DE1164680B - Process for the production of rod-shaped semiconductor bodies of high purity - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Cl .:

German KL: 4Od-3/02

Number: 1 164 680

File number: S 58305 VI a / 40 d

Filing date: May 21, 1958

Opening day: March 5, 1964

The invention relates to a method for producing rod-shaped semiconductor bodies of high purity with a defined course of the impurity conductivity and the conductivity type, practically over the entire Length of the body, in which the body is first made doped along its length and then is subjected to the zone melting process.

The cleaning process for semiconductor material known under the term zone melting is shared as is well known in two groups:

In the so-called zone cleaning (»zone refining«) one or more soluble substances are removed enriched at one end of the semiconductor rod so that the rest of the rod after the implementation of the process consists of very pure semiconductor material.

In the second group of zone melting for targeted and uniform distribution (»Zone-leveling«) is an even distribution of one or more soluble substances over a certain length of the semiconductor rod is achieved in that the rod has numerous melt zone passages is subjected in opposite directions.

The semiconductor rod produced after the zone cleaning has a uniform over part of its length Concentration of the dissolved substances, but at its ends there is a depletion or an enrichment of dissolved substances. With this method, a piece must always be closed at the end of the rod large or too low concentration of impurities due to the large conductivity inhomogeneity Deposit to be removed.

The second method requires a very large number of melt zone passes to make one over the entire length of the zone to produce a homogeneous distribution of defects. The present invention is based on the object of the enrichment occurring during zone cleaning at the rod ends and the associated loss of semiconductor material and after a few zone passes, if possible after only one zone pass, a defined, in particular to achieve a constant distribution of impurities practically over the entire length of the body.

In order to achieve this, it is proposed according to the invention that the body is initially overdoped and is then subjected to a crucible-free zone melting process in which to achieve the defined Defect distribution along the semiconductor body by evaporation the amount of defects per unit length of the body is changed.

Process for the production of rod-shaped
Semiconductor bodies of high purity

Applicant:

Siemens & Halske Aktiengesellschaft,

Berlin and Munich,

Munich 2, Witteisbacherplatz 2

Named as inventor:

Dipl.-Phys. Dr. Günther Ziegler, Erlangen

When pulling a semiconductor rod from a melt in a crucible, it is already known, by evaporation of an impurity, the concentration of this impurity in the melt to keep it as constant as possible while pulling. The pan-less zone pulling, however, is related with the exploitation of the evaporation of imperfections, there are significant technical advantages. These advantages lie mainly in the fact that ζ. B. the relatively large surface area of the melting zone compared to the volume the melt droplet allows better influencing of the doping of the finished rod.

In zone melting known per se, it is often assumed that

a) the diffusion of foreign matter in the solid part of the rod is negligible,

b) the concentration in the melting zone is uniform,

c) the distribution coefficient is constant,

d) there is no interaction with the crucible material and the atmosphere.

These assumptions are often only partially fulfilled. This applies above all to assumption b) more uniform Concentration in the pain and the interaction between crucible material and mentioned under d) The atmosphere. During a concentration enrichment at the solidification interface through introduction an effective distribution coefficient dependent on the test parameters is taken into account is this simple form of correction in the event of an interaction with crucible material or Atmosphere not possible. However, it is precisely these influences that are sometimes very disturbing. The influence of the crucible material By using crucible-free zone melting, an impurity from the gas atmosphere can be eliminated switch off by working in a high vacuum. However, there is then an additional

4Ü9 537/441

evaporation of the pollutants that superimpose the segregation and thereby the shape the concentration curves fundamentally changed.

In the mathematical treatment of the evaporation, it is known that for the zone melting in the crucible Differential equation established for the distribution of the impurities in a rod-shaped semiconductor body to be expanded by an evaporation link. The equations that are important in practice are given below.

The change over time in the concentration Cs in the melting zone caused by the evaporation is determined by the linear approach

dC _s

considered. The approach is synonymous with the assumption that the partial pressure of the dissolved Substance above the solution is proportional to the concentration Cs, which is probably the case with strong dilution should apply.

The concentration balance in the melting zone is then

for single crystal production occurs in that all impurities with the distribution factor / :, der is different from one, to be transported to the two ends of the rod, to be eliminated if one the zone length and the drawing speed is selected so that

_ i
v

k = 1

dCs =

C ₀ FdX kCsFdx
y γ

- * Cs dt, (1)

_a5

where C _{0 is} the initial concentration \ the evaporation coefficient depending on the geometry of the melting zone, F is the cross section of the semiconductor rod, V is the volume of the melting zone and k is the distribution coefficient. After some transformation, the differential equation for the solidified end is obtained

dC _f
dx ⁺ U
I. Cf- k
'Ί k Oi. U I. V Ί

C ₀ = O,

(2)

(3)

35

was set. C / is the concentration in the solid, solidified end, ν the speed with which the melting zone is guided along the rod, / the length of the melting zone.

The solution with the introduction of the corresponding boundary conditions Cs - C ₀ for χ = 0 reads with constant C ₀ and constant ν

Cf

C _n

= k-- e

k u

(4)

To illustrate the course of the curve, FIG. 1 some concentration curves with the same distribution coefficient k and different values of u, i.e. with different evaporation factors, are drawn. The starting point of the curves C / = C ₀ - k is independent of «. A stationary value then arises exponentially

55

Cf

C ₀ - k

60

a. _{As can be seen from FIG. 1} , an originally homogeneous concentration distribution, ie C 0 = constant, can be obtained homogeneously during zone melting (k · C ₀ ) if one makes 11 = 1 by a suitable choice of zone length and zone pulling rate.

It is therefore possible to determine the doping gradient along a semiconductor rod that occurs during zone melting

According to an embodiment of the invention, the zone melting process is carried out in a continuous manner evacuated vessel with small partial pressures of the Doticrungsstoff, d. H. with partial pressures of the im Rod contained dopants under the respective vapor pressures of these substances at the temperatures the melting zone are carried out.

However, the zone melting process can also be carried out in a protective gas atmosphere, e.g. B. in a hydrogen atmosphere, be performed. However, the evaporation is then less.

This is understandable because if a protective gas is present over the melting zone, a Gas layer with the partial pressure of the evaporating substance builds up, while in a vacuum the evaporating substance Particles can fly away unhindered. This gas layer is created by convection and diffusion significantly influenced. Since convection is dependent on the structure of the apparatus, it is also dependent on the apparatus of the evaporation coefficient when melting under protective gas.

The impurity distribution along the semiconductor body is according to the invention by choosing the Speed and / or the length and / or the temperature of the melting zone during a pull-through the melting zone through the body.

Any desired combination of evaporation and segregation can be used achieve defined concentration distributions of the doping substance along the rod. So it is z. B. possible by heating one zone of the rod longer to the melting temperature than that adjacent zones, steps in the concentration curve. In addition, a can only be used in places occurring concentration gradient can be leveled by longer evaporation times.

According to one embodiment of the invention, a rod-shaped semiconductor body with a stepwise Reduction of the conductivity along the rod axis produced and this reduction in stages held approximately constant with each passage through the melting zone.

If the speed and / or the length and / or the temperature of the melting zone are adjusted so that the value for u is somewhat greater than one, this can be done in the initial zone, i.e. in the melting zone adjoining the seed crystal at the beginning of the single crystal production The decrease in the concentration of impurities occurring during the zone passage is largely avoided and the desired impurity distribution along the rod can thus be obtained after the smallest possible number of passes if the melted zone is initially left in a stationary state.

The overdoped rod serving as the starting material can be prepared by known methods, e.g. B. by decomposition of a gaseous compound of the semiconductor material in the arc or by

Decomposition of a gaseous semiconductor compound take place on the surface of a heated substrate. In both cases, the simultaneous deposition of dopants from gaseous, Compounds of these substances mixed with the semiconductor compounds achieve the overdoping. According to the invention, it is ensured that the overdoping essentially extends over the length of the rod is constant, i.e. H. the deviations are no more than approximately ± 10% from the mean value over the entire rod.

Subsequently, the desired, in particular constant, impurity distribution is then achieved by means of crucible-free zone melting, in particular in a vacuum, optionally also in an inert gas, e.g. B. hydrogen, carried out at atmospheric pressure. The length of the melting zone and the drawing speed are, for. B. so coordinated that the damming up of the impurity substances causing the doping gradient is avoided and in particular the end doping is essentially constant over the rod length, ie the deviations are at most about ± 10% ^{from the} mean value over the entire rod.

In a special embodiment according to the invention, the rod is first n-overdoped and then in the crucible-free zone melting process to a desired, in particular constant Brought the course of the η conductivity.

The bars were z. B. overdoped with phosphorus and the crucible zone pulling was carried out under high vacuum (10 ~ ₅ Torr) with inductive heating of the melting zone, with one rod end, namely the one that solidified last while carrying out the process, was rotated at about 100 rpm to support the mixing of the melt and thus avoid the formation of a superficial depletion zone. In order to prevent the concentration of impurities from dropping at the beginning of the zone passage, the melted zone was initially left in a resting state for about 1.5 minutes and only then was the feed switched on.

A back doping by evaporating back the phosphor molecules from the apparatus parts is thereby avoided that the phosphor molecules in the rapidly on the directly vaporized parts of the apparatus growing silicon vapor deposition layers are incorporated and also partially in the applied dynamic vacuum can be sucked off immediately. It can also react with the there are traces of oxygen present in the vacuum.

The panless zone pulling used in the method according to the invention has opposite essential advantages of zone melting in the crucible. As already mentioned, this creates the disruptive influence of the crucible material avoided, and also in this process the free surface is opposite the volume of the melting zone is relatively large, so that a uniform evaporation and thereby also ensures increased uniformity of the concentration over the cross section of the semiconductor rod is. Drawing in a vacuum also has the advantage that the monocrystalline formation is disruptive Impurities such as oxygen also evaporate on the rod surface, so that only a few zone passages both for achieving the desired distribution of impurities and for the production of single crystals are necessary, which makes this process particularly economical. In addition, can then also the necessary initial concentration of the imperfections before the zone definition for a desired one It is easier to predict the final concentration, which also reduces the number of zone passes to one Minimum can be depressed.

In Fig. 2 is a semiconductor body produced by the method according to the invention, ζ. Β. the end Silicon, for rectifiers or transistors with

ίο z. B. two p-n junctions 2 and 3 shown at the η-conductive zone 1 of the silicon body to be manufactured from the by cutting one after the According to the method of the invention produced semiconductor rod obtained crystal obtained unchanged remains, while by alloying and / or diffusing p-zones in the according to the invention Method initially η-conductively produced silicon bodies are formed. When alloying effective as acceptors substances 4, 5 in the first

ao η-conductive silicon form the p-conductive recrystallization zones 41, 51. As shown in FIG. 2, it is advisable to use the cross-section of the as emitter effective zone transition 3 smaller than that of the collector 2.

Claims (12)

Patent claims: 1. Process for the production of rod-shaped semiconductor bodies of high purity with a defined Course of the impurity conductivity and the conductivity type over practically the entire length of the body, in which the body is first made doped along its length and then the Zone melting process is subjected, d a - 3: characterized in that the body is initially overdoped and then one Crucible-free zone melting process is subjected, whereby the amount of Defects per unit length of the body is changed. 2. The method according to claim 1, characterized in that the semiconductor body with constant Defect distribution is produced. 3. The method according to claim 1 or 2, characterized in that for the doping on the Rod length deviations of ± 10% of the mean value are permitted. 4. The method according to any one of claims 1 to 3, characterized in that the impurity distribution along the body by choosing the speed and / or the length and / or the Temperature of the melting zone during a passage of the melting zone through the body is set. 5. The method according to claim 4, characterized in that the change in speed and / or the length and / or the temperature of the melting zone during the passage of the Melt zone through the body according to the equation u = k + ix is carried out, where k is the segregation coefficient, α the evaporation factor, / the melting zone length, ν the drawing speed and u the resulting segregation evaporation coefficient. 6. The method according to claim 5, characterized in that u = 1 is made by appropriate choice of zone length and drawing speed. 7. The method according to any one of claims 1 to 6, characterized in that the crucible-free zone melting process is carried out in a continuously evacuated vessel with small partial pressures of the dopants. 8. The method according to any one of claims 1 to 7, characterized in that for the overdoping Deviations of ± 10% of the mean value are permitted over the length of the rod. 9. The method according to at least one of claims 1 to 8, characterized in that a Rod-shaped semiconductor body with a gradual reduction in conductivity along the Rod axis is produced and that this gradual reduction in conductivity, in particular by changing the pulling speed during a passage through the melting zone is kept roughly constant for each pass. 10. The method according to any one of claims 1 to 9, characterized in that the overdoped Semiconductor rod in the deposition process from compounds of the basic semiconductor material with simultaneous Deposition of dopants from gaseous, mixed with the semiconductor compounds Compounds of these substances is made. 11. The method according to any one of claims 1 to 10, characterized in that the rod is n-overdoped. 12. Semiconductor body made of silicon, ζ. Β. for rectifiers and transistors, with an or several p-n junctions, characterized in that the η-conductive zone of the silicon body to be manufactured from those produced by cutting up one of claims 1 to 11 Silicon rod obtained crystal remains unchanged, while the p-zone by alloying and / or diffusion of dopants effective as acceptors is formed. Considered publications:
German Auslegeschrift No. 1 018 558;
French Patent No. 1,158,205;
U.S. Patent No. 2,730,470. 1 sheet of drawings 409 537/441 2.64 © Bundesdruckerei Berlin