Fabrication of a guide block for measuring a device with fine pitch area-arrayed solder bumps

This paper describes the design and fabrication of a guide block and micro probes, which were used for a vertical probe card to test a chip with area-arrayed solder bumps. The size of the fabricated guide block was 10 mm × 6 mm. The guide block consisted of 172 holes to insert micro probes, 2 guide holes for exact alignment, and 4 holes for bolting between the guide block and the housing of a PCB. Pitch and size of the inserting holes were 80 μm, and 90 μm × 30 μm, respectively. A silicon on insulator wafer was used as the substrate of the guide block to reduce micro probes insertion error. The micro probes were made of nickel–cobalt (Ni–Co) alloy using an electroplating method. The length and thickness of the micro probes were 910 and 20 μm, respectively. A vertical probe card assembled with the fabricated guide block and micro probes showed good x–y alignment and planarity errors within ±4 and ±3 μm, respectively. In addition, average leakage current and contact resistance were approximately 0.35 nA and 0.378 ohm, respectively. The proposed guide block and micro probes can be applied to a vertical probe card to test a chip with area-arrayed solder bumps.     

Design and consideration of wafer level micropackaging for distributed RF MEMS phase shifters

A novel packaging structure which is performed using wafer level micropackaging on the thin silicon substrate as the distributed RF MEMS phase shifters wafer with vertical feedthrough is presented. The influences of proposed structure on RF performances of distributed RF MEMS phase shifters are investigated using microwave studio (CST). Simulation results show that the insertion loss (S₂₁) and return loss (S₁₁) of packaged MEMS phase shifters are −0.4–1.84 dB and under −10 dB at 1–50 GHz, respectively. Especially, the phase shifts have well linear relation at the range 1–48 GHz. At the same time, this indicated that the proposed pacakaging structure for the RF MEMS phase shifter can provide the maximum amount of linear phase shift with the minimum amount of insertion loss and return loss of less than −10 dB.     

A dielectrophoretic chip packaged at wafer level

The paper presents a dielectrophoretic chip, fully enclosed, with bulk silicon electrodes fabricated using wafer-to-wafer bonding techniques and packaged at the wafer level. The silicon electrodes, which are bonded to two glass dies, define in the same time the walls of the microfluidic channel. The device is fabricated from a silicon wafer that is bonded (at wafer level) anodically and using SU8 photoresist between two glass wafers. The first glass die includes drilled holes for inlet/outlet connections while the second glass die assure the electrical connections, through via holes and a metallization layer, between the silicon electrodes and a printing circuit board.

     

A dielectrophoretic chip packaged at wafer level

The paper presents a dielectrophoretic chip, fully enclosed, with bulk silicon electrodes fabricated using wafer-to-wafer bonding techniques and packaged at the wafer level. The silicon electrodes, which are bonded to two glass dies, define in the same time the walls of the microfluidic channel. The device is fabricated from a silicon wafer that is bonded (at wafer level) anodically and using SU8 photoresist between two glass wafers. The first glass die includes drilled holes for inlet/outlet connections while the second glass die assure the electrical connections, through via holes and a metallization layer, between the silicon electrodes and a printing circuit board.     

Thermally actuated microprobes for a new wafer probe card

A new type of MEMS microprobe was designed and fabricated which can be used for a nest generation wafer probe card. A prototype MEMS probe card consisting of an array of microprobes individually actuated by bimorph heating to make contact with the test chip was also fabricated. This probe card is called the CHIPP (Conformable, HIgh-Pin count, Programmable) card and can be designed to contact up to 800 I/O pads along the perimeter of a 1-cm² chip with a microprobe repeat distance of approximately 50 μm. Microprobes for a prototype CHIPP probe card have been fabricated with a variety of cantilever structures including Al-SiO₂, W-SiO₂ and Al-Si bimorphs, and with the resistive heater placed either inside or on the surface of the cantilever. Ohmic contacts between tips and bond pads were tested with contact resistance as low as 250 mΩ. The deflection efficiency varies from 5.23-9.6 μm/mW for cantilever lengths from 300-500 μm. The maximum reversible deflection is in the range of 280 μm. The measured resonant frequency is 8.16 kHz for a 50×500 μm device and 19.4 kHz for a 40×300 μm device. Heat loss for devices operating in air was found to be substantially higher than for vacuum operation with a heat loss ratio of about 2/1 for a heater inside the structure, and 4.25/1 for a structure with the heater as an outer layer of the cantilever     

Reliability study of hermetic wafer level MEMS packaging with through-wafer interconnect

In this paper, we developed a hermetic wafer level packaging for MEMS devices. Au–Sn eutectic bonding technology in a relatively low temperature is used to achieve hermetic sealing, and the vertical through-hole via filled with electroplated copper for the electrical connection is also used. The MEMS package has the size of 1 mm × 1 mm × 700 μm, and a square loop Au–Sn metallization of 70 μm in width for hermetic sealing. The robustness of the package is confirmed by several tests such as shear strength test, reliability tests, and hermeticity test. The reliability issues of Au–Sn bonding technology, and copper through-wafer interconnection are discussed, and design considerations to improve the reliability are also presented. By applying O₂ plasma ashing and fabrication process optimization, we can achieve the void-free structure within the bonding interface. The mechanical effects of copper through-vias are also investigated numerically and experimentally. Several factors which could induce via hole cracking failure are investigated such as thermal expansion mismatch, via etch profile, copper diffusion phenomenon, and cleaning process. Alternative electroplating process is suggested for preventing Cu diffusion and increasing the adhesion performance of the electroplating process.     

Characterization of intermediate In/Ag layers of low temperature fluxless solder based wafer bonding for MEMS packaging

Low temperature fluxless solder for wafer bonding has received a lot of attention due to its great potential in hermetic MEMS packaging. Previous research activities mainly deploy solder alloy of eutectic composition to achieve low bonding temperature. We proposed new intermediate bonding layers (IBLs) of rich Ag composition in In–Ag materials systems. In this study, we investigated the intermetallic compounds (IMCs) at the bonding interface with respect to the bonding condition, post-bonding room temperature storage and post-bonding heat treatment. With this IBL, the IMCs of Ag₂In and Ag₉In₄ with high temperature resist to post-bonding process are derived under process condition of wafer bonding at 180 °C, 40 min and subsequent 120–130 °C annealing for 24 h. Low melting temperature IMC phase of AgIn₂ is formed in the interface after long term room temperature storage or 70 °C aging treatment. This low melting temperature IMC phase can be completely converted into high melting temperature IMCs of Ag₂In and Ag₉In₄ after 120 °C additional annealing. Based on our results, we can design the packaging process flow so as to get reliable hermetic packaged MEMS devices by using low temperature fluxless In–Ag wafer bonding.     

Chirality assignment to carbon nanotubes integrated in MEMS by tilted-view transmission electron microscopy

Front side etching combined with sample tilting - instead of wafer through etching - allows for transmission electron microscopy (TEM) investigations on nanostructures integrated in microelectromechanical systems (MEMS). We present electron diffraction of an individual single-walled carbon nanotube (SWNT) suspended between sharp polycrystalline silicon tips as far as 165 μm away from the MEMS chip edge. This novel approach for transmission-beam characterization avoids complex wafer backside processing and facilitates alignment of the SWNTs to the focal plane using tips defined directly by photolithographic means. The demonstration of chirality assignment to the integrated SWNT paves the way for correlating experimentally measured response of the SWNT sensing element upon stimuli with the response predicted by theory.     

Geometrical strengthening and tip-sharpening of a microneedle array fabricated by X-ray lithography

A novel fabrication method for LIGA (from the German “Lithographie”, “Galvanik”, and “Abformung”) microneedles with through holes is presented. Such microneedles are in demand by most bio-medical MEMS applications and in some fluidic MEMS applications. We propose a technique that combines conventional deep X-ray lithography, plane-pattern to cross-section transfer (PCT) process, and alignment X-ray lithography. The technique provides precise hole alignment with ± 3 μm tolerance. Finite-element simulations on various hole locations were performed to determine the optimum position. We previously fabricated a microneedle with a 100-μm base and a 300-μm height by a right-triangular mask. The resultant microneedle had a very sharp tip but was excessively steep, and thus resulted in a very low strength. Improved strength and tip sharpness was consequently achieved by changing the mask-pattern from a triangular pattern to a polygonal mask and changing the dimensions of the microneedle to have a 300-μm base with various heights between 350 and 800 μm. Using the proposed technique, we could produce a total of 100 hollow microneedles on a 5 × 5 mm² chip. Moreover, we successfully fabricated sharpened microneedles that were stronger than that we have fabricated so far. The molding process or electroplating and the cost list of the LIGA microneedle will also be included.     

Slotted capacitive microphone with sputtered aluminum diaphragm and photoresist sacrificial layer

In this paper, we have fabricated a new microphone using aluminum (Al) slotted perforated diaphragm and back plate electrode, and photoresist (AZ1500) sacrificial layer on silicon wafer. The novelty of this method relies on aluminum diaphragm includes some slots to reduce the effect of residual stress and stiffness of diaphragm for increasing the microphone sensitivity. The acoustic holes are made on diaphragm to reduce the air damping, and avoid the disadvantages of non standard silicon processing for making back chamber and holes in back plate, which are more complex and expensive. Photoresist sacrificial layer is easy to deposition by spin coater and also easy to release by acetone. Moreover, acetone has a high selectivity to resist compared to silicon oxide and Al, thus it completely removes sacrificial resist without incurring significant damage silicon oxide and Al. The measured zero bias capacitance is 17.5 pF, and its pull-in voltage is 25 V. The microphone has been tested with external amplifier and speaker, the external amplifier was able to detect the sound waves from microphone on speaker and oscilloscope. The maximum amplitude of output speech signal of amplifier is 45 mV, and the maximum output of MEMS microphone is 1.125 μV.     

Development of a nanostructural microwave probe based on GaAs

In order to develop a new structural microwave probe, we studied the fabrication of an AFM probe on a GaAs wafer. A waveguide was introduced by evaporating Au film on the top and bottom surfaces of the GaAs AFM probe where a tip 7 μm high with a 2.0 aspect ratio was formed and the dimensions of the cantilever were 250 × 30 × 15 μm. The open structure of the waveguide at the tip of the probe was obtained by FIB fabrication. An AFM image and profile analysis for a standard sample, obtained by the fabricated GaAs microwave probe and a commercial Si AFM probe, indicate that the fabricated probe has a similar capability for measurement of material topography as compared to the commercial probe.     

Development of near hermetic silicon/glass cavities for packaging of integrated lithium micro batteries

A technology was developed for fabrication of very thin, chip-sized lithium secondary micro batteries. With help of wafer level processing the batteries can be directly integrated into silicon chips or MEMS devices. The batteries were packaged in 200 μm deep cavities of the silicon wafer and encapsulated with a glass substrate. Battery demonstrators were realized with 7 and 12 mm square and round foot prints. Near hermetic packaging was accomplished with the help of a UV curable epoxy seal that should ensure several years of battery lifetime. Bonding parameters, shear force and the water permeation rate of the adhesive were investigated. A capacity of 3 mAh/cm² and an energy density of 10 mWh/cm² were achieved. The electrical contact between the battery and the contact pads of the housing was investigated in detail. Electrical tests were made with encapsulated micro batteries and compared with macroscopic lithium polymer batteries. A reduction in capacity of approximately 10% was measured after 100 cycles.     

Stereolithography on silicon for microfluidics and microsensor packaging

 Stereolithography can be used to fabricate 3-D and high aspect ratio microstructures with low manufacturing cost and short fabrication time. Stereo lithography can customize the packages for microfluidics and microsensors to eliminate the dead volume of the reaction chamber. Fabrication of MEMS packages on a wafer level scale can decrease the manufacturing time and assembly time. In order to show the feasibility of integration of stereolithography with micromachined devices, alignment, cleaning, and dicing tests have been investigated. Stereolithography was applied to chemical sensors, interdigitated electrodes, and an AFM cantilever fluid cell package. After the cleaning process the devices were tested and passed a functional test. Received: 10 August 2001/Accepted: 24 September 2001 This project was supported by G. W. Woodruff School of Mechanical Engineering. Rapid Prototyping Manufacturing Institute and Microelectronic Research Center sponsored the experiment apparatus. Authors would like to thank Dr. James Gole and Lenward Seals for the test of the packaged chemical sensors, the help from Stan Halpern for dicing the wafer to individual measurement. Finally, authors would like to thank the helps from the MEMS research laboratory at Georgia Tech. This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Reduction of squeeze-film damping in a wafer-level encapsulated RF MEMS DC shunt switch

This paper presents the design, fabrication and characterization aspects of a wafer-level encapsulated RF MEMS shunt switch with a perforated base substrate and a corrugated diaphragm. A three-wafer stacking concept was proposed to achieve a RF MEMS shunt switch based on metal-metal contact. The introduction of damping holes in the base substrate wafer is proven to be an effective way to reduce squeeze-film damping and thus increase the switching speed of the switch. It is also demonstrated by analytical calculation that some factors play important roles on the damping characteristics, such as the physical location of damping holes in the base substrate, hole size, and number of holes per radius ring. By means of the implementation of damping holes, the pull-in and release time of the fabricated MEMS switch are significantly reduced by about 13 times, from 5.4 ms to 0.435 ms and 40.6 ms to 3.2 ms, respectively.     

Separation of beta-human chorionic gonadotropin from fibrinogen using a MEMS size exclusion chromatography column

This article reports a MEMS (Micro-Electro-Mechanical-Systems)-based column separator designed for potential integration with portable medical point-of-care testing (POCT) devices. The MEMS column uses size exclusion chromatography (SEC) to pre-separate raw samples by size, and is made of polydimethylsiloxane (PDMS) fabricated on a glass slide. The MEMS SEC column separates 300 ng/mL of beta-human chorionic gonadotropin (β-hCG), a cancer biomarker, from a fibrinogen-rich solution (20 μg/mL) in 10 min, showing 2.12 resolution and 0.036 mm plate height through fluorescent detection. Results are further verified by β-hCG and anti-β-hCG antibody conjugate using surface plasmon resonance (SPR). The collected β-hCG-rich eluent at 8 min shows 11 mDeg of angle shift. The fluorescent detection and SPR results demonstrate the complete discrimination of β-hCG from fibrinogen using the MEMS SEC column, and illustrate its viability for integrating a sample preparation stage in POCT devices to assist cancer screening and prognosis.     

Through-silicon via technologies for interconnects in RF MEMS

In this paper, we describe the application of through-silicon via (TSV) interconnects in Radio Frequency Micro-electro-mechanical systems (RF MEMS). Using TSV technologies as grounding connections, a Ku band miniature bandpass filter is designed and fabricated. Measured results show an insertion loss of 1.9 dB and a bandwidth of 20%. The chip size is 9.6 × 4 × 0.4 mm³. Using TSV as interconnections for 3 dimensional millimeter-wave integrated circuits, a silicon micromachined vertical transition with three layers is presented. TSV, alignment, bonding and wafer thinning technologies are used to fabricate the sample. This transition has an insertion loss of less than 6.7 dB from 26 to 34 GHz and its amplitude variation is less than 2 dB. The total size of the chip is 6.3 × 3.2 mm².     

Non-resonant electromagnetic wideband energy harvesting mechanism for low frequency vibrations

A novel non-resonant energy harvesting mechanism with wide operation frequency band is investigated for collecting energy from low frequency ambient vibration. A free-standing magnet is packaged inside a sealed hole which is created by stacking five pieces of printed circuit board substrates embedded with multi-layer copper coils. This device was tested under various acceleration conditions. Considering the air damping effect, two types of device structures with different covered plates are investigated. For type I, one covered acrylic plate with drilled air holes and another plate with no holes are used to package the moving magnet. For type II, the middle hole is sealed by two acrylic plates with drilled air holes. The output voltage of type II is better than the one of type I at the same acceleration. When the energy harvester of type II is shook at 1.9 g acceleration along longitudinal direction of the hole, the 9 mV output voltage with 40 Hz bandwidth, i.e., from 40 to 80 Hz, is generated. The maximum output power within the ranges of 40–80 Hz, i.e., operation bandwidth, is measured as 0.4 μW under matched loading resistance of 50 Ω. Experimental results show that type I device has wider frequency bandwidth, higher center frequency and smaller output voltage than type II device because type I device experiences severe damping influence.     

Fibre-optical MEMS switches based on bulk silicon micromachining

 Fibre-optical micro-electro-mechanical systems (MEMS) switches for optical communication systems require high-precision mechanical subassemblies due to the sensitivity of single-mode fibre coupling against misalignments. The fibre diameter of 125 μm also demands for actuators with at least ±62.5-μm travel range. Bulk micromachining based on wet anisotropic etching of crystalline silicon allows fabricating actuators and alignment structures with the required accuracy. Two concepts for lensless moving-fibre switches with thermo-mechanical and electrostatic Si-micromachined actuators with large displacements are demonstrated. Received: 14 January 2002/Accepted: 1 February 2002 This paper was presented at the Workshop “Optical MEMS and Integrated Optics” in June 2001.     

A microtranslation table with scratch-drive actuators fabricated from silicon-on-insulator wafer

From a silicon-on-insulator (SOI) wafer, a microtranslation table with scratch-drive actuator (SDA) has been fabricated. The device Si layer of SOI wafer is etched to form the plate of SDA, which is partially connected to the handle Si substrate by the SiO₂ layer. Dicing the handle Si substrate, a microtranslation table with the SDA array has been fabricated. Placing the microtranslation table upside down on the other Si substrate on which a thin conductive film is patterned for the electrical connection, the microtranslation table is moved by the SDA without carrying a metal wire. The moving velocity of 45.5 μm/s has been obtained by applying the voltage of 120 V at the operating frequency of 500 Hz.     

Glass frit bonding: an universal technology for wafer level encapsulation and packaging

This paper reports on glass frit wafer bonding, which is a universally usable technology for wafer level encapsulation and packaging. After explaining the principle and the process flow of glass frit bonding, experimental results are shown. Glass frit bonding technology enables bonding of surface materials commonly used in MEMS technology. It allows hermetic sealing and a high process yield. Metal lead throughs at the bond interface are possible, because of the planarizing glass interlayer. Examples of surface micromachined sensors demonstrate the potential of glass–frit bonding.