WO2002096799A2

WO2002096799A2 - Silicon subnitride method for production and use of said subnitride

Info

Publication number: WO2002096799A2
Application number: PCT/EP2002/005733
Authority: WO
Inventors: Rüdiger KNIEP; Jörg HABERECHT
Original assignee: Max-Planck-Gesellschaft Zur
Priority date: 2001-05-25
Filing date: 2002-05-24
Publication date: 2002-12-05
Also published as: WO2002096799A3; DE10125629A1

Abstract

The invention relates to a novel silicon subnitride, to a method for the production thereof, and to the use of the inventive subnitride.

Description

SILICIUMSUBNITRID

description

The invention relates to a new silicon subnitride, a process for its production and the use of the subnitride according to the invention.

The best known and most widely used silicon nitride is Si ₃ N ₄ , which has a wide range of uses as a ceramic material due to its resistance to temperature changes, strength and corrosion resistance.

In addition, a hydrolysis-sensitive silicon mononitride (SiN) _{x is} known, which forms when the polymer silicon imide [Si ₂ (NH) ₃ ] _x , which is accessible from Si ₂ CI ₆ and NH ₃ , is heated.

Furthermore, the production of trisilicon nitride (Si ₆ N ₂ ) _n by reacting powdered CaSi ₂ with NH ₄ Br at a temperature of 550 ° C. has been described in the literature (E. Hengge, Z. Anorg. Allg. Chemie. 31 5 ( 1 962), 298-304). In the production method described in this article using conventional, water-containing ammonium bromide, however, no pure silicon subnitride is obtained, but rather compounds which also contain oxygen in addition to silicon and nitrogen. In the analysis carried out according to Hengge, however, oxygen is classified as nitrogen, so that the presence of oxygen in the product was not recognized.

It was therefore an object of the present invention to provide a silicon subnitride which has semiconductor properties and is composed almost exclusively of the elements nitrogen and silicon. This object is achieved according to the invention by providing a silicon subnitride with the composition Si ₂ N.

Surprisingly, a topochemical reaction based on a silicide with a two-dimensional layer structure, in particular CaSi ₂ and an ammonium halide NH ₄ X, with X = F, Cl, Br or J, preferably NH ₄ Br, has made it possible to produce a silicon subnitride of the empirical formula Si ₂ N. In particular, a compound with two-dimensional infinite silicon layers is used as the silicide, preferably SrSi ₂ , BaSi ₂ , CaSi ₂ and alkali metal or alkaline earth metal silicides with two-dimensional Si structures. Suitable Si structures are described, for example, by J. Evers et al., Z. Naturforsch. B 34 (1 979), 358-359 and in J. Evers et al., Angew. Chem 89 (1 977), 673-674. The structure determination of the material obtained is based on chemical analyzes (in particular H, C, O, N), vibration and spectroscopy, NM R data, electron microscopic examinations, X-ray diffraction and thermoanalytical measurements.

The silicon subnitride according to the invention has material properties that are unique to semiconductors. In particular, it has areas with two-dimensional σ silicon. It particularly preferably includes

Structural units of the composition Si ° Si ^{3 +} N ^{3 "} , as shown in Figure 1, wherein a two-dimensional α-silicon region and

There are nitrogen atoms above and below the α-silicon layer. The two-dimensional α-silicon layer is thus by N ^{3 "} on the top and

Saturated or neutralized undersides of the silicon layers. While it is possible to produce monolayers of (Si ₂ N) _x , each one

Layer thickness of 5-7 A, preferably 6-7 A, on the other hand, a layer-like material, formed from several superimposed structures, as shown in Figure 1, can be formed. x can have values from 1 to practically any size, preferably values of at least 50, more preferably at least 1,000 and most preferably have at least 10,000 and up to 10 ³⁰ , more preferably up to 10 ²³ and particularly preferably up to 10 ¹⁰ .

The preferred two-dimensionally structured structure of the silicon subnitride Si ₂ N according to the invention thus consists of neutral, uncharged layers of about 6 A thickness. Measurements of the dielectric material properties show a dielectric constant e of about 4-6. The optical bandgap of the connection was determined by the diffuse reflection measurement to be 1.5 ± 0.2 eV and is therefore in the range of typical semiconductors.

The silicon subnitride according to the invention particularly preferably consists exclusively of the elements Si and N and is free of impurities, in particular free of carbon and oxygen. The content of elements other than Si and N is preferred, in particular the oxygen content is <1% by weight, more preferably <0.1% by weight and most preferably 0.01% by weight. The silicon subnitride can contain up to 0.001% by weight of hydrogen.

The invention further comprises a method for producing the silicon subnitride according to the invention described above, which is characterized in that a silicide, in particular calcium disilicide (CaSi ₂ ), is reacted with an ammonium halide, in particular ammonium bromide (NH ₄ Br). Surprisingly, it was found that by using high-purity and chemically and structurally characterized starting materials in the implementation of a silicide, in particular a silicide with a two-dimensional layer structure, such as calcium disilicide with an ammonium halide, in particular ammonium bromide, a silicon nitride of the empirical formula Si ₂ N can be produced. The silicon subnitride according to the invention is obtained in particular if the oxygen content and the content of other Impurities in the starting materials is ≤ 1% by weight, more preferably <0.5% by weight and most preferably <0.1% by weight.

The content of impurities in the calcium disilicide used is preferably <0.5% by weight, more preferably <1% by weight, for oxygen

Nitrogen preferably <0.5% by weight, more preferably ≤ 1% by weight and for

Carbon preferably <0.5% by weight, more preferably <0.2% by weight and most preferably 0.1% by weight. Other impurities are in the

Calcium disilicide preferably in a proportion of <1% by weight, more preferably <0.1% by weight. The calcium content in the educt

Calcium disilicide is> 40.5% by weight, more preferably> 41% by weight and most preferably 41.5% by weight (the theoretical content of calcium in

100% pure calcium disilicide is 41.7% by weight). The content of

Siiicium in the educt calcium disilicide is preferably> 57% by weight, more preferably> 57.5% by weight and most preferably> 58.1% by weight (the theoretical content of silicon in 100% pure calcium disilicide is

58.3% by weight).

The ammonium bromide used preferably has a total impurity content of <2% by weight, more preferably <1% by weight. The

The proportion of oxygen in the ammonium bromide is preferably <1.8 wt.

%, more preferably 1.5% by weight and most preferably <1% by weight.

The reaction proceeds according to the following equation (exemplified by the reaction of calcium silicide with ammonium bromide):

2 CaSi ₂ + 4 NH ₄ Br → 2 Si ₂ N + 2 CaBr ₂ + 2 NH ₃ t + 5 H ₂ t.

The CaBr ₂ formed in addition to the desired Si ₂ N or another salt when using other starting materials can, if desired, from the

Reaction mixture are removed. The CaBr ₂ (or another by-product formed) is preferably removed from the solid reaction mixture Extraction, for example with an organic solvent such as acetone or by sublimation, especially in vacuo, largely or completely removed.

The method is particularly preferably carried out in the absence of oxygen and in the absence of moisture. Excluding oxygen here means that the proportion of oxygen in the reaction medium is <1% by weight, more preferably <0.1% by weight. Excluding moisture means in particular that the reaction medium contains <1% by weight, more preferably <0.1% by weight of water or water vapor. The reaction is particularly preferably carried out under an inert gas, for example under an inert gas such as argon, or in a nitrogen atmosphere.

The reaction temperatures are preferably 1 150 ° C, more preferably> 200 ° C, even more preferably> 300 ° C and up to 900 ° C, more preferably up to 800 ° C, in particular up to 700 ° C and particularly preferably up to 500 ° C. Most preferably, the reaction is carried out at a reaction temperature of 330-370 ° C. It was found that in this temperature range the reaction takes place completely in a relatively short time (<0.5 hours).

Particularly good results are obtained if phase-pure silicide, for example calcium disilicide, is used, in particular calcium disilicide in the TR6 modification (cf. J. Evers, dissertation, LMU Munich, 1 974). In the TR6 modification, identity is achieved after six silicon layers. It is further preferred to use a silicide, for example calcium silicide, in the TR3 modification. It is particularly preferred to use a silicide, in particular a calcium disilicide, which contains less than 0.1% by weight, preferably less than 0.01% by weight, of impurities, in particular impurities from C, N and 0. It has been found in the context of the invention that the traces of water contained in conventional, commercially available ammonium halide, such as, for example, ammonium bromide, lead to a compound which, when reacted with a silicide, such as, for example, calcium disilicide, being obtained which contains the elements Si, Contains N and O and not the silicon subnitride described. To carry out the process according to the invention, the ammonia m al al en en, in particular ammonium bromide, is therefore used in pure form, in particular in anhydrous form. The oxygen content (oxygen serves as a reference value for H ₂ O) of the ammonium halide used according to the invention, for example ammonium bromide, is in particular <1% by weight, oxygen, more preferably 0 0.1% by weight, based on the weight of the ammonium halide used. Residual moisture or water can be removed from ammonium halide, in particular from ammonium bromide, for example by thermal sublimation and freezing out of the water obtained.

It is therefore essential for the process according to the invention to be carried out successfully that the starting components are pure, in particular that the silicide used, in particular calcium silicide, is free of oxygen and that the ammonium halide, in particular ammonimbromide used, is water-free, and that the reaction is carried out with the exclusion of oxygen and moisture.

To form the silicon subnitride according to the invention, the starting materials, that is to say the silicide and the ammonium halide, can both be mixed, for example, in powder form and then reacted at elevated temperature. In this way, the silicon subnitride according to the invention is obtained in powder form. However, it is also possible to use the silicide in crystal form, in particular as a single crystal, and the ammonium halide on the surface of the crystals aufzusublimieren. When reacting at elevated temperatures, the silicon subnitride is then formed epitaxially on the crystal surfaces of the silicide, for example the calcium disilicide. In this way it is possible to obtain layered material consisting of two-dimensionally ordered structures of silicon subnitride with a thickness of about 6-7 A per layer and a van der Waals distance between the layers. Both in the production from powders and in the production from single crystals, the silicon subnitride is formed in a topotactic reaction, i.e. in a reaction which leads to a material with structural orientations which, in connection with the crystal orientations in the starting product, i.e. in the silicide, in particular in the Calcium disilicide. In particular, the individual σ-silicon layers of silicide, for example CaSi ₂ , are found as two-dimensional layers of σ-Si in the silicon subnitride according to the invention.

Furthermore, it is also possible to carry out the reaction according to the invention on layers of a silicide, in particular of calcium disilicide, epitaxially applied to substrates. Suitable substrates are, for example, silicon surfaces, in particular silicon wafers. Calcium disilicides can be applied to substrates by methods known to those skilled in the art (see e.g. J.F. Morar et al., Physical Review B 37 (1 988), 261 8-2621).

When the silicon subnitride according to the invention is formed, starting from silicide layers, for example calcium disilicide layers, on substrates or from silicide single crystals, for example calcium disilicide single crystals, ordered layers of the silicon subnitride are obtained. When using silicide powders, for example calcium disilicide powders, an X-ray amorphous silicon subnitride is obtained as a highly disperse but essentially two-dimensionally ordered solid with a small particle size. The silicon subnitride according to the invention exhibits semiconductor properties and a high dielectric constant. It can therefore be used in particular in applications in which these properties are advantageous.

The invention therefore also includes the use of the silicon subnitride according to the invention as a dielectric. Due to its high dielectric constant, the silicon subnitride can be used in particular as a dielectric for memory modules, for example as a SiON replacement. Another area of application for the silicon subnitrides according to the invention is as a gate, in particular in CMOS technology. Here, the silicon subnitride can be used as a nano-insulator layer, for example by directly applying a silicide layer applied to a silicon substrate or another semiconductor, e.g. Calcium disilicide layer, is made with ammonium halide. Since monolayers with a layer thickness of about 6-7 A already have the desired material properties, in particular a high dielectric constant, very small components can be obtained here.

Another area of application for the silicon subnitride according to the invention is as a semiconductor in a nano or subnano transistor. The semiconductor layer usually used in such transistors, which consists, for example, of silicon with a thickness of 800 μm, can be replaced here by a monolayer of Si ₂ N, which contains a single layer of σ silicon. For reasons of stability, it is also possible to apply the silicon subnitride layer to a non-conductive substrate material.

In addition, the method according to the invention can also be used for the passivation of surfaces, in particular silicon surfaces, such as silicon wafers. The saturation occurs "dangling bonds" without the need to introduce carbon or any other metal. It is also advantageous that such a passivated surface can be converted directly to SiO ₂ layers with water vapor.

While the bulk material of the silicon subnitride according to the invention is stable to moist air, conversion to SiO ₂ layers can be carried out in a simple manner when treated with water vapor.

In addition to the applications in semiconductor technology, the silicon subnitride according to the invention is also outstandingly suitable for the production of ceramic materials. It can be used in particular as a precursor or precursor material for Si-N ceramics. To form the ceramics, the silicon subnitride according to the invention is sintered at elevated temperatures, in particular at temperatures> 900 ° C., more preferably> 1000 ° C. and preferably up to 2000 ° C., more preferably up to 1500 ° C. The silicon subnitride, which is stable at room temperature, is thermally decomposed. This leads to Si ₃ N ₄ , in particular a- and / or β-Si ₃ N ₄ , and Si under a protective gas atmosphere, for example argon or another noble gas, while pure Si ₃ N _{4 is} formed under a nitrogen atmosphere or nitrogen-containing atmosphere.

The invention is further illustrated by the attached figures and the examples below.

Figure 1 shows the structure of a monolayer of the invention

Silicon subnitride Si ₂ N. The layer thickness is 5.7 Å, whereby a monolayer of σ-Si (yellow circles) is surrounded by a layer containing Si and N at the top and bottom. The Si ₂ N layers can be arranged in accordance with the sphalerite type (top) or the rootite type (bottom). Between Layers there is a van der Waals distance or greater distance.

FIG. 2 illustrates the topochemical reaction starting from calcium disilicide to Si ₂ N according to the invention.

FIG. 3 shows SiN images of calcium disilicide (FIG. 3A) and the Si ₂ N obtainable therefrom by treatment with ammonium bromide at 350 ° C. (FIG. 3B). The layered structure of the silicon subnitride can be clearly seen.

FIG. 4 shows the dielectric material properties of Si ₂ N.

FIG. 5 shows the semiconductor properties of Si ₂ N, percent reflection versus the wavelength being plotted in nm.

FIG. 6 shows DTA / TG investigations on the course of the reaction and on thermal processes at higher temperatures, starting from CaSi ₂ and NH ₄ Br under argon.

FIG. 7 shows DTA / TG investigations on the course of the reaction and on thermal processes at higher temperatures starting from CaSi ₂ and NH ₄ Br under nitrogen.

Figure 8 shows the thermal resistance of the silicon subnitride

Air. Examples:

Example 1 Synthesis of Si ₂ N

1.1. Pure presentation of the educts

Calcium disilicide was produced from the elements in the induction furnace (glass carbon crucible; 60 seconds, 1000 ° C.) in a phase-pure manner in the TR6 modification (see J. Evers, dissertation, LMU Munich, 1974). The content of impurities from C, N and O in the calcium disilicide obtained is in each case less than 0.1% by weight.

Commercially available calcium disilicide usually contains, in addition to oxygen impurities (Table 1), the peripheral phases Siiicium and Caiciummonosilicid as well as possibly TaSi ₂ (this presumably comes from the crucible material).

Table 1: Oxygen analyzes of commercially available CaSi ₂ (carrier gas hot extraction, LECO TC 436 DR)

The CaSi ₂ that we synthesized consists solely of the CaSi ₂ -TR6 phase according to X-ray studies. The results of the chemical analyzes (Table 2) show that the self-made CaSi _{2 is} free of impurities. Table 2: Analysis results of self-synthesized CaSi ₂

* Detection limit of the measurement; ^θ carrier gas hot extraction; ^® analyzers from LECO

Before use in glass ampoules, ammonium bromide is purified by thermal sublimation and freezing of the water obtained with liquid nitrogen until the oxygen content is <1.5% by weight.

The ammonium bromide puratronic ^® from Merck is the cleanest commercially available NH ₄ Br. Table 3 shows the results of the oxygen analysis of commercially available and cleaned ammonium bromide. The oxygen content was reduced to 1% by repeated vacuum sublimation and freezing of the water in liquid nitrogen.

Table 3: Oxygen analyzes of NH ₄ Br (carrier gas hot extraction, LECO TC 436 DR)

1 .2 implementation to SUN

The starting materials thus produced were ground in a protective gas atmosphere in an argon box and ground to a homogeneous mixture by means of a vibration mill in an agate grinding jar. The chemical reaction was carried out in a niobium crucible at 350 ° C for 0.5 hours. The calcium bromide was separated off by continuous extraction with acetone for 8 hours under an argon atmosphere or by applying a vacuum, for example ¹⁰ -5 mbar for 5 days at 800 ° C.

An X-ray amorphous powdered silicon subnitride was obtained.

A reaction with the reaction parameters described above can also be carried out on single crystals of calcimdisilicide with previously sublimed ammonium bromide. The described reaction can also be carried out on layers of calcium disilicide epitaxially applied to substrates (e.g. α-Si).

1 .3 Analysis of the product

In contrast to the work of Hengge (Z. Anorg. Allg. Chem. 31 5 (1 962), 298-304), the oxygen content of the solid reaction product was determined directly in our investigations. This has a decisive influence on the actual total form! and the computational composition of the silicon subnitride. For comparison, analysis results of a reaction product from commercially available starting materials and from starting materials manufactured by us are shown (table 4). The equations underlying the reaction (1, according to Hengge) and (2, according to our own investigations) are as follows:

(1) 3 CaSi ₂ + 6 NH ₄ Br ^ ° l ^ ₂ ( _S i ₃ N) _∞ + 3 CaBr ₂ + 4 NH ₃ T + 6 H ₂ T

(2) 3 CaSi ₂ + 6 NH ₄ Br ^ Ξ- 3 (Si ₂ N) _∞ + 3 CaBr ₂ + 3 NH ₃ T + 7.5 H ₂ T

The two reaction equations (1) and (2) do not differ on the educt side. No errors were reported by Hengge for the analysis results of the reaction products. This results from the fact that the oxygen content is not taken into account.

Table 4: Results of the chemical analyzes of the reaction products (Si, Ca: lCP-AES after high-pressure microwave digestion; O / N carrier gas hot extraction)

The content and the calculated ratio of the elements calcium, silicon and bromine are constant in both reaction products. In conventional prepared silicon subnitride, the amount of oxygen (1.4%) contained in the preparation according to this invention is replaced by nitrogen,. At this point, the oxygen that is directly bound to silicon is replaced by nitrogen. The resulting increase in mass of nitrogen from 3.4% to 4.9% leads to a change in the Si / N ratio from 3: 1 to 2: 1.

1 .4 temperature range

The temperature range for the reaction of Si ₂ N and for the thermal decomposition of Si ₂ N was determined.

On the basis of thermal analysis tests it can be seen that the reaction begins at 150 ° C. and is completed at approximately 350 ° C. (FIGS. 6 and 7). The thermal decomposition / conversion of Si ₂ N begins in an inert gas atmosphere at about 900 ° C (Figures 6 and 7). Si ₂ N is stable in air up to about 800 ° C. (FIG. 8).

Claims

Expectations

1 . Silicon subnitride with the composition Si ₂ N.

2. Silicon subnitride according to claim 1, characterized in that it comprises areas with two-dimensional structural elements made of -Si.

3. Silicon subnitride according to one of claims 1 or 2, characterized in that it comprises structural units Si ° Si ^{3 +} N ^{3 "} , in which a two-dimensional σ-Si region and N atoms are present above and below the -Si layer.

4. Silicon subnitride according to the preceding claims, characterized in that it has an oxygen content of <1% by weight.

5. A process for producing a silicon subnitride according to one of claims 1 to 4, characterized in that a silicide, in particular a silicide with two-dimensional Si layers, is reacted with an ammonium halide.

6. The method according to claim 5, characterized in that calcium disilicide is reacted with ammonium bromide.

7. The method according to claim 6, characterized in that silicide with an oxygen content of <0.1 wt.% And ammonium halide with a water content of <1 wt.% Is used.

8. The method according to any one of claims 5 to 7, characterized in that salt formed in the reaction, in particular calcium bromide, by extraction or sublimation of the

Silicon subnitride is separated.

9. The method according to any one of claims 5 to 8, characterized in that the reaction at a temperature of> 1 50 ° C to

<900 ° C is carried out.

1 0. The method according to any one of claims 5 to 8, characterized in that the reaction with the exclusion of oxygen and under

Exclusion of moisture is carried out.

1 1. Method according to one of claims 5 to 1 0, characterized in that a silicide in TR6 modification is used.

1 2. The method according to any one of claims 5 to 1 1, characterized in that the silicide, in particular calcium disilicide as a powder, is used in crystal form, as a single crystal and / or as a layer on a substrate.

3. Silicon subnitride according to one of claims 1 to 4 or obtainable according to one of claims 5 to 1 2 in the form of a powder.

4. Silicon subnitride according to one of claims 1 to 4 or obtainable according to one of claims 5 to 1 1 in the form of a shaped body, in particular in the form of a layer located on a substrate.

5. Use of a silicon subnitride according to one of claims 1 to 4 or obtainable according to one of claims 5 to 1 1 as a semiconductor, as a dielectric, for passivating surfaces, in particular from

Silicon surfaces, in a sub-nano transistor, as nano-

Insulator layer and / or as a gate.

6. Use of a silicon subnitride according to one of claims 1 to 4 or obtainable according to one of claims 5 to 1 1 for

Manufacture of ceramic materials.

7. Use according to claim 1 6 as a precursor for silicon-nitrogen ceramics.

8. Use according to claim 1 6 as a precursor for silicon nitrogen coatings.