Up until now, although it was possible to manufacture miniscule (e.g. 175µm square) RFID and wireless sensor chips using advanced lithography processes, assembling and packaging such devices was impossible. But a novel transfer printing technology, developed by start-up Terapac Corp. of Waterloo, Ontario, stands ready to render the impossible possible. 3D InCites talked to Terapac’s CTO, Jayna Sheats, Ph.D., to learn more about this disruptive technology, and its implications for the future of micro-assembly from system-in-package (SiP) configurations to 3D IC integration.
What it isn’t and what it is
Not to be confused with traditional printed electronics, which fabricate transistors from the bottom up using fluid materials – one conductive material for source and drain, a semiconductor for channel and an additional conductor for the gate – Terapac’s roll-to-roll printing technology begin with traditionally fabricated ICs, and offers a means of handling these ultra-thin, microscopic bits of silicon from the wafer through to final assembly and packaging using a photo-controlled placement process.
How it works
The process flow begins with a fully processed and diced device wafer, in this case populated with RFID chips manufactured using state-of-the-art lithography to produce “vanishingly” small dice waiting to be placed precisely, accurately, and at top-speed on a flexible substrate already populated with ink-jet printed antennas. By means of a printing plate coated with “Digital Release Adhesive”™(DRA) , a proprietary photoactive polymer that converts quickly (<<1 sec.) from a sticky thermoplastic polymer film into a gas when a stimulus of heat and light is applied, dice are transferred en masse from the wafer to the substrate (Figure 1).
As Sheats explains it, Terapac’s technology replaces conventional pick-and-place processes mostly by focusing on the “pick” part. Rather than selecting die one by one, aligning them and placing them on the substrate, the printing plate picks up an entire array of components at once. Next, employing a concept much like that of proximity printing in microlithography, the appropriate proportions of heat and light are applied to the printing plate to vaporize the DRA, allowing the components to be transferred from the plate to the substrate at submicron accuracy, and at higher speeds than currently available pick-and-place tools allow for. Once placed, interconnects between components are made using printed interconnects rather than wire bonds.
Breaking the mold
As with any emerging technology destined to up-end the conventional apple cart, it’s best to test the waters. So the technology is being introduced to the market through simpler structures in 2D SiP configurations; namely RFID tags, smart cards, and wireless sensors. However, Sheats sees great promise for the process in the 3D integration process flow, where it could address the current conundrum of simultaneously achieving speed and accuracy for volume production die stacking for 3D ICs. “Our technique is beautifully suited to handle these ultra-thin chips at high speed and accuracy.” she said, adding while that it hasn’t been demonstrated yet, based on lab results she’s confident it can be handled.
Off to a good start
According to Sheats, the company’s current business model is one aimed at manufacturing devices themselves. The process requires a dedicated toolset, which Terapac has developed. Although Sheats says they’re not really interested in being equipment manufacturers, the idea is open for discussion. In the mean time, Terepac is beginning with basic HF/NFC and UHF embedded electronic products, and has signed agreements with two customers, who have just received the first samples of ultrathin, flexible, silicon-based products fresh off the pilot line. The company’s flagship product line, Microscale Circuit Cluster™, reportedly offer the advantages of a printed structure with the electrical performance of nano-structured silicon integrated circuits in an elegant, compact, rugged, low-cost package. Additionally, they have signed a collaborative agreement with a major research institute to fabricate and commercialize a flexible medical monitoring system. (I know who it is, but promised not to tell until its official).
For more on this innovative process, be sure to catch Jayna Sheats at IMAPS RF and Microwave Packaging Conference in San Diego, September 22-24, where she has been invited to present a technical update presentation.– F.v.T.