Method for heat process of silicon

Abstract: In a method for heat process of silicon, a single crystal silicon produced by the Czochralski process is thermally processed at a low temperature ranging from 400°C to 550°C . Outside this temperature range, the oxygen precipitate is not adequate. The result is that a predetermined oxygen precipitate can be obtained uniformly in the crystal growth direction without any reduction especially at the crystal bottom part. The resulting silicon is particularly suitable for manufacture of LSI.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to a method for heat process of a single crystal silicon produced by the Czochralski process, in which method target oxygen precipitation is obtained uniformly in the direction of silicon crystal growth. 2. Description of the Related Art: It has been common knowledge that a single crystal silicon produced by the Czochralski process contains supersaturated oxygen, which is precipitated to develop oxide precipitates during heat cycles in LSI manufacture. When they are introduced into a region remote from an electronic-device-forming region, these oxide precipitates serve as the center of gettering to attract to various kinds of impurities that are possibly introduced during manufacture of LSI, thus keeping the electronic-device-forming region clean. On the other hand, when the oxide precipitates are introduced into the electronic-device-forming region, they play a harmful role to the electronic devices, causing deterioration of the characteristic. Consequently, for manufacturing a LSI, it is important to control the amount of precipitated oxygen in the bulk of the single Crystal Silicon. The precipitation of oxygen contained in a Czochralski single crystal silicon highly depends on the initial oxygen concentration in the crystal and the heat history during the crystal growth ; even in one and the same Czochralski single crystal silicon, the heat history is different between the seed crystal side and the crystal bottom side. Namely, the oxygen precipitate is large at the seed side of the single crystal silicon and small at the bottom side of the crystal silicon, thus causing a non-uniform distribution of precipitation in the crystal growth direction. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a method to obtain a uniform precipitation in the direction of the silicon crystal growth of a single crystal silicon produced by the Czochralski process. This process is characterized in that it comprises a step of thermally processing side single crystal silicon at a low temperature ranging from 400°C to 550°C. Outside this temperature range, the oxygen precipitate is not adequate. The single crystal silicon may be a silicon wafer or a silicon ingot. According to another feature of the invention, the single crystal silicon produced by the Czochralski process is thermally processed at a high temperature ranging from 1200°C to 1350°C, and thereafter said low temperature process is taken effect. In an alternative form of the method of the invention, the single crystal silicon is thermally processed at a low temperature ranging from 400°C to 550°C over a period from 30 to 180 minutes under an atmosphere of dry oxygen in the furnace of a Czochralski crystal puller, as an after-heating step, after completion of pulling-up of the single crystal silicon. In this latter case, the single crystal silicon may be thermally processed at a temperature ranging from 1200°C to 1350°C and thereafter is processed at a low temperature ranging from 400°C to 550°C over a period from 30 to 180 minutes under an atmosphere of dry oxygen. When the high temperature treatment is carried out, it is carried out under dry oxygen, and the low temperature treatment is conducted over a period from 60 to 180 minutes under dry oxygen and thereafter an oxygen precipitation heat process is taken effect. After the above-mentioned high temperature and low temperature heat processes a normal oxygen precipitation heat process which may be taken effect in the manner described below so that a conductor integrated circuit can be produced with improved performance, thus causing an increased yield of manufacture of excellent IC products. In the Czochralski process, since a single crystal silicon grows from a molten silicon liquid, the crystal is cooled continuously from a melting point of silicon, i.e., 1,420°C to room temperature. Therefore, the local heat history is different between the head part and the botom part of the single crystal silicon. To this end, according to need, the heat history during the crystal growing is desirably initialized by taking a high-temperature process, and then a low-temperature treatment effective for oxygen precipitation is taken effect, thus realizing the uniform distribution of precipitation in the crystal growth direction. The above and other advantages, features and additional objects of this invention will be manifest to those skilled in the art upon making reference to the following detailed description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the fluctuations in oxygen precipitation in Example 1 and Comparative Example 1; FIG. 2 is a graph similar to FIG. 1, showing the fluctuations in oxygen precipitation in Examples 2 through 5; and FIG. 3 is a graph showing the fluctuations in oxgen precipitation with repect to the temperature change at a low-temperature heat process in the Experiment. DETAILED DESCRIPTION A general standard process to which the method of this invention is applied is as follows: (1) Heat process temperature: 1,280°C Heat process time: 60 minutes Heat process atmosphere: dry O₂ Step (1) is a heat process to initialize the heat history during the crystal growing. In each of Examples described below, this heat process was added in order to assess the operation and results of the method of this invention accurately. (2) Heat process temperature: 400°C-550°C Heat process time: 30-180 minutes Heat process atmosphere: dry O₂ Step (2) is a heat process to introduce the oxygen precipitation nuclei into a single crystal silicon uniformly and is the most significant process. (3) Heat process temperature: 800°C Heat process time: 4 hours Heat process atmosphere: N₂ (4) Heat process temperature: 1,000 °C Heat process time: 16 hours Heat process atmosphere: dry O₂ Steps (3) and (4) are oxygen precipitation heat processes, which are the conventional standard heat processes. Examples of this invention will now be described; this invention should by no means be limited to these specific examples. Example 1: [Improved Distribution of Oxygen Precipitation in Crystal Growth Direction] Used Sample was as follows: Conduction type: N type (Phosphorus doped) Crystal diameter (Wafer diameter): 6" ⊘ (150 mm) Resistivity: 10 (Ω · cm) Oxygen Concentration: 14-16 (x10¹⁷ atoms/cm³) Carbon Concentration: < 2.37(x10¹⁵ atoms/cm³) Thickness of samples to be thermally processed: 2.0 (mm) This sample was thermally processed as follows: (1) A heat process was conducted under the following conditions: Heat process temperature: 1,280°C Heat process time: 60 minutes Heat process atmosphere: dry O₂ (2) Then a low-temperature heat process was conducted under the following conditions: Heat process temperature: 450°C Heat process time: 60 minutes Heat process atmosphere: dry O₂ Thereafter the following heat processes were conducted. (3) Heat process temperature: 800°C Heat process time: 4 hours Heat process atmosphere: N₂ (4) Heat process temperature: 1,000°C Heat process time: 16 hours Heat process atmosphere: dry O₂ The quantities of precipitated oxygen were measured, and the results of these measurements are shown in FIG. 1, from which it is noted that the oxygen precipitates were within the narrow range from 7 to 9 (x10¹⁷ atoms/cm³) and in particular that the local oxygen precipitate at the crystal bottom part did not become lower. The oxygen precipitate was calculated by the following equation: Oxygen precipitate = Oxygen concentration before heat process - Oxygen concentration after heat process. The measurement of the oxygen concentrations was conducted by utilizing the infrared spectroscopic analysis. Comparative Example 1: Only the heat processes of steps (3) and (4) were conducted directly without having taken the preliminary heat processes of steps (1) and (2). The oxygen precipitates are measured, and the results of these measurements were shown in FIG. 1 along with the results of Example 1. From FIG. 1 it is noted that the oxygen precipitates ranged from 0.8 to 6.8 (x10¹⁷ atoms/cm³) and hence distributed non-uniformly and in particular that the local oxygen precipitate at the crystal bottom part became lower. Examples 2 through 5: [Uniform Distribution of Oxygen Precipitation in Crystal Growth Direction] Using the same sample as in Example 1, the heat process of step (2) were repeated four times under the same conditions as Example 1, except that the heat process time was selected to 0.5 hour, 1.0 hour, 2.0 hours and 3.0 hours, respectively. The oxygen precipitates were measured, and the results of these measurements are shown FIG. 2 and also in Table 1, in which the oxygen precipitates in Example 1 also are shown. From the foregoing results, it was confirmed that by suitably setting the heat process time at 450°C, it is possible to uniform a predetermined oxygen precipitate in the direction of crystal growth. Experiment : The heat processes of steps (1), (2), (3) and (4) were conducted under the same conditions as Example 1, except that as the temperature of the step (2) was changed at intervals of 50°C in the range from 300°C to 1,000°C and the process time of step (2) was selected constant two hours at each and every temperature change. The same sample was used as in Example 1, except that the oxygen concentration was 15.2x10¹⁷ (atoms/cm³). The oxygen precipitates were measured with respect to the temperature change in the heat process of step (2), and the results of this measurement are shown in FIG. 3. From these measured results it was proved that the preferred temperature range with respect to the oxygen precipitation in the heat process of step (2) was from 400°C to 550°C. As described above, according to this invention, it is possible to provide a method for heat process of a silicon with improved distribution of oxygen preciptation, in which a predetermined oxygen precipitation can be obtained uniformly in the crystal growth direction, thus causing an adequate yield of manufacture of excellent IC products.

Claims: Method for heat process a single crystal silicon produced by the Czochralski process characterized in that said single crystal silicon is thermally processed at a low temperature ranging from 400°C to 550°C over a period from 60 to 180 minutes to obtain predetermined uniform oxygen precipitation in the direction of silicon crystal growth. Method according to claim 1 characterized in that said single crystal silicon is a silicon wafer. Method according to claim 1, characterized in that said single crystal silicon is a silicon ingot. Method according to any one of the preceding claims, characterized in that said single crystal silicon produced by the Czochralski process is thermally processed at a high temperature ranging from 1200°C to 1350°C, and thereafter said low temperature process is taken effect. Method according to claim 1, characterized in that the low temperature process is taken effect in the furnace of a Czochralski crystal puller, as an after-heating step, after completion of pulling-up of said single crystal silicon. Method according to claim 5, characterized in that said single crystal silicon is thermally processed at a high temperature ranging from 1200°C to 1350°C, and thereafter said low temperature process is taken effect. Method according to claim 4 or 6, characterized in that said high temperature treatment is conducted under dry oxygen, and in that said low temperature treatment is conducted over a period from 60 to 180 minutes under dry oxygen, and in that thereafter an oxygen precipitation heat process is taken effect.

Inventors: ABE TAKAO, FUSEGAWA IZUMI, YAMAGSHI HIROTOSHI, ABE, TAKAO, FUSEGAWA, IZUMI, YAMAGSHI, HIROTOSHI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1990
Main classification: C30B15
IPC codes: C30B29, C30B33, C30B15, H01L21