A method for high boron diffusion of box method boron microcrystalline glass source

Abstract

The present invention provides a method for high-concentration boron diffusion of a box-type boron microcrystalline glass source, comprising: S1) performing high-temperature pretreatment on the boron microcrystalline glass to obtain the pretreated boron microcrystalline glass; S2) stacking the pretreated boron microcrystalline glass and the silicon wafer to be diffusion-treated, and heating for diffusion treatment to obtain the treated silicon wafer; the front surface of the silicon wafer to be diffusion-treated is in contact with the pretreated boron microcrystalline glass; S3) removing the borosilicate glass on the surface of the treated silicon wafer to obtain the silicon wafer subjected to boron diffusion treatment. Compared with the prior art, the present invention directly contacts the PWB source with the Si wafer, can achieve a high surface concentration while forming a shallow junction; secondly, the present invention can also make the upper and lower surfaces of the PWB source contact the doping surfaces of two layers of several Si wafers respectively, improving the diffusion efficiency; furthermore, the method provided by the present invention only uses one-step diffusion, without the need to take out the Si wafer for treatment in the middle, the process is simple and easy to operate, and unnecessary pollution is avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave semiconductors, and particularly relates to a method for high-concentration boron diffusion of a box-type boron microcrystalline glass source. Background Art

[0002] The microwave semiconductor diffusion process technology is divided into: constant surface source diffusion and limited surface source diffusion. In an ideal situation, during constant surface source diffusion, the impurity concentration distributes with an error function along the depth from the surface, and the surface concentration always remains unchanged and is independent of time; while for limited surface source diffusion, the impurity concentration is Gaussian-distributed. When the diffusion temperature is kept constant, as the diffusion time increases, the surface impurity concentration continuously decreases, but the total amount of diffused impurities remains unchanged.

[0003] Regarding the boron diffusion source of P-type dopants, it includes trimethyl borate, n-propyl borate liquid source, boron nitride (BN) powder source, BN flake source, and boron microcrystalline glass source (PWB). Among them, the PWB diffusion source has the advantages of convenient use, simple operation, good diffusion uniformity and repeatability, long service life and easy control. In the PWB source, B2O3 is uniformly distributed from the inside to the outside. It does not require source activation by burning before use, has low water absorption of the source wafer, does not require special storage, is not easy to stick to the boat, and has a relatively wide temperature range for use, with a high temperature reaching above 1100 °C. Its diffusion sheet resistance has a relatively large controllable range. Therefore, it is an ideal diffusion source. The diffusion principle of the PWB source is as follows: at high temperature, B2O3 in the PWB uniformly volatilizes, and part of it reacts with trace amounts of H2O in the atmosphere to generate HBO2 (metaboric acid). The HBO2 vapor and B2O3 gas deposit on the Si surface with a certain concentration gradient, and then react with Si to generate B. Finally, the generated B diffuses into the Si according to Fick's law.

[0004] When the thin-film epitaxial material is used in the diffusion process of microwave diodes, the PWB source is often used as the diffusion source. The open-tube diffusion method is adopted, and the two-step method is used for diffusion, namely pre-deposition and impurity redistribution. Pre-deposition is generally carried out at a lower temperature by adopting a diffusion method with a constant surface concentration to deposit a layer of impurity atoms on the surface of the silicon wafer. Redistribution is to put the silicon wafer that has undergone the pre-deposition process into a diffusion furnace at a higher temperature for heating, so that the impurities diffuse into the silicon wafer. There is no supplement of foreign impurities during the diffusion process. Therefore, redistribution is a diffusion with a limited source. Nitrogen is introduced during this diffusion process. Specifically: The first step is pre-deposition. Insert the well-cleaned PWB source into the quartz boat and push it into the furnace at 970 °C for pretreatment for 20 - 30 minutes. The purpose is to fully volatilize B2O3 in the PWB in the furnace. Subsequently, pull the quartz boat to the furnace mouth. After drying the well-cleaned silicon wafer, take it out and place it on the quartz boat at the furnace mouth. Insert the Si wafer so that it is arranged at intervals with the PWB source. After preheating the quartz boat loaded with the source and Si wafer at the furnace mouth for 5 minutes, push it into the constant-temperature zone of the furnace for pre-diffusion. After 20 - 30 minutes, pull the quartz boat to the furnace mouth for cooling. Since the reaction temperature is relatively high and the concentration of the reactants on the surface of the Si wafer is relatively high, a thin layer of borosilicate glass will be formed on the surface. Therefore, the silicon wafer taken out after cooling needs to be rinsed with HF to remove the borosilicate glass on the surface; then carry out the second step of main diffusion. The main diffusion temperature is between 1100 °C and 1150 °C. Insert the processed silicon wafer into the quartz boat again, preheat it at the mouth of the heated diffusion furnace for 5 minutes, and push it into the constant-temperature zone for diffusion for 20 - 30 minutes. After completion, pull it to the furnace mouth for cooling again, and the entire diffusion process is completed.

[0005] However, the open-tube diffusion method has a long and complex process flow. Moreover, the main diffusion adopted is a limited surface source diffusion. When it is applied to the process of low-voltage PIN diodes, the junction depth can be controlled. However, when carrying out impurity redistribution (main diffusion), the surface concentration is not as high as that of the box method of concentrated boron diffusion (constant surface source diffusion), and it can only reach 6×10 18 cm -3 , and the insufficient surface concentration will affect the ohmic contact on the surface of the product, greatly affecting the product parameter performance indicators, and further resulting in affecting the parameter of the series resistance of the product in the PIN diode technical indicators. Summary of the Invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a method for realizing concentrated boron diffusion of a box method boron microcrystalline glass source in one step.

[0007] The present invention provides a method for concentrated boron diffusion of a box method boron microcrystalline glass source, including:

[0008] S1) Perform high-temperature pretreatment on the boron microcrystalline glass to obtain the pretreated boron microcrystalline glass;

[0009] S2) Stack the pre-treated boron microcrystalline glass and the silicon wafer to be diffusion-treated in layers, and heat them for diffusion treatment to obtain the treated silicon wafer; the front side of the silicon wafer to be diffusion-treated is in contact with the pre-treated boron microcrystalline glass;

[0010] S3) Remove the borosilicate glass on the surface of the treated silicon wafer to obtain the boron-diffused silicon wafer.

[0011] Preferably, in step S1), the boron microcrystalline glass is first subjected to cleaning treatment and baking treatment, and then subjected to high-temperature pretreatment; for the cleaning treatment, first wipe with acetone and then wash with water; the temperature of the baking treatment is 1000 °C to 1200 °C; the time of the baking treatment is greater than or equal to 24 h.

[0012] Preferably, the baking treatment is carried out under the condition of introducing a protective atmosphere; the flow rate of the protective atmosphere is 0.5 - 2 L / min.

[0013] Preferably, in step S1), the temperature of the high-temperature pretreatment is 1000 °C to 1200 °C; the time of the high-temperature pretreatment is 10 - 60 min.

[0014] Preferably, in step S1), the high-temperature pretreatment is carried out under the condition of introducing a protective atmosphere; the flow rate of the protective atmosphere is 0.5 - 2 L / min.

[0015] Preferably, in step S2), after stacking the pre-treated boron microcrystalline glass and the silicon wafer to be diffused, preheat at the diffusion furnace inlet, and then heat in the diffusion furnace for diffusion treatment; the preheating time is 3 - 10 min.

[0016] Preferably, in step S2), the temperature of the diffusion treatment is 1100 °C to 1150 °C; the time of the diffusion treatment is 15 - 30 min.

[0017] Preferably, in step S3), removing the borosilicate glass on the surface of the treated silicon wafer specifically includes: first etching the treated silicon wafer in hydrofluoric acid, washing with water, and then performing electrolytic etching.

[0018] Preferably, the concentration of the hydrofluoric acid is not less than 40%; the etching time is 1 - 3 min; the water washing specifically includes rinsing with cold water, hot water, and cold water in sequence.

[0019] Preferably, the electrolytic etching uses a saturated copper sulfate solution as the electrolyte; the voltage during the electrolytic etching is 2 - 5 V.

[0020] The present invention provides a method for high-concentration boron diffusion using a box-type boron microcrystalline glass source, comprising: S1) performing high-temperature pretreatment on the boron microcrystalline glass to obtain the pretreated boron microcrystalline glass; S2) stacking the pretreated boron microcrystalline glass and the silicon wafer to be diffusion-treated, and heating for diffusion treatment to obtain the treated silicon wafer; the front side of the silicon wafer to be diffusion-treated is in contact with the pretreated boron microcrystalline glass; S3) removing the borosilicate glass on the surface of the treated silicon wafer to obtain the silicon wafer subjected to boron diffusion treatment. Compared with the prior art, the present invention directly contacts the PWB source with the Si wafer, can achieve a high surface concentration while forming a shallow junction; secondly, the present invention can also make the upper and lower surfaces of the PWB source respectively contact the doping surfaces of two layers of a plurality of Si wafers, improving the diffusion efficiency; furthermore, the method provided by the present invention only uses one-step diffusion, without the need to take out the Si wafer for treatment midway, the process is simple and easy to operate, and unnecessary pollution is avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the diffusion method provided by the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] The present invention provides a method for high-concentration boron diffusion using a box-type boron microcrystalline glass source, comprising: S1) performing high-temperature pretreatment on the boron microcrystalline glass to obtain the pretreated boron microcrystalline glass; S2) stacking the pretreated boron microcrystalline glass and the silicon wafer to be diffusion-treated, and heating for diffusion treatment to obtain the treated silicon wafer; the front side of the silicon wafer to be diffusion-treated is in contact with the pretreated boron microcrystalline glass; S3) removing the borosilicate glass on the surface of the treated silicon wafer to obtain the silicon wafer subjected to boron diffusion treatment.

[0024] Among them, the present invention does not have special restrictions on the sources of all raw materials, and they can be commercially available.

[0025] In the present invention, it is preferred to first perform a cleaning treatment and a baking treatment on the boron microcrystalline glass; the cleaning treatment is preferably specifically to first wipe with acetone and then wash with water; the washing with water is preferably sequentially carried out with a large amount of cold water, hot water and cold water; the water used for washing is preferably deionized water to avoid introducing other impurity ions; after the cleaning treatment, it is preferably dried first and then subjected to a baking treatment; the drying is preferably carried out using an infrared lamp; the drying time is preferably 20 - 40 min, more preferably 25 - 35 min, still more preferably 30 min to dry the water traces on the surface of the boron microcrystalline glass; the baking temperature is preferably 1000 °C - 1200 °C, more preferably 1050 °C - 1150 °C, still more preferably 1100 °C; the baking time is preferably greater than or equal to 24 h; the baking treatment is preferably carried out under the condition of introducing a protective atmosphere; the protective atmosphere can be the protective atmosphere well-known to those skilled in the art without special limitations, and is preferably nitrogen in the present invention; the flow rate of the protective atmosphere is preferably 0.5 - 2 L / min, more preferably 0.5 - 1.5 L / min, still more preferably 0.8 - 1.2 L / min, and most preferably 1 L / min; the baking treatment is preferably carried out in a diffusion furnace, and more preferably the boron microcrystalline glass is placed in a quartz boat in a quartz box, and then the quartz box is placed in the diffusion furnace for baking treatment.

[0026] Then, the boron microcrystalline glass after the cleaning treatment and the baking treatment is subjected to a high-temperature pretreatment to obtain the boron microcrystalline glass after pretreatment; the high-temperature pretreatment temperature is preferably 1000 °C - 1200 °C, more preferably 1050 °C - 1150 °C, still more preferably 1100 °C; the high-temperature pretreatment time is preferably 10 - 60 min, more preferably 20 - 50 min, still more preferably 30 - 40 min; the high-temperature pretreatment is preferably carried out under the condition of introducing a protective atmosphere; the protective atmosphere can be the protective atmosphere well-known to those skilled in the art without special limitations, and is preferably nitrogen in the present invention; the flow rate of the protective atmosphere is preferably 0.5 - 2 L / min, more preferably 0.5 - 1.5 L / min, still more preferably 0.8 - 1.2 L / min, and most preferably 1 L / min; in the present invention, it is preferred to place the boron microcrystalline glass in a quartz box for high-temperature pretreatment; after the high-temperature pretreatment, it is preferably cooled to obtain the boron microcrystalline glass after pretreatment.

[0027] The pre-treated boron microcrystalline glass and the silicon wafer to be diffusion-treated are stacked. In the present invention, it is preferred to place the pre-treated boron microcrystalline glass in the middle, and silicon wafers to be diffusion-treated are arranged on both its upper and lower surfaces; the front side of the silicon wafer to be diffusion-treated is in contact with the pre-treated boron microcrystalline glass, that is, the doping surface of the silicon wafer is in contact with the pre-treated boron microcrystalline glass; then preferably the stacked pre-treated boron microcrystalline glass and the silicon wafer to be diffusion-treated are placed in a closed quartz box, heated for diffusion treatment to obtain the treated silicon wafer; see Figure 1 , Figure 1 which is a schematic diagram of the diffusion method provided by the present invention; in the present invention, it is preferred to preheat at the diffusion furnace inlet first, and then heat for diffusion treatment; during preheating, the temperature required for diffusion has been reached in the diffusion furnace; the preheating time is preferably 3 - 10 min, more preferably 4 - 8 min, and still more preferably 5 - 6 min; the temperature of the diffusion treatment is preferably 1100 °C - 1150 °C; the time of the diffusion treatment is preferably 15 - 30 min; the specific temperature and time of the diffusion treatment can be selected according to the product process. Specifically, it can be calculated according to the following process: An important parameter describing the diffusion rate of impurities in a crystal is the diffusion coefficient D. The larger the diffusion coefficient of the impurities, the faster their diffusion rate, and in the same time, they diffuse deeper in the crystal. The diffusion coefficient D changes exponentially with the reciprocal of the absolute temperature, and its mathematical expression is:

[0028]

[0029] K: Boltzmann constant, 8.62×10 -5 ev / K;

[0030] T: absolute temperature;

[0031] D0: the apparent value of the diffusion coefficient when the temperature T is infinite;

[0032] △E: the activation energy of intrinsic diffusion.

[0033] The commonly used diffusion temperature for low-pressure silicon PIN diodes is 1120 °C ± 20 °C, and the diffusion coefficient calculated by the formula is: 4.95×10 -12 cm 2 / s;

[0034] For the diffusion of a constant surface source, its surface impurity concentration N s is basically determined by the solid solubility of the impurity at the diffusion temperature (900 °C - 1200 °C), and the solid solubility changes little with temperature.

[0035] Ideally, the surface concentration after boron diffusion of the PIN diode can reach the solid solubility of the impurity in silicon at the diffusion temperature, and only the constant surface source diffusion can meet the ideal situation. For the diffusion with a constant surface concentration, the impurity concentration has an error function distribution. That is:

[0036]

[0037] Substituting N (xj,t) = N B into the above formula, the expression of the junction depth xj can be solved:

[0038]

[0039] In the formula, erfc -1 : the complementary error function;

[0040] N B : the surface concentration of the epitaxial layer of the silicon wafer before diffusion;

[0041] N s : the surface concentration of the diffused impurity, which depends on the maximum solid solubility of the impurity in silicon;

[0042] D: the diffusion coefficient;

[0043] t: the diffusion time.

[0044] According to the required junction depth, the diffusion time can be deduced inversely.

[0045] After the diffusion is completed, it is preferably cooled, and then the borosilicate glass on the surface of the processed silicon wafer is removed to obtain a silicon wafer treated by boron diffusion; the cooling is preferably carried out at the mouth of the diffusion furnace; the cooling time is preferably 3 - 8 min, more preferably 5 min; in the present invention, removing the borosilicate glass on the surface of the processed silicon wafer specifically includes: first etching the processed silicon wafer in hydrofluoric acid, washing it with water, and then performing electrolytic etching; the mass concentration of the hydrofluoric acid is preferably 30% - 40%; the etching time is preferably 1 - 3 min; the water washing is specifically carried out with cold water, hot water and cold water in sequence; each washing is preferably carried out more than 20 times; to avoid introducing other ions, the water washing is preferably carried out with deionized water; the electrolytic etching preferably uses a saturated copper sulfate solution as the electrolyte; the voltage during the electrolytic etching is preferably 2 - 5 V, more preferably 3 - 4 V.

[0046] In the present invention, directly contacting the PWB source with the Si wafer can achieve a high surface concentration while forming a shallow junction; secondly, the present invention can also achieve that the upper and lower surfaces of the PWB source are respectively in contact with the doping surfaces of two layers of several Si wafers, improving the diffusion efficiency; furthermore, the method provided by the present invention only uses one-step diffusion, without the need to take out the Si wafer for treatment in the middle, the process is simple and easy to operate, and unnecessary pollution is avoided.

[0047] To further illustrate the present invention, the following is a detailed description of a method for high-concentration boron diffusion using a box-type boron microcrystalline glass source according to the present invention in combination with embodiments.

[0048] All reagents used in the following examples are commercially available.

[0049] Example

[0050] The high-concentration boron diffusion technology (constant surface source diffusion) using a box-type boron microcrystalline glass source (PWB) is similar to closed-tube diffusion but without sealing, different from A S Diffusion requires complete sealing.

[0051] For the used solid-state source PWB, first process the boron source. The processing steps are as follows: repeatedly wipe the boron source with acetone, then rinse with a large amount of cold, hot, and cold deionized water. After cleaning, put it into an infrared lamp oven to dry the water traces on the surface of the source for 30 minutes. Heat the diffusion furnace to 1100 °C, and pass N2 through the diffusion furnace at a flow rate of 1 L / min. Place the PWB source flat on the quartz boat in a small-volume quartz box for diffusion, cover the quartz box, and push it into the constant-temperature zone of the diffusion furnace and bake for more than 24 hours before it can be used.

[0052] Steps for high-concentration boron diffusion using a box-type boron microcrystalline glass source (PWB): Heat the diffusion furnace to 1100 °C, pass N2 at a flow rate of 1 L / min, push the quartz box containing the boron source into the constant-temperature zone for 30 minutes of pretreatment of the source. After completion, pull the quartz box to the furnace mouth for cooling, and at the same time adjust the diffusion temperature between 1100 °C and 1150 °C (determined according to the product process); when the diffusion temperature is constant at the specified value, take out the quartz box cooled at the furnace mouth, take out the PWB source and place it on the quartz tray in the mounting rack. Place half of the cleaned and dried silicon wafers (including the companion wafers) flat on the boat in the quartz box, with the front side facing up, then cover the PWB source on the silicon wafers, and place the remaining silicon wafers flat on the PWB source. Note that at this time, the silicon wafers should face the source, that is, the front side is facing down. Then cover the quartz box with a quartz glass sheet to seal the quartz box, put it into the diffusion furnace mouth for preheating for 5 minutes and then push it into the constant-temperature zone of the diffusion furnace for diffusion for 15 - 30 minutes. After diffusion is completed, pull it to the furnace mouth for cooling for 5 minutes. Take out the silicon wafers and put them into a plastic cup, pour in hydrofluoric acid with a concentration of not less than 40% and corrode for 1 - 3 minutes, then rinse with cold, hot, and cold deionized water more than 20 times each, and then electrolytically corrode the boron-silicate glass on the surface of the silicon. Dissolve copper sulfate in deionized water to form a saturated solution. Connect the negative pole of a 3V DC power supply to the copper sheet and the positive pole to the silicon wafer, and put them into the copper sulfate solution at the same time until the oxide layer on the surface of the silicon wafer is electrolytically cleaned and then taken out, thus completing the entire diffusion process.

[0053] After the diffusion is completed, use a four-probe test bench to measure the sheet resistance R of the silicon wafer, and measure the junction depth by beveling the reference wafer. Polish one of the reference wafers smoothly with 302#, 304#, and 308# emery successively, place the ground edge in the prepared etching solution (etching solution formula: 50 g of CrO3 is dissolved in 100 ml of water: HF = 1:1), for 50 - 60 s, at room temperature, and use a measuring microscope to test the results. Table 1 shows the test results under three different process conditions for the same material. Serial numbers 1 - 3 are for the high-concentration boron diffusion of the box method boron microcrystalline glass source (PWB), and serial number 4 is for the two-step limited-source diffusion.

[0054] As shown in Table 1, the sheet resistance of the diffused silicon wafer obtained for serial number 1 is 5.1 Ω, and the diffusion junction depth x j is 1.2 μm, meeting the product design requirements; converting the concentration, the surface concentration can reach 1×10 20 cm -3 . In the manufacturing process of PIN diode products, the higher the diffusion surface concentration, the better the ohmic contact of the product, manifested as a smaller series resistance of the electrical parameters. It can be seen that the series resistance R s of the final device using the diffusion process of the method of the present invention is better than that of the existing process (two-step diffusion method) limited surface source diffusion.

[0055] Table 1 Test results under different diffusion process conditions

[0056]

[0057]

Claims

1. A method for high boron diffusion of a box method boron microcrystalline glass source, characterized in that Including: S1) Perform high-temperature pretreatment on the boron microcrystalline glass to obtain the pretreated boron microcrystalline glass; S2) Stack the pretreated boron microcrystalline glass and the silicon wafer to be diffusion-treated, heat for diffusion treatment to obtain the treated silicon wafer; the front side of the silicon wafer to be diffusion-treated is in contact with the pretreated boron microcrystalline glass; S3) Remove the borosilicate glass on the surface of the treated silicon wafer to obtain the silicon wafer with boron diffusion treatment; In step S1), the temperature of the high-temperature pretreatment is 1000°C to 1200°C; the time of the high-temperature pretreatment is 10 to 60 minutes; In step S1), the high-temperature pretreatment is carried out under the condition of introducing a protective atmosphere; the flow rate of the protective atmosphere is 0.5 to 2 L / min; In step S2), after stacking the pretreated boron microcrystalline glass and the silicon wafer to be diffused, preheat at the diffusion furnace inlet, and then heat in the diffusion furnace for diffusion treatment; the preheating time is 3 to 10 minutes; In step S2), the temperature of the diffusion treatment is 1100°C to 1150°C; the time of the diffusion treatment is 15 to 30 minutes; In step S1), the boron microcrystalline glass is first subjected to cleaning treatment and baking treatment, and then high-temperature pretreatment; for the cleaning treatment, wipe with acetone first and then wash with water; the temperature of the baking treatment is 1000°C to 1200°C; the time of the baking treatment is greater than or equal to 24 hours; The baking treatment is carried out under the condition of introducing a protective atmosphere; the flow rate of the protective atmosphere is 0.5 to 2 L / min; In step S3), removing the borosilicate glass on the surface of the treated silicon wafer specifically means: first corrode the treated silicon wafer in hydrofluoric acid, wash with water, and then perform electrolytic corrosion; The concentration of the hydrofluoric acid is not less than 40%; the corrosion time is 1 to 3 minutes; the water washing specifically means rinsing with cold water, hot water, and cold water in sequence; The electrolytic corrosion uses a saturated copper sulfate solution as the electrolyte; the voltage during the electrolytic corrosion is 2 to 5 V.

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