DE3010340A1 - Vacuum deposition system for thin film transistors - uses flash vaporisation by discharging tablets of coating material into heated cylinder - Google Patents

Abstract

The vacuum deposition system for coating thin film transistors, uses a flash evaporation method. A reservoir formed by a rotating disc with pockets, dispenses small tablets of the coating material. Each travels in turn down a sloping chute to fall into a heated cylindrical tile, to be converted instantly into a vapour. This is discharged upwards inside the vacuum chamber for deposition on the substrate above, which carries a perforated mask. The inside surface of the vaporising tile can be of sintered corundum or of borium nitride. The tablets can be made of two different materials, delivered alternatively. The two materials are of respectively SiO2 rich and SiO2 poor mixtures of Al2O3 and SiO2.

Description

Description:

The manufacture of thin film transistors (TFTs) has been so much so far unknown disturbances afflicted that these in and of themselves tempting Could not yet introduce components in general. The results were seldom the same of 2 successive runs of each other, even if the same apparatus, the same Materials used by the same technician.

It was therefore the main goal of our becoming in many publications and inventions precipitating activity, a robust, reproducible TFT production process to develop. Only when this point has been clarified do they open up for this Switching elements the practically unlimited possibilities voi so often spoken of has been.

The present invention represents a further step in this direction that was already mentioned in our previous patent applications, such as "Optimal combination of materials for thin-film transistors ", and '[vacuum deposited insulator layers for thin-film transistors" has been started.

The starting point of the present invention is our finding that our new material for vapor-deposited insulating layers, namely the mixture of Al203 and SiO (mullite), which has an extremely high dielectric strength, at nåedrigerer Temperature melts and evaporates, namely 1500 C resp.

1800 C, than the pure Al 03 used previously. When used alone before Al 03 used for the insulating layers we were forced to use electron beam vapor deposition to be used, because this material is difficult to make from electrically heated metal Ships (such as tungsten) evaporate due to the formation of WO, which the layers weak and requires frequent replacement of the W-boat.

Electron beam heating of the vapor deposition material has, however, for the large-scale Production application many disadvantages. Apart from the high purchase price and the high susceptibility of the system to failure, it is a major deficiency in rotary pan plate systems, that the rotary feedthrough of the water-cooled copper crucible plate is a constant source of water leaks in the vacuum recipient. Occurs with every rotation of the crucible Gush of water vapor and forms monomolecular covering layers, which are particularly on the CdSe: In-semiconductor layer are disturbing, we have this deficiency with our invention "Gãl lium-metal-cooled crucible plate in an electron beam evaporation system" Overcome, but other disadvantages remain. One such is one-sided distortion the lobe of the vapor deposition beam through the laterally incident electron beam ("Elektronind"). Instead of a thickness distribution of the vapor deposition layer, this being in the center above the Source is thickest and radially outward in concentric lines of equal thickness decreases, a pear-shaped layer thickness distribution is obtained. This affects unpleasant in the case of large evaporation layers. You can do this through magnification reduce the distance between source and substrate, but this increases material consumption, and thus the deposition time, quadratic to Another disadvantage of electron beam deposition is the charging of the insulator layers by spray electrons, by secondary electron emission, and by charged-arriving vapor particles. In the case of thin film transistors, this produces which are field-effect components, disruptive effects that are difficult to control.

A very serious disadvantage of electron beam evaporation is the associated need to control the layer thickness achieved and the evaporation rate to use a controller that uses a quartz crystal as a sensor used. We found this to be a constant source of glitches, errors and failures is because the quartz crystal has either been vaporized too thickly or has slipped, or makes bad contact, etc.

Avoidance of these disturbances requires the use of high quality Personnel, which is not the case in practice. Our goal is yes, the plant of to let an unskilled person operate it. Therefore we # rSrif'en eager to get through our new Al 0 .SiO -Isolator offering an opportunity; from electron beam evaporation bbzgo.

Our new system is illustrated in Figure 1.

The figures mean: 1 vapor deposition material tablet 1 vapor deposition jet 2 rotary table -Reservoir for many tablets, enough for a Run 3 perforated plate (at least 24 holes) 4 tablet chute, consisting e.g. of NiCr 5 hot crucible (Al203 or BN) 6 ceramic fittings 7 radiation reflector 8 cooled outer jacket 9 thermocouple (Pt - Pt / Rh, or W - Re) 10 thermal insulation (e.g. pieces of corundum) 11 substrate plate (Glass) 12 perforated mask (see our patent application for this) 13 mask changer (only schematically indicated) 14 wall of the vacuum vessel 15 bottom plate of the vacuum vessel, e.g. cooled aluminum plate.

To explain Fig. 1 it should be stated that we have to vaporize the three substances required metal. (Ni), semiconductor (CdSe: In) and insulator (mullite) 2 (see our relevant patent application) only a single one, through 1 R-Stromwärme indirectly heated ceramic crucible 5 (replaceable) need.

The vapor deposition material is compressed into pills or tablets 1 of the same size3 and these pills are in a reservoir in the order in which they are vaporized 2. From this reservoir, constructed here as a circular turntable, the Pills, in accordance with that required for the particular material on substrate 12 Perforated mask 13, dropped one after the other into the crucible 5, where they evaporate completely. By choosing very small pills you get the favorable effect of ~ shallow evaporation ", which are particularly preferable for compounds with a tendency to decompose to be vaporized is. (This is the case with our fabrics). This makes layers more homogeneous Preserved composition.

The tablets of different materials can be of different sizes but if you use tablets of the same size you can do all of them with the same Tablet press are manufactured. The pieces of metal are not called pills but rather added as wire sections of equal length. So one uses e.g. for a to be vaporized Thin film transistor matrix in the following order: 1. 3 mullite pills one after the other for the "covering layer under everything", which is steamed onto the glass substrate, to create an alkali-free, pristine surface.

2. 1 wire section, for the first gate and the connecting lines.

3. 5 mullite pills in a row, for the 1st gate insulators top cross insulators and capacitor-Diel ektri ka. 1 4. 1 CdSe: in-pill for the semiconductor 5. 1 wire section for source and drain electrodes and connections.

6. 5 mullite pills one after the other, for 2nd gate insulator, upper cross insulator and capacitor di el ektri ka.

7. 1 wire section, for the top gate and metal connections.

8. 4 mullite pills one after the other, for the "protective layer over everything".

So that the reservoir 2 can hold enough batches, it is as Turntable with 24 holes on the circumference. By one from the outside by magnetic force simple mechanism to be triggered (not shown), similar to a restlessness a weight clock works, the disc rotates one position at a time further, so that the respective pill through the perforated plate 3 via the chute 4 in the crucible 5 falls and passes into steam 1 '.

Matching the reservoir steps with opening and closing the vapor deposition screen (not shown) and the replacement of the respective hole mask (not shown), as well as the regulation of the respective crucible temperature and monitoring all other parameters (maximum allowable pressure) will, as already published by us, fully automatically controlled by microprocessors (see e.g. A. Fischer, Nachrichtenentechnische Journal 33, February 1980, page 87, image 16, 18.) so that no human Operating errors can occur.

This new single-pot flash evaporation method with complete, successive evaporation of many small, calibrated batches, resulting in layer thicknesses are automatically controlled, i.e. based on the presence of crucible materials, which of the insulating material, which is the greatest aggressiveness - of the 3 required substances has not to be attacked. For all previously known good insulators there was no such crucible material.

We found that our new mullite vapor-deposition insulator can be made from sintered corundum crucibles 5, e.g. the Degussit crucible from Friedrichsfeld. It is important that with our method no plague of the vapor deposition material remains in the crucible, otherwise the crucibles would break when they cool down.

Still, the crucibles need to be replaced every now and then, but they are cheap.

The crucible material that also evaporates is Al203, so it is not a foreign substance, and is built into the layer. Such crucibles can be used up to 1800 C.

For higher temperatures and thus higher evaporation rates, use crucibles made of boron nitride BN according to the invention, preferably those from the company Union Carbide supplied which are low in B O binders, or the pyrolytically produced, which even contain painful oxidic binders. These are more expensive than that Al2O3 crucibles, but last longer.

As an external heating of this crucible up to approx. 1750 0C one can use porous Use sintered corundum tubes 7 with thread-like grooves for the wire wrapping (Morganite or Carborundum (o) being molybdenum wire from Thyssen-Edelstahlwerke ideally suited.

At higher temperatures, however, a disruptive chemical reaction occurs between the metal of the Heizdrj; resound the .'Xorund ceramic. Especially for The BN crucibles, which can be used up to 2100 C, are therefore needed as heaters Graphite cylinder (Schunck & Ebe, Giessen), either heated by eddy currents, generated by induction from a high-frequency generator coil (generator 5 KW from Hüttinger, Freiburg), or by high-ampere currents (80 A) in a zigzag shape designed graphite coils (Schunck & Ebe, Giessen) with massive, cooled Copper leads from a step-dow transformer.

So that the heat radiation does not unduly affect other parts of the vacuum system heats up, a cooled metal jacket 9, here made of aluminum, should be used will.

According to the invention, this is suitable for our "layered" insulator layers this vapor deposition method as well. For this purpose, the insulating material pills, which successively in each case almost to the point of being vaporized and the vapor-deposition layer build up, alternately pressed from low-SiO and from SiO2-rich Al O + SiO mixture. By overlapping the evaporation of the previous pill with the new pill, graduations are achieved Transition layers possible, which prevent tension.

With the method described here, it is possible to make a more production-friendly Ways better thin film transistors in a cheaper way, more uniform and reproducible than ever possible before.

Claims (4)

Claims: Production process for vacuum-deposited thin-film transistor circuits, characterized in that calibrated tablets of the various vapor deposition materials from a tablet reservoir one after the other, in cooperation with interchangeable Shadow masks on the substrate, delivered in a single electrically heated crucible, there, urtii completely evaporate various superimposed vapor deposition layers on the substrate of the desired thickness. 2) Manufacturing process for vacuum deposited thin film transistor circuits according to claim 1, characterized in that as a material for particularly puncture-resistant Insulating layers alternate with tablets of a SiO2-rich Al 0 + SiO - ~ mixture and Tablets of a low-SiO Al 0 + SiO mixture are vapor-deposited, with the total amount; of the isolator consists of many approx. 10 - 50 mm thick, continuously merging into one another Layers is built up. 3) Production method according to claim 1 and / or 2, characterized in that that the inner wall of the vapor deposition crucible consists of sintered corundum. 4) Production method according to claim 1 and / or 2, characterized in that that the inner wall of the vapor deposition crucible consists of boron nitride.