A METHOD TO DETERMINE THE EXTENT OF OXYGEN PRECIPITATE IN SILICON

Abstract: The method means that one measures with a spectrometer for the infrared interval the absorption coefficients for one or more of the wave numbers 728+/-1, 734+/-1, 740+/-1, 975+/-1, 988+/-1, 1000+/-1, 1006+/-1 and 1012+/-1<cm-1>, using the standard method that is used i.a. to determine oxygen atoms in interstitial position and carbon atoms in substitutional position when the measurement is carried out at the wave numbers 1106 and 605<cm-1>. The method means carrying out the absorption measurements at room temperature, standardizing the measurement results with respect to the thickness of the sample and multiplying them with a known calibration constant, whereupon the respective absorption coefficient for the different measured wave numbers are a measure of the concentration of different configurations of oxygen clusters.

Description: A METHOD TO DETERMINE THE EXTENT OF OXYGEN PRECI¬ PITATE IN SILICON : The present invention relates to a method to determine the extent of oxygen clusters in silicon. Crystalline silicon that is used to produce integrated circuits is usually so called Cz- silicon. These silicon crystals are produced according to the Czochralski method and contains relatively high concentrations of oxygen. The presence of oxygen has both advantages and disadvantages. Advantages are a more mechanically stable material and the so called gettering effect, which means that precipitates of oxygen can be used to absorb other undesirable contaminations. Disadvantages are i.a. that areas of different kinds of silicon oxides can develop within the crystals and that electrically active oxygen defects, so called thermal donors, can be created in the temperature range 350-500 °C, a range that is of interest today for producing integrated circuits of very high packaging density. The oxygen in the Cz-silicon originates from the walls of the quarts container con¬ taining the silicon melt and amounts typically to a concentration of 4 • 10 π - MO 18 atoms/cm 3 . This concentration corresponds to the solubility of oxygen atoms in melted silicon. At lower temperatures the solubility is lower and the supersaturation of oxygen that will therefore occur results i local clusters of silicon-oxygen com- pounds, Si x O v , where different values of x and y is conceivable. These clusters have other electrical properties than silicon and can therefore disturb the function¬ ing of a IC-circuit. The development towards smaller and smaller active elements in the silicon technology means that these clusters are able to affect the function and yield of circuit manufacturing more and more. In spite of the fact that the thermal donors created during heat treatment processes in the range 350-500 °C have been studied since the middle of the 1950's, no method exists up to now that directly measures the content of the silicon-oxygen complexes that are the underlying cause. The present invention solves this problem and gives an image of the extent of micro clusters and in which configurations they exist by being designed in the way that is evident from the following independent claim. The method according to the inven¬ tion is an important complement to the standard methods existing today to deter- mine the concentrations of individual oxygen and carbon atoms in silicon and are based on knowledge from optical and electrical measurements, by which a correla¬ tion between vibrational absorption of oxygen atoms and different electrically active centres have been established. The measurement is carried out at room temperature in a spectrometer for the infra¬ red interval and means that the intensity of the absorption bands corresponding to the wave numbers 728±1 , 734±1 , 740±1 , 975±1 , 988+1 , 1000±1 , 1006±1 and 1012 ±1 cm"1 are measured and standardized for the thickness of the sample. From a technical point of view the method of measurement is identical to the one used as a standard method for determining the oxygen and carbon content in . silicon, when the intensity of two absorption bands corresponding to the wave numbers 1106 and 605 cm" ' ' are measured, whereupon a correction is made for the thickness of the samples and a multiplication with known calibration constants. The known method measures the content of oxygen atoms in an interstitial position and carbon atoms in a substitutional position. Oxygen and carbon atoms in other posi¬ tions are not detected with this method. The difference in comparison to the known method is in principle that one measures at different wave numbers. The absorption coefficient is a direct measure of the number of oxygen atoms and can be used to determine the number of thermal donors. The absorption peak at the respective wave numbers corresponds to oxygen clus¬ ters of a certain configuration. By measuring at several wave numbers it is possible to get an image of the extent of oxygen clusters and of the distribution between different types of oxygen clusters. At the calculations it is possible to use the same calibration constant as is used in determining interstitial oxygen. In the continued development of the method it is possible that improved values can be achieved with a slightly adjusted calibration constant. The calibration constant for oxygen gives, however, a good estimation of the content and distribution of different types of oxygen clusters. The measurements can be carried out with different kinds of spectrometers for the infra-red interval. A modern Fourier Transform jnfra-Red spectrometer, FTIR, is very suitable in this connection.

Claims: Claims: 1. A method to determine the extent of oxygen clusters in silicon, characterized in that one measures with a spectrometer for the infra-red interval the absorption coefficients for one or more of the wave numbers 728+1 , 734±1 , 740±1 , 975±1 , 988 ±1 , 1000+1 , 1006±1 and 1012±1 cm -1 ' using the standard method that is used i.a. to determine oxygen atoms in interstitial position and carbon atoms in substitutional position when the measurement is carried out at the wave numbers 1106 and 605 cm "" ' - which method means carrying out the absorption measurements at room temperature, standardizing the measurement results with respect to the thickness of the sample and multiplying them with a known calibration constant, whereupon the respective absorption coefficient for the different measured wave numbers are a measure of the concentration of different configurations of oxygen clusters. 2. A method according to claim 1 , characterized in that one uses the same calibration constants is used in determining interstitial oxygen. 3. A method according to claim 1 or 2, characterized in that one uses a Fourier Transform jnfra-Red spectrometer, FTIR.

Inventors: HALLBERG TOMAS, LINDSTROEM LENNART, HALLBERG, TOMAS, LINDSTROEM, LENNART
Applicants: FOERSVARETS FORSKNINGSANSTALT, HALLBERG TOMAS, LINDSTROEM LENNART, HALLBERG, TOMAS, LINDSTROEM, LENNART
Application year: 1995
Main classification: G01N21
IPC codes: G01N21, H01L21