DE1644041B1 - METHOD FOR PRODUCING A CARRIER GAS CONTAINING DOCTANT VAPOR FOR THE TREATMENT OF SEMI-CONDUCTOR MATERIAL - Google Patents

Description

In the production of semiconductor bodies for use in components, often low Quantities of dopants are passed into treatment chambers, where they are in a body or in on layers to be deposited are introduced into a body. Such amounts of dopant must be in the Order of magnitude fewer parts per billion in the flow of the carrier gas containing the dopant be present, which is passed into the treatment chambers. The regulation of the addition of the dopant at these low levels is very difficult. Also for setting higher doping levels a precise and reliable control of the addition of the dopant is fraught with problems.

From the French patent 1410 745 a method is known in which a carrier gas over the dopant source is passed, thereby absorbing dopant vapor and to the precipitation area transported. In this known method, however, the dopant is not diffused into the Carrier gas introduced.

In another method known from French patent specification 1,345,266, a bundle of capillaries is used used as a diffusion barrier for liquid transition. A concentration gradient across the Capillary path from the inlet to the outlet side is not provided in this known method; rather, there is only a transport of liquid there by capillary action.

However, none of the known methods is able to ensure a control accuracy that for extremely low levels of dopants in carrier gases is necessary.

In a method for producing a carrier gas intended for the treatment of semiconductor material, in which a predetermined, small amount of dopant vapor is contained, the dopant vapor is introduced into the carrier gas in a mixing chamber in which both the carrier gas and the dopant vapor are introduced, but is a very precise and reliable concentration control of the dopants is possible at extremely low dopant contents in a carrier gas if according to the invention the dopant vapor into the mixing chamber through a tube of one of the dopants containing vessel diffused at constant speed and the carrier gas speed so is set so that no turbulence occurs in the pipe. The suitable conditions for the invention provided diffusion of dopant vapor are achieved in that the dopant source as well like the introduction tube and at least part of the mixing chamber under the same operating conditions, e.g. B. at the same temperature. Along the leading the dopant vapor to the mixing chamber Rohrs there is a significant concentration gradient, so that at the outlet end of the tube Dopant concentration of a few parts per billion can be achieved, although on the input side At the end of the tube (at the dopant source) the concentration of the carrier gas is of the order of magnitude of 30%. By using the principle of gas or vapor diffusion, an extreme small, practically constant amount of dopant are fed to the mixing chamber, which is practically independent on the flow rate or the amount of carrier gas supplied to the mixing chamber is. Therefore, the dopant concentration can be adjusted by controlling the amount of material supplied to the mixing chamber. comparatively large amount of carrier gas can be set.

In the following the invention is explained in more detail in connection with the drawing. This shows in schematic form a diffusion cell arrangement and a reaction chamber for epitaxial deposition of a material from a reaction gas onto a heated substrate, it being possible to add dopant to the reaction gas in a metered manner. The diffusion cell consists of a reservoir 14 which contains a dopant, e.g. B. arsenic trichloride, contains in liquid form. The space 25 above the liquid dopant 15 contains a gas mixture with the liquid dopant at a partial pressure p _ls which is determined by the temperature of the liquid 15. A feed pipe 16 with a control valve 17 is provided for the addition of liquid dopant. For the sake of simplicity, a closure cap 26 is provided in order to be able to introduce additional amounts of dopant with the aid of an injection syringe.

The gas chamber 25 above the liquid 15 is connected to a mixing chamber 11 by means of a tubular feed line 13. The mixing chamber 11 is also by means of the carrier gas tube 12 with the control valve 18 with carrier gas, for. B. hydrogen charged. In order to avoid turbulence and mixing in the feed 13, the gas is introduced from the feed pipe 12 into the mixing chamber 11 through radial openings 23 in the carrier gas pipe 12. The partial pressure P ₂ prevails in the mixing chamber . The outlet pipe 20 from the mixing chamber 11 via the control valve 19 lets the gas mixture carrying the dopant into the chamber 30 for the epitaxial deposition via the inlet pipe 29.

The supply pipe 31 with a control valve 32 allows a flow of hydrogen, which is a small amount

Silicon tetrachloride contains (SiCl ₄ ). This gas flow is combined with the flow carrying the dopant. The mixture flows through the deposition chamber 30. Inside this chamber 30, a disk 34 made of semiconducting silicon is supported by a frame 33 and heated by means of an induction coil 35. As is well known, a decomposition reaction occurs at suitable temperatures and results in the deposition of silicon on the wafer 34. In the particular case, if the original wafer 34 is monocrystalline, the deposited silicon will also be monocrystalline and have a conductivity which largely depends on the doping provided by the diffusion cell of the present invention. The carrier gas, which contains unreacted material and reaction products, escapes from the reaction chamber 30 by means of the exhaust line 36.

For the arrangement of the diffusion cell with the mixing chamber 30, the storage container 14 and the Connecting pipe 13, the following dimensions are exemplary. The mixing chamber 11 has an outside diameter of 20 mm. The inlet openings 23 have four equally spaced holes, each 2 mm in diameter. The connecting tube 13 is a capillary with an outer diameter of about 7 mm and an inside diameter of about 1 mm. The storage container 14 has an outer diameter of 16 mm, and the filling tube 16 has an outer diameter of approximately 7 mm and an inner diameter of 1 mm. The connecting pipe 13 is about 100 mm long, and the distance from the tip of the connecting pipe 13 to the center of the Opening 23 is 10 mm. The total length of the mixing chamber is approximately 70 mm. The various parts including the diffusion cell are expediently made of high-melting glass.

In a specific embodiment of the invention, the gas sent to the mixing chamber was hydrogen and both arsenic trichloride (AsCl ₃ ) and arsenic tribromide (AsBr ₃ ) were used as dopants.

In this connection it should be mentioned that arsenic tribromide is solid at room temperature, insofar as it is it has a melting point of 328 ° C. For convenience, additions are made to the stock batch made in liquid form by raising the temperature of the device above the melting point.

The part of the apparatus within the outline 40 was immersed in a thermostat which was kept at a controlled temperature during the test runs, with which the vapor pressure P _{1 was} determined. The flow rate of the hydrogen gas was kept at 100 and 220 ml per minute and fixed for various experiments.

The diffusion of the arsenic trichloride molecules or arsenic tribromide molecules from the chamber 25 through the capillary tube 13 into the mixing chamber 11 takes place because the partial pressure p _u, which corresponds to the vapor pressure of the doping, is greater than the partial pressure p ₂ in the mixing chamber. At acceptable flow rates of the hydrogen gas, in particular greater than about 10 ml per minute, it can be shown that p _{2 is} so small compared to P ₁ that it can be neglected when calculating the diffusion rate of the doping through the capillary tube. As a result, the amount of doping transported through the connecting pipe 13 to the mixing chamber 11 is independent of the flow rate of the hydrogen gas. In particular, the diffusion rate of the doping through the capillary 13 is given by the following equation:

φ =

ADP LRT

In

P-Pz

Herein means

φ = diffusion rate (mol / sec),

A = cross section of the capillary (cm ² ),

D = diffusion coefficient (cm ² / sec),

P = total pressure in M (atm),

R = gas constant (82.06 atm cm ³ / g mol ⁰ K),

T = temperature ( ⁰ K),

L = length of the capillary (cm),

P ₁ = vapor pressure of the doping (atm),

p ₂ = partial pressure of the doping in the exhaust gas (atm).

In a series of experiments using arsenic trichloride as a dopant and a mixture of 8 percent by volume of hydrogen in helium as the carrier gas, the results tabulated below were obtained obtain:

Table 1
AsCl ₃ in 8% H ₂ in He

attempt Γ Flow Measured X ₁₀ -! 0 steam 40 No. swiftly Diffusion X lO-io pressure P ₁ ro speed H ₂ -He speed X ₁₀ -io for AsCl ₃ 1 0.0 (ml / min) (Mol / sec) X ₁₀ -io (mmHg) 2 17.3 220 0.27 X _lo -io 2.2 45 3 17.9 106 0.82 X lO-io 6.6 4th 18.5 106 0.85 X lO-io 7.0 5 18.7 106 0.97 X lO-io 7.2 6th 19.0 106 0.95 X ₁₀ -io 7.2 50 7th 22.5 106 1.1 X I ₀ -IO 7.6 8th 25.0 106 1.4 X ₁₀ -io 9.5 9 30.0 220 1.6 X lO-io 11.0 10 30.0 106 2.4 X lO-io 14.0 55 11th 30.0 220 2.6 X lO-io 14.0 12th 50.0 106 2.4 X lO-io 14.0 13th 50.0 106 8.1 X ₁₀ -io 39.0 14th 50.0 220 8.5 X lO-io 39.0 60 15th 50.0 220 8.3 X ₁₀ -io 39.0 16 50.0 106 8.0 X ₁₀ -io 39.0 17th 71.0 220 8.5 X ₁₀ -io 39.0 18th 71.0 220 22nd X lO-io 100.0 r- _c 19th 71.0 106 22nd 100.0 05 20th 97.0 220 22nd 100.0 21 120.0 220 83 265.0 220 230 560.0

In a similar series of experiments using arsenic tribromide as a doping impurity the following results were obtained:

Table 2
AsBr ₃ in 8% H ₂ in He

T Flow Measured steam attempt swiftly Diffusion. pressure P ₁ No. Γ C) speed H ₂ -He speed for AsBr ₃ 34.0 (ml / min) (Mol / sec) (mm Hg) 22nd 55.0 106 0.068 χ 10 " ¹⁰ 0.48 23 85.0 106 0.23 χ ΙΟ " ¹⁰ 2.1 24 100.0 106 1.38 χ ΙΟ " ¹⁰ 10.5 25th 120.0 106 2.7 χ 10 " ¹⁰ 19.0 26th 106 7.1 χ 10 " ¹⁰ 42.0

IO

The diffusion rate was determined by chemical analysis of the exhaust gas from the exhaust pipe 20 measured using a coulometric method.

It can be seen from the tabulated results above that for a given temperature the diffusion rate of the doping is independent of the flow rate of the carrier gas. For example, tests 12 and 16 of Table 1 may be compared. Both experiments were carried out at a temperature of the impurity store of 50 ^° C. and practically the same measured diffusion rate was determined. However, for Run 12 the diluent gas flow rate was 106 ml / min and for Run 16 the flow rate was 220 ml / min. The concentration of the impurity in the carrier gas stream of experiment 16 was about half the impurity concentration in the gas stream of experiment 12. The change in the diffusion rate with temperature can be observed from the various experiments set out in the preceding tables.

Accordingly, the relative amount of doping can be adjusted by changing the hydrogen volume if the diffusion rate of the doping is kept constant. When the temperature the diffusion cell is changed, the absolute amount of the carrier gas flow delivered Endowment can be changed.

1 sheet of drawings

Claims (3)

Patent claims: 1. A method for producing a carrier gas intended for the treatment of semiconductor material, in which a predetermined, small amount of dopant vapor is contained, the dopant vapor is introduced into the carrier gas in a mixing chamber in which both the carrier gas and the dopant vapor are introduced, characterized in that the dopant vapor diffused into the mixing chamber through a tube from a vessel containing the dopant at a constant rate and the carrier gas velocity is adjusted so that no turbulence occurs in the pipe. 2. The method according to claim 1, characterized in that a tube is used, whose Length compared to its cross section is much greater. 3. The method according to claims 1 and 2, characterized in that the carrier gas in the Mixing chamber is introduced radially.