Jiandong Fang

Controlled multibatch self-assembly of microdevices

A technique is described for assembly of multiple batches of micro components onto a single substrate. The substrate is prepared with hydrophobic alkanethiol-coated gold binding sites. To perform assembly, a hydrocarbon oil, which is applied to the substrate, wets exclusively the hydrophobic binding sites in water. Micro components are then added to the water, and assembled on the oil-wetted binding sites. Moreover, assembly can be controlled to take place on desired binding sites by using an electrochemical method to deactivate specific substrate binding sites. By repeatedly applying this technique, different batches of micro components can be sequentially assembled to a single substrate. As a post assembly procedure, electroplating is incorporated into the technique to establish electrical connections for assembled components. Important issues presented are: substrate fabrication techniques, electrochemical modulation by using a suitable alkanethiol (dodecanethiol), electroplating of tin and lead alloy and binding site design simulations. Finally, we demonstrate a two-batch assembly of silicon square parts, and establishing electrical connectivity for assembled surface-mount light emitting diodes (LEDs) by electroplating.

Wafer-level packaging based on uniquely orienting self-assembly (the DUO-SPASS processes)

A wafer-level packaging strategy for micro device chips based on uniquely orienting self-assembly is presented with the following steps: 1) bulk parts are uniquely face- oriented and spread in a single layer; 2) Parts are palletized onto an alignment template having an array of receptor sites; 3) Parts are anchored one-to-one to the receptor sites; 4) Each anchored part is fixed to a unique in-plane orientation. We demonstrate all of these steps with two different self-organizing parallel assembly (SPASS) processes: a semidry uniquely orienting process (semi-DUO-SPASS) and a dry uniquely orienting (DUO-SPASS) process. The semidry process exploits: 1)An agitated air/water interface to uniquely face-orient bulk parts having a single hydrophobic face; 2) A hydrophobic carrier wafer to palletize the parts in an air environment; 3) Orbital shaking to drive the parts until they are anchored to receptor sites; 4) Gravity to uniquely align the parts. Experiments show that 2-mm square silicon parts are correctly registered on a 4-in alignment template having 164 receptor sites with a defect rate of /spl sim/1% after 3min orbital shaking. The dry process utilizes: 1) Asymmetry in dynamic stability to uniquely face-orient bulk parts having protruding features on one face; 2) Orbital shaking to drive the parts until they are first anchored to receptor sites and then fixed in well-defined in-plane orientations by two-stage shape recognition. In our experiments, 1-mm square silicon parts are assembled with a defect rate of /spl sim/2% in 10min on each of two 4-in alignment templates having, respectively, 397 and 720 receptor sites.A wafer-level packaging strategy for micro device chips based on uniquely orienting self-assembly is presented with the following steps: 1) bulk parts are uniquely face- oriented and spread in a single layer; 2) Parts are palletized onto an alignment template having an array of receptor sites; 3) Parts are anchored one-to-one to the receptor sites; 4) Each anchored part is fixed to a unique in-plane orientation. We demonstrate all of these steps with two different self-organizing parallel assembly (SPASS) processes: a semidry uniquely orienting process (semi-DUO-SPASS) and a dry uniquely orienting (DUO-SPASS) process. The semidry process exploits: 1)An agitated air/water interface to uniquely face-orient bulk parts having a single hydrophobic face; 2) A hydrophobic carrier wafer to palletize the parts in an air environment; 3) Orbital shaking to drive the parts until they are anchored to receptor sites; 4) Gravity to uniquely align the parts. Experiments show that 2-mm square silicon parts are correctly registered on a 4-in alignment template having 164 receptor sites with a defect rate of /spl sim/1% after 3min orbital shaking. The dry process utilizes: 1) Asymmetry in dynamic stability to uniquely face-orient bulk parts having protruding features on one face; 2) Orbital shaking to drive the parts until they are first anchored to receptor sites and then fixed in well-defined in-plane orientations by two-stage shape recognition. In our experiments, 1-mm square silicon parts are assembled with a defect rate of /spl sim/2% in 10min on each of two 4-in alignment templates having, respectively, 397 and 720 receptor sites.

Parallel micro component-to-substrate assembly with controlled poses and high surface coverage

We demonstrate a novel parallel micro assembly process based on both shape recognition and capillary-driven self-assembly in an air environment. Mechanically diced 790 µm square silicon parts with flat or step edges were used for proof-of-concept demonstrations. Each part had only one hydrophobic 790 µm × 790 µm face and its other faces were hydrophilic. On a vibrating plate, tumbling parts were captured by cavities having an opening clearance that only admitted a single part standing vertically. The trapped parts were then transferred to a substrate having an array of receptor sites covered with water droplets. The flat-edge parts attached vertically to these sites and then capillary forces from water condensate turned them to face the substrate with their 790 µm × 790 µm hydrophilic faces. The step-edge parts attached at a tilted angle due to their featured edges and then a pressing plate laid them down. This process assembled micro parts to 1000 densely packed receptor sites in about 2 min with a defect rate of ~1%. A single batch assembly process achieved 31% surface coverage, and a second batch doubled the ratio to 62%.

Self-assembly of PZT actuators for micropumps with high process repeatability

In this paper, we report a novel capillary-driven self-assembly technique which proceeds in an air environment and demonstrate it by assembling square piezoelectric transducer (PZT) actuators for 28 diffuser valve micropumps on a 4-inch pyrex/silicon substrate: on the substrate, binding sites are wells of 24 mum in depth and the only hydrophilic areas; on the bonding face of the PZT actuator, the central hydrophilic area is a square identical in size to the binding site, and the rim is hydrophobic; acrylate-based adhesive liquid is dispensed across the substrate and wets only the binding sites; the hydrophilic areas on the introduced PZT actuators self-align with the binding sites to minimize interfacial energies by capillary forces from the adhesive droplets; the aligned PZT actuators are pressed to contact the gold coated substrate by their rims and the adhesive is polymerized by heating to 85 degC for half an hour, so permanent mechanical and electrical connections are established, respectively, at the center and rim of each PZT actuator. These pumps perform with high uniformity, which is indicated by a small standard deviation of their resonant frequencies to pump ethanol: the average resonant frequency is 6.99 kHz and the standard deviation is 0.1 kHz. Compared with the conventional bonding process with highly viscous silver epoxy, this assembly method has several major advantages: highly accurate placement with self-alignment, controllable adhesive thickness, tilt free bonding, low process temperature and high process repeatability

High yield batch packaging of micro devices with uniquely orienting self-assembly

We demonstrate a high yield wafer level packaging technique for micro devices on the basis of uniquely orienting self-assembly with 2mm square diced silicon parts. Each silicon part has one hydrophobic thiolated gold face and one circular peg, offset from the center of mass, on the opposite face. A receptor site on an alignment template has a circular trap hole. The whole assembly process consists of five major steps: (1) bulk parts are oriented to face the same direction; (2) parts are palletized onto the alignment template; (3) parts are one-to-one distributed to receptor sites; (4) parts self-align to receptor sites with a unique in-plane orientation; and (5) parts are bonded to a chip carrier template. The experimental results indicate that step 1-4 have yields close to 100%. We skip step 5, a well established process widely used in the IC industry. This packaging strategy can be applied for any shape of silicon or non-silicon parts, and the assembly mechanism itself imposes no upper limit on the size of the assembly templates.

Uniquely orienting dry micro assembly by two-stage shape recognition

We demonstrate a completely dry, uniquely orienting and parallel micro assembly method on the basis of two-stage shape recognition between complementary features on parts and receptor sites. A dry environment benefits assembly of chips with either exposed movable microstructures or materials sensitive to liquid environments; uniquely orienting self-alignment based on complementary features works for any part shape; parallel assembly greatly increases throughput for mass production. In our experiments, 1 mm square dummy silicon parts were assembled with a defect rate of /spl sim/2% in 10 minutes on each of two assembly templates having respectively 397 and 720 receptor sites. This assembly technique enables either wafer level packaging of micro device chips or easy part feeding and palletizing for robotic assembly systems without constraints of part shapes or materials.

Vertical and Horizontal Parallel Mounting of Micro Components on a Substrate with High Surface Coverage

We demonstrate a novel parallel micro assembly process based on both shape recognition and capillary-driven self-assembly in an air environment. Mechanically diced silicon parts with dimensions of 790 µ m × 790 µ m × 330 µ m are used for proof-of-concept demonstrations. Each part has only one hydrophobic 790 µ m × 790 µ m face and its other faces are hydrophilic. On a vibrating plate, tumbling parts are captured by cavities having an opening clearance that only admits a single part standing vertically with a 790 µ m × 330 µ m footprint. The trapped parts are then transferred to a substrate having an array of receptor sites covered with water droplets. Initially the parts are vertically attached, but capillary forces from water condensate turn them to face the substrate with their 790 µ m × 790 µ m hydrophilic faces. This process assembles micro parts to densely packed 1000 receptor sites in about 2 minutes with a defect rate ~ 1%. A single batch assembly process achieves 31% surface coverage, and a 2ndbatch doubles the ratio to 62%.

1.14 – Self-Assembly

Self-assembly, a process typically based on interfacial energy minimization, enables rapid fabrication and packaging of microdevices. Microcomponents pose significant challenges for traditional handling techniques such as robotic pick and place because adhesion forces (electrostatic, capillary, and van der Waals interactions) dominate over gravitational forces in microdomains. This chapter reviews virtually all the published self-assembly methods that deal with sticky microcomponents. These self-assembly methods can be classified into four major categories: electrostatic-, magnetic-, inertial-, and capillary force-driven self-assembly. Electrostatic attraction can assemble neutrally charged and electrically polarizable microcomponents on an array of charged sites or holes with fringing electric fields; electrostatic repulsion, due to the similar charges on microcomponents and a carrier substrate, can construct 3D microstructures. Magnetic forces, short-range interactions, can attach microcomponents to magnetized sites on a substrate and can build 3D microstructures by varying the driving magnetic fields and by employing locking features. Inertial forces successfully applied to self-assembly are gravitational and centrifugal forces: gravitational forces anchor microcomponents to recessed sites on a horizontal substrate and centrifugal forces flip the hinged microplates to form 3D structures. Capillary forces from droplets on a substrate attract and anchor microcomponents in flat or 3D tilted poses. Employing two or more different self-assembly mechanisms in sequence can achieve an assembly of microcomponents to satisfy more stringent requirements such as unique face and in-plane orientations of microcomponents. At the end of this chapter, a physical model for developing a theoretical understanding of the self-assembly processes as well as for designing tools to optimize the processes and to establish the theoretical and practical bounds on their performance is given.