Released micromachined beams utilizing laterally uniform porosity porous silicon
© Sun et al.; licensee Springer. 2014
Received: 18 May 2014
Accepted: 12 August 2014
Published: 22 August 2014
Suspended micromachined porous silicon beams with laterally uniform porosity are reported, which have been fabricated using standard photolithography processes designed for compatibility with complementary metal-oxide-semiconductor (CMOS) processes. Anodization, annealing, reactive ion etching, repeated photolithography, lift off and electropolishing processes were used to release patterned porous silicon microbeams on a Si substrate. This is the first time that micromachined, suspended PS microbeams have been demonstrated with laterally uniform porosity, well-defined anchors and flat surfaces.
81.16.-c; 81.16.Nd; 81.16.Rf
KeywordsPorous silicon Photolithography Microbeams Release
Porous silicon (PS), which is normally formed via the partial electrochemical dissolution of crystalline silicon in a HF/ethanol solution , has gained significant attention due to its biocompatibility and stability. With a large surface area and easily tunable porosity (which directly determines the refractive index), PS has been demonstrated in applications including light emitting diodes , sensors [3, 4] and photo detectors [4, 5]. However, previously reported PS tunable microelectromechanical system (MEMS) devices for gas sensors , biological sensors  and optical filters [8, 9] have mainly been fabricated through a predefined patterning process utilizing a defined pattern or mask on Si prior to anodization, resulting in unwanted under-mask etching and very low lateral uniformity in PS films. The predefined patterning technique limits complementary metal-oxide-semiconductor (CMOS) compatibility of the process for making further complex structures , limiting PS use as a separate material in MEMS device fabrication.
PS-suspended structures can provide increased sensitivity in MEMS devices through the large surface area and the ability to use porosity to control mechanical properties [10–12]. Sensing using released microbeams has been studied for a variety of materials, including Si, Si3N4 and AlN [13–15]. Suspended PS structures have previously been fabricated and released [12, 16], but the porosity of those films was not uniform, leading to significant bending from internal stress, made worse by the very low stiffness of the material. Furthermore, previous PS MEMS have been large or poorly defined [7, 8]. This negates a significant advantage of MEMS, which is that their small size provides both robustness against inertial effects and high resonance, the latter being essential for high sensitivity biosensors . Uniform porosity and well-defined porous silicon patterning is required to achieve a high-quality MEMS technology. Furthermore the process must be compatible with a high-volume (scalable) manufacture process. Lai et al. demonstrated a process based on N2 annealing which reduced oxidation in ambient air and made the films compatible with standard CMOS photolithography . This approach makes PS a suitable platform for creating patterned structures of uniform porosity, and allows multistep processing through repeated anodization, annealing and photolithography to be performed.
In this work, we demonstrate that well-defined, laterally uniform porosity PS microbeams can be successfully fabricated and released. A process based on anodization, annealing, RIE, repeated photolithography, lift off and electropolishing is presented, which is designed with CMOS compatibility in mind. Process yield along with length of microbeam was studied, and surface profilometry of fabricated structures of PS microbeams was performed. The surface profile shows that this approach yields PS microbeam with small surface variation, showing well-defined PS structures were fabricated.
The wafer material used was moderately doped p-type (100) silicon with resistivity of 0.08 to 0.10 Ω · cm. Room temperature anodization was performed in a 15% HF/ethanol solution, unless otherwise specified. PS films in this paper were anodized using a current density of 10 mA/cm2 for 403 s and subsequently annealed in N2 atmosphere at 600°C for 6 min, to create low-temperature annealed porous silicon films with porosity P = 81% and a physical thickness of t = 2.45 μm. The annealing process is critical as it makes the PS film suitable for direct photolithography processing using alkaline developers . This type of PS was used in the work reported here, as its characterization and annealing has been previously comprehensively studied [19, 20]. However, as part of the investigations, it was confirmed that PS films with different porosity and thickness are also suitable. The PS microbeams under investigation here were designed and fabricated with dimensions L × W × 2.45 μm, where 80 μm < L < 1,000 μm and 20 μm < W < 50 μm.
After that, a second standard photolithographic process using negative photoresist AZ2070 (MicroChemicals, 6.8-μm thick) was employed to define a metal mask pattern up to the anchor, as shown in Figure 1d. A Cr/Au (10/200 nm) layer was subsequently deposited on to anchor regions with a lift-off process based on the second photolithography, as shown in Figure 1e. The negative photoresist was removed by a 15-min N-methyl-2-pyrrolidone (NMP) or dimethyl sulfoxide (DMSO) dip and a 5-min acetone dip in the lift-off process. The metal region over the PS was important to define the anchors during electropolishing as described later. Electropolishing with HF-based electrolyte was carried out to etch the Si, and the electrolyte ensured any residual SOG in the pores was removed. Electropolishing was carried out using a similar process to anodization, but with different electrolyte (a 3% HF/DI solution) and electrical conditions (20 mA/cm2, 180 s). After electroplishing, PS microbeams suspended on top of Si substrate were formed which were kept submerged until release. The samples were rinsed in DI water wash and transferal to a methanol bath during the critical point drying process used to release the PS doubly clamped microbeams illustrated in Figure 1f.
Results and discussion
On the contrary, SOG can be used to form a layer of SiO2 to fill the pores of PS at step I of Figure 6, which is not removed during the developing process at step II. This guarantees the accurate control of developing time for the photoresist layer, resulting in well-patterned PS structures at step III, as shown in Figure 6c. Our tests showed a 10-s dip in 10% HF/DI is sufficient to remove all SOG in an exposed PS film (where there was no photoresist) up to 2.45-μm thick. The short dip resulted in an optical thickness change of less than 4.4%, suggesting the short dip had very little effect on the PS layer. In this work which used PS layers of 2.45-μm thickness, SOG as a pore filling layer was more advantageous than ProLIFT and was used as described.
These results show a complete MEMS fabrication process using a single material system can be achieved using combination of anodization and electropolishing. No sacrificial layer was required to achieve release of the beams. This is fundamentally different from traditional MEMS processing and has the potential to resolve interface compatibility issues such as differences in thermal coefficient of expansion. The thickness of the PS beam (2.45 μm) and porosity (81%) were chosen to achieve the same rigidity as an a-Si beam of thickness 0.6 μm. This allowed us to demonstrate the fabrication process on extremely high-porosity meso-porous silicon, which is well suited to sensing applications due to its very large surface area [3, 32]. The high porosity and high thickness balance to produce an expected resonant frequency in the range of 16 to 400 kHz for microbeams with length of 100 to 500 μm. Variation of porosity and thickness are also options to adjust frequency of beams (not detailed in this work). Residual and stress gradients in the films need to be studied to allow both doubly clamped and cantilever structures to be fabricated, as these are the basis on most MEMS devices. We are aware that the use of Au as part of the metallisation scheme would prevent implementation in some CMOS foundries. Our investigations have been limited to metals currently available in our facility; however, alternative metallisation or doping could be used to replace the Cr/Au layers for the electropolishing steps to achieve a completely CMOS-compatible process.
This work has demonstrated micromachined, suspended PS microbeams with laterally uniform porosity and structurally well-defined beams. We have demonstrated repeated photolithographic processing on PS films that is compatible with CMOS processes; however, for complete CMOS integration, a different metallisation may be required to avoid use of Cr/Au. A deposited metal mask layer was used during electropolishing to ensure a uniform electric field and minimal underetching of the PS layer. A new pore filling technique using SOG allowed the use of thick (2.45 μm) films. The surface profile of the released microbeams indicated well-defined structures. This approach demonstrates a method of fabricating complex PS structures using a scalable PS-MEMS technology.
XS received the B.Sc. degree and the M.Sc. degree in optics from Xi’an Jiaotong University, Xi'an, China, in 2005 and 2008. In 2008, he joined the State Intellectual Property Office of China, working on extensive examination of patent applications in the areas of measuring devices and microelectromechanical systems. Since 2012, he has been working toward the Ph.D. degree in microelectronic engineering at The University of Western Australia, Perth, Australia. His thesis focuses on micromachining applications based on porous silicon. GP received the B.S. degree in Chemistry in 1995 and the bachelors and M.Sc. degrees in Electronic Engineering in 1995 and 1997, respectively, all from The University of Western Australia, Perth, and the Ph.D. degree in Electrical Engineering in 2001, from the University of California, Santa Barbara. She joined The University of Western Australia as an Australian Postdoctoral Fellow in 2001 and is now a professor at the same institution. Her main research interests are III-V nitride and porous silicon materials and devices. Specific interests within these areas currently include development of processing technology, transport studies and development of novel chem- and bio-sensors. AK received the bachelors and Ph.D. degrees in Electrical/Electronic Engineering in 1990 and 1995, respectively, from the University of Melbourne. He worked as a post-doctoral fellow at NTT (Musashinoshi, Japan) from 1996 and joined the UC Santa Barbara (USA) in 1998. He joined Calient Networks, Santa Barbara in 1999 as the Fiber Optics Technology Manager. In 2004, he joined the University of Western Australia as a research fellow and became an assistant professor in 2007 and a professor in 2010. He received the DSTO Eureka Prize for Outstanding Science in Support of Defence or National Security in 2008 for his contributions to the development of a MEMS microspectometer, and his current research interests include porous silicon for micromachined devices, optical MEMS biosensors, and microfluidics.
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This work was supported by The University of Western Australia. The authors acknowledge the support from the Australian Research Council, Western Australian Node of the Australian National Fabrication Facility, and the Office of Science of the WA State Government. The authors acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy, Characterization and Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.
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