Protective layer for PECVD graphite boats

Abstract

An improved protective layer is provided for PECVD graphite boats for receiving wafers and for transporting the wafers in or through PECVD coating systems, in particular in the photovoltaics industry. A more homogeneous antireflection layer on silicon substrates is achieved by virtue of the PECVD boat of graphite being provided with an electrically conductive hard material coating of at least boron carbide (B.sub.4C).

Claims

1. A PECVD boat composed of graphite and devoid of a silicon nitride covering for receiving and transporting a plurality of wafers in or through a PECVD coating plant, the PECVD boat including retaining means for holding individual wafers of said plurality spaced apart to facilitate a uniform flow of gases through all interspaces between the individual wafers during a PECVD coating process, wherein the PECVD boat has an electrically conductive hard-material coating which comprises at least boron carbide (B.sub.4C) to ensure formation of a uniform plasma between the wafers during the PECVD coating procedure.

2. The PECVD boat as claimed in claim 1, wherein thickness of the hard-material coating is between 0.1 m and 5 m.

3. The PECVD boat as claimed claim 1, wherein the hard-material coating comprises a combination of titanium nitride and boron carbide.

4. The PECVD boat as claimed in claim 1, wherein the hard-material coating comprises a combination of titanium nitride, titanium carbide and boron carbide.

5. The PECVD boat as claimed in claim 1, wherein the hard-material coating has a multilayer construction comprising boron carbide and at least one of titanium carbide and titanium nitride.

6. The PECVD boat as claimed in claim 1, wherein the retaining means comprises at least one of wafer pockets or retaining pins.

7. A PECVD boat composed of graphite and devoid of a silicon nitride covering for receiving and transporting a plurality of substrates in or through a PECVD coating plant to have an antireflective layer applied to the substrates in a PECVD coating procedure, the PECVD boat including retaining means for holding individual substrates of said plurality spaced apart to facilitate a uniform flow of gases through all interspaces between the individual substrates during the PECVD coating procedure, wherein the PECVD boat has an electrically conductive hard-material coating which comprises at least boron carbide (B.sub.4C) to ensure formation of a uniform plasma between the substrates during the PECVD coating procedure and to ensure homogeneity of the antireflective layer applied to the substrates during the PECVD coating procedure.

Description

BRIEF SUMMARY OF THE INVENTION

(1) The invention, then, has for its object to provide a protective layer for PECVD boats which does not suffer the disadvantages described and which simultaneously makes it possible to produce a better and more homogeneous antireflective layer on silicon substrates.

(2) It has been found that, surprisingly, the object of the present invention can be achieved simply by covering the PECVD boat composed of graphite or carbon with an electrically conductive hard-material coating.

DETAILED DESCRIPTION

(3) Hard-material coatings composed of titanium nitride (TiN), boron carbide (B.sub.4C) or else silicon carbide (SiC), with which particularly dense coatings may be produced, have proven especially suitable.

(4) The total layer thickness of the coating should be between 0.1 m and 5 m.

(5) In a further embodiment of the invention the hard-material coating may be composed of a combination of titanium nitride, titanium carbide and boron carbide.

(6) Alternatively the hard-material coating may have a multilayer construction, composed of titanium nitride and/or titanium carbide and/or boron carbide. Up to about 20 of these layers may be arranged one above the other.

(7) With such a coating the particularly good or at least improved electrical conductivity of the surface of the PECVD boats achieves a particularly uniform formation of the plasma, in particular also between the wafers, and the antireflective coating of silicon nitride to be produced on the wafers therefore exhibits markedly improved homogeneity.

(8) Even when the PECVD boats configured in accordance with the invention are exposed to other processes comprising oxygen or nitrous oxide the hard-material coating according to the invention exhibits particularly good durability.

(9) In particular, the hard-material coating according to the invention is particularly etch-resistant and said coating is therefore not attacked during the necessary back-etching with HF which results in a marked increase in the service life of the wafer boats and also the availability of the PECVD plant.

(10) Further advantages are that the previously required precovering of the PECVD boat with silicon nitride and wafers required therefor are no longer necessary.

(11) Furthermore, the PECVD boats and the retaining pins in particular are subject to reduced wear on account of the markedly greater hardness of the hard-material coating according to the invention compared to the hitherto customary coating with silicon nitride.

(12) The resulting cost savings during operation of a PECVD plant may run into seven figures.

(13) Finally, the hard-material coating according to the invention with titanium nitride, titanium carbide or boron carbide on the PECVD boat may be effected in the same PECVD plant in which for example application of the antireflective coating to the wafers also takes place.

(14) Depending on the materials used for the hard-material coating on the PECVD boats, titanium nitride, titanium carbide or boron carbide, the steps to be implemented in the PECVD plant are slightly different.

(15) It is first necessary to introduce into the PECVD plant either new PECVD boats or PECVD boats which have previously been used for producing antireflective coatings and have been etched back.

(16) It is subsequently necessary for the production of a titanium nitride coating to heat the PECVD plant to about 500 C. and to evacuate it. After introduction of for example an organic titanium compound with a carrier gas and admixing of NH.sub.3 as reductant the plasma necessary for producing the hard-material coating can be ignited.

(17) The duration of this procedure depends on the thickness of the hard-material coating to be produced and on the number of layers to be applied.

(18) By contrast the production of a hard-material coating made of boron carbide (B.sub.4C) is somewhat simpler because this requires neither a vacuum nor a plasma.

(19) The coating of the PECVD boats may be undertaken in a PECVD plant or at atmospheric pressure in a suitable oven in which the PECVD boats are initially heated to a temperature of about 1000 C. Carbon and boron are then introduced into the oven from suitable sources, a reaction to afford boron carbide then taking place on the surface of the PECVD boats.

(20) The production of a titanium carbide coating may be effected via a customary CVD process.

(21) If coating was undertaken in a PECVD plant, cleaning of the vacuum/reaction chamber should be undertaken after removal of the PECVD boats and this may be effected for example by introduction of hydrofluoric acid.