NL7905560A - CARRIER WITH A COATING WITH SPECIAL ELECTRICAL PROPERTIES. - Google Patents

Description

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Rockwell International Corporation of EL Segundo, California,

Far. St .v. America

Coated carrier with special electrical properties

The invention relates to a support with a coating thereon which exhibits special electrical properties.

Layers, coatings and thin films of metal oxides that are electrically conductive are well known. These have been extensively used in the manufacture of, for example, displays such as those with liquid crystals, some of the electrons having to be transparent so as not to obstruct an observer's visibility of the displayed information.

For an example of this application, reference is made to United States Patent Specification 3.81¼.501. The most commonly encountered metal oxides for this purpose are oxides of indium, tin, or of indium and tin together (indium-tin oxide or ITO). US patent 3,81¼,501 discloses cartridges of transparent electrically conductive material in a liquid crystal display wherein the conductive material is indium oxide or tin oxide or a combination of the two.

Another application for transparent electrical conductors is that as electrons for control structures that are mounted on semiconductor devices such as charge coupled image projectors.

In both one application and the other, practice has hitherto been to form the conductor pattern as a separate stand-alone unit. Typically, the pattern was formed by depositing metal oxide material and then using photo-etching techniques to shape the metal oxides into the desired conductor pattern, although other methods were also used, such as, for example, a printing technique. For an example of the latter, reference is made to German inspection document 2. Ml. 872.

In the case where a conductor pattern of, for example, 790 55 60 * 2 ITO is formed as a separate entity on an image projection or image display device, at least two separate kinds of light paths are created for light incident on the device.

In one type of light path, a light beam passes through the ITO once or twice in the direction of its thickness depending on whether or not the light beam was reflected. In the other type of light path that does not pass through an ITO layer, the light typically travels a long distance through an air-filled space. The difference between the refractive index of the air and that of the ITO, and the difference in the number of interfaces passed through a light beam depending on the distance traveled, produces a distortion of the image being recorded or reflected. The present invention significantly reduces these differences, thereby also reducing the resulting distortion.

US Pat. No. 2,932,590 describes the controlled deposition of metallic indium and tin and their subsequent thermal oxidation to yield within transparent and electrically conductive coatings. However, this U.S. patent does not describe the formation of a pattern of 20 electrical conductors contained in an electrically insulating film as will be described below,

The invention provides a pattern of transparent electrically conductive material that is deposited on a substrate, a semiconductor device or the like, wherein the pattern of electrically conductive material is incorporated in an otherwise continuous layer of transparent, but electrically insulating material, the refractive index of which is substantially the same is that of the electrically conductive material, the layer everywhere having at least substantially the same thickness. Light passing through the conductive material and light passing through the electrically insulating material follows the same light path because the distance differences and the propagation time differences for light rays passing through the two different types of material are made small.

In the preferred embodiment, the electrically insulating material is an indium oxide, in particular indium sesquoxide (ingO4).

790 55 60 3%

In order to make the ratio between the specific spring position of the electrically insulating material and the specific spring position of the electrically conductive material large, the electrically insulating vacuum is preferably made relatively free of impurities and at least substantially stoichiometric. Pollutants and oxygen ion deficiencies provide free charge carriers and are therefore unscrupulous.

Areas of the layer of electrically insulating material are modified to make these areas electrically conductive in a pattern corresponding to the corresponding electrically conductive pattern. In the preferred embodiment, tin is used as a dopant for modifying the electrically insulating material.

As much tin as the indium are initially produced. deposited on the substrate in elemental or metallic form. The tin 15 is placed in the first pattern for the electrical conductors. The tin is then diffused or alloyed in the indium and the whole is oxidized;

Elemental materials can be obtained with a considerably higher purity than, for example, is the case with 20 metal oxides. By exploration with initially metallic tin and indium, the purities of the materials utilized in the practice of the invention can be made consistent with the requirements of the semiconductor technique thereby reducing even minor impurity levels, as is known, to deteriorate properties of the resulting device.

A second advantage of the invention is that naive line widths of electrical conductors can be realized than hitherto possible conventional ones. This is because the conductor pattern is first formed in a thin layer of modifying material, such as the above-mentioned tin, which is considerably thinner. than the ultimately formed conductors. Thin layers can be forgotten to a relatively naive line width because the shorter time required for vegetating significantly reduces flooding.

The invention is explained below with a description of a number of preferred embodiments according to description 790 55 60

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w k refers to a drawing.

Pig. 1A to 1E are in the form of sketches of a sectional view in steps of the formation of conductors in accordance with a preferred embodiment of the invention.

Fig. 1A shows a carrier 10 on which a layer 12 of a modifying material has been applied. The carrier 10 may be, for example, a glass substrate used as a cover for a liquid crystal display device where transparent electrodes are desirable. Another example is that the carrier 10 may be an integrated semiconductor circuit, for example a CCD imaging device using transparent conductors in the gate construction. In the preferred embodiment, the applied modifying material is metallic tin.

High purity tin may be preferred in those cases where the support is, for example, a semiconductor device, as mentioned above, and where the properties of the semiconductor device could be adversely affected by impurities in the tin. Metallic tin with a purity of at least 99.999% can be easily obtained and used in the practice of the invention.

In the preferred embodiment, the metallic tin is preferably deposited using the normal sputtering techniques known to those skilled in the art. A suitable power for the high-frequency atomization of tin is about 150 watts. A suitable gas pressure for the argon gas contained in the atomization chamber is 8 + 2 microns.

However, instead of sputtering, any of the various other common metal deposition techniques can be used to deposit the layer 12 of tin if desired. These other techniques include, for example, plating, plasma coating, vacuum evaporation, fire coating, thermal evaporation and electron beam evaporation.

A thickness for a finished layer of the transparent material formed in accordance with the invention is preferably about 1000 angstroms. This thickness represents a compromise 35 that provides a sufficiently high electrical conductivity for the electrical conductive areas of the finished layer, as well as a sufficiently high specific spring resistance for the electrical insulating areas of the finished layer in electronic applications, such as in display or image devices, together with high light transmittance. For a finished layer thickness of 1000 angstroms, the thickness of the applied layer 12 of tin is preferably 75 + 25 angstroms. Thus, it appears that in the preferred embodiment, the modifying material layer 12 has a thickness of only 10% or even less of the thickness of the finished layer.

After the layer 12 of modifying material has been applied to the support 10, as shown in Fig. 1A, the layer 12 is formed or marked into a pattern corresponding to the desired pattern. The result is drawn in Fig. 1B with individual elements 1¼ of modifying material drawn 15 mounted on the support 10 and placed thereon at the desired locations for electrical conductors. In Fig. 1B, a number of individual elements 1¾ are drawn solely for the purpose of explanation. It will be apparent to those skilled in the art that the invention also applies to cases where a desired conductor pattern has only a single one-piece conductor structure.

In the case where the modifying material is tin, the individual elements 1U are preferably adapted to the desired conductor pattern by the ordinary photo-etching techniques known to those skilled in the art. For example, a suitable photoresist 25 for this purpose is manufactured and marketed under the trademark Kodak 809 by the Eastman Kodak Company, Rochester, New York. The photoresist is preferably baked at a relatively low temperature, for example no higher than about 90 ° C, to prevent diffusion of tin into the photoresist. A suitable etchant for the tin 30 is a mixture of 12.5 g of ammonium fluoroborate, 2.25 liters of concentrated nitric acid, 0.2 liters of concentrated fluoroboric acid and 150 liters of deionized water. Using this etchant, elements 14 can be formed in the tin layer in about 20 seconds.

It is well known that cartridges drawn by photo-etching techniques are limited in their definition 790 55 60 6 due to flooding caused by the etchants. However, in the method of forming the desired electrical conductor pattern described so far, the pattern is formed in a layer 12 having a thickness that is 10% or less of the thickness of the finished layer of transparent material in which the pattern of electrical conductors will eventually be included as discussed below. It is then clear that a conductor pattern with a finer definition (narrower line widths and a shorter line-to-line distance) can be obtained photolithographically in connection with the application of the invention than can be obtained in the case where a pattern is to be formed in a layer whose thickness is greater than that of the finished transparent material layer. The latter case represents the prior art situation in which layers of electrically conductive metal oxides are etched to form the desired pattern of electric conductors.

Although photoetching techniques are preferred for forming the elements 11 to form the desired electrical conductor pattern, other methods which are in accordance with the invention may be used. A first example is that the modifying material 20 can be deposited through a mask, so that the elements 1U of Figure 1B are immediately formed. This method may be more desirable if the elements 1 ^ are to be mounted on a support with sharp corners or steps to which the elements 1U are to conform. Yet another example is that the modifying material is deposited in an ink which is screen printed on the support according to the desired pattern. Using either of these two techniques for forming the elements 1U, the result may be that the number of steps required to form a finished layer is smaller since it is not necessary to first draw an unmarked layer of modifying material, such as the layer 12 in FIG. 1A.

Fig. 1C shows that the elements 14 of modifying material after being formed are covered with a layer 16 of a material which can both be oxidized to a transparent electrical insulating material when it is relatively pure, and which can be 790 5 5 60 τ oxidized to a transparent electrically conductive material before it is combined with a suitable modifying material.

Metallic indium with a purity of at least about 99.9% by weight is the preferred oxidizable material to be used in conjunction with 5 tin in the practice of the invention. As with the tin used to form a layer 12 of modifying material in Fig. 1A, greater purity of the indium may be preferable when used in cases where the support is, for example, a semiconductor device and where the properties of the semiconductor device can be adversely affected by impurities in the indium. Metallic indium with a purity of at least 99.999% by weight can be easily obtained and used in the practice of the invention.

There is another reason for using indium 15 of relatively high purity. Oxidized indium is to serve as an electrical insulator in a finished layer of transparent material according to the invention. Contaminants in the indium will produce free charge carriers and thereby decrease the value of the electrical resistance of the indium oxide. The extent to which a reduction in the specific spring resistance of the electrically insulating indium oxide due to impurities is a problem depends in part on the application in view of which the invention is applied.

In the event that the finished material system is relatively large with the desired pattern for its conductive regions providing relatively wide conductors relatively far apart, a satisfactory performance can be obtained with a relatively low ratio of the resistivity of the electrical insulating material to that of the electrically conductive material. In contrast, it is preferred for applications involving microelectronic circuits and a very precise definition of narrow conductors to make the ratio of the resistivity of the insulating material to that of the conducting material as high as practicable.

For the latter situation, the higher the purity of the metallic indium used, the greater the ratio of the resistances. A ratio between the resistivity of 790 55 60 8 5 of the insulating material and that of the conductive material of

at least 10 ^ in the practice of the invention has been obtained using yaar-. . M

making indium of the purity was about 99.999% by weight.

The preferred embodiment, the metallic indium 5 is preferably deposited using ordinary sputtering techniques known to those skilled in the art. A suitable power for high-frequency atomization of indium is 175 + 25 watts. Suitable gas pressure for the argon contained in the atomization chamber is 8 + 2 microns.

Instead of sputtering, one of several other common metal deposition techniques can be used to deposit the indium layer 16. As stated above in connection with the discussion of the tin deposition techniques, these other techniques include, for example, plating, plasma coating, vacuum evaporation, fire coating, thermal evaporation and electron beam evaporation.

For the preferred embodiment where the finished transparent material layer has a thickness of about 1000 angstroms and wherein the tin layer 12, as shown in Fig. 1A, is 75 + 25 angstroms thick, the thickness of the indium layer 16 is preferably about 20 700 angstrom.

The indium layer 16 must cover the entire surface on the support 10 which is required to be coated with a transparent material. Insofar as the application of the layer 16 of indium may have covered parts of the support 10 over which no coating of transparent material is desired, photo-etching techniques can be used to remove the indium from these parts. This will be the case, for example, where access is made to contact pads on an underlying semiconductor device to make them available for connection to intermediate switches 30 not provided by the procedures of the invention. In this case, the indium layer 16 is preferably formed using the same photoresist and the same etchant described above in conjunction with the discussion of forming the elements 1¼ as shown in Fig. 1B. For etching away the layer of 35 indium using the said etchant is about 2 min.

790 55 60 9 required.

After the elements 1U of the modifying material are covered with a coating of oxidisable material 16, as shown in Fig. 1C, the modifying material is infused into the oxidisable material. This can be accomplished in an oven equipped with either an inert or a reducing atmosphere. In the case of the materials, the preferred dopant is tin and the preferred oxidizable material indium, the diffusion or alloying is preferably performed by heating in a reducing atmosphere of about 85% nitrogen and 15% hydrogen.

Fig. 1B shows the result of the diffusion of the tin into the indium in regions 18 over the entire thickness of the layer 20.

The regions 18 of the layer 20 correspond to the desired pattern 15 for the electrical conductors, while the remaining parts of the layer 20 are intended to be occupied by electrically insulating material.

The definition of the electrical conductor pattern that the regions 18 of the alloyed material must be able to meet is limited by the need to maintain a certain minimum distance for electrical insulation between the conductors. The intermediate regions 18 of alloyed material depend on both the distance present between the elements of modifying material, as shown in Figs. 1B and 1C, and on the manner in which the diffusion of the modifying material into the oxidizable material is executed. The distance between adjacent edges of the modifying material elements 14, as shown in Figures 1B and 1C, is preferably at least about 10 times the thickness with which the layer 16 of oxidizable material is deposited.

For a layer 16 with a thickness of, for example, TOO angstrom, preferably the distance between the elements 1U is at least 7000 angstroms or about 1 µm. This lower limit of the distance is consistent with a minimum line-to-line distance that can be reasonably obtained with a layer thickness of TOO angstrom by photo-etching techniques.

The extent to which the modifying material is diffused into the oxidisable material is partly determined by the 790 5 5 60 3

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10 parameters of the heat treatment used to complete the diffusion process. In the preferred embodiment, the alloying of the tin and the indium is effected at a temperature of 200 + 20 ° C for 30 + 10 minutes. The diffusion process must remain limited since heating for too long a time or at too high a temperature or both will eventually result in electric conductors with too little spacing or even in a situation where the modifying material is evenly distributed over the entire layer 20 in Fig. 1D. In both cases the desired conductor pattern will be effectively eliminated. Heat treatment at a lower temperature or for a shorter time, or taking both conditions into account, can prevent sufficiently good mixing of the indium and tin in the regions 18 of the layer 20.

It will be clear to a person skilled in the art that, although the above-described procedure in manufacturing the system shown in Fig. 1D is preferred, it does not have to be the only mode of operation. For example, the layer 16 of indium can be deposited first, i.e. for the layer 12 of tin, directly on the support 10. After the layer 16 of indium is formed, the layer 12 of tin can be deposited and photo-etched formed on top of the layer 16 of indium using known exposure techniques and a photo-etching mask to protect the indium from the etchant. Once the tin has diffused into the indium, the result will be as shown in Figure 1D.

The structure shown in Figure 1D is converted to that of Figure 1E by heating the layer 20 in oxygen or air to oxidize the layer. The unmodified oxidizable material is converted to transparent insulating material in the finished layer 22 of Figure 1E. The modified material of d * regions 18 in Fig. 1D 30 is converted into transparent electrically conductive material in regions 2k of Fig. IE. In case the oxidizable material is the preferred metal indium, the formed transparent insulating material is an indium oxide. Complete oxidation in order to yield stoichiometric indium sesquioxide (O)) is preferred because it allows the highest value of resistivity to be obtained. In the 790 5 5 60 * 11 case, the modifying material is tin, the indium tin alloy is converted to indium tin oxide (ITO), a generally illustrated transparent electrical conductor in which the tin acts as a dopant for the otherwise electrically insulating indium oxide.

The stoichiometry of the indium sesquoxide is considered to be ensured by heating for a sufficiently long time at a sufficiently high temperature. Heating in air at U8o + 20 ° C for a time between 30 and 120 minutes »" appears to give satisfactory results.

The transparent layer 22 of metal oxide material thickens during oxidation. For a metal layer originally about 700 to 800 angstroms thick, the thickness of an oxide layer 22 of Fig. 1E will be about 1000 angstroms. The thickness growth of the metal to metal oxide film represents a factor between about 1.35 and about 1.5.

The aim is to control the values of the thickness of the indium layer and the tin layer so that after the treatment a composition of the ITO is obtained which contains about 90 mol% indium oxide and thus about 10 mol% tin oxide. This is the generally accepted composition of the ITO in its most conductive form.

Typical measured values obtained for the refractive index of the indium oxide appear to fall between 1.7 and 1.8.

Measured values for the refractive index of the ITO are between 1.9 and 2.0. Measured resistivity values for the indium oxide are found to be between 1000 ohm-cm and 10,000 ohm-cm, while the ITO resistivity created in the process described herein is between 0.01 ohm- cm and 0.02 ohm-cm.

A preferred embodiment of the invention has been described, but it goes without saying that a different way of working is possible.

For example, tin, when relatively pure, can be used as the oxidizable material to form transparent electrically insulating material. Antimony can be used as a dopant for modifying the tin oxide to become an electrically conductive material. Another example is the use of 35 cadmium as a modifying material which is diffused into tin 790 55 60

Claims (9)

1. Molded product, comprising a support, characterized by an at least substantially continuous layer of transparent material applied to the support, said layer of transparent material comprising a first region of electrically insulating material and a second region of electrically conducting material. Shaped product according to claim 1, characterized in. that the electrically insulating material is an oxide of the first metal. Molded product according to claim 2, characterized in that. that the electrically conductive material is an oxidation product of the first metal modified by alloying it with a modifying material. k. Molded product according to claim 3, characterized in. 15 that the first metal is indium and that the modifying material is tin. Molded product according to claim 3, characterized in that the first metal is tin and that the modifying material is antimony. 6. Molded product according to claim 2, characterized in. The electrically conductive material is a compound of the first metal and a modifying material, the compound being formed by oxidation of an alloy of the first metal with the modifying material. Molded product according to claim 6, characterized in. 25 that the first metal is tin and that the modifying material is cadmium. 8. A method of forming a desired pattern of transparent electrical conductors on a substrate, characterized by: forming a layer of modifying material on the support in a pattern according to the desired pattern of transparent electrical conductors; forming a layer of an oxidizable material to form a transparent electrically insulating material against the layer of modifying material on the substrate; Diffusing the modifying material into the oxidisable material; and oxidizing the oxidizable material to form a substantially continuous layer of oxidized material containing a transparent electrically conductive region corresponding to the desired pattern, as well as a transparent electrically insulative region, Method according to claim 8, characterized in that the oxidisable material is a metal. 10. A method according to claim 9, characterized in that the oxidisable material is indium or tin. A method according to claim 10, characterized in that the modifying material is tin, antimony or cadmium. 790 55 60