TW201736648A - Method for adjusting the resistivity of a semiconductor ingot during its manufacture - Google Patents

Abstract

The invention concerns a method for manufacturing ingots made of semiconductor material, comprising the following steps-crystallising, under particular pulling conditions, a first ingot, called the reference ingot, from a first molten charge containing oxygen; - measuring the interstitial oxygen concentration in different areas distributed along the reference ingot; - measuring, in the different areas of the reference ingot, the concentration of thermal donors formed during crystallisation of the reference ingot; - determining the effective times of a thermal donors formation annealing, undergone by the different areas of the reference ingot during its crystallisation, from measurements of the interstitial oxygen concentration and of the thermal donor concentration; - calculating the thermal donor concentration values to be obtained in order for a second ingot to have, after crystallisation, an axial electrical resistivity according to a target profile; - determining an axial profile of the interstitial oxygen concentration corresponding to the axial resistivity target profile, from thermal donor concentration values and from the effective times of the thermal donor formation annealing; - crystallising under said particular pulling conditions the second ingot from a second molten charge containing oxygen, where the oxygen concentration of the second molten charge is adjusted as the crystallisation progresses, in order to obtain, in the second ingot, the axial profile of the interstitial oxygen concentration.

Description

Method of adjusting the resistance of a semiconductor ingot in manufacturing

The present invention relates to a method of making an ingot made of a semiconductor material. In particular, the present invention relates to a manufacturing method capable of adjusting the axial resistance of an ingot.

The Czochralski method is generally used to form a single crystal germanium ingot. It involves melting a certain amount of ruthenium (called a filler or batch) in a crucible and crystallizing the crucible from the single crystal seed crystal. The seed crystal (which is aligned on the crystal axis of the formed crystal) is first immersed in the ruthenium melt. Then slowly rotate it and pull it up. In this way, the solid ruthenium ingot is gradually grown by feeding from the melt.

The crucible is usually doped to adjust its resistance. Prior to crystallization, dopants such as boron and phosphorus are incorporated into the molten filler or in the filler prior to the melting step.

With the diesel-type pull-up method, the dopant tends to accumulate in the tantalum melt due to segregation. The dopant concentration in the region where the ingot begins to solidify is lower than the region in which the solidification ends. In other words, the dopant concentration in the bismuth ingot increases as crystallization progresses. This creates a variation in resistance at the height of the ingot.

Tantalum ingots with varying electrical resistance may be difficult to use in their entirety. For example, the manufacture of photovoltaic cells requires a specific range of electrical resistance. Therefore, the highest or lowest end of the resistance of the ingot is usually discarded. In order to save enthalpy, it is therefore envisaged to form a bismuth ingot having a uniform electrical resistance on a solid portion of the height of the ingot.

Document US 2007/0056504 describes a technique for forming a crucible having a uniform axial electrical resistance such that the dopant concentration in the crucible melt remains fixed. The control of the resistance is obtained by adding germanium and a dopant at regular intervals.

This technique requires a lot of effort because the ingot must be removed from the melt in each addition step and someone must wait for the dopant and the crucible to be completely melted. The dopant is introduced in the form of a highly doped cerium powder or plate. In these cases, the addition of dopants also causes the ruthenium to be contaminated by other impurities, particularly metal impurities, which is not conducive to application in photovoltaic applications. Finally, if a uniform resistance is not obtained after pulling the ingot, it is discarded or regenerated.

Patent FR 2 997 096 describes a method of generating a hot donor by annealing so that the electrical resistance of the antimony ingot can still be corrected after it has been crystallized. The hot donor is an agglomerate formed by the interstitial oxygen contained in the crucible (i.e., the oxygen atoms occupy a gap in the crystal lattice) when heated to a temperature between 350 ° C and 550 ° C. Each thermal donor produces two free electrons that cause a change in the resistance of the crucible. The lifetime of charge carriers is not affected by the formation of hot donors because they do not recombine.

The method specifically comprises the steps of measuring the interstitial oxygen concentration along the ingot; calculating the desired thermal donor body in each portion of the crucible The concentration is to reach the target resistance value; the annealing temperature to be applied in each portion of the crucible is calculated to produce the thermal donor; and the annealing is applied in a furnace containing regions of different temperatures.

Since niobium is an excellent thermal conductor, the temperature gradient in the niobium ingot will vary greatly from the temperature gradient established in the furnace. Thus, the concentration of the hot donor produced in the various parts of the ingot does not match the previously calculated concentration. For this reason, it is preferred to separate the portions by cutting the ingot and distribute them in various temperature regions of the furnace. In this way, the desired heat donor concentration is obtained in each part.

This method makes the axial resistance of the ingot uniform. However, since each ingot must complete the cutting step and the annealing step, the implementation of the method is lengthy and expensive.

It is an object of the present invention to provide a fast and economical method for fabricating a semiconductor ingot having a predetermined axial resistance. The "axial" resistance of the terminology is interpreted to mean the electrical resistance along the longitudinal axis of the ingot. Since the shape of the ingot is generally cylindrical, the longitudinal axis of the ingot may coincide with the axis of rotation of the cylinder.

According to the present invention, this object is achieved by providing a method of making an ingot made of a semiconductor material, the method comprising the steps of crystallizing from a first molten filler containing oxygen under specific conditions of drawing. a first ingot referred to as a reference ingot; the interstitial oxygen concentration is measured in different regions distributed along the reference ingot; Measuring the concentration of the hot donor formed during the crystallization of the reference ingot in different regions of the reference ingot; determining the interstitial oxygen concentration and the thermal donor concentration during the crystallization of the reference ingot The effective time for forming the annealing of the hot donor body experienced by the different regions; calculating the thermal donor concentration value required for the second ingot to have the axial resistance according to the target graph after crystallization; by the hot donor concentration And an effective time for forming an annealing by the hot donor, determining an axial curve of the interstitial oxygen concentration corresponding to the axial resistance target graph; and under the specific pull condition, the second containing oxygen The molten filler crystallizes the second ingot, wherein the oxygen concentration of the second molten filler is adjusted as the crystallization progresses to obtain an axial plot of the interstitial oxygen concentration in the second ingot.

When the ingot is being crystallized, the temperature of the semiconductor material is slowly lowered, for example, from 1414 ° C (melting point of hydrazine) to room temperature (about 25 ° C). Between 350 ° C and 550 ° C, the interstitial oxygen contained in the semiconductor material forms a thermal donor.

The steps of the method for the reference ingot are for determining the formation kinetics of the hot donor for a given draw formulation, and particularly during the formation of the hot donor in the reference ingot during crystallization (ie, the The temperature of the semiconductor material is between 350 ° C and 550 ° C). This period can be considered as "the effective time for forming the annealing of the hot donor" because the solidification of the semiconductor material (from 550 ° C to 350 ° C) produces a hot donor in the same manner as conventional annealing. The ingots crystallized by the same pull-up formulation will then have the same effective time to form the hot donor.

Since the effective time to form the hot donor varies depending on the longitudinal position in the ingot (because the temperature drops somewhat somewhat depending on the position), these steps are carried out in several regions distributed along the reference ingot.

After these steps, referred to as the step of characterizing the pull-up recipe, it is possible to directly obtain an ingot having a target axial resistance profile after the pull-up. Indeed, the axial resistance can be adjusted to the target profile by adjusting the amount of thermal donor generated in each region of the ingot during crystallization. The amount of hot donor produced is adjusted by controlling the amount of oxygen that is incorporated into the various regions of the ingot at a given effective time to form a hot donor in this region.

In this manner, the manufacturing method according to the present invention can obtain an ingot having a predetermined axial resistance without using a cutting step and a subsequent annealing step. Therefore, the manufacturing method according to the present invention can be implemented faster and less expensively than the prior art method.

For each pull-up formulation, the crystallization of the reference ingot and the measurements made on the ingot were performed only once. The cost of these steps can be spread over a large number of ingots that are crystallized by the same pull-up formula. Thus, these preliminary steps did not result in a significant increase in the manufacturing cost of the ingot.

In a preferred embodiment of the invention, the target axial resistance profile is fixed. In other words, it is sought to obtain an ingot with a uniform axial resistance.

According to the method of the invention, it is also possible to consider more than one of the following features, individually or in all technically possible combinations: The second ingot is subjected to formation annealing of an additional hot donor during a predetermined period of time, such that when determining an axial plot of the interstitial oxygen concentration, the additional annealing time is added to the effective time; The concentration of oxygen in the second molten filler is adjusted by immersing at least one segment of the ring disposed around the second ingot with the second molten filler, the article being formed of an oxygen-containing material, such as quartz. Or yttrium oxide; and the method further comprises: determining the axial curve of the dopant concentration in the second ingot prior to calculating the hot donor concentration value when the first and second molten fillers contain dopants The steps of the diagram. When the first and second molten fillers contain the same concentration of dopant, the axial profile of the dopant concentration in the second ingot can be measured by measuring the dopant concentration in different regions of the reference ingot To decide, or to use the Scheil-Gulliver equation to calculate.

20‧‧‧ Melt

21‧‧‧ objects

22‧‧‧ vertical axis

23‧‧‧Top/isosceles triangle

24‧‧‧ Direction

Further features and advantages of the present invention are illustrated by the following description, taken in conjunction with the accompanying drawings, in which: FIG. 1 shows steps S1 to S7 of a method of manufacturing a semiconductor ingot according to the present invention; Figure 2 shows a preferred embodiment of the step S7 of crystallizing the second ingot, which increases the interstitial oxygen concentration in the second ingot; and Figures 3A to 3H illustrate an exemplary implementation of the manufacturing method according to the present invention. example.

For the sake of clarity, the same or similar elements are designated by the same element symbols throughout the drawings.

Figure 1 is a block diagram showing the different steps S1 to S7 of a method of manufacturing an ingot made of a semiconductor material and having a specific axial resistance profile. The semiconductor material of the ingot is, for example, tantalum.

During the first step S1, the first bismuth ingot referred to below as the reference ingot is crystallized from the cerium melt. The melt also contains oxygen, which is obtained by melting the ruthenium filler in a crucible. The oxygen can be obtained from ruthenium, which is usually formed of quartz or ruthenium oxide because it is partially dissolved under the effect of temperature, and/or ruthenium filler (i.e., solid ruthenium).

In addition to oxygen, the tantalum melt may also contain dopants such as boron and/or phosphorus. These dopants are introduced into the melt in the form of highly doped cerium powder or wafer prior to pulling the ingot, or are initially included in the filler. After the crystallization, the dopant is unevenly distributed in the reference ingot, which results in substantial variation of the axial resistance, for example, a factor of 10 between the ends of the ingot.

Different pull-up techniques can be used to crystallize the reference ingot. In particular, a wood-based process can be used to obtain a single crystal germanium ingot, or a Bridgman process can be used to obtain a polycrystalline germanium ingot. The reference ingot is crystallized under specific conditions of lift suitable for the nature of the filler and the selected draw technique. In the case of using the ingot crystallized by the woodworking process, these conditions are, for example, the pulling speed of the ingot relatively perpendicular to the crucible, the rotational speed of the crucible, and the rotational speed of the ingot.

During the crystallization of this step S1, when the temperature of the crucible is between 350 ° C and 550 ° C, the crucible of the reference ingot incorporates oxygen atoms at the interstitial position during the cooling of the crucible. The period during which the hot donor is formed Depending on the pull-up condition, and in particular on the pull-up speed of the ingot. Indeed, the faster this pull-up, the faster the temperature of the solid helium drops. Furthermore, the amount of oxygen incorporated into the ingot is also dependent on the conditions of the drawing, and in particular on the gas pressure in the furnace, and the speed of rotation of the crucible and the ingot. Therefore, each pull-up formulation has its own formation kinetics of the hot donor.

Steps S2 to S4 characterize the formation kinetics of the hot donor of the pull-up formulation used to crystallize the reference ingot.

In step S2, the concentration of the interstitial oxygen [Oi] _i incorporated into the reference ingot is measured in different regions distributed along the reference ingot (in other words, different positions along the longitudinal axis of the ingot). Therefore, an axial (longitudinal) graph of the interstitial oxygen concentration [Oi] _i in the reference ingot is obtained.

The longitudinal measurement of the interstitial oxygen concentration [Oi] _i can be performed by Fourier transform infrared spectroscopy (FTIR) on a thick wafer (>2 mm thickness) taken from the ingot perpendicular to the longitudinal axis and ground. . Beneficially, the number of measurement areas is equal to the number of wafers cut from the reference ingot.

In the preferred embodiment of step S2, the interstitial oxygen concentration [Oi] _i is measured over the entire ingot. The concentration [Oi] _i can be measured on the scale of the ingot using infrared spectroscopy techniques (generally referred to as "full stick FTIR"). This technique, derived from Fourier Transform Infrared Spectroscopy (FTIR), involves scanning the ingot with an infrared beam that moves in parallel with the longitudinal axis of the ingot. The infrared beam is absorbed by the ingot to determine the interstitial oxygen concentration which is averaged over the ingot diameter.

Another technique, based on the formation of an additional hot donor, determines the oxygen concentration [Oi] _{i in the} crucible. This technique has been described in detail in the patent FR 2 964 459 for the oxygen mapping of germanium wafers. It is also possible to apply this technique on the scale of the ingot.

After the initial resistance in each region of the reference ingot has been measured, the ingot is annealed to additionally form a thermal donor in addition to the thermal application formed during crystallization. The temperature of this annealing is preferably uniform and between 350 ° C and 550 ° C. Then, the annealed resistance was measured in each region of the reference ingot. Since the variation of the electrical resistance can be attributed to the formation of an additional thermal donor, the concentration of the thermal donor formed by the annealing can be inferred therefrom. The oxygen concentration [Oi] _i in each measurement zone is determined by the concentration of the hot donor newly generated during the annealing between 350 ° C and 550 ° C.

The latter technology is precise and extremely simple to use. Even if it is applied to a wafer sampled by the reference ingot, it is advantageous because it does not require polishing of the wafer and is not limited to the thickness of the wafer unlike FTIR technology.

In step S3, the concentration [DT] _i of the hot donor generated during the crystallization of the reference ingot is measured along the reference ingot in the same region as described above. This batch of measurements constitutes an axial plot of the hot donor concentration [DT] _i .

The concentration [DT] _{i is} preferably obtained by variation in electrical resistance or variation in charge carrier concentration caused by high temperature annealing (≧600 ° C). This high temperature anneal (typically 30 minutes at 650 ° C) causes the hot donor formed during crystallization of the reference ingot to be destroyed. The resistance can be measured by a four-point method or a Van der Pauw method (before and after destructive annealing) or by measurement of eddy currents. The charge carrier concentration can be measured by the Hall effect or by the CV measurement.

Even when the technique of the patent FR 2 964 459 is used to measure the interstitial oxygen concentration [Oi] _i , the step S3 can be carried out after the step S2. In this case, it is sufficient to consider the initial resistance (or initial charge carrier concentration) of the reference ingot, ie after crystallization but before the formation of an additional hot donor is annealed. It is also possible to carry out step S3 before step S2, in which case there is no longer a hot donor when annealing is carried out between 350 ° C and 550 ° C.

In other words, the manufacturing method is not limited to the order of steps S2 and S3. Patent Application FR3009380 gives more details on the manner in which step S2 of measuring the interstitial oxygen concentration [Oi] _i and step S3 of measuring the hot donor concentration [Oi] _i are combined.

After steps S2 and S3, the values of the pairs [Oi] _i and [DT] _i corresponding to the different measurement areas of the reference ingot are obtained.

In step S4, the effective time t _eff of annealing between 350 ° C and 550 ° C experienced by each of the measurement regions during the crystallization of the reference ingot is determined. This annealing can form a hot donor of concentration [DT] _i from the oxygen concentration [Oi] _i . Therefore, the effective time t _eff in the region of the reference ingot can be determined by the values of [Oi] _i and [DT] _i measured in the same region in steps S2 and S3, respectively.

For example, the effective time t _eff can be obtained by using the article ["The formation kinetics of oxygenated heat donors in sputum", Wijranakula CA et al, Appl. Phys. Lett. 59 (13), pp. 1608, 1991] The relationship is calculated. This article describes the kinetics of forming a hot donor in bismuth by annealing at 450 °C. Then, the time t _eff is considered to correspond to the period of annealing at 450 ° C, which must be used to obtain a hot donor concentration equal to [DT] _i from the oxygen concentration equal to [Oi] _i .

According to the above article, the initial hot donor concentration [DT] _i , the initial interstitial oxygen concentration [Oi] _i and the annealing time at 450 ° C are linked by the following relationship:

Wherein D ₀ is the interstitial oxygen diffusion coefficient at 450 ° C (D ₀ = 3.5.10 ^-19 cm ² /s).

The time t calculated in this way is a good approximation of the effective time t _eff , that is, the time it takes for the crucible to be in the range of 350 to 550 ° C during the solidification of the reference ingot.

In order to calculate the time t _eff , the above relation (1) is preferred because the temperature of 450 ° C is the temperature at which the formation kinetics of the hot donor is most understood. Annealing at 450 °C has been the subject of much research because it represents a good balance between the rate of formation of the hot donor and the maximum concentration obtained.

Alternatively, the effective time t _eff can be determined by a graph of the hot donor concentration [DT] _i giving a function of the time t annealed at 450 ° C for various values of the oxygen concentration [Oi] _i .

For the annealing temperature outside 450 °C, the relationship (1) and the chart can be modified, especially thanks to the article ["At the temperature between 350 and 500 ° C, the oxygen concentration on the formation kinetics of the hot donor in the crucible The influence of "Londos CA et al., Appl. Phys. Lett. 62 (13), pp. 1525, 1993]. This article also describes the formation kinetics of the hot donor in the crucible, but for the annealing temperature comprised between 350 ° C and 500 ° C.

When the reference ingot has been cut into wafers and has a number of pairs of [Oi] _i and [DT] _i for a given measurement area (ie, a given wafer), an annealing at 450 ° C can be calculated. Average time. This average will then represent a better indicator of the effective time t _eff associated with this region.

Knowledge of the effective hot donor formation time (t _eff ) along the reference ingot can be used to control the amount of hot donor formed during the subsequent steps of crystallization in accordance with the same pull-up formulation. Steps S5 to S7 of the method of Fig. 1 implement control of the amount of this hot donor to achieve a target axial resistance profile in the second bismuth ingot.

Step S5 of Figure 1 includes calculating a value of the target hot donor concentration [DT] _tg such that the target axial resistance profile is reached during crystallization of the second ingot, given the basic resistance of the second ingot. The term "substantial resistance" is interpreted to mean the inherent resistance of yttrium and can be increased by the addition of a receptor and/or a donor dopant (without a hot donor). The target hot donor concentration [DT] _tg is calculated for the portion of the second ingot corresponding to the reference ingot measurement region (i.e., those portions located at the same position on the longitudinal axis of the ingot).

In the following, a way of calculating the target hot donor concentration [DT] _tg is given.

The resistance ρ in a semiconductor material such as erbium varies according to two parameters: the main free charge carrier concentration m and the mobility μ of these carriers, depending on the concentration of the thermal donor present in the enthalpy [DT] ]. Its general formula is as follows: Where q is the basic charge (q = 1.6.10 ^-19 C). The total concentration of N _A/D ionized acceptor dopants N _A and/or donor dopants N _D (eg, boron and phosphorus, respectively).

The main free charge carrier concentration m is equal to the addition (in the case of n-type doped germanium) or the subtraction (in the case of p-type doped germanium) twice the hot donor concentration [DT] (per hot The donor has a net doping of the two electrons produced (the difference between the acceptor and donor dopant concentrations).

The rate of movement of the main charge carriers in the crucible depends on the temperature T of the crucible and the dopant concentration (both donor and/or acceptor). Considering the hot donor body (which is a "double" dopant in the donor form), the mobility μ (unit: cm ² .V ^-1 .S ^-1 ) can be expressed by the following relationship:

The temperature at which T _n is normalized with respect to room temperature (T _n = T/300).

For the two forms of the main charge carriers in the crucible, the parameters μ _max , μ _min , N _ref , α, β1, β2, β3 and β4 are given in the table below.

Using the above formulas (2) and (3), the corresponding value of the target hot donor concentration [DT] _tg can be calculated for the target resistance value ρ _tg . Indeed, when the second tantalum ingot will have no dopant (N _A/D =0), the only unknown number of equation (2) is [DT] _tg . Conversely, when the second ingot will be doped (N _A/D > 0), the same equation and the value of N _A/D are used to determine the concentration [DT] _tg . Thus, the concentration m is equal to: m ([DT], N _{A / D} )=| N _A - N _D -2. [DT]|

The shift rate μ is calculated as a function of the conductivity morphology of the ingot at the crystal end, but with a non-zero value of N _A/D (compare: relation (3)).

In this way, the value of the target concentration [DT] _tg at which the target axial resistance map can be reached is determined by assigning the target resistance value ρ _tg to each region of the second ingot. The target resistance value ρ _tg of the batch for different measurement regions constitutes a target axial resistance curve, and the batch [DT] _tg value constitutes an axial curve of the concentration of the hot donor body formed during crystallization by analogy. .

In a preferred embodiment of the manufacturing method, the object is to obtain a second tantalum ingot having a uniform axial resistance. Thus, the target axial resistance graph is fixed. In other words, a single target value ρ _tg is used for the entire area of the second ingot.

When the enthalpy of the second ingot is doped, for example, in order to obtain a p-type or n-type enthalpy, or a compensated enthalpy, it is advantageous to determine the ionization along the second ingot before solving equation (2). Dopant concentration N _A/D . Two methods can be used to determine the axial profile of the dopant concentration N _A/D : after annealing to destroy the hot donor ([DT] = 0), by the resistance from the reference ingot The longitudinal measurement of ρ is calculated (compare equation (2), the charge carrier concentration m depends on N _A/D ); or it is calculated by using the Schel-Grelev equation.

The Schell-Grefeld equation gives the distribution of dopants along the semiconductor ingot, which is caused by the phenomenon of dopant segregation. It is written as follows: Wherein, N _charge is added to the concentration of the dopant of the semiconductor filler (or contained in the filler), and the acceptor dopant (N _A ) or the donor dopant (N _D ) is effective in the k-system problem. The coefficient is assigned, and f _S is the fraction of the solidification of the ingot (expressed as % of the total length of the ingot).

The first method for determining the concentration N _A/D means that the first ingot (or reference ingot) and the second ingot (or subsequent ingot) are doped in the same manner, ie the same chemistry is utilized in the filler And the same dopant concentration. Conversely, a second method can be used without the doping of the second ingot.

If the hot donor body provides electrons ("body" role), the formation of the hot donor during crystallization will inevitably reduce the resistance of the n-doped germanium ingot and increase the resistance of the p-doped germanium ingot.

In step S6, an axial curve of the interstitial oxygen concentration [Oi] _{tg is} determined, which is a target axial curve obtained by obtaining the hot donor concentration [DT] _tg in the second ingot and thus obtaining a target The axial resistance curve ρ _tg is required. The axial profile of the oxygen concentration [Oi] _tg contains several values for the region, with the target hot donor concentration [DT] _tg value and the effective time t _eff value. These values are preferably calculated using the above relation (1), using the corresponding value of [DT] _tg calculated in step S5 and the corresponding effective time t _eff determined in step S4.

The dopant concentration of the first ingot and the second ingot may be different. When they are both below 2.10 ¹⁶ cm ^-3 , it is preferred to use the relation (1) of Wijaranakula CA et al., both of which calculate the time t _eff (reference ingot) and determine the interstitial oxygen concentration [Oi] _tg (p. Two spindles) because this model is extremely precise. In contrast, when the second ingot is to be doped to a level higher than 2.10 ¹⁶ cm ^-3 , the Kazumi Wada model ["a unified model for the formation dynamics of oxygenated donors in the sputum", Physical Review B, Vol. 30, N. 10, pp. 5885-5895, 1984] is more suitable for calculating the concentration [Oi] _{tg of} the second ingot. If the reference ingot is also doped to be higher than 2.10 ¹⁶ cm ^-3 , the model can also be used to calculate the effective time t _eff .

It is possible to judge a value (or a number of values) of the interstitial oxygen concentration [Oi] _tg calculated in step S6 to be too high, either because it is higher than a limit value determined by the specification of the ingot manufacturer, or It is because it is above the solubility limit of oxygen in the sputum (about 2.10 ¹⁸ cm ^-3 ). In this case, it is advantageous to apply annealing of the formation of the hot donor to the entire second ingot after its crystallization (step 7 described below). In this manner, the effective time t _eff at which the hot donor is formed increases the annealing time in each region of the second ingot. Thus, the calculation of the interstitial oxygen concentration [Oi] _tg value in step S6 takes into account the new effective time value t _eff .

This annealing allows a lower interstitial oxygen concentration to be used in the second ingot while ensuring that the hot donor concentration calculated in step S5 will be achieved. The annealing temperature over the entire length of the second ingot is uniform and preferably equals about 450 °C. The period may be between one minute and several hours. If the effective time for forming the hot donor is estimated at a temperature other than 450 ° C, the annealing after crystallization is completed at the same temperature.

Finally, in step S7, the second ingot is crystallized from the ruthenium melt in accordance with the pull-up formulation used to crystallize the reference ingot (i.e., using the same pull-up parameters). This melt system is obtained by melting a second ruthenium filler having the same mass as the first ruthenium filler in a crucible. The lanthanide of the second ingot is the same as the lanthanum of the first ingot, and is preferably made of quartz or yttria. Like the first filler, the second molten ruthenium filler contains oxygen and may have a dopant.

In each step of crystallization of the ingot, most of the oxygen (about 99%) contained in the helium melt escapes into the air of the crystallizing furnace and is agitated by the gas stream of the inert gas. A small portion (about 1%) of oxygen is incorporated into the ingot. The concentration of interstitial oxygen incorporated into the ingot depends on the oxygen concentration at the interface between the melt and the ingot. It also depends on the oxygen partition coefficient, which is a value close to one. The oxygen concentration of the melt is primarily dependent on the pull-up parameters, the air of the furnace, and the nature of the crucible.

In step S7, the oxygen concentration of the ruthenium melt is adjusted as the crystallization progresses to obtain the axial curve [Oi] _tg determined in step S6 in each of the solidified proportions of the second ingot. Consistent interstitial oxygen concentration. The oxygen concentration of the melt can be controlled by various mechanisms.

For example, a magnetic field can be generated around the crucible to locally slow down the convection of the crucible within the melt. This magnetic field is usually used to contain oxygen in the molten crucible around the crucible, and the ingots formed in this way are called "magnetic-CZ" ingots, and they are slightly contaminated with oxygen (because they It is formed by a molten crucible located in the center of the crucible. Conversely, the magnetic field can be constructed to homogenize the melt and accelerate the dissolution of the crucible around it to enrich the melt with oxygen.

According to the same principle, ruthenium (made of quartz or yttria) having a porosity that varies with depth can be used to change the rate of dissolution of the ruthenium and thus the rate at which oxygen is incorporated into the melt. The portion of the crucible that is most porous dissolves faster and thus increases the oxygen content of the melt. Conversely, the molten system is slowly enriched with oxygen when the molten helium is in contact with the least portion of the pores of the crucible.

The control of the rotational speed of the crucible is another technique that can change the rate of dissolution of the crucible. As in the foregoing techniques, it can increase or decrease the oxygen concentration of the melt.

The oxygen concentration of the melt can also be controlled by adjusting the gas flow of the inert gas in the air of the furnace or the pressure of the inert gas. As the gas flow increases or as the pressure of the gas decreases, as this contributes to the evaporation of oxygen (in the form of SiO), the melt gets less oxygen. Conversely, when the gas flow is reduced or the pressure is increased, the oxygen content of the melt increases.

Other techniques are based on the addition of external oxygen. In particular, another molten tantalum filler can be injected into the crucible of the second ingot during the crystallization of step S7. The additional filler placed in the auxiliary crucible contains more or less oxygen than the crucible contained in the main crucible (ie, the crucible that pulls out the second ingot), depending on the desired increase or decrease in the melt. Oxygen concentration. The oxygen concentration of the additional filler can be increased by incorporation of a quartz/yttria article or by maintaining the crucible for a longer period of time, which contributes to a higher dissolution of the auxiliary crucible. Conversely, in order to obtain an additional filler having a low oxygen content, the auxiliary crucible can be made of graphite instead of quartz/yttria.

In the preferred embodiment of step S7 shown in Fig. 2, the oxygen concentration of the tantalum melt 20 is obtained by immersing the article 21, preferably made of quartz or yttria, at a variable depth h. Body 20 to adjust. The article 21 is mounted and rotated relative to the translation of the crucible, preferably at the same speed and in the same direction as the crucible. The article 21 includes at least one segment of the ring disposed about the second ingot. It may comprise several different segments of the same ring or, as shown in Figure 2, formed by the entire ring. This second ingot is symbolically represented by its longitudinal axis 22. The segment(s) of the ring or the entire ring has a cross section with a tip end 23 (for example in the shape of a triangle). The top end 23 is oriented toward the bottom of the crucible in a direction 24 parallel to the longitudinal axis 22 of the ingot. In this manner, the article 21 does not significantly interrupt the convection of the molten helium in the crucible and can gradually increase the oxygen concentration. The radial uniformity of the second ingot is also maintained by the annular shape.

The triangular cross section as shown in Fig. 2 allows the area of the article 21 in contact with the melt 20 to be finely changed, and thus the oxygen concentration of the melt is precisely controlled. In the case of an isosceles triangle 23 (having the same length on both sides corresponding to the non-flat surface of the ring), the area of the article 21 in contact with the tantalum melt 20 is written as: Wherein the depth of the h-ring 21 soaked (measured relative to the surface of the tantalum melt 20), R1 is the intermediate diameter of the ring (measured at the center of the bottom of the isosceles triangle 23), R2 is the inner diameter of the ring and the A system The height of the isosceles triangle 23 (measured parallel to the longitudinal axis 22). For example, the sizes R1, R2, and A of the article 21 are equal to 20 cm, 16 cm, and 1 cm, respectively.

The above various techniques can be combined to further enhance the control of the oxygen concentration of the melt during step S7.

In this manner, after the drawing recipe is characterized in steps S1 to S4, the manufacturing method of Fig. 1 can obtain an ingot having a predetermined axial resistance in a small number of steps (steps S5 to S7). It is not necessary to have the ingot produced after the reference ingot be cut or subjected to annealing with a temperature gradient.

Due to the role of the "electron donor" of the hot donor, it is not necessary to add an additive such as phosphorus in order to obtain an n-type antimony ingot. In this case, contamination problems with the addition of dopants in the form of highly doped powders or wafers are avoided. The life of this charge carrier is particularly high in these n-type ingots. These high metallurgical quality ingots have advantages in many applications, particularly in the field of photovoltaics, such as in the manufacture of high efficiency photovoltaic cells.

Therefore, for the molten ruthenium filler, it is preferred to crystallize the reference ingot and the subsequent ingot without a dopant. This also contributes to the control of the electrical resistance, so that it is based solely on the formation of the thermal donor.

Now, an exemplary embodiment of the manufacturing method of Fig. 1 will be explained with reference to Figs. 3A to 3H.

In this example, the goal is to fabricate an n-type ingot B having a uniform resistance equal to 5 Ω.cm. Both the ingot B and the ingot A used as a reference are used according to a given pulling formula (the pulling pressure is equal to 14 Torr, the argon flow rate is 60 liters/min, the rotational speed of the crucible is equal to 8 rpm and the rotational speed of the seed crystal is equal to 10 rpm) A single crystal bismuth ingot crystallized by a wood-based process (measuring a diameter of 8 吋). Each ingot was crystallized from a molten ruthenium filler of about 50 kg and slightly doped with phosphorus (N _charge = 10 ¹⁴ cm ^-3 ).

Figures 3A and 3B show the measurement of the hot donor concentration [DT] _i along its length and the measurement of the interstitial oxygen concentration [Oi] _i after the crystallization of the reference ingot A, respectively. They are obtained by measuring the three series of electrical resistances segmented by annealing at about 450 ° C and 650 ° C in the manner described in the patent application FR3009380.

The longitudinal position of the measurement area is given by the solidification ratio section f _{s of the} ingot A (the abscissa in the 3A and 3B drawings). It is expressed as a percentage of the total length of the ingot. The measurement area is distributed almost entirely over the entire length of the ingot A, so they are at 0%, 10%, 20%, 50%, 70%, 80%, and 95% of the entire length of the ingot A.

According to the step S4, using the relation (1) and these measurements, the effective time t _eff of the annealing at 450 ° C applied during the crystallization of the ingot A is calculated for each of these longitudinal positions. The resulting effective time value t _eff is also effective for the ingot B that is subsequently produced because the ingot B will use the same pull-up formulation and the same portion of filler. The calculation of the effective time t _eff is the curve 31 of the 3C chart (the curve having the legend "without additional annealing").

Further, the 3D graph shows an axial graph of the dopant (phosphorus) concentration N _D in the ingots A and B obtained using the Schel-Gleevec equation (k (phosphorus partition coefficient) equal to 0.35).

When the measured ratio for each segment f _s, which is known N _D dopant content and the target resistance value (5Ω.cm), the axis of the decided thermal donor concentration in the ingot is required to produce the B The graph [DT] _tg (f _s ) is plotted (step S5). This axial plot is shown in Figure 3E.

Curve 32 of Figure 3F (with the legend "no additional annealing") shows a first axial plot [Oi] _tg (f _s ) of the interstitial oxygen concentration required to obtain ingot B with uniform electrical resistance. It uses the relation (1) and uses the effective time and the necessary annealing at 450 °C given by each of the measured proportional segments f _s and by curve 31 (Fig. 3C; "no additional annealing") The concentration of the resulting hot donor [DT] _tg (Fig. 3E) is determined.

It is observed in the 3F diagram that the target interstitial oxygen concentration [Oi] _tg is particularly high for the high value ratio segment f _s of the ingot B (i.e., for the final solidified region of the ingot). This is because the effective time t _{eff is} low in these same areas (compare Fig. 3C). However, such an oxygen concentration can impair the quality of the tantalum wafer to be produced by the ingot B. Therefore, it is preferred to select an additional annealing at 450 ° C, which must be completed after the ingot B is crystallized. As can be seen in Figure 3F (curve 33, "with additional annealing"), the effect of this annealing is to reduce the target interstitial oxygen concentration [Oi] _tg along the length of ingot B because of the effective annealing at 450 °C. The time t _{eff is} then increased by the period of the additional anneal, for example 2 hours (compare Fig. 3C; curve 34, "with additional annealing").

In this exemplary embodiment, a second target profile of oxygen concentration [Oi] _tg (curve 33, "with additional annealing") is achieved by dipping the quartz ring in the crucible melt in a controlled manner. As shown in Figure 2. Taking the interstitial oxygen concentration [Oi] _{i of the} ingot A (ie, the oxygen concentration after the pull-up when no additional oxygen source is used) as the reference, and then calculating for each proportional segment of the ingot B The oxygen concentration [Oi] _supp required to achieve the target interstitial oxygen concentration [Oi] _tg . Figure 3G shows a plot of this additional concentration [Oi] _supp , expressed as a percentage of the base concentration [Oi] _i .

Since the lanthanide used to crystallize the ingot B is formed of the same quartz as the ring, the 3G map corresponds to an increase in the quartz/ruthenium contact region S which must be formed by immersing the ring in the melt. According to the above relation (5), the increase of the contact region S is achieved by changing the depth h of the ring in the manner shown in Fig. 3H.

As described above, the manufacturing method can make the axial resistance of the ingot uniform by selecting a fixed target resistance graph. However, it is also possible to obtain an ingot having a variable axial resistance. For example, a crucible in which the conductivity type is changed from p to n during crystallization can be produced by the method of Fig. 1. Indeed, the amount of hot donor formed during crystallization can compensate for the initial doping of the p-type cerium filler under the specific conditions described above (oxygen concentration [Oi] _tg and effective time t _eff ). Thus, the axial resistance graph has more than one resistance peak at the change in the conductivity type. There are many applications for this type of ingot, such as the manufacture of high voltage photovoltaic cells.

Although the present manufacturing method is described using a bismuth ingot, it can be applied to other semiconductor materials such as tantalum or niobium-niobium alloys. Tantalum is a potential alternative material because when the ruthenium is crystallized, an oxygenated heat donor is also formed.

Claims (7)

A method of manufacturing an ingot made of a semiconductor material, the method comprising the steps of: crystallizing (S1) a first ingot called a reference ingot from a first molten filler containing oxygen under a specific pulling condition (A); measuring (S2) the interstitial oxygen concentration ([Oi] _i ) in different regions distributed along the reference ingot (A); measuring (S3) at the reference ingot in different regions of the reference ingot The concentration of the hot donor formed during the crystallization of (A) ([DT] _i ); determined by the measurement of the interstitial oxygen concentration ([Oi] _i ) and the thermal donor concentration ([DT] _i ) S4) an effective time (t _eff ) of formation annealing of the hot donor body experienced by the different regions during the crystallization of the reference ingot (A); (S5) is calculated in order to have the second ingot (B) after crystallization The hot donor concentration value ([DT] _tg ) required to obtain the axial resistance of the target graph; the effective value of the hot donor concentration ([DT] _tg ) and the annealing of the hot donor (t _eff ), determining (S6) an axial graph of the interstitial oxygen concentration corresponding to the axial resistance target graph ([Oi] _tg ); under the specific pull condition, the second containing oxygen The molten filler crystallizes (S7) the second ingot (B), wherein an oxygen concentration of the second molten filler is adjusted as the crystallization proceeds to obtain an axial graph of the interstitial oxygen concentration in the second ingot (B) ( [Oi] _tg ). The method of claim 1, wherein the resistance target graph is fixed. The method of claim 1 or 2, wherein the second ingot (B) is subjected to formation annealing of an additional hot donor during a predetermined period of time, and then an axial graph of the interstitial oxygen concentration is determined ( When [Oi] _tg ), the additional annealing time is added to the effective time (t _eff ). The method of any one of claims 1 to 3, wherein the concentration of oxygen in the second molten filler is by dipping the article (21) comprising at least one section of the ring disposed around the second ingot (22) Adjusted by the second molten filler, the article (21) is formed from an oxygen-containing material. The method of any one of claims 1 to 4, wherein the first and second molten fillers contain a dopant, the method further comprising: determining the hot donor concentration value ([DT] _tg ) before determining The step of the axial plot of the dopant concentration (N _D ) in the second ingot (B). The method of claim 5, wherein the axial profile of the dopant concentration (N _D ) in the second ingot (B) is when the first and second molten fillers contain the same concentration of dopant This is determined by measuring the dopant concentration (N _D ) in different regions of the reference ingot (A). The method of claim 5, wherein the axial plot (N _D ) of the dopant concentration in the second ingot (B) is calculated using a Scheil-Gulliver equation.