DD288845A5 - METHOD FOR PRODUCING DEFINED CONTINUOUS METAL STEAM STREAMS - Google Patents

Abstract

The invention relates to a process for the production of defined, continuous Metalldampfstroemen for the production of polycrystalline metal chalcogenides, which are used as IR optical media, inter alia, for the production of optics for high power laser, infrared and multispectral cameras or night vision devices, as well as window material for Kuevetten and detectors application , The aim of the invention is the generation of defined, continuous Metalldampfstroemen for quality synthesis of polycrystalline metal chalcogenides by chemical vapor deposition in large Maszstab. The method is characterized in that the metal to be vaporized is fed continuously to an evaporator during the process, whereby the metal mass supplied in the unit time can be controlled and controlled and the supplied metal is continuously completely evaporated, so that the mass produced always equals metal vapor the metal mass supplied is and there is only an insubstantial volume of liquid metal in the evaporator. {chalcogenides, chemical vapor deposition; metering; IR optical media; Metal vapor stream; Metal evaporation; Regulation}

Description

For this 2 pages drawings

Field of application of the invention

The invention relates to a process for producing definedon continuous metal vapor stream in vacuum operation in an inert carrier gas.

The maintenance of a constant metal vapor stream over a period of a few hundred hours in a reaction chamber is z. As an indispensable prerequisite for the production of polycrystalline metal chalcogenides on the process of chemical vapor deposition according to the equation

Me ₁₀ ) + H ₂ X (O) -> MeX + H ₂ | _g) (Me = Zn, Cd)

(X = S, Se)

on an industrial scale.

Polycrystalline ZnS and ZnSe produced in this way are IR-optical media which, among others, a. for the production of optics for high-power lasers, infrared and multispectral cameras or night-shift devices as well as window material for IR cuvettes and detectors.

Characteristic of the known state of the art

From the literature it is known that for the production of ZnS and ZnSe via the process of Chornish vapor deposition, the provision of zinc vapor by thermal evaporation .'on liquid zinc at temperatures around 600 ⁰ C and transport of the vaporous zinc with a stream of an inert carrier gas, preferably Argon, into the reaction chamber into it. All processes, including the deposition of zinc chalcogenides in the reaction chamber, are in the low pressure range (2 to 1OkPa).

The deposition is preferably carried out at temperatures around 600 ⁰ C.

(P.MILES: Proc. Symp. Mater., S. Aspects Thin Film Syst., Sol., Energy Convers., 1974, 402-418; RLTAYLOR, JF CONOLLY, RF DONADIO, RN DONADIO: Photonics Spectra 1982, 61-62; RL TAYLOR, RNDONADIO: Laser Focus 17 [1981], 41-43).

Of particular importance for the production of quality semi-finished products from ZnS or ZnSe is the feeding of the vapor or gaseous starting materials zinc and sulfur or hydrogen selenide with respect to a certain molar ratio

While the defined feed of the gaseous components under normal conditions by means of proven in semiconductor gas metering systems with mass flow controllers as a central unit is unproblematic, the previously known solutions for the technical realization of a defined zinc vapor supply still have deficiencies.

In the known solution for laboratory equipment is the zinc vapor source - a heatable crucible with the required amount for the process Zinkes- and the reaction chamber in a qemeinsamen reactor shell.

The Ermittluing introduced with the carrier gas flow per unit time in the Re action chamber amount of zinc vapor is problematic because of the highly corrosive gas atmosphere in Reaktorinr.enraum and the high temperatures.

Optical windows in the reactor envelope for the determination of the zinc vapor concentration with spectroscopic external methods are in a short time covered by a product layer according to the equation given above.

In some cases, the rate of evaporation of zinc is determined by the continuous determination of the wet weight loss of the zinc seal (P.BRAUDEAU, G. KELLER, JPTORRE: Journal de Physique 47 [1986], C1 -193-6; IASAVAGE, KL LEWIS, AM PITT, RHLWHITEHOUSE: SPIE, 505, Advances in Optical Materials (1984) 47-50).

A regulation of the zinc vapor stream in these technical solutions is not provided.

There is hitherto only a technical solution for the preparation before, metal chalcogenides by the process of chemical vapor deposition on a large scale known (GB 2173512Λ).

Again, the entire amount of metal needed for the process is in an evaporator crucible, which is laterally heated.

The coarse division of the metal vapor stream should be done by adjusting the vapor pressure of the liquid metal over the temperature.

Dor Metalldampf becomes with oinom Inorton inert gas, whose Massonstrom is controllable, from Dom steam flow over the surface of the melt into the Roktlonskammer gotragon. Dor in the Rooktionskommor flioßendo Massonstrom dos Metalldampfos is identical to the Metallverdampfungsrato or with change of Tlogolmosso with the Zoit.

For the measurement of the crucible is stored on pressure measuring. The average measured value is compared in oinom Meßwertverarboitungssystom with the respective target value. A major disadvantage of these technical solutions is that for coarse adjustment of the Motalldampfstromos the entire Metallmonge, large plants are at Prozoßboginn up to Mehroren kilogram, over the entire Prozdßdauer muston on a constant, depending on the desired vapor pressure Tomporatur firmly held.

Due to the large volume of the melt, especially at the beginning of the process, the requirements for the design of the tombstone field of the V jrdampfortiogol are raised.

However, the evaporation heat required for the pre-evaporation process is removed from the melt primarily at the surface, however, according to GB 2173512 veins crucible fed laterally, which leads to Tomperaturgradienten in the melt, which can be up to several tens of Kelvin and are naturally very large at high melt volumes. (H.-

A. FRIEDRICHS, O. KNACKE: Z. Motallkde. 60 (1969) 635-637).

Dioso temperature gradients cause thermal convection in the melt, so that the constant temperature conditions on the Schmelzoberflächo required for a stable adjustment of the vapor pressure are not guaranteed, especially not because even the Konvektionsverhältnisso in the melt because of the constant reduction of Schmolzvolumons not during the process are stationary.

By lowering the melt surface during dos evaporation Prozoscos continue to change the flow conditions in the gas space above the melt, so that wordon can not assume that under otherwise identical conditions with the same amount of carrier gas at the beginning and end dos Prozessos also the same amount of metal vapor in the reaction chamber is carried.

The Verdampfungsbodingungen continue to change during the duration of the process also characterized in that the inert carrier gas constantly Spurenverunroinigungen, eg oxygen, are brought to the surface of the melt odor that by back diffusion traces of the gaseous component H ₂ X get into the vapor space dos evaporator.

These substances react with the surface of the liquid metal and thereby change their properties so that the evaporation rate is sensitive affected. To compensate for the above-mentioned instabilities of the evaporation conditions, to maintain a constant flow of metal vapor into the reaction chamber, continuous measurement and control of that current during the entire process time of up to several hundred hours is required.

The disadvantages of measuring the metal vapor flow through the continuous determination of the mass change of the evaporator according to GB 2173512A consist in the following points:

For the purpose of unimpeded power transmission for accurate mass determination must be maintained between the evaporator and the gas distribution system for introducing the inert carrier gas and for introducing the metal vapor-laden carrier gas into the reaction chamber, a clear distance.

Despite minimizing the distance and purging this annular gap with an additional inert gas, an undefined leakage of metal vapor from this gap can not be excluded.

This leads at least to a falsification of the actual value for the metal vapor stream into the reaction chamber and, in the worst case, to an increase in the gap with MeX due to the reaction of the metal vapor with H ₂ X from the surrounding atmosphere. In this case, a reasonably accurate determination of the mass is no longer possible.

It can be expected that the measurement of relative to the absolute mass (500 kg) relatively small mass change (0.5 to 2 kg / h) at relatively small probing times (1 to 10min) in the proposed measurement principle by means of pressure doses by internal and external vibrations and shocks is superimposed and thus incorrect commands to the actuators can be triggered for control.

Because of the large mass of metal in the evaporator a fast response control of the metal vapor stream by temperature change of the melt and thus the vapor pressure is not feasible and provided in GB 2173512A only in the second place.

The regulation takes place primarily via the change in the mass flow of the carrier gas. The disadvantage of this approach is that in the course of a deposition cycle, the carrier gas flow rates may change significantly due to changes in evaporation conditions, thereby significantly affecting the flow conditions in the reaction chamber.

Since the flow pattern of the gases has a significant influence on the quality and yield of the deposited material (e.g.

G.WAHL: Thin Solid Films, 40 (1977) 13-26), can from dom principle of the regulation of the metal vapor flow exclusively

By changing the mass flow of the carrier gas, significant deviations from the optimal deposition conditions result.

Object of the invention

The aim of the invention is the generation of defined, continuous metal streams for the high-quality synthesis of polycrystalline Metallchalkogniden by chemical vapor deposition on a large scale.

Explanation of the essence of the invention

The object of the invention is the generation of defined, continuous metal vapor streams for the production of polycrystalline metal chalcogenides by chemical vapor deposition by a method by which a reliable regulation of the introduced into the reaction chamber metal vapor streams is possible.

Erfindiingsgomäß the object is achieved in that the metal to be evaporated during dos Prozossos continuously

an evaporator is supplied, wherein in the unit time supplied Motallmasso is measured and regulated and that the supplied metal in the evaporator is continuously completely evaporated, so that the mass produced metal vapor is always glolchder supplied metal mass and only an insignificant continuous Volumon dos metal in the evaporator bofindot.

The Motallvorrat and the device for dosing to be introduced into the Vordampfer Metallmonge are

outside the recipient, dor don Vordampfor and the reaction chamber for deposition of polycrystalline

Wrapped in metal halides. In this way, the amount of engine required in the time unit can be measured under such conditions vvordon, the dio Use of Dosiorvorrichtungon, preferably difforontialdoslerwaagen orlaubt. The introduction of the predetermined and measured Motallmengo in clon evaporator from the outside through dio wall dos Recipient sound boi maintain the operating conditions in the recipient, caused by hoho temperature, low Pressure and working under inert gas, preferably argon, are marked. The evaporator is heated to a temperature so high that the metal introduced into it is almost complete

evaporated. The metal vapor is then carried into the reaction chamber by means of a defined stream of carrier gas, where it reacts with the chalcogen hydrogens to form the polycrystalline metal halides.

The defined dosage of the metal vapor takes place with the metering device outside the recipient. To dio Temperature homogeneity and tempature accuracy of the evaporator are much more stringent than planned

at a dosage of the metal vapor on the evaporation rate einor melt.

Ausführungsbelsplel Dio invention is to be explained on an embodiment:

For the production of polycrystalline zinc selenide via the process of chemical vapor deposition at a temperature of 750-850 ⁰ C and a pressure of 1 to 4kPa in the reaction chamber is a constant zinc vapor flow of 1 to 2kg zinc per hour by means of an inert carrier gas over a period up to to bring in a few hundred hours.

Fig. 1: shows the evaporation scheme and Fig. 2: eino schematic diagram of the evaporator.

The starting material is zinc in granulated form. It is located in a storage bunker (1). The zinc granules will be

continuously via a differential loader scale (2) into a melting vessel (3), where sio are melted at 45O ⁰ C.

Storage bunker (1), differential dosing scale (2) and melting vessel (3) are located in an inert gas box (4) underneath one

constant argon pressure of 0.1 MPa.

The Vorbindung between dom crucible (3) and Dom evaporator (5), which in the same recipient (6) as the Reaction chamber (7) is formed eino heated line (8) via which the feed dos evaporator (5) via a Overflow (9) takes place. The temperature of the entire line (8) must be chosen so that the zinc only in the liquid state

is present (450 to 500 ⁰ C). The liquid zinc in the line (8) hermetically separates the gas spaces in the inert gas box (4) and the

Recipient (6) in dom a pressure of only 3kPa prevails. Dio pressure difference between Inertgasbox (4) and exit point of the line (8) irr. Predator (5) determines the required Height difference between the surface of the melt in the melting vessel (3) and the overflow (9). The overflow (9) passing liquid zinc flows in the evaporator (5) aufo on 65O ⁰ C heated plate (10), the so

is dimensioned that a liquid film can form with a large surface area. This is due to a gap-shaped opening of the

Conduit (8) having a cross-sectional area corresponding to the area of the otherwise circular quorric surface of the conduit (8)

corresponds, supports.

From the heated surface (10) each evaporates as much zinc, as just emerges from the line (8) or by means Differential dosing scale (2) was conveyed into the melting vessel (3). About the plate (10) is a heated to about 650 ⁰ C hood (11) with two openings. Through the opening (12) is argon

introduced as a carrier gas for the zinc vapor. The gaseous argon / zinc mixture is through the opening (13) in the

Reaction chamber (7) brought. For the separation of zinc droplet and zinc oxide particles, this opening is provided with a filter. With this method, a defined, continuous stream of zinc vapor can be generated, which is the prerequisite for the

quality production of polycrystalline zinc selenide is. After completion of the deposition process of the

Recipient (6) flooded with argon. The column of liquid zinc from the overflow (9) in the direction of the melting vessel (3) is pressed

and the levels of the melt in the conduit (8) and the melting vessel (3) are the same.

After switching off the heating, the zinc solidifies in line (8) and forms a tight seal. This tight seal is

the prerequisite for the preparation of the plant for the next deposition process.

After hermetizing and inertizing the recipient (6) for the next cycle and setting the required Process temperatures, the line (8) is reheated, the zinc melts and after adjusting the required Underpressure in the recipient (8), a defined, continuous stream of zinc vapor can be generated again.

Claims (2)

1. A process for the production of defined, continuous metal vapor streams for the production of polycrystalline metal chalcogenides by chemical vapor deposition, characterized in that the metal to be evaporated during the process is continuously fed to an evaporator, wherein the supplied in the unit time Motallmasse is measured and regulated and that the supplied metal is continuously evaporated completely in the evaporator, so that the mass produced metal vapor is always equal to the supplied metal mass and there is only a negligible constant volume of liquid metal in the evaporator. 2, Method according to claim 1, characterized in that granulated metal is continuously brought via a metering device, preferably a differential dosing, in a melting vessel, where it is melted under protective gas, preferably argon, at a temperature of about 50 K above the melting temperature and that the feed of the evaporator with liquid metal from this melting vessel via an overflow.