DE102013225146A1 - Producing a silicon single thin rod comprises welding of two thin rods of polycrystalline silicon to a longer thin rod by induction welding an induction coil - Google Patents

Abstract

Producing a silicon single thin rod comprises providing two thin rods of polycrystalline silicon, and welding of the two thin rods to a longer thin rod by induction welding an induction coil, where in the step of welding of two thin rods for longer thin rod, the temperature is measured at a weld and a thermal output of an induction coil which heats the two thin rods by thermal radiation, is time regulated.

Description

The invention relates to a method for producing a silicon thin rod.

Silicon thin rods are used to deposit polycrystalline silicon.

Polycrystalline silicon (polysilicon in short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by zone melting (floatzone or FZ process). This monocrystalline silicon is cut into slices (wafers) and used for a variety of mechanical, chemical and chemo-mechanical processes in the semiconductor industry for the manufacture of electronic components (chips).

In particular, however, polycrystalline silicon is increasingly required for the production of monocrystalline or multicrystalline silicon by means of drawing or casting processes, this monocrystalline or multicrystalline silicon being used to produce solar cells for photovoltaics.

The polycrystalline silicon, often called polysilicon for short, is usually produced by means of the Siemens process. In this case, in a bell-shaped reactor ("Siemens reactor") thin rods of silicon are heated by direct current passage and a reaction gas containing a silicon-containing component and hydrogen is introduced.

The silicon thin rods usually have an edge length of 3 to 15 mm.

Examples of silicon-containing components are silicon halide compounds such as silicon chloro compounds, in particular chlorosilanes. The silicon-containing component is introduced into the reactor together with hydrogen. At temperatures of more than 1000 ° C silicon is deposited on the thin rods. This finally results in a rod with polycrystalline silicon. In DE 1 105 396 the basic principles of the Siemens process are described.

Regarding the production of thin rods is out DE 1 177 119 B known to deposit silicon on a support body made of silicon (= thin rod), then separated from a part and to use this separated part in turn as a carrier body for the deposition of silicon. The separation can mechanically, z. B. by sawing, or electrolytically by means of a liquid jet.

In the mechanical separation of thin rods whose surface is contaminated with metals and with boron, phosphorus, aluminum and arsenic compounds. The surface contamination with metals after mechanical separation with metal-bonded saw blades is 0.1-1 ppmw (parts per million by weight) or higher. The average load of B, P, Al and As is about 1 ppba (parts per billion atomic) or more.

Therefore, it is usually necessary to subject the thin rods to surface cleaning before they can be used to deposit silicon. DE 1 177 119 B discloses in this regard a cleaning by mechanical means, for. B. by sandblasting, or by chemical means by etching.

By treating the thin rods in an etching tank of low-contamination material, eg. As plastic, using a mixture of HF and HNO ₃ , the surface contamination can be significantly reduced, for metals up to 300 pptw and less, for B, P, Al and As to less than 15 pptw.

In EP 0 548 504 A2 For example, a cleaning process is described in which HF and HNO _{3 are used} to purify silicon.

Another cleaning process is over DE 195 29 518 A1 , known. In this case, polycrystalline silicon is first cleaned with a mixture of aqua regia (mixture of HCl and HNO ₃ ) and then subjected to additional purification with HF.

EP 0 905 796 A1 discloses a method for producing semiconductor material having a low metal concentration, characterized in that polycrystalline silicon is washed in a pre-cleaning in at least one stage with an oxidizing cleaning solution, in a main cleaning in a further stage is washed with a cleaning solution containing HNO ₃ and HF and, in a hydrophilization, washing in yet another stage with an oxidizing cleaning liquid. This purification process reduces the iron and / or chromium content on the surface of the silicon from 1.332 × 10 ^-8 g / cm ² (after working with metal tooling) to less than 6.66 × 10 ^-11 g / cm ² .

In order to increase the yield in the silicon deposition, it would also be desirable to be able to use longer thin rods. Longer thin rods can in principle be produced by welding shorter thin rods.

WO 02/070184 A1 describes a method in which two silicon workpieces are joined together without cracking by welding. First, the workpieces are heated to a temperature of at least 600 ° C, preferably on a hot plate of silicon. Then the workpieces z. B. connected by means of electrical, plasma or laser welding.

However, this method is difficult to handle for thin workpieces. In addition, the silicon workpieces are constantly in direct contact with air during welding, which is disadvantageous in terms of contamination.

US 6573471 B1 also describes a method by which two silicon workpieces can be joined together by welding. The main difference to the procedure after WO 02/070184 A1 is that a negative pressure of at most 0.05 Torr is generated before the two workpieces are connected.

US 6852952 B1 describes a method in which two silicon workpieces are joined together by arc welding. For this purpose, a plasma is generated between two electrodes, in the vicinity of which the silicon workpieces to be joined are brought. This preferably takes place in argon atmosphere.

Also the procedure after US 6852952 B1 However, is expensive and disadvantageous for the welding of thin rods.

Another conceivable method is the induction welding. This usually plastic and metal parts are welded under air atmosphere.

The use of induction welding to join silicon workpieces would result in the formation of a SiN layer since silicon reacts with the nitrogen from the ambient air due to the high temperatures of more than 1500 ° C. Since SiN does not dissolve in a silicon melt and results in dislocations in the single crystal as particles, the use of such polycrystalline silicon for the production of silicon single crystals by means of crucible pulling or zone melting is not suitable.

For long thin rods, the currently available caustic tanks represent another problem.

The size of the etching tanks for cleaning systems made of pure plastic is in fact limited by design. From a certain dimension of the etching basin, the system becomes unstable. Additional beam bracing could allow for enlarging the caustic basins. However, the use of steel is critical because it can not be ruled out that acid from the etching tank will get near the steel braces due to stress cracks and the acid will be contaminated with metals.

• 1) Bearbeitung der Verbindungsflächen zu schweißender Siliciumstäbe zu glatten Oberflächen oder Oberflächen mit leichten Wölbungen;
• 2) Platzieren der Verbindungsflächen der zu schweißenden Siliciumstäbe auf der Oberseite bzw. Unterseite der Wärmeschmelzöffnung in der Mitte der Hochfrequenz-Induktionsheizspule im Siliciumstab-Ofen;
• 3) Ingangsetzen des Siliciumstab-Ofens und der Hochfrequenz-Induktionsheizspule
• 4) Wärmeschmelzen, nämlich den unteren oder oberen Siliciumstab in Richtung der Unterseite bzw. Oberseite der Wärmeschmelzöffnung in der Mitte der Hochfrequenz-Induktionsheizspule führen, dann ganz langsam in die Wärmeschmelzöffnung in der Mitte der Hochfrequenz-Induktionsheizspule einführen, ohne dass es dabei aber zu einem Kontakt mit der Hochfrequenz-Induktionsheizspule kommt, anschließend den entsprechenden Gegenstück-Siliciumstab ganz langsam in die Wärmeschmelzöffnung in der Mitte der Hochfrequenz-Induktionsheizspule einführen; zwischen den beiden Siliciumstäben verbleibt ein Spalt, die Temperatur in der entstandenen Schmelzzone der beiden Siliciumstäbe in der Wärmeschmelzöffnung in der Mitte der Hochfrequenz-Induktionsheizspule wird bis Erreichen des geschmolzenen Zustands gesteigert, woraufhin dann durch Steuern der Bewegung der Siliciumstäbe von oben nach unten bzw. von unten nach oben die Schmelzschweißverbindung der beiden Siliciumstäbe hergestellt wird;
• 5) Fortsetzung des vorangehenden Schritts bei Steuerung der Hochfrequenz-Induktionsheizspule in der Weise, dass es in der Schmelzzone der beiden Siliciumstäbe zum Zonenschmelzen kommt, so dass die beiden Siliciumstäbe an der Schweißstelle vollständig verschweißt und zu einer Einheit werden;
• 6) Im Anschluss an den vorigen Schritt wird durch Reduzieren des Stromflusses in der Hochfrequenz-Induktionsheizspule die Temperatur Schritt für Schritt bis Erreichen der Raumtemperatur gesenkt, wonach dann die verschweißten Siliciumstäbe aus der Wärmeschmelzöffnung in der Mitte der Hochfrequenz-Induktionsheizspule entnommen werden;
• 7) Wiederverwendung oder Verwahrung der zu einer Einheit verschweißten Siliciumstäbe.

CN 101746761 A discloses a method of fusion welding silicon rods, comprising

• 1) machining the bonding surfaces to be welded silicon rods to smooth surfaces or surfaces with slight bulges;
• 2) placing the bonding surfaces of the silicon rods to be welded on top of the heat-melting opening in the middle of the high-frequency induction heating coil in the silicon rod furnace;
• 3) Starting the silicon rod furnace and the high frequency induction heating coil
• 4) heat melting, namely the lower or upper silicon rod in the direction of the underside or top of the heat fusion opening in the middle of the high-frequency induction heating coil, then very slowly into the heat-melt opening in the middle of the high-frequency Induktionsheizspule introduce, but without it to a Contact with the high frequency induction heating coil, then insert the appropriate counterpart silicon rod very slowly into the heat fusion port in the middle of the high frequency induction heating coil; a gap remains between the two silicon rods, the temperature in the resulting fusion zone of the two silicon rods in the heat-fusing opening in the middle of the high-frequency induction heating coil is increased until reaching the molten state, whereupon by controlling the movement of the silicon rods from top to bottom or from down to the top of the fusion-welded joint of the two silicon rods is produced;
• 5) continuing the foregoing step in controlling the high frequency induction heating coil so that zone melting occurs in the melting zone of the two silicon rods so that the two silicon rods are completely welded and unified at the weld;
• 6) Following the previous step, by reducing the current flow in the high-frequency induction heating coil, the temperature is lowered step by step until it reaches room temperature, after which the welded silicon rods are removed from the heat-fusing opening in the middle of the high-frequency induction heating coil;
• 7) Reuse or storage of the silicon rods welded into a single unit.

US 2011220283 A1 discloses a device for manufacturing a silicon rod, in which two silicon rods are connected by welding to a silicon rod, comprising a holder for the upper silicon rod, a A lower silicon rod lifting device vertically movable thereby, an induction heating coil at the lower end of the upper silicon rod, a preheating ring below the induction heating coil, and a movable device for passing the preheating ring between a heating position at the preheating ring at the upper end of the lower silicon rod the induction heating coil is heated, and to move to a waiting position remote from the heating position.

• a) Bereitstellen eines Stabes aus polykristallinem Silicium, von dem wenigstens zwei Dünnstäbe mit gegenüber dem Stab aus polykristallinem Silicium reduziertem Querschnitt abgetrennt werden;
• b) Reinigung der wenigstens zwei abgetrennten Dünnstäbe durch Behandlung mit einem Material abtragenden flüssigen Medium;
• c) Verschweißen von wenigstens zwei der gereinigten Dünnstäbe zu einem längeren Dünnstab;
• d) Verpacken des längeren Dünnstabs in eine Schlauchfolie.

US 2012060562 A1 discloses a method of making thin silicon rods comprising the steps

• a) providing a rod of polycrystalline silicon, are separated from the at least two thin rods with respect to the rod of polycrystalline silicon reduced cross-section;
• b) cleaning the at least two separated thin rods by treatment with a material-removing liquid medium;
• c) welding at least two of the cleaned thin rods to a longer thin rod;
• d) Pack the longer thin rod into a tubular film.

The starting point of the method is a rod made of polycrystalline silicon, produced by depositing silicon on a thin rod, preferably by means of the Siemens process. This rod of polycrystalline material is cut into thin rods. The separation of the thin rods is preferably carried out mechanically by means of sawing. The separated thin rods are then chemically cleaned. Preferably, exactly one cleaning step takes place before the welding of the thin rods. This cleaning step is preferably carried out in a clean room of a cleanroom class 100 or lower (according to US FED STD 209E, superseded by ISO 14644-1 ). Preferably, the chemical cleaning is carried out by means of a HF / HNO ₃ mixture. Then the thin rods are welded. The welding of the cleaned thin rods is preferably carried out under inert gas. Preferably, the welding takes place by means of an induction method.

By welding sawn, but not previously cleaned thin rods at the weld metal concentration at the surface is increased to more than 10 ¹⁶ at / cm ² . Due to the high temperature during welding of more than 500 ° C, metallic and other particulate impurities diffuse into the bulk of the silicon thin rod. Such impurities in bulk can not be removed by surface cleaning. By the method described above and the mandatory cleaning of the thin rods before welding this is avoided.

However, it has been found that despite all efforts in the prior art, the welds and the welded rods produced are unsatisfactory. For example, often nose-like bulges form at the weld. The welded rods have in some cases high thermal stresses, which can lead to problems in downstream processes, in the worst case to material failure.

From this problem, the task of the invention resulted.

The object of the invention is achieved by a method for producing a silicon thin rod, comprising providing two thin rods of polycrystalline silicon, welding the two thin rods to a longer thin rod by induction welding by means of an induction coil, characterized in that in the welding of the two thin rods to longer thin rod, the temperature measured at a weld and a heating power of an induction coil, which heats the two thin rods by thermal radiation, timed.

Preferred embodiments of the method can be found in the dependent claims.

The inventors have recognized that when welding silicon thin rods, the parameters of temperature and time are important factors for successful welding and for a clean weld.

For exact control of these two variables, the temperature is preferably measured directly at the weld by means of a Pyrometermessgeräts in this process.

This non-contact surface temperature measurement also serves as a controlled variable in this process. By means of the radiation measurement, the exact temperature on the silicon rod can be determined and regulated according to this. This exact regulation prevents the formation of a nose at the weld.

The measuring device (radiation pyrometer) is preferably arranged and focused so that it detects the temperature directly at the joint of the two rod pieces.

The mechanical construction preferably ensures that the joint is located in the center of the induction coil (both vertically and horizontally).

By means of the radiation measurement, the exact temperature on the silicon rod can be determined and the heating power can be regulated according to this size.

Setpoints are applied to the high-frequency generator (HF generator) via a PID controller, which thereby changes its power output (regulation via anode power) at the induction coil and thus varies the energy input into the medium to be heated.

• a) Aufheizphase: Aufheizen der Siliciumstäbe bis die gewünschte Temperatur erreicht ist;
• b) Schweißphase: Halten der Temperatur für einen gewissen Zeitraum;
• c) Abkühlphase: langsames und stufenweise Herunterfahren der Heiztemperatur, um eine möglichst schonende Abkühlung der Schweißnaht zu erhalten.

In the rule process, the following process steps are preferably carried out:

• a) heating phase: heating the silicon rods until the desired temperature is reached;
• b) welding phase: keeping the temperature for a certain period of time;
• c) Cooling phase: slow and gradual shutdown of the heating temperature in order to obtain as gentle as possible a cooling of the weld.

example 1

The lower rod piece is clamped, followed by the upper rod piece. Then the welding process is started.

The following steps are automatically executed by a program stored in the temperature controller:
The coal sinks on high-frequency induction coil and is aligned by the mechanical guidance to the coil. The temperature setpoint is between 800 ° C and 1600 ° C. The high-frequency generator regulates to a maximum set power (30% <P <100%) and heats up the Kohlering (heating phase) by induction.

By Kohlering the two rod pieces are heated indirectly at the junction of the thin rods by the heat radiation.

After a certain time (material-dependent constant), the rod is preheated so far by the coal that the high frequency can couple directly into the silicon rod.

Due to the defined maximum power (generator power), a short temperature drop (approx. 5-60 ° C) occurs when the high frequency is coupled into the rod, which is detected by the pyrometer. This temperature drop serves as an indicator for the next process step.

In the next process step, the coal ring is lifted because it is no longer needed for the further process. The control program switches to a new setpoint (800 ° C <T <1600 ° C) to compensate for the temperature drop as quickly as possible.

Once the predetermined temperature is reached, the set point for the fusion of the rods is switched and regulated to this temperature (melting temperature of silicon) for a certain time (5 s <t <300 s).

During this time, the rod is completely annealed until the silicon at the weld is completely liquid.

Subsequently, several program steps are run successively in which the temperature setpoints are reduced stepwise (reduction per cooling step between 5 ° C and 1000 ° C).

Each individual setpoint is held for a defined time (1 s <t <300 s) before the next setpoint is activated (→ selective cooling of the weld to minimize "nose" and tension in the material).

During this cooling phase, the liquid silicon at the weld again solidifies.

After the last program step, the welding generator is switched off.

The welded thin rod is automatically removed via a handling system and stored in a buffer section for further cooling.

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

• DE 1105396 [0007]
• DE 1177119 B [0008, 0010]
• EP 0548504 A2 [0012]
• DE 19529518 A1 [0013]
• EP 0905796 A1 [0014]
• WO 02/070184 A1 [0016, 0018]
• US 6573471 B1 [0018]
• US 6852952 B1 [0019, 0020]
• CN 101746761 A [0025]
• US 2011220283 A1 [0026]
• US 2012060562 A1 [0027]

Cited non-patent literature

• ISO 14644-1 [0028]

Claims (9)

A method of making a thin silicon rod, comprising providing two polycrystalline silicon rods, welding the two rods to a longer rod by induction welding using an induction coil, characterized in that welding the two rods to the longer rod measures the temperature at a weld and a heating power of an induction coil, which heats the two thin rods by thermal radiation, is regulated in time. The method of claim 1, wherein the temperature is measured directly at the weld by means of a pyrometer. The method of claim 2, wherein the pyrometer is arranged and focused to detect the temperature immediately at the point of contact of the two thin rods. The method of claim 3, wherein the point of contact of the two thin rods is horizontally and vertically in the center of the induction coil. Method according to one of claims 1 to 4, wherein setpoint values of the temperature are given to a high-frequency generator via a PID controller, thereby changing its power output to the induction coil. A method according to any one of claims 1 to 5, wherein the heating of the two thin rods is effected indirectly via a carbon heater, the carbon heater being induction heated and the carbon heater each heating one end of the two thin rods at the weld above the melting temperature of Silicon heated, forming a drop of liquid silicon, after a few minutes, the induction coil is turned off, the two thin rods grow together to form a longer thin rod. Method according to one of claims 1 to 5, wherein the two thin rods are heated at the weld initially to a certain temperature, this temperature is maintained during the welding of the two thin rods, said temperature is then lowered slowly and gradually. The method of any one of claims 1 to 7, wherein the welding of the two thin rods to the longer thin rod in an inert atmosphere. Method according to one of claims 1 to 8, wherein the two provided thin rods are subjected to a treatment with a material erosive liquid medium prior to welding.