RU2475883C1 - Method for diffusion of boron in silicon - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: according to the method for diffusion of boron in silicon, ox darted BN plates, after storage in the atmosphere of air containing non saturated water vapours and prior to placement of such plates alongside with silicon plates in a silica boat, are etched in concentrated HF for 30-60 sec, washed in a flow of deionised water for 10-15 sec for removal of most hygroscopic boron compounds, annealed first in a neutral medium at a boron diffusion temperature within the range of 930°C-1000°C for at least 30 min and then - in dry oxygen atmosphere, repeatedly annealed at the same atmosphere for at least 30 min; treated boron plates and silicon plates are loaded into the boat and diffusion of boron into silicon is performed.

EFFECT: reproducibility of the process of boron diffusion into silicon.

1 tbl

Description

The present invention relates to a technology for creating semiconductor devices.

A known method of diffusion of boron into silicon with the preparation of a source for boron diffusion, which consists in oxidizing BN plates in dry oxygen and storing them before diffusion of boron in a diffusion furnace tube in a neutral gas atmosphere at a moderately elevated temperature [1]. Despite the fact that in dry oxygen, boron nitride should be converted into В ₂ О ₃ during oxidation, it was shown in [1] that the boron carrier formed is HBO ₂ pairs, while the vapor pressure of В ₂ О ₃ when using real temperatures diffusion is negligible.

The disadvantage of this method is the inefficiency of using diffusion equipment in which oxidized BN plates are stored between boron diffusion processes.

A more fundamental drawback is that the high uniformity and reproducibility of the Rs value of the diffusion layer is realized in a narrow range of diffusion modes. In particular, after carrying out a rather high-temperature and long-term diffusion regime, the reproducibility of the Rs value in the specified standard boron diffusion mode is violated, which is expressed in the increase in the Rs value achieved in this mode.

The prototype of the invention is a boron diffusion method described in [2]. It differs in that it does not use any specific storage method for oxidized BN plates. BN plates were stored in a normal atmosphere of air containing unsaturated water vapor. Before the process of boron diffusion, the BN plates were oxidized in dry oxygen at a temperature of 950 ° C without any other treatment after the previous boron diffusion process. As a result of boron diffusion, a high uniformity of the Rs values of the diffusion layers was achieved.

However, irreproducibility of the Rs value exists, especially with relatively low-temperature diffusion processes. This is due to the experimental fact that with an increase in the storage time of oxidized BN wafers in an atmosphere of air containing unsaturated water vapor, the thickness of the layer of boron carrier deposited on the surface of silicon wafers in the standard process of boron diffusion increases. This completely contradicts the idea of the authors of [2] that the carrier of boron is B ₂ O ₃ . Unlike the analogue [1], the storage of oxidized BN plates in moist air leads to the absorption of moisture from the air. In this case, various compounds can form:

B ₂ O ₃ + H ₂ O → 2HBO ₂

2B ₂ O ₃ + H ₂ O → H ₂ B ₄ O ₇

At high temperatures, there are three stable carriers of boron: B ₂ O ₃ , H ₂ B ₄ O ₇ and HBO ₂ . In this case, the vapor pressure of В ₂ О ₃ at a high temperature is negligible, the vapor pressure of HBO ₂ is the highest, and the vapor pressure of H ₂ B ₄ O ₇ is of intermediate value, that is, the vapor pressure of В ₂ О ₃ can be neglected, which is confirmed in [1 ], and the vapor pressure of H ₂ B ₄ O _{7 is} not so small that it can be neglected.

Thus, the presence of practically two boron carrier vapors of different elasticity (H ₂ B ₄ O ₇ and HBO ₂ ) should be a potential cause of the irreproducibility of the total vapor pressure of a mixture of these molecules when the ratio of their concentrations in the evaporated boron carrier changes.

The technical result of the invention is the reproducibility of the process of diffusion of boron into silicon.

The proposed method for the diffusion of boron into silicon using oxidized BN platinum provides the transfer of boron to silicon wafers mainly by H ₂ B ₄ O ₇ molecules, eliminates the sticking of oxidized BN wafers and silicon wafers in a quartz boat, and allows the formation of p ⁺ layers in silicon wafers with Rs values from 30 Ohm / □ to 100 Ohm / □ with uniformity and reproducibility of the Rs value with a deviation from the average value of not more than ± 5% with a minimum amount of time spent on the whole process.

The technical result is achieved by the fact that after storing the oxidized BN plates in an atmosphere of air containing unsaturated water vapor, before placing these plates and silicon plates in a quartz boat, the oxidized BN plates are etched in concentrated HF for 30-60 seconds, washed in a stream of deionized water for 10-15 seconds, removing the most hygroscopic boron compounds (HBO ₂ and H ₃ BO ₃ ), anneal in a neutral medium at a diffusion temperature of boron, in the range of 930-1000 ° C, for at least 30 minutes, then anneal in dry oxygen again at the same temperature for at least 30 minutes, load silicon wafers into a boat with processed BN plates and diffuse boron into silicon.

Obviously, the reproducible value of the vapor pressure of the boron carrier in the source, which is the result of oxidation of the surface of the BN plates, can be achieved with this method of preparing the BN plates, when the vapor pressure of the boron carrier is provided by one type of boron carrier molecules. If we take into account that the BN plates obtained by pressing have a porous structure that can absorb atmospheric moisture, then the only boron carrier obtained by high-temperature heating, free from the presence of a second boron carrier, can be H ₂ B ₄ O ₇ , since HBO when heated ₂ evaporates significantly faster than H ₂ B ₄ O ₇ .

During actual storage of oxidized BN plates in an atmosphere containing unsaturated water vapor, on the surface of BN plates contains not only a mixture of HBO ₂ and H ₂ B ₄ O ₇ molecules, but also H ₃ BO ₃ formed at normal temperature in accordance with the reaction

HBO ₂ + H ₂ O → H ₃ BO ₃

During the heat treatment of oxidized BN plates at a temperature above 185 ° C, H ₃ VO ₃ decomposes and releases water, which creates HBO ₂ molecules at the boundary with B ₂ O ₃ , increasing the concentration of these molecules, and water molecules evaporate from the surface of the oxidized layer. For this reason, during prolonged storage of oxidized BN plates in this atmosphere, the concentration of HBO ₂ molecules increases when the plates are heated. In this case, there is an increase in the vapor pressure of HBO ₂ at high temperature and an increase in the thickness of the boron carrier layer deposited on silicon wafers and on the surface of the quartz boat both during oxidation of the stored BN wafers and during diffusion of boron. The enhanced vapor deposition of HBO ₂ on a quartz boat and on silicon wafers leads to sticking of silicon wafers and BN wafers in the boat, which is unacceptable.

In the proposed method for the diffusion of boron into silicon, boron compounds such as H ₃ BO ₃ are removed from the surface of the BN wafers by etching; HBO ₂ and H ₂ B ₄ O ₇ disposed on the sublayer B ₂ O _3, and then forming a layer composed mainly of molecules H ₂ B ₄ O _7, located on the sublayer B ₂ O _3.

The novelty of the invention lies in the use of a previously unknown carrier of boron H ₂ B ₄ O ₇ instead of its mixture with HBO ₂ with their arbitrary ratio, created due to the fact that oxidized BN plates are etched in concentrated HF for 30-60 seconds, washed in a stream of deionized water 10-15 seconds, removing the most hygroscopic boron compounds (HBO ₂ and H ₃ BO ₃ ), anneal in a neutral medium at a diffusion temperature of boron, in the range of 930-1000 ° C, for at least 30 minutes, then anneal in dry oxygen again at the same temperature for at least 30 minutes, load silicon wafers into a boat with treated BN plates and diffuse boron into silicon.

The inventive step of the invention is confirmed by the fact that it was completely unobvious for specialists to transfer boron from the surface of oxidized BN plates by any compound other than HBO ₂ or B ₂ O ₃ .

The reliability of the invention follows from a comparison of the behavior of silicon wafers after diffusion of boron and their discharge into the atmosphere containing unsaturated water vapor.

If during standard preparation of oxidized BN wafers, when stored for unlimited time in an atmosphere of air containing unsaturated water vapors and subsequent oxidation in dry oxygen, the surface of the silicon wafers was covered with “dew” in the form of H ₃ BO ₃ for 1 minute resulting from the reaction :

HBO ₂ + H ₂ O → H ₃ BO ₃ ,

then after the diffusion of boron using the proposed method for a long time on the surface of the silicon wafers, the appearance of “dew” is not observed, which indicates a rather low concentration of HBO ₂ molecules in the layer of boron carrier deposited on silicon wafers, including on the surface of SiO ₂ .

The effectiveness of the invention follows from the fact that for reproducibility of the results of boron diffusion when using oxidized BN plates as a source of boron, in contrast to the analogue, it is not necessary to use their storage of oxidized plates around the clock in a diffusion furnace at a certain temperature and when applying a gas medium, this result of the current diffusion process does not depend on the diffusion mode of the previous process, in contrast to the prototype, the process of boron diffusion is reproducible at different th temperature conditions.

The use of the present invention with boron transfer mainly by H ₂ B ₄ O ₇ molecules with lower vapor pressure than when transferred by HBO ₂ molecules, as in the analogue and a mixture of HBO ₂ with H ₂ B ₄ O ₇ at a noticeable concentration of HBO ₂ , also irreproducible , as in the prototype, allows diffusion processes with vapor pressure of the boron carrier sufficient to ensure uniformity of Rs across the plates and reproducibility of Rs, but much less excessive than in known cases, which leads to a decrease in the thickness of the layer of borosilicate glasses on the surface of silicon and SiO ₂ on the surface, and this is very important, since the thickness of a bleed layer SiO ₂ is reduced at least twice.

The lower vapor pressure of H ₂ B ₄ O ₇ compared with HBO ₂ makes it less likely that the silicon wafers and BN wafers stick together in a quartz boat.

Example. Various modes of boron diffusion were tested using a quartz tube with an internal diameter of 95 mm, a diameter of BN plates of 75 mm, and a diameter of silicon wafers of 60 mm in a diffusion furnace. The optimal consumption of neutral gas (argon) during diffusion is 70 l / h. The results of diffusion processes are given in the table.

The table shows that the temperature, noticeably lower than 930 ° C, can hardly be used to form p ⁺ layers, since the Rs of the diffusion layers is excessively large and has a large scatter due to the excessively small thickness of borosilicate glass on the silicon surface due to the small vapor pressure H ₂ B ₄ O ₇ .

At temperatures above 1000 ° C, in particular at 1050 ° C, p ⁺ layers are realized normally, but using this mode creates an excess thickness of borosilicate glass, and reducing the diffusion time to reduce the glass thickness reduces the accuracy of the diffusion process due to the need to control the time heating and cooling time of the plates with the boat.

As regards the etching regimes in HF and washing the plates in water after their storage in the atmosphere, the indicated range of times does not require an exact determination of the boundaries of these values due to inexpediency.

Indeed, the lower limits of the boundaries are so low that the study of the possibility of their further reduction does not give a useful result. And the upper limit, in principle, cannot be clearly defined, since with an increase in the etching and washing time, the moistening of the porous structure of the BN plates increases, which is harmful, that is, an increase in the upper limits does not make sense. The indicated upper limits are sufficient to obtain satisfactory results of the formation of p ⁺ regions for relatively short annealing times indicated in the claims, as follows from the table.

The optimal gas flow rate, depending on the cross-sectional area of the gas flow between the walls of the diffusion pipe and the edges of the oxidized BN plates, is chosen based on the fact that, at a low gas flow rate, when the pressure in the gas stream in the specified section and between the BN plates differs little, it accumulates at the edges of the plates excessive concentration of boron carrier molecules, which leads to an increase in the thickness of the deposited layer of boron carrier in a significant part of the peripheral region of silicon wafers. With an increase in gas flow, the pressure in its flow decreases and the excess concentration of boron carrier molecules in the region of the periphery of the silicon wafers. The optimal gas flow rate is chosen from the condition when the region of noticeable non-uniformity of the thickness of the deposited layer of the boron carrier narrows to a width of ≤3 ÷ 4 mm, where p ⁺ regions are not really formed.

Literature

1. Rupprecht and Stach, J. Electrocem. Soc. V.120, No. 9 Sept 1973. Oxidized plates of boron nitride as a source for the diffusion of boron into silicon.

2. N. Goldsmith, J. Olmstead, J. Scott Jr. RCA Rev. V.XXVIII, No. 2, 1967, pp. 344. Boron nitride as a source for diffusion into silicon.

Table No. T _diff (° C) t _process (min) t _tr sec t _prom , sec R _s (Ohm / □) ΔR _s / R _s (%) W _b.s. (μm) Optimum gas flow (l / h) one 850 twenty twenty 10 370 ± 30 ≤0.05 70 2 930 thirty thirty 10 one hundred ± 5 0.1 70 3 930 thirty thirty fifteen 80 ± 5 0.11 70 four 950 thirty thirty fifteen 55 ± 4 0.12 70 5 1000 thirty 60 fifteen thirty ± 4 0.15 70 6 1050 thirty 80 twenty 16 ± 2 0.22 70

Claims (1)

The method of boron diffusion into silicon, including preliminary storage in the atmosphere of air containing unsaturated water vapor of oxidized BN wafers used in the previous boron diffusion process, placing them and silicon wafers in a quartz boat and conducting diffusion of boron into silicon wafers, characterized in that the oxidized BN plates before their placement are etched in concentrated HF for 30-60 s, washed in a stream of deionized water for 10-15 s, removing the most hygroscopic boron compounds, annealed in a neutral medium at a temperature e diffusion of boron in the range 930 ° C-1000 ° C for at least 30 minutes, then annealed at the same temperature for at least 30 minutes in an atmosphere of dry oxygen.