrode and at a certain value the electrostatic torque exceeds the mechanical restoring torque and the micromirror pulls in. The corresponding voltage value is called the pull-in voltage. After pull-in, one edge of the micromirror touches the landing electrode. Once the micromirror and the landing electrode come into contact, the adhesion force between both surfaces starts affecting the operational characteristic. As the applied voltage decreases, from a certain value the mechanical restoring torque again exceeds the combination of the electrostatic torque and the torque generated by the adhesion force, and thus the mirror releases. This occurs at a voltage which is called pullout voltage. A typical pull-in/pull-out characteristic is shown in Fig. 3. Fig. 2. Top-view SEM picture of (a) a single micromirror (20 20 lm), (b) a micromirror array and (c) schematic view of a single micromirror.
construct the anchors. A 330 nm optimized poly-SiGe structural layer is deposited by CVD at 450 °C followed by a CMP step, resulting in 300 nm thick flat mirrors. The micromirrors are laid out in arrays of either 40 40 (8 lm) or 16 16 (20 lm) devices with two different mirror sizes. The 300 nm thick micromirror is attached to two anchors by narrow torsion hinges. Under each micromirror there are two actuation and two landing electrodes with a height of 0.6 or 0.4 lm, resulting in a gap between the mirror and the electrodes of 0.4 and 0.6 lm, respectively. The length of the hinge is 1.5 lm, 3 lm or 4 lm. A schematic view of a single mirror is shown in Fig. 2. Detailed parameters are listed in Table 1. When the micromirror is electrostatically actuated by an applied voltage, it tilts and the angle of rotation depends on the actuation voltage applied between the micromirror and bottom electrodes. The mechanical restoring torque is a linear function of the tilt angle; the electrostatic force is a quadratic function of the same angle. In static equilibrium, the electrostatic torque and
2.3. Laser Doppler vibrometer The characterization of the micromirror array was performed in vacuum (1 mbar) at 125 °C. The micromirror array is actuated with a bipolar triangular waveform of frequency 1 kHz. The capacitance change of a single micromirror within an array cannot be measured by a regular electrical method. Hence the out-of-plane movements of individual mirrors were recorded by a Polytec laser-Doppler
Fig. 4. Experimental setup for tracking and recording device displacement.
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vibrometer (LDV). The laser is pointed at a spot near the edges of the mirrors, where the largest displacement occurs. From these optical measurements, the displacement-versus-voltage characteristic can be recorded. A schematic view of the measurement setup is shown in Fig. 4.
3. Results and discussion 3.1. Surface characterization The freshly released micromirror arrays were inspected by an optical microscope to check for any contamination or defects. Before testing, each device was also inspected for any curvature or buckling. Since it is not possible to measure the roughness of the bottom electrodes without destroying the micromirror, several micromirror arrays from the same wafer but different dies were removed in order to inspect the electrodes underneath as well as the backside of the micromirrors. Typical AFM topographic images (1 1 lm2) are presented in Fig. 5. The average root mean square (RMS) roughness of the reference thick and thin electrodes and the backside of micromirror are 2.16 nm, 6.53 nm and 0.39 nm, respectively. As described before, after formation of the thick and thin electrode, a 1200 nm
sacrificial oxide layer is deposited. Such a layer is then planarized by chemical–mechanical polishing (CMP). The CMP stops on top of the thick electrode. Because of this CMP step, the thick electrode exhibits a lower roughness than the thin electrode. Similar RMS values were measured on the arrays after they had been exposed to glass frit outgassing. This suggests that most of the solvent and organic additives in the glass frit were indeed burned out during the two pre-anneal steps. To further analyze the composition of the surfaces, samples were inspected by X-ray photoelectron spectroscopy (AR-XPS). The measurements were carried out in AR-mode using a Theta 300 system. Sixteen spectra were recorded at exit angles between 22° and 78° as measured from the perpendicular direction of the sample. The measurements were performed using a monochromated Al Ka X-ray source (1486.6 eV) and a spot size of 400 lm. Fig. 6 shows the C and Pb spectra of the sample surface before and after exposure to glass frit outgassing. The atomic concentration measured at 21.88° from the perpendicular direction is shown in Table 2. It is clearly shown that more C and some Pb contamination were found after exposure to glass frit outgassing. The increasing amount of C and Pb can be attributed to further outgassing of organic additives together with Pb from the glass frit at higher temperature. Thus these contaminations can land on the surface and possibly lead to unwanted attracting forces during
Fig. 5. Comparison of AFM topographic images (1 1 lm2) of the landing electrodes (a), (b) and the backside of micromirror, (c) before and after being exposed to glass frit outgassing. The average RMS roughness of (a)–(c) are 2.16 nm, 6.53 nm and 0.39 nm respectively.
Fig. 6. Overlay Pb4f (a), C1s (b) spectra of the outgassing and reference surfaces.
F.Z. Ling et al. / Microelectronics Reliability 52 (2012) 2256–2260 Table 2 Concentration measured at 21.88° from the perpendicular direction. A: sample after outgassing; B: reference sample. Sample
O1s
Si2p
Si2pO
C1s
Ge3d
Ge3d (GeO)
Ge3d (GeO2)
Pb4f
A B
22.19 37.32
1.89 7.19
5.37 9.24
48.82 13.12
8.88 19.08
2.23 3.63
8.29 10.41
2.32 0.00
contact which may increase the adhesion force. To check this, the pull-out behavior of the mirrors is investigated as discussed in the following paragraph.
3.2. Optical measurement As reported in our previous work, we analyze stiction of the mirrors by studying the hysteresis in the displacement-versusvoltage characteristics of electrostatically actuated micromirrors. The anomalous widening of the ‘‘pull-in window’’, caused by a stiction-induced delay of the pull-out, is used as an indication of the amount of mirror-to-surface stiction. The differences in the pullout voltage of mirrors within an array indicate varying levels of stiction from device to device. This stiction is mostly induced by the capillary forces that originate from moisture absorbed from the atmosphere. It can be significantly reduced by either applying a hydrophobic self-assembled-monolayer or annealing the devices in vacuum at an elevated temperature [10].
2259
In order to decouple the effect of moisture-induced stiction described above from the effect of glass frit induced stiction, the micromirror stiction sensor sample was first annealed in a vacuum chamber at 125 °C and measured in situ inside the chamber at 1 mbar. Eight to sixteen micromirrors within array A were tested in each measurement. As shown in Fig. 7a, the displacement of eight micromirrors from type A array are plotted as a function of the applied voltage. It was observed that all the micromirrors within this array consistently pulled in at 33.2 V and pulled out at 21.7 V with a relatively small spread. The displacements from the same micromirrors after glass frit outgassing are shown in Fig. 7b. Although the pull-in voltage (33.1 V on average) of all the micromirrors is equal to the value that was measured on the clean devices (i.e. before exposure of the sample to glass frit during the bonding step), the pull-out voltage shifts down by 2.5 V to 18.2 V, thus showing a widening the ‘‘pull-in window’’ as indicated by the arrows in Fig. 7. Since both measurements were performed in the vacuum chamber at an elevated temperature, the cause of the difference in the pull-out voltages cannot be due to capillary forces but only due to the effect of glass frit outgassing. Fig. 8 shows the results obtained from micromirrors (array C) that are much more sensitive to the adhesion forces due to their weaker torsional hinges and larger contact areas. The spread of the pull-out voltage increases from 0.18 V to 0.55 V before and after the outgassing experiment, respectively. The average pullout voltage shifts down by 0.5 V and some of the micromirrors become permanently stuck. Again, the significant decrease of the
Fig. 7. Measured displacement-versus-voltage characteristic from array A: (a) before glass frit outgassing and (b) after glass frit outgassing.
Fig. 8. Measured displacement-versus-voltage characteristic from array C: (a) before glass frit outgassing and (b) after glass frit outgassing.
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Table 3 Detailed results of tested samples and standard deviation (l is the mean value; r is the standard deviation, Cv is the coefficient of variation). No.
Pull-in (±) [V]
l
r
Pull-out (±) [V] Cv (%)
l
r
Cv (%)
0.54 0.59 0.80
21.65 5.10 3.65
0.12 0.23 0.18
0.54 4.50 5.05
0.56 0.62 0.95
18.19 3.55 3.1
0.11 0.34 0.55
0.62 9.54 17.74
Before glass frit outgassing A B C
33.15 7.75 5.05
0.18 0.05 0.04
After glass frit outgassing A B C
33.12 7.75 4.98
0.18 0.05 0.05
outgassing. This indicates that the adhesion forces do indeed increase, and may lead to stiction. AR-XPS analysis on the contacting surfaces after exposure to the glass frit bonding step shows an increased amount of C and Pb which can be the cause of the raising adhesion force. Acknowledgments The authors would like to thank Alain Moussa for doing the AFM measurements and the support the SBO project ‘GEMINI’ (‘Generic Electronics and Microsystems INtegration Initiative’), IWT-nr 60046, 2007–2011. The authors also would like to thank IMEC REMO and MEMS team for their kind help. References
average pull-out voltage is attributed to the increased adhesion forces between the backside of the micromirrors and the landing electrodes. The increased spread of the pull-out voltage is attributed to random distribution of outgassed contaminants, as well as to the random nature of the bottom electrode roughness and effective contact area between mirrors and bottom electrodes. A detailed summary of the results is shown in Table 3. 4. Conclusions Poly-SiGe micromirror arrays were used as a ‘‘stiction sensor’’ to study the effect of adhesion forces induced by glass frit bonding outgassing. The results show that thermal treatment of the glass frit before bonding can indeed remove most of the solvent and organic additives. Nonetheless, this does not guarantee that no extra elements will outgas during the compressive bonding step at a higher temperature. A clear widening of the ‘‘pull-in window’’ is observed on micromirrors that have been exposed to the glass frit
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