Filip Granek is the CEO of XTPL.

28 September

The challenge in additive manufacturing for modern microelectronics is printing ultra-thin and highly conductive interconnects. The Polish company XTPL has developed an ultra-precise deposition technology that tackles this. CEO Filip Granek explains the technology. He’ll also give a talk during the Bits&Chips Sysarch webinar on 13 October.

Additive manufacturing is an indispensable tool for prototyping and fabricating next-generation microelectronic devices. The printed structures on devices like 3D ICs, displays, sensors, antennas and biomedical devices should possess excellent electrical conductivity and complete adhesion to the substrate, including non-planar substrates and different materials.

In this area, however, commonly used additive manufacturing technologies like inkjet printing and aerosol jet printing have their limitations. Inkjet has a low printing resolution and the printed structures are thin even after multiple passes. It also has limited use on 3D substrates. Aerosol jet resolution goes down to 10 µm after a single pass, but multiple passes are needed for a high height-to-width aspect ratio. Furthermore, the line density is limited due to satellite droplets – as jet technology sprays the ink on the substrate, droplets are falling around the intended path, resulting in irregular printed structures, with the risk of short circuits. Neither inkjet nor aerosol jet allows depositing conductive traces directly on vertical slopes.

XTPL UPD features
UPD’s distinguishing features: high aspect ratios, even for 3D interconnectors, a high printing resolution on 3D topographies and a high printing resolution for microdots.

UPD

With its ultra-precise deposition (UPD) technology, XTPL offers a versatile alternative approach to printing micrometric conductive and non-conductive structures on various rigid and flexible substrates. UPD allows maskless deposition of highly-concentrated silver, copper and gold pastes, up to 85 weight percent of solid content. The printed feature size can be as small as 1 micrometer and the maximum electrical conductivity obtained in this range is around 45 percent of the bulk material.

XTPL UPD printing system
The printing system implementing the UPD technology.

The key parameters for controlling the UPD printing process are the internal opening of the nozzle, the process pressure and the printing speed. These three parameters determine the mass flow or the amount of material deposited on the substrate per unit area, which impacts the printed structures’ width and height. Another parameter to control the process is the time delay between turning on the pressure and the movement of the nozzle. This delay impacts the homogeneity and continuity of the printed structures. Finally, the distance between the nozzle opening and the substrate has to be precisely controlled.

The UPD structures as small as 1 micron remain uniform regardless of the wetting properties of the substrate. Therefore, it’s possible to print on materials such as oxides, nitrides, metals, glass and foils as well as on junctions of metal, semiconductors and insulators.

High-frequency applications

Microstrip antennas on large surfaces such as stickers (microstrip patch antennas) don’t meet the needs of today’s communication systems. For example, a microstrip antenna for FM radio at 100 MHz must be about 1 meter long. For AM radio at 1,000 kHz, the patch must be a soccer field in size. Rather impractical.

The commonly used additive manufacturing technologies have their issues when it comes to high-frequency applications. Aerosol jet printing, for example, is limited to a 20-µm gap size because of the overspray. Moreover, the satellite droplets around the printed signal line will produce radiation on the substrate. Other common problems are the high roughness of deposited structures, limiting the transmission frequency, and low adhesion to the substrate. UPD deals with these issues: it prints smooth silver lines and has very high adhesion to a wide range of substrates like glass, silicon and flexible foils.

XTPL silver lines on PEN foil
An example set of silver lines printed on a PEN foil. The line width is 3.2 µm and the interline distance is 0.7 µm. The inset shows a more distant view of the sample.

Its features make UPD well suited for high-frequency applications, giving it competitive advantages over other additive manufacturing technologies. The high surface smoothness, the constant line width and the constant line space limit high-frequency signal losses. The resulting printed structures are tailored for signals above 300 GHz; for interconnections above 330 GHz, we believe UPD is the only option.

Displays

UPD also allows depositing 3D interconnections for advanced packaging. This includes hybrid electronics that combine printed electronics and silicon technologies. Its capability to print on steps satisfies the requirements for the fabrication of interconnects in microelectronic devices like micro-LED arrays.

XTPL smooth silver lines
A set of continuous printed silver lines with a width of 15 µm printed on a step ten times higher (150 µm) than the width of the lines.

Another capability is printing microdots for electrical contacts. UPD produces a smooth, lens-like shape, which helps to deposit additional layers. Such a shape is very different from microdots obtained using lithography, which generally have a rectangular shape.

XTPL UPD printed microdots
UPD-printed microdots (the bar corresponds to 100 μm), together with a magnification of a single microdot. The dots have a diameter of around 13 µm and a height of approximately 2.5 µm.
XTPL UPD mass production
UPD for mass production. This shows part of the sample of 7,500 printed segments for transistors with a line width of 4 μm.

We’ve also demonstrated UPD for printing arrays of source-drain structures for printed flat panel displays. The technology’s critical features here are the line width and the ability to reduce the interline distance to single micrometers. Moreover, the shape to be printed can be defined arbitrarily, which supports lean manufacturing.

Recently, a Taiwanese semicon equipment builder integrated UPD into an industrial machine. This marks a step toward the next phase of industrializing our technology in production lines for future generations of electronics.

Filip Granek will be presenting at the Bits&Chips Sysarch webinar on 13 October. This is a condensed version of an article that was published in Nature Scientific Reports.