Printed Electronics and Spiders

What on earth have spiders got to do with printed electronics?  Well, as is often the case, nature shows us how to do really difficult tasks in quite spectacular ways.  Spiders are well known for their webs, approximately 40,000 species of spider exist many of which produce thin silk fibres spun from pressurized abdominal sacs containing a polymeric solution. These sacs eject tiny jets that dry in-flight to produce the fibres with diameters of approximately 2.5 to 4.0 microns and are used to make the intricate patterned webs we see all around us.  Studying these natural processes has led to the development of microfluidic liquid jet printing and hydrodynamic printing concepts.

A recent patent application US20160129634 was published on 12 May 2016 by inventors David Keicher and Adam Cook of the Sandia Corporation and describes how this process can be used to create very thin inkjets suitable for use in direct printed electronic applications.

JetSummary:  The present invention is directed to an apparatus for two-fluid hydrodynamic printing, comprising a coaxial tube assembly, comprising an inner tube having an exit orifice for flowing an ink stream therethrough and an annular outer tube for flowing a sheath fluid therethrough wherein the sheath fluid has a higher velocity than the ink stream such that the ink stream is hydrodynamically focused by the outer sheath fluid upon exiting from the exit orifice of the inner tube. The apparatus can further comprise a focusing nozzle downstream from the exit orifice of the inner tube for further focusing of the ink stream therein. The apparatus can further comprise means for removing the sheath fluid from the ink stream downstream from the exit orifice of the inner tube and a recirculation channel for recirculating the removed sheath fluid. For example the ink can comprise polyvinylpyrrolidone and the sheath fluid can comprise water, alcohol, or a combination thereof. The focused ink stream can be deposited on a substrate.

Print results using two-fluid hydrodynamic focusing yielded a 30 μm wide by 0.5 μm tall line that suggests that the cross-section of the printed feature from the print head was approximately 2 μm in diameter. Printing results also demonstrated that complete removal of the sheath fluid is not necessary for all material systems. For example, hydrodynamic printing of two fluids enables printing of insulated conductors and clad optical interconnects.

The inventor D. Keicher also has a granted patent US8919899 which describes the same basic principle and in my view is relevant prior art to this application.

Phil’s Comments:

Printed electronics requires some very tight specifications for line width and resolution if the resultant printed tracks are to work reliably and meet the designs required by modern electronic circuits and components.  The conductive tracks often require liquids with specific properties which are not always suitable for inkjet nozzles or the typical methods of ink droplet formation.  The technology described here produces very narrow liquid lines on the substrate and is an interesting alternative for direct printed electronics.


Printed Electronics Patents Granted in 2011 – Kovio Inc

This is the third in the series covering patents granted during 2011 in the printed electronics field.

US7977240: Metal Inks For Improved Contact Resistance

Joerg Rockenberger, Yu Chen, Fabio Zürcher, Scott Haubrich

Kovio, Inc.

Filing date: 13 Feb 2009; Issue date: 12 Jul 2011


Metal ink compositions, methods of forming such compositions, and methods of forming conductive layers are disclosed. The ink composition includes a bulk metal, a transition metal source, and an organic solvent. The transition metal source may be a transition metal capable of forming a silicide, in an amount providing from 0.01 to 50 wt. % of the transition metal relative to the bulk metal. Conductive structures may be made using such ink compositions by forming a silicon-containing layer on a substrate, printing a metal ink composition on the silicon-containing layer, and curing the composition. The metal inks of the present invention have high conductivity and form low resistivity contacts with silicon, and reduce the number of inks and printing steps needed to fabricate integrated circuits.

Phil’s Comments:

Kovio Inc. is one of the leading companies involved in printed electronics devices and have made significant progress in printing complex electronic devices such as RFID’s in which not only the antenna but also the associated electronics is printed by a solution deposition process.

Here is an extract from the patent which explains the problem faced in any printing method for electronic devices:

In integrated circuits, the devices (e.g., TFT, capacitors, diodes, etc.) are generally connected to each other with metal lines (i.e., interconnects). Integrated circuits with good performance generally include interconnects with low resistivity, and thus not all metals are suitable for use as interconnects. Typical examples of suitable metals are Al, Cu, Au and Ag. Often, metals used for interconnects in integrated circuits do not form low resistivity contacts with the device electrodes (e.g., gate and source/drain electrodes), which are usually made with n+ and/or p+ doped silicon. Therefore, in order to fabricate integrated circuits with good performance, a contact layer formed between the n+/p+ silicon and the metal lines often provides relatively low resistivity between the devices and the interconnects. Typically, silicides are the preferred contact layers used in microelectronic devices, because they can provide ohmic contacts to heavily doped semiconductors (e.g. n+/p+ silicon and/or germanium).

This patent describes metal ink compositions that have high conductivity and low contact resistance and can therefore reduce the number of inks and printing steps required to manufacture a device.  The patent also describes a method which comprises (a) forming a layer comprising silicon and/or germanium on a substrate, (b) depositing (e.g., by printing) a metal ink composition on the silicon-containing layer, and (c) curing the metal composition. In general, the metal ink compositions comprise a bulk metal and a silicide-forming transition metal source.

More details are available in the full patent specification which can be found here.  Email me or leave a comment if you would like any further details.

Printed Electronics Patents Granted in 2011 – Conductive Ink

This is the second in the series covering patents granted during 2011 in the printed electronics field.

US7968011: Conductive Ink

Inventor(s): Gerardus Cornelis Overbeek, Michael Arnoldus Jacobus Schellekens, Alfred Jean Paul Bückmann

Assignee(s): DSM IP Assets B.V.

Filed: 6 Mar 2007; Issued: 28 Jun 2011

Abstract: A conductive ink comprising 10 to 75 wt % of at least one solvent comprising ≦20 wt % water; 0 to 50 wt % of at least one radiation curable material having a Mn in the range of from 50 to 10,000 g/mol; 5 to 70 wt % of at least one polyurethane having a Mw in the range of from 4,000 to 70,000 g/mol, 0 to 5 wt % of isocyanate-reactive component(s) bearing ionic or potentially ionic water-dispersing groups and a free isocyanate group content <0.5 wt % and 20 to 85 wt % of a conductive material.

The claims provide more details around the various components but rather than copying the claims I’ll explain in simple English.

The problem being solved by this invention is the lack of a common binder that can be used across a range of different conductive ink materials.  The inventors have provided a polyurethane based binder which they have found can be used to create inks suitable for flexography, gravure and ink-jet printing.  The method does not require high temperature curing and leads to good conductivity. Claim 1 requires that a UV radiation curing material is also included with the binder and this gives additional improvements in film properties and faster printing speeds.

The conductive materials are described as any form of conductive particle and the scope is very broad, including nanoparticles of silver or copper.  The particles can be flakes, fibres, nanotubes or mixtures, the particles can be silver coated.

The system requires a solvent which can be alcohol based (e.g. ethanol, iso-propanol, etc.) or any from a broad range of esters, ethers, ketones, etc. and preferably contains less than 10% water.

Any reader interested in the full formulation should refer to the patent for details of the preferred formulations.

Printed Electronics Patents granted in 2011

I recently reviewed the patents that are granted in the field of printed electronics and a recent search indicated that during 2011 there have already been nearly 100 granted patents in this technology sector.  It is not possible to provide a comprehensive overview in this format but over my next few posts I will provide brief details of a few of the patents that caught my eye.  (If anyone is interested in a more thorough analysis or any particular technology field then please leave a comment or get in touch with me and I can provide details of how to obtain the data.)

US7879688 – Methods For Making Electronic Devices With A Solution Deposited Gate Dielectric

Issued: 1st Feb 2011

Inventors: James C. Novack, Dennis E. Vogel, Brian K. Nelson

Assignee: 3M Innovative Properties Company

Abstract:  A method of making an electronic device comprises solution depositing a dielectric composition onto a substrate and polymerizing the dielectric composition to form a gate dielectric. The dielectric composition comprises a polymerizable resin and zirconium oxide nanoparticles.

Granted claim 1:

1. A method of making an electronic device comprising: (a) solution depositing a dielectric composition onto a substrate, the dielectric composition comprising an ultraviolet radiation polymerizable resin comprising tris-(2-hydroxy ethyl) isocyanurate triacrylate, an ultraviolet photoinitiator, and zirconium oxide nanoparticles; and (b) polymerizing the dielectric composition via at least ultraviolet radiation to form a gate dielectric.

The additional claims generally expand on the ZrO2 nanoparticles and dielectric composition which is preferably ink-jet printed.  Further claims elaborate on a method for making the thin-film transistor.

The claims are fairly self explanatory and the method is said to allow a higher charge carrier mobility for a solution deposited semiconductor.  Obviously, the patent can be circumvented by using other types of metal oxide but for those exploring its potential as a dielectric this is a key patent.