DE102013107188A1 - Blank of silicon, process for its preparation and use thereof - Google Patents

Abstract

The invention relates to the use of monocrystalline or quasi-monocrystalline silicon, which is preferably produced by the directional solidification method, for functional components in systems for semiconductor processing, such as for showerheads or sputtering targets. According to the invention, the blanks have a concentration of individual dislocations in the range from 102 to 106 cm −2 in their volume free of both dislocation clusters and small-angle grain boundaries and a specific resistance of 0.001 to 0.2 Ωcm, can contain foreign grains, twin grain boundaries or clusters of dislocations or small-angle grain boundaries and can therefore be produced more cost-effectively and efficiently than blanks made of monocrystalline silicon using the Czochralski or float zone processes.

Description

Field of the invention

The invention relates generally to a blank made of monocrystalline or quasi-monocrystalline silicon, and to the production and use thereof, and in particular to the use of mono- or quasi-monocrystalline silicon, which is preferably produced by the method of directional solidification, for functional components in semiconductor processing plants, such as for example for showerheads or sputtering targets.

State of the art

Monocrystalline low-resistance silicon sputtering targets are currently obtained without exception from ingots produced by the Czochralski (CZ) process or the Float Zone (FZ) process. Here, low resistance refers to the range of resistivity of 0.001 Ωcm to 0.2 Ωcm, and the used range of these ingots is cylindrical. The diameter corresponds to that of ingots which are used for the production of semiconductor wafers, which are used for the production of integrated circuits. The diameters of such ingots are typically limited to 150 mm, 200 mm, 300 mm or 450 mm by the limitations of the CZ method or FZ method. At the maximum, the CZ method has so far reached diameters of up to 550 mm.

The same geometric limitations also apply to the production of low-resistance monocrystalline showerheads, as they are, for. B. in plasma etching, plasma cleaning or coating equipment can be used.

Since the production of CZ ingots larger than 320 mm raw diameter is extremely expensive and hardly available, this material is used for such functional components only in exceptional cases. Often it is necessary to resort to inexpensive multicrystalline material, which, however, has disadvantages in the application.

The quality requirements placed on such functional components hitherto always involved intrinsic material quality requirements with regard to the content of metallic impurities, nitrogen and oxygen.

Depending on the application is still a doping (p-type or n-type), an electrical resistance z. 0.001 Ωcm to 0.2 Ωcm, a particular crystallographic orientation in connection with the format (round or rectangular) and a certain purity (eg 6 N) are required (see for example the requirements for a sputtering target) http://www.spm.li/deutsch/sputtering-targets/silizium/ ).

Since no other monocrystalline material of corresponding doping is available on the market in addition to CZ or FZ material, it was never questioned whether the high quality of the expensive CZ material or the even more expensive FZ material for functional components in semiconductor processing equipment such as low-impedance showerheads or low-impedance sputtering targets is really imperative.

In the photovoltaic industry, the electrical efficiency of the solar cells used is the most important quality criterion for their use. It is known that monocrystalline solar wafers produced by the CZ process can achieve higher efficiencies than multicrystalline solar wafers. This is due, in particular, to recombinations of charge carriers generated under solar irradiation at the large number of grain boundaries present in the material.

According to the state of the art, a method of directional solidification of ingots was developed for the photovoltaic industry in order to be able to produce uniform (100) -oriented solar cells without grain boundaries and thus more powerful solar cells (cf. EP 2028292 . WO 2007/084934 . WO 2009/014957 ). In this method, arranged on the bottom of the crucible monocrystalline seeds use that initiate directional solidification and should lead. However, the applicants of these patents have not further developed the described production methods and the methods of material evaluation for other applications, since they have neither considered nor considered these applications possible. This is exemplified by the statements in [0002] EP 2028292 or [003] in WO 2007/084934 or [002] in WO 2009/014957 ,

In these crystallization processes developed especially for the photovoltaic industry, the entire ingot is further processed into solar wafers after the separation of an edge region, which inevitably results from the standardized crucible size, and the separation of a lid and bottom region.

SUMMARY OF THE INVENTION

The invention aims to provide a mono- or quasi-monocrystalline silicon material for low-resistance mono- or quasi-monocrystalline functional components for use in semiconductor processing equipment, which is much less expensive to produce than monocrystalline material originating from the CZ or FZ process and that only such technical requirements for these functional components such. As low-impedance sputtering or low-impedance showerheads met, which are actually mandatory for the application.

This object is achieved by a blank (blank) according to claim 1, by a use according to claim 12 and by a method according to claim 16. Further advantageous embodiments are the subject of the dependent claims.

Examples of functional components in the context of the present application are in particular sputtering targets and showerheads. Showerheads like you B. are used in plasma etching, are synonymous often referred to as electrode plates or shower head electrodes or gas distribution plates.

The term "blank", as used in the present application, refers to a blank or a semi-finished product which is produced from a crude crystal produced after a cultivation in order to be able to evaluate the material quality of this blank on its surface and / or in its interior , A blank deemed usable for the intended application undergoes further processing after its evaluation to adjust the final geometry and surface quality of the component.

The mono- or quasi-monocrystalline silicon crystal material according to the invention for functional components is only as good as really necessary and can thus be produced more cost-effectively than conventionally. In other words, according to the invention, cost advantages can be realized in that only the properties absolutely necessary for a satisfactory functional component are maintained. It has surprisingly been found that the degree of manifestation of certain defects and certain defects themselves, which may or may not occur in semiconductor silicon usable material, do not or do not significantly affect the performance of functional components for use in semiconductor processing equipment. For functional components of a size which can not be produced or are only extremely expensive to produce using the conventional crystallization methods for monocrystalline silicon (CZ, FZ), it is even possible for the first time to monolithically produce these from the comparatively more defect-resistant monocrystalline or quasi-monocrystalline silicon according to the present invention. Until now, these large functional components either had to be manufactured as a whole part from multicrystalline material or assembled from monocrystalline or multicrystalline material segments. However, functional components made of multicrystalline material have far shorter service lives. Segmented functional components have a high production cost.

Surprisingly, it has also been found by the inventors that planar determination of carrier lifetime (or physical quantity correlating therewith) with silicon and other semiconductor materials such as germanium, gallium arsenide and other so-called compound semiconductors is a very simple, fast and nondestructive method to improve tool life To roughly evaluate the wear (removal of material by etching gas or generation of interfering particles) of the functional components under operating conditions. Areas with only isolated dislocations can thus be distinguished from those with dislocation clusters (critical in certain electrical applications). The measuring technique, actually developed for evaluating the electrical quality of solar silicon or for determining the achievable efficiency of solar cells, can be used with the method described here for evaluating the structural quality or chemical stability of mono or quasi-monocrystalline silicon. Thus, simple, inexpensive and quick to perform evaluation procedures are available.

In this case, the surface of the workpiece to be examined is investigated using, for example, the following methods: μ-PCD (Microwave-Detected Photo-Conductance Decay Measurement), MWT (Microwave Detected Photoconductivity), PL (Photoluminescence), or similar scanning or imaging measurement techniques.

All these measurement techniques determine a value which is proportional to the charge carrier lifetime in the area of the surface. Here, the spatial resolution of the respective measurement techniques is different; it can range from a few to a few mm. In the vicinity of closely spaced dislocation lines - ie in the region of dislocation clusters - the carrier lifetime decreases greatly, since the dislocation lines or small angle grain boundaries represent places of very high charge carrier recombination and the distances of the dislocation lines go below the diffusion length of the charge carriers. The only condition for the material is that no other recombination mechanisms are allowed to superimpose the recombination on the dislocation lines. That is, a massive contamination z. B. with evenly distributed in volume metallic impurities prevents this type of detection of dislocation clusters. In particular, transition metals such as Fe, Cr, Co, Ni, Ti and the like should not exceed a concentration of 0.1 ppm.

A known grain-based method of directional solidification of silicon for the production of quasi-monocrystalline Si ingots for the photovoltaic industry is modified according to the present invention and supplemented in particular by an allocation and evaluation process such that a volume fraction of the resulting ingot is defined and evaluated so that it the production of low-resistance functional components for use in semiconductor processing equipment, eg. B. sputtering targets or showerheads with a resistivity in the range of 0.001 Ωcm to 0.2 Ωcm can be used.

However, the allotment and evaluation method can be used expressly for differently manufactured silicon crystal material. These include silicon material, which was produced by the known Czochralski method and contains unwanted twins and thus can not be used for semiconductor wafers, or silicon material, which is produced by a method based on the Kyropoulos method for growing silicon without crucible contact (see. Nakajima et al., J. Cryst. Growth 372 (2013) 121-128 ,

• – Herstellen eines mono- oder quasimonokristallinen Ingots durch gerichtete Erstarrung in einem Schmelztiegel;
• – Bodenschnitt, Mantelschnitt und Deckelschnitt am Ingot nach dessen Abkühlung und Entformung aus dem Schmelztiegel, um einen Ingotkern zu erhalten;
• – Festlegen einer monokristallinen Prüfoberfläche auf der Oberfläche des Ingotkerns oder einer von diesem abgetrennten Scheibe;
• – Prüfung und Bewertung der Prüfoberfläche hinsichtlich Gehalt, Verteilung und/oder Qualitätsrelevanz von zumindest einer der Größen: Fremdkörnern und Zwillingsgrenzen, Cluster von Versetzungen oder Kleinwinkelkorngrenzen, durch Anfertigung eines Scans der Prüfoberfläche und flächenmäßiger Bestimmung der Ladungsträgerlebensdauer oder einer daraus abgeleiteten physikalischen Größe zur Identifizierung von zumindest einem potentiellen Blank; und
• – Heraustrennen des Blanks, so dass das herausgetrennte Blank in Bereichen auf seiner Ober- oder Unterseite, die keine Cluster von Versetzungen oder Kleinwinkelkorngrenzen enthalten, eine Konzentration von Einzelversetzungen im Bereich von größer 1 × 10² bis kleiner 1 × 10⁶ cm^–2 aufweist und wobei das herausgetrennte Blank einen spezifischen Widerstand von 0,001 bis 0,2 Ωcm aufweist.

In the case of using a germ-based culture method in a crucible according to the method of directional solidification for the production according to the invention of mono or quasi-monocrystalline silicon in a Bridgman-type cultivating plant or a Vertical Gradient Freeze (VGF) plant, the following process steps are carried out in principle:

• - Producing a mono- or quasi-monocrystalline ingot by directional solidification in a crucible;
• - Bottom section, mantle cut and lid cut on the ingot after its cooling and demolding from the crucible to obtain a Ingotkern;
• - determining a monocrystalline test surface on the surface of the ingot core or a slice separated therefrom;
• Testing and evaluation of the test surface with regard to content, distribution and / or quality relevance of at least one of the following: foreign and twin boundaries, clusters of dislocations or small angle grain boundaries, by making a scan of the test surface and areal determination of the carrier lifetime or a physical quantity derived therefrom to identify at least one potential blank; and
• Separating out the blank such that the blank removed in areas on its top or bottom containing no clusters of dislocations or small angle grain boundaries has a concentration of discrete dislocations in the range of greater than 1 × 10 ² to less than 1 × 10 ⁶ cm ^-2 and wherein the separated blank has a resistivity of 0.001 to 0.2 Ωcm.

This process consists of further detailed steps:
It is the crucible bottom of a crucible with one or more mono- or quasi-monocrystalline occupied, preferably with heavily doped silicon nuclei.

The crucible is then further filled with silicon raw material with the addition of substantially more dopant than required for the production of solar cells, ie, for example, for the above-mentioned range of resistivity of 0.001 Ωcm to 0.2 Ωcm and the use of one or more already suitable doped nuclei, a large amount of dopant is added, resulting in an initial concentration in the melt of about 1.2 × 10 ²⁰ atoms / cm ³ to 1.0 × 10 ¹⁷ atoms / cm ^{3 of} the dopant boron (B) or ca 7.4 × 10 ¹⁹ atoms / cm ³ to 3.1 × 10 ¹⁶ atoms / cm ^{3 of} the dopant phosphorus (P).

For directed solidification, the filled crucible is placed in a technically modified system for the directional solidification of silicon, in particular a VGF plant.

This is followed by melting of the raw materials, melting of the germ or germs without keying the phase boundary, followed by a directional solidification of the liquid silicon to form an ingot.

After cooling, removal and demolding of the ingot, the cutting of the ingot soil from the ingot can be followed for preferential reuse as seed material. The quality of the cut makes it possible, if necessary, to visually recognize multicrystalline edge regions on both opposite cut surfaces and, if appropriate, separate them from the germplate. Subsequently, the cutting off of the cladding layer from the ingot can take place in a predetermined thickness or a thickness which is visually recognizable as non-monocrystalline at the cut surface. Subsequently, a division of the remaining ingot into slices can be made according to the desired thickness to blanks for functional components such. B to make sputtering targets or showerheads. Possibly. For example, a surface treatment of the wafer surfaces may be carried out to better ensure the visual distinctness of multicrystalline and quasi-monocrystalline regions of the processed surfaces and to mark the quasi-monocrystalline surface region. Subsequently, the marked quasi-monocrystalline surface region (test surface) on the disk or on the ingot can be checked for the presence and the location of detected foreign grains or detected twin boundaries.

Examination of a selected quasi-monocrystalline surface area (test surface) of the wafer can be made by one of three preferred electrical methods for quantifying the content of this test surface to clusters of dislocations or small angle grain boundaries. In the process, it is detected whether and at which location on the test surface pixels with clusters of dislocations or small-angle grain boundaries are present. Subsequently, a determination is made of the percentage of clusters of dislocations or small-angle grain boundaries of adhered pixels on the test surface, as well as a specification of one or more new test surfaces of the disk, in terms of their content with respect to the percentage of clusters of dislocations or small angle grain boundaries Foreign grains and, in terms of their content of twin boundaries, the geometry and specification requirements for one or more blanks. Subsequently, a separation of the new test surface can be made, which is identical to the surface of the blank.

Mono- or quasi-monocrystalline material according to this invention and regardless of whether it was prepared by a method of directional solidification, with a CZ or FZ process or a method based on the Kyropoulos method for silicon without crucible contact, contains a mean concentration of isolated dislocations between 10 ² cm ^-2 and 10 ⁶ cm ^-2 , typically between 10 ³ cm ^-2 and 10 ⁵ cm ^-2 . Herein, the mono- or quasi-monocrystalline silicon material according to the invention differs from monocrystalline silicon, produced e.g. B. with the Czochralski or the floating zone method with the intended use as a semiconductor wafer for the production of microelectronic devices. This use requires a concentration of isolated dislocations of usually below 10 ² cm ^-2 and is typically even wholly or nearly free of dislocations. However, monocrystalline silicon from the Czochralski or the floating zone process can also have very high concentrations of dislocations if disruptions occur in the crystallization process. However, such material is atypical and, above all, because of the then very high mechanical stresses also difficult to work (Czochralski and floating zone crystals with locally high dislocation densities tear easily).

In contrast to the method according to the invention, the microbes used in the methods according to the prior art for the production of p-doped quasi-monocrystalline solar cells have a comparatively very high resistance (typically 1 Ωcm to 3 Ωcm for B doping). In contrast, for the production of low-resistance functional components according to the present invention, it is possible to use germs which have a lower, or even significantly lower resistance.

Also, in the prior art method, the silicon raw material disposed above the seed (s) is additionally added with a smaller amount of boron dopant than in the present invention. According to the prior art, an increased resistance range of 1 Ωcm to 3 Ωcm is set.

FIGURE OVERVIEW

In more detail, the aforementioned process steps using the directional solidification method for producing the mono- or quasi-monocrystalline silicon will be described below with reference to the accompanying drawings. The following descriptions and figures relate to the VGF process as one of the methods of directional solidification. Silicon material produced by this process is referred to as VGF mono-silicon. Show it:

1 a vertical section through a generation 4 Si ingot, which is prepared by a crystallization process according to the present invention, with exemplary distribution of the material in monocrystalline and multicrystalline and a possible fragmentation of, for example, horizontally herausschneidbarer slices for obtaining blanks for functional components, eg. For sputtering targets or showerheads;

2 a further vertical section through a size 5 Si ingot, which is prepared by a crystallization process according to the present invention, with exemplary distribution of the material in monocrystalline and multicrystalline and a possible fragmentation, for example, horizontally herausschneidbarer slices to obtain blanks for functional components, eg. For sputtering targets or showerheads;

3a and 3b a photograph of a cut out of such an ingot at mid-height slice with monocrystalline region A and multicrystalline region B exemplified blanks for z. Sputtering targets or showerheads;

4a and 4b PL (left) and μPCD images (right) of a rough-cut wafer surface of monocrystalline dislocation-free silicon material produced by a CZ process (ring-shaped structures are visible);

4c and 4d corresponding PL (left) and μPCD images (right) of a rough-sawn wafer surface of dislocation monocrystalline silicon material prepared by a VGF process according to the present invention (no ring-shaped structures visible, no clusters of dislocations, no foreign grains or twin boundaries );

5a and 5b corresponding PL and μPCD images of a rough-cut wafer surface of monocrystalline silicon material originating from a bottom-near region of an ingot made by a VGF process according to the present invention, wherein clusters of dislocations or small-angle grain boundaries are visible;

6a and 6b corresponding PL and μPCD images of a rough-cut wafer surface of monocrystalline silicon material originating from a capped region of an ingot made by a VGF process according to the present invention, wherein clusters of higher surface area dislocations or small-angle grain boundaries than those of US Pat 5a and 5b are visible;

7 a PL image of a VGF monocrystalline test surface from area A of an ingot.

DETAILED DESCRIPTION

As a crucible according to the present invention, in principle, a ready-to-use crucible coated inside by the crucible manufacturer or a crucible coated by the user himself (eg a quartz or fused quartz crucible) or a graphite crucible or a Si ₃ N ₄ crucible may be used become. The crucible base depends on the size of the functional components to be produced therein. It can have square crucibles of size G4 (720 mm × 720 mm), G5 (880 mm × 880 mm), G6 (1050 mm × 1050 mm), intermediate sizes between G4, G5 and G6, or even larger crucibles currently not in use to 3000 mm × 3000 mm, round crucibles with a diameter greater than 450 mm or even rectangular crucibles with a minimum base surface edge length of 450 mm or a maximum base edge length of 3000 mm can be used. A crucible height below 250 mm is just as meaningless as a crucible height (possibly including crucible attachment) of greater than 800 mm. Total crucible heights of 450 mm, 550 mm, 650 mm, 780 mm or sizes in between have proved to be very practical.

As a germ, a surface germ filling almost the entire inner surface of the crucible is particularly useful. This can be obtained by a bottom cut from a previous ingot. For reasons of effectiveness, the germ should not be unnecessarily thick, but also not too thin, so that there is no risk of it completely melting in the later germination phase. Seed thicknesses of 15 mm, 30 mm, 45 mm, 50 mm, 70 mm, 80 mm or intermediate sizes have proven useful, with 40 mm or 45 mm being preferred.

For the production of low-resistance functional components, the orientation of the germs is not always important. The desired (100) orientation of the germs in photovoltaics is often desired for the production of showerheads, not necessarily for sputtering targets, but for breeding-technical considerations out the preferred germ orientation. Depending on the objective, however, it can also be bred based on (111) - or (110) - or still differently oriented germs.

However, the above-mentioned and preferred area germ does not limit the potential for germination. If the bottom of the crucible is to be laid out over smaller areas or over a small area, it is advisable to allow the individual germs to collide with each other without any joints, and none at all To allow gaps. Germs can be made square out of parts of a surface germ, a round crystal made by a CZ method, in the above seed thickness. Germs may also be obtained from longitudinal sections of the desired above-mentioned thickness from a crystal made by a CZ method. Germs can also be cut out vertically from a previously ingame-based directionally solidified ingot as vertical boards or ingots in the desired thickness mentioned above.

It is advantageous but not compulsory for all types of seed production for use in the cultivation of low-resistance ingots that the seed already has a high doping which fits the desired range of the specific resistance of 0.001 Ωcm to 0.2 Ωcm. Also, the germ must not be contaminated by foreign atoms, especially metallic impurities, otherwise the purity typically required for functional components can not be guaranteed (eg purity 6 N for sputtering targets).

After placement of the seeds, the remaining crucible volume is filled with virgin or recycled Si raw material (at least in the purity of solar grade) and dopant. When filling the crucible or provided with a crucible top crucible, the resulting total height is used as possible. A filling height of less than 250 mm is just as meaningless as a filling height (possibly including crucible attachment) of more than 800 mm. Total crucible heights of 350 mm, 450 mm, 550 mm, 650 mm and 780 mm or intermediate sizes have proved to be very practical.

To get into the desired range of resistance from 0.001 Ωcm to 0.2 Ωcm, z. For example, for a specific resistance of 0.02 ohm-cm, as much boron as dopant is added, such that an initial concentration of 3.25 × 10 ¹⁸ atoms / cm ³ in the melt or for a specific resistance of 0.005 ohm-cm boron for one concentration of 2.01 × 10 ¹⁹ atoms / cm ^{3 are} added, whereby other dopants such. B. with Al or P are possible (with adapted for each dopant concentrations).

The filled crucible is then placed in a Bridgman-type crystal growing facility or in a VGF facility. The latter is equipped with different heater configurations depending on the system type. Commonly used are systems with only ceiling heaters, systems with ceiling and floor heaters, systems only with jacket heaters, systems with ceiling and jacket heaters or systems with ceiling, jacket and bottom heaters. According to the invention, the raw material is melted from above. By a suitable temperature control of the heater with simultaneous cooling of the seed layer is ensured both that the added raw material is completely melted, the germ or the germs are not completely melted, but necessarily melted.

In order to be able to represent the melting of the seed layer in an understandable way, each individual area fraction of the seed layer is defined with a size of about 1 cm ² . Each of these individual areas should be melted to a defined height above the crucible bottom. This height coordinate, up to which the melting of each area fraction takes place, is in the range of 20% to 90% of the thickness of the inserted seed layer. Depending on the size of the pot, the type of plant and the seed thickness, areas are found for the desired height coordinate, which are preferred. For example, with a seed thickness of 40 mm, an area of the height coordinate of 35% to 75% of the original seed thickness or, with a seed thickness of 45 mm, a range of the height coordinate of 30% to 80% of the original seed thickness is preferred. By means of a suitable temperature / time control of the heaters, it is achieved that a horizontal phase boundary forms in the flatness in the interior of the crucible, which makes it possible to melt the seed layer without a key for determining the position of the phase boundary. Advantageously, the measurement of the temperature is not made in the center of symmetry on the underside of the plate, which is referred to as a crucible mounting plate. The measurement is instead carried out at a location further out, which is located near the edge of the crucible and provides representative and accurate measurement results by this layer in order to prevent a complete melting of the seed layer near the crucible wall. This danger is due to the fact that the system tends to form heat flows in the direction of the center by means of germ cooling, so that the temperature in the seed layer center is colder than at the edge of the seed layer. Important parameters for counteracting this are the cooling capacity and the geometry of the cooling arrangement. The cooling capacity must be equal to the heater temperatures be adapted that sets in particular during Ankeimprozess a flat phase boundary. The geometry, or rather the cooled surface, must be approximately equal to the area of the crucible. The position of the temperature measuring point must be selected so that it detects the edge area under the germ, but is not distorted by the cooling.

When the melting of the seed layer is completed, by further increasing the heat dissipation through the crucible mounting plate, further melting of the seed is stopped and directional solidification is initiated.

• – mit der gerichteten Erstarrung von unten nach oben
• – mit Keimstößen die überlappen und nicht senkrecht zur Phasengrenze stehen, um die Zwillingshäufigkeit zu senken
• – mit einem thermischen Regime, welches jegliche Sprünge in der Temperatur-, Druck- und Positionsregelung vermeidet, um die Zwillingshäufigkeit zu senken
• – mit einer konvexen Phasengrenze, welche das Einwachsen von Fremdkeimen, gebildet an der Tiegelwand, in die Ingotmitte verhindert
• – mit einer konvexen Phasengrenze deren Durchbiegung gering genug ist, die durch diese Durchbiegung induzierte thermische Spannung im Kristall geringer als die kritische Schubspannung von 1,6 MPa zu halten, um keine Versetzungsvervielfachung zu erzeugen.

As shown in the exemplary embodiments, the melting of the seed layer takes place without mechanically touching the position of the phase boundary. Instead, the procedure is such that the temperature of the underside of the crucible mounting plate is measured at a defined distance from the center of symmetry of the crucible bottom, specifically at the periphery of the crucible mounting plate of good heat-conducting graphite, by means of a pyrometer or thermocouple. The heaters and the cooling are controlled in such a way that a temperature minimum develops at this measuring point over time, which is absolutely determined and stored in each process. If the temperature measured at this point has risen again after passing through the minimum by a temperature difference of 5 K to 25 K above the minimum, the crystallization process, ie the directed solidification, is initiated by increased heat dissipation through the crucible mounting plate. It is advantageous to make the phase boundary solid-liquid convex in the crystallization phase, in order to curb the propagation of crystal grains of other orientation, which always occur due to foreign nucleation on the crucible wall in the direction of ingot center. At the same time, the convex deflection of the phase boundary must not be too large in order not to excite the dislocation formation caused by thermal stress. The crystallization of an ingot takes place according to the invention:

• - with the directed solidification from bottom to top
• - with seed bumps that overlap and are not perpendicular to the phase boundary to reduce the twin frequency
• - With a thermal regime, which avoids any jumps in the temperature, pressure and position control to reduce the twin frequency
• - With a convex phase boundary, which prevents the ingrowth of foreign nuclei formed on the crucible wall, in the Ingotmitte
• - With a convex phase boundary whose deflection is low enough to keep the induced by this deflection thermal stress in the crystal less than the critical shear stress of 1.6 MPa, so as not to produce dislocation multiplication.

After the finished cultured ingot has been cooled to a temperature close to room temperature according to a standard program, it is taken out of the crystal growing plant together with the crucible and demoulded. If one cuts a round ingot along the axis of symmetry or a cuboid ingot along a parallel to an edge extending and extending through the center of symmetry of the ingot line virtually, we obtain a fictional sectional surface, is illustrated at where the unmelted seed layer is located and where multi- and monocrystalline regions are present, as they are for the production of functional components, such. B. sputtering targets or showerheads, with in the 1 and 2 exemplified sizes or dimensions are suitable. The 1 and 2 show how such a ingot z. B. can be cut horizontally to win blanks for various end products. Photographs of such a horizontal section show the 3a and 3b , which will be explained in more detail below.

Such a basic cross-section of a generation 4 ingot (630 mm × 630 mm base and height 430 mm) is shown in FIG 1 and a generation 5 ingot (780 mm × 780 mm base and height 430 mm) shows the 2 , In both figures, areas are marked which can be used by way of example for two frequently required dimensions of sputtering targets (⌀ 450 mm × 10 mm or 320 mm × 100 mm × 8 mm). Basically, there are a variety of other products with different dimensions; these possibilities are in the 1 and 2 exemplified as a blank.

• a) eine Bodenschicht abgesägt, was meist mit einer Bandsäge erfolgt. An den sägerauen Schnittflächen ist unter geeignetem Lichteinfall und verschiedenen Betrachtungsrichtungen visuell genau erkennbar, wo der monokristalline Zentrumsbereich endet und ein multikristalliner Randbereich beginnt. Nachdem der Abstand des vertikalen Schnittes von jedem Ingotrand, der z. B zwischen 2 und 10 cm betragen kann, konkret definiert ist, erfolgt
• b) das Absägen der vier Seitenflächen bzw. einer Mantelschicht.

The further processing of an ingot takes place in the procedure described below by way of example:
First is

• a) sawed off a bottom layer, which usually takes place with a band saw. At the sawn cut surfaces it is possible to see visually exactly under suitable incidence of light and different viewing directions, where the monocrystalline center region ends and a multicrystalline edge region begins. After the distance of the vertical section of each Ingotrand, z. B can be between 2 and 10 cm, is concretely defined takes place
• b) the sawing of the four side surfaces or a cladding layer.

Alternatively, the four side surfaces or the cladding layer can also be sawed off in a predefined thickness first. What thickness is necessary, results from experience, d. H. from previous processes, or from whether the crucible bottom was laid over the entire surface or with the omission of a narrow edge area with germs. The soil layer to be sawn off after this step is slightly smaller in this procedure than in the procedure according to a). Thus, in the case of reuse of the soil layer as a germ in the subsequent process of this does not fill the crucible bottom to 100%.

As a next step, horizontal slices are cut out of the remaining ingot volume, which have a thickness that is still an allowance for the further processing steps for the production of z. B. sputtering targets, showerheads or other blanks possess. Typical final thicknesses for sputtering targets are 1 mm to 50 mm, with 3 mm to 6 mm or 8 mm to 10 mm being the most common final thickness ranges.

The determination of the specific resistance is expediently carried out in two steps: First, the resistance at the inner cut surface of a side surface along a vertical line is determined pointwise. This is to check at which height coordinates the ingot reaches the required resistivity for one or more types of final products. After cutting out horizontal slices from the ingot, specimens cut adjacent to the area selected for the final product can additionally determine the resistivity in higher accuracy in the form of a surface scan across the specimen. However, it is also possible to perform a planar raster measurement of the specific resistance over the entire interesting surface area of the slice, in order to specify the mean value and also the fluctuation range of the measured values within the slice. Similar to the latter case, this is also possible directly on the blank to be delivered. In most cases, this effort is not required.

Essentially, the methods or devices that can be used for the measurement differ only in what range of resistance is expected and whether it is necessary to record the measured values over a wide area. So the measurement z. B. offline and punctiform or scanning in the range up to 30 Ωcm done with an eddy current measuring method (eg., With the RT 100 from Semilab / Hungary) or pointwise or raster with a 4-tip measuring method done (z. Example of the Automatic Four-Point Probe, Model 280SI Series from the company Four Dimensions Inc./Hayward, CA, USA for the measurement of previously defined suitable sample geometries or the 4PP system for ingots of the same company).

The 3a shows a photograph of the top view of a cut from the ingot slice in middle ingot height. Although in this case the side surfaces have already been separated, a multicrystalline region (B) can be seen, which has grown in from the edge in the direction of the monocrystalline center (A) and is separated in this image by a hand-drawn line from the monocrystalline Gutbereich. In the monocrystalline center (A) three round blanks of different diameters are exemplarily drawn.

The 3b shows the same disc, in which a contour is drawn, which corresponds to a manufactured from a single piece Racetrack sputtering target of the maximum external dimensions about 700 mm × 123 mm. The clear distance between the two mutually parallel legs is about 42 mm. It is noteworthy that in the case of a production, this entire contour from a CZ ingot previously had to be made from two halves. For the first time, production from a VGF mono-silicon material allows one-piece production or, as in the past, two halves.

In the 3a and 3b Plotted sample is representative of various sample geometries and sample positions. This can be used for the above resistance measurements, transmission measurements, FTIR measurements or for contamination determination (AAS, GDMS, ICP-MS, ...).

Within the scope of the monocrystalline good range, which is considered from a visual point of view, an attempt is now being made to utilize the largest possible volume for end products. For this purpose, the monocrystalline Gutbereich must be examined in more detail, if not the location and especially local strong concentrations of clusters dispose of dislocations and small angle grain boundaries, foreign or twin boundaries, certain areas for the intended use. As a result of these tests described below, the size and position of the blanks obtainable from a disk must be finally determined.

Such slices - especially their monocrystalline region - are now assessed for content, distribution and quality relevance of a first group of crystal defects known as "dislocation clusters", "dislocation agglomerations", "small angle grain clusters" clusters or the like. This group has been referred to in the preceding text and is referred to in the following text as "clusters of dislocations or small angle grain boundaries" or simply as dislocation clusters or clusters. The assessment of this group of crystal defects is based on the following relationships and should be described as follows:
VGF mono-silicon material resulting from directional solidification contains an average concentration of isolated dislocations greater than 1 x 10 ² cm ^-2 and less than 1 x 10 ⁶ cm ^-2 , typically between 10 ³ cm ^-2 and 10 ⁵ cm ^-2 . Here, VGF mono-silicon differs from the ideal monocrystalline silicon made e.g. B. with the Czochralski or the floating zone method. Silicon produced by the latter method is usually below 10 ² cm ^-2 in displacement ^content and is typically even wholly or almost free from dislocations. However, monocrystalline silicon from the Czochralski or the floating zone process can also have very high concentrations of dislocations due to dislocation multiplication if disruptions occur in the crystallization process. Such material is untypical. It does not meet the specification requirements of the applications of material produced by this process, is typically recycled, and is also difficult to process because of the high mechanical stresses involved (Czochralski and floating zone crystals with locally high dislocation densities are easily broken).

As long as the dislocations are isolated, d. H. unless they are partially aligned to dislocation lines, small angle grain boundaries, and clusters thereof, the local orientation of the crystal lattice is not macroscopically affected by the presence of the dislocations. The optical appearance of a machined, ground or polished workpiece surface of monocrystalline and quasi-monocrystalline silicon workpieces of different dislocation concentration does not differ. Transmission and reflection behavior are identical.

This is different if dislocations during the crystallization or even during the cooling to arrange dislocation lines and accumulations of dislocation lines, the aforementioned dislocation clusters. The linear dislocations may also extend to small angle grain boundaries. The latter then enclose crystal regions which, compared to the rest of the matrix of the workpiece, can have a significant tilt of a few arc minutes to, in the extreme case, a few degrees, so-called subgrains. Typically, such dislocated small and minute subgrains are adjacent in dislocation clusters of this type, with distances and dimensions ranging from μm to cm.

Areas of greater tilt (sub-grains with a tilt angle of greater than about 20 °) are comparable in appearance to foreign grains and can be visually recognized with unaided eye and under good lighting on the mechanically machined surface, if they are sufficiently large. They represent a second group of crystal defects in VGF mono-silicon. Also the third group of crystal defects in VGF mono-silicon, the twin boundaries, are visually recognizable like the second group.

It is important in visual detection that the angle of incidence of the illumination to the surface to be assessed covers the angle range from 10 ° to 75 ° and the light source rotates azimuthally in 10 ° increments over 360 °. The viewing direction of the area to be assessed by the appraiser must be from the direction of incidence and the same incidence angle range from 10 ° to 75 °. In addition to the viewing direction of exactly opposite the direction of incidence of the light source, an azimuthal viewing angle range of -90 ° to + 90 ° to the viewing direction must be covered. Sanded surfaces are suitable for this, better are sandblasted surfaces. However, it has been found that a roughly ground surface quality typically achieved by wire-cutting lapping or on a band saw is also sufficient for the evaluation.

However, areas of greater tilting can also be very small and thus avoid visual recognition. These very small defects are not included in the corresponding material classes defined below.

These small undetectable and the larger detectable foreign grains, twin boundaries or areas of greater tilt are always in a monocrystalline matrix.

Monocrystalline matrix is defined as a monocrystalline region that embeds a foreign metal all around.

Klasse FK1:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material keine Fremdkörner aufweist. Bestimmungskriterium: Sichtbarkeit in der Prüfoberfläche. Die Prüfoberfläche ist entweder die Scheibenoberfläche oder der Blankoberfläche, die an Finalbauteilen geringer Dicke in Richtung der Belastung bzw. des chemischen Angriffs zeigt (funktionale Fläche). Geringe Dicke bedeutet in diesem Zusammenhang, dass das als Platte vorliegende Blank bzw. die aus dem Ingot herausgeschnittene Scheibe eine Dicke von mindestens 5 mal geringer als seine größte Flächendimension hat.

Klasse FK2:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material maximal 1 Fremdkorn je dm² in einer Größe kleiner als 50 mm² aufweist. Bestimmungskriterium: Sichtbarkeit in der Prüfoberfläche. Die Prüfoberfläche ist entweder die Scheibenoberfläche oder der Blankoberfläche, die am Finalbauteil in Richtung der Belastung bzw. des chemischen Angriffs zeigt (funktionale Fläche). Die am Finalbauteil vorliegende Richtung der Belastung bzw. des chemischen Angriffs bedeutet die Seite des Bauteils, die der Belastung ausgesetzt ist wie z. B. die Fläche, die sich in der Nähe eines Plasmas befindet oder die reaktiven Gasen ausgesetzt ist. Die Größe „Anzahl Fremdkorn je dm²” bedeutet die Flächendichte von Fremdkörnern auf der bewerteten Oberfläche.

Klasse FK3:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material maximal 2 Fremdkörner je dm² in einer Größe kleiner als 50 mm² aufweist, Bestimmungskriterium: Sichtbarkeit in der Prüfoberfläche. Die Prüfoberfläche ist entweder die Scheibenoberfläche oder in einer der Bewertung zugänglichen Blankoberfläche. Die einer Bewertung zugängliche Blankoberfläche ist die Außenfläche eines Blanks, durch die geeignete Messverfahren Aufschluss geben über eine für das Volumen repräsentative Oberflächenqualität bzw. die nach der Finalbearbeitung erreichbare Oberflächenqualität.

Klasse FK4:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material maximal 5 Fremdkörner je dm² in einer Größe kleiner als 50 mm² aufweist. Bestimmungskriterium: Sichtbarkeit in der Prüfoberfläche. Die Prüfoberfläche ist entweder die Scheibenoberfläche oder in einer der Bewertung zugänglichen Blankoberfläche.

Klasse FK5:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material Fremdkörner ohne zahlenmäßige Begrenzung jedoch mit einer Größe von je kleiner als 50 mm² aufweist. Bestimmungskriterium: Sichtbarkeit in der Prüfoberfläche. Die Prüfoberfläche ist entweder die Scheibenoberfläche oder in einer der Bewertung zugänglichen Blankoberfläche.

As a result of the visual evaluation, the material classification for foreign grains takes place in 5 classes:

Class FK1:

Single-dislocation silicon material in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small-angle grain boundary-free volume having no foreign grains on a cut surface through the material. Determination criterion: Visibility in the test surface. The test surface is either the disk surface or the blank surface, which points to final components of small thickness in the direction of stress or chemical attack (functional surface). In this context, a small thickness means that the blank or the slice cut out of the ingot has a thickness of at least 5 times less than its largest surface dimension.

Class FK2:

Silicon material with individual dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Versatzungscluster- or kleinwinkelkorngrenzenfreien volume on a cut surface through the material a maximum of 1 foreign particles per dm ² in one Size smaller than 50 mm ² has. Determination criterion: Visibility in the test surface. The test surface is either the disk surface or the blank surface, which points to the final component in the direction of stress or chemical attack (functional surface). The direction of loading or chemical attack present at the final component means the side of the component which is exposed to the load, such as a For example, the area that is near a plasma or exposed to reactive gases. The quantity "number of foreign particles per dm ² " means the area density of foreign grains on the assessed surface.

Class FK3:

Silicon material with discrete dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Versatzungscluster- or kleinwinkelkorngrenzenfreien volume on a sectional surface through the material a maximum of 2 foreign grains per dm ² in one Size smaller than 50 mm ² , Determination criterion: Visibility in the test surface. The test surface is either the disk surface or blank surface accessible to the evaluation. The blank surface accessible for evaluation is the outer surface of a blank, through which suitable measuring methods give information about a surface quality representative of the volume or the surface quality that can be achieved after final processing.

Class FK4:

Silicon material with individual dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Versatzungscluster- or kleinwinkelkorngrenzenfreien volume on a sectional surface through the material a maximum of 5 foreign grains per dm ² in one Size smaller than 50 mm ² has. Determination criterion: Visibility in the test surface. The test surface is either the disk surface or blank surface accessible to the evaluation.

Class FK5:

Silicon material with discrete dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Verklungscluster- or kleinwinkelkorngrenzenfreien volume on a cut surface by the material foreign grains without numerical limitation, but with a size of less than 50 mm ² each. Determination criterion: Visibility in the test surface. The test surface is either the disk surface or blank surface accessible to the evaluation.

The method described above for the determination of foreign grains on the respective test surface is also suitable for the determination of twin limits. A twin boundary is defined as a coherent grain boundary, preferably as a Σ3 grain boundary.

Klasse Z1:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material keine Zwillingsgrenzen aufweist

Klasse Z2:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material maximal 4 Stück Zwillingsgrenzen einer Gesamtlänge von maximal. 0,5 m pro dm² aufweist.

Klasse Z3:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material maximal 10 Stück Zwillingsgrenzen mit einer Gesamtänge von 1,4 m pro dm² aufweist.

Klasse Z4:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material maximal 100 Stück Zwillingsgrenzen mit einer Gesamtlänge von 14 m pro dm² aufweist.

Klasse Z5:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material mehr als 100 Stück Zwillingsgrenzen pro dm² aufweist.

As a result of the visual evaluation, the material classification with respect to twin boundaries takes place in 5 classes:
The above-mentioned determination criteria for foreign grains in each class 1-5 apply analogously to twin limits of classes 1-5.

Class Z1:

Single dislocation silicon material in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small-angle grain boundary-free volume having no twin boundaries on a cut surface through the material

Class Z2:

Silicon material with discrete dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Versatzungscluster- or small angle grain boundary volume, the maximum on a cut surface by the material maximum 4 twin boundaries of a total length of maximum , 0.5 m per dm ² .

Class Z3:

Silicon material with discrete dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Verklungscluster- or kleinwinkelkorngrenzenfreien volume on a cut surface through the material a maximum of 10 pieces of twin boundaries with a total length of 1.4 m per dm ² .

Class Z4:

Silicon material with discrete dislocations in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, Verklungscluster- or kleinwinkelkorngrenzenfreien volume on a cut surface through the material a maximum of 100 pieces of twin boundaries with a total length of 14 m per dm ² .

Class Z5:

Single dislocation silicon material in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small-angle grain boundary-free volume that has more than 100 twins boundaries per dm ² on a cut surface through the material having.

Visually not at all conspicuous are all dislocation clusters with sub-grains or areas of only slight tilt, regardless of their size. However, they represent structural inhomogeneities, which affect their function in reflective or transmissive optical components at too high a concentration. Also in other VGF mono-silcium material manufacturable products, such as sputtering targets, showerheads, focus rings, and other components needed in the semiconductor industry, these clusters may be undesirable and are to be judged in terms of their area fraction of the overall surface of the device. It is therefore necessary to detect the content of such clusters of dislocations or small-angle grain boundaries already on the partially machined workpiece or blank in order to be able to carry out a suitable allocation on the basis of this detection.

Since direct, visual detection of the clusters of dislocations or small-angle grain boundaries within the VGF monocrystalline crystal volume is not certainly possible as described, the detection must be done indirectly. Surprisingly, it has been found that a planar determination of the charge carrier lifetime (or a physical quantity correlating therewith) with silicon and other semiconductor materials such as germanium, gallium arsenide or other so-called compound semiconductors is a very simple, fast and non-destructive method, from which a predicted lifetime or the wear (Material removal by Ätzgaseinwirkung or generation of interfering particles) of the functional components under operating conditions rough estimate.

Areas with only isolated dislocations can thus be distinguished from those with dislocation clusters (critical in certain electrical applications). The measurement technique, actually developed for assessing the electrical quality of solar silicon or for determining the achievable efficiency of solar cells, can be used with the method described here for assessing the structural quality of mono or quasi-monocrystalline silicon. Thus, simple, inexpensive and quick to perform evaluation procedures are available.

In this case, the surface of the workpiece to be examined is investigated using, for example, the following methods: μ-PCD (Microwave-Detected Photo-Conductance Decay Measurement), MWT (Microwave Detected Photoconductivity), PL (Photoluminescence), or similar scanning or imaging measurement techniques. For all these measuring methods, there are already manufacturers of commercial measuring instruments such. Semilab / Hungary, Freiberg Instruments / Germany or Hennecke / Germany.

All these measurement techniques determine a value which is proportional to the charge carrier lifetime in the area of the surface. Here, the spatial resolution of the respective measurement techniques is different; it can range from a few μm to a few mm. In the vicinity of closely spaced dislocation lines - ie in the range of dislocation clusters - the carrier lifetime decreases greatly, since the dislocation lines or the small-angle grain boundaries represent places of very high charge carrier recombination and the distances of the dislocation lines are less than the diffusion length of the charge carriers. The only condition for the material is that no other recombination mechanisms are allowed to superimpose the recombination on the dislocation lines. That is, a massive contamination z. B. with evenly distributed in volume metallic impurities prevents this type of detection of dislocation clusters. In particular, transition metals such as Fe, Cr, Co, Ni, Ti and the like should not exceed a concentration of 0.1 ppm.

Furthermore, efficient use of the method benefits that dislocation clusters once present in the VGF monosilicon material continue to propagate and spread in the course of directional solidification, but never disappear or dissolve. So it is sufficient for a simple classification to examine the side of the workpiece, which is finally solidified. This corresponds to the measurement of the total area shown in FIG. 3 or the minimum of the monocrystalline area A marked in this image.

The evaluation or classification of the examination-accessible side of a cuboidal, round, annular or other shaped blank takes place according to the area fraction of dislocation clusters found. The determination of this area proportion takes place by means of image evaluation of the areal images (raster measurements or camera shots, depending on the measuring technique) of individual sides of the workpiece (possibly only the last solidified side). The measured values for the carrier lifetime in the region of the dislocation clusters are clearly below the mean of the measured values outside the dislocation clusters. Although the absolute values and the resolution depend on the chosen measuring technique, the type of mechanical surface treatment, the electrical conductivity of the material and the content of metallic impurities, the areas of dislocation clusters are always distinguished from the unloaded areas by significantly lower measured values. It can then z. B. by the definition of a threshold value for the carrier lifetime or a correlated with this carrier lifetime measure surface portions of areas with dislocation clusters of such areas without dislocation clusters separately or area-wise quantified.

If low-resistance material is to be evaluated, the PL method is preferred. However, the following description of the basic procedure is based on the μPCD method.

The 4a and 4c show each PL recordings and the 4b and 4d in each case show the μ-PCD mapping of a respective rough-cut wafer surface of monocrystalline dislocation-free CZ material ( 4a and 4b ) and VGF mono-dislocation silicon ( 4c and 4d ). In both cases, there are no dislocation clusters (defect area fraction 0%). The materials are very similar.

The color gradient in the μPCD images is due to thickness variations of the wire-trimmed wafer and has nothing to do with quality differences. These pictures are merely intended to demonstrate that the defect type individual dislocation can not be determined with the named measuring methods and this is also not necessary for the quality assessment according to the present invention.

The 5a shows a PL recording and the 5b Figure 3 shows μ-PCD mapping of a rough-sawn surface of VGF monosil silicon material in the geometric shape of a wafer. The sample contains dislocation clusters, which can be evaluated by means of image evaluation with respect to the area fraction. The area fraction interspersed with dislocation clusters and small-angle grain boundaries is low in this case.

The 6a shows a PL recording and the 6b shows a μ-PCD mapping of a VGF monosilicon material in the geometric shape of a wafer. The area fraction interspersed with dislocation clusters and small-angle grain boundaries is high in this case.

The 7 shows a surface element (inspection surface) of a larger blanks as a PL area image of size 156 mm × 156 mm. The larger blank can be further processed into a sputtering target and made from the VGF monocrystalline region A of an ingot.

The entire surface of the monocrystalline region A or of the blank to be evaluated is recorded either as a full image and evaluated or from a plurality of individual images (for example surface elements as in FIG 7 shown) and evaluated. If a statement is to be made about a specific blank geometry in the sense of a quality classification in the five classes C1 to C5 mentioned below, a contour (new test surface) is included in the evaluated area or in the composite image, within which the area proportion of the total area determined by dislocation clusters and small-angle grain boundaries.

In the DE 102011056404 , whose content is hereby expressly included in the reference, the method of determining the area fraction of areas with dislocation clusters on a total area is described on the basis of the measuring method MDP. This procedure is suitable when using the method μ-PCD or PL is in the same way.

Specifically, the charge carrier lifetime or a value correlated with the charge carrier lifetime of the material is always determined for a pixel of the test surface by means of the aforementioned rastering or imaging measurement techniques (preferably PL in the case of low-resistance material). This is also done for all adjacent pixels in a subarea of the total area to be evaluated. This subarea may be square, rectangular, circular or elliptical. From the pixels completely enclosed in this area with a center pixel (border pixels are not allowed), an averaged image is generated. This can be z. From an area of 11x11 pixels, i. H. 121 pixels, or any defined number of pixels such. B. greater than 50 pixels or greater than 100 pixels are obtained. Since the charge carrier lifetime is significantly lower in a dislocation cluster or pixels containing small-angle grain boundaries than in dislocation cluster-free or small-angle grain boundary-free good domains, a difference between the average value is formed from the defined number of neighboring pixels and the center pixel measurement value. If this difference exceeds a previously defined amount, the center pixel is evaluated as a bad pixel. This threshold value has to be specified according to the measuring method, the range of the specimen resistivity and the doping (p- or n-type). For medium-resistance material and the measurement method MDP z. For example, a threshold of 0.22 μs is a suitable value.

Corresponding to that in [0059] to [0062] of the referenced DE 102011056404 The cluster content in% is determined as follows:

The cluster content in% in the total area to be evaluated or in the stored contour (eg the blank surface or new test surface) is used to make a quality rating.

Klasse C1:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material einen Flächenanteil von 0% Clustern aufweist, der von Versetzungsclustern oder Kleinwinkelkorngrenzen durchsetzt ist.

Klasse C2:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material einen Flächenanteil von kleiner oder gleich 25% aufweist, der von Versetzungsclustern und Kleinwinkelkorngrenzen durchsetzt ist.

Klasse C3:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material einen Flächenanteil von kleiner oder gleich 50% aufweist, der von Versetzungsclustern und Kleinwinkelkorngrenzen durchsetzt ist.

Klasse C4:

Silizium-Material mit Einzelversetzungen im Bereich von größer 1 × 10² cm^–2 bis kleiner 1 × 10⁶ cm^–2 in seinem monokristallinen, versetzungscluster- oder kleinwinkelkorngrenzenfreien Volumen, das auf einer Schnittfläche durch das Material einen Flächenanteil von kleiner oder gleich 80% aufweist, der von Versetzungsclustern und Kleinwinkelkorngrenzen durchsetzt ist.

Klasse C5:

Silizium-Material, das auf einer Schnittfläche durch das Material einen Flächenanteil von kleiner oder gleich 100% aufweist, der von Versetzungsclustern und Kleinwinkelkorngrenzen durchsetzt ist.

Low-resistance monocrystalline VGF silicon material for functional components such. B. sputtering targets or showerheads is divided into the following five quality classes:

Class C1:

Single-dislocation silicon material in the range of greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small-angle grain boundary-free volume having an area fraction of 0% clusters on a slice through the material; interspersed with dislocation clusters or small angle grain boundaries.

Class C2:

Silicon material with discrete dislocations in the range from greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small angle grain boundary-free volume which has an area fraction of less than or equal to 25% on a sectional surface through the material. which is interspersed by dislocation clusters and small angle grain boundaries.

Class C3:

Silicon material with discrete dislocations ranging from greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small angle grain boundary-free volume which has an area fraction of less than or equal to 50% on a sectional surface through the material. which is interspersed by dislocation clusters and small angle grain boundaries.

Class C4:

Silicon material with discrete dislocations in the range from greater than 1 × 10 ² cm ^-2 to less than 1 × 10 ⁶ cm ^-2 in its monocrystalline, dislocation cluster or small angle grain boundary-free volume which has an area fraction of less than or equal to 80% on a sectional surface through the material. which is interspersed by dislocation clusters and small angle grain boundaries.

Class C5:

A silicon material having an area fraction of less than or equal to 100% on a cut surface through the material interspersed with dislocation clusters and small-angle grain boundaries.

When classifying mono or quasi-monocrystalline silicon for applications, the classes FK1 to FK5, Z1 to Z5 and C1 to C5 shall be used for the assessment.

Depending on the application, different material behavior under operating conditions can be observed and therefore material of a particular class must be selected.

• – der flächige Ätzabtrag pro Zeiteinheit (insbesondere z. B. in Plasmaätzanlagen)
• – die Homogenität des Ätzabtrages (insbesondere z. B. in Plasmaätzanlagen)
• – die Partikelfreisetzung unter Einsatzbedingungen (insbesondere z. B. in Plasmaätzanlagen)
• – die Standzeit des Bauteiles

An important material behavior is for example:

• The areal etching removal per unit of time (in particular, for example, in plasma etching plants)
• The homogeneity of the etching removal (in particular, for example, in plasma etching plants)
• Particle release under conditions of use (in particular, for example, in plasma etching plants)
• - the service life of the component

Klassen Z:

vergleichsweise schwach negative Wirkung

Klassen C:

vergleichsweise moderat negative Wirkung

Klassen FK:

vergleichsweise stark negative Wirkung

With regard to the strength of the influencing of the aforementioned material behavior, the defined classes have different strengths:

Classes Z:

relatively weak negative effect

Classes C:

relatively moderately negative effect

Classes FK:

comparatively strong negative effect

Regardless of whether a rated according to the present invention, low-resistance silicon material in one of the classes Z, C or FK was grouped and for the production of functional components in etching equipment (whether with or without plasma support), CVD systems (whether with or without plasma support) or other equipment used to process semiconductor devices, it has always had better operational characteristics than traditional low resistance multicrystalline material.

For the semiconductor manufacturing suitable low-resistance monocrystalline silicon material from the CZ or FZ method is the low-resistance silicon material according to the present invention, only weakly superior in terms of use characteristics.

Embodiment 1

A crucible with crucible attachment with a total height of 760 mm is placed in a G4 furnace. The crucible is a monocrystalline seed plate with a length × width of 600 mm × 600 mm and a height of 40 mm, polycrystalline silicon raw material weighing a total of 450 kg and 21 g of boron The germ plate is positioned on the bottom of the crucible so that a more uniform Gap between germ plate and crucible wall remains. The furnace system has three heating zones: ceiling heater, jacket heater and bottom heater. In the lower part of the system is an active cooling arrangement, which consists of water-cooled copper plate and an overlying highly heat-conductive graphite block identical geometry shape. The cooling arrangement can be moved vertically by means of a lifting mechanism and contacts the crucible installation plate via the graphite block. The water-cooled copper plate and the graphite each have a hole at the edge through which a pyrometer can see directly from below on the crucible mounting plate. This pyrometer is used to control the germplasm temperature. At the beginning of the melting phase, the cooling plate is in the lower position and all heaters are active. The crystalline silicon is melted from above. From a certain temperature, the cooling plate is partially moved up and reduces the performance of the floor heater. At this time, the cooling plate still has no contact with the overlying crucible mounting plate. The bottom heater encloses the graphite cylinder when fully driven upwards. During the melting phase, the soil is simultaneously cooled and heated to not completely melt the seed but to minimize heat losses at the edge of the crucible. The cooling capacity must be adjusted to the heater temperatures in such a way that, especially during the seeding process, a flat phase boundary is established. The temperature determined by the pyrometer at the measuring location below the crucible setting plate goes through a minimum in each process, which is determined and stored as an absolute value. The temperature difference between the current temperature value after passing through the minimum and the previously determined minimum is used to determine the height of the seeding point. This corresponds to a temperature difference between measured value and temperature minimum of 20 K. If this difference is reached, the crystallization is initiated. The Ankeimstelle lies with a germ with a height of 40 mm and then in the range of 15-25 mm above the crucible bottom. To start the crystallization, the cooling plate is fully moved up to the crucible mounting plate. Contact with the crucible mounting plate increases the heat dissipation downwards and the phase boundary moves upwards. During crystallization, all heater temperatures are additionally reduced according to a temperature-time profile. When crystallization is complete, the cooling phase is initiated. The cooling rates are a maximum of 100 K / h.

Thereafter, the ingot is removed from the crystallization unit and removed from the crucible.

On a band saw the ingot soil is separated in a thickness of 45 mm. In this way, a plate is recovered, which is used again as a germ after a sandblasting process with suitable material and subsequent cleaning. From the remaining ingot side parts are now sawn off so that the resulting ingot base area is reduced to 630 mm × 630 mm.

By measuring the resistivity profile according to the 4-peak measurement method on a sample from a side part along the growth direction, it is confirmed that the course predicted by the doping has actually been achieved.

The ingot is placed on a side surface, and there is a lid cut, which removes the experience according to segregation contaminated with impurities and therefore useless material of appropriate thickness. This shows how large the monocrystalline area is in the center of the ingot both on the bottom and on the lid.

Based on this result and according to the specific geometric requirements, slices with a corresponding thickness allowance are cut out of the ingot, which are sufficient to cut out blanks for functional components.

Under obliquely incident light of a source of illumination is a marking of the monocrystalline region (center region) of the disc. The outer edge region is therefore the multicrystalline region of the disc.

As already described above, the monocrystalline center region is now examined in more detail as to which number or location within the monocrystalline matrix extraneous grains or twin boundaries could be present and in which class FK1 to FK5 or Z1 to Z5 this center region or correspondingly predetermined areas from it, for the production of functional components such. B. blanks for showerheads or round or rectangular sputtering targets the dimensions ⌀ 450 mm × 10 mm or 320 mm × 100 mm × 8 mm or an area of about 420 mm × 170 mm × about 12 mm for two halves of a Racetrack Target appear appropriate, to be classified. Furthermore, this center area or corresponding predetermined areas thereof, which appear suitable for the production of functional components, by means of scanning or imaging measurement techniques (such as MDP, μ-PCD or low-resistance material preferably PL) examined and divided into the classes C1 to C5.

If necessary, a sample is also cut out of the monocrystalline center region to control the resistivity using a 4-tip measuring station, i. H. to confirm that he z. B. within the required range of 0.001 Ωcm to 0.2 Ωcm.

Corresponding to the requirements for the blank, the measured resistivity and the selection of a region of the monocrystalline center region A suitable for dimensions and classification (FK1-FK5, Z1-Z5 and C1-C5) (see example in FIGS 3a or 3b ) for the blank is now working out of the blanks from the disc. The blank surface now corresponds to a newly defined test surface for which the classification applies.

Blanks are required for several identical or different components, is accordingly 1 . 2 and 3 an optimization based on the geometrical dimensions and locally different classification results of the monocrystalline regions removed slices.

Embodiment 2

In a G5-VGF furnace plant, a crucible with crucible attachment with a total height of 780 mm is introduced. The crucible contains a monocrystalline silicon seed plate with a height of 45 mm, polycrystalline silicon raw material with a total weight of 700 kg and 33 g of boron. The large-area seed plate obtained from a previous process is dimensioned and positioned on the bottom of the crucible so that there is a gap all around of about 20 mm remains. Their surface is roughened in a sandblasting process with suitable material. Above and into the gap between germ plate and crucible wall crystalline silicon raw material is filled. The kiln plant is a multi-zone kiln plant with a total of four temperature-controlled heating zones: ceiling heaters, overhead heaters, bottom heaters and bottom heaters. Under the crucible mounting plate is an active cooling device. The cooling medium used is gaseous nitrogen. The dimensions of the cooling device (length, width) correspond at least to the dimensions of the crucible mounting plate. Below the crucible mounting plate in the region near the edge of the crucible is a thermocouple for Control of germplasm temperature. This thermocouple is guided from below in a protective tube in the cooling device, which consists of a good thermal conductivity graphite. The protective tube abuts the overlying graphite plate from below. On top of this graphite plate are the crucible mounting plate made of graphite and above it the crucible.

The melting process is designed so that the silicon raw material is melted from above and the silicon seed plate is only partially melted. For this purpose, a temperature profile is set, which typically has a higher temperature on the ceiling heater than on the bottom heater. During the melting phase of the bottom heater is only initially in operation and is turned off later to prevent melting of the monocrystalline nucleus. From reaching about 1400 ° C on the side heater, the gas cooling is activated. The cooling capacity must be adjusted to the heater temperatures in such a way that, especially during the seeding process, a flat phase boundary is established. Even after activation of the gas cooling, the polycrystalline silicon raw material continues to melt above the germplate. In the further course, the heater temperatures are kept at a predefined temperature value. Also in this state, the polycrystalline silicon raw material continues to melt. The temperature at the measuring point below the crucible setting plate goes through a minimum in each process, which is determined as an absolute value and stored. The total heating power continues to decrease continuously over the same period of time. At the temperature of the thermocouple or more precisely by the temperature difference between the current measured value and the previously stored temperature minimum, the height of the Ankeimstelle can be determined. In the case of a germ of 45 mm height, the germination site is ideally in the range of 25 mm to 35 mm above the crucible bottom. This corresponds to a temperature difference between the measured value and the temperature minimum of 8 K-12 K. If this difference is reached, the crystallization is initiated. It can be several hours between temperature minimum and start of crystallization. The crystallization is initiated on the one hand by increasing the cooling capacity of the gas cooling and on the other hand via a controlled temperature-time profile of the active heater. Here, the cooling capacity is increased rapidly, z. B. from 5 kW to 20 kW to prevent melting of the germ. The temperatures of the heating zones are slowly reduced. The cooling rates are in the range of -0.4 K / h to -15 K / h. By appropriate selection of the heater temperatures and the cooling capacity, a convex phase boundary is set in the center, which pushes the polycrystalline edge region further outward or supports a vertical columnar growth there. When the ingot has finished crystallizing, the cooling phase begins. During the cooling phase, the heaters are controlled by a further temperature-time profile. The cooling rates are -10 K / h to -80 K / h.

Thereafter, the ingot is removed from the crystallization unit and removed from the crucible.

On a band saw the ingot soil is separated in a thickness of 45 mm. In this way, a plate is recovered, which is used again as a germ after a sandblasting process with suitable material and subsequent cleaning. From the remaining ingot side parts are now sawn off so that the resulting ingot base area is reduced to 780 mm × 780 mm.

By measuring the profile of the resistivity at a side part along the growth direction, it is confirmed that the course predicted by the doping has actually been achieved. The ingot is placed on a side surface, and there is a lid cut, which removes the experience according to segregation contaminated with impurities and therefore useless material of appropriate thickness. This shows how large the monocrystalline area is in the center of the ingot both on the bottom and on the lid.

Based on this result and in accordance with the specific geometric requirements, slices with a corresponding thickness allowance are cut out of the ingot, which are sufficient to produce, for. B. round or rectangular sputtering targets of dimensions ⌀ 450 mm × 10 mm or 320 mm × 100 mm × 8 mm or an area of about 700 mm × about 123 mm × about 12 mm for a one-piece Racetrack Target to manufacture. Under obliquely incident light of a source of illumination is a marking of the monocrystalline region (center region) of the disc. The outer edge region is therefore the multicrystalline region of the disc.

As already described above, the monocrystalline center region is now examined in more detail as to which number or location within the monocrystalline matrix extraneous grains or twin boundaries could be present and in which class FK1 to FK5 or Z1 to Z5 this center region or correspondingly predetermined areas from it, which appear suitable for the production of functional components, be classified. Furthermore, this center area or corresponding predetermined areas thereof, which appear suitable for the production of functional components, by means of rastering or imaging Measuring techniques (such as MDP, μ-PCD or low-resistance material preferred PL) and divided into the classes C1 to C5.

If necessary, a sample is also cut out of the monocrystalline center region to control the resistivity by means of a 4-tip measuring station, i. H. to confirm that he z. B. within the required range of 0.001 Ωcm to 0.2 Ωcm.

Corresponding to the requirements for the blank, the measured resistivity and the selection of a region of the monocrystalline center region A suitable for dimensions and classification (FK1-FK5, Z1-Z5 and C1-C5) (see example in FIGS 3a or 3b ) for the blank is now working out of the blanks from the disc. The blank surface now corresponds to a newly defined test surface for which the classification applies.

Blanks are required for several identical or different components, is accordingly 1 . 2 and 3 an optimization based on the geometrical dimensions and locally different classification results of the monocrystalline regions removed slices.

In summary, therefore, a blank, as described above, a method for its preparation, as described above, and its use for functional components of mono- or quasi-monocrystalline silicon, such as. Sputtering targets or showerheads as described above.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2028292 [0009, 0009]
• WO 2007/084934 [0009, 0009]
• WO 2009/014957 [0009, 0009]
• DE 102011056404 [0087, 0089]

Cited non-patent literature

• http://www.spm.li/deutsch/sputtering-targets/silizium/ [0006]
• Nakajima et al., J. Cryst. Growth 372 (2013) 121-128 [0020]

Claims (20)

Blank of mono- or quasi-monocrystalline silicon material for use in functional components in semiconductor processing equipment, in particular as a sputtering target, or showerhead, wherein - the blank in areas on its top or bottom containing no clusters of dislocations or small-angle grain boundaries has a concentration of Individual dislocations in the range of greater than 1 × 10 ² to less than 1 × 10 ⁶ cm ^-2 and - the blank has a specific resistance of 0.001 to 0.2 Ωcm. Blank according to claim 1, further comprising areas containing clusters of dislocations or small angle grain boundaries on its top or bottom in addition to the areas not containing clusters of dislocations or small angle grain boundaries. Blank according to claim 1 or 2, having on its top or bottom a predetermined areal proportion of pixels containing clusters of dislocations or small angle grain boundaries and evaluated as bad pixels. Blank according to one of the preceding claims, which has on its top or bottom foreign grains of a size smaller than 50 mm ² . Blank according to one of the preceding claims, which has on its top or bottom a predetermined number of foreign grains of a size smaller than 50 mm ² Blank according to one of the preceding claims, which has twin boundaries on its top or bottom. Blank according to one of the preceding claims, having a predetermined number of twin boundaries on its top or bottom. Blank according to one of the preceding claims, having on its top or bottom side a combination of classes of defects clusters of dislocations or small angle grain boundaries, foreign grains and twin boundaries. Blank according to claim 8, having on its top or bottom side combinations of all three classes of defects from clusters of dislocations or small angle grain boundaries, foreign grains and twin boundaries. Blank according to one of claims 1 to 9, which has a diameter or a diagonal of greater than 320 mm or whose shorter edge length is greater than 320 mm. Blank according to one of claims 1 to 9, which has a diameter or a diagonal of greater than 470 mm or whose shorter edge length is greater than 470 mmmm. Use of the blank according to one of the preceding claims for the production of functional components in systems for semiconductor processing, in particular of sputtering targets or showerheads. Use according to claim 10, wherein the blank has a concentration of discrete dislocations in the range of 10 ² to 10 ⁶ cm ^-2 in its dislocation cluster and thus small angle grain boundary free volume and / or a resistivity of 0.001 to 0.2 Ωcm. Use according to claim 10 or 11, wherein the blank on its largest top or bottom has a defined area fraction, which is interspersed by clusters of dislocations or Kleinwinkelkorngrenzen. Use according to claim 10 or 11, wherein there is no intensive local accumulation of clusters of dislocations or small angle grain boundaries on the top or bottom of the blank. Process for the production of blanks acc. one of claims 1-11 of mono- or quasi-monocrystalline silicon material for the production of functional components in systems for semiconductor processing, in particular of sputtering targets or showerheads, with the following process steps: - Producing a mono- or quasi-monocrystalline ingot by directional solidification in a crucible; - Bottom section, mantle cut and lid cut on the ingot after its cooling and demolding from the crucible to obtain a Ingotkern; - determining a monocrystalline test surface on the surface of the ingot core or a slice separated therefrom; Testing and evaluation of the test surface with regard to content, distribution and / or quality relevance of at least one of the following: foreign and twin boundaries, clusters of dislocations or small angle grain boundaries, by making a scan of the test surface and areal determination of the carrier lifetime or a physical quantity derived therefrom to identify at least one potential blank; Separating out the blank such that the blank removed in areas on its top or bottom containing no clusters of dislocations or small angle grain boundaries has a concentration of discrete dislocations in the range of greater than 1 × 10 ² to less than 1 × 10 ⁶ cm ^-2 and wherein the separated blank has a resistivity of 0.001 to 0.2 Ωcm. The method of claim 14 or 15, further comprising the method steps: - Measurement of the course of the resistivity in the direction of crystallization at the ingot core; - Slicing the ingot in a thickness which allows the production of blanks of desired geometric dimensions. Method according to one of claims 14 to 16, wherein the evaluation of the test surface and areal determination of the carrier lifetime or the physical quantity derived therefrom is carried out by determining a surface portion and by image evaluation of areal images of the test surface, wherein measured values for the carrier lifetime in areas with dislocation clusters an average of the measured values of the entire test surface. The method of claim 17, wherein the definition of a threshold value for the carrier lifetime or a measured variable correlated with this carrier lifetime is used to separately or area-wise quantify area proportions of areas with dislocation clusters from areas without dislocation clusters. The method of claim 17 or 18, wherein a content of dislocation clusters in% is determined as follows:

wherein for pixels of the areal images of the test surface, the charge carrier lifetime or a value correlated with the charge carrier lifetime of the material is always repeated for all adjacent pixels in a partial area of the total area to be evaluated, from the pixels completely enclosed in this total area with a center pixel generating an average image, respectively forming a difference between an average of the adjacent pixels and the respective center pixel reading, and judging the center pixel as a bad pixel when the difference exceeds a predetermined threshold, and the center pixel being evaluated as a good pixel when the difference is the predetermined one Threshold does not exceed.

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