MEMS Optical Acoustic Sensors Manufactured in Laminates Yang Zhang, Jonas Tsai, G. P. Li, Mark Bachman Department of Electrical Engineering University of California, Irvine
[email protected] Abstract Latest advancement in printed circuit board (PCB) technologies and discoveries of novel laminate materials has enabled a series of laminate MEMS devices to be designed, fabricated, and productized. To demonstrate the benefit in design and fabrication using the Laminate MEMS technology, we introduce embedded MEMS optical sensors fabricated directly within PCB layers. Free-standing polymeric waveguides translate acoustic signals into optical ones with varied frequency response, directionality and dynamic range. Fiber optics components such as optical transmission lines, mirrors and beam-shaping lens are integrated in the same multi-layer electrical packages. Standard multi-mode, multichannel optical fiber connectors are adopted to seamlessly transfer optical signals to a linear photo detector array. Besides being low-cost and easy to integrate, this inline waveguide sensor design is completely EMI free, ideal for insitu monitoring of acoustic emission from microelectronics packaging processes. Introduction 2010 was a year of prosperity for the MEMS industry to experience a double digit market1. Shipments of existing MEMS components, for mobile handsets alone, are expected to reach three billion units by 2011. These MEMS devices are exclusively single-packaged sensors and actuators, such as motion sensors, RF devices, microphones, micro mirrors and displays, which are key components for human-computer interaction and automatic sensing and controlling. To manufacture smarter cell phones and automobiles, an increasing number of MEMS components are being integrated into the same product. The simple way is to integrated individually packaged MEMS chips directly on the board. Current cell phones have adopted as many as tens of MEMS chips on their multilayer boards. MEMS manufacturers are also exploring multiple MEMS sensors contained in a single package2. This solution eases on-board signal processing and to some extent reduces packaging cost. However, as the demand of sensors and actuators booms, the complexity and cost of integration can no longer be resolved by these nonscalable solutions. The true multi-MEMS solution cannot come from traditional MEMS industry. Reviewing the history of traditional MEMS technology and industry, one can see most infrastructures inherited directly from the semiconductor industry: silicon wafer based design approach, fabrication tools and processes, and first-level packaging. Silicon-based MEMS design approaches are very limited in 3D structures and the overall height is typically below ten microns. This puts a cap on figure-of-merit requiring large design space in the z direction. Secondly, available materials to traditional bulk machining or surface micromachining are limited to
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polymers, thin metals and ceramics. Devices need custom processes for special materials to meet requirements such as RF transmission loss and bio-chemical compatibility. Furthermore, the cost of silicon substrates accounts for 70% of the total manufacturing cost and scales with the manufacturing volume. Last but not the least, packaging of MEMS devices has always been the biggest challenge for speedy product development, single or multiple MEMS alike. Industry and research groups have been working to address this issue from microelectronics packaging point of view. Many electronics manufacturing and packaging communities, such as the IMAPS (International Microelectronics and Packaging Society) and the iNEMI (International electronics manufacturing initiative), have included MEMS sensors and actuators in their roadmaps for semiconductor industry. Their vision for the future electronic system, such as ITRS (International Technology Roadmap for Semiconductors), includes 3D integration of MEMS devices on the same board with ICs. Research groups also proposed visions to enclose MEMS devices, in which MEMS, actives and passives are integrated on the same SoP (System-onPackage)3. However, none of these solutions are truly scalable as MEMS devices become pervasive.
Figure 1. A laminate MEMS electro-optical board The Laminate MEMS Technology4 can manufacture multiple MEMS devices directly on the same platform similar to a multilayer printed circuit board, using tools, materials and processes that are compatible with PCB manufacture facilities. Laminate MEMS also refers to singulated or an ensemble of additively micromachined devices that are built on substrates and base materials in advanced printed circuit boards. Figure 1 illustrates typical MEMS sensors and actuators packaged in electro-optical laminates. MEMS optical structures are integrated with polymer optical elements, such as mirrors, lens and waveguides. PCB layers functions as packaging substrates for MEMS optical components, active and passive electronic components, and mechanical connections to standard optical fiber connectors. The three-dimensional design approach can utilize a limited number of optical waveguides or air paths to achieve integration in the z direction, similar to via holes across PCB layers5, 6.
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Polymer waveguide based MEMS sensors have been reported to convert physical7, chemical and biological signals8 into optical signals by modulating the intensity (amplitude), phase (frequency) and polarization of light. With the example of a novel inline acoustic sensor, the rest of this article further discusses the advantages in design and fabrication of MEMS devices utilizing the laminate MEMS technology. Laminate inline optical sensor design As microprocessors for PCs and servers become ever more powerful, the intra-motherboard data bandwidth is approaching the bottleneck defined by electrical signal lines. Embedded optical interconnects, or “hybrid PCBs”, have been developed to carry the high speed communication between processors or peripheral chips. Meanwhile, increasing demand of over 10 GBps data streaming speed has driven worldwide R&D efforts to develop multimode, multichannel fiber optics interconnects for the next generation fiber network. A variety of optical fiber connectors have been developed for quick connect/disconnect and have been used widely in server rooms and audio, video streaming devices. Since the inline optical sensor signals are transmitted directly in the optical fiber, it is crucial for the final packaged device to be compatible with existing standard connectors. Traditional MU, SC, and LC connectors found in datacom and telecom industry are not suitable for our application and do not provide multiplex interfacing in a footprint that will fit the size of our devices. Novel passive fiber optics adaptor in PCBs has been reported in [9,10, 11, 12, 13] to use the MTP® optical connector standard, which is currently the most popular for high fiber-density applications. The MTP® standard supports up to six rows of 12-channel fiber arrays at 250 um pitch, with 50/65 um multimode fibers (Figure 2).
Figure 2. Close-up image of MTP 12-channel fiber connector showing a Gaussian laser beam distribution (left) and white LED light (right) as the light source. We have first adopted the single-row 12-fiber standard to our device design (Figure 3). The 100 um wide and 75 um high optical cantilevers are designed at the same 0.25mm pitch as the fiber ferrule. The excess cross-sectional area over a 50 um fiber offers extra positioning tolerance when joining optics and PCB. The two stainless steel guiding pins and other mechanical features for passive alignment are incorporated in the laminate layer design. Figure 3 shows male connecting features and female MTP connectors at both ends of the laminate device. The transduction mechanism for the acoustic sensor is based on intensity modulation of light signals through freestanding cantilever waveguides above an open channel for sound wave [7]. With identical cross-sectional area for light
conduction, cantilevers of different lengths above a cavity resonate with acoustic signal at different frequencies. Equation (1) describes the dependence of the fundamental resonate frequency on the Young’s modulus of the waveguide material (E), the density (ρ) as well as the geometry (t: thickness; l: Length).
f0
t 4l
E 2
(1)
The independent cantilever sensors are flexible in the choice of core material, cladding as well as geometrical features. This design enables parallel sensing of environmental information at different frequency and amplitude.
Figure 3 Inline optical acoustic sensor array design compatible with MPO connectors. a) Top view: multichannel fiber optics connected with acoustic sensor array by mechanical alignment pins. b) Section view showing MEMS structures and fiber optics embedded within multi-layer PCB. Linear photo detector array chip can be directly coupled to the MTP fiber connector (Figure 3). We use the TAOS TSL201R-LF 64 x 1 linear sensor array with 125 um pitch between individual photo sensitive pixels. Each pixel measures 120 um by 70 um which is ideal for detecting high speed optical signal from an MTP connector. Figure 4 demonstrates light spots from the MTP fiber array illuminate the linear photo detection array. Extra coverage of the array (8 mm) offers excellent tolerance to the connection. Digital optimization of the incident signal can be realized through controlling the chip interrogation cycles. The 5 MHz refreshing rate is ready for acoustic signals up to 1 MHz.
Figure 4. Photo detector array chip showing illuminated pixels by a fiber optics array
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Polymeric optics fabrication PCB embedded optical waveguides [14, 15, 16, 17, 18, 19, 20] have been reported intensively over the past few years due to the high impact on inboard optical communication. Since optics is highly scalable down to a few microns, almost all optical elements can be achieved on a thin film polymer with most of the features in the x-y plane. Micro polymeric optical devices have been fabricated widely using photo lithographical methods [7, 8, 21]. This rather complicated process generally involves polymer components mixing, thin film spinning, baking, UV exposure, developing and final annealing. The most significant drawback of this approach is that, due to internal stress accumulated during various heating and wetting cycles, freestanding features tend to deform significantly. Majority of optical MEMS devices involve moving components and even slight misalignment would cause loss in optical signal. Post processing of waveguides include mechanical, chemical or ion etching to form mirrors [20, 22].
excimer laser allowing organic designs and structures with features sizes previously unobtainable. To produce the multichannel cantilever waveguides and connected optics where optimized cutting quality is desired, we used a 5 um diameter laser beam from the excimer laser. A 72 um aperture is used to process the sections outside of the optical path. The energy level, pulse width, repetition rate of the laser was carefully adjusted to balance the total cutting time and overall cut quality (Table 1). To achieve maximum surface quality of the optics, a low-power ablation cycle is suggested to be applied after a high-power cutting run [24]. We found this method helpful at a cost of twice the total processing time for each piece. Only critical surfaces that are prone to loss are processed using the refinement run, such as the micro-bend and lens sytems. Energ y (mJ)
Pulse width (ns) 20 20
Repetitio n rate (Hz) 100 30
Feed rate (mm/s) 0.01 0.01
Duratio n (hr) 2 2
Dia. 5 um 30 Dia. 5 um 10 (refinement ) Dia. 72 um 30 20 100 0.08 0.5 Table 1. Recommended parameters for excimer laser micromachining of polymer optics.
The final processed cantilever array is inspected under the SEM to have good surface condition and most importantly, no signs of any deformation caused by stress. Figure 6 a) shows SEM image of six 100 um x 75 um x 5 mm acetate polymer waveguides. Beam sidewalls and spaces in between are well preserved after laser cutting. High magnification image in Figure 6 b) shows sub micron surface smoothness at the interface. Figure 5. Thin film polymeric optical elements. a) Y split; b) Interface; c) Mirror; d) Lens. In search of a stress-free fabrication technique, we have explored Diode, CO2 and Excimer laser cutting systems that are largely available to high-end PCB manufacturers now. Optical waveguides, beam-shaping lens, mirrors and MEMS cantilevers have been micromachined on polymeric layers using laser ablation tools [23, 24], such as polyester, fluoropolymers and acetate. Figure 5 showcases the SEM pictures of these devices, where a) is a “Y” splitter from multichannel waveguide input, b) is the interfacing lens between emitting and receiving waveguides, c) is a micro reflector, and d) is a single lens for beam shaping. The laminate waveguides and sensing elements were produced in a class 1,000 clean room using the Resonetics Rapid XTM 250 ArF excimer laser. The excimer laser can produce features sizes with resolution of 2µm. Advantages of this system over traditional photolithographic processes are: high throughput, fast prototyping, and repeatability. The laminate layer processed with the excimer laser shows the least amount of residue and sub 1um surface smoothness. Curvilinear features can also be easily fabricated using the
Figure 6. Excimer laser processed micro cantilever array under SEM. a) Wide angle view of the stress-free array; b) Close-up view of individual waveguide at the 200 um gap. Experimental results Collimated 650 nm light from a bench top laser as well as miniature laser diodes are used to test the fabricated optics. An in-plane reflecting mirror is demonstrated in Figure 7 a) to show that a significant amount of light is reflected at the polymer-air interface. Multiple in-plane reflectors also have been tested. This configuration can also be applied to waveguide mirrors in the z direction. On the other hand, Figure 7 a) shows an array of six micro cantilevers illuminated by a single laser beam with Gaussian distribution.
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Polymer waveguides are proven to be good quality light transmitters. The loss due to multiple narrow air gaps appears negligible.
length of the cantilever design. The second and third harmonics are revealed in the spectrogram but at least 10 dB weaker than the fundamentals.
Figure 7. Optical test of polymeric waveguides. a) 90 degree bend using reflecting mirror; b) light coupling between cantilevers and receiving waveguide. The fully assembled inline acoustic sensor is first tested for alignment with multichannel MTP connectors. We installed the stainless steel alignment pins in the laminate channel routed by a high precision automatic routing tool (AutoLab MITS, Japan). Then the optical cantilevers are matched with the substrate by registration holes at four corners. 12-channel MTP connectors are then plugged in with the alignment pins. Figure 8 a) shows a microscopic image of the acoustic sensor taken with ambient light and top PCB removed. The image is focused on the section where cantilevers join with optical fibers from the MTP adaptor. A very good alignment is achieved by the simple mechanical alignment. Another image is taken without ambient light and focused on the other end of the cantilever array where individual channels are independently illuminated.
Figure 8. MEMS cantilever waveguides aligned with MTP fiber connector. a) with ambient light, focus on cantilever/fiber interface; b) without ambient light, focus on air gap at the end of cantilever array. The acoustic testing system consisted of laser diode driver, pigtailed laser diode, input and output optical fiber, polymeric cantilever array, photo diode, function generator, speaker, and sound level meter. The function generator sweeps the output frequency to drive the speaker sound. The modulated optical signal was converted to electrical signal by the optical detector. The sound level meter was placed near the cantilevers to measure the active and background sound level. Within a measured 30 dB sound level environment, the acoustic sensor has up to 40 dB signal-to-noise. Figure 9 is a spectrogram for a typical frequency sweep test. The sound level of the speaker output is measured to be at a constant 70 dB throughout the frequency sweep. Three resonant peaks are observed at frequencies according to the
Figure 9. Spectrogram of inline optical MEMS acoustic sensor. Three resonant frequencies can be identified to be according to three different cantilever lengths. Second and third harmonics are observed.
Figure 10. Frequency response of three resonating waveguides. When the resonance peaks are compared side by side, we observe very uniform sensitivity for all three resonant frequencies. The Q10 values of cantilevers are 5.6, 10.0 and 13 for resonant frequencies at 975, 1253 and 1602 Hz, respectively. They are very close to those that obtained with direct measurement of the basilar membrane vibration in a normal mammalian cochlea. This is a very important feature for acoustic sensing as microphone devices. Conclusions Multichannel cantilever-based inline MEMS optical acoustic sensors have been design, fabricated and tested. The device is proven to be compatible with standard multilchannel fiber optical connectors. Free-standing cantilevers of different lengths resonate with acoustic signal at different frequencies,
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resulting in different optical coupling to the fixed optical guides. By varying the width and thickness of the cantilevers, directionality and amplitude dynamic range can be adjusted. Inline cantilever arrays have been tested to produce over 40 dB dynamic range over a wide frequency band. Future work will include multilayer optical PCB design with up to six rows of 12-channel fiber optics connectors, multi-sensor configuration including thermal, acoustic and chemical sensing capabilities, and acoustic detection in low frequency (vibration) and ultrasonic range. Acknowledgments The authors would like to thank Mr. Ruisheng Chang and the staff of in the Integrated Nanosystems Research Facility and Dr. Lifeng Zheng in the Bion Laboratory at the University of California, Irvine for microfabrication assistance. This work was supported by funds from the University of California, Irvine. References
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