IMPROVEMENTS TO PULLING PROCESSES AND EQUIPMENT FOR GROWING SILICON CRYSTALS HAVING HIGH AND CONTROLLED CARBON CONTENT

Abstract: Silicon crystals with high and controlled carbon content can be grown by controlling the oxygen leakage during the melting in standard Czochralski silicon crystal pullers. Carbon concentration profile of grown crystals can be deduced from the carbon monoxide concentration [CO] real time monitoring, through its integral taken during the whole pulling duration.

Description: 1. Field of Invention The present invention relates to the growth of silicon crystals according to the Czochralski method, and more particularly to improvements to pulling processes and related equipment for growing silicon crystals having high and controlled carbon content. 2. Prior Art The present trend in microelectronics towards highly integrated semiconductor devices, requires more and more a good knowledge of the crystalline structure of the substrates used. In general, these substrates result from the slicing of monocrystalline silicon crystals obtained from the Czochralski pulling technique. It is generally admitted that the presence of oxygen atoms in the silicon substrate is beneficial. These atoms incorporated into the crystal during the pulling process, tend to diffuse during high temperature processing and to cluster, providing both valuable hardening and gettering effects. Hardening is a mechanical strength enhancement, due to the fact that oxygen precipitates impede dislocations and slips. Gettering is due to high microstresses at the vicinity of oxygen precipitates, which allow the capture of fast diffusing impurities, mainly metallic impurities. US Patent 4220483, assigned to the same assignee as of the present invention, describes in detail the gettering phenomenon. Recently, it has been demonstrated that carbon atoms, also incorporated in silicon crystals during the crystal growth, can play a significant role in the oxygen precipitation kinetics as well, when the substrates are submitted to subsequent thermal treatments. At that point, it is believed that the fundamentals of carbon (and oxygen) incorporation mechanisms must be understood before going further. The carbon incorporation in silicon crystals is assumed to occur from carbon monoxide (CO) absorption by the melt. Since the carbon vapor pressure is extremely low, it is admitted that, once a CO molecule is trapped by the melt, carbon will stay and become incorporated later on solid silicon according to the freezing segregation coefficient. Figure 1 shows a schematic cut view of conventional crystal puller equipment 10 of the atmospheric pressure type. A monocrystalline rod-shaped silicon crystal 11 is grown from a silicon melt 12 contained in a quartzliner 13. The crucible 14 consists of a rotating graphite holder. It is surrounded by an electric resistor 15. Other heating means may be used. A shield 16 . The seed 17 is affixed on a rotating mandrel 18. All the pulling device per se, is enclosed in a chamber 19, which is continuously filled with a neutral gas such argon. Seals 20 and 21 are necessary at crystal and crucible rotation shaft apertures to prevent leakage. In addition, the inside pressure is slightly above atmospheric pressure to avoid contamination. Thermochemical reactions that are likely to occur in pullers, to produce CO, are understood to be as follows. reactions between silica crucible and graphite holder : reactions between SiO and graphite : reactions between graphite and oxygen : The subscripts s, l, g denote solid, liquid and gaseous phases respectively. It is to be noticed that uncontrolled CO generation may give rise to silicon carbide production via reaction : Solid silicon carbide particles, if produced by reaction between the silicon melt and the carbon monoxide over the melt, can be driven by melt flow towards the freezing interface. Due to the presence of these defect particles, an interface breakdown occurs and is followed by the formation of polycrystalline grains. In such a case, also called structure loss, the crystal has to be remelted, to be pulled again in good condition. Should such a rework operation not be possible, the crystal would have to be considered as rejectable for integrated circuit manufacturing purposes. There is good background literature on this topic. We can refer for instance to a publication from F. Schmid et al. J. Electrochem. Soc. 126.6 (1979). Sometimes, as taught in the IBM Technical Disclosure Bulletin, Vol. 25, No. 4, Sept. 1982, pages 1905-1906 in an article entitled :"Low carbon Czochralski crystal growth" by R.C. Guggenheim et al, carbon is considered as an indesirable impurity, and care is exercised to reduce as much as possible, the carbon content in the grown silicon crystals. However, there is a strong tendancy to consider now that carbon atoms are beneficial to the gettering phenomenon, in cooperation with oxygen atoms. A good example of that approach may be found in European Patent application No. 84109528.4, entitled :"Silicon wafer and its application in producing integrated circuit devices", by D.C. Ahlgreen et al, and assigned to the same assignee as of the present invention. Disclosed in this patent application is a silicon wafer material, having a high carbon concentration in the range between 0.5 ppma and the solid solubility limit of carbon in silicon, and a low oxygen concentration in the range between about 15 ppma and 28 ppma. The wafer is capable of providing improved integrated circuit devices with significantly improved yields, due to large defect-free zones having a high quality crystal lattice formed therein during device fabrication. Another disclosed aspect is a manufacturing process of sorting the wafers formed from a melt-grown silicon crystal, by characterizing the wafers. The process is comprised of : measuring the concentrations of carbon and oxygen therein and grouping the wafers into classes in accordance with their oxygen and carbon concentrations. The wafers having high carbon and low oxygen concentrations are further processed through a device production line for obtaining high yield and high quality integrated circuit devices. According to the more detailed teachings of this patent application, once the silicon crystal rod has been pulled, the wafers are obtained from dicing the rod as standard, then characterized and individually sorted by measuring their concentration of carbon. It may happen, that only a small portion of the rod has the desired carbon concentration, so resulting in an important waste of costly wafers. In addition to its unexpected results, the suggested technique is complex and long. However, this application has the advantage of clearly emphasizing the key role of carbon on the final product yields. As it may be understood from Table I of above patent application, with wafers having the same oxygen concentration (e.g. 31ppma) the percentage of wafers having a leakage current of less than 20pA raises from 23% to an astonishing 73% when the carbon content slightly raises from about 1 to 3 ppma. On the other hand, the principle of introducing small amounts of carbon to Czochralski-grown silicon crystals is known, as described in the IBM Technical Disclosure Bulletin, Vol 25, No. 3A, August 1982, pages 962-963, an article entitled "Increasing carbon content in Czochralski-grown crystals by D.C. Ahlgreen et al. It is obvious from that disclosure, that no means is suggested on how to obtain controlled and reproducible carbon content. The secondary effects of introducing oxygen in the chamber, on the oxygen content in the silicon crystal are not analyzed. Lastly, according to that process, there is no indication on the actual amount of carbon introduced in the silicon crystal. In others words there is no control of carbon incorporation in the silicon crystal. In a recent publication, Y. Endo et al (J. Electrochem. Soc. Vol.126,No.8,1979,pages 1422-1425, proposed a combined oxygen-carbon control in silicon crystals by means of CO monitoring during crystal pulling. They assumed reactions (1) and (6) to occur, and an added reaction : They also defined an apparent equilibrium constant K. [C] si and [O] si being oxygen and carbon content in the silicon crystal at a location in the crystal rod corresponding to the time the carbon monoxide concentration was measured. Without any theoretical justification, K was found to be close to the equilibrium constant of reaction : derived from different experiments and computations. Experiments carried out by the inventors of the present invention, using Endo et al approach, concluded that no relationship could be established between carbon monoxide concentration [CO], measured during crystal pulling, and the corresponding product of the carbon content and oxygen concentration in silicon, [C] si x [O] si , for well defined locations in crystal rod, although [CO] values are in the same range of 10 to 50 ppma observed by Endo et al. This is demonstrated in Figs. 2a to 2e. In addition to that, no correlation could be made between oxygen present in a molecular state in the puller atmosphere, given by the oxygen concentration (O2), and oxygen concentration in silicon : [O] si (see Fig. 2f). We can explain the failure of this approach by the fact that carbon and oxygen are incorporated into a silicon crystal through completely separated independent mechanisms. It is generally admitted that oxygen incorporation in silicon results from silicon crucible disolution in the silicon melt via reactions : where {O} denotes dissolved oxygen in the silicon melt. Part of this oxygen is incorporated into the growing solid, while the most important fraction leaves the melt under SiO form via reactions : So, clearly, no relationship can be established between carbon generation (reactions (1) to (5)) and oxygen generation (reactions (9) to (11)), especially in the case where reactions (4) and (5) dominate. Consequently, a simple [CO] recording during the crystal pulling cannot be used to monitor carbon and oxygen concentrations in crystals on a real time basis. A process for providing silicon rods with controlled carbon content is given in Patent abstract of Japan, Vol 8, No. 102 (C-222) 1539, 12 May 1984. According to that disclosure before the silicon raw material is completely melted, a fixed amount (flow rate x time) of a carbon-containing gas such as CO gas is introduced from the inlet at the top of the furnace to it, and it is reacted with the silicon raw material melt. After the reaction is over, the silicon raw material is completely melted, the temperature of the melt is lowered to the seed contact temperature, and a single crystal rod is pulled up by the seed crystal. Consequently, the carbon impurtiy concentration in the silicon single crystal can be securely controlled. OBJECTS OF THE INVENTION It is a first object of the invention to provide improvements, including monitoring capabilities, to pulling processes and equipment for producing high carbon content silicon crystals, having controlled and reproducible characteristics. It is another object of the invention to provide improvements to pulling processes for producing high carbon content silicon crystals, based on an oxygen partial-pressure enhancement, achieved only during the melting in order to avoid formation of polycrystalline silicon grains at the melt interface. It is still another object of the invention to provide improvements to pulling processes and equipment for producing high carbon content silicon crystals, to render them fully operable in a manufacturing environment and, in addition, to be compatible with all known processes and equipment for growing silicon crystals with controlled oxygen characteristics. It is still another object of the invention to provide improvements to pulling equipment for producing high carbon content silicon crystals, which only requires minor design changes to the puller. SUMMARY OF THE INVENTION Desired and reproducible amounts of carbon in the silicon crystal may be obtained through the monitoring of : S = ∫ t o [CO]dt Carbon monoxide concentration [CO] in the puller atmosphere is continuously recorded during crystal pulling. A good correlation is pointed out between surface S and carbon content in silicon. A controlled amount of oxygen is introduced in the puller only during the melting, and will react with the carbon (graphite) pieces of the puller equipment. In a preferred embodiment, where conventional pullers are employed, the seals at the rotating shaft apertures are removed and the oxygen partial pressure is enhanced during a short and controlled time by creating a pressure depression. This depression may be indirectly caused by different argon flow settings to control oxygen leakage from ambient. As a result, because argon flow is of prime incidence on oxygen and carbon content in crystal, high carbon content crystals can be produced very simply this way. Finally, reaction between oxygen and the carbon parts may be enhanced by a special design of the graphite crucible, with respect to the quartz liner, in order to expose a greater area of the former. The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, forming a material part of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematical cross sectional view of a conventional crystal puller of the atmospheric pressure type. Figs. 2a to 2e are plots showing carbon monoxide concentration [CO] versus the product of carbon content and oxygen concentration in silicon [C] si x [O] si . Fig. 2f is a plot showing the oxygen concentration in silicon [O] si versus oxygen concentration [O2], all above plots resulting from previous works. Fig. 3a shows a plot of the carbon monoxide concentration [CO] during the various steps of the crystal pulling operation; it also points out surface Figs. 3b and 3c show respectively the graph of surface S, and carbon content in silicon [C] si as a function of the time t during the pulling operation. Fig. 3d shows the graph of surface S versus the carbon content [C] si during the pulling operation. Figure 4 shows the conventional crystal puller of Fig. 1 after modification, including the monitoring set-up and said special design of the graphite crucible, according to the present invention. Figure 5 shows a plot depicting surface (S) as a function of carbon content in a grown silicon crystal [C] si for a first experiment. Figure 6 shows a graph depicting surface (S) as a function of carbon content in a grown silicon crystal [C] si for another experiment. Figure 7 shows still another plot depicting surface (S) as a function of carbon content in a grown silicon crystal [C] si in an experiment where silicon crystals with high carbon content were desired. Figure 8 shows various plots depicting the carbon content in a grown silicon crystal [C] si as a function of the location in the rod, when different design changes are implemented in the crystal puller. DESCRIPTION OF THE INVENTION The chemistry of oxygen and carbon in silicon crystals will be now briefly reviewed, and more particularly, the interaction of oxygen and carbon monoxide in the gas phase of a crystal puller. From simple gas kinetics theory, we know that the number of particles striking a unit area per unit time, dN / dt, is given by : dN / dt = ¼ n . v where n is the particle density and v the average velocity. At a given temperature v is kept constant and n is proportional to the pressure, P. So that one can write that dN / dt proportional to P. dN / dt = α P where α is a proportionality constant. Then, any number, N, of particles having struck the same area within a given time interval, Δt, will be given by : N = α ∫ Δt P dt If we now apply the relation to our purpose, it is reasonable to assume that the total amount of dissolved carbon in the silicon melt, Q, will then be related to carbon monoxide concentration by : where Δt can be written as : Δt = t melt + t stab + t pull where t melt is the melting time duration starting from time t o till time t₁; t stab is the duration to cool the melt down from time t1 to pulling set time t₂; t pull is the overall duration of crystal growth starting from said pulling set time t₂; and, Δt is the total duration of the pulling process. Fig. 3 shows the graph of carbon monoxide concentration [CO]as a function of the total pulling duration Δt. The curve points out the different stages of the pulling operation : melting (I), stabilization (II) and growth (III). It is interesting to note that Q, as defined above, is proportional to the hatched surface S, as shown in Fig. 3a, where Then, since carbon does not evolve from silicon melt, one can expect that the carbon content in the grown silicon crystal will be related to surface S, as shown schematically in Fig. 3d, which is obtained from combining the graphs of Figs. 3b and 3c. Because the value of surface S is mainly determined by the preliminary stages of melting and stabilization of the melt, and is only slightly modified during the pulling operation itself , as clearly demonstrate by Fig. 3d, those preliminary stages, and in particular the initial carbon monoxide concentration [CO] o , is of key importance. The present invention is based on this observation, and controlled amounts of carbon will be introduced in the silicon crystal during its growth through the monitoring of surface S. According to the invention, a controlled amount of oxygen, or air, is introduced in the puller atmosphere only during the melting step. This oxygen is then converted into carbon monoxide via reactions (4) and (5) with hot graphite parts. The melting step is the most favorable time for incorporating carbon into the silicon melt for several reasons. As a matter of fact, during the melting of the polycrystalline load, the power provided to the heating resistor is maximum, and therefore the graphite heater temperature is the highest, enhancing reactions (4) and (5). There is, therefore, a maximum generation of CO in the chamber. On the other hand, for reasons previously given, it is not recommended to enhance CO generation during crystal growth. CO pressure is continuously recorded and the corresponding electronic output can be used to control the amount of oxygen introduced. Also, use of the same electronic output enables prediction of carbon content profile in the crystal. It also enables detection of any departure from given points of a crystal puller, and then the taking of corrective action to eliminate or minimize loss of defective substrate, say substrates with carbon content not in the specified range. Figure 4 shows the conventional puller of Figure 1, which has been modified according to the teachings of the present invention, the same elements bearing the same references. The main change in the puller, consists of the addition of a means for the controlled introduction of an oxygen-containing compound during the melting step, and a monitoring set-up 25 to monitor surface S through the continuous determination of [CO]. According to a first embodiment, the said means includes port 22, valve 23 and feeding pipe 24, connected to an oxygen source (not represented). There are others ways of introducing oxygen. According to an another embodiment, which is preferred, the hermetic seals, 20 and 21, are removed and a controlled amount of air is simply introduced by applying an argon depression. This depression may result from backstream, depending on chamber pressure with respect to ambient. The monitoring set-up 25 will now be described in detail. It is comprised of a means for extracting carbon monoxide from the chamber and integrating means to determine surfaces. Extracting means includes first a gas sample outlet 26, located on the top plate just above the melt. The sample outlet location is of prime importance for the validity of the results. It is reasonable to admit that not all the total amount of carbon monoxide, produced in the chamber, will be trapped by the silicon melt, since the flow of argon drives both SiO and CO away from the melt. For instance, measurements performed at the argon exhaust indicated [CO] values up to 150 to 200 ppm during crystal growth, compared with 20 ppm near the melt. It also includes a rockwool filter 27, used to trap silicon monoxide particles, so that, at the output of the filter, only carbon monoxide will be present. Carbon monoxide concentration [CO] is measured by IR absorption, with a Cosma diamant 6000 model analyzer, referenced 28. Before each measurement, the analyzer is calibrated with a reference gas through valve 29. A direct recording diagram is made available on a graphic plotter and, meanwhile, the output signal is recorded on a computer tape, and further electronically corrected for baseline drift. 82.5 mm diameter crystals have been first pulled with two different conventional pullers, referenced A and B, then with the improved equipment of Fig. 4. [CO] was recorded on a point to point basis during the melting operation and continuously thereafter. Starting from seed, cone wafers were sliced at 0,5 inch (13 mm) steps, corresponding to 10 minutes intervals of pulling time. The corresponding [CO] values were selected, providing unambiguous coincidence between a given [CO] value and carbon and oxygen concentrations in the crystal, as measured thru FTIR (Fourier Transform Infra Red) spectrophotometer. Concentrations of oxygen and carbon are given on the basis of ASTM standards F 121-1979 and F 123-1976 respectively. Typical results are plotted in Figures 5 and 6 for standard pullers A and B respectively, when no oxygen is introduced during the melting step. A good correlation is pointed out between surface S in ppm x min. and [C] Si in ppma. It is to be pointed out that plots of Figures 5 and 6 indicate a good agreement of numerical values.For example, with both pullers A and B, with S = 34 x 10³ ppm min., the carbon content in the silicon crystal is about 2 ppma. In both cases, the S versus [C] Si curve is not linear due to the segregation coefficient of carbon. Fig. 7 shows the same S versus [C] si plot, with the improved equipment, when air has been introduced during melting to achieve a [CO] level of 800 ppm, turning to a normal 15 ppm level during crystal growth. Higher values of [C] si are noticed starting from 2 ppma at the seed end portion of the crystal, ending at values near solubility limit at the tail end (about 10 ppma). These results were obtained under the following experimental conditions : A lower level of [C] si could be obtained by reducing the puller pressure below ambient for a shorter time. The S surface and therefore [C] si values would become consequently smaller. So the desired [C] si level could be reached by adjusting the S surface through appropriate settings of the two key parameters : depression amplitude and duration of depression among those mentioned above. Fig. 7 shows that if high carbon content silicon crystal are needed for example 4 ppma ≦ [C] si ≦ 10 ppma, the S factor should be at least equal to 37 x 10³ ppm x min. The dominating factor will be [CO] o in ppm as explained above (in conjunction with Figs. 2a to 2f). If the puller equipment of Fig. 4 is manually operated, the operator knows normal values for t melt , t stab and t pull , depending the results displayed by the analyzer, he will adjust the argon depression to reach the desired [CO] o level appropriate to achieve the desired S value. This again demonstrates that simply recording the [CO] value during crystal growth will not reflect the actual carbon concentration in the crystal, since a steady level in the 15 to 20 ppm range was observed during growth of either low or high carbon containing crystals. Since no major change has been implemented on a classical puller, it is possible to adjust the oxygen concentration profile on the crystal by employing techniques such as described in US-A-4417943 (IBM) for instance, to definitely meet the desired range of carbon content coupled with the desired range of oxygen concentration, necessary for gettering purposes with no loss of substrates. Graphite package geometry may also be arranged in such a way as to adjust, and in particular, to increase the graphite-oxygen exchange area near the melt, where CO is most likely to be trapped. Experiments have shown that to be effective, the design change to the package geometry must take place at the vicinity of the melt interface. Silica quartz liner 13 has 1 cm cut in height with respect to the crucible 14 to expose the upper part of it and so the available exchange area for oxygen reaction with graphite part was enlarged. This design change has been introduced in the equipment as shown in Figure 4. However, such a configuration, by itself does not provide reproducible results, as it will be shown in Fig. 8. Turning now to Fig. 8, curve I represents the carbon content in silicon crystal [C] si as a function of the location in the crystal rod, from seed to tail, for a crystal grown with the conventional puller equipment shown in Fig. 1. With the same equipment, two crystals have been grown with the above described design change. Curves II and III demonstrates there is a significant increase in the carbon content in the grown crystals. However, one can see that for the same apparent process condition, the carbon concentration profiles for the two silicon crystals are somewhat dispersed and therefore the design change could not be used alone without the controlable introduction of oxygen in an industrial manufacturing environment. Standard atmospheric pressure pullers, after having been modified according to the teachings of the present invention, appear to be flexible tools to produce silicon crystals with high and controlled carbon content. Simple action on chamber pressure, along with a monitoring of carbon monoxide partial pressure, result in a fully controlled process.

Claims: A crystal puller (10) for growing a silicon crystal (11) having reproducible, high and controlled carbon content including: a chamber (19) provided with hermetic seals (20, 21) at rotation shaft apertures; a quartz liner (13) fitted in a graphite crucible (14) adapted to contain the melted source of silicon (12) for the crystal to be grown; a heater (15) concentrically surrounding said crucible and operative to render and to maintain the source material in a molten state; said graphite crucible and heater being positioned within the said chamber; means for introducing and exhausting an inert gas to and from the chamber; means (18) for introducing a seed (17) in the molten source (12) and for pulling it to grow said crystal (11); means (26) for extracting carbon monoxide from the chamber located near the melt surface; means (28) for measuring carbon monoxide concentration (CO) so extracted; integrating means to monitor the quantity wherein Δt is the total pulling duration (Δt = t melt + t stab + t pull ) , so that S is equal to a selected value which corresponds to the desired carbon content in the grown crystal; and, means for the controlled introduction of an oxygen-containing gas into the chamber during the melting step, responsive to said integrating means ; said gas reacting with the crucible to produce carbon monoxide. A crystal puller (10) for growing a silicon crystal (11) having reproducible, high and controlled carbon content including: a chamber (19) without the provision of hermetic seals at rotation shaft apertures; a quartz liner (13) fitted in a graphite crucible (14) adapted to contain the melted source of silicon (12) for the crystal to be grown; a heater (15) concentrically surrounding said crucible and operative to render and to maintain the source material in a molten state; means for introducing and exhausting an inert gas into and from the chamber; means for providing a controlled depression of the innert gas during the melting; means (18) for introducing a seed (17) in the molten source (12) and for pulling it to grow said crystal (11); means (26) for extracting carbon monoxide from the chamber located near the melt surface; means (28) for measuring carbon monoxide concentration (CO) so extracted; integrating means to monitor the quantity wherein Δt is the total pulling duration (Δt = t melt + t stab + t pull ) so that S is equal to a selected value which corresponds to the desired carbon content in the grown crystal; and, means for the controlled introduction of an oxygen-containing gas into the chamber during the melting step, responsive to said integrating means ; said gas reacting with the crucible to produce carbon monoxide. Crystal puller according to claim 1 or 2, wherein, the flanges of graphite crucible (14) are exposed with respect to the quartz liner (13). A pulling process for growing a silicon crystal having reproducible high and controlled carbon content according to the Czochralski technique including the steps of: providing an inert atmosphere in a chamber (19) and melting silicon material (12) in a quartz liner (13) fitted in a graphite crucible (14); characterized in that it further includes the steps of: measuring the carbon monoxide concentration (CO) near the melt using measuring means ; controllably introducing an oxygen-containing gas into the chamber during the melting step, which forms carbon monoxide (CO) by reaction with the hot graphite crucible, until a determined initial carbon monoxide concentration (CO) o is attained ; monitoring the quantity determined by integration means wherein Δ t is the total pulling duration (Δt = t melt + t stab + t pull ) , so that said quantity S corresponds to the carbon content in the grown crystal that is finally desired. A pulling process according to claim 4 wherein said oxygen containing gas is selected in the group comprising oxygen and air.

Inventors: CARLE, JEAN-FRANCOIS, PHILIPPOT, PATRICK, Carle, Jean-François
Applicants: COMPAGNIE IBM FRANCE, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1984
Main classification: C30B15
IPC codes: C30B15, H01L21, C30B29