Hewlett-Packard Co. pioneered microelectromechanical systems (MEMS) for printheads in the 1980s. By combining integrated electronics with microfluidic channels, HP realized what it calls the "thermal-inkjet Moore's Law." Today's printheads feature Pentium-class microcontrollers on the front side of the chip, with the back side covered with microfluidic channels supporting up to 3,900 inkjet nozzles.
Now HP is applying its expertise in MEMS and CMOS integration to extend the semiconductor Moore's Law, from seismic-grade sensors to maskless lithography to nanotechnology approaching the atomic scale. Tim Weber, direc- tor of research and development for HP Labs' Technology Development Operation, described that work to EE Times contributing editor R. Colin Johnson.
EE Times: What processing approach did HP use for its first MEMS products?
Tim Weber: HP first invested in MEMS technology for thermal inkjet printers. We introduced our first MEMS-based product as early as 1984.
We use a resistor to heat a drop of ink on one side of the chip, then define an orifice leading to the other side, where we create microfluidic channels for the ink to flow under the control of circuitry. In the old days that circuitry was direct drive, but today it's a complete integrated circuit that sequences power pulses to all the resistors in the printhead, causing droplets of ink to eject.
EE Times: So you use both surface and bulk-silicon micromachining?
Weber: On one side, we use surface micromachining to define the microfluidic channels. Then we use bulk micromachining to get the ink from the back side of the substrate to the front side.
EE Times: How do you cap your chips--at the wafer level or with a hermetically sealed package?
Weber: We used to laminate what we called an orifice plate onto our device, but today our printheads are completely monolithically built in the wafer fab. Everything is built up using photolithography and different materials.
EE Times: So there's no glass cap?
Weber: They're actually capped with a photo-imageable polymer--a thick film that can be used in the fab just like other photolithographic materials.
EE Times: Do you use the same fabs that make your regular CMOS circuits?
Weber: We started out with the fab used for HP calculators. But we have done quite a few modifications on it, and now it's pretty well dedicated to MEMS devices. We also have a few other fabs that are pretty well dedicated to MEMS.
EE Times: How much circuitry do you integrate on a chip that also holds microfluids?
Weber: If you're talking about our scalable printhead technology, which we introduced in 2005, then our printheads have the equivalent of a Pentium processor on one side--roughly half a million transistors at Pentium 1 line widths, dedicated to managing the 3,900 nozzles on the printhead.
We went from just a few nozzles in 1984 to thousands of nozzles today in steps that make an interesting comparison to Moore's Law for the semiconductor industry. Every 18 months, we doubled the number of drops per second fired from our printheads [via] a combination of more nozzles and faster firing frequencies. We call this the thermal-inkjet Moore's Law.
EE Times: Interesting comparison.
Weber: Not only did we put on more nozzles every 18 months, but we also could sequence the nozzles faster.
EE Times: The more nozzles you have, the higher the resolution?
Weber: The higher the resolution, the faster you can run--and the less ink you use per droplet. Our first products used big drops that you could see with the naked eye--about 85 picoliters. But today's nozzles make drops smaller than the naked eye can see--1,200 droplets per inch for our scalable technology.
EE Times: Describe your ASIC.
Weber: We are managing 3,900 nozzles, each of which can fire at 48 kHz. We take the data from the printer and use digital processing to spread it out across all the nozzles. And since the resistors that heat the ink are fairly high-power components, we use transistors that are very high-voltage [up to 40 volts]. We also have mixed-signal circuitry for energy-, temperature- and quality-management algorithms that run in the analog domain so we can make sure our printheads are consistent. You don't want the customer to see variations.
EE Times: Do you use a pulse code to control each squirt?
Weber:Yes, per each firing event--which lasts only about 1 microsecond, though the heat transferred at the resistor's surface is greater than the the heat transfer at the surface of the sun. [The process] literally vaporizes a layer of ink instantaneously, making a bubble that then acts like a piston. That's how we get uniform ejection, over and over.
We have to protect all our circuitry from being contaminated with ink, so from a MEMS standpoint, we have to be much more careful about what materials we use. Often, we can't use the standard CMOS temperatures and heat.
EE Times: What are the design rules?
Weber: Typically, for our CMOS ASIC, we only need 0.8 micron, which is roughly a Pentium 1 integration level.
EE Times: Can HP adapt its MEMS capabilities to other applications?
Weber: We have developed a sensing technology and integrated it into an accelerometer that provides a dynamic range of six orders of magnitude with a demonstrated noise floor equivalent to seismic grade. Our testing and modeling lead us to believe this same sensor will achieve navigation grade for inertial guidance. Consequently, we are looking at a number of applications areas.
EE Times: Like what?
Weber: We are developing a gyro, based on the same sensing technology, that will be able to sense rotations very keenly. When this gyro is integrated with our accelerometer, we will have a sensor capable of measuring precise locations through dead reckoning. This would have applications in the consumer space.
EE Times: Have you presented any papers on this work?
Weber: No, this is the first time we have revealed how well our work on accelerometers and gyros is going.
EE Times: Many consumer applications--such as cell phones that scroll by being tipped and smart GPS--could use MEMS gyros. Even videogame controllers are predicted to use MEMS gyros one day. Those are big markets.
Weber: We are looking at other ways to apply our technologies also. We are using our thermal-inkjet technology itself to directly print electronic circuits--a form of maskless lithography with new materials. Here too, we think we are uniquely positioned. For instance, many labs have announced inkjet techniques to directly write organic semiconductor materials, but we have developed jettable inorganic metal oxides that can enable higher-performance circuits.
We have been supporting the HP Labs nanotech groups on several projects. Through prototyping, we were able to contribute to HP's [nanowire] crossbar switch announcement last month. We contributed to the switch's fabrication using our CMOS capabilities. We hope to help HP Labs extend Moore's Law further than many have thought possible.