Method for manufacturing transparent conductive indium-tin oxide layers
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of manufacturing transparent, conductive indium-tin oxide layers used as electrodes for image sensor arrays by means of low pressure deposition on to an amorphous, hydrogen-containing silicon layer and subsequent heat treatment in an oxygen atmosphere. 
     2. Decription of the Prior Art 
     It is known that a cover electrode for image sensor arrays must exhibit a high transparency on the order of at least 85% for visible light and must simultaneously exhibit a low electrical sheet resistance of less than 300 ohms/square. Such specifications can only be met by indium-tin oxide (ITO) layers that have either been deposited at temperatures above 200° C. or have been heat treated at temperatures above 440° C. (see the report by Hamberg, et al., J. Appl. Phys. 60 (11) 1986, R 123 thru R 129). 
     For heating of the indium-tin oxide layer in the manufacture of image transducers, the following must be taken into consideration during the layer deposition or during the subsequent heat treatment: 
     1. In order to be able to structure the ITO layer photolithographically after deposition, the deposited material must not be exposed to a temperature higher than about 150° C. A higher temperature treatment would reduce the chemical solubility of the ITO such that a residue-free etching of the structure is no longer possible. In the alternative method for structuring, what is referred to as a lift-off technique, the substrate must be covered with a structured photoresist during the coating and can no longer be further worked after exceeding a temperature of about 100° C., i.e., it can no longer be lifted off by organic solvents. 
     2. Above 300° C., the photoconductive, amorphous, hydrogen-containing silicon underneath the ITO layer loses its hydrogen content which is critical for its semiconducting properties. The heat treatment at 440° C. that could lead to meeting the required specifications in the case of cold-deposited and completely structured ITO layers can therefore not be applied (see the report by Raviendrad, et al., Physica Status Solidi (a) 88 (1985) K83 through K86, and the previously cited report by Hamberg, et al.). 
     In addition to the noted physical restrictions, a heating and subsequent cooling of substrates in the course of manufacturing the same represents a considerable complication in view of the costs and the reduction in throughput times. 
     When the standard ITO manufacturing methods are modified to meet the restrictions, the specifications of the ITO are rendered noticeably poorer. The performance capability of the image sensors is thereby noticeably decreased. 
     A method which is likewise concerned with this problem is set forth, for example, in European Patent Application No. 0 217 095. In order to improve the values of resistance (300 ohms per square) with unaltered transparency (90%) the heat treating in this method is performed in two steps at a maximum of 200° C., the first heat treating being carried out in an oxygen atmosphere for achieving the transparency and secondly in a plasma-activated forming gas atmosphere for determining the sheet resistivity. 
     SUMMARY OF THE INVENTION 
     The present invention resolves the problems of the prior art in a different way and provides a method in which an unheated substrate is used for the coating, the partial oxygen pressure in the treating vessel is temporarily reduced during the coating, the electrode structure being processed are etched after the coating and the heat treatment is carried out in an oxygen atmosphere or in air at tempeatures below 250° C. 
     In a preferred embodiment of the present invention, the partial oxygen pressure is in the range from 10 -4  to 10 -2  mbar at the beginning of the coating process and is lowered to less than 1×10 -5  mbar in a second stage after about one-third of the desired layer thickness has been deposited in a first stage. The reduction in oxygen pressure can be effected by shutting off the oxygen feed. After another third of the desired layer thickness has been deposited, the original oxygen partial pressure ia again restored by resuming the feed of oxygen. 
     Typically, the reduced partial oxygen pressure in the second stage can be maintained for a period of from 75 to 450 seconds. When the indium-tin oxide layer is applied by electron beam evaporation, the deposition may occur at a rate of about 0.1 nm/sec in the first stage, with the second stage commencing after the coating thickness reaches about 30 nm. 
     The method of the present invention makes it possible to manufacture ITO layers that exhibit an electrical sheet resistance of 200 ohms per square (corresponding to a specific resistance of 20×10 -4  ohm cm) and a transparency of 90% for visible light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the invention will be set forth below with reference to a particular embodment. In the diagram constituting the drawing, a single figure shows the dependency of the transparency in percent and the dependency of the sheet resistivity in ohms per square on the down time in seconds, i.e., on the time the flow of oxygen is interrupted. Curve I indicates the transparency values and curve II indicates the film resistivity values. As may be seen from these curves, the optimum values for transparency and sheet resistivity are obtained when the shut-off time for the oxygen is in the range from 150 to 345 seconds. 
    
    
     The following specific example illustrates the method of the present invention very specifically. 
     A reactive evaporation process was carried out wherein an oxidic initial material (In 2  O 3  having 16 weight percent SnO 2 ) was vapor-deposited on to an unheated substrate of an amorphous, hydrogen-containing silicon layer with an electron beam gun. At the beginning of the vapor deposition, the partial oxygen pressure in the chamber was set to 8×10 -4  mbar. A deposition rate of 0.1 nm/sec was maintained by controlling the power of the electron beam gun. After 30 nm had been deposited on the substrate, the oxygen feed was shut-off while continuing the vapor deposition, and the oxygen pressure dropped to below 1×10 -5  mbar. After the oxygen feed had been disconnected for 300 seconds, the oxygen pressure was again boosted to the original value of 8×10 -4  mbar and the vapor deposition was continued until a layer thickness of about 100 nm was reached. The coated substrates were taken from the deposition vessel and heat treated at 200° C. for an hour in a furnace while flowing oxygen through the vessel. 
     Microscopic investigations of polished sections of the layers of ITO manufactured according to this invention exhibit a noticeably denser structure than layers which have been manufactured without an interruption of the oxygen feed. 
     The chemical etchability of the layers for photolithographic structuring after vapor deposition and before heat treating was extremely good. No etching residues were observed. 
     ITO layers manufactured with a sputtering method exhibit the same advantageous effects. The same is true of ITO layers which were manufactured by using metallic initial materials instead of the oxides in the described process. 
     It will be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.