Printable dielectrics are required for many thin film devices. In some applications such as crossovers, low permittivity and reasonable insulation is required and in others such as gate insulators in printed field effect transistors they are one of the most critical materials. Here it is not a simple matter of insulation and it can affect both the size and the performance of the device.
Thin, high permittivity/ dielectric constant dielectrics are required for low voltage operation and higher current between source and drain and a low loss, room temperature printed dielectric is required for flexible OFETs. It must be pinhole free, with low surface roughness because this is critical for subsequent layer printing.
Low permittivity is relatively easy with polymers, examples of polymeric dielectrics being PMMA, polyimide and epoxy, but high permittivity is not possible with pure polymers so combined formulations are being explored, sometimes using conventional inorganic high K capacitor materials such as barium titanate. A challenge is to retain high breakdown strength and low leakage. They are frequently deposited on flexible substrates, as demanded by the largest potential markets, though usually by low temperature sputtering and other non-printed technologies.
Solution processed barium titanate nanocomposite
At first, inorganic compounds look a poor fit for these needs beyond the high permittivity requirement because they are non-solution, fragile and high temperature processed. We refer to traditional high permittivity inorganics such as BaTiO3, SrTiO3, BST, SiNx, SiO2 and other metal oxides.
However, promising advances are being made such as Motorola using solution processed high K polymer-ceramic nanocomposite for a printed FET gate insulator in the laboratory. As reported at a recent IDTechEx conference on Printed Electronics, it employs ceramic fillers - 1000 and 200 nm barium titanate (BTO) particles, thermally cured epoxy and propylene glycol methyl ether acetate solvent.
It exhibits low processing temperature, it is flexible, with low moisture absorption and a curing reaction that is initiated by a thermal agent or photosensitizer. These epoxies are also known for their frequency stability at normal temperature ranges. A ten fold increase in source-drain current was achieved by utilizing the BTO nanocomposite dielectric gate insulator. Device operating voltage was reduced compared to prior generation devices.
Alternative inorganic dielectrics HafSOx etc
Honeywell has a high K spin on dielectrics and NanoMas Technologies, , Dow Corning and others are working on high K dielectrics for printed devices. JSR, Dow Corning, TOK and others offer dielectric and passivation inks and formulations but there is more to do to make them work when thinner, to get the costs down and to improve the parameters.
Oregon State University is investigating a promising HafSOx dielectric film initially prepared by spin coating from acidic aqueous solution followed by thermal annealing. There has been considerable work in Japan and the USA - eg at Micron Technology - on equivalent inorganic oxide thin films to improve the performance of silicon chips.
Hafnium oxide - latest work
In the UK, in April 2007, researchers reported the following at the Nanomaterials conference in Newcastle Gateshead:
Hafnium oxide has been investigated as a dielectric material for thin film transistors incorporating zinc oxide as the channel material. In particular, there is a motivation to be able to deposit all transistor layers at temperatures below 150 ºC onto plastic. rf magnertron sputtering from a metallic hafnium target has been used for deposition.
Depositions of HfO, were performed in a CCR Technology GmbH sputter coating system onto clean, n-type silicon wafers. A shroud was placed around the 99.999% pure Hf target which extended down to the substrates to reduce contamination. MIS structures were fabricated by thermally evaporating aluminium contacts onto the samples through a shadow mask. CV and IV analysis was performed on an HP4192A LF impedance analyzer and HP414OB picoammeter/voltage course.
They concluded the following:
The optimized HfO, has a high relative permittivity of 18.2 and a high resistivity of (3.8±0.3)13 Ohms cm. Under moderate electric fields, Poole-Frenkel conduction dominates. Breakdown occurs under an applied electric field of 3.5 MV cm-1. The microstructure consists of small crystals ~10 nm in diameter, embedded within an amorphous matrix. This is consistent with Poole-Frenkel conduction where grain boundaries act as trap states.
HfO can be deposited at room temperature by rf magnetron sputtering from a metallic target through careful control of the oxidation state of the target surface with high resistivity and breakdown strength.
Hybrid inorganic dielectrics - zirconia
Hewlett Packard is a leader in printing inorganic semiconductors and dielectrics. As reported at a recent IDTechEx conference, it has obtained interesting results from hybrid structures and dual dielectrics in its experimental transistors produced by thermal ink jet printing, with laser ablation of the source drain gap.
The hybrid process improved performance, a device mobility of 0.3cm²/Vs being achieved in the first trials with an on-off ratio of 400,000 and a threshold voltage of 8V. The dual dielectric construction initially achieved a gate breakdown field of over 1MV/cm, maximum gate leakage of under 5 nA/cm², an excellent peak channel mobility of 5-15 cm²/Vs and a good on off ratio of 106 to 107.
Aluminium, lanthanum and other oxides
In Japan, a collaboration of Tokyo Institute of Technology, Japan Science and Technology Corporation, Advanced Materials Laboratories, National Institute for Materials Science and Tokyo Institute of Technology has reported another approach.
They said they had fabricated Al2O3, LaAl O3 (LAO), CaHf O3 (CHO) and CaZr O3 (CZO) thin films for the dielectric layers of field-effect transistors (FETs) by pulsed laser deposition (PLD). The films exhibited very smooth surfaces with root-mean-squares (rms) roughnesses of 1.3 Angstrom as evaluated by using atomic force microscopy (AFM).
The breakdown electric fields of Al2O3, LAO, CHO and CZO films were 7, 6, 10 and 2 MV/cm, respectively. The magnitude of the leak current in each film was low enough to operate an FET.
They performed a comparative study of pentacene FETs fabricated using these oxide dielectrics as gate insulators. High field-effect mobility of 1.4 cm²/Vs and on/off current ratio of 107 was obtained in the pentacene FET using LAO gate insulating film. Use of the LAO films as gate dielectrics has been found to suppress the hysteresis of pentacene FET operation. The LAO films are relevant to the dielectric layer of organic FETs. But all this is only part of the story .......
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