> Rich Duncombe, Distinguished Technologist

Contact: Imaging and Printing Group Hewlett Packard Company Corvallis, Oregon 97330

Ink jet printhead technology has been around for many years now and HP has long been at the forefront of this technology. From your position in this industry, what have been the most significant advances in the evolution of this product?

In 1984, HP’s calculator division introduced the first thermal inkjet (TIJ) printer, the HP ThinkJet; designed for use in small, battery-powered systems. The next generation printers, the HP DeskJets, and printheads were designed for desktop applications. The DeskJet designs became increasingly more sophisticated and capable. The glass substrates in the cartridges were replaced with silicon and new methods of fabricating the orifice plates and nozzles enabled higher print resolutions and performance. The development of new black and color inks paralleled the evolution of the printer and printhead providing products that produced prints that rivaled photographic realism and archival properties.

Over the last decade, HP engineers have focused on everincreasing integration of the printhead functions to provide higher performance, print quality, and lower cost. By leveraging and adapting advances in thin film semiconductor processing, silicon micromachining and semiconductor packaging, it became possible to integrate both the power electronics and per-nozzle interconnects to enable higher throughput and color resolution without excessive system cost and size. The development of compact firing chamber architectures enabled smaller ejected drop volumes and higher nozzle packing densities. The smaller drops required less firing energy per drop for increased frequency and higher throughput. Furthermore, the smaller drops provided more colors per dot, lighter tones and photo-quality printing on a wide variety of media. A timeline of HP’s TIJ technology development is shown in Figure 1.

 

What has been the specific impact of advances in MEMS technology?

HP’s latest generation of printhead technology, ink chambers, fluidic features and nozzles are formed monolithically using photo-imageable thick-film polymers. All-lithographic patterning coupled with new advances in silicon surface and bulk micromachining techniques eliminates the need for mechanical alignment processes and results in fluidic features that have the precision and integration scale of semiconductor devices and circuits. Examples of lithographically defined fluidic features are shown in Figure 2. Addtionally, Scalable Printing Technology (SPT)-based printheads combine Pentium-class addressing circuitry, high voltage mixed-signal driver electronics, dense electrical interconnects, and up to 3900 high-precision microfluidic devices. These devices operate at high temperatures in the presence of aggressive inks and are expected to last for more than five years.

Over the last 25 years, HP has continually pushed the limits of thermal inkjet printhead performance as shown in Figure 3. Moving from 12 nozzles in 1984 to thousands of nozzles in the current generation of products, HP has roughly doubled the number of drops-per-second ejected from a printhead every 18 months similar to Moore’s Law in the semiconductor industry.

 

What are the latest trends in technology development, especially since the launch of the Scalable Printing Technology (SPT)?

As HP’s thermal inkjet printhead technology has evolved, new processing, fabrication and manufacturing methods have been required. Furthermore, the extreme operating conditions of the thermal inkjet ejection process have required the development of new robust materials systems and innovative packaging technologies. As a consequence of meeting these demands, HP has established a unique expertise in integrating MEMS, microfluidics, novel materials, and semiconductor devices and circuitry. Over the last decade, HP has been leveraging these new capabilities and expertise to enable the invention and development of new MEMS-based technologies.

In the late 1990’s, HP launched a program to develop a new, high-density probe-based memory technology called Atomic Resolution Storage (ARS). The ARS device was a thumbnail-size device with storage densities greater than one terabit per square inch. Based on advances in atomic probe microscopy, the system used a dense array of electron field emitters to bombard the surface of a novel phase change material locally modifying the medium to change its state (in a bit-wise fashion) from polycrystalline to amorphous or vice-versa. Interrogation was done with lower electron currents by detecting either back-scattered electrons or by changes in the semiconducting properties of the storage media. A key requirement of the ARS technology was ability to physically scan the media surface at nanometer scale resolution and at access times that were superior to current hard disk drives. In order to accomplish these objectives a novel MEMS-based electrostatic micro-mover was developed and fabricated.

Leveraging the innovative bulk and surface micromachining methods that were incorporated into the latest generations of TIJ printheads and by developing state-of-the-art deep silicon etching processes and wafer bonding techniques, the HP micro-mover provided unprecedented performance with a step size of 0.17 nanometers (approximately the width of three hydrogen atoms) and with nanoscale positioning accuracy and repeatability in both the x- and y-axis. Micrographs showing the micro-mover plate and plate flexures are show in Figure 4.

More recently, HP has adapted the MEMS-based electrostatic micro-mover developed for ARS into a new mechanical sensor technology. Exploiting the nano-scale precision of the ARS electrostatic mover, researchers at HP Labs and at the HP Technology Development Lab in Corvallis Oregon have developed a compact chip-scale accelerometer with a dynamic range of more than six orders of magnitude and with a demonstrated noise floor equivalent to seismic grade. Testing and modeling of this technology suggest that this same sensor will achieve navigation grade performance making it applicable to inertial guidance applications.

As discussed above, the thermal inkjet ejection process places extreme demands on printhead materials and interfaces. Working at the leading edge of material science and nanotechnology, scientists and engineers in HP’s printhead development groups have made significant progress in the development of new thermally and mechanically robust materials, adhesives, thin film coatings, wafer bonding techniques, and hermetic packaging. These new materials and assembly processes are also extending HP’s MEMS technology platform are enabling the development of new MEMS-based applications that range from novel optical switching devices to high performance electrical and sensing devices and systems.

 

For the print head industry, what will be the impact of the emergence of new inkjet technology for industrial applications? What are the current challenges to overcome for MEMS thermal inkjet heads? How will HP react to these changes?

The critical issue for thermal inkjet technology in industrial and commercial printing markets is to expand the operating specifications of the system to serve these markets. The MEMS printhead is certainly a key part of the system and HP continues to invest in SPT to broaden the applicability of thermal inkjet. However, successfully transitioning analog print markets to digital requires a full system solution. Driving thermal inkjet systems into commercial and industrial markets requires broadening the range of inks, increasing throughput, lowering operating costs and improving printhead durability. Given HP’s fundamental position in thermal inkjet technology and systems, HP has successfully extended inkjet into a number of new applications. Four examples are included to illustrate the broad applicability of thermal inkjet: Retail Publishing Solutions, Inkjet Web Press, Dual-Drop-Volume Technology and Thermal Inkjet Latex Inks.

Retail publishing solutions
Historically, retail photo printing was dominated by silver halide technology. The historical view was that inkjet prints did not have adequate image quality or permanence and the system could not meet the throughput or cost of silver halide. HP changed that perception with a continuous wave of innovation and introductions into the retail market starting in 2006.

In 2006, HP introduced a suite of retail photo printing systems at the Photo Marketing Association (PMA) show. At that time, the HP Photosmart Express station was the most advanced standalone, self-service photo kiosk dedicated to producing 4 x 6-inch prints quickly – in as fast as five seconds. The HP Photosmart Studio was the first and only in-store solution where consumers can design creative products in about five minutes – without having to crop or place photos into individual templates – and pick them up in one hour or less. HP won wide press accolades and best in show awards for these innovations.

In 2008, HP introduced the Photosmart ml1000 Minilab printer, a high-performance dry inkjet retail photo printing system. It delivers lab quality photos as fast as 1,500 prints per hour in up to 15 different sizes in gloss and matte finishes. The system uses specially designed inks and papers to create smudge-, water-, and fade-resistant prints that retain vivid, accurate colors for generations. Redefining retail photo printing, the HP Photosmart ml1000 Minilab printer produces the longest lasting photos in retail—five times longer than traditional photos, delivering the ―best overall image permanence of any consumer 4x6-inch prints in the entire 130-year history of color photography, according to Henry Wilhelm, Wilhelm Imaging Research, Inc.

HP Inkjet web press
In May 2008, HP introduced the revolutionary HP Inkjet Web Press, the company’s first digital printing platform based on the SPT designed to increase print productivity and lower the cost of printing for high-volume commercial markets. Revitalizing wasteful, low-margin printing processes, the press offers an unprecedented combination of print width, color quality, productivity and cost for graphic arts industry professionals in the direct mail, transactional and transpromotional printing, book publishing and newspaper industries.

HP Inkjet Web Press technology offers a number of technological advances that bolster productivity, including a design innovation that allows 10,560 nozzles per printhead and a bonding agent that secures the press’s pigment inks to the surface of the paper. The high nozzle count improves performance in terms of image quality, productivity and fault tolerance. The bonding agent pre-treats the media only where ink is applied using the same printheads, and increases color vibrancy while minimizing show-through on lightweight media.

The press’s 762-mm size enables a full range of flexible imposition options, such as 12- and 16-page book signatures, 760-mm wide newspaper broadsheets and 178 x 356-mm (7 x 14-inch) transactional documents printed four-up. For standard A4 or U.S. letter-size documents, the HP Inkjet Web Press can produce up to 2,600 pages per minute.

Dual-drop-volume technology
HP’s dual-drop-volume cartridge technology eliminates the need for lighter, photo dye-load inks, reducing image grain and increasing the available color gamut for photos. With HP dual-drop-volume cartridge technology, customers can get photo quality comparable to six-color inkjet printers with a five ink printing system. Dual-drop-volume printing offers customers exceptional versatility by allowing the printer to produce light color shades and rich photographic black essential for lab-quality photos and vibrant color for graphics—all from a single set of ink supplies.

In dual-drop-volume modes, the printer can eject color and photo black ink droplets as small as 1.3 pl and as large as 5.2 pl. About 3.8 billion of these 1.3 picoliter droplets would fit in a teaspoon. The printer can fire any combination of these small or large ink droplets on a given print position— small and large droplets together, just small drops or just large. The size and combinations of ink droplets are determined by the printer in order to provide the optimum print quality and speed. Dual-drop-volume printing is used when printing photos and color graphics in Normal, Best and Maximum-dpi print modes on select HP photo and brochure papers.

Thermal inkjet latex inks
HP Latex Inks deliver long-lasting, durable prints withstand the elements; sharp, vivid image quality stands up to close inspection. These water-based inks produce great results across a wide range of media including most low-cost, uncoated solvent-compatible media and eye-catching, consistent results with Original HP large-format media designed together with HP Latex Inks. Broad outdoor and indoor application versatility gives print service providers more possibilities to meet customer needs.

Outdoor prints produced with HP Latex Inks achieve display permanence up to 3 years unlaminated and up to 5 years laminated and are scratch, smudge, and water resistant on a range of media performance comparable to solvent-ink technology. Indoor prints produced with HP Latex Inks achieve display permanence in-window up to 5 years unlaminated and up to 10 years laminated on a range of media.

HP Latex Inks together with HP Wide Scan Printheads produce a wide color gamut comparable to solvent-ink technology for sharp, vivid image quality. The HP Wide Scan Printhead design also makes it possible to produce high quality output at fast print speeds, supporting an ink drop size as small as 12 picoliters.

HP Latex Inks produce odorless prints, providing the best of both worlds. Prints are durable enough for demanding applications such as outdoor display but they lack the noticeable odor that can limit indoor applications a characteristic more typical of prints produced with solventink technology.

Prints produced with HP Latex Inks offer print service providers the opportunity to expand the variety of outdoor and indoor applications they offer to environmentally conscious customers. HP Latex Inks are compatible with most solvent compatible media and are expected to provide good image quality, display permanence, and durability. Odorless prints produced with HP Latex Inks emit extremely low levels of VOCs (volatile organic compounds). There is no special ventilation required to meet occupational exposure limits and there are not requirements for air discharge permitting, facilitating an improved printing environment.