David Gomez

Pressure activated interconnection of micro transfer printed components

Micro transfer printing and other forms of micro assembly deterministically produce heterogeneously integrated systems of miniaturized components on non-native substrates. Most micro assembled systems include electrical interconnections to the miniaturized components, typically accomplished by metal wires formed on the non-native substrate after the assembly operation. An alternative scheme establishing interconnections during the assembly operation is a cost-effective manufacturing method for producing heterogeneous microsystems, and facilitates the repair of integrated microsystems, such as displays, by ex post facto addition of components to correct defects after system-level tests. This letter describes pressure-concentrating conductor structures formed on silicon (1 0 0) wafers to establish connections to preexisting conductive traces on glass and plastic substrates during micro transfer printing with an elastomer stamp. The pressure concentrators penetrate a polymer layer to form the connection, and...

Miniaturized LEDs for flat-panel displays

Inorganic light emitting diodes (LEDs) serve as bright pixel-level emitters in displays, from indoor/outdoor video walls with pixel sizes ranging from one to thirty millimeters to micro displays with more than one thousand pixels per inch. Pixel sizes that fall between those ranges, roughly 50 to 500 microns, are some of the most commercially significant ones, including flat panel displays used in smart phones, tablets, and televisions. Flat panel displays that use inorganic LEDs as pixel level emitters (μILED displays) can offer levels of brightness, transparency, and functionality that are difficult to achieve with other flat panel technologies. Cost-effective production of μILED displays requires techniques for precisely arranging sparse arrays of extremely miniaturized devices on a panel substrate, such as transfer printing with an elastomer stamp. Here we present lab-scale demonstrations of transfer printed μILED displays and the processes used to make them. Demonstrations include passive matrix μILED displays that use conventional off-the shelf drive ASICs and active matrix μILED displays that use miniaturized pixel-level control circuits from CMOS wafers. We present a discussion of key considerations in the design and fabrication of highly miniaturized emitters for μILED displays.

Heterogeneous integration of microscale compound semiconductor devices by micro-transfer-printing

Integrating microscale electronic devices onto non-native substrates enables new kinds of products with desirable functionalities and cost structures that are inaccessible by conventional means. Micro assembly technologies are the practical ways to make such microscale heterogeneous device combinations possible. Elastomer stamp micro-transferprinting technology (μTP) is a widely-demonstrated form of micro assembly, having demonstrated applicability in optical communications, magnetic storage, concentrator photovoltaics and display technologies. Here we describe new experiments designed to assess the useful lifetime of the viscoelastic elastomer transfer stamp, and also describe the methodology and results for heterogeneous integration of microscale compound semiconductor devices onto non-native substrates using μTP.

Miniature Heterogeneous Fan-Out Packages for High-Performance, Large-Format Systems

High-throughput assembly of miniature wafer-fabricated packages onto panel substrates provides a manufacturing framework for high-performance multi-functional displays and other large-format systems. Control circuits, light emitters, sensors, and other micro-components formed in high-density arrays on wafers use a variety of processes and materials that do not easily translate to large-format panel processing. Systems assembled from some or all of those components can therefore exhibit combinations of properties and performance characteristics that are difficult to achieve by panel processes only. Here, we demonstrate hierarchical assembly strategies for fabricating high-performance systems using elastomer stamp micro-transfer-printing. In this work, red, green and blue microscale inorganic LEDs (µILEDs) are fabricated on their respective native wafer substrates and then assembled onto non-native intermediate silicon wafers. The intermediate silicon wafer, populated with heterogeneous µILEDs, then undergoes conventional wafer-level processes, such a dielectric depositions and thin-film metallization, to form miniature fan-out packages. Here, we will demonstrate three heterogeneous µILEDs integrated within a 75 µm × 35 µm fan-out package. We will present how this microscale package can be undercut and then micro-transfer-printed directly onto large-format application substrates. The print-compatible packages also include sharp pressure-concentrating conductor structures which allow the heterogeneous fan-out packages to be electrically interconnected to large-format substrates during the printing operation. We will present functional µILED displays that have been fabricated using these assembly techniques. We will report on the benefits of using intermediate packaging substrates for manufacturing of high-performance large-format systems, such as displays. We will also demonstrate strategies for repairing large multi-functional systems.

Pressure-Activated Electrical Interconnection During Micro-Transfer-Printing

Sharp electrically conductive structures integrated into micro-transfer-print compatible components provide an approach to forming electrically interconnected systems during the assembly procedure. Silicon micromachining techniques are used to fabricate print-compatible components with integrated, electrically conductive, pressure-concentrating structures. The geometry of the structures allow them to penetrate a polymer receiving layer during the elastomer stamp printing operation, and reflow of the polymer following the transfer completes the electrical interconnection when capillary action forces the gold-coated pressure-concentrator into a metal landing site. Experimental results and finite element simulations support a discussion of the mechanics of the interconnection.

Process Capability and Elastomer Stamp Lifetime in Micro Transfer Printing

Elastomer stamp based micro assembly or micro-transfer printing is a practical method for heterogeneous integration of micro-scale devices onto non-native substrates. In this paper, we evaluate the effect of stamp lifetime on performance and assess the useful lifetime of a stamp, both key metrics for using this technology in a manufacturing environment. We also review the performance of micro transfer-printing in several applications where >99% print yields and precise placement has been demonstrated.

Scalability and Yield in Elastomer Stamp Micro-Transfer-Printing

Elastomer stamp micro-transfer-printing is a highly scalable method for the assembly of microscale components onto non-native substrates. One of the key value propositions of micro-transfer-printing is that the transfer stamp can be scaled to wafer-dimensions and can transfer tens to thousands of micro-devices in a single step, equating to multiple millions of units per hour. Here, we report on the results of systematically scaling the stamp from 12.8 mm × 12.8 mm to a full 150 mm stamp, capable of transferring all the required devices to a 150 mm receiving wafer in one operation. The 150 mm stamp is designed to transfer more than 80,000 chips in one print cycle. This study was carried out using silicon nitride test vehicles that were specially designed for this project. We will discuss how stamp scaling impacts transfer yield and the implications for ultra-high throughput assembly of micro-devices. In addition, we will explore the capability to transfer very small devices down to 3 µm × 3 µm.