DE10146393A1 - Non-magnetic sintered body based on SiC and its use - Google Patents

Abstract

Ceramic sintered body based on SiC with a density> 95%, which is particularly suitable for magnetically relevant applications such as hard disk storage substrates or read / write heads, characterized in that it has a saturation magnetization of Ms <1.2 memu / cm · 3 · and was produced by one of the following processes: DOLLAR A a) pressureless solid sintering with sintering aids based on Al / C, boron / carbon or aluminum / boron / carbon with or without post-treatment by hot isostatic pressing DOLLAR A b) pressurized solid sintering with sintering aids based on boron / carbon or aluminum / boron / carbon DOLLAR A c) Liquid phase sintering with sintering aids based on rare earths (especially Y¶2¶O¶3¶) plus Al¶2¶O¶3¶ and / or AlN.

Description

The invention relates to a non-magnetic sintered body Base of silicon carbide, its manufacture and its Use in magnetically relevant applications, e.g. B. as Substrate or as a read / write head in magnetic Hard disk storage.

According to relevant textbooks and tables such. B. Gmelin, Handbook of Chemistry, does not have SiC Ferromagnetism but only diamagnetism, or possibly depending on Doping, paramagnetism. As from the following list can be seen, is also in the patent literature such. B. about Hard disk substrates made of SiC the ferromagnetism in SiC Sintered bodies have not yet been described or completely ignored, although in this application the magnetism of the substrate particularly annoying.

US 4,598,017 / T. Bayer et al. (IBM) discloses two reaction-bonded SiC ceramic disks with silicon are infiltrated. These become one with a polymer core Disk substrate connected. About ferromagnetism in SiC says nothing.

The patent US 4,738,885 / T. Matsumoto (Kyocera) describes a replenishment of pressure-free sintered Al ₂ O ₃ ceramic in order to reduce the porosity for use as a hard disk. SiC is mentioned in the description as another candidate for such an application. The magnetic properties of this SiC are not discussed.

The patent US 5,480,695 / M. Tenhover et al. describes the Manufacture of substrates from solid-sintered SiC after Regulations from US 4,312,954, which with amorphous SiC for Sputtered to produce a non-porous surface. Neither this patent also mention US 4,312,954 Ferromagnetism of SiC ceramics.

The patent US 5,358,685 / A. Ezis (Cercom) claims one particularly contaminated hot pressed SiC quality in the Application as z. B. ROM substrate. The iron content is at 0.01-2%. Nothing is said about magnetism.

J 61 013434 / T. Matsumoto (Kyocera) describes ceramic hard disk substrates that are coated with thin layers of SiC and Si ₃ N _{4 in} order to obtain pore-free surfaces. As a ceramic substrate, silicon carbide is mentioned, among others, that was produced according to typical recipes for solid sintering. Nothing is said about the magnetic properties of this ceramic.

J 60 229224 T. Wada et al. (Sumitomo) describes a hard disk substrate made of SiC ceramic, which is coated with thin sputter layers made of Al ₂ O ₃ , SiO ₂ , or Si ₃ N ₄ , in order to obtain pore-free surfaces. For the production of the ceramic it is stated that the usual and known methods can be used, including hot pressing, hot isotate pressing, SiSiC etc. There is no statement in this document about the magnetism of SiC.

J 63 070919 K. Kawakami et al (Hitachi) describes a hard disk substrate made of ceramic, coated with a thin glass layer in order to obtain non-porous surfaces. ZrO ₂ , AlN, Y ₂ O ₃ , and SiC with at least 5% Al ₂ O ₃ addition are proposed as ceramics. Nothing is said about the magnetic properties of SiC.

U.S. 5,487,931 Annacone et al. reveals one Hard disk substrate made of SiC with a thin Layer of silicon or TiC, B, TiN, TiCN is coated to to obtain pore-free and smooth surfaces. For the Manufacturing the SiC uses a wide range of methods suggested u. a. the (depressurized) sintering and Hot isostatic pressing. About the magnetic properties of the SiC ceramics say nothing.

As can be seen, the patent literature saves the aspect that in SiC substrate ferromagnetism can occur completely and thus mirrors those in textbooks and manuals (e.g. Gmelin) again held that SiC at best is diamagnetic.

However, work by the applicant clearly shows that ferromagnetism occurs in a conventional SiC ceramic, which leads to problems in the applications mentioned and depends on trace contaminants in the SiC. Fig. 1 shows the total by the applicant relationship between the content of iron, cobalt and nickel in SiC and the magnetization for various commercial SiC types again. The picture shows various commercially available SiC ceramics and gives an overview of the magnetic properties of known SiC ceramics. The methods and powder mixtures used to produce the sintered bodies examined correspond to the customary prior art for solid sintering, liquid phase sintering and reaction sintering of SiC ceramics. These measurements clearly show that ferromagnetism occurs in SiC ceramic, which is superimposed on a basic portion of diamagnetism. This affects magnetically relevant applications in general, wherever the natural field of the SiC interferes with functional magnet sizes, e.g. B. with substrates or read / write heads in hard disks. However, the impairment is not limited to these two examples, but occurs wherever sensitive magnet sizes are disturbed by the SiC's own field.

Such an impairment is discussed in detail using the example of substrates for hard disk use:
As shown in "Magnetic Disk Drive Technology" (ed. Kanu G. Ashar, IEEE Press, New York 1997) (p. 190), there is a need for technological reasons, the substrate materials used today aluminum and glass by harder and stiffer materials to replace to allow higher speeds, thinner discs, greater shock resistance, better polishes for lower flying height, higher sputtering temperatures, etc. In summary, such improvements serve the technological trend for higher information density and shorter access times. Silicon carbide is at the top of the list of "alternative" substrates.

The above-mentioned book also explains the process technology with which the information carrier, namely a thin layer (approx. 30 nm) made of magnetic cobalt alloys, is applied to the substrate. The magnetic parameters important for use as a storage disk, such as saturation magnetization, remanence and coercive force of this layer, are then measured using a "Vibrating Sample Magnetometer" or SQUID magnetometer. An approximately 1 cm ² piece is cut out of the coated substrate and its magnetization curve is recorded in the magnetometer. In detail, the sample is exposed to a static magnetic field "H", which is varied from 0 to about 10,000 Oersted. With increasing field "H" the magnetization "M" of the cobalt layer increases according to its susceptibility χ. With the vibration magnetometer, the magnetization is measured via a voltage signal, which is generated by vibration of the sample in a measuring coil. The signal is proportional to the magnetic moment of the sample, i.e. to the product of magnetization M (a pure material parameter) and the volume of the sample. Because of the extremely small layer thickness (approx. 30 nm), the volume is very small and accordingly the voltage signal, so that extreme demands are placed on the measurement accuracy. With the SQUID magnetometer too, the measured signal is proportional to the magnetic moment of the sample.

Since the area of the measured samples is usually kept constant at 1 cm ² , it has become common practice to use only the product of magnetization and layer thickness "t" as a measure of the magnetic moment. The above-mentioned book (p. 173) gives a typical value of Mr = 537 emu / cm ³ in the case of remanent magnetization "Mr", so that a remanent thickness product of Mr.t = 1 with t = 30 nm, 61 memu / cm ² (memu = milli electromagnetic units of magnetization) is obtained.

In another area of the disk, namely when reading of the information stored, this product is Mr.t also important because the voltage signal when reading is proportional to this product.

During the applicant's work on coating ceramic SiC substrates with cobalt alloys, it has now been found that SiC substrates themselves have a low magnetization, although with values around Mr = 5 memu / cm ³ for z. B. Commercially available SiC is about 5 orders of magnitude lower than that of the cobalt layer, but this results in a comparable Mr.t product in the product with the much larger slice thickness of 0.08 cm:
Cobalt layer: Mr.t = 1.61 memu / cm ²
SiC substrate: Mr.t = 0.4 memu / cm ²

This is the measurement of the magnetization curve Cobalt layer falsified by the substrate and that The result leaves no statement about the quality of the Co layer to.

Problems can also occur when reading the storage disk, especially if the magnetization of the SiC substrate is local fluctuates. Even when using a SiC sintered body as There is a substrate material for read / write heads Impairment of the quality of the read and written Signal through the self-field of the head substrate.

In US 5,770,324 dummy wafers for the Semiconductor process technology described, which are manufactured by hot pressing and specially designed for a very low iron content, to meet the specifications of the semiconductor industry. The Iron content of the purest SiC sinter powder used there was 10 ppm, Ni and Co content are not for the powder specified. Since the measured Fe impurities in the hot-pressed dummy wafers were well below these 10 ppm (e.g. 1.5 ppm), one has to conclude that the process of Hot pressing has an additional cleaning effect. The magnetic properties of these wafers are not known. Use of such materials for magnetically relevant Applications, e.g. B. as hard disk space is not mentioned.

The object of the invention is a ceramic sintered body based on SiC, which is available for magnetic relevant applications such as Hard disk storage substrate or read / write heads especially suitable is.

• a) druckloses Feststoffsintern mit Sinterhilfsmitteln auf Basis von Al/C, Bor/Kohlenstoff oder Aluminium/Bor/Kohlenstoff mit oder ohne eine Nachbehandlung durch Heißisostatisches Pressen
• b) druckbehaftetes Feststoffsintern mit Sinterhilfsmitteln auf Basis Bor/Kohlenstoff oder Aluminium/Bor/Kohlenstoff
• c) Flüssigphasensintern mit Sinterhilfsmitteln auf der Basis von Seltenen Erden (insbesondere Y₂O₃) plus Al₂O₃ und/oder AlN.

The object is achieved by a sintered body based on SiC with a density> 95% and a saturation magnetization Ms <1.2 memu / cm ³ , preferably <0.6 memu / cm ^3, which was produced by one of the following processes:

• a) pressureless solid sintering with sintering aids based on Al / C, boron / carbon or aluminum / boron / carbon with or without an aftertreatment by hot isostatic pressing
• b) pressurized solid sintering with sintering aids based on boron / carbon or aluminum / boron / carbon
• c) Liquid phase sintering with sintering aids based on rare earths (in particular Y ₂ O ₃ ) plus Al ₂ O ₃ and / or AlN.

The use of an SiC sintered body as a storage plate substrate is only useful if the ferromagnetism in the SiC is reduced below a value which, as a saturation thickness product, is less than about 5% of the analog product for the cobalt layer. Only then is it possible to characterize the cobalt layers magnetically without falsification, and only then will the interference and falsification of the signal read in the hard disk be sufficiently low. For the saturation magnetization Ms of the substrate, this means a value below 1.2 memu / cm ³ . To solve the problem, it is not enough to consider only the remanent magnetization "Mr" mentioned, because it depends on too many unknown factors and is therefore not really tangible. The "Ms" saturation magnetization is much more suitable because it is a material constant and because it includes the remanence as an upper limit for the magnetism of the substrate. From the already cited textbook "Magnetic Disk Drive Technology" it can be seen that the so-called squareness Mr / Ms = 0.8 for conventional cobalt layers. Thus Mr.t = 1.61 memu / cm ^{2 of} the above typical cobalt layer gives the value Ms.t = 2 memu / cm ² . In order to measure this value correctly, the interference from the substrate must not exceed 5%, i.e. Ms.t (substrate) <0.1 memu / cm ² . With a substrate thickness of 0.08 cm, this results in a maximum permissible saturation magnetization for the substrate of Ms = 1.2 memu / cm ³ .

The sintered body according to the invention preferably has a content an Fe <10 ppm, an Ni <3 ppm, an Co <1 ppm, particularly preferred Fe <5, Ni + Co <2 ppm.

It is surprising to the person skilled in the art that iron and nickel impurities are present in the sintered SiC in a ferromagnetically relevant manner. Ferromagnetic connections between iron and silicon or nickel and silicon are known, but according to Hansen (Constitution of Binary Alloys, McGraw-Hill 1956) these connections must be rich in metals (e.g. Fe ₂ Si). The form in which such compounds are present in sintered SiC is unknown, since the corresponding phase diagrams are not known over the entire range of thermal treatment during sintering. In addition, silicon is offered as a component of the SiC to the trace impurities iron and nickel in extreme excess, so that a reaction between Fe and Si (or Ni and Si) would rather be expected to be a silicon-rich and therefore non-ferromagnetic compound.

In the SiC sintered body according to the invention, iron, nickel and Cobalt bound in the form of non-magnetic compounds are present, preferably in the form of non-magnetic silicides.

Surprisingly, it has been shown that ferromagnetism in sintered SiC depends on the iron, nickel and cobalt content and reduced by reducing these impurities or can be eliminated. This applies to a wide range of Manufacturing methods for densely sintered SiC.

To sintered bodies according to the invention, which are used as substrates for Hard disks are used to manufacture, it is preferred the content of iron in the SiC powder below 10 ppm, the content of Nickel below 3 ppm and the cobalt content below 1 ppm humiliate. The Fe content should preferably be <5 ppm, and Ni + Co content can be <2 ppm. The necessary Purity conditions are also met if it applies that in Sintered body the sum of the content of Fe, Ni and Co <13 ppm, is preferably <7 ppm.

The sintering aids Al / C, B / C, Al / B / C and Y ₂ O ₃ + (Al ₂ O ₃ and / or AlN) are used in the usual amounts.

• a) drucklos gesinterte SiC-Sinterkörper mit den Sinterhilfsmitteln
  B/C oder
  Al/B/C oder
  Al/C
• b) druckbehaftet gesinterte SiC-Sinterkörper mit den SinterhilfsmittelnQ
  B/C oder
  Al/B/C
• c) flüssigphasengesinterte SiC-Sinterkörper mit den Sinterhilfsmitteln
  AlN/Y₂O₃ oder
  AlN/YAG
  (YAG = Yttrium Aluminium Granat)

The SiC sintered bodies according to the invention can be divided into the following groups:

• a) SiC sintered bodies sintered without pressure with the sintering aids
  B / C or
  Al / B / C or
  Al / C
• b) pressurized sintered SiC sintered bodies with the sintering aids Q
  B / C or
  Al / B / C
• c) liquid-phase sintered SiC sintered bodies with the sintering aids
  AlN / Y ₂ O ₃ or
  AlN / YAG
  (YAG = yttrium aluminum garnet)

Table 1 is an example of a typical, in no way exhaustive, selection of typical sintering additives and Sintering conditions specified for the above sintered bodies. she reflect the state of the art, there being countless Variations there. There are only those sintering additives listed, which have technically prevailed today. In addition, there are others with whom you can also can produce non-magnetic SiC. Is essential to the invention not the sintering aid, but the purity of the SiC Starting powder of iron, cobalt and nickel.

The sintering additives are weights. The C content depends on the oxygen content of the SiC powder. It should preferably be slightly overstoichiometric in order to react the oxygen according to:

SiO ₂ + 3C = SiC + 2CO.

Usual amounts of O in the SiC are 0.5-1.5% by weight, so that preferably an approximately similar amount of C (i.e. 0.5-1.5% by weight) is added. The amounts are preferably chosen so that between 0.3 and 0.8 wt .-% free carbon in solid-sintered sintered body remain. At the Carbon sintering is not necessary for liquid phase sintering. Carbon is therefore not intentionally added there, but instead only a small carbon content results from the addition organic binder.

The sum of the sintering additives and the oxygen content of the SiC powder and the SiC weight is 100%. Table 1 typical sintering aid weights in% and rough sintering conditions

An SiC sintered body according to the invention can preferably be produced using the starting powders mentioned using the known methods already mentioned:
Pressureless solid sintering with sintering aids based on Al / C, boron / carbon or aluminum / boron / carbon with or without aftertreatment by hot isostatic pressing; Pressurized solid sintering with sintering aids based on boron / carbon or aluminum / boron / carbon; Liquid phase sintering with sintering aids based on rare earths (in particular Y ₂ O ₃ ) plus Al ₂ O ₃ and / or AlN.

Possibly. the sintered body can still be pressurized be densified.

At SiSiC, a porous molded body made of SiC (porosity approx. 20%) is infiltrated with liquid silicon, i.e. at temperatures above 1400 ° C. All superficially adhering iron, nickel and cobalt should react with the excess silicon to form silicon-rich, i.e. non-magnetic silicides (e.g. FeSi ₂ ). The finding in Figure 1 confirms this assumption, because SiSiC proves to be almost non-magnetic despite a very high Fe + Ni + Co doping of 287 ppm. This means that SiSiC requires only a comparatively minor cleaning of the starting material in order to keep this sintered body completely non-magnetic.

The SiC sintered bodies according to the invention are particularly suitable as a substrate for hard disk storage or as a substrate for Read-write heads.

The following examples serve to further explain the Invention.

Examples

A basic process is described for all examples is valid and in which according to Table 1 the respective Sintering additives are used. The respective sintering process can also be found in Table 1.

basic process

• a) Alpha-SiC sinter powder with a specific surface of 12 m ² / gr and an average particle size of 0.7 µm is in a dissolver with an organic binder (e.g. PVA) and with water-soluble carbon donors such as sugar or starch and further mixed intensively wet with a pressing aid (fatty acid derivative) and with the corresponding sintering additives according to Table 1. The sintering additives have the following grain size:
  AlN D50 = 1.5 µm
  YAG D50 = 1.5 µm
  Y ₂ O ₃ D50 = 1.5 µm
  Boron <1 µm (amorphous)

For examples according to the invention (they are not shown in Tab. 2 and Fig. 1, but are all below 1.2 memu / cm ³ ), the SiC starting powder is specially cleaned of iron, cobalt and nickel, so that: Fe < 10 ppm, Ni <3 ppm, Co <1 ppm. For this purpose, the SiC starting powder is boiled for a long time (2 days) in hydrochloric acid at pH 0-1 and then washed out in a number of cycles (10-15) to the isoelectric point.

The comparative examples are the Ekasic® D, Ekasic® T and Ekasic® BM sintered bodies shown in Tab. 2 and Fig. 1 (commercially available from Wacker Chemie GmbH, Munich) and the ones not specifically named in Fig. 1 but listed in Tab. 2 Sintered body BM-A2, BM-P2, BM-C2, BM-Q, Vers-B22, BM-O2 (all EKasic® BM). They have a residual magnetism> 1.2 memu / cm ³ . The starting material was cleaned less intensively and less labor-intensive. In particular, the hydrochloric acid was not heated and less acid excess was used. This reduced the number of subsequent washes to a few.

The comparative examples Hexoloy SA and SiC / C & C in Fig. 1 are further confirmations for the relationship between purity and magnetic properties.

• a) Die homogenisierte Mischung wird dann in einem Sprühtrockner getrocknet, wobei ein rieselfähiges Pulver anfällt.

The SiSiC sample in FIG. 1 is proof that an excess of Si in the sintered body succeeds in bringing the iron, cobalt or nickel contamination into a silicon-rich and therefore non-magnetic compound.

• a) The homogenized mixture is then dried in a spray dryer, a free-flowing powder being obtained.

• a) Das Pulver wird entweder in einer Gummihülle bei ca. 1200 Bar kaltisostatisch verpresst oder in einer Trockenpresse verdichtet. Nach diesen Prozessen liegt eine Dichte von ca. 60% vor.
• b) Dann wird der verdichtete Pulverpressling grünbearbeitet und anschließend bei höherer Temperatur (z. B. 700°C) entbindert.
• c) Der nächste Schritt ist das Sintern über einen Zeitraum von 0,5 bis zu einigen Stunden bei den in Tabelle 1 angegebenen Temperaturen und Drucken.
• d) Zum Schluß wird eine kleine Probe von z. B. 1 × 1 × 0,1 cm aus dem Sinterkörper herausgeschnitten und in einem "Vibrating Sample Magnetometer" auf seine magnetischen Eigenschaften hin untersucht.

Typical additional quantities: sinter additives: see table 1 carbon: according to table 1, but depending on the oxygen content of the SiC sinter powder. Binder / pressing aids: 1-4% each

• a) The powder is either cold isostatically pressed in a rubber sleeve at approx. 1200 bar or compacted in a dry press. After these processes there is a density of approx. 60%.
• b) Then the compacted powder compact is processed green and then debindered at a higher temperature (e.g. 700 ° C).
• c) The next step is sintering over a period of 0.5 to a few hours at the temperatures and pressures given in Table 1.
• d) Finally, a small sample of e.g. B. 1 × 1 × 0.1 cm cut out of the sintered body and examined in a "Vibrating Sample Magnetometer" for its magnetic properties.

The measurement results are shown in Tab. 2 and Fig. 1. EKasic® BM, Ekasic® D, Ekasic® T and SiC / C & C were treated according to the basic process described. Hexoloy SA is obtained from Carborundum. The impurity content (Fe + Ni + Co) was determined analytically. The weight of 0.7% boron was taken from the Carborundum homepage. Table 2

BM-A2, BM-P2, BM-C2, BM-Q, Vers-B22, BM-O2: various EKasic® BM samples
SiC / CC: SiC sintered body commercially available from Wacker Ceramics' French plant in Bazet.
Ekasic® D: SiC sintered body commercially available from Wacker Chemie GmbH (Munich),
Ekasic® T: SiC sintered body commercially available from Wacker Chemie GmbH (Munich),
Ekasic® BM: SiC sintered body commercially available from Wacker Chemie GmbH (Munich),
Hexoloy SA: SiC sintered body commercially available from the American company Carborundum
SiSiC: liquid phase sintered SiC,

Claims (6)

1. Ceramic sintered body based on SiC with a density> 95%, which is particularly suitable for magnetically relevant applications such as hard disk memory substrate or read / write heads, characterized in that it has a saturation magnetization of Ms <1.2 memu / cm ³ and was manufactured using one of the following processes: a) pressureless solid sintering with sintering aids based on Al / C, boron / carbon or aluminum / boron / carbon with or without aftertreatment by hot isostatic pressing b) pressurized solid sintering with sintering aids based on boron / carbon or aluminum / boron / carbon c) Liquid phase sintering with sintering aids based on rare earths (in particular Y ₂ O ₃ ) plus Al ₂ O ₃ and / or AlN. 2. Ceramic sintered body according to claim 1, characterized in that it has a saturation magnetization of <0.6 memu / cm ³ . 3. Ceramic sintered body according to claim 1 or 2, characterized characterized in that the content of Fe <10 ppm, of Ni <3 ppm Co is <1 ppm. 4. A method for producing a sintered body according to one of the Claims 1 to 3, characterized in that an SiC Sinter powder with a content of Fe <10 ppm, of Ni <3 ppm and Co <1 ppm is used. 5. Use of a sintered body according to one of claims 1 to 5 as a substrate for a hard disk storage. 6. Use of a sintered body according to one of claims 1 to 5 as a substrate for a read / write head.