FR2471668A1 - Diffusing phosphorus into semiconductors via silicon phosphide - which is made by heating mixt. of silicon and phosphorus powders in sealed tube - Google Patents

Abstract

Diffusion is undertaken in a sealed tube, in which the dopant is SiP, and esp. a powder contg. SiP plus Si, using a doping temp. of 1150-1280, esp. 1225 deg.C. The dopant is pref. made by placing a mixt. of P and Si powders in a sealed tube, where both powders have a particle size below 100 microns. The tube is heated in a furnace at a rate below 150 deg.C/h. between 350 deg.C. and 1150 deg.C., then held 15-20 hr. at ca. 1150 deg.C., followed by cooling slowly, then naturally, and finally quenching in water. The SiP is used e.g. for doping silicon semiconductors when mfg. power transistors or thyristors, where the doped layers will withstand high voltages.

Description

 The present invention relates to a method for diffusing phosphorus in. a semiconductor using as a dopant source silicon phosphide as well as a process for obtaining silicon phosphide.

For the manufacture of electronic components, comprising semiconductor layers of alternating types, one of the means commonly used to modify the type of conductivity or the concentration of impurity in a substrate consists in proceeding by diffusion (other methods include in particular epitaxy, ion implantations ...)
Diffusion processes can be divided very generally into two categories o l-es diffusions in sealed tube and diffusions in open tube. These two techniques are well known and will not be repeated in detail here.

One of the drawbacks of the open-tube diffusion process, in particular in the case of phosphorus diffusion, is that a more or less doped phosphor glass is initially formed on the surface of the semiconductor wafer and then, in only a second step, after having removed the highly doped glass, the impurity is penetrated into the wafer during a so-called redistribution step
However, during this second stage, the quantity of phosphorus available for diffusion is initially fixed by the pre-deposit. It is therefore limited and, for very deep diffusions, it is not possible to reach significant surface concentrations or very flat nrofils. Another drawback is in the vicinity of the surface. is that, during pre-deposition, phosphorus chemically reacts with layers of
SiO2 ventue. '# 1ement formed to serve as a mask; and this in terdit. to carry out localized deep diffusions.

 It would therefore be desirable, at least for obtaining certain types of structures and in particular when the N-type layer must be deep and localized, to be able to carry out phosphorus diffusions in a sealed tube, in the absence of oxygen. . In sealed tube diffusion processes, dopant impurities are often used. pure, which limits impurities with low vapor pressure such as gallium and aluminum. In the case. of phosphorus, the sealed tube process seems a priori to be discarded since the vapor pressure of phosphorus exceeds 10 atmospheres beyond 3500. Diffusions having to take place around 1250, a burst of the quartz tube would occur before the diffusion temperature is reached.

In the prior art, however, attempts have been made to distribute phosphorus in a sealed tube using silicon doped with phosphorus as the dopant source. This method, which proves to be applicable in cases where it is desired to make only diffusions of shallow depth or of low surface concentration, cannot be used for applications to deep diffusions. with a high surface concentration of phosphorus. Indeed, the quantity of available phosphorus is then limited and, even with very large quantities of source, which is already a drawback, one cannot reach saturated surface concentrations and significant diffusion depths (greater than a few tens On the other hand, sources of highly phosphorus-doped silicon are difficult to obtain.

It is therefore an object of the present invention to provide a new method for diffusing phosphorus in a sealed tube.

 To achieve this object, the present invention provides for using as a source of. doping a silicon alloy. and phosphorus or silicon phosphide (PSi).

 Another object of the present invention is to provide a method for manufacturing silicon phosphide.

 According to this process, the silicon phosphide is itself obtained in sealed form from a mixture of phosphorus powder and silicon powder. This mixture is gradually raised in temperature, kept at a maximum temperature, then the temperature is gradually reduced.

 With the manufacturing method according to the present invention, it has been possible to obtain sources of silicon phosphide comprising a mixture of silicon phosphide and silicon with more than 90% of phosphorus. On the other hand, with the diffusion method according to the present invention , we were able to obtain a localized diffusion of phosphorus in. a silicon wafer, the diffusion depth being greater than 200 microns. and the concentration being maximum and saturated (slightly less than 10 1 at / cm3) over a depth of the order of 130 microns.

These objects, characteristics and advantages as well as others of the present invention will be explained in detail in the following description of particular embodiments made in relation to the attached figures, among which:
Figure 1 shows a schematic view of a diffusion furnace used in the method according to the present invention;
FIG. 2 represents the variation in temperature as a function of time applied to an oven # for obtaining silicon phosphide according to the method of the present invention
Figures 3 and 4 show. diffusion profiles (concentration (C) in atoms / cm3 as a function of the depth of diffusion (x) in microns) illustrating results obtained by the method according to the present invention; and
Figures 5 and 6 illustrate applications. of the process according to the present invention
The process for obtaining a source of silicon phosphide will be described below in relation to FIGS. 1 and 2. An intimate mixture of silicon powders and phosphorus 1 is placed in a cup 2. The silicon powders and phosphorus are ground to be as fine as possible and have a diameter of less than. 100 microns. The cup 2 is placed in a sealed tube 3, commonly made of quartz. This tube slides inside another tube 4, generally also made of quartz placed inside a diffusion furnace 5, so that the sealed tube 3 is in a temperature zone which is homogeneous with the inside the oven.

The oven is initially at a temperature of around 3000C.

This temperature is gradually led to believe during a first period between instants t0 and tl until a maximum temperature T1 has been reached, for example of the order of 115toc. After that, this maximum temperature T1 is maintained for a second period between the instants tl and t2. The sealed tube 3 is then caused to cool during a third period between instants t2 and t5. This third period can be broken down into 3 stages: a first stage of slow cooling between the instants t2 and t3, to go for example from the temperature of 11500C to a temperature of the order of 9000C; a second stage of cooling at a greater speed between the instants t3 and td, to go for example from the temperature of 900 C to a temperature of 6000C; a third very rapid cooling step resulting, for example, from a water quenching of the sealed tube 3 to descend to ambient temperature. During the first period, the temperature rise must be gradual, this period being able, for example, to be spread over twelve hours (one night) and the temperature variation speed being less than 1500C per hour. If the temperature rise were too rapid, the silicon phosphide would not form quickly enough and the vapor pressure of the phosphorus could help dangerous overpressures. The duration of the second period, or phase of maintaining at the maximum temperature T1, or phase of homogenization, will be greater than twelve hours, for example 18 hours as in the case of the example shown in FIG. 2.

 By using this process of forming silicon phosphide in a sealed tube, it is possible to obtain relatively high yields. which could not be obtained by the applicant by trying to use other methods such as sealed tube methods in which the silicon and phosphorus powders were separated or open tube methods in which a phosphorus vapor was caused to circulate over a bed of silicon powder. The examples below: illustrate results which can be obtained by the process according to the present invention. In these examples, it is considered that one has placed oneself in the general conditions above and only certain particular values will be exposed.

EXAMPLE 1
3 g of powdered silicon were mixed with 4 g of powdered phosphorus and placed in a crucible placed in an oven, in a sealed tube. The maximum temperature T1 was of the order of 11500C. The duration of the second period or period of homogenization between the instants tl and t2 was 17 hours. Between instants t2 and t5, the sealed tube 3 was cooled with compressed air. A total mass was thus obtained in the crucible after treatment of 5.80 g of a mixture of silicon phosphide and silicon, the mass of silicon phosphide being 5.42 g, ie a yield of 86% (this yield corresponding to the mass of SiP obtained in reality relative to that which should have been obtained if, stoichiometrically, 3 g of silicon had reacted with 3.308 g of phosphorus). Note that, in this example, the amount of phosphorus is greater than the stoichiometric amount, which would have been as we said above of 3.308 g. Indeed, part of the phosphorus is fixed by the quartz constituting the enclosure of the sealed tube
EXAMPLE 2
5.002 g of powdered silicon were mixed with 6.1146 g of powdered phosphorus (whereas the stoichiometric proportion would have been 5.516 g of phosphorus). The maximum temperature was 11500C; the duration of homogenization was 18 hours; cooling was carried out at 300C per hour between 11500C and 8750C, then naturally between 8750C and 6000C and finally, by water quenching between 6000C and room temperature. This gave 10.3357 g of mixture of silicon phosphide and silicon in the form of a very fine powder, dark brown, mat, in which was included 10.1982 g of phosphorus, ie a yield of 96.3%.

EXAMPLE 3
Under the same conditions as Example 2, we chose
If a homogenization time, between instants tl and t2, of 53 hours. The yield was only 81.9%. It emerges from this example 3 that when the duration of the homogenization temperature increases, the yield decreases, probably by fixing the phosphorus with the quartz surface of the enclosure.

 The method of diffusion in a sealed tube by means of a source of silicon phosphide as obtained by the method described above is implemented using the conventional techniques of diffusion in a sealed tube. Very schematically, it can be considered that this diffusion is achieved by using an apparatus as illustrated in FIG. 1, the sealed quartz enclosure 3 comprising on the one hand the powdered silicon phosphide placed in a crucible, on the other hand a number of silicon wafers.

In the case where it is desired to carry out localized diffusions, it will be possible to use as diffusion mask silica or preferably silicon nitride.

FIG. 3 illustrates various diffusion profiles obtained in silicon wafers using the method according to the present invention. Tests were carried out by placing 5 initially doped silicon wafers of the type
P with a concentration of the order of 1014 at / cm.

 In the case of curve 10, the amount of source was of the order of 1 mg; one then obtains a surface concentration of the order of 3 x 1020 at / cm3 and a profile similar to that which one would obtain by other diffusion methods, for example by the open tube method or by the tube method sealed using phosphorus doped silicon as the source. But, of course, in the case of the use of such a source of silicon doped with phosphorus in a sealed tube, the quantity of source should be much greater (of the order of 50 times according to the tests carried out by the plaintiff).

In the case of curves 11 and 12; the source quantities were respectively of the order of 50 and 100 mg, respectively. In the case of these curves, a particular diffusion profile will be noted. In fact, in the upper area shown in dotted lines, there is an abnormal diffusion with a very profile. dish. The source of silicon phosphide can be said to be supersaturated, and dislocations are likely to occur in the network in the supersaturated area shown in dotted lines, but this is of little practical importance since these N-type layers are not intended to form junctions but to resume contacts and only their conductivity characteristics matter as we will see in the application examples below
This supersaturation phenomenon is even better represented in an experiment illustrated in FIG. 4 during which the diffusion was continued at 12,600 for a period of 350 hours. A diffusion depth of the order of 220 microns was thus obtained, the supersaturated zone having a thickness of the order of 135 microns and the diffusion # being particularly abrupt. This abrupt character of the diffusion can be explained by the fact that , as a result of. dislocations introduced into the surface part of the silicon, diffusion is particularly rapid there, which modifies the profile.

 We will give below some examples of applications in which the possibility of obtaining diffusion profiles of the type represented in FIG. 4 and in FIG. 3 curves 11 and 12, finds particular advantages which were not obtained by any of the phosphorus diffusion methods known in the prior art.

The first application illustrated in relation to FIG. 5 relates to obtaining power transistors.

Power transistors are semiconductor structures with 3 layers of alternating conductivity types, the profile of which is shown in FIG. 5 The first surface layer 20 is of type N, it is followed by a layer of type P 21, of a layer N 22 and a layer of type N + 23.

The conventional and simplest method of forming such structures consists in starting from an N-type substrate (22), forming on one of its faces a P-type diffusion (21), then in. perform N-type diffusions on both sides. However, if one seeks to optimize the dimensions of the layers, it will be noted that the depth xl of the P-type scattering must be of the order of 40 microns and that, for power transistors having to withstand relatively high voltages, the thickness of the N-type layer or substrate layer must be of the order of 20 to 60 microns for voltage withstand from 200 to 400 volts. Thus, if this method were followed, since the methods of previous diffusion did not allow to reach depths of diffusion higher than about sixty microns, one obtained 40 microns for the layer P 21 + 30 to 60 microns for the layer
N 22 + 60 microns for the N + 23 layer, ie a total thickness between 130 and 160 microns. Such a thickness is in fact not usable industrially because the too thin wafers would be too fragile, especially since it is a medium power device whose various manufacturing steps include a sampling step which further weakens the This fragility of platelets is all the more annoying as there is currently a tendency to manufacture electronic components on plates of increasingly large diameters (up to 15 cm or 6 inches). It would therefore be desirable for the N + type layer 23 to be thicker. To achieve this result, in the prior art, a manufacturing process was used consisting of starting from an N + type substrate 23 on which an epitaxial N type layer was formed which should have a thickness of the order of 7G at 100 microns. Producing such thick epitaxial layers is technologically very delicate. Thus, the method according to the present invention which makes it possible to start from an N 22 type substrate and to diffuse a deep layer 23 which can have a diffusion depth of up to 150 at 250 microns solves the technological problem that arose.

Another application of the method according to the present invention is illustrated in FIG. 6 which very schematically represents a thyristor structure with reverse conduction. The left part of the figure includes a 4-layer structure of alternating conductivity types 31, 32, 33 and 34
To obtain the reverse conduction, it is sought to diffuse in a part of the lower surface of the wafer, starting from layer 34, a diffused zone of the same type as layer 33 (type N). This layer must have a depth of the order of 60 microns. This is achievable by the method according to the present invention as illustrated by the various examples of application explained above. In this particular case, the importance of the compatibility of the diffusion method according to the present invention with the provision of masking layers will be emphasized.

 Of course, many other applications of the method according to the present invention may be envisaged.

Obtaining the particular profiles illustrated in curves 11 and 12 in FIG. 3 and in FIG. 4 constituting only one of the advantages of the present invention. Even in the case as illustrated by curve 10 in FIG. 3 where the method according to the present invention provides results analogous to those of the methods of the prior art, this method could be considered preferable. Indeed, in comparison with the open tube processes, it will allow the use of simpler equipment, and in comparison with the previous sealed tube process with the use of a phosphorus doped silicon source, it has the advantage that the source silicon phosphide is simpler and faster to obtain and can be used in much smaller quantities.

The present invention is not limited to the embodiments which have been explicitly described above, it includes the various variants and generalizations thereof included in the field of claims below.

Claims (10)

 1. A method of diffusing phosphorus in a semiconductor in a sealed tube, characterized in that it consists in using as a dopant source silicon phosphide. 2. Method according to claim 1, characterized in that the silicon phosphide is introduced into the diffusion tube in the form of a powder essentially comprising silicon phosphide and silicon. 3. Method according to claim 1, characterized in that the diffusion in a sealed tube is carried out at a temperature between 1150 and 12800C.  4. Method according to claim 3, characterized in that the diffusion in a sealed tube is carried out at a temperature close to 1225 C. 5. Method for obtaining silicon phosphide, characterized in that it comprises the following steps  - place in a diffusion oven at homogeneous temperature a sealed tube containing a mixture of powdered phosphorus and powdered silicon, - gradually raise the oven temperature to a maximum temperature,  - maintain this temperature for a certain period, - gradually reduce the temperature.  6. Method according to claim 5, characterized in that the silicon and phosphorus powders have a grain of less than 100 microns. 7. Method according to claim 5, characterized in that said maximum temperature is of the order of 11500C.  8. Method according to revendi.cation 5, ca-characterized in that said certain duration is included in the range of 15 to 20 hours 9. Method according to claim 5, characterized in that the temperature rise is carried out at a speed lower than å 1500C per hour between 3500C and the maximum temperature.  10. Method according to claim 5, characterized in that the descent in temperature is carried out in three stages  - a first slow cooling step; - a second substantially natural cooling step  - a third cooling step by quenching in water