The present invention relates to a control process for a monocrystal pulling machine.
This process makes it possible to pull monocrystals from a bath of molten products. These monocrystals are essentially used for the production of integrated circuits. This process is performed by a pulling machine, in which the molten product bath is contained in a crucible and is kept at a temperature above the melting point of the product by appropriate heating means. The solid crystal or ignot is obtained by the growth of a monocrystalline nucleus of the same product, placed at the lower end of a vertically axed pulling pin or spindle, in accordance with the so-called Czochralski method, which is known in the art and will not be described in detail here.
Pulling machines operating according to the Czochralski method are fundamentally used in the crystallization of materials for which, in the crystallized solid state, said materials have the same composition as in the liquid state. It is then said that these materials have a congruent melt. For example, this is the case with certain oxides, as well as semiconductors of the silicon or germanium type.
In known machines making it possible to pull monocrystals according to the Czochralsky method, the monocrystalline nucleus is brought into contact with the liquid bath, whose temperature is adjusted in such a way that, at the solid-liquid interface, the conditions are such that at any time the quantity of crystallized atoms is identical with the quantity of atoms returning into the molten state. Growth is obtained by exerting a regular vertical translation movement at a speed which is generally between 1 and 30 mm per hour on a pulling spindle or pin, one end of which makes it possible to fix the nucleus. In order to check the shape of the solid-liquid interface, a rotary movement of the pin about its axis at a speed generally between 0 and 100 revolutions per minute is superimposed on the translatory movement thereof.
The upward displacement of the solid-liquid interface bring the latter into a colder zone, which has the effect of crystallizing the bath on the nucleus and consequently of growing the monocrystal, with the aim of forming therefrom a cylindrical ingot, which has a diameter which is as constant as possible.
The need for obtaining monocrystals with a regular geometry has led machine designers to seek control processes leading to monocrystal diameter regularity during the pulling thereof. This regularity is generally also obtained on the basis of measurements of the weight of the monocrystal during pulling. To this end, a weight transducer is generally placed at one end of the pulling pin and transmits signals representing weight values to a processing and control assembly, which make it possible to check the translation and rotation of the pin. The known machines are not completely automatic, because the processes performed by them do not use adequate processing of the weight information during pulling and information regarding the geometry of the crystal to be obtained. Thus, the regulation of the heating means containing the crucible, the fast and then slow approach of the surface of the molten product contained in the crucible, the contact between the nucleus and the surface of the molten product, the pulling of the monocrystal after contact has taken place and then the cooling of the furnace at the end of pulling are not completely automatic operations and these are often performed in an empirical manner. Thus, the results obtained are of a random nature, which is a serious disadvantage, because the monocrystals to be obtained are very expensive products.