DE102017211320A1 - Component formed with beta-silicon nitride material and a method for its production - Google Patents

Abstract

The component formed with β-silicon nitride material has a thermal conductivity> 80 W / mK and a dielectric puncture resistance> 35 KV / mm. In production, powdery SiN is formed into a block-shaped member and compacted at a temperature of at least 1700 ° C in a heat treatment in a nitrogen atmosphere. Subsequent to subsequent cooling from the block-shaped element, carrier elements are obtained as disk-shaped elements with a thickness of the individual carrier elements in the range of 100 μm to 3 mm by means of a wire sawing process.

Description

The invention relates to components formed with β-silicon nitride material and to a method for their production. The components are preferably ceramic carrier elements with high thermal conductivity. These support elements are provided in particular for electrical and / or electronic components.

Ceramic substrates as supports for in particular high-performance electronic components are made of either Al ₂ O ₃ (inexpensive, but low thermal conductivity (WLF)), AlN (high WLF, relatively high cost, insufficient long-term stability due to insufficient strength). There are silicon nitride substrates that have significantly better strength, and therefore longer life, WLF around 60-80 W / mK, but higher costs.

All known substrates are produced by means of film casting. Si ₃ N ₄ films are more difficult to sinter than AlN and Al ₂ O ₃ materials because they form decomposition zones on the surface, which limit the reproducibility of the properties and also the absolute values of the strength and the dielectric properties. In particular, in order to achieve high thermal conductivities, long sintering times and high temperatures have to be used, which increase these disadvantages. The necessary density sintering of the films at the lowest possible temperatures (in order to minimize decomposition phenomena) requires the highest possible proportion of additives. What tends to have a negative impact on the WLF.

There are a number of known technical solutions for substrates of Si ₃ N ₄ . However, production via film casting leads to unsatisfactory results in terms of electrical breakdown resistance, surface quality, flatness and mechanical strength, in particular with regard to variations in properties.

It is therefore an object of the invention to provide options for the production of components consisting essentially of Si ₃ N ₄ , which have sufficient mechanical strength, dielectric strength and a correspondingly high thermal conductivity even at a small thickness of the components, in particular of carrier elements and can be produced inexpensively.

According to the invention this object is achieved with a component having the features of claim 1. These components can be produced by a method according to claim 7. Advantageous embodiments and further developments of the invention can be realized with features described in the subordinate claims.

In the method of the present invention, powdery Si ₃ N _{4 is formed} into a block-shaped member and sintered at a temperature of at least 1700 ° C. in a heat treatment in a nitrogen atmosphere. Subsequent to subsequent cooling, carrier elements are obtained from the block-shaped element as disk-shaped elements with a thickness of the individual carriers in the range of 100 μm to 3 mm by means of a wire sawing process.

The starting powder used was an initial powder having a β-Si 3 N 4 content of 2% by volume to 15% by volume, preferably 5% by volume to 10% by volume. For this purpose, Si ₃ N ₄ starting powder β-Si ₃ N ₄ nuclei can be added to the Si. β-Si ₃ N ₄ is also formed during the sintering-leading heat treatment by phase transformation from the metastable α-modification.

The sintering should, if possible, be achieved at low nitrogen pressures, ie at pressures between 0.05 MPa and 0.3 MPa-0.5 MPa, in the temperature range from 1100 ° C to 1700 ° -1750 ° C, until closed porosity is reached. The lower pressures should be used at lower temperatures where less decomposition is expected. Too low an N ₂ pressure can lead to the decomposition of the sintering additives. At 1100 ° C, the decomposition is kinetically impeded, so in this range the pressure is not so critical.
The determination of the point in time of the pore closure can be done via abortive or dilatometric measurements. After achieving the closed porosity, which is typically achieved at densities corresponding to 95% of the theoretical density, it is advantageous to increase the pressure to 3 MPa to 5 MPa. A further increase to 10 MPa nitrogen pressure then brings only a smaller effect.
In the heat treatment carried out for sintering, a pressure greater than 5 MPa, preferably up to 10 MPa, should be maintained.

The pressure should therefore advantageously be increased gradually during the heat treatment leading to the sintering. In this case, the pressure and the temperature can be gradually increased, wherein the maximum pressure at a temperature in the range 1800 ° C to 1950 ° C and reached over a period of 1 h - 10 h, preferably 3 h - 5 h should be maintained.

Particularly advantageous is the sintering of large blocks of Si ₃ N ₄ , which then effectively by means of wire saws into individual components, in particular support elements (substrates) with predetermined any thickness can be cut between 100 microns and 3 mm as needed.

As a result, less sinterable inexpensive Si ₃ N ₄ starting powders having a higher β-Si ₃ N ₄ content> 3% by volume, advantageously> 5% by volume or particularly advantageously> 10% by volume, can be used. These starting powders, which are usually produced by a direct nitriding process, have a low oxygen content and cause greater grain growth, which is usually disadvantageous for sintering. But it helps to shorten the sintering times until high WLF is reached.

The sintering additives can be selected primarily in terms of high WLF and high mechanical properties. The boundary conditions that are determined by the sintering are much less restrictive than for the sintering of films.

Suitable sintering additives are mixtures of MgO, rare earth oxide, preferably an oxide of yttrium, lanthanum and / or a lanthanoid, and CaO with ZrO ₂ . The sintering additives should consist of a mixture in which MgO and rare earth oxide in the ratio between 20:80 and 60:40 (mol%: mol%) are added to the starting powder.

To achieve better processing in aqueous solution, the MgO and CaO may be added as silicate or else carbonate (e.g., Mg 2 SiO 4, magnesite, dolomite).

ZrO ₂ is bad at first as an additive in highly thermally conductive materials, because it has a low WLF. In addition, under sintering conditions, ZrO ₂ can be reduced to Zr (C, N, O) _1-x (0≤x≤0.5Zikoncarbonitrid, carbide, or even - carbooxinitride) and thereby adversely affect the dielectric strength. Due to the particular technology of sintering and the maintenance of the contents of stabilizing elements, firstly the reduction can be prevented and secondly a faster sintering and grain growth can be achieved by reducing the melt viscosity and the minimum melt formation temperature. As a result, a higher thermal conductivity can be achieved despite the admixing of a less thermally conductive component.

In the surface areas of the blocks in the sintered state, the cubic phase forms with NaCl structure (Zr (C, N, O)). However, this extends to a maximum of 100 μm - 200 μm in depth from the surface and is thus not relevant for the properties of the discs. In the case of single film sintering, this area with poor dielectric strength and poor oxidation and corrosion resistance would each account for approximately 25% -50% of the thickness, thereby minimizing the electrical breakdown and partial-current strength of the device. Too low an N2 pressure during the heat-up phase of sintering can lead to the preferential formation of (Zr (C, N, O)) below 1100 ° C the kinetically impaired decomposition, therefore the selected nitrogen pressure is not so important in this range.

Therefore, the sintering should be done so that the decomposition of ZrO ₂ in Zr (C, N, O) _1-x below the surface layer is prevented. This can be prevented by still small residual amounts of SiO ₂ in the volume of the component are present, they react immediately with possibly locally formed ZrN, oxidize it and form Si ₃ N ₄ . Therefore, there is a possibility to control the activity of SiO ₂ in the melt in this way. This can also be done both via the control of the N ₂ pressure and the CO partial pressure in the oven. The effect of these parameters can be estimated by thermodynamic calculations.

Since the ZrO _{2 is} in itself a poor conductor of heat, the content of ZrO ₂ should be limited to <5% by mass. The ZrO ₂ can be partially or completely replaced by HfO ₂ . As additional sintering additives, a total of 1% by mass - 5% by mass ZrO ₂ and / or HfO ₂ can be added to the starting powder.

The ZrO ₂ can be present as fully stabilized ZrO ₂ or, with slow cooling, as a cubic phase partially with ordered oxygen defects in the fluorite structure as Y ₄ Zr ₃ O ₁₂ . ZrO _{2 also} cleans the grain boundaries because the amount of residual amorphous glass phase can be minimized because the ZrO ₂ phase can crystallize as separate particles. Therefore, a certain minimum proportion for achieving the properties is advantageous (≥ 1 mass%).

By pressing / isostatic pressing in the molding higher green densities can be achieved, as in the film casting, which positively influences the sintering and the mechanical properties.

The pressing or isostatic pressing is typically in the range between 100 MPa and 250 MPa. However, higher pressures are also possible (in typical industrial plants, for example, up to 400 MPa can be realized). Before sintering, a green or white processing is possible to meet certain dimensions.

In addition, such shaped bodies can also be produced by extrusion or other common shaping methods.

There are also extremely thin substrates for the individual components / support elements with thicknesses in the Range 100 microns - 200 microns realized that can not be made with film casting or only with great effort.

For separating, which can be achieved in particular by wire sawing, a SiC suspension containing SiC particles with 20% by mass - 50% by mass with the grain size F320 - F800 can be used as a liquid in polyethylene glycol. Alternatively, a suspension containing B ₄ C, diamond and / or SiC particles may be used. Alternatively, a wire equipped with diamond grains can also be used.

After sawing and / or further process steps (metallization, printing, joining of active elements), the substrate slices can be separated by laser and other mechanical methods.

The result is a material with extremely improved properties. An electrical breakdown strength of> 35 KV / mm (measured on 200 μm thick substrates by standard methods) is achieved. The state of the art is at maximum values in the range 20 KV / mm - 25 KV / mm.

Strengths (ball on 3 ball)> 800 MPa can be achieved on substrates with a thickness of 0.5 mm and strengths of> 950 MPa measured on 200 μm thick substrates with a Weibull modulus> 15. It has a thermal conductivity> 80 W / mK. The thermal conductivity depends on the sintering temperature and sintering time. Higher temperatures and times lead to higher WLF, but also to a limitation of the strength due to larger grains.

There is no altered chemical composition on the surface, as is typically the case with the substrates produced by film casting, in which, depending on the sintering conditions, edge layer thicknesses of 20 .mu.m-30 .mu.m for short sintering times and up to several 100 .mu.m for long sintering times and high Temperatures are the case. As a result, high electrical breakdown strengths can be achieved. A prerequisite for these values is also a porosity <1, preferably <0.5%.

It can be a surface roughness <0.5 mm (Ra) and a total thickness variation <10 microns can be achieved.

Due to the cutting process, the substrates have no warping, as is often the case with sintered films. Deviations from the flatness of 75 mm * 75 mm substrates of <10 μm - 15 μm can be achieved.

In the additives and in the starting powder, the content of aluminum should be <0.1% by mass, since increased incorporation of Al and O into the Si ₃ N ₄ lattice leads to a reduction of the WLF.

It is advantageous that a powder with an oxygen content in the powder <1.5% is used. The higher the SiO ₂ content, the higher the amount of oxygen incorporated into the Si ₃ N ₄ lattice, thereby potentially reducing the WLF.

It can be added to improve the WLF β-Si ₃ N ₄ nuclei, which may be aligned isotropic or parallel or perpendicular to the substrate surface depending on the production used. Anisotropic properties, in particular an anisotropic WLF, can be achieved with β-Si ₃ N ₄ nuclei aligned parallel or perpendicular with respect to the surface of a block-shaped element or carrier element. Alignment of the β-Si ₃ N ₄ nuclei can be achieved by uniaxially pressing the powder mixture, or selectively filling the molds for cold isostatic pressing, for example, by slowly filling on the vibrating table and / or intermediate swaging by vertical vibration or the combination of uniaxial pressing and Isostatic recompression can be achieved. However, a high number of anisotropic films can also be produced and then laminated one on top of the other at high pressure.

The β-Si ₃ N ₄ nuclei should have an average particle size of 3 to 10 times the average particle size of the starting powder so as to allow the desired effect of texture coarsening and anisotropy.

The content of free or bound carbon in the sintered material should not exceed 0.5 mass%, preferably 0.1 mass%, ideally 0.05 mass%. The carbon typically forms SiC under sintering conditions, which is semiconductive and therefore negatively affects the electrical properties. In addition, carbon can locally reduce the sintering activity (reduction of the additives). This then leads to a reduction in strength through the formation of pores or pore clusters.

The invention will be explained in more detail by way of example in the following.

A starting powder consisting of Si ₃ N ₄ obtained by direct nitration, (β-Si ₃ N ₄ content based on the total Si ₃ N ₄ content of 12.5% by volume; Al content <0.05% by weight). % Oxygen content 0.6% by mass) was mixed with the sintering additives Y ₂ O ₃ (particle size d ₅₀ = 0.5 μm) MgO (d ₅₀ = 0.5 μm) and ZrO ₂ (TZ3Y from Tosoh) in a stirred ball mill mixed and post-shredded (average particle size, d ₅₀ = 1 μm). The additive content was 1 mass% MgO, 7 mass% Y ₂ O ₃ and 1 mass% ZrO ₂ .

The suspension is then spray dried. The average granule size was 61 μm. The maximum granule size was adjusted by sieving at 200 μm. Higher granule sizes are undesirable because of possible larger defects. To achieve a better processing in aqueous solution, the MgO can be added as silicate or carbonate.

The granules were placed in a rubber mold and pressed isostatically at 250 MPa and baked to 550 ° C. The average heating rate was 15 K / h - 30 K / h.

The block was then heated at ₂ bar in N ₂ atmosphere at ca. 2K / min to 1875 ° C. From 1750 ° C, the pressure was increased to 10 bar parallel to the heating. During the hold time at 1875 ° C, the pressure was increased to 60 bar and held for 5 h.

The dimensions of the sintered block were 75 × 75 × 150 mm ³ .

After cooling, the resulting block is severed by cutting, which preferably takes place by means of a suspension-based multi-wire sawing process. In this case, a steel wire and a suspension consisting of a carrier medium (eg PEG-200) and loose abrasive particles (SiC) are used. The feed rate is adjusted to the hardness and toughness of the Si ₃ N ₄ material and can be up to 0.5 mm / min. The wire speed is regulated according to the feed used and is up to 20 m / s. With a wire diameter of 110 μm-120 μm and SiC particle sizes of 6 μm-30 μm, a minimal loss of unusable Si ₃ N ₄ can be achieved.

The support elements obtained after separation had a thickness of 200 .mu.m a strength determined by ball on 3 ball analogous to the method of Danzer of 920 MPa + 65 (Institute of Structural and Functional Ceramics Montanuniversität Leoben, Ball on 3 balls test (web-app : http://www.isfk.at/en/960/); and an electrical breakdown strength of 43KV / mm and a thermal conductivity of 82 W / mK.

Claims (23)

Component which is formed with β-silicon nitride material and has a thermal conductivity> 80 W / mK and an electrical breakdown strength> 35 KV / mm. Component after Claim 1 , characterized in that the β-silicon nitride material has a strength> 900 MPa, a Weibullmodul> 15, a surface roughness <0.5 microns (Ra), a total thickness variation of <15 microns preferably <10 microns. Component according to one of the preceding claims, characterized in that the β-silicon nitride material contains 0.5 vol .-% to 15 vol .-% grain boundary phase consisting of SiO ₂ and the sintering additives and 1 mass% - 5 mass% cubic ZrO ₂ or HfO ₂ stabilized with CaO / MgO and rare earths. Component according to one of the preceding claims, characterized in that a ratio of the sum of MgO, CaO; R ₂ O ₃ to HfO ₂ / ZrO ₂ > 10: 90 (mol / mol), where R is selected from yttrium; Lanthanum and lanthanides. Component according to one of the preceding claims, characterized in that a ratio CaO to MgO <1 (mol / mol) is maintained. Component according to one of the preceding claims, characterized in that the porosity is <1%, preferably <0.5%. Process for the production of components, in particular ceramic carrier elements (substrates) according to one of the preceding claims, characterized in that powdery Si ₃ N _{4 is formed} into a block-shaped element and compacted during a heat treatment in a nitrogen atmosphere at a temperature of at least 1700 ° C and following a subsequent cooling from the block-shaped element, carrier elements are obtained as disk-shaped elements with a thickness of the individual carrier elements in the range of 100 μm to 3 mm by means of a wire sawing process. Method according to the preceding claim, characterized in that the block-shaped element is sintered in such a way that β-Si ₃ N ₄ is formed. Method according to one of the two preceding claims, characterized in that a starting powder with a β-Si ₃ N ₄ content 2 vol .-% -15 vol .-%, preferably 5 vol .-% - 10 vol .-% is used. Method according to one of the three preceding claims, characterized in that a starting powder having an oxygen content in the powder <1.5% is used. Method according to one of the four preceding claims, characterized in that the Si ₃ N ₄ starting powder 1 vol .-% - 10 vol .-% β-Si ₃ N ₄ - Germs having an average particle size 3 to 10 times greater than the average particle size of the starting powder may be added. Method according to one of the five preceding claims, characterized in that a proportion of aluminum in the starting powder and in the additives <0.1% by weight, preferably <0.05% by weight is maintained. Method according to one of the six preceding claims, characterized in that a content of free or bonded carbon in the sintered material <0.5% by mass, preferably <0.1% by mass, particularly preferably <0.05% by mass is maintained , Method according to one of the seven preceding claims, characterized in that the sintering additives consist of a mixture of MgO and rare earth oxide, in particular an oxide of yttrium; Lanthanum, a lanthanoid or a mixture thereof in the ratio between 20: 80 and 60: 40 (mol%: mol%) are added to the starting powder. Method according to one of the eight preceding claims, characterized in that MgO and CaO as silicate or carbonate, in particular Mg ₂ SiO ₄ ; Magnesite or dolomite are added to the starting powder. Method according to one of the nine preceding claims, characterized in that as additional sintering additives in total 1% by mass - 5% by mass of ZrO ₂ and / or HfO ₂ are added to the starting powder. Method according to one of the ten preceding claims, characterized in that a ratio of the sum of MgO, CaO; R ₂ O ₃ to HfO ₂ / ZrO ₂ > 10: 90 (mol / mol) is maintained, where R is selected from yttrium; Lanthanum and lanthanides. Method according to one of the eleventh preceding claims, characterized in that in the heat treatment carried out for sintering, a maximum pressure of greater than 3 MPa, preferably> 5 MPa is maintained. A method according to any one of the preceding claims, characterized in that the pressure and the temperature are gradually increased during the heat treatment for sintering, the maximum pressure being reached at a temperature in the range of 1700 ° C to 1950 ° C and over a period of 1 h - 10 h, preferably 3 h - 5 h is maintained. Method according to one of the thirteen preceding claims, characterized in that cubic ZrO ₂ and / or Y ₄ Zr ₃ O _{12 is} formed in the sintered material during the heat treatment leading to the sintering. Method according to one of the fourteen preceding claims, characterized in that a SiC suspension in which SiC particles of 20 mass% - 50% by mass with the grain size F320 - F800 are included, is used in polyethylene glycol for wire sawing. Method according to one of the fifteenth preceding claims, characterized in that a suspension is contained in the B ₄ C, diamond and / or SiC particles / are used for wire sawing. Method according to one of the sixteen preceding claims, characterized in that a diamond-tipped wire is used for sawing.