WO2013021149A2 - Methods for forming an organic layer on a substrate - Google Patents

Abstract

A dry method for forming a single organic layer on a substrate, the method involving: depositing organic molecules by vapour deposition on the substrate, wherein some of the molecules are bonded to the substrate and others are held together via intermolecular interaction, and removing using a dry process the organic molecules that are held together by breaking the intermolecular interaction, leaving a monolayer of organic molecules bonded to the substrate.

Description

Methods for Forming an Organic Layer on a Substrate

Introduction

The present invention relates to a dry-fabrication method for the production of a monolayer of an organic compound. The method can be applied to the manufacture of organic thin film transistors.

Background

To date high-performance organic thin film transistors (OTFT) based on conjugated small-molecules combine dry and wet procedures. Metals, inorganic dielectrics and organic small-molecule semiconductors are typically deposited by vacuum techniques (dry procedures), such as thermal or electron beam evaporation, sputtering, etc. On the other hand, organic dielectrics and self-assembled organic monolayers are typically deposited from solutions (wet procedures). A few attempts have been made to develop dry organic dielectrics, but these resulted in thick layers suppressing higher electrical currents (problematic for low-voltage transistors) or required high process temperatures, which induce polymerization between two monomers (incompatible with plastic substrates). Figure 1 shows the structure of an Organic Thin-Film Transistor (OTFT). Formed on a substrate is an elongate gate that is covered with a dielectric insulating layer. On the dielectric layer is a semiconductor layer on which source and drain contacts are formed. Alternatively, the deposition sequence of the semiconducting layer and source/drain contacts can be switched. Between the source and drain contacts a conducting channel is defined in the semiconductor layer. Since the conducting channel is formed in the semiconductor layer next to the dielectric/semiconductor interface, any surface roughness and/or surface energy of the dielectric, as well as any interface charges or dipole moments, affect the charge conduction in the channel. For a thin-film transistor, the capacitance of the dielectric is directly proportional to the drain-source current. The higher the capacitance, the higher is the drain-source current and the lower is the threshold voltage. There are two ways of increasing the capacitance: (a) by choosing dielectric materials with high relative permittivity, and (b) by making the dielectric layer as thin as possible while maintaining low leakage current. A suitable dielectric material is Al₂0₃ with relative permittivity of ~ 7-8. Al₂0₃ has a high surface energy, polarizability, and leakage current for ultra-thin layers. To reduce these effects and to enhance the transistor performance, self-assembled organic monolayers have been employed. These are commonly formed by immersing or spin coating a substrate in a solution containing an organic compound. These organic compounds typically have linear alkyl chains with one hydrophilic end group that can attach to the surface of an inorganic dielectric. Using organic monolayers also enhances the dielectric properties of the gate dielectric. For example, the introduction of an organic self-assembled monolayer has been shown to reduce the density of charge trapping states at the semiconductor-dielectric interface leading to improved subthreshold slope and threshold voltage in pentacene OTFTs [Applied Physics Letters, (2006) volume 88, article 073505]. Solution processed alkylphosphonic acid self assembled monolayers have also been used to obtain a mobility of 0.6 cm² V.s for p-channel and 0.02 cm² V.s for n-channel organic thin film transistors for implementation into logic circuits, also demonstrating low voltage pentacene OTFTs fabricated using dielectric monolayers based on phosphonic acids [Nature, (2007) volume 445, page 745-748]. Different types of organic self-assembled monolayers and multilayers are available for OTFTs.

US 7202547 describes a capacitor with a dielectric including a self-organized monolayer of an organic compound. The self organized monolayer is formed by immersing the substrate with a defined electrode in a solution of organic compound with concentration ranging from 10^"4 to 1% by weight, followed by rinsing of the substrate with a pure process solvent. Alternatively, if the organic compound is brought into contact with the electrode from the gas phase, the pressure is preferably between about 1x10^"4 Pa and about 40x10³ Pa, depending upon the volatility of the organic compound.

Summary of the invention

According to a first aspect of the invention, there is provided a dry method for forming a single organic layer on a substrate, the method involving: depositing organic molecules by vapour deposition on the substrate, wherein some of the molecules are bonded to the substrate and others are held together via intermolecular interaction, and removing using a dry process the organic molecules that are held together by breaking the intermolecular interaction, leaving a monolayer of organic molecules bonded to the substrate. Depositing the organic molecules may be performed using a source of organic material which is solid at room temperature. Depositing the organic molecules may be performed in vacuum.

Depositing the organic molecules and removing the molecules that are held together by intermolecular interaction may be done whilst maintaining a vacuum. It may be done in the same chamber or in a series of linked chambers. Maintaining the vacuum may involve maintaining a pressure in the chamber between 1x10-⁶ Pa and 1x10⁵ Pa.

The organic molecules may be deposited to form a layer that is thicker than a monolayer, for example more than two times the thickness of the monolayer; or more than nine times the thickness of the monolayer.

The intermolecular interaction may be van der Waals interaction.

Each organic molecule may have an anchor group and a tail. The anchor group may bind with the substrate by forming a chemical bond, for example a covalent bond, of characteristic strength and the tail may bind to the substrate and/or to the organic molecules with a strength less than the characteristic strength. The anchor group may be hydrophilic and the tail may be hydrophobic. Breaking the intermolecular interaction may involve heating the molecules. Heating may be done at or above the sublimation temperature of the said organic molecules. Heating may be performed for at least one minute, for example at least twenty minutes, preferably at least 30 minutes. Breaking the intermolecular interaction may involve using light waves and/or intermolecular resonant vibrations.

The substrate may be a metal oxide or may comprise a metal oxide. The metal oxide may comprise more than one metal. The metal oxide may be selected from: a silicon oxide; a titanium oxide; a silver oxide; an aluminium oxide, a chromium oxide, a zinc oxide, an indium tin oxide, a hafnium oxide, a magnesium oxide, a calcium oxide or a gold oxide. Thus the metal oxide may be an oxide of one or more element selected from the group consisting of silicon, titanium, silver, aluminium, chromium, zinc, indium, tin, hafnium, magnesium, calcium and gold.

The organic molecules may be an organosilane precursors or an organic acid. For example the organic molecules may be organic acids selected from the group consisting of: carboxylic acids, phosphonic acids, thiocarboxylic acids, sulphonic acids and derivatives thereof.

The organic molecules may be of the form R-X wherein. X is the organic acid function or derivative thereof; and R is a group selected from the group consisting of substituted or unsubstituted alkyl, for example n-alkyl that may be unsaturated (for example C1- C20 or even C2-C18); substituted or unsubstituted oligo-aromatics or oligo- heteroaromatics such as oligothiophenes and oligophenylenes (OPs); substituted or unsubstituted oligo(phenylenevinylenes) (OPVs); and substituted or unsubstituted oligo(phenyleneethynylenes) (OPEs).

The organic molecules may form a self assembled monolayer on the substrate.

The organic molecules may be alkane-phosphonic acid or carboxylic acid molecules. For example the organic molecules may be 1-octylphosphonic acid.

According to another aspect of the invention there is a method involving: forming a metal gate; creating a metal oxide layer on the gate; depositing a single organic layer on the metal oxide layer using the method of the first aspect of the invention; depositing a semiconductor layer, and forming source and drain contacts. The metal gate may be formed on a biodegrable substrate. The semiconductor layer may be deposited on the single organic layer. The semiconductor layer is deposited between the source and drain contacts. The source and drain contacts may be formed on the semiconductor layer.

The gate may comprise a metal oxide. The metal oxide may comprise more than one metal. The metal oxide may be selected from: a silicon oxide; a titanium oxide; a silver oxide; an aluminium oxide, a chromium oxide, a zinc oxide, an indium tin oxide, a hafnium oxide, a magnesium oxide, a calcium oxide or a gold oxide. Thus the metal oxide may be an oxide of one or more element selected from the group consisting of silicon, titanium, silver, aluminium, chromium, zinc, indium, tin, hafnium, magnesium, calcium and gold.

The semiconductor may be a conjugated polymer. The semiconductor may be a conjugated polymer that sublimes under vacuum. The conjugated polymer may be pentacene.

The semiconductor may be Copper(ll)1 , 2,3,4, 8,9, 10, 11 , 15,16, 17, 18,22,23, 24,25- hexadecaf luoro-29H, 31 H-phthalocy anine .

The source and /or drain contact may comprise a metal, for example one or more of gold, silver or aluminium.

According to yet another aspect of the invention, there is provided an electronic device formed on biodegradable substrate that includes a monolayer made according to the first aspect of the invention.

Brief Description of the Drawing

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:

Figure 2 is a flow diagram of a method for producing a SAM organic layer;

Figure 3a is a cross-sectional view of an Al₂0₃ capacitor;

Figure 3b is a cross-sectional view of an Al₂0₃ capacitor in which the Al₂0₃ is coated with a monolayer of 1-octylphosphonic acid;

Figure 4a is a cross-sectional view of a box type capacitor (crossover structure);

Figure 4b is a cross-sectional view of a dot type capacitor (planar structure); Figure 5 shows leakage current densities of various capacitors with varied capacitor areas and UV/ozone exposure times at a voltage of 1 V;

Figure 6 shows capacitance per unit area of Al₂0₃ capacitors prepared by using different VV/ozone exposure times with and without 1 -octylphosphonic acid layer; Figure 7 shows breakdown voltages of Al₂0₃ and Al₂0₃ coated with 1- octylphosphonic acid capacitors where Al₂0₃ is prepared by using different UV/ozone exposure times;

Figure 8 is a flow diagram of a totally-dry fabrication process for making an organic thin-film transistor;

Figure 9 shows the steps of the UV/ozone treatment process for Al₂0₃ formation;

Figure 10 shows the transfer (left) and output (right) characteristics of pentacene transistors without (a) and with (b) 1 -octylphosphonic acid;

Figure 11 shows the gate dielectric capacitance of three sets of transistors prepared with different Al₂0₃ thicknesses and UV/ozone exposure times;

Figure 12 shows the gate dielectric breakdown voltage of three sets of transistors prepared with different Al₂0₃ thicknesses and UV/ozone exposure times;

Figure 13 shows threshold voltages of three sets of transistors prepared with different Al₂0₃ thickness and UV/ozone exposure times;

Figure 14 shows inverse subthreshold slopes of three sets of transistors prepared with different Al₂0₃ thickness and UV/ozone exposure time;

Figure 15 shows field-effect mobility of three sets of transistors prepared with different Al₂0₃ thicknesses and UV/ozone exposure times;

Figure 16 shows the transistor off-current of three sets of transistors prepared with different Al₂0₃ thicknesses and UV/ozone exposure times;

Figure 17 shows the gate dielectric capacitance per unit area of a capacitor containing Al₂0₃ and octyl phosphonic acid (C₈PA) bi-layer deposited between the electrodes, measured as a function of the C₈PA desorption time;

Figure 18 shows the thickness of the octyl phosphonic acid layer obtained on the capacitor of Figure 17, measured as a function of desorption time;

Figure 19 shows the leakage current densities of bare Al₂0₃ and Al₂0₃/C₈PA dielectrics at various desorption times;

Figure 20 shows the breakdown voltage of Ab yCaPA dielectrics at various desorption times;

Figure 21 (a) is an AFM image of bare Al₂0₃;

Figure 21 (b) is an AFM image of the C₈PA layer immediately after its deposition on Al₂0₃;

Figure 21 (c) is an AFM image of the C₈PA layer after 210 minutes of desorption; Figure 22 shows the transfer characteristics of a pentacene transistor prepared with a desorption time of 210 minutes;

Figure 23 shows the output characteristics of a the transistor of Figure 22;

Figure 24 (a) shows the field effect mobility measured as a function of desorption time;

Figure 24 (b) shows the threshold voltage measured as a function of desorption time;

Figure 24 (c) shows the subthreshold slope measured as a function of desorption time, and

Figure 24 (d) shows the off-current measured as a function of desorption time.

Detailed description of the invention

Figure 2 shows steps in a method for constructing a single organic layer on a substrate. The dielectric properties of the layer are influenced by the molecular structure of the organic compound, which is a linear molecule with a reactive anchor group and a hydrophobic tail. Here, the reactive anchor group attaches the molecule to the surface of the previously deposited layer preferably by a covalent bond, which results in particularly high thermal, mechanical and chemical stability bond. The hydrophobic tail, which is preferably formed by an n-alkyl chain, brings about a virtually orthogonal alignment, and consequently a dense packing of the molecules.

The method involves deposition of the organic molecules on the substrate in an evacuated chamber. The source organic material is solid at room temperature (although the method could be adjusted to accommodate liquid sources) and must be heated above the room temperature in order to produce measurable evaporation rate. The substrate onto which the organic material is deposited can be held at a temperature in a range typically from 0°C to 200°C. The source organic material can be placed in a heating element with a crucible. Deposition of the organic material is typically done at a rate in the range of 0.01 A s to 100 A s and at a pressure in the range from 1x10^"6 Pa to 50x10³ Pa. Growth of the organic material resembles that of polycrystalline growth and the grain structure of the organic layer depends on the chosen materials, the substrate temperature, the rate of growth, and the pressure. Since the anchor group and the material on which the organic molecules are vapour deposited may vary, the thermodynamics may induce 2D and/or 3D growth of the organic layer. The growth of a first layer may not be completed before the growth of subsequent layers starts.

To create a complete and well ordered continuous layer, a thicker than desired layer of organic compound is deposited. For example, the layer may have a thickness that is two or more times the thickness of the desired monolayer. As a specific example, the deposited layer may be ten times the desired thickness, e.g. for a monolayer of thickness 10 A, the layer deposited may be circa 100 A. As shown in Figure 2, some molecules form a strong covalent bond with the substrate and some are physisorbed, i.e. held by a weak van der Waals interaction with adjacent molecules. Once the organic material is deposited, the source organic material may be cooled to suppress additional vapour formation and the substrate-layer is heated to a temperature allowing desorption of the physisorbed organic molecules. Only molecules that are bonded with the surface of the substrate remain, leaving an organic layer that is a single molecule thick, of the order of 10 A, with dielectric properties, i.e. a dielectric monolayer.

Capacitors based on (a) Al₂0₃ and (b) Al₂0₃ coated with an organic monolayer, in particular an organic monolayer of 1-octylphosphonic acid, were fabricated on clean quartz wafers. Figure 3a shows a cross-section of an Al₂0₃ capacitor and Figure 3b shows a cross-section of a basic capacitor based on Al₂0₃ coated with a monolayer of 1-octylphosphonic acid. Two structurally different capacitors were fabricated. These are shown in Figure 4.

Figure 4a shows a cross sectional view of a box type capacitor (crossover structure) and Figure 4b of the dot type capacitor (planar structure). Capacitors of each type were fabricated with and without phosphonic acid to study the effects of topography and the role of phosphonic acid on the properties of the capacitors.

Box capacitors without an organic monolayer were fabricated by vacuum deposition of 40 nm of aluminium at a pressure of 8*10^"5 Pa using a shadow mask containing slits of 0.5 mm by 5 mm. The substrate was then cut into six pieces to allow different UV/ozone exposure of aluminium. Previous experiments have shown that exposure of aluminium to ozone leads to aluminium oxide formation. The samples were exposed for 30, 60, 90, 120, and 180 minutes to study the effect of UV/ozone treatment on the devices and to identify the optimum conditions. After the dielectric layer was formed, 50 nm of gold was vacuum evaporated at a pressure of 1 x 10^"" Pa through the shadow mask containing slits of 0.775 mm by 0.5 mm to form the top electrodes of the capacitors. The capacitors had an average area of 0.39 mm². Dot capacitors without an organic monolayer were fabricated by covering a quartz substrate by a 40-nm thick layer of aluminium evaporated at a pressure of 8* 10^"5 Pa without using any shadow mask. It was then cut into several pieces for exposure to UV/ozone at different times of 30, 60, 90, 120, and 180 minutes. After the dielectric layer was formed, a 50-nm thick layer of gold was thermally evaporated at a pressure of 1 * 10^"" Pa using shadow masks with 150 pm and 500 μπι diameter holes. The gold dots form the top electrode and the continuous aluminium layer is the bottom electrode. These capacitors had corresponding average areas of 0.023 and 0.28 mm².

A second series of capacitors that had an organic monolayer was prepared in a similar manner. However, in each case, the aluminium oxide was coated with a monolayer of 1 -octylphosphonic acid before the top gold layer was deposited. Contrasting the previous research, the monolayer was thermally evaporated at a pressure of ~ Ι ^χ Ι Ο^" Pa following the process described with reference to Figure 2. Fabricated Al₂0₃ and A^Os/l-octylphosphonic acid capacitors were characterised by recording the electrical currents as function of voltage, capacitances at different frequencies and the breakdown voltages. All samples were subjected to the same measurement procedure. The measurements were done on Signatone probe station with Agilent B1500A semiconductor device analyzer.

Figure 5 shows the leakage current densities of various capacitors with varied capacitor areas and ozone exposure times at a voltage of 1 V. The triangular points indicate the leakage current densities of Al₂0₃ capacitors and the circular points indicate the leakage current densities of AI₂0₃/1 -octylphosphonic acid capacitors. Three graphs correspond to three different capacitor areas. Each point on the graph represents an average measurement from 5 capacitors.

Figure 5a represents the leakage current densities of the capacitors with an area of 0.023 mm² - capacitors with the smallest area. The leakage current density for Al₂0₃ capacitors was (2-8)* 10^"5 A/cm² and Al₂0₃ /1-octylphosphonic acid capacitors had a current density of (1-8)*10^"6 A/cm². There is a decrease of an order of magnitude of the leakage current density when 1-octylphosphonic acid is added. The ozone exposure time does not have significant effect on the leakage current densities. Figure 5b represents the characteristics of the capacitors with an area of 0.28 mm² which is an order of magnitude bigger than the ones shown in Figure 5a. The current density of Al₂0₃ capacitors was 8-15 A/cm² and AI₂0₃/1-octylphosphonic acid capacitors had current density of (2-6)* 10^"6 A/cm². The current density decreased by six to seven orders of magnitude after adding the monolayer of 1-octylphosphonic acid.

Figure 5c shows the leakage current density of the box capacitors with an area of 0.39 mm², which is slightly larger than that of capacitors in Figure 5b. In this case the current density of Al₂0₃ capacitors was 3-8 A/cm² and Al₂0₃/1-octylphosphonic acid capacitors had a current density of (8-11)*10^"6 A/cm². The current density decreased by 6 orders of magnitude by adding the 1-octylphosphonic acid deposited in the same way like the one in Figure 5b. The leakage current densities of the box capacitors of Figure 5c are slightly higher than those of Figure 5b. This is most likely caused by imperfect step coverage and unoptimized deposition of phosphonic acid. The results show that the thermally evaporated monolayer of 1-octylphosphonic acid exceptionally suppresses the leakage current, while the UV/ozone exposure of Al₂0₃ does not have much effect.

Figure 6 shows the capacitance per unit area of various capacitors. Capacitances were measured by varying the frequency from 1 kHz to 5 MHz with a DC bias of 1 V and an AC oscillation of 50 mV. The impedance, phase angle and dissipation were recorded as well and capacitance was calculated using both serial and parallel models. Values of capacitance were extracted at frequencies where the devices functioned as ideal capacitors and both models led to the same value of capacitance. Each point represents an average measurement from five different capacitors. The measured capacitance indicates that the area and the structure of the capacitor have only small effect on the capacitance per unit area. The Al₂0₃ capacitors were leaky and the capacitance was difficult to measure. By adding 1-octylphosphonic acid the leakage current was suppressed by many orders of magnitude and the yield substantially increased. There is not much effect of the UV/ozone exposure time on the capacitance. Similar capacitances suggest that the ratio between the relative permittivity and the thickness of the oxide layer does not change with the UV/ozone exposure time.

Figure 7 represents the breakdown voltage measurements of the fabricated capacitors. Breakdown voltage is the voltage at which the dielectric develops a non-recoverable highly conducting path and the current suddenly increases by many orders of magnitude. By varying the voltage from 0 to 10 V the breakdown voltage is extracted when the leakage current reaches 100 μΑ. The breakdown voltage is increased from 2- 3 V to ~ 4.7 V when a monolayer of 1 -octylphosphonic acid is added on top of Al₂0₃. Data for many Al₂0₃ capacitors could not be extracted due to high leakage currents. Ozone exposure time has no effect on the breakdown voltage of the capacitors with 1- octylphosphonic acid.

The results indicate that the leakage currents of Al₂0₃ capacitors are suppressed by many orders of magnitude when they are treated with vacuum evaporated 1- octylphosphonic acid. The results also show that the leakage current density of Al₂0₃ capacitors substantially increases when the capacitor area reaches a certain value. This indicates an existence of low-resistive defects, e.g. pinholes. By adding a single layer of 1-octylphosphonic acid these pinholes are covered up and the leakage current decreases by many orders of magnitude. This dramatic decrease in the leakage current indicates plausible dense packing of the organic molecules within the monolayer. Capacitance measurements indicate that the thickness of 1- octylphosphonic acid is ~ 1 nm, indicating a single layer growth. This method of fabrication of an organic self assembled monolayer can be incorporated into the manufacturing process of organic thin-film transistors, where the presence of a self assembled monolayer separating the gate from the semiconductor layer can be used to enhance the transistor performance. Figure 8 shows steps in a method for fabricating an organic thin-film transistor using a totally-dry fabrication process. As an example, P-channel pentacene thin-film transistors with aluminium oxide coated with alkylphosphonic acid were fabricated on glass substrates. In each case, a 3*3 inch glass substrate Eagle 2000 was cleaned before transistor fabrication. A layer of aluminium 30 nm thick was thermally evaporated through a shadow mask as a gate contact. One side of the gate was coated with a 20-nm-thick layer of gold thermally evaporated through a shadow mask. The gold layer caps a part of the gate electrode to prevent its oxidation. An aluminium oxide gate dielectric was then prepared. Its preparation was varied between the samples - this is described below. After this, a - 0-A-thick layer of 1-octylphosphonic acid was vacuum deposited on the aluminium oxide (Al₂0₃) by heating a solid form of the acid to cause evaporation. Deposition of the 1-octylphosphonic acid helps reduce high surface energy and suppresses leakage current. Next, a 50-nm-thick layer of pentacene was vacuum deposited on top of the 1-octylphosphonic acid. Finally, a 50- nm-thick gold layer was thermally evaporated using shadow mask to create the top source and drain contacts. After fabrication, the samples were stored in vacuum.

Figure 9 shows the steps of the UV/ozone treatment process for aluminium oxide (Al₂0₃) formation. The aluminium gate electrode was first exposed to UV/ozone to create Al₂0₃, then a 15-A-thick Al layer was deposited on top of the created Al₂0₃ and UV/ozone oxidized for the same time as the gate electrode. The 15-A-thick Al layer was selected based on previous experiments indicating that the UV/ozone exposure led to complete oxidation of such a thin layer. It is possible to deposit a number "n" of such thin Al layers sequentially, oxidising them one by one, and gradually build up the Al₂0₃ thickness to a desired thickness. This basic process was used for the formation of all of the Al₂0₃ layers of all of the test sets.

To test the effect of the organic monolayer on the transistor performance 24 transistors were manufactured with an Al₂0₃ layer corresponding to four 15-A-thick Al layer depositions, each exposed for 60 min to UV/ozone. Half of the transistors were processed to include an organic monolayer following the method described with reference to Figure 2. The other half did not include an organic monolayer.

Figure 10 compares the transfer (left) and output (right) characteristics of transistors with and without phosphonic acid monolayer. The gate dielectric capacitance was measured at a frequency of 1 MHz for samples with and without phosphonic acid. The Al₂0₃ layer exhibited capacitance of 3.52x 10^'7 F/cm², while the Al₂0₃ coated with a monolayer of phosphonic acid had a capacitance 2.76x 10^"7 F/cm². These values are slightly lower than the values of capacitance given above. This is because the Al₂0₃ layer is slightly thicker than the one used in the capacitor structures discussed above. If it is assumed that the relative permittivity of 1-octylphosphonic acid is ~ 2, these two capacitance values can be used to calculate the thickness of 1-octylphosphonic acid. The calculated thickness is 13.8 A. This value is very close to the length of a single 1- octylphosphonic acid molecule, suggesting that a single layer was formed. XRD measurements of phosphonic acid monolayers deposited from solutions have shown that the organic molecules are ordered almost perpendicular to the surface of the Al₂0₃ [Japanese Journal of Applied Physics, (2002), Volume 41 , L1474-L1477], The thermally evaporated 1-octylphosphonic acid suppresses the gate leakage current by one order of magnitude and raises the source-drain current by a factor of 5. Since both transistors have the same dimensions, the 5-fold increase in the source-drain current leads to a 5-fold increase in the field-effect mobility.

Three sets of transistors (A, B, C) were fabricated to optimize the preparation of the aluminium oxide/ organic SAM dielectric layer. Set A was fabricated to investigate the effect of UV/ozone exposure time with a fixed dielectric thickness. Set A (6 samples) was fabricated by exposing the aluminium gate to UV/ozone for 2, 5, 10, 20, 40 and 60 minutes to form AI2O3, each sample undergoing different UV/ozone treatment time. Afterwards, a 15-A-thick layer of aluminium was deposited on top of the Al₂0₃ and UV/ozone oxidized for the same time as the gate electrode.

Set B was fabricated by varying the AI₂O₃ thickness at a fixed UV/ozone exposure time. Set B (6 samples) was fabricated by fixing the UV/Ozone exposure time of each Al layer to 60 minutes and increasing the number of 15-A-thick Al layers from 1 to 6. Similar to set A, the Al gate contact is oxidised for 60 minutes before the deposition of the first 15-A-thick Al layer commences.

Set C was fabricated in a similar way to set A except that the Al₂0₃ thickness of set C is approximately double of that of set A. Set C (6 samples) was fabricated like set A except in set C six 15-A-thick Al layers were deposited and oxidised sequentially, leading to a maximum total thickness of about 20 nm. The same UV/ozone exposure times like in set A were used. Figure 11(a) shows the capacitance C as a function of UV/ozone exposure times for set A. In this case the Al₂0₃ layer consists of oxidized aluminium gate plus additional oxidized 15-A-thick Al layer. Figure 1 (b) shows the capacitance as a function of Al₂0₃ thickness produced by 60 minute UV/ozone exposure time for set B. Here the number of oxidized 15-A-thick Al layers is varied from 1 to 6. From this, it can be seen that as the thickness of the AI2O₃ layer increases the capacitance decreases. The change in capacitance also indicates that the Al₂0₃ thickness increases in approximately equal increments from sample to sample.

Figure 11(c) shows the capacitance as a function of UV/ozone exposure for set C. This set is similar to set A except the thickness of Al₂0₃ is approximately double of that of set A. The capacitance of set A is about 0.45 pF/cm² and there is a very slight decrease in capacitance with increasing UV/ozone exposure time. For set C, the capacitance is around 0.30 pF/cm² and independent of UV/ozone exposure time. The average gate dielectric capacitance of set C is smaller than that of set A because the AI2O3 thickness is higher for set C capacitors. All samples within set A (Figure 1 (a)) have approximately the same Al₂0₃ thickness as the sample with thinnest Al₂0₃ layer in set B (the left-most data points in Figure 11(b)), also indicated by similar capacitance values. Similarly, the thickness of Al₂0₃ in set C (Figure 1 (c)) is the same as the one in sample with the thickest Al₂0₃ layer in set B (the right-most data points in Figure 11(b)). In other words, the. samples represented by the right-most data points in Figure 11 (a) and the left-most data points in Figure 11(b) were prepared according to the same fabrication recipe. Similarly the samples represented by the right-most data points in Figure 11(b) and the left-most data points in Figure 11(c) were prepared according to the same fabrication recipe. Figure 12 represents the gate dielectric breakdown voltage of the three sets of transistor samples. The electric breakdown voltage does not change much with increasing UV/ozone exposure time, although it reaches slightly higher values for longer UV/ozone times (see Figures 12(a) and (c)). For set C 20-minute UV/ozone exposure of aluminium leads to the same breakdown voltage as for all longer exposures. Consequently, the 20-minute UV/ozone exposure time is sufficient to make dielectric with improved electric breakdown voltage. Figure 12(b) shows a gradual increase in the breakdown voltage with increasing AI2O3 thickness. The breakdown voltage rises from ~5 V to -12 V as the Al₂0₃ thickness increases from 93 A to 194 A. The right-most point in Figure 12(b) and the left-most point in Figure 2(c) correspond to transistor samples prepared according to the same recipe. Similarly, the right-most point in Figure 12(a) and the left-most point in Figure 12(b) correspond to transistor samples prepared according to the same recipe. This indicates good reproducibility of the transistor process between different growth runs. In all cases the electric breakdown field is at least 5 MV/cm.

All transistors have been evaluated using the general MOSFET equations in linear and saturation regimes to calculate the field-effect mobilities, threshold voltages and inverse subthreshold slopes. In the linear regime: |V_gs - V,| > |V_ds| and l_ds = (W/L) μ_ρ C, [(V_gs - V.) V_ds - V_ds ²/2] Eq. (1)

In the saturation regime:

| _gs - V_t| < |V_ds| and l_ds = (W/2L) μ_ρ C, (V_9S - V_t)² Eq. (2)

Here V_gs is the gate-to-source voltage, V_t the threshold voltage, V_ds the drain-to-source voltage, l_ds the drain-to-source current, W the channel width, L the channel length, μ_ρ the hole field-effect mobility, and C, is the capacitance per unit area of the gate dielectric.

Figure 13 represents the threshold voltages calculated using Eq (2) for sets A, B and C. The figure includes data for OTFTs with different channel lengths of L = 30, 50, 70, and 90 μηι. Figure 13(a) (set A) shows that as the UWozone exposure time increases the threshold voltage decreases. This behaviour is independent of the channel length of the transistor. The decrease in the threshold voltage is faster for shorter exposure times and reduces after 20-minute UV/ozone exposure time. Figure 13(b) (set B) gives the threshold voltage as a function of Al₂0₃ thickness for different channel lengths. There is a slight decrease in the threshold voltage as the Al₂0₃ thickness increases, dropping from -1.45 V to -1.35 V. Different channel lengths of the transistors show approximately the same behaviour.

Figure 3(c) represents the threshold voltages for thickest Al₂0₃ layers as a function of UV/ozone exposure time. Here the UV/ozone exposure time has similar effect on the threshold voltage of the transistors as in Figure 13(a). However, the data points are shifted downward by 0.2 V.

The right-most data points in Figure 3(a) and the left-most data points in Figure 13(b) correspond to samples prepared according to the same recipe at different times. Similarly, the right-most data points in Figure 13(b) and the left-most data points in Figure 3(c) correspond to samples prepared according to the same recipe at different times. This indicates good reproducibility of our fabrication process.

To summarize, the threshold voltage varies between ~ -1.2 V and ~ -1.9 V as a function of the Al₂0₃ preparation conditions. Lower operating threshold voltages are desirable for implementation of the transistors in the circuits, indicating that longer exposure times and thicker Al₂0₃ layers would be needed. On the other hand, shorter oxidation times lead to faster fabrication and therefore may be of interest.

Figure 14 shows the inverse sub-threshold slopes of the three sets of transistors. The values are more scattered than the threshold voltages in Figure 13. It seems that the subthreshold slopes are independent of the UV/ozone exposure times (see Figures 14(a) and 14(c)). However, the mean value for set C (Figure 14(c)) is about 50 mV/decade higher than the mean for set A (Figure 14(a)). From Figure 14(b), it can be seen that the sub-threshold slope is decreasing with decreasing Al₂0₃ thickness. As the Al₂0₃ thickness decreases from 194 A to 93 A, the sub-threshold slope decreases from ~ 250 m decade to ~ 170 mV/decade. To achieve a thinner dielectric layer with lower sub-threshold slopes for faster switching of the TFTs, the best results are obtained for the thinnest Al₂0₃ layer and the shorter UV/ozone exposure time.

Figure 15 represents the field-effect mobility of the three sets of transistors. In set A (Figure 15(a), thinnest Al₂0₃ layer) transistors with the shortest UV/ozone exposure time of two minutes exhibit the highest mobility of up to ^« 0.09 cm² V.s. When the thickness of the gate dielectric is increased (Figure 15(b), 60-minute UV/ozone exposure time) the mobility remains more-less unchanged. Similarly, for thicker Al₂0₃ layers, the UV/ozone time does not have much effect on the hole mobility. The values are more scattered for UV/ozone exposure times less than 20 minutes. Based on Figure 15, there is no clear correlation between the hole mobility and the channel length.

Figure 16 shows the off-currents of the three sets of transistors. There is a larger spread in the values of the off-currents for samples with thinner AI2O3 layer (Figure 16(a)) as compared to the thicker one (Figures 16(b)(c)). Shorter UV/ozone exposure times have a similar effect to a lesser degree (see Figure 16(c)). From Figure 16(b), it is clear that the change in the Al₂0₃ thickness does not affect the off-currents of the TFTs and Figure 16(c) suggests that longer UV/ozone exposure time might lead to better transistor uniformity. In addition, as mentioned above with reference to Figure 12, a thicker Al₂0₃ layer leads to higher breakdown voltage of the transistors.

Further experiments have been done to validate the results obtained above and to characterise the self assembled organic monolayer. Metal-insulator-metal (MIM) or capacitor structures were fabricated by vacuum deposition of 20-nm-thick aluminium electrodes on a glass substrate (Eagle 2000). One end of the electrode was capped by a gold layer to prevent its oxidation. This is to leave an area to contact the capacitor for the measurement. The aluminium surface was converted to aluminium oxide (AI2O3) by exposing it to UV/ozone for 1 hour. The UV/ozone exposure was performed in UVOCS UV/ozone cleaner enclosed under a Hepa filter. To increase the thickness of Al₂0₃ and suppress the leakage current, an additional 1.5 nm thick Al layer was deposited on top of Al₂0₃ and UV/ozone exposed as described above. A 10-nm-thick C₈PA layer was thermally evaporated on top of Al₂0₃. The substrate temperature was subsequently set to 200°C to remove physisorbed C₈PA molecules during a series of desorption times set at 25, 60, 90, and 210 minutes. Finally, 50-nm-thick gold contacts were evaporated to complete the MIM structures. Several MIM structures with an area of ~ 0.2 mm² were fabricated for each desorption time. In addition, MIM structures with no C₈PA layer were prepared as a reference. Bottom-gate, top-contact OTFTs based on thermally evaporated pentacene followed the same fabrication procedure. After C₈PA desorption, a 50-nm-thick pentacene layer was deposited at a rate of 0.24 A/s at room temperature. Pentacene purification was performed in a three-zone tube furnace at a pressure of 1x10^'3 Pa. The transistors were completed by evaporating gold source and drain contacts. All fabrication steps were completed by using shadow masks. The fabricated transistors have channel lengths (L) of 30, 50, 70, and 90 μηι and a channel width (W) of 1000 μητ To minimize the effect of extending electric field lines, that could lead to a substantial overestimate of the field-effect mobility, the W/L was chosen to be larger than 10. Transistors and capacitors were fabricated side by side and all thermal evaporation steps were conducted in inispectros (Kurt J. Lesker) high vacuum system (1 x10 ⁵ Pa).

All capacitor and transistor measurements were performed with Agilent B1500A semiconductor device analyzer under ambient environmental conditions. The gate dielectric capacitance of MIM structures was measured between 1 kHz and 1 MHz. The IM current density was measured between -3 and 3 V. In addition, the current-voltage measurement was extended to higher values to measure the dielectric breakdown voltage. The breakdown voltage is defined as voltage at which the current through the MIM structure is 1 mA/cm². The transfer and output characteristics of the OTFTs were measured in a sweep mode from positive to negative gate voltage. Multiple MIM structures and transistors were measured for each C₈PA desorption time. Mean values and standard deviations were calculated for all parameters. In addition, the topography of the gate dielectric surfaces was studied by atomic force microscopy (AFM) using the tapping mode. The scanned images are of 1 *1 pm² area.

The dielectric properties of bare Al₂0₃ and Al₂0₃ coated with n-octyl phosphonic acid (Al₂0₃/C₈PA) were investigated as a function of C₈PA desorption time.

Figure 17 shows the average capacitance per unit area (F/cm²) at 100 kHz. The capacitance of bare Al₂0₃ is 0.63±0.01 uF/cm² Al₂0₃/C₈PA capacitance value increases from 0.41 to 0.46 pF/cm² as the C₈PA desorption time rises from 25 to 210 minutes. The total gate dielectric capacitance (C_gd) consists of the capacitance of the oxide (CAIO) and the phosphonic acid layer (CCSPA) connected in series and is given by 1 Cgd = CAIO ⁺ /Cc₈pA- CAIO is the same in all capacitors since their aluminium oxide was prepared at the same time. Measured values of C_gd and C_Aio allowed us to calculate C_C8PA for each desorption time. The thickness of the C₈PA layer on A)₂0₃ is 10 nm (before desorption), as determined by AFM.

Figure 18 shows the thickness of the octyl phosphonic acid layer measured as a function of desorption time. The C₈PA thicknesses were calculated from the total capacitance values using a relative permittivity of 2.1 for C₈PA.

Figure 9 shows the leakage current densities of bare Al₂0₃ and AfeC CePA dielectrics at various C₈PA desorption times. The leakage current density through bare Al₂0₃ and Al₂0₃/C₈PA dielectric is 6x10^"7 and 10^"7 AJcw² for a voltage of -3 V, respectively. Some Al₂0₃/C₈PA structures exhibit leakage current density as low as 4 10^"8 A/cm².

Figure 20 shows the dielectric breakdown voltages. The mean breakdown voltage of Al₂0₃ and Al₂0₃/C₈PA is 5 and 6.3 V, respectively. The breakdown voltage and the leakage current density of Al₂0₃/C₈PA MIM structures do not change significantly with applied desorption time. The electric breakdown field of Al₂0₃ and AbC CePA calculated from the above layer thicknesses is 5 and 6 MV/cm, respectively.

Figure 21(a) shows an AFM image of bare Al₂0₃ with average RMS surface roughness of 0.45 nm. Figure 21 (b) and 21(c) show AFM images of C₈PA immediately after its deposition and after 210 minutes desorption, respectively. The deposited layer of C₈PA on Al₂0₃ exhibits very high RMS surface roughness of 4.5 nm. RMS surface roughness of 0.38 nm is achieved after 210-minute desorption. This value is just slightly lower than that of bare Al₂0₃ surface. One could also notice similar features on the surface of bare Al₂0₃ and the surface of C₈PA after 210 minutes desorption. The similarities in AFM topography in Figures 21 (a) and 21 (c) also indicate that the desorption procedure did not lead to Al crystallization.

These measurements confirm the presence of n-octyl phosphonic acid monolayer formed by thermal evaporation. The deposited 10-nm-thick C₈PA layer is reduced down to a monolayer (1 nm) by desorbing the non-chemically bonded C₈PA molecules via thermal heating. The measurements show that although this process is completed in approximately 30 minutes, the monolayer is present even after 3 hours of additional heating. C₈PA monolayer thermally evaporated on top of Al₂0₃ lowers the leakage current density, increases the breakdown voltage and the breakdown field, and slightly improves the RMS surface roughness.

Figure 22 shows the transfer characteristics of a pentacene transistor prepared with a desorption time of 210 minutes for drain-to-source voltage of -0. and -2 V.

Figure 23 shows the output transfer characteristics of the transistor of figure 22 for gate-to-source voltage of 0, -0.5, -1.0, -1.5, and -2.0 V. Figure 24 (a) shows the field effect mobility measured as a function of desorption time. The field-effect mobility is calculated using standard MOSFET equations. It is extracted from the linear fit of /_Ds (or Ji^) versus VGS and the intercept on the voltage axis corresponds to the threshold voltage. Figure 24 (b) shows the threshold voltage measured as a function of desorption time. Figure 24(c) shows the subthreshold slope measured as a function of desorption time. The subthreshold slope S is calculated from the inverse of slope of log(f_Ds) versus V_Gs. Figure 24(d) shows the off-current measured as a function of desorption time. The on and off-currents are the maximum and minimum drain-to-source currents at _Ds = -2 V. With increasing C₈PA desorption time the field-effect mobility increases from 0.02 cm² Vs to 0.04 cm²/Vs, the threshold voltage rises from -1.2 V to -1.4 V, the subthreshold slope decreases from 120 mV/decade to 80 mV/decade, and the off-current decreases from 5^χ10^"12 A to 1 x10^"12 A. The reaction between alkyl phosphonic acid and aluminium oxide can be given by R-PO(OH)₂ + OH-AI-→ R-(OH)OP-0-AI- + H₂0. The data in Figures 17, 18 and 21 confirm the formation of n-octyl phosphonic acid monolayer on the surface of Al₂0₃. The layer remains on the surface of the aluminium oxide even after prolonged heating of the surface. The C₆PA layer reaches a thickness of about 1 .1 nm after 90 minutes desorption time. No change in the C₈PA thickness is observed after another 120 minutes of heating. Hence, it can be concluded that the C_aPA monolayer prepared by thermal evaporation is covalently attached to the surface of Al₂0₃.

Depending on the desorption time, the total thickness of the Al₂0₃/C₈PA is 10-12 nm. The current density of Al₂0₃/C₈PA is almost the same for each desorption time with slightly smaller value at 210 minutes. Al₂0₃/C₈PA layers exhibit about an order of magnitude lower leakage current densities than bare Al₂0₃. The breakdown voltage of Al₂0₃/C₈PA dielectric is about 1.3 volts higher than that of Al₂0₃. The difference in AFM topography in Figure 21(a) and 21(b) reveals a very rough layer of deposited C₈PA. The surface morphology and RMS roughness changes rapidly after the desorption of non-bonded C₈PA molecules, slightly surpassing the RMS roughness of Al₂0₃ after 210 minutes desorption time. The A^ Cs A has an RMS surface roughness of 0.38 nm after 210 minutes desorption time which is the lowest value for alkyl phosphonic acids reported to date. Such small surface roughness is comparable to that achieved with polymer dielectric. This value could be a result of improved packing density of C₈PA molecules. The reduction in the leakage current density, increase in the breakdown voltage and improvement in the surface roughness show that dry C₈PA monolayer provides good step coverage and offers alternative path to alkyl phosphonic acid monolayers prepared from solution.

Performance of the OTFTs fabricated using the dry process of the invention is comparable to OTFTs that implement alkyl phosphonic acids assembled from solution. The source-to-drain current is three orders of magnitude higher than the gate-to-source current, demonstrating good insulating behaviour of the dielectric bilayer. The maximum field-effect mobility is 0.04 cm² Vs. The threshold voltage is between -1.1 and -1.4 V and hence, OTFTs can be operated in the voltage range of 2-3 V. Previous results have shown that low threshold voltage can result in higher off-current and, consequently, lower on/off current ratio. The lowest subthreshold slope of 80 mV/decade was achieved after 210 minutes desorption. The maximum on/off current ratio of 2.5 * 10⁵ was obtained for 210 minutes desorption. The low operating voltage and low fabrication temperature make the OTFTs suitable for plastic substrates.

The organic molecules employed to prepare a single layer (monolayer) on a substrate as described herein can be of the types such as are well known in the art for forming self assembled monolayers (SAMs) by conventional techniques. Specific examples of processing with suitable molecules are described in more detail hereafter.

Suitable molecules are described, for example, in documents such as: "Molecular Self- Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin-Film Transistor Applications", DiBenedetto et al, Advanced Materials, 2009, 21, 1407-1433; and US7202547 (discussed above). The contents of both these documents are incorporated herein by reference. In general, the molecules employed include an anchor group that is selected to bind, typically by means of chemical bond formation (e.g. covalent bonding), to the substrate being treated. The anchor group is connected to a 'tail' part of the molecule that is typically elongate. The anchor and tail combination is selected to permit good binding to the selected substrate, aid the formation of a suitably packed monolayer and to provide the desired modification to the physical properties of the device being fabricated.

In some cases, the tail may include one or more reactive or potentially reactive functional groups. Such additional functional groups may be for modifying the physical properties of the monolayer. Alternatively, these groups may be for use in subsequent modification by chemical reaction of the monolayer or they may be used for reaction with a subsequently applied layer of material in the fabrication procedure for the device.

In general, the molecules used in the methods described herein may take the form R-X wherein X is the anchor group and R represents the tail. In some cases (for example the disulfide molecules described below) the general structure may be of the form R-X- R, where the group X attaches at least one of the two groups R to the substrate in use of the methods described herein. In some examples (typically when forming a layer on a metal surface) the structure of molecule employed is X-R-X where an anchor group is provided at each end of an elongate molecule.

Illustrative examples of groups R such as are employed in manufacture of electronics are shown in Scheme 1 below, where the anchor group employed in each example is indicated by arrows from X.

Thiol.disulfide and thioacetyl Anchor Groups

10 11 12 13

Silane derivative, carboxylic acid and phosphonic acid Anchor Groups

Scheme 1

When forming a layer on a metal surface (e.g. a substrate with a Au, Ag, Cu, Pd or Pt surface), a thiol (-SH) or other reactive sulfur group may be employed as the anchor group binding to metal. Illustrative examples are shown in Scheme 1 above. Thus alkane thiols 1 and dithiols 2 may be used to form a monolayer on metal, for example in fabricating electronic devices. Other suitable molecules that can be usefully added to metal include thiols and dithiols of oligo-aromatics or oligo-heteroaromatics such as oligothiophenes (OTs) 3 and oligophenylenes (OPs) 4, 5, 6, oligo(phenylenevinylenes) (OPVs) 7 and oligo(phenyleneethynylenes) (OPEs) 8. Other reactive sulfur anchor groups that can be present for attaching a molecule to metal include disulfide and thiocarboxylic acid (-C(O)SH) for example. Where the anchor group is disulphide 9, group X is _-S-S- and two R groups, which may be the same or different, are present. Such a molecule may provide two moieties (R-S-) each of which can bind to the substrate.

In general, the tail groups R of organic molecules employed in the methods described herein may be selected from the group consisting of substituted or unsubstituted alkyl, (or alkylene), for example n-alkyl or (n-alkylene) that may be unsaturated (for example C1-C20 or C2-C18); substituted or unsubstituted oligo-aromatics or oligo- heteroaromatics such as oligothiophenes and oligophenylenes (OPs); substituted or unsubstituted oligo(phenylenevinylenes) (OPVs); and substituted or unsubstituted oligo(phenyleneethynylenes) (OPEs).

Where the tail groups R are substituted, they may be substituted for example once, twice, or three times, e.g. once, i.e. by formally replacing one or more hydrogen atoms of an alkyl, alkylene, aryl or heteroaryl group. Examples of such substituents are halo (e.g. fluoro, chloro, bromo and iodo), SF₅, CF₃ , aryl, aryl hydroxy, nitro, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, carbamate and the like. Where the substituent is amino it may be NH₂, NHR' or NR'₂, where the substituents R' on the nitrogen may be alkyl, aryl or heteroaryl (for example substituted or unsubstituted C1-C20 or even C1-C10). The tail groups R may also include cycloalkyl groups such as cyclopentyl or cyclohexyl.

The tail groups R may also include one or more ether or thioether linkages. For example, the tail groups R may include polyether chains having repeat units -CH₂-0- or -O-CH₂-CH₂-. There may be from 2 to 20 of the -CH₂-0- repeat units, or from 2 to 20 of the -O-CH₂-CH₂- repeat units.

Where the tail groups R are substituted or unsubstituted oligo-aromatics or oligo- heteroaromatics such as oligothiophenes and oligophenylenes (OPs); substituted or unsubstituted oligo(phenylenevinylenes) (OPVs); or substituted or unsubstituted oligo(phenyleneethynylenes) (OPEs), the number of aromatic or heteroaromatic rings in the chains may be from 2 to 10. These chains of aromatic or heteroaromatic rings may be attached to anchor groups X by short linker groups such as alkylene linker groups, for example methylene.

When forming a layer on a substrate including an oxide surface or layer, the tail groups R may be as discussed above with respect to the molecules that may be added to a metal surface.

Where the layer is formed on an oxide surface to improve dielectric performance of e.g. a capacitor or a transistor, convenient tail groups include n-alkyl, or substituted n-alkyl groups, as discussed above. For example substituted n-alkyl groups could be substituted with F, for example partially fluorinated alkyl chain. For example, unsubstituted n-alkyl groups of from 2 to 20 C atoms, such as n-octyl may be employed. For further example the molecules described in US7202547 where alkyl or polyether groups (both described as linker groups therein), that are attached to an anchor group and are terminated by aromatic groups (described as head groups therein). See the example 10 in Scheme 1 above where an n-alkyl chain is terminated by phenyloxy.

When forming a layer on a substrate that includes an oxide surface, different anchor groups, from those employed on a metal surface, may be employed to bond to the oxide.

Exemplary oxide surfaces include oxides of at least one of silicon, aluminium, titanium, silver or gold. Other examples may include a chromium oxide, a zinc oxide, a hafnium oxide, a magnesium oxide, a calcium oxide, and an indium tin oxide (ITO) and other oxides comprising more than one metal. In some cases the oxide surface on a substrate may be a naturally occurring (native) oxide layer that forms e.g. on an otherwise substantially pure metal.

For oxides such as, for example, Si0₂, Al₂0₃, and ITO reactive organosilane precursors may be employed as the molecule that reacts with the substrate. The organosilane precursors may take the general form RSiY₃ where -SiY₃ is the anchor group X. Each Y may be the same or different. Typically all groups Y are the same and are reactive to the substrate, for example groups Y may be halogen (e.g. chlorine - see 10, 11 in Scheme 1 above) or alkoxy (e.g. methyoxy or ethyoxy). However up to two of the Y groups may be non-reactive alkyl (for example C1 to C5, such as methyl or ethyl). Other silane precursors can include hexamethyldisilizane (HMDS) and 3- mercaptotrimethoxysilane (MPTMS).

Organic acids are conveniently employed as molecules in the method when forming a layer on an oxide. Suitable acid groups acting as anchor groups include carboxylic acid and phosphonic acid groups. Alternative acid groups may include thiocarboxylic acids (-C(O)SH) and sulphonic acids (-S(=0)₂OH). The combination of organic acid anchor groups and substituted or unsusbstituted n-alkyl tail groups (12, 13 in Scheme 1) is convenient when manufacturing devices such as capacitors or transistors including an oxide substrate layer.

As an alternative to the acid groups themselves, molecules having acid derivatives as anchor groups may be employed in the method to add a surface, such as an oxide surface. For example acid anhydrides, acid halides (e.g. carboxylic acid chlorides) or other suitably reactive acid derivatives may be employed. The use of aldehydes (-CHO) or aldehyde derivatives and even alkenes (for example X is _(-CH₂=CH₂)_may also be contemplated.

Other suitable anchor groups, for a given substrate surface, may be attached to chosen tails if required. For example as discussed in DiBenedetto et al (referenced above) hydrogen passivated silicon surfaces can be made when the oxide layer that forms naturally on silicon is not desired. Molecules such as the organosilane precursors discussed above (RSiY₃) may not react smoothly with such a surface and alternative molecules for example molecules that have unsaturated anchor groups (that attach to the surface by cycloaddition reaction) may be employed.

The method of the invention offers the possibility of manufacturing electronic devices using an all dry process. Also, it opens the possibility for manufacturing electronic devices that include an organic monolayer in a vacuum, without requiring breaking vacuum to deposit the monolayer. In particular, the method is suitable for large scale manufacturing of OTFTs for example by using roll-to-roll fabrication. B2012/000626

27

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, although the description has focused on heating the over-deposited layer to form the monolayer, it will be appreciated that other techniques could be used for breaking the intermolecular interaction, such as light waves or resonant vibrations. Also, whilst the method for producing OTFTs described uses Pentacene semiconductors, alternative semiconductors could be utilised such as:

Copper(ll) 1 ,2,3,4,8,9, 10, 11 , 15, 16, 17, 18,22,23,24,25-hexadecafluoro-29H,31 H phthalocyanine. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. A dry method for forming a single organic layer on a substrate, the method involving:

depositing organic molecules by vapour deposition on the substrate, wherein some of the molecules are bonded to the substrate and others are held together via intermolecular interaction, and

removing using a dry process the organic molecules that are held together by breaking the intermolecular interaction, leaving a monolayer of organic molecules bonded to the substrate.

2. A method as claimed in claim 1 , wherein depositing the organic molecules is performed using a source of organic material which is solid at room temperature.

3. A method as claimed in claim 1 or claim 2 wherein depositing the organic molecules by vapour deposition is done in vacuum.

4. A method as claimed in any of the preceding claims, wherein depositing the organic molecules and removing the molecules that are held together by intermolecular interaction is done whilst maintaining a vacuum.

5. A method as claimed in any of the preceding claims wherein depositing the organic molecules and removing the molecules that are held together by intermolecular interaction is done in the same chamber or in a series of linked chambers.

6. A method as claimed in claim 4 or claim 5, wherein maintaining vacuum involves maintaining a pressure in the chamber between 1x10^"6 Pa and 1x10⁵ Pa.

7. A method as claimed in any of the preceding claims wherein the organic molecules are deposited to form a layer that is thicker than a monolayer, for example more than two times the thickness of the monolayer; or more than nine times the thickness of the monolayer.

8. A method as claimed in any of the preceding claims, wherein the intermolecular interaction is van der Waals interaction.

9. A method as claimed in any of the preceding claims, wherein each organic molecule has an anchor group and a tail.

10. A method as claimed in claim 9, wherein the anchor group binds with the substrate by forming a chemical bond of characteristic strength and wherein the tail may bind to the substrate and/or to the organic molecules with a strength less than the characteristic strength.

11. A method as claimed in claim 10 wherein the chemical bond is a covalent bond.

12. A method as claimed in claim 9, wherein the anchor group is hydrophilic and the tail is hydrophobic.

13. A method as claimed in any of the preceding claims, wherein the dry process comprises heating the molecules.

14. A method as claimed in any of the preceding claims, wherein the dry process comprises using light waves and/or intermolecular resonant vibrations.

15. A method as claimed in claim 13, wherein heating is done at or above the sublimation temperature of the said organic molecules.

16. A method as claimed in claim 15, wherein heating is performed for at least one minute, for example at least twenty minutes, preferably at least 30 minutes.

17. A method as claimed in any of the preceding claims, wherein the substrate comprises a metal oxide.

18. A method as claimed in claim 17, wherein the metal oxide is an oxide of one or more element selected from the group consisting of silicon, titanium, silver, aluminium, chromium, zinc, indium, tin, hafnium, magnesium, calcium and gold.

19. A method as claimed in claim 17 or claim 18 wherein the organic molecules are organosilane precursors or an organic acid.

20. A method as claimed in claim 19 wherein the organic molecules are organic acids selected from the group consisting of: carboxylic acids, phosphonic acids, thiocarboxylic acids, sulphonic acids and derivatives thereof.

21. A method as claimed in claim 20 wherein the organic molecules are of the form R-X wherein:

X is the organic acid function or derivative thereof; and

R is a group selected from the group consisting of substituted or unsubstituted alkyl, for example n-alkyl that may be unsaturated (for example

C1-C20 or even C2-C 8); substituted or unsubstituted oligo-aromatics or oligo- heteroaromatics such as oligothiophenes and oligophenylenes (OPs); substituted or unsubstituted oligo(phenylenevinylenes) (OPVs); and substituted or unsubstituted o)igo(phenyleneethynylenes) (OPEs).

22. A method as claimed in any of the preceding claims, wherein the organic molecules form a self assembled monolayer on the substrate.

23. A method as claimed in any of the preceding claims, wherein the organic molecules are alkane-phosphonic acid or carboxylic acid molecules.

24. A method as claimed in claim 23 wherein the organic molecules are

1-octylphosphonic acid.

25. A method for constructing an electronic device, the method involving:

forming a metal gate,

creating a metal oxide layer on the gate,

depositing a single organic layer on the metal oxide layer using the method of any of claims 1 to 24,

depositing a semiconductor layer, and

forming source and drain contacts 31

26. A method as claimed in claim 25 wherein the metal gate is formed on a biodegrable substrate.

27. A method as claimed in claim 25 or claim 26 wherein the semiconductor layer is deposited on the single organic layer.

28. A method as claimed in claim 25 or claim 26, wherein the semiconductor layer is deposited on the source and drain contacts.

29. A method as claimed in claim 25 or claim 26, wherein the source and drain contacts are formed on the semiconductor layer.

30. A method as claimed in any of claims 25 to 29, wherein the gate contact comprises a metal oxide.

31. A method as claimed in claim 30, wherein the metal oxide is an oxide of one or more element selected from the group consisting of silicon, titanium, silver, aluminium, chromium, zinc, indium, tin, hafnium, magnesium, calcium and gold.

32. A method as claimed in any of claims 24 to 3 , wherein the semiconductor is a conjugated polymer that sublimes under vacuum.

33. A method as claimed in claim 32, wherein the conjugated polymer is Pentacene.

34. A method as claimed in any of claims 25 to 31 , wherein the semiconductor is Copper(ll)1,2,3,4,8,9,10,11 , 15,16,17, 18,22,23,24,25-hexadecafluoro-29H,31 V- phthalocyanine.

35. A method as claimed in any of claims 25 to 34, wherein the source and /or drain contact comprises metal.

36. A method as claimed in claim 35, wherein the metal comprises one or more of gold, silver or aluminium. 32

37. An electronic device that includes a monolayer made according to any of claims 1 to 24, wherein optionally the electronic device is formed on a biodegradable substrate.