Conductive metal oxide based layer

Details Conductive metal oxide based layer Conductive metal oxide based layer

2000-02-04

2001-10-23

Zarabian, Amir (Department: 2824)

Semiconductor device manufacturing: process

Coating with electrically or thermally conductive material

To form ohmic contact to semiconductive material

C438S608000, C427S126300

Reexamination Certificate

active

06306747

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of conductive layers based on metal oxides.
BACKGROUND OF THE INVENTION
Conductive layers deposited on a flexible transparent substrate have found major applications in various electronic and opto-electronic devices. A particular useful type of transparent conductive layer is based on metal oxides. These metal oxides can be applied to a substrate by different rather cumbersome methods. A review is given in K. L. Chopra et al, “Transparent Conductors—A Status Review”,
Thin Solid Films,
102, (1983), p. 1-46.
The most important metal oxides used in conductive layers are non-stoichiometric and doped oxides of tin, indium, cadmium, zinc and their various alloys. Well known examples of the latter category include tin oxide (TO) doped with antimony (ATO) or fluorine (FTO), indium oxide (IO) doped with tin (ITO), and zinc oxide doped with indium (IZO). These metal oxides exhibit high transmittance in the visible spectral region, high reflectance in the IR region and nearly metallic conductivity. The electrical as well as the optical properties of these materials can be tailored by controlling the deposition parameters.
Applications of these transparent conductive layers on flexible substrates can be divided in:
highly conductive layers (<0.5 k&OHgr;/sq) with applications in displays (electroluminescent displays, Liquid Crystal Devices, Plasma Display Panels), touchscreens, solar cells and smart windows;
conductive layers (>0.5 k&OHgr;/sq) with applications in EMI-shielding foils, electroluminiscent lamps and membrane switches.
Other applications include heating elements for aircraft and automobile windows, heat-reflecting mirrors, antireflection coatings and gas sensors.
Examples of references dealing with the various applications of conductive layers based on metal oxides include:
in solar cells the use of TO layers is disclosed in JP-A 05-′218477, JP-A 05-218476, EP 290345, the use of FTO layers in JP-A 05-017878;
for electroluminiscent displays the use of various metal oxide layers is disclosed in e.g. JP-A 09-024574, JP-A 08-281857, FR 2728082, JP-A 08-031572, and JP-A 01-081112;
A first main category of deposition techniques to form metal oxide layers on a substrate is evaporation. This technique can be further subdivided in post-oxidation of metal films, reactive evaporation, activated reactive evaporation, and direct evaporation. A second main category is sputtering, which can be further subdivided in reactive sputtering of metallic targets, direct sputtering of oxide targets and ion beam sputtering. Still further types of deposition techniques include reactive ion plating, chemical vapour deposition, spray pyrolysis, dip technique, chemical solution growth, reactive triode sputtering, and glow discharge composition.
In the scientific literature (M. Watanabe,
Jpn. J. Appl. Phys
., 9(1970) 1551; T. Nishino and Y. Hamakawa,
Jpn. J. Appl. Phys
., 9(1970) 1085; G. Bauer,
Ann. Phys
. (Paris), 30(1937) 433; G. Rupprecht,
Z. Phys
., 139(1954) 504; M. Watanabe,
Jpn. J. Phys
., 9(1970) 418) it is disclosed that a very thin (max. 50 nm) Sn layer (or In or Zn) deposited by evaporation can be oxidized whereby a transparent electrically conductive layer is formed. A serious drawback of this method is that with thicker layers the oxidation cannot be performed completely. Probably, due to the continuous phase nature of the metal film the passage of oxygen through this film is insufficient.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a process for the preparation of a conductive metal oxide layer on a substrate which is less cumbersome and more economic than the existing methods described above.
It is a further object of the invention to provide a process which allows for the formation of thicker metal oxide layers than is possibel with the prior art methods.
SUMMARY OF THE INVENTION
The objects of the present invention are realized by providing a process for the preparation of a conductive metal oxide based layer on a substrate comprising the following steps, in order :
(a) preparing an aqueous medium containing at least one type of metal salt,
(b) chemically reducing said metal salt by a reducing agent to form a dispersion of metal particles,
(c) washing said dispersion of metal particles,
(d) coating said washed dispersion onto a substrate, thereby obtaining a coated layer containing metal particles,
(e) subjecting said coated layer to an oxidizing treatment to form a conductive layer containing metal oxide particles.
In a most preferred embodiment the metal oxide is tin oxide or a mixture of the oxides of tin and another metal. The oxidizing treament is preferably a heat treament in an oxygen containing atmosphere.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
The process for preparing a conductive metal oxide layer on the transparent support will now be explained on the hand of the preferred embodiment wherein the metal oxide is tin oxide.
In a first step (a) an aqueous solution of a tin(IV) salt is prepared. As most suitable tin salt tin(IV) chloride is chosen. In order to maintain a sufficient amount of tin ions in solution, it is desirable to add a complexing agent. A preferred complexant is the well-known ethylenediaminetetraacetic acid (EDTA) or a homologous compound or a salt thereof. Another preferred one is citrate, e.g. triammonium citrate. Other suitable complexants include diethylenetriamine-pentaacetic acid (DTPA), trans-1,2-diaminocyclohexane-N, N,N′,N′-tetraacetic acid (CDTA), ethyleneglycol-O,O′-bis(2-aminoethyl)-N,N,N′,N′-tetraacetic acid (EGTA), N-(2-hydroxyethyl) ethylenediamine-N,N,N′-triacetic acid (HEDTA), etc.
In a following step the tin ions in the solution are reduced to highly dispersed metallic tin particles by means of the addition of a reducing agent. A preferred reducing agent is KHB
4
. Another suitable reducing agent is sodium hyposulphite. Others include glucose, formaldehyde and hypophosphorous acid. The reducing agent can be added to the original tin salt solution as a solid powder. On the other hand the reducing agent can be dissolved separately in a second aqueous medium and added to the tin salt solution according to a single jet or a double jet procedure. Preferably, according to the double jet principle, the aqueous medium containing the tin salt and the second solution containing the reducing agent are added together to a third aqueous medium.
In order to keep the metallic tin particles formed by reduction in colloidal dispersion a protective binder is preferably added to any of the three aqueous solutions involved. Preferably, this protective binder is added to the third aqueous medium wherein both others are jetted. A particularly preferred protective binder is carboxymethylcellulose (CMC), preferably of the high viscosity type. Other possible binders include gelatin, arabic gum, poly(acrylic acid), polyvinyl alcohol, cellulose derivatives and other polysaccharides.
When the reduction is substantially completed the superfluous ions present in the aqueous medium are removed from it by a washing process (step c), preferably involving ultrafiltration and/or diafiltration. This washing process may be preceeded ,by ultracentrifugation. In any of the solutions involved in the preparation a so-called dispersing aid can be present. In a preferred embodiment this compound is added to the diafiltration liquid at the last stage of the preparation. Suitable dispersing aids in the case of tin are pyrophosphates, more particularly a hexametaphosphate such as sodium hexametaphosphate. Due to the high affinity of the phosphate to metal oxides the phosphate will absorb immediately to the freshly formed metal oxide surface layer, which will be formed upon removing the excess of reducing agent during the first stage of the washing step. This absorption of phosphates' controls the further oxidation/passivat

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