AJP Applications

As already mentioned, the AJP technique is a relatively simple process, characterized by low cost, low waste of materials and great versatility including adaptability to large area fabrication. These characteristics, combined with a wide range of processable materials, a max resolution <8 μm and the ability to print on non-conventional materials make AJP a very promising technique in microelectronics applications. Aerosol deposition has been successfully used to print inorganic materials for the manufacturing of antennae ¹⁶⁸, extended to a great variety of organic and bio-organic systems to fabricate resistors and capacitors ³⁹, interconnection ¹⁶⁹, ZnO-based photodetector ¹⁷⁰, Organic Field Effect Transistors ^107,156 and bio-molecular materials for biosensing. ¹⁵⁰

The first research publications featuring AJP technique became to emerge around 2001-2002 when Marquez et al ¹⁶⁰ directly printed several bio-materials (cells, enzymes and proteins) directly on a culture media for the first time by an aerosol jet printer. After deposition, viability studies demonstrated the good viability of printed bio-materials.

Starting from this milestone, Grunwald et al ¹⁷¹ demonstrated the ability to print DNA strands and enzymes without any kind of damages in their biological activity. An agarose gel electrophoresis was used to investigate the DNA integrity before, during and after the printing process. Both pneumatic and ultrasonic atomizer were used showing that only the pneumatic atomizer preserves DNA integrity as shown in Fig.2.8

Figure 2.8: Comparison of printed DNA by UA and PA atomizers. In a) electrophoretic analysis for DNA printed with UA is reported. Before the ultrasonic wave application, the DNA is stable after printing it loses his biological integrity. In b) PA is used and it is evident that DNA maintain

his biological integrity ¹⁵⁰ through the printing process. c) shows the magnification of the DNA spot after deposition.

Marquez and Grunwald demonstrate the suitability of AJP for printing biomolecules paving the way to a new era for microarray bio-fabrication.

Applications of the AJP to print conductive traces designed to interconnect two or more electronic components are widely reported in literature. Because of their simplicity these components have been the primary structure developed to explore the huge potential of this technique in microelectronics applications.

The first application of the AJP to print interconnects is reported in an article of 2007 where highly conductive traces for silicon solar cell metallization were made by a silver nanoparticle ink¹⁷². Padovani et al ¹⁶⁹ deposited silver ink traces (35 µm) on a glass substrate to interconnect a LED array on a transparent display.

The z-axis easy positioning of the DH has allowed the production of vertical interconnection. For example Zhan et all ¹⁷³ reached a good coverage of trapezoidal and reverse trapezoidal holes to interconnect the top and bottom silicon parts of MEMS device as shown in Fig 2.9b.

One of the most advantages of AJP is its ability to produce fine features with demonstrated line width of 10 µm reported by Cai et al ¹⁷⁴. The ability to print fine lines is used by Kopola et al ¹⁷⁵ to improve device performances by decreasing the width of tracks in the current collection grid of a reverse ITO solar cell.

Figure 2.9: In a) the silver printing interconnections are reported with a 35 µm width for the line and 160 µm for the pad. The ability of AJP to produce fine structure is reported. Also, in a) is reported the top view and the rear view of the silver backbone in contact with the LED devices ¹⁶⁹.

In b) an example of 3D structure covered by AJP is reported. The simple way with trapezoidal structure and the difficult way reverse trapezoidal structure are shown ¹⁷³.

As already mentioned, another appealing feature of AJP technique is the ability to print a wide range of materials. Gupta et al ³⁹ used this technique for resistor and capacitors productions. Silver ink is printed on a Kapton substrate and several couples of silver pads with different distances are prepared.

After silver deposition, PEDOT:PSS is printed between the two contacts. In this way an array of resistors is obtained. The resistance increase with the length of the PEDOT:PSS channel. In the same article is reported the first full-printed capacitor. Silver ink traces are printed along a SU-8 dielectric medium. A linear increase of capacitance is obtained with the increase of the overlap between the printed silver lines and the dielectric medium.

In the 2008 Cho et al ¹⁴⁷ reported a gel organic thin film transistor (GEL-OTFT) where all the essential part of the devices were AJP printed over a Kapton substrate. They showed, for the first time, the performance achievable with a printed ion gel to fabricate devices achieving a switching speeds up to 10 kHz.

Xia et al¹⁷⁶ have published a GEL-OTFT fully-printed operating at a voltage below 2 V where the major key to obtain the low operating voltage was the ion gel used. The device fabrication was quite reliable as demonstrated by the repeatability of the process over the several printed devices including other substrates such as PEN.

Figure 2.10: a) schematics of the first fully-printed GEL-TFT.¹⁷⁶ All the essential parts of the transistor are made by AJP printing: the source and drain electrodes are printed from an Au ink;

for the gate electrode a PEDOT:PSS ink was used while P3HT and Ion gel inks were respectively used to print the channel and the electrolyte. a) shows also the array of devices as printed on

Kapton. b) schematic of protein sensor produced by Cantù et al ¹⁵⁹.

Recently, Cantù et al ¹⁵⁹ proposed a full-printed electrochemical sensor for the detection of Interlokin-8 where a silver nanoparticle ink is printed to produce the conductive traces, a carbon ink is used to print both the working (WE) and the counter (CE) electrodes, a silver chloride ink for the reference electrode (RE) and finally NOA 81, a UV-curable passivation polymer ¹³⁶, is printed to create delimiting edges to border liquid samples on the active channel. A PA was used to print all the essential part of the device. They used an Anodic stripping Voltammetry for protein detection achieving a LOD of 0,3 ng/L by coating the WE with multiwall carbon nanotubes (MWCNT).

In summary AJP has already demonstrated to be a viable and quite promising technique for the fabrication of devices integrated on polymeric substrates even though the field is still at the early stage of prototype demonstration. On this basis the present work is aimed at contributing at further developing the method both using different substrates and inks in particular for electrochemical devices on flexible and potentially stretchable substrates.